Physics & Astronomy

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High-precision Tests Of The Standard Model Begin At LHC

Z Boson

Two Rochester physicists, Professor Arie Bodek and Research Associate Jiyeon Han, are among the leaders of a high-statistics study of the decay of the Z boson at the Compact Muon Solenoid (CMS) detector at CERN.  This work, submitted to Physics Letters B in April and highlighted this week at the Fermilab Today newsletter, provides measurements of unprecedented precision of the decay of the Z.  The data disagree with theoretical expectations, likely because of insufficiently precise theoretical calculations.  As a result, the study has generated great interest among theoretical physicists and will motivate more precision measurements at the Large Hadron Collider (LHC).

The Z boson is one of the best studied of the subatomic particles. Z bosons decay in many ways, but one of the cleanest and easiest to study is when it decays into two muons, which are heavy cousins of electrons. You can do an analysis in any reference frame, from one in which the particle is moving to one that is stationary. If you do your analysis properly, you will draw the same conclusions in any frame. So Bodek, Han, and other CMS scientists picked the easiest frame in which the Z boson was stationary. In such a frame, only a few properties define the particle. The Z boson is electrically neutral and massive. It also decays only via the weak nuclear force. Given the measured mass of the Z boson and decay muons, many of the details of how the muons are emitted are determined completely via energy and momentum conservation. The muons are emitted back to back and with a specific energy.

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