π
+
π
+
π
+ correlations from sulphur-lead collisions at 200 GeV/c per nucleon are presented as measured by the focusing spectrometer of experiment NA44 at CERN. We have investigated the three-pion ...correlation function at mid-rapidity and found that a genuine three-body correlation is suppressed. A possible interpretation of this result is that the emission of particles is partially coherent.
We show that the formalism commonly used to implement Bose-Einstein correlations in Monte-Carlo simulations can lead to values of the two-particle correlator significantly smaller than unity, in the ...case of sources with strong position-momentum correlations. This is more pronounced when the phase space of the emitted particles is strongly reduced by experimental acceptance or kinematic analysis selections. It is inconsistent with general principles from the coherent state formalism according to which the Bose-Einstein correlator is larger than unity. This inconsistency seems to be rooted in the fact that quantum mechanical localization properties are not taken into account properly.
Correlations of two protons emitted near midrapidity in
p+
Pb collisions at 450 GeV/
c and
S+
Pb collisions at 200
A 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.
Results on transverse mass spectra of neutral pious measured at central rapidity are presented for impact parameter selected 158-A GeV Pb + Pb-1 and Pb + Nb collisions. The distributions cover the ...range 0.5 GeV/c(2) less than or equal to MT - Mo less than or equal to 4 GeV/c(2). The change of the spectral shape and the multiplicity with centrality is studied in detail. In going from p+p to semi-peripheral Pb+Pb collisions there is a nuclear enhancement increasing with transverse mass similar to the well known Cronin effect, while for very central collisions this enhancement appears to be weaker than expected.
Measurements of direct photon production in p+Pb and p+C collisions at $\sqrt{s_\mathrm{NN}} = 17.4\mathrm{GeV}$ are presented. Upper limits on the direct photon yield as a function of $p_\mathrm{T}$ ...are derived and compared to the results for Pb+Pb collisions at $\sqrt{s_\mathrm{NN}} = 17.3$ GeV. The production of the $\eta$ meson, which is an important input to the direct photon signal extraction, has been determined in the $\eta \rightarrow 2\gamma$ channel for p+C collisions at $\sqrt{s_\mathrm{NN}} = 17.4\mathrm{GeV}$.
π+π+ and π−π− correlations from 200 GeV per nucleon S + Pb collisions and π+π+ correlations from 450 GeV p + Pb collisions measured by the focussing spectrometer of CERN experiment NA44 are ...presented. The large data set which emphasizes small values of momentum difference allows multi-dimensional analysis along with the more traditional one-dimensional parameterization to characterize the pion emission source. It is found that the three radius parameters are similar and large compared to the projectile radius. This can be explained by pion scattering in the final state hadronic system.
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