Particle multiplicity & ET measurements with ATLAS

1. Particle multiplicity & ET measurements with ATLAS For the ATLAS Heavy Ion Group: S. Aronson, K. Assamagan, B. Cole, M. Dobbs, J. Dolejsi, H. Gordon, F. Gianotti, S. Kabana, M. Levine, F. Marroquin, J. Nagle, P. Nevski, A. Olszewski, L. Rosselet, P. Sawicki, H. Takai, S. Tapprogge, A. Trzupek, M.A.B. Vale, S. White, R. Witt, B. Wosiek, K. Woźniak and …… Andrzej Olszewski Institute of Nuclear Physics, Kraków, Poland

2. Outline of the Talk INTRODUCTION • Global observables in experiments at LHC • Global measurements in Atlas detector ATLAS MEASUREMENTS OF Nch and ET • Monte Carlo Simulations • Detector Occupancies • Global Measurements • Event Characterization • Tracking with ATLAS ID(Si) Andrzej Olszewski

3. Heavy Ions at the LHC Study of QCD matter at extremely high energy densities and ~vanishing baryon chemical potential. • deconfinement • restoration of the chiral symmetry, • physics of parton densities close to saturation Quantitative studies of a QGP properties: • Hot/dense matter effects should dominate over initial and final state effects. • The effects may be seen already in global observables like particle multiplicity and ET Andrzej Olszewski

4. The ATLAS Detector Andrzej Olszewski

5. ATLAS as a Heavy Ion Detector • Si Tracker • Large coverage up to || < 2.5 • Finely segmented pixel and strip detectors • Good momentum resolution • High Resolution Calorimeters • Hermetic coverage up to || < 4.9 • Fine granularity (with longitudinal segmentation) • Large Acceptance Muon Spectrometer • Coverage up to || < 2.7 Physics: Global event characterization Tracking particles with pT 1.0 GeV/c High pT probes, heavy quarks, quarkonia ... Andrzej Olszewski

6. Heavy Ion Interactions in Atlas Number of signals in Si Tracker Geometry of a collision Binary NN approximation Etot in EM and HAD calorimeters Nch, Etot,ET Parton distributions Detector AA collision ET in EM and HAD calorimeters Nuclear modifications Number of tracks reconstructed Soft & Hard processes Andrzej Olszewski

7. Simulation Tools • Event generator HIJING • Based on PYTHIA and Lund fragmentation scheme • with nuclear effects: nuclear shadowing, jet quenching • GEANT3 detector simulation • Full GEANT3 ATLAS detector simulations • High Geant cuts: 1 MeV tracking/10 MeV production • Only particles within |y| < 3.2 • Sample generated • 5,000 events in each of 5 impact parameter bins: • b = 0-1, 1-3, 3-6, 6-10, 10-15 fm • Detectors used in analysis Silicon Pixel, SCT. EM and HAD Calorimeters Andrzej Olszewski

8. Zoom on Atlas ID & Calorimeters Hadronic Tile Calorimeters Silicon Tracker in Inner Detector EM Accordion Calorimeters Forward LAr Calorimeters Hadronic LAr End Cap Calorimeters Andrzej Olszewski

9. Average Occupancies Central Collision Events b=0-1 fm • Occupancies still reasonable in all Si Detectors: below 2% in Pixels and below 20% in Strips (after accounting for local fluctuations in the data with low GEANT cuts) • TRT unusable – too high occupancy Andrzej Olszewski

10. Global Measurements DAY-ONE MEASUREMENTS! Nch, dNch/d, ET, dET/d, b • Constrain model prediction • Indispensable for all physics analyses Predictions for Pb+Pb central collisions at LHC (dNch/d)0 Model/data ~ 6500 HIJING:with quenching, with shadowing ~ 3200 HIJING:no quenching, with shadowing ~ 2300 Saturation Model (Kharzeev & Nardi) ~ 1500 Extrapolation from lower energy data Andrzej Olszewski

11. Measurements of Nch(|| < 3) Based on the correlation between measurable quantity Q and the true number of charged primary particles: Q = f(Nch) Q: Nsig (all Si detectors,except PixB-B) EtotEM, EtotHAD ETEM , ETHAD • Caution: • Consistency between the measured signals • and the simulated ones • Monte Carlo dependency Andrzej Olszewski

12. Calibration <Nhits> vs. <Nch> <Nhits> vs. <Nch> Zoom Linear calibration: Quadratic calibration: Andrzej Olszewski

13. Measurements of Nch(|| < 3) Relative reconstruction errors: |Nrec-Nch|/Nch Reconstructed multiplicity distribution (Nsig) Histogram – true Nch Points – reconstructed Nch Uncertainty up to 10% at low Nch, less than 3% at high Nch Andrzej Olszewski

14. Reconstruction of dNch/d Motivation: shape of the dNch/d distribution is sensitive to dynamical effects like e.g. quenching and shadowing. • Analysis is based on signals only from Pixel barrel layers • (done separately for each layer). • Clusterization procedure i.e. merging of hits in neighbor pixels is applied (particle traverses more than one pixel when   0). • Correction factors need to be applied to account for the • excess of clusters at large ||: • double hits from overlapping sensors • magnetic field effects (low pT particles bending back) • production of secondary particles Andrzej Olszewski

15. dNhits/d vs. dNch/d dNhits/d R= 5cm R= 9cm R=12cm dNch/d true-primary Andrzej Olszewski

16. Maximal Cluster Size Define the expected maximal size of the cluster: • In Z-direction the number of pixels to be merged • depends on the Z-coordinate of the hit: e.g. for R=5cm, Zhit=40cm Npixel  6 -7 • In -direction the number of traversed pixels depends on pT. • For a track with a curvature r, an angle at which particle • enters the sensor is cos()=R/2r (assuming that sensors form • an ideal tube). • Taking r = 15cm (corresponding to pT=90 MeV/c): Npixels = 4 – 6 for R=12cm Npixels = 3 – 4 for R= 5cm Andrzej Olszewski

17. Reconstructed dNch/d Comparison of the reconstructed dNch/d distributions with the true one of charged primary particles. One single correction function C() calculated froma sample of central events is used. Single Pb+Pb event, b =0-1fm 5 peripheral collision, b =10-15fm Reconstruction errors ~5% Reconstruction errors ~13% Correction factors are ~ centrality independent! Andrzej Olszewski

18. Reconstructed dNch/d Single Pb+Pb HIJING event with jet quenching, b =0-1fm 100 p+p events at s=200 GeV Different shape and higher density are correctly reproduced!! Correction factors are ~insensitive to the detailed properties of generated particles! Andrzej Olszewski

19. Estimate of the Collision Centrality Nsig ET - EM ET - HAD Based on the correlation between the measurable quantity Q and the centrality parameter: b, (Npart, Ncoll) Monotonic relation between Q and b allows for assigning to a certain fraction of events selected by cuts on Q, a well defined average impact parameter. Andrzej Olszewski

20. Stability of the Centrality Estimate Comparison of centrality cuts based on Nsig, Etot, ET Remark: A better approach would be to use a quantity measured outside the mid-rapidity region,e.g. energy in forward calorimeters, which is less sensitive to dynamical effects. Andrzej Olszewski

21. Resolution of the Centrality Estimate Resolution as a function of cut width Loss of resolution relative to pure b cuts High resolution available by using narrow cuts on centrality correlated quantity Andrzej Olszewski

22. Track Reconstruction Pixel and SCT detectors – ATLAS xKalman algorithm • pT threshold for reconstructable • tracks is 1 GeV (reduce CPU). • Tracking cuts are optimized to • get a decent efficiency and • low rate of fake tracks. • Vertex constraints applied • At least 10 measurements per track • Maximum two shared measurements • 2/ndf  4 • Tracking in the || < 2.5 • For pT: 1 - 15 GeV/c: efficiency ~ 70; fake rate<10% • Fake rate at high pT can be reduced by matching with calorimeter Andrzej Olszewski

23. Track Reconstruction Momentum resolution Efficiency versus rapidity ~3% for pT up to 20 GeV/c (for || < 2.5) Flat dependency for |y| < 2 Higher in EC (more layers) Tracking in HI events looks promising, still can be optimized! Andrzej Olszewski

24. Summary • Global measurements are needed for detailed studies of heavy ion interaction properties • ATLAS detector is capable of providing measurements of the total charged multiplicity and transverse energy as well as their rapidity densities using very simple reconstruction procedures • Precision of the measurements seems to be high and reasonably independent of the true collision properties • These results, available already in the first days of LHC Heavy Ion run, may provide on their own crude verification of some ideas in heavy ion models Andrzej Olszewski

25. BACKUPS Andrzej Olszewski

26. Zoom on Atlas ID & Calorimeters Hadronic Tile Calorimeters Silicon Tracker in Inner Detector EM Accordion Calorimeters Forward LAr Calorimeters Hadronic LAr End Cap Calorimeters Andrzej Olszewski

27. Detector Occupancies Examples of occupancy versus z and Nch (high GEANT thresholds) Occ Occ z z Nch Nch Pix1 SCT1 Andrzej Olszewski

28. Detector occupancies Central collisions b=0-1 fm, low GEANT thresholds Pixel Detector Silicon Tracker Andrzej Olszewski

29. Trigger DAQ For Pb+Pb collisions the interaction rate is 8kHz, a factor of 10 smaller than LVL 1 bandwidth. We expect further reduction to 1kHz by requiring central collisions and pre-scaled minimum bias events (or high pT jets or muons). The event size for a central collision is ~ 5 Mbytes. Similar bandwidth to storage as pp at design L implies that we can afford ~ 50 Hz data recording. ~200 Hz Andrzej Olszewski

30. Correction Factors Correction factors are defined as: C() calculated from the sample of 50 central (b=0-1fm) Pb+Pb events, and then parameterized. Correction function for the inner most barrel layer. Andrzej Olszewski

31. Cluster Formation • Choose seeds  large signals > 10,000 electrons • Start with the seed with the largest signal • Attached to it a signal in the adjacent pixel as long as: • There is a signal in a pixel • One of the closest neighbor pixels already belongs to the cluster • The distance from the seed to the pixel is not larger than the expected maximal size of the cluster (in both Z and  directions) up to 6 pixels in Z (depending on Zhit) and 3 pixels in  (depending on R) Andrzej Olszewski