SCIENCE
Higgs signal?
R. RAMACHANDRAN
| Physicists hope that they are closing in on Higgs boson, the crucial missing link in the subatomic world of elementary particles. |
ROLF HEUER, CERN director-general, flanked by Fabiola Gianotti (left), ATLAS experiment spokesperson, and Guido Tonelli, CMS experiment spokesperson, at a news conference at CERN at Meyrin, near Geneva, on December 13.
JUST four months ago, at the International Lepton Photon Symposium in Mumbai (LP2011), the widely held expectation that new data from the high-energy proton-proton collision experiments with the Large Hadron Collider (LHC) at CERN, the European Organisation for Nuclear Research in Geneva, would reveal the existence of Higgs boson, a particle that has been sought after by physicists for over three decades, had been belied. Although the data did not rule out its existence, the general feeling that prevailed at the end of the conference was that the particle perhaps did not exist ( Frontline, September 23).
Higgs is the crucial missing piece in the otherwise highly successful theoretical framework that describes the subatomic world of elementary particles. Higgs, which can be likened to an ether-like all-pervading force-field in the universe, is postulated to exist by this theory, which is known as the Standard Model (SM), as particles in the theory can acquire mass (as they should to describe correctly the real world) only through their interaction with the Higgs field. The Higgs boson is the particle associated with such an all-pervading force-field. The aim of particle physicists in the last nearly four decades has been to find evidence for its existence even as experiments have verified all predictions to great precision made with the SM with the assumption of the existence of Higgs.
The model itself does not predict a value for the mass of Higgs; there are indirect constraints on the Higgs mass from phenomenological considerations based on other processes governed by the SM. These constrain its mass to be less than 161 gigaelectronvolt (GeV) of energy (in accordance with Einstein's E=mc2 relation). A proton has a mass-energy of about 1 GeV. Such a Higgs is termed as 'low mass Higgs' because there are other theoretical models which allow Higgs to be much heavier, up to 600 GeV. This 'low mass Higgs' is, in fact, referred to as the 'Standard Model Higgs Boson'. But the SM Higgs has eluded searches in experiments at various accelerators so far.
HIGGS DECAYING INTO two photons.
The LHC is designed to explore the new energy domain in the teraelectronvolt (TeV) scale and is expected to show up such a low-mass Higgs, if it exists. The LHC is currently operating at a peak energy of 3.5 TeV per beam, which means a total of 7 TeV is available in every collision event for particle production. The high expectations in summer 2011 that evidence for Higgs, one way or the other, would soon be found arose with the high rate of data gathering, much higher than the target that had been set for 2011 during the early months of the LHC's operation.
By excluding its existence in vast regions of energy, the data presented at the Mumbai conference by the two CERN experiments, ATLAS and CMS, had narrowed the limits for the Higgs mass to a small window of 115 GeV-140 GeV. ATLAS and CMS are nearly identical multipurpose experiments with the search for Higgs as one of their primary objectives. Earlier, at the European High Energy Conference at Grenoble, the two experiments had already excluded Higgs from existing in the range 150-450 GeV. Higgs with mass below 115 GeV had already been ruled out by the earlier experiments with the Large Electron-Positron Collider (LEP) at CERN itself and, more recently, the Tevatron accelerator experiments at Fermilab, United States, had ruled out the region 156-177 GeV. Though Tevatron scientists felt that they had a chance of seeing Higgs by doing an intense data analysis in the region 115-155 GeV, the U.S. Department of Energy decided in September to shut down the machine for ever
Vivek Sharma, a physicist from the University of California at San Diego (UCSD) associated with the experiment CMS, had said at the time of LP2011: "We will triple the data set by end of October. And if you combine both CMS and ATLAS data…we will know if Higgs doesn't exist. We will know if [Higgs] was indeed science fiction by the end of the year." In principle, of course, from an experimentalist's perspective, the experiment should also search in the range beyond 450 GeV for a non-SM Higgs.
A TYPICAL CANDIDATE event including two high-energy photons whose energy (depicted by red towers) is measured in the CMS electromagnetic calorimetre (ECAL). The yellow lines are the measure tracks of other particles produced in the collision.
In the first week of December, rumours abounded in physics blogs that these experiments had found evidence for Higgs. They claimed that a Higgs signal at a mass-energy of 125 GeV had been seen. That is, Higgs is 125 times heavier than a proton, which has a mass of about 1 GeV energy equivalent. The evidence, the rumours said, was based on a slight excess of events (over a zero signal) seen in the search for Higgs by both the experiments in the channel in which Higgs decayed into two photons.
Higgs, according to the SM, is a very short-lived particle with a fleeting lifetime of about 10 - 2 2second; that is, ten-thousandth of a billionth of a billionth of a second. Once created, it would immediately decay into several channels. The experiments analyse these different channels that the SM allows Higgs to decay into. In this particular channel, in which two photons fly apart in opposite directions, there is a clean signal for clear identification in the debris of a multitude of particles that high-energy proton-proton collisions in particle colliders produce (Figure 1a & 1b). For example, in this decay mode, the CMS experiment will detect these photons by its detector called the Electromagnetic Calorimeter (ECAL). The ECAL, according to the CMS website, is able to tell the mass of the particle to better than 1 per cent if the Higgs is relatively light, below about 140 GeV. The most distinctive signature for Higgs in a lightly higher mass range, between 150 and 180 GeV, would have been its decay into two W bosons, the carriers of the weak nuclear force, which then decay into two leptons (particles like the electron that include the muon and the tau) and two neutrinos. So you do not see Higgs itself but detect the particles it decays into and see if the decay parameters are in accordance with the SM predictions and such decays are in sufficient numbers to be statistically significant for it to be reckoned as a discovery.
The task is to sift data from trillions of collisions and look for the Higgs signal. A signal for Higgs (or any new particle) among such data means that in a plot of events observed in the experiment, a peak clearly sticks out over the background from other particle processes governed by the SM that mimic the decay of Higgs into two photons. But such excess of events should be statistically significant to be ascribed to a new entity such as Higgs. That is, it has to be ensured that the excess seen is not due to statistical fluctuation in the background and are indeed events ascribable to processes involving Higgs.
HIGGS DECAYING INTO two Z bosons (carriers of weak force) which in turn decay into four muons.
Perhaps prompted by these rumours, on December 6, CERN announced a public seminar to be held on December 13 in which ATLAS and CMS would present the status of their searches for the SM Higgs. In any case such a seminar was due as, even in its normal course of operation, the LHC was scheduled for a maintenance shutdown for a few months after Christmas. A CERN press release of December 6 said: "These results will be based on the analyses of considerably more data than presented at the summer conferences [at Grenoble and Mumbai] sufficient to make significant progress in the search for the Higgs boson, but not enough to make any conclusive statement on the existence or non-existence of the Higgs" (emphasis added). The data analysed included the entire data sample of proton-proton collisions collected up to the end of 2011 run.
To put the increased data in perspective, at the Mumbai conference the analyses were based on a technical parameter of 'one inverse-femtobarn (fb - 1)' required for being able to see statistically significant physics results at 7 TeV. This is equivalent to data of 70 trillion proton-proton collisions events. This means that the LHC recorded this much of data since it began operations in March 2010. In fact, this was the target that had been set for the end of 2011. But because of extremely good performance of the machine, increase in the intensity of the beams attained by the machine has been much faster and hence a higher event rate. As of end-2011, the total data amount to 4.7 fb - 1. This means nearly five times the data gathered until summer. That is indeed fantastic performance. According to the CMS group, with this amount of data the experiment can study Higgs production in almost the entire mass range above the LEP limit of 114 GeV and up to 600 GeV and, as we shall see, the CMS has set limits on the non-existence of Higgs in that high-mass region.
The italicised part in the CERN release was clearly to scotch the rumours that were flying all around of Higgs having been discovered. But given the claims made in the rumours, there was considerable excitement all around and the seminar naturally got all the media hype. Indeed, it was a big draw even within the physics community as well, with physicists around the world catching the presentation live on the video streaming from the overflowing large auditorium at CERN.
A release from CERN after the seminar stated the main finding up front, which was a tighter limit on the real estate now available for Higgs to be present. As against the window of 115-140 GeV set at Mumbai, the main conclusion was that the SM Higgs boson, if it exists, is most likely to have a mass in the range 115-130 GeV. But, more significantly, it added the following: "Tantalising hints have been seen by both experiments in the same mass region, but these are yet not strong enough to claim a discovery." This marked a clear change in tone from the Mumbai conference. It was a positive statement. It also added a more specific statement.
Both ATLAS and CMS had analysed several decay channels – not just the two photon channel as the rumours had said – and the experiments see small excesses in the low mass region in the past few weeks that has not yet been excluded. "Taken individually," the CERN release said, "none of these excesses is any more statistically significant than rolling a die and coming up with two sixes in a row. What is interesting is that there are multiple independent measurements pointing to the region of 124 to 126 GeV." It also quoted the ATLAS spokesperson Fabioal Gianotti as saying, "This excess may be due to a statistical fluctuation, but it could also be something more interesting. We cannot conclude anything at this stage. We need more study and more data. Given the outstanding performance of LHC this year, we will not need to wait long for enough data and can look forward to resolving this puzzle in 2012."
ATLAS EVENT CONTAINING four muons. This event is consistent with coming from two Z particles decaying. Both Zs decay into two muons each. This view is a zoom into the central part of the detector. The four muons are shown as red tracks. Other tracks and deposits of energy in the calorimetres are shown in yellow.
The CMS spokesman echoed similar views. "We cannot exclude the presence of the SM of the SM Higgs boson between 115 and 127 GeV because of a modest excess of events in this mass region that appears quite consistently in five independent channels," said Guido Tonelli. "The excess," he added, "is most compatible with a SM Higgs in the vicinity of 124 GeV and below but the statistical significance is not large enough to say anything conclusive. As of today, what we see is consistent either with a background fluctuation or with the presence of a boson."
The slightly more detailed press release from the CMS group said: "Our results were achieved by combining results in a number of predicted Higgs 'decay channels' including pairs of W or Z [another carrier of the weak force besides the W], which decay into four leptons, pairs of heavy quarks, pairs of tau leptons and pairs of photons (Figures 2a & 2b)
In June 2011, the LHC attained the key data milestone 'one inverse-femtobarn (fb - 1)' required for being able to see statistically significant physics results at 7 TeV, which is equal to 70 trillion proton-proton collisions events. This was the target that the LHC had set in 2010 for the entire 2011 runs but this was achieved within a record time of just three-four months. The CMS experiment excludes the existence of Higgs in the mass range 128-525 GeV at 99 per cent 'confidence level' (CL). A mass is said to be excluded at '95 per cent CL' if the chance of SM Higgs boson showing up in the excluded mass range is 5 per cent of the time in a set of repeated experiments. That means their exclusion of Higgs beyond 128 GeV is now far more stringent than it was in Mumbai. What CMS does not exclude now is the region 115-127 GeV as Tonelli had said.
"If we explore the hypothesis that our observed excesses could be the first hint of the presence of SM Higgs, we find the production rate ('cross section' in high-energy physics terminology relative to the SM prediction) for each decay channel is consistent with expectations, albeit with large uncertainties. However, the low statistical significances mean that these excesses can reasonably be interpreted as fluctuations in the background. More data, to be collected in 2012, will help ascertain the origin of this excess."
Statistical significance is measured in terms of what is called standard deviation (called sigma). For any discovery in particle physics, the signal should be at least at '5 sigma (σ) level' over the background, which is equivalent to a CL of being wrong only one part in about 30 million. Current levels of excesses in both the experiments are still in the region of 2.4-3.6 σ, which is still far from the 'Golden Rule' for a discovery in particle physics (Figures 3a, 3b and 3c). The statistical significance, particularly in this tantalising region of around 125 GeV where both experiments seem to see an excess of events, would be greatly improved if a completely statistical analysis is done using the data sets of both the experiments together for each individual channel.
However, such a detailed exercise would take considerable amount of time, and probably one can expect such an analysis carried out during the period of LHC shutdown for a couple of months from now.
COMBINATION OF LEP (phase II) + Tevatron + CMS + ATLAS data. Peak sits at a Higgs mass of about 125 GeV.
However, the physics community outside the experimental groups is not waiting. One theoretical physicist, Philip Gibbs, has already carried out an approximate analysis of this kind in his blog and he sees a clear signal for a Higgs with mass at around 125 GeV with sufficiently improved statistical significance (3σ). Gibbs has combined the results of LEP, Tevatron, CMS and ATLAS where the signal strength fits neatly on 125 GeV (Figure 4). Indeed, some would like to believe that Higgs has already been seen with this new data. The higher the number of sigma, the more incompatible the data are with having only background and no Higgs. In his blog, Tommaso Dorigo of the CMS group has even ventured to term it as firm evidence for Higgs. The general physics community will, of course, wait until it hits the bar of 5σ to say that Higgs has been found. According to Gibbs, the experiments would need to achieve data mark of 25 fb - 1, which means five times more collisions. This should be achievable by the LHC in 2012, and that is precisely why the statements have been cautious in making claims and have said that a conclusive picture should emerge by 2012.
So the wait for its clear evidence, despite the fact that its discovery appears now more imminent than ever before, has to continue, until the end of 2012. In case a signal does show up with sufficient statistical significance, what we can say is, 'yes, there is a new particle, which looks like the SM Higgs'. But is it the SM Higgs? Much more work will have to be done to check if that particle has exactly the same characteristics of the SM Higgs. That will take many more years of data from the LHC. It could also happen that it does not turn out to be an SM Higgs. But there are non-SM Higgs models which would open doors to new physics beyond the SM. Either way there is exciting particle physics in store in the years to come.
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