Low Recoil Region Research Articles

The {Lambda}_bto {Lambda}^{ast }(1520)left(to Noverline{K}right){mathrm{ell}}^{+}{mathrm{ell}}^{-} decay at low-recoil in HQET

In this paper we discuss the Standard Model and new physics sensitivity of the {Lambda}_bto {Lambda}^{ast }(1520)left(to Noverline{K}right){mathrm{ell}}^{+}{mathrm{ell}}^{-} decay at low-recoil, where ℓ± are massive leptons and Noverline{K}=left{{pK}^{-},n{overline{K}}^0right} . We provide a full angular distribution with a set of operators that includes the Standard Model operators and their chirality flipped counterparts, and new scalar and pseudo-scalar operators. The resulting angular distribution allows us to construct observables that we study in the Standard Model and in model-independent new physics scenarios. To reduce the hadronic effects emanating from the Λb→ Λ* (1520) transition form factors, we exploit the Heavy Quark Effective theory framework valid at low Λ∗(1520) recoil, i.e., large dilepton invariant mass squared {q}^2sim mathcal{O}left({m}_b^2right) . Working to the leading order in 1/mb and including mathcal{O}left({alpha}_sright) corrections, we compute the ‘improved Isgur-Wise relations’ between the form factors. The relations correlate the form factors and thereby allow the description of this decay at the low-recoil region with a smaller number of independent form factors.

Ethylene Intersystem Crossing Caught in the Act by Photofragment Sulfur Atoms

Ethylene, C2H4, the simplest π-bonded molecule, is of enormous fundamental and commercial importance. Its lowest triplet state, in which the CH2 moieties occupy perpendicular planes, is well known from theory, but there has been no definitive experimental observation of this species. Here, velocity map imaging of the sulfur atoms in ethylene sulfide (c-C2H4S) photodissociation at 217 nm is used to reveal the internal state distribution of co-product ethylene. While both S (1D) and S (3P) translational energy distributions display three distinct regions that find their origins in singlet and triplet excited states of c-C2H4S, respectively, the S (3P) distribution is dominated by a fourth, low-recoil region. In this region, the distribution is fully isotropic at a recoil of 9 ± 1 kcal/mol, corresponding to the opening of the triplet ethylene channel. Multireference calculations suggest that this photodissociation pathway is mediated by a hot, transient biradical CH2CH2S that strongly favors CH2-hindered rotations in the predissociated complex. This photochemical ring-opening mechanism is invoked to account for the vibrational features observed in this low-recoil region, which are attributed to triplet ethylene relaxing to the torsional saddle point on the ground-state singlet surface. This study thereby gives for the first time the experimental confirmation of an adiabatic singlet-triplet splitting of 66 ± 1 kcal/mol and a torsional barrier height of 64 ± 1 kcal/mol in ethylene.

RareBmeson decays on the lattice

We discuss a framework for the measurement of the B → K * transition form factors in lattice simulations, when the K * eventually decays. The possible mixing of πK and ηK states is considered. We reproduce the two-channel analogue of the Lellouch-Luscher formula, which allows one to extract the B → K * l + l − decay amplitude in the low-recoil region. Since the K * is a resonance, we provide a procedure to determine the form factors at the complex pole position in a process-independent manner. The infinitely-narrow width approximation of the results is also studied.

Rare Decays at LHCb

Rare decays of beauty and charm hadrons provide an effective method of testing the Standard Model and probing possible new physics scenarios. The LHCb experiment has published a variety of interesting results in this field, some of which are presented here. In particular the measurements of the branching fractions of B(s) 0 → μ + μ − which, in combination with CMS, resulted in the first observation of the Bs 0 → μ + μ − decay. Other topics include searches for the rare decay D 0 → μ + μ − , the lepton flavour violating decays B(s) 0 → e ± μ ∓ , and the observation of the ψ (4160) resonance in the region of low recoil in B + → K + μ + μ − decay. New results on the angular analysis of the decay B 0 → K *0 μ + μ − with form factor independent observables are also shown.

New physics effects and hadronic form factor uncertainties inB→K*ℓ+ℓ−

It is well known that new physics can contribute to weak decays of heavy mesons via virtual processes during its decays. The discovery of new physics, using such decays is made difficult due to intractable strong interaction effects needed to describe it. Modes such as $B\ensuremath{\rightarrow}{K}^{*}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ offer an advantage as they provide a multitude of observables via angular analysis. We show how the multitude of ``related observables'' obtained from $B\ensuremath{\rightarrow}{K}^{*}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$, can provide many new ``clean tests'' of the standard model. The hallmark of these tests is that several of them are independent of the unknown universal form factors that describe the decay in heavy quark effective theory. We derive a relation between observables that is free of form factors and Wilson coefficients, the violation of which will be an unambiguous signal of new physics. We also derive relations between observables and form factors that are independent of Wilson coefficients and enable verification of hadronic estimates. We show how form factor ratios can be measured directly from helicity fraction with out any assumptions what so ever. We find that the allowed parameter space for observables is very tightly constrained in standard model, thereby providing clean signals of new physics. We examine in detail both the large-recoil and low-recoil regions of the ${K}^{*}$ meson and point out special features and derive relations between observables valid in the two limits. In the large-recoil regions several of the relations are unaffected by corrections to all orders in ${\ensuremath{\alpha}}_{s}$. We present yet another new relation involving only observables that would verify the validity of the relations between form factors assumed in the low-recoil region. The several relations and constraints derived will provide unambiguous signals of new physics if it contributes to these decays.

Cornering new physics in b → s transitions

Abstract We derive constraints on Wilson coefficients of dimension-six effective operators probing the b → s transition, using recent improved measurements of the rare decays B s → μ + μ −, B → Kμ + μ − and B → K * μ + μ − and including all relevant observables in inclusive and exclusive decays. We consider operators present in the SM as well as their chirality-flipped counterparts and scalar operators. We find good agreement with the SM expectations. Compared to the situation before winter 2012, we find significantly more stringent constraints on the chirality-flipped coefficients due to complementary constraints from B → Kμ + μ − and B → K * μ + μ − and due to the LHCb measurement of the angular observable S 3 in the latter decay. We also list the full set of observables sensitive to new physics in the low recoil region of B → K * μ + μ −.

More benefits of semileptonic rare B decays at low recoil: CP violation

We present a systematic analysis of the angular distribution of Bbar -> Kbar^\ast (-> Kbar pi) l^+ l^- decays with l = e, mu in the low recoil region (i.e. at high dilepton invariant masses of the order of the mass of the b-quark) to account model-independently for CP violation beyond the Standard Model, working to next-to-leading order QCD. From the employed heavy quark effective theory framework we identify the key CP observables with reduced hadronic uncertainties. Since some of the CP asymmetries are CP-odd they can be measured without B-flavour tagging. This is particularly beneficial for Bbar_s,B_s -> phi(-> K^+ K^-) l^+ l^- decays, which are not self-tagging, and we work out the corresponding time-integrated CP asymmetries. Presently available experimental constraints allow the proposed CP asymmetries to be sizeable, up to values of the order ~ 0.2, while the corresponding Standard Model values receive a strong parametric suppression at the level of O(10^-4). Furthermore, we work out the allowed ranges of the short-distance (Wilson) coefficients C_9,C_10 in the presence of CP violation beyond the Standard Model but no further Dirac structures. We find the Bbar_s -> mu^+ mu^- branching ratio to be below 9*10^-9 (at 95% CL). Possibilities to check the performance of the theoretical low recoil framework are pointed out.

The benefits of $ \bar{B} \to {\bar{K}^*}{l^{+} }{l^{-} } $ decays at low recoil

Using the heavy quark effective theory framework put forward by Grinstein and Pirjol we work out predictions for \( \bar{B} \to {\bar{K}^*}{l^{+} }{l^{-} } \), l = e, μ, decays for a softly recoiling \( {\bar{K}^*} \), i.e., for large dilepton masses \( \sqrt {{{q^2}}} \) of the order of the b-quark mass m b . We work to lowest order in Λ/Q, where \( Q = \left( {{m_b}\sqrt {{{q^2}}} } \right) \) and include the next-to-leading order corrections from the charm quark mass m c and the strong coupling at \( \mathcal{O}\left( {{{{m_c^2}} \left/ {{{Q^2},{\alpha_s}}} \right.}} \right) \). The leading Λ/m b corrections are parametrically suppressed. The improved Isgur-Wise form factor relations correlate the \( \bar{B} \to {\bar{K}^*}{l^{+} }{l^{-} } \) transversity amplitudes, which simplifies the description of the various decay observables and provides opportunities for the extraction of the electroweak short distance couplings. We propose new angular observables which have very small hadronic uncertainties. We exploit existing data on \( \bar{B} \to {\bar{K}^*}{l^{+} }{l^{-} } \) distributions and show that the low recoil region provides powerful additional information to the large recoil one. We find disjoint best-fit solutions, which include the Standard Model, but also beyond-the-Standard Model ones. This ambiguity can be accessed with future precision measurements.

Exclusive rareB→K*ℓ+ℓ−decays at low recoil: Controlling the long-distance effects

We present a model-independent description of the exclusive rare decays $\overline{B}\ensuremath{\rightarrow}{K}^{*}{e}^{+}{e}^{\ensuremath{-}}$ in the low recoil region (large lepton invariant mass ${q}^{2}\ensuremath{\sim}{m}_{b}^{2}$). In this region the long-distance effects from quark loops can be computed with the help of an operator product expansion in $1/Q$, with $Q={{m}_{b},\sqrt{{q}^{2}}}$. Nonperturbative effects up to and including terms suppressed by $\ensuremath{\Lambda}/Q$ and ${m}_{c}^{2}/{m}_{b}^{2}$ relative to the short-distance amplitude can be included in a model-independent way. Based on these results, we propose an improved method for determining the Cabibbo-Kobayashi-Maskawa matrix element $|{V}_{ub}|$ from a combination of rare and semileptonic $B$ and $D$ decays near the zero recoil point. The residual theoretical uncertainty from long-distance effects in this $|{V}_{ub}|$ determination comes from terms in the operator product expansion of order ${\ensuremath{\alpha}}_{s}(Q)\ensuremath{\Lambda}/{m}_{b}$, ${\ensuremath{\alpha}}_{s}^{2}(Q)$, ${m}_{c}^{4}/{m}_{b}^{4}$, and duality violations, and is estimated to be below 10%.