Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Design for an eRHIC detector based on the sPHENIX detector†
K. Boyle∗1 for the PHENIX Collaboration
The PHENIX experiment has recently submitted aplan1) for a detector design at eRHIC, a version of theElectron Ion Collider (EIC) planned at Brookhaven
National Laboratory (BNL) which makes use of oneof the current RHIC hadron rings. The EIC will collide polarized electrons with heavy nuclei and polarized
protons, with its primary purpose to explore the gluon(the strong force carrier). eRHIC is expected to turnon in 2025.
A detailed description of the EIC physics case has
been laid out in the recent White Paper2) . eRHIC isexpected to probe through polarized electron-proton
collisions the properties of (sea)quarks and gluons in
the nucleon, such as spin, orbital motion and spacial
distributions. The kinematic coverage of ePHENIX in
parton momentum fraction x and 4-momentum transfer Q2 is compared to that of other measurements inFig. 1. Measurements of the gluon helicity over a wide
kinematic range will allow unprecedented constraintsof the gluon polarization, ∆g. In Semi-Inclusive scattering, correlations between transverse momentum ofgluons and quarks and the proton spin will be fully explored. Through Deeply Virtual Compton Scattering,eRHIC will measure the orbital angular momentum
contributions to the proton spin.
As it will also be able to scatter electrons off nuclei,
eRHIC will be able to explore the nature of gluons
at high density, where the effects of gluon saturation(when gluon splitting and recombination balance) areexpected. By varying kinematics as well as the nucleispecies, we will be able to vary the path length of astruck quark through the nuclei, and probe the natureof hadronization in and out of nuclear matter.
The ePHENIX detector design is shown in Fig. 2,and makes use of the BABAR solenoid3) and thesPHENIX detector upgrade4) being planned for later
this decade. We plan to add a high resolution electromagnetic calorimeter in the electron-going direction for
precision measurement of the scattered electron. GEM
based trackers will allow for charged sign identificationand hadron rejection based on energy to momentumcuts.
In addition to the sPHENIX electromagnetic andhadronic calorimetry, we plan to add a Time Projection Chamber (TPC) for tracking and a Detector of Internally Reflected Čerenkov radiation (DIRC)
for hadron particle identification (PID), based on theBABAR DIRC detector. PID in the central barrel allows for measurements of sea quark spin and transverse
momentum distributions at low momentum fraction, x.
In the hadron-going direction, new electromagnetic∗1and hadronic calorimeters are planned, as well as additional trackers and PID detectors. The combinationof an Aerogel-based Ring Imaging Čerenkov (RICH)
detector for low momentum tracks and a gas-based
RICH detector for tracks up to ∼ 60 GeV/c will allow for measurements at highest and moderate x overthe full available Q2 range.
ePHENIX will be capable of doing the physics possible with eRHIC. Efforts on fully simulating the detector is currently underway.

Fig. 1. Kinematic coverage (blue and red bands) expected at eRHIC with the ePHENIX detector for inclusive measurements in electron-proton scattering. Also
shown are current world data.

Fig. 2. Design of the ePHENIX detector at eRHIC. The
proton/Nuclei beam enters from the left, and the electron beam enters from the right.

References
1) A. Adare et al. [PHENIX Collaboration], arXiv:1402.1209[nucl-ex].
2) A. Accardi, et al., arXiv:1212.1701 [nucl-ex].
3) B. Aubert et al. [BaBar Collaboration], Nucl. Instrum.
Meth. A 479, 1 (2002).
4) C. Aidala, et al., arXiv:1207.6378 [nucl-ex].

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