Protons, Deuterons and Flow

Proton and deuteron mT spectra and rapidity densities at y = 1.9− 2.3 are presented for central SS, SPb and PbPb collisions. The inverse slopes increase with system size and mass. The radius parameter, RG, extracted from ratios of the spectra increases with system size. RG ∝ 1/ √ mT consistent with transverse flow and NA44’s HBT results. Experiment NA44 NA44 measures particle spectra and correlations, [1]. It triggers on rare particles and has excellent momentum resolution of σp/p = 0.2%. The mean occupancy is about 0.5 particles per central PbPb collision. Changing angles and momentum settings scans transverse mass, mT ≡ √ p2T +m 2 and rapidity. This paper compares deuterons to protons at the same velocity. Results Figure shows proton and deuteron mT spectra at y=1.9-2.3. Protons from the weak decays of Λ and Σ are subtracted from the spectra using GEANT and RQMD, resulting in systematic errors ≤ 2% on the slopes and ≤ 6% on the dN/dy values. The spectra flatten as the particle mass or the system gets heavier as noted in [2] for π, K, p and p̄. For larger systems more baryons are pushed into mid-rapidity. The energy they deposit produces particles that suffer secondary collisions which may produce transverse flow and explain the 1 trends in the slopes. Figure 2 shows that (dNd/dy)/(dNp/dy) increases from SS to PbPb while (dNd/dy)/(dNp/dy) 2 decreases. In the coalescence model deuterons form when p − n pairs are close in phase space. However, a 3rd particle must absorb the excess energy of the p−n pair. This is easier in PbPb collisions. In a spherically symmetric thermal model , [3], with a gaussian source ρ(r) = exp −r 2 2R G the radius in the p− n rest frame is R G = 3 4 (π)(h̄c) md m2p · (p Np dp ) Ed·dNd dp (1) The cross sections are evaluated at the same velocity for proton and deuterons and we assume Nn = Np. RG must be devided by √ 3 to compare it to NA44’s 3d HBT radius parameters. Figure 3 shows that RG falls with mT . Collective expansion creates position-momentum correlations with correlation lengths ∝ 1/ √ mT , [4, 5]. The data are consistent with this form. Figure 4 compares NA44 HBT radii, [1], with RG/ √ 3 for SPb. All SPb radii agree with 2/ √ mT . Conclusions Proton and deuteron mT spectra at y=1.9-2.3 are exponential with inverse slopes that increase with mass and system size. The ratio (dNd/dy)/(dNp/dy) increases From SS to PbPb but (dNd/dy)/(dNp/dy) 2 decreases. The radius parameter RG extracted from proton and deuteron spectra using Eqn. (1) increases from SS to PbPb and falls with mT . The increase of inverse slopes with mass and the 1/ √ mT dependence of RG are consistent with radial flow. References [1] H. Becker et al., Phys. Rev. Lett. 74 3340 (1995). [2] I. Bearden et al., Phys. Rev. Lett. 78 2080c (1997). [3] W. J. Llope et al., Phys. Rev. C. 52 2004 (1995). [4] T. Csörgő, B. Lörstad, Phys. Rev. C 54 1390 (1996) [5] Y. M. Sinyukov, Nucl. Phys. A 566 589c (1994). John