Search

We study measurement-induced phases of free fermion systems with U(1) symmetry. Following a recent approach developed for Majorana chains, we derive a field theory description for the purity and bipartite entanglement at large space and time scales. We focus on a multi-flavor one-dimensional chain with random complex hoppings and continuous monitoring of the local fermion density. By means of the replica trick, and using the number of flavors as a large parameter controlling our approximations, we derive an effective field theory made up of a SU(N) non-linear sigma model (NL$\sigma$M) coupled to fluctuating hydrodynamics. Contrary to the case of non-interacting Majorana fermions, displaying no U(1) symmetry, we find that the bipartite entanglement entropy satisfies an area law for all monitoring rates, but with a nontrivial scaling of entanglement when the correlation length is large. We provide numerical evidence supporting our claims. We briefly show how imposing a reality condition on the hoppings can change the NL$\sigma$M and also discuss higher dimensional generalizations., Comment: 22 pages, 3 figures; v2: references added and minor revision

We perform a systematic symmetry classification of the Markov generators of classical stochastic processes. Our classification scheme is based on the action of involutive symmetry transformations of a real Markov generator, extending the Bernard-LeClair scheme to the arena of classical stochastic processes and leading to a set of up to fifteen allowed symmetry classes. We construct families of solutions of arbitrary matrix dimensions for five of these classes with a simple physical interpretation of particles hopping on multipartite graphs. In the remaining classes, such a simple construction is prevented by the positivity of entries of the generator particular to classical stochastic processes, which imposes a further requirement beyond the usual symmetry classification constraints. We partially overcome this difficulty by resorting to a stochastic optimization algorithm, finding specific examples of generators of small matrix dimensions in six further classes, leaving the existence of the final four allowed classes an open problem. Our symmetry-based results unveil new possibilities in the dynamics of classical stochastic processes: Kramers degeneracy of eigenvalue pairs, dihedral symmetry of the spectra of Markov generators, and time reversal properties of stochastic trajectories and correlation functions., Comment: 28 pages, 2 figures

There are serious prospects that the next $\gamma$-ray space mission could use a telescope based on silicon pixel detectors. I characterize the potential of such active targets for polarimetry with gamma-ray conversions to pairs and find it excellent both in terms of selection efficiency and of effective polarization asymmetry., Comment: 7 pages, 7 figures

We introduce a new class of large structured random matrices characterized by four fundamental properties which we discuss. We prove that this class is stable under matrix-valued and pointwise non-linear operations. We then formulate an efficient method, based on an extremization problem, for computing the spectrum of subblocks of such large structured random matrices. We present different proofs -- combinatorial or algebraic -- of the validity of this method, which all have some connection with free probability. We illustrate this method with well known examples of unstructured matrices, including Haar randomly rotated matrices, as well as with the example of structured random matrices arising in the quantum symmetric simple exclusion process. tured random matrices arising in the quantum symmetric simple exclusion process., Comment: 30 pages main text, 2 pages appendix

I present a study of the several contributions to the single-photon angular resolution of pair telescopes in the MeV energy range. I examine some test cases, the presently active {\sl Fermi} LAT, the ``pure-silicon'' projects ASTROGAM and AMEGO-X, and the emulsion-based project GRAINE., Comment: Presented to Session 14 "Future spaceborne MeV detectors and related astrophysics'' of the 2023 International Conference of Deep Space Sciences, April 2023, Hefei, China

Entanglement properties of driven quantum systems can potentially differ from the equilibrium situation due to long range coherences. We confirm this observation by studying a suitable toy model for mesoscopic transport~: the open quantum symmetric simple exclusion process (QSSEP). We derive exact formulae for its mutual information between different subsystems in the steady state and show that it satisfies a volume law. Surprisingly, the QSSEP entanglement properties only depend on data related to its transport properties and we suspect that such a relation might hold for more general mesoscopic systems. Exploiting the free probability structure of QSSEP, we obtain these results by developing a new method to determine the eigenvalue spectrum of sub-blocks of random matrices from their so-called local free cumulants -- a mathematical result on its own with potential applications in the theory of random matrices. As an illustration of this method, we show how to compute expectation values of observables in systems satisfying the Eigenstate Thermalization Hypothesis (ETH) from the local free cumulants., Comment: 10 pages main text; 10 appendices, including an application to the Eigenstate Thermalization Hypothesis (ETH)

We derive field theory descriptions for measurement-induced phase transitions in free fermion systems. We focus on a multi-flavor Majorana chain, undergoing Hamiltonian evolution with continuous monitoring of local fermion parity operators. Using the replica trick, we map the dynamics to the imaginary time evolution of an effective spin chain, and use the number of flavors as a large parameter for a controlled derivation of the effective field theory. This is a nonlinear sigma model for an orthogonal $N\times N$ matrix, in the replica limit $N\to 1$. (On a boundary of the phase diagram, another sigma model with higher symmetry applies.) Together with known results for the renormalization-group beta function, this derivation establishes the existence of stable phases -- nontrivially entangled and disentangled respectively -- in the physically-relevant replica limit $N\to 1$. In the nontrivial phase, an asymptotically exact calculation shows that the bipartite entanglement entropy for a system of size $L$ scales as $(\log L)^2$, in contrast to findings in previously-studied models. Varying the relative strength of Hamiltonian evolution and monitoring, as well as a dimerization parameter, the model's phase diagram contains transitions out of the nontrivial phase, which we map to vortex-unbinding transitions in the sigma model, and also contains separate critical points on the measurement-only axis. We highlight the close analogies as well as the differences with the replica approach to Anderson transitions in disordered systems., Comment: Version accepted for publication; added discussion of the phase diagram for more than one flavor. 33 pages, 9 figures

Studies of random unitary circuits have shown that the calculation of Renyi entropies of entanglement can be mapped to classical statistical mechanics problems in spacetime. In this paper, we develop an analogous spacetime picture of entanglement generation for random free or weakly interacting fermion systems without conservation laws. We first study a free-fermion model, namely a 1D chain of Majorana modes with nearest neighbour hoppings, random in both space and time. We analyze the Nth Renyi entropy of entanglement using a replica formalism, and we show that the effective model is equivalent to an SO(2N) Heisenberg spin chain evolving in imaginary time. By applying a saddle-point approximation to the coherent states path integral for the N = 2 case, we arrive at a semiclassical picture for the dynamics of the entanglement purity, in terms of two classical fields in spacetime. The classical solutions involve a smooth domain wall that interpolates between two values, with this domain wall relaxing diffusively in the time direction. We then study how adding weak interactions to the free-fermion model modifies this spacetime picture, reflecting a crossover from diffusive to ballistic spreading of information., Comment: 25 pages, 12 figures

We present a new description of the known large deviation function of the classical symmetric simple exclusion process by exploiting its connection with the quantum symmetric simple exclusion processes and using tools from free probability. This may seem paradoxal as free probability usually deals with non commutative probability while the simple exclusion process belongs to the realm of classical probability. On the way, we give a new formula for the free energy -- alias the logarithm of the Laplace transform of the probability distribution -- of correlated Bernoulli variables in terms of the set of their cumulants with non-coinciding indices. This latter result is obtained either by developing a combinatorial approach for cumulants of products of random variables or by borrowing techniques from Feynman graphs., Comment: 40 pages, submitted to the special issue of the Annales H. Poincare in memory of Krzysztof Gawedzki

The polarimetry of gamma rays converting to an $e^+e^-$ pair would open a new window on the high-energy gamma-ray sky by, among other things, providing insight into the radiation mechanism in pulsars (curvature or synchrotron) or deciphering the composition of the gamma-ray emitting jets in blazars (leptonic or lepto-hadronic). The performance of polarimeters based on homogeneous active targets (gas detectors (MeV, HARPO) or emulsions (GeV, GRAINE) has been studied both with simulation and by the analysis of data collected with telescope prototypes on linearly-polarised gamma-ray beams, and found to be excellent. The present (Fermi LAT), AGILE and future project (AMEGO, ASTROGAM) gamma-ray missions, though, are using active targets based on silicon strip detectors (SSD). No demonstration of a non-zero effective polarisation asymmetry with SSDs has been published to date, be it only with simulated data, and sensitivity estimations were obtained from an assumed value of the effective polarisation asymmetry. I present a characterisation of the potential of SSD-based active targets for polarimetry with gamma-ray conversions to pairs and the development of various methods to improve on the sensitivity. This work could pave the way to providing the polarimetry of the brightest gamma-ray sources of the sky from the decade of data collected by the Fermi LAT and by AGILE, and to guiding the design of future missions., Comment: Submitted to Nucl. Instrum. Meth. A

The detection of photons with energies greater than a few tenths of an MeV, interacting via Compton scattering and/or pair production, faces a number of difficulties. The reconstruction of single-scatter Compton events can only determine the direction of the incoming photon to a cone, or an arc thereof and the angular resolution of pair-conversion telescopes is badly degraded at low energies. Both of these difficulties are partially overcome if the density of the interaction medium is low. Also no precise polarization measurement on a cosmic source has been obtained in that energy range to date. We present the potential of low-density high-precision homogeneous active targets, such as time-projection chambers (TPC) to provide an unambiguous photon direction measurement for Compton events, an angular resolution down to the kinematic limit for pair events, and the polarimetry of linearly polarized radiation., Comment: Invited chapter for the Handbook of X-ray and Gamma-ray Astrophysics

While the scientific potential of high-energy X-ray and gamma-ray polarimetry has long been recognized, measuring the polarization of high-energy photons is challenging. To date, there has been very few significant detections from an astrophysical source. However, recent technological developments raise the possibility that this may change in the not-too-distant future. Significant progress has been made in the development of Gamma-ray Burst (GRB) polarimeters and polarization sensitive Compton telescopes. A second-generation dedicated GRB polarimeter, POLAR-2, is under development for launch in 2024, and COSI a second-generation polarization sensitive Compton Telescope has been selected by NASA for launch in 2025. This chapter reviews basic concepts and experimental approaches of scattering polarimetry of hard X-rays to MeV {\gamma}-rays, and pair production polarimetry of higher-energy photons, Comment: 44 pages

Quantum coherences characterise the ability of particles to quantum mechanically interfere within some given distances. In the context of noisy many-body quantum systems these coherences can fluctuate. A simple toy model to study such fluctuations in an out-of-equilibrium setting is the open quantum symmetric simple exclusion process (Q-SSEP) which describes spinless fermions in one dimension hopping to neighbouring sites with random amplitudes coupled between two reservoirs. Here we show that the dynamics of fluctuations of coherences in Q-SSEP have a natural interpretation as free cumulants, a concept from free probability theory. Based on this insight we provide heuristic arguments why we expect free probability theory to be an appropriate framework to describe coherent fluctuations in generic mesoscopic systems where the noise emerges from a coarse-grained description. In the case of Q-SSEP we show how the link to free probability theory can be used to derive the time evolution of connected fluctuations of coherences as well as a simple steady state solution., Comment: 17 pages main text, 13 pages appendices. This paper has been submitted simultaneously with "Eigenstate Thermalization Hypothesis and Free Probability" by S. Pappalardi, L. Foini and J. Kurchan where the relation between these two frameworks is discussed

The S-matrix of the well-studied sinh-Gordon model possesses a remarkable strong/weak coupling duality $b \to 1/b$. Since there is no understanding nor evidence for such a duality based on the quantum action of the model, it should be questioned whether the properties of the model for $b>1$ are simply obtained by analytic continuation of the weak coupling regime $01$. Namely, we propose that in this region one needs to introduce a background charge $Q_\infty = b + 1/b -2$ which differs from the Liouville background charge by the shift of $-2$. We propose that in this regime the model has non-trivial massless renormalization group flows between two different conformal field theories. This is in contrast to the weak coupling regime which is a theory of a single massive particle. Evidence for our proposal comes from higher order beta functions. We show how our proposal correctly reproduces the freezing transitions in the multi-fractal exponents of a Dirac fermion in $2+1$ dimensions in a random magnetic field, which provides a strong check since such transitions have several detailed features. We also point out a connection between a semi-classical version of this transition and the so-called Manning condensation phenomena in polyelectrolyte physics., Comment: Version 2: published version in JHEP; 17 pages, 3 figures

We consider critical one dimensional quantum systems initially prepared in their groundstate and perturbed by a smooth noise coupled to the energy density. By using conformal field theory, we deduce a universal description of the out-of-equilibrium dynamics. In particular, the full time-dependent distribution of any $2$--pt chiral correlation function can be obtained from solving two coupled ordinary stochastic differential equations. In contrast with the general expectation of heating, we demonstrate that the system reaches a non-trivial and universal stationary state characterized by broad distributions. As an example, we analyse the local energy density: while its first moment diverges exponentially fast in time, the stationary distribution, which we derive analytically, is symmetric around a negative median and exhibits a fat tail with $3/2$ decay exponent. We obtain a similar result for the entanglement entropy production associated to a given interval of size $\ell$. The corresponding stationary distribution has a $3/2$ right tail for all $\ell$, and converges to a one-sided Levy stable for large $\ell$. Our results are benchmarked via analytical and numerical calculations for a chain of non-interacting spinless fermions with excellent agreement., Comment: 9 pages, 5 figures, Supplemental material (16 pages)

The Macroscopic Fluctuation Theory is an effective framework to describe transports and their fluctuations in classical out-of-equilibrium diffusive systems. Whether the Macroscopic Fluctuation Theory may be extended to the quantum realm and which form this extension may take is yet terra incognita but is a timely question. In this short introductory review, I discuss possible questions that a quantum version of the Macroscopic Fluctuation Theory could address and how analysing Quantum Simple Exclusion Processes yields pieces of answers to these questions., Comment: 28 pages plus references, 5 figures. Prepared as a contribution to a special volume of Journal of Physics A on "Hydrodynamics of Low-Dimensional Quantum Systems"

We consider the dynamics of fluctuations in the quantum asymmetric simple exclusion process (Q-ASEP) with periodic boundary conditions. The Q-ASEP describes a chain of spinless fermions with random hoppings that are induced by a Markovian environment. We show that fluctuations of the fermionic degrees of freedom obey evolution equations of Lindblad type, and derive the corresponding Lindbladians. We identify the underlying algebraic structure by mapping them to non-Hermitian spin chains and demonstrate that the operator space fragments into exponentially many (in system size) sectors that are invariant under time evolution. At the level of quadratic fluctuations we consider the Lindbladian on the sectors that determine the late time dynamics for the particular case of the quantum symmetric simple exclusion process (Q-SSEP). We show that the corresponding blocks in some cases correspond to known Yang-Baxter integrable models and investigate the level-spacing statistics in others. We carry out a detailed analysis of the steady states and slow modes that govern the late time behaviour and show that the dynamics of fluctuations of observables is described in terms of closed sets of coupled linear differential-difference equations. The behaviour of the solutions to these equations is essentially diffusive but with relevant deviations, that at sufficiently late times and large distances can be described in terms of a continuum scaling limit which we construct. We numerically check the validity of this scaling limit over a significant range of time and space scales. These results are then applied to the study of operator spreading at large scales, focusing on out-of-time ordered correlators and operator entanglement., Comment: 66 pages, 12 figures

The measurement of exclusive $e^+e^-$ to hadrons processes is a significant part of the physics program of $BABAR$ experiment, aimed to improve the calculation of the hadronic contribution to the muon $g-2$ and to study the intermediate dynamics of the processes. We present the most recent results obtained by using the full data set of about 470\,fb$^{-1}$ collected by the BABAR detector at the PEP-II $e^+e^-$ collider at a center-of-mass energy of about 10.6 GeV. In particular, we report the results on $e^+e^-$ annihilation into six- and seven-pion final states. The study of the very rich dynamics of these processes can help to understand the difference seen between the QCD prediction and the sum of exclusive cross sections in the energy region around 2 GeV, thus improving the precision on the total hadronic cross section and of the $g-2$ calculation. Additionally, we report the results on a dedicated study to shed light on the resonant states production in the energy region around 2.2 GeV, which is presently rather unclear. We measure the reaction $e^+e^-\to K_SK_L$ with data collected with the $BABAR$ detector and analyse these data in conjunction with published BESIII data on $e^+e^-\to K^+K^-$ and $BABAR$ data on $e^+e^-\to K^+K^-$, $\pi^+\pi^-$, $\pi^+\pi^-\eta$, $\pi^+\pi^-\omega$. This study supports the existence of an isovector resonance $\rho(2230)$ consistent with the resonance observed by BESIII., Comment: Presented at 55th Rencontres de Moriond on QCD and High Energy Interactions (Moriond QCD 2021); On behalf of the BaBar Collaboration

We study the probability distribution of entanglement in the Quantum Symmetric Simple Exclusion Process, a model of fermions hopping with random Brownian amplitudes between neighboring sites. We consider a protocol where the system is initialized in a pure product state of $M$ particles, and focus on the late-time distribution of R\'enyi-$q$ entropies for a subsystem of size $\ell$. By means of a Coulomb gas approach from Random Matrix Theory, we compute analytically the large-deviation function of the entropy in the thermodynamic limit. For $q>1$, we show that, depending on the value of the ratio $\ell/M$, the entropy distribution displays either two or three distinct regimes, ranging from low- to high-entanglement. These are connected by points where the probability density features singularities in its third derivative, which can be understood in terms of a transition in the corresponding charge density of the Coulomb gas. Our analytic results are supported by numerical Monte Carlo simulations., Comment: 6 pages, 3 figures; v2: minor revision

We examine the dielectron invariant mass spectrum and the variation of the polarization asymmetry as a function of that mass in the event generator of the five-dimensional Bethe-Heitler differential cross section of the conversion of linearly polarized photons that we have implemented recently as a Geant4 Physics Model. We compare the results obtained with simulated samples to analytical expressions. Studies of $b \to s \gamma$ decays at LHC and Belle II experiments could provide hints of physics beyond the standard model in flavor-changing neutral current processes; issues have been reported in the simulation of the properties of the background induced by genuine $\gamma$-ray conversions in their detectors. Our results demonstrate that using the five-dimensional model would help solve these issues., Comment: v2 : complemented [7] with most recent LHCb result

I re-examine a recent work by G. Landi and G. E. Landi. [arXiv:1808.06708 [physics.ins-det]], in which the authors claim that the resolution of a tracker ca vary linearly with the number of detection layers, $N$, that is, faster than the commonly known $\sqrt{N}$ variation, for a tracker of fixed length, in case the precision of the position measurement is allowed to vary from layer to layer, i.e. heteroscedasticity, and an appropriate analysis method, a weighted least squares fit, is used.

The Quantum Symmetric Simple Exclusion Process (Q-SSEP) is a model for quantum stochastic dynamics of fermions hopping along the edges of a graph with Brownian noisy amplitudes and driven out-of-equilibrium by injection-extraction processes at a few vertices. We present a solution for the invariant probability measure of the one dimensional Q-SSEP in the infinite size limit by constructing the steady correlation functions of the system density matrix and quantum expectation values. These correlation functions code for a rich structure of fluctuating quantum correlations and coherences. Although our construction does not rely on the standard techniques from the theory of integrable systems, it is based on a remarkable interplay between the permutation groups and polynomials. We incidentally point out a possible combinatorial interpretation of the Q-SSEP correlation functions via a surprising connexion with geometric combinatorics and the associahedron polytopes., Comment: 46 pages, 3 figures

Blazars are known for their variability on a wide range of timescales at all wavelengths. Most studies of TeV gamma-ray blazars focus on short timescales, especially during flares. With a decade of observations from the Fermi-LAT and VERITAS, we present an extensive study of the long-term multi-wavelength radio-to-gamma-ray flux-density variability, with the addition of a couple of short-time radio-structure and optical polarization observations of the blazar 1ES 1215+303 (z=0.130), with a focus on its gamma-ray emission from 100 MeV to 30 TeV. Multiple strong GeV gamma-ray flares, a long-term increase in the gamma-ray and optical flux baseline and a linear correlation between these two bands are observed over the ten-year period. Typical HBL behaviors are identified in the radio morphology and broadband spectrum of the source. Three stationary features in the innermost jet are resolved by VLBA at 43.1, 22.2, and 15.3 GHz. We employ a two-component synchrotron self-Compton model to describe different flux states of the source, including the epoch during which an extreme shift in energy of the synchrotron peak frequency from infrared to soft X-rays is observed., Comment: Accepted for publication in the Astrophysical Journal

We introduce the asymmetric extension of the Quantum Symmetric Simple Exclusion Process which is a stochastic model of fermions on a lattice hopping with random amplitudes. In this setting, we analytically show that the time-integrated current of fermions defines a height field which exhibits a quantum non-linear stochastic Kardar-Parisi-Zhang dynamics. Similarly to classical simple exclusion processes, we further introduce the discrete Cole-Hopf (or G\"artner) transform of the height field which satisfies a quantum version of the Stochastic Heat Equation. Finally, we investigate the limit of the height field theory in the continuum under the celebrated Kardar-Parisi-Zhang scaling and the regime of almost-commuting quantum noise., Comment: 5 pages, 2 figures

We introduce a model of effective conformal quantum field theory in dimension $d=1+1$ coupled to stochastic noise, where Kardar-Parisi-Zhang (KPZ) class fluctuations can be observed. The analysis of the quantum dynamics of the scaling operators reduces to the study of random trajectories in a random environment, modeled by Brownian vector fields. We use recent results on random walks in random environments to calculate the time-dependent entanglement entropy of a subsystem interval, starting from a factorized state. We find that the fluctuations of the entropy in the large deviation regime are governed by the universal Tracy-Widom distribution. This enlarges the KPZ class, previously observed in random circuit models, to a family of interacting many body quantum systems., Comment: 5 pages, plus references, plus Supp. Mat. (15 pages), 3 figures

I describe a five-dimensional, polarised, Bethe-Heitler event generator of $\gamma$-ray conversions to $\mu^+\mu^-$, based on a generator for conversion to $e^+e^-$ developed in the past. Verifications are performed from close-to-threshold to high energies., Comment: 8 pages, 6 figures

For a quantum system in a macroscopically large volume $V$, prepared in a pure state and subject to maximally noisy or ergodic unitary dynamics, the reduced density matrix of any sub-system $v\ll V$ is almost surely totally mixed. We show that the fluctuations around this limiting value, evaluated according to the invariant measure of these unitary flows, are captured by the Gaussian unitary ensemble (GUE) of random matrix theory. An extension of this statement, applicable when the unitary transformations conserve the energy but are maximally noisy or ergodic on any energy shell, allows to decipher the fluctuations around canonical typicality. According to typicality, if the large system is prepared in a generic pure state in a given energy shell, the reduced density matrix of the sub-system is almost surely the canonical Gibbs state of that sub-system. We show that the fluctuations around the Gibbs state are encoded in a deformation of the GUE whose covariance is specified by the Gibbs state. Contact with the eigenstate thermalisation hypothesis (ETH) is discussed., Comment: 5 pages + 5 pages Supplementary Material

The All-sky Medium Energy Gamma-ray Observatory (AMEGO) is a probe class mission concept that will provide essential contributions to multimessenger astrophysics in the late 2020s and beyond. AMEGO combines high sensitivity in the 200 keV to 10 GeV energy range with a wide field of view, good spectral resolution, and polarization sensitivity. Therefore, AMEGO is key in the study of multimessenger astrophysical objects that have unique signatures in the gamma-ray regime, such as neutron star mergers, supernovae, and flaring active galactic nuclei. The order-of-magnitude improvement compared to previous MeV missions also enables discoveries of a wide range of phenomena whose energy output peaks in the relatively unexplored medium-energy gamma-ray band., Comment: Astro2020 APC White Paper Updated to make small change to author list in metadata

We introduce and solve a model of fermions hopping between neighbouring sites on a line with random Brownian amplitudes and open boundary conditions driving the system out of equilibrium. The average dynamics reduces to that of the symmetric simple exclusion process. However, the full distribution encodes for a richer behaviour entailing fluctuating quantum coherences which survive in the steady limit. We determine exactly the system state steady distribution. We show that these out of equilibrium quantum fluctuations fulfil a large deviation principle and we present a method to recursively compute exactly the large deviation function. On the way, our approach gives a solution of the classical symmetric simple exclusion process using fermion technology. Our results open the route towards the extension of the macroscopic fluctuation theory to many body quantum systems., Comment: 5 pages + SM, 2 figures

Enabled by the Fermi Large Area Telescope, we now know young and recycled pulsars fill the gamma-ray sky, and we are beginning to understand their emission mechanism and their distribution throughout the Galaxy. However, key questions remain: Is there a large population of pulsars near the Galactic center? Why do the most energetic pulsars shine so brightly in MeV gamma rays but not always at GeV energies? What is the source and nature of the pair plasma in pulsar magnetospheres, and what role does the polar cap accelerator play? Addressing these questions calls for a sensitive, wide-field MeV telescope, which can detect the population of MeV-peaked pulsars hinted at by Fermi and hard X-ray telescopes and characterize their spectral shape and polarization., Comment: 8 pages, 4 figures, Astro2020 Science White Paper submitted to the National Academies of Sciences

A number of techniques are being developed to solve the monstrous sensitivity gap that exists between the energy ranges of good sensitivity of the Compton and of the pair telescopes and to extend polarimetry to the MeV gamma-ray world. I characterize the properties of an active target, detailing the various contributions to the angular resolution, using a full five-dimensional event generator of the Bethe-Heitler differential cross section. With the same tool I also examine the dilution of the polarization asymmetry induced by multiple scattering in the conversion wafer., Comment: Proceedings of the 12th INTEGRAL conference and 1st AHEAD Gamma-ray Workshop, Geneva (Switzerland), 11-15 February 2019, Ed. C. Ferrigno, E. Bozzo, P. von Balmoos

We extend beyond the Euler scales the hydrodynamic theory for quantum and classical integrable models developed in recent years, accounting for diffusive dynamics and local entropy production. We review how the diffusive scale can be reached via a gradient expansion of the expectation values of the conserved fields and how the coefficients of the expansion can be computed via integrated steady-state two-point correlation functions, emphasising that PT-symmetry can fully fix the inherent ambiguity in the definition of conserved fields at the diffusive scale. We develop a form factor expansion to compute such correlation functions and we show that, while the dynamics at the Euler scale is completely determined by the density of single quasiparticle excitations on top of the local steady state, diffusion is due to scattering processes among quasiparticles, which are only present in truly interacting systems. We then show that only two-quasiparticle scattering processes contribute to the diffusive dynamics. Finally we employ the theory to compute the exact spin diffusion constant of a gapped XXZ spin-1/2 chain at finite temperature and half-filling, where we show that spin transport is purely diffusive., Comment: 57 pages + bibliography. 4 figures

We show that hydrodynamic diffusion is generically present in many-body interacting integrable models. We extend the recently developed generalised hydrodynamic (GHD) to include terms of Navier-Stokes type which lead to positive entropy production and diffusive relaxation mechanisms. These terms provide the subleading diffusive corrections to Euler-scale GHD for the large-scale non-equilibrium dynamics of integrable systems, and arise due to two-body scatterings among quasiparticles. We give exact expressions for the diffusion coefficients. Our results apply to a large class of integrable models, including quantum and classical, Galilean and relativistic field theories, chains and gases in one dimension, such as the Lieb-Liniger model describing cold atom gases and the Heisenberg quantum spin chain. We provide numerical evaluations in the Heisenberg spin chain, both for the spin diffusion constant, and for the diffusive effects during the melting of a small domain wall of spins, finding excellent agreement with tDMRG numerical simulations., Comment: 5 pages + SM, 1 figure. Definition of \mathfrak{D} added

No $\gamma$-ray polarimeter sensitive above 1 MeV has ever flown space. $\gamma$-ray polarimetry would be a new window on the radiative processes at work in cosmic sources, processes that produce linearly polarised emission, each of which with different polarisation fractions. The HARPO Collaboration has designed, built and characterised on beam a gas-TPC active target with which we have demonstrated for the first time the polarimetry of a linearly polarised MeV $\gamma$-ray beam, from the analysis of the conversions to $e^+ e^-$ pairs., Comment: Presented at PM2018, 14th Pisa Meeting on advanced detectors, 27 May - 02 June 2018, Isola d'Elba, Italy

We describe a new version of the 5D, exact, polarised, Bethe-Heitler event generator of $\gamma$-ray conversions to $e^+e^-$, developed in the context of the HARPO project, that is able to simulate successive events with different photon energies and on different atomic targets without any substantial CPU overhead. The strong correlation between kinematic variables in the divergence of the five-dimensional differential cross section are mitigated by performing each step of the conversion in the appropriate Lorentz frame. We extend the verification range down to 1 keV above threshold and up to 1 EeV. This work could pave the way to the precise simulation of the high-performance $\gamma$-ray telescopes and polarimeters of the post-Fermi-LAT area., Comment: Submitted to Nucl. Instrum. Meth. A

We present here the results of the analysis of the $\gamma$-ray blazar, 1ES 1215+303, over a 10-year period, from 2008 to 2017, measured at high energies (HE; 200 MeV $< E <$ 100 GeV) by the Fermi Large Area Telescope (LAT) and at very high energies (VHE; $E >$ 100 GeV) by Fermi-LAT and VERITAS. This is the longest temporal study of this high-frequency-peaked BL Lac object (HBL) at $\gamma$-ray energies to date. The spectrum follows a log parabola over this time period, and its HE and VHE spectra are well-connected. Its flux is sufficiently strong at HE to allow us to bin the Fermi-LAT data in 3-day intervals, enabling us to investigate the temporal evolution of the flux in unprecedented detail. Several flaring episodes were detected and evidence for an overall trend of increasing flux over the span of the 10 years was observed. These light curves, in addition to the spectra, are presented. This unique data set will help us to advance our understanding of the underlying physical processes in blazar jets., Comment: Presented at the 7th Fermi Symposium 2017, 15-20 October 2017, Garmisch-Partenkirchen, Germany. Submitted to Proceedings of Science (PoS(IFS2017)116)

Novel gamma-ray telescope schemes (silicon wafer stacks, emulsions, gas detectors) are being developed so as to bridge the sensitivity gap between Compton and pair-creation telescopes. The lower average density with respect to the tungsten/silicon active target of the Fermi-LAT makes large effective-area telescopes voluminous objects, for which the photon energy measurement by conventional means (calorimeter, magnetic spectrometer, transition radiation detector) is a challenge for the mass budget of the space mission. We present an optimal measurement of track momentum by the multiple measurement of the angular deflections induced by multiple scattering in the active target itself, using a Bayesian analysis of the filtering innovations of a series of Kalman filters applied to the track. For a silicon-wafer-stack telescope, the method yields meaningful results up to a couple of GeV/c., Comment: Presented at the 7th Fermi Symposium 2017, 15-20 October 2017, Garmisch-Partenkirchen, Germany. Submitted to Proceedings of Science (PoS(IFS2017)126)

$\gamma$ rays are emitted by cosmic sources by non-thermal processes that yield either non-polarized photons, such as those from $\pi^0$ decay in hadronic interactions, or linearly polarized photons from synchrotron radiation and the inverse-Compton up-shifting of these on high-energy charged particles. Polarimetry in the MeV energy range would provide a powerful tool to discriminate among "leptonic" and "hadronic" emission models of blazars, for example, but no polarimeter sensitive above 1\,MeV has ever been flown into space. Low-$Z$ converter telescopes such as silicon detectors are developed to improve the angular resolution and the point-like sensitivity below 100 MeV. We have shown that in the case of a homogeneous, low-density active target such as a gas time-projection chamber (TPC), the single-track angular resolution is even better and is so good that in addition the linear polarimetry of the incoming radiation can be performed. We actually characterized the performance of a prototype of such a telescope on beam. Track momentum measurement in the tracker would enable calorimeter-free, large effective area telescopes on low-mass space missions. An optimal unbiased momentum estimate can be obtained, in the tracker alone, based on the momentum dependence of multiple scattering, from a Bayesian analysis of the innovations of Kalman filters applied to the tracks., Comment: Presented at the Polarised Emission from Astrophysical Jets Conference, June 12-16, 2017, Ierapetra, Greece

We revisit the precision of the measurement of track parameters (position, angle) with optimal methods in the presence of detector resolution, multiple scattering and zero magnetic field. We then obtain an optimal estimator of the track momentum by a Bayesian analysis of the filtering innovations of a series of Kalman filters applied to the track. This work could pave the way to the development of autonomous high-performance gas time-projection chambers (TPC) or silicon wafer gamma-ray space telescopes and be a powerful guide in the optimisation of the design of the multi-kilo-ton liquid argon TPCs that are under development for neutrino studies., Comment: 39 pages, 12 figures