arXiv subjects

Yi Liao

Publications and source records attributed to Yi Liao.

At least 145 records · Page 8Linked to original sources

arXiv2006

Active-sterile neutrino mixing in the absence of bare active neutrino mass

We investigate a minimal extension of the standard model in which the only new ingredient is the sterile neutrinos. We do not introduce extra Higgs multiplets or high dimensional effective operators to induce mass terms for the active neutrinos, and the model is renormalizable in itself. We show for arbitrary numbers of generations and sterile neutrinos that the independent physical parameters in the leptonic sector are much less than previously anticipated. For instance, with three active and two sterile neutrinos, there are four mixing angles and three CP phases in addition to four non-vanishing neutrino masses. We study phenomenological implications for tritium beta decay, neutrinoless double beta decay and neutrino oscillations. For the most natural see-saw parameters, we find that it is difficult to accommodate in the model the best-fit values of masses and mixing parameters from oscillation data no matter whether we include or not the null short-baseline experiments together with the LSND result. This implies that if the LSND result is confirmed by MiniBooNE, the see-saw parameter region of the model with two sterile neutrinos could be largely excluded.

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arXiv2005

Lepton Mixing Matrix in Standard Model Extended by One Sterile Neutrino

We consider the simplest extension of the standard electroweak model by one sterile neutrino that allows for neutrino masses and mixing. We find that its leptonic sector contains much less free physical parameters than previously realized. In addition to the two neutrino masses, the lepton mixing matrix in charged current interactions involves (n-1) free physical mixing angles for n generations. The mixing matrix in neutral current interactions of neutrinos is completely fixed by the two masses. Both interactions conserve CP. We illustrate the phenomenological implications of the model by vacuum neutrino oscillations, tritium beta decay and neutrinoless double beta decay. It turns out that, due to the revealed specific structure in its mixing matrix, the model with any n generations cannot accommodate simultaneously the data by KamLAND, K2K and CHOOZ.

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arXiv2005

On Evaluation of Nonplanar Diagrams in Noncommutative Field Theory

This is a technical work about how to evaluate loop integrals appearing in one loop nonplanar (NP) diagrams in noncommutative (NC) field theory. The conventional wisdom says that, barring the ultraviolet/infrared (UV/IR) mixing problem, NP diagrams whose planar counterparts are UV divergent are rendered finite by NC phases that couple the loop momentum to the external NC momentum ρ^μ=θ^{μν}p_ν. We show that this is generally not the case. We find that subtleties arise already on Euclidean spacetime. The situation is even worse in Minkowski spacetime due to its indefinite metric. We compare different prescriptions that may be used to evaluate loop integrals in ordinary theory. They are equivalent in the sense that they always yield identical results. However, in NC theory there is no a priori reason that these prescriptions, except for the defining one built in Feynman propagator, are physically justified. Employing them can lead to ambiguous results. For ρ^2>0, the NC phase can worsen the UV property of loop integrals instead of always improving it in high dimensions. We explain how this surprising phenomenon comes about from the indefinite metric. For ρ^2<0, the NC phase improves the UV property and softens the quadratic UV divergence in ordinary theory to a bounded but indefinite UV oscillation. We employ a cut-off method to quantify the new UV non-regular terms. For ρ^2>0, these terms are generally complex and thus also harm unitarity. As the new terms are not available in the Lagrangian, our result casts doubts on previous demonstrations of one loop renormalizability based exclusively upon analysis of planar diagrams, especially in theories with quadratic divergences.

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arXiv2004

On Double Gauging of U(1) Symmetry on Noncommutative Space

We point out that a field ϕcharged under a global U(1) symmetry generally allows for a starred localized extension with the transformation rule, ϕ\to U_L\starϕ\star U_R^{-1}. This results in a double gauging of the global U(1) symmetry on noncommutative space. We interpret the gauge theory so obtained in terms of the gauge fields that in the commutative limit appear naturally and are respectively the gauge field responsible for the charge and a decoupled vector field. The interactions are shown to be very different from those obtained by assigning a transformation rule of ϕ\to U\starϕor ϕ\star U^{-1}.

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arXiv2004

Note on CKM Matrix Renormalization

A simple inspection of the one loop quark self-energy suggests a prescription of the CKM matrix renormalization in the standard model. It leads to a CKM matrix counterterm which is gauge parameter independent and satisfies the unitarity constraint, and renormalized physical amplitudes which are gauge parameter independent and smooth in quark mass difference. We make a point that caution should be practiced when interpreting the CKM martix counterterm in terms of those of parameters in a given representation due to rephasing effects from renormalization. We show how this can be done using the degrees of freedom in the on-shell renormalization scheme.

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arXiv2002

Some Phenomenological Consequences of the Time-Ordered Perturbation Theory of QED on Noncommutative Spacetime

A framework was recently proposed for doing perturbation theory on noncommutative (NC) spacetime. It preserves the unitarity of S matrix and differs from the naive, popular approach already at the lowest order in perturbation when time does not commute with space. In this work, we investigate its phenomenological implications at linear colliders, especially the TESLA at DESY, through the processes of e^+e^-\toμ^+μ^-,H^+H^-,H^0H^0. The results are indeed found to be very different from the ones obtained in the naive approach. The first two get corrected at tree level as opposed to the null result in the naive approach, while the third one coincides with the naive result only in the low energy limit. The impact of the Earth rotation is incorporated. The NC signals are generally significant when the NC scale is comparable to the collider energy. If this is not the case, the nontrivial azimuthal angle distribution and day-night asymmetry of events due to Lorentz violation and the Earth rotation will be useful in identifying signals. We also comment briefly on the high energy behaviour of the cross section that grows up linearly in the center of mass energy squared and argue that it does not necessarily contradict some statements, e.g., the Froissart-Martin bound, achieved in ordinary theory.

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arXiv2002

Spectral Representation and Dispersion Relations in Field Theory on Noncommutative Space

We study the spectral representation and dispersion relations that follow from some basic assumptions and the reduced spacetime symmetries on noncommutative (NC) space. Kinematic variables involving the NC parameter appear naturally as parametric variables in this analysis. When subtractions are necessary to remove ultraviolet divergences, they are always made at the fixed values of these NC variables. This point is also illustrated by a perturbative analysis of self-energies. Our analysis of the reduced spacetime symmetries suggests a weaker microcausality requirement. Starting from it, we make a first attempt at dispersion relations for forward scattering. It turns out that the attempt is hampered by a new unphysical region specified by a given motion in the NC plane which does not seem to be surmountable using the usual tricks. Implications for a possible subtraction and renormalization scheme for NC field theory in which the ultraviolet-infrared (UV/IR) mixing is removed are also briefly commented on.

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arXiv2002

Time-ordered Perturbation Theory on Noncommutative Spacetime: Basic Rules

Assuming the S-matrix on noncommutative (NC) spacetime can still be developped perturbatively in terms of the time-ordered exponential of the interaction Lagrangian, we investigate the perturbation theory of NC field theory. We first work out with care some typical Green functions starting from the usual concepts of time-ordering and commutation relations for free fields. The results are found to be very different from those in the naive approach pursued in the literature. A simple framework then appears naturally which can incorporate the new features of our results and which turns out to be the usual time-ordered perturbation theory extended to the NC context. We provide the prescriptions for computing S-matrix elements and Green functions in this framework. We also emphasize that the naive seemingly covariant approach cannot be reproduced from the current one, in contrast to the field theory on ordinary spacetime. We attribute this to the phase-like nonlocal interaction intrinsic in NC field theory which modifies the analytic properties of Green functions significantly.

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arXiv2002

Time-ordered Perturbation Theory on Noncommutative Spacetime II: Unitarity

We examine the unitarity issue in the recently proposed time-ordered perturbation theory on noncommutative (NC) spacetime. We show that unitarity is preserved as long as the interaction Lagrangian is explicitly Hermitian. We explain why it makes sense to distinguish the Hermiticity of the Lagrangian from that of the action in perturbative NC field theory and how this requirement fits in the framework.

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arXiv2002

Validity of Goldstone Theorem at Two Loops in Noncommutative U(N) Linear Sigma Model

The scalar theory is ultraviolet (UV) quadratically divergent on ordinary spacetime. On noncommutative (NC) spacetime, this divergence will generally induce pole-like infrared (IR) singularities in external momenta through the UV/IR mixing. In spontaneous symmetry breaking theory this would invalidate the Goldstone theorem which is the basis for mass generation when symmetry is gauged. We examine this issue at two loop level in the U(N) linear sigma model which is known to be free of such IR singularities in the Goldstone self-energies at one loop. We analyze the structures in the NC parameter (θ_{μν}) dependence in two loop integrands of Goldstone self-energies. We find that their coefficients are effectively once subtracted at the external momentum p=0 due to symmetry relations between 1PI and tadpole contributions, leaving a final result proportional to a quadratic form in p. We then compute the leading IR terms induced by NC to be of order p^2\ln(θ_{μν})^2 and p^2\ln\tilde{p}^2 (\tilde{p}_μ=θ_{μν}p^ν) which are much milder than naively expected without considering the above cancellation. The Goldstone bosons thus keep massless and the theorem holds true at this level. However, the limit of θ\to 0 cannot be smooth any longer as it is in the one loop Goldstone self-energies, and this nonsmooth behaviour is not necessarily associated with the IR limit of the external momentum as we see in the term of p^2\ln(θ_{μν})^2.

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arXiv2002

One Loop Renormalizability of Spontaneously Broken Gauge Theory with a Product of Gauge Groups on Noncommutative Spacetime: the U(1) x U(1) Case

A generalization of the standard electroweak model to noncommutative spacetime would involve a product gauge group which is spontaneously broken. Gauge interactions in terms of physical gauge bosons are canonical with respect to massless gauge bosons as required by the exact gauge symmetry, but not so with respect to massive ones; and furthermore they are generally asymmetric in the two sets of gauge bosons. On noncommutative spacetime this already occurs for the simplest model of U(1) x U(1). We examine whether the above feature in gauge interactions can be perturbatively maintained in this model. We show by a complete one loop analysis that all ultraviolet divergences are removable with a few renormalization constants in a way consistent with the above structure.

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arXiv2001

One Loop Renormalization of Spontaneously Broken U(2) Gauge Theory on Noncommutative Spacetime

We examine the renormalizability problem of spontaneously broken non-Abelian gauge theory on noncommutative spacetime. We show by an explicit analysis of the U(2) case that ultraviolet divergences can be removed at one loop level with the same limited number of renormalization constants as required on commutative spacetime. We thus push forward the efforts towards constructing realistic models of gauge interactions on noncommutative spacetime.

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arXiv2001

Pair Production of Neutral Higgs Bosons through Noncommutative QED Interactions at Linear Colliders

We study the feasibility of detecting noncommutative (NC) QED through neutral Higgs boson (H) pair production at linear colliders (LC). This is based on the assumption that H interacts directly with photon in NCQED as suggested by symmetry considerations and strongly hinted by our previous study on π^0-photon interactions. We find the following striking features as compared to the standard model (SM) result: (1) generally larger cross sections for an NC scale of order 1 TeV; (2) completely different dependence on initial beam polarizations; (3) distinct distributions in the polar and azimuthal angles; and (4) day-night asymmetry due to the Earth's rotation. These will help to separate NC signals from those in the SM or other new physics at LC. We emphasize the importance of treating properly the Lorentz noninvariance problem and show how the impact of the Earth's rotation can be used as an advantage for our purpose of searching for NC signals.

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arXiv2001

Anomalous C-violating Three Photon Decay of the Neutral Pion in Noncommutative Quantum Electrodynamics

We show that a simple and reasonable generalization of the anomalous interaction between the neutral pion and two photons can induce the C-violating three photon decay of the neutral pion in noncommutative quantum electrodynamics. We find that it is mandatory for consistency reasons to include simultaneously the normal neutral pion and photon interaction in which the neutral pion transforms under U(1) in a similar way as in the adjoint representation of a non-Abelian gauge theory. We demonstrate that the decay has a characteristic distribution although its rate still seems too small to be experimentally reachable in the near future. We also describe how to manipulate phase space integration correctly when Lorentz invariance is lost.

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arXiv2001

Photon-3-Pion And Pion-2-Photon Form Factors From Dynamical Constituent Quarks

We study the form factors of the low energy anomalous pion-2-photon and photon-3-pion processes in the nonlocal chiral quark model which incorporates the momentum dependence of the dynamical quark mass and realizes correctly the chiral symmetries. The obtained slope parameter for pion-2-photon is in reasonable agreement with the direct experimental results but smaller than the ones invoking vector meson dominance. Our result for the photon-3-pion form factor interpolates between the two extremes of theoretical approaches, with the largest one provided by the vector meson dominance and the smallest one by the Schwinger-Dyson approach. But all of them are well below the single data point available so far. This situation will hopefully be clarified by the experiments at CEBAF and CERN.

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arXiv2001

Non-unitarity of CKM matrix from the vector singlet quark mixing and neutron electric dipole moment

In the standard model (SM) the lowest order contribution to the quark electric dipole moment (EDM) occurs at the three loop level. We show that the non-unitarity of the CKM matrix in models with an extended quark sector typically gives rise to a quark EDM at the two loop level which has no GIM-like suppression factors except the external quark mass. The induced neutron EDM is of order 10^{-29} e cm and can be well within the reach of the next generation of experiments if it is further enhanced by long distance physics as happens in the SM.

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arXiv2000

Associated production of Higgs bosons and heavy quarks at photon colliders

The idea of fermion mass generation through Yukawa couplings between Higgs bosons and fermions may be tested by their associated production at future linear colliders (LC). We study the production mechanism in the minimal supersymmeric standard model at photon colliders that can be realized by the laser back-scattering technique at the LC. We find that the cross sections for the production of the light Higgs-top pair, heavy Higgs-bottom pair and pseudoscalr Higgs-bottom pair can reach the level of \sim 1.0 fb or higher for phenomenologically favoured values of the parameters in the Higgs sector. Since this mechanism is not affected by the appearance of Higgs resonances as in electron collisions, it may provide a more sensitive way to determine the Yukawa couplings between Higgs bosons and heavy quarks.

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arXiv1999

Enhanced contribution to quark and neutron electric dipole moments with small mixing of right-handed currents and CKM CP violation

We study the light quark and the neutron electric dipole moments (EDMs) under the assumptions that the CP source is still in the usual CKM matrix and that there is a small mixing of right-handed charged currents in the quark sector. We find that the EDMs arise already at two loop order that are much larger than the standard model (SM) result even for a small mixing.

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