First measurement of the ratio of branching fractions Br(lambdab to lambdac mu nu)/Br(lambdab to lambdac pi) at CDF II.

摘要

We present the first measurement of the ratio of branching fractions B&parl0;Lb→L+ cm-n&d1; m&parr0;/B&parl0;Lb→ L+cp-&parr0; based on 171.5 pb-1 of pp¯ collisions at s = 1.96 TeV taken with the CDF-II detector. In addition, we present measurements of B&parl0;B0→D*+ m-n&d1; m&parr0;/B&parl0;B0 →D*+p-&parr0; and B&parl0;B0→D+m -n&d1;m &parr0;/B&parl0;B0→D +p-&parr0; , which serve as control samples to understand the data and Monte Carlo used for the Lb analysis. We find the relative branching fractions of the control samples to be: B B0→D* +m-nm B B0→D *+p- =17.7+/-2.3stat +/-0.6syst +/-0.4BR +/-1.1UBR, and B B0→D+ m-nm B B0→D +p- =9.8+/-1.0stat +/-0.6syst +/-0.8BR +/-0.9UBR, which are consistent with the ratios obtained by the Particle Data Group at the 0.7 and 1.1 sigma level, respectively. Finally, we obtain the relative Lb branching fraction to be: B&parl0;L b→L+cm- n&d1;m&parr0; B&parl0;Lb→L +cp-&parr0;=20.0 +/-3.0stat+/-1.2 syst+0.7-2 .1BR +/-0.5UBR. The uncertainties of the three relative branching fractions are from statistics, CDF internal systematics, external measured branching ratios and unmeasured branching ratios, respectively. We present a method to derive B&parl0;Lb→L+ cp-&parr0; using previous CDF measurements and obtain B&parl0;Lb →L+cp-&parr0; =0.41+/-0.19stat⊕syst +0.06-0.08&parl0;PT&parr0; %, where the last uncertainty is due to the measured Lb PT spectrum. Combining B&parl0;Lb→L+ cp-&parr0; with our result, we determine the exclusive semileptonic branching fraction for the Lb ; B&parl0;Lb →L+cm- n&d1;m&parr0;= 8.1+/-012stat+1. 1-1.6syst +/-4.3&parl0;B&parl0;Lb→ L+cp-&parr0;&parr0; %.