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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 Mbianda1 for the Baryon (E01-002) Collaboration

1 University of the Witwatersrand, Johannesburg

Exclusive Electroproduction of π+ and η mesons at High Momentum Transfer

  • 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


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.

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)


MX2 = Mη2 = (pi + ki – pp - kf)2for process (2)

q = (ν,q)

PX = (EX, PX)


Pi = (MP, 0)


Pπ, p= (Eπ,p , Pf)

experimental setup
Experimental Setup



LH2 target


angular coverage for
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

particle identification
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


Enorm = 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





exclusive and m x 2 cut
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



Data in Blue

SIMC in Red

2 pion background





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)}



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

some results of exclusive electroproduction
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


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




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

conclusion 1
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
conclusion 2

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