Low x QCD with CMS at LHC

Transcription

1 Low x QCD with CMS at LHC Quark Matter'06 Shanghai, Nov. 17th 2006 David d'enterria CERN, PH EP, Geneva CMS Heavy Ion Groups: Athens, Auckland, Budapest, CERN, Chongbuk, Colorado, Cukurova, Iowa, Kansas, Korea, Los Alamos, Lyon, Maryland, Minnesota, MIT Moscow, Mumbai, Rice, Seoul, Vanderbilt, UC Davis, UI Chicago, Yonsei, Zagreb 1/22

2 Overview Introduction: Parton structure & evolution at low x Gluon saturation & non linear evolution = Color Glass Condensate (CGC) Experimental probes of low x PDFs Low x QCD (Pb, p) with CMS at the LHC: CMS experimental capabilities Four case studies: 5.5 TeV: (1) dnch/dη, (2) ϒ e+e,µ+µ photoprod. in UPC PbPb 14 TeV: (3) Forward jets, (4) Mueller Navelet dijets Summary 2/22

3 Parton structure at low x DIS ep collisions probe partonic distributions in the proton: Q2 = resolving power Bjorken x = momentum fraction carried by parton Q2 F1,F2 = proton structure functions. HERA: strong rise at low x of F2(x,Q2) ~ sea quarks, lnf2 / lnq2 ~ gluons 3/22

4 Parton (x,q2) evolution at low x Q2 DGLAP (parton branch.): F2(Q2) ~ αsln(q2/q02)n, Q02 ~1 GeV2 x BFKL (parton emission ordered in pl): F2(x) ~ αsln(1/x)n Linear equations (single parton radiation/splitting): Cannot work at low x! (i) High gluon density : nonlinear g g fusion will balance parton branchings (ii) pqcd (collinear & kt) factorization should break (no incoherent parton scattering) (iii) Violation of unitarity even for Q2>>Λ2 (too large perturbative cross sections) CGC = effective field theory for low x QCD limit: Hadrons = classical fields below saturation scale Qs (enhanced in nucleus) Gluons overlap for momenta ~Qs Non linear JIMWLK, BK evolution eqs. 4/22

5 Experimental probes: low x PDFs (pp,pa,aa) Perturbative processes: Drell Yan, (di)jets, prompt γ, heavy Q _ Diffractive QQ (γp, γa): (di)jets (y=4) Forward production lower x pt x2 s/2 x1 s/2 x2min ~ pt/ s e y= xt e y Every 2 units of y, xmin decreases by ~10 5/22

6 CMS TOTEM at the LHC HF,CASTOR,ZDC + TOTEM: Quasi full acceptance at LHC [5.2 < η < 6.6] CASTOR HF η = TOTEM HF (z = ±140 m) ZDC [η > 8.3 neutral] Detection capabilities within η < 6.7 (and η > 8.1, neutral). Hard scattering measurements (jets, high pt hadrons, DY) possible down to x~10 6 in pp, pa, AA. 6/22

7 CMS PbPb: low x nuclear PDF studies 5.5 TeV, 8.8 TeV: (i) Very high s x = 2pT/ s ~ 10 3 ~30 45 times lower than RHIC! (ii) Saturation momentum (at y=0): Qs ~ 0.2 A1/6 ( s ey)0.25 ~ 3 GeV (iii) Very large perturbative cross sections. Armesto, J.Phys.G32:R367 (2006) Ratio of gluon densities in Pb to p:? ea, pa Nuclear xg(x,q2) basically unknown for x<10 3! Dd'E,J.Phys.G G30 (2004) S767 7/22

8 Case study I: Total PbPb hadron multiplicity Final A+A multiplicity Initial number of released gluons : CGC: + local parton hadron duality (1 gluon = 1 final hadron) CMS dn/d ( <2.5) via hit counting a la PHOBOS (pixels pulse height) 3 layers. η <2.5, full φ radius=4.4 cm Pixels (z,r): 100 µmx150 µm 32 Mpixels in 3 layers 8/22 Occupancy <2% for dnch/dη=5000

9 Case study I: Total PbPb hadron multiplicity Final A+A multiplicity Initial number of released gluons : CGC: + local parton hadron duality (1 gluon = 1 final hadron) CMS dn/d ( <2.5) via hit counting a la PHOBOS (pixels pulse height) 9/22

10 Case study I: Total PbPb hadron multiplicity Final A+A multiplicity Initial number of released gluons : CGC: + local parton hadron duality (1 gluon = 1 final hadron) CMS dn/d ( <2.5) via hit counting a la PHOBOS (pixels pulse height) Gluon saturation C.Smith,CMS AN03 15 reduced dn/d =0~ % PbPb HIJING (no quench) simulated primary tracks reconstructed (ptmin~50 MeV) KLN, NPA747 I(2005) /22

11 Case study II: ϒ photoprod. (γ Pb) in UPC PbPb High energy heavy ions 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.: Max. γ energy: Eγmax ~ 80 GeV (PbPb LHC) γpb: max. sγpb 1. TeV sγp(hera) QQ diffractive photoproduction sensitive to the gluon density squared: y=0 : x(ϒ) = y~2 : x(ϒ)~x(y=0) e y~ /22

12 e± and µ± measurement in CMS ( η <2.4) Tracking + ECAL + muon chambers ϒ e+e ϒ + Si TRACKER Silicon Microstrips and Pixels CALORIMETERS ECAL HCAL PbWO4 Plastic Sci/Steel sandwich 12/22 MUON BARREL Drift Tube Chambers (DT) Resistive Plate Chambers (RPC)

13 ϒ and ℓ+ℓ cross sections in UPC (γ Pb, γ γ ) STARLIGHT MC predictions [J. Nystrand, S.Klein, NPA752(2005)470] Signal Background Pb Pb ℓ ℓ Pb Pb ~50% of UPC interactions have soft EM interactions leading to nuclear breakup w/ fwd. neutron (Xn) emission (important for triggering) 13/22

14 UPC via fwd. neutron tagging in ZDC ( η > 8.3) ZDC = tungsten+quartz fibre sampling Cerenkov calorimeter with HAD & EM sections: Downstream1st beam separator dipoles (140 m from IP5) ZDC Forward neutral signals (n,γ): available at L1 trigger CMS IP T1/T2, Castor 5.32 η 6.86 ZDC RPs@150m RPs@220m Acceptance (neutral): θ<~(5.cm) /140.m <~400 μrad η > 8.3, pt < O(2. GeV/c) beams 14/22 TAN

15 Full simulation reconstruct. ϒ + ℓ+ℓ continuum Full CMS simulation+digi+hit+reconstruction: ϒ e+e DdE, A.Hees, CMS AN ϒ + tracker+ecal tracker+µ chambers Peak position: ~9.35 GeV/c2 Peak position: ~9.52 GeV/c2 Mass resolution: ~150 MeV/c2 Mass resolution: ~90 MeV/c2 _ Excellent µµ mass resolution: higher mass bb states (ϒ',ϒ'') can be resolved (not in MC now) 15/22

16 Expected ϒ yields (PbPb 1 year luminosity) Backgd. subtracted dn/dminv for ℒ dt =0.5 nb 1 PbPb 5.5 TeV (t=106 s) (error bars = stat. uncertainties for integrated luminosity) DdE, A.Hees, CMS AN ϒ e+e ϒ + Syst. uncertainties (dominated by backgd. subtraction, 5% lumin.): ~10% Final total rates : N(ϒ e+e ) ~ 220±15(stat)±10%(syst) N(ϒ µ+µ ) ~ 180±13(stat)±10%(syst) Large stats. for detailed studies (including y dependence) of gluon PDF 16/22

17 CMS pp: low x proton PDF studies 14 TeV : (i) At y=0, x=2pt/ s ~10 3 (domain HERA,Tevatron). Go for x<10 4 (ii) Saturation momentum: Qs ~ 0.2 ( s ey)0.25 ~ 1 GeV (y=0), 3 GeV(y=5) (iii) Very large perturbative cross sections: Prompt γ Drell Yan Jets _? Heavy Flavours ep, pp W,Z production LHC forward rapidities e.g. y ~ 5, Q2 ~ 10 GeV x down to 10 5! 17/22

18 Hadronic Forward Calorimeter (3 < η < 5) 11.2 m from IP m absorber depth.. Steel+quartz fibre Cerenkov (EM,HAD) calorimeter Hamamatsu R7525 rad hard PMTs 2x1200 towers: Δηx ΔΦ ~ 0.18 x 0.18 Excellent jet position ~10%, energy ~15% resolutions Large jet reco efficiency: ~90% Jet candidates in level 1 trigger (for VBF,qq >qqh) 18/22

19 Hadronic Forward Calorimeter (3 < η < 5) 11.2 m from IP m absorber depth.. Steel+quartz fibre Cerenkov (EM,HAD) calorimeter Hamamatsu R7525 rad hard PMTs 2x1200 towers: Δηx ΔΦ ~ 0.18 x 0.18 Excellent jet position ~10%, energy ~15% resolutions Large jet reco efficiency: ~90% Jet candidates in level 1 trigger (for VBF,qq >qqh) 19/22

20 Case study III: Forward jets in HF (3 < η < 5) Inclusive forward low ET jet (ET ~ GeV) production: p + p jet1 + jet2 + X Sensitive to partons with: x2 ~ 10 4, x1 ~ 10 1 Inclusive fwd. jet reconstruction: PYTHIA 6.4. min bias (hard&soft QCD) MC level proof of principle only (no det. response!) HF grid: η φ = Iterative cone, R=0.5, Ethresh=10 GeV, Eseed=3 GeV Missing important corrections: underlying evt. (PYTHIA CMS Tune), hadronization (cluster vs. Lund) Large yields. Low ET uncertainties to be determined. 20/22 PYTHIA ~ NLO [W. Vogelsang]

21 Case study IV: Mueller Navelet dijets in HF Mueller Navelet dijets separated by large Δy: very sensitive to non DGLAP evolution jet1 suppressed ratio sat./bfkl pp s = 14 TeV increasing y~10 jet2 rapidity C.Marquet, Royon, hep ph/ (MC level study only!) Dijet rates ET~20 40 GeV: enough stat. for detailed η1= η2= [3. 3.5] studies of y evolution η1= η2= [4.5 5.] 21/22

22 Summary Gluon saturation and non linear evolution must set in at (some) low x in hadronic wave functions Fundamental info on high energy limit of QCD Hints in ep HERA, da,aa RHIC: stronger signals LHC (Qs~5 GeV) CMS = unique experiment to study high parton density /evolution in proton&pb down to x~10 5 using fwd. detectors and perturbative processes: (di)jets, ϒ,... 22/22

23 Backup slides 23/22

24 Case study III: DY in CASTOR T2 (5.2 < η < 6.6) Drell Yan feasibility studies with CMS (CASTOR) + TOTEM (T2): Sensitive to low x quark densities PDF parametrizations 5.2 < η < 6.6 Log10(x) 7< η <9 5.5< η <7.8 Kimber Martin Ryskin Log10(x) T2 tracker+ CASTOR calo needed to deal w/ large QCD (and QED) bckgd. 24/22

25 TOTEM 2 and CASTOR (5.2 < η < 6.6).. TOTEM GEM ( Gas Electron Multiplier ) telescope detector: electron polar angle HF, TOTEM, CASTOR Platforms T2 telescope CASTOR (W/ Q fiber calo): electromag. shower identification CASTOR Tungsten plates + quartz fibres Cherenkov sampling calorimeter Light guides + APDs readout Azimuth segmented (8 octants) EM section: 11.2 cm ~ 19 X0 HAD+EM sections: 136 cm~ 10 λi 192 channels 25/22

26 Fully reconstructed ϒ distributions Input & output ϒ rapidity and pt distributions: electrons muons Better acceptan at y ~2 Better acceptance Dip at y=0 (lowest at y=0 total muon momentum) Very similar reco momentum pt responses slightly boosted. 26/22

27 UPC Level 1 trigger: signature & rates Use of following L1 primitives: Veto ('OR') on simultaneous activity in HF+/ (3< η <5): 1 or 2 rap gaps Neutron signal in ZDC+ or ZDC ( η >8.3): nuclear breakup tagging Isolated ECAL tower above 3 GeV: ϒ decay electron Signal in muon RPCs ( η <2.1) or CSCs (0.8< η <2.4): ϒ decay muon (>4 GeV) 2 UPC L1 triggers: L1 trigger (background) rates (ℒ=0.5 mb 1s 1) ~ 2 4 Hz Nother ~ (other γpb, IP Pb, γγ X hard processes)~ 2 Hz 27/22

28 Acceptance x Efficiencies ϒ Efficiencies acceptances versus y and pt : Total Acc. Effic. : ~21% ( + ), 28% (e+e ) ϒ e +e ϒ + Acc Effic(pT): ~8% at pt=0 GeV/c Electron and muon analyses have ~10% at peak of prod. (pt~50 GeV/c) complementary rapidity acc. effic: increasing with pt. Overefficiency above + : ~60% at y ~ MeV/c unimportant (very low yields) e+e : ~60% at y <0.5 28/22

29 CMS+TOTEM forward detectors (detail) 140 m CASTOR ZDC TOTEM T2 (Cal) HF TOTEM T1 T2 HF T1 (Track) (Cal) (Track) IP 29/22

30 Case study II: low x via γ Pb QQbar (and in general, hard) diffractive photoproduction sensitive to the gluon density squared in the nucleus: Small x probed in γa collisions at LHC: y=0 : x(ϒ) = y~2 : x(ϒ)~x(y=0) e y~10 4 Unexplored (x,m2) regime of the nucleus wave function. Gluon saturation non linear QCD evolution expected Suppressed RHIC Fixed target hard photoproduction. 30/22

31 HF, TOTEM (T1,T2), CASTOR.. HF, TOTEM, CASTOR Platforms 31/22

32 TOTEM T2 Tracker (5.3 <η< 6.7) TOTEM GEM ( Gas Electron Multiplier ) charged particle telescope detector: VF AT AP V Shielding envelope ~ 50 0m m HV GEM half telescope 32/22

33 Zero Degree Calorimeter ( η > 8.1 neutral) ZDC = Tungsten plates+quartz fibres sampling Cerenkov calorimeter with HAD and EM sections: Beams ZDC: forward neutral energy (n,γ) detection: Beam pipe splits ~140 m from IR (Follows closely RHIC experience). EM section: x33 2mm W cells (~19X0) HAD sect: x24 15mm W cells (~5.6λ0) PMTs: R7525 (as HF) Rad hard to ~20 Grad (AA, pp low lum.) Energy resolution (n,γ): σ/e~10% Position resolution: ~2 mm (EM sect.) 33/22 Signals available for L1

34 Forward Detectors in CMS/ATLAS CMS + TOTEM: TOTEM T2 HF (Iron Q fiber calo): 3 < η < 5 HF TOTEM T1 IP TOTEM T1: 3.1 < η < 4.7 TOTEM T2 (GEM telescope): 5.3 < η < 6.7 CASTOR (W/ Q fiber calo): 5.25 < η < 6.5 ZDC (W/Q fiber calo): η > 8.5 (neutral) Forw. Cal.: 3 < η < 5 ATLAS: LUCID (Cerenkov Counter): 5.4 < η < 6.1 ZDC (W/Q fiber calo): η > 8.5 (neutral) 34/22

35 CASTOR forward calorimeter (5.1 <η < 6.6) T2 Tungsten plates + quartz fibres Cherenkov sampling calorimeter Light guides + APDs readout Azimuth segmented (8 octants) EM section: 11.2 cm ~ 19 X0 HAD+EM sections: 136 cm~ 10 λi 192 channels 35/22

36 PDF (x,q2) experimental maps: proton, nucleus Kinematical (x,q2) domains covered experimentally: much less nuclear PDF data available: ea, pa (_) ep, pp Dd'E,nucl ex/ /22

37 Small x Forward rapidities 2 2 parton kinematics: y = 0: x1~x2 ~ xt = 2pT/ s x1 s/2 x2 s/2 pt x1 s/2 x2 s/2 Every 2 units of y, xmin decreases by ~10 e.g. LHC, pt = 10 GeV/c θ <10 3 (η>7): x< (gluon fusion) kinematics: even much lower x allowed in CGC CGC: x(y=4) ~ 10 4 pqcd: x(y=4) ~ 10 2 Momentum balanced by the medium ( gluon ladder ) (RHIC) Accardi, nucl th/ /22

38 RHIC: Total AA hadron multiplicity (I) AuAu (200 GeV) 0 5% central collisions: ~ 700 charged particles per unit rapidity at y=0 38/22 Armesto Pajares IJMP A 14 (2000) 2019 dnch/dη

39 Non linear QCD evolution equations Quantum evolution of classical fields governed by JIMWLK eqs. Non linear, all twist equations in saturation regime Generalized Fokker Planck eq. (diffussion of wave function) Large Nc limit BK Weak field limit BFKL. [Jalilian Marian, Iancu, McLerran, Kovner, Leonidov, Weigert ] Additional quantum corrections: lead to anomalous dimension in extended scaling region: extended window of applicability outside CGC! 39/22

40 RHIC: forward dau pt spectra Suppression of hadron spectra: p ~ 2 4 GeV/c at forward rapidities T (reduced number of partonic scattering centers at low x): Ratio dau to pp (200 GeV) y = 3.2 y dependence: pqcd collinear factorization + LT shadowing Kharzeev, Tuchin, Kovchegov, Plus, CGC can explain other pa phenomenology... hep ph/ (1) Leading twist shadowing : extended scaling region (2) Cronin enhancement at y=0: gluons redistributed from low to higher x Qs ~ 1 2 GeV/c too close evidences to non perturbative regime.p However: RHIC/HERA saturation at moderate T 40/22

41 HERA: Geometric scaling of low x F2(x,Q2) Saturation predicts low x structure dependence on single scale Q2 s DIS inclusive cross section shows scaling for all x < 0.01, < Q2 < 450 GeV2 σγ*p Golec Biernat Wusthoff PRD (1999), Q0 = 1 GeV, λ = 0.3 described by dipole cross section model (derivable within CGC): up to relatively large Q2 ( extended scaling region): 41/22

42 ZDC tagged (hard) photoproduction dσ/dy, mb Quarkonia: γ+a J/Ψ,ϒ +A very sensitive to nuclear gluon density: No shadowing FGS shadowing σ(j/ψ,ϒ) ~ xg(x,q2) 2 [FGS: Frankfurt et al. 2001] ~30% reduction of G(x,Q2) 0.5 σj/ψ,ϒ y Dijet: via gluon exchange (well described in QCD & HERA) Wider range of Q2 than QQbar. Photon jet (~1% of dijet rate) has clear signature ttbar possible in pa collisions (measure charge of top quark) 42/22

43 Backup slides 43/22

44 Colour Glass Condensate (CGC) CGC = EFT in high energy (small x) QCD limit: [McLerran, Venugopalan, Kharzeev, Kovchegov, Jalilian Marian, Mueller, Iancu, Gelis, Tuchin, Hadrons = classical fields below saturation scale Qs: Ikatura, Dumitru,... ] Gluons overlap for momenta below: saturated gluon wave function (Bremsstrahlung) Saturation enhanced in nucleus: ~6 Qs hard enough perturbative calc. (strong Fµν, weak coupling): p,a+a = Collision of gluon wave function(s): resums all multiple scatts. Color Glass (quarks ~ static color sources) Condensate (high gluon occupation) 44/22

45 . McLerran, RV; Kovchegov; Jeon, RV Effective action describes a weakly coupled albeit non perturbative system 45/22

46 Mechanism for parton saturation Competition between attractive bremsstrahlung and repulsive recombination effects. Maximal phase space density => Saturated for wee parton cloud Valence partons In infinite momentum frame (IMF), Construct effective theory of wee parton modes 46/22

47 Hadron at high energies is a Color Glass Condensate. Gluons are colored Random sources evolving on time scales much larger than natural time scales very similar to spin glasses Bosons with large occupation # ~ form a condensate Typical momentum of gluons is 47/22

48 Forward Detectors in ALICE/LHCb ALICE, LHCb forward muon spectrometers: 2.5< η< 4 2<η<5 Good capabilities for heavy Q, QQbar measurements at low x: 48/22