arXiv subjects

J. Takahashi

Publications and source records attributed to J. Takahashi.

25 records · Page 2Linked to original sources

arXiv2010

A Non-extensive Equilibrium analysis of $π^{+}$ $p_{T}$ Spectra at RHIC

By analyzing the dynamical properties of particle production in relativistic heavy ion collisions, it is possible to characterize the final stage of the equilibration process occurring in the collision fireball. In this work, we use the Hagedorn model coupled with non-extensive statistics to evaluate the transverse momentum spectra of positive pions for various event centrality classes in Au+Au collisions at RHIC with center-of-mass energies of 62.4GeV/A and 200GeV/A. We find that, by assuming an energy distribution that incorporates particle correlations, it is possible to explain the entire $π^{+}$ $p_{T}$ spectrum as measured by RHIC. We find that spectra from central collisions, when compared to peripheral collisions, are consistent with a system that has smaller values of the non-extensivity parameter "q" and higher values of temperature. Comparison between different beam energies also shows a variation of the "q" parameter. The result is discussed using the interpretation that the "q" parameter is a measure of particle correlations within the system. Under this assumption, our results show that more central collisions are consistent with a system with less particle correlations.

hep-ph↗

arXiv2009

Topology studies of hydrodynamics using two particle correlation analysis

The effects of fluctuating initial conditions are studied in the context of relativistic heavy ion collisions where a rapidly evolving system is formed. Two particle correlation analysis is applied to events generated with the NEXSPHERIO hydrodynamic code, starting with fluctuating non-smooth initial conditions (IC). Results show that the non-smoothness in the IC survives the hydro-evolution and can be seen as topological features of the angular correlation function of the particles emerging from the evolving system. A long range correlation is observed in the longitudinal direction and in the azimuthal direction a double peak structure is observed in the opposite direction to the trigger particle. This analysis provides clear evidence that these are signatures of the combined effect of tubular structures present in the IC and the proceeding collective dynamics of the hot and dense medium.

nucl-th↗

arXiv2008

Strangeness production in STAR

We present a summary of strangeness enhancement results comparing data from Cu+Cu and Au+Au collisions at sqrt(SNN) = 200GeV measured by the STAR experiment. Relative yields in central Cu+Cu data seem to be higher than the equivalent sized peripheral Au+Au collision. In addition, strange particle production from these two systems is compared in terms of a statistical model, applying a Grand-Canonical ensemble and also applying a canonical correlation volume for the strange particles. Thermal fit results from the Grand-Canonical formalism shows little dependence on the system size but, when considering a strange canonical ensemble, strangeness enhancement shows a strong dependency on the correlation volume.

nucl-ex↗

arXiv2007

Statistical methods applied to composition studies of ultrahigh energy cosmic rays

The mass composition of high energy cosmic rays above $10^{17}$ eV is a crucial issue to solve some open questions in astrophysics such as the acceleration and propagation mechanisms. Unfortunately, the standard procedures to identify the primary particle of a cosmic ray shower have low efficiency mainly due to large fluctuations and limited experimental observables. We present a statistical method for composition studies based on several measurable features of the longitudinal development of the CR shower such as $N_{max}$, $X_{max}$, asymmetry, skewness and kurtosis. Principal component analysis (PCA) was used to evaluate the relevance of each parameter in the representation of the overall shower features and a linear discriminant analysis (LDA) was used to combine the different parameters to maximize the discrimination between different particle showers. The new parameter from LDA provides a separation between primary gammas, proton and iron nuclei better than the procedures based on $X_{max}$ only. The method proposed here was successfully tested in the energy range from $10^{17}$ to $10^{20}$ eV even when limitations of shower track length were included in order to simulate the field of view of fluorescence telescopes.

astro-ph↗

arXiv2005

Incomplete fusion versus breakup competition with weakly bound projectiles

The importance of the breakup channel in the vicinity of the Coulomb barrier (1<Ec.m./Vb<2) is investigated for the medium weight 6Li+59Co system. Three-body final-state analysis of light-particle coincident data was carried out to disentangle, for the first time, the breakup contributions from other competiting mechanisms. alpha-d angular correlations show incomplete fusion components as significant as that from breakup process. Their strong coupling to total fusion is discussed within a comparison with predictions of continuum discretized coupled-channel calculations.

nucl-ex↗

arXiv2004

How does breakup influence near-barrier fusion of weakly bound light nuclei ?

The influence on the fusion process of coupling to collective degrees of freedom has been explored. The significant enhancement of he fusion cross setion at sub-barrier energies was understood in terms of the dynamical processes arising from strong couplings to collective inelastic excitations of the target and projectile. However, in the case of reactions where breakup becomes an important process, conflicing model predictions and experimental results have been reported in the literature. Excitation functions for sub- and near-barrier total (complete + incomplete) fusion cross sections have been measured for the $^{6,7}$Li + $^{59}$Co at the Vivitron facility and at the 8UD Pelletron tandem facility using standard $γ$-ray techniques. The data extend to medium-mass systems previous works exploring the coupling effects in fusion reactions of both lighter and heavier systems. Results of continuum-discretized coupled channel (CDCC) calculations indicate a small enhancement of total fusion for the more weakly bound $^{6}$Li at sub-barrier energies, with similar cross sections for both reactions at and above the barrier. A systematic study of $^{4,6}$He induced fusion reactions with the CDCC method is in progress. The understanding of the reaction dynamics involving couplings to the breakup channels requires th explicit measurement of precise elastic scattering data as well as yields leading to the breakup itself. Recent coincidence experiments for $^{6,7}$Li + $^{59}$Co are addressing this issue. The particle identification of the breakup products have been achieved by measuring the three-body final-state correlations.

nucl-ex↗

arXiv2004

Effect of hydrostatic pressure on irreversible thermal transformations in a polymer glass at low temperatures

Irreversible broadening of spectral holes in chlorin-doped polystyrene glass was studied for the first time in the temperature cycling experiments under high pressure (by raising the temperature from 5 K to various magnitudes up to 18 K and turning back to 5 K at several fixed pressures between 0 and 5 kbar). At all pressures the increment in the hole width observed after completing a temperature cycle exhibits a slightly superlinear (proportional to T to the power 3/2) dependence on the cycling temperature. The magnitude of this increment is essentially reduced under high pressure (e.g., at 4.9 kbar it makes up less than 2/3 of its initial value obtained at ambient pressure). The residual broadening of holes is interpreted as a result of irreversible thermally induced spectral diffusion arising from interaction of the electronic transition in a dopant molecule with two-level systems (TLSs) which perform thermally activated overbarrier jumps between two possible states of a TLS. The pressure effects are treated theoretically within the scope of the soft anharmonic potential model with asymmetric distribution of the cubic anharmonicity parameter. It is shown that more abundant are TLSs with such double-well potentials where the minimum, corresponding to a larger glass volume, is placed at a higher energy than the minimum, corresponding to a smaller volume. In this case, applied pressure reduces the number of almost symmetric TLSs (having very small energy difference between the two minima), which provide the largest contribution to the residual broadening of spectral holes after a temperature cycle. Our earlier results on isothermal hole burning at various fixed pressures [V. Hizhnyakov et al., Phys. Rev. B 62, 11296 (2000)] also qualitatively fit into this picture.

cond-mat.dis-nn↗