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Particles and interactions

Particles and interactions

Has LHCb spotted physics beyond the Standard Model?

02 Aug 2013 Hamish Johnston
Photograph of the LHCb experiment
Is physics beyond the Standard Model lurking in LHCb? Courtesy: Maximilien Brice/CERN)


An analysis of data from the LHCb experiment at the CERN particle-physics lab suggests that the B-meson could decay in a way not predicted by the Standard Model of particle physics, according to theoretical physicists in Spain and France. The researchers believe that the deviation from the Standard Model has been measured with a confidence of 4.5σ – which is approaching the gold standard of 5σ required for a discovery in particle physics.

One of seven experiments at the Large Hadron Collider (LHC), the LHCb experiment focuses on the physics of B-mesons – those particles containing the bottom (or beauty) quark – produced during proton collisions. One process of great interest is the decay of a B-meson into a kaon (K*) and two muons: B →  K*μ+μ. This is a relatively rare decay and according to the Standard Model it occurs only because of the subtle effects of heavier particles – W and Z bosons – that mediate the weak force. As a result, particles that are not described by the Standard Model may be contributing to the decay and so their effects could be measured by LHCb. Evidence that this decay happens in a manner that the Standard Model cannot explain could point the way to “new physics”.

Now, Sébastien Descotes-Genon of the University of Paris together with Joaquim Matias and Javier Virto of the Autonomous University of Barcelona have carefully studied the LHCb data associated with this decay and believe that the results are not as predicted by the Standard Model.

Coherent deviations

The team analysed the full set of parameters associated with the decay, including experimental measurements of the angular distribution of the K* and muons. The researchers calculated how these observables deviate from the predictions of the Standard Model. The analysis suggests that these deviations are not random but rather show a “coherent” pattern, which allows the researchers to pinpoint where the deviations originate. According to Matias, when all the observables are considered collectively, the results point to a 4.5σ deviation from the Standard Model predictions. This is close to the 5σ mark, which would be heralded as a discovery.

In a paper on the arXiv preprint server, the trio describes the coherent nature of the deviations as “pointing towards specific [new physics] models”. The next step, according to Matias, is for physicists to work out which of the various models of physics beyond the Standard Model best describes the results.

A new boson?

One possibility is the existence of a new particle called the Z-boson, which would be similar to the existing Z-boson but more massive. Tim Gershon of the University of Warwick in the UK told that he remains cautious. “I for one would not bet my house on this being a first sign of new physics, but it is certainly very interesting,” he says. Gershon, who is a member of the LHCb collaboration, points out that this recent work relies on data from the 2011 LHCb experimental run and that many more data from 2012 have yet to be analysed. “They will more than triple the size of the sample,” he says, adding that it could take up to a year to analyse the 2012 data.

A preprint of the research is available on arXiv.

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