Isolating chirality-breaking SMEFT operators with Drell-Yan angular analysis

TL;DR

This work develops a high-precision, beyond-interference strategy to isolate chirality-breaking SMEFT operators in Drell–Yan by exploiting Lam–Tung violation in the lepton-pair angular distribution. The authors show that dipole and scalar/tensor four-fermion operators contribute to the angular observable $A_0 - A_2$ in a way that grows with energy and does not interfere with the SM, allowing a clean probe of these operators via transverse-momentum and invariant-mass analyses. Using precise SM predictions at NNLO for DY and analytic SMEFT contributions, they project 95% CL bounds on several Wilson coefficients for LHC Run 3 and HL-LHC scenarios, highlighting that angular observables provide an independent handle that helps break degeneracies with chirality-preserving operators. The study demonstrates the complementary value of angular analyses alongside differential cross-sections in global SMEFT fits and informs future experimental analyses targeting chirality-breaking directions in the EFT parameter space.

Abstract

We present a comprehensive strategy to isolate the effect of a class of chirality-breaking interactions in the Standard Model Effective Field Theory (SMEFT) by exploiting Drell-Yan angular analysis and the violation of the Lam-Tung relation. Unlike most SMEFT interpretation of Drell-Yan measurements, dominated by growing-with-energy effects generated by the interference of SMEFT-induced and SM amplitudes, this method isolates operators that contribute only quadratically in the Wilson coefficients, allowing for an independent probe of non-interfering operators. Denoting with $v$ the electroweak vev, with $\sqrt{s}$ the center-of-mass energy, and with $Λ$ the scale of new physics, the non-interfering contributions to the amplitude generated by the chirality-breaking operators can be proportional to $v\sqrt{s}/Λ^{2}$ or $s/Λ^{2}$. We argue that these two classes can be further distinguished by analyzing the angular observables of the lepton pair in the transverse momentum and in the invariant mass distribution of the lepton pair. We therefore present an analysis of the lepton-pair angular observables in both these distributions. Based on a precise estimate of the Standard Model contribution to the relevant observables for the $pp\to l^{+}l^{-}+X$ process up to $O(α_{S}^{2})$, we present realistic projections for the sensitivity of the LHC with $300$ fb$^{-1}$ and for the HL-LHC with $3$ ab$^{-1}$ to chirality-breaking interactions, demonstrating that angular observables provide an independent and clean handle on SMEFT effects, especially in regions where the Standard Model contribution is naturally suppressed thanks to the Lam-Tung relation. This analysis becomes crucial to go beyond single parameter global fits, since it helps breaking degeneracies with chirality preserving operators and to disentangle overlapping directions in the EFT parameter space.

Figures (8)

• Figure 1: Standard Model prediction for the $p_{T}^{\ell\ell}$ (left panel) and $m_{\ell\ell}$ (right panel) spectra in the DY process at the LHC at $13$ TeV with $300$ fb$^{-1}$. Uncertainties are too small to be visible, so the result is visually the same for both $300$ fb$^{-1}$ and $3$ ab$^{-1}$ of integrated luminosity. The exact numerical values (both central values and uncertainties) corresponding to $300$ fb$^{-1}$ and $3$ ab$^{-1}$ of integrated luminosity are given in Tables \ref{['tab:ptxsdistributions']} and \ref{['tab:mllxsdistributions']} of Appendix \ref{['app::Tables']}.
• Figure 2: Standard Model prediction for the $p_{T}^{\ell\ell}$ dependence of the angular observables $A_{0}$ (upper left), $A_{2}$ (upper right), and $A_{0}-A_{2}$ (lower left) in the DY process at the LHC at $13$ TeV with $300$ fb$^{-1}$ and $3$ ab$^{-1}$ both inclusive in rapidity. The lower right panel gives a combined view of the result. The numerical values of the observables appearing in the plots are given in Tables \ref{['tab:ptangcoefficientsA0']}, \ref{['tab:ptangcoefficientsA2']}, and \ref{['tab:ptangcoefficientsA0mA2']} of Appendix \ref{['app::Tables']}.
• Figure 3: Standard Model prediction for the $m_{\ell\ell}$ dependence of the angular observables $A_{0}$ (upper left), $A_{2}$ (upper right), and $A_{0}-A_{2}$ (lower left) in the DY process at the LHC at $13$ TeV with $300$ fb$^{-1}$ and $3$ ab$^{-1}$ both inclusive in rapidity. The lower right panel gives a combined view of the result. The numerical values of the observables appearing in the plots are given in Tables \ref{['tab:mllangcoefficientsA0']}, \ref{['tab:mllangcoefficientsA2']}, and \ref{['tab:mllangcoefficientsA0mA2']} of Appendix \ref{['app::Tables']}.
• Figure 4: Comparison between the contribution of the different SMEFT operators to the transverse momentum (first row) and invariant mass distribution (second row) of the $A_0-A_2$ observable. The left column shows scalar/tensor four-fermion operators, the right one shows the dipole operators. For each distribution, the Wilson coefficients are fixed equal to the largest positive value consistent with the bounds from the corresponding kinematic distribution at $3000\text{ fb}^{-1}$ (Table \ref{['tab:pT_bounds']} for $p_{T}^{\ell \ell}$ and Table \ref{['tab:mll_bounds']} for $m_{\ell \ell}$).
• Figure 5: Comparison between the 95% CL boundaries obtained looking at the $A_0-A_2$ observable (blue) and the cross-section (orange) in the $p_{T}^{\ell\ell}$ distribution. Left: 300 fb$^{-1}$. Right: 3000 fb$^{-1}$