Direct photon production as a probe of quarks chromoelectric and chromomagnetic dipole moments at the LHC

Abstract

In this paper we show that the $\gamma+$jet invariant mass distribution in proton-proton collisions at the LHC is significantly sensitive to the quarks chromoelectric ($\kappa$) and chromomagnetic ($\tilde{\kappa}$) dipole moments. It is shown that the presence of $\kappa$ or $\tilde{\kappa}$ leads to an increment of the cross section of $\gamma+$jet process in particular in the tail of $\gamma+$jet invariant mass distribution. Using the measured $\gamma+$jet invariant mass distribution by the CMS experiment at the center-of-mass energy of 8 TeV, we derive bounds on the quarks chromoelectric and chromomagnetic dipole moments. In extraction of the limits, we consider both theoretical and systematic uncertainties. The uncertainties originating from variation of the renomalization/factorization scales and the choice of proton parton distribution functions are taken into account as a function of the $\gamma+$jet invariant mass. We exclude $\kappa$ or $\tilde{\kappa}$ above $10^{-5}$ at 95$\%$ confidence level. This is the most stringent direct upper limit on $\kappa$ or $\tilde{\kappa}$.