Threshold Effects in the Decay of Heavy b' and t' Quarks

TL;DR

This paper investigates the decay behaviors of hypothetical heavy fourth-generation quarks t' and b', highlighting threshold effects and proposing methods to measure CKM mixing elements through decay branching fractions.

Contribution

It introduces a convolution method for analyzing complex decay channels and emphasizes the importance of threshold effects in measuring CKM matrix elements for heavy quarks.

Findings

Threshold effects significantly influence decay branching ratios.

Measurement of V_{t'b} and V_{tb'} is feasible via decay branching fractions.

Heavy-to-heavy transitions are not suppressed by quark mixing.

Abstract

A sequential fourth generation is still viable, but the t' and b' quarks are constrained to be not too far apart in mass. The t'{\to}bW and b'{\to}tW decay channels are still being pursued at the Tevatron, which would soon be surpassed by the LHC. We use a convolution method with up to five-body final state to study t' and b' decays. We show how the two decay branches for m_{b'} below the tW threshold, b'{\to}tW^* and t^*W, merge with b'{\to}tW above the threshold. We then consider the heavy-to-heavy transitions b'{\to}t^{\prime(*)}W^{(*)} (or t'{\to}b^{\prime(*)}W^{(*)}), as they are not suppressed by quark mixing. We find that, because of the threshold sensitivity of the branching fraction of t'{\to}b'W^* (or b'{\to}t'W^*), it is possible to measure the strength of the CKM mixing element V_{t'b} (or V_{tb'}), especially when it is rather small. We urge the experiments to pursue and separate the t'{\to}b'W^* (or b'{\to}t'W^*) decay in their search program.