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Particle physics is the study of the fundamental forces of nature as they are embodied in the interactions of elementary and composite particles at high energies and short time and distance scales. DO NOT USE THIS TAG for point particles in classical mechanics.

4 votes
2 answers
578 views

What formula is used to make exclusion plot in the direct detection of dark matter?

The idea of direct detection of dark matter is that a dark matter particle striking some underground target will cause the target nucleus to recoil. From the recoil, one can determine the scattering c …
4 votes
1 answer
245 views

How are the dark matter (DM) thermalization rates derived in singlet DM model?

This well-cited paper talks about a minimal renormalizable extension to the Standard Model (SM) to incorporate particle dark matter (DM) into it by adding a real scalar field $S$ which (unlike the Hig …
4 votes
1 answer
659 views

Understanding CP-violation from a toy model of two fermions and a scalar boson

Consider a field theory given by the following Lagrangian $$\mathcal{L}_{int}=y\overline{\psi_1}\psi_2\phi+y^*\overline{\psi}_2\psi_1\phi^\dagger$$ where $\phi$ is a complex scalar field, and $\psi_1, …
6 votes
2 answers
474 views

Measuring and calculating free quark masses

Particle data book contains masses of the free quarks. I wonder, how do experimentalists determine the masses of the free quarks even though they are trapped inside hadrons (except perhaps in quark-gl …
3 votes
3 answers
345 views

Why don't the nuclear fusion processes inside the sun produce electron antineutrinos $(\bar\...

Why don't the nuclear fusion processes inside the sun produce $\bar\nu_e$ despite having the same mass as $\nu_e$? Is the reason as simple as "there is no production channel for $\bar\nu_e$s." ?
4 votes
1 answer
509 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 natur …
5 votes
1 answer
290 views

A terminology question: What are active neutrinos and why?

The mass term for the type-I seesaw is given by $$\mathcal{L}_{mass}=-m_D\overline{\nu_L}N_R+M_R\overline{(N_R)^c}N_R+h.c.$$ where the right-chiral fields $N_R$ are electroweak singlets. Since they do …
6 votes
1 answer
788 views

Why Majorana phases cannot be removed?

Why is the extra two Majorana phases in the PMNS matrix cannot be removed if neutrinos are Majorana fermions? Or in other words, why are the Majorana phases cannot be absorbed into the redefinition of …
3 votes
1 answer
192 views

Can we derive the formula $Q=I_3+\frac{1}{2}(B+S)$ instead of accepting it as an empirical r...

The electric charge of a quark or lepton, $Q$, is related to the third component of the weak Isospin $T_3$ and weak hypercharge $Y$ according to the formula $$Q=T_3+\frac{Y_W}{2}.\tag{1}$$ This, in a …
2 votes
0 answers
191 views

Anomalous baryon current in the Standard Model (SM) and the stability of free protons within...

In the Standard Model, the baryon number is not exactly conserved due to anomaly but the decay rate is extraordinarily small at ordinary temperatures. Does this make free protons unstable in the Stand …
3 votes
2 answers
2k views

How to tell whether a Feynman diagram is $t$-channel or $s$-channel by looking?

By looking at a diagram, how does one tell whether it represents a $s$-channel process or a $t$-channel process i.e., without finding the amplitude? I'm familiar with Mandelstam variables but I've tro …
1 vote

How to tell whether a Feynman diagram is $t$-channel or $s$-channel by looking?

For a two-body scattering process denoted by $$a_1(p_1)+a_2(p_2)\to a_3(p_3)+a_4(p_4)$$ the tree-level Feynman diagrams can be classified into three categories. A tree-level scattering diagram is call …
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5 votes
3 answers
369 views

How are the sterile neutrinos $\nu_s$ different from the heavy right-handed fields $N_R$ (or...

To generate three light neutrino mass eigenstates via type-I seesaw, we include a set of $n$ (need not be equal to 3) heavy$^1$ right-handed fields $N_R$ in addition to three $\nu_L$ fields to the Sta …
0 votes
1 answer
452 views

Physical meaning of the form factors $F_1(q^2)$ and $F_2(q^2)$ for $q^2\neq 0$

The charge form factor $F_1(q^2)$ and the anomalous magnetic moment form factor $F_2(q^2)$ have clear interpretations at the value $q^2=0$ i.e. $F_1(0)$ is equal to the charge of the electron and $F_2 …
3 votes
1 answer
1k views

What is the correct definition of the Jarlskog invariant?

In this lecture on neutrino physics, Prof. Feruglio defines the Jarlskog invariant as $$J=\text{Im}(U_{\alpha i}^{*} U_{\beta i}^{\,} U_{\alpha j}^{\,} U_{\beta j}^{*})\tag{1}$$ where $U$ is the neutr …

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