LHC study hints at physics beyond the Standard Model

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After analyzing LHC data, researchers are cautiously optimistic that they have found evidence for physics that doesn’t fit the Standard Model. The decay of particles apparently does not proceed as that model predicts.

Physics that do not fit within the Standard Model of particle physics, the theory that describes how all forces and particles relate to each other, could lead to answers to fundamental physics questions. For example, the current model notoriously cannot explain how gravity works and whether so-called dark matter exists and what it consists of. The researchers who have observed phenomena from the data from the Large Hadron Collider at the CERN research institute that do not fit within that model are therefore excited.

Experiments with particle accelerators are being carried out in the Large Hadron Collider, in which, among other things, protons collide with high speed and energy. At such energy levels, very short-lived unstable particles are formed that decay into more stable elementary particles, according to a process that also took place at the origin of the universe. The researchers from the universities of Bristol, Imperial College and Nikhef, among others, analyzed the decay of specific particles, so-called B mesons, which, according to the standard model, have to decay into a K meson and two leptons. Those leptons, a collective term for electrons, muons and tau leptons, would all interact equally with other particles. That would also mean that they would be produced in equal proportions in the decay process.

Meson decay in leptons (image: CERN / LHCb / PA)

The analysis of collisions in the LHCb detector of the particle accelerator showed that this ratio was not equal, and more electrons than muons were counted. That would suggest that virtual particles play a role in the decay of the B mesons that don’t fit the Standard Model. The data that has been analyzed so far does not provide a definitive answer as to whether the observation is actually significant. The uncertainty, or standard deviation, in the observations is still fairly large: the researchers report a sigma of 3.1. The chance that the observation is based on chance is roughly one in a thousand. It is only at a sigma of 5, where the chance of coincidence is in the millions, that particle scientists are generally more certain in their claim.

One possible explanation would be that a theoretical particle, a leptoquark, causes differences in interaction with one of the fundamental forces of nature during the decay. Other possible explanations could be sought in the influence of a boson, extra dimensions or supersymmetry. One of the other detectors at the Large Hadron Collider, CMS, is also looking in collected data for evidence of the discrepancy in lepton decay. If the LHC can be turned on again next year, the LHCb experiment, with an improved detector, will also continue to look for evidence.

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