Search for Low-Mass Leptoquarks using the $llqq$ final states in Pb-Pb Ultra-Peripheral Collisions

TL;DR

This paper proposes a novel search method for low-mass leptoquarks in Pb-Pb ultra-peripheral collisions, leveraging high photon flux and low background, to improve detection prospects and set exclusion limits.

Contribution

It introduces a model-independent search strategy for low-mass leptoquarks in heavy-ion collisions, expanding the experimental approaches beyond traditional proton-proton collision methods.

Findings

100 GeV scalar LQ excluded at 95% CL with 4 pb$^{-1}$ data

High photon flux enhances low-mass LQ production rates

Method applicable to high-mass LQ searches in proton-proton collisions

Abstract

After a review over past experiments and theoretical requirements from low-energy flavour physics, we argue that the possibility of low-mass leptoquarks (LQ) cannot be fully excluded due to the assumptions made in the measurements. Therefore we propose to search for the pair production of low-mass leptoquarks in Pb-Pb ultra-peripheral collisions with the least model dependence (assuming a small coupling constant $\lesssim0.1$). There are a couple of advantages: 1) high photon flux provides high production rate for low mass LQs; 2) the background contamination is much lower than that in $p$-$p$ collisions. The analysis strategy permits a leptoquark to decay to all possible lepton-plus-quark modes. Taking the scalar LQ, $S_3$, with an electric charge $|q|=\frac{4}{3}e$, as example, the mass point of 100 GeV can be excluded at the 95~\% confidence level using a dataset of 4~pb$^{-1}$ Pb-Pb ultra-peripheral collisions at $\sqrt{s}=5.02$ TeV and the performance of the ATLAS detector in Run~2. The proposed method also applies to searching for high-mass LQs in $p$-$p$ collisions as long as the LQ pair production mechanism dominates.