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First W+W- Event from LEP?

The W and Z were first discovered with the SPS proton collider (now used as an injector for LEP) in 1983, and won their discoverers the Nobel Prize. However precision measurements using this method were difficult, due to the complexity of the production mechanism and difficulty of unambiguously identifying all the products of the W decay. In contrast LEP events (Z0, and now W+W-) are very clean and should allow many detailed tests of the W's properties (and thus test the physics of the standard model), just as has been done for the Z0.

LEP first achieved 160 GeV beams at 03:13am on the morning of 9th July 1996 (I was there!). DELPHI was lucky enough to be the first of the four the LEP experiments to see a W event. Of course luck comes to those prepared, and it was no doubt due to the excellent state of the DELPHI Slow Controls (OK, the whole online system) that we detected the event and identified it so quickly.

These are pictures from the DELPHI event display. They show two representations of the detector (from the side, or z view, and end-on, or RPhi view, respectively) and the results of that first W event. Assuming this is indeed a W+W- event (and it is of such a classic topology that it would be suprising if it weren't), it is the first time a W-boson has been created in an electron collider.

The electron and positron collide with sufficient energy to create a W+ and W-. Each W decays into a quark and antiquark (one up-type and one down-type), each of which produces a jet of particles. The four jets are seen by the detector, shown in red, green, blue, and black.

If a colour printer or postscript previewer is available, the event is probably best viewed from the original postscript file.

[z view] [RPhi view]

Details: run 67777 event 16923 (12:14pm, 9th July 1996):-

[ preformatted table for pre-HTML 3.2 browsers also available ]
Beam energy, Ecms= 80.7 GeV
Reconstructed jet1-jet2 mass, Mj1,j2= 74 GeV/c2 ( compare W mass, MW= 80 GeV/c2 )
Reconstructed jet3-jet4 mass, Mj3,j4= 45 GeV/c2 ( much of the discrepancy probably due to jet4 particles lost down the beampipe )
Total visible energy, Evis= 124 GeV
Total visible mass, Mvis= 125 GeV/c2
Missing transverse momentum, Pt,miss= 2.2 GeV/c
Missing longitudinal momentum, Pl,miss= 17.5 GeV/c ( down beam pipe )

If you liked these pictures, have a look at some other events from DELPHI: either fully reconstructed (as above, though not nearly as pretty as what I made!), or straight out of the online system.

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Tim Adye, <T.J.Adye@rl.ac.uk>