717 research outputs found
Multiplicities in ultrarelativistic proton-(anti)proton collisions and negative binomial distribution fits
Likelihood ratio tests are performed for the hypothesis that charged-particle
multiplicities measured in proton-(anti)proton collisions at = 0.9
and 2.36 TeV are distributed according to the negative binomial form. Results
indicate that the hypothesis should be rejected in the all cases of ALICE-LHC
measurements in the limited pseudo-rapidity windows, whereas should be accepted
in the corresponding cases of UA5 data. Possible explanations of that and of
the disagreement with the least-squares fitting method are given.Comment: 14 pages, clarified version, reference added. To appear in
International Journal of Modern Physics
Forward-Backward Multiplicity Correlations in Au+Au Collisions at = 200 Gev
The study of correlations among particles produced in different rapidity
regions may provide understanding of the mechanisms of particle production.
Correlations that extend over a longer range are observed in hadron-hadron
interactions only at higher energies. Results for short and long-range
multiplicity correlations (Forward-Backward) are presented for Au+Au collisions
at = 200 GeV. The growth of long range correlations are
observed as a function of the pseudorapidity gap in central Au+Au collisions.
The Dual Parton model and Color Glass Condensate phenomenology have been
explored to understand the origin of long range correlations.Comment: 8 pages, 4 figures, IWCF06, Hangzhou, China, Nov. 21-24, 200
Expectation of forward-backward rapidity correlations in collisions at the LHC energies
Forward-backward correlation strength () as a function of pesudorapidity
intervals for experimental data from non-singly diffractive
collisions are compared to PYTHIA and PHOJET model calculations. The
correlations are discussed as a function of rapidity window ()
symmetric about the central rapidity as well as rapidity window separated by a
gap () between forward and backward regions. While the correlations
are observed to be independent of , it is found to decrease with
increase in . This reflects the role of short range correlations
and justifies the use of to obtain the accurate information about
the physics of interest, the long range correlations. The experimental
value shows a linear dependence on with the maximum value of
unity being reached at = 16 TeV, beyond the top LHC energy. However
calculations from the PYTHIA and PHOJET models indicate a deviation from linear
dependence on and saturation in the values being reached
beyond = 1.8 TeV. Such a saturation in correlation values could have
interesting physical interpretations related to clan structures in particle
production. Strong forward-backward correlations are associated with cluster
production in the collisions. The average number of charged particles to which
the clusters fragments, called the cluster size, are found to also increase
linearly with for both data and the models studied. The rate of
increase in cluster size vs. from models studied are larger
compared to those from the data and higher for PHOJET compared to PYTHIA. Our
study indicates that the forward-backward measurements will provide a clear
distinguishing observable for the models studied at LHC energies.Comment: 15 pages, 14 Figures, accepted for publication in International
Journal of Modern Physics
Energy dependence of transverse mass spectra of kaons produced in p+p and p+pbar interactions.A compilation
The data on m_T spectra of K0S K+ and K- mesons produced in all inelastic p+p
and p+pbar interactions in the energy range sqrt(s)NN=4.7-1800GeV are compiled
and analyzed. The spectra are parameterized by a single exponential function,
dN/(m_T*dm_T)=C exp(-m_T/T), and the inverse slope parameter T is the main
object of study. The T parameter is found to be similar for K0S, K+ and K-
mesons. It increases monotonically with collision energy from T~30MeV at
sqrt(s)NN=4.7GeV to T~220MeV at sqrt(s)NN=1800GeV. The T parameter measured in
p+p and p+pbar interactions is significantly lower than the corresponding
parameter obtained for central Pb+Pb collisions at all studied energies. Also
the shape of the energy dependence of is different for central Pb+Pb
collisions and p+p(pbar) interactions.Comment: more differential analysis adde
Surface drift at the western edge of the Agulhas Bank
The Agulhas Bank is a wide continental shelf that forms the southern tip of the African continent. On the eastern side of this shelf the flow of water is dominated by the adjacent Agulhas Current. On its western border, the movement is more complex. It is influenced by the Benguela Current, by the occasional presence of products from the Agulhas Current such as Agulhas rings, Agulhas filaments and by lee eddies. Understanding the flow on this western side of the Agulhas Bank is of considerable ecological importance because it has been assumed that a shelf edge jet carries immotile or weakly motile fish larvae and eggs from the spawning region on the bank to the biologically productive regions of the Benguela upwelling regime. We have used the tracks of a set of surface drifters to study the movement at the western edge of the bank, and show that on average the movement is indeed equatorward along this shelf edge, but that this movement is not persistent in direction or magnitude. Instead, this movement appears to be driven entirely by mesoscale turbulence created at the termination of the Agulhas Current
The Conrad Rise as an obstruction to the Antarctic Circumpolar Current
The Antarctic Circumpolar Current (ACC) carries water freely around the whole continent of Antarctica, but not without obstructions. Some, such as the Drake Passage, constrict its path, while others, such as mid-ocean ridges, may induce meandering in the current's cores and may cause the genesis of mesoscale turbulence. It has recently been demonstrated that some regions that are only relatively shallow may also have a major effect on the flow patterns of the ACC. This is here shown to be particularly true for the Conrad Rise. Using the trajectories of surface drifters, altimetry and the simulated velocities from a numerical model, we show that the ACC bifurcates at the western side of this Rise. In this process it forms two intense jets at the two meridional extremities of the Rise with a relatively stagnant water body over the Rise itself. Preliminary results from a recent cruise provide compelling support for this portrayal
Ocean currents south of Africa from drifters
The ocean currents and their variability south of Africa are known to cover a wide spectrum, spatially and possibly temporally. Hydrographic observations in this vast ocean region are logistically demanding and expensive. In recent years the prevalence of drifting buoys has allowed one to infer certain current characteristics for the region that otherwise would be difficult. Observations from satellite-tracked drifters drogued at a depth of 15 m, collected between 1988 and 2005, were used to infer the mean surface circulation and kinetic energy distributions of the surface flow in the African sector of the Southern Ocean between 30 and 60°S. Regions of intensified flow and of higher levels of eddy kinetic energy were identified and agree fairly well with those established from remote sensing products. These results confirm the value of these observations and indicate the increasing usefulness of this data set as the number of drifter tracks increases
Sea-surface temperatures at the sub-Antarctic islands Marion and Gough during the past 50 years
Sea-surface temperatures (SSTs) have been measured at Marion and Gough islands for nearly 50 years. These are some of the longest records of their kind in the sub-Antarctic. We present the trend, the seasonal cycle, and the time-frequency characteristics of the SST for both islands, which rose by 1.4°C at Marion Island and by 0.5°C at Gough Island over the 50-year period. Intermittent temperature oscillations, with periods of between 1 and 5 years, were observed throughout the record. A 5-year periodicity, compatible with an Antarctic CircumpolarWave signature, dominated after 1990 in both records. We also observed a strong low-frequency component with a period varying from 9.3 to 11.4 years at Marion Island, and a much weaker component with a period varying from 9.9 to 11.8 years at Gough Island
Physical and biological coupling in eddies in the lee of the South-West Indian Ridge
Eddies have some decisive functions in the dynamics of the Southern Ocean ecosystems. This is particularly true in the Indian sector of the Southern Ocean, where a region of unusually high-mesoscale variability has been observed in the vicinity of the South-West Indian Ridge. In April 2003, three eddies were studied: eddy A, a recently spawned anticyclone south of the Antarctic Polar Front (APF),; eddy B, an anticyclone north of lying between the Subantarctic Front and the APF; and eddy C, a cyclone north of the APF west of the ridge. Elevated concentrations of total Chl-a coincided with the edges of the cyclonic eddy, whereas both anticyclonic eddies A and B were characterised by low total Chl-a concentrations. Biologically, the two anticyclonic eddies A and B were distinctly different in their biogeographic origin. The zooplankton community in the larger anticyclonic eddy A was similar in composition to the Antarctic Polar Frontal Zone (APFZ) community with an addition of some Antarctic species suggesting an origin just north of the APF. In contrast, the species composition within the second anticyclonic eddy B appeared to be more typical of the transitional nature of the APFZ, comprising species of both subantarctic and subtropical origin and thus influenced by intrusions of water masses from both north and south of the Subantarctic Front. Back-tracking of these features shows that the biological composition clearly demarcates the hydrographic origin of these features
Decay of eddies at the South-West Indian Ridge
The South-West Indian Ridge in the Indian sector of the Southern Ocean is a region recognised for the creation of particularly intense eddy disturbances in the mean flow of the Antarctic Circumpolar Current. Eddies formed at this ridge have been extensively studied over the past decade using hydrographic, satellite, drifter and float data and it is hypothesised that they could provide a vehicle for localised meridional heat and salt exchange. The effectiveness of this process is dependent on the rate of decay of the eddies. However, in order to investigate eddy decay, logistically difficult hydrographic monitoring is required. This study presents the decay of cold eddies at the South-West Indian Ridge, using outputs from a highresolution ocean model. The model's representation of the dynamic nature of this region is fully characteristic of observations. On average, 3-4 intense and well-defined cold eddies are generated per year; these eddies have mean longevities of 5.0±2.2 months with average advection speeds of 5±2 km/day. Most simulated eddies reach their peak intensity within 1.5-2.5 months after genesis and have depths of 2000 m - 3000 m. Thereafter they dissipate within approximately 3 months. The decay of eddies is generally characterised by a decrease in their sea surface height signature, a weakening in their rotation rates and a modification in their temperature-salinity characteristics. Subantarctic top predators are suspected to forage preferentially along the edges of eddies. The process of eddy dissipation may thus influence their feeding behaviour
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