[Paper Review] Hints from Tevatron, a prelude to what?

V. E. Ozcan, S. Sultansoy+1 more|ArXiv.org|Aug 3, 2008

Historical, Religious, and Philosophical Studies15 references3 citations

TL;DR

The paper investigates a 2.9σ excess in the W+jets channel at the Tevatron, suggesting it may indicate new physics beyond the Standard Model. It proposes that degenerate fourth-family quarks or E6-GUT-like iso-singlet quarks with masses around 400 GeV could explain the excess, with model predictions yielding 3–5 signal events—close to the observed 7 events—under favorable assumptions about QCD scale and PDFs.

ABSTRACT

We comment on the recent results from the Tevatron experiments in the W+jets channel and consider some models as the possible underlying physical theories. We also list some channels for further studies.

Motivation & Objective

• To investigate the origin of a 2.9σ excess in the W+jets channel at the Tevatron, observed as 7 events between 375–500 GeV with a background expectation of 1.8 events.
• To evaluate whether this excess can be explained by new physics models, particularly fourth-family quarks or E6-GUT-inspired iso-singlet quarks.
• To assess the compatibility of theoretical cross sections from these models with the observed event count using 2.8 fb⁻¹ of integrated luminosity.
• To propose new search channels—such as $t'\bar{t}' \to WjZj$ and $h \to \nu\nu \to 2W2\mu$—to distinguish between competing models and enhance discovery potential.
• To position the Tevatron excess as a prelude to new physics discoveries at the LHC, especially for 400 GeV-scale new quarks or Higgs bosons in extended models.

Proposed method

• Used the significance estimator $\mathcal{S} = \sqrt{2\times[(s+b)\ln(1+s/b)-s]}$ to quantify the observed excess, yielding 2.9σ significance.
• Calculated reconstruction efficiency $\epsilon$ from the 95% CL exclusion limit (81 fb at 400 GeV) and signal requirement (3 events for 1.96σ), yielding $\epsilon \approx 0.01$.
• Evaluated theoretical cross sections for $p\bar{p} \to Q\bar{Q}$ in fourth-family, E6-GUT, and Little Higgs models using CTEQ6L1 PDFs and QCD scale set to $m_Q$ or $2m_W$.
• Applied NLO corrections via a $k$-factor of 1.2–1.5 to convert LO cross sections to NLO estimates, improving agreement with data.
• Assessed the $t' \to Z + \text{jet}$ channel as a discriminator between fourth-family and E6-GUT models, given its 33% branching fraction and expected 2–3 events at 2.8 fb⁻¹.
• Modeled the $h \to \nu\nu \to 2W2\mu$ channel for Higgs searches in 4-family models, using a 4×4 MNS matrix and BR($\nu \to W\mu$) = 0.68 to predict >100 $WW\mu\mu$ events at 2.8 fb⁻¹.

Experimental results

Research questions

• RQ1 Can the observed 2.9σ excess in the Tevatron W+jets channel (7 events vs. 1.8 expected) be explained by new physics beyond the Standard Model?
• RQ2 Do fourth-family quarks with degenerate masses near 400 GeV, or E6-GUT-like iso-singlet quarks, produce sufficient cross sections to account for the observed event excess?
• RQ3 Can the $t' \to Z + \text{jet}$ decay channel serve as a clean discriminator between fourth-family and E6-GUT models?
• RQ4 What is the expected event yield in the $h \to \nu\nu \to 2W2\mu$ channel under a 4-family extension of the SM, and can it be observed at the Tevatron with 2.8 fb⁻¹?
• RQ5 How soon could the LHC discover 400 GeV-scale new quarks or Higgs bosons predicted by these models, based on luminosity projections?

Key findings

• The observed 7-event excess in the 375–500 GeV W+jets channel has a 0.2% probability under background-only hypothesis, corresponding to a 2.9σ significance.
• The tree-level cross section for a 400 GeV $t'$ quark with SM-like couplings yields only 0.5 signal events after efficiency and luminosity corrections, insufficient to explain the excess.
• With NLO corrections ($k$-factor ≈ 1.5) and QCD scale choice of $2m_W$, the predicted signal yield increases to 2.4 events, and to 4.2 events for $m_{t'} = 375$ GeV and $m_{b'} = 400$ GeV.
• For $m_{t'} = 375$ GeV and $m_{b'} = 400$ GeV, the Poisson probability of observing 7 or more events rises to 28%, making the excess plausible under such mass splitting.
• The $t' \to Z + \text{jet}$ channel is predicted to yield 2–3 events at 2.8 fb⁻¹, offering a clean way to distinguish E6-GUT models from fourth-family models.
• In the 4-family model with $m_H = 200$ GeV and $m_\nu = 90$ GeV, the $h \to \nu\nu \to 2W2\mu$ channel predicts over 100 $WW\mu\mu$ events, with ~50 same-sign muon pairs, making it a promising discovery channel.