Production of a photon in association with a heavy quark ... · Production of a photon in...

Production of a photon in association with a heavy quark jet in pA and AA

collisions

I. SchienbeinLPSC Grenoble/Univ. Grenoble

9th International Workshop on High-pT Physics at LHCLPSC Grenoble, 24 - 28 September 2013

EAOM LPSC, Serge Kox, 20/9/2012

CampusUniversitaire

Laboratoire de Physique Subatomique et de Cosmologie

Monday, October 8, 12

Saturday, September 28, 13

Introduction


γ+Q production is a versatile process!

• in pp collisions:

• probe of heavy quark PDFs

• intrinsic charm (IC)/ intrinsic bottom (IB)

• in pA collisions:

• probe of nuclear PDFs (gluon, heavy quarks)

• in AA collisions:

• probe of heavy quark energy loss


γ+Q production at NLODirect Photon and Heavy Quarks - hardscattering

how are they produced?

Leading Order - O(↵↵s

) - Only one hard-scattering subprocess

Compton Subprocess g + Q ! Q + �

Next-to-Leading Order - O(↵↵2

s

)

Real Corrections - 2 ! 3 body scattering subprocesses

g + g ! Q + Q̄ + � Q + Q ! Q + Q + �g + Q ! g + Q + � Q + Q̄ ! Q + Q̄ + �Q + q ! q + Q + � q + q̄ ! Q + Q̄ + �Q + q̄ ! Q + q̄ + �

Virtual Corrections - interference between LO Born diagram andvirtual diagrams

T. Stavreva Probing IC through � + Q production

[T. Stavreva, J. Owens, PRD79(2009)054017]


Fragmentation contributionDirect Photon and Heavy Quarks - fragmentation

LO: include all 2 ! 2 subprocesses ⇠ O(↵2

s

),O(↵2

s

)⌦ D�/q,g ⇠ ↵2

s

↵/↵s

= ↵↵s

Q

Q̄

Q

Q̄

Also need to include NLO fragmentation contributions - convoluteall 2 ! 3 ⇠ O(↵3

s

) with � FF

Q

Q̄

Reduced due to isolation requirements

minimizes background from photons : ⇡0 ! ��



Final state collinear singularity in qq̄ ! QQ̄+ �Final State Collinear Singularity: qq̄ ! QQ̄�

q

q̄

Q

Q̄

1)

q

q̄

Q

Q̄

2)

Unlike for inclusive direct photon the annihilation subprocess doesnot appear at LO

q

q̄

Jet observed in final state (not meson ! no HQ FF)

Regulate singularity by retaining HQ mass: (pQ

+ p

¯

Q

)2 > 4m2

Q

T. Stavreva Probing IC through � + Q productionSaturday, September 28, 13

γ+Q in pp collisions

References:T. Stavreva, J. Owens, PRD79(2009)054017Bednyakov, Demichev, Lykasov, Stavreva, Stockton,arXiv:1305.3548


Is there charm in the nucleon?

• Standard approach: Charm entirely perturbative

• Heavy Flavour Schemes

• FFNS: charm not in the protonkeep logs(Q/m) in fixed order

• VFNS: charm PDF in the protonresum logs(Q/m)

• Different Heavy Flavour Schemes = different ways to organize the perturbation series

• What is structure? What is interaction?Freedom to choose the factorization scale

• However, charm not so much heavier than Lambda_QCD

• There could be a non-perturbative intrinsic charm component

• Important to test the charm PDF experimentally

LO NLO N2LO N3LO

Full ACOT

ACOT Extension to Higher Orders 29

Based on the Collins-Wilczek-Zee (CWZ) Renormalization Scheme… hence, extensible to all orders

DGLAP kernels & PDF evolution are pure MS-BarSubtractions are MS-Bar

ACOT: mÆ 0 limit yields MS-Bar

with no finite renormalization

LO NLO N2LO N3LO

Full ACOT

ACOT Extension to Higher Orders 29

Based on the Collins-Wilczek-Zee (CWZ) Renormalization Scheme… hence, extensible to all orders

DGLAP kernels & PDF evolution are pure MS-BarSubtractions are MS-Bar

ACOT: mÆ 0 limit yields MS-Bar

with no finite renormalizationSaturday, September 28, 13

How to Access Heavy Flavor Components Directly??? 23

c g→ c γb g→ b γ

s g→ c W

c g→ b W

How to access the heavy quark PDFs directly?


How to Access Heavy Flavor Components Directly??? 23

c g→ c γb g→ b γ

s g→ c W

c g→ b W

How to access the heavy quark PDFs directly?


Results from TevatronComparison between theory & data

Measurements by DØ Collaboration [arXiv:0901.0739]

0 50 100 150 200

pTγ(GeV)

0.0001

0.001

0.01

0.1

1

dσ/d

p Tγ(p

b/G

eV)

yγyQ>0

0 50 100 150 200

yγyQ<0

p+p -> γ+b+X √$S = 1.96 TeV

0 50 100 150 200

pTγ(GeV)0.0001

0.001

0.01

0.1

1

10

dσ/d

p Tγ(p

b/G

eV)

yγyQ>0

0 50 100 150 200

yγyQ<0

p+p -> γ+c+X √$S = 1.96 TeV

Really good agreement for � + b

Not so for � + c

Given this: Possible explanation - existence of intrinsic charm ratherthan higher order corrections



Intrinsic Charm effect on γ+c

Intrinsic Charm e↵ect on � + c

50 100 150pTγ

0

0.5

1

1.5

2

2.5

3

3.5

Data/Theory

µ=pTγµ=0.5pTγµ=2pTγBHPSSea-like

yγyQ>0

50 100 150pTγ

0

0.5

1

1.5

2

2.5

3

3.5

Data/Theory

µ=pTγµ=0.5pTγµ=2pTγBHPSSea-like

yγyQ<0

Sealike - overshoots data at low pT and undershoots at high pT

BHPS - the cross section grows at large pT, but still below dataResult inconclusive -

New Measurements - Tevatron - CDF & DØTest at pp Colliders - RHIC & LHC


•BHPS IC: cross section grows at large pT but still below data

•Results inconclusive:•new measurements at Tevatron: CDF & D0• test at pp colliders: RHIC & LHC


New results from D0

new results DØ

(GeV)!

Tp

0 50 100 150 200 250 300

Ra

tio

to

NL

O

-1

0

1

2

3

4

5

6

Data / NLOSHERPA / NLOPYTHIA / NLO

fact./ NLOTk

Scale uncertaintyPDF uncertainty

Sea-like IC/CTEQBHPS IC/CTEQ

-1DØ, L = 8.7 fb

|<1.0!

|y

>15 GeVjet

T|<1.5, p

jet|y

(GeV)!

Tp

0 50 100 150 200 250 300

(p

b/G

eV

)! T

/dp

" d

-410

-310

-210

-110

1

| < 1.0!

data, |y

| < 2.5!

data, 1.5 < |y

NLO (Stavreva, Owens) fact. (Lipatov, Zotov)Tk

SHERPA, v1.3.1

PYTHIA, v6.420

>15 GeVjet

T|<1.5, p

jet|y

(x0.3)

-1DØ, L = 8.7 fb

� + c - left - arXiv:1210.5033� + b - right - arXiv:1203.5865Even higher discrepancy - now consider all leading jets


Even larger discrepancy now!


• Tevatron: q-qbar channel

• effectively LO

• dominant at pT > 70 GeV

• large NNLO corrections!?

• LHC:

• g & Q initiated subprocesses dominate (>80%)

• expect no anomaly

• important to measure, probe of IC

• For many more details (predictions for RHIC, LHC, used cuts, γ+b, etc.) see:

• Talk T. Stavreva at DIS 2013

• arXiv:1305.3548

Discussion: Is there a QCD anomaly?Subprocess Contributions

50 100 150 200pTγ(GeV)

1e-08

1e-06

0.0001

0.01

1

dσ/d

p Tγ(p

b/G

eV)

NLOqq->QQγqQ->qQγgQ->gQγLO+gg->QQγQQ->QQγ

p+p -> γ+b+X√$ S =1.96 TeV

At pT� ⇠ 70 GeV qq̄ ! QQ̄� starts to dominate

Abundance of q & q̄ at pp̄ colliders


Subprocess Contributions in pp Collisions

50 100 150

pTγ(GeV)0.1

1

10

100

1000

10000

dσ/d

p Tγ(p

b/G

eV) NLO

LO+gg->QQγgQ->gQγqq->QQγqQ->qQγ ; QQ->QQγ

p+Pb-> γ +c +X √$ S = 8800 GeV, nCTEQ

g & Q initiated subprocesses dominate ( > 80% ) ) sensitivity togluon and HQ PDFs.




• effectively LO



• LHC:






• arXiv:1305.3548


50 100 150 200pTγ(GeV)

1e-08

1e-06

0.0001

0.01

1

dσ/d

p Tγ(p

b/G

eV)


p+p -> γ+b+X√$ S =1.96 TeV





50 100 150

pTγ(GeV)0.1

1

10

100

1000

10000

dσ/d

p Tγ(p

b/G

eV) NLO







• effectively LO



• LHC:






• arXiv:1305.3548


50 100 150 200pTγ(GeV)

1e-08

1e-06

0.0001

0.01

1

dσ/d

p Tγ(p

b/G

eV)


p+p -> γ+b+X√$ S =1.96 TeV





50 100 150

pTγ(GeV)0.1

1

10

100

1000

10000

dσ/d

p Tγ(p

b/G

eV) NLO






γ+Q in pA collisions

References:T. Stavreva et al., JHEP1101(2011)152


Nuclear PDFs: needed for pA collisionsGlobal fits

Global fit analyses of nuclear parton densities

DIS and Drell-Yan data [ EKS98, HKM, nDS, nDSg, nCTEQ ]

. . . and hadron production at RHIC [ EPS09, DSSZ ]

QuestionHow to probe small-x gluon shadowing at LHC ?

which observables

why prompt photons look promising

Francois Arleo (LLR & LAPTh) ! and !+Q production in p A collisions Trento Workshop pA@LHC 4 / 25

Global fits

Global fit analyses of nuclear parton densities

DIS and Drell-Yan data [ EKS98, HKM, nDS, nDSg, nCTEQ ]

. . . and hadron production at RHIC [ EPS09, DSSZ ]

0.20.40.60.81.01.21.4

10-4 10-3 10-2 10-1 1

EPS09NLOnDSHKN07

G(,

2 =1.69GeV

2 )Pb

[ EPS09 Eskola, Paukkunen, Salgado 0902.4154 ]


gluongluon nCTEQ decut3gx fits nCTEQ decut3gx fits

x

A=1,2,4,9,12,27,56,108,207

● This still underestimates the true uncertainty

● Some curves lie outside the error bands of EPS'09 and/or HKN'07!

Friday, April 6, 12

[EPS09, Eskola, Paukkunen, Salgado, 0902.4154] [nCTEQ, 0907.2357 & 1012.1178]


nCTEQ PDFs with uncertainties

13

nCTEQ nCTEQ nuclear correction factors

with uncertainties

different solution for d-valence & u-valence

compared to EPS09

larger uncertainty @ gluon nuclear correction

factor & bigger low-x suppression

sea quark nuclear correction factors similar to

EPS09

Ri(Pb) =f

Pbi (x,Q)

f

pi (x,Q)

Q2 = 100 GeV2Pb

PREL

IMINA

RY

Q2 = 100 GeV2@

nuclear correction factors depend largely on

underlying proton baseline

13

nCTEQ nCTEQ nuclear correction factors

with uncertainties

different solution for d-valence & u-valence

compared to EPS09

larger uncertainty @ gluon nuclear correction

factor & bigger low-x suppression

sea quark nuclear correction factors similar to

EPS09

Ri(Pb) =f

Pbi (x,Q)

f

pi (x,Q)

Q2 = 100 GeV2Pb

PREL

IMINA

RY

Q2 = 100 GeV2@

nuclear correction factors depend largely on

underlying proton baseline Q2=100 GeV2

Pb

Preliminary

work in progress, see [K. Kovarik et al, 1307.3454]


• Jets

• Tevatron incl. jet data already used

• Prompt Photons

• Arleo et al, JHEP1104(2011)055

• d’Enterria, Rojo, NPB860(2012)311

• Virtual Photons

• see talk by M. Klasen

• Photons + Heavy Quarks

• T. Stavreva et al., JHEP1101(2011)152 (this talk)

• Open heavy quarks?

• Heavy Quarkonia?

How to probe the gluon distribution?

Kinematical range

1

10

10 2

10 3

10 4

10 5

10 6

10 -6 10 -5 10 -4 10 -3 10 -2 10 -1

photons

heavy bosonsjets

NMCQs

2

x

Q2 (G

eV2 )

(x ,Q2) domain covered at the LHC

Photons and jets are clearly complementary

Photons cover small Q2 where shadowing should be large


LHC kinematics


• Jets

• Tevatron incl. jet data already used

• Prompt Photons

• Arleo et al, JHEP1104(2011)055

• d’Enterria, Rojo, NPB860(2012)311

• Virtual Photons

• see talk by M. Klasen

• Photons + Heavy Quarks

• T. Stavreva et al., JHEP1101(2011)152 (this talk)

• Open heavy quarks?

• Heavy Quarkonia?

How to probe the gluon distribution?

Kinematical range

1

10

10 2

10 3

10 4

10 5

10 6

10 -6 10 -5 10 -4 10 -3 10 -2 10 -1

photons

heavy bosonsjets

NMCQs

2

x

Q2 (G

eV2 )

(x ,Q2) domain covered at the LHC

Photons and jets are clearly complementary

Photons cover small Q2 where shadowing should be large


LHC kinematics


Predictions in p A collisions

NLO calculations assuming that charm PDF is radiatively generated(c(x , µ = mc) = 0)

dc(x ,Q)

dt=

!s

2"

!

dy

y[c(x/y)PQ!Q(y) + g(x/y)Pg!Q (y)]

in the variable flavour number scheme

Probing charm nPDF allows for constraining the gluon sector

0

0.5

1

1.5

2

RgPb

nCTEQEPS09HKN07

0

0.5

1

1.5

RcPb

0.001 0.01 0.1x0

0.5

1

1.5

Ru v

Pb

u

0

0.5

1

1.5

2

RcPb/RgPb

nCTEQEPS09HKN07

0.01 0.1x0

0.5

1

1.5

RcPb/Ru v

Pb


Predictions in pA


Predictions in p A collisions

NLO calculations assuming that charm PDF is radiatively generated(c(x , µ = mc) = 0)

dc(x ,Q)

dt=

!s

2"

!

dy

y[c(x/y)PQ!Q(y) + g(x/y)Pg!Q (y)]

in the variable flavour number scheme

Predictions at RHIC and LHCRHIC: d Au collisions at

!sNN

= 200 GeVLHC: p Pb collisions at

!sNN

= 8.8 TeV

Calculation of quenching factors

R!QpA =

# (pA " $ Q X)

A # (pp " $ Q X)

using di!erent nPDF sets then available: nCTEQ, EPS09, HKN07


Predictions in pA


Spectra and rates

Absolute cross sections

10 15 20 25

pTγ(GeV)

1

10

100

1000

10000

dσ/d

p Tγ(p

b/G

eV) NLO


d+Au-> γ +c +X √$ S = 200 GeV, nCTEQ

50 100 150

pTγ(GeV)0.1

1

10

100

1000

10000

dσ/d

p Tγ(p

b/G

eV) NLO



Cross section dominated by Compton scattering

Important NLO corrections


Spectra and rates


Spectra and rates

Expected rates at RHIC (PHENIX kinematics)

Assuming Lyear = 0.74 pb!1

Photon + charm

N ! 3" 104 for pT!> 7 GeV

N ! 102 for pT!> 20 GeV

Photon + bottom

< 100 events for pT!> 17 GeV


Spectra and rates


Spectra and rates

Expected rates at LHC (ALICE kinematics)

Assuming Lyear = 0.1 pb!1

!pPb!+Q NpPb

!+Q

" + c PHOS 22700 pb 2270" + b PHOS 3300 pb 330" + c EMCal 119000 pb 11900" + b EMCal 22700 pb 2270

Large for " + c and " + b events at LHC


Spectra and rates


Constraining the gluon nPDF

LHC

50 100

pTγ(GeV)0.75

0.8

0.85

0.9

0.95

1

1.05

1.1

1.15

1.2

1.25

Rσγ+

c

EPS09HKN07nCTEQ

p+Pb->γ+c+X / p+p->γ+c+X √$ S = 8800GeV

0.01x0.75

0.8

0.85

0.9

0.95

1

1.05

1.1

1.15

1.2

1.25

RgPb

EPS09HKN07nCTEQ

Nuclear Modifications for gp/Pb(x,Q=50 GeV)

R!+cpPb follows RPb

g very closely

Almost no overlap between EPS09 and HKN07, and nCTEQ decut3

Measurements with su!ciently small error bars should disentangle thevarious nPDF sets



Main result



RHIC

8 10 12 14 16 18 20

pTγ(GeV)0.8

1

1.2

1.4

1.6

1.8

RdA

uγ+c

nCTEQnCTEQ - no nuclear corrections to dEPS09HKN07

d+Au->γ+c+X / p+p->γ+c+X √# S = 200 GeV

0.1 x0.8

1

1.2

1.4

1.6

1.8

RgA

u

nCTEQEPS09HKN07

Nuclear Modifications for gp/Au(x, Q = x √#S / 2 ~ pT)

Same observation than at LHC : R!+cdAu ! RAu

g

Kinematic region probed at RHIC (x = 10!1–2" 10!1)complementary to that at LHC (x = 5" 10!3–2 " 10!2)



Main result


Conclusions I

• g+Q initiated subprocesses dominate

• In standard approach: Heavy quark PDF dynamically generated; clean probe of gluon PDF

• No isospin effects! Quenching factor R=1 if no nuclear effects

• Cross sections large enough at RHIC and LHC

•

• Baseline for photon+Q production in AA collisions


LHC

50 100

pTγ(GeV)0.75

0.8

0.85

0.9

0.95

1

1.05

1.1

1.15

1.2

1.25

Rσγ+

c

EPS09HKN07nCTEQ

p+Pb->γ+c+X / p+p->γ+c+X √$ S = 8800GeV

0.01x0.75

0.8

0.85

0.9

0.95

1

1.05

1.1

1.15

1.2

1.25

RgPb

EPS09HKN07nCTEQ

Nuclear Modifications for gp/Pb(x,Q=50 GeV)

R!+cpPb follows RPb

g very closely

Almost no overlap between EPS09 and HKN07, and nCTEQ decut3

Measurements with su!ciently small error bars should disentangle thevarious nPDF sets



γ+Q in AA collisions

References:T. Stavreva, F. Arleo, IS, JHEP1302(2013)072


Probing (massive) parton energy loss in QGP

Energy loss of massive partons

Heavy quark mass acts as a collinear cuto! for medium-induced gluonradiation, just like in vacuum (dead cone) [ Doskhitzer Kharzeev 2001 ]

Clear hierarchy expected!

"E"

"

g>#

"E"

"

q> "E

"

"

c> "E

"

"

b








"E"

"

g>#

"E"

"

q> "E

"

"

c> "E

"

"

b

[ ALICE ]








"E"

"

g>#

"E"

"

q> "E

"

"

c> "E

"

"

b

! + Q unique tool to probe Q energy loss in the plasma

γ γ c γ bq




Analysis in A A collisions

Calculations performed at NLO accuracy at!s = 5.5 TeV

Heavy quark energy loss !Qestimated on an event-by-event basis from

the quenching weight (probability distribution) obtained perturbatively[ Armesto Dainese Salgado Wiedemann 2005 ]

Various observables investigatedPhoton–jet energy asymmetry AJ

AJ =ET1 " ET2

ET1 + ET2,!" > #/2

Momentum imbalance z!Q

z!Q = "$pT! .$pTQp2T!

Photon–jet pair momentum q!

q! = |pT! + pTQ |Francois Arleo (LLR & LAPTh) ! and !+Q production in p A collisions Trento Workshop pA@LHC 22 / 25

Analysis in AA collisions


Transverse momentum distributions

[ Stavreva FA Schienbein 2013 ]

Comparing photon and jet p! spectra

No (or little) modifications of the photon p!

Stronger e!ects on the heavy quark jet spectrum





[ Stavreva FA Schienbein 2013 ]

Nuclear production ratios RAA

Sensitive to the amount of energy loss assumed in the calculation

Needs to be compared to !+ inclusive jet production




Pair momentum distribution

Why q! distribution

q! ! !Qat LO accuracy if the photon is produced directly

The shift in the q! distributions should reflect the strength of heavyquark energy loss in the medium


Pair momentum distribution


Conclusions II

• AA→γ+Q very interesting processAllows to study heavy quark energy loss in the QGP calibrated against the photon

• Performed first exploratory study at NLO for the LHC


Merci!


INTRINSIC CHARM

Intrinsic charm:c(x , µ0) 6= 0 at initial scale µ0 = mc

Models implemented in CTEQ 6.5C (PRD75, 2007)global fit allows average momentum hxic+c̄ or order 1 %

1 Light-cone Fock-space picture (Brodsky et al.), concentrated at large xhxic+c̄ = 0.57, 2.0 %

2 Meson-cloud model (Navarra et al.)hxic+c̄ = 0.96, 1.8 %

3 Phenomenological model: sea-like charm, broad in xhxic+c̄ = 1.1, 2.4 %

H. Spiesberger (Mainz) DIS, 27. 3. 2012 28 / 37

A colleague: “If QCD is right, there has to be IC”(which normalization?)

Intrinsic Charm

Recent CTEQ-TEA analysis of IC: 1309.0025


Results for LHC - ATLAS

LHC - ATLASExperimental Cuts

p

T

Rapidity Isolation Cuts

Photon p

min

T ,� = 45 GeV |y� | < 1.37 R = 0.4, ET

= 7 GeVp

max

T ,� = 1000 GeV 1.52 < |y� | < 2.37Heavy Jet p

min

T ,Q = 20 GeV |yQ

| < 2.4 R

jet

> 0.4,RQ� > 1


� + c Central Rapidity

IC shows up at much higher pT

dependence on rapidity


γ+c, central rapidity


Results for LHC - ATLAS

LHC - ATLASExperimental Cuts

p

T


Photon p

min

T ,� = 45 GeV |y� | < 1.37 R = 0.4, ET

= 7 GeVp

max

T ,� = 1000 GeV 1.52 < |y� | < 2.37Heavy Jet p

min

T ,Q = 20 GeV |yQ

| < 2.4 R

jet

> 0.4,RQ� > 1


γ+c, forward rapidity

� + c Forward Rapidity

x ⇠ p

TpS

(ey1 + e

y

2)

Consider forward rapidities at LHC



Results for RHIC - PHENIX

RHIC - PHENIXExperimental Cuts

p

T


Photon⇤ p

min

T ,� = 7 GeV |y� | < 0.35 R = 0.5, ✏ < 0.1E�

Heavy Jet p

min

T ,Q = 5 GeV |yQ

| < 0.8 ——


Intrinsic Charm at RHIC

10 15 20 25pTγ(GeV)

0.01

0.1

1

10

100

dσ/d

p Tγ(p

b/G

eV)

NLOLOBHPSsea-like

pp-> γ +c +X √$ S = 200GeV, CTEQ6.6M

10 15 20 25pTγ

0

1

2

3

4

(dσBHPS;sea-like;LO/dp T

γ)/(dσCTEQ6.6M/dp T

γ)

BHPSsea-likeLO

Small center of mass energy probes high x ⇠ p

T

/pS

Cross section is very sensitive to IC - especially BHPS



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