Electro-Production of Pi0 near Electro-Production of Pi0 near the Thresholdthe Threshold

andand

the E04the E04 – Experiment– Experiment

Miha MihoviloviMiha Mihovilovič & doc. dr. Simon Šircač & doc. dr. Simon Širca

PresentPresent

Also Starring:Also Starring:

Quantum Chromo Dynamics Quantum Chromo Dynamics

- Basic constituents of hadrons are quarksquarks and gluons gluons

- Strong interactionStrong interaction is the fundamental force between them

- Theory describing this interaction is Quantum Chromo DynamicsQuantum Chromo Dynamics (QCD)

- Basic LagrangianLagrangian:

Quark Fields Gluon Fields Interaction term

Symmetries of QCD Symmetries of QCD

QCD Lagrangian doesn’t change under symmetry transformations:

- Discrete ChargeCharge, ParityParity and Time reversalTime reversal symmetries

- Approximate Flavor symmetry:Flavor symmetry: ( u -> d, d -> s, etc.)

- Approximate Chiral Symmetry:Chiral Symmetry:

These symmetries hold in the limit of limit of

massless quarksmassless quarks

Invariance under separate unitary global transformations of left- and right-handed quark fields.

In the limit of massless quarks the Axial and Axial and

Vector currents Vector currents are conservedare conserved

Emma Noether

Confinement Confinement

- Strong coupling constant measures the strength of interactions

- At high-high-energiesenergies quarks and gluons interact weakly.

- Asymptotic Asymptotic freedomfreedom allows employment of perturbative methods for analysis

-At low-low-energiesenergies forces between quarks becomes stronger

- Known as ConfinementConfinement

- Beyond the reach of perturbative treatment

- Development of effective Field effective Field theoriestheories

Chiral Chiral Perturbation Perturbation

TheoryTheory

Chiral Perturbation Theory - ChPT Chiral Perturbation Theory - ChPT

- The main idea is to devise a Lagrangian which includes allall 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:

CCii – Low energy constants

(Determined by experiment)

ππ Electroproduction in the ChPT Electroproduction in the ChPT

- The ChPT Lagrangian for the electroproduction of a pion:

Firs

t O

rder

Seco

nd

Ord

er

For π0 productio

n

ccii,d,dii,l,lii – Low energy

constants for π electroproduction

Leading Feynman Graph

The momentum transfer 4-vector is determineddetermined by the momenta of incident and scattered electron:

Three independent scalars:

Threshold energy for π0 production:

ωωLLThresholdThreshold(0)(0) = 144.69MeV144.69MeV

Kinematics Kinematics

-Unpolarized cross-section for π0 production in one photon exchange:

- The Transverse and Longitudinal component of the cross-section:

The Cross-Section for p(e,e’p)The Cross-Section for p(e,e’p)ππ00

- EEl±l± (electric), MMl±l± (magnetic) LLl±l± (longitudinal) are pion production pion production multipolesmultipoles.

- They determine the interactioninteraction of a given multipole component of the photon with the hadron systemhadron system.

Multipole threshold behaviour:

The Measurements and their problems #1The 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 EE0+0+ agree well with the ChPT.

- ChPT does not provide ChPT does not provide good representation of Lgood representation of L0+0+

datadata.

- High-precision measurements of the total cross-sectiontotal cross-section for the ππ00 production production as function of ΔΔWW and QQ22

- Striking differenceStriking difference between the calculation and measurements from Mainz.

The Measurements and their problems #2The Measurements and their problems #2

Mainz DataMainz Data

ChPT PredictionChPT Prediction

AnotherModelAnotherModel

- Differential longitudinal-transverse cross-section for Q2 = 0.05[GeV]2 and different ΔW above threshold.

- Difference between the ChPT ChPT prediction and measured datameasured data gets worse as ΔW increases.

The Measurements and their problems #3The Measurements and their problems #3

ChPT predictionChPT prediction

Mainz DataMainz Data

Possible reasons and solutionsPossible reasons and solutions

1.) Include a larger larger number of termsnumber of terms in the Chiral in the Chiral expansionexpansion i.e. increasing the number of LEC’s to be determined from data.

3.) Maybe one or more data data points are points are incorrectincorrect.

2.) S-waves are approx. constant at Threshold, while P-waves increase with the energy. This suggest problems problems with P-waveswith P-waves.

4.) Something is basically wrongbasically wrong with the formulation of the ChPTChPT

5.) Something is wrong with the wrong with the QCDQCD.

New, more New, more preciseprecise datadata

Need new Need new experiment to experiment to measure new datameasure new data

Threshold Threshold ππ00 experiment at JLab experiment at JLab

E04 – E04 – ExperimentExperiment

High precision measurements of p(e,e’p)π0 near threshold on a fine grid

of Q2 and ΔW

We would like to extendextend and re-examinere-examine

existing measurements

This experiment will provide a strong test of Chiral strong test of Chiral

QCD dynamicsQCD dynamics

The Threshold Pi0Threshold Pi0 experiment was performed by

the Hall A CollaborationHall A Collaboration using High-resolution

spectrometers in coincidence with the BigBite Spectrometer

TThomas homas JJefferson efferson NNational ational AAccelerator ccelerator FFacilityacility

- CEBAFCEBAF center at JLabJLab 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 QCDnon-perturbative QCD. Beam is delivered independently to three experimental Halls A, B and C.

Experimental Setup in Hall A Experimental Setup in Hall A

Target SystemTarget System

Cry

ogenic

targ

ets

Solid targets Solid

targets

Cryogenic targets

-The target systemtarget system consists of 6 cryo targets (LH2, LD2) and 5 solid targets used mostly for the calibration

Targ

ets

are

movin

g u

p

Targ

ets

are

movin

g u

p

an

d d

ow

n t

he lad

der.

an

d d

ow

n t

he lad

der.

High Resolution SpectrometersHigh Resolution Spectrometers

2 Quadrupoles

Quadru

pole

Dipole

Detector package

BigBite SpectrometerBigBite Spectrometer

- Single normal-conducting dipole magnet spectrometer

- Combines a large solid large solid angleangle with a large large momentum acceptance.momentum acceptance.

- Main characteristics:

BigBite ElectronicsBigBite Electronics

Radiological HazardRadiological Hazard

Current ResultsCurrent Results

- The experiment E04-E04-007007 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.

ConclusionsConclusions

- The nuclear reactions at low-energies are impossibleimpossible to describe in terms of perturbative QCDperturbative QCD.

- Therefore we use phenomenological modelsphenomenological models to describe reactions at low-energieslow-energies.

- These theories (ChPT) are firmly grounded in the symmetries of QCD.

- Current measurements show disagreementdisagreement with the ChPT predictions

- There are many possible explanations of these inconsistencies.

- To resolve these issues we need new experimentsneed new experiments, such as E04-007 at JLab.

““I think I think he got he got

the the point.”point.”

The End – Thank YouThe End – Thank You

Target System #2Target System #2

Temperature of the Liquid Liquid

Hydrogen targetHydrogen target is approx. 20K20K

Pressure of the Liquid Liquid

Hydrogen targetHydrogen target is approx. 1.6bar1.6bar

Heat loadIn the Liquid Liquid

Hydrogen targetHydrogen target is approx. 50W50W

All about Eve – My workAll about Eve – My work

- EveEve 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 BagHelium Bag

Full Helium bag:

Almost empty Helium bag:

Measurements:

- To minimize stragglingminimize straggling of low low energy protonsenergy protons through the BigBite we installed helium bagshelium bags in the empty gaps between the target and detector package.

Beam-Lock ProblemBeam-Lock Problem

-The beam energy in Hall-A was not lockednot locked.

- Beam energy was drifting drifting aroundaround.

- These beam energy changes cause shifts in the measured shifts in the measured datadata.