Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Measurement of longitudinal double-spin asymmetries of J/ψ√production in polarized p+p collisions at s=500 GeV for 2012 run
X. Wang,∗1,∗2 H. Yu,∗2,∗3,∗4 and M. Liu∗42000
1800
1600

1 N ++ − RN +−,Pb Py N ++ + RN +−

1000
800
600
400

∗2∗3∗4N sig. = 2.386e+04

2500

N bkg. = 8.257e+03

2000

N sig.
= 2.889
N bkg.
N bkg.
r== 0.257
N sig.+N bkg.

1500
1000
500

20002345067M(µ + µ -) GeV/c234567M(µ + µ -) GeV/cFig. 1. Invariant mass distribution of dimuons atGeV for data obtained during the 2012 run.

√s = 500

J/ψ

Aincl
LL is related to ALL by
BG
Aincl
LL − r · ALL,
1−r

BG 2
22(δAincl
LL ) + r · (δALL )J/ψδALL =,
1−r
J/ψ

(3)

ALL =(4)where r is the background fraction, and ABG
LL is thebackground asymmetry, which is measured using likesign dimuons under the J/ψ peak and the side-band
unlike-sign dimuons.
Fig. 2 shows the sensitivity of J/ψ Aincl
LL vs. pT
based on data obtained during the 500 GeV polarized
p+p run in 2012. The analysis is in progress and weare working towards preliminary results for J/ψ crosssection and its longitudinal double-spin asymmetry.

(1)

ALLJ/ψ

χ2 / ndf

0.2

North

0.15

(2)

where Pb and Py are the beam polarizations for blue
and yellow beams, respectively; N ++ (N +− ) is the J/ψ
yield from the same (opposite) helicity beam collisions;and R = L++ /L+− is the relative luminosity measured using beam beam counter (BBC) and zero degreecalorimeter (ZDC) at very forward rapidity.
The invariant mass distribution of dimuons is shownin Fig. 1. Invariant mass distribution is fitted using
a third-order polynomial and two Gaussian functions.
The number of J/ψ is calculated on the basis of he fitting with a 2σ cut. The measured inclusive asymmetry∗1N sig.
= 2.473
N bkg.
N bkg.
r== 0.288
N sig.+N bkg.

1200

where ∆g(x)(g(x)) is the (un)polarized gluon distribuQ̄
tion, and agg→Q
is the partonic asymmetry.
LLThe J/ψ production have been measured by thePHENIX muon spectrometers at forward and backward rapidities (1.2 < |η| < 2.4), where two muons go
into the same arm.
The longitudinal double-spin asymmetry ALL canbe measured according to the following equation:
ALL =N bkg. = 6.889e+03

1400

σJ/ ψ = 0.188

3000

N sig. = 1.704e+04

p0

6.072e-027 / 2
2.588e-015 ± 0.03321

J/ψ∆g(x1 ) ∆g(x2 )Q̄
×× agg→Q,LLg(x1 )
g(x2 )

σJ/ ψ = 0.199

2200

0.10.050.05

00-0.05-0.05-0.1

-0.1

-0.15

-0.15

12345678

South0.15

RUN 12 p+p 510GeV, 42pb-1

0.1

-0.2
0

χ2 / ndf

0.2

ALLALL ∼Di-muon mass, South arm

Di-muon mass, North arm

Understanding the contribution of polarized gluons
to the proton spin is a key step for resolving the protonspin puzzle. A number of different channels have beenused to study gluon polarization, including final state
hadrons1) and jets2) . Some rarer process involving
direct photons or heavy-flavor production will allow
us to measure the gluon contribution at the leadingorder, but their significantly lower production rates
limit their impact on the ∆G constraints. At Relevistic Heavy Ion Collider (RHIC) energies, heavy-quark
production is dominated by the gluon–gluon interaction; thus, measurements of the longitudinal doublespin asymmetry in heavy-flavor production in the polarized p+p collisions will allow us to study the polarized gluon distributions. J/ψ is a bound state of a c
and c̄ pair. Here, we report the status of longitudinal
double-spin asymmetries in J/ψ production in polarized p+p collisions in the PHENIX experiment at theRHIC for data collected during 2012.
In the case of the heavy-quark production at RHIC,
the asymmetry is proportional to the gluon polarization at the leading order:

910p (GeV)
T-0.2
0p0

1.025e-040 / 2-2.727e-022 ± 0.02694

RUN 12 p+p 510GeV, 42pb-1

12345678910p (GeV)
TFig. 2. Sensitivity of ALL vs. pT in J/ψ production at√s = 500GeV for data obtained during 2012.

References1) PHENIX collaboration: Phys. Rev. D 83, 032001(2011); Phys. Rev. Lett. 103, 012003 (2009); Phys.
Rev. D 79, 012003 (2009); Phys. Rev. D 73, 091102(R)(2006).
2) PHENIX collaboration: Phys. Rev. D 84, 012006(2011).

RIKEN Nishina Center
Department of Physics, New Mexico State University
Department of Physics, Peking University
P-25, Los Alamos National Laboratory- 87 -