Small-x physics at the LHC

Transcription

1 Small-x physics at the LHC EMMI GSI Workshop GSI, Darmstadt, 24th Nov David d'enterria CERN 1/26

2 Overview Introduction: - Uncertainties on parton structure & evolution at low-x. - Measurements of (n)pdfs: processes, kinematic domains,... Perspectives of low-x PDF constraints at the LHC: 14 TeV (forward):, DY, (jets, W, Z) _ -Pb in ultraperipheral Pb-Pb 5.5 TeV: QQ _ p-pb 8.8 TeV:, QQ, (jets, W,Z) Summary 2/26

3 Motivation (I): Low-x PDFs DIS collisions probe distributions of partons inside hadrons: Q 2 = resolving power Bjorken x = momentum fraction carried by parton Q2 F2,F3,FL= proton structure functions, (y = inelasticity). Gluons dominate but only indirectly constrained via F2 scaling violations : xg(x,q2) Q2 = 2 GeV2 J. Rojo et al. NPB 809, 1 (2009) 3/26

4 Motivation (II): low-x QCD evolution Q2 x DGLAP (kt-order'd emission): F2(Q2)~α sln(q2/q02)n,q02 ~1 GeV2 [LT,coll.factoriz.] - BFKL (pl-ordered emission): Linear F2(x) ~ α sln(1/x)n [updfs, kt-factoriz.] equations (single parton radiation/splitting) cannot work at low-x: (i) Too high gluon density: nonlinear gluongluon fusion balances branchings (ii) pqcd (collinear & kt) factorization assumptions invalid (HT, no incoherent parton scatt.) (iii) Violation of unitarity even for Q2>> 2 (too large perturbative cross-sections) Saturation enhanced in multi-parton systems (nuclei): Qs2 ~ A1/3 ~ 6 4/26

5 Experimental access to low-x gluon PDF DdE, DIS'07 arxiv: Perturbative processes: _ e( )-p, e( )-A F2, FL, F2c, excl. QQ (di)jets (y=4) p-p, p-a,, heavy-q, jets: Forward production: x1 s/2 x2 s/2 x2min ~ pt/ s e-y= xt e-y Every 2-units of y, xmin decreases by ~10 5/26

6 Low-x gluon PDF (proton) Kinematical (x,q2) domain covered so far experimentally: Low-x gluons mostly indirectly from F2 scaling violations. R.D. Ball et al. arxiv: e-p, p-p Q2 = 2 GeV2 6/26 Large uncertainties below x~10-2 at moderate Q2

7 Low-x gluon PDF (nucleus) Current knowledge of low-x gluons from: F2 (e, -A), Drell-Yan (p-a), high-pt hadrons (d-au). x<0.01: very few measurements (non-perturbative): huge uncertainties! Eskola-Kolhinen-Salgado JHEP04 (2009) 065 (NLO) e-a, p-a DdE,JPG30 (05)S767 Phys. Rept. 240 (94) 301 xg(x,q2) virtually unknown below x~10-2! 7/26

8 Improvements before future ndis facilities? [see N.Armesto's talk] DdE, arxiv: Ultimately: F2A, FA2,charm, FLA,... erhic: (e-) 20 GeV (A) 100 GeV s~60 GeV, L~1033 cm-2s-1 LHeC: (e-) 70 GeV (p,a) 2.75, 7 TeV s~0.9,1.4 TeV, L~1033 cm-2s-1 Huge increase in nuclear (x,q2) kinematical reach! Q2 ~ GeV2 x down to ~ /26

9 Jets & : ATLAS/CMS calorimeters < 5 Note: also LHCb calos within 2 < <5 but limited pt range pt ( )< 15 GeV/c pt (jets)< 50 GeV/c < 5, 5< <6.6 9/26

10 Heavy-flavour & DY: LHCb & ALICE Forward muon spectrometers: 2.5 < < 4 2< <5 Excellent _ capabilities for heavy-q, QQ, DY fwd. measurements at low-x: (It would be very interesting if LHCb runs p-a) 10/26

11 Low-x proton PDFs from p-p collisions at the LHC 11/26

12 LHC forward detectors Measurements exploiting forward detectors: x2min ~ pt/ s e-y = xt e-y (every 2-units of y, xmin decreases by ~10) DdE, arxiv: Jets, photons in fwd. calorimeters: CMS/ATLAS: within <5 LHCb: 2 < < 5 (pt < 15,50 GeV) _ Muons (DY, heavy-q, QQ) in fwd. -spectrometers: ALICE: 2.5 < < 4 (but no 2nd vtx.) LHCb: 2 < < 5 Fwd. trackers/pid: LHCb: 2< <5 12/26

13 Low-x studies at the LHC: proton At y=0, x=2pt/ s~10-3 (probed at HERA,Tevatron). Go fwd. for x<10-4 Very large pqcd cross-sections: Prompt Drell-Yan Jets Heavy-Q W,Z? Forward rapidities: (e.g. y~5, M~2.5 GeV) x down to 10-6! But barely touching sat. momentum: Qs2 ~ 0.6 GeV2 (y=0), 3 GeV2 (y=5) 13/26

14 Example I: Forward in LHCb (2 < < 5) R. Ichou-DdE, PRD82, (2010) Isolated photons (ET ~5-15 GeV) ~80% from quark-gluon Compton: Spectrum dependence on PDF choice: x probed down to ~ /26

15 Example II: fwd EWK in LHCb (2 < < 5) PDF uncertainty on forward EWK x-sections: [de Lorenzi, DIS'10] Impact of 1 fb-1 DY data on sea, glue (after/before): ~3% (Z), ~30% (low-mass DY) PDF uncertainties improved up to ~70% 15/26

16 Low-x nuclear PDFs from A-A collisions at the LHC _ -Pb (UPC Pb-Pb at 5.5 TeV) QQ 16/26

17 Photoproduction in A-A collisions at the LHC High-energy heavy-ions produce strong E.M. fields due to coherent action of ZPb = 82 protons: Ultra-Peripheral Collisions Various UPCs in 5.5 TeV: E max ~ 80 GeV -Pb max. s Pb 1 TeV 3 s p (HERA) processes in (x, Q2) plane: _ -Pb QQ -Pb dijets 17/26

18 _ xg(x,q2) from exclusive QQ photoprod. at HERA p J/Ψ,ϒ + p is sensitive to gluon distribution squared: NLO pqcd analysis: Discrimination of different xg(x,q2) parametrizations Martin, Nockles, Ryskin, Teubner PLB 662, (2008) /26

19 _ xg(x,q2) from exclusive QQ photoprod. at RHIC Au J/Ψ + Au at W A~24 GeV [1] [2] [3] [4] PHENIX Collab. PLB 679, 321 (2009) dσ/dy y=0 = 76± 31 (stat) ± 15 (syst) µb [1] PRL 89 (2002) [2] PLB 626 (2005) 72 [3] arxiv: [4] arxiv: Model comparisons: - Starlight (coherent): HERA data parametrization - Strikman et al (coherent & incoherent): color-dipole + σ J/ψN = 3mb [Filho et al, PRC78 (2008)044904] - Gonçalves-Machado (coherent): color-dipole + Glauber-Gribov shadowing - Kopeliovich et al. (coherent & incoherent): color-dipole + gluon saturation - Filho, Gonçalves, Griep (coherent): DGLAP nuclear PDFs 19/26

20 Example: Pb-Pb Pb J/Ψ, ϒ Pb Theoretical predictions: ϒ J/Ψ Impulse: σ = 133 mb LT shadowing: σ = 78 mb CGC: σ 40 mb Pb J/Ψ,ϒ + Pb in ALICE, CMS: ~500 ϒ / 0.5 nb-1 DdE, NPB 184, 158 (08) ~500 J/Ψ / 0.5 nb-1 ϒ ee - + J/Ψ e-e+ J. Nystrand NPA 787, 29 (07) tracker+ecal 20/26 ϒ - + tracker+µ-chambers

21 Low-x nuclear PDFs from p-a collisions at the LHC p-pb at 8.8 TeV jets,, *, heavy-q, W/Z 21/26

22 (x,q2) coverage in p-a at the LHC [C.Salgado et al., in preparation] Central rapidities: jets,, h±, heavy-q Forward rapidities: Heavy-quarks (D,B) 22/26

23 Example I: Isolated in ATLAS/CMS ( < 3) p-a X at 8.8 TeV, pt ~ GeV/c at <3 probe glue at x2 ~10-3 Nuclear modification factor RpPb(xT): (y=0) (y=2.5) [ Arleo-Gousset, PLB2008 ] 23/26

24 _ Example II: Forward QQ in ALICE (2.5 < < 4) J/ measurement in fwd -spectrometer: [ D. Stocco ] Sensitive to xg(x) down to x2~10-5 Expected RpPb(y,pT) in p-pb at 8.8 TeV : EKS98 EPS08 EKS98 CGC CGC [ C. Hadjidakis] 24/26

25 Summary: low-x QCD at the LHC (proton) Improvements of low-x PDFs expected using forward detectors:? In particular, low mass DY (y~5, M~2.5 GeV): Reduction of sea/glue uncertainty by up to 70%. But LHC barely touches Qs2(x). LHeC needed for non-linear evolution studies in proton. 25/26

26 Summary: low-x QCD at the LHC (nucleus) Current knowledge of low-x nuclear gluon density (& evolution?) is as bad or worst! than for the proton ~15 years ago (pre-hera). In order to reach present-day proton PDF precision we would need a machine like LHeC. '94-'99 Gluon density in proton recent pre-hera LHC xg(x,q2) constraints: -Pb 5.5 TeV 8.8 TeV Plus, lots of low-x (saturation) dynamics studies possible too at the LHC 26/26

27 Backup slides 27/26

28 Example: Forward jets in CMS (3 < < 6.6) [S.Cerci-DdE, arxiv: ] Forward jets (ET ~ GeV) sensitive to low-x PDFs: Jets in HF (3< η <5) probe: x2 ~10-4 Jets in CASTOR (5.1< η < 6.6): x2 ~10-5 varying PDFs: x2 ~ 10-4 log10(x1,2) 28/26

29 Example: Forward * in LHCb (2 < < 5) Drell-Yan forward : Sensitive to low-x quark densities (trigger on low-p muons: p>8gev, pt>1gev) 14 TeV (expected cross sections) x < /26

30 Example: Mueller-Navelet dijets in CMS ( ~10) Mueller-Navelet dijets with large y separation very sensitive to low-x QCD evolution (testing ground for BFKL): jet1 y~10 BFKL: extra radiation between the 2 jets will smooth out back-to-back topology jet2 A.H.Mueller, H.Navelet, NPB282 (1987)727 (partially compensated by gluon saturation?) _ pp s = 1.8 TeV Increased azimuthal decorrelation with increasing y (w.r.t. DGLAP collinear-factorization): NLO BFKL [DelDuca, Schmidt], [Orr, Stirling] [A.Sabio-Vera, F.Schwennsen] [C.Marquet, Royon] [E. Iancu et al.] 30/26 [D0 Collab, PRL77(97)595]

31 Example: Mueller-Navelet dijets in CMS ( ~10) Mueller-Navelet dijets with large y separation very sensitive to low-x QCD evolution (testing ground for BFKL): jet1 y~10 BFKL: extra radiation between the 2 jets will smooth out back-to-back topology jet2 A.H.Mueller, H.Navelet, NPB282 (1987)727 (partially compensated by gluon saturation?) Increased azimuthal decorrelation with increasing y (w.r.t. DGLAP collinear-factorization): BFKL (LHC) [DelDuca, Schmidt], [Orr, Stirling] [A.Sabio-Vera, F.Schwennsen] [C.Marquet, Royon] [E. Iancu et al.] 31/26 ~80% decorrelation

32 Example: Mueller-Navelet dijets in CMS ( ~10) [S.Cerci-DdE, arxiv: ] Mueller-Navelet dijets with large y separation very sensitive to low-x QCD evolution (testing ground for BFKL): jet1 y~10 BFKL: extra radiation between the 2 jets will smooth out back-to-back topology jet2 A.H.Mueller, H.Navelet, NPB282 (1987)727 (partially compensated by gluon saturation?) Increased azimuthal decorrelation with increasing y (w.r.t. DGLAP collinear-factorization): PYTHIA/HERWIG vs. NLL-BFKL [DelDuca, Schmidt], [Orr, Stirling] [A.Sabio-Vera, F.Schwennsen] [C.Marquet, Royon] [E. Iancu et al.] 32/26