Rarest Bs decays: the final word from CDF

A comparison of current limits from ATLAS, DZero, CMS, CDF (this paper) and LHCb at 95 percent confidence limit. The SM prediction, at 68 percent confidence limit, is shown as a gray vertical band.

You may remember the excitement in July 2011 surrounding searches by CDF for extremely rare decays of Bs mesons. Bs mesons are composed of an anti-b quark and an s quark. Their decays into pairs of muons (Bs → μ+μ) are predicted in the Standard Model to be so infrequent that only a couple of them would be detected in the 500 trillion proton-antiproton collisions that occurred in Run II of the Tevatron.

This makes for an ideal testing ground for new physics theories like supersymmetry because new particles could allow this decay to happen more frequently. The CDF experiment pioneered searches for evidence of Bs → μ+μ decays back in Run I (data taken in 1992-93, results published in 1996). More recently, both CDF and DZero, as well as the LHC experiments, have been trying to shed light on these important decays. The results from 2011 revealed a slight excess in the data compared to the Standard Model, maybe the first hints of this elusive decay.

CDF physicists have now updated their results using the full CDF data set of 10 inverse femtobarns. The measured decay rate is slightly larger than, but not inconsistent with, the Standard Model prediction. It is also consistent with other results, for example, the most recent measurement by the LHCb experiment at CERN. These measurements indicate that there is no strong enhancement of this decay rate, which puts very tight constraints on many new physics theories.

The CDF result is the culmination of a program spanning nearly 20 years. The sensitivity for our search is better than the pioneering measurement by more than a factor of 800! This impressive achievement is the product of the ingenuity and perseverance of many people—not only at CDF, but also in the Accelerator Division and throughout the laboratory. Thank you, Fermilab!

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edited by Andy Beretvas

These physicists were responsible for this analysis. Top row from left: Douglas Glenzinski (Fermilab), Matt Herndon (Wisconsin), Walter Hopkins (Cornell). Second row from left: Teruki Kamon (Texas A&M/Kyungpook National University, Korea), Dae Kong (Kyungpook National University, Korea), Slava Krutelyov (University of California, Santa Barbara). Bottom row from left: Chang-Ju Lin (Lawrence Berkeley National Laboratory), Julia Thom (Cornell), Satoru Uozumi (Kyungpook National University, Korea).