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Option J: Particle physics J5 Evidence for quark and standard model

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J.5.1State what is meant by deep inelastic scattering.

J.5.2Analyze the results of deep inelastic scattering.

J.5.3Describe what is meant by asymptotic freedom.

J.5.4Describe what is meant by neutral current.

J.5.5Describe how evidence of a neutral current is evidence for the standard model.

Option J: Particle physicsJ5 Evidence for quark and standard model

State what is meant by deep inelastic scattering.

●Deep inelastic scattering consists of bombarding nucleons (like protons or neutrons) with high-energy leptons and analyzing the momentums and the scattering angles of the collisions, in much the same way as Rutherford’s atomic scattering.

●In early experiments performed on protons, low- energy electrons did not penetrate the internal structure of the proton. They scattered at shallow angles. Thus protons appeared to be elementary particles.

●High-energy electrons (20 GeV) were shown by Feynman and Bjorken (in 1968) to scatter in such a way that an internal structure of the proton was revealed.

Option J: Particle physicsJ5 Evidence for quark and standard model

FYI

Elementary particles have no internal structure.

Analyze the results of deep inelastic scattering.

●Evidence for the existence of quarks (and verification of the standard model) is threefold.

●From deep inelastic scattering experiments it is possible to deduce

a) Nucleons are made of smaller point charges (later called “quarks”).

b) At even higher energies

i) baryons scatter electrons in a way that is consistent with the baryon having three point charges within.

ii)Mesons scattered electrons consistent with having two point charges within.

c) The total momentum of the “quarks” was more than expected, implying the existence of another particle within the nucleon — the “gluon.”

Option J: Particle physicsJ5 Evidence for quark and standard model

Describe what is meant by asymptotic freedom.

●We know that the strong force has the property that it is strong enough to overcome the coulomb repulsion between two protons in a nucleus, but has no large scale effects.

●This would lead one to suppose that the closer the nucleons (and quarks) are, the stronger the force holding them together. But if this were the case, all nuclei (and quark combinations) would collapse to nothing!

●Therefore, it is necessary for the strong force to decrease to zero when the nucleons are sufficiently close together.

●We call the property that the force between quarks decreases to zero at very short range asymptotic freedom.

Option J: Particle physicsJ5 Evidence for quark and standard model

Describe what is meant by asymptotic freedom.

●An “asymptote” in mathematics is a number which a function gets closer and closer to but never attains. In this context, the force approaches zero as the quarks get closer together.

●The “freedom” part comes from the fact that at very close ranges the quarks in a hadron do not feel any force and can thus move around freely.

●Analyzing the results of deep inelastic scattering shows that the quarks in hadrons move easily around in asymptotic freedom.

Option J: Particle physicsJ5 Evidence for quark and standard model

EXAMPLE: Perhaps you have experienced the squishy eyeball sack Halloween toy.

●If you squeeze the toy (nucleon), the eyeballs (quarks) slide freely (grossly) around in the sack (asymptotic freedom).

●Try to pull two eyeballs (quarks) apart! No way!

There are only 3 quarks in a nucleon…

Describe what is meant by neutral current.

●Consider the Feynman diagram for - decay:

●Essentially, a neutron loses a negative charge and becomes a proton.

●That negative charge is transferred between particles, and this transfer occurs via the W- boson (of the weak interaction).

●The W- constitutes a negative current. And - decay is called a negative current interaction.

Option J: Particle physicsJ5 Evidence for quark and standard model

Describe what is meant by neutral current.

●Consider the Feynman diagram for + decay:

●Essentially, a proton loses a positive charge and becomes a neutron.

●That positive charge is transferred between particles, and this transfer occurs via the W+ boson (of the weak interaction).

●The W+ constitutes a positive current. And + decay is called a positive current interaction.

Option J: Particle physicsJ5 Evidence for quark and standard model

FYI

Negative and positive current interactions were observed from the very beginning of particle physics experimentation.

Describe what is meant by neutral current.

●Consider the Feynman diagram for electron-neutrino scattering:

●In this interaction, nothing loses charge: All that happens is that a neutrino and an electron collide and ricochet.

●That “neutral” charge is “transferred” between particles, and this transfer occurs via the Z0 boson (of the weak interaction).

●The Z0 constitutes a neutral current.

Option J: Particle physicsJ5 Evidence for quark and standard model

FYI

Neutral current interactions were not observed for 30 years even though there was active search.

●Physicists began to worry that the standard model’s prediction of such currents would be found to be wrong, and that the model was flawed.

Describe how evidence of a neutral current is evidence for the standard model.

●Probably since the beginning, physicists have been searching for “elegance” in the idea that perhaps the four fundamental forces are merely different aspects of a single force.

●In 1961 Sheldon Glashow showed how the weak force could be explained by Feynman diagrams and linked with the electromagnetic force. Using the diagrams and the standard model he predicted the need for a neutral current Z0 weak interaction for this to be possible.

Option J: Particle physicsJ5 Evidence for quark and standard model

FYI

One strength of a model is its ability to predict new observations.

Describe how evidence of a neutral current is evidence for the standard model.

●In 1983 all three weak interaction bosons were observed in CERN’s UA1 (Underground Area 1) by Rubbia and van der Meer.

●The Z0 discovery was predicted by the standard model - and found!

Option J: Particle physicsJ5 Evidence for quark and standard model

Describe how evidence of a neutral current is evidence for the standard model.

●Once the neutral current had been established the link between the weak force and the electromagnetic force was more promising – the Z0 particle in many was similar to a photon (the exchange particle for the electromagnetic force).

●A big problem with the weak force was that the W and Z exchange particles are very massive, and the photon is massless. In fact, any theory of exchange particles seemed to require that they be massless.

●The Higgs field, theorized by Salam and Weinberg, was the way around this obstacle. The interaction of this field with the weak exchange particles would give otherwise massless particles the mass observed in experiments.

Option J: Particle physicsJ5 Evidence for quark and standard model

Solve problems involving quarks and the standard model.

Option J: Particle physicsJ5 Evidence for quark and standard model

n p + e- + e

Q: 0 1 + -1+ Q(e)

Q: 0 0 + Q(e)

●Thus Q(e) = 0.

●The weak force interaction.

●Their lepton numbers are opposite..

Solve problems involving quarks and the standard model.

Option J: Particle physicsJ5 Evidence for quark and standard model

●The strong force interaction.

●The weak force interaction.

●The gluon.

●The W bosun.

Solve problems involving quarks and the standard model.

Option J: Particle physicsJ5 Evidence for quark and standard model

●In the first interaction L = -1 on the left and the right. Charge is conserved. Baryon number is conserved.

●In the second interaction L = -1 on the left L = +1 on the right. Lepton number is not conserved.

Solve problems involving quarks and the standard model.

Option J: Particle physicsJ5 Evidence for quark and standard model

●The weak force interaction.

Solve problems involving quarks and the standard model.

Option J: Particle physicsJ5 Evidence for quark and standard model

●The anti electron neutrino.

Solve problems involving quarks and the standard model.

Option J: Particle physicsJ5 Evidence for quark and standard model

●The neutron loses a negative charge to become a positive proton.

●The W- carries it away.

Solve problems involving quarks and the standard model.

Option J: Particle physicsJ5 Evidence for quark and standard model

●A particle that appears as an intermediate particle in a Feynman diagram.

●It is short-lived and not actually observed.

Solve problems involving quarks and the standard model.

Option J: Particle physicsJ5 Evidence for quark and standard model

+2/3

-3/3

-1/3

-3/3

-3/3

0

●Look for conservation of charge at the vertices:

●It must be a -1 and thus the W- boson.

Solve problems involving quarks and the standard model.

Option J: Particle physicsJ5 Evidence for quark and standard model

●Use the range formula R h/(4mc).

mc2=(100109 eV)(1.610-19 J/eV) = 1.610-8

mc =1.610-8/310-8 = 5.3310-17.

R h/(4mc)

R 6.6310-34/[4(5.3310-17)]

R 9.910-19= 110-18 m.

Analyze the results of deep inelastic scattering.

Option J: Particle physicsJ5 Evidence for quark and standard model

●Very high energy electrons bombard protons and the scattering patterns and momentum changes are analyzed.

●Analysis of scattering pattern showed that the internal structure of hadrons consisted of three point charges named quarks.

●Analysis of momentum changes showed that all of it couldn’t be accounted for in the quarks alone. Thus massive neutral particles named gluons were postulated.

Analyze the results of deep inelastic scattering.

Option J: Particle physicsJ5 Evidence for quark and standard model

●As the energy of the bombarding electrons was increased it was found that the strength of the force between the quarks decreased.

●The closer they are to each other, the less the force and the freer the quarks were to move around within the hadron.

●This observation was termed “asymptotic freedom.”