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Check Your Understanding What is the lepton number of an electron-positron pair?

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Strangeness conservation

In the late 1940s and early 1950s, cosmic-ray experiments revealed the existence of particles that had never been observed on Earth. These particles were produced in collisions of pions with protons or neutrons in the atmosphere. Their production and decay were unusual. They were produced in the strong nuclear interactions of pions and nucleons, and were therefore inferred to be hadrons; however, their decay was mediated by the much more slowly acting weak nuclear interaction. Their lifetimes were on the order of 10 −10 to 10 −8 s , whereas a typical lifetime for a particle that decays via the strong nuclear reaction is 10 −23 s . These particles were also unusual because they were always produced in pairs in the pion-nucleon collisions. For these reasons, these newly discovered particles were described as strange . The production and subsequent decay of a pair of strange particles is illustrated in [link] and follows the reaction

π + p Λ 0 + K 0 .

The lambda particle then decays through the weak nuclear interaction according to

Λ 0 π + p ,

and the kaon decays via the weak interaction

K 0 π + + π .
Figure a shows a photograph with a black background and a white pattern of swirls and lines on it. There is a bright white spot on the top left. Figure b shows the same pattern as a line drawing. It is labeled in various places with names of particles.
The interactions of hadrons. (a) Bubble chamber photograph; (b) sketch that represents the photograph.

To rationalize the behavior of these strange particles, particle physicists invented a particle property conserved in strong interactions but not in weak interactions. This property is called strangeness    and, as the name suggests, is associated with the presence of a strange quark. The strangeness of a particle is equal to the number of strange quarks of the particle. Strangeness conservation requires the total strangeness of a reaction or decay (summing the strangeness of all the particles) is the same before and after the interaction. Strangeness conservation is not absolute: It is conserved in strong interactions and electromagnetic interactions but not in weak interactions. The strangeness number for several common particles is given in [link] .

Strangeness conservation

(a) Based on the conservation of strangeness, can the following reaction occur?

π + p K + + K + n .

(b) The following decay is mediated by the weak nuclear force:

K + π + + π 0 .

Does the decay conserve strangeness? If not, can the decay occur?

Strategy

Determine the strangeness of the reactants and products and require that this value does not change in the reaction.

Solution

  1. The net strangeness of the reactants is 0 + 0 = 0 , and the net strangeness of the products is 1 + ( −1 ) + 0 = 0 . Thus, the strong nuclear interaction between a pion and a proton is not forbidden by the law of conservation of strangeness. Notice that baryon number is also conserved in the reaction.
  2. The net strangeness before and after this decay is 1 and 0, so the decay does not conserve strangeness. However, the decay may still be possible, because the law of conservation of strangeness does not apply to weak decays.

Significance

Strangeness is conserved in the first reaction, but not in the second. Strangeness conservation constrains what reactions can and cannot occur in nature.

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Check Your Understanding What is the strangeness number of a muon?

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Summary

  • Elementary particle interactions are governed by particle conservation laws, which can be used to determine what particle reactions and decays are possible (or forbidden).
  • The baryon number conservation law and the three lepton number conversation law are valid for all physical processes. However, conservation of strangeness is valid only for strong nuclear interactions and electromagnetic interactions.

Conceptual questions

What are six particle conservation laws? Briefly describe them.

Conservation energy, momentum, and charge (familiar to classical and relativistic mechanics). Also, conservation of baryon number, lepton number, and strangeness—numbers that do not change before and after a collision or decay.

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In general, how do we determine if a particle reaction or decay occurs?

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Why might the detection of particle interaction that violates an established particle conservation law be considered a good thing for a scientist?

It means that the theory that requires the conservation law is not understood. The failure of a long-established theory often leads to a deeper understanding of nature.

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Problems

Which of the following decays cannot occur because the law of conservation of lepton number is violated?

( a ) n p + e ( e ) π e + υ e ( b ) μ + e + + υ e ( f ) μ e + υ e + υ μ ( c ) π + e + + υ e + υ μ ( g ) Λ 0 π + p ( d ) p n + e + + υ e ( h ) K + μ + + υ μ

a, b, and c

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Which of the following reactions cannot because the law of conservation of strangeness is violated?

( a ) p + n p + p + π ( e ) K + p Ξ 0 + K + + π ( b ) p + n p + p + K ( f ) K + p Ξ 0 + π + π ( c ) K + p K + + ( g ) π + + p Σ + + K + ( d ) π + p K + + ( h ) π + n K + Λ 0

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Identify one possible decay for each of the following antiparticles:

(a) n , (b) Λ 0 , (c) Ω + , (d) K , and (e) Σ .

a. p e + v e ; b. p π + or p π 0 ; c. Ξ 0 π 0 or Λ 0 K + ; d. μ v μ or π π 0 ; e. p π 0 or n π

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Each of the following strong nuclear reactions is forbidden. Identify a conservation law that is violated for each one.

(a) p + p p + n + p (b) p + n p + p + n + π + (c) π + p Σ + + K (d) K + p Λ 0 + n

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Questions & Answers

why we have physics
Anil Reply
because is the study of mater and natural world
John
because physics is nature. it explains the laws of nature. some laws already discovered. some laws yet to be discovered.
Yoblaze
is this a physics forum
Physics Reply
explain l-s coupling
Depk Reply
how can we say dirac equation is also called a relativistic equation in one word
preeti Reply
what is the electronic configration of Al
usman Reply
what's the signeficance of dirac equetion.?
Sibghat Reply
what is the effect of heat on refractive index
Nepal Reply
As refractive index depend on other factors also but if we supply heat on any system or media its refractive index decrease. i.e. it is inversely proportional to the heat.
ganesh
you are correct
Priyojit
law of multiple
Wahid
if we heated the ice then the refractive index be change from natural water
Nepal
can someone explain normalization condition
Priyojit Reply
please tell
Swati
yes
Chemist
1 millimeter is How many metres
Darling Reply
1millimeter =0.001metre
Gitanjali
The photoelectric effect is the emission of electrons when light shines on a material. 
Chris Reply
What is photoelectric effect
Amit Reply
it gives practical evidence of particke nature of light.
Omsai
particle nature
Omsai
photoelectric effect is the phenomenon of emission of electrons from a material(i.e Metal) when it is exposed to sunlight. Emitted electrons are called as photo electrons.
Anil
what are the applications of quantum mechanics to medicine?
Neptune
application of quantum mechanics in medicine: 1) improved disease screening and treatment ; using a relatively new method known as BIO- BARCODE ASSAY we can detect disease-specific clues in our blood using gold nanoparticles. 2) in Genomic medicine 3) in protein folding 4) in radio theraphy(MRI)
Anil
Quantam physics ki basic concepts?
Laxmikanta Reply
why does not electron exits in nucleaus
Kabbo Reply
electrons have negative
YASH
Proton and meltdown has greater mass than electron. So it naturally electron will move around nucleus such as gases surrounded earth
Amalesh
.......proton and neutron....
Amalesh
excuse me yash what negative
Rika
coz, electron contained minus ion
Manish
negative sign rika shrestha ji
YASH
electron is the smallest negetive charge...An anaion i.e., negetive ion contains extra electrons. How ever an atom is neutral so it must contains proton and electron
Amalesh
yes yash ji
Rika
yes friends
Prema
koantam theory
Laxmikanta
yes prema
Rika
quantum theory tells us that both light and matter consists of tiny particles which have wave like propertise associated with them.
Prema
proton and nutron nuclear power is best than proton and electron kulamb force
Laxmikanta
what is de-broglie wave length?
Ramsuphal
plot a graph of MP against tan ( Angle/2) and determine the slope of the graph and find the error in it.
Ime Reply
expression for photon as wave
BARISUA Reply
Practice Key Terms 3

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Source:  OpenStax, University physics volume 3. OpenStax CNX. Nov 04, 2016 Download for free at http://cnx.org/content/col12067/1.4
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