Physics for Scientists and Engineers, 6e

Chapter 46 - Particle Physics and Cosmology

Given the identification of the particles in Figure b, what is the direction of the external magnetic field in the bubble chamber shown in Figure a?

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1. into the page

2. out of the page

3. impossible to determine

The right-hand rule for the positive particle tells you that this is the direction that leads to a force directed toward the center of curvature of the path.

Consider the following decay: n → π + + π – + μ + + μ

–. What conservation laws are violated by this decay?

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1. energy and electric charge

2. electric charge and baryon number

3. baryon number and angular momentum

4. no conservation laws

There is a baryon, the neutron, on the left of the reaction, but no baryon on the right. Therefore, baryon number is not conserved. The neutron has spin 1/2. On the right side of the reaction, the pions each have integral spin, and the combination of two muons must also have integral spin. Therefore, the total spin of the particles on the right-hand side must be integral and angular momentum is not conserved.

Consider the following decay: n -> p + π –. What conservation laws are violated by this decay?

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1. energy

2. electric charge

3. baryon number

4. angular momentum

5. no conservation laws

The sum of the proton and pion masses is larger than the mass of the neutron, so energy conservation is violated.

Consider the following decay: π0 → μ – + e + + vμ.

What conservation laws are violated by this decay?

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1. energy, electric charge, and baryon number

2. angular momentum, electron lepton number, and muon lepton number

3. electric charge, tau lepton number, and electron lepton number

4. no conservation laws

The pion on the left has integral spin while the three spin-1/2 leptons on the right must result in a total spin that is half-integral. Therefore, angular momentum is not conserved. Electron lepton number is zero on the left and – 1 on the right. There are no muons on the left, but a muon and its neutrino on the right (both with Lμ = +1). Therefore, muon lepton number is

not conserved.

Suppose a claim is made that the decay of the neutron is given by n -> p + e –. What conservation laws are violated by this decay?

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1. energy and electric charge

2. angular momentum and electron lepton number

3. baryon number and muon lepton number

4. no conservation laws.

There is one spin-1/2 particle on the left and two on the right, so angular momentum is not conserved. There are no leptons on the left and an electron on the right, so electron lepton number is not conserved

A student claims to have observed a decay of an electron into two electron neutrinos, traveling in opposite directions. What conservation laws would be violated by this decay?

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1. energy, baryon number , and muon lepton number

2. angular momentum, electric charge, and electron lepton number

3. no conservation laws

Angular momentum is not conserved because there is one spin-1/2 particle before the decay and two spin-1/2 particles afterward. Electric charge is not conserved because the negative charge on the electron disappears. Electron lepton number is not conserved, because there is an electron with Le = 1 before the

decay and two neutrinos, each with Le = 1,

afterward.

Doubly charged baryons, such as the Δ+ +, are known to exist. Doubly charged mesons also exist.

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1. true

2. false

Because the charges on quarks are +2e/3 and – e/3, the maximum possible charge of a combination of a quark and an antiquark is ±e.