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
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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.