We report on measurements of the top quark mass by the CDF experiment at the FermiLab Tevatron collider. The top quark is produced in {ital p{anti p}} collisions at {radical}{ital s} = 1.8 TeV. The analyses use an integrated luminosity of 110 pb{sup -1}. Results are presented from three decay channels, (i) one lepton plus jets, (ii) dilepton, (iii) all hadronic. The smallest error on the mass comes from (i), with a preliminary top mass value of 175.6 {+-}4.4 (stat.) {+-} 4.8 (syst.) GeV. Results from the other channels are consistent with this value

CDF Collaboration

Lys, J.

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Fermi National Accelerator Laboratory FERMILAFWonf-96/409-E CDF Top Mass Measurement at CDF J. Lys For the CDF Collaboration Lawrence Berkeley National Laboratory Berkeley, California 94720 Fermi National Accelerator Laboratory P.O. Bm 500, Batavia, Illinois 60510 November 1996 Published Proceedings of the 28th International Conference on High Energy Physics (ICHEP%), Warsaw, Poland, July 25-31, 1996. 3 operated by Universities Fbseatd~ A OsDdation Inc. under ConM No. DE-ACO2-76CHBOOO witi the United States Depabnmt of Energy Disclaimer This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof nor any of their employees, makes any warranty, express or implied. or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process or service by trade name, trademark, manufacturer or otherwise, does not necessarily constitute or imply its endorsement, recommendation or favoring by the United States Government or any agency thereof The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof Distribution Approved for public release: further dissemination unlimited. CDF/ANAL/TOP/PuBLIc/3927 FERMILAB-CONF-96/409-E TOP MASS MEASUREMENTS AT CDF 3. LYS Lawrence Berkeley ,Vaiionnl Laboratory, Berkeley, Ca 94720 (For the CDF Collaboration) We report on measurements of the top quark mass by the CDF experiment at the Fermilab Tevatron collider. The top quark is produced in pp collisions at &- = 1.8 TeV. The analyses use an integated luminosity of 110 pb-‘. Results are presented from three decay channels, (i) one lepton plus jets, (ii) dilepton, (iii) all hadronic. The smallest error on the mass comes from (i), with a preliminary top mass value of 175.6 +Z 4.4 (stat.) f 4.8 (syst.) GeV. Results from the other channels are consistent with this value. 1 Introduction In pp collisions the Standard Model predicts that high-mass top quarks will be produced predomi- nantly in pairs. That is, as pp b tt + X. The model also predicts that the top quark will decay almost exclusively into a W boson plus a b quark. Therefore top quark events are classified accortl- ing to the decay modes (hadronic or leptonic) of the two W bosons: (i) lepton plus jets, one lep- tonic and one hadronic W decay; (ii) dilepton, two leptonic W decays; (iii) all hadronic, two hadronic W decays. The expected percentages in the three channels are 30%, 5%, and 44% respectively, if tau-lepton decays are excluded. Events where one or both decays of the W includes a tau-lepton can contribute to all of these channels. CDF has made top mass measurements for events in each of these three channels. The events come from 110 pb-’ of pp collisions at &- = 1.8 TeV at the Fermilab Tevatron Collider. 2 Lepton plus jets analysis 2.1 Event selection The event selection here requires (i) an isolated, high transverse momentum ( PT > 20 GeV/c) elec- tron or muon, (ii) a large missing transverse energy (J?, > 20 GeV), (iii) at least 3 jets with uncor- rected transverse energy ET > 15 GeV and pseudo rapidity magnitude 1~1 < 2.0, (iv) at least 4 jet,s with uncorrected ET > 8 GeV and 1~1 < 2.4. A total of 163 events pass these cuts. 2.2 Kinematic fit To extract a top mass, a kinematic fitting method is used l. The fit considers 5 vertices, the pro- duction vertex, two top decay vertices, and two W decay vertices. The 4 jets with the highest ET values in an event are assigned to the two b quarks from top decay and the two quarks from hadronic W decay, initially with all possible per- mutations. Jet directions and corrected jet ener- gies, with errors, are assigned to the quark-level entities, together with masses of 5.0 (b-quarks) or 0.5 GeV. The masses of the two top quarks in an event are required to be equal. The net result is a 2-constraint fit. In general there are two solu- tions per jet assignment permutation, correspond- ing to two solutions for the neutrino longitudinal momentum. Therefore, with all possible permu- tations there may be 24 fit solutions for a given event. The number of solutions may be reduced by t.agging b jets. A jet is tagged as a b-jet if there is evidence that the jet contains a bhadron de- cay. Two types of tagging ’ are used, (a) SVX, making use of the silicon vertex detector to find tracks that originate from a vertex downstream from the event’s primary vertex; (b) SLT, ob- serving a soft lepton that is consistent with com- ing from a semileptonic decay of a bhadron or a daughter charm hadron. Then, only those permu- tations that have a tagged jet assigned as a b jet need be considered. Kinematic fits are required to have x2 < 10. Of the 163 events passing the cuts, 153 have at least one passing kinematic fit. Of these 153 events, 34 events have one or two b-tags. Pub. Proceedings 28th International Cohference on High Energy Physics (ICHEP'96), Warsaw, Poland, July 25-31, 1996 2.3 Monte Carlo simulation A Monte Carlo simulation of the kinematic fitting is necessary for several reasons. Firstly, there is experimental smearing, predominantly in the jet energy. Secondly, there is the need to relate the jet energies to parton energies. Thirdly, one (or more) of the four highest ET jets may not be directly re- lated to the partons in the decay chain (and in particular could come from hard gluon radiation from inititial or final state partons). Fourthly, the fit solution with the lowest x2 may not have the correct jet assignment or correct neutrino longitu- dinal momentum solution. The Monte Carlo programs used are HER- WIG 2 for top production and decay, and VEC- BOS3 for background processes, together with the standard CDF detector simulation, which includes modelling of the b tagging. The distributions of many kinematic vari- ables of the observed events have been compared with the predictions from HERWIG plus VEC- BOS events in the expected ratios. Generally good agreement is observed. 2.4 Background estimate The estimate of the background (i.e., non-top) fraction of the event samples comes from the oh- served numbers of &tags of different types and the estimated tagging efficiencies for top and non- top events. Then background fraction estimates are approx. 66% of the 153-event sample, and approx. 20% of the 34-event tagged sample. 2.5 Likelihood Fit The top mass estimate usea a likelihood fit to the Mt distribution. Here, M1 is the top mass esti- mate from the kinematic fit (i.e., the reconstructed mass), one A4r value per event. The Mt value is taken from the lowest chisquare solution that has jet assignments consistent with observed b-tags. The predicted distributions for the likelihood fit come from Monte Carlo events, at several Al,,,, values, run through the same kinematic fitting as the data. Finite Monte Carlo statistics give a sta- tistical error to the likelihood at each MtoP value. A quadratic fit is made to a limited portion of the negative-log- likelihood versus MtoP plot. The true MtoP estimate is given by the minimum of the quadratic, and the statistical error by where M I 175.6 k S.‘l(rut) t 5.9(yt) GeV IO Figure 1: Lepton plus jets with 2 1 tag, observed (solid) and predicted (dashed) reconstructed mass, and likelihood fit. the quadratic has increased by 0.5 units from the minimum. 2.6 Tagged event results Results for the 34-event sample with b tags are shown in Fig. 1. The histograms show the ob- served Mt distribution and that predicted for top events (Mt, = 175 GeV) plus background events. The likelihood fit gives MtoP = 175.6 f 5.7 GeV (stat.error). The estimated systematic error is f 5.9 GeV. Compared to what was presented ear- lier 4 the systematic errors have been reevaluated. The leading systematic error sources are soft gluon plus jet energy scale uncertainties (f 3.8 GeV), hard gluon uncertainties (k 3.6 GeV), and kine- matic and likelihood fitting method uncertainties (31 2.0 GeV). 2.7 Optimized tagged plus no-tagged samples Monte Carlo studies indicate a reduced error if the t.agged events are subdivided into different tag- ging classes and a subset of the no-tagged events is added. Therefore, the following four exclusive subsamples were used: (i) events with just one SVX tag (15 events), [ii) events with two SVX t,ags (5 events), (iii) events with a SLT tag(s) but no SVX tag (14 events), (iv) events with no tag and with all four leading jets having ET > 15 GeV (48 events). The first three subsamples comprise the 34-event tagged sample considered above. For each subsample, an expected background fraction was estimated, and predicted M* (re- 2 Figure 2: Lepton plus jets optimized subsamples, observed (shaded) and predicted (unshaded) re- constructed masses, and likelihood fits. Figure 3: Lepton plus jets optimized subsamples, combined observed (solid) and predicted (dashed) reconstructed mass, and product likelihood. constructed mass) distributions for both top and background events were obtained from Monte Carlo events at several Mtop values. Then a like- lihood fit was made for each subsample, shown in Fig. 2. At a given top mass value, the likelihood val- ues from the four subsamples are essentially inde- pendent, so a combined result is obtained by tak- ing the product of the four values. The sums of the A4t distributions, and the likelihood product versus top mass plot, are shown in Fig. 3. From the likelihood product plot the top mass estimate is 176.8 f 4.4 (stat) GeV. Pseudeexperiments were used to check this procedure. For each pseudo-experiment, Monte Figure 4: Lepton plus jets optimized subsamples, top mass values and statistical errors from 1000 Monte Carlo experiments. Carlo events were used, with the same number of events in each subsample as observed in the data and with the appropriate background fraction in each subsample. Then the same likelihood fit as used on the data was applied. Results are in Fig. 4. The median top mass estimate of 175.3 GeV agrees with the input Monte Carlo value of 175 GeV. The median statistical error of 5.4 GeV is somewhat larger than the value from the data of 4.4 GeV, but a value smaller than 4.4 GeV occurs in nearly 10% of the pseudo-experiments. The total systematic error is f 4.8 GeV. The largest contribution (5 3.6 GeV) comes from soft gluon plus jet ET scale uncertainties, that is, from uncertainty in estimating the true jet energy from the observed energy in a cone of radius 0.4 (in q- 4 space). The hard gluon effects systematic error (& 2.2 GeV) reflects uncertainty in how often one of the four leading jets is associated with a hard gluon rather than directly with a parton in top decay. All other contributions are 5 1.5 GeV. 3 Dilepton events The event selection requires (i) two central, oppositely-charged leptons (electron or muon) with transverse energy ET > 20 GeV, (ii) large missing transverse energy (1, > 25 GeV), (iii) at least two jets with ET > 10 GeV, (iv) the sum of the ET’S of the leptons and the jets plus the J!!, > 170 GeV, (v) the two-lepton invariant mass not in the 2 region. If 1, < 50 GeV there is a cut on 3 the angular separation between the g, direcrion and the closest jet or lepton. A total of 8 event.s pass these cuts. The expected background (i.e.. non-top) contribution is - 1 event. The two neutrinos present preclude straight- forward kinematic fitting in this channel. Instead, the observed distribution in a kinematic variable whose distribution is dependent on the top mass can be compared to the predicted distribution as a function of top mass. Earlier4 a result using the lepton (electron or muon) energy distribution was shown. A second analysis has used the invariant mass-squared of a lepton plus a jet. For each event the average value of the pairing that gives the smaller value is chosen. Comparison to predicted distributions leads to an Miop estimate of 162 f21 (stat) ‘li L (syst.) GeV. 4 All hadronic events Events here are required to have at least 6 ,jets with ET > 15 GeV and 171 < 2.0, and no more than 8 such jets. The minimum jet-jet separation in 7-4 space must be > 0.5. The total transverse energy must be > 200 GeV and > 0.75 6. There is also a transverse energy-dependent cut on apla- narity. Finally, events must have at least one jet with an SVX tag. The final event sample contains 142 events. The calculated number of background ( i.e., non-top) events in the sample is 113 events with an error of 6.5%. A kinematic fit is performed, using the 6 lead- ing jets of each event. The fit has each top parton decaying to a W plus a b parton, and each W de- caying to two jets, and has 3 constraints. After requiring the tagged jet to be assigned as a b jet, there are 30 possible jet assignments, and so 30 possible solutions. The solution with lowest l2 value is chosen, and that value is required to be < 10. The predicted background mass distribution is taken to be given by netag events that other- wise pass the selection cuts. These events are run through the same kinematic fit as the tagged sam- ple. The predicted mass distribution for top events is obtained from HERWIG Monte Carlo events. A likelihood fit, with the background contribution constrained to the calculated fraction, gives a top maSS estimate of 187 f8 (stat.) GeV. The system- atic error is ‘:; GeV, with largest contributions Figure 5: All hadronic channel, observed (solid circles), background (shaded), and predicted top (185 GeV) plus background reconstructed mass. coming from the jet energy scale and hard gluon uncertainties. The best fit top mass distribution is shown in Fig. 5 , together with the predicted mass distri- butions from background events and background plus top events with Mtop = 185 GeV. 5 Conclusion The CDF experiment has measured the top quark mass in each of three topologies. The most accurate measurement comes from the lepton plus jets topology, using 34 tagged and 48 no-tag events. The result is 176.8 f4.4 (stat.) 3~4.8 (syst.) GeV. The dilepton topology result, from the lepton- plus-jet effective mass, is 162 f21 f7 GeV. The all-hadron topology kinematic fitting re- sult is 187 f8 f12 GeV. References 1. F. Abe et al (CDF Collaboration), Phys. Rev. D 50, 2966 (1994). 2. G. Marchesini et al, Comput. Phys. Comm. 67, 465 (1992). 3. F. A. Behrends et al, Nucl. Phys. B 357, 32 (1991). 4. A. BarbareGaltieri (CDF Collaboration), FNAL-Conf-96/146-E, Presented at XXXIst Rencontres de Moriond, March 1996. 4 

Top mass measurements at CDF

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Top mass measurements at CDF

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