The Endgame Homing in on the Higgs boson

The Higgs boson cannot be seen or detected directly. Being a heavy particle (about 125 times as heavy as a proton), it decays to lighter particles immediately after being created in a collision. Some of these decay products may decay to yet lighter particles. Depending on the final decay products, a number of techniques have been developed to search for the Higgs. There are five important search channels, with the Higgs decaying into:
two photons (diphoton channel),
two Z bosons, each of which decays into two electrons or two muons (4-lepton channel),
two W bosons, each of which decays into an electron or a muon and a corresponding neutrino (lvlv channel),
two tau leptons, which can decay in various ways (tautau channel),
two bottom quarks, with the Higgs being produced in association with a W or a Z boson (bb channel).
Of these, the first two channels are the most sensitive, in that they provide the cleanest signatures of Higgs-like events. In each of these two channels, a number of events were found that looked very Higgs-like.
But when and how was it decided that a particle had been discovered?
In high-energy physics, a specific quantitative measure is used to determine whether or not a discovery has occurred. This is necessary because, for example, events that are not from Higgs boson decays can look like Higgs decays, thus generating a fake Higgs signal. A particular set of events constitutes a discovery when the probability of these events being fakes is less than about 3 parts in 10 million. This is a very small number, and means that the analyzers are highly confident that what they see is a real signal. This probability is interpreted in terms of a significance: the significance necessary to claim a discovery is 5 sigma.
With the data collected by June 2012, ATLAS found that while the diphoton and 4-lepton channels individually had significances of less than 5 sigma, a combination of the two yielded a significance of 5 sigma. At that point, ATLAS was ready to declare a discovery. Meanwhile, the CMS experiment had been analyzing their own data, and they also found that a combination of the diphoton and 4-lepton channels gave 5 sigma significance. So the ATLAS results were independently confirmed. Moreover, the mass of the Higgs boson measured in each of the two channels by each experiment was around 125 GeV, which strengthened our conviction that we are all seeing the same particle. For comparison, the mass of a proton is a little less than a GeV.
The two experiments announced their discoveries in a heavily attended seminar at CERN on the morning of July 4, 2012. I have written about my emotions and experiences on that day previously in a short article, so I will not repeat them, except to say that this was the event of a lifetime in a very true sense.
Progress since the discovery
The Higgs discovery was not the end of the story by any means, since much remained to be done. For a starter, it is not known for certain that the discovered particle is in fact the Higgs boson predicted by the Standard Model: there are many models of particle physics that go beyond the Standard Model and predict one or more Higgs or Higgs-like particles. To ascertain whether this is the Standard Model Higgs, it has to be observed in all five of the decay modes listed in the previous section.
Since July 2012, a large amount of additional data has been collected, and the search for the Higgs in the lvlv, tautau and bb channels has continued in addition to the two sensitive channels. None of the less sensitive channels is able to discover the Higgs on its own as yet, but some of them show hints of a Higgs signal. With all channels combined, the significance currently stands at 7 sigma, meaning that the probability of the discovery being spurious is 1 part in a trillion!
Another important step is the measurement of the spin and parity quantum numbers of the discovered particle. Any Higgs-like particle must have spin 0, while the Standard Model Higgs must have even parity. These measurements have already started. At this point, it is definitely starting to look like the Standard Model Higgs, or at least a very Standard Model-like Higgs.
What lies ahead…
2012 has been a phenomenal year for us; Higgs search and measurement results using the full 2012 dataset will be presented at major conferences in the spring and summer of 2013. It is expected that these measurements will go a long way toward answering the vital question: is this the Standard Model Higgs?
Notwithstanding, a full closure of the issue will require much more data than we have now. After a two-year break for upgrades, the LHC will start taking data again in 2015, at the substantially higher collision energy of 13 TeV. The event rate will also be higher, leading to a faster accumulation of Higgs-like events. We will be prepared to analyze these events, increase the accuracy of our measurements and explore the Higgs sector in detail.
Throughout all the Higgs excitement, we firmly keep in mind that this is only one of the principal goals of the LHC. High-energy physics at this crossroads faces a number of intriguing questions: What constitutes the so-called Dark Matter? Is SuperSymmetry a fuller explanation of nature than the Standard Model? Are quarks and leptons really elementary particles, or are they composed of something even more fundamental? We will be looking to answer these questions and more as the LHC program continues through the 2010s, the 2020s and possibly the 2030s.

The writer did his undergraduate studies at Yale University, received a PhD in Physics from Harvard University, and is now a postdoctoral researcher with CERN/University of Wisconsin-Madison. He is based at CERN in Geneva, Switzerland.