We examine pair mass dependence near threshold as a means to measure the spin
of the top quark in hadron collisions, and we discuss the possibility that a
top squark signal could be hidden among the top events.Comment: 9 pages, RevTeX, 3 figures. Contribution to the Thinkshop on
  Top-Quark Physics for the Tevatron Run II, Fermilab, October 16 - 18, 199

Berger, Edmond L.

Tait, Tim M. P.

English

arXiv

We examine pair mass dependence near threshold as a means to measure the spin
of the top quark in hadron collisions, and we discuss the possibility that a
top squark signal could be hidden among the top events

Berger, Edmond L

Tait, Tim MP

eScholarship - University of California

Top Spin and Experimental Tests

The authors examine pair mass dependence near threshold as a means to measure the spin of the top quark in hadron collisions, and they discuss the possibility that a top quark signal could be hidden among the top events

Berger, E. L.

Tait, T. M. P.

UNT Digital Library

The submitted manuscript has been createdby the University of Chicago as Operator of“Argonne National Laboratory (“Argonne”)under Contract No. W-31-109-ENG-38 withthe U.S. Department of Energy. The U.S.. Government retains for itself, and others act-ing on its behalf, a paid-up, nonexclusive,irrevocable worldwide license in said atilcleto reproduce, prepare derivative works, dis-tribute copies to the public, and perform pub-licly and display publicly, by or on behalf ofthe Government...000Nz “ hep-ph/OO02305ANL-HEP-CP-OO-014Top Spin and Experimental Tests*Edmond L. Berger and Tim M. P. TaitHigh Eneqy Physics Division, Argonne National LaboratoryAqonne, Illinois 60489(February29, 2000)AbstractWe examine pair mass dependence near threshold as a meansto measurethespin of the top quark in hadron collisions, and we discuss the possibility thata top squarksignal could be hidden among the top events.I. INTRODUCTIONEvidence to date is circumstantial that the top events analyzed in Tevatron experimentsare attributable to a spin-1/2 parent. The evidence comes primarily from consistency of thedistribution in momentum of the decay products with the pattern expected for the weakdecay -t~ h+ W, with W ~ 1+ v or W ~ jets, where the top t is assumed to have spin-1/2.It is valuable to ask whether more definitive evidence for spin-1/2 might be obtained infuture experiments at the Tevatron and LHC, We take one look at this question by studyingthe differential cross section do/dJ4tz in the region near production threshold. Here kf~~is the invariant mass of the tf pair. We contrast the behavior of it production with that*Contributionto the Thinkshop on TopQuark Physicsfor the TevatronRun II, Fermilab,October16-18, 1999.1DISCLAIMERThis report was prepared as an account of work sponsoredby an agency of the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warranty, express orimplied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or reflect those of the United StatesGovernment or any agency thereof.DISCLAIMERPortions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument.expected for production of a pair of spin-O objects. We are motivated by the fatt” that inelectron-positron annihilation, et + e- + q + ?, there is a dramatic difference in energydependence of the cross section in the near-threshold region for quark spin assignments of Oand 1/2.For definiteness, we compare top quark t and top squark ~ productionphenomenology exists for top squark pair production, obviating the needsince a consistentto invent a modelof scalar quark production. Moreover, top squark decay may well mimic top quark decay.Indeed, if the chargino ~ is lighter than the light top squark, as is true in many modelsof supersymmetry breaking, the dominant decay of the top squark is ~ ~ b + ~+. If thereare no sferrnions lighter than the chargino, the chargino decays to a W and the lightestneutralino ~. In another interesting possible decay mode, the chargino decays into a Ieptonand slepton, ~+ ~ l+fi. The upshot is that decays of the top squark may be very similar tothose of the top quark, but have larger values of missing energy and softer momenta of thevisible decay products. A recent study for Run II of the Tevatron [1] concluded that evenwith 4 fb-l of data at the Tevatron, and including the LEP limits on chargino masses, thesedecay modes remain open (though constrained) for top squarks with mass close to the topquark mass .II. CALCULATION AND RESULTSAt the energy of the Fermilab Tevatron, production of t; pairs and of ;; pairs is dom-inated by q~ annihilation, where the initial light quarks q are constituents of the initialhadrons. The subprocess proceeds through a single intermediate gluon at leading-order inQCD perturbation theory. The analogy to e~ ~ q? through an intermediate virtual photonis evident. We choose to work at leading-order in the tf and i!’;partonic cross sections.In Fig. l(a) we dkplay the partonic cross sections ~(@ as functions of the partonicsubenergy W for the @ channel, where q represents a single flavor of massless quark. Weuse the nominal value mt = 175 GeV for the mass of the top quark. We select m; =2z . 165 GeV for the mass of the top squark so that the maximum values of the partonic crosssections occur at about the same value of W in tfand ;; production. Although the couplingstrengths g, where as = g2/(4n), are the same in the amplitudes for ti and ;~ production,the magnitude of the ~~partonic cross section is a factor of = 0.015 smaller at the peak. Thereduction comes in part from the final-state sum over spins and in part from the momentumdependence of the p-wave coupling for ;~ production.If we ignore relative normalization, the very different threshold energy dependence ofthe ti$and ~~cross sections in Fig, l(a) suggest that spin discrimination might be possibleif one could study the pair mass distribution, du/dMtf. However, in hadron reactions, oneobserves the cross section only after convolution with parton densities.In Fig. l(b), we display the hadronic cross sections for p~ + tfX and p~ + RX atproton-antiproton center-of-mass energy 2 TeV as a function of pair mass. We use theCTEQ5L parton densities [2] with the factorization scale p chosen equal to the top quarkmass for t; production, and the top squark mass for ~?production. We include the relativelysmall contributions from the glu~glue initial state. The parton luminosities fall steeplywith subenergy so the tails at high pair mass evident in Fig. 1(a) are cut-off sharply inFig. l(b). Indeed, the convolution with parton densities sharpens the peak of the i; pairmass distribution significantly and makes it resemble a background that is similar to t;production. The top squark cross section is approximately 12% of the top cross section.The smaller value is due in part to the fact that the p–wave top squark production reducesthe partonic cross section for low A&, where the parton luminosities are largest.At the energy of the CERN LHC, production of ti?pairs and of ~ pairs is dominated by ggsubprocess, and the threshold behaviors in the two cases do not differ as much as they do forthe qij incident channel. In Fig. 2(a), we show the partonic cross sections 6(W) as functionsof the partonic subenergy W for the gg channel. In Fig. 2(b), we display the hadronic crosssections for pp + tfX and pp + ;~X at proton-proton center-of-mass energy 14 TeV tia function of pair mass. We include the relatively small contributions from the qij initialstate. After convolution with parton densities, the shape of the i%pair mass distribution is3remarkably similar to that of the ti case.III. DISCUSSIONBased on shapes and the normalization of cross sections, it is difficult to exclude thepossibility that some fraction (on the order of 10%) of top squarks with mass close to 165GeV is present in the current t~ sample. The invariant mass distribution of the producedobjects, lkft~,is quite different at the partonic level for the q@ initial state (dominant atthe Tevatron), but much less so for the gg initial state (dominant at the LHC). However,after one folds with the parton distribution functions, the difference in the q~ channel at theTevatron is reduced to such an extent that the Jfti distribution is not an effective means toisolate top squarks from top quarks.Ironically, the good agreement of the absolute rate for tf production with theoreticalexpectations [3,4] would seem to be the best evidence now for the spin-1/2 assignment inthe current Tevatron sample.A promising technique to isolate a top squark with mass close to mt would be a detailedstudy of the momentum distribution of the top quark decay products (presumably in thetop quark rest frame). One could look for evidence of a chargino resonance in the missingtransverse energy and charged lepton momentum, or for unusual energy or angular distri-butions of the decay products owing to the different decay chains. One could also look fordeviations from the expected correlation between angular dktributions of decay productsand the top spin [5].As a concrete example of an analysis of this type, in Fig. 3 we present the distributionin the invariant mass X of the bottom quark and charged lepton, with X2 = (pb + pl+ )2,where the bottom quark and lepton are decay products of either a top quark with mt = 175GeV or a top squark ; ~ ~+b ~ W+jj”b a l~vl~ob, with m: = 165 GeV, mx+ = 130GeV, mgo = 4(I GeV, and mb = 5 GeV. The X distribution is a measure of the degree ofpolarization of the W boson in top quark decay [6], and the figure shows that the different4dynamics responsible for top squark decay result in a very different distribution, peaked atmuch lower X. The areas under the curves are normalized to the inclusive tf and i!’; ratesat the Tevatron and LHC, respectively. At the LHC there is relatively more top squark inthe sample, and thus the difference is more prominent.In this simple demonstration potentially important effects are ignoredextract the t; signal from its backgrounds, detector resolution and efficiency,in identifying the correct b with the corresponding charged lepton fromsuch as cuts toand ambiguitiesa single decay.Detailed simulations would be required to determine explicitly how effective this variablewould be in extracting a top squark sample from top quark events. Nevertheless, suchtechniques, combined with the large t; samples at the Tevatron Run II and LHC, shouldprove fruitful in ruling out the possibility of a top squark with mass close to the top quarkmass, or alternatively, in discovering a top squark hidden inWork inby the U.S.ACKNOWLEDGMENTSthe top sample.the High Energy Physics Division at Argonne National Laboratory is supportedDepartment of Energy, Division of High Ener~ Physics, under Contract W-31-109-ENG-38.5.REFERENCES ,.[1] R. Demina, J. Lykken, K. Matchev, and A. Nomerotski, hep-ph/9910275.[2] H. L, Lai, J. Huston, S. Kuhlmann, J. Morfin, F. Olness, J. Pumplin, andhep-ph/9903282.Wu-kl Tung,[3] E. L. Berger and H. Contopanagos, Phys. Rev. D57, 253 (1998) and references therein.[4] R. Bonciani, S. Catani, M. L. Mangano, and P. Nason, Nucl. Phys. B519, 424 (1998).[5] G. Mahlon and S. Parke, Phys. Lett B 411,173 (1997).[6] D. O. Carlson, Ph.D. Thesis, Michigan State University, MSUHEP 050727.6.0.40.350.3FIGURESl“’I’’’ 1’’’ 1’’’ 1’’’ 1’”1’ “1’ ’’11’1:(a) ~L----stopanti-stop 480.250 0.20.150.10.0500 200 400 600 800 100012001400160018002000M (GeV)1.41.231tJ 0.80.60.40.20i I I I # 1 I I I I 1 I [ 1 1 I 1 1 I:— t tbar (b)2------ stopanti-stopJI1 1 I I I 1 I ! 10 200 400 600 800 1000M (GeV)FIG. 1. (a) Partonic cross sections 6(M) as functions of partonic subenergy lt4 for the qijchannel. (b) Hadronic cross sections du/dIkf in proton-antiproton collisions at 2 TeV as functionsof pair mass. The top quark mass mt = 175 GeV, and the top squark (stop) mass m~= 165 GeV.0*/.0.40.350.3z 0.25b 0.20.150.10.05o0 200 400 600 800 1000 1200 1400 1600 18002000M (GeV)e“’I’’’ l’’’ l’’’ ”1”’l’’’l’”l” ‘1’’’ 1’”9104[-- .StopSnti-stop @) j~03 —*+~--;;2 /’--=.> 1—!. Lucu— 1. .l-J I ‘- --a-10-2-----------1010:10-510 t 1 , ,1,,,1,,,1,,,1,,11,,,1,,,1 ,f, n,!--0 200 400 600 800 1000 1200 1400 1600 1800 2000M (GeV)FIG.2. (a) Partonic cross sections 6(M) as functions of partonic subenergy A4 fortheggchannel. (b) Hadronic crose sections du/dMin proton-proton collisions at 14 TeV as functions ofpair mass. The top quark mass mt = 175 GeV, and the top squark (stop) mass m;= 165 GeV.80.060.050.04g 0.03Gm 0.020.0100.060.050.04$0.03Lm 0.020.010E“’I’’’ l’’’ l’’’I’’’ l’’’I’’’ l’’’l’’’l’”irtop decay----stop decayf-Ii(a)I------ ---11 ,11,,1-0 20 40 60 80 100 120 140 160 180 200X (GeV)k“’I’’’ l’’’I’’’ l’’’I’’’ l’’’l’’’l’’’l’”~E-topdecay (b)----stop decay IL J1 ,1,11,1-0 20 40 60 80 100 120 140 160 180 200X (GeV)FIG.3. Distribution of theinvariant m-ofabottom quark andchargedlepton (X) for atopquark or top squark decay, with relative size normalized to the cross sections at (a) the TevatronRun II and (b) the LHC. The top squarkdecay and sparticle massesare discussed in the text.

Top spin and experimental tests

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Top Spin and Experimental Tests

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