60 research outputs found
Weak radiative hyperon decays
The problem of weak radiative hyperon decays (WRHD) is reviewed. With the
recent measurement of the 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 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 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 Decays
A method parametrizing all inelastic final state interactions (FSI) in decays is presented ( - 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 from the data. It is furthermore pointed
out that the size of FSI effects cannot be determined from
decays in a satisfactory way. The case of SU(3)-violating FSI is also
discussed. When fits to the branching ratios of all decays are
performed with all inelastic FSI included, the extracted value of is
shifted down by some when compared to the no-FSI analyses, and
becomes consistent with the Standard Model value of .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 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
Inelastic rescattering effects in B decays into a pair of pseudoscalar
mesons ( or ) are discussed. It is assumed that
meson decays through a short-distance tree-diagram process into two jet-like
states composed of low-mass resonances which rescatter into . The
rescattering of resonance pair into the final state is assumed
to proceed through a Regge flavour exchange. Since such processes constitute a
fraction,diminishing with increasing energy, of total inelastic
scattering, the inelastic rescattering contribution should die out for . At , however, explicit estimates show that
rescattering corrections could be substantial, leading to long-distance
corrections to , 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
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