PROBING GLUON NPDF WITH PHOTON+HEAVY QUARK … · HEAVY QUARK PDF 6 dc(x,Q) dt = α s 2π dy y " c(x/y)P QQ(y)+g(x/y)P Qg (y) 1x 1e-06 1 xc(x,Q) CTEQ6.6M BHPS Meson Cloud Sea-like

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