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

Born diagram for the processes The missing mass reconstructions are obtained from the expressions: M X 2 = M n 2 = (p i + k i – p π - k f ) 2 for process (1) and M X 2 = M η 2 = (p i + k i – p p - k f ) 2 for process (2) k i = (E, K i ) k f = (E’,K f ) ν q = ( ν,q) X W P X = (E X, P X ) P π, p = (E π,p, P f ) P i = (M P, 0) Incident electronScattered electron

Experimental Setup Beam SOS HMS LH2 target

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

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 N p.e > 0.5 and E norm = E/P > 0.7

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 protons pions ( GeV ) (ns)

Exclusive π + and M X 2 cut  We deal exclusively with π + electroproduction by introducing an M X 2 cut  The detected π + at the HMS could come from nucleon resonance decay processes such as: N * → n + π + and N * → n + π o + π +  Introducing the cut 0.8 < M X 2 < 1 thereby avoids the multipion background that begins at an M X 2 threshold of about 1.16 GeV 2 W MX2MX2 MX2MX2 2 pion background Data in Blue SIMC in Red (GeV 2 ) (GeV)

Exclusive π + Cross-Sections -black line is the MAID 2003 cross-section extrapolated at 5.5 GeV 2, using the dipole form factor G= (1 - q 2 /0.71) -2 -green points are the extracted differential cross-sections averaged over all φ cm 2 nd Resonance Region {e.g. P 11 (1440)} 3 rd Resonance Region {e.g. F 15 (1680)} PRELIMINARY

Insignificant φ cm dependence on the cross-section as illustrated for the most forward cosθ cm bin

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

differential cross-sections for Q 2 = 5.7 GeV 2 fitted with d 2 σ/dΩ* = A + B cosθ + C cos 2 θ* + D sinθ*cosφ* + E cosθ*sinθ*cosφ* + F sin 2 θ*cos 2 φ* A-F are truncated multipole expansions M. Dalton

Exclusive η total Production Cross-Section Fit of Breit-Wigner to data with a very small linear non- resonant background term indicates the dominance of S 11 (1535) at its resonance mass. W R = ± GeV Γ R = ± GeV σ R = ± GeV W R = ± GeV Γ R = ± GeV σ R = ± GeV GeV 2 M. Dalton

The helicity Amplitude, A 1/2, appears to begin scaling as 1/Q 3 at photon momentum transfer of Q 2 ~ 5 GeV 2 as predicted by pQCD. M. Dalton

Conclusion (1)  The cross sections of the processes e + p → e’ + π + + n was measured at Q 2 = 5.5 GeV 2  Preliminary results are sensitive to high-mass baryon resonances and diverge significantly from the MAID2003 extrapolation of the lower Q 2 data  Combined with world data, the Q 2 dependence of the transition form factors into higher resonances could be measured.  Paper in progress

 The cross sections of the processes e + p → e’ + p + η was measured at Q 2 = 5.7 and 7.0 GeV 2  From threshold to S 11 ( 1535 ) resonance mass, the differential cross-section is largely isotropic – consistent with previous measurements  The extracted helicity-conserving amplitude, A 1/2, suggests the onset of scaling in this reaction  Paper in progress