Two significant anomalies have been recently observed in the emission of electron-positron
pairs in the 7Li(p,e− e+)8Be and 3H(p,e− e+)4He reactions [1, 2]. These anomalies have
been interpreted as the signature of the existence of a boson (hereafter referred to as X17)
of mass M_X17 = 16.8 MeV that could be a mediator of a fifth force, characterised by a
strong coupling suppression of protons compared to neutrons (protophobic force). Beyond
the importance of such a discovery - if confirmed -, this scenario could explain, at least
partially, the long-standing (recent) anomaly on the muon (electron) magnetic moment.
More in general, the possible existence of a new particle is of paramount importance in
particle physics and in cosmology (dark matter). Therefore, the ATOMKY claim [1, 2],
clearly calls for new experimental studies.
We are carrying on an experiment at n_TOF, where the excited levels of 4He, 8Be
can be populated via the conjugated 3He(n,e− e+)4He and 7Be(n,e− e+)8Be reactions. This
approach has two relevant advantages: (i) for the first time X17 existence is investigated
through neutron induced reactions; and (ii) the experimental setup is completely different
with respect to the one used by the ATOMKY group. More in detail, the ATOMKI
experimental setup used for the 3H(p,e− e+)4He reaction (a very similar one was used
for the other reaction) consists of a tritium target adsorbed on Ti layer, bombarded
with a 900 keV proton beam with a current of about 1 μA.
The main limitations of ATOMKI measurement are: (i) a monochromatic beam of 900 keV, i.e. no information about the X17 production at different energies is available; (ii) no tracking and vertex
recognition, (iii) only particles produced orthogonally to the beam line are detected; (iv)
no charge and particle identification, i.e. the ejectiles are only deduced to be e− e+ pairs.
Our approach aims to realise a suited detection setup for the determination of particle
kinematics and able to discriminate particles, i.e. the reaction ejectiles (assumed to be e− e+ pairs) in a wide energy range. If the existence of X17 is confirmed, with the here-
proposed experimental setup it will be possible to establish quantum numbers and mass of the X17 boson, and to shed light on the so-called protophobic nature of a fifth force. In fact, state-of-the-art ”ab-initio” calculations are in good agreement with present literature data (in particular for the ”few body” 4He nucleus) and would provide quantitative
predictions to establish the X17 nature, e.g. if it is a scalar, pseudoscalar, vector or axial boson and to get information on the interaction of the X17 boson with quarks and gluons.
The study of the 3He(n,e− e+)4He and 7Be(n,e− e+)8Be reaction can be performed at the EAR2 station of the n_TOF facility at CERN. In fact, the facility provides a pulsed neutron beam in a wide energy range, which broadly covers the region of interest for this experiment, i.e. 10^3 < E_n(eV) <10^7 . In addition, count-rate estimations have demonstrated that the neutron intensity at EAR2 is high enough to carry on a conclusive experiment within about 1 month of measurement. We present and discussthe project of this measure at EAR2: the detection setup and the final goals in order to say a definitive word about X17 puzzle.
 A.J. Krasznahorkay et al., Phys. Rev. Lett. 116, 042501 (2016)
 A.J. Krasznahorkay et al., arXiv:1910.10459