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Julia Velkovska

Selected CMS Results from pp collisions 27th Winter Workshop on Nuclear Dynamics Winter Park, Colorado Feb 6-13, 2011. Julia Velkovska. Motivation . To get insights into multi-particle production in high energy pp collisions soft vs hard production

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Julia Velkovska

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  1. Selected CMS Results from pp collisions 27th Winter Workshop on Nuclear Dynamics Winter Park, Colorado Feb 6-13, 2011 Julia Velkovska

  2. Motivation • To get insights into multi-particle production in high energy pp collisions • soft vs hard production • Description of multi-parton interactions • Non-linear effects at high gluon densities • Collective effects at high energy densities • String fragmentation and color neutralization J. Velkovska

  3. Results: pp collisions at √s = 0.9, 2.36 and 7 TeV • Charged Hadrons • spectra, <pT>, dN/dη, Event-by-event multiplicity • Correlations • Azimuthal and Long-range rapidity correlations • Bose-Einstein Correlations • Sensitive to the detailed implementation of string fragmentation, multi-parton interactions, interplay between hard-scatterings and the underlying event, collective dynamics • Strangeness production • spectra, <pT>, dN/dη • Jet shapes • Sensitive to partonradiation and fragmentation scheme • Proposed as a model-discriminating observable in jet-quenching J. Velkovska

  4. The CMS Si Tracker Acceptance: |η|<2.4 and full azimuthal coverage 66M Pixels: ● 150x100 μm2, closest to the interaction point ● 3 barrel layers (4, 7 and 10 cm radii) and 2 endcaps on each side 9.6 M Strips: ● 10 layers: Larger silicon modules, refine momentum resolution J. Velkovska

  5. Charge hadron reconstruction Full tracks • Use all pixel and strip hits, provide ηand pT • Robust and least sensitive to bg • pT > 100 MeV/c, | η | < 2.4 • Used for dN/dη and spectra Tracklets (2 hits) • Form hit pair - calculate η • Data-driven bg subtraction • pT > 50 MeV/c, | η | < 2 • Used for dN/dη Tracklets (2 hits) Form hit pairs, calculate { Data-driven background subtraction { pT > 50 MeV/c, jj < 2 Pixel hit counting (1 hit) Using the primary vertex, calculate ηfor each cluster Immune to detector mis-alignment, simplest pT > 30 MeV/c, | η | < 2 Used for dN/dη J. Velkovska

  6. Charged hadron spectra and dN/dη Average all methods & Symmetrize JHEP 02 (2010) 041 and PRL 105 (2010) 022002 Spectra are well described by Tsallis fit Rapidity density nearly flat in the measured range: good agreement between experiments J. Velkovska

  7. Comparison to models Most PYTHIA tunes underestimate dN/dη PHOJET ( based on dual-parton model – multiple soft strings) underpredictsdN/dη, but does well on <pT> Saturation models do relatively well J. Velkovska

  8. <pT> and dN/dηvs √s compared to models Most models fail to describe both quantities simultaneously. Why do these quantities rise and why faster than ln(√s) ? What is the role of hard processes and that of soft multi-parton interactions ? J. Velkovska

  9. Event-by-event multiplicity ArXiv: 1011.5531 JHEP, submitted pT >500 MeV/c pT >0 PYTHIA 8 describes the total multiplicity well, but produces too many charged hadrons with high transverse momentum J. Velkovska

  10. Correlate <pT> and event-by-event multiplicity ArXiv: 1011.5531 JHEP, submitted the rise of the average transverse momentum with the multiplicity is roughly energy-independent J. Velkovska

  11. Di-hadron Correlations • More detailed information about the dynamics • Short range rapidity correlations ~ |∆η | < 2 are sensitive to string fragmentation models • Bose-Einstein Correlations ( very short range in η and φ ) – reflect the space-time distribution of the source • Long range rapidity “Away-side” (Δφ ~ π) typically come from back-to-back jets • A new long range correlation was observed on the near side (Δφ ~ 0) in high multiplicity pp events at √s = 7 TeV J. Velkovska

  12. Here is how the data look like: JHEP 09 (2010) 091 Min bias 1 < PT< 3 GeV/c Min bias PT> 0.1 GeV/c High Multiplicity PT> 0.1 GeV/c High Multiplicity 1 < PT< 3 GeV/c “Ridge” structure J. Velkovska

  13. The Ridge Yield evolution pT Associated yield is largest in 1<pT <3 GeV Increases with multiplicity J. Velkovska

  14. Source parameters from BEC Phys. Rev. Lett. 105 (2010) 032001: √s  =0.9 and 2.36 TeV arXiv:1101.3518 : √s  =0.9 and 7 TeV STAR: ArXiv 1004.0925 J. Velkovska Source radii depend on multiplicity, but not much on colliding energy

  15. Source radii vs pair transverse momentum kT At high NchtheSource radius depends on kT Radial expansion ? Similar observation in 1.8 TeV data : Phys. Rev. D 48, 1931–1942 (1993) J. Velkovska

  16. The Invariant Mass Peaks Peak shape and S/B: Excellent agreement between data and simulation J. Velkovska

  17. Strange hadron spectra The spectra are well described by Tsallis Fit J. Velkovska

  18. Mean pT and dN/dy CMS PAS: QCD-10-007 J. Velkovska

  19. Strange Hadron Rapidity Density Yield under predicted by a factor of 3 Yield significantly larger than MC predictions Discrepancy increases with energy and hadron mass J. Velkovska

  20. Jet shapes • Jets are characterized by : • charged particle multiplicity in a jet,Nch • Charge particle transverse jet shape • Integrated jet shape – the fraction of energy contained in a disk of radius r<R J. Velkovska

  21. Jet Shapes in Pythia and data CMS PAS: QCD-10-014 • Nch and transverse shape sensitive to quark/gluon fraction • In PbPb events we expect different quenching of quark and gluon jets • Sensitivity to underlying jet-quenching mechanism J. Velkovska

  22. Integrated Jet shapes (calo and tracking): data vs models CMS PAS: QCD-10-014 Low pT High pT Agreement is worse at low pT J. Velkovska

  23. Conclusion • Detailed CMS measurements of QCD observables in pp collisions at √s = 0.9, 2.36 and 7 TeV • Insights into soft and hard particle production mechanisms and the role of multi-parton interactions • Possible observation of collective effects in high-multiplicity pp collisions – needs further experimental and theoretical studies • Important baseline measurements for HI observables have been carried out • Stay tuned for upcoming HI results J. Velkovska

  24. J. Velkovska

  25. Identified Strange Hadrons Fit pair of oppositely charged tracks to common vertex ● Daughter tracks: Of good quality and Not from primary vertex ● Secondary vertex – Far “enough” from the primary vertex ● Require V0 point back to primary vertex Reconstruct Λ ● pair it with a negatively charged track ● Secondary vertex within 5σ from primary ● Require Ξ- to point to primary ● Ξ- decay vertex well separated from primary J. Velkovska

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