We present CDF results on the branching fractions and time-integrated direct
CP asymmetries for Bd, Bs and Lb decay modes into pairs of charmless charged
hadrons (pions, kaons and protons). The data-set for these measurements amounts
to 1fb^{-1} of pbar-p collisions at a center of mass energy 1.96TeV. We report
on the first observation of the Bs->Kpi, Lb-ppi and Lb->pK decay modes and on
the measurement of their branching fractions and direct CP asymmetries.Comment: 4 pages, 1 figure, 2 tables, proceedings of ICHEP200

Morello, M. J.

English

arXiv

We present CDF results on the branching fractions and time-integrated direct CP asymmetries for Bd, Bs and Lb decay modes into pairs of charmless charged hadrons (pions, kaons and protons). The data-set for these measurements amounts to 1fb{sup -1} of p{bar p} collisions at a center of mass energy 1.96TeV. We report on the first observation of the Bs-&gt;Kpi, Lb-ppi and Lb-&gt;pK decay modes and on the measurement of their branching fractions and direct CP asymmetries

Morello, Michael Joseph

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34th International Conference on High Energy Physics, Philadelphia, 2008Charmless b-hadrons decays at CDFM. J. Morello (for the CDF collaboration)University and I.N.F.N. of Pisa, Building C, Polo Fibonacci, Largo B. Pontecorvo, 3 - 56127 Pisa, ItalyWe present CDF results on the branching fractions and time-integrated direct CP asymmetries for B0, B0s andΛ0bdecay modes into pairs of charmless charged hadrons (pions, kaons and protons). The data-set for these measurementsamounts to 1 fb−1 of p¯p collisions at a center of mass energy 1.96 TeV. We report on the first observation of theB0s → K−pi+, Λ0b → ppi− andΛ 0b → pK− decay modes and on the measurement of their branching fractions anddirect CP asymmetries.1. INTRODUCTIONThe interpretation of the CP violation mechanism is one of the most controversial aspects of the Standard Model.Many extensions of Standard Model predict that there are new sources of CP violation, beyond the single Kobayashi-Maskawa phase in the quark-mixing matrix (CKM). Considerations related to the observed baryon asymmetry ofthe Universe imply that such new sources should exist. The non-leptonic decays of b hadrons into pairs of charmlesscharged hadrons are effective probes of the CKM matrix and sensitive to these potential new physics effects. Thelarge production cross section of b hadrons of all kinds at the Tevatron allows extending such measurements to B0sandΛ 0b decays, which are important to supplement our understanding of B0 meson decays.The branching fraction of B0s → K−pi+ decay mode could be used to measure γ [1] and the measurement of itsdirect CP asymmetry could be a powerful model-independent test of the source of direct CP asymmetry in the Bsystem [2]. This may provide useful information to solve the current discrepancy between the direct CP asymmetriesobserved in the neutral B0 → K+pi− decay mode and charged B+ → K+pi0 decay mode [3]. The B0s → pi+pi− andB0 → K+K− decay modes proceed through annihilation and exchange topologies, which are currently poorly knownand a source of significant uncertainty in many theoretical calculations [4, 5]. A measurement of both decay modeswould allow a determination of the strength of these diagrams [6]. CP violating asymmetries inΛ 0b → ppi− andΛ0b → pK− decay modes may reach significant size O(10%) in the Standard Model [8]. Measurements of asymmetriesand branching fractions of these modes would rule out (or allow) some extensions of the Standard Model [9].Throughout this paper, C-conjugate modes are implied and branching fractions indicate CP-averages unless oth-erwise stated.2. CDF IICDF II is a multipurpose magnetic spectrometer surrounded by calorimeters and muon detectors [7]. A siliconmicro-strip detector (SVXII) and a cylindrical drift chamber (COT) situated in a 1.4 T solenoidal magnetic fieldreconstruct charged particles in the pseudo-rapidity range |η| < 1.0. The SVXII consists of five concentric layers ofdouble-sided silicon detectors with radii between 2.5 and 10.6 cm, each providing a measurement with 15 µm resolutionin the azimuthal (φ) direction and 70 µm along the beam (z) direction. The COT has 96 measurement layers, between40 and 137 cm in radius, organized into alternating axial and ±2◦ stereo “super-layers”. The transverse momentumresolution is σpT /pT " 0.15% pT/(GeV/c) and the observed mass-widths are about 14 MeV/c2 for J/ψ → µ+µ−decays, and about 9 MeV/c2 for D0 → K−pi+ decays. The specific energy loss by ionization (dE/dx) of chargedparticles in the COT is measured from the amount of charge collected by each wire. An average separation power of1.5 Gaussian-equivalent standard deviation is obtained in separating pions and kaons with momentum larger than2 GeV/c.1FERMILAB-CONF-08-447-E34th International Conference on High Energy Physics, Philadelphia, 20083. MEASUREMENTS OF H0b → h+h′− DECAYSThe Collider Detector at Fermilab (CDF) experiment analysed an integrated luminosity∫ Ldt " 1 fb−1 sampleof pairs of oppositely-charged particles with pT > 2 GeV/c and pT (1) + pT (2) > 5.5 GeV/c, used to form hadroncandidates. The trigger required also a transverse opening angle 20◦ < ∆φ < 135◦ between the two tracks, to rejectbackground from particle pairs within the same jet and from back-to-back jets. In addition, both charged particleswere required to originate from a displaced vertex with a large impact parameter d0 (100 µm < d0(1, 2) < 1 mm),while the b-hadrons candidate was required to be produced in the primary p¯p interaction (d0 < 140 µm) and tohave travelled a transverse distance $LT > 200 µm. A sample of about 14,500 H0b → h+h′− decay modes (whereH0b = B0, B0s or Λ0b and h = K or pi) was reconstructed after the off-line confirmation of trigger requirements. In theoﬄine analysis, an unbiased optimization procedure determined a tightened selection on track-pairs fit to a commondecay vertex. The selection cuts were chosen minimizing directly the expected uncertainty of the physics observablesto be measured (through several “pseudo-experiments”). Just two different sets of cuts were used in the analysis,respectively optimized to measure the CP asymmetry ACP(B0 → K+pi−) (loose cuts) and to improve the sensitivityfor discovery and limit setting [10] of the not yet observedB0s → K−pi+ mode (tight cuts). For the Λ0b measurements,the additional requirement pT (H0b ) > 6 GeV/c was applied to allow easy comparison with other Λ0b measurements atthe Tevatron, that are only available above this threshold [11].In addition to tightening the trigger cuts, in the oﬄine analysis the discriminating power of the H0b isolation andof the information provided by the 3D reconstruction capability of the CDF tracking were used, allowing a greatimprovement in the signal purity. Isolation is defined as I(H0b ) = pT(H0b )/[pT(H0b ) +∑i pT(i)], in which the sumruns over every other track (not from the H0b hadron) within a cone of unit radius in the η − φ space around theH0b hadron flight direction. By requiring I(H0b ) > 0.5, the background was reduced by a factor four while keepingalmost 80% of signal. The 3D silicon tracking allowed multiple vertices to be resolved along the beam direction andthe rejection of fake tracks, reducing the background by a factor of two, with only a small efficiency loss on signal.The resulting pipi-mass distributions (see Figure. 1) show a clean signal of H0b → h+h′− decays. In spite of a goodmass resolution (≈ 22MeV/c2), the various H0b → h+h′− modes overlap into an unresolved mass peak.]2-mass[GeV/c!!Invariant 5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.82Candidates per 20 MeV/c020040060080010002Candidates per 20 MeV/c-1=1 fbintCDF Run II  Preliminary L-!+ K" 0B+!- K" 0B-K+ K"   s0B/s0B-!+! " 0B/0B-!+ K" s0B ++!- K" s0B+Kp " b0#+ - pK" b0# +!p " b0#+ -! p" b0#Combinatorial backg.Three-body B decays Figure 1: Invariant mass distribution of H0b → h+h′− candidates passing tight cuts selection, using a pion mass assumptionfor both decay products. Cumulative projections of the likelihood fit for each mode are overlaid.The resolution in invariant mass and in particle identification (dE/dx) is not sufficient for separating the individualH0b → h+h′− decay modes on an event-by-event basis, therefore a maximum likelihood fit was performed. Thiscombines kinematic and particle identification information to statistically determine both the contribution of eachmode, and the relative contributions to the CP asymmetries.234th International Conference on High Energy Physics, Philadelphia, 2008Three separate fits were performed: one on the sample selected with loose cuts, one on the sample selected withtight cuts and one on the sample selected with tight cuts plus the requirement pT(H0b ) > 6 GeV/c. Significant signalsare seen for B0 → pi+pi−, B0 → K+pi−, and B0s → K+K−, previously observed by CDF [12]. Three new rare modeswere observed for the first time B0s → K−pi+, Λ0b → ppi− and Λ0b → pK−, with a significance respectively of 8.2, 6.0and 11.5 Gaussian-equivalent standard deviation estimated using a p-value distribution on pseudo-experiments. Noevidence was obtained for B0s → pi+pi−or B0 → K+K− mode.Table I: Branching fractions results. Absolute branching fractions are normalized to the the world–average valuesB(B0 → K+pi−) = (19.4± 0.6)× 10−6 and fs/fd = 0.276± 0.034 and fΛ/fd = 0.230± 0.052 [3]. The first quoted uncertaintyis statistical, the second one is systematic.Mode Quantity Measurement B(10−6)B0 → pi+pi− B(B0→pi+pi−)B(B0→K+pi−) 0.259 ± 0.017 ± 0.016 5.02 ± 0.33 ± 0.35B0s → K+K− fsfd ×B(B0s→K+K−)B(B0→K+pi−) 0.324 ± 0.019 ± 0.041 24.4 ± 1.4 ± 3.5B0s → K−pi+ fsfd ×B(B0s→K−pi+)B(B0→K+pi−) 0.071 ± 0.010 ± 0.007 5.0 ± 0.7 ± 0.8Λ0b → pK− fΛfd ×B(Λ0b→pK−)B(B0→K+pi−) 0.066 ± 0.009 ± 0.008 5.6 ± 0.8 ± 1.5Λ0b → ppi− fΛfd ×B(Λ0b→ppi−)B(B0→K+pi−) 0.042 ± 0.007 ± 0.006 3.5 ± 0.6 ± 0.9B0s → pi+pi− fsfd ×B(B0s→pi+pi−)B(B0→K+pi−) 0.007 ± 0.004 ± 0.005 0.49 ± 0.28 ± 0.36 (< 1.2 @ 90% CL)B0 → K+K− B(B0→K+K−)B(B0→K+pi−) 0.020 ± 0.008 ± 0.006 0.39 ± 0.16 ± 0.12 (< 0.7 @ 90% CL)Table II: CP asymmetries results. The first quoted uncertainty is statistical, the second one is systematic.Quantity MeasurementB(B0→K−pi+)−B(B0→K+pi−)B(B0→K−pi+)+B(B0→K+pi−) -0.086 ± 0.023 ± 0.009B(B0s→K+pi−)−B(B0s→K−pi+)B(B0s→K+pi−)+B(B0s→K−pi+)0.39 ± 0.15 ± 0.08B(Λ0b→pK+)−B(Λ0b→pK−)B(Λ0b→pK+)+B(Λ0b→pK−)-0.37 ± 0.17 ± 0.03B(Λ0b→ppi+)−B(Λ0b→ppi−)B(Λ0b→ppi+)+B(Λ0b→ppi−)-0.03 ± 0.17 ± 0.05fdfs× Γ(B0→K−pi+)−Γ(B0→K+pi−)Γ(B0s→K+pi−)−Γ(B0s→K−pi+)-3.00 ± 1.50 ± 0.22The relative branching fractions are listed in Table I while the CP asymmetries are listed in Table II, where fd,fs and fΛ indicate the production fractions respectively of B0, B0s and Λ0b from fragmentation of a b quark in p¯pcollisions. An upper limit is also quoted for modes in which no significant signal is observed. Absolute results arealso listed in Table I, they are obtained by normalizing the data to the world–average of B(B0 → K+pi−) [3].The branching fraction of the newly observed mode B0s → K−pi+ is in agreement with the latest theoreticalexpectation [13], which is lower than the previous predictions [4, 14]. This mode offers an unique opportunityto probe the direct CP violation in the B0s mesons system. For the first time, the direct CP asymmetry in theB0s → K−pi+ decay mode was measured and its central value favors a large CP violation (different from 0 at2.3 Gaussian-equivalent standard deviation), although it is also compatible with zero. In Ref. [2] a robust test ofthe Standard Model or a probe of new physics is suggested by the comparison of the direct CP asymmetries inB0s → K−pi+ and B0 → K+pi− decays. Using the external input for fs/fd = 0.276± 0.034 [3] it is also possible toquote the following interesting quantity Γ(B0→K−pi+)−Γ(B0→K+pi−)Γ(B0s→K−pi+)−Γ(B0s→K+pi−)= 0.83± 0.41± 0.12, which is in agreement withthe Standard Model expectation of unity. Assuming this relationship true (equal to unity), and using the external334th International Conference on High Energy Physics, Philadelphia, 2008inputs for the B(B0s → K−pi+), the world–average for the direct CP violating asymmetry in the B0 → K+pi− decaymode [3], the B(B0 → K+pi−) [3], it is possible to estimate the expected value for the direct CP violating asymmetryin the B0s → K−pi+ decay mode (≈ 0.40) which is in agreement with the CDF measurement presented here.The branching fraction of B0s → K+K− decay mode is in agreement with the latest theoretical expectation [15, 16]and with the previous CDF measurement [12].The results on the B0 sector are in agreement with world–average values [3]. The direct CP violating asymmetryin the B0 → K+pi− is competitive with the current B-Factories measurements [3].The results on the Λ0b sector are in agreement with Standard Model expectations. The absolute branching frac-tions exclude O(10−4) values indicated for R-parity violating Minimal Supersymmetric extensions of the StandardModel [9]. The measurements of the direct CP violating asymmetries in the b-baryon decays, presented here, are thefirst such measurements in this sector. The statistical uncertainty dominates the resolution and prevents a statementon the presence of asymmetry, whose measured value deviates from 0 at 2.1 Gaussian-equivalent standard deviationlevel in the Λ0b → pK− decay mode and is fully consistent with 0 in the Λ0b → ppi− decay mode.With full Run II samples (5 − 6 fb−1 by year 2010) CDF collaboration expects a measurement of the direct CPviolating asymmetry in the B0 → K+pi− mode with a statistical plus systematic uncertainty at 1% level; observationof the direct CP violating asymmetry in the B0s → K−pi+ mode (or alternatively the possible indication of non-SMsources of CP violation); more precise measurements of direct CP violating asymmetries in the Λ0b charmless decays;and improved limits, or even observation, of annihilation modes B0s → pi+pi− and B0 → K+K−. In addition to theabove, time-dependent measurements will be performed for B0 → pi+pi− and B0s → K+K− decay [17]. See [18–21]for more details.References[1] M Gronau and J. L. Rosner, Phys. Lett. B482, 71 (2000), hep-ph/0003119.[2] H. J. Lipkin, Phys. Lett. B621, 126 (2005), hep-ph/0503022.[3] C. Amsler et al. Phys. Lett. B667,1 (2008).[4] M. Beneke and M. Neubert, Nucl. Phys. B675, 333 (2003), hep-ph/0308039.[5] Y. D. H. Yang et al., Eur. Phys. J., C44, 243 (2005), hep-ph/0507326.[6] A. J. Buras et al., Nucl. Phys. B697, 133 (2004), hep-ph/0402112.[7] D. Acosta et al. (CDF Collaboration), Phys. Rev˙D71, 032001 (2005), hep-ex/0412071.[8] R. Mohanta, A.K. Giri, M.P. Khanna, Phys. Rev. D63, 074001 (2001), hep-ph/0006109.[9] R. Mohanta, Phys. Rev. D63, 056006 (2001), hep-ph/0005240.[10] G. Punzi, Talk given at PHYSTAT2003, eConf C030908, MODT002, physics/0308063.[11] A. Abulencia et al. (CDF Collaboration), Phys. Rev. Lett. 98, 122002 (2007), hep-ex/0601003.[12] A. Abulencia et al. (CDF Collaboration), Phys. Rev. Lett. 97, 211802 (2006), hep-ex/0607021.[13] A.R. Williamson, J. Zupan, Phys. Rev. D74, 014003 (2006), hep-ph/0601214.[14] Xian-Qiao Yu et al., Phys. Rev. D71, 074026 (2005), hep-ph/0501152.[15] S. Descotes-Genon, J. Matias and J. Virto, Phys. Rev. Lett. 97, 061801 (2006), hep-ph/0603239.[16] S. Baek, D. London, J. Matias and J. Virto, J. High Energy Phys. 612 (2006), hep-ph/0610109.[17] G. Punzi, Talk given at 4th International Workshop on the CKM Unitarity Triangle (CKM 2006), Nagoya,Japan, hep-ex/0703029.[18] M.J. Morello, Nucl. Phys. Proc. Suppl. 170, 39− 45 (2007), hep-ex/0612018.[19] M.J. Morello, Ph.D. Thesis, Scuola Normale Superiore, Pisa (2007), (Fermilab report No. FERMILAB-THESIS-2007-57).[20] CDF Public Note 8579, http://www-cdf.fnal.gov/physics/new/bottom/060921.blessed-bhh_1fb/.[21] CDF Public Note 9092, http://www-cdf.fnal.gov/physics/new/bottom/071018.blessed-ACP_Lambdab_ph/.4

Charmless b-hadrons decays at CDF

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Charmless b-hadrons decays at CDF

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