HIP Theory Project: Laws of Nature and Condensed Particle Matter @ LHC

Detailed research topics:

1. Beyond standard model phenomenology

• Supersymmetry: It is widely believed that a new symmetry linking bosons and fermions, so-called supersymmetry exists.  In order to solve the problems in the Standard Model Higgs sector, the particles predicted by the supersymmetric models should not be heavier than of the order of 1 TeV. Supersymmetry is also an integral element in attempts to unify the theories of particle physics, both if one considers the unification of Standard Model gauge couplings and if one wishes to unify all the forces of Nature, including gravity, as an essential ingredient of superstring models. Of prime importance in supersymmetric theories is the R-parity violation and its phenomenological consequnces. Many supersymmetric models either allow explicit R-parity breaking or it is spontaneously broken in the model.  The phenomenology may be quite different in R-parity preserving models.  A major motivation for future accelerators is supersymmetry.  Therefore, it is important to understand the phenomenological differences in the models. Another important topic in the supersymmetric theories is the method of supersymmetry breaking.  Lately an interesting possibility has emerged, namely te breaking via gauge mediation.  The advantage of this method is that flavour changing interactions are naturally suppressed.  Viable models and their consequences at colliders are examined.
• Electroweak symmetry breaking:  One of the main objectives for future experiments is to study the electroweak symmetry breaking.  In the Standard Model, a Higgs boson
  emerges due to the breaking of gauge symmetry.  The mass of the Higgs is one of the most important parameters in view of the experiments. It is of great interest that the effects of symmetry breaking should become available at the latest in the TeV energy range, which is exactly where the experiments now under construction or planning are supposed to function.  If found, the Higgs boson will be the first particle of its kind ever detected.  Properties of Higgs boson in view of the current and coming experiments are studied. Since there is no direct evidence for the symmetry breaking through a Higgs boson, one has to consider the possibility that some other mechanism is responsible for the symmetry breaking.  This is being studied by using effective theories, e.g. at LEP the anomalous couplings are utilised.
• Extended gauge models: It may well be that the minimal supersymmetric standard model (MSSM) is in real world extended.  For instance, the arbitrariness of the R-parity in MSSM is hinting towards a more fundamental theory.  It would be advantageous to have a natural explanation for R-parity. One such explanation is that B-L is gauged, in which case Lagrangian
  automatically conserves R-parity.  This way one is lead to consider the left-right symmetric models, based on the gauge group SU(2)_L x SU(2)_R x U(1)_(B-L).  Left-right models and their supersymmetric versions have also many other motivations, e.g. in these models a light neutrino appears naturally  through the so-called see-saw mechanism.
• Technicolor: We have introduced minimal walking technicolor (MWTC) theories, and shown that these models provide viable candidate theories for dynamical electroweak symmetry breaking. They have shown to be compatible with current precision data, and hence will be put to a further test at LHC. Despite the minimal particle content, only two techniflavors together with a new generation of leptons to cancel the Witten anomaly associated with weak interactions, these models provide very rich low energy phenomenology. The spectrum of technihadrons has been worked out and progress is being made to cartograph their collider signatures. These models also provide candidates for weakly interacting dark matter particles (WIMP).

2. Strong interactions and pQCD

   • Phenomenology of URHIC: Relativistic hydrodynamics provides an appealing method to study the evolution of a locally thermalized expanding system and phenomena like electromagnetic emission during the expansion stage. Strong features in its favour are the implementation of conservation laws and the QCD phase transition through the equation of state. In addition to the hydrodynamic modelling itself, one of our pioneering specialities is the calculation of the QGP initial densities in a (nearly) closed framework of pQCD minijet production and gluon saturation. In fact, we have been among the few groups in the position of making hydrodynamic predictions for future colliders. Based on the successful tests at RHIC, we have predicted the bulk hadron pT-spectra, multiplicities and net-proton number in central Pb+Pb collisions at the LHC, and the amount of elliptic flow in noncentral collisions. These results were presented e.g. in the CERN workshop Last Call for LHC Predictions in 5/07. The latest improvements here are a dynamical decoupling procedure, based on comparing the rates of QCD-matter expansion and pion scattering, and the study of the correlation of final flow and the yet uncertain initial energy density profile. The next step will be viscous hydrodynamics.
   • Phases and properties of strongly interacting matter: We have studied effective theories for QCD at finite temperature and density to obtain description of QCD thermodynamics at temperature regions relevant for URHIC. Our results, to be published soon, demonstrate how phenomenological effective theories can be constrained by lattice data, vacuum properties and analytic results from perturbation theory at asymptotically high temperatures. We are in progress to study explicit chiral symmetry breaking in these theories in order to understand the corresponding features in the lattice data.
   • pQCD dynamics: Phenomenology of QGP and URHIC requires a good knowledge of reference cross sections of inclusive hard processes, such as direct photon and high-pT hadron production. These can be computed through the QCD factorization theorem, provided that nuclear parton distribution functions (nPDFs) are available. Previously, we have pioneered a global analysis, where the QCD-evolving nPDFs are extracted from nuclear hard-process data and conservation laws. Our EKS98 parameterization is in a worldwide use and a standard reference in the field. We have now, after extensive further work, published a statistical error analysis for the obtained nPDFs. For the very first time in such global analyses, we have also been able to include constraints given by the RHIC high-pT hadron data from d+Au collisions, and obtain much better controlled gluon distributions than before. These results are to be published soon. Further data constraints from direct photon and heavy quark production at RHIC, as well as the extension to next-to-leading order pQCD, will be considered in the future.