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Elementary Particle Physics

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The research is conducted in particle physics and cosmology using both experimental and theoretical methods.  For particle physics the year 2011 has been remarkable. The Large Hadron Collider at CERN, Geneva, continued very successful operation of both the accelerator and the experiments and interesting results were produced.

Theoretical particle physics

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

In theoretical cosmology, the scale dependence of non-gaussianities of the primordial perturbation and other observables in curvaton models have been studied. Rare massive galaxy clusters and their implications for non-gaussianity have been considered and issues in models of Higgs inflation have been addressed.

The highlight of the work of the quantum field theory group was the standing question of  the relation between Lorentz invariance and CPT theorem, due to the interpretation of recent data on neutrino oscillations. The group demonstrated that the violation of the CPT theorem does not necessarily lead to Lorentz invariance violation. Other studied topics modified gravitational theories for dark matter and energy, a cosmological model with E(6) unification, the problem of noncommutative time in QFT, the Dirac  monopole quantization in noncommutative space-time and the hypothesis of gravity as an entropic force. In collaboration with researchers at Nordita, quantum entanglement has been studied as a measure of entropy and approach to thermal equilibrium in a strongly interacting non-relativistic quantum field theory. Thermalization times for a system thermalizing from a far-from-equilibrium initial state have been calculated.

In the computational field theory group the properties of the SU(2) gauge field theory with various number of fermion flavours have been studied.  For the first time, clear indication for a conformal window (infrared fixed point) extending from 7 up to 10 flavours has been observed.  The 6 flavour case is seen to be very interesting: the theory may have an infrared fixed point, but it also may be the first example of a theory with almost-fixed-point behaviour, needed for technicolor theories. Also the properties of SU(N) gauge theories at high temperatures, especially the large-N scaling of the Polyakov line have been studied.

In beyond the Standard Model phenomenology emphasis was in supersymmetric models and models with higher dimensions. It was found that still a large parameter space exists for the susy partner of top to be the next-to-lightest supersymmetric particle, and with no discovery of supersymmetry so far, this option has become more attractive. Use of top polarization as means to detect beyond the Standard Model physics was considered. Constraints for a typical particle of higher dimensions, were found from mesonic and leptonic processes.

In hadron physics the focus has been on understanding hadron dynamics based on Quantum  Chromodynamics. A method developed by the group for measuring the transverse shape of  scattering processes is being applied to experimental data. For the first time equal-time bound state wave functions in different reference frames have been related. The Lorentz contraction of relativistic wave functions turns out to depend on the interaction strength. In "softer" hadron physics meson interactions, especially pionic physics and relations between eta-nuclear scattering and possible bound states, are investigated. Also breaking of the mirror symmetry (parity nonconservation) is an object of studies in laboratories around the world, and various features and phenomena at low and intermediate energies in this field are theoretically pursued in our division.

Experimental Particle Physics

All the experimental activities are in collaboration with the Helsinki Institute of Physics, which has the coordinating role in particle physics in Finland. Researchers in the University of Helsinki and Helsinki Institute of Physics are participating in the CMS and TOTEM experiments at the Large Hadron Collider (LHC) at CERN, Geneva.  Researchers in Helsinki have also participated in the CDF experiment at Tevatron at Fermilab, USA.

CMS

The main scientific goals of CMS are detailed investigations of particles and interactions at a new energy regime, understanding the origin of electroweak symmetry breaking (search for Higgs bosons), and search for direct or indirect signatures of new physics beyond the Standard Model of particle physics.

The performance of the LHC during the year 2011 exceeded all expectations. The target set for the collected integrated luminosity in 2011, 1 fb-1, was reached already in June, and by the end of the proton-proton run, end of October, LHC had delivered a total integrated luminosity of 5.7 fb-1 of pp collisions at 7 TeV centre-of-mass energy. The record peak instantaneous luminosity was 3.6 x 1033 cm-2s-1, achieved with 1380 bunch operation per beam colliding with 50 ns separation.

The CMS experiment operated stably and with a high efficiency during the whole year. The total number of papers published by CMS on collision data is now 85 and rapidly increasing. Highlights during the year were new results on Higgs, supersymmetry and rare B decays.

CMS has already conducted and presented Higgs searches with the full 2011 dataset, and the results are indeed intriguing. The preliminary results exclude the existence of the Standard Model Higgs boson in a wide range of possible Higgs boson masses, from 127 to 600 GeV at 95% confidence level. There is an excess of events in the mass region between 115 and 127 GeV (see Fig. 1), that appears, quite consistently, in five independent channels. The excess is most compatible with a Standard Model Higgs hypothesis in the vicinity of 124 GeV and below, but with a statistical significance of less than two standard deviations from the known backgrounds, once the so-called Look-Elsewhere Effect has been taken into account. The ATLAS experiment sees a similar excess roughly at the same mass, although ATLAS has not yet analysed all the Higgs decay modes with the full 2011 data, but only a subset of decay modes. In 2012, the total amount of data is expected to be quadrupled, which should enable CMS and ATLAS to ascertain the origin of the excess.

Fig 1. a)Fig. 1. a) Standard Model Higgs exclusion limit at 95% confidence level for 4.7 fb-1 proton-proton data collected by CMS in 2010 and 2011, showing the lower mass region, as of 13 December 2011.

Fig 1. b)Fig. 1. b) A typical Higgs candidate event including two high-energy photons.

CMS Supersymmetry searches extend the Tevatron exclusion region by a large margin. CMS has also produced new limits on the rare decay modes Bs,d µ+µ-, which together with similar LHCb results put tight constraints on new physics models with large tanβ.

Strong contributions to the search for the charged Higgs boson, to B-physics analyses, and to jet physics have been made. The working group for exclusive B decays was co-convened by P. Eerola, and M. Voutilainen co-convened the jet energy corrections group. The Helsinki research groups contributed also to the operation of the CMS experiment – operation of the experiment (shifts), data certification, development and maintenance of general software, calibration, etc..

Forward physics

In 2011, the Forward Physics project concentrated on: (1) running in-the Helsinki built T2-spectrometer, (2) the physics analysis activities of the TOTEM/CMS experiment, (3) the CDF based analysis on exclusive gamma-gamma interactions, top quark studies in all-hadronic channel, and the Higgs analyses in WH channel and in VBF channel should be completed.

TOTEM experiment

The main responsibility of the Helsinki group in TOTEM centers around the T2 spectrometer and, in particular, its GEM detectors. The TOTEM team concentrates on both hardware and software (reconstruction) contributions to T2. After a running-in period, the T2 spectrometer and its trigger and signal processing electronics is now mostly understood and the detector exhibits excellent efficiencies. The team has since long worked on leading proton detection at the LHC and can be considered as the world expert on the subject. It is in group’s interest to continue the work on leading proton detectors and studies on their performance vs. different LHC optics scenarios.

The 40 Gas Electron Multiplier (GEM) detectors of T2 were completed by the end of 2007 in the Kumpula Detector Laboratory. Since commissioning of the T2 spectrometer in 2009 large volumes of data has been collected and is currently used for first physics analyses of inelastic diffraction scattering. During spring 2012 the VFAT based read-out test system is of special importance in studying the signal formation in T2.

With the T2 spectrometer fully commissioned, the focus of the group’s research activity has shifted towards the forward physics program of TOTEM, with important analysis prospects already at a very early stage of LHC running. The physics scenario of TOTEM is based on (1) short special high statistics runs which begun during the running-in stages of the machine and (2) forward physics runs in conjunction with the CMS experiment with the nominal low-β* machine conditions. Investigations on QCD can be carried out at relatively modest luminosities, and large statistics gluon studies will become available at the beginning of TOTEM runs. In Fall 2011, the first measurements of elastic cross section were carried out, first at large t, then at lower values of t. Together with the total and inelastic cross section measurements (based on CMS measured luminosity), TOTEM achieved its first goals.

To reach the physics goals, TOTEM will be integrated in the trigger and data recording systems of the CMS experiment, and in case of the signatures depending on the central CMS detectors (e.g. the CED Higgs process), a common plan of utilization of the two experimental set-ups is being prepared.

 The TOTEM differential elastic proton-proton cross-section vs the momentum transfer squared t at the LHC center-of-mass energy of 7 TeV confirming the measurements at ISR 40 years ago at 100 times the energy of the increase of the size of the proton with increasing center-of-mass energy.

CDF-experiment

In the end of October 2011, the Tevatron runs were finished and the Helsinki group now concentrates in finalizing its analysis tasks connected to four PhD theses. In a number of areas the CDF based analysis provides important support for the physics analysis activities now beginning at the LHC. The Helsinki CDF group has three major responsibilities in CDF analysis: (1) precision measurement of the top quark mass, (2) Higgs search in associated production with the W-boson and in VBF analysis, and (3) exclusive gamma-gamma analysis.

CLIC

To the Compact Linear Collider (CLIC) feasibility study for a future e+e− linear collider, the University of Helsinki contributes to high precision assembly and production of its RF structures, development of methods to measure dynamically the vacuum inside its RF structures, thermo-mechanical modeling of the complete CLIC module, as well as industrialization and cost study for its RF structure, all in close collaboration with several Finnish academic and industrial collaborators, notably VTT.

Observational Cosmology

The Planck satellite has continued observing the sky. It began its fifth full-sky survey in August 2011. The coolant for the last cooling stage, required for observations at the higher frequencies (100 GHz and above), is estimated to run out at the beginning of 2012, but observations at the three lowest frequencies (30, 44, and 70 GHz) will continue for most of 2012. We have been responsible for producing the sky maps for the three lowest frequencies, as well as a number of related tasks, including calibration, estimation of residual noise correlations on the maps, and producing large Monte Carlo simulations (performed at CSC - IT Centre for Science in Finland) of the data.  First scientific results, dealing with astrophysics, were published in 2011.  They include observation of the cosmic infrared background and discovery of many galaxy clusters via the Sunyaev-Zeldovich effect (scattering of cosmic microwave photons to higher frequencies).  The Early Release Compact  Source Catalogue from Planck was also released.  It contains more than 15000 astronomical objects observed by Planck: radio galaxies, blazars, infrared-luminous galaxies, features in the galactic interstellar medium, cold molecular cloud cores, stars with dust shells, galaxy clusters and unidentified objects. First cosmological results are expected in early 2013.

The galaxy cluster Abell 2319 seen by Planck.  At frequencies below 217 GHz, the cluster appears as a shadow in the cosmic microwave background, because it scatters photons to higher frequencies, where the cluster therefore appears as a hot spot.  From: http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=48227.   Credit: ESA/ LFI & HFI Consortia.

Detector Laboratory

Detector Laboratory is a joint laboratory between Helsinki Institute of Physics and the Department of Physics. It provides premises, equipment and know-how for research groups developing silicon and gas detectors. Presently, the Laboratory hosts projects related to CERN (CMS and TOTEM), FAIR (Super-FRS), and EU FP7 (Electric Sail by ERL). The Laboratory also has an active role in teaching and societal interaction. The highlights of the year 2011 were the update of the Laboratory infrastructure and the installation of clean room air dryer system.

Highlights of research

CMS Higgs search

The Large Hadron Collider (LHC) at CERN, Geneva, delivered a total integrated luminosity of 5.7 fb-1 of proton-proton collisions at 7 TeV centre-of-mass energy in 2011. The record peak instantaneous luminosity was 3.6 x 1033 cm-2s-1. The CMS experiment operated stably and with a high efficiency during the whole year. The total number of papers published by CMS on collision data is now 85 and rapidly increasing. Highlights during the year were in particular new results on Higgs, and also results on Supersymmetry and rare B decays.

The CMS results for the Standard Model Higgs show an excess of events in the mass region between 115 and 127 GeV in five independent channels. The excess is most compatible with a Standard Model Higgs hypothesis in the vicinity of 124 GeV and below, but with a statistical significance of less than two standard deviations from the known backgrounds. The ATLAS experiment sees a similar excess roughly at the same mass. The statistical significance of these results is not yet conclusive, and the results can also be reasonably explained as statistical fluctuations. More data, which will be collected in 2012, will resolve the question of the existence of Higgs.

Exclusion limit on the mass of the Standard Model Higgs boson at 95% confidence level (black data points below red line). The analysis is based on 4.7 fb-1 of proton-proton data collected by CMS in 2010 and 2011. The hatched bands show the mass regions excluded by previously by LEP and Tevatron, and now by CMS. The dashed line and the green and yellow bands show the average expected CMS sensitivity corresponding to the actual amount of data analysed.

TOTEM total cross-section measurement

The TOTEM result of the total proton-proton cross-section of (98 +- 3) mb at the LHC center-of-mass energy of 7 TeV, published in Europhysics Letters 96 (2011) 21002. The result, obtained by combining the optical theorem with a differential cross-section measurement of elastic scattering, confirms the rise of the total proton-proton cross-section with increasing energy as already shown by measurements at lower center-of-mass energies.

The TOTEM measurement of the total, inelastic and elastic proton-proton cross-sections at the 7 TeV centre-of-mass energy are shown together with results from previous collider measurements and results deduced from cosmic ray measurements.