Final Report Summary - TEV EXPECTATIONS (Predictions of theories beyond the Standard Model for TeV scale new physics)
This project aimed to develop predictive theories beyond the standard model (SM) of particle physics, in particular those connected with electroweak (EW) symmetry breaking at the TeV scale. We pursued two main research directions, reaching substantially new results:
(1) the unification of gauge couplings in the framework of composite-Higgs models;
(2) a new promising candidate for sub-GeV dark matter.
(1) The motivation for new physics associated with the EW scale is the hierarchy problem.
The EW scale is unstable against radiative corrections; therefore, it is the smallness with respect to the gravity scale is not natural. A possible solution is provided by a new strongly-interacting sector that generates the EW scale by condensation, in analogy with the generation of the quantum chromodynamics (QCD) scale. An elegant way to be compatible with EW precision tests is to identify the Higgs with a pseudo-Nambu-Goldstone boson of the strongly-interacting sector, that is, a composite state significantly lighter than the condensation scale.
We investigated composite-Higgs models where the evolution of the SM gauge couplings can be predicted at leading order, because the global symmetry of the composite sector is a simple group G that contains the SM gauge group. It turns out that, if the right-handed top quark is also composite, precision gauge unification can be achieved with precision comparable to the minimal supersymmetric SM. We built minimal consistent models for a composite sector with these properties, thus demonstrating how composite grand unified theories (GUTs) may represent an alternative to supersymmetric GUTs.
In this scenario, the G group structure and the requirement of proton stability determine the set of light composite states accompanying the Higgs and the top quark: a coloured triplet scalar and several vector-like fermions with exotic quantum numbers. It will be very interesting to look for the signatures of these composite partners at the Large Hadron Collider (LHC): distinctive final states contain multiple top and bottom quarks, either alone or accompanied by a heavy stable charged particle or by missing transverse energy.
(2) We have proposed a new dark matter (DM) candidate, naturally associated with the EW scale, and characterised by a minimal number of free parameters.
The existence of DM has been established because of its gravitational effects. However, we do not know the basic properties of the DM particles: mass, spin, couplings to other particles. A class of naturally light new particles is provided by pseudo-Nambu-Goldstone bosons (pNGBs), associated with global symmetries broken spontaneously at very large energy scales. These particles may get a small mass from explicit symmetry breaking at much smaller scales.
We argue that a pNGB is an appealing DM candidate:
(i) Its mass is induced by an energy scale already present in the visible sector, and the DM relic density is determined by the same couplings that generate its mass.
(ii) A pNGB is stable on cosmological time scales, because its decays to the visible sector are suppressed by the large scale of spontaneous symmetry breaking.
We proposed a concrete realisation of the pNGB-DM scenario, which is closely associated with the origin of light neutrino masses. The DM and the Higgs boson, responsible for the EW symmetry breaking, are feebly coupled via neutrino interactions. As a consequence, the DM mass is connected to the EW scale in a natural way. The DM decays are suppressed by the smallness of the neutrino masses. Such pNGB is only partially thermalised in the early Universe, through the interactions with heavy neutrinos and Higgs bosons, a mechanism known as freeze-in.
We find that the correct DM relic density is generated as long as the DM mass lies in the keV-MeV range.
Today, the late DM decays into light neutrinos and electrons produce a flux that could be observed in cosmological and astrophysical measurements.
(1) the unification of gauge couplings in the framework of composite-Higgs models;
(2) a new promising candidate for sub-GeV dark matter.
(1) The motivation for new physics associated with the EW scale is the hierarchy problem.
The EW scale is unstable against radiative corrections; therefore, it is the smallness with respect to the gravity scale is not natural. A possible solution is provided by a new strongly-interacting sector that generates the EW scale by condensation, in analogy with the generation of the quantum chromodynamics (QCD) scale. An elegant way to be compatible with EW precision tests is to identify the Higgs with a pseudo-Nambu-Goldstone boson of the strongly-interacting sector, that is, a composite state significantly lighter than the condensation scale.
We investigated composite-Higgs models where the evolution of the SM gauge couplings can be predicted at leading order, because the global symmetry of the composite sector is a simple group G that contains the SM gauge group. It turns out that, if the right-handed top quark is also composite, precision gauge unification can be achieved with precision comparable to the minimal supersymmetric SM. We built minimal consistent models for a composite sector with these properties, thus demonstrating how composite grand unified theories (GUTs) may represent an alternative to supersymmetric GUTs.
In this scenario, the G group structure and the requirement of proton stability determine the set of light composite states accompanying the Higgs and the top quark: a coloured triplet scalar and several vector-like fermions with exotic quantum numbers. It will be very interesting to look for the signatures of these composite partners at the Large Hadron Collider (LHC): distinctive final states contain multiple top and bottom quarks, either alone or accompanied by a heavy stable charged particle or by missing transverse energy.
(2) We have proposed a new dark matter (DM) candidate, naturally associated with the EW scale, and characterised by a minimal number of free parameters.
The existence of DM has been established because of its gravitational effects. However, we do not know the basic properties of the DM particles: mass, spin, couplings to other particles. A class of naturally light new particles is provided by pseudo-Nambu-Goldstone bosons (pNGBs), associated with global symmetries broken spontaneously at very large energy scales. These particles may get a small mass from explicit symmetry breaking at much smaller scales.
We argue that a pNGB is an appealing DM candidate:
(i) Its mass is induced by an energy scale already present in the visible sector, and the DM relic density is determined by the same couplings that generate its mass.
(ii) A pNGB is stable on cosmological time scales, because its decays to the visible sector are suppressed by the large scale of spontaneous symmetry breaking.
We proposed a concrete realisation of the pNGB-DM scenario, which is closely associated with the origin of light neutrino masses. The DM and the Higgs boson, responsible for the EW symmetry breaking, are feebly coupled via neutrino interactions. As a consequence, the DM mass is connected to the EW scale in a natural way. The DM decays are suppressed by the smallness of the neutrino masses. Such pNGB is only partially thermalised in the early Universe, through the interactions with heavy neutrinos and Higgs bosons, a mechanism known as freeze-in.
We find that the correct DM relic density is generated as long as the DM mass lies in the keV-MeV range.
Today, the late DM decays into light neutrinos and electrons produce a flux that could be observed in cosmological and astrophysical measurements.