using photons in p A and A A collisions

Probing parton densities and energy loss processes using photons in p A and A A collisions François Arleo LAPTH, Annecy High p Probes of High Density QCD May 2011 Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 1 / 27

Introduction Motivations Hard QCD processes in nuclear collisions Single γ production in p A collisions Probing nuclear parton densities Predictions at RHIC and LHC γ +Q production in A A collisions Probing heavy-quark energy loss in quark-gluon plasma Preliminary estimates at LHC References FA, K. Eskola, H. Paukkunen, C. Salgado, JHEP 04 (2011) 055 [arxiv:1103.1471] T. Stavreva et al., JHEP 01 (2011) 152 [arxiv:1012.1178] + work in progress Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 2 / 27

Hard processes in QCD media Hard processes are ideal tools to probe the hot QCD medium Can be computed in perturbative QCD Can be compared systematically to p p collisions Sensitive to parton energy loss processes p p + ε A A Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 3 / 27

Hard processes in QCD media Schematically two classes of processes Medium sensitive Jets Large p hadrons Heavy-quarkonia and open heavy flavour Medium insensitive Prompt photons W ± /Z 0 bosons Drell-Yan pair production Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 3 / 27

Jet quenching Significant suppression of large-p hadrons in Au Au collisions at RHIC One of the most important discoveries in heavy-ion collisions Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 4 / 27

First LHC measurements Strong hadron suppression seen by ALICE Significant asymmetries in jet production reported in central Pb Pb collisions by ATLAS and CMS Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 5 / 27

First LHC measurements A robust interpretation of the data requires a quantitative understanding of hard processes in nuclear collisions Probing nuclear parton densities (npdfs) Essential to predict benchmark predictions in p A and A A collisions Probing energy loss processes Variety of observables to investigate Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 5 / 27

Part I Probing nuclear parton densities Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 6 / 27

Present constraints on npdfs Ratio of PDFs in nuclei over that in a proton R i (x,q 2 ) = f p/a i (x,q 2 )/f p i (x,q 2 ) poorly constrained especially at small x and in the gluon sector Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 7 / 27

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 ] 2 =1.69 GeV 2 ) Pb G (, EPS09NLO nds HKN07 1.4 1.2 1.0 0.8 0.6 0.4 0.2 10-4 10-3 10-2 10-1 1 [ EPS09 Eskola, Paukkunen, Salgado 0902.4154 ] Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 8 / 27

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 ] 2.5 2 Nuclear Modifications for g p/pb (x,q o =1.3 GeV) ncteq decut3 ncteq decut3g3 ncteq decut3g9 1.5 R g Pb 1 0.5 0 0.01 0.1 1 x [ ncteq Schienbein et al. 0710.4897 & 0907.2357 ] Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 8 / 27

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 ] Question How to probe small-x gluon shadowing at LHC? which observables why prompt photons look promising Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 8 / 27

Comparing observables Advantages and limitations Jets high rates, rich phenomenology, forward rapidities large scales Q 2 10 3 GeV 2 Heavy-bosons constraints on sea-quark shadowing large scales Q 2 10 4 GeV 2 Prompt photons low Q 2 10 10 3 GeV 2, rich phenomenology parton-to-photon fragmentation process Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 9 / 27

Comparing observables [ Aurenche et al. 2006 ] Very good description of isolated / inclusive photon world-data Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 9 / 27

Kinematical range Q 2 (GeV 2 ) 10 6 10 5 10 4 10 3 10 2 photons jets heavy bosons NMC Q s 2 10 1 10-6 10-5 10-4 10-3 10-2 10-1 x (x,q 2 ) domain covered at the LHC Photons and jets are clearly complementary Photons cover small Q 2 where shadowing should be large Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 10 / 27

Dynamics At leading order Compton scattering q( q)g q( q) γ Annihilation process q q g γ At high energy: Compton Annihilation Simple relationship between prompt photon production and parton densities! Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 11 / 27

Nuclear production ratio Definition R pa (x ) = 1 A d 3 σ / d 3 σ dy d 2 (p +A γ +X) p dy d 2 (p +p γ +X) p Most naive estimates [ FA Gousset 2008 ] Around mid-rapidity R pa (p,y) 0.5 [ ] R F2 (x e y ) + R G (x e y ) At (very) forward rapidity R pa (p,y) R G (x e y ) At (very) backward rapidity R pa (p,y) R F2 (x e y ) Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 12 / 27

Caveats Relationship between photon momenta and parton kinematics spoiled by fragmentation processes and higher-order corrections Photon fragmentation contribution This component very much reduced using isolation criteria E had E max for particles in a cone (η η γ ) 2 +(φ φ γ ) 2 R 2 Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 13 / 27

Caveats Relationship between photon momenta and parton kinematics spoiled by fragmentation processes and higher-order corrections Photon fragmentation contribution Next-to-leading order (NLO) corrections 3-body kinematics in the final state Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 13 / 27

Checking the approximation Using ndsg in p A collisions at the LHC 1.2 1 0.8 R ppb (x ) 0.6 0.4 0.2 0 s = 8.8 TeV y = 0 isolated γ NLO w/ ndsg 0.5 ( R F2 + R G ) 10-3 10-2 x Excellent matching between R pa and nuclear density ratios at the LHC < 2 3% at y = 0 5% at y = 2.5 Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 14 / 27

Set-up Prompt photon suppression R pa and R AA computed systematically NLO accuracy Using most recent npdf sets available nds, EPS09, HKN At RHIC and LHC RHIC: d Au and Au Au at s NN = 200 GeV LHC: p Pb at s NN = 8.8 TeV and Pb Pb at s NN = 5.5 TeV Full error analysis uncertainty computed from the 31 EPS09 error sets Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 15 / 27

Isospin effect Due to QED coupling σ(ug uγ)/σ(dg dγ) = e 2 u /e2 d = 4 When valence quarks dominate (large x 1) σ(nn γ X) < σ(pn γ X) < σ(pp γ X) 1 AA σ(aa γ X) < 1 Aσ(pA γ X) < σ(pp γ X) Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 16 / 27

Isospin effect Due to QED coupling σ(ug uγ)/σ(dg dγ) = e 2 u /e2 d = 4 When valence quarks dominate (large x 1) σ(nn γ X) < σ(pn γ X) < σ(pp γ X) 1 AA σ(aa γ X) < 1 Aσ(pA γ X) < σ(pp γ X) Consequence R pa (x ) < 1 at large x large transverse momentum: 2p / s NN 1 backward rapidity: e y 1 Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 16 / 27

RHIC At mid-rapidity (y = 0) R dau 1.4 1.2 1.0 0.8 y <0.35 s 1/2 =200 GeV proton PDF EPS09 nds HKN 0.6 0.4 0 5 10 15 20 25 30 35 40 45 50 p T [GeV] Interplay between anti-shadowing and EMC effect Large differences between the various npdf sets Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 17 / 27

RHIC At forward rapidity (y = 3) R dau 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 y=3 s 1/2 =200 GeV proton PDF EPS09 EPS09 + + - 0.5 3.0 4.0 5.0 6.0 7.0 8.0 p T [GeV] Important isospin effect from the deuteron projectile Interplay between shadowing and anti-shadowing Effects rather similar in the pion channel Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 17 / 27

LHC At y = 0 R ppb 1.3 1.2 1.1 1.0 y <0.5 s 1/2 =8.8 TeV proton PDF EPS09 nds HKN 0.9 0.8 0 20 40 60 80 100 120 140 160 180 200 p T [GeV] Interplay between shadowing and anti-shadowing (like at RHIC at forward rapidity) Future data should discriminate between the various predictions Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 18 / 27

Counting rates in p A collisions d /dp T dy (pb/gev) 10 8 10 6 10 4 10 2 s 1/2 =200 GeV y=0 s 1/2 =8.8 TeV y=0 1 0 10 20 30 40 50 60 70 80 90 100 p T (GeV) RHIC-II (assuming L int = 0.7 pb 1 ) N > 100/GeV for p 35 GeV LHC (assuming L int = 0.1 pb 1 ) N > 100/GeV for p 100 GeV Statistical accuracy in a year much better than the present spread of theoretical predictions Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 19 / 27

Prompt photons in A A collisions npdf effects magnified in A A collisions, roughly R AA R 2 (RA ) 2 pa i Ideal collisions to probe npdf R ppb 1.3 1.2 1.1 y <0.5 s 1/2 =8.8 TeV proton PDF EPS09 nds HKN R PbPb 1.4 1.3 1.2 1.1 y <0.5 s 1/2 =5.5 TeV proton PDF EPS09 nds HKN 1.0 1.0 0.9 0.9 0.8 0.8 0 20 40 60 80 100 120 140 160 180 200 p T [GeV] 0.7 0 20 40 60 80 100 120 140 160 180 200 p T [GeV] Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 20 / 27

Prompt photons in A A collisions npdf effects magnified in A A collisions, roughly R AA R 2 (RA ) 2 pa i Ideal collisions to probe npdf Caveat Prompt photons may be sensitive to hot medium effects Jet-to-photon conversion in QGP due to Compton scattering Medium-induced photon emission due to parton multiple scattering Photon quenching due to the suppression of the fragmentation component Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 20 / 27

Prompt photons in A A collisions npdf effects magnified in A A collisions, roughly R AA R 2 (RA ) 2 pa i Ideal collisions to probe npdf R AuAu 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 y <0.35 s 1/2 =200 GeV EPS09 EPS09 quenched 0.0 0 5 10 15 20 25 30 35 40 45 50 p T [GeV] R PbPb 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 y <0.5 s 1/2 =5.5 TeV EPS09 EPS09 quenched 0.0 0 20 40 60 80 100 120 140 160 180 200 p T [GeV] Stronger quenching at small p where frag. processes are larger Effects extend to very large p at the LHC Caution: separation between direct and fragmentation not physical! Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 20 / 27

Part II Probing energy loss processes Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 21 / 27

Probing (massive) parton energy loss in QGP Energy loss of massive partons Heavy quark mass acts as a collinear cutoff for medium-induced gluon radiation, just like in vacuum (dead cone) [ Doskhitzer Kharzeev 2001 ] Hierarchy expected ( E ) g > E q > E c > E b Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 22 / 27

Probing (massive) parton energy loss in QGP Energy loss of massive partons Heavy quark mass acts as a collinear cutoff for medium-induced gluon radiation, just like in vacuum (dead cone) [ Doskhitzer Kharzeev 2001 ] Hierarchy expected ( E ) g > E q > E c > E b γ q γ c γ b The photon may be used to tag the energy of the massive parton γ +Q unique tool to probe Q energy loss in the plasma Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 22 / 27

Comparison between theory and Tevatron data Measurements by DØ Collaboration [ 0901.0739 ] compared to NLO theory [ Stavreva Owens 0901.3791 ] Excellent agreement between data and theory in the γ +b channel γ +c data above theory at large p Tγ hint for intrinsic charm? Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 23 / 27

γ +Q in p A collisions γ +Q in p A collisions will also probe the charm and the gluon npdfs 1.25 1.2 1.15 1.1 p+pb->γ+c+x / p+p->γ+c+x S = 8800GeV EPS09 HKN07 ncteq 1.25 1.2 1.15 1.1 Nuclear Modifications for g p/pb (x,q=50 GeV) EPS09 HKN07 ncteq R σ γ+c 1.05 1 0.95 0.9 0.85 0.8 R g Pb 1.05 1 0.95 0.9 0.85 0.8 0.75 50 100 p Tγ (GeV) follows RPb g very closely Almost no overlap between EPS09 and HKN07, and ncteq decut3 R γ+c ppb Measurements with sufficiently small error bars should disentangle the various npdf sets Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 24 / 27 0.75 x 0.01

Analysis in A A collisions Calculations performed at LO accuracy Heavy quark energy loss ǫ Q estimated on an event-by-event basis from the quenching weight (probability distribution) obtained perturbatively [ Armesto Dainese Salgado Wiedemann 2005 ] Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 25 / 27

Analysis in A A collisions Calculations performed at LO accuracy Heavy quark energy loss ǫ Q estimated on an event-by-event basis from the quenching weight (probability distribution) obtained perturbatively Various observables investigated Photon jet energy asymmetry A J Momentum imbalance z γq A J = E T1 E T2 E T1 +E T2, φ > π/2 z γq = p Tγ. p TQ p 2 Tγ Photon jet pair momentum q q = p Tγ +p TQ Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 25 / 27

Pair momentum distribution Why q distribution q ǫ Q at LO accuracy if the photon is produced directly Preliminary result in Pb Pb collisions at s NN = 5.5 TeV Significant medium modifications reported Stronger effects in γ +c than γ +b due to the larger energy loss Needs to be compared to γ + inclusive jet production Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 26 / 27

Pair momentum distribution Why q distribution q ǫ Q at LO accuracy if the photon is produced directly Preliminary result in Pb Pb collisions at s NN = 5.5 TeV Significant medium modifications reported Stronger effects in γ +c than γ +b due to the larger energy loss Needs to be compared to γ + inclusive jet production Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 26 / 27

Summary Prompt photons in nuclear collisions extremely useful Single photons in p A collisions Sensitive probe of the gluon PDF in nuclei NLO calculations performed using various npdf sets Large rates expected at LHC γ +Q production in A A collisions Access to the mass hierarchy of parton energy loss in QCD plasma Promising preliminary results at the LHC Francois Arleo (LAPTH) Prompt γ in p A and A A collisions HPHD 2011 May 2011 27 / 27