Journal of Physics G: Nuclear and Particle Physics

Particles beyond the Standard Model (SM) can generically have lifetimes that are long compared to SM particles at the weak scale. When produced at experiments such as the Large Hadron Collider (LHC) at CERN, these long-lived particles (LLPs) can decay far from the interaction vertex of the primary proton–proton collision. Such LLP signatures are distinct from those of promptly decaying particles that are targeted by the majority of searches for new physics at the LHC, often requiring customized techniques to identify, for example, significantly displaced decay vertices, tracks with atypical properties, and short track segments. Given their non-standard nature, a comprehensive overview of LLP signatures at the LHC is beneficial to ensure that possible avenues of the discovery of new physics are not overlooked. Here we report on the joint work of a community of theorists and experimentalists with the ATLAS, CMS, and LHCb experiments—as well as those working on dedicated experiments such as MoEDAL, milliQan, MATHUSLA, CODEX-b, and FASER—to survey the current state of LLP searches at the LHC, and to chart a path for the development of LLP searches into the future, both in the upcoming Run 3 and at the high-luminosity LHC. The work is organized around the current and future potential capabilities of LHC experiments to generally discover new LLPs, and takes a signature-based approach to surveying classes of models that give rise to LLPs rather than emphasizing any particular theory motivation. We develop a set of simplified models; assess the coverage of current searches; document known, often unexpected backgrounds; explore the capabilities of proposed detector upgrades; provide recommendations for the presentation of search results; and look towards the newest frontiers, namely high-multiplicity ‘dark showers’, highlighting opportunities for expanding the LHC reach for these signals.

The nature of dark matter and properties of neutrinos are among the most pressing issues in contemporary particle physics. The dual-phase xenon time-projection chamber is the leading technology to cover the available parameter space for weakly interacting massive particles, while featuring extensive sensitivity to many alternative dark matter candidates. These detectors can also study neutrinos through neutrinoless double-beta decay and through a variety of astrophysical sources. A next-generation xenon-based detector will therefore be a true multi-purpose observatory to significantly advance particle physics, nuclear physics, astrophysics, solar physics, and cosmology. This review article presents the science cases for such a detector.

We review the description of tunnelling phenomena in the semi-classical approximation in ordinary quantum mechanics and in quantum field theory. In particular, we describe in detail the calculation, up to the first quantum corrections, of the decay probability per unit time of a metastable ground state. We apply the relevant formalism to the case of the standard model of electroweak interactions, whose ground state is metastable for sufficiently large values of the top quark mass. Finally, we discuss the impact of gravitational interactions on the calculation of the tunnelling rate.

This white paper is the result of a collaboration by many of those that attended a workshop at the facility for rare isotope beams (FRIB), organized by the FRIB Theory Alliance (FRIB-TA), on ‘Theoretical Justifications and Motivations for Early High-Profile FRIB Experiments’. It covers a wide range of topics related to the science that will be explored at FRIB. After a brief introduction, the sections address: section 2: Overview of theoretical methods, section 3: Experimental capabilities, section 4: Structure, section 5: Near-threshold Physics, section 6: Reaction mechanisms, section 7: Nuclear equations of state, section 8: Nuclear astrophysics, section 9: Fundamental symmetries, and section 10: Experimental design and uncertainty quantification.

The main objectives of the KM3NeT Collaboration are (i) the discovery and subsequent observation of high-energy neutrino sources in the Universe and (ii) the determination of the mass hierarchy of neutrinos. These objectives are strongly motivated by two recent important discoveries, namely: (1) the high-energy astrophysical neutrino signal reported by IceCube and (2) the sizable contribution of electron neutrinos to the third neutrino mass eigenstate as reported by Daya Bay, Reno and others. To meet these objectives, the KM3NeT Collaboration plans to build a new Research Infrastructure consisting of a network of deep-sea neutrino telescopes in the Mediterranean Sea. A phased and distributed implementation is pursued which maximises the access to regional funds, the availability of human resources and the synergistic opportunities for the Earth and sea sciences community. Three suitable deep-sea sites are selected, namely off-shore Toulon (France), Capo Passero (Sicily, Italy) and Pylos (Peloponnese, Greece). The infrastructure will consist of three so-called building blocks. A building block comprises 115 strings, each string comprises 18 optical modules and each optical module comprises 31 photo-multiplier tubes. Each building block thus constitutes a three-dimensional array of photo sensors that can be used to detect the Cherenkov light produced by relativistic particles emerging from neutrino interactions. Two building blocks will be sparsely configured to fully explore the IceCube signal with similar instrumented volume, different methodology, improved resolution and complementary field of view, including the galactic plane. One building block will be densely configured to precisely measure atmospheric neutrino oscillations.

The Physics Beyond Colliders initiative is an exploratory study aimed at exploiting the full scientific potential of the CERN’s accelerator complex and scientific infrastructures through projects complementary to the LHC and other possible future colliders. These projects will target fundamental physics questions in modern particle physics. This document presents the status of the proposals presented in the framework of the Beyond Standard Model physics working group, and explore their physics reach and the impact that CERN could have in the next 10–20 years on the international landscape.

The detection of gravitational waves from mergers of tens of Solar mass black hole binaries has led to a surge in interest in primordial black holes (PBHs) as a dark matter candidate. We aim to provide a (relatively) concise overview of the status of PBHs as a dark matter candidate, circa Summer 2020. First we review the formation of PBHs in the early Universe, focussing mainly on PBHs formed via the collapse of large density perturbations generated by inflation. Then we review the various current and future constraints on the present day abundance of PBHs. We conclude with a discussion of the key open questions in this field.

The existence of nonzero neutrino masses points to the likely existence of multiple Standard Model neutral fermions. When such states are heavy enough that they cannot be produced in oscillations, they are referred to as heavy neutral leptons (HNLs). In this white paper, we discuss the present experimental status of HNLs including colliders, beta decay, accelerators, as well as astrophysical and cosmological impacts. We discuss the importance of continuing to search for HNLs, and its potential impact on our understanding of key fundamental questions, and additionally we outline the future prospects for next-generation future experiments or upcoming accelerator run scenarios.

This document summarizes the discussions and outcomes of the Facility for Rare Isotope Beams (FRIB) Theory Alliance topical program ‘The path to Superheavy Isotopes’ held in June 2024 at FRIB. Its content is non-exhaustive, reflecting topics chosen and discussed by the participants. The program aimed to assess the current status of theory in superheavy nuclei (SHN) research and identify necessary theoretical developments to guide experimental programs and determine fruitful production mechanisms. This report details the intersection of SHN research with other fields, provides an overview of production mechanisms and theoretical models, discusses future needs in theory and experiment, explores other potential avenues for SHN synthesis, and highlights the importance of building a strong theory community in this area.

The equation of state for a degenerate gas of fermions at zero temperature in the non-relativistic case is a polytrope, i.e. $p\sim {\rho }^{5/3}/{m}_{F}^{8/3}$. If dark matter is modeled by such a non-interacting fermion, this dependence on the mass of the fermion m_F explains why, if dark matter is very heavy, the effective pressure of dark matter is negligible. Nevertheless, if the mass of the dark matter is very low, the effective pressure can be very large, and thus a system of self-gravitating fermions can be formed. In this work we model the dark matter halo of the Milky Way by solving the Tolman–Oppenheimer–Volkoff equations, with the equation of state for a partially degenerate ultralight non-interacting fermion. We found that to fit the rotational velocity curve of the Milky Way, the mass of the fermion should be in the range 31.5 eV < m_F < 35 eV at a 90% confidence level. Moreover, the central density is restricted to be in the range of 1.2 < ρ₀ < 1.7 GeV cm^–3 at a 90% confidence level. The fermionic dark matter halo has a very different profile from the standard Navarro–Frenk–White profile, thus the possible indirect signals for annihilating dark matter may change by orders of magnitude. We found bounds for the annihilation cross section in this case by using the Saggitarius A* spectral energy distribution.

In this work, we investigate the properties of the flux tube produced between static quark–antiquark pairs using the energy–momentum tensor (EMT) in SU(3) pure gauge theory at finite temperature. We compute the width of the flux tube at multiple transverse cross-sections for various quark separations at two temperatures below the deconfined phase, T = 0.86T_c and T = 0.95T_c, as well as at two temperatures above the critical temperature, T = 1.03T_c and 1.44T_c. The gradient flow method Lüschera and Weisz (2011 J. High Energy Phys. JHEP02(2011) 051); Suzuki (2013 Prog. Theor. Experimental Phys.2013 083B03); Asakawa et al (2014 Phys. Rev. D90 011501) was used to define the EMT on the lattice and to suppress the statistical fluctuation of the gauge configuration in numerical simulations. We analyze the symmetry and temperature dependence of the flux tube profile, and compare our results with those of previous studies based on the action density Bakry (2016 (Springer) chap 6).

In order to comprehend the process underlying mirror energy differences (MEDs) in mirror pairs, we have performed shell-model calculations for T_z = ±2 sd-shell nuclei in the mass range A = 20–36 and neutron number varying from N = 8 to 20. Isospin-symmetry breaking is responsible for the MED of excited states. We have investigated the isospin non-conserving interactions: USDC and USDCm to explore the low-lying energy spectra, MEDs, isoscalar (M₀), isovector (M₁) matrix elements, E2 transition probability, magnetic (μ), and quadrupole moments (Q) of mirror-pair and compared them with their available experimental data. The impact of single-particle states on weakly bound and unbound nuclear states are investigated, especially those of the s-wave. We have also analyzed single proton/neutron separation energies and proton/neutron occupancy for (T_z = −2)/(T_z = +2) sd-shell nuclei.

This study demonstrates the application of supervised machine learning (ML) techniques to distinguish between isotropic and jet-like event topologies in heavy-ion collisions via the spherocity observable. State-of-the-art ML algorithms, optimized through systematic hyperparameter tuning, are employed to predict both the traditional transverse spherocity S₀ and the unweighted transverse spherocity ${S}_{0}^{{p}_{{\rm{T}}}=1}$ directly from raw event data. Moreover, the results of this study demonstrate that our approach remains largely model-independent, underscoring its potential applicability in future experimental heavy-ion physics analyses.

The investigation of quartic gauge couplings provides a crucial test of the Standard Model and serves as a potential window into new physics at higher energy scales. Within the framework of Effective Field Theory, deviations from the SM can be parameterized through dimension-8 operators. In this study, we analyze the process pp → γγjj at the High-Luminosity Large Hadron Collider (HL-LHC) and the Future Circular Collider in hadron mode (FCC-hh) to probe the sensitivity to anomalous quartic gauge couplings (aQGCs), particularly f_T8/Λ⁴ and f_T9/Λ⁴. Monte Carlo simulations of signal and relevant backgrounds are performed using MadGraph for event generation, Pythia for parton showering and hadronization, and Delphes for detector simulation. A multivariate analysis based on Boosted Decision Trees is employed to optimize the signal-to-background discrimination, incorporating a comprehensive set of kinematic and reconstructed variables of the final state particles. Additionally, we evaluate unitarity-violating effects associated with dimension-8 operators by imposing energy cutoffs on the di-photon invariant mass. The expected exclusion and discovery significances are computed, accounting for systematic uncertainties to ensure a realistic assessment of collider reach. Our findings indicate that the FCC-hh offers significantly improved sensitivity compared to the HL-LHC and current experimental results by LHC, reinforcing its potential for probing aQGCs. Notably, even under a 10% systematic uncertainty, our projected limits for FCC-hh at 95% confidence level surpass the current best constraints reported by the ATLAS and CMS collaborations, highlighting the enhanced discovery prospects at future high-energy colliders.

This document summarizes the discussions and outcomes of the Facility for Rare Isotope Beams (FRIB) Theory Alliance topical program ‘The path to Superheavy Isotopes’ held in June 2024 at FRIB. Its content is non-exhaustive, reflecting topics chosen and discussed by the participants. The program aimed to assess the current status of theory in superheavy nuclei (SHN) research and identify necessary theoretical developments to guide experimental programs and determine fruitful production mechanisms. This report details the intersection of SHN research with other fields, provides an overview of production mechanisms and theoretical models, discusses future needs in theory and experiment, explores other potential avenues for SHN synthesis, and highlights the importance of building a strong theory community in this area.

The kaon physics programme, long heralded as a cutting-edge frontier by the European Strategy for Particle Physics, continues to stand at the intersection of discovery and innovation in high-energy physics (HEP). With its unparalleled capacity to explore new physics at the multi-TeV scale, kaon research is poised to unveil phenomena that could reshape our understanding of the Universe. This document highlights the compelling physics case, with emphasis on exciting new opportunities for advancing kaon physics not only in Europe but also on a global stage. As an important player in the future of HEP, the kaon programme promises to drive transformative breakthroughs, inviting exploration at the forefront of scientific discovery.

Genetic Programming (GP) is an evolutionary algorithm that generates computer programs, or mathematical expressions, to solve complex problems. In this Guide, we demonstrate how to use GP to develop surrogate models to mitigate the computational costs of modeling atomic nuclei with ever increasing complexity. The computational burden escalates when uncertainty quantification is pursued, or when observables must be globally computed for thousands of nuclei. By studying three models in which the mean field depends on the total particle density self-consistently, we show that by constructing reduced order models supported by GP one can speed up many-body computations by several orders of magnitude with a negligible loss in accuracy.

Processes that violate baryon number, most notably proton decay and $n\bar{n}$ transitions, are promising probes of physics beyond the Standard Model (BSM) needed to understand the lack of antimatter in the Universe. To interpret current and forthcoming experimental limits, theory input from nuclear matrix elements to UV complete models enters. Thus, an interplay of experiment, effective field theory, lattice QCD, and BSM model building is required to develop strategies to accurately extract information from current and future data and maximize the impact and sensitivity of next-generation experiments. Here, we briefly summarize the main results and discussions from the workshop ‘INT-25-91W: Baryon Number Violation: From Nuclear Matrix Elements to BSM Physics,’ held at the Institute for Nuclear Theory, University of Washington, Seattle, WA, 13–17 January 2025.

This paper reports on the possible role of tritium-induced reactions of light nuclei, which may influence nucleosynthesis in short-lived environments such as the third minute of the Big Bang. They may also play a role during the emergence of the neutrino-driven shock front in core collapse supernovae or merging neutron stars at extreme densities. The production of tritium requires a very dynamic and neutron-rich environment; under such conditions tritium-induced reactions are expected to play an important role in the development of specific reaction patterns that could lead to a delayed release of neutrons influencing the associated nucleosynthesis. Here, we summarize different possible reaction sequences and discuss the strength and impact of tritium cluster resonances that occur near the tritium threshold in the respective compound systems.

We propose a minimal inverse seesaw framework based on $A_4$ modular symmetry. We have studied the neutrino oscillation parameters in our work and our model excludes some $3 \sigma$ values of the mixing angle $\theta_{23}$. Also, there is a clear linear relation between the mixing angles $\theta_{12}$ and $\theta_{23}$ found in the allowed $3 \sigma$ region. We also examine whether the parameter points consistent with neutrino oscillation data simultaneously comply with the experimental limits on lepton flavor violating (LFV) decays, specifically: $\mu \longrightarrow e \gamma$, $\tau \longrightarrow e \gamma$, and $\tau \longrightarrow \mu \gamma$. We have also investigated the matter-antimatter asymmetry of our universe via the resonant leptogenesis mechanism. Here, we present the contribution of lepton number conserving scattering processes mediated by the \( Z' \) boson in the context of leptogenesis.

Through the application of a molecular model to band-head states in ⁹B, a prediction of the Coulomb energy difference relative to the 1/2⁺ analogue state in ⁹Be is calculated. Under such treatment, a positive shift in energy of 0.605 MeV is found to emerge from molecular structures defined by replication of experimental rotational band parameters. Given the relationship between the spatial arrangement of clusters within the nucleus and the obtained Coulomb energy, it is also possible to examine nuclear structures using known Thomas-Ehrman shifts. Molecular structures built using the α-α separation, 2β, required to give Thomas-Ehrman shifts for both a 1.84 and 0.8 MeV candidate of the ⁹B(1/2⁺) analogue state (β = 1.89 fm and β = 3.18 fm respectively) are used to calculate state inertial parameters (A). From these structures, it is found that a 1.84 MeV state (A = 0.407 MeV) agrees with the experimental K = 1/2⁺ rotational band inertial parameter (A = 0.41 ± 0.01 MeV) and a 0.8 MeV state does not (A = 0.191 MeV). Thus, due to the large α-α spacing required to lower the Coulomb energy relative to 9 Be, the existence of a 0.8 MeV ⁹B(1/2⁺) analogue state is highly unlikely. Indeed, excitation energies less than ≈1.25 MeV for a 1/2⁺ state, corresponding to a normal Thomas-Ehrman shift, can also be ruled out.

We present a generalisation of our previous approach of a renormalon-motivated resummation of the QCD observables. Previously it was applied to the spacelike observables whose perturbation expansion was D(Q 2 ) = a(Q 2 )+O(a 2 ), where a(Q 2 ) ≡ αs(Q 2 )/π is the running QCD coupling. Now we generalise the resummation to spacelike quantities D(Q 2 ) = a(Q 2 ) ν 0 + O(a ν 0 +1 ) and timelike quantities F(σ) = a(σ) ν 0 + O(a ν 0 +1 ), where ν0 is in general a noninteger number (0 < ν0 ≤ 1). We evaluate with this approach a timelike quantity, namely the scheme-invariant factor of the Wilson coefficient of the chromomagnetic operator in the heavy-quark effective Lagrangian, and related quantities.

We examine the relationship between the Asymptotic Normalization Coefficient (ANC) of $^6$Li and other low-energy observables in the $\alpha$-deuteron system. Our analysis uses a set of calculations carried out within the {\it ab initio} No Core Shell Model with Continuum (NCSMC) using a variety of inter-nucleon &#xD; interactions and basis sizes, and yielding ${}^6$Li deuteron separation energies between 1.3 and 1.8 MeV~\cite{PhysRevLett.129.042503}. These NCSMC calculations show that the square of the ANC is strongly correlated with the separation energy over this range. In this work, we investigate the origin of this correlation using the phenomenological $R$-matrix, a single-channel potential and a perturbative approach. We show that this correlation occurs because the depth of the $\alpha$-deuteron central potential changes by only a small relative amount as the separation energy varies. We then investigate if the ANC can be accurately extracted from $\alpha$-deuteron phase shifts in an ideal case in which low-energy data are available and there are no experimental errors. We find that both $R$-matrix and Coulomb-modified effective-range theory (CM-ERE) yield extracted ANCs close to, although not exactly equal to, the NCSMC value, &#xD; provided the extrapolation is constrained by the known position of the bound-state pole and at least three terms are included in the fit function. The $R$-matrix approach converges faster than the CM-ERE as the number of parameters increases and is also more robust against the inclusion of low-energy and high-energy phase shift data. Finally, our study also shows that a naive quantification of uncertainties by comparing different truncations used in both theories is not accurate, and suggests the accuracy of ANCs extracted from phase shift data needs further investigation.

In the framework of a macroscopic α-cluster model, the structural properties and the spectroscopy of the ²⁴Mg nucleus are investigated. Special attention is devoted to the electromagnetic selection rules imposed by the point-symmetry group D_4h that leaves invariant the adopted 6α equilibrium configuration, a square bipyramid. The analysis entails the application of group-theoretical identities and character tables, in a way familiar to quantum chemists. The results show that the occurrence of interband E0, E2, and M1, M2, M3 transitions is strictly regulated by the transformation properties of the excited vibrational modes to which the states in the process belong. Unlike the ¹²C nucleus in the D_3h-symmetric 3α arrangement, M1 transition channels are active between states corresponding to a single quantum of vibrational excitation. Conversely, the measured E1 strengths in the ²⁴Mg spectrum are attributable to the excitation of single-nucleon degrees of freedom, as E1 transitions are forbidden by the model. The implications can be relevant for the spectroscopic analysis of&#xD;²⁴Mg, namely for the γ transitions between collective states ascribed to different&#xD;rotational bands.

The equation of state for a degenerate gas of fermions at zero temperature in the non-relativistic case is a polytrope, i.e. $p\sim {\rho }^{5/3}/{m}_{F}^{8/3}$. If dark matter is modeled by such a non-interacting fermion, this dependence on the mass of the fermion m_F explains why, if dark matter is very heavy, the effective pressure of dark matter is negligible. Nevertheless, if the mass of the dark matter is very low, the effective pressure can be very large, and thus a system of self-gravitating fermions can be formed. In this work we model the dark matter halo of the Milky Way by solving the Tolman–Oppenheimer–Volkoff equations, with the equation of state for a partially degenerate ultralight non-interacting fermion. We found that to fit the rotational velocity curve of the Milky Way, the mass of the fermion should be in the range 31.5 eV < m_F < 35 eV at a 90% confidence level. Moreover, the central density is restricted to be in the range of 1.2 < ρ₀ < 1.7 GeV cm^–3 at a 90% confidence level. The fermionic dark matter halo has a very different profile from the standard Navarro–Frenk–White profile, thus the possible indirect signals for annihilating dark matter may change by orders of magnitude. We found bounds for the annihilation cross section in this case by using the Saggitarius A* spectral energy distribution.

Through the application of a molecular model to band-head states in ⁹B, a prediction of the Coulomb energy difference relative to the 1/2⁺ analogue state in ⁹Be is calculated. Under such treatment, a positive shift in energy of 0.605 MeV is found to emerge from molecular structures defined by replication of experimental rotational band parameters. Given the relationship between the spatial arrangement of clusters within the nucleus and the obtained Coulomb energy, it is also possible to examine nuclear structures using known Thomas-Ehrman shifts. Molecular structures built using the α-α separation, 2β, required to give Thomas-Ehrman shifts for both a 1.84 and 0.8 MeV candidate of the ⁹B(1/2⁺) analogue state (β = 1.89 fm and β = 3.18 fm respectively) are used to calculate state inertial parameters (A). From these structures, it is found that a 1.84 MeV state (A = 0.407 MeV) agrees with the experimental K = 1/2⁺ rotational band inertial parameter (A = 0.41 ± 0.01 MeV) and a 0.8 MeV state does not (A = 0.191 MeV). Thus, due to the large α-α spacing required to lower the Coulomb energy relative to 9 Be, the existence of a 0.8 MeV ⁹B(1/2⁺) analogue state is highly unlikely. Indeed, excitation energies less than ≈1.25 MeV for a 1/2⁺ state, corresponding to a normal Thomas-Ehrman shift, can also be ruled out.

We present a generalisation of our previous approach of a renormalon-motivated resummation of the QCD observables. Previously it was applied to the spacelike observables whose perturbation expansion was D(Q 2 ) = a(Q 2 )+O(a 2 ), where a(Q 2 ) ≡ αs(Q 2 )/π is the running QCD coupling. Now we generalise the resummation to spacelike quantities D(Q 2 ) = a(Q 2 ) ν 0 + O(a ν 0 +1 ) and timelike quantities F(σ) = a(σ) ν 0 + O(a ν 0 +1 ), where ν0 is in general a noninteger number (0 < ν0 ≤ 1). We evaluate with this approach a timelike quantity, namely the scheme-invariant factor of the Wilson coefficient of the chromomagnetic operator in the heavy-quark effective Lagrangian, and related quantities.

We examine the relationship between the Asymptotic Normalization Coefficient (ANC) of $^6$Li and other low-energy observables in the $\alpha$-deuteron system. Our analysis uses a set of calculations carried out within the {\it ab initio} No Core Shell Model with Continuum (NCSMC) using a variety of inter-nucleon &#xD; interactions and basis sizes, and yielding ${}^6$Li deuteron separation energies between 1.3 and 1.8 MeV~\cite{PhysRevLett.129.042503}. These NCSMC calculations show that the square of the ANC is strongly correlated with the separation energy over this range. In this work, we investigate the origin of this correlation using the phenomenological $R$-matrix, a single-channel potential and a perturbative approach. We show that this correlation occurs because the depth of the $\alpha$-deuteron central potential changes by only a small relative amount as the separation energy varies. We then investigate if the ANC can be accurately extracted from $\alpha$-deuteron phase shifts in an ideal case in which low-energy data are available and there are no experimental errors. We find that both $R$-matrix and Coulomb-modified effective-range theory (CM-ERE) yield extracted ANCs close to, although not exactly equal to, the NCSMC value, &#xD; provided the extrapolation is constrained by the known position of the bound-state pole and at least three terms are included in the fit function. The $R$-matrix approach converges faster than the CM-ERE as the number of parameters increases and is also more robust against the inclusion of low-energy and high-energy phase shift data. Finally, our study also shows that a naive quantification of uncertainties by comparing different truncations used in both theories is not accurate, and suggests the accuracy of ANCs extracted from phase shift data needs further investigation.

In the framework of a macroscopic α-cluster model, the structural properties and the spectroscopy of the ²⁴Mg nucleus are investigated. Special attention is devoted to the electromagnetic selection rules imposed by the point-symmetry group D_4h that leaves invariant the adopted 6α equilibrium configuration, a square bipyramid. The analysis entails the application of group-theoretical identities and character tables, in a way familiar to quantum chemists. The results show that the occurrence of interband E0, E2, and M1, M2, M3 transitions is strictly regulated by the transformation properties of the excited vibrational modes to which the states in the process belong. Unlike the ¹²C nucleus in the D_3h-symmetric 3α arrangement, M1 transition channels are active between states corresponding to a single quantum of vibrational excitation. Conversely, the measured E1 strengths in the ²⁴Mg spectrum are attributable to the excitation of single-nucleon degrees of freedom, as E1 transitions are forbidden by the model. The implications can be relevant for the spectroscopic analysis of&#xD;²⁴Mg, namely for the γ transitions between collective states ascribed to different&#xD;rotational bands.

The investigation of quartic gauge couplings provides a crucial test of the Standard Model and serves as a potential window into new physics at higher energy scales. Within the framework of Effective Field Theory, deviations from the SM can be parameterized through dimension-8 operators. In this study, we analyze the process pp → γγjj at the High-Luminosity Large Hadron Collider (HL-LHC) and the Future Circular Collider in hadron mode (FCC-hh) to probe the sensitivity to anomalous quartic gauge couplings (aQGCs), particularly f_T8/Λ⁴ and f_T9/Λ⁴. Monte Carlo simulations of signal and relevant backgrounds are performed using MadGraph for event generation, Pythia for parton showering and hadronization, and Delphes for detector simulation. A multivariate analysis based on Boosted Decision Trees is employed to optimize the signal-to-background discrimination, incorporating a comprehensive set of kinematic and reconstructed variables of the final state particles. Additionally, we evaluate unitarity-violating effects associated with dimension-8 operators by imposing energy cutoffs on the di-photon invariant mass. The expected exclusion and discovery significances are computed, accounting for systematic uncertainties to ensure a realistic assessment of collider reach. Our findings indicate that the FCC-hh offers significantly improved sensitivity compared to the HL-LHC and current experimental results by LHC, reinforcing its potential for probing aQGCs. Notably, even under a 10% systematic uncertainty, our projected limits for FCC-hh at 95% confidence level surpass the current best constraints reported by the ATLAS and CMS collaborations, highlighting the enhanced discovery prospects at future high-energy colliders.

To understand the dynamics of jet–medium interaction in small systems such as proton–proton (pp) and proton–lead (p–Pb) collisions at $\sqrt{{s}_{{\rm{NN}}}}$ = 5.02 TeV, particle production is studied in three distinct topological regions defined with respect to the charged particle with the highest transverse momentum in the event (${p}_{{\rm{T}}}^{{\rm{trig}}}$). The jet-like yield is defined by the particle density in the toward region (∣Δφ∣ < π/3) after subtracting that in the transverse region (π/3 < ∣Δφ∣ < 2π/3). The activity on the transverse side is used as a proxy for medium-like effects. Three different Monte Carlo event generators—Pythia8, a multiphase transport (AMPT) model, and EPOS4—are employed to investigate particle yields as a function of ${p}_{{\rm{T}}}^{{\rm{trig}}}$ in the interval 0.5–20 GeV/c. Calculations are performed for the p_T threshold of 0.5 GeV/c at mid-rapidity (∣η∣ < 0.8). The jet-like yield in the toward region for pp collisions show interesting dynamics; they are significantly affected by the medium-like effects in the low to intermediate ${p}_{{\rm{T}}}^{{\rm{trig}}}$ (<8 GeV/c) which is studied through color reconnection and hydrodynamics in Pythia8 and EPOS4, respectively. However, the results from AMPT show that the jet-like yield is medium-like modified throughout the entire ${p}_{{\rm{T}}}^{{\rm{trig}}}$ range. The jet-like yield in p–Pb collisions using AMPT is also studied. Notably, a dip structure that is observed in the jet-like signal ratio of pp to p–Pb at low ${p}_{{\rm{T}}}^{{\rm{trig}}}$ in ALICE data, is reproduced by AMPT model with SM on, pointing to possible medium-like behavior in small systems. The results of this article also underscore the importance of high-${p}_{{\rm{T}}}^{{\rm{trig}}}$ (${p}_{{\rm{T}}}^{{\rm{trig}}}\gt$ 8 GeV/c) for minimizing underlying event biases in jet-related studies.

This document summarizes the discussions and outcomes of the Facility for Rare Isotope Beams (FRIB) Theory Alliance topical program ‘The path to Superheavy Isotopes’ held in June 2024 at FRIB. Its content is non-exhaustive, reflecting topics chosen and discussed by the participants. The program aimed to assess the current status of theory in superheavy nuclei (SHN) research and identify necessary theoretical developments to guide experimental programs and determine fruitful production mechanisms. This report details the intersection of SHN research with other fields, provides an overview of production mechanisms and theoretical models, discusses future needs in theory and experiment, explores other potential avenues for SHN synthesis, and highlights the importance of building a strong theory community in this area.

Machine learning methods, in particular deep learning methods such as artificial neural networks (ANNs) with many layers, have become widespread and useful tools in nuclear physics. However, these ANNs are typically treated as ‘black boxes’, with their architecture (width, depth, and weight/bias initialization) and the training algorithm and parameters chosen empirically by optimizing learning based on limited exploration. We test a non-empirical approach to understanding and optimizing nuclear physics ANNs by adapting a criticality analysis based on renormalization group flows in terms of the hyperparameters for weight/bias initialization, training rates, and the ratio of depth to width. This treatment utilizes the statistical properties of neural network initialization to find a generating functional for network outputs at any layer, allowing for a path integral formulation of the ANN outputs as a Euclidean statistical field theory. We use a prototypical example to test the applicability of this approach: a simple ANN for nuclear binding energies. We find that with training using a stochastic gradient descent optimizer, the predicted criticality behavior is realized, and optimal performance is found with critical tuning. However, the use of an adaptive learning algorithm leads to somewhat superior results without concern for tuning and thus obscures the analysis. Nevertheless, the criticality analysis offers a way to look within the black box of ANNs, which is a first step towards potential improvements in network performance beyond using adaptive optimizers.

We investigate the role of short-range correlations (SRC) in the transverse nuclear response within the quasielastic peak region, focusing on the one-particle one-hole (1p1h) channel. The calculation is performed in nuclear matter by solving the Bethe–Goldstone equation with the realistic Granada 2013 nucleon–nucleon potential, including both one-body and two-body meson-exchange currents (MEC) of seagull, pion-in-flight, and Δ types. We find that MEC produce a sizable enhancement of the transverse response in the 1p1h channel when acting on correlated nucleon pairs with high-momentum components generated by SRC. This is in contrast to the uncorrelated case, where MEC—dominated by the Δ current—can even yield a negative effect. The results are consistent with previous findings based on the correlated basis function approach, supporting the interpretation that SRC play a central role in the transverse response enhancement from MEC.

The detection of gravitational waves from mergers of tens of Solar mass black hole binaries has led to a surge in interest in primordial black holes (PBHs) as a dark matter candidate. We aim to provide a (relatively) concise overview of the status of PBHs as a dark matter candidate, circa Summer 2020. First we review the formation of PBHs in the early Universe, focussing mainly on PBHs formed via the collapse of large density perturbations generated by inflation. Then we review the various current and future constraints on the present day abundance of PBHs. We conclude with a discussion of the key open questions in this field.

I describe a framework for the presentation of search results which is motivated by frequentist statistics. The most well-known use of this framework is for the combined search for the Higgs boson at LEP. A toy neutrino oscillations experiment is used to illustrate the rich information available in the framework for exclusion and discovery. I argue that the so-called CL_s technique for setting limits is appropriate for determining exclusion intervals while the determination of confidence intervals advocated by Feldman and Cousins' method is more appropriate for treating established signals, i.e. going beyond discovery to measurement.

(From the workshop ‘Advanced Statistical Techniques in Particle Physics’, 18–22 March 2002)

The main objectives of the KM3NeT Collaboration are (i) the discovery and subsequent observation of high-energy neutrino sources in the Universe and (ii) the determination of the mass hierarchy of neutrinos. These objectives are strongly motivated by two recent important discoveries, namely: (1) the high-energy astrophysical neutrino signal reported by IceCube and (2) the sizable contribution of electron neutrinos to the third neutrino mass eigenstate as reported by Daya Bay, Reno and others. To meet these objectives, the KM3NeT Collaboration plans to build a new Research Infrastructure consisting of a network of deep-sea neutrino telescopes in the Mediterranean Sea. A phased and distributed implementation is pursued which maximises the access to regional funds, the availability of human resources and the synergistic opportunities for the Earth and sea sciences community. Three suitable deep-sea sites are selected, namely off-shore Toulon (France), Capo Passero (Sicily, Italy) and Pylos (Peloponnese, Greece). The infrastructure will consist of three so-called building blocks. A building block comprises 115 strings, each string comprises 18 optical modules and each optical module comprises 31 photo-multiplier tubes. Each building block thus constitutes a three-dimensional array of photo sensors that can be used to detect the Cherenkov light produced by relativistic particles emerging from neutrino interactions. Two building blocks will be sparsely configured to fully explore the IceCube signal with similar instrumented volume, different methodology, improved resolution and complementary field of view, including the galactic plane. One building block will be densely configured to precisely measure atmospheric neutrino oscillations.

The observation of electromagnetic radiation from radio to γ-ray wavelengths has provided a wealth of information about the Universe. However, at PeV (10¹⁵ eV) energies and above, most of the Universe is impenetrable to photons. New messengers, namely cosmic neutrinos, are needed to explore the most extreme environments of the Universe where black holes, neutron stars, and stellar explosions transform gravitational energy into non-thermal cosmic rays. These energetic particles have millions of times higher energies than those produced in the most powerful particle accelerators on Earth. As neutrinos can escape from regions otherwise opaque to radiation, they allow an unique view deep into exploding stars and the vicinity of the event horizons of black holes. The discovery of cosmic neutrinos with IceCube has opened this new window on the Universe. IceCube has been successful in finding first evidence for cosmic particle acceleration in the jet of an active galactic nucleus. Yet, ultimately, its sensitivity is too limited to detect even the brightest neutrino sources with high significance, or to detect populations of less luminous sources. In this white paper, we present an overview of a next-generation instrument, IceCube-Gen2, which will sharpen our understanding of the processes and environments that govern the Universe at the highest energies. IceCube-Gen2 is designed to:

(a) Resolve the high-energy neutrino sky from TeV to EeV energies

(b) Investigate cosmic particle acceleration through multi-messenger observations

(c) Reveal the sources and propagation of the highest energy particles in the Universe

(d) Probe fundamental physics with high-energy neutrinos

IceCube-Gen2 will enhance the existing IceCube detector at the South Pole. It will increase the annual rate of observed cosmic neutrinos by a factor of ten compared to IceCube, and will be able to detect sources five times fainter than its predecessor. Furthermore, through the addition of a radio array, IceCube-Gen2 will extend the energy range by several orders of magnitude compared to IceCube. Construction will take 8 years and cost about $350M. The goal is to have IceCube-Gen2 fully operational by 2033.

IceCube-Gen2 will play an essential role in shaping the new era of multi-messenger astronomy, fundamentally advancing our knowledge of the high-energy Universe. This challenging mission can be fully addressed only through the combination of the information from the neutrino, electromagnetic, and gravitational wave emission of high-energy sources, in concert with the new survey instruments across the electromagnetic spectrum and gravitational wave detectors which will be available in the coming years.

We provide an updated recommendation for the usage of sets of parton distribution functions (PDFs) and the assessment of PDF and PDF+${\alpha }_{s}$ uncertainties suitable for applications at the LHC Run II. We review developments since the previous PDF4LHC recommendation, and discuss and compare the new generation of PDFs, which include substantial information from experimental data from the Run I of the LHC. We then propose a new prescription for the combination of a suitable subset of the available PDF sets, which is presented in terms of a single combined PDF set. We finally discuss tools which allow for the delivery of this combined set in terms of optimized sets of Hessian eigenvectors or Monte Carlo replicas, and their usage, and provide some examples of their application to LHC phenomenology. This paper is dedicated to the memory of Guido Altarelli (1941–2015), whose seminal work made possible the quantitative study of PDFs.

The Jiangmen Underground Neutrino Observatory (JUNO), a 20 kton multi-purpose underground liquid scintillator detector, was proposed with the determination of the neutrino mass hierarchy (MH) as a primary physics goal. The excellent energy resolution and the large fiducial volume anticipated for the JUNO detector offer exciting opportunities for addressing many important topics in neutrino and astro-particle physics. In this document, we present the physics motivations and the anticipated performance of the JUNO detector for various proposed measurements. Following an introduction summarizing the current status and open issues in neutrino physics, we discuss how the detection of antineutrinos generated by a cluster of nuclear power plants allows the determination of the neutrino MH at a 3–4σ significance with six years of running of JUNO. The measurement of antineutrino spectrum with excellent energy resolution will also lead to the precise determination of the neutrino oscillation parameters ${\mathrm{sin}}^{2}{\theta }_{12}$, ${\rm{\Delta }}{m}_{21}^{2}$, and $| {\rm{\Delta }}{m}_{{ee}}^{2}|$ to an accuracy of better than 1%, which will play a crucial role in the future unitarity test of the MNSP matrix. The JUNO detector is capable of observing not only antineutrinos from the power plants, but also neutrinos/antineutrinos from terrestrial and extra-terrestrial sources, including supernova burst neutrinos, diffuse supernova neutrino background, geoneutrinos, atmospheric neutrinos, and solar neutrinos. As a result of JUNO's large size, excellent energy resolution, and vertex reconstruction capability, interesting new data on these topics can be collected. For example, a neutrino burst from a typical core-collapse supernova at a distance of 10 kpc would lead to ∼5000 inverse-beta-decay events and ∼2000 all-flavor neutrino–proton ES events in JUNO, which are of crucial importance for understanding the mechanism of supernova explosion and for exploring novel phenomena such as collective neutrino oscillations. Detection of neutrinos from all past core-collapse supernova explosions in the visible universe with JUNO would further provide valuable information on the cosmic star-formation rate and the average core-collapse neutrino energy spectrum. Antineutrinos originating from the radioactive decay of uranium and thorium in the Earth can be detected in JUNO with a rate of ∼400 events per year, significantly improving the statistics of existing geoneutrino event samples. Atmospheric neutrino events collected in JUNO can provide independent inputs for determining the MH and the octant of the ${\theta }_{23}$ mixing angle. Detection of the ⁷Be and ⁸B solar neutrino events at JUNO would shed new light on the solar metallicity problem and examine the transition region between the vacuum and matter dominated neutrino oscillations. Regarding light sterile neutrino topics, sterile neutrinos with ${10}^{-5}\;{{\rm{eV}}}^{2}\lt {\rm{\Delta }}{m}_{41}^{2}\lt {10}^{-2}\;{{\rm{eV}}}^{2}$ and a sufficiently large mixing angle ${\theta }_{14}$ could be identified through a precise measurement of the reactor antineutrino energy spectrum. Meanwhile, JUNO can also provide us excellent opportunities to test the eV-scale sterile neutrino hypothesis, using either the radioactive neutrino sources or a cyclotron-produced neutrino beam. The JUNO detector is also sensitive to several other beyondthe-standard-model physics. Examples include the search for proton decay via the $p\to {K}^{+}+\bar{\nu }$ decay channel, search for neutrinos resulting from dark-matter annihilation in the Sun, search for violation of Lorentz invariance via the sidereal modulation of the reactor neutrino event rate, and search for the effects of non-standard interactions. The proposed construction of the JUNO detector will provide a unique facility to address many outstanding crucial questions in particle and astrophysics in a timely and cost-effective fashion. It holds the great potential for further advancing our quest to understanding the fundamental properties of neutrinos, one of the building blocks of our Universe.

The Physics Beyond Colliders initiative is an exploratory study aimed at exploiting the full scientific potential of the CERN’s accelerator complex and scientific infrastructures through projects complementary to the LHC and other possible future colliders. These projects will target fundamental physics questions in modern particle physics. This document presents the status of the proposals presented in the framework of the Beyond Standard Model physics working group, and explore their physics reach and the impact that CERN could have in the next 10–20 years on the international landscape.

The existence of dark matter as evidenced by numerous indirect observations is one of the most important indications that there must be physics beyond the Standard Model of particle physics. This article reviews the concepts of direct detection of dark matter in the form of Weakly Interacting Massive Particles in ultra-sensitive detectors located in underground laboratories, discusses the expected signatures, detector concepts, and how the stringent low-background requirements are achieved. Finally, it summarizes the current status of the field and provides an outlook on the years to come.

The existence of nonzero neutrino masses points to the likely existence of multiple Standard Model neutral fermions. When such states are heavy enough that they cannot be produced in oscillations, they are referred to as heavy neutral leptons (HNLs). In this white paper, we discuss the present experimental status of HNLs including colliders, beta decay, accelerators, as well as astrophysical and cosmological impacts. We discuss the importance of continuing to search for HNLs, and its potential impact on our understanding of key fundamental questions, and additionally we outline the future prospects for next-generation future experiments or upcoming accelerator run scenarios.

The nEXO neutrinoless double beta (0νββ) decay experiment is designed to use a time projection chamber and 5000 kg of isotopically enriched liquid xenon to search for the decay in ¹³⁶Xe. Progress in the detector design, paired with higher fidelity in its simulation and an advanced data analysis, based on the one used for the final results of EXO-200, produce a sensitivity prediction that exceeds the half-life of 10²⁸ years. Specifically, improvements have been made in the understanding of production of scintillation photons and charge as well as of their transport and reconstruction in the detector. The more detailed knowledge of the detector construction has been paired with more assays for trace radioactivity in different materials. In particular, the use of custom electroformed copper is now incorporated in the design, leading to a substantial reduction in backgrounds from the intrinsic radioactivity of detector materials. Furthermore, a number of assumptions from previous sensitivity projections have gained further support from interim work validating the nEXO experiment concept. Together these improvements and updates suggest that the nEXO experiment will reach a half-life sensitivity of 1.35 × 10²⁸ yr at 90% confidence level in 10 years of data taking, covering the parameter space associated with the inverted neutrino mass ordering, along with a significant portion of the parameter space for the normal ordering scenario, for almost all nuclear matrix elements. The effects of backgrounds deviating from the nominal values used for the projections are also illustrated, concluding that the nEXO design is robust against a number of imperfections of the model.