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Measurement of charm production in DIS at HERA

Measurement of charm production in DIS at HERA. Marcello Bindi University of Bologna. XIII LNF Spring school, 12-16 May 2008, Frascati. ZEUS. 0.5 fb -1 per experiment. HERA. Collider ep; end of data taking in June 2007 2 general purpose experiments: ZEUS & H1. e ± (27.6 GeV).

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Measurement of charm production in DIS at HERA

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  1. Measurement of charm production in DIS at HERA Marcello Bindi University of Bologna XIII LNF Spring school, 12-16 May 2008, Frascati. XIII LNF Spring School 2008, Frascati.

  2. ZEUS 0.5 fb-1 per experiment HERA Collider ep; end of data taking in June 2007 2 general purpose experiments: ZEUS & H1 e± (27.6 GeV) p (820/920 GeV) E c.m. = 300/318 GeV • HERA I (1992-2000):190 pb-1 • fascio e± unpolarized • HERA II (2002-2007):560 pb-1 • e± long. polarized beam~30/40 % • Lumin.Upgrade:1.57.5x 1031cm-2s-1 • Run at low Ec.m. for FL measurement XIII LNF Spring School 2008, Frascati.

  3. p e± ZEUS Calorimeter Muons Chambers Central tracking detector Microvertex detector (HERA II) XIII LNF Spring School 2008, Frascati.

  4. Deep Inelastic Scattering at HERA (k’) (k) (k) (k’) Virtuality of the exchanged boson (q) (q) Neutral Currents Charged Currents Bjorken variable Inelasticity • Kinematic determined by 2 variables • Q2 and x mostly used • HERA range: Q2 = 10-1 - 105 GeV2 x = 10-6 - ~0.5 XIII LNF Spring School 2008, Frascati.

  5. Motivations • Heavy flavour production at HERA can be studied for different kinematic regions, from Photoproduction to DISand for different values of transverse momentum of the heavy quark. • In DIS regime the heavy quarks are produced mainly by the Boson-Gluon-Fusion process. • This process is directly sensitive to gluon content inside the proton. • Factorization in pQCD: • s = parton distr. x hard scatt. x fragm./hadron. • Important test of pQCD at different scales (MQ, pTQ,Q2). XIII LNF Spring School 2008, Frascati.

  6. Charm tagging D mesons reconstruction in the final state • D*, D0 mesons: tagged by invariant mass M (Kp) and DM = M(Kpp) - M(Kp). • D+ meson: being a long lived particle, the • secondary vertex of the decay can be • reconstructed and used for a better • discrimination of the signal. Both these cases, need an extrapolation factor to the full space phase to obtain the F2cc Inclusive or Lepton Impact Parameter tagging • Measure of the impact parameter with • respect to primary vertex (beamspot) XIII LNF Spring School 2008, Frascati.

  7. D* production in DIS Golden mode : can be double tagged by the slow pion and the D0 in the final state. HERA II 2003-2005 162 pb-1 • Kinematic Region • 5 < Q2 < 1000 GeV2 • 0.02 < y < 0.7 • |h (D*)| < 1.5 • 1.5 < pT (D*) < 15 GeV • Selection of D* candidates • D*± (2010)  D0 p±slow (BR: 67.7 %) • with D0 K±p±(BR: 3.8 %) • |h| < 1.75 (candidate tracks) • pT > 0.4 GeV (tracks from D0) • pT > 0.12 GeV (slow pion from D*) XIII LNF Spring School 2008, Frascati.

  8. D* differential cross sections in DIS XIII LNF Spring School 2008, Frascati.

  9. Muon vs D-mesons • Measurements based on reconstructed D (D*) mesons suffer from: - low branching ratio B(cD*) x B(D*±D0 ±) x B(D0K±±) ~ 0.6% - angular acceptance restricted to good tracks detector region ||<1.5 - measurement limited to Q2 <~ 500 GeV2 due to limited statistics • Muons advantages : - larger branching ratio : B(c )~ 10% - larger angular acceptance of muon chamber -2 <  < 2.3 • Muons disadvantages : - background from fake muons and beauty - need to require large muon momentum (Pt > 1.5 GeV) to reach muon chambers • Muons less precise at low Q2 ( where D* statistics is large) due to worse control of background and extrapolation in pT. • Muons should be better at large Q2 due to larger statistics. XIII LNF Spring School 2008, Frascati.

  10. Semi-leptonic decay to muon • Kinematic Region • 20 < Q2 < 10000 GeV2 • 0.02 < y < 0.95 • -1.6 < |hμ| < 2.3 • pTμ > 1.5 GeV HERA II 2005 135 pb-1 • ≥ 1 muon from prim. vtx. • Rear/Barrel:-1.6 ≤ < 1.3, PT >1.5 GeV • Forward: 1.475 ≤ < 2.3, PT >1.5 GeV DIS TLTs trigger chain • Electron finder : • - P(ele)> 0.001, Ee> 10 GeV , • ECONE<5 GeV • yJb > 0.02 , yel<0.95 • Q2Σ>20 GeV2 • |Zvtx| < 30 cm • 80 > E-Pz > 40GeV Muon anti-isolation : Econe > 0.5 GeV Muon-jet association; (lab. frame KTclus) massivesch. PTjet >2.5 Gev; -3 < ηjet< 3 XIII LNF Spring School 2008, Frascati.

  11. Discriminating variables Neutrino missing PT parallel to the muon : PT= ( PTmiss.PT ) / |PT| PTREL (GeV) IP(cm) PT (GeV) XIII LNF Spring School 2008, Frascati.

  12. Cross sections: Q2,x,pTμ,ημ,Y1,Y2 XIII LNF Spring School 2008, Frascati.

  13. F2cc extraction: ZEUS • Subtraction of the beauty contribution by using the RAPGAP MC • predictions for each bin. • ZEUS extracts F2cc from D meson cross sections using HVQDIS • to extrapolate to the full meson phase space: • The extrapolation is performed at the centre of gravity of the bin. • ZEUS-S PDF were used throughout. XIII LNF Spring School 2008, Frascati.

  14. F2cc at HERA • The measurements can distinguish between • different gluon parameterizations. • ZEUS and H1 measurements are in good agreement. XIII LNF Spring School 2008, Frascati.

  15. Conclusions and outlook • The charm contribution to the proton structure functions, • F2, is measured at HERA using different techniques, • based on D meson tagging and lifetime measurements. • The two collaborations, ZEUS and H1, using very • different methods for the analysis, implying different • extrapolations factors, agree on the results. • The precision of the measurement is improving, and the • measurement appears to be able to distinguish between • the different gluon parameterizations provided by theorists. • The use of the whole HERA data sample can help in • constraining the gluon parameterization in the proton XIII LNF Spring School 2008, Frascati.

  16. BACKUP - SLIDES XIII LNF Spring School 2008, Frascati.

  17. Theoretical models at NLO PhP : Frixione et al., FMNR DIS : Harris & Smith, HVQDIS fully differential NLO program Massive approach (Fixed Flavour Number Scheme): • c & b massive full massive matrix elements; • scale MQ ; • appropriate for Q2 ~ MQ2 c & b produced dinamically in the hard subprocess (not part of the proton or photon; 3 active flavours in proton: u, d, s ) Massless approach (Zero Mass Variable Flavour Number Scheme): • c & b massless  resums [aS ln (Q2/M2Q)]n ; • scale Q2, pTQ ; • appropriate for Q2 >> MQ2 c & b present in proton and photon PhP : Kniehl et al. DIS : only inclusive calculation of F2cc available • Combined approach (Generalized Mass Variable Flavour Number Scheme): • massive at smallQ2 • massless at highQ2 PhP: Cacciari et al. DIS : only F2cc available XIII LNF Spring School 2008, Frascati.

  18. R=F = ZEUS: NLO QCD predictions • The HVQDIS program has been used, which evaluates • cross sections for heavy quark production at NLO in the • Fixed Flavour Number Scheme. • Biggest uncertainty from Charm mass ~10-20 % XIII LNF Spring School 2008, Frascati.

  19. D± production in DIS • D± long lifetime  displaced secondary vertex (MVD). • Signed 2-D decay length significance (SDL) with respect to the beamspot • Better S/N ratio by cutting on the SDL of the decay products. 1/3 HERA II: 2005 data 135 pb-1 Kinematic Region • 5 < Q2 < 1000 GeV2 • 0.02 < y < 0.7 • |η(D±)| < 1.6 • 3 < PT(D±) < 20 GeV D± Selection Cuts • PT(K) > 0.7 GeV • PT(π,π) > 0.5 GeV • |η(K,π,π)| < 1.6 • D* and Ds reflections subtracted. With SDL > 3, statistical error 7.7%  3.8% XIII LNF Spring School 2008, Frascati.

  20. D± cross sections XIII LNF Spring School 2008, Frascati.

  21. D0 production in DIS • Kinematic Region • 5 < Q2 < 1000 GeV2 • 0.02 < y < 0.7 • h |(D)| < 1.6 • 3 < pT (D) < 20 GeV 1/3 HERA II: 2005 data 135 pb-1 • Selection of D0 candidates • |h(K,p)| < 1.6 • pT (K) > 0.7 GeV ; pT (p) > 0.3 GeV • SL > 1, c2Dvtx < 8 • DM tagged and untagged D0 signals are fitted simultaneously XIII LNF Spring School 2008, Frascati.

  22. D0 cross sections in DIS XIII LNF Spring School 2008, Frascati.

  23. F2ccwith D± • Previous results combined measurements of F2cc from 3 mesons to reduce uncertainty. • F2cc determined with D±atHERA II (1/3 lumi) has comparable precision to that of the whole HERA I measurement with D0, D± and Ds mesons. XIII LNF Spring School 2008, Frascati.

  24. Inclusive Impact Parameter tagging • A powerful tool to separate beauty, charm and • background is the significance of the (signed) impact parameterS = d / s(d) Kinematic Region • 12 < Q2 < 650 GeV2 • 0.0002 < x < 0.032 • 0.07 < y < 0.65 HERA II: 2006 e- 54 pb-1 see talk A. Metha for technical details… XIII LNF Spring School 2008, Frascati.

  25. Pc,b,l are the fractions of charm, beauty and light flavour from the fit and is the inclusive reduced cross section. F2cc extraction: H1 H1 measures charm inclusively Structure function evaluated after small corrections for the longitudinal structure functions FLcc XIII LNF Spring School 2008, Frascati.

  26. Inclusive measurements vs D mesons • Good agreement between different techniques • Agreement with NLO QCD within the spread of different theoretical approaches • GM-VFNS MRST04 • CTEQ6HQ • FFNS CCFM XIII LNF Spring School 2008, Frascati.

  27. H1 inclusive charm reduced cross sections • HERA I and II agree • Combined results • agree with NLO • theoretical expectation • GM-VFNS & MRST04 XIII LNF Spring School 2008, Frascati.

  28. XIII LNF Spring School 2008, Frascati.

  29. High Q2HERA analyses XIII LNF Spring School 2008, Frascati.

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