Highly cited researchers: physics, chemistry and astronomy

Paula Eerola (b. 1962), programme director at the Helsinki Institute of Physics and professor of experimental elementary particle physics, has spent most of her career studying the bottom quark and its antiparticle, the bottom antiquark.  She has participated in the massive project to develop measuring equipment for the detectors located on the ring of the Large Hadron Collider (LHC) at the European Organization for Nuclear Research CERN ever since its inception.

After completing her doctoral dissertation at the University of Helsinki, Eerola worked as a researcher at CERN from 1991 until 1997. At first, she used what was then the LEP collider to study the decay of Z0 bosons into bottom quarks and developed research methods relating to the LEP collider.

It was already known that the LEP collider would be discontinued over the course of the coming years to make way for the larger and more modern LHC. Eerola joined the research into what kinds of experiment settings and measurement equipment would be best suited for studying the phenomena on the ring of the LHC, and participated in the planning for the ATLAS detector.

Between 1998 and 2008, Eerola worked at the University of Lund as a special researcher and professor. There she continued to develop the equipment and experiment plans for the ATLAS detector.

In 2008, Eerola was appointed professor of elementary particle physics at the University of Helsinki and she began to coordinate the University's cooperation with the CMS detector located on the ring of the LHC. Bottom quarks were still the focus of her research.

Why bottom quarks?

The bottom quark and the bottom antiquark are studied so that we may understand the differences between particles and antiparticles.

In the Standard Model for particle physics, the particle and antiparticle are considered identical but with opposite electrical charges. This is not, however, a viable explanation, as the universe could not have been formed in such complete symmetry - all particles and antiparticles would have just annihilated each other back into nothingness.

Some asymmetries have since been discovered between particles and antiparticles, but not sufficiently to explain the physical structure of the universe.

Bottom quarks and antiquarks are the best way to study these asymmetries, since as heavy quarks, their decay processes are more extensive and distinct than can be observed in any other quarks. The decay products of bottom quarks are also easier to identify among other decaying quarks.

In bottom quark experiments in the LHC, two proton bunches are accelerated to near light speed and collided in the beamline. The collisions can produce many different results, but Eerola and her team focus on the collisions that generate a bottom quark and a bottom antiquark which are then hurled in different directions.

On the way, they decay into B mesons, which then decay primarily into pions, muons and electrons, which then continue to move through the detectors. They, in turn leave traces from which researchers try to piece together what the original bottom quarks and antiquarks were like and how they decayed.

Thanks to her participation in the CMS consortium, Eerola is also associated with the much-cited 2012 paper reporting the discovery of the Higgs boson.

However, ever since her dissertation and later, during her work as head of her research group at the University of Helsinki, Eerola herself has focused on a different kind of Higgs particle, namely, the charged Higgs boson. The Higgs boson discovered in 2012 carried no electrical charge, but it is conceivable that charged sibling-Higgs bosons might exist.

Discovering such a particle would be a major scientific breakthrough which would require extensive updates to the theories on the Higgs boson.

At the time of writing, no such sibling-Higgs has been observed. The CMS consortium has published zero-result studies conducted in Helsinki – to confirm that they hadn’t found anything. In science in general and particle physics in particular, such zero-result studies are relevant, as these elementary particles cannot be observed in any conventional medium, and all their properties or lack thereof must be established through complicated experiments.

Throughout her international research career, Eerola has been the single parent of one child.


Participating in the European Space Agency’s famous Planck satellite project, and its cosmology papers in particular, has made Mika Juvela (b. 1967), university lecturer of observational astronomy, one of the University of Helsinki’s most cited researchers.

Ever since his dissertation, which was approved in 1997, Juvela has specialised in the phenomena of our galaxy, the Milky Way: interstellar dust and how interstellar clouds form stars.  

The measuring equipment for observing cosmic dust has developed in leaps and bounds during Juvela’s career, and he has been among the first researchers to take advantage of the new instruments and techniques.

In 2008, Juvela and his colleagues realised that interstellar clouds emit infrared radiation which is scattered by dust particles. In further studies, the same phenomenon was discovered on mid-infrared wavelengths, i.e., wavelengths of a few micrometres. This means that the dust cloud contains particles at least a few micrometres in diameter, as particles can only scatter light which has a lower wavelength than their size.

However, in some dust cloud cores such scattered light has not been observed, which suggests that their particles are under one micrometre in size. These observations indicate that space contains dust clouds at different stages of the contraction process. 

According to prevailing hypotheses, once the contraction proceeds past a certain point, the enormous gravitational forces in the core of the cloud trigger nuclear reactions, and a star is born. However, until that point, the cloud core keeps getting colder as it becomes denser. Heat radiation from stars cannot penetrate the dense cloud, and as the cloud becomes denser, its molecules collide more, leading to some of their motion energy being transformed into molecular excitation. When this excitation is released, the molecule launches a photon into space, further cooling the cloud.

From Earth’s surface, the atmosphere prevents the observation of such radiation of cold matter, but the Planck satellite with its delicate sensors was able to detect it. For Juvela, the most significant result from the Planck mission is the list of approximately 13,000 cold objects in space approximately one light year in diameter, published in 2015. These objects are probably undergoing a contraction reaction, which will ultimately lead to the creation of stars.

The most well-known discovery from the Planck project was the 2013 microwave map. This map, one of unprecedented detail, describes the cosmic background radiation emitted when the universe was 380,000 years old. At this time, the first atoms were formed and the universe was transitioning to its current transparent state after being full of opaque plasma.

Nevertheless, the 2013 cosmic microwave background was not the whole, complex truth, as it was calculated using only the total brightness of light. In cosmic radiation, the polarisation of light also contains information about the early universe.

The radiation emitted by the dust clouds in the Milky Way is also polarised. This means that an observation of polarisation has implications for both the cosmic background radiation and the dust filter through which it passes.

However, determining the dust filter is slow, and it was only in 2015 that the Planck consortium was able to publish a slightly more exact map, with cosmic background radiation differentiated from the impact of cosmic dust on it.

Not only was the dust considered as an interference factor, but valuable additional information was gained on the structure and polarisation of dust clouds.

In addition to observations, Juvela also makes computer models. He simulates the radiation emitted by the clouds and how it should appear in a telescope. The simulations can be made more exact by comparing them to genuine telescope observations. In this process, the simulations can ultimately reveal real information about the nature of the dust clouds and how stars are born.

Researcher Mika Juvela: Publications, Projects, Activities

Kai Nordlund (b. 1969), professor of computational materials physics, specialises in molecular dynamics, i.e., the simulation of atomic movements through computer programmes. Nordlund has been the first to create such simulations in several areas of materials physics.

Nordlund’s most cited paper is from 1996, when he was a postdoctoral researcher at the University of Illinois. There he used molecular dynamics to simulate how radiation from a particle accelerator erodes metals and semiconductors in different ways. Semiconductors must endure 10–100 times more damage than metals, because the atoms in metals are packed in more tightly.

Around the turn of the millennium, Nordlund and his colleagues used simulations to demonstrate how the hot particles leaking from the 100,000,000-degree plasma generated in a fusion reactor damaged its carbon-based wall material. Before this, it was hypothesised that plasma leaks should not erode carbon, even though in experiments they always did.

Nordlund discovered a new kind of physiochemical reaction, in which a hydrogen atom comes between two carbon atoms and breaks their bond, allowing the upper carbon atom to escape. In hindsight this explanation seems obvious, but at the time no one had thought of it.

Nordlund determined that the erosion of the carbon walls in a fusion reactor is unavoidable. Consequently, contemporary fusion reactors use tungsten and other more durable materials for their inner walls.

A few years ago, Nordlund noticed in the course of his simulations that the size distribution of the erosion of the tungsten wall follows a simple power law, which was later empirically proven.

With the exclusion of his postdoctoral years in the US, Nordlund has worked his whole career in the same room at the particle accelerator laboratory in Kumpula since 1992 when he began his dissertation research. Nordlund’s wife, Docent Flyura Djurabekova, works in the neighbouring office. The couple often work 10- to 12-hour days.

To balance his stationary work in Kumpula, Nordlund accrues approximately one hundred international travel days per year. His visits and conference trips help him build cooperation with other top-level research groups in materials physics.

Researcher Kai Nordlund: Publications, Projects, Activities