Nuclear & Particle Physics - Standard Model

According to the Standard Model of elementary particles, which of the following is NOT a composite object?

A. Muon
B. Pi-meson
C. Neutron
D. Deuteron
E. Alpha particle

(GR9677 #63)

Solution:

A.TRUE
Muon is a lepton. Leptons, along with quarks, are considered the fundamental particles.

B. FALSE
Pi-Meson consists of a quark and its antiparticle. Moreover, a pi-meson is a hadron. Hadrons interact with the strong-force, and all of them are composed of combinations of quarks. (The fundamental particles are classified as quarks and leptons.)

C. FALSE
A neutron is made up of 3 quarks.

D. FALSE
A deuteron consists of a proton and a neutron. (tritium is two neutrons and a proton, while regular Hydrogen is just an electron and proton).

E. FALSE
An alpha particle consists of electrons and protons and neutrons.

Answer: A

Nuclear & Particle Physics - Cosmic Rays

In experiments located deep underground the two types of cosmic rays that most commonly reach the experimental apparatus are

A. alpha particles and neutrons
B. protons and electrons
C. iron nuclei and carbon nuclei
D. muons and neutrinos
E. positrons and electrons

(GR9277 #25)

Solution:

Since the experiment is located deep underground, the particles must be essentially massless and able to pass through or do not interact with matter easily → muons and neutrinos.

Muon:

• "heavy" electron (200 times the mass of the electrons)
• highly penetrating

Neutrinos:

• almost no mass
• travel close to the speed of light
• weakly interact with other

Answer: D



Notes:

• Alpha particles and neutrons have high kinetic energy but very short penetrating depth because they are quite massive, can't go through the Earth.
• Protons and electrons don’t penetrate much as they interact easily with matter. Protons are also pretty massive.
• Iron and carbon nuclei are very heavy and interact very easily with matter.
• Positrons and electrons have the same mass and are highly interacting with matter. Positron also has a very short life span.

Nuclear & Particle Physics - Muon

Two horizontal scintillation counters are located near the Earth’s surface. One is 3.0 meters directly above the other. Of the following, which is the largest scintillator resolving time that can be used to distinguish downward-going relativistic muons from upward-going relativistic muons using the relative time of the scintillator signals?

A. 1 picosecond
B. 1 nanosecond
C. 1 microsecond
D. 1 millisecond
E. 1 second

(GR9277 #49)

Solution:

Muons can travel with maximum speed near the speed of light:
v = 3 × 10⁸ m/s

Given s = 3 m,

t = s/v = 3/(3 × 10⁸) = 10⁻⁸

Micro = 10⁻⁶
Nano = 10⁻⁹
Pico = 10⁻¹²

10⁻⁸ second is close to 1 nanosecond.

Answer: B

Nuclear & Particle Physics - Muon

Except for mass, the properties of the muon most closely resemble the properties of the

A. electron
B. graviton
C. photon
D. pion
E. proton

(GR8677 #16)

Solution:

(A) TRUE
Muon and electron are elementary particles → fermions (half-integer spin) → leptons (one unit charge)

(B) FALSE
Graviton is gravity force carrier → boson (integer spin)

(C) FALSE
Photon is EM force carrier → boson (integer spin)

(D) FALSE
Pion is composite particle → meson (consist of quark-antiquark)

(E) FALSE
Proton is composite particle → baryon (consist of 3 quarks)

click image to enlarge

Answer: A

Nuclear & Particle Physics - Conservation Laws

The muon decays with a characteristic lifetime of about 10⁻⁶ second into an electron, a muon neutrino, and an electron antineutrino. The muon is forbidden from decaying into an electron and just a single neutrino by the law of conservation of

A. charge
B. mass
C. energy and momentum
D. baryon number
E. lepton number

(GR0177 #78)

Solution:

If  μ → e⁻ + ν

A. FALSE. Conservation of charge: − 1 → − 1 + 0 is not violated.

B. FALSE. Conservation of mass cannot be violated in any decay/interactions.

C. FALSE. Conservation of energy and momentum cannot be violated in any decay/interactions.

D. FALSE. Muon is a lepton. It follows the Conservation of Lepton Number, not Baryon Number.

E. TRUE. Lepton Numbers: 1 → 1 + 1 is violated.

Answer: E

Nuclear & Particle Physics - Muon Decay

Which of the following is the principal decay mode of the positive muon μ⁺?

A. μ⁺ → e⁺ + ν_e
B. μ⁺ → p + ν_μ
C. μ⁺ → n + e⁺ + ν_e
D. μ⁺ → e⁺ + ν_e + v̄_μ
E. μ⁺ → π⁺ + v̄_e + ν_μ

(GR0877 #98)

Solution: 

(A) FALSE
μ⁺ → e⁺ + ν_e  violates lepton number: 1 → 1 + 1

(B), (C) FALSE
Muon is elementary particle (lepton), p and n are composite particle (bosons). Elementary particle cannot decay into composite particle, (B), (C) FALSE

(D) TRUE

(E) FALSE
π is pion, which a boson, not a lepton.

Answer: D