A Search for Rare Decay D0 --> mu+mu - High Energy Physics UPRM

ŽuŽd’ ‘ “”“ “–• — —˜— —– “’‘ŽcŽs’ ‘ “”“ “–• — —˜— —– “’‘ŽtŽbŽeŽ›š‰Ž7žŽ›šŒ1.1 The Standard Model 8to the quarks. Here also the weak interaction can turn one flavor into another. However,it is not true to say that the weak force cannot act across families. It can, but with a much`reduced effect.QuarksLeptonsŽgœ(a)(b)Žš‹Figure 1.2: Transformations permmited by the Standard Model. (a) The effect of theweak force on the quarks. Transformations such as b u are also possible. (b)The effect of the weak force on the leptons. Transformations within families arerepresented by vertical arrows and across families are represented by horizontal anddiagonal arrows.ŸFor quarks, the electric charge changing transformations within families and acrossfamilies are dominant (the vertical transformations are more probable and the diagonaltransformations are less probable) as seen in Figure 1.2.This does not mean thattransformations due to neutral currents cannot occur. Quark transitions such as c u,in which the charge is the same, could occur through a neutral current . What happensis that transitions such as this one have a small probability of occurrence compared to theelectric charge changing transitions. In Section 1.3, this will be explained in more detail.©As mentioned before, the weak interactions of quarks as well as leptons are mediatedby the W , W and Z© . In the Standard Model leptons cannot have cross transformationsdirectly. Nevertheless, the Standard Model permits processes such as&3&