First Z bosons ever produced and reconstructed

The first Z boson candidate reconstructed in heavy ion collisions. The figure shows the calorimetric and charged particle tracking response to a lead-lead collision in the CMS detector plus the two muon tracks from the Z0 decay.

The Z boson is an electrically neutral subatomic particle that mediates the weak nuclear force. It has a mass 182,000 times that of the electron. Unlike the other weak force mediator (the W boson), the Z boson does not change particles it interacts with into other types of particles.

In November, Los Alamos researchers participated in data collection at the Compact Muon Solenoid (CMS) experiment. The CMS experiment consisted of lead-lead collisions at the unprecedented energy of 2.76 GeV (a factor 14 higher than at the Relativistic Heavy Ion Collider) at the Large Hadron Collider at CERN. Their goal is to understand the quark gluon plasma on a quantitative level.

On November 9, the CMS heavy ion dilepton group, convened by Subatomic Physics postdoctoral researcher Catherine Silvestre, found the first Z boson ever produced and reconstructed in heavy ion collisions. The Los Alamos team was responsible for including the Z0 measurements in the CMS heavy ion Technical Design Report, enabled by Laboratory Research and Development (LDRD) funding that began in 2006. Preparation for the current dilepton analysis was made possible by a second LDRD project begun in 2009. Part of the funding supports theoretical predictions of Z0 and hadronic jet productions. A CERN seminar on the first results from heavy ion collisions was held on December 2nd, 2010. The CMS talk featured a preliminary dimuon mass spectrum with 27 identified Z bosons. The manuscript in preparation, “First Unambiguous Measurement of Jet Fragmentation and Energy Loss in the Quark Gluon Plasma,” includes LDRD Principal Investigator Gerd Kunde.

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