Characterization of the pion source at the AGS Mike Lisa The Ohio State University Motivation and Measurement Systematics of HBT in E895 Existing problems - failed consistency check Summary APS Meeting March 1999

AGS

Motivation 1) fully characterize HBT systematics as fctn of collision conditions: E beam, b, y, m T,  rp 3) test dynamical models that provide “hadronic baseline”: can they extrapolate to RHIC? 2) watch for “anomolous” E observable AGSSPSRHIC “”“” with transition cc E observable AGSSPSRHIC 3D hydro model Rischke & Gyulassy NPA 608, 479 (1996) ~ emission timescale Signatures may be subtle

M max  180 (320) at 2 (8) AGeV N   M  p/p  1%  5% e - contamination

1D  - HBT Excitation Function Smooth evolution of source parameters

Bertsch-Pratt projections at y cm for central collisions

Bertsch-Pratt projections at y cm for central collisions data and RQMD v2.3 with and without meanfield

Bertsch-Pratt & Yano-Koonin Fits - central collisions at y cm RQMD with or without meanfield: E beam dependence weaker than observed At low energy... Effective spatial extent underpredicted Effective temporal extent overpredicted

AGS: apparent size underpredicted SPS:  - size overpredicted NA44 RQMD R out 4.88   0.14 R side 4.45   0.20 R long 6.03   0.21 Extrapolate to RHIC? I.G. Bearden et al (NA44) PRC58, 1656 (1998) D. Hardtke, Ph.D. thesis (1997)

Well reproduced by RQMD increases w/ m T due to decreased contrib from  ’s... m T falloff in R long greatest m T falloff in R out least x (fm) y (fm) x (fm) 8 AGeV m T dependence m T dependence sensitive to space-momentum correlations, flow

Well reproduced by RQMD increases w/ m T due to decreased contrib from  ’s... m T falloff in R long greatest m T falloff in R out least x (fm) y (fm) x (fm) 8 AGeV p T dependence p T dependence sensitive to space-momentum correlations, flow

6 AGeV m T dependence “again CERN-type systematics”

6 AGeV p T dependence “again CERN-type systematics”

4 AGeV m T dependence

4 AGeV p T dependence

2 AGeV m T dependence

2 AGeV p T dependence

Rapidity dependence Strongest at lower energy (y beam = for 2 AGeV) Asymmetry about y cm may indicate major problem!?! Spurious y-dependent effect = spurious E beam -dependent effect?

Centrality dependence p T = , y=y cm  0.4 Ignoring shadowing, flow, lifetime... Overlap region RMS changes ~35% between b=2, b=7 Gentle M ch dependence: Transverse radii vary by 15-35% strongest variation at 2 AGeV

b  0  anisotropy in momentum space (elliptic flow) visible in coordinate space?? P. Danielewicz et al, PRL 81, 2438 (1998) C. Pinkenburg et al (E895), submitted to PRL see also JB 12.02

Nonspherical source (w/ no physics) “b=7 fm”  RMS x =3.5 fm RMS y =6.0 fm

RQMD (w/meanfield) prediction of effect at y cm b=4-9 fm (perfect  rp resolution) Elliptic flow of baryons 0.5 fm difference consistent w/RQMD freezeout geometry Rs and Ro oscillate against each other  3D HBT more sensitive than 1D

b = 7 x and y from RQMD

2 and 4 AGeV, b=4-7 fm: promising signal in Rside, but... 2 AGeV, y=(y cm -0.5) - y beam p T = MeV/c 4 AGeV, y=ycm  0.4 p T = MeV/c

6 AGeV midrapidity - no signal so far...

Transverse source size varies with viewing angle with respect to reaction plane in peripheral collisions target projectile overlap region reaction plane Transverse size small when viewed at 90 o

Modulation of x-p correlation with  rp enhances geometric effect stronger flow effects out of plane reduce R side further. Meanfield required at low energy to give squeeze-out in momentum space Modulation decreases with beam energy