5 Jul 2012

"This is just the beginning of a long journey"

R. Ramachandran

“As a layman, we have it,” is what Rolf Heur, Director-General of CERN, said at the end of the seminar
Yes. The long-sought Higgs boson, dubbed by lay media as ‘God Particle’ much to the dislike and discomfiture of scientists, may have been discovered. This essentially was the take home message of the much publicised and keenly awaited seminar on Wednesday at CERN, the Geneva-based European Centre for Nuclear Research. CERN houses the huge underground 27 km-long ringed particle accelerator called the Large Hadron Collider (LHC) with the search for the Higgs particle as one of its main goals.
The Higgs boson, hypothesised in the 1960s by three groups of scientists (Peter Higgs and five others) independently, is the crucial missing piece in an otherwise enormously successful theoretical framework called the Standard Model which accurately describes the fundamental particles and forces of nature. The masses of fundamental particles of nature are determined by the strengths of their interaction with Higgs. Without the Higgs particle matter in the universe will have no mass. So its existence is a vital cornerstone for describing the universe correctly.
For the lay public at large, the uncertainty signified by the use of ‘may’ above does not really matter as it arises purely from technical considerations. The two key experiments at the LHC – CMS and ATLAS – designed to look for the hypothesised Higgs particle have found unambiguous signals for the observation of a new boson. This new particle waddles like a Higgs and quacks like a Higgs. But is it the Higgs particle of the Standard Model that scientists have been searching for the last nearly four decades? At this point of time, however, LHC scientists would like to duck the question. They would like to ascertain other characteristics of the new particle before definitively dubbing it as the Standard Model Higgs.
“As a layman, we have it,” is indeed what Rolf Heur, the Director-General of CERN, said at the end of the seminar. But more correctly, he added: “[We have] observed a new particle consistent with a Higgs boson. There is a lot of work ahead of us. We also now know which direction to go. This is just the beginning of a long journey.” At the press conference that followed the seminar, Joe Incandela, the spokesperson for the CMS experiment clearly stated: “[Before the LHC shuts down for maintenance three months later], it would be difficult to say definitively that it is the Standard Model Higgs.”
While the LHC is designed to accelerate protons up to 7 TeV per beam (total collision energy of 14 TeV), since its commissioning in 2010, the beam energy has been gradually ramped up. After operating at 7 TeV (3.5 TeV per beam) till end-2011, it was increased to 4 TeV per beam (8 TeV of total energy) in April. This increase in energy, along with unprecedented performance of the accelerator, the detectors, improved techniques of analysis and the intense computation on the worldwide LHC computational grid established, is what has enabled this landmark finding within a short time of two and a quarter years since LHC went into operation.
In terms of numbers, till the end of 2011, the experiments took data from about 400 trillion proton-proton collisions at 7 TeV of total energy, when there were already tantalising hints of a Higgs-like signal at around a mass value of 125 G(iga) eV. (At relativistic energies, mass and energy are interchangeable in accordance with Einstein’s E=mc2 relation. So masses of particles are measured in units of energy. The mass of a proton is about 1 GeV.) But it was not statistically significant to call it a discovery of a particle. The statistical significance of the bump or excess of events around 125 GeV at end-2011 was only at the level of about 3 sigma, which means one-in-750 chance of being due to statistical fluctuation, which is far from the ‘golden rule’ for discovery of 5 sigma, which means there is only one-in-3.5 million chance of being wrong.
But the data taken in just 3 months (from April 5 to June 18) at 8 GeV of collision energy — from about 500 trillion proton-proton collisions — has surpassed the data taken in all of 2011 at 7 TeV. Moreover, the Standard Model predicts that at 8 TeV there is an enhancement in the probability of producing a Standard Model Higgs by a factor of 1.27, Aleandro Nisati of the ATLAS experiment said in an email response. So if you do the arithmetic of combining the effect of increased energy and that of higher proton-proton collision rate because of better accelerator performance, scientists now have 2.6 more Higgs-like events at 8 TeV for every Higgs-like event at 7 TeV in 2011.
All these have contributed to improved statistics in the data gathered, in particular for two of the five important decay channels available to Higgs — namely its decay into two photons and to four charged leptons (electrons/muons) — whose data were presented on Wednesday. These channels are considered important because these allow the Higgs mass to be measured with greater precision. The later is, in fact, called the Golden Channel because it is much cleaner compared to the other channels. The analysis had also been particularly optimised to pick Higgs-like events decaying into these final states, according to Nisati.
According to the presentations made on Wednesday by Joe Incandela and Fabiola Gianotti, the respective spokespersons of CMS and ATLAS respectively, both the experiments, which have worked entirely independently of each other, observed a “new particle” in the mass region around 125 – 126 GeV at 5 sigma level. The world physicist community had not really expected that the 5 sigma level would be attained so quickly. Both presented results from their analyses only of 2 photon and 4 lepton channels.
Specifically, the mass value given by CMS to this Higgs-like particle from is 125.6 GeV with an error window of 0.6 GeV, and that given by ATLAS is 125.3 GeV with an error bar of 0.6 GeV. It is clear that both the results are consistent with each other within experimental errors. The immediate next step for these experiments is first to analyze data from the remaining three channels of Higgs decay and check for consistency LHC before shuts down for maintenance. The larger task in months ahead is to ascertain whether the other properties of this new Higgs-like particle fit the predicted properties of the Higgs boson or is it something entirely different.

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