Probing dark gauge boson via Einstein-Cartan portal

Abstract

Einstein-Cartan gravity which is an alternative formulation of general relativity introduces new degrees of freedom contained in the torsion field which encodes the torsion feature of spacetime. Interestingly, the torsion field couples to all fermions through its axial-vector mode with a universal coupling $\eta=1/8$ which is possible to change under the quantum effects. We argue that Einstein-Cartan gravity provides a significant portal to probe $A'$ dark gauge boson which resides in dark sector existing as an invisible world parallel to our own and couples to the standard model (SM) particles through only the kinetic mixing. For the (very) small kinetic mixing, searches for the $A'$ from Drell-Yan processes are insensitive due to the suppressed production cross-section and the considerable SM backgrounds. However, through the mediation of torsion field the $pp$ collisions produce dark-sector fermions which would significantly produce the $A'$ due to unsuppressed dark gauge coupling. We explore the potential production modes of the $A'$ through bremsstrahlung off dark-sector fermion and the cascade decays. Einstein-Cartan gravity suggests the torsion mass $\gtrsim\mathcal{O}(4)$ TeV for $\eta$ varying around the classical value since the present scenarios would tend to produce the $A'$ with the high boost and large missing transverse momentum from dark-sector fermions where the SM backgrounds are low. On the other hand, the $A'$ search via Einstein-Cartan portal can reach even for the signal events to be not large and is also sensitive to the (very) small kinetic mixing as long as the decay channels of the $A'$ to dark-sector particles are inaccessible.