Observation of the X17 anomaly in the decay of the Giant Dipole Resonance of ⁸Be

We published very challenging experimental results in 2016 kr16 indicating the electron-positron ($e^{+}e^{-}$) decay of a hypothetical new light particle. The $e^{+}e^{-}$ angular correlations for the 17.6 MeV and 18.15 MeV transitions in ⁸Be were studied and an anomalous angular correlation was observed for the 18.15 MeV transition kr16 . This was interpreted as the creation and decay of an intermediate bosonic particle with a mass of $m_{X}c^{2}$=16.70$\pm$0.35(stat)$\pm$0.5(sys) MeV, which is now called X17.

Our data were first explained with a vector gauge boson, X17 by Feng and co-workers fe16 ; fe17 ; fe20 , which would mediate a fifth fundamental force with some coupling to standard model (SM) particles. The possible relation of the X17 boson to the dark matter problem triggered an enormous interest in the wider physics community ins . New results will hopefully be published soon on the X17 particle from a few different experiments al23 .

We also observed a similar anomaly in ⁴He kr21 . It could be described by the creation and subsequent decay of a light particle during the proton capture process on ³H to the ground state of ⁴He. The derived mass of the particle ($m_{X}c^{2}=16.94\pm 0.12$(stat.)$\pm 0.21$(syst.) MeV) agreed well with that of the proposed X17 particle.

Recently, we have studied the E1 ground state decay of the 17.2 MeV J^π = 1^- resonance in ¹²C kr22 . The angular correlation of the $e^{+}e^{-}$ pairs produced in the ¹¹B(p,$\gamma$)¹²C reaction were studied at five different proton energies around the resonance. The gross features of the angular correlations can be described well by the Internal Pair Creation (IPC) process following the E1 decay of the $1^{-}$ resonance. However, on top of the smooth, monotonic distribution, we observed significant peak-like anomalous excess around 155-160^∘ at four different beam energies. The $e^{+}e^{-}$ excess can be well-described by the creation and subsequent decay of the X17 particle. The invariant mass of the particle was derived to be ($m_{X}c^{2}=17.03\pm 0.11$(stat.)$\pm 0.20$(syst.) MeV), in good agreement with our previously published values.

However, despite the consistency of our observations, more experimental data are needed to understand the nature of this anomaly. For this reason, many experiments all over the world are in progress to look for such a particle in different channels. Many of these experiments have already put constraints on the coupling of this hypothetical particle to ordinary matter. Others are still in the development phase, but hopefully they will soon contribute to a deeper understanding of this phenomenon as concluded by the community report of the Frascati conference al23 .

Very recently, Barducci and Toni published an updated view on the ATOMKI nuclear anomalies ba23 . They have critically re-examined the possible theoretical interpretation of the observed anomalies in ⁸Be, ⁴He and ¹²C anomalies in terms of a BSM boson X with mass $\approx$17 MeV. Their results identify an axial vector state as the most promising candidate to simultaneously explain all three anomalous nuclear decays, while the other spin/parity assignments seems to be disfavored for a combined explanation.

At the same time, the NA62 collaboration was searching for K⁺ decays to the $\pi^{+}e^{+}e^{-}e^{+}e^{-}$ final state and excluded the QCD axion as a possible explanation of the 17 MeV anomaly NA23 . Hostelt and Pospelov reanalysed some old pion decay constraints po23 , ruled out the vector-boson explanations and set limits on axial-vector ones.

The aim of this paper is to use a simpler geometry of the spectrometer to avoid non-trivial possible artefacts, which may be connected to the spectrometer itself al21 .

With such a new spectrometer, we studied the X17 creation and the $e^{+}e^{-}$ pair emission from the decay of the Giant Dipole Resonance (GDR) fi76 ; sn86 ; ha01 excitations of ⁸Be.

The experiments were performed in Debrecen (Hungary) at the 2 MV Tandetron accelerator of ATOMKI, with a proton beam energy of E_p= 4.0 MeV.

Owing to the rather large width of the GDR($\Gamma$ = 5.3 MeV fi76 ), a 1 mg/cm² thick ⁷Li₂O target was used in order to maximize the yield of the $e^{+}e^{-}$ pairs. The target was evaporated onto a 10 $\mu$m thick Ta foil. The average energy loss of the protons in the target was $\approx$100 keV.

$\gamma$ radiations were detected by a 3”x3” LaBr3 detector monitoring also any potential target losses. The detector was placed at a distance of 25 cm from the target at an angle of 90 degrees to the beam direction.

A typical $\gamma$ energy spectrum is shown in Fig 1. The figure clearly shows the transitions from the decay of GDR to the ground and first excited states in ⁸Be. The cosmic ray background is also visible on the high energy side of the spectrum, but it is reasonably low.

The intensity ratio of the peaks was found to be: I(GDR$\rightarrow$ g.s.)/I(GDR$\rightarrow 2_{1}^{+}$)=0.18$\pm$0.02 at E_p= 4.0 MeV bombarding energy.

We used Double-sided Silicon Strip Detectors (DSSD) and plastic scintillators as “particle telescopes” to determine the hit positions and the energy of the electrons and positrons, respectively. In our previous experiments, the spectrometers were built up of five and six particle telescopes, both having different acceptances as a function of the $e^{+}e^{-}$ correlation angle. A detailed description of the spectrometers can be found in Ref. kr21 . However, in the present experiment, only two telescopes were used, placed at an angle of 110^∘ with respect to each other. The diameter of the carbon fiber tube of the target chamber has been reduced from 70 mm to 48 mm to allow a closer placement of the telescopes to the target. Thus, we could cover a solid angle around 110^∘ with the two telescopes much larger than with the previous setups. Also, in this setup the efficiency function has only one maximum as a function of the $e^{+}e^{-}$ opening angle. This angular dependence can be simulated and calibrated more reliably. Another advantage of this setup is that its sensitivity to the cosmic background is significantly less. Since the vertical angles of telescopes were -35^∘ and -145^∘, the cosmic rays coming mostly vertically, have a very small chance of firing both telescopes at the same time.

The energy calibration of the telescopes, the energy and position calibrations of the DSSD detectors, the Monte Carlo (MC) simulations as well as the acceptance calibration of the whole $e^{+}e^{-}$ coincidence pair spectrometer were explained in Ref. kr21 . Good agreement was obtained between the experimental acceptance and the results of the MC simulations, as presented in Fig. 2. Due to the very tight geometry, the DSSD position data and therefore the $e^{+}e^{-}$ angular distribution experiences an enhanced dependence on the beam spot size and position. According to previous measurements and MC simulations of the present setup we could take into account this effect properly.

At the proton energy of E_p= 4.0 MeV, the (p,n) reaction channel is open (E_thr= 1.88 MeV) and generated neutrons and low-energy $\gamma$ rays with a large cross section. (Other reaction channels are also open, but their cross sections are much smaller and their influence on our experiment is much weaker.) The maximum neutron energy E${}_{n}=1.6$ MeV, which induces only a 300 keV electron equivalent signal in the plastic scintillator due to the quenching effect. Such a small signal fell well below the CFD thresholds that we used.

The low-energy neutrons did not produce any measurable signal in the DSSD detectors either since the maximum energy that can be transferred in elastic scattering on Si atoms is only $\approx$50 keV, which is below the detection threshold.

A single energy spectrum measured by the scintillators and gated by “multiplicity=2” events in the DSSD detector, which means that both the electron and positron coming from the internal pair creation are detected in the same telescope, is used for energy calibration. Such a calibration spectrum is shown in Fig. 3 for telescope 1.

As shown, the energy resolution for the ground-state transition is reasonably good ($\approx 14\%$). The intensity ratio of the GDR to ground state and the GDR to the 2${}_{1}^{+}$ state is determined to be: I(GDR$\rightarrow$g.s.)/I(GDR$\rightarrow$2${}_{1}^{+}$)=0.25$\pm 0.03$.

Unfortunately, the gain of the PMT connected to the second plastic scintillator was less stable than the first one and its energy resolution was somewhat worse. This is represented by the worse resolution of the energy sum spectrum of the two telescopes as shown in Fig. 4.

The angular correlation spectra of the $e^{+}e^{-}$ pairs for the different energy sum regions were then obtained for symmetric $-0.5\leq\epsilon\leq 0.5$ pairs, where the energy asymmetry parameter $\epsilon$ is defined as $\epsilon=(E_{1}-E_{2})/(E_{1}+E_{2})$, where $E_{1}$ and $E_{2}$ denote the kinetic energies of the leptons measured in telescope 1 and telescope 2, respectively.

The angular correlation gated by the low energy-sum region (below 14 MeV), as marked in Fig. 4, is shown in Fig. 5. The measured counts were corrected for the acceptance obtained from the raw data collected for the whole experiment in the similar way as described previously kr21 . It is a smooth distribution without showing any anomalies. It could be described by assuming E1 + M1 multipolarities for the IPC process and a constant distribution, which may originate from cascade transitions of the statistical $\gamma$ decay of the GDR appearing in real coincidence. In such a case, the lepton pair may come from different transitions, and thus their angles are uncorrelated. This smooth curve reassured us that we were able to accurately determine the efficiency of the spectrometer.

The angular correlation of the $e^{+}e^{-}$ pairs gated by the GDR energy region (above 14 MeV), as marked in Fig. 4, is shown Fig. 6.

The experimental data corrected for the acceptance of the spectrometer is shown as red dots with error bars. The simulated angular correlation for the E1 internal pair creation is indicated as a black curve. Significant deviations were observed. First of all, a peak-like deviation at 120^∘, but also an even stronger deviation at larger angles.

The measured angular correlation was fitted from 70 degrees to 160 degrees with the sum of simulated E1, M1 and X17 contributions calculated for both the GDR to ground state and for the GDR to 2${}_{1}^{+}$ state transitions. The simulations concerning the decay of the X17 boson in the transition to the ground state of ⁸Be were carried out in the same way as we did before kr16 ; kr21 ; kr22 and could describe the anomaly appearing at around 120^∘.

However, based on Figures 3 and 4 and previous measurements fi76 , the $\gamma$-decay of GDR to the first excited state is much stronger than its decay to the ground state. According to that, we assumed that the X17 particle was created also in the decay of GDR to the ground state and to the first excited state. Based on the energy of that transition (17.5 MeV), we would expect a peak around 150 degrees. However, the first excited state is very broad ($\Gamma$=1.5 MeV), so the shape of the expected anomaly is significantly distorted. The simulations were then performed as a function of the X17 mass from 10 MeV/c² to 18 MeV/c² for both transitions.

To derive the invariant mass of the decaying particle, we carried out a fitting procedure for both the mass value and the amplitude of the observed peaks. The fit was performed with RooFit Verkerke:2003ir in a similar way as we described before kr21 ; kr22 .

The experimental $e^{+}e^{-}$ angular correlation was fitted with the following intensity function (INT):

As shown in Fig. 7, the simulation can describe the experimental distributions from $\Theta=70^{\circ}$ to 160^∘ well. The significance of the fit is larger than 10$\sigma$.

The measured invariant mass of the hypothetical X17 particle obtained from the fit is: 16.94 $\pm$ 0.47 MeV(stat)/c² and the intensity ratio of the X17 particle was found to be:

We reported on a new direction of X17 research. For the first time, we successfully detect this particle in the decay of the Giant Dipole Resonance (GDR). Since this resonance is a general property of all nuclei, the study of GDR may extend these studies to the entire nuclear chart.

We have studied the GDR (J^π =1^-) E1-decay to the ground state (J^π =$0^{+}$) and to the first excited state (J^π=$2_{1}^{+}$) in ⁸Be. The energy-sum and the angular correlation of the $e^{+}e^{-}$ pairs produced in the ⁷Li($p$,e⁺e^-)⁸Be reaction was measured at a proton energy of E_p= 4.0 MeV. The gross features of the angular correlation can be described well by the IPC process following the decay of the GDR. However, on top of the smooth, monotonic distribution of the angular correlation of $e^{+}e^{-}$ pairs, we observed significant anomalous excess at about 120^∘ and above 140^∘.

The $e^{+}e^{-}$ excess can be well-described by the creation and subsequent decay of the X17 particle, which we have recently suggested kr16 ; kr21 ; kr22 . The invariant mass of the particle was measured to be ($m_{\mathrm{X}}c^{2}=16.95\pm 0.48$(stat.) MeV), which agrees well with our previous results.

The present observation of the X17 particle in an E1 transition supports its vector/axial-vector character.

We wish to thank Z. Pintye for the mechanical and J. Molnar for the electronic design of the experiment. This work has been supported by the GINOP-2.3.3-15-2016-00034 and GINOP-2.3.3-15-2016-00005 grants.