1,694 research outputs found
On BLM scale fixing in exclusive processes
We discuss the BLM scale fixing procedure in exclusive electroproduction
processes in the Bjorken regime. We show that in the case of vector meson
production the usual way to aplly the BLM method fails due to singularities
present in equations fixing the BLM scale. We argue that the BLM scale should
be extracted from the squared amplitudes which are directly related to
observables.Comment: accepted for the publication in Eur.Phys.J.
Towards a SDLCQ test of the Maldacena Conjecture
We consider the Maldacena conjecture applied to the near horizon geometry of
a D1-brane in the supergravity approximation and present numerical results of a
test of the conjecture against the boundary field theory calculation using
DLCQ. We previously calculated the two-point function of the stress-energy
tensor on the supergravity side; the methods of Gubser, Klebanov, Polyakov, and
Witten were used. On the field theory side, we derived an explicit expression
for the two-point function in terms of data that may be extracted from the
supersymmetric discrete light cone quantization (SDLCQ) calculation at a given
harmonic resolution. This yielded a well defined numerical algorithm for
computing the two-point function. For the supersymmetric Yang-Mills theory with
16 supercharges that arises in the Maldacena conjecture, the algorithm is
perfectly well defined; however, the size of the numerical computation
prevented us from obtaining a numerical check of the conjecture. We now present
numerical results with approximately 1000 times as many states as we previously
considered. These results support the Maldacena conjecture and are within
of the predicted numerical results in some regions. Our results are
still not sufficient to demonstrate convergence, and, therefore, cannot be
considered to a numerical proof of the conjecture. We present a method for
using a ``flavor'' symmetry to greatly reduce the size of the basis and discuss
a numerical method that we use which is particularly well suited for this type
of matrix element calculation.Comment: 10 pages, 1 figur
Piezoelectric-based apparatus for strain tuning
We report the design and construction of piezoelectric-based apparatus for
applying continuously tuneable compressive and tensile strains to test samples.
It can be used across a wide temperature range, including cryogenic
temperatures. The achievable strain is large, so far up to 0.23% at cryogenic
temperatures. The apparatus is compact and compatible with a wide variety of
experimental probes. In addition, we present a method for mounting
high-aspect-ratio samples in order to achieve high strain homogeneity.Comment: 8 pages, 8 figure
The Pion Light-Cone Wave Function Phi_pi on the lattice: a partonic signal?
We determine the conditions required to study the pion light-cone wave
function Phi_pi with a new method: a direct display of the partons constituting
the pion. We present the preliminary results of a lattice computation of Phi_pi
following this direction. An auxiliary scalar-quark is introduced. The
spectroscopy of its bound states is studied. We observe some indications of a
partonic behavior of the system of this scalar-quark and the anti-quark.Comment: 3 pages, 4 figures, Lattice2001(matrixelement
Extraction of the pion distribution amplitude from polarized muon pair production
We consider the production of muon pairs from the scattering of pions on
longitudinally polarized protons. We calculate the cross section and the single
spin asymmetry for this process, taking into account pion bound state effects.
We work in the kinematic region where the photon has a large longitudinal
momentum fraction, which allows us to treat the bound state problem
perturbatively. Our predictions are directly proportional to the pion
distribution amplitude. A measurement of the polarized Drell-Yan cross section
thus allows the determination of the shape of the pion distribution amplitude.Comment: 13 pages, using revtex, two figures added separately as one uuencoded
Z-compressed fil
Light-Front-Quantized QCD in Covariant Gauge
The light-front (LF) canonical quantization of quantum chromodynamics in
covariant gauge is discussed. The Dirac procedure is used to eliminate the
constraints in the gauge-fixed front form theory quantum action and to
construct the LF Hamiltonian formulation. The physical degrees of freedom
emerge naturally. The propagator of the dynamical part of the free
fermionic propagator in the LF quantized field theory is shown to be causal and
not to contain instantaneous terms. Since the relevant propagators in the
covariant gauge formulation are causal, rotational invariance---including the
Coulomb potential in the static limit---can be recovered, avoiding the
difficulties encountered in light-cone gauge. The Wick rotation may also be
performed allowing the conversion of momentum space integrals into Euclidean
space forms. Some explicit computations are done in quantum electrodynamics to
illustrate the equivalence of front form theory with the conventional covariant
formulation. LF quantization thus provides a consistent formulation of gauge
theory, despite the fact that the hyperplanes used to impose
boundary conditions constitute characteristic surfaces of a hyperbolic partial
differential equation.Comment: LaTex, 16 page
Electroproduction of Charmonia off Nuclei
In a recent publication we have calculated elastic charmonium production in
ep collisions employing realistic charmonia wave functions and dipole cross
sections and have found good agreement with the data in a wide range of s and
Q^2. Using the ingredients from those calculations we calculate exclusive
electroproduction of charmonia off nuclei. Here new effects become important,
(i) color filtering of the c-cbar pair on its trajectory through nuclear
matter, (ii) dependence on the finite lifetime of the c-cbar fluctuation
(coherence length) and (iii) gluon shadowing in a nucleus compared to the one
in a nucleon. Total coherent and incoherent cross sections for C, Cu and Pb as
functions of s and Q^2 are presented together with some differential cross
sections. The results can be tested with future electron-nucleus colliders or
in peripheral collisions of ultrarelativistic heavy ions.Comment: 21 pages of Latex including 14 figures; few misprints are fixe
The Perils of `Soft' SUSY Breaking
We consider a two dimensional SU(N) gauge theory coupled to an adjoint
Majorana fermion, which is known to be supersymmetric for a particular value of
fermion mass. We investigate the `soft' supersymmetry breaking of the discrete
light cone quantization (DLCQ) of this theory. There are several DLCQ
formulations of this theory currently in the literature and they naively appear
to behave differently under `soft' supersymmetry breaking at finite resolution.
We show that all these formulations nevertheless yield identical bound state
masses in the decompactification limit of the light-like circle. Moreover, we
are able to show that the supersymmetry-inspired version of DLCQ (so called
`SDLCQ') provides the best rate of convergence of DLCQ bound state masses
towards the actual continuum values, except possibly near or at the critical
fermion mass. In this last case, we discuss improved extrapolation schemes that
must supplement the DLCQ algorithm in order to obtain correct continuum bound
state masses. Interestingly, when we truncate the Fock space to two particles,
the SDLCQ prescription presented here provides a scheme for improving the rate
of convergence of the massive t'Hooft model. Thus the supersymmetry-inspired
SDLCQ prescription is applicable to theories without supersymmetry.Comment: 11 pages, Latex; 2 figures (EPS); Numerical results extended;
conclusions revise
Light-Front Holography, Light-Front Wavefunctions, and Novel QCD Phenomena
Light-Front Holography, a remarkable feature of the AdS/CFT correspondence,
maps amplitudes in anti-de Sitter (AdS) space to frame-independent light-front
wavefunctions of hadrons in physical space-time. The model leads to an
effective confining light-front QCD Hamiltonian and a single-variable
light-front Schrodinger equation which determines the eigenspectrum and the
light-front wavefunctions of hadrons for general spin and orbital angular
momentum. The coordinate z in AdS space is identified with a Lorentz-invariant
coordinate zeta which measures the separation of the constituents within a
hadron at equal light-front time and determines the off-shell dynamics of the
bound-state wavefunctions and the fall-off in the invariant mass of the
constituents. The soft-wall holographic model, modified by a positive-sign
dilaton metric, leads to a remarkable one-parameter description of
nonperturbative hadron dynamics -- a semi-classical frame-independent first
approximation to the spectra and light-front wavefunctions of meson and
baryons. The model predicts a Regge spectrum of linear trajectories with the
same slope in the leading orbital angular momentum L of hadrons and the radial
quantum number n. The hadron eigensolutions projected on the free Fock basis
provides the complete set of valence and non-valence light-front Fock state
wavefunctions which describe the hadron's momentum and spin distributions
needed to compute measures of hadron structure at the quark and gluon level.
The effective confining potential also creates quark- antiquark pairs. The
AdS/QCD model can be systematically improved by using its complete orthonormal
solutions to diagonalize the full QCD light-front Hamiltonian or by applying
the Lippmann-Schwinger method to systematically include the QCD interaction
terms. A new perspective on quark and gluon condensates is also presented.Comment: Presented at LIGHTCONE 2011, 23 - 27 May, 2011, Dallas, T
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