40 research outputs found
Holographic Flavor Transport in Arbitrary Constant Background Fields
We use gauge-gravity duality to compute a new transport coefficient
associated with a number Nf of massive N=2 supersymmetric hypermultiplet fields
propagating through an N=4 SU(Nc) super-Yang-Mills theory plasma in the limits
of large Nc and large 't Hooft coupling, with Nf << Nc. We introduce a baryon
number density as well as arbitrary constant electric and magnetic fields,
generalizing previous calculations by including a magnetic field with a
component parallel to the electric field. We can thus compute all components of
the conductivity tensor associated with transport of baryon number charge,
including a component never before calculated in gauge-gravity duality. We also
compute the contribution that the flavor degrees of freedom make to the
stress-energy tensor, which exhibits divergences associated with the rates of
energy and momentum loss of the flavor degrees of freedom. We discuss two
currents that are free from these divergences, one of which becomes anomalous
when the magnetic field has a component parallel to the electric field and
hence may be related to recent study of charge transport in the presence of
anomalies.Comment: 27 page
Spinning Dragging Strings
We use the AdS/CFT correspondence to compute the drag force experienced by a
heavy quark moving through a maximally supersymmetric SU(N) super Yang-Mills
plasma at nonzero temperature and R-charge chemical potential and at large 't
Hooft coupling. We resolve a discrepancy in the literature between two earlier
studies of such quarks. In addition, we consider small fluctuations of the
spinning strings dual to these probe quarks and find no evidence of
instabilities. We make some comments about suitable D7-brane boundary
conditions for the dual strings.Comment: 25 pages, 4 figures; v2 refs added; v3 to appear in JHEP, clarifying
comment
Critical Exponents from AdS/CFT with Flavor
We use the AdS/CFT correspondence to study the thermodynamics of massive N=2
supersymmetric hypermultiplet flavor fields coupled to N=4 supersymmetric
SU(Nc) Yang-Mills theory, formulated on curved four-manifolds, in the limits of
large Nc and large 't Hooft coupling. The gravitational duals are probe
D-branes in global thermal AdS. These D-branes may undergo a topology-changing
transition in the bulk. The D-brane embeddings near the point of the topology
change exhibit a scaling symmetry. The associated scaling exponents can be
either real- or complex-valued. Which regime applies depends on the
dimensionality of a collapsing submanifold in the critical embedding. When the
scaling exponents are complex-valued, a first-order transition associated with
the flavor fields appears in the dual field theory. Real scaling exponents are
expected to be associated with a continuous transition in the dual field
theory. For one example with real exponents, the D7-brane, we study the
transition in detail. We find two field theory observables that diverge at the
critical point, and we compute the associated critical exponents. We also
present analytic and numerical evidence that the transition expresses itself in
the meson spectrum as a non-analyticity at the critical point. We argue that
the transition we study is a true phase transition only when the 't Hooft
coupling is strictly infinite.Comment: 31 pages, 21 eps files in 12 figures; v2 added one reference and one
footnote, version published in JHE
Universal Holographic Chiral Dynamics in an External Magnetic Field
In this work we further extend the investigation of holographic gauge
theories in external magnetic fields, continuing earlier work. We study the
phenomenon of magnetic catalysis of mass generation in 1+3 and 1+2 dimensions,
using D3/D7- and D3/D5-brane systems, respectively. We obtain the low energy
effective actions of the corresponding pseudo Goldstone bosons and study their
dispersion relations. The D3/D7 system exhibits the usual
Gell-Mann--Oakes--Renner (GMOR) relation and a relativistic dispersion
relation, while the D3/D5 system exhibits a quadratic non-relativistic
dispersion relation and a modified linear GMOR relation. The low energy
effective action of the D3/D5 system is related to that describing magnon
excitations in a ferromagnet. We also study properties of general Dp/Dq systems
in an external magnetic field and verify the universality of the magnetic
catalysis of dynamical symmetry breaking.Comment: 41 pages, 11 figures, references adde
Toward a Holographic Model of Superconducting Fermions
We use the AdS/CFT correspondence to study N=4 supersymmetric SU(Nc)
Yang-Mills theory, in the limits of large Nc and large 't Hooft coupling,
coupled to a number Nf of massless hypermultiplet fields in the fundamental
representation of the gauge group. We identify a U(1) subgroup of the
R-symmetry under which the fermions in the hypermultiplet are charged but the
scalars are not. All the hypermultiplet fields are also charged under a U(1)
baryon number symmetry. We introduce an external magnetic field for the baryon
number U(1), which triggers the spontaneous breaking of the U(1) R-symmetry,
and we then introduce a chemical potential for the U(1) R-charge, producing a
state with a nonzero density of the U(1) R-charge. The system should then
exhibit superconductivity of the U(1) R-charge. The dual supergravity
description is a number Nf of D7-branes in AdS5 x S5 with angular momentum on
the S5 and a worldvolume magnetic field. We study the zero-temperature
thermodynamics of the system, and find that for sufficiently large magnetic
field the system prefers to be in the symmetry-broken phase. For smaller
magnetic fields we find a discontinuous free energy, indicating that our
gravitational setup does not capture all equilibrium states of the field
theory.Comment: 32 pages, 22 eps files in 9 figure
Adding Flavor to AdS4/CFT3
Aharony, Bergman, Jafferis, and Maldacena have proposed that the low-energy
description of multiple M2-branes at a C4/Zk singularity is a (2+1)-dimensional
N=6 supersymmetric U(Nc) x U(Nc) Chern-Simons matter theory, the ABJM theory.
In the large-Nc limit, its holographic dual is supergravity in AdS4 x S7/Zk. We
study various ways to add fields that transform in the fundamental
representation of the gauge groups, i.e. flavor fields, to the ABJM theory. We
work in a probe limit and perform analyses in both the supergravity and field
theory descriptions. In the supergravity description we find a large class of
supersymmetric embeddings of probe flavor branes. In the field theory
description, we present a general method to determine the couplings of the
flavor fields to the fields of the ABJM theory. We then study four examples in
detail: codimension-zero N=3 supersymmetric flavor, described in supergravity
by Kaluza-Klein monopoles or D6-branes; codimension-one N=(0,6) supersymmetric
chiral flavor, described by D8-branes; codimension-one N=(3,3) supersymmetric
non-chiral flavor, described by M5/D4-branes; codimension-two N=4
supersymmetric flavor, described by M2/D2-branes. Finally we discuss special
physical equivalences between brane embeddings in M-theory, and their
interpretation in the field theory description.Comment: 60 pages, 1 figure; v2: minor corrections, added two references,
version published in JHE
Transport Properties of Holographic Defects
We study the charge transport properties of fields confined to a
(2+1)-dimensional defect coupled to (3+1)-dimensional super-Yang-Mills at
large-\nc and strong coupling, using AdS/CFT techniques applied to linear
response theory. The dual system is described by \nf probe D5- or D7-branes
in the gravitational background of \nc black D3-branes. Surprisingly, the
transport properties of both defect CFT's are essentially identical -- even
though the D7-brane construction breaks all supersymmetries. We find that the
system possesses a conduction threshold given by the wave-number of the
perturbation and that the charge transport arises from a quasiparticle spectrum
which is consistent with an intuitive picture where the defect acquires a
finite width. We also examine finite- modifications arising from
higher derivative interactions in the probe brane action.Comment: 54 pages, 22 figures, references added, minor changes to figures and
comments, final version published in JHE
Novel experimental setup for megahertz X-ray diffraction in a diamond anvil cell at the High Energy Density (HED) instrument of the European X-ray Free-Electron Laser (EuXFEL)
The high-precision X-ray diffraction setup for work with diamond anvil cells (DACs) in interaction chamber 2 (IC2) of the High Energy Density instrument of the European X-ray Free-Electron Laser is described. This includes beamline optics, sample positioning and detector systems located in the multipurpose vacuum chamber. Concepts for pump-probe X-ray diffraction experiments in the DAC are described and their implementation demonstrated during the First User Community Assisted Commissioning experiment. X-ray heating and diffraction of Bi under pressure, obtained using 20â
fs X-ray pulses at 17.8â
keV and 2.2â
MHz repetition, is illustrated through splitting of diffraction peaks, and interpreted employing finite element modeling of the sample chamber in the DAC
The LHCb upgrade I
The LHCb upgrade represents a major change of the experiment. The detectors have been almost completely renewed to allow running at an instantaneous luminosity five times larger than that of the previous running periods. Readout of all detectors into an all-software trigger is central to the new design, facilitating the reconstruction of events at the maximum LHC interaction rate, and their selection in real time. The experiment's tracking system has been completely upgraded with a new pixel vertex detector, a silicon tracker upstream of the dipole magnet and three scintillating fibre tracking stations downstream of the magnet. The whole photon detection system of the RICH detectors has been renewed and the readout electronics of the calorimeter and muon systems have been fully overhauled. The first stage of the all-software trigger is implemented on a GPU farm. The output of the trigger provides a combination of totally reconstructed physics objects, such as tracks and vertices, ready for final analysis, and of entire events which need further offline reprocessing. This scheme required a complete revision of the computing model and rewriting of the experiment's software