D0 bags evidence for semileptonic dibosons October 24, 2008Posted by dorigo in news, physics, science.
Tags: D0, hadronic signals, Higgs boson, Tevatron, weak decays
A week ago I discussed here the recently approved analysis by which CDF shows a small hint of WW/WZ signal in their Run II data, with one W boson decaying to a lepton-neutrino pair, and the other boson (either a W or a Z) producing a pair of hadronic jets.
Such a process is very hard to put in evidence in hadronic collisions, due to the large irreducible background of events due to one leptonic W decay accompanied by QCD radiation from the initial state of the collision. In fact, despite having been sought by many in Run I, no appreciable signal had surfaced in Tevatron data either from CDF or D0.
Now, D0 has really bagged it. They used a more performant selection method than the one used by CDF, and were a bit more bold in their use of Monte Carlo simulations. The result is that they find a very significant excess, amounting roughly to 960 events, in a total of nearly 27,000.
I encourage those readers who are unfamiliar with the basics of vector boson production at hadron colliders to read the introductory part of the former post on this topic, which I linked above. Here I will avoid repeating that introduction, and concentrate instead on the analysis details.
D0 uses a total of 1.1 inverse femtobarns of 1.96 TeV proton-antiproton collisions, acquired during Run II of the Tevatron. The samples of data are collected by triggers selecting a signal of a high-energy electron or muon, and a further requirement that a transverse energy imbalance of 20 GeV or more is requested, thus characterizing the leptonic decay of one vector boson. Finally, the transverse mass of the lepton-missing transverse energy system has to be larger than 35 GeV, reducing backgrounds from non-W events.
[The transverse mass is computed by neglecting the z-component of the particles momenta: if both particles are emitted perfectly transverse to the beam direction, transverse and total mass coincide. This is forced by the absence of a z-measurement of the neutrino momentum, since the energy imbalance it creates by escaping the detector cannot be measured along the proton-antiproton axis.]
Besides characterizing the leptonic W decay, two jets with transverse energy above 20 GeV are required. After this selection, the data contain a non-negligible amount of non-W backgrounds, constituted by QCD multijet events where the leptonic W is a fake; but the bulk is due to W+jets production, where the jets arise from QCD radiation off the initial partons participating in the hard interaction. Several Monte Carlo samples are used to model the latter background process, while the former is handled by loosening the lepton identification criteria in the data: the looser the lepton requirement, the larger this contamination, such that for really loose electron and muon candidates the samples are almost purely due to QCD multijet events.
Signal and backgrounds are separated using a multivariate classifier to combine information from several kinematic variables. This is the Random Forest algorithm, which I had the occasion to discuss in the past (two years ago a student of mine used it to discriminate hadronic top events in a similar dataset in CDF). The Random Forest output is highest (close to one) for signal events, while backgrounds are given a value closer to zero. The result of the classification is shown below: the excess for high values of RF output are due to the diboson signal (in red the signal content estimated by the fit).
A fit to the RF output provides the normalization of the signal and the background components, as shown above. Notice the blue “envelope” in part (b) of the plot: it is the systematic uncertainty due to background RF templates. Of course, the level of the blue curve is deceiving, since shape uncertainties are totally correlated among themselves; but the signal does stand out on top of it.
A plot of the dijet mass distribution confirms the interpretation, as shown below. The bottom part shows the data subtracted by background contributions (points with error bars), which compares well with the shape of the expected diboson contribution. D0 finds a combined WV (WW+WZ) cross section of , where the first uncertainty is statistical, the second is systematic, and the third relates to the integrated luminosity uncertainty of the base of data used in the search. This compares well with the theoretical prediction of .
In the plot above, the combined W/Z signal peaks at about 80 GeV, with a resolution of roughly 15 GeV; the background template uncertainty is again in blue, again underlining the difficulty of this measurement, which finds a signal excess exactly where the backgrounds peak.
One question I often hear asked in plots such as the one above is “why do W and Z boson peak at the same mass value ? They have a 10.7 GeV mass difference after all”. True, but the dijet mass resolution of the D0 detector is insufficient to tell the two signals apart, and what one observes is the combined shape. To be more precise, one should also add that the Z contribution in the plot is much smaller than the W one (about one third). Further, one should also point out that the heavy flavors produced by the Z boson will produce a underestimated Z mass reconstruction, due to the neutrinos often produced in the semileptonic decay of b- and c-quark jets. It is a fact that the decay will peak at about 83 GeV after calibration of generic jet response, due to that effect alone…
I like to let the authors point out that “This work further provides a validation of the analytical methods used in searches for Higgs bosons at the Tevatron”, as in the conclusion of their paper. Indeed, the advanced methodologies by which the Tevatron experiments are setting more and more stringent limits on Higgs boson production are perceived by some as a bit uncautious. Things appear to be well under control, it instead transpires, once one can demonstrate that a signal known to be there can be indeed extracted from samples which have a very small signal-to-background ratio, as is the case of all Higgs searches.