Status Report on Universal Extra Dimensions After LHC8

Although they do not address the hierarchy problem, models with Universal Extra Dimensions have attracted a lot of attention as simple benchmark models characterized by small mass splittings and a dark matter WIMP played by the Lightest Kaluza-Klein particle (LKP). We review their status, with emphasis on the minimal implementation in 5 dimensions (MUED) in which the LKP is a massive hypercharge gauge boson. In this case, the mass range accounting for the correct dark matter abundance (around 1.4 TeV) remains untouched by LHC8 and is out of reach of present DM direct detection experiments. However, LHC14 can probe the relevant region in the 3-lepton channel.Comment: Invited review for special issue on "Extra dimensions vs Collider Physics" to appear in IJMPA. 14 pages, 9 figures; v2: Figure 5 clarified and references added; v3:comment added on low cutoff cas

Higgsogenesis

In addition to explaining the masses of elementary particles, the Higgs boson may have far-reaching implications for the generation of the matter content in the Universe. For instance, the Higgs plays a key role in two main theories of baryogenesis, namely electroweak baryogenesis and leptogenesis. In this letter, we propose a new cosmological scenario where the Higgs chemical potential mediates asymmetries between visible and dark matter sectors, either generating a baryon asymmetry from a dark matter asymmetry or vice-versa. We illustrate this mechanism with a simple model with two new fermions coupled to the Higgs and discuss associated signatures.Comment: 5 pages, 2 figures; v2: Intro and conclusion improved, clarifications added, results unchanged. Compared to the PRL version, this arxiv version contains two extra plots, one additional table and a slightly longer conclusio

Gravitational Waves from Phase Transitions at the Electroweak Scale and Beyond

If there was a first order phase transition in the early universe, there should be an associated stochastic background of gravitational waves. In this paper, we point out that the characteristic frequency of the spectrum due to phase transitions which took place in the temperature range 100 GeV - 10^7 GeV is precisely in the window that will be probed by the second generation of space-based interferometers such as the Big Bang Observer (BBO). Taking into account the astrophysical foreground, we determine the type of phase transitions which could be detected either at LISA, LIGO or BBO, in terms of the amount of supercooling and the duration of the phase transition that are needed. Those two quantities can be calculated for any given effective scalar potential describing the phase transition. In particular, the new models of electroweak symmetry breaking which have been proposed in the last few years typically have a different Higgs potential from the Standard Model. They could lead to a gravitational wave signature in the milli-Hertz frequency, which is precisely the peak sensitivity of LISA. We also show that the signal coming from phase transitions taking place at T ~ 1-100 TeV could entirely screen the relic gravitational wave signal expected from standard inflationary models.Comment: 18 pages, 24 figure

QCD-induced Electroweak Phase Transition

Phase transitions associated with nearly conformal dynamics are known to lead to significant supercooling. A notorious example is the phase transition in Randall-Sundrum models or their CFT duals. In fact, it was found that the phase transition in this case is first-order and the tunneling probability for the radion/dilaton is so small that the system typically remains trapped in the false vacuum and the phase transition never completes. The universe then keeps expanding and cooling. Eventually the temperature drops below the QCD scale. We show that the QCD condensates which subsequently form give an additional contribution to the radion/dilaton potential, an effect which had been ignored so far. This significantly reduces the barrier in the potential and allows the phase transition to complete in a substantially larger region of parameter space. Due to the supercooling, electroweak symmetry is then broken simultaneously. This class of models therefore naturally leads to an electroweak phase transition taking place at or below QCD temperatures, with interesting cosmological implications and signatures.Comment: 33 pages, 5 figure

The Serendipity of Electroweak Baryogenesis

The origin of the matter antimatter asymmetry of the universe remains unexplained in the Standard Model of particle physics. The origin of the flavour structure is another major puzzle of the theory. In this article, we report on recent work attempting to link the two themes through the appealing framework of electroweak baryogenesis. We show that Yukawa couplings of Standard Model fermions can be the source of CP-violation for electroweak baryogenesis if they vary at the same time as the Higgs is acquiring its vacuum expectation value, offering new avenues for electroweak baryogenesis. The advantage of this approach is that it circumvents the usual severe bounds from Electric Dipole moments. These ideas apply if the mechanism explaining the flavour structure of the Standard Model is connected to electroweak symmetry breaking, as motivated for instance in Randall-Sundrum or Composite Higgs models. We compute the resulting baryon asymmetry for different configurations of the Yukawa coupling variation across the bubble wall, and show that it can naturally be of the right order.Comment: ArXiv version of the contribution to the "Higgs cosmology" issue published in Philosophical Transactions A, associated with the Theo Murphy meeting which took place at the Royal Society Kavli Centre at Chicheley Hall on March 27-28 2017. 12 pages, 6 figure

Extra Dimensions at the LHC

We discuss the motivation and the phenomenology of models with either flat or warped extra dimensions. We describe the typical mass spectrum and discovery signatures of such models at the LHC. We also review several proposed methods for discriminating the usual low-energy supersymmetry from a model with flat (universal) extra dimensions. (For the official website of the book, see http://cambridge.org/us/catalogue/catalogue.asp?isbn=9780521763684 .)Comment: 33 pages, 17 figure

CP-violation for Electroweak Baryogenesis from Dynamical CKM Matrix

We show that the CKM matrix can be the source of CP violation for electroweak baryogenesis if Yukawa couplings vary at the same time as the Higgs acquires its vacuum expectation value. This offers new avenues for explaining the baryon asymmetry of the universe. These ideas apply if the mechanism explaining the flavour structure of the Standard Model is connected to electroweak symmetry breaking. We compute the resulting baryon asymmetry for various low-scale flavour models and different configurations of the Yukawa coupling variation across the bubble wall, and show that it can naturally be of the right order.Comment: 43 pages, 17 figures; v2: replaced mismatched plot in Figure 8 and corrected a typo in the caption; v3: JCAP published version, clarifications added, results unchange