# Tagged Questions

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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### Are fields intuitive choices to explain forces?

Fields are used to explain all the forces with respective particle being their facilitators. However, are fields intuitive choice to explain the forces? The physical significance is not very apparent. ...
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### Number of free parameters in a Standard Model with Neutrino oscillation

In two related questions, it was asked how many free parameters the standard model has. Without neutrino oscillation, there are 19 free parameters. Now it turned out that neutrinos have a mass aswell, ...
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### What does a physical surface look like at a subatomic level?

At the macroscopic level, we are all quite familiar with the concept of a physical surface. From wikipedia: https://en.wikipedia.org/wiki/Surface_(topology) In mathematics, a surface is a ...
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### Difference between measurement and expectation value?

Does measurement mean simply acting on a wave function by a physical observable? Or finding the expectation value of the physical observable? I understand the result is same if the function is an ...
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### How does the LHC explore extra dimensions?

The Large Hadron Collider (LHC) has been smashing particles for a long time and sometimes people say that it has found new dimensions. How is it even possible for a particle accelerator to find new ...
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### Resource Recommendations: General relativity, local tetrads and particle physics

I'm still self-learning general relativity. I have been a huge fan of Andrew Hamilton's amazing lecture notes on GR, black holes and cosmology. He goes through GR in pretty much full tetrad formalism. ...
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### Interesting question to figure it out [on hold]

A 50-cm-long iron rod is heated from 20 °C to 100 °C. The linear coefficient of expansion for iron is $1.2 × 10^{-5} K^{-1}$. How to determine the change in length of the rod. By which formula?
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### Why can we approximate massive particles as massless or vice versa?

Our descriptions of massless and massive particles are very different. For example: Massless particles have only two polarizations, which we call helicities. Spin projection on axes different than ...
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### What is the decay width and why is it given in energy units?

I'm reading Thomson, Modern Particle Physics, and in chapter 16 author says that the decay width of the Z boson is $\Gamma_Z =2.452 \pm 0.0023 \,\mathrm{GeV}$. He also says the total width of the ...
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### What's smaller: a neutrino, or a string from string theory [on hold]

I've recently read an article that stated "If an atom were as big as the solar system, a neutrino would be the size of a golf ball". I watch the science channel, and on (I believe) the show How the ...
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### Can neutrinos interact by the EM interaction and gravity?

A definition of a lepton is: A particle that does not interact by the strong force but does by the 3 other forces.$^1$ Neutrinos are leptons, so from the above definition one would expect it to ...
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### Does Compressing Energy over a Multiverse Dimensional Rupre Create Mass? [closed]

my question is only on a specific method of creating mass through super collapsation of enormous amount of energy in at a singular point or point of singularity.
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### Doesn't quantum uncertainty disprove string theory? [duplicate]

String theory states that the oscillations of little strings are responsible for all the particles in and the evolution of the universe. The specific type of particle created by a string depends on ...
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### Sufficient conditions for a interaction to be classified as weak, strong, …?

Let us say I have been given the equation of a interaction/decay/etc. between particles: $$X+Y\rightarrow A+B$$ Are their any sufficient conditions that we can use to determine the type of interaction ...
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### The interpretation of bubble and oyster Feynman diagrams?

I am reading 'A Guide To Feynman Diagrams in the Many Body Problem' By R.D.Mattuck, in which their is reference to oyster and bubble Feynman diagrams, shown respectively below. In these diagrams I ...
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### How to understand CP-violation in kaon systems with Feynman diagrams and matrix elements?

I am trying to understand CP-Violation in the Kaon system using feynman diagrams and matrix element. Here is a slide from Mark Thomson corresponding exactly to what I am looking for (http://www.hep....
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### Internal structure of electrons and quarks

I have some questions about experiments to probe the internal structure of electron and quarks. What type of experiments were done, in the recent years, to probe the internal structure of the ...
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### Alpha Particle moving through a magnetic field [closed]

How would I find the acceleration of an alpha particle moving through a magnetic field given the force of the magnetic field, the charge, the initial velocity and the strength of the magnetic field.
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### Running of $\alpha$ and scattering amplitudes

Consider a QED scattering process $e^-+e^-\rightarrow e^-+e^-$. The scattering crosssection at the tree-level depends on the square of the fine-structure constant $\alpha$ (apart from the electron ...
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### How would cold neutrinos get trapped by stars?

Continuing on from the cool physics Q&A'd on the threads Where are all the slow neutrinos?, Is it possible that all "spontaneous nuclear decay" is actually "slow neutrino" ...
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### Connection between Veneziano amplitude and Regge amplitude

I have tried to read about Regge theory, and I continue to run my head against the Veneziano formula, which is said to produce correct Regge trajectories by eg. t'Hooft at page 6 here: http://www....
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### Can we give particle with no mass, mass? [duplicate]

Is it possible to take a particle with no mass and give it mass. For example light? Or increase mass?
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### Definition of a meson? [closed]

I am looking for a definition of a meson that does not include the quark model. After some research I have come across this definition: A meson is a particle that is (1) believed to be ...
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### Particle Data Group book

Many PDG data books have 'July' marked on their covers, does this mean they are published (and made available on their website) in July? When is it expected to be available this year? (date or month ...
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### Which theoretical models are there between quantum mechanics and cosmology? [closed]

I'm an enthusiast/hobbyist right now and I'm quite curious about the subject of understanding which scales come between the quantum scale (ab initio/first principles) and the macroscopic scale. After ...
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### If particles are points, then aren't atoms empty space?

Zero dimensional points do not take up space, so then wouldn't everything in the universe be literally empty? Or is there something that I'm missing?
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### Why Do Glueballs Have Mass, When Individual Gluons Are Massless?

From Wikipedia Glueballs Glueballs are predicted by quantum chromodynamics to be massive, notwithstanding the fact that gluons themselves have zero rest mass in the Standard Model. Glueballs with ...
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### Is it possible that all “spontaneous nuclear decay” is actually “slow neutrino” induced?

This thought was inspired by a comment from the current leading answer, by @Sentry, to the question Where are all the slow neutrinos? This [slow-neutrino induced nuclear decay] will still be an ...
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### Do massless particles really exist? [duplicate]

I was in doubt, so I went to wikipedia. There it says "the photon has zero rest mass", but on the side description it says the mass is $<1.10^{-18} \:\mathrm{eV}/c^2$. So is the mass of the photon ...
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### Where are all the slow neutrinos?

The conventional way physicists describe neutrinos is that they have a very small amount of mass which entails they are traveling close to the speed of light. Here's a Wikipedia quote which is also ...
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### How can fields interaction give rise to particles?

We say light a matter-wave, meaning along with its wave property it shows particle nature. But how can fields interaction (electric and magnetic) give rise to particles (photon)? I wish someone could ...
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### Can we see/detect things which don't have electrons

Whenever we sees a thing the process happening at atomic level is the electrons of that stuff absorbs the energy from packets and goes to higher state and then comes to ground state and emits ...
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### Why Weyl fermion in Weyl semimetals(WSM) have high mobility only at low temperature?

I read several papers reporting high Weyl fermion with very high mobility in WSMs such as TaAs, NbAs, WTe2 and so on. However, this high mobility looks like (=Weyl fermion) always appears at only low ...
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### What is meant by interactions being mediated by force-carrier particles?

When a photon interacts with an electron, what is observed to happen? Force-carrier particles are described as the mediators of these interactions. What does this mean and how is this concluded?
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### Do photons interact with everything?

Suppose you shoot a beam of photons in a particle collider. Are there any particles in which the photons do not interact with? Q2: What is an interaction between particles?
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### Why isn't proton radiation by decay on earth known in nature?

Perhaps asking for why isn't appropriate in physics, but as there is neutron and alfa radiation what causes proton radiation not to be very common in nature (in laboratory it is seen although)