The most immediate goal on the particle physics roadmap as part of the Energy Frontier campaign is to fully understand the unification of the electromagnetic and weak nuclear interactions into a single, “electroweak” force. Scientists expect this to occur at an energy scale of about one trillion electron volts (TeV), or the terascale. The Standard Model has successfully explained almost all particle physics below 1 TeV in energy. Above that energy range, however, a new physical mechanism must be present to confirm Standard Model predictions.
Physicists call this mechanism electro-weak symmetry breaking. The need for this mechanism arose from a problem with the Standard Model. Theoretical calculations indicated that certain so-called gauge bosons (force carriers such as the W and Z bosons) had zero mass, which obviously wasn’t the case as their masses had been measured. Still, the Standard Model was very successful in predicting many other physical phenomena. So the question became: How can gauge bosons gain rest mass, as we already know they have, without violating other parts of the theory that successfully explain many experimental observations? This problem was tackled in the 1960s by a number of theoreticians who postulated a new field and a new boson as a quantum of that field. By convention, these became known as the Higgs field and the Higgs boson, named after the theorist Peter Higgs.
For many years, research at Fermilab’s Tevatron accelerator led the Energy Frontier in search of this theoretically postulated particle. Many significant discoveries and precision measurements were made at this historical facility, such as the discovery of the heaviest quark, known as the top quark, in 1995; direct observation of CP violation in the decay of neutral kaons in 1998; and precision measurements of top quark and W boson masses. In 2012, the Tevatron saw hints of a new particle, but statistics weren’t strong enough to claim that a new particle had indeed been found.(Click here for a Tevatron timeline.)
The Tevatron, shutdown in September 2011, collided protons and antiprotons at an energy of 2 TeV. The LHC collides protons. In March 2012, the LHC achieved center of mass collision energies of 8 TeV and will ultimately reach 14 TeV. After more than two decades of intensive search, physicists may have at last found the long sought-after have Higgs boson. On July 4, 2012, the ATLAS and CMS experiments at the Large Hadron Collider (LHC) at CERN jointly announced their results to the world. Both experiments observed a new particle in the mass region around 125-126 GeV.
If the new particle proves to be the Higgs then this implies the existence of the Higgs field. Because mass is everywhere in the universe, then the Higgs field must be everywhere. Unlike gravitational and electric fields, the strength of which decrease with distance, the strength of the Higgs field must be constant. It has been likened to a giant vat of molasses spread throughout the universe through which particles wade. If a specific elementary particle is heavier than another, then it is more strongly coupled to the Higgs field. The Higgs boson, as the quantum of the Higgs field (analogous to a photon for the electromagnetic field), mediates the interaction between the particle and the field. The strength of this interaction decides the particles mass. If the new particle is not the Higgs, then some other explanation for electroweak symmetry breaking must be found and experimentally verified.
Originally scientists proposed that a single Higgs boson is the answer, but newer theories such as supersymmetry and extra hidden dimensions, suggest that multiple Higgs bosons could solve the TeV scale conundrum in the Standard Model. No matter which of these theories is proven to be correct, it will provide a deeper understanding of the fundamental nature of matter, energy, space and time. One thing is clear: the terascale will unlock a new world of physics for scientists to explore.