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Bryan Webber Cavendish Laboratory University of Cambridge

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Bryan Webber Cavendish Laboratory University of Cambridge QCD and Event Generation for the Large Hadron Collider Bryan Webber Cavendish Laboratory University of Cambridge Bryan Webber 1 Burg Liebenzell, Sept 2014 Multiparton Interaction Model (PYTHIA/JIMMY) For small pt min and high energy inclusive parton—parton cross section is larger than total proton—proton cross section. !UnderlyingMore than one parton—parton scatter Event per proton—proton MultipleNeed a model parton of spatial interactions distribution inwithin same proton collision • ! Perturbation theory gives n-scatter distributions LHC Simulations 3 Bryan Webber ✤ Depends on density profile of proton • Assume QCD 2-to-2 secondary collisions ✤ Need cutoff at low pT • Need to model colour flow ✤ Colour reconnections are necessary Bryan Webber 2 Burg Liebenzell, Sept 2014 Underlying Event 3 Data (2012) ATLAS Preliminary Data (2010) - trans-max ATLAS Preliminary PYTHIA6 AUET2B CTEQ6L1 4 PYTHIA6 AUET2B CTEQ6L1 - trans-max PYTHIA6 DW -1 Data (2010) - trans-min -1 Lint = 37 pb , s = 7 TeV Lint = 37 pb , s = 7 TeV 2.5 Pythia8 AU2 CT10 PYTHIA6 AUET2B CTEQ6L1 - trans-min HERWIG/JIMMY AUET2 LO** Data (2010) - trans-diff > [GeV] Herwig++ UE7000 > [GeV] 3 PYTHIA6 AUET2B CTEQ6L1 - trans-diff φ Alpgen + HERWIG/JIMMY AUET1 φ d 2 d η η 2 leading jet /d /d T 1.5 T p p ∑ ∑ 1 2 2 1 <d <d 0 jet jet jet jet 0.5 N ≥ 1 (p > 20 GeV, |y | < 2.8) N ≥ 1 (p > 20 GeV, |y | < 2.8) Transverse region jet T jet T ch ch ch ch Inclusive jet p > 500 MeV, |η | < 2.5 -1 Inclusive jet p > 500 MeV, |η | < 2.5 ∆⇤ ∆⇤ 0 T T 2 2 − 1.2 10 1.2 10 1.1 1.1 1 1 towards 0.9 0.9 MC/Data 0.8 MC/Data 0.8 ∆⇤ < 60⇤ | | 20 30 40 100 200 300 20 30 40 100 200 300 plead [GeV] plead [GeV] T T transverse transverse > 2 Data (2012) > 3 Data (2010) - trans-max 60⇤ < ∆⇤ < 120⇤ 60⇤ < ∆⇤ < 120⇤ φ PYTHIA6 AUET2B CTEQ6L1 ATLAS Preliminary φ PYTHIA6 AUET2B CTEQ6L1 - trans-max ATLAS Preliminary d 1.8 PYTHIA6 DW L = 37 pb-1, s = 7 TeV d Data (2010) - trans-min L = 37 pb-1, s = 7 TeV | | | | η int η int Pythia8 AU2 CT10 2.5 PYTHIA6 AUET2B CTEQ6L1 - trans-min /d 1.6 HERWIG/JIMMY AUET2 LO** /d Data (2010) - trans-diff ch Herwig++ UE7000 ch 2 PYTHIA6 AUET2B CTEQ6L1 - trans-diff Alpgen + HERWIG/JIMMY AUET1 N N 2 2 1.4 away 1.5 ∆⇤ > <d 1.2 <d 120⇤ 1 | | 1 0.5 0.8 jet jet 0 jet jet 0.6 N ≥ 1 (p > 20 GeV, |y | < 2.8) N ≥ 1 (p > 20 GeV, |y | < 2.8) Transverse region jet T jet T pch > 500 MeV, |ηch| < 2.5 -0.5 pch > 500 MeV, |ηch| < 2.5 0.4 Inclusive jet T Inclusive jet T 2 2 1.2 10 1.2 10 1.1 1.1 1 1 0.9 0.9 MC/Data 0.8 MC/Data 0.8 Figure 1: Definition of regions in the azimuthal angle with respect to the leading jet. The balancing20 30 40 parts 100 200 300 20 30 40 100 200 300 plead [GeV] plead [GeV] of the jet system are indicated with green arrows, compatible with the dominant dijet event topology. T T Multijet topologies, encountered in the inclusive jet event selection, are expected to contribute more ATLAS CONF-2012-164 substantially to the transverse regions than the geometry shown here. Bryan Webber Figure 2:3 Profiles of charged particle pT (topBurg row) Liebenzell, and charged Sept 2014 multiplicities (bottom row) against lead pT , for the inclusive jet event selection. The left column shows the result for the total transverse region in the ATLAS calorimeters, due to interactions with material upstream of theand calorimeters several MC and bendingmodels for comparison, with the data error bars indicating the statistical uncertainty and in the magnetic field. the shaded area showing the combined statistical and systematic uncertainty. The right column plots These detector-level objects have been identified [10] with true hadron-level quantities in terms of compare the trans-max/min/diff observables to each other and the Pythia 6 AUET2B CTEQ6L1 MC primary particles, i.e. particles with a mean proper lifetime ⇥ 0.3 10 10 s either directly produced ⇥ − in the pp interactions or in the decay of particles with a shorter lifetime. Themodel. selected tracksThe error correspond bands on the top plots show the combined systematic and statistical uncertainty, while to primary charged particles with p > 0.5 GeV and η < 2.5, and ATLAS clustersthe grey are equivalent band in (when the ratio plots shows the maximum combined statistical and systematic uncertainty T | | summed over) to primary charged particles with momentum p > 0.5 GeV oramong primary the neutral three particles regions. with p > 0.2 GeV. Lower momentum particles are not included as they are unlikely to reach the ATLAS calorimeters due to material interactions and bending in the magnetic eld. fact, Herwig/Jimmy AUET2 LO gives the best description of all models considered here for inclusive The observables used in this study, defined in Table 1, employ the conventional UE azimuthal division ∗∗ of events into regions relative to the direction of the “leading” object in thejet event. events The with leadingNch object 15. in this case is defined by the calorimeter-based anti-kT [11] jet with a radius of RFinally,= 0.4 and the having ATLAS the tunes of both Pythia 6 and Pythia 8 are seen to undershoot this data somewhat largest pT, after application of jet selection criteria as described in Section 4. Thefor azimuthal low N , regions particularly used in the inclusive jet sample, but describe the p of higher-multiplicity events ch ⇥ T⇤ are defined with respect to the ⇤ of the leading jet (i.e. the jet with the largestwellpT, for which both is denoted event selections. by As both these tunes incorporated the equivalent of this observable in plead): a 120 “towards” region surrounds the leading jet, an “away” region of the same size is azimuthally T ⇤ the ATLAS leading charged particle UE analysis [4], the flaws in their data description seen here are opposed to it and two “transverse” regions each of 60⇤ are defined orthogonal to the leading jet direction [2]. This is illustrated in Figure 1, with the azimuthal angular di⇥erence fromunexpected, the leading jet and defined use of as this data in future tunes may substantially change the MPI model parameters. ∆⇤ = ⇤ ⇤ . | | | − lead jet| As the towards region is dominated by the leading jet and in the dominant dijet configuration the away region is dominated by the balancing jet, the transverse regions are the most sensitive to accompanying particle flow, i.e. the UE. In addition, the transverse regions may be distinguished event-by-event based 2 10 4 A C Fig. 4 Formation of clusters, which we represent by ovals here. Colour lines are dashed. The left D diagram shows colour-singlet clus- ters formed according to the dom- inating colour structure in the 1/Nc expansion. The right di- agram shows a possible colour- B reconnected state: the partons of the clusters A and B are arranged in new clusters, C and D. – C and D are no colour octets. p 4. If at least one reconnection possibility could be found in step 3, select the one which results in the v v smallest sum of cluster masses, mC + mD. Accept v s s¯ this colour reconnection with an adjustable proba- bility preco. In this case replace the clusters A and v g s¯ s B by the newly formed clusters C and D. 5. Continue with the next quark in step 2. Fig. 3 For the hard subprocess a valence quark v is extracted from the proton. Since the valence quark parton distribu- The parameter preco steers the amount of colour recon- 7 tion functions dominate at large momentum fractions x and nection in the PCR model. Because of the selection rule small scales Q2,theinitial-stateshower,whichisgenerated to remark that the fraction of WW events with non- 1.0 in step 4, the PCR model tends to replace the heaviest vanishingbackwards colour length drop starting is slightly from higher the than partonic for scatter,n-type commonly ter- i-type the dijet case. Nevertheless, the vast majority of WW 0.8 minates on a valence quark. This situationh-type is shown in the clusters by lighter ones. A priori the model is not guar- events is not a↵ectedColour by colour reconnection, too. Reconnection leftmost figure. If the perturbative evolution still terminates 0.6 anteed to be generally valid because of the following on a sea (anti)quark or a gluon, asi indicated in the other 8 f reasons: The random ordering in the first step makes 3.2 Classificationfigures, of oneclusters or two additional non-perturbative0.4 splittings are performed to force the evolution to end with a valence quark. 8 this algorithm non-deterministic since a di↵erent or- 0.2 1 1 The grey-shaded area indicates this non-perturbative0 region, 1 der of the initial clusters, generally speaking,1 0 leads to − − 10 − − 10 whereas the perturbative parton shower0.0 happens in the region 0di.35↵erent reconnectiondefault possibilities clusters being tested.1 More- default clusters 100 101 102 103 10− 0.35 1 GeV h-type clusters GeV below. 10− mcut/GeV / / 2 cl over, apparently quarks and antiquarks arecl treated dif- GeV h-type clusters GeV 0.30 i-type clusters 10− / / n type Fig. 8 Cluster fraction2 functions, defined in Eq.M (6), for LHC M 3 − d n-type clusters d cl cl ferently in the algorithm described above.10− 0.30 i-type clusterscluster dijet events10 at− 7 TeV.
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