LHC Experiments ATLAS and CMS to update their Higgs boson hunt results

CERN has announced that the two experiments leading the search for the Higgs boson, ATLAS and CMS will update their results concerning the search for the Higgs boson tomorrow on July 4th.

Last December the ATLAS and CMS experiments reported they excluded a Higgs boson in the mass range above 130 GeV and up to 500 GeV and observed a modest excess of collisions compatible with a Higgs boson at about 125 GeV, but with a low statistical significance.

The probability for the observed December 2011 excess to be the result of a statistical fluctuation (rather than a Higgs boson) is about one chance in a thousand.

With current computer technologies, physicists can easily look at thousands of distributions while trying to find a handful potential Higgs boson events among billions of proton-proton collisions. In clear: there is always a chance that a few mundane proton proton collisions will look like collisions in which a Higgs boson was produced and then decayed. It is for this reason that particle physics needs to have very strict criteria to assert whether an effect is real or is just a fluke of statistics. To assert with certitude that a certain outcome is not just the result of a statistical fluctuation we require that the probability that a fluctuation would explain the observation to be lower than one in a few millions.

That’s what physicists refer to as “5 sigma”. If what was observed last year is a fluctuation then its significance or number of sigmas combined with 2012 data will decrease, because it will become diluted with the new data.

After a short winter maintenance period the LHC restarted swiftly its operation in march and delivered to date more data than for the entire year 2011 and with a higher energy of 8 TeV available in the center of mass of the protons. The ATLAS and CMS collaborations have spent the spring frantically analysing this data to see whether last year’s finding can be confirmed. An update by the summer was promised and now or rather tomorrow is the time, so of course there is excitement building up.

Whether the Higgs signal is confirmed or infirmed, it will have huge implications for the field. If it is not confirmed one would still be able to argue that it could be there but has not been found yet because it has a lower production rate than expected, this would thus point indirectly to theories beyond the standard model. Several theories could accomodate that and the search will have to continue after the long 2013-14 LHC shutdown, that would probably be a blow to the standard model, but since we all hope for something new “beyond the standard model”, it would not be entirely bad!

On the other hand many theories of beyond the standard model physics simply rely on the presence of a Higgs boson. One of our favourite theories at the Oskar Klein Center for cosmo-particle physics is Supersymmetry since it can provide for a very plausible dark matter particle candidate. An absence of Higgs confirmation tomorrow would probably not kill supersymmetry. Nevertheless one of the primary motivations for Supersymmetry is to “fix” the mass of the Higgs boson. Without doing anything special with the current standard model of particle physics, the Higgs boson mass has the bad tendency to become extremely high when calculated in theory. This is called the “hierarchy problem”. So if there is no Higgs, clearly we lose one of the main motivations for supersymmetry which is to stabilise the mass of the Higgs boson.

But the Higgs boson fulfills other functions, for instance in absence of a Higgs boson, the WW scattering cross section starts to become infinite at the energy of the LHC unless some new exotic particle was exchanged. Without the Higgs one would have to rethink all that, and imagine what should be there instead. But ATLAS and CMS will have the capability to study this problematic in detail.

If tomorrow the LHC experiments confirm what was observed last year, it will instead validate the theory which unifies the electromagnetic and weak interaction, so called “electroweak theory”, and provide a mechanism by which the elementary particles get their mass. That’s quite a big step forward in understanding the Universe, that would tell us that the particles were born without a mass at the time of the Big Bang and would tell us exactly how they got their mass shortly after.

Quite fascinating that we could potentially understand this kind of things “just” by building an experiment under Geneva. But this theory would nevertheless still be incomplete, since it does not allow us to predict the mass of the particles and in fact the mass of the Higgs boson would remain a puzzle in itself since it has a tendency to become infinite in the theory.

And after all we have no idea why just quarks and leptons were created in the Big Bang and not something entirely different, so there would be in any case still a lot to learn.

Whatever the outcome of the ATLAS and CMS Higgs boson hunt, it is great that the LHC and its experiments allow us to tackle these questions experimentally in real life.

Event display of a real proton-proton collision viewed by the ATLAS detector where 4 muons are observed, one of which has a mass compatible with the Z boson mass. The Higgs boson could theoretically decay for example into two Z bosons leading to four muons such as in this collision. In this collision the invariant mass of the four leptons is m4l = 145.8 GeV. The masses of the lepton pairs are 94.3 GeV and 29.7 GeV respectively. ©CERN

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