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Science Bordering on Science Fiction
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Science Bordering on Science Fiction
June 17, 2010   
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This replaces the previous version of an interview with Prof. Zbigniew Szadkowski. After reading the originally published version, Prof. Szadkowski requested that some of his comments be deleted and revised. We apologize to our readers for any inconvenience this may have caused.

Prof. Zbigniew Szadkowski from the Department of High Energy Astrophysics at the University of £ód¼, talks to Danuta Gruszczyńska.

International scientists taking part in the Pierre Auger astrophysics project are studying ultra-high energy cosmic rays, also known as cosmic accelerators. These cosmic rays are one of the greatest mysteries of contemporary astrophysics. What is the aim of this endeavor?
At the moment, the topic of the highest energy cosmic rays remains a puzzle. Controversy surrounds measurements of the energy spectrum, composition and anisotropy, and the proposed models cannot be tested without a significant improvement in the observations. Studying the sources of the universe’s highest energy cosmic particles is in any case a vital discipline that justifies a sensitive full-sky exposure in a large area detector.

Ultra high-energy cosmic rays with energies reported beyond 1020 eV have been investigated by several experiments, but their origin is still unknown, imposing upon us a great challenge, while providing a unique opportunity to explore new physics, astronomy and cosmology. The largest exposure experiments to date, AGASA and HiRes, both in the Northern Hemisphere, report an intriguing disagreement in the energy spectrum and clustering of cosmic ray arrival directions near the GZK energy threshold. This very fact indicates that we do need more accurate and larger-scale experiments to investigate this question unambiguously.

The statistics of registered events with an energy of about 1020 eV is insufficient due to an extremely low flux estimated at less than 0.5 per square kilometer per century per steradian. This means that only detectors of an immense size will be able to observe a significant number of these extraordinary cosmic ray events.

To obtain full sky coverage, the construction of two nearly identical air shower detectors has been planned, the first in the Southern Hemisphere (Malargüe, Argentina, Mendoza province, already completed—1,600 surface detectors spread over an area of 3,000 sq km) and the second in the Northern Hemisphere (Colorado—4,000 surface detectors spread over an area of 10,000 sq km). Atmospheric fluorescence telescopes placed on the boundaries of the surface array will record showers that strike the array. The two air shower detector techniques working together form a powerful instrument for these studies.

To achieve the scientific goals of the Pierre Auger experiment, several issues need to be explained:
- What is the origin and nature of the observed cosmic rays with the highest energies (E > 1020 eV)? How and where do they acquire such enormous energies?
- Mass spectrum: we have virtually no information as to their fundamental nature. Are they protons? Nuclei? Or perhaps something exotic?
- Sources: Are the highest CR coming from particularly energetic astrophysical objects? Do they exhibit any directional anisotropy?
- GZK cut-off: does it have a galactic or extragalactic origin?
- Can we go beyond the current limitations of physics?

In this international research project, you are developing electronics for space particle detectors…
Actually, I studied theoretical physics and developed a model that makes it possible to calculate the Kobayashi-Maskawa quarks mixing matrix in a spontaneous breaking of the chiral SU6 x SU6 symmetry. But in the Pierre Auger Observatory I started to work in electronics. When I was younger electronics was my hobby and that has come in handy now.

In 1998-1999, I cooperated with the Forschungszentrum Karlsruhe in Germany, designing the prototype of a Second-Level Trigger (SLT) for fluorescence detectors. The algorithm of the SLT is being used in all 24 fluorescence telescopes equipped with 10,560 photo-multiplier tubes.

In 1999, I was invited to Michigan Technological University (MTU), where in 1999-2002 I developed the first and second generation of the First Level Trigger (FLT) for the Surface Detector based on Programmable Logic Devices, which has just appeared on the market. A total of 800 surface detectors have been equipped with the second-generation FLT based on ACEX PLD.

In 2002-2003, I worked at College de France optimizing the second generation of the FLT.

In 2003-2006, I developed the third generation of the FLT and coordinated the manufacture of 800 Front-End Boards based on Cyclone FPGA at Wuppertal University in Germany.

What is a trigger specifically?
It is an algorithm or a device that triggers recording. Imagine a system that analyzes millions or even billions of events per second. When some parameters of the system match what you are expecting to happen, that is, a pattern you have come up with, then these certain system parameters get recorded over an arbitrary time interval. Third-generation triggers perform the analysis relying on a single FPGA microchip with around 12,000 logic blocks, which is a total of around half a million logic gates. When unformatted, this FPGA circuit is like a box of Lego building blocks jumbled together. You have to put the blocks in order, that is, program the structure of the internal connections so they carry out a certain algorithm. Each of the 1,600 detectors of the Auger experiment implements one Front-End electronic board. Second-generation FPGA boards of the ACEX range are mounted in 800 detectors, while more modern, third-generation FPGA boards of the Cyclone range are in use in the other 800 detectors. I have just designed a new, fourth-generation board based on an FPGA circuit of the Cyclone III range with 40,000 logic blocks, including Digital Signal Processing (DSP) blocks. The DSP blocks are ultra-fast multipliers that implement a spectral trigger based on the Discrete Cosine Transform algorithm and recognize horizontal air showers by the shape of signals from ADC converters. The electromagnetic component of particle showers, which enter the atmosphere tangent to the Earth surface, gets damped down and the only “survivors” are muons that create a narrow front, which in the surface detectors generates very slim peaks in ADC traces. Consequently, even though air showers pass through hundreds of detectors, they are only detected by a few. Spectral triggers enable much more efficient detection. New and advanced signal processing algorithms are of particular importance to the Auger North Observatory, where the standard coincident technology cannot be applied any longer, because unlike Auger South, which uses three photo-multipliers, Auger North uses only one.

What are the “charged particles?”
The Pierre Auger experiment, the Japanese experiment AGASA, and the American Fly’s Eye experiment detect cosmic particles with gigantic energies reaching 1020 eV. This is equivalent to the energy of a brick falling from the third floor, but concentrated in a microscopic object such as a proton, an iron nucleus or a gamma quantum. No contemporary physical theory can explain how particles with such high energies are actually created. We do not know if it is a bottom-up mechanism where the particles are accelerated by cosmic accelerators, or a top-down mechanism where super massive particles break down as envisioned by the Unified Field Theories.

In 2007, the Pierre Auger Collaboration team announced that the directions from which ultra-high energy particles came were strongly correlated with the location of active cores of galaxies, which lay not farther than several hundred million light years from the Earth. The coefficient of correlation was 69 percent, which is a lot, but the statistics were relatively poor, at a mere 34 events. We have now gathered more events but the coefficient of correlation has declined to 31 percent. We are convinced, however, that the anisotropy we found is really there, because while the statistics keep growing, the coefficient of correlation has stabilized and stopped declining.

Just 34 cases in 1,600 detectors?
The flux of high-energy particles is estimated at one particle per square km per 100 years. The surface detector array of the Pierre Auger experiment is sized 3,000 sq km and so, statistically speaking, we should detect around 30 events every year. The signals are extremely rare and the background is overwhelming. This necessitates sophisticated algorithms to eliminate the background while maintaining high reliability in the potential detection of extremely rare events.

What we observe are not just individual particles that strike the atmosphere. A particle from space collides with particles of the air and generates a shower of billions, trillions of particles that diffuse onto dozens or even hundreds of square kilometers. The particles of this secondary air shower reach dozens of detectors simultaneously. Electronic devices in all detectors are synchronized by GPS, and when they detect a signal in several or more detectors simultaneously then it is clear that the signal was started by one and the same particle.

How were particles with such gigantic energies discovered?
A particle with an energy of 1020 eV was found in 1962, but back then scientists were skeptical about the energy estimate that was presented. This is so improbable, they said, that we should better wait with any interpretation. When you deal with just one particle per square km per century, it would have to be a truly extraordinary occurrence to detect the particle using such relatively ordinary technology. And as we all know, when something is out of the ordinary, it is usually dismissed as an error of interpretation or equipment malfunction.

The first reports of these ultra-high energies were thus taken with a grain of salt. The two experiments, AGASA and Fly’s Eye, conducted independently of each other, did detect ultra-high energy particles, but discrepancies occurred in findings for energies above 1019 eV. When high-energy particles propagate through space, they start colliding with the microwave background radiation and they are believed to lose energy in the process, which is called the GZK effect. The AGASA experiment established that the flux of high energies kept escalating, while the results from Fly’s Eye indicated that such a flux should remain below the GZK limit. Consequently, even if sources of high-energy particles existed, they should be located quite close to the Earth, between 50 and 100 megaparsecs away. Otherwise, the energy would get diffused in collisions and lost during the propagation. These major discrepancies became the basis for the Pierre Auger experiment, designed to find out if the GZK limit, proposed in the 1960s, really existed. The experiment seems to indicate it does. The measurements are becoming increasingly precise thanks to theoretical research groups like those led by Prof. Maria Giller in £ód¼ and Prof. Henryk Wilczyński in Cracow. The groups are working to make sure that all sorts of side effects are taken into account, such as the Cherenkov light, which in some configurations can significantly distort the measurement. All events detected since 2004 are being analyzed again to reject dubious ones and base the research on very clear ones, so that the margin of error is as narrow as possible.

What do the detectors look like?
The surface detector on the pampas plains in Argentina is a huge plastic cylinder filled with 12 tons of ultra-clean water. The water has to be so clean to prevent the growth of any microorganisms, as the detector has to work for 20 years nonstop. Air showers of high-energy particles travel through the water and since they are ultra-relativistic particles, they generate what is known as Cherenkov radiation, detected by photo-multipliers. The radiation is simply visible light. Very fast electronic devices then process electric signals from photo-multipliers. The Auger South Observatory comprises an array of 1,600 such detectors scattered on 3,000 sq km of the pampas. The detectors are always on, and the effective working time of each detector approximates 97 percent.

The electronic component, the Frond-End board with FPGA integrated circuits working as triggers, is installed in all 1,600 detectors. The board has proved to be one of the most reliable parts of the detectors. Each detector operates autonomously, powered by solar panels. The detectors communicate with the Central Data Acquisition Station via radio connection.

Additional fluorescence detectors have been placed on the edges of the 3,000-sq-km area, fitted with huge parabolic reflectors that monitor the atmosphere at zenith angles of 0-30 degrees. Diffused fluorescent light generated along the trajectory of a particle hits the reflectors and is focused on the sensors of photo-multipliers. The second-level triggers then identify the topological patterns of the traces on the sensors, recognizing those of air showers, which develop in the atmosphere, and rejecting ones that are deemed random.

What have you been working on lately?
I’ve been trying to make the detection systems more “intelligent,” so that they could learn to identify the most interesting events and focus on those. I want the systems to be able to spontaneously adapt to changing conditions. We would like to equip the systems with a neural network of some kind.

I am also involved in the AERA experiment, where I am developing a trigger and signal filtration system for 150 measurement systems designated to detect geo-synchrotron radiation. The systems work with and support the surface and fluorescence detectors in the Auger experiment.

Do the detectors require constant maintenance?
Each participating research center signs a clause under which it is obligated to take maintenance shifts. The point is for all to be actively involved in the experiment, including the practical side of it. Theoreticians are frequently detached from reality. While they do not necessarily ignore the precision factor in experiments, they often fail to realize that some data needs to be approached very critically.

I often travel to Argentina and spend three weeks of moonless nights operating the fluorescence detectors or a device called LIDAR, which is used to survey the atmosphere.

Why moonless nights?
A cascade of particles emits fluorescent light in the UV range, which gets captured by the huge reflectors. Even the faintest light from the Moon could blind the fluorescent detectors that watch the sky. When thunderstorms happen, even far away from the Surface Array, the equipment shuts down automatically, because thunderbolts blind the detectors as well. Besides, these are very sensitive detectors and could be easily damaged during a storm.

There is more to the maintenance shifts than just the technical side of the experiment. For example, we constantly check if all the recorded events make sense, we see to it that the calibration is correct and so on. This is complicated and vulnerable equipment.

My colleagues in Karlsruhe have recently been developing modules to enable remote control of the detectors in Argentina from any spot on the Earth. Consulting colleagues by phone is far easier to do from Germany or France than it is from the plains of Argentina, as telecommunications in that area leaves much to be desired. Work to lay a fiber-optic line is only beginning. So far, we have had to lease a traditional microwave line from an oil company, but the bandwidth of the line is insufficient.

Whenever I can I try to combine my journeys with the installation of new equipment. Any such operation takes a 24-hour trip to the plains in an off-road vehicle that in such terrain travels with an average speed of 5-10 kilometers per hour. After new electronics have been installed in the detector and calibrated, I have to check if the detector communicates well with the control center and if the preliminary measurements fit within the acceptable range. If they do, then long-term tests have to be launched to verify the stability of the detector’s work in temperatures that can vary by 40 degrees Celsius in one day. A normal 24-hour shift comprises a night watch over a fluorescence detector and hours spent during daytime on installing new electronics out in the open.

At the end of the 19th century, physics was considered to be a more or less closed chapter, but two phenomena had not been fully explained then: the black body spectrum and the transformation of electromagnetic waves. Planck’s elucidation of the black body spectrum sparked the rise of quantum mechanics with all the consequences of the process, such as the A-bomb and H-bomb, but also transistors, computers, the internet and so on. The Lorentz transformation for electromagnetic waves, in turn, became the foundation for Einstein’s Special and General Theory of Relativity.

Nowadays, we observe particles that carry phenomenal energies. Currently available theories are incapable of explaining how such ultra-high energy particles are created. Different theories propose explanations such as topological space-time defects or interference between our world and parallel universes, and some suggest that the particles are relics of the Big Bang. We like to joke that we need a new Einstein.

How is the Pierre Auger experiment organized?
There is no formalized structure as such. The collaboration is mostly based on gentlemen’s agreements without any arbitrary task assignments. We meet every six months in Argentina at Collaboration Meetings during which each participating unit declares what tasks it can perform given its financial resources, equipment and research potential. There is an awful lot of satisfaction, when such cooperation is founded on enthusiasm, exceptionally friendly relations and curiosity about the fundamental secrets of the universe.

Do you sometimes wonder about the purpose and meaning of the research?
I think the worst thing that can happen to a scientist is to accomplish your goal and feel totally satisfied with it, because that’s when you cease to be a scientist. The fascinating thing about science is that the answer to one question leads to a succession of new questions, so you never fall into routine, as if reconnecting with the inner, curious child. The ideas behind our experiment? To figure out the origins of the world perhaps? The things we do sometimes border on science fiction. Right now it is very fundamental research, but who knows, 10 years or a century from now we might see the advent of some brand-new physics of which we do not have the slightest idea at present. Could people 100 years ago even imagine the global village that we live in today? Computers, the internet… Back then, novels by Jules Verne seemed so extremely futuristic that people considered his ideas too far-fetched and improbable.

The Pierre Auger experiment is a multinational scientific project that investigates one of the greatest puzzles of contemporary astrophysics: ultra-high energy cosmic rays known as cosmic accelerators. The research is based on the Pierre Auger Observatory built in Argentina in 2008. It is an array of 1,600 surface detectors placed on an area of 3,000 sq km and aided by 24 fluorescence telescopes located at four stations on the edges of the surface array. A second such observatory, Auger North, will be constructed in the U.S. state of Colorado and consist of 4,000 detectors on around 22,000 sq km. Used to detect and watch charged particles from outer space, Auger North will be the world’s largest hybrid detector making simultaneous use of two types of detectors: fluorescence telescopes and a surface network of particle counters.

The idea behind the Pierre Auger project was developed during a number of working meetings in Paris (1992), Adelaide (1993), Tokyo (1993) and then at the Fermi National Accelerator Laboratory (Fermilab) in Chicago (1995). The technical design for the experiment was ready in 1995. In November that year, the Pierre Auger Collaboration, a group of more than 250 physicists from 17 countries, was formally established at the UNESCO headquarters in Paris. At the same time, a decision was made that two Auger laboratories would be set up in Malargüe, Argentina and in Millard County, Utah, in the United States. The latter location was subsequently changed to Colorado.

The first leaders of the project were Nobel Prize-winning physicist Prof. James Cronin from the University of Chicago in the United States and Prof. Alan Watson from the University of Leeds, Britain. The Pierre Auger project participants come from Argentina, Australia, Brazil, Britain, the Czech Republic, France, Germany, Italy, Mexico, the Netherlands, Poland, Slovenia, Spain, and the United States.

“We have taken a big step forward in solving the mystery of the nature and origin of the highest-energy cosmic rays, first revealed by French physicist Pierre Auger in 1938,” said Cronin. “We find the Southern Hemisphere sky as observed in ultra-high-energy cosmic rays is non-uniform. This is a fundamental discovery. The age of cosmic-ray astronomy has arrived.”
The Polish scientists working on the project are grouped in two teams, one led by Prof. Maria Giller at the University of £ód¼ and the other headed by Prof. Henryk Wilczyński at the Institute of Nuclear Physics in Cracow. Both teams are dealing with theoretical calculations. The £ód¼ team includes Prof. Zbigniew Szadkowski from the University of £ód¼’s Department of High Energy Astrophysics, who is working to develop electronic systems for space particle detectors.
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