Proton Decay Problem Research Articles

Dark matter in universal extra dimension models: Kaluza-Klein photon and right-handed neutrino admixture

We show that in a class of universal extra dimension models (UED), which solves both the neutrino mass and proton decay problem, an admixture of Kaluza-Klein (KK) photon and KK right-handed neutrinos can provide the required amount of cold dark matter (CDM). This model has two parameters ${R}^{\ensuremath{-}1}$ and ${M}_{{Z}^{\ensuremath{'}}}$ ($R$ is the radius of the extra space dimensions and ${Z}^{\ensuremath{'}}$ the extra neutral gauge boson of the model). Using the value of the relic CDM density, combined with the results from the cryogenic searches for CDM, we obtain upper limits on ${R}^{\ensuremath{-}1}$ of about 400--650 GeV and ${M}_{{Z}^{\ensuremath{'}}}\ensuremath{\le}1.5\text{ }\text{ }\mathrm{TeV}$, both being accessible to the LHC. In some regions of the parameter space, the dark matter-nucleon scattering cross section can be as high as ${10}^{\ensuremath{-}44}\text{ }\text{ }{\mathrm{cm}}^{2}$, which can be probed by the next round of dark matter search experiments.

Why Black Hole Production in Scattering of Cosmic Ray Neutrinos Is Generically Suppressed

It has been argued that neutrinos originating from ultrahigh energy cosmic rays can produce black holes deep in the atmosphere in models with TeV-scale quantum gravity. Such black-hole events could be observed at the Auger Observatory. However, any phenomenologically viable model with a low scale of quantum gravity must explain how to preserve protons from rapid decay. We argue that the suppression of proton decay will also suppress lepton-nucleon scattering and hence black-hole production by scattering of ultrahigh energy cosmic ray neutrinos in the atmosphere. We discuss explicitly the split fermion solution to the problem of fast proton decay.

Split supersymmetry in an orbifold GUT

We propose a supersymmetric SU(5) orbifold GUT in 5 dimensions in which supersymmetry breaking at the boundaries give rise to split supersymmetry spectrum. The extra dimension is compactified on a $S^1/Z_2 \times Z_2^\prime$ orbifold having two fixed points. The standard model fermions are localized in the standard model brane, where supersymmetry is broken by the nonvanishing of the F-term of a superfield X at some intermediate scale. All scalar superpartners acquires mass at this stage. The standard model gauginos and higgsinos remain light, since their masses are protected by R-symmetry and induced only by loop contributions and contact interaction in our brane. Usual features of orbifold GUTs are preserved in this scenario and proton decay and neutrino mass problems are solved.

Supersymmetric unification without low energy supersymmetry and signatures for fine-tuning at the LHC

The cosmological constant problem is a failure of naturalness and suggests that a fine-tuning mechanism is at work, which may also address the hierarchy problem. An example -- supported by Weinberg's successful prediction of the cosmological constant -- is the potentially vast landscape of vacua in string theory, where the existence of galaxies and atoms is promoted to a vacuum selection criterion. Then, low energy SUSY becomes unnecessary, and supersymmetry -- if present in the fundamental theory -- can be broken near the unification scale. All the scalars of the supersymmetric standard model become ultraheavy, except for a single finely tuned Higgs. Yet, the fermions of the supersymmetric standard model can remain light, protected by chiral symmetry, and account for the successful unification of gauge couplings. This framework removes all the difficulties of the SSM: the absence of a light Higgs and sparticles, dimension five proton decay, SUSY flavor and CP problems, and the cosmological gravitino and moduli problems. High-scale SUSY breaking raises the mass of the light Higgs to about 120-150 GeV. The gluino is strikingly long lived, and a measurement of its lifetime can determine the ultraheavy scalar mass scale. Measuring the four Yukawa couplings of the Higgs to the gauginos and higgsinos precisely tests for high-scale SUSY. These ideas, if confirmed, will demonstrate that supersymmetry is present but irrelevant for the hierarchy problem -- just as it has been irrelevant for the cosmological constant problem -- strongly suggesting the existence of a fine-tuning mechanism in nature.

Warped supersymmetric unification with a nonunified superparticle spectrum

We present a new supersymmetric extension of the standard model. The model is constructed in warped space, with a unified bulk symmetry broken by boundary conditions on both the Planck and TeV branes. In the supersymmetric limit, the massless spectrum contains exotic colored particles along with the particle content of the minimal supersymmetric standard model (MSSM). Nevertheless, the model still reproduces the MSSM prediction for gauge coupling unification and does not suffer from a proton decay problem. The exotic states acquire masses from supersymmetry breaking, making the model completely viable, but there is still the possibility that these states will be detected at the LHC. The lightest of these states is most likely A_5^XY, the fifth component of the gauge field associated with the broken unified symmetry. Because supersymmetry is broken on the SU(5)-violating TeV brane, the gaugino masses generated at the TeV scale are completely independent of one another. We explore some of the unusual features that the superparticle spectrum might have as a consequence.

Supersymmetric [formula omitted] models inspired by deconstruction

We consider 4-dimensional N = 1 supersymmetric SO ( 10 ) models inspired by deconstruction of 5-dimensional N = 1 supersymmetric orbifold SO ( 10 ) models and high-dimensional non-supersymmetric SO ( 10 ) models with Wilson line gauge symmetry breaking. We discuss the SO ( 10 ) × SO ( 10 ) models with bi-fundamental link fields where the gauge symmetry can be broken down to the Pati–Salam, SU ( 5 ) × U ( 1 ) , flipped SU ( 5 ) × U ( 1 ) ′ or the Standard Model like gauge symmetry. We also propose an SO ( 10 ) × SO ( 6 ) × SO ( 4 ) model with bi-fundamental link fields where the gauge symmetry is broken down to the Pati–Salam gauge symmetry, and an SO ( 10 ) × SO ( 10 ) model with bi-spinor link fields where the gauge symmetry is broken down to the flipped SU ( 5 ) × U ( 1 ) ′ gauge symmetry. In these two models, the Pati–Salam and flipped SU ( 5 ) × U ( 1 ) ′ gauge symmetry can be further broken down to the Standard Model gauge symmetry, the doublet–triplet splittings can be obtained by the missing partner mechanism, and the proton decay problem can be solved. We also study the gauge coupling unification. We briefly comment on the interesting variation models with gauge groups SO ( 10 ) × SO ( 6 ) and SO ( 10 ) × flipped SU ( 5 ) × U ( 1 ) ′ in which the proton decay problem can be solved.

Low energy six-dimensional N = 2 supersymmetric SU(6) models on T2 orbifolds

We propose low energy six-dimensional N = 2 supersymmetric SU(6) models on M4 × T2/(Z2)3 and M4 × T2/(Z2)4, where the orbifold SU(3)C × SU(3) model can be embedded on the boundary 4-brane. For the zero modes, the six-dimensional N = 2 supersymmetry and the SU(6) gauge symmetry are broken down to the four-dimensional N = 1 supersymmetry and the SU(3)C × SU(2)L × U(1)Y × U(1)′ gauge symmetry by orbifold projections. In order to cancel the anomalies involving at least one U(1)′, we add extra exotic particles. We also study the anomaly free conditions and present some anomaly free models. The gauge coupling unification can be achieved at 100–200 TeV if the compactification scale for the fifth dimension is 3–4 TeV. The proton decay problem can be avoided by putting the quarks and leptons/neutrinos on different 3-branes. And we discuss how to break the SU(3)C × SU(2)L × U(1)Y × U(1)′ gauge symmetry, solve the μ problem, and generate the Z − Z′ mass hierarchy naturally by using the geometry. The masses of exotic particles can be at the order of 1 TeV after the gauge symmetry breaking. We also forbid the dimension-five operators for the neutrino masses by U(1)′ gauge symmetry, and the realistic left-handed neutrino masses can be obtained via non-renormalizable terms.

Precision electroweak data and unification of couplings in warped extra dimensions

Warped extra dimensions allow a novel way of solving the hierarchy problem, with all fundamental mass parameters of the theory naturally of the order of the Planck scale. The observable value of the Higgs vacuum expectation value is redshifted, due to the localization of the Higgs field in the extra dimension. It has been recently observed that, when the gauge fields propagate in the bulk, unification of the gauge couplings may be achieved. Moreover, the propagation of fermions in the bulk allows for a simple solution to potentially dangerous proton decay problems. However, bulk gauge fields and fermions pose a phenomenological challenge, since they tend to induce large corrections to the precision electroweak observables. In this article, we study in detail the effect of gauge and fermion fields propagating in the bulk in the presence of gauge brane kinetic terms compatible with gauge coupling unification, and we present ways of obtaining a consistent description of experimental data, while allowing values of the first Kaluza-Klein mode masses of the order of a few TeV.

Lifting a realisticSO(10)grand unified model to five dimensions

It has been shown recently that the problem of rapid proton decay induced by dimension five operators arising from the exchange of colored Higgsinos can be simply avoided in grand unified models where a fifth spatial dimension is compactified on an orbifold. Here we demonstrate that this idea can be used to solve the Higgsino-mediated proton decay problem in any realistic SO(10) model by lifting that model to five dimensions. A particular SO(10) model that has been proposed to explain the pattern of quark and lepton masses and mixings is used as an example. The idea is to break the SO(10) down to the Pati-Salam symmetry by the orbifold boundary conditions. The entire four-dimensional SO(10) model is placed on the physical SO(10) brane except for the gauge fields, the 45 and a single 10 of Higgs fields, which are placed in the five-dimensional bulk. The structure of the Higgs superpotential can be somewhat simplified in doing so, while the Yukawa superpotential and mass matrices derived from it remain essentially unaltered.

GUT breaking on M4×T2/(Z2×Z2′)

We construct two SU(5) models on the space-time $M^4 \times T^2/(Z_2 \times Z_2^{\prime})$ where the gauge and Higgs fields are in the bulk and the Standard Model fermions are on the brane at the fixed point or line. For the zero modes, the SU(5) gauge symmetry is broken down to $SU(3) \times SU(2) \times U(1) $ due to non-trivil orbifold projection. In particular, if we put the Standard Model fermions on the 3-brane at the fixed point in Model II, we only have the zero modes and KK modes of the Standard Model gauge fields and two Higgs doublets on the observable 3-brane. So, we can have the low energy unification, and solve the triplet-doublet splitting problem, the gauge hierarchy problem, and the proton decay problem.

Proton stability in TeV-scale GUTs

We discuss the proton decay problem in theories with low gravity and/or GUT scales. We pointed out that the gravity induced proton decay can be indeed suppressed up to a desired level, while the GUT origin of the proton instability is rather problematic. To solve this problem we suggest the GUT model where the proton is stable in all orders of perturbation theory. This can be simply achieved by the replication of quark-lepton families with ordinary quarks and leptons residing in different GUT representations and by an appropriate dimensional reduction. The model predicts extra mirror states which along with the GUT particles and the excitations of extra dimensions could be observable at high-energy colliders providing the unification scale is in the TeV range.

The μ and soft masses from the intermediate scale brane with non-factorizable geometry

The proton decay problem and the negative brane tension problem in the original Randall–Sundrum model can be resolved by interpreting the Planck scale brane as the visible sector brane. The hierarchy problem is resolved with supersymmetry, and the TeV scales for soft masses and μ in supersymmetric models are generated by the physics at the intermediate scale (∼10 11–13 GeV) brane.

Split fermions in extra dimensions and exponentially small cross sections at future colliders

We point out a dramatic new experimental signature for a class of theories with extra dimensions, where quarks and leptons are localized at slightly separated parallel ``walls'' whereas gauge and Higgs fields live in the bulk of the extra dimensions. The separation forbids direct local couplings between quarks and leptons, allowing for an elegant solution to the proton decay problem. We show that scattering cross sections for collisions of fermions which are separated in the extra dimensions vanish exponentially at energies high enough to probe the separation distance. This is because the separation puts a lower bound on the attainable impact parameter in the collision. We present cross sections for two body high energy scattering and estimate the power with which future colliders can probe this scenario, finding sensitivity to inverse fermion separations of order 10--70 TeV.

Phenomenology of 3-family grand unified string models

In the 3-family grand unified string models constructed so far, there is only one adjoint (and no higher dimensional representation) Higgs field in the grand unified gauge group. In this preliminary analysis, we address the proton-decay problem in the 3-family E_6 and related SO(10) string models. In particular, we analyze the doublet-triplet splitting (within certain assumptions about non-perturbative dynamics). It appears that generically some fine-tuning is necessary to arrange for a pair of Higgs doublets to be light, while having color Higgs triplets superheavy. We also discuss charge-2/3 quark mass matrix that generically also seems to require some fine-tuning to have rank 1.

Light colour-triplet Higgs is compatible with proton stability: An alternative approach to the doublet-triplet splitting problem

It is usually assumed that the proton stability requires the coloured triplet partner of the electroweak Higgs doublet to be superheavy (with a mass ∼ M GUT). We show that this is a very model-dependent statement and that the colour triplet can be as light as the weak doublet without leading to the proton decay problem. This implies an alternative approach to the doublet-triplet splitting problem: instead of using the mass difference the splitting can occur between the doublet and triplet Yukawa coupling constants so that the light Higgs triplet can appear decoupled from the quarks and leptons and cannot lead to the proton decay. In this scenario the GUT symmetry breaking automatically induces an extremely strong suppression ∼ M W M GUT of the coloured Higgs effective Yukawa coupling; this happens without any fine-tuning, just because of the Clebsch factors. Conceptual differences of the above picture are: (1) an essentially stable proton: both d = 5 and d = 6 proton decay mediating operators are suppressed by the same factors ∼ ( M w M GUT ) 2 ; (2) the possibility of solving the μ problem by the light gauge singlet field (this fact would lead to the destabilization of the hierarchy in the standard case); (3) the existence of the long-lived, light, coloured and charged supermultiplet in the 100 GeV to 1 TeV mass region, which can be the subject of an experimental search. We construct two explicit SO(10) examples with the above properties, with superpotentials most general under the symmetries.

Proton stability in superstring derived models

I discuss the problem of proton decay, from dimension-four, -five and -six operators, in superstring derived standard-like models. I classify the sectors that produce color triplet superfields which may generate proton decay from dimension-five and -six operators. I show that for two of these sectors there exists a unique superstring doublet-triplet splitting mechanism that projects out the color triplets by means of the GSO projection, while leaving the electroweak doublets in the physical spectrum. I investigate possible proton decay due to additional color triplets in the massless spectrum and due to non-renormalizable terms I show that there exist models in which the dimension-four, -five and -six operators, that lead to proton decay, are forbidden by the symmetries of the string models, to all orders of nonrenormalizable terms.

Short-time behavior of unstable systems in field theory and proton decay.

The short-time behavior of an unstable particle is examined in a realistic field theoretical model. The onset of linear decreasing of the nondecay probability is shown to be extremely rapid, so rapid as to exclude any relevance of regimes quadratic in time (``Zeno'' quantum paradox). The result applies to super-renormalizable, renormalizable, and nonrenormalizable cases, particularly to the proton decay problem. We discuss also the deviations from the exponential decay law in small-Q-value decays.

Superstring models from the second realization of Yau's three-generation manifold

We classify the models obtained upon compactification of the E 8 × E 8 heterotic superstring on the second realization of Yau's three-generation manifold according to their discrete symmetries and study the most symmetric case. We find that the presence of R-symmetries, in spite of the absence of acceptable matter parities, leads to considerable improvement, compared with most models based on the first realization of the three-generation manifold, as far as the proton decay problem is concerned.

Phenomenological constraints on superstring models with intermediate-mass scales

The mechanism for generating radiatively the breaking of the gauge symmetry at an intermediate-mass scale, in the framework ofE 6 superstring-inspired models, is discussed in the case where the light standard-model singlets from the additional $$b_{1,1} \left( {\underline {27} + \overline {\underline {27} } } \right)$$ pairs interact with the fields of then g generations of27, through both renormalizable and non-renormalizable couplings. The minimization of the effective scalar potential is worked out in a specific model, based on the gauge groupSU(3)c×SU(2)L×U(1) Y ×U(1) E . The resultant mixings among the various fields are analysed, pointing out the consequences for the problem of proton decay: it is shown that this problem cannot be solved together with that of the neutrino mass, without involving some discrete symmetry.

On proton decay

The problem of proton decay is considered taking into account that in actual experiments there is an interaction of the proton with its environment which could imply an increase of its theoretical lifetime. It is seen that, by application of the time-energy uncertainly relation, no prolongement of the lifetime is obtained in this case.