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Search for New Particles Decaying to Dijets, B Bar B, and T Bar T At F Fermi National Accelerator Laboratory FERMILAB-Conf-95/152-E CDF Search for New Particles Decaying to Dijets, bbÅ and ttÅ at CDF Robert M. Harris For the CDF Collaboration Fermi National Accelerator Laboratory P.O. Box 500, Batavia, Illinois 60510 June 1995 Proceedings of the 10th Topical Workshop of Proton-Antiproton Collider Physics, Fermi National Accelerator Laboratory, Batavia, Illinois, May 9-13, 1995 Operated by Universities Research Association Inc. under Contract No. DE-AC02-76CHO3000 with the United States Department of Energy Disclaimer This report was preparedasanaccount 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, expressed 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 speci c commercial product, process, or servicebytrade 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 re ect those of the United States Government or any agency thereof. 1 Fermilab-CONF-95/152-E CDF/PUB/EXOTIC/PUBLIC/3192 June 15, 1995 Search for New Particles Decaying to Dijets, bb, and tt at CDF CDF Collab oration Presented by Rob ert M. Harris Fermilab MS 318 Batavia, IL 60510 1 We present three searches for new particles at CDF. First, using 70 pb of data we search the dijet mass sp ectrum for resonances. There is an upward uctuation 2 near 550 GeV/c (2.6 ) with an angular distribution that is adequately describ ed by either QCD alone or QCD plus 5% signal. There is insucient evidence to claim a signal, but we set the most stringent mass limits on the hadronic decays 0 0 of axigluons, excited quarks, technirhos, W ,Z, and E6 diquarks. Second, using 1 19 pb of data we search the b-tagged dijet mass sp ectrum for bb resonances. 2 Again, an upward uctuation near 600 GeV/c (2 ) is not signi cant enough to claim a signal, so we set the rst mass limits on top color b osons. Finally, using 1 67 pb of data we search the top quark sample for tt resonances like a top color 0 Z . Other than an insigni cant shoulder of 6 events on a background of 2.4 in 2 the mass region 475-550 GeV/c , there is no evidence for new particle pro duction. 0 Mass limits, currently in progress, should b e sensitive to a top color Z near 600 2 GeV/c . In all three searches there is insucient evidence to claim new particle pro duction, yet there is an exciting p ossibili ty that the upward uctuations are the rst signs of new physics b eyond the standard mo del. I. SEARCH FOR NEW PARTICLES DECAYING TO DIJETS As in our previous analysis of Run 1A data (1), we conduct a general search for new particles with a narrow natural width that decay to dijets. In addition, we search for the following particles summarized in Fig. 1: axigluons (2) from chiral QCD (A ! q q), excited states (3) of comp osite quarks (q ! qg), color o ctet technirhos (4) ( ! g ! q q; g g ), new T 0 0 c ,Z ! q q), and scalar E diquarks (5) (D ! u d and D ! ud). gauge b osons (W 6 Using four triggers from run 1A and 1B, we combine dijet mass sp ectra ab ove a mass of 2 2 2 2 150 GeV/c , 241 GeV/c , 292 GeV/c , and 388 GeV/c with integrated luminosities of New Particles that Decay to Dijets 2 Model Particle Production/Decay JP(color) Reference & Γ σ. & /2 B(500 GeV) q q + A 1 (8) Bagger, Schmidt Chiral Color Axigluon & King.1988 SU(3)L x SU(3)R A q q .05M 210 pb + Composite Excited q q 1/2 (3) Baur, Hinchliffe Fermions Quark q* & Zeppenfeld q* 1987 g g .02M 40 pb q q ρ T - gg 1 (8) Eichten & Lane Technicolor Technirho q q 1994 Lane & Ramana ρ gg 1991 T ρ T gg .01M 13 pb g g Extended New q q 1 (1) Standard Gauge Gauge W', Z' Model Models. Bosons Couplings SU(2) x SU(2) W', Z' L R q q .01M 5 pb, 4 pb E6, etc u( u) u( u) + - c 0 (3) Hewett & Rizzo Superstring D (D ) Inspired Diquarks 1989 c E Models D, D d( d) d( d) For each new particle that decays to dijets we list the mo del name, the particle name, 6 FIG. 1. .004M 4 pb the Feynman diagram, and a 22 text grid containing the quantum numb ers, a reference, 2 the half-width and the cross section at a mass of 500 GeV/c . The pro duction and decay couplings for the rst three particles are strong, for new gauge b osons the coupling is weak, and for E 6 diquarks the coupling is electromagnetic. 3 4 ] 10 1 ) 2 0.8 10 3 pb/(GeV/c (Data-Fit)/Fit [ 0.6 10 2 /dM σ 0.4 d 10 0.2 1 0 -1 10 -0.2 -2 -0.4 10 -0.6 -3 10 -0.8 -4 10 -1 0 200 400 600 800 1000 0 200 400 600 800 1000 2 TWO JET MASS (GeV/c ) Two Jet Mass (GeV/c2) FIG. 2. The dijet mass data (solid p oints) is compared to a parameterization t to the data (curve). The logarithmic plot also shows a QCD simulation (op en b oxes). 1 1 1 1 .089 pb , 1.92 pb , 9.52 pb , and 69.8 pb resp ectively. Jets are de ned with a xed cone clustering algorithm (R=0.7) and then corrected for detector resp onse, energy lost outside the cone, and underlying event. We take the two highest P jets and require that they have T pseudorapidity j j < 2 and a CMS scattering angle j cos j = j tanh [( )=2]j < 2=3. 1 2 The cos cut provides uniform acceptance as a function of mass and reduces the QCD background which p eaks at j cos j = 1. In Fig. 2 the dijet mass distribution is presented as a di erential cross section in bins of the mass resolution ( 10%). At high mass the data is systematically higher than a prediction from PYTHIA plus a CDF detector simulation, similar to the inclusive jet E sp ectrum (6). To search for new particles we determine the T QCD background by tting the data to a smo oth function of three parameters (7); Fig. 2 2 shows the fractional di erence b etween the data and the t ( =D F =1:43). We note 2 2 2 upward uctuations near 200 GeV/c (2:4 ), 550 GeV/c (2:6 ) and 850 GeV/c (1 ). For narrow resonances it is sucient to determine the mass resolution for only one typ e of new particle b ecause the detector resolution dominates the width. In Fig. 3 we show the mass resolution for excited quarks (q*) from PYTHIA plus a CDF detector simulation; the long tail at low mass comes from gluon radiation. For eachvalue of new particle mass in 50 2 GeV/c steps, we p erform a binned maximum likeliho o d t of the data to the background parameterization and the mass resonance shap e. In Fig. 3 we display the b est t and 95% 2 con dence level upp er limit for a 550 GeV/c resonance. For the mass region 517 <M <625 2 GeV/c , there are 2947 events in the data, 2810 53 events (2:6 ) in the background for the t without a resonance, 2765 53 events (3:4 ) in the background for the t that includes the resonance, and the value of the resonance cross section from the t is 5:8 2:9 pb (statistical). In Fig. 4 we study the angular distribution of the uctuation in the mass region 517 < 2 . The angular distribution is compatible with b oth QCD alone, and with M<625 GeV/c 4 0.5 0.4 (Data-Fit)/Fit 0.3 0.2 0.1 0 -0.1 -0.2 200 300 400 500 600 700 800 900 1000 1100 Two Jet Mass (GeV/c2) FIG. 3. left: CDF dijet mass resolution for narrow resonances like excited quarks, including the e ects of radiation and detector resolution. right: The dijet mass data (solid p oints) t 2 with a background (solid line) and a 550 GeV/c resonance (dashed hist). QCD + 5% excited quark (b est t). This amount of excited quark is coincidentally the same as found in the mass t. Although the uctuation is interesting, we conclude it is not yet statistically signi cant, and pro ceed to set limits on new particle pro duction. 800 800 700 700 600 600 500 500 Number of Events 400 Number of Events 400 300 300 200 200 100 100 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0 0.1 0.2 0.3 0.4 0.5 0.6 * Cos θ Cos θ* 0.2 0.2 0.15 0.15 0.1 0.1 0.05 0.05 0 0 -0.05 (Data-Theory)/Theroy -0.05 (Data-Theory)/Theroy -0.1 -0.1 -0.15 -0.15 -0.2 -0.2 0 0.1 0.2 0.3 0.4 0.5 0.6 0 0.1 0.2 0.3 0.4 0.5 0.6 * Cos θ Cos θ* 2 FIG.
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