Gaz, Alessandro

UNT Digital Library

PoS(EPS-HEP2011)156Recent BABAR measurements of hadronic Bbranching fractionsAlessandro GAZ∗University of ColoradoE-mail: gaz@slac.stanford.eduWe present recent BABAR measurements of branching fractions of B mesons to final states con-taining at least one charmed hadron. These decays are useful to test our knowledge on the CKMMatrix paradigm and verify the predictions provided by different theoretical frameworks. More-over, the decays of B mesons to baryons are still poorly understood, so measuring as many finalstates as possible may lead to the development of more reliable phenomenological models. Allresults have been obtained using the final BABAR dataset, which consists of ∼ 467 million BBpairs.The 2011 Europhysics Conference on High Energy Physics-HEP 2011,July 21-27, 2011Grenoble, Rhône-Alpes France∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/Work supported by US Department of Energy under contract DE-AC02-76SF00515 and HEP.SLAC-PUB-15915PoS(EPS-HEP2011)156Recent BABAR measurements of hadronic B branching fractions Alessandro GAZ1. Common analysis toolsIn all the analyses presented here it is possible to fully reconstruct the hadronic final state. Two(almost completely uncorrelated) variables, exploiting the very precise knowledge of the kinemat-ics in a e+e− collider, are used to separate signal events from the combinatorial background: theenergy-substituted mass of the B meson candidate mES ≡√s/4−p2B and the difference betweenthe energy of the reconstructed B candidate and the expected one: ∆E ≡ EB−√s/2 (√s representsthe energy of the e+e− system in the center of mass frame). A significant source of backgroundcomes from e+e− → qq (q = u,d,s,c) events, which typically exhibit a jet-like topology, opposedto the more spherical B decays. This background can be effectively suppressed combining severalvariables sensitive to the event shape by means of a Fisher discriminant or a Neural Network.2. Final states containing a D(∗) mesonThe decays of B mesons to final states containing at least one D(∗) meson are useful to probevery suppressed amplitudes and in general verify the patterns predicted by different theoreticalframeworks.The interest of the B+→ D+K(∗)0 decays consists in the fact that neither of the (anti)quarkspresent in the B meson appear in the final state, thus these decays must proceed through annihilationdiagrams. The branching fractions are expected to be of the order of 10−6, but rescattering, partic-ularly through the Dsπ0 channel, may play a significant role, enhancing them by almost one orderof magnitude. We reconstruct candidate signal events combining K0S → π+π−, K∗0→ K+π−, andD+→ K−π+π+,K0S π+,K−π+π+π0,K0S π+π0 candidates. B+→ D+ρ0 decays constitute a sourceof peaking background, which is suppressed with a cut on the helicity of the π+π− system. Thesignal yield is extracted through a maximum likelihood (ML) fit on the mES and Fisher variables.No significant signal is found (see e.g. left plot of Figure 1), and we derive the upper limits (at 90%CL): B(B+→D+K0)< 2.9×10−6, B(B+→D+K∗0)< 3.0×10−6, which are the most stringentlimits set to date [1].The decays B → D(∗)D(∗)K have sizable branching fractions, adding up to a few percent.Several amplitudes contribute to the 22 sub-modes, so these decays offer a unique opportunity toverify in a coherent way the pattern predicted by different theoretical models. From the experi-mental point of view, one of the main challenges of the analysis is to properly take into accountthe crossfeed (mostly generated by failing to reconstruct the soft pion from the decays of the D∗)among the different submodes. In that respect, the signal yields are extracted from a ML fit to themES variable with an iterative procedure, in which the branching fractions of the crossfeed modesfor the channel under study are taken from the previous iteration of the fit. This procedure ensuresthat all the 22 submodes are treated in a consistent way. Depending on the particular mode, thesignal significance varies between 2.2 and 13.4 standard deviations [2]. No evidence of breakingof the isospin symmetry has been found. Summing separately the charged and the neutral modes,we obtain:B(B0→ D(∗)D(∗)K) = (3.68±0.10±0.24)% (2.1)B(B+→ D(∗)D(∗)K) = (4.05±0.11±0.28)%. (2.2)2PoS(EPS-HEP2011)156Recent BABAR measurements of hadronic B branching fractions Alessandro GAZThe decays B0→D(∗)0h0 (where h0 = π0,ω,η ,η ′) are color suppressed, and thus particularlysuited to show effects from non-factorizable contributions. Different theoretical predictions areavailable for these modes; in particular SCET predicts that the ratio R ≡B(B→ D∗0h0)/B(B→D0h0) is ∼ 1 (ω modes excluded), and HQET computes that the longitudinal polarization fractionfL for the D∗0ω mode be ∼ 90%. The B−→ D(∗)0ρ− channel (which also constitutes a source ofpeaking background for the analysis) is widely used as a control sample to calibrate the detectorresponse. The signal yield is extracted through a ML fit to the ∆E variable and again the crossfeedamong the different submodes is estimated using an iterative procedure. The results [3] are in goodagreement with the previous measurements and show significant disagreement with predictionsbased on QCD factorization. We confirm the SCET prediction on the ratio R being consistent with1 (see right plot of Figure 1) and the measurement of fL = (66.5±4.7±1.5)% for D∗0ω , in pooragreement with HQET predictions, may give an indication of sizable long-range contributions.Figure 1: Left: projection plot on the mES variable of the ML fit extracting the signal yield for the channelD+K0S , with D+→ K−π+π+. The black solid line represents the signal component, the magenta dotted lineis the BB combinatorial, the green dotted is the continuum, and the red dotted is the peaking background:no signal is found. Right: ratio B(B→ D∗0h0)/B(B→ D0h0) for the B0→ D(∗)0h0 analysis. The resultsconfirm the predictions by SCET.3. Final states containing baryonsB meson decays with baryons in the final state account for ∼ 7% of the total, but the sumof the currently measured modes reaches only ∼ 1%. These decays are poorly described by thetheory, so more experimental information can be useful to refine the phenomenological models. Inseveral cases enhancements at threshold or discrepancies with respect to the predictions based onphase-space arguments have been observed.We report the first observation of the decay B0→Λc p̄π0 [4]. Signal candidates are searched forby combining p̄ and π0 candidates with a Λc→ pK−π+ candidate. A ML fit to the mES distributionsextracts a signal yield of 273±23 events, which corresponds to B(B0→Λc p̄π0) = (1.94±0.17±0.14± 0.50)× 10−4, where the first quoted uncertainty is the statistical error, the second is thesystematic, and the third arises from the uncertainty on the branching fraction of the Λc decay usedin this measurement. We observe (see Figure 2) an enhancement at the threshold of the invariantmass of the Λc p̄ system. Finally, we find no evidence of the B0→ Σc p̄ decay.3PoS(EPS-HEP2011)156Recent BABAR measurements of hadronic B branching fractions Alessandro GAZFigure 2: Left plot: invariant mass of the Λc p̄ in the B0 → Λc p̄π0 analysis. The points with error barsrepresent the data, the histogram is the simulation generated according to the phase-space assumption; aclear enhancement at threshold is visible. Right: invariant mass of the ΛcK− in the B0→ ΛcΛ̄K− analysis.The points with error bars represent the data, while the histogram is the phase-space Monte Carlo: a yet tobe explained structure is visible at a mass of ∼ 3.9 GeV/c2.The decay B0 → ΛcΛ̄K− is searched for using a strategy similar to the analysis presentedabove. The decays B0→ Λc p̄π+K−, having the same final state of the mode under investigation,constitute a source of peaking background, which is suppressed imposing a cut on the flight-lengthof the Λ̄ candidate. The signal is extracted with a ML fit to the ∆E variable and yields 51±9 events,which correspond to a branching fraction B(B0→ ΛcΛ̄K−) = (3.8±0.9±0.2±1.0)×10−5 [5],where the first error is statistical, the second is systematic, and the third derives from the uncertaintyon the Λc→ pK−π+ branching fraction. The analysis of the invariant mass of the ΛcK− system,shows a structure around 3.9 GeV/c2 (see Figure 2) which is inconsistent with the phase-space MCand is not explained by any known resonance.4. ConclusionsAfter more than three years from the end of the data-taking the BABAR Collaboration continuesto exploit its rich dataset. The latest measurements on hadronic B meson decays provide new con-straints for the existence of New Physics contributions and will help theorists in the developmentof more reliable phenomenological models.References[1] BABAR Collaboration, P. del Amo Sanchez et al., Phys. Rev. D 82, 092006 (2010).[2] BABAR Collaboration, P. del Amo Sanchez et al., Phys. Rev. D 83, 032004 (2011).[3] BABAR Collaboration, J. P. Lees et al., arXiv:1107.5751 [hep-ex], submitted to Phys. Rev. D.[4] BABAR Collaboration, B. Aubert et al., Phys. Rev. D 82, 031102(R) (2011).[5] BABAR Collaboration, J. P. Lees et al., arXiv:1108.3211 [hep-ex], submitted to Phys. Rev. D.4

Recent BaBar Measurements of Hadronic B Branching Fractions

https://digital.library.unt.edu/ark:/67531/metadc868770/m2/1/high_res_d/1122514.pdf

Recent BaBar Measurements of Hadronic B Branching Fractions

Abstract

Similar works

Full text

Available Versions

UNT Digital Library