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Leptoquark Production and Identification in High Energy + , E+E and E Collisions + + e e e Leptoquark Production and Identification in High Energy , and Collisions 2 Michael A. Doncheski 1 and Stephen Godfrey 1 Department of Physics, Pennsylvania State University, Mont Alto, PA 17237 USA 2 Ottawa-Carleton Institute for Physics Department of Physics, Carleton University, Ottawa CANADA, K1S 5B6 ~ S S 1 ABSTRACT Buchm¨uller, R¨uckl and Wyler (BRW) [17]: 1 , (scalar, iso- ~ R R S 2 3 singlet); 2 , (scalar, iso-doublet); (scalar, iso-triplet); + e e We study single leptoquark production in e , ,and ~ ~ U U V V U 1 2 2 3 1 , (vector, iso-singlet); , (vector, iso-doublet); + collisions due to the quark content of the photon. Lepto- (vector, iso-triplet). The production and corresponding decay quarks can be produced in substantial numbers for masses very signatures are quite similar, though not identical, and have been close to the centre of mass energy of the collider which results in studied separately by many authors. The question arises as to equivalently high discovery limits. Using polarization asymme- how to differentiate between the different types. In the sec- tries in an e collider the ten different types of leptoquarks listed ond part of this report we show how a polarized e collider can by Buchm¨uller, R¨uckl and Wyler can be distinquished from one be used to differentiate the LQs. (ie. a polarized e beam, like another for leptoquark masses essentially up to the kinematic SLC, in conjunction with a polarized-laser backscattered photon limit. Thus, if a leptoquark were discovered an e collider could beam.) This process takes advantage of the hadronic component play a crucial role in determining its origins. of the photon which is important and cannot be neglected [18]. I. INTRODUCTION II. LEPTOQUARK PRODUCTION There is considerable interest in the study of leptoquarks (LQs) — colour (anti-)triplet, spin 0 or 1 particles which carry The process we are considering is shown if Fig. 1. The parton both baryon and lepton quantum numbers. Such objects ap- level cross section for scalar leptoquark production is trivial, pear in a large number of extensions of the standard model such given by: 2 p m as grand unified theories, technicolour, and composite mod- e (^s)= (M s^) s (1) M els. Quite generally, the signature for leptoquarks is very strik- s p ing: a high p lepton balanced by a jet (or missing bal- T T where we have followed the convention adopted in the anced by a jet, for the q decay mode, if applicable). Searches literature[13] where the leptoquark couplings are replaced by for leptoquarks have been performed by the H1 [1] and ZEUS a generic Yukawa coupling g which is scaled to electromag- [2] collaborations at the HERA ep collider, by the D0 [3] and 2 g =4 = netic strength em . The cross section for vector p CDF [4] collaborations at the Tevatron p collider, and by the leptoquark production is a factor of two larger. We only con- ALEPH [5], DELPHI [6], L3 [7], and OPAL[8] collaborations sider generation diagonal leptoquark couplings so that only lep- + e at the LEP e collider. Discovery limits have also been ob- toquarks which couple to electrons can be produced in e (or tained for the LHC[9]. In this report we consider single lep- + + e e ) collisions while for the collider only leptoquarks + + e e e toquark production in , ,and collisions which which couple to muons can be produced. Convoluting the par- utilizes the quark content of either a backscattered laser pho- ton level cross section with the quark distribution in the photon ton for the e case or a Weizacker-Williams photon radiating one obtains the expression + + e e off of one of the initial leptons for the or cases [10, 11, 12, 13, 14, 15, 16]. This process offers the advantage of Z 2 (s) = f (z; M )^(^s)dz q= a much higher kinematic limit than the LQ pair production pro- s cess, is independent of the chirality of the LQ, and gives similar 2 2 em 2 2 ) =s; M f (M : results for both scalar and vector leptoquarks. In the first part = q= (2) s s of this report we use single leptoquark production to estimate s + + e e e the discovery limits at future high energy , ,and The cross section depends on the LQ charge since the photon colliders. d has a larger u quark content than quark content. We note Although the discovery of a leptoquark would be dramatic that the interaction Lagrangian used in Ref. [14] associates a evidence for physics beyond the standard model it would lead p = 2 factor 1 with the leptoquark-lepton-quark coupling. Thus, to the question of which model the leptoquark originated from. one should compare our results with to those in Ref. [14] with Given the large number of leptoquark types it would be imper- 2 . We give results with chosen to be 1. + ative to measure its properties to answer this question. There + e e For e , ,and colliders the cross section is ob- are 10 distinct leptoquark types which have been classified by tained by convoluting the expression for the resolved photon This research was supported in part by the Natural Sciences and Engineer- contribution to e production of leptoquarks, Eqn. (2), with, as ing Research Council of Canada. appropriate, the backscattered laser photon distribution [19] or 933 1 2 Q the Weizs¨acker-Williams effective photon distribution: the d quark content which can be traced to the larger of the q u-quark. There exist several different quark distribution func- Z 1 2 p 2 dx em + tions in the literature [21, 22, 23, 24, 25]. For the four different (` ` ! XS) = f (x; s=2) =` s x 2 leptoquark charges we show curves for three different distribu- M =s s 2 2 tions functions: Drees and Grassie (DG)[23], Gl¨uck, Reya and ): f (M =(xs);M q= (3) s s Vogt (GRV)[24], and Abramowicz, Charchula and Levy (LAC) Before proceeding to our results we consider possible back- set 1[25]. The different distributions give almost identical re- Q = 1=3; 5=3 grounds [20]. The leptoquark signal consists of a jet and elec- sults for the LQ leptoquarks and for the Q = 2=3; 4=3 tron with balanced transverse momentum and possibly activ- LQ leptoquarks give LQ cross sections that ity from the hadronic remnant of the photon. The only serious vary by most a factor of two, depending on the kinematic region. background is a hard scattering of a quark inside the photon by In the remainder of our results we will use the GRV distribution functions [24] which we take to be representative of the quark ! eq the incident lepton via t-channel photon exchange; eq .By comparing the invariant mass distribution for this background distributions in the photon. to the LQ cross sections we found that it is typically smaller Comparing the cross sections for the two collider modes we than the LQ signal by two orders of magnitude. Related to this see that the kinematic limit for the e mode is slightly lower process is the direct production of a quark pair via two photon fusion + ! e + q +q: e (4) However, this process is dominated by the collinear divergence which is actually well described by the resolved photon process ! eq eq given above. Once this contribution is subtracted away the remainder of the cross section is too small to be a concern [20]. Another possible background consists of ’s pair produced via various mechanisms with one decaying leptonically and the other decaying hadronically. Because of the neutrinos in the p final state it is expected that the electron and jet’s T do not in general balance which would distinguish these backgrounds from the signal. However, this background should be checked in a realistic detector Monte Carlo to be sure. The remaining backgrounds originate from heavy quark pair production with one quark decaying semileptonically and only the lepton being observed with the remaining heavy quark not being identified as such. All such backgrounds are significantly smaller than our signal in the kinematic region we are concerned with. III. LEPTOQUARK DISCOVERY LIMITS p + =1 e e In Fig. 2 we show the cross sections for a s TeV operating in both the backscattered laser e mode and in the + e e mode. The cross section for leptoquarks coupling to the d u quark is larger than those coupling to the quark. This is due to the larger u quark content of the photon compared to 1 m The effective photon distribution from muons is obtained by replacing e m with . Figure 2: The cross sections for leptoquark production due to resolved photon contributions in e collisions, with chosen to X be 1. In the top figure the photon beam is due to laser backscat- q p + = 1000 e e S tering in a s GeV collider. In the bottom figure the the photon distribution is given by the Weizs¨acker-Williams p + = 1000 e e effective photon distribution in a s GeV col- e lider. In both cases the solid, dashed, dot-dashed line is for re- solved photon distribution functions of Abramowicz, Charchula Figure 1: The resolved photon contribution for leptoquark pro- andLevy[25],Gl¨uck, Reya and Vogt [24], Drees and Grassie duction in e collisions. [23], respectively. 934 + e than the e mode. This is because the backscattered laser either directly or by using a left-right asymmetry measurement: mode has an inherent energy limit beyond which the laser pho- + 2 2 C C + R tons pair produce electrons.
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