Evidence of the Higgs boson in LHC data?--UPDATE

UPDATE – Batavia, Ill. and Upton, NY – Aug. 22, 2011 – Two experimental collaborations at the Large Hadron Collider, located at CERN laboratory near Geneva, Switzerland, announced on August 22 that they have significantly narrowed the mass region in which the Higgs boson could be hiding.

The ATLAS and CMS experiments excluded with 95 percent certainty the existence of a Higgs over most of the mass region from 145 to 466 GeV. They announced the new results at the biennial Lepton-Photon conference, held this year in Mumbai, India.

“Each time we add new data to our analyses, we close in more on where the Higgs might be hiding,” said Darin Acosta, a University of Florida professor and deputy physics coordinator for the CMS experiment.  Read more...

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July 26, 2011 – Analysis of the latest data from experiments at CERN's Large Hadron Collider and Fermilab's Tevatron Collider were presented at the International Europhysics Conference on High Energy Physics (EPS) in Grenoble, France last week. A buzz of excitement was generated by the results that were presented by the ATLAS and CMS collaborations on searches for Higgs boson production in high energy (7-TeV center-of-mass energy) proton-proton collisions.

The Higgs boson, which has been predicted by the standard model of particle physics but never experimentally observed, is believed to give mass to known particles in the process of spontaneous electroweak symmetry breaking. Despite decades of experiments at ever-more-powerful accelerators and higher collision energies, the Higgs remains the only standard model particle not directly observed.

While the standard model does not predict the exact mass of the Higgs, scientists can put bounds on its mass through direct searches for its production and decay and by observing subtle effects it has on other observables in electroweak theory.

Searching for the Higgs

“It’s like detective work,” said UI physicist Mark Neubauer. “Just as a technician examines the scene of a crime and concludes that the suspect had to be at least 6 ft tall, right-handed, and employed as a bricklayer based on the forensic evidence, physicists look at the massive data sets generated in collisions at high-energy particle accelerators and look for traces of the elusive Higgs.”

The results presented at EPS by the ATLAS and CMS collaborations show sensitivity to a previously unexplored region of high Higgs mass (>200 GeV) and place strong constraints on the Higgs mass over 120-600 GeV. The ATLAS experiment excludes a Higgs boson having mass in the 155–190-GeV and the 295–450-GeV ranges, while the CMS experiment rules out a Higgs boson in the ranges 149–206 GeV and 300–440 GeV at 95% confidence level.

“Knowing where the Higgs is not helps us concentrate our search,” said Neubauer. “Intriguingly, both experiments observe a modest excess of events in the 130-150-GeV mass range, as compared with what would be expected in the absence of a Higgs boson, as shown in this plot,” said Neubauer.

“The excess depends on the experiment and the exact mass hypothesis being tested, but largest measured excess is in the 2.7 to 2.9 standard deviations (σ) range. While these significances are below the 3σ (99.7%) threshold required to claim “evidence” in particle physics and we must carefully consider the probability of observing a similar level of excess at any of the hypothesized masses just by chance, the combined ATLAS and CMS analysis will likely exceed the 3σ evidence threshold,” said Neubauer. He also noted that in the ATLAS result, contributions to the excess come from three independent search channels.

The ATLAS Group at Illinois

Physicists from the University of Illinois have been a part of the ATLAS collaboration since Professor of Physics Steve Errede started the group in 1994, and UI researchers contibuted directly to the Higgs results shown at EPS. The group includes Professors Tony Liss, Mark Neubauer, and Research Professor Deborah Errede.

Specifically, Neubauer and members of his group (postdoc Viviana Cavaliere, graduate student Markus Atkinson, and undergraduate Matthew Feickert) played a leading role in the search for a high mass (240-600 GeV) Higgs boson decaying 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.

Other Neubauer group members (graduate student Phillip Chang and undergraduate Brian Wang) are currently investigating the 130-150 GeV excess in ATLAS data using sophisticated multivariate analysis techniques that combine available information in the Higgs candidate events.

What's next?"

"If there is a Higgs boson in the low mass region that is suggested by the direct searches using LHC data and also analysis of precision electroweak data collected by other expirements, then the modest excesses seen by multiple experiments and in multiple search channels is exactly the type of behavior we would expect at this stage," said Neubauer. "More sensitive analysis techniques and additional data to be collected by the end of this year should enable a definitive statment on a potential Higgs signal in this mass range. It’s really great to see the LHC experiments making such a big splash at this early stage in its physics program."

In other news, Cavailiere presented at the EPS conference new results showing that the significance of the unexpected bump in the dijet invariant mass spectrum at 150 GeV (unrelated to the Higgs search, see http://physics.illinois.edu/news/story.asp?id=1231) grows to 4.1σ with additional data from CDF and that an analysis of data from the Dzero experiment does not confirm the CDF signal (the two results are compatible at the 2σ level).