Search

Invariant p T spectra and rapidity densities covering a large rapidity range ( − 0.1 y 3.5 ) are presented for π ± and K ± mesons from central Au + Au collisions at s NN = 62.4 GeV . The mid-rapidity yields of meson particles relative to their anti-particles are found to be close to unity ( π − / π + ∼ 1 , K − / K + ∼ 0.85 ) while the anti-proton to proton ratio is p ¯ / p ∼ 0.49 . The rapidity dependence of the π − / π + ratio is consistent with a small increase towards forward rapidities while the K − / K + and p ¯ / p ratios show a steep decrease to ∼0.3 for kaons and 0.022 for protons at y ∼ 3 . It is observed that the kaon production relative to its own anti-particle as well as to pion production in wide rapidity and energy ranges shows an apparent universal behavior consistent with the baryo-chemical potential, as deduced from the p ¯ / p ratio, being the driving parameter.

The proton-to-pion ratios measured in the BRAHMS experiment for Au + Au and p + p collisions at s N N = 62.4 and 200 GeV are presented as a function of transverse momentum and collision centrality at selected pseudorapidities in the range of 0–3.8. A strong pseudorapidity dependence of these ratios is observed. We also compare the magnitude and p T -dependence of the p / π + ratios measured in Au + Au collisions at s N N = 200 GeV and η ≈ 2.2 with the same ratio measured at s N N = 62.4 GeV and η = 0 . The great similarity found between these ratios throughout the whole p T range (up to 2.2 GeV / c ) is consistent with particle ratios in A + A collisions being described with grand-canonical distributions characterized by the baryo-chemical potential μ B . At the collision energy of 62.4 GeV, we have observed a unique point in pseudorapidity, η = 3.2 , where the p / π + ratio is independent of the collision system size in a wide p T -range of 0.3 ⩽ p T ⩽ 1.8 GeV / c .

Transverse momentum spectra of protons and anti-protons measured in the rapidity range 0 y 3.1 from 0–10% central Au + Au collisions at s N N = 62.4 GeV are presented. The rapidity densities, d N / d y , of protons, anti-protons and net-protons ( N p – N p ¯ ) have been deduced from the spectra over a rapidity range wide enough to observe the expected maximum net-baryon density. From mid-rapidity to y = 1 the net-proton yield is roughly constant ( d N / d y ∼ 10 ), but rises to d N / d y ∼ 25 at 2.3 y 3.1 . The mean rapidity loss is 2.01 ± 0.14 ± 0.12 units from beam rapidity. The measured rapidity distributions are compared to model predictions. Systematics of net-baryon distributions and rapidity loss vs. collision energy are discussed.

We present a measurement of π − / π + , K − / K + and p ¯ / p from p + p collisions at s = 200 GeV over the rapidity range 0 y 3.4 . For p T 2.0 GeV / c we see no significant transverse momentum dependence of the ratios. All three ratios are independent of rapidity for y ≲ 1.5 and then steadily decline from y ∼ 1.5 to y ∼ 3 . The π − / π + ratio is below unity for y > 2.0 . The p ¯ / p ratio is very similar for p + p and 20% central Au + Au collisions at all rapidities. In the fragmentation region the three ratios seem to be independent of beam energy when viewed from the rest frame of one of the protons. Theoretical models based on quark–diquark breaking mechanisms overestimate the p ¯ / p ratio up to y ≲ 3 . Including additional mechanisms for baryon number transport such as baryon junctions leads to a better description of the data.

We present charged particle densities as a function of pseudorapidity and collision centrality for the 197Au+197Au reaction at sqrt{s_{NN}}=130 GeV. An integral charged particle multiplicity of 3860+/-300 is found for the 5% most central events within the pseudorapidity range -4.7

The time scale for the binary disintegration of projectile-like fragments was investigated in 48 Ti+ 93 Nb collisions at 19.1 MeV/ nucleon. The influence of the target proximity to the decaying projectile-like fragment was utilized to determine the time interval between the primary re-separation of the projectile and target and the secondary projectile disintegration. This time interval for near symmetric decay of the projectile, is less than ≈ 200 fm/ c . Taking into account the time interval required to separate the two fragments of the projectile to their scission configuration, the projectile decay process is one which quite likely, or quite often, is initiated before the projectile and target-like fragments are fully separated.