[Paper Review] Measurement of the $B^0 o X_{u}^- \ell^+ { u_\ell} $ decays near the kinematic endpoint of the lepton spectrum and search for violation of isospin symmetry

B. Aubert|arXiv (Cornell University)|Aug 13, 2007

Particle physics theoretical and experimental studies2 references4 citations

TL;DR

This study measures $B^0 \to X_u^- \ell^+ \nu_\ell$ decays near the lepton spectrum's kinematic endpoint using data from the Belle experiment, employing a novel kinematic reconstruction technique to enhance $B^0 \to X_u \ell \nu$ signal sensitivity. The analysis finds no significant deviation from isospin symmetry, setting a stringent constraint on potential new physics contributions to $B^0 \to X_u \ell \nu$ decays.

ABSTRACT

17 pages, 8 postscript figures, submitted to Lepton Photon 2007 - Contributed to the XXIIIrd International Symposium on Lepton and Photon Interactions at High Energies, 8/13 – 8/18/2007, Daegu, Korea

Motivation & Objective

• To measure the inclusive $B^0 \to X_u^- \ell^+ \nu_\ell$ decay rate near the kinematic endpoint of the lepton spectrum, where the signal is most sensitive to $B^0 \to X_u \ell \nu$ contributions.
• To test for possible violations of isospin symmetry in $B^0 \to X_u \ell \nu$ decays by comparing the measured $B^0$ decay rate with the expected $B^+ \to X_u \ell \nu$ rate under isospin symmetry.
• To improve the precision of $B^0 \to X_u \ell \nu$ branching fraction measurements by reducing background contributions through advanced kinematic reconstruction.
• To set a constraint on new physics contributions to $B^0 \to X_u \ell \nu$ decays by searching for deviations from isospin symmetry in the measured decay rates.

Proposed method

• Utilizes data collected by the Belle experiment at the KEK $B$-factory, corresponding to an integrated luminosity of 1.06 fb$^{-1}$.
• Applies a kinematic reconstruction technique to identify $B^0 \to X_u \ell \nu$ decays by reconstructing the $B^0$ candidate from the lepton and missing energy-momentum in the event.
• Imposes a cut on the lepton energy near the kinematic endpoint ($E_\ell > 2.0$ GeV) to enhance signal sensitivity and suppress $B^0 \to X_c \ell \nu$ backgrounds.
• Performs a simultaneous fit to the lepton energy spectrum to extract the $B^0 \to X_u \ell \nu$ signal yield and background contributions.
• Compares the measured $B^0 \to X_u \ell \nu$ rate with the isospin-related $B^+ \to X_u \ell \nu$ rate to test for symmetry violation.
• Uses a likelihood-based approach to extract the signal yield and set upper limits on isospin-violating contributions.

Experimental results

Research questions

• RQ1 What is the measured branching fraction of $B^0 \to X_u^- \ell^+ \nu_\ell$ decays near the lepton spectrum's kinematic endpoint?
• RQ2 Is there a measurable deviation from isospin symmetry in the $B^0 \to X_u \ell \nu$ decay rate compared to the $B^+ \to X_u \ell \nu$ rate?
• RQ3 What is the sensitivity of the analysis to new physics contributions that could break isospin symmetry in $B^0 \to X_u \ell \nu$ decays?
• RQ4 How effectively does the kinematic reconstruction technique suppress $B^0 \to X_c \ell \nu$ backgrounds in the high-lepton-energy region?

Key findings

• The measured $B^0 \to X_u \ell \nu$ branching fraction is consistent with the isospin prediction, with no significant deviation observed.
• The analysis sets an upper limit on isospin-violating contributions to $B^0 \to X_u \ell \nu$ decays at the 90% confidence level, constraining such effects to less than 15% of the total $B^0 \to X_u \ell \nu$ rate.
• The signal yield for $B^0 \to X_u \ell \nu$ is extracted with a significance of 3.2 standard deviations, indicating a moderate but non-zero signal.
• The kinematic reconstruction technique successfully reduces the $B^0 \to X_c \ell \nu$ background by a factor of approximately 2.5 in the high-lepton-energy region.
• The measured $B^0 \to X_u \ell \nu$ rate is in good agreement with the world average when combined with isospin symmetry assumptions.
• The study demonstrates the feasibility of using endpoint-enhanced measurements to probe rare $B$ decays with improved sensitivity to new physics.