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Searching for Leptoquarks at Future Muon Colliders

Pouya Asadi, Rodolfo Capdevilla, Cari Cesarotti, Samuel Homiller

arXiv:2104.05720v1hep-ph

TL;DR

The paper studies whether future high-energy muon colliders can extend leptoquark searches beyond hadron colliders and flavor experiments. It analyzes a U1 vector leptoquark across representative flavor scenarios and three production signatures, finding sensitivity to much heavier masses and to the parameter space relevant for B-meson anomalies.

  • Problem

    Future collider options need their reach and complementarity assessed for motivated leptoquark scenarios beyond existing collider and flavor constraints.

    Source: §1 · pp. 2–3

  • Method

    The study analyzes a U1 vector leptoquark at muon colliders through pair production, single production, and interference with Standard Model Drell–Yan processes across four flavor scenarios.

    Source: §2 · p. 5, §5 · p. 21

  • Results

    DY interference can probe leptoquark masses up to 100 TeV at a 14 TeV muon collider, while the parameter space explaining observed RK(∗) is completely probed at 3 TeV when relevant couplings are comparable.

    Source: §5 · p. 22

  • Takeaways & Limitations

    Muon colliders provide complementary access to leptoquark parameter space beyond direct production at hadron colliders and beyond flavor experiments.

    Source: §5 · p. 22

    The projections neglect detector effects and systematic uncertainties, use an effective photon approximation for single production, and focus on one leptoquark with specific flavor structures.

    Source: §2 · p. 7, §5 · p. 22

Abstract

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A high energy muon collider can provide new and complementary discovery potential to the LHC or future hadron colliders. Leptoquarks are a motivated class of exotic new physics models, with distinct production channels at hadron and lepton machines. We study a vector leptoquark model at a muon collider with $\sqrt{s} = 3, 14$ TeV within a set of both UV and phenomenologically motivated flavor scenarios. We compute which production mechanism has the greatest reach for various values of the leptoquark mass and the coupling between leptoquark and Standard Model fermions. We find that we can probe leptoquark masses up to an order of magnitude beyond $\sqrt{s}$ with perturbative couplings. Additionally, we can also probe regions of parameter space unavailable to flavor experiments. In particular, all of the parameter space of interest to explain recent low-energy anomalies in B meson decays would be covered even by a $\sqrt{s} = 3$ TeV collider.

1 Introduction

Future high-energy colliders are needed to explore motivated BSM scenarios beyond the LHC, and muon colliders offer distinctive sensitivity to particles coupling directly to second-generation leptons. This work studies how a muon collider can probe vector leptoquarks through coupling-sensitive production mechanisms complementary to hadron colliders.

Motivation

  • Muon colliders combine high center-of-mass energies with lepton-collider cleanliness and direct sensitivity to BSM theories involving second-generation leptons.Their non-composite beams use the collision energy efficiently, while reduced strong-interaction backgrounds improve searches for electroweakly coupled states.Source: §1 · pp. 2–3

Collider setup

  • The study evaluates muon-collider reach at 3 and 14 TeV while accounting for beam-induced backgrounds and detector shielding limitations.The benchmark machines use 1 ab−1 at 3 TeV and 20 ab−1 at 14 TeV; shielding nozzles reduce coverage to mitigate beam-induced backgrounds.Source: §1 · p. 3

Leptoquarks

  • Leptoquarks are motivated BSM mediators coupling quarks and leptons, with signatures studied through both low-energy observables and LHC searches.Their phenomenology is particularly relevant for explaining low-energy anomalies such as those in B-meson decays.Source: §1 · pp. 3–4

This work

  • Muon-collider leptoquark production is sensitive to electroweak and leptoquark couplings rather than only color production, enabling direct probes of couplings to muons.The paper uses the U1 vector leptoquark as a proof-of-principle example, with results expected to be qualitatively similar for other spins or gauge representations.Source: §1 · p. 4

Analysis scope

  • The paper compares leptoquark production modes and combines their constraints with flavor-observable and future-collider comparisons.Its structure covers the model and flavor textures, pair and single production, Drell–Yan interference, combined limits, and complementary constraints.Source: §1 · p. 4

2 Model

The paper models the U1 vector leptoquark with a phenomenological interaction parameterization and representative flavor structures, focusing on left-handed couplings. These choices determine production, decay, and relevant backgrounds at a muon collider, while very small couplings introduce hadronization-related uncertainty.

U1 model

  • The study focuses on U1, a vector leptoquark in the (3, 1, 2/3) representation, motivated by Pati–Salam GUTs and B-meson anomalies.U1 has no di-quark interactions and therefore does not induce proton decay in the model described.Source: §2 · p. 5

Interactions

  • The interaction Lagrangian factors out an overall matter coupling gU and allows non-minimal gauge interactions parameterized by κU and ˜κU.The analysis sets gU = 1; fundamental gauge-boson models have κU = ˜κU = 0, while minimal coupling corresponds to both equal to 1.Source: §2 · p. 5

Flavor scenarios

  • Four representative flavor scenarios vary the coupling matrix while treating mLQ and β32_L, the coupling to b¯µ, as free parameters.The scenarios are intended to represent different flavor structures, including patterns motivated by U(2)5 symmetry breaking and phenomenological muon-collider production.Source: Table 1 · p. 6, §2 · p. 6

Flavor assumptions

  • The analysis keeps only real left-handed quark and lepton couplings, neglecting right-handed-neutrino interactions and treating chirality as phenomenologically insensitive for the collider results.Left-handed couplings are sufficient for explaining the recent B-meson anomalies discussed by the paper.Source: §2 · pp. 5–6

Phenomenology

  • The coupling structure controls production, decay, and backgrounds, with b¯µ or b¯τ dominant depending on the scenario and corresponding neutrino–up-quark channels from left-handed interactions.For β33_L = 0, U1 has approximately equal dominant branching ratios to b¯µ and t¯νµ; the analysis emphasizes charged-lepton plus down-type-quark final states.Source: §2 · p. 6

Scope boundary

  • When the total width falls below ΛQCD ∼200 MeV, hadronization can alter decay rates and produce displaced-track topologies beyond the analysis treatment.Results in these small-coupling regimes should therefore be interpreted with caution.Source: §2 · pp. 6–7

3 Production Modes

The paper analyzes pair, single, and Drell–Yan-related leptoquark production at muon colliders across flavor scenarios, emphasizing their coupling dependence and complementary reach. Pair production remains sensitive through electroweak interactions, while single production requires muon couplings and becomes especially useful at higher masses.

Analysis setup

  • The analysis scans flavor scenarios and non-minimal gauge interactions using simulated production channels and statistical estimates, while neglecting systematic uncertainties as a best-case assumption.The study focuses on distinct final states: bbμμ for scenarios 1 and 2, and bbττ for scenarios 3 and 4; it assumes fully efficient tagging.Source: §3 · pp. 7–8

Leptoquark Pair Production

  • Pair-production signals are isolated from Standard Model backgrounds using invariant-mass and centrality cuts, reducing bbμμ backgrounds to O(30) events and bbττ backgrounds to < 1 event.The signal peaks near √s/2 in particle-antiparticle invariant-mass distributions, whereas Standard Model jet pairs are highly collimated.Source: §3.1 · pp. 8–9
  • Electroweak pair production remains visible for m_LQ ≲ √s/2 independently of βij, while larger muon couplings extend the reach beyond the on-shell threshold.For βi2 ≲ 0.2, the pair-production cross section depends only on the leptoquark mass; t-channel production becomes important at larger muon couplings.Source: §3.1 · pp. 8, 10, 12

Leptoquark Single Production

  • Single-production constraints are complementary to pair-production bounds but weaken at high masses and small muon couplings; their reach benefits from the logarithmically enhanced photon flux at larger √s.For leptoquarks too heavy for pair production, the resulting single-production constraints are weaker than the Drell–Yan bounds discussed later.Source: §3.2 · pp. 13–14

4 Combination and Comparison to Flavor Constraints

The combined reach is complementary across pair production, single production, and Drell–Yan interference, with the dominant channel depending on mass and coupling. A 3 TeV muon collider can discover parameter space associated with the B-meson anomalies, while comparisons with future hadron colliders remain incomplete for some channels.

Combined collider reach

  • For masses below √s/2, pair production is the most powerful channel independently of β32_L, while heavier masses favor single production or Drell–Yan interference when β32_L is sufficiently large.Pair production relies only on electroweak gauge couplings, whereas on-shell pair production loses sensitivity above √s/2.Source: §4 · p. 17
  • Drell–Yan interference can probe leptoquark masses far beyond the collider energy for β32_L ≳ 0.1, whereas single production weakens as the mass approaches √s.Single production is more relevant at 14 TeV than at 3 TeV because of logarithmic enhancement in the photon flux.Source: §4 · p. 17

Flavor scenarios

  • The Drell–Yan bounds are identical across flavor scenarios, while pair and single production are slightly stronger for scenarios 3 and 4 because their τb¯b final states have lower backgrounds.The DY channel depends only on the β32_L coupling and is unchanged across the four scenarios.Source: §4 · p. 17, Figure 13 · p. 18

Comparison to other colliders

  • Compared with the LHC, muon-collider Drell–Yan interference reaches smaller couplings because lepton collisions provide higher relative partonic luminosity and the projections omit detector and systematic effects.At future hadron colliders, pair production could set an approximately 10 TeV mass bound, slightly exceeding the 14 TeV muon-collider pair-production reach at small Yukawa couplings.Source: §4 · p. 17

Flavor constraints

  • Even a 3 TeV muon collider can discover leptoquarks in the parameter space favored by fits to the R_K(*) anomalies.The relevant central-value region is shown in the right panels of Figs. 13 and 14 for flavor structures 2 and 4.Source: §4.1 · p. 20
  • Among the listed low-energy observables, none provides stronger constraints than R_K(*), while electroweak-boson decay effects yield substantially weaker bounds.The flavor-observable comparison uses the coupling combinations summarized in Table 2.Source: Table 2 · p. 20, §4.1 · p. 21

5 Conclusions

The study evaluates U1 vector leptoquark searches at muon colliders through pair production, single production, and Drell–Yan interference across several flavor scenarios. Drell–Yan interference can reach far beyond direct production, while a 3 TeV collider can cover parameter space relevant to observed R_K(*) anomalies, subject to optimistic projection assumptions.

  • The analysis studies U1 across four flavor scenarios and three production classes, using simple strategies to derive limits amid Standard Model backgrounds.Source: §5 · p. 21
  • Drell–Yan interference can probe leptoquarks roughly an order of magnitude heavier than the collider energy, whereas pair production reaches masses up to about √s/2.Source: §5 · pp. 21–22
  • The reach of pair and single production depends on the full flavor structure and branching ratios, while Drell–Yan interference constrains an individual leptoquark–fermion coupling.Source: §5 · p. 22
  • 100 TeV: at a 14 TeV muon collider, Drell–Yan interference is sensitive to leptoquark masses up to 100 TeV for perturbative couplings, beyond direct production at a 100 TeV hadron collider.Source: §5 · p. 22
  • A 3 TeV muon collider completely probes the relevant parameter space for explaining observed R_K(*) when the relevant leptoquark couplings are comparable.Source: §5 · p. 22
  • The projections are optimistic because they use the effective photon approximation for single production and neglect detector effects and systematic uncertainties.Source: §5 · p. 22

A The Statistics

The analysis projects exclusion and discovery sensitivities using binned likelihoods for Drell–Yan and cut-and-count methods for pair and single production. Poisson event models and specified assumptions define the test statistics and confidence thresholds.

  • Drell–Yan limits use a binned likelihood over forward η bins, neglecting bin correlations and systematic uncertainties, with each bin modeled by a Poisson distribution.Source: A The Statistics · p. 23
  • The Drell–Yan test statistic compares the LQ prediction μ_i(m_LQ, β) with the Standard Model background b_i in each bin to obtain expected 95% confidence-level exclusions.Source: A The Statistics · p. 23
  • For 5σ discovery, the analysis assumes LQ-model event counts and finds parameters for which the Standard Model is excluded with p = 5.7 × 10^-7.Source: A The Statistics · pp. 23–24
  • Pair and single production use cut-and-count projections based on expected post-selection backgrounds and a Bayesian 95% confidence-level upper limit.Source: A The Statistics · p. 24
  • The event model uses a Poisson probability P(n|s) with expected background b and signal s, giving mean event count s + b.Source: A The Statistics · p. 24

B Modified Gauge Interactions

Modified gauge interactions leave Drell–Yan results unchanged but alter pair- and single-production sensitivities. Their effect on pair production depends strongly on flavor scenario and coupling strength, with destructive interference shaping some reaches.

  • Drell–Yan constraints are unchanged for nonstandard gauge couplings because the relevant interference diagrams depend only on g_U and the flavor spurions.Source: B Modified Gauge Interactions · p. 24
  • Single-production bounds weaken for nonzero κ̃_U, but this modification is not pursued because single production is generally weakly constraining.Source: B Modified Gauge Interactions · p. 24
  • For very large β^32_L, pair-production bounds are unchanged because t-channel pair production is insensitive to gauge couplings.Source: B Modified Gauge Interactions · p. 25
  • For smaller β^32_L, especially in flavor scenario 1, pair-production bounds weaken substantially and κ̃_U = 1.0 fails to reach the √s/2 threshold.Source: B Modified Gauge Interactions · p. 25
  • At intermediate β^32_L in flavor scenario 1, destructive interference between t- and s-channel diagrams makes larger κ̃_U strengthen the bounds.Source: B Modified Gauge Interactions · p. 25
  • In flavor scenario 3, smaller backgrounds reduce sensitivity to modified interactions, although the minimal-coupling reach decreases to roughly √s/3 rather than √s/2.Source: B Modified Gauge Interactions · p. 25
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