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Universal couplings in two-body strong and electromagnetic meson decays


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Jones, NR 1997 , 'Universal couplings in two-body strong and electromagnetic meson decays', PhD thesis, University of Tasmania.

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The strong and electromagnetic interactions of the ground state hadrons
have, since the early 1970's, been known to comply with the tree level approximations
given by effective Lagrangians. In more recent years the weak
decays of heavy hadrons have been successfully calculated by use of the Heavy
Quark Effective Theory (HQET). It is suited to hadrons containing a heavy
quark (c, b, t) with light quark partner(s) (u, d, s) and uses the approximation
that the hadron momentum is carried by the heavy quark. This resembles a
group theoretical approach of constructing relativistic hadron wavefunctions
in the mid 1960's, based on U(2N1f) ⊗ U(2Nf) ⊂ U(4Nf ) symmetry group,
producing wavefunctions whose structure implies that the constituent quarks
must be moving collinearly and with the same velocity as the hadron. If one
quark is much heavier than the others then it will have most of the momentum,
as is the case for HQET. The main difference between the schemes
is in the treatment of the light quark momentum, but this is a higher order
correction leading only to minor changes.
Given the similarity between the models, but with the present favouritism
shown for HQET, we re-investigate the U(4Nf) scheme because of its many -
advantages. Firstly, since the scheme is a spin extension of the SU(Nf)
symmetry, the known mesons (including excited states) are classified into
spin-flavour supermultiplets which naturally leads to the reduction of the
number of free parameters compared to simple flavour SU(Nf). Secondly,
the scheme's interaction Lagrangian is applicable to two-body strong decays
with one only needing a single coupling constant for each parent orbital
angular momentum state. Vector meson dominance is easily incorporated to
enable study of the electromagnetic processes. In this thesis the U(4Nf) x 0(3, 1)L scheme has been applied to the twobody
strong and radiative decays of ground and excited mesons (from L = 0
to L = 3). We use known decay widths and meson masses to calculate the
universal coupling constant involved. Uniformity in the value of supports the supposed supersymmetry. There is high quality experimental data on
the ground states decays and our analysis finds reasonably constant coupling,
but there is some small symmetry breaking associated with the parent
meson mass. Nonetheless, the breaking is quantifiable and leads to predictions
for the D* and B* decay rates and branching fractions. From the less
accurate experimental data available on excited decays we find good uniformity
in the results, especially when taking into account the uniquely large
number of processes considered in this work. Thus I have determined that
the U(4Nf) scheme appears well suited to describing meson interactions and
their excitations and should be useful in studying the weak interactions after
appropriate manipulation, as well.
As a contrast to U(4Nf) we develop a quark triangle scheme involving
a Feynman graph which provides quark level universal couplings to account
for the symmetry breaking effects of unequal quark masses. Due to its complexity
it is only applied to the ground state radiative decays, but with
considerable success and over a range of processes far exceeding any similar
work. Also, whereas other research has used approximations in the derivation
of the decay amplitude, ours is exact and lends itself to study of all applicable
channels regardless of the mesons involved. Unfortunately it is difficult
to apply the method to higher excitations as the quark triangle involved is
divergent, and so techniques of renormalization would have to be used.

Item Type: Thesis - PhD
Authors/Creators:Jones, NR
Keywords: Hadrons, Mesons
Copyright Holders: The Author
Copyright Information:

Copyright 1997 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1997. Includes bibliographical references

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