LHC finds another hint of violation of the Standard Model

LHC finds another hint of violation of the Standard Model

Fig. one. One of the variants of the B-meson decay caused by the quark transformation (b to s ell ^ + ell ^ – ), in which once again hints of deviations from the Standard Model were found.

Recently, news came from CERN that another measurement of the Large Hadron Collider is at odds with the predictions of the Standard Model. The result, unveiled by the LHCb collaboration after more than four years of analysis, is indeed different from theoretical predictions, but not so significant that one can speak of a discovery. However, the most important thing here is that this deviation is not one. It fits into a number of other anomalies in decays B-mesons, increasing their collective discrepancy with the Standard Model. Everything looks as if physicists really found some “painful point” of the microworld and are now trying to extract maximum information from this.

Riddles of lovely mesons

The main task of modern physics of elementary particles is to discover the New Physics, a certain more fundamental layer in the description of the microworld on which the Standard Model rests. Physicists are sure of its existence, too many indirect signs indicate it, but it has not yet been possible to reliably detect direct discrepancies with the Standard Model. The Large Hadron Collider, the main tool in modern particle physics, aims to do just that. It carries out hundreds of options for analyzing the accumulated data in search of reliable indications of a crack in the current understanding of how the microcosm works.

We have already repeatedly said that the search for New Physics can be carried out not only by the “brute force” method, trying to directly generate new heavy particles, but also by “cunning”, through precise measurements of rare phenomena. The most popular class of such processes is decays B-mesons, or adorable mesons, as they are called in physical jargon. They contain a heavy b-quark that can decay into any lighter quark, and this opens up a huge wealth of possibilities. All such decays can be studied experimentally, and, as it turned out in recent years, some of them give surprises: their probabilities or other characteristics differ markedly from the predictions of the Standard Model.

Among all these transformations, bs accompanied by a muon or electron-positron pair: (b to s ell ^ + ell ^ – ), where ( ell ^ + ell ^ – ) denotes e+e or μ+μ… A concrete example of decay B-meson caused by such a quark transformation is shown in Fig. 1. This is a rather rare process, it occurs with a probability of less than one millionth. This smallness arises because, within the framework of the Standard Model, there is no particle that would be able to change the type of quark without changing the charge. Therefore, this process takes place in two stages, bts, and requires the help of heavy virtual particles. But since the contribution of the Standard Model is so small, it means that the weak effects of hypothetical New Physics can intervene and significantly change the probability of this process or the angular distribution of the scattering particles.

Over the past few years, primarily through the efforts of the LHCb collaboration, deviations have indeed been found in these decays. Yes, they are not yet so statistically significant that one can confidently speak about the discovery of New Physics. But there are already several of them, and when more and more new data arrives, they do not disappear. On the collider puzzle page, we keep a close eye on two of them: this is a strong difference in the probability of decays BK*μμ and Bsφμμ (by about 3.5σ) and one hint of violation of the lepton universality of the weak interaction (2.6σ). We also add that an equally serious deviation from lepton universality was found in the b → c transformation, from which no one expected such tricks.

Finally – and this is a very inspiring moment! – all these deviations do not give the impression of something disorderly. The measured values ​​do not deviate anywhere, but in one general direction. A more precise formulation is as follows. If you theoretically “dig” into the depths of the process (b to s ell ^ + ell ^ – ) and add hypothetical interactions of a certain type, which are forbidden in the Standard Model, then the predictions for all deviating quantities in general are shifted there, where the data points to. It is this consistency of several small deviations that has attracted the attention of physicists in recent years.

New result

The result announced the other day is another deviation from the same series. On April 18, at a seminar at CERN dedicated to hot results from the collider, the LHCb collaboration spoke about testing lepton universality in (B to K ^ * ell ^ + ell ^ – ) decays. Lepton universality is the property of a weak interaction to act equally (universally) on leptons of different kinds. Let theorists not be able to accurately calculate the probability of decays (B to K ^ * mu ^ + mu ^ – ) (it is just shown in Fig. 1) and (B to K ^ * e ^ + e ^ – ), but they know that their attitude is [ R_{K^*} = {B to K^* mu^+mu^- over B to K^* e^+e^-} ] should, within the Standard Model, be very close to one. Moreover, this unit should be fulfilled not only for the total decay probabilities, but also for differential ones, that is, in the distribution over the invariant mass of the lepton pair q2

However, the LHCb found that, according to their measurements, RK* significantly less than unity (Fig. 2).

Fig.  2. The ratio of the decay probabilities of  (B  to K ^ *  mu ^ +  mu ^ - ) and  (B  to K ^ * e ^ + e ^ - ) for different invariant masses of a lepton pair

Fig. 2. The ratio of the decay probabilities of (B to K ^ * mu ^ + mu ^ – ) and (B to K ^ * e ^ + e ^ – ) for different invariant masses of a lepton pair. Black dots with errors – LHCb data, colored symbols – predictions based on the Standard Model made by different groups. Image from the report under discussion

The collaboration measured this ratio separately for small invariant masses of lepton pairs q2 <1 GeV2 (that is, when a lepton pair flies out with a high recoil and a small angle of spread) and for averages, up to q2 = 6 GeV2 (the maximum in this process is 19 GeV2). In these two areas RK* turned out to be equal:

small q2: (R_ {K ^ *} = 0 {,} 660 ^ {+ 0.110} _ {- 0.070} pm 0 {,} 024 ), the difference from the SM by 2.2σ,
average q2: (R_ {K ^ *} = 0 {,} 685 ^ {+ 0.113} _ {- 0.069} pm 0 {,} 047 ), the difference from the CM is 2.4σ.

In both cases, the first of the indicated errors is statistical, the second is systematic. The fact that the statistical error still dominates is not surprising. It is much more difficult to register this process with electrons than with muons – in fact, this is why there has not been a reliable comparison so far. LHCb experimenters managed to collect only a few dozen events with an electron-positron pair (versus hundreds with a muon pair), and they became the main source of uncertainties.

There are two important additions to this result.

First, two decays – with electrons and with muons – are recovered by the detector with very different efficiency. When measuring them, their own characteristic sources of systematic errors arise, and it is very difficult to compare them directly with each other. Therefore, a reasonable question arises: why are physicists so sure that this discrepancy is not an instrumental artifact caused by imperfect registration, but something real?

The answer is that the LHCb Collaboration has done several cross-checks of this efficiency in other processes that have been studied much more precisely – and everything was everywhere within the expected range. Perhaps the strongest argument is that there is always a pivotal process at hand in this analysis: decay B-meson on K* and J/ψ-meson, and J/ψ further decays into the same lepton pair. Such a decay occurs a hundred times more often, it is captured by the same selection algorithm, but in it guaranteed no surprises, in which the experimenters were convinced for greater reliability. Therefore, the value RK* measured in reality through double attitude:

[ R_{K^*} = {Br(B to K^* mu^+mu^-)/Br(B to K^* J/psi[to mu^+mu^-]) over Br (B ​​ to K ^ * e ^ + e ^ -) / Br (B ​​ to K ^ * J / psi[to e^+e^-])}. ]

In this respect, many sources of error associated with both theoretical description and experimental registration have been reduced – and it is to this that the above numbers apply.

The second point is this. The results published now were obtained only on the statistics of the Run 1. The dataset was completed back in 2012, but their analysis, due to its complexity, required more than four years of work. Now the Run 2 session is in full swing, which will last until the end of 2018, and as a result, several times more data will be collected. The number of captured decays of this type will also increase, the statistical errors will significantly decrease, and the difference from the SM, if it persists, will become much more impressive.

Recall also that two years ago LHCb reported a very similar deviation in another pair of decays: (B ^ + to K ^ + ell ^ + ell ^ – ), see news for details LHCb sees an unexpected deviation from the lepton universality and on the page Violation of lepton universality in decay B+K+ll… The ratio measured at that time by 2.6σ differed from unity, and also downward. The main difference between that work and the new LHCb result is that K-mesons at the end are different: before it was an ordinary kaon with spin 0, now it is an excited kaon with spin 1. Different spins of the final meson are a very significant moment, because of it, quarks are added in different ways to hadrons, and a lepton pair is emitted in account of a slightly different mechanism, especially in the area of ​​small q2… As a consequence, different decays allow one and the same mysterious quark process (b to s ell ^ + ell ^ – ) to be viewed from different angles. And the fact that LHCb sees the same deviations in both of them reinforces the belief that we are not dealing with a statistical fluctuation, but something more tangible. The only thing that is still unclear is whether this is an indication of a real New Physics or a flaw that has not yet been caught in the analysis of data or in theoretical calculations.

The situation today

So, it seems that through the joint efforts of several experiments, and in particular LHCb, we found a certain “pain point” in the world of elementary particles. It has been holding for several years, does not disappear, and even intensifies with the arrival of new data and the refinement of theoretical predictions. We already have several different decays B-mesons, in which something goes wrong – but what exactly is not yet clear. Moreover, judging by the theoretical analysis, all these anomalies are generally consistent with each other and collectively diverge from the SM predictions by 4σ and higher, depending on the details of the comparison.

The new result fully fits into this picture and further enhances the deviation. A few days have passed since the announcement of this result, but more than a dozen theoretical articles have already appeared in the archive of electronic preprints, including this observation in the general analysis. Now theorists are already talking about the cumulative difference from the Standard Model at the 5σ level. In the coming years, the situation will continue to worsen. Now the main source of uncertainty is the statistical error in the LHCb experiment. In a few years, when a significant part of the Run 2 statistics is processed, this error will decrease by a factor of two or three – and then what now seems to be a hint may develop into a full-fledged discovery. Anyway, thanks to the LHCb experiment – I must say, the only one of the LHC experiments that regularly delivers positive results! – we will definitely not be bored.

A source: S. Bifani (for LHCb Collaboration). Search for new physics with bsl+l decays at LHCb // talk at the LHC Seminar on April 18, 2017.

Additional materials:

1) Popular review of LHCb results.
2) LHCb finds new hints of possible deviations from the Standard Model // CERN press release.
3) The standard model is in question: is lepton universality fulfilled in the decays of pretty mesons? // a note on the website of the INP SB RAS (Novosibirsk).

Igor Ivanov

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