6,627 research outputs found
Electroweak structure of light nuclei within chiral effective field theory
We review the results of the most recent calculations for the electromagnetic
structure of light nuclei, the weak muon capture on deuteron and 3He and the
weak proton-proton capture reaction at energies of astrophysical interest,
performed within the chiral effective field theory framework.Comment: 6 pages, 3 figures, submitted to Few-Body System
The hep reaction and the solar neutrino problem
The results of a new calculation of the astrophysical S-factor for the proton
weak capture on 3He are here reviewed. The methods used to obtain very accurate
initial and final state wave functions and to construct the nuclear weak
current operator are described. Finally the implications of these results for
the Super-Kamiokande solar neutrino data are discussed.Comment: 10 pages, 1 figure, XVII European Few-Body plenary talk, Evora,
September 200
Road Pricing: Old Beliefs, Present Awareness and Future Research Patterns
The theoretical evolution of academic beliefs and practical policymakers' perceptions of road pricing (from now on rp) as an instrument of efficient and equitable allocation of resources are described and analysed. The aim of the paper is to reconstruct the logical evolution of the theory behind rp in order to understand why there has been scarce policy impact in spite of a long theoretical tradition. In so doing I try to bring to the fore the fundamental issues that will have to be tackled by future research in order to generate consensus around this policy instrument. The paper is structured in four parts. In the first part the fundamental issues of a typical rp model are considered. Among the most important aspects one recalls: first-best/second-best environment, short/long term analysis, homogeneous/heterogeneous time evaluation, perfect/imperfect information, efficiency/equity analysis, use/non-use of resources generated, private/public transportation provision. In the second part the characterising parameters have been interpreted in the light of the Smeed Report of 1964 that can be considered representative of the "old belief". In the third part the "present awareness" is expressed by an analysis of the main contents of the book Internalising the Social Costs of Transport of 1993. In the fourth part some reflections on the most promising research areas for rp implementation and acceptance are put forward. Specific research will have to be conducted concerning social acceptability and feasibility, simultaneous cost internalisation, behavioural assumptions, information and pricing interconnections.Road pricing, Social Acceptability, Congestion, Congestion charging
Mechanosensing in myosin filament solves a 60 years old conflict in skeletal muscle modeling between high power output and slow rise in tension
Almost 60 years ago Andrew Huxley with his seminal paper \cite{Huxley1957}
laid the foundation of modern muscle modeling, linking chemical events to
mechanical performance. He described mechanics and energetics of muscle
contraction through the cyclical attachment and detachment of myosin motors to
the actin filament with ad hoc assumptions on the dependence of the rate
constants on the strain of the myosin motors. That relatively simple hypothesis
is still present in recent models, even though with several modifications to
adapt the model to the different experimental constraints which became
subsequently available. However, already in that paper, one controversial
aspect of the model became clear. Relatively high attachment and detachment
rates of myosin to the actin filament were needed to simulate the high power
output at intermediate velocity of contraction. However, these rates were
incompatible with the relatively slow rise in tension after activation, despite
the rise should be generated by the same rate functions. This discrepancy has
not been fully solved till today, despite several hypotheses have been
forwarded to reconcile the two aspects. Here, using a conventional muscle
model, we show that the recently revealed mechanosensing mechanism of
recruitment of myosin motors \cite{Linarietal2015} can solve this long standing
problem without any further ad-hoc hypotheses
Muon capture on deuteron and the neutron-neutron scattering length
We study the capture rate in the doublet hyperfine initial state for the muon
capture reaction \mu^- + \,^2{\rm H} \rightarrow \nu_\mu + n + n ()
and the total capture rate for the reaction \mu^- + \,^3{\rm He} \rightarrow
\nu_\mu + \,^3{\rm H} (). We investigate whether and
could be sensitive to the -wave scattering length
(). To this aim, we consider nuclear potentials and weak currents
derived within EFT. We employ the N3LO chiral potential with cutoff
=500 MeV, but the low-energy constant (LEC) determining is
varied so as to obtain =-18.95 (the present empirical value), -16.0,
-22.0, and +18.22 fm. The last value leads to a bound state with a binding
energy of 139 keV. The LECs and , present in the three-nucleon
potential and axial-vector current, are fitted to reproduce the binding
energies and the triton Gamow-Teller matrix element. The capture rate
is found to be 399(3) s for =-18.95 and -16.0 fm; and
400(3) s for =-22.0 fm. For =+18.22 fm, we obtain 275(3)
s (135(3) s), when the final system is unbound (bound). The
rate is found to be 1494(15), 1491(16), 1488(18), and 1475(16)
s for =-18.95, -16.0, -22.0, and +18.22 fm, respectively. The
theoretical uncertainties are due to the fitting procedure and radiative
corrections. Our results seem to exclude the possibility of constraining a
negative with an uncertainty of less than 3 fm through an
accurate determination of the muon capture rates, but the uncertainty on the
present empirical value will not complicate the interpretation of the
(forth-coming) experimental results for . Finally, a comparison with
the already available experimental data discourages the possibility of a bound
state.Comment: 5 pages, 2 figures; revisited version accepted for publication on
Phys. Rev.
Muon capture on light nuclei
This work investigates the muon capture reactions 2H(\mu^-,\nu_\mu)nn and
3He(\mu^-,\nu_\mu)3H and the contribution to their total capture rates arising
from the axial two-body currents obtained imposing the
partially-conserved-axial-current (PCAC) hypothesis. The initial and final A=2
and 3 nuclear wave functions are obtained from the Argonne v_{18} two-nucleon
potential, in combination with the Urbana IX three-nucleon potential in the
case of A=3. The weak current consists of vector and axial components derived
in chiral effective field theory. The low-energy constant entering the vector
(axial) component is determined by reproducting the isovector combination of
the trinucleon magnetic moment (Gamow-Teller matrix element of tritium
beta-decay). The total capture rates are 393.1(8) s^{-1} for A=2 and 1488(9)
s^{-1} for A=3, where the uncertainties arise from the adopted fitting
procedure.Comment: 6 pages, submitted to Few-Body Sys
The Demand Potential of an Urban Freight Consolidation Centre
Urban Transport, stated preferences, service contract.
Revisiting the empirical evidence on firmsĂâ money demand
In this paper we estimate the demand for liquidity by US non financial firms using data from COMPUSTAT database. In contrast to the previous literature, we consider firm-specific effects, such as cost-of-capital and wages. From the balanced and unbalanced panel estimations we infer that there are economies of scale in money demand by US business firms, because estimated sales elasticities are smaller than unity. In particular, they are lower than in previous empirical studies, suggesting that economies of scale in the demand for money are even bigger than formerly thought. In addition, it emerges that labor is not a substitute for money.Panel Data, Liquidity, Demand for Money, COMPUSTAT
DISSECTING PREFERENCE HETEROGENEITY IN CONSUMER STATED CHOICES
This paper investigates alternative methods to account for preference heterogeneity in choice experiments. The main interest lies in assessing the different results obtainable when investigating heterogeneity in various ways. This comparison can be performed on the basis of model performance and, more interesting, by evaluating willingness to pay measures. Preference heterogeneity analysis relates to the methods used to search for it. Socioeconomic variables can be interacted with attributes and/or alternative-specific constants. Similarly one can consider different subsets of data (strata variables) and estimate a multinomial logit model for each of them. Heterogeneity in preferences can be investigated by including it in the systematic component of utility or in the stochastic one. Mixed logit and latent class models are examples of the first approach. The former, in its random variable specification, allows for random taste variations assuming a specific distribution of the attribute coefficients over the population and permit to capture additional heterogeneity by consenting parameters to vary across individuals both randomly and systematically with observable variables. In other words it accounts for heterogeneity in the mean and in the variance of the distribution of the random parameters due to individual characteristics. Latent class models capture heterogeneity by considering a discrete underlying distribution of tastes. The small number of mass points are the unobserved segments or behavioral groups within which preferences are assumed homogeneous. The probability of membership in a latent class can be additionally made a function of individual characteristics. Alternatively, heterogeneity can be incorporated in terms of the random component of utility. The covariance heterogeneity model adopts the second approach representing a generalization of the nested logit model and can be used to explain heteroscedastic error structures in the data. It allows the inclusive value parameter to be a function of choice alternative attributes and/or individual characteristics. An alternative method refers to an extension of the multinomial logit model in which the integration of unobserved heterogeneity is performed through random error components distributed according to a tree. An interesting improvement in modeling preference heterogeneity is related to its simultaneous inclusion in both systematic and stochastic parts. A valid example is the inclusion of an error component part in a random coefficient specification of the mixed multinomial logit model. The empirical data used for comparing the various methods tested relates to departure airport choice in a multi-airport region. The area of study includes two regions in central Italy, Marche and Emilia-Romagna, and four airports: Ancona, Rimini, ForlĂŹ and Bologna. A fractional factorial experimental design was adopted to construct a four alternative choice set and five hypothetical choice exercises in each questionnaire. The selection of the potentially most important attributes and their relative levels was developed on the basis of previous research.heterogeneity, airport choice, stated preferences, discrete choice model.
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