Heavy ion physics with the PytHIa generator

Transcription

1 Heavy ion physics with the PytHIa generator Christian Bierlich With L. Lönnblad and G. Gustafson Lund University Aug 21, 2017 Novel tools and observables for jet physics in heavy-ion collisions Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 1 / 24

2 PytHIa what, how and why? What? Extending Pythia8 to also handle heavy ion collisions. Previously models in the Dipsy generator. Jet universality: Same models should describe e + e to AA. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 2 / 24

3 PytHIa what, how and why? What? Extending Pythia8 to also handle heavy ion collisions. Previously models in the Dipsy generator. Jet universality: Same models should describe e + e to AA. How? Extending the Multiparton Interactions framework with Glauber model. Interactions of Lund strings provides QGP effects works well for pp. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 2 / 24

4 PytHIa what, how and why? What? Extending Pythia8 to also handle heavy ion collisions. Previously models in the Dipsy generator. Jet universality: Same models should describe e + e to AA. How? Extending the Multiparton Interactions framework with Glauber model. Interactions of Lund strings provides QGP effects works well for pp. Why? Smooth transistion from small to large systems points to common origin. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 2 / 24

5 PytHIa what, how and why? What? Extending Pythia8 to also handle heavy ion collisions. Previously models in the Dipsy generator. Jet universality: Same models should describe e + e to AA. How? Extending the Multiparton Interactions framework with Glauber model. Interactions of Lund strings provides QGP effects works well for pp. Why? Smooth transistion from small to large systems points to common origin. Benefit from existing infrastructure for e.g. hard processes. Tuned to small systems only built from several elements. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 2 / 24

6 PytHIa what, how and why? What? Extending Pythia8 to also handle heavy ion collisions. Previously models in the Dipsy generator. Jet universality: Same models should describe e + e to AA. How? Extending the Multiparton Interactions framework with Glauber model. Interactions of Lund strings provides QGP effects works well for pp. Why? Smooth transistion from small to large systems points to common origin. Benefit from existing infrastructure for e.g. hard processes. Tuned to small systems only built from several elements. Glauber Gribov CF Dipsy initial state model Multiparton interactions Proton+Pomeron PDFs Parton shower Colour reconnection Hadronization Rope formation String shoving Predictions for pa and AA pp semi-inclusive cross sections ep and pp data e + e data pp data Tuning Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 2 / 24

7 Particle production: MPIs (Sjöstrand and van Zijl: Phys.Rev. D36 (1987) 2019) Starting point: Pythia8 MPI model (pp). Several partons taken from the PDF. Hard sub-collisions with 2 2 ME: dσ 2 2 dp 2 α2 s (p 2 ) p 4 α2 s (p 2 + p2 0 ) (p 2 + p2 0 )2. Figure T. Sjöstrand Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 2 / 24

8 Particle production: MPIs (Sjöstrand and van Zijl: Phys.Rev. D36 (1987) 2019) Starting point: Pythia8 MPI model (pp). Several partons taken from the PDF. Hard sub-collisions with 2 2 ME: dσ 2 2 dp 2 α2 s (p 2 ) p 4 α2 s (p 2 + p2 0 ) (p 2 + p2 0 )2. Momentum conservation and PDF scaling. Ordered emissions: p 1 > p 2 > p 4 >... from: [ P(p = p i ) = 1 dσ 2 2 exp σ nd dp p i 1 p 1 σ nd Figure T. Sjöstrand dσ dp dp Number distribution narrower than Poissonian (momentum and flavour rescaling). Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 1 / 24 ]

9 Extrapolating pp: Glauber + partonic picture from Dipsy Corrections to Glauber-Gribov based on Dipsy partonic picture. Q: Reproduce centrality forward particle production? Wounded nucleons including fluctuations in target and projectile. Notation: Optical theorem in b-space I(A el ) = 1 2 ( A el 2 + P abs ); T ia el dσ el d 2 b = T (b) 2, dσ tot d 2 b dσ abs d 2 b = 2 T (b) = 2 T (b) T (b) 2 Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 2 / 24

10 The wounded cross section (CB et al.: arxiv: [hep-ph]) Fluctuations related to diffractive excitations: Good-Walker. dσ tot d 2 b = 2 T t,p, dσ el d 2 b = T 2 t,p, dσ SD,(p t) d 2 b = T 2 (t p) T (p t) 2 p,t dσ DD d 2 b = T 2 T p,t 2 t p T 2 p + T t 2 p,t Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 3 / 24

11 The wounded cross section (CB et al.: arxiv: [hep-ph]) Fluctuations related to diffractive excitations: Good-Walker. dσ tot d 2 b = 2 T t,p, dσ el d 2 b = T 2 t,p, dσ SD,(p t) d 2 b = T 2 (t p) T (p t) 2 p,t dσ DD d 2 b = T 2 T p,t 2 t p T 2 p + T t 2 p,t The wounded cross section is the sum of: Wounded cross section dσ w d 2 b = dσ abs d 2 b + dσ SD,t d 2 b + dσ DD d 2 b = 2 T p,t T 2 t. p Contributions to centrality observable: absorptively wounded, diffractively wounded, NOT elastically scattered. We need now to calculate T (b). Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 3 / 24

12 The Dipsy model (Flensburg et al. arxiv: [hep-ph]) Partonic model in impact parameter space: Dipole evolution in Impact Parameter Space and rapidity. LL-BFKL with some corrections built on Mueller dipole model [Mueller and Patel arxiv:hep-ph/ ]. Proton/Nucleus structure built up dynamically from dipole splittings: dp dy = 3α s 2π 2 d 2 ( x y) 2 [ ( z ( x z) 2 ( z y) 2, f ij = α2 s ( xi y j ) 2 ( y i x j ) 2 )] 2 log 8 ( x i x j ) 2 ( y i y j ) 2 ( Optical theorem gives: T (b) = 1 exp ) ij f ij Will serve as an initial state truth for parametrization development. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 4 / 24

13 Glauber-Gribov fluctuations (GG or GGCF) Parametrization of cross section fluctuations in Glauber-Gribov formalism (Alvioli and Strikman: arxiv: [hep-ph]): Parametrization of total cross section distribution: σ tot = dσσp tot (σ) = dσρ σ2 exp [ (σ/σ 0 1) 2 ] σ + σ 0 Ω 2 Normal usage: With black disk, scale to total inelastic σ in = λσ tot. From arguments above, should be σ w BUT! σ Glauber = σ w in GG/GGCF is not enough. Lack of information wrt. Dipsy calculates full T (b). Assume semi-transparent disk: T (pp) (b, σ) = T 0 Θ ( ) σ b 2πT 0 Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 5 / 24

14 Fit to semi-inclusive cross sections. Log-normal distribution fits Dipsy better. σ tot = d 2 b dσp tot (σ)2t (pp) (b, σ), σ el = d 2 b dσp tot T (pp) 2 (b, σ), ( ) σ winc = d 2 [ b dσp tot (σ) 2T (pp) (b, σ) T (pp) ] 1 (b, σ), P tot (σ, b) = Ω 2π exp log2 (σ/σ 0 ) 2Ω DIPSY GG Ω =0.37 GG Log-normal Ω =0.25 GG Log-normal Ω = P winc (σ) σ [mb] Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 6 / 24

15 Types of wounded nucleons We can now fit to pp cross sections and obtain: 1 The number of wounded nucleons inc. diffractive excitation. 2 T (b) assumption+good Walker which are which! ( ) 2 α P(diff w incl ) = Θ σgg /π (r 1 r 2 ) b 2 αc. We now have input for a model for particle production Black Disk GG + target fluct. Ω =0.82 GG Log-normal + target fluct. Ω =0.43 2x2 model Black Disk GG Ω =0.82 GG Log-normal Ω =0.43 2x2 model P(N w ) P(N w ) N t w abs N t w inc Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 7 / 24

16 Full final states: Revival of Fritiof One absorptive collision contributes to full rapidity span. The rest contributes similarly to diffractive excitation (plus a colour exchange). Implementation in Pythia8 (FritiofP8), but idea is general. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 8 / 24

17 Results [Data: ATLAS: [hep-ex]] Very good agreement with centrality observable. Absorptive overshoots. Measuring the exact region where diffractive excitation is important. dn/(ndσe ) [GeV 1 ] Sum E, 4.9 <η < 3.2, p >0.1 GeV FritiofP8 Absorptive DIPSY ΣE [GeV] Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 9 / 24

18 Multiplicity Reproducing central collisions well. Does better than Dipsy in central collision. Comparison with Rivet see talk by J. F. Grosse-Oetringhaus. dn/dη Centrality 0-1% Data FritiofP8 Absorptive DIPSY dn/dη Centrality 10-20% Data FritiofP8 Absorptive DIPSY MC/Data MC/Data Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 10 / 24

19 Transverse momentum (Data: CMS: [nucl-ex]) Low-p region improved from Absorptive model. Large uncertainties from pdf in this observable. (Dipsy not in figure does poorly for high p ). 1/(2πp )d 2 N/dp /dη (GeV 2 c 2 ) MC/Data TeV, Inclusive charged 1.3 < η < Data FritiofP8 Absorptive p [GeV] 1/(2πp )d 2 N/dp /dη (GeV 2 c 2 ) MC/Data TeV, Inclusive charged 1.3 < η < Data FritiofP8 Absorptive p [GeV] Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 11 / 24

20 Preliminary AA Preliminary comparison to 200 GeV/nn. Centrality dependence at mid-rapidity Normalization? 1 dn N dy y= Multiplicity centrality dependence, p > 0 GeV Data phenix MC/Data Centrality Validated data comparison needed (see next talk). Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 12 / 24

21 Partial summary Including diffractive excitation is important for centrality observables. Reproducing charged particle spectra well. Functional Underlying Event for heavy ions. Signal processes can be added out of the box. Available in Pythia8 soon. Now: Microscopic model for QGP effects: modelled as interactions between Lund strings. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 13 / 24

22 Concepts of String Hadronization hep-ph/ Linear confinement potential V (r) κr, confirmed on the lattice. Valid for large distances for small distances perturbation theory should be valid. Figure credit: Torbjörn Sjöstrand Very simple, but powerful, picture. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 14 / 24

23 Lund String Hadronization (See e.g. hep-ph/ ) Non-perturbative phase of final state. Confined colour fields strings with tension κ 1 GeV/fm. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 15 / 24

24 Lund String Hadronization (See e.g. hep-ph/ ) Non-perturbative phase of final state. Confined colour fields strings with tension κ 1 GeV/fm. ( ) Breaking/tunneling with P exp πm2 κ gives hadrons. Lund symmetric fragmentation function ( ) f (z) z 1 (1 z) a bm exp. z a and b related to total multiplicity. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 15 / 24

25 Lund String Hadronization (See e.g. hep-ph/ ) Non-perturbative phase of final state. Confined colour fields strings with tension κ 1 GeV/fm. ( ) Breaking/tunneling with P exp πm2 κ gives hadrons. Lund symmetric fragmentation function ( ) f (z) z 1 (1 z) a bm exp. z a and b related to total multiplicity. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 15 / 24

26 Lund String Hadronization (See e.g. hep-ph/ ) Non-perturbative phase of final state. Confined colour fields strings with tension κ 1 GeV/fm. ( ) Breaking/tunneling with P exp πm2 κ gives hadrons. u d d s s ud ud u ū ū Lund symmetric fragmentation function ( ) f (z) z 1 (1 z) a bm exp. z a and b related to total multiplicity. Flavours determined by relative probabilities ρ = P strange P u or d, ξ = P diquark P quark Probabilities related to κ by Schwinger equation. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 15 / 24

27 Busy events QGP effects, or...? Interactions between strings must be treated. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 16 / 24

28 Busy events QGP effects, or...? Interactions between strings must be treated. 1 Strings produced with no transverse width, expands with c. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 16 / 24

29 Busy events QGP effects, or...? Interactions between strings must be treated. 1 Strings produced with no transverse width, expands with c. 2 Strings propagate according to parton p. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 16 / 24

30 Busy events QGP effects, or...? Interactions between strings must be treated. 1 Strings produced with no transverse width, expands with c. 2 Strings propagate according to parton p. 3 Overlapping strings will push each other. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 16 / 24

31 Busy events QGP effects, or...? Interactions between strings must be treated. 1 Strings produced with no transverse width, expands with c. 2 Strings propagate according to parton p. 3 Overlapping strings will push each other. 4 At t 2 fm/c strings will hadronize. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 16 / 24

32 Shoving: A microscopic model for QGP dynamics (CB, Gustafson and Lönnblad, arxiv: [hep-ph].) Idea: The overlapping regions will generate a transverse pressure. First suggested by Abramovsky et al. in (Pisma Zh. Eksp. Teor. Fiz. 47, 281 (1988)) b y b x t = t 1 t = t 2 t = t 3 t = t 4 Initial study focuses on soft effects. Non-trivial effect on jets, affected only at earliest times. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 17 / 24

33 Local pressure, dynamically generated Pressure will vary from string to string, slice to slice. Slices treated independently during shoving. Greatly affected by event-by-event fluctuations. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 18 / 24

34 In the transverse plane Shoving resolved pair-wise, p conservation. Practically done by adding a small excitation (gluon) to the string in each slice. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 19 / 24

35 The shoving pressure p push on string segment, length δl, time interval δt. If everything starts in a point at t = 0 then δl = tδy. δp 12 = f 12 δlδt = f 12 tδyδt The force is f ; chromoelectric field of effective dual s.c. (lattice). Approximate with Gaussian: ( E l = C 0 exp x 2 ) 2R 2 Interaction energy between two vortex lines: U 12 E l f 12 = U 12 x 12 ( = Cx 12 exp x 12 2 ) 2R 2 (Cea et al. arxiv: [hep-lat]) Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 20 / 24

36 Two particle correlations Gives a centrality dep t ridge (and thus flow). Model still not fully understood especially connection to hard production. S( η, φ)/b( η, φ) Pythia8 default Shoving 20 Shoving 5 S( η, φ)/b( η, φ) Pythia8 default Shoving 20 Shoving Ratio φ Ratio φ Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 21 / 24

37 Modifications to hadronization Strings still overlapping at hadronization time creates rope. Modifies string tension changes hadrochemistry. Strange suppression is detemined by: ( ρ = exp π(m2 s mu) 2 ). κ Effective parameter ρ ξ ã b Enhancement from SU(3) multiplet structure. h = κ κ = C 2(multiplet) C 2 (singlet) h (Enhancement of string tension) Also in accordance with lattice QCD. Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 22 / 24

38 Flavour composition more strange quarks Investigated inclusively next step: Jet hadrochemistry. Jets escaping at early times 2K 0 s /π ± (Λ + Λ)/π ± 6φ/π ± 6(Ξ + Ξ)/π ± ratio to π + π (Ω + Ω)/π ± e + e DIPSY pp Pythia8 + ropes DIPSY ppb DIPSY PbPb N ch, 2 < η <4, p > 0.15 GeV Implemented in DIPSY; (CB and Christiansen: arxiv: [hep-ph]) Pythia8 by plugin: (CB: arxiv: [hep-ph]) (ALICE: arxiv: [nucl-ex]) Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 23 / 24

39 Conclusions and Outlook New general purpose event generator for HI built on: 1 Pythia model for MPIs. 2 Extrapolation model based on GGCF. 3 Microscopic collectivity: Rope Hadronization and shoving. Microscopic models: QGP effects from interactions between strings. Available in Pythia8 for pp primo October. Rope hadronization: good results for strangeness in pp through AA inclusively. Interest in taking it to eg. jet hadrochemistry, provided data exists. Shoving: Currently soft effects non-trivial interplay with jets. Point to take home: General purpose MC for heavy ions rapidly developing solid comparison to data is neccesary. Interested? Opportunities for eg. short term (funded!) projects for Ph.D. students. Contact me. Thank you for your attention! Christian Bierlich (Lund) PytHIa Aug 21, CERN TH 24 / 24