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

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 $\Lambda$ the scale of new physics, the non-interfering contributions to the amplitude generated by the chirality-breaking operators can be proportional to $v\sqrt{s}/\Lambda^{2}$ or $s/\Lambda^{2}$. 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 $O(\alpha_{S}^{2})$, 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.