ep in Pythia 8 Poetic-8 Satellite Workshop on Monte Carlo Event Generators Ilkka Helenius March 23rd, 2018

Transcription

1 ep in Pythia 8 Poetic-8 Satellite Workshop on Monte Carlo Event Generators Ilkka Helenius March 23rd, 208 Tübingen University Insititute for Theoretical Physics

2 Outline A (biased) overview on recent Pythia 8 developments regarding to ep collisions Outline. DIS in Pythia 2. Photoproduction in Pythia 3. Hard diffractive photoproduction 4. Summary & Outlook

3 Pythia 8 A general-purpose Monte-Carlo event generator Current version 8.230, next week Main focus has been in pp, now extensions to ee, ep, pa, AA Team: Torbjörn Sjöstrand Christian Bierlich Nishita Desai Nadine Fischer Ilkka Helenius Philip Ilten Leif Lönnblad Stephen Mrenna Stefan Prestel Christine O. Rasmussen Lund University Lund University CNRS-Universite de Montpellier Monash University Tübingen University University of Birmingham Lund University Fermi National Accelerator Laboratory Fermi National Accelerator Laboratory Lund University Peter Skands Monash University 2

4 Event classes in ep Virtuality of photon related to scattering angle of the lepton Q 2 2E 2 l ( x)( cos θ) Deep inelastic scattering (DIS) High virtuality, Q 2 > a few GeV 2 Hard process + Parton showers Photoproduction (PhP) Low virtuality, Q 2 GeV 2 Intermediate photon may fluctuate into hadronic state Resolved γ Factorize γ flux, set up γp collision Also multiparton interactions (MPIs) possible 3

5 DIS in Pythia

6 DIS in Pythia Hard processes Neutral current, γ /Z exchange, also separate contributions Charged current, W ± exchange Parton shower New dipole-shower option SpaceShower:dipoleRecoil B. Cabouat and T. Sjöstrand [arxiv: [hep-ph]] Alternative to the default global recoil approach, keeps the scattered lepton momentum intact suitable for DIS Also linking with Dire shower is a valid option for DIS S. Höche and S. Prestel [Eur.Phys.J. C75 (205) 46] 4

7 0 3 Comparison.0. to DIS data from HERA.0. 5 η MC/Data /N de deec /dη / GeV /dω MC/Data deec p /dω (a) Transverse energy energy flow as acorrelation function offor rapidity, x < 0 3 x > B. Cabouat and T. Sjöstrand [arxiv: [hep-ph]] Data New (c) (b) η Data from H, [Z. Phys. (b) C63, (994)] ηω Transverse p 2 as a function energy energy of x correlation for x > 0 3 L all W Data Data Below data for energy-energy 0 New correlations 2 New 0 Results based on existing default tune with global recoil MC/Data deec /dω MC/Data Transverse energy energy correlation for x > 0 3 Data New Reasonable agreement for single-particle properties /σ dσ/dx L (d) η ω 5

8 Photoproduction

9 Photoproduction in ep Direct processes Photon initiator of the hard process No MPIs but FSR and ISR for hadron Resolved processes Photon fluctuates into a hadronic state (VMD and anomalous) Partonic structure described with PDFs FSR and ISR for both sides, also MPIs 6

10 MPI and parton shower generation in Pythia 8 Common evolution scale (p T ) for FSR, ISR and MPIs ( dp dpmpi = dp T dp T [ exp + dp ISR + ) dp FSR dp T dp T p max T p T dp T ( dpmpi dp T where exp[...] is a Sudakov factor Simultaneous partonic evolution + dp ISR dp T + ) ] dp FSR dp T. Start the evolution from a scale related to the hard process 2. Sample p T values for each P i, pick one with highest p T 3. Continue from the sampled p T until reach p Tmin Λ QCD 7

11 MPIs in Pythia 8 Probability for MPIs from 2 2 QCD processes Partonic cross section diverges at p T 0 Regulate the divergence with screening parameter p T0 dσ 2 2 dp 2 T α s(p 2 T ) p 4 T α s(p 2 T0 + p2 T ) (p 2 T0 + p2 T )2 pp: Power-law in s p T0 ( s) = p ref T0 ( s/7 TeV) α = 2.28 GeV/c, α = 0.25 p ref T0 (Monash tune) γγ: Logarithmic in s p T0 ( s) = p ref T0 + α log ( s/00 GeV ) p ref T0 =.52 GeV/c, α = 0.43 (I.H., T. Sjöstrand, in prep.) Parametrization for γp? pt0( s) [GeV/c] γγ pp s [GeV] 8

12 Parton showers in photoproduction DGLAP equations for photons Additional term due to γ qq splittings f γ i (x, Q2 ) log(q 2 ) = α em 2π e2 i P iγ(x) + α s(q 2 ) 2π Add corresponding term to ISR probability dp a b = dq2 Q 2 j x dz z P ij(z) f j (x/z, Q 2 ) α s x f γ a (x,q 2 ) 2π xf γ b (x,q2 ) P a bc(z) dz + dq2 α em e 2 Q 2 b P γ bc(x) 2π f γ b (x,q2 ) Corresponds to finding the beam photon during evolution No further ISR No further MPIs No need for beam remnants 9

13 Charged particle p T spectra in ep collisions at HERA d 2 σ/dηdp 2 T [nb] ratio to Pythia H Pythia resolved direct p ref T0 = 3.00 GeV/c η < pt [GeV/c] [H: Eur.Phys.J. C0 (999) ] H measurement E p = 820 GeV, E e = 27.5 GeV < W γp > Q 2 γ < 0.0 GeV GeV Comparison to Pythia 8 Resolved contribution dominates Good agreement with the data using p ref T0 = 3.00 GeV/c MPI probability between pp and γγ 0

14 Charged particle p T spectra in ep collisions at HERA d 2 σ/dηdp 2 T [nb] MC/Data H p ref T,0 = 2.28 GeV, χ 2 /n = 9.90 p ref T,0 = 2.70 GeV, χ 2 /n =.85 p ref T,0 = 3.00 GeV, χ 2 /n = 0.79 p ref T,0 = 3.30 GeV, χ 2 /n =.69 MPI off, χ 2 /n = p T [GeV/c] [H: Eur.Phys.J. C0 (999) ] H measurement E p = 820 GeV, E e = 27.5 GeV < W γp > Q 2 γ < 0.0 GeV GeV Comparison to Pythia 8 Resolved contribution dominates Good agreement with the data using p ref T0 = 3.00 GeV/c MPI probability between pp and γγ 0

15 Charged particle p T spectra in ep collisions at HERA d 2 σ/dηdp 2 T [nb] pt0( MC/Data s) [GeV/c] γγ γp pp H p ref T,0 = 2.28 GeV, χ 2 /n = 9.90 p ref T,0 = 2.70 GeV, χ 2 /n =.85 p ref T,0 = 3.00 GeV, χ 2 /n = 0.79 p ref T,0 = 3.30 GeV, χ 2 /n =.69 MPI off, χ 2 /n = p T [GeV/c] H measurement E p = 820 GeV, E e = 27.5 GeV < W γp > Q 2 γ < 0.0 GeV GeV Comparison to Pythia 8 Resolved contribution dominates Good agreement with the data using p ref T0 = 3.00 GeV/c MPI probability between pp and γγ s [GeV] 0

16 Dijet photoproduction in ep collisions at HERA ZEUS dijet measurement Q 2 γ <.0 GeV 2 34 < W γp < 277 GeV E jet T E jet2 T > 4 GeV, > GeV < η jet,2 < 2.4 Different contributions Define x obs γ = Ejet T e ηjet + E jet2 T e ηjet2 2yE e to discriminate direct and resolved processes (=x in γ at LO parton level) dσ/dx obs γ [pb] ratio to Pythia ZEUS Pythia resolved direct 7 < E jet T < 25 GeV x obs γ [ZEUS: Eur.Phys.J. C23 (2002) 65-63] At high-x obs γ direct processes dominate

17 Hard diffractive photoproduction

18 Hard diffraction in ep e + (k) e + (k ) e + (k) e + (k ) p(p) γ * (q) z IP x IP (u) (v) jet jet remnant p(p') M 2 X(P ) X [Figure: H: (a) JHEP 505 (205) 056] Diffractive dijets p(p) γ* (q) x γ (u) jet jet M 2 X(P ) X z IP (v) rapidity gap remnant between proton and x IP Pomeron remnant (b) p(p') Factorized cross section in hard diffraction Figure : Leading order diagrams of the direct a) and resolved b) diffractive dijet production. DIS: dσ 2jets = f 3 Factorisation in Diffractive i IP (z IP, µ 2 ) f p Dijet IP (x IP, t) dσ ie 2jets Production PhP: dσ 2jets = fγ e (x γ ) fi IP (z IP, µ 2 ) f p IP (x IP, t) dσ iγ 2jets In the QCD where factorisation f p isapproach Pomeron the diffractive flux dijet andcross f IP section diffractive is given bypdf the convolution (dpdf) of partonic cross sectionsdˆσ IP with diffractive parton distributionsf i i/p D : Photon interacts with Pomeron remnant from proton which produce jets Can be DIS or photoproduction Signature: scattered proton or 2

19 ] Breaking of factorization Diffractive dijet cross section in DIS and photoproduction H VFPS data H VFPS data H VFPS data AFG γ -PDF [pb] dσ/dz IP 00 H DIS 50 NLO H2006 Fit-B 0.83 (+δ ) hadr [pb] dσ/dx IP 4000 H DIS NLO H2006 Fit-B NLO 0.83 H2006 (+δfit-b) 0.83 (+δ ) hadr hadr [pb] dσ/dz IP 500 H γp [pb] dσ/dx IP ratio to NLO dσ/dy [pb] z IP H VFPS edata ηjet + E jet2 e ηjet2 z obs IP = Ejet T T ratio to NLO NLO H2006 Fit-B 2x IP 0.83 E p (+δ ) NLO H2006 Fit-B 0.83 (+δ ) hadr hadr Good agreement with 0 H data and NLO pqcd inh DIS 2 [pb/gev H VFPS data DIS 500 DIS γ NLO 00 overshoots the data by factor of 2 in photoproduction ratio to NLO dσ/dy [pb] NLO H2006 Fit-B 0.83 (+δ ) hadr H p ratio to NLO x IP z IP [H: VFPS JHEP data 505 AFG (205) γ -PDF 056] [pb] dσ/dx γ

20 Hard diffraction in Pythia 8 X(P ) X Dynamical rapidity gap survival Originally introduced for pp e + (k) C. O. Rasmussen and T. Sjöstrand [JHEP 602 (206) 42] p(p) γ* (q) x IP xγ z IP (b) e + (k ) (u) (v) remnant jet jet M 2 X(P ) X remnant p(p') [Figure: H: JHEP 505 (205) 056] s of the direct a) and resolved b) diffractive dijet production. ep implementation I.H., C. O. Rasmussen and T. Sjöstrand Select diffractive events based on dpdfs (γ or proton) Check whether MPIs between (resolved) photon and proton Reject events where MPIs shroud the diffractive signature ffractive Dijet Production 4

21 Comparison to ZEUS measurement (preliminary) ZEUS diffractive dijets Q 2 γ <.0 GeV < y < 0.85 E jet T E jet2 T > 7.5 GeV, > 6.5 GeV.5 < η jet,2 <.5 Pythia setup Pomeron flux: H Fit B Diffractive PDF: H Fit B LO PDF selection overshoots the data by 40% Good agreement with MPI selection dσ/dm X [pb] MC/Data Mass of the hadronic system ZEUS PDF selection MPI selection M X [ZEUS: Eur.Phys.J. C55, 77 9 (2008)] 5

22 Comparison to ZEUS measurement Diffractive PDF: H Fit B LO dσ/dz P obs [pb] ZEUS PDF selection MPI selection Diffractive PDF: H Fit A NLO dσ/dz P obs [pb] ZEUS PDF selection MPI selection MC/Data MC/Data zp obs zp obs [ZEUS: Eur.Phys.J. C55, 77 9 (2008)] Some distributions not that well described Results very sensitive to Pomeron PDFs (and flux) Promising results but large uncertainties from dpdfs 6

23 Summary & Outlook Current ep capabilities of Pythia DIS with new SpaceShower:dipoleRecoil shower Photoproduction including Automatic mixing of direct and resolved processes Full parton-level evolution including MPIs for resolved γp Possible to use photon flux from nuclei Next release (8.235, next week) Soft diffraction for γγ and γp Hard diffractive photoproduction in ep based on Diffractive PDFs Dynamical rapidity gap survival based on MPIs 7

24 Summary & Outlook Future work Soft diffraction in ep Hard diffraction in DIS Smooth merging of photoprodcution and DIS Combine photoproduction framework with Angantyr heavy-ion model [C. Bierlich] MC for ea collisions Capability to simulate γa interactions in ultra-peripheral heavy ion collisions with a proper nuclear target For MCEG development Rivet analyses for HERA data would be very welcome 8