PROBING GLUON NPDF WITH PHOTON+HEAVY QUARK
PRODUCTION
Karol Kovařík
in collaboration with T. Stavreva, I.Schienbein, F.Arleo, F.Olness, J.Owens, J.Y.Yu
arXiv:1012.1178 [hep-ph]
Institute for Theoretical Physics, KIT Karlsruhe, Germany
OUTLINE
1. Direct photon & heavy quark production
2. Constraining heavy quark PDF in pp collisions
3. Constraining nuclear gluon PDF
4. Conclusions and Outlook
2
DIRECT PHOTON - LO
3
Leading order Compton hard-scattering subprocess O(!s!)
g + Q! Q + !
Direct photons
Direct photons & heavy quark jets
- any photon produced during the hard scattering process or via fragmentation
- photons escape confinement & probe the hard scattering
- ideal to determine gluon PDFs
- photons + heavy quark jets simpler than with light quark jets - less contributing processes
- more direct access to the gluon PDF
- access to the heavy quark PDF
4
Photon fragmentation contribution to leading order
O(!2s)!D!/q,g ! !2
s!
!s= !s!
DIRECT PHOTON - LO
Photon isolation suppresses fragmentation
!0 ! ""
R =!
!!2 + !"2
- photons emitted collinear to a quark → singularity
- singularities absorbed in the fragmentation function
& logs resumed via DGLAP
D!/q,g(x, µF )
- photon coupling to quark
D!/q,g(x, µF ) ! O(!/!s)
contributes already at leading order
- isolation criteria necessary to separate direct photons from photons
from hadronic decays e.g.
- hadronic energy , separation radiusEh < Eiso
5
DIRECT PHOTON - NLO NLO real contributions
gg ! QQ!
gQ! Qg! Qq ! Qq!
Qq ! Qq!
QQ! QQ!
QQ! QQ!qq ! QQ!
O(!2s!)
0 50 100 150 200
pT!
(GeV)
0
0.1
0.2
0.3
0.4
"(N
LO
fra
g)/"
(LO
Fra
g)
-1
no isolationisolation
p+p -> !+b+X#$ S =1.96 TeV
Fragmentation contributions to NLO
- fragmentation function convoluted with 2→3 process
O(!3s)!D!/q,g " O(!2
s!)
- without isolation NLO fragmentation increases
cross-section by 30%
- isolation reduces NLO fragmentation
to a few %
HEAVY QUARK PDF
6
dc(x,Q)dt
=!s
2"
!dy
y
"c(x/y)PQQ(y) + g(x/y)PQg(y)
#
0.001 0.01 0.1 1
x
1e-06
1
xc(x
,Q)
CTEQ6.6M
BHPSMeson CloudSea-like
Charm PDFsQ=40 GeV
Heavy quark PDF
- usually no intrinsic heavy quark PDF at the initial scale
- heavy quark PDF at higher scale is generated via DGLAP from the gluon PDF
c(x, µ=mc) = 0
- some data (EMC charm structure function at high x) suggest presence of intrinsic
charm in nucleons
- non-perturbative models of intrinsic charm quark
BHPS model (CTEQ6.6C0, CTEQ6.6C1) [Brodsky et.al]
Meson Cloud model (CTEQ6.5C2, CTEQ6.5C3)
Sea-like model (CTEQ6.6C2, CTEQ6.6C3)
c(x) ! d(x) + u(x)
0 50 100 150 200
pT!
(GeV)
0.0001
0.01
1
d"
/dp
T!(
pb/G
eV)
#!#
Q>0
p+p -> !+c+X$% S =1.96 TeV
0 50 100 150 200
#!#
Q<0
0 50 100 150 200
pT!
(GeV)
0.0001
0.01
1
d"
/dp
T!(
pb/G
eV)
#!#
Q>0
p+p -> !+b+X$% S =1.96 TeV
0 50 100 150 200
#!#
Q<0
TEVATRON-D0
7
Direct photon in association with charm / bottom quark jets @ D0
- comparison of NLO theory predictions with D0 measurements
- bottom quark agrees well but charm quark theory is off
- discrepancy in photon+charm description allows for testing models of intrinsic charm
[arXiv:0901.3791, arXiv:0901.0739]
0 50 100 150 200
pT!
(GeV)
0.0001
0.01
1
d"
/dp
T!(
pb/G
eV)
#!#
Q>0
p+p -> !+c+X$% S =1.96 TeV
0 50 100 150 200
#!#
Q<0
0 50 100 150 200
pT!
(GeV)
0.0001
0.01
1
d"
/dp
T!(
pb/G
eV)
#!#
Q>0
p+p -> !+b+X$% S =1.96 TeV
0 50 100 150 200
#!#
Q<0
TEVATRON-D0
7
0 50 100 150 200
pT!
(GeV)
0.0001
0.01
1
d"
/dp
T!(
pb
/GeV
)
#!#
Q<0
CTEQ 6.6M
BHPS IC modelSea-like IC model
p+p -> !+c+X$% S =1.96 TeV
0 50 100 150 200
pT!
(GeV)
0.0001
0.01
1
d"
/dp
T!(
pb
/GeV
)
#!#
Q>0
CTEQ 6.6M
BHPS IC modelSea-like IC model
p+p -> !+c+X$% S =1.96 TeV
Direct photon in association with charm / bottom quark jets @ D0
- comparison of NLO theory predictions with D0 measurements
- bottom quark agrees well but charm quark theory is off
- discrepancy in photon+charm description allows for testing models of intrinsic charm
[arXiv:0901.3791, arXiv:0901.0739]
LHC-CMS
8
50 100 150pT!(GeV)
0.001
0.01
0.1
1
10
100
d"/d
p T!(p
b/G
eV)
NLOLOBHPSsea-like
pp-> ! +c +X #$ S = 7 TeV, CTEQ6.6M
1.566<|y!|<2.5
1.5<|yQ|<2
p!T
d!/d
p! T(p
b/G
eV)
50 100 150pT!
1
1.5
2
2.5
(d"
BHPS
/dp T!
)/(d"
CTEQ
6.6M
/dp T!
)
|y!|<1.4442, |yQ|<0.5
|y!|<1.4442, 1.5<|yQ|<2
1.566<|y!|<2.5, |yQ|<0.5
1.566<|y!|<2.5, 1.5<|yQ|<2
p!T
d!B
HP
S
dp! T
/d!
CT
EQ
dp! T
Direct photon in association with charm / bottom quark jets @ CMS
- CMS cuts on photon & HQ transverse momentum, rapidity & isolation cuts
pT min Rapidity Isolation
Photon 20 GeV |yγ|<1.4442 R=0.4, pT = 4.2GeV
1.56<|yγ|<2.5
Heavy Jet 18 GeV |yQ|<2.0 ---
[CMS notes: CMS PAS EGM-10-005, CMS PAS BPH-10-009]
NPDFS
9
CTEQ framework for nuclear PDF - based on CTEQ6M proton fit
ck ! ck(A) " ck,0 + ck,1
!1#A!ck,2
", k = {1, . . . , 5}
- coefficients with A-dependance (reduces to proton for A=1)
x fk(x,Q0) = c0 xc1(1! x)c2ec3x(1 + ec4x)c5 k = uv, dv, g, u + d, s, s
d(x,Q0)/u(x,Q0) = c0 xc1(1! x)c2 + (1 + c3x)(1! x)c4
- functional form for bound protons same as for free proton PDF (restrict x to 0<x<1)
- PDF for a nucleus with A-nucleons out of which Z-protons
f (A,Z)i (x,Q) =
Z
Afp/A
i (x,Q) +A! Z
Afn/A
i (x,Q)
Note: PDF of neutron are related to the proton by isospin symmetry
- Input scale and other input parameters as in CTEQ6M proton analysis
Q0 = mc = 1.3GeV !s(mZ) = 0.118mb = 4.5 GeV
[arXiv:0907.2357]
NUCLEAR GLUON
10
Nuclear gluon PDF is badly constrained by data
- only constrained by scale variations in Sn/C DIS data
- uncertainty larger than indicated by error PDF of different nPDF analysis
0.0001 0.001 0.01 0.1 1x
0
2
R=
gP
b/g
p
EPSHKNCTEQ
Nuclear Modifications for gPb
(x,Q=50 GeV)
RP
b=
gPb(x
,Q0)
gp(x
,Q0)
x
x x x
g(x,Q
0)
g(x,Q
0) DECUT3
DECUT3G3 DECUT3G8 DECUT3G9
DECUT3G4DECUT3G2
- vary gluon nPDF assumptions & parametrizations
nCTEQ estimate of gluon nPDF uncertainty
- large uncertainty for low x<0.1 in nCTEQ framework
- need further data to constrain gluon nPDF
11
Direct photon+heavy quark in pPb collisions - very sensitive to gluon nPDF
- no intrinsic heavy quark PDF in nuclear case & gluon nuclear PDF is unconstrained
- direct photon initial state depends almost exclusively on gluon nPDF
0
0.5
1
1.5
2
nCTEQ
EPS
HKN
x0
0.5
1
1.5
f(x,Q)
g
c
RP
bg
(x,Q
=50
GeV
)R
Pb
c(x
,Q=
50G
eV)
x
- proton-Pb collisions at LHC ideal to help constrain gluon nPDF
NUCLEAR GLUON
LHC-ALICE
12
50 100 150pT!(GeV)
0.001
0.01
0.1
1
10
100
d"/d
p T!(p
b/G
eV)
NLOLO
p+Pb-> ! +c +X #$ S = 8800GeV, nCTEQ
p!T
d!/d
p! T(p
b/G
eV)
50 100 150pT!(GeV)
0.0001
0.001
0.01
0.1
1
10
d"/d
p T!(p
b/G
eV)
NLOLO
p+Pb-> ! +b +X #$ S = 8800GeV, nCTEQ
p!T
d!/d
p! T(p
b/G
eV)
Direct photon in association with charm / bottom quark jets @ ALICE
- ALICE cuts on photon & HQ transverse momentum, rapidity & isolation cuts
- Total cross-sections EMCAL: & PHOS: 130.5 pb850 pb
pT min Rapidity Φ Isolation
PHOS 20 GeV |yγ|<0.12 220°<Φ<320° R=0.2, pT= 2GeV
EMCAL 20 GeV |yγ|<0.7 60°<Φ<180° R=0.2, pT= 2GeV
Heavy Jet 15 GeV |yQ|<0.7 --- ---
LHC-ALICE
13
50 100 150
pT!
(GeV)
0.0001
0.001
0.01
0.1
1
10
100
d"
/dp
T!(
pb/G
eV)
NLO
LO+gg->QQ!gQ->gQ!
qq->QQ!qQ->qQ!
QQ->QQ!
p+Pb -> !+c+X#$ S =8800 GeV,CTEQ nPDFs
p!T
d!/d
p! T(p
b/G
eV)
Direct photon - contributing NLO subprocesses
- most of the cross-section is dominated by gluon/charm quark initiated subprocesses (80%)
- direct photon + HQ @ LHC is sensitive to gluon nuclear PDF
LHC-ALICE
14
Direct photon - contributing NLO subprocesses
- most of the cross-section is dominated by gluon/charm quark initiated subprocesses (80%)
- direct photon + HQ @ LHC is sensitive to gluon nuclear PDF
0.0001 0.001 0.01 0.1 1x
0
2
R=
gP
b/g
p
EPSHKNnCTEQ
Nuclear Modifications for gPb
(x,Q=50 GeV)
RP
b=
gPb(x
,Q0)
gp(x
,Q0)
x50 100
pT!
(GeV)
0.75
0.8
0.85
0.9
0.95
1
1.05
1.1
1.15
1.2
1.25
R("
(p+
Pb
)/"
(p+
p))
EPSHKNnCTEQ
p+Pb->!+c+X / p+p->!+c+X #$ S = 8800GeV
p!T (GeV)
R!
!(p
+P
b)!(p
+p)
"
- direct photon + HQ @ ALICE test nuclear gluon PDF at x ! 0.01
CONCLUSIONS
15
Direct photon in association with a heavy quark jet is a useful tool
TEVATRON - D0 → good distinction between different IC models
can test BHPS, Sea-like IC models
LHC - CMS → better sensitivity to gluon & HQ PDF than Tevatroncan test IC models
LHC - ALICE → pPb collisions one of few possibilities out there
to constrain nuclear gluon PDF
Direct photon in association with a heavy quark jet can be used to
investigate jet quenching in PbPb collisions