1. BONUS (Barely Off-Shell Nucleon Structure) Experiment Update Thia Keppel CTEQ Meeting November 2007

2. Large x Parton Distribution Functions - Large Uncertainties Where can we turn for help…. u(x)d(x)

3. Textbook Physics, Major Problem Quark-Parton Model Without a free neutron we are forced to extract F 2 n from a bound system. However, a large dependence on correct modeling of the deuteron comes into play for x >~0.5 F 2 n /F 2 p

4. Many More Sophisticated Predictions F 2 n /F 2 p d/u SU(6) 2/3 1/2 Scalar Diquark1/4 0 H-P Quark Model 1/4 0 pQCD 3/7 1/5 Counting Rules 3/7 1/5 Review Articles : Isgur, Phys. Rev. D59, 34013 (1999) Brodsky et al., Nucl. Phys. B441, 197 (1995) Melnitchouk and Thomas, Phys. Lett. B377, 11 (1996)

6. Bound Neutron Structure Functions Simple subtraction (deuteron- proton) yields nonsense Kinematic shift of the effective Bjorken variable x or of W 0.700.69 0.800.78 0.900.85 1.000.90 + Binding (off-shell) effects, coherent scattering, final state interactions, non- nucleonic degrees of freedom in the ground state, nucleon structure modification (“EMC”-effect)

7. Can we reduce model dependence? Yes! Within the nuclear impulse approximation. The virtual photon interacts with the neutron on a short enough time scale that the proton doesn’t know what happened. The spectator continues on unperturbed w/ momentum p s = -p γ*(q)γ*(q) d(p d ) n(p) p(p s ) X F 2 n(eff) S

8. Can we reduce model dependence? γ*(q)γ*(q) d(p d ) n(p) p(p s ) X F 2 n(eff) S Yes! Within the nuclear impulse approximation. The virtual photon interacts with the neutron on a short enough time scale that the proton doesn’t know what happened. The spectator continues on unperturbed w/ momentum p s = -p BoNuS Region

9. Can we reduce model dependence? γ*(q)γ*(q) d(p d ) n(p) p(p s ) X F 2 n(eff) S Yes! Within the nuclear impulse approximation. The virtual photon interacts with the neutron on a short enough time scale that the proton doesn’t know what happened. The spectator continues on unperturbed w/ momentum p s = -p BoNuS Region We focus on VIPs (Very Important Protons) where R n > 99%

10. n e p e p n Detect very important low momentum protons. If the proton is also going backwards in the lab frame it is almost guaranteed to be only a spectator. n e p proton targetneutron target neutron target ! BONUS: An Effective Free Neutron Target from Deuterium….(e- + d  Spectator proton e-

11. BoNuS Radial TPC Design 3 cm He φ, z from pads r from time dE/dx from charge along track (particle ID) 3 cm Helium/DME at 80/20 ratio Stagger pads in z to improve theta angle reconstruction

12. Big Picture rTPC inside Solenoid inside CEBAF Large Acceptance Spectrometer (CLAS) Track scattered e - in CLAS Locate e - interaction point in target. Electron tracked in CLAS provides trigger to BoNuS radial TPC Link p spectator with electron vertex. CLAS (taking data since 1997) is made of Drift Chambers, Time of Flight Scintillators, Cerenkov counters and Electromagnetic Calorimeters for tracking, momentum determination, and PID

13. Solenoid Magnet BoNuS BoNuS in CLAS RTPC viewed during insertion into CLAS (above), and from the solenoid downstream end (left)

14. Proton Tracks in rTPC e - beam coming out of page Fit to track e - beam traversing target RTPC entrance window RTPC readout plane Proton track identifiable by consecutive charge deposit

15. BoNuS RTPC in action The z coordinate of the trigger electron’s vertex is compared with the z vertex of the track in the RTPC. A clear peak around Δz = 0 is seen riding on top of accidental coincidences.

16. ∆p/p Compare proton momentum measured by the RTPC to CLAS prediction σ ≈ 20% BoNuS RTPC in action The z coordinate of the trigger electron’s vertex is compared with the z vertex of the track in the RTPC. A clear peak around Δz = 0 is seen riding on top of accidental coincidences.

17. ∆p/p Compare proton momentum measured by the RTPC to CLAS prediction σ ≈ 20% BoNuS RTPC in action The z coordinate of the trigger electron’s vertex is compared with the z vertex of the track in the RTPC. A clear peak around Δz = 0 is seen riding on top of accidental coincidences. The measured dQ/dx is compared w/ Bethe- Bloch prediction for a proton having the measured momentum. protons heavy nuclei background

18. Status of BoNuS Analysis p The spectator proton’s four momentum: p n μ = -(E s – M D, p s ) Light-cone momentum fraction: α s = (E s – p s z )/M E = 4.223 GeV

19. Status of BoNuS Analysis model for σ n / σ D by P. Bosted Choose the QUASI-ELASTIC region (0.8 < W < 1.07 GeV) to make a first approximation of the tagging efficiency Tagging Efficiency (%)

20. Status of BoNuS Analysis So far… RTPC effectively reconstructed and identified slow protons spectator tagging technique works VERY PRELIMINARY results for n/d and n/p What the future holds… compare experimentally determined tagging efficiency w/ MC Complete final CLAS and RTPC corrections and calibrations, including radiative corrections Finish acceptance and charge measurement studies in order to extract absolute cross-sections preliminary

21. Kinematic Coverage E = 5.262 GeV E = 4.223 GeV E = 2.140 GeV W (GeV) Q 2 (GeV 2 ) cos(θ pq ) p spec (MeV)