Papers

We revisit aspects of particle production during cosmological first-order phase transitions, and show that the expansion of true-vacuum bubbles can be a copious source of particle production. Specifically, for a massive spin-0 field linearly coupled to the bubble profile, spherical bubbles expanding even at constant radial velocity efficiently produce particles until the local-rest-frame radius of curvature of the bubble wall exceeds the particle Compton wavelength – contrary to the expectation that walls moving at constant speed cannot radiate. We compute the momentum spectrum of the produced particles for both accelerated and constant-velocity expansion histories, identify the regimes of coherent and incoherent production, quantify the validity of the perturbative treatment, and show that this mechanism can parametrically dominate other production processes of feebly coupled particles, including freeze-in. Our results apply to many models of light, feebly coupled particles studied in the literature, providing new sources of dark radiation and dark matter. In particular, a first-order deconfinement-confinement transition in a hidden Yang-Mills sector can abundantly produce axion-like particles, leading to dark radiation and/or dark matter signatures over large regions of parameter space.

We develop a partial-wave formalism providing the missing spin-resolved framework for multipositivity among massless planar amplitudes. We present a systematic procedure for constructing kinematic charts realizing the complex-forward limit. Applying it to obtain the four-, five-, and six-point charts, we show how to derive the one-to-two and one-to-three wavefunctions and construct the associated partial waves. The resulting formalism places higher-point multipositivity on the same footing as conventional partial-wave analyses of two-to-two scattering.

Muon decays in a muon collider ring would produce TeV neutrino and antineutrino beams of electron and muon flavor. We study this flux in the forward $\mu^+$ and $\mu^-$ beam directions at a 10 TeV muon collider and introduce \mint, a dedicated Monte Carlo simulation to model neutrino fluxes accounting for muon beam dynamics. We find that a benchmark detector at 5 km from the interaction point would see about $\mathcal{O}(10^{9})$ neutrino interactions per year in a $\sim3$~tonne fiducial volume with a beam spot size of $\mathcal{O}(1)$~meter. We calculate the number of secondary muons and neutrinos generated by neutrino interactions in the rock upstream of the forward detectors and find that about two secondary high-energy and highly polarized muons from the rock would cross each detector per bunch crossing. Neutrino productions of charmed mesons and taus in the rock generate a small $\nu_\tau+\bar\nu_\tau$ secondary flux, with $\mathcal{O}(0.2)$ events per year in the detectors, likely too small to be observed. Wrong-sign neutrinos from similar processes, such as $\nu_e+\bar\nu_\mu$ in the $\mu^-$ beam, are more numerous but still a fraction no larger than $\mathcal{O}(10^{-8})$ of the number of TeV neutrino interactions. Finally, we outline how the large forward neutrino exposure can be used to search for beyond-the-Standard-Model particles produced in neutrino interactions, with examples of heavy neutral leptons coupled to electron and muon flavors through mixing or electromagnetic dipole operators.

While muon colliders are primarily considered precision machines for new physics searches at the energy frontier, they are also intense sources of high-energy neutrinos. Large fluxes of electron and muon neutrinos are produced in the decay of beam muons, especially along the straight sections around the experiments. In this work, we re-evaluate these neutrino fluxes using the latest design of a \SI{10}{TeV} muon collider and find that fluxes around the interaction region are almost two orders of magnitude higher than earlier estimates. We also compute the fluxes of neutrinos produced in $\mu^+\mu^-$ collisions, electromagnetic showers induced by electrons from muon decay, and interactions of neutrinos close to the collider ring. We find that these additional neutrino sources are non-negligible and would lead to a sizable number of neutrino interactions of all flavors, including several tau neutrino events, in a ton-scale detector placed in the forward direction. We discuss the physics opportunities offered by muon collider neutrinos in the context of QCD and nuclear physics, electroweak precision measurements, and searches for new physics.

We study whether the observed hierarchy of quark and lepton masses can be described by a small set of simple relations between the three fermion generations. Using the nine charged-fermion Yukawa couplings at the common scale M_Z, we distinguish genuine mass relations from those that arise simply because a model has enough freedom to reproduce the data. A systematic scan of projector-based models identifies a particularly accurate relation between the first- and third-generation mass slopes. We then construct more restrictive models that also constrain the second generation and test them by leave-one-out reconstruction. Several six-parameter models predict all nine charged masses with errors of only a few percent; the best fixed models reduce the worst leave-one-out error to about 4% without adding continuous parameters. We also give a possible microscopic interpretation of the most successful integer relations. Two mass-independent combinations of Standard-Model quantum numbers reproduce the required charged-sector pattern, and simple twofold or threefold matching classes fix the relative coefficient before the masses are used. In a separate two-loop running test, a down-sector ratio close to 1/15 near 6 x 10^6 GeV evolves to a value close to 1/14 at M_Z. The same framework gives neutrino-mass sums near 0.059-0.061 eV for normal ordering, while one family also allows inverted ordering near 0.103 eV. These results do not prove preons or fermion compositeness, but provide concrete targets for future microscopic models.

In this contribution I present the first exact calculation of the leading-colour two-loop QCD amplitude for the associated production of a top-anti-top pair and a W boson. I discuss strategies to address the complexity of the computation, which involves complicated analytic structures, such as nested square roots, elliptic functions, and expressions with a high degree of algebraic complexity. The final result is expressed in terms of a set of special functions, which are evaluated using the method of differential equations, and rational coefficients, evaluated via finite field techniques.

One can define three levels of ambiguities for the accelerator experiments: worse than 0.3%, around 0.3% and better than 0.3%. I will address issues important, for the last case, when Monte Carlo programs that simultaneously account for theoretical and experimental effects are necessary. To complete efforts, simultaneous effort in many specialities over years was necessary. The monumental projects of Bryan Lynn, Robin Stuart, Dima Bardin, Wolfgang Hollik and contributions of S. Jadach in the domain of precision physics need to be mentioned, they provide theoretical foundations. Incomplete lists of methodology domains and projects, usually left into references, appendices, and private notes include: (i) Phase space–> symmetries (ii) matrix element preparation –> factorizations (iii) program and development process design (iv) testing strategies (v) user interaction (vi) software tools (vii) partners and competitors. Usually we were publishing our own projects, let me mention incomplete lists of methodology domains and projects, which were usually left aside into references, appendices, and private notes: The work started on the basis of previous efforts which can be listed following names of the programs: (i) FOWL, (ii) GENRAP, (iii) Mustraal, (iv) Koralb, (v) Lesko, (vi) Tauola, (vii) KoralZ, (viii) Lumlog, (ix) Oldbab, (x) Bhlumi, (xi) Bhwide and (xii) KKMC. I will focus on some of these points. Others, hopefully, are covered in other talks. In particular I do not need to cover exponentiation and some issues of factorization; see contributions to the proceedings by B.F.L. Ward and A. Tapadar. Developments took years and did not follow a straight line; that is why there are simplifications, biases. Also a review of literature could not be completed.

Within a lattice Boltzmann-Uehling-Uhlenbeck transport model coupled to a kinetic approach for light-cluster formation, we investigate the impact of explicit light-cluster degrees of freedom on collective flows in Au+Au collisions at FOPI energies with beam energies $E_{\rm beam}$= $120$–$1500 A$ MeV by using a density-, momentum-, and isospin-dependent N$5$LO Skyrme pseudopotential. We first benchmark the kinetic approach by comparing the calculated light-cluster yields with FOPI data in central Au+Au collisions. We then analyze the collective flows of protons and light nuclei (deuterons, tritons, $^{3}\mathrm{He}$, and $^{4}\mathrm{He}$) in mid-central collisions. For protons, calculations with and without dynamical light-cluster degrees of freedom are compared to quantify the influence of dynamical cluster formation on proton directed ($v_1$), elliptic ($v_2$), triangular ($v_3$), and quadrangular ($v_4$) flows. We find that the dynamical light-cluster effect appreciably modifies proton $v_1$–$v_4$ flows at $E_{\rm beam}=120$–$150 A$ MeV, remains visible at $E_{\rm beam}=250$–$400 A$ MeV, and gradually weakens at $E_{\rm beam}\gtrsim 600 A$ MeV. For light nuclei, the kinetic approach captures the overall beam-energy dependence of the FOPI flow data, with better agreement for $E_{\rm beam}\geq 400 A$ MeV. We further examine the nucleon-number scaling of $v_2/A$ in both model calculations and experimental data, finding that the kinetic light-cluster formation approach qualitatively reproduces the observed scaling behavior. These results highlight the importance of a dynamical treatment of light-cluster formation for interpreting collective flows in heavy-ion collisions below about $600 A$ MeV, although the clustering effects on proton flows are minor at higher collision energies.

We investigate the phase diagram of the (2+1)-dimensional Gross-Neveu-Yukawa (GNY) model at finite temperature, density, and magnetic field beyond mean-field, using the Functional Renormalization Group (FRG) in the local potential approximation. Large magnetic fields result in magnetic catalysis, a dimensional reduction of the system, and enhancement of chiral symmetry breaking. We employ a hydrodynamical algorithm to solve the FRG flow equation for the effective potential, which allows for go further into the infrared region than with previously used methods. We find that the chiral condensate exhibits non-trivial behavior in various regions of the phase diagram: several first-order phase transitions and de Haas – van Alphen oscillations at small magnetic field and large chemical potential, as well as a critical endpoint which shifts to higher temperature with increasing magnetic field.

We study the diffusion and shear viscosity coefficients of hot isospin asymmetric strange hadronic matter. The effects due to the baryon density, isospin, strangeness, and temperature on the nucleons and hyperons are studied within a chiral SU(3) model. The medium modifications of the baryons arise due to interactions with the mean scalar and vector fields within the model. The thermodynamic and transport properties are studied in the hot strange hadronic matter. The diffusion matrix associated with the multiple charges (baryon number, isospin, and strangeness), as well as the coefficient of shear viscosity, are computed from the Boltzmann equation using first-order Chapman–Enskog expansion within the relaxation time approximation. There are observed to be significant effects from the isospin asymmetry as well as the strangeness of the medium on the diffusion coefficients. The coefficient of shear viscosity, $\eta$ is observed to have a large enhancement in the presence of finite strangeness in the medium due to additional contributions from the hyperons. The effects due to isospin asymmetry on the shear viscosity coefficient is however observed to be marginal both in nuclear and hyperonic matter. The present study can be relevant for the experimental observables of asymmetric relativistic heavy-ion collisions, e.g., in the compressed baryonic matter (CBM) experiment at the FAIR facility at GSI as well as in the future J-PARC-HI program.

This review summarizes recent theoretical progress on the QCD equation of state near the critical point and developments in the maximum entropy freeze-out framework, which provides a systematic connection between hydrodynamic fluctuations and hadronic multiplicity cumulants. We also discuss recent applications of this framework, together with advances in the dynamical evolution of critical fluctuations.

We derive the necessary and sufficient conditions for nonlinear causality in viscous anisotropic hydrodynamics (VAH) within the approximation that neglects small correction terms. Relativistic hydrodynamics provides a successful description of the space-time evolution of the matter produced in relativistic heavy-ion collisions, yet the earliest stage at which a hydrodynamic description becomes valid remains an open question. VAH has been proposed as an extension of conventional viscous hydrodynamics (VH) that can accommodate the large pressure anisotropies of the early-time dynamics. However, in such far-from-equilibrium regimes, the nonlinear causality of the theory is not guaranteed. By analyzing the characteristic velocities of the VAH equations of motion, we derive a set of inequalities that ensures causal signal propagation in all directions. The resulting conditions take a remarkably simple form and admit a clear physical interpretation in terms of the characteristic modes of the anisotropic medium. These results establish the regime of validity of VAH and provide a foundation for its application to the early-time dynamics of relativistic heavy-ion collisions.

The quality of the QCD axion can be protected by a gauge symmetry. This work considers a gauged chiral $U(1)_{B-L}$. The vacuum expectation values of two complex scalars break $U(1)_{B-L}$. One linear combination of their two phase degrees of freedom is eaten by the $U(1)_{B-L}$ gauge boson, while the other becomes the QCD axion. Since the PQ symmetry is accidental, there is no ambiguity in the PQ charge assignment. High axion quality implies a large anomalous coupling between the axion and the $U(1)_{B-L}$ gauge boson (or gauged Majoron). A `quality floor', namely a minimal coupling determined by the axion quality, arises for the coupling between the axion and SM fermions, extending the parameter space of QCD axions toward improved testability.

We investigate neutrino mass and thermal leptogenesis in the Babu-Nandi-Tavartkiladze (BNT) model featuring a scalar quadruplet ($\Phi$) and a pair of vector-like fermion triplets ($\Sigma$). In this framework, neutrino masses are generated via an effective dimension-7 operator $LLHH(H^{\dagger}H)/\Lambda^3$ at the tree level and via the dimension-5 operator $LLHH/\Lambda$ at the one-loop level. It naturally accommodates sub-eV neutrino masses even if the new physics scale $\Lambda$ is $\mathcal{O}(\rm TeV)$, thus making the model a compelling target for experimental searches. We explore the viability of thermal leptogenesis in this model, which is distinct from the canonical seesaw-based leptogenesis due to the presence of vector-like fermions. We find that leptogenesis is viable for $M_\Sigma \gtrsim 10^{7}$ GeV for a hierarchical spectrum of fermion triplets. However, in the quasi-degenerate regime, resonant enhancement of the $CP$ asymmetry lowers this scale down to $\mathcal{O}({\rm TeV})$, reconciling successful leptogenesis with the originally motivated TeV-scale phenomenology and testability of the model at colliders.

The trilinear Higgs self-coupling determines the shape of the Higgs potential, and its measurement is a central goal of future colliders. We assess the sensitivity of multi-TeV muon colliders to the coupling modifier $\kappa_3$ in Higgs boson pair production via vector boson fusion, using the $b\bar{b}b\bar{b}$ final state at $\sqrt{s}=3$ TeV with $1$ ab$^{-1}$ and at $10$ TeV with $10$ ab$^{-1}$. Events are analyzed in two complementary regions, a resolved region with four jets and a boosted region with two large-radius jets. To extract the signal from backgrounds a few orders of magnitude larger, we combine a supervised jet-to-Higgs pairing network based on the SPANet approach, topological data analysis of the event energy flow, and two dedicated classifiers, $D_{\rm HH}$ for the signal-to-background separation and $D_{\kappa_3}$ for the $\kappa_3$ shape information. The coupling is extracted from a two-dimensional likelihood fit to the distribution of the two classifier outputs. Combining the two regions, we obtain $0.80<\kappa_3<1.29$ at $3$ TeV and $0.96<\kappa_3<1.05$ at $10$ TeV at $68\%$ confidence level. The $10$ TeV determination reaches the few-percent level in this statistics-limited projection, which surpasses by a large amount the precision projected for the HL-LHC.

We investigate how the gluon condensate modifies the thermal spectral functions and melting patterns of heavy vector mesons, specifically charmonium and bottomonium, using an improved soft-wall AdS/QCD model. The framework is extended to finite temperature via a dilaton black hole geometry that consistently incorporates the backreaction from the gluon condensate. We numerically track the evolution of spectral resonance peaks as functions of both temperature and gluon condensate strength. Our calculations reveal that increasing temperature systematically broadens and suppresses spectral peaks, signaling in-medium dissociation. In contrast, a stronger gluon condensate considerably mitigates peak broadening and enhances the spectral weight of both ground and excited states. This behavior indicates that the gluon condensate hinders thermal dissociation, thereby stabilizing heavy quarkonia within the quark-gluon plasma. These findings are consistent with existing studies and provide new holographic evidence for the stabilising role of the gluon condensate from the perspective of thermal spectral functions.

We revisit the possibility of detecting the Odderon exchange through high-energy neutral-kaon regeneration, focusing on the in-matter $K^0_L\to K^0_S$ conversion of $(\sim\!0.2-2)$~TeV $K^0_L$ mesons originating from $pp$ collisions at $\sqrt{s}=13.6$~TeV, and on the practical constraints of realizing such a measurement in the very-forward region of an LHC interaction point. The analysis has two complementary parts. First, we reproduce the original coherent-forward-regeneration estimates for a liquid-hydrogen regenerator and extend them to realistic C, Cu and Pb regenerators of an LHC-compatible geometry, including neutral-kaon attenuation. We show that an Odderon-induced regeneration phase produces a measurable distortion of the $K^0\to\pi^0\pi^0$ decay-vertex distribution at TeV kaon energies, but that the survival of primary $K^0_S$ mesons from the interaction point imposes severe baseline requirements, while a competing electromagnetic $C=-1$ (photon-exchange) amplitude limits the interpretation of the coherent mode as a clean Odderon measurement. Second, we examine non-forward (diffractive) regeneration at lower kaon energies of $0.2-0.8$~TeV, where the primary-$K^0_S$ contamination is naturally suppressed, and estimate the competing Odderon, Pomeron–Odderon-cut, $\omega$-Reggeon and photon-exchange contributions to the regeneration amplitude. We identify neutron-induced neutral-only strangeness production and inelastic Regge backgrounds as the dominant limitations, quantify the background suppression required for an observable Odderon signal, and formulate the ingredients of an active double-regenerator subtraction strategy.

Energy-energy correlators are usually reported as ensemble averages and therefore do not specify how different angular regions fluctuate together from jet to jet. We retain the binned two-point correlator for each jet and study the distribution and covariance of its angular-shell weights. In a high-statistics Pythia sample, the shell variables are sparse and strongly non-Gaussian, and their covariance contains nonzero cross-shell structure spread over many modes. Regressing each shell on five jet-level summaries leaves 84-92% of the covariance trace, while shell-wise permutation and constituent-angle shuffling do not reproduce the observed off-diagonal correlations. We then compare separately generated parton- and hadron-level samples in Pythia and Herwig. Across 24 generator-radius-momentum configurations, the hadron-level covariance trace is smaller and the average neighboring-shell correlation over 0.30 <= r/R < 1 is larger. The size and angular dependence of the change differ between the generators, and a characteristic-position analysis does not identify a common scale. Jet-by-jet EEC covariance thus provides information on the parton-to-hadron transition that is absent from the mean spectrum.

Cosmological collider signals are usually discussed for isolated massive particles, whose exchange produces characteristic logarithmic oscillations in primordial correlators. In this work, we study how this signal is modified when the exchanged states form a continuous mass spectrum. We first develop a spectral representation for inflationary correlators mediated by a continuum field. In the soft limit, the non-analytic part of the seed function is expressed as a Fourier–Laplace transform of the spectral weight with respect to logarithmic momentum variables. This representation shows that a continuum superposes clock signals with different frequencies and can dephase the sharp oscillation associated with a single massive particle. We then realize this mechanism in an RS2-like inflationary braneworld, where the inflaton is localized on a de Sitter brane in a five-dimensional AdS bulk. The tensor sector contains a localized massless graviton and a continuum of Kaluza-Klein gravitons starting at \(m=3H/2\). We derive the KK wavefunctions and identify the brane spectral weight fixed by the continuum wavefunction on the brane. Applying this weight to the inflaton four-point function, we find a smooth seed function rather than a persistent logarithmic clock oscillation. This behavior follows from the fact that the continuum starts at zero clock frequency, while the KK spectral weight vanishes near threshold. Our results provide a concrete higher-dimensional example in which deviations from the standard cosmological collider signal encode the structure of a continuum high-energy spectrum.

We compute the modification to the quark-neutral pion vertex, induced by a constant and uniform magnetic field, using the Linear Sigma Model with quarks as a low energy effective theory of QCD. The vertex is modified even at tree-level since, due to the loss of translational invariance, the calculation should be carried out in configuration space, with the quark described by Ritus wave functions instead of plane waves. We also compute the one-loop vertex modification. These modifications are found for arbitrary Landau levels occupied by the quark. We illustrate the result for the case where the quark occupies the lowest Landau level. To check the result, we also compute the one-loop vertex modification using the Schwinger proper-time method with the quark occupying the lowest Landau level and find the same result as in the case where the Ritus formalism is used.