A new anomaly observed in 4^4He supporting the existence of the hypothetical X17 particle

Recently, we observed an anomalous peak-like excess of internal $e^+e^-$ pairs at around 140° for the M1 transition depopulating the 18.15 MeV isoscalar $1^+$ state in $^8$Be. The deviation from the theoretical prediction can be described by GEANT simulations assuming the creation and subsequent decay of a new, light boson with a mass of 16.7 MeV/$c^2$. In order to reduce the possible systematic errors from the experimenntal data, we re-investigated the $^8$Be anomaly with an improved setup and confirmed the anomaly within the statistical uncertainties. We also studied the angular correlation of the electron-positron pairs created in the M0 transition depopulating the 21.01 MeV 0$^-$ state in $^4$He, and observed an anomalous excess of $e^+e^-$ pairs at a significantly smaller angle of 115°. Since the transition energy was higher in this case, the observed anomaly could be described by assuming the creation and subsequent decay of the same light particle in the simulations

Confirmation of the existence of the X17 particle

In a 2016 paper, an anomaly in the internal pair creation on the M1 transition depopulating the 18.15 MeV isoscalar 1+ state on 8Be was observed. This could be explained by the creation and subsequent decay of a new boson, with mass mXc2 = 16.70 MeV. Further experiments of the same transition with an improved and independent setup were performed, which constrained the mass of the X17 boson (mXc2) and its branching ratio relative to the γ-decay of the 8Be excited state (BX), to mXc2 = 17.01(16) MeV and BX = 6(1) 10−6, respectively. Using the latter setup, the e+e− pairs depopulating the 21 MeV Jπ = 0− 0+ transition in 4He were investigated and a resonance in the angular correlation of the pairs was observed, which could be explained by the same X17 particle, with mass mXc2 = 16.98 ± 0.16(stat) ± 0.20(syst) MeV

Improving constraints on gluon spin-momentum correlations in transversely polarized protons via midrapidity open-heavy-flavor electrons in p↑+pp^{\uparrow}+p collisions at s=200\sqrt{s}=200 GeV

Polarized proton-proton collisions provide leading-order access to gluons, presenting an opportunity to constrain gluon spin-momentum correlations within transversely polarized protons and enhance our understanding of the three-dimensional structure of the proton. Midrapidity open-heavy-flavor production at $\sqrt{s}=200$ GeV is dominated by gluon-gluon fusion, providing heightened sensitivity to gluon dynamics relative to other production channels. Transverse single-spin asymmetries of electrons and positrons from heavy-flavor hadron decays are measured at midrapidity using the PHENIX detector at the Relativistic Heavy Ion Collider. These charge-separated measurements are sensitive to gluon correlators that can in principle be related to gluon orbital angular momentum via model calculations. Explicit constraints on gluon correlators are extracted for two separate models, one of which had not been constrained previously

Transverse momentum dependent forward neutron single spin asymmetries in transversely polarized p+pp+p collisions at s\sqrt {s} = 200 GeV

International audienceIn 2015, the PHENIX collaboration has measured very forward (η>6.8) single spin asymmetries of inclusive neutrons in transversely polarized proton-proton and proton-nucleus collisions at a center of mass energy of 200 GeV. A previous publication from this dataset concentrated on the nuclear dependence of such asymmetries. In this measurement the explicit transverse momentum dependence of inclusive neutron single spin asymmetries for proton-proton collisions is extracted using a bootstrapping unfolding technique on the transverse momenta. This explicit transverse momentum dependence will help improve the understanding of the mechanisms that create these asymmetries