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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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Quantum wave function evolution and time dilation [closed]

We know that spin state evolves with time...but in non relativistic QM time dilation is not accounted ...so in Dirac equation does evolution of spin state with time depend on speed...i.e does time ...
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2answers
169 views

How do we know that gluons travel at the speed of light?

Since gluons are located within nucleons and immediately outside of them, how do experiments determine parameters like their speed? Is it possible we could be assuming they travel at the speed of ...
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Transformation of opening angle in different frame of rerefence in special relativity

I have to find the minimum opening angle for the decay $\pi^0 \rightarrow \gamma \gamma$ given the energy and rest mass of the $\pi^0$. My question is that: since the center-of-mass frame's opening ...
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Particle collision

At the end of Chapter 3 in Griffiths "Introduction to elementary particles", he mentions that one can benefit from a change in frames, in this case, center-of-mass (CM) frame and particle-rest frame. ...
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35 views

Is Quantum spin greater than $1$ possible? [duplicate]

I know that fermions have a spin of half of and bosons have a spin of 1 but at many places I have seen that their is written that spin for a boson can be 0,1,2.... Is there any physical meaning of ...
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1answer
35 views

Collider Luminosity basics

In a collider experiment, the Luminosity is defined as the proportionality factor between the interaction rate and the interaction cross section $dN/dt = L\times\sigma$, with units of $cm^{-2}s^{-1}$. ...
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146 views

Is there such a thing as an anti-boson?

Can there be an anti-boson that when interacting with normal bosons, creates matter, like when anti-matter creates energy when interacting with matter? I know that anti-particles can be considered ...
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2answers
125 views

Why is the parity of the spatial wavefunction $(-1)^{\ell}$?

Consider a composite particle state $|\psi\rangle$ (like a hadron or a meson) that is an eigenstate of some Hamiltonian (e.g. the QCD Hamiltonian). Since the Hamiltonian is invariant under rotations ...
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1answer
36 views

Why does the upper component of a $SU(2)$ doublet has $T^3=1/2$ and lower component $T^3=-1/2$ and not the opposite?

For a $SU(2)$ doublet, why does the upper component have $T^3=1/2$ and lower component $T^3=-1/2$? I know that this can be answered in the Standard Model by using $Q=T^3+Y/2$. But that is because we ...
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1answer
116 views

Light neutrino number from the invisible decay width of $Z$ boson, and precluding heavy neutrinos as dark matter candidate

The $Z$ boson decays into pairs of quarks and leptons. While the decays to quarks pairs and charged lepton pairs can be observed, the decays to $\nu\bar\nu$ are cannot be. By subtracting the visible ...
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Why can't there be a direct absorption of a photon between Raman active states?

My question pertains to the fundamental difference between Raman spectroscopy and IR spectroscopy. Simply put, the Raman active transitions correspond to a change in the polarizability of the molecule ...
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2answers
57 views

How do photons travel and what does the frequency of a photon mean?

I am pretty much confused about all this. I have researched a lot about this and many sites say that photons are particles, but this guy CuriousOne here doesn't even regard photons as particles and ...
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2answers
58 views

Charged and neutral current in Feynman diagrams

I am stuck on finding the 'right' mediator in weak interactions. Consider the following reactions. 1: $$\mu^+\rightarrow \bar{\nu}_\mu + e^+ +\nu_e$$ This is mediated by the vector boson $W^+$. 2: ...
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1answer
41 views

How to interpret the radioactivity for a radioactive material that generate isotopes?

I have a Strontium-90 radioactive source. The radioactivity of this source is $0.1\mu\ \rm Ci$. This source $\beta$−decays into Yttrium-90. Then Yttrium-90 $\beta$−decays into Zirconium-90 with a ...
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58 views

What would happen to 2 separate photons on an uninterrupted path

So I am a middle schooler and from what i understand, photons can interact with surrounding particle via gravity. If this is true, would 2 photons on an uninterrupted and completely isolated path ...
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45 views

Interaction Lagrangian for specific channels

I'm trying to build an interaction Lagrangian (${\cal L}_{int}$) with 2 fields, a complex $\varphi$ and another $\chi$ that can be real or complex. The key is to do it granting $U(1)$ symmetry and in ...
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1answer
22 views

In a Radioisotope betavoltaic cell, does the amperage or voltage decay?

In a tritium-powered betavoltaic cell, the power output obviously decreases as the tritium decays. Is it the voltage, amperage, or both that would decay?
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1answer
53 views

Calculation of maximum decay length

Consider the following decay: $$\Upsilon(4S) \rightarrow B^0 + \bar{B}^0.$$ I have to determine the maximum decay length given the lifetime $\tau$ of a $B$-meson (as measured in its rest frame). I ...
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2answers
41 views

How to measure masses of decay products $B^0$-meson?

Say we have a decay of $B^0$ -meson to some charged particles. Is there a way of determining the masses of these particles? I know that we can for example measure the momentum of the particles using ...
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1answer
66 views

Permanent electric dipole moment parity violation

In particle physics we deal with parity transformations and in particular when we regard weak interactions. While learning this subject I found something that needs more clarification for me. Namely, ...
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3answers
1k views

Why don't we use positrons from beta plus radiation as a source of anti-electrons for energy?

Why can't we use the positrons (let off by beta plus radiation) as a source of anti-matter, such that we can collide it with an electron for 100% efficient mass to energy conversion?
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1answer
39 views

Description of particles using co-moving frame in accelerator

I am learning particle accelerators and there is a basic concept as to description of particle beams in accelerators which confused me a lot. Moving orthogonal, right-handed coordinate system (x, y, ...
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ATLAS jet energy corrections

I've been reading this paper and on page 3 at the beginning of section 3 it states: 'After the events are recorded, the trigger-level jet energy and direction are corrected to those of simulated ...
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2answers
123 views

How does the exchange of pions result in the strong force?

I understand that the residual strong force is a result of an exchange of pions. But I fail to understand how this exchange results in a force that holds nuclei together! May this query please be ...
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1answer
54 views

weak interaction and Parity violation

It is said that the Weak Interaction only couples to left-handed particles which a negative spin (left-handed). However some sources say that spin or helicity is dependent on the observer's position ...
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Invisible energy in a particle collision

I read a lot about the so-called "missing" energy in hadronic collisions. I always thought that it was mainly due to neutrinos and also nuclear deformations, but today I read that "on average, 30% of ...
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2answers
62 views

Orbital parity of simple bound states in atomic and particle physics

The parity operator commutes with the Hydrogen atom Hamiltonian. The energy eigenfunctions are parity eigenstates with orbital parity $(-1)^\ell$ which follows from the fact that $Y_{\ell m}(\theta,\...
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1answer
173 views

So is it not $CP$ instead of $C$ that is responsible for changing a particle to its antiparticle?

The charge conjugation operator $C$ reverses the charge of a state. But it may or may not convert a particle to its antiparticle. For example, consider a neutrino which is charge-neutral and left-...
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1answer
44 views

How to show that the charge conjugation reverses the charge of a state?

How to show that the charge conjugation operator reverses the charge(s) of a (fermionic or bosonic) state? Let us consider a spin-$\frac{1}{2}$ fermionic state of momentum $\textbf{k}$ and spin ...
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0answers
16 views

Ratio between neutrino scattering cross-sections

I would like to calculate the ratio between the cross-section of the processes: $$\frac{\nu_\mu+e^- \rightarrow \nu_e+\mu^-}{\bar{\nu}_e+e^- \rightarrow \bar{\nu}_\mu + \mu^-}$$ I found this set of ...
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2answers
63 views

Can a particle radiate in vacuum without external forces applied?

I was reading about possible and forbidden processes involving photons. First of all, the annihilation process of a pair electron and positron cannot result into a single photon because it would be ...
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2answers
65 views

Theoretical definition and pratical mesurement of differential cross section

In Sakurai's book, the definition of differential cross section is: $$d\sigma/d\Omega= transition \;rate / probability\; flux $$ However this def doesn't contain any information about the thickness ...
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1answer
176 views

Eigenstates in QFT and amplitude of a field operator

I've seen in different posts (such as here) that given a field $\hat{\phi}(x)$, its eigenstates $|\phi\rangle$ are of the form: $$|\phi\rangle\ = e^{\int dx\phi(x)\hat{\phi}(x)}|0\rangle\tag1$$ I ...
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3answers
122 views

Are particles and waves separate entities? [closed]

Can this be proven incorrect: Particles and waves are separate entities. Waves are the energy mechanism that transports the particle but not part of the particle itself. If a photon gets created the ...
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1answer
58 views

Does the nucleus have less inertial mass than sum of inertial masses of constitute protons and neutrons?

In nuclear fusion part of nucleons' rest mass goes into radiation energy. Does it mean that inertial mass of result nucleus decreased?
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1answer
117 views

Integration of the splitting function

I have a problem performing the following integration provided in the paper by Catani and Seymour (arXiv: hep-ph/9605323) page 27. Given is the integral $$ \mathcal{V}=\int_0^1 (z(1-z))^{-\epsilon} \...
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4answers
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Does a moving electrically charged particle have a “magnetic charge”?

I have feeling that the force on a moving charged particle from an external field is due to the interaction of the external magnetic field with the magnetic field produced by the charged particle. I ...
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4answers
102 views

Momentum of an ultrarelativistic electron

I am aware of the relativistic equation: $E^2 = (pc)^2 + (mc^2)^2$ And if we are dealing with a massless particle then $E = pc$ However I am doing some work in Astrophysics and have been told that ...
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3answers
268 views

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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0answers
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Integration by parts in dimensional regularisation

I have a question concerning integration by parts identities in dimensional regularisation. Appearently, almost every textbook about dimensional regularisation claims that $$\int d^Dl_1...d^Dl_L \...
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0answers
62 views

Why isn't energy conserved in time-ordered diagrams?

I'm new to particle physics, and I'm reading chapter 5 of Prof. Mark A. Thompson's "Modern Particle Physics", which talks about Time-ordered perturbation theory vs QED. However, in page 119 he wrote: ...
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1answer
196 views

How much energy does a neutrino have? [closed]

Neutrinos were theorized when some energy was noticed to be 'missing' from beta decays, correct? But until recently, they were thought to be massless. So, how much energy does a single neutrino have?...
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1answer
61 views

Coefficients of Color Potential in Quarks [duplicate]

I was wondering if there was a relation between the fractional coefficients $$2/3, 1/3$$ obtained when calculating color potential for quarks and the value of electric charge for the up and down quark....
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0answers
35 views

Particle creation, energy density and the Compton wavelength

The Compton wavelength of a particle is given by $$\lambda=\frac{h}{mc}.$$ One can construct an expression for the energy density $\rho$ of a particle of mass $m$ given by $$\rho = \frac{mc^2}{\lambda^...
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2answers
47 views

Is it possible to survive the destruction of a planet (earth)? [closed]

I recently watched Star Trek VI: The Undiscovered Country again. The main story evolves around the destruction of a moon of the klingon empire which is their main ...
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1answer
58 views

Mathematical proof that the centre of mass energy is the smallest energy in a collision

I recently answered this question about how the energy of a system of colliding particles is minimised in the centre of mass frame. I'm happy with the qualitative answer that treating the collection ...
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1answer
93 views

Why is the laboratory frame energy always greater than the center of mass frame energy for collisions? [duplicate]

I looked through lots of sources to answer the question, 'Why is lab frame energy (total energy) always greater than the center of mass frame energy?' Many of them provided lots of mathematical ...
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3answers
619 views

Why is the laboratory frame energy always greater than the center of mass frame energy?

I have been looking for an answer to 'Why is the laboratory frame energy always greater than the center of mass frame energy during collisions?'. A lot of resources provided mathematical explanations....
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1answer
44 views

Why use $\ln$ in definition of pseudorapidity? [duplicate]

Why is pseudorapidity defined as $\eta = -\ln[\tan(\frac{\theta}{2})]$ as opposed to just $\eta = \tan(\frac{\theta}{2})$?
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6answers
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What is the experimental evidence that the nucleons are made up of three quarks?

What is the experimental evidence that the nucleons are made up of three quarks? What is the point of saying that nucleons are made of quarks when there are also gluons inside it?