Origins of and PYTHIA experience with. Colour Reconnection

Transcription

1 Origins of and PYTHIA experience with Colour Reconnection Torbjörn Sjöstrand Department of Astronomy and Theoretical Physics Lund University Sölvegatan 14A, SE Lund, Sweden Satellite meeting on Colour Reconnection, PSR18 Workshop, Lund, 7 June 2018

2 Reconnection in B decays Colour operators in B decay some η c : A. Ali, J.G. Körner, G. Kramer, J. Willrodt, Z. Phys. C1 (1979) 269 B J/ψ µ + µ good way to find B mesons: H. Fritzsch, Phys. Lett. B86 (1979) 164, soon confirmed by experiment g cc J/ψ production mechanism in pp ( colour octet ) H. Fritzsch, Phys. Lett. B67 (1977) 217 more complicated to test (at the time, later confirmed ) Torbjörn Sjöstrand Origins of and PYTHIA Experience with Colour Reconnection slide 2/21

3 rl &2 g & I I l I I I I I I I I 1 I s I I I I I 1 multiplicities is affected by the noninclusion of (double-) diffractive events. ] The range of populated n,h values is obviously increasing, and so is the (n,h), so there is a steady increase with energy of the (pz. ) of all charged particles, Fig. 30. The trend is enhanced if only central particles, 2, are included. ~ Reconnection at SppS CL ~ 04/ O.C I I I I I I I I I I I I I I I I I I I I I t I I I may account for part of this effect, but not all. Multiple interactions cannot solve the problem as long as all collisions are assumed to be equivalent, ' but the introduction of variable impact parameters offers a solution, as follows. Since the average number of jets is larger in central collisions than in peripheral ones, the sample of events containing a hard interaction is biased towards central collisions. These events are then likely to contain additional jet activity. We will now proceed to show that our standard multiple-interaction scenario, with a double Gaussian matter distribution, indeed gives a satisfactory agreement with data. I CL 0.60 I I I I I I I 1 I \ I I I I 1 1 a i i&l I I I I I I I I o. so 0.40 I I I l 1 I t 5 IO I ~ ~ noh FIG. 28. Average transverse momentum of charged particles in as a function of the multiplicity, SFM data points at 63 and 31 GeV (Ref. 52) vs model curves; solid line, at 63 GeV; and dashed line, at 31 GeV. p (n ch ) decreasing for E CM 50 GeV l ~ ~ I I I I I I I (GeV) FIG. 30. Average transverse momentum as a function of the c.m. energy, solid line is all charged particles, dashed line is charged particles with 10 TS and M. van Zijl, Phys.Rev. D36 (1987) 2019 Torbjörn Sjöstrand Origins of and PYTHIA Experience with Colour Reconnection slide 3/21

4 Multiparton Interactions and Colour Reconnection If MultiParton Interactions (MPIs) occur independently then many MPIs is more of the same. Colour Reconnections (CR) introduce some kind of collectivity. Important in all pp generators. Torbjörn Sjöstrand Origins of and PYTHIA Experience with Colour Reconnection slide 4/21

5 Short vs. long strings: the λ measure λ is intended to gauge phase space for hadron production, λ n hadrons n charged. For simple qq string: ( ) m 2 qq λ = ln with m 0 1 GeV measure of hadronic mass scale. For qqg instead ( ) ( m 2 qqg λ = ln m0 2 + m2 qgmqg 2 pq m0 4 ln p ) ( g ) 2 pq m0 2 +ln p ) g ) 2 m0 2 with simplification in limit p g m 0. More generally, for q 0 g 1 g 2 g n q n+1, with factor 1 2 for gluon momentum, ( ) ( ) n m 2 i,i+1 n λ ln m0 2 ln 1 + m2 i,i+1 m0 2 i=0 m 2 0 i=0 Torbjörn Sjöstrand Origins of and PYTHIA Experience with Colour Reconnection slide 5/21

6 The rise of transverse momentum p (n ch ) increasing from ISR energies upwards. [GeV] T 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) Ratio to ATLAS N ch p (n ch ) is sensitive to colour flow: long1 strings to remnants much n ch /MPI p (n ch ) flat mcplots.cern.ch Rivet 1.8.2, 200k events short strings (more central) less n ch /MPI p (n ch ) rising 0.5 Torbjörn Sjöstrand 0 50 Origins of and 100PYTHIA Experience with Colour Reconnection slide 6/21

7 Charged multiplicity dependence on CR CR fewer hadrons, fewer more per extra MPI. Not quite straight line also without CR: 1) Energy momentum conservation 2) Assumed colour correlations in beam remnants reduces number of strings stretched all the way to the beams. Torbjörn Sjöstrand Origins of and PYTHIA Experience with Colour Reconnection slide 7/21

8 Interconnection at LEP 2 e + e W + W q 1 q 2 q 3 q 4 reconnection limits m W precision! perturbative δm W 5 MeV : negligible! (killed by dampening from off-shell W propagators) nonperturbative δm W 40 MeV : favoured. (but more extreme models from other authors ruled out) Bose-Einstein δm W 40 MeV : only little allowed. V.A. Khoze & TS, PRL 72 (1994) 28; L. Lönnblad & TS, EPJ C6 (1999) 271 Torbjörn Sjöstrand Origins of and PYTHIA Experience with Colour Reconnection slide 8/21

9 Colour rearrangement models for LEP 2 Colour rearrangement studied in several models, e.g. Scenario II: vortex lines. Analogy: type II superconductor. Strings can reconnect only if central cores cross. Scenario I: elongated bags. Analogy: type I superconductor. Reconnection proportional to space time overlap. q q g q W q W g W + W + q q q q In both cases favour reconnections that reduce total string length. g g (schematic only; nothing to scale) Torbjörn Sjöstrand Origins of and PYTHIA Experience with Colour Reconnection slide 9/21

10 Colour rearrangement results at LEP 2 Best LEP2 fit 2013 (topology + mass): 51% of 189 GeV events reconnected in SKI model. No-CR excluded at 99.5% CL. Torbjörn Sjöstrand Origins of and PYTHIA Experience with Colour Reconnection slide 10/21

11 Comparison of CR models at LEP authors Khoze Todorova Gustafson Lönnblad Webber Sjöstrand Häkkinen reference [52] [59] [53, 66] [57] [58] based on Pythia Ariadne HERWIG reconnection space time overlap (I) string length cluster space time criterion or crossing (II) of strings reduced sizes reduced reconnection I: free parameter free partly free probability II: partly predicted parameter predicted parameter model of yes yes no yes yes all events space time picture implemented for ( = not applicable) Wvertices yes yes no no yes parton shower no yes yes fragmentation yes yes multiple no yes yes yes yes reconnections reconnection no yes yes yes yes inside W/Z change of event almost small but visible, needs large, needs properties invisible visible retuning retuning Table 14: Survey of colour reconnection models. The information should be taken as indicative; only the original literature gives the ideological motivation behind the choices. + some more, notably Ellis, Geiger: space time hadronization model m W 400 MeV Torbjörn Sjöstrand Origins of and PYTHIA Experience with Colour Reconnection slide 11/21

12 Diffraction and Colour Reconnection Rapidity gaps observed at HERA: diffractive DIS conventionally Pomeron explanation, but... Ingelman et al. (Uppsala): SCI Soft Colour Interactions 1. The Extended string configuration to pp/pp andin e + ae DIS, forboson-gluon-fusion quarkonium, W, Higgs, event: (a) conventio nnection gaps ofbetween partons, (Tevatron) and (b,c) after jets, reconnection diffraction, etc. due to soft colour interactions Rathsman: GAL Generalized Area Law: P rec = 1 exp( b A) where A m 2 unlike λ ln m 2 = ln m 2 into non-pertubative quarks and gluons and the formation of the confinin e in between them. ingtorbjörn a proper Sjöstrand understanding Origins of andsuch PYTHIA Experience non-perturbative with Colour Reconnection QCD processes,wec slide 12/21

13 PYTHIA CR models for the LHC Space time models too complicated and uncertain at the LHC simplified (in Pythia) Common aspect: reduce string length λ = ln(m. ij 2/m2 0 ) multiplicity In total 12 scenarios in Pythia 6, mainly annealing 2 4 m13 m 24 < m 12 m 34 Also add-ons, like Ingelman & Rathsman Generalized Area Law Pythia 8 currently three options ColourReconnection:mode = 0: default MPI-based scheme (slide 14) = 1: QCD-based model (slide 15) = 2: gluon-move model (slide 17) = 3,4: SKI and SKII models (e + e only; slide 9) examples/main29.cc further straw man models (slide 17) 1 3 Torbjörn Sjöstrand Origins of and PYTHIA Experience with Colour Reconnection slide 13/21

14 The default MPI-based CR model Basic mechanism that of fusing two MPIs: the partons of the lower-p system are added to the strings defined by the higher-p system, in such a way as to give the smallest total string length. Optimal for inserting gluons on existing strings, while (the rarer) quarks are handled more approximately. Tends to give fewer but longer separate string systems. P recon = r 2 p 2 0 /(r 2 p p2 ) where p is lower-mpi scattering scale and r =ColourReconnection:range. A low-p MPI is less localized, and so can easier overlap! Reconnections are defined from lowest-p system upwards, but then executed from highest-p system downwards. By default most systems tend to be reconnected. Torbjörn Sjöstrand Origins of and PYTHIA Experience with Colour Reconnection slide 14/21

15 The new QCD-based CR model Possible reconnections New model relies on two main principles SU(3) colour rules give allowed reconnections Ordinary string reconnection Double junction reconnection (qq: 1/9, gg: 1/8, model: 1/9) (qq: 1/3, gg: 10/64, model: 2/9) Triple junction reconnection Zipping reconnection (qq: 1/27, gg: 5/256, model: 2/81) (Depends on number of gluons) minimal λ measure gives preferred reconnections J.R. Christiansen & P.Z. Skands, JHEP 1508, 003 Jesper Roy Christiansen (Lund) Non pertubative colours November 3, 10 / 15 Torbjörn Sjöstrand Origins of and PYTHIA Experience with Colour Reconnection slide 15/21

16 The top mass uncertainty from CR Γ t 1.5 GeV Γ W 2 GeV Γ Z 2.5 GeV cτ 0.1 fm b t W t Decays occur when p pancakes have passed, after MPI/ISR/FSR with p 2 GeV, but inside hadronization colour fields. Experimentalists achieve impressive m t precision, e.g. CMS m t = ± 0.16 ± 0.48 GeV (PRD93 (2016) ), whereof CR ±0.10 GeV from Pythia 6.4 Perugia 2011 CR - nocr Is this realistic? Torbjörn Sjöstrand Origins of and PYTHIA Experience with Colour Reconnection slide 16/21

17 g New t CR models for top studies W Late q t decay: first ordinary CR (existing model) as if t stable, j j then CRl between g s from n l t&w decays and g s fromn rest of event, in 5 variants, some straw-man, e.g. random ( λ increases) Early t decay: new gluon-move.solidlinesindicatetheoriginal model for whole event 1) move: remove gluon and insert on other string if reduces λ g t b g r i m b i k i k m (ij, lm) =min ij,lm [ im + m lj ( ij + lm )] apple cut is selected m for a flip. Here g systems W that r have undergone one flip are not allowed any further ones, or else the q ure leads to the formation jof many low-mass gg systems, thus j markedly reducing l n l n arged particle multiplicity. While the normal string has a color and an anticolor at 2) flip: cross two chains if reduces λ ( swing) 2. In the move model, a gluon j originally attached to string piece ik can be moved to a b i k i t string piece lm if it leads to a smaller m total string length.solidlinesindicatetheoriginal m k ration and dashed g W r lines indicate the resulting configuration after moving the gluon. q l j n l j n (ij, lm) =min ij,lm [ im + lj ( ij + lm )] apple cut is selected for a flip. Here systems3) that (swap: have undergone interchange one flip are two notgluons allowed any if reduces further ones, λ) or else the ure 3. leads Illustration to theof formation the effect of the many flip low-mass model. The gg systems, same process thusasmarkedly in figure 1, reducing shown erged underlying particlestring multiplicity. configuration. While The thesolid normal lines string indicatehas theainitial colorconfiguration and an anticolor and the at lines represent a flip in the string pieces ij and lm, resultingfromtheexchangeofoneof or indices g t between gluons j and m. The figure represents a case where a flip reduces the ring length. Wenotethataftertheflip, theb quark from the top decay (endpoint k) is k i k m 2. In the move model, a gluon j originally attached to string piece ik can be moved to a t string piece lm if it leads to a smaller total string length ration and dashed lines indicate the resulting configuration after moving the gluon. g t S. Argyropoulos & TS, JHEP 1411, 043; P. Skands et al. earlier Torbjörn Sjöstrand Origins of and PYTHIA Experience with Colour Reconnection slide 17/21

18 Effects on top mass after (re)tuning No publicly available measurements of UE in top events (then). 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 Excluding most extreme (unrealistic) models m max top mmin top 0.50 GeV (in line with Sandhoff, Skands & Wicke) m top relative to no CR. But m top 0 in QCD-based model Studies of top events could help constrain models: jet profiles and jet pull (skewness) underlying event Torbjörn Sjöstrand Origins of and PYTHIA Experience with Colour Reconnection slide 18/21

19 CR and collective effects Mean transverse momentum vs. mass at 7 TeV, y < 0.5 p [GeV/c] 1 π + ALICE data Gaussian p off Gaussian p ColRec Gaussian p HadScat Gaussian p NrString K 0 K + p φ Ξ Σ ± Ξ 0 Ω MC/Data m [GeV/c 2 ] Transversely boosted strings collective-flow-like effect. N. Fischer, TS, JHEP 1701, 140 Torbjörn Sjöstrand Origins of and PYTHIA Experience with Colour Reconnection slide 19/21

20 Fragmenting system size dependence on CR r m CR off CR on CR off tuned n Had r m = median radius (fm) density = n hadrons πrm 2 y ( ) τ y τ = 1 2 ln t+z t z no CR: r m rather flat, steep increase of density. n Had π r 2 y τ CR off CR on CR off tuned with CR: r m increases with n hadrons, density reasonably flat but still large n Had S. Ferreres-Solé, TS, in prep. Torbjörn Sjöstrand Origins of and PYTHIA Experience with Colour Reconnection slide 20/21

21 Summary and outlook Evidence for Colour Reconnection in LEP data. CR proven at pp colliders by p (n ch ) rise. Few other constraints, so many models possible. Some signs of collectivity partly explained by CR. CR part of scenario for alternative to Quark Gluon Plasma! Or does QGP partly replace CR? (But recall LEP.) CR can influence other observables, like m top. CR part of ongoing development in Lund framework. Very much an open field. Torbjörn Sjöstrand Origins of and PYTHIA Experience with Colour Reconnection slide 21/21