The birth process of ttH is finally open, but no longer arouses the enthusiasm of theorists

The birth process of ttH is finally open, but no longer arouses the enthusiasm of theorists

Fig. one. The statistics of events of the production of a top-quark-antiquark pair and two high-energy photons shows a clear peak in the distribution over the invariant mass of two photons at the Higgs boson mass, which allows us to confidently speak about the registration of the ttH process. Image from the article: ATLAS Collaboration. Observation of Higgs boson production in association with a top quark pair at the LHC with the ATLAS detector

After several years of searching, the ATLAS and CMS collaboration finally reported the reliable discovery of the Higgs boson production process accompanied by a top-quark-antiquark pair. The birth rate, within the margin of error, agrees with the predictions of the Standard Model. A few years ago, there was a stir around this process, but by now all passions have subsided, and theorists greeted CMS and ATLAS messages very cool.

In the physics of elementary particles, among the huge variety of reactions of creation and decay of very different particles, there is a small but very important class of sign, key, cornerstone processes for the further development of one or another branch of particle physics. These processes stand apart for both theorists and experimenters. They are usually difficult to register experimentally; in some cases, physicists have been hunting them for decades. On the other hand, it is through them that you can first feel a new facet of how the Universe works at the microcosm level. They are extremely attractive to theorists as well: after all, if in these processes there is a significant deviation from the expectations of the Standard Model (SM), this could be a harbinger of fireworks of discoveries and the beginning of a new era in particle physics.

Several such processes have already been detected at the Large Hadron Collider. These are, first of all, the creation of the Higgs boson, as well as some super-rare decays of B-mesons. Another process involving the Higgs boson – its simultaneous creation together with a top-quark-antiquark pair – the creation of the ttH system can be attributed to the same group. The other day, after four years of searching, surprises, hopes and disappointments, this process was finally declared open, finally and irrevocably. Both of the largest detectors of the Large Hadron Collider, ATLAS and CMS, see a clear indication of this process at a statistical significance level above 5 sigma, which in particle physics is considered the threshold for a discovery claim.

The CMS collaboration reported this back in April with a preprint that was published in the magazine in early June Physical Review Letters… Their result was based on 2016 statistics combined with Run 1 data (2010-2012), giving the combined signal significance of 5.2σ. The ATLAS collaboration completed its analysis a little later, publishing the results only at the beginning of June. But she also included the 2017 data, bringing the total integrated luminosity to 80 fb-one, and statistical significance – up to 6.3σ. This work was one of the very first publications, which finally began to take into account the 2017 record-breaking statistics. The signal intensity – that is, the number of reported births of ttH compared to the expectations of the CM – was μ (ttH) = (1.26 ^ {+ 0.31} _ {- 0.26} ) according to the CMS and μ (ttH ) = (1.32 ^ {+ 0.28} _ {- 0.26} ) according to ATLAS data. In both cases, the result is consistent with the Standard Model within the margin of error.

A story about these studies appeared on the CERN website, in the journals Physics and CERN Courier and on collaboration sites. In the note New ATLAS result establishes production of Higgs boson in association with top quarks, you can find an animation of how, as the data accumulated, the ttH system birth signal appeared in the ATLAS detector. The news also spread to many media outlets, once again stirring up public interest in the results of the collider. But it should be noted that those stamped solemn formulations in which the message about this discovery is now clothed hide from the readers the most interesting part of this story, which lasted several years and became a source of inspiration for hundreds of theorists. You can even say this: if three years ago there was a real excitement around this process, by now all passions have subsided, and theorists are now meeting CMS and ATLAS messages coolly, if not indifferently.

Let us briefly recall this story, which we have been following since 2015 on the Top-Anti-Top-Higgs Combination page. The first hint of the birth of ttH appeared in a 2014 CMS publication. The very fact that traces of this rare process were visible in the Run 1 session caused astonishment among physicists – according to SM estimates, the Run 1 statistics should not have been enough for this. Having estimated the likelihood of this process from the data, the CMS collaboration received an almost three-fold increase in comparison with the Standard Model! Theorists happily began to explain this anomaly in different models of New Physics. The fact is that the top quark, due to its abnormally large mass and, as a result, its intense connection with the Higgs boson, has always aroused suspicion among theorists. Therefore, the deviation in the birth of the ttH system was quite consistent with the expectations that there is room for non-standard effects. Many then felt that physicists were about to find a “pain point” of the Standard Model.

ATLAS data appeared a little later and did not clarify the situation. In September 2015, when the Higgs data from the two detectors were officially merged, the ttH birth anomaly persisted and continued to excite the imagination. In addition, everyone was waiting for the first data from Run 2 – after all, with an increase in collision energy up to 13 TeV, this birth process should sharply, fourfold in comparison with the Run 1 session. It was clear that 2016 would either bring a big sensation or close the anomaly. In addition, in the first half of 2016, physicists experienced the strongest shock in recent decades, when a two-photon burst at 750 GeV appeared out of nowhere and disappeared into nowhere. Barely recovering from the blow, physicists said that at least the ttH anomaly is still holding, because the first data of 2016 still retained some intrigue. However, throughout 2017, hopes were inexorably melting, and by December it became clear that, alas, there was no need to catch anything here.

By the beginning of 2018, the statistical significance of this signal was already 4σ. In contrast, as the evidence for him grew more reliable, the enthusiasm of theorists waned. The intensity of the interaction of the Higgs boson with the top quarks turned out to be completely standard, and it was clear that there was still quite a bit of pressure left, and the process would be officially opened. This is exactly what happened the other day. Of course, this is an important process that physicists have been hunting for for so many years; this is a kind of “checkpoint” of the Higgs collider program. But it no longer evokes the former enthusiasm of theorists.

From the point of view of the experimental result, it is important not only the reliable discovery of this process in itself, but also the fact that physicists have learned to cope with such a complex system for analysis (Fig. 2). After all, all three created particles – top quarks and the Higgs boson – decay, and into a variety of final states. In addition, their decay products can greatly interfere with each other and make it difficult to select events.

Fig.  2. Event - a candidate for the birth of the ttH system with a certain decay channel

Fig. 2. The event is a candidate for the birth of the ttH system with a specific decay channel. Image from home.cern

Actually, the 2016 data had to be processed for so long precisely because of the exceptional entanglement of the generated set of particles. In 2017, demonstrating at conferences preliminary conflicting results for individual decay channels, the experimenters honestly admitted: apparently, we are still poor at unraveling such complex processes. Now they have coped with this task, which means that now they can swing at other, even more complex reactions of birth.

Sources:

1) CMS Collaboration. Observation of ttH production // Physical Review Letters 120, 231801 (2018). A preprint of the article is also available as arXiv: 1804.02610.
2) ATLAS Collaboration. Observation of Higgs boson production in association with a top quark pair at the LHC with the ATLAS detector // arXiv preprint: 1806.00425.

Igor Ivanov

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