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.

• The paper examines the theoretical and experimental aspects of leptoquarks, focusing on constraints from low-energy precision experiments and signatures at particle colliders.
• Precision experiments constrain leptoquark properties through measurements of observables like atomic parity violation and meson decays, offering model-independent bounds.
• Particle colliders like the LHC search for leptoquarks via direct production and decay signatures, providing complementary limits to those from low-energy probes.

Overview of "Physics of leptoquarks in precision experiments and at particle colliders"

The paper "Physics of leptoquarks in precision experiments and at particle colliders" offers a detailed examination of the theoretical and experimental aspects of leptoquarks (LQs), which are hypothetical particles that mediate interactions between quarks and leptons. The analysis spans various scenarios involving scalar and vector leptoquarks within extensions of the Standard Model (SM) that preserve proton stability and comply with low-energy precision constraints.

Theoretical Background

Leptoquarks are anticipated in several extensions of the SM that endeavor to unify the elementary particle interactions, suggesting a deeper connection between quarks and leptons. Their existence could provide insights into unresolved issues such as neutrino masses and the origin of matter's parity violation.

The paper categorizes LQs based on their spin and interaction properties, detailing the possible scalar ($S_3$, $R_2$, $\tilde{R}_2$, $\tilde{S}_1$, $S_1$, and $\bar{S}_1$) and vector ($U_3$, $V_2$, $\tilde{V}_2$, $\tilde{U}_1$, $U_1$, and $\bar{U}_1$) LQ states. It further explores interactions afforded by these LQs in both the gauge framework of the SM and potential grand unified theories, specifying the impact on proton stability.

Constraints from Low-Energy Experiments

The search for leptoquarks has considerably relied on indirect constraints derived from precision experiments. These include measurements of electric and magnetic dipole moments, atomic parity violation, and the decay rates of mesons. Specific leptoquark couplings can induce significant deviations from SM predictions in these observables, serving as indirect evidence or bounds for their properties.

The study investigates the sensitivity of these measurements to leptoquark interactions, offering a model-independent analysis of constraints. It emphasizes that parameters such as Yukawa couplings can be tightly constrained by data from flavor physics experiments and precision measurements in atomic systems.

Collider Signatures

Leptoquarks can be explored through direct production at particle colliders like the LHC, where their decay products can lead to distinct experimental signatures. The paper assesses the production mechanisms and associated cross-sections at both hadron colliders and proposed lepton-collider scenarios.

Importantly, it discusses how current limits on leptoquark mass and decay channels from collider experiments provide complementary bounds to those derived from low-energy experiments. The direct searches for LQs target typical decay modes that include lepton-quark pairs, with branching ratios that could vary based on specific interaction models.

Theoretical and Practical Implications

The exploration of leptoquarks has implications for both theoretical physics and experimental strategies. Theoretically, finding a leptoquark would indicate physics beyond the SM and provide clues towards grand unification theories. Practically, designing experiments and interpreting data with the potential presence of LQs requires enhanced detectors and innovative data analysis techniques to identify subtle signals amidst extensive backgrounds.

Future Directions

The paper speculates on future developments with respect to both precision measurements and collider experiments. As data accumulation increases, particularly from next-generation colliders and improved precision experiments, the sensitivity to potential leptoquark-induced effects will enhance. This could potentially allow for the exclusion of broad classes of models, or, optimistically, for the discovery of leptoquarks themselves.

In conclusion, the paper presents a comprehensive analysis of the status and prospects concerning the search for leptoquarks, leveraging both theoretical advances and experimental innovations as a concerted approach to understanding their role within particle physics.