The deep structure of the proton

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The deep structure of the proton
and why it matters!
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1 Particle Physics an overview
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Particle Physics?
The study of the fundamental constituents of
matter in the Universe, and the forces that
operate between them
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What are we made of ?

• Democritus of Abdera (5c BC) and others ? atoms
• John Dalton (early 19c)
• ? atomic theory
• Ernest Rutherford (early 20c)
• ? atomic nuclear structure

Rutherford Geiger Marsden
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protons, neutrons,

• in the early 20c, experiments showed that the
  nucleus is made of smaller particles, protons
  and neutrons, bound together by a strong force
• by the mid-20c, many more hadrons had been
  discovered, through cosmic ray and particle
  collisions experiments S, L, X, ?, K, ...
• as for elements, patterns among hadrons emerged,
  and these led to the discovery of a new layer of
  substructure...

Hadrons particles that experience strong
interactions
Mp 1.673 ? 10-27 kg 0.938 GeV/c2 ? 1 GeV
Mpc2
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not the anti-electron
The question then arises to the physical
interpretation of the negativeenergy states,
which on this view really exist. We should expect
the uniformly filled distribution of
negativeenergy states to be completely
unobservable to us, but an unoccupied one of
these states, being something exceptional, should
make its presence felt as a kind of hole. It was
shown that one of these holes would appear to us
as a particle with a positive energy and a
positive charge and it was suggested that this
particle should be identified with a proton.
Subsequent investigations, however, have shown
that this particle necessarily has the same mass
as an electron and also that, if it collides with
an electron, the two will have a chance of
annihilating one another much too great to be
consistent with the known stability of matter. It
thus appears that we must abandon the
identification of the holes with protons and must
find some other interpretation for them.
Paul Dirac
P.A.M. Dirac, Proc. Roy. Soc. (London) A 133, 60
(1931)
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the structure of the proton (1960s and 1970s)
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Quarks!

• in 1964 Murray Gell-Mann (and independently
  George Zweig) suggested that the hundreds of
  hadron particles found could all be considered as
  combinations of just three quarks
• which he called up, down and strange
• proton (uud), neutron (ddu), S0 (uds), ?-
  (sss), etc

George Zweig
Murray Gell-Mann
helium atom
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Three quarks for Muster Mark! Sure he hasn't
got much of a bark And sure any he has it's all
beside the mark. But O, Wreneagle Almighty,
wouldn't un be a sky of a lark   To see that
old buzzard whooping about for uns shirt in the
dark And he hunting round for uns speckled
trousers around by Palmerstown Park? Hohohoho,
moulty Mark! You're the rummest old rooster ever
flopped out of a Noah's ark And you think you're
cock of the wark. Fowls, up! Tristy's the spry
young spark That'll tread her and wed her and
bed her and red her Without ever winking the tail
of a feather And that's how that chap's going to
make his money and mark!
James Joyce, Finnegans Wake, p.383
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more quarks and leptons

• 1897 electron
• 1933 neutrino
• 1937 muon
• 1964 up, down, strange
• 1974 charm
• 1975 tau
• 1977 bottom
• 1996 top

Six quarks for Muster Mark!
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The Standard Model Lagrangian
and beyond?
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Standard Model
6 quarks (u,d,s,c,b,t) 6 leptons
(e,?,?,?e,??,??) gauge bosons (?,W?,Z,g) Higgs
boson
10-18m D4
supersymmetry?
particle ? sparticle
dark matter?
bottom up
string, brane theory?
M-Theory?
quarks and gluons confined in hadrons baryons
(p,n), mesons (?)
10-35m D11?
Theory of Everything?
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Quantum Chromodynamics the strong interaction
field theory

• non-abelian gauge field theory with SU(3)
  symmetry
• responsible for the binding of quarks and gluons
  into hadrons

gluon
gS
gluon
gluon
?S gS2/4?
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Asymptotic Freedom What this year's Laureates
discovered was something that, at first sight,
seemed completely contradictory. The
interpretation of their mathematical result was
that the closer the quarks are to each other, the
weaker is the 'colour charge'. When the quarks
are really close to each other, the force is so
weak that they behave almost as free particles.
This phenomenon is called asymptotic freedom.
The converse is true when the quarks move apart
the force becomes stronger when the distance
increases.
Nobel citation
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summary of experimental measurements of ?s
Bethke
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calculating with QCD

• static properties of hadrons (masses, magnetic
  moments, decay rates) can be calculated using
  Lattice QCD brute force numerical calculation of
  path integrals made finite by discretizing space
  and time
• but this approach fails for dynamic quantities,
  e.g. scattering amplitudes

Christine Davies Cavendish Physical Society,
March 2007
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2 Partons
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Quarks!

• in 1964 Murray Gell-Mann (and independently
  George Zweig) suggested that the hundreds of
  hadron particles found could all be considered as
  combinations of just three quarks
• which he called up, down and strange
• proton (uud), neutron (ddu), S0 (uds), ?-
  (sss), etc
• but are quarks real? are they
• fundamental objects, like the electron?
• can they be detected in isolation?

Murray Gell-Mann
helium atom
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Deep Inelastic Scattering

• in the late 1960s, experiments at Stanford fire
  very fast electrons at stationary proton targets,
  to study possible proton sub-structure
• elastic scattering (ep?ep) dominates at low beam
  energy, but at high beam energy, the proton is
  blasted apart and electrons are scattered at
  wide angles (cf. Rutherford)
• the protons have a hard core (or hard cores.)!

Nobel Prize for Physics, 1990 Taylor, Friedman,
Kendall
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deep inelastic scattering

• variables
• Q2 4 E E sin2?/2
• x Q2 /2Mp(E E)

electron
E
E
photon
proton
( y 1 E/E )

• Q2 measures resolution
• modern experiments measure Q2 lt 105 GeV2
• ? ? gt 10-18 m rp/1000

• x measures inelasticity
• x 1 ? elastic
• 0 lt x lt 1 ? inelastic

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structure functions and scaling

• in general, we can write
• where F1,2(x,Q2) are called the structure
  functions of the proton
• experimentally,
• for Q2 gt 1 GeV2
• Fi(x,Q2) ? Fi(x)
• Bjorken scaling

40 years of Deep Inelastic Scattering measurements
F2(x,Q2)
x
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the parton model (Feynman 1969)

• photon scatters incoherently off massless,
• pointlike, spin-1/2 quarks
• probability that a quark carries fraction ? of
  parent protons momentum is q(?) (0lt ? lt 1),
  then

• the functions u(x) and d(x) are called parton
  distribution functions (pdfs) - they encode
  information about the protons deep parton
  structure
• we extract them from structure function
  measurements

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extracting pdfs from experiment

• different beams (e,?,?,) targets (H,D,Fe,)
  measure different combinations of quark pdfs
• thus the individual q(x) can be extracted from a
  set of structure function measurements
• quarks and antiquarks account for only about 1/2
  of the protons momentum the rest is carried by
  gluons!

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quarks as partons!

• and, indeed, up quark and down quark partons
  are observed in the proton, and their
  distribution functions measured..

• however, they only appear to carry about 30 of
  the protons momentum what carries the
  remainder?!
• answer a sea of quark and antiquark pairs (up,
  down, strange, charm, )

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sea quarks and gluons

• the strong force field inside the proton causes
  quark-antiquark pairs to fluctuate out of the
  vacuum, and become candidate partons

• but valence (u,d) quarks and sea quarks still
  only account for about 50 of the momentum the
  rest is carried by gluons

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the MRS/MRST/MSTW project
Alan Martin (1987) Durham Richard Roberts
(19872005) James Stirling (1987)
Cambridge Robert Thorne (1998) UCL Graeme
Watt (2007) CERN

• since 1987, the aim is to produce
  state-of-the-art pdfs
• combine experimental data with theoretical
  formalism to perform global fits' to data to
  extract the pdfs in user-friendly form to the
  particle physics community
• currently widely used at HERA and the Fermilab
  Tevatron, and in physics simulations for the
  future LHC proton-proton collider
• 50 published papers,
• 140 citations/paper

http//projects.hepforge.org/mstwpdf/
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partons valence quarks sea quarks gluons
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40 years of Deep Inelastic Scattering experiments
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DIS and QCD
Q2 gt Q1
Q1
quarks emit gluons!
DGLAP equations, at LO, NLO, NNLO, pQCD
Dokshitzer Gribov Lipatov Altarelli Parisi
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testing QCD

• QCD-improved parton model fits to recent Deep
  Inelastic Scattering data

• precision test of QCD
• measurement of the strong coupling
• ?SNNLO(MZ) 0.117 0.003
• (MSTW 2008, from global fit)

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HERA (DESY, Hamburg)
e, e? (28 GeV)
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a deep inelastic scattering event at HERA
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what we have learned

• the proton consists of pointlike partons
  valence (uud) quarks, gluons, and a sea of
  quarkantiquark pairs
• the sea has interesting quark flavour structure,
  some of which is not understood, i.e. heavier
  quarks are less likely, but why anti-u ? anti-d?

• the small-x partons are predominantly gluons, and
  they play an important role in LHC physics (see
  next part)
• the observed scale (Q2) dependence of the
  distributions is beautifully described by the QCD
  theory
• we know the distributions to few accuracy over
  most of the x range

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3 Partons at the LHC
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the Large Hadron Collider at CERN
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and so in proton-proton collisions
quark or gluon parton
proton
x2P
? Eparton ?(x1x2) Ecollider ? Ecollider
relativistic kinematics
this collision energy distribution is just a
convolution of the two parton probability
distribution functions f(x1)f(x2)
Eparton
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and to calculate event rates (scattering cross
sections)
large transverse momentum hadronic jets
Higgs production by gluon fusion
Z production by quark-antiquark annihilation
Higgs production with a Z boson
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producing and detecting a Higgs Boson

• at the LHC, two protons collide

• two gluon partons fuse to create a Higgs boson,
  which rapidly decays to two Z0 bosons, each of
  which decays into a muon and antimuon pair

• the detector registers an event with four muon
  tracks, together with the debris from the rest
  of the collision
• we can accurately predict the Higgs production
  rate using our pdfs

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simulation of Higgs Boson production in the ATLAS
detector at the LHC
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H i g g s B o s o n
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W and Z production at the Tevatron collider
comparing theory with experiment
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Higgs hunting at the Fermilab Tevatron proton
antiproton collider!
Tevatron ?s 1.96 TeV, cf. LHC ?s 7 14 TeV
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summary

• protons are complex objects they are made up of
  point-like constituents called partons, i.e.
  quarks, antiquarks and gluons no further
  substructure has been revealed down to lt 10-18 m
• we learn about this structure from Deep Inelastic
  Scattering experiments
• Parton Distribution Functions (pdfs) encode how
  the energy of a fast-moving proton is shared
  among its partons
• an accurate knowledge of these functions is
  needed in order to be able to predict very
  precisely what happens when protons collide at
  the LHC
• the UK is a world leader is producing pdfs

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Milky Way
100 m
lt 10-18 m
quarks
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t
c
b
W,Z
H, SUSY,?
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