Elsing, M

Kluit, P M

Lesiak, T

Liebig, W

CERN Document Server

OPEN-99-40015/06/1999HEP'99 # 6 219 DELPHI 99-99 CONF 286Submitted to Pa 6 15 June 1999Pl 6A measurement of the cross-sectionratios for avour tagged events atenergies of 183 and 189 GeVPreliminaryDELPHI CollaborationPaper submitted to the HEP'99 ConferenceTampere, Finland, July 15-21A measurement of the cross-section ratios for avourtagged events at energies of 183 and 189 GeVPreliminaryDELPHI collaborationP.M. KluitNIKHEF, AmsterdamT. Lesiak1Institute of Nuclear Physics, KrakowM. ElsingCERN, CH-1211 Geneva 23, SwitzerlandW. LiebigGesamthochschule Wuppertal Bergische UniversitatAbstractA measurement of the cross-section ratios for avour tagged events in e+e an-nihilation at centre-of-mass energies of 183 and 189 GeV has been performed withthe DELPHI experiment at LEP, including the data taken in 1997 and 1998 cor-responding to an integrated luminosity of 53 and 158 pb 1, respectively. Hadronicevents were selected and classied as bottom, charm and light quark events on thebasis of the silicon tracker information. The cross-section ratios, dened as theratio of the cross-section for one quark avour to the total hadronic cross-section,measured for bottom and uds quark events were compared to the Standard Modelexpectations.1partially supported by the KBN grant 2 P03B 111 16 and SPUB/P03/178/9821 IntroductionSince the autumn of 1995 the centre-of-mass energy of the LEP accelerator was up-graded, thus allowing to test the Standard Model predictions in an unexplored energyregion between 130 and 189 GeV. In particular an observation of any deviation betweenthe measured and predicted cross-sections for the production of quark-antiquark pairsof denite avour would indicate some new physics beyond the Standard Model like thecontact interactions [1] or the production of an exotic heavy particle [2].The present study was devoted to the measurement of the cross-section ratios of avourtagged qq pairs in the energy range between 183 and 189 GeV (the analogue resultsconcerning the energy range 130-173 GeV have been published in [3]). The informationprovided by the silicon tracker about the presence of long-lived particles coming from thedecays of heavy avours allowed to split the data into subsamples enriched in bottom,charm and light quark-pair events. From the fraction of bottom (uds quark) tagged eventsthe ratio Rb= b=h(Ruds= uds=h) was extracted, respectively (hdenotes the totalhadronic cross-section).2 Data sampleThe DELPHI detector is described in [4]. The data sets which have been analyzed cor-respond to the full sample of 53 pb 1collected in 1997 in the centre-of-mass energy of183 GeV and of 158 pb 1gathered in 1998 withps = 189 GeV.3 Track selectionCharged particles were selected if they satised the following criteria: polar angle with respect to the beam axis between 20 and 160 degrees; momentum greater than 0.4 GeV; relative error on the momentum measurement less than 100 %; distance of track's closest approach to the beam-spot less than 4 cm in the planeperpendicular to the beam axis and less than 10 cm along it; track length larger than 30 cm.While neutral particles were demanded to pass the following cuts: energy of the shower detected in calorimeters (either hadronic or electromagnetic orSmall Angle Tile Calorimeter (STIC)) of more than 1.5 GeV; energy deposits of showers in the STIC calorimeter to be concentrated in more thanone cell.34 Event selectionThe selection of hadronic events coming from the process e+e ! qq() is based on theglobal event features which were deduced from the set of selected particles. The selectioncriteria were as follows: at least nine charged particles; at least seven charged particles seen in the silicon tracker; at least 18 % of the centre-of-mass energy was carried by charged particles; the rejection of Bhabba events in the forward region was obtained by the requirementthat the variableqE2A+ E2Cwas less than 90 % of the beam energy where EAandECwere the energies in the respective endcaps A and C of the forward calorimeter; the polar angle of the thrust axis of the event had to be in the range between 25 and155 degrees (the thrust axis was evaluated from both charged and neutral particlesof the event); most of the hadronic events coming from the process e+e ! W+W was removedby requiring at most three jets reconstructed in the event (using the LUCLUSalgorithm with the cut on the invariant mass set at 5 GeV); in addition only events in which all jets contained at least three particles wereaccepted; the rejection of the events with so called Z0radiative return was obtained by the cuton the eective centre-of-mass energyps0, calculated using the SPRIME package [5]:ps0=ps > 0:85. These events come from the radiative corrections to the qq produc-tion in e+e collisions where, due to the photon radiation in the initial state, theinvariant mass of the fermion pair is close to the mass of the Z0.This procedure yielded 1051 (2481) events selected atps=183 (189) GeV, respectively.The simulated data samples of the processes e+e ! qq() and e+e ! W+W havebeen subjected to the same selection criteria. They have been generated by the PYTHIA5.7 generator [6] with DELPHI tuning [7], simulated in the DELPHI detector [8] andpassed the same algorithm of event reconstruction as the data. The number of simulatedevents was 15 times larger than the data. The yield of W+W events in the data samplewas estimated from the simulation to be of the order of 3.8 (4.1) % for the energy of 183(189) GeV, respectively. The contribution of other processes like the production of ZZpair or  interaction was found negligible.5 Tag of events with dierent quark avoursDue to the relatively long lifetime of charmed and beauty particles (typically 0.4 and1.5 ps, respectively) the events in which the heavy avours were produced are characterizedby the presence of secondary vertices. The information provided by the silicon tracker [9]of DELPHI was used to split the data into three subsamples enriched in bottom, charm4and light quarks, respectively. The variable used to discriminate among these three datasets was the probability P+Efor the hypothesis that all tracks detected in the silicontracker come from the primary vertex [10]. It was evaluated per event and using onlycharged tracks with a positive lifetime. The sample enriched with bottom quarks whichwill be called since now b-tagged had to satisfy the requirement P+E< 0:002. For the oneclassied as c-tagged the P+Eprobability had to cover the range between 0.002 and 0.1.Events of the third sample containing mostly the light quarks and labelled as L-taggedhad to fulll P+E> 0:1 (in the following the light quarks u, d and s will be noted L). Thecuts were chosen to maximize the separation between the quark avours. The respectiveeciencies and purities of the dierent tags are summarized in Table 1.The dierences between data and simulation for the dierent tags were studied usingthe data collected the same year at a centre-of-mass energy corresponding to the Z0peak.At the Z0the Standard Model fractions for bottom, charm and light quark events wereassumed giving rise to the correction factors collected in Table 2 which were applied tothe simulation sample at high energy. The Z0data were also used to check the mistagrates (a mistag is e.g. a charm (light quark) event that is tagged as a bottom). Thiswas done dividing the events into two hemispheres and applying the b, c and uds tag perhemisphere. In this way data and simulation were compared selecting a b (c, uds) tag inone hemisphere and looking at the bottom, charm and light quark tags in the other one.It was found out that all the mistag rates agree to better than 5 %.6 Cross section ratios for dierent avoursThe numbers of events belonging to the dierent tagged categories were divided by thetotal number of selected hadronic events in order to obtain the cross-section ratios. Theywere denoted as Rxywhere the upper index distinguishes between the data and the sim-ulation of the process e+e ! qq() and the lower one labels dierent subsamples. Itwas assumed that the cross-section ratios obtained for the simulation sample correspondto the values expected in the Standard Model. The predictions from the simulationwere obtained using the correction factors given in the Table 2. The variable dened as(Rdata RSM)=RSMwas dened for each tag separately as a measure of possible deviationsfrom the Standard Model. Its values are presented in Table 3.The systematic error had four components. The rst one came from the statisticaluncertainty in the estimation of the correction factors (shown in Table 2). The seconduncertainty was the largest one and came from the modelling of the mistag rates discussedin the previous section. A systematic error on the mistag eciency of 10 % was propagatedinto the expected number of events. The last two (less pronounced) components were dueto the presence of theW+W background and from the systematic eects coming from thereconstruction of the eective centre-of-mass energyps0(contamination of non-radiativesample from double radiative events which where misinterpreted by the SPRIME package).The quadratic sum of these four components is quoted (cf Table 3) as the total systematicerror which turned out to be small with respect to the statistical uncertainties.As an additional check of consistency the cross-section ratios were evaluated in thesame way for the data corresponding to the Z0radiative return (0:43 <ps0=ps < 0:57)(cf Table 3). No signicant discrepancy between the cross-section rates resulting from thedata and the Standard Model predictions has been found both for non-radiative and Z05radiative return eventsThe cross-section ratios Rband RL(dened as the ratio of the quark cross-section qto the total hadronic cross-section h) were extracted from the event rates measured foravour-tagged subsamples according to the formula:Rq=qh= RSMq(1 +Nobsq NexpqNexpqPq) (1)where q denotes b and uds quarks, RSMqrefers to the Standard Model expectation as givenby ZFITTER version 6.10 [11], Nobsq(Nexpq) to the observed (expected) number of eventsfor a given q enriched tag and Pqto the q purity for a q enriched tag (cf Tables 1 and 3).The result for the 183 GeV can be summarized as:Rb= 0:153  0:017(stat+ syst)  0:18 (Rc  0:250)   0:03 (Ruds  0:581) (2)Ruds= 0:576  0:031(stat+ syst)  0:07 (Rb  0:169)   0:64 (Rc  0:250): (3)while the one for the 189 GeV reads:Rb= 0:167  0:012(stat+ syst)  0:21 (Rc  0:252)   0:03 (Ruds  0:581) (4)Ruds= 0:571  0:022(stat+ syst)  0:07 (Rb  0:167)   0:63 (Rc  0:252): (5)The cross-section Rcwas not given as the c-enriched sample was not pure/ecient enoughto perform a direct measurement (it can be recuperated from the relation Rc= 1 Rb Ruds). The results for Rband Rudsare summarized in Table 4 and plotted in Fig 1together with the predictions from ZFITTER (Fig 1 presents results in the full energyrange between 130 and 189 GeV including earlier results from [3]).7 SummaryThe cross-section ratios have been measured in qq hadronic events collected at centre-of-mass energies of 183 and 189 GeV. A preliminary comparison with the Standard Modeldid not show any signicant deviation from its predictions.AcknowledgementsT.L. wishes to thank the IReS laboratory for its hospitality while he was working inStrasbourg and acknowledges partial support from the KBN (Polish State Committee forScientic Research) grant 2 P03B 111 16 and SPUB/P03/178/98.References[1] F. Caravaglios, preprint CERN-TH/97-104 (1997);V. Barger et al., preprint MADPH-97-991, hep-ph/9703311 (1997).[2] G. Altarelli et al., preprint CERN-TH/97-40, hep-ph/97033276 (1997).[3] P. Abreu et al., (DELPHI collaboration) accepted by E. Phys. J. C, preprint CERN{EP/99{005 (1999).6[4] P. Aarnio et al., (DELPHI collaboration) Nucl. Instr. Meth. A378 (1996) 57.[5] P Abreu et al., (DELPHI collaboration) internal note 96-124 PHYS 632.[6] T. Sjostrand, preprint CERN-TH 7112/93 (1993).[7] P. Abreu et al., (DELPHI collaboration) internal note 95-80 PHYS 515.[8] P. Abreu et al., (DELPHI collaboration) internal notes 89-67 PROG 142 and 89-68 PROG143.[9] P. Abreu et al., (DELPHI collaboration) internal note 97-121 CONF 103.[10] G. Borisov, (DELPHI collaboration) internal notes 97-94 PHYS 716and 94-125 PROG 208;G. Borisov, C. Mariotti (DELPHI collaboration) internal note 95-142 PHYS 567.[11] D. Bardin et al., preprint CERN-TH 6443 (1992) and references therein.Table 1: Eciences and purities (in %) of quark avours for the two subsamples denedin the text. The quoted errors are statistical from the simulated sample.Eciency (%)ps (GeV) 183 189b enriched bb events (bbb) 78:1  1:1 81:0  1:2cc events (bcc) 14:1  0:3 16:7  0:4LL events (bLL) 1:9 0:1 2:3 0:1c enriched bb events (cbb) 15:8  0:4 13:8  0:4cc events (ccc) 34:4  0:5 34:7  0:5LL events (cLL) 16:4  0:2 17:8  0:2L enriched bb events (Lbb) 6:0 2:0 5:3 2:0cc events (bcc) 51:5  0:7 48:7  0:7LL events (LLL) 81:7  0:6 79:9  0:6Purity (fraction) (%)ps (GeV) 183 189b enriched bb events (fbbb) 74:2 1:1 71:1  1:0cc events (fbcc) 19:7 0:4 22:1  0:5LL events (fbLL) 6:1 0:2 6:8 0:2c enriched bb events (fcbb) 13:1 0:3 10:9  0:3cc events (fccc) 41:7 0:7 41:6  0:7LL events (fcLL) 45:2 0:7 47:6  0:7L enriched bb events (fLbb) 1:7 0:1 1:5 0:1cc events (fLcc) 21:2 0:3 21:1  0:3LL events (fLLL) 77:1 0:6 77:4  0:67Table 2: The correction factors accounting for the dierence in the distribution of thetagging variable P+E. The quoted errors are statistical from the Z0data sample.ps (GeV) 183 189b enriched sample 1:011  0:013 1:006  0:010c enriched sample 1:029  0:014 1:052  0:011L enriched sample 0:989  0:005 0:984  0:004Table 3: Summary of deviations in the cross-section ratios (rx= (Rdatax  RSMx)=RSMx,x = b; L) between the data and the Standard Model predictions as given by simulationsplitted into avour tagged subsamples. The upper (lower) tables correspond to non-radiative events and Z0radiative return, respectively.non radiativeqs0=s > 0:85data simulationtag obs. exp. stat. syst. rxevents events error errorps = 183 GeVb enriched 179 192.4 1.8 3.5  0:070 0:064  0:006c enriched 242 224.7 1.9 4.6 +0:077  0:062  0:010L enriched 630 633.9 3.2 5.3  0:006 0:025  0:006ps = 189 GeVb enriched 481 480.4 4.4 8.8 +0:001  0:042  0:008c enriched 577 557.9 4.7 12.8 +0:034  0:039  0:014L enriched 1423 1442.7 7.6 12.1  0:014 0:017  0:008Z0radiative return 0:43 <qs0=s < 0:57data simulationtag obs. exp. stat. syst. rxevents events error errorps = 183 GeVb enriched 412 400.4 2.6 3.5 +0:029  0:046  0:007c enriched 394 362.8 2.5 4.8 +0:086  0:050  0:010L enriched 1223 1265.9 4.6 6.9  0:034 0:018  0:006ps = 189 GeVb enriched 832 855.0 6.0 10.1  0:027 0:031  0:008c enriched 786 823.0 5.9 13.5  0:045 0:032  0:013L enriched 2780 2720.1 10.8 14.9 +0:022  0:012  0:0088Table 4: The derived quark cross-section ratios for bottom and uds quarks at energies183 and 189 GeV withqs0=s > 0:85 (the total error is given).ps RbRuds(GeV) exp. obs. (stat. + syst.) exp. obs. (stat. + syst.)183 0.169 0:153  0:017 0.581 0:576  0:031189 0.167 0:167  0:012 0.581 0:571  0:02200.10.20.30.40.50.60.70.80.9180 100 120 140 160 180 200√s (GeV)RqDELPHIRudsudsRbbFigure 1: The measured cross-section ratios for bottom and uds quarks as a function ofcentre-of-mass energy, together with the prediction from ZFITTER9

A measurement of the cross-section ratios for flavour tagged events at energies of 183 and 189 GeV

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