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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 2012 has been remarkable: the Higgs boson which has been searched for tens of years, was discovered at the Large Hadron Collider at CERN, Geneva, which also otherwise continued very successful operation of both the accelerator and the experiments. In 2012, the University of Helsinki and the Academy of Finland both evaluated the field very positively.

Theoretical particle physics and cosmology

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.

The focus in theoretical cosmology has been twofold. On one hand, the possible decay mechanisms of inflaton and curvaton models have been studied in detail, with an emphasis on non-perturbative phenomena. These may have important consequences for stochastic gravitational waves, isocurvature perturbations, and non-gaussianities. As a highlight one may mention the technically very involved calculation of gravitational waves due to out-of-equilibrium fermions produced at the end of inflation. On the other hand, issues related to formation of non-linear structures have been addressed both within the context of backreaction, with a major review article, and within the context of rare massive galaxy clusters. The latter are relevant for testing ΛCDM cosmology against observations of the integrated Sachs-Wolfe effect.

In mathematical physics, focus has been on studying dynamics of strongly coupled systems far from equilibrium, by using holographic methods. For some materials, one interesting quantity is the time dependent spectral function, which can be measured by time-resolved version of angle-resolved photoemission spectroscopy. Using holographic methods, we have defined time-dependent spectral functions and generalized notions of mean occupation number, by using Wigner transform. We have developed methods for computing them in conformal theories holographically dual to black holes formed from null collapse.

The quantum field theory (QFT) group continued research on Lorentz invariant CPT violating theories, with a mechanism of mass generation for neutrinos by a CPT-violating mechanism alternative to seesaw, with right-handed neutrino in the standard model, without spoiling the gauge symmetry. A neutrino-inspired mass splitting between particle and antiparticle is also proposed. The suggested CPT violation opens a new path to analyze baryon asymmetry. Other topics are: Hamiltonian analysis of modified gravitational theories; a mirror-world model in the framework of Plebanski's gravity; construction of extended three dimensional supersymmetric Poincare representations/anyons, and a QFT description of Casimir-Polder effect of atoms with graphene, proposed for quantum reflection experiments to distinguish between different models for graphene.

The computational field theory group continued pushing the boundaries of applications of quantum field theory simulations. In the lattice technicolor project, a careful analysis of the appropriate Schrödinger functional boundary conditions for fermions in various gauge group representations was done; an important ingredient in precision numerical simulations. In cosmological applications, the rate of the baryon number violation as a function of the temperature in the Standard Model was calculated using the recently discovered Higgs mass, thus finally determining an important property of the Standard Model. The possibility of a strong first order cosmological phase transition was also investigated in the minimal supersymmetric extension of the Standard Model, MSSM. In these calculations the leading expertise of the group in simulations using effective theories to study electroweak physics was fully utilized.

In the studies of beyond the Standard Model phenomenology the new experimental constraints due to the detected Higgs-like boson were taken into account. The results seem to favour supersymmetric scenarios with a light stop. Such scenarios have been investigated earlier by the group, and studies were continued by developing a novel way to test such a scenario at the Large Hadron Collider. Another interesting feature of supersymmetric models, the existing candidate for a dark matter particle, was considered in the case of a model with spontaneous CP violation. Benchmark points for both sneutrino and neutralino dark matter were found, and signals at the LHC discussed. Also generic extensions of the MSSM were studied, especially the lightest supersymmetric particles in them. Constraints for a typical particle of higher dimensions were found from mesonic and leptonic processes.

In hadron physics work has continued on a systematic derivation of relativistic bound states in QCD. A novel boundary condition is imposed on the Coulomb field implied by the quark charges, giving a linear potential which is of zeroth order in the QCD coupling. Properties of the bound states formed by this potential were studied, and the formulation of a complete perturbative expansion around these solutions indicated. 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 the research area.

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 Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) has been in operation at CERN since 2010. By now, the experiment has published or submitted to publication already almost 200 papers, including the discovery of a new Higgs-like particle in mid-2012. Finnish particle physicists have been essential contributors in the CMS experiment ever since the initial phases in the beginning of 1990's. 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 (Higgs bosons), and search for direct or indirect signatures of new physics beyond the Standard Model of particle physics.

Observation of a new Higgs-like boson with a mass of about 125 GeV (The CMS Collaboration, ”Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC”, Phys. Lett. B 716 (2012) 30-61) was undoubtedly the great highlight of the year in 2012. The result was achieved with 2012 data collected up to June, combined with the data set collected in 2011. Signals reaching five standard deviation significances were observed both by ATLAS and by CMS. The CMS results are shown in Fig. 1a (updated with more 2012 data) and Fig. 1b.

Fig 1. a)Fig. 1. a) The observed local p-value for the five decay modes and the overall combination as a function of the SM Higgs boson mass. The dashed line shows the expected local p-values for a SM Higgs boson with a mass mH.

Fig 1. b)Fig. 1. b) Values of σ/σ_SM for the combination (solid vertical line) and for individual decay modes (points). The vertical band shows the overall σ/σ_SM value 0.80 ± 0.22. The horizontal bars indicate the ±1 standard deviation uncertainties in the σ/σ_SM values for individual modes; they include both statistical and systematic uncertainties.

The CMS paper has already been cited 506 times (according to Inspire) in just 6 months (4 July 2012 - 4 January 2013), which is one indication of the far-reaching impact of the result. The Science magazine nominated the results by CMS and ATLAS as "the scientific breakthrough of the year", see

 http://www.sciencemag.org/site/special/btoy2012/index.xhtml  

There were plenty of other exciting new physics results as well in 2012: observation of new unexplained particle states in decays of the B+ meson, observation of a new beauty baryon Ξb*0, high-accuracy measurements of top quarks, observation of melting of Upsilon particles in heavy ion collisions, and so on - see more in  http://cms.web.cern.ch/

Researchers in the University of Helsinki and Helsinki Institute of Physics made strong contributions to the search for the charged Higgs boson, to B-physics analyses, and to jet physics. 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. P. Eerola was chosen as a member of the CMS Management Board, representing the small CERN member states. She also was a member of the Collaboration Board Chair Advisory Group, and the vice-chair of the B-physics Publication Committee.

A public event was organized in Kumpula in connection to the announcement of the Higgs results on July 4th, and consequently the results obtained a very wide media coverage. Other outreach activities consisted of participating in joint PR-events in Kumpula, such as the alumni and new student days in 2012, as well as blog writing, giving interviews and presenting public lectures.

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 2012, the Forward Physics Group concentrated on: (1) Completing the CDF/Tevatron physics analyses, (2) the physics analysis activities of the TOTEM/LHC experiment, (3) constructing the remaining reserve GEM detectors for the Helsinki built highly succesful T2-spectrometer, and (4) participating in the TOTEM Upgrade phase development and detector construction work.

TOTEM experiment

The TOTEM experiment has rapidly gained reputation as a frontier experiment in elastic scattering and total cross section measurements in proton-proton collisions. The first rate LHC data collected during the low luminosity special runs in 2011 and 2012 have yielded four major publications with strong impact to the both experimental and theoretical community in the field. The performance of the TOTEM detectors is by now well understood, and the data collection efficiencies are at the design levels.

Fig 1. b)Figure 2. Elastic, inelastic and total cross section measurement points at 7 TeV and 8 TeV center of mass energy.

The first physics results include the first total cross section measurement at the LHC, measurement of the forward charged particle pseudorapidity density, and the differential cross-section for elastic proton-proton scattering. Several physics results are either submitted for publication (inelastic cross section at 7 TeV and total cross section at 8 TeV) or being finalized for publication (single, double and central diffractive cross sections as well as the measurement of the ratio of the imaginary and real value of elastic amplitude). See. Fig. 2.

The Helsinki Group has been one of the key contributors in the design and R&D of the forward detector systems at the LHC. The Group has excellent potential for major contributions in the physics of inelastic diffractive scattering and central exclusive production, where the ultimate goal is to measure the spin-parity properties of the Higgs boson.

CDF-experiment

In the end of October 2011, the Tevatron run II was finished and the Helsinki group members now concentrate on finalizing their thesis based on CDF analysis. In 2012, the Forward Physics group was responsible for a number of frontier Phys. Rev. publications on (1) observation of the exclusive gamma-gamma process, (2) Weak Boson Fusion production limits of the Higgs boson, (3) on WH associated production of the Higgs boson, and (4) all-hadronic decays of the top quark. The CDF based analyses have been a key factor in boosting the physics analysis at the LHC.

CLIC

The Compact Linear Collider (CLIC) study is a feasibility study for a future electron-positron linear collider for the post-LHC era. During 2012, the study completed its Conceptual Design Report that describes the CLIC concept and its feasibility for the collision energy range up to 3 TeV, the physics and the detectors as well as the work plan for the current project implementation phase (2012-16). The University of Helsinki contributes to the high precision assembly, industrialization and cost study for the CLIC RF structures as well as the assembly and thermo-mechanical modeling of the complete CLIC module, all in close collaboration with several Finnish academic and industrial collaborators, notably VTT and Loval Oy. In addition, UH develops methods to measure the internal shape and stresses of the RF structures after assembly and dynamically the vacuum inside the RF structures during operation.

Observational Cosmology

The Planck High Frequency Instrument ceased observations after five full-sky surveys in January 2012, as planned. The Low Frequency Instrument (LFI) is expected to continue until August 2013, to complete eight full-sky surveys. We have been responsible for producing the sky maps for the three LFI frequencies (30, 44 and 70 GHz) as well as a number of related tasks, including null tests on the maps, estimation of their residual noise correlations, and producing large Monte Carlo simulations (at CSC - IT Center for Science in Finland) of the data. The second set of astrophysical results, the Planck Intermediate Results, have been published over the course of the year. They include the first all-sky map of the distribution of carbon monoxide, a detection of a mysterious diffuse microwave emission from central regions of our galaxy - called Galactic Haze (Fig. 3), and the discovery of a filament of hot gas linking two galaxy clusters. First cosmological results from Planck are expected in early 2013.

Figure 3. Galactic “haze” - radiation of unknown origin, possibly related to supernovae or dark matter annihilation, coming mainly from near the central regions of our galaxy. The Galactic Center is at the center of the image. Regions where other kinds of radiation are too strong for a reliable separation of the haze component are in black. Credit: ESA/Planck Collaboration.

The next cosmology mission after Planck will be Euclid, with launch in 2020. Euclid (Fig. 4) was adopted in the European Space Agency program in June 2012. Euclid will address some of the main open questions in cosmology, in particular the mystery of dark energy: what is causing the accelerated expansion of the universe? Euclid will observe the last three quarters - about 10 billion years - of the history of the universe; complementing Planck, whose cosmological measurements are mainly from the 400 000 year old early universe. We participate in the development of data analysis methods for Euclid and will eventually analyze a part of the Euclid data.

Figure 4. Artist’s impression of the Euclid satellite. Credit: ESA/C. Carrea

Detector Laboratory

Helsinki Detector Laboratory is infrastructure specialized in the instrumentation of particle and nuclear physics. It is a joint laboratory between Helsinki Institute of Physics (HIP) and the Department of Physics of the University of Helsinki (UH/Physics). The Laboratory provides premises, equipment and extensive know-how for research projects developing detector technologies. The personnel of the Laboratory have extensive expertise in the design, construction and testing of silicon and gas-filled detectors. The Laboratory is also active in education and outreach.

All the projects present in the Detector Laboratory have the objective to provide reliable instruments for large international experiments. Therefore, special effort is being put on component testing and long-term reliability, as well as on detector assembly. In 2012, the Laboratory hosted several projects concentrating on the CMS and TOTEM experiments at CERN, and the NUSTAR/SUPER-FRS experiment at FAIR. The Laboratory is also participating actively in the development of detector technologies in the framework of CERN RD39, RD50 and RD51 collaborations.

In addition, the Laboratory collaborates with the Electronics Research Laboratory, supporting especially their activities in optical imaging techniques and ultrasonic interconnection technologies. Furthermore, the cooperation with the Materials Physics researchers of the Department’s Accelerator Laboratory is traditionally strong in the field of radiation hard silicon detectors. Additionally, the connection was strong with the Accelerator Laboratory of the University of Jyväskylä and with the Micronova/Nanofab facility of the School of Electronics of the Aalto University.

The Laboratory participates actively in teaching and societal interaction. In 2012, the Laboratory offered laboratory exercises and special assignments of detector technologies for students from UH/Physics. In addition, several students perform continuously their doctoral and master studies in the Laboratory. Groups of high-school students and teachers visited monthly the Laboratory for demonstrations about detector technologies. The Laboratory also participates actively in the common outreach efforts taking place in the University. Consultancy, based on own expertise, is frequently given to research groups from other universities and research institutes.

The versatile infrastructure of the Laboratory forms a strong basis for the research activities. The infrastructure was significantly improved during 2012. Firstly, the Laboratory obtained new ultrasonic wire-bonding device that is crucial for manufacturing semiconductor detector modules. Secondly, the Laboratory was able to purchase several new, fast devices for detector characterization and quality control.

Highlights of research

Observations of a new Higgs-like boson

Observation of a new Higgs-like boson with a mass of about 125 GeV (The CMS Collaboration, “Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC”, Phys. Lett. B 716 (2012) 30-61) was the great highlight of the year in 2012. Signals reaching five standard deviation significances were observed both by ATLAS and by CMS.

Event recorded with the CMS detector in 2012 at a proton-proton centre of mass energy of 8 TeV. The event shows characteristics expected from the decay of the SM Higgs boson to a pair of photons (dashed yellow lines and green towers).

TOTEM forward charged particle pseudorapidity density measurement

The TOTEM measurement of the forward charged particle pseudorapidity density at the LHC center-of-mass energy of 7 TeV, published in Europhysics Letters 98 (2012) 31002, extends the existing measurements at LHC to the previously unexplored forward region. The measurement, based entirely on the T2 telescope made up of the Helsinki-built GEM-chambers, reveals the lack of ability of both high energy physics and cosmic ray event generators to adequately describe such a basic property of inelastic proton-proton collisions as the average charged particle production in the forward region.

Charged-particle pseudorapidity density distribution for proton-proton collisions at 7 TeV center-of-mass energy. Black squares, red triangles, blue circles and orange diamonds represent, respectively, the TOTEM measurement, and Phojet, Pythia8 and Sherpa event generator predictions for charged particles with transverse momentum larger than 40 MeV/c in events with at least one charged particle in the 5.3 < |η| < 6.5 range.

CLIC Conceptual Design Report

The Compact Linear Collider (CLIC) study completed its Conceptual Design Report in 2012 of a multi-TeV linear electron positron collider based on the CLIC two-beam concept. The first volume “A Multi-TeV linear collider based on CLIC technology” published as CERN report CERN-2012-007 contains the feasibility and description of a 3 TeV collider based on the CLIC technology. The second volume “Physics and Detectors at CLIC”, CERN-2012-003, describes the physics and the detectors with their performance at such a collider. The last volume, CERN-2012-005, entitled “The CLIC Programme: towards a staged e+e− Linear Collider exploring the Terascale” describes a LHC physics output dependent staging scenario of a CLIC concept based linear collider including its layout, performance, cost estimate and power consumption.