Beam Dump Experiments Research Articles

Axion baryogenesis puts a new spin on the Hubble tension

We show that a rotating axion field that makes a transition from a matterlike equation of state to a kinationlike equation of state around the epoch of recombination can significantly ameliorate the Hubble tension, i.e., the discrepancy between the determinations of the present-day expansion rate H0 from observations of the cosmic microwave background on one hand and type Ia supernovae on the other. We consider a specific, UV-complete model of such a rotating axion and find that it can relax the Hubble tension without exacerbating tensions in determinations of other cosmological parameters, in particular the amplitude of matter fluctuations S8. We subsequently demonstrate how this rotating axion model can also generate the baryon asymmetry of our Universe, by introducing a coupling of the axion field to right-handed neutrinos. This baryogenesis model predicts heavy neutral leptons that are most naturally within reach of future lepton colliders, but in finely tuned regions of parameter space may also be accessible at the high-luminosity LHC and the beam dump experiment SHiP. Published by the American Physical Society 2024

Diphoton signals of muon-philic scalars at DarkQuest

We analyze the capability of the DarkQuest proton beam-dump experiment at Fermilab to discover new light resonances decaying into photons. As an example model, we focus on muon-philic scalar particles that decay to photons. This is one of the few minimal models that can address the (g−2)μ anomaly at low mass. These scalars can be copiously produced by meson decays and muon bremsstrahlung. We point out that thanks to DarkQuest’s compact geometry, muons can propagate through the dump and efficiently produce dark scalars near the end of the dump. This mechanism enables DarkQuest to be sensitive to both long-lived and prompt scalars. At the same time, diphoton signatures are generically not background free, and we discuss in detail the different sources of background and strategies to mitigate them. We find that the backgrounds can be sufficiently reduced for DarkQuest to test currently viable (g−2)μ parameter space. Published by the American Physical Society 2024

Warm inflation with a heavy QCD axion

We propose the first model of warm inflation in which the particle production emerges directly from coupling the inflaton to Standard Model particles. Warm inflation, an early epoch of sustained accelerated expansion at finite temperature, is a compelling alternative to cold inflation, with distinct predictions for inflationary observables such as the amplitude of fluctuations, the spectral tilt, the tensor-to-scalar ratio, and non-gaussianities. In our model a heavy QCD axion acts as the warm inflaton whose coupling to Standard Model gluons sources the thermal bath during warm inflation. Axion-like couplings to non-Abelian gauge bosons have been considered before as a successful microphysical theory with emerging thermal friction that can maintain finite temperature during inflation via sphaleron heating. However, the presence of light fermions charged under the non-Abelian group suppresses particle production, hindering a realization of warm inflation by coupling to QCD. We point out that the Standard Model quarks can be heavy during warm inflation if the Higgs field resides in a high-energy second minimum which restores efficient sphaleron heating. A subsequent large reheating temperature is required to allow the Higgs field to relax to its electroweak minimum. Exploring a scenario in which hybrid warm inflation provides the large reheating temperature, we show that future collider and beam dump experiments have discovery potential for a heavy QCD axion taking the role of the warm inflaton.

Tracking system performance for the proposed SHADOWS experiment at CERN

The proposed SHADOWS experiment, Search for Hidden And Dark Objects With the SPS, is a proton beam-dump experiment at the CERN SPS. Positioned off-axis to suppress background, SHADOWS can profit from the high intensity ECN3 proton beam line, to explore the existence of a wide range of feebly-interacting particles (FIPs). The detector, with dimensions of 2.5 × 2.5 m2 and a length of 34 m, is designed for optimal tracking and timing performance, facilitating the identification and reconstruction of various visible final states of FIP decays. Despite the excellent outcome of the CERN SPS Committee review, the CERN Research Board decided to stop the project in March 2024. In this article we present the tracking system performance study based on simulation as presented in the Proposal. We show that using straw drift-tubes of 1 cm diameter, we can achieve a few mm vertex resolution and 1% mass resolution.

Dark matter-electron scattering and freeze-in scenarios in the light of Z′ mediation

We investigate dark matter (DM-)electron scattering in a minimal U(1)X extension of the Standard Model (SM), where the DM can appear as a Majorana fermion, a complex singlet scalar, or a Dirac fermion. To study bounds on the U(1)X gauge coupling (gX) and new gauge boson mass (MZ′), from DM-electron scattering, we consider several direct search experiments like CDMS, DAMIC, SENSEI, PandaX-II, DarkSide-50, and XENON1T-S2 for different U(1)X charges. In this setup, we consider DM production via freeze-in in both radiation-dominated and modified cosmological background to project sensitivities onto gX−MZ′ plane satisfying observed relic abundance. DM-electron scattering could provide comparable, or even stronger, bounds compared to those obtained from the electron/muon (g−2), low-energy scattering, and intensity frontier experiments within 0.01 GeV≲MZ′≲0.1 GeV. Constrains from freeze-in could provide stronger sensitivities for MZ′≳O(1) GeV; however, these limits are comparable to those obtained from LHCb and LEP experiments for O(10) GeV≲MZ′≲150 GeV. In the future, electron-muon scattering (MUonE), proton (FASER and DUNE), and electron/positron (ILC) beam-dump experiments could probe these parameters. Published by the American Physical Society 2024

The SHADOWS calorimeter — A pointing calorimeter for future beam dump experiments

This contribution presents advancements in the design and testing of an electromagnetic calorimeter intended for the detection of feebly interacting particles in future beam dump experiments. The calorimeter is designed to accurately reconstruct particle decays into photons, focusing on achieving precise energy and direction measurements. We propose a plastic scintillator-based calorimeter capable of providing the necessary resolution for these measurements. The pointing capability has been validated through GEANT4 simulations, optimizing scintillator granularity for effective shower direction reconstruction. Additionally, we detail the prototyping efforts and the first results of test beam measurements conducted at DESY.

Search by the SENSEI Experiment for Millicharged Particles Produced in the NuMI Beam.

Millicharged particles appear in several extensions of the standard model, but have not yet been detected. These hypothetical particles could be produced by an intense proton beam striking a fixed target. We use data collected in 2020 by the SENSEI experiment in the MINOS cavern at the Fermi National Accelerator Laboratory to search for ultrarelativistic millicharged particles produced in collisions of protons in the NuMI beam with a fixed graphite target. The absence of any ionization events with 3 to 6 electrons in the SENSEI data allow us to place world-leading constraints on millicharged particles for masses between 30 to 380MeV. This work also demonstrates the potential of utilizing low-threshold detectors to investigate new particles in beam-dump experiments, and motivates a future experiment designed specifically for this purpose.

Searching for a new light gauge boson with axial couplings in muon beam dump experiments

We present a formalism for new U(1) interactions involving weak hypercharge, baryon, and lepton numbers, and a possible axial symmetry generator FA in the presence of a second Brout-Englert-Higgs doublet. The resulting U boson, after mixing with the Z, interpolates between a generalised dark photon, a dark Z, and an axially coupled gauge boson. We especially focus on the axial couplings originating from FA or from mixing with the Z, determined by the scalar sector via parameters like tan β and the v.e.v. of an extra dark singlet.We explore the distinctive features of axially coupled interactions, especially in the ultrarelativistic limit, where the U boson behaves much as an axion-like particle, with enhanced interactions to quarks and leptons. This enhancement is particularly relevant for future muon beam dump experiments, since the muon mass considerably increases the effective coupling, proportional to 2mμ/mU, compared to analogous experiments with electrons.We also analyse the shape of the expected beam dump exclusion or discovery regions, influenced by U boson interactions and the experiment geometry. Different situations are considered, limited in particular by cases for which the U decays before reaching the detector, or has too small couplings to produce detectable events. We also compare to vectorially coupled bosons and axion-like pseudoscalars, highlighting the importance of understanding the parameter space for future experiment design and optimisation.

The SHADOWS experiment at the CERN SPS

SHADOWS (Search for Hidden And Dark Objects With the SPS) is a proposed proton beam-dump experiment for the search of a large variety of Feebly-Interacting Particles (FIPs) at the CERN SPS. It will exploit the potential for searches and discoveries at the intensity frontier offered by the upgrade of the ECN3 beam line.SHADOWS will be located off-axis, which allows the optimisation of the signal-to-background ratio, and will collect data from up to 5×1019 protons of 400 GeV on target in 4 years of operations.The experiment has a transversal size of 2.5×2.5 m2 and is composed by an upstream veto, a 20 m long decay volume and a spectrometer with a tracking system in a dipole magnet, a timing detector, a calorimeter and a muon system. The conceived detector offers excellenttracking and timing performance for the identification and reconstruction of most of the visible final states of FIP decays. SHADOWS will allow to explore a large parameter space region of many FIPs, like light dark scalars, axion-like particles and heavy neutral leptons, with masses ranging between 0.1 and 10 GeV.This paper reports about the status of the proposal of the SHADOWS experiment, with focus on the detector challenges.

Unveiling time-varying signals of ultralight bosonic dark matter at collider and beam dump experiments

Unveiling time-varying signals of ultralight bosonic dark matter at collider and beam dump experiments

Self-interacting forbidden dark matter under a cannibally co-decaying phase

In a usual dark matter (DM) model without a huge mass difference between the DM and lighter mediator, using the coupling strength suitable for having the correct relic density, the resulting self-interaction becomes several orders of magnitude smaller than that required to interpret the small-scale structures. We present a framework that can offer a solution for this point. We consider a model that contains the vector DM and a heavier but unstable Higgs-like scalar in the hidden sector. When the temperature drops below ~ mDM, the hidden sector, which is thermally decoupled from the visible sector, enters a cannibal phase, during which the DM density is depleted with the out-of-equilibrium decay of the scalar. The favored parameter region, giving the correct relic density and the proper size of self-interactions, shows the scalar-to-DM mass ratio ∈ [1.1, 1.33] and the scalar mass ∈ [9, 114] MeV. A sizable parameter space still survives the most current constraints and can be further probed by the near future NA62 beam dump experiment.

The new physics case for beam-dump experiments with accelerated muon beams

As the field examines a future muon collider as a possible successor to the LHC, we must consider how to fully utilize not only the high-energy particle collisions, but also any lower-energy staging facilities necessary in the R&D process. An economical and efficient possibility is to use the accelerated muon beam from either the full experiment or from cooling and acceleration tests in beam-dump experiments. Beam-dump experiments are complementary to the main collider as they achieve sensitivity to very small couplings with minimal instrumentation. We demonstrate the utility of muon beam-dump experiments for new physics searches at energies from 10 GeV to 5 TeV. We find that, even at low energies like those accessible at staging or demonstrator facilities, it is possible to probe new regions of parameter space for a variety of generic BSM models, including muonphilic, leptophilic, Lμ − Lτ, and dark photon scenarios. Such experiments could therefore provide opportunities for discovery of new physics well before the completion of the full multi-TeV collider.

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

The photon content of the neutron

In this work, we complete our CT18qed study with the neutron’s photon parton distribution function (PDF), which is essential for the nucleus scattering phenomenology. Two methods, CT18lux and CT18qed, based on the LUXqed formalism and the DGLAP evolution, respectively, to determine the neutron’s photon PDF have been presented. Various low-Q2 non-perturbative variations have been carefully examined, which are treated as additional uncertainties on top of those induced by quark and gluon PDFs. The impacts of the momentum sum rule as well as isospin symmetry violation have been explored and turned out to be negligible. A detailed comparison with other neutron’s photon PDF sets has been performed, which shows a great improvement in the precision and a reasonable uncertainty estimation. Finally, two phenomenological implications are demonstrated with photon-initiated processes: neutrino-nucleus W-boson production, which is important for the near-future TeV–PeV neutrino observations, and the axion-like particle production at a high-energy muon beam-dump experiment.

Leptonic ALP portal to the dark sector

We discuss the leptonic axionlike particle (ALP) portal as a simple scenario that connects observed discrepancies in anomalous magnetic moments to the dark matter relic abundance. In this framework an axionlike particle in the multi-MeV range couples to SM leptons and a dark matter (DM) fermion, with mass above the ALP mass but below 1 GeV. The ALP contributes to (g−2)μ and (g−2)e dominantly through two-loop Barr-Zee diagrams, while the DM abundance is generated by p-wave annihilation to ALP pairs. Constraints from beam-dump experiments, colliders, and cosmic microwave background probes are very stringent, and restrict the viable parameter space to a rather narrow region that will be tested in the near future. Published by the American Physical Society 2024

Z′ mediated right-handed neutrinos from meson decays at the FASER

We investigate the pair production of right-handed neutrinos mediated by a Z′ mainly from the meson decays at the FASER detector of the HL-LHC. The Z′ can be the additional gauge boson in either the U(1)B−L or sterile ν-specific U(1)s model. Taking the gauge coupling or the kinetic mixing at the current limits, we analyze the sensitivity to the masses of the heavy neutrinos, MN, and active-sterile mixing, |VlN|2, of FASER2. In a background-free scenario, for the U(1)B−L case, FASER2 is able to probe |VlN|2≈10−8 when MN∼0.3 GeV, which is comparable to the current limits from the beam dump experiments. When comes to the U(1)s model, FASER2 can probe |VlN|2≈10−10, which is better than the current limits for at least one magnitude, in all three flavors. A proposed long-lived particle detector, FACET, is also studied, but no significant difference from FASER2 is derived. Published by the American Physical Society 2024

Reconstructing axion-like particles from beam dumps with simulation-based inference

Axion-like particles (ALPs) that decay into photon pairs pose a challenge for experiments that rely on the construction of a decay vertex in order to search for long-lived particles. This is particularly true for beam-dump experiments, where the distance between the unknown decay position and the calorimeter can be very large. In this work we use machine learning to explore the possibility to reconstruct the ALP properties, in particular its mass and lifetime, from such inaccurate observations. We use a simulation-based inference approach based on conditional invertible neural networks to reconstruct the posterior probability of the ALP parameters for a given set of events. We find that for realistic angular and energy resolution, such a neural network significantly outperforms parameter reconstruction from conventional high-level variables while at the same time providing reliable uncertainty estimates. Moreover, the neural network can quickly be re-trained for different detector properties, making it an ideal framework for optimizing experimental design.

Improving the potential of BDF@SPS to search for new physics with liquid argon time projection chambers

Beam dump experiments proposed at the SPS are perfectly suited to explore the parameter space of models with long-lived particles, thanks to the combination of a large intensity with a high proton beam energy. In this paper, we study how the exploration power may be augmented further by installing a detector based on liquid argon time projection chamber technology. In particular, we consider several signatures of new physics particles that may be uniquely searched for with such a detector, including double bang events with heavy neutral leptons, inelastic light dark matter, and millicharged particles.

Sub-GeV dark matter search at ILC beam dumps

Light dark matter particles may be produced in electron and positron beam dumps of the International Linear Collider (ILC). We propose an experimental setup to search for such events, the Beam-Dump eXperiment at the ILC (ILC-BDX). The setup consists of a muon shield placed behind the beam dump, followed by a multi-layer tracker and an electromagnetic calorimeter. The calorimeter can detect electron recoils due to elastic scattering of dark matter particles produced in the dump, while the tracker is sensitive to decays of excited dark-sector states into the dark matter particle. We study the production, decay and scattering of sub-GeV dark matter particles in this setup in several models with a dark photon mediator. Taking into account beam-related backgrounds due to neutrinos produced in the beam dump as well as the cosmic-ray background, we evaluate the sensitivity reach of the ILC-BDX experiment. We find that the ILC-BDX will be able to probe interesting regions of the model parameter space and, in many cases, reach well below the relic target.

The MESA physics program

In the recent past, a comprehensive experimental program has been worked out at the Mainz Energy-Recovery Superconducting Accelerator, MESA, at the Institute of Nuclear Physics in Mainz. MESA is a high-intensity, low-energy electron accelerator presently under construction and will thereby provide great opportunities to perform a new generation of high-precision scattering experiments. The versatile MAGIX experiment will use MESA’s innovative energy recovery technique, with a science focus on the study of hadron structure and few-body systems, dark sector searches, and investigations into reactions pertinent to nuclear astrophysics. An external beam line will supply spin-polarized electrons to the P2 experiment, enabling the performance of sensitive tests of the Standard Model through parity-violating electron scattering. The DarkMESA beam dump experiment, situated behind P2, is dedicated to the search for light dark matter particles.