Particle Physics Tutorial 10 - Classification of Particles (Baryons)

Baryons

These are the heavyweights of particle physics, and include the familiar proton and neutron.

 Let us look at the properties of the baryons:

 

Name 

Symbol

Q

B

S

Lifetime (s)

Anti-particle

Proton

p

+1 

1 0

stable

p

Neutron

n

0

1 0

898

n

Lambda

L0

0

1 0

2.6 ◊ 10-10

L0

Sigma 

S+

+1

1 -1

0.8 10-10

S-

 

S0

1 -1

7.4 10-20   

S0

 

S-

-1

1 -1

1.5 10-10

S+

Omega

W-

-1

1 -3

0.8 10-10

W+

NOTE:

In many books you will see the anti-particles with a bar over the symbol, for example, ū, ("u-bar") for anti-up.  However it is not easy to produce these in this particular web editor.  Therefore I will represent antiparticles by using a white letter on a black background.  So d is to be read as "d-bar", meaning anti-down.

 

In the exam, you will only be asked about protons, neutrons, and possibly the sigma.  So donít spend ages learning these baryons off-by-heart! 

The diagram shows the quarks for the proton and the neutron:

In baryons, particles and antiparticles are NEVER found together in the same baryon.

The baryons feel all four of the fundamental interactions. 

The proton is the only baryon that is stable in isolation.  The neutron on its own decays to a proton by beta minus decay after about 12 minutes.  The decay is as a result of the weak interaction that occurs within nucleons. 

Question 1

This question is about the neutron.

(a) What is the quark combination of a neutron?

(b) Show that the charge is 0.

(c) Explain how the neutron has a baryon number of 1.

(d) An isolated neutron lasts about 12 minutes.  What does it decay to?

 

Answer

 

Anti-baryons

The anti-proton and anti-neutron are shown:

 

There are other anti-baryons, for example, the anti-sigma minus, S-, which is the antiparticle to the sigma plus (S+).  It does NOT have the same quark composition as the sigma minus.

Question 2 State two ways that an anti-neutron is similar to a neutron.

How is it different to a neutron?

An anti-neutron will decay.  What will it decay to?

Answer

It is possible to make simple anti-atoms, like anti-hydrogen:

Anti-hydrogen, H ("H-bar") has been made at CERN, but the atoms annihilate as soon as they touch matter.  Powerful magnetic fields are needed to keep the little brutes in place.  Recently (June 2011) 300 anti-hydrogen atoms were reported to have been made, some lasting about 1000 s (17 minutes).  In a perfect vacuum, there is no reason why the anti-atoms shouldn't last for ever.  When CERN is fully operational, they reckon to be able to make 107 hydrogen anti-atoms every second.  At that rate, they will make 1 mole (6 ◊ 1023 atoms) in about 100 000 million years.  That's a long time. 

There may be parts of the universe where there is complex antimatter material, but there is certainly no evidence for it.

 

Strange baryons

Strange baryons have one or more strange quarks.  The strange baryons still have a baryon number of 1, and must have a charge of  -1, 0, +1.  Strange anti-baryons will have the opposite charge of +1, 0, -1.   The strangeness number can be -1 (1 strange quark), or -2 (2 strange quarks), or -3 (3 strange quarks).

Here are a picture of baryons containing one strange quark:

Question 3 The sigma baryons have a strangeness number of -1.

(a) How many strange quarks do they have?

(b) What is the quark composition of the sigma plus (S+) baryon?

(c) What is the quark composition of the sigma minus anti-baryon (S-).

(d) How does the S- differ from the S-?

Answer

The Xi-baryon (X) has two strange quarks.  The strange symbol is "Xi", a Greek upper case letter 'X'.  The X- baryon has a quark composition of dss, while the X0 baryon is uss.  The W- ("omega-minus") baryon has 3 strange quarks, sss.

If the strangeness number is conserved during interactions involving strange baryon, the interaction is mediated by the strong force.  If the strangeness is NOT conserved during the interaction, the interaction proceeds (provided that other quantum numbers are conserved) but it's through the weak interaction.

There is a whole zoo of different particles that contain other types of quark.