On the Trail of the Higgs Boson

Scientists at CERN, a multinational research center based in Geneva, Switzerland, announced on July 4 that they had found a new elementary subatomic particle that has properties consistent with the hypothesized Higgs boson, which supposedly gives mass to all other particles. The finding also appears to validate the Standard Model of particle physics, the overarching theory describing the dynamics of subatomic particles that has underpinned the field for a half-century.

To create the conditions to find the elusive particle, scientists forced protons to smash into each other at extraordinarily high speeds in the Large Hadron Collider (LHC) at CERN, and then had to sift through trillions of such collisions looking for evidence of the particle.

Tufts scientists were part of the large, international team that made the discovery, in particular working in the ATLAS group, one of two searching for evidence of the Higgs boson. The Tufts team comprises professors Krzysztof Sliwa and Austin Napier, both of whom specialize in experimental high-energy physics; Hugo Beauchemin, an assistant professor of physics; postdoctoral associates Sharka Todorova-Nova and Evelin Meoni, both based at CERN; and graduate students Samuel Hamilton and Jeffrey Wetter.

Tufts Now asked Napier about the finding, the role he and other Tufts scientists played and what lies ahead.

Tufts Now: Can you explain the findings related to the Higgs boson at CERN?

Austin Napier: Discovering the Higgs particle is like finding the Holy Grail of physics—it has taken 45 years. One of the primary goals of the Large Hadron Collider (LHC) at CERN was to discover the Higgs boson, the only particle in the Standard Model of particle physics that has remained undetected since its prediction back in 1964. The mass of the Higgs boson is not predicted by the Standard Model, but once that mass is known, the theory can be used to calculate the probability of producing the Higgs particle per proton-proton collision.

The mass of the Higgs particle was expected to be very large for subatomic particles—at least 120 times the mass of a proton, which implies a low rate of production. Therefore, very high-energy particle beams were needed to produce it, and very large numbers of interactions were needed to accumulate a small number of candidate events.

The LHC, at 27 kilometers in circumference—or more than 16 miles—is the largest machine in the world. The $3.5 billion device uses counter-rotating beams of protons to recreate, in miniature, the conditions that existed right after the Big Bang. The protons in each beam are accelerated to 4 tera-electron-volts (TeV) to produce 8 TeV proton-proton collisions inside the four large particle detectors. After collecting data for three years, the two large general-purpose particle detectors, ATLAS and CMS, finally accumulated enough collision events to independently claim discovery of a new particle with a mass of about 126 giga-electron-volts (GeV), which so far appears to have the properties expected for the Higgs particle in the Standard Model.

Why was it so difficult to find this particle?

Since the Higgs mass was not predicted by the Standard Model, physicists had to search over a very wide mass range. We expected the Higgs particle to be heavy and to decay into many different final states, as established by previous unsuccessful searches over many years. The new particle is observed to often decay into two gamma rays, and this final state has the highest statistical significance at present.

Both the ATLAS and CMS detectors have good resolution for the detection of gamma rays, but hundreds of trillions of collisions were required to get enough events to claim discovery of this new particle.

What does the finding mean?

If the new particle is indeed the Higgs particle of the Standard Model, it will be a triumph for that model. It is remarkable that the Standard Model works as well as it does, and it is important to measure the decay modes of the new particle to check consistency with the Standard Model. Any deviations from the Standard Model predictions would be clues suggesting that the Standard Model might need to be revised.  

The LHC running period has been extended for several months to allow additional Higgs candidate events to be collected. If further study shows the properties of the new particle to be inconsistent with the Standard Model’s Higgs boson, then new theoretical work will be needed to understand the discrepancies.

For example, there is the minimal supersymmetric extension to the Standard Model, or MSSM, a variant on the Standard Model that includes new supersymmetric particles, none of which has yet been detected. The MSSM requires the existence of several Higgs particles, some possessing electric charge and some neutral, and the particle just discovered might be the least massive member of a family of Higgs bosons. Such extensions to the Standard Model could explain the existence of dark matter, which appears to dominate much of the mass in the universe. After the present run ends in late 2013, the LHC will be upgraded by 2015 to double the collision energy and increase the likelihood of new discoveries, such as one or more supersymmetric particles, if the MSSM is valid.

What was the role of Tufts scientists in the research effort at CERN? 

Tufts University is a member of the Boston Muon Consortium that was formed in the 1990s and includes Boston University, Brandeis, Harvard, MIT and UMass-Amherst. This collaboration designed and built monitored drift tubes for the ATLAS muon detector, which was used for study of the four-lepton final state decays of the new particle. 

Our group provides expertise in the study of top quarks, bottom quarks and quark jets, and in the search for exotic final states. The bottom and top quarks are the heavier quarks, which are elementary subatomic particles. The other quarks are named up, down, charm and strange quarks. Each quark has its own anti-quark. When a proton-proton collision produces a quark–anti-quark pair, the energetic quark will produce a “jet” of particles, both charged and neutral, which can be identified in the detector. Careful studies have resulted in reliable ways to identify the type of quark that has produced a given jet. Exotic particles are those particles not predicted by the Standard Model.  

The Tufts UIT group has provided access to the extensive computing resources and network support necessary for our continuing ATLAS research efforts. The Tufts group is a strong team player in the large ATLAS collaboration of 3,000 physicists, and we are excited to be a part of this new discovery and the continuing studies that may yet reveal new surprises.