Jet production in ultra-peripheral collisions with Pythia 8 COST workshop on collectivity in heavy-ion collisions Ilkka Helenius February 28th, 2019 In collaboration with Christine O. Rasmussen and Torbjörn Sjöstrand
Outline Motivation • Ultra-peripheral collisions (UPCs) allows to study γ p and γ A , complementary to pp and p A (collectivity?) • Provide a Monte-carlo event generator for UPCs validated against HERA data • Model the factorization-breaking effects for diffractive dijets in photoproduction [I.H. and C.O.R., arXiv:1901.05261 [hep-ph]] Outline 1. Event generation in Pythia 8 2. Photoproduction and ultra-peripheral collisions 3. Dynamical rapidity gap survival model for hard diffraction 4. Summary & Outlook 1
Pythia 8 • A general-purpose Monte-Carlo event generator • Use theory where available (perturbative QCD), add phenomenological models where not Authors (release 8.240): • Torbjörn Sjöstrand Lund University • Christian Bierlich Lund University & Niels Bohr Institute • Nishita Desai CNRS-Universite de Montpellier • Ilkka Helenius University of Jyväskylä • Philip Ilten University of Birmingham • Leif Lönnblad Lund University • Stephen Mrenna Fermi National Accelerator Laboratory • Stefan Prestel Lund University • Christine O. Rasmussen Lund University • Peter Skands Monash University 2
������� ���� ������ �� � �� �� ��� ��� ����� �������� ���������� ���� �� �� ��������� ���� ���� � ��������� �� ���� ���� ������ ������������ ���� ��������� ������ ������ Event generation in Pythia 8 1. Hard scattering • Convolution of partonic cross sections and PDFs 2. Parton showers • Generate Initial and Final State Radiation (ISR & FSR) using DGLAP evolution 3. Multiparton interactions (MPIs) • Use regularized QCD 2 → 2 cross sections [Figure: S. Prestel] 4. Beam remnants • Minimal number of partons to conserve colour and flavour 5. Hadronization • Using Lund string model with color reconnection • Decays into stable hadrons 3
Ultra-peripheral heavy-ion collisions b > 2 R A Photon flux from equivalent photon approximation • Described with a flux of quasi-real (low- Q 2 ) photons ⇒ Corresponds to photoproduction in ep collisions • Flux in impact-parameter space from b min ( ≈ R A + R B ) γ ( x ) = 2 α EM Z 2 ξ K 1 ( ξ ) K 0 ( ξ ) − ξ 2 [ )] f A K 2 1 ( ξ ) − K 2 ( 0 ( ξ ) x π 2 Z is nuclear charge, ξ = b min xm , m (per-nucleon) mass 4
Event generation in photoproduction Direct processes b b • Cross section from convolution x k d σ b p → kl + X = f b γ ( x ) ⊗ f p i ( x p , µ 2 ) ⊗ d σ γ i → kl l x p p remn. Resolved processes • Convolute also with photon PDFs b b d σ b p → kl + X = f b j ( x γ , µ 2 ) remn. γ ( x ) ⊗ f γ x x γ k ⊗ f p i ( x p , µ 2 ) ⊗ d σ ij → kl l x p p remn. • Sample photon kinematics and setup γ p sub-system with W γ p • Evolve the sub-system as any hadronic collision (incl. MPIs) 5
b b b b b b b b b b b b b b b b b Dijet photoproduction in ep collisions at HERA ZEUS dijet measurement [pb] 2000 • Q 2 γ < 1 . 0 GeV 2 ZEUS d σ/ d x obs Pythia 8.226 γ resolved • 134 < W γ p < 277 GeV 1500 direct 17 < E jet1 < 25 GeV • E jet1 T > 14 GeV, T 1000 E jet2 > 11 GeV T 500 • − 1 < η jet1 , 2 < 2 . 4 Different contributions 0 1.4 ratio to Pythia 1.3 • Define 1.2 T e η jet1 + E jet2 1.1 e η jet2 = E jet1 1.0 x obs T 0.9 0.8 2 yE e γ 0.7 0.6 0.5 to discriminate direct and 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 x obs γ resolved processes [ZEUS: Eur.Phys.J. C23 (2002) 615-631] (= x γ in γ at LO parton level) • At high- x obs direct processes dominate 6 γ
Dijets in ultra-peripheral collisions by ATLAS [ATLAS-CONF-2017-011] Event selection 6 10 b / GeV ] • anti- k T with R = 0 . 4 ATLAS Preliminary anti- k R =0.4 jets t -1 lead 2015 Pb+Pb data, 0.38 nb p > 20 GeV 4 T 10 s = 5.02 TeV m > 35 GeV NN jets • p lead µ > 20 GeV, [ T 42 < H < 50 GeV A T 2 x 10 ∼ σ p jets d > 15 GeV, | η jets | < 4 . 4 2 T -1 d 50 < H < 59 GeV ( × 10 ) H T T d 1 59 < H < 70 GeV ( 10 -2 ) × T Event-level variables: -3 70 < H < 84 GeV ( × 10 ) 2 10 − T √ (Σ i E i ) 2 − � 2 -4 84 < H < 100 GeV ( × 10 ) • m jets = p i T � Σ i ⃗ � � 4 10 − -5 100 < H < 119 GeV ( × 10 ) T ( Σ i E i +Σ i p zi ) • y jets = 1 − 6 10 2 log 119 < H < 141 GeV ( 10 -6 ) × T Σ i E i − Σ i p zi -7 141 < H < 168 GeV ( × 10 ) − 8 T 10 Data • H T = Σ i p T i Pythia+STARlight -8 168 < H < 200 GeV ( × 10 ) scaled to data T − 10 • x A = m jets 10 √ s e − y jets Not unfolded for detector response − 12 10 3 10 − 10 − 2 10 − 1 1 x • Preliminary data compared to Pythia 6 where events A reweighted with photon flux from STARlight • In Pythia 8 photon flux can be set by the user 7
Dijets in ultra-peripheral collisions with Pythia 8 Dominant contributions PbPb, √ s NN = 5 . 5 TeV NNPDF2.3 • Large x A : resolved anti- k T , R = 0 . 4 EPPS16 p lead > 20 GeV/c Resolved T • Small x A : direct m jets > 35 GeV Direct d σ/ d x A [nb] • Weak dependence on γ PDF Sensitivity to nPDFs • Data not public, estimate the statistical uncertainty at Ratio to NNPDF2.3 different luminosities • Potential to constrain nPDFs L = 0 . 38 nb − 1 GRV L = 10 nb − 1 SaSgam down to x ∼ 10 − 3 x A • With lower p jets can extend T [I.H., arXiv:1811.10931 [hep-ph]] the low- x reach further [see also Guzey, Klasen, arXiv:1902.05126 [hep-ph]] 8
Factorization breaking in hard diffraction • Factorization-based H1 VFPS data AFG -PDF γ NLO H12006 Fit-B × 0.83 × (1+ δ ) calculation overshoot the hadr [pb] H1 data in photoproduction γ p IP 1000 /dz σ regime by a factor of two d 500 • But good agreement in DIS [CDF: PRL 84 (2000) 5043-5048] ratio to NLO 1.5 1 0.5 0 0.2 0.4 0.6 0.8 z IP [H1: JHEP 1505 (2015) 056] • Factorization breaking observed at Tevatron • Similar results from pp collisions at the LHC 9
2. Reject events where MPIs in p system (MPI selection) 3. Evolve IP system, allow MPIs for this subsystem Hard diffraction in photoproduction Starting point: Assume factorization of the cross section • Direct: P , µ 2 ) ⊗ f p d σ 2jets = f b γ ( x ) ⊗ d σ γ j → 2jets ⊗ f I P j ( z I P ( x I P , t ) I • Resolved: d σ 2jets = f b P , µ 2 ) ⊗ f p i ( x γ , µ 2 ) ⊗ d σ ij → 2jets ⊗ f I P γ ( x ) ⊗ f γ j ( z I P ( x I P , t ) I Direct: Resolved: b b b b remn. γ γ jet jet ✓ jet jet ✗ remn. remn. P P p p p p Dynamical rapidity gap survival for resolved events 1. Generate diffractive events with dPDFs (PDF selection) 10
3. Evolve IP system, allow MPIs for this subsystem Hard diffraction in photoproduction Starting point: Assume factorization of the cross section • Direct: P , µ 2 ) ⊗ f p d σ 2jets = f b γ ( x ) ⊗ d σ γ j → 2jets ⊗ f I P j ( z I P ( x I P , t ) I • Resolved: d σ 2jets = f b P , µ 2 ) ⊗ f p i ( x γ , µ 2 ) ⊗ d σ ij → 2jets ⊗ f I P γ ( x ) ⊗ f γ j ( z I P ( x I P , t ) I Direct: Resolved: b b b b remn. γ γ jet jet ✓ jet jet ✗ remn. remn. P P p p p p Dynamical rapidity gap survival for resolved events 1. Generate diffractive events with dPDFs (PDF selection) 2. Reject events where MPIs in γ p system (MPI selection) 10
Hard diffraction in photoproduction Starting point: Assume factorization of the cross section • Direct: P , µ 2 ) ⊗ f p d σ 2jets = f b γ ( x ) ⊗ d σ γ j → 2jets ⊗ f I P j ( z I P ( x I P , t ) I • Resolved: d σ 2jets = f b P , µ 2 ) ⊗ f p i ( x γ , µ 2 ) ⊗ d σ ij → 2jets ⊗ f I P γ ( x ) ⊗ f γ j ( z I P ( x I P , t ) I Direct: Resolved: b b b b remn. γ γ jet jet ✓ jet jet ✗ remn. remn. P P p p p p Dynamical rapidity gap survival for resolved events 1. Generate diffractive events with dPDFs (PDF selection) 2. Reject events where MPIs in γ p system (MPI selection) 3. Evolve γ IP system, allow MPIs for this subsystem Originally for pp in [C.O. Rasmussen and T. Sjöstrand, JHEP 1602 (2016) 142] 10
Comparisons to HERA data H1 2007: ZEUS 2008: [EPJC 55 (2008) 177] [EPJC 51 (2007) 549] • Q 2 < 0 . 01 GeV 2 • Q 2 < 1 GeV 2 , 0 . 2 < y < 0 . 85 • x IP < 0 . 03 • x IP < 0 . 025 • E jet1 > 5 . 0, E jet2 • E jet1 > 7 . 5, E jet2 > 4 . 0 GeV > 6 . 5 GeV T T T T • − 1 . 0 < η jet1,2 < 2 . 0 • − 1 . 5 < η jet1,2 < 1 . 5 Observables Default Pythia setup • W γ p (H1) • dPDFs from H1 fit B LO • M X (ZEUS) • γ PDFs from CJKL jet ( E jet + p jet • p ref ∑ z ) T0 = 3 . 00 GeV/ c • z obs = IP i ∈ X ( E i + p i ∑ z ) (Tuned to HERA γ p data) jet ( E jet − p jet ∑ z ) • x obs = i ∈ X ( E i − p i γ ∑ z ) 11
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