Project proposals 2016

Laboratoire de Physique Théorique et Hautes Energies (LPTHE)

Institut d'Astrophysique de Paris (IAP)

Laboratoire de Physique Nucléaire et Hautes Energies (LPNHE)

Fundamental aspects of supergravity and string theory and applications in High-energy physics

Laboratory: Laboratoire de Physique Théorique et Hautes Energies (LPTHE)
Contact person: Ignatios Antoniadis /// antoniad[at]lpthe.jussieu.fr /// T +33 1 44 27 78 84
Website: www.lpthe.jussieu.fr

High energy physics enters a new era with the Large Hadron Collider (LHC) at CERN searching for new particles and forces at energies ten times bigger than previously, creating conditions similar to those of the early Universe, just after the Big Bang. At the same time, several new observations arise from experiments in astrophysics and cosmology which complete our understanding of the same physics, while the direct and indirect search of dark matter continues. There is a real opportunity to extract the underlying microscopic theory, beyond the current standard models of particle physics and cosmology. The subject of this thesis is to explore several theoretical ideas and proposals, based on supersymmetry, string theory, extra dimensions and holography, in order to make progress in this direction. Indeed, supersymmetry is a new spacetime symmetry motivated by theoretical arguments and experimental indications. On the other hand, string theory renders compatible two fundamental theories: quantum mechanics and general relativity, by replacing the notion of point particles by extended objects. Two important consequences are the existence of extra dimensions and the possible braneworld description of our Universe.

Several research directions are possible. An important problem in string theory, necessary for making low-energy predictions, is the stabilisation of the so-called moduli scalar fields, whose vacuum expectation values determine the parameters of the compactification, such as the size and shape of the internal manifold. A simple and efficient method for this stabilisation, without breaking necessarily supersymmetry, is based on the presence of internal magnetic fields along the compactified directions. In this context, several open problems remain, such as the derivation and study of the effective low-energy field theory and the total or partial breaking of supersymmetry.

Another important question is how string theory is realised in Nature and what are possible predictions that may be tested either at LHC, or in observations of the sky. This realisation should be related to the mechanism of supersymmetry breaking and to the values of the fundamental parameters of the theory, such as the size of the string and of the extra dimensions. An extraordinary possibility is that some of these sizes are sufficiently large to be explored experimentally, for instance at LHC. Their discovery would drastically change our concept of physics at short distances and would have spectacular consequences, such the possibility of studying gravitational phenomena in particle accelerators.

Another possible direction is on the formalism of the theory, the compactification methods, mechanisms of supersymmetry breaking, the structure of the effective supergravity and its properties, the computation of non-perturbative effects, the propagation of strings in curved spacetime, and the study of gravitational phenomena in regions of strong curvature where the effective field theory approach breaks down.

The expected duration of the thesis is three years. During the first months, the candidate should rather study the literature in this highly competitive subject, in order to improve her/his level and to be able to choose the precise research direction depending on personal interests.

A Master internship is possible, prior to the start of the PhD thesis.

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Phenomenology of extensions of the Standard Model

Laboratory: Laboratoire de Physique Théorique et Hautes Energies (LPTHE)
Contact persons:
Karim Benakli /// kbenakli[at]lpthe.jussieu.fr /// T +33 1 44 27 41 05
Mark Goodsell /// goodsell[at]lpthe.jussieu.fr /// T +33 1 44 27 41 26
Pietro Slavich /// slavich[at]lpthe.jussieu.fr /// T +33 1 44 27 28 51
Website: www.lpthe.jussieu.fr

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.

Interested candidates should contact one of the members of the team.

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High-energy astroparticles and pulsars

Laboratory: Institut d'Astrophysique de Paris (IAP)
Contact person: Kumiko Kotera /// kotera[at]iap.fr /// T +33 1 44 32 81 95
Website: www.iap.fr

Cosmic rays are mostly charged nuclei that constantly bombard the Earth. Since their discovery more than a hundred years ago, they have provided a well of excitement and enigma for particle physicists and astrophysicists, with their colossal energies, and their mysterious extra-solar origins. A connection between cosmic rays and pulsars was suggested in the decades following the discovery of the first pulsar, but was never deeply investigated, in particular at the highest energies. The recent collection of multi-messenger data related to cosmic-rays and the boom in pulsar observations make it timely to dig into this matter now. During this Ph.D., the candidate will explore the links between astroparticles and pulsars at high and ultrahigh energies. Recent works have shown that the production of high and ultrahigh energy cosmic rays in these objects could give a picture that is surprisingly consistent with the latest observational data. The candidate will calculate the signatures associated to this original and promising source model in terms of other messengers (neutrinos and photons at different wavelengths), analytically and with a numerical propagation code. Some of these signatures reveal how astroparticles interact with the neutron star surrounding medium, and our novel approach will be to use them as tools to diagnose the nature of pulsars and their winds.

The neutrino signatures that will be calculated will be a good target to be probed by the Giant Radio Array for Neutrino Detection (GRAND) project, supported by ILP, and in which K. Kotera is heavily involved. GRAND aims at detecting high-energy neutrinos with a 100'000 km2 radio antenna array in the Tianshan mountains in China. The candidate will thus be able to take part in this experimental project by participating in the definition of the Science Case.

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The origin of the Hubble sequence

Laboratory: Institut d'Astrophysique de Paris (IAP)
Contact persons:
Yohan Dubois /// dubois[at]iap.fr /// T +33 1 44 32 81 34
Christophe Pichon /// pichon[at]iap.fr
Website: www.iap.fr

The morphological variety of galaxies observed in the local Universe illustrated by the presence of galaxies with disc, ellipsoidal or irregular shapes is also witnessed at higher redshift during the peak epoch of star formation activity as more and more data accumulates. Theoretical models suggest that the morphology of galaxies is acquired by a subtle combination of galaxy mergers and cosmic gas accretion. However, how the anisotropy of this infall impacts the galaxy properties have received little attention, even though galaxies are stemmed from the highly anisotropic filamentary cosmic web. This question is especially important for high-redshift galaxies since large-scale cosmic filaments manage to penetrate in a cold and collimated fashion down to the center of halos hosting galaxies. Therefore, it is crucial to study how these cold filamentary streams of gas connect to galaxies, how they exchange angular momentum with their environment and whether the galactic activity (supernovae explosions, active galactic nuclei from supermassive black holes) can regulate the arrival of gas into the galaxy, and, finally, what are the consequences on the galaxy morphologies (disc rebuilding, galaxy compaction, enhanced gas turbulence).

In order to tackle this problem, the PhD student will have to perform hydrodynamical simulations of the formation of galaxies in a complex cosmological environment, for which he will have to use some of the already existing numerical tools (RAMSES code, Teyssier, 2002). With this numerical code, the student will be able to scrutinise how cold filamentary gas connects to galaxies and transfer its angular momentum with it surroundings, as well as the role of galaxy mergers onto the cataclysmic change of galaxy properties. The PhD student will also have to design new physical models for the feedback from galaxies (such as feedback from supernovae with cosmic ray physics, feedback from supermassive black holes including radiation and cosmic-ray driven jets, dust formation and destruction in order to couple the infrared radiation to the gas, etc.) in order to understand how the galactic outflows modify the anisotropic infall of gas onto galaxies. We expect the student to develop, design, run and analyse the set of simulations required to fulfil the proposed project.

This thesis will be directed by Yohan Dubois and co-directed by Christophe Pichon both at IAP. This scientific topic is one of the key topic in IAP as well as numerical simulations. This topic is the core of the ANR grant 'cosmic origin of Hubble sequence' (PI C. Pichon), and the student will benefit from the strong expertise in IAP for developing physical models for state-of-the-art hydrodynamical cosmological simulations as well as characterising the large-scale structures of the cosmic web (the horizon simulation). The student will also have the opportunity to work with our close collaborator, Julien Devriendt, in Oxford.

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Numerical and theoretical studies of particle acceleration in relativistic astrophysical outflows

Laboratories: Institut d'Astrophysique de Paris (IAP) /// 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

The physics of particle acceleration in powerful relativistric astrophysical objects, such as gamma-ray bursts, blazars, pulsar winds etc. represents one of the cornerstones of modern astroparticle physics and high energy astrophysics. Acceleration mechanisms are indeed responsible for the production of high energy charged particles, which in turn produce high energy photons and/or neutrinos through their interactions with the surrounding fields. Similarly, the challenge of particle acceleration in astrophysical sources also lies at the root of the enigma of the origin of the highest energy cosmic rays.

According to the standard scenario, acceleration is achieved through the dissipation of the bulk energy of a relativistic flow into a gas of accelerated particles, thanks to the interaction between these particles and fast electromagnetic modes of the plasma. The non-linear and multi-scale interaction between these particles and the electromagnetic turbulence renders this problem rather complex. Substantial progress has been achieved in recent years, notably thanks to high performance computing using particle-in-cell (PIC) numerical simulations, which permit the ab initio simulation of the dynamics of a collisionless plasma (Fig. 1). Nevertheless, many questions remain open on the theoretical level. Furthermore, the introduction of PIC techniques in high energy astrophysics is quite recent, so that this field of research is expanding rapidly.

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 (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 (CALDER) 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.

FIG. 1 – Positron density profile in a relativistic unmagnetized shock, as simulated by the CALDER PIC code. The formation of filamentary structures and their subsequent destabilization from kink-like processes near the shock front are clearly seen.

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 of the shock front and as a function of the degree of magnetization of the ambient plasma. Simulations for mildly relativistic flows 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 magnetized turbulence, which gives rise to the acceleration process, can be excited efficiently in these shocks. This turbulence is usually produced through beam-plasma instabilities, whose nature and growth rate depend sensitively on the shock parameters. For particular configurations in which the growth of such instabilities is not expected, other mechanisms will be considered and analyzed, such as magnetic reconnection. 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.

This competitive field of research involves high performance computing, relativistic hydrodynamics, plasma physics and radiative interaction physics. The PhD candidate should possess some notions on at least some of these topics.

PhD candidates may also apply for an internship on this topic during the Spring of 2016.

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Secular evolution of galaxies induced by cosmic environment

Laboratory: Institut d'Astrophysique de Paris (IAP)
Contact person: Christophe Pichon /// pichon[at]iap.fr /// T +33 1 44 32 81 35
Website: cosmicorigin.org

The most serious problems encountered by the hiearchical models of structures formation, all arise on galactic scales. To deal with these difficulties, one has to study the effects of this paradigm on galactic evolution. On galactic scales, interactions with the circum-galactic environment may either be constructive (adiabatic gas accretion) or destructive (satellite infall, warp, bars). This depends on the detailed nature of the accretion processes and on the gravitational susceptiblity of the central galaxy. From a dynamical point of view, the cosmic environment of a given galaxy acts as a "boundary condition", whose statistical impact remains to be characterised. Only a detailed analysis of the associated processes (impact parameter, nature of the accreted objects, accreation orientation, etc.) may allow us to determine the dominant mechanism.

The main objective of this Ph.D. is to address the pressing question of "nature vs. nurture" in establishing galaxies' morphological properties. By studying the statistical propagation of the properties of the haloes' cosmic environments to their internal regions, one may quantify the subsequent mean secular diffusion induced on the central galaxy.

This research work will rely on the conjugation of analytical methods (calculation of response operators, writing and implementation of diffusion equations such as Fokker-Planck or Balescu-Lenard (Weinberg 2001, Pichon & Aubert 2006, Heyvaerts 2010, Fouvry 2015 a-e)) and of numerical analysis of simulations (to validate analytical methods and quantify the cosmic environment; cf horizon-simulation.org, in collaboration with Y. Dubois, IAP). The perturbations' properties will be statistically constrained in the circum-galactic medium thanks to the analysis of such simulations.

The Ph.D. student will identify the respective roles of the flow of virialised sub-structures and of diffuse accretion, so as to analyse the "mean" dynamical consequences induced by the environment (warps, spirals, thick discs). As the secular diffusion coefficients involve the auto-correlation of the potential perturbations, this formalism will allow for the calculation of "environmental" relaxation times, and for the prediction of the functional form of asymptotic solutions (and therefore "universal" density profiles induced by this relaxation process). These investigation will provide an appropriate description of the phase of secular relaxation, during which the dark matter halo and the stellar disc restructure themselves on cosmic time through dressed potential fluctuations. This Ph.D. will present new clues for the relative importance of constructive and destructive processes on the the secular evolution of galaxies.

This work could be in part in the continuation of J.B. Fouvry's PhD. His web page offers more details on a possible context for this Ph.D. It is also the topic of an upcoming workshop organized at the IAP.

The formation and evolution of galaxies in the cosmological context, as well as numarical simulations, are both research priorities of the ILP/IAP. This topic also lies within the themes of the ANR cosmicorigin.org. Galactic modelling is a national priority during the production phase of the GAIA spacecraft, the MUSE instrument, and ALMA Observatory. This internship/Ph.D. may be suitable for a co-supervision with Pr. J. Magorrian (Oxford), and Dr. S. Prunet (Hawaii).

1. Aubert, et al. 2004, MNRAS, 352, 376
2. Aubert, D., Pichon, C 2007, MNRAS, 374, 877
3. Chavanis, P.-H. (2012). Physica A: , 391(14), 3680–3701.
4. Codis, S., et al., 2012, MNRAS, 427, 3320
5. Codis, S; Pichon, C; Pogosyan, D in press. MNRAS
6. Fouvry, J.-B., Pichon, C., Magorrian, J., Chavanis, P.-H, 2015, A&A
7. Fouvry, J.B.; Pichon, C. Chavanis, P. H. in press. A&A
8. Fouvry, J-B; Binney, J; Pichon, C, 2015, Astrophysical Journal, 806 117F
9. Fouvry, J-B; Pichon, C, , MNRAS, 2015, 449, 1982F
10. Fouvry, J-B; Pichon, C, Prunet, S., MNRAS, 2015, 449, 1967F
11. Kimm, T; et al 2011 arXiv1106.0538K
12. Heyvaerts J. (2011) , MNRAS, 1–36.
13. Ocvirk, P. et al. 2008, MNRAS, 390, 1326
14. Pichon, C., Aubert, D., , 2006, MNRAS, 368, 1657
15. Tillson et al., 2012, ArXiv e-prints, arXiv:1211.3124
16. Weinberg M. D., 2001a, MNRAS, 328, 321

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Fundamental cosmology from ultra deep sky surveys

Laboratory: Institut d'Astrophysique de Paris (IAP)
Contact persons:
Guilhem Lavaux /// lavaux[at]iap.fr /// T +33 1 44 32 55 13
Benjamin Wandelt /// wandelt[at]iap.fr /// T +33 1 44 32 81 43
Website: www.iap.fr

Recent years have seen the dawn of many deep observations of the sky. Among these, the Planck mission has given a large wealth of information on the content of the Universe like the amount of Dark Matter, the amount of Baryonic Matter and the global curvature of the Universe, using the Cosmic Microwave Background. More information on cosmology is available from other probes, notably the distribution of galaxies and quasars on large scales. Among the data-sets available in other wavelengths, 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. Such large scales hold promising information on gravitational waves, inflation and the assembly history of the most massive objects in the Universe. Combining all these pieces of information together will give us unique insight on the physics of the early and late Universe. Notably, the largest scales will give us access to the physics of inflation, while the intermediate scales will constrain the equation of state of Dark Energy.

Unfortunately, such deep surveys are impeded by many foreground and boundary effects, which limit 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 by analyzing data using an appropriate probabilistic model. Such models can incorporate both fundamental physical problems (small scale dynamics, Universe expansion) and observational issues (detectability, noise, foregrounds) in a single package. However such an approach requires additional data to learn from the surveys how to disentangle all the effects that give rise to the observed distributions of galaxies and quasars.

Desired skills include background in physics (formal education in astronomy/astrophysics is welcomed but not necessary) and experience in analytical and numerical calculations and software development. The PhD student will be hosted at the IAP and will work under the direct supervision of Guilhem Lavaux. The successful applicant is expected to do theoretical modeling, software development, real data handling and cosmological measurements.

Travel funds are allocated to organize visits at the Excellence Cluster (TUM), an official partner of the Institut Lagrange de Paris, and the nearby Max Planck Institute for Astrophysics to visit members of the collaboration. The student will have ample opportunity to interact with the many members of these world-class institutes.

Some useful links and papers:
• Home page of Guilhem Lavaux: www.iap.fr/users/lavaux
• The SDSS3 project: www.sdss3.org
• ARES algorithm: 2010MNRAS.406...60J, 2013ApJ...779...15J
• Fast ARES: 2014arXiv1402.1763J

Potential internship: Spring 2016 at IAP

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High order correlation functions in the large-scale structure of the universe - exploitation of the Euclid mission

Laboratory: Institut d'Astrophysique de Paris (IAP)
Contact persons:
Francis Bernardeau /// francis.bernardeau[at]iap.fr /// T +33 1 44 32 81 60
Karim Benabed /// benabed[at]iap.fr /// T +33 1 44 32 80 45
Website: www.iap.fr

The european space mission Euclid, to be launched in 2020, will provide us with observations of the large-scale structure of the Universe with unprecedented accuracy. The main scientific objective of this projet is to get insights into the properties of the dark energy which accounts for the largest fraction of energy density of the universe but whose nature is totally unknown! This component, which is responsible of the late time acceleration of the expansion of the universe, diminishes the efficiency of the gravitational collapse driving the formation of the large-scale structure. The measurement of the statistical properties of the latter can then give constraints on dark energy models. The probes that are usually exploited are the second order structure functions — such as spectra of the density field —but it is now well known that, with the non-linear growth of the structure, it will be necessary to go beyond and use more elaborate observables. Natural candidates are then third order structure functions. However, whereas the performances of the second order structure functions are well known, this is not at all the case for higher order structure functions. Trying to asses the efficiency of such observable is the aim of this thesis.

The proposed thesis work can cover different aspects of this problem: how to build optimal estimators in order to extract all cosmological information from data sets? What is the covariance of these estimators? What are the specific parameters that can be improved from such measurements? Large surveys such as Euclid will provide us with galaxy catalogues as well as cosmic shear measurements, each with specific observational biases. How then to best combine these probes?

This thesis will make use mainly of theoretical and semi-analytical tools. It could however require the exploitation of numerical simulations of large-scale structure of the universe. Besides, it might be necessary to run calculations with the help of dedicated Monte-Carlo techniques. The supervision of the thesis will be done by Francis Bernardeau, specialist in theoretical studies of the non-linear evolution of the large-scale structure, and Karim Benabed, expert in weak lensing measurements in particular in the context of cosmic-microwave background observations. Both are members of the Euclid mission.

Note that the Institut d’Astrophysique de Paris where this thesis will take place hosts a world's leading group in cosmological numerical simulations. IAP is also strongly involved in the Euclid mission. It hosts Yannick Mellier, lead of the mission. The laboratory is also in charge of the data analysis of one of the key instrument, the VIS camera, as well as the organisation of the End-to-End simulations.

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Constraining the nature of dark matter by probing galaxies all the way down to dwarf spheroidals

Laboratory: Institut d'Astrophysique de Paris (IAP)
Contact persons:
Gary Mamon /// gam[at]iap.fr /// T +33 1 44 32 81 15
Joe Silk /// silk[at]iap.fr /// T +33 1 44 32 81 54
Website: www.iap.fr/users/gam/research.html

Dark matter (DM) is understood to contribute 85% of the mass density of the Universe, but its nature is unknown. The distribution of DM, still poorly known, is an important reference for modeling, and serves as a test of several ideas. DM-only cosmological simulations of the growth of structure in the Universe indicate that DM structures (halos) should have steep inner density profiles (cusps), unless the DM particle is self-interacting. However, when baryons (gas and stars) are included in the simulations, the dissipative nature of gas leads to energy and angular momentum loss leading to even steeper inner density profiles, while the effects of intermittent energetic outflows from exploding stars (supernovae) and from jets originating from supermassive central black holes (SMBHs), as well as major galaxy mergers, can lead to more homogeneous (cored) inner density profiles. Finally, if DM is in the form of Weakly Interacting Massive Particles (WIMPs), and if its annihilation cross-section is roughly 100 times greater than its thermal freeze out value, it should be detectable from gamma rays indirectly produced by these annihilations, but the precise radial distribution of DM, reduced to the line-of-sight integral of the square density (J-factor), is required to calibrate the cross-section.

In additional to gravitational lensing and X-ray observations, one can probe the distribution of DM in galaxies from the motions of their stars (internal kinematics or IK). This old field has progressed rapidly in recent years and 1) is expected to reach more accurate DM density profiles than the other two methods, and 2) is the only method available for very low mass satellites of our Milky Way galaxy, known as dwarf spheroidals (dSphs), which are gas-poor and DM-dominated, hence the cleanest probes of DM, and some of which have very detailed kinematic observations.

This doctoral thesis can follow two paths (or some combination of both), according to the interests of the candidate.
1) The generalization of the Bayesian MAMPOSSt IK method to non-spherical geometries, more realistic 3D velocity distributions, possible proper motion data, extensions to modified gravity theories, and combination with lensing and/or X-ray modeling. The aim is to a) determine the normalization, concentration, inner and outer slopes of DM, as well as the shapes of the orbits in the inner and outer regions of galaxies, all as a function of galaxy mass from dSphs to clusters of galaxies (using a host of observations, including Integral Field Spectroscopy, i.e from WHT/SAURON, VLT/MUSE and SDSS/MANGA); b) predict the J-factors for the gamma-ray detections or upper limits, in particular in the context of the forthcoming Cerenkov Telescope Array (CTA). This analysis will involve thorough testing on mock data sets.
2) Hydrodynamical (HD) simulations of the evolution of dSphs, using an Adaptive Mesh Refinement Technique, where contrary to standard practice, the refinement will not be just on density, but on the combination of density and small mass structures. This will allow a much better understanding of the history of star formation and mechanisms of gas ejection in the progenitors of dSphs. These simulations will also lead to the 1st predictions of the present-day SMBH masses of dSph galaxies on one hand, and of the DM density profiles of dSphs (hence their J-factors), which depend on the history of mergers and of gas ejection from SMBHs and supernovae on the other. We will thus be able to predict realistic J factors, especially after including a toy model for the accumulation of DM very close to the SMBH.

Collaborators:
Part 1: A. Biviano (Trieste), M. Limousin (Marseille), E. Moulin (Saclay), A. Romanowsky (Santa Cruz), R. Wojtak (Stanford);
Part 2: Y. Dubois (IAP), M. Volonteri (IAP)

Some useful links and publications:
MAMPOSSt algorithm: 2013MNRAS.429.3079M
Review on kinematic modeling methods: 2014RvMP...86...47C (chapter 5)
CTA telescope: portal.cta-observatory

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Jet calibration, cross section measurements and New Physics searches with the ATLAS experiment

Laboratory: Laboratoire de Physique Nucléaire et Hautes Energies (LPNHE) - Thematic team: 'Masses et Interactions Fondamentales' - Experiment: ATLAS
Contact persons:
Bogdan Malaescu /// bogdan.malaescu[at]lpnhe.in2p3.fr /// T +33 1 44 27 91 30
Mélissa Ridel /// melissa.ridel[at]lpnhe.in2p3.fr /// +33 1 44 27 37 68
Website: lpnhe.in2p3.fr

The Large Hadron Collider (LHC) at CERN (Geneva) is currently taking data at a center of mass energy of 13 TeV, the largest energy ever achieved in laboratory up to now. This data-taking period will last until 2018, covering the first two years of this PhD proposal.

The ATLAS experiment, installed at the LHC, has been designed to explore, due to the very large amount of data provided by the LHC, phenomena taking place at these very high energies. In particular, ATLAS, together with the other mainstream experiment at the LHC, CMS, were able to discover in 2012 the Higgs boson, responsible for dynamically generating a mass for elementary particles in Standard Model.

The ATLAS LPNHE team has expertise on the jet reconstruction and calibration, as well as on electrons and photons. In particular the group has been involved in the Run I data analyses, like the discovery of the Higgs boson, precision measurements with jets in the final state, or precision measurements of Standard Model parameters. The group is also involved in the first Run II analyses, exploiting the luminosity that became available up to now.

Due to their similar experimental signatures, New Physics searches require an excellent understanding of Standard Model processes. Being able to measure as precisely as possible the energy of the objects reconstructed in the detector is of outmost importance for making precision measurements, as well as for enhancing the sensitivity of physics analyses to signals beyond the Standard Model.

The proposed PhD subject deals with these points by studying events with one or several jets in the final state, recorded using the ATLAS detector. On one side these events are used to improve the jet energy calibration, extremely important for analyses using jet counting or veto. Indeed, the new data-taking conditions at Run II require redefining the jet calibration strategy. Several improvements are possible, with a direct impact on numerous ATLAS physics results. On the other side, the differential cross section of multijet events is measured as a function of numerous observables and compared with various theoretical predictions. These measurements will be used to test QCD predictions, while also being sensitive to signals beyond the Standard Model like SUSY particles potentially explaining dark matter, or yet to new types of interactions.

Discussions with experts on the phenomenological aspects of this project, like Matteo Cacciari of LPTHE, will be organized.

An internship with the supervising team will allow the student to get familiarized to the topics covered by the project, as well as to the tools necessary for its implementation.

Regular trips to CERN (Geneva) are foreseen.

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Measurement of the top quark mass in dilepton channels with 13 TeV data with the ATLAS experiment at the LHC

Laboratory: Laboratoire de Physique Nucléaire et Hautes Energies (LPNHE)
Contact persons:
Frédéric Derue /// derue[at]lpnhe.in2p3.fr /// T +33 1 44 27 47 03
Tristan Beau /// beau[at]in2p3.fr /// T +33 1 44 27 41 94
Website: lpnhe.in2p3.fr/atlas

The PhD subject is about the measurement of the top quark mass in in dilepton channels with 13 TeV data with the ATLAS experiment at the LHC. The student will work on the all data taken during the Run2 with an energy in the centre of mass of 13 TeV. He/She will work in a dedicated working group of the collaboration. Stress will be put on the decrease of the systematic uncertainties by using different measurement methods and by specific studies on the reconstruction of jets or hadronization of b-quarks.

The ATLAS experiment is one of the major experiments at the CERN LHC proton-proton collider. A first data taking period (Run 1) occured 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. After a shutdown in 2013-14, needed to upgrade the accelerator, LHC restarted collisions with an energy of 13 TeV (Run 2) in march 2015. In 2015, already ~3-4 fb-1 of integrated luminosity were taken.
It will continue data taking until 2018 for a total integrated luminosity expected to be in the 100 fb-1 range.

The group of LPNHE is made of 28 members among which 18 are seniors. it has a strong expertise in the domain of electromagnetic calorimetry and tracking. It participated to the development and the analysis of data taken during test beams, commissioning and colliding periods. The group is now involved in the upgrade of the pixel detector to be put in place during the very high luminosity period, and to the development of computing and storage facility for the collaboration. The group is participating to the study of the electroweak symmetry breaking through studies on the Higgs boson, precision measurements on the Standard Model and search for New Physics. In particular, a group of five senior physicists and a post-doc are working on the top quark and jet physics, such as the measurement of production cross section of top-antitop pairs and the top quark mass.

The quark top is the only elementary fermion with a mass of the order of the electroweak scale. It is thus an important sector to study the electroweak symmetry breaking. The mass of the top quark is a fundamental parameter of the Standard Model. The radiative corrections to the top quark mass are sensitive to the mass of the W boson, the Higgs boson but also to the predictions of the stability of the Universe. It is thus important to be able to compare with a high precision the different mass measurements to search for evidence of possible effects not predicted by the Standard Model.

The PhD project will initially focus on the measurement of the top quark mass with the 2015 data. With an increased cross section production at 13 TeV, it is already half of the statistics of top-antitop pairs available during the Run 1. The measurement of the top quark mass being limited in precision by systemtic uncertainties an early measurement will be done using the template method, based on the comparison between data and simulation of observables sensitive to the top quark mass. During this first year, the student will work also on the jet calibration or hadronization phenomena of b-quarks, both being among the largest systematic uncertainties on the mass measurement. This part of the work will be qualifying for ATLAS authorship in agreement with the collaboration management.

The PhD work will then include the analysis of the all data accumulated during the Run 2. The sample of top-antitop events will be about twenty times larger than the one accumulated during the Run 2. This will allow to do stricter selection cuts to minimize the systematic uncertainties. Another method to measure the mass measurement will be also explored, based on a method giving an event-by-event probability based on the calculation of the associated matrix element and cross-section. This method is potentially more precise but is very computing demanding. The development of the method on parallel computing ressources such as a supercomputer UV2000 of the University UPMC and/or GPU/XeonPhi processors will be explored.

The PhD project will include all necessary parts for a good understanding of the mass measurement:
• Good understandind of the detector and combined reconstruction of physical objects (leptons, jets, b-jets);
• Selection (and optimization) of events based on a priori knowledge based on simulation of the expected signal and background events;
• Measurement of the top quark mass with different methods;
• Quantification of the systematic uncertainties, in particular the jet calibration, hadronization of b-jets, initial and final state radiation etc...

Possible internship in spring 2015 at CERN and LPNHE Paris.

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Search for Dark Matter with the DarkSide Liquid Argon detector at LNGS

Laboratory: Laboratoire de Physique Nucléaire et Hautes Energies (LPNHE)
Contact person: Claudio Giganti /// claudio.giganti[at]lpnhe.in2p3.fr /// T +33 1 44 27 39 05
Website: lpnhe.in2p3.fr

The ~20% of the Universe is composed by yet undiscovered matter, the so-called “Dark Matter”. Its existence is inferred by cosmological data but no direct evidences for it exist yet. Among the possible Dark Matter candidates, one of the most appealing and actively searched nowadays are the WIMP (Weakly Interacting Massive Particles).

One of the most promising experiments searching for WIMPs is the DarkSide experiment, a double phase Liquid Argon Time Projection Chamber (LArTPC) installed at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy.
Liquid Argon is an excellent candidate for WIMPs searches because of its capability of distinguishing electron-recoils (ER) produced by β or γ from nuclear recoils (NR) produced by WIMPs (or by neutrons). This is possible in LAr thanks to the different shape of the scintillation signals produced by ER and NR.

Currently DarkSide-50 (DS-50), a 50 kg LArTPC is taking data at LNGS using depleted Liquid Argon as a target and it will continue the data taking for the next 3 years. Depleted Argon is extracted from underground sources and is naturally depleted in 39Ar, a β emitter that contribute to the largest electron-recoils backgrounds in detectors based on atmospheric LAr that are searching for WIMPs.

In its first two physics runs DS-50 has demonstrated a discrimination power between ER and NR larger than 1.5x107 and has measured a 39Ar depletion factor of 1400.

Thanks to the excellent performances already demonstrated, the collaboration is working towards the conception and the construction of the next generation experiment, a 20-ton LArTPC (DS-20K).
DS-20K is expected to be background free for an exposure that would allow to put the world best limits in the search of high masses WIMPs.

The LPNHE DarkSide group is deeply involved in both, the analyses of DS-50 and the conceptual design of DS-20K and has a leading role in the simulation of the experiments.
French groups participating in DarkSide are also proposing to build and operate a small TPC prototype to be exposed at the IPNO Licorne neutron beam to study the liquid argon scintillation and ionization response, investigating the possibility of reconstructing the direction of the incoming particle from the amount of observed ionization and scintillation.

The PhD will work on the following areas:
• Operations of DS-50 detector, analysis of the data and the development of the Monte Carlo simulation of the experiment;
• Optimization of the design of the TPC prototype to be exposed at the IPNO neutron beam and analysis of the data;
• Simulations, R&D and construction of DS-20K.

An internship before the starting of the PhD is planned (March-July 2016) at LPNHE.

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

Laboratory: Laboratoire de Physique Nucléaire et Hautes Energies (LPNHE) - Cosmology Group
Contact persons:
Julien Guy /// guy[at]lpnhe.in2p3.fr
Christophe Balland /// balland[at]lpnhe.in2p3.fr
Website: lpnhe.in2p3.fr

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 (several 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~2.3 with the SDSS3/BOSS survey, with a precision of 2%. 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). The thesis subject consists in pursuing this analysis with the SDSS-IV/eBOSS survey high-redshift quasars, where an improvement of 40% is expected. The subject will consists in complementing the now standard Lyman-alpha forest analysis with data in the wavelength region between the Lyman-beta and Lyman-lines in the quasar frame, allowing improved statistics on the BAO at higher redshift.

Potential internship: Spring 2016 at LPNHE, UPMC, Paris

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Search for New Physics with Muons: precision experiments for physics beyond the Standard Model

Laboratory: Laboratoire de Physique Nucléaire et Hautes Energies (LPNHE)
Contact persons:
Frédéric Kapusta /// frederic.kapusta[at]lpnhe.in2p3.fr /// T +33 1 44 27 63 15
Wilfrid da Silva /// wilfrid.dasilva[at]lpnhe.in2p3.fr /// T +33 1 44 27 41 29
Website: lpnhe.in2p3.fr

The search for New Physics goes through the detection of rare events such as the conversion of a muon to an electron in an aluminum stopping target or a new precise measurement of fundamental quantities such as the anomal magnetic moment of the muon.
These two approaches are currently the goal of two experiments in Japan at JPARC respectively : COMET (E21) and g-2/EDM (E34).

This thesis requires a good phenomenological knowledge and the mastering of informatics tools to contribute to the simulation and the tracking, both in COMET and g-2/EDM.
Specific topics to be studied within COMET are the optimization of the muon stopping target and its possible extension to an active one, together with the study of physics processes to be tested with the Phase-I data.

Documentation: comet.kek.jp /// g-2.kek.jp
Trips to Imperial College London, JPARC, KEK are foreseen.

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Search for light dark matter with CCDs

Laboratory: Laboratoire de Physique Nucléaire et Hautes Energies (LPNHE)
Contact persons:
Antoine Letessier-Selvon /// Antoine.Letessier-Selvon[at]in2p3.fr /// T +33 1 44 27 73 31
Mariangela Settimo /// mariangela.settimo[at]lnphe.in2p3.fr /// T +33 1 44 27 82 36
Website: lpnhe.in2p3.fr

The DAMIC (DArk Matter In CCDs) experiment uses fully depleted, high resistivity CCDs to search for dark matter particles. With an energy threshold ∼50 eVee, and excellent energy and spatial resolutions, the DAMIC CCDs are well-suited to identify and suppress radioactive backgrounds, having an optimal sensitivity to WIMPs with masses <6 GeV. Early results motivated the construction of a 100 g detector, DAMIC100, currently being installed at SNOLAB.

The candidate PhD will participate to the collection and analyses of the DAMIC 100 data. The prospects for physics results after one year of data taking is to be able to directly test the CDMS-Si signal. Further studies developments to construct and operate an 1Kg detector will also be conducted.

Trips to Fermilab, University of Chicago (USA) and SNOLAB (Canada) are foreseen.

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