4,442 research outputs found
Two-loop representations of low-energy pion form factors and pi-pi scattering phases in the presence of isospin breaking
Dispersive representations of the pi-pi scattering amplitudes and pion form
factors, valid at two-loop accuracy in the low-energy expansion, are
constructed in the presence of isospin-breaking effects induced by the
difference between the charged and neutral pion masses. Analytical expressions
for the corresponding phases of the scalar and vector pion form factors are
computed. It is shown that each of these phases consists of the sum of a
"universal" part and a form-factor dependent contribution. The first one is
entirely determined in terms of the pi-pi scattering amplitudes alone, and
reduces to the phase satisfying Watson's theorem in the isospin limit. The
second one can be sizeable, although it vanishes in the same limit. The
dependence of these isospin corrections with respect to the parameters of the
subthreshold expansion of the pi-pi amplitude is studied, and an equivalent
representation in terms of the S-wave scattering lengths is also briefly
presented and discussed. In addition, partially analytical expressions for the
two-loop form factors and pi-pi scattering amplitudes in the presence of
isospin breaking are provided.Comment: 57 pages, 12 figure
Radiative corrections to neutral pion-pair production
We calculate the one-photon loop radiative corrections to the neutral
pion-pair photoproduction process . At leading
order this reaction is governed by the chiral pion-pion interaction. Since the
chiral contact-vertex depends only on the final-state
invariant-mass it factors out of all photon-loop diagrams. We give analytical
expressions for the multiplicative correction factor
arising from eight classes of contributing one-photon loop diagrams. An
electromagnetic counterterm has to be included in order to cancel the
ultraviolet divergences generated by the photon-loops. Infrared finiteness of
the virtual radiative corrections is achieved (in the standard way) by
including soft photon radiation below an energy cut-off . The
radiative corrections to the total cross section vary between and
for center-of-mass energies from threshold up to . The finite part of
the electromagnetic counterterm gives an additional constant contribution of
about , however with a large uncertainty.Comment: 10 pages, 6 figures, submitted to Eur. Phys. J.
The Politics of Service Delivery Reform
This article identifies the leaders, the supporters and the resisters of public service reform. It adopts a principal–agent framework, comparing reality with an ‘ideal’ situation in which citizens are the principals over political policy-makers as their agents, and policy-makers are the principals over public service officials as their agents. Reform in most developing countries is complicated by an additional set of external actors — international financial institutions and donors. In practice, international agencies and core government officials usually act as the ‘principals’ in the determination of reforms. The analysis identifies the interests involved in reform, indicating how the balance between them is affected by institutional and sectoral factors. Organizational reforms, particularly in the social sectors, present greater difficulties than first generation economic policy reforms
The isospin symmetry breaking effects in decays
The Fermi-Watson theorem is generalized to the case of two coupled channels
with different masses and applied to final state interaction in
decays. The impact of considered effect on the phase of the scattering
is estimated and shown that it can be crucial for scattering lengths extraction
from experimental data on decays
Soft-photon corrections in multi-body meson decays
The effects due to soft-photon emission (and the related virtual corrections)
in multi-body decays of B, D, and K mesons are analysed. We present analytic
expressions for the universal O(alpha) correction factors which can be applied
to all multi-body decay modes where a tight soft-photon energy cut in the
decaying-particle rest-frame is applied. All-order resummations valid in the
limit of small and large velocities of the final-state particles are also
discussed. The phenomenological implications of these correction factors in the
distortion of Dalitz-plot distributions of K -> 3 pi decays are briefly
analysed.Comment: 8 pages, 2 figures (v2: minor modifications - published version
K -> 3 pi Final State Interactions at NLO in CHPT and Cabibbo's Proposal to Measure a_0-a_2
We present the analytical results for the K -> 3 pi final state interactions
at next-to-leading order (NLO) in CHPT. We also study the recent Cabibbo's
proposal to measure the pi-pi scattering lenghts combination a_0-a_2 from the
cusp effect in the pi^0-pi^0 energy spectrum at threshold for K^+ -> pi^0 pi^0
pi^+ and K_L -> pi^0 pi^0 pi^0$, and give the relevant formulas to describe it
at NLO. For that, we use the NLO CHPT expression to fit the real part of K -> 3
pi to data while the pi-pi scattering lenghts are treated non-perturbatively.
Using them, we make a quantitative estimate of the theoretical uncertaintity of
the a_0-a_2 determination at NLO in our approach and obtain that it is not
smaller than 5 % if added quadratically and 7 % if linearly for K^+ -> pi^0
pi^0 pi^+. One gets similar theoretical uncertainties if the neutral K_L ->
pi^0 pi^0 pi^0 decay data below threshold are used instead. For this decay,
there are very large theoretical uncertainties above threshold due to
cancellations and data above threshold cannot be used to get the scattering
lenghts. All the numbers we present are in the isospin limit apart of two-pion
phase space factors which are physical. We compare our results for the cusp
effect with Cabibbo and Isidori's results and discuss the differences and
agreements.
We also comment on the apperance of the singularity at the K -> 3 pi
pseudo-threshold s=(m_K-m_pi)^2 in the discontinuity that defines the cusp.Comment: 31 pages, 8 figures. v2=v3 Added the full contributions to the cusp
from the real part of the discontinuity. v4 Improved text. Matches published
versio
The electromagnetic effects in decay
The final state interaction of pions in decay allows one to obtain
the value of the isospin and angular momentum zero scattering length
.We take into account the electromagnetic interaction of pions and
isospin symmetry breaking effects caused by different masses of neutral and
charged pions and estimate the impact of these effects on the procedure of
scattering length extraction from decay.Comment: 8 pages,1 figur
Final state interaction in kaons decays
The kaons decays to the pairs of charged and neutral pions are considered in
the framework of the non-relativistic quantum mechanics. The general
expressions for the decay amplitudes to the two different channels accounting
for the strong interaction between pions are obtained. The developed approach
allows one to estimate the contribution of terms of any order in strong
interaction and correctly takes into account the electromagnetic interaction
between the pions in the final state.Comment: 8 page
Couplings of light I=0 scalar mesons to simple operators in the complex plane
The flavour and glue structure of the light scalar mesons in QCD are probed
by studying the couplings of the I=0 mesons and to the
operators , and to two photons. The Roy dispersive
representation for the amplitude is used to determine the
pole positions as well as the residues in the complex plane. On the real axis,
is constrained to solve the Roy equation together with elastic
unitarity up to the K\Kbar threshold leading to an improved description of
the . The problem of using a two-particle threshold as a matching
point is discussed. A simple relation is established between the coupling of a
scalar meson to an operator and the value of the related pion form-factor
computed at the resonance pole. Pion scalar form-factors as well as two-photon
partial-wave amplitudes are expressed as coupled-channel Omn\`es dispersive
representations. Subtraction constants are constrained by chiral symmetry and
experimental data. Comparison of our results for the couplings with
earlier determinations of the analogous couplings of the lightest I=1 and
scalar mesons are compatible with an assignment of the ,
, , into a nonet. Concerning the gluonic operator
we find a significant coupling to both the and the
.Comment: 31 pages, 5 figure
Bremsstrahlung simulation in K to pi l^pm nu_l (gamma) decays
In physics simulation chains, the PHOTOS Monte Carlo program is often used to
simulate QED effects in decays of intermediate particles and resonances. The
program is based on an exact multiphoton phase space. In general, the matrix
element is obtained from iterations of a universal kernel and approximations
are involved. To evaluate the program precision, it is necessary to formulate
and implement within the generator the exact matrix element, which depends on
the decay channel. Then, all terms necessary for non-leading logarithms are
taken into account.
In the present letter we focus on the decay K to pi l^pm nu_l and tests of
the PHOTOS Monte Carlo program. We conclude a 0.2% relative precision in the
implementation of the hard photon matrix element into the emission kernel,
including the case where approximations are used.Comment: 1+20 pages, 8 figure
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