Beyond the Standard Model
In particle physics, Beyond the Standard Model generally refers to developments in the fields theoretical and high energy physics that have taken place since the initial formulation of the Standard Model (SM) in the mid 1970s. The Standard Model, the first complete theoretical description of all subatomic processes, is often hailed as one of the crowning achievements of twentieth century theoretical physics. It consists of the union ofquantum electrodynamics (QED), electroweak theory, quantum chromodynamics (QCD), and Einstein’s theory of general relativity. Coupled with theLambda-CDM model of cosmology, this provides the first consistent scientific model of the universe.
However, there are several areas where the Standard Model does not adequately describe, nor align with experimental observation. For instance, certain formulas in the Standard Model require that neutrinos be massless; however, over the past two decades experiments have shown thatneutrinos possess a small, but non-zero mass. Thus, some modification to the Standard Model becomes necessary. Another problem lies within the Standard Model itself: the mathematical framework of the quantum theories of the Standard Model is inconsistent with that of general relativity, the SM’s theory of gravitation, to the point that both theories break down in their descriptions under certain conditions (e.g. The Big Bang, within the event horizons of black holes, etc). Moreover, while Lambda-CDM accurately models these cosmological phenomena to within observation, the Standard Model offers no explanation for the source of Dark Energy, nor the identity of Dark Matter.
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 believe that such theories are the next theoretical step toward a true Theory of Everything. Until recently, theories of quantum gravity such as loop quantum gravity and others were believed 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, in August 2009 an experiment conducted aboard the Fermi Gamma-ray Space Telescope experimentally ruled out all existing theories of spacetime-quantized gravity beyond their theoretical tolerances. As all said hypotheses of the time predicted the existence of non-minute Lorentz violations, violations of a consequence of special relativity known as Lorentz covariance, this experiment had the effect of invalidating all theories of quantized spacetime to an extraordinarily high statistical power (with a pβ greater than 12). A paper that was coauthored by over two hundred associated physicists announced this confirmation ofLorentz covariance was submitted to and featured in Nature and posted on arXiv.
Thus, at the moment string theory is the only remaining body for the redress of shortcomings of the Standard Model, and currently the only route in the search for viable candidate theories of Everything. Among such candidates, M-theory, whose mathematical existence was first proposed at a String Conference in 1995, is believed by many to be a proper “ToE” candidate, notably public faces 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 alternative 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.
There are several areas where “Beyond the Standard Model” physics focuses:
- the hierarchy problem
- the missing matter problem (dark matter and dark energy)
- the cosmological constant problem
- the strong CP problem
In addition to these subjects, there are also attempts at relating different phenomena and parameters to a more fundamental theory. A partial classification of these attempts are:
- gauge coupling unification
- a theory of quark masses and mixings
- a theory of neutrino masses and mixings
