Download QCD and the origin of proton

Spin is everywhere…
Evidence for
dark matter in
the universe…
Why do cats
always land
on their feet?
Even they were puzzled…
N. Bohr
W. Pauli

… but this is purely classic!
Spin in quantum mechanics……
G. Uhlenbeck
O. Klein
A. Goudsmit
P. Ehrenfest (1925):
“This is a good idea.
Your idea may be wrong,
but since both of you are
so young without any
reputation, you would
not loose anything making
a stupid mistake.”
…. spin as a fundamental property of elementary particles:
Proton spin
• The value: S =
1
2

• The original idea:
• The applications:
p  
 S p  S quark  12 
Example: MRI Alzheimer’s disease
© VUMC Alzheimercentrum
The origin of proton spin
• European Muon Collaboration (EMC) @ CERN (1985):
Quarks carry only a small fraction of the proton spin
• ZEUS and H1 experiments @ DESY (early ’90’s):
In the proton abundant
quark-antiquark pairs
and gluons are found.
gluon
Quark-antiquark pair
Search the carriers of proton spin
• Three possible sources:
 quark-antiquarks
(sea quarks)
 gluons
 orbital momentum
• Mathematically:
½ = ½ Sq + DG + Lq
~ 30%
(Valence  Sea Quarks?)
?
?
HERA e-p collider at DESY
6.2 km circumference ring,
27 GeV e+ & 920 GeV p’s
The HERMES experiment at HERA:
Outlook
• Spin is everywhere
• The origin of proton spin:
– gluons & orbital motion
• Importance QCD physics:
mquark  5 MeV  938 MeV  m proton
The of mass and spin of the
constituents of the visible
universe are rooted in QCD
dynamics