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The BoNuS Experiment at Jefferson Lab’s CLAS .

Svyatoslav Tkachenko University of South Carolina for the CLAS collaboration. The BoNuS Experiment at Jefferson Lab’s CLAS. Structure functions and parton distribution functions. Structure Functions and Moments. Precise PDFs at large x needed as input for LHC

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The BoNuS Experiment at Jefferson Lab’s CLAS .

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  1. Svyatoslav Tkachenko University of South Carolina for the CLAS collaboration The BoNuS Experiment at Jefferson Lab’s CLAS.

  2. Structure functionsand parton distribution functions

  3. Structure Functions and Moments • Precise PDFs at large x needed as input for LHC • Large x, medium Q2 evolves to medium x, large Q2 • Moments can be directly compared with OPE (twist expansion), Lattice QCD and Sum Rules • All higher moments are weighted towards large x qup(x)‏ qdown(x)‏ Q2=3.15 (GeV/c)2 Q2=3.15 (GeV/c)2 Ratio to CTEQ6

  4. Structure Functions and Resonances Precise structure functions in Resonance Region constrain nucleon models[Separate resonant from non-resonant background; isospin decomposition] Needed as input for spin structure function data, radiative corrections,… Compare with DIS structure functions to test duality

  5. d(x) and u(x) as x  1 • Valence structure of the nucleon - sea quarks and gluons don’t contribute • SU(6)-symmetric wave function of the proton in the quark model: • In this model: d/u = 1/2, u/u*) = 2/3, d/d = -1/3 for all x • Hyperfine structure effect (1-gluon exchange): S=1 suppressed d/u = 0, u/u = 1, d/d = -1/3 for x 1 • pQCD: helicity conservation (qp) d/u = 1/5, u/u = 1, d/d = 1 for x  1 • Wave function of the neutron via isospin rotation: replace u  d and d  u => using experiments with protons and neutrons one can extract information on u, d, u and d in the valence quark region. *) helicity q = (q - q) for Nucleon N

  6. To extract d/u ratio, we need neutron data. Extracting structure function ratio is model dependent and the results from the same data set might differ a lot depending on the model applied for analysis.

  7. Large x - Large Nuclear Effects • Even simple “Fermi Smearing” leads to significant dependence on D wave function • Different models for off-shell and “EMC” effects lead to large additional variations • Contributions from MEC, (1232) and “exotic” degrees of freedom unknown • FSI?

  8. Bound neutron… Free neutron… • How can we study free neutron structure without free neutrons available? • Emulate them with nuclear targets: • In 3He, due to fortuitous cancellation of proton spins, we can study neutron spin structure. • If we can find observables that are mostly sensitive to the low-momentum part of the deuteron wave function, we can treat the nucleons as quasi-free and thus study neutrons.

  9. Spectator tagging(akapinpointing the low-momentum part of the deuteron wave function)

  10. e n p SpectatorTagging E = 4.223 GeV <Q2> = 1.19 (GeV/c)2 *

  11. “Rules” for the spectator.Final state interactions. Ciofi degli Atti and Kopeliovich, Eur. Phys. J. A17(2003)133 The momentum and angular dependence of the ratio of spectral functions with and without FSI effects. Blue boxes mark preferred kinematics – regions where FSI have smaller effect.

  12. “Rules” for the spectator.“Off-shellness” depends on the spectator momentum magnitude. Ratio of the bound to free F2 neutron structure functions vs spectator momentum. Model by W.Melnitchouk.

  13. Modification of the off-shell scattering amplitude (Thomas, Melnitchouk et al.) Color delocalizationClose et al. Suppression of “point-like configurations”Frankfurt, Strikman et al. Deviations from free structure function: Off-shell Effects [should depend on (ps), x, Q2] pT = 0 “Off-shell” mass of the nucleon M* 939 MeV 905 MeV 823 MeV 694 MeV … plus 6-quark bags, , MEC… And of course FSI! Ps = 0 0.09 0.17 0.25 0.32 0.39 GeV/c

  14. Rules for the spectator.Summary. Low momentum spectators PS < 100 MeV/c Minimize uncertainty due to the deuteron wave function and on-shell extrapolation. O (1%) correction. Minimize effects from FSI and target fragmentation. O (5%) correction. Backward kinematics θqp > 110o

  15. Validation of the spectator tagging method(BoNuS experiment) • Check angular dependence of effective (bound) • structure functions in comparison with PWIA • spectator model • Check spectator momentum dependence of • effective (bound) structure functions in • comparison with PWIA spectator model

  16. Low Spectator Momenta - Nearly Free Neutrons ? 20% The Experiment BoNuS Region VIPs 0.07 0.2 GeV/c CLAS e- backwards p Radial TPC (view from downstream) *BoNuS = Bound Nucleon Scattering **RTPC = Radial Time Projection Chamber

  17. Bonus Radial Time Projection Chamber.(Detector system for slow protons)‏ • Thin-walled gas target (7 atm., room temperature)‏ • Radial Time Projection Chamber (RTPC) with Gaseous Electron Multipliers (GEMs)‏ • 4 - 5 Tesla longitudinal magnetic field (to suppress Möller electrons and to measure momentum)‏ • 3-dimensional readout of position and energy loss (“pads”)‏

  18. Particle ID (after gain calibration of each channel) RTPC Performance e- reconstructed in CLAS & RTPC Gain constants for every channel (RTPC-Right on top) – red (blue) indicates “hotter” (“colder”) than average pads =8mm Out-of-time track suppression z =4º =1.4º  

  19. 80 MeV/c 100 MeV/c 80 MeV/c 100 MeV/c 120 MeV/c 140 MeV/c 120 MeV/c 140 MeV/c Spectator momentum dependence (preliminary) Ratio to simulation Effective F2n Backwards angles (cos θpq < -0.25) data are shown Simulation uses PWIA spectator model, radiative effects, full model of RTPC and CLAS. P. Bosted and M.E. Christy F2n model is used.

  20. Angular dependence(preliminary) Q2 = 1.66 (GeV/c)2 W* = 1.73 GeV 80 MeV/c 100 MeV/c • No significant deviations • from PWIA (ps<100 MeV/c) • Possible θ dependence • at higher momenta 120 MeV/c 140 MeV/c

  21. ExtractedF2n (analyses comparison)(preliminary) ▼ - Analysis 1 ▲ - Analysis 2 ___ Simulation in PWIA spectator picture - - - CTEQ6X calculation

  22. Extracted F2n/F2p(N. Baillie)(preliminary)

  23. Extracted F2n(N.Baillie)(preliminary) 1.6 < Q2 < 1.9 1.9 < Q2 < 2.2 2.7 < Q2 < 3.2 “Free” neutron structure function compared with a model by P. Bosted and M.E. Christy

  24. Cross Section Fitting (J.Zhang I) + A1 Cosf* + A2 Cos2f* = A0 24

  25. BoNuS Vs Models, 5 GeV, W = 1.525 (J.Zhang II) preliminary 25 MAID 07 SAID 08 D(e,ep-pRTPC)p D(e,ep-pCLAS)p

  26. Plans for 12 GeV BoNuS CLAS12 Central Detector E12-06-113 • Data taking of 35 days on D2 and 5 days on H2 with L = 2·1034 cm-2 sec-1 • Planned BoNuS detector DAQ and trigger upgrade • DIS region with • Q 2 > 1 GeV2/c 2 • W *> 2 GeV • ps< 100 MeV/c • pq > 110° • Largest value for x* = 0.80 (bin centered x* = 0.76) • Relaxed cut of W *> 1.8 GeV gives max. x* = 0.83

  27. Conclusions • Preliminary analysis does not contradict spectator model • Technically different analyses of BoNuS data converge • Analysis note underway • BoNuS12 proposal re-submission in preparation

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