i ' Do^y^W-S- 2 Progress report 2.1 The D0 Experiment The D0 detector had its first data run in the period May 1992 till June 1993. The first two thirds of the run period were characterized by low luminosity and frequent machine development periods. The luminosity gradually climbed, and the machine ultimately delivered 31 pb_1 to the experiment. The detector performed admirably well, and data analysis tracked data taking closely. The overall D0 efficiency was 54% and 16.7 pb_1 of data were collected; due to high backgrounds during Main Ring injection and transition, and beam gas interactions in the MR pipe that intersects our calorimeter, triggers in D0 were vetoed for about 29% of the store time; data acquisition busy (7%), experiment down time (5%), and run startup overhead (5%) added another 17% to the inefficiency. Many cycles of reconstruction software upgrade were done as experience with real data was gained, bugs were corrected, and new data corrections were added. First results were presented at the Spring 1993 APS meeting, the Summer conferences, and at the Fall 1993 pp symposium in Tsukuba. Several papers (See Attached [LeptoQuark PRL, RapGap PRL, Top PRL]) have appeared in print, a lepto-quark search, a rapidity-gap study, and. a lower mass limit on the Standard Model top quark where we also present a spectacular event that has laxge $y, a high energy electron and a high momentum muon, both isolated, and two sizable jets. When interpreted as a it event, a top mass around 145 GeV (with a large 30 GeV error) is obtained. The current run began in December 1993, and machine efficiency started to improve only recently. At this date about 5 pb-1 has been collected. 2.1.1 The D0 detector, Calibrations, Maintenance and Data Taking Our group is active in many areas. We are contributing to the running and maintenance of the detector: Guida leads the calorimeter group, responsible for all aspects of the operation of the calorimeter. Guida is participating in an effort to reduce the MR veto time by making it an active veto. She has responsibility for maintaining the calorimeter online database, and its associated software. We are contributing to online software for the trigger: Claes, Rajagopalan, and H. Li have been contributing to online hit finding, hit block size compression (tracking information represents about 80% of the raw event data), fast tracking, and online vertex finding with the Stony Brook central drift chamber (CDC). Claes is responsible for the maintenance, documentation, and simulation of all Level 2 trigger code. Trigger simulators are of utmost importance in estimating efficiencies and background rejec• tion for a large variety of physics processes. Over the course of Run la, the trigger code evolved through more than two dozen versions, and for the current run we are already at the eighth version of the Level 2 code. All versions, together with parameters, constants, and related simulation code have to be maintained as long as the data will be used. Special trigger runs, coordinated and analyzed by Claes, ensure proper operation. We are involved in the online calibration. Guida has cross-calibrated the online pulser system. Yanagisawa is working on a program to measure calorimeter pedestals from "calibration events" taken in between collider bunches, and compares this pedestal data with pedestal data from special MASTER -••••-".>; i~CN OF TH;3 DOCUMENT 18 UNLfMfTS DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. runs with no bunches in the machine, to check for luminosity dependent effects. Rajagopalan, working on the online calibration, has produced effective tools to allow the reconstruction packages quick access to compressed run calibration data. Thompson, until his graduation, had responsibility for the CDC built at Stony Brook by the NSF and DOE supported groups. Feher, with Stony Brook support, constructed the scintillating fiber calibration detector that is used for tQ and drift velocity calibration of the CDC and covers a 17° section of the azimuth. This detector uses a single Multi-anode PMT for readout and time- multiplexes its signals into sixteen spare channels of the D0 FADC data acquisition system. In addition, Feher was responsible for the determination of the tracking alignment parameters with minimum bias events. H. Li has been involved with the offline reconstruction passes of all the data. This was a major effort, as the wealth of data, and its tremendous size (3000 8mm raw data tapes), make this a bookkeeping nightmare. The latest reconstruction passes were done on UNIX clusters, which entailed porting the code developed under VAX CMS to the IBM and Silicon Graphics Unix environment, and the development of monitoring and bookkeeping tools. 2.1.2 Search for the Top Quark Although the discovery of the top quark was recently announced at Fermilab, the evidence for it in the data is still marginal. CDF has reported a 2.8 a effect in the cross section of several decay channels combined, while D0 sees no statistically significant effect. In Figure 1 the it cross sections measured by D0 are shown as function of mtop- The D0 95% CL contour is indicated as well, together with the recently announced CDF measurement (scaled to the total pp cross section at 1.8 TeV used by D0), and the NNLO QCD calculation by Laenen et ai.. However, D0 has a few spectacular candidates. We are strongly involved in the top analysis. Two graduate students recently defended their theses on searches for the top quark in the '92-'93 data. Both students have accepted postdoctoral positions in High Energy Physics. Cochran was instrumental in calculating the backgrounds in the e/x decay mode of the it pair. His thesis, "Search for Truth in the ep channel at D0" (Dec 1993), was the main ingredient in our top publication in PRL that establishes a lower limit of 131 GeV on the top mass at 95% CL. Thompson graduated in February 1994 with a thesis titled "Search for the Top Quark in the Muon + Jets Channel at D0". This work contributed to raise the top mass limit significantly by 8 GeV compared to the limit derived from the other top decay modes. In the fi + jets channel two events were found with a calculated background of 0.5 ± 0.3 events. Both events are spectacular and a it mass fit yields a miop value in the neighborhood of 170 GeV. Work is continuing on lepton plus jets with a /z-tag (a soft muon close to a jet). One of these events, an event with a /n-tag is shown in Figure 2. The threefold increase in statistics expected from the current run should settle the question of the top's existence in the 160-190 GeV mass range. Reports of recent results from SLC are somewhat in conflict with a LEP plus SM prediction of the top mass, raising the lower top mass limit above 200 GeV. It is clear that only a direct measurement with better statistics at the FNAL collider will be decisive. Feher is investigating multivariate approaches to the search for it production in the all-jets 3 DJ Preliminary Top Limit and Cross Section - 50 Ln 40 - NNLO (Central) 30 r- 110 120 130 140 150 160 170 180 190 200 Top Mass (GeV/cJ) Figure 1: D0 Preliminary limit on the it cross section. The single measurement labeled "CDF" is the recent CDF measurement scaled to use the same total pp cross section as D0. decay channel. Topological variables, like the generalized sphericity tensor, may improve QCD background reduction. An online multi-jet trigger is in development for run lb which aims to improve signal efficiency and reduce background rates. The current multi-jet filter selects events with 5 and more jets. Efficiency and rejection power of this filter are shown in Figure 3.a. The cross section as a function of the ET of the 5th jet for the data sample (dark boxes) and for a simulated it {mtop = 150 GeV) sample (open boxes) is shown. This filter is more than 80% efficient at -ErQets) > 10 GeV, where the signal-to-noise ratio is about 7 X 10-4. A sum of jet transverse energy, ^,\ET\, requirement offers additional discrimination power (a factor two for a threshold at 125 GeV) without complicating the triggering logic. Figure 3.b shows the cross section for events passing the existing multi-jet filter as function of the J2 \ET\ threshold. Figure 3.c shows the signal efficiency vs. background cross section for various multi-jet ET thresholds including a J2 \ET\ threshold requirement. 2.1.3 Precision Determination of the W Mass The physics of W's and Z's is at the heart of the Collider program. The precision measurement of the W mass is an important goal of D0 because it will, together with a good mass determination of the top, limit the mass range available to the Higgs boson and constrain possible extensions of the Standard Model. The identification of W's and Z's in the electron and muon channels is crucial for the top search in leptonic channels. Looking for energetic isolated direct photons produced together with the W or Z tells us about the strength of the trilinear vector boson vertices WW7, ZZ-j and Z77 that are all precisely predicted in the SM, but are allowed "anomalous" values in various CP conserving and CP-non-conserving extensions. These trilinear couplings are directly related to the W magnetic 4 30 Toe View L0-3EP-1392 IZr.l '•• ?.u.-.