WorldCat Identities

Universitat de Barcelona Departament de Física Quàntica i Astrofísica

Overview
Works: 24 works in 41 publications in 2 languages and 41 library holdings
Publication Timeline
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Most widely held works by Universitat de Barcelona
Study of the extreme gamma-ray emission from Supernova Remnants and the Crab Pulsar by Daniel Galindo Fernández( Book )

2 editions published in 2018 in English and held by 2 WorldCat member libraries worldwide

In our Galaxy, supernova remnants and pulsars are the two most numerous populations of non-thermal objects. The goal of this thesis is to study the extreme gamma-ray emission from these two astrophysical objects with Fermi -LAT and MAGIC. In particular, supernova remnants Cassiopeia A and SNR G24.7+0.6 and the Crab pulsar. Cassiopeia A, one of the historical supernova remnants and the prime candidate of its class to be a PeVatron accelerator, has been discarded as so since we provided the first measurement of a turn-off in the gamma-ray spectrum at 3 TeV, implying the emission observed is produced by the decay of neutral pions, produced in proton- proton interactions of a parent population of accelerated protons with an energy cut-off at about 10TeV. Such a maximum energy of accelerated cosmic rays in Cassiopeia A falls short to explain the high energy end ( PeV) of the Galactic cosmic ray spectrum. Considering that Cassiopeia A was the main PeVatron candidate, the results obtained in this work challenge the existence of supernova remnants as galactic Pevatrons and therefore the popular conviction that supernova remnants are the main source of Galactic cosmic ray up to the knee. In the case of SNR G24.7+0.6, the remnant is evolving in a dense medium and might be interacting with the CO-rich surrounding. The observations performed allowed us to detect for the first time the counterpart of the radio emission, MAGIC J1835–069, from 60MeV up to 5TeV. This very high energy emission results from proton-proton interactions between the runaway protons from the supernova remnant and a nearby molecular cloud. These observations of the field of view of SNR G24.7+0.6, also resulted in the detection of another new source, MAGIC J1837–073, that is likely to be associated with a stellar cluster as suggested by its localization in a region rich in molecular content and crowded of sources. The total energy obtained in accelerated protons can be explained assuming a quasi-continuous injection of cosmic rays during the cluster lifetime. The second part of this thesis is focused on the study and understanding of the Crab pulsar, the young and most energetic pulsar in our galaxy. Observations carried out with MAGIC resulted in the first ever detection of very energetic pulsed emission from a pulsar, reaching up to about 1.5 TeV. Moreover, the light curve of the Crab above 400 GeV shows two peaks synchronized with those measured at lower energies. Such extremely energetic pulsed emission has to be produced by electrons with very high Lorentz factor scattering low energy photons in the vicinity of the light cylinder, either inside or outside of it. Currently, none of the postulated models is yet capable of reproducing at the same time the light curve and the spectral shape for both peaks above 400 GeV
Wave propagation in metamaterials mimicking spacetime geometry: black holes and cosmic strings by Isabel Fernández Núñez( Book )

2 editions published in 2018 in English and held by 2 WorldCat member libraries worldwide

In physics, it is common to find different phenomena being described by similar equations. A good analogy can make us look at a problem from a different point of view. In that way, ideas may be transferred from one field of science to another, allowing to model new phenomena after previous, well-studied ones. In the case of the field of analogue gravity, systems that mimic certain aspects of the physics of curved spacetimes are studied. In this thesis, we are interested in the analogy between geometry and media. It has been known for several decades that light propagation in a gravitational field is formally equivalent to that in a bianisotropic medium. On the one hand, ray paths are bent due to spacetime curvature. On the other hand, spatial variations of the permittivity and permeability of a material can make light follow curved trajectories. These two phenomena can be related mathematically in the context of transformation optics, which provides the tools to determine the medium parameters necessary to mimic a certain coordinate transformation. Materials with these specific properties are not naturally occurring, therefore, the emergence of metamaterial science at the beginning of the century was needed to realize them. Metamaterials are artificial composite materials with sub-wavelength constitutive elements that exhibit exotic properties. They have been one of the hot topics of the past years given the variety of opportunities they offer: negative refraction, superlenses, indefinite dispersion, invisibility, among many others. In this thesis we study the analogues of two static spacetimes from the point of view of transformation optics: one with spherical symmetry and one with conical geometry. Both cases are inspired by solutions to Einstein's equations: the Schwarzschild black hole and the cosmic string, respectively. For each case, we derive the permittivity and permeability of the analogous material using Plebanski's formulation of the electromagnetic constitutive equations. We solve numerically the wave equation in the metamaterial and compare the results with analytical theories. We find that the spherically symmetric spacetime can be mimicked by either an anisotropic or isotropic medium due to its rotational symmetries. This is achieved by performing a coordinate transformation of the general metric to a conformally flat form. We obtain the medium parameters for both cases and apply the results to the case of the Schwarzschild black hole. We simulate the propagation of a Gaussian beam in the two materials and compare the numerical results with the null-geodesics in the Schwarzschild spacetime, finding a good agreement. The cosmic string is an example of a topological defect with conical geometry. A conical space can be interpreted as flat space with a wedge removed. We make use of this transformation to study the wave equation in the cosmic string background. We apply asymptotic diffraction theories to obtain analytical models that describe wave propagation of electromagnetic or gravitational waves (in a certain gauge). We find that our expressions reproduce accurately the results of the numerical simulations in the analogous metamaterial. Moreover, with our models, we can understand the observed diffraction pattern as the interference of four characteristic waves. With this interpretation we can introduce the Fresnel observation zones, which are related to the diffraction maxima. They help localize the regions – in either space or frequency – where the wave effects are more significant. In fact, in the diffraction by a non-compact object such as the cosmic string, we find that the contribution to the field of wave effects such as interference or diffraction can be of the same order as the geometrical optics terms. Furthermore, the conical topology also appears in condensed matter systems as disclinations or wedge dislocations, therefore we expect our results to be applicable in those systems as well
Study of b-hadron decays into two hadrons and a photon at LHCb and first observation of b-baryon radiative decays by Vicente José Rives Molina( Book )

2 editions published in 2017 in English and held by 2 WorldCat member libraries worldwide

"This thesis is divided in different sections. After a short introduction about the thesis contents, the theory chapter presents the Standard Model of Particle Physics (SM), with an emphasis on those properties of flavour physics, such as oscillations. In this chapter a brief comment on radiative decays, the main topic through the thesis, is given, to present the specific theoretical description of these processes. The European Centre of Nuclear Research (CERN) is introduced in the second chapter, where the nature of such organization and the different experiments it holds are explained. Special emphasis is given for the LHCb experiment, which is the one that collected the data used in the analysis. The third chapter describes the monitorization and re-calibration of the SPD sub- detector, which is the one in charge of the discrimination between photons and other electromagnetic particles. During the Run 1 (2011-2012), this sub-detector suffered from ageing due to the presence of radiation inside the LHCb cavern. This ageing led to a drop in the SPD cell efficiencies, which could imply a loss in the data quality. The SPD is key in the context of radiative decays due to the presence of photons in the radiative final states and due to the fact that the main sources of background for radiative decays are related to the mis-identification of other particles as photons. With the work presented in this thesis, the performance of the SPD sub-detector reached the same status than it did for the beginning of Run 1, which corresponds to an almost flat cell efficiency around 95%. This recovery in efficiencies was performed by applying a correction factor calculated through the study of cosmic ray data and collision data from 2015. The fourth chapter describes a software tool developed within the radiative decays context. This tool is aimed for a better description of the photon/neutral pion separation variable, which is built making use of the electromagnetic clusters that these particles leave in the LHCb electromagnetic calorimeter (ECAL). This tool builds a variable that separates between the two particles but discrepancies between the data and the simulation distributions are observed. Because of this, two additional tools were developed to improve the agreement between them. These tools trust that the separation variable can be binned in terms of the particle transverse momentum and pseudorapidity. The first of the tools, "efficiency table tool", assigns the efficiency for a certain 2-dimensional bin as a weight to the simulation sample, so the distribution is corrected and matches the one for data. The "resampling tool", however, creates a new variable from calibration histograms built from background- subtracted distributions, trusting all radiative decays to have a similar behaviour in the 2-dimensional bins. The two tools can also be applied to neutral pions selection. The main chapter of the thesis is related to the measurement of the ratio of branching fractions for Bs decaying into phi (which decays into two kaons) and a photon and Lambda_b, decay into a Lambda bayron (which decays into a kaon and a proton) and a photon with respect to the best controlled radiative decay, Bd decaying into a K* (which decays into a kaon and a pion) and a photon, as well as the measurement of the direct CP asymmetry for the Lambda_b and Bd decays. This analysis also implies the first observation of a b-baryon radiative decay and constitutes the best measurement of the observables presented above. The analysis is performed over the whole Run 1 LHCb dataset (3 inverse fb). Special care is applied to the Lambda_b decay since it had never been done before; taking into account the different resonances that may intervene (that cannot be distinguished since they overlap and interfere). A 2- dimensional binning (on the resonance mass and on the proton angle) is defined for the study of the efficiencies, as recommended by the theory approaches to this decay and trusting (and confirmed afterwards) the different resonance decays to give a similar efficiency for the offline selection for a given bin. The thesis concludes with an overview of the whole work presented, given the calculated results for the physical observables and comparing them to the previously measured ones in the LHCb"--TDX
Exploring the Universe with Quasar Absorption Spectra : correlations among tracers of the mass density field and the impact of ionizing background intensity fluctuations by Satya Gontcho A Gontcho( )

2 editions published in 2017 in Spanish and English and held by 2 WorldCat member libraries worldwide

Chemical and dynamical analysis of Open Clusters in the context of the Milky Way disc by Laia Casamiquela Floriach( Book )

2 editions published in 2018 in English and held by 2 WorldCat member libraries worldwide

Multipartite entanglement and quantum algorithms by Daniel Alsina Leal( Book )

2 editions published between 2017 and 2018 in Spanish and English and held by 2 WorldCat member libraries worldwide

"Quantum information science has grown from being a very small subfield in the 70s until being one of the most dynamic fields in physics, both in fundamentals and applications. In the theoretical section, perhaps the feature that has attracted most interest is the notion of entanglement, the ghostly relation between particles that dazzled Einstein and has provided fabulous challenges to build a coherent interpretation of quantum mechanics. While not completely solved, we have today learned enough to feel less uneasy with this fundamental problem, and the focus has shifted towards its potential powerful applications. Entanglement is now being studied from different perspectives as a resource for performing information processing tasks. With bipartite entanglement being largely understood nowadays, many questions remain unanswered in the multipartite case. The first part of this thesis deals with multipartite entanglement in different contexts. In the first chapters it is studied within the whole corresponding Hilbert space, and we investigate several entanglement measures searching for states that maximize them, including violations of Bell inequalities. Later, focus is shifted towards hamiltonians that have entangled ground states, and we investigate entanglement as a way to establish a distance between theories and we study frustration and methods to efficiently solve hamiltonians that exhibit it. In the practical section, the most promised upcoming technological advance is the advent of quantum computers. In the 90s some quantum algorithms improving the performance of all known classical algorithms for certain problems started to appear, while in the 2000s the first universal computers of few atoms began to be built, allowing implementation of those algorithms in small scales. The D-Wave machine already performs quantum annealing in thousands of qubits, although some controversy over the true quantumness of its internal workings surrounds it. Many countries in the planet are devoting large amounts of money to this field, with the recent European flagship and the involvement of the largest US technological companies giving reasons for optimism. The second part of this thesis deals with some aspects of quantum computation, starting with the creation of the field of cloud quantum computation with the appearance of the first computer available to the general public through internet, which we have used and analysed extensively. Also small incursions in quantum adiabatic computation and quantum thermodynamics are present in this second part." -- TDX
Coupling fluid-dynamics and non-thermal processes to study sources of high-energy emission by Víctor Moreno de la Cita( Book )

2 editions published in 2017 in English and held by 2 WorldCat member libraries worldwide

In this thesis we have developed a tool for the computation of non-thermal emission in astrophysical sources. The code reads the hydrodynamic data coming from relativistic hydrodynamic simulations and compute the injection, evolution and radiation of non-thermal particles. The emission channels explored in the thesis are mainly Inverse Compton (IC) and synchrotron. Making use of this tools, we have studied the following sources: Star-jet interactions in agn Some galaxies with an Active Galactic Nucleus (AGN) present two jets of relativistic particles. The pressence of this jets is linked with the merge of galaxies, as well as the posibility of starbursts. Therefore, it is plausible that an important amount of stars could interact with the relativistic jets. We have expored that situation performing simulations of an encounter between the jet and an isolated star. The result or our simulations is a potentially observable gamma-ray emission related with this interactions. Pulsar wind interacting with a clumply stellar wind Gamma-ray binaries are two-body systems in which one of the objects is compact (either a neutron star or a black hole) and the other one is a star, with their spectral energy distribution (SED) peaking above 1 MeV (gamma rays). Here we focus on the cases in which the companion star is massive, presenting strongly inhomogeneous winds with clumps. The interaction of one of these clumps with a pulsar wind is simulated, and the results obtained showed that this type of inhomogeneities enhance the gamma radiation. An specific study of the source PSR B1259 − 63 was performed. Clumpy wind-jet interactions in hmmqs High-mass microquasars (H07Qs) are binaries compound by a high-mass star and a compact object accerting material. The compact object form two collimated jets and here we explore the possibility that the jet interacts with a stellar wind inhomogeneity, or clump. We found that for the clump with the studied characteristics, the inhomogeneity is able so survive enough time to emit gamma rays before disruption. Moreover, the luminosity levels found could be enough to explain the observations of Cyg X- 1 and Cyg X-3 adopting conservative parameters. A specific study for Cyg X-1 was carried out
Collapse scenarios in magnetized star-forming regions by Carmen Juárez Rodríguez( Book )

2 editions published in 2018 in English and held by 2 WorldCat member libraries worldwide

"Turbulence, magnetic fields and gravity driven flows are important for the formation of new stars. Although magnetic fields have been proven to be important in the formation of stars, only a few works have been done combining magnetic field and kinematic information. Such studies are important to analyze both gravity and gas dynamics and be able to compare them with the magnetic field. In this thesis we will combine dust polarization studies with kinematic analysis towards different star-forming regions. We aim to study the physical properties at core scales (<0.1 pc) from molecular line and dust emission, and study the role of the magnetic field in their dynamic evolution. For this, we will use millimeter and submillimeter observational data taken towards low- and high- mass star-forming regions in different environments and evolutionary states. The first project is the study of the physical, chemical and magnetic properties of the pre-stellar core FeSt1-457 in the Pipe nebula. We studied the emission of the molecular line N2H+(1-0) which is a good tracer of dense gas and therefore describes well the structure of the core. In addition, we detected more than 15 molecular lines and found a clear chemical spatial differentiation for molecules with nitrogen, oxygen and sulfur. Using the ARTIST radiative transfer code (Brinch & Hogerheijde 2010, Padovani et al., 2011, 2012, Jørgensen et al., 2014), we simulated the emission of the different molecules detected and estimated their abundance. In addition, we estimated the magnetic field properties of the core (using the Chandrasekhar-Fermi approximation) from polarization data previously obtained by Alves et al., (2014). Finally, we found interesting correlations between the polarization properties and the chemistry in the region. The second project is the study of a high-mass star-forming region called NGC6334V. NGC6334V is in a more advanced evolutionary state and in an environment surrounded by other massive star-forming regions. During the project we studied the magnetic field from the polarized emission of the dust and also the kinematics of the gas from the molecular line emission of the different tracers of dense gas. From the molecular emission of the gas tracing the envelope of the dense core, we see two different velocity structures separated by 2 km/s and converging towards the potential well in the region. In addition, the magnetic field also presents a bimodal pattern following the distribution of the two velocity structures. Finally, we compared the observational results with 3D magnetohydrodynamic simulations of star-forming regions dominated by gravity. The last project is the study of a lower-mass star-forming region, L1287. From the data obtained with the SMA, the dust continuum structure shows six main dense cores with masses between ̃0.4 and 4 solar masses. The dense gas tracer DCN(3-2) shows two velocity structures separated by 2-3 km/s, converging towards the highest-density region, the young stellar object IRAS00338+6312, in a similar scenario to the one observed in the higher-mass case of NGC6334V. Finally, the studies of the pre-stellar core FeSt1-457 and the massive region NGC6334V, show how the magnetic field has been overcome by gravity and is not enough to avoid the gravitational collapse. In addition, NGC6334V and the lower- mass region L1287 present very similar scenarios with the material converging from large scales (̃0.1 pc) to the potential wells of both regions at smaller scales (̃0.02 pc) through two dense gas flows separated by 2-3 km/s. In a similar scenario, FeSt1-457 is located just in the region where two dense gas structures separated by 3 km/s appear to converge." -- TDX
Studies of Black Hole Horizons by Marina Martínez Montero( Book )

2 editions published between 2016 and 2017 in English and held by 2 WorldCat member libraries worldwide

"This thesis has focused entirely on classical and thermodynamical aspects of black hole physics. We have developed four different projects involving different kinds of black holes. 1 BLACK BRANES IN A BOX Neutral black branes with extended horizons are dynamically unstable to long wavelength perturbations along their horizons; this instability is known as the Gregory-Laflamme instability. In some regimes, the dynamics of black branes can be captured by an effec¬tive hydrodynamic description. We have studied the effective hydrodynamics of neutral black branes inside a cylindrical cavity to investigate their dynamic and thermodynamic instabilities. We have used the size of the box as a control parameter for stability (smaller cavities increase rigidity and contribute to the stability of the solutions); we have ob¬served that both instabilities disappear at the same critical value of the cavity radius. We have discussed the Correlated Stability Conjecture, which relates thermodynamic and dynamic instabilities in these objects and we have argued that its correct interpre¬tation is given by the Correlated Hydrodynamic Stability (CHS). The CHS relates the presence of unstable hydrodynamic modes to the local thermodynamic instability; this is transparent in our approach. In the effective fluid description we have computed the specific quantities that characterize the fluid. Finally we have studied the system close to the critical point at which the instability disappears and we have obtained that the wavenumber that marks the onset of the instability vanishes with a critical behaviour ruled by a critical exponent of 1/2. 2 BLACK STRING FLOW We have constructed an event horizon describing a heat flow, that remains constant in time, between to asymptotic regions at constant temperature. This horizon is the smooth interpolation between the horizon of a black string and a planar acceleration horizon. This was the first exact description of a flowing horizon connecting a stringlike horizon with a planar one (this can also be an infinitely big spherical black hole); the construction is valid for any number of dimensions greater than four. We obtained the horizon generators as well as the exact geometry and we showed that this horizon resembles that of flowing funnels. We computed a surface gravity that approaches on one end, the black string's surface gravity, and on the other, the infinite black hole's surface gravity which is 0. We also computed the expansion associated to the horizon generators and it vanishes in both asymptotic regions; thus reflecting the property that the black string flow horizon interpolates between two asymptotic horizons, each of which is asymptotically in equilibrium at different temperature. This construction shows that stationary black holes with non-killing horizons are possible with non-AdS asymptotics. 3 BUMPY BLACK HOLES We have constructed numerically three new families of stationary black holes with a single angular momentum. These black holes have spherical topology but they differ from the Myers Perry solution (higher dimensional generalisation of Kerr solution) in that the radius of the sphere transverse to rotation varies non-monotonically with the polar angle. We have seen that half of these solutions connect, in the space of solutions, the Myers Perry family with other families featuring non-spherical topology such as the black ring, the black saturn, etc. We found strong evidence for the presence of cones in the horizons of solutions close to the topological transition in solution space. The other half of the solutions spread widely in the rotation plane and develop a singularity along their equator. These probably do not connect to other stationary black hole branches. We have also studied stability properties of all branches. 4 BLACK HOLE MERGER We have described in an exact analytic way the event horizon of a black hole merger in the extreme mass ratio (EMR) limit; we have done it for four and five dimensions. Curiously numerical computation in which the ratio of the masses is large are difficult and not very well studied. We hope our exact result can serve as check/guide for future results in the area. We constructed the event horizon of this dynamical process by computing its null generators. We extracted a number of parameters that characterise the merger. We identified the line of caustics, the critical radius at which both horizons touch, the big horizon relaxation timescale among other things. We showed that our hypersurface describes all possible mergers, in the EMR limit, for which the small black hole is non-rotating. Finally we analysed the instants shortly before and after the pinch-on and found evidence for critical behaviour in the forming of the cusp and in the initial growth of the throat."
Meson-Baryon interactions from effective Chiral Lagrangians by Albert Feijoo Aliau( Book )

2 editions published in 2017 in Spanish and English and held by 2 WorldCat member libraries worldwide

New insights into holography from supersymmetric localization by Genís Torrents Verdaguer( Book )

2 editions published in 2016 in English and held by 2 WorldCat member libraries worldwide

Maldacena's conjecture, often known as the holographic duality or the AdS/CFT correspondence, proposes an equivalence between gravitational theories in a hyperbolic space of a certain dimensionality and gauge theories living on its boundary. The manner in which this connection is established makes the duality specially remarkable: both sides are thought to describe the very same string theoretic physics, but the validity regimes of the two descriptions are disjoint, and one expects either framework to be in its regime of validity the appropriate way to effectively reexpress the physics of the other. The nonintersection of these applicability regimes makes the duality very useful, but also very hard to verify and materialize. Notice the potential implications of this framework for theoretical physics: one direction, strongly coupled quantum field theories become in a certain regime describable as semiclassical gravitational spacetimes, while on the other direction certain string theories without semiclassical background obtain a clean and workable definition as gauge field theories. As a consequence of these facts, holography has played a central role in research since its appearance, almost two decades ago. However, despite numerous efforts devoted to its characterization, general understanding of the duality has only been majoritarely achieved around the regimes where the gravitational description becomes semiclassical. Consequently, the gauge-to-gravity direction of the duality is far less exploited than the opposite one, despite its conceptual relevance. Having available results for strongly coupled gauge theories would be of a great help in addressing holography in this comparatively underdevelopped direction, and they would set a fertile ground to test, refine and understand the holographic conjecture. These type of predictions are hard to come by, but not inexistent: non-renormalized magnitudes constitute their most paradygmatical example, and recently different techniques have obtained exact results at arbitrary coupling for specific obserable sectors. This thesis studies specifically one of these techniques, known as supersymmetric localization, and its role in shoring AdS/CFT. In particular, it restricts its analysis to a specific type of theory: Lagrangian N. = 2 SYM, and for specific results: half-BPS Wilson circular loops. Several interesting insights are put forward by its results. A first observation is that the exact functional dependence we obtain from localization offer a guide on how to extend holographic predictions from their validity regime to a finite gauge range which produces plausible results, although a great care has to be taken in this process. Complementarily, the study of this parametrical dependence for gauge N. = 4 theories with gauge Lie algebras presents two suggestive patterns: On the one hand, 't Hooft's topological expansion presents, at least for charges in fundamental representations, an underlying structure that relates sectors with different number of crosscaps among themselves. On the other hand, the matrix model structure obtained in the localization process can be interpreted in terms of a fermionic quantum mechanics, which at the 't Hooft limit matches the "bubbling geometry" structure of Lin, Lunin and Maldacena, but which persists at finite gauge group range. Finally, the comparison of localization results within a more general type of construction is pre-sented. The specific set of theories considered contains both examples with semiclassical holographic duals and examples where this type of geometry is precluded. Supersymmetric field predictions in this case differentiate both groups with qualitatively different behaviours in the matrix model. This suggests a possible connection between the matrix model structure and the semiclassical spacetime codification in the dual field theory. Similar observations have been made in the literature. This thesis, therefore, explicits a wide list of suggestions for holography that are motivated by lo-calization results in different regimes, even though the latter have been severely restricted to particular examples
Dynamics of isotopically pure he droplets doped with atomic impurities by Antonio Jesús Leal Martín( Book )

2 editions published between 2016 and 2017 in English and held by 2 WorldCat member libraries worldwide

Helium is the second lightest element and the second most abundant element in the universe. It is named for the Greek god of the sun, Helios, and it was first discovered by French astronomer Jules Janssen as an unknown yellow spectral line in sunlight during a solar eclipse in 1868. Helium has a very simple atomic structure: two electrons around a nucleus formed of two protons and two neutrons (the case of He-4) or two protons and a single neutron (for He-3). Looking at the corresponding phase diagrams of both isotopes, it can be seen that they both have the unique property of maintaining the liquid phase down to T=0 K, thanks to their zero point energy, which is large enough to avoid solidification (due to their low mass and the weak He-He interaction). The zero point energy is the lowest possible energy that a quantum mechanical physical system may have; it is the energy of its ground state. The existence of this energy is a prediction of Quantum Mechanics with no classical equivalent, and plays the role of a kinetic energy present even when there is no "motion" in the classical sense. The first liquefaction of helium was achieved in 1908, by Dutch physicist Heike Kamerlingh Onnes. Furthermore, under certain temperature conditions (below 2.17 K for He-4 and 2.7 mK for He-3) helium becomes superfluid, that is, its viscosity is almost zero and it can flow without any friction. This remarkable feature was first discovered by Pyotr Kapitsa, John F. Allen, and Don Misener in 1937. Helium nanodroplets have been extensively studied in cluster physics and physical chemistry for more than 20 years, as they have the ability to capture atoms and molecules they collide with. This property, along with the weak interaction of superfluid He-4 with atoms and molecules, makes them ideal nanometric scale matrices for spectroscopic studies of molecules and other structures. Once captured, the impurities (also known as dopants) may so inside the droplet or, in some cases, remain in a dimple on the surface. The five papers presented in this thesis are organized in three different sections: Chapter 2 addresses the dynamic study of Ba+ impurities at He-4 nanodroplets by two publications. The first paper focuses on the dynamic study of Ba+ upon ionization of Ba in a dimple on the surface of a droplet and the second paper studies the evolution of Ba+ in the bulk portion of the droplet when it is excited to 2P or 2D states. Chapter 3 presents the dynamic study of alkali impurities (Rb and Cs) at the same droplets. This chapter is also represented by two papers, where the first one addresses the ejection of these impurities from the surface of He-4 nanodroplets upon excitation to the 6s state (for rubidium) or 7s state (in the case of cesium), and the second paper studies the dynamic evolution of Rb+ and Cs+ after the corresponding neutral atom is ionized on the surface of a droplet. Lastly, Chapter 4 presents an article that focuses on the capture of Cs atoms by He-4 nanodroplets. We have studied this process by making cesium atoms collide with He droplets using different projectile velocities and impact parameters
Jets as probes of strongly coupled quark-gluon plasma by Daniel Pablos Alonso( Book )

2 editions published between 2016 and 2017 in English and held by 2 WorldCat member libraries worldwide

In this thesis we have studied how high energetic excitations propagate through a non- abelian strongly coupled plasma. This new state of matter is produced at heavy ion collisions in our accelerators and allows us to study a stage of the evolution of our Universe that occurred during the first microseconds after the Big Bang. In this extreme conditions of temperature and density the ordinary matter that we are made of behaves as a an almost perfect fluid, the most perfect known by mankind up to now in fact. The theory of strong interactions is tested at an energy scale that even though it is high enough to melt hadrons, it does not get to the point where the coupling constant is low enough to allow a perturbative description. In the plasma, the partonic field content, the quarks and gluons, cease to be the relevant degrees of freedom and a microscopic description in terms of quasi-particles is not possible. A very useful tool to put to test the actual behaviour of this strongly coupled fluid is the analysis of jet modifications as a result of their interactions with the plasma. In a first introductory part we have given the concepts needed to picture how heavy ion collisions develop as we are able to understand it today. At weak coupling, the main mechanism responsible for energy loss is induced gluon emission and interesting interference phenomena occur that lead to a dependence on path length of as the squared distance. These are known as coherence effects and their study becomes richer by considering multi-gluon emission, as it is done done in Part III. The strongly coupled picture uses holography to map a dressed excitation moving through a strongly coupled plasma into a string propagating in a higher dimensional space containing a black hole. Since the non-abelian theory in which the calculation is done is not QCD, but N = 4 SYM, we take these results as an insight to describe energetic parton propagation in a model of jet quenching in heavy ion collisions. Even though we assume that the exchanges with the medium are soft enough to include non-perturbative effects, as described by gauge/gravity duality, the energetic partons that are produced in the collision generally have a high virtuality which they relax by successive splittings. The latter occur at length scales that are not resolvable by the medium, and they should proceed as in vacuum. This observation motivates us to adopt a hybrid description for the interplay between the multi scale jet and the QGP, using each description at the scale it is supposed to be valid. This phenomenological description has proven to be very successful in describing dijet and photon-jet data at different centralities, and predictions have been made for a wide range of observables for the coming data from run 2 of LHC, including a new observable, the ratio of the fragmentation functions of the leading and subleading jet in a dijet pair, which is highly sensitive to the specific energy loss mechanism. In the next part of the work we extend our hybrid model by the inclusion of two effects, broadening and medium response, which should help us better describe intra-jet observables. The first effect, broadening, is due to the Brownian motion that probes experience in a thermal bath, and it will tend to broaden the distribution of particles within the jet. As it turns out, the observable quantifying such modifications, the jet shapes, are rather insensitive to the inclusion of this effect. However, by restricting the range of the tracks entering this analysis, we have been able to produce a new observable which shows a remarkable dependence on the precise strength of the broadening mechanism. The second effect involves overall energy-momentum conservation. The rapidly thermalized energy deposited by the energetic partons modifies the plasma, inducing temperature and velocity fluctuations in the surrounding fluid cells. This perturbation propagates long distances in the form of a wake and eventually decays into soft hadrons, whose orientations keep a correlation with the jet direction and therefore produce a net effect even after background subtraction. The observable consequences are best noticed in intra-jet measurements such as jet shapes and fragmentation functions, where it is clearly seen that the inclusion of such physics is in good agreement with the observed experimental trend, and it becomes simply unavoidable when comparisons against global measurements are performed. Finally, we compute the inclusive two gluon stimulated emission within the context of perturbative QCD. By studying the full answer in different kinematical limits we arrive to the conclusion that jet propagation is perceived from the point of view of the plasma as a set of effective emitters depending on the resolution power, which for a thin plasma it is of the order of the Debye screening mass. This physics is a missing piece of the Monte Carlo jet quenching model presented in this thesis and its inclusion is expected to have important consequences for the more differential observables, a task that will be undertaken in future work. These are very exciting times for the physics of strong nuclear interactions. We have seen how the very fundamental questions about the nature of the high temperature, strongly coupled phase of ordinary matter can be addressed by the study of jet quenching and its observable consequences. This thesis represents an effort in the confrontation of the seductive ideas of holography with experiments. Having the means to quantitatively confront new ideas, as we have done throughout the presented work, new observables, and new data is critical if we are eventually to understand the properties of the strongly coupled liquid quark-gluon plasma that Nature has served us
Holographic collisions and non-conformal dynamics by Miquel Triana Iglesias( )

2 editions published between 2017 and 2018 in Spanish and English and held by 2 WorldCat member libraries worldwide

"The gauge/gravity duality has proven to be a very useful tool in the understanding of quantum field theories outside the perturbative regime. In particular, holography has been able to shed light not only on generic mechanisms of strongly coupled theories, but also on processes occurred in experimental set-ups, such as the heavy ion collisions. Experimental observations such as small viscosities or fast hydrodynamization find a natural explanation when the problem is expressed in terms of gravity and black holes. Despite the successes, however, it is important to bear in mind that holography provides computational tools for toy models rather than for QCD itself, and that these models are usable only under certain assumptions. Nature is very often far more nuanced than the models physicists use to describe it. In the case of heavy ion experiments and QCD there are many features that are commonly coarse grained in the holographic computations. For instance, non-trivial RG flows or baryon currents have not been included in the holographic models until very recently, although these are very relevant to experiments, and fundamental in critical phenomena. In this thesis we present a series of works in the topics field theory and heavy ion collisions that use applied holography and numeric GR as computational tools. The unifying factor among them is that they consider gravitational set-ups beyond pure gravity to describe the physics of conserved currents, non-trivial RG flows and phase transitions. In chapter 2 we use an Einstein-Maxwell set-up to compute the collision of two shock-waves with a conserved current and the hydrodynamization of the subsequent plasma. This conserved current is used to model the baryonic charge deposition by rapidity, observed in the experiments. The simulations are done with and without including the backreaction of the Maxwell field into the metric, which corresponds to the quenched approximation for the effects of the baryon charge on the gluons. In chapter 3 we present a one parameter family of non-conformal models. By adding an scalar field with a polynomial potential to the pure gravity set-up, we can achieve a non-trivial RG flow between two fixed points in the dual field theory. In this work we compute the thermodynamics and the quasi-normal modes spectra for the homogeneous states, being the latter one of the main results of the chapter. In chapter 4 we present the first holographic shock-wave collisions in a non-conformal model. To do so, we use the model introduced in chapter 3. In non-conformal models the average pressure in equilibrium is not fixed by symmetry, but by the equation of state. Out of equilibrium the average pressure might take any value, giving a new probe for the equilibration of the system. When the plasma's average pressure is well approximated by the equation of state value, we say that the system has "EoSizied". In this chapter we show that the EoSization can indeed happen before the plasma has hydrodynamized. Finally, in chapter 5 we explore a holographic model that can contain phase transitions. This model is the same as the one presented in chapter 3, but now taking pure imaginary numbers for the controlling parameter. In an effort to understand the instabilities present in models with phase transitions, we trigger and evolve a spinoidal instability to its inhomogeneous end state. This is done by adding a small perturbation to a uniform black brane in a locally unstable branch, triggering a Gregory-Laflamme type instability in the gravity side. The most remarkable result found in the simulation is that both the evolution and the final result are well described by second order hydrodynamics."
Vacuum Energy in Quantum Field Theory and Cosmology by Adrià Gómez Valent( Book )

2 editions published in 2017 in English and held by 2 WorldCat member libraries worldwide

Numerical Relativity studies in Anti-de Sitter spacetimes: Gravitational Collapse and the AdS/CFT correspondence by Daniel Santos-Oliván( Book )

2 editions published in 2018 in English and held by 2 WorldCat member libraries worldwide

"In this thesis we study several open problems using Numerical Relativity on asymptotically Anti-de Sitter (AdS) spacetimes. The understanding of the dynamics of AdS is interesting not only because of pure theoretical reasons but also because of its importance in the correspondence gauge/gravity. In the thesis we present three different topics. The first is our research on the gravitational collapse of massless scalar fields in 18dS spacetimes. We have developed a new method that combines two different formulations of the Einstein Field Equations to get closer and with more accuracy to the collapse. The simulation starts with a Cauchy evolution with pseudo-spectral methods and when the collapse is taking place, it performs a change of coordinates to a characteristic one to track the formation of the apparent horizon. The collapse of the scalar field happens after a number of bounces with the critical points being the separation between the different branches. We have numerical evidence that in the separation of the branches there is a power law for subcritical configurations in addition to the one for supercritical ones. This new power law confirms that there is a gap in the mass of the apparent horizon. In the second part, we introduce a shock waves model in AdS to study the far-from-equilibrium regime in the heavy ion collisions through the holographic correspondence in a non-conformal theory. Holographic collisions have attracted a lot of attention in the last few years because of the possibility of simulating strongly coupled systems but, as a drawback, we do not know yet the exact dual of the QCD that should explain the phenomena. In the models used until now, the shock waves correspond to conformal gauge theories while QCD is not conformal. In order to get closer to a description of the actual physical collisions we present the first shock wave collisions in a non-conformal theory. With this, we show how the non-conformality increases the hydrodynamisation time and also that this can happen before the equation of state is fulfilled. In the last part, we propose the use of spectral methods as a very strong option for high precision computations. Arbitrary precision arithmetic has two main problems. The first is the necessity of increasing a lot the discretisation units to reach the precision we want. The other one is the slowing down in the computational performance due to the fact that we need to emulate the fundamental operations with software because current processors are not adapted to carry out computations with precision different from the standard one. The exponential convergence of spectral methods can approximate functions to a very high accuracy with a few hundred terms in our spectral expansion while in other numerical methods it would be a few orders of magnitude larger. This makes these methods very attractive because they facilitate the accessibility to very small error simulations, removes the bottleneck of the memory demand and also help in the computational speed because fewer points are needed for the computation. We have tested this idea with the ANETO library for simulations in AdS spacetimes and the gravitational collapse in an asymptotically flat spacetime with very promising results. This library has been developed as a direct result of this thesis and that can be downloaded as Free Software." -- TDX
The cross-correlation among tracers of the underlying large-scale mass distribution in the universe by Ignasi Pérez Ràfols( Book )

2 editions published between 2016 and 2017 in English and held by 2 WorldCat member libraries worldwide

Exploring Signatures of New Physics in Cosmology by Nicola Bellomo( )

1 edition published in 2020 in English and held by 1 WorldCat member library worldwide

Neutron-rich matter in atomic nuclei and neutron stars by Claudia González Boquera( )

1 edition published in 2020 in English and held by 1 WorldCat member library worldwide

The physics of rotational atomic and photonic quantum fluids by Albert Gallemí Camacho( Book )

1 edition published in 2017 in English and held by 1 WorldCat member library worldwide

In this thesis, we will study the superfluidity of condensed atomic and photonic systems, through the manipulation of rotational states, such as vortices or persistent currents. We will study Bose-Einstein condensates both in the strongly-correlated regime, where models based on second quantization, like Bose-Hubbard model, will be required; and in weakly-interacting systems, where mean-field approximations will be accurate enough, and the system is described by means of the Gross-Pitaevskii equation. We will start with the analysis of the fundamental properties of Bose gases trapped in few-site lattices, such as the phase diagram, the condensed fractions and the entanglement. Concerning the phases, we will study the properties of the transitions between them and, in particular, their characteristic critical exponents. Afterwards, we will consider the sites of a lattice constituting a ring geometry and study the effect of manipulating the tunnelling rate between two of the wells. This kind of tunable link is called weak link, and we will analyze what happens in the mean-field approximation, in comparison with the strongly-correlated case. In both regimes we will observe that the weak link behaves as a key element in the system in order to generate superpositions of flow states. Moreover, in the mean-field case, we can identify an energy barrier that separates two current states (also known as winding number states), where solitonic states, i.e. states characterized by the presence of topological singularities, live. Such a barrier will be the origin of the appearance of a hysteresis cicle in processes of transfer between different winding number curves, called phase slips. After that, we will study two coherently-coupled components of a toroidally-trapped Bose-Einstein condensate. We will see that when we imprint a persistent current in one of the components, there is an angular momentum transfer between both components. This transfer can be identified as a phase slip event, and the tunability of the system allows it to behave as a robust qubit, due to the fact that states supported by currents are less fragile. In two-component condensates, it is possible to find a particular solitonic state called Josephson vortex. This state is characterized by a density depletion around a point with nonzero currents. Moreover, these states are energetically more favourable than dark soliton states, whose main difference with respect to Josephson vortices is the fact that dark solitons do not present currents. However, when spin-orbit coupling is added, dark soliton states are no longer possible, but Josephson vortices persist. In this thesis, we will see that these states decay through transversal excitations (i.e. snake instability), producing vortex-antivortex pairs, and their subsequent dynamical evolution depends on the initial orientation of the Josephson vortex. Finally, we will move to the field of polariton condensates. Polaritons are quasiparticles product of the coupling between photons and excitons (which are electron-hole excitations) in semiconductor microcavities. These particles can constitute an out-of-equilibrium (due to the short lifetime of polaritons) Bose-Einstein condensate described by the Gross-Pitaevskii-like equation for two components, because of the two polarization components inherent to the photonic nature of polaritons. The cavities where these condensates are created generate a spin-orbit coupling between the two polariton components, in such a way that current states with different orbital angular momentum are coupled. This yields to a phenomenon of spin-to-orbital angular momentum conversion that we will study in ring-trapped polariton condensates. At the end of this thesis, we will probe the superfluid properties of polariton condensates, by analyzing the response of the generated currents against the presence of disorder
 
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Universitat de Barcelona. Facultat de Física. Departament de Física Quàntica i Astrofísica

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