22 research outputs found
Adjoint Trapping: A New Phenomenon at Strong 't Hooft Coupling
Adding matter of mass m, in the fundamental representation of SU(N), to N=4
supersymmetric Yang-Mills theory, we study ``generalized quarkonium''
containing a (s)quark, an anti(s)quark, and J massless (or very light) adjoint
particles. At large 't Hooft coupling >> 1, the states of spin <= 1
are surprisingly light (Kruczenski et al., hep-th/0304032) and small
(hep-th/0312071) with a J-independent size of order . This
``trapping'' of adjoint matter in a region small compared with its Compton
wavelength and compared to any confinement scale in the theory is an unfamiliar
phenomenon, as it does not occur at small . We explore adjoint
trapping further by considering the limit of large J. In particular, for J >>
>> 1, we expect the trapping phenomenon to become unstable.
Using Wilson loop methods, we show that a sharp transition, in which the
generalized quarkonium states become unbound (for massless adjoints) occurs at
. If the adjoint scalars of N=4 are massive and
the theory is confining (as, for instance, in N=1* theories) then the
transition becomes a cross-over, across which the size of the states changes
rapidly from ~ to something of order the confinement scale ~
.Comment: Clarified transition with a better figure and improved presentation;
added careful discussion of the small regime of validity of the
Born-Oppenheimer computation and adjusted some remarks appropriately; also
added two reference
The Pomeron and Gauge/String Duality
The traditional description of high-energy small-angle scattering in QCD has
two components -- a soft Pomeron Regge pole for the tensor glueball, and a hard
BFKL Pomeron in leading order at weak coupling. On the basis of gauge/string
duality, we present a coherent treatment of the Pomeron. In large-N QCD-like
theories, we use curved-space string-theory to describe simultaneously both the
BFKL regime and the classic Regge regime. The problem reduces to finding the
spectrum of a single j-plane Schrodinger operator. For ultraviolet-conformal
theories, the spectrum exhibits a set of Regge trajectories at positive t, and
a leading j-plane cut for negative t, the cross-over point being
model-dependent. For theories with logarithmically-running couplings, one
instead finds a discrete spectrum of poles at all t, where the Regge
trajectories at positive t continuously become a set of slowly-varying and
closely-spaced poles at negative t. Our results agree with expectations for the
BFKL Pomeron at negative t, and with the expected glueball spectrum at positive
t, but provide a framework in which they are unified. Effects beyond the single
Pomeron exchange are briefly discussed.Comment: 68 pages, uses JHEP3.cls, utphys.bst; references added, typos
corrected, and clarifying remarks adde
Hard Scattering and Gauge/String Duality
We consider high-energy fixed-angle scattering of glueballs in confining
gauge theories that have supergravity duals. Although the effective description
is in terms of the scattering of strings, we find that the amplitudes are hard
(power law). This is a consequence of the warped geometry of the dual theory,
which has the effect that in an inertial frame the string process is never in
the soft regime. At small angle we find hard and Regge behaviors in different
kinematic regions.Comment: 4 page
On the Couplings of Vector Mesons in AdS/QCD
We address, in the AdS/CFT context, the issue of the universality of the
couplings of the rho meson to other hadrons. Exploring some models, we find
that generically the rho-dominance prediction f_\rho g_{\rho H H}=m_\rho^2 does
not hold, and that g_{\rho H H} is not independent of the hadron H. However, we
prove that, in any model within the AdS/QCD context, there are two limiting
regimes where the g_{\rho H H}, along with the couplings of all excited vector
mesons as well, become H-independent: (1) when H is created by an operator of
large dimension, and (2) when H is a highly-excited hadron. We also find a
sector of a particular model where universality for the rho coupling is exact.
Still, in none of these cases need it be true that f_\rho g_\rho=m_\rho^2,
although we find empirically that the relation does hold approximately (up to a
factor of order two) within the models we have studied.Comment: 28 pages, 3 figures. ver 2: Comments about the commutability of two
universal limits in the D3/D7 case corrected. Typos corrected. ver 3:
Substantive revisions of certain calculations, with improved conventions,
correction of typos, clarifications, new formulas, new figures; no changes in
essential results or conclusion
Quarkonium from the Fifth Dimension
Adding fundamental matter of mass m_Q to N=4 Yang Mills theory, we study
quarkonium, and "generalized quarkonium" containing light adjoint particles. At
large 't Hooft coupling the states of spin<=1 are anomalously light (Kruczenski
et al., hep-th/0304032). We examine their form factors, and show these hadrons
are unlike any known in QCD. By a traditional yardstick they appear infinite in
size (as with strings in flat space) but we show that this is a failure of the
yardstick. All of the hadrons are actually of finite size ~ \sqrt{g^2N}/m_Q,
regardless of their radial excitation level and of how many valence adjoint
particles they contain. Certain form factors for spin-1 quarkonia vanish in the
large-g^2N limit; thus these hadrons resemble neither the observed J/Psi
quarkonium states nor rho mesons.Comment: 57 pages, LaTeX, 5 figure
Parton picture for the strongly coupled SYM plasma
Deep inelastic scattering off the strongly coupled N=4 supersymmetric
Yang-Mills plasma at finite temperature can be computed within the AdS/CFT
correspondence, with results which are suggestive of a parton picture for the
plasma. Via successive branchings, essentially all partons cascade down to very
small values of the longitudinal momentum fraction x and to transverse momenta
smaller than the saturation momentum Q_s\sim T/x. This scale Q_s controls the
plasma interactions with a hard probe, in particular, the jet energy loss and
its transverse momentum broadening.Comment: 4 pages, Talk given at Quark Matter 2008: 20th International
Conference on Ultra-Relativistic Nucleus Nucleus Collisions (QM 2008),
Jaipur, India, 4-10 Feb 200
Confining strings in SU(N) gauge theories
We calculate the string tensions of -strings in SU() gauge theories in
both 3 and 4 dimensions. In D=3+1, we find that the ratio of the string
tension to the fundamental string tension is consistent, at the level, with both the M(-theory)QCD-inspired conjecture and with
`Casimir scaling'. In D=2+1 we see a definite deviation from the MQCD formula,
as well as a much smaller but still significant deviation from Casimir scaling.
We find that in both D=2+1 and D=3+1 the high temperature spatial -string
tensions also satisfy approximate Casimir scaling. We point out that
approximate Casimir scaling arises naturally if the cross-section of the flux
tube is nearly independent of the flux carried, and that this will occur in an
effective dual superconducting description, if we are in the deep-London limit.
We estimate, numerically, the intrinsic width of -strings in D=2+1 and
indeed find little variation with . In addition to the stable -strings we
investigate some ofthe unstable strings, finding in D=2+1 that they satisfy
(approximate) Casimir scaling. We also investigate the basic assumption that
confining flux tubes are described by an effective string theory at large
distances. We estimate the coefficient of the universal L\"uscher correction
from periodic strings that are longer than 1 fermi, and find in
D=3+1 and in D=2+1. These values are within of the
simple bosonic string values and are inconsistent with other simple effective
string theories.Comment: 57 pages, 11 figures. Errors on fits reduced by altering the analysis
to a standard one. Conclusions unchanged; note addedchanged. Some typos
correcte
On The Pomeron at Large 't Hooft Coupling
We begin the process of unitarizing the Pomeron at large 't Hooft coupling.
We do so first in the conformal regime, which applies to good accuracy to a
number of real and toy problems in QCD. We rewrite the conformal Pomeron in the
-plane and transverse position space, and then work out the eikonal
approximation to multiple Pomeron exchange. This is done in the context of a
more general treatment of the complex -plane and the geometric consequences
of conformal invariance. The methods required are direct generalizations of our
previous work on single Pomeron exchange and on multiple graviton exchange in
AdS space, and should form a starting point for other investigations. We
consider unitarity and saturation in the conformal regime, noting elastic and
absorptive effects, and exploring where different processes dominate. Our
methods extend to confining theories and we briefly consider the Pomeron kernel
in this context. Though there is important model dependence that requires
detailed consideration, the eikonal approximation indicates that the Froissart
bound is generically both satisfied and saturated.Comment: 63 pages, 7 figures; published version: references updated and
several typos correcte
The AdS/QCD Correspondence: Still Undelivered
We consider the particle spectrum and event shapes in large N gauge theories
in different regimes of the short-distance 't Hooft coupling, lambda. The
mesons in the small lambda limit should have a Regge spectrum in order to agree
with perturbation theory, while generically the large lambda theories with
gravity duals produce spectra reminiscent of KK modes. We argue that these
KK-like states are qualitatively different from QCD modes: they are deeply
bound states which are sensitive to short distance interactions rather than the
flux tube-like states expected in asymptotically free, confining gauge
theories. In addition, we also find that the characteristic event shapes for
the large lambda theories with gravity duals are close to spherical, very
different from QCD-like (small lambda, small N) and Nambu-Goto-like (small
lambda, large N) theories which have jets. This observation is in agreement
with the conjecture of Strassler on event shapes in large 't Hooft coupling
theories, which was recently proved by Hofman and Maldacena for the conformal
case. This conclusion does not change even when considering soft-wall
backgrounds in the gravity dual. The picture that emerges is the following:
theories with small and large lambda are qualitatively different, while
theories with small and large N are qualitatively similar. Thus it seems that
it is the relative smallness of the 't Hooft coupling in QCD that prevents a
reliable AdS/QCD correspondence from emerging, and that reproducing
characteristic QCD-like behavior will require genuine stringy dynamics to be
incorporated into any putative dual theory.Comment: 32 pages, 15 figures; references added, minor changes, history
clarifie
On The Interaction Of D0-Brane Bound States And RR Photons
We consider the problem of the interaction between D0-brane bound state and
1-form RR photons by the world-line theory. Based on the fact that in the
world-line theory the RR gauge fields depend on the matrix coordinates of
D0-branes, the gauge fields also appear as matrices in the formulation. At the
classical level, we derive the Lorentz-like equations of motion for D0-branes,
and it is observed that the center-of-mass is colourless with respect to the
SU(N) sector of the background. Using the path integral method, the
perturbation theory for the interaction between the bound state and the RR
background is developed. We discuss what kind of field theory may be
corresponded to the amplitudes which are calculated by the perturbation
expansion in world-line theory. Qualitative considerations show that the
possibility of existence of a map between the world-line theory and the
non-Abelian gauge theory is very considerable.Comment: LaTeX, 28 pages, 4 eps figures. v2 and v3: eqs. (3.18) and (B.2) are
corrected, very small change