60 research outputs found
Future Small x Physics with ep and eA Colliders
The interaction of spatially small dipoles with nucleons, nuclei is
calculated in the DGLAP approximation at the top of HERA energies and found to
be close to the -channel unitarity limit in the case of the color octet
dipoles. The DGLAP analyses of the current diffractive data appear to support
this conclusion as they indicate a probability of the gluon
induced diffraction for GeV. The need for the high-precision
measurements of the -dependence of inclusive and exclusive diffraction for
pinpointing higher twist effects in the gluon sector is emphasized.The
collisions at HERA would provide a strong amplification of the gluon densities
allowing to reach deep into the regime of nonlinear QCD evolution. Connection
between the leading twist nuclear shadowing and leading twist diffraction in
scattering is explained. The presented model independent results for the
nuclear shadowing for light nuclei indicate much larger shadowing for the gluon
sector than for the sea quark sector.It is argued that HERA in mode would
be able to discover a number of new phenomena including large gluon shadowing,
large nonlinearities in parton evolution, small color transparency in the
vector meson production followed by color opacity at , large
probability of inclusive diffraction. Implications for the nucleus-nucleus
collisions at LHC are discussed as well.Comment: 17 pp, 12 fig. Plenary talk to be published in the proceedings of 7th
International Workshop on Deep Inelastic Scattering and QCD (DIS 99),
Zeuthen, Germany, 19-23 Apr 1999. Submitted to Nucl.Phys.Proc.Supp
Soft Gluon Approach for Diffractive Photoproduction of J/psi
We study diffractive photoproduction of by taking the charm quark as
a heavy quark. A description of nonperturbative effect related to can
be made by using NRQCD. In the forward region of the kinematics, the
interaction between the -pair and the initial hadron is due to
exchange of soft gluons. The effect of the exchange can be studied by using the
expansion in the inverse of the quark mass . At the leading order we find
that the nonperturbative effect related to the initial hadron is represented by
a matrix element of field strength operators, which are separated in the moving
direction of in the space-time. The S-matrix element is then obtained
without using perturbative QCD and the results are not based on any model.
Corrections to the results can be systematically added. Keeping the dominant
contribution of the S-matrix element in the large energy limit we find that the
imaginary part of the S-matrix element is related to the gluon distribution for
with a reasonable assumption, the real part can be obtained with
another approximation or with dispersion relation. Our approach is different
than previous approaches and also our results are different than those in these
approaches. The differences are discussed in detail. A comparison with
experiment is also made and a qualitative agreement is found.Comment: 25 pages, 6 figures. Tiny changes in two figures, conclusion and text
unchanged, accpeted by Nucl. Phys.
Non-equilibrium initial conditions from pQCD for RHIC and LHC
We calculate the initial non-equilibrium conditions from perturbative QCD
(pQCD) within Glauber multiple scattering theory for AGeV and
ATeV. At the soon available collider energies one will
particularly test the small region of the parton distributions entering the
cross sections. Therefore shadowing effects, previously more or less
unimportant, will lead to new effects on variables such as particle
multiplicities , transverse energy production , and the
initial temperature . In this paper we will have a closer look on the
effects of shadowing by employing different parametrizations for the shadowing
effect for valence quarks, sea quarks and gluons. Since the cross sections at
midrapidity are dominated by processes involving gluons the amount of their
depletion is particularly important. We will therefore have a closer look on
the results for , , and by using two different
gluon shadowing ratios, differing strongly in size. As a matter of fact, the
calculated quantities differ significantly.Comment: typo in ref's removed, ack's added, no change in result
Study of High Energy Heavy-Ion Collisions in a Relativistic BUU-Approach with Momentum-Dependent Mean-Fields
We introduce momentum-dependent scalar and vector fields into the Lorentz
covariant relativistic BUU- (RBUU-) approach employing a polynomial ansatz for
the relativistic nucleon-nucleon interaction. The momentum-dependent
parametrizations are shown to be valid up to about 1 GeV/u for the empirical
proton-nucleus optical potential. We perform numerical simulations for
heavy-ion collisions within the RBUU-approach adopting momentum-dependent and
momentum-independent mean-fields and calculate the transverse flow in and
perpendicular to the reaction plane, the directivity distribution as well as
subthreshold K+-production. By means of these observables we discuss the
particular role of the momentum-dependent forces and their implications on the
nuclear equation of state. We find that only a momentum-dependent parameter-set
can explain the experimental data on the transverse flow in the reaction plane
from 150 - 1000 MeV/u and the differential K+-production cross sections at 1
GeV/u at the same time.Comment: 27 pages, figures can be obtained from the authors, UGI-93-0
Mathematical stories: Why do more boys than girls choose to study mathematics at AS-level in England?
Copyright @ 2005 Taylor & FrancisIn this paper I address the question: How is it that people come to choose mathematics and in what ways is this process gendered? I draw on the findings of a qualitative research study involving interviews with 43 young people all studying mathematics in post-compulsory education in England. Working within a post-structuralist framework, I argue that gender is a project and one that is achieved in interaction with others. Through a detailed reading of Toni and Claudiaâs stories I explore the tensions for young women who are engaging in mathematics, something that is discursively inscribed as masculine, while (understandably) being invested in producing themselves as female. I conclude by arguing that seeing âdoing mathematicsâ as âdoing masculinityâ is a productive way of understanding why mathematics is so male dominated and by looking at the implications of this understanding for gender and mathematics reform work.This work is funded by the ESR
Collective modes of asymmetric nuclear matter in Quantum HadroDynamics
We discuss a fully relativistic Landau Fermi liquid theory based on the
Quantum Hadro-Dynamics () effective field picture of Nuclear Matter
({\it NM}).
From the linearized kinetic equations we get the dispersion relations of the
propagating collective modes. We focus our attention on the dynamical effects
of the interplay between scalar and vector channel contributions. A beautiful
``mirror'' structure in the form of the dynamical response in the
isoscalar/isovector degree of freedom is revealed, with a complete parallelism
in the role respectively played by the compressibility and the symmetry energy.
All that strongly supports the introduction of an explicit coupling to the
scalar-isovector channel of the nucleon-nucleon interaction. In particular we
study the influence of this coupling (to a -meson-like effective field)
on the collective response of asymmetric nuclear matter (). Interesting
contributions are found on the propagation of isovector-like modes at normal
density and on an expected smooth transition to isoscalar-like oscillations at
high baryon density. Important ``chemical'' effects on the neutron-proton
structure of the mode are shown. For dilute we have the isospin
distillation mechanism of the unstable isoscalar-like oscillations, while at
high baryon density we predict an almost pure neutron wave structure of the
propagating sounds.Comment: 18 pages (LATEX), 8 Postscript figures, uses "epsfig
Relating parton model and color dipole formulation of heavy quark hadroproduction
At high center of mass energies, hadroproduction of heavy quarks can be
expressed in terms of the same color dipole cross section as low Bjorken-x deep
inelastic scattering. We show analytically that at leading order, the dipole
formulation is equivalent to the gluon-gluon fusion mechanism of the
conventional parton model. In phenomenological application, we employ a
parameterization of the dipole cross section which also includes higher order
and saturation effects, thereby going beyond the parton model. Numerical
calculations in the dipole approach agree well with experimental data on open
charm production over a wide range of energy. Dipole approach and next to
leading order parton model yield similar values for open charm production, but
for open bottom production, the dipole approach tends to predict somewhat
higher cross sections than the parton model.Comment: 16 pages, 4 figure
Color coherent phenomena on nuclei and the QCD evolution equation
We review the phenomenon of color coherence in quantum chromodynamics (QCD),
its implications for hard and soft processes with nuclei, and its experimental
manifestations. The relation of factorization theorems in QCD with color
coherence phenomena in deep inelastic scattering (DIS) and color coherence
phenomena in hard exclusive processes is emphasized. Analyzing numerically the
QCD evolution equation for conventional and skewed parton densities in nuclei,
we study the onset of generalized color transparency and nuclear shadowing of
the sea quark and gluon distributions in nuclei as well as related phenomena.
Such novel results as the dependence of the effective coherence length on
and general trends of the QCD evolution are discussed. The limits of the
applicability of the QCD evolution equation at small Bjorken are estimated
by comparing the inelastic quark-antiquark- and two gluon-nucleon (nucleus)
cross sections, calculated within the DGLAP approximation, with the dynamical
boundaries, which follow from the unitarity of the matrix for purely QCD
interactions. We also demonstrate that principles of color coherence play an
important role in the processes of soft diffraction off nuclei.Comment: 58 pages, 19 figures, Revtex. Minor editor's changes, final version
published in J.Phys. G27 (2001) R23-6
Coherent QCD phenomena in the Coherent Pion-Nucleon and Pion-Nucleus Production of Two Jets at High Relative Momenta
We use QCD to compute the cross section for coherent production of a di-jet
(treated as a moving at high relative transverse momentum,). In the target rest frame,the space-time evolution of this reaction is
dominated by the process in which the high component of
the pion wave function is formed before reaching the target. It then interacts
through two gluon exchange. In the approximation of keeping the leading order
in powers of and all orders in
the amplitudes for other processes are
shown to be smaller at least by a power of . The resulting dominant
amplitude is proportional to ( is the fraction
light-cone(+)momentum carried by the quark in the final state) times the skewed
gluon distribution of the target. For the pion scattering by a nuclear target,
this means that at fixed (but ) the nuclear process in which there is only a single interaction is the
most important one to contribute to the reaction. Thus in this limit color
transparency phenomena should occur.These findings are in accord with E971
experiment at FNAL. We also re-examine a potentially important nuclear multiple
scattering correction which is positive and . The
meaning of the signal obtained from the experimental measurement of pion
diffraction into two jets is also critically examined and significant
corrections are identified.We show also that for values of achieved
at fixed target energies, di-jet production by the e.m. field of the nucleus
leads to an insignificant correction which gets more important as
increases.Comment: 23 pages, 9 figure
Employability and studentsâ part-time work in the UK: does self-efficacy and career aspiration matter?
Amidst a growing focus on graduate employability, this study examines the relationship between students' part-time work, career aspirations and self-efficacy, in a survey of 357 UK students from two post-92 universities. The results suggest a positive and significant relationship between part-time work and career aspiration. Students who work part-time, and value this opportunity, are likely to have a high career aspiration and strive to enhance their employability agenda. Self-efficacy (students' belief in their ability to succeed) is significantly associated with career aspiration. No significant gender differences were found in our sample for all study variables. Finally, students' level of study and malleable self-theories (the belief that people are changeable and with effort can achieve more) were found to be the strongest predictors of part-time work, while self-efficacy is the strongest predictor of career aspiration. These findings confirm the importance of individual self-efficacy in the value attached to part-time working among students in higher education (HE). Nonetheless, those students who do not work part-time whilst studying, do so mainly because they do not want to detract from their study. The concluding part of the paper discusses relevant application and policy implications of these findings
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