[Paper Review] Leptoquark searches in CMS

Y. Takahashi|arXiv (Cornell University)|Jan 11, 2019

Particle physics theoretical and experimental studies1 references4 citations

TL;DR

This paper presents updated CMS searches for scalar and vector leptoquarks (LQs) in proton-proton collisions at √s = 13 TeV, using 35.9 fb⁻¹ of integrated luminosity. It focuses on second- and third-generation LQs decaying to di-muons, di-taus, or final states with missing transverse energy, employing advanced multivariate and cut-based analyses to set stringent exclusion limits, with second-generation LQs excluded up to 1530 GeV for β=1.0 and third-generation LQs excluded up to 750 GeV at 95% CL, constraining models that explain B-physics anomalies.

ABSTRACT

Leptoquarks (LQs) are hypothetical bosons, each coupling to a lepton and a quark of a given generation. They recently got particular attention, as the "TeV-scale LQ" might explain observed anomalies reported by dedicated B physics experiments. In these proceedings, the recent progress of the direct searches on LQs at CMS experiment is presented. The results are based on a data sample of proton-proton collisions at a center-of-mass energy of 13 TeV, recorded with the CMS detector corresponding to an integrated luminosity of 35.9 fb-1.

Motivation & Objective

• To search for scalar and vector leptoquarks (LQs) in proton-proton collisions at √s = 13 TeV, motivated by anomalies in B-physics decays.
• To test the viability of TeV-scale LQs as a solution to observed B-physics anomalies in B → K*μ⁺μ⁻ decays.
• To improve sensitivity to LQs by reinterpreting existing searches for squarks and gluinos in the context of LQ pair and single production.
• To extend the reach of LQ searches to final states involving top, bottom, and tau quarks, including those with missing transverse energy and b-jets.
• To set 95% confidence level exclusion limits on LQ masses and Yukawa couplings, particularly for models explaining B-physics anomalies.

Proposed method

• The analysis uses a cut-and-count method for the μμjj and μνjj channels, applying optimized kinematic cuts on ST^μμjj, Mμμ, and M^min_μj to enhance signal-to-background ratio.
• For the ννjj, ννbb, and ννtt final states, the search reinterprets a prior squark/gluino analysis, assuming similar acceptance for LQ pair production.
• In the ττtt and ττbb channels, the analysis uses hadronically decaying τ leptons (τh) with isolation and charge requirements, and requires at least one b-tagged jet to enhance top/bottom quark signatures.
• Backgrounds are estimated using data-driven techniques for QCD multijet and simulated samples for t¯t and Z+jets, with a binned maximum likelihood fit used for signal extraction.
• The analysis combines results from multiple final states (eτh, μτh, τhτh, and eμ control region) to improve systematic uncertainty constraints and sensitivity.
• Exclusion limits are set at 95% confidence level for LQ masses and branching fractions β, with sensitivity dependent on the Yukawa coupling λ.

Experimental results

Research questions

• RQ1 What is the sensitivity of CMS to second-generation leptoquarks decaying to di-muons or muon-plus-neutrino plus jets, and what mass limits can be set?
• RQ2 To what extent can reinterpretations of existing squark and gluino searches constrain scalar and vector leptoquark pair production?
• RQ3 How do the limits on third-generation leptoquarks decaying to ττ final states compare with the parameter space preferred by B-physics anomalies?
• RQ4 What are the exclusion limits for singly produced leptoquarks decaying to τ and b quarks, and how do they constrain the Yukawa coupling λ?
• RQ5 How do the combined results from multiple final states improve the overall sensitivity to leptoquark models?

Key findings

• Second-generation scalar leptoquarks with mass below 1530 GeV are excluded at 95% confidence level for β = 1.0, and below 1285 GeV for β = 0.5.
• Third-generation scalar leptoquarks with mass below 750 GeV are excluded at 95% confidence level for λ = 1 and β = 1, based on the ττtt final state.
• The reinterpretation of the squark/gluino search extends constraints on scalar and vector LQ pair production to higher masses, with acceptance similar to squarks.
• The search in the ττbb final state sets limits on LQs decaying to τ and b quarks, improving sensitivity in the 1–2 TeV mass range.
• The observed data are consistent with the Standard Model background in all channels, with no significant excess observed across the LQ mass range studied.
• The combined results begin to constrain the parameter space of scalar leptoquarks that could explain the B-physics anomalies, particularly in the λ ≈ 0.95 ± 0.25 × m_LQ (TeV) region.