This paper presents results from studies on observability of Z' particles and Randall-Sundrum gravitons in the dimuon channel in the CMS detector. Full detector simulation and reconstruction were carried out to derive both the discovery potential and results for model discrimination. Effects of systematic uncertainties on the expected reach of the CMS experiment are also presented

Traczyk, Piotr

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Available on CMS information server CMS CR 2006/056CMS Conference Report8 September 2006Search for Z-primes and Randall-SundrumGravitons with dimuons in CMSP. TraczykSoltan Institute for Nuclear Studies, Warsaw, Polandfor the CMS collaborationPresented at Physics at LHC July 2006 Krak·ow, PolandAbstractThis paper presents results from studies on observability of particles and Randall-Sundrum gravi-tons in the dimuon channel in the CMS detector. Full detector simulation and reconstruction werecarried out to derive both the discovery potential and results for model discrimination. Effects ofsystematic uncertainties on the expected reach of the CMS experiment are also presented.1 IntroductionA new heavy neutral boson decaying into lepton pairs can provide a striking signal of new physics at the LHC.Two types of particles giving such signature are currently considered - a spin-2 graviton excitation and a spin-1extra gauge boson (  ). is a generic name for new neutral gauge bosons appearing in many models, including models of dynamicalsymmetry breaking [1], little Higgs [2] models, also superstring-inspired [3] and grand unied theories [4]. Excitedgravitons appear in models with extra dimensions, in particular a narrow  -like resonance is predicted by theRandall-Sundrum (RS) [5] model.The couplings of the  are xed by the choice of the theoretical framework, and the only free parameter is itsmass. Experimental lower limits on this mass are of the order of 600-900 GeV [6], while theory currently does notprovide any reliable predictions.The phenomenology of the Randall-Sundrum model is governed by two parameters. The rst is the mass ofthe rst graviton excitation, and the second is 	 determining the couplings and width of the graviton.Experimental and theoretical bounds on these parameters form a closed allowed region in parameter space, withmasses up to 4 TeV, and   between 0.01 and 0.1 [7].2 Signal and background processesSignal and background samples were generated with PYTHIA [8] 6.227 (with the CTEQ6L set of parton distribu-tion functions [9]) and processed through full CMS detector simulation and reconstruction software.RS gravitons were simulated with mass values of 1, 1.5, 2, 3 and 4 TeV, and values of   	 ﬀﬁﬂ ﬃ ﬁﬂ !ﬂ"ﬂ#$ . particles vere simulated with masses of 1,3 and 5 TeV, with six different sets of couplings corresponding to sixdifferent models predicting an extra neutral gauge boson: %$%& , the Sequential Standard Model (SSM)  , withcouplings the same as the Standard Model ' ; ( , ) and * , arising in E + and SO(10) GUT groups; ,.- & and0/,.-& , appearing in the so-called left-right and alternative left-right models. Further details on the varioustypes of  and their couplings are given in Ref. [10].The Standard Model (SM) background for the processes under study consistis mainly of Drell-Yan dimuon produc-tion. Other possible sources of high energy muon pairs ( 1$21 , 3 23 and Z and W boson pair production) have been foundto have a contribution an order of magnitude lower than Drell-Yan, and can be further suppressed by requiring thatthe reconstructed muons be isolated. Hence, only Drell-Yan was considered in this work.3 Search for a resonanceSignal observability was estimated by performing maximum-likelihood ts to reconstructed dimuon invariant massspectra and calculating the value of the signicance estimator 4657	98 ﬃ;:=<6>@?BADCEFG?BEﬁH , where ?IE and ?IAJCE are thebest-t likelihoods of the pdf’s corresponding to the null hypothesis K'(no signal present) and the alternativehypothesis KML (signal plus background), respectively. The relative normalization of the signal and backgroundprocesses is a free parameter of the t - the analysis exploits the difference in shape between the signal (a quasi-gaussian peak) and the background (an exponentially falling continuum). A more in-depth look at this and othermethods of estimating discovery signicance is given in Refs [12] and [10].3.1 Treatment of systematic uncertaintiesThere are two types of sources of systematic uncertainty in this analysis. The rst are theoretical uncertainties onthe signal and background cross-sections, due mainly to parton distribution function (pdf), hard process scale un-certainties and higher order QCD and electroweak corrections. Since the analysis does not rely on any assumptionsabout signal and background normalization, these uncertainties affect only the reach estimate, and will not haveany direct impact on signicance calculation based on real data. These uncertainties were combined in quadrature,and translated into ON uncertainty in the predicted discovery reach.The second type of systematic uncertainties are uncertainties resulting from imperfect knowledge of the experimen-tal condidions. These include pileup, tracker and muon system misalignment, magnetic eld and muon detectorcalibration uncertainty. In this study, misalignment and pileup were taken into account during detector simulation,while other effects were found to be negligible.2The results for CMS 5 P discovery reach are shown in Figs 1 and 2.)2Z’ mass (TeV/c1 2 3 4 5 6)   -1Int. luminosity (fb-210-110110210310 ψZ ηZχZLRMZSSMZALRMZ)2Z’ mass (TeV/c1 2 3 4 5 6)   -1Int. luminosity (fb-210-110110210310 ψZALRMZFigure 1: Integrated luminosity needed to reach 5 P signicance as a function of Q6R mass. The left plot showsresults with statistical uncertainties only, the right plot shows results with systematics, for two of the consideredmodels. Solid lines show the best estimates, dashed lines correspond to SUT$P theoretical uncertainty.4 Model discriminationAfter a new resonance has been discovered, its identity has to be established by measuring its various properties.A graviton can be distinguished from a QR because of it’s spin-2 nature. Different QR particles originating fromdifferent models can be distinguished by measuring the forward-backward asymmetry ( VXWZY ) of the nal statemuons.4.1 Spin discriminationThe spin of the observed resonance manifests itself in the angular distributions of its decay products:subprocess angular distribution[\[^]`_a$bdceabR]gf\f hi0jTBklmZn oZpq$r[\[^]tsp]gf\f uijTwvyxzlmZn{oZpqBk}|~lmn o{pFGrZ]tsp]gf\fui0jTwvylmZn oZpOrwhere lmZn opis the angle between the incident quark or gluon and the outgoing lepton. In an experimental situationthe transverse momenta of the incoming partons are not known, and optimal results are achieved by calculatinglmn oZp in the Collins-Soper frame [13]. In order to reduce the contamination from the Drell-Yan background, onlyevents in a .P mass window around the resonance peak were used in the analysis.The analysis closely follows the method described in Ref. [11], where an unbinned likelihood is calculated for thegraviton and b R , under the two hypotheses considered. The test statistic used to distinguish the two hypotheses isthe likelihood ratio: v;#wŁ;=dwBvy=IqThe test was constructed to treat the two hypotheses equally, ie. with equal probabilities of Type-I and Type-IIerrors. The CMS reach for 2 P discrimination in the  -  a  plane for different values of integrated luminosityis shown in Fig. 33500 1000 1500 2000 2500 3000 35000,010,11fb-1300fb-1100fb-1  k/MPLGraviton Mass, GeV/c2allowed regionLp<10 TeV 10fb-1CMS Discovery Limit of Randall-Sundrum GravitonG  - >  m mFigure 2: Theoretical and experimental constraints on the RS model parameters, with systematic uncertaintiestaken into account. The region to the left of the curves corresponds to 5  discovery reach, for different values ofintegrated luminosity.Graviton Mass (GeV)1000 1500 2000 2500 3000 3500 4000    Plk/M-210-110 Spin-1 Explusion (10 fb-1)σ2 Spin-1 Exclusion (100 fb-1)σ2 Spin-1 Explusion (300 fb-1)σ2Figure 3: CMS reach for 2  discrimination between spin-1 and spin-2 hypotheses in the Randall-Sundrum modelparameter space, for different values of integrated luminosity. The accessible region lies to the left of the curves.44.2 z measurementIn the   ¡ﬀ¢£¥¤¦6¤e§ channel, ¨©Zª arises from the combination of parity-conserving couplings (from the ) and parity-violating couplings (from the ¢£ ) in the interaction Lagrangian. A ¢« contributes another parity-violating term, shifting the value of ¨ ©Zª , especially in the mass range around the peak, as shown in Fig. 4.Mass (GeV)   400 500 600 700 800 900 1000 1100 1200 1300 1400  FBA-1-0.8-0.6-0.4-0.2-00.20.40.60.81 vs Mass, 1 TeV (Pythia)FBA(a) SSMψ χ η  LRM ALRM vs Mass, 1 TeV (Pythia)FBAMass (GeV)   1500 2000 2500 3000 3500 4000 4500  FBA-1-0.8-0.6-0.4-0.2-00.20.40.60.81 vs Mass, 3 TeV (Pythia)FBA(b) SSMψ χ η  LRM ALRMFigure 4: ¨ ©ª vs ¬}­e­ for ®®Ł`¡¢0£O¡¢«°¯¦;¯;§ generated events with ¢« mass set to (a) 1 TeV and (b) 3 TeV.As opposed to the spin discrimination case, for ¨ ©Zª measurement the terms odd in ±²³ ´ µ are important. Sincethe angle ´ is measured with respect to the direction of the incoming quark, the study of ¨ ©Zª in proton-protoninteractions is troublesome. This is due to the fact that a quark can originate with equal probability from eitherproton, and the sign of ±²³ﬂ´{µ is not directly measurable. Here it is assumed that the longitudinal motion of thedimuon system is in the direction of the proton contributing the annihilating quark, since a quark in a protontypically carries a larger momentum fraction ¶ than does an antiquark. This introduces an uncertainty in theasymmetry measurement.The value of ¨©Zª is extracted from data by performing a multi-dimensional maximum likelihood t, taking intoaccount the probability of incorrect assignment of the sign of ±²Z³ ´ﬂµ , the detector acceptance, and resolution. Therapid variation of ¨ ©ª with ¬­e­ shown in Fig. 4 will be hard to observe with real data, due to limited statisticsand mass resolution. In this work only the mass peak region is considered, to investigate to what extent differentmodels can be distinguished.Fig. 5 contains a graphical representation of the results. The vertical axis is split into six regions, one for each ofthe six models studied. On the horizontal axis, the value ¨ ©ª for each model is represented by an asterisk and avertical dotted line spanning the plot; a triangle marker with error bars indicates the expected error on ¨^·@¸J¹©ªwith 10fb §º (for a 1 TeV ¢« mass shown in (a)) or 400 fb §º (for a 3 TeV ¢« mass shown in (b)) of integrated luminosity.Solid vertical lines are drawn halfway between the adjacent values of ¨»©Zª , forming the boundaries of the criticalregions, ¨w¹J¼½©Zª, for the respective pairs of models.Table 1 gives a summary of the values of the signicance level for pairwise comparisons of ¢6« models at ¬}¾e¿~ÀÁTeV and with 10 fb §º of integrated luminosity. The left column species the model taken as the null hypothesisÂ£, which is tested against the alternative of each of the other ve models in the adjacent columns.Model ¢0ÃÄ.ÅÆ ¢Ç ¢È ¢É ¢ÊÊ$Æ ¢0Ä.ÅÆ¢0ÃÄ.ÅÆ  0.0 5.3 6.6 7.6 9.4¢Ç 0.0  3.7 4.6 5.3 6.6¢È 2.7 2.6  0.7 1.2 2.1¢É 3.3 3.3 0.7  0.5 1.4¢ÊÊ$Æ 6.8 6.8 2.1 0.9  1.6¢0Ä.ÅÆ 6.8 6.8 3.0 2.1 1.3 Table 1: Signicance level Ë (expressed in equivalent number of Ì ’s) for pairwise comparisons of ¢Í« models at¬}¾¿ÀÁTeV, for 10 fb §º of integrated luminosity.5FBA-0.6 -0.4 -0.2 0 0.2 0.4 0.6FBA-0.6 -0.4 -0.2 0 0.2 0.4 0.6Model of Generated SampleALRM    c    h    ySSM LRM ALRM    c    h    ySSM LRM , 1 TeVrecs and countFBOn-peak A(a)countFBA lum.-1 w/ 10 fbrecs, 1 TeVrecs and countFBOn-peak AFBA-0.6 -0.4 -0.2 0 0.2 0.4 0.6FBA-0.6 -0.4 -0.2 0 0.2 0.4 0.6Model of Generated SampleALRM    c    h    ySSM LRM ALRM    c    h    ySSM LRM , 3 TeVrecs and countFBOn-peak A(b)countFBA lum.-1 w/ 400 fbrecs, 3 TeVrecs and countFBOn-peak AFigure 5: On-peak ÎÏÐ with expected statistic alerrors, for (a) ÑÒeÓIÔ 1 TeV and (b) ÑÒeÓBÔ 3 TeV. The dottedvertical lines and asterisks show the ÎÕÏZÐ for each model. The solid vertical lines are halfway between the adjacentÎÏÐ . The error bars on the triangle markers correspond to (a) 10 fb Ö× and (b) 400 fb Ö× of integrated luminosity.5 ConclusionsThe above results show that high energy muon pairs provide a promising signature for searches for new heavybosons. Even with an integrated luminosity of only 0.1-1 fb Ö× and non-optimal alignment a 5 Ø disovery is possiblefor particles with mass of 1-2 TeV. For a luminosity of 100 fb Ö× and optimal alignment, the discovery reach is inthe range between 3.9 and 4.8 TeV for a ÙÚ and 1.7 to 4.1 TeV for a RS graviton.Spin-1 and spin-2 hypotheses can be discriminated at 2 Ø level for gravitons with mass up to 1.1 TeV for ÛÜÔÝÞ Ýßand 2.5 TeV for ÛÔ ÝÞ=ß , with an integrated luminosity of àUáãâﬀä = 100 å@æ Ö× . With 400 å@æ Ö× of data, themeasurement of forward-backward asymmetry can be used to distinguish between either Ù6çè.é0ê or Ùë and one ofthe other four studied models ( Ùìì$ê , Ùí , Ùî , or Ùè.éê ) up to a Ñ ÒÓbetween 2.0 and 2.7 TeV. The Ùì$ìê , Ùí , Ùî ,and Ù0è.éê are distinguishable with the same level of signicance only up to Ñ ÒÓÔß1.5 TeV, and Ùçè.é0ê andÙë are indistinguishable for masses over 1 TeV.Furter details of the analyses presented in this paper can be found in the corresponding CMS Notes [14, 15, 16].This work was partially nanced by Polish Ministry of Science in 2006-2007 as a research project.References[1] For a review, see C.T. Hill and E.H. Simmons, Strong Dynamics and Electroweak Symmetry Breaking,Phys. Rep. 381 (2003) 235.[2] See, for example, T. Han et al., Phenomenology of the Little Higgs Model, Phys. Rev. D 67 (2003) 095004.[3] M. Cvetic and P. Langacker, Implications of Abelian Extended Gauge Structures From String Models,Phys. Rev. D 54 (1996) 3570; New Gauge Bosons From String Models, Mod. Phys. Lett. A 11 (1996)1247.[4] For a review, see A. Leike, The Phenomenology of Extra Neutral Gauge Bosons, Phys. Rep. 317 (1999)143.[5] L. Randall and R. Sundrum, A large mass hierarchy from a small extra dimension, Phys. Rev. Lett. 83(1999) 3370; An alternative to compactication, Phys. Rev. Lett. 83 (1999) 4690.[6] Review of Particle Properties, K. Hagiwara et al., Phys. Rev. D 66 (2002) 328,The LEP Collaborations, the LEP Electroweak Working Group, the SLD Electroweak and Heavy FlavourGroups, A Combination of Preliminary Electroweak Measurements and Constraints on the Standard Model,CERN-PH-EP/2004-069, hep-ex/0412015.6[7] H. Davoudiasl, J.L. Hewett and T.G. Rizzo, Experimental Probes of Localized Gravity: On and Off theWall, Phys. Rev. D63 (2001) 075004.[8] T. Sj¤ostrand et al., High-Energy-Physics Event Generation with PYTHIA 6.1, Comput. Phys. Commun.135 (2001) 238.[9] J. Pumplin et al., New Generation of Parton Distributions with Uncertainties from Global QCD Analysis,JHEP 07 (2002) 012.[10] R. Cousins, J. Mumford, V. Valuev, Detection of ïð Gauge Bosons in the Dimuon Decay Mode in CMS,CMS Note 2005/002.[11] R. Cousins et al., Spin Discrimination of New Heavy Resonances at the LHC, JHEP 11 (2005) 046.[12] V. Bartsch and G. Quast, Expected Signal Observability at Future Experiments, IEKP-KA/2003-30, CMSNote 2005/004.[13] J. Collins and D. Soper, Angular Distribution of Dileptons in High-Energy Hadron Collisions, Phys. Rev.D 16 (1977) 2219.[14] R. Cousins, J. Mumford and V. Valuev, Detection of ñ ð Gauge Bosons in the Dimuon Decay Mode in CMS,CMS Note 2006/062.[15] R. Cousins, J. Mumford, V. Valuev, Forward-Backward Asymmetry of Simulated and Reconstructed ïÍðòóôó;õ Events in CMS, CMS Note 2005/022.[16] I.Belotelov et al., Search for Randall-Sundrum Graviton Decay into Muon Pairs, CMS Note 2006/104.7

Search for Z-primes and Randall-Sundrum Gravitons with dimuons in CMS

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