arXiv subjects

Brett McInnes

Publications and source records attributed to Brett McInnes.

At least 37 records · Page 2Linked to original sources

arXiv2019

Applied Holography of the AdS$_5$-Kerr Spacetime

Asymptotically Anti-de Sitter Kerr black holes (we focus here on the five-dimensional case) are associated holographically with matter at conformal infinity which has a non-zero angular momentum density. It is natural to attempt to associate this angular momentum with the recently discovered vorticity of the plasmas produced in peripheral heavy-ion collisions. We assume that an AdS$_5$-Kerr black hole with angular momentum to mass ratio $\mathcal{A}$ is dual to boundary matter with an angular momentum density to energy density ratio also equal to $\mathcal{A}$. With this assumption, we find that, for collisions corresponding to a given value of $\mathcal{A}$, there is a maximal possible angular velocity (well below the maximal value permitted by causality) for such matter at infinity, and that this value is in approximate agreement with the experimentally reported value of the average plasma vorticity produced in typical peripheral collisions of heavy ions.

hep-ph↗

arXiv2019

Cosmic Censorship for AdS$_5$-Kerr

We show that cosmic censorship takes an exceptionally complex and interesting form in the case of five-dimensional AdS-Kerr black holes, due to the unusually distant relation obtaining in that case between the black hole parameters and the physical mass and angular momentum. One finds that, in this case, censorship is less restrictive than one might hope: it apparently allows some rather bizarre behaviour, and in particular does not prohibit arbitrarily large angular momenta. We find however that most of the unwelcome geometries permitted by censorship can be eliminated by requiring stability against pair-production of branes. We suggest that the small set of surviving problematic cases can be eliminated in a natural way by imposing a certain (holographic) bound on the physical mass.

hep-th↗

arXiv2019

Complexity vs. Vorticity

In the study of "holographic complexity", upper bounds on the rate of growth of the (specific) complexity of field theories with holographic duals have attracted much attention. Underlying these upper bounds there are inequalities relating the parameters of the dual black hole. We derive such an inequality in the case of the five-dimensional AdS-Kerr black hole, dual to a four-dimensional field theory with a non-zero angular momentum density. We propose to test these underlying inequalities "experimentally", by using the conjectured analogy of the field theory with phenomenological models of the Quark-Gluon Plasma. The test consists of comparing data for the parameters of the QGP with the upper bound on the relevant combination of black hole parameters. The bound in the non-rotating case passes the test: in this sense, it is confirmed "experimentally". In the rotating case, the inequality makes predictions regarding the entropy density of the vortical plasma, recently observed by the STAR collaboration.

hep-th↗

arXiv2019

Holography of Low-Centrality Heavy Ion Collisions

Large vorticities in the Quark-Gluon Plasma produced in peripheral collisions studied by the STAR collaboration at the RHIC facility have been deduced from observations of polarizations of $Λ$ and $\overlineΛ$ hyperons. Recently, the STAR collaboration has reported on the dependence of these polarizations on centrality, at impact energy 200 GeV and relatively large centralities $\mathcal{C} \geq 20\%$. The polarizations increase slowly with centrality, up to perhaps $\mathcal{C} = \,60 - 70\%$. Here we use a holographic model of the vortical QGP to make predictions regarding these polarizations for smaller centralities, ranging from $5 - 20\%$. The model predicts that as one moves downwards from $20\%$, $Λ/\overlineΛ$ polarizations at first decrease but then \emph{increase} sharply, in a characteristic pattern which should be readily detected if collisions can be studied at impact energies below 200 GeV and centrality as low as $5 - 10\%$. The effect should be most evident at moderate impact energies below 200 GeV, so we give predictions for impact energy 27 GeV.

hep-th↗

arXiv2018

Viscosity vs. Vorticity in the Quark-Gluon Plasma

We show that, in a holographic or gauge-gravity approach to the study of the Quark-Gluon Plasma, the specific entropy density of the plasma produced in a peripheral heavy-ion collision can be strongly suppressed relative to its value for central collisions at the same impact energy; this is connected with the strong vorticity fields which are now known to characterize the peripheral case. This implies, within the holographic model, that the shear viscosity of `peripheral plasmas' may be considerably lower than that of the corresponding `central plasmas', with potentially important consequences for other aspects of QGP physics, such as the centrality dependence of jet quenching.

hep-th↗

arXiv2018

The Effect of Vorticity on QGP Thermodynamics

Currently there is intense interest in the properties of the Quark-Gluon Plasma produced in peripheral collisions of heavy ions at various facilities, such as the RHIC. In particular, it is essential to understand the difference between such plasmas and their more readily understood counterparts produced in nearly central collisions. The differences arise primarily from the intense magnetic and vorticity fields generated in the QGP in the peripheral case. It has been argued that the magnetic fields might have a profound effect on QGP thermodynamics. Here we will argue, using a gauge-gravity model incorporating the recently proposed holographic vorticity bound, that vorticity also has important consequences for the plasma thermodynamics, in particular, for the entropy density at a given impact energy. A crucial point in our analysis is the need to determine the fate of bulk gravitational parameters when the duality translates them to the boundary.

hep-th↗

arXiv2018

How Does the Quark-Gluon Plasma Know the Collision Energy?

Heavy ion collisions at the LHC facility generate a Quark-Gluon Plasma (QGP) which, for central collisions, has a higher energy density and temperature than the plasma generated in central collisions at the RHIC. But sufficiently peripheral LHC collisions give rise to plasmas which have the \emph{same} energy density and temperature as the "central" RHIC plasmas. One might assume that the two versions of the QGP would have very similar properties (for example, with regard to jet quenching), but recent investigations have suggested that \emph{they do not}: the plasma "knows" that the overall collision energy is different in the two cases. We argue, using a gauge-gravity analysis, that the strong magnetic fields arising in one case (peripheral collisions), but not the other, may be relevant here. If the residual magnetic field in peripheral LHC plasmas is of the order of at least $eB\,\approx \,5\,m^2_π$, then the model predicts modifications of the relevant quenching parameter which approach those recently reported.

hep-th↗

arXiv2017

Jet Quenching in The Most Vortical Fluid: A Holographic Approach

The STAR collaboration at the RHIC facility has recently announced the exciting discovery of direct evidence for extremely large vorticity in the Quark-Gluon Plasma generated in peripheral collisions, seen in the form of global polarization of $Λ$ and $\barΛ$ hyperons. This prompts the question: does this vorticity have any effect on other observed phenomena, such as jet quenching? Using a simple gauge-gravity model, we suggest that such an effect may be detectable, in data from near-future experiments, as a reduction in the jet quenching parameter. This is predicted to be most prominent in collisions corresponding to a narrow range of centralities around $10 \%$. The relative reduction (compared to collisions at zero or, alternatively, large centrality) is predicted to behave in an unexpected and characteristic manner: the model suggests that it is independent of the impact energy.

hep-ph↗

arXiv2017

Holography of the QGP Reynolds Number

The viscosity of the Quark-Gluon Plasma (QGP) is usually described holographically by the entropy-normalized dynamic viscosity $η/s$. However, other measures of viscosity, such as the kinematic viscosity $ν$ and the Reynolds number $Re$, are often useful, and they too should be investigated from a holographic point of view. We show that a simple model of this kind puts an upper bound on $Re$ for nearly central collisions at a given temperature; this upper bound is in very good agreement with the observational lower bound (from the RHIC facility). Furthermore, in a holographic approach using only Einstein gravity, $η/s$ does not respond to variations of other physical parameters, while $ν$ and $Re$ can do so. In particular, it is known that the magnetic fields arising in peripheral heavy-ion collisions vary strongly with the impact parameter $b$, and we find that the holographic model predicts that $ν$ and $Re$ can also be expected to vary substantially with the magnetic field and therefore with $b$.

hep-th↗

arXiv2017

On The Existence of a Holographic Description of the LHC Quark-Gluon Plasmas

Peripheral collisions of heavy ions can give rise to extremely intense magnetic fields. It has been suggested that these fields might invalidate the holographic description of the corresponding quark-gluon plasmas, assuming that these can be modelled by strongly coupled field theories. In the case of the plasmas produced in collisions at the RHIC facility (including in the beam energy scans), it is known how to deal with this problem: one has to take into account the large angular momenta generated in these plasmas, and the effects of the baryonic chemical potential. But this does not work for the plasmas produced in peripheral collisions at the LHC. However, these results neglect some (less significant) aspects of bulk physics; could it be that the problem is resolved by taking into account these lower-order effects? Here we use a bulk dilatonic field (fully compatible with boundary data, as well as with the asymptotically AdS character of the bulk geometry) as a model of these effects, and show that this is unlikely to be the solution. Thus, the existence of a consistent holographic description of the most extreme LHC plasmas remains open to question.

hep-th↗

arXiv2016

Isoperimetric Inequalities and Magnetic Fields at CERN

We discuss the generalization of the classical isoperimetric inequality to asymptotically hyperbolic Riemannian manifolds. It has been discovered that the AdS/CFT correspondence in string theory requires that such an inequality hold in order to be internally consistent. In a particular application, to the systems formed in collisions of heavy ions in particle colliders, we show how to formulate this inequality in terms of measurable physical quantities, the magnetic field and the temperature. Experiments under way at CERN in Geneva can thus be said to be testing an isoperimetric inequality.

hep-th↗

arXiv2016

Field Theories Without a Holographic Dual

In applying the gauge-gravity duality to the quark-gluon plasma, one models the plasma using a particular kind of field theory with specified values of the temperature, magnetic field, and so forth. One then assumes that the bulk, an asymptotically AdS black hole spacetime with properties chosen to match those of the boundary field theory, can be embedded in string theory. But this is not always the case: there are field theories with no bulk dual. The question is whether these theories might include those used to study the actual plasmas produced at such facilities as the RHIC experiment or the relevant experiments at the LHC. We argue that, \emph{provided} that due care is taken to include the effects of the angular momentum associated with the magnetic fields experienced by the plasmas produced by peripheral collisions, the existence of the dual can be established for the RHIC plasmas. In the case of the LHC plasmas, the situation is much more doubtful.

hep-th↗

arXiv2016

A Rotation/Magnetism Analogy for the Quark-Gluon Plasma

In peripheral heavy ion collisions, the Quark-Gluon Plasma that may be formed often has a large angular momentum per unit energy. This angular momentum may take the form of (local) rotation. In many physical systems, rotation can have effects analogous to those produced by a magnetic field; thus, there is a risk that the effects of local rotation in the QGP might be mistaken for those of the large genuine magnetic fields which are also known to arise in these systems. Here we use the gauge-gravity duality to investigate this, and we find indeed that, with realistic parameter values, local rotation has effects on the QGP (at high values of the baryonic chemical potential) which are not only of the same kind as those produced by magnetic fields, but which can in fact be substantially larger. Furthermore, the combined effect of rotation and magnetism is to change the shape of the main quark matter phase transition line in an interesting way, reducing the magnitude of its curvature; again, local rotation contributes to this phenomenon at least as strongly as magnetism.

hep-th↗

arXiv2016

Inverse Magnetic/Shear Catalysis

It is well known that very large magnetic fields are generated when the Quark-Gluon Plasma is formed during peripheral heavy-ion collisions. Lattice, holographic, and other studies strongly suggest that these fields may, for observationally relevant field values, induce "inverse magnetic catalysis", signalled by a lowering of the critical temperature for the chiral/deconfinement transition. The theoretical basis of this effect has recently attracted much attention; yet so far these investigations have not included another, equally dramatic consequence of the peripheral collision geometry: the QGP acquires a large angular momentum vector, parallel to the magnetic field. Here we use holographic techniques to argue that the angular momentum can also, independently, have an effect on transition temperatures, and we obtain a rough estimate of the relative effects of the presence of both a magnetic field and an angular momentum density. We find that the shearing angular momentum reinforces the effect of the magnetic field at low values of the baryonic chemical potential, but that it can actually decrease that effect at high chemical potentials.

hep-th↗

arXiv2016

The Trajectory of the Cosmic Plasma Through the Quark Matter Phase Diagram

Experimental studies of the Quark-Gluon Plasma (QGP) focus on two, in practice distinct, regimes: one in which the baryonic chemical potential $μ_B$ is essentially zero, the other in which it is of the same order of magnitude as the temperature. The cosmic QGP which dominates the early Universe after reheating is normally assumed to be of the first kind, but recently it has been suggested that it might well be of the second: this is the case in the theory of "Little Inflation". If that is so, then it becomes a pressing issue to fix the trajectory of the Universe, as it cools, through the quark matter phase diagram: in particular, one wishes to know where in that diagram the plasma epoch ends, so that the initial conditions of the hadronic epoch can be determined. Here we combine various tools from strongly coupled QGP theory (the latest lattice results, together with gauge-gravity duality) in order to constrain that trajectory, assuming that Little Inflation did occur.

hep-th↗

arXiv2015

A Note on Physical Mass and the Thermodynamics of AdS-Kerr Black Holes

As with any black hole, asymptotically anti-de Sitter Kerr black holes are described by a small number of parameters, including a "mass parameter" $M$ that reduces to the AdS-Schwarzschild mass in the limit of vanishing angular momentum. In sharp contrast to the asymptotically flat case, the horizon area of such a black hole increases with the angular momentum parameter $a$ if one fixes $M$, this appears to mean that the Penrose process in this case would violate the Second Law of black hole thermodynamics. We show that the correct procedure is to fix not $M$ but rather the "physical" mass $E=M/(1-a^2/L^2)^2$, this is motivated by the First Law. For then the horizon area decreases with $a$. We recommend that $E$ always be used as the mass in physical processes: for example, in attempts to "over-spin" AdS-Kerr black holes.

gr-qc↗

arXiv2015

When Is Holography Consistent?

Holographic duality relates two radically different kinds of theory: one with gravity, one without. The very existence of such an equivalence imposes strong consistency conditions which are, in the nature of the case, hard to satisfy. Recently a particularly deep condition of this kind, relating the minimum of a probe brane action to a gravitational bulk action (in a Euclidean formulation), has been recognised; and the question arises as to the circumstances under which it, and its Lorentzian counterpart, are satisfied. We discuss the fact that there are physically interesting situations in which one or both versions might, in principle, \emph{not} be satisfied. These arise in two distinct circumstances: first, when the bulk is not an Einstein manifold, and, second, in the presence of angular momentum. Focusing on the application of holography to the quark-gluon plasma (of the various forms arising in the early Universe and in heavy-ion collisions), we find that these potential violations never actually occur. This suggests that the consistency condition is a "law of physics" expressing a particular aspect of holography.

hep-th↗

arXiv2015

Holography of Little Inflation

For several crucial microseconds of its early history, the Universe consisted of a Quark-Gluon Plasma. As it cooled during this era, it traced out a trajectory in the quark matter phase diagram. The form taken by this trajectory is not known with certainty, but is of great importance: it determines, for example, whether the cosmic plasma passed through a first-order phase change during the transition to the hadron era, as has recently been suggested by advocates of the "Little Inflation" model. Just before this transition, the plasma was strongly coupled and therefore can be studied by holographic techniques. We show that holography imposes a strong constraint (taking the form of a bound on the baryonic chemical potential relative to the temperature) on the domain through which the cosmic plasma could pass as it cooled, with important consequences for Little Inflation. In fact, we find that holography applied to Little Inflation implies that the cosmic plasma must have passed quite close to the quark matter critical point, and might therefore have been affected by the associated fluctuation phenomena.

hep-th↗