What does "applying a cut" mean in terms of particle physics? I often came across the phrase "to apply a cut" in terms of data analysis in context of particle physics. Could someone explain what it means?
 A: To "apply a cut" means to apply a certain selection criterion on a data set. For example, in searches for heavy particles decaying into lighter ones, the lighter particles often have a relatively high transverse momentum. In statistical analyses, one could apply a cut on this transverse momentum, e.g. that $p_T > 10 \,\mathrm{GeV}$, in order to increase the fraction of interesting events. If the particles in a collision event do not fulfill this requirement, the event is not considered within the subsequent analysis. With this method you can "cut away" unwanted data.
A: It just means that you have applied selection criteria to a data set: the phrase 'after cuts' which you have probably read means after some particular selection criteria have been applied to remove events which are not interesting.
For example, you might be looking for a peak when re-constructing the invariant mass of a particle which then decayed into other particles.  Typically, you would apply cuts on the transverse momentum to make the peak structure more defined and get rid of uninteresting events : you would simply specify in the code that you only fill the histogram with events or print values if the transverse momentum is above a particular value.  You would then say that the results are with a transverse momentum cut of 30 GeV, it's essentially jargon.
