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    LHCb: Evidence For Rare Decay Bs To Dimuons
    By Tommaso Dorigo | November 12th 2012 10:26 AM | 9 comments | Print | E-mail | Track Comments
    About Tommaso

    I am an experimental particle physicist working with the CMS experiment at CERN. In my spare time I play chess, abuse the piano, and aim my dobson...

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    It has taken a while, but the rare decay of B_s mesons (particles composed of a bottom and an anti-strange quark) to muon pairs has finally been seen. The authors of the find -we cannot yet call it an observation given the scarce statistical significance of the signal- are the members of the LHCb collaboration, one of the four experiments working with the proton-proton collisions delivered by the Large Hadron Collider at CERN.

    The decay of the B mesons to muon pairs are quite rare -of the order of one in a billion- but very important, because they proceed via loop quantum diagrams within which existing particles may circulate. And since these loops are virtual, even very massive particles, even ones we do not know yet about, would produce a significant contribution. So measuring the rate of the rare decays allow us to gauge whether there is new physics in store for us, or whether there is just a desert of Standard Model physics awaiting us at the high-energy frontier.

    The CDF and DZERO collaborations at the Tevatron collider have sought for these rare decays for two decades, and now the CERN experiments have taken over. The LHCb detector is quite well suited to spot these decays, since it looks at very forward-going proton-proton collisions, ones which produce B quarks in large amounts. Eventually CMS and ATLAS will take over (they have collected a larger integrated luminosity), so now is the time for LHCb to show their stuff. And they do.

    The figure below, hot off the press, shows the LHCb data in a reconstructed dimuon mass distribution. A B_s signal seems to be there (the red component; the blue line is the total fit, which includes backgrounds), although the statistical significance is still in the range where fluctuations might be the real source of the bump. LHCb estimates it as a 3.5 standard deviations effect.



    For more information on the analysis, see here. Tomorrow a seminar at CERN will provide all the details on this new search. What can be noted already, despite the fact that the decay has not been "observed" as canonically required with five standard deviations of more, is the fact that many new physics theories receive a hard blow by the very good match between Standard Model prediction and observed rate.

    Comments

    Tommaso,

    I read somewhere that this is considered a blow to SUSY... is that right? Why would it be such a blow?

    Thanks!

    "The decay of the B mesons to muon pairs are quite rare -of the order of one in a billion- but very important, because they proceed via loop quantum diagrams within which existing particles may circulate. And since these loops are virtual, even very massive particles, even ones we do not know yet about, would produce a significant contribution. So measuring the rate of the rare decays allow us to gauge whether there is new physics in store for us, or whether there is just a desert of Standard Model physics awaiting us at the high-energy frontier."

    SUSY is one of the 'new physics' models that could increase the rate of this decay by some of its SUSY particles. And since there is no increase observed, this measurement will eliminate some of the possible SUSY models.

    Thanks! So SUSY may include some additional heavy particles that would decay and increase that observed probability.

    Hi there,
    we (the MasterCode collaboration) took the latest results of LHCb on BR(Bs -> mu mu) into account in our SUSY fits, but the effect is not large, as can be seen here: http://mastercode.web.cern.ch/mastercode/news.php .

    Cheers, Sven

    dorigo
    Hi Sven, thanks for the link to your plots. One question I have is the following:

    You show the "best fit" points  in the M0-M12 plane accounting for all measurements.
    Then we update a measurement, and you show the new best-fit result. This does not
    change much.

    My question is: can we plot these delta-chisquare contours with respect to the old
    minimum ? Or equivalently, state what is the best-chisquare in the two cases.

    Cheers,
    T.
    Hi Tommaso,
    now we have a few more plots on our web page that go in this direction, e.g. plots showing the values for BR(Bs -> mu mu) in the m0-m1/2 plane. Since the change with the new LHCb result is small, one can estimate the chi^2 contribution.

    Cheers, Sven

    ust for the record, in order to warn any non-westerners:

    "The cost [...] has been evaluated, taking into account realistic labor prices in different countries. The total cost is X (with a western equivalent value of Y) [where Y>X]

    source: LHCb calorimeters : Technical Design Report

    ISBN: 9290831693 http://cdsweb.cern.ch/record/494264

    http://cdsweb.cern.ch/record/1127343?ln=en

    Hi
    When will LHC update its higgs search result.?
    Thanks.

    dorigo
    ATLAS and CMS will present updates at the HCP conference this week.

    Cheers,
    T.