A Quantum Diaries Survivor

A few days ago I discussed here the meaning of jet energy scale and jet energy resolution… See the relevant post in http://dorigo.wordpress.com/2006/06/16/jet-energy-scale-for-total-beginners/ if you must.

Today, I would like to discuss with some detail why I have been spending lots of my time on the issue of improving the resolution of CDF measurements of the energy of b-quark jets.

Actually, if a little auto-biography is allowed here, let me make a diversion for a moment, to say I have been working at improving the measurement of hadronic-decaying resonances for the whole of my career as a physicist:

- Sought a signal for top decays into six jets, 1992-1995. Found a signal, published "First observation of the all-hadronic decay of ttbar pairs", 1997.

- Sought a signal for Z decays to b-quark pairs, 1996-1998. Found a signal, got PhD thesis, 1999. Not published - moved to more pressing issues with CMX construction

- Sought a signal for associated production of Upsilon mesons and W/Z bosons (including the jet decay of the bosons). Found no signal, demonstrated that the jet decay was the one yielding highest sensitivity. Published "Search for associated production of Upsilon and Vector boson",  2002.

- Took on problem of improving the resolution of H->bb decay in the Higgs Sensitivity Working Group. Devised algorithm that achieves 10% resolution on mass of resonance.

- Sought a signal of Z->bb decays in Run II data. Found a signal, blessed it. Signal now being used to extract a measurement of the b-jet energy scale. Expect determination for late summer 2006.

- Managed Jet Energy and Resolution Working Group in CDF, September 2004- September 2005. Devised b-specific jet energy corrections with Hyperball algorithm. Corrections now under testing on Z->bb signal.

Ok. Now that the picture of why I care for the jet energy measurement is clear, if you got this deep in the post you deserve some insight on Why the hell am I so interested in improving the b-jet energy resolution.

The answer lies in the figure above. It shows three sets of curves in a plot where the x axis is the unknown mass of the Higgs boson, and the y axis is the total integrated luminosity CDF and D0 wish they will collect during Run II. The two curves for each color show predictions of 1998 (the thick ones) and 2003 (the thin ones). The purple curves indicate what luminosity y is needed to exclude the existence of a Higgs boson with a mass lower than x. The green curves indicate the luminosity y' needed for a first indication of the existence of the Higgs boson at the mass x; and the blue ones indicate the luminosity y'' needed to claim discovery, again if the mass is x. 

The plot is the result of a lot of work by several people in CDF and D0 during 1998-1999, and again by a smaller committee in 2003. The 2003 results basically ended up confirming the previous ones (the thinner bands show more precise determination of luminosity thresholds, but no systematic uncertainties were considered to derive them - basically for lack of time), and are credible since they are based not on an extrapolation of the predicted behavior of the detectors, but on real data actually collected by CDF and D0 until spring 2003.

Ok, so how about b-jet resolution ? Well. In order to push those curves down to the low luminosity values you see in the plot - low enough that the Tevatron still hopes to discover the Higgs boson, there being a chance that before 2009 the Tevatron will deliver 8 fb-1 to each experiment (check what mass corresponds to 8 fb-1 in the green band above) - CDF and D0 needed to demonstrate that the resolution for a resonance decaying to a pair of b-quark jets was of the order of 10%.

Which leads us to my own plot - my contribution to the HSWG in 2003. With the hyperball algorithm, together with two other steps of jet corrections, we did get to a 10% mass resolution, as can be seen in the bottom right quadrant.

In the plot, the red points show the mass reconstructed from a set of simulated Higgs boson decays (for a Higgs mass of 120 GeV), and the other histograms describe the various backgrounds that one has to cope with when trying to identify the Higgs boson in the production process ppbar -> WH -> l u bb , that is when a proton-antiproton collision at the Tevatron yields a W boson and a Higgs, and the former decays to a lepton-neutrino (lu) pair, while the Higgs decays to two b-quarks (which hadronize into two b-jets).

Amusingly, soon after producing the plot above, CDF issued a first analysis of their W+jets data in the search for a Higgs boson, and sported a 17% mass resolution… But that was before any optimization! In fact, that less-than-satisfactory result was what spurred the creation of the Jet Energy and Resolution Working Group, which collected people working in four different smaller working groups in CDF.

By the end of the summer, CDF will have something to say about their predictions for a 10% resolution: it will be a by-product of the determination of the jet energy scale, by analyzing the Z->bb signal. Stay tuned!