Weak radiative hyperon decays

The problem of weak radiative hyperon decays (WRHD) is reviewed. With the recent measurement of the $\Xi ^0 \to \Lambda \gamma$ asymmetry confirming Hara's theorem, implications from its violation in low-energy theoretical approaches are discussed. It is shown how an underlying symmetry link should be formulated for a successful description of both nonleptonic and radiative weak hyperon decays. The sign of the $\Xi ^0 \to \Lambda \gamma$ asymmetry and the overall size of parity-violating WRHD amplitudes together lead to the resolution of the old S:P problem in nonleptonic decays.Comment: 5 pages, uses espcrc2.sty; invited talk at the 5th International Conference on Hyperons, Charm and Beauty Hadrons (BEACH2002), University of british Columbia, Vancouver, June 25-29,200

Weak Radiative Decays of Hyperons: Quark Model and Nonlocality

It is proved that symmetry structure of the parity-viol. (p.v.) amplitudes of weak radiative hyperon decays in the VMD approach, and the violation of Hara's theorem in particular, are also obtained when direct coupling e_q \bar{q}\gamma_{\mu}q A^{\mu} of photon to quarks is used in place of VMD. Thus, violation of Hara's theorem in VMD-based models does not result from the lack of gauge invariance. It is further shown that, in the static limit of the quark model, the CA commutator term in the p.v. amplitudes of nonleptonic hyperon decays (NLHD) and the parity-violating \Sigma ^+ \to p \gamma decay amplitude are proportional to each other. As a result, Hara's theorem may be satisfied in this limit if and only if the contribution from the CA commutator in NLHD is zero. Violation of Hara's theorem is traced back to the nonlocality of quark model states in the static limit. It is argued that the ensuing intrinsic baryon nonlocality does not have to be unphysical. It is stressed that the \Xi ^0 \to \Lambda \gamma asymmetry will provide very important information concerning the presence or absence of nonlocal features in p.v. photon coupling to baryons at vanishing photon momentum. If the \Xi ^0 \to \Lambda \gamma asymmetry is found negative, Hara's theorem is satisfied but the gauge-invariant quark model machinery predicting its violation must miss some contribution, or be modified. If experiment confirms positive \Xi ^0 \to \Lambda \gamma asymmetry, then, most likely, Hara's theorem is violated. Although positive \Xi ^0 \to \Lambda \gamma asymmetry admits of the possibility that Hara's theorem is satisfied, this alternative is in disagreement with hints suggested by the similarity of photon and vector-meson couplings and the observed size of p.v. nuclear forces.Comment: Revised with minor changes; 31 LaTeX pages, 1 .ps figure; to appear in Acta Phys. Pol.

Hara's theorem, quark model, and gamma_5-dependent renormalization constants

We discuss the applicability of gamma_5-dependent field renormalization as a means of renormalizing away the apparent violation of Hara's theorem observed in the quark model. It is pointed out that a result totally analogous to the violation of Hara's theorem is predicted by the quark model also for neutral baryons. For neutral baryons, however, such a result cannot be renormalized away. This proves that gamma_5-dependent renormalization does not provide a proper way for a hadron-level understanding of the violation of Hara's theorem observed in the quark model.Comment: 8 pages, LaTe

On the Origin of the Violation of Hara's Theorem for Conserved Current

I elaborate on the argument that the violation of Hara's theorem for conserved current requires that the current is not sufficiently well localized. It is also stressed that whatever sign of asymmetry is measured in the $\Xi ^0 \to \Lambda \gamma$ decay, one of the following three statements must be incorrect: 1) Hara's theorem is satisfied, 2) vector meson dominance is applicable to weak radiative hyperon decays, and 3) basic structure of our quark-model description of nuclear parity-violation is correct.Comment: LaTeX, 14 pages, 1 ps figur

Weak Radiative Hyperon Decays: Questioning the Basics

Main theoretical approaches to weak radiative hyperon decays are briefly reviewed. It is emphasized that only approaches with great predictive power should be seriously considered when seeking a resolution of the puzzle presented by observed large negative asymmetry alpha (Sigma ^+ --> p gamma). In such cases, asymmetry in the Xi ^0 --> Lambda gamma decay is always large while its sign is positive (negative) if Hara's theorem is violated (satisfied). Measuring this asymmetry is therefore crucial for determining whether the large value of alpha (Sigma ^+ --> p gamma) is due to large SU(3) breaking or to some deeper reason. Some arguments suggesting that violation of Hara's theorem might be a feature of Nature, and hints as to its possible origin are also given.Comment: 8 LaTeX pages, uses hyperon99_paper.sty which is included; Talk given at Hyperon 99: Hyperon Physics Symposium, Batavia, IL, 27-29 Sep 199

Inelastic Final State Interactions in B→PPB \to PP Decays

A method parametrizing all inelastic final state interactions (FSI) in $B\to PP$ decays is presented ($P$ - pseudoscalar meson). The method explicitly shows how rescattering leads to the replacement of the short-distance amplitudes with the effective quark diagram amplitudes, and how it affects the extraction of the unitarity triangle angle $\gamma$ from the data. It is furthermore pointed out that the size of FSI effects cannot be determined from $B^0_d \to K^+ K^-$ decays in a satisfactory way. The case of SU(3)-violating FSI is also discussed. When fits to the branching ratios of all $B \to PP$ decays are performed with all inelastic FSI included, the extracted value of $\gamma$ is shifted down by some $20^o-30^o$ when compared to the no-FSI analyses, and becomes consistent with the Standard Model value of $65^o \pm 7^o$.Comment: Talk presented at the Sixth International Conference on Hyperons, Charm and Beauty Hadrons, IIT, Chicago, June 27--July 3 200

The Concept of Mass, Quarks, and Phase Space

We point out the conceptual problems related to the application of the standard notion of mass to quarks and recall the arguments that there should be a close connection between the properties of elementary particles and the arena used for the description of classical macroscopic processes. Motivated by the above and the wish to introduce more symmetry between the coordinates of position and momentum we concentrate on the classical nonrelativistic phase space with ${\bf {p}}^2+{\bf {x}}^2$ as an invariant. A symmetry-based argument on how to generalize the way in which mass enters into our description of Nature is presented and placed into the context of the phase-space scheme discussed. It is conjectured that the proposed non-standard way of relating "mass" to the variables of the classical phase space is actually used in Nature, and that it manifests itself through the existence of quarks. Some properties of this proposal, including unobservability of "free quarks" and the emergence of mesons, are discussed.Comment: 26 pages. to be published in Concepts of Physics, Vol.3, 2006 (http://www.merlin.fic.uni.lodz.pl/concepts/index.html

Radiative and nonleptonic hyperon decays in broken SU(3)

We report on the recently proposed joint resolution of two long-standing puzzles in weak radiative (WR) and nonleptonic (NL) hyperon decays (HD). First, a good VMD-based description of WRHD is presented. In particular, the large negative asymmetry observed in the \Sigma^+ \to p \gamma decay is explained through a calculably large SU(3)-breaking effect in the relevant parity-violating amplitude. Second, the achieved description of the parity-violating WRHD amplitudes permits the extraction, via the SU(2)_W + VMD route, of the non-soft-pion correction term in the parity-violating NLHD amplitudes. The latter subtracts a substantial amount from the current-algebra commutator term, thus leading to the resolution of the old S:P discrepancy in NLHD.Comment: Talk given at 7th International Conference on Hyperons, Charm and Beauty Hadrons (BEACH2006), Lancaster, U.K., 2 - 8 July 2006, ; 4 pages, uses espcrc2.st

Inelastic Rescattering in B→ππ,KKˉB \to \pi \pi, K \bar{K}

Inelastic rescattering effects in B decays into a pair $PP$ of pseudoscalar mesons ($PP = \pi \pi$ or $K\bar{K}$) are discussed. It is assumed that $B$ meson decays through a short-distance tree-diagram process into two jet-like states composed of low-mass resonances $M_1 M_2$ which rescatter into $PP$. The rescattering of resonance pair $M_1 M_2$ into the final $PP$ state is assumed to proceed through a Regge flavour exchange. Since such processes constitute a fraction,diminishing with increasing energy, of total inelastic $PP$ scattering, the inelastic rescattering contribution should die out for $m_B \to \infty$. At $m_B = 5.2 ~GeV$, however, explicit estimates show that rescattering corrections could be substantial, leading to long-distance corrections to $B^0 \to K\bar{K}$, comparable to short-distance penguin contributions.Comment: 11 LaTeX pages, 3 .ps figure files, presented at the Cracow Epiphany Conference on b Physics and CP Violation, Cracow, Poland, January 5-7, 200

On the Pattern of Asymmetries in the Pole Model of Weak Radiative Hyperon Decays

We study the question whether the pole-model VMD approach to weak radiative hyperon decays can be made consistent with Hara's theorem and still yield the pattern of asymmetries characteristic of the quark model. It is found that an essential ingredient which governs the pattern of asymmetries is the assumed off-shell behaviour of the parity-conserving 1/2^- 1/2^+ gamma amplitudes. It appears that this behaviour can be chosen in such a way that the pattern characteristic of the quark model is obtained, and yet Hara's theorem satisfied. As a byproduct, however, all parity-violating amplitudes in weak radiative and nonleptonic hyperon decays must then vanish in the SU(3) limit. This is in conflict with the observed size of weak meson-nucleon couplings