1. Long-Range, Near-Side Angular Correlations in Proton-Proton Interactions in CMS Kajari Mazumdar LHC Physics Seminar, DHEP, TIFR October 23, 2010. First observation in p-p or p-pbar collisions !

2. ArXiv:1009.4122 (hep-ex), CERN-PH-EP-2010-031 Submitted to Journ. of High Energy Physics

3. Prelude Trying to understand the analysis and its implications. We have not participated in this analysis and would like to know more about the analysis. Several slides are taken from presentations by G.Tonelli (Spokesperson, CMS collaboration) and G. Roland (leader of the analysis) at CERN on September 21, 2010. http://indico.cern.ch/conferenceDisplay.py?confId=10744 Additionally materials collected from various presentations by Wei Li and private communications. CMS press release can be obtained from http://cms.cern.chhttp://cms.cern.ch Finally, we are proud to be in CMS collaboration, to have various colleagues with expertise in various aspects.

4. Summary of the seminars on 21.9.2010 Short-range and long-range angular collisions studied in pp collision with CMS at LHC Observed long-range, near-side correlations in high-multiplicity events. -- signal grows with event multiplicity -- effect is maximal in transverse momentum range of 1- 3 GeV/c Long-range, near-side correlation is not seen in low multiplicity events and generators, but resembles effects seen in heavy-ion collisions at high energies. The feature has survived all possible checks for various effects. Collaboration decided to submit the paper to expose our findings to the scrutiny of the scientific community at large. Since there are a number of potential explanations, today’s presentation is focussed on the experimental evidence in the interest of fostering a broader discussion on the subject. The incoming Heavy Ion run of LHC during end of the year will be an additional important test bench.

5. Starting November 2009, LHC delivered proton-on-proton collisions at 900 GeV, 2,360 GeV before moving on to 7000 GeV from March 30, 2010. We have few more weeks with pp collision at 7 TeV, before heavy ion collision. The sample of hard scattered events, till now is too small to really see anything new at high energy scale. Integrated luminosity at present is about 3.5 /pb. However, even with low luminosity, lower energy operation, LHC is already turning out to be a no-loose machine, albeit, in a different sense! Physics commissioning studies in CMS experiment have shown that, in general, the detector is behaving extremely well, and the results match Monte Carlo predictions till now. The level of preparedness of the experiment is very high. No significant discrepancy with known theory observed. The first physics with hadron collision: minimum bias events  mainly soft particles (pions, kaons of average momentum few hundred MeV/c).  measurements have to be explained by mostly phenomenological models rather than by perturbative QCD, and the description can be improved with new data.  study basic properties at the new kinematic regime offered by LHC.  Need high resolution, highly efficient detector to “track” soft particles. Preliminaries

6. TOB TID TIB TEC PD Coverage up to |  |<2.5; extremely high granularity, due to the small cell size and high longitudinal segmentation, to keep low occupancy (~ a few%) also at LHC nominal luminosity. It is the largest Silicon Tracker ever built: Strips: 9.3M channels; Pixels: 66M channels. Operational fractions: strips 98.1%; pixel 98.3% CMS All Silicon Tracker : detection of charged particles (Pixel Detector, Tracker Inner and Tracker Outer, Tracker Endcap,..) PD TIB TOB  view, beam out of/into the plane  - θ view, beam parallel to horizontal axis

7. Pixel detector: 3 barrel layers at radii between 4.4 and 10.2 cm Single point resolution: 10  m in , 25  m in z Silicon detector: 10 barrel layers upto 1.1 m Resolution of transverse momentum for particles with 1 GeV/c: 0.7% at  = 0, 2% at |  |=2.5 Primary vertex (requires atleast 3 tracks) to lie within 4.5 cm of nominal collision point along Z and 0.15 cm in direction transverse to beam. Track counting done with dz, dxy error ~ 100  m

8.   angle between two tracks in the transverse plane   angle between two tracks in the longitudinal plane

9. Strategy: make the best possible use of the large eta coverage of CMS detector, the redundancy of the apparatus and the flexibility of trigger system. High multiplicity events are rare. Special trigger needed to collect large statistics sample of high multiplicity events. Probing new energy frontier starts with the study of minimum bias events in the new energy regime: charge multiplicity, average momentum distribution, etc. Charged hadron multiplicity in mimimum bias events at different energies. For primary/online data collection implemented event trigger condition in 2 steps: total transverse energy in the event > 60 GeV the multiplicity of charged tracks > 70/85 (above pt>400 MeV/c, |  |<2, within dz < 0.12 cm of a single vertex, with z < 10 cm) Statistics for high multiplicity events enhanced by O(10 3 ) Compared to min.bias events.

10. The particle densities in the high multiplicity events of proton-proton collisions at 7TeV begin to approach those in high-energy collisions of nuclei such as copper. It is natural to study the two particle angular correlations in LHC and compare the results with the ones obtained in relativistic heavy ion colliders like RHIC. High multiplicity events

11. The schematization of the collision is cut into pieces and modeled in different ways, though, actually, the pieces are correlated. 2-particle correlation probes the connection between various pieces. clusters

12. Various correlations have been studied extensively in previous experiments at ISR, SPS, RHIC. STAR, PHOBOS experiments at RHIC has reported observation of long- range (high |  |) angular correlation at near side (|  | ~ 0). Heavy Ion collisions at LHC by end of 2010, will provide more opportunity to study the effect in detail. Independent cluster model depicts: 1. independently produced clusters from the initial interaction. 2. Isotropic decay of these clusters in their CM system into hadrons. Clusters: jets, resonances, strings, …, having short range correlation. The Past and The Future 2-particle correlation probes essentially the mechanism of multiparticle production in high energy collision of hadrons. At high energy collisions, the mechanism of hadronisation and possible collective effects due to high particle densities can be studied,

13. Short-range correlations in minimum bias events has a typical width of  ~ 1. The correlation strength and extent can be parametrized in terms of a simple cluster model and the strength quantified in terms of cluster size (average no. of particles in a cluster) and width (separation of particles in pseudorapidity)  However, this does not really lead to basic understanding of the process. Angular correlation can be both short- and long-range which characterizes QCD in the energy ranges encountered in the experiment. Types of correlation The long range correlation, for high |  | values, may be significantly affected by the presence of hot and dense matter formed in high energy collisions of hadronic matter. Possible to study till now only in heavy-ion facilities, but now, may be already in proton-proton collisions at LHC. It could as well be manifestation of some jet properties at high energies. The physical origin of long range correlation is not yet well-understood.

14. Steps to make 2-particle correlation for a given momentum bin Signal: pairs from same event Take each event and make all possible 2-particle pair combination. For each pair calculate  and  and fill a histogram. Then average over all events Normalization of the histo not too important. However, the distribution is normalised to unit integral in the present analysis. Background: essentially product of 2 single particle distributions. take random combinatorial pairs, (using event mixing procedure to kill any correlation) Select 2 events randomly and make pairs using one particle from each event having similar vertex and multiplicity. Take  always positive and fill other quadrants by reflection  symmetry around  = 0

15. Correlation function

16. R is a measure of correlated pairs/pairticle/event. Calculate ratio for each multiplicity bin first and then weight by average multiplicity of each bin. Multiplicity weighting factor: N-1 = total number of pairs/particle/event

17. Features of correlation plot:1

18. Features of correlation plot: 2

19. Features of correlation plot: 3

20. 2-particle correlation in minimum bias data

21. Result from real data:1 Cut the region of |  |<0.06, |  |<0.06, to reduce secondary effects (tracks from photon conversions, weak decays, event not rejected by impact parameter cut.

22. Result from real data: 2

23. Result: 3, the novel feature!

24. Result from Monte Carlo generated events

25. Multiplicity and Pt dependence: Turn on of the Ridge

26. High Multiplicity and Pt dependence: 2 MC has minimum at  = 0 Data has local maximum at  = 0

27. Systematic uncertainties Negligibly small statistical uncertainty. However the signal is subtle and unexpected.  Estimate systematic uncertainties Is there a way to fake the signal qualitatively?  Check for effects due to: Pile-up + beam background, Detector noise, acceptance, efficiency Trigger efficiency, bias Reconstruction efficiency, fakes Bugs in analysis code. Apply data-driven methods No indication of effect that would fake ridge signal.

28. Like-sign vs. unlike sign No dependence on relative charge sign Correlations calculated separately for pairs of same sign and opposite signs

29. Analysis code

30. Track reconstruction code

31. Trigger

32. Events background: 1

33. Events background: 2

34. Event pileup Pileup effects are suppressed due to excellent resolution.

35. Analysis with tracks paired with photons

36. Conclusion CMS experiment has measured 2-dimensional angular correlations between particles with high |  | over full range of  for proton-proton collision at cm energies of 0.9, 2.36 and 7 TeV. A variety of features are observed due to short- and long-range correlations. The most interesting feature is the first observation of near-side, long-range correlation in high-multiplicity events at 7 TeV which resembles the observation at RHIC experiments. The physical origin of the correlation is not yet understood. Further detailed studies are needed. CMS decided to report about the finding to the HEP community.

37. Backup

38. Event and track selection

39. Minimum Bias Data sample at different energies Nevent integrated luminosity energy 168,854 3.3  b -1 900 GeV 10,902 0.2  b -1 2360 GeV 150086 3.0  b -1 7000 GeV Event multiplicity corrected for all detector and reconstruction algorthm Data at 7 TeV For high multiplicity analysis data corr. To int. lumi = 980 /nb

40. Short range correlation vs. sqrt(s)

41. Quantifying cluster properties and their energy dependence On average every 2-3 charged particles, typically pions are produced in a correlated Fashion  cluster mass ~ 1GeV Clusters are also getting narrower with increasing energy.

42. 2-particle correlation

43. p T >1.0GeV/c |  |<1.0 o 20<N<35 90<N<110 Tracking performance in high multiplicity events p T >1.0GeV/c |  |<1.0 o 20<N<35 90<N<110 The CMS Tracker feels at home in high charged particles multiplicity environment. It has been designed to tackle thousand of tracks per event as it will happen with LHC running in pp at nominal luminosity. It is foreseen to provide good tracking performance also for the imminent Heavy Ion running of LHC where we expect to have order of 10 4 tracks per event.

44. Corrections Event selection efficiency: low for low multiplicity events Triggering vertexing

45. Tracking/acceptance efficiency correction Overall efficiency 76%, for pT ~ 100 MeV/c efficiency = 55% No significant change after correction, since event multiplicity is high

46. Clusters are partially lost at the edge of Acceptance, correlation affected by 20-25

47. Track impact parameter significance for track pT between 100 to 200 MeV/c Very good agreement between data and monte carlo down to soft tracks  Use MC to determine efficiency of track selection Track Selection:

49. Energy dependence of cluster properties

50. Dedicated high multiplicity trigger in the two steps. Level 1 (L1): Sum of the total transverse energy E T (ECAL, HCAL, and HF) > 60 GeV. High-level trigger (HLT): number of online tracks built from the three layers of pixel detectors >70 (85). Data Collection in p-p collision Statistics for high multiplicity events enhanced by O(10 3 ). Total datasets corresponding to 980nb -1

51. The particle densities in the high multiplicity events of proton-proton collisions at 7TeV begin to approach those in high-energy collisions of nuclei such as copper. It was considered natural to study the two particle angular correlations in LHC and compare the results with the ones obtained in relativistic heavy ion colliders like RHIC. p T >0.1GeV/c high multiplicity pp 7TeV comparable to ~18 nucleon pairs, each colliding at 62.4GeV in CuCu CMSPHOBOS  -2+2

53. Zero Yield At Minimum (ZYAM) Strength of the near side ridge and its dependence on pt and multiplicity Can be quantified by calculating the associated yield: number of other Particles correlated with a specific particle. ZYAM uses R(Dj) integrated between |Dh|=2.4 and 4.8. First fit a polynomial in the range of 0.1 to 2.0 and find the minimum, FZYAM. Integrated R(Dj) between o to FZYAM and multiply with background integrated over Dh between 2.0 to 4.8 Assume that away side jet contribution, the background, negligible. The uncertainty in fitted minimum gives the uncertainty of the associated yield

55. Detector