384 research outputs found
New measurement of the scattering cross section of slow neutrons on liquid parahydrogen from neutron transmission
Liquid hydrogen is a dense Bose fluid whose equilibrium properties are both
calculable from first principles using various theoretical approaches and of
interest for the understanding of a wide range of questions in many body
physics. Unfortunately, the pair correlation function inferred from
neutron scattering measurements of the differential cross section from different measurements reported in the literature are
inconsistent. We have measured the energy dependence of the total cross section
and the scattering cross section for slow neutrons with energies between
0.43~meV and 16.1~meV on liquid hydrogen at 15.6~K (which is dominated by the
parahydrogen component) using neutron transmission measurements on the hydrogen
target of the NPDGamma collaboration at the Spallation Neutron Source at Oak
Ridge National Laboratory. The relationship between the neutron transmission
measurement we perform and the total cross section is unambiguous, and the
energy range accesses length scales where the pair correlation function is
rapidly varying. At 1~meV our measurement is a factor of 3 below the data from
previous work. We present evidence that these previous measurements of the
hydrogen cross section, which assumed that the equilibrium value for the ratio
of orthohydrogen and parahydrogen has been reached in the target liquid, were
in fact contaminated with an extra non-equilibrium component of orthohydrogen.
Liquid parahydrogen is also a widely-used neutron moderator medium, and an
accurate knowledge of its slow neutron cross section is essential for the
design and optimization of intense slow neutron sources. We describe our
measurements and compare them with previous work.Comment: Edited for submission to Physical Review
The Nab Experiment: A Precision Measurement of Unpolarized Neutron Beta Decay
Neutron beta decay is one of the most fundamental processes in nuclear
physics and provides sensitive means to uncover the details of the weak
interaction. Neutron beta decay can evaluate the ratio of axial-vector to
vector coupling constants in the standard model, , through
multiple decay correlations. The Nab experiment will carry out measurements of
the electron-neutrino correlation parameter with a precision of and the Fierz interference term to
in unpolarized free neutron beta decay. These results, along with a more
precise measurement of the neutron lifetime, aim to deliver an independent
determination of the ratio with a precision of that will allow an evaluation of and sensitively
test CKM unitarity, independent of nuclear models. Nab utilizes a novel, long
asymmetric spectrometer that guides the decay electron and proton to two large
area silicon detectors in order to precisely determine the electron energy and
an estimation of the proton momentum from the proton time of flight. The Nab
spectrometer is being commissioned at the Fundamental Neutron Physics Beamline
at the Spallation Neutron Source at Oak Ridge National Lab. We present an
overview of the Nab experiment and recent updates on the spectrometer,
analysis, and systematic effects.Comment: Presented at PPNS201
The Importance of Correlations and Fluctuations on the Initial Source Eccentricity in High-Energy Nucleus-Nucleus Collisions
In this paper, we investigate various ways of defining the initial source
eccentricity using the Monte Carlo Glauber (MCG) approach. In particular, we
examine the participant eccentricity, which quantifies the eccentricity of the
initial source shape by the major axes of the ellipse formed by the interaction
points of the participating nucleons. We show that reasonable variation of the
density parameters in the Glauber calculation, as well as variations in how
matter production is modeled, do not significantly modify the already
established behavior of the participant eccentricity as a function of collision
centrality. Focusing on event-by-event fluctuations and correlations of the
distributions of participating nucleons we demonstrate that, depending on the
achieved event-plane resolution, fluctuations in the elliptic flow magnitude
lead to most measurements being sensitive to the root-mean-square, rather
than the mean of the distribution. Neglecting correlations among
participants, we derive analytical expressions for the participant eccentricity
cumulants as a function of the number of participating nucleons,
\Npart,keeping non-negligible contributions up to \ordof{1/\Npart^3}. We
find that the derived expressions yield the same results as obtained from
mixed-event MCG calculations which remove the correlations stemming from the
nuclear collision process. Most importantly, we conclude from the comparison
with MCG calculations that the fourth order participant eccentricity cumulant
does not approach the spatial anisotropy obtained assuming a smooth nuclear
matter distribution. In particular, for the Cu+Cu system, these quantities
deviate from each other by almost a factor of two over a wide range in
centrality.Comment: 18 pages, 10 figures, submitted to PR
Evidence of Final-State Suppression of High-p_T Hadrons in Au + Au Collisions Using d + Au Measurements at RHIC
Transverse momentum spectra of charged hadrons with 6 GeV/c have
been measured near mid-rapidity (0.2 1.4) by the PHOBOS experiment
at RHIC in Au + Au and d + Au collisions at . The spectra for different collision centralities are compared to collisions at the same energy. The resulting nuclear modification
factor for central Au + Au collisions shows evidence of strong suppression of
charged hadrons in the high- region ( GeV/c). In contrast, the d +
Au nuclear modification factor exhibits no suppression of the high-
yields. These measurements suggest a large energy loss of the high-
particles in the highly interacting medium created in the central Au + Au
collisions. The lack of suppression in d + Au collisions suggests that it is
unlikely that initial state effects can explain the suppression in the central
Au + Au collisions.Comment: 3 pages, 4 figures, International Europhysics Conference on High
Energy Physics EPS (July 17th-23rd 2003) in Aachen, German
Centrality dependence of charged antiparticle to particle ratios near mid-rapidity in d+Au collisions at sqrt(s_NN)=200 GeV
The ratios of the yields of charged antiparticles to particles have been
obtained for pions, kaons, and protons near mid-rapidity for d+Au collisions at
sqrt(s_NN) = 200 GeV as a function of centrality. The reported values represent
the ratio of the yields averaged over the rapidity range of 0.1<y_pi<1.3 and
0<y_(K,p)<0.8, where positive rapidity is in the deuteron direction, and for
transverse momenta 0.1<p_(T)^(pi,K)<1.0 GeV/c and 0.3<p_(T)^(p)<1.0 GeV/c.
Within the uncertainties, a lack of centrality dependence is observed in all
three ratios. The data are compared to results from other systems and model
calculations.Comment: 6 pages, 4 figures, submitted to PR
Centrality and pseudorapidity dependence of elliptic flow for charged hadrons in Au+Au collisions at sqrt(sNN) = 200 GeV
This paper describes the measurement of elliptic flow for charged particles
in Au+Au collisions at sqrt(sNN)=200 GeV using the PHOBOS detector at the
Relativistic Heavy Ion Collider (RHIC). The measured azimuthal anisotropy is
presented over a wide range of pseudorapidity for three broad collision
centrality classes for the first time at this energy. Two distinct methods of
extracting the flow signal were used in order to reduce systematic
uncertainties. The elliptic flow falls sharply with increasing eta at 200 GeV
for all the centralities studied, as observed for minimum-bias collisions at
sqrt(sNN)=130 GeV.Comment: Final published version: the most substantive change to the paper is
the inclusion of a complete description of how the errors from the hit-based
and track-based analyses are merged to produce the 90% C.L. errors quoted for
the combined results shown in Fig.
System size and centrality dependence of charged hadron transverse momentum spectra in Au+Au and Cu+Cu collisions at sqrt(s) = 62.4 and 200 GeV
We present transverse momentum distributions of charged hadrons produced in
Cu+Cu collisions at sqrt(s) = 62.4 and 200 GeV. The spectra are measured for
transverse momenta of 0.25 < p_T < 5.0 GeV/c at sqrt(s) = 62.4 GeV and 0.25 <
p_T < 7.0 GeV/c at sqrt(s) = 200 GeV, in a pseudo-rapidity range of 0.2 < eta <
1.4. The nuclear modification factor R_AA is calculated relative to p+p data at
both collision energies as a function of collision centrality. At a given
collision energy and fractional cross-section, R_AA is observed to be
systematically larger in Cu+Cu collisions compared to Au+Au. However, for the
same number of participating nucleons, R_AA is essentially the same in both
systems over the measured range of p_T, in spite of the significantly different
geometries of the Cu+Cu and Au+Au systems.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let
System Size, Energy and Centrality Dependence of Pseudorapidity Distributions of Charged Particles in Relativistic Heavy Ion Collisions
We present the first measurements of the pseudorapidity distribution of
primary charged particles in Cu+Cu collisions as a function of collision
centrality and energy, \sqrtsnn = 22.4, 62.4 and 200 GeV, over a wide range of
pseudorapidity, using the PHOBOS detector. Making a global comparison of Cu+Cu
and Au+Au results, we find that the total number of produced charged particles
and the rough shape (height and width) of the pseudorapidity distributions are
determined by the number of nucleon participants. More detailed studies reveal
that a more precise matching of the shape of the Cu+Cu and Au+Au pseudorapidity
distributions over the full range of pseudorapidity occurs for the same
Npart/2A value rather than the same Npart value. In other words, it is the
collision geometry rather than just the number of nucleon participants that
drives the detailed shape of the pseudorapidity distribution and its centrality
dependence at RHIC energies.Comment: Submitted to Physical Review Letter
Identified charged antiparticle to particle ratios near midrapidity in Cu+Cu collisions at sqrt(s) = 62.4 and 200 GeV
Antiparticle to particle ratios for identified protons, kaons and pions at
sqrt(s) = 62.4 and 200 GeV in Cu+Cu collisions are presented as a function of
centrality for the midrapidity region of 0.2 < eta < 1.4. No strong dependence
on centrality is observed. For the / ratio at ~ 0.51 GeV/c, we
observe an average value of 0.50 +/- 0.003_(stat) +/- 0.04_(syst) and 0.77 +/-
0.008_(stat) +/- 0.05_(syst) for the 10% most central collisions of 62.4 and
200 GeV Cu+Cu, respectively. The values for all three particle species measured
at sqrt(s) = 200 GeV are in agreement within systematic uncertainties with that
seen in both heavier and lighter systems measured at the same RHIC energy. This
indicates that system size does not appear to play a strong role in determining
the midrapidity chemical freeze-out properties affecting the antiparticle to
particle ratios of the three most abundant particle species produced in these
collisions.Comment: 5 Pages, 4 figures Made changes to the figures to include the panel
numbers. Slight changes to the text. Updated data points from other
experiment
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