# Particle physics Definition and 199 Discussions

Particle physics (also known as high energy physics) is a branch of physics that studies the nature of the particles that constitute matter and radiation. Although the word particle can refer to various types of very small objects (e.g. protons, gas particles, or even household dust), particle physics usually investigates the irreducibly smallest detectable particles and the fundamental interactions necessary to explain their behaviour.
In current understanding, these elementary particles are excitations of the quantum fields that also govern their interactions. The currently dominant theory explaining these fundamental particles and fields, along with their dynamics, is called the Standard Model. Thus, modern particle physics generally investigates the Standard Model and its various possible extensions, e.g. to the newest "known" particle, the Higgs boson, or even to the oldest known force field, gravity.

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1. ### A Vacuum Transitions and Lorentz Symmetry Breaking

There are several "bumblebee" models [1], [2] where Lorentz invariance is violated usually resulting from a local vector or tensor field acquiring a nonzero vacuum expectation value We do not know whether we are in the true vacuum state or in a "false"/metastable vacuum state that could decay...
2. ### Quantum machine learning in particle physics: Signal efficiency and background rejection

Hello forum, I am reading this article on quantum machine learning. At one point in the article (page 7) they plot the ROC curve as background rejection vs. signal efficiency. Researching these concepts (since I did not understand them fully), I read that ROC curves should be plotted as TPR...
3. ### A HQET Feynman rules

Several texts state the vertex HQET Feynman rule as : igTijvμ\frac{1}{1+\slashed{v}} with the reasoning for vμ presented as being due to the QCD interaction vertex being between two heavy quark propagators, which are \frac{1+\slashed{v}}{2v.k}, giving ...
4. ### A The Factorization Theorem in Particle Physics

I have been tasked with calculating amplitudes of a B meson decaying to a photon and lepton/lepton anti-neutrino pair ,upto one loop and have pretty much never seen this thing before. I will ask my questions along the way as I describe what I am doing. This factorization theorem seems to go thus...
5. ### A Isospin for Antiparticles

In Quarks & Leptons: An Introductory Course in Modern Particle Physics by Halzen and Martin page 42 reads: I do not understand what the issue is, however. What do they mean by "we want... to transform in exactly the same way"? Didn't they just show that they do transform in exactly the...
6. ### Programs MSE --> Chemistry (BS)-->?

Just to keep this from being a long post: I went to a local CSU for Materials Science and Engineering in the fall of 2018 after graduating high school in the spring of 2018. I really like learning about everything that makes up the world around me, and why things are the way they are, and in...
7. ### I What would a hypothetical quark-quark collision yield?

As seen in the summary, my question is purely hypothetical and I understand that it would most likely be impossible to happen (or I just haven't read enough). The concept that quarks and leptons are the fundamental particles of the universe has existed for a while now - therefore we know that...
8. ### Momentum transfer in electron-proton collision

In a head-on collision between the proton and electron, what is the squared 4-momentum transfer between the two particles. Starting with the difference in momentum of the electron with the 4-vectors before and after the event: $$(P-P')^2=P^2+P'^2-2P\cdot P'$$ The circumstances are such that the...
9. ### A Some basic questions about effective field theory (EFT)

I had been studying this topic from various textbooks and review articles and I am confused about some terminologies and concepts. The question I have typed in this PDF. [Mentor Note -- Added screenshot of PDF contents]
10. ### A Why does D(1,1) representation of SU(3) give baryon octet?

The question may be ambiguous but it's really simple. One says that the baryon octet is the D(1,1) representation of SU(3), but then uses the same one for mesons. D(1,1) means one quark and one antiquark, which corresponds perfectly to mesons. But how can it explain baryons? My information and...
11. ### I Question about how the plots are made in particle physics books and articles

Usually in papers there are many plots, and sometimes I do not understand how they plot them, with which kind of software or program they are plotted. I just attached three of the plots, I would be very thankful if you guide me, any of them is plotted with using which method, software or...

36. ### Physics Leaving Academia While Still Contributing to Physics Research

There is a Theoretical Physicist by the name of Garret Lisi. He gave a ted Talk on one of his unified physics theories that received a lot of attention. The reason why I am bringing him up is because he left academia after he finished his phd and moved to Maui. During his time there he found...
37. ### A Average transverse momentum as a function of the longitudinal momentum

Hello everybody! I am working on a code in which I need to study the dependence of ##<p_T>## vs ##p_L## (the average transverse momentum and the longitudinal momentum of a particle). I am looking for references, papers, books, etc. concerning this topic, but I have not been so lucky. My...
38. ### A Madame Wu Experiment

Hi all, I have some doubts regarding the experiment of Madame Wu. I know a strong magnetic field is used to polarise the ##^{60}Co## nuclei, then we have an anthracene scintillator on the top of the sample which will detect the electron produced in the decay: ##^{60}Co \rightarrow...
39. ### Proton structure

Hi all. I'd personally consider the De Broglie wavelength λ=h/p, with p being the momentum of the electron beam. I get $$\lambda \simeq 0.6 \times 10^{-18} m$$ and since the radius of a quark is ## \leq 10^{-19} m ##, the proton structure can't be resolved. I'm quite sure there's something...

43. ### Energy of Two Gamma Rays Produced by Pion Decay

So I worked out the first part and obtained ##E_1 = 478.8MeV##, ##E_2 = 459.4MeV## and ##p = 0.49 MeV/c## but I can't quite wrap my head around the second part. Normally, I'd use the equation for s but I'm confused since I don't know the angle between the gamma rays.

Hi all, I am in a bit of a funny situation where I need to pick up at least a cursory knowledge of QFT and particle physics in the space of two weeks. I borrowed "QFT and the Standard Model" by Schwartz but I have no idea how I should approach it. Ideally I'd pour through every page, but I...
45. ### How to find the decay rate of processes: a→ b+c

As mentioned above I'm unable to begin because I don't know the relevant equations or method.
46. ### Can we have a proton and an antiproton in the 1^S_0 atomic orbital

I have a belief from something I read on the internet that the superscript 1 means that this is a singlet state so it can only hold one particle but then say 3^S_0 is a triplet so it could hold three. Then the first state I mentioned couldn't have a particle and an antiparticle in it , but the...
47. ### Initial states ppbar can proceed to npi^0 with parity conserved

I'm working on some stuff for particle physics and I had a few questions I wanted to ask . Heres the outline of the problem : Establish which initial states of the ppbar system amongst 1^S_0, 3^S_1, 1^P_1, 3^P_0, 3^P_1, 3^P_2, 1^D_2, 3^D_1, 3^D_2, 3^D_3 the reaction ppbar->npi^0 can...
48. ### How to determine particle energies in center of momentum frame?

That said, my approach was to determine the energies and 3-momenta at the center of momentum reference frame for each particle, with a fixed s, and check it corresponds to each one of the above, but I'm having some trouble proving that, for example, E_A=\frac{s+m^2_A-m^2_B}{2\sqrt{s}}. I've...
49. ### I Particle-in-a-box problem

It is interesting to consider why sufficiently slow work does not tend to increase entropy. We can model atoms as a collection of quantum particle-in-a-box’s; Compression work will tend to squeeze the size of these quantum boxes, reducing L. Now, suppose instead that you left this quantum...
50. ### Minimum energy positron electron collision producing Z-boson

Homework Statement Homework Equations m2 = E2 - p2 ([/B] The Attempt at a Solution I got stuck by not knowing how to handle the frames and I thought you can view the Z boson (CoM) frame, but then you simply get that mz = 2me