Transcript Quarks-oldx - JLab Computer Center

The Discovery of the Quark
Mac Mestayer, Jlab
• alchemy  elements  atoms
• the “discovery” of quarks
– evidence for charged “partons” inside the proton
– properties ( frac. charge, spin, momentum )
– evidence for gluons  development of QCD
• the continuing search
– constituent quark and flux-tube model
• what are the “effective degrees of freedom” ?
– quark-pair creation
April 30, 2010
Quarks: search for the smallest
1
Alchemy  elements
– Egyptians – metallurgy, embalming, cosmetics
– Greeks – theory
• Khemeia (magic & science): suppressed by Christians
– Moslems (7th – 12th centuries)  al Kimiya
– Crusades, Renaissance  back to Europe
Experiments  looking at chemical change
searching for changeless
Chemical Element
April 30, 2010
Quarks: search for the smallest
2
Alchemy  Chemistry & Physics
Alchemy: transmutation
( common  rare; iron, lead  gold; spirit  enlightenment )
– Successful ? YES ! !
• many of the hallmarks of experimental science
–
–
–
–
rigorous methods
recording of data
jargon (communication, mnemonics)
discoveries (elements & compounds)
– Alchemy itself transmuted;
 chemistry, physics, medicine
April 30, 2010
Quarks: search for the smallest
3
Chemical Element  Atom
(1800) John Dalton  combined chemical element plus atom
(smallest part)
• explained Proust’s law: 8g oxygen + 1g hydrogen  9g water
• Dalton guessed that water was a molecule formed of one atom
of hydrogen and one atom of oxygen (wrong !!, but brilliant !!)
Avagadro & Gay-Lussac  measured combination ratios by
volume in gases
(1808) Gay-Lussac measured volumes of H & O to form water;
 found ratio to be 2 to an accuracy of 0.1%
April 30, 2010
Quarks: search for the smallest
4
Development of Newton’s laws
•
•
•
•
Brahe: precise measurements of planetary positions
Kepler: saw regularities  elliptical orbits, 3 laws
Galileo, Descartes  force and acceleration
Hooke, Halley, Wren  circular motion 
acceleration
• Isaac Newton “Principia Mathematica” (1687)
•
•
•
•
•
April 30, 2010
sun-centered solar system
Kepler’s laws
centripetal force
inverse-square law
universal – affects all matter
Quarks: search for the smallest
Fgravity 
gM1M 2
r2
F  ma
5
… meanwhile …
Advance of Electricity & Magnetism
(1780)
(1785)
(1800)
(1820)
(1826)
(1831)
(1860)
Luigi Galvani - animal electricity
Charles de Coulomb - 1/r2 force } like gravity ??
Giuseppe Volta - first battery
Hans Oersted - current  magnetic field
Georg Ohm - V = I R
Michael Faraday, Joseph Henry – electromagnetic induction
James Clerk Maxwell
E  
dE
 B 
j
dt
B  0
dB
 E 
dt
April 30, 2010
Theory  forces in the atom
are electromagnetic
Equipment generators to
vacuum tubes to accelerators
Quarks: search for the smallest
6
Discovery of the electron
1870’s - Wm. Crookes  “Crookes Tube”
Observation:
cathode rays - stopped by metal objects
- pass through thin metal
- bent by magnets
Interpretation:
British - microscopic, negatively-charged particles
German – non-material waves, like light
Controversy:
Always good for funding !
April 30, 2010
Quarks: search for the smallest
7
Thomson discovers the electron
(1897) J. J. Thomson published his discovery of the electron.
Electric and magnetic fields deflect “cathode rays”  charged particle.
Values of E and B  charge to mass ratio and velocity were measured.
Thomson: “I can see no escape from the conclusion that they are
charges of negative electricity carried by particles of matter”
Newton’s equations + Maxwell’s laws + experimental equipment
April 30, 2010
Quarks: search for the smallest
8
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’
(1904) Nagaoka - suggested that atom was like solar system; ignored
(1907) at University of Manchester; use a-particles as a beam
Rutherford, Geiger, Marsden: (professor) (post-doc) (grad student)
but- a few at large-angle !
‘backscatters’ due to small,
heavy nucleus
April 30, 2010
Quarks: search for the smallest
9
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
10
Electron Scattering - Bigger & Better
(1950’s) Cornell & Stanford 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
Quarks: search for the smallest
11
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
… and Coulomb’s
law holds.
April 30, 2010
Quarks: search for the smallest
12
Elastic  inelastic scattering
If the object stays intact elastic.
golf-ball struck by club: elastic
snow-ball striking the back of your head: inelastic
electron
eP  eP : elastic
ep  eNp+: inelastic
E&M  exchange of
a photon
April 30, 2010
pion
Neutron
photon
electron
Quarks: search for the smallest
Proton
13
Momentum & energy transfer
for elastic scattering
Relativistic equations for
momentum and energy
exchange from electron to
photon to proton.
electron
April 30, 2010
P’
q
photon
electron
4-momentum transfer
squared, Q2, and energy
transfer,  are proportional
M (mass of the final state)
Proton
P
Proton
q  P  P' (conservation of 4 - momentum)
 Q 2  2mv  m 2  m 2
Q 2  2m
Quarks: search for the smallest
14
Momentum & energy transfer
for inelastic scattering
W (mass of the final state)
pion
electron
Relativistic equations for
momentum and energy
exchange from electron to
photon to proton.
q
P’
Neutron
photon
P
Proton
electron
4-momentum transfer squared,
Q2, and energy transfer,  are
NOT proportional
q  P  P'
(conservation of 4 - momentum)
 Q 2  2mv  m 2  W 2
Q 2  2m  m 2  W 2
April 30, 2010
Quarks: search for the smallest
15
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
by “hadronization”  Q2 now proportional to  again !
April 30, 2010
Quarks: search for the smallest
16
“Elastic” scattering from a parton
Electron scatters from charged components
 exchange of a virtual photon
How is x defined?
q  xP  xP
 Q 2  2 xm  x 2 m 2  x 2 m 2
Excited State
mass = W
electron
x  Q 2 / 2m
Structure-less components:
scattering amplitude is the product of
•momentum distribution: f(X)
•charge (squared) of the component
     f i ( x)  q
2
i
photon
electron
Proton
i
April 30, 2010
Quarks: search for the smallest
17
“Bjorken scaling”
“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)
April 30, 2010
Quarks: search for the smallest
18
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  become function of x;  x = Q2 / 2m
cross-section depends only on the x-distribution and charge
“Scaling” occurs whenever the cross-sections (for different Q2 and )
becomes a function of their ratio, x, only.
April 30, 2010
Quarks: search for the smallest
19
Scaling seen  partons inside proton
W2 plotted versus
ratio of 2m/q2
Many different energies
and angles overplotted,
but they lie on one curve
if plotted versus w
April 30, 2010
Quarks: search for the smallest
20
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 T dominates
April 30, 2010
Quarks: search for the smallest
21
L/T is small  partons are spin 1/2
April 30, 2010
Quarks: search for the smallest
22
“jets” are distributed like fermions
April 30, 2010
Quarks: search for the smallest
23
Quarks !!
fractionally charged, spin ½ partons
 Quarks are discovered
… but many mysteries remain
- what carries the rest of the proton’s momentum ?
- why do quarks seem to have variable mass ?
- also, does ‘scaling’ hold exactly ?
April 30, 2010
Quarks: search for the smallest
24
Properties of partons
• fractional charge
• momentum distribution
connected, we measure q2 * f(x)
2
q
  x  f ( x)  dx  0.18
If quark charges were unity, the average fraction of the proton’s
momentum carried by the quarks is only 0.18 !
For fractional charges (2/3, 1/3) the
momentum fraction is about 0.5.
What carries the remainder?
April 30, 2010
Quarks: search for the smallest
25
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)
26
Evidence for QCD
• Missing momentum !
– half the momentum carried by neutrals
• Pattern of scaling violation
– maxim: “first discover scaling, THEN the violation”
• Explained by “gluon radiation”
– analogous to bremsstrahlung (“braking radiation”)
• How can the field quanta carry half the momentum ?
– long story  development of QCD
April 30, 2010
Quarks: search for the smallest
27
asymptotic freedom & QCD
If you probe the proton at small distances (high Q2), the quark responds as if it is not
bound (free), but as it pulls away to larger distances (fm’s), it feels the attractive force.
“for the discovery of asymptotic freedom in the theory of the strong interaction”
2004 Nobel Prize in Physics
April 30, 2010
Alleluia !!
Quarks: search for the smallest
28
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
10 meters
Quarks: search for the smallest
29
CLAS12 Simulation
April 30, 2010
Quarks: search for the smallest
30
wire chamber: gas ionization by particles
tube
gas
wire
(at positive
high-voltage)
cosmic ray
~1 ionization/ 300 mm
1 - 10 electrons / ionization
~ 100 electrons/cm
July 2, 2009
What’s a Drift Chamber?
Mac Mestayer
“drifting” of the electrons
wire at positive voltage
•electrons drift to the wire
•strike a molecule every 2 mm
•velocity ~ 50 mm/ns (max)
July 2, 2009
What’s a Drift Chamber?
Mac Mestayer
the “avalanche”
wire
radius at
which
E = i / mfp
i ~ 20 eV
mfp ~ 2 mm
close-up
of wire
Ecrit ~ 200 kV/cm
every mfp
the number
of electrons
doubles
here comes
the electron!
July 2, 2009
What’s a Drift Chamber?
Mac Mestayer
how tracking works
hit wires
shown in
yellow
minimize rms between
track and calculated distance
July 2, 2009
What’s a Drift Chamber?
Mac Mestayer
detailed simulations done
Simulation ‘knows’ blue hits are good
Red hits are “background”
How to distinguish ?
Plenty of opportunity for clever
pattern-recognition software.
April 30, 2010
Quarks: search for the smallest
35
many aspects to analysis
Rotate F of view-plane
Blue hits “line up”
Swim a trial
trajectory
April 30, 2010
Quarks: search for the smallest
36
First, we have to build them, ~1995
April 30, 2010
Quarks: search for the smallest
37
… and now
April 30, 2010
Quarks: search for the smallest
38
Quarks: what next?
QCD is well-established as the theory of the strong
interactions defining the forces between quarks and
gluons;
BUT, because it is a strongly-interacting field it is very
difficult to SOLVE the equations;
INSTEAD, we have to experimentally find patterns and
come up with models which suggest new experiments
…
one avenue is to try to understand quark-pair creation
April 30, 2010
Quarks: search for the smallest
39
Quark “Confinement”
No single quark has been ‘seen’ in a detector
Why not ?
Is the binding force infinite?
-no; the string tension (linear potential) binding heavy
QQ systems agrees with energy level spacings if its
value is ~0.9 GeV/Fermi (i.e. you gain 1 GeV of
potential energy if you stretch the tube by 1 fm)
16 tons of force: large, but not infinite
similar to Zen riddle: the sound of one hand clapping
April 30, 2010
Quarks: search for the smallest
40
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
April 30, 2010
tons of force
Quarks: search for the smallest
41
April 30, 2010
Quarks: search for the smallest
42
Quark-pair creation
‘flux-tube’
broken by the
creation of a qq pair !
An ‘escaping’
quark always
gets a partner
anti-quark !
April 30, 2010
Quarks: search for the smallest
43
Quark-pair creation
‘flux-tube’
broken by the
creation of a qq pair !
An ‘escaping’
quark always
gets a partner
anti-quark !
April 30, 2010
note spin
correlation
Quarks: search for the smallest
44
e p  K+ L
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
March 26, 2007
U.Conn PAN Seminar
Mestayer
Mac
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
Quarks: search for the smallest
46
L Polarization
Transfer
• xyz system
defined in electron plane
z along g direction
• Polarization transfer
near maximal along z
~ 75%
~0 along x direction
• Models are only “ok”
but, not tuned
sensitive to polarization
Simpler in quark picture ?
March 26, 2007
Carman et al,
PRL90. 131804
(2003)
U.Conn PAN Seminar
Mac Mestayer
L Polarization
Transfer
e’
unpolarized
p
Simple
Phenomenology
e
-or-
L polarized ~ g direction
•for all K+ angles
•for all W
K+
L
L
K+
March 26, 2007
U.Conn PAN Seminar
Mac Mestayer
Quark Spins: Transferred Polarization
polarized
electron
g polarized
virtual photon
u p
u
d
u
after absorption of
photon’s momentum
u
d
L polarization
in direction of g
if s and s have
opposite spins !
u
d
March 26, 2007
u-quark polarized
by photon’s spin:
helicity conserved
K+
u
s s
L
s spin selected
opposite u-quark’s
U.Conn PAN Seminar
Mac Mestayer
Two model explanations …
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 the
two models !
April 30, 2010
Quarks: search for the smallest
50
it takes all types …
experimenters
theorists
April 30, 2010
Quarks: search for the smallest
detector builders
51
advice to the young scientist
•
•
•
•
•
•
ask questions
graph your results:
follow your intuition:
compare complex with simple
know your strengths & weaknesses and seek out a mentor
have fun
April 30, 2010
Quarks: search for the smallest
52
The discovery of the quark
•
•
•
•
elastic e-p cross-section deviates from 1/sin4(q/2)  proton has finite size
inelastic e-p scattering shows ‘scaling’ behavior  “partons”
smallness of L/T & ‘jets’ “partons” are spin 1/2
sum of momentum carried by partons  fractional charge
QCD can explain the scattering data with “colored”, fractionally
charged, spin ½ quarks and a gluonic force-field.
Questions remain*: effective mass of quarks, nature of multigluonic forces, spin state of quark-pair creation…
“It does no harm to the mystery to understand a little about it.”
- Richard Feynman
April 30, 2010
Quarks: search for the smallest
53