The HIBEAM Experiment ^†

Abstract

The violation of baryon number is an essential ingredient for baryogenesis—the preferential creation of matter over antimatter—needed to account for the observed baryon asymmetry in the universe. However, such a process has yet to be experimentally observed. The HIBEAM/NNBAR program is a proposed two-stage experiment at the European Spallation Source to search for baryon number violation. The program will include high-sensitivity searches for processes that violate baryon number by one or two units as follows: free neutron–antineutron oscillation via mixing, neutron–antineutron oscillation via regeneration from a sterile neutron state, and neutron disappearance; the effective process of neutron regeneration is also possible. The program can be used to discover and characterize mixing in the neutron, antineutron, and sterile neutron sectors. The experiment addresses topical open questions such as the origins of baryogenesis and the nature of dark matter, and it is sensitive to scales of new physics that substantially exceed those available at colliders. A goal of the program is to open a discovery window to neutron conversion probabilities (sensitivities) by up to three orders of magnitude compared with previous searches, which is a rare opportunity. A conceptual design report for NNBAR has recently been published.

1. Introduction

There are several outstanding questions in modern physics that are not explained by the Standard Model of particle physics, including the observed matter/antimatter asymmetry, i.e., there is an overabundance of matter compared to antimatter in the universe. In order for the matter/antimatter asymmetry to arise, a set of conditions, specified by Sakharov as follows, must be met [1]: (1) the accidental conservation of baryon number, B, in the Standard Model must be violated; (2) the charge, C-, and charge-parity, $CP$-, symmetries must be violated; and (3) these interactions must occur outside of thermal equilibrium.

The first Sakharov condition, the baryon number violation, $BNV$, can arise with or without the corresponding lepton number, L, violation, $LNV$, and one may give rise to the other. Mechanisms for this may involve the sphaleron processes, grand unification models, supersymmetry, hidden sector processes, etc., and they may be probed, for example, via neutrinoless double beta decay searches ($\Delta B=0$, $\Delta L\ne 0$, $\Delta \left[B-L\right]\ne 0$), proton decay searches ($\Delta B\ne 0$, $\Delta L\ne 0$, $\Delta \left[B-L\right]=0$), or searches for neutron conversion into antineutrons or hidden sector neutrons ($\Delta B\ne 0$, $\Delta L=0$, $\Delta \left[B-L\right]\ne 0$) [2,3]. The latter was last probed in the 1990s [4] and is the subject of the proposed HIBEAM/NNBAR program at the European Spallation Source [5].

The European Spallation Source (ESS) ERIC is a research infrastructure under construction in Lund, Sweden, which will provide the world’s highest intensity neutron source for studies in, for example, material science, life science, or fundamental physics [6]. The ESS is jointly hosted by Sweden and Denmark, with 13 member states from around Europe, and with plans of hosting 22 instrumented beamlines (of which 15 have been decided).

The ESS neutrons will be produced through the spallation process on a rotating tungsten target wheel using a 2 GeV pulsed proton beam, with 14 Hz repetition, 3 ms pulses, and an initial (final) power of 2 (5) MW. The cold neutrons are then guided by moderators to the experimental beamlines, which are accessed through several beam ports around the circumference of the target area. Three standard beam ports have been merged to form one larger beam port (dubbed the large beam port) to allow maximum neutron flow, which can be used in fundamental physics studies with the planned NNBAR program [7].

2. The HIBEAM/NNBAR Program

The HIBEAM/NNBAR program [5,7,8,9] will search for neutrons n, produced at the ESS that convert into antineutrons $\overline{n}$ or sterile/mirror neutrons $n’$ in a two-stage program. First, the HIBEAM experiment will increase the discovery potential for these processes by up to one order of magnitude compared to the last search, followed by the NNBAR experiment, which will increase this discovery potential by a factor greater than 1000 [10]. The NNBAR studies have been conducted as part of the Horizon2020-funded HighNESS program, and NNBAR was detailed in a recently released conceptual design report [7].

Figure 1 shows the layout of the ESS experimental hall, including the proposed placements for the HIBEAM and NNBAR beamlines. NNBAR is placed at the large beam port.

2.1. The NNBAR Experiment

Figure 2 shows the layout of the NNBAR experiment, from the neutron source at the large beam port, through to the neutron reflector optics used for beam focusing, into the oscillation tunnel, and finally incident on a carbon target foil inside the annihilation detector. The 200 $\mathrm{m}$-long oscillation tunnel must be under vacuum and magnetic field-free (<$10\mathrm{nT}$) in order to allow the oscillation between neutrons and antineutrons or mirror neutrons. This will be achieved using a passive shield of mu-metal. In order to maximize the oscillation time, the neutrons will propagate very slowly, ≲$1000$ m s⁻¹. When an oscillated neutron reaches the target foil, it will annihilate with a neutron in a carbon nucleus into 4–5 pions (charged and neutral), with a center-of-mass energy of $\sqrt{s}=2m_n\approx 1.9$ GeV. The pions, spread isotropically, will each carry an energy of ≲500 MeV (peaking around 100 MeV), and be detected with the surrounding detector, consisting of a time-projection chamber, a lead-glass electromagnetic calorimeter, and a scintillator-based hadronic range calorimeter. Detector prototypes have been constructed and will be tested [11].

Multiple studies have been conducted to demonstrate the ability to reconstruct the pion masses and multiplicity, the system’s invariant mass, as well as to separate signal pions from background protons and cosmic rays, using the NNBAR detector [12,13]. The neutron–antineutron (or mirror neutron) oscillation would be a very rare process, which requires an excellent separation between signal and background events. This is achieved through highly effective event selection, which yields a rejection of background events by more than a factor of 10⁹ while maintaining a signal efficiency of ~70% [7].

Figure 3 shows the resulting estimated sensitivity along with previous experiments (circles) and future experiments (triangles). Indirect searches (blue) are shown along with direct searches (red) [7]. The currently leading indirect search for neutron–antineutron oscillation was conducted in the Super-Kamiokande experiment [14] and will be superseded by the future DUNE experiment [15] (denoted by “SuperK-2” and “DUNE” in Figure 3). These searches rely on detecting neutron conversion in nucleus-bound neutrons in large neutrino detectors, which are water Cherenkov-based and liquid Argon-based, respectively. The neutron conversion process is highly suppressed for neutrons bound in nuclei, and the analysis requires large corrections that rely heavily on the parameters of the chosen nuclear model, therefore carrying large uncertainties.

The most recent direct search for neutron–antineutron oscillation, i.e. making use of free neutrons, was conducted at the ILL neutron source (denoted by “ILL-2” in Figure 3) [4]. This search used a neutron beam directed at an annihilation detector corresponding to the setup of the NNBAR experiment, with an effective running time of 2.4 × 10⁷ s. The discovery potential of these experiments is proportional to the neutron beam’s intensity, the effective efficiency of the neutron reflector, the length of the neutron propagation path, and the runtime of the experiment. For NNBAR, these are improved by factors of >2, 40, 5, and 2 with respect to the ILL search, compounding to a discovery potential gain of a factor of >1000.

The sensitivity and final upper limit are proportional to the squareroot of the discovery potential, which results in the NNBAR sensitivity surpassing the ILL-2 result by 1.5 orders of magnitude and the SuperK-2 result by one order of magnitude [7].

2.2. The HIBEAM Experiment

The HIBEAM experiment is the precursor of NNBAR. It will make use of a magnetically shielded ∼$50 \mathrm{m}$ beamline at the ESS, and it can be operated in four distinct search modes as follows: (a) direct neutron–antineutron oscillation, $n\to \overline{n}$; (b) neutron–mirror neutron oscillation, $n\to n’$; (c) neutron–mirror neutron–neutron oscillation, $n\to n’\to n$; and (d) neutron–mirror neutron–antineutron oscillation, $n\to n’\to \overline{n}$. Search modes (c) and (d) make use of a mechanical beam stop in the middle of the beamline to stop the large flow of neutrons while allowing the non-interacting mirror neutrons to pass. Modes (a) and (d) operate with neutron annihilation detectors using a carbon foil (similar to NNBAR), either with a bespoke detector or the WASA crystal calorimeter, while modes (b) and (c) make use of a neutron counting detector. This will result in a discovery potential improvement of a factor up to 10 compared with previous results, both for the antineutron and mirror neutron oscillation [8].

Additionally, HIBEAM will be sensitive to direct axion detection over a broad range of possible axion masses [16]. Assuming that the local dark matter density ($\rho$ ≈ 0.4 GeV cm⁻³) is made up of axions, the ambient axions act as a pseudo-magnetic field to the neutrons in the beamline, and therefore change their Larmor frequency (magnetic moment precession) as they propagate through the beampipe, which can be detected through a Ramsey interferometry experiment. This requires several adjustments to the experimental apparatus, including polarizing the neutrons at the start of the beamline, measuring the polarization at the end of the beamline, as well as 2–3 spin flippers (by $\left[{\displaystyle \frac{\pi }{2}},{\displaystyle \frac{\pi }{2}}\right]$ or $\left[{\displaystyle \frac{\pi }{2}},\pi ,{\displaystyle \frac{\pi }{2}}\right]$) over the beamline length. The HIBEAM sensitivity to direct axion detection with a 1 $\mathrm{yr}$ runtime is comparable to the indirect supernova energy-loss limit for axion masses from 10⁻²² eV to 10⁻¹⁶ eV, which is an improvement of more than two orders of magnitude compared to previous direct searches (see Figure 4).

3. Summary and Outlook

Discovering $BNV$ in the neutron sector, as neutron oscillation into antineutrons or mirror neutrons, may explain the matter–antimatter asymmetry observed in the universe. Searches for this require an enormous and highly controlled flux of cold neutrons, which can be achieved at the European Spallation Source. Utilizing this, the HIBEAM and NNBAR programs are poised to open a new discovery window to neutron oscillation that surpasses previous discovery potentials by more than three orders of magnitude, while also opening up ancillary searches for, e.g., axions.