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Associated ZZ production measured! April 5, 2007

Posted by dorigo in news, physics, science.
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Particle physics these days is a dirty business. One does not simply observe a process and measure its characteristics: by having gone to extremely high energies, trying to explore extremely low probability processes, we have gotten accustomed to the fact that we have to cope with large backgrounds – processes which are much more frequent, and which mimic the signature of the signal we try to find.

That means that whatever process we claim to be observing can never be associated with any particular event we detect in our apparata. Statistical inference is used as a premise to any claim, and the strength of a inconfutable “observation” -such as that of looking at a photographic plate of a bubble chamber interaction, as it used to happen during most of the past century- is replaced with sizing up the number of standard deviations of the effect you see.

However, every now and then we do get to study something that allows us to feel like we are back to the early days of particle physics, when observing one event would mean a sure observation of a process and a first determination of its cross section, and observing two would allow some inference on angular distributions – and therefore a determination of the all-important angular momentum characteristics of the observed particle decay!

That is indeed the case of observing such a rare and yet so strikingly unmistakable process as associated production of two Z bosons, in proton-antiproton collisions. To give you an idea, only once in sixty billion collisions you can produce a pair of Z bosons in Tevatron collisions, and only one every ten or so of those pairs decays in a way that allows us to spot them.

CDF had recently reported about the observation of a candidate ZZ event, decaying to a pair of electrons and a pair of muons (see picture on the left, which shows the tracks of electrons and muons in a transverse cut-away view of the CDF detector). However, that was a curiosity: the event was part of a dataset used to measure WZ production, another rare process which is easier to detect. Now, a full-blown analysis has allowed to provide a first measurement of the cross section for ZZ production, by taking into account many different final states, including the one to which the above event belongs. Granted, all the considered signatures involve leptonic decays of Z bosons, but now the analysis includes the signature of two charged electrons or muons from one Z and neutrinos from the other.

Neutrino pairs! The neutrino is a quite special particle. It is practically massless, and it interacts so weakly with matter that it escapes the 5000-ton CDF detector without breaking a sweat or leaving the faintest trace. So when a Z boson -a hefty particle weighting about as much as 95 protons- decays in two neutrinos, we see nothing. However, when two Z bosons are produced, and only one of them decays to neutrinos, we see a large imbalance in the energy flow produced transversely to the proton-antiproton beams: two leptons from one Z traveling in one direction, and nothing in the other direction!

A picture is worth a thousand words. Imagine two heavy objects produced in the collision. They must be traveling in opposite directions, due to the law of physics called “conservation of impulse”. You see one of them materializing in two electrons (see picture on the left): by measuring the electron energies and directions in the calorimeter you determine that the pair does come from a Z boson due to their “invariant mass”. Then, you search for a signal of the other Z boson, and you find none. No particles, no energy detected from neutral bodies in the calorimeter, nothing. You are left with measuring the “transverse energy imbalance”, the arrow in the picture.

The spectacular events shown above are so infrequently happening due to alternative processes (what we call “backgrounds”) that, despite the quantum nature of the involved processes, we would be willing to bet that what we see is what we describe as ZZ production. However, while the one event CDF observed with two electrons and two muons is really several times more probable to come from ZZ production than from backgrounds, the few events with energy imbalance are easier to mimic by backgrounds. I would bet money on the first, not on the second. The former reminds me of particle searches at bubble chambers of the fifties or sixties of the last century, the latter is a more complex business.

Anyway, the analysis leading to a cross section measurement is not so emotional. It entails a careful determination of all possible background sources, a determination of signal efficiency, and a comparison of event count to predictions for either background alone and background plus signal.

The result ? CDF measures a production cross section of 0.71 picobarns for ZZ pairs. The evidence is only at the level of 3-sigma, but it is quite robust, and the larger dataset already collected will grant a more solid result in the course of 2007.

If you are interested in more detail, please visit the public page of CDF: http://fcdfwww.fnal.gov/physics/ewk/2007/ZZ/  .

I also recently reported on a search for these events by D0 in https://dorigo.wordpress.com/2007/03/11/a-zz-candidate-by-d0-no-wait-zgamma/. And I mentioned the ZZ event found by CDF in the four-charged leptons final state in my discussion of the WZ observation in https://dorigo.wordpress.com/2006/10/26/wz-production-discovered/ .

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