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