[Paper Review] Linear Collider Physics Resource Book for Snowmass 2001 - Part 3: Studies of Exotic and Standard Model Physics

K. Abe|ArXiv.org|Jun 13, 2001

Cryospheric studies and observationsEarth and Planetary Sciences19 citations

TL;DR

This paper proposes a high-precision $e^+e^-$ linear collider program to probe new physics beyond the Standard Model, focusing on top quark, QCD, two-photon, and exotic particle physics, with particular emphasis on polarized $Z$ boson decays and $\Lambda_b$ baryon decays. Using $10^9$ to $10^{10}$ $Z$ bosons, especially with polarized positron beams, it enables sensitive tests of $V-A$ structure, chirality, and CP violation through angular asymmetries in $\Lambda_b \to \Lambda\gamma$ and $\Lambda_b \to l^+l^-X$, achieving $5\sigma$ sensitivity to right-handed couplings with $r \lesssim 1.9$.

ABSTRACT

This Resource Book reviews the physics opportunities of a next-generation e+e- linear collider and discusses options for the experimental program. Part 3 reviews the possible experiments on that can be done at a linear collider on strongly coupled electroweak symmetry breaking, exotic particles, and extra dimensions, and on the top quark, QCD, and two-photon physics. It also discusses the improved precision electroweak measurements that this collider will make available.

Motivation & Objective

• To explore new physics beyond the Standard Model using a high-luminosity $e^+e^-$ linear collider, particularly through precision measurements of $Z$ boson decays and $\Lambda_b$ baryon decays.
• To test the $V-A$ nature of weak interactions with unprecedented accuracy using polarized $\Lambda_b$ baryons.
• To search for direct CP violation in $b$-baryon decays via $T$-odd correlations and CP-conjugate measurements.
• To constrain exotic couplings, such as right-handed $W$ couplings, through angular asymmetries in rare decays like $\Lambda_b \to \Lambda\gamma$.
• To evaluate the impact of positron beam polarization on $m_W$ determination and electroweak precision observables.

Proposed method

• Utilize $10^9$ to $10^{10}$ $Z$ bosons produced via $e^+e^-$ collisions at a linear collider to enable high-statistics studies of rare decays.
• Employ polarized positron beams to enhance sensitivity to left-right asymmetries ($\Delta A_{LR}$) and improve precision in $m_W$ measurements.
• Measure angular asymmetries in $\Lambda_b \to \Lambda\gamma$ decay using the spin quantization axis defined by the $\Lambda_b$ polarization and the photon momentum in the $\Lambda_b$ rest frame.
• Construct $T$-odd correlation observables such as $C_T = \langle \vec{\sigma}_{\Lambda_b} \cdot (\vec{p}_A \times \vec{p}_B) \rangle$ to probe direct CP violation in three-body decays.
• Compare $\Lambda_b \to l^- (p+X)_{\text{no charm}}$ and $\overline{\Lambda}_b \to l^+ (\bar{p}+X)_{\text{no charm}}$ to search for CP asymmetries.
• Use the theoretically clean nature of $\Lambda_b \to \Lambda\gamma$ decay to extract the ratio $r$ of left- to right-handed couplings, with sensitivity limited primarily by statistics.

Experimental results

Research questions

• RQ1 Can angular asymmetries in $\Lambda_b \to \Lambda\gamma$ decay detect deviations from the Standard Model prediction of $r \lesssim 0.04$ for right-handed couplings?
• RQ2 To what extent can $T$-odd correlations in three-body $\Lambda_b$ decays like $\Lambda_b \to \Lambda_c^+ \pi^- \pi^0$ reveal direct CP violation?
• RQ3 How does positron beam polarization improve the precision of $m_W$ determination and $\Delta A_{LR}$ measurements?
• RQ4 What is the sensitivity of $\Lambda_b \to \Lambda\gamma$ to new physics contributions with opposite chirality to the SM, particularly in penguin amplitudes?
• RQ5 Can semileptonic $\Lambda_b$ decays with $\tau$ leptons enhance the sensitivity to CP-violating effects due to the lepton mass dependence?

Key findings

• With roughly 750 fully reconstructed $\Lambda_b \to \Lambda\gamma$ events, the measurement achieves $5\sigma$ sensitivity to values of the coupling ratio $r$ between 0.5 and 1.9.
• The angular asymmetry in $\Lambda_b \to \Lambda\gamma$ is theoretically clean and primarily limited by statistical uncertainty, making it ideal for testing $V-A$ structure.
• The $T$-odd correlation $C_T$ in three-body decays such as $\Lambda_b \to \Lambda_c^+ \pi^- \pi^0$ can detect direct CP violation if $C_T \neq \overline{C}_T$ in the CP-conjugate process.
• Semileptonic decays like $\Lambda_b \to l^+l^-X$ allow for inclusive polarization studies, though they are theoretically less clean than $\Lambda_b \to \Lambda\gamma$.
• The $\Lambda_b \to \Lambda\gamma$ program offers a unique probe of four-quark penguin operators with chirality opposite to the SM, providing a consistency check on CP and chirality structure.
• Positron beam polarization is crucial for achieving a $5-6$ MeV determination of $m_W$ via threshold scanning, and its absence could significantly degrade this precision.