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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
2/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
Experimental studies of gluonsaturation: from RHIC/HERA to LHC
http://wwwspht.cea.fr/Images/Pisp/fgelis/Workshops/Trento2007/
3/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
4/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
5/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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)
...
6/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
⇒
7/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
(_)
8/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
Saturation effects at HERA (ep) and RHIC (dA,AA)
9/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
10/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
11/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
12/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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:
14/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
16/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
Lowx QCD at LHC
17/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
?
18/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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_
?
19/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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)
20/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
21/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
22/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
23/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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]
24/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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)
25/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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 ➞ +
26/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
27/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
28/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
29/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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)
_
30/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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)
31/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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)
32/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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 ?]
33/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
}
34/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
35/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
36/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
37/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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 !”
38/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
Backup slides
39/37ECT* Highenergy QCD, 12/01/07 David d'Enterria (CERN)
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
RHIC Fixedtarget