Quick question - Help!! Im working on branching ratios of quark jets, the book is pulling fractions out of nowhere and im wondering how its getting them! for example with a beam of e+ and e- colliding you get quark jets, the book says the ratio of the crossections of the collsions going to dd* compared to muons (where* designates the anti-particle) is (1/3)^2 and the ratio given for uu* created is (2/3)^2. Therefore total = 5/9 The notation the book uses is σ(ee --> dd*)/σ(ee -->mu mu) = (1/3)^2 As the centre of mass energy increases other heavier quarks can become possible... Can anyone shed light on where these fractions come from? Im sure there easily calculated.