Minireview of Leptoquark Searches

Leptoquarks are hypothetical bosons which couple to a lepton and a quark via a Yukawa coupling (denoted λ). In the Standard model, both quarks and leptons occur in left-handed SU(2) doublets and right-handed SU(2) singlets. The symmetry between quarks and leptons leads to the cancellation of triangle anomalies which make the SM renormalizable. Leptoquarks appear in theories[1] in which this symmetry is more fundamental. Leptoquarks (LQs) are color triplets, which would be pair produced in either qq̄ or gg interactions at pp̄ or pp colliders. Because they carry electroweak charge, they would also be pair produced in ee or μμ collisions. Only standard model gauge couplings are involved in pair production; therefore the cross sections depend neither on the quark-lepton-LQ Yukawa coupling nor on the quark and lepton generations to which the leptoquark couples. In contrast, leptoquarks would be singly produced via the Yukawa coupling in a lepton-quark collision. Searches at electron-proton colliders are sensitive only to LQs which couple to electrons and the sensitivity to LQs which couple to second and third generation quarks is far below that of first-generation LQs. Leptoquarks are usually (but not always) assumed to be generation diagonal. Models in which LQs couple to more than one generation of quarks or leptons would induce flavor-changing neutral currents[2,3] or lepton flavor violation[2,4] respectively. The model of Buchmüller, Rückl and Wyler[5] (BRW), in which leptoquarks couple to a single generation of SM fermions via chiral Yukawa couplings which are invariant under SU(3)×SU(2)× U(1) is often used to classify possible leptoquark species. In the BRW model baryon and lepton numbers (B and L) are conserved; there exist ten possible leptoquark species characterized by the chirality of the coupling, the the spin (J = 0 or 1), the weak isospin (T =0, 1/2, or 1), and the fermion number, F = 3B + L = 0 or 2. Four of the species could have separate couplings to rightand left-handed leptons. A leptoquark which coupled first generation quarks to both leftand right-handed electrons would mediate π → eν decay[6]. The agreement of the measured decay rate with SM predictions motivates the conventional assumption that only one of the couplings can be nonzero. In the most commonly used notation[7] F = 0 scalar (S) and vector (V ) species are denoted S 1/2, S R 1/2, S̃ L 1/2, V L 0 , V R 0 , Ṽ R 0