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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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1answer
49 views

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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91 views

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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2answers
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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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50 views

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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46 views

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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1answer
46 views

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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55 views

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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1answer
39 views

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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1answer
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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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3answers
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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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2answers
105 views

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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1answer
488 views

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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1answer
101 views

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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1answer
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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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2answers
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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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1answer
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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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1answer
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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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1answer
75 views

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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1answer
25 views

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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1answer
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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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1answer
27 views

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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40 views

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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1answer
97 views

Politzer, Gross & Wilczek running formula

I've been told that for any group of SM, the running of the corresponding coupling constant, $g$, is given by: $$ \frac{dg}{d(\ln{Q})} = b·g^3/(16\pi^2) $$ Where $$ b = -\frac{11}{3}C_2(A) + \sum\...
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2answers
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Can someone explain the $ttH$ Coupling?

IS the $ttH$ an actual particle? If so, how can it form? I though the life of the top quark was way to short to form a particle. If it is not a particle, what is meant by coupling.
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3answers
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How to find the corrsponding expression after working with natural units $\hbar=c=1$?

If one does long calculations in natural units how does one find the right expression in let's say SI units in the end? I know that natural units make the calculations easier and also help to show ...
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1answer
48 views

Sum over real photon polarizations. The minus sign

Ok for real photons there is the formula when summing over the polarizations: $$ \sum_{\lambda=\pm}\epsilon^{*\mu}_\lambda\epsilon^\nu_\lambda = -\eta^{\mu\nu}$$ But if I have a matrix element of ...
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Weak hypercharge physical existence

How do we know that electrons are chiral? Even if left-handed electron carries weak hypercharge, it's only for an infinitesimally small time or is it predicted by QFT?
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1answer
67 views

Is the electron a bound state of two Weyl fermions?

The massive Dirac fermion is strange in that it transforms under the Poincare group as a direct sum of two spin-1/2 representations of the group $(\frac{1}{2},0)\oplus (0,\frac{1}{2})$. This suggests ...
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How many quark flavor quantum numbers are really needed?

Several authors I've consulted recently use six separate flavor quantum numbers for the six observed flavors of quarks when constructing their QCD Lagrangians That seems excessive when three "family" ...
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Correlation diagram for related branching ratios. How to interpret?

I wonder how to interpret the following diagram and how one can see where to place the diagonal lines? Source:https://arxiv.org/pdf/0809.0608.pdf So my thoughts on this: The two axis are two ...
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3answers
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Embedding of $SU(2)_L \times U(1)_Y$ into $SU(2)_L \times SU(2)_R$ in electroweak chiral effective theories

In the context of chiral effective theories we usually deal with the pion field \begin{equation} U= ie^{\frac{\pi^a \sigma^a}{2f}} \end{equation} where $\pi^a=\big(\pi^1, \pi^2, \pi^3\big)$ are the ...
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1answer
56 views

What determines the specific value of the order parameter in spontaneous symmetry breaking?

Three examples in the spontaneous symmetry breaking that occurs at a phase transitions: A ferromagnet below the Curie temperature chooses an axis of quantisation along which all the spins align, ...
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1answer
99 views

Is there an equation for the weak force?

I have read this question: Is there an equation for the strong nuclear force? Weak force: attractive or repulsive? And yes, I have read the comment, where is says that the weak and the strong ...
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4answers
215 views

Does the weak force have an attractive/repulsive force observable in everyday life like the other forces?

After the correct comments, this question is not here to compare gravity's and EM's long range forces' energetics and amplitudes to microscopic scattering amplitudes of such forces as weak and strong. ...
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1answer
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Lorentz decomposition of electromagnetic current Cheng&Li $\mu\rightarrow e\gamma$ p.421

I've been struggeling with this for a while, should be very easy probably.... In Cheng and Li p. 421 they consider the transition $$T(\mu \rightarrow e\gamma) = \epsilon^\lambda<e|J^{em}_\lambda|\...
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1answer
98 views

How can weak interactions not conserve strangeness if the SM always conserves energy?

I have read these questions: Weak Interaction and Strangeness transformation Weak decay iff or if strangeness changes? Understanding type of force interaction in particle decays The decay of the ...
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Why does the conclusion that protons are monopoles has led us to believe that they specifically may have a half-life? [closed]

from Zeldovich, Ya. B.; Khlopov, M. Yu. (1978). "On the concentration of relic monopoles in the universe": "The majority of particles appearing in any quantum field theory are unstable, and they ...
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1answer
185 views

Decay width of the tau lepton

The total decay width of a particle, $\Gamma$, is proportional to the fifth power of its mass, $m$, $$ \Gamma \propto m^5$$ We also know that the dominant decays of the tau lepton are $$\tau^- \to \...