What is Pion: Definition and 127 Discussions

In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: π) is any of three subatomic particles: π0, π+, and π−. Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more generally, the lightest hadrons. They are unstable, with the charged pions π+ and π− decaying after a mean lifetime of 26.033 nanoseconds (2.6033×10−8 seconds), and the neutral pion π0 decaying after a much shorter lifetime of 85 attoseconds (8.5×10−17 seconds). Charged pions most often decay into muons and muon neutrinos, while neutral pions generally decay into gamma rays.
The exchange of virtual pions, along with vector, rho and omega mesons, provides an explanation for the residual strong force between nucleons. Pions are not produced in radioactive decay, but commonly are in high-energy collisions between hadrons. Pions also result from some matter–antimatter annihilation events. All types of pions are also produced in natural processes when high-energy cosmic-ray protons and other hadronic cosmic-ray components interact with matter in Earth's atmosphere. In 2013, the detection of characteristic gamma rays originating from the decay of neutral pions in two supernova remnants has shown that pions are produced copiously after supernovas, most probably in conjunction with production of high-energy protons that are detected on Earth as cosmic rays.The pion also plays a crucial role in cosmology, by imposing an upper limit on the energies of cosmic rays surviving collisions with the cosmic microwave background, through the Greisen–Zatsepin–Kuzmin limit.

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  1. James1238765

    I Is the superposition of mesons (such as pion) a new particle?

    There are 36 hadron composites composed of 2 quarks selectable from the set ##[u, d, c, s, t, b, \bar u, \bar d, \bar c, \bar s, \bar t, \bar b]## satisfying the condition of having total charge = ##[-1, 0, 1]##. However, the superposition states of pure hadrons are sometimes also listed as new...
  2. Y Y Kim

    I What is the meaning of 'CP violation' in the Cronin and Fitch experiment?

    Below are the statements in the "David Griffiths, Introduction to Elementary Particles 2nd Edition, 147~148 p.": Evidently, the long-lived neutral kaon is not a perfect eigenstate of CP after all, but contains a small admixture of K1. The coefficient epsilon is a measure of nature's departure...
  3. Mr_Allod

    Probability of finding a pion in a small volume of pionic hydrogen

    Hello, I am trying to figure out the right way to approach this. First of all, other than the different Bohr radius value, does the change to a negative pion make any other difference to calculating the probability? Also what would be the correct way to apply the "small volume"? What I'm...
  4. JD_PM

    I Pion Decay Rate: Verifying Decay Rate Formula

    I am studying the following process: pion decays (mediated by the charged ##W## boson) into a muon and anti-muon neutrino (i.e. ##\pi^- \to \mu + \bar{\nu}_{\mu}##). The Feynman diagram associated to it is And its Feynman amplitude is (where we of course neglected...
  5. Physicsphysics

    Pion annihilation Feynman diagram

    π+ + π- → γ + γ How do you represent this in a Feynman diagram showing the individual quark? I am very confused please help! [Moderator's note: Moved from a technical forum and thus no template. Own effort below.]
  6. TheBigDig

    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.
  7. Q

    B Neutral pion quark composition help

    Hi guys, Merry Christmas to you all! I wanted to know whether a neutral pion can be made up of a strange quark and an anti-strange quark. I know that the kaon is the only strange meson and all variations contain an s quark but wouldn't the strangeness be zero in an s quark/anti-s quark pair as...
  8. T

    Axial Transformation of a pion

    Homework Statement I'm considering a non-linear chiral theory where the Lagrangian is in terms of the field #\Sigma = e^{\frac{2i\pi}{f}}# where #\pi# is my pion matrix containing pion, kaon, and #\eta#. I need to calculate the transformation of #\pi# up to order #\pi^2# under an axial...
  9. S

    I Time period in which an interaction with a π-meson happens?

    The longer-range inter-nucleon force is not a direct result of the gluon field, but is indirectly affected by a quark-antiquark pair (meson). If this ‘residual’ strong interaction between nucleons is mediated by a π-meson, then what is the maximum time period in which the interaction takes...
  10. Cocoleia

    Lambda decay, momentum of the pion and proton

    Homework Statement I have a lambda decaying into a pion and a proton. The lambda is moving with velocity 0.9c and I know the mass of the lambda as well as the pion and proton (these are known constants). I need to find the momentum of the pion and the proton after the decay happens. Homework...
  11. Gene Naden

    I Pion decaying to two neutrons demonstrates odd parity

    Does anyone have a reference to a good explanation of this experiment. I am looking at https://quantummechanics.ucsd.edu/ph130a/130_notes/node323.html I am unable to comprehend the reasoning by which it determines the parity of the two neutrons in the final state. Particularly when it says the...
  12. M

    Why do 2 d not decay into alpha particle and neutral pion?

    Homework Statement The reaction dd → απ0 (where α is the Helium-4 nucleus and d denotes the deuteron) has never been observed. Why? Homework EquationsThe Attempt at a Solution [/B] So I checked first the masses to make sure energy/momentum are conserved and found out that mass of 2d is...
  13. A

    I Pion+/- decay - what *Exactly* is happening?

    I have seen vague explanations and (to me) misleading diagrams, and I am still not completely sure what is supposed to be happening in pion+/- decay. This Feynman diagram is a starting point. https://en.wikipedia.org/wiki/Pion#/media/File:PiPlus_muon_decay.svg What is happening? What happens to...
  14. L

    Are pions produced in pairs?

    I thought only strange particles were produced in pairs via the strong interaction, however in my textbook it states 'kaons, like pions are produced in pairs via the strong interaction'. Can particles that are not strange also be produced in this way?
  15. M

    Issue calculating the velocity of pions from K-meson decay

    Homework Statement We were supplied with several images of hydrogen bubble chamber tracks, from which we measured the length of the (invisible) K0 track, angles of departure from the K0 meson's track and curvature. As a way to limit the amount of data we're talking about, I'll only discuss the...
  16. H

    Relativity: Solving Pion Homework Problem

    Homework Statement After being produced in a collision between elementary particles, a positive pion (π+) must travel down a 1.00 km -long tube to reach an experimental area. A π+ particle has an average lifetime (measured in its rest frame) of 2.60×10−8s; the π+ we are considering has this...
  17. S

    I Matrix Elements via Feynman Diagrams

    Hello everyone, I am currently trying to understand how we can use feynman diagrams to estimate the matrix element of a process to be used in fermi's golden rule so that we can estimate decay rates. I am trying to learn by going through solved examples, but I am struggling to follow the logic...
  18. A

    What should the speed of a pion be

    Homework Statement The speed of a particle in Earth's frame is 0.4 c. A second particle goes away from the first one with speed 0.6 c. What is the speed of the second particle in Earth's frame? Homework Equations u=(u'+v)/(1+u'v/c^2) The Attempt at a Solution I think v=0.4 c and u'=-0.6, but...
  19. B

    Neutral Pion Mass from Its Decay into Two Photons

    Homework Statement A ##\pi^0## meson decays predominantly to two photons. Suppose the energies (E1, E2) and angle (##\theta##) between the emitted photons are measured. Find an expression for the ##\pi^0## mass in terms of E1, E2, and ##\theta##. Homework EquationsThe Attempt at a Solution...
  20. A

    Pion Photoproduction Homework: Proton Energy Calculation

    I have been given the following problem on a homework sheet; "The CMB has an (almost) uniform temperature of (almost) 3K. If we take the energy of a photon to be E=3kT, what minimum energy must a proton have in order to interact with a CMB photon in order to produce neutral Pi Mesons?"...
  21. khelzor

    I Pion Detection: Finding a Film for Interference Experiment

    I'm a student in high school asking for suggestions for an experiment proposal. So far, our idea has been to recreate Young's interference experiment, but instead of using electrons, we will use a heavier particle. Charged pions have been the first choice so far. Aluminum is the current delegate...
  22. A

    I Dimensions of the pion decay constant

    The pion decay constant f_{\pi} is defined by <0|\overline{d}\gamma^{0}\gamma^{5}u|\pi^{+}>=f_{\pi}m_{\pi} where I have set the momentum \boldsymbol{p}=0 (and used the temporal component of the axial vector). Now, at the right-hand-side the dimension is two (the decay constant and the mass are...
  23. J

    I How Is Spin and Parity Determined in Pion Decay?

    Hi, I'm recently reading Krane's nuclear physics textbook, and in the meson physics chapter there is a section about the spin and parity of pions. He demonstrated a way to find out the parity of the pion by investigating a pion decay(as in the attached images). I think I understand what he's...
  24. S

    B Neutral Pion Decay: Exploring Spin Conservation

    Hi everyone, I am curious about how a neutral pion decays in two photons. I don't understand how it is possible since the pion has spin = 1 and the photons have spin = 1 as well: doesn't this mean that spin is not conserved in this decay? What am I missing? Another question I have is about...
  25. P

    Momentum density of states for pion decay.

    Homework Statement I am trying to calculate the ratio of the density of states factor, ##\rho(p)##, for the two decays: $$\pi^+\rightarrow e^++\nu_e~~$$ and $$\pi^+\rightarrow \mu^++\nu_{\mu}~~$$ Homework Equations ##\rho(p)~dp=\frac{V}{(2\pi\hbar)^3}p^2~dp~d\Omega## Which is the number...
  26. P

    Feynman diagram; ##\pi^0+\pi^0\rightarrow \pi^++\pi^-##

    Homework Statement For the reaction below draw three Feynman diagrams, one that proceeds through exchange of a gluon, one through a photon and one exchanging a weak W-Boson. ##\pi^0+\pi^0\rightarrow \pi^++\pi^-## Which diagram provides the dominant contribution for this reaction? Explain how...
  27. Dilatino

    Confused about isospin conservation in rho to pion decays

    The decay \rho^0 \rightarrow \pi^- \pi^+ occurs with a probability of 100\%, whereas the decay \rho^0 \rightarrow \pi^0 \pi^0 does not occur in nature, due to isosphin conservation. I don't understand this. Looking at the Isospin and its third component ¦I,I_3\rangle we have in the first...
  28. RyanH42

    New Theory: Dark Matter Acts Like Pion - Evidence Found

    I was searching Dark Matter and I found this site is that can be true ? Dark matter can be a act like a pion ? http://right.is/space/2015/07/deja-vu-new-theory-says-dark-matter-acts-like-well-known-particle-2117.html
  29. B

    How Does Special Relativity Affect Pion Decay Measurements?

    Homework Statement A beam of pion mesons travels down a tube at the CERN accelerator moving at v/c=0.95 with respect to the laboratory. (i) Compute the γ factor for these pions. (ii) The pion lifetime is 2.6 × 10−8 s, what is the lifetime measured at the laboratory? (iii) The pions travel 50...
  30. tg85

    K-Long and K-Short pion decay cross sections

    In the text-book example of CP violation in the kaon system, the usual statement is that the KS decays much faster than the KL because the phase space is larger for a decay into two pions, compared to three pions. (See, for example, Griffiths or "Modern Particle Physics" by Thomson.) I was...
  31. T

    Left-Handed Pion: Direction of Flight vs Spin

    Can you have a left handed pion? i.e. it's spin is opposite to its direction of flight
  32. M

    Energy of neutral pion in proton's rest mass frame

    Hello, my problem is as follows I've tried finding the invariant mass of the positron and pion as follows M^2=(E_e+E_{\pi})^2-(\mathbf{p_e}+\mathbf{p_{\pi}})\\ =E_e^2+E_{\pi}^2+2E_eE_{\pi}-p_e^2-p_{\pi}^2-2\mathbf{p_ep_{\pi}}\\ =m_e^2+m_{\pi}^2-2(E_eE_{\pi}+\mathbf{p_ep_{\pi}})\\...
  33. O

    Pion Decay into Tau and Tau Neutrino: Energy Considerations

    This may be a silly question, but what prevents the decay of a Pion into a (anti) Tau and (anti)Tau neutrino. I can see the process for the electron but not for the tau.
  34. J

    Maximum kinetic energy of a Pion produced via a kaon decay

    Homework Statement A possible decay mode a of a positive Kaon in the production of three Pions as shown below:. K+ → π+ + π+ + π− Whats is the maximum Kinetic energy that anyone of the pions can have? The kaon is a rest when...
  35. ORF

    Mesonic effects in the nuclear structure

    Hello The textbooks about nuclear physics I have read don't explain too much about the Yukawa's model. For example, the textbook Structure of the Nucleus -by Preston and Bhaduri- finish the Yukawa's model with open questions about the "mesonic effects" by virtual pions. And my doubt: are the...
  36. R

    Decay of a proton into a positron and a neutral pion

    p -> e+ + pi° is >10^33 years. what conservation law does this limit test? p is uud pi° is uu or dd where the second quark of each is an antiquark baryon number is not conserved 1 = 0 + 0 charge is conserved lepton number is not conserved 0 = -1 + 0 not sure what this...
  37. H

    How Fast Must a Pion Travel to Cover 9 Meters Before Decaying?

    How fast must a pion be moving, on average, to travel 9.0 m before it decays? The average lifetime, at rest, is 2.60 ✕ 10-8 s. (Answer in terms of c) I'm not exactly sure what equations I have to use but I believe it relates to time dilation. I originally didn't realize it was a time...
  38. C

    Pion Decay into Gamma rays angle

    So i have a question regarding a homework question I'm working on which suggest a neutral pion traveling with velocity v decays into two gamma rays of equal theta to the normal and they of course have velocity c. It then asks to prove that v= cos theta Which kind of confuses me. I mean if...
  39. R

    How to determine the decay mode probability of a pion

    Hi, Given the initial energy of a pion is it possible to calculate the probability and mode of its decay? Ive looked everywhere for a relevant formula but everywhere simply states the probability and mode... Wiki "The primary decay mode of a pion, with probability 0.999877, is a purely...
  40. R

    Is it possible to calculate the rest mass of a pion given only its E ?

    The TOTAL energy of a pion is given, no other information is given so we don't know how fast it is moving, is it possible to calculate its mass? I'm assuming it's relativistic and we need GAMMA (the lorentz factor) Also if E is it's total energy what would it's kinetic energy be? wouldn't its...
  41. R

    What is the energy of photons in pion decay at rest?

    Apologies, this is going to be a bit disjointed, I don't want to write the full question down as I don't want anyone to give me a solution as its an assignment question. 1. am i correct in assuming when a pion decays at rest its energy will be given by: E^2 = p^2 c^2 + m^2 c^4 which...
  42. T

    Particle Physics - decay of a neutral pion

    Particle Physics -- decay of a neutral pion Homework Statement Consider the decay of a neutral pion that has a momentum of 10 GeV/c into two photons. What is the minimum energy that a photon from this decay can have? In terms of the pion mass and pion momentum. What about Maximum Energy...
  43. L

    Solving for Momentum in Pion Decay

    Homework Statement We are the given the rare decay: π+ → e+ + ve Also, we are given mπ = 139.6 MeV/c2, mv ≈ 0, me+ = 0.511 MeV/c2 And we are given that the decay starts with the pion at rest, so I took that to mean that pπ = 0 Solve for the momentum of the positron. Homework Equations...
  44. G

    Is the W boson in this decay a vector boson based on parity nonconservation?

    Homework Statement "(a) Draw a quark-flow diagram for the weak decay \pi^{-}\rightarrow\mu^{-}+\overline{\nu_{\mu}} . Explicitly include the appropriate intermediate vector boson. (b) By considering the production of \mu^{-} and \overline{\nu_{\mu}} in the rest frame of the vector boson...
  45. G

    Why is the production of pions in D-D fusion not allowed?

    Homework Statement "Assuming that the incident deuteron has sufficient energy, why is the reaction d+d \rightarrow He^{4}_{2}+\pi^{0} not allowed?" Homework Equations The Attempt at a Solution I've gone through the various conservation laws that apply in a strong reaction...
  46. S

    Decay of pion on electron and antineutron. Energy of electron?

    Homework Statement Negative pion with kinetic energy 100 Mev decays to electron and antineutron. What is the kinetic energy of electron , which will move in the same direction as pion did?Homework Equations ##p^{\mu }=(\frac{E}{c},\vec{p})## ##E=T+mc^2=\sqrt{p^2c^2+m^2c^4}##The Attempt at a...
  47. websterling

    What is the meaning behind the quark content of the neutral pion?

    The quark content of the neutral pion is listed as \frac{u\overline{u}-d\overline{d}}{\sqrt{2}} What is the proper way to interpret this?
  48. M

    Classical mechanics exercise, pion decay

    Homework Statement If anyone could help me with this classical mechanics exercise I would be very grateful! The exercise is as follows: The muon (μ) is a particle with mass mμ=207me, with me being the electron mass. The pion (∏) has a mass of m∏=273me. The pion can decay into a muon...
  49. B

    Neutral pion decay: JPC conservation

    \pi^0s decay to two photons via the EM interaction. The J^{PC} of the pion is 0^{-+} and of a \gamma is 1^{--}. \gamma\gamma therefore has J^{PC} = 0^{++}, 1^{-+}, 2^{++}. This does not match the pion, so how can this decay occur?
  50. U

    Calculating Energies of Pion - Seeming trouble with constants?

    Homework Statement Calculate the energy for 1S->2P transition in lithium and carbon. We had to calculate the energy and eigenvalue equations (though I verified these are correct) Homework Equations E = mc^2/sqrt(1+(Za/lambda)^2) lambda = n + 1/2 + sqrt((l+1/2)^2- (Za)^2) The...