First ALICE Results from Proton-Proton Collisions

1. EPJC65:111 (2010) First ALICE Results from Proton-Proton Collisions Jan Fiete Grosse-Oetringhaus, CERN/PH for the ALICE collaboration Rencontres de Moriond 14. – 19. March 2010, La Thuile, Italy

2. 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) First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

3. Detector Acceptance x y SPD Layer 1 SPD Layer 2 Detectors • V0 scintillator array • Trigger • 2.8 < h < 5.1 and -1.7 < h < -3.7 • 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) • Trigger & Tracking • |h| < 1.4; pT > 50 MeV/c • 9.8 M channels • Tracklet: 2 points + vertex • During 2009 data-taking 80% active h ALICE Performance work in progress Efficiency SPD physics efficiency for primaries (2009 configuration 80% active) pT (GeV/c) First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

4. Trigger for MB Physics • ALICE measures MB properties for all inelastic (INEL) and non single-diffractive (NSD) events • Inclusive trigger ("MB1") for INEL: central pixel hit (SPD) or forward scintillator (V0) • One particle in 8 h units • (Trigger-)sensitive to 95-97% of the inelastic x-section • Two-arm trigger ("V0AND") for NSD • Both forward scintillators • One particle in 2.8 < h < 5.1 and one in -1.7 < h < -3.7 ALICE Performance work in progress Trigger efficiency Pythia 0.9 TeV Generated multiplicity in |h| < 1 First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

5. Analysis based on 900 GeV: 186k collision events out of 430k total 2.36 TeV: 41k collision events MC adapted to mean vertex position and spread MC Data Entries ALICE Performance work in progress Tracklet Df Data Sample Events ALICE Performance work in progress z vertex distribution (cm) inefficient sector  no significant effect on result ALICE Performance work in progress Data MC Ratio Data / MC second layer Cluster yield yield in first and second layer compatible within 0.5% ALICE Performance work in progress first layer SPD module First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

6. all events all events all events no trigger no trigger no trigger no trigger no vertex no vertex no vertex no vertex no vertex dNch/dh Measurement • Basically if the detector was perfect • But… there is • Detector acceptance, tracking efficiency • Decay, conversions, stopping, etc. • Vertex reconstruction efficiency/bias, trigger efficiency/bias • Low momentum cut-off • Three corrections needed • Track-to-particle correction • Vertex reconstruction correction • Trigger bias correction Primary particles = charged particles produced in the collision and their decay products excluding weak decays from strange particles First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

7. dNch/dh Measurement (2) Beam-induced background • Use the number of triggered events without reconstructed vertex to constrain the "0 bin" • Assess background from control triggers (single bunch passing, no bunch passing) • Background with activity < 0.02% • At 2.36 TeV no control triggers • Use 0 bin from MC larger syst. error • Average correction factors • Tracking: 1.53 • Events (INEL): 1.05 • Events (NSD): 1.02 [+8.3% (NSD) – 6.8% (SD)] • Subtraction of SD tracks for NSD: 3% ALICE Performance work in progress dNch/dh h First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

8. dNch/dh NB: CMS data points do not include charged leptons  ~1.5 % difference dNch/dh dNch/dh ALICE, CMS: stat. and syst. uncertainty added quadratically ALICE, CMS: stat. and syst. uncertainty added quadratically h h * only stat. error First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

9. Larger increase from 0.9 to 2.36 TeV at mid-rapidity as in MC generators dNch/dh (2) points at the same energy slightly shifted ALICE, CMS: stat. and syst. uncertainty added quadratically First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

10. Multiplicity Distributions • Efficiency, acceptance • Resolution vs. bin size  bin flow • Correction by 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 |h| < 1 ALICE Performance work in progress measured multiplicity true multiplicity First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

11. Unfolding using c2-Minimization • One free parameters per bin for unfolded spectrum Ut • Regularization • Prefer constant locally • Prefer linear function locally • Use number of triggered events without vertex to estimate vertex efficiency in “0 bin” from data • Systematic uncertainties evaluated by using modified response matrices • Iterative Bayesian unfolding as cross-check Regularizations Blobel, CERN Comp.School 1984:0088 Nucl.Instrum.Meth.A362:487-498,1995 First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

12. Multiplicity Distributions • Distributions in limited h-regions • Vertex range reduced a region in which every event has full h-acceptance • Consistent with UA5 results for NSD at 0.9 TeV UA5: ZP C43,357 First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

13. Phojet provides a good description at 900 GeV, but fails at 2.36 TeV Distribution at 2.36 TeV closer to Pythia with Atlas CSC tune Qualitatively same conclusions for NSD MC generators do not describe the energy dependence of the tail of the distribution correctly Multiplicity Distributions First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

14. Summary • The pseudorapidity density and multiplicity distributions at 0.9 and 2.36 TeV have been presented • Consistent with UA5 and CMS • MC generators do not describe the energy-dependence of the average multiplicity as well as the tail of the distributions correctly • Many other analyses on the way, including • Charged particle pT, identified particles pT (π, K, p), strangeness production (K0, Λ, Ξ, Φ), baryon-antibaryon asymmetry, Bose-Einstein correlations, π0 spectra, event structure, azimuthal correlations, -φ correlations Other opinions on MB physics: National Geographic News (4th Dec) ‘….a machine called ALICE.... found that a proton-proton collision recorded on November 23rd created the precise ratio of matter and antimatter particles predicted from theory..’ See talks by Jacek Otwinowski, Francesco Noferini First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus