Probing Invisible Vector Meson Decays with NA64 and LDMX

TL;DR

This paper investigates how electron fixed target experiments like NA64 and LDMX can detect invisible vector meson decays via photoproduction processes, significantly improving constraints on dark matter models and mediators.

Contribution

It introduces a novel detection channel through vector meson decays in fixed target experiments, enhancing sensitivity to dark matter interactions beyond traditional methods.

Findings

Existing NA64 data constrains invisible vector meson decays.

Future LDMX runs could improve constraints by up to 5 orders of magnitude.

Sensitivity to thermal relic dark matter with mass > 0.1 GeV is significantly increased.

Abstract

Electron beam fixed target experiments such as NA64 and LDMX use missing energy-momentum to detect the production of dark matter and other long-lived states. The most studied production mechanism is dark Bremsstrahlung through a vector mediator. In this work, we explore a complementary source of missing energy-momentum signals: Bremsstrahlung photons can convert to hard vector mesons in exclusive photoproduction processes, which then decay to dark matter or other invisible particles, such as neutrinos. We find that existing NA64 data can improve the leading constraints on invisible light vector meson decays, while a future run of LDMX could improve them by up to $5$ orders of magnitude. For the examples of a dark photon and a $U(1)_B$ gauge boson mediator, accounting for meson decays substantially enhances these experiments' sensitivity, especially to thermal relic dark matter of mass $m_\chi \gtrsim 0.1\ \mathrm{GeV}$.