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Register a new userPion-pion and pion-proton correlations - new results from CERES
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Results of a new two-particle correlation analysis of central Pb+Au collision data at 158 GeV per nucleon are presented. The emphasis is put on pion-proton correlations and on the dependence of the two-pion correlation radii on the azimuthal emission angle with respect to the reaction plane.
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Recent results from the CERES experiment at the SPS are reviewed. Emphasis is put on the centrality and beam energy dependence, and the results are put into perspective with findings at lower and higher beam energies. The rather weak beam energy dependence of the HBT radii maybe understood in terms of a transition from baryon to pion dominated freeze-out. The observed short lifetimes end emission durations are presently in contradiction to results from model calculations.
The scattering lengths of a two pion system are the {it golden magnitudes} to test the QCD predictions in the low energy sector. The DIRAC (PS-212) experiment at CERN will obtain a particular combination of the S-wave isospin 0 and 2 scattering lengths by measuring the lifetime of pionium, the hydrogen-like $pi^+ pi^-$ atom. This measurement tests the accurate predictions of the Chiral Perturbation Theory. The most recent experimental results are presented.
With the aim of extracting the pion charge radius, we analyse extant precise pion+electron elastic scattering data on $Q^2 in [0.015,0.144],$GeV$^2$ using a method based on interpolation via continued fractions augmented by statistical sampling. The scheme avoids any assumptions on the form of function used for the representation of data and subsequent extrapolation onto $Q^2simeq 0$. Combining results obtained from the two available data sets, we obtain $r_pi = 0.640(7),$fm, a value $2.4,sigma$ below todays commonly quoted average. The tension may be relieved by collection and similar analysis of new precise data that densely cover a domain which reaches well below $Q^2 = 0.015,$GeV$^2$. Considering available kaon+electron elastic scattering data sets, our analysis reveals that they contain insufficient information to extract an objective result for the charged-kaon radius, $r_K$. New data with much improved precision, low-$Q^2$ reach and coverage are necessary before a sound result for $r_K$ can be recorded.
We use detailed balance for a hadron composed of quark and gluon Fock states to obtain parton distributions in the proton and pion on the basis of a simple statistical model.
We consider the pion structure in the region of low and moderately high momentum transfers: at low $Q^2$, the pion is treated as a composite system of constituent quarks; at moderately high momentum transfers, $Q^2=10div25;GeV^2$, the pion ff is calculated within perturbative QCD taking into account one--gluon hard exchange. Using the data on pion ff at $Q^2<3;GeV^2$ and pion axial--vector decay constant, we reconstruct the pion wf in the soft and intermediate regions. This very wave function combined with one--gluon hard scattering amplitude allows a calculation of the pion ff in the hard region $Q^2=10div25;GeV^2$. A specific feature of the reconstructed pion wf is a quasi--zone character of the $qbar q$--excitations. On the basis of the obtained pion wf and the data on deep inelastic scattering off the pion, the valence quark distribution in a constituent quark is determined.
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