The most accurate measurement of the decay of mesons. Standard Model still arch-accurate

Scientists studying meson decays at the LHCb experiment at CERN hoped to find a breakthrough in the Standard Model. And here – as if out of spite – the theory again turned out to coincide surprisingly with experience.

The Standard Model consists of theoretical tools constructed in the 1970s. ub. age to describe phenomena occurring in the world of elementary particles. But he, like any other physical theory, is always only a simplified representation of reality and, consequently, has a certain range of applicability.

Many scientistsów working on the LHCb experiment at CERN hoped that the just-achieved, exceptional accuracy of measurementsóin the rare decay of the beautiful meson Bs0 will finally allow to determine the limits of the Standard Model and reveal the first phenomena unknown wspón modern physics. Meanwhile, the spectacular result of the latest analysis has only expanded the Model’s applicability. The research is summarized in a press release from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Krakow, Poland.

Mesons are intractable particles formed m.in. as a result of proton collisionsóin the LHC gas pedal at CERN near Geneva. Amongód physicistów there is a belief that in certain very rare decays of these particles, processes can potentially take place thatóre would lead science on the trail of new physics, occurring with the participation of hitherto unknown elementary particles. Scientists participating in the LHCb experiment at the Large Hadron Colliderów have been looking at one such decay for a long timeów: the decay of the beautiful meson Bs0 into muon and antimuon. The latest analysis, conducted for a much larger number of cases than beforeów, has achieved exceptional accuracy for this kind of observation. Her results are in excellent agreement with the predictions of the Standard Model.

– The latest measurement is a spectacular victory, except that somewhat. pyrrhic. This is because it is one of the few cases of theów when such a high correspondence between experience and theory, instead of rejoicing, slowly begins to worry. As the accuracy of the measurement improvesóin decayóin the mesonóat Bs0 we expected to see new phenomena, going beyond the Standard Model, about which theóWe know that it is certainly not a definitive theory. However, instead of rejoicing at the discovery of the harbinger of a scientific revolution – we have only demonstrated that the Model is more accurate than we originally thought! – mówi prof. dr hab. Mariusz Witek of the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Krakow, Poland.

An extremely rare phenomenon

Decay of the beauty meson Bs0 into a muon and a positively charged antimuon occurs extremely rarely. During the period of the LHCb detector’s operation covered by the analysis, hundreds of trillionóin proton collisionsów. Whole cascades of further decaying derivative particles were recorded during each of them. From such a huge number of events, the multi-step selection process managed to select only a dozen or so cases ofóin the sought-after decay. One of them can be viewed in three dimensions at the following link.

In the latest analysis of the teamół of the LHCb experiment has taken into account data not only from the first, but róAlso from the second phase of the LHC gas pedal operation. The augmented statistics made it possible to achieve an exceptional accuracy of the measurement of the decay of the beautiful meson into muon and antimuon, amounting to as much as 7.8 standard deviations (popularly denoted by the Greek letter sigma). In practice, this value means that the probability of registering a similar result due to random fluctuation is less than one to the power of more than 323 trillionów.

– The spectacular measurement of the decay of the beauty meson into a muon-antimuon pair agrees with the predictions of the Standard Model with an accuracy as high as nine decimal places! – emphasizes prof. Witek.

"Instead of getting closer, the new physics knowów is moving away"

The Standard Model emerged victorious from another confrontation with reality. Nevertheless, physicists are sure that it is not a perfect theory. This belief is based on a number of factsów. Model in ogóle does not take into account the existence of gravity, does not explain the dominance of matter over antimatter in coóof the modern Universe, offers no explanation for the nature of dark matter, fails to answer the question of why fermions form three groups, called families of. In addition, for the Standard Model to work, it needs to include more than 20 empirically selected constants, m.in. The masses of the postólunar particles.

– Latest analysis from LHCb significantly narrows parameter valuesów, whichóre should adopt some currently proposed extensions of the Standard Model, e.g. supersymmetric theories. They assume that every type of elementary particle to date has its more massive counterpart – superpartner. Now, as a result of the measurement in question, supersymmetry theorists have fewer and fewer ways to match their theory to reality. Instead of getting closer, the new physics znów is moving away,” concludes Prof. Witek.

However, physicists do not intend to give up on studying the decay of the Bs0 meson into a pair of muon and antimuon. Indeed, there still remains the possibility that the new physics is indeed at work here, only that its effects are smaller than expected and are still lost in measurement errors.

Sourceóbackground: PAP – Science in Poland, fot. IFJ PAN, CERN, The LHCb Collaboration