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Particle Physics > Definitions > Flashcards

Definitions Flashcards

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1
Q

What is particle physics?

A

Particle physics is the search for the fundamental building blocks and forces of nature. As pieces get smaller, more energy is needed to break them apart.

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2
Q

What is the purpose of the Large Hadron Collider (LHC)?

A

LHC smashes protons at 99.999999% of the speed of light, probing distances 1/1000th the size of a proton.

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3
Q

What is the Standard Model of particle physics?

A

The SM is a quantum field theory, a merger of quantum mechanics and special relativity, but it has limitations and deficiencies.

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4
Q

How do scientists search for new physics beyond the Standard Model?

A

Two approaches: top-down (guess a new theory and derive consequences) and bottom-up (data-driven precision).

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5
Q

What is the size range of particles in particle physics?

A

Particle physics spans from 10^(-15)m to 10^(-35)m in size.

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6
Q

Why are natural units used in particle physics instead of SI units?

A

Natural units are based on the energy of accelerated charged particles due to the small distances and large energies in particle physics.

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7
Q

What are the postulates of special relativity?

A

(1) Physics is the same in all inertial frames. (2) The speed of light is the same in all frames.

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8
Q

What do Lorentz transformations do in special relativity?

A

Lorentz transformations symmetrically convert coordinates between inertial reference frames, defining proper length and proper time.

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9
Q

What are some deficiencies of the Standard Model (SM)?

A

No gravity integration with quantum field theory.
Massless neutrinos (known mass cannot equal zero).
Minor discrepancies in weak force (flavor physics).
Incomplete explanation for dark matter (23% of the universe) and dark energy (73% of the universe).

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10
Q

What is an invariant interval?

A

An invariant interval specifies the spacetime separation of two events.

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11
Q

Define spacelike separation in the context of particle physics.

A

Spacelike separated events have a spacetime interval where (Δx)^2 < 0. Such events are causally disconnected and cannot affect each other.

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12
Q

Define timelike separation in the context of particle physics

A

timelike separated events have a spacetime interval where (Δx)^2 > 0.

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13
Q

Define lightlike separation in the context of particle physics

A

lightlike separated events have a spacetime interval where (Δx)^2 = 0.

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14
Q

Why do we use relativistic kinematics?

A

particles that are travelling at speeds close to the speed of light must include effects of special relativity. This requires using relativistic kinematics.

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15
Q

How do 4-vectors encode the rules of special relativity?

A

4-vectors provide a standard way to deal with relativistic kinematics, ensuring the dot product is Lorentz invariant.

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16
Q

What does invariant mass represent in particle physics?

A

Invariant mass is the total energy available in the center-of-momentum frame, a crucial measure in collisions.

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17
Q

How is particle physics probed experimentally?

A

Particle physics is probed by colliding particles, studying the outcomes to understand fundamental properties.

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18
Q

What are the two main types of particle physics experiments?

A

Fixed Target (target at rest, beam collides) and Colliding Beam (two accelerated and focused beams collide).

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19
Q

How does the treatment of forces and particles differ in classical mechanics compared to quantum mechanics?

A

In classical mechanics, forces and particles are fundamentally different. In quantum mechanics, forces are associated with particles via quantum field theory (QFT), blurring the line between them.

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20
Q

What does the Spin-Statistics Theorem state?

A

It states that any particle with integer spin is a boson, while any particle with half-integer spin is a fermion. Fermions obey the Pauli Exclusion Principle.

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21
Q

How is the wave function effected by indistinguishable bosons?

A

you can swap any two indistinguishable bosons in a system and the wave function describing that system is unchanged.

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22
Q

What is the probability of having two electrons with all the same ‘quantum numbers’

A

The probability of having two electrons with all the same ‘quantum numbers’ is zero as according to Pauli’s exclusion principle two indistinguishable fermions cannot occupy the same state.

23
Q

How are forces and particles subdivided in quantum field theory?

A

In quantum field theory, forces and particles are both treated quantum mechanically. Hadrons are divided into baryons (spin 1/2) and mesons (spin 0 or 1). Leptons include electrons, muons, electron neutrinos, and muon neutrinos.

24
Q

What did Dirac propose in 1928, and how are antiparticles related to time?

A

Dirac proposed the universe having energy E₀ in a giant potential well. Electrons with E < E₀ fill the potential well’s energy levels, and antiparticles have E < 0 but travel backward in time, as proposed by Feynman and Stueckelberg.

25
Q

What are the four fundamental forces in nature?

A

Gravity (not in SM, always attractive), 2. Electromagnetism (Quantum Electrodynamics - QED), 3. Weak Force (can change particle types, mediated by W⁺, W⁻, Z⁰ bosons), 4. Strong Force (Quantum Chromodynamics - QCD, mediated by gluons).

26
Q

In quantum field theory what is the treatment of electricity and magnetism called?

A

Treatment of electricity and magnetism is called Quantum Electrodynamics (QED)

27
Q

How is Quantum Electrodynamics (QED) defined, and what is its gauge symmetry?

A

QED is based on gauge symmetry (U(1)). QED is defined by the Lagrangian density which contains information about electromagnetic interactions. The Lagrangian has a gauge symmetry that ensures invariance under local gauge transformations.

28
Q

How does the Weak Force differ from QED, and what is Electroweak Symmetry Breaking (EWSB)?

A

The Weak Force is based on SU(2) symmetry. EWSB breaks this symmetry, unifying QED and the Weak Force before breaking into distinct forces.

29
Q

How is the strong force described in quantum mechanics, and what is Quantum Chromodynamics (QCD)?

A

Our understanding of the strong force is analogous to QED and is called Quantum Chromodynamics (QCD). The Lagrangian has a gauge symmetry SU(3). Gluons mediate the strong force. Confinement prevents the observation of free colour charges.

30
Q

What are matter fields in particle physics?

A

Fermion fields are often referred to as matter fields. Fermiom fields interact by exchanging gauge bosons.

31
Q

Why is the Higgs boson necessary, and when was it discovered?

A

The Higgs boson is a byproduct of Electroweak Symmetry Breaking (EWSB) and was discovered in 2012 at the LHC. It explains the masses of W⁺, W⁻, and Z⁰ gauge bosons.

32
Q

What is the Pauli Exclusion Principle?

A

The Pauli Exclusion Principle states that no two identical fermions can occupy the same quantum state simultaneously.

33
Q

What is an Abelian Group?

A

An Abelian Group is a mathematical group in which the order of combining two elements does not affect the result.

the U(1) gauge symmetry in Quantum Electrodynamics (QED) is an example of an Abelian Group.

34
Q

What does Flavor Changing Charged Current (FCCC) refer to in particle physics?

A

FCCC interactions allow quarks and leptons to change their type (flavor). This phenomenon is governed by a part of the weak force Lagrangian

35
Q

What is the Quark Model?

A

The Quark Model is a predecessor to Quantum Chromodynamics (QCD), employing a group theory approach to understand the particle zoo.

36
Q

What is Flavour SU(3) in the Quark Model?

A

Flavour SU(3) involves the u, d, and s quarks. The assumed symmetry arises from quarks having the same strong force, despite differences in flavor. The model accounts for the lightness of u, d, and s quarks.

37
Q

What are Charmness (c) and Bottomness (B) in the Quark Model?

A

Charmness (c) is the difference between the number of c quarks and c antiquarks. Bottomness (B) is similarly defined for b quarks. These parameters were introduced onto the flavour SU(3) without additional symmetry, and their values are not conserved, representing a historic accident.

38
Q

Why was hypercharge (Y) introduced in the Quark Model?

A

Hypercharge (Y) was introduced to accommodate the s quark, addressing its unique properties within the framework of the model.

39
Q

What is the Path Integral in quantum field theory?

A

The Path Integral is a transition between states a and b, considering all possible paths. Each path represents a process altering quantum numbers. The total probability is the squared magnitude of probability amplitudes for all paths.

40
Q

How is Perturbation Theory related to the Path Integral?

A

Perturbation Theory simplifies the Path Integral via a power series in the coupling constant. Valid as long as coupling constants are small. In QCD at low energy, this becomes problematic, leading to nonperturbative approaches.

41
Q

What are Feynman Diagrams, and how are they used in quantum field theory?

A

Feynman Diagrams are graphical representations of particle interactions. Feynman Rules assign mathematical expressions to each component of the diagram.

42
Q

What are some of the rules of a Feynman Diagram?

A

Fermions have arrows forward in time, antifermions have arrows backward, Electroweak gauge bosons are drawn with solid wavy lines, gluons with curly spring-like lines, and Higgs bosons with straight dashed lines.

43
Q

How do continuous spacetime symmetries relate to conserved quantities in the Standard Model?

A

Continuous spacetime symmetries in the Standard Model lead to conserved quantities through Noether’s theorem. Examples include energy-momentum conservation due to translational symmetry.

44
Q

What discrete symmetries are built into the Standard Model interactions?

A

Charge, baryon number, and lepton number are built into SM interactions. Baryon number relates to the flow of quarks, while lepton number involves the number of incoming and outgoing leptons.

45
Q

How does Time Reversal Symmetry relate to weak interactions?

A

Time reversal is anti-unitary and requires complex conjugation. It is not respected by weak force, introducing an unspecified factor in the probability amplitude, which must be measured.

46
Q

What is the structure of Vcmk in quantum field theory?

A

Vcmk has a structure that is nearly diagonal and symmetric.

47
Q

What is the significance of the phase angle in quantum field theory?

A

the phase angle is is called the action and it is calculated from the Legrangian

48
Q

How are hadrons classified?

A

Hadrons, made of quarks, are classified into baryons (3 quarks) and mesons (quark-antiquark pairs). Quarks explain hadron characteristics like electric charge and spin.

49
Q

How are baryons classified?

A

Baryons are classified in terms of quarks which requires a new quantum number known as colour.

50
Q

What are parity transformations?

A

Parity transformation reverses spatial coordinates, and weak interactions do not conserve parity.

51
Q

What is charge conjugation?

A

Charge conjugation changes the sign of the charge of a particle. It is not respected by the weak force.

52
Q

What suppresses the probability?

A

CKM suppression and loop supression

53
Q

Describe a virtual particle.

A

Virtual particles often violate energy-momentum conservation

Particle Physics (2 decks)

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