Measuring | V t d | at the LHC

Abstract

We propose a direct measurement of the Cabibbo-Kobayashi-Maskawa element Vtd at the LHC. Making use of the imbalance between d and d quark content in the proton, we show that a nonzero Vtd induces a charge asymmetry in the tW associated production. The main backgrounds to this process - tt production, and tW-associated production mediated by Vtb - give charge-symmetric contributions at leading order in QCD. Therefore, using specific kinematic features of the signal, we construct a charge asymmetry in the dilepton final state which - due also to a reduction of systematic uncertainties in the asymmetry - is potentially sensitive to Vtd suppressed effects. In particular, using signal and background simulations up to the detector level, we show that this new observable could already improve the current direct upper bound on |Vtd| with existing LHC data. We also project that |Vtd| values down to ∼10 times the standard model prediction could be probed at the high-luminosity LHC.