Electro-Production of Pi0 near the Threshold and the E04 – Experiment

1. Electro-Production of Pi0 near the Threshold and the E04– Experiment Miha Mihovilovič & doc. dr. Simon Širca Present Also Starring:

2. Quantum Chromo Dynamics • Basic constituents of hadrons are quarks and gluons • Strong interaction is the fundamental force between them • Theory describing this interaction is Quantum Chromo Dynamics (QCD) • Basic Lagrangian: Quark Fields Gluon Fields Interaction term

3. Symmetries of QCD • QCD Lagrangian doesn’t change under symmetry transformations: • Discrete Charge, Parity and Time reversal symmetries • ApproximateFlavor symmetry: • ( u -> d, d -> s, etc.) • ApproximateChiral Symmetry: These symmetries hold in the limit of massless quarks Invariance under separate unitary global transformations of left- and right-handed quark fields. In the limit of massless quarks the Axial and Vector currents are conserved Emma Noether

4. Confinement • Strong coupling constant measures the strength of interactions • At high-energies quarks and gluons interact weakly. • Asymptotic freedom allows employment of perturbative methods for analysis • At low-energies forces between quarks becomes stronger • Known as Confinement • Beyond the reach of perturbative treatment • Development of effective Field theories Chiral Perturbation Theory

5. Chiral Perturbation Theory - ChPT • The main idea is to devise a Lagrangian which includes all terms consistent with the symmetries of the QCD. • There is ∞number of such terms but at low energies only few are relevant. • From the dimensional analysis it follows: • The Lagrangian can be written as: Ci – Low energy constants (Determined by experiment)

6. π Electroproduction in the ChPT • The ChPT Lagrangian for the electroproduction of a pion: For π0production First Order Leading Feynman Graph Second Order ci,di,li – Low energy constants for π electroproduction

7. Kinematics The momentum transfer 4-vector is determined by the momenta of incident and scattered electron: Three independent scalars: Threshold energy for π0production: ωLThreshold(0)= 144.69MeV

8. The Cross-Section for p(e,e’p)π0 • Unpolarized cross-section for π0production in one photon exchange: • The Transverse and Longitudinal component of the cross-section: • El± (electric),Ml±(magnetic) Ll± (longitudinal)are pion production multipoles. • They determine the interaction of a given multipole component of the photon withthe hadron system. Multipole threshold behaviour:

9. The Measurements and their problems #1 • - Measurements of the pion electroproduction were made in Mainz and at NIKHEF • Near the threshold cross-section is dominated by s-wave amplitudes E0+, L0+. • Other multipoles are zero. • Measurements of E0+agree well with the ChPT. • ChPT does not provide good representation of L0+data.

10. The Measurements and their problems #2 • High-precision measurements of the total cross-section for the π0 production as function of ΔW and Q2 • Striking difference between the calculation and measurements from Mainz. Mainz Data ChPT Prediction AnotherModel

11. The Measurements and their problems #3 • Differential longitudinal-transverse cross-section for • Q2 = 0.05[GeV]2 and different ΔW above threshold. • - Difference between the ChPT prediction and measured data gets worse as ΔW increases. Mainz Data ChPT prediction

12. Possible reasons and solutions 3.) Maybe one or more data points are incorrect. 1.) Include a larger number of termsin the Chiral expansion i.e. increasing the number of LEC’s to be determined from data. 4.) Something is basically wrong with the formulation of the ChPT New, more precise data 5.) Something is wrong with the QCD. 2.) S-waves are approx. constant at Threshold, while P-waves increase with the energy. This suggest problems with P-waves. Need new experiment to measure new data

13. E04 – Experiment Threshold π0 experiment at JLab High precision measurements of p(e,e’p)π0near threshold on a fine grid of Q2 and ΔW We would like to extend and re-examine existing measurements The Threshold Pi0 experiment was performed by the Hall A Collaboration using High-resolution spectrometers in coincidence with the BigBite Spectrometer This experiment will provide a strong test of Chiral QCD dynamics

14. Thomas Jefferson National Accelerator Facility • CEBAF center at JLab was built to investigate the structure of nuclei and hadrons at intermediate energies and underlying fundamental forces. • 6 GeV polarized continuous beam is an ideal probe for the study of non-perturbative QCD. Beam is delivered independently to three experimental Halls A, B and C.

15. Experimental Setup in Hall A

16. Target System • The target system consists of 6 cryo targets (LH2, LD2) and 5 solid targets used mostly for the calibration Targets are moving up and down the ladder. Cryogenic targets Cryogenic targets Solid targets Solid targets

17. High Resolution Spectrometers Detector package Quadrupole Dipole 2 Quadrupoles

18. BigBite Spectrometer • Single normal-conducting dipole magnet spectrometer • Combines a large solid angle with a large momentum acceptance. • Main characteristics:

19. BigBite Electronics

20. Radiological Hazard

21. Current Results • The experiment E04-007 was running in April and May 2008 • Data analysis is at the moment focused on the calibration of the BigBite spectrometer. • BigBite has been used for the first time in this configuration. • It is very important to understand the optical properties of the spectrometer well before analyzing real data. • Expect first preliminary results on cross-sections by early 2009.

22. Conclusions • The nuclear reactions at low-energies are impossible to describe in terms of perturbative QCD. • Therefore we use phenomenological models to describe reactions at low-energies. • These theories (ChPT) are firmly grounded in the symmetries of QCD. • Current measurements show disagreement with the ChPT predictions • There are many possible explanations of these inconsistencies. • To resolve these issues we need new experiments, such as • E04-007 at JLab.

23. “I think he got the point.” The End – Thank You

24. Target System #2 Temperature of the Liquid Hydrogen target is approx. 20K Pressure of the Liquid Hydrogen target is approx. 1.6bar Heat load In the Liquid Hydrogen target is approx. 50W

25. All about Eve – My work • Eve is a event display for the BigBite spectrometer. • Displays hits in scintillation planes and wire-chambers and shows possible particle trajectories thorough the detector package.

26. Helium Bag Measurements: Full Helium bag: Almost empty Helium bag: - To minimize straggling of low energy protons through the BigBite we installed helium bags in the empty gaps between the target and detector package.

27. Beam-Lock Problem • The beam energy in Hall-A was not locked. • Beam energy was drifting around. • These beam energy changes cause shifts in the measured data.