Questions tagged [standard-model]

A model of the basic particles and forces featuring six quarks, three charged leptons, three massless neutral leptons and four fundamental force carrying bosons. The twelve fermions are arranged into three generations, while the bosons serve to explain the electromagnetic interaction plus the strong and weak nuclear forces (and the Higgs mechanism). Do NOT use this tag for the standard model of cosmology, etc..

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Strangeness Conservation

I am stuck as to how to prove that strangeness is conserved in electromagnetic and strong interactions, while it is not conserved in weak interactions. I know that this can be shown by studying the ...
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Why can't two real photon, gluon, graviton, and $W$ and $Z$ fields interact by means of their virtual counterparts (the mediators of the process)?

It is a fact that two real (massless) photons, gluons, or gravitons can't react by means of their virtual counterparts (for example, two external photons that interact via one of these massless ...
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Could $\nu_e+p\rightarrow e^-+\pi^++p$ occur via the weak interaction? If so, how would be the Feynman diagram?

I checked if the reaction conservate baryon number, charge and leptonic number and it seems like it does. But I tried to draw the Feynman diagram and I don't understand how the proton interact, so ...
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How lepton numbers and baryon numbers of a particle are decided?

Suppose a new particle is discovered how will it be assigned a lepton and baryon number? Moreover, what difference would it make if the electron had +1 lepton number and a positron -1? The same ...
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How to understand the makeup of neutral pi and eta mesons?

I know that mesons are bosons made up of quark-antiquark pairs. But when I see the list of mesons, I can see that the makeup of neutral pions and eta mesons are noted in a strange way. $$\pi^0=(u\bar{...
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Which sterile neutrino are we looking for?

I am trying to wrap my head around sterile neutrinos. As far as I understand it, sterile neutrino is a name for every particle that feels only gravitational force. The most notable example is a right-...
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How to determine which force mediates an interaction?

I have been given an assignement gives me a number of interactions, asks me to determine which, among these interactions are permitted under conservation laws (Charge, Baryon number, Leptonic numbers, ...
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66 views

An invariant of the gauge group $G$ that is totally symmetric with three indices in the adjoint representation

In Ch.19 of the textbook An Introduction to Quantum Field Theory by Peskin and Schroeder, on P.680 the property of a quantity $$\mathcal{A}^{abc}=\mathrm{tr}\left[t^a\{t^b,t^c\}\right]\tag{19.132}$$ ...
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Sizes of Elementary Particles

Present observation shows that elementary particles have no internal structure, and have no real size as they are described by wavefunction. Something that therefore confuses me is that on a lot of ...
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Dark matter composition [closed]

I assume that dark matter is made of heavy neutrinos.But wouldnt they be converted into neutrinos with less mass throygh weak interaction?So dark matter isnt made of neutrinos.I think there is truly ...
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Why does charmonium (and phi mesons) not decay via quark and antiquark annihilation?

The decay of heavy quark/antiquark pairs (say $c\bar{c}$, $s\bar{s}$) is supposedly 'suppressed because of the Zweig/OZI rule', see for instance Phi meson. And they certainly have a longer lifetime ...
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Custodial Symmetry in SM Higgs

this excercise is about the custodial $SU(2)_R$ symmetry in the SM Higgs Mechanism. Step by step I have to develop the theory. I get the generel idea of the global custodial symmetry and why we need ...
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Why is the relation $M_W=M_Z\cos\theta_W$ true only at tree-level?

In Glashow-Weinberg-Salam electroweak theory, the relation $$M_W=M_Z\cos\theta_W\tag{1}$$ is said to be remain true only at the tree-level; it receives corrections from the loop diagrams. See here. ...
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why do quantum fields exist? [closed]

Why did the big bang give rise to the quantum fields it did, could the big bang have created the universe any other way? and why not more elementary particles?
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How can pure leptonic interactions be mediated by the weak force?

$\mu^+ \rightarrow e^+ + \nu_e + \bar \nu_\mu$ Consider the above decay, it is given in my lecture notes as a weak force interaction. I thought the weak force was only involved when quarks change ...
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What are direct experimental evidences that quarks exist? [duplicate]

Now we have very established model of quarks explaining fundamental strong interaction. What are experimental proofs for existence of quarks and what is the name of physicist which made them?
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When is a decay mediated by the weak force

If a $\pi ^+ \to \nu_\mu+\mu^+$, here quark flavour is lost entirely. I am aware the weak force mediates decays where quark flavour is not conserved. But If I consider a $p+\bar p$ annihilating, here ...
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Most of the momentum of a proton is carried by the quarks

I saw this in wiki https://en.wikipedia.org/wiki/Antiproton#Modern_experiments_and_applications "the valence quarks in the proton, and the valence antiquarks in the antiproton, tend to carry the ...
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How to figure out whether a force mediator is $W^+$ or $W^-$ in Feynman diagram?

How to figure out whether a force mediator is w+ or w- in feynman diagram? I always make sure each vertex is zero but sometimes I get my w+ or w- worng.
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Conservation of lepton

I was reading a bit about nuclear/particle physics when I came up to Cours et problèmes corrigés de physique nucléaire et de physique des particules by Philippe Miné in 2016 ISBN 2340011566, which is ...
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Difference between expected $p_0$-value and observed $p_0$-value?

In the ATLAS paper for the higgs discovery, they used 2 kinds of different $p_0$-values. One is the expected $p_0$-value, the other one is observed $p_0$-value. What's the difference between the ...
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Why do we have 3 quantum colours?

I understand the need to invoke colour as another quantum state to explain the observation of uuu ddd and sss baryons. I just wanted to know if there was some other property which explains why we only ...
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How does one calculate branching ratio of Higgs particle decaying into virtual particle?

Very often when people talk about decay they use branching ratio to describe a particular decay channel, for example: $$Z\rightarrow e^{+}+e^{-}$$ If both Z and the electron and positon are one-...
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How Is Are Parton Distribution Functions (PDFs) Determined In Practice And In Theory?

I have put my actual questions in bold normal font, and the rest of this question clarifies what I mean to be asking, especially in terms of the depth and kinds of information I am looking for. ...
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Why can't we have a massive photon in the Standard Model of Particle Physics

I've heard that in the Standard Model of Particle Physics you can´t have a massive photon whatever you do but I'm having a few problems showing that. I understand that the trick to make this work is ...
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How is the charge conserved at the up to anti-down vertex?

I can see how the charge is conserved in the overall diagram but I can not understand how it is conserved at the up to anti-down vertex.
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How does a field know when to make how many particles?

Suppose I dump a certain amount of energy into a field. When does it make one particle and when does it make 2 with lower energies?
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What is the dimension of the weak gauge field couplin constant NOT in natural units?

What is the dimension of g and g', NOT in natural units, but in terms of mass, length, time, and permittivity?
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Is there violation of energy conservation when a particle transforms to its anti-particle, for example $B^0\to\bar{B}^0$, or $K^0\to\bar{K}^0$

additive information : $B^0$ and $\bar{B}^0$ don't have the same mass, $K^0$ and $\bar{K}^0$ don't have the same mass. is there energy violation? if yes, is the key explanation of this coming from the ...
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Why does the reconstructed mass of a particle have a fairly wide distribution?

In HEP experiments, a particle of interest (e.g. Higgs) is reconstructed using information about decay products, momentum, energy deposits, angle of those particles, etc. The invariant mass of the ...
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How are branching ratios for the Higgs boson calculated?

I'm somewhat familiar with how to calculate branching ratios. I know that they are calculated as the ratio between the decay width of a specific decay process and the total decay width of the particle ...
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What is $U(1)$ symmetry?

I saw there are three intrinsic symmetries in physics,U(1),SU(2) and SU(3).What's the U(1) symmetry talking about?I would appreciate it if you can give me some explaination.
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Mass of the mesons in a universe with massless quarks

As I propose in this post, About the mass of the particles, imagine a universe with massless quarks due to Higgs' VEV is exactly zero. In our universe, where quarks are massive, we have consider ...
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If the gauge symmetry is not broken by spontaneous symmetry breaking, what symmetry is broken?

In this post, the answer by buzhidao showed that the $U(1)$ gauge symmetry is not broken by spontanous symmetry broken and Higgs mechanism. What role does "spontaneously symmetry breaking" ...
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Is the (range of the) strong force the same for nucleons and quarks?

In high school, I learnt that the strong force held the nucleus together. It had a very short range and was repulsive at small separation distances to prevent the nucleons from collapsing. Now I am ...
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Why do we conclude $SU(2)\times U(1)$ is broken into $U(1)_{em}$ when local gauge symmetry is not broken?

Recently, I spent some time reading some articles on the concept of SSB and I found that after SSB the local gauge symmetry will not be broken.What are broken are the global symmetries by non-zero VEV ...
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Can muons decay into quarks?

Muon decays are almost always written as $$\mu^- \rightarrow e^-+ \bar\nu_e +\nu_\mu.$$ The reason given on wikipedia is that one of the product neutrinos of muon decay must be a muon-type neutrino ...
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Lifetime of up quark

I am taking an introductory particle physics course, and from having studied about the breit wigner cross section and propagator terms: $$\frac{1}{(E-E_0)^2+ \frac{\Gamma^2}{4}}$$ where $E$ is the ...
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About the mass of the particles

Studying Higgs mechanism in EW theory and QCD I have a couple of questions that I would like to clarify: 1) The quark mass term in QCD Lagrangian should come from Higgs mechanism in EW sector of SM. ...
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Motivation for Weinberg angle in electroweak gauge interaction?

Suppose I have the following lagrangian If we only focus on the neutral current in the lagrangian: Where $L$ is defined as: And $Y_L$,$Y_{R}^{\nu}$,$Y_{R}^{e}$, are the hypercharge values of the ...
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Besides the up and down quark, what other quarks are present in daily matter around us?

Protons and neutrons, which are found in everyday matter around us, compose of up and down quarks. Are the other two generations of quarks, i.e. $c,s,t,b$ quarks found in everyday matter around us? I ...
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What gives an electron its charge? [duplicate]

What exactly gives electrons a charge? I understand how in molecules, an imbalance between electrons and protons give ions charges and I also understand that there is really no positive or negative ...
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Branching fractions for the 125 GeV Higgs boson

Calculating the branching fractions of a Higgs boson $m_h=125\,\text{GeV}$ into a muon and an anti-muon, and a $b$-quark and an anti-$b$-quark from the decay formula $$\Gamma=\frac{\alpha_{w}}{8 \...
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Feynman diagram and vertices in a hadron decay

This question is about Feynman diagram and vertices in a hadron decay, which comes from Problem 89.5 in Srednicki's textbook Quantum Field Theory. The reaction involved is \begin{equation} d\...
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Why is anti-neutrino produced along side with electron in beta minus decay? [duplicate]

so i get that when there is an excess of neutrons, neutron is converted into a proton. An electron is produced to neutralise the +1 charge of proton. But why is an anti neutrino produced?
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Why Aren't Hadron Masses More Useful In Determining Standard Model Constant Values?

Some of the most exquisitely precise experimental measurements in all of physics are the masses of the various hadrons. Consider these examples: the proton mass is known to eleven significant digits. ...
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A question on the strong interaction and charm number

I know that the particle $P_c(4380)^+$, has quark content $\bar{c}cuud$. Furthermore I know that the reaction $K^-+P_c(4380)^+\rightarrow K^-+J/\psi+p$, is strong and the quark content on the left ...
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Detemining whether a particle is stable with respect to the electromagnetic, EM or strong force

If we have some beautiful hadron , how can we decide based on the quantum numbers and masses of the other beautiful hadrons whether it is stable under the electromagnetic, weak or strong interaction . ...
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Is there a better name for flavour of particles apart from generation number? [closed]

If we assume the standard model falls into 3 generations ordered by mass. (This needn't necessarily be true.) We call these "generation 1", "generation 2" and "generation 3". So the property of a ...
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Why is the decay $\eta'\rightarrow \gamma + \gamma$ suppressed?

So the $\eta$ meson's dominant decay mode is $\eta \rightarrow \gamma + \gamma$ whereas the dominant mode for $\eta'$ is $\rho+\gamma $ or $\eta + \pi+\pi$. Why does it not decay to two photons ...

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