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 ...

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76 views

Charge of $W$-bosons in Feynman diagrams

When looking at Feynman diagrams of particle decays, how would I be able to find out the charges of the $W$-bosons involved in the decay?
2
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1answer
114 views

How unique are the quantum numbers we commonly use?

We use the eigenvalues of the Cartan generators (=diagonal generators) of a given gauge group as quantum numbers in physics. Are these numbers somehow fixed and if not, what transformations are ...
2
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0answers
53 views

Physical proceses before the breaking of $SU(2)_L\times U(1)_Y$ symmetry

The energy scale which the electromagnetic and the weak interaction were unified, there were 4 massless gauge bosons: 3 corresponding to the unbroken generators of $SU(2)_L$, say $W_{\mu}^{1,2,3}$ ...
5
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3answers
242 views

The Makeup of the Pentaquark

Why is it that when they have the artist's rendition of the Pentaquark it shows two downs, two ups, and one anti-strange quark? Is this or is this just for show? Follow up to this question: if this ...
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2answers
278 views

Radioactive decay as pure radiation of neutrinos and antineutrinos?

Is it possible with radioactive decay as pure radiation of neutrinos and antineutrinos? From a theoretical point of view? I'm not asking for processes since I'm convinced there are no known processes ...
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0answers
111 views

What are the differences between the Jetphox, Pythia and Herwig event generators?

I know Jetphox is a parton-level event NLO generator program. But I want to know more about other generator programs such as Pythia and Herwig. What are the differences? I am undergraduate student so ...
3
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1answer
58 views

A pedagogical exposition of the hadron physics?

I am looking for a textbook/lecture notes/etc. on the basics of hadron physics. I wish to understand how to construct the effective Lagrangian for pions and nucleons starting from the QCD Lagrangian. ...
2
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0answers
107 views

Isospin In Kaon Decay

The decay $ K \to \pi \pi $ at zero-strong interaction level is calculated by considering the matrix element of the operator $ Q_2 = (\bar{s}u)_{V-A} (\bar{u}d)_{V-A} $ for two kinds of processes: ...
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1answer
109 views

Why didn't we have inflation when the theory of everything and GUTS broke symmetry?

I asked this in astronomy and they suggested I ask it here. Inflation seems to have occurred when the symmetry breaking of the electroweak occurred. But do we know of any reason why we did not have ...
2
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1answer
69 views

Why are bare mass terms for W-bosons forbidden, but coupling terms to Higgs doublets allowed?

The $W$ bosons live in the adjoint rep of $SU(2)$, which is three dimensional. The standard model Higgs lives in a $SU(2)$ doublet, i.e. the two dimensional rep. The $W$ bosons get their mass ...
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1answer
2k views

What parity has an electron?

I couldn't find anything about the parity of an electron. Neither in the german, nor in the spanish and nor in the english version of Wikipedia. I only found one sentence in the parity article of ...
3
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1answer
141 views

All of Physics! [duplicate]

In several of Neil Turok's talks, he talks about this equation that encompasses all of physics. Here it is: How much of it is true? If it isn't, then is it possible to put all of our knowledge of ...
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3answers
2k views

Shape of the Higgs branching ratio to ZZ

I've been looking at the, now very popular, graph of the SM Higgs decay branching ratios: You see that the ZZ branching ratio has a funny dip around the $170\, GeV$, very different from the WW ...
3
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1answer
114 views

Representation of the Standard Model group $SU(3) \times SU(2) \times U(1)$

As the gauge group of the Standard Model is $SU(3) \times SU(2) \times U(1)$, would the associated fermions fields be the product of a triplet, a doublet and a singlet, for all particles, or is that ...
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1answer
149 views

How is the Full Standard Model group representation displayed?

I have often seen, on YouTube lectures and textbooks, the direct product gauge group representation listed below and it is often accompanied with a statement to the effect that "this is how we sum ...
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1answer
97 views

Are all elementary interactions arising from a gauge theory?

The standard model of particle physics is based on the gauge group $U(1) \times SU(2) \times SU(3)$ and describes all well-known physical interactions but with exception that gravity isn't involved. ...
1
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1answer
46 views

Transformations of electroweak gauge field $W_\mu$ under $U(1)_{e.m.}$

As the vector boson field $W_\mu$ is, together with $Z^0$, the gauge field for the Standard electroweak model, I know it transforms as a connection under the $SU(2)\times U(1)_Y$ group. But, when this ...
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3answers
938 views

What do the four components of Dirac Spinors represent in the Standard Model?

I've been trying to get my head around the formalisms used in the Standard Model. From what i've gathered Dirac Spinors are 4 component objects designed to be operated on by Lorentz Transformations ...
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1answer
124 views

Poincaré' lemma and EM potential $A^{\mu}$

My lecturer said that given the sourceless Maxwell's equations $$ \partial_{\mu}\, ^ *F^{\mu\nu} = 0 $$, we can find a solution $$ F^{\mu\nu} = \partial_{\mu}A_{\nu} - \partial_{\nu}A_{\mu},$$ that ...
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0answers
97 views

Implications of dark matter imprints on Cosmic Microwave Background Radiation?

Looking at this link, CMB Anisotropy, I have two questions regarding the possible creation and properties of dark matter: If dark matter has left it's imprint on the CMB, that to me, would imply ...
3
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1answer
97 views

How to compute scattering amplitude $\gamma\pi^+\to\pi^+\pi^0$

I wish to find the amplitude for process $\gamma\pi^+\to\pi^+\pi^0$ at low energies. I am familiar with the basic concepts and techniques of QFT but have never dealt with the scattering processes ...
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1answer
164 views

Are gauge theories always renormalizable?

Speaking of quantum field theories. Is one of the following implications correct? gauge theory (gauge invariant) => renormalizable renormalizable => gauge theory (gauge invariant) If yes do you ...
3
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1answer
258 views

Vacuum stability in quantum field theory

What exactly do people mean when they talk about the scale dependence of the effective potential ($V$)? I explain the motivation for my question (and hence my confusion) below. Please correct me as ...
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1answer
64 views

What does “Standard Model” really refer to?

I am reading the arXiv published paper by Frank Wilczek called "Physics in 100 Years". Apparently, this paper is the printed form of a talk given at Brown University earlier this year (2015). Right ...
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0answers
13 views

Chemical potentials of SM particles at high temperatures

Recently I've heard the statement that chemical potentials of SM particles on $T > 1 \text{ GeV}$ are very small, so that Gibbs potential reduces to free energy. How to prove this statement? Does ...
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2answers
282 views

If mesons were stable could they form atoms?

if there were stable enough mesons similar to protons and neutrons could they capture electrons to form exotic elements. if not why is this not possible?
5
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0answers
51 views

Why are $W$ bosons massless above electroweak scale? [duplicate]

Because of the Higgs mechanism, one must replace the Higgs field $\phi$ with $\phi_0 + \phi_1$ where $\phi_0$ is the vacuum expectation value. As far as I understand, the $\phi_0$ gives the mass term ...
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1answer
161 views

Chiral Fermion Problem and the String Net Model

In Xiao-Gang Wen's book "Quantum Field Theory of Many-Body Systems", he mentions that (the string-net condensation picture)...has a problem: we do not yet know how to produce the $SU(2)$ part of ...
0
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1answer
40 views

colliding point particles

when I draw e.g. the diagram of compton scattering I assume that the electron of given momentum gets 'hit' by a photon and interacts with it. How close does the photon have to get to the electron that ...
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0answers
37 views

What are the parity of particles? [duplicate]

When looking to see if particle collisions/decays are possible and what force they act through, how do you know the parity of particles to know whether they act through weak force? Is there a grouping ...
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0answers
225 views

One loop effective potential of Standard Model

The one loop Coleman-Weinberg contribution of a scalar field to the effective potential (in MSbar scheme) is: \begin{equation} const. \times m^4(\phi_c) \left( log \left( ...
2
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0answers
81 views

Are mass terms forbidden in the Lagrangian because of parity violation or because fermions live in a complex representation?

Normally one argues that we can't write down Lorentz AND gauge invariant mass terms, because of parity violation, i.e. l-chiral and r-chiral fields transform differently. This means that mass terms ...
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1answer
88 views

Help for project on the basics of the Higgs field

I have a project for my university class on the Higgs fields and how it impacts the standard model. Also I was going to add some information on how the Higgs particle is formed and decays into ...
3
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0answers
80 views

Is Witten's claim that gauge group representations get exchanged with its dual under parity correct?

I'm currently reading Physics and Geometry by Witten, which I really liked up to the point where he claimed that we exchange representations $R$ and $\tilde R$ under parity transformations, where $R$ ...
3
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1answer
530 views

Can bosons have anti-particles?

Can bosons have anti-particles? In the past, I would have answered this question with a yes, primarily because I can imagine writing down a QFT for complex scalars that has a U(1) symmetry that ...
2
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1answer
216 views

Why does the electromagnetic and weak coupling strength do not meet at the electroweak scale?

The running of the coupling strengths is usually visualized on a logarithmic scale like here What surprises me is that the weak and the electromagnetic coupling strength do not meet before the GUT ...
2
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1answer
75 views

Experimental determination of $\Lambda_{QCD}$

I have a question about $\Lambda_{QCD}$, the energy scale at which there is a transition from the regime of perturbative QCD to quark confinement. How it is measured experimentally?
5
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3answers
2k views

Why does the Standard Model predict Neutrinos are massless?

Why are neutrinos massless in the Standard Model? Is it connected with experimental fact that neutrinos always have only one direction of projection of spin on motion direction?
2
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1answer
54 views

For Charmonium, why does the spin-spin interaction mostly affect the $L = 0$ states?

For Charmonium, why does the spin-spin interaction mostly affect the $L = 0$ states? My textbook states that this is because "only then is the wave function at the origin non-vanishing". Could anyone ...
5
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1answer
116 views

Where would the dark matter particle fit inside a picture of the standard model of particle physics?

Where would the dark matter particle fit inside a picture of the standard model of particle physics? While I am particularly fond of the Sterile Neutrino, I have not been able to find any example of ...
1
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1answer
391 views

How to tell if a particle interaction is strong/weak?

Problem: Prove that the reaction $p+p\rightarrow p+K^+$ is impossible, where $p=\text{proton}$, $K^+=\text{kaon}$. (Side question: Is there a specific name for a positive Kaon, just like a "positive ...
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1answer
50 views

Higgs mass and EW precision tests

I'm trying to understand how the Higgs mass can influence EW precision tests. In order to do that I'm using the following document (section 4.3): http://arxiv.org/pdf/0706.0684v1.pdf There are a ...
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3answers
4k views

Why do quarks have a fractional charge?

I am aware that evidence exists that strongly suggests the existence of quarks and do not doubt it. It is just simply really weird to me that they can have a fractional charge. While other ...
5
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1answer
230 views

Is the exact form of the Higgs potential known?

Usually the Higgs potential is given as $$ \frac{1}{2}\mu^2\phi^2 - \frac{1}{4}\lambda^2\phi^4 $$ but I never quite understood if this just serves to give us an idea of how symmetry breaking works, or ...
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2answers
197 views

What is the property of neutrino? [closed]

If mass of a particle is due to Higgs boson, spin is due to Z boson and translational energy is due to photons, what exactly is the innate property of a neutrino? If none exist, is neutrino truly a ...
17
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1answer
580 views

Why do we need complex representations in Grand Unified Theories?

EDIT4: I think I was now able to track down where this dogma originally came from. Howard Georgi wrote in TOWARDS A GRAND UNIFIED THEORY OF FLAVOR There is a deeper reason to require ...
3
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3answers
250 views

Why are complex fields in the Lagrangian?

I know that a complex field has twice the number of degrees of freedom of a real field, and that fields (in QFT) aren't observables so we don't really care if they are real. But why the need for ...
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0answers
32 views

Wu experiment and masses of neutrino

Wu experiment have shown that there are only left-handed neutrinos (and right-handed antineutrinos) take part in weak interactions. My question is about the significance of this experiment in a ...
93
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4answers
13k views

Why do electrons, according to my textbook, exist forever?

Does that mean that electrons are infinitely stable? The neutrinos of the three leptons are also listed as having a mean lifespan of infinity.
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3answers
638 views

What is the difference between the Higgs Boson particle and an electron moving through the Higgs field?

I am watching a lecture by Sean Caroll titled "Particles, Fields, and the Future of Physics". I am not a physicist by any means but enjoy the subject in my spare time hoping to understand it. This ...