291 research outputs found
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
Rare top decays t -> c gamma, t -> c g and CKM unitarity
Top flavour-changing neutral decays are extremely suppressed within the
Standard Model (SM) by the GIM mechanism, but can reach observable rates in
some of its extensions. We compute the branching ratios for t -> c gamma and t
-> c g in minimal SM extensions where the addition of a vector-like up or down
quark singlet breaks the unitarity of the 3 x 3 Cabibbo-Kobayashi-Maskawa (CKM)
matrix. The maximum rates obtained indicate to what extent present experimental
data allow 3 x 3 CKM unitarity to be broken in these models, and are too small
to be observed in the near future. As a by-product, we reproduce the
calculation of these branching ratios in the SM, and with an improved set of
parameters we obtain values one order of magnitude smaller than the ones
usually quoted in the literature. We study the CP asymmetries between the decay
rates of the top quark and antiquark, which can be much larger than in the SM,
also as a consequence of the partial breaking of 3 x 3 CKM unitarity.Comment: LaTeX, 15 pages, 12 PS figures, minor changes. To be published in PL
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
Multilepton production via top flavour-changing neutral couplings at the CERN LHC
Zt and gamma t production with Z -> l+ l- and t -> Wb -> l nu b provides the
best determination of top flavour-changing neutral couplings at the LHC. The
bounds on tc couplings eventually derived from these processes are similar to
those expected from top decays, while the limits on tu couplings are better by
a factor of two. The other significant Z and W decay modes are also
investigated.Comment: 30 pages, 23 PS figures. Uses epsfig.sty and elsart.sty. Added some
references and corrected some typos. Added more comments about statistics.
Using elsart.sty reduces the size to 30 pages. Published in Nucl. Phys.
Top flavour-changing neutral coupling signals at a linear collider
We present an analysis of the sensitivity of the TESLA e+ e- collider to top
flavour-changing neutral couplings to the Z boson and photon. We consider the
cases without beam polarization, with only e- polarization and with e- and e+
polarization, showing that the use of the latter substantially enhances the
sensitivity to discover or bound these vertices. For some of the couplings the
expected LHC limits could be improved up to an order of magnitude for equal
running times.Comment: 12 pages, 5 PS figures. Added some references and corrected a typo.
Final version to appear in PL
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
Combined analysis of Z' -> t tbar and Z' -> t tbar j production for vector resonance searches at LHC
We have implemented a code for Z' + n jets production in ALPGEN, with Z'
decays into several final states, including l+ l- and t tbar. The MLM
prescription is used for matching the matrix element with the parton shower,
including in this way the leading soft and collinear corrections. In order to
demonstrate its capabilities, we perform a combined analysis of Z' -> t tbar
and Z' -> t tbar j production for a heavy leptophobic gauge boson. It is found
that the effect of the extra jet cannot only be accounted for by a K factor
multiplying the leading-order cross section. In fact, the combined analysis for
Z' -> t tbar and Z' -> t tbar j presented improves the statistical significance
of the signal by 25% (8.55 sigma versus 6.77 sigma for a Z' mass of 1 TeV),
compared with the results of an inclusive analysis carried out on the same
sample of t tbar + t tbar j events.Comment: LaTeX 17 pages, 13 PS figure
Electroweak constraints on new physics
We briefly review the limits on new interactions implied by electroweak
precision data. Special attention is payed to the bounds on the Higgs boson
mass. We also comment on the required cancellation among the new contributions
to precisely measured electroweak observables in any Standard Model extension,
if the new particles have to evade the indirect constraints on their couplings
and masses but still remain at the LHC reach.Comment: Latex 4 pages, 1 table, 2 figures. To appear in the proceedings of
10th Hellenic School and Workshops on Elementary Particle Physics and Gravity
(Corfu 2010), Corfu, Greece, Aug 29 - Sep 19, 201
Heavy neutrino signals at large hadron colliders
We study the LHC discovery potential for heavy Majorana neutrino singlets in
the process pp -> W+ -> l+ N -> l+ l+ jj (l=e,mu) plus its charge conjugate.
With a fast detector simulation we show that backgrounds involving two
like-sign charged leptons are not negligible and, moreover, they cannot be
eliminated with simple sequential kinematical cuts. Using a likelihood analysis
it is shown that, for heavy neutrinos coupling only to the muon, LHC has 5
sigma sensitivity for masses up to 200 GeV in the final state mu+- mu+- jj.
This reduction in sensitivity, compared to previous parton-level estimates, is
driven by the ~ 10^2-10^3 times larger background. Limits are also provided for
e+- e+- jj and e+- mu+- jj final states, as well as for Tevatron. For heavy
Dirac neutrinos the prospects are worse because backgrounds involving two
opposite charge leptons are much larger. For this case, we study the
observability of the lepton flavour violating signal e+- mu-+ jj. As a
by-product of our analysis, heavy neutrino production has been implemented
within the ALPGEN framework.Comment: Latex 36 pages, 49 PS figures. Major extension incorporating analysis
for e+- e+-, e+- mu+- and e+- mu-+ final states. Final version to appear in
JHE
Top effective operators at the ILC
We investigate the effect of top trilinear operators in t tbar production at
the ILC. We find that the sensitivity to these operators largely surpasses the
one achievable by the LHC either in neutral or charged current processes,
allowing to probe new physics scales up to 4.5 TeV for a centre of mass energy
of 500 GeV. We show how the use of beam polarisation and an eventual energy
upgrade to 1 TeV allow to disentangle all effective operator contributions to
the Ztt and gamma tt vertices.Comment: LaTeX 13 pages. Typos corrected. Final version in JHE
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