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6th August 2013

First experimental signs of a "New Physics" beyond the Standard Model

New data from the Large Hadron Collider (LHC) – based on measurements of the B meson – could offer the first direct glimpse of new physics beyond the Standard Model.

 

LHC

 

A team of physicists from the Universitat Autònoma de Barcelona (UAB) and the French CNRS has predicted deviations in the probability of one of the B meson decays that have been detected experimentally in the LHC accelerator at CERN. Confirmation of these results would be the first direct evidence of "new physics", a more fundamental theory than the current Standard Model.

The Standard Model has given us the most complete explanation of the universe so far. This model has gaps, however, and cannot explain phenomena like dark matter or gravitational interactions between particles. Physicists are therefore seeking a more fundamental theory called "New Physics" – but until now, there has been no direct proof of its existence, only indirect observations of dark matter, as deduced, among other things, from the movement of galaxies.

A team of physicists – formed by the professor of Physics at Universitat Autònoma de Barcelona (UAB) Joaquim Matias, Javier Virto, postdoctoral researcher at the same university, and Sebastien Descotes Genon, from the Centre National de la Recherche Scientifique (CNRS) / Université Paris-Sud – has predicted that New Physics would imply the existence of deviations in the probability of a very specific decay of a particle, the B meson. Detecting these small deviations through an experiment would be the first direct proof of the existence of this fundamental theory.

 

lhc
@ CERN

 

At the recent EPS 2013 conference on particle physics held in Stockholm, scientists at the LHCb detector – one of the large experiments being conducted by the LHC – presented the results of experimental measurements of B meson decay. Their measurements showed deviations with respect to the predictions of the Standard Model that were previously calculated by UAB and CNRS researchers. The team of scientists have demonstrated that all these deviations show a coherent pattern and that has allowed them to identify their origin from a unique source.

The results of their analysis point to a deviation from the Standard Model prediction of 4.5 sigmas. If confirmed, this is a major event, as scientists regard 3 sigmas as "scientific proof" of New Physics and 5 sigmas as a "discovery."

"We must be prudent, because more studies and more experimental measurements will be needed for confirmation," explains Joaquim Matias, "but if they are confirmed this is the first direct proof of New Physics, a more general theory than the current Standard Model. If the Higgs completed the Standard Model puzzle, these findings could be the first piece in an even bigger puzzle."

The researchers claim that one of the New Physics models that could explain these results would be the one that postulates the existence of a new particle named Zprima, "but there could be lots of compatible models," says Matias.

The findings are so interesting that scientists at the other main LHC experiment – the CMS detector – want to take these measurements as well. The CMS has invited Dr. Matias to explain the theoretical details in a seminar to see if the results can be corroborated.

 

lhc diagram

Credit: Arpad Horvath (CC BY-SA 2.5)

 

For years, particle physicists have known that the Standard Model – despite being a very successful model in all tests carried out so far – has significant deficiencies. These include the lack of a candidate for dark matter, the huge discrepancy between aspects of the weak nuclear force and gravity (two of the four fundamental forces in the universe), and the matter-antimatter asymmetry that occurred at the moment of the Big Bang.

Two of the central goals for the LHC at CERN are finding the Higgs boson and finding what is called New Physics, a more fundamental and general theory than that of the Standard Model in which the latter would be just one particular case. Just a year ago, the Higgs boson was discovered, but the particle seems to fit perfectly into the Standard Model and currently gives us no clues regarding New Physics.

At CERN there are four experiments, four large detectors (ATLAS, CMS, LHCb and Alice) that record collisions between particles so that scientists can study their behaviour. The LHCb detector is designed to study the behaviour of quarks and what are known as rare decays, which are very infrequent.

Dr. Matias presented theoretical predictions of his research team on one of these decays: that of a B meson, formed by a b quark and d antiquark, into a pair of muons and a particle called K*. The UAB and CNRS researchers calculated and predicted how this decay should work and how it should change in different New Physics scenarios.

Shortly afterwards, an experimental physicist from the LHCb detector, Nicola Serra, presented at the same conference the first completed experimental results of that decay. Surprisingly, the experimental measurements were consistent with the deviations predicted by Joaquim Matias and his collaborators. For the first time, deviations of this type were consistent with theoretical predictions based on the presence of contributions that transcend the Standard Model. LHCb is now adding new data to improve the statistics and will confirm the measurements in March 2014.

 

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