Leptoquark Search at e+e− Colliders

We investigate the possibility of detecting a scalar leptoquark, coupling to the electron and the top, at a linear collider. For coupling strength equalling the weak coupling constant, the present mass bounds are of the order of 300 GeV. We demonstrate that at the NLC, one could detect such particles if their mass were less than a few TeV’s. ∗[email protected], [email protected] In the otherwise glorious success of the Standard Model (SM), there remains the “aesthetic drawback” of the arbitrary assignment of quark and lepton fields. Theories that venture beyond the SM do offer hints of a pattern [1, 2] albeit at the cost of new fields that mediate new interactions between the quarks and leptons. Leptoquarks are but a kind. Transforming as the fundamental representation of the SU(3)c gauge group, these can be either (pseudo-)scalars or (axial-)vectors. Some phenomenogical constraints (at least for leptoqaurks coupling to the first two generations) are obvious. The stability of the proton dictates that if the mass (m) and the coupling (f) be such that m/f < ∼ 10 GeV, then a leptoquark may not have a diquark coupling[3]. Bounds[4] from flavour changing neutral current processes (m/f > ∼ 10 GeV) are evaded only if we assume that these couple to only one flavour of leptons and quarks each[5]. Furthermore, the requirement that the leptonic decay modes of the pseudoscalar mesons be not enhanced stipulates that either m/f ∼ 10 GeV or the leptoqaurk couples chirally[6]. Bounds from atomic parity violation are somewhat weaker[5]. Inspite of such constraints, there has been a recent surge of interest in the subject. The reasons are twofold. On the one hand, there do exist symmetries and models wherein the above constraints are comfortably evaded while allowing a rich phenomenology; on the other, recent experiments have begun to provide us with direct probes[7]. For coupling strengths similar to the weak gauge coupling, these searches give lower bounds of the order of a few hundred GeV. There could exist though one class of leptoquarks that would easily evade the aforementioned constraints without any further assumptions. We focus here on a scalar particle (S) that couples the electron only with the quarks of the third generation. In the SM, the ordinary Yukawa coupling for the top quark is almost the same as the weak gauge coupling. If this be the case here too, one could look forward to interesting new features. In a recent work, Bhattacharyya et al. [8], have investigated such a scenario. They conclude that the For a representative list of recent work, please see ref.[8]