Spin is everywhere…

Why do catsalways landon their feet?

Evidence fordark matter inthe universe…

Even they were puzzled…

W. Pauli

N. Bohr

… but this is purely classic!

Spin in quantum mechanics……

O. Klein

G. Uhlenbeck

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 makinga stupid mistake.”

…. spin as a fundamental property of elementary particles:

Proton spin

• The value: S =

• The original idea:

• The applications:

21

21 quarkp SSp

Example: MRI Alzheimer’s disease

© VUMC Alzheimercentrum

The origin of proton spin

• European Muon Collaboration (EMC) @ CERN (1985):

• ZEUS and H1 experiments @ DESY (early ’90’s):

Quarks carry only a small fraction of the proton spin

In the proton abundantquark-antiquark pairs and gluons are found.

gluon

Quark-antiquark pair

Search the carriers of proton spin

½ = ½ q + G + Lq

• Three possible

sources:

quark-antiquarks

(sea quarks) gluons orbital momentum

• Mathematically:

~ 30% ? ?

(Valence Sea Quarks?)

HERA e-p collider at DESY

The HERMES experiment at HERA:

6.2 km circumference ring,

27 GeV e+ & 920 GeV p’s

Outlook

• Spin is everywhere• The origin of proton spin:

– gluons & orbital motion

• Importance QCD physics:

The of mass and spin of the constituents of the visible universe are rooted in QCD dynamics

MeV 938 MeV 5 protonquark mm