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

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 2 WWND 2/9/2011

3. J. Velkovska 3 WWND 2/9/2011 Results: pp collisions at √s = 0.9, 2.36 and 7 TeV Charged Hadrons  spectra,, 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,, dN/dη Jet shapes Sensitive to parton radiation and fragmentation scheme Proposed as a model-discriminating observable in jet-quenching

4. The CMS Si Tracker J. Velkovska 4 WWND 2/9/2011 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

5. Charge hadron reconstruction Pixel hit counting (1 hit) Using the primary vertex, calculate η for each cluster Immune to detector mis- alignment, simplest p T > 30 MeV/c, | η | < 2 Used for dN/d η J. Velkovska 5 WWND 2/9/2011 Tracklets (2 hits) Form hit pairs, calculate { Data-driven background subtraction { pT > 50 MeV/c, jj < 2 Tracklets (2 hits) Form hit pair - calculate η Data-driven bg subtraction p T > 50 MeV/c, | η | < 2 Used for dN/d η Full tracks Use all pixel and strip hits, provide η and p T Robust and least sensitive to bg p T > 100 MeV/c, | η | < 2.4 Used for dN/d η and spectra

6. Charged hadron spectra and dN/dη Spectra are well described by Tsallis fit Rapidity density nearly flat in the measured range: good agreement between experiments J. Velkovska 6 WWND 2/9/2011 JHEP 02 (2010) 041 and PRL 105 (2010) 022002 Average all methods & Symmetrize

7. Comparison to models Most PYTHIA tunes underestimate dN/dη PHOJET ( based on dual-parton model – multiple soft strings) underpredicts dN/dη, but does well on Saturation models do relatively well J. Velkovska 7 WWND 2/9/2011

8. and dN/dη vs √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 8 WWND 2/9/2011

9. Event-by-event multiplicity PYTHIA 8 describes the total multiplicity well, but produces too many charged hadrons with high transverse momentum J. Velkovska 9 WWND 2/9/2011 ArXiv: 1011.5531 JHEP, submitted p T >0 p T >500 MeV/c

10. Correlate and event-by-event multiplicity the rise of the average transverse momentum with the multiplicity is roughly energy-independent J. Velkovska 10 WWND 2/9/2011 ArXiv: 1011.5531 JHEP, submitted

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 11 WWND 2/9/2011

12. Here is how the data look like: J. Velkovska 12 WWND 2/9/2011 JHEP 09 (2010) 091 Min bias P T > 0.1 GeV/c Min bias 1 < P T < 3 GeV/c High Multiplicity P T > 0.1 GeV/c High Multiplicity 1 < P T < 3 GeV/c “Ridge” structure

13. The Ridge Yield evolution Associated yield is largest in 1<p T <3 GeV Increases with multiplicity J. Velkovska 13 WWND 2/9/2011 pTpT

14. Source parameters from BEC J. Velkovska 14 WWND 2/9/2011 Source radii depend on multiplicity, but not much on colliding energy Phys. Rev. Lett. 105 (2010) 032001Phys. 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

15. Source radii vs pair transverse momentum k T At high N ch the Source radius depends on k T Radial expansion ? Similar observation in 1.8 TeV data : Phys. Rev. D 48, 1931– 1942 (1993) J. Velkovska 15 WWND 2/9/2011

16. The Invariant Mass Peaks Peak shape and S/B: Excellent agreement between data and simulation J. Velkovska 16 WWND 2/9/2011

17. Strange hadron spectra The spectra are well described by Tsallis Fit J. Velkovska 17 WWND 2/9/2011

18. Mean p T and dN/dy J. Velkovska 18 WWND 2/9/2011 CMS PAS: QCD-10-007

19. Strange Hadron Rapidity Density Yield significantly larger than MC predictions Discrepancy increases with energy and hadron mass J. Velkovska 19 WWND 2/9/2011 Yield under predicted by a factor of 3

20. Jet shapes Jets are characterized by :  charged particle multiplicity in a jet,N ch  Charge particle transverse jet shape  Integrated jet shape – the fraction of energy contained in a disk of radius r<R J. Velkovska 20 WWND 2/9/2011

21. Jet Shapes in Pythia and data N ch 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 21 WWND 2/9/2011 CMS PAS: QCD-10-014

22. Integrated Jet shapes (calo and tracking): data vs models Agreement is worse at low p T J. Velkovska 22 WWND 2/9/2011 CMS PAS: QCD-10-014 Low p T High p T

23. J. Velkovska 23 WWND 2/9/2011 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

24. J. Velkovska 24 WWND 2/9/2011

25. Identified Strange Hadrons J. Velkovska 25 WWND 2/9/2011 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 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