Fast Embedding of Jets in HeavyIon Collisions for

Fast Embedding of Jets in Heavy-Ion Collisions for Background Studies with ALICE Bastian Bathen Institut für Kernphysik, University of Münster for the ALICE Collaboration 6 th Int. Workshop High-p. T Physics at LHC Utrecht, April 5 th 2011

Outline • Introduction to Jets • ALICE • Jets in ALICE • Jet Algorithms • Jets • in p+p Collisions • in Pb+Pb Collisions • Background in HI Events • Background Fluctuations • Fast Embedding • Random Cones • Conclusion and Outlook Bastian Bathen, Münster 6 th High-p. T Workshop, Utrecht April 5 th 2011 2

Introduction to Jets • parton scattering with large • quarks / gluons achieve a high transverse momentum • Q 2 due to confinement it results in a spray of observable hadrons (with high p. T) Proton-Proton collision with hard scattered partons Jet collimated spray of hadrons Proton Underlying Event hard scattered partons “jet” Proton initial state radiation multiple parton interactions semi-hard interactions beam-beam remnants based on sketch by Alison Lister “jet” Bastian Bathen, Münster NB: for A+A multiple nucleon-nucleon collisions, strong increase of underlying event 6 th High-p. T Workshop, Utrecht April 5 th 2011 3

A Large Ion Collider Experiment Electro Magnetic Calorimeter (EMCal) Time-of-Flight (TOF) Transition Radiation Detector (TRD) Time Projection Chamber (TPC) Inner Tracking System (ITS) High Momentum Particle Identification (HMPID) Photon Spectrometer (PHOS) list of detectors not complete Solenoid magnet (L 3) Central barrel Bastian Bathen, Münster 6 th High-p. T Workshop, Utrecht Dipole Magnet Forward Muon Arm April 5 th 2011 4

Jets in ALICE Di-jet event in Pb+Pb @ 2. 76 Te. V *) Fast. Jet package: Phys. Lett. B 641 (2006) [hep-ph/0512210] Bastian Bathen, Münster 6 th High-p. T Workshop, Utrecht April 5 th 2011 5

Jet Finder Algorithms *) original, not in general Bastian Bathen, Münster 6 th High-p. T Workshop, Utrecht April 5 th 2011 6

p+p Collisions p+p collisions • give a ‘clean’ sample of jets (underlying event is small) • allow to understand the – detector response – jet finder characteristics – other uncertainties and systematics start with raw jet spectrum in p+p • track quality cuts applied, aimed for high-p. T after detailed studies • correct spectrum by unfolding for – smearing in p. T: due to track momentum resolution – shift in p. T: due to charged tracks only and tracking efficiency – yield of reconstructed jets: fake jets / cosmics, tracking efficiency • characteristics of jet finder algorithm – cone size – combination scheme Bastian Bathen, Münster 6 th High-p. T Workshop, Utrecht April 5 th 2011 7

Jets in p+p Collisions ~ 33% of full statistics (2010) • uncorrected jet spectrum, needs to be unfolded Ø good agreement between different jet finders above 20 Ge. V/c Bastian Bathen, Münster 6 th High-p. T Workshop, Utrecht April 5 th 2011 8

Jets in Pb+Pb Collisions Motiviation • use hard scattered partons as hard probes • scattered partons interact with the Quark-Gluon Plasma (before fragmentation) – for high-p. T tracks RAA – jets: modification of jet structure e. g. momentum distribution in jets Challenge • underlying event (increases with nb. of participants) • background subtraction • region-to-region fluctuations (mainly Poissonian) Pb + Pb Jet as hard probe in the QGP jet Pb + Pb ? peripheral 50 -80% Bastian Bathen, Münster central 0 -10% 6 th High-p. T Workshop, Utrecht April 5 th 2011 9

Background in HI Events Jet with R=0. 4 has an area of 0. 5 Centrality Multiplicity track p. T > 150 Me. V/c Bastian Bathen, Münster 6 th High-p. T Workshop, Utrecht April 5 th 2011 10

Jets in Pb+Pb Collisions raw charged jet spectrum from Pb+Pb collisions @ 2. 76 Te. V - in different centralities ~ 15% of full statistics (2010) • not corrected for – background fluctuations – tracking efficiency and resolution – neutral energy fraction – fake jets / cosmics § will be unfolded Ø tools for jet reconstruction incl. background subtraction in place Ø detailed knowledge of background fluctuations essential Bastian Bathen, Münster 6 th High-p. T Workshop, Utrecht April 5 th 2011 11

Fast Embedding Method: • embed a known probe: single high-p. T track or full p+p jet event (real or MC) • into a HI event (mainly soft background) Bastian Bathen, Münster 6 th High-p. T Workshop, Utrecht April 5 th 2011 12

Fast Embedding (cont’d) Bastian Bathen, Münster 6 th High-p. T Workshop, Utrecht April 5 th 2011 13

Background Fluctuations centrality 0 -10% tail from jets Bastian Bathen, Münster 6 th High-p. T Workshop, Utrecht April 5 th 2011 14

Background Fluctuations (cont’d) centrality 0 -10% tail from jets random cones • alternative method for background studies • place randomly cones with fixed area • exclude leading jets tail on RHS suppressed • different methods help to understand tail on RHS Bastian Bathen, Münster 6 th High-p. T Workshop, Utrecht April 5 th 2011 15

Centrality Dependence • 0 -10% 10 -30% peripheral central • background subtraction is independent from background fluctuation decreases from central to peripheral events 30 -50% 50 -80% (mean of the fit around 0) Bastian Bathen, Münster 6 th High-p. T Workshop, Utrecht April 5 th 2011 16

Different Track-p. T Cuts influence of track-p. T cuts on “measured” background ALICE Physics Performance Report II J. Phys. G: Nucl. Part. Phys. 32, 1295 (2006) 0. 15 Ge. V/c cut 1. 0 Ge. V/c cut 2. 0 Ge. V/c cut background HIJING quenched for b < 5 fm track p. T cut average ρ 0. 15 Ge. V/c 136 Ge. V/c 1. 0 Ge. V/c 61 Ge. V/c 2. 0 Ge. V/c 13 Ge. V/c Bastian Bathen, Münster • reduction of soft background by track p. T cut also visible in background fluctuations • methods work also with high-p. T cut off 6 th High-p. T Workshop, Utrecht April 5 th 2011 17

Different Track-p. T Cuts influence of track-p. T cuts on “measured” background ALICE Physics Performance Report II J. Phys. G: Nucl. Part. Phys. 32, 1295 (2006) 0. 15 Ge. V/c cut 2. 0 Ge. V/c cut track p. T cut average ρ 0. 15 Ge. V/c 136 Ge. V/c 1. 0 Ge. V/c 61 Ge. V/c 2. 0 Ge. V/c 13 Ge. V/c Bastian Bathen, Münster • reduction of soft background by track p. T cut also visible in background fluctuations • methods work also with high-p. T cut off 6 th High-p. T Workshop, Utrecht April 5 th 2011 18

Conclusion & Outlook Conclusion • tools for jet reconstruction incl. background subtraction in place • underlying event in Pb+Pb is large (50 – 100 Ge. V/c for typ. jet area in centrality 0 -10%) • use fast embedding and random cones for background studies • background subtraction works independent from amount of background • background fluctuations about 10 Ge. V/c (0 -10% most central events, lowest p. T cut) Outlook • • • embed quenched jets background fluctuation as function of multiplicity (fluctuation) final parameterization of – fluctuations and – tail on RHS • • unfolding and physics statement important for all jet studies to understand the background fluctuations jet reconstruction tools basis for lots of impressive studies (e. g. momentum distribution in jets) Bastian Bathen, Münster 6 th High-p. T Workshop, Utrecht April 5 th 2011 19

Thank you for your attention! … and many thanks to the ALICE jet group Thanks to specified “background” people: Jochen Klein Christian Klein-Bösing Leticia Cunqueiro Mendez Marta Verweij Bastian Bathen, Münster 6 th High-p. T Workshop, Utrecht April 5 th 2011 20

Backup Bastian Bathen, Münster 6 th High-p. T Workshop, Utrecht April 5 th 2011 21

Collision Centrality PRL 106, 032301 (2011) ar. Xiv: 1012. 1657 v 2 Pb + Pb peripheral 50 -80% Bastian Bathen, Münster Pb + Pb central 0 -10% 6 th High-p. T Workshop, Utrecht April 5 th 2011 22

Pb+Pb @ 2. 76 Te. V jet 1 51. 5 Ge. V/c jet 2 50. 0 Ge. V/c after background subtraction jet event in p+p @ 7 Te. V Bastian Bathen, Münster 6 th High-p. T Workshop, Utrecht April 5 th 2011 23

ALICE Physics Performance Report II J. Phys. G: Nucl. Part. Phys. 32, 1295 (2006) Bastian Bathen, Münster 6 th High-p. T Workshop, Utrecht April 5 th 2011 24