What is Particle physics: Definition and 516 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. cemtu

    I K-Shell, Low-Energetic Photons & Photoelectric effect

    At low photon energies, the probability of the photoelectric effect to occur increases, but the probability of the photoelectric effect to happen also increases when going towards most inner shells like K shell but inner shells require much more photon energies to be broken, so isn't there a...
  2. Rayan

    Determining Form factor from density distribution

    So my first thought was that I can just use Fourier trick and integrate: $$ F(q^2) = \int_V \rho(r) \cdot e^{ i \frac{ \vec{q} \cdot \vec{r} }{h} } d^3r $$ $$ F(q^2) = 2\pi \rho_0 \int_0^{\infty} r^2 \cdot e^\frac{-r}{R} dr \cdot \int_0^{\pi} \sin{\theta} \cdot e^{ -i \frac{q \cdot r...
  3. cemtu

    I Thomson Scattering when low-intensity light meets an orbital electron

    Can you explain to me the reason why Thomson Scattering can not explain what happens when light meets an electron at low intensity, and what does that have to do with light being a wave or particle or relativistic/QM effects? https://en.wikipedia.org/wiki/Compton_scattering
  4. Rayan

    Momentum in a Head-on collision

    An electron and muon collide head-on, with energies 35 GeV and 50 GeV, the following reaction takes place: $$ e^- + \mu^+ \rightarrow \nu_e + \nu_{\vec{\mu}} $$ If the electron neutrino has energy of 25 GeV, and collides at angle 20 with respect to incoming electron direction, what is the...
  5. deuteron

    I What exactly are resonances?

    I am currently learning about the coalescence model and femtoscopy, and am very confused about what resonances are. I read here (https://www.quantamagazine.org/how-the-physics-of-resonance-shapes-reality-20220126/?utm_source=pocket_saves) that they are, as all particles, an excitation of the...
  6. S

    I All possible models to explain the hierarchy problem?

    There is an interesting paper by Arkani-Hamed and collaborators (https://arxiv.org/abs/1607.06821) to address the hierarchy problem. There, they consider many possible models of fundamental particle physics where they all have an exact copy of the Standard Model but with different Higgs...
  7. cemtu

    I How does a photon interact with the EM field of a nucleus?

    How does a photon interact with EM field of a nucleus thus exchange momentum and recoil the nucleus when pair production happens?
  8. Frigorifico9

    I Who was the first to make a Stern-Gerlach experiment with two magnets?

    I'm trying to fill a conceptual gap I have in the history of physics In 1922 Stern and Gerlach make their experiment, proving that electrons have intrinsic angular momentum, however it takes a while for people to understand this. At first they think this is somehow caused by quantization of...
  9. cemtu

    I Why is bremsstrahlung radiation greater than acceleration radiation?

    Why is breaking radiation stronger than accelerating radiation? Why is it that when an electron comes accelerating toward a nucleus radiate weaker than when it goes decelerating away from the nucleus? Is it because when it decelerates, at the same time it changes direction? Or is it because it...
  10. F

    Programs Postdoc in Particle Physics with a PhD in Nanoscience Nanotechnology

    I already have a PhD in Nanoscience and Nanotechnology and I will start an M.S. in Physics at SJSU next Monday (August 21 2023). I am interested in Particle Physics. A professor at my institution working in the field told me the following "For you, since you already have a PhD, I don’t think it...
  11. cemtu

    A Thomson - Rayleigh Scattering: Rotational & Vibrational Energy Levels

    We know from molecular spectroscopy that incoming light on a molecule can change a molecule's rotational, vibrational and electronic energy levels. If the incoming light is, on the far-infrared and microwave region the molecule gets rotational energy.(microwave spectra) on the near-infrared...
  12. cemtu

    A Why KE of Annihilation Electrons Differs?

    We know that when a high energy gamma ray(E >= 1022 keV because the total energy of 1 electron at rest and 1 positron at rest is 511 keV) passes near a high Z(atomic weight) atomic nucleus interacts with the electrical field of the nucleus and there is a probability that this high energetic...
  13. D

    I Compton scattering amplitude and propagator

    Hi everyone How can i prove that the propagator is And then
  14. N

    Feynman diagram for ##\mu^+\mu^-## production in ##p\bar{p}## reaction

    I was able to solve b) but I am confused for a). I understand that in the proton-antiproton collision, only two quarks (one from proton and other from anti-proton) can be combined to get a virtual photon that in turn creates muon and anti-muon. I don't understand what would happen to the other...
  15. O

    I Question on Baryons and Specific Decays

    1) We know in case of beta decay if there is excess of protons or neutrons in the nucleus beta decay take place. Force involved - weak interaction Reason involved - to stabilize, to obtain proton neuton 1:1 ratio in the nucleus. E. g., Beta minus - Carbon 14 transforms into Nitrogen 14 2) Case...
  16. D

    I Equation which is related with the Lorentz invariant quantities

    Hi every one.How can i prove the below equation? And then that it's Lorentz invariant quantitude ? Thanks for your help
  17. rogdal

    Isospin and Partons model in a pentaquark

    TL;DR Summary: Given a pentaquark: (a) Determine the isospin multiplet it belongs to. (b) Calculate a kind of a Gottfried Sum Rule for this pentaquark-neutrino or -antineutrino scattering. Hello everybody, I'm having a bit of a trouble with the exercise below as it deals with a pentaquark and...
  18. M

    I want to learn particle physics

    How did you find PF?: Google Search I am in 8th grade and 10 hours a day free time and i am highly motivated to learn some particle physics. What should i start with?
  19. Someon332

    Schools What are some good science fair project ideas or resources?

    I am currently a sophomore in HS, and I have a science fair project next year. I want to do something interesting- I don't want to put in barely any effort and do something that everyone has done, like 'growing plants in different light levels'. I would ideally like to do something with...
  20. malawi_glenn

    High Energy (free kindle) Gauge Theories in Particle Physics by Aitchison & Hey

    Free Kindle via Amazon, both part 1 and part 2 Part 1: https://www.amazon.com/dp/B00OD49Z96/?tag=pfamazon01-20 Part 2: https://www.amazon.com/dp/B00OD49Z00/?tag=pfamazon01-20 Enjoy
  21. S

    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...
  22. D

    ROC Curve: Background Rejection vs. Signal Efficiency

    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...
  23. Jamestein Newton

    Particle Better Textbooks than Griffiths for Particle Physics 101

    Griffiths "introduction to elementary particles" serves a great introduction as a first course on particle physics for undergraduate students. But what are the possible alternatives of this textbook? Like being more beneficial in the long run, more time-consuming, for aspired students. But it...
  24. E

    A HQET Feynman Rule: QCD Interaction Vertex Explained

    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 ...
  25. E

    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...
  26. S

    Looking for a book with an Overview of Nuclear and Particle Physics

    Hi, I am doing my PHD in Nuclear/Particle Physics and I am getting all of the information I am using from papers, which are very specific. I feel like I am lacking general knowledge, which is likely to be tested in the defense. Do you know any book where I can get a better overview on Nuclear...
  27. orochi

    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...
  28. G

    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...
  29. Auxirius

    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...
  30. TheMercury79

    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...
  31. E

    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]
  32. S

    I Why does particle physics predict cosmo constant is 10^120 too big?

    I was reading this article at Wikipedia that says particle physics predicts that the cosmological constant is 10^120 larger than per observation: https://en.wikipedia.org/wiki/Anthropic_principle
  33. joneall

    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...
  34. K

    I Plotting Data in Papers: A Guide

    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...
  35. RicardoMP

    A How to reduce an integral in phase space to a one-dimensional form?

    I've been trying for a very long time to show that the following integral: $$ I_D=2{\displaystyle \int} \, {\displaystyle \prod_{i=1}^3} d \Pi_i \, (2\pi )^4\delta^4(p_H-p_L-p_R) |{\cal M}({e_L}^c e_R \leftrightarrow h^*)|^2 f_{L}^0f_{R}^0(1+f_{H}^0). $$ can be reduced to one dimension: $$ I_D...
  36. debesta

    Job Skills Nuclear physics vs particle physics job prospects

    Which experimental physics branch has better job prospects (both inside and outside academia) - particle physics or nuclear physics? Is the difference very big?
  37. Mikkel

    Ultra high energy cosmic ray deflection angle causes by magnetic field

    I'm given an ultra-high energy cosmic ray with energy 10^20 eV. It is coming from a source 10 Mpc away with an extragalactic magnetic field with strength B = 10^-9 G. I am to determine the maximum angular deflection of this cosmic ray, so it hits Earth. I don't have an attempt of the solution...
  38. L

    I C-parity π+ π- π0: Is (-1)^L (+1) Correct?

    The C-parity of π+ π- alone is simply (-1)^L, where L is the angular momentum quantum number for the system. But then what is C-parity of π+ π- π0? Is it simply (-1)^L (+1), where L is the angular momentum quantum number for the π+ π- subsystem (which isn't necessarily the angular momentum of...
  39. JD_PM

    High Energy Books on the Standard Model of elementary particle physics & beyond

    Hello! I am taking a course on Electroweak & Strong Interactions (you could equally call it Standard Model I) and I find it absolutely fascinating! 😍 We studied how weak interactions violate parity, introduction to QCD, flavor physics (CKM matrix, CP violation, …) and neutrino physics...
  40. L

    I Physical parameters for spin 1/2 particles

    I am having trouble to understand what it means by "physically relevant real parameters" and how does it help us to specify a quantum system. Let say, we have a state of k half spin electrons? My guess is about the local phase of the spin, and this would make it 2^k parameters since each...
  41. L

    I Books about Particle Physics and some clarifications

    Hi hi, I would like clarify this, I'm looking for models that can support to a particles have several properties, I know classic physics, my main problem, if we have a fluid with movement forces and heat, the classic model don't support this, only independent from each other... So, a lot of ppl...
  42. Y

    Physical Valentine's Day -- quotes from particle physics magazine

    Here are the quotes https://www.symmetrymagazine.org/article/show-your-affection-with-physics-valentines , I find them funny except for the last one is hurtful, I shouldn't say such a thing for someone on a valentine's day because they would care about the valentine's day, personally I find it...
  43. R

    I Video lectures in Nuclear and Particle physics? (Undergraduate)

    Hi everyone, I was hoping the internet would be filled with video lectures since lots of universities have been forced to conduct online teaching. However, that doesn't seem to be the case. Therefore I was hoping that some of you know of some great video lectures for nuclear and particle...
  44. J

    Water analogy in Particle Physics

    There is something unusual about water. Without atmosphere, water can't exist. It's as if water is part of atmosphere. In vacuum, liquid water can't exist. What other things like water where it needs other aspects like atmosphere to exist? And what is the analogy of water in particle physics...
  45. John Greger

    A What are some example Feynman diagrams in Yang-Mills theory?

    Hi! So I have just been studying Yang-Mills theory advanced quantum field theory. In chapter 72 of Srednicki's book Quantum Field Theory they list the Feynman rules for non-abelian gauge theory. I was asked if I could show some sample allowed diagrams but I could not.. In standard particle...
  46. DanielR1

    I Confused on the premise behind De Broglie's hypothesis

    I'm confused on how De Broglie's hypothesis works. I've attached my thinking in a pdf file below. I'm not an expert in the field of particle physics(I'm only in high school), so there may be some error in my logic(I really think so, or else De Broglie was wrong!(hint:he isn't)) I've also read...
  47. Adams2020

    Mass difference due to electrical potential energy

    I do not really know the relationship between potential energy and mass difference. Isn't the difference in mass of protons and neutrons due to their quarks? (the neutron is made of two down quarks and an up quark and the proton of two up quarks and a down quark.) Please help.
  48. Adams2020

    I Loss of electron & proton energy due to radiation

    Can you compare the energy loss of electrons and protons due to the radiation they emit? In fact, I want to know which of the two loses more energy when it emits radiation.
  49. S

    A High energy symmetry breaking and laws of physics?

    In some models of the beginning of the universe, like for example in chaotic inflation, space would stop expanding in some points, creating Hubble volumes that could experience different spontaneous symmetry breaking, which would result in different properties, such as different physical...
  50. mjmnr3

    Why does a symmetric wavefunction imply the angular momentum is even?

    I looked in the instructor solutions, which are given by: But I don't quite understand the solution, so I hope you can help me understand it. First. Why do we even know we are working with wavefunctions with the quantum numbers n,l,m? Don't we only get these quantum numbers if the particles...