hadron talk - AGS freezeout parameters
NIX-CHAPMAN FREEZEOUT PARAMETERS FOR THE AGS DATA SET
After feeding the model all the E802 singles and pairs data, it settles at a
chisquare minimum with parameter values as shown in the table below. One of
the beauties of the model is that it produces error bars. This allows
us to see how well a data set can constrain a particular parameter (even if
you disagree with this particular implementation).
| parameter |
value and uncertainty
at 99% confidence |
|
|
| Nuclear temperature T |
92.9 +- 4.4 MeV |
| Baryon chemical potential mu-b/T |
5.97 +- 0.56 |
| Pion incoherence fraction lambda-pi |
0.65 +- 0.11 |
| Transverse freezeout radius R |
8.0 +- 1.6 |
| Transverse freezeout velocity v-t |
0.683 +- 0.018 c |
| Transverse freezeout coefficient alpha-t |
-0.86 +0.37 -0.14 |
| Source rapidity y |
1.355 +- 0.066 |
| Longitudinal spacetime rapidity eta-0 |
1.47 +- 0.13 |
| Longitudinal freezeout proper time tau-f |
8.2 +- 2.2 fm/c |
|
|
There are a couple of surprises here:
- The temperature seems low in light of the 140 or so MeV temperature
derived from the systematics of hadron slopes, as shown on
preceeding slides (#4,5,6,7,8)
Note that the velocity is correspondingly high, see slide #3.
- The pion incoherence factor is not ==1.0. Note that we experimentalists
ascribe the non-unity lambda to the decay of long-lived resonances. However,
in the Nix-Chapman model, these are already taken into account, and the
parameter in this table is the true coherence that comes on top of the
resonance-decay effect.
Reference: Phys.Rev. C54 (1996) 866, or a copy on the web
HvH, Phenix hadron workshop, March '97, Costa mesa
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