# How experimentalists put bounds on new physics at the LHC?

I would like to understand how experimentalists search for new physics at the LHC. Lets imagine I want to use the LHC data to put a bound on the coupling of some new physics effective operator, say, for example a flavor changing Yukawa interaction of the form $t_R c_LH$ (top-charm-Higgs).

What I have understood up to now is the following (correct me if I'm wrong): one must first look at the interesting channels that involve these new interactions e.g. $p p \to t \bar t$ followed by the new decay $t\to Hc$. Next, assume the new effective coupling is such that the Branching ratio lies within the current experimental uncertaintes $Br(t\to Hc)\sim 1\%$. Then you have to simulate (with Monte Carlo) the relevant processes, count the number of expected events (after subtracting the SM background) and finally compare it to the real Data. This should give an upper bound on the effective coupling.

Can someone give me a detailed description of these type of LHC searches? I am particularly interested in understanding the role of the MC simulations (MadGraph, Pythia, etc). Thanks!

• Ok, so the role of the MC simulations is to model how the detector will respond to your new interaction, i.e. it tells you the value of the efficiency $\epsilon$. I am not sure if I agree with what you say after. I don't see why 3rd parties can't use the CMS and ATLAS searches to give improved bounds on new physics. We can also use the same detector simulations they use. Sometimes they base their analysis on a given number of processes and leave out other possible competing processes that, if included, could give better bounds on effective operators. – Dar Dec 20 '14 at 10:52