The Discovery of the Quark
Mac Mestayer, Jlab
• the discovery of the nucleus - “Rutherford scattering”
– method: measure scattering rates vs. angle
• the discovery of quarks
– evidence that the proton is not a ‘point’ particle
– evidence for charged “partons” inside the proton
– properties ( frac. charge, spin, momentum )
• the continuing search
– details of quark-pair creation
April 30, 2010
Quarks: search for the smallest
1
Atomic structure
(1897) electron discovered
 how is it arranged with the positive charge?
(1902) Lord Kelvin - “raisin pudding” model
 electrons are ‘raisins’ embedded in a positive ‘pudding’
(1907) at University of Manchester; use a-particles as a beam
Hans Geiger
Ernest Rutherford
Rutherford, Geiger, Marsden: (professor) (post-doc) (undergrad)
but- a few at large-angle !
‘backscatters’ due to small,
heavy nucleus
April 30, 2010
Ernest Marsden
Quarks: search for the smallest
2
relation between rates and angle
“beams-eye” view
side-view  scattering angle
distant approach  small angle
impact parameter “DOCA”
q
measure at 4
angle settings
4 rings of approach distance
More area for small-angle scattering  higher rates
April 30, 2010
Quarks: search for the smallest
3
The “Rutherford scattering”* experiment
* done by Geiger and Marsden
Rutherford did calculations
like orbital mechanics ;
using 1/r2 electrostatic
forces and a massive
charged center.
Knowing the charge of the
nucleus and the alpha
particle, he estimated that
the nucleus was smaller
than 10-12 cm.
April 30, 2010
Quarks: search for the smallest
4
Electron Scattering - Bigger & Better
(1950’s) Cornell & Stanford Univ’s built electron accelerators to study the
structure of the nucleus, and even of the proton.
Electron scattering from Hydrogen
 deviation from 1 / sin4(q/2)
 proton is NOT a point particle
 radius (proton) ~ 10-13 cm1 m.
April 30, 2010
Quarks: search for the smallest
5
Proton has a finite size
Electron scattering from proton, Hofstadter, McAllister (1955)
a two-page paper !
Experimentalists
defer to future
theory, BUT make
a conjecture !
… that they are
measuring the
proton’s size;
~ 10-13 cm radius
Robert Hofstadter
April 30, 2010
Quarks: search for the smallest
… and Coulomb’s
law holds.
6
Elastic  inelastic scattering
If the object stays intact elastic.
one pool ball hitting another: elastic
snow-ball striking the side of the house: inelastic
electron
eP  eP : elastic
eP  eNp+: inelastic
p+
Proton
Neutron
photon
electron scattering
 exchange of a photon
April 30, 2010
electron
Quarks: search for the smallest
Proton
7
Momentum
energytransfer
transfer
Momentum &
& energy
for
scattering
forinelastic
elastic scattering
electron
Relativistic equations for
momentum and energy
exchange from electron to
photon to proton.
p+
P’
q
photon
Proton
Neutron
P
Proton
electron
4-momentum
4-momentumtransfer
transfersquared,
2, and energy
Q
squared,
Q2, and
transfer,
energy are
NOT
proportional
transfer,
 are proportional
Q 2  4 EE ' sin 2 q / 2
  E  E'
April 30, 2010
W (mass of the final state)
M (mass of the final state)
q  P  P' (conservat
(conservat
ionof
of44--momentum)
momentum)
ion
 Q 2  2mv  m 2  W
m 22
Q  2m  m  W
2
Quarks: search for the smallest
2
2
8
Deep inelastic scattering  “elastic scattering”
(off partons)
Excited State
mass = W
electron
electron
pion
Neutron
photon
photon
electron
Proton
electron
Proton
Inelastic scattering  elastic scattering from “parton” followed
Richard
Feynman
by “hadronization”
Q2 now proportional to  again !
April 30, 2010
Quarks: search for the smallest
9
“Elastic” scattering from a parton
How is x defined?
Excited State
mass = W
q  xP  P'
 Q 2  2 xm  x 2 m 2  x 2 m 2
electron
x  Q 2 / 2m
P’
q
Proton’s structure revealed by
scattering rate which depends on:
• charge (squared) of the components
• momentum distribution: f(x)
F   f2i ( x)  q
2
i
photon
xP
electron
Proton
Rate ~ f(Q
,v)  f(x)
i
as Q2,   large
April 30, 2010
Quarks: search for the smallest
10
“Bjorken scaling”
Richard Taylor
James Bjorken
“scaling”: function of two variables becomes a function of their ratio.
April 30, 2010
Quarks: search for the smallest
11
Big detectors to look for small objects
April 30, 2010
Quarks: search for the smallest
12
Scaling seen  partons inside proton
F (x)
Data from many different
Jerry Friedmanenergies (4.5 - 18 GeV) and
three angles (18, 26, 340)
overplotted, but they lie on one
curve if plotted versus 1/x.
Henry Kendall
1/x
April 30, 2010
Richard Taylor
Quarks: search for the smallest
13
Discovery of “partons”
• “Scaling” observed: functions of Q2 and 
become function of x only, where x = Q2 / 2m.
• Explained by electron scattering elastically off
‘point’ particles which carry a fraction (x) of the
proton’s 4-momenta (pq = x P).
• “Partons” discovered, what is spin, charge?
April 30, 2010
Quarks: search for the smallest
14
angle of “jets”  quarks are spin 1/2
Gail Hanson
April 30, 2010
Marty Perl
Quarks: search for the smallest
15
Other properties of partons
Experiment measures charge & momentum distribution
experiment  q
2
  x  f  x   dx  0.18
• Quark model of 1964 proposed the new particles
(excited protons) were composed of three “quarks”
with charge 2/3 or -1/3  total charge: 2,1,0,-1
• If partons are quarks, they carry only 60% of the
Murray
proton’s momentum
!! Gell-Mann
What carries the remainder ?
April 30, 2010
Quarks: search for the smallest
16
Quarks discovered!!
fractionally charged, spin ½ partons
 Quarks are discovered
… but many mysteries remained
- what carries the rest of the proton’s momentum ?
- does ‘scaling’ hold exactly ?
- let’s see
April 30, 2010
Quarks: search for the smallest
17
F2(x,q2)
Pattern of scaling violation
Structure function is NOT a function
of x only; depends on Q2.
•Small-x values INCREASE with Q2.
•Large-x values DECREASE with Q2.
quarks are radiating energy !
(probability increases with Q2)
WHAT are they radiating ?
-quanta of the strong color field
GLUONS
This pattern of scale-breaking can be
calculated using QCD.
April 30, 2010
Quarks: search for the smallest
Q2 (GeV2)
18
Evidence for QCD
• Missing momentum & pattern of scaling violation
– Explained by “gluon radiation”
– analogous to bremsstrahlung (X-ray machines)
• How can electrons scatter from quarks elastically?
– they act like free particles, but are bound in the proton !
If you probe the proton at small distances (high Q2), the quark
responds as if it is not bound (free), but as it moves away to
larger distances, it feels the attractive force (like a rubber band).
This is not like electromagnetism !!
April 30, 2010
Quarks: search for the smallest
19
asymptotic freedom & QCD
David Gross
David Politzer
Frank Wilczek
“for the discovery of asymptotic freedom in the theory of the
strong interaction” 2004 Nobel Prize in Physics
April 30, 2010
Quarks: search for the smallest
20
Quarks: what next?
• QCD: well-established as the theory of the strong
interactions  forces between quarks
• BUT, it’s a strongly-interacting field theory  very
difficult to SOLVE the equations
• INSTEAD, people GUESS solutions based on
qualitative aspects of QCD … and work out the
consequences.
April 30, 2010
Quarks: search for the smallest
21
Gluons: the strong force-field
Because of self-interactions the field lines compress into a tube.
The field energy grows linearly with separation  constant force
~ 1 GeV/fm
(16 TONS !!)
April 30, 2010
Quarks: search for the smallest
22
Nathan Isgur
April 30, 2010
Quarks: search for the smallest
23
A Modern Particle Detector
CLAS detector:
-magnetic spectrometer
(curvature ~ 1/p)
-drift chambers (tracking)
-scintillators (timing)
-calorimeters (energy, e/p)
-Cerenkov (e/p)
-------------------------------Fast: > 2000 evts/sec
Large acceptance > 2p sr
April 30, 2010
Quarks: search for the smallest
24
Geiger counter: gas ionization by particles
cosmic ray
tube
gas
~1 ionization/ 300 mm
wire
(at high voltage,
~ 2000 V)
1 - 10 electrons / ionization
~ 100 electrons/cm
April 30, 2010
Quarks: search for the smallest
25
“drifting” of the electrons
wire at positive voltage
•electrons drift to the wire
•strike a molecule every 2 mm
•velocity ~ 50 mm/ns
signal
New Idea - increase the accuracy
Time
Difference
of the tube by measuring the time
difference between the wire signal
and another prompt signal
signal
Georges Charpak
Timing counter
April 30, 2010
Quarks: search for the smallest
26
how tracking works
wires with signals shown
in yellow;
circle radius ~ drift time
Wire chamber
- looking along the wires
April 30, 2010
minimize rms between
track and calculated distance
27
Quarks: search for the smallest
First, we had to build them, ~1995
April 30, 2010
Quarks: search for the smallest
28
Now, we can analyse the data
Analysis:
• Detect Electron
•Cerenkov with C4F10
•e.m. shower counter
• Identify Kaon & Proton
•time of flight: ~100 ps
• p/K separation to 2 GeV/c
• Missing-mass for L
•good resolution: 0.5% dp/p
•separate L from S0
e p  K+ L : experiment at CLAS
April 30, 2010
Quarks: search for the smallest
29
L polarization probes quark-pair creation
‘flux-tube’
broken by the
creation of a qq pair !
An ‘escaping’
quark always
gets a partner
anti-quark !
note spin
correlation
April 30, 2010
Quarks: search for the smallest
30
Two model explanations …
Two views of how the L is
polarized:
top: u-quark polarized; sbar
polarization selected opposite; ssbar in spin-0 state
bottom: s and s-bar polarized directly
by photon
Both can explain L polarization !
On-going studies to distinguish
between the two models.
April 30, 2010
Quarks: search for the smallest
31
it takes all types …
experimenters
theorists
detector builders
April 30, 2010
Quarks: search for the smallest
32
Summary: the discovery of the quark
• Rutherford conceives scattering experiments
modern detectors
– measures rate vs. angle
are bigger and better
– nuclear radius less than 10-12 cm
• elastic e-p scattering rate deviates from 1/sin4(q/2)  proton has finite size
• inelastic e-p scattering ‘scales’  point-like “partons” in proton
• angular distribution of ‘jets’  partons have spin 1/2
• earlier quark model suggested charge 2/3, -1/3  partons are quarks !!
• asymptotic freedom explained  quarks act free, but cannot escape alone
}
Questions remain:
• nature
of flux-tube,
oftoquark-pair
“It does
no harm todynamics
the mystery
understandcreation…
a little about it.”
- Richard Feynman
April 30, 2010
Quarks: search for the smallest
33
Polarized photon scattering  parton spin
Electron scatters from charged
partons;
 exchange of a virtual photon
 virtual photon is polarized
(carries spin-transfer from electron)
transverse polarization
( electric field is transverse )
 spin along momentum vector
 spin 1/2 if sT dominates
April 30, 2010
Quarks: search for the smallest
34
sL/sT is small  partons are spin 1/2
April 30, 2010
Quarks: search for the smallest
35
relation between rates and angle
More area for small-angle scattering  higher rates
April 30, 2010
Quarks: search for the smallest
36
Two model explanations …
Two views of how the L is
polarized:
K*+
top: u-quark polarized; sbar
polarization selected opposite; ssbar in spin-0 state
bottom: s and s-bar polarized directly
by photon
On-going studies to distinguish
between the two models.
 measure L polarization for
production of K*+ L final state
April 30, 2010
Quarks: search for the smallest
K*+
37
Quark Pair Creation
• Quark-pair creation: “kernel” of exclusive production
• What field couples to the q-q current?
N
PL
ss produced
From flux-tube
s-quark  L K+ final state
ss produced from photon+
s
d-quark  N p final state
K+
u
u-quark  P p0 final state
s
u
du
Sept. 26, 2009
K0++
p
-measure ratio of rates
-different ratios
Mac Mestayer 38
s
u
d
L
Hadron Spectroscopy Meeting
Using Exclusive Production
to Study Quark Pair Creation
• Lund model: successful phenomenology for hadron
production; e.g. in e+e- reactions
• color flux-tube broken by qq production
– production rate depends on constituent quark mass
– uu : dd : ss ~ 1 : 1 : 0.2
• Vector meson dominance: photon fluctuates into a
virtual qq meson
– production rate depends on quark charge
– uu :dd : ss ~ 1: 0.25 : 0.25
Sept. 26, 2009
Mac Mestayer 39
Hadron Spectroscopy Meeting
Kaon Identification
Hyperon Missing Mass
Kaon candidates after timing cut
L, S0
Missing Mass (GeV)
e p g e’ K+ (X)
Mass = P / g b (GeV)
October 15, 2004
Spin2004
Mac
Mestayer
April 30, 2010
Quarks: search for the smallest
41
Scientific “belief”
• what does it mean to “believe in quarks”? – the role
of evidence, proof, intuition, belief
• “when you believe in things you don’t understand,
you’re in trouble” – Stevie Wonder
• “Shall I refuse my dinner because I do not fully
understand the process of digestion?” -O. Heaviside
• How can we say we have “discovered” quarks when
we have never seen evidence for a quark existing
alone and singly?
– “what is the sound of one hand clapping?”
April 30, 2010
Quarks: search for the smallest
42