Top quark results from the Tevatron

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Nuclear PhysicsB (Pros. Suppl.) 66 (1998) 502-505

PROCEEDINGS SUPPLEMENTS

Top Quark Results from the Tevatron Herbert Greenlee a for the CDF and DO Collaborations aFermilab, MS 357, P.O. Box 500, Batavia, IL 60510, USA The CDF and DO collaborations have collected top quark data samples consisting of several dozen events each. Using these data samples, the two Teratron experiments have measured the top quark mass and pair production cross section in a wrlety of decay channels, including the so-called dilepton, lepton plus jets and all jets channels. The combined top quark mass from both experiments in the lepton plus jets channels is mt = 175.6 4- 8.5 GeV/c ~. Additionally, upper limits have been obtained for certain rare or nonstandard (FCNC and charged ttiggs) top quark decays.

The discovery of the top quark was announced by the CDF and DO collaborations in 1995 [1, 2]. Since time of the discovery, the integrated luminosity available to the two collaborations has approximately doubled, to about 100 pb -1. The sophistication of the top quark analyses by the two experiments has increased as well. At the Tevatron, top quarks are pair produced in V~ = 1.8 TeV p/~ collisions. In the Standard Model, top quarks decay 100% of the time to a W boson and a b quark, with subsequent decay of the W boson into either a charged lepton and a neutrino, or a quark-antiquark pair. Top quark signatures are classified on the basis of the decay of the two W bosons into the dilepton (ee, e/~, and/~/~), lepton (e or/~) plus jets, and all jets decay channels. 1. T o p Quark M a s s in t h e L e p t o n plus J e t s Channel

The lepton plus jets channel has produced the most precise measurement of the top quark mass to date [3,4]. The analysis method used by the two experiments is very similar. Events are required to have one charged lepton (e or p), four or more jets and missing E~.. Both experiments make use of information that identifies or "tags" certain jets as b quark jets. Jets can be b-tagged by means of a displaced vertex in the SVX (CDF) or by the presence of a soft lepton near the jet (CDF and DO). For untagged events, DO uses a 0920-5632/98/$19.00© 1998ElsevierScienceB.V. All fights reserved. PII S0920-5632(98)00095-4

Table 1 Top quark mass results. If two errors are specified, the first is statistical and the second is systmatic. Exp. Method rn~ (GeV/c ~) CDF l + jets 176.8 4- 4.4 4- 4.8 U (Eb) 159+24-22~-'L17 U (rnzb) 162 4- 21 4- 7 all jets 186 4- 10 -t- 12 DO l + jets 173.3 4- 5.6 -4- 6.2 U 168.4 4- 12.3 ± 3.7 U, l + jet8 172.04- 5.1 4- 5.5 CDF+DO l + jets 175.64- 5.5

multivariate event shape requirement to improve the signal to background ratio. Events are reconstructed using the four leading jets with a 2C constrained fit to the top quark pair hyi~othesis. A top quark mass likelihood function is derived from the observed distribution of reconstructed masses using signal and background "templates." A template is the expected distrubution reconstructed top quark mass for background events or top quark events of a particular assumed mass. The results from the lepton plus jets channels are summarized in Table 1, along with all other top quark mass results.

H. Greenlee/Nuclear Physics B (Proc. Suppl.) 66 (1998) 502-505

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2. Top Quark Mass in the Dilepton Channel

in Table 1.

Dilepton event selection requires two charged leptons (e or ~), two or more jets, and missing E~,. The dilepton channel differs from the lepton plus jets channel in that there is not enough kinematic information to reconstruct the event. Therefore, the experiments have resorted to using mass estimators other than the reconstructed mass. In principle, any quantity that is correlated with the top quark mass can be used as an estimator. In all cases, the same template technique that was used for the lepton plus jets mass analysis is used to calculate a top quark mass likelihood in the dilepton mass analysis. The two experiments use different quantities as the top quark mass estimators. CDF has done two analyses using the b quark jet energy (Eb), and the lepton-b invariant mass (mzb) as the mass estimator [3]. DO uses a "weight curve" method, in which a weight is derived as a function of an assumed top quark mass that somehow describes how likely each candidate event is to have come from a particular mass top quark. A separate weight curve is derived for each candidate event. In principle, the whole curve is the mass estimator, although DO actually uses only four numbers extracted from the curve [5]. DO has also combined its dilepton and lepton plus jets mass measurements into a single top quark mass measurement. The results of the dilepton mass analyses can b e found in Table 1.

4. Top Quark Pair Production Cross Section

3. Top Quark Mass in the All Jets Channel CDF has obtained a top quark mass measurement in the all jets channel [6]. The event selection for the all jets channel consists of six or more jets, an SVX b-tag, plus further event shape requirements. The event sample consists of 136 events with an expected background of 108-4-9. This measurement uses the reconstructed top quark mazs based on a 3C fit to the top quark pair hypothesis as its mass estimator, with likelihood derived using the same template method that is used for other top quark mass measurements. The all jets top quark mass result is shown

Both experiments have measured the top quark production cross section using a variety of channels. The main results quoted by both experiments are based on channels that contain at least one charged lepton (e or ~), basically the dilepton and lepton plus jets channels described above [7,8]. Both experiments have also measured the top quark cross section using the all jets channel [6,9]. Other channels are the t r (e or plus a hadronically decaying ~') dilepton channel (CDF) [7] and the so-called e~ channel (DO) [8], which requires one charged lepton, two or more jets, plus very high missing E~, and ev transverse mass.

Event selection sometimes varies slightly between the top quark mass and cross section analyses. For example, both experiments only require three jets in the lepton plus jets cross section analysis, compared to four in the mass analysis. Cross section results are summarized in Table 2. The CDF and DO top quark mass and cross section results are shown in Fig. 1, together with three theoretical predictions [10].

5. Non-Standard Model Top Quark Decays CDF has made a direct search for top decaying to charged Higgs using the decay chain t ~ H+b, H + --. ru, r --* hadrons [11]. The branching ratios of top to charged Higgs, and charged Higgs to r depend on the parameter tan/~, which is the ratio of the vacuum expectation values for the two Higgs doublets. These branching ratios approach one, and maximum sensitivity is acheived, for large tan/~. The result is expressed as an exclusion region in the mH+ vs. tan// plane (see Fig. 2). For tariff = oo, the excluded region is mH+ < 147(158) GeV/c 2 at 95% CL assuming m~ = 175 GeV/c 2 and at~ = 5.0(7.5) pb. CDF has also searched for the flavor changing neutral current (FCNC) top quark decays ~ ---, q7 and ~ --4 qZ [12]. In each case, one top quark is assumed to decay by an FCNC decay mode,

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H. Greenlee/Nuclear Physics B (Proc. Suppl.) 66 (1998) 502-505

Table 2 Top quark cross section results from CDF (rat = 175 Exp. Channel e x B R (%) CDF U 0.744- 0.08 l + jets (SVX b-tag) 3•5 4- 0.7 l + jets (soft lepton b-tag) 1.7 4- 0.3 Combined leptonic CDF All jets (single SVX b-tag) 4.4 4- 0.9 All jets (double SVX b-tag) 3.0 4- 0.9 Combined all jets CDF t'r 0.124- 0.014 DO tA, ev 0.914-0.17 l + jets (no b-tag) 2.27 4- 0.46 t + jets (soft # b-tag) 0.96 4- 0.15 Combined leptonic 4.144- 0.69 DO All jets (single soft # b-tag) 1.8 4- 0.4

GeV/c 2) and DO (mr Data Background 9 2.1 4- 0.4 34 8•0 4- 1•4 40 24.3 4- 3.5 83 33.4 187 142 4- 12 157 120 4- 18 4 9 19 11 39 44

2.0 4- 0.4 2.64-0.6 8.7 4- 1.7 2.4 4- 0.5 13.74- 2.2 25.3 4- 3.1

= 172 GeV/c2). ¢r,t (pb) a ¢+4.44 u•t~_s. 6 • 8 +2.s -1.8 8 •0 +4.4 ~-~ 7.5+~::. 9 • 6 +4.4 -s.6 11• 5 +7.7 -7.0 10 • 1+4"6 -3,6 15.6S~ 6.44-3.4 4.1 4- 2.1 8.3 4- 3.6 5.64- 1.8 7•9 4- 3•5

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Figure 2. Excluded region in the mu+ vs. t a n 3 plane• Figure 1. Top quark cross section vs. mass•

H. Greenlee/Nuclear Physics B (Proc. Suppl.) 66 (1998) 502-505

while the other top quark decays in the standard way L --. Wb. The Standard Model predicts that FCNC decays of the top quark should be very small. A positive result in these searches would be evidence for non-Standard Model physics. The photon FCNC decay search uses both the lepton plus photon and photon plus jets (with SVX b-tag) signatures. One #7 event is observed with an expected background of less than half of an event. The non-background-subtracted upper limit on the photon branching ratio of the top quark is BR(t ---, q~f) < 2.9% at 95% CL. In the g FCNC decay search, the Z was required to decay into a lepton pair (ee or ##), and the W was required to decay into quarks, giving a g plus four jets signature. One ## event was observed with an expected background of 1.2 events, giving a non-background-subtracted upper limit on the g branching ratio of BR(* ---, qg) < 44% at 90% CL. 6. Other R e s u l t s

We conclude with one final result from CDF on the ratio [7] Rtb

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This ratio should be one in the three-generation Standard Model. The analysis uses the oberved number of events with zero, one, and two btags, in the dilepton and lepton plus jets channels. Rt~ is varied to maximize likelihood of the observed number of events, taking into account the expected background, acceptance, and tagging efficiency. The result is R t b = ]. . .9~+ . . o.37 0.st or

I: 7. C o n c l u s i o n s and F u t u r e P r o s p e c t s

Top quark decays have been observed, and found to be consistent with Standard Model expectations, in a large number of channels. The current world average top quark mass, based on combining the CDF and DO lepton plus jets masses is rn~ - 175.6 4-5.5 GeV/c ~.

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The next Tevatron collider run is scheduled to begin around the year 2000 with increased luminosity and energy. The experiments expect to increase their integrated luminosity by about a factor of 20 (from 100 pb -1 to 2 fb-X). Combined with the increased Tevatron energy (from 1.8 to 2.0 TeV) and detector improvements, the number of observed top quark events should increase by about a factor of 40. REFERENCES

1. CDF Collaboration, F. Abe eL al., Phys. Rev. Lett., 74, 2626 (1995). 2. DO Collaboration, S. Abachi et al., Phys. Rev. Lett., 74, 2632 (1995). 3. J. Anto~ for the CDF Collaboration, Fermilab-Pub-97/058-E. 4. DO Collaboration, S. Abachi et al., Phys. Rev. Lett., 79, 1197 (1997). 5. DO Collaboration, B. Abbott eL al., Fermilab-Pub-97/172-E. 6. CDF Collaboration, F. Abe et al., FermilabPub-97/O75-E. 7. M. Gallinaro for the CDF Collaboration, Fermilab-Conf-97/004-E; 8. DO Collaboration, S. Abachi et al., Phys. Rev. Lett., 79, 1203 (1997). 9. N. Amos for the DO Collaboration, presented at 12th Hadron Collider Physics Symposium, Stony Brook, New York, June 5-11, 1997. 10. E. Laenen, J. Smith, and W. van Neerven, Phys. Left. 321B, 254 (1994); E. Berger and H. Contopanagos, Phys. Lett. 361B, 115 (1995) and Phys. Rev. D 54, 3085 (1996); S. Catani, M.L. Mangano, P. Nason, and L. Trentadue, Phys. Lett. 378B, 329 (1996). 11. CDF Collaboration, F. Abe et al., Phys. Rev. Lett. 79, 357 (1997). 12. T.J. LeCompte for the CDF Collaboration, Fermilab-Conf-97/10O-E.