Searching for New Particles Hidden under Known Resonances: A Heavy-Ion Diagnostic

TL;DR

This paper proposes using heavy-ion collision observables like $R_{AA}$ and $v_2$ to detect hidden new particles that are nearly degenerate with known resonances, addressing a potential loophole in traditional searches.

Contribution

It introduces a model-independent two-component framework and sensitivity maps to identify or constrain hidden states in heavy-ion collision data, especially near quarkonium peaks.

Findings

Heavy-ion observables can reveal hidden states under known resonances.

A two-component model helps distinguish signals from backgrounds.

Sensitivity maps guide experimental measurements to detect hidden particles.

Abstract

Searches for new physics typically rely on proton-proton collisions, where isolated mass bumps are the primary signatures. However, when a new particle is nearly degenerate in mass with a known Standard Model resonance, it can be partially or fully absorbed into the primary signal template. We investigate this generic loophole by proposing that heavy-ion collisions can provide complementary diagnostics for such hidden states. By utilizing observables sensitive to the quark-gluon plasma, such as the nuclear modification factor ($R_{AA}$) and elliptic flow ($v_2$), a hidden component can manifest as correlated biases in the extracted kinematics. We formulate a model-independent two-component framework, focusing on quarkonium peaks and using the $\Upsilon(1S)$ mass region as a concrete stress-test example. The same template-level issue can in principle arise in other precision dimuon resonances, including electroweak channels such as $Z\to\mu^+\mu^-$, although those cases involve different intrinsic widths, backgrounds, and systematic uncertainties. Treating the mass resolution as an experimental nuisance parameter, we present sensitivity maps identifying the measurements required to constrain or reveal such hidden components.