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Questions tagged [particle-physics]

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.

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Upper limits from Bayesian inference [closed]

While setting an upper exclusion limit for the new physics interaction with Bayesian inference, how do people decide on whether to claim 90% or 95% upper limit from the posterior marginalized pdf? ...
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1answer
48 views

Difference between 'anything' and 'X' in Particle Data Group publications' decay modes?

In the PDG publications, (like M. Tanabashi et al. (Particle Data Group) Phys. Rev. D 98, 010001 (2018) ) the decay modes of particles are listed. From the $B^0$ meson chapter, I took this ...
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Positron beams and crystallography

I wad mainly reading about impact and uses of positron beam to determine conductivity and the structure of material . Do can anyone just describe how positron beams can determine conductive of ...
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3answers
108 views

What are the Basic Properties of a Photon?

I want to grasp the idea of a photon. While researching, I have come upon many different ways of describing a photon, but have found "quantum of the electromagnetic field" to be most satisfying. ...
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52 views

Retrodiction of Particle Masses

While researching baryon asymmetry, I came across this article discussing problems with the Standard Model, where it is noted that: The Standard Model did not and cannot predict the masses of the ...
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94 views

Neutron Degeneracy Pressure Calculations

Today I was having a discussion with a colleague about what would happen if you took a 1 cm cube of neutron star matter and set it on the earth. He thought it would fall through and I was trying to ...
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Breit–Wheeler experimental evidence?

has there been any experimental evidence of the Breit–Wheeler process? Either in it's full or weak form, would you have some links to back this up? Can we say that all matter is merely energy ...
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27 views

Can space be considered as a grid of cubes of planck length, or is it continuous? [duplicate]

I don't know how to describe it exactly, please try to understand. The question is if space is continuous or gridded. Consider a particle of planck length (or whatever is the smallest possible ...
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30 views

Experimental justification for modelling electron as point instead of charged shell? [duplicate]

Down to what size is there experimental justification for modelling the electron as a point like particle without volume? Asked in another way, at what size scale would it be more correct to model the ...
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1answer
133 views

What is a String Wall?

This question has nothing to do with sound waves, physical strings or walls... I am reading: https://arxiv.org/abs/1709.07091 It states "In the post-inflationary PQ symmetry breaking scenario, ...
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1answer
41 views

Lack of knowledge about ambient neutrinos below a certain energy?

A colleague and I were discussing the fact that beta decay can emit neutrinos with arbitrarily low energies. We have neutrino detectors that can detect solar neutrinos -- and maybe extrasolar ...
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2answers
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Is the interaction $\gamma p \to \pi^+n$ allowed?

I'm doing an undergraduate lecture course in particle physics and I'm still getting to grips with the basics of interactions. One of the example interactions I've been given in an exercise is $\gamma ...
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1answer
71 views

How to calculate the distance that 1-10 KeV x-rays have to travel in air to lose 90% of its energy?

I'm trying to calculate the distance of air that can strip a beam of 1-10 KeV x-rays from 90% of its original energy. I came across this graph which shows the mass attenuation coefficient of air, but ...
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33 views

Naive question about particles and spinor fields [duplicate]

What is the difference between the "real" particle electron (for instance) and the spinor field of electron? I mean, which means that the electron have been described by a spinor field?My question is ...
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2answers
67 views

What is the difference between an interaction and a decay in particle physics?

While studying secondary cosmic rays, I have encountered sentences like: Charged pions and kaons can either initiate further interactions or decay. or Because of the low area density at large ...
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2answers
66 views

What elementary particles have properties that vary by spin direction?

This table of elementary particles indicates that electron spin = 1/2. My understanding is that electron spin may proceed in two directions which may be denoted by spin sign and that electron spin ...
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Why particles annihilate only with their antiparticles? [duplicate]

Why particles annihilate only with their antiparticles? Why can't a particle annihilate with any other antiparticle? Is it because both the particle and the antiparticle have to be excitations of the ...
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1answer
26 views

Is it necessary to know the spin state of a particle to find the relative branching ratios?

I have this problem: $\Sigma^{*0}$ has an isospin state of $|10>$. Assume it can only decay to the final states $\Sigma^+\pi^-,\Sigma^0\pi^0,$ and $\Sigma^-\pi^+$. I need to find the relative ...
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1answer
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How can matter and anti-matter coexist in a meson?

Whilst learning about the nature of subatomic particles, I came along a pion zero meson. What really stumbled me was that the quark configuration of the particles was up and anti-up or down and anti-...
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3answers
69 views

Why Neutrino is a ghost particle?

why neutrinos are called ghost particle.why it is not affected by strong magnetic field. why it does not interact with matter. why it does not interact with gravitational field? I am unable to ...
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Vacuum energy in continuous phase transitions

In a first-order cosmological phase transition, the tunneling from the false to the true vacuum (see left picture below) releases false vacuum energy corresponding to the potential difference $\Delta ...
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1answer
56 views

If a particle decay has quark annihilation will it always emit a photon?

From what I can gather the decay $\pi^0 \rightarrow \gamma \gamma $ emits two photons because $\pi^0 $ has quark content $(u\bar u-d\bar d)/\sqrt{2} $, and the $u$ annihilates with $\bar u$, and $d$ ...
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1answer
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Layperson wondering about The power within one protium atom (1H) or H

Within 1 hydrogen atom (1H) protium we have 1 electron the half life of this stable isotope is 10 to the 34th power years. It can fuse into Helium if 4 atoms fuse when the Coulomb force is overcome ...
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2answers
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Electric charge of the Higgs field

The Higgs field is \begin{equation} \Phi = \left( \begin{array}{cc} \phi_{1} + i\phi_{2} \\ \phi_{3} + i\phi_{4} \end{array} \right) \tag{1} \end{equation} with $\phi_{1}$ and $\phi_{2}$ ...
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2answers
61 views

Layperson can not get his head around Force Carriers

I read all I can but, this is vexing me. It realtes to Quantum-Mechanics, and I believe Quantum Field Theory, and maybe even Quantum Chromodynamics. Before the Higgs the Model was 16 instead of 17 we ...
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1answer
28 views

Can muons exist in space?

Since muons produce when cosmic rays crash and collide into air molecules in the atmosphere, one would think that there would not be any muons in space. Is it true?
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66 views

What is the minimal preliminary knowledge required for a PhD in particle physics? [closed]

Currently, I am doing a master in mathematical physics. I am interested in particles and field theory and want to apply a PhD in this field. But I am not sure whether I can.... I just learned a ...
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5answers
133 views

Why are matter fields predominantly fermions?

Apart from the Higgs, all matter is made up of fermions. Is there any obvious reason why that is?
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1answer
61 views

Muons - how are we even able to detect them? [closed]

muons have a very small half life comparable to 2.5 μs or so. But we know that it has to cover a very large distance from upper atmospheric layers to reach the particle detectors installed at earth's ...
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3answers
43 views

Half-life and muon decay

Imagine that we have a single muon. It has a life span of 2.2 microseconds on average. So normally, after that amount of time, the muon will decay. Now imagine we have two groups of muons. Group A has ...
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22 views

Proton-Proton, CNO, or white dwarf matter fusion reactor: Is it vaguely possible?

I understand that all efforts at present to make fusion work as an energy source have used light nuclei other than protons due to the fact that the cross section of the proton-proton reaction is just ...
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0answers
34 views

Bosonic commutation relations for force carriers?

Why are force carriers bosons? The easiest answer that I can give myself is that the gauge field $A_\mu$ is introduced like this: $$ \partial_\mu \rightarrow D_\mu = \partial_\mu+ieA_\mu, $$ so it ...
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4answers
118 views

Can a proton and an electron annihilate in a gravitational field?

According to this Physics.SE comment, it is gravitationally allowed, though very unlikely, for a proton and an electron to annihilate yielding two photons. Is that correct? If so, why? (In ...
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Charge operator applied to matrix multiplets

In the context of SM ($SU(3)_C\otimes SU(2)_L\otimes U(1)_Y$) the charge operator is $Q_{SM} = T_3 + \frac{Y}{2}\mathbb{I}_2$ and gives us the fermions charges. Here $T_3=\frac{1}{2}\sigma_3$ is the ...
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1answer
55 views

Is every particle sustained ripple in its respective field?

This is neither a homework nor a calculational question, but more of a conceptual one. I was wondering can every particle; that is, the ones indivisible (because divisible ones can be broken down ...
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Why can't large momentum effective theory be applied to fragmentation function?

Large momentum effective theory was proposed in 2013 by Prof. Xiang-Dong Ji. The idea is to numerically solve PDF in lattice QCD via so-called quasi-PDF in finite momentum frame. Such method is ...
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2answers
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Finding the value of $r$ for which the radial function, $P(r)$, has a maximum? [closed]

In my (university) particle physics course, I am asked to find the values of $r$ for which the function $P(r)$ of a $2s$ Hydrogen electron has its maximum values. Here, $r$ denotes the distance in ...
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1answer
36 views

What is $\mathbb{Z}_2$ Parity?

While reading about exotic decays of Higgs boson one of the simplest interaction that we come up with which leads to BSM decays is: $$\Delta L = \frac{\zeta}{2}s^{2}|H|^{2}.$$ This is the ...
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1answer
50 views

Inverse Compton scattering

Given a collision between a high energy cosmic ray proton (~$10^{20}$ eV) and a CMB photon, I want to calculate the final energy of the photon. I know to proceed, I use the following conservation of ...
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35 views

How to find energy levels formula for symmetric triangular potential well [closed]

As some of you already know that the asymmetric triangular potential well has the energy level where an here refers to the airy function a1 =2.338, a2=4.088, a3=5.521 and a4= 6.787 and V=eFz (F here ...
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0answers
52 views

Quantization during phase transition

Consider a scalar field $\phi(t,\vec{x})$ in $\mathbb{R}^{1,3}$ with the following lagrangian $$ \mathcal{L} = \frac{1}{2}\partial_\mu\phi\partial^\mu\phi - V(\phi) $$ where $V(\phi)$ is such that ...
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1answer
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Why can't the electrons in Lamb-Retherford Experiment transition from 2S to 2P(j=1/2)

This experiment reveals metastable state and uses the fact that an electron in 22S1/2 can't go to the ground state as ∆l=0 is restricted. But I was thinking why doesn't electron go to 22P1/2 and then ...
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53 views

Quark flow diagram for $\rho^0 \rightarrow \pi^0+\gamma$

So $\pi^0=\frac{1}{\sqrt{2}}(u\bar{u}-d\bar{d})$ and $\rho^0=\frac{1}{\sqrt{2}}(u\bar{u}-d\bar{d})$ except with a higher angular momentum, this angular momentum is taken by the photon. How do I ...
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3answers
118 views

Difference between $W^-$ and $\pi^-$

Maybe it's a very naif question, but what is the difference between a $W^-$ and a $\pi^-$? I mean they both change a $d$ into a $u$ right? $d \rightarrow u W^- \quad \text{and} \quad d \rightarrow ...
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0answers
24 views

Why Proca Term forbidden in Schwinger Model?

In my QFT Lecture we considered the Schwinger model with a Proca term. Solving the eom for the Stueckelberg field and plugging it back into the original Lagrangian, we receive an effective Lagrangian ...
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39 views

Antiproton production threshold

Why is the antiproton threshold on cern 6* m_p and on uspas it is 7 * m_p?
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1answer
49 views

Maximal Parity violation in Weak interactions

In 1956 Lee and Yang proposed parity violation of the weak interactions to explain the $\theta-\tau$ puzzle. The following year, 1957, Madam Wu and collaborators found that in the $\beta$ decay of ...
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4answers
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How does natural unit make sense? [duplicate]

Both the fundamental constants $\hbar$ and $c$ have dimensions. In particular, $[\hbar]=ML^2T^{-1}$ and $[c]=LT^{-1}$. But in natural units, we make them dimensionless constants of equal magnitude. ...
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2answers
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Why can't a particle interaction $A + B \rightarrow C$ conserve energy and momentum at the same time?

I learned that in the Feynman diagram for electron photon scattering, the intermediate particle represented by the middle solid line must be a virtual particle because it violates energy-momentum ...
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Why my cloud chamber won't work?

let me briefly introduce myself, I'm a highschool student from Indonesia, I'm currently making a cloud chamber for my science fair project, I did what the videos on youtube told me, but for some ...