Leptonic decays of baryons

We study the weak interaction axial form factors of the octet baryons, within the covariant spectator quark model, focusing on the dependence of four-momentum transfer squared, Q 2 . In our model the axial form factors GA(Q 2 ) (axial-vector form factor) and GP (Q 2 ) (induced pseudoscalar form factor), are calculated based on the constituent quark axial form factors and the octet baryon wave functions. The quark axial current is parametrized by the two constituent quark form factors, the axial-vector form factor g q A (Q 2 ), and the induced pseudoscalar form factor g q P (Q 2 ). The baryon wave functions are composed of a dominant S-state and a P -state mixture for the relative angular momentum of the quarks. First, we study in detail the nucleon case. We assume that the quark axial-vector form factor g q A (Q 2 ) has the same function form as that of the quark electromagnetic isovector form factor. The remaining parameters of the model, the P -state mixture and the Q 2 -dependence of g q P (Q 2 ), are determined by a fit to the nucleon axial form factor data obtained by lattice QCD simulations with large pion masses. In this lattice QCD regime the meson cloud effects are small, and the physics associated with the valence quarks can be better calibrated. Once the valence quark model is calibrated, we extend the model to the physical regime, and use the low Q 2 experimental data to estimate the meson cloud contributions for GA(Q 2 ) and GP (Q 2 ). Using the calibrated quark axial form factors, and the generalization of the nucleon wave function for the other octet baryon members, we make predictions for all the possible weak interaction axial form factors GA(Q 2 ) and GP (Q 2 ) of the octet baryons. The results are compared with the corresponding experimental data for GA(0), and with the estimates of baryon-meson models based on SU (6) symmetry.

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“…For the discussions about the sign conventions see Refs. [4,6,24,124,125]. Also in Table V one F + D).…”

Section: Su (3) Baryon-meson Modelmentioning

confidence: 99%

Axial form factors of the octet baryons in a covariant quark model

Ramalho

Tsushima

2016

Phys. Rev. D

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“…The a, A, and B correlation coefficients in this order, which we denote with a "0" subscript, are functions only of λ ≡ g A /g V > 0 in the SM, where g A and g V are the weak axial-vector and vector coupling constants; this is tantamount to the V − A law. Note that for consistency with earlier work [23][24][25], we employ a λ > 0 sign convention, so that the sign of the γ 5 terms in the weak currents are chosen opposite to that of the Particle Data Group [22]. We also assume that λ is real, which has been established beyond our assumed level of sensitivity in the observables we consider [26,27].…”

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confidence: 99%

Framework for maximum likelihood analysis of neutronβdecay observables to resolve the limits of theV−Alaw

Gardner

Plaster

2013

Phys. Rev. C

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We assess the ability of future neutron β decay measurements of up to O(10 −4 ) precision to falsify the standard model, particularly the V − A law, and to identify the dynamics beyond it. To do this, we employ a maximum likelihood statistical framework which incorporates both experimental and theoretical uncertainties. Using illustrative combined global fits to Monte Carlo pseudodata, we also quantify the importance of experimental measurements of the energy dependence of the angular correlation coefficients as input to such efforts, and we determine the precision to which ill-known "second-class" hadronic matrix elements must be determined in order to exact such tests.

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“…We adopt the historic (1 + γ 5 ) sign convention in order to retain manifest consistency with earlier work [25][26][27]. Lorentz invariance and translation invariance implies that the nucleon weak current p|J µ (0)| n has six terms:…”

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confidence: 99%

Sharpening Low-Energy, Standard-Model Tests via Correlation Coefficients in NeutronβDecay

Gardner

Zhang

2001

Phys. Rev. Lett.

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The correlation coefficients a, A, and B in neutron β-decay are proportional to the ratio of the axial-vector to vector weak coupling constants, gA/gV , to leading recoil order. With the advent of the next generation of neutron decay experiments, the recoil-order corrections to these expressions become experimentally accessible, admitting a plurality of Standard Model (SM) tests. The measurement of both a and A, e.g., allows one to test the conserved-vector-current (CVC) hypothesis and to search for second-class currents (SCC) independently. The anticipated precision of these measurements suggests that the bounds on CVC violation and SCC from studies of nuclear β-decay can be qualitatively bettered. Departures from SM expectations can be interpreted as evidence for non-V − A currents.Precision nuclear β-decay measurements have played an important role in the rise of the Standard Model (SM), giving strong credence to the conserved-vector-current (CVC) hypothesis, as well as to the absence of secondclass currents (SCC). We show that upcoming neutrondecay experiments can sharpen tests of the CVC hypothesis and of the absence of SCC significantly, eliminating assumptions inherent to the nuclear studies.Searches for CVC violation and SCC in nuclear β-decay experiments have spanned decades of effort. We consider a CVC test originally suggested by GellMann [1]: the strength of the "weak magnetism" term of the nucleon weak current ought be given by the strength of the corresponding electromagnetic M1 transition. The SM test realized from such a comparison constrains a combination of the weak magnetism and induced tensor terms of the nucleon weak current. The induced tensor term is a "second class" current and thus is zero in the SM [2], save for isospin-violating effects engendered by the differing mass and charge of the u and d quarks. In tests of this sort, the CVC hypothesis is tested if SCC are assumed to be zero, or, alternatively, the non-existence of SCC is tested if the CVC hypothesis is assumed to be valid.Historically, the best constraints on the non-existence of SCC and CVC violation are realized in the mass 12 system [3,4]. The CVC hypothesis can be tested through the comparison of the spectral shape correction parameters a ∓ measured in 12 B → 12 C and 12 N → 12 C transitions with the strength of the electromagnetic M1 transition from the analog state of 12 C. This procedure yields a test of the CVC hypothesis at the 10% level [3][4][5]. In order to realize a SCC test, the decays of spin-aligned 12 B and 12 N nuclei are studied. For purely aligned 1 + → 0 + transitions [6], the e ∓ angular distribution for 12 B (−) and 12 N (+) decay is given by [4]where p e and E e are the momentum and energy of the electron (positron), E max e is the endpoint energy, θ is the angle between p e and the spin orientation axis, and A is the nuclear alignment. The difference α − − α + is sensitive to the weak magnetism term as well as to the induced tensor term in the nucleon weak current. Unfortunately, it is also sensitive t...

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Leptonic decays of baryons

Cited by 51 publications

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Axial form factors of the octet baryons in a covariant quark model

Axial form factors of the octet baryons in a covariant quark model

Framework for maximum likelihood analysis of neutronβdecay observables to resolve the limits of theV−Alaw

Sharpening Low-Energy, Standard-Model Tests via Correlation Coefficients in NeutronβDecay

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