The two-photon peak disappeared in the new collider data – Elements

«- You see a gopher
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No – I do not see. And he is »
Two theoretical physicists are looking at new data
Collider and they can not believe

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The conference ICHEP 2016 published new results of the LHC on the mysterious two-photon peak at a mass of 750 GeV, hints of which were six months ago. Now, based on four times larger statistics, collaboration ATLAS and CMS have taken out an unambiguous verdict: in the new data is no hint of this peak is not. What excites the theoretical physicists of recent months, it was a statistical fluctuation.

The end of the tale

The two-photon surge at a mass of 750 GeV, which put on the ears of the scientific community in the physics of elementary particles and became the subject for more than 500 theoretical papers – turned out to be a mirage, a fairy tale. The beautiful, believable, thrilling fantasy – but a fairy tale. She left us with strong feelings and warm memories, but we need to move on.

On the morning of 5 August passing these days conference ICHEP 2016, the main event of the year in elementary particle physics, the representatives of the ATLAS and CMS have shown the long-awaited new the results of the Large hadron collider on the creation of two high-energy photons. Last year when the collider was just beginning his second season after a long pause, the statistics has been set up is very modest, it is consistent with the luminosity of a little more than 3 fb ^-1 . This year the collider came to a record rate of the data set, so that the results presented at the conference for the year 2016 were prepared on the luminosity of about 12 fb ^-1 . Last session dataset caught in this sample was on the night of 14 July 15,

In March, the deviation from the Standard Model, it seemed very important -. Look at Fig. 2 and 3 of the March news – and have reason to hope that the five-fold increase in the statistics lead to the guaranteed opening. Provided, of course, that the observed deviation then -. Actually, it’s not a statistical fluctuation

But the new data destroyed these hopes. Fig. 2 shows the same schedule that the last news, but only with the new statistics. From spike at 750 GeV was gone. Moreover, the statistics exactly evil whim at this value there is even a shortage of some events, but not exceeding. In the whole area of ​​the invariant mass of up to 2500 GeV, as there is no significant deviation from the Standard Model background (color curves in Fig. 2). Fluctuations up and down in the data are present, but they look exactly the way from normal statistical fluctuations and expect. Even some semblance of spike in the CMS database (. Figure 2, right) in the 600-650 GeV is not drawn to any significant deviation – the more so because it is not supported in the ATLAS experiment

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Comparison of accumulated data with the background is best shown in Fig. 3 ATLAS experiment example for moderately wide scalar resonance; CMS have observed a similar picture. Here are the local p are the values, then it is likely that only one background processes of the Standard Model, with no new physics may be due to statistical attacks to give the masses at a given value, this (or even more) deviation, which saw in physics experiment. When p is the value drops to very small quantities of the order of one-thousandth or less, it gives reason to suspect physicists background that one can not do here, that perhaps there is a new effect.

As well as the data was in 2015: the blue dashed line at a value of 750 GeV plummets down. However, the data in 2016 (red dotted line) nothing like this occurs. Even if the two samples to combine, the total local effect falls to 2σ, and then only by the old data. From the perspective of the experiment – it is nothing; fluctuations of this magnitude will inevitably occur in any way large statistics.

In order to really dot the «i», CMS collaboration showed how should look like a signal of a new particle, if it were real and its mass and cross section would be such, which pointed out the March analysis. It is shown in Fig. 4 in the area of ​​the invariant mass of 600-900 GeV. The peak would be standing proudly over a background of nearly 4 standard deviations. Instead – failure

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Conclusion unambiguous and inexorable:. two-photon spike at 750 GeV is closed . What we have seen in last year’s data, was only a statistical fluctuation. The new findings are consistent with the expectations of the Standard Model.

Negative search resonances, as in such cases is allowed physicists to set upper limit on the cross section of a hypothetical particle. Fig. 5 shows the ATLAS data limitations. Almost the entire mass limit the scope increased by 2-4 times compared with the old data, and in particular, they have closed the cross section at a mass of 750 GeV, which was estimated in March (5-10 fb)

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Issues and lessons

brewing was a sensation disappeared as lightning, as it came. But she left behind a question, in a sense, coming out even from a purely scientific plane: What was that? How is the result? ?

Are expectations were justified

First of all, we emphasize once again that neither in the experiment, even more so in the behavior of experimenters no puncture there. Accusations that the participants in the experiments allegedly wishful thinking – are groundless. The approach to the analysis of the data was weighed and impartial analysis of the results reported in the most conservative terms, and in communicating with the media theorists and experimenters stressed that it is too early to draw conclusions, we need new data. The strongest statement that the collaboration officially issued in March, after the new results confirm sounded « Now we can feel cautiously optimistic ». The word “discovery” of the experimenters did not tell even close. And it is very different from the situation with superluminal neutrinos, when all the hype has arisen due to a defective cable, and by hasty posts BICEP2 of registration of primary gravitational waves in the CMB polarization – it turned out that was not correctly assessed the galactic background

<. p> Here, apparently, with the experiment, everything was in order from beginning to end. The difference in the results could be attributed to the fact that in the last analysis of the physics of something not taken into account, and now taken into account. But the method of analysis, in fact, remained the same, only now it is a new dataset has been processed (and again, by the way, revalidated old). It turns out that this data is played with physicists cruel joke – and soon in both experiments. The coincidence of strange, no doubt, but no other explanations from representatives of collaboration is not (Fig. 6). They even have estimated the probability that the same physical situation may give different results so in two successive data samples. The probability of getting at least one percent (2,7σ, ATLAS) and a couple of percent (2,4σ, CMS). The probability of such a joke of nature right in both experiments was not shown

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The enormous – and, as it turned out, groundless – the expectations are, rather, on the conscience of theorists, including many people with big names, who from day one thrown remarkable strength to the full discussion of deviations in terms of new physics and thereby lead by example for many others. Source ATLAS and CMS analysis showed deviation level (unofficially combined) statistical significance of about 3σ. For this, in general, a modest deviation of the response of physicists was unduly optimistic. Now, of course, there is a backlash, including not only a joke, but also direct accusations among other theorists. Therefore, if talk about the “loss of contact” is not between physicists and the press, but rather between experimentalists and theorists. It is possible, even, that the outcome of this story will be reviewed, how much weight should be given to deviations in the 4-5 Sigma, especially when they occur in a previously unexplored area.

In this story there are, however, some positive points . First, the effort of thousands (!) Of theorists will not be lost as a minimum because there were universal phenomenological developments in the case of new unexpected spike in only one of the channels without confirmation in the other. Secondly, well, that the anomaly has disappeared quickly and unconditionally. It would be much worse, if such a spike showed up at the turn of the first session of the collider in 2012: then it would be three years Physics amused ourselves with illusions. Equally unpleasant situation would have been if the new data did not close the deflection, and kept him on the edge of the curious. Physicists have recently passed through the situation with painful uncertainty, when the decay of the Higgs boson into two photons (again, the two-photon channel!) For a long time seemed unusual.

In general, the scientific community in elementary particle physics wakes up and returns to the weighted , justified the theoretical interpretation of the data. Fortunately, most of these data will now be a lot. Collider is working properly, statistics even triple by the end of this year – and then grow back in several times to the end of the session Run 2. Finally, the 2020s will be pleased with another approximately ten-fold increase in the statistics compared to the whole sample Run 2. This program has already been approved by the CERN and it will be implemented regardless of the developments in the coming years. So far, the accumulated less than percent of the total amount of data, which is calculated the LHC, and in anticipation of new horizons, this percentage should be to clearly and carefully consider.

Links

ATLAS Collaboration:

CMS Collaboration:

Igor Ivanov

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