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Carlo Ewerz

Publications and source records attributed to Carlo Ewerz.

At least 19 recordsLinked to original sources
arXiv2024

Quantum Turbulence in a Three-Dimensional Holographic Superfluid

We report real-time simulations of far-from-equilibrium dynamics of a holographic superfluid in three dimensions. The holographic duality maps a strongly coupled superfluid to a weakly coupled theory with gravity in a higher-dimensional curved space which we study numerically. Starting from configurations of dense tangles of quantized vortex lines, the superfluid exhibits turbulent behavior during its evolution towards thermal equilibrium. Specifically, we observe two subsequent universal regimes of quasi-stationary scaling in the occupation number spectrum, the first of which is Kolmogorov-like. Indications for the strongly dissipative nature of vortex dynamics in the three-dimensional holographic superfluid are found in the rapid shrinking and annihilation of small vortex rings that emerge from frequent reconnections of vortex lines during the evolution.

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arXiv2020

Dynamics of a Vortex Dipole in a Holographic Superfluid

We use holography to investigate the dynamics of a vortex-anti-vortex dipole in a strongly coupled superfluid in 2+1 dimensions. The system is evaluated in numerical real-time simulations in order to study the evolution of the vortices as they approach and eventually annihilate each other. A tracking algorithm with sub-plaquette resolution is introduced which permits a high-precision determination of the vortex trajectories. With the increased precision of the trajectories it becomes possible to directly compute the vortex velocities and accelerations. We find that in the holographic superfluid the vortices follow universal trajectories independent of their initial separation, indicating that a vortex-anti-vortex pair is fully characterized by its separation. Subtle non-universal effects in the vortex motion at early times of the evolution can be fully attributed to artifacts due to the numerical initialization of the vortices. We also study the dependence of the dynamics on the temperature of the superfluid.

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arXiv2020

Vortex motion quantifies strong dissipation in a holographic superfluid

Holographic duality provides a description of strongly coupled quantum systems in terms of weakly coupled gravitational theories in a higher-dimensional space. It is a challenge, however, to quantitatively determine the physical parameters of the quantum systems corresponding to generic holographic theories. Here, we address this problem for the two-dimensional holographic superfluid, known to exhibit strong dissipation. We numerically simulate the motion of a vortex dipole and perform a high-precision matching of the corresponding dynamics resulting from the dissipative Gross-Pitaevskii equation. Excellent agreement is found for the vortex core shape and the spatio-temporal trajectories. A further comparison to the Hall-Vinen-Iordanskii equations for point vortices interacting with the superfluid allows us to determine the friction parameters of the holographic superfluid. Our results suggest that holographic vortex dynamics can be applied to experimentally accessible superfluids like strongly coupled ultracold Bose gases or thin helium films with temperatures in the Kelvin range. This would make holographic far-from-equilibrium dynamics and turbulence amenable to experimental tests.

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arXiv2019

The Tensor Pomeron and Small-x Deep Inelastic Scattering

We apply the tensor-pomeron model to small-$x$ deep-inelastic lepton-proton scattering and photoproduction. Our model includes a soft and a hard tensor pomeron as well as a reggeon contribution. Data with c.m. energies $6 < \sqrt{s} < 318$ GeV and virtualities $Q^2 < 50 \,\mbox{GeV}^2$ are considered. Our fit gives a very good description of the available data in this kinematic region, including the latest HERA data for $x < 0.01$. In particular, the transition region from low to high $Q^2$ is well described. Within the errors, the hard pomeron is absent in photoproduction. The intercepts of the soft and hard pomeron in the two-tensor-pomeron model are found to be $1.0935\, ({}^{+76}_{-64})$ and $1.3008 \,({}^{+73}_{-84})$, respectively. We argue that a vector pomeron would not give any contribution to photoproduction.

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arXiv2019

The Tensor Pomeron and Low-x Deep Inelastic Scattering

The tensor-pomeron model is applied to low-x deep-inelastic lepton-nucleon scattering and photoproduction. We consider c.m. energies in the range 6 - 318 GeV and Q^2 < 50 GeV^2. In addition to the soft tensor pomeron, which has proven quite successful for the description of soft hadronic high-energy reactions, we include a hard tensor pomeron. We also include f_2-reggeon exchange which turns out to be particularly relevant for real-photon-proton scattering at c.m. energies in the range up to 30 GeV. The combination of these exchanges permits a description of the absorption cross sections of real and virtual photons on the proton in the same framework. In particular, a detailed comparison of this two-tensor-pomeron model with the latest HERA data for x < 0.01 is made. Our model gives a very good description of the transition from the small-Q^2 regime where the real or virtual photon behaves hadron-like to the large-Q^2 regime where hard scattering dominates. Our fit allows us, for instance, a determination of the intercepts of the hard pomeron as 1.3008(+73,-84), of the soft pomeron as 1.0935(+76,-64), and of the f_2 reggeon. We find that in photoproduction the hard pomeron does not contribute within the errors of the fit. We show that assuming a vector instead of a tensor character for the pomeron leads to the conclusion that it must decouple in real photoproduction.

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arXiv2018

Free Energy of a Heavy Quark-Antiquark Pair in a Thermal Medium from AdS/CFT

We study the free energy of a heavy quark-antiquark pair in a thermal medium using the AdS/CFT correspondence. We point out that a commonly used prescription for calculating this quantity leads to a temperature dependence in conflict with general properties of the free energy. The problem originates from a particular way of subtracting divergences. We argue that the commonly used prescription gives rise to the binding energy rather than the free energy. We consider a different subtraction procedure and show that the resulting free energy is well-behaved and in qualitative agreement with results from lattice QCD. The free energy and the binding energy of the quark pair are computed for N = 4 supersymmetric Yang-Mills theory and several non-conformal theories. We also calculate the entropy and the internal energy of the pair in these theories. Using the consistent subtraction, we further study the free energy, entropy, and internal energy of a single heavy quark in the thermal medium for various theories. Also here the results are found to be in qualitative agreement with lattice QCD results.

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arXiv2016

Helicity in Proton-Proton Elastic Scattering and the Spin Structure of the Pomeron

We discuss different models for the spin structure of the nonperturbative pomeron: scalar, vector, and rank-2 symmetric tensor. The ratio of single-helicity-flip to helicity-conserving amplitudes in polarised high-energy proton-proton elastic scattering, known as the complex r_5 parameter, is calculated for these models. We compare our results to experimental data from the STAR experiment. We show that the spin-0 (scalar) pomeron model is clearly excluded by the data, while the vector pomeron is inconsistent with the rules of quantum field theory. The tensor pomeron is found to be perfectly consistent with the STAR data.

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arXiv2015

Holography for Heavy Quarks and Mesons at Finite Chemical Potential

We study the properties of heavy quarks as probes of strongly coupled plasmas with and without chemical potential by means of the gauge/gravity (AdS/CFT) duality. We compute the screening distance of a heavy quark-antiquark pair, its free energy, and the running coupling in large classes of non-conformal models arising as deformations of pure AdS space. We further investigate the energy loss of a heavy quark moving on a circular orbit as an example of an accelerated motion. These observables exhibit universal features independent of the deformation, pointing to strong-coupling universality. Our results should be relevant for processes involving heavy quarks and their bound states in the quark-gluon plasma, including the case of finite net baryon density.

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arXiv2015

Non-Thermal Fixed Point in a Holographic Superfluid

We study the far-from-equilibrium dynamics of a (2+1)-dimensional superfluid at finite temperature and chemical potential using its holographic description in terms of a gravitational system in 3+1 dimensions. Starting from various initial conditions corresponding to ensembles of vortex defects we numerically evolve the system to long times. At intermediate times the system exhibits Kolmogorov scaling the emergence of which depends on the choice of initial conditions. We further observe a universal late-time regime in which the occupation spectrum and different length scales of the superfluid exhibit scaling behaviour. We study these scaling laws in view of superfluid turbulence and interpret the universal late-time regime as a non-thermal fixed point of the dynamical evolution. In the holographic superfluid the non-thermal fixed point can be understood as a stationary point of the classical equations of motion of the dual gravitational description.

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arXiv2014

Photoproduction of pi+ pi- pairs in a model with tensor-pomeron and vector-odderon exchange

We consider the reaction (gamma p) to (pi+ pi- p) at high energies. Our description includes dipion production via the resonances rho, omega, rho-prime and f2, and via non-resonant mechanisms. The calculation is based on a model of high energy scattering with the exchanges of photon, pomeron, odderon and reggeons. The pomeron and the C=+1 reggeons are described as effective tensor exchanges, the odderon and the C=-1 reggeons as effective vector exchanges. We obtain a gauge-invariant version of the Drell-Soeding mechanism which produces the skewing of the rho-meson shape. Starting from the explicit formulae for the matrix element for dipion production we construct an event generator which comprises all contributions mentioned above and includes all interference terms. We give examples of total and differential cross sections and discuss asymmetries which are due to interference of C=+1 and C=-1 exchange contributions. These asymmetries can be used to search for odderon effects. Our model is intended to provide all necessary theoretical tools for a detailed experimental analysis of elastic dipion production for which data exist from fixed target experiments, from HERA, and are now being collected by LHC experiments.

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arXiv2013

Heavy Probes in Strongly Coupled Plasmas With Chemical Potential

We study the properties of heavy probes moving in strongly coupled plasmas at finite chemical potential. Using the gauge/gravity duality we consider large classes of gravity models consisting in deformed AdS5 spacetimes endowed with Reissner-Nordström-type black holes. We report on our analysis of the screening distance of a quark-antiquark pair, its free energy, and the running coupling. These observables show a certain insensitivity as to which model and deformation is used, pointing to strong-coupling universal behavior. Thus, the results may be relevant for modeling heavy quarkonia traversing a quark-gluon plasma at finite net baryon density, and their suppression by melting.

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arXiv2013

A Model for Soft High-Energy Scattering: Tensor Pomeron and Vector Odderon

A model for soft high-energy scattering is developed. The model is formulated in terms of effective propagators and vertices for the exchange objects: the pomeron, the odderon, and the reggeons. The vertices are required to respect standard rules of QFT. The propagators are constructed taking into account the crossing properties of amplitudes in QFT and the power-law ansaetze from the Regge model. We propose to describe the pomeron as an effective spin 2 exchange. This tensor pomeron gives, at high energies, the same results for the p-p and p-pbar elastic amplitudes as the standard Donnachie-Landshoff pomeron. But with our tensor pomeron it is much more natural to write down effective vertices of all kinds which respect the rules of QFT. This is particularly clear for the coupling of the pomeron to particles carrying spin, for instance vector mesons. We describe the odderon as an effective vector exchange. We emphasise that with a tensor pomeron and a vector odderon the corresponding charge-conjugation relations are automatically fulfilled. We compare the model to some experimental data, in particular to data for the total cross sections, in order to determine the model parameters. The model should provide a starting point for a general framework for describing soft high-energy reactions. It should give to experimentalists an easily manageable tool for calculating amplitudes for such reactions and for obtaining predictions which can be compared in detail with data.

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arXiv2013

Applications of Holography to Strongly Coupled Plasmas

We study several observables related to heavy quarks in strongly coupled plasmas using the gauge/gravity correspondence. Besides the AdS_5 space dual to N=4 supersymmetric Yang-Mills theory we consider large classes of theories obtained from various deformations of the AdS_5 space. Among them are theories that solve equations of motion of a 5-dimensional Einstein-Hilbert-scalar action. Specifically, we calculate the screening distance of a heavy quark-antiquark pair moving at constant velocity through the plasma, the running coupling defined via the free energy of such a static pair, and the energy radiation from a heavy quark forced into a circular motion in the plasma. We find that these observables show universal behaviour in large classes of theories. The screening distance in these classes of theories, that is the maximal distance for which a heavy quark-antiquark pair is bound, is found to be bounded from below by its value in N=4 super Yang-Mills theory.

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arXiv2012

The New F_L Measurement from HERA and the Dipole Model

From the new measurement of F_L at HERA we derive fixed-Q^2 averages . We compare these with bounds which are rigorous in the framework of the standard dipole picture. The bounds are sharpened by including information on the charm structure function F_2^(c). Within the experimental errors the bounds are respected by the data. But for 3.5 GeV^2 <= Q^2 <= 20 GeV^2 the central values of the data are close to and in some cases even above the bounds. Data on F_L/F_2 significantly exceeding the bounds would rule out the standard dipole picture at these kinematic points. We discuss, furthermore, how data respecting the bounds but coming close to them can give information on questions like colour transparency, saturation and the dependencies of the dipole-proton cross section on the energy and the dipole size.

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arXiv2010

On the Energy Dependence of the Dipole-Proton Cross Section in Deep Inelastic Scattering

We study the dipole picture of high-energy virtual-photon-proton scattering. It is shown that different choices for the energy variable in the dipole cross section used in the literature are not related to each other by simple arguments equating the typical dipole size and the inverse photon virtuality, contrary to what is often stated. We argue that the good quality of fits to structure functions that use Bjorken-x as the energy variable - which is strictly speaking not justified in the dipole picture - can instead be understood as a consequence of the sign of scaling violations that occur for increasing Q^2 at fixed small x. We show that the dipole formula for massless quarks has the structure of a convolution. From this we obtain derivative relations between the structure function F_2 at large and small Q^2 and the dipole-proton cross section at small and large dipole size r, respectively.

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arXiv2010

Screening in Strongly Coupled Plasmas: Universal Properties from Strings in Curved Space

We use the gauge/gravity correspondence to study the screening of a heavy quark-antiquark pair in various strongly coupled plasmas. Besides N=4 super Yang-Mills theory and the corresponding AdS_5 space we also study theories obtained as deformations of AdS_5, among them in particular a class of deformations solving supergravity equations of motion. We consider the dependence of the screening distance on the velocity and the orientation of the pair in the plasma. The value of the screening distance in N=4 SYM is found to be a minimum in the class of theories under consideration for all kinematic parameters.

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arXiv2009

High Energy Behavior of a Six-Point R-Current Correlator in N=4 Supersymmetric Yang-Mills Theory

We study the high energy limit of a six-point R-current correlator in N=4 supersymmetric Yang-Mills theory for finite N_c. We make use of the framework of perturbative resummation of large logarithms of the energy. More specifically, we apply the (extended) generalized leading logarithmic approximation. We find that the same conformally invariant two-to-four gluon vertex occurs as in non-supersymmetric Yang-Mills theory. As a new feature we find a direct coupling of the four-gluon t-channel state to the R-current impact factor.

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