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B New Findings from LHC?

  1. Mar 9, 2016 #1
  2. jcsd
  3. Mar 10, 2016 #2
    Update: It's now morning, and I'm no longer lazy. I can't figure out how to delete/edit the above post, but I do want to hear some insight on how significant this may or may not be, and when we might know more.
  4. Mar 10, 2016 #3


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    It is interesting, but the significance is not that large, and it is in a variable where theory predictions depend on messy QCD calculations. LHCb had a similar significance with 1/3 of that dataset. As a result, theorists investigated that more closely and found some issues with previous calculations, so the central value shifted a bit and the theory uncertainty went up. Now the experimental uncertainty went down so we are back at 3.4 sigma...

    A while ago there was a similar story with ##\Delta A_{CP}## in charm mixing. Theory predictions for this parameter are tricky, but the general consensus was "the variable should be about 0, maybe ##\pm##0.1%". The world average from previous experiments was something like -0.2% ##\pm## 0.2%, when LHCb measured -0.82%, 3.5 sigma away from 0 (arXiv). It triggered a lot of discussion, theorists looked at it more carefully and figured out that the value could be larger (in magnitude) than -0.1%. Well, it turned out to be just a statistical fluctuation, with more analyzed channels and larger datasets the measured value got much closer to zero again.

    The angular analysis here becomes more interesting if you combine it with a few other 2-3 sigma effects in B-physics - there are some BSM ideas that could explain multiple of them at the same time. On the other hand, some experimental systematics are correlated between the analyses, so a common origin there cannot be excluded either.

    More data will certainly help - 2016 and 2017 will increase the datasets significantly.
  5. Mar 10, 2016 #4
    Hi all, I've been following the "b->s ll" transition measurements closely these past few years.

    The rare decay B->K*mumu, allows to probe these transitions in an exclusive way (you have a specific kaon in the final state). The problem with exclusive decays is that theoretically you rely on having knowledge about how the quarks (the b, s and the spectator quarks) are really in hadronic states.

    However, it is not really clear if the different theoretical approaches are really the right approach. For example, a general approach is QCD factorisation, and maybe there are corrections which cannot be factorised which are large. A scenario where this might be true is near the charm resonance. Unfortunately, the interpretation of the angular analysis as evidence for new physics has exactly the same type of signal as an underestimated non factorisable correction near the charm resonances. This is where the data deviates from the "standard model" is largest.

    In view of this, I take the approach that believing the angular analysis is not a safe observable to look for new physics (particularly near the charm resonances, this is less so true at extreme values of dilepton mass).

    I would say it's necessary to use different observables (like the muon vs electron final state ratios) and wait for measurements of inclusive observables (b -> s ll, where you look at all final states with a strange quark). These are theoretically clean.

    If new physics really is there, it will show up for these observables. Like always, "wait for more data and more measurements"
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