Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Status of π 0 pair ALL analysis in RHIC-PHENIX experimentK.Hashimoto,∗1,∗2 R.Seidl,∗1 and Y.Goto∗1 for the PHENIX collaboration
1Introduction

and x2 of the dijet production at the mid-rapidity region have a similar value. We can not mesure total energy of scattered partons. We measured the π 0 pair instead of the dijet(or partons). If we select back-to-backhadron pair production at the mid-rapidity region, theBjorken-x of two incoming partons should be almostbalanced. If values of two Bjorken x are not balanced,
the produced particles system is boosted, and theseparticles should move to the exterior of the PHENIX
central arm acceptance, where the rapidity region is|∆η| < 0.35. Figure 1 is a result of PYTHIA6.4 simulation. The vertical the horizontal axes are the log of
Bjorken-x distribution. The colors denote the numberof events. We accepted all events in which the pt ofπ 0 , which decays gamma fire trigger is up to 2.0 GeV
and in which the pt of π 0 , which is produced in the opposite direction of the triggered π 0 is up to 1.5 GeV.
The events in which two Bjorken-x’s are similar are selected. The selection of back-to-back hadron pair production at a mid-rapidity region can suppress eventsin which two Bjorken-x’s are Bjorken-x1>>Bjorken-x2
or Bjorken-x1<<Bjorken-x2.

The proton has a spin of 1/2 that originates frominternal quarks and gluons. Results from deep inelastic scattering (DIS) experiments1) show that the quark
spin contribution to the proton spin is only about 25%. In the PHENIX experiment, the gluon-spin contribution to the proton spin has been studied for more than
10 years. In recent years, double helicity asymmetries,
ALL , have been measured in several types production(π 0 , π ± , direct photon, etc). In the case of single inclusive π 0 production, ALL is defined as follow:
0AπLL ≡∑

ab→cX ab→cX π 0âLLDc∑
ab→cX D π 0ffσ̂
ca,b,c a ba,b,c ∆fa ∆fb σ̂

(1)where fa,b represent unpolarized parton distributionfunctions (PDFs) of partons a and b and ∆fa,b repre0sent polarized PDFs, Dcπ is a fragmentation function(FF) of parton c to π 0 , σ̂ ab→cX and âab→cXdenoteLL
the cross section and ALL of the partonic subprocess
ab → cX respectively.
Experimentally, the ALL for π 0 production is determined as0AπLL =N++ − RN+−
1L++;R =< PB PY > N++ + RN+−L+−3

We calculated ALL as well as AL for π 0 pair production , ALL for the single inclusive π 0 production
etc, for cross checking. We determined te our analysis
passed these tests. We are also checking for consistencyin statistics between experimental data and simulated
data with the PYTHIA 6.4 event generator.
This analysis is being preformed at RIKEN-CCJ2) ,and we are grateful for its smooth operation.

(2)

where N++(+−) is the number of π 0 s and R is the relative luminosity between bunches with the same andopposite helicities. < PB PY > are the averaged beampolarizations.
2

Current status of this study

Simulation study on kinematics coverage

Bjorken-x is a fraction of a proton’s longitudinal momentum. Let us imagine a reaction parton a interacting with parton b and producing jets(or partons) c andd: parton(a)+parton(b) → jet(c)+jet(d). In this case,
Bjorken-x can be determined by using pseudorapidity
and the transverse momentum of scatterted partons cand d1
x1 = √ (pT,c eηc + pT,d eηd )s(3)1x2 = √ (pT,c e−ηc + pT,d e−ηd )s(4)Fig. 1. Bjorken-x distribution for the π 0 pair production.

If both partons c and d are produced in the midrapidity region, eηc,d and e−ηc,d are similar. Hence, x1∗1∗2References1) http://arxiv.org/abs/hep-ph/0101224v1
2) http://ccjsun.riken.jp/

RIKEN Nishina Center
Department of Physics, Rikkyo University- 86 -