Study of the Z boson polarization asymmetry in processes with Z + b quark interaction

Mehdi Hamoumi
Aug 2013

Thesis Type:

The Standard Model of particle physics has been very successful in describing three out of the four fundamental interactions. It contains several parameters which must be precisely measured and compared to their prediction in order to show the consistency of the theory. One of these parameters, the b quark forward-backward asymmetry $A^{0,b}_{FB}$, has a measurement that deviates in a non negligible way from the prediction ($2.8 \sigma$ measured at LEP). My work consisted in showing how to measure at the LHC the Z boson polarization asymmetry, $A_Z = \frac{\sigma_R - \sigma_L}{\sigma_R + \sigma_L}$ which is related to $A^{0,b}_{FB}$ as it was shown in a related work. I have studied $A_Z$ by looking at the angular distribution of the antimuon produced in the Z decay in Monte-Carlo simulations, for possible use with CMS data. This distribution provides enough information to compute the Z polarization fractions (right, left and transverse), which I did by two methods. The first one was found to have limitations with real detector data. The second one consists in reweighting the Monte-Carlo events to get the samples of the purely right, left and transverse polarized Z bosons, and then fit these contributions to the data to obtain the respective polarization fractions. This method is more robust since it gives relevant results after applying selections on detector level (transverse momentum of the muons, b-jets...). It also provides the uncertainty on AZ which will eventually allow us to determine whether measurements at LHC can confirm or invalidate the deviation on $A^{0,b}_{FB}$ measured at LEP. An analysis of systematic errors is also done, mainly concerning the background estimation (processes with no Z + b interaction), and the ability to experimentally distinguish between $b$ and $\bar{b}$.