Searching for an elusive particle using an elusive signature

The x-axis, which goes from zero to one, shows that a Higgs signal has a peak at a value of 0.85. The backgrounds peak at a value of 0.2.

The Higgs boson has been a very elusive particle: It has escaped observation for more than 48 years since theorists postulated its existence in 1964. The particle observed at the LHC with a mass of about 125 GeV/c2 is most likely the long-sought Standard Model Higgs boson. Scientists need to conduct more studies to verify that.

In contrast with the LHC experiments, those at the Tevatron are particularly sensitive to the decay into a pair of bottom quarks, which is the most likely decay scenario for a Higgs boson with a mass close to 125 GeV/c2. The CDF data set contains about one event involving the Higgs boson for every 10 billion background events. Clearly CDF physicists need to significantly reduce the sample in order to proceed.

A first step is to look at a subset of the Higgs events, particularly one that has a much smaller background. This subset consists of events where the Higgs boson is produced together with a W or Z boson. An interesting and challenging way to look at these events is to analyze W and Z bosons whose decay products are not directly identified in the detector – though the two bottom quark jets are often detected, other decay products are not. And though these decay products are not observed directly, the W or Z particles are inferred from a large transverse energy imbalance in the event.

Using specific event selection requirements and a carefully designed procedure illustrated in the top figure, the CDF team rejects a large portion of the largest background, jet production from QCD. This latter step is crucial in making this analysis the second-most sensitive at the Tevatron. (To learn about the most sensitive analysis, see this result of another Higgs search.)

The techniques developed for this analysis are at the forefront of small-signal data analysis and are now commonly used at the LHC. The CDF group responsible for this analysis worked very carefully in developing these tools and in ensuring that all backgrounds were estimated properly.

The second figure shows both the expected and observed 95 percent confidence limits for a Higgs particle being produced in association with a W or Z particle. While the excess of events observed in the data is not significant enough to claim a discovery, it suggests that the particle observed at the LHC is very much like the Standard Model Higgs boson.

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—edited by Andy Beretvas

Observed and expected 95 percent confidence level upper limits on (Z or W) + Higgs cross section times the branching ratio for Higgs → bb divided by the Standard Model prediction, as a function of the Higgs mass.
These CDF physicist contributed to this data analysis.
Top row from left: Daniela Bortoletto (Purdue), Oscar Gonzales (Madrid, Spain), Ben Kilminster (Fermilab)
Second row from left: Mike Kirby (Fermilab), Kyle Knoepfel (Fermilab), Qiu Guang Liu (Purdue)
Bottom row from left: Fabrizio Margaroli (Rome, Italy), Karolos Potamianos (Purdue/LBNL), Homer Wolfe (The Ohio State University)