December 05
ATLAS and CMS: on the verge of discovery or a statistics game?
A mysterious two-photon peak at the mass of 750 GeV, recorded on the Large Hadron Collider a year ago, has excited the scientific community and raised a hope, that the new physics at the LHC will be finally opened. Scientists have written more than a hundred theoretical articles that explained this peak. But hopes were dashed: accumulating new experimental data, physicists did not see in them any evidence of the existence of two-photon peak and came to the conclusion that this was just a game of statistics. Some hope for the discovery of new physics still remains, because an anomaly in one of the few channels of the Higgs boson has not disappeared.
The standard model of elementary particles - is an extremely successful theory. Despite the efforts, the Large Hadron Collider has not yet revealed any deviations from the predictions of the standard model. The model contains a relatively small number of phenomenological parameters (mass of particles, the parameters of the matrix of the Cabibbo-Kobayashi-Maskawa, etc. -.. all in all nineteen parameters), but explains a huge variety of the experimental data. Some of its predictions, such as the anomalous magnetic moments of the electron and muon, experimentally tested with dazzling accuracy. For example, for an electron agreement between theoretical predictions and experimental result is 11 decimal places. Despite this, none of the physicists, doubts, that the Standard Model is an incomplete theory, which is a low-energy approximation of something more fundamental.

LHC does not see any signs of supersymmetry, or any other things of the new physics. All anomalies, which are still observed at the LHC, turned out to be the game statistics and were ,eventually, "resorbed". But there is one anomaly that has not yet disappeared.

This is an abnormally high appearance probability of the Higgs boson with a couple of t-quark and an anti-t-quark. Here is how the ATLAS detector shows the birth of tt¯H intermediate state, followed by the collapse of the produced particles:

For an uninformed person it is unclear how physics extract useful information from a this "garbage" of the registered secondary particles. But for the specialists relevant procedures are well known and have been successfully tested many times. So far, all the results of experiments on the Large Hadron Collider are consistent with the predictions of the standard model, but at the last session of experiments, physicists were surprised to find the excess over the Standard Model prediction.

Both ATLAS and CMS collaborations have seen this excess, and this value is their combined result. It was decided not to attract public attention to this anomaly, since the selection signal tt¯H is a rather complicated procedure: t-quarks at these energies are abundantly born also through other processes, and the researchers were not sure that a small signal was correctly isolated from this background. Physicists have decided not to risk their reputation and wait for new experimental data to confirm the truth of the anomaly.

New experimental season was held at higher LHC collider energy (13 TeV center of mass). In the transition from 8 to 13 TeV cross section tt¯H increased almost four times, so experimental statistics on this process increases faster. In early August, at a conference ICHEP-2016, both collaborations presented new results on the birth of Higgs in tt¯H canal, and it became obvious that the anomaly has not disappeared. Below is the result of the ATLAS detector.

The first three rows correspond to different channels of decay of the Higgs boson, the fourth line shows the combined result of 2016, the last line - the ATLAS experimental results from the past season. As can be seen, ATLAS approximately reproduced his old result.

CMS detector also sees tt¯H anomaly at about the same level. It is naive to combine the results of both experimental seasons, but if we do, we'll get an impressive result. But we don't need to rush with the statement about the opening. Systematic error is largely unknown and the reduced combined mistake, which should not be taken too literally. There is still much to be done in order to reliably establish the existence of anomalies or refute it.