Arnold, O.

Fabbietti, L.

GSI Repository

Exploring two-particle correlations in p+Nb reactions∗O. Arnold1,2, L. Fabbietti1,2, and the HADES collaboration1,21Physik Department E12, Technische Universita¨t Mu¨nchen, James-Franck-Str. 1, 85748 Garching, Germany;2Excellence Cluster ’Origin and Structure of the Universe’, Boltzmannstr. 2, 85748 Garching, GermanyThe HADES collaboration measured in 2008 the colli-sion of a proton with a niobium nucleus (p+93Nb), wherethe kinetic energy of the proton was Ekin = 3.5 GeV. ThispA system offers the possibility to study two-particle cor-relations in an interesting environment, namely in a smallsystem at rather low energies which means low multiplic-ity. On the one hand side we can compare the results withtrends established in heavy-ion collisions at much largerenergies to see similarities or find differences and their ex-planations and on the other hand the measurements are freefrom correlation-disturbing effects like mini-jets which areproduced at LHC energies. The latter point is a good reasonto investigate the interaction of particles where the interac-tion strength is not well established (e.g. between Λ hyper-ons and protons). It was found in [1] that the pΛ correlationfunction is sensitive to the size of the particle emission re-gion because the strong interaction between both particlesis large enough that they can interact in their final state.Two-particle correlations are experimentally determined bythe ratio:C(k) =A(k)B(k). (1)Here A(k) is the distribution of particles from the sameevent as a function of the relative momentum in the pairrest frame k = 12 |pa − pb| and B(k) is the correspondingdistribution of pairs from mixed event. This ratio is sensi-tive to the spatial extension of the particle emitting regionfor small relative momenta k < 100 MeV/c. We startedin the analysis to establish features of the source e.g. itssize and shape. For this purpose we used particles wherethe interaction is well known: protons and pions. For pi-ons it is also possible to extract a three dimensional sourceinformation by measuring the correlations in the longitu-dinally comoving system (LCMS). In the proton case wecan apply the Koonin model with a Gaussian source as-sumption to the data, which describes the proton-protoncorrelation function theoretically. We extracted a sourcesize of 2.078 ± 0.010 (stat) fm. For negatively chargedpions we extracted a slightly smaller Gaussian source sizeof 1.797±0.016 (stat) fm. In a next step we compared ourmeasurements to predictions of transport models (UrQMD[2]) and we calculated the correlation function with help ofthe correlation afterburner CRAB [3]. The result is shownin Figure 1, which displays the comparison of the UrQMDresult to the experimental measurement. UrQMD is able todescribe the measurements quite well. For this reason wetrust UrQMD that it also describes the momentum and con-∗Work supported by HIC4FAIRFigure 1: Proton-proton correlation function. The open cir-cles show the experimental correlation function correctedfor all inefficiencies. The (red) solid line is the correlationfunction determined from UrQMD simulations.figuration space of Λp, which allowed us to determine thesource function from UrQMD simulations. We found thatforΛp pairs the source size is 1.24 times smaller comparedto the proton-proton case. We will use this information toextract the (spin averaged) scattering length and effectiverange of the Λp interaction. With help of CRAB we canalso test different two-particle interaction models and con-strain their parameter space.To summarize, we measured two-particle correlations in apA system. We extracted the source size for like-sign pi-ons and for protons. The measurement of the proton sourcesize helped us to pin-down the interaction region of the Λppair. With this information we are able to study the inter-action between these particles and test different interactionhypothesis with help of the correlation afterburner CRAB.References[1] F. Wang, S. Pratt, , “Lambda-Proton Correlations in Relativis-tic Heavy Ion Collisions”, Phys. Rev. Lett. 83, 3138[2] S. A. Bass et. al., “Microscopic Models for UltrarelativisticHeavy Ion Collisions”, Prog. Part. Nucl. Phys. 41 (1998) 225-370[3] S. Pratt, “Correlation afterburner CRAB version 3”,http://www.pa.msu.edu/∼pratts/freecodes/crab/home.htmlGSI SCIENTIFIC REPORT 2014 MU-NQM-HADES-06DOI:10.15120/GR-2015-1-MU-NQM-HADES-06 7

Exploring two-particle correlations in p+Nb reactions

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Exploring two-particle correlations in p+Nb reactions

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