In the minimal supersymmetric standard model (MSSM) properties of the third
generation sfermions are important from the viewpoint of discriminating the
SUSY breaking models and in the determination of the Higgs boson mass. If
gluinos are copiously produced at CERN LHC, gluino decays into tb through stop
and sbottom can be studied using hadronic decays of the top quark. The
kinematical endpoint of the gluino decays can be evaluated using a W sideband
method to estimate combinatorial backgrounds. This implies that fundamental
parameters related to the third generation squarks can be reliably measured.
The top-quark polarization dependence in the decay process may also be
extracted by looking at the b jet distribution near the kinematical endpoint.Comment: 4 pages in PRL format, 4 Postscript figures, uses revtex

A. E. Nelson

A. G. Cohen

A. Pomarol

H. Bachacou

H. E. Haber

H. P. Nilles

J. Hisano

J. R. Ellis

Junji Hisano

Kiyotomo Kawagoe

M. Battaglia

M. Carena

M. Dine

Mihoko M. Nojiri

R. Barbieri

Ryuichiro Kitano

S. Dimopoulos

T. Kobayashi

Y. Okada

English

arXiv

Crossref

Scenery from the top: Study of the third generation squarks at the CERN LHC

In the minimal supersymmetric standard model (MSSM) properties of the third generation sfermions are important from the viewpoint of discriminating the SUSY breaking models and in the determination of the Higgs boson mass. If gluinos are copiously produced at CERN LHC, gluino decays into tb through stop and sbottom can be studied using hadronic decays of the top quark. The kinematical endpoint of the gluino decays can be evaluated using a W sideband method to estimate combinatorial backgrounds. This implies that fundamental parameters related to the third generation squarks can be reliably measured. The top-quark polarization dependence in the decay process may also be extracted by looking at the b jet distribution near the kinematical endpoint

Hisano, J

Kawagoe, K

Kitano, R

Nojiri, M M

CERN Document Server

arXiv:hep-ph/0204078 v1   8 Apr 2002SceneryfromtheTop:StudyoftheThirdGenerationSquarksatCERNLHCJunjiHisano,1,2KiyotomoKawagoe,3RyuichiroKitano,2,4andMihokoM.Nojiri51ICRR,UniversityofTokyo,Kashiwa277-8582,Japan2IPNS,KEK,Tsukuba305-0801,Japan3DepartmentofPhysics,KobeUniversity,Kobe657-8501,Japan4DepartmentofPhysics,TohokuUniversity,Sendai980-77,Japan5YITP,KyotoUniversity,Kyoto606-8502,Japan(Dated:January23,2003)Intheminimalsupersymmetricstandardmodel(MSSM)propertiesofthethirdgenerationsfermionsareimportantfromtheviewpointofdiscriminatingtheSUSYbreakingmodelsandinthedeterminationoftheHiggsbosonmass.IfgluinosarecopiouslyproducedatCERNLHC,gluinodecaysintotbthroughstopandsbottomcanbestudiedusinghadronicdecaysofthetopquark.ThekinematicalendpointofthegluinodecayscanbeevaluatedusingaWsidebandmethodtoestimatecombinatorialbackgrounds.Thisimpliesthatfundamentalparametersrelatedtothethirdgenerationsquarkscanbereliablymeasured.Thetop-quarkpolarizationdependenceinthedecayprocessmayalsobeextractedbylookingatthebjetdistributionnearthekinematicalendpoint.PACSnumbers:12.60.Jv,14.80.LyTheminimalsupersymmetricstandardmodel(MSSM)isoneofthepromisingextensionsofthestandardmodel(SM).Themodelrequiressuperpartnersofordinarypar-ticles(sparticles),andLHCatCERNmightconrmtheexistenceofthesenewparticles[1].Sincethesparti-clemassesarerelatedtotheSUSYbreakingmechanism,measurementofthemassesprovidesawaytoprobetheoriginofSUSYbreakinginnature.Themassesandmixingsofstops(~t1;2)andsbottoms(~b1;2)aresensitivetotheavorstructureofscalarmasses.First,thediagonalmassesinthestopandsbottommassmatrices,m~Q3,m~tR,andm~bR,arepredictedtobelessthanthoseofothersquarksintheminimalsupergravity(MSUGRA)model[2]becauseoftheYukawaRGErun-ningeects.SomeSUSYbreakingmodels,suchastheavorU(2)model[3]orthedecouplingsolution[4],andthesuperconformalmodel[5]fortheSUSYavorprob-lem,havealsospecialimprintsontheseparameters.The~tL-~tR-HiggstrilinearcouplingAtattheweakscalehasalargecoeÆcientproportionaltomt,resultinginalargeleft-rightmixingofstops;m2LR=mt(At cot).IntheMSUGRAAtisproportionaltotheuniversalgaug-inomassMattheGUTscaleMGUT,andinsensitivetotheAparameteratMGUT[6].Indeed,thisisoneoftherobustpredictionsofSUSYbreakingatMGUT.Thesere-lationisnotguaranteediftheSUSYbreakingmediationscaleismuchsmallerthanMGUT.ItshouldbestressedthatthestopmassesandthemixingareveryimportantparameterstopredictthelightHiggsmass[7].ItispossibletostudythestopandsbottomatLHCthroughthegluino(~g)decaymodeslistedbelow.I)~g!b~b1!bb~02!bbl+l ~01;II)~g!t~t1!tt~01;III)~g!t~t1!tb~ 1;IV)~g!b~b1!tb~ 1:(1)Inpreviousliteratures[1,8],thethirdgenerationsfermionsareoftenstudiedusingthemodeI)(bbl+l channels).Thismodeisimportantwhen~02hassubstan-tialbranchingratiosintoleptons.Measurementofthekinematicalendpointsofthesignaldistributionstellsusthesparticlemasses.Unfortunately,thebranchingratioscouldbeverysmall,andthismodeisinsensitivetothestop.WetriedtostudythemodeII)inEqs.(1),buttheresultwasunsuccessfulbecauseofthesmallbranchingratiointheMSUGRA.InthispaperwetrytomeasuretheendpointsofthemodesIII)andIV)inEqs.(1).ThedecaymodesareexpectedtobedominantintheMSUGRAsince~ 1islikelytobeSU(2)Lgaugino-likeandBr(~b1;~t1!~ 1)couldbeaslargeas60%.Wefocusonthereconstructionofhadronicdecayofthetopquark,becausethedistribu-tionofthetbinvariantmassmtbmakesaclearendpointinthiscase.Thepartonleveldistributionofthetbnalstateinvari-antmassisexpressedasafunctionofm~g,m~t1,m~b1,andcharginomassm~ 1:d =dmtb/mtb,andtheendpointsMtbforthemodesIII)andIV)arewrittenasfollows;M2tb(III)=m2t+m2~t1 m2~ 12m2~t1n(m2~g m2~t1 m2t)+q(m2~g (m~t1 mt)2)(m2~g (m~t1+mt)2)o;M2tb(IV)=m2t+m2~g m2~b12m2~b1n(m2~b1 m2~ 1+m2t)+q(m2~b1 (m~ 1 mt)2)(m2~b1 (m~ 1+mt)2)o:Notethattheendpointofthenalstatetb~ 1shouldbesensitivetoboth~t1and~b1.Inordertodemonstratetheendpointreconstruction,wetakeanMSUGRApointwiththescalarmassm=2m~gm~t1m~b1m~b2 m~gm~t1m~b1m~b2A1 707 427 570 613 26 A2 706 496 587 614 25G1 807 427 570 613 18 G2 806 496 587 614 18T1 707 327 570 613 30 T2 707 477 570 612 25B 609 402 504 534 56 C 931 636 771 805 5G 886 604 714 763 7 I 831 571 648 725 10E1 515 273 521 634 77 E2 747 524 770 898 8TABLE I: The sparticle masses in GeV and the total SUSYcross sections in pb for the parameter points studied in thispaper.100GeV, the gaugino mass M = 300GeV, the A param-eter  300GeV at the GUT scale, tan  = 10 and  > 0.This corresponds to the sample point A1 in Table 1. Themasses and mixings of sparticles are calculated by ISAS-USY/ISASUGRA [9]. The Monte Carlo SUSY events aregenerated by PYTHIA [10] using the masses and mix-ings, and then passed through a fast detector simulationprogram for the ATLAS experiment [11]. Jets are recon-structed by a cone-based algorithm with R = 0:4. Theb and  tagging eÆciencies are set to be 60% and 50%,respectively.In our study we apply the following selection for thetb signal; 1) EmissT> 200 GeV, 2) me> 1000 GeV(me= EmissT+PallpjetT), 3) two and only two b-jetswith pT> 30 GeV, 4) 4  njet 6 (njet; number ofadditional jets with pT> 30 GeV and jj < 3:0). Inaddition, events with leptons are removed to reduce thebackground from tt production. At this stage the numberof remaining tt events is rather small, about 10% of theremaining SUSY events for the point A1.To reconstruct the hadronic decay of the top quark,we rst take a jet pair consistent with a hadronic Wboson decay with a cut on the jet pair invariant massmjj; jmjj  mWj < 15GeV. The invariant mass of thejet pair and one of the b jets, mbjj, is then calculated.All possible combinations are tried in an event, and thecombination which minimizes jmbjj mtj is chosen. Thejet combination is regarded as a top candidate if jmbjj mtj < 30GeV. The energy and momentum of the jet pairare then rescaled so that mjj= mW.The expected tb endpoint is not clearly visible in themtbdistribution shown in Fig. 1(a). As there are 7 to 8jets on average in a selected event, events with a fake Wboson (and a fake top quark) dominate the distribution.The distribution of the fake W events is estimated fromthe events that contain jet pairs with the invariant massin the ranges A) jmjj (mW 15GeV)j < 15 GeV and B)jmjj (mW+15GeV)j < 15 GeV; `theW sidebands'. Theenergy and momentum of the jet pairs are then rescaledlinearly to be in the range jmjj  mWj < 15GeV. Thefake top candidates are reconstructed from the rescaledjet pairs and b jets in the events.The estimated background distribution is shown inFIG. 1: (a) The signal mtbdistribution for the sample pointA1 in Table 1, (b) the estimated background distribution fromthe sideband events, (c) (a) (b), and (d) the mtbdistributionfor the modes III) and IV) in Eqs. (1), and a decay modeirreducible to the mode III).Fig. 1(b), which is obtained by averaging distributionsfrom the sidebands A) and B). The estimation is basedon an assumption that most of the jets in the eventsdo not have signicant correlation with the b jets in theevents. The estimated background distribution is sub-tracted from the signal distribution in Fig. 1(c). Theestimated correct signal distribution (c) shows the betterendpoint compared to (a). Fig. 1(d) is the same distri-bution as Fig. 1(c) but for the events which contain themode III), the mode IV), or a decay mode irreducible tothe mode III); ~g !~b1b !~t1(Wb) ! b~ 1(bW ). Notethat if (bW ) has an invariant mass consistent to a top,the decay is kinematically equivalent to the mode III).Fig. 1(d) shows the expected clear edge at the right place(Mtb(III) = 476GeV and Mtb(IV) = 420 GeV), demon-strating that the sideband method works well. Here, thenumber of the generated SUSY events is 3  106, whichcorresponds to an integrated luminosity of 120fb 1. Theplots do not include the SM backgrounds.Note that the signals from the modes III) and IV) inEqs. (1) are signicant in the total selected events. Wet the total distribution shown in Fig. 1(c) by a simpletting function, which is described as a function of theendpoint Mttb, the edge height h, and the smearing pa-rameter  from the jet energy resolution. We assume thatthe signal distribution is sitting on a linearly-decreasingbackground. The Mttbis compared with the weightedendpoint Mwtbdened byMwtb=Br(III)Mtb(III) + Br(IV)Mtb(IV)Br(III) + Br(IV);3FIG. 2: (a) A t to the mtbdistribution (point A1), and (b)comparison of Mttband Mwtbfor the sample parameters givenin this paper.where Br(III) and Br(IV) are branching ratios for themodes III) and IV), respectively. The t is shown inFig. 2 (a). We obtain Mttb= 443:2 7:4 GeV, which isconsistent with Mwtb= 459GeV. The Mttbchanges mod-erately when one changes the mtbrange used for the t.We choose the range so that the signicance of the heightS = h=h is at maximum. For the t shown in Fig. 2(a),S(max) = 196:9=15:2 = 13:0 is obtained.In order to check the availability of the endpoint mea-surement, we study twelve sample points in total, includ-ing the previous point, shown in Table 1, and compareMttbandMwtb. We choose two reference MSUGRA pointsas A1 and A2, where m = 100 GeV, M = 300 GeV,tan  = 10,  > 0 and A = 300 GeV. We also studypoints with dierent mass spectrums from the MSUGRApredictions; two points with gluino masses heavier thanthe reference points, (G1, G2), and two points with mod-ied stop masses (T1,T2). Furthermore, we include theMSUGRA points selected from [12] (B,C,G,I) and twonon-SUGRA points E1 and E2 where the gluino decaysexclusively into~t1. The result is summarized in Fig. 2(b)[14]. The error bars represent the statistical errorsfor 3  106SUSY events at each point, and the system-atic error of the jet energy scale (1%) is not included(see Table 1 for SUSY cross sections). This plot showsan impressive linearity between the expectation and theMC ts, although Mttbis systematically lower than Mwtb.This is reasonable since some of particles are always outof the cone to dene jets. This eect may be correctedby comparing distributions with dierent jet denitions.Another uncertainty may come from the jet fragmen-tation. If events are generated by ISAJET for the pointA1, the reconstructed endpoint is smaller by 10%, andthe number of events after the sideband subtraction issmaller by a factor of 1.5. The dierence comes from thedierent jet fragmentation schemes. ISAJET radiatesmore soft jets for a parton, resulting in more backgroundand smeared endpoint distribution. The event genera-tors must be tuned carefully to extract the kinematicalinformation from the signal distribution.We now discuss the physics that might be studied withthe tb endpoint measurement.We cannot determine all of the relevant mass param-eters from only the tb endpoint measurement, since theendpoint Mtbdepends on m~g, m~b1, and m~t1. Thus, thestudy of bbl+l nal state would be important to singleout the possible~t and~b contributions to the tb nal stateand to proceed to a model-independent study. For ex-ample, let us assume that errors of the endpoints Mbblland Mbllare 10 GeV and that of the endpoint Mb1lis30 GeV in the measurement. Here, b1is one of the twob jets for which the invariant mass mbllis larger. Whenwe generate m~g, m~b1, ~02, and ~01randomly around thereference point A1 xing the endpoint Mll(which is ex-pected to have a very small error), and require 2Pi(Mi MA1i)2=M2i< 1(9) (i runs over the possibleendpoints), the deviation Mtb(IV)  MA1tb(IV) is alwaysless than 15(45)GeV. For the point A1, Mtb(IV) = 420GeV and Mwtb 460 GeV. The dierence of Mtb(IV) andMwtbtherefore may be statistically distinguishable.The measurement of the SUSY breaking parameters indierent sectors might reveal an overall inconsistency ofthe SUSY breaking mediation models. The distributionof the invariant mass formed by combining the highestPTjet and a same-avor and opposite-sign lepton pair(jll channels) is sensitive to m~qand m~l, and this maylead to the determination of m and M in the MSUGRA[1, 13]. Once M and m are xed, they strongly constrainthe endpoint Mwtbin the MSUGRA { by comparing itwith the measured value, one should be able to check ifm~bor m~tare consistent to the MSUGRA predictions.Note also that the formulae ofMtborMbbllinvolvem~g,while the rst generation squark mass is lower boundedby m~gin the model. If m~g m~qis established by com-bining the squark mass scale determined through a jllanalysis [1] and the analysis of the nal state involvingb jets, we can show that some new physics should occurbelow the scale where m2~q< 0. Note that, in the pointsG1 and G2 in Table 1, where the gluino mass is increasedby 100 GeV from the MSUGRA predictions, m2becomesnegative at the GUT scale. We study the jll distributionsfor the point G2 in a similar way as given in Ref. [13] andnd that the jll endpoints are successfully reconstructed.Therefore the information on m~qshould be obtained inthis case.In the framework of the MSUGRA, the measurementof Mtbis sensitive to the GUT scale A parameter. Thiseect is large when M  A < 0 as can be seen in Fig. 3.Here we take M  m and m = 230 GeV, and vary Aso that jAj < 3m. The Mwtband m~t1vary by 50 GeVand 150 GeV, respectively, and the changes are againdetectable. Note that the ~02decay into~ll is closed in thiscase, therefore the information from the bbl+l channelsis not available. If m = 100 GeV and A is varied from 300 GeV to 300 GeV, Mwtband m~t1change by 30 GeV4FIG. 3: Dependence of Mtb, m~t1, and m~b1on the GUT scaleA parameter in the MSUGRA.00.0020.0040.0060.0080.010 100 200 300 400 500~tRm   (GeV) M   (III)-50GeV < m   < M   (III)~tLtbbbtb tbFIG. 4: The parton level distribution of mbbfor point A1.The two histograms show the distribution for the event withMtb(III)   50GeV < mtb< Mtb(III) in cases~t1=~tRand~t1=~tL.and 70 GeV, respectively.Finally, we discuss top-quark polarization dependencein the gluino decay. Naturally all top quarks comingfrom ~g,~t1, and~b1decays are polarized, depending on thesfermion and chargino mixings. For example, when~t1ismostly right(left)-handed stop, t of ~g! t~t1is tR(L). Thispropagates into the average top-quark helicity, if the topquark is relativistic enough in the gluino rest frame. Ifthe tb invariant mass is close to the endpoint Mtbof themode III) in Eqs. (1), the t and b go to the opposite direc-tion in the gluino rest frame. Because the bottom quarkfrom the top-quark decay tends to go to the opposite di-rection to the top-quark helicity, the distribution of theinvariant mass mbbfor events with mtbclose to Mtb(III)depends on the average top-quark helicity. The parton-level distributions of mbbare plotted in Fig. 4 for thepoint A1. The solid (dotted) curve shows the distributionfor the events with Mtb(III)   50GeV < mtb< Mtb(III)in the case of~t1=~tR(~t1=~tL). One can read fromFig. 1 that roughly 200 signal events are available nearthe endpoint. The statistical dierence between the twodistributions is 3  for 100 signal events without takingcare of the background [15].In this paper, we try to reconstruct nal states whichconsisting of hadronic jets at LHC. This was considereddiÆcult due to the large combinatorial background, butis overcome by a W sideband method developed to esti-mate the background. We reconstruct the tb nal statefrom the event containing two b jets. The reconstructedendpoint provides us an access to the gluino and the thirdgeneration sparticle masses without relying on the decaymodes including leptons. The correct reconstruction ofthe events also allows us to consider the top-quark polar-ization dependence of the distribution. This informationis important to determine the radiative correction to theHiggs mass, as well as the origin of SUSY breaking.It is a pleasure to thank Dr. Kanzaki and Mr. Toyafor the construction of a simulation environment. Weacknowledge ICEPP, Univ. of Tokyo, for providing uscomputing resources. This work is supported in part bythe Grant-in-Aid for Science Research, Ministry of Ed-ucation, Science and Culture, Japan (No. 12047127 forMMN).[1] ATLAS: Detector and physics performance technical de-sign report, CERN-LHCC-99-14.[2] See references in H. P. Nilles, Phys. Rept. 110, 1 (1984).[3] R. Barbieri, G. R. Dvali and L. J. Hall, Phys. Lett. B377, 76 (1996).[4] See references in J. Hisano, K. Kurosawa and Y. Nomura,Phys. Lett. B 445, 316 (1999).[5] A. E. Nelson and M. J. Strassler, JHEP 0009, 030 (2000);T. Kobayashi, H. Nakano, T. Noguchi and H. Terao, hep-ph/0202023.[6] M. Carena and C. E. Wagner, Nucl. Phys. B 452, 45(1995).[7] H. E. Haber and R. Hemping, Phys. Rev. Lett. 66, 1815(1991); Y. Okada, M. Yamaguchi and T. Yanagida, Prog.Theor. Phys. 85, 1 (1991); J. R. Ellis, G. Ridol andF. Zwirner, Phys. Lett. B 257, 83 (1991); R. Barbieriand M. Frigeni, Phys. Lett. B 258, 395 (1991).[8] I. Hinchlie, F. E. Paige, E. Nagy, M. D. Shapiro,J. Soderqvist and W. Yao, LBNL-40954.[9] H. Baer, F. E. Paige, S. D. Protopopescu and X. Tata\ISAJET 7.48: a Monte Carlo event generator for pp, pp,and e+e reactions", hep-ph/0001086.[10] T. Sjostrand, L. Lonnblad and S. Mrenna \PYTHIA 6.2:physics and manual", hep-ph/0108264.[11] E.Richter-Was et al., ATLFAST2.21, ATLAS InternalNote PHYS-NO-079.[12] M. Battaglia et al., Eur. Phys. J. C 22, 535 (2001).[13] H. Bachacou, I. Hinchlie and F. E. Paige, Phys. Rev. D62, 015009 (2000).[14] For the point C where SUSY decays contain many lep-tons, we omit the lepton veto and reduce the tt back-ground by requiring mbl> 140 for any lepton withpT> 10 GeV.[15] For events with mtb< Mtb(III)   50 GeV the gluinodecay into~b1starts to contribute.

Scenery from the Top: Study of the Third Generation Squarks at CERN LHC

Scenery from the Top: Study of the Third Generation Squarks at CERN LHC

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