DETECTOR . James Robinson 1 1 DESY Introduction ATLAS has measured - PowerPoint PPT Presentation

MPI@LHC 2015, on behalf of the ATLAS Collaboration 23nd November 2015 STUDIES OF THE UNDERLYING EVENT AND PARTICLE PRODUCTION WITH THE ATLAS DETECTOR . James Robinson 1 1 DESY

Introduction ATLAS has measured particle production and the underlying event PRD 91 (2015) 032004 PLB 749 (2015) 242-61 Exclusive dilepton production accepted by PLB Dijet production with large rapidity gaps EPJC 74 (2014) 3195 Underlying event in inclusive Z -boson production EPJC 74 (2014) 2965 Underlying event in jet events ATL-PHYS-PUB-2015-019 13 TeV! Detector-level underlying event distributions Too much to discuss in full, so I will show only most recent results: > using various different hard processes J.E.M. Robinson | Underlying event and particle production | 23/11/15 | Page 1/23 > at several centre-of-mass energies Transverse polarisation of Λ and ¯ Λ hyperons

Underlying event

What is the underlying event? Any hadronic activity not associated with hard scattering process > Unavoidable background to collision events Typically modelled with > Multiple parton interactions > Initial/final-state radiation > Colour reconnection with beam remnants Impossible to unambiguously assign particles to hard scatter or UE > Measurements must not be dependent on details of model used J.E.M. Robinson | Underlying event | 23/11/15 | Page 2/23 > Non-perturbative effects dominate → not well-predicted

Underlying event topology > Identify a “hard scatter” using a reference object (eg. jet/ Z /track) > Define azimuthal regions with respect to this leading object > Toward and transverse regions most sensitive to the underlying event > Transverse region can be further divided into trans-max and trans-min depending on the amount of activity J.E.M. Robinson | Underlying event | 23/11/15 | Page 3/23 > High p T recoil important in away region → perturbative QCD

Underlying event observables Reconstruct kinematics: calorimeter deposits and charged tracks Densities and averages Particle spectra J.E.M. Robinson | Underlying event | 23/11/15 | Page 4/23 > Average p T of charged particles: ⟨ p T ⟩ > Number density of charged particles: N ch / δηδϕ > p T density of charged particles: ∑ p T / δηδϕ > E T density of all particles: ∑ E T / δηδϕ > Charged particle p T spectrum > Charged particle multiplicity spectrum

Underlying event in 7 TeV jet events EPJC 74 (2014) 2965 lessens sensitivity to pQCD > Veto on extra hard activity trans-min regions flat in p T > Both trans-max and > Colour connection to jet? > Increasing activity for treat UE activity as constant J.E.M. Robinson | Underlying event | 23/11/15 | Page 5/23 Exactly 2 jets ≥ 1 jet > [GeV] > [GeV] Data 2010 - trans-max Data 2010 - trans-max ATLAS 2.5 ATLAS 5 Pythia6 Perugia 2011 - trans-max Pythia6 Perugia 2011 - trans-max L = 37 pb -1 , s = 7 TeV L = 37 pb -1 , s = 7 TeV Data 2010 - trans-min Data 2010 - trans-min int int Pythia6 Perugia 2011 - trans-min Pythia6 Perugia 2011 - trans-min Exclusive dijet Data 2010 - trans-diff Data 2010 - trans-diff 2 4 Pythia6 Perugia 2011 - trans-diff Pythia6 Perugia 2011 - trans-diff φ φ δ Inclusive jet δ 1.5 η η 3 δ δ / / T T 1 2 p p ∑ ∑ < < 1 0.5 1.2 1.2 MC/Data MC/Data 1.1 1.1 1 1 0.9 0.9 0.8 0.8 20 30 40 100 200 300 20 30 40 100 200 300 p lead [GeV] p lead [GeV] T T > Trans-min flat (at hard enough scales) → trans-max → pQCD

Underlying event in 7 TeV jet events EPJC 74 (2014) 2965 > Compare to different Monte Carlo models and tunes J.E.M. Robinson | Underlying event | 23/11/15 | Page 6/23 Inclusive jet selection Exclusive dijet selection > [GeV] > [GeV] Data 2010 ATLAS Data 2010 ATLAS 6 Pythia8 AU2 CT10 Pythia8 AU2 CT10 4 Pythia6 Perugia 2011 L = 37 pb -1 , s = 7 TeV Pythia6 Perugia 2011 L = 37 pb -1 , s = 7 TeV int int Pythia6 DW Pythia6 DW Transverse region Herwig++ UE7-2 MRST LO** Herwig++ UE7-2 MRST LO** 5 3.5 Herwig + Jimmy AUET2 LO** Herwig + Jimmy AUET2 LO** Exclusive dijet φ φ Alpgen + Herwig + Jimmy AUET1 Alpgen + Herwig + Jimmy AUET1 δ δ Powheg + Pythia6 Perugia 2011 Powheg + Pythia6 Perugia 2011 η η 3 4 δ δ / / cent ch+neut cent ch+neut 2.5 3 2 2 Transverse region 1.5 T T E E ch(neut) Inclusive jet ch(neut) p > 500(200) MeV, | η ch,neut | < 2.5 p > 500(200) MeV, | η ch,neut | < 2.5 ∑ ∑ < < 1.2 1.2 MC/Data MC/Data 1.1 1.1 1 1 0.9 0.9 0.8 0.8 20 30 40 100 200 300 20 30 40 100 200 300 p lead [GeV] p lead [GeV] T T > Similar distributions for ∑ E T from calorimeter clusters > Best agreement given by PYTHIA 6 with Perugia 2011 tune

Underlying event in 7 TeV Drell-Yan events EPJC 74 (2014) 3195 sensitive to higher N jets > Toward and transverse regions > Away region dominated by Z +j > Tune non-perturbative models > Measurement of toward UE! J.E.M. Robinson | Underlying event | 23/11/15 | Page 7/23 3 > > 1.2 ATLAS s = 7 TeV, 4.6 fb -1 Toward region ATLAS s = 7 TeV, 4.6 fb -1 Away region φ φ δ δ 1.1 η η 2.5 δ δ / / 1 ch ch 2 <N <N 0.9 0.8 1.5 0.7 Alpgen+Herwig+Jimmy AUET2 Alpgen+Herwig+Jimmy AUET2 Data Data 1 Sherpa Sherpa Pythia8 AU2 Pythia8 AU2 0.6 Pythia6 Perugia2011C Pythia6 Perugia2011C Powheg+Pythia8 AU2 Herwig++ UE-EE-3 Powheg+Pythia8 AU2 Herwig++ UE-EE-3 MC/Data MC/Data 1.15 0 20 40 60 80 100 120 140 160 180 200 1.15 0 20 40 60 80 100 120 140 160 180 200 1.1 1.1 1.05 1.05 1 1 0.95 0.95 0.9 0.9 0.85 0.85 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 p Z [GeV] p Z [GeV] T T > 1.4 ATLAS s = 7 TeV, 4.6 fb -1 Transverse region φ δ 1.3 η δ 1.2 / 1.1 ch <N 1 0.9 0.8 Alpgen+Herwig+Jimmy AUET2 Data 0.7 with low p T region Sherpa Pythia8 AU2 Pythia6 Perugia2011C 0.6 Powheg+Pythia8 AU2 Herwig++ UE-EE-3 MC/Data 1.15 0 20 40 60 80 100 120 140 160 180 200 1.1 1.05 1 0.95 0.9 0.85 0 20 40 60 80 100 120 140 160 180 200 p Z [GeV] T

J.E.M. Robinson | Underlying event | 23/11/15 | Page 8/23 EPJC 74 (2014) 3195 > Provide further discrimination between Monte Carlo models Z -boson selection Jet selection Particle p T and multiplicity φ φ 1 ATLAS ATLAS s = 7 TeV, 4.6 fb -1 Trans-min region δ δ 1 L = 37 pb -1 , s = 7 TeV η η 20 GeV < p Z < 50 GeV int ev δ δ dN / T dN Dressed Leptons / 10 -1 T 10 -1 p ch dN ∑ Data Transverse region d Pythia8 AU2 10 -2 -2 10 ev Sherpa Inclusive jet 1 1 N N Pythia6 Perugia2011C -3 10 Data 2010 Powheg+Pythia8 AU2 10 -3 Pythia8 AU2 CT10 Herwig++ UE-EE-3 Pythia6 Perugia 2011 10 -4 Alpgen+Herwig+Jimmy AUET2 Pythia6 DW Herwig++ UE7-2 MRST LO** MC/Data 1.8 10 -4 Herwig + Jimmy AUET2 LO** Alpgen + Herwig + Jimmy AUET1 1.6 Powheg + Pythia6 Perugia 2011 1.4 1.4 0 0.5 1 1.5 2 2.5 3 3.5 1.2 MC/Data 1.2 1 0.8 1 0.6 0.8 0.4 0.6 0 0.5 1 1.5 2 2.5 3 3.5 10 -1 1 10 p / δ η δ φ [GeV] N / δ η δ φ ∑ ch T > Double differential charged particle multiplicity and p T spectra > Current models do not describe these observables well

Universality of MPI model EPJC 74 (2014) 3195 > How well does the MPI model extrapolate to higher energies?… > Good agreement between jet and Z -boson measurements J.E.M. Robinson | Underlying event | 23/11/15 | Page 9/23 > Compare UE measurements with different hard scatters > Qualitative test of MPI universality in different hard processes 1.6 > ATLAS s = 7 TeV Transverse region φ 10 φ ATLAS δ s = 7 TeV Trans-min region δ 1.4 Data 2011: Z events (4.6 fb -1 ) η ev Data 2011: Z events (4.6 fb -1 ) η δ Data 2010: Minimum bias and jet events (168 µ -1 , 37 pb -1 ) dN / δ Data 2010: Jet events (137 pb -1 ) / 1.2 ch 1 Z, leadjet 20 GeV < p < 60 GeV dN ch T <N ev 1 1 N 10 -1 0.8 1.2 > φ δ η 1 δ 0.6 / 10 -2 ch 0.8 <N 0.6 0.4 Minimum bias events 0.4 -3 10 Z events Jet events 0.2 0.2 Jet events Z events 0 5 10 15 20 25 30 35 40 45 50 p [GeV] T 10 -4 0 1 2 3 4 5 6 0 50 100 150 200 250 300 350 400 450 500 leadjet p Z or p leadtrack or p [GeV] N / δ η δ φ T T T ch → especially for trans-min (most sensitive to MPI)

Leading track underlying event at 13 TeV ATL-PHYS-PUB-2015-019 > Good agreement with data in toward region > Detector-level only (preliminary result) J.E.M. Robinson | Underlying event | 23/11/15 | Page 10/23 7 > > [GeV] ATLAS Preliminary φ 2.5 Toward region ATLAS Preliminary Toward region d 6 η s = 13 TeV s = 13 TeV /d p > 0.5 GeV, | |< 2.5 p > 0.5 GeV, | η |< 2.5 η 2 5 T ch T φ N lead d p lead > 1 GeV p > 1 GeV 2 η T T 4 <d 1.5 /d T p 3 Σ 1 2 <d 2 DATA (uncorrected) EPOS DATA (uncorrected) EPOS 0.5 1 PYTHIA 8 A14 PYTHIA 8 A2 PYTHIA 8 A14 PYTHIA 8 A2 HERWIG++ EE5 PYTHIA 8 Monash HERWIG++ EE5 PYTHIA 8 Monash 0 0 5 10 15 20 25 30 5 10 15 20 25 30 1.3 1.3 1.2 1.2 MC/Data MC/Data 1.1 1.1 1 1 0.9 0.9 0.8 0.8 0.7 0.7 5 10 15 20 25 30 5 10 15 20 25 30 p lead [GeV] p lead [GeV] T T