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an abnormal and “exciting” particle decay at CERN


The CERN particle accelerator, at Echenevex (Ain), February 6, 2020.

Is a serious crack appearing in the beautiful edifice of particle physics? More specifically, has the current theory, known as the standard model, which has so far described all the phenomena of the infinitely small perfectly well, been faulted for the first time? The international LHCb experiment, installed at CERN in Switzerland on the LHC underground particle accelerator, has, in any case, just published the most precise image of an index pointing in this direction.

But is it just the plaster that cracks or the wall itself that moves? It is too early to tell, because these results could just as well be explained by chance: drawing an abnormal succession of double sixes does not necessarily imply that the dice are loaded. “There is a one in 1000 chance that our result will remain fully compatible with the standard model”, confirms Chris Parkes, spokesperson for LHCb, who outlined his conclusions during a remote conference at CERN, Tuesday 23 March.

The researchers therefore observed an anomaly in their detector 20 meters long and 4,500 tonnes. The LHC accelerator violently propels packets of protons, the main components of ordinary matter, against each other. Among the debris, in 2012, CERN had identified the Higgs boson, the last particle to complete the Standard Model. But other particles are produced. Some, called mesons, can themselves decay into other particles that the LHCb experiment counts.

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And, even more rarely, on the order of once in a million collisions, one of these mesons can disappear in a particular way. Either by emitting a pair of electrons and anti-electrons. Or a pair of muons and antimuons (a cousin of the electron twice as heavy).

“Lepton universality”

The Standard Model predicts that these two fates are equally probable and specialists even speak “Leptonic universality” (electrons and muons belonging to the lepton family). But experience shows that the first is 15% more frequent than the second … And this difference, if it is confirmed, can therefore only be explained by adding new laws, new forces or new particles, completely. outside the current framework. It would then be the foundations that would move.

“I am cautiously excited”, summarizes Chris Parkes, who recalls that other experiments such as BaBar in the United States, Belle in Japan, or the other experiments at CERN, Atlas and CMS, have already found clues in the same direction, although less precise. “It is more a satisfaction than an excitement, because indeed these results go in the direction of other data, recalls Sébastien Descotes-Genon, deputy director of the Irène-Joliot-Curie Physics Laboratory of the Two Infinities (Orsay). But, with this precision, a psychological threshold has perhaps been crossed. “

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