192 research outputs found
A membrane model for cytosolic calcium oscillations. A study using Xenopus oocytes
Cytosolic calcium oscillations occur in a wide variety of cells and are involved in different cellular functions. We describe these calcium oscillations by a mathematical model based on the putative electrophysiological properties of the endoplasmic reticulum (ER) membrane. The salient features of our membrane model are calcium-dependent calcium channels and calcium pumps in the ER membrane, constant entry of calcium into the cytosol, calcium dependent removal from the cytosol, and buffering by cytoplasmic calcium binding proteins. Numerical integration of the model allows us to study the fluctuations in the cytosolic calcium concentration, the ER membrane potential, and the concentration of free calcium binding sites on a calcium binding protein. The model demonstrates the physiological features necessary for calcium oscillations and suggests that the level of calcium flux into the cytosol controls the frequency and amplitude of oscillations. The model also suggests that the level of buffering affects the frequency and amplitude of the oscillations. The model is supported by experiments indirectly measuring cytosolic calcium by calcium-induced chloride currents in Xenopus oocytes as well as cytosolic calcium oscillations observed in other preparations
Two-Proton Correlations near Midrapidity in p+Pb and S+Pb Collisions at the CERN SPS
Correlations of two protons emitted near midrapidity in p+Pb collisions at
450 GeV/c and S+Pb collisions at 200A GeV/c are presented, as measured by the
NA44 Experiment. The correlation effect, which arises as a result of final
state interactions and Fermi-Dirac statistics, is related to the space-time
characteristics of proton emission. The measured source sizes are smaller than
the size of the target lead nucleus but larger than the sizes of the
projectiles. A dependence on the collision centrality is observed; the source
size increases with decreasing impact parameter. Proton source sizes near
midrapidity appear to be smaller than those of pions in the same interactions.
Quantitative agreement with the results of RQMD (v1.08) simulations is found
for p+Pb collisions. For S+Pb collisions the measured correlation effect is
somewhat weaker than that predicted by the model simulations, implying either a
larger source size or larger contribution of protons from long-lived particle
decays.Comment: 10 pages (LaTeX) text, 4 (EPS) figures; accepted for publication in
Phys. Lett.
Strange Meson Enhancement in PbPb Collisions
The NA44 Collaboration has measured yields and differential distributions of
K+, K-, pi+, pi- in transverse kinetic energy and rapidity, around the
center-of-mass rapidity in 158 A GeV/c Pb+Pb collisions at the CERN SPS. A
considerable enhancement of K+ production per pi is observed, as compared to
p+p collisions at this energy. To illustrate the importance of secondary hadron
rescattering as an enhancement mechanism, we compare strangeness production at
the SPS and AGS with predictions of the transport model RQMD.Comment: 11 pages, including 4 figures, LATE
Nuclear Clusters as a Probe for Expansion Flow in Heavy Ion Reactions at 10-15AGeV
A phase space coalescence description based on the Wigner-function method for
cluster formation in relativistic nucleus-nucleus collisions is presented. The
momentum distributions of nuclear clusters d,t and He are predicted for central
Au(11.6AGeV)Au and Si(14.6AGeV)Si reactions in the framework of the RQMD
transport approach. Transverse expansion leads to a strong shoulder-arm shape
and different inverse slope parameters in the transverse spectra of nuclear
clusters deviating markedly from thermal distributions. A clear ``bounce-off''
event shape is seen: the averaged transverse flow velocities in the reaction
plane are for clusters larger than for protons. The cluster yields
--particularly at low at midrapidities-- and the in-plane (anti)flow of
clusters and pions change if suitably strong baryon potential interactions are
included. This allows to study the transient pressure at high density via the
event shape analysis of nucleons, nucleon clusters and other hadrons.Comment: 38 pages, 9 figures, LaTeX type, eps used, subm. to Phys. Rev.
Two-kaon correlations in central Pb + Pb collisions at 158 A GeV/c
Two-particle interferometry of positive kaons is studied in Pb + Pb
collisions at mean transverse momenta and 0.91 GeV/c. A
three-dimensional analysis was applied to the lower data, while a
two-dimensional analysis was used for the higher data. We find that the
source size parameters are consistent with the scaling curve observed in
pion correlation measurements in the same collisions, and that the duration
time of kaon emission is consistent with zero within the experimental
sensitivity.Comment: 4 pages incl. 1 table and 3 fig's; RevTeX; accepted for publication
in PR
Charged Particle Pseudorapidity Distributions in Au+Al, Cu, Au, and U Collisions at 10.8 AGeV/c
We present the results of an analysis of charged particle pseudorapidity
distributions in the central region in collisions of a Au projectile with Al,
Cu, Au, and U targets at an incident energy of 10.8~GeV/c per nucleon. The
pseudorapidity distributions are presented as a function of transverse energy
produced in the target or central pseudorapidity regions. The correlation
between charged multiplicity and transverse energy measured in the central
region, as well as the target and projectile regions is also presented. We give
results for transverse energy per charged particle as a function of
pseudorapidity and centrality.Comment: 31 pages + 12 figures (compressed and uuencoded by uufiles), LATEX,
Submitted to PR
One and two dimensional analysis of 3pi correlations measured in Pb+Pb interactions
3pi- correlations from Pb+Pb collisions at 158 GeV/c per nucleon are
presented as measured by the focusing spectrometer of the NA44 experiment at
CERN. The three-body effect is found to be stronger for PbPb than for SPb. The
two-dimensional three-particle correlation function is also measured and the
longitudinal extension of the source is larger than the transverse extension
Centrality dependence of pi^[+/-], K^[+/-], p and p-bar production from sqrt(s_NN)=130 GeV Au + Au collisions at RHIC
Identified pi^[+/-] K^[+/-], p and p-bar transverse momentum spectra at
mid-rapidity in sqrt(s_NN)=130 GeV Au-Au collisions were measured by the PHENIX
experiment at RHIC as a function of collision centrality. Average transverse
momenta increase with the number of participating nucleons in a similar way for
all particle species. The multiplicity densities scale faster than the number
of participating nucleons. Kaon and nucleon yields per participant increase
faster than the pion yields. In central collisions at high transverse momenta
(p_T greater than 2 GeV/c), anti-proton and proton yields are comparable to the
pion yields.Comment: 6 pages, 3 figures, 1 table, 307 authors, accepted by Phys. Rev.
Lett. on 9 April 2002. This version has minor changes made in response to
referee Comments. Plain text data tables for the points plotted in figures
for this and previous PHENIX publications are publicly available at
http://www.phenix.bnl.gov/phenix/WWW/run/phenix/papers.htm
Centrality Dependence of Charged Particle Multiplicity in Au-Au Collisions at sqrt(s_NN)=130 GeV
We present results for the charged-particle multiplicity distribution at
mid-rapidity in Au - Au collisions at sqrt(s_NN)=130 GeV measured with the
PHENIX detector at RHIC. For the 5% most central collisions we find
. The results,
analyzed as a function of centrality, show a steady rise of the particle
density per participating nucleon with centrality.Comment: 307 authors, 43 institutions, 6 pages, 4 figures, 1 table Minor
changes to figure labels and text to meet PRL requirements. One author added:
M. Hibino of Waseda Universit
Charged kaon and pion production at midrapidity in proton nucleus and sulphur nucleus collisions
The NA44 collaboration has measured charged kaon and pion distributions at midrapidity in sulphur and proton collisions with nuclear targets at 200 and 450 GeV/c per nucleon, respectively. The inverse slopes of kaons are larger than those of pions. The difference in the inverse slopes of pions, kaons and protons, all measured in our spectrometer, increases with system size and is consistent with the buildup of collective flow for larger systems. The target dependence of both the yields and inverse slopes is stronger for the sulphur beam suggesting the increased importance of secondary rescattering for SA reactions. The rapidity density, dN/dy, of both K+ and K- increases more rapidly with system size than for pi+ in a similar rapidity region. This trend continues with increasing centrality, and according to RQMD, it is caused by secondary reactions between mesons and baryons. The K-/K+ ratio falls with increasing system size but more slowly than the pbar/p ratio. The pi-/pi+ ratio is close to unity for all systems. From pBe to SPb the K+/p ratio decreases while K-/pbar increases and ({K+*K-}/{p*pbar})**1/2 stays constant. These data suggest that as larger nuclei collide, the resulting system has a larger transverse expansion, baryon density and an increasing fraction of strange quarks.The NA44 collaboration has measured charged kaon and pion distributions at midrapidity in sulphur and proton collisions with nuclear targets at 200 and 450 GeV/c per nucleon, respectively. The inverse slopes of kaons are larger than those of pions. The difference in the inverse slopes of pions, kaons and protons, all measured in our spectrometer, increases with system size and is consistent with the buildup of collective flow for larger systems. The target dependence of both the yields and inverse slopes is stronger for the sulphur beam suggesting the increased importance of secondary rescattering for SA reactions. The rapidity density, dN/dy, of both K+ and K- increases more rapidly with system size than for pi+ in a similar rapidity region. This trend continues with increasing centrality, and according to RQMD, it is caused by secondary reactions between mesons and baryons. The K-/K+ ratio falls with increasing system size but more slowly than the pbar/p ratio. The pi-/pi+ ratio is close to unity for all systems. From pBe to SPb the K+/p ratio decreases while K-/pbar increases and ({K+*K-}/{p*pbar})**1/2 stays constant. These data suggest that as larger nuclei collide, the resulting system has a larger transverse expansion, baryon density and an increasing fraction of strange quarks.The NA44 collaboration has measured charged kaon and pion distributions at midrapidity in sulphur and proton collisions with nuclear targets at 200 and 450 GeV/c per nucleon, respectively. The inverse slopes of kaons are larger than those of pions. The difference in the inverse slopes of pions, kaons and protons, all measured in our spectrometer, increases with system size and is consistent with the buildup of collective flow for larger systems. The target dependence of both the yields and inverse slopes is stronger for the sulphur beam suggesting the increased importance of secondary rescattering for SA reactions. The rapidity density, dN/dy, of both K+ and K- increases more rapidly with system size than for pi+ in a similar rapidity region. This trend continues with increasing centrality, and according to RQMD, it is caused by secondary reactions between mesons and baryons. The K-/K+ ratio falls with increasing system size but more slowly than the pbar/p ratio. The pi-/pi+ ratio is close to unity for all systems. From pBe to SPb the K+/p ratio decreases while K-/pbar increases and ({K+*K-}/{p*pbar})**1/2 stays constant. These data suggest that as larger nuclei collide, the resulting system has a larger transverse expansion, baryon density and an increasing fraction of strange quarks
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