Precision Measurement of the Electroproduction of p0 Near Threshold: Khem Chirapatpimol University of Virginia July 31st, 2012 This is proposal 04-007 - an update to E01-014. 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

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)

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

ChPT Lagrangian first three lowest order

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

In term of S and P wave Multipoles Calculate in ChPT

p0 photoproduction ChPT with one loop corrections

Mainz Data p0 electroproduction Harald Merkel 6th International Workshop on Chiral Dynamics July 6-10 2009 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

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

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

-Left High Resolution Spectrometer(LHRS) -electron Beam 1.193 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

HRS

HRS

BigBite

BigBite

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

Beam energy drift

LHRS sieve pattern

Ta -elastics ΔW(MeV)

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

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

Time of flight physical time electronic time

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

Efficiency of run number 4760

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

BigBite magnetic field from TOSCA

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

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

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

EXCLURAD internal radiation

Cross section Beam helicity

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

Cross section in Legendre polynomials instead of

Cross section measurement

Some results

Differential cross sections black points are data

Legendre fits

Total Cross section

Conclusion