A QCD measurement and why you should care about it August 25, 2008Posted by dorigo in news, physics, science.
Tags: CDF, Higgs boson, QCD
Quantum ChromoDynamics, the theory of strong interactions, is admittedly not considered the most exciting branch of particle physics at colliders these days. QCD processes make up 99.99% of what happens in hadronic collisions at the Tevatron, or what will happen at the LHC starting this fall: they are usually backgrounds to those much more interesting reactions involving electroweak bosons and leptons, or to the searches for the Higgs boson.
I would like to point out that QCD is in truth a wonderfully complex and beautiful theory, and that QCD measurements are very important. Only by understanding strong interactions in detail can we hope to find new phenomena lying underneath. And don’t even get me started on the need for more studies on low-energy strong interactions -they are really not well understood yet, and in fact precise measurements of strong interaction cross sections are direly needed in cosmology. But let me go back to high-energy high-energy physics.
Today I would like to discuss a precise measurement by CDF which will prove very useful when somebody -me and Mia, for instance- will start studying CMS data in search for the decay : the production of a higgs boson and its subsequent decay into a pair of Z bosons, with a final state including one leptonic Z very easy to identify, and a second one which can be separated from backgrounds by the identification of b-quark jets.
The signal is buried in a large background, namely production where the pair of b-quarks is not coming from a Z boson decay. How large ? Well, we have theoretical calculations, Monte Carlo simulations incorporating them, detector simulations… We have a pretty good idea, but unless we check that these calculations are precise, we are stuck with large systematic uncertainties. One good part of these is due to our limited knowledge of the probability to find a b-quark inside the proton, the b-quark PDF.
A recent result which improves matters has been obtained on the cross section for production by Andrew Mehta and Beate Heinemann, two very skilled colleagues from Liverpool and Berkeley, respectively. The comparison of the result with theoretical predictions provides a nice confirmation that the latter are in the right ball-park, and an estimate of the level of trust we can put in them. Let me try and describe very briefly how the measurement is produced.
Events with a leptonic Z boson decay are selected from 2.0 inverse femtobarns of proton-antiproton collisions produced by the Tevatron 1.96 TeV synchrotron in the core of the CDF detector. Both and decays are selected, for a total of about 200,000 events. Among these, the analysis selects those events containing one hadronic jet which has a secondary vertex reconstructed with its charged tracks: the vertex is the signal of the decay of a B-hadron, which contains the long-lived b-quark. By selecting jets with a secondary vertex, their b-purity is increased tremendously.
Below you can see the Z mass peak for events containing a b-quark jet accompanying the dilepton system. The black points are CDF data, the black line is the total of the various contributions, which include, together with the signal, few small backgrounds.
To compute a cross-section for production, there remains one step (ok, I am making things simpler than they really are, for the sake of clarity and space): understanding the fraction of these jets really due to b-quark hadronization. This can be accomplished by studying the invariant mass of all the measured charged tracks originating from the secondary vertex: the mass is larger for real b-quark jets and smaller for charm-quark jets or jets due to lighter quarks or gluons, for which the secondary vertex is due to a random mismeasurement of tracks rather than the true decay of a long-lived particle.
Above you can see a fit to the secondary vertex mass distrbution, with the three components. The cyan histogram represents the b-jet fraction, which has a larger vertex mass and amounts for 40% of the total. By measuring the fraction of b-jets one can proceed to measure the cross section, if one knows the efficiency of the selection of Z boson decays and the efficiency of the vertex-finding b-tag algorithm. What I am talking about is the following formula:
Don’t be scared: the ingredients have all been introduced to you already. is the cross-section of the process, i.e. the thing that is measured in the analysis. is the fraction of b-jets among those with a secondary vertex, and is extracted by the figure shown above. is the fraction of Z bosons which are detected and reconstructed from two observed muons or electrons; is the efficiency of finding the b-jet with the required energy and with a secondary vertex inside it. Finally, is the integrated luminosity of the data used, 2.0/fb.
What is the result ? CDF finds , a small number -eight times smaller than the cross-section for producing a pair of top quarks! Theory calculations at Next-to-Leading-Order (a good level of precision for this calculation) predict , a figure smaller but not utterly incompatible with the data.
Maybe the most interesting part of the measurement is the ratio between the measured cross section and the cross section for production of one Z boson alone. It is shown in the plot below as a function of the transverse energy of the b-jet, compared to three different Monte Carlo calculations. As you can see, the fraction of Z bosons which are produced together with a b-jet is tiny! The reason has to do with the smallness of the b-quark PDF.