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Summary

  • All particles of matter have an antimatter counterpart that has the opposite charge and certain other quantum numbers as seen in [link] . These matter-antimatter pairs are otherwise very similar but will annihilate when brought together. Known particles can be divided into three major groups—leptons, hadrons, and carrier particles (gauge bosons).
  • Leptons do not feel the strong nuclear force and are further divided into three groups—electron family designated by electron family number L e size 12{L rSub { size 8{e} } } {} ; muon family designated by muon family number L μ size 12{L rSub { size 8{μ} } } {} ; and tau family designated by tau family number L τ size 12{L rSub { size 8{τ} } } {} . The family numbers are not universally conserved due to neutrino oscillations.
  • Hadrons are particles that feel the strong nuclear force and are divided into baryons, with the baryon family number B size 12{B} {} being conserved, and mesons.

Conceptual questions

Large quantities of antimatter isolated from normal matter should behave exactly like normal matter. An antiatom, for example, composed of positrons, antiprotons, and antineutrons should have the same atomic spectrum as its matter counterpart. Would you be able to tell it is antimatter by its emission of antiphotons? Explain briefly.

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Massless particles are not only neutral, they are chargeless (unlike the neutron). Why is this so?

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Massless particles must travel at the speed of light, while others cannot reach this speed. Why are all massless particles stable? If evidence is found that neutrinos spontaneously decay into other particles, would this imply they have mass?

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When a star erupts in a supernova explosion, huge numbers of electron neutrinos are formed in nuclear reactions. Such neutrinos from the 1987A supernova in the relatively nearby Magellanic Cloud were observed within hours of the initial brightening, indicating they traveled to earth at approximately the speed of light. Explain how this data can be used to set an upper limit on the mass of the neutrino, noting that if the mass is small the neutrinos could travel very close to the speed of light and have a reasonable energy (on the order of MeV).

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Theorists have had spectacular success in predicting previously unknown particles. Considering past theoretical triumphs, why should we bother to perform experiments?

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What lifetime do you expect for an antineutron isolated from normal matter?

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Why does the η 0 size 12{η rSup { size 8{0} } } {} meson have such a short lifetime compared to most other mesons?

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(a) Is a hadron always a baryon?

(b) Is a baryon always a hadron?

(c) Can an unstable baryon decay into a meson, leaving no other baryon?

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Explain how conservation of baryon number is responsible for conservation of total atomic mass (total number of nucleons) in nuclear decay and reactions.

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Problems&Exercises

The π 0 size 12{π rSup { size 8{0} } } {} is its own antiparticle and decays in the following manner: π 0 γ + γ size 12{π rSup { size 8{0} } rightarrow γ+γ} {} . What is the energy of each γ size 12{γ} {} ray if the π 0 size 12{π rSup { size 8{0} } } {} is at rest when it decays?

67.5 MeV

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The primary decay mode for the negative pion is π μ + ν - μ size 12{π rSup { size 8{ - {}} } rightarrow μ rSup { size 8{ - {}} } + { bar {ν}} rSub { size 8{μ} } } {} . What is the energy release in MeV in this decay?

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The mass of a theoretical particle that may be associated with the unification of the electroweak and strong forces is 10 14 GeV/ c 2 size 12{"10" rSup { size 8{"14"} } `"GeV/"c rSup { size 8{2} } } {} .

(a) How many proton masses is this?

(b) How many electron masses is this? (This indicates how extremely relativistic the accelerator would have to be in order to make the particle, and how large the relativistic quantity γ size 12{γ} {} would have to be.)

(a) 1 × 10 14 size 12{1 times "10" rSup { size 8{"14"} } } {}

(b) 2 × 10 17 size 12{1 times "10" rSup { size 8{"17"} } } {}

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The decay mode of the negative muon is μ e + ν - e + ν μ size 12{μ rSup { size 8{ - {}} } rightarrow e rSup { size 8{ - {}} } + { bar {ν}} rSub { size 8{e} } +ν rSub { size 8{μ} } } {} .

(a) Find the energy released in MeV.

(b) Verify that charge and lepton family numbers are conserved.

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The decay mode of the positive tau is τ + μ + + ν μ + ν - τ size 12{τ rSup { size 8{+{}} } rightarrow μ rSup { size 8{+{}} } +ν rSub { size 8{μ} } + { bar {ν}} rSub { size 8{τ} } } {} .

(a) What energy is released?

(b) Verify that charge and lepton family numbers are conserved.

(c) The τ + size 12{τ rSup { size 8{+{}} } } {} is the antiparticle of the τ size 12{τ rSup { size 8{ - {}} } } {} .Verify that all the decay products of the τ + size 12{τ rSup { size 8{+{}} } } {} are the antiparticles of those in the decay of the τ size 12{τ rSup { size 8{ - {}} } } {} given in the text.

(a) 1671 MeV

(b) Q = 1, Q = 1 + 0 + 0 = 1. L τ = 1; L τ = 1; L μ = 0; L μ = 1 + 1 = 0

(c) τ μ + v μ + v ¯ τ μ   antiparticle of  μ + v μ   of  v ¯ μ v ¯ τ   of  v τ alignl { stack { size 12{ \( a \) " 1671"`"MeV"} {} #size 12{ \( b \) " "Q=1,~Q'= - 1+0+0=1 "." ~L rSub { size 8{τ} } = - 1;} {} # L rSub { size 8{μ} } rSup { size 8{'} } +L rSub { size 8{v rSub { size 6{μ} } } } +L rSub { {overline {v}} rSub { size 6{τ} } } size 12{ {}= - 1 - 1+1= - 1} {} #\( c \) τ rSup {-{}} size 12{ rightarrow μ rSup { - {}} } size 12{+v rSub {μ} } size 12{+ {overline {v}} rSub {τ} } {} # size 12{ drarrow μ rSup { - {}} size 12{`"antiparticle"`"of"`μ rSup {+{}} } size 12{;`v rSub {μ} } size 12{`"of"` {overline {v}} rSub {μ} } size 12{;` {overline {v}} rSub {τ} } size 12{`"of"`v rSub {τ} }} {} } } {}

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The principal decay mode of the sigma zero is Σ 0 Λ 0 + γ size 12{Σ rSup { size 8{0} } rightarrow Λ rSup { size 8{0} } +γ} {} .

(a) What energy is released?

(b) Considering the quark structure of the two baryons, does it appear that the Σ 0 size 12{Σ rSup { size 8{0} } } {} is an excited state of the Λ 0 size 12{Λ rSup { size 8{0} } } {} ?

(c) Verify that strangeness, charge, and baryon number are conserved in the decay.

(d) Considering the preceding and the short lifetime, can the weak force be responsible? State why or why not.

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(a) What is the uncertainty in the energy released in the decay of a π 0 size 12{π rSup { size 8{0} } } {} due to its short lifetime?

(b) What fraction of the decay energy is this, noting that the decay mode is π 0 γ + γ size 12{π rSup { size 8{0} } rightarrow γ+γ} {} (so that all the π 0 size 12{π rSup { size 8{0} } } {} mass is destroyed)?

(a) 3.9 eV

(b) 2 . 9 × 10 8 size 12{2 "." 9 times "10" rSup { size 8{ - 8} } } {}

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(a) What is the uncertainty in the energy released in the decay of a τ size 12{τ rSup { size 8{ - {}} } } {} due to its short lifetime?

(b) Is the uncertainty in this energy greater than or less than the uncertainty in the mass of the tau neutrino? Discuss the source of the uncertainty.

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Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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