4,318 research outputs found
Cross Sections for Charm Production in Collisions: Massive versus Massless Scheme
The next--to--leading order inclusive cross section for large-
photoproduction of charm quarks at HERA is calculated in two different
approaches. In the first approach the charm quarks are treated as massive
objects which are strictly external to the proton and the photon while in the
second approach the charm mass is neglected and the quark is assumed to be
one of the active flavours in the proton and photon structure functions. We
present single-inclusive distributions in transverse momentum and rapidity
including direct and resolved photons. The cross section in the massless
approach is found to be significantly larger than in the massive scheme. The
deviation originates from several contributions which are disentangled. We
argue that large- photoproduction of charm quarks at HERA will be
sensitive to the charm content of the photon structure function.Comment: 11 pages, Latex, epsfig, 6 figures appended as uuencoded file
(hardcopy can be obtained upon request from [email protected]
Magnetotransport of lanthanum doped RuSr2GdCu2O8 - the role of gadolinium
Strongly underdoped RuSr_1.9La_0.1GdCu_2O_8 has been comprehensively studied
by dc magnetization, microwave measurements, magnetoresistivity and Hall
resistivity in fields up to 9 T and temperatures down to 1.75 K. Electron
doping by La reduces the hole concentration in the CuO2 planes and completely
suppresses superconductivity. Microwave absorption, dc resistivity and ordinary
Hall effect data indicate that the carrier concentration is reduced and a
semiconductor-like temperature dependence is observed. Two magnetic ordering
transitions are observed. The ruthenium sublattice orders antiferromagnetically
at 155 K for low applied magnetic field and the gadolinium sublattice
antiferromagnetically orders at 2.8 K. The magnetoresistivity exhibits a
complicated temperature dependence due to the combination of the two magnetic
orderings and spin fluctuations. It is shown that the ruthenium magnetism
influences the conductivity in the RuO2 layers while the gadolinium magnetism
influences the conductivity in the CuO2 layers. The magnetoresistivity is
isotropic above 4 K, but it becomes anisotropic when gadolinium orders
antiferromagnetically.Comment: 7 pages, 9 figures, submitted to European Physical Journal
Dependence of the BEC transition temperature on interaction strength: a perturbative analysis
We compute the critical temperature T_c of a weakly interacting uniform Bose
gas in the canonical ensemble, extending the criterion of condensation provided
by the counting statistics for the uniform ideal gas. Using ordinary
perturbation theory, we find in first order , where T_c^0 is the transition temperature of the corresponding
ideal Bose gas, a is the scattering length, and is the particle number
density.Comment: 14 pages (RevTeX
Supersymmetric NLO QCD Corrections to Resonant Slepton Production and Signals at the Tevatron and the LHC
We compute the total cross section and the transverse momentum distribution
for single charged slepton and sneutrino production at hadronic colliders
including NLO supersymmetric and non-supersymmetric QCD corrections. The
supersymmetric QCD corrections can be substantial. We also resum the gluon
transverse momentum distribution and compare our results with two Monte Carlo
generators. We compute branching ratios of the supersymmetric decays of the
slepton and determine event rates for the like-sign dimuon final state at the
Tevatron and at the LHC.Comment: 14 pages, LaTeX, 8 figures, uses REVTex
Complementarity of Resonant Scalar, Vector-Like Quark and Superpartner Searches in Elucidating New Phenomena
The elucidation of the nature of new phenomena requires a multi-pronged
approach to understand the essential physics that underlies it. As an example,
we study the simplified model containing a new scalar singlet accompanied by
vector-like quarks, as motivated by the recent diphoton excess at the LHC. To
be specific, we investigate three models with -doublet, vector-like
quarks with Yukawa couplings to a new scalar singlet and which also couple
off-diagonally to corresponding Standard Model fermions of the first or third
generation through the usual Higgs boson. We demonstrate that three classes of
searches can play important and complementary roles in constraining this model.
In particular, we find that missing energy searches designed for superparticle
production, supply superior sensitivity for vector-like quarks than the
dedicated new quark searches themselves.Comment: References added; small bug found in model and analysis
implementation, numerical results slightly modified, conclusions unchange
Inclusive Photoproduction of D* Mesons with Massive Charm Quarks
We have calculated the next-to-leading order cross sections for the inclusive
production of D* mesons in gamma-p collisions at HERA in two approaches using
massive or massless charm quarks. The usual massive theory for the direct cross
section with charm quarks only in the final state was transformed into a
massive theory with MS-bar subtraction by subtracting the mass divergent and
additional finite terms calculated earlier in connection with the process
gamma+gamma -> D*+X. This theory approaches the massless theory with increasing
transverse momentum. The difference between the massive and the massless
approach with MS-bar subtraction is studied in detail in those kinematic
regions relevant for comparison with experimental data. With these results and
including the resolved cross section which is dominated by the part originating
from the charm in the photon, we compute the fully inclusive D* cross section
and compare it with preliminary data from the ZEUS collaboration at HERA. We
find on average good agreement.Comment: 18 pages, 9 figures, figures modified to include statistical and
systematic experimental error
Rotation of an atomic Bose-Einstein condensate with and without a quantized vortex
We theoretically examine the rotation of an atomic Bose-Einstein condensate
in an elliptical trap, both in the absence and presence of a quantized vortex.
Two methods of introducing the rotating potential are considered -
adiabatically increasing the rotation frequency at fixed ellipticity, and
adiabatically increasing the trap ellipticity at fixed rotation frequency.
Extensive simulations of the Gross-Pitaevskii equation are employed to map out
the points where the condensate becomes unstable and ultimately forms a vortex
lattice. We highlight the key features of having a quantized vortex in the
initial condensate. In particular, we find that the presence of the vortex
causes the instabilities to shift to lower or higher rotation frequencies,
depending on the direction of the vortex relative to the trap rotation.Comment: 15 pages, 8 figure
Color-Octet Contributions to Photoproduction
We have calculated the leading color-octet contributions to the production of
particles in photon-proton collisions. Using the values for the
color-octet matrix elements extracted from fits to prompt data at the
Tevatron, we demonstrate that distinctive color-octet signatures should be
visible in photoproduction. However, these predictions appear at
variance with recent experimental data obtained at HERA, indicating that the
phenomenological importance of the color-octet contributions is smaller than
expected from theoretical considerations and suggested by the Tevatron fits.Comment: 10 pages, LaTeX, epsfig, 4 figure
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