Elementary Particle Physics

The research is conducted in particle physics and cosmology using both experimental/observational and theoretical methods. In particle physics, the discovery of the Higgs boson at the LHC in 2012 led to the Nobel prize in 2013 for the developers of the concept of the method and the associated particle. In cosmology, the important Planck results were published in March 2013. The update of the European strategy was formulated in 2013, and the Division participated strongly in that work. Professor Paul Hoyer and university lecturer Jouni Niskanen retired, and they were succeeded by professor Oleg Lebedev and university lecturer Kimmo Tuominen.

Experimental Particle Physics

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

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. Finnish particle physicists have been essential contributors to the CMS experiment ever since its initial phases in the beginning of 1990's.

The first phase of LHC operation, so-called Run 1 (2010-2012), has now been completed. Run 1 started in 2010 with 7 TeV proton–proton collisions, and continued in 2011 with the same centre-of-mass but with increased luminosity. In 2012 the centre-of-mass energy was raised to 8 TeV, and the peak instantaneous luminosity reached over 7.7x1033 cm-2 s-1, 77% of the original design luminosity. The total integrated luminosity collected during Run 1 was about 25 fb-1. In addition, CMS collected data from the short lead-lead and proton-lead LHC runs.

LHC and the experiments are having a service break 2013–2014, during which the accelerator and the experiments are going through reparations and maintenance. The LHC Run 2 will start in spring 2015. The collision centre-of-mass energy will be raised to 13 TeV, and luminosity will be further improved. By the end of 2013 the experiment has published or submitted to publication 285 papers, including the discovery of a new Higgs-like particle in mid-2012 (Phys. Lett. B 716 (2012) 30-61). Further analysis with the whole Run 1 data sample has now confirmed that all the properties of the new particle (spin, parity, couplings, production probability and decays modes) correspond to those of a Higgs particle. The public CMS Higgs results are available at the CMS Higgs results twiki-page.

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Higgs spin: model distributions of the test statistics comparing the signal JP hypotheses 0+ (standard model prediction, yellow) and 2+m(gg) (blue) in the best fit to the data. The observed value is indicated by the arrow and disfavours the 2+m(gg) signal hypothesis with a CLs value of 0.6%. Higgs production and decays: values of μ̂ = σ/σSM for the sub-combinations by decay mode. The horizontal bars indicate the ±1σ uncertainties on the μ̂ values for individual channels; they include both statistical and systematic uncertainties. The vertical dashed line indicates the prediction for a standard model Higgs boson.

The Higgs discovery paper had by the end of 2013 collected 2100 citations (according to Inspire) in one year and a half. The significance of the observation was crowned by the Nobel prize in physics 2013, given to Francois Englert and Peter Higgs "for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN's Large Hadron Collider".

There were plenty of other exciting physics results, which showed up in 2013 when the whole Run 1 data set had been analyzed. Maybe the biggest news in 2013 was the observation of a very rare decay of the B0s meson to a pair of muons. CMS observed an excess of Bs → μμ events over the background-only expectation, corresponding, with a significance of 4.3σ, to a decay rate of 3.0+1.0–0.9 x 10–9. This is consistent with the standard model prediction. This decay mode has been searched for over 25 years, because the decay mode is sensitive to contributions from eventual new particles predicted by theories beyond the standard model. Also LHCb experiments saw a similar signal with a significance of 4.0σ. CMS results are shown here below in Fig. 2. More CMS physics results can be found at the CMS public website.

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The di-muon mass distribution. The purple and red curves show the B0 and Bs signals, respectively, while the dashed line, and the green and black shapes show three different types of background. The solid curve shows the sum of the fit components. A candidate Bs → μμ event recorded in the CMS detector in 2012, produced in proton-proton collisions at 8 TeV.

Researchers in University of Helsinki and HIP contributed to the search for the charged Higgs boson, to B-physics analyses, and to jet physics. Mikko Voutilainen continued to co-convene the jet energy corrections group. The Helsinki research groups contributed also to service tasks of the CMS experiment, for example, development and maintenance of general software, and calibration. Paula Eerola continued as a member of the CMS Management Board, representing the small CERN member states. She was also the vice-chair of the B-physics Publication Committee.

In 2013, several important physics analyses were completed by the Helsinki group based on the data collected in the CDF experiment at the Tevatron (Fermilab), and in the TOTEM/CMS experiment at the LHC (CERN). As physics highlights of CDF studies, a study was completed of the exclusive gamma-gamma production - the first observation of the process. Comprehensive analyses of Higgs production were completed, as well as the study on all hadronic decay mode of the top quark.

Since 2012, the TOTEM experiment has published several important physics results including 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.

In the TOTEM/CMS experiment, the Helsinki group has had a key role in constructing and operating the T2 spectrometer detectors (GEM detectors). The success of these detectors is based on the novel method of quality assurance developed in Helsinki. The first precision measurement of doubly diffractive pp cross section was carried out. Studies on proton-proton inelastic processes were completed. Physics analyses on single diffractive cross section are well advanced. Novel multidimensional analyses and event classification approaches have been introduced and tested.

In close connection with its research activities, the Helsinki group carries out educational programs both at the undergraduate and graduate level. Within the past four years, six PhD and five MSc degrees have been completed in the group. Importantly, these former students of the group have rapidly been recruited to important positions in research institutions, notably at CERN, and in industries. Domestic summer student and technical trainee programs, tailored for university and polytechnical students, are continued at CERN.

The global CTF3/CLIC collaboration hosted by CERN studies the Compact Linear Collider (CLIC) as an option for a future electron-positron linear collider for the post-LHC era. The University of Helsinki plays a leading role in the collaboration of the development of a model for the phenomena that causes breakdowns in the CLIC RF structures and hence limits the CLIC gradient. Based on the understanding obtained, the CLIC RF structures are being optimized to reduce the breakdown rate during their operation. In addition, UH together with several Finnish academic and industrial partners is developing the high precision assembly of the RF structures and the complete CLIC module to obtain viable and cost-effective procedures applicable to industrialized manufacturing. The first complete CLIC test module was finalized and first measurements of its thermo-mechanical behaviour were performed in 2013. Furthermore, 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. Both methods will become important quality assurance tests of the series of test structures being currently produced.

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.

In theoretical cosmology the dynamics of spectator fields during inflation was studied. These include fields like the curvaton and the Higgs, which could also play an important role in the reheating of the universe after inflation. A detailed calculation of the curvaton decay into Higgs particles was performed, accounting for the thermal history of the universe. The non-perturbative decay of the Higgs field condensate formed during inflation was also considered. The constraints implied by the Planck results on mixed curvaton and inflaton models were found.

A highlight of the research was construction of the first exact statistically homogeneous and isotropic cosmological solution in which inhomogeneties, in the form of symmetric holes, have a significant backreaction effect on the expansion rate. The holes, dubbed as Tardis regions, are larger on the inside than the corresponding background domain. Studies of large-scale structure included an investigation of the observed anomalous hemispherical power asymmetry in the cosmic microwave background on small angular scales and found that it is really just a coincidence of relativistic power modulation, edge effects from the mask applied in observations, and inter-scale correlations.

The mathematical physics group has continued its study of holographic methods applied to the physics of strongly coupled quantum liquids. An example is given by the quark-gluon matter created in ultrarelativistic heavy ion collisions at the LHC and RHIC collider experiments. Recent data suggests that inhomogeneities associated by event-by-event fluctuations are important for interpreting the experimental signatures. The group has studied time evolution of inhomogeneities in a holographic model, and found that it supports the common assumption that early dynamics can be described by free streaming followed by a rapid onset of hydrodynamic evolution.

The quantum field theory group has extended its activity towards quantum aspects of gravity. We have calculated subleading quantum corrections to the Bekenstein-Hawking black hole entropy, with all its quantum corrections to the gravity partition function being expressible in a closed form. For the problem of ghosts, the Hamiltonian analysis of various gravitational models was continued, including the Weyl conformal gravity and the TeVeS theory proposed by J. Bekenstein as a relativistically covariant formulation of the Modified Newton Dynamics (MOND), and as alternative to dark matter. The issue of CPT violating Lorentz invariant theories has also been pursued further, with the final aim of finding an alternative paradigm to Sakharov's for the generation of matter-antimatter asymmetry in the Universe.

The computational field theory group has studied new applications for numerical field theory methods. As highlights, the generation of gravitational waves in the first order cosmological phase transitions was simulated. Here the critical new ingredient is that for the first time fully hydrodynamical simulations were used. Another highlight is the first study of the non-perturbative contributions to the jet quenching coefficient. This is an important quantity used to measure the properties of the hot quark-gluon plasma in heavy ion collisions.

In the beyond the Standard Model phenomenology, the Higgs sector with scalars in triplet representations have been studied and the full radiatively corrected Higgs mass has been calculated. The rare B-meson decays in the model with vanishing hypercharge triplet were considered. Charge and parity violating version of the minimal supersymmetric standard model was tested by searching several benchmark points and considering specific signals in those. As a highlight of the research, a new method was developed to test theoretically favoured light stop squark at the LHC (Phys. Rev. Lett. 110 (2013) 141801).

Observational Cosmology

The Planck satellite ceased operations in October 2013 after over four years of observing the microwave sky. We have been responsible for producing the sky maps for the three Low Frequency Instrument (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. We have also contributed to development of improved calibration methods for LFI and fitted cosmological models of primordial isocurvature perturbations to Planck data.

Cosmological results based on the first 15 months of Planck observations were released in March 2013. They include a high-resolution temperature map of the cosmic microwave background (CMB) and its angular power spectrum. The current 5-parameter "standard cosmological model", ΛCDM, is in remarkable agreement with these data.

A number of features theoretically predicted by the standard cosmological model, but not observed previously, were detected by Planck for the first time: the (non-primordial) non-Gaussianity of the CMB due to the correlation between gravitational lensing of the CMB and its redshift variations (integrated Sachs-Wolfe effect) due to the gravitational effect of matter concentrations by which the CMB radiation has travelled, the correlation between this lensing of the CMB and the cosmic infrared background; and the effect of the motion of the solar system on higher multipoles (than the dipole) of the CMB sky.

At the largest scales the CMB temperature variations have some puzzling features, most of them already observed by WMAP and now confirmed by Planck. The most obvious is that at one hemisphere, mostly north of the ecliptic plane, these variations are weaker than expected and weaker than on the other hemisphere. There is no plausible physical explanation for this. Since the primordial perturbations were produced by quantum fluctuations, this could just be an accidental result from their random nature, but the probability for this appears low.

The next cosmology mission after Planck will be Euclid, with launch in 2020. The contracts for building the satellite were signed between the industry and the European Space Agency in summer 2013. 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. There will be at least eight national Euclid Science Data Centers (SDC), one of them in Finland, among which the Euclid data will be divided. A prototype of SDC-Finland is now running as a virtual machine at the CSC Kajaani Data Center, and we have participated in and passed the two first "SDC challenges" devised by the Euclid system team.

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The temperature variation of the cosmic microwave background over the entire celestial sphere, shown in ecliptic coordinates. This map is obtained by analyzing the microwave maps of all 9 Planck frequencies and separating the cosmic microwave background from other sources of microwave radiation. The Northern hemisphere (north of the orbital plane of the Earth) is on the left, and the southern hemisphere on the right. The color scale ranges from -300 microkelvin (blue) to +300 microkelvin (red) representing deviations from the average CMB temperature of 2.7255 K. The "smeared" features on the map are regions where the "foreground" microwave radiation from galaxies was so strong that the CMB could not be properly separated from it: the most obvious ones are the band of the Milky Way and the Large Magellanic Cloud near the South Ecliptic Pole. Planck Legacy Archive: http://www.sciops.esa.int/index.php?project=planck&page=Planck_Legacy_Archive

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The angular power spectrum of the cosmic microwave background, showing how strong the structure is at each angular scale on the sky. The red dots are the measurements by Planck; the green curve is the prediction of the standard ΛCDM cosmological model with best-fit values of its parameters. The light green band represents cosmic variance, the expected scatter due to the random nature of the origin of the CMB structure. (Planck Collaboration: Planck 2013 results I. Overview of products and scientific results, submitted to Astronomy & Astrophysics (2013)).

Detector Laboratory

Helsinki Detector Laboratory is an infrastructure specialized in the instrumentation of particle and nuclear physics. It is a joint laboratory between the Department of Physics of the University of Helsinki (UH/Physics) and Helsinki Institute of Physics (HIP). The Laboratory provides premises, equipment and extensive know-how for research projects developing detector technologies for large international physics experiments. The personnel of the Laboratory have extensive expertise in the design, construction and testing of silicon and gas-filled detectors. In addition, the Laboratory organizes teaching for UH/Physics students and demonstrations for visiting high school students.

All present projects 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 2013, the Laboratory hosted several HIP projects concentrating on CMS, TOTEM and ALICE experiments at CERN, and the NUSTAR/SUPER-FRS experiment at FAIR. To maintain the outstanding expertise of the Laboratory, new detector technologies are actively developed in the framework of CERN CMS, RD39, RD50 and RD51 collaborations. The Laboratory supports several UH/Physics research activities. The connection to the Division of Elementary Particle Physics is naturally very tight. 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 Division of Material Physics Accelerator Laboratory is strong in the field of radiation hard silicon detectors. Additionally, the connection is strong with University of Jyväskylä Accelerator Laboratory and with Aalto University Micronova/Nanofab facility.

Teaching and societal interaction are an essential part of the Laboratory. Exercises and special assignments of detector technologies are organized for UH/Physics students. In addition, several students work continuously with their doctoral and master studies in the Laboratory. Furthermore, groups of high-school students and teachers visit monthly the Laboratory for demonstrations of detector technologies. The Laboratory also participates actively in University outreach efforts. 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. In 2013, the infrastructure was significantly improved. The Laboratory obtained several devices for the needs of the characterization and quality control of novel, fast detector technologies.