The context
In order to understand our world in terms of elementary building blocks (particles & fields) larger and larger particle accelerators are constructed. The latest success in that direction was achieved in 2012 at the Large Hadron Collider (LHC) by discovering the last missing particle of the Standard Model of Particles (SM) [1, 2], the Higgs boson. Nevertheless the SM does not provide explanation for masses of neutrinos [3], misses any accounts of dark matter (DM) and dark energy [4], nor explains the apparent matter-antimatter asymmetry in the Universe [5]. To explain any of these we need to go beyond the SM.
Concentrating on the DM puzzle, there were quite a few suggestions in the past as to what DM could consist of (e.g., axions, WIMPs) as well as by what means it interacts with the SM particles. Although DM particles and forces are typically expected to be heavy and short-range (hence accessible to high-energy colliders), there are still other viable options providing motivation for high-statistics searches far from the energy frontier such as, for example, in the MeV-energy range. For those, low-energy accelerators with fixed targets are the right tool. Since our proposal directs towards detection of a new boson particle mediating force between the SM and DM sectors with the expected mass of around 17 MeV, we opt for the latter type of constellation, too.
Recent experimental studies on excited states of 8Be [6] and 4He [7] showed significant 6.8 and 7.2 sigma discrepancies, respectively, from the theoretical expectation in the process called internal pair production (IPP; or alternatively IPC for ‘creation’), i.e., in the process of nuclear de-excitation via a concurrent emission of an electron and a positron. SM predicts that the distribution of the angle between the electron and positron in such an event peaks at 0° and drops monotonically and without features as the angle increases, Fig. 1, left.
FIG. 1 Left: SM prediction of internal pair creation correlation (IPCC) for different electromagnetic multipolarities and a transition energy of Eγ=17.6 MeV as a function of the opening angle Θ between the electron and positron of a pair. Right: Measured angular correlation (red) of the e+e− pairs originating from the decay of the 8Be 17.6-MeV resonance, compared to the SM prediction (blue) and prediction employing the new boson (green) [8].
The experiment, however, sees an extra structure, Fig. 1, right. Introducing a new massive neutral particle X with a coupling to neutrons, protons, or both as well as to the electron-positron pair, adds an extra decay channel and can explain the feature: first, the new particle is emitted and only afterwards it decays to the e+e− pair, Fig. 2. In the rest system of X, the pair members are emitted antiparallel to each other, which in the laboratory system translates into a peak at intermediate angles. The peak not only suggests the existence but its position (at about 140°, 150°, and 115° for 8Be(18.15 MeV), 8Be(17.64 MeV), and 4He(20.21 MeV), respectively) also provides a means of estimating the mass of X (mX ≈ 2E sin(Θpeak/2) at least when both the electron and the positron carry about the same energy E).
FIG. 2 Schematic view of obtaining an excited 8Be state by proton bombardment of 7Li, and its de-excitation by emitting an X particle, which subsequently decays into an e+e− pair to be detected [10].
The hypothetical X particle must be a boson since it can decay into an e+e− pair (assuming no other dark/invisible particle is produced). The mass as inferred from the mentioned experiments mutually nicely agrees: 16.70 ± 0.35 (stat) ± 0.5 (syst) MeV for 8Be(18.15 MeV state) [6]; 17.0 ± 0.2 (stat) ± 0.5 (syst) MeV for 8Be(17.64 MeV state) [8]; and 16.84 ± 0.16 (stat) ± 0.2 (syst) MeV for 4He(21.01 MeV state) [7]. If parity is conserved in the discussed X production, it cannot be a scalar; otherwise no strict restriction is known [9], with pseudoscalar and vector cases most often discussed. The mean decay length of the X boson, in the experiments created with velocity of about 0.4c, is probably less than 1 cm in the laboratory frame [10].
Regarding the measurements necessary to observe these effects, magnetic spectrometers represent a very useful tool for the measurement of positron-electron pairs in coincidence. Some setups used interesting baffle systems to improve the energy resolution, at the expense of their efficiency [11, 12]. The adaptation of the Debrecen superconducting magnetic spectrometer to solenoid transporter in ATOMKI, Debrecen, Hungary [13], increased the efficiency of signal observation to around 35% and energy resolution of 0.5% at 2.2 MeV. The PEPSI spectrometer [14] used a special configuration of permanent magnets as the filter for e+e− pairs, covering the whole solid angle and a wide range of energies (5–20 MeV). The energy resolution was 8.0% and 5.5% at 5 and 9 MeV, respectively, with an efficiency dependent on the energy, with a maximum around 30% for 5 MeV electrons. One of the first setups using a phoswitch array of plastic scintillators is described in [15], measuring energies between 2 and 30 MeV with a resolution of 13% at a 20 MeV transition for 29% of the total solid angle, with an efficiency above 1%. A more sophisticated array of six detectors that could be moved along different angles, plus two larger detectors, providing measurements for 15 different angles simultaneously, between 20º and 130º claimed that this setup was “...well suited to perform searches for an elusive short-lived neutral boson with a mass between 6 and 15 MeV/c2...” [16].
The use of position sensitive detectors improved the resolution needed in measurements of angular anomalies. Experiments in ATOMKI used multiwire proportional counters (MWPC) and silicon strip detectors for the measurement of the angular correlation [8, 17] with the angular resolution ~7º. Other groups are building their own experiments, such as the Particle Physics Group of the University of Montreal, in Canada, which is building a spectrometer with thick and long plastic scintillators surrounding the target and a cylindrical multiwire proportional chamber with three detecting layers, recovered from the DAPHNE detector [18].
The work at UTEF's Van de Graaff
The IEAP is collaborating with the University of Montreal on building some instrumentation based on plastic scintillators. It has also mounted a setup based on Timepix3 (Tpx3) detectors. Three of these detectors were placed in a packed triangular shape around the target inside a vacuum chamber (to avoid scattering) at the IEAP’s Van de Graaff (VdG) accelerator, taking advantage of the high resolution of the pixelated detectors. The position of the target could be precisely determined (including its alignment) and also the direction of each track, with subsequent calculation of the angular correlations between coincidences. The first, preliminary results of the calibration of the system with a fluorine target are shown in Fig. 3 and are ready to be presented in the next international conference.
Members of the team have also been developing theoretical work in the study of the decay of the excited levels of nucleus 8Be and 4He within their activities in the IEAP [19].
FIG. 3 Experimental setup in a vacuum chamber at the VdG accelerator (left), where the data from the Tpx3 detectors was used (center) to reconstruct the position of the target (right) and calculate the angular correlations of e+e− pairs.
The applicant has experience with Micropattern Gaseous Detectors (MPGD) (applications and R&D), construction of imaging detectors for X-rays, charged particles and neutrons. His PhD thesis was devoted to the development of a novel MPGD-based neutron detector concept for imaging [20]. He has been a member of CERN RD51 since its beginning in 2008; he has collaborated with the Weizmann Institute of Science, Israel, on the development of MPGD for hadron calorimeters [21], with several test beam campaigns at CERN, and was a member of the NEXT experiment [22]. Recently he performed R&D work in the optimization of detectors within the ALICE Time Projection Chamber (TPC) upgrade. During that time, he also developed neutron position-sensitive detectors and supervised a master thesis on a gaseous detector for X-ray fluorescence imaging (see applicant’s CV).
The experiment
The measurement of e+e− pair emission requires high efficiency, good angular, energy and timing resolution and a good particle identification for gamma and cosmic background suppression. This suggests the use of a gas for the energy measurement. The geometry of the problem, where the events are expected at every direction from a point-like source, clearly suggests a TPC [34]. This type of detector reconstructs in 3D the tracks of free electrons generated by the particles, using the projections in the readout plane and the drift time of the electron-ion pairs in the gas. Using strong magnets in the gas volume, the momentum of the charged particles can be calculated from their curvature. The measurement of the energy loss in the gas provides the particle identification.
The TPC is very popular in high energy physics experiments, such as ALICE [35] or STAR [36] and will be part of the PANDA-FAIR [37]; in neutrino Physics experiments such as NEXT [38] or ICARUS [39]; in the search for dark matter such as XENON [40] and LUX-ZEPLIN [41]. Smaller TPCs are also very common in the study of nuclear reactions induced by secondary beams, like ACTAR-TPC [42], AT-TPC [43], or many other small setups at universities and research facilities aimed not only at fundamental science studies, but also at the study of gas properties, charge mobility and detector R&D.
In this project, we propose the finalization of an existing spectrometer composed of six small TPCs equipped with multiwire proportional counters (MWPC) [44] at the entrance windows, and its upgrade with an inner tracker based in Tpx3 detectors. It consists of a cylindrical vessel, divided in sextants, separated by strong permanent magnets. The spectrometer can easily be installed or removed from the beam line.
Each sextant of the spectrometer can be operated as a separate detector, and will be composed of three sensitive layers. The first layer is a Tpx3 detector with a very thin Si sensor (50 µm) and its ASIC. The 55 µm spatial resolution will provide an excellent angular resolution and a very precise determination of the source position. The second layer is inside the gas volume and consists of a MWPC that provides some redundancy in the angular determination of the system, but will also be used to correct the energy as a function of the scattering of the particles in the Tpx3 detectors and the vacuum tube wall.
FIG. 4 Left: Simulation of 8 MeV e+ in the magnetic field after passing through the vacuum tube wall. The energy is determined from the radius of curvature of the tracks. Right: Two sextants of the spectrometer, detecting one e+e− pair produced in the experiment.
Finally, the trajectory of the particles in the magnetic field will be bent in the gas volume, allowing a determination of the energy. Fig. 4 shows a simulation of positrons entering the gas volume in a magnetic field of 0.3 T and a scheme of two sextants of the experiment. The spectrometer will have an angular resolution below 0.5º and an energy resolution of 4%.
The e+e− pairs will be produced by bombardment of a fixed target with protons of suitable energy and current, originating from a VdG accelerator. Targets containing 7Li will be used to study pairs coming from the excited 8Be levels of 17.64 and 18.15 MeV (corresponding to 441 and 1030 keV of incident proton energy, respectively). Tests have been made with LiF material of various thicknesses evaporated onto a thin (10 μm) Al foil. This material is convenient in the preparatory phase since it contains both 7Li transmuting to 8Be to be eventually studied and 19F, which transmutes to 16O, whose first excited 6.05 MeV state cannot decay by emission of a single photon, but produces e+e− pairs instead (E0 transition) that are known to not display any anomaly, can be produced in large numbers, and will serve in calibration studies. Later, to suppress background, pure Li target will be employed, evaporated on a submicron carbon foil. Targets for 4He studies (the wide, Г=0.84 MeV, 21.01 MeV state; incident proton energy of about 900 keV to stay below the threshold for neutron creation) will be of tritium implanted into a titanium substrate. These targets will be actively cooled (with water or liquid nitrogen) to keep tritium bound to the substrate. Any used targets will be positioned by holders to the accelerator beamline axis passing through the detector system. The vacuum tube (ie., beampipe) separating the accelerator vacuum from the detectors will be made of thin carbon to minimize undesired scattering.
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Last update: May 2021