Atul Gurtu


“4 July 2012: The most expensive and painstaking hunt in fundamental science is over and a new chapter is beginning. The Higgs boson is finally pinned down; the so-called ‘God Particle’ unveiled. Elation, not only among the practitioners of particle physics, but headlines the world over.” (Atul Gurtu: INSAF Bulletin 125 September 2012).


Discovery of the “Higgs-like” particle at CERN was announced down-under at the High Energy Physics Conference at Melbourne in July 2012. But scientists were looking for statistically more clinching evidence by study of additional data. Thereafter the Large Hadron Collider at CERN, Geneva, Switzerland, continued to run beautifully and the two experiments ATLAS and CMS were able to collect as much more data as was available at the time of the July 2012 announcement. Results were presented recently, on 6th March 2013, during the “Rencontres de Moriond” conference at La Thuile in the Aosta Valley, Italy.


So what were the scientists looking for this time? They wanted to increase the significance of the signal they were seeing, they wanted to determine more accurately its branching ratios (how frequently it decays into various possible combinations of particles) and finally, they wanted to determine its spin and parity. Every fundamental particle has an intrinsic spin associated with it and an additional, more esoteric, property called parity which determines its behaviour if one views its mirror image. It’s only by comparing these properties with that expected of a Higgs that one can draw firm conclusions about the nature of the new particle.


Both ATLAS and CMS presented preliminary findings based on the additional data collected. The statistical significance of the mass peak has now increased as expected, so now one can say even more emphatically that a new particle at 125 GeV mass has indeed been discovered. The ATLAS and CMS results for decay into ZZ are given in figure 1 below where each Z decays into an electron or muon pair (leptons).


Decays of this particle into a pair of gamma rays, into a pair of Z particles, into W+W_, into τ+ τ_, and into bb̅ have now been studied. The last two decay modes are important as the τ (tau lepton) and the b (quark) are spin 1/2 particles whereas the gamma, Z and W are spin 1. The branching fractions so determined are completely in accordance with expectations from the standard model Higgs particle. Furthermore, the analyses for determination of its spin-parity indicate agreement with its standard model Higgs nature.


Summary and Outlook: The newly discovered particle looks more and more exactly like the standard model Higgs. Final analyses of the experimental results will be published over the course of time, with the next update due at the time of the summer conferences in July/August 2013. The LHC has now entered a long shut-down mode during which its energy will be upgraded from 8 to 13 TeV and the all the detectors will be augmented/upgraded. It will come back on-line in 2015 and many unanswered questions will be much better and much more quickly addressed during its next run. But for the present one can celebrate the discovery of a new particle, the much heralded Higgs boson.


[Atul Gurtu, retired from TIFR, Mumbai, was Indian Spokesperson in CMS, 2003-2011; Currently Adjunct Professor, University of Petroleum & Energy Studies, Dehradun, India]

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