Speaker
Description
Modeling angular distributions of Z-decay leptons at the Large Hadron Collider (LHC) is important in precision electroweak measurements. Z production at the LHC is dominated by gluon processes and the quark-antiquark ($q \bar q$) process accounts for only $40\%$ of the cross section. We investigate the theoretical predictions (using the POWHEG-MiNNLOPS 2.0 event generator) for the difference in the angular distributions of $pp\to Z\to \mu \mu$ events produced via $q \bar q$, guark-Gluon ($qG$), and gluon-gluon ($GG$) processes. The simulation is done for a CMS-like detector for proton-proton collisions at the large hadron collider at $\sqrt{s}$=13 TeV. We investigate the angular coefficients $A_0$ and $A_2$ (for $\theta$ and $\phi$ distributions in the Collins-Soper frame) for the different processes and for different categories of events as a function of the $Z$ boson rapidity and transverse momentum ($P_T$). For the $q \bar q$ process $A_0^{ q\bar q }=P_T^2/(P_T^2+M^2)$ (where $M$ is the mass of the dilepton pair) is expected from purely geometrical considerations. For the $qG$ process $A_0$ is larger and dependent on patron distribution functions. Previous theoretical calculations find $A_0^{qG}\approx 5P_T^2/(5P_T^2+M^2)$. Our full simulation with the POWHEG-MiNNLOPS Monte Carlo (MC) indicates that this approximation is an overestimate. We show that the validity of this approximation can also be tested experimentally by measuring the angular coefficients for events with a single b-quark jet in the final state (which is primarily produced via the $qG$ process). In our simulation we also find that at 13 TeV the violation of the Lam-Tung (LT) relation $A_0=A_2$ is much smaller for events with only zero or one jet in the final state indicating that the violation of the LT relation originates from events with two or more jets in the final state. This can also be checked experimentally by measuring $A_0$ and $A_2$ for events with only zero or one jet in the final state.