Wikipedia

Physics beyond the Standard Model

Physics beyond the Standard Model(BSM) refers to the theoretical developments needed to explain the deficiencies of theStandard Model,such as the inability to explain the fundamental parameters of the standard model, thestrong CP problem,neutrino oscillations,matter–antimatter asymmetry,and the nature ofdark matteranddark energy.[1]Another problem lies within themathematical frameworkof the Standard Model itself: the Standard Model is inconsistent with that ofgeneral relativity,and one or both theories break down under certain conditions, such asspacetime singularitieslike theBig Bangandblack holeevent horizons.

Theories that lie beyond the Standard Model include various extensions of the standard model throughsupersymmetry,such as theMinimal Supersymmetric Standard Model(MSSM) andNext-to-Minimal Supersymmetric Standard Model(NMSSM), and entirely novel explanations, such asstring theory,M-theory,andextra 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 aTheory of Everything,can only be settled via experiments, and is one of the most active areas of research in boththeoreticalandexperimental physics.[2]

Problems with the Standard Model

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Despite being the most successful theory of particle physics to date, the Standard Model is not perfect.[3]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

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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 agravitonto 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.[4][b][5][a]
  • Dark matter.Assuming thatgeneral relativityandLambda CDMare true, cosmological observations tell us the standard model explains about 5% of the mass-energy present in the universe. About 26% should be dark matter (the remaining 69% being dark energy) 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.[citation needed]
  • Dark energy.As mentioned, the remaining 69% of the 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 ofvacuum energyof the standard model lead to a mismatch of 120 orders of magnitude.[6]
  • Neutrino oscillations.According to the Standard Model,neutrinosdo not oscillate. However, experiments and astronomical observations have shown thatneutrino oscillationdoes occur. These are typically explained by postulating that neutrinos have mass. Neutrinos do not have mass in the Standard Model, and 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. There are also other extensions of the Standard Model for neutrino oscillations which do not assume massive neutrinos, such asLorentz-violating neutrino oscillations.
  • 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.[7]

Experimental results not explained

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No experimental result is accepted as definitively contradicting the Standard Model at the 5σlevel,[8]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:

  • B meson decay etc.– results from aBaBar experimentmay suggest a surplus over Standard Model predictions of a type of particle decay(B→ D(*)τ−ντ).In this, an electron and positron collide, resulting in aB mesonand an antimatterBmeson, which then decays into aD mesonand atau leptonas well as atau antineutrino.While the level of certainty of the excess (3.4σin 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.[9]In 2015,LHCbreported observing a 2.1σexcess in the same ratio of branching fractions.[10]TheBelle experimentalso reported an excess.[11]In 2017 ameta analysisof all available data reported a cumulative 5σdeviation from SM.[12]
  • Neutron lifetime puzzle- Free neutrons are not stable but decay after some time. Currently there are two methods used to measure this lifetime ( "bottle" versus "beam"[13][c]) that give different values not within each other's error margin. Currently the lifetime from the bottle method is at[14][15]with a difference of 10 seconds below the beam method value of.[16][17]

Theoretical predictions not observed

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Observation atparticle collidersof all of the fundamental particles predicted by the Standard Model has been confirmed. TheHiggs bosonis predicted by the Standard Model's explanation of theHiggs 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,CERNscientists using theLarge Hadron Colliderannounced the discovery of a particle consistent with the Higgs boson, with a mass of about126GeV/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.[18]

A fewhadrons(i.e. composite particles made ofquarks) 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"[19](i.e. composite particles made ofgluons) 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

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  • Koide formula– an unexplainedempirical equationremarked upon byYoshio Koidein 1981, and later by others.[20][21][22][23]It relates the masses of the three chargedleptons:.The Standard Model does not predict lepton masses (they are free parameters of the theory). However, the value of the Koide formula being equal to 2/3 within experimental errors of the measured lepton masses suggests the existence of a theory which is able to predict lepton masses.
  • TheCKM matrix,if interpreted as a rotation matrix in a 3-dimensional vector space, "rotates" a vector composed of square roots of down-type quark massesinto a vector of square roots of up-type quark masses,up to vector lengths, a result due to Kohzo Nishida.[24]
  • The sum of squares of the Yukawa couplings of all Standard Model fermions is approximately 0.984, which is very close to 1. To put it another way, the sum of squares of fermion masses is very close to half of squared Higgs vacuum expectation value. This sum is dominated by thetop quark.
  • The sum of squares of boson masses (that is, W, Z, and Higgs bosons) is also very close to half of squared Higgs vacuum expectation value, the ratio is approximately 1.004.
  • Consequently, the sum of squared masses of all Standard Model particles is very close to the squared Higgs vacuum expectation value, the ratio is approximately 0.994.

It is unclear if these empirical relationships represent any underlying physics; according to Koide, the rule he discovered "may be an accidental coincidence".[25]

Theoretical problems

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Some features of the standard model are added in anad hocway. These are not problems per se (i.e. the theory works fine with thead hocinsertions), but they imply a lack of understanding. These contrived features have motivated theorists to look for more fundamental theories with fewer parameters. Some of the contrivances are:

  • Hierarchy problem– the standard model introduces particle masses through a process known asspontaneous symmetry breakingcaused by theHiggs field.Within the standard model, the mass of theHiggs particlegets some very large quantum corrections due to the presence ofvirtual particles(mostly virtualtop quarks). These corrections are much larger than the actual mass of the Higgs. This means that thebare massparameter of the Higgs in the standard model must befine tunedin such a way that almost completely cancels the quantum corrections.[26]This level of fine-tuning is deemedunnaturalby many theorists.[who?]
  • Number of parameters– the standard model depends on 19 parameter numbers. Their values are known from experiment, but the origin of the values is unknown. Some theorists[who?]have tried to find relations between different parameters, for example,between the masses of particlesin differentgenerationsor calculating particle masses, such as inasymptotic safetyscenarios.[citation needed]
  • 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 theLandau poleproblem.[27]A possible solution is that the renormalized value could go to zero as the cut-off is removed, meaning that the bare value is completely screened by quantum fluctuations.
  • Strong CP problem– it can be argued theoretically that the standard model should contain a term in thestrong interactionthat breaksCP symmetry,causing slightly different interaction rates for matter vs.antimatter.Experimentally, however, no such violation has been found, implying that the coefficient of this term – if any – would be suspiciously close to zero.[28]

Additional experimental results

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Research from experimental data on thecosmological constant,LIGOnoise,andpulsar timing,suggests it's very unlikely that there are any new particles with masses much higher than those which can be found in the standard model or theLarge Hadron Collider.[29][30][31]However, this research has also indicated thatquantum gravityorperturbativequantum field theorywill become strongly coupled before 1 PeV, leading to other new physics in the TeVs.[29]

Grand unified theories

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Main article:Grand Unified Theory

The standard model has threegauge symmetries;thecolourSU(3),theweak isospinSU(2),and theweak hyperchargeU(1)symmetry, corresponding to the three fundamental forces. Due torenormalizationthe coupling constants of each of these symmetries vary with the energy at which they are measured. Around1016GeVthese 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 asimplegauge group, and just one coupling constant. Below this energy the symmetry isspontaneously brokento the standard model symmetries.[32]Popular choices for the unifying group are the special unitary group in five dimensionsSU(5)and the special orthogonal group in ten dimensionsSO(10).[33]

Theories that unify the standard model symmetries in this way are calledGrand 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 ofmagnetic monopolesin the early universe,[34]and instability of theproton.[35]Neither of these have been observed, and this absence of observation puts limits on the possible GUTs.

Supersymmetry

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Main article:Supersymmetry

Supersymmetry extends the Standard Model by adding another class of symmetries to theLagrangian.These symmetries exchangefermionicparticles withbosonicones. Such a symmetry predicts the existence ofsupersymmetric particles,abbreviated assparticles,which include thesleptons,squarks,neutralinosandcharginos.Each particle in the Standard Model would have a superpartner whosespindiffers by 1/2 from the ordinary particle. Due to thebreaking of supersymmetry,the sparticles are much heavier than their ordinary counterparts; they are so heavy that existingparticle collidersmay not be powerful enough to produce them.

Neutrinos

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In the standard model, neutrinos cannotspontaneously change flavor.Measurements however indicated that neutrinos do spontaneously change flavor, in what is calledneutrino oscillations.

Neutrino oscillations are usually explained using massive neutrinos. In the standard model,neutrinoshave exactly zero mass, as the standard model only containsleft-handedneutrinos. With no suitable right-handed partner, it is impossible to add a renormalizable mass term to the standard model.[36] 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 oftritiumdecay, providing an upper limit 2 eV, which makes them at least five orders of magnitude lighter than the other particles in the standard model.[37] 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.[38]

One approach to add masses to the neutrinos, the so-calledseesaw mechanism,is to add right-handed neutrinos and have these couple to left-handed neutrinos with aDirac massterm. The right-handed neutrinos have to besterile,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 aMajorana massterm. 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.[39] 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.[40] 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 bedark matter,[41] they can have a mass in theLHCenergy range[42][43] and lead to observablelepton numberviolation,[44] or they can be near the GUT scale, linking the right-handed neutrinos to the possibility of a grand unified theory.[45][46]

The mass terms mix neutrinos of different generations. This mi xing is parameterized by thePMNS matrix,which is the neutrino analogue of theCKM quark mi xing matrix.Unlike the quark mi xing, which is almost minimal, the mi xing of the neutrinos appears to be almost maximal. This has led to various speculations of symmetries between the various generations that could explain the mi xing patterns.[47] The mi xing 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 asleptogenesis.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.[33]

The light neutrinos are disfavored as an explanation for the observation of dark matter, based on considerations of large-scale structure formation in the early universe. Simulations ofstructure formationshow that they are too hot – that is, their kinetic energy is large compared to their mass – while formation of structures similar to the galaxies in our universe requirescold dark matter.The simulations show that neutrinos can at best explain a few percent of the missing mass in dark matter. However, the heavy, sterile, right-handed neutrinosarea possible candidate for a dark matterWIMP.[48]

There are however other explanations for neutrino oscillations which do not necessarily require neutrinos to have masses, such asLorentz-violating neutrino oscillations.

Preon models

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Severalpreonmodels 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 theRishon model.[49][50][51]

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

Theories of everything

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Main article:Theory 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 withGeneral Relativityin a theory ofquantum 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 aresupersymmetry,loop quantum gravity,andString theory.

Supersymmetry

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Main article:supersymmetry

Loop quantum gravity

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Main article:loop quantum gravity

Theories ofquantum gravitysuch asloop quantum gravityand 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.[52]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.[53]

String theory

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Main article:String theory

Extensions, revisions, replacements, and reorganizations of the Standard Model exist in attempt to correct for these and other issues.String theoryis one such reinvention, and many theoretical physicists think that such theories are the next theoretical step toward a trueTheory of Everything.[52]

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 physicistsBrian GreeneandStephen Hawking.Though a full mathematical description is not yet known, solutions to the theory exist for specific cases.[54]Recent works have also proposed alternate string models, some of which lack the various harder-to-test features ofM-theory(e.g. the existence ofCalabi–Yau manifolds,manyextra dimensions,etc.) including works by well-published physicists such asLisa Randall.[55][56]

See also

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Footnotes

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  1. ^ab "One can find thousands of statements in the literature to the effect thatgeneral relativity and quantum mechanics are incompatible.These are completely outdated and no longer relevant.
    Effective field theoryshows that general relativity and quantum mechanics work together perfectly normally over a range of scales and curvatures, including those relevant for the world that we see around us. However, effective field theories are only valid over some range of scales. General relativity certainly does have problematic issues at extreme scales. There are important problems which the effective field theory does not solve because they are beyond its range of validity. However, this means that the issue of quantum gravity is not what we thought it to be: Rather than a fundamental incompatibility of quantum mechanics and gravity, we are in the more familiar situation of needing a more complete theory beyond the range of their combined applicability.
    The usual marriage of general relativity and quantum mechanics is fine at ordinary energies, but we now seek to uncover the modifications that must be present in more extreme conditions. This is the modern view of the problem of quantum gravity, and it represents progress over the outdated viewof the past. "— Donoghue (2012)[5]
    See also contrary contemporary quote[b]from Sushkov, Kim,et al.(2011).[4]
  2. ^ab "It is remarkable that two of the greatest successes of 20th century physics, general relativity and the standard model, appear to be fundamentallyincompatible. "— Sushkov, Kim,et al.(2011)[4]
    But see contrary quote[a]from Donoghue (2012).[5]
  3. ^ "When physicists strip neutrons from atomic nuclei, put them in a bottle, then count how many remain there after some time, they infer that neutrons radioactively decay in 14m39s,on average. But when other physicists generate beams of neutrons and tally the emerging protons—the particles that free neutrons decay into—they peg the average neutron lifetime at around 14m48s.The discrepancy between the 'bottle' and 'beam' measurements has persisted [ever] since both methods of gauging the neutron's longevity began yielding results in the 1990s. At first, all the measurements were so imprecise that nobody worried. Gradually, though, both methods have improved, and still they disagree. "[13]

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Further reading

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External resources

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