2011 QCD and High Energy Interactions - Rencontres de Moriond ...

Pb
R g
2
Nuclear Modifications for g
nCTEQ
EPS09
HKN07
p/Pb (x,Q=50 GeV)
0
0.0001 0.001 0.01 0.1 1
x
P b
Figure 1: a) R
γ+c
R pPb
1.25
1.2
1.15
1.1
1.05
1
0.95
0.9
0.85
0.8
0.75
Figure 2: a) R γ+c
pP b
g (x, Q = x √ S/2 ∼ pT ) for nCTEQ decut3, decut3g3, decut3g9, EPS09 + error band, HKN07 +
P b
error band b) Rg (x, Q = x √ S/2 ∼ pT ) for different nCTEQ decut3, decut3g1-decut3g9.
p+Pb->γ+c+X / p+p->γ+c+X √
nCTEQ decut3
nCTEQ decut3g3
nCTEQ decut3g9
EPS09
HKN07
⎯ S = 8800 GeV
50 100
p Tγ (GeV)
Pb
R g
1.25
1.2
1.15
1.1
1.05
1
0.95
0.9
0.85
0.8
0.75
Nuclear Modifications for g
nCTEQ decut3
nCTEQ decut3g3
nCTEQ decut3g9
EPS09
HKN07
p/Pb (x, Q = x √ ⎯ S / 2 ~ p )
T
at LHC within ALICE PHOS acceptances, using nCTEQ decut3, decut3g3, decut3g9, EPS09
P b
+ error band, HKN07 + error band. b) Rg (x, Q = x √ S/2 ∼ pT ) in the x region probed at the LHC.
7 , EPS09 8 ) is shown. Fig. 1b) furthermore shows different fits with equally good χ2 , whose
spread represents a lower limit on the uncertainty associated with the nCTEQ seta . The need
for measurements of processes sensitive to the gluon nPDF is evident. Here we point out that
γ + Q production is an excellent probe of gA (x, Q2 ), and can serve as one such process, as
evidenced by Fig. 9 and Fig. 10 in Ref. 3 . Fig. 9 shows the differential cross-section for both
γ + c and γ + b at √ sNN = 8.8 TeV for p − P b collisions at ALICE EMCal acceptances. The
anticipated event rate (before experimental efficiencies) is sufficiently large for a measurement
pP b
pP b
b
(Nγ+c = 11900, Nγ+b = 2270). In Fig. 10 the subprocess contributions to dσpP γ+c /dpT γ are
presented, with g − Q and g − g being the dominant ones; for more details see Ref. 3 . The
sensitivity to the gluon nPDF further shows up in the nuclear modification factor to the crosssection,
R γ+c 1 dσ/dpT γ(pPb→γ+c+X)
pP b = 208
in Fig. 2a), when compared to RP b
dσ/dpT γ(pp→γ+c+X) g (x, Q) in Fig. 2b). It
can clearly be seen by juxtaposing Fig. 2a) and Fig. 2b) that R γ+c
pP b follows closely RP
b
g in the
region of x probed at the LHC for each nPDF set. Therefore we can conclude that this process
is an excellent candidate for constraining the gluon nuclear distribution as a measurement of the
prompt photon + heavy jet process with appropriately small error bars will be able to distinguish
between the three different nPDF sets. This process is thus particularly complementary to the
measurement of single inclusive photons in order to constrain parton distributions in nuclei 9 .
a These fits are available at http://projects.hepforge.org/ncteq/ .
0.01
x