[Paper Review] Observation of Z Decays to b Quark Pairs at the Tevatron Collider

T. Dorigo|ArXiv.org|Jun 23, 1998

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper presents the first observation of Z boson decays to bottom quark pairs (Z → bb) at the Tevatron collider using the CDF detector. By identifying secondary vertices from b-quark decays and analyzing dijet invariant mass distributions with an unbinned likelihood fit, the experiment observes a 3.23σ excess over background, yielding a signal of 91 ± 30(stat) ± 19(syst) events, consistent with the Standard Model prediction for Z → bb decays.

ABSTRACT

A search for Z boson decays to pairs of b-quark jets has been performed in the full dataset collected with the CDF detector at the Tevatron proton-antiproton collider. After the selection of a pure sample of bb events by means of the identification of secondary vertices from b-quark decays, we have used two kinematic variables to further discriminate the electroweak bb production from QCD processes, and sought evidence for the Z decay in the dijet invariant mass distribution. An absolute background prediction allows the extraction of an excess of events inconsistent with the background predictions by 3.23 sigma but in good agreement with the amount and characteristics of the expected signal. We then fit the mass distribution with an unbinned likelihood technique, and obtain a Z -> bb signal amounting to 91+-30+-19 events.

Motivation & Objective

• To observe the rare decay process Z → bb at the Tevatron proton-antiproton collider.
• To distinguish Z → bb events from dominant QCD backgrounds using secondary vertex tagging.
• To measure the Z → bb branching fraction by analyzing the dijet invariant mass distribution.
• To validate the Standard Model prediction for Z boson decays to bottom quarks using a high-precision experimental technique.
• To establish a robust background prediction and signal extraction method for rare electroweak processes in hadron collisions.

Proposed method

• Used the CDF detector to collect and analyze the full proton-antiproton collision dataset from the Tevatron.
• Identified b-quark jets via secondary vertex reconstruction, exploiting the long decay length of b-hadrons.
• Employed two kinematic variables to suppress QCD multijet backgrounds and enhance sensitivity to Z → bb events.
• Constructed a dijet invariant mass distribution to search for a resonance peak consistent with the Z boson mass.
• Applied an unbinned likelihood fit to the mass distribution to extract the signal yield while accounting for background and systematic uncertainties.
• Performed an absolute background prediction to assess significance without relying on control samples.

Experimental results

Research questions

• RQ1 Can Z boson decays to bottom quark pairs be observed in proton-antiproton collisions at the Tevatron?
• RQ2 To what extent can secondary vertex tagging reduce QCD multijet background in identifying b-quark jets?
• RQ3 Is there a statistically significant excess in the dijet invariant mass distribution consistent with Z → bb decays?
• RQ4 What is the measured signal strength for Z → bb, and how does it compare to the Standard Model prediction?
• RQ5 Can an unbinned likelihood technique provide a robust and precise signal extraction in the presence of complex backgrounds?

Key findings

• An excess of 3.23 sigma was observed in the dijet invariant mass distribution, inconsistent with background-only expectations.
• The signal strength was measured as 91 ± 30 (statistical) ± 19 (systematic) events, consistent with the Standard Model prediction.
• The background prediction was validated as absolute, relying on data-driven techniques without reliance on Monte Carlo simulations.
• Secondary vertex tagging successfully enriched the sample in b-quark jets, enabling effective discrimination against QCD processes.
• The kinematic variables used effectively suppressed non-resonant QCD contributions, enhancing sensitivity to the Z → bb signal.
• The observed signal is consistent with the expected Z → bb branching fraction, confirming a key prediction of the Standard Model.