Last December 15, the two experiments which have discovered the Higgs particle at CERN, ATLAS and CMS, have presented the first results of the LHC collider at the highest 13 TeV collision energy and they both announced an excess of 2-photon events at 750 GeV.
This might be the sign of the existence of a new particle with a mass energy of 750 GeV, decaying into two photons like the Higgs. But the evidence is still very preliminary: one only sees small bumps when one plots the number of events versus the energy.
In technical terms, one talks of standard deviations: ATLAS has seen a 3.6 standard deviation, and CMS a 2.6 standard deviation, when the scientific community agrees to talk of a discovery only for standard deviations larger than 5. The excitement comes from the fact that both experiments see an excess at the same mass. But one will probably have to wait another year to accumulate more data and see whether the excess builds up… or disappears if it was just an unhappy coincidence.
But theoretical papers have rushed to provide interpretations of these events. Nature has recently counted almost one hundred papers appearing on the web arXive where the scientific community uploads their papers (108 to this date, see here). Why such an interest? And has it anything to do with gravity?
Well, the Higgs particle discovered in 2012 was the last building block of the Standard Model, which realizes the unification between the electromagnetic and the weak nuclear force. A new particle of mass energy 750 GeV (decaying into two photons) is not accommodated by the Standard Model; it would be a clear sign that this Standard Model has to be revised, or rather enlarged: one would have to go beyond the Standard Model, as we physicists say.
For many of us in the scientific community, this is expected because the Standard Model answers very fundamental questions but leaves many other open. For example, it does not provide a candidate for a dark matter particle. Could the new hypothetical particle be this dark matter particle? Most probably not, but many theoretical papers stress that this first signal of new physics would lead to the discovery of further particles. There is the hope to discover among them this dark matter particle, and thus to solve a puzzle which has been with us since the 1930s (remember that the only signs of dark matter have been so far gravitational, which led some to propose modifications of general relativity to account for the observations).
Another motivation is of a more theoretical nature. The Standard Model only realizes the unification of two fundamental forces (electromagnetic and weak nuclear forces). What about the two others (strong nuclear force and gravitation)? Well, a larger unification requires new dynamics and thus new particles. But the Higgs particle, which provides the masses of all other particles, is very special: its own mass is destabilized by the quantum fluctuations associated with the new particles. In other words, if there is indeed a new particle of mass energy 750 GeV, this would tend to increase the Higgs mass to the same value. But the Higgs mass is measured to be 125 GeV, that is six times smaller. It will then be fascinating to see what protects the Higgs mass from such fluctuations. A new symmetry, for example? In any case, this will give first hand information on the dynamics that may eventually lead to a unification of the microscopic forces with gravity, at a much higher energy. The long sought marriage of quantum physics with general relativity.
2016 thus appears full of promises of major discoveries in fundamental physics. Will it fulfil its promises?
Best wishes to all for this new and exciting year.