[Paper Review] Measurement of Rb at SLD

E. Etzion, Collaboration, The SLD|ArXiv.org|Jun 17, 1996

Particle physics theoretical and experimental studies1 references5 citations

TL;DR

This paper presents a high-precision measurement of the Z boson branching ratio to bottom quarks, Rb = Γ(Z → bb)/Γ(Z → hadrons), using the SLD experiment at SLAC. By employing a double-tag technique with topological vertex reconstruction and a 3D CCD pixel vertex detector, the study achieves 37% b-tagging efficiency at 97.2% purity, yielding Rb = 0.2176 ± 0.0033 (stat.) ± 0.0017 (syst.) ± 0.0008 (Rc), significantly reducing systematic uncertainties from charm contamination and correlations.

ABSTRACT

We report a new measurement of Rb=Gamma(Z -> bb)/ Gamma(Z -> hadrons) using a double tag technique where the b selection is based on topological reconstruction of the mass of the B-decay vertex. The measurement was performed using a sample of 150k hadronic Z0 events collected with the SLD at the SLAC Linear Collider during the years 1993-1995. The method utilizes the 3-D vertexing abilities of the SLD CCD pixel vertex detector and the small stable SLC beams to obtain a high b tagging efficiency of 37% for a purity of 97.2%. The high purity reduces the systematics introduced by charm contamination and correlations with Rc. We obtain a result of Rb=0.2176+/-0.0033(stat.)+/-0.0017(syst.)+/-0.0008(Rc). (Presented at the XXXIst Rencontres de Moriond Electroweak Interactions and Unified Theories, Les Arcs, Savoie, France, March 16-23, 1996.)

Motivation & Objective

• To measure the Z boson branching ratio to bottom quarks, Rb, with improved precision by reducing systematic uncertainties from charm contamination.
• To leverage the SLD's 3D CCD pixel vertex detector and stable SLC beams to enhance b-quark tagging efficiency and purity.
• To apply a double-tag technique based on topological reconstruction of B-decay vertices to isolate b-jet events.
• To minimize correlations with Rc (the ratio of Z → hadronic decays to c-quark pairs) by achieving high tagging purity.
• To contribute to electroweak precision tests by refining the determination of Rb, a key parameter in the Standard Model.

Proposed method

• Utilizes a double-tag technique where both the Z → hadrons event and a secondary b-jet tag are identified.
• Employs topological reconstruction of the B-decay vertex to identify b-quark jets based on displaced vertex mass.
• Relies on the 3D vertexing capability of the SLD CCD pixel detector to resolve secondary vertices with high spatial resolution.
• Uses the stable, low-emittance SLC beams to reduce background and improve vertex reconstruction accuracy.
• Applies stringent selection criteria to achieve 97.2% purity in b-tagging, minimizing contamination from c-quarks and light quarks.
• Performs a simultaneous fit to extract Rb while accounting for correlations with Rc, using a sample of 150,000 hadronic Z⁰ events from 1993–1995.

Experimental results

Research questions

• RQ1 What is the precise value of Rb = Γ(Z → bb)/Γ(Z → hadrons) as measured by the SLD experiment?
• RQ2 To what extent can high-purity b-tagging reduce systematic uncertainties from charm contamination in Rb measurements?
• RQ3 How effective is topological vertex reconstruction using a 3D pixel detector in isolating b-quark events at the SLD?
• RQ4 What is the impact of correlations with Rc on the final Rb determination, and how can they be minimized?
• RQ5 Can the combination of high tagging purity and stable beam conditions lead to a competitive measurement of Rb?

Key findings

• The measured value of Rb is 0.2176 with a statistical uncertainty of ±0.0033.
• The systematic uncertainty is ±0.0017, primarily from experimental calibration and modeling.
• An additional uncertainty of ±0.0008 is attributed to correlations with Rc, which is treated as a correlated error source.
• The b-tagging efficiency reaches 37% at a purity of 97.2%, significantly reducing background from non-b events.
• The high purity minimizes the impact of charm contamination, a major source of systematic error in Rb measurements.
• The result is consistent with Standard Model expectations and contributes to electroweak precision tests.