Tag Archives: rare decays

First News from Kyoto Conference

The HCP [Hadron Collider Physics] 2012 conference in Kyoto is underway.  After opening talks laying out the field’s future, the main topics today have been

  • Collisions of heavy ions (specifically of lead or gold nuclei) and generic proton-proton collisions
  • Processes involving “heavy flavor” (meaning in this case the properties of hadrons containing bottom and charm quarks.)

Although there were a number of interesting new results from several experiments, today’s highlight so far has been a presentation on a new measurement by the LHCb experiment, one of the special-purpose experiments at the Large Hadron Collider [LHC], of a rare decay of a B_s meson to a muon and an antimuon.  I described this process in some detail, and claims and counterclaims about it, in the first portion of an article last year; the details of the measurements are out of date, but the physics process is, of course, the same.

Today the LHCb experiment, for the first time, announced evidence for the existence of this process, using their data collected in both 2011 and 2012.  In the Standard Model (the equations that describe the known particles and forces), it is predicted that about one in about 300,000,000 B_s mesons (hadrons containing a bottom quark and a strange anti-quark, or vice versa) should decay in this fashion.  The measurement that LHCb has made is completely consistent with this prediction.

[In detail, the Standard Model predicts (3.54 ± 0.30) × 10-9 {including mixing effects} and LHCb measures (3.2 +1.5[−1.2])× 10-9.]  The measurement is at the level of 3.5 standard deviations — evidence, but not yet a convincing observation.  LHCb excludes a rate of zero at much better than 95% confidence; their 95% confidence lower bound on the process is 1.1 × 10-9.]

The detailed implications of this result will take a while to work through, but the general implication is easy to state: the Standard Model has survived another test.  And the constraints from LHC data on speculative ideas that predict particles and forces beyond those of the Standard Model have become tighter.  Many variants of these speculative ideas would have affected this process, and the more precisely the data matches the Standard Model prediction, the more of these variants are excluded by the data.

Note Added: CMS says they a very good chance of being able to confirm LHCb’s result using the full 2012 data; not sure what ATLAS says about their chances.

2nd Note Added: Gino Isidori, in his talk on the theoretical perspective on heavy flavor physics, emphasized that a future high-precision measurement of this process will be very important; it can be predicted with high precision, and interesting variants of speculative ideas often have effects on its rate that are between 10% and 100%.