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$\Lambda_b \to \Lambda \ell^+\ell^-$ and $\Lambda_b \to \Lambda^{\ast} \ell^+\ell^-$ decays with polarized leptons

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Diganta Das, Vedant Parekh, Ria Sain, Dargi Shameer

High Energy Physics - Phenomenology · Submitted 19 Oct 2026 · 90 references

Pith's verdict: First polarization predictions for Λ_b→Λ*ℓℓ; uncertainty treatment in the Λ* channel is fragile at current precision.

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arXiv ID2610.27015 (version 1)
Pith ID6CCYM2HD, Pith's permanent record of this paper and its review
CitationDargi Shameer, Diganta Das, Ria Sain, Vedant Parekh. "$\Lambda_b \to \Lambda \ell^+\ell^-$ and $\Lambda_b \to \Lambda^{\ast} \ell^+\ell^-$ decays with polarized leptons." arXiv:2610.27015 (2026). https://pith.science/paper/6CCYM2HD
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The paper

The paper claims

This paper presents the first calculation of lepton polarization observables in Λ_b → Λ* ℓ⁺ℓ⁻ decays and identifies the longitudinal-normal asymmetry P_LN as the most sensitive double-polarization probe of weak phases.

The reading

This paper attempts to establish a complete set of lepton polarization observables for the rare decays Λ_b → Λ ℓ⁺ℓ⁻ and, for the first time, for Λ_b → Λ* ℓ⁺ℓ⁻. The authors derive single and double lepton polarization asymmetries and polarized forward-backward asymmetries using an effective Hamiltonian and helicity formalism. They find that the longitudinal-normal asymmetry P_LN in the Λ channel is the most sensitive double-polarization probe of a weak phase in the Wilson coefficients, able to distinguish the standard model from scenarios with complex phases. In the Λ* channel, the observables show more modest sensitivity but provide complementary constraints. The work provides new theoretical tools for future searches for physics beyond the standard model at LHCb and Belle II.

What carries the argument

The analysis uses the helicity method to express the decay amplitudes in terms of transversity amplitudes, which decompose the hadronic matrix elements into vector, axial-vector, tensor, and axial-tensor form factors. For Λ_b → Λ the decomposition uses 10 lattice QCD form factors; for Λ_b → Λ* it uses 14. The observables are rational combinations of these transversity amplitudes. The key observable P_LN arises from the imaginary part of the interference between timelike and longitudinal transversity amplitudes, specifically from terms such as Im[$A_L^{{∥t}}$ $A_L^{{∥0*}}$ - $A_R^{{∥t}}$ $A_L^{{∥0*}}$ + ...], which selects the combination sensitive to a weak phase in C_{10} and $C_9^{{eff}}$.

Extended reading notes

Core claim

The central discovery is that the longitudinal-normal asymmetry P_LN in Λ_b → Λ ℓ⁺ℓ⁻ is the most sensitive double-polarization observable for detecting a weak phase in the effective Wilson coefficients, because its numerator isolates the imaginary part of the interference between timelike and longitudinal transversity amplitudes, which is directly proportional to Im[C_{10}($C_9^{{eff}}$ + C_{10})^*]. In the Λ_b → Λ* ℓ⁺ℓ⁻ channel, the corresponding asymmetry P*_LN is computed for the first time; it has the opposite sign and smaller distinguishing power, but still deviates from the standard model in new physics scenarios with complex phases. The full set of single and double lepton polarization asymmetries and polarized forward-backward asymmetries is provided for both channels, enabling experimental tests of the chiral structure of the b→sℓ⁺ℓ⁻ transition.

Load-bearing premise

The accuracy of the lattice QCD form factors from [58] and [86] fully captures the hadronic uncertainties in the transversity amplitudes; any unaccounted errors in these form factors could shift the predicted asymmetries and degrade their discriminating power.

Editorial extensions

If this is right

  • These polarization observables provide new tests of the chiral structure of b→sℓ⁺ℓ⁻ effective operators, complementing existing tests from B-meson decays.
  • The P_LN asymmetry in Λ_b → Λ ℓ⁺ℓ⁻ offers a clean handle on CP-violating new physics, as it can separate scenarios with complex Wilson coefficients from the standard model.
  • The Λ* channel extends the kinematic reach and provides cross-checks via the opposite sign of P*_LN relative to P_LN.
  • Binned predictions in Tables 2 and 3 serve as direct targets for experimental analyses at LHCb, especially if muon polarization can be partially reconstructed.
  • The relations between observables, such as A_LNFB ∝ P_NT and A_NTFB ∝ P_LN, reduce the number of independent measurements needed to extract the short-distance parameters.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If lattice QCD form factor uncertainties are reduced, especially for the Λ* channel, the distinguishing power of several asymmetries (e.g., P_TT and P_NN) will improve significantly.
  • The T-odd asymmetries (e.g., P_NT, P_LN) could be combined with CP-odd differences between Λ_b and anti-Λ_b to isolate genuine CP violation beyond the standard model, as suggested by the authors.
  • The framework can be extended to other baryon decays such as Ξ_b → Ξ ℓ⁺ℓ⁻ and Ω_b → Ω ℓ⁺ℓ⁻ to test the universality of polarization observables across different baryon sectors.
  • Experimental feasibility studies at LHCb should investigate whether lepton-polarization reconstruction, though difficult for muons, can be achieved in the full cascade decay Λ_b → Λ(→ N π) ℓ⁺ℓ⁻.
Original abstract

We present an analysis of lepton polarization observables in the rare flavor-changing neutral current decays $\Lambda_b \to \Lambda\ell^+\ell^-$ and $\Lambda_b \to \Lambda^*\ell^+\ell^-$. These decays are driven by the quark-level transition $b \to s\ell^+\ell^-$ and are sensitive to short-distance physics within and beyond the Standard Model. We work within an effective Hamiltonian framework and derive single lepton polarization asymmetries, double lepton polarization asymmetries, and polarized forward-backward asymmetries using the helicity method. The observables are expressed in terms of transversity amplitudes and evaluated using state-of-the-art lattice QCD form factors. We analyze the $q^2$ dependence of several observables for both the ground-state $\Lambda$ and the excited $\Lambda^*(1520)$ final states and discuss sensitivities of the observables to chiral structure of the effective operators and complex Wilson coefficients.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-10-21: 1 request, 4,496 input tokens.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.07 · human participants 0.01 · controlled comparison 0.03 · randomized assignment 0.01 · states quantitative result 0.04 · compares with prior results 0.07 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

We present an analysis of lepton polarization observables in the rare flavor-changing neutral current decays $\Lambda_b \to \Lambda\ell^+\ell^-$ and $\Lambda_b \to \Lambda^*\ell^+\ell^-$. These decays are driven by the quark-level transition $b \to s\ell^+\ell^-$ and are sensitive to short-distance physics within and beyond the Standard Model. We work within an effective Hamiltonian framework and derive single lepton polarization asymmetries, double lepton polarization asymmetries, and polarized forward-backward asymmetries using the helicity method. The observables are expressed in terms of transversity amplitudes and evaluated using state-of-the-art lattice QCD form factors. We analyze the $q^2$ dependence of several observables for both the ground-state $\Lambda$ and the excited $\Lambda^*(1520)$ final states and discuss sensitivities of the observables to chiral structure of the effective operators and complex Wilson coefficients.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 We present an analysis of lepton polarization observables in the rare flavor-changing neutral current decays $\Lambda_b \to \Lambda\ell^+\ell^-$ and $\Lambda_b \to \Lambda^*\ell^+\ell^-$. contribution 1.00 0.95 0.08 asserted 0.96
s1 These decays are driven by the quark-level transition $b \to s\ell^+\ell^-$ and are sensitive to short-distance physics within and beyond the Standard Model. background 1.00 0.06 0.04 asserted 1.00
s2 We work within an effective Hamiltonian framework and derive single lepton polarization asymmetries, double lepton polarization asymmetries, and polarized forward-backward asymmetries using the helicity method. method 0.97 0.96 0.05 asserted 1.00
s3 The observables are expressed in terms of transversity amplitudes and evaluated using state-of-the-art lattice QCD form factors. method 0.99 0.88 0.05 asserted 1.00
s4 We analyze the $q^2$ dependence of several observables for both the ground-state $\Lambda$ and the excited $\Lambda^*(1520)$ final states and discuss sensitivities of the observables to chiral structure of the effective operators and complex Wilson coefficients. contribution 0.93 0.94 0.15 asserted 0.98

What kind of paper the classifier reports this is: theoretical result 1.00 · evidence from proof or derivation 0.96.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. Single lepton polarization asymmetries P − L and P − N as a function of q 2 for Λb → Λµ +µ − decay in the SM and beyond. The longitudinal polarization asymmetries P ± L satisfy the exact relation P − L = −P + L . The numerator is proportional to p q 2 p q 2 − 4m2 ℓ P λ (|AR λ | 2−|AL λ | 2 ), (see Appendix B for analytical expressions) making P ± L a direct measure of the chiral asymmetry be￾tween left and right handed transversity amplitudes. In the massless limit, mℓ → 0, 8
Figure 2. Single lepton polarization asymmetries P − T and P + T as a function of q 2 for Λb → Λµ +µ − decay in the SM and beyond. The normal polarization asymmetries P ± N are proportional to imaginary parts of interference terms between transversity amplitudes, P ∓ N ∝ mℓ q q 2 − 4m2 ℓ Im ±A R ∥1 (A L ⊥1 ) ∗ + (A L ∥t − A R ∥t )((A L ∥0 ) ∗ − (A R ∥0 ) ∗ ) + {∥↔⊥} . (21) In the second term (AL ∥t − AR ∥t )((AL ∥0 ) ∗ − (AR ∥0 ) ∗ ), the imaginary part cancels and P − N = −P + N . (22) Therefore, in
Figure 3. Double lepton polarization asymmetries PLL,PT T and PNN of Λb → Λµ +µ − decay as a function of q 2 in the SM and new physics scenarios
Figure 4. Double lepton polarization asymmetries PNT and PLN as a function of q 2 for Λb → Λµ +µ − in the SM and beyond. In the massless limit, the longitudinal-longitudinal asymmetry PLL → −1 at both low and high q 2 independently of the Wilson coefficients. In this limit the vector and axial-vector currents produce ℓ − and ℓ + only with opposite helicities, so that their spin projections along ˆp− and ˆp+ respectively, always have opposite signs. In the massive limit the correction is of the order of O(m2 ℓ /q2 ) and significant only at low q 2 . Therefore, PLL carries essentially
Figure 5. Double lepton polarization asymmetries PT L and PLT as a function of q 2 for Λb → Λµ +µ − in the SM and beyond. The asymmetry PLN is, like PNT , T-odd. Its numerator is the imaginary part of the same timelike–longitudinal combination whose real part splits P − T from P + T . In the SM it is small but non-zero, (4.14 ± 1.65) × 10−3 and (5.61 ± 0.37) × 10−3 in the low- and high-q 2 bins. For C ′ 10 = 0, the numerator is combination of Im C10(C eff 9 ± C ′ 9 + 2κiC eff 7 ) ∗
Figure 6. Doubly polarized forward-backward asymmetries
Figure 7. Doubly polarized forward-backward asymmetry
Figure 8. Single lepton polarization asymmetries P ∗− L and P ∗− N as a function of q 2 for Λb → Λ ∗µ +µ − decay in the SM and beyond
Figure 9. Single lepton polarization asymmetry P ∗− T and P ∗+ T as a function of q 2 for Λb → Λ ∗µ +µ − decay in the SM and beyond. 6.2 Double lepton polarization asymmetries The asymmetry P ∗ LL is close to −1 throughout the kinematic range, with bin value (−9.97 ± 0.004) × 10−1 . As in the Λ channel, P ∗ LL → −1 in the massless limit for any q 2 and independently of the Wilson coefficients, because vector and axial-vector currents always produce the two leptons with opposite helicities. The deviation from - 1 comes from the m2 ℓ terms in Eq.(90) and here m2 µ /q2 ≈ 7×10−4 . The NP…
Figure 10. Double lepton polarization asymmetries P ∗ LL,P ∗ T T ,P ∗ NN as a function of q 2 for Λb → Λ ∗µ +µ − in the SM and beyond. (see Appendix C for analytical expressions), so that P ∗ T T ≈ P ∗ NN in the massless limit, as in the Λ channel. The SM predictions are close to zero in the bin: P ∗ T T = (−8.30 ± 2.82) × 10−3 and P ∗ NN = (−7.58 ± 3.16) × 10−3 . The two numbers differ by about 7 × 10−4 , which is the order of O(m2 µ /q2 ) correction. Both observables show clear NP sensitivity, comparable to the Λ channel at similar q 2 . The benchmark scenario I produce a well-separated…
Figure 11. Double lepton polarization asymmetries P ∗ NT and P ∗ LN as a function of q 2 for Λb → Λ ∗µ +µ − in the SM and beyond
Figure 12. Double lepton polarization asymmetries P ∗ T L and P ∗ LT as a function of q 2 for Λb → Λ ∗µ +µ − in the SM and beyond. The asymmetry PNT is proportional to Im(ARAL∗ ) with no explicit mℓ suppres￾sion beyond the phase-space factor. For Λ∗ the expression includes the B-amplitude contribution: P ∗ NT ∝ p q 2 q q 2 − 4m2 ℓ Im 2A R ∥0A L∗ ∥0 − A R ∥1A L∗ ∥1 − B R ∥1B L∗ ∥1 + {∥↔⊥} . (32) The SM and the three New Physics benchmark scenarios produce a small but nonzero signal at O(10−2 ), arising from the absorptive imaginary parts of the charm-loop contributions to the effective Wilson…
Figure 13. Doubly polarized forward-backward asymmetries
Figure 14. Doubly polarized forward-backward asymmetries

Figures reproduced from arXiv:2610.27015 by the authors.

References

Discussion

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