The Squishiness of the Proton

Thomas Jefferson National Accelerator Facility
Hall-A Collaboration Experiment 93-050
Data Taking March 12 - April 13, 1998

 

Squishiness = How hard do we have to squeeze something to change its shape or size.

squeezing a cubical proton Squeeze box with mouse

How do we squeeze a proton?

What are the strongest electric forces we can produce in the laboratory?
 
Static electricity E=106 Volt/meter
Use Electromagnetic Waves:
Radio-Frequency (TJNAF): E=107 V/m
Lasers: E=1012 V/m
Gamma-rays E=1017 V/m
High energy electron 

10-15 m away

 E=1021 V/m
  How do we detect a momentary shift of 1 part per million in the proton mass?
Virtual Compton Scattering:  Hit the proton, then take a picture.

The electromagnetic force from a high energy electron passing by a proton acts like a mallet hitting a chime.  the proton and all its internal constituents (quarks, gluons, pions...) start to ring (oscillate).

An oscillating electric charge always radiates light at the frequency of vibration.  The intensity of the light is porportional to the square  of the amplitude of the oscillation.

The intensity of the light radiated by the proton after it is shaken by the passing electron is proportional
to the disturbance of the proton by the passing electron.

This is the squishiness of the proton.

How to gently squish a proton, in 6 [not so] easy steps.

1) Prepare a beam of electrons, each with energy 10,000 times greater than the rest mass energy of the electron.
 

2) Put lots of protons in front of each electron. 
At 700 Watts, the Hall-A target is the world's highest power liquid hydrogen target. 3) Collide individual electron in beam with individual proton in target.  Only one electron in 100 billion in the beam hits one of the mole (6.E23) of  protons in the target hard enough to ricochet the electron into our detector.

4) Detect scattered electron.  This determines the number of protons hit and how hard each proton was hit.

5) Detect recoil proton within a 6 nano-second coincidence time-window relative to the electron.

6) Use Conservation of energy and momentum to measure the mass of everything left over. 
After the proton is hit by the electron, it can either radiate a high energy gamma-ray or break up into a proton plus other sub-atomic particles. 
The intensity of the peak around zero  in the figure below tells us the squishiness of the proton.  The peak near 20,000 are events that broke the proton apart.

 Who built the equipment, conceived the experiment, carried out the measurements, and  analyzed the results?

Jefferson Lab Hall-A Collaboration
 

Why Physics?
The 19th century global economy was dominated by the practical and theoretical understanding of Thermodynamics: the science of heat & motion.

The 20th century economy has been dominated by Quantum Mechanics: the understanding of the wave nature of matter.

The 21st century economy will be dominated by the atomic manipulation of matter.

Scientific Publications from this experiment:
.
Electromagnetic form factors of the nucleon and Compton scattering.
 Charles Earl Hyde, Kees de Jager  Ann.Rev.Nucl.Part.Sci. 54 (2004) 217-267   e-Print: nucl-ex/0507001
Virtual Compton Scattering and the Generalized Polarizabilities of the Proton at Q^2=0.92 and 1.76 GeV^2.
 (H. Fonvieille  et al.).   e-Print: arXiv:1205.3387 [nucl-ex]
Virtual Compton Scattering and Neutral Pion Electroproduction in the Resonance Region up to the Deep Inelastic Region at Backward Angles.
 (G. Laveissiere   et al.)  Phys.Rev. C79 (2009) 015201   e-Print: hep-ex/0406062
Measurement of the generalized polarizabilities of the proton in virtual Compton scattering at Q^2 = 0.92 GeV^2 and 1.76 GeV^2. 
(G. Laveissiere et al.).  Phys.Rev.Lett. 93 (2004) 122001  e-Print: hep-ph/0404243

This page created by Charles Earl Hyde

Last modified:  17 May 2012