- INFN & Universita' Padova 1 CMS results on Hadron Production at LHC The 5th International Nordic "LHC and Beyond" Workshop "The First LHC Physics and Major Spin-Offs" St.Petersburg 8-11 June 2010

- INFN & Universita' Padova 2 Outline ● Motivations ● The Detector ● The Environment ● The measurements : – Charged Hadron Spectra – Angular Correlations – Bose Einstein Correlations – Underlying Event

- INFN & Universita' Padova 3 Motivations ● Understanding Soft Hadron Dynamics: – Interesting per-se (model validation – several SHD measurements are TOP-cited) – Mandatory to reduce uncertainties in Hard Processes and discovery Physics – Benchmark for Heavy Ions Physics Program – Benchmark validating the detector performances ● CMS is delivering wide variety of results on SHD

- INFN & Universita' Padova 4 4 The Compact Muon Solenoid

- INFN & Universita' Padova 5 5 The Compact Muon Solenoid

- INFN & Universita' Padova 6 6 CMS Tracker ● Acceptance |  | < 2.5 ● ≥ 3 P T > 100 MeV ● fine pixel granularity ensures two track separation for  ≥ o(1 mrad) Pixel Detector

- INFN & Universita' Padova 7 7 The Compact Muon Solenoid Hadron Forward

- INFN & Universita' Padova 8 8 Hadron Forward Calorimeter ● steel – Cherenkov quartz fiber ● fast light readout ● ~ 11 m from I.P. ● 2.9 <  < 5.2 ●  segmentation: X ● embedded triggering Beam Scintillation Counters

- INFN & Universita' Padova 9 9 Minimum Bias Trigger ● Dec data collision rate ~ 10 Hz (no pileup) ● Minimum Bias Trigger : – signal in BSC – in time with beam crossing at interaction point 175 m from interaction point)

- INFN & Universita' Padova 10 Inelastic p-p collisions Inelastic collisions grouped (exp. cuts - Pythia) as : ● Single Diffractive ● Double Diffractive ● Non-Diffractive – Hard Scattering Hard, high P T, parton

- INFN & Universita' Padova 11 Inelastic p-p collisions Inelastic collisions grouped (exp. cuts - Pythia) as : ● Single Diffractive ● Double Diffractive ● Non-Diffractive – Hard Scattering Hard, high P T, parton

- INFN & Universita' Padova 12 Diffractive Event Rejection ● >3 GeV total energy on each HF side ● Beam Halo rejection (BSC) ● Dedicated beam background rejection ● Collision vertex from ≥ 3 reconstructed tracks Sample composition from fit to HF energy deposit (shapes from PYTHIA, PHOJET) Efficiencies: NSD:~86 % SD:~27 % DD:~34 %

- INFN & Universita' Padova 13 Inelastic p-p collisions Inelastic collisions grouped (exp. cuts - Pythia) as : ● Non-Diffractive – Hard Scattering Hard, high P T, parton Track multiplicity, , P T distributions Inter-particle correlations : - Angular - Bose Einstein Correlations

- INFN & Universita' Padova 14 Inelastic p-p collisions Inelastic collisions grouped (exp. cuts - Pythia) as : ● Non-Diffractive – Hard Scattering Hard, high P T, parton Study of the Underlying Event

- INFN & Universita' Padova 15 The Measurements ● Charged Particle Spectra ● Two Particles Angular Correlations ● Bose Einstein Correlations ● Underlying Event in Hard Scattering 0.9, 2.36, 7 TeV 0.9, 2.36 TeV JHEP 02 (2010) 041, arXiv: CMS PAS QCD (2010) arXiv: CMS PAS QCD (2010) arXiv:

- INFN & Universita' Padova 16 The Measurements ● Charged Particle Spectra ● Two Particles Angular Correlations ● Bose Einstein Correlations ● Underlying Event in Hard Scattering

- INFN & Universita' Padova 17 Charged Particle Spectra Three (redundant) algos : ● Full track reco (pixel+strips), P T >100 MeV ● Tracklet method (pixel only), P T >50 MeV ● Single hit (pixel only), P T >30 MeV

- INFN & Universita' Padova 18 Track Iterative algorithm ● ≥ 3 hits in pixel + strips ● compatibility with primary vertex low fake rate (ghosts < 1%) allows for measurement of P T low  at low P T sensitive to alignment, beamspot low P T power high P T t

- INFN & Universita' Padova 19 ● does not measure P T ● need MC for acceptance, subtraction of secondaries, weak decays, pixel splitting Tracklet ● Pair hits from different pixel layers, ● Look for compatibility along  ● Subtract background using  sidebands (1<  <2) extend acceptance P T ~50 MeV

- INFN & Universita' Padova 20 Single Pixel ● Count pixel hits in single layer ● Pixel size  |sinh  | ● Independent meas. in each pixel layer extend  to 30 MeV short clusters (loopers, sec.) removed does not measure P T. ● need MC to subtract secondaries, weak decays, loopers

- INFN & Universita' Padova 21 Results ● Average using uncorrelated errors ● Results symmetrized around  = 0 arXiv: submitted to PRL JHEP 02 (2010) 041

- INFN & Universita' Padova 22 Energy Dependence ● Model tuning ● Steep multiplicity increase at 7 TeV

- INFN & Universita' Padova 23 The Measurements ● Charged Particle Spectra ● Two Particles Angular Correlations ● Bose Einstein Correlations ● Underlying Event in Hard Scattering

- INFN & Universita' Padova 24 Angular Correlations ● Particles produced in clusters ● Clusters characterized by: – “size” : # of particles in the cluster – “width” : angular separation between particles ● Quantitative comparisons wrt model predictions

- INFN & Universita' Padova 25 The Measurement Signal events Reference uncorrelated sample (event mixing) ● Fit  projections in the near (  ) and far (  ) sides with a Gaussian function ● Measure size (K eff ) and width (  ) K ef f  Excluded from fit

- INFN & Universita' Padova 26 Results ● Cluster widths comparable in the near and far sides ● Well described by PYTHIA ● Near cluster size increasing with energy ● Only qualitative agreement with PYTHIA

- INFN & Universita' Padova 27 Results ● Cluster widths comparable in the near and far sides ● Well described by PYTHIA ● Near cluster size increasing with energy ● Only qualitative agreement with PYTHIA ● Confirm trends observed at lower energies

- INFN & Universita' Padova 28 The Measurements ● Charged Particle Spectra ● Two Particles Angular Correlations ● Bose Einstein Correlations ● Underlying Event in Hard Scattering

- INFN & Universita' Padova 29 Bose-Einstein Correlations ● Bose statistics enhances production of identical particles with comparable 4-momentum p ● Correlation function : r r r r : source size  : strenght ( 0 ÷ 1) C : normalization (~1)  : long range effects (~0)

- INFN & Universita' Padova 30 Bose-Einstein Correlations ● Bose statistics enhances production of identical particles with comparable 4-momentum p ● Correlation function : r r r

- INFN & Universita' Padova 31 Reference Samples Geometrical transformation of 2 nd track parameter: ● space inversion ● (xy) rotation Opposite-charge tracks Mix events ● randomly ● comparable dN/d  ● comparable invariant mass dN/dQ sig dN/dQ ref BEC ?

- INFN & Universita' Padova 32 Non Identical Particles Study correlations for : ● opposite charge tracks ● same charge non identical particles, selected by tracker dE/dx Coulomb attraction (described by Gamow factors Opposite Charges BEC !

- INFN & Universita' Padova 33 Double Ratios ● No perfect reference sample ● Reduce bias computing double ratios: ● improve fit quality ( P ~1÷10%) ● reduce spread among result (RMS spread ±15 %)

- INFN & Universita' Padova 34 Combined Result ● Combined reference sample : sum of all the others ● Bad Gaussian fit ● Good fits with Levy, exponential :

- INFN & Universita' Padova 35 Dependencies & Comparisons ● r grows with N ch ● : slight decrease

- INFN & Universita' Padova 36 Dependencies & Comparisons ● r grows with N ch ● : slight decrease ● consistent with previous observations

- INFN & Universita' Padova 37 Dependencies & Comparisons ● r grows with N ch ● : slight decrease ● consistent with previous observations Caveat ● different environments (fixed taget, ) ● different treatment of reference samples

- INFN & Universita' Padova 38 The Measurements ● Charged Particle Spectra ● Two Particles Angular Correlations ● Bose Einstein Correlations ● Underlying Event in Hard Scattering

- INFN & Universita' Padova 39 Multi-Parton Interaction: ● hard parton scattered  high P T track/Jet ● soft interactions, low P T tracks at large angles from scattered parton (Underlying Event) ● proper understanding of U.E. needed to tune background in high pile up environment Underlying Event

- INFN & Universita' Padova 40 Definitions Select high P T track/jet : ● “Toward” (|  |<60 0 ): hard scattering ● “Away” (|  |>120 0 ): balancing jet ● “Transverse” : U.E.

- INFN & Universita' Padova 41 Measurement Tight track selection ● |  | 0.5 GeV Hard parton: ● Track/Jet with P T > threshold ( 1/2/3 GeV ) Measure aggregate properties (multiplicity, momentum flow,etc.) of tracks in the transverse region

- INFN & Universita' Padova 42 Results ● UE activity ( N ch,  P T ) increase with leading Jet P T cut ● similar behavior w.r.t. leading track Multiplicity P T sum

- INFN & Universita' Padova 43 Comparison with MC ● Models fail to represent data, both in absolute values and as for trends ● Shown here : data / MC vs Jet P T threshold Multiplicity P T sum PTPT

- INFN & Universita' Padova 44 Conclusions ● Outstanding take-off of CMS detector: – Decade-long preparation paying off first dividends ● Several Physics results after first months of run – Fast, efficient chain from data-taking to final publication ● First measurements test Soft Hadron Dynamics, reveal breaches in phenomenological models, allow MC tuning. ● Soon many other measurements to come ● Ready to face Hard Scattering Processes