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D0 bags evidence for semileptonic dibosons October 24, 2008

Posted by dorigo in news, physics, science.
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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 W \to l \nu_l 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 20.2 \pm 1.4 \pm 3.6 \pm 1.2 pb, 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 \sigma(WV)=16.1 \pm 0.9 pb.

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 Z \to b \bar b 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.

When increasing the collider energy does not pay off July 14, 2008

Posted by dorigo in physics, science.
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Run 1C at the Tevatron lasted only a few months between 1995 and 1996. The data which had been instrumental to discover the top quark and measure its mass had been already collected, and a long shutdown was awaiting to allow a massive enhancement of the accelerator complex and the upgrade and commissioning of the improved CDF and D0 detectors in view of Run II.

I was reminded of that small dataset -a handful of inverse picobarns, which fruited only a few papers and studies- while making a summary of past searches for heavy particles (W, Z, top quarks) in hadronically triggered datasets at hadron colliders. Hadronic signals of electroweak decays are hard to extract in hadron collisions, where they get swamped by strong interaction processes: the signal to noise ratio is usually smaller than a thousandth before any selection.

In 1987 the UA2 experiment used 4.7 inverse picobarns of proton-antiproton collisions collected at a center-of-mass energy of 630 GeV to show that W and Z bosons did indeed also decay to jet pairs (the signal is the small bump on the lower plot on the right). It was not a real surprise -nobody doubted that weak currents coupled to quarks as much as they coupled to leptons- but it was indeed a significant achievement, because it showed that the comparatively tiny signal could be extracted.

Then, we had to wait until 1996 for the next hadronically-triggered weak decay to emerge: it was the one of top quark pairs with a totally hadronic decay into six jets. My group in Padova isolated the signal after five years of studies, to which I participated actively from the start: it was my first achievement in CDF. We were later also able to measure the top mass from the same sample, and our measurement was surprisingly good, given the premises.

The next news of a hadronic signal was the observation of Z decays to b-quark pairs, the topic of my Ph.D. thesis, in 1998. I have written several times about my work on this topic, so I will not indulge in details here.

Nothing else of the same kind has emerged in CDF and D0 since then (I repeat: hadronically-triggered events yielding a signal of weakly-decaying heavy objects). The next possible signal could be that of supersymmetric particles showing up in multijet datasets with large missing energy, but I try to keep believing, against all odds, that it will not be the case.

Of the three signals discussed above, the top quark pair decay to hadronic jets was later also found in D0 with Run I data, and then in Run II by both CDF and D0. So was the Z decay to b-quark pairs (no published paper there by D0 though, while a publication by CDF is coming out in NIM very soon). Instead, the 1987 signal of W and Z decays to jet pairs found by UA2 was not replicated. Why, you might well ask.

The reason is that while UA2 collected those 4.7 inverse picobarns of data with a dedicated trigger collected by a very low transverse energy requirement, no such thing was attempted during Run 1A or Run 1B, because triggers then were tuned to collect events with leptons, which yielded much cleaner signals of W and Z decays; the hadronic decay of W and Z bosons was not considered very interesting any more, and the large fraction of the bandwidth required by hadronic triggers discouraged their collection.

We thus arrive at Run IC. In the fall of 1995, after some weeks of running, a wire of the central tracking chamber of CDF broke, and with it a whole sector of the tracking had to be turned off. Rather than having to cope with biases hard to deal with, CDF decided to stop collecting data based on the presence of tracks, and to give a much larger bandwidth to calorimeter-based triggers: in the menu, a very low-energy jet trigger was then inserted, to see if the UA2 signal of hadronic W and Z bosons could be replicated.

The W and Z signal was however not found in more than two inverse picobarns of data collected in the remainder of Run 1C (later a portion of the data was collected with the Tevatron running at the lowered energy of 630 GeV, to compare UA2 and CDF cross sections for QCD events; but the triggers had then changed again). The question again is, why couldn’t CDF replicate the UA2 result ?

CDF is the better detector, and the cross-section for W and Z production is three times larger at 1.8 TeV than it is at 630 GeV. But for once, the larger energy of the Tevatron is a disadvantage! In fact, while signals increase their rate threefold, the background from strong interactions yielding two jets with the same characteristics as those emitted by the weak decay of W and Z bosons grows by more than one order of magnitude.

(Above you can see the calculated and measured trend with collision energy of the W and Z cross sections. The data point correspond to the S p \bar p S and Tevatron energies)

The different behavior of strong and weak interactions as a function of energy has to do with the inner structure of the processes yielding jets. In electroweak processes, one has to produce a heavy vector boson by quark-antiquark annihilation. That means finding in the proton and antiproton quarks carrying a fraction of about 15% (at the S p \bar p S) or 5% (at the Tevatron) of the parent hadron.

In contrast, jet pair production occurs for the largest part by gluon initial states. So, since other things are equal (strong versus electroweak coupling constant, branching ratios of W and Z to jet pairs), the question really is: how much larger is the probability to produce gluon interactions than q \bar q annihilations at a total energy equal to 15% and at 5% of the total proton-antiproton energy ?

Posed that way, the larger energy of the Tevatron appears to have been derated to asking for smaller energy partons to produce the studied processes. It turns out that, as one decreases the energy fraction, gluons start dominating. In contrast, when a larger fraction of the parent energy is required, the partons one finds are increasingly likely to be quarks. Because of the gluon domination at lower energy, the background is much larger at the Tevatron than it is at the Sp \bar p S.

In conclusion, a larger center-of-mass energy is not necessarily the better choice in the search of rare processes! The features of parton distribution functions complicate the matter