2.4 - quarks and antiquarks

what are strange particles?

particles that decay into:

• pions

• pions and protons

what are kaons?

strange particles that decay into pions only

which strange particles decay into pions only?

kaons

how are other strange particles (such as sigma) different to kaons?

• kaons decay into pions only (they don’t decay into protons as well)

• otheres have different rest masses which are always greater than the proton’s rest mass

• others decay either in sequence or directly into protons and pions

whhat do kaons decay into?

only pions (i.e., not pions and protons)

what are some examples of strange particles?

• kaons

• sigma

what is the rest masses of strange particles (excluding kaons) like?

• each have different rest masses

• always greater than the a proton’s rest mass

rest masses of strange particles

other strange particles (including sigma) > proton > kaons

what is the rest mass of other strange particles (excluding kaons) always larger than?

the rest mass of a proton, and therefore also the rest mass of a kaon (as protons > kaons)

how do other strange particles (excluding kaons) decay?

either in sequence or directly into pions + protons

via which interaction do strange particles decay?

weak nuclear

in what quantity are strange particles created?

created in 2s, as a quark-antiquark pair

what must be conserved in strange changes?

strangeness

how is strangeness conserved in strong interactions?

always conserved

how is strangeness conserved in weak interactions?

it may change by 0, +1, or -1

how can we explain why hadrons have the properties they do (charge, strangeness, rest mass)?

by assuming they’re composed of smaller particles called quarks and antiquarks

quarks and antiquarks

• up (u)

• down (d)

• strange (s)

• anti-up (s^-)

• anti-down (d^-)

• anti-strange (s^-)

charge, strangeness, baryon number of quarks and antiquarks

what is the charge of an up quark?

+ 2/3

what is the charge of a down quark?

- 1/3

what is the charge of a strange quark?

- 1/3

what is the strangeness of an up quark?

0

what is the strangeness of a down quark?

0

what is the strangeness of a strange quark?

-1

what is the baryon number of an up quark?

+ 1/3

what is the baryon number of a down quark?

+ 1/3

what is the baron number of a strange quark?

+ 1/3

what is the charge of an up antiquark?

- 2/3

what is the charge of a down antiquark?

+ 1/3

what is the charge of a strange antiquark?

+ 1/3

what is the strangeness of an up antiquark?

0

what is the strangeness of a down antiquark?

0

what is the strangeness of a strange antiquark?

+1

what is the baryon number of an up antiquark?

- 1/3

what is the baryon number of a down antiquark?

- 1/3

what is the baryon number of a strange antiquark?

- 1/3

what is the quark composition for a baryon?

3 quarks

what is the quark composition for an anti-baryon?

3 antiquarks

what are sigma particles (Σ)?

baryons with a strange or anti-strange quark in any amount

what is a baryon with a strange / anti-strange particle in any amount?

a sigma (Σ)

how many strange / anti-strange particles must there be to constitute as a sigma particle?

any quantity

quark composition of a proton

uud

charge = +2/3 +2/3 -1/3 = +1

baryon number = +1/3 +1/3 + 1/3 = +1

quark composition of a neutron

udd

charge = +2/3 -1/3 -1/3 = 0

baryon number = +1/3 +1/3 +1/3 = +1

quark composition of an antiproton

u^-d^-d^-

charge = -2/3 -2/3 +1/3 = -1

baryon number = -1/3 -1/3 -1/3 = -1

what is the only stable baryon? why?

a proton, because it’s the only one with both a charge and baryon number of +1

what does a free neutron decay into?

a proton, releasing an electron and electron antineutrino (i.e., β^- decay)

n → p + e^- + v_e

what is the decay of a free neutron the same as?

β- decay

what is the quark composition for mesons?

• quark-antiquark pair

• 9 combinations

which hadrons is composed of a quark-antiquark pair, and which are composed of 3 quarks / antiquarks?

• mesons = quark-antiquark pair

• baryons = 3 quarks / antiquarks

how many quark-antiquark combinations can there be for mesons?

9

what is a meson when strangeness = 0?

pion

what is a meson when strangeness doesn’t = 0?

kaon

what is this combination?;

u + u^-

• charge = +2/3 -2/3 = 0

• baryon number = +1/3 -1/3 = 0

• strangeness = 0 + 0 = 0

no charge, not a baryon, and no strangeness, so π⁰

what is this combination?;

d + d^-

• charge = -1/3 +1/3 = 0

• baryon number = +1/3 -1/3 = 0

• strangeness = 0 + 0 = 0

no charge, not a baryon, and no strangeness, so π⁰

what is this combination?;

s + s^-

• charge = -1/3 +1/3 = 0

• baryon number = +1/3 -1/3 = 0

• strangeness = -1 +1 = 0

no charge, not a baryon, and strangeness cancels out, however this is not a pion as it decays much faster

why is s + s^- not a π⁰ meson?

because it decays much faster than a pion

what is this combination?;

u + d^-

• charge = +2/3 +1/3 = +1

• baryon number = +1/3 -1/3 = 0

• strangeness = 0 + 0 = 0

charge of +1, not a baryon, and no strangeness, so π⁺

what is this combination?;

d + u^-

• charge = -1/3 -2/3 = -1

• baryon number = +1/3 -1/3 = 0

• strangeness = 0 + 0 = 0

charge of -1, not a baryon, and no strangeness, so π^-

what is this combination?;

u + s^-

• charge = +2/3 +1/3 = +1

• baryon number = +1/3 -1/3 = 0

• strangeness = 0 +1 = +1

charge of +1, not a baryon, and strangeness of +1, so K⁺

*strangeness included, so a kaon

what is this combination?;

d + s^-

• charge = -1/3 +1/3 = 0

• baryon number = +1/3 -1/3 = 0

• strangeness = 0 +1 = +1

no charge, not a baryon, and strangeness of +1, so K⁰

*strangeness included, so a kaon

*if a down quark is involved with strangeness, then charge cancels out

what is this combination?;

s + d^-

• charge = -1/3 +1/3 = 0

• baryon number = +1/3 -1/3 = 0

• strangeness = -1 + 0 = -1

no charge, not a baryon, and strangeness of -1, so K⁰

*strangeness included, so a kaon

*if a down quark is involved with strangeness, then charge cancels out

what is this combination?;

s + u^-

• charge = -1/3 -2/3 = -1

• baryon number = +1/3 -1/3 = 0

• strangeness = -1 + 0 = -1

charge of -1, not a baryon, and strangeness of -1, so K^-

*strangeness included, so a kaon

what is the antiparticle of a meson? why?

antiparticle of a meson is a meson, because mesons are uncharged

what happens to the quark composition during β⁺ decay?

• proton (uud) turns to a neutron (udd)

• up quark turns to a down quark

• up quark releases a W⁺ boson, which releases a down quark that turns the proton to a neutron

what happens to the quark composition during β^- decay?

• neutron (udd) turns to a proton (uud)

• down quark turns to a up quark

• down quark releases a W^- boson, which releases a up quark that turns the neutron to a proton