Journal of Physics G: Nuclear and Particle Physics

The European spallation source (ESS) will be the world's brightest neutron source and will open a new intensity frontier in particle physics. The HIBEAM collaboration aims to exploit the unique potential of the ESS with a dedicated ESS instrument for particle physics which offers world-leading capability in a number of areas. The HIBEAM program includes the first search in thirty years for free neutrons converting to antineutrons and searches for sterile neutrons, ultralight axion dark matter and nonzero neutron electric charge. This paper outlines the capabilities, design, infrastructure, and scientific potential of the HIBEAM program, including its dedicated beamline, neutron optical system, magnetic shielding and control, and detectors for neutrons and antineutrons. Additionally, we discuss the long-term scientific exploitation of HIBEAM, which may include measurements of the neutron electric dipole moment and precision studies of neutron decays.

We consider the electromagnetic form factors ratios in the Rosenbluth and polarization methods. We explore what impact adding new particles as mediators in electron-proton scattering has on these ratios. We show that such new particles can compensate for the difference between the two methods and potentially solve this paradox. Consequently, we find some bounds on the scalar coupling as α_sc ∼ 10⁻⁵ for m_sc ∼  5 MeV–2 GeV and α_sc ∼ 10⁻⁴ − 10⁻³ for m_sc ∼  2–10 GeV. Meanwhile, the vector coupling is bounded as α_v ∼ 10⁻⁵ for m_v ∼  5 MeV–1.1 GeV and α_v ∼ 10⁻⁴ − 10⁻³ for m_v ∼  1.2–10 GeV. These constraints are in complete agreement with those found in other independent terrestrial experiments.

The XLZD collaboration is developing a two-phase xenon time projection chamber with an active mass of 60–80 t capable of probing the remaining weakly interacting massive particle-nucleon interaction parameter space down to the so-called neutrino fog. In this work we show that, based on the performance of currently operating detectors using the same technology and a realistic reduction of radioactivity in detector materials, such an experiment will also be able to competitively search for neutrinoless double beta decay in ¹³⁶Xe using a natural-abundance xenon target. XLZD can reach a 3σ discovery potential half-life of 5.7 × 10²⁷ years (and a 90% CL exclusion of 1.3 × 10²⁸ years) with 10 years of data taking, corresponding to a Majorana mass range of 7.3–31.3 meV (4.8–20.5 meV). XLZD will thus exclude the inverted neutrino mass ordering parameter space and will start to probe the normal ordering region for most of the nuclear matrix elements commonly considered by the community.

Maximizing the discovery potential of increasingly precise neutrino experiments will require an improved theoretical understanding of neutrino-nucleus cross sections over a wide range of energies. Low-energy interactions are needed to reconstruct the energies of astrophysical neutrinos from supernovae bursts and search for new physics using increasingly precise measurement of coherent elastic neutrino scattering. Higher-energy interactions involve a variety of reaction mechanisms including quasi-elastic scattering, resonance production, and deep inelastic scattering that must all be included to reliably predict cross sections for energies relevant to DUNE and other accelerator neutrino experiments. Refined nuclear interaction models in these energy regimes will also be valuable for other applications, such as measurements of reactor, solar, and atmospheric neutrinos. This manuscript discusses the theoretical status, challenges, required resources, and path forward for achieving precise predictions of neutrino-nucleus scattering and emphasizes the need for a coordinated theoretical effort involved lattice QCD, nuclear effective theories, phenomenological models of the transition region, and event generators.

Inspired by the discrepancies observed in the b → sℓ⁺ℓ⁻ neutral current decays, we study the decay channel ${B}_{c}\to {D}_{s}^{(* )}\,{{\ell }}^{+}{{\ell }}^{-}$(ℓ = μ, τ), which is based on the same flavor changing neutral current transition at the quark level. The current study shows that this decay channel can provide a useful probe for physics beyond the standard model. We use the helicity formalism while employing the effective theory approach where we include the effects of vector and axial vector 'new' physics (NP) operators. In this study, we have computed the branching ratio ${{ \mathcal B }}_{r}$, the ${D}_{s}^{* }$ helicity fraction f_L, the lepton forward–backward asymmetry ${{ \mathcal A }}_{\mathrm{FB}}$, and the lepton flavor universality ratio (LFU) ${R}_{{D}_{s}^{* }}^{\tau \mu }$. In addition, as a complementary check on the LFU, we also calculate the various other LFU observables, ${R}_{i}^{\tau \mu }$ where i = A_FB, f_L. We assume that the NP universal coupling is present for both muons and tauons, while the non-universal coupling is only present for muons. Regarding these couplings, we employ the latest global fit to the b → sℓ⁺ℓ⁻ data, which is recently computed in Algueró etal [Eur. Phys. J. C83 648 (2023)]. We give predictions of some of the mentioned observables within the SM and the various NP scenarios. We have found that not only are the considered observables sensitive to NP but are also helpful in distinguishing among the different NP scenarios. These results can be tested at the LHCb, HL-LHC, and FCC-ee, and therefore, a precise measurements of these observables not only deepens our understanding of the b → sℓ⁺ℓ⁻ process but also provides a window of opportunity to possibly study various NP scenarios.

Maximizing the discovery potential of increasingly precise neutrino experiments will require an improved theoretical understanding of neutrino-nucleus cross sections over a wide range of energies. Low-energy interactions are needed to reconstruct the energies of astrophysical neutrinos from supernovae bursts and search for new physics using increasingly precise measurement of coherent elastic neutrino scattering. Higher-energy interactions involve a variety of reaction mechanisms including quasi-elastic scattering, resonance production, and deep inelastic scattering that must all be included to reliably predict cross sections for energies relevant to DUNE and other accelerator neutrino experiments. Refined nuclear interaction models in these energy regimes will also be valuable for other applications, such as measurements of reactor, solar, and atmospheric neutrinos. This manuscript discusses the theoretical status, challenges, required resources, and path forward for achieving precise predictions of neutrino-nucleus scattering and emphasizes the need for a coordinated theoretical effort involved lattice QCD, nuclear effective theories, phenomenological models of the transition region, and event generators.

Collective modes emerge as the relevant degrees of freedom that govern low-energy excitations of atomic nuclei. These modes—rotations, pairing rotations, and vibrations—are separated in energy from non-collective excitations, making it possible to describe them in the framework of effective field theory. Rotations and pairing rotations are the remnants of Nambu–Goldstone modes from the emergent breaking of rotational symmetry and phase symmetries in finite deformed and finite superfluid nuclei, respectively. The symmetry breaking severely constrains the structure of low-energy Lagrangians and thereby clarifies what is essential and simplifies the description. The approach via effective field theories exposes the essence of nuclear collective excitations and is defined with a breakdown scale in mind. This permits one to make systematic improvements and to estimate and quantify uncertainties. Effective field theories of collective excitations have been used to compute spectra, transition rates, and other matrix elements of interest. In particular, predictions of the nuclear matrix element for neutrinoless double beta decay then come with quantified uncertainties. This review summarizes these results and also compares the approach via effective field theories to well-known models and ab initio computations.

In modern high energy physics (HEP) experiments, triggers perform the important task of selecting, in real time, the data to be recorded and saved for physics analyses. As a result, trigger strategies play a key role in extracting relevant information from the vast streams of data produced at facilities like the large hadron collider (LHC). As the energy and luminosity of the collisions increase, these strategies must be upgraded and maintained to suit the experimental needs. This whitepaper presents a high-level overview and reviews recent developments of triggering practices employed at the LHC. The general trigger principles applied at modern HEP experiments are highlighted, with specific reference to the current trigger state-of-the-art within the ALICE, ATLAS, CMS and LHCb collaborations. Furthermore, a brief synopsis of the new trigger paradigm required by the upcoming high-luminosity upgrade of the LHC is provided. This whitepaper, compiled by Early Stage Researchers of the SMARTHEP network, is not meant to provide an exhaustive review or substitute documentation and papers from the collaborations themselves, but rather offer general considerations and examples from the literature that are relevant to the SMARTHEP network.

Collective anisotropic flow, where particles are correlated over the entire event, is a prominent phenomenon in relativistic heavy-ion collisions and is sensitive to the properties of the matter created in those collisions. It is often measured by two- and multi-particle correlations and is therefore contaminated by nonflow, those genuine few-body correlations unrelated to the global event-wise correlations. Many methods have been devised to estimate nonflow contamination with various degrees of successes and difficulties. Here, we review those methods pedagogically, discussing the pros and cons of each method, and give examples of ballpark estimate of nonflow contamination and associated uncertainties in relativistic heavy-ion collisions. We hope such a review of the various nonflow estimation methods in a single place would prove helpful to future researches.

Heavy-ion collisions provide a window into the properties of many-body systems of deconfined quarks and gluons. Understanding the collective properties of quarks and gluons is possible by comparing models of heavy-ion collisions to measurements of the distribution of particles produced at the end of the collisions. These model-to-data comparisons are extremely challenging, however, because of the complexity of the models, the large amount of experimental data, and their uncertainties. Bayesian inference provides a rigorous statistical framework to constrain the properties of nuclear matter by systematically comparing models and measurements. This review covers model emulation and Bayesian methods as applied to model-to-data comparisons in heavy-ion collisions. Replacing the model outputs (observables) with Gaussian process emulators is key to the Bayesian approach currently used in the field, and both current uses of emulators and related recent developments are reviewed. The general principles of Bayesian inference are then discussed along with other Bayesian methods, followed by a systematic comparison of seven recent Bayesian analyses that studied quark-gluon plasma properties, such as the shear and bulk viscosities. The latter comparison is used to illustrate sources of differences in analyses, and what it can teach us for future studies.

Neutron physics is one of the oldest branches of the experimental nuclear physics,
but the investigation of the spontaneous neutron emission from the ground state along the neutron dripline is still at its beginning, in spite of the crucial importance for nuclear astrophysics. The proton dripline is much better investigated and a systematics of spontaneous proton half lives corected by the centrifugal barrier (monopole transitions) is given by the Geiger-Nuttall law $\log_{10}T\sim\chi$, 
where $\chi\sim ZQ^{-1/2}$ is the Coulomb parameter characterizing the outgoing Coulomb-Hankel wave in terms of the daughter charge $Z$ and Q-value.
Our purpose is to propose a similar simple systematics of spontaneous neutron half lives, but in terms of the nuclear reduced radius $\rho=\kappa R\sim A^{1/3}Q^{1/2}$, characterizing the "neutral" outgoing spherical Hankel wave.
It turns out that the half life in emission of neutral particles is governed by the scaling law $T\sim\rho^{-2}\sim A^{-2/3}Q^{-1}$ for monopole transitions.
We evidence the important role of the angular momentum carried by the emitted neutron. The influence of the neutron wave function generated by a Woods-Saxon nuclear mean field is also analyzed.

In heavy-ion collisions, elliptic flow (\( v_2 \)) quantifies the azimuthal anisotropy in particle emission, reflecting the medium's response to initial spatial anisotropies. The presence of Electromagnetic fields produced by the fast moving protons in the nucleus can modify this flow, causing a splitting of \( v_2 \) between the produced particles and antiparticles. Hence in this study, we explore this effect, emphasizing the dominant role of electric fields in the charge splitting of elliptic flow, \( \Delta v_2 \), as a function of transverse momentum (\( p_T \)). 
The velocity and temperature profiles of quark-gluon plasma (QGP) medium is described through thermal model calculations. The electromagnetic field evolution is however determined from the solutions of Maxwell's equations, assuming constant electric and chiral conductivities. We find that the slower decay of the electric fields compared to the magnetic fields makes its impact on the splitting of the elliptic flow more dominant.
We further estimated that the maximum value of \( |\langle eF \rangle| \), evaluated by averaging the field values over all spatial points on the hypersurface and across all field components, is approximately \( (0.01 \pm 0.0002) \, m_{\pi}^2 \) for \( \sqrt{s_{\text{NN}}} = 7.7 \, \text{GeV} \), which could describe the splitting of elliptic flow data within the current experimental uncertainty reasonably well.

In this work, we investigate the fluctuations of the net-proton
 number in Au+Au collisions at √sNN = 14.5 and 19.6 GeV by calculating
 cumulants and their ratios within the framework of the Ultra-relativistic Quantum
 Molecular Dynamics (UrQMD) model. Our results are consistent with STAR data of C2/C1 and
C3/C2. Furthermore, we also calculated the sixth-order cumulants of net-proton
at √sNN = 14.5, 16.5, and 19.6 GeV. The results reveal the cumulant ratios
approach zero as the centrality increases. We also analyze the energy dependence
of the net-proton distribution at eight different energies and compare these results
with STAR experiment data. The analysis shows a monotonic trend for C2/C1
and C3/C2, while the UrQMD model cannot describe the non-monotonic energy
dependence for C4/C2 for the most central Au+Au collisions measured by STAR.
Finally, we investigated the energy dependence of proton factorial cumulants and
found a sign change, as well as fluctuations at low energies. Although the UrQMD
simulations do not incorporate the physics of the QCD critical point, the results
provide a baseline for the search for the QCD critical point.

 
 

We present a systematic study of the PYTHIA 8.311 model under various hadronization mechanisms to investigate proton, charged pion, and charged kaon production in inelastic proton-proton collisions at CERN SPS energies ranging from 20 to 158 GeV/c. Specifically, we compare four hadronization scenarios: (i) the default popcorn mechanism, which permits extended string configurations with intermediate meson formation between baryon-antibaryon pairs; (ii) a diquark scenario utilizing a QCD-based color reconnection scheme (CR1) designed to minimize string lengths, thereby restricting baryon production exclusively to baryon-antibaryon pairs; (iii) a junction-based scenario where baryon number is carried by non-perturbative QCD string junctions; and (iv) a thermal model featuring string tension fluctuations, leading to a thermal-like transverse momentum distribution. Our results indicate that the default popcorn approach inadequately describes proton stopping and hadron distributions at mid-rapidity, especially at lower collision energies. In contrast, the diquark scenario significantly enhances the accuracy of proton rapidity distributions, while the thermal model further improves the reproduction of pion spectra at lower transverse momenta and captures essential features of strangeness production, specifically suppressing $K^-$ at central rapidity ($y < 1$) and enhancing $K^+$ yields. Conversely, the junction-based scenario provides no substantial improvement at higher SPS energies. These findings underscore the necessity of refined hadronization descriptions for accurately modeling particle production at SPS energies.

Nuclear power reactors are the most intense man-made source of antineutrino's and have long been recognized as promising sources for coherent elastic neutrino-nucleus scattering (CE$\nu$NS) studies. Its observation and the spectral shape of the associated recoil spectrum is sensitive to a variety of exotic new physics scenarios and many experimental efforts are underway. Within the context of the reactor antineutrino anomaly, which initially indicated eV-scale sterile neutrino's, the modeling of the reactor antineutrino spectrum has seen a significant evolution in the last decade. Even so, uncertainties remain due to a variety of nuclear structure effects, incomplete information in nuclear databases and fission dynamics complexities. Here, we investigate the effects of these uncertainties on one's ability to accurately distinguish new physics signals. For the scenarios discussed here, we find that reactor spectral uncertainties are similar in magnitude to the projected sensitivities pointing towards a need for $\beta$ spectroscopy measurements below the inverse $\beta$ decay threshold.

The European spallation source (ESS) will be the world's brightest neutron source and will open a new intensity frontier in particle physics. The HIBEAM collaboration aims to exploit the unique potential of the ESS with a dedicated ESS instrument for particle physics which offers world-leading capability in a number of areas. The HIBEAM program includes the first search in thirty years for free neutrons converting to antineutrons and searches for sterile neutrons, ultralight axion dark matter and nonzero neutron electric charge. This paper outlines the capabilities, design, infrastructure, and scientific potential of the HIBEAM program, including its dedicated beamline, neutron optical system, magnetic shielding and control, and detectors for neutrons and antineutrons. Additionally, we discuss the long-term scientific exploitation of HIBEAM, which may include measurements of the neutron electric dipole moment and precision studies of neutron decays.

Neutron physics is one of the oldest branches of the experimental nuclear physics,&#xD;but the investigation of the spontaneous neutron emission from the ground state along the neutron dripline is still at its beginning, in spite of the crucial importance for nuclear astrophysics. The proton dripline is much better investigated and a systematics of spontaneous proton half lives corected by the centrifugal barrier (monopole transitions) is given by the Geiger-Nuttall law $\log_{10}T\sim\chi$, &#xD;where $\chi\sim ZQ^{-1/2}$ is the Coulomb parameter characterizing the outgoing Coulomb-Hankel wave in terms of the daughter charge $Z$ and Q-value.&#xD;Our purpose is to propose a similar simple systematics of spontaneous neutron half lives, but in terms of the nuclear reduced radius $\rho=\kappa R\sim A^{1/3}Q^{1/2}$, characterizing the "neutral" outgoing spherical Hankel wave.&#xD;It turns out that the half life in emission of neutral particles is governed by the scaling law $T\sim\rho^{-2}\sim A^{-2/3}Q^{-1}$ for monopole transitions.&#xD;We evidence the important role of the angular momentum carried by the emitted neutron. The influence of the neutron wave function generated by a Woods-Saxon nuclear mean field is also analyzed.

The XLZD collaboration is developing a two-phase xenon time projection chamber with an active mass of 60–80 t capable of probing the remaining weakly interacting massive particle-nucleon interaction parameter space down to the so-called neutrino fog. In this work we show that, based on the performance of currently operating detectors using the same technology and a realistic reduction of radioactivity in detector materials, such an experiment will also be able to competitively search for neutrinoless double beta decay in ¹³⁶Xe using a natural-abundance xenon target. XLZD can reach a 3σ discovery potential half-life of 5.7 × 10²⁷ years (and a 90% CL exclusion of 1.3 × 10²⁸ years) with 10 years of data taking, corresponding to a Majorana mass range of 7.3–31.3 meV (4.8–20.5 meV). XLZD will thus exclude the inverted neutrino mass ordering parameter space and will start to probe the normal ordering region for most of the nuclear matrix elements commonly considered by the community.

Maximizing the discovery potential of increasingly precise neutrino experiments will require an improved theoretical understanding of neutrino-nucleus cross sections over a wide range of energies. Low-energy interactions are needed to reconstruct the energies of astrophysical neutrinos from supernovae bursts and search for new physics using increasingly precise measurement of coherent elastic neutrino scattering. Higher-energy interactions involve a variety of reaction mechanisms including quasi-elastic scattering, resonance production, and deep inelastic scattering that must all be included to reliably predict cross sections for energies relevant to DUNE and other accelerator neutrino experiments. Refined nuclear interaction models in these energy regimes will also be valuable for other applications, such as measurements of reactor, solar, and atmospheric neutrinos. This manuscript discusses the theoretical status, challenges, required resources, and path forward for achieving precise predictions of neutrino-nucleus scattering and emphasizes the need for a coordinated theoretical effort involved lattice QCD, nuclear effective theories, phenomenological models of the transition region, and event generators.

Lattice QCD calculations of the 2s radial excitation of the nucleon place the state at an energy of approximately 1.9 GeV, raising the possibility that it is associated with the N1/2⁺(1880) and N1/2⁺(1710) resonances through mixing with two-particle meson-baryon states. The discovery of the N1/2⁺(1880) resonance in pion photoproduction but not in πN scattering and the small width of the N1/2⁺(1710) resonance suggest that a state associated with these resonances would be insensitive to the manner in which pions are permitted to dress it. To explore this possibility, we examine the spectrum of nucleon radial excitations in both 2 + 1 flavour QCD and in simulations where the coupling to meson-baryon states is significantly modified through quenching. We find the energy of the 2s radial excitation to be insensitive to this modification for quark masses close to the physical point. This invariance provides further evidence that the 2s radial excitation of the nucleon is associated with the N1/2⁺(1880) and N1/2⁺(1710) resonances.

It is well known that the coupling of an axion-like particle with a photon modifies the Maxwell equations. One of the main consequences of these modifications is the conversion of axions into photons. Little has been said about other possible effects. In this paper we show that the trajectory of an electron can be significantly altered because of the emergence of an electric field due to the dark matter background of the axion-like particles in this modified axion-electrodynamics. Different dark matter densities and magnetic field strengths are considered and it is shown that an axion-like particle with a mass m_a ∼ 10⁻²² eV generates an electric field that can significantly change the trajectory of an electron in these scenarios.

Collective modes emerge as the relevant degrees of freedom that govern low-energy excitations of atomic nuclei. These modes—rotations, pairing rotations, and vibrations—are separated in energy from non-collective excitations, making it possible to describe them in the framework of effective field theory. Rotations and pairing rotations are the remnants of Nambu–Goldstone modes from the emergent breaking of rotational symmetry and phase symmetries in finite deformed and finite superfluid nuclei, respectively. The symmetry breaking severely constrains the structure of low-energy Lagrangians and thereby clarifies what is essential and simplifies the description. The approach via effective field theories exposes the essence of nuclear collective excitations and is defined with a breakdown scale in mind. This permits one to make systematic improvements and to estimate and quantify uncertainties. Effective field theories of collective excitations have been used to compute spectra, transition rates, and other matrix elements of interest. In particular, predictions of the nuclear matrix element for neutrinoless double beta decay then come with quantified uncertainties. This review summarizes these results and also compares the approach via effective field theories to well-known models and ab initio computations.

Starting from the measurements conducted in 2004 with the ADAMO silicon spectrometer, we have performed a 2D fit procedure to diverse combinations of muon flux datasets documented in the literature. The fit employed three different formulas describing the energy and angular distribution of atmospheric muons at the Earth's surface. The analysis revealed that some measurements showed discrepancies in their compatibility with the expected results or between them with variations greater than a few sigmas. For example, vertical measurements expand over a wide range of flux values even for similar setup experiments. However, we have identified a formula capable of furnishing comprehensive spectrum coverage utilizing a singular set of parameters. Despite inherent limitations, we have achieved a satisfactory agreement with the Guan parameterization. Subsequently, we have integrated this optimized parameterization into the EcoMug library and a Geant4-based muon generator. The model's predictive capabilities have been validated by comparing it with experimental flux measurements and against other measurements in the literature. Through this process, we have shown and tested the reliability and accuracy of different muon flux modelling, facilitating advancements in domains reliant on the precise characterization of atmospheric muon phenomena.

Transverse momentum correlations were recently measured by the ALICE collaboration at the LHC [Acharya etal (2020 Phys. Lett. B804 135375)]. A long-range structure in terms of relative pseudorapidity of particle pairs is observed. This may imply some signal of the initial state owing to the sheer spread of the correlation. However, the fluctuations inside a thermally equilibrated medium have to be taken into account, serving as motivation for this paper. Using lattice Quantum Chromodynamics constraints, we predicted the development and spread of balancing correlations caused by energy-momentum conservation. At the same time, we propagated the Gaussian-shaped initial correlation using hydrodynamics to estimate its effects. Our findings suggest that the resulting correlation, known as 'the ridge,' is sensitive both to flucutations seeded in the pre-equilibrium stage and to those seeded in the equilibrated medium. This can provide important insight into the early stages of the collision.

In modern high energy physics (HEP) experiments, triggers perform the important task of selecting, in real time, the data to be recorded and saved for physics analyses. As a result, trigger strategies play a key role in extracting relevant information from the vast streams of data produced at facilities like the large hadron collider (LHC). As the energy and luminosity of the collisions increase, these strategies must be upgraded and maintained to suit the experimental needs. This whitepaper presents a high-level overview and reviews recent developments of triggering practices employed at the LHC. The general trigger principles applied at modern HEP experiments are highlighted, with specific reference to the current trigger state-of-the-art within the ALICE, ATLAS, CMS and LHCb collaborations. Furthermore, a brief synopsis of the new trigger paradigm required by the upcoming high-luminosity upgrade of the LHC is provided. This whitepaper, compiled by Early Stage Researchers of the SMARTHEP network, is not meant to provide an exhaustive review or substitute documentation and papers from the collaborations themselves, but rather offer general considerations and examples from the literature that are relevant to the SMARTHEP network.