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Gravitational wave and collider implications of electroweak baryogenesis aided by non-standard cosmology
Received: November
Gravitational wave and collider implications of electroweak baryogenesis aided by non-standard cosmology
Michal Artymowski 0 2 5
Marek Lewicki 0 2 3 4
James D. Wells 0 1 2 4
Open Access 0 2
c The Authors. 0 2
0 ul. Pasteura 5 , 02-093 Warsaw , Poland
1 Deutsches Elektronen-Synchrotron DESY, Theory Group
2 Lojasiewicza 11 , 30-348 Krakow , Poland
3 Faculty of Physics, University of Warsaw
4 Michigan Center for Theoretical Physics, University of Michigan
5 Jagiellonian University
We consider various models realizing baryogenesis during the electroweak phase transition (EWBG). Our focus is their possible detection in future collider experiments and possible observation of gravitational waves emitted during the phase transition. We also discuss the possibility of a non-standard cosmological history which can facilitate EWBG. We show how acceptable parameter space can be extended due to such a modi cation and conclude that next generation precision experiments such as the ILC will be able to con rm or falsify many models realizing EWBG. We also show that, in general, collider searches are a more powerful probe than gravitational wave searches. However, observation of a deviation from the SM without any hints of gravitational waves can point to models with modi ed cosmological history that generically enable EWBG with weaker phase transition and thus, smaller GW signals.
cosmology; Cosmology of Theories beyond the SM; Higgs Physics; Solitons Monopoles
1 Introduction 2 3 4
Modifying The Standard Model
Higgs precision measurements
Details of the phase transition
Modi cation of the cosmological history
Evolution of primordial inhomogeneities
Cosmological modi cation of the sphaleron bound
Gravitational waves detection
Discovery of the Higgs boson, with a mass of 125 GeV at the Large Hadron Collider
(LHC) [1, 2]
nally con rmed that the electroweak symmetry is broken due to a
vacuum expectation value of an elementary scalar. This discovery also marks the beginning of
a new era of precision measurements of the Higgs properties as a probe for physics beyond
the standard model. Another very important recent discovery of the rst gravitational
wave signal [3] opened a new way of probing violent events in the history of our universe
through observation of the gravitational waves they would leave behind. With these
experimental prospects it is very interesting to re-examine paradigms that predict observable
e ects in both these areas.
In this paper, we wish to study electroweak baryogenesis [4{7] in which a strong rst
order electroweak phase transition (EWPT) is responsible for the observed baryon
asymmetry of the universe. In the Standard Model (SM) the phase transition is second order
with the observed Higgs mass [8, 9] and so a modi cation is required. We will study a
simple toy model where a single new scalar is added to the SM, and we will consider several
possible charge assignments for this new particle [10{12]. Such a modi cation creates a
barrier between the symmetric minimum and the new electroweak symmetry breaking
minimum which develops as the temperature of the universe drops, making the phase transition
more strongly rst order. This has two e ects. First, modi cation of the high temperature
potential inevitably leads to a modi cation of the zero temperature Higgs potential which
we can probed in colliders. And second, a more violent phase transition (i.e., stronger rst
order) results in larger production of gravitational waves
The main point we wish to make comes from the fact that early cosmological
evolution of the universe is rather poorly constrained by experiments. To be more speci c, in
our discussion we will include the possibility that the early universe energy density was
dominated by a new contribution not interacting with the SM which red-shifted away
before nucleosynthesis. This scenario is much di erent from the standard assumption that
the universe was dominated by radiation; however, as we will show, no currently available
experimental data can exclude this possibility.
The necessary condition for baryogenesis we will address comes from the fact that the
same sphaleron processes that can be responsible for creation of the asymmetry can also
wash it away when the universe goes back to thermal equilibrium and the sphalerons are
not su ciently decoupled. As mentioned already we will discuss not only how generating
a larger potential barrier helps in damping the sphaleron processes but also discuss how
their cosmological freeze-out can help ameliorate the situation [13{16].
While we will not discuss generation of the baryon asymmetry during the phase
transition, additional problems can appear when considering the CP violation that is also needed
for the asymmetry. Helpful sources of CP violation are limited by increasingly accurate
experimental EDM constraints [17, 18], which in turn requires a stronger rst-order phase
transition for the asymmetry to develop [19]. This (...truncated)