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ALICE: The Experiment and its Performance

This article discusses the ALICE experiment and its performance in studying ultrarelativistic heavy-ion collisions and providing insights into the phase transition of ordinary hadronic matter to a Quark Gluon Plasma. The article also covers the ALICE physics program, including pp collisions and minimum bias inclusive measurements.

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ALICE: The Experiment and its Performance

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  1. ALICE: The Experiment and its Performance The ALICE experiment ALICE performance ALICE First Physics Chiara Zampolli for the ALICE Collaboration V workshop italiano sulla fisica p-p ad LHC Perugia, 30 Gennaio - 2 Febbraio, 2008

  2. The ALICE Experiment Chiara Zampolli – ALICE

  3. ALICE Physics Program • ALICE is the LHC experiment dedicated to ultrarelativistic heavy ion collisions: • insights in the phase transition predicted by QCD from ordinary hadronic matter to a Quark Gluon Plasma • The ALICE physics program also includes pp collisions: • serving as a reference to heavy-ion • providing unique information about low-pT pp physics • specific QCD measurements • Minimum Bias inclusive measurements Chiara Zampolli – ALICE

  4. The ALICE Experiment TOF TRD HMPID ITS PMD Muon Arm ALICE Set-up PHOS TPC Chiara Zampolli – ALICE

  5. TPC SPD SSD/SDD ALICE Inner Tracking System – ITS Six Layers of silicon detectors for precision tracking in ||< 0.9 Sliding the SSD/SDD over the SPD Three technologies: SPD - Silicon Pixel SDD - Silicon Drift SSD - Silicon Strip • 3-D reconstruction (< 100mm) of thePrimary Vertex • Secondary Vertex finding (Hyperons, D and B mesons) • Particle Identificationvia dE/dx for momenta < 1 GeV • Tracking+Stand-alone Reconstruction of very low momentum tracks Chiara Zampolli – ALICE

  6. ALICE Time Projection Chamber – TPC Conventional TPC optimized up to dNch/dy = 6000 densities • Efficient (>90%) tracking in • || < 0.9 • Momentum resolution • s(p)/p < 2.5% up to 10 GeV/c • Two-track resolution < 10 MeV/c • PID with dE/dx resolution < 10% picture by A. Keiditsch picture by A. Saba • Space-Point resolution 0.8 (1.2) mm in xy,(z) • Occupancy from 40% to 15% Chiara Zampolli – ALICE

  7. ALICE Transition Radiation Detector – TRD 6 layer TRD, covering the whole azimuthal central region (|| < 0.9 and full φ) with 18 sectors and 5-fold z-segmentation 1st TRD Supermodule installation TRD Supermodule • Electron ID in the momentum range p > 1 GeV/c • Fast trigger for high-pT particles • Hadron PID Chiara Zampolli – ALICE

  8. ALICE Time Of Flight – TOF Large array at R ~ 3.7 m, covering || < 0.9 and full φ with 18 sectors (supermodules), with a total active area of ~160 m2 • TOF basic element: double-stackMultigap RPCstrip • Occupancy < 15% (O(105) readout channels) • Separation π/K, K/p @ low/intermediate p, better than 3σfrom ~0.5 GeV/c up to a few GeV/c picture by A. Saba TOF MRPC cross section 122 cm 2x5 gas gaps of 250mm Readout pads 3.5x2.5 cm2 picture by A. Saba TOF Supermodule • Intrinsic Resolution ~ 40 ps • Efficiency> 99% Chiara Zampolli – ALICE

  9. ALICE High Momentum Particle ID Detector – HMPID SINGLE-ARM proximity-focus RICH, at 4.7 m, with 7 modules for an active area of ~11 m2 • 15 mm layer of C6F14 liquid radiator (n = 1.2989 • @ λ= 175 nm, pth = 1.21 m (GeV/c) • CsI thin films deposited onto a cathode plane of MWPC (O(105) readout channels), the largest scale application • Hadron ID @ high momenta, 1 < pT < 3 GeV/c for π and K, 1 < pT < 5 GeV/c for p picture by A. Saba HMPID installation Chiara Zampolli – ALICE

  10. ALICE Performance Chiara Zampolli – ALICE

  11. ALICE PbPb Event Display pT > 1 GeV Chiara Zampolli – ALICE

  12. ALICE Tracking • Track Reconstruction has to be performed in a high flux environment • Reconstruction at low pT very delicate (multiple scattering and energy loss) Tracking based on aKalman Filtering procedure • Simultaneous reconstruction and fitting • Rejection of incorrectly associated space points “on the fly” • Algorithm faster than global methods • Simpler handling of multiple scattering and energy loss effects • Easy extrapolation from one detector to the other Chiara Zampolli – ALICE

  13. ALICE Tracking Strategy After cluster finding, start iterative process through all central tracking detectors, ITS+TPC+TRD: dN/dy =8000 (slice: 2o in q) • Primary Vertex Finding in ITS • Track seeding in outer TPC HMPID • Propagation to the vertex, tracking in ITS TOF • Back-propagation in TPC and in the TRD TRD • Extrapolation and connection with outer PID detectors TPC • Final refit inwards ITS Chiara Zampolli – ALICE

  14. ALICE Tracking Performance Tracking Efficiency / Fraction of Fake Tracks for dN/dy = 2000, 4000, 6000, 8000 ITS + TPC • FordN/dy = 2000÷4000: • efficiency > 90%, • fake track probability < 5%!!! Chiara Zampolli – ALICE

  15. pp ALICE Tracking Physical Efficiency Efficiency normalized to the number of generated particles (to take into account dead zones, electronics inefficiencies..) Central PbPb collisions dNch/dη = 6000 pp collisions fraction with usable TRD points Chiara Zampolli – ALICE

  16. ALICE Transverse Momentum Resolution • At low pT: ionization loss + multiple scattering • At high pT: space-point measurement precision + align + calib Central PbPb collisions dNch/dη = 6000 pp collisions Chiara Zampolli – ALICE

  17. ALICE Particle Identification Identification of stable hadrons (π, K, p), leptons (e, μ), photons and π0 Separation: @ 3s @ 2s ALICE uses almost all the known techniques Chiara Zampolli – ALICE

  18. ITS TPC stand-alone stand-alone TOF ITS & TPC & TOF Bayesian combined PID!! stand-alone ALICE Combined PID Performance Central Pb + Pb HIJING events – kaon case Combiningthe PID information from different detectors allows a weaker momentum dependence of the efficiency (contamination) which stays higher (lower) or at least equal than with stand-alone detectors!!! p dependence of: efficiency contamination Chiara Zampolli – ALICE

  19. ALICE First Physics Chiara Zampolli – ALICE

  20. ALICE First Physics with pp • Nominal luminosity ∫Ldt = 3·1030 cm-2 s-1, running for 107 s, σgeom = 0.07 b → Npp collision = 2·1012 collisions • Startup configuration: • Complete - fully installed & commissioned: ITS, TPC, TOF, HMPID, MUONS, PMD, V0, T0, FMD, ZDC, ACORDE, DAQ • Partially complete: TRD (25% - to be completed by 2009), PHOS (60% - to be completed by 2010), HLT (30% - to be completed by 2009), EMCAL (0% - to be completed by 2010/11) Chiara Zampolli – ALICE

  21. ALICE First Physics with pp – motivations • ALICE is not a pp experiment, but still... • Tracking has even better performance than in HI • Efficient trigger on MB... • ...with which to perform inclusive studies and try to tune MC generators (underlying event) • It has a lower pT cut-off wrt CMS and ATLAS (because of the low B and especially of the low material budget) → QCD measurements • ...and with a pT reach up to 100 GeV/c, can compare to them • Excellent PID • pp ideal for commissioning. • Herein, some examples of the first physics measurements, and heavy flavour and quarkonia production later on (A. Dainese and D. Stocco) Chiara Zampolli – ALICE

  22. µ arm Charged Particle Multiplicity – pp 14 TeV • Minimum Bias triggerprovided by a coincidence between V0 countersand by ITS-SPD(-3.7 < η < -1.7 and 2.8 < η <5.1for V0 and │η│ < 1.95 for ITS-SPD). • We expect a dN/dη excellent measurement in the central region thanks to the ITS + TPCdetector • V0 and FMD in the forward region. Chiara Zampolli – ALICE

  23. ALICE pT reach – pp • With 20k events, we can reach 10 GeV/c • With 70M events, we can reach 50 GeV/c (requiring ~30 entries above) Chiara Zampolli – ALICE

  24. A -0.9 y=0 0.9 ASYMMETRY Baryon Transport Number – pp • Baryon junction mechanism may explain the transport of the baryon number over wide rapidity gaps. This may involve valence quarks (G.C. Rossi and G. Veneziano, Nucl. Phys. B123 (1977) 507), or gluonic fields (B.Z. Kopeliovich and B. Zahkarov, Z. Phys. C43 (1989) 241). • Thanks to the very wide rapidity gap between the incoming protons, the two different models can be tested at LHC . string j. B quark Chiara Zampolli – ALICE

  25. Baryon Transport – Systematics Transport code: Reconstruction Efficiency Systematic error due to the spread of the cross section values for real data. +,proton -,antiproton P. Christakoglou, M. Oldenburg Material Budget: Systematic Error on AB Systematic error below 1% for a material uncertainty of 15% (p > 0.5 GeV/c). antiproton proton Chiara Zampolli – ALICE

  26. Summary and Conclusions • ALICE is a very versatile experiment... • with many different experimental techniques • with excellent tracking and PID capabilities • ...meant to study Heavy Ion collisions but with pp physics capabilities: • crucial for commissioning (calibration + alignment) • complementary to CMS & ATLAS in the low-pT region • first physics aimed at inclusive measurements (multiplicity, pT distribution, baryon transport...) • crucial as a reference for PbPb • The detector installation is going on, waiting for the first pp collisions • First global commissioning run in December 2007, with a second one in few days (February-March 2008). Chiara Zampolli – ALICE

  27. `Take some more tea,’ the March Hare said to Alice, very earnestly. `I've had nothing yet,' Alice replied in an offended tone, `so I can't take more.’ `You mean you can't take LESS,' said the Hatter: `it's very easy to take MORE than nothing.’ L. Carroll, Alice in Wonderland Chiara Zampolli – ALICE

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