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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Is there any way to distinguish experimentally gauge mediation from gravity mediation in an unambiguous way?

There are lots of models of gravity mediated SUSY breaking with various spectra as well as various general gauge mediation models. Are there any "smoking gun" experimental singnatures that could ...
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status of +4/3 scalar as explanation of $t\bar t$ asymmetry

One of the early proposals for the Tevatron asymmetry on $t \bar t$ was a "fundamental diquark" with a charge (and hypercharge) +4/3, either in a triplet or a sextet colour. I am interested on the ...
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Why might $B$ meson decays violate lepton universality?

Scientists from various institutions have recently discovered that there might be a break in the application of the Standard Model, particularly with a fundamental principle called the lepton ...
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Integration & bremsstrahlung calculation

In this paper (relevant pdf section) that I'm reading, involving the calculation of bremsstrahlung in electron proton scattering (diagram below), the author calculates the integral over outgoing ...
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On the naturalness problem

I know that there are several questions about the naturalness (or hierarchy or fine-tunning) problem of scalars masses in physics.stackexcange.com, but I have not found answers to any of the following ...
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How well is the $\rho$ and $\omega$ coupling universality measured?

Is there any good recent experimental data that tests whether the $\rho$ coupling constant depends only on the isospin multiplet of the produced particle? EDIT: I got a downvote, so I should explain ...
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8 votes
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Why are particles either bosons or fermions in spatial dimensions $d>2$ (in Wigner classification)

This questions might have been asked several times, but I haven't seen a mathematical point of view, so here it is. Based on Wigner classfication: A particle is a representation, because any theory ...
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Up and down quark content of neutral pion

If isospin were a perfect symmetry, then the neutral pion would have equal $u\bar u$ and $d\bar d$ content, but since up quarks are slightly lighter than down quarks, the neutral pion, being the ...
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Is it possible for quarks and charged leptons to 'oscillate' analogously to neutrinos?

In a hypothetical scenario where the coupling of the fermions to the gauge fields were extremely small, would it be possible to observe quark and charged leptons oscillate in the same manner as we ...
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Axial and vector resonances in composite Higgs models

Is there a reason to believe that the axial resonances are heavier than the vector resonances in the composite Higgs models? For instance, in this article, to have zero tree level contributions to S ...
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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 the spin 3/2 particle equation would violate causality?

I've recently come around the study of the so called Rarita-Schwinger equation for elementary particles of spin $3/2$. The point it the article is really short, and no book treats the topic in a very ...
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Large Hadron Collider 2015 upgrade, what may we discover?

I realise that the initial answer to my question that may come to mind is, "we don't know yet, obviously" But my question is hopefully not opinion based. For example, does this upgrade have a ...
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Could sphaleron-induced proton decay also cause vacuum decay?

I will say right away that I don't mean standard-model sphalerons, I mean the sphalerons of some extension of the standard model. The reason to even think about this is last year's paper by Frampton ...
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What statistical test should I use?

I have two differential cross sections $d\sigma_{1,2}/dM$ ($M$ is some invariant mass) corresponding to different hypothesis. What I want is to calculate how well the appropriate experiment would do ...
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Where in fundamental physics are Lie groups actually important (and not just Lie algebras)?

I was wondering where in fundamental physics the global structure of a Lie group actually makes a difference. Most of the time physicists are sloppy and don't distinguish groups and algebras ...
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Categorizing solutions to Hierarchy problem

We know that no gauge symmetry can prevent a term $m_\phi^2|\phi|^2$ for a scalar field, and that, given the quadratic loop corrections, the natural scale is $m_\phi \sim M_P$. This is related to the ...
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Final-state baryons in $p \bar p$ collisions in Pythia

I'm trying to simulate inclusive $\chi_c$ production in $p \bar p$ collisions at very low energies (~ 5.5 GeV) using Pythia8 event generator. Leaving aside problems bounded with applicability of ...
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Seeking chiral anomaly EFT example

If an effective field theory has a chiral anomaly it means that chiral symmetry isn't a symmetry of the underlying theory which has been cut off to make the EFT. My question is whether there's a good ...
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5 votes
2 answers
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Can fundamental particles have magnetic/electric quadrupoles, octopoles, and higher-order moments?

Fundamental particles come with magnetic and electric charge, which makes the particles into a monopole source for the magnetic and electric fields. Of course, the magnetic charge is zero for all ...
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How would a quark star maintain degeneracy near the surface?

I'm still not exactly understanding how a quark star could possibly exist for more than a fraction of a second. Neutron stars are already hypothesized to have degenerate quark matter at their cores, ...
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How Is Are Parton Distribution Functions (PDFs) Determined In Practice And In Theory?

I have put my actual questions in bold normal font, and the rest of this question clarifies what I mean to be asking, especially in terms of the depth and kinds of information I am looking for. ...
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$2\nu\beta\beta$ vs. $2\nu$ECEC

Considering the recent observation of two-neutrino double electron capture ($2\nu$ECEC) by the XENON Experiment: https://arxiv.org/abs/1904.11002 I was wondering why the half-life of this process is ...
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5 votes
2 answers
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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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Representation Theory of $SL(2, \mathbb{C})$

I'm a PhD. in mathematics (working mainly in complex algebraic geometry), but I'm looking for a "convincing" answer concerning the various applications of representation theory of the group $SL(2, \...
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What is the difference between selection efficiency and acceptance in high energy physics?

Often in papers the product of these two quantities is mentioned, but I'm not sure what exactly they mean separately. My guess would be that one of these is the fraction of the events that you are ...
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5 votes
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What is the difference between Breit-Wigner mass and pole mass in PDG?

In Review of Particle Physics 2014, page 81, in the baryon summary table, there are usually noticeable differences between Re(pole position) and Breit-Wigner mass, for example, for $N(1440)$, $M_{BW}=...
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Intrinsic parity

When we apply parity on a field two times, we demand that we should get back the same field. This gives us, $P^{2} =1$, which implies, $ P \psi = e^{i \theta} \psi$ . This extra phase factor is ...
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Smallest minimum feature size possible for transistors printed by photolithography

I'm doing a project about Moore's Law, one of the subtopics I've come to is photolithography. The way I understand it is that the MOSFET transistors are currently printed on a silicon wafer by ...
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Classic mass predictions from Left-Right models with discrete symmetries?

I am covering the classic literature on predictions of Cabibbo angle or other relationships in the mass matrix. As you may remember, this research was a rage in the late seventies, after noticing that ...
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Is there precision experimental evidence for Furry's theorem -- that only even degree VEVs are non-zero?

Is there precision experimental evidence for or contradicting Furry's theorem -- that only even degree VEVs are non-zero, specifically for the EM field?
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Can a massless electric charge particle exist in a de Sitter universe?

Because of the cosmological constant, our universe may reach a de Sitter universe in the far future. If we suppose this, the most basic group of particle physics will be the Sitter, rather than ...
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Fermilab $g-2$ muon experiment: How to obtain $a_{\mu}$?

In the $g−2$ muon experiment, we measure the anomalous precession frequency $$\omega_a = \omega_s − \omega_c,$$ where $$\omega_s = \frac{g_{\mu}\left\vert q\right\vert B}{2m} + \frac{\left( 1-\gamma\...
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Vacuum catastrophe: why isn't a large length (1mm) cutoff plausible?

I've been trying to get a better grasp of the vacuum catastrophe problem, that is vacuum energy as calculated in QFT vs the observed value in cosmology differs by 120 orders of magnitude. The issue ...
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Which explanation of the OZI rule is correct (or most important)?

The OZI rule states that a QCD diagram is suppressed if it can be cut into two by only cutting gluon lines (the precise wording is addressed in the question What is the precise statement of the OZI ...
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4 votes
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Zee Chapter VII.5: Can the fermion $10$ be the composite of fermion $5^*$?

In Zee Chapter VII.5 QFT book: He showed that the standard model fermions form $5^*$ and $10$ representations of $SU(5)$: $$ 5^* \oplus 10. $$ In particular, the $10$ is the anti-symmetric matrix ...
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Figure of $SO(10)$ grand unified theory in $E_6$ from Wikipedia

In this figure from Wikipedia, it shows that the representations of particles of $SO(10)$ grand unified theory, their patterns of charges for particles in the $SO(10)$ model, rotated to show the ...
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4 votes
1 answer
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How does cobalt-60 beta decay experiment also violate charge conjugation?

I understand why the cobalt-60 decay experiment shows that parity is violated in the weak interaction. However, my lecture notes also say that 'Note that the outcomes of this experiment can also be ...
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Adjusting the rate of proton decay in the standard $\rm SU(5)$ grand unified theory

The proton decay rate in the standard $SU(5)$ grand unified theory is given by $$ \Gamma \sim \left(\frac{g^2}{M_x^2}\right)^2 m_p^5 =\frac{g^4}{M_x^4}m_p^5 $$ Naively we could push up the bound for ...
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4 votes
1 answer
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What is the highest-energy event "naturally" happening in the current state of the Universe?

I know the question sounds a bit broad, but I will specify it a bit more. I also don't think there is a right answer necessarily, I am just interested in the scales of different processes happening ...
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How solitons are related to particle physics?

Recently, I read a paper about introduction to solitons. Author said that the solutions of sine-Gordon equation can be candidate for modeling elementary particles and there are some applications in ...
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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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Does the background shift affects the renormalization group equations?

In Section 21 of "Quantum Field theory" by Mark Srednicki, it is shown that there are two equivalent ways to get the quantum action of the shifted field $\phi'= \phi-\tilde{\phi}$, where $\phi$ is the ...
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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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Is the intrinsic parity the only contribution to parity for single-particle systems?

In the book A Introductory Course Of Particle Physics by Palash B. Pal, the author claims the following (on page 167, section 6.4.3) For single-particle systems, intrinsic parity is the only ...
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2 answers
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Is it sensible to talk about protons at the time when the universe was a quark-gluon plasma?

I'm currently studying from "Particle Astrophysics" by Perkins. On page 132 it says: After $kT$ fell below the strong quantum chromodynamics (QCD) scale parameter ∼ $200$ MeV, the remaining ...
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4 votes
1 answer
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How are the dark matter (DM) thermalization rates derived in singlet DM model?

This well-cited paper talks about a minimal renormalizable extension to the Standard Model (SM) to incorporate particle dark matter (DM) into it by adding a real scalar field $S$ which (unlike the ...
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  • 24k
4 votes
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Beta function for non-Abelian gauge theory with both fermionic and bosonic matter fields

Consider a non-Abelian gauge theory with both fermionic $\psi$ and bosonic $\phi$ matter fields in 4D, which may be called a QCD-Higgs model, $$L=-\frac{1}{4 g^2}F_{\mu\nu}F^{\mu\nu}+\bar{\psi}i\gamma^...
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4 votes
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How can I test whether my cloud chamber is supersaturated or not?

Me and my friend are building a diffusion cloud chamber for a school project. In stead of using dry ice or a peltier, we have used the phase changer of a freezer to cool our plate. We were able to ...
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4 votes
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Why are spin 1/2 baryons less massive/energetic than spin 3/2 ones?

I am just wondering why the lower spin state multiplets for both baryons (when looking at $L=0$ and considering only the u d and s quarks) are lower in energy i.e. the 1/2+ multiplet is lower than the ...
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