# Quark Matter 2012

12-18 August 2012
US/Eastern timezone

## Direct photon capabilities of the proposed MPC-EX detector at PHENIX

16 Aug 2012, 16:00
2h

Poster Experiment upgrades, new facilities, and instrumentation

### Speaker

Sarah Campbell (I)

### Description

The proposed MPC-EX detector is a Si-W preshower extension to PHENIX's existing Muon Piston Calorimeter (MPC). The MPC-EX consists of eight layers of alternating W absorber and Si mini-pad sensors. Located at large rapidities, 3.1 < |$\eta$| < 3.8, the MPC-EX and MPC access low-x partons in the Au nucleus in d+Au collisions. With the addition of the MPC-EX, the neutral pion reconstruction range extends to energies > 80 GeV, a factor of four improvement over current capabilities. Not only will the MPC-EX strengthen PHENIX's existing forward $\pi^0$ and jet measurements, it also provides the necessary $\pi^0$ rejection to make a direct photon measurement feasible. With this $\pi^0$ rejection, direct photon yields at high $p_{T}$, $p_{T}$ > 3 GeV, can be statistically extracted using a double ratio method. The direct photon $R_{dAu}$ measured with the MPC-EX will quantify the level of gluon shadowing or saturation in the Au nucleus at low-x, x ~ 10^-3, with a projected systematic error band a factor of four smaller than EPS09’s current allowable range. Direct photons at forward rapidities are optimally sensitive to the gluon distribution because, unlike pions, direct photons are only produced by processes that are directly sensitive to the gluon distribution at leading order. A measurement of the forward direct photon $R_{dA}$ will cleanly access and greatly expand our understanding of the gluon nuclear parton distribution functions and provide important information about the initial state in heavy ion collisions.

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