319 NEW PERSPECTIVE ON B SEMILEPTONIC DECAY RATE PROBLEM FROM CLEO Youngjoon Kwon University of Rochester, Rochester, NY 14627 Abstract By studying D-lepton angle correlations, CLEO observed first evidence for has fJ-+DX decay and measured r(fJ -+ fJX)/r(fJ -+ DX) = 0.107 ± 0.029 ± O.Q18. In addition, the D-lepton correlation study enabled us to set an upper limit for inclusive charmless hadronic B decay: B(b -+ 8.5%(@90% CL). These results give us a new under­ sg) < standing of the B semileptonic decay problem. 320 Table 1: Current status of B semileptonic decay problem. b decay Branching Ratio (%) modes bsL = 10.5 ± 0.5 b-+c(u)ev, c(u)µv f>sL 2 (10.5 ± 0.5) x b-+c(u)rv �bsL 2.6 ± 0.1 b-+c(u)ud rud bsL 42.0 ± 2.0 ± 4.2 b-+c(u)c s D, 10.0 ± 2.7 (cc) 3.0 ± 0.5 baryons 1.1 ±0.8 upper vertex 150, D- O?? b-+s(d)g, s(d)qq 1?? [ Total 80.7 ± 4.2 ± 4.2 1 Introduction For several years it has been realized that the B semileptonic decay branching ratio is lower than theoretical expectations. Last year, a CLEO measurement with dilepton events1) removed the substantial model dependence of existing measurements based on single lepton events and confirmed the low value for semileptonic branching ratio. A possible explanation of the low value is that the process b-+ces has a higher rate than expected. That explanation, however, B-+ predicts a higher rate fortotal charm production, B-+DX + DX, than is observed. To gain further understanding, one would like to measure separately the rates for B -+ DX and B -+ DX, rather than measuring only their sum. By studying D-lepton angle correlations, CLEO has carried out these measurements. 2 Motivation Table 1 shows one way to view the current status of B semileptonic decay ratio problem. We list all the separate components of B decays, give their individual branching ratios based ( on measurement and/or theory), and see if they sum to 100%. We express several of the components in terms of bsL, the B semileptonic decay branching ratio, whose measured value1) is (10.49 ± 0.46)%. For the b cti.d'and b uud' we use a QCD calculation by Bagan et -+ -+ . aJ.3), which gives Tud :: r(b -+ cti.d')/r(b -+ cev) = 4.0 ± 0.4 . (1) We take the contribution from b-+cud' , b-+uud' as equaling rudbsL, thereby assuming that 321 the b piece has a multiplier similar to the b --t c piece. --t u We split the b ces contribution into several parts: (a)upper vertex 1 D,; (b) char­ --t monium; (c) baryons from b ces decay; (d) upper vertex D0, n+ . The numbers quoted for --t (a)-(c) are from existing CLEO measurements. For the baryons, we quoted the central value of the measurement in the spirit of making best educated guess, while CLEO does not claim a signal for it. Same thing is true for the estimation of lower vertex D, production. Traditionallythe upper vertex D 2 contributions have been taken aszero without making serious experimental attempt. The contributions from rare decays b s(d)g, s(d)qif. has not --t been experimentally measured, nor its inclusive upper limit has been set. Theorists have estimated it to be from the Standard Model. � 1 % Adding these all up, we come up with (80.7 ± 4.2 ± 4.2) %, which is far less than the desired 100 %. Most experimental uncertainties are in the upper vertex D production and b sg 3 decays. In this paper, we present new results of CLEO experiment on these two --t issues. 3 D-lepton Correlation Technique The CLEO collaboration has developed a meson-lepton correlation technique to measure the inclusive B decay branching ratio to a given meson, separately to particle and antiparticle final states.4) We use this technique to address the issues of both upper vertex D production and b sg decay. The principle underlying the technique is that if one B from a BB pair decays --t semileptonically, with a high momentum lepton, then the other decay products from that B will have substantial angular correlations with the lepton, tending to come off back-to-back to it, while the decay products from the other B have negligible angular correlations with the lepton. The lepton tags the flavor of its parent B, and thus also the other B (with a small correction needed formixin g). Therefore by simultaneously studying the charge combination of lepton and D, and their angular distributions, we can separately measure the inclusive decay branching ratios of B DX (lower vertex) and B DX (upper vertex). --t --t 4 Experimental Procedures and Results We use the BB events of £, 2.41 fb-1 , collected with CLEO II detector. Signal leptons are = required to have momenta between 1.5 and 2.8 GeV /c. D mesons are identified through the For a charm meson produced in B decay, we it an 'upper vertex' charm meson it is produced fromW 1 call if fragmentation, and a 'lower vertex' charm meson if it from a hadronization of the quark produced primarily is c in b-+cW 2 From now on, we will use D to inclusive represent n° and n+, but not D., unless explicitly specified. 3From now on, we denote to generically include and decays b-+sg b-+s(d)g b-+s(d)qq 322 1.00•101 (o) D/1- (b) D/1+ 120000 750000 80000 500000 40000 250000 0 ..... 0 -1.0 -0.5 0.0 0.5 1.0 -1.0 -0.5 0.0 0.5 1.0 Figure 1: Distribution of the angle between n and lepton: (a) forn° Ie- and n+ I e- combined; (b) for 15°/ e- and n-/ e- combined. decay modes n° K-7r+ and n+ K-7r+7r+. To avoid the ambiguity between n° and 15°, ---t ---t we use a tight hadron identification using dE/dx and Time-of-flight and uniquely determine whether a given K7r state is K-7r+ or K+7r-. We determine n° and n+ detection efficiencies as a function of n momentum and polar angle and correct for the efficiency event by event. This allows us to combine the yield of n° and n+ for a combined analysis. Figure 1 shows the combined n° and n+ yield with a tagging lepton of Pt > 1.5 GeV /c, as a fu nction of cosine of the angle between the n and lepton. Figure 1 (a) is for e-/ n° and e-/ n+ combinations. This is expected mostly for lepton and n from the same B, hence a strong peaking behavior towards cos !Jn t -1 (back-to-back n and l). We decomposed the - � distribution into "same B" and "other B" components, based on the shapes determined from Monte-Carlo. Note that while the same B component dominates the distribution, there is a non-zero excess of other B component shown with an almost isotropic fitting function. Similarly, Fig. 1 (b) is for e-I 15° and e-In- combinations. We expect this n-lepton charge correlation only when the n and lepton are produced by decays of two different B's. Naturally, the distribution is mostly explained by an isotropic fitting function (other B com­ ponent) and the contribution of same B component is consistent with zero, as expected. Table 2 summarizes the same B and other B decompositions of n-lepton yields in two charge configurations. 5 n° and n+ in the Upper Vertex Let x be the ratio of the other B component of n / e- yield to the other B component of D/ e­ yield: N(e-;n, other B) x _ = N(e-/ n, other B) " 323 Table 2: Results D-lepton yields from "same B" and "other B" D, e-) "same B" (276.0 ± 7.5) x103 ( "other B" (48.6 ± 6.l)x103 e-) "same B" (2.9 ± 5.5) x 103 (fJ, "other B" (225.4 ± 7.3) x 103 We obtain the ratio of upper vertex D production to lower vertex D production using the following formula: f(B DX) (x - Fm) --t (2) f(B DX) (1 -xFm )' --t where Fm is a correction factor for BB mixing and lepton-mistag probabilities (Fm � 0.11). Using the results in Table 2, we have measured x = 0.216 ± 0.028 and consequently, f(B DX) --t 0.107 ± 0.029 ± 0.018 (3) f(B DX = --t ) where the first error is statistical and the second error is systematical. Using the relation DX) (B D X) (B D X) baryons (B DX B(B bsL(2.25 + - ; - (B --t + ) --t = + rud) --t ; --t ) --t and taking = 4.0 ± 0.4 as in Table 1, we solve for B(B DX) and obtain rus --t B(B DX) (8.1 ± 2.6) %. (4) --t = 6 Search b Decays for --t sg Let us define Pgen to be the probability of lower vertex D meson production in a generic B decay, and to be the probability of lower vertex D production in a semileptonic B decay Ps L 4• We expect Pgen and PsL to be close to 100 % except forthe following corrections: Pgen 1 - 2 D,; low. vtx.) - (ce) - baryons - (b (5) IVub/Vcbl2 -( --t sg) 3 1 Ps L 1 - x 2 x - D,; low. vtx. 2 IV..b/Vcbl2 2( ) (6) Using the results in Table 2, we have measured N(e- other B) / fJ, 0.834 ± 0.034 N(f- D, same B) = / 4With a requirement of 1.5 2.8 GeV /c.