Umut Gürsoy - Independent Researcher (original) (raw)
Papers by Umut Gürsoy
SciPost Physics
We study relativistic hydrodynamics in the presence of a non vanishing spin potential. Using a va... more We study relativistic hydrodynamics in the presence of a non vanishing spin potential. Using a variety of techniques we carry out an exhaustive analysis, and identify the constitutive relations for the stress tensor and spin current in such a setup, allowing us to write the hydrodynamic equations of motion to second order in derivatives. We then solve the equations of motion in a certain dynamical spin limit and in a perturbative setup and find surprisingly good agreement with measurements of global \LambdaΛ-hyperon polarization carried out at RHIC.
Journal of High Energy Physics
We investigate the combined effects of anisotropy and a magnetic field in strongly interacting ga... more We investigate the combined effects of anisotropy and a magnetic field in strongly interacting gauge theories by the gauge/gravity correspondence. Our main motivation is the quark-gluon plasma produced in off-central heavy-ion collisions which exhibits large anisotropy in pressure gradients as well as large external magnetic fields. We explore two different configurations, with the anisotropy either parallel or perpendicular to the magnetic field, focusing on the competition and interplay between the two. A detailed study of the RG flow in the ground state reveals a rich structure where depending on which of the two, anisotropy or magnetic field, is stronger, intermediate geometries with approximate AdS4 × ℝ and AdS3 × ℝ2 factors arise. This competition is also manifest in the phase structure at finite temperature, specifically in the dependence of the chiral transition temperature on anisotropy and magnetic field, from which we infer the presence of inverse magnetic and anisotropic...
Holography and Magnetically Induced Phenomena in QCD
SpringerBriefs in Physics
Gauge-Gravity Duality
SpringerBriefs in Physics
Improved Holographic QCD with Magnetic Field
SpringerBriefs in Physics
Improved Holographic QCD at Finite Density
SpringerBriefs in Physics
Basic Features of QCD
SpringerBriefs in Physics
Hydrodynamics and Transport Coefficients
SpringerBriefs in Physics
SpringerBriefs in Physics
AdS/CFT and holography ♠ The large N c approximation to QCD has promised a string theory descript... more AdS/CFT and holography ♠ The large N c approximation to QCD has promised a string theory description of the color singlet sector of gauge theories. 't Hooft ♠ The nature of this string theory became more palpable with the formulation of the AdS/CFT correspondence for N = 4 4 sYM.
Thermodynamics and the Confinement/Deconfinement Transition
SpringerBriefs in Physics
The European Physical Journal A
We review the holographic approach to electromagnetic phenomena in large N QCD. After a brief dis... more We review the holographic approach to electromagnetic phenomena in large N QCD. After a brief discussion of earlier holographic models, we concentrate on the improved holographic QCD model extended to involve magnetically induced phenomena. We explore the influence of magnetic fields on the QCD ground state, focusing on (inverse) magnetic catalysis of chiral condensate, investigate the phase diagram of the theory as a function of magnetic field, temperature and quark chemical potential, and, finally discuss effects of magnetic fields on the quark–anti-quark potential, shear viscosity, speed of sound and magnetization.
Physical Review C
We introduce a model for heavy ion collisions named TRAJECTUM, which includes an expanded initial... more We introduce a model for heavy ion collisions named TRAJECTUM, which includes an expanded initial stage with a variable free streaming velocity v fs and a hydrodynamic stage with three varying second order transport coefficients. We describe how to obtain a Gaussian emulator for this 20-parameter model and show results for key observables. This emulator can be used to obtain Bayesian posterior estimates on the parameters, which we test by an elaborate closure test as well as a convergence study. Lastly, we employ the optimal values of the parameters found in Nijs et al. [G. Nijs, W. van der Schee, U. Gürsoy, and R. Snellings, Phys. Rev. Lett. 126, 202301 (2021)] to perform a detailed comparison to experimental data from PbPb and pPb collisions. This includes both observables that have been used to obtain these values as well as wider transverse momentum ranges and new observables such as correlations of event-plane angles.
Journal of High Energy Physics
We point out minor errors appearing in the published version of our paper [1].
Charge-dependent flow as evidence of strong electromagnetic fields in heavy-ion collisions
Modern Physics Letters A
The extremely large electromagnetic fields generated in heavy-ion collisions provide access to no... more The extremely large electromagnetic fields generated in heavy-ion collisions provide access to novel observables that are expected to constrain various key transport properties of the quark-gluon plasma and could help solve one of the outstanding puzzles in QCD: the strong CP problem. In this review we present a brief overview of the theoretical and experimental characterization of these electromagnetic fields. After reviewing the current state, emphasizing one of the observables — the charge-dependent flow — we discuss the various discrepancies between the measurements and theoretical predictions. Finally, to help resolve the discrepancies, we suggest new measurements and theoretical ideas.
Physical Review D
We continue the study initiated in [P. Betzios, U. Gürsoy, M. Järvinen, and G. Policastro, Phys. ... more We continue the study initiated in [P. Betzios, U. Gürsoy, M. Järvinen, and G. Policastro, Phys. Rev. D 97, 081901 (2018).] of the fluctuations of a strongly coupled nonconformal plasma described holographically by Einstein gravity coupled to a dilaton with an exponential potential. The plasma approaches a critical point of a continuous phase transition in a specific limit, where the metric becomes a linear-dilaton background. This results to an analytic description of the quasinormal mode spectrum, that can be extended perturbatively in the deviation away from the critical point. In the previous paper we showed that at criticality the quasinormal frequencies coalesce into a branch cut on the real axis. In this paper we give a more extended and complete discussion of these results. We compare in detail the numerical and analytical approximations in order to confirm their validity; we study (numerically and in a Wentzel-Kramers-Brillouin approximation) the momentum dependence of the modes, in order to determine the crossover scale that limits the validity of the hydrodynamic approximation, and which becomes arbitrarily low at the critical point; and we discuss in detail the procedure we use to complete the theory in the UV by gluing a slice of AdS geometry, and the extent to which it should provide a good approximation to a smooth UVcomplete situation.
Physical Review D
We study fluctuations around equilibrium in a class of strongly interacting nonconformal plasmas ... more We study fluctuations around equilibrium in a class of strongly interacting nonconformal plasmas using holographic techniques. In particular, we calculate the quasinormal mode spectrum of black hole backgrounds that approach Chamblin-Reall plasmas in the IR. In a specific limit, related to the exact linear-dilaton background in string theory, we observe that the plasma approaches criticality and we obtain the quasinormal spectrum analytically. We regulate the critical limit by gluing the IR geometry that corresponds to the nonconformal plasma to a part of AdS space-time in the UV. Near criticality, the spectrum can still be computed analytically and we find two sets of quasinormal modes, related to the IR and UV parts of the geometry. In the critical limit, the quasinormal modes accumulate to form a branch cut in the correlators of the energy-momentum tensor on the real axis of the complex frequency plane.
Journal of High Energy Physics
Bifurcation analysis is used to study an effective model of QCD4 with four-fermi interactions. Ou... more Bifurcation analysis is used to study an effective model of QCD4 with four-fermi interactions. Our analysis supports the scenario of a fixed point merger at the lower edge of the conformal window. This indicates square root scaling of the anomalous scaling dimensions of the fermion fields just above the lower edge and exponential scaling just below. We also predict existence of new fixed points in this model whose (dis)appearance may indicate transitions of the flow within the conformal window. Furthermore, we make new predictions for the critical value (N f /N c )crit at the lower edge. We also obtain exotic spiraling flows that are generated by complex scaling dimensions of the effective four-fermi interactions. Finally, we extend the model by adding a scalar field that couples with a Yukawa interaction term and study the modifications it causes to the RG-flow.
Journal of High Energy Physics
We investigate the effects of anisotropy on the chiral condensate in a holographic model of QCD w... more We investigate the effects of anisotropy on the chiral condensate in a holographic model of QCD with a fully backreacted quark sector at vanishing chemical potential. The high temperature deconfined phase is therefore a neutral and anisotropic plasma showing different pressure gradients along different spatial directions, similar to the state produced in noncentral heavy-ion collisions. We find that the chiral transition occurs at a lower temperature in the presence of anisotropy. Equivalently, we find that anisotropy acts destructively on the chiral condensate near the transition temperature. These are precisely the same footprints as the “inverse magnetic catalysis” i.e. the destruction of the condensate with increasing magnetic field observed earlier on the lattice, in effective field theory models and in holography. Based on our findings we suggest, in accordance with the conjecture of [1], that the cause for the inverse magnetic catalysis may be the anisotropy caused by the pre...
Physical Review Letters
We investigate the phase diagram of QCD-like gauge theories at strong coupling at finite magnetic... more We investigate the phase diagram of QCD-like gauge theories at strong coupling at finite magnetic field B, temperature T and baryon chemical potential µ using the improved holographic QCD model including the full backreaction of the quarks in the plasma. In addition to the phase diagram we study the behavior of the quark condensate as a function of T , B and µ and discuss the fate of (inverse) magnetic catalysis at finite µ. In particular we observe that inverse magnetic catalysis exists only for small values of the baryon chemical potential. The speed of sound in this holographic quark-gluon plasma exhibits interesting dependence on the thermodynamic parameters.
SciPost Physics
We study relativistic hydrodynamics in the presence of a non vanishing spin potential. Using a va... more We study relativistic hydrodynamics in the presence of a non vanishing spin potential. Using a variety of techniques we carry out an exhaustive analysis, and identify the constitutive relations for the stress tensor and spin current in such a setup, allowing us to write the hydrodynamic equations of motion to second order in derivatives. We then solve the equations of motion in a certain dynamical spin limit and in a perturbative setup and find surprisingly good agreement with measurements of global \LambdaΛ-hyperon polarization carried out at RHIC.
Journal of High Energy Physics
We investigate the combined effects of anisotropy and a magnetic field in strongly interacting ga... more We investigate the combined effects of anisotropy and a magnetic field in strongly interacting gauge theories by the gauge/gravity correspondence. Our main motivation is the quark-gluon plasma produced in off-central heavy-ion collisions which exhibits large anisotropy in pressure gradients as well as large external magnetic fields. We explore two different configurations, with the anisotropy either parallel or perpendicular to the magnetic field, focusing on the competition and interplay between the two. A detailed study of the RG flow in the ground state reveals a rich structure where depending on which of the two, anisotropy or magnetic field, is stronger, intermediate geometries with approximate AdS4 × ℝ and AdS3 × ℝ2 factors arise. This competition is also manifest in the phase structure at finite temperature, specifically in the dependence of the chiral transition temperature on anisotropy and magnetic field, from which we infer the presence of inverse magnetic and anisotropic...
Holography and Magnetically Induced Phenomena in QCD
SpringerBriefs in Physics
Gauge-Gravity Duality
SpringerBriefs in Physics
Improved Holographic QCD with Magnetic Field
SpringerBriefs in Physics
Improved Holographic QCD at Finite Density
SpringerBriefs in Physics
Basic Features of QCD
SpringerBriefs in Physics
Hydrodynamics and Transport Coefficients
SpringerBriefs in Physics
SpringerBriefs in Physics
AdS/CFT and holography ♠ The large N c approximation to QCD has promised a string theory descript... more AdS/CFT and holography ♠ The large N c approximation to QCD has promised a string theory description of the color singlet sector of gauge theories. 't Hooft ♠ The nature of this string theory became more palpable with the formulation of the AdS/CFT correspondence for N = 4 4 sYM.
Thermodynamics and the Confinement/Deconfinement Transition
SpringerBriefs in Physics
The European Physical Journal A
We review the holographic approach to electromagnetic phenomena in large N QCD. After a brief dis... more We review the holographic approach to electromagnetic phenomena in large N QCD. After a brief discussion of earlier holographic models, we concentrate on the improved holographic QCD model extended to involve magnetically induced phenomena. We explore the influence of magnetic fields on the QCD ground state, focusing on (inverse) magnetic catalysis of chiral condensate, investigate the phase diagram of the theory as a function of magnetic field, temperature and quark chemical potential, and, finally discuss effects of magnetic fields on the quark–anti-quark potential, shear viscosity, speed of sound and magnetization.
Physical Review C
We introduce a model for heavy ion collisions named TRAJECTUM, which includes an expanded initial... more We introduce a model for heavy ion collisions named TRAJECTUM, which includes an expanded initial stage with a variable free streaming velocity v fs and a hydrodynamic stage with three varying second order transport coefficients. We describe how to obtain a Gaussian emulator for this 20-parameter model and show results for key observables. This emulator can be used to obtain Bayesian posterior estimates on the parameters, which we test by an elaborate closure test as well as a convergence study. Lastly, we employ the optimal values of the parameters found in Nijs et al. [G. Nijs, W. van der Schee, U. Gürsoy, and R. Snellings, Phys. Rev. Lett. 126, 202301 (2021)] to perform a detailed comparison to experimental data from PbPb and pPb collisions. This includes both observables that have been used to obtain these values as well as wider transverse momentum ranges and new observables such as correlations of event-plane angles.
Journal of High Energy Physics
We point out minor errors appearing in the published version of our paper [1].
Charge-dependent flow as evidence of strong electromagnetic fields in heavy-ion collisions
Modern Physics Letters A
The extremely large electromagnetic fields generated in heavy-ion collisions provide access to no... more The extremely large electromagnetic fields generated in heavy-ion collisions provide access to novel observables that are expected to constrain various key transport properties of the quark-gluon plasma and could help solve one of the outstanding puzzles in QCD: the strong CP problem. In this review we present a brief overview of the theoretical and experimental characterization of these electromagnetic fields. After reviewing the current state, emphasizing one of the observables — the charge-dependent flow — we discuss the various discrepancies between the measurements and theoretical predictions. Finally, to help resolve the discrepancies, we suggest new measurements and theoretical ideas.
Physical Review D
We continue the study initiated in [P. Betzios, U. Gürsoy, M. Järvinen, and G. Policastro, Phys. ... more We continue the study initiated in [P. Betzios, U. Gürsoy, M. Järvinen, and G. Policastro, Phys. Rev. D 97, 081901 (2018).] of the fluctuations of a strongly coupled nonconformal plasma described holographically by Einstein gravity coupled to a dilaton with an exponential potential. The plasma approaches a critical point of a continuous phase transition in a specific limit, where the metric becomes a linear-dilaton background. This results to an analytic description of the quasinormal mode spectrum, that can be extended perturbatively in the deviation away from the critical point. In the previous paper we showed that at criticality the quasinormal frequencies coalesce into a branch cut on the real axis. In this paper we give a more extended and complete discussion of these results. We compare in detail the numerical and analytical approximations in order to confirm their validity; we study (numerically and in a Wentzel-Kramers-Brillouin approximation) the momentum dependence of the modes, in order to determine the crossover scale that limits the validity of the hydrodynamic approximation, and which becomes arbitrarily low at the critical point; and we discuss in detail the procedure we use to complete the theory in the UV by gluing a slice of AdS geometry, and the extent to which it should provide a good approximation to a smooth UVcomplete situation.
Physical Review D
We study fluctuations around equilibrium in a class of strongly interacting nonconformal plasmas ... more We study fluctuations around equilibrium in a class of strongly interacting nonconformal plasmas using holographic techniques. In particular, we calculate the quasinormal mode spectrum of black hole backgrounds that approach Chamblin-Reall plasmas in the IR. In a specific limit, related to the exact linear-dilaton background in string theory, we observe that the plasma approaches criticality and we obtain the quasinormal spectrum analytically. We regulate the critical limit by gluing the IR geometry that corresponds to the nonconformal plasma to a part of AdS space-time in the UV. Near criticality, the spectrum can still be computed analytically and we find two sets of quasinormal modes, related to the IR and UV parts of the geometry. In the critical limit, the quasinormal modes accumulate to form a branch cut in the correlators of the energy-momentum tensor on the real axis of the complex frequency plane.
Journal of High Energy Physics
Bifurcation analysis is used to study an effective model of QCD4 with four-fermi interactions. Ou... more Bifurcation analysis is used to study an effective model of QCD4 with four-fermi interactions. Our analysis supports the scenario of a fixed point merger at the lower edge of the conformal window. This indicates square root scaling of the anomalous scaling dimensions of the fermion fields just above the lower edge and exponential scaling just below. We also predict existence of new fixed points in this model whose (dis)appearance may indicate transitions of the flow within the conformal window. Furthermore, we make new predictions for the critical value (N f /N c )crit at the lower edge. We also obtain exotic spiraling flows that are generated by complex scaling dimensions of the effective four-fermi interactions. Finally, we extend the model by adding a scalar field that couples with a Yukawa interaction term and study the modifications it causes to the RG-flow.
Journal of High Energy Physics
We investigate the effects of anisotropy on the chiral condensate in a holographic model of QCD w... more We investigate the effects of anisotropy on the chiral condensate in a holographic model of QCD with a fully backreacted quark sector at vanishing chemical potential. The high temperature deconfined phase is therefore a neutral and anisotropic plasma showing different pressure gradients along different spatial directions, similar to the state produced in noncentral heavy-ion collisions. We find that the chiral transition occurs at a lower temperature in the presence of anisotropy. Equivalently, we find that anisotropy acts destructively on the chiral condensate near the transition temperature. These are precisely the same footprints as the “inverse magnetic catalysis” i.e. the destruction of the condensate with increasing magnetic field observed earlier on the lattice, in effective field theory models and in holography. Based on our findings we suggest, in accordance with the conjecture of [1], that the cause for the inverse magnetic catalysis may be the anisotropy caused by the pre...
Physical Review Letters
We investigate the phase diagram of QCD-like gauge theories at strong coupling at finite magnetic... more We investigate the phase diagram of QCD-like gauge theories at strong coupling at finite magnetic field B, temperature T and baryon chemical potential µ using the improved holographic QCD model including the full backreaction of the quarks in the plasma. In addition to the phase diagram we study the behavior of the quark condensate as a function of T , B and µ and discuss the fate of (inverse) magnetic catalysis at finite µ. In particular we observe that inverse magnetic catalysis exists only for small values of the baryon chemical potential. The speed of sound in this holographic quark-gluon plasma exhibits interesting dependence on the thermodynamic parameters.