Quarks & Antiquarks (AQA A Level Physics): Revision Note

A K^- particle has a strangeness of –1. Determine the quark structure of this particle.

[2]

Answer:

Step 1: Determine the types of quarks in this particle

• The kaon is a meson, so is made up of a quark and an anti-quark. 

Step 2: Determine the quark that gives a strangeness of −1

• The strange quark has a strangeness of −1, so this kaon must include an s quark [1 mark]

Step 3: Determine the quark that gives an overall charge of −1

• The kaon has an overall charge of −1

• The strange quark has a charge of $-\frac{1}{3}$.

• Adding the charges of the quarks together gives the overall charge:

$-\frac{1}{3}+x=-1$

• Hence, the missing charge $x=-\frac{2}{3}$

• The anti-up quark ($\stackrel{-}{\mathrm{u}}$) has a charge of $-\frac{2}{3}$

Step 4: State the quark structure of the particle

• Strange anti-up = $\mathrm{s}\stackrel{-}{\mathrm{u}}$ [1 mark]

You will not be expected to remember the charge of each quark as this information is provided on the datasheet. So, instead of memorising the charges of anti-quarks too, just remember they are identical but with opposite signs.

In the nucleus of ${}_{26}{}^{56}\mathrm{Fe}$, how many 'up' quarks are there?

[2]

Answer:

Step 1: Calculate the number of protons

             The number of protons is from the proton number = 26 protons

Step 2: Calculate the number of neutrons

    Number of neutrons = nucleon number – proton number

    Number of neutrons = 56 - 26 = 30 neutrons [1 mark]

Step 3: Up quarks in a proton

             Protons are made up of uud quarks = 2 up quarks

Step 4: Up quarks in a neutron

             Neutrons are made up of udd quarks = 1 up quark

Step 5: Total number of up quarks

    26 protons × 2 up quarks = 52 up quarks

    30 neutrons × 1 up quark = 30 up quarks

    Total number of up quarks = 52 + 30 = 82 up quarks [1 mark]

The quark combinations for protons, neutrons, pions and kaons will be expected to be memorised for the exam.