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LHCb and CMS see rare decay of the strange B meson

A strange B meson decay event as seen by CMS (Courtesy: CMS)

A strange B meson decay event as seen by CMS. (Courtesy: CERN)

By Hamish Johnston

It’s a story with a hint of both “man bites dog” and “dog bites man” about it.

Physicists working on the CMS and LHCb experiments at CERN have independently seen an incredibly rare decay of a particle – a strange B meson decaying into two muons. The odds of this meson decaying in this particular way is about one in a billion, making the joint discovery a triumph of experimental particle physics. And it is officially a discovery. That’s because when data from the two experiments are combined the observation has a statistical significance of greater than 5σ, which is the gold standard in particle physics.

So much for “man bites dog” – the “dog bites man” aspect of this discovery is that the decay occurs just as predicted by the Standard Model (SM) of particle physics. Some physicists had been hoping that its discovery could point to physics beyond the SM, perhaps offering up tantalizing clues of supersymmetry for example.

The combined LHCb and CMS results are presented in this report published by both experiments.

Continuing with trite analogies, if you are a “glass half empty” type of person you might see this as more evidence that the LHC is failing in one of its key missions: to find physics beyond the SM. I prefer the “glass half full” option and look forward to the LHC restarting at 13 TeV, when it might just push the SM to the breaking point.

In the meantime, blogger Tommaso Dorigo has written an intriguing post about another rare B meson decay that could point beyond the SM.  It seems that certain aspects of the decay as measured by LHCb don’t agree with the SM – and the certainty of these observations could be as high as  3.7σ. The glass is slowly filling!

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  1. Jack Miller

    I can’t help buy think that, as the SM predicts decay after decay to within experimental accuracy, that we should perhaps put a bit more pride in the giant Lagrangian that is the closest thing to a god I believe in. It really is phenomenal that we can predict quantities such as $g$ to one part in 10^14.

    I know that there are many good reasons why we know that the SM can’t really be the whole story — and I look forward to seeing what the answer will be (SuSy is beautiful) — but it’s still worthwhile pointing out that this paper’s basically going ‘Hey guys, we measured something INSANELY accurately, and it bloody well agreed with the theory AGAIN…’.

    I just wish my research had such problems!

  2. M. Asghar

    The low decay probability of 1 in 3.10^9 of the strange B meson into two muons comes about, because the decay process is not simple, but it has to go through a guantum loop, where the beyond-SM particles such as those of SUSY, may come in and effect the result. However, even here the SM as an effective field theory stands firm. Here, one may ask: Does it mean that beyond the SM, there is nothing but empty desert for physics or that the energy needs for the things beyond the SM are beyond our reach with the earthly machines?

  3. G Dixon

    Preface: I have never found supersymmetry beautiful. Many do. Many of those will hold on to the notion that there is enough leeway/fudge-room in the model to allow for adjustments to this – and probably many more – experimental results at odds with SS. Supposing there is some success with this, a question needs then to be asked – the big question all theorists should carry in the back of their minds at all times: if your model is shown to account for everything (or even just a lot), how will you explain how it’s success was inevitable, and the alternatives impossible? (I am a big believer in the notion that were we given a real ToE, if we could understand it, we would be blown away by its unique inevitability.)

  4. Emily Bug

    I recently read a physics book(written by Sean Carroll). He talked about some of the stuff(I don’t like using the word “stuff”, but it is the only word I could think of) mentioned in this article, and I could understand the article and what this author was referencing! I had one of those happy moments where I applied new knowledge to something I read!


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