Theory

The research of the Theory Group covers a very wide array of subjects, ranging from the infinitely small to the infinitely large: from the properties of the smallest constituents of matter, to questions pertaining to the large-scale structure of the universe. The ultimate goal is to provide theoretical models to explain the experimental observations, or make theoretical predictions that may be tested experimentally. We can classify our activities in three broad axes: high-energy physics, nuclear and hadronic physics and formal theoretical physics.

High-energy physics studies the fundamental interactions of elementary particles. While the currently accepted theory of fundamental interactions (known as the Standard Model) has been extremely accurate in describing the smallest constituents of matter accessible to us through experiment, several reasons lead to believe that it is not a complete theory of Nature. Our research includes the study of various theories that go beyond the Standard Model.

At a larger scale, nuclear and hadronic physics is the study of the properties of the nuclei and their constituents, the quarks and the gluons. Some of the phenomena we address occur at low energies, while others arise at extreme conditions, e.g. in the interior of very hot and dense stars.

Formal theoretical physics includes the study of various mathematical aspects of physical theories, as well as subjects that are not necessarily directly amenable to experimental tests at the present time. The latter include quantum gravity, a theory which combines Einstein’s theory of General Relativity with Quantum Mechanics.

Our research spans a large spectrum of topics: nuclear structure, few-body physics, QCD, high-energy particle physics, LHC phenomenology, astroparticle physics, physics beyond the Standard Model, quantum information, non-commutative spaces, differential and algebraic geometry, supergravity and string theory. We may classify our research into three broad axes across which numerous collaborations exist.

• Nuclear and hadronic physics
  • Nuclear EDF methods
  • Neutrino-nucleus interaction and hadronuclear physics
  • Hadronic matter and quark-gluon plasma
  • Multiquarks
  • Few-body systems
  • Oscillations neutron-antineutron
  • Compact stars properties: equation of state, thermal emissivity, neutrino scattering, gravitational waves.
  • Quark polarimetry and Electromagnetism
  • On-going activities
• Particle physics
  • Higgs physics
  • Collider phenomenology
  • Physics Beyond the Standard Model
  • Nanotubes and extra dimensions
  • Astroparticles
  • Black holes, Big-Bang nucleosynthesis, gravitational waves
  • On-going activities
• Formal theoritical physics
  • Superspace, higher-order corrections and fermionic condensates
  • Flux compactification, NATD and consistent truncations
  • Little strings and U-duality
  • Monodromies in one-loop string amplitudes
  • Field theoretical models in ordinary and noncommutative spaces
  • Weyl-Heisenberg algebra and quantum information
  • On-going activities

Nuclear and hadronic physics

Nuclear EDF methods

The activities in nuclear structure physics address the description of low-energy phenomena with energy density functionals (EDFs). During the past few years, we focused on three aspects of these methods: the construction of generalized forms of the EDF and the set-up of advanced fit protocols for the adjustment of their parameters; the construction of numerical tools to describe ground- and low-lying states of complex nuclei, and to characterize the effective interaction; the application of the available parametrizations and codes to questions of experimental interest.

There are several motivations, both phenomenological and formal, to investigate more general forms of the EDF. Several directions are presently explored. In order to improve the performance of contact interactions of Skyrme type, their generalization including terms with four (N2LO) and six (N3LO) gradients is considered. To overcome certain formal problems, a new type of non-local EDF based on finite-range generators has been proposed. New formal and numerical developments and exploratory fits have been achieved.

A second major axis of this activity is the development of tools to study properties of complex finite nuclei. One of these is a new Cartesian 3D coordinate-space code for self-consistent mean-field calculations. The code supersedes codes we developed in the past, and offers several major improvements, including numerical accuracy, a major reduction of computational time, together with a decrease of the need for fine-tuning numerical parameters. The second tool that is also constantly maintained and improved is a code for generator-coordinate-method calculations based on angular-momentum and particle-number projected mean-field states.

Another activity in our group is related to the development of relativistic effective Lagrangians. The first finite temperature Relativistic Hartree-Fock-Bogoliubov calculation has been performed by our group and applied to understand pairing properties in finite nuclei, such as pairing persistence or pairing re-entrance, predicted in 48Ni and 48Si drip-line nuclei. The latter was also predicted to be a doubly closed-shell and doubly bubble nucleus.

Neutrino-nucleus interaction and hadronuclear physics

One recognized expertise of our group concerns the interface between the nuclear many-body problem and the physics of its constituents. Our nuclear model for the neutrino-nucleus interaction, incorporates the n particle-n hole interactions. This proposal is now recognized as a decisive breakthrough. We compared our results to those of the “Continuum Random Phase Approximation” (CRPA), and successfully tested our model on the MiniBooNE or T2K data.

We have solved another outstanding problem concerning the determination, in a given event, of the true neutrino energy from the information provided by the characteristics of the emitted lepton: its energy and its emission angle. We have introduced the distribution of the true energy around this reconstituted value and shown the existence of a low-energy shift induced by the multi-nucleon excitations.

Another subject concerns the coupling between the strong interaction many-body problem and the nucleon structure. In a chiral approach including the response of the nucleon to the nuclear vector and scalar fields, an EDF has been constructed, with parameters constrained by non-perturbative QCD and hadron phenomenology.

Hadronic matter and quark-gluon plasma

The study of the hot and dense phases of QCD and the search for a chiral critical point is one of the main goals of current research, together with the formal understanding of deconfinement. Our approach is based on effective quark models of QCD. We have developed statistical tools to examine the parameterization of effective models and their predictive power. We develop improved effective potentials for Polyakov loop effective models, and we quantitatively assess the predictive power of the models.

We are developing a diagrammatic approach facilitating the computation of correlation coefficients used in the analysis of elliptic flow, and we are contributing to the calculation of four-body combinatorial background. We also work on the parallelization of the GNU Scientific Library (GSL) quadrature routines.

Multiquarks

We analyzed the spectroscopy of string-inspired models, and studied new flavor configurations for tetraquarks, pentaquarks and di-baryons with heavy quarks. We are currently working on heavy quark correlations.

Few-body systems

We have shown that certain hyperon-nucleon and hyperon-hyperon interaction models allow for the existence of novel light hypernuclei with S=−2 strangeness. Our recent work includes the first study of a three-body exotic atom, and a review of Hall-Post inequalities with new developments and applications.

We performed the first hypernuclear charts in three-dimension (N,Z,S) based on a new density functional approach, and studied the strangeness composition of these hypernuclei.

Oscillations neutron-antineutron

The neutron-antineutron oscillations in 40Ar were revisited, in connection with members of the DUNE collaboration. The spatial distribution of the antineutron cloud around the 39Ar core and its subsequent annihilation was estimated, and used in Monte-Carlo simulations for the DUNE experiment.

Compact stars properties: equation of state, thermal emissivity, neutrino scattering

The inner core of neutron stars can reach high densities where a phase transition to deconfined quarks may occur. We have started an ambitious research program aiming at confronting observations to a large set of nuclear equations of state, setting-up a meta-modeling for nuclear matter inspired by the EDF approach.

Neutrino diffusion in neutron stars is crucially linked to the neutrino spectrum observed on Earth. We have analyzed the neutrino coherent scattering in non-uniform matter, in order to clarify if a coherent effect could enhance the scattering, thereby reducing the neutrino mean free path.

The detection of gravitational waves provided the first firm observational constraint on the radius of neutron stars, through the tidal deformability measured during the last orbits of the inspiral merger phase. We have found that modern nuclear-physics-based calculations of the equation of state of dense neutron-rich matter predict radii that are compatible but more restrictive than the one from GW170817. We have also determined how improved constraints from future observations can provide new insights into dense matter and possible phase transitions in the neutron-star core.

Quark polarimetry and Electromagnetism

We are developing a recursive model for Monte Carlo simulation of the fragmentation of a polarized quark, reproducing the Collins effect. A simplified version, where only pseudoscalar mesons are emitted, has been implemented in a Monte Carlo program interfaced with PYTHIA. The simulations are in qualitative agreement with data from COMPASS and BELLE. In a stand-alone program, vector mesons have been included. We found an analogue of the Collins effect in atomic physics.

We studied the light created in an optical fiber when a charged particle passes through or near the fiber. We pointed out the common properties of synchrotron radiation and light leaking from a bent optical fiber. We participate in the research on positron sources assisted by channeling radiation.

On-going activities

The development, implementation, adjustment and validation of generalized nuclear energy density functionals for nuclear structure studies will be continued, with the long-term goal of widening their range of applicability at the mean-field (and possibly beyond mean-field) level and enhancing their predictive power. One particular focus will be the improvement of the description of properties of very heavy and super heavy nuclei, which most notably requires better control over the single-particle spectra in deformed nuclei. Calculations of charge radii, electromagnetic moments, characteristics of rotational bands, and other observables will be used in the evaluation of future experiments to be performed at ISOLDE/CERN, GANIL, and elsewhere.

Our group is investigating the links between bare nuclear interactions, such as the Bonn-type one-boson exchange potential, and relativistic effective Lagrangians developed for finite nuclei and uniform matter. Our project aims at improving the boson exchange potential and establishing a bridge from the bare nuclear potential to the effective approaches used in finite-nuclei. The ultimate goal of our project is to determine whether a novel meson exchange Lagrangian, adjusted on nucleon-nucleon scattering and complemented with off-shell interaction couplings, could bridge the gap between few-body and many-nucleon systems. Applications of this new model to the physics of the neutron star crust will also be considered. This approach can be extended to the strange sector including hyperon interactions.

We shall pursue and refine the construction of generalized Nambu-Jona-Lasinio models incorporating simultaneously chiral symmetry breaking and confinement, using either the Field Correlator Method or the Coulomb gauge in QCD. One aim is to improve the calculation of the condensates using an RPA-like method, but the main aim will be to derive an effective theory for nuclear physics or neutron star matter generating microscopically the nuclear vector and scalar fields, and the in-medium nucleon polarization, starting from pure QCD parameters (string tension and correlation length).

A better characterization of the properties of the merging stars can be translated into an improved knowledge of the equation of state of dense matter, with potential hints concerning phase transitions, as well as for the conditions for heavy element nucleosynthesis. We plan to implement the state-of-the-art microphysics into a global simulation for neutron star mergers, initially provided by D. Radice.

Within the next five years we plan to produce kilonovae simulations with state-of-the-art nuclear input. This will allow us to investigate the multi-messenger signals emitted by kilonovae, such as GW. We will also have a new tool to investigate continuous GW emission from neutron stars. Such calculations provide a perfect framework for a close collaboration with the Virgo and observational cosmology groups at IP2I. The modeling of the r-process nucleosynthesis existing in the ejected matter offers a possibility to link with the expertise of the nuclear theoreticians and the nuclear experimentalists of IP2I. The questions related to dense matter also allow a cross-fertilization between nuclear and hadron physics. More globally, the modeling of kilonovae and, possibly in the future, of core-collapse supernovae, could animate an active research program in the laboratory, including also astro-particle physics and cosmology.

Other on-going activities include the work on light hypernuclei so as to include charmed baryons; the systematic study of the weak decays of stable tetraquarks, in order to predict their lifetime and provide hints for the discovery channels; simulation models of jets of polarized quarks and their polarimetry: the search of efficient estimators of quark polarization, and the simulation of “spin entanglement” of two jets in e+e- annihilation.

Particle physics

Higgs physics

The discovery of the Higgs particle gave us new tools for the study of the physics of the spontaneous breaking of electroweak symmetry. We have proposed a new parameterization of the Higgs couplings, which allows to directly extract the loop contributions of the New Physics. We have extended our studies with the inclusion of a second Higgs boson, and are collaborating with CMS in the interpretation of the low mass results. We are working on the characterization of the supersymmetric Higgs sector and the prospects for future colliders.

Collider phenomenology

High energy proton collisions continue to furnish new high-precision results in energy regions hitherto unexplored. These results can be used to test theories beyond the Standard Model. We have developed international level expertise in the phenomenology of heavy vector quarks and supersymmetry, and have developed several numerical tools for the detection of such particles at LHC. We have implemented vector quarks with generic couplings in Monte Carlo tools such as MadGraph, allowing for model independent studies. NLO effects in QCD are also included and our tools are routinely used by experimental collaborations, such as CMS, for the production of Monte Carlo data. We have developed XQCAT, a tool used in extracting the mass constraint on heavy vector quarks, from experimental data.

Physics Beyond the Standard Model

We have developed expertise in various classes of models: extra dimensions, composite models, extended scalar sectors, dark matter. The recruitment of A. Arbey and N. Mahmoudi has given us expertise in flavor physics and supersymmetry. In particular, the public code SuperIso for the calculation of flavor physics observables has been vastly improved in recent years. We have proposed new models with extra dimensions for dark matter candidates arising from geometric symmetries. We have developed composite Higgs, and dark matter models. We have studied “Gauge-Higgs Unification” models, and showed the possibility of gauge-Yukawa unification. We have studied the flavor constraints on composite models and models with extra dimensions. In 2017, after five years of efforts, we completed the code GAMBIT, resulting in 5 publications.

Nanotubes and extra dimensions

The properties of carbon nanotubes are usually modelled by numerical simulations at the atomic level. We proposed a different approach based on relativistic field theory in the continuous approximation. This has the advantage of simplifying the calculation of the properties of the electron-hole modes propagating in the nanotube, reducing the system to a 2+1 model with one compactified dimension. We are developing the relevant formalism and testing its predictions.

Astroparticles

Our activities focus on searches for new particles by direct or indirect detection, relic density of dark matter, and the links with collider physics. The code SuperIso Relic was developed in order to provide a calculational tool for different observables in connection with dark matter and particle physics. Until recently devoted to supersymmetry, the code is currently being developed to allow a flexible and generic implementation of all types of scenarios of BSM physics. Moreover, we have studied the links with primordial cosmology and shown that the discovery of new particles will allow to obtain information on the content of the universe before primordial nucleosynthesis, despite the fact that this era is currently unobservable.

Black holes, Big-Bang nucleosynthesis, gravitational waves

Our research includes the development of the public code AlterBBN, devoted to the study of alternative cosmological models. Our studies cover two directions: the consequences of the presence of cosmological scalars, and the existence of primordial black holes during primordial nucleosynthesis. The latter subject requires the study of Hawking radiation of Schwarzschild and Kerr black holes, and a calculational code of primary and secondary spectra has been developed, and will soon become public. It is a code with unique functionalities. Black holes are also studied in the context of gravitational waves and the possibility of particular gravitational signatures in certain exotic models (e.g. boson stars) or alternative gravity theories (LQG, tensor-scalar theories, etc).

On-going activities

The theory group is involved in the scientific preparation of future colliders, and is eagerly awaiting the start of the HL-LHC. Different new physics scenarios are studied by the different members, such as supersymmetry, models with extended Higgs sectors, composite models and extra-dimensional scenarios. In addition, in recent years, flavor physics has become an emerging sector to probe new phenomena, mainly due to the appearance of several discrepancies with the Standard Model in the semileptonic decays of B mesons. The theory group will continue to play a major role in this respect, and to further develop the SuperIso code, which is a public tool to reinterpret the experimental results in new physics models. Furthermore, the links with astroparticle physics and cosmology are of great importance to understand the properties of physics beyond the SM, in particular through its relation with dark matter. A new extension of SuperIso Relic, which will allow for a flexible new physics implementation, is currently under development, and will provide an automatic calculation of dark matter observables in any new physics scenarios. In case of discovery of new physics at colliders or in astroparticle experiments, it will allow us to be at the forefront of characterization of new physics scenarios, and to derive constraints on the cosmological properties of the early Universe.

The BlackHawk code, which is the first program allowing an automatic calculation of the Hawking radiation of Schwarzschild and Kerr black holes, will be made public in the near future. It will allow us to pursue studies in the domain of cosmology, in particular for primordial black holes, and of astroparticle physics, with the design of new analysis to observe the Hawking radiation with astroparticle physics experiments. Combined with the AlterBBN code, it will also allow us to study the impact of primordial black holes or cosmological scalar fields on Big-Bang nucleosynthesis.

Other on-going activities include the study of phenomenological models with hyperbolic extra-dimensional geometries, such as nilmanifolds, and the implications for gauge-Higgs unification scenarios.

Formal theoretical physics

Superspace, higher-order corrections and fermionic condensates

The effective action of string/M-theory admits an infinite tower of derivative corrections, playing a crucial role in areas such as black holes, cosmology and the AdS/CFT duality. We have applied superspace methods to constrain the form of M-theory invariants. The tools developed were used in order to determine the quartic fermion terms of IIA supergravity and their impact on the search for de Sitter solutions.

Flux compactification, NATD and consistent truncations

Flux compactification (FC) refers to the most general setup in which the various tensor fields of string theory are turned on, thus resolving the “problem of moduli”. We used Generalized geometry and G-structures to uncover certain general periodicities and features of FC backgrounds. These tools were also used to shed light on non-abelian T-duality, and in constructing consistent truncations admitting de Sitter solutions.

Little strings and U-duality

Little string theories are a class of interacting, non-local, ultraviolet complete quantum theories in six dimensions (or lower). In a series of works, we have shown that various incarnations of string U-duality lead to remarkable dualities and symmetries among these gauge theories, which are intrinsically non-perturbative in nature. These symmetries allow us a better understanding of the gauge theories, as well as the construction of new theories.

Monodromies in one-loop string amplitudes

N-point tree-level scattering amplitudes in open string theory are described as correlation functions on the world-sheet disk. Monodromy relations can be established which have been instrumental in the study (and solution) of tree level string scattering amplitudes. A generalization of these relations to 1-loop amplitudes has been performed, giving new relations among one-loop string scattering amplitudes.

Toroidally compactifying supergravity theories to lower dimensions reveals the presence of exceptional symmetries, which cannot be explained by Riemannian geometry of the internal manifold alone (or any other “conventional’’ symmetries within SUGRA itself). They are remnants of the U-duality of string (or M-) theory, hinting at more complex structures at higher energy scales. Since these exceptional symmetries are difficult to explain within the framework of standard SUGRA theories, it has been attempted to extend the higher dimensional versions of the latter in such a way as to make the appearance of the exceptional symmetries manifest upon compactification. The resulting theories are called exceptional field theories. The basic idea behind these constructions is an extended space-time, which allows for a geometric realization of the U-duality group. We are exploring various possibilities to formulate the (3,1) and (4,0) supergravity theories within the framework of exceptional field theories.

Field theoretical models in ordinary and noncommutative spaces

We gave an improvement procedure to construct a gauge-invariant, symmetric energy-momentum tensor. We generalized Wong’s equations to non-commutative space and determined the properties of the energy-momentum tensor of gauge fields coupled to matter. We studied field theoretical models on deformed spaces and obtained local conservation laws as well as balance equations for interacting fields on these spaces.

Weyl-Heisenberg algebra and quantum information

Mutually unbiased bases in the Hilbert space Cd play an important role in quantum information, quantum cryptography and quantum tomography. We systematized the construction of mutually unbiased bases in Cd. We introduced a new parameter, the perma-concurrence, that generalizes the concurrence for a symmetrical state of N=2 qubits.

On-going activites

In recent years, the group has studied supersymmetric field theories in six dimensions or less. A large class of such theories lack a Lagrangian description and are therefore notoriously difficult to describe with purely field theoretic methods alone. However, their connection to string theory provides us with many new approaches and tools which allow to analyze non-perturbative aspects of these theories. We will continue this approach, focusing in particular on non-perturbative dualities among theories with different matter and gauge content.

Other on-going activities include: the study of low-energy effective actions in the presence of fermionic condensation, beyond the simplest “universal’’ models of Calabi-Yau compactifications, and the implications for the problem of de Sitter space in string theory; a systematic investigation of the string effective action beyond tree level in the string coupling, and its implications for cosmological models.

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