Appel à candidatures 2015

traduction en français à venir

Applications for Lagrange Thesis Fellowships are now closed!

The ILP invites applications for the fourth class of Lagrange thesis fellowships. These fellowships will include a net stipend of approximately EUR 1,400/month (approx. $1,900/month)* for three years and generous travel and research funds. The new Lagrange thesis fellows are expected to start in the Fall semester of 2015.

Successful candidates will have demonstrated academic excellence, outstanding potential for creative research, and leadership qualities.

Lagrange thesis fellows will be immersed in an internationally visible, world-class research environment in terms of intellectual, and computational resources, an extensive visitors' program, and significant involvement in the world's leading astronomical and (astro-)particle physics projects, such as Planck, Herschel, Euclid, CFHTLS, TeraPix, BOSS, SDSS III, VIPER, Square Kilometer Array, LHC, HESS, Auger, etc.

The Lagrange thesis fellows will be enrolled in a doctoral school of the Université Pierre et Marie Curie, part of Sorbonne Universités, with access to intensive short courses in cutting edge areas of the field, such as dark matter and dark energy research, theoretical and experimental (astro-)particle physics, recent advances in quantum field theory and string theory. Special short courses will be offered in emerging research methods such as astrostatistics, discovery in petascale data sets, etc. The Lagrange Institute will support yearly summer or winter schools where thesis fellows can meet their future colleagues. All courses will be offered in English and taught by the leading experts in each field - either members of ILP or guest professors, including the Lagrange award holders.

The list of thesis project proposals written by members of the Institute is now available! Candidates are invited to contact their potential advisors directly before submitting their application.

Applications should include academic history, course work, scientific interests, scholarships and honors, outside interests, purpose and goals. Candidates should arrange for 3 letters of reference to be uploaded to the web application.


*with a salary supplement of approximately EUR270/month in case of teaching activities.


Project proposals 2015

- Phenomenology of extensions of the Standard Model
- BOSS and eBOSS: Reconstruction of the Intergalactic Medium
- Numerical and theoretical studies of particle acceleration in relativistic astrophysical outflows
- Testing Lorentz invariance in gravity and cosmology
- Clusters of galaxies in the cosmic web
- Bayesian dark matter & orbit modeling of galaxies and clusters: including non-sphericity, proper motions, and modified gravity
- Search for physics beyond the Standard Model in rare B → K(*)+(')- decays with the LHCb experiment
- Exploring the Cosmic Dawn and the Epoch of Reionization with the 21 cm signal
- Bayesian 3d power spectrum reconstruction from Quasar and LRG surveys
- Observation of the Higgs boson decaying to b quark pairs and measurement of the Higgs Yukawa coupling to Bottom and Top with the ATLAS experiment at the LHC Run 2
- Baryon acoustic oscillations in the Lyman-alpha forest with the quasar spectra of the SDSS-IV/eBOSS survey
- Numerical Methods for the prediction of Gravitational Lensing Signal as a probe of the mass content of the Universe

Phenomenology of extensions of the Standard Model

Laboratory: LPTHE
Contact persons:
Karim Benakli /// kbenakli[at]lpthe.jussieu.fr
Mark Goodsell /// goodsell[at]lpthe.jussieu.fr
Pietro Slavich /// slavich[at]lpthe.jussieu.fr
Website: www.lpthe.jussieu.fr

The Beyond the Standard Model research team - consisting of Karim Benakli, Mark Goodsell and Pietro Slavich - will have a PhD position starting in Autumn 2015.

The Standard Model provides neither a Dark Matter candidate nor a mechanism to generate matter-anti-matter asymmetry, while extensions of it are constrained by data from experiments at the LHC, dark matter searches, astrophysical and cosmological observations. In particular, when confronted with the experimental measurements, a precise theoretical computation of the Higgs boson mass and of the branching ratios of its decays restricts the parameter space of the Minimal Supersymmetric Standard Model, and motivates new approaches which have recently been receiving a growing interest.

The subject of this thesis is the investigation of various aspects of these extensions of the Standard Model. Depending also on the preferences of the candidate, the focus of the thesis may vary from model-building aspects to precision computations and analysis of expected collider signatures.

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BOSS and eBOSS: Reconstruction of the Intergalactic Medium

Laboratory: Institut d'Astrophysique de Paris
Contact person: Patrick Petitjean /// petitjean[at]iap.fr
Website: www.iap.fr
Internship: A subject has been proposed on the website of SF2A gathering all M2 internships

A high fidelity 3D map of the high redshift Universe would be a useful cosmological tool. It could, for example, aid in ruling out cosmological theories or refining Baryon Acoustic Oscillation (BAO) measurements. A prominent obstacle in the ability to reconstruct a map is that distant objects must be very luminous for us to observe them. Fortunately, HI density fluctuations in the IGM, which are known to trace the dark matter density structures on scales above a pressure smoothing scale, can be inferred by analyzing the Lyman-alpha absorption features (called the Lyman-alpha forest) they imprint in quasar spectra. By correlating the Lyman-alpha forests observed along adjacent lines of sight (from pairs or groups of quasars), it is possible to reconstruct the properties of the IGM. If enough quasars are available, it is possible to reconstruct a 3-D image of the IGM.

The SDSS-BOSS and eBOSS surveys are providing us with this dense network of sources. Starting from the analysis of pairs of quasars, the PhD will be interested in these reconstructions to trace the underlying density field and analyse, not only large scale structures, but also the connection between the IGM and galaxies.

Besides the wealth of low spectral resolution data available from SDSS, follow-up VLT-XSHOOTER high resolution data are available for pairs and groups of quasars to start with. Although the subject is mostly observational, interaction with theorists working in this field in IAP will be encouraged. In particular, hydro-simulations suitable to analyse the data are available in IAP.

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Numerical and theoretical studies of particle acceleration in relativistic astrophysical outflows

Laboratories: Institut d'Astrophysique de Paris /// CEA, DAM, DIF
Contact persons:
Martin Lemoine /// lemoine[at]iap.fr /// +33 1 44 32 80 50
Laurent Gremillet /// laurent.gremillet[at]cea.fr /// +33 1 69 36 73 61
Website: www.iap.fr

Mechanisms of particle acceleration in powerful astrophysical sources represent one of the cornerstones of modern astroparticle physics. Acceleration physics indeed plays a key role in the interpretation of observational data of non-thermal sources such as pulsar winds, active galactic nuclei, gamma-ray bursts, just as it plays a key role in understanding the origin of very high energy cosmic rays. Important progress has been achieved in this field in recent years, notably through massive particle-in-cell (PIC) simulations, which allow a direct ab initio simulation of the dynamics of collisionless plasmas.

This PhD thesis proposes to study the mechanisms of particle acceleration in relativistic outflows, through the conjunction of PIC simulations and analytical studies. This PhD will be co- supervised by Laurent Gremillet (Direction des Applications Militaires du CEA, Bruyères le Chatel), who is specialized in laser-plasma interactions and PIC simulations, and Martin Lemoine (Institut d'Astrophysique de Paris, Paris), astrophysicist. The PhD will thus follow a double sided approach with, on the one hand, the development and the conduction of PIC simulations of astrophysical sources and, on the other hand, the analytical study of acceleration physics and its astrophysical consequences. The PIC code is already available, but it has been developped for use in laser-plasma interaction studies, hence it needs to be optimized for its application to astrophysical topics.

An in-depth simulation of a relativistic and magnetized collisionless shock wave, as found in gamma-ray bursts outflows or in active galactic nuclei, represents a major objective of this thesis. The simulations must characterize how and when acceleration takes place as a function of the velocity βc of the shock front, more commonly expressed in terms of the shock Lorentz factor Γ= (1-β2)-1/2, and as a function of the degree of magnetization of the ambient plasma. Simulations for a moderate Γ~2-10 are particularly interesting because such shocks correspond to internal shocks in a relativistic outflow, and because their physics remain at present largely unexplored. In particular, one does not know whether acceleration takes place, and if yes, up to which energy. Analytical studies will complement the PIC simulations, by defining the most pertinent simulations, and by extracting astrophysical consequences both in terms of radiative signatures and high energy particle production.

Numerous other applications of such simulations can be envisaged in the course of this PhD, since PIC codes have just made their way into the community of high energy astrophysics and astroparticles.

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Testing Lorentz invariance in gravity and cosmology

Laboratory: Institut d'Astrophysique de Paris
Contact persons:
Enrico Barausse /// barausse[at]iap.fr
Luc Blanchet /// blanchet[at]iap.fr
Website: www.iap.fr

Lorentz symmetry allows reconciling Maxwell's theory of electromagnetism with the principle of relativity. This reconciliation was the basis of Einstein's theory of Special Relativity. Einstein later formulated General Relativity as a Lorentz symmetric completion of Newtonian gravity. Today, much of theoretical physics is built on Lorentz symmetry. Given how embedded this symmetry is in our understanding of Nature, any hint of its violation would shake theoretical physics at its core.

In spite of its importance, constraints on Lorentz symmetry violations in the gravitational sector are still weak and partial. In particular, all available tests involve weak gravitational fields and/or velocities v that are small compared to the speed of light c. Therefore, gravitational theories (and Lorentz symmetry) remain essentially untested in the strong-field/highly dynamical v ~ c regime, where high-energy corrections may appear. Because strong-field regimes are naturally associated with binary systems containing black holes and neutron stars, which are powerful sources of gravitational radiation, upcoming gravitational-wave experiments such as Advanced LIGO, Advanced Virgo and KAGRA will provide an unprecedented opportunity to test Lorentz symmetry in gravity in highly dynamical regimes.

Another promising testing ground for Lorentz violations in gravity is given by cosmological observations such as those of the CMB and of the large scale structure. This is because certain classes of Lorentz-violating gravity theories can be shown to reproduce certain aspect of the Dark Matter phenomenology (e.g. the rotation curves of galaxies) without any actual Dark Matter.

According to the candidate profile, the goal of this PhD project will be either
1) To derive predictions for binary systems of black holes and neutron stars in Lorentz violating gravity theories, using analytical approximations (Post-Newtonian formalism, perturbation theory) for the early inspiral of the system and the late-time ringdown of the merger remnant, as well as numerical-relativity techniques to study the highly relativistic, strong-field merger phase.
2) To compare the class of Lorentz-violating theories giving a Dark-Matter phenomenology on the scale of galaxies with both local tests of gravity (binary pulsars, solar system tests) and more general cosmological observables (e.g. BBN, CMB and large scale structure).

A combination of the two project may also be possible.

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Clusters of galaxies in the cosmic web

Laboratories: Institut d'Astrophysique de Paris /// LAM, Marseille
Contact persons:
Florence Durret /// durret[at]iap.fr
Christophe Adami /// christophe.adami[at]lam.fr
Website: www.iap.fr

Clusters of galaxies are the largest gravitationally bound structures in the universe, and in the framework of hierarchical structure formation in the universe they are believed to be formed by the grouping of galaxies at redshifts around z=2. In clusters, the galaxies are embedded in a very hot and tenuous gas emitting in X-rays. Observations show that galaxies are distributed along filaments, at the intersection of which clusters are located, in agreement with numerical simulations of large scale structure formation. In some cases, filaments of galaxies have been detected, and there is evidence that clusters are still accreting galaxies along preferential directions. A very small number of filaments between clusters have also been detected in X-rays, and through weak gravitational lensing. The best way to search for galaxy filaments is to obtain hundreds of spectroscopic redshifts for galaxies in a large field surrounding each cluster, but this requires large amounts of telescope time. Another method is to obtain deep large field imaging in several photometric bands covering a wide zone around the cluster. By computing photometric redshifts it is possible to pre-select galaxies located roughly at the same distance as the cluster, and to identify groups and clusters that may be in the cluster environment. Since the galaxies belonging to the cluster have formed more or less at the same epoch, they define a sequence in a colour-magnitude diagram (the “red sequence”). The concentrations of galaxies selected in the cluster field (in filaments, groups or small clusters) that are at the same distance as the cluster and may therefore be linked to the cluster should follow approximately the same sequence. By the study of their spatial distribution and the computation of the galaxy density maps of the galaxies thus selected, it should be possible to detect large numbers of filaments joining clusters, as well as groups of galaxies, and to analyze their properties.

We have tested this method on nine clusters of the DAFT/FADA survey (see cencos.oamp.fr/DAFT/), and detected extended structures several Mpc in length in at least three clusters. This work was done during the Master 2 internship of A. Acebron in March-June 2014 and preliminary results will be published in the Proceedings of the SF2A 2014 meeting (Acebron et al. 2014).

The PhD proposed here is aimed at automatizing the method to detect filaments (and possibly groups) and to apply it to the CFHTLS (Canada France Hawaii Telescope Legacy Survey) in which our team has used photometric redshifts to search for clusters (see Adami C., Durret F. et al. 2010, A&A 509, 81 ; Durret F., Adami C. et al. 2011, A&A 535, 65) and detected several thousand candidate clusters up to redshift z=1.15. The CFHTLS is a very high quality survey and its properties at very large scale around clusters can give important constraints on cluster formation and evolution. Besides, the analysis of numerical simulations by C. Pichon and his team at IAP has recently shown that the orientations of galaxies located along filaments was perpendicular to that of galaxies outside filaments. Tests on observations have until now been scarce and inconclusive, and could bring very interesting results linked to cluster formation and evolution. A collaboration with the group of C. Pichon is foreseen, probably during the last year of the PhD, to compare observations to simulations.

All the data are already reduced and available, as well as photometric redshifts for millions of galaxies. Besides, part of the CFHTLS coincides with other surveys such as VIPERS (large spectroscopic survey) or XXL (X-ray survey). For the clusters which are in common in various surveys it will be possible to characterize more precisely the environments of clusters and to search for an evolution of the properties found as a function of redshift and cluster richness.

The work can be started on a small part of the Canada France Hawaii Telescope Legacy Survey data during a 3-month Master 2 internship at IAP in April-June 2015.

Collaborations are foreseen with several members of the IAP.

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Bayesian dark matter & orbit modeling of galaxies and clusters: including non-sphericity, proper motions, and modified gravity

Laboratory: Institut d'Astrophysique de Paris
Contact person: Gary Mamon /// gam[at]iap.fr
Website: www.iap.fr/users/gam/research.html
Internship: A Masters-2 project has been submitted along similar (but much less ambitious) lines.

Dark matter is understood to contribute 85% of the mass density of the Universe, but its distribution is still poorly known, despite progress with gravitational lensing, X-ray observations and internal kinematics of galaxies and clusters. While it is fairly well accepted that the dark or total matter profiles are not very different from the NFW-like profiles arising in the halos within dissipationless cosmological N-body simulations, the details of the dark matter normalization, concentration and inner slope, and their variations with the mass and type of the structure, which serve as important constraints to models of galaxy formation, are still very poorly measured. At the same time, there has been little work so far on testing modified theories of gravity or inertia using state-of-the-art kinematic modeling of galaxies and clusters.

This thesis proposal aims to strongly improve the efficiency of the mass modeling of cosmic structures (recovering the dark matter component after subtracting off the visible components). The aim is to adapt new, state-of-the-art mass modeling techniques using internal motions, in particular the rapid and fairly accurate Bayesian MAMPOSSt algorithm (which has performed extremely well in an ongoing cluster mass challenge involving 25 competing algorithms), in various directions: non-sphericity, proper motions, and modified gravity, as well as generalizing MAMPOSSt from discrete velocity tracers (i.e. stars in galaxies) to the spatially resolved line-of-sight velocity distribution functions now measured thanks to the recent advances in Integral Field Spectroscopy (Atlas3D, CALIFA, MANGA). The generalization of MAMPOSSt to non-spherical clusters is expected to allow mass measurements with up to 5 times more accuracy, and even more once MAMPOSSt is joined with another Bayesian mass modeling tool based upon strong gravitational lensing. This will render clusters competitive in measuring the dark energy parameter and its variation with cosmic time (i.e. with the future Euclid mission). Mass modeling also provides new constraints on the masses of black holes that may lie at the centers of globular clusters and dwarf spheroidal galaxies, thus providing a new fundamental constraint on the unknown growth mechanism of supermassive black holes. The measured stellar masses will be used to constrain the initial stellar mass function (IMF) of galaxies. Bayesian evidence will be used to distinguish modified and standard gravity. Methods using internal motions (such as MAMPOSSt) allow one to measure, at the same time, constraints on the anisotropy of the velocity tensor in the inner and outer regions, itself linked to the orbital shapes of stars in galaxies and galaxies in clusters, which permit to indirectly observe the formation process of structures (infall leading to radial orbits, while major mergers transfer sufficient orbital angular momentum into internal motions to produce tangential orbits). Finally, there are obvious extensions to mapping the dark matter in the Milky Way at large (shape) and small (dark clumps) scales with the ongoing Gaia mission, and planning future proper motion observations of local group dwarf spheroidals with the THEIA mission (followup to Gaia, soon to be proposed to ESA).

Collaborators: A. Biviano (Trieste), R. Wojtak (Stanford), A. Romanowsky (Santa Cruz), M. Limousin (Marseille), M. Volonteri (IAP)

Some useful links and publications:
Gaia mission: sci.esa.int/gaia/40846-galactic-structure
Theia mission: centra.tecnico.ulisboa.pt/network/sim/news/?id=1963
MAMPOSSt algorithm: 2013MNRAS.429.3079M
Review on kinematic modeling methods: 2014RvMP...86...47C (chapter 5)

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Search for physics beyond the Standard Model in rare B → K(*)+(')- decays with the LHCb experiment

Laboratory: LPNHE
Contact person: Francesco Polci /// polci[at]lpnhe.in2p3.fr ///+33 1 44 27 53 07
Website: lpnhe.in2p3.fr
Internship: An internship before the starting of the PhD is planned

The unprecedented large quantity of data collected by the LHCb experiment at the Large Hadron Collider LHC at CERN allows for the first time searching for, or studying in details, some rare
B → K(*)+(')- decays, where ℓ is a lepton. These decays have the strong potential to unveil the presence of new physics beyond the Standard Model, or at least constrain alternative models proposed to solve open issues in our understanding of the universe, like, for example, the nature of the dark matter. This is one of the main research lines of the LHCb collaboration, and on this path the LHCb group at LPNHE proposes a thesis focused on the analysis of two channels with large new physics discovery potential: B → K*μ+ μ- and B → K(*)e+μ-.

The B → K*μ+ μ- decay proceeds, in the Standard Model, via a flavor changing neutral current, described by box diagrams and loop diagrams called « electroweak penguins ». Yet undiscovered particles could appear in the loops, modifying the angular distributions predicted by the Standard Model. The LPNHE group has already been involved in the first LHCb measurements, where a good agreement with the Standard Model has been observed for all angular observables, except P5', different from the prediction by 3.7 standard deviations. Several interpretations of this discrepancy have been proposed, some of them involving the presence of a new interaction mediated by a new vector boson Z'. Therefore, the analysis is currently, and for the next years with more data to come, at the core of the LHCb physics program and of the theoretical discussions. LHCb will resume data taking in 2015. The data that will be collected during the next years and that the PhD student will have the opportunity to analyze, would allow confirming or not the discrepancy, and possibly clarifying its nature.

In synergy with the B → K*μ+ μ- analysis, the student will also search for the B → K(*)e+μ - charged lepton flavor violating decay. The observation of neutrino oscillations implies that the lepton flavor number is not conserved. On the other hand, a charged lepton flavor violation process has never been observed. Since they are expected to be extremely small in the Standard Model, the evidence for decays like B → K(*)e+μ- would be an outstanding sign of new physics, with many possible implications: on the seesaw mechanism and heavy neutrinos, natural candidates for dark matter; on the grand unified theories involving lepto-quarks; on the physics behind the matter–antimatter asymmetry of the universe.
The search for lepton flavor violating channels has risen in priority since the recent observation of a 2.6 standard-deviations difference from unity of the ratio RK = Br(B+→ K+e+e-)/Br(B+ → K+μ+ μ-). If confirmed, this result implies the violation of the lepton universality, necessarily associated to a new lepton flavor violating interaction that would increase the rates of B → K(*)e+μ- decays. Theoretical approaches relating the P5' and RK observed deviations to a common source are already being explored, and the observation or not of the B → K(*)e+μ- decays would be a crucial additional information.

In order to continue collecting data after 2018, strengthening the discovery potential in this type of searches, the LHCb experiment is planning an upgrade of its detector. The LPNHE group works on the new Scintillating Fiber tracker. The student will also participate in the effort to improve the timing and the efficiency of the track reconstruction algorithms.

In summary the thesis gives the opportunity to develop analysis and technical skills, joining a dynamic collaboration working on physics topics with potentially large impact on some of the most fundamental open questions in physics.

The student will have the opportunity to interact with Robert Ziegler, an ILP postdoc at LPTHE with experience in lepton flavor violation, as well as with other theorists of the domain.

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Exploring the Cosmic Dawn and the Epoch of Reionization with the 21 cm signal

Laboratory: LERMA, Observatoire de Paris
Contact person: Benoit Semelin /// benoit.semelin[at]obspm.fr
Website: lerma.obspm.fr

From 2020 on, the Square Kilometer Array radio-interferometer (SKA) will start observing the Cosmic Dawn and the Epoch of Reionization (EoR). Following recombination at z~1000, the universe is cold and neutral. Only with the formation of the first stars at z~20-30 (Cosmic Dawn) are ionizing photons emitted again. As star clusters grow into primordial galaxies, bubbles of ionized IGM form around them and expand into the intergalactic medium until they overlap. Around z~6 (1 billion years after the Big Bang), the universe is completely reionized. During these first billion years, patches of cold and neutral hydrogen exist in the IGM and emit 21 cm photons. The tremendous sensitivity and collecting area of the SKA will allow us to perform a 3-dimensionnnal mapping of the IGM with this faint signal. To design the instrument a vast effort has been going on worldwide in modeling the 21-cm signal with state of the art numerical simulations.

The predictions of these simulations, however, are impacted by a number of unknown astrophysical parameters such as the relative contribution of sources of different nature (massive stars, quasars, X-ray binaries, etc...), the star formation efficiency, the escape fraction of ionizing photons and more, in the same way as the CMB signal depends on the cosmological model parameters. Defining a parameter space and exploring systematic methods to constrain the parameters from the observational data is, in the case of the 21 cm signal, a largely unexplored field and the subject of this PhD project. It will require building a large database of 21 cm signal light cones by running a number of numerical simulations, and then exploring various methods to estimate the astrophysical parameters in an unknown data cube using the information in the database: such algorithms as neural network, principal component analysis will be explored.

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Bayesian 3d power spectrum reconstruction from Quasar and LRG surveys

Laboratory: Institut d'Astrophysique de Paris
Contact persons:
Guilhem Lavaux /// lavaux[at]iap.fr
Jens Jasche /// jasche[at]iap.fr
Website: www.iap.fr
Internship: The internship would be focused on the quasar data model, built from the DEUSX simulation covering a huge fraction of the mock-observable universe. This subject is a seamless prelude to the PhD subject leading to the analysis of real data.

The Sloan Digital Sky survey has given us access to both a wide and deep view of the Universe, from the most local dwarf galaxies to the distant Quasars (QSOs), the most massive compact objects of the Universe. In particular QSOs hold the promise to probe the largest scales of the Universe, giving us access to information on gravitational waves, inflation and the assembly history of the most massive objects. Additionally, we would have the largest redshift lever arm from redshift zero to two to check on tracer evolution. Unfortunately, the survey is impeded by many foreground and boundary effects, which limits the acquisition of statistical information from the clustering of the Large Scale Structure of the Universe (LSS) on Horizon scales.

Many of these problems can be alleviated using a proper data model within a Bayesian framework. To achieve this, we propose to use the ARES framework. ARES has been developed to measure accurately the statistics of the density fluctuations assuming Gaussian statistics, self-consistently modeling the galaxy bias and edge effects. Building upon these foundations, the Bayesian inference framework shall be extended to incorporate foreground effects, typically affecting the detectability of galaxies in deep surveys. The structure of such foregrounds can be derived from complementary data, such as the density of stars, the dust column density or the sky brightness, from which we may build sky templates for foregrounds. The student shall develop a data model and implement it in the ARES code. Ultimately, it will perform the data analysis on the available SDSS-QSOs dataset.

A student will gain experience in all sub areas of scientific working. It involves theoretical modeling, software development, data handling as well as actual cosmological inference. Proficiency in C/C++ is preferable, as well as basic astronomy knowledge. The project will be hosted at the IAP and supervised by Guilhem Lavaux in close collaboration with Jens Jasche, who was the first author of the ARES software.

Jens Jasche is currently in Germany. The student will have many opportunities to interact with the many members of the IAP, the Excellence Cluster (TUM) and the Max Planck Institute for Astrophysics (MPA/Garching).

Some useful links and papers:
* DEUS: www.deus-consortium.org
* The SDSS3 project: www.sdss3.org
* ARES algorithm: 2010MNRAS.406...60J, 2013ApJ...779...15J, 2014arXiv1402.1763J

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Observation of the Higgs boson decaying to b quark pairs and measurement of the Higgs Yukawa coupling to Bottom and Top with the ATLAS experiment at the LHC Run 2

Laboratory: LPNHE (Paris) /// Group: ATLAS
Contact person: Sandro de Cecco /// sandro.dececco[at]lpnhe.in2p3.fr
Website: lpnhe.in2p3.fr
Internship in spring 2015 at CERN and LPNHE Paris

Higgs boson physics: properties and couplings. H → bb decay and VH, VBF, ttH. production processes. New analysis and trigger (FTK) techniques for the LHC Run 2. Measurement of Higgs couplings to fermions and perspectives studies for the high luminosity LHC phase (HL-LHC) and for future e+e accelerators (ILC).

The study of spontaneous electro-weak symmetry breaking in the Standard Model together with the search for signs of new physics are the main pillars for Large Hadron Collider physics. The recent Higgs boson discovery with the LHC run 1 data, with a mass of 125 GeV is a major milestone for high energy physiscs field. The observed properties of the Higgs are in good agreement with the Standard Model predictions. At this mass value, several Higgs decays final states are experimentally accessible and allow testing the predictions for Higgs couplings to fermions and gauge bosons. The observation of the Higgs boson has been dominated by the decay modes in boson pairs (photons, W and Z). More recently, also an evidence for tau lepton pair decay was reported. With the LHC Run 1 dataset, the high BR decay mode in b quark pairs has not yet been observed by the LHC experiments and the top quark Yukawa Higgs coupling was assessed only indirectly through the gluon fusion Higgs production process. The first goal in Higgs physics at the LHC Run2 will then be the observation of the H → bb decay mode produced in association with a vector gauge boson (VH) and the search for Higgs production in association with a top quark pair (ttH). Both searches will contribute significantly to the profiling of Higgs boson Yukawa couplings to matter. This PhD project will develop along this important LHC physics topic.

The ATLAS experiment is one of the major experiments at the CERN LHC proton-proton collider. A first data taking period with an energy in the center of mass of 7 TeV in 2010-11, of 8 TeV in 2012 collected respectively 5 fb-1 and 20 fb-1 of integrated luminosity. The current shutdown in 2013-14 is needed to upgrade the accelerator. LHC is now ready to restart collisions with an energy of 13 TeV, the Run 2 will start in march 2015 and continue data taking until 2018 for a total integrated luminosity expected to be in the 100 fb-1 range.

The LPNHE ATLAS group contributed significantly to the Higgs boson discovery mainly in the two photon channel with the Run 1 data and is currently active in the H→bb decay search analysis and in the detector and trigger improvements such as the Pixel vertex detector sensors and the new online tracking device FasTracK (FTK) as part of the Atlas trigger system. These activities will have a significantly impact on physics signatures involving b-jets in the final state like the H→bb decay channel.

The PhD project will initially focus on the search for Higgs boson produced in association with a W or Z vector boson with the Higgs decaying in b quark pairs initiating b-jets. The vector boson final state signature will include 0 and 1 lepton (electron or muon) and transverse missing energy. Another interesting signature to be eventually searched for is the fully hadronic Higgs production through Vector Boson Fusion (VBF) with H→bb where only jets are present in the final state. The new FTK track trigger performances on this mode will be evaluated in terms of the potential improvement of the signal acceptance.

This work will be developed within the ATLAS Higgs working group and in collaboration with a net of French labs, among which LPNHE, which was recently granted an ANR 2014 project Hbb+ttH@LHC for this research topic.

The PhD work will include several analysis ingredients from the compound objects reconstruction and trigger (b-jets, missing energy, …) to the Higgs coupling fit and combination with other channels. From the point of view of the detector performances and operations the focus will be put on b-jet trigger and in particular on the performances of the new FastTrack (FTK) system which will be commissioned and installed in early Run 2. This part of the work will be qualifying for ATLAS authorship in agreement with the collaboration management.

Finally, in the last part of the PhD, a contribution to the perspective studies for the Higgs physics and coupling determination with the high luminosity HL-LHC or with the future e+e- linear collider ILC, could be considered.

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Baryon acoustic oscillations in the Lyman-alpha forest with the quasar spectra of the SDSS-IV/eBOSS survey

Laboratory: Laboratory: LPNHE (Paris) /// Cosmology group
Contact person: Julien Guy /// guy[at]lpnhe.in2p3.fr
Website: lpnhe.in2p3.fr
Potential internship : Spring 2015 at the LBNL, Berkeley, USA

The measurement of the baryon acoustic peak position is a powerful probe of the expansion history of the universe. At a comoving scale of about 100 Mpc/h, it offers a unique standard ruler. The theoretical prediction for this distance scale is derived from physics of the early universe that has been extensively tested and validated with the cosmic microwave background data. From the experimental stand-point, its measurement requires to trace the matter density field in 3D (angles and redshifts) over an extremely large volume of the universe (typically 10 Gpc cube). It has been successfully measured in the correlation function of galaxy redshift catalogs up to a redshift z~0.6, and in the Lyman-alpha forest at z~2.4 with the SDSS3/BOSS survey. Whereas the former provides a complement to type Ia supernovae for the measurement of the puzzling acceleration of the expansion, the latter allows a unique measurement in a redshift window where the expansion is expected to be decelerated in the standard cosmological model (LCDM). Current measurements are in tension with the standard model, however new data and a careful study of both astrophysical and instrumental systematics are needed to confirm this result. The thesis subject consists in pursuing this analysis, complementing the data set with the SDSS-IV/eBOSS survey high-redshift quasars.

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Numerical Methods for the prediction of Gravitational Lensing Signal as a probe of the mass content of the Universe

Laboratory: Institut d'Astrophysique de Paris
Contact persons:
Raphael Gavazzi /// gavazzi[at]iap.fr
Christophe Pichon /// pichon[at]iap.fr
Website: www.iap.fr
Potential internship :

With the advent of high performance computing, it has now become possible to address the problem of the complex interplay of baryons and dark matter on all relevant scales. In the context of the future cosmological experiments, such as the Euclid mission, the gravitational lensing signal (either weak or strong) needs to be calibrated and characterized with state-of-the-art hydrodynamical cosmological simulations. Two key issues, in which gas physics may play a crucial role for cosmology, are the so-called missing satellite problem and halo profile problem on the one hand, and the intrinsic alignment of galaxies and the effect of galactic feedback on the estimation of the equation of state of the Dark Energy on the other hand.

The student will make use of existing large-scale hydrodynamical cosmological simulations and will undertake the production of high resolution zoomed simulations before carrying out the ray-tracing through well resolved structures (galaxies, clusters). The student will then build and validate non parametric estimates of the aforementioned lensing observables.

This work will take place at IAP under the supervision of Dr. Raphael Gavazzi and will be immersed into a close collaboration involving Dr. Yohan Dubois, Prof. Christophe Pichon, Dr. Sebastien Peirani and Dr. Karim Benabed.

Some Useful links and publications:
- projet-horizon
- spine-public.projet-horizon
- 2013ApJ...777...98S
- 2013MNRAS.433.3297D
- 2008MNRAS.390..945P
- 2010MNRAS.408.1969V

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