Double Helicity Asymmetry of Inclusive pi0 Production in Polarized pp
  Collisions at sqrt(s)=62.4GeV
Double Helicity Asymmetry of Inclusive π0
Production in Polarized pp Collisions at√
s = 62.4 GeV
K. Aoki for the PHENIX Collaboration
Department of Physics, Kyoto University, Kyoto, Kyoto, Japan, 606-8502
Abstract. The proton spin structure is not understood yet and there has remained large uncertainty
on ∆g, the gluon spin contribution to the proton. Double helicity asymmetry (ALL) of π0 production
in polarized pp collisions is used to constrain ∆g. In this report, preliminary results of ALL of π0
in pp collisions at
s = 62.4 GeV measured by PHENIX experiment in 2006 is presented. It can
probe higer x region than the previously reported π0ALL at
s = 200 GeV thanks to the lower center
of mass energy.
Keywords: Spin, Proton spin structure
PACS: 14.20.Dh, 13.85.Ni
INTRODUCTION
The so-called “proton spin crisis,” initiated by the results from the polarized deep
inelastic scattering experiments, has triggered wide effort towards the understanding
of proton spin. Despite the wide efforts, there has remained large uncertainty on ∆g,
the gluon spin contribution to the proton. RHIC, the world’s first polarized proton-
proton collider, provides us an opportunity to directly probe gluons in the proton.
Double helicity asymmetry (ALL) of inclusive π0 production in polarized pp collisions
is sensitive to ∆g because π0 production is dominated by gluon-gluon and quark-gluon
interactions in the measured pT range. PHENIX has previously reported π0ALL in pp
collisions at
s = 200 GeV [1] which is based on the data taken in 2005 (Run5) and
it indicates that ∆g is not large.[2] But a large uncertainty remains for large Bjorken
x (> 0.1) and more statistics are needed. During the run in 2006 (Run6), one-week data
taking was performed at
s = 62.4 GeV. Spin rotator commissioning was successful
and we had longitudinally polarized collisions. [3] Even in this short data taking with a
small integrated luminosity of 60 nb−1 and the average polarization of 48%, the data has
a big advantage to cover the larger x region thanks to the lower center of mass energy.
According to a pertubative QCD (pQCD) calculation, collisions at
s = 62.4 GeV has
∼ 300 times larger cross-section than that at
s = 200 GeV at fixed xT = 2pT/
s. It
corresponds to 10 times larger statistics than the previously reported π0ALL which is
based on the integrated luminosity of 1.8 pb−1 with average polarization of 47%.
ALL is defined as
ALL =
σ++−σ+−
σ+++σ+−
http://arxiv.org/abs/0704.1369v1
where σ++(+−) is the production cross-section in like (unlike) helicity collisions. Ex-
perimentally, ALL is calculated as
ALL =
|PB||PY |
N++−RN+−
N+++RN+−
, R =
where PB(Y ) denotes the beam polarization, N
++(+−) is the π0 yield and L++(+−) is the
luminosity in like (unlike) helicity collisions. R is the relative luminosity.
EXPERIMENT
The stable polarization direction of RHIC beam is transverse. Then it is rotated to get
longitudinally polarized collisions just before the PHENIX interaction point. PHENIX
local polarimeter[3] confirms that the beam is longitudinal by measuring AN of forward
neutrons.
PHENIX has Beam-Beam Counter (BBC) which covers 3.0 < |η| < 3.9 and Zero
Degree Calorimeter (ZDC) which covers very forward angle (±2mrad).[4] These two
detectors serve as independent luminosity measure. We used BBC counts to measure
relative luminosity R in equation (2) and its uncertainty is estimated by comparing to
ZDC counts. It is found to be δR = 1.3×10−3. This corresponds to δALL = 2.8×10−3
which is less than the statistical uncertainty.
PHENIX has the ability to clearly identify π0 through its gamma decay by using
an Electro-Magnetic Calorimeter (EMCal) which covers the central rapidity region
(|η| < 0.35) and half in azimuth angle. [4] PHENIX also has an excellent gamma
triggering capability (the threshold is 0.8 GeV or 1.4 GeV) which makes high-statistics
π0 measurement feasible.[5] EMCal based trigger without coincidence with BBC is used
because the collision trigger efficiency based on BBC is low at
s = 62.4 GeV.
The systematic uncertainty is evaluated by the bunch shuffling technique,[6] and it is
found to be negligible.
ALL CALCULATION
π0ALL (Aπ
LL) is calculated by subtracting A
LL from A
π0+BG
LL . A
π0+BG
LL is the asymmetry
for the diphoton invariant-mass range of 112 MeV/c2-162 MeV/c2 (under the π0 peak).
ABGLL is the asymmetry for the range of 177 MeV/c
2-217 MeV/c2 (higher side band).
Figure 1 shows the diphoton invariant mass spectra. The lower mass peak corresponds
to background from hadrons and cosmic particles, which induce EMCal clusters with
more complicated structure, each of them are then splitted on several ones. This peak
roughly corresponds to two EMCal cell separation between two clusters, which moves
to higher mass with increasing cluster pair pT . Since we used EMCal based trigger
without coincidence with collision trigger at
s = 62.4 GeV, the cosmic background is
prominent unlike in data at
s = 200 GeV. The contribution of such background under
π0 peak is negligible in the measured pT range. Since it does affect the lower side band,
the Aπ
LL estimation was done based only on the higher side band. The subtraction is
FIGURE 1. Diphoton invariant mass spectra.
done by using the following formula.
LL − rABGLL
where r is the background fraction.
RESULTS
Figure 2 shows the Run 6 results of π0ALL as a function of pT . ALL is consistent with
zero over the measured pT region. Detailed offline analysis on beam polarization is not
provided yet by the RHIC polarimeter group. Thus online values are used and systematic
uncertainty of 20% is assigned for a single beam polarization measurement. It introduces
scaling uncertainty of 40% on ALL. Theory curves based on pQCD using four proton
spin models are also shown.[7] The theory is based on pQCD; thus it is important to test
pQCD applicability at
s = 62.4 GeV. To test pQCD applicability, analysis on π0 cross-
section is on-going. With our cross section result, we will be able to discuss our ALL
result further by comparing with pQCD calculations. Figure 3 shows the Run 6 results
of π0ALL as a function of xT together with Run 5 results. A clear statistical improvement
can be seen in the large xT region.
SUMMARY
During the RHIC run in 2006, π0ALL at
s= 62.4 GeV was measured with the PHENIX
detector. Preliminary results of π0ALL at
s = 62.4 GeV with integrated luminosity of
60 nb−1 and the average polarization of 48% are presented. There is a clear statistical
improvement in the large xT regin compared to the Run5 preliminary results at
200 GeV with integrated luminosity of 1.8pb−1 and the average polarization of 47%.
To extract the gluon spin contribution to the proton, it is important to test pQCD
applicability at
s = 62.4 GeV. Analysis on cross-section is on-going to test pQCD
at this energy. With our cross section result, we will be able to discuss our ALL result
further by comparing with pQCD calculations.
FIGURE 2. π0ALL as a function of pT . The error bar denotes statistical uncertainty. Gray band denotes
systematic error from relative luminosity.
FIGURE 3. π0ALL as a function of xT .
REFERENCES
1. K. Boyle, AIP Conf. Proc. 842, 351–353 (2006), nucl-ex/0606008.
2. M. Hirai, S. Kumano, and N. Saito, Phys. Rev. D74, 014015 (2006), hep-ph/0603213.
3. M. Togawa, et al., RIKEN Accel. Prog. Rep. to be published 40 (2007).
4. K. Adcox, et al., Nucl. Instrum. Meth. A499, 469–479 (2003).
5. K. Okada, et al., RIKEN Accel. Prog. Rep. 36, 248 (2003).
6. S. S. Adler, et al., Phys. Rev. Lett. 93, 202002 (2004), hep-ex/0404027.
7. B. Jager, A. Schafer, M. Stratmann, and W. Vogelsang, Phys. Rev. D67, 054005 (2003),
hep-ph/0211007.
nucl-ex/0606008
hep-ph/0603213
hep-ex/0404027
hep-ph/0211007
	Introduction
	Experiment
	ALL calculation
	Results
	SUMMARY
