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  1. The First Physics from ALICE O. Villalobos Baillie School of Physics and Astronomy The University of Birmingham Report from the First ALICE Physics week Erice, Sicily, December 4th-9th 2005

  2. Structure of Talk • Introduction • The ALICE detector and the LHC • ALICE Physics • Detector measurement capabilities • Numbers of participants and multiplicities • Hadronic Spectra • Heavy flavour • Quarkonia • Jets • Photons • The first few days of ALICE • Summary O.Villalobos Baillie - First Physics from ALICE-

  3. Introduction • In December 2005, the ALICE collaboration, having finished its Physics Performance Report, had its first physics week at Erice, in Sicily, where the first physics goals for the experiment were discussed. • There are now about 1½ years to go before LHC start-up. It became clear we should start to make priorities as to what we shall do, and when. • We are told to expect the first collisions in summer 2007 (nominally July 1st 2007) O.Villalobos Baillie - First Physics from ALICE-

  4. Scenario for LHC machine commissioning • The first proton beam should be injected through TI8 starting in Nov. 2006, up to a beam dump just before Point 7: Start of LHC commissioning! O.Villalobos Baillie - First Physics from ALICE-

  5. Time to produce 2x104 events 140s 20s 5s Beam characteristics (LHC-OP-BCP-0001 rev 1.) • The very first collisions at LHC will be proton-proton collisions at a center of mass energy of 900 GeV. Then highest possible beam energy, but with a small number of bunches, and low intensity. Beam conditions will be ideal for ALICE. 936 75 Only 3 minutes to collect sample … O.Villalobos Baillie - First Physics from ALICE-

  6. Perhaps first Pb-Pb collisions? This is the period during which ALICE must collect the first few minutes of data Machine time scale (as envisaged today) • T0 (= 1st of July 2007 as of today) • One month to get the machine ready for beams (T0 + 1 month) • Three months to commission the machine with beams (T0 + 4 months)=>possibility for ALICE to collect the first pp data sample for first paper! • One month of rather stable operations, interleaved with machine development with 43 and 156 bunches, with the possibility of collisions for physics during nights (~ 20 shifts of 10 hours each L ~ 1030cm–2s–1) (T0 + 5 months)=>possibility for ALICE to collect the first large pp data sample! • Perhaps first Pb-Pb collisions. • Shutdown (T0 + 8 to 9 months): today machine people talk about 3 to 4 months. It will depend on requirements by experiments. If T0 = 1st of July, start of shutdown will coincide with the Christmas holidays. Stable beams Preparation …First collisions……. Shutdown 3 to 4 months? July Nov. Feb. Mar Aug. Sept. Oct. Dec. Jan. O.Villalobos Baillie - First Physics from ALICE-

  7. First LHC Conditions • This schedule indicates that the first few months of LHC running will provide pp collisions at two or more different energies, from 900 GeV up to the design energy. • Luminosities will be low, and the number of filled bunch-crossings will be low. • These are favourable conditions for ALICE. Full LHC luminosity is too high and so beams will have to be defocused later on in order to take LHC data • Crucial to get physics out as soon as possible, and to aim to be complementary to the other LHC experiments. • ALICE has some advantages in studying Minimum Bias collisions • Low pt threshold • Excellent particle identification capabilities • There is still the possibility of Pb-Pb collisions in 2007, as the accelerator team are also keen to ensure that everything is working before the first long shut-down. It was stressed that it is important that ALICE should already be producing physics in the early pp running days in order to encourage this additional step to be taken. O.Villalobos Baillie - First Physics from ALICE-

  8. HMPID PID (RICH) @ high pt TOF PID TRD Electron ID PMD g multiplicity TPC Tracking, dEdx ITS Low pt tracking Vertexing MUON m-pairs PHOS g,p0 The ALICE Experiment ACORDE cosmics V0, T0 Trigger + multiplicity O.Villalobos Baillie - First Physics from ALICE-

  9. Experimental conditions Estimated charged particle rapidity density at mid-rapidity – for central Pb – Pb dNch/dη = 2600 dNch/dη = 1200 ALICE detector designed for charged-particle densities up todNch/dη = 4000 ALICE tracking and PID performance is checked up to dNch/dη = 8000 Physics observables studied up to dNch/dη = 6000 with HIJING Particle densities to be explored by ALICE range from pp values dNch/dη < 10 at the commissioning, through pA values (few times higher), up to Pb–Pb O.Villalobos Baillie - First Physics from ALICE-

  10. TPC all detectors Tracking efficiency Challenge in high-particle density environment • Tracking done using parallel Kalman filter • Starting in outer part of TPC • Unfold overlap clusters during tracking • Merging tracks to ITS • Refit tracks outwards – merging to TRD • Refit to primary vertex • Several passes, with/out vertex constraint For realistic particle densities dN/dy = 2000 – 4000 combined efficiency well above 90% and fake track probability below 5% O.Villalobos Baillie - First Physics from ALICE-

  11. central Pb – Pb TPC pp Physical efficiency Efficiency normalized to number of generated particles at primary vertex within the central acceptance |η|<0.9 • protons – large absorption • kaons – decays on flight At lower pt : TRD inclusion drops the efficiency due to unavoidable material between TRD sectors O.Villalobos Baillie - First Physics from ALICE-

  12. central Pb–Pb pp central Pb–Pb pp Tracking – momentum resolution at low momentum dominated by - ionization-loss fluctuations - multiple scattering at high momentum determined by - point measurement precision - and the alignment & calibration (which is here assumed ideal) O.Villalobos Baillie - First Physics from ALICE-

  13. pp low multiplicity pp high multiplicity Impact parameter precision Impact parameter resolution is crucial for the detection of short-lived particles - charm and beauty mesons and baryons At least one component has to be better than 100 mm (ct for D0 meson is 123 mm) central Pb–Pb For low-multiplicity events (i.e. pp) the contribution from primary-vertex resolution is not negligible Full reconstruction with primary tracks has to be used O.Villalobos Baillie - First Physics from ALICE-

  14. central Pb–Pb TPC pp p limit central Pb–Pb Secondary tracks & vertices Reconstruction efficiency for V0 topology (K0s and L) better than 50% Reconstruction of cascades X and W Kink topologies O.Villalobos Baillie - First Physics from ALICE-

  15. dE/dx spectra at 0.5 GeV/c Gaussian fits low momentum high momentum separation power central Pb–Pb pp dE/dx resolution to 50 GeV Charged particle PID – TPC dE/dx O.Villalobos Baillie - First Physics from ALICE-

  16. Charged particle PID – TOF central Pb–Pb p K p TOF with 80 ps resolution will identify kaons up to 3 GeV/c and protons up to 5 GeV/c kaons 0.5 < p < 3 GeV/c kaons 0.5 < p < 3 GeV/c O.Villalobos Baillie - First Physics from ALICE-

  17. Stable hadrons (p, K, p): 100 MeV < p < 5 GeV (few 10 GeV) dE/dx in silicon (ITS) and gas (TPC) + Time-of-Flight (TOF) + Cerenkov (RICH) Decay topology (K0, K+, K-, L) K and L decays up to at least 10 GeV Leptons (e, m), photons, p0, h electrons in TRD: p > 1 GeV, muons: p > 5 GeV, p0 in PHOS: 1 < p < 80 GeV Particle Identification Alice uses ~ all known techniques! O.Villalobos Baillie - First Physics from ALICE-

  18. Identified Particle Spectra • Identified particle spectra feasible over a broad range of momenta for many different particle species. O.Villalobos Baillie - First Physics from ALICE-

  19. 3 Global event characterization in Pb-Pb Centrality determination brec(fm) sb ~ 1fm Event by event determination of the centrality Zero degree hadronic calorimeters (ZDC) + electromagnetic calorimeters (ZEM) EZDC , EZEM Nspec Npart impact parameter (b) bgen (fm) Correlations between ZDC and ZEM EZDC (TeV) Events reconstructed generated sNpart ~15 O.Villalobos Baillie - First Physics from ALICE- Npart EZEM (GeV) Npart

  20. Global event properties in Pb-Pb 4 Generated Tracklets Multiplicitydistribution (dNch/dh) in Pb-Pb Energy density Silicon Pixel Detector (SPD) : -1.6 < h < +1.6 + Forward Multiplicity Detector (FMD): h -5, +3.5 (dN/dh)|h|<0.5 dN/dh % centrality (Npart) Fraction of particles produced in hard processes Generated Tracklets (dN/dh)|h|<0.5 1 central Hijing event O.Villalobos Baillie - First Physics from ALICE- Npart

  21. Identified particle spectra 6 Particle reconstruction and identification capabilities: unique to ALICE Global tracking (ITS-TPC-TRD) + dE/dx (low pT + relativ. rise), TOF, HMPID, PHOS, … Invariant mass, topological reconstruction Acceptance / efficiency / reconstruction rate (e) / contamination pT range (PID or stat. limits) for 107 central Pb-Pb and 109 min. bias pp For ~ 20 particle species for -1 < y < +1 and -4 < y < +2.5 p, K, p: 0.1- 0.15 50 GeV Weak or strong decaying particles: until 10-15 GeV Mid-rapidity p PID in the relativistic rise K p Pb-Pb Pb-Pb O.Villalobos Baillie - First Physics from ALICE- pT (GeV/c)

  22. Topological identification of strange particles 7 Statistical limit : pT ~11 - 13 GeV for K+, K-, K0s, L, 7 - 10 GeV for X, W Secondary vertex and cascade finding pT dependent cuts -> optimize efficiency over the whole pT range Pb-Pb central L 300 Hijing events Reconst. rates: X: 0.1/event W: 0.01/event pT: 1 7-10 GeV 13 recons. L/event 11-12 GeV About the same pT limit for 109 pp Identification of K+, K- via their kink topology K mn 6x104 pp collisions X pp collisions O.Villalobos Baillie - First Physics from ALICE- Limit of combined PID

  23. Resonances (r, f, K*, …) 8 Time difference between chemical and kinetic freeze-out In medium modifications of mass, width, comparison between hadronic and leptonic channels partial chiral symmetry restoration Invariant mass reconstruction, background subtracted (like-sign method) mass resolutions ~ 1.5 - 3 MeV and pT stat. limits from 8 (r) to 15 GeV (f,K*) r0(770)p+p- 106central Pb-Pb K*(892)0 K p 15000 central Pb-Pb Mass resolution ~ 2-3 MeV Invariant mass (GeV/c2) Generated & reconstructed f for 107 central Pb-Pb Mass resolution ~ 1.2 MeV f (1020) K+K- O.Villalobos Baillie - First Physics from ALICE-

  24. Heavy Quarks in AA coll. • Small xBj (10-3 –10-5) • DtQ ~ 0.04 –0.15 fm/c • DtQ << tQGP • Shadowing • HQ energy loss in QGP • HQ coalescence (low pT) L.O. O.Villalobos Baillie - First Physics from ALICE-

  25. J.F. Gunion and R. Vogt, N.P. B 492 (1997) 301 • J/ suppression & regeneration? • c suppression (J/ TD > 1.5 Tc)? enhanced regeneration enhanced suppression H. Satz, CERN Heavy Ion Forum, 09/06/05 30 SPS RHIC LHC Measuring s in AA collisions at the LHC • (1s) melts only at LHC • (2s)/(1s) vs. pt sensitive to system temp. & size • (2s) can unravel J/ suppression vs. regeneration • TLHC >> J/ TD ~ (2s) TD • C.-Y. Wong, PRC 72 (2005) 034906 • W.M. Alberico, hep-ph/0507084 •  regeneration is small • L. Grandchamp et al., hep-ph/0507314 O.Villalobos Baillie - First Physics from ALICE-

  26. Physics Analysis • Hadronic decays: • D0K p, D+-K p p, DsK K*, Dsfp, … • Leptonic decays: • B l (e or m) + anything. • Invariant mass analysis of lepton pairs: BB, DD, BDsame, J/Y, Y’,  family, B J/Y + anything. • BB m m m (J/Ym). • e-m correlations. • W+-  l+-n, Z0l+l-. In red channels studied in the PPR In black under study O.Villalobos Baillie - First Physics from ALICE-

  27. 1<pT<2 GeV/c D0Kp channel • High precision vertexing, better than 100 mm (ITS) • High precision tracking (ITS+TPC) • K and/or p identification (TOF) 109 pp 108 pPb 107 PbPb 107 central PbPb D Kp O.Villalobos Baillie - First Physics from ALICE-

  28. High Precision charm measurement pp at 14 TeV Sensitivity to PDF’s Central PbPb Shadowing + kT + energy loss Shadowing region O.Villalobos Baillie - First Physics from ALICE-

  29. Open Beauty from single electrons • Electron Identification (TRD+TPC) • High precision vertexing (ITS) • Subtraction of the open charm contribution. 107 central PbPb O.Villalobos Baillie - First Physics from ALICE-

  30. Quarkonia  e+e-,m+m- PbPb cent, 0 fm<b<3 fm Yields for baseline • (1S) & (2S) : 0-8 GeV/c • J/Y high statistics: 0-20 GeV/c • Y’ poor significance • ’’ ok, but 2-3 runs will be needed. O.Villalobos Baillie - First Physics from ALICE-

  31. CMS  ~ 80 MeV ALICE dielectrons ALICE dimuons background level 1 = 2 HIJING evts with dNch/d = 6000 @  = 0 each Mass Resolution(  100 MeV @ M ~ 10 GeV is needed to separate the  sub-states) ATLAS  > 120 MeV • ALICE (& CMS) can measure the  sub-states • warning: ≠ simulation frameworks & inputs O.Villalobos Baillie - First Physics from ALICE- ATLAS CERN/LHCC/2004-009, CMS NOTE 2000-060 (updated)

  32. Suppression scenarios • Suppression-1 • Tc =270 MeV • TD/Tc=1.7 for J/Y • TD/Tc= 4.0 for . • Suppression-2 • Tc=190 MeV • TD/Tc=1.21 for J/Y • TD/Tc= 2.9 for . PRC72 034906(2005) Hep-ph/0507084(2005) Good sensitivity J/Y, (1S) & (2S) O.Villalobos Baillie - First Physics from ALICE-

  33. gluon radiation Physics motivation • High energy partons, from an initial hard scattering, will create a high energy collimated spray of particles → jets • Partons traveling through a dense colour medium are expected to lose energy via medium induced gluon radiation, “jet quenching”, and the magnitude of the energy loss depends on the gluon density of the medium • Total jet energy is conserved, but “quenching” changes the jet structure and fragmentation function Measurement of the parton fragmentation products reveals information about the QCD medium O.Villalobos Baillie - First Physics from ALICE-

  34. jet parton nucleon nucleon RHIC: finding the leading particle Find this… … here: “leading particle” p+p (STAR@RHIC) Au+Au (STAR@RHIC) O.Villalobos Baillie - First Physics from ALICE-

  35. gluon radiation nucl-ex/0406012 PRL91, 072304 (2003) ● PHENIX (π0) 1/NtriggerdN/d() x5 Results from RHIC Evidence for partonic energy loss in heavy ion collisions High-pT suppression in central AuAu collisions High-pT hadrons of recoiling jet suppressed in AuAu but not in dAu O.Villalobos Baillie - First Physics from ALICE-

  36. Eskola et al., hep-ph/0406319 Full jet reconstruction Leading Particle Leading particle becomes fragile as a probe • Surface emission: • Small sensitivity of RAA to medium properties. • For increasing in medium path length L, the momentum of the leading particle is less and less correlated with the original parton 4-momentum. Reconstructed Jet Ideally, the analysis of reconstructed jets will allow us to measure the original parton 4-momentum and the jet structure. → Study the properties of the medium through modifications of the jet structure: • Decrease of particles with high z, increase of particles with low z • Broadening of the momentum distribution perpendicular to jet axis O.Villalobos Baillie - First Physics from ALICE-

  37. 100 Jet rates at the LHC • Huge jet statistics from ET ~10 GeV to ET~100 GeV • Jets with ET > 50 GeV will allow full reconstruction of hadronic jets, even in the underlying heavy-ion environment. • Multijet production per event extents to ~ 20 GeV O.Villalobos Baillie - First Physics from ALICE-

  38. Jet reconstruction in ALICE In pp-collisions jets: excess of transverse energy within a typical cone of R = 1. In heavy-ion collisions • jets reconstructed using smaller cone sizes • subtract energy from underlying event Main limitations: • Background energy. Reduced by: • reducing the cone size (R = 0.3-0.4) • transverse momentum cut (pT = 1-2 GeV/c) • Background energy fluctuations: • event-by-event fluctuations • Poissonian fluctuations of uncorrelated particles • fluctuations of correlated particles • Collimation: ~ 80% energy around jet axis in R < 0.3 • Background energy in cone of size R is ~R2 and background fluctuations ~R. O.Villalobos Baillie - First Physics from ALICE-

  39. ET = 100 GeV, R = 0.4 background — no pT cut — pT >1 GeV/c — pT >2 GeV/c E(R) [GeV] (TPC+EMCal) (TPC) ETjet 150 GeV (TPC – like RHIC) 100 GeV 50 GeV 30 GeV R Intrinsic performance limits • Energy contained in a subcone of radius R reduced by: • reducing the cone size • cutting on pT • Limited cone size leads to a low energy tail • Charged reconstruction (TPC) dominated by charged to neutral fluctuations O.Villalobos Baillie - First Physics from ALICE-

  40. Reconstructed jet 107 central events R = 0.4 Charged jets • Study properties of the medium through the modifications on the transverse jet structure • Jet shape (dE/dr) and jet particle momentum perpendicular to jet axis (jt) vs. reconstructed energy • Study hard processes with low pT observables by measuring the fragmentation function to low pT. Energy loss and radiated energy • Decrease of hadrons in the high-z part and increase of hadrons in the low-z region of fragmentation function (z = pT/ETjet) O.Villalobos Baillie - First Physics from ALICE-

  41. N. Borghini, U. Wiedemann Increase on # of particles with low z pT ~ 1.8 GeV/c Statistical error 10,000 jets ETcone > 70 GeV Decrease on # of particles with high z Jet-structure observables Representing the fragmentation function: Hump-backed Plateau. Charged jets. Particles from medium induced gluon radiation in ξ ~ 4-6 For ET ~ 100 GeV, S/B ~ 10-2 Leading Particles S/B > 0.1 O.Villalobos Baillie - First Physics from ALICE-

  42. Dominant processes: g + q → γ+ q (QCD Compton) q + q → γ + g (Annihilation) pT > 10 GeV/c non-quenched g quenched jet Photon-tagged jets g-jet correlation • Eg = Ejet • Opposite direction • Direct photons are not perturbed by the medium • Parton in-medium-modification through the fragmentation function O.Villalobos Baillie - First Physics from ALICE-

  43. signal x5 Identifying prompt gsin ALICE They carry information about the hard processes in the dense medium • pT > 10 GeV/c • Direct photons g + q →g + q / q + q →g + g • unaffected by the medium • g-jet correlations • normalization of hard processes (nuclear PDF’s) • Fragmentation photons • may be affected by the medium • Identified event-by-event through: • isolation criteria (w/o hadronic activity) • shower shape analysis O.Villalobos Baillie - First Physics from ALICE-

  44. Identifying thermal photons They carry information about the thermal evolution of the system • They are produced • in the QGP phase by softer partons → pT ~ 1-5 GeV/c • in the hadronic phase. • Identified on a statistical basis • Contamination from hadrons below 5% for pT > 1.5 GeV/c O.Villalobos Baillie - First Physics from ALICE-

  45. First ALICE Physics – Four Measurements Pseudorapidity density dN/dη pT spectrum unidentified hadrons CDF: Phys. Rev. D41, 2330 (1990) CDF: Phys. Rev. Lett. 51, 1819 (1988) Multiplicity distribution Mean pT vs multiplicity CDF: Phys. Rev. D65,72005(2002) UA5: Z. Phys 43, 357 (1989) O.Villalobos Baillie - First Physics from ALICE-

  46. Multiplicity Distributions true –  events (norm.) true – 106 events measured Unfolding (measured true) is not a simple problem. see: Anykeev et al, Nucl. Instr. Meth.A303, 350 (1991) d’Agostini, DESY 94-099, June 1994. C. Jorgensen, talk at ALICE p+p meeting, Oct 7, 2005 O.Villalobos Baillie - First Physics from ALICE-

  47. Unfolding Problem Nmeas=100 Ntrue=100 measured true true measured • Probability that true is j • when measured is i P(j|i) • model dependent Probability to measure i when true is j P(i|j)  from detector simulation (Response matrix) O.Villalobos Baillie - First Physics from ALICE-

  48. The Response Matrix Probability P(i|j) to measure i when the true multiplicity is j. Generated from 50000 (biased) Pythia (flat multiplicity distribution). We need a way to generate high stat (>106 events) response matrix. • Using fake (realistic) responses with with ~20000 events per multiplicity. O.Villalobos Baillie - First Physics from ALICE-

  49. Unfolding – Methods • Unfolding by X2 minimization • Sum of differences between measured and • guess μ smeared with response (Rij = P(i|j)) • Regularization term R(μ) adds “smoothness” • Minuit used for minimization X2 calculation • Bayesian unfolding • Iterative method using Bayes theorem • 1) P(j|i) is calculated assuming prior P0 • 2) Guess is calculated from P(j|i) and measured • 3) Prior is updated (set to normalized guess) • 4) Go to 1 Bayes theorem (d’Agostini, DESY 94-099, June 1994) (in collaboration with J. Conrad) O.Villalobos Baillie - First Physics from ALICE-

  50. Unfolding – Procedure measured smoothed • Smoothing of measured histogram • Getting rid of fluctuations Unfolding  Χ2 minimization  Bayesian method Estimate end of true spectrum • Where the true starts to contribute to the end of measured spectrum (first j where ΣjP(j|imax) > 0.01) measured true contr. i=100 imax=375 O.Villalobos Baillie - First Physics from ALICE-