New bounds on leptoquarks

Abstract

We derive new bounds on scalar leptoquark couplings from K°-K°, D°-D°, and B°'B° mixing. Although leptoquarks contribute to these processes only at one loop, their contribution is large, due to the lack of Glashow-Iliopoulos-Maiani cancellation. Our bounds have two important features: (i) They bound g 4 /M 2 , in contrast to the hitherto known bounds on g 2 /M 2 , and are consequently stronger at high masses, (ii) The bound from D°-D° mixing is the first flavor changing neutral current bound in the up sector for chirally coupled leptoquarks, and is similar in strength to the K°-K° and B°-B° bounds. Together, these bounds strongly constrain any leptoquark that couples to left-handed quarks.

Naturally light leptoquarks

“…We wish however to stress that one is not likely to do better in any model which is natural and supersymmetric. This is due to two reasons: (i) The minimal bound in (5.2) is achieved by fine tuned balance between FCNC processes in the down and up sector [7,8]. Since we are interested in presenting a model which is natural and not fine-tuned, we cannot have such a balance.…”

“…The FCNC bound on s arises from D −D mixing. There are two contributions to this process: One of them arises from a leptoquark-electron box diagram and was already discussed in [7,8]. The other contribution arises from a sleptoquark-selectron loop.…”

“…The Pati-Salam scale bounds are also avoided: the bounds from leptonic π decays is circumvented because nonchiral couplings are very severely suppressed. The bounds from FCNC processes are also avoided because the couplings in the lepton sector are exactly diagonal, in the down quark sector the deviation from diagonality is so small that it can safely be ignored, and in the up quark sector nondiagonal couplings are suppressed by sin θ C , and consequently the FCNC bounds from this sector are at the TeV scale (see [7,8]). Now that we are assured that the bounds on the s leptoquark are at the "unavoidable scale", we go into a more detailed test: We will compare the actual values of the bounds on s in our model with the unavoidable bounds that apply to any leptoquark which is in the (3, 1) 1 3 representation of SU(3) C × SU(2) W × U(1) Y and couples to LH quarks.…”

“…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.…”