Comprehensive study of leptoquark bounds

Abstract

We make a comprehensive study of indirect bounds on scalar leptoquarks that couple chirally and diagonally to the first generation by examining available data from low energy experiments as well as from high energy e + e − and pp accelerators.The strongest bounds turn out to arise from low energy data: For leptoquarks that couple to right-handed quarks, the most stringent bound comes from atomic parity violation. For leptoquarks that couple to left-handed quarks, there are two mass regions: At low masses the bounds arise from atomic parity violation or from universality in leptonic π decays. At masses above a few hundred GeV's the dominant bounds come from the FCNC processes that are unavoidable in these leptoquarks: The FCNC bound of the up sector, that arises from D 0 −D 0 mixing, combines with the FCNC bounds from the down sector, that arise from rare K decays and K 0 −K 0 mixing, to a bound on the flavour conserving coupling to the first generation.The bounds restrict leptoquarks that couple with electromagnetic strength to lie above 600 GeV or 630 GeV for leptoquarks that couple to RH quarks, and above 1040 GeV, 440 GeV, and 750 GeV for the SU(2) W scalar, doublet and triplet leptoquarks that couple to LH quarks. These bounds are considerably stronger than the first results from the direct searches at HERA. Our bounds also already exclude large regions in the parameter space that could be examined by various methods proposed for indirect leptoquark searches.

Naturally light leptoquarks

“…The FCNC bounds are circumvented by demanding that the leptoquarks couple "diagonally", namely, they couple to a single generation in the lepton sector and to a single generation in the quark sector. The diagonality requirement cannot always be fully satisfied [7,8]: In the case of leptoquarks that couple to left-handed quarks, the CKM rotation always induces some nondiagonality in the quark sector, leading to an unavoidable FCNC bound M/g 2 ∼ > few TeV. (iii) Enhancement of the leptonic decays of the pion and other pseudoscalars.…”

“…The search for low-lying leptoquarks is supported theoretically by many beyond-standard models which predict their existence [2]. However, close phenomenological studies show that leptoquarks are troublesome [3][4][5][6][7][8][9]: They can induce proton decay and various FCNC processes, and they can enhance leptonic decays of pseudoscalar mesons by many orders of magnitude. Up to now, these potential problems with leptoquarks have been circumvented simply by demanding that the leptoquarks couplings obey a list of strong constraints.…”

Constraints on leptoquark models from IceCube data

“…These arise in grand unified theories like SO (10), SU (5) or Pati-Salam SU (4) and are expected to have masses in the GUT scale in order to not cause rapid proton decay. Models with low scale leptoquarks have been considered [1][2][3][4] which do not lead to rapid proton decays.…”

“…Leptoquark can be produced, singly or doubly, at colliders (hadron, e + e − or e ± p) and signals (like the jj from the decay of a leptoquark pair) and other important properties and constraints can be studied [8][9][10][11][12][13][14][15][16][17][18][19]. A search for pair-production of first and second generation scalar LQs has been performed with 19.6 fb −1 of data by CMS [20,21].…”

Minireview of Leptoquark Searches

“…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.…”