We report on D0 searches for scalar quarks (tilde-q) and gluinos (tilde-g),
the superpartners of quark and gluons, in topologies involving jets and missing
transverse energies. Data samples obtained with D0 detector from proton
anti-proton collisions at a center-of-mass energy of 1.96 TeV corresponding to
an intergrated luminosities of 1-4 inverse-fb were analyzed. No evidence for
the production of such particles were observed and lower limits on squarks and
gluino masses were set.Comment: To be published in the proceedings of DPF-2009, Detroit, MI, July
  2009, eConf C09072

U., /Northern Illinois

Uzunyan, Sergey A.

English

arXiv

We report on D0 searches for scalar quarks ({tilde q}) and gluinos ({tilde g}), the superpartners of quark and gluons, in topologies involving jets and missing transverse energies. Data samples obtained with D0 detector from p{bar p} collisions at a center-of-mass energy of 1.96 TeV corresponding to an integrated luminosities of 1-4 fb{sup -1} were analyzed. No evidence for the production of such particles were observed and lower limits on squarks and gluino masses were set

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Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009 1Search for Squarks and Gluinos with the D0 detectorSergey A. Uzunyan∗Department of Physics, Northern Illinois University, DeKalb, IL 60115, USAWe report on D0 searches for scalar quarks (q̃) and gluinos (g̃), the superpartners of quark and gluons, intopologies involving jets and missing transverse energies. Data samples obtained with D0 detector from pp̄collisions at a center-of-mass energy of 1.96 TeV corresponding to an intergrated luminosities of 1–4 fb−1 wereanalyzed. No evidence for the production of such particles were observed and lower limits on squarks and gluinomasses were set.1. IntroductionSupersymmetric (SUSY) extensions of the standardmodel (SM) assign a bosonic superpartner to everySM fermion and vice-versa. The theory that consideronly the SM particles, their corresponding superpart-ners, and two Higgs doublets is known as the min-imal supersymmetric standard model (MSSM) [1]).The MSSM Lagrangian depends on 105 parametersthat makes it difficult to apply for searches involv-ing complex production and decay of the SUSY parti-cles. Analyses described in section 3 were done in thethe minimal supergravity model with a reduced setof the parameters (mSUGRA [2]). In mSUGRA thesparticles masses and couplings are defined by a setof only five parameters: m0 and m1/2, respectivelythe universal scalar mass and the universal gauginomass at ΛGUT ; A0, the universal trilinear couplingat ΛGUT ; tan β the ratio of the vacuum expectationvalue of the two Higgs fields; and µ, the sign of Higgs-mixing mass parameter. Searches for stop and sbot-tom quarks (section 4) do not depend on SUSY pa-rameters other then the masses of the sparticles. Inall presented analysis R-parity is assumed conservedand thus the lightest supersymmetric particle (LSP)is stable and all SUSY particles are produced in pairs.2. The D0 experimentThe D0 experiment was proposed in 1983 to studyhigh mass states and large pT phenomena in proton-antiproton collisions at the Fermilab Collider Com-plex. During 1996-2001 both the Tevatron and theD0 detector [3] were significantly upgraded. In Run II(which started March 2001) the Fermilab collideroperates at an increased center-of-mass energy of1.96 TeV and at higher instantaneous luminosity (inrange from 1.6 × 1032 cm−2s−1 in the beginning ofRun II up to 2.8× 1032 cm−2s−1 after 2006). The D0detector upgrade included new central tracking andnew forward muon systems and an improved central∗On behalf of the D0 collaborationmuon system. In 2006 the D0 tracking system wasupgraded and the trigger system was improved. Thisreport describes the analyses of the Run II D0 datasets collected between April 2002 and December 2008.3. Searches for squarks and gluinosThese searches were done using the mSUGRAmodel ignoring the production of scalar top quarks.Thus below the “squark mass” is the average of allother squarks.3.1. Search for squarks and gluinos inthe jets plus /ET final stateA search was made in 2.1 fb−1 of data for theq̃˜̄q/q̃q̃/q̃g̃/g̃g̃ → jets + /ET . The analysis explored(m0, m1/2) plane of mSUGRA parameters with fixedvalues of A0 = 0, tan β=3 and µ < 0.The minimum number of jets in the final state couldbe two (pp̄ → q̃¯̃q → qχ̃01q̄χ̃01, pp̄ → q̃q̃ → qχ̃01qχ̃01), ifthe squark mass is less then the mass of the gluino;three (pp̄ → q̃g̃ → qχ̃01qq̄χ̃01), at mq̃ ≈ mg̃; or four(pp̄ → g̃g̃ → qq̄χ̃01qq̄χ̃01), if mq̃ > mg̃. For the di-jet analysis events were required to pass an acopla-nar dijet trigger, and a multijet and /ET trigger wasused for selection of events with higher number ofjets. The jets transverse energies were required tobe ET > 35 GeV for the first three leading jets andET > 20 GeV for the fourth jet. In addition, the/ET > 40 GeV, the leading jet acoplanarity < 165◦,the azimutal angles between the /ET and first jet∆φ( /ET , jet1) > 90◦ and between the /ET and sec-ond jet were ∆φ( /ET , jet1) > 50◦ were required in allanalyses. A veto on isolated electrons or muons withpT > 10 GeV was also applied in all analyses to re-ject backrounds events with a lepton from W bosondecay. In the dijet analysis a cut on the minimum az-imutal angle ∆φmin( /ET , jets) > 40o between the /ETand any jet with ET > 15 GeV was applied to sup-press the QCD background. The two final cuts on /ETand HT =∑jets ET were optimized for each of theanalyses by minimizing the expected upper limit onthe cross section in the absence of signal.FERMILAB-CONF-09-4752 Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009Table I shows the number of data and backgroundevents after all selections. The background is dom-inate by W/Z+jets and tt̄ processes. No excess ofdata events were found against the SM predictions inall three analysis and the 95% C.L. limits on squark-gluino production cross-section were set. The QCDcontribution was found to be small and was not ac-counted in the limit calculations.Table I Number of data and background events after allselections for each of the analyses.Analysis (HT , /ET ) GeV Data Background>=2 jets (325,225) 11 11.1±1.2+2.9−2.3>=3 jets (375,175) 9 10.0±0.9+3.1−2.1>=4 jets (400,100) 20 17.7±1.1+5.5−3.3Figure 1(a) show the excluded regions in the squark-gluino mass plane. The observed 95% C.L. limits onsquark and gluino masses of 392 GeV and 327 GeVare the most constraining direct limits to date. Fig-ure 1(b) shows the excluded regions in the (m0, m1/2)mSUGRA parameter plane. The limits from LEP2SUSY searches are improved for m0 in the range 70–300 GeV and for m1/2 values between 125 and 65 GeV.3.2. Search for squarks production in thejets+tau leptons+ /ET final stateA search explored the region of mSUGRA param-eter space (tan β=15, A0 = −2m0, µ < 0, m0 andm1/2 are varied) where squark and gluino cascade de-cays could lead to final state with at least one τ lep-tons, two or more jets and large missing transverseenergy from the undetected neutralino. The complexcascade decays giving such a signature are only pos-sible within a narrow region of (m0,m1/2) parameterspace and have not previously been studied at SUSYsearches at Tevatron. The data sample correspondsto an integrated luminosity of 1 fb−1.The event selection are similar to the inclusivesearch and were optimized for “tau-dijet” or “tau-multijet” signatures depending on whether a dijet ormultijet trigger were required. The analysis tuned tohadronically decaying tau leptons, which appear inthe detector as narrow isolated jets. Jets with trans-verse energy greater then 15 GeV were considered astau-candidates if they were not already identified asone of the two highest pT jets. A neural net trainedon simulated Z → τ τ̄ signal was used to separatetau candidates from quark and gluon jets. The sig-nal selection was optimized (against the expected up-per limit on the cross section) with cuts on /ET andST = pjet1T +pjet2T +EtauT . After all selections 3 eventswere observed in data in good agreement with the SMprediction of 2.3 ± 0.4(stat) ± 0.3(syst). The back-ground is dominated by top quark production andW (→ lν)+jets events. The QCD multijet backroundcontribution was found to be negligible. Because thehadronic tau lepton can be reconstructed as a jet theobtained result was combined with the 2.1 fb−1 di-jet and multijet searches for squark production in the/ET +jets topology.Figure 2(a) shows the expected and observed ex-cluded regions in the (m0, m1/2) plane for the com-bination of all squark analysis. The result extendsthe exclusion region from LEP SUSY searches. Thehighest excluded squark mass at 95% C.L. limits is410 GeV, as illustrated in Figure 2(b).4. Searches for pair production of thetop and bottom squarksThe supersymmetric quarks (squarks) are mixturesof the superpartners q̃L and q̃R of the SM quark he-licity states qL and qR. The theory permits a massdifference between squark mass eigenstates q̃1 and q̃2that gives the possibility that the lightest states ofthe stop and sbottom quarks are lighter than the firsttwo generation squarks and thus have the largest pro-duction cross section. The presented analysis also as-sumes that the sneutrino (stop search) or the neu-tralino (sbottom analysis) is the LSP.4.1. Search stop pairs in 2 b-jets+ eµ+ /ETeventsA search was performed for three body decays oftop squark pairs when each stop decays leptonically,t̃ → b + ν̃ + l± with a 100% branching fraction. Thusthe stop pair production final state corresponds to thee± + µ∓ + 2b + /ET topology. The analysis studied3.1 fb−1 of data.The events were required to have an isolated elec-tron and a muon of the opposite charge with pTgreater then 15 GeV and 8 GeV respectively, accom-panied by jets with transverse energy ET > 15 GeVand /ET . No explicit trigger requirements were ap-plied, as the trigger efficiency is high for this combina-tion of lepton pT . The largest backrounds are Z → τ τ̄ ,QCD, WW, and top quark pairs. Cuts on angular dif-ferences between leptons and /ET and /ET > 18 GeVwere applied to remove Z and QCD events.The signal final state kinematic is determined by thesize of the mass difference between squark and sneu-trino ∆(mt̃1 , mν̃). Events with a large ∆(mt̃1 , mν̃)have more missing energy, higher jets energies andhigh charge leptons pT , while signals with ∆(mt̃1 , mν̃)have softer kinematics. The “hard” and “soft” signalefficiencies after the /ET cut are 15% and 3% respec-tively. To prevent futher losses of the signal accep-Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009 3Gluino Mass (GeV)0 100 200 300 400 500 600Squark Mass (GeV)0100200300400500600 -1DØ, L=2.1 fb<0µ=0, 0=3, AβtanUA1UA2LEPCDF IBDØ IADØ IBDØ IIno mSUGRAsolution±χ∼LEP2  ±l~LEP2   (GeV)0m0 100 200 300 400 500 600 (GeV)1/2m050100150200250 (GeV)1/2m-1DØ, L=2.1 fb <0µ=0, 0=3, Aβtanno EWSB±χ∼LEP2  ±l~LEP2  DØ II(a) (b)Figure 1: The 95% C.L. excluded regions in the gluino and squark mass plane in the mSUGRA framework with tan β=3,A0=0, µ <0 in the (a) gluino and squark mass plane and (b) in the (m0, m1/2) plane. The thick (dotted) line is thelimit of the observed (expected) excluded region for signal cross section calculated with renormalization and factorizationscale µ= Q, with Q equal mq̃, mg̃, (mq̃ + mg̃)/2 for g̃g̃, q̃˜̄q/q̃q̃ and g̃g̃ production, respectively. The yellow band and theband delimited the two dotted lines correspond to the parton distribution functions and µ uncertainties. Red shadingrepresent the D0 Run II results. Light shaded regions are excluded by previous experiments.tances, a technique that isolates the remaining back-ground was applied: signal and backround events weredistributed into a two dimensional histogram in theHT (the scalar sum of the pT of the jets) and theST (the scalar sum of /ET , the electron pT and themuon pT ) plane. The bin edges were selected suchthat the background events are concentrated into afew bins while the remaining bins have a significantsignal to background ratio. This method allows use ofthe same set of selections for the signals with different∆(mt̃1 , mν̃).Table II shows the number of data, backgroundsand signal events after all selections. No significant ex-cess above the standard model predictions were found.Figure 3 shows the 95% C.L. exclusion limits in the(mt̃1 , mν̃) mass plane. Stop masses up to 200 GeVwere excluded for signals with ∆(mt̃1 , mν̃) > 30 GeVand mν̃ < 110 GeV.Table II Number of data, background and signal eventsafter all selections for different stop signals.(mt̃1 ,mν̃) GeV Data Background Signal(acpt,%)(110,80) 288 303+16−20 89+11−13 (3)(150,50) 288 303+16−20 122+14−16 (15)4.2. Search sbottom pairs in 2 b-jets+ /ETeventsA search considered the region of SUSY parameterspace where the only possible decay of the sbottomquark’s lightest state is b̃1 → bχ̃01, mb̃1 > mb + mχ̃01and mb̃1 < mt + mχ̃±1, where the neutralino χ̃01andchargino χ̃±1are the lightest SUSY partners of theelectroweak bosons. These requirements result inbχ̃01b̄χ̃01final state. The corresponding detector signa-ture are two acoplanar b-jets from the scalar bottomquarks and the missing energy due to escaping neu-tralinos. The data sample was collected using jet plusmissing energy triggers and corresponds to an inte-grated luminosity of 4 fb−1.Events with two or three jets with EjetT > 20 GeVand the /ET > 40 GeV were selected. For the two lead-ing jets acoplanarity < 165◦ was required. To reducethe contribution from W → lν decays, events with iso-lated electrons or isolated muons with pT > 15 GeVwere vetoed. To suppress the instrumental back-ground events where the /ET direction overlapped ajet in φ were removed. Also events where the di-rection of /ET is not aligned with the missing track/pT , calculated as the negative of the vectorial sum ofcharged particles transverse momenta were removed.A neural net b-tagging tool was used to identify theheavy-flavor jets while significantly reducing the SMand QCD backgrounds which are dominated by light4 Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009 (GeV)0m60 80 100 120 140 160 (GeV)1/2m100120140160180200DØ -10.96-2.1 fbTau combin.<0µ, 0=-2m0)=15, Aβtan(LEP2 sleptonprel. limitLEP2 chargino prel. limittau corridorObserved limitand error bandExpected limitSquark Mass (GeV)250 300 350 400 450Cross Section (pb)0.10.20.30.40=-2m0=80 GeV, A0m<0µ)=15, β          tan(DØ -10.96-2.1 fbNLOσtau corridorExpected limit jet-combinationExpected limit tau-combinationObserved limit tau-combination(a) (b)Figure 2: a) The 95% C.L. excluded regions in the (m0, m1/2) plane in the mSUGRA framework with tan β=15,A0 = −2m0, µ < 0. The green region represents the (m0, m1/2) space in which the final states with tau’s are possiblewith the shaded part excluded by the combination of all squark analysis. b) The 95% C.L. upper limits on squark andgluino pair production cross section for m0 = 80 GeV.flavor jets. As the final state consist of two high ET b-jets, the Xjj ≡ (Ejet1T +Ejet2T )/(ΣjetsET ) variable wasused as a discriminant against top quark processes,requiring Xjj > 0.9. Finally the cuts on Ejet1T , /ETand HT were optimized for the different (mb̃1 , mχ̃01)signals. These selections are tighter for the sbottomsignals with large /ET . For the regions with a lowmb̃1−mχ̃01 , the /ET and jet energies in the signal eventsare lower and relaxed thresholds were used.After all selections the number of data eventsagreed with the expectations from the simulatedSM processes (dominated by the tt̄ production andW/Z+heavy flavor jets events) and the QCD estima-tion from data. The later is significant for the selec-tions with soft requirement on /ET . Table III showsthe number of data, backround and signal events afterall selections applied in searches for the signals withlow and high /ET .Table III Number of data,background and signal eventsafter all selections for low and high /ET sbottom signals.(mb̃1 ,mχ̃01) GeV Data Background Signal(acpt,%)(100,60) 483 493±12 610±29 (1.0)(240,0) 7 7.1±0.4 11.4±0.2 (3.6)Stop mass (GeV)60 80 100 120 140 160 180 200Sneutrino mass (GeV)406080100120140 excludedILEP IILEP excluded) ν∼) = M(b) + M(1t~M( Preliminary Result∅D)-1+ee Obs (1.1 fbµe )-1+ee Exp (1.1 fbµe )-1 Obs (3.1 fbµe )-1 Exp (3.1 fbµeFigure 3: Excluded 95% C.L regions for the stop pair pro-duction in the dilepton plus /ET events. Points below theblue lines are excluded by the presented 3.1 fb−1 analysis.Also shown the D0 1.1 fb−1 exclusion countour from theeµ and ee channels (the red line) and LEP I and LEP II ex-cluded regions (shaded green and orange regions). Dashedlines represent the expected exclusion contours.Proceedings of the DPF-2009 Conference, Detroit, MI, July 27-31, 2009 5Figure 4 shows the 95% C.L. and excluded regionin the sbottom mass versus neutralino mass plane.Production of sbottom quarks with mb̃1 < 253 GeVis excluded for mχ̃01= 0. The limits from previousTevatron searches are improved and the exclusion re-gion extends up mχ̃01= 95 GeV for mb̃1 in the range150–200 GeV.Sbottom Mass (GeV)0 50 100 150 200 250Neutralino Mass (GeV)020406080100120Neutralino Mass (GeV)Neutralino Mass (GeV)ObservedExpected)-1D0 Run II Preliminary (4 fbD0Run I-192 pbCDFRun I-188 pbCDFRun IIa-1295 pbD0Run IIa-1310 pbLEP=208 GeVs)10χ) = M(b) + M(b~M(Neutralino Mass (GeV)Figure 4: The 95% C.L. exclusion contour in (mb̃1 , mχ̃01)mass plane. Also presented results from previous searchesat LEP and at the Tevatron.5. SummarySearches for squarks and gluinos are performed in1–4 fb−1 D0 data samples for different regions ofMSSM/mSUGRA parameter space. All presentedanalysis are in good agreement with the SM predic-tions. As no signs of SUSY were observed a set of95% C.L. limits on squark and gluino masses and onmSUGRA parameters have been obtained improvingprevious Tevatron results. More details on the pre-sented analyses can be found at [4].References[1] P. Fayet, Phys. Rev. Lett. 69, 489 (1977).[2] H. P. Nilles, Phys. Rep. 110, 1 (1984).[3] D0 Collaboration, V. Abazov et al., Nucl. Instrum.Methods A 565, 463 (2006).[4] http://www-d0.fnal.gov/Run2Physics/WWW/results/np.htm

Search for Squarks and Gluinos with the D0 detector

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