ZEUS Novosibirsk, Russia 15-19 June, 2015 O U T L I N E - PowerPoint PPT Presentation

Heavy flavour production at HERA Uri Karshon Weizmann Institute of Science, ISRAEL On behalf of the H1 and ZEUS Collaborations PHOTON-2015 Conference Budker Institute of Nuclear Physics, ZEUS Novosibirsk, Russia 15-19 June, 2015 O U T L I N E Introduction and experimental set-up Theory of heavy quark production D ∗± photoproduction at 3 center-of-mass energies Charm fragmentation fractions in photoproduction D ± production in deep inelastic scattering HERA charm data combination in DIS Combination of D ∗± differential cross sections in DIS Beauty production in DIS Summary

Introduction and experimental set-up s = ( P + k ) 2 e + (k) e + (k') e ± ( k ) + p ( P ) → e ± ( k ′ ) + X ; Photon virtuality: Q 2 = − q 2 = − ( k − k ‘ ) 2 γ * (q) Q 2 Inelasticity: y = q · P Bjorken x: x = 2 q · P ; c k · P Q 2 = sxy ; W = γ ∗ p CM energy W 2 Photoproduction (PHP): Q 2 ≃ 0 GeV 2 ( e ± undetected) _ c Deep Inelastic Scattering (DIS): Q 2 > 1 or 5 GeV 2 ( e ± detected) g (x g P) proton (P) BGF: Dominant process for c,b production in DIS Direct probe of gluon density in proton; Sensitivity to c,b quark masses HERA: unique e ± p collider with E ( e ± , p ) = 27 . 6 , 820 / 920 GeV Halle NORD (H1) Hall NORTH (H1) Hall nord (H1) 2 main experiments: H1, ZEUS � HERA 2 run periods: HERA I, HERA II Halle OST (HERMES) Hall EAST (HERMES) Hall est (HERMES) 1995-2000 2003-2007 Halle WEST (HERA-B) Hall WEST (HERA-B) Hall ouest (HERA-B) Elektronen / Positronen √ s Electrons / Positrons Electrons / Positons � Protonen 318 (300) 318 GeV Protons� Protons� � HASYLAB � Synchrotronstrahlung Synchrotron Radiation DORIS Rayonnement Synchrotron 7 · 10 31 cm − 2 s − 1 1 . 5 · 10 31 DESY L PETRA Halle SÜD (ZEUS) Hall SOUTH (ZEUS) Hall sud (ZEUS) 373 pb − 1 L int 126 HERA II data taken ≈ half e + p and half e − p In 2007 two short runs at lower p energies: E p = 575 GeV; E p = 460 GeV Heavy Flavour production at HERA U. Karshon 2

Theory of heavy quark production Several QCD NLO schemes for heavy quark (Q=c or b) production: 1)Massive scheme: Q 2 ≈ m 2 Q Fixed flavour number scheme (FFNS) • 3 active flavours in proton; Q-quark not considered as parton in p • c or b produced perturbatively in hard scattering (see p.2) • Mass effects correctly included Spoiled by large logs of Q 2 /m 2 • Q , p t /m Q ... 2)Massless scheme: Q 2 >> m 2 Q Zero-mass variable flavour number scheme (ZM-VFNS) • c or b treated as massless parton Resummation of large logarithms of Q 2 /m 2 • Q • ⇒ c or b density added as 4th flavour like the light quarks At intermediate Q 2 the 2 schemes should be merged 3) General-mass variable flavour number scheme (GM-VFNS) Equivalent to FFNS for Q 2 ≤ m 2 Q and to ZM-VFNS for Q 2 > m 2 • Q • Interpolation in between (various schemes interpolate differently) • Used in parton density function (PDF) fits (useful at LHC) Heavy Flavour production at HERA U. Karshon 3

D ∗± photoproduction at 3 CM energies Clear D ∗± signals seen in M ( K − π + π + s ) − M ( K − π + ) distributions at 3 different CM energies: √ s = 318 , 251 , 225 GeV in the kinematic region: 1 . 9 < p D ∗ < 20 GeV ; | η D ∗ | < 1 . 6 ; T Q 2 < 1 GeV 2 ; 0 . 167 < y < 0 . 802 JHEP 10 (2014) 003 HER: L = 144 pb − 1 MER: L = 6 . 3 pb − 1 LER: L = 13 . 4 pb − 1 ZEUS ZEUS ZEUS ZEUS ZEUS ZEUS 240 Entries Entries Entries 6000 → π π → π π D* K → π π D* K D* K s 220 s s 400 -1 -1 ZEUS 144 pb ( s = 318 GeV) -1 ZEUS 6.3 pb ( s = 251 GeV) ZEUS 13.4 pb ( s = 225 GeV) 200 5000 Wrong-sign combinations Wrong-sign combinations 350 Wrong-sign combinations 180 Signal region Signal region Signal region 300 Background fit (correct-sign) 160 Background fit (correct-sign) 4000 Background fit (correct-sign) Background fit (wrong-sign) Background fit (wrong-sign) 140 Background fit (wrong-sign) 250 120 3000 200 100 80 150 2000 60 100 1000 40 50 20 0 0 0 0.14 0.145 0.15 0.155 0.16 0.165 0.17 0.14 0.145 0.15 0.155 0.16 0.165 0.17 0.14 0.145 0.15 0.155 0.16 0.165 0.17 π π π M(K )-M(K ) (GeV) π π π M(K )-M(K ) (GeV) π π π M(K )-M(K ) (GeV) s s s N ( D ∗ ) = 12256 ± 191 N ( D ∗ ) = 417 ± 37 N ( D ∗ ) = 859 ± 49 Background estimated by fitting simultaneously correct- and wrong-sign distributions in the range ∆ M < 0 . 168 GeV Heavy Flavour production at HERA U. Karshon 4

D ∗± photoproduction at 3 CM energies Visible D ∗ PHP cross sections obtained from: σ vis ( D ∗ ) = N data ( D ∗ ) A · BR ·L BR = B ( D ∗ → D 0 π ) · B ( D 0 → Kπ ) = 0 . 0263; A = acceptance σ i Ratio of visible cross sections: R σ = σ HER ; i = HER, MER, LER yields higher precision of E-dependence of cross section since some syst. uncertainties in data and theory cancel Data compared to FFNS NLO predictions: ZEUS 1.2 Total syst. uncertainty ≈ 5% in data σ R 192 GeV 〈 〉 W = 1 152 GeV few % in theory 136 GeV 0.8 < W > = mean W from generated MC 0.6 2 2 Q < 1 GeV MER/LER cross sections similar → η ZEUS ep D* X D* | | < 1.6 0.4 D* 1.9 < p < 20 GeV NLO QCD T 0.167 < y < 0.802 HER cross section higher 0.2 0 240 260 280 300 320 s (GeV) Cross sections increase with increasing ep CM energy This increase is predicted by NLO QCD Heavy Flavour production at HERA U. Karshon 5

Charm fragmentation fractions in PHP Fragmentation fractions of c-quarks into charm hadrons: Probability of c quark to hadronise into a given charm hadron f ( c → charm hadron ) = σ ( charm hadron ) /σ ( total charm production ) Needed to go from partonic QCD to hadronic cross sections No QCD predictions; crucial to compare pQCD with measurements Are they the same for c-quarks produced in e + e − , ep, pp collisions ? Test fragmentation universality by measuring all of them Measurements performed in PHP regime: Q 2 < 1 GeV 2 Charm hadrons reconstructed in the range: p T > 3 . 8 GeV, | η | < 1 . 6, 130 < W < 300 GeV Charm hadrons measured: D 0 → K − π + , D + → K − π + π + D ∗ + → D 0 π + s → K − π + π + s D + s → φπ + , Λ + c → K − pπ + σ tot = σ eq ( D 0 ) + σ eq ( D + ) + σ ( D + s ) + 1 . 14 σ (Λ + c ) Full HERA II data: 372 pb − 1 JHEP 09 (2013) 058 Heavy Flavour production at HERA U. Karshon 6

Charm fragmentation fractions in PHP Silicon-strip detector used for charm vertices ⇒ Clear charm hadron signals for all channals 0.7 Charm fragmentation fractions Charm fragmentation fractions: 0.6 Results (left column) in good 0.5 agreement with previous results: 0 → f (c D ) 0.4 + → f (c D ) ZEUS PHP, ZEUS DIS, H1 DIS, e − e − + → f (c D* ) 0.3 → f (c D ) s → Λ f (c ) 0.2 c 0.1 γ γ p p - ep DIS ep DIS e + e 0 ZEUS ZEUS H1 ZEUS HERA II HERA I HERA I Precision of charm f.f. competitive with combined e + e − LEP results Fragmentation fractions of c -quarks independent of production Support hypothesis of universality of heavy-quark fragmentation Universality supported also by new LHC pp data (ALICE + LHCb) Heavy Flavour production at HERA U. Karshon 7

D ± production in DIS Full HERA II data: 354 pb − 1 ZEUS ZEUS 1 ) 2 ) (nb/GeV) (nb/GeV + -1 ZEUS D 354 pb + -1 JHEP 05 (2013) 023 ZEUS D 354 pb + -1 ZEUS D 133.6 pb + -1 ZEUS D 133.6 pb -1 10 HVQDIS 1 HVQDIS Clean D + signal 2 + /dQ ZEUS -2 (D 10 Combinations / 10 MeV 4500 T σ 4000 /dp d 3500 N ( D + ) = 8356 ± 198 -3 3000 10 σ -1 10 2500 d 2000 1500 ZEUS D + 354 pb -1 1000 -4 10 500 mod Gauss + background 0 D + differential cross sections 1.7 1.75 1.8 1.85 1.9 1.95 2 2.05 2.1 π π M(K ) (GeV) -2 10 2 3 10 10 10 2 3 4 5 6 7 8 9 10 2 2 + Q (GeV ) p (D ) (GeV) T w.r.t Q 2 , y, p T ( D + ) , η ( D + ) ZEUS ZEUS 20 /dy (nb) 2.4 ) (nb) + -1 in kinematic region 18 ZEUS D 354 pb 2.2 2 16 + HVQDIS (D 5 < Q 2 < 1000 GeV 2 , 1.8 σ 14 d η 1.6 /d 12 σ 1.4 d 10 1 . 5 < p T ( D + ) < 15 GeV, 1.2 8 1 0.8 6 | η ( D + ) | < 1 . 6, + -1 0.6 ZEUS D 354 pb 4 + -1 ZEUS D 133.6 pb 0.4 2 HVQDIS 0.2 0 . 02 < y < 0 . 7 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 -1.5 -1 -0.5 0 0.5 1 1.5 y η + (D ) NLO QCD predictions based on FFNS describe data well up to Q 2 ≈ 1000 GeV 2 Similar agreement for double differential cross sections dσ/dy for different Q 2 ranges Heavy Flavour production at HERA U. Karshon 8