Crete High Energy Physics: Research

Members of the group are actively pursuing research in the following areas of Field Theory, String Theory and Cosmology

The Center publications since 2005 from SPIRES

Publications for 2014

Publications for 2013

Publications for 2012

Publications for 2011

Publications for 2010

Publications for 2009

Publications for 2008

Publications for 2007

Publications for 2006

Publications for 2005



: Cosmological measurements have become increasingly accurate in recent years and provide stringent constraints on theories of cosmological evolution. The focus is on the construction of such theories that are valid below the Planck scale, are minimal in the sense of the dynamical degrees of freedom needed to achieve realistic evolution, and are free of fine tunings.

With this in mind, a systematic analysis of gravitation on the quantum infrared level has been done for inflationary spacetime backgrounds. Topics under current study, within the framework of these theories, are: post-inflationary evolution, primordial density perturbations, coupling to matter, etc.

Some relevant articles are:

[1] WMAP Three Year Results: Implications for Cosmology

[2] Lectures on the Theory of Cosmological Perturbations

[3] The Quantum Gravitational Back-Reaction On Inflation

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: String theory has provided several new paradigms for fundamental physics. Some, like extra dimensions of spacetime and the concept of branes existed before, but obtained a new impetus and necessity in the context of the theory. Other effects as the geometrization of the gauge interactions via D-branes, were novel and led to ground-breaking new concepts. These include the counting of black-hole microstates explaining the semiclassical Bekenstein-Hawking entropy, the emerging concept of holography and the bulk-boundary correspondence as exemplified by the AdS/CFT correspondence and its generalizations. Several of the directions of current research here involve directly or indirectly such concepts, and they will be detailed further in the appropriate places.

Some simple explanations, textbooks and more advanced material can be found at this address.

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: If our universe is localized on a p-brane as might be the case in string theory, cosmology both at early and late times can be very different from the standard cosmology. There are several frameworks for studying the associated problems. They include branes in almost flat bulk space-times, or in the opposite case in strongly curved bulk space-times. A well known example in the second case is the Randall-Sundrum context. Branes may back-react on the geometry of the bulk or they may be treated as "light" probes. Cosmology on the branes is then analyzed with the goal of providing new contexts and mechanisms for important cosmological problems as the causality and flatness problems, as well as the origin of structure in the universe and the nature of dark matter.

A set of recent reviews are:

[1] Brane cosmology: An Introduction.

[2] Lectures on string/brane cosmology.

[3] D-branes in standard model building, gravity and cosmology.

[4] Brane-world gravity

[5] String Cosmology: A Review.

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One of the most surprising observational clues we have obtained from cosmology in the last ten years is that the universe is currently accelerating. This in turn translates into the fact that about 70% of the resent energy of the universe is of unknown origin, behaving more or less as a cosmological constant. A cosmological constant so small is at odds with our present understanding of Quantum Field Theory and/or string theory (although solutions of the type of the string theory landscape are also considered).

Another approach involves a modification of gravity at large (cosmological) scales. The simplest such modification is a graviton mass, and more sophisticated modifications have been studied. A class of such modifications use higher dimensions and involve brane induced gravity. It seems that there is a general difficulty with changing gravity in the IR showing up as a non-linear instability (ghosts), strong couplings problems, and fine tuning. Current research is focusing in finding out if this is an omnipresent feature, or if it can be evaded and at what cost. Some recent reviews include

[1] Dynamics of dark energy.

[2] Dark Energy and Dark Gravity

[3] Infrared-modified gravities and massive gravitons.

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STRING THEORY AND ITS IMPLICATIONS FOR BEYOND THE STANDARD MODEL PHYSICS

String theory contains promising features to serve as a unifying theory of all forces. Apart from the fact that it always includes quantum gravity it also includes gauge and Yukawa interactions, known to describe the other three known forces (electroweak and strong). Efforts to find vacua in string theory that at low energy resemble the Standard Model (SM) of particle interactions go back to the mid eighties. Although a lot of progress has been done since then the problem remains open to this day. One of the lessons learned in this direction is that our technical control and understanding of string theory is still in its infancy, and we are still at odds with several deeper question on the theory.

Heterotic string theory, was the first to be analyzed in the detail, searching for a SM vacuum, due to its promising gauge group in ten dimensions and the fact that it was a theory of closed oriented strings, something that made its consistency conditions simpler to implement. Several vacua were found that were reasonable close in structure to the SM (gauge group, low energy interactions and a reasonable structure of mass spectra). The typical problem with heterotic vacua is that they would need a strong string coupling to accommodate observables like gauge coupling unification or supersymmetry breaking.

In the mid '90s, evidence surfaced that different-looking string theories are probably related by non-perturbative dualities. This motivated the analysis of open string theories, and their non-perturbative incarnations as heterotic M-theory and F-theory. Orientifold vacua and open strings have the advantage that a bottom-up construction of the SM seems more suited, giving the hope that the SM search might be simpler than in the heterotic case.

Although the problem has not been solved yet, a large a effort is in place in order to study various aspects of the task. These include:

1. The construction of distinct classes of orientifold vacua.

2. The algorithmization of such constructions in order to perform large scale computer searches.

3. The study of supersymmetry breaking and moduli stabilization with a combination of fluxes and non-perturbative effects.

4. The study of D-instanton effects and their contributions to the respective low energy effective actions.

5. The study of the geometry of compactification manifolds and their bundles as well as the stability conditions for D-branes, populating the associated vacua.

6. The study of the space of vacua of the theory that seems enormous, termed "the landscape", and its implications for physics.

7. The study of generic signatures at LHC of classes of string vacua. The most interesting case seems to be that of anomalous Z' gauge bosons.

The Center is part of an international effort, the "String Vacuum Project" whose goal is to bring together techniques from physics, mathematics and computer science in order to tackle the mysteries of the string theory vacua and their relevance for the real world.

Recent reviews on the subjects above are

[0] D-brane primer.

[1] Lectures on heterotic M-theory.

[2] D-branes in standard model building, gravity and cosmology.

[3] Toward realistic intersecting D-brane models.

[4] Orientifolds, hypercharge embeddings and the Standard Model.

[5] Four-dimensional String Compactifications with D-Branes, Orientifolds and Fluxes

[6] Physics of String Flux Compactifications

[7] Overlooking the String Theory Landscape.

[8] D-brane Instantons in Type II String Theory.

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AdS-CFT CORRESPONDENCE AND APPLICATIONS TO THE PHYSICS OF QCD

The AdS/CFT correspondence emerged in string theory during the effort of counting black-hole microstates. It lead to a revolutionary view of the large-N limit in gauge theories. It also provided a novel look at non-perturbative aspects of string theory.

At its simplest and best understood, the AdS/CFT correspondence asserts the equivalence of 4d N=4 supersymmetric Yang Mills theory on one hand and 10d IIB string theory living on AdS5xS5. At strong 't Hooft coupling the gauge theory description is intractable, while the string theory description can be approximated by two-derivative effective supergravity. This has lead to new insights into the physics of strongly coupled gauge theories. The correspondence was generalized in many directions and its is now called the bulk-boundary correspondence, or the holographic correspondence. Non-conformal theories can be involved as well as theories in other dimensions.

An important effort was channeled towards finding the string theory dual of real world QCD. Phenomena that are common in QCD like confinement, a discrete spectrum of glueballs and a mass gap could be described in several string theories that are dual to gauge theories that are confining in the IR. Similarly chiral symmetry breaking and low energy meson dynamics could also be described in related gauge theory duals. However, real world QCD has been so far a challenge, due to the fact that the coupling becomes strong in the UV, and their the geometry becomes stringy. And stringy geometries with RR backgrounds we still have trouble controlling.

Around the same time, experimental efforts at the RHIC Collider have put forward the first convincing evidence that the (thermalized) deconfinement phase of QCD was observed. Together came the realization that this state of matter near the phase transition is strongly coupled, and perturbative approaches cannot describe well its properties. Non-perturbative lattice calculations, although well suited for static properties, are not very useful to calculate dynamical properties including transport coefficients like viscosity.

The viscosity "measured" at RHIC is very close to a conjectured lower bound, found in AdS duals, a fact that sparked interest in AdS/CFT type techniques.

The experimental effort is on-going and will be continued in LHC, primarily with the ALICE experiment but also the others. Using holographic techniques to calculate in QCD both at zero and finite temperature is still un open problem.

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AdS-CFT CORRESPONDENCE AND APPLICATIONS TO CONDENSED MATTER PROBLEMS

The AdS/CFT correspondence provides a collection of string/gravitational theories living on asymptotically AdS spaces whose gauge theory duals should correspond to strongly coupled large-N theories. As such it provides a rather tractable description of conformal theories even in dimensions (like 6) where we know no such (weakly coupled) examples at all.

The particular case of three dimensions (dual to AdS4 vacua) seems well suited to describe quantum systems with essentially two-dimensional physics or three dimensional statistical systems. The fact that such theories are at strong coupling explains why the thermodynamics obtained is not always similar to other critical theories. Moreover, in three dimensions there are essentially two alternatives when it comes to scale-invariant theories.

This is the reason why AdS systems have been used to model condensed matter systems of various sorts, ranging from systems with High-Tc superconductivity behavior to strongly interacting cold fermion gases, to systems with strong disorder (spin glasses).

There are no reviews on this subject as it is very young, but some representative papers are given below.

[1] Toward an AdS/cold atoms correspondence: A Geometric realization of the Schrodinger symmetry.

[2] Gravity duals for non-relativistic CFTs.

[3] Disordered Systems and the Replica Method in AdS/CFT.

[4] Gravity Duals of Lifshitz-like Fixed Points.

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HIGHER-SPIN THEORIES AND TENSIONLESS STRINGS

Constructing consistent interactions for higher-spin (HS) gauge fields is an old and notoriously difficult problem. HS gauge fields are believed to be relevant for the UV completion for string theory. However, a surprising development took place some years ago by Fradkin and Vasiliev who have shown the consistency of HS interactions in (A)dS spaces. Since then, the study of HS gauge theories has enjoyed a remarkable renaissance and a wealth of new and interesting results have appeared. See [1] for some recent review articles. One of the approaches to the interaction of HS gauge fields is based on the BRST-like methods. This is particularly appealing as it resembles similar constructions in string field theory. However in the general case of interacting massless HS fields there is no analog of overlap conditions such as that is present in the Open String Field Theory and therefore one has to consider a general polynomial of the corresponding matter and ghost oscillators.

Local research efforts focused in the recent past on finding the general cubic coupling of any three HS gauge fields in flat and AdS spaces. In [2] a systematic method based on the triplet construction was presented. In [3] the method above was applied to the simplest case; the interaction between one HS triplet and two massive free scalars. Despite its apparent simplicity this is still a highly nontrivial case since it requires the construction of an infinite number of conserved currents, made out of scalars, that couple properly to HS gauge fields. This is also an important case since it elucidates the emergence of HS gauge fields via the gauging of higher derivative symmetries of free matter Lagrangians.

Current research focuses on the holographic consequences of HS interactions via the AdS/CFT correspondence.

[1] M. A. Vasiliev, Fortsch. Phys. 52, 702 (2004) [arXiv:hep-th/0401177].
X. Bekaert, S. Cnockaert, C. Iazeolla and M. A. Vasiliev, [arXiv:hep-th/0503128].
D. Sorokin, AIP Conf. Proc. 767, 172 (2005) [arXiv:hep-th/0405069].
N. Bouatta, G. Compere and A. Sagnotti, [arXiv:hep-th/0409068].

[2] I. L. Buchbinder, A. Fotopoulos, A. C. Petkou and M. Tsulaia, Phys. Rev. D 74 (2006) 105018 [arXiv:hep-th/0609082].

[3] A. Fotopoulos, N. Irges, A. C. Petkou and M. Tsulaia, Higher-Spin Gauge Fields Interacting with Scalars: The Lagrangian Cubic Vertex, JHEP 0710 (2007) 021 [arXiv:0708.1399 [hep-th]].

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