The PRIMEX Experiment: A Fundamental Test of the Chiral

Anomaly Prediction in QCD

Erik Minges

April 23rd, 2010

Outline • Symmetry and conservation Laws

– Overview and examples

• PRIMEX physics motivation

– The Chiral Anomaly

• Experimental Setup

• PRIMEX first run results

• Prototype detector beamtest

– Energy calibration, shower profile, position resolution

– Conclusion

Symmetry – What is it?

• Generalized to mean invariance

– A system which remains unchanged under a certain kind of transformation.

• Rotational symmetry of a pencil

Rotational Symmetry

Symmetry Breaking

Symmetry cont. • Powerful tool in physics

– In 1905, Amalie Noether determined the connection between symmetry and conservation laws

• Noether’s theorem – For every continuous mathematical symmetry, there is a corresponding conservation law.

• “it is only slightly overstating that physics is the study of symmetry.” – Philip Warren Anderson

– American Physicist and Nobel Laureate

• Contributions to theories of localization, antiferromagnetism, and high-temperature superconductivity

PRIMEX Experiment

• The PRIMEX experiment at Thomas Jefferson National Lab is to measure the lifetime of the π0 particle with high precision. This experiment is a very important test of Quantum Chromodynamics (the theory to describe the strong interaction).

PRIMEX Physics Motivation

• Prediction of the decay width in Chiral Anomaly is exact in the Chiral limit.

– However quarks have mass.

– Leads to correction in theory.

• Neutral Pion decay measured to test fundamental prediction of low energy QCD.

– High precision measurement of decay rate, dominated by axial anomaly.

 0

 0

Chiral Anomaly • Axial symmetry of the Classical Lagrangian of

Quantum Chromodynamics is broken in the Chiral limit.

– Chiral limit is when the up, down quark masses vanish.

• This symmetry breaking is of pure Quantum mechanical origin due to quantization.

• The Lagrangian has continuous symmetry, but no conservation law associated with it

– i.e. - no conserved current

Confirmation of Quark Color • In the chiral limit, the predicted decay amplitude

– Where is the number of quark colors in QCD and is the Pion decay constant.

• Precise predicted decay width

• Number of quark colors will be directly measured by this experiment

– Number colors most fundamental parameter in Physics

1210513.2 )3(

 GeV f

N A C



 



MeVf 42.92

3CN

  eV

Am 725.7

64

23

 



Primakoff Effect

• Virtual photon from nucleus of target interacts with tagged photon from incident photon beam.

– Virtual photon and tagged photon create neutral Pi meson

• Photopion production

from Coulomb field of nucleus.

• Pion decay in forward direction, so

need good central resolution.

– Pi zero has high branching ratio to decay into 2 gamma rays, which HYCAL can detect.

Experimental Setup

Electron beam passing through

nucleus field and change momentum,

produce Bremsstrahlung radiation

Measure energy and momentum

of scattered electrons yield energy

and momentum of gamma produced

Radiator

Physics target

(Primakoff)

Magnet measure pair production to monitor

photon flux and sweep out charged particles

Helium bag less particle interaction

than air and cheaper than vacuum

Status of PRIMEX

PrimEx-I

() = 7.82eV2.2%2.1%

Leading Order

Chiral Anomaly

Next toLeading Order

Chiral Anomaly

Analysis Results for the 6x6 PbWO4 Crystal Prototype

Detector Beam Test

Center Crystals PbWO4

Surrounding Lead Glass

My Project for PRIMEX

• What is the need for an energy calibration? – 36 PbWO4 crystals detect incident beam energy via ADC

– Meaningful average pulse height signal from ADC requires calibration of each crystal using known incident beam energies

• How is the incident beam energy found? – x-y coordinate detector with 60 scintillating fibers on each axis

• Detects position of incident electrons.

• Each fiber approximately 2 mm in diameter

– Dipole magnet’s field yields incident energy of electrons • Incident electron beam energy as a function of X – fiber number is

e

e

       36241 1011645.0*1046112.0*1015153.05004.3 XXXXEbeam  

Relative Energy Calibration • Alignment

– Each crystal needs to have the same response

– Align each individual crystal center

• Assume all incident energy is deposited at center of crystal

• Assume no energy is lost due to leakage into surrounding crystals

• Find mean ADC at center of crystal, divide by known incident energy

Ebeam=A*(mean ADC)

i.e. For run 435 (centered on crystal 26)

Ebeam = E(29) = 3.98145729105

(mean ADC) = 5413

... so A = 7.35536171E-04!!!

adc(1) for run 435 cut at (X=29) and (Y=31)

mean ADC = 5413

Global Energy Calibration • Absolute scale from 5x5 cluster size

• Shower interacts in several crystal – Sum all deposited energy in surrounding crystals to account for leakage

Ebeam = B*[ ∑ Ai * (mean ADC)i ]

419

i.e. For run 419 (centered on crystal 15)

Ebeam = E(29) = 3.98145729105

∑ Ai * (mean ADC)I = 5.095

... so B = .781444021796 !!!

with σ/Ere = .018637880275

Now just take the average value of the four center crystals

Bavg = .781858818128

Results Error bars for (Ere/Ebeam)associated with run 413

21 22 23

- Ratio of Energies Deposited in a single module

Minimum deposited energy is at boundary of crystal. (As expected)

Maximum deposited energy is at center of crystal.

Results

21 22 23

Error bars for (σ/Ere)associated with run 413

21 22 23

- Energy Resolution of the Prototype

Energy resolution at center is greater than at boundaries of crystal. (As expected)

Results - Shower Profile The shower profile measures the dependence of the

average pulse-height ratio for different crystal channels

1.) Adjacent Channel ratio

Ai/Ai+1

2.) Adjacent Row ratio

Aj/Aj+1

3.) Separated Channel ratio

Ai+1/Ai-1

Results - Adjacent Channel Ratio

Run 435 for E26/E27

2726

1

Boundary

Results - Adjacent Row Ratio Run 422 E3,9,15/E4,10,16

3, 9, 15 4, 10, 16

1

Boundary

Results - Separated Channels Ratio Run 435 for E28/E26

26 27

Center: 27

28

1

Reconstructed Position Method

• Center of Gravity Method

Weight factor for cell

• Linear Weight

where,

• Logarithmic Weight

)(

*)(

i

N

i

i

ii

N

i

i

cal

E

xE

x



 





)( ii E

T

i ii

E

E E )( 

N

i

iT EE

  

  



  

 

  



 

  

 

T

i ii

E

E WE ln,0max 0

thi

Linear - Reconstructed Position

3x3 – run 389 3x3 – run 422

8

9

10

9

10

11

Linear - Reconstructed Position 5x5 – run 413

21

22

S shape indicates some systematic shift from the method incorporated.

Linear Position Resolution

3x3 – run 389 3x3 – run 422

8 9 10

9 10 11

Linear Position Resolution 5x5 – run 413

21 22 Position resolution worst at boundaries and best at center

Logarithmic Reconstructed Position

3x3 – run 389 3x3 – run 422

8

9

10

9

10

11 ~ %3 error (leakage)

~ %3 error (leakage)

Logarithmic Reconstructed Position 5x5 – run 413

21

22

(No systematic shift)

Logarithmic Position Resolution

3x3 – run 389 3x3 – run 422

8 9 10 9 10 11

3% error

Logarithmic Position Resolution 5x5 – run 413

21 22

1% error

Li