MPI in PYTHIA. 1. Brief overview 2. Color reconnection and the top mass

Transcription

1 MPI in PYTHIA 1. Brief overview 2. Color reconnection and the top mass Torbjörn Sjöstrand Department of Astronomy and Theoretical Physics Lund University Sölvegatan 14A, SE Lund, Sweden 2014, Krakow, 3 7 November 2014

2 1987: the (almost) original PYTHIA model Seek unified description of hard jets, UE and MB. Perturbative origin p d.o.f. essential (unlike multi-pomeron models at the time). Screening dp 2 /p4 dp2 /(p2 + p2 0 )2 with p GeV finite MPI number. p -ordered generation, Sudakov/shower style. Hardest MPI standard PDFs, softer modified. Tuneable impact-parameter picture. Colour reconnection needed. Makes use of existing PYTHIA/JETSET components, such as Lund string fragmentation, and initial- and final-state parton showers. TS & M. van Zijl, Phys.Rev. D36 (1987) 2019 Torbjörn Sjöstrand MPI in PYTHIA slide 2/16

3 1987: the experimental evidence MPI signals included width of charged multiplicity, forward backwards correlations, and jet pedestal effect: CR signal from p (n ch ): Torbjörn Sjöstrand MPI in PYTHIA slide 3/16

4 Today: basic generation of MPI Basic ideas remain: screening, p -ordered generation, all events contain at least one perturbative interaction. Still allow for many different impact-parameter profiles. Screening p 0 energy-dependent for post-hera PDFs. Two three basic generation possibilities: 0 no separate hard interaction minbias events, 1 start from fixed hard interaction underlying event, or 2 select two hard interactions, e.g. W W. More sophisticated rescaled PDF s, taking into account momentum and flavour correlations. Possible to kick out several valence quarks ( junctions), and to have more complicated bream remnants. Each MPI associated with its ISR and FSR activity. MPI machinery also for diffractive events. Torbjörn Sjöstrand MPI in PYTHIA slide 4/16

5 Interleaved evolution MPI ordered in p from onset. Now also p -ordered parton showers for ISR and FSR. Allows interleaved evolution for MPI, ISR and FSR: ( dp dpmpi = + dp ISR + ) dp FSR dp dp dp dp ( p max ( dpmpi exp p dp + dp ISR dp Ordered in decreasing p using Sudakov trick. + ) ) dp FSR dp dp Corresponds to increasing resolution of partonic final state: smaller p fill in details of basic picture set at larger p. Torbjörn Sjöstrand MPI in PYTHIA slide 5/16

6 Rescattering Rescattering Often assume that MPI =...but should also include Same order in α s, same propagators, but one PDF weight less smaller σ one jet less QCD radiation background 2 3 larger than 2 4 will be tough to find direct evidence. Rescattering grows with number of previous scatterings: Tevatron LHC Min Bias QCD Jets Min Bias QCD Jets Normal scattering Single rescatterings Double rescatterings R. Corke & TS, JHEP 01 (2010) 035 Torbjörn Sjöstrand MPI in PYTHIA slide 6/16

7 An x-dependent proton size Reasonable to assume that low-x partons are more spread out: ρ(r, x) 1 ( a 3 (x) exp r 2 ) ( a 2 with a(x) = a a 1 ln 1 ) (x) x a tuned to rise of σ ND a 0 tuned to value of σ ND, given PDF, p 0,... b 2 eik [fm] a1 = 0.00 a1 = 0.15 a1 = 1.00 (b) E CM [GeV] norm (1 / N) dn / db MPI (a) SG DY Z 0 Z norm b MPI Consequence: collisions at large x will have to happen at small b, and hence further large-to-medium-x MPIs are enhanced. a 1 > 0 not favoured by tunes so far! Torbjörn Sjöstrand MPI in PYTHIA slide 7/16

8 Colour reconnection T p (n ch ) effect alive and kicking: [GeV] p GeV pp Soft QCD (mb,diff,fwd) Average p T ATLAS Pythia 8 Pythia 8 (no CR) vs N ch (N ch > 1, p T ATLAS_2010_S Pythia > 0.5 GeV/c) multiplicity N ch Ratio to ATLAS 1.5 Colour reconnection (CR): reduce total string length 1 reduce hadronic mcplots.cern.ch Rivet 1.8.2, 200k events Prob(n) multiplicities in nondiffractive events (8 TeV LHC) strings crossing y = 0 primary hadrons in y < 0.5 charged particles in y < String width hadronic width Overlap factor 10! Larger for hard collisions (small impact parameter) n Torbjörn Sjöstrand MPI in PYTHIA slide 8/16

9 A top mass puzzle Γ t 1.5 GeV Γ W 2 GeV Γ Z 2.5 GeV cτ 0.1 fm : puzzle p pancakes about m top have passed, MPI/ISR/FSR for p 2 GeV, inside hadronization colour fields. t t b W Experiment m top [GeV] Error due to CR Reference World comb ± MeV (40%) arxiv: CMS ± MeV (20%) CMS-PAS-TOP D ± MeV (13%) arxiv: (S. Argyropoulos) 1. Great job in reducing the errors. 2. CR is one of the dominant systematics. 3. Why is the CR uncertainty going down when there are no advances in theoretical understanding, and no measurements to constrain it? 1. Great job in reducing the errors 2. CR is one of the dominant systematics 3. Why is the CR uncertainty going down when there are - no advances on the theoretical understanding Torbjörn Sjöstrand MPI in PYTHIA slide 9/16

10 Top mass shift in Pythia 6 Studies for the Tevatron. Green bands: old virtuality-ordered showers. Blue bands: new p -ordered showers. In total ±1.0 GeV, whereof ±0.7 GeV perturbative, and ±0.5 GeV nonperturbative. NON-PERTURBATIVE QCD EFFECTS AND THE TOP MASS AT THE TEVATRON Fit scaled: Jet Energy Scaling. m fit top {}}{ Pythia v6.416 Tune A Tune A-CR Tune A-PT Tune DW Tune BW S0 S1 S2 NoCR m scaled top {}}{ Δm t (M.Sandhoff and P.Z Skands, FERMILAB-CONF T;) Fig. 5. Comparison of calibration offsets obtained for each model. The column o D. Wickeshow and the P.Z. results Skands, obtained EPJ C52 before(2007) JES rescaling, 133, Nuovo the right Cim. column B123 (squares) (2008) S1 after statistical precision due to the finite number of generated events is at the level Torbjörn Sjöstrand MPI in PYTHIA slide 10/16

11 Pythia 8.1 CR model Only one CR model: Starting from lowest-p MPI and moving upwards define its P rec (p T ) = (R rec p T 0 ) 2 (R rec p T 0 ) 2 + pt 2, with any higher-p MPI. R rec one free parameter of model. Find colour dipoles of highest-p MPI. Consecutively attach each gluon of each lower-p MPI to be reconnected where it increases the string length λ the least. Repeat for lower-p MPIs that form separate systems. End result: fewer but bigger systems, with reduced total λ. Three CR options for top: no CR at all late resonance decays: t/w decays after CR early resonance decays: t/w decays before CR Torbjörn Sjöstrand MPI in PYTHIA slide 11/16

12 Further Pythia 8.2 CR models S. Argyropoulos & TS: arxiv: [hep-ph] JHEP Basic idea: produce range of models to study how big m top could be without contradicting data. Top CR as afterburner: toy / stealth models forced random forced nearest forced farthest forced smallest λ smallest λ Top CR on equal footing: more sophisticated / fragile swap move swap + flip move + flip so as to reduce λ The λ measure of an event is approximated by λ λ approx = ( ) ln 1 + m2 ij m0 2 with m 0 m hadronic 1 GeV. dipoles Torbjörn Sjöstrand MPI in PYTHIA slide 12/16

13 Some ways to perform a reconnection swap: t b W i j k l m n g t g r q q i m j l n k move: t b W i j k l m n g t g r q q i k l j n m flip: t b W i j k l m n g t g r q q i l k n m j Torbjörn Sjöstrand MPI in PYTHIA slide 13/16

14 Effects on top mass before tuning 1/NdN/dmtop[GeV 1 ] Reconstructedtopmass,m W [75,85]GeV,p T (jets) >40GeV CRoff default forced random m top[gev] m top relative to no CR: model m top m top [GeV] rescaled default (late) default early forced random Asymmetric spread: m top < 0 easy, m top > 0 difficult. Parton showers already prefer minimal λ. Main effect from jet broadening, some from jet jet angles. Torbjörn Sjöstrand MPI in PYTHIA slide 14/16

15 Effects on top mass after tuning No publicly available measurements of UE in top events. Afterburner models tuned to ATLAS jet shapes in tt events high CR strengths disfavoured. Early-decay models tuned to ATLAS minimum bias data maximal CR strengths required to (almost) match p (n ch ). model m top rescaled default (late) forced random swap m max top mmin top 0.80 GeV Excluding most extreme (unrealistic) models down to m max top mmin top 0.50 GeV m top relative to no CR. (in line with Sandhoff, Skands & Wicke) Studies of top events could help constrain models: jet profiles and jet pull (skewness) underlying event Torbjörn Sjöstrand MPI in PYTHIA slide 15/16

16 Summary and Outlook MPI key PYTHIA component since almost 30 years. Original concepts still hold: screening with p 0 2 GeV, p -order, n pert 1, reconnection, strings,... Many aspects gradually becoming more sophisticated, notably interleaved evolution MPI + ISR + FSR. Everything mixed up experimental tests indecisive, e.g. rescattering and x-dependent proton size. Colour reconnection one of big known unknowns. Experimental m top CR error out of control? Need dedicated experimental studies of CR in top events. New CR model/framework by J.R. Christiansen and P. Skands coming up (next). Torbjörn Sjöstrand MPI in PYTHIA slide 16/16