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All hail the Standard Model, once again


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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  1. M. Asghar

    Although a good result on the decay of Bs and B^0 with a probability of the order of one per billion, the SM is still smugly all smiles of authority waiting for the 13 TEV thrashing.

  2. John Duffield

    The Standard Model is much trumpeted, Hamish, but it isn’t nearly as good as people make out. Have a read of Science or Fiction? by Ofer Comay for some of the issues. I reviewed it. NB: re dark matter and the “mystery” of the missing antimatter, the Standard model won’t tell you that inhomogeneous space has its vacuum energy which has a mass equivalence, or that positronium is like light hydrogen.

  3. M. Asghar

    The so-far-known four fundamental interactions of Nature and their manifestation in the micro- to the macro-worlds has been, is and will be the essence of physics. Anything else is just fiddling around.

  4. MJ Bridger

    Perhaps you would like to elucidate then.
    What are particle physicists looking for (with the LHC or bigger) that they think will put them on a path to a theory of everything?
    They should have some idea.

    • Abed Peerally

      The ultimate physics debate seems to be getting hotter. Both Hamish and Bridger raise useful points. So far the SM is useful for having endowed us with a model which tells us what we can presently scientifically advocate. It does not pretend it is the last word in physics reality. One of the saddest issues in Physics is the disproportionate belief that a Theory of Everything (TOE) is in the corner just waiting to be seen. This view is a huge exaggeration and basically a wrong belief which the Nature paper highlighted by Hamish seems to insinuate. To start with the SM as it so now, with a Higgs boson having found a still dodgy place, is what we have to deal with inevitably. There will have to come a new chapter, the post QT chapter, which will give a new interpretation to what the SM currently stands for. The scientific breakthrough will emerge not so much from higher and higher energy accelerators as Arkani-Hamed believes but by increasingly better theoretical physics breakthrough on what could be our ultimate realities. The whole physical reality behind the Higgs, the origin of the universe, the nature of elementary particles and many physics concepts like Bell’s theorem, relativity and QT etc etc require to be better understood. It is not true that particles physics work in accelerators of the kind of relatively low energies we have currently and with some higher energies as planned in China and the US in the coming years will reveal a lot more. The talk about particles like the B mesons, neutrinos changing from one type into another for instance in CPT violations require a lot more research. But what is clear to me is that the SM is not our last word and there is new physics which underpins QT and which will also underpin the whole subject of Physics and of the whole domain of science and even of philosophy. Hamish referred to “Particle physics discovery raises hope for a theory of everything”. This kind of interpretation will not the least lead to a TOE which is a hugely complicated affair and there is nothing currently in our knowledge which even gets near to a TOE. You only need to look at the various concepts about QT and of the origin of the universe to see we are nowhere getting nearer to the explanation of our ultimate realities. However it is not impossible to unravel the TOE I believe.


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