[Paper Review] Observation of the X17 anomaly in the decay of the Giant Dipole Resonance of $^8$Be

A. Krasznahorkay, A. Krasznahorkay+7 more|arXiv (Cornell University)|Aug 12, 2023

Nuclear physics research studiesPhysics and Astronomy3 citations

TL;DR

This paper reports the first observation of the X17 particle in the decay of the Giant Dipole Resonance (GDR) in $^8$Be, using angular correlation measurements of $e^{+}e^{-}$ pairs from the $^7$Li($p$, $\gamma$) $^8$Be reaction at 4.0 MeV proton energy. The data show anomalous excesses at ~120° and >140°, well described by the creation and decay of a hypothetical 16.95 ± 0.48 MeV/c² particle, consistent with the X17 boson previously observed in $^8$Be and $^4$He transitions.

ABSTRACT

Angular correlation spectra of $e^+e^-$ pairs produced in the $^{7}$Li($p$,$γ$)$^{8}$Be nuclear reaction were studied at a proton beam energy of $E_p$~=~4.0~MeV, which corresponds to the excitation energy of the Giant Dipole Resonance (GDR) in $^8$Be. The spectra measured show a peak like anomaly at 120$^\circ$ and a broader anomaly also above 140$^\circ$. Both anomalies could consistently be described by assuming that the same hypothetical X17 particle was created both in the ground-state transition and in the transition going to the broad ($Γ$=1.5~MeV), first excited state in $^8$Be. The invariant mass of the particle, which was derived to be $m_Xc^2 = 16.95 \pm 0.48$(stat.)~MeV, agrees well with our previously published values.

Motivation & Objective

• To investigate the existence of the X17 particle in the decay of the Giant Dipole Resonance (GDR) in $^8$Be.
• To test whether the X17 anomaly, previously observed in $^8$Be and $^4$He transitions, also appears in the E1-decay of the GDR, a broader resonance with $J^\pi = 1^-$.
• To determine the invariant mass of the X17 particle in this new decay channel and assess its consistency with prior measurements.
• To evaluate the spin/parity assignment of the X17 particle by observing its decay in a different nuclear transition with distinct multipolarity and width characteristics.

Proposed method

• Performed the $^7$Li($p$, $\gamma$) $^8$Be reaction at 4.0 MeV proton energy to excite the GDR in $^8$Be.
• Used a simplified spectrometer with two particle telescopes at 110° to measure $e^{+}e^{-}$ pair angular correlations.
• Measured $\gamma$-ray spectra with a LaBr3 detector to confirm GDR transitions to the ground state and first excited state ($2_1^+$).
• Fitted the $e^{+}e^{-}$ angular correlation data using a combination of internal pair creation (IPC) and signal contributions from X17 decay, modeled via RooFit.
• Simulated probability density functions (PDFs) for E1 and M1 transitions (IPC) and for X17 decay to the ground state and $2_1^+$ state as a function of X17 mass (10–18 MeV/c²).
• Applied a fitting function (INT) that included background (E1, M1) and signal (X17) contributions, with free parameters for signal amplitude and X17 mass, while fixing the X17 mass to be the same in both decay channels.

Figure 1: Tipical $\gamma$ -ray spectrum measured for the 7 Li( $p$ , $\gamma$ ) 8 Be nuclear reaction at $E_{p}$ = 4.0 MeV

Experimental results

Research questions

• RQ1 Does the X17 anomaly manifest in the E1 decay of the GDR in $^8$Be, a broad resonance with $\Gamma = 5.3$ MeV?
• RQ2 Can the observed $e^{+}e^{-}$ angular correlation excess at ~120° and >140° be explained by the decay of a single hypothetical X17 particle?
• RQ3 Is the invariant mass of the X17 particle in the GDR decay consistent with previous measurements in $^8$Be and $^4$He transitions?
• RQ4 What is the relative branching ratio of X17 production in GDR decays to the ground state versus the $2_1^+$ excited state?
• RQ5 Does the observation in a different nuclear transition support the vector or axial-vector nature of the X17 particle?

Key findings

• Anomalous excesses in the $e^{+}e^{-}$ angular correlation were observed at approximately 120° and above 140°, indicating the presence of a new particle.
• The data were well described by a fit including internal pair creation (IPC) and a signal component from the decay of a hypothetical X17 particle.
• The invariant mass of the X17 particle was measured to be $m_Xc^2 = 16.95 \pm 0.48$ MeV (statistical uncertainty), consistent with previous results.
• The intensity ratio of X17 decays to the GDR ground state versus the $2_1^+$ state was found to be $\alpha_{\text{ground}}/(1 - \alpha_{\text{ground}}) = 0.08 \pm 0.19$, consistent within $1\sigma$ with the $\gamma$-ray intensity ratio of 0.18 ± 0.02.
• The significance of the fit, including the X17 signal, exceeded 10$\sigma$, indicating a highly significant observation.
• The observation of X17 in the E1-decay of the GDR supports its interpretation as a vector or axial-vector boson, as the decay pattern matches expectations for such a state.

Figure 2: Experimental acceptance of the spectrometer as a function of correlation angle ( $\theta$ ) for consecutive, uncorrelated e + e - pairs (red line histogram) compared with the results of the MC simulations (blue line histogram) as explained in the text.