Watching for Bs to mu+mu-
On Tuesday, November 13th, Matteao Palutan representing the LHCb Collaboration will report new results on the search for the extremely rare decay Bs→μ+μ–.
At tree level, this decay is forbidden in the standard model. It can occur through a loop diagram, however, involving a top quark and W bosons that are far, far off mass shell:
The SM prediction is really very small: the branching ratio B(Bsmumu) = (3.2±0.2)×10-9.
Since this decay is almost completely absent in the standard model, it provides a very good opportunity for new physics to appear — any observation of this decay above the SM rate would be a clear signal for new physics. Indeed, many models of new physics allow for branching ratios a factor of ten or one hundred higher than the SM value. Chief among theses is generic Supersymmetry, which predicts large enhancements when tanβ is large (20 to 50) and when MA (the mass of the pseudoscalar Higgs boson) is not too large (less than 200 GeV).
This opportunity has enticed experimentalists for nearly twenty years, and a series of searches by CDF and D0 put more and more stringent bounds during the 1990s and 2000s. See, for example, a discussion of a D0 result in 2010 by Tommaso Dorigo. The Tevatron limits were about an order of magnitude above the SM branching ratio.
Early in 2012, the CDF Collaboration reported a two-sided confidence interval for B(Bsmumu), meaning that they had evidence for a signal although they did not use those words. They used an artificial neural network to categorize the events. Using the very best candidates, they reported B(Bsmumu) = 1.3+0.9-0.7×10-8. This result generated some interest and much discussion (e.g., Tommaso’s blog),
needless to say. (For information, see this cdf web page).
The advent of the LHC opened new opportunities to observe this decay. Early results from CMS and LHCb excited experts. The superior capabilities of the CMS and especially the LHCb detectors make the searches for this decay more effective. The luminosity and higher center-of-mass energy deliver much larger data samples than the Tevatron collaborations enjoyed. To see how good the data are, here is a beautiful event from the LHCb Collaboration:
The two pink tracks are the muons, and the blue track shows how the Bs flow our from the primary vertex. More event displays of this type can be viewed at the LHCb web page.
The CMS Collaboration recently published a result based on 5 fb-1 of data taken in 2011 at √s = 7 TeV (arXiv:1203.3976, March 2012): B(Bsmumu) < 7.7×10-9 at 95% CL.
At the same time, the LHCb Collaboration published a slightly more stringent result (arXiv:1203.4493 March 2012): B(Bsmumu) < 4.5×10-9, based on 1 fb-1.
Combining the results from LHCb, CMS and ATLAS, the upper limit is B(Bsmumu) < 4.2×10-9 (combination note).
These limits are rather close to the SM value, so defining the expected limit is tricky: does one make a calculation assuming no signal, or does one assume that the SM process will indeed produce events? These graphs from the combination note make plain that the two calculations are very different:
The plot on the left is calculated assuming the SM contribution, while the plot on the right assumes no contribution from any source.
It seems clear that the LHC experiments are on the verge of observing a signal for this process, if only at the level the SM predicts. CMS has approximately 20 fb-1 at 8 TeV, while LHCb has a data sample more than twice the size of the one used for March’s publication. So it should be very interesting to listen to the presentation by M. Palutan on Tuesday.
The INDICO web page for this seminar is: https://indico.cern.ch/conferenceDisplay.py?confId=216344, and there will be a web retransmission.
Let’s see whether LHCb reports the first observation of this important decay mode…
Update: The news from LHCb will be presented on the first day of the HCP Conference by Johannes Albrecht, at 16:10 in Tokyo which is 8:10 in Geneva (1:00 am in Chicago). The INDICO page for HCP is http://kds.kek.jp/conferenceDisplay.py?confId=9237.
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