arXiv subjects

Jean-François Fortin

Publications and source records attributed to Jean-François Fortin.

At least 19 recordsLinked to original sources

arXiv2026

Polarization Formalism for Photon-Gravitational Wave Mixing Around Magnetars

The Gertsenshtein effect can be used to probe the stochastic gravitational wave background at high frequencies, well above the range of standard cosmological sources. In this paper, we revisit the conversion between electromagnetic and gravitational waves in the magnetosphere of magnetars by solving the evolution equations of the associated Stokes parameters. In the process, we point out that the adiabatic approximation usually taken in the literature is not generally justified in the context of the Gertsenshtein effect. To derive analytical results, we focus our attention on two specific geometries where the adiabatic approximation is valid. From these, we derive a lower bound on the stochastic gravitational wave background from the conversion of magnetar electromagnetic emission into gravitational waves, and an upper bound by requiring that the conversion of background gravitational waves into electromagnetic radiation does not exceed the observed magnetar flux in the X-ray band. Our results demonstrate that gravitational waves generated through the Gertsenshtein conversion of magnetar electromagnetic emission produce a negligible stochastic background, as anticipated.

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arXiv2026

Virasoro OPE and Conformal Blocks from the Inverse Shapovalov Form

We derive expressions for the Virasoro OPE and four-point conformal blocks on the sphere via the resolution of identity recently determined in [Phys. Rev. D 111, 085010 (2025), arXiv:2409.12224]. Even though the resolution of the identity depends on Virasoro singular vectors, our expression for the blocks does not depend on their precise form, but just on their well-known conformal weights. We verify that our expression is compatible with -- but differs from -- Zamolodchikov's $h$-recursion relation and we also examine the impact of various large central charge limits in our formula. A Mathematica notebook with a simple implementation of our expression for the Virasoro conformal blocks is provided as an ancillary file.

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arXiv2026

The generic basis and flavour non-universal SMEFT

Whenever an anomaly in the flavour sector appears, analyses are performed examining whether it can be explained by adding a small number of carefully-chosen flavour non-universal four-fermion SMEFT operators. These analyses are typically carried out in the down or the up basis, i.e., it is assumed that the weak and mass eigenstates are the same for the left-handed down-type or up-type quarks. In these bases, there is no dependence on the matrices that transform from the weak to the mass basis, and which are unmeasurable in the Standard Model. In this paper, we argue that it is better to use a generic weak basis, in which no assumptions about the alignment of weak and mass eigenstates are made. The analysis now directly includes elements of the transformation matrices. By doing a fit to the data, it is possible to both determine if the flavour anomaly can be explained and extract the transformation matrices. In principle, this can be extended to a complete reconstruction of the Yukawa matrices.

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arXiv2025

Conformal Bootstrap Equations from the Embedding Space Operator Product Expansion

We describe how to implement the conformal bootstrap program in the context of the embedding space OPE formalism introduced in previous work. To take maximal advantage of the known properties of the scalar conformal blocks for symmetric-traceless exchange, we construct tensorial generalizations of the three-point and four-point scalar conformal blocks that have many nice properties. Further, we present a special basis of tensor structures for three-point correlation functions endowed with the remarkable simplifying property that it does not mix under permutations of the external quasi-primary operators. We find that in this approach, we can write the $M$-point conformal bootstrap equations explicitly in terms of the standard position space cross-ratios without the need to project back to position space, thus effectively deriving all conformal bootstrap equations directly from the embedding space. Finally, we lay out an algorithm for generating the conformal bootstrap equations in this formalism. Collectively, the tensorial generalizations, the new basis of tensor structures, as well as the procedure for deriving the conformal bootstrap equations lead to four-point bootstrap equations for quasi-primary operators in arbitrary Lorentz representations expressed as linear combinations of the standard scalar conformal blocks for spin-$\ell$ exchange, with finite $\ell$-independent terms. Moreover, the OPE coefficients in these equations conveniently feature trivial symmetry properties. The only inputs necessary are the relevant projection operators and tensor structures, which are all fixed by group theory. To illustrate the procedure, we present one nontrivial example involving scalars $S$ and vectors $V$, namely $\left\langle SSSV\right\rangle$.

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arXiv2025

Casimirs of the Virasoro Algebra

We explicitly solve a recurrence relation due to Feigin and Fuchs to obtain the Casimirs of the Virasoro algebra in terms of the inverse of the Shapovalov form. Combined with our recent result for the inverse Shapovalov form, this allows us to write the Casimir operators as linear combinations of products of singular vectors.

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arXiv2025

The Virasoro Completeness Relation and Inverse Shapovalov Form

In this work, we introduce an explicit expression for the inverse of the symmetric bilinear form of Virasoro Verma modules, the so-called Shapovalov form, in terms of singular vector operators and their conformal dimensions. Our proposed expression also determines the resolution of the identity for Verma modules of the Virasoro algebra, and can be thus employed in the computation of Virasoro conformal blocks via the sewing procedure.

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arXiv2024

First Constraints on the Photon Coupling of Axion-like Particles from Multimessenger Studies of the Neutron Star Merger GW170817

We use multimessenger observations of the neutron star merger event GW170817 to derive new constraints on axion-like particles (ALPs) coupling to photons. ALPs are produced via Primakoff and photon coalescence processes in the merger, escape the remnant and decay back into two photons, giving rise to a photon signal approximately along the line-of-sight to the merger. We analyze the spectral and temporal information of the ALP-induced photon signal, and use the Fermi-LAT observations of GW170817 to derive our new ALP constraints. We also show the improved prospects with future MeV gamma-ray missions, taking the spectral and temporal coverage of Fermi-LAT as an example.

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arXiv2023

One- and two-dimensional higher-point conformal blocks as free-particle wavefunctions in AdS$_3^{\otimes m}$

We establish that all of the one- and two-dimensional global conformal blocks are, up to some choice of prefactor, free-particle wavefunctions in tensor products of AdS$_3$ or limits thereof. Our first core observation is that the six-point comb-channel conformal blocks correspond to free-particle wavefunctions on an AdS$_3$ constructed directly in cross-ratio space. This construction generalizes to blocks for a special class of diagrams, which are determined as free-particle wavefunctions in tensor products of AdS$_3$. Conformal blocks for all the remaining topologies are obtained as limits of the free wavefunctions mentioned above. Our results show directly that the integrable models associated with all one- and two-dimensional conformal blocks can be seen as limits of free theory, and manifest a relation between AdS and CFT kinematics that lies outside of the standard AdS/CFT dictionary. We complete the discussion by providing explicit Feynman-like rules that can be used to work out blocks for all topologies, as well as a Mathematica notebook that allows simple computation of Casimir equations and series expansions for blocks, by requiring just an OPE diagram as input.

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arXiv2023

Polarization Formalism for ALP-induced X-ray Emission from Magnetars

Missions like NASA's Imaging X-ray Polarimetry Explorer (IXPE) are poised to provide an unprecedented view of the Universe in polarized X-rays. Polarization probes physical anisotropies, a fact exploited by particle physicists to look for the anisotropic $a\boldsymbol{E}\cdot\boldsymbol{B}$ operator in the axion-like-particle (ALP) Lagrangian. Such studies have typically focused on polarization in the radio and microwaves, through local or cosmic birefringence effects. To such polarization studies we add X-rays emanating from magnetars -- a class of neutron stars with near-critical strength magnetic fields -- that are important targets for IXPE. ALPs produced in the neutron star core convert to X-rays in the magnetosphere; such X-rays are polarized along the direction parallel to the dipolar magnetic field at the point of conversion. We develop the full theoretical formalism for ALP-induced polarization in the presence of dipolar magnetic fields. For uncorrelated photon and ALP production mechanisms, we completely disentangle the ALP contributions to the Stokes parameters in terms of the ALP intensity, the ALP-to-photon conversion probability, and the ALP-induced birefringence. In the proper limit, our results demonstrate that the inclusion of ALPs suppresses the observed degree of circular polarization compared to its pure astrophysical value. Our results can also be used to impose limits on ALP couplings with IXPE polarization data from magnetars 4U 0142+61 and 1RXS J170849.0-400910, the subject of upcoming work.

astro-ph.HE↗

arXiv2022

Conformal Blocks in Three Dimensions

We derive expressions for conformal blocks involving operators with arbitrary spins in 3-dimensional CFTs. We use previous results on the action of the OPE in the embedding space to derive the conformal blocks. The blocks are given as explicit power series in terms of the conformal cross ratios.

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arXiv2022

Feynman Rules for Scalar Conformal Blocks

We complete the proof of "Feynman rules" for constructing $M$-point conformal blocks with external and internal scalars in any topology for arbitrary $M$ in any spacetime dimension by combining the rules for the blocks (based on their Witten diagram interpretation) with the rules for the construction of conformal cross ratios (based on OPE flow diagrams). The full set of Feynman rules leads to blocks as power series of the hypergeometric type in the conformal cross ratios. We then provide a proof by recursion of the Feynman rules which relies heavily on the first Barnes lemma and the decomposition of the topology of interest in comb-like structures. Finally, we provide a nine-point example to illustrate the rules.

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arXiv2022

Magnetar Hard X-Ray Emission from Axion-like Particle Conversion

We explore the possibility that axion-like-particles (ALPs), which would be produced in the core of magnetars and would then convert in the magnetosphere into photons, can explain magnetar hard X-ray spectra. We remark that this scenario would also provide answers to some questions related to magnetar heating. Indeed, considering that magnetars have: 1) hard X-ray spectra that are difficult to explain with known mechanisms; 2) large photon luminosities that force high core temperatures; 3) high core temperatures that imply large neutrino emissivities; 4) and large neutrino emissivities that lead to small magnetar lifetimes in contradiction to observations -- explaining the hard X-ray spectra with ALPs could decrease the core temperatures and thus the neutrino emissivities, allowing for longer magnetar lifetimes as expected from observations. In this work, we initiate the study of this scenario for three magnetars with extreme luminosities, and conclude that the general idea is likely worth investigating in more detail.

astro-ph.HE↗

arXiv2022

Photon-Dark Photon Conversions in Background Electromagnetic Fields

The mixing of photons with light pseudoscalars in the presence of external electromagnetic fields has been used extensively to search for axion-like-particles. A similar effect for dark photon propagating states is usually not considered due to the Landau-Yang theorem. We point out that mixing between photon and dark photon propagating states in background electromagnetic fields can indeed occur, in non-linear QED, through a four-photon vertex by integrating out the electron box diagram. Starting from the Schwinger Lagrangian, we derive the equations of motion for dark photons interacting with the Standard Model photon through gauge kinetic terms. We provide expressions for the perpendicular and parallel refractive indices in series expansions in the critical field strength, valid both in the strong and weak background field limits. We then consider mixing between the photon-dark photon propagating system in the presence of pure electric and magnetic background fields, and work out the probability of conversion when the background fields are homogeneous. We indicate outlines of the calculation in the inhomogeneous case, and finally express our results in the active-sterile basis, where we find that the mixing induced by background fields can lead to corrections to the tree-level mixing in the zero field limit that is usually considered to probe such systems. Our results may find applications for probing photon-dark photon conversions in the vicinity of neutron stars and in table-top petawatt laser experiments.

hep-ph↗

arXiv2022

The Forward Physics Facility at the High-Luminosity LHC

High energy collisions at the High-Luminosity Large Hadron Collider (LHC) produce a large number of particles along the beam collision axis, outside of the acceptance of existing LHC experiments. The proposed Forward Physics Facility (FPF), to be located several hundred meters from the ATLAS interaction point and shielded by concrete and rock, will host a suite of experiments to probe Standard Model (SM) processes and search for physics beyond the Standard Model (BSM). In this report, we review the status of the civil engineering plans and the experiments to explore the diverse physics signals that can be uniquely probed in the forward region. FPF experiments will be sensitive to a broad range of BSM physics through searches for new particle scattering or decay signatures and deviations from SM expectations in high statistics analyses with TeV neutrinos in this low-background environment. High statistics neutrino detection will also provide valuable data for fundamental topics in perturbative and non-perturbative QCD and in weak interactions. Experiments at the FPF will enable synergies between forward particle production at the LHC and astroparticle physics to be exploited. We report here on these physics topics, on infrastructure, detector, and simulation studies, and on future directions to realize the FPF's physics potential.

hep-ex↗

arXiv2021

Light scalars in neutron star mergers

Due to their unique set of multimessenger signals, neutron star mergers have emerged as a novel environment for studies of new physics beyond the Standard Model (SM). As a case study, we consider the simplest extension of the SM scalar sector involving a light CP-even scalar singlet $S$ mixing with the SM Higgs boson. These $S$ particles can be produced abundantly in neutron star mergers via the nucleon bremsstrahlung process. We show that the $S$ particles may either be trapped in or stream freely out of the merger remnant, depending on the $S$ mass, its mixing with the SM Higgs boson, and the temperature and baryon density in the merger. In the free-streaming region, the scalar $S$ will provide an extra channel to cool down the merger remnant, with cooling timescales as small as ${\cal O}$(ms). On the other hand, in the trapped region, the Bose gas of $S$ particles could contribute a larger thermal conductivity than the trapped neutrinos in some parts of the parameter space, thus leading to faster thermal equilibration than expected. Therefore, future observations of the early postmerger phase of a neutron star merger could effectively probe a unique range of the $S$ parameter space, largely complementary to the existing and future laboratory and supernova limits. In view of these results, we hope the merger simulation community will be motivated to implement the effects of light CP-even scalars into their simulations in both the free-streaming and trapped regimes.

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arXiv2021

Magnetars and Axion-like Particles: Probes with the Hard X-ray Spectrum

Quiescent hard X-ray and soft gamma-ray emission from neutron stars constitute a promising frontier to explore axion-like-particles (ALPs). ALP production in the core peaks at energies of a few keV to a few hundreds of keV; subsequently, the ALPs escape and convert to photons in the magnetosphere. The emissivity goes as $\sim T^6$ while the conversion probability is enhanced for large magnetic fields, making magnetars, with their high core temperatures and strong magnetic fields, ideal targets for probing ALPs. We compute the energy spectrum of photons resulting from conversion of ALPs in the magnetosphere and then compare it against hard X-ray data from NuSTAR, INTEGRAL, and XMM-Newton, for a set of eight magnetars for which such data exists. Upper limits are placed on the product of the ALP-nucleon and ALP-photon couplings. For the production in the core, we perform a calculation of the ALP emissivity in degenerate nuclear matter modeled by a relativistic mean field theory. The reduction of the emissivity due to improvements to the one-pion exchange approximation is incorporated, as is the suppression of the emissivity due to proton superfluidity in the neutron star core. A range of core temperatures is considered, corresponding to different models of the steady heat transfer from the core to the stellar surface. For the subsequent conversion, we solve the coupled differential equations mixing ALPs and photons in the magnetosphere. The conversion occurs due to a competition between the dipolar magnetic field and the photon refractive index induced by the external magnetic field. Semi-analytic expressions are provided alongside the full numerical results. We also present an analysis of the uncertainty on the axion limits we derive due to the uncertainties in the magnetar masses, nuclear matter equation of state, and the proton superfluid critical temperature.

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arXiv2021

Axions: From Magnetars and Neutron Star Mergers to Beam Dumps and BECs

We review topics in searches for axion-like-particles (ALPs), covering material that is complementary to other recent reviews. The first half of our review covers ALPs in the extreme environments of neutron star cores, the magnetospheres of highly magnetized neutron stars (magnetars), and in neutron star mergers. The focus is on possible signals of ALPs in the photon spectrum of neutron stars and gravitational wave/electromagnetic signals from neutron star mergers. We then review recent developments in laboratory-produced ALP searches, focusing mainly on accelerator-based facilities including beam-dump type experiments and collider experiments. We provide a general-purpose discussion of the ALP search pipeline from production to detection, in steps, and our discussion is straightforwardly applicable to most beam-dump type and reactor experiments. We end with a selective look at the rapidly developing field of ultralight dark matter, specifically the formation of Bose-Einstein Condensates (BECs). We review the properties of BECs of ultralight dark matter and bridge these properties with developments in numerical simulations, and ultimately with their impact on experimental searches.

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arXiv2020

Conformal Conserved Currents in Embedding Space

We study conformal conserved currents in arbitrary irreducible representations of the Lorentz group using the embedding space formalism. With the help of the operator product expansion, we first show that conservation conditions can be fully investigated by considering only two- and three-point correlation functions. We then find an explicitly conformally-covariant differential operator in embedding space that implements conservation based on the standard position space operator product expansion differential operator $\partial_μ$, although the latter does not uplift to embedding space covariantly. The differential operator in embedding space that imposes conservation is the same differential operator $\mathcal{D}_{ijA}$ used in the operator product expansion in embedding space. We provide several examples including conserved currents in irreducible representations that are not symmetric and traceless. With an eye on four-point conformal bootstrap equations for four conserved vector currents $\langle JJJJ\rangle$ and four energy-momentum tensors $\langle TTTT\rangle$, we mostly focus on conservation conditions for $\langle JJ\mathcal{O}\rangle$ and $\langle TT\mathcal{O}\rangle$. Finally, we reproduce and extend the consequences of conformal Ward identities at coincident points by determining three-point coefficients in terms of charges.

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