1. International School of Subnuclear Physics - Erice, August 31, 2003 Andrea Dainese 1 Probing the Quark-Gluon Plasma with charm at LHC Andrea Dainese Universita` degli Studi di Padova cc c D0D0 K-K- ++ c

2. International School of Subnuclear Physics - Erice, August 31, 2003 Andrea Dainese 2 Outline Quick introduction: physics of hot and dense QCD matter (see lectures by F.Karsch and T.W.Ludlam) Heavy ion physics at the LHC Parton energy loss in QGP the case for heavy quarks (charm) Experimental feasibility study Can we measure the energy loss of c quarks in the QGP?

3. International School of Subnuclear Physics - Erice, August 31, 2003 Andrea Dainese 3 Physics of hot and dense QCD matter (  F.Karsch) Temperature Baryonic density Lattice QCD (  B ~ 0) estimates the phase transition at: T c ~ 170 MeV and  c ~ 1 GeV/fm 3 ~ 5  nucleus QCD phase diagram hadronic matter exists in different states at high energy density (high temperature or high density) nuclear matter undergoes a phase transition to a deconfined Quark-Gluon Plasma (QGP)

4. International School of Subnuclear Physics - Erice, August 31, 2003 Andrea Dainese 4 Create the Little Bang in the lab In high energy heavy ion collision large energy densities (> 2-3 GeV/fm 3 ) are reached over large volumes (> 1000 fm 3 ) Evidence for the QGP formation collected by the 7 experiments of CERN-SPS programme (up to: Pb-Pb, ) Now BNL-RHIC is confirming/extending these results at (see lecture by T.W. Ludlam and talk by D. D’Enterria) Next step: LHC with Pb-Pb @ large production of hard partons and heavy quarks excellent tools to study properties of QGP

5. International School of Subnuclear Physics - Erice, August 31, 2003 Andrea Dainese 5 Hard partons probe the medium Partons travel ~ 5 fm in the high colour-density medium Energy loss by gluonsstrahlung (more details follow) modifies momentum distributions depends on medium properties PROBE medium probe IN (known from pp, pA + pQCD) probe OUT IN vs. OUT

6. International School of Subnuclear Physics - Erice, August 31, 2003 Andrea Dainese 6 Parton Energy Loss Due to medium-induced gluon emission Average energy loss (BDMPS model): hard parton QCD process: emitted gluon radiates itself   E  L 2 path length L Casimir coupling factor: 4/3 for quarks 3 for gluons Medium transport coefficient  gluon density and momenta R.Baier, Yu.L.Dokshitzer, A.H.Mueller, S.Peigne' and D.Schiff, (BDMPS), Nucl. Phys. B483 (1997) 291. C.A.Salgado and U.A.Wiedemann, arXiv:hep-ph/0302184. cold matter  E in 5 fm

7. International School of Subnuclear Physics - Erice, August 31, 2003 Andrea Dainese 7 Heavy quarks with momenta < 20-30 GeV/c  v < c Gluons cannot be radiated at angles < m Q /E Q Due to distructive interference (same mechanism that determines angular ordering in parton showers) This ‘dead cone’ implies lower energy loss Heavy Quarks: dead cone Dead cone in vaccum: Yu.L.Dokshitzer, V.A.Khoze and S.I.Troyan, J. Phys. G17 (1991) 1602. Dead cone in medium: Yu.L.Dokshitzer and D.E.Kharzeev, Phys. Lett. B519 (2001) 199. Q D mesons almost not quenched at all Ratio D/hadrons enhanced and sensitive to medium properties

8. International School of Subnuclear Physics - Erice, August 31, 2003 Andrea Dainese 8 How to measure it? Most direct way to study quenching is by comparing p t distributions in pp and AA Nuclear modification factor: Energy loss R AA < 1 D 0  K -   decay allows the exclusive reconstruction of open charm mesons and a direct measurement of their p t distribution

9. International School of Subnuclear Physics - Erice, August 31, 2003 Andrea Dainese 9 D 0  K  detection strategy with ALICE STRATEGY: invariant mass analysis of fully-reconstructed topologies originating from (displaced) secondary vertices Measurement of Impact Parameters (Silicon vertex) Measurement of Momenta (TPC + silicon) Particle identification (High resolution Time of Flight) to tag the two decay products (K/  separation)

10. International School of Subnuclear Physics - Erice, August 31, 2003 Andrea Dainese 10 Results. Example: Pb-Pb p t -integrated (K,  ) Invariant Mass distribution (p T –integrated) (corresponding to 10 7 Pb-Pb events ~ 1 month of ALICE) Statistical Significance of the Signal: after background subtraction Details on selection strategy in: N.Carrer, A.D. and R.Turrisi, J. Phys. G29 (2003) 575. analysis for Pb-Pb and pp done in bins of p t and main errors estimated

11. International School of Subnuclear Physics - Erice, August 31, 2003 Andrea Dainese 11 R AA for D mesons ‘High’ p t (6-15 GeV/c) here energy loss can be studied (it’s the only expected effect) Low p t (< 6-7 GeV/c) Nuclear shadowing

12. International School of Subnuclear Physics - Erice, August 31, 2003 Andrea Dainese 12 R AA with Quenching standard quenching (no dead cone) increasing density R AA ~ 0.7-08 decreasing at high p t R AA ~ 0.4-05 increasing at high p t medium, Dead cone: low quenching but increasing with p t (typical signal of mass effect)

13. International School of Subnuclear Physics - Erice, August 31, 2003 Andrea Dainese 13 D/hadrons ratio Ratio expected to be enhanced because: D comes from (c) quark, while , K, p come mainly (~80%) from gluons, which lose twice energy as quarks dead cone for heavy quarks D/h ratio: R D/h = R D AA / R h AA R D/h ~ 2-3 in hot QGP sensitive to medium density

14. International School of Subnuclear Physics - Erice, August 31, 2003 Andrea Dainese 14 Summary LHC allows to investigate deconfined QCD matter by means of hard probes and their medium-induced quenching Exclusive reconstruction of D 0 decays with the ALICE detector provides sensitive tool for heavy quark ‘colourimetry’ of QGP

15. International School of Subnuclear Physics - Erice, August 31, 2003 Andrea Dainese 15 Back-up slides

16. International School of Subnuclear Physics - Erice, August 31, 2003 Andrea Dainese 16 Heavy ion Physics at LHC LHC: factor 30 jump in w.r.t. RHIC hotter bigger study of: longer-living ‘drops’ of QGP 4-101.5- 4.0< 1  QGP (fm/c) 2x10 4 7x10 3 10 3 V f (fm 3 ) 15- 403.52.5  (GeV/fm 3 ) 2- 8 x10 3 700430dN ch /dy 550020017s 1/2 (GeV) LHCRHICSPS Central collisions  3  5 -10 What are the properties of deconfined matter? ‘Deep de-confinement’ Lattice QCD,  B =0  closer to ‘ideal’ QGP  easier comp. with theory F. Karsch

17. International School of Subnuclear Physics - Erice, August 31, 2003 Andrea Dainese 17 Heavy Ion Physics at LHC Pb-Pb @ Main novelty of the LHC: large cross section for hard processes (high-p t partons and heavy quarks) Hard processes are extremely useful tools happen at t = 0 (initial stage of the collision) have large virtuality Q and small “formation time”  t  1/ Q initial production not sensitive to properties of medium formed in the AA collision Initial yields and p t distributions in AA can be predicted using pp measurements + pQCD + collision geometry + “known” nuclear effect (e.g. shadowing  suppression of low-x (low-p t ) production) Deviations from such predictions are due to the medium q q Q Pb Ideal to study the properties of deconfined matter

18. International School of Subnuclear Physics - Erice, August 31, 2003 Andrea Dainese 18 Shadowing Nuclear initial state effect Bjorken-x: fraction of the momentum of the proton ( ) carried by the parton entering the hard scattering At the LHC Pb ion @ LHC ~ 10 5 -10 6 partons (mainly gluons) q q Q xaxa xbxb x a +x b g Pb (x)/g p (x) Shadowing: reduces initial hard yield at low p t (low x) scales trivially from pA to AA “they are so close that they fuse”