Version Of String Research Articles

Emergent magnetic monopoles, disorder, and avalanches in artificial kagome spin ice (invited)

We study artificial spin ice with isolated elongated nanoscale islands arranged in a kagome lattice and solely interacting via long range dipolar fields. The artificial kagome spin ice displays a phenomenology similar to the microscopic pyrochlore system, where excitations at sub-Kelvin temperatures consist of emergent monopole quasiparticles that are connected via a solenoidal flux line, a classical and observable version of the Dirac string. We show that magnetization reversal in kagome spin ice is fundamentally different from the nucleation and extensive domain growth scenario expected for a generic 2D system. Here, the magnetization reverses in a strictly 1D fashion: After nucleation, a monopole-antimonopole dissociates along a 1D path, leaving a (Dirac) string of islands with reversed magnetization in its wake. Since the 2D artificial spin ice spontaneously decays into a 1D subsystem, magnetization reversal in kagome spin ice provides an example of dimensional reduction via frustration.

A Note on q-Deformed Two-Dimensional Yang–Mills and Open Topological Strings

In this note we make a test of the open topological string version of the OSV conjecture in the toric Calahi–Yau manifold X = O(–3) → P2 with background D4-hranes wrapped on Lagrangian suhmanifolds. The D-brane partition function reduces to an expectation value of some inserted operators of a q-deformed Yang–Mills theory living on a chain of P1 's in the base P2 of X. At large N this partition function can be written as a sum over squares of chiral blocks, which are related to the open topological string amplitudes in the local P2 geometry with branes at both the outer and inner edges of the toric diagram. This is in agreement with the conjecture.

Mozart K.448 and epileptiform discharges: Effect of ratio of lower to higher harmonics

Certain music has been shown to improve mental function, leading to what is known as the Mozart effect. This study measured the impact of Mozart's Sonata for two pianos in D major, K.448, on different epileptic foci of epileptiform discharge in Taiwanese children (n=58) with seizure disorders and investigated the characteristics of the musical stimulus presented that resulted in epileptiform discharge reduction. We examined the relationship between the number of discharges with the foci of epileptiform discharge (n=6), sleep state, gender, and mentality. A continuous electroencephalogram was recorded before, during and after exposure to Mozart's Sonata for two pianos in D major, K.448 (piano K.448), and the frequencies of discharges were compared. The study was repeated a week later using digitally computerized string version of the same musical stimulus (string K.448), in patients who responded to piano K.448 with the largest reduction in interictal discharges (n=11). Interictal discharges were reduced in most (81.0%) patients and varied greatly (33.10+/-28.33%) as they listened to the piano K.448 (more fundamental tones and lower harmonics). Patients with generalized or central discharge showed the most improvement. In most patients (76.1%), the decrease in epileptiform discharges continued after the music ended. The state of wakefulness, gender and mentality did not affect the results. Although the string K.448 had a larger number of higher harmonics in the spectrogram analysis, the discharges were not reduced at all when listening to this music. These results suggest that listening to Mozart K.448 for two pianos reduced epileptiform discharges in children with epilepsy. This study suggests that it is possible to reduce the number of epileptiform discharges in some patients by optimizing the fundamental tones and minimizing the higher frequency harmonics.

The Baikal Experiment – from Megaton to Gigaton

We review the status of the Lake Baikal Neutrino Experiment. Preparation towards a km3-scale Gigaton Volume Detector (GVD) in Lake Baikal is currently a central activity. As an important milestone, a km3-prototype string comprising 6 optical modules and based on a completely new technology, has been installed and was put into operation together with NT200+ in April, 2008. An upgraded version of the prototype string, which comprises 12 optical modules, was put into operation in April 2009. We also present new results from the long-term operation of NT200, including an improved limit on the diffuse astrophysical neutrino flux.

D-branes and doubled geometry

We define the open string version of the nonlinear sigma model on doubled geometry introduced by Hull and Reid-Edwards, and derive its boundary conditions. These conditions include the restriction of D-branes to maximally isotropic submanifolds as well as a compatibility condition with the Lie algebra structure on the doubled space. We demonstrate a systematic method to derive and classify D-branes from the boundary conditions, in terms of embeddings both in the doubled geometry and in the physical target space. We apply it to the doubled three-torus with constant H-flux and find D0-, D1-, and D2-branes, which we verify transform consistently under T-dualities mapping the system to f-, Q- and R-flux backgrounds.

Conformal supergravity tree amplitudes from open twistor string theory

We display the vertex operators for all states in the conformal supergravity sector of the twistor string, as outlined by Berkovits and Witten. These include ‘dipole’ states, which are pairs of supergravitons that do not diagonalize the translation generators. We use canonical quantization of the open string version of Berkovits, and compute N-point tree level scattering amplitudes for gravitons, gluons and scalars. We reproduce the Berkovits–Witten formula for maximal helicity violating (MHV) amplitudes (which they derived using path integrals), and extend their results to the dipole pairs. We compare these trees with those of Einstein gravity field theory.

Duration and context speech rate as cues to lexical perception and word segmentation.

Duration and speech rate are traditionally assumed to be filtered out before lexical lookup takes place, although these factors are known to influence phoneme perception. Here, the hypothesis was investigated that duration can affect both perceived lexical identity, as well as the perceived number and implied locations of word boundaries relative to the speech signal. Experiment 1 was a production study which investigated durations of vocalic portions of phonetically similar versions of target word strings which differed in their number of syllables (e.g., cease versus see us); these target word strings were spoken in semantically neutral context sentences. As expected, vocalic durations in target strings with fewer syllables were shorter than those with more syllables. In Experiment 2, the relative durations of vocalic portions of target strings in sentences from Experiment 1, as well as sentential context speech rate, were manipulated using speech resynthesis. Relative duration and context speech rate both affected the words that participants heard, as well as the implied number of phonemes and imputed locations of word boundaries. These findings indicate that duration plays a significant and underinvestigated role in spoken word recognition and word segmentation.

String/flux tube duality on the light cone

The equivalence of quantum field theory and string theory as exemplified by the AdS/CFT correspondence is explored from the point of view of light cone quantization. On the string side we discuss the light cone version of the static string connecting a heavy external quark source to a heavy external antiquark source, together with small oscillations about the static string configuration. On the field theory side we analyze the weak/strong coupling transition in a ladder diagram model of the quark-antiquark system, also from the point of view of the light cone. Our results are completely consistent with those obtained by more standard covariant methods in the limit of infinitely massive quarks.

Comments on MHV tree amplitudes for conformal supergravitons topological b-model

We use the twistor-string theory on the B-model of CP^{3|4} to compute the maximally helicity violating(MHV) tree amplitudes for conformal supergravitons. The correlator of a bilinear in the affine Kac-Moody current(Sugawara stress-energy tensor) can generate these amplitudes. We compare with previous results from open string version of twistor-string theory. We also compute the MHV tree amplitudes for both gravitons and gluons from the correlators between stress-energy tensor and current.

Random fractal strings: Their zeta functions, complex dimensions and spectral asymptotics

In this paper a string is a sequence of positive non-increasing real numbers which sums to one. For our purposes a fractal string is a string formed from the lengths of removed sub-intervals created by a recursive decomposition of the unit interval. By using the so-called complex dimensions of the string, the poles of an associated zeta function, it is possible to obtain detailed information about the behaviour of the asymptotic properties of the string. We consider random versions of fractal strings. We show that by using a random recursive self-similar construction, it is possible to obtain similar results to those for deterministic self-similar strings. In the case of strings generated by the excursions of stable subordinators, we show that the complex dimensions can only lie on the real line. The results allow us to discuss the geometric and spectral asymptotics of one-dimensional domains with random fractal boundary.

Large {$N$} duality for compact Calabi-Yau threefolds

We study geometric transitions for topological strings on compact Calabi-Yau hypersurfaces in toric varieties. Large N duality predicts an equivalence between topological open and closed string theories connected by an extremal transition. We develop new open string enumerative techniques and perform a high precision genus zero test of this conjecture for a certain class of toric extremal transitions. Our approach is based on a) an open string version of Gromov-Witten theory with convex obstruction bundle and b) an extension of Chern-Simons theory treating the framing as a formal variable.

Non-commutativity in a time-dependent background

We compute a time-dependent non-commutativity parameter in a model with a time-dependent background, a spacetime metric of the plane wave type supported by a Neveu–Schwarz two-form potential. This model is the open string version of the WZW model based on a non-semi-simple group previously studied by Nappi and Witten. Like its closed string counterpart, it is exactly conformally invariant to all orders in α′. We quantize the sigma-model in light-cone gauge, compute the worldsheet propagator, and use it to derive the non-commutativity parameter.

Specification search in nonlinear time-series models using the genetic algorithm

The Genetic Algorithm (GA) is used to estimate dynamic nonlinear time-series models from nonstationary data. Specification search takes place at three different levels: between competing covariates, between different dynamic specifications, and across functional forms. A variation of GA is developed that operates on strings representing functional forms. Although the dimensionality of the specification space is very large, we show that GA succeeds in estimating strings that have straightforward economic interpretations. The nonstationarity of the data gives rise to the problem of spurious fitness in strings obtained by GA. We suggest the use of stationarity tests on the residuals obtained from static versions of dynamic strings to determine whether the underlying relationship is cointegrated. We use data on “Lotto” sales in Israel to illustrate the application of GA. Finally, we compare models estimated by artificial intelligence (GA) with models estimated by conventional specification search.

The asymptotics of waiting times between stationary processes, allowing distortion

Given two independent realizations of the stationary processes $\mathbf{X} = {X_n;n \geq 1}$ and $\mathbf{Y} = {Y_n;n \geq 1}$, our main quantity of interest is the waiting time $W_n(D)$ until a D-close version of the initial string $(X_1, X_2,\dots, X_n)$ first appears as a contiguous substring in $(Y_1, Y_2, Y_3,\dots)$, where closeness is measured with respect to some "average distortion" criterion. We study the asymptotics of $W_n(D)$ for large n under various mixing conditions on X and Y. We first prove a strong approximation theorem between $\logW_n(D)$ and the logarithm of the probability of a D-ball around $(X_1, X_2,\dots, X_n)$. Using large deviations techniques, we show that this probability can, in turn, be strongly approximated by an associated random walk, and we conclude that: (i) $n^{-1} \log W_n(D)$ converges almost surely to a constant R determined byan explicit variational problem; (ii) $[\log W_n(D) - R]$, properly normalized, satisfies a central limit theorem, a law of the iterated logarithm and, more generally, an almost sure invariance principle.

Pattern recognition of strings with substitutions, insertions, deletions and generalized transpositions

We study the problem of recognizing a string Y which is the noisy version of some unknown string X chosen from a finite dictionary, H. The traditional case which has been extensively studied in the literature is the one in which Y contains substitution, insertion and deletion (SID) errors. Although some work has been done to extend the traditional set of edit operations to include the straightforward transposition of adjacent eharacters [see J. Assoc. Comput. Mach. 22, 177–183 (1975)] the problem is unsolved when the transposed characters are themselves subsequently substituted, as is typical in cursive and typewritten script, in molecular biology and in noisy chain-coded boundaries. In this paper we present the first reported solution to the analytic problem of editing one string X to another, Y, using these four edit operations. A scheme for obtaining the optimal edit operations has also been given. Both these solutions are optimal for the infinite alphabet case. Using these algorithms we present a syntactic pattern recognition scheme which corrects noisy text containing all these types of errors. The paper includes experimental results involving sub-dictionaries of the most common English words which demonstrate the superiority of our system over existing methods.

Boosting some type-D metrics

We present a general solution to the classical Einstein - Maxwell dilaton - axion equations starting from the general metric of type-D without a conformal factor. Namely, this stringy solution is the result of a transformation on a vacuum type-D solution to the Einstein equations which was studied in detail some years ago. This method also allows us to obtain the string version of the so-called C-metric.

Linking information reconciliation and privacy amplification

Information reconciliation allows two parties knowing correlated random variables, such as a noisy version of the partner's random bit string, to agree on a shared string. Privacy amplification allows two parties sharing a partially secret string about which an opponent has some partial information, to distill a shorter but almost completely secret key by communicating only over an insecure channel, as long as an upper bound on the opponent’s knowledge about the string is known. The relation between these two techniques has not been well understood. In particular, it is important to understand the effect of side-information, obtained by the opponent through an initial reconciliation step, on the size of the secret key that can be distilled safely by subsequent privacy amplification. The purpose of this paper is to provide the missing link between these techniques by presenting bounds on the reduction of the Rényi entropy of a random variable induced by side-information. We show that, except with negligible probability, each bit of side-information reduces the size of the key that can be safely distilled by at most two bits. Moreover, in the important special case of side-information and raw key data generated by many independent repetitions of a random experiment, each bit of side-information reduces the size of the secret key by only about one bit. The results have applications in unconditionally secure key agreement protocols and in quantum cryptography.

Perturbative supersymmetry breaking in orbifolds with Wilson line backgrounds

A way to break supersymmetry in perturbative superstring theory is the string version of the Scherk-Schwarz mechanism. There, the fermions and bosons have mass splitting due to different compactification boundary conditions. We consider the implementation of this mechanism in abelian orbifold compactifications with Wilson line backgrounds. For Z N and Z N × Z M orbifolds, we give the possible U(1) R-symmetries which determine the mass splitting, and thus, the supersymmetry breaking at the perturbative level. The phenomenlogical viability of this mechanism implies some dimension(s) to be as large as the TeV scale. We explain how the lighter Kaluza-Klein states associated with the extra-dimension(s) have quantum numbers depending on the Wilson lines used.

Phase transition in lattice surface systems with gonihedric action

We prove the existence of an ordered low-temperature phase in a model of soft-self-avoiding closed random surfaces on a cubic lattice by a suitable extension of Peierls contour method. The statistical weight of each surface configuration depends only on the mean extrinsic curvature and on an interaction term arising when two surfaces touch each other along some contour. The model was introduced by F.J. Wegner and G.K. Savvidy as a lattice version of the gonihedric string, which is an action for triangulated random surfaces.

Classical dynamics of the rigid string from the Willmore functional

A new approach for investigating the classical dynamics of the relativistic string model with rigidity is proposed. It is based on the embedding of the string world surface into a space of constant curvature. It is shown that the rigid string in flat space-time is described by the Euler-Lagrange equation for the Willmore functional in a space-time of constant curvature K = − γ/(2 α), where γ and α are constants in front of the Nambu-Goto term and the curvature term in the rigid string action, respectively. For simplicity the Euclidean version of the rigid string in three-dimensional space-time is considered. The Willmore functional (the action for the “Willmore string”) is obtained by dropping the Nambu-Goto term in the Polyakov-Kleinert action for the rigid string. Such a “reduction” of the rigid string model would be useful, for example, by applying some results about the Nambu-Goto string dynamics in the de Sitter universe to the rigid string model in the Minkowski space-time. It also allows us to use numerous mathematical results about Willmore surfaces in the context of the physical problem.