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Acknowlegements:
Betsy Beise, Abhay Deshpande, Naomi Makins, Kent Paschke, Bernd Surrow...
www.particleadventure.org
What is the Proton
Made of?
Krishna S. Kumar
University of Massachusetts, Amherst, USA
A H is to rica l Pe rs pe ct ive
PANIC11 Pedagogical Lectures
MIT, July 24 2011
1
Fundamental Questions
What are we made of?
What holds us together?
An early attempt
What is the Proton made of?
Current understanding
2
Krishna Kumar, PANIC11, July 24 2011
Fundamental Particles
& Fundamental Forces
Visible matter
made up of first
generation
quarks and
leptons
ee-
strong
interaction
electromagnetic
(EM) interaction
Are there other forces? Is there a single superforce?
Do quarks and leptons have structure?
How do protons, neutrons and nuclei get built from quarks and gluons?
…
What is the Proton made of?
3
Krishna Kumar, PANIC11, July 24 2011
Atomic Substructure:
Length Scales
Helium Atom
Alpha
Particle
What is the Proton made of?
4
Krishna Kumar, PANIC11, July 24 2011
Outline
Rutherford Scattering and the Atomic Nucleus
Electron Scattering and Nuclear Size
Electron Scattering and Proton Size
What is inside the Proton?
Current Research
What is the Proton made of?
5
Krishna Kumar, PANIC11, July 24 2011
Rutherford and the Atom
Parity-Violating Electron Scattering
6
Krishna Kumar, Mainz, Feb. 25 2011
Rutherford Scattering
~ 1910
Thomson’s
“plum
pudding”
model
0.1 nm
Rutherford devised a
scattering experiment
involving a beam of
alpha particles
scattering off a gold foil
An
alternative
hypothesis
testable by
experiment
What is the Proton made of?
7
Krishna Kumar, PANIC11, July 24 2011
Rutherford Scattering
~ 1910
Thomson’s
“plum
pudding”
model
0.1 nm
Rutherford devised a
scattering experiment
involving a beam of
alpha particles
scattering off a gold foil
An
alternative
Atom’s positive charge: static EM potential
hypothesis
testable by alpha particles & gold atom’s positive charge:
experiment
interaction governed by Maxwell’s equations
What is the Proton made of?
7
Krishna Kumar, PANIC11, July 24 2011
Rutherford Scattering
~ 1910
Thomson’s
“plum
pudding”
model
0.1 nm
Rutherford devised a
scattering experiment
involving a beam of
alpha particles
scattering off a gold foil
An
alternative
Atom’s positive charge: static EM potential
hypothesis
testable by alpha particles & gold atom’s positive charge:
experiment
interaction governed by Maxwell’s equations
differential
cross
section
�
�
dσ
dΩ
× ∆Ω
# alphas scattered into
solid angle per unit time
Incident beam intensity
What is the Proton made of?
7
Krishna Kumar, PANIC11, July 24 2011
Geiger-Marsden Data
Differential Cross Section
�
� �
�2
dσ
Z1 Z2 α
=
2
2
dΩ
2mv sin (θ/2)
•Established that atoms have a tiny nuclear core ~ 10-14 m, much
smaller than the atomic size of 10-10 m
•Revolutionized experimentation: particle scattering as a microscope
What is the Proton made of?
8
Krishna Kumar, PANIC11, July 24 2011
Electron Scattering and
the Size of the Nucleus
Parity-Violating Electron Scattering
9
Krishna Kumar, Mainz, Feb. 25 2011
A femtoscope
Electron Scattering
Electron interactions well understood
At nuclear scales, particles
behave mostly like waves
Particle
Accelerators
Wave mechanics or
quantum mechanics
How to produce femtometer
wavelengths in the laboratory?
What is the Proton made of?
10
Krishna Kumar, PANIC11, July 24 2011
A femtoscope
Electron Scattering
Electron interactions well understood
At nuclear scales, particles
behave mostly like waves
Particle
Accelerators
Wave mechanics or
quantum mechanics
How to produce femtometer
wavelengths in the laboratory?
e
e
pp
p
p
Electron scattering: electromagnetic interaction,
described as an exchange of a virtual photon.
What is the Proton made of?
10
Krishna Kumar, PANIC11, July 24 2011
A femtoscope
Electron Scattering
Electron interactions well understood
At nuclear scales, particles
behave mostly like waves
Particle
Accelerators
Wave mechanics or
quantum mechanics
How to produce femtometer
wavelengths in the laboratory?
e
e
pp
p
p
Electron scattering: electromagnetic interaction,
described as an exchange of a virtual photon.
4-momentum transfer
θ
q = −4 EE ʹ′ sin
2
2
€
2
Q2: -(4-momentum)2
of the virtual photon
Increase momentum transfer -­‐> shorter wavelength -­‐> higher resolu6on -­‐> smaller scales
What is the Proton made of?
10
Krishna Kumar, PANIC11, July 24 2011
1930’s and 40’s
Mott Scattering
Quantum Electrodynamics
µ
Free fermions fields are solutions to the Dirac equation (iγ µ∂ − m)ψ = 0
Feynman Rules: emission and absorption of virtual
photons by fermion electromagnetic current
current
γ
€
What is the Proton made of?
11
Krishna Kumar, PANIC11, July 24 2011
1930’s and 40’s
Mott Scattering
Quantum Electrodynamics
µ
Free fermions fields are solutions to the Dirac equation (iγ µ∂ − m)ψ = 0
Feynman Rules: emission and absorption of virtual
photons by fermion electromagnetic current
current
γ
€
E’
E
θ
Heavy, spinless
N
nucleus
What is the Proton made of?
Differential Cross Section
� dσ �
4Z 2 α2 E 2
dΩ
11
M ott
=
q4
Krishna Kumar, PANIC11, July 24 2011
1930’s and 40’s
Mott Scattering
Quantum Electrodynamics
µ
Free fermions fields are solutions to the Dirac equation (iγ µ∂ − m)ψ = 0
Feynman Rules: emission and absorption of virtual
photons by fermion electromagnetic current
current
γ
€
E’
E
θ
Heavy, spinless
N
nucleus
Differential Cross Section
� dσ �
4Z 2 α2 E 2
dΩ
M ott
=
q4
If Q sufficiently large, nuclear size modifies formula
dσ
=
dΩ
�
dσ
dΩ
�
Mott
�
�2
� F (q) �
F (q) =
The point-­‐like sca:ering probability modified: account for Finite Target Extent with a “form factor”
What is the Proton made of?
11
�
eiqr ρ(r)d3 r
Form factor is the Fourier transform of charge distribu6on
Krishna Kumar, PANIC11, July 24 2011
Form Factors & Nuclear Size
208Pb
1
What is the Proton made of?
12
2
3
q (fm)-­‐1
Krishna Kumar, PANIC11, July 24 2011
Form Factors & Nuclear Size
208Pb
few 100 MV of acceleration
1
What is the Proton made of?
12
2
3
q (fm)-­‐1
Krishna Kumar, PANIC11, July 24 2011
Electron Scattering and
the Size of the Proton
Parity-Violating Electron Scattering
13
Krishna Kumar, Mainz, Feb. 25 2011
Proton Form Factors
e
e
p
p
p
p
For a point-like target, accounting for target recoil:
Function of (E,θ).
�
�
dσ
dσ
2
=
1 + 2 τ tan (θ/2)
dΩ
dΩ Mott
Cross-section for infinitely
heavy, fundamental target
What is the Proton made of?
τ=Q2/4M2
14
Krishna Kumar, PANIC11, July 24 2011
Proton Form Factors
e
e
p
p
p
p
For a point-like target, accounting for target recoil:
Function of (E,θ).
�
�
dσ
dσ
2
=
1 + 2 τ tan (θ/2)
dΩ
dΩ Mott
Cross-section for infinitely
heavy, fundamental target
τ=Q2/4M2
If proton is not point-like: The electric and magnetic form factors GE
and GM parameterize the effect of proton structure.
dσ
dσ
E�
=
dΩ
dΩ Mott E
�
�
2
2
(GE + τ GM )
+ 2 τ G2M tan2 (θ/2)
1+τ
If the proton were like the electron:
GE = 1 (proton charge)
e
e
p
p
GM = 1 (magnetic moment = 1 Bohr magneton).
What is the Proton made of?
14
Krishna Kumar, PANIC11, July 24 2011
1950’s
Hofstadter Data
Otto Stern (1932) measured the proton
magnetic moment µp~ 2.5 µBohr
(first indication that the proton was not
a point-like particle, Nobel prize 1943)
What is the Proton made of?
15
Krishna Kumar, PANIC11, July 24 2011
1950’s
Hofstadter Data
Precise e-p cross section measurements at various scattering angles
Otto Stern (1932) measured the proton
magnetic moment µp~ 2.5 µBohr
(first indication that the proton was not
a point-like particle, Nobel prize 1943)
Stanford U. Mark III Accelerator
McAllister and Hofstadter, Physical
Review 102 (1956) 851.
What is the Proton made of?
15
Krishna Kumar, PANIC11, July 24 2011
1950’s
Hofstadter Data
Precise e-p cross section measurements at various scattering angles
Otto Stern (1932) measured the proton
magnetic moment µp~ 2.5 µBohr
(first indication that the proton was not
a point-like particle, Nobel prize 1943)
Stanford U. Mark III Accelerator
McAllister and Hofstadter, Physical
Review 102 (1956) 851.
What is the Proton made of?
15
Krishna Kumar, PANIC11, July 24 2011
1950’s
Hofstadter Data
Precise e-p cross section measurements at various scattering angles
Otto Stern (1932) measured the proton
magnetic moment µp~ 2.5 µBohr
(first indication that the proton was not
a point-like particle, Nobel prize 1943)
Stanford U. Mark III Accelerator
McAllister and Hofstadter, Physical
Review 102 (1956) 851.
What is the Proton made of?
15
Krishna Kumar, PANIC11, July 24 2011
1950’s
Hofstadter Data
Precise e-p cross section measurements at various scattering angles
Otto Stern (1932) measured the proton
magnetic moment µp~ 2.5 µBohr
(first indication that the proton was not
a point-like particle, Nobel prize 1943)
Stanford U. Mark III Accelerator
McAllister and Hofstadter, Physical
Review 102 (1956) 851.
What is the Proton made of?
15
Krishna Kumar, PANIC11, July 24 2011
1950’s
Hofstadter Data
Precise e-p cross section measurements at various scattering angles
Otto Stern (1932) measured the proton
magnetic moment µp~ 2.5 µBohr
(first indication that the proton was not
a point-like particle, Nobel prize 1943)
Stanford U. Mark III Accelerator
McAllister and Hofstadter, Physical
Review 102 (1956) 851.
It isn’t Mott, nor Dirac, nor modified
Dirac with GE=1 and GM=2.79…
What is the Proton made of?
15
Krishna Kumar, PANIC11, July 24 2011
1950’s
Hofstadter Data
Precise e-p cross section measurements at various scattering angles
Otto Stern (1932) measured the proton
magnetic moment µp~ 2.5 µBohr
(first indication that the proton was not
a point-like particle, Nobel prize 1943)
Stanford U. Mark III Accelerator
McAllister and Hofstadter, Physical
Review 102 (1956) 851.
It isn’t Mott, nor Dirac, nor modified
Dirac with GE=1 and GM=2.79…
the proton has finite size!
Robert Hofstadter –
Noble Laureate 1961
What is the Proton made of?
15
Krishna Kumar, PANIC11, July 24 2011
What is inside the
proton?
Parity-Violating Electron Scattering
16
Krishna Kumar, Mainz, Feb. 25 2011
Early 60’s
Quark Model
Late 40’s to the late 70’s, explosion of new “particle” discoveries
Made spectacular prediction for new
bound state which was subsequently found
with precisely the predicted properties
Gell-Mann & co-workers:
Quark hypothesis
Explained known bound-states with 3 degrees of freedom: u, d and s quarks
Example:
u and d
flavors:
SU(2)
symmetry
Gell-Mann:
1969 Nobel Prize
What is the Proton made of?
17
Krishna Kumar, PANIC11, July 24 2011
Deep Inelastic Scattering
Late 1960’s
e-
eγ*
p
What is the Proton made of?
X
In the mid-60s, electron accelerators achieved
electric field gradients that made 10 Billion
Volts of acceleration possible
•proton absorbs energy and breaks up
•total observed energy no longer conserved
•need to measure both scattering angle and
scattered momentum
18
Krishna Kumar, PANIC11, July 24 2011
Deep Inelastic Scattering
Late 1960’s
e-
eγ*
p
X
In the mid-60s, electron accelerators achieved
electric field gradients that made 10 Billion
Volts of acceleration possible
•proton absorbs energy and breaks up
•total observed energy no longer conserved
•need to measure both scattering angle and
scattered momentum
W1 and W2 are structure functions
What is the Proton made of?
x is fraction of proton momentum
carried by struck fragment
18
Krishna Kumar, PANIC11, July 24 2011
Deep Inelastic Scattering
Late 1960’s
e-
eγ*
p
X
In the mid-60s, electron accelerators achieved
electric field gradients that made 10 Billion
Volts of acceleration possible
•proton absorbs energy and breaks up
•total observed energy no longer conserved
•need to measure both scattering angle and
scattered momentum
W1 and W2 are structure functions x is fraction of proton momentum
carried by struck fragment
SLAC: Stanford Linear
Accelerator Center
Electron Spectrometer
Friedman,
Kendall & Taylor:
Nobel Prize 1990,
Measurement at
SLAC
What is the Proton made of?
18
Krishna Kumar, PANIC11, July 24 2011
Scaling and Partons
W = Mass of recoiling fragments
As W increased, the structure function
became surprisingly independent of Q2
electrons are hitting structureless
objects that have negligible size!
discovery of proton constituents called partons:
associate with quarks
What is the Proton made of?
19
Krishna Kumar, PANIC11, July 24 2011
Quark Model & QCD
Quantum Chromodynamics
✦QCD naturally introduces the concept of “color”
charge: necessary ingredient of Quark Model
✦There are 3 types of “color”: red, green and blue
✦QCD bound states are “color neutral”: individual
constituents cannot be observed (confinement)
✦The exchanged force carrier, the gluon, itself
carries color; gluons can interact with gluons
✦The interaction strength decreases with distance
(asymptotic freedom)
Gross, Politzer, Wilzcek: Nobel Prize 2004
What is the Proton made of?
20
Krishna Kumar, PANIC11, July 24 2011
Scaling Violations & QCD
1980’s & 90’s
10-18 m
What is the Proton made of?
21
Krishna Kumar, PANIC11, July 24 2011
Scaling Violations & QCD
1980’s & 90’s
10-18 m
What is the Proton made of?
21
Krishna Kumar, PANIC11, July 24 2011
Scaling Violations & QCD
1980’s & 90’s
10-18 m
What is the Proton made of?
21
Krishna Kumar, PANIC11, July 24 2011
Current Research
Parity-Violating Electron Scattering
22
Krishna Kumar, Mainz, Feb. 25 2011
Some Open Questions
✦Basic bulk properties of the proton are not easily
calculable from first principles QCD
•
theoretical progress ongoing from Lattice QCD
•
Ongoing research to obtain new, precise data on structure of
protons; and associated theoretical research to interpret data
★ electron, muon and proton beams in many different accelerator facilities
✦Several predicted quark bound states have not
yet been found
•
In particular, do quark-gluon or pure gluon states exist?
What is the Proton made of?
23
Krishna Kumar, PANIC11, July 24 2011
What carries the Spin of the
Proton?
✦ Spin: Fundamental quantum-mechanical property
of elementary particles; no classical analog
✦ all matter particles have two substates; moving
particles can have spin aligned or anti-aligned
✦ Proton has spin-1/2: how is that built up by the
constituents (quarks, gluons and orbital motion)?
quark contribution is only 25% ± 10%; where is the rest?
must be gluons or orbital motion
What is the Proton made of?
24
Krishna Kumar, PANIC11, July 24 2011
Do Gluons and Sea Quarks
affect Proton Bulk Properties?
For example: u, d & s quarks and anti-quarks are
known to appear and disappear in the proton
curve from
precise data
collected over
past 20 years
This can be studied by teasing
out the small component of
the weak force between
electrons and protons
Quantitative understanding leads to
deeper knowledge about the dynamics
of QCD at long distance scales
A complete separation of the u,d & s contributions to the charge,
magnetism and spin of the proton is ongoing with precise experiments
What is the Proton made of?
25
Krishna Kumar, PANIC11, July 24 2011
Take-Away Message
Saclay, France
NIKHEF, Netherlands
MIT-Bates, USA
SLAC, USA
DESY, Germany
CERN, Switzerland
Jefferson Lab, USA
Mainz, Germany
RHIC-Spin, USA
J-PARC, Japan
✦ Quark Model and QCD provide qualitative understanding of the
structure of the proton
✦ Theoretical and experimental research focus on quantitative
understanding to gain further insights into the dynamics of QCD
at the full relevant range of length scales
✦ Newer, improved accelerator facilities play an essential role:
a new polarized electron light-ion collider (EIC) is motivated in
the future
What is the Proton made of?
26
Krishna Kumar, PANIC11, July 24 2011
Related Talks at PANIC11
✦ Morning Plenary Sessions
•
•
•
Radial Distribution of the Proton and its Constituents (Haiyan Gao)
•
Probing Hadron Structure and New Physics with Parity-Violating
Electron Scattering (Jeff Martin)
•
•
Light Baryon Spectroscopy (Volker Crede)
•
Future Directions in Accelerator-Based HEP & NP Facilities (Thomas
Roser)
The Quark-Gluon Structure of the Nucleon (Naomi Makins)
Glimpsing the Fly in the Cathedral: Marking the First Centennial of
the Description of the Atomic Nucleus (Brian Cathcart)
Recent Results in Particle and Nuclear Physics from Lattice QCD
(Tetsuo Hatsuda)
✦ Public Lectures by Brian Cathcart and Jerome Friedman
What is the Proton made of?
27
Krishna Kumar, PANIC11, July 24 2011