Study of hadron properties in cold nuclear matter with HADESPavel Tlustý, Nuclear Physics Institute, Řež, Czech Republic

for the HADES Collaboration

, . p - beams

W. Weise

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M. Post et al., NPA 741 (2004) 81

e-

e+

tot

eemesoneeeeN

Partial decay branch might be suppressed by collisional broadening:

HIC and cold matter experiments: Ceres, NA60, Phenix KEK-E325, Clas, CBELSA/TAPS ..-results mostly consistent with broadening of mass distribution, no indication of mass shift except line shape results of KEK-E325 - some experiments (Clas, KEK-E325) not sensitive to low monenta mesons - no complete picture yet

Dileptons from p+p and p+Nb at 3.5 GeV Charged mesons from p+Nb at 3.5 GeV

Motivation and MethodModification of hadronic properties in nuclear matter predicted by theory – already at normal nuclear density:

Effect most prominent at low momenta – spectral function:

Experimental method: reconstruction of the vector meson (VM) mass distribution from their e+e- pair decay + no final state interaction - low yields

Advantage over nucleus-nucleus collisions (HIC):No density evolution in “cold” nuclear matter, controlled conditions

Direct measurements of the ρ:

NA60, nucleus-nucleus collisionscentrality dependent broadening, no shiftR.Arnaldi et al. PRL 96 (2006)

Indirect measurements of ω-width:CBELSA/TAPS: N reaction yield not affected by secondariesstrong broadening (factor ≈16)M.Nanova arXiv:1109.4029 [nucl-ex]

HADES - operates on beam of SIS18 heavy-ion synchrotron at GSI Darmstadt- measures e+e- pairs from p,,and heavy-ion induced reactions with large acceptance in solid angle and momentum - important for reconstruction of low momenta mesons where (1) medium modifications are predicted (2) most mesons decay inside the nucleus

- p+Nb at 3.5 Gev measured, with p+p at the same energy as a reference

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:

HADES acceptance allows cut on low momenta, even in the VM mass region!:

Momentum cut

PreliminaryScaling of pp and pNb data:two methods consistentmethod 1 – reaction cross section * number of participantsmethod 2 – 0 yield

Invariant mass distributions: meson peak at 0.782 GeV clearly identified Scaled data: no obvious difference

Measured data

Preliminary

HSDcalculations :

Fast pairs Slow pairs

Preliminary

Preliminary

“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 peak (bands represent sys. errors)

Invariant mass distributions for low (p < 0.8 GeV) and high (p > 0.8 GeV) momenta pairs

Preliminary

- 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

ω-mesons:

identified ω's show also a flat behavior

No feeding from secondary collisions due to strong broadening and decreased partial branching ratio..?

ρ-mesons:

No strong decrease of the partial branching ratio expected since it is already broad in vacuum

Due to the ρ coupling to baryonic resonances a solid theoretical description needed to extract possible broadening or mass shifts

Preliminary

Momentum dependence of various sources Conclusion

HADES data: s = M * stot, scaling constant to HARP-CDP data is stot = 848 ± 14 mb

Normalization to HARP-CDP data

Motivation

HADES measures particle multiplicities per reaction, which have to be recalculated to cross sections to compare data from pp and pA systems.

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 section s = M * 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 thermalized pions from a baryonic matter, when increasing the atomic number of the target nucleus.

Summary

200 < p < 1000 MeV/c

30 < < 90

closest system to

p+Nb at 3.5 GeV

p+A at 3,5,8,12 GeV/c

Bolshakova A. et al. HARP-CDP Collaboration EPJ C63 (2009) 549-609., EPJ C64 (2009) 181-241.

Existing data: cross sections from pA

4 closest system to p+Nb at 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+Nb at 3 GeV via interpolation in 2D energy – atomic number space.

same as on the left side, in the log scale:

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 as stot = 848 mb, 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.

Preliminary

Electron pair production in “cold” nuclear matter:

Measured data

Multiplicity of - measured in various polar angle regionsStatistical errors are negligible, systematic errors are 15% (not shown).

Preliminary

Preliminary