Using a sample of $26$ b\=b events, produced in $350\,\hbox{GeV}/c$ $\pi^-$ interactions in a copper target, which includes $13$ events where the decays of both $B$ and $\overline{B}$ are well reconstructed, we measure the differential distributions with respect to $x_F$ and $p_T^2$ as well as some two-particle kinematic variables. We also compare our results with a previous experiment and with predictions based on perturbative QCD

Alexandrov, Yu A

Angelini, C

Barberis, D

Ceradini, F

Cianfarani, C

Dameri, M

Darbo, G

Duane, A

Flaminio, Vincenzo

French, Bernard R

Gemme, C

Harrison, K

Hurst, R B

Lazzeroni, C

Malferrari, L

Martellotti, G

Mazzanti, P

Nechaeva, P Yu

Osculati, B

Penso, G

Rossi, L

Websdale, David M

Zanello, L

Zavertyaev, M V

Physics Letters B

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN{EP/99{71May 21, 1999Measurement of the kinematic variables of beautyparticles produced in 350 GeV/c pi−-Cu interactionsThe BEATRICE CollaborationY. Alexandrov5), C. Angelini6), D. Barberis3), F. Ceradini8), C. Cianfarani1),M. Dameri3), G. Darbo3), A. Duane4), V. Flaminio6), B.R. French2), C. Gemme3),K. Harrison6), R. Hurst3), C. Lazzeroni6), L. Malferrari1), G. Martellotti7), P. Mazzanti1),P. Nechaeva5), B. Osculati3), G. Penso7), L. Rossi3), D.M. Websdale4), L. Zanello7) andM. Zavertyaev5).AbstractUsing a sample of 26 bb¯ events, produced in 350GeV/c pi− interactions in a cop-per target, which includes 13 events where the decays of both B and B are wellreconstructed, we measure the differential distributions with respect to xF and p2Tas well as some two-particle kinematic variables. We also compare our results witha previous experiment and with predictions based on perturbative QCD.submitted to Physics Letters B1) Universita` di Bologna and INFN, Bologna, Italy.2) CERN, Geneva, Switzerland.3) Universita` di Genova and INFN, Genoa, Italy.4) Blackett Laboratory, Imperial College, London, United Kingdom.5) Lebedev Physical Institute, Moscow, Russian Federation.6) Universita` di Pisa and INFN, Pisa, Italy.7) Universita` di Roma “La Sapienza” and INFN, Rome, Italy.8) Universita` di Roma “Roma Tre” and INFN, Rome, Italy.Hadroproduction of heavy quarks is an important testing ground for Quantum Cro-modynamics (QCD). In fact the production of a quark having a mass, mQ, much largerthan the QCD parameter QCD can be calculated perturbatively via a power-series expan-sion in the strong coupling constant s(mQ2)  1= ln(mQ2=QCD2). In the case of beautyproduction (mQ = mb  4:75 GeV=c2, QCD  140 MeV), the perturbative calcula-tions, currently evaluated to next-to-leading order (NLO) [1], are expected to be reliable.Nonetheless, the theoretical predictions have uncertainties arising from a dependence onthe chosen renormalization and factorization scales, the b-quark mass, the parton den-sity functions and QCD. Other uncertainties are due to the fragmentation of quarks intohadrons and to the interacting partons’ momentum, kT, transverse to the beam direction.For some quantities, such as the production cross-section, the uncertainties in the theo-retical predictions are large. Smaller uncertainties aect the prediction of the kinematicdistributions.At xed-target energies, the beauty production cross-section is of the order of fewnanobarns and amounts to  10−7 of the inelastic cross-section. In experiment WA92,performed at the CERN Super Proton Synchrotron, we identied 26 beauty events [2], thelargest sample so far obtained in a xed-target environment. We previously reported re-sults on the beauty production cross-section [2] and on the azimuthal angle  between as-sociated beauty particles [3]. In this letter we complete our studies of beauty hadroproduc-tion by reporting on the measurement of kinematic variables. These measurements requireknowledge of the beauty-particle momenta which must be either measured or estimated.The measured variables include the p2T and xF of the beauty particles (single-dierentialdistributions) and some correlation variables such as the invariant mass M(BB), theFeynman-x of the BB system and the Feynman-x dierence jxF j = jxF (B) − xF (B)j(double-dierential distributions). We compare the measurements with the results fromexperiment E653 [4], where the decays of 9 beauty pairs were reconstructed in nuclearemulsion.Data taking for the WA92 experiment was performed at the CERN Ω0 spectrom-eter in 1992 and 1993, with a 350 GeV=c − beam incident on a 2 mm copper target.The number of events recorded was  108, corresponding to an integrated luminosity of8:1 (nb per Cu nucleus)−1.Full details of the experimental apparatus and trigger are given elsewhere [5]. Thepresent analysis exploits the imaging capabilities of the high-resolution tracking system.This was formed from silicon-microstrip planes, arranged as a decay detector (DkD) anda vertex detector (VxD). The DkD, covering the rst 3.2 cm downstream of the target,consisted of 17 silicon planes with 10 m pitch and analogue readout. The VxD consistedof 12 silicon planes of 25 m pitch and 5 planes of 50 m pitch. Tracking further fromthe target was performed using multiwire proportional chambers (58 planes in a 1:8 Tmagnetic eld) and drift chambers (8 planes). A muon detector, based on resistive-platechambers, was positioned downstream of the tracking detectors.Use was made of a combination of several independent triggers [5], the aim beingto keep the acceptance for beauty events high, with acceptable dead-time. An event waswritten to tape if an interaction trigger was satised in coincidence with any two of thefollowing: a high pT trigger, obtained using a pair of butterfly-shaped hodoscopes, crossedonly by particles with transverse momentum greater than 0:6 GeV=c; a muon trigger,requiring detection in the resistive-plate chambers of a muon consistent with an origin inthe target; a secondary-vertex trigger, which used information from a silicon microstrip1beam hodoscope and from the VxD.The trigger acceptance was  2% for inelastic interactions and  30% for beautyevents. Assuming a linear A-dependence, the total number of beauty events written ontape is of the order of 150 per nb of cross-section.For the acceptance calculations, and for studies of backgrounds in the beauty search,we fully simulated bb, cc and minimum-bias events. The last-mentioned were generatedusing Fluka [6]. Events with heavy quarks were generated using Pythia 5.4 [7] and Jetset7.3 [8] to describe the hard process and quark fragmentation, and using Fluka to determinethe characteristics of all other interaction products. Tracking of particles through theexperimental apparatus was performed using Geant 3.21 [9].Analysis of the data yielded a sample of 26 bb events, with an estimated backgroundof 0:6  0:6 events; this sample includes 13 events where the decays of both B and B arewell reconstructed. A complete account of the analysis strategy can be found elsewhere [2,3]. Here we give only a brief outline of the procedure followed. Secondary vertices weresearched for in the region between 0.3 cm and 6 cm from the primary vertex. Takingadvantage of the analogue readout of the DkD, we disregarded secondary vertices close tolarge energy releases, which were usually due to hadronic interactions in the silicon planes.We then selected three categories of events, consistent with beauty-decay topologies:i. events containing  1 secondary  of pT  1 GeV=c and  2 secondary verticeswith minimum mass  0:55 GeV=c2, at least one of which with minimum mass 2:3 GeV=c2;ii. events containing a vertex consistent with the hadronic decay of a charmed mesonnot originating in the primary interaction (impact parameter relative to primaryvertex  30 m) and at least one other secondary vertex;iii. events containing  3 secondary vertices and satisfying a high-pT-track requirement(one track with pT > 1:5 GeV=c, or one track with pT > 0:8 GeV=c and one trackwith pT > 0:6 GeV=c).To calculate a vertex’s minimum mass, we assumed that it corresponded to the decay ofa particle from the primary vertex and that the energy-momentum conservation of thedecay was ensured by an unobserved zero-mass particle. All detected decay products wereassigned the pion mass.Events from categories ii.) and iii.) above were ltered by a neural network [10],taking into account topology and kinematics through a set of 16 input variables. We nextscanned events individually using a graphical display program, selecting 26 bb events;each one contains at least one vertex interpreted as a beauty-particle decay.The second step of the analysis consisted of the identication of the decay vertexof the associated beauty particle. First of all we used an interactive version of our re-construction program [5, 11] to improve the vertex reconstruction accuracy. With theinteractive program we were able to correct wrong assignments of hits to tracks, and oftracks to vertices, thus improving both the vertex position and our knowledge of the totalmomentum.In our 26 bb events, the majority of the secondary vertices are due to decays ofcharm and beauty. As a result, a candidate secondary vertex for the decay of the associatedbeauty was initially required to satisfy only minimal selection criteria:i. to be outside the target, within 6 cm of the target edge, and with a distance fromthe primary vertex of at least six times the vertex separation uncertainty;ii. not to be compatible with a hadronic interaction, and hence to be far from any large2energy release in the DkD;iii. not to be compatible with the decay of a 0 or K0, or with a photon conversion intoan e+e− pair;iv. not to be compatible with the decay of the charmed hadron from the beauty-particledecay already identied;v. to have a transverse decay length of > 50 m.In particular this last requirement, together with a similar cut of > 20 m on the trans-verse decay length for the other beauty particle, improved the resolution on the measuredvariables.In order to avoid confusing the associated beauty vertex with its secondary charmdecay, we introduced two further kinematic conditions:i. at least one outgoing track with a momentum component > 1 GeV=c transverse tothe line joining the vertex to the primary-interaction point;ii. a minimum mass > 2 GeV=c2.With these selection criteria, we identied the associated beauty-decay vertex in13 events (double-dierential sample); we have thus a total of 39 beauty decays (single-dierential sample). The background, already low (0:6  0:6) for the 26 bb events [2],becomes negligible for the double-dierential sample; therefore in the present analysis weneglected the background for both samples. We identied the secondary charmed hadronin half of the single-dierential sample. This fraction, as well as the charge distributionof the beauty particles, was well reproduced by a sample of simulated bb events passingthe same selection criteria.The study of the kinematics of our samples requires an estimator for the momentumof the partially reconstructed vertex, taking into account the unseen decay products. Weused an estimator very similar to the one already used by this collaboration for the analysisof charm pair production [12]: a combination of two methods that are based on dierentphysical considerations. The rst method consists in closing the beauty decay by adding aneutral pion balancing the visible decay momentum on the beauty line of flight. Imposingthe beauty mass on the decay vertex and requiring that its total momentum vector pointto the primary vertex, leads to two possible choices for the beauty momentum. Thesecond method was used by other experiments [4] and is based on the assumption thatin the rest frame of the beauty particle the unseen momentum is emitted on average in adirection perpendicular to the beauty laboratory momentum. The Lorentz boost γB fromthe beauty rest frame to the laboratory frame is then given by γB = Evis=√M2vis + p2Tviswhere Evis is the visible energy, pTvis is the visible transverse momentum, and Mvis isthe mass of the visible system, calculated assigning the pion mass to all the detectedparticles. A comparison between these two methods and the simulation indicates thatboth give useful and independent information; however the rst method systematicallyunderestimates the momentum while the second one systematically overestimates it. Thebest estimator for the beauty momentum is a weighted average of the results of the twomethods. The possible presence of a charmed particle from the beauty decay was taken intoaccount since the charmed hadron usually carries a large fraction of its parent’s momentum(mean momentum fraction  50%). The same estimator was adopted also for the charmmomentum when the charm decay was not completely reconstructed. If more than onesecondary vertex could be identied as the charmed hadron, all the possibilities weretaken into account and the solutions for the beauty momentum were equally weighted.3Using the charm momentum, the accuracy of the beauty momentum wasp=p = 0:017 + 1:7  10−4 pwhile if the charm decay coming from the beauty was not reconstructed the accuracy ofthe beauty momentum wasp=p = 0:057 + 4:3  10−4 pwhere p is in GeV/c.Acceptance distributions for single- and double-dierential variables are shown inFig. 1. Acceptance corrections for these variables were determined from a simulationof 13500 bb events, corresponding to about 4.5 times the bb events produced in thedata-taking period. These simulated events were processed with the same analysis andselection chain as the experimental data. After scanning, a sample of 116 events survived;we identied the decay vertex of the associated beauty particle in 72 events, obtaininga total of 188 beauty decays. Comparisons of the simulated and reconstructed valuesshowed that the experimental precision in each variable is smaller than the bin sizeschosen, therefore we calculated the acceptance corrections by dividing each distributionof reconstructed events by the corresponding distribution of generated events. For thesquare of the transverse momentum of the BB pair, p2T (BB), the comparison betweensimulated and reconstructed values would impose too large a bin size with respect to ourstatistics, therefore we do not have a signicant measurement of this variable.The p2T and xF distributions of the single-dierential sample, corrected for accep-tance, are plotted in Fig. 2a)-b) together with the results of experiment E653 and thepredictions based on NLO QCD calculations. The theoretical distributions were obtainedtaking mb = 4:75 GeV=c2; QCD = 140 MeV; the factorization scale F and the renor-malization scale R are equal to√mb2 + pT2, where pT is the transverse momentum of theb-quark. We used the SMRS2 [13] parton density set for the beam pion, and the MRSD-[14] parton density set for target nucleons. Within the large statistical uncertainties, ourdata are consistent both with the result of experiment E653 and with the theoretical pre-dictions. The consistency with the theoretical predictions is not surprising since, thanksto the large b-quark mass, the NLO QCD calculations are expected to be reliable and thecorrections due to fragmentation, intrinsic transverse momentum of the incoming partonsor higher order contributions are small. The single-dierential distributions were ttedwith the usual parametric functions exp(−bp2T ) for the p2T and (1− jxF j)n for the xF ; tparameters are b = (0:14  0:03) (GeV/c)−2 and n = 3:1  0:9; xF > 0. The meanvalues of the distributions are reported in Table 1.Using the double-dierential sample, we measured the invariant mass M(BB), theFeynman-x of the BB system and the Feynman-x dierence jxF j = jxF (B) − xF (B)j.Fig. 2c)-e) show these kinematic correlation variables, corrected for acceptance, alongwith results from experiment E653, when available, and the predictions based on NLOQCD calculations. The theoretical distributions were obtained taking the same param-eter values used for the single-dierential distributions, apart from xing R = F =√mb2 +12(pT2 + pT2), where pT and pT are the transverse momenta of the b-quark andthe b-antiquark respectively. In the calculation of the invariant mass of the BB system,the b-quark mass was taken to be mb = 5:3 GeV=c2, in order to reproduce the physicalthreshold for B-mesons production. Again, within the large statistical uncertainties, our4data are consistent both with the results of experiment E653 and with the theoretical pre-dictions. Table 1 reports the mean values found by the present analysis for all measuredcorrelation variables.A study of beauty particle hadroproduction was performed in the WA92 experiment,in which a beam of 350 GeV=c − particles interacted in a 2-mm-thick copper target. TheWA92 experiment recorded 26 events featuring production of beauty particles; in 13 ofour 26 bb events we identied the decay vertex of the second beauty particle, giving atotal of 39 beauty decays. Using this sample of beauty decays and optimising an estimatorfor the momentum of the partially reconstructed vertices, we measured the distributionswith respect to xF and p2T for the beauty particles as well as some kinematic correlationvariables. Our results, which are consistent with the only previous measurements for 9 bbevents, were compared to NLO QCD predictions.The authors are indebted to Dr. Giovanni Ridol for supplying the Fortran programfor the NLO QCD calculation and for the fruitful discussions on the interpretation of theresults.References[1] S. Frixione et al., Heavy Flavours II (Advanced Series on Directions in High EnergyPhysics - Vol. 15), eds. A.J. Buras and M. Lindner (World Scientic, Singapore,1998), p. 609 and references therein [preprint CERN-TH/97-16 (1997)].[2] M. Adamovich et al., Nucl. Phys. B495 (1997) 3.[3] Y. Alexandrov et al., Phys. Lett. B433 (1998) 217.[4] K. Kodama et al., Phys. Lett. B303 (1993) 359.[5] M. Adamovich et al., Nucl. Instr. and Meth. A379 (1996) 252.[6] A. Fasso et al., Proc. IV International Conference on Calorimetry and their Appli-cations, La Biodola, Italy (World Scientic, 1994) 493.[7] H.-U. Bengtsson and T. Sjo¨strand, Comput. Phys. Commun. 46 (1987) 43.[8] M. Bengtsson and T. Sjo¨strand, Comput. Phys. Commun. 43 (1987) 367.[9] GEANT Detector Description and Simulation Tool, CERN Program Library LongWriteup W5013 (1994).[10] L. Malferrari, Nucl. Instr. and Meth. A368 (1995) 185.[11] J.C. Lassalle et al., Nucl. Instr. and Meth. 176 (1980) 371.[12] M. Adamovich et al., Phys. Lett. B385 (1996) 487.[13] A.D. Martin et al., Phys. Rev. D45 (1992) 2349.[14] A.D. Martin et al., Phys. Rev. D43 (1991) 3648.5Table 1: Mean values of the single and double-differential variables. Theoretical expectionsare shown for comparison.WA92 NLO QCD< p2T > (GeV/c)2 7.9  1.0 6.2< xF > 0.06  0.04 0.096< M(BB) > (GeV/c) 12.4  0.3 12.3< xF (BB) > 0.16  0.08 0.21< jxF (B)− xF (B)j > 0.28  0.04 0.27Figure 1: Acceptances of beauty particles as a function of a) p2T and b) xF ; acceptances ofbeauty pairs as a function of c) BB invariant mass, d) Feynman-x of the BB system ande) jxF (B)− xF (B)j.6Figure 2: Distributions of a) p2T and b) xF for the beauty particles; distributions of c) BBinvariant mass, d) Feynman-x of the BB system and e) jxF (B)− xF (B)j. All the distri-butions are corrected for the experimental acceptances. Present results (solid circles) arecompared with NLO QCD predictions (solid line) and E653 measurements (open squares).7

Measurement of the kinematic variables of beauty particles produced in 350 GeV/c $\pi^-$-Cu interactions

Using a sample of 26 b (b) over bar events, produced in 350 GeV/c pi(-) interactions in a copper target, which includes 13 events where the decays of both B and (B) over bar are well reconstructed, we measure the differential distributions with respect to x(F) and p(T)(2) as well as some two-particle kinematic variables. We also compare our results with a previous experiment and with predictions based on perturbative QCD. (C) 1999 Published by Elsevier Science B.V. All rights reserved

Y. Alexandrov

C. Angelini

D. Barberis

F. Ceradini

C. Cianfarani

M. Dameri

G. Darbo

A. Duane

V. Flaminio

B. R. French

C. Gemme

K. Harrison

R. Hurst

C. Lazzeroni

L. Malferrari

G. Martellotti

P. Mazzanti

P. Nechaeva

B. Osculati

L. Rossi

D. Websdale

L. Zanello

M. Zavertyaev

PENSO, Gianni

Archivio della ricerca- Università di Roma La Sapienza

Measurement of the kinematic variables of beauty particles produced in 350 GeV/c pi(-)-Cu interactions

G. Penso

BARBERIS, DARIO

OSCULATI, BIANCA MARIA

Archivio istituzionale della ricerca - Università di Genova

Measurement of the kinematic variables of beauty particles produced in 350 GeV/c π−-Cu interactions

Measurement of the kinematic variables of beauty particles produced in 350 GeV/c $\pi^-$ -Cu interactions

Abstract

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CERN Document Server

Archivio della ricerca- Università di Roma La Sapienza

Archivio istituzionale della ricerca - Università di Genova

Measurement of the kinematic variables of beauty particles produced in 350 GeV/c π−\pi^-π−-Cu interactions

Abstract

Similar works

Full text

Available Versions

CERN Document Server

Archivio della ricerca- Università di Roma La Sapienza

Archivio istituzionale della ricerca - Università di Genova

Measurement of the kinematic variables of beauty particles produced in 350 GeV/c $\pi^-$ -Cu interactions