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All known leptons are listed in the table given above. There are only six leptons (and their antiparticles), and they seem to be fundamental in that they have no apparent underlying structure. Leptons have no discernible size other than their wavelength, so that we know they are pointlike down to about 10 18 m size 12{"10" rSup { size 8{ - "18"} } m} {} . The leptons fall into three families, implying three conservation laws for three quantum numbers. One of these was known from β size 12{β} {} decay, where the existence of the electron's neutrino implied that a new quantum number, called the electron family number     L e size 12{L rSub { size 8{e} } } {} is conserved. Thus, in β size 12{β} {} decay, an antielectron's neutrino v - e size 12{ { bar {v}} rSub { size 8{e} } } {} must be created with L e = 1 size 12{L rSub { size 8{e} } = - 1} {} when an electron with L e =+ 1 size 12{L rSub { size 8{e} } "=+"1} {} is created, so that the total remains 0 as it was before decay.

Once the muon was discovered in cosmic rays, its decay mode was found to be

μ e + v - e + v μ , size 12{μ rSup { size 8{ - {}} } rightarrow e rSup { size 8{ - {}} } + { bar {v}} rSub { size 8{e} } +v rSub { size 8{μ} } ","} {}

which implied another “family” and associated conservation principle. The particle v μ size 12{L rSub { size 8{μ} } } {} is a muon's neutrino, and it is created to conserve muon family number     L μ size 12{L rSub { size 8{μ} } } {} . So muons are leptons with a family of their own, and conservation of total L μ size 12{L rSub { size 8{μ} } } {} also seems to be obeyed in many experiments.

More recently, a third lepton family was discovered when τ size 12{τ} {} particles were created and observed to decay in a manner similar to muons. One principal decay mode is

τ μ + v - μ + v τ . size 12{τ rSup { size 8{ - {}} } rightarrow μ rSup { size 8{ - {}} } + { bar {v}} rSub { size 8{u} } +v rSub { size 8{τ} } "."} {}

Conservation of total L τ size 12{L rSub { size 8{μ} } } {} seems to be another law obeyed in many experiments. In fact, particle experiments have found that lepton family number is not universally conserved, due to neutrino “oscillations,” or transformations of neutrinos from one family type to another.

Mesons and baryons

Now, note that the hadrons in the table given above are divided into two subgroups, called mesons (originally for medium mass) and baryons (the name originally meaning large mass). The division between mesons and baryons is actually based on their observed decay modes and is not strictly associated with their masses. Mesons are hadrons that can decay to leptons and leave no hadrons, which implies that mesons are not conserved in number. Baryons are hadrons that always decay to another baryon. A new physical quantity called baryon number     B size 12{B} {} seems to always be conserved in nature and is listed for the various particles in the table given above. Mesons and leptons have B = 0 size 12{B=0} {} so that they can decay to other particles with B = 0 size 12{B=0} {} . But baryons have B =+ 1 size 12{B"=+"1} {} if they are matter, and B = 1 size 12{B= - 1} {} if they are antimatter. The conservation of total baryon number    is a more general rule than first noted in nuclear physics, where it was observed that the total number of nucleons was always conserved in nuclear reactions and decays. That rule in nuclear physics is just one consequence of the conservation of the total baryon number.

Forces, reactions, and reaction rates

The forces that act between particles regulate how they interact with other particles. For example, pions feel the strong force and do not penetrate as far in matter as do muons, which do not feel the strong force. (This was the way those who discovered the muon knew it could not be the particle that carries the strong force—its penetration or range was too great for it to be feeling the strong force.) Similarly, reactions that create other particles, like cosmic rays interacting with nuclei in the atmosphere, have greater probability if they are caused by the strong force than if they are caused by the weak force. Such knowledge has been useful to physicists while analyzing the particles produced by various accelerators.

Questions & Answers

A golfer on a fairway is 70 m away from the green, which sits below the level of the fairway by 20 m. If the golfer hits the ball at an angle of 40° with an initial speed of 20 m/s, how close to the green does she come?
Aislinn Reply
cm
tijani
what is titration
John Reply
what is physics
Siyaka Reply
A mouse of mass 200 g falls 100 m down a vertical mine shaft and lands at the bottom with a speed of 8.0 m/s. During its fall, how much work is done on the mouse by air resistance
Jude Reply
Can you compute that for me. Ty
Jude
what is the dimension formula of energy?
David Reply
what is viscosity?
David
what is inorganic
emma Reply
what is chemistry
Youesf Reply
what is inorganic
emma
Chemistry is a branch of science that deals with the study of matter,it composition,it structure and the changes it undergoes
Adjei
please, I'm a physics student and I need help in physics
Adjanou
chemistry could also be understood like the sexual attraction/repulsion of the male and female elements. the reaction varies depending on the energy differences of each given gender. + masculine -female.
Pedro
A ball is thrown straight up.it passes a 2.0m high window 7.50 m off the ground on it path up and takes 1.30 s to go past the window.what was the ball initial velocity
Krampah Reply
2. A sled plus passenger with total mass 50 kg is pulled 20 m across the snow (0.20) at constant velocity by a force directed 25° above the horizontal. Calculate (a) the work of the applied force, (b) the work of friction, and (c) the total work.
Sahid Reply
you have been hired as an espert witness in a court case involving an automobile accident. the accident involved car A of mass 1500kg which crashed into stationary car B of mass 1100kg. the driver of car A applied his brakes 15 m before he skidded and crashed into car B. after the collision, car A s
Samuel Reply
can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?
Joseph Reply
Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time
Joseph
Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing
Joseph
"Generation of electrical energy from sound energy | IEEE Conference Publication | IEEE Xplore" ***ieeexplore.ieee.org/document/7150687?reload=true
Ryan
what's motion
Maurice Reply
what are the types of wave
Maurice
answer
Magreth
progressive wave
Magreth
hello friend how are you
Muhammad Reply
fine, how about you?
Mohammed
hi
Mujahid
A string is 3.00 m long with a mass of 5.00 g. The string is held taut with a tension of 500.00 N applied to the string. A pulse is sent down the string. How long does it take the pulse to travel the 3.00 m of the string?
yasuo Reply
Who can show me the full solution in this problem?
Reofrir Reply
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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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