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

Quantum Chromo Dynamics quarks gluons - Basic constituents of hadrons are quarks and gluons Strong interaction - Strong interaction is the fundamental force between them Quantum Chromo Dynamics - Theory describing this interaction is Quantum Chromo Dynamics (QCD) Lagrangian - Basic Lagrangian: Quark FieldsGluon FieldsInteraction term

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

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

Chiral Perturbation Theory - ChPT all - 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: C i C i – Low energy constants (Determined by experiment)

π Electroproduction in the ChPT - The ChPT Lagrangian for the electroproduction of a pion: First Order Second Order For π 0 production c i,d i,l i c i,d i,l i – Low energy constants for π electroproduction Leading Feynman Graph

determined The momentum transfer 4-vector is determined by the momenta of incident and scattered electron: Three independent scalars: Threshold energy for π 0 production: ω L Threshold (0) MeV ω L Threshold (0) = MeV Kinematics

-Unpolarized cross-section for π 0 production in one photon exchange: - The Transverse and Longitudinal component of the cross-section: The Cross-Section for p(e,e’p) π 0 E l± M l± L l± pion production multipoles - E l± (electric), M l± (magnetic) L l± (longitudinal) are pion production multipoles. interaction hadron system - They determine the interaction of a given multipole component of the photon with the hadron system. Multipole threshold behaviour:

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 E 0+, L Other multipoles are zero. E 0+ - Measurements of E 0+ agree well with the ChPT. ChPT does not provide good representation of L 0+ data - ChPT does not provide good representation of L 0+ data.

total cross-section π 0 production ΔWQ 2 - High-precision measurements of the total cross-section for the π 0 production as function of ΔW and Q 2 Striking difference - Striking difference between the calculation and measurements from Mainz. The Measurements and their problems #2 Mainz Data ChPT Prediction AnotherModel

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

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

Threshold π 0 experiment at JLab E04 – E04 –Experiment High precision measurements of p(e,e’p) π 0 near threshold on a fine grid of Q 2 and ΔW extend re-examine We would like to extend and re-examine existing measurements strong test of Chiral QCD dynamics This experiment will provide a strong test of Chiral QCD dynamics Threshold Pi0 Hall A Collaboration The Threshold Pi0 experiment was performed by the Hall A Collaboration using High- resolution spectrometers in coincidence with the BigBite Spectrometer

Thomas Jefferson National Accelerator Facility CEBAFJLab - CEBAF center at JLab was built to investigate the structure of nuclei and hadrons at intermediate energies and underlying fundamental forces. non-perturbative QCD - 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.

Experimental Setup in Hall A

Target System Cryogenic targets Solid targets Solid targets Cryogenic targets target system -The target system consists of 6 cryo targets (LH 2, LD 2 ) and 5 solid targets used mostly for the calibration Targets are moving up and down the ladder.

High Resolution Spectrometers 2 Quadrupoles Quadrupole Dipole Detector package

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

BigBite Electronics

Radiological Hazard

Current Results E The experiment E was running in April and May 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.

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

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

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

All about Eve – My work Eve - 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.

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

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