# Presentation: 818052

## Wounded nucleons and binary collisions

- nuclear physics doesn’t vary with IR
- but global detectors do vary (the common ZDC’s help limit this variation a bit) and sensible discussion of Npart, Ncoll depends on detector specifics
- basically ...
- determining average values not impossible
- determining fluctuations about those averages is harder
- determining reliable systematic errors consumes most of the effort

## Numerical approaches

- we have code which implements optical approximation
- Woods-Saxon nuclear density
- TA(b), TAB(b), pdf’s, statistical fluctuations all accessible
- but still leaves open the question of how to draw correspondence with real collisions as characterized by available detectors

## Slide 3

## Monte Carlo methods

- this is the approach used in current analyses
- work driven (and largely done) by Klaus Reygers
- similar to work done for WA98
- chose to build up from simple code rather than pare down more complex one (in part because it gives us knobs we know how to twiddle)

## Nuclear geometry

- had used parameterization as in VENUS: 6.65fm, 0.54fm
- used also by Date, Gyulassy, Sumiyoshi, PRD32 (1985) 619
- slightly, um, murky provenance
- MJT dug heartily into the topic
- back to Hofstadter, Phys. Rev. 101 (1956) 1131, down through the ages
- now using 6.38fm, 0.54fm as Woods-Saxon params
- effect is covered by systematic errors; had already investigated effect of varied geometry on results

## Building a nucleus

- distribute nucleons according to Woods-Saxon
- straight-line trajectories
- collisions when nucleons within distance governed by NN
- blue and red nucleons, just like PHOBOS

## Layout of PHENIX

- ZDC

- zero-degree calorimeter (ZDC)

- beam-beam counters (BBC)

- beam pipe

- central arm

- muon arm

## Forward neutrons and the ZDC

- based on measure of free neutron yield in NA49 setup, Eur. Phys. J. A2 (1998) 383
- gives some guidance out to mid-centrality
- 20% systematic error still leaves lots of uncertainty
- extrapolation to peripheral collisions largely unconstrained
- comparison to JAM for central events agrees
- JAM fine for most central 30% or so
- can reproduce shape well, overall scale 20-30% off, treated as a calibration factor

## Reproducing the ZDC spectrum

## Simulating the BBC

- Beam-Beam Counter (BBC) roughly homologous to PHOBOS paddle counter
- assumed response approximately linear with Npart; tried different relationships to map systematic errors
- separately, convoluted Pythia p+p events through GEANT to study efficiency for detecting min bias Au+Au

## Event classes

- ZDC distribution is double-valued as a function of centrality, so need add’l information
- 2D cuts in BBC vs ZDC
- cuts perpendicular to ridge line of distribution
- “clock face” cuts
- events near edges of bins move from one class to another depending on exact prescription used, but doesn’t affect extracted results significantly

## From the experiment ...

- Solid line indicates (*Q**BBC*,*E**ZDC*) centroid positions for fixed Pad Chamber multiplicity
- Centrality classes were defined with cuts perpendicular to centroid-position curve

- from K. Reyger’s seminar

## From the model ...

- Define centrality classes as in analysis of real data
- Extract mean of *N**part  *and* **N**coll* distribution for each class
- Estimate errors by varying model assumptions:

- from K. Reyger’s seminar

## Estimating systematic errors

- default calculation (R = 6.38, a = 0.54, nn = 40 mb)
- varied nn from 37 to 43 mb, R from 6.25 to 6.65, a from 0.53 to 0.55
- BBC with and without saturation
- overlapping nucleons, hard core (0.4 fm) nucleons
- different neutron loss parameterizations, BBC response functions, experimental and theoretical centrality selections
- lots of work by Jamie Nagle, Sean Kelly, Mickey Chiu, others to study influence of fluctuations, detector response, other effects