Search for Sub-GeV Axion-Like Particles at EBES Pilot Run Using 4 GeV Positron Beam at KEK LINAC

TL;DR

This paper reports a search for sub-GeV axion-like particles using a 4 GeV positron beam at KEK LINAC, setting new limits in parameter space with no observed events.

Contribution

First search for sub-GeV ALPs at KEK LINAC using a positron beam and a beam dump setup, extending experimental constraints.

Findings

No events observed in the signal region.

Derived 90% CL upper limits on ALP coupling strength.

Extended the parameter space coverage beyond previous experiments.

Abstract

We report the results of a search for sub-GeV axion-like particles (ALPs) using pilot run data from the Electron Beam-dump Experiment at KEK LINAC Switching Yard 3 (EBES). The data were collected in December 2023 with a 4 GeV positron beam and correspond to $1.3\times10^{14}$ positrons on target. In the pilot run setup, a tungsten beam dump and a single PbO calorimeter were used. We consider ALP production via the Primakoff process induced by bremsstrahlung photons in the beam dump, followed by the decay $a\to\gamma\gamma$. The background was estimated with a data-driven method, and a signal region was defined such that the expected background yield is below 0.1 events. No events were observed after unblinding. Upper limits at the 90% confidence level were derived in the ALP mass-coupling plane, extending the experimental coverage into a region of parameter space not explored by previous searches.