8,439 research outputs found
The Small-x Behaviour of the Singlet Polarized Structure Function g_2 in the Double Logarithmic Approximation
The small-x behavior of the singlet contributions to the polarized structure
function g_2(x,Q^2) is calculated in the double-logarithmic approximation of
perturbative QCD. The dominant contribution is due to the gluons which, in
contrast to the unpolarized case, mix with the fermions also in the small-x
domain. We find a power-like growth in 1/x in the odd-signature parts of the
amplitude with the same power as in the singlet function g_1(x,Q^2) at x<< 1.Comment: 20 pages, latex, 3 ps figure
Exclusive diffractive processes at HERA within the dipole picture
We present a simultaneous analysis, within an impact parameter dependent
saturated dipole model, of exclusive diffractive vector meson (J/psi, phi and
rho) production, deeply virtual Compton scattering and the total gamma* p cross
section data measured at HERA. Various cross sections measured as a function of
the kinematic variables Q^2, W and t are well described, with little
sensitivity to the details of the vector meson wave functions. We determine the
properties of the gluon density in the proton in both longitudinal and
transverse dimensions, including the impact parameter dependent saturation
scale. The overall success of the description indicates universality of the
emerging gluon distribution and proton shape.Comment: 48 pages, 28 figures, the final version to appear in Physical Review
The Process \gamma^{*}_L+ q \to q\bar{q}g + q: Real Corrections to the Virtual Photon Impact Factor
We calculate, for the longitudinally polarized virtual photon, the cross
section of the process \gamma^{*}+q\to (q\bar{q}g)+q at high energies with a
large rapidity gap between the fragmentation system q\bar{q}g and the other
quark. This process provides the real corrections of the virtual photon impact
factor in the next-to leading order. Evidence is given for the appearance of a
new q\bar{q}g Fock-component of the photon state.Comment: 17 pages, 4 figures, revte
Inclusive production of a pair of hadrons separated by a large interval of rapidity in proton collisions
We consider within QCD collinear factorization the inclusive process , where the pair of identified hadrons, , having large
transverse momenta is produced in high-energy proton-proton collisions. In
particular, we concentrate on the kinematics where the two identified hadrons
in the final state are separated by a large interval of rapidity . In
this case the (calculable) hard part of the reaction receives large higher
order corrections . We provide a theoretical input
for the resummation of such contributions with next-to-leading logarithmic
accuracy (NLA) in the BFKL approach. Specifically, we calculate in NLA the
vertex (impact-factor) for the inclusive production of the identified hadron.
This process has much in common with the widely discussed Mueller-Navelet jets
production and can be also used to access the BFKL dynamics at proton
colliders. Another application of the obtained identified-hadron vertex could
be the NLA BFKL description of inclusive forward hadron production in DIS.Comment: 29 pages, 9 figures; corrected few typos and added an acknowledgment;
version to be published on JHEP. arXiv admin note: substantial text overlap
with arXiv:1202.108
The dipole representation of vector meson electroproduction beyond leading twist
We link the recent computation beyond leading twist of the impact factor of
the transition gamma*T -> rhoT performed in the light-cone collinear approach,
to the dipole picture by expressing the hard part of the process through its
Fourier transform in coordinate space. We show that in the Wandzura-Wilczek
approximation the impact factor up to twist 3 factorises in the wave function
of the photon combined with the distribution amplitudes of the rho-meson and
the colour dipole scattering amplitude with the t-channel gluons. We show also
that beyond the Wandzura-Wilczek approximation, the hard contribution of the
amplitude still exhibits the signature of the interaction of a single colour
dipole with the t-channel gluons. This result allows a phenomenological
approach of the helicity amplitudes of the leptoproduction of vector meson, by
combining our results to a dipole/target scattering amplitude model.Comment: 63 pages, 18 figure
NLO Corrections to the \gamma* Impact Factor: First Numerical Results for the Real Corrections to \gamma*_L
We analytically perform the transverse momentum integrations in the real
corrections to the longitudinal \gamma*_L impact factor. The resulting
integrals are Feynman parameter integrals, and we provide a MATHEMATICA file
which contains the integrands. The remaining integrals are carried out
numerically. We perform a numerical test, and we compute those parts of the
impact factor which depend upon the energy scale s_0: they are found to be
negative and, with decreasing values of s_0, their absolute value increases.Comment: 27 pages, 5 figures, uses revtex4; misprint in fig.4,5 and typos
corrected; for associated files, see
http://www.desy.de/uni-th/smallx/kyrie/ImpFactor/index.htm
Geometric Scaling in Inclusive Charm Production
We show that the cross section for inclusive charm production exhibits
geometric scaling in a large range of photon virtualities. In the HERA
kinematic domain the saturation momentum stays below the hard
scale , implying charm production probing mostly the color
transparency regime and unitarization effects being almost negligible. We
derive our results considering two saturation models which are able to describe
the DESY ep collider HERA data for the proton structure function at small
values of the Bjorken variable . A striking feature is the scaling on
above saturation limit, corroborating recent
theoretical studies.Comment: 4 pages, 2 figures. Version to be published in Physical Review
Letter
Astronomical spectrograph calibration with broad-spectrum frequency combs
Broadband femtosecond-laser frequency combs are filtered to
spectrographically resolvable frequency-mode spacing, and the limitations of
using cavities for spectral filtering are considered. Data and theory are used
to show implications to spectrographic calibration of high-resolution,
astronomical spectrometers
Ultralow phase noise microwave generation with an Er:fiber-based optical frequency divider
We present an optical frequency divider based on a 200 MHz repetition rate
Er:fiber mode-locked laser that, when locked to a stable optical frequency
reference, generates microwave signals with absolute phase noise that is equal
to or better than cryogenic microwave oscillators. At 1 Hz offset from a 10 GHz
carrier, the phase noise is below -100 dBc/Hz, limited by the optical
reference. For offset frequencies > 10 kHz, the phase noise is shot noise
limited at -145 dBc/Hz. An analysis of the contribution of the residual noise
from the Er:fiber optical frequency divider is also presented.Comment: 4 pages, 3 figure
Unifying approach to hard diffraction
We find a consistency between two different approaches of hard diffraction,
namely the QCD dipole model and the Soft Colour Interaction approach. A
theoretical interpretation in terms of S-Matrix and perturbative QCD properties
in the small regime is proposed.Comment: 4pages, 1 figure, letter submitted for publicatio
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