Measurement of Underlying Event Observables with the ATLAS detector

Transcription

1 Measurement of Underlying Event Observables with the ALAS detector Róbert Astaloš (Comenius University Bratislava) on behalf of the ALAS Collaboration LHC 6 VIII International Worksho on Multile Partonic Interactions at the LHC San Cristóbal de las Casas, Chiaas, Mexico, 8 November - December 6 November 8, 6

2 Overview Measurement of charged-article distributions sensitive to underlying event in s = 3 ev roton-roton collisions with the ALAS detector at the LHC Preliminary results Measurement of event-shae observables in Z l + l events in collisions at s = 7 ev with the ALAS detector at the LHC Eur. Phys. J. C. (6) 76:375, arxiv:6.898

3 Motivation Underlying Event = soft rocesses unavoidably accomanying hard arton-arton scatterings in collisions with a high momentum transfer interactions between roton 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 couling strength diverges and ertubative methods of QCD lose redictivity described by henomenological models, imlemented in MC event generators contain many free arameters which are needed to be constrained by measurements. 3

4 Measurement of Underlying Event η, ϕ lane divided into regions around leading (the highest ) object (track, calo. cluster, jet...): ϕ < 6 - toward 6 < ϕ < - transverse ϕ > - away towards and away regions dominated by article roduction from the hard rocess relatively insensitive to the softer UE transverse region more sensitive to UE φ transverse (min) 6 < φ < leading charged article towards φ < 6 away φ > φ transverse (max) 6 < φ < further subdivision of the observables on an event-by-event basis deending on which side of the event is more activity: trans-max: observables in the more-active transverse region (higher ) includes both MPI and hard-rocess contamination trans-min: observables in the less-active transverse region (lower ) most sensitive to MPI effects (edestal) trans-diff: difference of trans-max and trans-min clearest measure of hard-rocess contamination 4

5 Measured Observables Observable binned variables lead φ Descrition ransverse momentum of the leading charged article Absolute difference in article azimuthal angle from the leading article unbinned variables Nch /δηδφ Mean number of charged articles er unit η φ (in radians) /δηδφ Mean scalar sum of charged articles er unit η φ (in radians) δφ = π/3 for toward, away an transverse regions π/3 for the single-sided trans-min and trans-max regions π/n bins for each of the n bins equally-sized bins in φ distributions δη = 5 in all cases mostly deendences of these quantities on the lead : low high lead smooth transition: minimum bias hard scattering regime 5

6 Event and rack Selection s = 3 ev data taken in a secial configuration of the LHC: low beam currents, reduced beam focusing, roducing a low mean number of interactions er bunch (.3 µ.3) trigger: one or more MBS counters above treshold on either side of the detector integrated luminosity of.6 nb events: required to contain reconstructed vertex from tracks with > MeV required to contain at least one track with lead > GeV corrected to the article level, including a correction for leading article realignment 66 million data events assed the trigger and vertex selection track selection criteria: >.5 GeV; η <.5 6

7 Leading charged article and Angular distributions ] - [GeV lead / d dn ev 3 lead > GeV >.5 GeV, η <.5 PYHIA 8 A4 PYHIA 8 A ALAS s Preliminary - = 3 ev,.6 nb PYHIA 8 Monash Herwig7 Eos / δη δφ ch N >.5 GeV, η <.5 lead lead > GeV > GeV ALAS Preliminary - s = 3 ev,.6 nb PYHIA 8 A4 PYHIA 8 Monash Herwig7 Eos [GeV] / δη δφ Σ >.5 GeV, η <.5 lead lead > GeV > GeV ALAS Preliminary - s = 3 ev,.6 nb PYHIA 8 A4 PYHIA 8 Monash Herwig7 Eos /N ev 3 4 Model / lead [GeV] MC / MC / lead. > GeV.9.8 lead > GeV φ [degrees] MC / MC / lead. > GeV.9.8 lead > GeV φ [degrees] N ev vs lead : steely falling distribution with a change of sloe for lead 5 GeV broadly modelled by all generators, best descrition by EPOS and PYHIA 8 A4 lead > GeV lead > GeV transition from relatively isotroic minimum bias scattering to the emergence of hard artonic scattering structure and a dominant axis of energy flow, no clear best MC: more inclusive selection ( lead > GeV) EPOS hard-scattering selection ( lead > GeV) HERWIG7 and Pythia 8 Monash 7

8 N ch and densities in azimuthal regions / δη δφ ch N.5.5 >.5 GeV, η <.5 ALAS Preliminary s = 3 ev,.6 nb - [GeV] / δη δφ Σ >.5 GeV, η <.5 owards region ransverse region Away region ALAS Preliminary s = 3 ev,.6 nb owards region ransverse region Away region lead [GeV] lead [GeV] general shae: first very raid rise in activity 3 regions not strongly distinguished abrut transition at lead 5 GeV, above it distinct behavior of 3 regions different shae of the transverse region: almost comletely lateaus after lead 5 GeV hard rocess dominates the towards and away regions, which continue to increase in activity as the hard rocess scale grows, but transverse region is relatively unaffected lead > 7 GeV: away region with highest multilicity, desite not containing lead the towards region is the most active by for all lead values track 8

9 densities in trans-min/max/diff regions [GeV] / δη δφ Σ.6.4. rans-min region >.5 GeV, η <.5 lead > GeV ALAS s Preliminary - = 3 ev,.6 nb [GeV] / δη δφ.8 Σ 3.5 rans-max region >.5 GeV, η <.5 lead > GeV ALAS s Preliminary - = 3 ev,.6 nb [GeV] / δη δφ.5 Σ.6.4. rans-diff region >.5 GeV, η <.5 lead > GeV ALAS s Preliminary - = 3 ev,.6 nb PYHIA 8 A4 PYHIA 8 A PYHIA 8 Monash Herwig7 Eos.5 PYHIA 8 A4 PYHIA 8 A PYHIA 8 Monash Herwig7 Eos.6.4. PYHIA 8 A4 PYHIA 8 A PYHIA 8 Monash Herwig7 Eos Model /.8 Model /.8 Model / lead [GeV] lead [GeV] trans-min: best descrition by PYHIA 8 Monash and Herwig7 (in the lateau region) PYHIA 8 A (mild but broad undershoot extending u to lead GeV) and Herwig7 (severe undershoot for lead < 5 GeV) mismodel the transition trans-max: similar, undershoot of PYHIA 8 A slightly better trans-diff: best descrition by PYHIA 8 Monash and A tunes lead [GeV] EPOS not able to model the level of underlying event activity for higher lead PYHIA 8 A4 used much for the hard rocess simulation in ALAS redicts activity % below the data some re-tuning for 3 ev event modelling may yield erformance benefits 9

10 N ch densities in trans-min/max/diff regions.6.4 / δη δφ ch N.4..8 rans-min region >.5 GeV, η <.5 lead > GeV ALAS s Preliminary - = 3 ev,.6 nb / δη δφ ch N rans-max region >.5 GeV, η <.5 lead > GeV ALAS s Preliminary - = 3 ev,.6 nb / δη δφ ch N rans-diff region >.5 GeV, η <.5 lead > GeV ALAS s Preliminary - = 3 ev,.6 nb PYHIA 8 A4 PYHIA 8 A PYHIA 8 Monash Herwig7 Eos.6.4. PYHIA 8 A4 PYHIA 8 A PYHIA 8 Monash Herwig7 Eos.3. PYHIA 8 A4 PYHIA 8 A PYHIA 8 Monash Herwig7 Eos Model /.8 Model /.8 Model / lead [GeV] lead [GeV] lead [GeV] trans-min: same as in case of trans-max: models cluster together more tightly roviding good descrition for > GeV excet EPOS lead trans-diff: mostly flat % overshoots from all models excet EPOS no obvious best model for all observables: PYHIA 8 Monash agrees well excet of trans-diff N ch density & Herwig7 has comarable erformance for lead > 5 GeV

11 Event shae observables in Z l + l events at 7 ev Event Shaes = observables that describe the atterns, correlations, and origins of the energy flow in an interaction sensitive to UE roerties quantities that are exerimentally easy to access enable detailed tests of henomenological QCD models inut for tuning MC generators ratios of final state observables reduced sensitivity to theoretical and exerimental uncertainties events containing Z e + e or Z µ + µ Z -boson without colour charge does not affect hadronic activity in the collision observables calculated using charged articles excluding the Z -boson decay roducts observables measured in different ranges of the Z -boson transverse momentum (l + l ): 6; 6 ; 5; 5 GeV small (l + l ) values low jet activity from the hard rocess high sensitivity to UE high (l + l ) values at least one high jet recoiling against the l + l system reasonably described by ertubative calculations of the hard rocess

12 Event shae observables Normalized distributions: (/N ev )dn/do N ev - number of all selected events O are following observables: N ch charged article multilicity scalar sum of transverse momenta of selected charged articles in the event he Beam thrust B = e η sum over all selected charged articles of transverse momentum weighted by raidity contributions from forward and backward articles suressed and B have different sensitivities to hadronic activity from initial-state radiation

13 ransverse hrust, Sherocity S and F-arameter S = π 4 = max ˆn min n=(n x,n y,) i ˆn i i i,i n i i,i O dijet isotroic /π S F F = λ λ ; λ < λ - two eigenvalues of the transverse momentum tensor M lin : M lin = ( ) x,i x,i y,i,i i x,i y,i y,i the sum over the,i of all charged articles in the event ˆn - the unit vector of the thrust axis maximizing the exression found iteratively n - vector in the transverse lane which minimises the exression coincides with one of the transverse momentum vectors,i 3

14 Event and rack Selection s = 7 ev data collected in requiring a Z boson candidate decaying to an e + e or µ + µ air - restricted to a subsamle with mean number of collisions er bunch crossing 5 and not > 7 to reduce PU (integrated luminosity. fb ) events were required to contain a rimary vertex the vertex with the highest ( trk ) to reject events from cosmic-ray muons and other non-collision backgr. vertex must have at least one track with > 4 MeV selected electrons and muons were required > GeV and η <.4 for electrons.37 < η <.5 also excluded assive detector material in ECAL Z l + l signal events, when m l + l [66, 6] GeV.6 5 events in electron channel and 4. 5 in muon channel assed track selection criteria: >.5 GeV; η <.5 4

15 Corrections Leton track removal: e ± can interact with material in front of the ECAL bremsstrahlung & hoton conversion multile tracks tracks not used if they fell inside a cone of R e,trk =. around any selected e ± alied also to the muon channel to treat two channels as similarly as ossible Pile-u correction: Hit Backsace Once More (HBOM) aroach: arxiv:.54, New J. Phys. 3, 5333 () Background treatment: only for multijet events with misidentified leton candidates estimated from data Unfolding: O corrected for contributions from non-rimary articles, detector efficiency and resolution effects, Bayesian aroach, PYHIA 8 and SHERPA 5

16 ransverse hrust, Sherocity and F-Parameter /Nev dn/d 6 4 ALAS s = 7 ev,. fb (ee) 6 GeV (ee) 6 GeV (ee) 5 GeV (ee) > 5 GeV /Nev dn/ds ALAS s = 7 ev,. fb (ee) 6 GeV (ee) 6 GeV (ee) 5 GeV (ee) > 5 GeV S /Nev dn/df ALAS s = 7 ev,. fb (ee) 6 GeV (ee) 6 GeV (ee) 5 GeV (ee) > 5 GeV F same results in the muon channel lower (l + l ) ranges: sherical events revalence (l + l ) > GeV: shift to less sherical events (l + l ) 6

17 Observables deending exlicitly on N ch /Nev dn/dn ch /Nev dn/db [GeV ].3.. ALAS s = 7 ev,. fb (µµ) 6 GeV (µµ) 6 GeV (µµ) 5 GeV (µµ) > 5 GeV N ch.8 ALAS s = 7 ev,. fb.6.4. (µµ) 6 GeV (µµ) 6 GeV (µµ) 5 GeV (µµ) > 5 GeV B [GeV] /Nev dn/d [GeV ].3.. ALAS s = 7 ev,. fb (µµ) 6 GeV (µµ) 6 GeV (µµ) 5 GeV (µµ) > 5 GeV [GeV] same results in the electron channel as (l + l ) rises, i.e. as recoiling jets emerge, N ch increases, as do and beam thrust (l + l ) 7

18 Event Shae Observables for (l + l ) < 6 GeV /Nev dn/d 6 s = 7 ev,. fb, (ee) 6 GeV ALAS /Nev dn/ds s = 7 ev,. fb, (ee) 6 GeV ALAS /Nev dn/df s = 7 ev,. fb, (ee) 6 GeV ALAS 4.4 Pythia 8. Shera. Herwig 7..4 Pythia 8. Shera. Herwig 7..4 Pythia 8. Shera. Herwig 7. MC/.6 MC/.6 MC/ S F (l + l ) < 6 GeV bin: exected to be characterised by low jet activity articularly sensitive to UE characteristics PYHIA 8 shows very good agreement with the data very similar results in the muon channel 8

19 Observables deending on N ch for (l + l ) < 6 GeV /Nev dn/dn ch.4.3. s = 7 ev,. fb, (ee) 6 GeV ALAS Pythia 8. Shera. Herwig 7. /Nev dn/d [GeV ].4.3. s = 7 ev,. fb, (ee) 6 GeV ALAS Pythia 8. Shera. Herwig 7. /Nev dn/db [GeV ]. s = 7 ev,. fb, (ee) 6 GeV ALAS Pythia 8. Shera. Herwig MC/.6 MC/.6 MC/ N ch [GeV] B [GeV] none of the generators succeeding fully, very similar results in the muon channel best agreement for HERWIG7, followed by PYHIA 8 low N ch and values: challenging region for all 3 generators sensitive to the way beam-remnant interactions are modelled in the MC better agreement for B where tracks with larger η trk are suressed 9

20 Observables deending on N ch for (l + l ) > 5GeV. /Nev dn/dn ch.3. s = 7 ev,. fb, (ee) > 5 GeV ALAS Pythia 8. Shera. Herwig 7. /Nev dn/d [GeV ].. s = 7 ev,. fb, (ee) > 5 GeV ALAS Pythia 8. Shera. Herwig 7. /Nev dn/db [GeV ] s = 7 ev,. fb, (ee) > 5 GeV ALAS Pythia 8. Shera. Herwig MC/.6 MC/.6 MC/ N ch [GeV] B [GeV] (l + l ) > 5 GeV bin: exected to contain at least one jet of high transverse momentum recoiling against the Z boson well described by the hard matrix element better agreement than for (l + l ) < 6 GeV, but still significant deviation best agreement for HERWIG7, followed by PYHIA 8

21 Summary Underlying event at s = 3 ev: no obvious best model for all observables: PYHIA 8 Monash agrees well excet of trans-diff N ch density & Herwig7 comarable for lead > 5 GeV trans-diff N ch density: mostly flat % overshoots from all models but EPOS EPOS articular discreant features for higher lead PYHIA 8 A4 redicts activity 5 % below the data excet trans-diff N ch density Event shae observables in Z l + l events at s = 7 ev: better redictions of all 3 MC generators at high (l + l ) and for the observables that are less sensitive to the number of charged articles in the event (transverse thrust, sherocity, and F-arameter) PYHIA 8 best significant differences from data at low values of N ch, and beam thrust in certain (l + l ) regions