1. Precision Measurement of the Electroproduction of p0 Near Threshold:
Khem Chirapatpimol University of Virginia
July 31st, 2012
This is proposal an update to E
It is a threshold measurement of neutral pion production off the proton at low Q2 to test ChPT.
The idea is to detect the electron using the HRS at 6 degree in order to get a measurement of W and Q2 near threshold with excellent resolution.
Such a forward angle provides low Q and with maximum count rate needed for threshold measurements.
The proton scatters into a narrow cone of a few degrees about q. Using the large acceptance BigBite spectrometer shipped to JLab from NIKHEF several years ago, we can detect the entire phase of the proton in one kinematic setting near threshold.
The experiment was presented and approved by PAC 19 in Jan 2001 for 16 days. It is a Hall A and BigBite collaboration experiment.
One reason why it has not been scheduled and we are in jeapordy is because it requires substantial equipment construction including instrumenting the detector plane of BigBite.
I would like to show that we are are well underway with building the new equipment and the motivation for doing the experiment is still very important. 1

2. Quantum Chromodynamics (QCD)
-the fundamental theory of strong interactions of quarks and gluons
-can be treated perturbatively at high energy
-very successful in describing high energy scattering processes
Problem is
-at low energy, the strong coupling increases
-cannot use perturbation theory, because higher order terms cannot be ignored.
Solution (in theory)
-Chiral Perturbation Theory (ChPT)

3. The basic idea of ChPT
-expand the Lagrangian in terms of pion and baryon fields, while retaining the symmetries of QCD.
->Scattering or production processes are described in terms of small quantities Q/M, and mp /M ,
Q= four momentum transfer
M=nucleon mass
mp=pion mass
-near threshold dominated by S and P wave
-The coefficients in Lagrangian, called low energy constants (LEC), are obtained from experimental data.
-Once the LECs are determined the theory posseses predictive power

4. ChPT
Lagrangian first three lowest order

5. Differential cross section
The differential cross section for pion electro-production using an polarized
electron beam can be written as

6. In term of S and P wave Multipoles
Calculate in ChPT

7. p0 photoproduction
ChPT with one loop corrections

8. Mainz Data p0 electroproduction
Harald Merkel
6th International
Workshop on
Chiral Dynamics
July
Bern, Switzerland
Q2=0.1 (GeV/c)2
Distler PRL 80, 2294 (1998)
Q2=0.05 (GeV/c)2
Merkel et al.
PRL 88, 1230 (2002)
W - cm energy of the
pion-nucleon system
HBChPT was fitted to old data set up to Q2=0.1 (GeV/c)2 8

9. This experiment repeat the Mainz experiment -with more Q2 points
-extended to higher W

10. Experimental Setup and Kinematics
Target
6 cm Liquid Hydrogen( LH2)
Electron Beam
1-5 μA

11. -Left High Resolution Spectrometer(LHRS)
-electron Beam GeV
-target
Liquid hydrogen for pion production
Carbon, tantalum, aluminium, liquid deuterium for calibration
Spectrometer
-Left High Resolution Spectrometer(LHRS)
magnet for bending particles QQDQ
two Vertical Drift Chambers for tracking
two scintillator planes for timing and trigger
a gas Cerenkov for particle ID
-BigBite
dipole magnet
two multi-wire drift chambers for tracking
two scintillator planes for trigger + particle ID

12. HRS

13. HRS

14. BigBite

15. BigBite

16. Beam energy determination
Elastic scattering using various targets, various scattering angles

17. Beam energy drift

18. LHRS sieve pattern

19. Ta -elastics
ΔW(MeV)

20. Missing mass In terms of 4 momentum
So pion is created if missing mass = pion mass = 135 MeV/c2

21. Missing mass BigBite optics was calibrated using hydrogen elastics
and pion production
(GeV/c2)

22. Time of flight
physical time
electronic time

23. after accidental background subtraction
Missing mass
after accidental background subtraction
p0 mass =
0.135 GeV/c2
Coincidents from aluminium target window

24. Efficiency
of run number 4760

25. Simulation Simulation takes care of these effects -use GEANT3
-magnet field calculated using TOSCA
-event generator used cross sections from MAID and DMT
Simulation takes care of these effects
-beam energy drift
-bin migration
-external radiation

26. BigBite magnetic field from TOSCA

27. Alignment data and simulation
Hydrogen elastics
Data
BB.tr.th- vertical
bending angle
BB.tr.x- vertical position
at BB focal plane
Q_th- Q vertical angle
Simulation

28. the current while taking pion data
Simulation
Hydrogen elastics
Four Bigbite magnet
current settings show
good agreement with data
the current while taking pion data

29. Acceptance cuts Use the well defined region of spectrometers
Target coordinate – rotate
from Lab frame to
face spectrometer
th -vertical angle
ph-horizontal angle
dp=p-p0
p0=central momentum setting

30. EXCLURAD
internal radiation

31. Cross section
Beam helicity

32. Cross section measurement
The cross section is measured in
transform
by Jacobian

33. Cross section in Legendre polynomials
instead of

34. Cross section measurement

35. Some results

36. Differential cross sections
black points are data

37. Legendre fits

38. Total Cross section

39. Conclusion