Inclusive dielectron production in ArKCl collisions at 1

Inclusive di-electron production in Ar+KCl collisions at 1. 76 A Ge. V with HADES Filip Krizek, NPI Rez for the HADES collaboration § Motivation § HADES detector setup § Results from Ar+KCl run § Discussion and summary 1

Introduction Baryonic matter @ 1 -2 A Ge. V (√s. NN = 2. 3 - 2. 7 Ge. V ): - r/r. N = 1 – 3 , T < 80 Me. V, t ~ 12 – 14 fm/c - nucleons, baryonic resonances, ( mesons < 10%) - very little strangeness The main HADES goal: Properties of r, w in nuclear medium HADES @ SIS via e+e- decay channel (BR 10 -4 - 10 -5 ) - at SIS energies vector mesons produced below or close to threshold - di-electrons emitted over full history of a collision 2

High Acceptance Di-Electron Spectrometer • • GSI Darmstadt SIS accelerator facility. (HI beams 1 -2 A Ge. V) Geometry: • • full azimuthal angle, polar angle 18 – 85 deg pair acceptance ~ 35% Particle identification: • hadron blind Cherenkov detector RICH • time of flight TOF, TOFINO • Pre-Shower Momentum measurement: • superconducting toroidal magnet • multi-wire drift chambers • inv. mass resolution in w region 2. 5% FW 3

HADES trigger Two level on-line trigger: § LVL 1 trigger – charge particle multiplicity § LVL 2 trigger – single electron trigger The case of Ar+KCl run: - Charged part. mult. > 16 - LVL 1 enhanced mean pion mult. 2× w. r. t. MB (MB = Minimum bias) - <Apart> ~ 38. 5 - 34% of events out of MB were selected - total number of LVL 1 events 2. 1× 109 impact parameter distribution of Ar+KCl collisions (Ur. QMD simulation) 4

Ar+KCl @ 1. 76 AGe. V • efficiency corr. spectrum in HADES acceptance normalized to Np 0 • average of two analyses (different pid method: MVA, hard-cut ) Preliminary systematic errors (red ticks) - efficiency corr. ~ 20 % - CB subtraction - p 0 normalization ~ 11% - diff. between both analyses Se+e- = Ne+e- – CB CB : - “like-sign” method - first observation of w production in heavy ion collisions at SIS energies - inv. mass resolution in w region ~ 3% - “mixed-event” technique 5

HSD prediction for Ar+KCl @ 1. 76 AGe. V HSD transport code prediction E. L. Bratkovskaya, W. Cassing, Nucl. Phys. A 807 (2008) 214 -250. • vacuum properties Preliminary • long-lived sources p 0, h, w • pair sources from early collision phase: D → N e+ e- , NN-bremsstrahlung, p. N –bremsstrahlung, r → e+ e 6

HSD prediction for Ar+KCl @ 1. 76 AGe. V HSD transport code prediction E. L. Bratkovskaya, W. Cassing, Nucl. Phys. A 807 (2008) 214 -250. medium scenario for r, w simultaneous broadening and mass dropping Preliminary 7

Ar+KCl @ 1. 76 AGe. V PLUTO event generator I. Fröhlich et al. (2007) ar. Xiv: 0708 2382 [nucl-ex] Preliminary cocktail A - long-lived components - at thermal freeze out (T = 75 Me. V) p 0 and h : - mult. constrained by TAPS R. Averbeck et al. , Z. Phys. A 359 (1997), p. 6573. R. Averbeck et al. , Phys. Rev. C 67 (2003), 024903. -anisotropic polar angle distribution Missing part comes from short-lived sources from early collision stage. w : – mult. from m. T scaling - isotropic decay pattern 8

Lighter systems measured by HADES Phys. Lett. B 663 (2008) 43 Pluto event generator Cocktail A: long-lived components p 0 and h mult. TAPS w mult. from m. T scaling ansatz Phys. Rev. Lett. 98 (2007) 052302 Low mass resolution (in w region ~ 10%) 9

Elementary reactions measured by HADES Preliminary p+p 1. 25 A Ge. V, h subthreshold d+p 1. 25 A Ge. V, quasi-free p+n reaction selection (fast spectator p in FW) p 0 fixed by our measurement normalized to pp elastic scatt. p 0 fixed by our measurement h mult. H. Calen et al. , Phys. Rev. C 58 (1998), 2667 -2670. normalized to elastic scatt. 10 Nontrivial isospin dependence of pair emission for Mee> 0. 15 Ge. V/c 2 !

Signal with subtracted h component F~3 Preliminary e+e- yield in HADES acceptance. In (0. 15, 0. 45) Ge. V/c 2, di-electron yield scales with Ebeam like pions ! Preliminary - excess above elementary cocktail - nonlinear scaling of the di-electron yield with Apart. pp+pn (~ first chance collisions) within error bars same as CC 11

Summary § Preliminary results from Ar+KCl @ 1. 76 AGe. V run. § Clear w signal at SIS energies seen for the first time. § Systematic investigation of di-electron yield from early stage sources in the range 0. 15 - 0. 45 Ge. V/c 2 : - C+C data suggest, that the yield scales with beam energy like pion production. Overlay with elementary cocktail within error bars. - Ar+KCl data exhibit excess above elementary cocktail. The yield grows faster than linearly with A part. - next year heavier systems (Ni+Ni or Au+Au) 12

collaboration 13

Backup slides 14

Experimental campaigns Anomalous excess of e+e- pairs in 0. 15< Mee< 0. 5 Ge. V/c 2 (DLS puzzle) NN-bremsstrahlung D Dalitz decays first chance collisions C+C 1. 0 AGe. V p+p 1. 25 Ge. V C+C 2. 0 AGe. V d+p 1. 25 AGe. V Resonance (D, N*) production Form-factors and studies of p/h Dalitz decays (helicity angles) p+p 2. 2 Ge. V Vector mesons r/w in-medium p+p 3. 5 Ge. V p+Nb 3. 5 Ge. V cold nuclear matter Ar+KCl 1. 76 AGe. V Strangeness (L, K, f) production in HI 15

Ar+KCl hard-cut and MVA analyses Comparison of two analyses which used different particle identification methods. MVA = Multi Variate Analysis - neural network based pid hard-cut – sharp cuts e. g. in beta versus p*q 16

Ar+KCl signal to background ratio Hard-cut analysis Se+e- = Ne+e- – CB CB : - “like-sign” method Signal to background ratio (hard-cut analysis) 17 - “mixed-event” technique

HSD versus Ar+KCl @ 1. 76 AGe. V Preliminary HSD transport code prediction E. L. Bratkovskaya, W. Cassing, Nucl. Phys. A 807 (2008) 214 -250. Preliminary medium scenario for r, w simultaneous broadening and mass dropping 18

C+C @ 1 A Ge. V, pp & pn @ 1. 25 Ge. V § Spectra normalized to p 0 yield in C+C and NN § Absolute scale C+C @ 1 A Ge. V <Mp>/Apart = 0. 056 ± 0. 007 NN @ 1. 25 (A)Ge. V <MNNp>/Apart ~ 0. 093 but for NN @ 1 AGe. V <MNNp>/Apart ~ 0. 06 h. C+C constrained by data from TAPS R. Averbeck et al. , Z. Phys. A 359 (1997), p. 6573. R. Averbeck et al. , Phys. Rev. C 67 (2003), 024903 hpp+np contrained by data from Celsius/COSY H. Calen et al. , Phys. Rev. C 58 (1998), 2667 -2670 Elementary cross-sections: S. Teis et al. Z. Phys. A 356(1997) 421 -435 19

Ar+KCl 1. 75 AGe. V and C+C 2 AGe. V • h component subtracted • spectra shown in HADES acceptance • C+C low mass resolution mode • Ar+KCl high mass resolution 20

Meson multiplicity systematics Data taken from: § TAPS § Kao. S § Bevalac exp. 21

Electron/Positron identification Selection criteria for single electrons/positrons: 1) track is associated with a Cherenkov ring 2) velocity constraints 3) shower condition velocity vs charge times momentum e+e- pair analysis : • close pair cuts (opening angle > 9 deg ) • correction on reconstruction efficiencies charge times momentum e- e+ 22

p+p @ 3. 5 Ge. V Invariant mass resolution at w pole mass 2. 5%. 23

Pair Acceptance in HADES 24

DLS comp. of di-electron yield from pp and pd W. K. Wilson et al. (DLS collaboration), Phys. Rev. C 57 (1998), 1865 -1878. 25

Comparison with DLS: Mee • HADES data + extrapolation in DLS acceptance • Different choice of extrapolation • Multiplicities converted to cross sections by multiplying with the total C + C reaction cross section W. -Q. Shen et al. : Nucl. Phys. A 491 (1989) 130 DLS Data: R. J. Porter et al. : PRL 79(1997)1229 HADES and DLS Data agree Direct confirmation of DLS results 26