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.