The mechanisms that drive the development of gravitational instabilities, leading to the formation of the large-scale structure of the universe, is not yet understood in its full details. Yet, with the arrival of a new generation of projects of observational cosmology, such as the LSST and EUCLID, that aim at measuring dark energy properties from large-scale structure observations, it is now necessary to characterize its properties with a large precision and in a controlled manner. N-body simulations bring answers to those issues but only for a very limited number of models and for a limited range of cosmological parameters. The scientific cases of the project aforementioned rely however heavily on our ability to make such predictions for large classes of models. It is therefore necessary to sharpen our theoretical knowledge on the growth of gravitational structures. This project aims at developing tools for predicting and computing cosmic density spectra and bispectra (three-point correlation function) for a large set of cosmological models that include non-standard effects such as massive neutrinos or clustering dark energy. More precisely we wish to build theoretical tools for predicting those quantities analytically and with a controlled precision in the quasilinear regime - therefore in a regime that defines the validity of the linear regime and extends it - and develop robust and fast numerical codes for computing a set of well defined observables such as those related to cosmic shear observations, redshift space density field, etc. We also wish to construct more phenomenological models that explore the relationships between the density field (and its various components) and the halo density. The approaches we favor in this project make use of computation techniques that have been recently put forward in which re-summations of large classes of diagrams can be taken into account. These approaches explicitly, or implicitly, take advantage of the so-called eikonal approximation. Those approaches allow to develop perturbation theory calculations in a controlled way and for a large class of observables such as spectra, bispectra etc. Our project aims at writing and releasing packages - in fortran to make its portability to different systems easier - for the fast computation of perturbation theory spectra and bispectra beyond linear theory. More precisely, we wish to develop codes that compute spectra up to 2 loops (NNLO) and bispectra up to 1 loop (NLO).

Large-N approaches are powerful methods to study quantum many body problems such as those encountered in solid state physics and correlated electron problems. These methods consist in generalizing the problem to an arbitrary number, N, of particle flavors. When N goes to infinity the different flavors decouple and the interaction terms become solvable (quadratic and mean-field like). The task is then to expend the physical properties in 1/N to approach the initial model of interest. When applied to quantum spin systems in dimension greater than one, this approach has proven to be very fruitful and has provided the basis of our current understanding of the highly quantum phases these systems, and spin liquids in particular. However, one cannot make reliable quantitative predictions about a given microscopic model without including the fluctuation (i.e. finite-N) effects, some of which are known to be related to gauge degrees of freedom. In this project we will revisit and expand one particular large-N limit of antiferromagnetic Heisenberg model, that based on the Schwinger boson representation [Sp(2N)]. The idea is to start from some systematic numerical exploration of the low-energy landscape defined by the self-consistent mean-field solutions. For N=infinity the system is “locked” into the global energy minimum of this landscape. On the other hand, for finite N, the system can explore the low-energy regions of this landscape, and local minima and saddle-points in particular. Our idea is to gain some insight about the finite-N physics from the detailed knowledge of the mean-field energy landscape. As a first concrete application of this idea, we plan to study some frustrated spin models which have a Z2 spin liquid phase for small enough value of the spin (and large enough N), such as the triangular or kagome Heisenberg models. Such Z2 spin liquids are known to host some particular elementary excitations, dubbed visons, and which correspond to Pi-flux vortices for the (emergent) Z2-gauge field. We will look for these vortex states as saddle-points in the mean-field landscape, study their correlations and energetics (gap). Characterizing the excited mean-field states in magnetically ordered phases is also part of our program. Then, by computing numerically some (imaginary) time-dependent saddle-points of the large-N theory we will have access to the dynamical properties of these visons. Indeed, such instanton calculations should give the tunneling amplitudes for a vison to hop from one lattice plaquette to another, and thus their dispersion relation. This will provide an effective Ising-gauge theory model describing the non-perturbative finite-N fluctuations, and which will allow to address quantitatively some important questions such as the possible critical value of N below which the vison may condense and give rise to quantum phase transition (typically toward a valence-bond crystal).

Weak gravitational lensing is a key probe of the distribution of dark matter in the Universe; it is one of the main focuses of stage-IV galaxy surveys such as Euclid and the Legacy Survey of Space and Time. The standard weak-lensing measure – cosmic shear – consists in observing the apparent alignment of galaxies that results from the coherent deflection of light by the large-scale structure of the Universe. This technique is nevertheless limited by our ignorance of the intrinsic shape and orientation of galaxies. The ELROND project proposes to use strong-lensing Einstein rings as “standard shapes” to measure cosmic shear without the systematic errors that inevitably occur when using galaxies as sources. It will pave the way to a new independent measurement of the cosmological S8 parameter and offer novel probes of the distribution of dark matter on small scales. The fundamental idea behind ELROND is well motivated by prior theoretical analyses (Birrer et al. 2017, Fleury et al. 2021b) and its feasibility has already been demonstrated on mock images (Hogg et al. 2023). The project has three mostly independent objectives. Firstly, we shall assess the cosmological potential of Einstein rings as weak-lensing probes by forecasting the associated gain in precision and accuracy of S8 measurements by stage-IV surveys. Secondly, we shall confirm the measurability of the line-of-sight shear on real strong-lensing images, and apply it to the James Webb Space Telescope COSMOS-Web survey. Thirdly, we shall explore the detectability of line-of-sight perturbations to Einstein rings beyond shear, such as flexion, and study their ability to unveil small-scale properties of the dark-matter distribution.

For decades we have tried to explain the value of the Higgs boson mass in terms of symmetry. We have expected new symmetries and the new particles realizing them, to appear, first at LEP, then at the Tevatron and finally at the LHC. After more than 40 years we have not observed them and the origin of the scale of weak interactions remains mysterious. In this proposal I argue for a complete change of perspective on the problem. The origin of the weak scale can be found at early times in the history of the Universe, but it leaves non-trivial traces in the laboratory today. I discuss how the value of the Higgs boson mass can be tied to the evolution of the Universe, developing a program to fully explore the experimental consequences of this possibility. The impact of such a change of perspective is far reaching: it changes sharply our understanding of the origin of the weak scale. It offers a completely new motivation for current and future cosmological experiments. The impact is profound also on the high energy physics experimental program since this class of ideas points to a number of new experiments and signatures, ranging from probes of long-range forces to new signatures at the LHC and at future colliders.

The Heavy Photon Search (HPS) is a fixed target experiment planned to run intermittently in 2014-2015 at the Jefferson Laboratory (JLab), based in Virginia (USA). We will be using the high intensity electron accelerator facility available there to search for a new vector boson, called "heavy photon" in the mass range from 20 to 1000 MeV. The search for such a boson is motivated by the introduction of a new interaction, mediated by such gauge boson, which could help to solve several important puzzles of contemporary physics. For instance are concerned the (g-2)µ anomaly, the muonic hydrogen Lamb shift in atomic physics and the excess of electrons and positrons observed in cosmic ray data. Moreover, by coupling to dark matter, this force could also explain the discrepancies observed between the various direct dark matter searches (DAMA/LIBRA and CDMS for instance). This project seeks to produce the heavy photons by Bremsstrahlung-like emission of 2.2 and 6.6 GeV electrons sent on a high Z target. The heavy photon would decay into charged lepton pairs (mainly e+-e-) and give a twofold signature: a sharp peak in the invariant mass spectra of the e+-e- system above the QED (Quantum Electro-Dynamics) background and a secondary decay vertex distinct from the interaction point; the latter being possible when the coupling is weak enough to generate detectable life time effects. By removing most of the QED background, this latter method provides unparalleled sensitivity in the mass range 20-250 MeV making HPS unique among other experiments searching for a heavy photon. The HPS is also the occasion to discover true muonium (µ+-µ-) a predicted bound QED state that was never observed. The very compact structure of the true muonium makes this measurement an important precision test for QED calculations involving muons. The project is led in IPN Orsay by a group of young scientists, fitting particularly well with the objectives of the JCJC program. Indeed, with this project R. Dupré will take first responsibilities in a new and original field for the laboratory with the support of slightly more experienced colleagues. The HPS project also permits important interdisciplinary collaborations and raises multidisciplinary interests (atomic, nuclear, particle and astroparticle physics communities). Negative result for the search would also give important insight in these domains by reducing drastically the available phase space for the existence of a new gauge boson. Therefore, it will be an important constraint for the models including new forces in order to solve the physics problems highlighted previously. The HPS experiment has been approved by the JLab PAC (Program Advisory Committee) in 2012 with the highest rating, "A". In the very positive report the committee stated that "This experiment has the potential to make a revolutionary discovery if carried out in a timely manner". The IPN Orsay seeks the financial help of the ANR to take responsibility in the collaboration with a contribution to the electromagnetic calorimeter construction, an essential piece of equipment of the experiment. We request in this grant 257 kEuros to finance equipment on the calorimeter (for front end electronics and a monitoring system), a 2-year postdoc (for the development, construction and installation of the monitoring system) and travel money needed for this project. Although the experiment takes place in the US, because JLab is the only facility in the world offering the appropriate beam characteristics, the funding will be invested in France. The construction and assembly of the equipment as well as our part of the data analysis will be carried out in IPN Orsay.