Small ripple shakes a roomful of physicists

After the exciting results announced by CERN physicists at the EPS conference, the CERN Quantum Diaries blog gave an insightful recap of the news. Here's what blogger, Pauline Gagnon, reported...


The CMS collaboration combined results for the Higgs boson search covering a possible Higgs in the region from 110 to 600 GeV.

This Friday afternoon, the 750 physicists attending the European Physics Society meeting in Grenoble, France, were pleasantly surprised. The audience was waiting with some anticipation to see the first important set of results from the two large LHC experiments, ATLAS and CMS on the search for the Higgs boson. In fact, for the past two days, results had been shown from both experiments as well as from the Tevatron experiments in various individual channels. But today, the latest combined results from each experiment were shown in public for the first time. Of course, all physicists belonging either to the CMS or ATLAS experiment had had a chance to see their own results in advance, since they had been widely distributed and discussed. A small excess was seen in ATLAS, but nothing particularly convincing. In physicist’s jargon, we refer to these as mildly significant excesses. This was found in one of the Higgs decay channels, namely when the Higgs boson decays into two W bosons.

But unbeknown to the ATLAS people, the CMS collaboration was also observing a similar excess that would correspond to a Higgs with the same mass and is seen in the same channel. So taken alone, none of these small ripples were compelling but once they show up in two completely different detectors, it starts being intriguing. Both experiments also reported small excesses for a Higgs to two Z bosons when the Z themselves decay into an electron or muon pair. What might turn out to be the first hint of a much sought after particle also occurred in a mass range not excluded by the Tevatron experiments.

It might not seem like much but this could be the first interesting bite we have in some decades. We are all researchers. This could turn us into “finders”, something very few of us had a chance to live in their career. Hence the lively discussions that followed this session during the coffee break.

But at the same time, we all know it is way too early to get excited. We need more data to be able to say something conclusive, something we do not risk regretting a few months down the line. Today, both experiments showed what they had after analyzing one inverse femtobarn of data (counted in the unit we use to see how much data we have). We already have another 0.4 inverse femtobarn ready to be analyzed. As soon as we can look at them in the coming weeks, we will see if the trend is maintained. The other important missing ingredient at this point is a full fledge combination of both results, taking into account all the common uncertainties. For example, we both use the same simulations to describe our backgrounds. Even though we crosscheck these simulations with real data, there is always a chance a small inaccuracy would trick us both in the same way. The combination team is getting to work on this combination tonight but it will take up weeks to complete this task.

Everybody agrees: We need more data and the combined results. But it is already interesting enough to keep staring in that direction. We might be seeing the caravan appearing in the far horizon. In just a few months, we will see it clearly or discover we all had sand in our eyes…

Another major conference is planned in late August in Mumbai, India. For sure, this combination and hopefully, more new analyzed data will be shown there.

But why do we care? Very simple. As much as we like to say we know about the particles that make up all matter we see around us, to this day, we, physicists, have no clue how the particles that make up all matter get their mass. Roughly speaking, that means we have no idea why any basic constituents of matter has a mass, (or weight), when we all know every object we grab has a mass! That Higgs boson, if it exists, would explain it all.


Friday 22 July, Pauline Gagnon for CERN's Quantum Diaries Blog.