Annual Report 2005

The scientific activities of the Theory Unit (PH-TH) of CERN’s Department of Physics cover a wide spectrum of research, reaching from phenomenological topics related to the physics potential of the LHC to formal aspects of string theory. Let us, before highlighting some of the achievements that were obtained in 2005, give a brief statistical overview of the publications. In 2005, 269 papers appeared with a registered CERN-PH-TH number, whereas in total 303 publications submitted to the e-print archives had at least one author with a CERN affiliation. The distribution over the individual archives is very similar to that of 2004 and reads as follows: hep-ph (172), hep-th (66), astro-ph (28), hep-lat (18), gr-qc (10), nucl-th (8), and quant-ph (1). If we group these papers by their main subjects, we obtain the following statistics: Standard Model (SM) phenomenology (37%), physics beyond the SM (22%), string theory and formal aspects of field theory (22%), particle astrophysics and cosmology (13%), and field theories on the lattice (6%). In these categories, also some more specific areas of research are included. For instance, SM phenomenology contains also heavy-ion and flavour physics, while “beyond SM” includes papers with a pronounced phenomenological cut and others dealing with more field-theoretical aspects of the extensions of the SM. It should be emphasized that also several papers in the SM category are actually driven by the search for new physics, which requires a solid understanding of the SM picture.

Let us now have a closer look at the activities of the Theory Unit in 2005. With the approach of the LHC operation the focus of the collider physics community, at CERN and elsewhere, is naturally moving more and more towards preparations for its exploitation. The work in PH-TH has ranged from more accurate calculations of fundamental observables, such as the Higgs boson mass and decay rates, to proposals of new techniques for analysing the LHC data. Since the LHC is a hadron collider, every collision of interest is initiated by coloured partons. Therefore all processes suffer large corrections from QCD effects, both perturbative and non-perturbative. Many observables are important either for precision studies of the SM or as backgrounds for new processes for which the expected experimental uncertain-ties are smaller than those in QCD calculations. Thus the QCD corrections are blunting our sensitivity to any new physics that may lie hidden in these observables. The Theory Unit has played a leading role in pushing back this frontier on several fronts. For instance, calculations at the next-to-leading order in α_s were performed, including for the first time several processes involving the associated production of gauge bosons and heavy quarks, and five-parton processes including a heavy quark–antiquark pair. Moreover, new calculational techniques and tools were developed. It has recently been realized that at high momentum transfers even electroweak corrections could be important on the scale of the experi-mental uncertainties, and the Theory Unit has been active in calculating such corrections. Our long-term event generator projects have also continued, with the existing version of HERWIG effectively frozen but still receiving a high level of user support and physics exploitation, and with a new and improved generation of programs, ALPGEN and Herwig++ under active development. The Theory Unit was also involved in the third meeting of the “TeV4LHC Workshop”, in April 2005, which explores the use of the data and experience from the Tevatron for the preparation of the LHC.

This new machine will also allow the investigation of the physics of strongly interacting matter in nucleus–nucleus collisions, lying a factor of 30 above the centre-of-mass energies that can be studied at RHIC (BNL). The small PH-TH heavy-ion group has continued to work on the novel theoretical challenges entailed. One focus of the research was on “hard probes”, such as jets, open charm and beauty, and their sensitivity to the properties of the dense state produced in the collision. In particular, we pursued detailed studies on heavy-flavour production and photon production in heavy-ion collisions, as well as photon tagging of jets and medium modifications of the internal jet structure. Thus, on the one hand, the group contributed to the understanding of data from RHIC, where first measurements of the high-p_t photon production and electron production became available. On the other hand, the group devised strategies to go beyond what could be done at RHIC by exploiting the much greater kinematical coverage of the LHC; there was a continuation of the preparation of the theoretical tools for an adequate treatment of the LHC data. Moreover, a topical workshop on proton–nucleus collisions at the LHC was organized in May 2005.

At the Theory Unit, there is also a group exploring the non-perturbative regime of strong inter-actions with the help of lattice QCD, which allows, in principle, the calculation of the basic properties of the light hadrons using numerical simulations. The simulations tend to become unpractical, however, at small lattice spacings and when the quark masses are scaled towards their physical values. A new algorithm, based on classical domain-decomposition ideas, now makes such simulations significantly faster. Somewhat paradoxically, the simulations turn out to be easier the larger the lattice is, as a result of a thermodynamical effect that stabilizes the low end of the spectrum of the Dirac operator. Extensive simulations of two-flavour QCD are now in progress, partly on a dedicated 32-node PC cluster that was installed at CERN in 2005. Hadronic transition matrix elements (the kaon decay amplitudes for exam-ple) is another topic where lattice QCD can give important qualitative clues and quantitative results. In general, these calculations are complicated by operator mixings, final-state interactions, and the need for non-perturbative renormalization. Formulations of lattice QCD with special symmetry properties were explored in this context, as well as the possible use of the so-called z-regime of QCD. A relatively easy case is the kaon mixing parameter B_K , which was accurately determined, with all systematic errors under control, except for the effects of the light sea quarks.

The B_K parameter enters also the fits of the unitarity triangle of the Cabibbo–Kobayashi–Maskawa (CKM) matrix, which allows us to accommodate CP violation in the SM. This topic and quark-flavour physics, with a focus on the B system and very rare K decays, is another branch of research at PH-TH. Thanks to the B factories, CP violation is now well established in the B system and several strategies to explore this phenomenon – including methods that were developed at the Theory Unit – can now be confronted, for the first time, with experimental data. The global consistency of the CKM fits obtained within the SM is remarkable. However, also potential discrepancies are indicated by the data. For instance, the observables of B → πK decays with sizeable contributions from electroweak penguin topologies show a puzzling pattern. These effects, which could be indications of new sources of CP violation in the electroweak penguin sector, were studied in detail. A key aspect of these studies are also correlations with the observables of rare decays such as K → πνν¯ and B_s,d → µ⁺µ⁻, which show patterns that are characteristic of different new-physics scenarios. Another hot topic studied here are b → d penguin processes, which are now coming within reach of the B factories. The first signals were observed in the summer of 2005, in accordance with bounds derived at the Theory Unit. Finally, the B_s-meson system remains an important research topic, which can be fully exploited at the LHC, in particular by the LHCb experiment. The final goal of flavour studies is to find indications of new physics and to explore its properties. On the other hand, the ATLAS and CMS detectors of the LHC should give us direct evidence for physics beyond the SM. The investigation of the interplay and synergy between flavour and high-Q physics at the LHC is the subject of the workshop on “Flavour in the era of the LHC”, which was initiated at the Theory Unit, held its first meeting at CERN in November 2005, and will run until early 2007.

The flavour physics of the charged lepton and neutrino sectors is another important field of research at PH-TH. The established neutrino oscillations point to physics beyond the SM and have received a lot of attention in the particle physics community. Besides several activities at the Theory Unit, a particularly symmetric model of neutrino mixings was discussed in 2005, where, with good accuracy, the atmospheric mixing angle θ₂₃ is maximal, θ₁₃ = 0, and the solar angle satisfies sin²(θ₁₂) = 1/3. Using a suitable discrete symmetry group and a formulation with extra dimensions, a model could be constructed where such a mixing pattern is exactly obtained in a first approximation without imposing ad hoc relations among parameters. Within this context, it was also shown that the observed hierarchy of charged-lepton masses can be obtained by assuming a larger flavour symmetry; it was pointed out that, under general conditions, a maximal θ₂₃ can never arise from an exact flavour symmetry. Another detailed analysis explored the absolute neutrino mass scale, the neutrino mass spectrum, and Majorana CP violation in the lepton sector that can be obtained with the help of future neutrinoless double-beta decay experiments.

In 2005, the Theory Unit continued various studies of supersymmetry (SUSY) phenomenology. Here a key issue is now how we can exploit the data to extract the maximum information about the underlying theory. PH-TH has participated in a variety of studies of this type, addressing, for exam-ple, the question of how to distinguish the Minimal SUSY Standard Model (MSSM) from non-minimal models. A major issue is the complementarity of different types of experiment, such as high-energy searches versus precision low-energy measurements, collider physics versus cosmology, or hadron ver-sus lepton colliders, where the Theory Unit was involved in the corresponding debates. Moreover, the relation of precision electroweak observables to the underlying parameters of the theory was investigated, where PH-TH contributed through complete two-loop SUSY calculations, and a number of new search or measurement channels were proposed and analysed in Monte Carlo studies. Another topic was the production and decay of a coupled system of mixed neutral MSSM Higgs bosons in exclusive double-diffractive processes at the LHC, including non-vanishing CP phases in the soft supersymmetry-breaking gaugino masses and third-generation trilinear squark couplings. The three neutral Higgs bosons are nat-urally nearly degenerate, for large values of tan β, when the charged Higgs boson weighs around 150 GeV. Measurements of the polarization of the tau leptons coming from the Higgs-boson decays could offer a direct and observable signal of CP violation in the Higgs sector.

Concerning the more theoretical aspects of SUSY and model building, various different topics were studied at the Theory Unit in 2005. Progress could be made towards a fully calculable scenario of gravity-mediated SUSY breaking, where the mixing between the hidden and visible sectors arises from quantum corrections at the compactification scale and is dominated by the “universal” field theoretic limit of gravity. This scenario allows an interesting prediction of the amount of flavour violation, in particular for the µ → eγ rate. Another topic was SUSY dark matter, which is usually considered as one success of supersymmetry. Instead, it was argued that – after LEP constraints are taken into account – the correct prediction for the dark-matter density, at a quantitative level, is no longer a natural consequence of supersymmetry, but requires special relations among parameters, which are highly sensitive to small variations. In the general parameter space of low-energy SUSY, one of the most plausible solutions to reproduce the correct value of the dark-matter density is the well-tempered neutralino, which corresponds to the boundary between a pure Bino and a pure Higgsino or Wino. The properties of well-tempered neutralinos as well as their realization within split supersymmetry were studied. Moreover, also the neutron and electron electric dipole moments (EDMs) were analysed in this scenario, where the leading contributions arise at two loops. Interestingly, the predictions for the EDMs are typically within the sensitivities of the next generation of experiments, and a correlation between the electron and neutron EDMs was found, which would provide a robust test of split supersymmetry. Furthermore, various technical aspects of this scenario were explored and novel strategies for model building pointed out. In order to prepare for the physics interpretation of the first LHC data and their implications for the extension of the SM, the “LHC Olympics Workshop” was held at the Theory Unit in July 2005, and will be continued in 2006.

Another research area at PH-TH is particle astrophysics and cosmology. In this field, constraints on the decaying or annihilating light dark matter particles and on the shape of the Milky Way dark halo were obtained from the recent measurements of the positron annihilation line in the Galactic centre by ESA’s INTEGRAL spectrometer SPI. Moreover, also the effects of the unstable particles on the produc-tion and destruction of the primordial light elements were studied. In SUSY theories with neutralino or gravitino dark matter, constraints on the model parameters were obtained and the signatures of the axino and gravitino dark-matter candidates at the LHC and ILC were analysed. Another topic was a class of modified gravity models, where the Friedmann equation acquires an extra dark-energy component lead-ing to an accelerated expansion of the Universe. Here non-relativistic gravitational waves can comprise the cold dark matter and would be detectable by future gravitational wave searches. Furthermore, grav-itational and hydrodynamical perturbations were analysed in a relativistic plasma containing a mixture of interacting fluids characterized by non-negligible bulk viscosity coefficients. It was shown that, for typical length scales larger than the Hubble radius, the fluctuations of the bulk viscosity coefficients and of the decay rate provide source terms for the evolution of both the curvature and the entropy fluctu-ations; a detailed investigation of the scalar perturbations in a generic model for a bouncing Universe was performed. Moreover, a model for the origin of the galactic and intercluster magnetic fields was constructed, where they are produced by highly relativistic jetted ejecta from supernovae. It was also shown that the behaviour of the early X-ray afterglows of long-duration gamma-ray bursts, observed at the X-Ray telescope of the SWIFT satellite, is consistent with the predictions of the cannonball model, and robust limits on the scale of the Lorentz violation from gamma-ray bursts were obtained.

In 2005, there was also a considerable amount of work done at PH-TH in string theory and related areas such as supergravity and supersymmetric Yang–Mills theory. A major fraction of activities focused on string compactifications of various sorts, and the effective actions they produce in four dimensions. This includes the interplay of supergravity gaugings and background fluxes, in particular on twisted tori, as well as the analysis of moduli stabilization in the Kachru–Kallosh–Linde–Trivedi scenario and other models, such as brane worlds. Another field of activity in this area concerned compactifications on Einstein–Sasaki manifolds and related issues. On the more phenomenological front, there has been activities in model building and realizations of split supersymmetry, as noted above. Another important direction of research concerned string theory descriptions of supersymmetric gauge theories. One type of approach describes gauge theories in terms of geometric configurations of D-branes; this allows us to study non-perturbative dualities and interesting phenomena such as duality cascades. A useful tool for addressing questions of this kind are so-called brane tilings. On the other hand, one of the most fascinating aspects of supersymmetric gauge theories is their relation to string theories via the AdS/CFT correspondence. There has been a lot of work done in this direction at the Theory Unit in the recent years, and also in 2005. Specifically, work was done on the relation to integrable models and spin chains, and gravity duals of deformations of certain N = 4 supersymmetric theories have been studied. A third major line of research was dealing with various aspects of topological strings, which allow us to do exact computations of certain terms in the effective field theory. The activities include the counting of BPS states on certain manifolds, the study of D-branes and tachyon condensation in terms of boundary Landau–Ginzbug models, as well as supersymmetry breaking via gaugino masses. Apart from this, there has been work on a large number of quite different areas, such as string cosmology, and black holes at finite temperature. On the more formal side, there has been considerable activity in further developing the pure spinor formalism, and extending it to non-critical superstrings. Morever, a highlight of 2005 was the analytical solution of tachyon condensation in open string field theory.

Besides being a centre of excellence in the research areas described above, many other activities took place at the Theory Unit in 2005, as in previous years. An important role was again played by the PH-TH visitors programme, which now has a global character, thereby allowing the world-wide commu-nity of theorists to discuss and collaborate with the Staff and Fellows, and among one another. In 2005, the Theory Unit was again involved in the work of the Particle Data Group and in several teaching activ-ities, including the Academic Training, the Summer Student and High-School Teacher programmes, as well as the European and Latin American Schools of High-Energy Physics. The Theory Unit started also successfully a new PhD doctoral student programme within the European Commission 6th framework project EUROTHEPHY. In addition to the workshop activities described above, the Theory Unit was involved in various research programmes, including INTAS , as well as the “Integrated Large Infrastruc-tures for Astroparticle Science” and the “Quest for Unification” networks. Finally, CERN theorists were also involved in study groups for CLIC and the physics opportunities for future proton accelerators.