Online Event Pictures from LEP-2 Running

Events from the First 189 GeV Runs

Data-taking at 189 GeV centre-of-mass energy began on 23 May 1998 and will continue through until Fall 1998 for continued "production" running at LEP2. We are now substantially above the ZZ (both Z's real and "on mass shell") threshold. Along with all these exciting new electroweak processes, we have good old QED processes plus being able to search for even more massive new particles (i.e., if new particles are produced in pairs, can reach masses of one-half the centre-of-mass energy, in this case, masses of 91.5 GeV). It just doesn't get much more fun than this (well, 192 or 200 GeV centre-of-mass energy will be nice...)

Event Pictures:

Z⁰Z⁰ candidates from the 189 GeV run

W⁺W^- candidates from May 1998

The first observation (probably) of e+e- ->WW ->qqqq at roots = 189 GeV

Other electroweak process events from May 1998

Events from the First 183 GeV Runs

Data-taking at 183 GeV centre-of-mass energy began on 31 July 1997 and will continue through until Fall 1997 for our first big "production" run at LEP2. The cross-section for W⁺W^- production at this energy is even higher than at 172 or 161 GeV and only two weeks of running resulted in large numbers of W⁺W^- candidates. This is also the first crossing of the ZZ (both Z's real and "on mass shell") threshold. Along with all these exciting new electroweak processes, we have good old QED processes plus being able to search for even more massive new particles (i.e., if new particles are produced in pairs, can reach masses of one-half the centre-of-mass energy, in this case, masses of 91.5 GeV). It just doesn't get much more fun than this (well, 192 or 200 GeV centre-of-mass energy will be nice...)

Event Pictures:

Z⁰Z⁰ candidates from the 183 GeV run

At 183 GeV, we have crossed a new threshold where we can now pair-produce two real Z bosons (and at threshold, both have low momentum [approx. zero momentum if the Z had no width]).

A "gold-plated" Z⁰Z⁰ candidate with one Z boson decaying clearly into an electron and positron leaving large electromagnetic clusters while the other is most likely decaying into a tau⁺ and a tau^- lepton. More information superimposed on the event itself.
This event, is consistent with one Z decaying into a back-to-back pair of neutrinos that carry off missing energy, but leaves the total xy and z momentum balanced. Finding the "missing mass" or "recoil mass" of this invisible system one finds 93.5 GeV consistent with the Z mass. The remaining Z then decays into a quark and anti-quark with an emission of a hard gluon giving rise to three jets. The mass of this visible, hadronic system is 89.3 GeV, again consistent with the Z mass. Unfortunately in this decay channel there is still significant background due to double ISR photons allowing for radiation down to the Z peak but which balance each other in momentum.

W⁺W^- candidates from July - August 1997

Candidate for the process e⁺e^- -> W⁺W^- -> four quarks, observed in the detector as four jets of hadrons (i.e., each W decays into a pair of different quarks): one candidate, another candidate.
A candidate for the process e⁺e^- -> W⁺W^- -> two quarks, an electron and a neutrino, observed in the detector as two jets of hadrons and a separated high momentum electron from W decay. The red track pointing to the yellow "blob" is the electron showing its large deposition of energy in our electromagnetic calorimeter. The arrow is our estimate of the direction of the elusive neutrino that handily escapes the detector without a trace.
A candidate for the process e⁺e^- -> W⁺W^- -> two quarks, a muon and a neutrino, observed in the detector as two jets of hadrons and a separated high momentum electron from W decay. The red track pointing to the small yellow and purple "blobs" is the muon showing only a small energy deposit in our calorimeters. The small red arrow indicates "hits" in our muon chambers, convincing us that it is a muon. The arrow is our estimate of the direction of the elusive neutrino that handily escapes the detector without a trace.
A candidate for the process e⁺e^- -> W⁺W^- -> e nu mu nu; where one W decays into an electron (see above for how we can tell) and a neutrino, and the other into a muon (again, see above for how we can tell) and a neutrino. The neutrinos carry away undetected energy and momentum to make the event appear "unbalanced".
A candidate for the process e⁺e^- -> W⁺W^- -> e nu e nu; where one W decays into an electron (see above for how we can tell) and a neutrino, and the other into an anti-electron (positron) and a neutrino. The neutrinos carry away undetected energy and momentum to make the event appear "unbalanced". When we search for new particles, this event is also a candidate for new particles: a supersymmetric scalar electron and scalar positron, decaying into an electron or positron and "invisible" neutralinos. However, for a certain new particle mass, we don't see enough of these type of events above regular W⁺W^- processes.

Other electroweak process events from July - August 1997:

A candidate for the process e⁺e^- -> Z gamma* -> nu nu(bar) q q; where the Z decays into a pair of neutrinos and the gamma* to a pair of quarks. The neutrinos escape giving rise to this "monojet" topology.
A candidate for the process e⁺e^- into a neutrino and an anti-neutrino (both escape) and a photon that subsequently undergoes "pair conversion" into an electron and positron. These two tracks are "boosted" strongly in the direction of the photon and are almost right on top of each other.
Not everything is unbalanced. This candidate is probably e⁺e^- -> mu mu gamma , where the gamma or photon is the "blob" without a charged track pointing to it, everything balancing nicely.
Similarly, this candidate is probably e⁺e^- -> electron positron gamma.
Now it's a game. This candidate looks fairly common, is balanced in momentum, but the total visible energy is only roughly half of the centre-of-mass energy. A number of possibilities: e⁺e^- -> W e nu (single W production, e down the beampipe); Z Z, where one Z decays to neutrinos, the other almost at rest into a pair of quarks; or Z gamma gamma, where the photons both go down the beampipe and are balanced. a pair of neutrinos and the gamma* to a pair of quarks.

Events from the First 172 GeV Runs

Data-taking at 172 GeV centre-of-mass energy began just after midnight on 19 October 1996. The higher cross-section for W⁺W^- production at this energy than at 161 GeV was evident in the first fills, when several W⁺W^- candidates were recorded.