1333797812.2679Meson Physics.pdf

7/28/2019 1333797812.2679Meson Physics.pdf

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Meson Physics

Meson (Greek: medium)


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Exchange of mesons from one nucleonto another is responsible for a major componentof nuclear binding. As a part of the model, weregard the nucleon as surround by a cloud ofvirtual mesons that are continually beingemitted and absorbed. The maximum distance

these mesons can travel before they must beabsorbed determines the range of the nuclearforce and the size of a nucleon. Other mesons,including and , contribute to the short-range

nuclear interaction, particularly the tensor, spin-orbit, and repulsive core terms.


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The mesons (pions) are the lightest

members of the meson family, one of the

three major groupings of particles (leptons{(, +), , + , (, +) and their (, )},

baryons {nucleons, other two families}).

The mesons are particles that have integerspin and interact with nucleons through the

strong force.

Mesons are composite particles made up of aquark and an antiquark.

There are no stable mesons.


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Yukawas Hypothesis In 1935, Yukawa proposed a mathematical

potential to represent the nucleon-nucleon

interaction. =

where is a

constant that represents the strength of the

pion field.

If m represents the rest mass of the

exchanged particle, then a virtual particle can

be created and exist for a time t without

violating conservation of energy.


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The exchange of mesons with mass m

leads to the Yukawa-potential


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The uncertainty principle:

The greatest distance the particle can move is:

For a range of 1fm, the mass of the exchange

particles is of order 200 MeV/c

2

. For photon infinite range.

In 1949, Powell showed evidence for two distinct

mesons from the emulsion tracks, a heavier

meson (~150 MeV) decaying to the lighter one

(~100 MeV). It is the heavier meson that is

Yukawas particle, now known as the meson

and the lighter particle is a meson .

2 2

2/2, ,t E E mc tmc t

mc

2 2

200 MeVcx ct

mc mc

0m x


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Properties of Mesons Electric Charge: pions can carry electric charge

of +e,0,-e corresponding to +, , . The pions are set of three particles +, ,

whose antiparticles are , , + ,

respectively. Isospin: The family forms from the multiplicity

2T+1. This demands T=1 for pions. The

member with the maximum electric chargehave the highest projection, we have

3 = 1,0, 1 for , , +,respectively.


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Mass: The mass of the is determined with

great precision from the energies of-mesic X

rays, which are emitted when a

is capturedinto an atomic orbit and cascades down

toward the nucleus.

Mass(MeV/c2)Pion

139.5675134.9745139.5658+


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Spin and Parity: The decay of into two s and theproduction of a single pion from nucleon-nucleon collisionssuch as:

+

immediately shows that the pion musthave integer spin, as do all mesons. The most direct indicationof the pion spin comes from the study of +and its inverse +

If nature is symmetric with respect to time reversal, the direct

and inverse come cross sections should be identical, exceptfor statistical factors and kinematics. This is called the

principle of detailed balance. That is:

where is the

spin-dependent statistical factor.()

()=

3 2+

2(

)2

= (1)= = 1, = 0Thus = 1 requires =even, while = 1 requires=odd.


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Decay Modes: The pion is the lightest meson

and therefore the lightest strongly interacting

particle. It must decay by the much slowerelectromagnetic or weak interaction, with

consequently a much longer lifetime.

The

decays electromagnetically:

Photoproduction:

Dalitz decay mode: + with a

branching intensity of 1.2%.The +and lifetimes are identical =26.02ns

The charged pions decay by weak interaction.

0A A

Z ZX X


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Production: Pions are most produced fromcollisions of protons with nuclear targets. & +

&

Note that the initial state and the final state bothhave two nucleons; this is a consequence of baryon

conservation, the nucleons being the lightestmembers of the baryon family.

Unlike spin-1/2 fermions such as leptons andbaryons, there is on law requiring conservation of

the number of integral-spin particles such asmesons. Therefore, nucleon-nucleon (N-N)reactions can produce any number of mesons,consistent with energy and charge conservation.


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At a threshold of about 600 MeV, two-pion

production becomes possible:

+

+

+ +

Gamma rays incident upon nucleons can also

produce pions:

+

In the meson factories, the pion production

targets are solids of relatively low-Z materials.


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Pion-Nucleon ReactionsThere are three types of reactions: charge

exchange, elastic and inelastic scattering. Elasticscattering as:

+ + & + +

In the inelastic reaction, the target nucleus is leftin an excited state. In the case of pions, the

energy is deposited through the creation of new

pions:

+

+

+ + +

Charge exchange reactions are similar to (p,n)

reactions:


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The + cross section is dominated by a huge

resonance at a pion energy of about 200 MeV;

the same resonance occurs in the elasticand charge-exchange cross section as well.

Some resonances such as 1232 MeV (++, +,

,

) with isospin T= 3/2,3 =

3

2,

2,

2,

3

2,respectively.

For N state like nucleon T=1/2 for proton and

neutron with 3 = 2

, 2

, respectively.


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Meson ResonancesThe pions are the lightest members of the

meson family. As the incident energy is

increased, there is the possibility to produce

other mesons in proton-proton or pion-proton

reaction. All of these mesons have massesgreater than twice the pion mass, and because

there is no conservation law for the number of

mesons, they can decay into two or more pionsthrough the strong interaction on a time scale of

the order of 10-23s.


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+ + +

Annihilation: + +

The spin for (+, , ) meson = 1, = 1

meson (549 MeV), -meson (783 MeV),

meson (769 MeV)

See other meson resonances table (17.2) page682 in your textbook Krane.

+ + (90%) + +

+ (9%)

+ (1%) +

+ (6.7x10-3%)


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Strange Meson and Baryons

In the microworld of particle physics, processesoccur which have no analog in our ordinary

experience. Classifying and then understanding

those processes are the challenges faced by

nuclear and particle physicists. They have given

arbitrary names such as strangeness, color,

flavor, charm, and bottomness to classify

particles, but have absolutely no connection

with our ordinary use of those descriptions.


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Strangeness (S)

Strangeness is another additive quantum

number. It is conserved in STRONG and EMinteractions but not in WEAK interaction( = 1).

S = 0 for nonstrange particles (p,n, etc.).Example of strange particles are K mesons(kaons) m(K)= 500 MeV, S(+)=1& S() = 1

+ , strong interaction.

+ + weak interaction since S= -1.

:hyperons (strange baryons) S( )= -1

m( ) = 1116 MeV/c2

0 0

0


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This decay is forbidden byenergy conservation (the final mass energies

total more than 1400 MeV, compared withinitial mass 1116 MeV).

is also forbidden by thestrangeness rule (EM interaction).

The heavies baryon ,

m( )=1190 MeV , S( )= -1

See table(17.3):Strange Baryons page 689 Krane.

0 p K

0n

0( , , )

0, ,

0 0m( , or ) 1320 MeV & S( , or ) 2 - -m( ) 1673 MeV & S( ) 3

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1333797812.2679Meson Physics.pdf