The Pythia 8 Event Generator

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University Sölvegatan 14A, SE-223 62 Lund, Sweden LHC

1 The Pythia 8 Event Generator Torbjörn Sjöstrand Department of Astronomy and Theoretical Physics Lund University Sölvegatan 14A, SE Lund, Sweden LHC meets Cosmic Rays school (ISAPP 2018), CERN, 28 Oct 2 Nov 2018

2 The structure of an event An event consists of many different physics steps to be modelled: Torbjörn Sjöstrand The Pythia 8 Event Generator slide 2/29

3 Colliding beams e + e (e.g. LEP) pp & pp (e.g. LHC) e ± p (e.g. HERA), but either DIS or photoproduction ll & ll (l = e, µ, τ, ν) hh & hh (h = p, n, π ±,0, limited by PDFs) lh, γh, γγ, where γ direct or resolved pa, AA (Angantyr; recent) Limitations: Only one beam combination at a time. Only one CM energy at a time (or small range). E cm > 10 GeV. No air shower tracking. Torbjörn Sjöstrand The Pythia 8 Event Generator slide 3/29

4 Core processes Available hardcoded internally, almost freely mixable: Soft QCD: elastic, single diffractive, double diffractive, central diffractive, nondiffractive (including hard processes) Hard QCD: 2 2 (e.g. qg qg), open heavy flavours, charmonium, bottomonium, top, (2 3) Electroweak: ff γ /Z 0, ff W + W, qg qγ, ff γγ, lq lq, qγ qg, γγ ff,... Higgs in the SM and various extensions BSM: SUSY, new gauge bosons, left right symmetry, leptoquarks, compositeness, hidden valleys, extra dimensions, dark matter Beyond that: no internal ME generator, so then external input, e.g. MadGraph5 amc@nlo, PowHeg Box, AlpGen, typically using Les Houches Event Files exchange standard. Torbjörn Sjöstrand The Pythia 8 Event Generator slide 4/29

5 Parton Distribution Functions Trend towards NLO/NNLO parametrizations: need not be positive definite, notably small-x gluon at low Q 2, which is nuisance. Coming: NNPDF3.1sx+LHCb (N)NLO+NLLx QED. Internal implementation of several PDFs Can also p: 21 sets, from legacy to new (2017), mainly LO n: by isospin (watch out for QED) nuclear modification factors (EPS09 LO/NLO, EPPS16 NLO) π: GRV 92L (isospin for π + π 0 ) Pomeron (diffraction): 15 sets γ: CJKL l: QED (exponentiated) link to whole LHAPDF library, or read single.dat file of one PDF set/member. Torbjörn Sjöstrand The Pythia 8 Event Generator slide 5/29

6 Parton showers 1 2 n = (2 2) ISR FSR Dipole recoil (for FSR): r a r c b p b + p c + p r = p a + p r Based on DGLAP evolution equations: dp a bc = α s dq 2 2π Q 2 P a bc(z) dz (Sudakov) with p ordering, Q 2 = p evol 2 p2, and dipole recoils. ISR by backwards evolution from the hard interaction. Torbjörn Sjöstrand The Pythia 8 Event Generator slide 6/29

7 Parton showers 2 Currently three (main) parton shower options; Internal default SpaceShower + TimeShower; Vincia antenna shower plugin; Dire dipole shower plugin. Same basic structure, e.g. MPI + ISR + FSR interleaved evolution: ( dp dpmpi = + dp ISR + ) dp FSR dp dp dp dp ( p max ( dpmpi exp p Support the same facilities, like dp + dp ISR dp + ) ) dp FSR dp dp matching and merging with higher-order matrix elements, automated uncertainty band from factorization and renormalization scale choices, and finite splitting-kernel terms. Torbjörn Sjöstrand The Pythia 8 Event Generator slide 7/29

$2 α2 s (p 2 ) p 4 α2 s (p 2 0 + p2 ) (p 2 0 + p2 )2 with p 0 2 3 GeV 1/d. Semiperturbative 2 2 generates whole nondiffractive σ!? Torbjörn Sjöstrand The Pythia 8 Event Generator slide 8/29$

8 MultiParton Interactions 1 Hadrons are composite many partons can interact: Divergence for p 0 in perturbative 2 2 scatterings; tamed by unknown colour screening length d in hadron dˆσ dp 2 α2 s (p 2 ) p 4 α2 s (p p2 ) (p p2 )2 with p GeV 1/d. Semiperturbative 2 2 generates whole nondiffractive σ!? Torbjörn Sjöstrand The Pythia 8 Event Generator slide 8/29

9 MultiParton Interactions 2 Hadrons are extended, so dependence on impact parameter b. Overlap of protons during encounter is O(b) = d 3 x dt ρ 1 (x, t) ρ 2 (x, t) where ρ is (boosted) matter distribution in p, e.g. Gaussian or more narrow peak. Average activity at b proportional to O(b): central collisions more active P n broader than Poissonian; peripheral passages normally give no collisions finite σ tot. eter b O(b) At LHC n MPI 3 for all events, but 10 for central collisions. broader Preselected than Poissonian hard process central pedestal effect. o collisions ) finite tot later) Torbjörn Sjöstrand The Pythia 8 Event Generator slide 9/29

10 MPIs in PYTHIA MPIs are gererated in a falling sequence of p values; recall Sudakov factor approach to parton showers. Core process QCD 2 2, but also onia, γ s, Z 0, W ±. Energy, momentum and flavour conserved step by step: subtracted from proton by all previous collisions. Protons modelled as extended objects, allowing both central and peripheral collisions, with more or less activity. Colour screening increases with energy, i.e. p 0 = p 0 (E cm ), as more and more partons can interact. Colour connections: each interaction hooks up with colours from beam remnants, but also correlations inside remnants. Colour reconnections: many interaction on top of each other tightly packed partons colour memory loss? Torbjörn Sjöstrand The Pythia 8 Event Generator slide 10/29

The QCD potential In QCD, for large charge separation, field lines are believed to be compressed to tubelike region(s) string(s) Gives force/potential between a q and a q: F (r) const = κ V (r) κr κ

11 The QCD potential In QCD, for large charge separation, field lines are believed to be compressed to tubelike region(s) string(s) Gives force/potential between a q and a q: F (r) const = κ V (r) κr κ 1 GeV/fm potential energy gain lifting a 16 ton truck. Flux tube parametrized by center location as a function of time simple description as a 1+1-dimensional object a string. Torbjörn Sjöstrand The Pythia 8 Event Generator slide 11/29

12 String motion The Lund Model: starting point Use only linear potential V (r) κr to trace string motion, and let string fragment by repeated qq breaks. Assume negligibly small quark masses. Then linearity between space time and energy momentum gives de dz = dp z dz = de dt = dp z dt = κ (c = 1) for a qq pair flying apart along the ±z axis. But signs relevant: the q moving in the +z direction has dz/dt = +1 but dp z /dt = κ. -L/2 L12 X Fig The motion of q and ~ in t Torbjörn Sjöstrand The Pythia 8 Event Generator slide 12/29

13 The Lund Model Combine yo-yo-style string motion with string breakings! Motion of quarks and antiquarks with intermediate string pieces: time quark antiquark pair creation space A q from one string break combines with a q from an adjacent one. Gives simple but powerful picture of hadron production. Torbjörn Sjöstrand The Pythia 8 Event Generator slide 13/29

14 Where does the string break? Fragmentation starts in the middle and spreads outwards: Corresponds to roughly same invariant time of all breaks, τ 2 = t 2 z 2 constant, with breaks separated by hadronic area m 2 = m2 + p 2. Hadrons at outskirts are more boosted. Approximately flat rapidity distribution, dn/dy constant total hadron multiplicity in a jet grows like ln E jet. Torbjörn Sjöstrand The Pythia 8 Event Generator slide 14/29

15 How does the string break? String breaking modelled by tunneling: ( ) ( P exp πm2 q κ = exp πp2 q κ ) exp Common Gaussian p spectrum, p 0.4 GeV. Suppression of heavy quarks, ( πm2 q κ uu : dd : ss : cc 1 : 1 : 0.3 : Diquark antiquark simple model for baryon production. String model unpredictive in understanding of hadron mass effects many parameters, depending on how you count. Torbjörn Sjöstrand The Pythia 8 Event Generator slide 15/29 )

16 The Lund gluon picture gluon The most characteristic feature of the Lund model: quark antiquark string motion in the event plane (without breakups) Gluon = kink on string Force ratio gluon/ quark = 2, cf. QCD N C /C F = 9/4, 2 for N C No new parameters introduced for gluon jets! Torbjörn Sjöstrand The Pythia 8 Event Generator slide 16/29

17 Colour flow in hard processes One Feynman graph can correspond to several possible colour flows, e.g. for qg qg: while other qg qg graphs only admit one colour flow: Interference terms with indeterminate colour flow 1/N 2 C. Torbjörn Sjöstrand The Pythia 8 Event Generator slide 17/29

18 2. Colour rearrangement: many models, in general Colour Reconnection δm W < 40 MeV. e + e W + W q 1 q 4 q 2 q 3 π + π + BE 3. Bose-Einstein: At LEP 2 search forsymmetrization effects in e + e W of + Wunknown q 1 q 2 amplitude, wider spread W 5 MeV : negligible! q 3 q 4 : perturbative δm MeV among models, nonperturbative δm but realistically δm W W < 40 MeV : 40 MeV. favoured; no-effect option ruled out at 99.5% < CL. In sum: δm W tot < m π, δm W tot /m W 0.1%; a Best description for reconnection in 50% of the events. small number Bose-Einstein that becomes δm of interest only because W 100 MeV : full effect ruled out we aim for (while high models accuracy. with 20 MeV barely acceptable). Torbjörn Sjöstrand The Pythia 8 Event Generator slide 18/29

19 Colour (re)connections and p (n ch ) Torbjörn Sjöstrand The Pythia 8 Event Generator slide 19/29

20 reported here is two standard deviations below the COMPETE parameterization. Some other models prefer a somewhat slower increase of the total cross section with energy, predicting values below 95 mb, and thus Total agree slightly cross better with section the result reported here [59 61]. σ [mb] ATLAS TOTEM Lower energy pp Lower energy and cosmic ray pp Cosmic rays COMPETE RRpl2u ln(s) ln (s) σ tot σ el s [GeV] Several options for total and partial pp & pp cross sections: DL/SaS, MBR, ABMST, RPP2016. Figure 19: Comparison of total and elastic cross-section measurements presented here with other published measurements [11, 29, 55 58] andmodelpredictionsasfunctionofthecentre-of-massenergy. 33 Torbjörn Sjöstrand The Pythia 8 Event Generator slide 20/29

21 Diffraction Diffraction Ingelman-Schlein: Ingelman-Schlein: Pomeron Pomeron as as hadron hadron with with partonic partonic content content Diffractive Diffractive event event = (Pomeron = (Pomeron flux) flux) (IPp (IPp collision) collision) p p Used e.g. in POMPYT IP POMWIG p PHOJET 1) σ SD 1) σand SD, σ DD and taken σ CD from setexisting by Reggeon parametrization theory. or set by user. 2) Shape 2) f IP/p of(x Pomeron IP, t) diffractive distribution mass inside spectrum, a proton, p fof IP/p proton (x IP,t) out. gives3) diffractive Smooth transition mass spectrum from simple and scattering model low p masses of proton. IPp with 3) Atfull lowpp masses machinery: retainmultiparton old framework, interactions, with longitudinal parton showers, string(s). etc. Above 4) 10 Choice GeVbetween begin smooth different transition PomerontoPDFs. IPp handled with full pp machinery: 5) Free multiple parameter interactions, σ IPp neededparton fix n showers, interactions beam = σ jet remnants, /σ IPp ) Choice between 5 Pomeron PDFs. Torbjörn Sjöstrand The Pythia 8 Event Generator slide 21/29

22 for γγisr andwith γp physics photon beams f γ a(x, Q 2 ) f γ b (x, Q2 ) e 2 b P γ bc(x) Dual nature α s of photon: direct (pointlike) and resolved (hadronlike). DGLAP evolution 2π P a bc(z) dz + dq2 α em has additional term Q 2 from2π γ qq: f γ b (x, Q2 ) df γ i (x, Q 2 ) d ln Q 2 = α em(q 2 ) ei 2 P 2π i/γ (x)+ α s(q 2 ) 1 dz ( x ) 2π z f j P z i/j (z) to finding the beam photon during j evolution x SR ow the scale beam remnants so backwards evolution can find photon beam. Have implemented combined direct + resolved for γp and γγ, for hard and soft processes; elastic and diffractive to come. Also ep and e + e in quasi-real Equivalent Photon Approximation. To come: Photon flux from hadrons (p and A), nuclear PDFs. 2 Torbjörn Sjöstrand The Pythia 8 Event Generator slide 22/29

23 Heavy-ion collisions 1 Angantyr (from 19th century Norse-style poem, like Fritiof.) (a) (b) (a) (c) (b) pp: MPIs naively attach pd: similarly CR will reduce Figure 2: Schematic pictures of multi-parton interactions Figure 3: in aapp schematic collision. picture The y-axis (c.f. figure should2) beof multiple scattering between one projectile and interpreted as rapidity. All initial- and final-state radiation target nucleons has been (e.g. removed in a pdto collisions). avoid cluttering. In (a) the second interaction is directly colour connecte multiple Each gluon should colour be interpreted chains as having two tocolour the lines first associated one, whileactivity with in (b) it, the which second in inthe nucleon psubse- quent string hadronisation will contribute to the soft Both multiplicity. cases givein rise (a) tothe final colour stringlines configurations for both that will contribute in the same way to the fi hemisphere; only diffractively excited byapomeronexcha remnants, sub scatterings stretches but all the CRway used/needed out to the proton state remnants, hadron while distribution. as one normal and in (b) and (c) the secondary scattering is colour-connected to the primary one. to reduce activity at large y. one diffractive scattering. between the two sub-scatterings gives rise to a colour flow as if they were (perturbativ connected. In this way the multiple scatterings can give risetoincreasedaverage transve function using some assumption about the matter distribution in the colliding protons and momentum from the partons coming from extra sub-scattering, without increasing an assumed impact parameter. multiplicity of soft particles due to the strings stretched all the way out to the pro Torbjörn Sjöstrand In figure 2a there is an illustration of an event remnants. The with Pythia two 8sub-scatterings Event Generator(in red and slide 23/29

24 Heavy-ion collisions 2 (1/Nev) dn ch/dη (a) Centrality-dependent η distribution, ppb, SNN = 5TeV ATLAS Pythia8/Angantyr (generated centrality) Pythia8/Angantyr ( E Pb bins from data) (1/N ev )dn ) ch ch /dη (b) Centrality-dependent distribution, ppb, NN 5TeV (a) η PbPb, S NN = 5.02 TeV Pythia8/Angantyr Pytha8/Angantyr ( E Pb ALICE PbPb S percentiles) Pytha8/Angantyr (Impact NN = 5.02 TeV parameter) (1/N ev )dn ch /dη (b) C η η 0 Glauber formalism for geometry and number of NN collisions. Figure 12: Comparison between the average charged Figure multiplicity as a function of pseudo rapidity in percentile bins of centrality ppb collisions at 17: The centrality dependence charged multiplicit Good Walker formalism for diffractive at s NN s NN = = TeV. cross TeV In (a) (a) sections. data andfrom s NN ATLAS =2.76[31] TeV is (b). Bot compared to results from Angantyr. The lines correspond 20-30% %. to the percentile Data frombins ALICE in figure [62 64]. 11 (from Full MPI machinery for NN collisions, also diffractive. top to bottom: 0 1%, 1 5%,..., 60 90%). The red line is binned using percentiles of the generated E Pb dx, and the blue line according to the experimental distribution (c.f. the table in figure 11). Pom /x Pom spectrum for energyintaken order tofrom finishbeam the discussion remnant. on the centralit In (b) the red line is the same as in (a), but here the blue line uses percentile bins based on the generated Energy momentum flavour impact parameter in Angantyr. conservation. the ALICE results on the centrality dependence of th central pseudo-rapidity bin for PbPb collisions at s N Torbjörn Sjöstrand The Pythia 8 Event Generator slide 24/29

25 Heavy-ion collisions /(2πp 2 1 d 2 N )d N/dp /dηcm (GeV 2 c 2 N 2πp T AA,m dη [mb/gev2 ] ) MC/Data η ppb 2.0, 5.02 Centrality: TeV, Inclusive 0-5 pct. charged 1.0 < η CM < ATLAS CMS Pythia8/Angantyr p p [GeV] [GeV/c] η < 2.0, Centrality: pct. η < 2.0, Centrality: pct. re 14: The transverse momentum distribution of charged particles in the central pseudoity region in inclusive 10 1 ppb events. ATLAS Pythia8/Angantyr 1 dp dη [mb/gev2 ] ATLAS Pythia8/Angantyr most fluctuations 10 1 will average out. It is therefore reasonable to assume that 10 1 basically centrality observable 10 2 based on multiplicity or energy flow the nuclei directions 10 will which data can be explained without 2 QGP? ell correlated with 10 3 the number of wounded nucleons and the actualimpactparameter e we will now compare simulation to results from the ALICE experiment,wehave 10 No explicit collective effects, for 4 10 now. 4 rinciple to use the ALICE experimental definition of centrality, rather than the one ATLAS used in the previous chapter. In ALICE centrality is defined as percentiles 10 Under active evolution to improve 5 e amplitude distribution 6 10 agreement with data. obtained in the two V0 detectors, placed at 3.7 <η< <η< Since this amplitude is not unfolded to particle level, 1.4 and 7 cannot Torbjörn Sjöstrand 1.3 The Pythia 8 Event 1.3 Generator slide 25/ N 2πp a T AA,m d 2 N d 2 N N 2πp T AA,m dp dη [mb/gev2 ] MC/Data dp dη [mb/gev2 ] N 2πp a T AA,m d 2 N η < 2.0, Centrality: pct p [GeV/c] Possibility to preselect one trigger event, e.g. Z 0 production. No quark-gluon plasma, for better or worse: ATLAS Pythia8/Angantyr

26 The ALICE revelation: goodbye jet universality! Several unexpected collective effects at LHC pp, like ridge, and Signs of QGP in high-multiplicity pp collisions? If not, what else? A whole new game! Torbjörn Sjöstrand The Pythia 8 Event Generator slide 26/29

27 Ropes and shove DIPSY: initial-state dipole evolution in transverse coordinates and longitudinal momenta. Strong string overlap! Ropes: combination of several overlapping strings into higher colour multiplets higher string tension favour strangeness, notably multistrange baryons. Shove: overlap pushes strings apart ridge effects etc. Currently not in Pythia, but ropes and shove will come. Also other collective-event alternatives available or coming. Torbjörn Sjöstrand The Pythia 8 Event Generator slide 27/29

The Pythia collaboration Current members: Christian Bierlich, Nishita Desai, Ilkka Helenius, Philip Ilten, Leif Lo nnblad, Stephen Mrenna, Stefan Prestel, Christine Rasmussen, Torbjo rn Sjo strand,

28 The Pythia collaboration Current members: Christian Bierlich, Nishita Desai, Ilkka Helenius, Philip Ilten, Leif Lo nnblad, Stephen Mrenna, Stefan Prestel, Christine Rasmussen, Torbjo rn Sjo strand, Peter Skands... but many have other projects as their main research interest. Significant code pieces contributed by 30 more persons. Comments and bug reports from > 100 persons. Torbjo rn Sjo strand The Pythia 8 Event Generator slide 28/29

Summary and outlook Core Pythia program is small and self-contained: 160k lines code, 20 MB gzipped tarball. Quick & easy to install, well documented and many examples, download from http://home.thep.

29 Summary and outlook Core Pythia program is small and self-contained: 160k lines code, 20 MB gzipped tarball. Quick & easy to install, well documented and many examples, download from Feasible to do simple standalone analyses, e.g. with jet finders. Possible to link to various external libraries. Used by many other programs, notably string fragmentation. Steady progress, e.g. heavy ions, γ beams, NLL showers. Not well structured for complete cosmic ray shower. Torbjörn Sjöstrand The Pythia 8 Event Generator slide 29/29

The Lund Model. and some extensions. Department of Astronomy and Theoretical Physics Lund University Sölvegatan 14A, SE Lund, Sweden

The Lund Model. and some extensions. Department of Astronomy and Theoretical Physics Lund University Sölvegatan 14A, SE Lund, Sweden The Lund Model and some extensions Torbjörn Sjöstrand Department of Astronomy and Theoretical Physics Lund University Sölvegatan 4A, SE-223 62 Lund, Sweden Workshop on Collective effects in small collisions