Hard And Soft QCD Physics In ATLAS

Transcription

1 Hard And Soft QCD Physics In ALAS Stefanie Adomeit on behalf of the ALAS collaboration International Conference on New Frontiers in Physics - Crete, June -6,

2 QCD Measurements in ALAS at LHC every kind of new physics will begin with a QCD process QCD processes are an important background to new physics Soft QCD low momentum transfer inelastic interactions at LHC are dominated by soft processes pqcd breaks down phenomenological models this talk: minimum bias and underlying event measurements of soft QCD observables Hard QCD large momentum transfer test LO and NLO pqcd prescriptions this talk: jet cross-section measurements, results on jet substructure studies Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, /

3 Soft QCD Physics in ALAS here is more than just a pair of interacting partons... Minimum Bias: events passing a minimum set of requirements making sure an inelastic interaction has occurred dominated by soft multiple scattering pile-up events (=overlay of multiple proton-proton interactions per bunch-crossing) are minimum bias like measurements of minimum bias observables: charged particle distributions [New J. Phys. 3 () 5333] and correlations [arxiv:3.3549], [arxiv:3.3], [arxiv:3.49] Underlying Event: additional parton interactions in hard processes due to beam remnants and rest of the proton constituents measurements of underlying event characteristics using tracks [Phys. Rev. D83 () ], calorimeter clusters [Eur.Phys.J. C7 () 636], track jets Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, 3 /

4 Soft QCD Measurements in ALAS datasets recorded at three different center of mass energies: s int. Luminosity inelastic interactions 7 ev 9 µb > 6.36 ev. µb ev 7 µb > 3 5 use of Minimum Bias rigger Scintillator disks (6 counters on each side of ALAS) sensitive to charged particles make sure inelastic interaction has occurred data are only corrected for detector efficiencies and resolution no model dependent corrections use different phase space regions with varying contribution from different effects (diffractive events, hadronisation,...) Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, 4 /

5 Minimum Bias: Charged-Particle Distributions Study kinematic properties of charged particles in minimum bias events. average charged-particle multiplicity per unit of rapidity for η = as a function of the centre-of-mass energy [New J. Phys. 3 () 5333] data tend to show higher particle multiplicities than predicted by the Monte Carlo models however, except for AMB the data/mc comparison only includes pre-lhc tunes better agreement between the models in phase-space regions with higher cut on minimum p (less contribution from diffractive processes) / dη η = dn ch /N ev p ALAS Data PYHIA 6 AMB PYHIA 6 MC9 PYHIA 6 DW PYHIA 8 PHOJE > MeV, n 3 ch p > 5 MeV, n 6 ch p > 5 MeV, n ch 4 s [GeV] Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, 5 /

6 Minimum Bias: Forward-Backward Correlations Soft processes tend to produce lower particle multiplicities with weaker correlations over a wider pseudorapidity distance. est the strength of soft and hard contributions by studying the ALAS Data s = 7 ev particle multiplicity correlation p correlation ρ n fb = (n f n f ) (n b n b ) (n f n f ) (n b n b ) FB correlation forward η backward η.5 p > MeV n ch η <.5 ρ p fb = ( f p f p )( b p b p ) ( f p f p ) ( b p b p ) of charged particles in different pseudorapidity bins in the forward and backward region [arxiv:3.3]. ALAS simulation s FB correlation = 7 ev forward η backward η Pythia MC9.5 p > MeV n ch η <.5 Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, 6 /

7 Minimum Bias: (Forward-Backward) Correlations forward-backward multiplicity correlation in symmetrically opposite η intervals / ratio of the 9 GeV results to 7 ev results [arxiv:3.3] FB multiplicity correlation MC / data ALAS.9.8 Data Pythia 6 MC9 Pythia 6 DW Pythia 6 Perugia.7 Pythia 6 AMBB Pythia 8 4C Herwig++ p > MeV.6 n ch η < s = 7 ev η η ratio ALAS Pythia 6 MC9 Pythia 6 DW Pythia 6 Perugia Pythia 6 AMBB FB multiplicity correlations Ratio 9 GeV / 7 ev > MeV Pythia 8 4C Data n ch η < η p MC description of shapes in general good, however there are discrepancies w.r.t. strength of correlation between data and MC (up to 5% for some tunes at 7 ev) long-range correlation considerably higher at 7 ev w.r.t. 9 GeV Further minimum bias related correlation studies in ALAS: wo-particle angular correlations [arxiv:3.3549] Azimuthal ordering of charged hadrons [arxiv:3.49] Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, 7 /

8 Underlying Event Measurements in ALAS study observables sensitive to UE activity in transverse region (6 < φ particle φ jet < w.r.t. leading object) various studies in ALAS using tracks [Phys. Rev. D83 () ] calorimeter clusters [Eur.Phys.J. C7 () 636] track jets [to be submitted to Phys.Rev. D.] study of charged particle distributions in the transverse and away region as a function of the leading jet p jet using track jets in addition the R-dependence of these observables has been studied sensitive to fluctuations in UE activity Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, 8 /

9 Underlying Event Measurements in ALAS mean value of N ch and its ratio for various R-values in the transverse region as a function of the leading jet p jet using track jets reasonable data/mc agreement for mean UE activity and fluctuations Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, 9 /

10 Hard QCD Physics in ALAS testing the Standard Model at the shortest distance scales large number of studies/publications in ALAS, this talk: cross-section measurements, studies on jet substructure further QCD measurements with jets in ALAS: D*+/- meson production in jets [Phys. Rev. D85 () 55] properties of jets measured from tracks [Phys.Rev. D84 () 54] dijet production with a veto on additional central jet activity [JHEP 9 () 53] dijet azimuthal decorrelations [Phys.Rev.Lett. 6 () 7] Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, /

11 Jet Reconstruction in ALAS Combination of 3 dimensional, noise-suppressed calorimeter clusters according to anti-k t clustering algorithm. based on min(p i,p j ) scaled distance ij = (y i y j ) + (φ i φ j ) measurement combine hardest objects first until all objects are separated by R ij > R infrared and collinear safe standard jet reconstruction algorithm in ALAS with R=.4/.6 Apply calibration constants to restore the JES. [Cacciari, Salam, Soyez, JHEP 84:63,8] correct for non-compensating calorimeters, inactive material, out-of-cone effects and pile-up uncertainty [ALAS-CONF--3]: <.5% for central jets (p = GeV), <9 (4)% in endcap (forward) region uncertainty: reduced uncertainty thanks to in-situ measurements Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, /

12 Inclusive And Dijet Cross-Section Probe NLO perturbative QCD in a large, new kinematic regime... using full dataset: L=37 pb [arxiv.697, to be published by PRD] dijet cross section measurement redone for full dataset: L=4.8 fb [ALAS CONF--] Covering a large kinematic Range: inclusive: p = GeV -.5 ev and y < 4.4 m up to 5 ev and y = y -y / < 4.4 dominant experimental systematic uncertainties from jet energy scale σ/dm dy* [pb/ev] d Systematic uncertainties NLOJE++ (C, µ=p exp(.3 y*)) Non pert. corr. 8. y* <.5 ( ) 6.5 y* <. ( ) 4. y* <.5 ( ).5 y* <. ( ) y* <.5 ( ) 3 ALAS Preliminary anti k t jets, R =.4 s = 7 ev, L dt = 4.8 fb Data [ev] m Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, /

13 Inclusive and Dijet Cross-Section comparison to theoretical NLOJE++ predictions (with C, MSW 8, NNPDF., and HERAPDF.5) good data/mc agreement in central region, some overall shifts in forward region Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, 3 /

14 Inclusive and Dijet Cross-Section comparison to PowHeg good agreement between data and Monte-Carlo, deviations between data and PowHeg interfaced to Herwig Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, 4 /

15 Multi-Jet Cross-Section important background in searches for new physics: constrain uncertainty on multi-jet cross-section JES dominant source of experimental systematic uncertainty cross-section vs. inclusive jet multiplicity σ [pb] MC/Data 6 ALAS R=.4, L dt=.4 pb Data ( s=7 ev)+syst. ALPGEN+HERWIG AUE. PYHIA AMB.65 ALPGEN+PYHIA MC9. SHERPA Inclusive Jet Multiplicity data/mc agreement over the full multiplicity spectrum differential cross-section vs. leading jet p [pb/gev] d σ/d p MC/Data R=.4, L dt=.4 pb Data ( s=7 ev)+syst. ALAS ALPGEN+HERWIG AUE. PYHIA AMB.65 ALPGEN+PYHIA MC9. SHERPA.6 N jets p (leading jet) [GeV] differences between data and LO Monte-Carlo [Eur.Phys.J. C7 () 763] Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, 5 /

16 Inclusive and Dijet Cross-Section of b-jets b-jets represent substantial backgrounds in many searches for new physics. b-jes and b-jet tagging efficiency/purity dominant sources of experimental systematic uncertainty double differential inclusive cross-section dijet cross-section good agreement with PowHeg+Pythia discrepancies with good agreement with PowHeg+Pythia, [Eur.Phys.J.C 7 () 846] Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, 6 /

17 Jet Substructure Decay products of heavily boosted objects tend to be collimated in a small area and can thus be clustered into a single jet. Jets from two- or three body decays show a different subtructure w.r.t. quark and gluon initiated jets. Jet substructure studies thus provide a useful tool to suppress background from QCD jets when searching for new physics (Higgs). hey also provide a test of non-perturbative effects like fragmentation and hadronisation. ALAS results on jet substructure: Jet shapes [Phys.Rev. D83 () 53] Jet fragmentation [Eur.Phys.J.C 7 () 795] Jet mass and substructure variables [JHEP 5 () 8] Jets properties for boosted objects [ALAS-CONF--44] Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, 7 /

18 Jet Substructure Cambridge/Aachen Filtering reverse the clustering of large Cambridge-Aachen jets (R=.) in an iterative procedure by looking for hard (large mass drop) and symmetric splittings recluster remaining constituents with smaller R suitable for identification of two-body decays (H bb) normalized cross section as a function of mass [JHEP 5 () 8] better data/mc agreement after C/A filtering (right plot) [Phys. Rev. Lett., 4 (8)] GeV σ d σ dm ALAS Cambridge Aachen R=. 4 < p < 5 GeV N PV =, y < Data, Statistical Unc. otal Unc. Pythia Herwig++ L = 35 pb GeV σ d σ dm ALAS Cambridge Aachen R=. Split/Filtered with R >.3 qq 4 < p < 5 GeV N PV =, y < Data, Statistical Unc. otal Unc. Pythia Herwig++ L = 35 pb MC / Data Jet Mass [GeV] MC / Data Jet Mass [GeV] Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, 8 /

19 d Jet Substructure - Some Further Variables... GeV d σ d σ MC/Data.8 ALAS Data, L = 35 pb anti k t R=. 3 < p < 4 GeV N PV =, y < Statistical unc. otal unc. Pythia Herwig++ d [GeV] splitting scale: kinematic threshold for breaking jet into subjets [JHEP 5 () 8] d σ σ dτ MC / Data Data, L = 35pb Statistical Unc. otal Unc. Pythia Herwig++ ALAS N PV Cambridge-Aachen R=. jets.5 3 < p < 4 GeV =, y < N-subjettiness τ N-subjettiness: how well can the jet be described by n subjets d N d W N ALAS preliminary Data, L=35 pb Pythia AMB Pythia PERUGIA Herwig++.4. Anti k t jets, R =.6 p > 3 GeV η < MC/Data Width width: p -weighted distance of constituents to jet axis [ALAS-CONF--44] variables show reasonable data/mc agreement d N d E N MC/Data Data, L=35 pb Pythia AMB Pythia PERUGIA Herwig++.4. Anti k t jets, R =.6 p > 3 GeV η <.7 M > GeV ALAS preliminary Eccentricity eccentricity: deviation of jet profile from circle Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, 9 /

20 Conclusions Modelling of Soft Processes: several observables sensitive to minimum bias/underlying event activity have been investigated these studies have been carried out for different center of mass energies, in different phase space regions data/mc comparison shows there is still need to improve on the modelling of soft QCD processes Cross-Section: test pqcd in new kinematic regime very good agreement between data and NLO pqcd calculations Jet Substructure: exploit the substructure of jets to reduce background from quark-/gluon initiated jets in searches for new physics several discriminating variables have been tested in recent studies: reasonable data/mc agreement Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, /

21 Backup: Azimuthal Ordering of Charged Hadrons Modified Lund String Model: Formation of a helix-like structured gluon field at the end of the parton cascade leads to observable effects in the azimuthal ordering of direct hadrons. he azimuthal ordering of hadrons should be observable with the help of a power spectrum defined according to the expected structure of the helix field: S E (ω) = N ev S η(ξ) = N ev n ch n exp ( i ( )) ξη j φ j event ch j n ch n exp ( i ( )) ωx j φ j event ch j k<j X j =.5E j + k= E k Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, /

22 Backup: Azimuthal Ordering of Charged Hadrons S η (ξ).. ALAS s = 7 ev S η (ξ).8.6 n ch >, max(p )< GeV p >5 MeV, η <.5 Data PHOJE.4 PYHIA8 4C PYHIA6 AMBb. n ch >, max(p )< GeV p > MeV, η <.5 Data PHOJE PYHIA8 4C PYHIA6 AMBb HERWIG++ UE ALAS s = 7 ev HERWIG++ UE ξ ξ phase space is dominated by the production of low p particle hadronisation effects more evident MC predicts too little correlation low p depleted sample non-diffractive processes suppressed MC overestimates correlations [arxiv:3.49] Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, /

23 Backup: wo-particle Angular Correlations Correlations between final-state particles can indicate a common mechanism of particle production. foreground distribution F(n ch, η, φ): particle pairs from same event background correlation B(n ch, η, φ): particle pairs from different events two-particle angular correlation function: R( η, φ) = (n ch )F (n ch, η, φ) ch B(n ch, η, φ) n ch ch complex structure, various components reflect the contributions from different underlying processes [arxiv:3.3549] Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, 3 /

24 Backup: wo-particle Angular Correlations R( η) two-particle correlation function obtained by integrating the foreground and background distributions over φ between and π R( φ) two-particle correlation function obtained by integrating the foreground and background distributions over η between and R( η) 6 4 s = 7 ev, n ch < φ < π ALAS Data Pythia8 4C Pythia6 DW Pythia6 AMBB Pythia6 Perugia Herwig++ R( φ) 6 s = 7 ev, n ch Data Pythia8 4C 5 4 < η < ALAS Pythia6 DW Pythia6 AMBB Pythia6 Perugia 3 Herwig++ MC R( η) Data R( η) η MC R( φ) Data R( φ) 3 4 many of the Monte Carlo tunes reproduce the general features of the two-particle correlation function none of them provide a good quantitative description of the strength of the correlations [arxiv:3.3549] Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, 4 / φ

25 Backup: Cambridge/Aachen Filtering + Pile-Up Splitting and filtering procedure reduces the effective area of large jets and is therefore expected to reduce sensitivity to pile-up. impact of pile-up for anti-k jets as a function of R impact of pile-up for C/A jets R=., before and after filtering Mean Jet Mass [GeV] / PV 6 ALAS anti-k t jets, p - L = 35 pb > 3 GeV, y < R=.: d m / d N PV = 3. (. R=.6: d m / d N PV =.7 (. R=.4: d m / d N PV =. ( [JHEP 5 () 8] N pv Mean Jet Mass [GeV] / PV 6 ALAS Before Splitting/Filtering L = 35 pb dm dn PV =.9 ( dm dn PV dm dn PV.3 GeV =. ( = 4. (. GeV. GeV After Splitting/Filtering After Splitting Only Cambridge-Aachen R=. jets Split/Filtered with R qq p > 3 GeV, y < > N PV Stefanie Adomeit (LMU Munich) Hard And Soft QCD Physics In ALAS June -6, 5 /