Search Results (36 total)

We propose an atom-interferometry experiment based on the scalar Aharonov-Bohm effect which detects an atom charge at the 10^-28e level, and improves the current laboratory limits by 8 orders of magnitude. This setup independently probes neutron charges down to 10^-28e, 7 orders of magnitude below current bounds.

Long-lived gluinos are the trademark of split supersymmetry. They form R-hadrons that, when charged, efficiently lose energy in matter via ionization. Independent of R-spectroscopy and initial hadronization, a fraction of R-hadrons become charged while traversing a detector. This results in a large number of stopped gluinos at present and future detectors. For a 300 GeV gluino, 10⁶ will stop each year in LHC detectors, while several hundred stop in detectors during Run II at the Tevatron. The subsequent decays of stopped gluinos produce distinctive depositions of energy in calorimeters with no activity in either the tracker or the muon chamber. The gluino lifetime can be determined by looking for events where both gluinos stop and subsequently decay.

The unprecedented precision of atom interferometry will soon lead to laboratory tests of general relativity to levels that will rival or exceed those reached by astrophysical observations. We propose such an experiment that will initially test the equivalence principle to 1 part in 10¹⁵ (300 times better than the current limit), and 1 part in 10¹⁷ in the future. It will also probe general relativistic effects—such as the nonlinear three-graviton coupling, the gravity of an atom’s kinetic energy, and the falling of light—to several decimals. In contrast with astrophysical observations, laboratory tests can isolate these effects via their different functional dependence on experimental variables.

We propose a technique, using interferometry of Bose-Einstein condensed alkali atoms, for the detection of submicron-range forces. It may extend present searches at 1 micron by 6 to 9 orders of magnitude, deep into the theoretically interesting regime of 1000 times gravity. We give several examples of both four-dimensional particles (moduli), as well as higher-dimensional particles—vectors and scalars in a large bulk—that could mediate forces accessible by this technique.

The recently proposed theories with TeV-scale quantum gravity do not have the usual ultraviolet desert between ∼10³-10¹⁹ 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 problems with large flavor-changing neutral currents; (2) we reiterate that proton stability can arise if baryon number is gauged in the bulk; (3) we study limits on light gauge fields and scalars in the bulk coming from rare decays, astrophysics and cosmology; (4) we remark that the same ideas can be used to explain small neutrino masses, as well as hierarchical supersymmetry breaking; (5) we construct a theory with bulk technicolor, avoiding the difficulties with extended technicolor. There are also a number of interesting experimental signals of these ideas: (1) attractive or repulsive, isotope dependent sub-millimeter forces ∼10⁶ 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 γ or jet+ missing energy signals as in the case of bulk graviton production, with comparable or larger rates.

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 M_* to be as small as the weak scale M_*≃O(TeV) and the observed weakness of gravity at long distances is due the existence of new submillimeter 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 M_*. Such suppressed neutrino masses are not the result of a seesaw, 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 infrared desert: the space in the extra dimensions. Here, small Majorana neutrino masses are generated by a breaking lepton number on distant branes.

Generic classes of string compactifications include “brane throats” emanating from the compact dimensions and separated by effective potential barriers raised by the background gravitational fields. The interaction of observers inside different throats occurs via tunneling and is consequently weak. This provides a new mechanism for generating small numbers in nature. We apply it to the hierarchy problem, where supersymmetry breaking near the unification scale causes TeV sparticle masses inside the standard model throat. We also design naturally long-lived cold dark matter which decays within a Hubble time to the approximate conformal matter of a long throat. This may soften structure formation at galactic scales and raises the possibility that much of the dark matter of the universe is conformal matter. Finally, the tunneling rate shows that the coupling between throats, mediated by bulk modes, is stronger than a naive application of holography suggests.

If the scale of quantum gravity is near TeV, the CERN Large Hadron Collider will be producing one black hole (BH) about every second. The decays of the BHs into the final states with prompt, hard photons, electrons, or muons provide a clean signature with low background. The correlation between the BH mass and its temperature, deduced from the energy spectrum of the decay products, can test Hawking’s evaporation law and determine the number of large new dimensions and the scale of quantum gravity.

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 TeV and the observed weakness of gravity at long distances is due to the existence of new submillimeter 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 Λ̅ can stabilize the internal manifold against expansion, and that the value of Λ̅ 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.

We construct intersecting brane configurations in anit–de Sitter (AdS) space which localize gravity to the intersection region, generalizing the trapping of gravity to any number n of infinite extra dimensions. Since the 4D Planck scale M_Pl is determined by the fundamental Planck scale M_* and the AdS radius L via the familiar relation M_Pl²∼M_*²⁺ⁿLⁿ, we get two kinds of theories with TeV scale quantum gravity and submillimeter deviations from Newton's law. With M_*∼TeV and L∼submillimeter, we recover the phenomenology of theories with large extra dimensions. Alternatively, if M_*∼L^-1∼M_Pl, and our 3-brane is at a distance of ∼100M_Pl^-1 from the intersection, we obtain a theory with an exponential determination of the weak/Planck hierarchy.

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>~2 new dimensions of submillimeter size, with the SM fields localized 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 ∼30 TeV to avoid cooling SN 1987A 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>~2, with string scale m_s∼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 ∼10⁶–10⁸ times stronger than gravity at submillimeter distances, as well as 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.

The signatures for low energy supersymmetry breaking at the Fermilab Tevatron are investigated. It is natural that the lightest standard model superpartner is an electroweak neutralino, which decays to an essentially massless Goldstino and photon, possibly within the detector. In the simplest model of gauge-mediated supersymmetry breaking, the production of right-handed sleptons, neutralinos, and charginos leads to a pair of hard photons accompanied by leptons and/or jets with missing transverse energy. The relatively hard leptons and softer photons of the single e⁺e^-γγ+E_T event observed by CDF implies this event is best interpreted as arising from left-handed slepton pair production. In this case the rates for l^±γγ+E_T and γγ+E_T are comparable to that for l⁺l^-γγ+E_T.

The experimental signatures for low energy gauge-mediated supersymmetry breaking are distinctive since the gravitino is naturally the lightest supersymmetric particle. The next lightest supersymmetric particle (NLSP) can be a gaugino, Higgsino, or right handed slepton. For a significant range of parameters, decay of the NLSP to its partner plus the gravitino can be measured as a displaced vertex or kink in a charged particle track. In the case that the NLSP is mostly gaugino, we identify the discovery modes as e⁺e^-→γγ+E, and pp̅ →l⁺l^-γγ+E _T. If the NLSP is a right-handed slepton, the discovery modes are e⁺e^-→l⁺l^-+E and pp̅ →l⁺l^-+E _T. A NLSP which is mostly Higgsino is also considered.

We reanalyze the problem of fermion masses in supersymmetric SO(10) grand unified models. In the minimal model, both low energy Higgs doublets belong to the same 10 representation of SO(10), implying the unification not only of the gauge but also of the third generation Yukawa couplings. These models predict large values of tanβ∼50. In this paper we study the effects of departing from the minimal conditions in order to see if we can find models with a reduced value of tanβ. In order to maintain pre- dictability, however, we try to do this with the addition of only one new parameter. We still assume that the fermion masses arise from interactions of the spinor representations with a single 10 representation, but this 10 now only contains a part of the two light Higgs doublets. This enables us to introduce one new parameter ω=λ_b/λ_t. For values of ω≪1 we can in principle reduce the value of tanβ. In fact, ω is an overall factor which multiplies the down quark and charged lepton Yukawa matrices. Thus the theory is still highly constrained. We show that the first generation quark masses and the CP-violation parameter ε_K are sufficient to yield strong constraints on the phenomenologically allowed models. In the end, we find that large values of tanβ are still preferred.

A new approach for deducing the theory of fermion masses at the scale of grand unification is proposed. Combining SO(10) grand unification, family symmetries and supersymmetry with a systematic operator analysis, the minimal set of fermion mass operators consistent with low-energy data is determined. Exploiting the full power of SO(10) to relate up, down, and charged lepton mass matrices, we obtain predictions for seven of the mass and mixing parameters. The assumptions upon which the operator search and resulting predictions are based are stressed, together with a discussion of how the predictions are affected by a relaxation of some of the assumptions. The masses of the heaviest generation, m_t, m_b, and m_τ, are generated from a single renormalizable Yukawa interaction, while the lighter masses and the mixing angles are generated by nonrenormalizable operators of the grand unified theory. The hierarchy of masses and mixing angles is thereby related to the ratio of grand to Planck scales, M_G / M_P. An explicit realization of the origin of such an economical pattern of operators is given in terms of a set of spontaneously broken family symmetries. In the preferred models the top quark is found to be heavy, M_t=180±15 GeV, and tanβ is predicted to be very large. Predictions are also given for m_s, m_s / m_d, m_u / m_d, V_cb, V_ub / V_cb and the amount of CP violation. Stringent tests of these theories will be achieved by more precise measurements of M_t, V_cb, α_s, and V_ub / V_cb and by measurements of CP violation in neutral B meson decays.

The fermion mass and mixing angle predictions of a recently proposed framework are investigated for large b and τ Yukawa couplings. A new allowed region of parameters is found for this large tanβ case. The two predictions that are substantially altered, m_t and tanβ, are displayed, including the dependence on the input |V_cb|, m_c, m_b, and α_s. A simple restriction on this framework yields an additional prediction for |V_cb|. If the b, t, and τ Yukawa couplings are equal at the GUT scale then |V_cb| is predicted and the top quark mass is constrained to lie in the range m_t=179±4 GeV.

A framework for predicting charged fermion masses in supersymmetric grand unified theories is extended to make predictions in the neutrino sector. Eight new predictions are made: the two neutrino mass ratios and the three mixing angles and three phases of the weak leptonic mixing matrix. There are three versions of the theory which are relevant for producing MSW neutrino oscillations in the Sun. One of these is preferred by the combined solar neutrino observations. Another will be probed significantly by the searches for ν_μμ_τ oscillations at the NOMAD, CHORUS, and P803 experiments. In this second version ν_τ could be a significant component of the dark matter in the Universe.

Using supersymmetric grand unified theories, we have recently invented a framework which allows the prediction of three quark masses, two of the parameters of the Kobayashi-Maskawa matrix, and tanβ, the ratio of the two electroweak vacuum expectation values. These predictions are used to calculate ε and ε^′ in the kaon system, the mass mixing in the B_d⁰ and B_s⁰ systems, and the size of CP asymmetries in the decays of neutral B mesons to explicit final states of given CP.

In this paper we present a new Ansatz for fermion mass matrices in the context of supersymmetric grand unified theories. We are able to fit the 13 parameters, associated with quark and lepton masses and mixing angles, and the ratio of Higgs vacuum expectation values (VEV's) which enters any supersymmetric theory, in terms of 8 input parameters; hence, we have 6 predictions. The top quark is predicted to have a mass in the range 176 to 190 GeV, where the upper bound results from the assumption of perturbative unification, and the lower bound is sensitive to the experimental value of V_cb. Predictions are also made for m_s, m_s / m_d, |V_ub / V_cb|, the ratio of Higgs VEV's, and the CP-violating phase of the Kobayashi-Maskawa matrix.

The fermion masses and mixings are derived from a small number of input parameters. The resulting six predictions are consistent with data and have interesting consequences for future experiments. The top quark is heavy, near 188 GeV; its precise mass is sensitive to V_cb.

We constrain, from the observed properties of diffuse interstellar clouds, the interactions of halo particles with atomic hydrogen.

We discuss the possibility that the dark matter consists of strongly interacting massive particles (SIMP's) which have cross sections with ordinary matter which are larger than characteristic weak-interaction cross sections. We show that, while results from ββ decay, cosmic-ray detectors, galactic-halo stability, the cooling of molecular clouds, proton-decay detectors, and the existence of old neutron stars and the Earth constrain the interactions of the missing matter with ordinary matter over a broad range of parameter space, there still exist several windows for SIMP's. It is noteworthy that there are two regions of less than geometric cross sections: one with masses of 10⁵-10⁷ GeV and another with masses above 10¹⁰ GeV.

We consider the possibility that dark matter is in the form of charged massive particles. Several constraints are discussed: (a) the absence of heavy-hydrogen-like atoms in water; (b) the agreement between the observed cosmic abundance of the elements and standard big-bang nucleosynthesis predictions; (c) the observed properties of galaxies, stars, and planets; (d) their nonobservation in γ-ray and cosmic-ray detectors, and the lack of radiation damage to space-borne electronic components. We find that integer-charged particles less massive than 10³ TeV are probably ruled out as dark matter; but note briefly that there is a slim chance they could be blown out of the halo by supernovae. Above this mass the freeze-out abundance of these particles would overclose the Universe; thus their discovery would be evidence for inflation (or other late-time entropy dumping) below m_ch. We indicate where one should consider looking for charged massive dark matter.

In the standard minimal low-energy supersymmetric model, superpartners are produced only in pairs and the lightest superpartner is stable. At hadron colliders missing transverse energy is the most important signature for this model. There are two other minimal supersymmetric models: one has lepton-number violation, the other baryon-number violation. In both models superpartners can be singly produced and the lightest superpartner is unstable. At hadron colliders missing transverse energy is a poor signature for these models. However, there are several important signatures. The most spectacular signatures have two charged leptons without jets. There are also events with two or more isolated charged leptons and jets. Superpartner masses may be reconstructed from combinations of lepton-lepton or lepton-jet invariant masses and from jet spectroscopy. Cross sections are presented for the most important single and pair superparticle production mechanisms in p-p¯ collisions. Present limits from CERN collider data are given and a variety of signatures, events, and backgrounds at √s =2 TeV are discussed. For example, Drell-Yan fusion of a single superpartner gives a bump in the cross section for e⁺e^- pairs or jet pairs with invariant mass at the superpartner mass. Squark pair production could yield events with two jets and two isolated charged leptons. If the lightest superpartner is long lived, it can give rise to secondary vertices or to signatures in stable-particle searches. A run of 10 pb^-1 at √s =2 TeV will enable a large region of the parameter space to be explored.

An era of inflation in the hot big bang can lead to a dilution of axions. Such a dilution removes the upper bound on f_A, the symmetry-breaking scale at which axions are produced. The dilution may occur during or just after inflation, but in either case there is a restriction on the reheat temperature of the Universe after inflation. Hence, such a scenario for an invisible axion requires baryogenesis to occur at a low temperature.