String Physics Research Articles

The LHC string Hunter's companion

The mass scale of fundamental strings can be as low as few TeV / c 2 provided that spacetime extends into large extra dimensions. We discuss the phenomenological aspects of weakly coupled low mass string theory related to experimental searches for physics beyond the Standard Model at the Large Hadron Collider (LHC). We consider the extensions of the Standard Model based on open strings ending on D-branes, with gauge bosons due to strings attached to stacks of D-branes and chiral matter due to strings stretching between intersecting D-branes. We focus on the model-independent, universal features of low mass string theory. We compute, collect and tabulate the full-fledged string amplitudes describing all 2 → 2 parton scattering subprocesses at the leading order of string perturbation theory. We cast our results in a form suitable for the implementation of stringy partonic cross sections in the LHC data analysis. The amplitudes involving four gluons as well as those with two gluons plus two quarks do not depend on the compactification details and are completely model-independent. They exhibit resonant behavior at the parton center of mass energies equal to the masses of Regge resonances. The existence of these resonances is the primary signal of string physics and should be easy to detect. On the other hand, the four-fermion processes like quark–antiquark scattering include also the exchanges of heavy Kaluza–Klein and winding states, whose details depend on the form of internal geometry. They could be used as “precision tests” in order to distinguish between various compactification scenarios.

The Langlands program and string modular K3 surfaces

A number theoretic approach to string compactification is developed for Calabi–Yau hypersurfaces in arbitrary dimensions. The motivic strategy involved is illustrated by showing that the Hecke eigenforms derived from Galois group orbits of the holomorphic two-form of a particular type of K3 surface can be expressed in terms of modular forms constructed from the worldsheet theory. The process of deriving string physics from spacetime geometry can be reversed, allowing the construction of K3 surface geometry from the string characters of the partition function. A general argument for K3 modularity is given by combining mirror symmetry with the proof of the Shimura–Taniyama conjecture.

Pre-inflationary spacetime in string cosmology

Seiberg and Witten have shown that the non-perturbative stability of string physics on conformally compactified spacetimes is related to the behaviour of the areas and volumes of certain branes as the brane is moved towards infinity. If, as is particularly natural in quantum cosmology, the spatial sections of an accelerating cosmological model are flat and compact, then the spacetime is on the brink of disaster: it turns out that the version of inflationary spacetime geometry with toral spatial sections is marginally stable in the Seiberg–Witten sense. The question is whether the system remains stable before and after inflation, when the spacetime geometry is distorted away from the inflationary form but still has flat spatial sections. We show that it is indeed possible to avoid disaster, but that requiring stability at all times imposes non-trivial conditions on the spacetime geometry of the early universe in string cosmology. This in turn allows us to suggest a candidate for the structure which, in the earliest universe, forbids cosmological singularities.

Vacuum geometry and the search for new physics

We propose a new guiding principle for phenomenology: special geometry in the vacuum space. New algorithmic methods which efficiently compute geometric properties of the vacuum space of N=1 supersymmetric gauge theories are described. We illustrate the technique on subsectors of the MSSM. The fragility of geometric structure that we find in the moduli space motivates phenomenologically realistic deformations of the superpotential, while arguing against others. Special geometry in the vacuum may therefore signal the presence of string physics underlying the low-energy effective theory.

Detection and imaging of atmospheric radio flashes from cosmic ray air showers

The nature of ultrahigh-energy cosmic rays (UHECRs) at energies >10(20) eV remains a mystery. They are likely to be of extragalactic origin, but should be absorbed within approximately 50 Mpc through interactions with the cosmic microwave background. As there are no sufficiently powerful accelerators within this distance from the Galaxy, explanations for UHECRs range from unusual astrophysical sources to exotic string physics. Also unclear is whether UHECRs consist of protons, heavy nuclei, neutrinos or gamma-rays. To resolve these questions, larger detectors with higher duty cycles and which combine multiple detection techniques are needed. Radio emission from UHECRs, on the other hand, is unaffected by attenuation, has a high duty cycle, gives calorimetric measurements and provides high directional accuracy. Here we report the detection of radio flashes from cosmic-ray air showers using low-cost digital radio receivers. We show that the radiation can be understood in terms of the geosynchrotron effect. Our results show that it should be possible to determine the nature and composition of UHECRs with combined radio and particle detectors, and to detect the ultrahigh-energy neutrinos expected from flavour mixing.

Cosmic superstrings II

Modern string theory provides a rich array of objects which could potentially appear as cosmic strings in our observable four-dimensional universe. However, there are many possible decay modes which would prevent the appearance of such strings. We investigate the conditions under which metastable strings can exist, and we find that such strings are present in many models. Hence cosmic strings give a potentially large window into string physics. To cite this article: E.J. Copeland et al., C. R. Physique 5 (2004).

Cosmic F- and D-strings

Macroscopic fundamental and Dirichlet strings have several potential instabilities: breakage, tachyon decays, and confinement by axion domain walls. We investigate the conditions under which metastable strings can exist, and we find that such strings are present in many models. There are various possibilities, the most notable being a network of (p,q) strings. Cosmic strings give a potentially large window into string physics.

Application of logistic analysis to the history of physics

Recently, two analyses have tried to put technological progress in a larger context. One interpretation hypothesizes that technological progress is likely to continue at increasingly higher rates of change. Another interpretation, which includes data from the beginning of the universe to the present, suggests that the universe is approaching a transition point in a logistic development of complexity. This logistic development is similar to the way ideas or products diffuse in a population, i.e., the rate of discovery in a field of knowledge is proportional to the amount discovered and the amount to be discovered. To test a part of this hypothesis, a leading indicator field (fundamental physics) was identified and the events in the history of this field were analyzed. Twelve subfields were identified and grouped into six stages. Each stage seemed to demonstrate a logistic-like development. By analyzing both the median time of development and the characteristic time of development of these stages, the overall development of this one field was found to suggest logistic development. These data seem to indicate that development in fundamental physics is slowing down, with at least one subfield beyond string physics yet to be developed. The data tend to support the hypothesis that a knowledge field can develop logistically.

Quantization of the electromagnetic field outside static black holes and its application to low-energy phenomena

We discuss the Gupta-Bleuler quantization of the free electromagnetic field outside static black holes in the Boulware vacuum. We use a gauge which reduces to the Feynman gauge in Minkowski spacetime. We also discuss its relation with gauges used previously. Then we apply the low-energy sector of this field theory to investigate some low-energy phenomena. First, we discuss the response rate of a static charge outside the Schwarzschild black hole in four dimensions. Next, motivated by string physics, we compute the absorption cross sections of low-energy plane waves for the Schwarzschild and extreme Reissner-Nordstrom black holes in arbitrary dimensions higher than three.

New dimensions at a millimeter to a fermi and superstrings at a TeV

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.

K3 fibrations and softly broken N = 4 supersymmetric gauge theories

Global geometry of K3 fibration Calabi-Yau threefolds, with Hodge number h 2,1 = r + 1, is used to define N = 4 softly broken SU( r + 1) gauge theories, with the bare coupling constant given by the dual heterotic dilaton, and the mass of the adjoint hypermultiplet given by the heterotic string tension. The U( r + 1) Donagi-Witten integrable model is also derived from the K3 fibration structure, with the extra U(1) associated to the heterotic dilaton. The case of the SU(2) gauge group is analyzed in detail. String physics beyond the heterotic point particle limit is partially described by the N = 4 softly broken theory.

String physics and black holes

In these lectures we review the quantum physics of large Schwarzschild black holes. Hawking's information paradox, the theory of the stretched horizon and the principle of black hole complementarity are covered. We then discuss how the ideas of black hole complementarity may be realized in string theory. Finally, arguments are given that the world may be a hologram. Lectures delivered at ICTP Spring School on String Theory, Gauge Theory, and Quantum Gravity, 1995.

PHYSICS AT THE PLANCK LENGTH

Results of the conference, “String Quantum Gravity and Physics at the Planck Energy Scale,” organized by Dr. Norma Sanchez, in Erice, Sicily, June 21–28, 1992 will appear in Proceedings of the International Workshop of Theoretical Physics, 6th Session, World Scientific Publishing Co. Pte. Ltd., Singapore, 1993. The conference has given new occasion to ask what linkage, if any, ties together particle physics and the quantum theory of gravitation, where this fantastically short distance first displayed its relevance.

A SUPERSTRING MODEL WITH R SYMMETRIES AND LOW B−L BREAKING

The phenomenology of a three-generation superstring model arising from the compactification of the ten-dimensional heterotic superstring theory on the Tian-Yau manifold with a discrete Z2×Z3 symmetry is discussed. The Z2 is used to define a matter parity and the Z3 generates an R symmetry. Presence of the discrete R symmetry enforces the existence of exactly flat directions in the superpotential that motivate us to break the SU(3) 3 to SU(3) × SU(2) L× SU(2) R× U(1) B−L at a scale M X closely associated with the compactification scale MC~1018 GeV , since the R symmetry and B−L breaking need to arise from unknown string physics and for the successful suppression of proton decay rate due to certain dimension-five operators. When the R symmetry is broken at a scale MR~1014 GeV , the matter parity is broken spontaneously as well, guiding us to the scale of the breaking to the standard model, MB−L~103 GeV , making the additional Z boson accessible at accelerator energies. When the spectrum of particles is derived from the mass matrices, the model emerges as the first model with known particles having nonstandard Z2 matter-parity assignments. The transformation properties of the standard model fermions under the discrete symmetries of the theory give rise to an extremely interesting pattern for their mass generation. The model has acceptable properties vis à vis proton decay lifetime considerations and renormalization group considerations. This is a superstring model where the lightest supersymmetric particle, favored to be the photino, decays with a cosmologically interesting lifetime.

Nonperturbative many-body physics in the open bosonic string.

We use covariant string field theory to explore many-body string physics. A candidate nonperturbative vacuum is identified via a level-truncation scheme. The breaking of internal and Lorentz symmetries is investigated. We show that tree-level string physics exhibits similarities to loop-level particle physics. The effective string coupling runs at tree level, inducing asymptotic freedom. This is a feature of any string theory. The running coupling has significant effects on the spectrum in the nonperturbative vacuum, including the disappearance of states.

Conformal invariance of σ-models and classical string physics

We show that the identification of the conformal anomaly of the general bosonic two-dimensional non-linear σ-model as the generating functional for on-shell string scattering amplitudes is correct up toO(α′) terms. The absence, in the loop corrections to the spacetime effective action, of contributions from the explicit coupling to the dilaton field is suggested as a general feature for σ-models describing tree-level string physics.

Strings at superplanckian energies: in search of the string symmetry

The characteristic energy scale of superstring theory, which attempts to unify all the interactions of matter with gravity, is the Planck energy of 10 28 eV. Although this energy is 16 orders of magnitude higher than currently accessible energies, it is important to consider the nature of string physics in this region since it could shed light on the non-perturbative physics at the Planck scale, which determines the structure of the vacuum. In this paper I review some recent attempts to explore this domain. In particular, I discuss string scattering at very high energies, the indications of the existence of a large symmetry that is restored at short distances and the possible breakdown of our concepts of space-time at these energies.

Off-shell string physics from conformal field theory

The subject of off-shell conformal field theory is developed and is used to produce the off-shell amplitudes of string field theory. The method is also applied to the reparametrization ghosts.

Vertex operators in background fields

We show that vertex operators for strings propagating in background fields are composite operators of anomalous dimension two in a two-dimensional field theory defined on a curved world-sheet. We show how special features of renormalization theory in curved spacetime are responsible for a number of peculiar features of string physics, including the need for apparently unphysical fields to maintain gauge invariance in string field theory.

Modular forms and two-loop string physics

The two-loop path integral for the closed bosonic string is given by a genus-two modular form: the product of the even theta functions. Thus the two-loop vacuum amplitude, scattering amplitudes, and the one-loop free energy may be expressed simply in terms of theta functions.