113 research outputs found
STAR Barrel Electromagnetic Calorimeter Absolute Calibration Using "Minimum Ionizing Particles" from Collisions at RHIC
The procedure for the STAR Barrel Electromagnetic Calorimeter (BEMC) absolute
calibrations, using penetrating charged particle hits (MIP-hits) from physics
events at RHIC, is presented. Its systematic and statistical errors are
evaluated. It is shown that, using this technique, the equalization and
transfer of the absolute scale from the test beam can be done to a percent
level accuracy in a reasonable amount of time for the entire STAR BEMC.
MIP-hits would also be an effective tool for continuously monitoring the
variations of the BEMC tower's gains, virtually without interference to STAR's
main physics program. The method does not rely on simulations for anything
other than geometric and some other small corrections, and also for estimations
of the systematic errors. It directly transfers measured test beam responses to
operations at RHIC.Comment: 19 pages, 7 figures, Accepted to NIM-
High-field paramagnetic effect in large crystals of YBa2Cu3O7-δ
We observed the appearance of a paramagnetic moment in YBa2Cu3O7-δ single crystals after cooling in a strong field (3–7 T). It is found that the effect depends on the cooling rate and sample size, indicating that the paramagnetic moment can be induced by compression of the magnetic flux in the course of rapid cooling. After rapid cooling, the temperature dependence of the magnetization during the field-cooled warming process exhibits a very articulated negative dip between irreversibility temperature Tirr and Tc. The diamagnetic dip may result from escaping the compressed flux. Comparing the dip widths for a number of crystals with different irreversibility lines Hirr(T) we found that a remarkable ``fishtail'' effect appears in the temperature region below the dip. This suggests that the field-induced pinning observed by the fishtail effect is linked with the paramagnetic effect that appears after rapid cooling in the high field
The vortex depinning transition in untwinned YBaCuO using complex impedance measurements
We present surface impedance measurement of the vortex linear response in a
large untwinned YBCO crystal. The depinning spectra obtained over a broad
frequency range (100 Hz- 30 MHz) are those of a surface pinned vortex lattice
with a free flux flow resistivity (two modes response). The critical current in
the "Campbell" like regime and the flux flow resistivity in the dissipative
regime are extracted. Those two parameters are affected by the first order
transition, showing that this transition may be related to the electronic state
of vortices.Comment: to be published in the proceedings of M2S RI
Change in the order of the melting transition with oxygen content in YBa2Cu3O7-δ
The vortex phase transition was systematically studied in twin-free YBa2Cu3O7-δ for various δ. For 7-δ>~6.94, the first order transition was evidenced by an abrupt jump and a hysteresis of magnetization in the field-cooled (FC) cooling-warming cycles, which indicates the coexistence of vortex crystallites and liquid. For 7-δ<6.89 the jump was not sharp and FC magnetization was reversible, suggesting a second order transition. The first and the second order transitions merge at a critical point (CP) which goes to zero field for 7-δ<6.89. Three-dimensional XY scaling satisfactorily fits the data at 1–7 T for 7-δ<6.94 and for 7-δ<6.89, but does not fit for 6.89<7-δ<6.94, where the CP passes through this field range
The origin of paramagnetic magnetization in field-cooled YBa2Cu3O7 films
Temperature dependences of the magnetic moment have been measured in
YBa_2Cu_3O_{7-\delta} thin films over a wide magnetic field range (5 <= H <=
10^4 Oe). In these films a paramagnetic signal known as the paramagnetic
Meissner effect has been observed. The experimental data in the films, which
have strong pinning and high critical current densities (J_c ~ 2 \times 10^6
A/cm^2 at 77 K), are quantitatively shown to be highly consistent with the
theoretical model proposed by Koshelev and Larkin [Phys. Rev. B 52, 13559
(1995)]. This finding indicates that the origin of the paramagnetic effect is
ultimately associated with nucleation and inhomogeneous spatial redistribution
of magnetic vortices in a sample which is cooled down in a magnetic field. It
is also shown that the distribution of vortices is extremely sensitive to the
interplay of film properties and the real experimental conditions of the
measurements.Comment: RevTex, 8 figure
Hysteretic behavior of the vortex lattice at the onset of the second peak for HgBaCuO superconductor
By means of local Hall probe ac and dc permeability measurements we
investigated the phase diagram of vortex matter for the HgBaCuO superconductor in the regime near the critical temperature. The second peak
line, , in contrast to what is usually assumed, doesn't terminate
at the critical temperature. Our local ac permeability measurements revealed
pronounced hysteretic behavior and thermomagnetic history effects near the
onset of the second peak, giving evidence for a phase transition of vortex
matter from an ordered qausilattice state to a disordered glass
Demonstration of the temporal matter-wave Talbot effect for trapped matter waves
We demonstrate the temporal Talbot effect for trapped matter waves using
ultracold atoms in an optical lattice. We investigate the phase evolution of an
array of essentially non-interacting matter waves and observe matter-wave
collapse and revival in the form of a Talbot interference pattern. By using
long expansion times, we image momentum space with sub-recoil resolution,
allowing us to observe fractional Talbot fringes up to 10th order.Comment: 17 pages, 7 figure
Kaon Production and Kaon to Pion Ratio in Au+Au Collisions at \snn=130 GeV
Mid-rapidity transverse mass spectra and multiplicity densities of charged
and neutral kaons are reported for Au+Au collisions at \snn=130 GeV at RHIC.
The spectra are exponential in transverse mass, with an inverse slope of about
280 MeV in central collisions. The multiplicity densities for these particles
scale with the negative hadron pseudo-rapidity density. The charged kaon to
pion ratios are and
for the most central collisions. The ratio is lower than the same
ratio observed at the SPS while the is higher than the SPS result.
Both ratios are enhanced by about 50% relative to p+p and +p
collision data at similar energies.Comment: 6 pages, 3 figures, 1 tabl
Mid-rapidity anti-proton to proton ratio from Au+Au collisions at GeV
We report results on the ratio of mid-rapidity anti-proton to proton yields
in Au+Au collisions at \rts = 130 GeV per nucleon pair as measured by the
STAR experiment at RHIC. Within the rapidity and transverse momentum range of
and 0.4 1.0 GeV/, the ratio is essentially independent of
either transverse momentum or rapidity, with an average of for minimum bias collisions. Within errors, no
strong centrality dependence is observed. The results indicate that at this
RHIC energy, although the -\pb pair production becomes important at
mid-rapidity, a significant excess of baryons over anti-baryons is still
present.Comment: 5 pages, 3 figures, accepted by Phys. Rev. Let
Strange anti-particle to particle ratios at mid-rapidity in sqrt(s_NN)= 130 GeV Au+Au Collisions
Values of the ratios in the mid-rapidity yields of anti-Lambda/Lambda = 0.71
+/- 0.01(stat.) +/- 0.04(sys.), anti-Xi+/Xi- = 0.83 +/- 0.04(stat.) +/- 0.05
(sys.), anti-Omega+/Omega- = 0.95 +/- 0.15(stat) +/- 0.05(sys.) and K+/K- 1.092
+/- 0.023(combined) were obtained in central sqrt(s_NN) = 130 GeV Au+Au
collisions using the STAR detector. The ratios indicate that a fraction of the
net-baryon number from the initial system is present in the excess of hyperons
over anti-hyperons at mid-rapidity. The trend in the progression of the baryon
ratios, with increasing strange quark content, is similar to that observed in
heavy-ion collisions at lower energies. The value of these ratios may be
related to the charged kaon ratio in the framework of simple quark-counting and
thermal models.Comment: 6 pages, 3 figures, revtex4, now accepted by Physics Letters B. All
figures improved for clarity, fig. 2 now has kaon ratio separated by
technique, fig. 3 now has additional other RHIC data points. Minor
clarifications in text in response to referee comments. Updated ref
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