High-$p_T$ dilepton tails and flavour physics

Abstract: We investigate the impact of flavour-conserving, non-universal quark-lepton contact interactions on the dilepton invariant mass distribution in $p~p \to \ell^+ \ell^-$ processes at the LHC. After recasting the recent ATLAS search performed at 13 TeV with 36.1 fb$^{-1}$ of data, we derive the best up-to-date limits on the full set of 36 relevant four-fermion operators, as well as estimate the sensitivity achievable at the HL-LHC. We discuss how these high-$p_T$ measurements can provide complementary information to the low-$p_T$ rare meson decays. In particular, we find that the recent hints on lepton flavour universality violation in $b \to s \mu^+ \mu^-$ transitions are already in mild tension with the dimuon spectrum at high-$p_T$ if the flavour structure follows minimal flavour violation. Even if the mass scale of New Physics is well beyond the kinematical reach for on-shell production, the signal in the high-$p_T$ dilepton tail might still be observed, a fact that has been often overlooked in the present literature. In scenarios where new physics couples predominantly to third generation quarks, instead, the HL-LHC phase is necessary in order to provide valuable information.

High-$p_T$ Dilepton Tails and Flavour Physics: An Analytical Overview

This paper investigates the influence of flavour-conserving yet non-universal quark-lepton contact interactions on the dilepton invariant mass distribution in proton-proton collisions, specifically focusing on the process ( p~p \to \ell^+ \ell^- ) at the LHC. This analysis is pivotal given recent observations that suggest possible lepton universality violation, particularly in the process ( b \to s \mu^+ \mu^- ). The research presented sets current bounds on 36 notable four-fermion operators impacting these interactions, utilizing data from a 13 TeV ATLAS search with an integrated luminosity of 36.1~fb(^{-1}). Furthermore, the authors extend this foundation to predict the sensitivity achievable at the High Luminosity LHC (HL-LHC).

Key Numerical Results and Claims

• The study provides stringent constraints on the non-universal contact interactions by analyzing the high-momentum transfer regions (high-(p_T) tails) in dilepton invariant mass distributions.
• The ATLAS data is recast to limit these interactions at the present, setting a benchmark for future assessments at HL-LHC as integrated luminosity advances to 3000 fb(^{-1}).
• There is a noted tension between the hints of lepton flavour universality violation in (b \to s \mu^+ \mu^-) and the dimuon spectrum at high-(p_T), especially if the interactions follow minimal flavour violation structures.
• Notably, even for new physics situated beyond the kinematic scope for on-shell production, detectable signals could still appear within high-(p_T) dilepton tails, an aspect often neglected in existing literature.

Implications for Flavour Physics

The interplay between high energy and precision measurements is crucial when exploring new physics via flavour non-universal processes. The paper highlights that high-(p_T) dilepton tail studies furnish valuable insights, comparable in sensitivity to low-energy flavour experiments. This is particularly relevant when crossing borders between the Standard Model (SM) predictions and the effective field theory (EFT) limits imposed by such new physics scenarios.

Theoretical Considerations and Future Directions

The EFT framework is applied across the momentum transfer spectrum in the LHC's proton collisions, facilitating the exploration of beyond-SM physics via dimension-six operators, utilizing the Warsaw basis. Importantly, this involves dismissing chirality-flipping interactions due to the suppression by lightweight fermion Yukawa couplings, allowing a concentrated assessment on local four-fermion interactions.

The paper further creates a scaffold for examining distinguished flavours by studying deviations within the differential lepton flavour universality (LFU) ratio, offering a fine scale for the precision of mass-dependent assessments.

Conclusion

This comprehensive evaluation establishes foundational constraints on flavour-conserving quark-lepton interactions within high-energy physics, setting a trajectory for future observations at the LHC's HL phase. It accentuates the need to reconsider present models of new physics, ensuring alignment or revision based on emerging data from high-(p_T) measurements. Models adhering to minimal flavour violation face scrutiny, urging a broadened perspective in upcoming theoretical explorations and experimental strategies.