Event with 2 Jets and MET (Missing Et)
Here we have an unusual 2-jet event. Normally, 2 jets events have the 2 jets
back-to-back (in the COT azimuthal view), with one jet directly opposite
the other jet balancing their momentum.
This particular event have 2 jets which are not back-to-back,
indicating that there is a 3rd item off to the side to balance the momentum.
This 3rd item (expected to point in the 9 o'clock direction) is not
observed either as a high Et jet, nor muon nor electron (such a high Et e
or mu would leave a stiff non-bending track, which we do not see)
and is in the MET (Missing Et) catagory, suggesting a neutrino or other
particle(s) that does not interact with our detector (off around 9 o'clock).
So, what could the missing item be ?? Here's some possibilities
- This could have been a Z + 2 jet event, with the Z decaying into a
pair of neutrinos (roughly 1/5 of all Z decays into 2 neutrinos, 1/3 each
into electron, muon and tau neutrino pairs).
- The missing item could be a jet that happens to fragment mostly
into energetic neutrinos with only a few other soft particles observed,--
highly unlikely since we do not see any even low energy particles in that
- The jet off at one o'clock seems to have only 2-3 stiff tracks, so
could possibly be a tau jet--in which case, this could have been a
W + jet event, with the W decaying into a tau (at 1 o'clock) plus a
tau neutrino (at 9 o'clock).
- Other less likely old-physics possibilities
- New physics ? We need to collect a large samples (at least dozens,
perhaps hundreds) of such events, and then do a statistical analysis
determining all sources of old-physics, such as those named above as well
as others, to see if there are any excess that can not be explained by
old-physics, before we can investigate whether there are evidence for
new physics--a long, drawn-out, thorough process which takes man-years
of hard work by dedicated physicists !!!