Showing papers in "Physics Letters B in 1998"

Abstract: We suggest a means of obtaining certain Green's functions in 3+1-dimensional N =4 supersymmetric Yang-Mills theory with a large number of colors via non-critical string theory The non-critical string theory is related to critical string theory in anti-deSitter background We introduce a boundary of the anti-deSitter space analogous to a cut-off on the Liouville coordinate of the two-dimensional string theory Correlation functions of operators in the gauge theory are related to the dependence of the supergravity action on the boundary conditions From the quadratic terms in supergravity we read off the anomalous dimensions For operators that couple to massless string states it has been established through absorption calculations that the anomalous dimensions vanish, and we rederive this result The operators that couple to massive string states at level n acquire anomalous dimensions that grow as 2 ng YM 2N 1/2 for large `t Hooft coupling This is a new prediction about the strong coupling behavior of large N SYM theory

Abstract: We propose a new framework for solving the hierarchy problem which does not rely on either supersymmetry or technicolor. In this framework, the gravitational and gauge interactions become united at the weak scale, which we take as the only fundamental short distance scale in nature. The observed weakness of gravity on distances ≳ 1 mm is due to the existence of n ≥2 new compact spatial dimensions large compared to the weak scale. The Planck scale M Pl ∼ G N −1/2 is not a fundamental scale; its enormity is simply a consequence of the large size of the new dimensions. While gravitons can freely propagate in the new dimensions, at sub-weak energies the Standard Model (SM) fields must be localized to a 4-dimensional manifold of weak scale “thickness” in the extra dimensions. This picture leads to a number of striking signals for accelerator and laboratory experiments. For the case of n =2 new dimensions, planned sub-millimeter measurements of gravity may observe the transition from 1/ r 2 →1/ r 4 Newtonian gravitation. For any number of new dimensions, the LHC and NLC could observe strong quantum gravitational interactions. Furthermore, SM particles can be kicked off our 4 dimensional manifold into the new dimensions, carrying away energy, and leading to an abrupt decrease in events with high transverse momentum p T ≳ TeV. For certain compact manifolds, such particles will keep circling in the extra dimensions, periodically returning, colliding with and depositing energy to our four dimensional vacuum with frequencies of ∼10 12 Hz or larger. As a concrete illustration, we construct a model with SM fields localized on the 4-dimensional throat of a vortex in 6 dimensions, with a Pati-Salam gauge symmetry SU (4)× SU (2)× SU (2) in the bulk.

Abstract: Recently, a new framework for solving the hierarchy problem has been proposed which does not rely on low energy supersymmetry or technicolor. The gravitational and gauge interactions unite at the electroweak scale, and the observed weakness of gravity at long distances is due the existence of large new spatial dimensions. In this letter, we show that this framework can be embedded in string theory. These models have a perturbative description in the context of type I string theory. The gravitational sector consists of closed strings propagating in the higher-dimensional bulk, while ordinary matter consists of open strings living on D3-branes. This scenario raises the exciting possibility that the LHC and NLC will experimentally study ordinary aspects of string physics such as the production of narrow Regge-excitations of all standard model particles, as well more exotic phenomena involving strong gravity such as the production of black holes. The new dimensions can be probed by events with large missing energy carried off by gravitons escaping into the bulk. We finally discuss some important issues of model building, such as proton stability, gauge coupling unification and supersymmetry breaking.

Abstract: It is suggested that the fermion determinant for a vector-like gauge theory with strictly massless quarks can be represented on the lattice as det 1+V 2 , where V=X(X † X) −1/2 and X is the Wilson-Dirac lattice operator with a negative mass term. There is no undesired doubling and no need for any fine tuning. Several other appealing features of the formula are pointed out.

Abstract: We use a variational procedure to study finite density QCD in an approximation in which the interaction between quarks is modelled by that induced by instantons We find that uniform states with conventional chiral symmetry breaking have negative pressure with respect to empty space at all but the lowest densities, and are therefore unstable This is a precisely defined phenomenon which motivates the basic picture of hadrons assumed in the MIT bag model, with nucleons as droplets of chiral symmetry restored phase At all densities high enough that the chirally symmetric phase fills space, we find that color symmetry is broken by the formation of a 〈 qq 〉 condensate of quark Cooper pairs A plausible ordering scheme leads to a substantial gap in a Lorentz scalar channel involving quarks of two colors, and a much smaller gap in an axial vector channel involving quarks of the third color

Abstract: We find one-loop correction to the integral kernel of the BFKL equation for the total cross section of the high energy scattering in QCD and calculate the next-to-leading contribution to anomalous dimensions of twist-2 operators near j =1.

Abstract: It is shown that the Ginsparg-Wilson relation implies an exact symmetry of the fermion action, which may be regarded as a lattice form of an infinitesimal chiral rotation. Using this result it is straightforward to construct lattice Yukawa models with unbroken flavour and chiral symmetries and no doubling of the fermion spectrum. A contradiction with the Nielsen-Ninomiya theorem is avoided, because the chiral symmetry is realized in a different way than has been assumed when proving the theorem.

Abstract: We discuss the structure of infrared singularities in on-shell QCD amplitudes at two-loop order. We present a general factorization formula that controls all the ϵ-poles of the dimensionally regularized amplitudes. The dependence on the regularization scheme is considered and the coefficients of the 1/ϵ4,1/ϵ3 and 1/ϵ2 poles are explicitly given in the most general case. The remaining single-pole contributions are also explicitly evaluated in the case of amplitudes with a q q pair.

Abstract: In a previous publication [Phys. Lett. B 417 (1998) 141, hep-lat/9707022] I showed that the fermion determinant for strictly massless quarks can be written on the lattice as det D , where D is a certain finite square matrix explicitly constructed from the lattice gauge fields. Here I show that D obeys the Ginsparg-Wilson relation Dγ 5 D = Dγ 5 + γ 5 D .

Abstract: We introduce a new and well defined power counting for the effective field theory describing nucleon-nucleon interactions. Because of the large NN scattering lengths it differs from other applications of chiral perturbation theory and is facilitated by introducing an unusual subtraction scheme and renormalization group analysis. Calculation to subleading order in the expansion can be done analytically, and we present the results for both the 1 S 0 and 3 S 1 − 3 D 1 channels.

Abstract: We study the effects of extra spacetime dimensions at intermediate mass scales, as expected in string theories with large-radius compactifications, and focus on the gauge and Yukawa couplings within the Minimal Supersymmetric Standard Model. We find that extra spacetime dimensions naturally lead to the appearance of grand unified theories at scales substantially below the usual GUT scale. Furthermore, we show that extra spacetime dimensions provide a natural mechanism for explaining the fermion mass hierarchy by permitting the Yukawa couplings to receive power-law corrections. We also discuss how proton-decay constraints may be addressed in this scenario, and suggest that proton-decay amplitudes may be exactly cancelled to all orders in perturbation theory as a result of new Kaluza-Klein selection rules corresponding to the extra spacetime dimensions.

Abstract: Initial results are presented from CHOOZ 1 , a long-baseline reactor-neutrino vacuum-oscillation experiment. The data reported here were taken during the period March to October 1997, when the two reactors ran at combined power levels varying from zero to values approaching their full rated power of 8.5 GW (thermal). Electron antineutrinos from the reactors were detected by a liquid scintillation calorimeter located at a distance of about 1 km . The detector was constructed in a tunnel protected from cosmic rays by a 300 MWE rock overburden. This massive shielding strongly reduced potentially troublesome backgrounds due to cosmic-ray muons, leading to a background rate of about one event per day, more than an order of magnitude smaller than the observed neutrino signal. From the statistical agreement between detected and expected neutrino event rates, we find (at 90% confidence level) no evidence for neutrino oscillations in the ν e disappearance mode for the parameter region given approximately by Δm 2 >0.9 10 −3 eV 2 for maximum mixing and sin22θ>0.18 for large Δm2.

Abstract: We complete the calculation of the next-to-leading kernel of the BFKL equation by disentangling its energy-scale dependent part from the impact factor corrections in large- k dijet production. Using the irreducible part previously obtained, we derive the final form of the kernel eigenvalue and of the hard Pomeron shift for various scales. We also discuss the scale changes, the physical equivalence of a class of scales, and how to use the collinear safe ones.

Abstract: Supersymmetric extensions of the standard model generically contain stable non-topological solitons, Q-balls, which carry baryon or lepton number. We show that large Q-balls can be copiously produced in the early universe, can survive until the present time, and can contribute to dark matter.

Abstract: The fixed point Dirac operator on the lattice has exact chiral zero modes on topologically non-trivial gauge field configurations independently whether these configurations are smooth, or coarse. The relation n L − n R = Q FP , where n L ( n R ) is the number of left (right)-handed zero modes and Q FP is the fixed point topological charge holds not only in the continuum limit, but also at finite cut-off values. The fixed point action, which is determined by classical equations, is local, has no doublers and complies with the no-go theorems by being chirally non-symmetric. The index theorem is reproduced exactly, nevertheless. In addition, the fixed point Dirac operator has no small real eigenvalues except those at zero, i.e. there are no `exceptional configurations'.

Abstract: Recently, a new framework for solving the hierarchy problem was 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 letter, we study how the properties of black holes are altered in these theories. Small black holes—with Schwarzschild radii smaller than the size of the new spatial dimensions—are quite different. They are bigger, colder, and longer-lived than a usual (3+1)-dimensional black hole of the same mass. Furthermore, they primarily decay into harmless bulk graviton modes rather than standard-model degrees of freedom. We discuss the interplay of our scenario with the holographic principle. Our results also have implications for the bounds on the spectrum of primordial black holes (PBHs) derived from the photo-dissociation of primordial nucleosynthesis products, distortion of the diffuse gamma-ray spectrum, overclosure of the universe, gravitational lensing, as well as the phenomenology of black hole production. For example, the bound on the spectral index of the primordial spectrum of density perturbations is relaxed from 1.25 to 1.45–1.60 depending on the epoch of the PBH formation. In these scenarios PBHs provide interesting dark matter candidates; for 6 extra dimensions MACHO candidates with mass ∼0.1M⊙ can arise. For 2 or 3 extra dimensions PBHs with mass ∼104M⊙ can occur and may act as both dark matter and seeds for early galaxy and QSO formation.

Abstract: Independent of assumptions about the form of the quark-antiquark scattering kernel we derive the explicit relation between the pion Bethe-Salpeter amplitude, {Gamma}{pi}, and the quark propagator in the chiral limit; {Gamma}{pi} necessarily involves a non-negligible {gamma}{sub 5}{gamma} x P term (P is the pion four-momentum). We also obtain exact expressions for the pion decay constant, f{sub {pi}}, and mass, both of which depend on {Gamma}{sub {pi}}; and demonstrate the equivalence between f{sub {pi}} and the pion Bethe-Salpeter normalization constant in the chiral limit. We stress the importance of preserving the axial-vector Ward-Takahashi identity in any study of the pion itself, and in any study whose goal is a unified understanding of the properties of the pion and other hadronic bound states.

Abstract: We show that if the solar and atmospheric data are both described by maximal vacuum oscillations at the relevant mass scales then there exists a unique mixing matrix for three neutrino flavors. The solution necessarily conserves CP and automatically implies that there is no disappearance of atmospheric ν e , consistent with indications from the Super-Kamiokande experiment. We also investigate the consequences for three-neutrino mixing if the solar and atmospheric oscillations exhibit mixing that is large but not maximal. For non-maximal mixing ν e ↔ ν τ and ν e ↔ ν μ oscillations are predicted that may be observable in future long-baseline experiments.

Abstract: From an exposure of 25.5 kiloton-years of the Super-Kamiokande detector, 900 muon-like and 983 electron-like single-ring atmospheric neutrino interactions were detected with momentum p e >100 MeV/ c , p μ >200 MeV/ c , and with visible energy less than 1.33 GeV. Using a detailed Monte Carlo simulation, the ratio ( μ / e ) DATA /( μ / e ) MC was measured to be 0.61±0.03(stat.)±0.05(sys.), consistent with previous results from the Kamiokande, IMB and Soudan-2 experiments, and smaller than expected from theoretical models of atmospheric neutrino production.

Abstract: We consider the effects of neutralino-stau ( χ− τ ) coannihilations on the cosmological relic density of the lightest supersymmetric particle (LSP) χ in the minimal supersymmetric extension of the Standard Model (MSSM), particularly in the constrained MSSM in which universal supergravity inputs at the GUT scale are assumed. For much of the parameter space in these models, χ is approximately a U(1) gaugino B , and constraints on the cosmological relic density Ω B h 2 yield an upper bound on m B . We show that in regions of parameter space for which the cosmological bound is nearly saturated, coannihilations of the B with the τ , the next lightest sparticle, are important and may reduce significantly the B relic density. Including also B coannihilations with the e and μ , we find that the upper limit on m χ is increased from about 200 GeV to about 600 GeV in the constrained MSSM, with a similar new upper limit expected in the MSSM.

Abstract: We consider a free massive spinor field in Euclidean Anti-de Sitter space. The usual Dirac action in bulk is supplemented by a certain boundary term. The boundary conditions of the field are parametrized by a spinor on the boundary, subject to a projection. We calculate the dependence of the partition function on this boundary spinor. The result agrees with the generating functional of the correlation functions of a quasi-primary spinor operator, of a certain scaling dimension, in a free conformal field theory on the boundary.

Abstract: I present a definition of the cross section for the production of an isolated photon plus n jets which only depends upon direct photon production, and it is independent of the parton-to-photon fragmentation contribution. This prescription, based on a modified cone approach which implements the isolation condition in a smooth way, treats in the same way quarks and gluons and can be directly applied to experimental data in hadron-hadron, photon-hadron and e + e − collisions. The case of several, isolated photons in the final state can also be dealt with in the very same way.

Abstract: Using a proposal of Maldacena we compute in the framework of the supergravity description of N coincident D3 branes the energy of a quark anti-quark pair in the large N limit of U(N) N=4 SYM in four dimensions at finite temperature.

Abstract: Solar neutrino fluxes and sound speeds are calculated using a systematic reevaluation of nuclear fusion rates. The largest uncertainties are identified and their effects on the solar neutrino fluxes are estimated.

Abstract: We present a simple N=1 five-dimensional model where the fifth dimension is compactified on the orbifold S 1 / Z 2 . Non-chiral matter lives in the bulk of the fifth dimension (five dimensions) while chiral matter lives on the fixed points of the orbifold (four-dimensional boundaries). The massless sector constitutes the Minimal Supersymmetric Standard Model while the massive modes rearrange in N=2 supermultiplets. After supersymmetry breaking by the Scherk-Schwarz mechanism the zero modes can be reduced to the non-supersymmetric Standard Model.

Abstract: We study CP asymmetries in lepton-number violating two-body scattering processes and show how they are related to CP asymmetries in the decays of intermediate massive Majorana neutrinos. Self-energy corrections, which do not contribute to CP asymmetries in two-body processes, induce CP violating couplings of the intermediate Majorana neutrinos to lepton-Higgs states. We briefly comment on the implications of these results for applications at finite temperature.

Abstract: The flavor ratio of the atmospheric neutrino flux and its zenith angle dependence have been studied in the multi-GeV energy range using an exposure of 25.5 kiloton-years of the Super-Kamiokande detector. By comparing the data to a detailed Monte Carlo simulation, the ratio ( μ / e ) DATA /( μ / e ) MC was measured to be 0.66±0.06(stat.)±0.08(sys.). In addition, a strong distortion in the shape of the μ -like event zenith angle distribution was observed. The ratio of the number of upward to downward μ -like events was found to be 0.52 +0.07 −0.06 (stat.)±0.01(sys.), with an expected value of 0.98±0.03(stat.)±0.02(sys.), while the same ratio for the e -like events was consistent with unity.

Abstract: Recent calculations of heavy quark cross sections near threshold at next-to-next-to-leading order have found second-order corrections as large as first-order ones. We analyse long-distance contributions to the heavy quark potential in momentum and coordinate space and demonstrate that long-distance contributions in momentum space are suppressed as Λ QCD 2 / q 2 . We then show that the long-distance sensitivity of order Λ QCD r introduced by the Fourier transform to coordinate space cancels to all orders in perturbation theory with long-distance contributions to the heavy quark pole mass. This leads us to define a subtraction scheme – the `potential subtraction scheme' – in which large corrections to the heavy quark potential and the `potential-subtracted' quark mass are absent. We compute the two-loop relation of the potential-subtracted quark mass to the MS quark mass. We anticipate that threshold calculations expressed in terms of the scheme introduced here exhibit improved convergence properties.

Abstract: We show that Q-balls naturally exist in the Minimal Supersymmetric Standard Model (MSSM) with soft SUSY breaking terms of the minimal N = 1 SUGRA type. These are associated with the F-and D-flat directions of the scalar potential once radiative corrections are taken into account. We consider two distinct cases, corresponding to the "HuL" (slepton) direction with L-balls and the "u(c)d(c)d(c)" and "u(c)u(c)d(c)e(c)" (squark) directions with B-balls. The L-ball always has a small charge, typically of the order of 1000, whilst the B-ball can have an arbitrarily large charge, which, when created cosmologically by the collapse of an unstable Affleck-Dine condensate, is likely to be greater than 10(14). The B-balls typically decay at temperatures less than that of the electroweak phase transition, leading to a novel version of Affleck-Dine baryogenesis, in which the B asymmetry comes from Q-ball decay rather than condensate decay. This mechanism can work even in the presence of additional L violating interactions or B-L conservation, which would rule out conventional Affleck-Dine baryogenesis.

Abstract: A set of preliminary test data, collected with large mass highly radiopure NaI(Tl) detectors, has been analysed by a maximum likelihood method to search for the WIMP annual modulation signature.