Exploring the extremes of the Underlying Event Peter Skands (Monash U), with T. Martin & S. Farrington (Warwick U) pp 13000 GeV Strange things have been observed in ) MPI Number of parton-parton interactions Prob(n high-multiplicity minimum-bias events 1 ND UE ( p =20) Associated with small impact parameters <MB> T Z <UE> 1 − 10 tt Jet trigger → bias to small b (pedestal effect) Extreme UE ( → <UE> is larger than <MB>) − 2 10 Complementary studies can be carried out in UE. Fluctuates from event to event. 3 − 10 Studies as function of UE N ch (density) V I N C I A R O O T analogous to MB studies vs N ch (density) Pythia 8.227 Monash 2013 4 − 10 0 10 20 n MPI Based on Eur.Phys.J. C76 (2016) no.5, 299, arXiv:1603.05298 Collective effects in small collision systems CERN, June 2017
MIN-BIAS VS THE UNDERLYING EVENT ๏ Tautology: a jet trigger provides a bias (ed subsample of min-bias) ๏ Pedestal effect: <MB> <UE> • Events with a hard jet trigger are accompanied by a higher plateau of ambient activity (extending far from the jet cores) • MPI: interpreted as a biasing effect. Small impact parameters → larger matter overlaps → more MPI → higher chances for a hard one (and the trigger throws out any events that didn’t have at least one) 2 Peter Skands Monash University
DEFINING THE UNDERLYING EVENT ๏ Jet trigger (or other hard probe but want high statistics) • Consider event in transverse plane (x,y) Jet φ =0 - Δφ | Δφ |<60° + Δφ Towards Operational Look at 90 degrees to definition of UE Transverse 60°<| Δφ |<120° leading jet direction: = “Transverse Region” | Δφ |>120° Away Note: if your hard probe is a Z, you can also look in the “Towards” region If your hard probe is a ttbar pair, can (subtracting the decay leptons). do “Swiss Cheese” or jet median UE in Drell-Yan studies by ATLAS and approach (also generally applicable) Recoiling Jet CMS Cacciari, Salam, Sapeta, “On the characterisation of the underlying event,” JHEP 04(2010)065,arXiv:0912.4926 [hep-ph]. 3 Peter Skands Monash University
THE (AVERAGE) UE ๏ By now lots of measurements of the average properties of the underlying event, and of its (non)-evolution * with p Ttrigger note: PHOJET does not describe the rise of the UE NB: trigger can be: Hardest track if you don’t have (good) calorimetry Hardest track-jet Hardest calo-jet more inclusive … *: radiation spillover into the UE does provide a (slow) evolution with p Ttrigger 4 Peter Skands Monash University
FLUCTUATIONS OF THE UE ๏ ATLAS: UE fluctuates a lot from event to event mean level • (similarly to the large width of the N ch distribution in min-bias) Std Dev. Implies that there are “quiet” events with N TRNS << <N> Does the UE in those events look like min-bias? or LEP? Implies that there are “extreme UE” events with N TRNS >> <N> Does the UE in those events exhibit same effects as high-N ch min-bias? 5 Peter Skands Monash University
STRANGENESS IN THE UE ๏ CMS: average strangeness (K0) as a function of trigger jet p T • 10% - 20% strangeness deficit also in UE (Ideally, should show ๏ strangeness fraction to avoid concluding that e.g., the PHOJET result is just due to strangeness; recall PHOJET was low on total N as well) Do extreme events have even larger deficits? What about quiet ones? What about other PIDs? 6 Peter Skands Monash University
FLIPPING THE AXES ๏ Instead of plotting UE plateau as function of trigger jet p T , • ⇨ Plot salient quantities (e.g., strangeness) as function of event-by- event UE level, for some window of trigger jet p T Eur.Phys.J. C76 (2016) 5, 299, arXiv:1603.05298 We propose a window just 50 > [Trans.] Pythia 8.210 Monash 45 above the turn-on of the Pythia 8.210 Monash + New CR EPOS 1.3 LHC Study along this axis 40 plateau; maximises rates and DIPSY NoSwing 35 DIPSY Rope minimises contamination of Inc. 30 < N the UE by radiation 25 20 10 GeV < p Ttrigger < 30 GeV Window 15 10 5 s = 13 TeV Note: N inc is ATLAS jargon for a particular Monash 1.4 0 5 10 15 20 25 30 35 combination of charged tracks and long- 1.2 MC 1 lived strange hadrons that they can 0.8 reconstruct well. Think of it as N ch . 0.6 35 0 5 10 15 20 25 30 35 [GeV] Leading Track-Jet p [GeV] T 7 Peter Skands Monash University
WHAT MIGHT YOU SEE? ๏ Bear in mind: models only represent a subset of the possibilities in nature Generator Tune h N Inc . i σ P YTHIA 8 Monash 24.7 12.5 Ev P YTHIA 8 Monash + New CR 25.5 12.6 Here is basically an 1 − 1/N 10 LHC 24.2 14.6 EPOS D IPSY NoSwing 21.3 12.2 D IPSY Rope 25.1 12.0 N ch spectrum 2 − 10 (but for the UE) 3 − 10 Pythia 8.210 Monash 4 − Pythia 8.210 Monash + New CR 10 EPOS 1.3 LHC Use average to define DIPSY NoSwing DIPSY Rope 5 − 10 AVERAGE UE LEVEL 10 Jet p < 30 GeV, s = 13 TeV ≤ T Monash 10 0 10 20 30 40 50 60 70 80 90 100 and R T = N / <N> MC 1 (analogous to the 0 10 20 30 40 50 60 70 80 90 100 KNO z variable) N [Trans.] Inc. 8 Peter Skands Monash University
THEORY: UNDER THE HOOD Number of MPI Impact parameter (relative to average MB) Extreme UE 25 2 > > MPI MPI <n > vs R <b > vs R higher <b> than MPI T MPI T <n <b min-bias but Monash 13 Monash 13 20 have hard jets Average 4C 4C 1.5 4Cx 4Cx UE AU2-CT6L1 AU2-CT6L1 Quiet 15 <MB> UE 1 Quiet ~ MB 10 UE <UE> 0.5 V I N C I A R O O T V I N C I A R O O T 5 Extreme Pythia 8.216 Pythia 8.216 0 0 1.4 1.4 1.2 1.2 Ratio Ratio 1 1 0.8 0.8 0.6 0.6 1 0.5 0 0.5 1 0.5 0 0.5 − − − − log (R ) log (R ) T T 10 10 9 Peter Skands Monash University
THE UE ANALOGUE OF <PT>(NCH) ๏ Rising trend in minimum-bias taken as indicative of collectivity; how about in UE? > [GeV][Trans.] RMS > [GeV][Trans.] 14 1.2 Pythia 8.210 Monash Pythia 8.210 Monash Pythia 8.210 Monash + New CR Pythia 8.210 Monash + New CR EPOS 1.3 LHC EPOS 1.3 LHC 12 1.1 DIPSY NoSwing DIPSY NoSwing DIPSY Rope DIPSY Rope 1 10 0.9 8 T 0.8 < mean p 6 0.7 T 4 < p 0.6 s = 13 TeV s = 13 TeV 2 Monash Monash 1.4 MC 1.2 MC 1.2 1 1 0.8 1 1 − − 2 10 1 2 3 4 2 10 1 2 3 4 × × R R T T <pT> RMS 10 Peter Skands Monash University
STRANGENESS ! ๏ Significant power to separate different physics mechanisms (b) > [Trans.] ) > [Trans.] 0.18 0.058 Pythia 8.210 Monash Pythia 8.210 Monash Pythia 8.210 Monash + New CR Pythia 8.210 Monash + New CR EPOS 1.3 LHC EPOS 1.3 LHC 0.056 0.16 DIPSY NoSwing DIPSY NoSwing DIPSY Rope DIPSY Rope 0.054 0.14 Inc. S 0 )/N(K )/N 0.052 0.12 s = 13 TeV S 0 < N(K 0.05 φ < N( 0.1 s = 13 TeV 0.048 0.08 0.046 Monash Monash 1.1 2 MC MC 1 1.5 0.9 1 0.8 1 1 − − 2 10 2 3 4 2 10 2 3 4 1 1 × × R R T T Kaons Phi mesons (b) 11 Peter Skands Monash University
IT’S EVEN MORE FUN WITH BARYONS 0.08 0.9 > [Trans.] ) > [Trans.] Pythia 8.210 Monash Pythia 8.210 Monash 0.075 Pythia 8.210 Monash + New CR Pythia 8.210 Monash + New CR 0.8 EPOS 1.3 LHC EPOS 1.3 LHC 0.07 DIPSY NoSwing DIPSY NoSwing DIPSY Rope DIPSY Rope 0.065 0.7 Inc. - K 0.06 )/N + )/N(K 0.6 0.055 p < N(p 0.05 p s = 13 TeV 0.5 < N(p 0.045 0.04 0.4 s = 13 TeV Monash Monash 1.6 1.4 MC MC 1.4 1.2 1.2 1 1 0.8 0.8 − 1 − 1 2 10 2 3 4 2 10 2 3 4 × 1 × 1 R R T T (a) (b) 0.7 ) > [Trans.] ) > [Trans.] 0.18 Pythia 8.210 Monash Pythia 8.210 Monash 0.65 Pythia 8.210 Monash + New CR Pythia 8.210 Monash + New CR EPOS 1.3 LHC EPOS 1.3 LHC 0.6 0.16 DIPSY NoSwing DIPSY NoSwing DIPSY Rope DIPSY Rope 0.55 0.14 S Λ 0 s = 13 TeV 0.5 )/N(K Λ )/N( 0.12 0.45 Λ + 0.4 Ξ Λ 0.1 - < N( Ξ s = 13 TeV 0.35 < N( 0.08 0.3 Monash Monash 2.5 MC MC 1.5 2 1.5 1 1 1 1 − − 2 10 1 2 3 4 2 10 1 2 3 4 × × R R T T 12 Peter Skands Monash University
SUMMARY / OUTLOOK ๏ The UE provides a complementary phase-space region to min-bias which could well exhibit similar phenomena as high-mult min-bias • Hard trigger biases selection to small impact parameters But can find “ultra-quiet” UE levels with even less activity / higher b, than MB (LEP-like?) ๏ Can explore events with “extreme” UE levels → collectivity? ๏ • Models based on different principles predict qualitatively different trends for the various particle ratios, as functions of the UE level NB: so far, we only studied particle multiplicities ( ratios) and spectra; particle ๏ correlations would provide additional information ๏ Work is ongoing in ATLAS; but limited by PID capabilities • CMS similar? (but not aware of any measurement underway?) ๏ ALICE and LHCb have the PID to do it • What is the status of UE studies? Jet or hard-track triggers? Other hard trigger probes? ๏ 13 Peter Skands Monash University
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