Particle physics Definition and 537 Threads

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. KH_137

    Schools Help with shortlisting graduate programs in HEP-PH/TH

    I will be applying to graduate programs in theoretical particle physics this winter in the US (for the Fall 2025 intake) and I would appreciate some help with finding appropriate programs/schools. My interests (and experiences) are a weird mix of hep-th and hep-ph, so, I would like to look for a...
  2. AshanTharuka

    Finding a PhD program which is compatible with GPA 3.485

    I am looking for instructions from you to find out a good university to enroll to a PhD program in Astrophysics and particle physics or a similar study. I have completed my 4 year BSc(Hons) Special in Theoretical Physics with a grade of second class upper division in Sri lanka. My specialized...
  3. KH_137

    Admissions Feedback for my grad school SOP for theoretical particle physics

    Hi, I would like some feedback on my draft of a SOP for a grad school application for particle physics. I am mostly targeting places in the US. I am finding it difficult to identify areas I should focus on and areas that are not as important. I read that such a statement should focus on research...
  4. Giuseppino32

    I Does a certainty in the position imply infinite variation in speed?

    I would like to know if this thought makes any sence or if i'm missing something Heisenberg principle states that: ΔxΔρ ≥ ħ/2 ⇒ Δρ ≥ ħ /2Δx If we consider a scenario where we increase the precision of our measurement of position, we have Δx ⇒ 0 the principle implies: Δρ ≥ ħ/2Δx → Δρ ⇒ ∞...
  5. billtodd

    I Are There GUTs Where the Proton Never Decays?

    Are there any GUTs where the proton never decays? One should write such a theory, perhaps me... :oldbiggrin:
  6. S

    B Can energy be stored in a single particle indefinitely?

    Can energy be stored in a single particle without it being lost over time? I mean, photons would be an exampld in principle, but they get redshifted as the universe expands and become less energetic as time goes by We could store that energy in form of kinetic energy for individual...
  7. S

    I Can proton/neutron decay be avoided in some conditions?

    I was wondering whether the decay of neutrons and protons (if they happen to be able to decay, as it is predicted by some GUTs) could be avoided in some cases. Let's begin with neutrons: In principle neutrons have a very short time when they are isolated (around 10 minutes) and they suffer...
  8. VikasRajG

    A How Does the ss¯ Pair Affect the Partial Width Γ2 in Gluon Decays?

    How do we normally calculate the partial width Γi for a decay involving gluons.Consider the following example: in the first example, I know that one can estimate the partial width Γ1 using CKM matrix elements to get "Γ1 is directly proportional to V2cb V2cs ."Now, for the second diagram it is...
  9. zaramahdi

    A Can particles appear and disappear "with" a cause?

    The first thing we need for this is to define what a particle is... It is an object that has specific intrinsic properties and is described by a wave sign How to measure it? This is done by the interaction of the particle to be measured with the measurement system. When measuring, the wave...
  10. imgamedeving

    What is the CKM element for J psi and phi meson

    I am given the ckm matrix and told that B0s decay to Ds- and pi+ with a branching frcation of 3x10-3. I am then asked to find the branching fraction for other decays. I have done this easily enough for decays like B0s to Ds-K+ by using the ckm matrix elements. However the J/psi meson is a charm...
  11. physics32123

    Physics Realistic to switch from atomic to nuclear or high-energy theory?

    Is it realistic to switch from atomic theory to nuclear or high energy theory?
  12. hongseok

    I Why the binding energy per nucleon has specific pattern?

    The binding energy per nucleon for each element has a specific pattern. It increases from hydrogen to iron and then decreases again. What is the fundamental reason why the binding energy per nucleon has this pattern? According to my investigation, the range of action of the strong force is...
  13. C

    Schools Particle Physics Instrumentation Research: which UK university?

    Hello there! Recently, I was awarded a governmental scholarship to study for a PhD at the University of Cambridge. I am interested in particle detectors (development or characterisation) and am okay with analysis work. Is anyone here studying or has studied at the University of Cambridge and...
  14. M

    FLUKA: Why Does Neutron Flux Increase When Passing Through a Moderator?

    I have been banging my head against this for a few weeks now; tweaking the simulation, adjusting my calculations, and searching around the web to try to find an answer. I can't post questions on CERN's Fluka help forum so this seemed like the best option. I really appreciate any help you can...
  15. 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...
  16. 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...
  17. 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
  18. 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 muon...
  19. deuteron

    I What are resonances in quantum field theory?

    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...
  20. 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...
  21. 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?
  22. 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...
  23. 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...
  24. 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...
  25. 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...
  26. 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...
  27. D

    I Compton scattering amplitude and propagator

    Hi everyone How can i prove that the propagator is And then
  28. 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...
  29. 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...
  30. 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
  31. 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...
  32. 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?
  33. 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...
  34. 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
  35. 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...
  36. 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...
  37. 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...
  38. 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 ...
  39. 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...
  40. 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...
  41. orochi

    A The Symmetry of Antiparticle Isospin Doublets in Particle Physics

    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...
  42. thedubdude

    What can a retired electrical engineer learn about physics at 69 years old?

    I join as a 69 year old retired electrical engineer who is interested in physics. I have particular interest in particle physics and quantum mechanics. I don't expect to provide answers on this forum, but I do intend to ask questions.
  43. G

    Programs What can I do with a Chemistry BS and a minor in Physics?

    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...
  44. 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...
  45. 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...
  46. 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]
  47. 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
  48. 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...
  49. K

    I What Software Is Used for Creating Plots in Academic Papers?

    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...
  50. 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...
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