[Paper Review] Observation of structures at $\sim 17$ and $\sim 38$ MeV/c$^{2}$ in the $γγ$ invariant mass spectra in pC, dC, and dCu collisions at $ extit{p}_{lab}$ of a few GeV/c per nucleon

Kh.U. Abraamyan, Ch. Austin+5 more|arXiv (Cornell University)|Nov 30, 2023

High-Energy Particle Collisions Research4 citations

TL;DR

This study reports the observation of enhanced structures at invariant masses of ~17 MeV/c² and ~38 MeV/c² in diphoton invariant mass spectra from pC, dC, and dCu collisions at a few GeV/c per nucleon. Using Monte Carlo simulations and background subtraction, the authors conclude these signals are consistent with the decay of new light bosons—X17 and E38—into two photons, providing independent evidence for these exotic states in a different experimental setup and reaction channel.

ABSTRACT

The results of an analysis of the invariant mass spectra of photon pairs produced in dC, pC and dCu interactions at momenta of 2.75, 5.5 and 3.83 GeV/c per nucleon respectively, are presented. Signals in the form of enhanced structures at invariant masses of about 17 and 38 MeV/c2 are observed. The results of testing of the observed signals, including the results of the Monte Carlo simulation are presented. The test results support the conclusion that the observed signals are the consequence of detection of the particles with masses of about 17 and 38 MeV/c2 decaying into a pair of photons.

Motivation & Objective

• To search for narrow states in the diphoton invariant mass spectrum below the π⁰ mass (135 MeV/c²), particularly at ~17 and ~38 MeV/c².
• To test the hypothesis that the observed enhancements correspond to new light bosons (X17 and E38) decaying into two photons.
• To confirm the existence of these states using a different experimental setup and reaction dynamics (proton and deuteron beams on carbon and copper targets) compared to previous e⁺e⁻-based observations.
• To assess the robustness of the signals against background contributions and systematic effects through detailed Monte Carlo simulations and kinematic cuts.

Proposed method

• Performed high-energy pC, dC, and dCu collisions at 2.75, 5.5, and 3.83 GeV/c per nucleon using the SPHERE (PHOTON-2) two-arm gamma spectrometer at the JINR Nuclotron.
• Measured photon energies and positions in NaI(Tl) detectors to reconstruct the invariant mass of γγ pairs.
• Applied kinematic cuts on the opening angle (θγγ > 7° and >10°) to suppress background and improve signal clarity.
• Subtracted background contributions using Monte Carlo simulations and normalized spectra to the number of pairs in the (21, 33) MeV/c² range.
• Used amplitude spectra from individual detector modules to validate signal consistency and identify instrumental thresholds.
• Conducted systematic error analysis by varying kinematic cuts and comparing experimental and simulated spectra.

Experimental results

Research questions

• RQ1 Are there significant enhancements in the diphoton invariant mass spectrum at ~17 and ~38 MeV/c² in pC, dC, and dCu collisions?
• RQ2 Can the observed signals be attributed to new light bosons (X17 and E38) decaying into two photons, or are they due to background or detector effects?
• RQ3 How robust are the signals under kinematic cuts and background subtraction, and do they remain stable across different beam and target configurations?
• RQ4 Can the observed diphoton signals be consistently explained by the same theoretical framework as the e⁺e⁻-based X17 anomaly?
• RQ5 What is the role of systematic errors and detector response in the observed enhancements, particularly in the low-mass region (7–10 MeV/c²)?

Key findings

• Enhanced structures at ~17 MeV/c² and ~38 MeV/c² are observed in the γγ invariant mass spectra from pC, dC, and dCu collisions at beam momenta of 5.5, 2.75, and 3.83 GeV/c per nucleon.
• The signal at ~17 MeV/c² remains stable under increasing opening angle cuts (7° and 10°), indicating robustness against kinematic biases.
• Monte Carlo simulations support the conclusion that the observed signals are consistent with the decay of particles with masses of ~17 MeV/c² and ~38 MeV/c² into two photons.
• The results provide independent evidence for the X17 and E38 bosons in a different reaction environment (nucleon-nucleus collisions) compared to the original e⁺e⁻ anomaly observation.
• A preliminary indication of a possible enhancement in the 7–10 MeV/c² region is observed, though its physical origin remains uncertain due to potential noise and systematic effects.
• The simultaneous observation of π⁰, X17, and E38 in the same apparatus constrains theoretical models and supports a composite picture of these states as QED-bound q̃q̃ states.