1Distinguished Doctoral Dissertation Colloquia—May 21, 2008Kieran Boyle

What makes the Proton Spin?

Kieran Boyle

2Distinguished Doctoral Dissertation Colloquia—May 21, 2008Kieran Boyle

Outline• How can we study the proton• What’s inside the proton• How are the proton’s properties

composed of the stuff inside– Charge– Momentum– Spin

• Why we want to study this by colliding protons

• Results

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3Distinguished Doctoral Dissertation Colloquia—May 21, 2008Kieran Boyle

How can we look inside a proton?• Consider X-rays• With them, we can see our bone

structure, but cannot see individual atoms

• Why?– The wavelength, in this case of the

X rays, is too large• To get better resolution, shrink

the wavelength• There are many examples of this

– an electron microscope– Synchrotron light sources

• Can we use this to study the proton?

4Distinguished Doctoral Dissertation Colloquia—May 21, 2008Kieran Boyle

Looking inside the proton• Yes, if the wavelength is small

enough• Small wavelength=large energy• Therefore, shoot very high

energy electrons at protons.• The electron and proton interact

via a very high energy, small wavelength photon (light)

• If at high enough energy, this photon can actually resolve the structure in the proton

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5Distinguished Doctoral Dissertation Colloquia—May 21, 2008Kieran Boyle

What’s in a proton?• This experiment has been

done at numerous photon energies, and much is now known.

• The structure that is seen by the photon is made up of particles, called quarks.

• They are bound together by a force stronger than electromagnetism, and so was called the strong force.

• This force is propagated by a particle called a gluon.

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6Distinguished Doctoral Dissertation Colloquia—May 21, 2008Kieran Boyle

Sum of its Parts• We know a lot about the proton (charge, spin,

etc.)• Can we understand how these properties arise

from the quarks and gluons within?• Charge:

– Gluons carry no charge, and so add nothing– Therefore, we can effectively describe the charge as

the sum of the quark charges

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7Distinguished Doctoral Dissertation Colloquia—May 21, 2008Kieran Boyle

Momentum• Quick reminder:

– What is momentum?– Momentum tells how much impact something will have, and depends on

velocity (speed) and mass

Dependence on Velocity

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Dependence on Mass

fly truck

baseball

8Distinguished Doctoral Dissertation Colloquia—May 21, 2008Kieran Boyle

Momentum• How is the proton momentum the sum of its parts?

– Maybe the quarks share the momentum equally?• Actual answer is more complicated

– quarks are continuously exchanging gluons, and so the momentum is continuously changing.

• In fact, only half of the proton momentum is carried by the quarks, with the gluons carrying the rest.

9Distinguished Doctoral Dissertation Colloquia—May 21, 2008Kieran Boyle

Spin• What is spin?

– Quantum mechanical analogy to angular momentum

– Particles like the electron behave like there is something inside going around in a circle, but as far as we understand, there is no inside.

– Proton also has spin, but it does have structure

• How is the proton spin the sum of its parts?– Maybe the quarks carry all the spin,

and just balance to give the proton spin?

• Again, the answer is more complicated– When the quarks exchange gluons,

the gluons can carry spin

10Distinguished Doctoral Dissertation Colloquia—May 21, 2008Kieran Boyle

Earlier results• So what did we know?• Quark spin contribution

was well measured previously– Found to be only ~25%– expected to be ~65%

• Where did the spin go?– Maybe gluons?– From previous

measurements, this was not well known

Quarks

Gluons

11Distinguished Doctoral Dissertation Colloquia—May 21, 2008Kieran Boyle

What does the photon see?

• We want to see the gluons to understand how they affect the proton’s porperties.

• Therefore, instead of a photon, we should use quarks and gluon.• But quarks and gluons are bound in protons, and so we use another proton.

12Distinguished Doctoral Dissertation Colloquia—May 21, 2008Kieran Boyle

Colliding Polarized Protons• The idea is that we understand what went

in by understanding what came out

• We measure what comes out when the protons have the same or opposite spin.

• From this we can calculate an asymmetry, called ALL.

• ALL can then be studied to understand the effect of the gluon spin on the proton spin

G2 Gq q2

Hard Scattering Process

2P2 2x P

1P

1 1x P

0

ALL~ agg * G2 + bgq * G q + cqq q2

vs.

13Distinguished Doctoral Dissertation Colloquia—May 21, 2008Kieran Boyle

The Results• Data from 2005

and 2006.• Clearly tell us a

lot more about the gluon spin in the proton than previous measurements

14Distinguished Doctoral Dissertation Colloquia—May 21, 2008Kieran Boyle

So Does the Gluon Make the Proton Spin?

• Turn ALL into a constraint on the gluon spin contribution (G)

• The result are significantly better than previous measurement, and indicate a small gluon spin

previous uncertainty

15Distinguished Doctoral Dissertation Colloquia—May 21, 2008Kieran Boyle

Conclusions and Prospects• The proton is composite, and so its properties

must be the sum of its part.• The quark spins only contribute a small fraction

of the total proton spin.• ALL can access the gluon spin contribution,

which was previously not well known.• 2005 and 2006 offer a significant constraint, and

indicate a small gluon contribution, though it’s possible that the gluon does make up the missing spin.

• If it doesn’t, then we must understand how the quarks and gluons are moving around inside the proton, as this should make up the difference.