A Quantum Diaries Survivor

I am writing this single-handedly because of a stupid mistake. I am  spending this long week-end (1st of May is a holiday in Italy) in a nice mountain place in the austrian alps with family and friends, and I found a way to make things interesting…

I was carving a small wooden ship for Ilaria this afternoon, on the terrace of our hotel room, and I forgot that the blade should be always aimed in the opposite direction of your flesh. So as a hard piece of wood gave in all of a sudden, I found myself shocked, looking at a wide cut on the second finger of my left hand. The skin layer was parted, allowing a entertaining view of the bone, partly attacked by the violent blow. Blood was copious but fortunately not threatening.

After some initial trouble - arranging for a trip to an emergency room isn’t straightforward with two kids, one of them sleeping - things were straightened out. The tendon was not damaged, and four stitches did the job. I am still shocked, though, by how stupid I have been. In cases such as this, however, I tend to feel happy for the fact that things are easily repaired - bad luck could see me impaired in my playing the piano or typing for what’s left to live… And since I intend to live for a long while more, it would have really been disappointing!

Upon coming back from a shor vacation on the Alps, I rushed to connect my laptop to the internet. And one of the first things I did was to check for recent results by CDF. The experiment has been producing new beautiful results at an impressive pace during the last few months: it is as if the work of years of preparations, refining algorithms, tools, thinking hard at new methods, and a parallel strong push for the collection and processing of data had converged to a singularity, and now results are popping up like flowers in a garden.

My interest in the new analyses is boosted by the fact that in less than a month I will be describing them at PPC2008, a conference in Albuquerque where I am going to give a talk on new CDF results. So it is about time for me to start thinking about the organization of my talk.

As I browsed the recent talks in the Higgs Discovery Group, I found a new blessed (i.e., internally approved for public consumption) result that warmed my heart. It is the first Run II limit on associated Higgs boson production based on the 4-jet signature of WH or ZH decay. This signature arises when the Higgs boson is produced by the process called “higgs-strahlung” off a virtual W or Z boson, and both bosons then decay in a pair of hadronic jets (see picture). The Higgs, if it is lighter than 135 GeV, most of the times decays to a pair of b-quarks (in red), while W and Z bosons decay to all available quarks (in blue) more democratically.

Hadronic decays of vector bosons are the most common ones: W bosons decay to two quark jets 66% of the time, and Z bosons 70% of the time. So, with a large fraction of Higgs bosons also materializing into two jets, looking for four-jet final states to see a WH or ZH signal might look like a no-brainer. Quite the contrary!

Indeed, the 4-jet final state has always been considered absolutely hopeless. 4-jet events are among the most common final states of a proton-antiproton collision, and the kinematic handles one can use to try and discriminate associated WH or ZH production from generic QCD 4-jet production are absolutely insufficient. One can consider the invariant mass of pairs of jets, in the knowledge that W, Z, H all have a well-defined mass, while QCD produces jet pairs without any constraint on their common mass.

Hopeless, in particle physics, is a very attractive word for some of us. Out-smarting our colleagues is one of the highest forms of satisfaction in a scientific workplace… So, after my group demonstrated against all odds the possibility to see top pair decays in their 6-jet final state (one that arises when both W bosons emitted in the chain decay to jet pairs), in 1996, we started thinking at what would be the best way to exploit the experience we had formed in reconstructing high-mass states with jets.

One branch had already born fruit: my PhD was already in full swing, and I would show a first signal of decays soon thereafter. But that is another long story. Instead, in 1998 we started working at the idea of reconstructing the WH or ZH signal in events with four hadronic jets. In Run I the analysis had already been undertaken by Juan Valls and Jorge Troconiz, and they had indeed produced a fine piece of physics, with a limit on Higgs production which challenged those in the “golden” leptonic channels.

We aimed at Run II, and started working at the most critical issue: the one of triggering on 4-jet events with b-quarks. The multijet trigger which had been the basis of both the and the analyses was very inefficient on the latter signal, because of inefficiencies in the online jet reconstruction.

Enter the SVT (silicon vertex tracker), a fantastic device which measures online the impact parameter of tracks, allowing the collection of B-decays with high efficiency. SVT had been designed for B-physics purposes and was thus aimed at low-energy events, so we needed to verify it would work fine for 4-jet events too. This implied determining that those complicated, high-track-multiplicity events were reconstructable in the 20 microseconds available for a trigger decision at Level 2; and then designing a set of selection cuts that would allow the maximum efficiency on signal events while keeping the data acquisition rate at an acceptable level. In parallel, we also studied alternative strategies involving the semileptonic decay of B-hadrons, by combining jet signatures with soft lepton detection.

This job kept us busy for three years, and fruited a graduation to Giorgio Cortiana, a PhD to Luca Scodellaro and Mario Paolo Giordani (and I am certainly forgetting some other students). But as Run II started for real, and multijet events started being collected with high efficiency, we gradually lost interest: Luca Scodellaro’s analysis had shown that the signal was really, really hopeless. Too hopeless even for us - or maybe we were already growing old and disillusioned ?

The recent analysis by Song-Ming Wang, Rong-Shyang Lu, and Ankush Mitra (Academia Sinica), Daniel Whiteson (UC Irvine), and Aart Heijboer and Joe Kroll (University of Pennsylvania) shows otherwise. Sure, they do not reach a sensitivity sufficient to exclude Standard Model production of WH and ZH events in any region of Higgs masses, but they nevertheless extract an excellent result which will be successfully combined with the other searches, improving the global Tevatron limits on Higgs production. Since this post has become much longer than I wanted, I will only describe it shortly, and jump to the results.

The analysis selects events with four jets, two of which have to contain a signal of B-hadron decay, and then uses a Matrix-Element approach to determine the probability that the observed final state is the result of the decay of a WH or ZH pair, and the probability that it is instead due to background processes. The information is merged in a discriminant which separates the processes on a statistical basis. One thus ends up fitting the distribution of the discriminant as a sum of background and signal, as in the plot below.

To put in evidence the small contribution from top pair production (in blue), diboson and single top (in green), and WH/ZH processes (in red), a logarithmic plot is appropriate:

As you see, the signal would contribute mainly in the right part of the distribution, but with a tiny fraction of the events: Standard Model predicts a contribution of less than 10 events in a sample of more than 20,000.

The maximum amount of signal allowed by the fit determines a limit on the production cross-section of Higgs and vector bosons. The limit on the cross-section depends on the Higgs boson mass for two reasons: one is the increase in collection efficiency as the Higgs mass grows, and the other is the decrease in Higgs branching fraction to b-jet pairs. In the end, one obtains a limit on the ratio between cross section and SM expected cross section, as a function of Higgs mass. The limit is always larger than 1 -it actually is higher than 30- so no Higgs mass is excluded by this search. It is shown below with a red line; the limit the analysis would predict to set, based on pseudo-experiments, is shown by the hatched black line and 1-sigma and 2-sigma yellow and green bands.

This result really makes me feel that the work we did eight years ago was not wasted!