SN0605 : A polarized p+p and p+A program for the next years

Author(s):STAR Collaboration
Date:Jun. 1, 2014
File(s): pp.pA_.LoI_.pp_.pA_.v7.pdf

In this document, the STAR collaboration presents a science driven program for future polarized p+p and p+A beam operations at RHIC in the time period after Beam Energy Scan phase II and prior to the transition to eRHIC.  This document is in response to an October 2013 charge from the BNL Associate Laboratory Director Berndt Mueller, which is reproduced in the Appendix.  STAR has previously submitted a white paper and a Letter of Intent describing its plans for Beam Energy Scan phase II and eRHIC, respectively.

The major scientific goal of the polarized p+p and p+A program is to explore Quantum Chromo Dynamics (QCD) in the high and low Bjorken x domain, driven by four overarching questions,

·       What is the nature of the spin of the proton?

·       How do quarks and gluons hadronize into final-state particles?

·       How can we describe the multidimensional landscape of nucleons and nuclei?

·       What is the nature of the initial state in nuclear collisions?


It is centered around the unique capabilities afforded with the existing STAR detector, complemented with detector upgrades including the forward calorimetric system (FCS) and forward tracking system (FCS), which are required to carry out the proposed physics program at forward rapidities. The proposed FCS and FTS upgrades were first envisioned in the STAR Decadal Plan and represent a natural evolution of the growth for STAR scientific program. These upgrades will also be in an integral part of the eSTAR configuration at eRHIC area as outlined in our eSTAR LOI.

The envisioned beam operation periods and primary science objectives are:






Longitudinally polarized p+p

500 GeV

1 fb-1

Low x Gluon helicity

Transversely polarized p+p

500 GeV

1 fb-1

Transverse Momentum Dependent proton spin structure

transversely polarized p+A with 3 nuclei: C, Cu, Au

200 GeV

2.5 pb-1

Saturation and nuclear parton distributions


We propose measurements with forward photon, J/Psi, Drell-Yan, inclusive and di-jet, and hadron/jet correlation probes at both energies, as well as W and Z probes at top energy, and demonstrate measurement capability and sensitivity through detailed simulations. It will be shown that these probes allow addressing fundamental aspects of the nucleon partonic structure, which are still rather poorly determined by experiment. One is the nature of the nucleon spin; the other is go beyond our current simple one-dimensional picture of nucleons by correlating the information on the individual parton contribution to the spin of the nucleon with its transverse momentum and spatial distribution inside the nucleon.

Measuring these probes in p+A collisions give the unique opportunity to progress our quest to understand QCD processes in Cold Nuclear Matter (CNM) by studying the dynamics of partons at very small and very large momentum fractions x in nuclei, and at high gluon-density to investigate the existence of nonlinear evolution effects.

In the remainder of this document, we motivate polarized p+p and p+A science and outline the world-wide context. We also discuss the potential for discovery in the search of exotics, i.e. glueballs by combining STAR’s large detector acceptance with its small angle tagging capabilities. Section 2 contains our proposed measurements and section 3 outlines the necessary instrument upgrades, their cost and an envisioned construction schedule.

Keywords:pp, pA, spin, CNM, polarized beam