All Questions
15
questions
1
vote
2
answers
122
views
Confusion about Higgs mechanism
I am trying to understand the mass acquisition of particles in the Standard Model based on the book 'Fundamentals of Neutrino Physics and Astrophysics' by C. Giunti, and several doubts have arisen ...
- 161
0
votes
2
answers
122
views
What symmetry present in a low energy theory is broken or not exact at high energies?
The opposite is quite common such as EWSB, SUSY or GUT. Is there any example where a certain symmetry emerges from a low energy effective theory but is not present in the high energy theory?
- 195
1
vote
0
answers
49
views
Relation between matrix element of a process with its crossed processes
Consider a process in particle physics denoted by, $$(1) \quad a+b\to c+d$$ which is related to the reverse process $$(2)\quad c+d\to a+b.$$ By virtue of the hermiticity of the interaction Hamiltonian,...
- 11.8k
1
vote
2
answers
578
views
Field "Expansion"/Shift in Spontaneous Symmetry Breaking
I currently take a lecture called Symmetries in Particle Physics.
And when talking about the spontaneous breaking of a global continuous symmetry the lecturer said that the field is expanded around ...
- 2,196
0
votes
2
answers
97
views
Is it possible to falsify the $SU(2)_{lepton, left}*SU(2)_{quark, left}*U(1)$ symmetry group as an alternative candidate for GSW Model?
We know that the current symmetry of GSW is $SU(2)_{fermions, left}*U(1)$, and the correct representation of the $SU(2)_{fermions, left}$ is the $2+2$ representation.
I want to know what is the reason ...
- 1,268
1
vote
1
answer
174
views
What is the difference between local and global symmetries in terms of properties of the particle spectrum?
I know that a global symmetry implies the presence of a conserved charge but how it does affect the particle spectrum? and in this sense what is the difference with a gauge symmetry?
- 11
2
votes
0
answers
100
views
How is a T-violation inherited in a QFT?
CP violation
In quantum field theory (QFT), ${\rm CP}$ symmetry or ${\rm CP}$ violation is a property of the Lagrangian. For a ${\rm CP}$ violating QFT, in general, the absolute square of the Feynman ...
- 26.8k
2
votes
1
answer
260
views
Why is the approximate $\rm U(2)\times U(2)$ global symmetry of QCD that has a special importance?
I was looking at Peskin and Schroeder (Section 19.3, page $667-668$). They talk about $\rm U(2)\times U(2)$ symmetry for the QCD Lagrangian in the limit of massless $u$ and $d$ quarks. However, this ...
- 26.8k
1
vote
1
answer
704
views
How to show that the charge conjugation reverses the charge of a state?
How to show that the charge conjugation operator reverses the charge(s) of a (fermionic or bosonic) state?
Let us consider a spin-$\frac{1}{2}$ fermionic state of momentum $\textbf{k}$ and spin ...
- 26.8k
6
votes
2
answers
1k
views
What forbids off-diagonal elements in the kinetic terms of the Standard Model?
In the Standard Model, the gauge covariant kinetic term for the left-handed lepton doublets $\psi_{iL}=(\nu_{iL},e_{iL})^T$, for example, is given by $$i\overline{\psi_{iL}}\gamma^\mu D_\mu\psi_{iL}.$$...
- 26.8k
4
votes
1
answer
499
views
Flavour symmetries of neutrino and charged lepton mass matrices
The symmetry of neutrino mass matrix $M_\nu$ is often realized as $$G^TM_\nu G=M_\nu$$ where $G$ is an element of the corresponding symmetry group. Is this because the neutrinos are Majorana in nature?...
- 26.8k
4
votes
0
answers
183
views
What is so soft about soft symmetry breaking? [duplicate]
Explicit symmetry breaking terms are classified as "hard" and "soft". If the the explicit breaking is induced by a term whose dimension is less than four it is called "soft" breaking. Does soft ...
- 26.8k
1
vote
0
answers
211
views
Is it necessary that spin $0$ particles have intrinsic parity $\pm 1$?
For simplicity let's consider a simple example of real scalar free quantum field
$$ \phi(x) = \int \mathrm{d}\mu(p) \left( a(p) e^{-ipx} + a^{\dagger}(p)e^{ipx}\right), $$
where $\mathrm{d} \mu(p)$ is ...
- 2,191
3
votes
2
answers
349
views
Global symmetry and particle multiplets
In chapter 20, of Peskin and Schroeder's quantum field theory book, they start with a comment that a global symmetry that is manifest lead to particle multiplets with restricted interactions. Can ...
- 26.8k
6
votes
0
answers
227
views
Categorizing solutions to Hierarchy problem
We know that no gauge symmetry can prevent a term $m_\phi^2|\phi|^2$ for a scalar field, and that, given the quadratic loop corrections, the natural scale is $m_\phi \sim M_P$. This is related to the ...
- 15.2k