Experimental studies of gluon saturation: from RHIC/HERA ...€¦ · Experimental (ep,pp,AA) probes...

1/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)

Experimental studies of gluonsaturation: from RHIC/HERA to LHC

CERN, PH/EP

“Highenergy QCD: from RHIC to LHC”ECT*, January 12th 2007

David d'Enterria


Experimental studies of gluonsaturation: from RHIC/HERA to LHC

http://wwwspht.cea.fr/Images/Pisp/fgelis/Workshops/Trento2007/


Overview➢ Introduction: Gluon saturation (CGC) & nonlinear evolution (BKJIMWLK) Experimental (ep,pp,AA) probes of lowx PDFs: processes, kin. ranges

➢ Experimental status: Results at HERA (proton) and RHIC (nucleus)➢ Lowx QCD (Pb,p) studies @ the LHC: ALICE, ATLAS, CMS, LHCb experimental capabilities Six casestudies: [disclaimers: no pPb, CMScentric] PbPb @ 5.5 TeV: (1) dNch/dη, (2) QQbar→e+e,µ+µ in UPC (γPb), pp @ 14 TeV: (3) fwd QQbar, (4) fwd jets, (5) “MuellerNavelet” dijets, (6) fwd DY, ...

➢ Summary


Lowx gluon saturation – CGC

➢ Gluons overlap below “saturation scale” Qs(x)~x−λ

➢ Color Glass Condensate = effectivefield theory describing hadrons as classical fields around Qs (nonlinear BK/JIMWLK evolution eqs.).➢ Saturation effects enhanced in nuclear targets:

➢ Strong rise at lowx of gluons observed at HERA:➢ Radiation controlled by QCD evolution eqs.: Q2 DGLAP: F2(Q2) ~ αsln(Q2/Q0

2)n, Q02 ~1 GeV2

x BFKL: F2(x) ~ αsln(1/x)n Linear equations (single parton radiation/splitting) cannot work at lowx: Unitarity violated (even for Q2>>Λ2), collinear & kT factorization break down

λ~0.3

~ 6


Experimental probes: lowx PDFs (γp,pp,γA,AA)➢Perturbative processes:

‣ DrellYan, prompt γ (pp,AA)

‣ (Di)Jets (pp,AA):

‣ Diffractive QQ, heavyQ(γp, γA):_

(di)jets (y=4)

...


Smallx ➞ Forward rapidities

➢ 2 2 parton kinematics:

➢ 2 1 (gluon fusion) CGC kinematics: much lower x allowed (x2~x2min)

CGC: pQCD:

(RHIC) [Accardi, nuclth/0405046]

x(y=4) ~ 104 x(y=4) ~ 102

x1√s/2 x2√s/2pT

e.g. LHC, pT = 10 GeV/c θ ~103 ( ~η 7): xmin~106

x1√s/2 x2√s/2

y = 0: x1~x2 ~ xT = 2pT/√s

Every 2units of y,x2 decreases by ~10

⇒


PDF (x,Q2) experimental maps: proton, nucleus

➢ Kinematical (x,Q2) domains covered experimentally:

➢ Note: most existing lowx nPDFs measurements in the nonperturbative regime

ep, pp

eA, pAmuch less nuclear PDF dataavailable:

Dd'E,nuclex/0404018

(_)


Saturation effects at HERA (ep) and RHIC (dA,AA)


HERA: F2(x,Q2) at moderate Q2

➢ Saturation models describe well F2(x,Q2) in “transition region” of moderate/low Q2 (Note: also DGLAP, though at limit of applicability)

F2

F2

No sat. fits

x

CGCSat. fits

J.Forshaw,G.ShawJHEP 0412 (2004) 052


HERA: “Geometric scaling” of lowx F2(x,Q2)

➢ Saturation predicts lowx structure dependence on a single scale Q2s

DIS inclusive crosssection shows geo. scaling x < 0.01, 0.045 < Q2 < 450 GeV2

GolecBiernatWusthoffPRD60 114023 (1999)

Described by dipole model:

up to relatively large Q2 (“extended scaling” region): BK/JIMWLK evolution

σγ*p

, Q0 = 1 GeV, λ = 0.3


HERA: Diffractive structure function (dPDFs)

➢ Saturation models can describe consistently total γp xsection (F2) and DDIS ( xIPF2

D(3), Pomeron structure) and DVCS forward amplitudes:

xIP = fraction of p momentum carried by Pomeron

xIP

β

β = fraction of IP momentum carried by struck parton

J.Forshaw,G.ShawJHEP 0412 (2004) 052


HERA: Geometric scaling in diffractive DIS

➢ Geometric scaling also observed in diffractive observables: DVCS,

exclusive vectormeson:

C.Marquet, L. SchoeffelPLB639 (2006) 471

ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)

➢AuAu (200 GeV) 0-5% central collisions:

➢ “Reduced” multiplicity predicted by saturation models (gluon “fusion”

reduces incoming parton flux).

RHIC: Total AA hadron multiplicity (I)RHIC: Total AA hadron multiplicity (I)

dNch/dη~ 700 charged particles per unit rapidity at y=0

Predicted multiplicites:


RHIC: Total AA hadron multiplicity (II)

➢Final hadron multiplicity ∝ Initial multiplicity of released gluons ∝ Qs

➢Centrality & √sNN dependence well described:

Armesto, Salgado, WiedemannPRL94 (2005) 022002

+ “local partonhadron duality” (1 gluon = 1 final hadron)

Collision of 2 classical (saturated) fields

~

KLN, PLB507 (2001) 121

ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)

RHIC: Suppressed forward dAu pRHIC: Suppressed forward dAu pTT spectra spectra

η = 3.2

➢Hard hadrons at y=0 well described by coll. factoriz. + mild LT shadow.:

➢ But RHIC & HERA saturation “evidences” too close to nonperturbative range (Qs

2~1 GeV2). Much better conditions @ LHC (Qs2 ~5 GeV2, lower x, larger y)

CGC

pQCD➢ Suppressed fwd hadron spectra: pT ~ 2 – 4 GeV/c not described by standard pQCD but by CGC: reduced # of partonic scattering centers in Au at x~103


Lowx QCD at LHC


LHC PbPb: lowx nuclear PDF studies➢PbPb @ 5.5 TeV, pPb @ 8.8 TeV:

(i) Very high √s ⇒ x = 2pT/√s ~ 103 ~3045 times lower than AuAu,dAu @ RHIC !

(ii) Saturation momentum (A1/3~6): Qs2 ~ [5 GeV2]·e(0.3 y)

(iii) Very large perturbative crosssections.

Nuclear xG(x,Q2) basically unknown for x<103 !

Armesto, J.Phys.G32:R367 (2006)

Ratio of gluon densities in Pb to p:

eA, pA

Dd'E,J.Phys.G G30 (2004) S767

?


LHC pp: lowx proton PDF studies➢ pp @ 14 TeV :

(i) = ,At y 0 x=2pT/√s ~103 (domain probed @ HERA,Tevatron). Go fwd. for x<104

(ii) Saturation momentum: Qs2 ~ 1 GeV2 (y=0), 3 GeV2 (y=5)

(iii) Very large perturbative crosssections:

DrellYan

LHC forward rapidities e.g. y ~ 5, Q2 ~ 10 GeV x down to 105 !

Prompt γ

Jets

W,Z productionHeavy Flavours

ep, pp_

?


Forward detectors in CMS/ATLAS➢CMS + TOTEM:

➢ATLAS:

TOTEM T2IP

TOTEM T1HF

HF (IronQfiber calo): 3 < η < 5

TOTEMT1: 3.1 < η < 4.7TOTEMT2 (GEM telescope): 5.3 < η < 6.7

CASTOR (W/Qfiber calo): 5.25 < η < 6.5

ZDC (W/Qfiber calo): η > 8.5 (neutral)

Forw. Cal.: 3 < η < 5

LUCID (Cerenkov Counter): 5.4 < η < 6.1

ZDC (W/Qfiber calo): η > 8.5 (neutral)


CMSTOTEM at the LHC➢ HF,CASTOR,ZDC + TOTEM: Quasifull acceptance at LHC

➢ Detection capabilities within η < 6.7 (and η > 8.1, neutral).➢ Hard scattering measurements (jets, highpT hadrons, DY) possible down to x~106 in pp, pA, AA.

CASTORCASTOR TOTEMTOTEM

ZDCZDC

[5.2 < η < 6.6]

(z = ±140 m)

[ η > 8.3 neutral]

HFHF

η = 8 6 4 2 0 2 4 6 8


Forward Detectors in ALICE/LHCb➢ALICE, LHCb forward muon spectrometers:

➢Excellent capabilities for heavyQ, QQbar measurements at lowx:

2.5< η< 4

2 < η < 5


Casestudy I: Total PbPb hadron multiplicity

➢Final A+A multiplicity ∝ Initial number of released gluons :

➢ CMS dN/d (||<2.5) via hit counting à la PHOBOS (pixels pulse height)

+ “local partonhadron duality” (1 gluon = 1 final hadron)

● simulated primary tracks ✝ reconstructed (pT

min~50 MeV)

CGC:

Gluon saturation ⇒

reduced dN/d|=0~1800

010% PbPb HIJING (noquench)

C.Smith,CMSAN0315

KLN, NPA747 (2005) 609 N.Armesto,Pajares IJMPA15(00)2019


Parenthesis: Good dN/dη for CGC ⇒ “Bad” news for QGP

➢Expected reduced dN/d|=0~1800 ⇒(relatively) low T plasma:

➢LHC conditions still too close to sQGP regime ?

KLN, NPA747 (2005) 609N.Armesto,C.Pajares, IJMPA15(00)2019

Take equilibrated QGP at τ0~0.3 fm/c: ε0~(0.5·ρ0)4/3~[0.5 dN/dy/(τ0AT)]4/3~ ~ 100 GeV/fm3

T0~(ε0/12)1/4~0.52 GeV

LHC ?RHIC

✺[Note also: ~10% of dN/dy at RHIC is due to viscous effects]


Casestudy II: ϒ photoprod. (γ Pb) in UPC PbPb

➢ High energy heavyions produce strong electromagnetic fields due to the coherent action of Z = 82 protons:

➢ Equivalent flux of photons in EM (aka. Ultra Peripheral, bmin~ 2RA ~20 fm) AA colls.:

➢ QQ diffractive photoproduction sensitive to the gluon density squared:

Max. γ energy: Eγmax ~ 80 GeV (PbPbLHC)

γ Pb: max. sγPb ≈ 1. TeV 3. ≈ 4. × sγp(HERA)

y=0 : x(ϒ) = 2·103

y~2 : x(ϒ)~x(y=0)·ey~104unexplored xGA(x,Q2)


CMS: central e± and µ± measurement (|η|<2.4) Tracking + ECAL + muonchambers

Silicon Microstripsand Pixels

Si TRACKER CALORIMETERS

ECAL

HCALPlastic Sci/Steel sandwich

MUON BARRELDrift Tube Chambers (DT)

Resistive Plate Chambers (RPC)PbWO4

ϒ ➞ e+e

ϒ ➞ +


ϒ and ℓ+ℓ crosssections in UPC (γ Pb, γ γ )

➢ STARLIGHT MC predictions [J. Nystrand, S.Klein, NPA752(2005)470]

➢ ~50% of UPC interactions have soft EM interactions leading to nuclear breakup w/ forward neutron (Xn) emission (ZDC important for triggering)

Pb

Pb

Pb

Pb

ℓ

ℓ

Signal Background


Full simulationreconstruct. ϒ + ℓ+ℓ continuum

➢ Full CMS simulation+digi+hit+reconstruction:

➢ Excellent µµ mass resolution: higher mass bb states (ϒ',ϒ'') can be resolved

ϒ ➞ e+e ϒ ➞ +

Peak position: ~9.35 GeV/c2

Mass resolution: ~150 MeV/c2

Peak position: ~9.52 GeV/c2

Mass resolution: ~90 MeV/c2

tracker+ECAL tracker+µchambers

(not in MC now)

_

DdE, A.Hees, CMSAN06107


Expected ϒ yields (PbPb 1“year” luminosity)

➢ Backgd. subtracted dN/dminv for ∫ℒ dt =0.5 nb1 PbPb5.5 TeV (t=106 s)

➢ Syst. uncertainties (dominated by backgd. subtraction, 5% lumin.): ~10% ➢ Final total rates :

(error bars = stat. uncertainties for integrated luminosity)

N(ϒ ➞ e+e) ~ 220±15(stat)±10%(syst) N(ϒ ➞ µ+µ) ~ 180±13(stat)±10%(syst)

ϒ ➞ e+e ϒ ➞ +

➢ Enough stats. for detailed studies (including ydependence) of gluon PDF

DdE, A.Hees, CMSAN06107


Casestudy III: Forward QQ in ALICE (2.5 <| |η < 4)[D.Stocco,Gagliardi]

➢ Sensitive to x2~105 where xg(x) is not well constrained:

dσ/dy J/ :ψ NLO CEM w/ varying PDFs

QQbar: Interesting observable to test DGLAP versus CGC/BDL predictions (alsoheavyQ, see A. Dainese talk tomorrow)

_


CMS: Hadronic Forward Calorimeter (3 <| |η < 5)

• 11.2 m from IP5. 1.65 m absorber depth..• Steel+quartzfibre Cerenkov (EM,HAD) calorimeter • Hamamatsu R7525 radhard PMTs• 2x1200 towers: Δη x ΔΦ ~ 0.18 x 0.18• Excellent jet position ~10%, energy ~15% resolutions• Large jet reco efficiency: ~90%• Jet candidates in level1 trigger (for VBF,qq>qqH)


CMS: Hadronic Forward Calorimeter (3 <| |η < 5)

• 11.2 m from IP5. 1.65 m absorber depth..• Steel+quartzfibre Cerenkov (EM,HAD) calorimeter • Hamamatsu R7525 radhard PMTs• 2x1200 towers: Δη x ΔΦ ~ 0.18 x 0.18• Excellent jet position ~10%, energy ~15% resolutions• Large jet reco efficiency: ~90%• Jet candidates in level1 trigger (for VBF,qq>qqH)


Casestudy IV: Forward jets in CMSHF (3 <| |η < 5)

➢ Inclusive forward “lowET” jet (ET ~20100 GeV) production:

➢ Inclusive fwd. jet reconstruction:

Sensitive to partons with: x2 ~ 104 , x1 ~ 101

PYTHIA ~ NLO [Vogelsang]

p + p → jet1 + jet2 + X

• PYTHIA 6.4. minbias (hard&soft QCD)• MClevel proofofprinciple only (no det. response !)• HF grid: ∆η×∆φ = 0.175×0.175• Iterative cone, R=0.5, Ethresh=10 GeV, Eseed=3 GeV• Missing important corrections: underlyingevt.

(PYTHIA CMSTune), hadronization (cluster vs. Lund)• Large yields. LowET uncertainties to be determined.

[inside “extended scaling” region in proton ?]


Casestudy V: MuellerNavelet dijets in CMSHF➢ MuellerNavelet dijets separated by large Δy: very sensitive to nonDGLAP evolution

➢Proofofprinciple study in CMS: MClevel dijet reconstruction applying MN kinematics cuts to PYTHIA pp14 TeV:

jet1

jet2

∆y~10

C.Marquet, Royon, NPB739 (2006)131

suppressed ratio sat./BFKL

increasingrapidity

pp √s = 14 TeV

A.H.Mueller, H.Navelet, NPB282 (1987)727

}


Casestudy V: MuellerNavelet dijets in CMSHF➢ MuellerNavelet dijets separated by large Δy: very sensitive to nonDGLAP evolution

Stats in 1st pp run ℒ ~ 1 pb1

Dijet rates [ET~2040 GeV]~104

enough stat. for detailedstudies of ∆yevolution. Redo analysis w/ sat./BFKL predictions & full det. response

η1= η2= [4.55.]η1= η2= [3.3.5]

jet1

jet2

∆y~10

C.Marquet, Royon, NPB739 (2006)131

suppressed ratio sat./BFKL

increasingrapidity

pp √s = 14 TeV


HF, TOTEM, CASTOR Platforms

CMS: TOTEM2 and CASTOR (5.2 <| |η < 6.6)

➢ ..

T2 telescope

➢ TOTEM GEM (“Gas Electron Multiplier”) telescope detector: electron polar angle

CASTOR (W/Qfiber calo): electromag. shower identification

CASTOR • Tungsten plates + quartz fibres• Cherenkov sampling calorimeter• Lightguides + APDs readout• Azimuth segmented (8 octants)• EM section: 11.2 cm ~ 19 X0

• HAD+EM sections: 136 cm~ 10 λI

• 192 channels


Casestudy VI: DY in CASTORT2 (5.2 <| |η < 6.6)

7<|η|<9 5.5<|η|<7.8

Log10(x)

5.2 < |η| < 6.6

KimberMartinRyskin

➢ DrellYan feasibility studies with CMS (CASTOR) + TOTEM (T2):➢ Sensitive to lowx quark densities

T2 tracker+ CASTOR calo needed to deal w/ large QCD (and QED) bckgd.Log10(x)

PDF parametrizations


Summary➢ Gluon saturation and nonlinear evolution must setin at (some) lowx in hadronic wavefunctions → Fundamental info on highenergy limit of QCD➢ Hints in epHERA, dA,AARHIC: stronger signals expected @ LHC (Qs~5 GeV)

➢ LHC = unique laboratory to study high parton density / evolution in p,Pb down to x~106

using fwd. detectors and perturbative processes: (di)jets, QQbar, DY...

“We (interested LHC experimentalists) need quantitative predictions to plug into our MC !”


Backup slides


Casestudy II: lowx via γ Pb➢ QQbar (and in general, hard) diffractive photoproduction sensitive to

the gluon density squared in the nucleus:

Smallx probed in γA collisions at LHC:

➢ Unexplored (x,M2) regime of the nucleus wave function. Gluon saturation – nonlinear QCD evolution expected ⇒ Suppressed hard photoproduction.

y=0 : x(ϒ) = 2·103

y~2 : x(ϒ)~x(y=0)·ey~104

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