[Paper Review] Classically conformal B-L extended Standard Model and phenomenology

Yuta Orikasa|arXiv (Cornell University)|Apr 17, 2013

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper proposes a classically conformal B-L extended Standard Model where the Higgs potential is flat at the Planck scale, leading to radiative breaking of B-L symmetry at the TeV scale via the Coleman-Weinberg mechanism. The model predicts a TeV-scale Z′ boson, right-handed Majorana neutrinos, and a singlet Higgs, all within reach of the LHC and ILC, with the Z′ boson potentially discoverable through dilepton resonances and precisely measurable at the ILC despite its high mass.

ABSTRACT

Bardeen has argued that once the classically conformal invariance and its minimal violation by quantum anomalies are imposed on the SM, it can be free from the quadratic divergences and hence the gauge hierarchy problem. Under the hypothesis, We investigated the minimal B-L extended SM with a flat Higgs potential at the Planck scale. In this model, the B-L symmetry is radiatively broken at TeV scale. We studied phenomenology and detectability of the model at LHC and the ILC.

Motivation & Objective

• To resolve the gauge hierarchy problem by enforcing classical conformal invariance and minimal quantum anomaly violation in the SM.
• To construct a minimal B-L extension of the SM with a flat Higgs potential at the Planck scale to avoid quadratic divergences.
• To study the phenomenology of the model, particularly the detectability of new particles like the Z′ boson and right-handed neutrinos at the LHC and ILC.
• To demonstrate that the B-L symmetry breaking scale and Higgs mass are naturally generated via the Coleman-Weinberg mechanism.

Proposed method

• Imposing classical conformal invariance on the SM by setting the Higgs quartic coupling λH and mixing coupling λ′ to zero at the Planck scale.
• Using the Coleman-Weinberg mechanism to generate a small negative λ′(mEW) ≈ −O(10−3), leading to radiative breaking of U(1)B-L symmetry.
• Deriving the Z′ boson mass as M = √(m_h² / |λ′|), resulting in a TeV-scale breaking scale due to the small λ′.
• Calculating dilepton production cross sections at the LHC using parton distribution functions (CTEQ5M) and comparing with SM backgrounds to assess signal significance.
• Evaluating the ILC's sensitivity to Z′ couplings by computing deviations in e⁺e⁻ → μ⁺μ⁻ cross sections from the SM, integrated over angular acceptance.
• Using the flat potential assumption to link the B-L gauge coupling αB-L and Z′ mass, enabling a single-parameter prediction for collider searches.

Experimental results

Research questions

• RQ1 Can a classically conformal B-L extension of the SM naturally generate the electroweak and B-L breaking scales without fine-tuning?
• RQ2 What is the detectability of the Z′ boson at the LHC through dilepton resonances, given a TeV-scale mass and αB-L ≈ 0.01?
• RQ3 Can the ILC precisely measure Z′ properties such as couplings and mass, even when √s is below the Z′ resonance?
• RQ4 How do the LHC and ILC search sensitivities compare for the Z′ boson in this model?
• RQ5 What is the role of the singlet scalar Φ and right-handed neutrinos in generating Majorana masses and stabilizing the vacuum?

Key findings

• The Z′ boson mass is predicted to be around a few TeV, with M ≈ √(m_h² / |λ′|) and λ′ ≈ −10⁻³, consistent with a TeV-scale breaking scale.
• At the LHC with 100 fb⁻¹ luminosity, a Z′ boson with mZ′ = 2.5 TeV and αB-L = 0.008 produces ~560 signal events in the dilepton channel, far exceeding the SM background.
• The LHC can discover Z′ bosons with masses up to mZ′ ≲ 5 TeV if αB-L ≈ 0.01, based on 5-σ sensitivity with 100 fb⁻¹.
• The ILC with √s = 1 TeV can detect deviations of a few percent in the e⁺e⁻ → μ⁺μ⁻ cross section from the SM for mZ′ up to 10 TeV, enabling discovery even when √s < mZ′.
• The ILC's sensitivity exceeds that of the LHC for probing the Z′ boson, as shown by the 1% deviation reach extending beyond the LHC's 5-σ limit.
• The model predicts a viable phenomenology with all new particles—Z′, right-handed neutrinos, and the singlet Higgs—within the reach of future colliders, given the flat potential and radiative symmetry breaking.