The latest ALEPH measurements on heavy flavours are presented. In particular
the measurements of V_ub, the B(b -> s gamma) and a study on the width
difference between mass eigenstates in the B_s system are presented.Comment: 7 pages, RevTe

Palla, Fabrizio

English

arXiv

The latest ALEPH measurements on heavy flavours are presented. In particular the measurements of V_ub, the B(b -> s gamma) and a study on the width difference between mass eigenstates in the B_s system are presented.The latest ALEPH measurements on heavy flavours are presented. In particular the measurements of V_ub, the B(b -> s gamma) and a study on the width difference between mass eigenstates in the B_s system are presented

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arXiv:hep-ex/9905017v1  11 May 1999Recent Results on Heavy Flavours with ALEPHFabrizio PallaINFN sezione di Pisa, Via vecchia livornese 1291, I-56010 S. Piero a Grado (PI) ItalyThe latest ALEPH measurements on heavy flavours are presented. In particular the measurementsof |Vub|, the B(b → sγ) and a study on the width difference between mass eigenstates in the Bssystem are presented.I. INTRODUCTIONThe large sample of Z collected by ALEPH during the LEP1 consists of about 4.5 million hadronic decays. Out ofthem, about 1 million of bb¯ events allow to investigate rare processes which could help constraining the CKM mixingmatrix, before the start of the new experiments at the B factories and hadronic colliders.In this report I will present the measurements of the B(b→ ulνX) [1] which allows a measurement of the |Vub|CKM matrix element, the B(b → sγ) [2] and a study on the width difference between mass eigenstates in the Bssystem.II. MEASUREMENT OF |VUB |The measurement of |Vub| is performed by measuring the semileptonic branching ratio of B hadrons into charmlessfinal states (b→ ulνX ). In fact this rate is proportional to the |Vub|2 and can be computed in the framework of theHeavy Quark Expansion theory [3].Previous measurements of |Vub| were performed in both the exclusive and inclusive channels in the Υ(4S) [4].The exclusive measurements show large theoretical uncertainties in the transition amplitude computation. Inclusivemeasurements overcame this difficulty looking for an excess of events at the endpoint of the lepton momentumdistribution where the b → c contribution vanishes. At LEP both methods are not viable, either due to the smallbranching ratio and large background in one case or to the small accuracy in reconstructing the B hadron rest framein the other case.This measurement relies upon the fact that nearly 90% of the b→ ulνX decays are expected to have an invariantmass of the hadronic system MX below the charm threshold of 1.87 GeV/c2. By contrast, only in 10% of the casesthe lepton energy in the b rest frame, E∗, is above the kinematic boundary for b→ clνX transitions [3].A. Event SelectionEvents are selected by requiring the presence of an identified lepton with momentum larger than 3 GeV/c. In theopposite hemisphere a b lifetime tag is applied to reduce non–b contamination to less than 2%. The neutrino energyand direction is estimated from the missing momentum in the lepton hemisphere with a typical accuracy of 280 mradon its direction and 2 GeV on its energy.Particles coming from the charmless hadronic system X are reconstructed by means of two Neural Networks, oneto select photons and the other charged particles. The B hadron rest frame is then reconstructed by adding themomenta of the lepton, the neutrino and the selected particles. The total energy is determined by assigning a massof 5.38 GeV/c2 to the total system. The momentum and angular resolution, obtained from the simulation, are 4.5GeV/c and 60 mrad respectively.The discrimination of the b→ ulνX signal events from the background b → c transitions is made on a statisticalbasis and is based on the fact that the c quark is heavier with respect to the u quark, leading to different kinematicalproperties for the two final states. Because of resolution effects the separation that uses a single kinematical variable1such asMX can be considerably improved by combining more information characterizing the leptonic and the hadronicparts into a multivariate Neural Network.The choice of the variables is based not only in such a way to have a good discrimination between signal andbackground, but also on a reduced sensitivity to the specific composition of the Xu system.Among the Neural Network inputs, the ones with larger separating power are the invariant mass, the sphericityand the lepton momentum. The Neural Network output is close to 1 for signal events and close to 0 for backgroundevents.The signal Monte Carlo use a Hybrid model following Ramirez et al. [5]. At low hadronic energy (below 1.6 GeV),only resonant states are produced, while for large energy, non-resonant states are expected to contribute, which occursin 75% of the cases.Figure 1 shows the Neural Network output for the Monte Carlo and data after all the selection cuts. The signalis extracted from a binned likelihood fit to the Neural Network output between 0.6 and 1.0, which gives the smallesttotal relative error on the measurement.10 310 40 0.2 0.4 0.6 0.8 1b → u l nb → c l nb → c → lotherevents / 0.1a)NNbuALEPH050010001500200025000 0.2 0.4 0.6 0.8 1b)NNbu-10001002000 0.2 0.4 0.6 0.8 1Data - Monte Carlo c)NNbu-10001002000 0.2 0.4 0.6 0.8 1d)NNbuFIG. 1. Neural Net output: a) and b) comparison between data (points) and Monte Carlo (histogram), c) difference betweendata and Monte Carlo with no b → u transitions (points) compared to the b → u contribution (histogram), and d) differencebetween the data and Monte Carlo with the fitted value of b→ u.The Monte Carlo is normalized to the same number of events in the data to reduce the sensitivity to the assumedefficiencies of the analysis cuts. The first bin is excluded in this normalisation procedure, to minimise the effects ofuncertainties of the background events in the fit. In this way, the fit is effectively a fit to the shape of the signal andbackground distributions.B. Results and systematic checksAn excess of 303 ± 88 events is clearly seen in the region where the signal is expected, and is in good agreementwith the predicted Neural Network shape.From the fitted number of events one can extract the value of the branching ratio:B(b→ ulν) = (1.73± 0.55stat ± 0.51systb→c ± 0.21systb→u)× 10−3. (1)2The systematics come from the uncertainties in modelling the b→ c and the b→ ulνX transitions. For the b→ csystematic source, the main uncertainties are the ones which come from the knowledge of the charm topologicalbranching ratios and the statistical uncertainties on BR(b → l), BR(b → c → l) and < Xb > (the average energyfraction of the B hadron) which have been measured by ALEPH [6]. The systematic coming from the modelling ofthe inclusive decays of the b→ u transitions dominates the second systematic error.Extensive checks have been made to support the measurement. The fit is stable against variations on the fit intervalof the Neural Network output, as well as changing some variables in input of the Neural Network.Since neutral hadrons have not been considered when reconstructing the b hadron, a bad simulation of the b → cstates involving energetic neutral hadrons would alter the background in the region of high values of the NeuralNetwork output. The neutral hadronic energy distribution in a 300 cone around the lepton is different for final stateswith and without K0L, allowing a measurement of the inclusive production rate of K0L in D meson decays. Goodagreement is observed between data and simulation. The rate found is in very good agreement with the one measuredby MARKIII.Finally, since no vertexing information has been used in the selection, a common vertex between the lepton andthe charged hadronic system is searched for, with a cut on the χ2 probability larger than 0.2. The efficiency for thiscut is larger for b → u transitions with respect to the b → c, due to the presence of the charm vertex in the latter.This difference is further enhanced in the Neural Network region close to 1, due to the smaller charged multiplicityand poor vertex of the b → c transitions. The good agreement of vertexing efficiencies in data and Monte Carlo isshown in Fig.2, where the effect of the signal inclusion is evident.0.80.911.11.21.30.2 0.4 0.6 0.8 1Data / Monte CarloCut on NNbuALEPHFIG. 2. Ratio of the vertexing efficiencies between data and Monte Carlo as a function of the Neural Net output cut. Blackcircles are with b→ u transitions, while open circles without.From this measurement and the average b lifetime one can extract the value of the |Vub| matrix element, by usingthe relation obtained in the framework of the Heavy Quark Expansion Theory [3]:|Vub|2 = 20.98B(b→ Xulνl)0.0021.6psτB(1± 0.05pert ± 0.06mb)× 10−6 (2)where τb = (1.554± 0.013) ps is the average b hadron lifetime:|Vub|2 = (18.68± 5.94stat ± 5.94syst ± 1.45HQE)× 10−6. (3)corresponding to |Vub| = (4.16± 1.02)× 10−3.3III. MEASUREMENT OF THE B(b→ sγ)The b→ sγ decay is a flavour changing neutral current that in the Standard Model proceeds via an electromagneticpenguin diagram in which the photon is radiated from either the W or one of the quark lines and its branching ratiois predicted to be (3.76± 0.30)× 10−4 at the Z peak.Virtual particles in the loop may be replaced by non-Standard Model particles, such as charged Higgs bosons orsupersymmetric particles which could either enhance or suppress the decay rate, making it sensitive to physics beyondthe Standard Model [7].The b→ sγ decay is expected to be dominated by two body decays, leading to the presence of a very energeticphoton in the final state associated with a system of high momentum and rapidity hadrons, originating from adisplaced secondary vertex.The composition of the signal b→ sγ Monte Carlo is based primarily on predictions from the Heavy Quark EffectiveTheory as well as on the CLEO measurements of the inclusive b→ sγ branching ratio and the exclusive B → K∗(892)γ[8].A. Event SelectionThe event reconstruction starts from the requirement of a photon with an energy larger than 10 GeV which doesnot gives any pi0 when paired with other photons in the event. In that hemisphere, pi0 and Ks mesons are searched for.The remaining tracks were assigned a probability to come from the b→ sγ according to their momentum, rapiditywith respect to the B hadron direction and, in the case of charged tracks, their three dimensional impact parametersignificance. The probability functions were derived from the simulation.The reconstruction of the b→ sγ candidate is then made by the photon and the hadronic system in the samehemisphere resulting by adding the pi0 and Ks, charged tracks and neutral calorimetric objects in decreasingb→ sγ probability. The candidate is accepted if the jet mass lies within 700 MeV/c2 of the mean B meson mass, thehadronic system has a mass smaller than 4 GeV/c2 and a multiplicity smaller than eight objects.The hemisphere opposite to the candidate is required to be b-like using a lifetime b tag.After the preselection there remain 1560 hadronic events, which are then splitted into eight different categoriesdepending on :• the value of the length of the major axis of the shower ellipse, σl, of the photon candidate in the electromagneticcalorimeter• the energy of the hard photon, E∗γ• the b tag probability of the opposite hemisphere.The signal is extracted from a binned log-likelihood fit of the E∗γ distributions of the eight subsamples, using thecorresponding distributions for the signal and background simulations, taking into account the finite Monte Carlostatistics in each bin. The five parameters in the fit are Nb→sγ , NFSR, N(b→c)pi0 , N(non−b)pi0 and Nother, which are,respectively, the total number of signal, Final State Radiation, (b→ c)pi0, (non− b)pi0 and ’other’ background events.Figure 3 shows the E∗γ distribution for the purest signal sub-sample in data and Monte Carlo. The clear excess indata is nicely explained by the signal contribution.B. Results and systematic checksFrom the number of fitted events and correcting for the efficiency in the selection cuts the inclusive b→ sγ branchingratio is determined to be4B(b→ sγ) = (3.11± 0.80stat ± 0.72syst)× 10−4 (4)where the statistical error takes into account also the finite Monte Carlo statistics. The largest systematic error comesfrom the shape of the E∗γ background distributions as well as on the relative proportion of each background source ineach sub-sample. This is assessed by observing the change in the measured branching ratio as the boundaries betweenthe eight sub-classes are varied. The uncertainty coming from the energy calibration has been estimated by varyingboth the ECAL and HCAL calibrations. Finally the uncertainty of the baryonic b→ sγ decays has been assessed byrepeating the fit setting that fraction to zero.01020304050600.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3Eg*(GeV)Eg*(GeV)-10-505101520250.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3FIG. 3. The energy of the photon in the rest frame of the reconstructed jet. The top figure shows the data and the MonteCarlo background events in the region of high purity signal. The bottom figure shows the excess in data after the subtractionof the Monte Carlo background and the signal distribution from the fit.Checks have been performed which are consistent with the expected b→ sγ hypothesis:• there is evidence of lifetime in the same hemisphere as the photon• there is an excess of high momentum kaons• the shape of the excess σl distribution is characteristic of single photons.IV. A STUDY ON THE WIDTH DIFFERENCE BETWEEN MASS EIGENSTATES IN THE BS SYSTEMOne of the most challenging measurements in the Bs−B¯s system is the measurement of the mass difference betweenthe two mass eigenstates, ∆Ms, which allows to constrain the CKM unitarity triangle. Since ∆Ms is large [9] andtherefore difficult to measure, a complementary insight might come from the width difference ∆Γs, which is relatedto ∆Ms via the relation [10]:∆Γs∆Ms= −32pim2bm2tη∆ΓsQCDη∆MsQCD(5)where the ratio of the QCD correction factors (η) is expected to be of the order of unity, and does not depend on theCKM matrix elements [11]. The width difference is expected to be of the order of 10% of the total width.One of the simplest way to investigate the width difference is to measure directly one of the two components of theBs lifetime. The decay Bs → D(∗)+s D¯(∗)−s X is dominantly CP even [12] and, if one neglects CP violating effects, isthe short lifetime eigenstate.5The investigation is performed in the case when each of the two Ds decay into a φ. The φ are identified in theK+K− decay mode.A. Event selectionALEPH has undergone a refinement of its tracking performances of the data already taken at LEP1. Firstly abetter pattern recognition in the Silicon Vertex Detector allows to increase the vertexing efficiency of hadronic Bdecays of about 30%. In addition, thanks to the readout of the specific ionization induced by charged particles on theTPC pads, the dE/dx measurement efficiency has now increased from 85% to nearly 100%, without degrading verymuch the purity. This measurement is the first one which benefits from this upgrade.Each kaon candidate is required to have at least 1.5 GeV/c momentum and is identified by requiring the dE/dx tobe consistent with the kaon hypothesis and vetoing pions. The angle between the two kaons in the φ rest frame hasto be larger than -0.8. The φφ system is required to have at least 10 GeV/c in momentum and an invariant massbetween 2.0 and 4.5 GeV/c2. Most of the combinatorial and fragmentation background is thus removed.Besides the signal events there remain other sources of double φ events: one or both φ can originate from fragmen-tation or combinatorial background. Three physics background are also present in Z → bb¯ and Z → cc¯ events:1. B → DDs(X), Ds → φ1X , D → φ2X2. B(s) → D(s)X , D(s) → φ1X and φ2 from fragmentation3. D(s) → φ1X and φ2 from fragmentationThe first reproduces exactly the signal signature and can not be removed in the selection. However the D decay isCabibbo suppressed with respect to the signal process (Ds → φX ∼ 18%, D+ → φX ∼ 2.9%, D0 → φX ∼ 0.9%).To have a good tracking quality, each kaon must have at least one hit in the VDET. The two φ are then constrainedto a common vertex with the χ2 probability of the vertex greater than 1%. At this point it is possible to reconstructthe Bs decay length as the distance of the φφ vertex from the primary vertex projected along the momentum direction.To reject non b events a b-tag is also demanded.After this chain of cuts, the contribution due to φ from fragmentation is negligible.The global efficiency is about 8% with a b-purity of 84% in the final sample.B. ResultsThe Bs lifetime is determined from the proper decay time distribution of the φφ events. For each Bs candidate, theproper time is obtained from the decay length l of the φφ system and the Bs boost. The decay length is measuredin three dimensions by projecting the vector joining the interaction point and the φφ decay vertex onto the directionof flight of the φφ resultant momentum. The typical resolution of the φφ decay vertex along the direction of flight is200µm. The boost of the Bs is computed from the nucleated jet method [13] starting from the two φ tracks.The Bs is extracted from an unbinned likelihood fit to the proper time distribution of the Bs candidates. Thebackground events amount to 78% of the total, and their proper time distribution has been parametrised from theevents in the two φ sidebands.Figure 4 shows the result of the fit:τshortBs = (1.42± 0.23± 0.16)ps. (6)The main systematics comes from the combinatorial background shape parametrisation.60102030405060708090-2 -1 0 1 2 3 4 5 6ALEPHPRELIMINARYDataBs signalBackgroundEntries /(0.7 ps)Proper Time (ps)FIG. 4. Proper time distribution of the two φ candidates for events selected in the data.From this measurement one could extract the difference in the width of the two Bs mass eigenstates:∆ΓΓ= 21− τshortBsτ¯Bs= 0.24± 0.35 (7)where τ¯Bs is the average Bs lifetime (1.61± 0.10 ps) and the statistical and systematic errors are combined.[1] ALEPH Collaboration, Eur. Phys. J. C6, 555 (1998)[2] ALEPH Collaboration, Phys. Lett. B429, 169 (1998)[3] R.D. Dikerman and N.G. Uraltsev, Nucl. Phys. B 509, 378 (1998)I. Bigi, R.D. Dikerman and N.G. Uraltsev, Eur. Phys. J. C 4, 453 (1998)[4] ARGUS Collaboration, Phys. Lett. B 234, 409 (1990)ARGUS Collaboration, Phys. Lett. B 255, 297 (1991)CLEO Collaboration, Phys. Rev. Lett. 64, 16 (1990)CLEO Collaboration, Phys. Rev. Lett. 71, 4111 (1993)CLEO Collaboration, Phys. Rev. Lett. 77, 5000 (1996)[5] C. Ramirez, J.F. Donoghue and G. Burdman, Phys. Rev. D 41, 1496 (1990).[6] P. Perret, talk given at EPS-HEP95, Brussel, July 1995.[7] A. Buras et al., Nucl. Phys. B 424, 374 (1994).[8] CLEO Collaboration, Phys. Rev. Lett. 74, 2885 (1995)CLEO Collaboration, CLEO CONF 96-05, Contributed paper to the ICHEP-96, Warsaw (1996).[9] Particle Data Group, Eur. Phys. J. C 3 (1998).[10] J. Hagelin, Nucl. Phys. B 193, 123 (1981)M.B. Voloshin et al. Sov. J. Nucl. Phys. 46, 112 (1987).[11] M. Beneke, G. Buchalla and I. Dunietz, Phys. Rev. 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Recent Results on Heavy Flavours with ALEPH

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