40,894 research outputs found
Effective four-fermion operators in top physics: a roadmap
We write down a minimal basis for dimension-six gauge-invariant four-fermion
operators, with some operator replacements with respect to previous ones which
make it simpler for calculations. Using this basis we classify all four-fermion
operator contributions involving one or two top quarks. Taking into account the
different fermion chiralities, possible colour contractions and independent
flavour combinations, a total number of 572 gauge-invariant operators are
involved. We apply this to calculate all three-body top decay widths t -> d_k
u_i dbar_j, t -> d_k e_i+ nu_j, t -> u_k u_i ubar_j, t -> u_k e_j+ e_i-, t ->
u_k nu_j nu_i (with i,j,k generation indices) mediated by dimension-six
four-fermion operators, including the interference with the Standard Model
amplitudes when present. All single top production cross sections in pp, p pbar
and e+ e- collisions are calculated as well, namely u_i d_k -> d_j t, dbar_j
d_k -> ubar_i t, u_i dbar_j -> dbar_k t, u_i u_k -> u_j t, u_i ubar_j -> ubar_k
t, e+ e- -> ubar_k t and the charge conjugate processes. We also compute all
top pair production cross sections, ubar_i u_j -> t tbar, dbar_i d_j -> t tbar,
u_i u_j -> t t and e+ e- -> t tbar. Our results are completely general, without
assuming any particular relation among effective operator coefficients.Comment: LaTeX 43 pages, 1 PS figure. Cross sections for uu,cc -> tt correcte
Model-independent measurement of the top quark polarisation
We introduce a new asymmetry in the decay t -> W b -> l nu b, which is shown
to be directly proportional to the polarisation of the top quark along a chosen
axis, times a sum of W helicity fractions. The latter have already been
precisely measured at the Tevatron and the Large Hadron Collider. Therefore,
this new asymmetry can be used to obtain a model-independent measurement of the
polarisation of top quarks produced in any process at hadron or lepton
colliders.Comment: LaTeX 12 pages. Discussion expanded with a new plot, references
added. Final version to appear in PL
A minimal set of top anomalous couplings
We simplify the general form of the fermion-fermion-gauge boson interactions
generated by dimension-six gauge-invariant effective operators by using the
equations of motion to remove redundant operators. It is found that the most
general vertex for off-shell fermions fi, fj and an off-shell boson
V=W,Z,gamma,g only involves gamma^mu and sigma^{mu nu} q_nu terms, with
q=p_i-p_j. Examples are given for the Wtb, Ztt, gamma tt and gtt interactions,
whose general expression is greatly simplified with respect to previous results
in the literature. The same arguments apply to top flavour-changing neutral
interactions with the Z boson, the photon or the gluon, which can also be
parameterised in full generality with only gamma^mu and sigma^{mu nu} q_nu
couplings. Explicit expressions are given for these vertices in terms of
dimension-six gauge-invariant operators. We also discuss how effective operator
coefficients might be determined from eventual measurements of anomalous
couplings.Comment: LaTeX 29 pages, 3 PS figures. Minor comments added, final version to
appear in NP
A minimal set of top-Higgs anomalous couplings
We use the equations of motion to simplify the general form of
fermion-fermion-Higgs interactions generated by dimension-six gauge-invariant
effective operators. After removing redundant operators it is found that the
most general H f_i f_j vertex for off-shell fermions f_i, f_j and an off-shell
Higgs boson only involves scalar and pseudo-scalar terms, without derivatives.
Examples are presented for the Htt and Htq interactions, where q=u,c, giving
the explicit expressions of the vertices in terms of gauge-invariant operators.
The new operator equalities obtained here also reduce the number of operators
relevant for the Ztq vertices, although the general form of these interactions
is not simplified with respect to previous results.Comment: LaTeX 16 pages, 2 PS figures. Typos corrected. Final version which
appeared on the journa
Effective Field Theory for Nonstandard Top Quark Couplings
We present an effective-field-theory calculation of the effect of a
dimension-six operator involving the top quark on precision electroweak data
via a top-quark loop. We demonstrate the renormalizability, in the modern
sense, of the effective field theory. We use the oblique parameter U to bound
the coefficient of the operator, and compare with the bound derived from
top-quark decay.Comment: 10 pages, 3 figure
W polarisation beyond helicity fractions in top quark decays
We calculate the density matrix for the decay of a polarised top quark into a
polarised W boson and a massive b quark, for the most general Wtb vertex
arising from dimension-six gauge-invariant effective operators. We show that,
in addition to the well-known W helicity fractions, for polarised top decays it
is worth to define and study the transverse and normal W polarisation
fractions, that is, the W polarisation along two directions orthogonal to its
momentum. In particular, a rather simple forward-backward asymmetry in the
normal direction is found to be very sensitive to complex phases in one of the
Wtb anomalous couplings. This asymmetry, which indicates a normal W
polarisation, can be generated for example by a P-odd, T-odd transition
electric dipole moment. We also investigate the angular distribution of decay
products in the top quark rest frame, calculating the spin analysing powers for
a general Wtb vertex. Finally we show that, using a combined fit to top decay
observables and the tW cross section, at LHC it will be possible to obtain
model-independent measurements of all the (complex) Wtb couplings as well as
the single top polarisation. Implications for spin correlations in top pair
production are also discussed.Comment: LaTeX 36 pages, 51 EPS figures. Typos corrected. To appear in NP
Renegotiating Incomplete Contracts: Over and Under Investment of Concessioned Public Infrastructure.
Opportunistic Behavior and Legal Disputes in the Chilean Electricity Sector
In this paper I analyze post-contractual disputes in the newly privatized electricity sector in Chile. I discuss the presumption that opportunistic behavior and disputes arise due to inadequate market design, ambiguous regulation, and institutional weaknesses. I also assess the presumption that a large number of legal (public) disputes are inhibited by the nonexistence of institutions able to verify and enforce contracts. An in-depth analysis of 6 cases of open con‡ict provides support to such presumptions and highlights the crucial role of an adequate (preprivatization) market design. In addition, it concludes that the reduced number of open con‡icts observed in Chile is probably due to institutional weaknesses, which induces the parties to use private con‡ict resolution mechanisms.
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