# Centrality

**Centrality definitions for P18ih Au+Au 200 GeV**

**Status of centrality definitions:
** - low/mid luminosity - preliminary centrality definition complete

- high luminosity - no centrality definition

**Low/mid luminosity centrality definition**

Production: P18ih

Trigger sets: AuAu_200_production_low_2014, AuAu_200_production_mid_2014, AuAu_200_production_2014

Run IDs: 15076101-15167014

Cuts: |vz| < 30 cm, |vz - vpd vz| < 3 cm, zdcX < 60 kHz

**Links**

presentation

glauber model

centrality definition reference implementation

**StRefMultCorr procedure/parameters**

Luminosity correction:

Mean refmult as a function of ZDC coincidence rate is corrected for by a linear fit (see presentation)

parameters: 175.758, -0.307738

normalization point: 30 kHz

Vz correction:

The position of the high multiplicity tail in the refmult distribution changes as a function of Vz due to performance differences

throughout the TPC. The tail of the luminosity corrected refmult distribution is fit with an error function, and the mean of the

error function is plotted as a function of Vz. This distribution is fit with a sixth order polynomial. (see presentation)

parameters: 529.051, 0.192153, 0.00485177, -0.00017741, -1.44156e-05, 3.97255e-07, -6.80378e-10

normalization point: 0 cm

Glauber fit:

Glauber model parameters are shown in the presentation. The Glauber model is fit to the corrected refmult distribution by

optimizing the multilpicity model parameters (Npp, K, x) (see presentation for details). The best fit is selected by a chi2 test.

The centrality definition is defined by integrating out 5% bins of the Glauber distribution.

centrality bounds (refmult): 7, 10, 15, 22, 31, 43, 58, 77, 100, 129, 163, 203, 249, 303, 366, 441

The reweighting for trigger inefficiency is calculated by fitting the ratio of the corrected refmult over glauber.

weight function parameters: 1.22692, -2.04056, 1.53857, 1.55649, -0.00123008, 193.648, 1.30923e-06

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