At the time of the European Physics Conference in July, an intriguing small excess of events was reported in the search for the elusive Higgs boson. Yesterday, as the Lepton-Photon conference in Mumbai, India, opened, these signs appear to be less compelling. What happened?
All phenomena we study follow statistical laws and are therefore subject to statistical fluctuations. The signal can grow bigger, smaller or disappear. Nothing we can do about it but analyze more data to get a definitive answer. In times, either the signal emerges unambiguously if it was real or it vanishes if it is only due to a statistical fluctuation.
Fortunately with statistics, when you double the data sample size, the error bar or margin for statistical fluctuations goes down as the square root of the increase. This is why we are always trying to collect more data, to reduce the size of possible statistical fluctuations.
So where do we stand now? With twice as much data as we had in July, each experiment sees a small decrease in the potential signal. So it is less compelling than it was in July, but far from having vanished.
Most importantly, the two large LHC experiments, CMS and ATLAS, exclude a wide range of possible Higgs masses. This is just as crucial since we may soon simply prove that there is no Higgs! What is guaranteed is, given the rate at which the data are coming in, within a year we will have the final answer. If it is there, we’ll see it. If it does not exist, we will prove that, which is just as important as finding the Higgs.
As for what is happening at 145 GeV, where the first excess was initially spotted, it is still impossible to say if what we have been seeing could be due to a Higgs boson. As it is, we have been expecting this new particle to show up a bit like travellers waiting for their train. While checking in the direction of the tracks, we noticed something in the far distance looking much like that long awaited train (the Higgs boson in our case). Add a bit of fog and it becomes impossible to say if what we see is really our train.
The “fog” comes from all the other known processes that can mimic the Higgs boson, what we call the background. Even today, with the size of data sample at hand, it is too early for the train to be clearly visible in the distance. We still can not really distinguish between a Higgs boson and the background. All we see is an indistinct shape in the far distance, too small to be able to say if it is our much expected train or just a shape emerging from thick fog.
Once the two experiments combine their results, we will gain another factor of two in statistics. The combination has the advantage of taking into account all possible problems that are common to both experiments, ironing out fluctuations. However, this combination is tedious and requires that all individual measurements be well understood, which should be done in the coming weeks.
We are also expecting to double our data sample again before the end of the year. With four times as much data as in July and the combined results from the two experiments, it will be like looking at the train at a quarter of the distance. We will have a much better chance to say if this is our train or just an illusion
Pauline Gagnon
All Higgs masses excluded by the CMS collaboration after only eight months of data taking are shown in orange. The ATLAS group obtains similar results. This is to be compared to the blue zones excluded by the Tevatron experiments after 20 years of hard work. And that’s only the beginning…
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