CERN further tightens limits on elusive Higgs particle

New results with improved data on the search of the elusive Higgs boson, presented on Monday at the biennial international symposium on Lepton-Photon Interactions at the Tata Institute of Fundamental Research (TIFR) here, have excluded the existence of the Higgs particle over virtually all the mass regions from about 145 Giga electron-Volt (GeV) to 466 GeV with 95 per cent certainty; that is, there is less than one-in-twenty chance of Higgs having a mass in this range. The Higgs particle is the only missing and crucial piece in the otherwise enormously successful theory of fundamental particles and their interactions called the Standard Model (SM).

GeV is a unit of energy in which elementary particle masses are measured in accordance with Einstein's energy-mass relation and proton has a mass of about one GeV. As a CERN (European Organization for Nuclear Research) release said, “Higgs particle, if it exists, is running out of places to hide.” The only mass range where Higgs, if it exists, is hiding from experiments is 114-145 GeV on which much of the search in the ongoing experiments at the Large Hadron Collider (LHC) in Geneva will focus with much more improved data. LHC is the most powerful particle accelerator in the world today in which two high-energy proton beams at an energy of 3.5 TeV each collide head-on to produce a multitude of particles.

The SM model itself does not predict a value of the mass of Higgs but the existence of Higgs is important for the survival of the model. For, it is only with Higgs that one can explain the origin of masses of the various fundamental particles of the theory.

The data presented included those from the important ATLAS and CMS at the LHC. In the debris of the proton-proton collisions with the total available energy of 7 TeV, one looks for signatures of Higgs through its dominant decay modes. The difficulty is in picking out these signals from a large background from other known processes that could mimic those signals.

So far these searches have excluded the entire mass region allowed by the SM except for a tiny window between 114 GeV and 145 GeV, said Aleandro Nisati of the ATLAS collaboration and Vivek Sharma of the CMS collaboration. Early experiments, most notably the LEP at the CERN, ruled out Higgs in the mass range below 114 GeV given the lower available energy in those searches.

The current analysis is based on roughly twice the data sample presented at the Europhysics Symposium (EPS) on high-energy physics held at Grenoble, France, in July, where it was suspected that there were possibly some hints of sighting Higgs in some region, which the improved data have not been able to substantiate any more, if not exactly less. Even at Grenoble, the two experiments were conservative enough to stress that the excess events seen could be explained by statistical fluctuations.

The ATLAS, in particular, looked at nine different decay modes of Higgs and excluded a mass range wider than the results presented at Grenoble. Besides the mass range 115-146 GeV, the only mass ranges left for ATLAS to search for Higgs are in the following narrow windows: 232-256 GeV, 282-286 GeV and over 466 GeV. The present data have not seen any enhancement of the excess events seen at Grenoble in one region, widely reported in the media then as a possible hint for Higgs.

According to Aleandro, who presented the ATLAS data, by the end of 2011, this data sample may double again and increasing statistics may rule out the excess events as being due to Higgs at 95 per cent confidence level. However, if there is an enhancement, further analysis to confirm Higgs would take about a year more before the LHC shuts down for maintenance and repair for about 18 months. This shutdown period was earlier stated to be about 15 months but CERN Director-General Rolf Heuer said, on the sidelines of the symposium, it would be more like 18 months.

“If Higgs does not exist, by the end of this year we should be able to say definitely so,” said Mr. Sharma, who is heading the Higgs search in the CMS collaboration and presented the CMS results at the conference. Like ATLAS, CMS is also looking at different decay modes of Higgs. The number of collisions that the CMS studied between July and now marked an increase of about 60 per cent from 100 trillion proton-proton collisions studied until then, said Mr. Sharma.

CMS too has not observed any enhancement of events in the mass range between 110 and 600 GeV but can rule with 95 per cent certainty the existence of Higgs in the mass ranges, 145-216 GeV, 226-288 GeV and 310-400 GeV. In fact, CMS can exclude the mass region 144-440 GeV at 90 per cent confidence level.

Except for the mass range below 145 GeV other narrow windows would certainly go away as mere statistical fluctuations, said Mr. Sharma. Now the focus of CMS too will be on the mass range 114-145 GeV to see that the modest excess events seen over the expected are enhanced with improved data to be collected till end-2012 or can also be excluded as statistical fluctuations, he added.

  • New results on Higgs boson presented at Mumbai symposium
  • “If Higgs does not exist, by the end of this year we should be able to say definitely so”

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