Search for the Lepton Flavor Violating Decays Bs,d → eμ 

This web page summarizes the results of the search for the lepton flavor violating decays Bsd->eμ. These decays are forbidden within the original Standard Model of particle physics  in which leptons do not change flavor. For example, a muon would never turn into an electron. If this were observed, it would be a signal for new physics. The  observation of  neutrino oscillations has shown that lepton flavor changes actually occur in nature and therefore the  standard model needs to be modified to incorporate this. This analysis represents a search for lepton flavor changes in a different channel.  

CDF performed this measurement in Run-I  [PRL(81) 1998]. The Run I measurement was an absolute measurement using  an electron-muon triggered sample with an integrated luminosity of 102 pb-1 . For Run II no suitable electron muon trigger exists,  therefore we employ a different measurement strategy. In Run-II, we perform a  relative measurement using samples collected by the two-track SVT trigger and using candidate Bd->Kπ events collected with the same trigger path for a relative normalization.  The branching ratio Br( Bd->Kπ) is measured to be 19.4 +/- 0.6 x 10-6.  This way, many of the common reconstruction efficiencies due to tracking, triggering, and vertexing, cancel. The remaining effects that still have to be considered are the relative detector and kinematic acceptance including the mass window requirement and the electron and muon identification efficiencies. To see all the ingredients that need to be measured to perform this measurement and for the formula relating the number of observed candidate events to the branching ratio Br(Bs→eμ) click here
Once we obtain the limit on the branching ratios we can use this result to constrain the masses of leptoquarks  predicted by a specific theoretical model. The decays  Bsd->eμ are possible in some theoretic models containing lepton-flavor violation tree-level couplings  mediated by leptoquarks. The Grand Unification Theory (GUT) by J. Pati and A. Salam predicts  spin 1 gauge bosons the so called “Pati Salam Leptoquarks” ( PSLQ) that carry both color and lepton quantum numbers  [PRD 10,275 (1974)] (see Feynman diagram below). It is the simplest model based on  the symmetry group SU(4)c where the lepton number is the fourth “color” . Within the Pati-Salam model, one can derive a relationship between Br(Bs→eμ)  and  the leptoquark mass  (see G. Valencia, S. Willenbrock [PRD 50,(1994)]). We will use this relationship to constrain the leptoquark masses.  For the function relating the branching ratios to the Pati Salam leptoquark masses click Bs or Bd



Using the same technique as for the Bsd->eμ search (with the difference that both tracks are required to be identified as an electron) we also performed a search for the decay Bsd->e+e-. This decay can occur within the standard model at a very low rate. 

With 2fb-1 of CDF run II data we measure :

(all results are Bayesian limits given at 90 (95)% confidence level)

Br(Bs→eμ) < 2.0(2.6) x 10-7 
MLQ(Bs) > 47.7(44.6) TeV
Br(Bd→eμ) < 6.4(7.9) x 10-8
MLQ(Bd) > 58.6(55.7) TeV
Br(Bs→e+e-) < 2.8(3.7)x 10-7
Br(Bd→e+e-) < 8.3(10.6)x 10-8

A detailed description of the results can be found in this public note: (pdf)

This results have been presented for preblessing  May 22, 2008 and for blessing on June 12,2008. This results were also presented as Wine & Cheese seminar on Friday June 20 2008: "Recent Results in Rare Heavy Flavor Decays at CDF" pdf, ppt.

Below are the eps and gif versions of all figures meant for downloads.