Measurement of Underlying Event Observables with the ATLAS detector Róbert Astaloš (Comenius University Bratislava) on behalf of the ATLAS Collaboration MPI @ LHC 2016 – VIII International Workshop on Multiple Partonic Interactions at the LHC San Cristóbal de las Casas, Chiapas, Mexico, 28 November - 2 December 2016 November 28, 2016 1
Overview Measurement of charged-particle distributions sensitive to underlying event in √ s = 13 TeV proton-proton collisions with the ATLAS detector at the LHC – Preliminary results Measurement of event-shape observables in Z → ℓ + ℓ − events in pp collisions at √ s = 7 TeV with the ATLAS detector at the LHC Eur. Phys. J. C. (2016) 76:375, arXiv:1602.08980 2
Motivation Underlying Event = soft processes unavoidably accompanying hard parton-parton scatterings in pp collisions with a high momentum transfer interactions between proton remnants, MPI, initial and final state QCD radiation Soft interactions not reliably calculable by theory – dominated by low-scale QCD interactions, in which the strong coupling strength diverges and pertubative methods of QCD lose predictivity ⇒ described by phenomenological models, implemented in MC event generators ⇒ contain many free parameters which are needed to be constrained by measurements. 3
Measurement of Underlying Event leading charged particle η, ϕ plane divided into regions around leading (the highest p T ) object (track, calo. cluster, jet...): | ∆ ϕ | < 60 ◦ - toward − ∆ φ ∆ φ 60 ◦ < | ∆ ϕ | < 120 ◦ - transverse | ∆ ϕ | > 120 ◦ - away towards | ∆ φ | < 60 ◦ towards and away regions dominated by transverse (min) transverse (max) 60 ◦ < | ∆ φ | < 120 ◦ 60 ◦ < | ∆ φ | < 120 ◦ particle production from the hard process → relatively insensitive to the softer UE away | ∆ φ | > 120 ◦ transverse region more sensitive to UE further subdivision of the observables on an event-by-event basis depending on which side of the event is more activity: trans-max : observables in the more-active transverse region (higher � p T ) includes both MPI and hard-process contamination trans-min : observables in the less-active transverse region (lower � p T ) most sensitive to MPI effects (pedestal) trans-diff : difference of trans-max and trans-min clearest measure of hard-process contamination 4
Measured Observables Observable Description binned variables p lead Transverse momentum of the leading charged particle T | ∆ φ | Absolute difference in particle azimuthal angle from the leading particle unbinned variables � � N ch /δηδφ Mean number of charged particles per unit η − φ (in radians) �� p T /δηδφ � Mean scalar p T sum of charged particles per unit η − φ (in radians) δφ = 2 π/ 3 – for toward, away an transverse regions π/ 3 – for the single-sided trans-min and trans-max regions 2 π/ n bins – for each of the n bins equally-sized bins in | ∆ φ | distributions δη = 5 in all cases mostly dependences of these quantities on the p lead : T low → high p lead ∝ smooth transition: minimum bias → hard scattering regime T 5
Event and Track Selection √ s = 13 TeV data taken in a special configuration of the LHC: low beam currents, reduced beam focusing, producing a low mean � � number of interactions per bunch (0.003 ≤ µ ≤ 0.03) trigger: one or more MBTS counters above treshold on either side of the detector integrated luminosity of 1 . 6 nb − 1 events: required to contain 1 reconstructed vertex from ≥ 2 tracks with p T > 100 MeV required to contain at least one track with p lead > 1 GeV T corrected to the particle level, including a correction for leading particle realignment 66 million data events passed the trigger and vertex selection track selection criteria: p T > 0 . 5 GeV ; | η | < 2 . 5 6
Leading charged particle p T and Angular distributions 〉 [GeV] φ δ η η p > 0.5 GeV, | | < 2.5 ATLAS Preliminary p > 0.5 GeV, | | < 2.5 ATLAS Preliminary 3 η T T ] 10 -1 -1 -1 δ 10 s = 13 TeV, 1.6 nb s = 13 TeV, 1.6 nb 〉 [GeV / p lead > 1 GeV ATLAS Preliminary φ ch 10 2 δ T N p lead > 10 GeV PYTHIA 8 A14 p lead > 10 GeV PYTHIA 8 A14 η 10 η -1 p > 0.5 GeV, | | < 2.5 s = 13 TeV, 1.6 nb T T lead 〈 δ lead lead T p > 1 GeV PYTHIA 8 Monash / p > 1 GeV PYTHIA 8 Monash 10 T T / d p T p T Herwig7 Herwig7 Data PYTHIA 8 Monash Σ 1 PYTHIA 8 A14 Herwig7 〈 ev Epos Epos dN PYTHIA 8 A2 Epos − 10 1 ev 1/N 1 − 2 10 1 − 10 3 − 4 10 − 5 10 MC / Data MC / Data 1.1 lead 1.1 lead p > 10 GeV p > 10 GeV T T 1 1 Model / Data 1.4 0.9 0.9 1.2 0.8 0.8 MC / Data MC / Data 1 lead lead p > 1 GeV p > 1 GeV 1.2 1.2 T T 0.8 1 1 5 10 15 20 25 30 0.8 0.8 lead p [GeV] T 0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 180 | ∆ φ | [degrees] ∆ | φ | [degrees] N ev vs p lead : steeply falling distribution with a change of slope for p lead ≥ 5 GeV T T broadly modelled by all generators, best description by EPOS and PYTHIA 8 A14 p lead > 1 GeV → p lead > 10 GeV – transition from relatively isotropic minimum bias T T scattering to the emergence of hard partonic scattering structure and a dominant axis of energy flow, no clear best MC: more inclusive selection ( p lead > 1 GeV) – EPOS T hard-scattering selection ( p lead > 10 GeV) – HERWIG7 and Pythia 8 Monash T 7
N ch and � p T densities in azimuthal regions 8 〉 [GeV] 2.5 φ η η p > 0.5 GeV, | | < 2.5 ATLAS Preliminary p > 0.5 GeV, | | < 2.5 ATLAS Preliminary δ T T η -1 7 -1 s = 13 TeV, 1.6 nb s = 13 TeV, 1.6 nb δ 〉 / φ ch 2 δ N 6 η Towards region 〈 δ / Transverse region T 5 p Away region 1.5 Σ 〈 4 1 3 2 Towards region 0.5 Transverse region 1 Away region 0 0 5 10 15 20 25 30 5 10 15 20 25 30 lead lead p [GeV] p [GeV] T T general shape: first very rapid rise in activity – 3 regions not strongly distinguished abrupt transition at p lead ≈ 5 GeV, above it distinct behavior of 3 regions T different shape of the transverse region: almost completely plateaus after p lead ≈ 5 GeV T → hard process dominates the towards and away regions, which continue to increase in activity as the hard process scale grows, but transverse region is relatively unaffected p lead > 7 GeV: away region with highest multiplicity, despite not containing p lead track T the towards region is the most active by � p T for all p lead T values T 8
� p T densities in trans-min/max/diff regions 1.6 [GeV] 3 1.6 [GeV] [GeV] Trans-min region ATLAS Preliminary Trans-max region ATLAS Preliminary Trans-diff region ATLAS Preliminary 1.4 1.4 〉 η -1 p > 0.5 GeV, | | < 2.5 s = 13 TeV, 1.6 nb η -1 η -1 〉 2.5 p > 0.5 GeV, | | < 2.5 s = 13 TeV, 1.6 nb 〉 p > 0.5 GeV, | | < 2.5 s = 13 TeV, 1.6 nb φ T φ φ δ T T 1.2 p lead > 1 GeV δ δ lead 1.2 lead η p > 1 GeV p > 1 GeV T η η δ T T δ δ / 2 T / / 1 p T T 1 p p Σ Σ Σ 〈 0.8 〈 〈 1.5 0.8 0.6 0.6 1 0.4 Data PYTHIA 8 Monash 0.4 Data PYTHIA 8 Monash Data PYTHIA 8 Monash PYTHIA 8 A14 Herwig7 PYTHIA 8 A14 Herwig7 PYTHIA 8 A14 Herwig7 0.5 0.2 PYTHIA 8 A2 Epos 0.2 PYTHIA 8 A2 Epos PYTHIA 8 A2 Epos Model / Data Model / Data Model / Data 1 1 1 0.8 0.8 0.8 5 10 15 20 25 30 5 10 15 20 25 30 5 10 15 20 25 30 p lead [GeV] lead lead p [GeV] p [GeV] T T T trans-min: best description by PYTHIA 8 Monash and Herwig7 (in the plateau region) PYTHIA 8 A2 (mild but broad undershoot extending up to p lead ≈ 20 GeV) T and Herwig7 (severe undershoot for p lead < 5 GeV) mismodel the transition T trans-max: similar, undershoot of PYTHIA 8 A2 slightly better trans-diff: best description by PYTHIA 8 Monash and A2 tunes EPOS not able to model the level of underlying event activity for higher p lead T PYTHIA 8 A14 used much for the hard process simulation in ATLAS predicts activity ∼ 10 % below the data → some re-tuning for 13 TeV event modelling may yield performance benefits 9 trans-max: models cluster together more tightly providing good description for
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