Observation of an Excess in the Search for the Standard Model Higgs Boson at ALEPH

TL;DR

The paper reports a ~3 σ excess in the search for the Standard Model Higgs boson using ALEPH data at LEP energies up to 209 GeV, with the most signal-like events clustering in four-jet final states and a reconstructed mass near 114 GeV/c^2. The analysis uses dual streams (neural-network and cut-based) across multiple final states, employing 4C kinematic fits, b-tagging, and a likelihood ratio framework to quantify compatibility with signal. Systematic checks and high-purity subsets support the robustness of the excess, though the authors caution that the result is preliminary and requires confirmation from additional data or other experiments. Overall, the work provides a technically detailed, multi-channel Higgs search that highlights a potentially important hint consistent with SM Higgs production at m_H ~ 114 GeV/c^2.

Abstract

A search has been performed for the Standard Model Higgs boson in the data sample collected with the ALEPH detector at LEP, at centre-of-mass energies up to 209GeV. An excess of 3sigma beyond the background expectation is found, consistent with the production of the Higgs boson with a mass near 114GeV/c2. Much of this excess is seen in the four-jet analyses, where three high purity events are selected.

Figures (13)

• Figure 1: Feynman diagrams of Higgs boson production at LEP through the (a) Higgsstrahlung and (b) gauge boson fusion processes.
• Figure 2: The expected number of Standard Model Higgs boson events produced in the year 2000 data as a function of the Higgs boson mass (solid curve). The dashed curve shows the contribution of the boson fusion processes, including their interference with the Higgsstrahlung process.
• Figure 3: Distributions of the reconstructed Higgs boson mass for the data collected in 2000 (dots with error bars) and the expected background (histogram) for the (a) NN and (b) cut streams.
• Figure 4: The log-likelihood estimator $-2 \ln{Q}$ for the (a) NN and (b) cut streams as a function of the mass of the Higgs boson for the observation (solid) and background-only expectation (dashed). The light and dark grey regions around the background expectation represent the one and two sigma bands, respectively. The dash-dotted curves show the medians of the log-likelihood estimator as a function of the Higgs boson mass for the signal hypothesis.
• Figure 5: Observed (solid) and expected (dashed) CL curves for the background hypothesis as a function of the hypothesized Higgs boson mass for the (a) NN and (b) cut streams. The dash-dotted curves indicate the location of the median CL for a Higgs boson signal as a function of the Higgs boson mass.