UI Physicists contribute to discovery of Higgs-like particle

Urbana—University of Illinois physicists celebrated the announcement on July 4 from CERN, the European Organization for Nuclear Research based in Geneva, Switzerland, regarding the discovery of a new subatomic particle consistent with the long-sought Higgs boson—the final missing piece in the Standard Model (SM) of particle physics. Since 1994, University of Illinois researchers have been heavily involved in the design, building, commissioning, data taking, and data analysis at the ATLAS experiment at CERN.

Said Professor of Physics Steven Errede, “Major discoveries in high-energy physics such as this one do not come often, so I am certain this will be remembered by all of us for the rest of our lives.”

The SM describes the fundamental particles from which every visible thing in the universe is made and the forces that govern them. First theorized in 1964, existence of the Higgs boson would explain how subatomic particles acquire their mass. And while further analysis is needed before this discovery can be positively identified as the Higgs particle, the latest preliminary findings announced Wednesday are cause for excitement.

The discovery is the result of two years of analysis of proton-proton collisions at the Large Hadron Collider (LHC) at CERN. Two independent experiments, ATLAS and CMS, each observed the particle in the mass region around 125 GeV. Publication of the results is expected by the end of July.

Said Professor of Physics Mark Neubauer, “Before Tuesday's announcement, all particles in the SM have been definitively observed by scientists except for the Higgs boson. What physicists working on the ATLAS and CMS experiments have discovered is a new particle that looks very much like the elusive Higgs boson. The new particle is the heaviest boson ever produced with a mass of 125 GeV—that’s 133 times the mass of the proton, or approximately the mass of a single cesium atom.”

The discovery was long in coming because the particle itself cannot be directly observed. Since the particle decays a tiny fraction of a second after it is created, detectors at ATLAS and CMS look for secondary particles and sprays of particles called “jets” expected to result from a Higgs boson-type particle. Since other far more common processes can mimic Higgs decay, it has taken a multinational team of physicists years to sort through the data and arrive at this momentous announcement.

University of Illinois high energy physicists Steven Errede, Deborah Errede, Tony Liss, and Mark Neubauer, along with their many team members, have worked with the ATLAS collaboration since Steven Errede first joined the collaboration in 1994.

The contributions of the Illinois team have been substantial and far-reaching.

Steven Errede, Deborah Errede and their team (including three technicians and more than 20 physics undergraduate students) built and installed a major portion of the ATLAS detector, called the Scintillating Tile Hadron Calorimeter, or TileCal for short.

Neubauer’s group works on a part of the ATLAS detector called the “trigger,” a sophisticated assembly of electronics that analyzes the collisions in real time and determines which results are interesting enough to keep for further analysis. This team played a leading role in the search for a high mass (240-GeV to 600-GeV) Higgs boson that decays to W boson pairs that subsequently decay into a high-energy electron or muon, large missing transverse energy (from an unobserved neutrino), and two high-energy jets.

Neubauer’s group also investigated the 130-GeV to150-GeV excess in ATLAS data using sophisticated multivariate analysis techniques that make maximal use of the available information in the Higgs candidate events.

Starting in 2011, Neubauer also spearheaded a project with senior research physicist David Lesny to build a Tier-2 computing center at the UI in collaboration with the NCSA. With 15 petabytes of data generated by LHC experiments each year, it is an enormous technological challenge to process and share the data with thousands of physicists around the world.

The Tier-2 center at UI leverages the substantial expertise and infrastructure available to UI researchers in high-performance computing to help meet this challenge as a resource within the Worldwide LHC Computing Grid (WLCG). Online since last winter, the UI Tier-2 center contributed to the processing of the data used in the Higgs discovery.

The Liss group works on the ATLAS muon system, one of the key detector components for identifying the final states of some of the decay modes of Higgs boson. His group has been studying the top quark, the last big discovery in high-energy physics, which is interesting in its own right, and can mimic the Higgs signal and must be understood so it can be separated from the new particle.

Liss said, “The discovery is of absolutely fundamental importance. The SM has been enormously successful for 50 years and explains nearly every experimental measurement made over that period.  But the SM requires a mechanism for separating the electromagnetic and weak forces (weak forces are responsible for some kinds of radioactive decay—such as those that make the sun shine). This effect is called electroweak symmetry breaking (EWSB).  In the SM, EWSB occurs via the Higgs mechanism, and the Higgs mechanism requires a Higgs boson. The Higgs boson, in turn, is what gives mass to the fundamental particles.

“We know that electroweak symmetry is broken—this is a measurable fact of nature—the electromagnetic force is much stronger than the weak force. So something has to do it, and now we are on the verge of establishing what it is that does it. I say “on the verge” because even though we have discovered this new particle, we don’t know quite yet that it is the SM Higgs boson; it could be a more exotic Higgs boson. That would be the greatest thing! It would point the way to things beyond the SM, which we know must be there. But that’s another story.”

The next goal for the ATLAS and CMS teams will be positive identification of the new particle’s characteristics, and this will take considerable time and data.

Said Steven Errede, “In reality, the Higgs work is just beginning. In the near- and far-term future, we and our ATLAS collaborators will be working to obtain a more precise measurement of the Higgs boson mass. There are also many other properties of the Higgs boson that we will be measuring, to compare with their SM predictions.

“We will also ask, are there more Higgs bosons out there, perhaps at higher masses? The SM predicts only one such particle, but in other theoretical models, more than one Higgs-type boson are predicted. If another Higgs boson were to be found, then that would tell us that nature has a more complicated solution for EWSB than those proposed by the three original groups of Higgs particle theorists.”

Said Deborah Errede, “ We have anticipated the discovery of the Higgs particle, fundamental to the fabric of our SM of elementary particles physics, since its introduction by Steven Weinberg and Abdus Salam in 1967, for over forty years. The collaboration of nations accomplishing this achievement is truly remarkable and we at the University of Illinois are proud to have contributed to this.”

ATLAS collaborators at the UI Physics Department include Professors Steven Errede, Deborah Errede, Tony Liss and Mark Neubauer; visiting Associate Research Professors Nectarios Benekos, Viviana Cavaliere, and Irene Vichou; Senior Research Physicist David Lesny; systems programmer Larry Nelson; graduate students Markus Atkinson, Austin Basye, James Coggeshall, Philip Chang, Arely Cortes Gonzalez, Ki Lie, Hovhannes Khandanyan (graduated March, 2012) and Allison McCarn; and undergraduate UI students Ian Dayton, Matthew Feickert, Tony Thompson, Tim Thorp, and Brian Wang, and REU external student Julia Gonski .

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