UAlbany Physicists Help Analyze Higgs Boson

An event candidate for the production of a top quark and top antiquark pair in conjunction with a Higgs boson in CMS. (Graphic courtesy CMS/CERN)

ALBANY, N.Y. (Aug. 8, 2018) — New results from the ATLAS and CMS experiments at the Large Hadron Collider (LHC), CERN, Switzerland, reveal how strongly the Higgs boson interacts with the heaviest known elementary particle, the top quark, and UAlbany researchers are contributing to the analysis.

The Higgs boson is the keystone of the Standard Model of particle physics, which has been developed over the last 50-60 years. The Higgs boson gives mass to other elementary particles such as quarks and electrons, and its existence is necessary for the theory to give sensible results (i.e., the probability of processes to occur cannot be greater than 100 percent).

The Higgs boson is also seen as a portal to new physics, according to scientists taking part in the ATLAS and CMS research projects. ATLAS, or A Toroidal LHC ApparatuS, and CMS, or Compact Muon Solenoid, are the names given to the two general-purpose detectors in operation at the LHC. While both projects involve studying particle interactions, they use different detection methods in order to produce findings.

Named after Nobel Laureate Peter Higgs, who along with other physicists proposed the existence of the particle in 1964, the Higgs boson’s existence was not confirmed until 2012, when both ATLAS and CMS claimed observation of a new neutral scalar boson, with virtually identical conclusions on its mass.

Since this time, researchers at CERN have sought to determine the precise properties of this new particle, and how it interacts with other particles, most notably the “top quark,” which is the most massive of all observed elementary particles. This is done by observing the production of the Higgs boson in association with a top – anti-top quark pair. The top quark’s large mass (about that of a gold atom) implies that it interacts very strongly with the Higgs boson.

Associate Professor of Physics Vivek Jain
Associate Professor Vivek Jain and colleagues in the Department of Physics are contributing to analyses of the Higgs boson.

Observing this extremely rare process is a significant milestone for the field of High-Energy Physics. It allows physicists to test critical parameters of the Higgs mechanism in the Standard Model, and sets constraints on new physics.

“This measurement constitutes a landmark achievement in the exploration of the Higgs mechanism and the interaction of Standard Model particles with the Higgs boson,” said Karl Jakobs, ATLAS Spokesperson. “As only one percent of all Higgs bosons are produced in association with top quarks, its observation was extremely challenging. ATLAS physicists examined five years of collision data to arrive at this result.”

Physicists at the University at Albany have been members of the ATLAS experiment for more than 10 years, and have made significant contributions to the analyses that led to the latest result.

Ph.D. student Julian Bouffard, and post-doctoral researcher Stefan Guindon joined Associate Professors of Physics Jesse Ernst and Vivek Jain on a paper published in 2015 in Physical Review, where strict upper limits were set on the rate of this process.

Later, Guindon, Jain and Ph.D. student Stewart Swift collaborated on a paper published in 2017 in The European Physical Journal, which presented the first hints for this rare process.

ATLAS result exploits the full dataset delivered by the Large Hadron Collider and establishes the signal with a statistical significance of 6.3 standard deviations. It agrees with a recent observation by the CMS Collaboration with a statistical significance of 5.2 standard deviations using a smaller dataset -- a 5 standard deviation result implies that the probability that the signal is a statistical fluke is less than 1 in about 3.5 million, which is the standard criteria in High-Energy Physics for claiming an observation of a new effect. A result with 6.3 statistical significance implies that the probability of it being a fluke is less than 1 in 100 million.

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