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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?


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


  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.


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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  • 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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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

plot a graph of MP against tan ( Angle/2) and determine the slope of the graph and find the error in it.
Ime Reply
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Are beta particle and eletron are same?
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how can you confirm?
If they are same then why they named differently?
because beta particles give the information that the electron is ejected from the nucleus with very high energy
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atomic n.o
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it's a subject
it's a branch in science which deals with the properties,uses and composition of matter
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god particles is know as higgs boson, when two proton are reacted than a particles came out which is used to make a bond between than materials
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microscopic particles or gases
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Thanks for your contribution Areej.
no problem
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intensity seems to be directly proportional radius of slit
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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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