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Elementary Particle Physics

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Theoretical
particle physics

The research in particle theory covers a wide range of topics in quantum field theories, including phenomenology, computational field theory, string theory and non-commutative space-time.

In the mathematical physics group, amplitudes for different decay channels for unstable D-branes in string theory can be calculated using the tools of random matrix theory. We have discovered a double scaling limit where the calculations can be mapped to and solved as electrostatic problems of point charges in a background charge density in two dimensions. The same limit can then be used in other random matrix applications. We have also demonstrated how to use solitons to reveal that holographic superfluid models have features analogous to the BEC-BCS crossover in fermionic superfluids. In addition, we have investigated thermalization in a strongly coupled field theory following a quench. In holographic models at strong coupling we show that thermalization proceeds "top-down".

The quantum field theory group has continued works on noncommutative field and gauge theories, Very Special Relativity and Dirac's monopole quantization, modified Horava-Lifshitz gravity with its Hamiltonian analysis and implication on a way to accelerating FRW cosmology and unification of early-time inflation with late-time acceleration, astrophysical implications of superstring-inspired unification with shadow world, twistor formalism for chiral supermultiplets and stochastic description of particles clustering in turbulent flows by Wiener path integral.

In computational field theory group we are studying non-perturbative properties of theories for technicolor, which is one of the popular scenarios for beyond the standard model physics. The main achievement in 2010 is the derivation of an improved action, which enables significantly more accurate computations. We have also studied Electroweak baryogenesis in the early Universe and used AdS/CFT duality to study specific properties of hot quantum field theories.

In phenomenology group several studies for identifying the correct model beyond the Standard Model were conducted, mainly in the framework of supersymmetry. These investigations include sneutrino-antisneutrino mixing at LHC energies, implications of supersymmetry breaking mechanism on particle masses and decay cascades, as well as top polarization and B-meson decays. Possible structure of flavor in view of the experi-mental constraints on the electric dipole moments and tau decays was studied.

In hadron physics the focus was on understanding hadron dynamics based on Quantum Chromodynamics. A method was discovered for measuring the transverse shape of any process induced by virtual photons, which is expected to have many experimental applications. A new approach to relativistic bound states based on an expansion in Planck’s constant hbar was developed, which allows a systematic treatment of hadrons that is consistent with the laws of QCD.

Cosmology

In theoretical cosmology we have discussed the non-gaussianities of the primordial perturbation in self-interacting curvaton models and the implications of the observations of very high-mass galaxy clusters. We have also constructed a novel class of gravity theories that interpolate between the metric and Palatini formulations. The possibility for cold electroweak baryogenesis was demonstrated in a model where the phase transition is a fast quench.

The Planck satellite completed its originally planned 15-month observation program in November 2010, having covered the entire sky twice. The mission has been extended by another year, so observations continue, while we analyze the first 15 months of data. We have been responsible for producing the sky maps for the three lowest observation frequencies (30, 44, and 70 GHz), as well as a number of related tasks, including calibration, estimation of residual noise correlations on the maps, and producing large Monte Carlo simulations of the data. First scientific results dealing with astrophysics are expected to be published early 2011, but cosmological results will take longer.

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A full-sky image of the microwave sky from the first Planck full-sky survey. This image combines different frequencies so that the astrophysical foregrounds ones have been given the most weight, and therefore the cosmic microwave background, shown in red and orange is only visible far from the galactic plane. In other parts of the sky the image mainly shows emission from the gas and dust in our own galaxy. From http://www.esa.int/Planck . Credit: ESA/ LFI & HFI Consortia.

Experimental Particle Physics

The Large Hadron Collider (LHC) at CERN, Geneva, produced its first and long-awaited 7 TeV proton-proton collisions on March 30, 2010. By the end of the year over 40/pb of proton-proton data were collected, as well as lead-lead heavy-ion collisions during the last weeks of operation. The accelerator exceeded the goals set for the first year of operation, reaching an instantaneous proton-proton luminosity of 2x1032 cm-2s-1.

In 2010, the CMS scientific output consisted of 19 publications, making CMS the most productive of all the LHC experiments during the first year of operation. Researchers in the University of Helsinki and Helsinki Institute of Physics have made strong contributions to the success of the CMS experiment.

Another field of physics in which Helsinki is heavily involved in is Higgs searches. The amount of data collected in 2010 was not yet sufficient to reach sensitivity to Higgs particles, but a large fraction of the groundwork needed for the Higgs searches, such as investigation of backgrounds from the data, could be performed. Researchers in Helsinki made also many contributions to the operations of this large and complex experiment, and the Helsinki Tier-2 computing cluster has been an integrated part of the CMS world-wide computing and storage network.

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Full invariant-mass spectrum of opposite-sign muon pairs, using the whole CMS 2010 data.

 

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First ZZ->4 muon event observed in CMS.

In 2010, the forward physics project concentrated on: (1) Finalizing the CDF based analysis on exclusive gamma-gamma interactions, top quark studies in all-hadronic channel, and the Higgs analysis in WH and VBF channels at Fermilab Tevatron, and (2) running-in the Helsinki built T2-spectrometer and (3) commissioning the TOTEM experiment for the physics runs at the LHC at CERN.

The group is a leading group in the analysis of the 'all hadronic' decays of the top quark, and has introduced novel methods of background analyses for the Higgs searches. The group is developing multivariate analysis techniques together with students of artificial intelligence techniques of the Helsinki University of technology. The group has a major responsibility in the online operations of the SVX, the fine tuning of the SVX simulation software and the offline SVX calibration. By its contributions to the b-quark physics analysis of the CDF experiment the group has gained expertise in extracting the top quark and Higgs signals from the QCD backgrounds.

The TOTEM experiment at CERNs Large Hadron Collider took during 2010 at √s = 7 TeV with its Helsinki-built GEM-based T2 charged particle telescope and Roman Pot detectors its first physics grade data for upcoming measurements of elastic scattering in the 0.4 ≤ −t ≤ 5 GeV2 range and charged particle multiplicity in the 5.3 ≤ |η|≤ 6.5 range as well as a study of soft single and central diffraction.

The University of Helsinki contribution to the Compact Linear Collider (CLIC) feasibility study for a future e+ e− linear collider comprises high precision assembly and machining for its RF structures, thermo-mechanical modeling of the CLIC module as well as industrialization and cost study for its RF structure, all in close collaboration with Finnish academic and industrial partners notably VTT.

Detector Laboratory

Detector Laboratory provides premises, equipment and know-how for research projects developing silicon and gas detectors. Presently, the Laboratory hosts projects related to CERN (CMS and TOTEM), FAIR (SUPER-FRS) and EU FP7 (Electrical Solar Wind Sail by ERL). It is a joint laboratory between the Department of Physics of the University of Helsinki and the Helsinki Institute of Physics. The Laboratory takes part in teaching by organizing courses and in interaction with society by inviting high-school students and teachers.

Highlight of research

CMS results on two-particle correlations in high-multiplicity proton-proton collisions were the first truly unexpected physics phenomenon observed at the LHC. The paper, published in J. High Energy Phys. 09 (2010) 091, showed that quite contrary to expectations particle pairs exhibit long-range correlations, as if they were correlated already at the production point one way or another.

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The first unexpected physics phenomenon observed at LHC: long-range correlation of particles produced in high-multiplicity proton-proton collisions at CMS.