arXiv:0810.1952v3 [hep-ph] 21 Jul 2009
Deciphering the spin of new resonances in Higgsless models
Alexandre Alves∗and O. J. P.´Eboli†
Instituto de F´ ısica, Universidade de S˜ ao Paulo, S˜ ao Paulo – SP, Brazil.
M. C. Gonzalez–Garcia‡
Instituci´ o Catalana de Recerca i Estudis Avan¸ cats (ICREA),
Departament d’Estructura i Constituents de la Mat` eria,
Universitat de Barcelona, 647 Diagonal, E-08028 Barcelona, Spain and
C.N. Yang Institute for Theoretical Physics, SUNY at Stony Brook, Stony Brook, NY 11794-3840, USA
J. K. Mizukoshi§
Centro de Ciˆ encias Naturais e Humanas, Universidade Federal do ABC, Santo Andr´ e – SP, Brazil.
We study the potential of the CERN Large Hadron Collider (LHC) to probe the spin of new
massive vector boson resonances predicted by Higgsless models. We consider its production via
weak boson fusion which relies only on the coupling between the new resonances and the weak
gauge bosons. We show that the LHC will be able to unravel the spin of the particles associated
with the partial restoration of unitarity in vector boson scattering for integrated luminosities of
150–560 fb−1, depending on the new state mass and on the method used in the analyses.
PACS numbers: 12.60.Fr, 14.70.Pw
Despite the success of the Standard Model (SM) of
particle physics in describing electroweak physics below
∼ 100 GeV in terms of a non-abelian gauge theory with
spontaneously broken SU(2)L×U(1)Y gauge group, the
gauge symmetry does not predict the precise mechanism
of the electroweak symmetry breaking (EWSB). Indeed,
up to this moment, there is no direct experimental signal
of the mechanism of EWSB, being its search one of the
main goals of the LHC.
The EWSB mechanism plays an important role in the
high energy electroweak gauge boson scattering which
violates partial wave unitarity or becomes strongly inter-
acting at energies of the order of E ∼ 2 TeV, if there is
no new state to cut off its growth [1, 2]. In the context
of the SM, as well as in its supersymmetric realization,
electroweak symmetry is broken by the vacuum expecta-
tion value of some weakly coupled neutral scalar state(s),
the Higgs boson(s), which will contribute to electroweak
gauge boson scattering, preventing the unitarity violation
of the process.
Alternatively, Higgsless extensions of the SM [3, 4, 5]
have been proposed in which the electroweak symmetry
is broken without involving a fundamental Higgs field.
Generically on these models, the electroweak symmetry
∗Electronic address: firstname.lastname@example.org
†Electronic address: email@example.com
‡Electronic address: firstname.lastname@example.org
§Electronic address: email@example.com
is broken by boundary conditions in a higher dimensional
space.The originally proposed Higgsless models gave
large contributions to precision electroweak observables,
in particular to the S parameter  (ǫ3) [8, 9, 10, 11,
12, 13]. Such problems could be overcome, for example,
by appropriate modifications of the fermion sector. In
this way, a variety of Higgsless models have been con-
structed [14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24] which
ensure agreement with electroweak precision data.
From the point of view of unitarity, all Higgsless mod-
els share the common feature that new weakly interact-
ing spin-1 gauge bosons particles with the same quan-
tum numbers as the SM gauge bosons appear and they
are responsible for the partial restoration of unitarity
in vector boson scattering and for rendering a theory
weakly coupled to energies well above 2 TeV [25, 26, 27].
This property allows for an almost model independent
search for the lightest charged resonance V±
LHC through pp → V±
pp → V±
onance. The LHC experiments will be able to unravel the
existence of the charged state via these processes with
modest integrated luminosities of 10–60 fb−1. On the
contrary, the corresponding search for the neutral vector
resonance in gauge boson fusion is expected to be very
difficult, since a generic feature of this class of models
is the absence of coupling between the neutral resonance
and ZZ pairs. Reconstructing the heavy neutral vec-
tor resonance decaying into W+W−requires at least one
hadronic W decay, posing the challenge to dig it out from
the large SM backgrounds.
Once a clear signal of the charged resonance is observed
in the above channels, it is mandatory to study its spin
1W∓or via weak boson fusion
1qq [28, 29], as long as V±
1remains a narrow res-
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