Arkani-Hamed and Schmaltz (AS) have shown that proton stability need not originate from symmetries in a high energy theory. Instead the proton decay rate is suppressed if quarks and leptons are spatially separated in a compact extra dimension. This separation may be achieved by coupling five dimensional fermions to a bulk scalar field with a non-trivial vacuum profile and requires relationships between the associated quark and lepton Yukawa couplings. We hypothesise that these relationships are the manifestation of an underlying symmetry. We further show that the AS proposal may suggest that proton stability is the result of an underlying symmetry, though not necessarily the traditional baryon number symmetry.PACS numbers: 11.10. Kk, In recent years it has been proposed that the fundamental scale of nature may be much less than the Planck scale [1,2,3]. By introducing large extra dimensions one is able to reframe the hierarchy problem and remove the need to explain the disparity between the Planck scale and the electroweak scale. In the standard model (SM) it is known that proton decay proceeds at the non-renormalizable level via the dimension six operator Q 3 L/Λ 2 , where Q (L) generically denotes a quark (lepton) field operator and Λ is the SM cut-off. The stringent lower bound of 1.6 × 10 33 years on the decay mode p → e + π leads to the bound Λ 10 16 GeV [4]. In models with large extra dimensions the fundamental gravitational scale may be reduced to TeV energies, removing the order 10 16 GeV cut-off required to suppress the proton decay rate.Arkani-Hamed and Schmaltz (AS) [5] have suggested that proton longevity need not imply a conserved symmetry in the more fundamental theory [6]. They have shown that proton decay can be suppressed in models with a low fundamental scale if quarks and leptons are localised at different four dimensional slices of a five dimensional spacetime. If the fifth dimension forms an S 1 /Z 2 orbifold, maximal suppression of the proton decay rate results when quarks and leptons are localised at different fixed points. It is known that the zero mode of a five dimensional fermion, which may be identified with a SM fermion, can be localised at an S 1 /Z 2 orbifold fixed point by coupling the fermion to a bulk scalar field with a non-trivial vacuum profile [7]. The sign of the associated Yukawa coupling determines the fixed point at which the fermion zero mode is localised [7,8]. Thus proton decay may be suppressed by arbitrarily choosing different sign Yukawa couplings for quarks and leptons with a bulk scalar (see e.g. [9]).It is interesting to speculate that an underlying theory may possess symmetries which fix the relative bulk scalar Yukawa coupling signs between quarks and leptons. In this work we * Electronic address: a.coulthurst@physics.unimelb.edu.au † Electronic address: k.mcdonald@physics.unimelb.edu.au ‡ Electronic address: b.mckellar@physics.unimelb.edu.au ask if the separation of quarks and leptons required to achieve the AS proposal may itself be the manifestation of an u...

We construct a complete five dimensional Quark-Lepton symmetric model, with all fields propagating in the bulk. The extra dimension forms an S 1 /Z2 × Z ′ 2 orbifold with the zero mode fermions corresponding to standard model quarks localised at one fixed point. Zero modes corresponding to left(right)-chiral leptons are localised at (near) the other fixed point. This localisation pattern is motivated by the symmetries of the model. Shifting the right-handed neutrinos and charged leptons slightly from the fixed point provides a new mechanism for understanding the absence of relations of the type me = mu or me = m d in Quark-Lepton symmetric models. Flavour changing neutral currents resulting from Kaluza Klein gluon exchange, which typically arise in the quark sector of split fermion models, are suppressed due to the localisation of quarks at one point. The separation of quarks and leptons in the compact extra dimension also acts to suppress the proton decay rate. This permits the extra dimension to be much larger than that obtained in a previous construct, with the bound 1/R 30 TeV obtained.

A five dimensional model containing both left-right and quark-lepton symmetries is constructed, with the gauge group broken by a combination of orbifold compactification and the Higgs mechanism. An analysis of the gauge and scalar sectors is performed and it is shown that the 5d model admits a simpler scalar sector. Bounds on the relevant symmetry breaking scales are obtained and reveal that two neutral gauge bosons may appear in the TeV energy range to be explored by the LHC. Split fermions are employed to remove the mass relations implied by the quark-lepton symmetry and the necessary fermion localisation is achieved by introducing bulk scalars with kink vacuum profiles. The symmetries of the model constrain the Yukawa sector, which in turn severely constrains the extent to which realistic split fermion scenarios may be realized in the absence of Yukawa coupling hierarchies. Nevertheless we present two interesting one generation constructs. One of these provides a rationale for mt > m b , mτ and mν ≪ mt with Yukawa parameters which vary by only a factor of five. The other also suppresses the proton decay rate by spatially separating quarks and leptons but requires a Yukawa parameter hierarchy of order 10 2 .

The origin of the matter-antimatter asymmetry of the universe is one of the major unsolved problems in cosmology and particle physics. In this paper, we investigate the recently proposed possibility that split fermion models -extra dimensional models where the standard model fermions are localized to different points around the extra dimension -could provide a means to generate this asymmetry during the phase transition of the localizing scalars. After setting up the scenario that we consider, we use a single fermion toy model to estimate the reflection coefficients for scattering off the phase boundary using a more realistic scalar profile than previous work resulting in a different Kaluza Klein spectrum. The value we calculate for nB/s is consistent with the mechanism being the source of the baryon asymmetry of our universe provided the B − L violating processes have an efficiency of order 10 −5 . * acoul@physics.unimelb.edu.au 1 It should be noted that none of Sakharov's conditions are strictly necessary for baryogenesis to occur and models have been developed where each can be circumvented [3]. However, such models have thus far had limited success and at least for the type of models we consider here, all three conditions are necessary.