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Everyday science

Everyday science

All hail the Standard Model, once again

15 May 2015 Hamish Johnston


By Hamish Johnston

I am a condensed-matter physicist by training and sometimes I struggle to get excited by the latest breakthrough in particle physics – usually because most don’t seem much like breakthroughs to me. The latest hot paper from physicists working on the Large Hadron Collider (LHC) at CERN is a perfect example of what I am talking about.

Writing in Nature this week, physicists working on the CMS and LHCb experiments at CERN announced the discovery of a rare decay of the strange B-meson, as well as further information regarding an even rarer decay of the B0-meson. In both cases the decays produce two oppositely charged muons. An animation of how the strange B-meson decay is detected by the CMS appears in the video above.

The Standard Model (SM) of particle physics predicts that both processes are very unlikely, and this means that both decays should be very sensitive to the existence of physics beyond the SM. In other words, if the measured decay rates differ from those predicted by the SM, then this could provide important clues about physics mysteries such as dark matter and the dearth of antimatter in the universe. Indeed, a deviation could even be an important milestone on the long journey towards a “theory of everything” that reconciles the SM with the general theory of relativity.

But alas the SM is a tough nut to crack, and the combined CMS/LHCb decay rate for the strange B-meson is just as predicted by the SM. The B0 decay rate also appears to fall in line with the SM; however, it is not yet deemed a “discovery” because the statistical significance of the measurement is only about 3σ – well short of the required 5σ.

So much for that, you might think – or maybe not?

Writing in The Conversation, Vakhtang Kartvelishvili points out that the measured B0 decay rate is about four times larger than that predicted by the SM – while still being statistically compatible. If the universe is kind to particle physicists, this discrepancy will endure as more data are collected on the decay in the upcoming run of the LHC.

Kartvelishvili works on the ATLAS experiment at CERN and is based at the University of Lancaster in the UK. His article about the Nature paper is called “Particle physics discovery raises hope for a theory of everything” and its title reflects the hope in the particle-physics community that important clues about physics beyond the SM could soon be forthcoming in the next run of the LHC.

But what if the universe is very unkind? Some have called this the “nightmare scenario of particle physics” in which physicists build increasingly energetic accelerators but never reach energies high enough to see physics beyond the SM. With collisions at 13 TeV expected soon at the LHC, we won’t have long to wait to see if a breakthrough is forthcoming.

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