The Discovery of the Quark Mac Mestayer, Jlab the discovery of the nucleus - “Rutherford scattering” – method: measure scattering rates vs. angle the discovery

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


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


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.


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


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


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


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


“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


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


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


angle of “jets” quarks are spin 1/2

April 30, 2010

Gail Hanson Marty Perl


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


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


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


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.


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


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


First, we have to build them, ~1995

April 30, 2010


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


“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

26Quarks: search for the smallestApril 30, 2010

simulated tracking


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


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

29Quarks: search for the smallestApril 30, 2010

Nathan Isgur

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


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


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


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


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.


building the equipment !

April 30, 2010


it takes all types …

April 30, 2010

experimenters

detector builders

theorists


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


More area for small-angle scattering higher rates


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

October 15, 2004 Spin2004 Mac Mestayer

L, S0

Kaon Identification Hyperon Missing Mass

Mass = P / g b (GeV) Missing Mass (GeV)e p g e’ K+ (X)

Kaon candidates after timing cut

Documents

The Discovery of the Quark Mac Mestayer, Jlab the discovery of the nucleus - “Rutherford scattering” – method: measure scattering rates vs. angle the discovery