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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Degeneracy despite Interaction

Lifting of degeneracies of a composite system through interactions among its constituents is a generic theme of all physical systems - from splitting of $s-$orbital energies when two hydrogen atoms ...
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What is the difference between these mixing-angle equations?

In the following article the mixing angle of the neutrinos is given by: $$\sin^2 2\theta = \frac{\mu^2}{\mu^2 +[c\Gamma_\alpha E /M +M/2]^2} \tag{1}$$ where $\mu$ represents the Dirac mass , M ...
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A photon scatters an electron at an angle… Does it imply electron having an area greater then the photon's?

Even we don't know much about scattering areas of photons and electrons does the fact that a photon scattering an electron at an angle mean that the photon cross-section area hits only a small lateral ...
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Why is the square of the neutrino mass negative?

Why is the square of the neutrino mass negative? In arXiv:hep-ph/0009291 this is explained by giving the example of: $$m^2_{\nu_e}= -2.5 \pm 3.3 \text{eV}^2 \tag{1}$$ "Thee negative value of the ...
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Particle as wave, stable?

I've started reading about the wave-particle duality but, after a few steps, reached a dead end: Schrodinger equation solutions for a free particle is a sum of terms of the form: $$\psi(\mathbf{r}, ...
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Lagrangian of free particle relativistic case

Why must the covariant Lagrangian of a free particle be a first-order differential?
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How can I build up my knowledge in particle physics to the level that I can calculate the path of elementary particles in a perfect vacuum?

I have to write a research paper (don't know the exact english translation) for school. The question I want the paper to answer/discuss is: 'Can the universe be predicted at sub-atomic level?' The ...
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The strong force, gluons, and $m=E/c^{2}$: Is the equal sign replaced by “is”? [closed]

In his book Quantum Space page 82, Baggott says "In fact, elaborate QCD calculations demonstrate that about 95 percent of the mass of a proton or neutron is derived from the energy of massless ...
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Does the Hubble constant depends on the model of the universe used?

I am writing an equation using the Hubble constant. My work is based on Dodelson's article on sterile neutrinos which uses a C$+$HDM model.In it a value of $h=0.5$. But when reading the more recent ...
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Why would the damping factor of neutrino oscillations be negligible?

In this article the Boltzmann equation for the sterile neutrinos is simpified to this form: $$\frac{\partial}{\partial t} f_s (p,t) -H p \frac{\partial}{\partial p} f_s (p,t) \approx \frac{\Gamma_\...
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What is it about the mechanism for the strong force which creates a force?

So as I understand it, the mechanism for the strong force is that quarks emit/exchange gluons. Similarly, quark-anti-quark pairs are what pass between nucleons in an atom causing them to stick to each ...
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Is it possible to find the ground state of interacting $\phi^{4}$ theory with quartic interaction analytically?

Is it possible to find the ground state of interacting $\phi^4$ theory with quartic interaction analytically?
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How is the mixing of neutrinos related to eigenstates?

Can someone please explain to me why the following statement is true: "Neutrino mixing phenomena arise from the noncoincidence of energy-propagation eigenstate and the weak (interaction) ...
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Why is the mass of the proton such a precise value?

Why is the mass of the proton such a precise value? A proton is composed of 3 net valence quarks and what is often described as "binding energy" or "a zillion gluons and quarks and anti-...
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Which is the meaning of these masses?

When reading about the masses of neutrinos in the HDM (Hot Dark Matter) model of the universe, I came across the following equation: $$\tag{1} \sum_i m_{\nu_i} = 92 \Omega_\nu h^2 \text{eV}$$ My ...
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How do weak interactions maintain thermal equilibrium in the early universe?

When reading about why SM neutrinos are not a candidate for DM, it was stated: "Neutrinos are involved in weak interactions that keep them in thermal equilibrium in the early Universe down to the ...
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Do elementary particles last forever?

I have heard that not even black holes last forever, because of Hawking radiation. But what about elementary particles? Will an electron, for example, exist for all time?
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Derivations of Eq.(20) in Vogel & Beacom Paper

I was reading P. Vogel and J.F. Beacom, Phys. Rev. D 60, 5 (1999). I was trying to follow to use eq.(18) and eq.(19) to derive eq.(20) to no avail. Would someone help me? Thank you.
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What is a neutrino state if not a particle?

When reading about the 2015 Nobel prize and how this led to the possibility of the existence of sterile neutrinos I am told that: "(...) three active neutrinos $\nu_e$, $\nu_\mu$, $\nu_\tau$, are ...
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Are weak force particle transformations continuous or instantaneous?

The weak force transforms particles from one type to another. What can we say about the transition? Take this as an example: In the discrete view: Before junction A there are 2 up quarks and one ...
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Maximum Electron Speed Around an Atomic Nucleus [duplicate]

At what element do we hit a wall because of the electrons relativistic speed? i.e. the speed of the outer shells can go no faster?
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What is the atmospheric neutrino oscillation length scale associated to the LSND?

When reading Sterile neutrinos, hot, warm and cold dark matter, there is a discussion about framing $\nu$ (neutrino) oscillation solutions into a 3$\nu$ framework. And the following is stated: "...
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What is the meaning of short baseline experiments?

When trying to find the meaning of "short baseline" concept I came across the definition of a long baseline, described as "neutrinos are detected after traversing macroscopic distances&...
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Is there any reaction where electrons or protons get directly converted to energy, and are destroyed as a consequence?

This question is not about fission or fusion reactions, or changes in mass/energy as the speed of a particle changes. The number of protons (including protons within neutrons) and electrons pre- and ...
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Spin conservation in pair production

In QED, when two photons collide, they can turn into an electron and positron pair. We know from $U(1)$ gauge symmetry that the total charge of the initial and final states must be conserved. On the ...
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Electron-electron scattering into muon-muon state

It is common in QED that electron positron pair can scatter into different lepton family such as muon and antimuon pair. Is it possible that two energetic electrons can scatter into two muons ? I have ...
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What is the difference between alignment limit and decoupling limit?

In the context of Higgs physics, what is the difference between alignment limit and decoupling limit ?
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In experimental particle physics: Why and when can a combinatorial background be modeled with exponential functions?

A common model for the combinatorial background in an invariant mass distribution in experimental particle physics is an exponential distribution. Is there a physical reason why this is a good model ...
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Absorption of pi mesons on deuterium nuclei

In the process we have \begin{equation}\pi^{-}+d \longrightarrow n+n\end{equation} and by the parity conservation \begin{equation}\eta_{\pi} \eta_{d}(-1)^{\ell_{i}}=\eta_{n} \eta_{n}(-1)^{\ell_{f}}\...
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Detection of resonance particles

why we detect an increment in the number of events of the reactions for the energy of resonance particles? I read that cross section of this new unstable particle must be added to the cross section of ...
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Spin and force carrying bosons

Fundamental forces are mediated by gauge bosons. The known ones all have spin-1 but the graviton, if it actually exists, would have spin-2. How do they know this? How do the properties of the forces ...
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Why is a fourth type of active neutrinos not allowed by the invisible width of the $Z$-boson?

I don't understand the following statement from my notes: "A fourth active neutrino is not allowed by the invisible width of the $Z$-boson to which it would contribute as much as one active ...
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Is the decay of a neutral rho meson into a neutral pi meson and a photon forbidden?

is the decay of a neutral rho meson into a neutral pi meson and a photon forbidden? It doesn´t look like parity conservation or charge conservation is violated but I haven´t found this decay is ...
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Using an electric field to rip apart a nucleus

Can we use an electric field to rip apart a nucleus? . Let's take 3He which is a very stable isotope but unfortunately not commonly found. And let's assume we have placed an 3He inside a spherical box ...
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How would a monochromatic line in the $\beta$-decay spectrum neglect the existence of neutrinos?

When reading about the history of the discovery of the neutrino I came across the following paragraphs in my lecturer's notes: (...) physicists were puzzled by the continuous spectrum of $\beta$-...
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How are subatomic particles affected by gravity?

I‘ve been searching for it on many different websites and they all basically say the same thing. The subatomic particles’ mass is so small that other forces (as the weak force, strong force and ...
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Can we theoretically prove that for an elementary particle to carry a nonzero charge it must have a nonzero mass? [duplicate]

There is no massless elementary particle that carries a nonzero electric charge. It seems necessary that an elementary particle must have nonzero mass in order to carry a nonzero electric charge. Can ...
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Could gravity be much stronger (or weaker) at the atomic scale?

If gravity is mediated by particles and you are at a scale where those particles are relatively much larger does that perhaps imply that gravity can't work exactly the same way at very small scales as ...
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If a relativistic proton e.g. $\gamma=5$ hits a stationary proton will this collision take all its momentum?

In classical physics when a billiard ball hits a stationary one of same mass it transfers all its momentum to it so becoming itself stationary while the other gathers their whole momentum. But what ...
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Which properties of individual nucleons change depending on the nuclear environment?

Free neutrons are unstable (half-life $\sim 10$ min), but bound neutrons are (for many nuclei, including those which are stable) stable. Question 1: Are there other properties of the neutron / proton ...
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Exercise on local and global symmetry for a given Lagrangian density [closed]

The following Lagrangian density describes a charged scalar field. Determine the global U(1) symmetry. Make this global symmetry local and give the Lagrangian density. The Lagrangian density is given ...
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Would a highly degenerate cosmic neutrino background affect fusion reactions that proceed via the weak interaction?

This might seem like a really strange question, but here's my reasoning. A proton-proton fusion reaction proceeds in two steps: \begin{align*} p + p + \text{1.25 MeV} &\rightarrow {}^2_2\mathrm{He}...
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Why does energy come in the form of packets?

Photons are the packets of energy released by continuous oscillation of charges. But I have some questions about this. Since the electrons oscillate regularly while transitioning between orbitals ...
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Can electrons be electrically polarized, i.e., can they acquire an induced electric dipole moment?

A comment on a recent question raises an interesting point: Neutrons can have intrinsic electric dipole moments. Neutrons also have a nonzero electric polarizability, i.e., they acquire an induced ...
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Muonic hydrogen 21 cm line?

Is there any difference between electron and muon in spin change behaviour(like with mass)? In Muonic hydrogen the "21cm line" will have different frequency, but not of the spin difference, ...
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Relativistic momentum in pion and muon

So this is from Griffiths particle physics book about pion decaying into neutrino and muon. The part I have problem with is that it states ``` \begin{equation} p_{\pi}^2=m_{\pi}^2c^2 , p_{\mu}^2=m_{\...
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Hadrons as tensors of flavor symmetry even though flavor symmetry is broken?

I will briefly summarize what I know and then ask my questions. If you spot mistakes in my summary, please tell me. The idea of flavor symmetry is that massless QCD is invariant under SU(6) ...
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What is the relation between energy density ($\Omega$) and the number density of neutrinos ($n$)?

What is the numerical relation between energy density ( $\Omega_\text{s}, \Omega_\alpha$), and the number density of neutrinos (sterile - $n_s$ , active - $n_\alpha$)? Background info that might help ...
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Do the atomic emission spectra provide supporting evidence for the wave or particle nature of electrons?

My textbook states that the observation that a electric current passing through a gas causes characteristic emission spectrum to be observed gives supporting evidence for the wave nature of electrons. ...
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Question about kinematic variable transformation (in the Drell-Yan process)

I want to get the inverse transformation, or at least the Jacobian determinant in terms of the $q^0,\mathbf{q},\cos\theta_1, \cos\theta_2$ coordinates, of the following coordinate transformation $$ (\...

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