Physics of leptoquarks in precision experiments and at particle colliders

Abstract

We present a comprehensive review of physics effects generated by leptoquarks (LQs), i.e., hypothetical particles that can turn quarks into leptons and vice versa, of either scalar or vector nature. These considerations include discussion of possible completions of the Standard Model that contain LQ fields. The main focus of the review is on those LQ scenarios that are not problematic with regard to proton stability. We accordingly concentrate on the phenomenology of light leptoquarks that is relevant for precision experiments and particle colliders. Important constraints on LQ interactions with matter are derived from precision low-energy observables such as electric dipole moments, (g − 2) of charged leptons, atomic parity violation, neutral meson mixing, Kaon, B, and D meson decays, etc. We provide a general analysis of indirect constraints on the strength of LQ interactions with the quarks and leptons to make statements that are as model independent as possible. We address complementary constraints that originate from electroweak precision measurements, top, and Higgs physics. The Higgs physics analysis we present covers not only the most recent but also expected results from the Large Hadron Collider (LHC). We finally discuss direct LQ searches. Current experimental situation is summarized and self-consistency of assumptions that go into existing accelerator-based searches is discussed. A progress in making next-to-leading order predictions for both pair and single LQ productions at colliders is also outlined.

A leptoquark model to accommodate R K exp < R K SM and R K ∗ exp < R K ∗

“…Another set of diagrams, similar to those shown above is obtained by replacing t → c. µγ) < 4.4 × 10 −8 [37], we checked the expression derived in refs. [35,38] with which we agree, and write:…”

“…For a recent review of their properties see ref. [35]. In general, a leptoquark can be a scalar or a vector field and it may come as a SU(2) L -singlet, -doublet or -triplet [36].…”

“…Since the SM estimate of this quantity is not yet fully assessed [39,40], we require the leptoquark contribution to be smaller than 2σ error on ∆a exp µ . To do so we use the expression [35]:…”

Exotic colored scalars at the LHC

“…[84,85] compute the Wilson coefficient of these effective interactions for a general scalar. We present their results in table 6. Note that even though this list is not completely independent when the Higgs and gauge bosons equations of motion are considered, we find it convenient for our purposes to determine the oblique parameters and the Higgs couplings, as long as no redundancy is used when considering physical parameters.…”

“…[6]). Within SU(5) models, SU(2)-triplets are parts of the 35, 45 and 70 representations, and an SU(2) W -quartet is contained in the 70 representation [7].…”