A Quantum Diaries Survivor

I have been searching for Z decays to b-quark pairs in proton-antiproton collider data for the last 11 years. Of course that was not the only thing I did research-wise in this long time span, but it has been some sort of trademark of my research activity.

During my whole career I almost exclusively dealt with the reconstruction of particle decays in jet final states. I saw top quarks, W bosons, and Z bosons decays to jets -and in two cases, these were first-time observations. Let me describe what this is about in a few lines.

The heaviest known particles - W and Z bosons, plus of course the top quark, or the still-to-be-seen Higgs boson - can decay to quarks pairs (triplets, the top) carrying so much energy that the latter fragment into collimated streams of hadrons - particles themselves composed of quarks.

Hadronic jets are complicated objects. Their efficient reconstruction and precise measurement in present-day experimental apparata is not trivial. And the measurement of the mass of the originating particle, which disintegrated to yield the quarks and eventually the jets we observe, is very challenging.

I begun my career as a particle physicist in 1992, when I started collaborating with a group of physicists in Padova that was looking for the decay of top quark pairs into six hadronic jets - a final state apparently doomed by a huge background of QCD processes. Despite skepticism from our collaborators, my group eventually found a signal of that decay (you can see the top quark decay contribution as a white area in the plot, best fitting the black points if added on top of the yellow distribution describing the background shape), and in 1997 we published a “first observation” paper describing our result. 

From 1996 to 1998, as a PhD student, I decided I would search for the decay of the Z boson to pairs of b-quark jets. As hard as finding six-jet decays of top-antitop pairs had been, this is even more difficult: you only have two jets in the event, and your background is more than 1000 times larger than the expected signal, and almost impossible to tell apart. In fact, at the outset most people in my collaboration believed I was wasting my time -a few experts had tried and failed before I even started. But I was confident in myself, and I also knew that finding Z->bb decays was of paramount importance: even failure would not be a waste of time!

In fact, most of our hopes to find the Higgs boson at the Tevatron rely in the identification of its decay to pairs of b-quark jets, the very same final state of Z decay. And the Z is only slightly lighter than the Higgs is expected to be. But the Higgs has a much lower production rate! How, then, can we hope to see a Higgs if we do not first find the Z in the same final state ?

Moreover, a well-defined Z signal allows one to tune the response of our detector to b-jets. We can test whether our jet energy is measured correctly, because we KNOW what the Z mass is (the LEP experiments measured it with extreme accuracy), and what we reconstruct in the detector can be brought to match that knowledge, improving our measurements with b-jets - the top quark mass, or the Higgs mass if the latter is found.

Finally, the Z signal becomes a testing ground for any algorithm that attempts to increase the resolution of the b-jet energy - again, an issue of paramount importance in the search for the Higgs: if you are looking for a tiny little bump in a mass distribution, the better your resolution the higher your bump will stand on top of a flat background - so your discovery reach on the Higgs is a linear function of your energy resolution!

In 1998 I blessed the result shown on the left. I found a small signal, about 90 events (you can see the excess of red points over the blue histogram in the inset, shown as a function of the reconstructed Z boson mass, and a background-subtracted distribution in the main plot): not altogether so significant by itself, but an important assertion about the capability of hadronic colliders to use that signal to learn how to measure b-jets and search for a light Higgs boson.

Then, in 2005 Julien and I, together with other members of a small, dedicated group were able to bless the “public-relation” plot shown here, using four times more statistics and a much more efficient triggering procedure from Run II data. The green Z decay signal shown here amounts to about 3000 Z decays, but we did not use it in the determination of the b-jet energy scale, nor did we obtain a precise estimate of the production cross section.

Last year it was D0’s turn to show what they could do in this department. They found a signal of about 1200 events (see plot on the left), but they did not venture in a full measurement of the energy scale or the Z cross section either.

Ok, that was the past. Now, in little less than two weeks, my work of ten years will reach a significant milestone. The search for Z bosons in CDF Run II data will be made public by CDF via what is called a formal “blessing” - an internal meeting where the results of a full-blown analysis are carefully scrutinized and approved for public consumption. After that happens, you will see here the new plots and the results we obtain.

This post is becoming too long, so I will describe the analysis - still without giving out any classified details yet! - in another post tomorrow.