Study of hadron properties in cold nuclear matter with HADES

1. p+A at 3,5,8,12 GeV/c M. Post et al., NPA 741 (2004) 81 AT [GeV-2] e+  e- m [GeV] q [GeV] , . p - beams W. Weise • Study of hadron properties in cold nuclear matter with HADES PavelTlustý, NuclearPhysics Institute, Řež, CzechRepublic forthe HADESCollaboration Preliminary ω-mesons: identified ω's show also a flat behavior No feeding from secondary collisions due to strong broadening and decreased partial branching ratio..? 200 < p< 1000 MeV/c 30 <  < 90 Preliminary Preliminary Fast pairs Slow pairs closest system to p+Nb at 3.5 GeV - pions(Mee <0.15 GeV/c2) show flat behavior - higher invariant mass regions rise with decreasing momentum → feeding due to secondary reactions - omega : no dependence on momentum ρ-mesons: No strong decrease of the partial branching ratio expected since it is already broad in vacuum Due to the ρcoupling to baryonic resonances a solid theoretical description needed to extract possible broadening or mass shifts Motivation and Method Experimental method: reconstruction of the w/r vector meson (VM) mass distribution from their e+e- pair decay + no final state interaction - low yields HADES - operates on beam of SIS18 heavy-ionsynchrotronat GSI Darmstadt • measurese+e- pairs fromp,p,and heavy-ioninducedreactionswithlarge • acceptance in solid angle andmomentum • importantforreconstructionoflowmomentamesonswhere • (1) medium modificationsarepredicted • (2) mostmesonsdecayinsidethenucleus • - p+Nbat 3.5 Gevmeasured, withp+patthe same energyas a reference Modification of hadronic properties in nuclear matter predicted by theory – already at normal nuclear density: HIC andcold matter experiments: Ceres, NA60, Phenix KEK-E325, Clas, CBELSA/TAPS .. • resultsmostlyconsistentwithbroadeningofmassdistribution, • noindicationofmassshiftexceptr lineshaperesultsof KEK-E325 • someexperiments (Clas, KEK-E325) not sensitive tolowmonentamesons - nocompletepictureyet Bolshakova A. et al. HARP-CDP Collaboration EPJ C63 (2009) 549-609., EPJ C64 (2009) 181-241. Electron pair production in “cold” nuclear matter: Effect most prominent at low momenta – rspectral function: Partial decay branch might be suppressed by collisional broadening: Indirectmeasurementsofω-width: CBELSA/TAPS:gN reaction w yield not affected by secondaries strong broadening (factor≈16) M.NanovaarXiv:1109.4029 [nucl-ex] Directmeasurementsoftheρ: NA60, nucleus-nucleuscollisions centralitydependentbroadening, noshift R.Arnaldiet al. PRL 96 (2006) Advantage over nucleus-nucleus collisions (HIC): No density evolution in “cold” nuclear matter, controlled conditions Dileptons from p+p and p+Nb at 3.5 GeV Charged p mesons from p+Nb at 3.5 GeV Measured data Invariant mass distributions: w meson peak at 0.782 GeV clearly identified Scaled data: no obvious difference Measured data Motivation Scaling of pp and pNb data: two methods consistent method 1 – reaction cross section * number of participants method 2 – p0 yield Multiplicity of p- measured in various polar angle regions Statistical errors are negligible, systematic errors are 15% (not shown). HADESmeasuresparticlemultiplicities per reaction, whichhave to berecalculated to crosssections to compare data fromppandpAsystems. p+p reaction: measured pp elastic scattering yield is matched to known cross section. p+A reaction: measured charged pion multiplicity is matched to pion cross section from existing pA data scaling constant is the total reaction cross sectionsp= Mp * sttot Measured data contribute to the results from systematic studies of the pion production in the proton-nucleus collisions, and can be used for tuning of transport models (see e.g. K.Gallmeister, U.Mosel, arXiv:0901.1770 [hep-ex] ), in the region of transition of the pion source from simple NN collisions to emission of thermalizedpions from a baryonic matter, when increasing the atomic number of the target nucleus. Preliminary Preliminary Preliminary Momentum cut HADES acceptance allows cut on low momenta, even in the VM mass region! : Momentum cut important: Largest effect expected at low momenta (see above). Low momenta mesons decay mostly inside nucleus where the change of the line shape is expected: HSDcalculations : Invariant mass distributions for low (p < 0.8 GeV) and high (p > 0.8 GeV) momenta pairs Existing data: p cross sections from pA “fast” pairs - the distribution for pp and pNb is the same. “slow” pairs - visible difference, see zoomed picture on the right side: Fast Slow Slow zoomed into VM region: additional broad contribution below the w peak (bands represent sys. errors) Normalization to HARP-CDP data Preliminary 4 closest system to p+Nbat 3.5 GeV were used to compare with our data: p+Cu and p+Ta at 3 and 5 GeV/c (see below). Before the comparison, the HARP-CDP cross sections from these 4 systems were rescaled to expected values for p+Nbat 3 GeV via interpolation in 2D energy – atomic number space. same as on the left side, in the log scale: HADES data: sp= Mp * stot, scaling constant to HARP-CDP data is stot= 848 ± 14 mb Preliminary Preliminary Momentum dependence of various sources Conclusion Summary Multiplicities of charged pions from p+Nb at 3 GeV were measured. From comparison with the HARP-CDP data the p+Nb total cross section was extracted asstot = 848mb, with systematic error of 15%. This allows for an absolute normalization of the measured data, including dilepton production. Measured data together with results of systematic studies of pion production in p+p and p+A can be used for adjusting of transport models in the region between elementary p+p and proton-nucleus collisions.