11. DETECTION OF HIGGS BOSON
On 4 July 2012, the ATLAS and CMS experiments at CERN’s Large Hadron Collider (LHC) announced they had each observed a new particle in the mass region around 126 GeV. This particle is consistent with the Higgs boson predicted by the Standard Model. The Higgs boson, the simplest manifestation of the Higgs mechanism, was for a long time the last undiscovered particle in the Standard Model.
In this section we will discuss how Higgs bosons were created and detected at the LHC.
LHC is the largest machine ever built by human beings. This massive accelerator is a testimony of human ingenuity and perseverance. There are 10,000 people working at CERN, over a 100 nationalities that includes countries who do not look at each other eye to eye but all have physicists working together in harmony.
LHC is a collider where two counter-circulating proton beams are made to collide instead of making them impinge on a static target. Each proton beam has about 3000 bunches of particles, with each bunch containing about 1011 protons. The energy of each proton beam is 8 TeV, so the total collision energy is 16 TeV. LHC lives in a circular tunnel 330 feet underground, 17 miles in circumference, as it is cheaper to excavate a tunnel as opposed to building something on the surface after land acquisitions and other logistical impediments. Besides, the Earth’s crust provides good shielding for radiation. The pressure in the beam pipes of the LHC is about ten times lower than on the Moon. Low vacuum is necessary to avoid collision with air molecules. LHC power consumption is massive (120 MW) and the electric current in the wires generates a lot of heat. In order to prevent wires from melting down, they are cooled to a very low temperature 1.9 K (-271.3 ºC) resulting in them becoming superconductors.
It is important to talk about the approaching protons in the two colliding beams. Special relativity dictates that the shape of individual protons should be like pancakes (normally they are spherical) when their speeds are very close to that of light. When the protons interact, it is usually the constituent quarks that interact. The result is a messy spray of hadrons (Figure 11-4) emerging from the interaction vertex, sometimes more than hundreds in a single event. These hadrons which are remnants of these collisions have to be individually identified in order to look for the Higgs particles—much like detecting a needle in a haystack. That job belongs to the two biggest experiments at LHC, ATLAS (A Toroidal LHC ApparatuS) and CMS (Compact Muon Solenoid), located on the opposite sides of the LHC ring (Figure 11-1). ATLAS is positioned near the main CERN site and CMS on the other side of the border in France.
ATLAS is the largest detector ever built for a collider. It was designed to study a variety of topics in particle physics and astronomy, starting with the search for the Higgs boson to supersymmetry (SUSY). The main feature of the ATLAS detector is, it is capable of taking a snap-shot of every single collision of a billion of such collisions. The ATLAS detector consists of eight 25‑m long superconducting magnet coils, arranged to form a cylinder around the beam pipe through the center of the detector. The collaboration team consists of more than 1900 members from 164 institutes in 35 member countries (as of April 2007). For more information, visit: http://atlas.ch/.
CMS is a general‑purpose detector with the same physics goals as ATLAS, but has different technical solutions and design. It is built around a huge superconducting solenoid. This takes the form of a cylindrical coil of superconducting cable generates a magnetic field of 4 T, about 100,000 times that of the Earth. More than 2,000 people work for CMS, who represent 181 institutes in 38 member countries (as of May 2007). For more information, visit: http://cmsinfo.cern.ch/outreach/.
A typical proton-proton collision event is shown in Figure 11-4. This particular event was recorded in the CMS detector. Notice the hundreds of tracks emanating from the event vertex—the state-of-the-art instruments commissioned inside both detectors collect precise information about every particle that is responsible for creating those tracks. A single event results in about one megabyte of compressed data. With hundreds of millions of events generated per second, it is not practical to record all the events in computer disks. Only a small number of interesting events are filtered out using triggers which are combinations of hardware and sophisticated software programs. These handful of events (out of many millions produced per second) are the candidates for further analysis. The Higgs particle (rather its decay products, see below) has to be isolated from hundreds of other particle emerging from the event vertex (Figure 11-4).
The Higgs boson is a very short lived particle. The only chance of finding it is via its decay products. The Higgs boson decays into other particles almost instantly after it is produced. Figure 11-5 and the associated table shows the various decay modes and their relative occurrence. But the most common decay modes that we are capable of “seeing” or detecting are either the two-photon or pairs decaying to four-lepton (electrons or muons) final states. The ATLAS and CMS detectors have the best mass resolution corresponding to these channels. Notice the peaks in each of the plots in Figure 11-6 corresponding to the above decay channels.
The ATLAS experiment announced Higgs discovery in a paper that appeared on the arxiv. A portion of the abstract is quoted below (submitted on 31 Jul 2012):
…Individual searches in the channels , H→γγ and in the 8 TeV data are combined with previously published results of searches for , , and in the 7 TeV data and results from improved analyses of the and channels in the 7 TeV data. Clear evidence for the production of a neutral boson with a measured mass of GeV is presented. This observation, which has a significance of 5.9 standard deviations, corresponding to a background fluctuation probability of , is compatible with the production and decay of the Standard Model Higgs boson.
Similarly, CMS published their claim (submitted on 31 Jul 2012):
…The search is performed in five decay modes: , , , , and . An excess of events is observed above the expected background, a local significance of 5.0 standard deviations, at a mass near 125 GeV, signaling the production of a new particle. The expected significance for a standard model Higgs boson of that mass is 5.8 standard deviations. The excess is most significant in the two decay modes with the best mass resolution, and ; a fit to these signals gives a mass of GeV. The decay to two photons indicates that the new particle is a boson with spin different from one.
More than 50 years after it was predicted, the Higgs was finally discovered in LHC, although earlier accelerator experiments did provide tantalizing hints of its existence. A year after it was discovered, Higgs and Englert shared the 2013 Nobel Prize for Physics (Englert’s co-author Robert Brout had died in 2011 and the Nobel Prize is not usually awarded posthumously) for building on Nambu’s ideas to predict the existence of the Higgs boson that was subsequently identified in the ATLAS and CMS experiments at CERN. It was the ingenuity of the theoreticians who came up with the concepts of symmetry breaking and of Higgs mechanism that lead to the formulation of the hugely successful electroweak theory. In turn the accelerator engineers and experimental physicists were able to build a state-of-the-art machine and the associated detectors to identify a particle that in Leon Lederman’s words:
…is so central to the state of physics today, so crucial to our final understanding of the structure of matter, yet so elusive, that I have given it a nickname: the God Particle. Why God Particle? Two reasons. One, the publisher wouldn’t let us call it the Goddamn Particle, though that might be a more appropriate title, given its villainous nature and the expense it is causing. And two, there is a connection, of sorts, to another book, a much older one…