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					                      What makes the
                       Proton Spin?

                             Kieran Boyle




Kieran Boyle   Distinguished Doctoral Dissertation Colloquia—May 21, 2008   1
                               Outline
    • How can we study the proton                                  10-4 m




    • What’s inside the proton                                              hair



    • How are the proton’s properties
      composed of the stuff inside
        – Charge
        – Momentum
        – Spin
    • Why we want to study this by
      colliding protons
    • Results


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



Kieran Boyle    Distinguished Doctoral Dissertation Colloquia—May 21, 2008   3
               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)                                     e-
                                                     e-
    • If at high enough energy, this
      photon can actually resolve the
      structure in the proton




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


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

                                                         -
                                                               +
                                                     +

Kieran Boyle   Distinguished Doctoral Dissertation Colloquia—May 21, 2008   6
                           Momentum
    • Quick reminder:
        – What is momentum?
        – Momentum tells how much impact something will have, and depends on
          velocity (speed) and mass

                               Dependence on Velocity




        you              baseball                                   baseball
                                Dependence on Mass




                         fly                                         truck

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




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


Kieran Boyle     Distinguished Doctoral Dissertation Colloquia—May 21, 2008        9
                       Earlier results
    • So what did we know?                                   Quarks
    • Quark spin contribution
      was well measured
      previously
        – Found to be only ~25%
        – expected to be ~65%
                                                            Gluons
    • Where did the spin go?
        – Maybe gluons?
        – From previous
          measurements, this was
          not well known
Kieran Boyle   Distinguished Doctoral Dissertation Colloquia—May 21, 2008   10
          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.


Kieran Boyle     Distinguished Doctoral Dissertation Colloquia—May 21, 2008         11
             Colliding Polarized Protons
    • The idea is that we understand what went
      in by understanding what came out


                          Hard
        P                 Scattering   p0
         1

                          Process
                   x1P1




               x2 P2
        P2
                                                DG2   DGDq     Dq2

                       ALL~ agg * DG2 + bgq * DG Dq + cqq Dq2
    • We measure what comes out when the
      protons have the same or opposite spin.
    • From this we can calculate an                                     vs.
      asymmetry, called ALL.
    • ALL can then be studied to understand the
      effect of the gluon spin on the proton spin


Kieran Boyle           Distinguished Doctoral Dissertation Colloquia—May 21, 2008   12
                         The Results
    • Data from 2005
      and 2006.
    • Clearly tell us a
      lot more about
      the gluon spin in
      the proton than
      previous
      measurements



Kieran Boyle   Distinguished Doctoral Dissertation Colloquia—May 21, 2008   13
      So Does the Gluon Make the Proton Spin?




    • Turn ALL into a constraint
      on the gluon spin
      contribution (DG)
    • The result are
      significantly better than
      previous measurement,
      and indicate a small
      gluon spin
                                                 previous uncertainty
Kieran Boyle    Distinguished Doctoral Dissertation Colloquia—May 21, 2008   14
          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.

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

				
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