50 research outputs found
Heavy Quark Parton Distribution Functions
We present the CTEQ6HQ parton distribution set which is determined in the
general variable flavor number scheme which incorporates heavy flavor mass
effects; hence, this set provides advantages for precision observables which
are sensitive to charm and bottom quark masses. We describe the analysis
procedure, examine the predominant features of the new distributions, and
compare with previous distributions. We also examine the uncertainties of the
strange quark distribution and how the the recent NuTeV dimuon data constrains
this quantity.Comment: 5 pages, 2 figures; contribution to the XIII International Workshop
on Deep Inelastic Scattering (DIS 2005
QCD Hard Scattering and the Sign of the Spin Asymmetry A_LL^pi
Recent preliminary PHENIX data are consistent with a negative and sizable
longitudinal double-spin asymmetry A_LL^pi for pi^0 production at moderate
transverse momentum p_perp \simeq 1 - 4 GeV and central rapidity. By means of a
systematic investigation of the relevant degrees of freedom we show that the
perturbative QCD framework at leading power in p_perp produces at best a very
small negative asymmetry in this kinematic range.Comment: 4 pages, 3 figures, final version published in PRL (only minor
changes; note: title changed in published version
Open Heavy Flavor Production in QCD -- Conceptual Framework and Implementation Issues
Heavy flavor production is an important QCD process both in its own right and
as a key component of precision global QCD analysis. Apparent disagreements
between fixed-flavor scheme calculations of b-production rate with experimental
measurements in hadro-, lepto-, and photo-production provide new impetus to a
thorough examination of the theory and phenomenology of this process. We review
existing methods of calculation, and place them in the context of the general
PQCD framework of Collins. A distinction is drawn between scheme dependence and
implementation issues related to quark mass effects near threshold. We point
out a so far overlooked kinematic constraint on the threshold behavior, which
greatly simplifies the variable flavor number scheme. It obviates the need for
the elaborate existing prescriptions, and leads to robust predictions. It can
facilitate the study of current issues on heavy flavor production as well as
precision global QCD analysis.Comment: 13 pages, 10 figures, Proceedings of Ringberg Workshop: New Trends in
HERA Physics 2001, Munich, German
The Parton Structure of the Nucleon and Precision Determination of the Weinberg Angle in Neutrino Scattering
A recently completed next-to-leading-order program to calculate neutrino
cross sections, including power-suppressed mass correction terms, has been
applied to evaluate the Paschos-Wolfenstein relation, in order to
quantitatively assess the validity and significance of the NuTeV anomaly. In
particular, we study the shift of obtained in
calculations with a new generation of PDF sets that allow , enabled by recent neutrino dimuon data from CCFR and NuTeV, as
compared to the previous parton distribution functions like
CTEQ6M. The extracted value of is closely
correlated with the strangeness asymmetry momentum integral
. We also consider isospin violating effects
that have recently been explored by the MRST group. The results of our study
suggest that the new dimuon data, the Weinberg angle measurement, and other
data sets used in global QCD parton structure analysis can all be consistent
within the Standard Model.Comment: 4 page
Ultrahigh-Energy Neutrino-Nucleon Cross Sections and Perturbative Unitarity
Unitarity relates the total cross section for neutrino-nucleon scattering to
the neutrino-nucleon forward scattering amplitude. Assuming the validity of the
perturbative expansion of the forward amplitude in the {\em weak} coupling
constant, we derive a unitarity bound on the inelastic cross section. The
inelastic cross section saturates this bound at a typical neutrino energy
. This implies that calculations of the inelastic
cross section that use current parton distribution functions and lowest order
weak perturbation theory are unreliable above this energy.Comment: 11 pages, 3 figures, RevTeX, additional reference