Perhaps best known in the field of particle physics as the co-author of the Higgs Hunter’s Guide, Jack Gunion has been in the theoretical trenches of the search for the Higgs boson for several decades now. He is a senior professor and leader of the theoretical particle physics group at UC Davis, where he has been a member of the faculty for over 25 years. Here is a guest post from him on today’s big news from CERN.
Tuesday December 13 has been a very exciting day for particle physics. The ATLAS and CMS experiments at the Large Hadron Collider (LHC) announced today that they are both seeing hints of a Higgs boson with properties that are close to those expected for the Standard Model (SM) Higgs boson as originally proposed by Peter Higgs and others. While the “significance” of the signals has not yet reached “discovery level” (5 sigma in technical language) the two experiments both see signals that exceed 2 sigma so that there is less than a 5% chance that they are simply statistical fluctuations. Most persuasively, the signals in the channels with excellent mass determination (the photon-photon final decay state and the 4-lepton final state) are all consistent with a a Higgs boson mass of around 125 GeV IN BOTH EXPERIMENTS. This coincidence in mass between two totally independent experiments (as well as independent final states) is persuasive evidence that the photon-photon and 4-lepton excesses seen near 125 GeV are not mere statistical fluctuations.
Observation of the Higgs with approximately the SM-like rate suggests that to first approximation the Higgs is being produced as expected in the SM and that it also decays as predicted in the SM. Many theorists, including myself, have suggested that a Higgs might be produced as in the SM but might have extra decays that would have decreased the photon-photon and 4-lepton decay frequencies to an unobservable level, making the Higgs boson much harder to detect at the LHC. The level of the observed excesses argues against such extra decays being very important. The photon-photon and 4-lepton detection modes were originally proposed and shown to be viable for a SM-like Higgs boson by myself and collaborators (in particular, Gordy Kane and Jose Wudka) way back in 1986-1987. It has taken a long time (25 years) for the technology and funding to reach the point where these detection modes could be examined. I often joked that I was personally responsible for forcing each of the LHC collaborations to spend the 30 million dollars or so needed to build a photon detector with the energy resolution required. Fortunately, it seems that the money was well-spent and the ATLAS and CMS detectors both found ways to build the needed detectors, a real triumph of international collaboration and technical expertise. Also key is the very successful operation of the LHC that has produced the enormously large number of collision events needed to dig out the Higgs signal from uninteresting ‘background’ events. Until this summer produced the first very weak signs of the Higgs, I was beginning to wonder if the Higgs would be discovered during my lifetime. Fortunately, simplicity (i.e. a very conventional SM-like Higgs boson) seems to have prevailed and ended my wait.
Going forward, by the end of 2012 the levels of these excesses should reach the 5 sigma “discovery” level if the SM-like Higgs really does have the mass and decays indicated by current results. Further, we will begin to have some moderately precise (20%-30% or so?) measurements of the individual decay modes of the Higgs boson that might indicate just how precisely SM-like it is. Many theories beyond the Standard Model predict the possibility of deviations from the predictions of the purely SM Higgs boson. As data accumulate, looking for such deviations will be a major focus. Current data (weakly) hint at the possibility that the Higgs production rate might turn out to be modestly larger than predicted if the Higgs is that of the Standard Model and has mass of 125 GeV — the best-fit ATLAS cross section is about 1.5 times as large as the SM prediction, whereas the best-fit CMS cross section is very close to the SM prediction. Time (i.e. more accumulated data) will tell.
Of course, the current run of the LHC will be halted at the end of 2012, followed by a lengthy shut down for upgrades to the accelerator and to the detectors. After this upgrade, the LHC will operate at a much higher energy (14 TeV) compared to the current energy of 7 TeV, and, if all goes according to plan, have a much higher collision rate. At this point, precision studies of the Higgs boson will certainly be possible. If deviations are observed, then we will strongly suspect that the SM is incomplete. Even before that time, we are hoping that by the end of 2012 we will have seen new types of particles that do not fit into the Standard Model. This, for example, is predicted if the universe is supersymmetric. Supersymmetric models tend to predict a light Higgs boson that is fairly, but not completely, SM-like with mass in the range 110 GeV to 140 GeV and so are very consistent with what is being observed. If some supersymmetric particles (so called sparticles) are observed then their masses and properties constrain the Higgs mass in a given model and consistency of the entire theory can be nicely tested. Hopefully, some version of physics beyond the Standard Model will be directly observed at the LHC, in which case there will be at least a decade of exciting observations and analyses to determine the precise beyond-the-Standard-Model theory.
Still, the pure Standard Model with no new physics cannot be totally discarded. Although this would not allow a so-called “natural” explanation of the Z boson and Higgs boson masses, the pure SM is still internally consistent even at energies close to the Planck scale for a Higgs mass greater than or equal to about 125-130 GeV. In other words, the observed Higgs mass is on a borderline. Above 125-130 GeV, new physics below the Planck mass scale is not required for internal consistency of the SM. But, had the Higgs boson mass been significantly below this (the precise border being somewhat uncertain theoretically), the SM would necessarily break down at some energy scale below the Planck scale and at this lower energy scale new physics would have to enter. In short, a SM-like Higgs boson with mass near 125 GeV is maximally interesting from many theoretical perspectives.
In any case, theorists and experimentalists are all very relieved that the LHC appears to be observing a Higgs boson thereby ensuring an extremely interesting program of physics at the LHC for decades to come. Further, such a light SM-like Higgs boson provides strong motivation for a linear electron-positron collider of low center-of-mass energy. Studies suggest that only such a collider can easily measure the properties of such a light Higgs boson at the few percent level, although the LHC might not do that much worse depending upon future improvements and upgrades.