References - Bogoliubov Laboratory of Theoretical Physics - JINR

magnetic calorimeter (EMCal). Individual calorimeter towers are made of lead scintillator
(PbSc) or lead glass (PbGl) and subtend Δη × Δφ ≈ 0.01 × 0.01. This fine segmentation
allows the two photons from π 0 decay to be resolved up to pT of 12 GeV/c. Shower
shape analysis extends this range beyond 20 GeV/c, and allows the two photon invariant
mass peak from π 0 decay to be used for energy calibration. Fine segmentation is also
important for distinguishing rare direct photon events from the background from photons
originating from neutral meson decay. This allows the measurement of the theoretically
clean direct-photon asymmetry [2].
In addition to having well-understood detectors,
PHENIX has selective triggers for final
states of interest to the spin program,
including high pT pions and photons. Such
events can be recorded at rates above 5 kHz
with the PHENIX data acquisition system.
Data analysis is done in parallel at the RHIC
Computing Facility and the CCJ facility at
RIKEN in Wako, Japan.
4 Recent Physics Results
PHENIX acquires sensitivity to Δg by measuring
double helicity asymmetries in the production
of particular final states, such as π 0 .
Theasymmetriesaredefined:
A π0
LL = dσ++/dpT − dσ+−/dpT
dσ++/dpT + dσ+−/dpT
where σ++(σ+−) is the production cross section
for π 0 from pp collisions with like (unlike)
helicities. In leading order, this can be factor-
Figure 1: Beam and side views of the PHENIX
detector
ized as the sum of all partonic subprocesses ab → cX where parton c fragments into the
detected π 0 . In this framework, ALL can be understood as the convolution :
A π0
LL =
�
abc ΔfaΔfbˆσ(ab → cX)âLL(ab → cX)Dπ0 c
�
abc fafbˆσ(ab → cX)Dπ0 ,
c
where fa(Δfa) are the unpolarized (polarized) parton distribution functions, and D π c is
the fragmentation function of c into π. The spin-averaged partonic scattering cross-section
for ab → cX is denoted by ˆσ, andâLL is the analyzing power; both of which are calculable
at next-to-leading order [2].
The ALL(pp → π 0 X) analysis have been PHENIX’ most sensitive probes of Δg since
we can trigger on and reconstruct π 0 → 2γ with high efficiency. Also, at √ s = 200 GeV,
the cross-section is dominated by qg scattering for pTπ > 5 GeV, so the process is directly
sensitive to Δg.
In the analysis, the invariant mass spectrum is formed from all pairs of photons, and
the asymmetry is formed around the π 0 mass peak in a window from 112-162 MeV. The
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