Prepared for a challenge

This figure represents the energy depositions for the single four-lepton candidate event observed by DZero researchers. The di-electron plus di-muon event is most consistent with the signature of a Z boson plus an energetic photon, with the muon energies shown in green and the electron energies in red.

When it comes to finding leptons at the Tevatron, the age-old saying is true: Three is a crowd. Physicists at the DZero experiment have recently learned the next line in this saying: Four is a challenge. While studying the strength of the electro-weak force, these physicists found that detecting four leptons at once is indeed a tricky task. DZero researchers have used this rare four-lepton state to search for the production of Z boson pairs. This process is among the rarest studied at the Tevatron, because less than 2000 Z pairs were expected to have been produced at DZero in the first five years of Run II and less than 10 of those were expected to decay to four leptons. Missing even one of these rare events makes the job an even bigger challenge.

Particle physicists get a lot of mileage out of Z bosons. The mass of the Z helps in measuring many aspects of detector performance, from lepton identification to energy scales. Adding an additional Z boson increases that mileage significantly. Because the standard model doesn't allow self-couplings of Z bosons, the rate of Z pair production can reveal information about potential new physics from Higgs bosons to anomalous electro-weak couplings. By looking for Z/photon pairs decaying to four leptons (eeee, eeμμ, or μμμμ), DZero physicists are able to eliminate all but the rarest potential background processes from their search. But the cleanliness of this decay comes at the cost of a challenging job of finding four leptons in one event. The odds that all four leptons fall in the detector volume and are identified become very small unless large efforts are made to find them. By studying the process of identifying leptons, these researchers were able to carefully loosen the identification criteria without increasing background significantly. They were helped significantly by foresight in the construction of the DZero detector, providing for additional muon detection capability in the regions where the DZero detector is supported from below.

By analyzing the relative angles of the four leptons and the invariant masses of their possible pairings, the DZero physicists selected just one event in one inverse femtobarn of recorded data. They expected 1.71 +/- 0.15 signal events and a background of only 0.13 +/- 0.03 events. Though the candidate eeμμ event appears to be a Z boson and a virtual photon event, the analysis sets a 95 percent confidence level upper limit on this process at 4.4 picobarn, compatible with an expected cross section of 1.6 picobarn. This DZero search now stands as an example of how to overcome the large challenges faced by Tevatron physicists each day.

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The primary contributors to this analysis were Tom Diehl, Nick Hadley, Chad Jarvis, and Michiel Sanders.

Members of the DZero calorimeter operations and algorithms groups, who are responsible for operating, maintaining and calibrating the inter-cryostat detectors, have recently completed a push to enhance the performance of this important part of the DZero detector. Their efforts help to ensure calorimeter measurements, such as those of electrons, are as reliable and accurate as possible.

Result of the Week Archive

Dec. 3 - 5
- One store provided 9 hours and six minutes of luminosity
- A3 thermometry crate repaired
- MI ground fault due to cable
- Linac vacuum leak found

Read the Current Accelerator Update
Read the Early Bird Report
View the Tevatron Luminosity Charts

Have a safe day!

Education Office holiday sale Dec. 4-6
The Education Office will host its annual holiday sale Dec. 4-6, from 11 a.m. to 1:30 p.m., outside One West in Wilson Hall.

Dec. 14 deadline for The University of Chicago Tuition Remission Program
The deadline for applying for the tuition remission program at The University of Chicago for the Winter 2008 quarter is Dec. 14. More information and enrollment forms. Contact Nicole Gee at x3697 with any questions.

Ticket sales in the Recreation Office
Blackhawks @ the United Center -- Jan. 11, 16 and 24. Save up to 50 percent. Harlem Globetrotters at the Allstate Arena and Vlaparaiso Athletics Center -- Jan. 11, 12 and 13. Save $7 per ticket. Chicago Bulls at the United Center, Dec. 14. Half price on the $60 tickets. All tickets are ordered direct. More information and registration forms can be found in the Recreation Office or online. AMC movie tickets make great gifts and stocking stuffers. Tickets are $6.50 each (some restrictions) and have no expiration date. Tickets available in the Recreation Office.

Blood Drive Dec. 18, 19
Mark your calendars. Heartland Blood Centers will be here for the Fermilab Blood Drive on Dec. 18 and 19, from 8 a.m to 2 p.m. in the Wilson Hall Ground Floor NE Training Room. Appointments can be scheduled on the Web or by calling Diana at x3771 or Margie at x5680. More information can be found here.

Additional Activities