Electron Beam Dump Experiments Research Articles

Probing for chiral Z′ gauge boson through scattering measurement experiments

Motivated by the observation that tiny neutrino mass cannot be explained within the framework of the Standard Model, we consider extra gauge extended scenarios in which tiny neutrino masses are generated through the seesaw mechanism. These scenarios are equipped with a beyond the Standard Model neutral gauge boson called Z′ in the general U(1)X symmetry, which is a linear combination of U(1)Y and U(1)B−L. In this case, left- and right-handed fermions interact differently with the Z′. The Z′ gives rise to different processes involving neutrino-nucleon, neutrino-electron, electron-nucleus, and electron-muon scattering processes. By comparing with proton and electron beam-dump experiment data, recast data from searches for the long-lived and dark photon at , LHCb, and CMS experiments, the electron and muon g−2 data, and the data of the dilepton and dijet searches at the LEP experiment, we derive bounds on the gauge coupling and the corresponding gauge boson mass for different U(1)X charges and evaluate the prospective limits from the future beam-dump scenarios at DUNE, FASER (2), and ILC. We conclude that large parameter regions could be probed by scattering, beam-dump, and collider experiments in the future. Published by the American Physical Society 2024

Heavy neutral leptons at beam dump experiments of future lepton colliders

A new beam dump experiment that utilizes the beam of future high energy electron-positron colliders could be an excellent avenue to search for dark sector particles due to its unprecedented high energy and intensity. We consider heavy neutral leptons (HNLs) as a specific example to demonstrate the sensitivity of searches for dark sector particles at future electron-positron collider beam dump experiments. This includes the study of the reach at the International Linear Collider (ILC), the Cool Copper Collider (C3), and the Compact Linear Collider (CLIC). We comprehensively examine the HNL production and detector acceptance at these electron beam dump experiments. We show that these experiments will probe regions of HNL parameter space, not yet probed by past experiments, as well as by future approved experiments. Our study also motivates a more detailed analysis of heavy meson productions in high-energy electron-nucleon collisions in thick targets.

Axion-like particles as mediators for dark matter: beyond freeze-out

We consider an axion-like particle (ALP) coupled to Standard Model (SM) fermions as a mediator between the SM and a fermionic dark matter (DM) particle. We explore the case where the ALP-SM and/or the ALP-DM couplings are too small to allow for DM generation via standard freeze-out. DM is therefore thermally decoupled from the visible sector and must be generated through either freeze-in or decoupled freeze-out (DFO). In the DFO regime, we present an improved approach to obtain the relic density by solving a set of three stiff coupled Boltzmann equations, one of which describes the energy transfer from the SM to the dark sector. Having determined the region of parameter space where the correct relic density is obtained, we revisit experimental constraints from electron beam dump experiments, rare B and K decays, exotic Higgs decays at the LHC, astrophysics, dark matter searches and cosmology. In particular, for our specific ALP scenario we (re) calculate and improve beam dump, flavour and supernova constraints. Throughout our calculation we implement state-of-the-art chiral perturbation theory results for the ALP partial decay width to hadrons. We find that while the DFO region, which predicts extremely small ALP-fermion couplings, can probably only be constrained by cosmological observables, the freeze-in region covers a wide area of parameter space that may be accessible to other more direct probes. Some of this parameter space is already excluded, but a significant part should be accessible to future collider experiments.

Performance of BDX-MINI veto systems

This paper describes the veto system of the BDX-MINI detector installed at Jefferson Lab (US). The BDX-MINI experiment is the first electron beam-dump experiment specifically designed to search for Light Dark Matter (LDM) particles in the MeV-GeV mass range. The core of the BDX-MINI detector is a lead-tungstate electromagnetic calorimeter, for a total volume of 4 dm3. The calorimeter is surrounded by a multi-layer veto aimed at rejecting cosmic background: the innermost layer of the veto is made by a passive tungsten shielding for low energy radiation, while plastic scintillators make the middle and outer layers for charged cosmic particles rejection. Being located about 20 m downstream, the dirt between the beam dump and the detector provides sufficient shielding from the beam-related background. In 2019–2020, BDX-MINI was exposed for about six months to weakly interacting particles (neutrinos and, if existing, DM) produced by a 2.176 GeV electron beam incident on the beam dump of experimental Hall-A at Jefferson Lab.

Light vector dark matter with scalar mediator and muon g−2 anomaly

We study a model with a vector dark matter (DM) candidate interacting with the standard model (SM) charged leptons through a scalar portal. The dark matter candidate acquires mass when the complex scalar breaks an Abelian gauge symmetry spontaneously. The scalar interacts with the SM charged leptons through a dimension-6 operator. The scalar mediator induces elastic scattering of dark matter with electrons at tree level and also DM-nucleon interaction when the effects from scalar-Higgs mixing are taken into account. Given the recent results from Xenon1T upper bounds on DM-electron elastic scattering cross sections where the strongest sensitivity lies in the range $\ensuremath{\sim}\mathcal{O}(1)\text{ }\text{ }\mathrm{GeV}$, we find the viable space in the parameter space respecting constraints from the observed relic density, direct detection, muon $({g}_{\ensuremath{\mu}}\ensuremath{-}2)$ anomaly, ${e}^{+}{e}^{\ensuremath{-}}$ colliders, electron beam-dump experiments and astrophysical observables. It is shown that the current upper bounds of Xenon1T on DM-electron interactions are partially sensitive to the regions in the viable parameter space which is already excluded by the electron beam-dump experiment, Orsay. We also find that there are viable DM particles with masses $\ensuremath{\sim}\mathcal{O}(1)\text{ }\text{ }\mathrm{GeV}$ evading the direct detection but standing well above the neutrino floor. Almost the same viable regions are found when we apply the direct detection upper limits on the DM-proton spin-independent cross section.

Electron beam dump constraints on light bosons with lepton flavor violating couplings

We study constraints on light and feebly interacting bosons having charged lepton flavor violating couplings from electron beam dump experiments. Scalar, pseudoscalar, vector, and dipole type interactions of the bosons are analyzed, and excluded regions from the searches for decays into electron-positron pairs are derived. It is found that parameter regions unconstrained by flavor violating decays of muon can be excluded using the results of the E137 experiment. We also discuss the impact of the search for flavor violating decays of the light bosons in electron beam dump experiments.

Electron beam studies of light collection in a scintillating counter with embedded fibers

The light collection of several fiber configurations embedded in a box-shaped plastic scintillating counter was studied by scanning with minimum ionizing electrons. The light was read out by silicon photomultipliers at both ends. The light yield produced by the 855-MeV beam of the Mainz Microtron showed a strong dependence on the transverse distance from the beam position to the fibers. The observations were modeled by attributing the collection of indirect light inside of the counter and of direct light reaching a fiber to the total light yield. The light collection with fibers was compared to that of a scintillating counter without fibers. These studies were carried out within the development of plastic scintillating detectors as an active veto system for the DarkMESA electron beam-dump experiment that will search for light dark matter particles in the MeV mass range.

A light scalar explanation of (g \u2212 2)\u03bc and the KOTO anomaly

The KOTO experiment has recently performed a search for neutral Kaons decaying into neutral pions and a pair of neutrinos. Three events were observed in the KOTO signal region, with an expected background of about 0.05. Since no clear signal of systematic errors have been found, the excess of events in the decay KL→ π0ν overline{v} is quite intriguing. One possibility to explain this anomaly would be the presence of a scalar \U0001d719 with mass of the order of the pion mass and inducing decays KL→ π0\U0001d719 which mimic the observed signal. A scalar with mass of the order of the pion mass and a coupling to muons of the order of the Standard Model Higgs coupling could also explain the muon anomalous magnetic moment anomaly (g − 2)μ. We built on these facts to show that a light singlet scalar with couplings to the leptons and quarks as the ones induced by mixing with Higgs states in two Higgs doublet models may lead to an explanation of both anomalies. More specifically, we show that this is the case in the so-called type-X models in which leptons and quarks couple to two different Higgs doublets, and for scalar masses that are in the range between 40 and 70 MeV. Due to the relatively large coupling to leptons required to fit (g − 2)μ, the scalar lifetime accidentally falls into the sub-nanosecond range which is essential to evade the severe proton beam dump experiments and astrophysical constraints, though it becomes sensitive to constraints from electron beam dump experiments. The additional phenomenological properties of this model are discussed.

Eta decay and muonic puzzles

New physics motivated by muonic puzzles (proton radius and muon g−2 discrepancies) is studied. Using a light scalar boson ϕ, assuming Yukawa interactions, accounts for these muonic puzzles simultaneously. Our previous work limits the existence of such a scalar boson's mass mϕ from about 160 keV to 60 MeV. We improve this result by including the influence of all of the possible particles that couple to the ϕ in computing the decay rate. Doing this involves including the strong interaction physics, involving quarks, necessary to compute the ηπϕ vertex function. The Nambu-Jona-Lasinio model, which accounts for the spontaneous symmetry breaking that yields the constituent mass is employed to represent the relevant strong-interaction physics. We use the ηπϕ vertex function to reanalyze the electron beam dump experiments. The result is that the allowed range of mϕ lies between about 160 keV and 3.5 MeV. This narrow range represents an inviting target for ruling out or discovering this scalar boson. A possible UV completion of our phenomenological model is discussed.

Measurements of the muon flux produced by 10.6 GeV electrons in a beam dump

This paper presents the results of an experiment to assess the muon flux produced by the interaction of a 10.6GeV electron beam with the Hall-A beam dump at Jefferson Lab (JLab). The goal was to benchmark Monte Carlo simulations that are an essential tool for estimating beam-related backgrounds in beam-dump experiments aimed at searching for rare events, such as the Beam Dump eXperiment (BDX) planned at JLab. Beam-produced muons were measured with a CsI(Tl) crystal sandwiched between a set of segmented plastic scintillators placed at two different distances from the dump: 25.7m and 28.8m. At each location the muon flux was sampled at different vertical positions with respect to the beam height. Data have been compared with detailed Monte Carlo simulations using FLUKA for the muon production in the dump and propagation to the detector, and GEANT4 to simulate the detector response. The good agreement between data and simulations, within the uncertainties of the soil composition and density, demonstrate the validity of our simulation tools to predict the beam-related muon background in electron beam-dump experiments at ∼10GeV.

Probing leptophilic dark sectors at electron beam-dump facilities

Medium-energy electron beam-dump experiments provide an intense sources of secondary muons. These particles can be used to search for muon-coupling light dark scalars that may explain the $(g-2)_\mu$ anomaly. We applied this idea to SLAC E137 experiment deriving new exclusion limits and evaluated the expected sensitivity for the planned Jefferson Lab BDX experiment (in case of null result report). The calculation is based on numerical simulations that include a realistic description of secondary muons generation in the dump, dark scalar production, propagation, and decay, and, finally, the decay products (electrons, positrons, or photons) interaction with the detector. For both experiments, exclusion limits were extended to cover a broader area in the scalar-to-muon coupling vs. scalar mass parameter space. This study demonstrates that electron beam-dump experiments have an enhanced sensitivity to new physics in processes that are usually studied using proton beams.

Sgoldstino searches at CERN Electron Beam Dump

In the present paper we discussed light scalar sgoldstino in context of low scale SUSY breaking. We analyzed electromagnetic sector of interaction of sgoldstino with SM. We also took into account the impact on this interaction of the admixture of lightest MSSM Higgs scalar in the sgoldstino state. The signal rate of rare sgoldstino decays into SM particles at CERN electron beam dump experiment NA64 for the scalar sgoldstino has been estimated. We used expected signal rate to obtain some regions of sgoldstino parameter space (sgoldstino mass and SUSY breaking scale √F).

Muon anomalous magnetic moment through the leptonic Higgs portal

An extended Higgs sector may allow for new scalar particles well below the weak scale. In this work, we present a detailed study of a light scalar $S$ with enhanced coupling to leptons, which could be responsible for the existing discrepancy between experimental and theoretical determinations of the muon anomalous magnetic moment. We present an ultraviolet completion of this model in terms of the lepton-specific two-Higgs doublet model and an additional scalar singlet. We then analyze a plethora of experimental constraints on the universal low energy model, and this UV completion, along with the sensitivity reach at future experiments. The most relevant constraints originate from muon and kaon decays, electron beam dump experiments, electroweak precision observables, rare $B_d$ and $B_s$ decays and Higgs branching fractions. The properties of the leptonic Higgs portal imply an enhanced couplings to heavy leptons, and we identify the most promising search mode for the high-luminosity electron-positron colliders as $e^+{+}e^-\to\tau^+{+}\tau^-{+}S \to \tau^+{+}\tau^-{+}\ell{+}\bar \ell$, where $\ell =e,\mu$. Future analyses of existing data from BaBar and Belle, and from the upcoming Belle II experiment, will enable tests of this model as a putative solution to the muon $g-2$ problem for $m_S < 3.5$ GeV.

Strong constraints on sub-GeV dark sectors from SLAC beam dump E137.

We present new constraints on sub-GeV dark matter and dark photons from the electron beam-dump experiment E137 conducted at SLAC in 1980-1982. Dark matter interacting with electrons (e.g., via a dark photon) could have been produced in the electron-target collisions and scattered off electrons in the E137 detector, producing the striking, zero-background signature of a high-energy electromagnetic shower that points back to the beam dump. E137 probes new and significant ranges of parameter space and constrains the well-motivated possibility that dark photons that decay to light dark-sector particles can explain the ∼3.6σ discrepancy between the measured and standard model value of the muon anomalous magnetic moment. It also restricts the parameter space in which the relic density of dark matter in these models is obtained from thermal freeze-out. E137 also convincingly demonstrates that (cosmic) backgrounds can be controlled and thus serves as a powerful proof of principle for future beam-dump searches for sub-GeV dark-sector particles scattering off electrons in the detector.

New electron beam-dump experiments to search for MeV to few-GeV dark matter

In a broad class of consistent models, MeV to few-GeV dark matter interacts with ordinary matter through weakly coupled GeV scale mediators. We show that a suitable meter scale (or smaller) detector situated downstream of an electron beam dump can sensitively probe dark matter interacting via sub-GeV mediators, while B-factory searches cover the 1--5 GeV range. Combined, such experiments explore a well-motivated and otherwise inaccessible region of dark matter parameter space with sensitivity several orders of magnitude beyond existing direct detection constraints. These experiments would also probe invisibly decaying new gauge bosons (``dark photons'') down to kinetic mixing of $ϵ\ensuremath{\sim}{10}^{\ensuremath{-}4}$, including the range of parameters relevant for explaining the $(g\ensuremath{-}2{)}_{\ensuremath{\mu}}$ discrepancy. Sensitivity to other long-lived dark sector states and to new millicharge particles would also be improved.

Searching for Light Dark Matter with the SLAC Millicharge Experiment

New sub-GeV gauge forces ("dark photons") that kinetically mix with the photon provide a promising scenario for MeV-GeV dark matter and are the subject of a program of searches at fixed-target and collider facilities around the world. In such models, dark photons produced in collisions may decay invisibly into dark-matter states, thereby evading current searches. We reexamine results of the SLAC mQ electron beam dump experiment designed to search for millicharged particles and find that it was strongly sensitive to any secondary beam of dark matter produced by electron-nucleus collisions in the target. The constraints are competitive for dark photon masses in the ~1-30 MeV range, covering part of the parameter space that can reconcile the apparent (g-2)(μ) anomaly. Simple adjustments to the original SLAC search for millicharges may extend sensitivity to cover a sizable portion of the remaining (g-2)(μ) anomaly-motivated region. The mQ sensitivity is therefore complementary to ongoing searches for visible decays of dark photons. Compared to existing direct-detection searches, mQ sensitivity to electron-dark-matter scattering cross sections is more than an order of magnitude better for a significant range of masses and couplings in simple models.

New limits on hidden photons from past electron beam dumps

Hidden sectors with light extra U(1) gauge bosons, so-called hidden photons, have recently attracted some attention because they are a common feature of physics beyond the Standard Model like string theory and supersymmetry and additionally are phenomenologically of great interest regarding recent astrophysical observations. The hidden photon is already constrained by various laboratory experiments and presently searched for in running as well as upcoming experiments. We summarize the current status of limits on hidden photons from past electron beam dump experiments including two new limits from such experiments at the High Energy Accelerator Research Organization in Japan (KEK) and the Laboratoire de l'accel\'erateur lin\'eaire (LAL, Orsay) that have so far not been considered. All our limits take into account the experimental acceptances obtained from Monte Carlo simulations.

An unambiguous search for a light Higgs boson

A search for light Higgs bosons has been performed in an electron beam-dump experiment. No positive signal is observed which allows us to reject at 90% CL the existence of a standard Higgs in the range from 1.2 to 52 MeV. Non-standard Higgs bosons are also excluded in a large range of couplings. This search relies on the well controlled and calculable process of bremsstrahlung from electrons in the Coulomb field of large Z nuclei. Both production and decay are governed by the single coupling constant of Higgs to electrons.

Search for axion-like particles in electron bremsstrahlung

A search for weakly interacting particles with short lifetime has been performed in an electron beam-dump experiment. Comparing the rate of produced positrons behind a dump of variable geometry, limits are placed or a large range of lifetime. A 1.8 MeV particle, as hinted from recent results using heavy ion collisions, is ruled out for lifetimes between 6 × 10 −4 and 9 × 10 −11 s. These results, those previously published for longer lifetimes and the g - 2 agreement with QED rule out a simple particle interpretation of the heavy ion effect.