Future tests of Higgs compositeness: direct vs indirect
JHE
Future tests of Higgs compositeness: direct vs indirect
Andrea Thamm 0 1 2 3 4
Riccardo Torre 0 1 2 3 4
Andrea Wulzer 0 1 2 3 4
0 INFN , Sezione di Padova
1 55099 Mainz , Germany
2 Johannes Gutenberg University
3 via Marzolo 8, I-35131 Padova , Italy
4 [28] B. Bellazzini , C. Cs ́aki, J. Hubisz, J. Serra and J. Terning, Composite Higgs sketch, JHEP
We estimate the reach of the 14 TeV LHC and future hadronic and leptonic colliders in the parameter space of the minimal composite Higgs model, outlining the complementarity of direct resonance searches and indirect information from the measurements of the Higgs boson couplings. The reach on electroweak charged spin-one resonances, taken here as representative direct signatures, is obtained from the current 8 TeV LHC limits by an extrapolation procedure which we outline and validate. The impact of electroweak precision tests, and their possible improvement at future colliders, is also quantified.
Beyond Standard Model; Technicolor and Composite Models
1 Introduction 2 3 4
A A simple check of the extrapolation procedure
The LHC is about to restart operations at 13 TeV centre-of-mass energy, which will
precollected in the next runs, followed by a high-luminosity (HL-LHC) phase which should
eventually deliver 3 ab−1.
While the current priority clearly lies on profiting from this
experimental program, some effort should also be devoted to the design of future colliders,
planning the investigation of the energy frontier on the time-scale of several decades. This
may well be premature: the next LHC run could radically change the situation by
discovering new particles, in which case the priority would be on characterising their properties
and nature. However an assessment of future colliders’ capabilities on the basis of the
current theoretical understanding and experimental status might still be a useful exercise.
Proposed future machines come in two main classes, lepton (e.g. ILC [1], CLIC [2, 3],
TLEP [4], also referred to as FCC-ee) and hadron (such as the FCC-hh [5]) colliders, which
will search for New Physics (NP) from complementary sides.1 Experimental programs at
lepton colliders are more suited for indirect searches, thanks to the high precision of the
measurements. Hadron colliders reach higher energies and are thus more effective for direct
searches of new particles. Indeed, it is not by chance that the best current indirect and
direct limits on NP mostly come, respectively, from LEP and LHC data. Because of this
complementarity, a comparison between the reach of lepton and hadron colliders on NP is
a delicate issue, which cannot be performed in absolute terms and on completely
modelindependent grounds. Some theory bias is needed, in the form of one or several NP
scenarios, in order to display the reach of indirect and direct searches on the same parameter space.
Here we consider the Composite Higgs (CH) scenario in its minimal realisation [6–13].
1Here we will not consider the possibility of an electron-proton collider such as the FCC-he [5].
Aside from being a well-motivated theoretical possibility, CH is the ideal framework
for our investigation since it predicts both indirect and direct effects which could both
be sizeable enough to be detected. Telling which strategy could be more effective to test
the CH idea is non-trivial and requires dedicated studies. Indirect effects, in the form of
corrections to SM couplings or new BSM vertices [14–23], unavoidably emerge due to the
pseudo-Nambu-Goldstone boson nature of the Higgs leading to deviations proportional to
ξ ≡ v2/f 2 where f is the Goldstone boson Higgs decay constant and v the electroweak
symmetry breaking (EWSB) scale. Further corrections, but normally subdominant, come
from the virtual exchange of new heavy resonances mixing with the SM particles at tree
level, giving contributions of order m2SM/m2NP. The latter resonances can also be produced
at high enough energies, giving rise to a number of possible direct signatures. The most
studied and promising ones are the production of spin-one EW-charged vectors [9, 24–33]
and of the coloured partners of the top quark (shortly referred to as top partners) [34–36].
The strongest indirect constraints on CH models currently come from electroweak
precision tests (EWPT), where CH models could have already shown up in the form of
oblique corrections or modifications of the Zb¯b vertex [14, 19, 23]. Even restricting to
custodially symmetric cosets and to fermionic operator representations which implement
the so-called PLR protection symmetry for Zb¯b [37], EWPT are still the dominant indirect
constraint on the CH scenario. In spite of this, and in spite of the fact that we will discuss
them in detail in section 4, we will not take EWPT and their possible improvements at
future colliders as a central pillar of our investigation. The reason is that we judge their
impact too model-dependent to be quantified in a robust way.2 Namely, as known in the (...truncated)