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. DO NOT USE THIS TAG for point particles in classical mechanics.
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Why does electron-positron annihilation prefer to emit photons?
If gravitons are also massless, and neutrinos nearly so, why aren't pairs of either of them normally expected outcomes of electron-positron annihilations? Are they possible but simply unlikely, or is ...
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Why absence of electron is called hole?
I am having hard time in understanding the concept of holes:
If there is no electron than how can it be a hole?
For a moment lets assume absence of electron is termed as hole but how can this absent ...
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What is a particle? [closed]
I posted this elsewhere also and just found this place so copied it down but yeah. I've always wondered this cause I like wondering bout things but I wanna know and it's simple so I should. I got an ...
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Cosmic ray hazards
The Pierre Auger Observatory site mentions the detection of a 3E20 eV (48 J) cosmic ray whose energy, well above the GZK cutoff, was based on an analysis of its atmospheric shower. This was equivalent ...
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More on matter and anti-matter
Does every particle that has rest mass also have an anti-particle with which it would annihilate?
Does annihilation only occur between like particles? For example what happens if a antineutron (anti ...
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What is "charge discreteness"?
I assume it is some kind of quantity. Google only made things more confusing.
I get that it has something to do with circuits.
I also get what a discrete charge is. In fact, I thought charges were,...
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Can neutrons and protons have excited states?
Anything with a substructure has excited states. Molecules, atoms, nuclei all can absorb photons which causes them to make a transition to an excited state. Conversely, when they jump from an excited ...
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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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What is the highest accuracy of measuring time differences achievable today?
I was wondering if it would be possible to shorten the distance between detectors when measuring the speed of neutrinos to, say, 7m rather than the current ~700km? In this way the distance traveled ...
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Why isn't Hydrogen's electron pulled into the nucleus? [duplicate]
Possible Duplicate:
Why do electrons occupy the space around nuclei, and not collide with them?
Why don’t electrons crash into the nuclei they “orbit”?
From what I learned in chemistry, the ...
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Why can we apply the $E=hf$ equation for electrons?
So in my textbook, it states that the $E=hf$ equation applies to electrons, and all particles, not just photons.
But in order to prove this, wouldn't these particles need to have zero mass, to ...
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What distinguishes the behaviour of particle from its antiparticle: C violation or CP violation?
It is said that a CP violation would mean that the behaviour of the particle is different from the behaviour of antiparticle. Why is C violation not good/enough?
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Why does an electron have spin? [duplicate]
An electron is assumed to be a point particle that does not have structure and volume. Why does it have spin? Does this imply that the electron has volume? It is hard to imagine that a point (without ...
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Given the transformation of $SU(2)$ triplet $\vec{\phi}$ how to find the transformation of ${\Phi}\equiv\vec{\phi}\cdot\vec{\tau}$?
Given the transformation of a $SU(2)$ triplet $\vec\phi$ $$\phi\to \exp{(-i\vec{T}\cdot\vec{\theta})}~\vec\phi\tag{1}$$ (in the question here by @physicslover) how does obtain the transformation of $\...
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Largest Mass Diffraction
I have read "Matter-wave interference with particles selected from a molecular library with masses exceeding 10000 amu" which claims to observe diffraction patterns in objects of around 10'...
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Similar masses and lifetimes of the $\Delta$ baryons
Why do the four spin 3/2 $\Delta$ baryons have nearly identical masses and lifetimes despite their very different $u$ and $d$ quark compositions?
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$\beta^-$ decay: expression for maximum electron energy
I'm having some trouble finding an expression for the maximum electron energy in $\beta^-$ decay. In the frame where the neutron is initially at rest, conservation of momentum reads:
$$\vec{p}_p+\vec{...
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Is everything in the physical world composite?
In philosophy there is a principle that anything composite cannot have existed eternally, since it is preceded by its parts and whatever forces assembled it.
Is everything in the physical world ...
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The charge of an electron is a constant. In any case? [closed]
Since Millikan it is obvious that the charge of the electron can be measured as a result of the force exerted by an external electric field. What we get in detail is the charge from the excess of ...
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Lie theory, Representations and particle physics
This is a question that has been posted at many different forums, I thought maybe someone here would have a better or more conceptual answer than I have seen before:
Why do physicists care about ...
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Why is 7 TeV considered as a big amount of energy?
Considering that $7$ TeV is more or less the same kinetic energy as a mosquito flying, why is it considered to be a great amount of energy at the LHC?
I mean, a giant particle accelerator that can ...
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What do we see while watching light? Waves or particles?
I'm trying to understand quantum physics. I'm pretty familiar with it but I can't decide what counts as observing to cause particle behave (at least when it's about lights). So the question is what do ...
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Can you please show me a final atomic model which demonstrates movement of electrons inside it? [closed]
Is there any final model of an atom, of which we can say, “This is it”! Or is it still improving and physicists are not completely sure about it? I am particularly interested to know how exactly ...
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How does Positronium exist?
I've just recently heard of Positronium, an "element" with interesting properties formed by an electron and positron, and I was shocked to hear that physicists were actually working with this element, ...
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Terminology confusion - "particle"
I am confused about the word "particle" being used in academic contexts. Some professors at my university are adamant on the fact that particles do not exist, and only fields, as per QFT. One of them ...
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What is $p_T$? (transverse momentum?)
I've been looking at a few papers in experimental physics (from the ATLAS collaboration, for example) and I've often run across phrases such as "high-$p_T$ electron." What exactly is $p_T$? Is it ...
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What happened to the idea of tachyonic or other superluminal neutrinos?
While hunting around for information about the recent OPERA measurement that hints at superluminal neutrinos, I discovered that this idea was actually considered back in the 1980s. Wikipedia lists as ...
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Gauge fermions versus gauge bosons
Why are all the interactions particle of a gauge theory bosons.
Are fermionic gauge particle fields somehow forbidden by the theory ?
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Please explain the physics of a Cloud Chamber
A friend of mine was telling me about building a cloud chamber while he was in graduate school. As I understand it, this allows you to "see" interactions caused by high energy particles going through ...
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What are the main algorithms the LHC particle detectors use to reconstruct decay pathways?
I am just starting to look into how we understand the data from particle collisions.
My question is, what are the algorithms or ways that these detectors interpret the data? Are there standard ...
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Why are neutrinos more weakly interacting than light?
When people describe neutrino interactions they describe them as rare/infrequent due to the fact that the neutrinos are electrically neutral and have little mass, if any. Well why then is the photon ...
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Are there really left-chiral particles?
A chiral eigenstate is always a linear combination of a particle and an antiparticle state and a particle or antiparticle state is always a linear combination of chiral eigenstates. Now, how can we ...
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If atoms never "physically" touch each other, then how does matter-antimatter annihilation happen?
It is known that matter and antimatter annihilate each other when they "touch" each other. And as far as I know, the concept of "touching" as our brain gets it is not true on the atomic level since ...
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How can we 'see'/measure/detect particles during experiments?
I often read (high-level, conceptual) articles and news on the advances of particle physics. In these, statements are made along the lines of 'particle X splits into particles Y and Z which exist for ...
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Is there any theory for origination of charge?
We have a theory of a Higgs field that describes how a particle gets mass. Since mass and charge both are intrinsic properties of a particle, is there any similar theory for how particles get electric ...
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On the interpretation of Feynman diagrams
I am currently trying to find out about what exactly Feynman diagrams are, and up until now I have mainly used the lecture notes 'Mathematical ideas and notions of quantum field theory' by Etingof. In ...
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The delay between neutrinos and gammas in a supernova, and the absolute mass scale of neutrinos
In a supernova explosion (of some type), there is a huge amount of neutrinos and gamma rays produced by a runaway nuclear reaction at the stellar core. In a recent comment, dmckee noted that the ...
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Haag's comment on the relation between fields and particles
I am very confused by the statement made in Haag's, Local Quantum Physics: Fields, Particles, Algebras (page 46):
... the idea that to each particle there is a corresponding field and to each field a ...
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Why are there no elementary charged, spin-zero particles?
In the spirit of a related inquiry, I would like to know if there's a basis for understanding why there aren't any elementary particles that have non-zero electric charge but zero spin?
Can such a ...
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CP violation from the Electroweak SU(2)$_{weak,flavor}$ by $\int \theta F \wedge F $
Question: Why there is NO Charge-Parity (CP) violation from a potential Theta term in the electroweak SU(2)$_{weak,flavor}$ sector by $\theta_{electroweak} \int F \wedge F$?
(ps. an explicit ...
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Do photons generate gravitational waves since they affect with their energy the stress tensor? [duplicate]
The gravitational waves are fact. They are produced in a way predicted 100 years before by Einstein. Anything with energy affecting stress tensor of space time produces them. What does it happen with ...
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Could spontaneous symmetry breaking happen again in our universe?
It is generally believed that $10^{-35}$ seconds after the Big Bang, the symmetry of a GUT was broken and after $10^{-12}$ seconds the electroweak force was broken:
\begin{equation}
\mathrm{SU(2)} \...
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Concept of a point particle in quantum mechanics
A point particle is usually thought of as structureless and without dimension. However, given that Heisenberg's uncertainty principle prohibits us from knowing the position of a particle exactly, what ...
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Trace of stress tensor vanishes $\implies$ Weyl invariant
You often see in textbooks the statement that ${T^\mu}_\mu = 0$ implies Weyl invariance or conformal invariance. The proof goes like
$$\delta S \sim \int \sqrt{g} T^{\mu\nu} \delta g_{\mu\nu} \sim \...
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why is there no ninth gluon?
A teacher of mine told me once that there were no ninth gluon because such a one should be white and interact infinitely far, and no one has been observed. Is there also a theoretical reason?
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Standard Model Proton Decay Rate
The electro-weak force is known to contain a chiral anomaly that breaks $B+L$ conservation. In other words, it allows for the sum of baryons and leptons to change, but still conserves the difference ...
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Why an electron and a positron should have the same lifespan?
According to Particle Data Group:
source: data
Particles and their antiparticles (i.e. antimatter) have the same lifespan.
The electron/positron for example have a minimum of 6.6E28 yr. This was ...
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Why is the singlet state for two spin 1/2 particles anti-symmetric?
For two spin 1/2 particles I understand that the triplet states ($S = 1$) are:
$\newcommand\ket[1]{\left|{#1}\right>}
\newcommand\up\uparrow
\newcommand\dn\downarrow
\newcommand\lf\leftarrow
\...
user181173
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Free parameters in the Standard Model
From my understanding of the standard model, I understand that there are 19 or 20 free parameters that we need to put in by hand as, and I'm guessing here, there is as yet no theoretical basis for ...
user74893
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What are mass eigenstates?
According to Wikipedia
Neutrino oscillation arises from a mixture between the flavor and mass eigenstates of neutrinos. That is, the three neutrino states that interact with the charged leptons in ...
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