1 Sparsh Navin – University of Birmingham for the ALICE collaboration Trigger Efficiencies in ALICE and Diffraction in PYTHIA 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 Summary

3 A Large 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 1 st measurements:, multiplicity and momentum distributions and

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

5 5 Process Types True cross sections at LHC energies are not known Scaling of cross sections with energy is model dependent PHOJET Default fractions PYTHIA 0.134SD DD0.127 Figure from Torbjörn Sjöstrand, MCnet school, 2008.

6 66 V0AV0C Outgoing undiffracted p beam Diffractive system Single Diffraction (SD) Pseudorapidity gap

7 77 V0AV0C Diffractive system (2) Diffractive system (1) Double Diffraction (DD) Pseudorapidity gap

8 888 V0AV0C Non Diffractive (ND) No pseudorapidity gap

9 9 First Physics Triggers Minimum Bias Triggers MB1SPD or (V0A or V0C) MB3SPD and (V0A and V0C) Different efficiencies for different  Triggers (MB1, MB3)  Type of process (SD, DD, ND)  Event generator (Pythia and Phojet) Global Fast Or (GFO) is the trigger from the Silicon Pixel Detector (SPD) Actual trigger used – at least 2 pixels in coincidence with beams

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 ProcessSDDDND Fraction (f) Efficiency (e) ProcessSDDDND Fraction (f) Efficiency (e) MB1 efficiencies: Pythia: 92.9% Phojet: 96.4% Reason for difference: f – uncertainty in fractions e – uncertainty in kinematics Major difference is in diffractive events ~2-4% effect each

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 Pomeron couples to valence quarks Dominates at small Diffraction in “old” PYTHIA – stretching the string Version (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 Has a gluon (g) and quark (q) contribution Pomeron couples to gluon Dominates at large

19 Diffraction in PYTHIA – “new” 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 - H DPDF Fit Jets and H 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 Work done with Torbjörn Sjöstrand - MCnet

20 Energy dependent Only SS provides separate t spectrum for DD Pomeron Flux factor

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

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

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

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

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)