364 research outputs found
Quantum Phase Fluctuations Responsible for Pseudogap
The effect of ordering field phase fluctuations on the normal and
superconducting properties of a simple 2D model with a local four-fermion
attraction is studied. Neglecting the coupling between the spin and charge
degrees of freedom an analytical expression has been obtained for the fermion
spectral function as a single integral over a simple function. From this we
show that, as the temperature increases through the 2D critical temperature and
a nontrivial damping for a phase correlator develops, quantum fluctuations fill
the gap in the quasiparticle spectrum. Simultaneously the quasiparticle peaks
broaden significantly above the critical temperature, resembling the observed
pseudogap behavior in high-T_c superconductors.Comment: 5 pages, ReVTeX, 1 EPS figure; final version to appear in Physica
The low-energy phase-only action in a superconductor: a comparison with the XY model
The derivation of the effective theory for the phase degrees of freedom in a
superconductor is still, to some extent, an open issue. It is commonly assumed
that the classical XY model and its quantum generalizations can be exploited as
effective phase-only models. In the quantum regime, however, this assumption
leads to spurious results, such as the violation of the Galilean invariance in
the continuum model. Starting from a general microscopic model, in this paper
we explicitly derive the effective low-energy theory for the phase, up to
fourth-order terms. This expansion allows us to properly take into account
dynamic effects beyond the Gaussian level, both in the continuum and in the
lattice model. After evaluating the one-loop correction to the superfluid
density we critically discuss the qualitative and quantitative differences
between the results obtained within the quantum XY model and within the correct
low-energy theory, both in the case of s-wave and d-wave symmetry of the
superconducting order parameter. Specifically, we find dynamic anharmonic
vertices, which are absent in the quantum XY model, and are crucial to restore
Galilean invariance in the continuum model. As far as the more realistic
lattice model is concerned, in the weak-to-intermediate-coupling regime we find
that the phase-fluctuation effects are quantitatively reduced with respect to
the XY model. On the other hand, in the strong-coupling regime we show that the
correspondence between the microscopically derived action and the quantum XY
model is recovered, except for the low-density regime.Comment: 29 pages, 11 figures. Slightly revised presentation, accepted for
publication in Phys. Rev.
Au+Au Reactions at the AGS: Experiments E866 and E917
Particle production and correlation functions from Au+Au reactions have been
measured as a function of both beam energy (2-10.7AGeV) and impact parameter.
These results are used to probe the dynamics of heavy-ion reactions, confront
hadronic models over a wide range of conditions and to search for the onset of
new phenomena.Comment: 12 pages, 14 figures, Talk presented at Quark Matter '9
Search for a W' boson decaying to a bottom quark and a top quark in pp collisions at sqrt(s) = 7 TeV
Results are presented from a search for a W' boson using a dataset
corresponding to 5.0 inverse femtobarns of integrated luminosity collected
during 2011 by the CMS experiment at the LHC in pp collisions at sqrt(s)=7 TeV.
The W' boson is modeled as a heavy W boson, but different scenarios for the
couplings to fermions are considered, involving both left-handed and
right-handed chiral projections of the fermions, as well as an arbitrary
mixture of the two. The search is performed in the decay channel W' to t b,
leading to a final state signature with a single lepton (e, mu), missing
transverse energy, and jets, at least one of which is tagged as a b-jet. A W'
boson that couples to fermions with the same coupling constant as the W, but to
the right-handed rather than left-handed chiral projections, is excluded for
masses below 1.85 TeV at the 95% confidence level. For the first time using LHC
data, constraints on the W' gauge coupling for a set of left- and right-handed
coupling combinations have been placed. These results represent a significant
improvement over previously published limits.Comment: Submitted to Physics Letters B. Replaced with version publishe
Search for sterile neutrino oscillation using RENO and NEOS data
We present a reactor model independent search for sterile neutrino
oscillation using 2\,509\,days of RENO near detector data and 180 days of NEOS
data. The reactor related systematic uncertainties are significantly suppressed
as both detectors are located at the same reactor complex of Hanbit Nuclear
Power Plant. The search is performed by electron
antineutrino\,() disappearance between six reactors and two
detectors with baselines of 294\,m\,(RENO) and 24\,m\,(NEOS). A spectral
comparison of the NEOS prompt-energy spectrum with a no-oscillation prediction
from the RENO measurement can explore reactor oscillations
to sterile neutrino. Based on the comparison, we obtain a 95\% C.L. excluded
region of \,eV. We also obtain a 68\% C.L. allowed
region with the best fit of \,eV and
=0.080.03 with a p-value of 8.2\%. Comparisons of
obtained reactor antineutrino spectra at reactor sources are made among RENO,
NEOS, and Daya Bay to find a possible spectral variation.Comment: 6 pages, 5 figures: This manuscript has been significantly revised by
the joint reanalysis by RENO and NEOS Collaborations. (In the previous
edition, the RENO collaboration used publicly available NEOS data to evaluate
the expected neutrino spectrum at NEOS.
Measurement of the Lambda(b) cross section and the anti-Lambda(b) to Lambda(b) ratio with Lambda(b) to J/Psi Lambda decays in pp collisions at sqrt(s) = 7 TeV
The Lambda(b) differential production cross section and the cross section
ratio anti-Lambda(b)/Lambda(b) are measured as functions of transverse momentum
pt(Lambda(b)) and rapidity abs(y(Lambda(b))) in pp collisions at sqrt(s) = 7
TeV using data collected by the CMS experiment at the LHC. The measurements are
based on Lambda(b) decays reconstructed in the exclusive final state J/Psi
Lambda, with the subsequent decays J/Psi to an opposite-sign muon pair and
Lambda to proton pion, using a data sample corresponding to an integrated
luminosity of 1.9 inverse femtobarns. The product of the cross section times
the branching ratio for Lambda(b) to J/Psi Lambda versus pt(Lambda(b)) falls
faster than that of b mesons. The measured value of the cross section times the
branching ratio for pt(Lambda(b)) > 10 GeV and abs(y(Lambda(b))) < 2.0 is 1.06
+/- 0.06 +/- 0.12 nb, and the integrated cross section ratio for
anti-Lambda(b)/Lambda(b) is 1.02 +/- 0.07 +/- 0.09, where the uncertainties are
statistical and systematic, respectively.Comment: Submitted to Physics Letters
Search for new physics in events with opposite-sign leptons, jets, and missing transverse energy in pp collisions at sqrt(s) = 7 TeV
A search is presented for physics beyond the standard model (BSM) in final
states with a pair of opposite-sign isolated leptons accompanied by jets and
missing transverse energy. The search uses LHC data recorded at a
center-of-mass energy sqrt(s) = 7 TeV with the CMS detector, corresponding to
an integrated luminosity of approximately 5 inverse femtobarns. Two
complementary search strategies are employed. The first probes models with a
specific dilepton production mechanism that leads to a characteristic kinematic
edge in the dilepton mass distribution. The second strategy probes models of
dilepton production with heavy, colored objects that decay to final states
including invisible particles, leading to very large hadronic activity and
missing transverse energy. No evidence for an event yield in excess of the
standard model expectations is found. Upper limits on the BSM contributions to
the signal regions are deduced from the results, which are used to exclude a
region of the parameter space of the constrained minimal supersymmetric
extension of the standard model. Additional information related to detector
efficiencies and response is provided to allow testing specific models of BSM
physics not considered in this paper.Comment: Replaced with published version. Added journal reference and DO
Strengthening mechanisms in thermomechanically processed NbTi-microalloyed steel
The effect of deformation temperature on microstructure and mechanical properties was investigated for thermomechanically processed NbTi-microalloyed steel with ferrite-pearlite microstructure. With a decrease in the finish deformation temperature at 1348 K to 1098 K (1075 °C to 825 °C) temperature range, the ambient temperature yield stress did not vary significantly, work hardening rate decreased, ultimate tensile strength decreased, and elongation to failure increased. These variations in mechanical properties were correlated to the variations in microstructural parameters (such as ferrite grain size, solid solution concentrations, precipitate number density and dislocation density). Calculations based on the measured microstructural parameters suggested the grain refinement, solid solution strengthening, precipitation strengthening, and work hardening contributed up to 32 pct, up to 48 pct, up to 25 pct, and less than 3 pct to the yield stress, respectively. With a decrease in the finish deformation temperature, both the grain size strengthening and solid solution strengthening increased, the precipitation strengthening decreased, and the work hardening contribution did not vary significantly
Updated Nucleosynthesis Constraints on Unstable Relic Particles
We revisit the upper limits on the abundance of unstable massive relic
particles provided by the success of Big-Bang Nucleosynthesis calculations. We
use the cosmic microwave background data to constrain the baryon-to-photon
ratio, and incorporate an extensively updated compilation of cross sections
into a new calculation of the network of reactions induced by electromagnetic
showers that create and destroy the light elements deuterium, he3, he4, li6 and
li7. We derive analytic approximations that complement and check the full
numerical calculations. Considerations of the abundances of he4 and li6 exclude
exceptional regions of parameter space that would otherwise have been permitted
by deuterium alone. We illustrate our results by applying them to massive
gravitinos. If they weigh ~100 GeV, their primordial abundance should have been
below about 10^{-13} of the total entropy. This would imply an upper limit on
the reheating temperature of a few times 10^7 GeV, which could be a potential
difficulty for some models of inflation. We discuss possible ways of evading
this problem.Comment: 40 pages LaTeX, 18 eps figure
Use of SMS texts for facilitating access to online alcohol interventions: a feasibility study
A41 Use of SMS texts for facilitating access to online alcohol interventions: a feasibility study
In: Addiction Science & Clinical Practice 2017, 12(Suppl 1): A4
- âŚ