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A ZZ candidate by D0 – no, wait, Zgamma* March 11, 2007

Posted by dorigo in internet, news, physics, science.

The creation of a pair of Z bosons is a very rare process in the Standard Model. The proton-antiproton collisions of the Tevatron collider, at 1.96 TeV of energy, are expected to yield one such event every forty billions, so the detection of this process is indeed not easy from the outset. What’s worse, Z bosons are only detectable with clarity when they decay into electron-positron or muon-antimuon pairs, something that happens to them less than 7% of the time. The Feynman diagram below shows the creation of a ZZ pair and their decay:


Seven percent times seven percent is five times in a thousand. And then, one needs to take in account the fact that the detector has a limited acceptance to the electrons and muons. What’s more, additional kinematic and clean-up cuts must be applied to reduce backgrounds effectively. The result of the careful cuts applied by the D0 analysis are that 23% of four-lepton events are selected.

So let’s compute on the back of this virtual envelope how many ZZ events the Standard Model should yield in the D0 detector if they collect one inverse femtobarn of collision data – which is to say, 60 thousand billion proton-antiproton collisions.

Sixty thousand billion divided by forty billions (the chance of producing a ZZ pair) is 1,500. Take that times 0.005 (the fraction of ZZ pairs which yield four leptons of the electron or muon kinds) and you are left with 7.5. The latter, times the acceptance factor of 0.23, brings down to 1.71 events.

On the face of 1.71 ZZ events expected, only 0.17 events due to backgrounds are expected to survive the tight selection. The latter are partly due to top pair production, when both top quarks decay to a charged lepton, a neutrino, and a b-quark which in turn yields an energetic leading lepton: all in all, you get a quite similar signature to the ZZ decay, save that you expect more energy around the leptons (from the remainder of the b-quarks) and a transverse imbalance in the overall momentum due to the escaping neutrinos.

So what did D0 find ? They find one event, quite in line with expectations. It displays two energetic electrons (ok, one of them is a positron of course!) and two muons. It would indeed appear they saw a ZZ candidate, were it not for the fact that  the invariant mass of the object which decayed into the two muons is only 33.4 GeV! (The two electrons do make a well-behaved mass of 93.4 GeV, close to the Z mass value). The event display shows the electrons as red bars (their heights are proportional to the electrons’ transverse energy as measured in the D0 calorimeter) and muons as green bars (same proportionality to the measured momentum of muon tracks). The two coordinates in the plot are phi, the azimuthal angle in the plane orthogonal to the beam line,  versus eta – basically a monotonous function of the angle of the emitted particles with respect to the beam:

So, can we classify this as a ZZ event ? No. It is most likely a Z-gamma* event: one where along with one single Z boson, the collision produced a virtual photon (gamma*, the * symbol meaning that the particle is far off its typical mass value – that is, virtual) of 33.4 GeV energy, which materialized immediately in a muon pair. Since D0 did not explicitly apply an invariant mass cut on the lepton pair masses, they do expect some Z-gamma* contamination in their final selection, and that is what they appear to have seen.

I know I may raise a few eyebrows here in the less die-hard particle physicists: how can one tell a Z from a virtual photon, after all ? Well. The two are distinct entities, but it is true that they interfere quantum-mechanically, which is to say they are utterly impossible to tell apart. However, a M=33.4 GeV neutral electroweak carrier is made with lots of photon wavefunction and quite a tiny little Z content… So this is really better described as a Z-gamma* event. That, in fact, is the opinion of the  D0 preliminary paper itself .

So far, therefore, the only ZZ event seen at the Tevatron appears to be the one seen by CDF, which I mentioned a few months ago in a post about  the discovery of WZ production by CDF , and discussed in a recent proceedings paper .


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