Mono-chromatic single photon events at the muon collider

TL;DR

This paper explores the potential of a muon collider to detect signals of dark sector particles via mono-chromatic photon events, demonstrating sensitivity to very high energy scales of new physics.

Contribution

It introduces the use of mono-chromatic photon signatures at muon colliders to probe portal operators coupling the Standard Model to dark sector particles at unprecedented energy scales.

Findings

Detection of effective scales up to 375 TeV for axion-like particles.

Detection of effective scales up to 459 TeV for dark photons.

Signal distinguishability between pseudo scalar and spin-1 particles with about 500 events.

Abstract

The cross section for lepton pair annihilation into a photon and a dark photon or an axion-like particle is constant for large center-of-mass energies because some of the portal operators coupling Standard Model and dark sector are proportional to the energy. Feebly coupled though they are, these portal operators will be enhanced by the large center-of-mass energy made available by a muon collider and thus provide the ideal example of possible physics beyond the Standard Model to be studied with such a machine. We discuss the characteristic signature of the presence of these operators: mono-chromatic single photon events for the two benchmarks of having center-of-mass energies of 3 and 10 TeV and integrated luminosity of, respectively, 1 and 10 ab$^{-1}$. We find that an effective scale of the portal operator as large as $\Lambda=112$ TeV for an axion-like particle and $\Lambda=141$ TeV for a dark photon can be separated from the background with a confidence level of 95% in the first benchmark; these interaction scales can be raised to $\Lambda=375$ TeV and $\Lambda=459$ TeV in the case of the second benchmark. The signal for the pseudo scalar particle can be distinguished from that of the spin-1 with about 500 events. The response of the detector to high-energy photons is examined.