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Georgie Mbianda 1 for the Baryon (E01-002) Collaboration

Georgie Mbianda 1 for the Baryon (E01-002) Collaboration 1 University of the Witwatersrand, Johannesburg. Exclusive Electroproduction of π + and η mesons at High Momentum Transfer. Introduction. The differential cross-section of the process

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Georgie Mbianda 1 for the Baryon (E01-002) Collaboration

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  1. Georgie Mbianda1 for the Baryon (E01-002) Collaboration 1 University of the Witwatersrand, Johannesburg Exclusive Electroproduction of π+ and η mesons at High Momentum Transfer

  2. Introduction • The differential cross-section of the process e + p → e’ + π+ + n (1) has been measured at an average Q2 ~ 5.5 GeV2 in the low invariant mass range of 1.3 < W < 2 GeVat forward angles; and of the process e + p → e’ + p + η (2) at Q2 ~ 5.7 and 7.0 GeV2 for centre-of-mass energies of 1.5 < W < 1.8 GeV • These are the highest Q2 measurements of these exclusive processes to date.

  3. Incident electron Scattered electron kf = (E’,Kf) ki = (E, Ki) Born diagram for the processes The missing mass reconstructions are obtained from the expressions: MX2 = Mn2 = (pi + ki – pπ - kf)2for process (1) and MX2 = Mη2 = (pi + ki – pp - kf)2for process (2) q = (ν,q) PX = (EX, PX) X Pi = (MP, 0) W Pπ, p= (Eπ,p , Pf)

  4. Experimental Setup HMS SOS LH2 target Beam

  5. Angular Coverage for π+ • The pions were very forward in the centre-of-mass system; cosθcm > 0.6 • Parasitic data extracted from the experiment (E01-002) t-channel processes dominate

  6. Particle Identification In the SOS, electrons were separated from π-by the aid of the threshold gas Cerenkov detector and the lead-glass calorimeter The particle ID cuts to select electrons Np.e > 0.5 and Enorm = E/P > 0.7

  7. Particle Selection at HMS • TOF method was used to make either the pion or proton to appear at a specific time independent of momentum • So, an interval on the timing spectrum allowed for selection of pion or proton events (GeV) protons pions (ns)

  8. Exclusive π+ and MX2 cut • We deal exclusively with π+ electroproduction by introducing an MX2 cut • The detected π+ at the HMS could come from nucleon resonance decay processes such as: N*→ n + π+ and N*→ n + πo + π+ • Introducing the cut 0.8 < MX2 < 1 thereby avoids the multipion background that begins at an MX2 threshold of about 1.16 GeV2 W (GeV) Data in Blue SIMC in Red 2 pion background MX2 (GeV2) MX2

  9. Exclusive π+ Cross-Sections-black line is the MAID 2003 cross-section extrapolated at 5.5 GeV2 , using the dipole form factor G= (1 - q2/0.71) -2-green points are the extracted differential cross-sections averaged over all φcm 2nd Resonance Region {e.g. P11(1440)} 3rd Resonance Region {e.g. F15(1680)} PRELIMINARY

  10. Insignificant φcm dependence on the cross-section as illustrated for the most forward cosθcm bin

  11. Systematic Error Analysis- simulation was run with altered parameters and results compared with a set standard. Results do not change much. Standard analysis Target position Broadened resolution

  12. Some Results of Exclusive η Electroproduction differential cross-sections for Q2 = 5.7 GeV2 fitted with d2σ/dΩ* = A + B cosθ + C cos2θ* + D sinθ*cosφ* + E cosθ*sinθ*cosφ* + F sin2θ*cos2φ* A-F are truncated multipole expansions M. Dalton

  13. Exclusive η total Production Cross-SectionFit of Breit-Wigner to data with a very small linear non-resonant background term indicates the dominance of S11(1535) at its resonance mass. WR = 1.523 ± 0.001 GeV ΓR = 0.123 ± 0.010 GeV σR = 0.977 ± 0.024 GeV WR = 1.523 ± 0.001 GeV ΓR = 0.188 ± 0.121 GeV σR = 0.519 ± 0.058 GeV M. Dalton GeV2 GeV2

  14. The helicity Amplitude, A1/2 , appears to begin scaling as 1/Q3 at photon momentum transfer of Q2~ 5 GeV2 as predicted by pQCD. M. Dalton

  15. Conclusion (1) • The cross sections of the processes e + p → e’ + π+ + n was measured at Q2 = 5.5 GeV2 • Preliminary results are sensitive to high-mass baryon resonances and diverge significantly from the MAID2003 extrapolation of the lower Q2 data • Combined with world data, the Q2 dependence of the transition form factors into higher resonances could be measured. • Paper in progress

  16. Conclusion (2) The cross sections of the processes e + p → e’ + p + η was measured at Q2 = 5.7 and 7.0 GeV2 From threshold to S11(1535) resonance mass, the differential cross-section is largely isotropic – consistent with previous measurements The extracted helicity-conserving amplitude, A1/2 , suggests the onset of scaling in this reaction Paper in progress

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