Model building (particle physics)
Encyclopedia
In particle physics
, the term model building refers to a construction of new quantum field theories
beyond
the Standard Model
that have certain features making them attractive theoretically or for possible observations in the near future. If the model building physicist uses the tools of string theory, he or she is called "superstring model builder".
A model builder typically chooses new quantum fields and their new interactions, attempting to make their combination realistic, testable and physically interesting. In particular, an interesting
new model should address questions left unanswered in the Standard Model which has, including three massive neutrinos, 28 free parameters. A model which extends the Standard Model should predict one or more of these parameters or shed light on some other issue such as why there are three quark-lepton families or, the most common motivation, the naturalness or hierarchy problem
associated with the quadratic divergences appearing in the scalar sector.
are, although the distinction is often blurred in practice.
Model building is speculative because current particle accelerator
s can only probe up to a few hundred GeV, where physics is well described by the Standard Model. One result of renormalization group
theory is that at low energies, models flow toward universality classes and different models may flow to the same universality class so many models can coexist beyond the Standard Model.
Only experimental data will distinguish between them. All experimental attempts to look for irrelevant couplings beyond the Standard Model, such as those that give rise to proton decay
, flavor changing neutral current
s, Peskin-Takeuchi parameter
s and nonuniversal couplings have so far come up only with upper bounds. However, the observation of neutrino oscillation
s could be explained by adding a Majorana
neutrino mass, which is an irrelevant coupling that could arise, e.g., from the seesaw mechanism
.
There may be patterns among the marginal and relevant couplings
so that among all the possible models which give rise to the Standard Model universality class, one of them has fewer free parameters than the others allowing it to make predictions. It is an article of faith amongst some model builders in particle physics that there exists an elegant high-energy theory or UV completion
of the Standard Model.
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
, the term model building refers to a construction of new quantum field theories
Quantum field theory
Quantum field theory provides a theoretical framework for constructing quantum mechanical models of systems classically parametrized by an infinite number of dynamical degrees of freedom, that is, fields and many-body systems. It is the natural and quantitative language of particle physics and...
beyond
Beyond the Standard Model
Physics beyond the Standard Model refers to the theoretical developments needed to explain the deficiencies of the Standard Model, such as the origin of mass, the strong CP problem, neutrino oscillations, matter–antimatter asymmetry, and the nature of dark matter and dark energy...
the Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
that have certain features making them attractive theoretically or for possible observations in the near future. If the model building physicist uses the tools of string theory, he or she is called "superstring model builder".
A model builder typically chooses new quantum fields and their new interactions, attempting to make their combination realistic, testable and physically interesting. In particular, an interesting
new model should address questions left unanswered in the Standard Model which has, including three massive neutrinos, 28 free parameters. A model which extends the Standard Model should predict one or more of these parameters or shed light on some other issue such as why there are three quark-lepton families or, the most common motivation, the naturalness or hierarchy problem
Hierarchy problem
In theoretical physics, a hierarchy problem occurs when the fundamental parameters of some Lagrangian are vastly different than the parameters measured by experiment. This can happen because measured parameters are related to the fundamental parameters by a prescription known as renormalization...
associated with the quadratic divergences appearing in the scalar sector.
The work of model builders
Model builders constitute a group between experimentalists and "pure" theorists; model builders are theorists, but with an emphasis on using current tools to fit data, in addition to the more long-term pursuit of a more complete theory of nature. Model builders are one step closer to pure theorists than phenomenologistsParticle physics phenomenology
Particle physics phenomenology is the part of theoretical particle physics that deals with the application of theory to high-energy particle physics experiments. Within the Standard Model, phenomenology is the calculating of detailed predictions for experiments, usually at high precision...
are, although the distinction is often blurred in practice.
Model building is speculative because current particle accelerator
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
s can only probe up to a few hundred GeV, where physics is well described by the Standard Model. One result of renormalization group
Renormalization group
In theoretical physics, the renormalization group refers to a mathematical apparatus that allows systematic investigation of the changes of a physical system as viewed at different distance scales...
theory is that at low energies, models flow toward universality classes and different models may flow to the same universality class so many models can coexist beyond the Standard Model.
Only experimental data will distinguish between them. All experimental attempts to look for irrelevant couplings beyond the Standard Model, such as those that give rise to proton decay
Proton decay
In particle physics, proton decay is a hypothetical form of radioactive decay in which the proton decays into lighter subatomic particles, such as a neutral pion and a positron...
, flavor changing neutral current
Flavor changing neutral current
In theoretical physics, flavor-changing neutral currents are expressions that change the flavor of a fermion current without altering its electric charge. If they occur in the Lagrangian, they may induce processes that have not been observed in experiment...
s, Peskin-Takeuchi parameter
Peskin-Takeuchi parameter
In particle physics, the Peskin–Takeuchi parameters are a set of three measurable quantities, called S, T, and U, that parameterize potential new physics contributions to electroweak radiative corrections...
s and nonuniversal couplings have so far come up only with upper bounds. However, the observation of neutrino oscillation
Neutrino oscillation
Neutrino oscillation is a quantum mechanical phenomenon predicted by Bruno Pontecorvowhereby a neutrino created with a specific lepton flavor can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates...
s could be explained by adding a Majorana
Majorana fermion
In physics, a Majorana fermion is a fermion which is its own anti-particle. The term is used in opposition to Dirac fermion, which describes particles that differ from their antiparticles...
neutrino mass, which is an irrelevant coupling that could arise, e.g., from the seesaw mechanism
Seesaw mechanism
In theoretical physics, the seesaw mechanism is a mechanism within grand unification theory, and in particular in theories of neutrino masses and neutrino oscillation, where it can be used to explain the smallness of observed neutrino masses relative to those of quarks and leptons.There are several...
.
There may be patterns among the marginal and relevant couplings
Renormalization group
In theoretical physics, the renormalization group refers to a mathematical apparatus that allows systematic investigation of the changes of a physical system as viewed at different distance scales...
so that among all the possible models which give rise to the Standard Model universality class, one of them has fewer free parameters than the others allowing it to make predictions. It is an article of faith amongst some model builders in particle physics that there exists an elegant high-energy theory or UV completion
UV Completion
In theoretical physics, ultraviolet completion, or UV completion, of a quantum field theory is the passing from a lower energy quantum field theory to a more general quantum field theory above a threshold value known as the cutoff...
of the Standard Model.
Examples of models
- Minimal Supersymmetric Standard ModelMinimal Supersymmetric Standard ModelThe Minimal Supersymmetric Standard Model is the minimal extension to the Standard Model that realizes supersymmetry, although non-minimal extensions do exist. Supersymmetry pairs bosons with fermions; therefore every Standard Model particle has a partner that has yet to be discovered...
(MSSM) and extensions such as the NMSSMNMSSMIn particle physics, NMSSM is an acronym for Next-to-Minimal Supersymmetric Standard Model. It is a supersymmetric extension to the Standard Model that adds an additional singlet chiral superfield to the MSSM and can be used to dynamically generate the \mu term, solving the mu problem.For review... - Models with extra dimensionsExtra dimensionsSeveral speculative physical theories have introduced extra dimensions of space for various reasons:*Kaluza-Klein theory introduces extra dimensions to explain the fundamental forces other than gravity ....
, such as the Randall-Sundrum modelRandall-Sundrum modelIn physics, Randall–Sundrum models imagine that the real world is a higher-dimensional Universe described by warped geometry...
s and models with large extra dimensionLarge extra dimensionIn particle physics, models with universal extra dimensions propose that there are one or more additional dimensions beyond the three spatial dimensions and one temporal dimension that are observed...
s - Little HiggsLittle HiggsIn particle physics, little Higgs models are based on the idea that the Higgs boson is a pseudo-Goldstone boson arising from some global symmetry breaking at a TeV energy scale...
models - Grand unified theories