Kristian. L. McDonald
Current Organizations / Institutes
The organizations/institutes from the scholar's latest article
Research Subjects
Research Interests
Kristian. L. McDonald has a total of 3 co-authors. Here are the top co-authors by number of shared publications:
- Bruce H. J. McKellar
University of Melbourne, Parkville, Victoria 3052, Australia
8 shared publications - A. Coulthurst
School of Physics Research Centre for High Energy Physics The University of Melbourne 3010 Victoria Australia
4 shared publications - Alison Demaria
School of Physics, Research Centre for High Energy Physics, The University of Melbourne, Victoria 3010, Australia
3 shared publications
Latest Publications
- Open Access
Quark-Lepton Symmetry and Quartification in Five Dimensions
We outline some features of higher dimensional models possessing a Quark-Lepton (QL) symmetry. The QL symmetric model in five dimensions is discussed, with particular emphasis on the use of split fermions. An interesting fermionic geography which utilises the QL symmetry to suppress the proton decay rate and to motivate the flavor differences in the quark and leptonic sectors is considered. We discuss the quartification model in five dimensions and contrast the features of this model with traditional four dimensional constructs.
Open AccessQuark-Lepton Symmetry and Quartification in Five Dimensions
We outline some features of higher dimensional models possessing a Quark-Lepton (QL) symmetry. The QL symmetric model in five dimensions is discussed, with particular emphasis on the use of split fermions. An interesting fermionic geography which utilises the QL symmetry to suppress the proton decay rate and to motivate the flavor differences in the quark and leptonic sectors is considered. We discuss the quartification model in five dimensions and contrast the features of this model with traditional four dimensional constructs.
- Open Access
The Type-II Singular See-Saw Mechanism
The singular see-saw mechanism is a variation of the see-saw mechanism whereby the right-chiral neutrino Majorana mass matrix is singular. Previous works employing the singular see-saw mechanism have assumed a vanishing left-chiral Majorana mass matrix. We study the neutrino spectrum obtained under a singular see-saw mechanism when the left-chiral neutrinos possess a non-zero Majorana mass matrix. We refer to this as the type-II singular see-saw mechanism. The resulting neutrino spectrum is found to be sensitive to the hierarchy of the Dirac and Majorana mass scales used and we explore the phenomenological consequences of the candidate hierarchies. The compatibility of the resulting spectra with the body of neutrino oscillation data is discussed. It is found that neutrino mass matrices with this structure result in 3+1 or 2+2 neutrino spectra, making it unlikely that this mass matrix structure is realized in nature. If the left-chiral Majorana mass matrix is also singular we show that a type-II singular see-saw mechanism can realize a spectrum of one active-sterile pseudo-Dirac neutrino in conjunction with two active Majorana neutrinos effectively decoupled from the sterile sector. This realizes a scheme discussed in the literature in relation to astrophysical neutrino fluxes.
Open AccessThe Type-II Singular See-Saw Mechanism
The singular see-saw mechanism is a variation of the see-saw mechanism whereby the right-chiral neutrino Majorana mass matrix is singular. Previous works employing the singular see-saw mechanism have assumed a vanishing left-chiral Majorana mass matrix. We study the neutrino spectrum obtained under a singular see-saw mechanism when the left-chiral neutrinos possess a non-zero Majorana mass matrix. We refer to this as the type-II singular see-saw mechanism. The resulting neutrino spectrum is found to be sensitive to the hierarchy of the Dirac and Majorana mass scales used and we explore the phenomenological consequences of the candidate hierarchies. The compatibility of the resulting spectra with the body of neutrino oscillation data is discussed. It is found that neutrino mass matrices with this structure result in 3+1 or 2+2 neutrino spectra, making it unlikely that this mass matrix structure is realized in nature. If the left-chiral Majorana mass matrix is also singular we show that a type-II singular see-saw mechanism can realize a spectrum of one active-sterile pseudo-Dirac neutrino in conjunction with two active Majorana neutrinos effectively decoupled from the sterile sector. This realizes a scheme discussed in the literature in relation to astrophysical neutrino fluxes.
- 26 March 2007
- Published by American Physical Society (APS) in Physical Review D
- Vol. 75 (5) , 056006
Quartification on an orbifold
We investigate quartification models in five dimensions, with the fifth dimension forming an orbifold. The orbifold construction is combined with a boundary Higgs sector to break the quartified gauge group directly to a group which is operative at the electroweak scale. We consider and , where is the standard model gauge group, and find that unification occurs only when the remnant leptonic color symmetry remains unbroken. Furthermore, the demands of a realistic low-energy fermion spectrum specify a unique symmetry breaking route for the unifying case of . We contrast this with four-dimensional quartification models where unification may be achieved via a number of different symmetry breaking routes both with and without the remnant symmetry. The boundary Higgs sector of our model may be decoupled to achieve a Higgsless limit, and we show that the electroweak Higgs doublet may be identified as the fifth component of a higher-dimensional gauge field.- 26 March 2007
- Published by American Physical Society (APS) in Physical Review D
- Vol. 75 (5) , 056006
Quartification on an orbifold
We investigate quartification models in five dimensions, with the fifth dimension forming an orbifold. The orbifold construction is combined with a boundary Higgs sector to break the quartified gauge group directly to a group which is operative at the electroweak scale. We consider and , where is the standard model gauge group, and find that unification occurs only when the remnant leptonic color symmetry remains unbroken. Furthermore, the demands of a realistic low-energy fermion spectrum specify a unique symmetry breaking route for the unifying case of . We contrast this with four-dimensional quartification models where unification may be achieved via a number of different symmetry breaking routes both with and without the remnant symmetry. The boundary Higgs sector of our model may be decoupled to achieve a Higgsless limit, and we show that the electroweak Higgs doublet may be identified as the fifth component of a higher-dimensional gauge field.- Open Access
Quartification On An Orbifold
We investigate quartification models in five dimensions, with the fifth dimension forming an $S^1/Z_2\times Z_2'$ orbifold. The orbifold construction is combined with a boundary Higgs sector to break the quartified gauge group directly to a group $H\subset SU(3)^4$ which is operative at the electroweak scale. We consider $H=G_{SM}\otimes SU(2)_\ell$ and $H=G_{SM}$, where $G_{SM}$ is the standard model gauge group, and find that unification occurs only when the remnant leptonic colour symmetry $SU(2)_\ell$ remains unbroken. Furthermore, the demands of a realistic low energy fermion spectrum specify a unique symmetry breaking route for the unifying case of $H=G_{SM}\otimes SU(2)_\ell$. We contrast this with four dimensional quartification models where unification may be achieved via a number of different symmetry breaking routes both with and without the remnant $SU(2)_\ell$ symmetry. The boundary Higgs sector of our model may be decoupled to achieve a Higgsless limit and we show that the electroweak Higgs doublet may be identified as the fifth component of a higher dimensional gauge field.Open AccessQuartification On An Orbifold
We investigate quartification models in five dimensions, with the fifth dimension forming an $S^1/Z_2\times Z_2'$ orbifold. The orbifold construction is combined with a boundary Higgs sector to break the quartified gauge group directly to a group $H\subset SU(3)^4$ which is operative at the electroweak scale. We consider $H=G_{SM}\otimes SU(2)_\ell$ and $H=G_{SM}$, where $G_{SM}$ is the standard model gauge group, and find that unification occurs only when the remnant leptonic colour symmetry $SU(2)_\ell$ remains unbroken. Furthermore, the demands of a realistic low energy fermion spectrum specify a unique symmetry breaking route for the unifying case of $H=G_{SM}\otimes SU(2)_\ell$. We contrast this with four dimensional quartification models where unification may be achieved via a number of different symmetry breaking routes both with and without the remnant $SU(2)_\ell$ symmetry. The boundary Higgs sector of our model may be decoupled to achieve a Higgsless limit and we show that the electroweak Higgs doublet may be identified as the fifth component of a higher dimensional gauge field. - Open Access
Quark-Lepton Symmetry In Five Dimensions
We construct a complete five dimensional Quark-Lepton symmetric model, with all fields propagating in the bulk. The extra dimension forms an $S^1/Z_2\times Z_2'$ 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 $m_e=m_u$ or $m_e=m_d$ 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 $1/R\gtrsim30$ TeV obtained.Open AccessQuark-Lepton Symmetry In Five Dimensions
We construct a complete five dimensional Quark-Lepton symmetric model, with all fields propagating in the bulk. The extra dimension forms an $S^1/Z_2\times Z_2'$ 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 $m_e=m_u$ or $m_e=m_d$ 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 $1/R\gtrsim30$ TeV obtained. - Open Access
Suppressing Proton Decay By Separating Quarks And Leptons
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 \emph{is} the result of an underlying symmetry, though not necessarily the traditional baryon number symmetry.
Open AccessSuppressing Proton Decay By Separating Quarks And Leptons
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 \emph{is} the result of an underlying symmetry, though not necessarily the traditional baryon number symmetry.
- 26 December 2006
- Published by American Physical Society (APS) in Physical Review D
- Vol. 74 (12) , 127701
Suppressing proton decay by separating quarks and leptons
Arkani-Hamed and Schmaltz 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 nontrivial vacuum profile and requires relationships between the associated quark and lepton Yukawa couplings. We hypothesize that these relationships are the manifestation of an underlying symmetry. We further show that the Arkani-Hamed and Schmaltz proposal may suggest that proton stability is the result of an underlying symmetry, though not necessarily the traditional baryon number symmetry.
- 26 December 2006
- Published by American Physical Society (APS) in Physical Review D
- Vol. 74 (12) , 127701
Suppressing proton decay by separating quarks and leptons
Arkani-Hamed and Schmaltz 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 nontrivial vacuum profile and requires relationships between the associated quark and lepton Yukawa couplings. We hypothesize that these relationships are the manifestation of an underlying symmetry. We further show that the Arkani-Hamed and Schmaltz proposal may suggest that proton stability is the result of an underlying symmetry, though not necessarily the traditional baryon number symmetry.
- Open Access
Developments In 5D Quark-Lepton Symmetric Models
We outline some recent developments in higher dimensional Quark-Lepton (QL) symmetric models. The QL symmetric model in five dimensions is discussed, with particular emphasis on the use of split fermions. An interesting fermionic geography which utilises the QL symmetry to suppress the proton decay rate and to motivate the flavor differences in the quark and leptonic sectors is considered. The 5D quartification model is outlined and contrasted with 4D constructs.
Open AccessDevelopments In 5D Quark-Lepton Symmetric Models
We outline some recent developments in higher dimensional Quark-Lepton (QL) symmetric models. The QL symmetric model in five dimensions is discussed, with particular emphasis on the use of split fermions. An interesting fermionic geography which utilises the QL symmetry to suppress the proton decay rate and to motivate the flavor differences in the quark and leptonic sectors is considered. The 5D quartification model is outlined and contrasted with 4D constructs.
- Open Access
Orbifold Reduction Of The Quark-Lepton Symmetric Model
We investigate the quark-lepton symmetric gauge group in five dimensions, with the gauge symmetry broken by a combination of orbifold compactification of the extra dimension and the Higgs mechanism. The gauge sector of the model is investigated and contrasted with the four dimensional case. We obtain lower bounds on the mass of the exotic gauge bosons, the inverse compactification scale and the exotic leptons. Light neutrinos are obtained without requiring any scale larger than a TeV. However an ultra-violet cut-off of order $10^{11}$ GeV is required to suppress proton decay inducing non-renormalizable operators.Open AccessOrbifold Reduction Of The Quark-Lepton Symmetric Model
We investigate the quark-lepton symmetric gauge group in five dimensions, with the gauge symmetry broken by a combination of orbifold compactification of the extra dimension and the Higgs mechanism. The gauge sector of the model is investigated and contrasted with the four dimensional case. We obtain lower bounds on the mass of the exotic gauge bosons, the inverse compactification scale and the exotic leptons. Light neutrinos are obtained without requiring any scale larger than a TeV. However an ultra-violet cut-off of order $10^{11}$ GeV is required to suppress proton decay inducing non-renormalizable operators. - 7 April 2006
- Published by American Physical Society (APS) in Physical Review D
- Vol. 73 (7) , 073004
Systematic study of leptonic mixing in a class ofmodelsWe perform a systematic analysis of the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrices which arise when one assigns the three generations of leptons to the representation of a horizontal symmetry. This idea was explored previously by Kuchimanchi and Mohapatra. However, we assume (i) the neutrino mass matrix results from leptonic couplings to triplet scalar fields and (ii) hierarchies exist among lepton mass matrix elements which result from couplings to scalar fields with different charges. Of the 16 candidate PMNS matrices which result it is found that only one is both predictive and possesses a leading order structure compatible with experimental data. The relevant neutrino mass matrix displays the symmetry to leading order and we explore the perturbations required to produce a realistic lepton spectrum. The effective mass in neutrinoless double beta decay is required to lie in the range , which is just below current experimental bounds. is nonzero but not uniquely determined.- 7 April 2006
- Published by American Physical Society (APS) in Physical Review D
- Vol. 73 (7) , 073004
Systematic study of leptonic mixing in a class ofmodelsWe perform a systematic analysis of the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrices which arise when one assigns the three generations of leptons to the representation of a horizontal symmetry. This idea was explored previously by Kuchimanchi and Mohapatra. However, we assume (i) the neutrino mass matrix results from leptonic couplings to triplet scalar fields and (ii) hierarchies exist among lepton mass matrix elements which result from couplings to scalar fields with different charges. Of the 16 candidate PMNS matrices which result it is found that only one is both predictive and possesses a leading order structure compatible with experimental data. The relevant neutrino mass matrix displays the symmetry to leading order and we explore the perturbations required to produce a realistic lepton spectrum. The effective mass in neutrinoless double beta decay is required to lie in the range , which is just below current experimental bounds. is nonzero but not uniquely determined.