version 0.3, John Yoh, 12/2/04

Here, we have the event displays of an event which is a candidate of a pair of neutral Z Bosons (one of the 3 Intermediate Vector boson which carries the force of the weak interaction, which causes such phenomenom as radiative decays), which subsequently decays into 4 electrons (note : we use the "electron" as a generic designation for both electron and the positron, its antiparticle)--2 of the electron candidates are in the central well-measured region, with stiff tracks pointing to central EM calorimeter cluster satisfying the E vs. P match. The other 2 electrons candidates are in the Plug region, with tracks not well measured (one Plug electron candidate actually fails our stringent Plug electron criteria, which one would have expected of order 10% of real electrons--since we use tight selection cuts to choose only those "Golden" electrons). The 4 electrons do form 2 pairs of electrons with each with mass of around 90 GeV, consistant with Z mass, and thus this event is a candidate of a pair of Z decaying into 2 electron pairs. The Mass of the entire 4-electron system is about 190 GeV.

Figure 1 shows the end-view of the tracking system along with the calorimetry, indicating 4 stiff tracks (one that shows almost no bending, and thus is measured to have a very high Momentum). All tracks, which does not have other high energy tracks nearby (we call this "isolated" track) also point directly to energy deposits in the front EM ealorimetry--thus confirming that this is an electron (only electrons and photons deposit almost all their energies in the front EM calorimeter--most other particles such as pions, kaons, and protons would penetrate deeper and deposit much of their energies in the second calorimetry--the HAD (Hadron) calorimetry.

Figure 2 shows the Lego plot, where we see the 4 EM clusters, with practically no other significant energy deposit

INTERPRETATION --One event is difficult to interpret--there are many physics processes that a pair of Z could be produced.
(1) "Old Physics" --Standard model would give us some probability of such an event being observed in our data sample-- perhaps few tenth probability. So, that is the most likely scenario.
(2) On the other hand, there are many other possible interpretation for this event (though, at this moment, based on one event, it's mere speculation-- it would require at least dozens of such events, perhaps even hundreds, for us to untangle what these events might be--see below)
(2a) One possibility is the Higgs Particle--One of the cleanest way to observed the Higgs (though with very small branching ratio---so only a minute fraction of Higgs would decay into 2 Z and then into 4 electrons) is this particular mode of H --> ZZ -->4 e's. However, the most simple model of Higgs would suggest that we would see perhaps .001 such events for a 190 GeV Higgs. There are other models with enhanced Higgs production which might give us a higher probability of observing Higgs.
(2b) Other possibilities are other new objects decaying into a pair of Z's.
If this event is indeed a new heavy object (whether Higgs or others) decaying into 2 Z's, we would expect to see the other modes of Z decays-- equal number of 4 muons; twice as many eemumu; and 6 times as many each eeMET, mumuMET each. Thus, it may well be that these hypothesis could be elucidated when we have a few more time the data sample and thus more statistics.

Given we have dozens (or even hundreds) of such events, how do we tell whether it's "old physics" or a new particle ?? Well, we look at the mass distributions. First of all, we must verify that these are Double-Z events, by making sure that all (or at least most) of these events have a ZZ combination (that is, 2 pairs of leptons (e or mu) with each pair mass close to m(Z)).
Next, we can look at the mass distribution of the 4-lepton system-- which should be falling as a function of the M(4L). If the distribution should a peak at a particular mass (see a sample at left--for the Z mass) that would suggest that a new massive object is produced. Only when this evidence is beyond statistical error can one claim a discovery.