Origin of the high-asymmetry excess

Determine the origin of the rise in the corrected top–bottom asymmetry distributions above 0.85 observed in atmospheric-argon and underground-argon data but absent from the detector simulation.

Background

The analysis compares the corrected top–bottom asymmetry distributions of atmospheric-argon data, underground-argon data, and simulated 39Ar events. An excess is observed in both data sets for corrected asymmetry values above 0.85, whereas the simulation does not reproduce this feature. The authors conservatively treat the difference between the atmospheric-argon and underground-argon data in this region as a systematic uncertainty in the measured 39Ar specific activity, but do not identify the physical source of the excess.

References

We also noticed a rise in the AAr and UAr distributions above 0.85 for $A{\mathrm{corr}_{\mathrm{TB}$. The origin of this rise is not fully understood and is not observed in the simulation.

— Measurement of the $^{39}$Ar specific activity in atmospheric argon with DArT at the Canfranc Underground Laboratory  (2609.39400 - Agnes et al., 30 Sep 2026) in Section 4, subsection “Detector response with AAr and UAr,” paragraph discussing the corrected top–bottom asymmetry distribution