# Form Factor Counting and HQET Matching for New Physics in $Λ_b \to Λ_c^*lν$

**Authors:** Michele Papucci, Dean J. Robinson  
**arXiv:** [2105.09330](https://arxiv.org/abs/2105.09330)  
**Date:** 2021-05-19  
**Summary page:** https://www.summarizepaper.com/en/paper/2105.09330v2/

## Key points

- Study investigates form factors and decay rates of Λb → Λc*(2595)lν and Λb → Λc*(2625)lν processes
- Considers b → c l ν four-Fermi interactions within and beyond the Standard Model (SM)
- Calculations include nonzero charged lepton masses and terms up to order O(αs, 1/mc,b) in HQET
- Highlights subtlety related to representation of spin-parity 1/2+ → 3/2- antisymmetric tensor form factors
- Presents general method for counting physical form factors for any hadronic transition matrix element and matching onto HQET
- Performs preliminary fit using HQET-based parametrization of Λb → Λc* form factors at O(αs, 1/mc,b) to existing quark model for lepton universality ratios R(Λc*)
- Examines potential incompatibilities between lattice QCD results, heavy-quark expansion, and experimental data
- Contributes significantly to understanding of these decay processes within different theoretical frameworks
- Sheds light on potential new physics effects in these transitions

## Summary

In their study titled "Form Factor Counting and HQET Matching for New Physics in Λb → Λc*lν", authors Michele Papucci and Dean J. Robinson investigate the form factors and decay rates of the Λb → Λc*(2595)lν and Λb → Λc*(2625)lν processes. They consider all possible b → c l ν four-Fermi interactions, both within and beyond the Standard Model (SM). The calculations include nonzero charged lepton masses and terms up to order O(αs, 1/mc,b) in the heavy quark effective theory (HQET). The authors highlight a subtlety related to the representation of spin-parity 1/2+ → 3/2- antisymmetric tensor form factors, which is relevant not only to these specific transitions but also to other higher excited-state transitions. They present a general method for counting physical form factors for any hadronic transition matrix element and their matching onto HQET. To provide preliminary predictions for lepton universality ratios R(Λc*), beyond those predicted by the SM, they perform a preliminary fit using a simple HQET-based parametrization of the Λb → Λc* form factors at O(αs, 1/mc,b) to an existing quark model. Furthermore, they examine potential incompatibilities between recent lattice QCD results and expectations from the heavy-quark expansion as well as available experimental data. Overall, this study contributes significantly to our understanding of these specific decay processes within different theoretical frameworks; it sheds light on potential new physics effects that may manifest in these transitions.