The integrated luminosity at Run 2 of the Tevatron is approaching the Run 1
total, and data analysis is progressing. New results in searches for new
physics by the DO experiment are presented in a variety of channels,
demonstrating good performance of the detector and detailed understanding of
the data.Comment: 6 pages, proceedings of the 38th Rencontres de Morion

Brooijmans, G.

English

arXiv

The integrated luminosity at Run 2 of the Tevatron is approaching the Run 1 total, and data analysis is progressing. New results in searches for new physics by the D0 experiment are presented in a variety of channels, demonstrating good performance of the detector and detailed understanding of the data

Brooijmans, Gustaaf

UNT Digital Library

Fermilab FERMILAB-Conf-03/073-E  June 2003Searches for New Physics at DG. Brooijmansfor the D CollaborationFermi National Accelerator Laboratory, P.O. Box 500,Batavia, IL 60510, USA.The integrated luminosity at Run 2 of the Tevatron is approaching the Run 1 total, anddata analysis is progressing. New results in searches for new physics by the D experimentare presented in a variety of channels, demonstrating good performance of the detector anddetailed understanding of the data.1 IntroductionThe D experiment has been taking data at Run 2 of the Tevatron since March 2001. In theearly part of the run, the data was used for detector commissioning and calibration, and onlydata taken since April 2002, when the ber tracker instrumentation was completed, is used forphysics analysis. Between April 2002 and January 2003, the Tevatron delivered 135 pb 1 ofintegrated luminosity, of which 83 pb 1 were recorded by D. It should be noted that D'sdatataking eÆciency increased gradually during this period, and reached 90% at the end of 2002.The results presented here use up to 50 pb 1 from that data sample.2 Searches for SupersymmetrySignals for supersymmetry are searched for in a variety of signatures predicted by various modelsof supersymmetry breaking. A few of these are presented here. For a given superpartner mass,production cross-sections at the Tevatron are highest for colored particles and lowest for particlesthat only interact weakly. However, while the cross-sections are therefore highest for squarksand gluinos, these are expected to be signicantly heavier than the electroweak gauginos, suchthat for a given point in parameter space the production rates are similar 1.Figure 1: Distribution of jets + missing ET events as a function of missing ET . The data (points) is tted (line)at moderate missing ET to estimate the QCD contribution to the high missing ET region.Table 1: Expected and observed events for various cuts on the minimal event missing ET . The last line givesa limit on the new physics production cross-section of events with at least two hard jets (of which one haspT > 100 GeV ) and large missing ET .Cut: Missing ET > 70 GeV 80 GeV 90 GeV 100 GeVExpected Events 18.4  8.4 9.5  5.3 5.1  3.2 2.7  1.8Observed Events 7 6 4 395% C.L. Cross-Section Limit (pb) 4.2 3.8 3.1 2.72.1 Jets and Missing ETThe generic signature for production of squarks and/or gluinos in SUGRA-type models consistsof jets and missing transverse energy (missing ET ). These come from the cascade decays ofthe squarks and gluinos into quarks, gluons and the Lightest Supersymmetric Particle (LSP).In these models the latter is heavy (O(100 GeV )), neutral, stable and weakly interacting, suchthat it escapes the detector undetected, leaving as its only signature an imbalance in transversemomentum.This type of signature unfortunately suers from very signicant instrumental backgrounds,due to the tails of the dominant standard model production of jets (denoted QCD in the fol-lowing). A pilot analysis has been performed using 4 pb 1 of data in order to demonstrate thatthese backgrounds could be understood and reduced suÆciently. The analysis starts by applyinga series of \cleaning" cuts designed to reduce the instrumental background, followed by topo-logical cuts to increase the signal over background ratio. After that, the physics backgroundsare evaluated using simulation, and the QCD background is derived from the data at moderatemissing ET . This is illustrated in Figure 1. As can be seen from Table 1, the number of dataevents at high missing ET is in good agreement with the expectations and does not exhibit anyanomalous tails.2.2 Di- and TrileptonsThe cascade decays of the electroweak gauginos, which are predicted to be relatively light, oftenlead to signatures with multiple leptons. At hadron colliders, this is the SUGRA channel withthe lowest background, and is often called the \golden channel". It should be noted howeverthat good sensitivity to this signal requires both low lepton pT thresholds (because of the smallFigure 2: Distribution of expected (histogram) and observed (points) e events as a function of missing ET .Figure 3: Distribution of expected (histogram) and observed (points) ee events as a function of dielectron invariantmass.mass dierence between the gauginos and the LSP), and the ability to identify hadronic taudecays. This is due to the fact that at large tan the leptonic branching fractions for charginosand neutralinos are dominated by taus.Among the multilepton signatures, the channel with an electron and a muon has such lowbackgrounds that the analysis is pursued in a model-independent way. Events are required tohave at least one electron and one muon, both with pT > 15 GeV . The fake rates are estimatedfrom data, while the physics backgrounds are evaluated from simulation. The dominant physicsbackgrounds are Z !  at low missing ET , and WW and tt production at high missing ET .In approximately 30 pb 1 of data (the uncertainty on the luminosity is currently conservativelyestimated at 10%), the data agree well with the backgrounds as can be seen from Figure 2.Based on this, a model-independent upper limit on acceptance  cross-section is set for newphysics leading to events with an electron and a muon as a function of missing ET . This limitranges from about 400 to 100 fb for missing ET going from 0 to 35 GeV .The study of channels with two electrons and a third lepton starts with the study of thedielectron invariant mass spectrum. This is shown in Figure 3 and is used to check that trigger,reconstruction, simulation and the QCD fake background are well understood. While not perfectTable 2: Expected and observed events for successive cuts on dielectron events.Backgrounds DatapT (e1) > 15 GeV; pT (e2) > 10 GeV 3216  43.2 313210 GeV < Mee < 70 GeV 660.2  19.1 721MT > 15 GeV 96.4  8.1 123Additional Isolated Track, pT > 5 GeV 3.2  2.3 3Missing ET > 15 GeV 0.0  2.0 0Figure 4: Distribution of candidate Z ! X ! ehX events (points) as a function of reconstructed ditauinvariant mass. Also shown is the expectation from simulation (histogram).yet, the agreement is good. This analysis uses approximately 40 pb 1 of data, and the successivecuts, along with the expected and observed numbers of events are given in Table 2. The typicalselection eÆciency for SUGRA events at points in parameter space close to the current exclusionlimit is 2-4%, but the sensitivity is still about a factor of 7 away from extending the excluded area.In addition to increased integrated luminosity, eorts are underway to improve the sensitivityby improving the eÆciencies and adding other channels.One of these approaches involves identifying Z !  events where one of the taus decayshadronically. For the pp ! ZX ! X ! ehX channel, events with an electron with pT >12 GeV and a tau-like jet are selected from about 50 pb 1 of data. The tau-like requirementsare a cut on the jet width, and only one track with pT > 1:5 GeV compatible with the tau mass(i.e. starting from the leading track, the two-track invariant mass is calculated for all trackswith pT > 1:5 GeV and required to be below the tau mass). For each of the tau decay channels(with or without a neutral pion), a neural net using both calorimetric and tracking variablesfor inputs is used to discriminate between QCD and tau jets. After cutting on the neural netoutputs, the ditau invariant mass is reconstructed under the hypothesis that the tau directionis the same as the visible tau daughter direction. Finally, same-sign e events are subtractedfrom opposite sign events, leading to the histogram shown in Figure 4, indicating a clear signalwhich can be used to further improve the identication of hadronically decaying taus.2.3 Diphotons and Missing ETAs opposed to the SUGRA scenario, in Gauge Mediated Supersymmetry Breaking (GMSB) theLSP is a very light ( eV ) gravitino, and the model phenomenology is driven by the nature ofthe NLSP (Next-to-Lightest Supersymmetric Particle). In the case of a \bino-like" NLSP, eventswill have 2 energetic photons and large missing ET . This search uses about 50 pb 1 of data,requires two photons with pT > 20 GeV , and applies quality and topological cuts to reducethe instrumental backgrounds and increase the signal over background ratio. The remainingFigure 5: Distribution of diphoton events (points) as a function of missing ET . The histogram is the expectedbackground from QCD fakes.Figure 6: Feynman diagrams showing contributions of virtual graviton exchange to Drell-Yan processes.background is dominated by QCD fakes and is determined from data. The resulting missing ETdistribution is shown in Figure 5 and shows good agreement between data and backgrounds.A limit is set on GMSB using the Snowmass slope 2 with  as the only free parameter andM = 2; N5 = 1; tan = 15 and  > 0. The limit corresponds to a lower limit on the neutralinomass of 66 GeV at 95% C.L., very close to the Run 1 result.3 ExoticsOne of the more exotic signals for new physics D searches for is leptoquarks. The analysispresented here uses about 30 pb 1 of data to search for second generation leptoquarks decayingto a muon and a c or s quark. Events are required to have two opposite sign muons withpT > 15 GeV , two jets with pT > 20 GeV and M > 110 GeV . The dominant backgroundcomes from =Z !  + jets, and since no events are found a lower limit on the leptoquarkmass is set at 157 GeV , 43 GeV worse than the Run 1 limit.4 Large Extra DimensionsD searches for Large Extra Dimensions (LED) in the ADD 3 model assuming standard modelparticles are conned to a 3-brane, but gravity propagates in the extra dimensions. In theseanalyses, the signature is an excess of high-mass dielectron, diphoton or dimuon events due to thecoupling to Kaluza-Klein gravitons. Figure 6 shows Feynman diagrams of the standard modeland graviton contributions to the dilepton processes, which can be parametrized as follows:d2dMd cos = fSM + finterfG + fKK2G: (1)Here cos  is the scattering angle in the di-em or dimuon rest frame, and G measures thecontribution from gravitons. The dielectron and diphoton channels are treated simultaneously(I.e. no track requirement is imposed) so that events are required to have 2 electromagneticobjects with pT > 25 GeV , and missing ET < 25 GeV to ensure the events are well measured.Figure 7: Standard model expectation, data, extra dimension signal and QCD fake background in the invariantmass - cos plane for the di-em channel.Table 3: Lower limits in TeV on MS , the fundamental Planck scale, for various formalisms (see text).Formalism GRW HLZ, n=2 HLZ, n=7 Hewett,  = +1Di-Em ( 50pb 1) 1.12 1.16 0.89 1Dimuon ( 30pb 1) 0.79 0.68 0.63 0.71In the dimuon channel, two muons with pT > 15 GeV and M > 40 GeV are required. In bothcases, physics backgrounds are derived from simulation and instrumental backgrounds from data.The data distribution in the invariant mass - cos  plane is then tted to signal + background(see Figure 7) to evaluate an upper limit on G. This limit is translated into a lower limit onMS , the fundamental Planck scale, with results shown in Table 3 for various formalisms4. Thedielectron and diphoton channel yields a limit which is very close to the Run 1 result, while thedimuon channel is new at hadron colliders. Both results are competitive with LEP.5 ConclusionsD continues to search for many dierent signatures for new physics. The eects of increasedcenter-of-mass energy and an improved detector can now be seen in improved sensitivity com-pared to Run 1, and eorts continue on calibration and reconstruction to further extend theexperiment's reach. We are now entering uncharted territory, so signal could be just around thecorner.1. Report of the SUGRA Working Group for Run II of the Tevatron, hep-ph/0003154.2. P3 Summary Report, Proceedings of Snowmass 2001, Ed. N. Graf.3. N. Arkani-Hamed, S. Dimopoulos and G. Dvali, Phys. Lett. B429, 263 (1998).4. G. Giudice et al., Nucl. Phys. B544, 3 (1999); T. Han et al., Phys. Rev. D 59, 105006(1999); J. Hewett, Phys. Rev. Lett. 82, 4765 (1999).

Searches for New Physics at D0

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