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Sparsh Navin – University of Birmingham for the ALICE collaboration

Trigger Efficiencies in ALICE and Diffraction in PYTHIA. Sparsh Navin – University of Birmingham for the ALICE collaboration. Diffractive and electromagnetic processes at the LHC, Trento 2010. Outline. ALICE and pp trigger efficiencies ALICE detectors for first physics with protons

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Sparsh Navin – University of Birmingham for the ALICE collaboration

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  1. Trigger Efficiencies in ALICE and Diffraction in PYTHIA Sparsh Navin– University of Birmingham for the ALICE collaboration Diffractive and electromagnetic processes at the LHC, Trento 2010

  2. Outline ALICE and pp trigger efficiencies • ALICE detectors for first physics with protons • Process Types • First physics trigger efficiencies Diffraction and PYTHIA • Kinematic comparison – PYTHIA 6.4 and PHOJET • Diffraction in PYTHIA – then and now • Comparison of kinematic plots – PYHTIA 8.310 • Summary

  3. ALarge Ion Collider Experiment • Primary purpose – heavy ion detector for Pb-Pb collisions at 5.5 TeV • First physics programme is pp at 0.9, 2.36, 7 (maybe 10) and 14 TeV • - Time and space alignments • - pp bench mark to study genuine Pb-Pb effects • Special features for pp collisions: • - Low magnetic field => low pT cut-off, tracking resolution from 100 MeV/c • - Excellent PID over broad range of momenta • - Primary vertex resolution (100μm for pp, 10 μm for Pb-Pb) • pp Physics contributions: • - Physics cross checks with previous results • - Good description of underlying event and high multiplicity collisions • 1st measurements: , multiplicity and momentum distributions • and

  4. ALarge Ion Collider Experiment - 2 V0A z = 3.3m • Provide the V0A and V0C trigger signals • Time resolution better than 1ns V0C z=-0.9m • SPD – Inner layers - 3.9cm 7.6cm radii - better than 100μm resolution - provide the SPD trigger signal ITS (Inner Tracking System) TPC (Time Projection Chamber) • Main gas-filled tracking detector ZN 52 m underground 16 m high 26 m long 10,000 tonnes 0.5 T magnetic field ZDC at ZP ZEM at 7 m

  5. Process Types Figure from Torbjörn Sjöstrand, MCnet school, 2008. • True cross sections at LHC energies are not known • Scaling of cross sections with energy is model dependent 5

  6. Single Diffraction (SD) Outgoing undiffracted p beam Diffractive system V0A V0C Pseudorapidity gap 6 6

  7. Double Diffraction (DD) Diffractive system (2) Diffractive system (1) V0A V0C Pseudorapidity gap 7 7

  8. Non Diffractive (ND) V0A V0C No pseudorapidity gap 8 8 8

  9. First Physics Triggers Actual trigger used – at least 2 pixels in coincidence with beams Global Fast Or (GFO) is the trigger from the Silicon Pixel Detector (SPD) • Different efficiencies for different • Triggers (MB1, MB3) • Type of process (SD, DD, ND) • Event generator (Pythia and Phojet) 9

  10. Trigger Efficiencies and corrections • Need to know the fraction (f) and the efficiency (e) for each process. • Efficiency is process, trigger and generator dependent Eg: MB1 = SPD or V0A or V0C MB1 efficiencies: Reason for difference: f – uncertainty in fractions e – uncertainty in kinematics Pythia: 92.9% Phojet: 96.4% ~2-4% effect each Major difference is in diffractive events

  11. Kinematics – eta ND 7 TeV

  12. Kinematics – eta SD 7 TeV

  13. Kinematics – pT ND 7 TeV

  14. Kinematics – pT SD 7 TeV

  15. Kinematics – multiplicity ND 7 TeV

  16. Kinematics – multiplicity SD 7 TeV

  17. Diffraction in PYTHIA – “old” • Event Generation: • Diffractive cross sections given by model by Schuler and Sjöstrand (Phys. Rev. D 49, 2257 (1994)) • Diffractive mass ( ) and momentum transfer (t) generated according to: • Particle Production: • above mass of incoming particles => isotropic decay into 2-body state • More massive system treated as a string with quantum numbers of the original hadron

  18. Diffraction in “old” PYTHIA – stretching the string Has a gluon (g) and quark (q) contribution Pomeron couples to valence quarks Dominates at small Pomeron couples to gluon Dominates at large • Version 6.214 (Fortran) • q and g contributions are set by a user-defined fixed ratio • Version 8.1 (C++) • Slope p and normalisation N set by user • mass ( ) dependence • Gluonic domination at large

  19. Diffraction in PYTHIA 8.130 – “new” Work done with Torbjörn Sjöstrand - MCnet Event Generation: • Cross sections - same way as before • Diffractive mass ( ) and momentum transfer (t) picked by Pomeron flux model Particle Production: • Pomeron-p collisions • Pomeron PDF with - dependence from H1 data - H1 2007 DPDF Fit Jets and H1 2006 Fits A and B - Pion PDF also available • Standard PYTHIA machinery for multiple interactions, parton showers, hadronization Mass separation: • For non-perturbative description (as before) - longitudinally stretched strings • For perturbative

  20. Pomeron Flux factor • Energydependent • Only SS provides separate t spectrum for DD

  21. PYTHIA “old” vs “new” - eta 7 TeV

  22. PYTHIA “old” vs “new” - pT 7 TeV

  23. PYTHIA “old” vs “new” - multiplicity 7 TeV

  24. Comments In the massless limit Not known from first principles Multiple interactions => screening of diffractive rates Diffractive hard scattering cross section:

  25. Future Plans - PYTHIA LRG LRG • Include Central Diffraction • Does cut-off depend on the diffractive mass? • Introduce a screening factor to go from ep to pp collisions • Momentum sum of PDFs • H1 2006 DPDF LO fit • Tuning to data

  26. Summary • The ALICE detectors and trigger for the first pp physics programme were discussed • Trigger efficiencies are model dependent - PYTHIA and PHOJET • Earlier versions of PYTHIA had a primitive description of diffraction • No hard diffraction – caused the difference in pT and multiplicity tails compared to PHOJET • New version (8.130) has Pomeron description of diffraction • Hard collisions can be simulated • Better agreement with PHOJET

  27. Back up slides • ALICE first paper • Extraction of fractions from data - ALICE • Diffraction in Phojet

  28. ALICE first paper - arXiv: 0911.5430 [hep-ex] • Trigger – atleast 2 fired chips in coincidence with bunches – 284 events • Magnetic field off; SPD used for analysis; SDD, SSD and V0 used for cross checks and background ID and removal. • In the region • Trigger efficiency obtained from MC PYTHIA 6.141 D6T and PHOJET 1.12: • SD – 48% to 58%, DD – 53% to 76%, ND – 98% to 99% • Fractions taken from UA5: • SD – , DD - , ND – • Trigger efficiencies: Inelastic - 87% to 91%, NSD – 94% to 97% • Results obtained with PYTHIA, difference between PYTHIA and PHOJET used to estimate systematic uncertainty. • Inelastic • NSD

  29. Extraction of fractions - Z.Matthews 20/03/09, ALICE first physics meeting • Trigger on bunch crossing • Define 8 uncorrelated trigger types using SPD, V0A and V0C • Meausre measured calc • Program works out combinations of fractions to generate so as to minimise

  30. Central elements of Phojet - R. Engel workshop on soft diffraction at LHC 26/6/09 Two component Pomeron Only one pomeron with soft and hard contributions Topological identification of different terms (Dual parton model) Soft and hard partons differ in impact parameter distribution Application of existing parton density parametrisation Initial and final state radiation (leading logQ^2 parton showers)

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