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Stato della ricerca e prospettive in collisioni ultrarelativistiche nucleo-nucleo

Stato della ricerca e prospettive in collisioni ultrarelativistiche nucleo-nucleo

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Stato della ricerca e prospettive in collisioni ultrarelativistiche nucleo-nucleo

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  1. Stato della ricerca e prospettive in collisioni ultrarelativistiche nucleo-nucleo Federico Antinori INFN Padova

  2. Contenuto • Introduzione (QGP, SPS) • Risultati principali di RHIC • Prospettiva: “hard probes” (heavy flavour) • non tratterò di FAIR (GSI, “bassa” energia) FA - IFAE 2006 - Pavia, 20 aprile 2006

  3. QCD phase diagram ~ 10 ms after Big Bang LHC Early Universe RHIC Quark-Gluon Plasma Tc ~ 170 MeV SPS AGS Hadron gas Temperature Nuclear Neutron Star matter ~ 5 - 10 nuclear r Baryon density ec ~ 1 GeV/fm3 FA - IFAE 2006 - Pavia, 20 aprile 2006

  4. (Partial) Chiral Symmetry Restoration • Confined quarks acquire an additional mass (~ 350 MeV) dynamically, through the confining effect of strong interactions • M(proton)  938 MeV; m(u)+m(u)+m(d) = 1015 MeV • Deconfinement is expected to be accompanied by a restoration of the masses to the “bare” values they have in the Lagrangean • As quarks become deconfined, the masses would go back to the bare values; e.g.: • m(u,d): ~ 350 MeV  a few MeV • m(s): ~ 500 MeV  ~ 150 MeV FA - IFAE 2006 - Pavia, 20 aprile 2006

  5. Due predizioni storiche...

  6. Strangeness enhancement K+ s s s s s s s s d d u u d X- u p- u d d d d d d d d d d d s u u u u u u u u s s u d d d d p+ u u s u u u u p u u d d d s W+ u s s u u d d u s d L • restoration of csymmetry -> increased production of s • mass of strange quark in QGP expected to go back to current value • mS ~ 150 MeV ~ Tc • copious production of ss pairs, mostly by gg fusion [Rafelski: Phys. Rep. 88 (1982) 331] [Rafelski-Müller: P. R. Lett. 48 (1982) 1066] • deconfinement  stronger effect for multi-strange • can be built recombining uncorrelated s quarks produced in independent microscopic reactions • strangeness enhancement increasing with strangeness content [Koch, Müller & Rafelski: Phys. Rep. 142 (1986) 167] FA - IFAE 2006 - Pavia, 20 aprile 2006

  7. Charmonium suppression • QGP signature proposed by Matsui and Satz, 1986 • In the plasma phase the interaction potential is expected to be screened beyond the Debye length lD (analogous to e.m. Debye screening) • Charmonium (cc) and bottonium (bb) states with r > lD will not bind; their production will be suppressed For T ~ 200 MeV: lD ~ 0.1 – 0.2 fm (very) rough estimate... FA - IFAE 2006 - Pavia, 20 aprile 2006

  8. L’era dell’SPS...

  9. CERN heavy-ion programme • 1994 to 2003 • typically 4 - 6 weeks per year Pb nuclei @ 158 A GeV/c (40 GeV in 1999, 158 GeV In in 2003, + short 20, 30, 80 GeV runs) • 9 experiments: • WA97(silicon pixel telescope spectrometer: production of strange and multiply strange particles) • WA98 (photon and hadron spectrometer: photon and hadron production) • NA44 (single arm spectrometer: particle spectra, interferometry, particle correlations) • NA45 (e+e- spectrometer: low mass lepton pairs) • NA49 (large acceptance TPC: particle spectra, strangeness production, interferometry, event-by-event studies…) • NA50 (dimuon spectrometer: high mass lepton pairs, J/y production) • NA52(focussing spectrometer: strangelet search, particle production) • NA57 (silicon pixel telescope spectrometer: production of strange and multiply strange particles) • NA60 (dimuon spectrometer + pixels: dileptons and charm) FA - IFAE 2006 - Pavia, 20 aprile 2006

  10. A Pb-Pb collision at the SPS • “Busy” events! (thousands of produced particles) • High granularity detectors are employed (TPC, Si Pixels,...) FA - IFAE 2006 - Pavia, 20 aprile 2006

  11. Particle production • Rapidity distribution for negative hadrons (mostly pions) for central Pb-Pb collisions (from NA49) • ~ 2500 hadrons per collision • corresponding to an initial energy density of 2 – 3 GeV/fm3 FA - IFAE 2006 - Pavia, 20 aprile 2006

  12. Strangeness enhancement at the SPS • Enhancement relative to p-Be Enhancement is larger for particles of higher strangeness content (QGP prediction!) up to a factor ~ 20 for W So far, no hadronic model has reproduced these observations (try harder!) Actually, the most reliable hadronic models predicted an opposite behaviour of enhancement vs strangeness FA - IFAE 2006 - Pavia, 20 aprile 2006

  13. J/y suppression at the SPS • measured/expected J/y suppression vs estimated energy density • anomalous suppression sets in at e ~ 2.3 GeV/fm3 (b ~ 8 fm) • effect seems to accelerate at e ~ 3 GeV/fm3 (b ~ 3.6 fm) • this pattern has been interpreted as successive melting of the cc and of the J/y FA - IFAE 2006 - Pavia, 20 aprile 2006

  14. What have we learned at the SPS? • We create a strongly interacting fireball in a state of very large energy density, at which the very concept of individual, separated hadrons is not too meaningful • At freeze-out (when the reinteractions between the produced particles cease) the system is expanding at ~ ½ c • The properties of this state are not explained in terms of those of a conventional system of strongly interacting hadrons: we have found a new “regime” for strongly interacting system, which has to be investigated further • This state exhibits many features of the QGP, in particular the expected effects of deconfinement (J/y suppression) and chiral simmetry restoration (strangeness enhancement) are there. It looks like the partonic degrees of freedom are active • in order to better understand the properties of this state we need to go to higher energy  RHIC, LHC • conditions closer to ideal, harder probes FA - IFAE 2006 - Pavia, 20 aprile 2006

  15. The next step: RHIC • Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab (USA - NY) Au-Au collision in STAR • sNN = 200 GeV,  10 from SPS: • higher energy densities, temperatures, expected QGP lifetimes, volumes • QCD calculations more reliable  improve our understanding! • 10 units rapidity span: better separation of central and fragmentation regions • 4 dedicated, complementary experiments: • 2 larger (STAR: hadronic signals, PHENIX: leptonic, e.m. signals), • 2 smaller (PHOBOS, BRAHMS) FA - IFAE 2006 - Pavia, 20 aprile 2006

  16. RHIC Headlines • Rather low multiplicity • Gluon saturation? (“Colour Glass”) • Large Elliptic Flow • v2 • High pT Suppression • Rcp, RAA • Suppression of Away-Side Jets • Plus an New Structure in Away-Side? • Valence Quark Counting Rules • Recombination? FA - IFAE 2006 - Pavia, 20 aprile 2006

  17. Particle multiplicity • Multiplicity per nucleon-nucleon collision increases • Bjorken estimate of energy density:   5.5 GeV/fm3 • Extrapolation to LHC: dNch/dy ~ 2500 FA - IFAE 2006 - Pavia, 20 aprile 2006

  18. Colour Glass? • At small x, large A, saturation of gluon pdf’s? • would explain relatively low multiplicity FA - IFAE 2006 - Pavia, 20 aprile 2006

  19. Elliptic Flow • Non-central collisions are azimuthally asymmetric • The transfer of this asymmetry to momentum space provides a measure of the strength of collective phenomena • Large mean free path • particles stream out isotropically, no memory of the asymmetry • extreme: ideal gas (infinite mean free path) • Small mean free path • larger density gradient -> larger pressure gradient -> larger momentum • extreme: ideal liquid (zero mean free path, hydrodynamic limit)

  20. Azimuthal Asymmetry • at low pT: azimuthal asymmetry as large as expected at hydro limit • very far from “ideal gas” picture of plasma FA - IFAE 2006 - Pavia, 20 aprile 2006

  21. Rcp, RAA, RdAu Yield/collision in central collisions Yield/collision in peripheral collisions Yield/collision in nucleus-nucleus Yield/collision in proton-proton Yield/collision in deuteron-nucleus Yield/collision in proton-proton FA - IFAE 2006 - Pavia, 20 aprile 2006

  22. High pT suppression • High pT particle production expected to scale with number of binary NN collisions if no medium effects • Clearly does not work for more central collisions FA - IFAE 2006 - Pavia, 20 aprile 2006

  23. AA vs pp [David d’Enterria] • Differences can arise due to both initial-state and final-state effects FA - IFAE 2006 - Pavia, 20 aprile 2006

  24. Centrality Dependence Au-Au (“cold” matter and medium effects) dAu (only “cold” matter effects expected) • Opposite behaviour with centrality • Looks like suppression in AA is due to medium • Parton energy-loss in the medium? (“Jet Quenching”) Final Data Preliminary Data

  25. Azimuthal Correlations (pp) Adler et al., PRL90:082302 (2003), STAR away-side trigger near-side • In high energy collisions particles are correlated in azimuth due to jets • e.g.: at RHIC in proton-proton collisions from STAR • “trigger” particle: 4 < pT< 6 GeV/c • associated particles: pT > 2 GeV/c FA - IFAE 2006 - Pavia, 20 aprile 2006

  26. Azimuthal Correlations is this what happens? hadrons leading particle Adams et al., Phys. Rev. Let. 91 (2003) q q • away-side jet still present in dAu • but disappears in central AuAu • away-side jet quenched? • trigger bias on jets produced close to surface? FA - IFAE 2006 - Pavia, 20 aprile 2006

  27. Two peaks? STAR Preliminary Fit to near side: const. + gaussian + Borghini-cos(fixed) p+p Au+Au 5% free fit (1/Ntrig) dN/d(Df) stat. mom. conserv.Borghini et al. stat. mom. conserv.Borghini et al. Df Df • Disappearance of away jet in central collisions at RHIC • 4 < pT(trig) < 6 GeV/c • pT(assoc) > 2 GeV/c • if pT(assoc) is lowered, peak turns up again, but with strange shape: • 0.15 < pT(assoc) < 4 GeV/c [F. Wang: QM 2004] FA - IFAE 2006 - Pavia, 20 aprile 2006

  28. Similar results from PHENIX, too PHENIX STAR • Low pT associated particles seem to peak ~ 1 radian (~ 60O) away from p FA - IFAE 2006 - Pavia, 20 aprile 2006

  29. Shock wave? • Idea: [Casalderrey-Solana, Shuryak, Teaney: hep-ph/0411315] (see also [Stocker: Nucl.Phys. A750 (2005) 121]) • medium is dense • “away jet” parton travels through medium with speed ~ c • faster than speed of sound in medium: cs • shock wave (“sonic boom”) is emitted at angle: • with cs2 ~ 0.2 : FA - IFAE 2006 - Pavia, 20 aprile 2006

  30. Baryon puzzle @ RHIC • Central Au-Au: as many p- (K-) as p (L) at pT ~ 1.5  2.5 GeV • e+e-jet (SLD) • very few baryons from fragmentation! p K p H.Huang @ SQM 2004 FA - IFAE 2006 - Pavia, 20 aprile 2006

  31. Rcp • strange particles come to rescue! • if loss is partonic, shouldn’t it affect p and p in the same way? FA - IFAE 2006 - Pavia, 20 aprile 2006

  32. Quark Recombination K+ s s s s s s s s d d u u d X- u p- u d d d d d d d d d d d s u u u u u u u u s s u d d d d p+ u u s u u u u p u u d d d s W+ u s s u u d d u s d L • if hadrons are formed by recombination, features of the parton spectrum are shifted to higher pT in the hadron spectrum, in a different way for mesons and baryons  constituent quark counting S.Bass @ SQM`04 FA - IFAE 2006 - Pavia, 20 aprile 2006

  33. Where should recombination work? • Proponents say @ pT between 1 and 4 GeV (6 GeV) for mesons (baryons) • hydro below, fragmentation above (at RHIC energy) fragmenting parton: ph = z p, z<1 recombining partons: p1+p2=ph R.Fries @ QM`04 FA - IFAE 2006 - Pavia, 20 aprile 2006

  34. elliptic flow v2 STAR Preliminary • Recombination also offers an explanation for the v2 baryon puzzle... scaled with n(quarks) FA - IFAE 2006 - Pavia, 20 aprile 2006

  35. A few hiccups... • Strict recombination has a few theoretical problems... • it violates 2nd law of thermodynamics • reduction of the number of particles  lower disorder  entropy decreased • it actually also violates the 1st... • impossible to conserve energy and momentum simultaneously • what happens to gluons? FA - IFAE 2006 - Pavia, 20 aprile 2006

  36. Picture emerging from RHIC • System formed in AuAu collisions • presumably partonic • Strongly collective (~ liquid behaviour) • Very opaque medium (strong energy loss) • Indications for hadronization by recombination • How can we test this scenario? • Need to confirm this picture with a different probe • Heavy quarks FA - IFAE 2006 - Pavia, 20 aprile 2006

  37. Charm & beauty: ideal probes • calculable in pQCD; calibration measurement from pp • rather solid ground • caveat: modification of initial state effects from pp to AA • shadowing ~ 30 % • saturation? • pA reference fundamental! • produced essentially in initial impact • probes of high density phase • no extra production at hadronization • probes of fragmentation • e.g.: independent string fragmentation vs recombination FA - IFAE 2006 - Pavia, 20 aprile 2006

  38. Probing the medium • quenching vs colour charge • heavy flavour from quark (CR = 4/3) jets • light flavour from (pT-dep) mix of quark and gluon (CR = 3) jets • quenching vs mass • heavy flavour predicted to suffer less energy loss • gluonstrahlung: dead-cone effect • beauty vs charm • heavy flavour should provide a fundamental cross-check of quenching picture emerging from RHIC • at LHC: high stats and fully developed jets FA - IFAE 2006 - Pavia, 20 aprile 2006

  39. Heavy flavour production in AA • binary scaling: can be broken by: • initial state effects (modified PDFs) • shadowing • kT broadening • gluon saturation (colour glass) (concentrated at lower pT) • final state effects (modified fragmentation) • parton energy loss • violations of independent fragmentation (e.g. quark recombination) (at higher pT) FA - IFAE 2006 - Pavia, 20 aprile 2006

  40. Heavy flavour energy loss? Energy loss for heavy flavours is expected to be reduced: i) Casimir factor • light hadrons originate predominantly from gluon jets, heavy flavoured hadrons originate from heavy quark jets • CR is 4/3 for quarks, 3 for gluons ii) dead-cone effect • gluon radiation expected to be suppressed for q < MQ/EQ [Dokshitzer & Karzeev,Phys. Lett. B519 (2001) 199] [Armesto et al., Phys. Rev. D69 (2004) 114003] average energy loss distance travelled in the medium Casimir coupling factor transport coefficient of the medium  R.Baier et al., Nucl. Phys. B483 (1997) 291 (“BDMPS”) FA - IFAE 2006 - Pavia, 20 aprile 2006

  41. Large suppression at RHIC! scaled to M. Cacciari et al., hep-ph/0502203 [J.Bielcik @QM05] • n.p. electrons ~ as suppressed as expected for c only (no b) • yet, region above 3-4 GeV expected to be dominated by beauty... [Xin Dong@QM05] • disentangling c/b is a must! • e.g. full reconstruction of D vertices FA - IFAE 2006 - Pavia, 20 aprile 2006

  42. ALICE rf: 50 mm z: 425 mm PIXEL CELL  9.8 M Two layers: r = 4 cm r = 7 cm |h|<0.9: B = 0.4 T TOF TPC ITS with: - Si pixels - Si drifts - Si strips FA - IFAE 2006 - Pavia, 20 aprile 2006

  43. Track Impact Parameter • expected d0 resolution (s) FA - IFAE 2006 - Pavia, 20 aprile 2006

  44. LHC is a Heavy Flavour Machine! system shadowing NN x-sect (mb) total multiplicity pp 14 TeV 11.2/0.5 1/1 0.16/0.007 Pb-Pb 5.5 TeV (5% cent) 6.6 /0.2 0.65 /0.85 115 /4.6 • cc and bb rates • ALICE PPR (NTLO + shadowing) cc bb PbPb/pp PbPb/pp PbPb PbPb pp pp FA - IFAE 2006 - Pavia, 20 aprile 2006

  45. some prediction ... beauty charm [Armesto et al.: Phys.Rev. D71 (2005) 054027] FA - IFAE 2006 - Pavia, 20 aprile 2006

  46. D0 K-p+ statistical. systematic. • expected ALICE performance • S/B ≈ 10 % • S/(S+B) ≈ 40 (1 month Pb-Pb running)  similar performance in pp • (wider primary vertex spread) pT - differential FA - IFAE 2006 - Pavia, 20 aprile 2006

  47. RAA(D) in ALICE Low pT (< 6–7 GeV/c) Nuclear shadowing ‘High’ pT (6–15 GeV/c) • expected performance (1 month Pb-Pb, 9 months p-p) FA - IFAE 2006 - Pavia, 20 aprile 2006

  48. B  e± + X • Expected ALICE performance (1 month Pb-Pb) • e± identification from TRD and dE/dx in TPC • impact parameter from ITS S/(S+B) S per 107 central Pb-Pb events FA - IFAE 2006 - Pavia, 20 aprile 2006

  49. At LHC: real jets! 2 GeV20 GeV 100 GeV 200 GeV Mini-Jets 100/event 1/event 100k/month • Well visible event-by-event! e.g. 100 GeV jet + underlying event: • e.g.: study quenching with b-tagged jets! FA - IFAE 2006 - Pavia, 20 aprile 2006

  50. b tagging • e.g.: ATLAS u rejection (Ru) performance in Pb-Pb • H  bb, uu with MH = 400 GeV FA - IFAE 2006 - Pavia, 20 aprile 2006