Physics beyond the Standard Model

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https://en.wikipedia.org/wiki/Physics_beyond_the_Standard_Model 

 

Beyond the Standard Model
Simulated Large Hadron Collider CMS particle detector data depicting a Higgs boson produced by colliding protons decaying into hadron jets and electrons

Physics beyond the Standard Model (BSM) refers to the theoretical developments needed to explain the deficiencies of the Standard Model, such as the inability to explain the fundamental parameters of the standard model, the strong CP problem, neutrino oscillations, matter–antimatter asymmetry, and the nature of dark matter and dark energy. Another problem lies within the mathematical framework of the Standard Model itself: the Standard Model is inconsistent with that of general relativity, and one or both theories break down under certain conditions, such as spacetime singularities like the Big Bang and black hole event horizons.

Theories that lie beyond the Standard Model include various extensions of the standard model through supersymmetry, such as the Minimal Supersymmetric Standard Model (MSSM) and Next-to-Minimal Supersymmetric Standard Model (NMSSM), and entirely novel explanations, such as string theory, M-theory, and extra dimensions. As these theories tend to reproduce the entirety of current phenomena, the question of which theory is the right one, or at least the "best step" towards a Theory of Everything, can only be settled via experiments, and is one of the most active areas of research in both theoretical and experimental physics.

Problems with the Standard Model

Despite being the most successful theory of particle physics to date, the Standard Model is not perfect. A large share of the published output of theoretical physicists consists of proposals for various forms of "Beyond the Standard Model" new physics proposals that would modify the Standard Model in ways subtle enough to be consistent with existing data, yet address its imperfections materially enough to predict non-Standard Model outcomes of new experiments that can be proposed.

The Standard Model of elementary particles + hypothetical Graviton

Phenomena not explained

The Standard Model is inherently an incomplete theory. There are fundamental physical phenomena in nature that the Standard Model does not adequately explain:

• Gravity. The standard model does not explain gravity. The approach of simply adding a graviton to the Standard Model does not recreate what is observed experimentally without other modifications, as yet undiscovered, to the Standard Model. Moreover, the Standard Model is widely considered to be incompatible with the most successful theory of gravity to date, general relativity.
• Dark matter. Cosmological observations tell us the standard model explains about 5% of the energy present in the universe. About 26% should be dark matter, which would behave just like other matter, but which only interacts weakly (if at all) with the Standard Model fields. Yet, the Standard Model does not supply any fundamental particles that are good dark matter candidates.
• Dark energy. The remaining 69% of universe's energy should consist of the so-called dark energy, a constant energy density for the vacuum. Attempts to explain dark energy in terms of vacuum energy of the standard model lead to a mismatch of 120 orders of magnitude.
• Neutrino masses. According to the standard model, neutrinos are massless particles. However, neutrino oscillation experiments have shown that neutrinos do have mass. Mass terms for the neutrinos can be added to the standard model by hand, but these lead to new theoretical problems. For example, the mass terms need to be extraordinarily small and it is not clear if the neutrino masses would arise in the same way that the masses of other fundamental particles do in the Standard Model.
• Matter–antimatter asymmetry. The universe is made out of mostly matter. However, the standard model predicts that matter and antimatter should have been created in (almost) equal amounts if the initial conditions of the universe did not involve disproportionate matter relative to antimatter. Yet, there is no mechanism in the Standard Model to sufficiently explain this asymmetry.

Experimental results not explained

No experimental result is accepted as definitively contradicting the Standard Model at the 5 σ level, widely considered to be the threshold of a discovery in particle physics. Because every experiment contains some degree of statistical and systemic uncertainty, and the theoretical predictions themselves are also almost never calculated exactly and are subject to uncertainties in measurements of the fundamental constants of the Standard Model (some of which are tiny and others of which are substantial), it is to be expected that some of the hundreds of experimental tests of the Standard Model will deviate from it to some extent, even if there were no new physics to be discovered.

At any given moment there are several experimental results standing that significantly differ from a Standard Model-based prediction. In the past, many of these discrepancies have been found to be statistical flukes or experimental errors that vanish as more data has been collected, or when the same experiments were conducted more carefully. On the other hand, any physics beyond the Standard Model would necessarily first appear in experiments as a statistically significant difference between an experiment and the theoretical prediction. The task is to determine which is the case.

In each case, physicists seek to determine if a result is merely a statistical fluke or experimental error on the one hand, or a sign of new physics on the other. More statistically significant results cannot be mere statistical flukes but can still result from experimental error or inaccurate estimates of experimental precision. Frequently, experiments are tailored to be more sensitive to experimental results that would distinguish the Standard Model from theoretical alternatives.

Some of the most notable examples include the following:

• Anomalous magnetic dipole moment of muon – the experimentally measured value of muon's anomalous magnetic dipole moment (muon "g − 2") is significantly different from the Standard Model prediction.
• B meson decay etc. – results from a BaBar experiment may suggest a surplus over Standard Model predictions of a type of particle decay ( B  →  D^(*)  τ⁻  ν_τ ). In this, an electron and positron collide, resulting in a B meson and an antimatter B meson, which then decays into a D meson and a tau lepton as well as a tau antineutrino. While the level of certainty of the excess (3.4 σ in statistical jargon) is not enough to declare a break from the Standard Model, the results are a potential sign of something amiss and are likely to affect existing theories, including those attempting to deduce the properties of Higgs bosons. In 2015, LHCb reported observing a 2.1 σ excess in the same ratio of branching fractions. The Belle experiment also reported an excess. In 2017 a meta analysis of all available data reported a 5 σ deviation from SM.

Theoretical predictions not observed

Observation at particle colliders of all of the fundamental particles predicted by the Standard Model has been confirmed. The Higgs boson is predicted by the Standard Model's explanation of the Higgs mechanism, which describes how the weak SU(2) gauge symmetry is broken and how fundamental particles obtain mass; it was the last particle predicted by the Standard Model to be observed. On July 4, 2012, CERN scientists using the Large Hadron Collider announced the discovery of a particle consistent with the Higgs boson, with a mass of about 126 GeV/c². A Higgs boson was confirmed to exist on March 14, 2013, although efforts to confirm that it has all of the properties predicted by the Standard Model are ongoing.

A few hadrons (i.e. composite particles made of quarks) whose existence is predicted by the Standard Model, which can be produced only at very high energies in very low frequencies have not yet been definitively observed, and "glueballs" (i.e. composite particles made of gluons) have also not yet been definitively observed. Some very low frequency particle decays predicted by the Standard Model have also not yet been definitively observed because insufficient data is available to make a statistically significant observation.

Unexplained relations

• Koide formula - an unexplained empirical equation discovered by Yoshio Koide in 1981. It relates the masses of the three charged leptons: ${\displaystyle Q={\frac {m_{e}+m_{\mu }+m_{\tau }}{{\big (}{\sqrt {m_{e}}}+{\sqrt {m_{\mu }}}+{\sqrt {m_{\tau }}}{\big )}^{2}}}=0.666661(7)\approx {\frac {2}{3}}}$. Standard Model does not predict masses of leptons (they are free parameters of the theory). However, the value of Koide formula being equal to 2/3 within experimental errors of measured lepton masses suggests existence of theory which is able to predict lepton masses.

• CKM matrix, if interpreted as a rotation matrix in a 3-dimensional vector space, "rotates" vector composed of square roots of down-type quark masses ${\displaystyle ({\sqrt {m_{d}}},{\sqrt {m_{s}}},{\sqrt {m_{b}}}{\big )}}$ into vector of square roots of up-type quark masses ${\displaystyle ({\sqrt {m_{u}}},{\sqrt {m_{c}}},{\sqrt {m_{t}}}{\big )}}$, up to vector lengths. 

Theoretical problems

Some features of the standard model are added in an ad hoc way. These are not problems per se (i.e. the theory works fine with these ad hoc features), but they imply a lack of understanding. These ad hoc features have motivated theorists to look for more fundamental theories with fewer parameters. Some of the ad hoc features are:

• Hierarchy problem – the standard model introduces particle masses through a process known as spontaneous symmetry breaking caused by the Higgs field. Within the standard model, the mass of the Higgs gets some very large quantum corrections due to the presence of virtual particles (mostly virtual top quarks). These corrections are much larger than the actual mass of the Higgs. This means that the bare mass parameter of the Higgs in the standard model must be fine tuned in such a way that almost completely cancels the quantum corrections. This level of fine-tuning is deemed unnatural by many theorists.
• Number of parameters – the standard model depends on 19 numerical parameters. Their values are known from experiment, but the origin of the values is unknown. Some theorists have tried to find relations between different parameters, for example, between the masses of particles in different generations or calculating particle masses, such as in asymptotic safety scenarios.
• Quantum triviality – suggests that it may not be possible to create a consistent quantum field theory involving elementary scalar Higgs particles. This is sometimes called the Landau pole problem.
• Strong CP problem – theoretically it can be argued that the standard model should contain a term that breaks CP symmetry—relating matter to antimatter—in the strong interaction sector. Experimentally, however, no such violation has been found, implying that the coefficient of this term is very close to zero. This fine tuning is also considered unnatural.

Grand unified theories

The standard model has three gauge symmetries; the colour SU(3), the weak isospin SU(2), and the weak hypercharge U(1) symmetry, corresponding to the three fundamental forces. Due to renormalization the coupling constants of each of these symmetries vary with the energy at which they are measured. Around 10¹⁶ GeV these couplings become approximately equal. This has led to speculation that above this energy the three gauge symmetries of the standard model are unified in one single gauge symmetry with a simple gauge group, and just one coupling constant. Below this energy the symmetry is spontaneously broken to the standard model symmetries. Popular choices for the unifying group are the special unitary group in five dimensions SU(5) and the special orthogonal group in ten dimensions SO(10).

Theories that unify the standard model symmetries in this way are called Grand Unified Theories (or GUTs), and the energy scale at which the unified symmetry is broken is called the GUT scale. Generically, grand unified theories predict the creation of magnetic monopoles in the early universe, and instability of the proton. Neither of these have been observed, and this absence of observation puts limits on the possible GUTs.

Supersymmetry

Supersymmetry extends the Standard Model by adding another class of symmetries to the Lagrangian. These symmetries exchange fermionic particles with bosonic ones. Such a symmetry predicts the existence of supersymmetric particles, abbreviated as sparticles, which include the sleptons, squarks, neutralinos and charginos. Each particle in the Standard Model would have a superpartner whose spin differs by 1/2 from the ordinary particle. Due to the breaking of supersymmetry, the sparticles are much heavier than their ordinary counterparts; they are so heavy that existing particle colliders may not be powerful enough to produce them.

Neutrinos

In the standard model, neutrinos have exactly zero mass. This is a consequence of the standard model containing only left-handed neutrinos. With no suitable right-handed partner, it is impossible to add a renormalizable mass term to the standard model. Measurements however indicated that neutrinos spontaneously change flavour, which implies that neutrinos have a mass. These measurements only give the mass differences between the different flavours. The best constraint on the absolute mass of the neutrinos comes from precision measurements of tritium decay, providing an upper limit 2 eV, which makes them at least five orders of magnitude lighter than the other particles in the standard model. This necessitates an extension of the standard model, which not only needs to explain how neutrinos get their mass, but also why the mass is so small.

One approach to add masses to the neutrinos, the so-called seesaw mechanism, is to add right-handed neutrinos and have these couple to left-handed neutrinos with a Dirac mass term. The right-handed neutrinos have to be sterile, meaning that they do not participate in any of the standard model interactions. Because they have no charges, the right-handed neutrinos can act as their own anti-particles, and have a Majorana mass term. Like the other Dirac masses in the standard model, the neutrino Dirac mass is expected to be generated through the Higgs mechanism, and is therefore unpredictable. The standard model fermion masses differ by many orders of magnitude; the Dirac neutrino mass has at least the same uncertainty. On the other hand, the Majorana mass for the right-handed neutrinos does not arise from the Higgs mechanism, and is therefore expected to be tied to some energy scale of new physics beyond the standard model, for example the Planck scale. Therefore, any process involving right-handed neutrinos will be suppressed at low energies. The correction due to these suppressed processes effectively gives the left-handed neutrinos a mass that is inversely proportional to the right-handed Majorana mass, a mechanism known as the see-saw. The presence of heavy right-handed neutrinos thereby explains both the small mass of the left-handed neutrinos and the absence of the right-handed neutrinos in observations. However, due to the uncertainty in the Dirac neutrino masses, the right-handed neutrino masses can lie anywhere. For example, they could be as light as keV and be dark matter, they can have a mass in the LHC energy range and lead to observable lepton number violation, or they can be near the GUT scale, linking the right-handed neutrinos to the possibility of a grand unified theory.

The mass terms mix neutrinos of different generations. This mixing is parameterized by the PMNS matrix, which is the neutrino analogue of the CKM quark mixing matrix. Unlike the quark mixing, which is almost minimal, the mixing of the neutrinos appears to be almost maximal. This has led to various speculations of symmetries between the various generations that could explain the mixing patterns. The mixing matrix could also contain several complex phases that break CP invariance, although there has been no experimental probe of these. These phases could potentially create a surplus of leptons over anti-leptons in the early universe, a process known as leptogenesis. This asymmetry could then at a later stage be converted in an excess of baryons over anti-baryons, and explain the matter-antimatter asymmetry in the universe.

The light neutrinos are disfavored as an explanation for the observation of dark matter, due to considerations of large-scale structure formation in the early universe. Simulations of structure formation show that they are too hot—i.e. their kinetic energy is large compared to their mass—while formation of structures similar to the galaxies in our universe requires cold dark matter. The simulations show that neutrinos can at best explain a few percent of the missing dark matter. However, the heavy sterile right-handed neutrinos are a possible candidate for a dark matter WIMP.

Preon models

Several preon models have been proposed to address the unsolved problem concerning the fact that there are three generations of quarks and leptons. Preon models generally postulate some additional new particles which are further postulated to be able to combine to form the quarks and leptons of the standard model. One of the earliest preon models was the Rishon model.

To date, no preon model is widely accepted or fully verified.

Theories of everything

Theoretical physics continues to strive toward a theory of everything, a theory that fully explains and links together all known physical phenomena, and predicts the outcome of any experiment that could be carried out in principle.

In practical terms the immediate goal in this regard is to develop a theory which would unify the Standard Model with General Relativity in a theory of quantum gravity. Additional features, such as overcoming conceptual flaws in either theory or accurate prediction of particle masses, would be desired. The challenges in putting together such a theory are not just conceptual - they include the experimental aspects of the very high energies needed to probe exotic realms.

Several notable attempts in this direction are supersymmetry, loop quantum gravity, and string theory.

Supersymmetry

Main article: supersymmetry

Loop quantum gravity

Main article: loop quantum gravity

Theories of quantum gravity such as loop quantum gravity and others are thought by some to be promising candidates to the mathematical unification of quantum field theory and general relativity, requiring less drastic changes to existing theories. However recent work places stringent limits on the putative effects of quantum gravity on the speed of light, and disfavours some current models of quantum gravity.

String theory

Extensions, revisions, replacements, and reorganizations of the Standard Model exist in attempt to correct for these and other issues. String theory is one such reinvention, and many theoretical physicists think that such theories are the next theoretical step toward a true Theory of Everything.

Among the numerous variants of string theory, M-theory, whose mathematical existence was first proposed at a String Conference in 1995 by Edward Witten, is believed by many to be a proper "ToE" candidate, notably by physicists Brian Greene and Stephen Hawking. Though a full mathematical description is not yet known, solutions to the theory exist for specific cases. Recent works have also proposed alternate string models, some of which lack the various harder-to-test features of M-theory (e.g. the existence of Calabi–Yau manifolds, many extra dimensions, etc.) including works by well-published physicists such as Lisa Randall.

Strong interaction

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The nucleus of a helium atom. The two protons have the same charge, but still stay together due to the residual nuclear force

 

Standard Model of particle physics
Elementary particles of the Standard Model

In nuclear physics and particle physics, the strong interaction is the mechanism responsible for the strong nuclear force, and is one of the four known fundamental interactions, with the others being electromagnetism, the weak interaction, and gravitation. At the range of 10⁻¹⁵ m (1 femtometer), the strong force is approximately 137 times as strong as electromagnetism, a million times as strong as the weak interaction, and 10³⁸ times as strong as gravitation. The strong nuclear force holds most ordinary matter together because it confines quarks into hadron particles such as the proton and neutron. In addition, the strong force binds these neutrons and protons to create atomic nuclei. Most of the mass of a common proton or neutron is the result of the strong force field energy; the individual quarks provide only about 1% of the mass of a proton.

The strong interaction is observable at two ranges and mediated by two force carriers. On a larger scale (about 1 to 3 fm), it is the force (carried by mesons) that binds protons and neutrons (nucleons) together to form the nucleus of an atom. On the smaller scale (less than about 0.8 fm, the radius of a nucleon), it is the force (carried by gluons) that holds quarks together to form protons, neutrons, and other hadron particles. In the latter context, it is often known as the color force. The strong force inherently has such a high strength that hadrons bound by the strong force can produce new massive particles. Thus, if hadrons are struck by high-energy particles, they give rise to new hadrons instead of emitting freely moving radiation (gluons). This property of the strong force is called color confinement, and it prevents the free "emission" of the strong force: instead, in practice, jets of massive particles are produced.

In the context of atomic nuclei, the same strong interaction force (that binds quarks within a nucleon) also binds protons and neutrons together to form a nucleus. In this capacity it is called the nuclear force (or residual strong force). So the residuum from the strong interaction within protons and neutrons also binds nuclei together. As such, the residual strong interaction obeys a distance-dependent behavior between nucleons that is quite different from that when it is acting to bind quarks within nucleons. Additionally, distinctions exist in the binding energies of the nuclear force of nuclear fusion vs nuclear fission. Nuclear fusion accounts for most energy production in the Sun and other stars. Nuclear fission allows for decay of radioactive elements and isotopes, although it is often mediated by the weak interaction. Artificially, the energy associated with the nuclear force is partially released in nuclear power and nuclear weapons, both in uranium or plutonium-based fission weapons and in fusion weapons like the hydrogen bomb.

The strong interaction is mediated by the exchange of massless particles called gluons that act between quarks, antiquarks, and other gluons. Gluons are thought to interact with quarks and other gluons by way of a type of charge called color charge. Color charge is analogous to electromagnetic charge, but it comes in three types (±red, ±green, ±blue) rather than one, which results in a different type of force, with different rules of behavior. These rules are detailed in the theory of quantum chromodynamics (QCD), which is the theory of quark–gluon interactions.

History

Before the 1970s, physicists were uncertain as to how the atomic nucleus was bound together. It was known that the nucleus was composed of protons and neutrons and that protons possessed positive electric charge, while neutrons were electrically neutral. By the understanding of physics at that time, positive charges would repel one another and the positively charged protons should cause the nucleus to fly apart. However, this was never observed. New physics was needed to explain this phenomenon.

A stronger attractive force was postulated to explain how the atomic nucleus was bound despite the protons' mutual electromagnetic repulsion. This hypothesized force was called the strong force, which was believed to be a fundamental force that acted on the protons and neutrons that make up the nucleus.

It was later discovered that protons and neutrons were not fundamental particles, but were made up of constituent particles called quarks. The strong attraction between nucleons was the side-effect of a more fundamental force that bound the quarks together into protons and neutrons. The theory of quantum chromodynamics explains that quarks carry what is called a color charge, although it has no relation to visible color. Quarks with unlike color charge attract one another as a result of the strong interaction, and the particle that mediates this was called the gluon.

Behavior of the strong force

The fundamental couplings of the strong interaction, from left to right: gluon radiation, gluon splitting and gluon self-coupling.

The word strong is used since the strong interaction is the "strongest" of the four fundamental forces. At a distance of 1 femtometer (1 fm = 10⁻¹⁵ meters) or less, its strength is around 137 times that of the electromagnetic force, some 10⁶ times as great as that of the weak force, and about 10³⁸ times that of gravitation.

The strong force is described by quantum chromodynamics (QCD), a part of the standard model of particle physics. Mathematically, QCD is a non-Abelian gauge theory based on a local (gauge) symmetry group called SU(3).

The force carrier particle of the strong interaction is the gluon, a massless boson. Unlike the photon in electromagnetism, which is neutral, the gluon carries a color charge. Quarks and gluons are the only fundamental particles that carry non-vanishing color charge, and hence they participate in strong interactions only with each other. The strong force is the expression of the gluon interaction with other quark and gluon particles.

All quarks and gluons in QCD interact with each other through the strong force. The strength of interaction is parameterized by the strong coupling constant. This strength is modified by the gauge color charge of the particle, a group theoretical property.

The strong force acts between quarks. Unlike all other forces (electromagnetic, weak, and gravitational), the strong force does not diminish in strength with increasing distance between pairs of quarks. After a limiting distance (about the size of a hadron) has been reached, it remains at a strength of about 10,000 newtons (N), no matter how much farther the distance between the quarks. As the separation between the quarks grows, the energy added to the pair creates new pairs of matching quarks between the original two; hence it is impossible to create separate quarks. The explanation is that the amount of work done against a force of 10,000 newtons is enough to create particle–antiparticle pairs within a very short distance of that interaction. The very energy added to the system required to pull two quarks apart would create a pair of new quarks that will pair up with the original ones. In QCD, this phenomenon is called color confinement; as a result only hadrons, not individual free quarks, can be observed. The failure of all experiments that have searched for free quarks is considered to be evidence of this phenomenon.

The elementary quark and gluon particles involved in a high energy collision are not directly observable. The interaction produces jets of newly created hadrons that are observable. Those hadrons are created, as a manifestation of mass–energy equivalence, when sufficient energy is deposited into a quark–quark bond, as when a quark in one proton is struck by a very fast quark of another impacting proton during a particle accelerator experiment. However, quark–gluon plasmas have been observed.

Residual strong force

Contrary to the description above of distance independence, in the post-Big Bang universe it is not the case that every quark in the universe attracts every other quark. Color confinement implies that the strong force acts without distance-diminishment only between pairs of quarks, and that in compact collections of bound quarks (hadrons), the net color-charge of the quarks essentially cancels out, resulting in a limit of the action of the color-forces: From distances approaching or greater than the radius of a proton, compact collections of color-interacting quarks (hadrons) collectively appear to have effectively no color-charge, or "colorless", and the strong force is therefore nearly absent between those hadrons. However, the cancellation is not quite perfect, and a residual force (described below) remains. This residual force does diminish rapidly with distance, and is thus very short-range (effectively a few femtometers). It manifests as a force between the "colorless" hadrons, and is sometimes known as the strong nuclear force or simply nuclear force.

An animation of the nuclear force (or residual strong force) interaction between a proton and a neutron. The small colored double circles are gluons, which can be seen binding the proton and neutron together. These gluons also hold the quark/antiquark combination called the pion together, and thus help transmit a residual part of the strong force even between colorless hadrons. Anticolors are shown as per this diagram.

The nuclear force acts between hadrons, known as mesons and baryons. This "residual strong force", acting indirectly, transmits gluons that form part of the virtual π and ρ mesons, which, in turn, transmit the force between nucleons that holds the nucleus (beyond protium) together.

The residual strong force is thus a minor residuum of the strong force that binds quarks together into protons and neutrons. This same force is much weaker between neutrons and protons, because it is mostly neutralized within them, in the same way that electromagnetic forces between neutral atoms (van der Waals forces) are much weaker than the electromagnetic forces that hold electrons in association with the nucleus, forming the atoms.

Unlike the strong force itself, the residual strong force, does diminish in strength, and it in fact diminishes rapidly with distance. The decrease is approximately as a negative exponential power of distance, though there is no simple expression known for this; see Yukawa potential. The rapid decrease with distance of the attractive residual force and the less-rapid decrease of the repulsive electromagnetic force acting between protons within a nucleus, causes the instability of larger atomic nuclei, such as all those with atomic numbers larger than 82 (the element lead).

Although the nuclear force is weaker than strong interaction itself, it is still highly energetic: transitions produce gamma rays. The mass of a nucleus is significantly different from the summed masses of the individual nucleons. This mass defect is due to the potential energy associated with the nuclear force. Differences between mass defects power nuclear fusion and nuclear fission.

Unification

The so-called Grand Unified Theories (GUT) aim to describe the strong interaction and the electroweak interaction as aspects of a single force, similarly to how the electromagnetic and weak interactions were unified by the Glashow–Weinberg–Salam model into the electroweak interaction. The strong interaction has a property called asymptotic freedom, wherein the strength of the strong force diminishes at higher energies (or temperatures). The theorized energy where its strength becomes equal to the electroweak interaction is the grand unification energy. However, no Grand Unified Theory has yet been successfully formulated to describe this process, and Grand Unification remains an unsolved problem in physics.

If GUT is correct, after the Big Bang and during the electroweak epoch of the universe, the electroweak force separated from the strong force. Accordingly, a grand unification epoch is hypothesized to have existed prior to this.

Guano

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https://en.wikipedia.org/wiki/Guano 

 

The nest of the Peruvian booby is made of almost pure guano.

 

The Guanay cormorant has historically been the most important producer of guano.

 

Man-made Guano Island near Walvis Bay in Namibia

Guano (Spanish from Quechua: wanu) is the accumulated excrement of seabirds and bats. As a manure, guano is a highly effective fertilizer due to its exceptionally high content of nitrogen, phosphate, and potassium: key nutrients essential for plant growth. Guano was also, to a lesser extent, sought for the production of gunpowder and other explosive materials.

The 19th-century guano trade played a pivotal role in the development of modern input-intensive farming, but its demand began to decline after the discovery of the Haber–Bosch process of nitrogen fixing led to the production of synthetic fertilizers. The demand for guano spurred the human colonization of remote bird islands in many parts of the world, resulting in some of the first examples of U.S. colonialism and the expansion of the British Empire.

The guano mining process resulted in ecological degradation through the loss of millions of seabirds. Unsustainable guano mining in caves alters cave shape, causing bats to abandon the roost. Guano mining also involved the poor treatment and enslavement of workers such as Chinese immigrants, Native Hawaiians, and African diaspora.

Guano is ecologically important due to its role in dispersing nutrients. Cave ecosystems, in particular, are often wholly dependent on bats to provide nutrients via their guano, which supports bacteria, fungi, invertebrates, and vertebrates. The loss of bats from a cave can result in the extinction of species that rely on their guano. Guano also has a role in shaping caves, as its high acidity results in erosion.

Composition and properties

Bird guano

Bird guano has high levels of nutrients like nitrate and ammonium. By mass, it is 8–21% nitrogen; the nitrogen content is about 80% uric acid, 10% protein, 7% ammonia, and 0.5% nitrate. Some of bird guano's most common chemical elements are phosphorus, calcium, and magnesium. It may react with the rocky substrate of islands like basalt to form authigenic, phosphatic minerals including taranakite and leucophosphite.

Bat guano

The mineral whitlockite, which is found in bat guano

When freshly excreted, the guano of insectivorous bats consists of fine particles of insect exoskeleton, which are largely composed of chitin. Elements found in large concentrations include carbon, nitrogen, sulfur, and phosphorus. Through the action of bacteria and fungi, the fresh guano decays rapidly, usually losing its organic matter the fastest. Organic matter usually does not persist in a cave guano deposit at depths greater than a few centimeters. Fresh guano contains about 2.4–7 times as much carbon as nitrogen; the carbon-to-nitrogen ratio drops or remains similar when sampling older guano. Fresh guano has a pH of 5.1–7.3, making it neutral or somewhat acidic. However, as it ages, the guano becomes strongly acidic, reaching pH levels of 2.7–4.1. Similar to bird guano, the acidic properties of the guano and limestone of the cave can interact to create phosphatic minerals such as whitlockite, taranakite, variscite, spheniscidite, montgomeryite, and leucophosphite. Other minerals found in guano include quartz, graphite, gypsum, bassanite, and mica.

Guano composition varies among bat species with different diets. Comparing guano from insectivores (Mexican free-tailed bats), frugivores (Rodrigues flying foxes), and sanguivores (common vampire bats), a 2007 study found that the three did not differ significantly in proportions of organic matter or carbon in dry matter. The sanguivore's had elevated carbon in organic matter, sanguivores and insectivores had elevated nitrogen in organic and dry matter, and insectivore and frugivore had elevated phosphorus. Frugivores had the greatest carbon-to-nitrogen ratio, while sanguivores had the greatest nitrogen-to-phosphorus ratio and carbon-to-phosphorus ratio.

History of human use

Chincha Islands where guano was found in abundance. Mining was done on site and ships transported it to Europe.

Bird guano

Indigenous use

The word "guano" originates from the Andean indigenous language Quechua, which refers to any form of dung used as an agricultural fertilizer. Archaeological evidence suggests that Andean people collected guano from small islands and points off the desert coast of Peru for use as a soil amendment for well over 1,500 years and perhaps as long as 5,000 years. Spanish colonial documents suggest that the rulers of the Inca Empire greatly valued guano, restricted access to it, and punished any disturbance of the birds with death. The Guanay cormorant is historically the most abundant and important producer of guano. Other important guano-producing species off the coast of Peru are the Peruvian pelican and the Peruvian booby.

Western discovery and the Guano Age (1802–1884)

Advertisement for guano, 1884

In November 1802, Prussian geographer and explorer Alexander von Humboldt first encountered guano and began investigating its fertilizing properties at Callao in Peru, and his subsequent writings on this topic made the subject well known in Europe. Although Europeans knew of its fertilizing properties, guano was not widely used before this time. Cornish chemist Humphry Davy delivered a series of lectures which he compiled into an 1813 bestselling book about the role of nitrogenous manure as a fertilizer, Elements of Agricultural Chemistry. It highlighted the special efficacy of Peruvian guano, noting that it made the "sterile plains" of Peru fruitful. Though Europe had marine seabird colonies and thus, guano, it was of poorer quality because its potency was leached by high levels of rainfall and humidity. Elements of Agricultural Chemistry was translated into German, Italian, and French; American historian Wyndham D. Miles said that it was likely "the most popular book ever written on the subject, outselling the works of Dundonald, Chaptal, Liebig..." He also said that "No other work on agricultural chemistry was read by as many English-speaking farmers."

The arrival of commercial whaling on the Pacific coast of South America contributed to scaling of its guano industry. Whaling vessels carried consumer goods to Peru such as textiles, flour, and lard; unequal trade meant that ships returning north were often half empty, leaving entrepreneurs in search of profitable goods that could be exported. In 1840, Peruvian politician and entrepreneur Francisco Quirós y Ampudia negotiated a deal to commercialize guano export among a merchant house in Liverpool, a group of French businessmen, and the Peruvian government. This agreement resulted in the abolition of all preexisting claims to Peruvian guano; thereafter, it was the exclusive resource of the State. By nationalizing its guano resources, the Peruvian government was able to collect royalties on its sale, becoming the country's largest source of revenue. Some of this income was used by the State to free its more than 25,000 black slaves. Peru also used guano revenue to abolish the head tax on its indigenous citizens. This export of guano from Peru to Europe has been suggested as the vehicle that brought a virulent strain of potato blight from the Andean highlands that began the Great Famine of Ireland.

Soon guano was sourced from regions besides Peru. By 1846, 462,057 metric tons (509,331 short tons) of guano had been exported from Ichaboe Island, off the coast of Namibia, and surrounding islands to Great Britain. Guano pirating took off in other regions as well, causing prices to plummet and more consumers to try it. The biggest markets for guano from 1840–1879 were in Great Britain, the Low Countries, Germany, and the United States.

By the late 1860s, it became apparent that Peru's most productive guano site, the Chincha Islands, was nearing depletion. This caused guano mining to shift to other islands north and south of the Chincha Islands. Despite this near exhaustion, Peru achieved its greatest ever export of guano in 1870 at more than 700,000 metric tons (770,000 short tons). Concern of exhaustion was ameliorated by the discovery of a new Peruvian resource: sodium nitrate, also called Chile saltpeter. After 1870, the use of Peruvian guano as a fertilizer was eclipsed by Chile saltpeter in the form of caliche extraction from the interior of the Atacama Desert, close to the guano areas.

The Guano Age ended with the War of the Pacific (1879–1883), which saw Chilean marines invade coastal Bolivia to claim its guano and saltpeter resources. Knowing that Bolivia and Peru had a mutual defense agreement, Chile mounted a preemptive strike on Peru, resulting in its occupation of the Tarapacá, which included Peru's guano islands. With the Treaty of Ancón of 1884, the War of the Pacific ended. Bolivia ceded its entire coastline to Chile, which also gained half of Peru's guano income from the 1880s and its guano islands. The conflict ended with Chilean control over the most valuable nitrogen resources in the world. Chile's national treasury grew by 900% between 1879 and 1902 thanks to taxes coming from the newly acquired lands.

Imperialism

Islands claimed by U.S. via the 1856 Guano Islands Act in the Atlantic
1. Arenas Keys
2. Alacranes Island
3. Swan Islands
4. Serranilla Keys
5. Quita Sueño Island
6. Roncador Island
7. Serraña Key
8. Petrel Island
9. Morant Keys
10. Navassa Island
11. Alta Vela Island
12. Aves Island
13. Verd Key

 

Islands claimed by U.S. via the 1856 Guano Islands Act in the Pacific
1. Enderbury Island
2. McKean Island
3. Howland Island
4. Baker Island
5. Canton Island
6. Phoenix Islands
7. Dangerous Islands
8. Swains Atoll
9. Flint Island
10. Caroline Island
11. Maidens Island
12. Jarvis Island
13. Christmas Atoll
14. Starbuck Island
15. Fanning Island
16. Palmyra Island
17. Kingman Reef
18. Johnston Atoll
19. Clipperton Island

The demand for guano led the United States to pass the Guano Islands Act in 1856, which gave U.S. citizens discovering a source of guano on an unclaimed island exclusive rights to the deposits. In 1857, the U.S. began annexing uninhabited islands in the Pacific and Caribbean, totaling nearly 100. Several of these islands are still officially U.S. territories. Conditions on annexed guano islands were poor for workers, resulting in a rebellion on Navassa Island in 1889 where black workers killed their white overseers. In defending the workers, lawyer Everett J. Waring argued that the men could not be tried by U.S. law because the guano islands were not legally part of the country. The case went to the Supreme Court of the United States where it was decided in Jones v. United States (1890). The Court decided that Navassa Island and other guano islands were legally part of the U.S. American historian Daniel Immerwahr claimed that by establishing these land claims as constitutional, the Court laid the "basis for the legal foundation for the U.S. empire". The Guano Islands Act is now considered "America's first imperialist experiment".

Other countries also used their desire for guano as a reason to expand their empires. The United Kingdom claimed Kiritimati and Malden Island. Others nations that claimed guano islands included Australia, France, Germany, the Hawaiian Kingdom, Japan, and Mexico.

Decline and resurgence

In 1913, a factory in Germany began the first large-scale synthesis of ammonia using German chemist Fritz Haber's catalytic process. The scaling of this energy-intensive process meant that farmers could cease practices such as crop rotation with nitrogen-fixing legumes or the application of naturally derived fertilizers such as guano. The international trade of guano and nitrates such as Chile saltpeter declined as artificially synthesized fertilizers became more widely used. With the rising popularity of organic food in the twenty-first century, the demand for guano has started to rise again.

Bat guano

Aerial view of Guano Point. Old tramway headhouse is at the end of dirt road (right). Second tramway tower is more clearly visible, on skyline to right. Bat Cave mine is 2,500 feet (760 m) below, across the canyon.

In the U.S., bat guano was harvested from caves as early as the 1780s to manufacture gunpowder. During the American Civil War (1861–1865), the Union's blockade of the southern Confederate States of America meant that the Confederacy resorted to mining guano from caves to produce saltpeter. One Confederate guano kiln in New Braunfels, Texas had a daily output of 100 lb (45 kg) of saltpeter, produced from 2,500 lb (1,100 kg) of guano from two area caves. From the 1930s, Bat Cave mine in Arizona was used for guano extraction, though it cost more to develop than it was worth. U.S. Guano Corporation bought the property in 1958 and invested 3.5 million dollars to make it operational; actual guano deposits in the cave were one percent of predicted and the mine was abandoned in 1960.

In Australia, the first documented claim on Naracoorte's Bat Cave guano deposits was in 1867. Guano mining in the country remained a localized and small industry. In modern times, bat guano is used in low levels in developed countries. It remains an important resource in developing countries, particularly in Asia.

Paleoenvironment reconstruction

Coring accumulations of bat guano can be useful in determining past climate conditions. The level of rainfall, for example, impacts the relative frequency of nitrogen isotopes. In times of higher rainfall, ¹⁵N is more common. Bat guano also contains pollen, which can be used to identify prior plant assemblages. A layer of charcoal recovered from a guano core in the U.S. state of Alabama was seen as evidence that a Woodlands tribe inhabited the cave for some time, leaving charcoal via the fires they lit. Stable isotope analysis of bat guano was also used to support that the climate of the Grand Canyon was cooler and wetter during the Pleistocene epoch than it is now in the Holocene. Additionally, the climatic conditions were more variable in the past.

Mining

Workers load guano onto a cart in 1865

Process

Mining seabird guano from Peruvian islands has remained largely the same since the industry began, relying on manual labor. First, picks, brooms, and shovels are used to loosen the guano. The use of excavation machinery is not only impractical due to the terrain but also prohibited because it would frighten the seabirds. The guano is then placed in sacks and carried to sieves, where impurities are removed.

Similarly, mining in caves was and is manual. In Puerto Rico, cave entrances were enlarged to facilitate access and extraction. Guano was freed from the rocky substrate by explosives. Then, it was shoveled into carts and removed from the cave. From there, the guano was taken to kilns to dry. The dried guano would then be loaded into sacks, ready for transport via ship. Today, bat guano is usually mined in the developing world, using "strong backs and shovels".

Ecological impacts and mitigation

A large colony of Guanay cormorants on South Chincha Island of Peru in 1907

Bird guano

Peru's guano islands experienced severe ecological effects as a result of unsustainable mining. In the late 1800s, approximately 53 million seabirds lived on the twenty-two islands. As of 2011, only 4.2 million seabirds lived there. After realizing the depletion of guano in the Guano Age, the Peruvian government recognized that it needed to conserve the seabirds. In 1906, American zoologist Robert Ervin Coker was hired by the Peruvian government to create management plans for its marine species, including the seabirds. Specifically, he made five recommendations:

1. That the government turn its coastal islands into a state-run bird sanctuary. Private use of the island for hunting or egg collecting should be prohibited.
2. To eliminate unhealthy competition, each island should be assigned only one state contractor for guano extraction.
3. Guano mining should be entirely ceased from November to March so that the breeding season for the birds was undisturbed.
4. In rotation, each island should be closed to guano mining for an entire year.
5. The Peruvian government should monopolize all processes related to guano production and distribution. This recommendation was made with the belief that a single entity with a vested interest in the long-term success of the guano industry would manage the resource most responsibly.

Despite these policies, the seabird population continued to decline, which was exacerbated by the 1911 El Niño–Southern Oscillation. In 1913, Scottish ornithologist Henry Ogg Forbes authored a report on behalf of the Peruvian Corporation focusing on how human actions harmed the birds and subsequent guano production. Forbes suggested additional policies to conserve the seabirds, including keeping unauthorized visitors a mile away from guano islands at all times, eliminating all the birds' natural predators, armed patrols of the islands, and decreasing the frequency of harvest on each island to once every three to four years. In 2009, these conservation efforts culminated into the establishment of the Guano Islands, Isles, and Capes National Reserve System, which consists of twenty-two islands and eleven capes. This Reserve System was the first marine protected area in South America, encompassing 140,833 hectares (543.76 sq mi).

Bat guano

Unlike bird guano which is deposited on the surface of islands, bat guano can be deep within caves. Cave structure is often altered via explosives or excavation to facilitate extraction of the guano, which changes the cave's microclimate. Bats are sensitive to cave microclimate, and such changes can cause them to abandon the cave as a roost, as happened when Robertson Cave in Australia had a hole opened in its ceiling for guano mining. Guano mining also introduces artificial light into caves; one cave in the U.S. state of New Mexico was abandoned by its bat colony after the installation of electric lights.

In addition to harming bats by necessitating they find another roost, guano mining techniques can ultimately harm human livelihood as well. Harming or killing bats means that less guano will be produced, resulting in unsustainable mining practices. In contrast, sustainable mining practices do not negatively impact bat colonies nor other cave fauna. The International Union for Conservation of Nature's (IUCN) 2014 recommendations for sustainable guano mining include extracting guano when the bats are not present, such as when migratory bats are gone for the year or when non-migratory bats are out foraging at night.

Work conditions

Chinese laborers stand on a partially extracted guano deposit in 1865

Guano mining in Peru was at first done with black slaves. After Peru formally ended slavery, it sought another source of cheap labor. In the 1840s and 1850s, thousands of men were blackbirded (coerced or kidnapped) from the Pacific islands and southern China. Thousands of coolies from South China worked as "virtual slaves" mining guano. By 1852, Chinese laborers comprised two-thirds of Peru's guano miners; others who mined guano included convicts and forced laborers paying off debts. Chinese laborers agreed to work for eight years in exchange for passage from China, though many were misled that they were headed to California's gold mines. Conditions on the guano islands were very poor, commonly resulting in floggings, unrest, and suicide. Workers experienced lung damage by inhaling guano dust, were buried alive by falling piles of guano, and risked falling into the ocean. After visiting the guano islands, U.S. politician George Washington Peck wrote:

I observed Coolies shoveling and wheeling as if for dear life and yet their backs were covered with great welts...It is easy to distinguish Coolies who have been at the islands a short time from the new comers. They soon become emaciated and their faces have a wild desparing expression. That they are worked to death is as apparent as that the hack horses in our cities are used up in the same manner.

Hundreds or thousands of Pacific Islanders, especially Native Hawaiians, traveled or were blackbirded to the U.S.-held and Peruvian guano islands for work, including Howland Island, Jarvis Island, and Baker Island. While most Hawaiians were literate, they could usually not read English; the contract they received in their own language differed from the English version. The Hawaiian language contract was often missing information about the departure date, the length of the contract, and the name of the company for which they would be working. When they arrived at their destination to begin mining, they learned that both contracts were largely meaningless in terms of work conditions. Instead, their overseer, who was usually white, had nearly unlimited power over them. Native Hawaiian laborers of Jarvis Island referred to the island as Paukeaho, meaning "out of breath" or "exhausted". Pacific Islanders also risked death: one in thirty-six laborers from Honolulu died before completing their contract. Slaves blackbirded from Easter Island in 1862 were repatriated by the Peruvian government in 1863; only twelve of 800 slaves survived the journey.

On Navassa Island, the guano mining company switched from white convicts to largely black laborers after the American Civil War. Black laborers from Baltimore claimed that they were misled into signing contracts with stories of mostly fruit-picking, not guano mining, and "access to beautiful women". Instead, the work was exhausting and punishments were brutal. Laborers were frequently placed in stocks or tied up and dangled in the air. A labor revolt ensued, where the workers attacked their overseers with stones, axes, and even dynamite, killing five overseers.

Although the process for mining guano is mostly the same today, worker conditions have improved. As of 2018, guano miners in Peru made US$750 per month, which is more than twice the average national monthly income of $300. Workers also have health insurance, meals, and eight-hour shifts.

Human health

Histoplasmosis endemism map for the U.S.

Guano is one of the habitats of the fungus Histoplasma capsulatum, which can cause the disease histoplasmosis in humans, cats, and dogs. H. capsulatum grows best in the nitrogen-rich conditions present in guano. In the United States, histoplasmosis affects 3.4 adults per 100,000 over age 65, with higher rates in the Midwestern United States (6.1 cases per 100,000). In addition to the United States, H. capsulatum is found in Central and South America, Africa, Asia, and Australia. Of 105 outbreaks in the U.S. from 1938–2013, seventeen occurred after exposure to a chicken coop while nine occurred after exposure to a cave. Birds or their droppings were present in 56% of outbreaks, while bats or their droppings were present in 23%. Developing any symptoms after exposure to H. capsulatum is very rare; less than 1% of those infected develop symptoms. Only patients with more severe cases require medical attention, and only about 1% of acute cases are fatal. It is a much more serious illness for the immunocompromised, however. Histoplasmosis is the first symptom of HIV/AIDS in 50–75% of patients, and results in death for 39–58% of those with HIV/AIDS. The Centers for Disease Control and Prevention recommends that the immunocompromised avoid exploring caves or old buildings, cleaning chicken coops, or disturbing soil where guano is present.

Rabies, which can affect humans who have been bitten by infected mammals including bats, cannot be transmitted through guano. A 2011 study of bat guano viromes in the U.S. states of Texas and California recovered no viruses that are pathogenic to humans, nor any close relatives of pathogenic viruses. It is hypothesized that Egyptian fruit bats, which are native to Africa and the Middle East, can spread Marburg virus to each other through contact with infected secretions such as guano, but a 2018 review concluded that more studies are necessary to determine the specific mechanisms of exposure that cause Marburg virus disease in humans. Exposure to guano could be a route of transmission to humans.

As early as in the 18th century there are reports of travellers complaining about the unhealthy air of Arica and Iquique resulting from abundant bird spilling.

Ecological importance

The Ozark cavefish, a species that depends on bat guano as a source of food.

 

Cave cockroaches on guano

Colonial birds and their guano deposits have an outsize role on the surrounding ecosystem. Bird guano stimulates productivity, though species richness may be lower on guano islands than islands without the deposits. Guano islands have a greater abundance of detritivorous beetles than islands without guano. The intertidal zone is inundated by the guano's nutrients, causing algae to grow more rapidly and coalesce into algal mats. These algal mats are in turn colonized by invertebrates. The abundance of nutrients offshore of guano islands also supports coral reef ecosystems.

Cave ecosystems are often limited by nutrient availability. Bats bring nutrients into these ecosystems via their excretions, however, which are often the dominant energy resource of a cave. Many cave species depend on bat guano for sustenance, directly or indirectly. Because cave-roosting bats are often highly colonial, they can deposit substantial quantities of nutrients into caves. The largest colony of bats in the world at Bracken Cave (about 20 million individuals) deposit 50,000 kg (110,000 lb) of guano into the cave every year. Even smaller colonies have relatively large impacts, with one colony of 3,000 gray bats annually depositing 9 kg (20 lb) of guano into their cave.

Invertebrates inhabit guano piles, including fly larvae, nematodes, springtails, beetles, mites, pseudoscorpions, thrips, silverfish, moths, harvestmen, spiders, isopods, millipedes, centipedes, and barklice. The invertebrate communities associated with the guano depends on the bat species' feeding guild: frugivorous bat guano has the greatest invertebrate diversity. Some invertebrates feed directly on the guano, while others consume the fungi that use it as a growth medium. Predators such as spiders depend on guano to support their prey base. Vertebrates consume guano as well, including the bullhead catfish and larvae of the grotto salamander.

Bat guano is integral to the existence of endangered cave fauna. The critically endangered Shelta Cave crayfish feeds on guano and other detritus. The Ozark cavefish, a U.S. federally listed species, also consumes guano. The loss of bats from a cave can result in declines or extinctions of other species that rely on their guano. A 1987 cave flood resulted in the death of its bat colony; the Valdina Farms salamander is now likely extinct as a result.

Bat guano also has a role in shaping caves by making them larger. It has been estimated that 70–95% of the total volume of Gomantong cave in Borneo is due to biological processes such as guano excretion, as the acidity of the guano weathers the rocky substrate. The presence of high densities of bats in a cave is predicted to cause the erosion of 1 metre (3.3 ft) of rock over 30,000 years.

Cultural significance

There are several references to guano in the arts. In his 1845 poem "Guanosong", German author Joseph Victor von Scheffel used a humorous verse to take a position in the popular polemic against Hegel's Naturphilosophie. The poem starts with an allusion to Heinrich Heine's Lorelei and may be sung to the same tune. The poem ends however with the blunt statement of a Swabian rapeseed farmer from Böblingen who praises the seagulls of Peru as providing better manure even than his fellow countryman Hegel. This refuted the widespread Enlightenment belief that nature in the New World was inferior to the Old World. The poem has been translated by, among others, Charles Godfrey Leland.

English author Robert Smith Surtees parodied the obsession of wealthy landowners with the "religion of progress" in 1843. In one of his works featuring the character John Jorrocks, Surtees has the character develop an obsession with trying all the latest farming experiments, including guano. In efforts to impress the upper-class around him and to disguise his low-class origins, Jorrocks makes references to guano in conversation at every chance he can. At one point, he exclaims "Guano!" along with two other varieties of fertilizer, to which the Duke replies, "I see you understand it all!"

Guano is also the namesake for one of the nucleobases that comprise RNA and DNA: guanine. Guanine was first obtained from guano by Julius Bodo Unger [de], who described it as xanthine in 1844. After he was corrected, Bodo Unger published it with the new name of "guanine" in 1846.