arXiv subjects

Nima Arkani-Hamed

Publications and source records attributed to Nima Arkani-Hamed.

At least 127 records · Page 7Linked to original sources

arXiv2000

A Small Cosmological Constant from a Large Extra Dimension

We propose a new approach to the Cosmological Constant Problem which makes essential use of an extra dimension. A model is presented in which the Standard Model vacuum energy ``warps'' the higher-dimensional spacetime while preserving 4D flatness. We argue that the strong curvature region of our solutions may effectively cut off the size of the extra dimension, thereby giving rise to macroscopic 4D gravity without a cosmological constant. In our model, the higher-dimensional gravity dynamics is treated classically with carefully chosen couplings. Our treatment of the Standard Model is however fully quantum field-theoretic, and the 4D flatness of our solutions is robust against Standard Model quantum loops and changes to Standard Model couplings.

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arXiv2000

Solving the Hierarchy Problem with Exponentially Large Dimensions

In theories with (sets of) two large extra dimensions and supersymmetry in the bulk, the presence of non-supersymmetric brane defects naturally induces a logarithmic potential for the volume of the transverse dimensions. Since the logarithm of the volume rather than the volume itself is the natural variable, parameters of O(10) in the potential can generate an exponentially large size for the extra dimensions. This provides a true solution to the hierarchy problem, on the same footing as technicolor or dynamical supersymmetry breaking. The area moduli have a Compton wavelength of about a millimeter and mediate Yukawa interactions with gravitational strength. We present a simple explicit example of this idea which generates two exponentially large dimensions. In this model, the area modulus mass is in the millimeter range even for six dimensional Planck scales as high as 100 TeV.

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arXiv1999

Exponentially Small Supersymmetry Breaking from Extra Dimensions

The supersymmetric ``shining'' of free massive chiral superfields in extra dimensions from a distant source brane can trigger exponentially small supersymmetry breaking on our brane of order e^{-2 pi R}, where R is the radius of the extra dimensions. This supersymmetry breaking can be transmitted to the superpartners in a number of ways, for instance by gravity or via the standard model gauge interactions. The radius R can easily be stabilized at a size O(10) larger that the fundamental scale. The models are extremely simple, relying only on free, classical bulk dynamics to solve the hierarchy problem.

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arXiv1999

Manyfold Universe

We propose that our world is a brane folded many times inside the sub-millimeter extra dimensions. The folding produces many connected parallel branes or folds with identical microphysics - a Manyfold. Nearby matter on other folds can be detected gravitationally as dark matter since the light it emits takes a long time to reach us traveling around the fold. Hence dark matter is microphysically identical to ordinary matter; it can dissipate and clump possibly forming dark replicas of ordinary stars which are good MACHO candidates. Its dissipation may lead to far more frequent occurrence of gravitational collapse and consequently to a significant enhancement in gravitational wave signals detectable by LIGO and LISA. Sterile neutrinos find a natural home on the other folds. Since the folded brane is not a BPS state, it gives a new geometric means for supersymmetry breaking in our world. It may also offer novel approach for the resolution of the cosmological horizon problem, although it still requires additional dynamics to solve the flatness problem. Although there are constraints from BBN, structure formation, the enormity of galactic halos and the absence of stars and globular clusters with a discernible dark matter component, we show that the model is consistent with current observational limits. It presents us with a new dark matter particle and a new framework for the evolution of structure in our universe.

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arXiv1999

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.

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arXiv1999

Flavor at the TeV Scale with Extra Dimensions

Theories where the Standard Model fields reside on a 3-brane, with a low fundamental cut-off and extra dimensions, provide alternative solutions to the gauge hierarchy problem. However, generating flavor at the TeV scale while avoiding flavor-changing difficulties appears prohibitively difficult at first sight. We argue to the contrary that this picture allows us to lower flavor physics close to the TeV scale. Small Yukawa couplings are generated by ``shining'' badly broken flavor symmetries from distant branes, and flavor and CP-violating processes are adequately suppressed by these symmetries. We further show how the extra dimensions avoid four dimensional disasters associated with light fields charged under flavor. We construct elegant and realistic theories of flavor based on the maximal U(3)^5 flavor symmetry which naturally generate the simultaneous hierarchy of masses and mixing angles. Finally, we introduce a new framework for predictive theories of flavor, where our 3-brane is embedded within highly symmetrical configurations of higher-dimensional branes.

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arXiv1999

Logarithmic Unification From Symmetries Enhanced in the Sub-Millimeter Infrared

In theories with TeV string scale and sub-millimeter extra dimensions the attractive picture of logarithmic gauge coupling unification at $10^{16}$ GeV is seemingly destroyed. In this paper we argue to the contrary that logarithmic unification {\it can} occur in such theories. The rationale for unification is no longer that a gauge symmetry is restored at short distances, but rather that a geometric symmetry is restored at large distances in the bulk away from our 3-brane. The apparent `running' of the gauge couplings to energies far above the string scale actually arises from the logarithmic variation of classical fields in (sets of) two large transverse dimensions. We present a number of N=2 and N=1 supersymmetric D-brane constructions illustrating this picture for unification.

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arXiv1999

Infinitely Large New Dimensions

We construct intersecting brane configurations in Anti-de-Sitter space localizing gravity to the intersection region, with any number $n$ of extra dimensions. This allows us to construct two kinds of theories with infinitely large new dimensions, TeV scale quantum gravity and sub-millimeter deviations from Newton's Law. The effective 4D Planck scale $M_{Pl}$ is determined in terms of the fundamental Planck scale $M_*$ and the $AdS$ radius of curvature $L$ via the familiar relation $M_{Pl}^2 \sim M_{*}^{2+n} L^n$; $L$ acts as an effective radius of compactification for gravity on the intersection. Taking $M_* \sim $ TeV and $L \sim $ sub-mm reproduces the phenomenology of theories with large extra dimensions. Alternately, taking $M_* \sim L^{-1} \sim M_{Pl}$, and placing our 3-brane a distance $\sim 100 M_{Pl}^{-1}$ away from the intersection gives us a theory with an exponential determination of the Weak/Planck hierarchy.

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arXiv1999

Early Inflation and Cosmology in Theories with Sub-Millimeter Dimensions

We discuss early cosmology in theories where the fundamental Planck mass is close to the TeV scale. In such theories the standard model fields are localized to a (3+1)-dimensional wall with n new transverse sub-millimeter sized spatial dimensions. The topic touched upon include: early inflation that occurs while the size of the new dimensions are still small, the spectrum and magnitude of density perturbations, the post-inflation era of contraction of our world while the internal dimensions evolve to their final ``large'' radius, and the production of gravitons in the bulk during these two eras. The radion moduli problem is also discussed.

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arXiv1999

Hierarchies without Symmetries from Extra Dimensions

It is commonly thought that small couplings in a low-energy theory, such as those needed for the fermion mass hierarchy or proton stability, must originate from symmetries in a high-energy theory. We show that this expectation is violated in theories where the Standard Model fields are confined to a thick wall in extra dimensions, with the fermions "stuck" at different points in the wall. Couplings between them are then suppressed due to the exponentially small overlaps of their wave functions. This provides a framework for understanding both the fermion mass hierarchy and proton stability without imposing symmetries, but rather in terms of higher dimensional geography. A model independent prediction of this scenario is non-universal couplings of the Standard Model fermions to the ``Kaluza-Klein'' excitations of the gauge fields. This allows a measurement of the fermion locations in the extra dimensions at the LHC or NLC if the wall thickness is close to the TeV scale.

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arXiv1999

Rapid Asymmetric Inflation and Early Cosmology in Theories with Sub-Millimeter Dimensions

It was recently pointed out that the fundamental Planck mass could be close to the TeV scale with the observed weakness of gravity at long distances being due the existence of new sub-millimeter spatial dimensions. In this picture the standard model fields are localized to a $(3+1)$-dimensional wall or ``3-brane''. We show that in such theories there exist attractive models of inflation that occur while the size of the new dimensions are still small. We show that it is easy to produce the required number of efoldings, and further that the density perturbations $δρ/ρ$ as measured by COBE can be easily reproduced, both in overall magnitude and in their approximately scale-invariant spectrum. In the minimal approach, the inflaton field is just the moduli describing the size of the internal dimensions, the role of the inflationary potential being played by the stabilizing potential of the internal space. We show that under quite general conditions, the inflationary era is followed by an epoch of contraction of our world on the brane, while the internal dimensions slowly expand to their stabilization radius. We find a set of exact solutions which describe this behavior, generalizing the well-known Kasner solutions. During this phase, the production of bulk gravitons remains suppressed. The period of contraction is terminated by the blue-shifting of Hawking radiation left on our wall at the end of the inflationary de Sitter phase. The temperature to which this is reheated is consistent with the normalcy bounds. We give a precise definition of the radion moduli problem.

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arXiv1998

Field Theoretic Branes and Tachyons of the QCD String

Dvali and Shifman have proposed a field-theoretic mechanism for localizing gauge fields to "branes" in higher dimensional spaces using confinement in a bulk gauge theory. The resulting objects have a number of qualitative features in common with string theory D-branes; they support a gauge field and flux strings can end on them. In this letter, we explore this analogy further, by considering what happens when N of these "branes" approach each other. Unlike in the case of D-branes, we find a reduction of the gauge symmetry as the "branes" overlap. This can be attributed to a tachyonic instability of the flux string stretching between the branes.

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arXiv1998

Neutrino Masses from Large Extra Dimensions

Recently it was proposed that the standard model (SM) degrees of freedom reside on a $(3+1)$-dimensional wall or ``3-brane'' embedded in a higher-dimensional spacetime. Furthermore, in this picture it is possible for the fundamental Planck mass $\mst$ to be as small as the weak scale $\mst\simeq O(\tev)$ and the observed weakness of gravity at long distances is due the existence of new sub-millimeter spatial dimensions. We show that in this picture it is natural to expect neutrino masses to occur in the $10^{-1} - 10^{-4}\ev$ range, despite the lack of any fundamental scale higher than $\mst$. Such suppressed neutrino masses are not the result of a see-saw, but have intrinsically higher-dimensional explanations. We explore two possibilities. The first mechanism identifies any massless bulk fermions as right-handed neutrinos. These give naturally small Dirac masses for the same reason that gravity is weak at long distances in this framework. The second mechanism takes advantage of the large {\it infrared} desert: the space in the extra dimensions. Here, small Majorana neutrino masses are generated by breaking lepton number on distant branes.

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arXiv1998

New origin for approximate symmetries from distant breaking in extra dimensions

The recently proposed theories with TeV-scale quantum gravity remove the usual ultraviolet desert between $\sim 10^{3} - 10^{19}$ GeV where effective field theory ideas apply. Consequently, the success of the desert in explaining approximate symmetries is lost, and theories of flavor, neutrino masses, proton longevity or supersymmetry breaking, lose their usual habitat. In this paper we show that these ideas can find a new home in an infrared desert: the large space in the extra dimensions. The main idea is that symmetries are primordially exact on our brane, but are broken at O(1) on distant branes. This breaking is communicated to us in a distance-suppressed way by bulk messengers. We illustrate these ideas in a number of settings: 1) We construct theories for the fermion mass hierarchy which avoid large FCNC's. 2) We re-iterate that proton stability can arise if baryon number is gauged in the bulk. 3) We study experimental constraints on light gauge fields and scalars in the bulk. 4) We remark that the same ideas can be used to explain small neutrino masses, and hierarchical supersymmetry breaking. 5) We construct a theory with bulk technicolor, avoiding the difficulties with extended technicolor. There are also interesting experimental signals of these ideas: 1) Attractive or repulsive, isotope dependent sub-millimeter forces $\sim 10^6$ times gravitational strength, from the exchange of light bulk particles. 2) Novel Higgs decays to light generation fermions plus bulk scalars. 3) Collider production of bulk vector and scalar fields, leading to large $γ$ or jet+ missing energy signals.

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arXiv1998

Stabilization of Sub-Millimeter Dimensions: The New Guise of the Hierarchy Problem

A new framework for solving the hierarchy problem was recently proposed which does not rely on low energy supersymmetry or technicolor. The fundamental Planck mass is at a $\tev$ and the observed weakness of gravity at long distances is due the existence of new sub-millimeter spatial dimensions. In this picture the standard model fields are localized to a $(3+1)$-dimensional wall or ``3-brane''. The hierarchy problem becomes isomorphic to the problem of the largeness of the extra dimensions. This is in turn inextricably linked to the cosmological constant problem, suggesting the possibility of a common solution. The radii of the extra dimensions must be prevented from both expanding to too great a size, and collapsing to the fundamental Planck length $\tev^{-1}$. In this paper we propose a number of mechanisms addressing this question. We argue that a positive bulk cosmological constant $\barΛ$ can stabilize the internal manifold against expansion, and that the value of $\barΛ$ is not unstable to radiative corrections provided that the supersymmetries of string theory are broken by dynamics on our 3-brane. We further argue that the extra dimensions can be stabilized against collapse in a phenomenologically successful way by either of two methods: 1) Large, topologically conserved quantum numbers associated with higher-form bulk U(1) gauge fields, such as the naturally occurring Ramond-Ramond gauge fields, or the winding number of bulk scalar fields. 2) The brane-lattice-crystallization of a large number of 3-branes in the bulk. These mechanisms are consistent with theoretical, laboratory, and cosmological considerations such as the absence of large time variations in Newton's constant during and after primordial nucleosynthesis, and millimeter-scale tests of gravity.

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arXiv1998

Phenomenology, Astrophysics and Cosmology of Theories with Sub-Millimeter Dimensions and TeV Scale Quantum Gravity

We recently proposed a solution to the hierarchy problem not relying on low-energy supersymmetry or technicolor. Instead, the problem is nullified by bringing quantum gravity down to the TeV scale. This is accomplished by the presence of $n \geq 2$ new dimensions of sub-millimeter size, with the SM fields localised on a 3-brane in the higher dimensional space. In this paper we systematically study the experimental viability of this scenario. Constraints arise both from strong quantum gravitational effects at the TeV scale, and more importantly from the production of massless higher dimensional gravitons with TeV suppressed couplings. Theories with $n>2$ are safe due mainly to the infrared softness of higher dimensional gravity. For $n=2$, the six dimensional Planck scale must be pushed above $\sim 30$ TeV to avoid cooling SN1987A and distortions of the diffuse photon background. Nevertheless, the particular implementation of our framework within type I string theory can evade all constraints, for any $n \geq 2$, with string scale $m_s \sim 1$ TeV. We also explore novel phenomena resulting from the existence of new states propagating in the higher dimensional space. The Peccei-Quinn solution to the strong CP problem is revived with a weak scale axion in the bulk. Gauge fields in the bulk can mediate repulsive forces $\sim 10^6 - 10^8$ times stronger than gravity at sub-mm distances, and may help stabilize the proton. Higher-dimensional gravitons produced on our brane and captured on a different "fat" brane can provide a natural dark matter candidate.

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arXiv1998

Light active and sterile neutrinos from compositeness

Neutrinos can have naturally small Dirac masses if the Standard Model singlet right-handed neutrinos are light composite fermions. Theories which produce light composite fermions typically generate many of them, three of which can marry the left-handed neutrinos with small Dirac masses. The rest can serve as sterile states which can mix with the Standard Model neutrinos. We present explicit models illustrating this idea.

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arXiv1998

Composite Quarks and Leptons from Dynamical Supersymmetry Breaking without Messengers

We present new theories of dynamical SUSY breaking in which the strong interactions that break SUSY also give rise to composite quarks and leptons with naturally small Yukawa couplings. In these models, SUSY breaking is communicated directly to the composite fields without ``messenger'' interactions. The compositeness scale can be anywhere between 10 TeV and the Planck scale. These models can naturally solve the supersymmetric flavor problem, and generically predict sfermion mass unification independent from gauge unification.

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