Preparing ALICE for First Proton-Proton Minimum-Bias Physics

1. Preparing ALICE for First Proton-Proton Minimum-Bias Physics HCP 2009, Evian Jan Fiete Grosse-Oetringhaus for the ALICE collaboration CERN PH/ALICE

2. Content p+Si* collision * or other detector material • ALICE • Status • Physics performance from cosmics • First Measurements • Physics motivation • Predictions • Analysis procedures • Systematic uncertainties w.r.t first data 11.09.08 run 58338 ALICE detector global status Werner Riegler - Thu 9:05 Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

3. Proton Physics at the LHC • ATLAS and CMS (general-purpose detectors for p+p) • Higgs boson search + physics beyond the Standard Model • LHCb • CP-symmetry violating processes in b-quark systems • ALarge Ion Collider Experiment (ALICE) • ALICE has special features also for p+p collisions • Low momentum cut-off and low material budget (13% at outer wall of TPC) • Excellent particle identification (PID) • Primary vertex resolution 100 mm (p+p) and 10 mm (Pb+Pb) • ALICE can give an important contribution to p+p physics • Rare signals need good description of underlying event (bulk is soft, no pQCD) • Study of high-multiplicity collisions • Charm cross-section at low pT • Tuning of MC generators will be done using measured distributions from all experiments (joint effort of all experiments) • Reference for Pb+Pb (see session on Thursday morning) Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

4. inv. yield dNch/dh h pT (GeV/c) P(Nch) <pT> (GeV/c) JFGO, Reygers, Review on charged-particle multiplicity, to be published First Measurements • General event properties in collisions of hadrons • Important ingredients for the understanding of multi-particle production • LHC will be an energy realm where multiple parton interactions are in the bulk of the events • Constrain, reject and improve models • Measurements • Pseudorapidity density dNch/dh • h = - ln tan q/2 • Multiplicity distribution • Momentum distribution dNch/dpT • Correlation <pT> vs. Nch Skands, hep-ph/0905.3418 JFGO, Reygers, Review on charged-particle multiplicity, to be published charged multiplicity Nch Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

5. Predictions for LHC Energies • Extrapolations of trends at lower Ös • Just for average multiplicities • For the multiplicity distribution • Gluon saturation models (Armesto et al, Kharzeev et al) • Dual Parton Model / Quark-Gluon String Model • Monte Carlo generators • Pythia (pQCD + soft phenomenology) with all its tunes • Phojet (based on DPM/QGSM) • Epos (allows mini-plasma in p+p) dNch/dh @ 7 TeV predictions range from 4.3 to 6.4 (estimated systematic uncertainties < 10%) JFGO, Reygers, Review on charged-particle multiplicity, to be published Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

6. Predictions for Ös = 14 TeV Multiplicity distributions differ by a factor 2 up to an order of magnitude (estimated systematic uncertainties < 20%) full phase space full phase space JFGO, Reygers, Review on charged-particle multiplicity, to be published |h| < 1 |h| < 1 Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

7. full full EMCAL γ, π0, jets L3 Magnet T0/V0 Trigger ACORDE Cosmic trigger 36% 39% full HMPID PID (RICH) @ high pT TRD Electron ID (TR) TOF PID full full PMD γ multiplicity full TPC Tracking, PID (dE/dx) full full ITS Low pT tracking PID + Vertexing MUON μ-pairs full 60% FMD Charged multiplicity PHOS γ, π0, jets ALarge Ion Collider Experiment Dipole Not shown: ZDC (at ±116m) Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

8. Silicon Pixel Detector (SPD) • Two innermost layers of the Inner Tracking System (ITS) • Radii of 3.9/7.6 cm (|h| < 2.0/1.4) • 9.8 M channels • MB trigger (Fast OR) + High-multiplicity trigger • Tracklet: 2 points + vertex • Very efficient down to pT ~ 30 MeV/c • Nominal efficiency 99% • but ~ 12 % need hardware fixes Efficiency SPD efficiency (tracklets) full simulation pT in GeV/c Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

9. Time Projection Chamber (TPC) • Largest TPC in the world (90 m³) • 0.85 m < r < 2.5 m • |h| < 0.9 (1.5) • 560 K channels, up to 160 clusters per track • Tracking & PID • Laser system for calibration and alignment Efficiency TPC efficiency (tracks) full simulation pT in GeV/c Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

10. Minimum-Bias Trigger • Hit in central region (SPD) or in forward region V0 (either side) • Beam-gas background may motivate the use of different MB triggers • SPD and V0 (either side) • SPD and V0 (both sides) Trigger Efficiency 900 GeV Pythia Full detector simulation MB1 trigger charged multiplicity in |h| < 1.5 Trigger Efficiency 900 GeV Pythia Full detector simulation MB1 trigger ch. multiplicity in full phase space Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

11. 900 GeV Collisions • Assumed scenario • 2 on 2 bunches • For each orbit (and for each experiment) • 1 bunch-bunch encounter (bb) • 1 bunch-empty encounter (be) • 1 empty-bunch encounter (eb) • ALICE will initially trigger on bunch crossings • A few 1010 protons per bunch, β* = 10 m, µ ~ 1% • Most events will be empty (offline selection) • Trigger on empty-bunch / empty-empty for beam-gas / background / halo study • bb / eb / be / ee will be time-shared • Move to minimum-bias trigger + bunch-crossing trigger when background levels are understood delayed by 75ns Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

12. Constraining the Cross-sections • Combining the information from different (on/offline) triggers the cross-sections of non-, single- and double-diffractive events can be constrained • Use triggers from SPD, V0 (left/right), FMD, ZDC (left/right) • Input are the efficiencies derived from a MC simulation • Output are the process type fractions • Different MCs allow to assess the uncertainty on those fractions due to the model dependency of efficiency one equation per trigger combination Unknowns MC Data Constant Alner, PRD50,9,1994 Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

13. all events all events no trigger no trigger no vertex no vertex no vertex dNch/dh Measurement • Basically if the detector was perfect • But… there is • Detector acceptance, tracking efficiency • Decay, energy loss, stopping • Vertex reconstruction efficiency/bias, trigger efficiency/bias • Low momentum cut-off • Three corrections needed • Track-to-particle correction • pT cut-off (only TPC) • Vertex reconstruction correction • Trigger bias correction • 20,000 events sufficient for first measurement Primary Particles = charged particles produced in the collision and their decay products excluding weak decays from strange particles all events no trigger no trigger no vertex no vertex Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

14. Tracking Efficiency • Corrected as function of h, z-position of event vertex (vtx-z), pT (only TPC) TPC h h TPC pT (GeV/c) SPD vtx-z (cm) vtx-z (cm) h Acceptance clearly visible Average correction ~ 1.4 Secondaries: ~7% Average correction for pT > 0.20 GeV/c: 1.1 pT cut-off correction: ~ 1.24 Secondaries: ~8% Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

15. Systematic Uncertainties dNch/dh Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

16. |h| < 1 measured multiplicity true multiplicity Multiplicity Measurement • Efficiency, acceptance • Resolution vs. bin size  bin flow • Correction by unfolding • c2 minimization with regularization • Bayesian iterative unfolding • Detector response • Probability that a collision with the true multiplicity t is measured as an event with the multiplicity m • Vertex reconstruction, trigger bias correction • Like for dNch/dh, but in unfolded variables (true multiplicity) because it is applied after unfolding • 100,000 events sufficient for first measurement c2 minimization: Z. Phys.C69,1995,15-26 Bayesian: Nucl.Instrum.Meth.A362:487-498,1995 Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

17. Systematic Uncertainties • Uncertainty does not shift the whole spectrum • By definition the spectrum is normalized • Spectrum gets distorted • Uncertainty a function of multiplicity  depends on the true spectrum • Data is unfolded with modified response matrices that consider changes in cross-sections, particle composition, spectra, material budget, misalignment Ratio change/normal Efficiency change Nch Uncertainty (%) Example for Pythia distribution Nch Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

18. Systematic UncertaintiesMultiplicity Distribution Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

19. Transverse Momentum Distribution • Bin-by-bin correction procedure • Similar to dNch/dh analysis • Second analysis using unfolding being investigated • Loose quality criteria for tracks • Number of clusters + c2/cluster • Track-vertex matching: distance of closest approach ~ 2 - 3 cm • pT resolution ~ 7 % @ 10 GeV/c (cosmics) • 20,000 events sufficient for first measurement up to ~ 5 GeV/c pT (GeV/c) 2x104 pp @ 900 GeV Full ALICE simulation and correction Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

20. Systematic Uncertainties dNch/dpT Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

21. sDxy=52 mm   sspatial=14 mm (MC ideal geometry: sspatial=11 mm) sDxy=18 mm  sDxy=sspatial2  sspatial=14 mm (MC ideal geometry: sspatial=11 mm) not realigned realigned Track-to-track Dxy at y=0 Double cluster in same track Status of the Silicon Pixel Detector clusters from cosmics • 106 out of 120 modules stably running ~ 88% • Very few dead/noisy pixels < 0.15% • Aligned with about 50k cosmic tracks y (cm) x (cm) Dxy [cm] Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

22. Status of the Time Projection Chamber • About 0.15% non-active channels • Temperature stable within 0.1 K • Very low noise of 0.7 ADC counts (700 e) • Direct energy calibration using Krypton gas cosmic event DTRMS=53 mK Dtmin/max= 206 mK Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

23. TPC Calibration & Alignment pT resolution from cosmics • Performance measure by matching of upper and lower track • PID is a first physics topic! • 20.000 events sufficient for identified pT spectrum up to 1 GeV/c spT / pT 7 % @ 10 GeV/c pT (GeV/c) dE/dx from cosmics rel. dE/dx resolution Number of tracks TPC signal (a.u.) < 5% (Nclusters > 100) p (GeV/c) number of clusters Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus

24. Summary TPC cosmic event • ALICE is commissioned • Already good calibration and alignment • ALICE's first analyses are fully prepared • ALICE can measure with a few days of data basic properties of p+p collisions from 900 GeV to 14 TeV • This provides the reference and paves the way for a rich heavy-ion program that will study the QGP in detail ALICE detector global status Werner Riegler - Thu 9:05 Thank you for your attention! Preparing ALICE for First Proton-Proton Minimum-Bias Physics - Jan Fiete Grosse-Oetringhaus