Observation of Z decays to four leptons with the CMS detector at the LHC

Abstract

The first observation of the Z boson decaying to four leptons in proton-proton collisions is presented. The analyzed data set corresponds to an integrated luminosity of 5.02 fb −1 at √ s = 7 TeV collected by the CMS detector at the Large Hadron Collider. A pronounced resonance peak, with a statistical significance of 9.7 σ, is observed in the distribution of the invariant mass of four leptons (electrons and/or muons) with mass and width consistent with expectations for Z boson decays. The branching fraction and cross section reported here are defined by phase space restrictions on the leptons, namely, 80 < m 4 < 100 GeV, where m 4 is the invariant mass of the four leptons, and m > 4 GeV for all pairs of leptons, where m is the two-lepton invariant mass. The measured branching fraction is B(Z → 4 ) = 4.2

Measurement of theZZproduction cross section and search for the standard model Higgs boson in the four lepton final state inpp¯collisions

“…ZZÞ in 5:0 fb À1 of data at ffiffi ffi s p ¼ 7 TeV [14], and has observed the rare decay Z ! ' þ ' À ' 0þ ' 0À with a branching fraction in agreement with the SM prediction [15].…”

Search for an L − L gauge boson using Z → 4μ events in proton-proton collisions at s=13 TeV

“…The Z → 4µ process has been studied by the ATLAS and CMS Collaborations [20][21][22] and constraints on the L µ − L τ parameter space have been derived. However, these measurements are not optimized for the presence of a Z particle.…”

The Onset of Jet Quenching Phenomenon

“…; the nuclear modifications factor, etc.. (R AA , R CP , I AA , I CP ). However, the absence of different levels of suppression for light vs. heavy quarks [10,11], and quark vs. gluon jets [12], in contrast to the QCD predictions, has necessitated the need for more sensitive observables and differential studies in order to have better quantification for the formed medium properties, and accordingly better constraints for the medium parameters. Moreover, other interesting measurements that have shown similar phenomena in the high multiplicity events in proton-proton collisions as in central heavy ion collisions, e.g.…”