Atomic parity violation and the HERA anomaly

We show that the two scenarios able to explain the Hera anomaly — a new leptoquark coupling or a new contact interaction — predict new contributions to atomic parity violation. These corrections are sufficiently large and different that a feasible reduction in the dominant atomic theory uncertainty could give some hint in favour of one of the two scenarios. 1 The excess of events at large Q observed at Hera [1] could be due to new physics. In this case two different mechanisms can account for the Hera anomaly without contradicting present experimental bounds: • an effective four-fermion interaction [2, 3], related to a new physics scale or produced by an exchange of a particle in the t-channel; • the resonant s-channel exchange of a particle with ‘leptoquark’ couplings [3, 4, 5], that would give a characteristic peaked distribution for the invariant mass of the final states. The aim of this work is to show that the new physics would give a non negligible predictable extra contribution to atomic parity violation (APV) in both cases. This happens as follows. In the first scenario, the new contact interaction does not contribute resonantly to the Hera process, and affects significantly other physical observables. In particular, the correction to APV is the most interesting one: if a single new operator should explain the Hera anomaly, the correction to APV would be (10 ÷ 20) times larger than the present error on its determination. It is thus necessary to assume the existence of different contact operators which individually give a too large contribution to APV, but related in such a way that the total contribution cancels. Such cancellation, at tree-level, can be justified invoking appropriate global symmetries [6]. As discussed in section 2, however, radiative corrections from the standard gauge interactions upset the cancellation giving a predictable correction to APV.