Isospin-violating dark matter at the LHC

Abstract

We consider a toy model of dark matter with a gauge singlet Dirac fermion that has contact interactions to quarks that differ for right-handed up and down quarks. This is motivated by the isospin-violating dark matter scenario that was proposed to reconcile reported hints of direct dark matter detection with bounds from nonobservation of the signal in other experiments. We discuss how the effects of isospin violation in these couplings can be observed at the LHC. By studying events with large missing transverse momentum (E T ), we show that the ratio of monophoton and monojet events is sensitive to the ratio of the absolute values of the couplings to the up and down quarks, while a dedicated study of dijet plus E T events can reveal their relative sign. We also consider how our results are modified if, instead of a contact interaction, a particle that mediates the interaction is introduced. Our methods have broad applicability to new physics that involves unequal couplings to up and down quarks.

Null radiation zone at the LHC

“…The theorem predicts that, when the conditions eq. (5,6) are nearly satisfied, the cross section for uu → uuγ subprocess in pp collisions is strongly suppressed compared to the cross section estimated without the interference terms. On the other hand, the cross section for uū → uūγ subprocess in pp collisions is expected to be enhanced compared to the estimate without the interferene terms, because the up and anti-up quarks have opposite electric charges and hence the contributing diagrams have constructive interference.…”

“…In other words, when the conditions eqs. (5,6,7) are satisfied, the diagrams of Figure 1 have maximal destructive interference and their amplitudes sum up to zero. Consequently, if Q a = Q 1 and Q b = Q 2 hold, but Q a and Q b are not necessarily equal, the sum of the amplitudes is proportional to (Q a − Q b ) and the cross section to (Q a − Q b ) 2 in the kinematic region satisfying eqs.…”

“…[4]. The null radiation zone theorem in its general form [1,5] is described as follows. Consider the following process in which an Abelian vector current, V , is emitted:…”

“…Consequently, if Q a = Q 1 and Q b = Q 2 hold, but Q a and Q b are not necessarily equal, the sum of the amplitudes is proportional to (Q a − Q b ) and the cross section to (Q a − Q b ) 2 in the kinematic region satisfying eqs. (5,6,7). Were it not for interference terms between the left two and the right two diagrams of Figure 1, the cross section would be given by Q 2 a + Q 2 b .…”

“…We thus notice that, in the region satisfying eqs. (5,6,7), destructive interference occurs when Q a and Q b take the same sign and constructive interference occurs when they take the opposite signs.…”