WZ production discovered! October 26, 2006Posted by dorigo in news, physics, science.
Yesterday the CDF collaboration blessed a new result on electroweak physics: the measurement of cross section of the production of WZ pairs.
When protons and antiprotons accelerated by the Tevatron collider are smashed together in the core of CDF, they have a very slight chance of producing a W boson: this happens only once in three million collisions. Yet less frequent is the production of a Z boson: one in ten millions. Despite their rarity, the production of W and Z bosons is extremely important for the physics program of CDF. In fact, it is through the collection of these particles that we discovered the top quark, and now that the Higgs boson search is in full swing, the identification of W and Z decays is even more crucial. W and Z boson datasets, collected by identifying the charged leptons emitted in the leptonic decay, now amount to hundreds of thousands of events.
However small a one-in-ten-million process may seem to you, the search is now focusing on associated production of two of these particles together. The probability of producing a WW pair is one in six billions, but that process was observed as far back as in Run I by CDF. Still more exotic is the associated production of a WZ pair, predicted to be a one-in-20 billions process. And ZZ production runs at the staggering one-in-60-billions rate.
Why are we so interested in the production of pairs of these objects ? After all, we know W and Z bosons extremely well. The fact is, we want to test the Standard Model – the physics theory that allows us to understand processes between elementary particles and compute observable quantities – in al the available realm of predictions it makes. The rarer a process, the better the chances that something does not agree with the theory, in a way. Unexpected things might hide themselves thanks to the rarity of the processes involved.
There is more to it, of course. We think we may one day see Higgs bosons decaying to a pair of W bosons or, even more strikingly, to a pair of Z bosons. The latter would be an extremely clean signature for Higgs boson production. So we collect events with W and Z bosons, and search for a second decay signature. WZ production should yield three charged leptons plus a significant amount of missing energy: while Z-> lepton-lepton can be reconstructed easily, the W-> lepton-neutrino decay yields a neutrino that escapes the detector unseen.
CDF sifted their data in search of events with three charged leptons (electrons or muons) of high energy, and studied the distribution of the missing energy in these events. You can see the distribution in the plot on the left. It is apparent that if you do not add the the mixture of known “background” processes the signal of WZ production, you cannot explain the spectrum of collected events, particularly when large missing transverse energy is present.
Of course, tens of different kinematical distributions are studied to verify that what is observed is indeed what it is sought. For instance, you can see in the second plot below the transverse mass of lepton and missing energy (transverse mass is the mass of the body that produced the lepton and missing energy, computed in the plane transverse to the beam since the longitudinal component of the missing energy cannot be measured in hadron colliders). Everything agrees: CDF observes 16 trilepton events with significant missing energy, when 2.7 are expected if no WZ production is included in the tally.
Interestingly, CDF does observe also a quadri-lepton event, which is perfectly consistent with ZZ production (1.8 events expected from that process alone). The background from other processes should yield only a fraction of a hundredth of an event… So the best explanation for that one event is indeed ZZ production, with the subsequent decay to four muons. But one event is not enough to claim for discovery of ZZ production, so that will have to wait for more statistics!
Now Higgs seekers will jump up and down on their chair: what is the total mass of the four leptons ? Well, it is 312.4 GeV: far from the expectation for a standard model Higgs boson.
From a purely aesthetic standpoint, these events are beautiful to look at. In fact, what you observe in the detector is the clean signal of the four final state particles, plus basically nothing else – only a few low-energy tracks are produced in association with the two vector bosons. This has to be compared to the usual “mess” of strong-interaction events, when hadronic jets are produced which splash energy everywhere. Below you can see a clean WZ candidate event, in the usual transverse view (a cutaway orthogonal to the beams direction, showing a section of the CDF detector and the charged tracks reconstructed, plus their energy deposit in the outer shell of calorimeters).
It is fair to say at this point that our competitors in D0 also observed a few WZ candidates recently, and they also measured a cross section for the process from their excess. However, their result was only quoted as a “evidence” for the sought process, the probability of a background fluctuation being 3.4 sigma, that is a not overwhelmingly improbable occurrence. CDF establishes WZ production with its analysis. The measured cross section in 1.96TeV p-anti p collisions is measured at 5.0 +1.8 -1.6 pb, well in agreement with the standard model prediction of 3.7+-0.3 pb.
One last thing to note: for years, the ideas of finding Supersymmetry from the clean multi-lepton signals the cascade decays of SUSY particles has been advertised… Now that we start seeing the very rare standard model processes yielding multi-leptons, and we still see no new physics, we have one reason to frown… Or at least, one more reason for betting 1000 dollars on the absence of SUSY at LHC (see https://dorigo.wordpress.com/2006/09/04/this-1000-says-there-aint-new-physics-at-the-tev-scale/ )!