the discovery of the quark mac mestayer, jlab the discovery of the nucleus - “rutherford...
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1Quarks: search for the smallest
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
detectors
3Quarks: search for the smallest
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
Rutherford, Geiger, Marsden: (professor) (post-doc) (undergrad)
April 30, 2010
but- a few at large-angle !‘backscatters’ due to small, heavy nucleus
Hans Geiger Ernest Rutherford
Ernest Marsden
4
relation between rates and angle
April 30, 2010 Quarks: search for the smallest
More area for small-angle scattering higher rates
“beams-eye” view side-view scattering angle
Quarks: search for the smallest 5
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
6Quarks: search for the smallest
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 cm
April 30, 2010
1 m.
7Quarks: search for the smallest
Proton has a finite size
April 30, 2010
Electron scattering from proton, Hofstadter, McAllister (1955)Experimentalists defer to future theory, BUT make a conjecture !
… that they are measuring the proton’s size;
~ 10-13 cm radius
… and Coulomb’s law holds.
a two-page paper !
Robert Hofstadter
8Quarks: search for the smallest
Elastic inelastic scattering
April 30, 2010
If the object stays intact elastic.
one pool ball hitting another: elastic
snow-ball striking the side of the house: inelastic
eP eP : elastic
eP eNp+: inelastic
electron scattering
exchange of a photonProton
p+
Neutron
electron
photon
electron
9Quarks: search for the smallest
Momentum & energy transfer for elastic scattering
April 30, 2010
Protonelectron
photon
electron
q
P
Relativistic equations for momentum and energy exchange from electron to photon to proton.
mQ
mmmvQ
PPq
2
2
'
2
222
momentum)-4 ofion (conservat
4-momentum transfer squared, Q2, and energy
transfer, n are proportional
Proton
M (mass of the final state)P’
222
222
2
2
' momentum)-4 ofion (conservat
WmmQ
WmmvQ
PPq
4-momentum transfer squared, Q2, and energy transfer, n are
NOT proportional
W (mass of the final state)p+
Neutron
Momentum & energy transfer for inelastic scattering
'
2/sin'4 22
EE
EEQ
10Quarks: search for the smallest
Inelastic scattering elastic scattering from “parton” followed by “hadronization” Q2 now proportional to n again !
Deep inelastic scattering “elastic scattering” (off partons)
April 30, 2010
Proton
pion
Neutron
electron
photon
electron
Excited State mass = W
Protonelectron
photon
electron
Richard Feynman
Quarks: search for the smallest
“Elastic” scattering from a parton
April 30, 201011
2)( ii
i qxfF Protonelectron
photon
electron
Excited State mass = W
q
xP
P’
How is x defined?
Proton’s structure:“structure function” F is the product of
• momentum distribution: f(x)• charge (squared) of the component
Rate of interaction ~ F F ~ q2 * f(x)
mQx
mxmxxmQ
PxPq
2/
2
'
2
22222
12Quarks: search for the smallest
“Bjorken scaling”
April 30, 2010
“scaling” function of two variables becomes a function of their quotient
probability of scatter = probability that parton has fraction (x ) of proton’s momentum times probability of interaction (charge2)
Richard Taylor James Bjorken
13Quarks: search for the smallest
Evidence for “partons”
Hypothesis: proton made of “parts”• revealed in scattering experiments (like Rutherford’s discovery)
• carry a fraction (x) of the proton’s 4-momenta (pq = x P)
• assumed structure-less, so electron scatters elastically off partons• functions of Q2 and n become function of x; x = Q2 / 2mn• cross-section depends only on the x-distribution and charge
“Scaling” occurs whenever the cross-sections (for different Q2 and n) becomes a function of their ratio, x, only.
April 30, 2010
14Quarks: search for the smallest
Scaling seen partons inside proton
April 30, 2010
F2 plotted versus ratio of 2m /n q2.
Many different energies and angles overplotted, but they lie on one curve if plotted versus .w
F (
x)
1/x
Jerry Friedman
Henry Kendall
Richard Taylor
15Quarks: search for the smallest
angle of “jets” quarks are spin 1/2
April 30, 2010
Gail Hanson Marty Perl
16Quarks: search for the smallest
Other properties of partons
• fractional charge• momentum distribution
April 30, 2010
connected, we measure q2 * f(x)
18.0)( experiment 2 dxxfxq
Original quark model of 1964, proposed that many of the new particles (basically excited protons) were composed of three quarks. The quarks have charge 2/3 or -1/3; e.g. the proton has charge 1 = 2/3 + 2/3 - 1/3;while the neutron has charge 0 = 2/3 -1/3 -1/3.
For these charges the momentum fraction of the proton which is carried by the partons is only 50%.
What carries the remainder?
Murray Gell-Mann
17Quarks: search for the smallest
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
18Quarks: search for the smallest
Pattern of scaling violation
April 30, 2010
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.
F 2(x,q
2 )
Q2 (GeV2)
‘lines’ at
constant x
19Quarks: search for the smallest
Evidence for QCD
• Missing momentum & pattern of scaling violation– Explained by “gluon radiation”– analogous to bremsstrahlung (“braking radiation”)
• How can electrons scatter from quarks elastically?– they act like free particles, but are bound in the proton !
April 30, 2010
If you probe the proton at small distances (high Q2), the quark responds as if it is notbound (free), but as it pulls away to larger distances (fm’s), it feels the attractive force.
20Quarks: search for the smallest
asymptotic freedom & QCD
April 30, 2010
“for the discovery of asymptotic freedom in the theory of the strong interaction” 2004 Nobel Prize in Physics
David Politzer Frank WilczekDavid Gross
21Quarks: search for the smallest
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
Bryce
April 30, 2010 Quarks: search for the smallest
Geiger counter: gas ionization by particles
tube
gas
wire(at positivehigh-voltage,~ 2000 V
cosmic ray
~1 ionization/ 300 mm
1 - 10 electrons / ionization
~ 100 electrons/cm
23
April 30, 2010 Quarks: search for the smallest
“drifting” of the electrons
wire at positive voltage
•electrons drift to the wire•strike a molecule every 2 mm•velocity ~ 50 mm/ns (max)
24
April 30, 2010Quarks: search for the smallest
how tracking works
hit wiresshown in yellow
minimize rms betweentrack and calculated distance
25
Georges Charpak
27Quarks: search for the smallest
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
28Quarks: search for the smallest
Gluons: the strong force-field
April 30, 2010
Because of self-interactions the field lines compress into a tube.The field energy grows linearly with separation constant force
~ 1 GeV/fm
Quarks: search for the smallest 30April 30, 2010
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
31Quarks: search for the smallest
how to measure Lambda polarization
• Lambda is a spin ½ particle– decays to Proton (spin ½) and p-(spin 0)– two amplitudes: s-wave (L=0) and p-wave (L=1)– (A1+A2)2 ~ (1.+ a cosq)
• a = 0.61
measure the angular distribution of the decay proton relative to some axis and fit to (1. + P a cosq ) P is the polarization of the sample of Lambda’s
April 30, 2010
April 30, 2010 Quarks: search for the smallest
Simpler in quark picture ?
L PolarizationTransfer
• xyz systemdefined in electron planez along g direction
• Polarization transfer near maximal along z~ 75%~0 along x direction
• Models are only “ok”but, not tunedsensitive to polarization
Carman et al,PRL90. 131804 (2003)
32
33Quarks: search for the smallest
Quark-pair creation
‘flux-tube’ broken by the creation of a q-q pair !
An ‘escaping’ quark always gets a partner anti-quark !
April 30, 2010
note spin correlation
34Quarks: search for the smallest
Two model explanations …
April 30, 2010
Two views of how the L is polarized:
top: u-quark polarized; sbar polarization selected opposite; s-sbar 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.
36Quarks: search for the smallest
it takes all types …
April 30, 2010
experimenters
detector builders
theorists
37Quarks: search for the smallest
Summary: the discovery of the quark
How the quark was discovered:• scattering experiments: measure rate vs. angle, momentum• elastic e-p cross-section deviates from 1/sin4(q/2) proton has finite size• inelastic e-p scattering shows ‘scaling’ behavior “partons” in proton
Development of the theory:• QCD can explain the scattering data with fractionally charged, spin ½
quarks and a gluonic force-field.
Questions remain*: • dynamics of quark-pair creation…* “It does no harm to the mystery to understand a little about it.”
- Richard Feynman
April 30, 2010
38
relation between rates and angle
April 30, 2010 Quarks: search for the smallest
More area for small-angle scattering higher rates
39Quarks: search for the smallest
Two model explanations …
April 30, 2010
Two views of how the L is polarized:
top: u-quark polarized; sbar polarization selected opposite; s-sbar 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
K*+
K*+
K+su
Lud
Sss producedFrom flux-tube
Quark Pair Creation• Quark-pair creation: “kernel” of exclusive production• What field couples to the q-q current?
su
sud
K+
L
ss produced from photon
Sept. 26, 2009 Mac Mestayer 40Hadron Spectroscopy Meeting
p+
d
d
N
u
u
P
p0
s-quark L K+ final stated-quark N p+ final stateu-quark P p0 final state
-measure ratio of rates -different ratios
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– : : ~ 1 : 1 : 0.2
• Vector meson dominance: photon fluctuates into a virtual qq meson– production rate depends on quark charge– : : ~ 1: 0.25 : 0.25
uu dd ss
uu dd ss
Sept. 26, 2009 Mac Mestayer 41Hadron Spectroscopy Meeting
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