CMS observes spin-dependent asymmetry in heavy-ion

The CMS experiment at the Large Hadron Collider has measured a significant spin-dependent asymmetry in the decay of particles produced in ultra-relativistic heavy-ion collisions. The analysis is based on data from lead-lead collisions at a nucleon-nucleon center-of-mass energy of 5.36 TeV, corresponding to an integrated luminosity of 1.26 inverse nanobarns.

Researchers quantified the azimuthal asymmetry of muons from the decay of coherently photoproduced J/ψ mesons with respect to the second-order event plane. The asymmetry is characterized by the amplitude A₂ of the second-order harmonic modulation. The collaboration observed a negative A₂ value of -0.194, with statistical and systematic uncertainties of ±0.022 and ±0.018, respectively. This result has a significance exceeding five standard deviations.

Measurement details and theoretical context

The measurement represents the first observation of this effect with such high statistical confidence. According to the CMS collaboration, the A₂ values show no significant dependence on the J/ψ meson's rapidity or on the degree of overlap between the colliding lead ions. A theoretical model in which the coherently photoproduced J/ψ meson inherits the linear polarization of the incident photon is consistent with the collected data.

This finding provides a direct observation of spin dependence in the azimuthal asymmetry of decay products from coherently photoproduced vector mesons. The result gains further context when considered alongside recent measurements of coherent ρ⁰ meson photoproduction, which decays to π⁺π⁻ pairs, in ultraperipheral collisions.

Implications for reaction plane determination

The observed correlation between the muon asymmetry and the event plane suggests a potential new method for determining the reaction plane in future heavy-ion collision experiments. The CMS collaboration notes that this correlation "may enable a novel approach to reaction plane determination in the future."

The analysis specifically studied coherent photoproduction, a process where the entire nucleus participates in the photon interaction without being disrupted. This differs from incoherent processes where individual nucleons interact. The measurement was performed using the full capabilities of the CMS detector to track muon pairs from J/ψ decays.

Broader significance in nuclear physics

This result advances the understanding of photon-induced processes in the strong electromagnetic fields generated by ultra-relativistic heavy ions. The successful measurement of the spin-dependent asymmetry provides a new observable for testing quantum chromodynamics in the color-transparent regime. It also offers a fresh perspective on the polarization transfer from photons to vector mesons within the extreme environment of a quark-gluon plasma.

The work, submitted to the arXiv preprint server by the CMS Collaboration, contributes to a growing body of evidence detailing the complex interplay between electromagnetic and strong forces in high-energy nuclear collisions. The collaboration's report concludes by framing the discovery within the broader experimental program studying photoproduction phenomena at the LHC.