Editing Standard Model (section)

== Challenges ==
{{See also|Physics beyond the Standard Model}}
{{unsolved|physics|
* What gives rise to the Standard Model of particle physics?
* Why do particle masses and [[coupling constant]]s have the values that we measure?
* Why are there three [[Generation (particle physics)|generations]] of particles?
* Why is there more matter than [[antimatter]] in the universe?
* Where does [[dark matter]] fit into the model? Does it even consist of one or more new particles?
}}

Self-consistency of the Standard Model (currently formulated as a non-[[abelian group|abelian]] gauge theory quantized through path-integrals) has not been mathematically proved.  While regularized versions useful for approximate computations (for example [[lattice gauge theory]]) exist, it is not known whether they converge (in the sense of S-matrix elements) in the limit that the regulator is removed.  A key question related to the consistency is the [[Yang–Mills existence and mass gap]] problem.

Experiments indicate that [[neutrinos]] have [[mass]], which the classic Standard Model did not allow.<ref>
{{cite web
 |date=31 May 2010
 |title=Particle chameleon caught in the act of changing
 |url = http://press.cern/press-releases/2010/05/particle-chameleon-caught-act-changing
 |publisher=[[CERN]]
 |access-date=2016-11-12
}}</ref> To accommodate this finding, the classic Standard Model can be modified to include neutrino mass, although it is not obvious exactly how this should be done.

If one insists on using only Standard Model particles, this can be achieved by adding a non-renormalizable interaction of leptons with the Higgs boson.<ref>
{{cite journal
 |author=S. Weinberg
 |year=1979
 |title=Baryon and Lepton Nonconserving Processes
 |journal=[[Physical Review Letters]]
 |volume=43 |issue=21 |pages=1566–1570
 |bibcode=1979PhRvL..43.1566W
 |doi=10.1103/PhysRevLett.43.1566
}}</ref> On a fundamental level, such an interaction emerges in the [[seesaw mechanism]] where heavy right-handed neutrinos are added to the theory.
This is natural in the [[left-right symmetry|left-right symmetric]] extension of the Standard Model<ref name="Minkowski1977">
{{cite journal
 |author=P. Minkowski
 |year=1977
 |title=μ → e γ at a Rate of One Out of 10<sup>9</sup> Muon Decays?
 |journal=[[Physics Letters B]]
 |volume=67 |issue=4 |pages=421–428
 |bibcode=1977PhLB...67..421M
 |doi=10.1016/0370-2693(77)90435-X
}}</ref><ref name="MohapatraSenjanovic1980">
{{cite journal
 |author1=R.N. Mohapatra |author2=G. Senjanovic |year=1980
 |title=Neutrino Mass and Spontaneous Parity Nonconservation
 |journal=[[Physical Review Letters]]
 |volume=44|issue=14|pages=912–915
 |bibcode = 1980PhRvL..44..912M
 |doi=10.1103/PhysRevLett.44.912
}}</ref> and in certain [[grand unified theory|grand unified theories]].<ref name="Gell-Mann1979">
{{cite book
 |author1=M. Gell-Mann, P. Ramond  |author2=R. Slansky
 |name-list-style=amp |year=1979
 |pages=315–321
 |title=Supergravity
 |editor1=F. van Nieuwenhuizen |editor2=D.Z. Freedman |publisher=[[North Holland]]
 |isbn=978-0-444-85438-4
}}</ref> As long as new physics appears below or around 10<sup>14</sup> [[electronvolt|GeV]], the neutrino masses can be of the right order of magnitude.

Theoretical and experimental research has attempted to extend the Standard Model into a [[unified field theory]] or a [[theory of everything]], a complete theory explaining all physical phenomena including constants. Inadequacies of the Standard Model that motivate such research include:
* The model does not explain [[gravitation]], although physical confirmation of a theoretical particle known as a [[graviton]] would account for it to a degree. Though it addresses strong and electroweak interactions, the Standard Model does not consistently explain the canonical theory of gravitation, [[general relativity]], in terms of [[quantum field theory]]. The reason for this is, among other things, that quantum field theories of gravity generally break down before reaching the [[Planck scale]]. As a consequence, we have no reliable theory for the very early universe.
* Some physicists consider it to be ''ad hoc'' and inelegant, requiring 19 numerical constants whose values are unrelated and arbitrary.<ref name="BlumhoferHutter1997">
{{cite journal
 |author1=A. Blumhofer |author2=M. Hutter |year = 1997
 |title = Family Structure from Periodic Solutions of an Improved Gap Equation
 |journal = Nuclear Physics
 |volume =  B484
 |issue=1 |pages = 80–96
 |doi = 10.1016/S0550-3213(96)00644-X
 |bibcode = 1997NuPhB.484...80B
 |arxiv = hep-ph/9605393
}}</ref> Although the Standard Model, as it now stands, can explain why neutrinos have masses, the specifics of neutrino mass are still unclear. It is believed that explaining neutrino mass will require an additional 7 or 8 constants, which are also arbitrary parameters.<ref>
{{cite arXiv|eprint=hep-ph/0606054|last1=Strumia|first1=Alessandro|title=Neutrino masses and mixings and...|year=2006}}</ref>
* The Higgs mechanism gives rise to the [[hierarchy problem]] if some new physics (coupled to the Higgs) is present at high energy scales. In these cases, in order for the weak scale to be much smaller than the [[Planck scale]], severe fine tuning of the parameters is required; there are, however, other scenarios that include [[Asymptotic safety in quantum gravity|quantum gravity]] in which such fine tuning can be avoided.<ref>
{{cite journal |title=Agravity |journal=Journal of High Energy Physics|volume=2014 |issue=6 |page = 080 |url = http://inspirehep.net/record/1286134 |arxiv = 1403.4226 |bibcode = 2014JHEP...06..080S |doi=10.1007/JHEP06(2014)080|pmid = 31258400|pmc = 6560704|last1 = Salvio|first1 = Alberto|last2 = Strumia|first2 = Alessandro|year = 2018}}</ref> There are also issues of [[quantum triviality]], which suggests that it may not be possible to create a consistent quantum field theory involving elementary scalar particles.<ref name="TrivPurs">
{{cite journal |author=D.J.E. Callaway |year=1988
|title=Triviality Pursuit: Can Elementary Scalar Particles Exist? |journal=[[Physics Reports]]
|volume=167 |issue=5 |pages=241–320 |doi=10.1016/0370-1573(88)90008-7
|bibcode = 1988PhR...167..241C |author-link=David J E Callaway
}}</ref>
* The model is inconsistent with the emerging [[Lambda-CDM model]] of cosmology. Contentions include the absence of an explanation in the Standard Model of particle physics for the observed amount of [[cold dark matter]] (CDM) and its contributions to [[dark energy]], which are many orders of magnitude too large. It is also difficult to accommodate the observed predominance of matter over antimatter ([[matter]]/[[antimatter]] [[Baryon asymmetry|asymmetry]]). The [[isotropic|isotropy]] and [[Homogeneity (physics)|homogeneity]] of the visible universe over large distances seems to require a mechanism like cosmic [[Inflation (cosmology)|inflation]], which would also constitute an extension of the Standard Model.

Currently, no proposed [[theory of everything]] has been widely accepted or verified.