Construction of a telescope that will map the sky in a new way has begun on a mountain in Chile. The Large Synoptic Survey Telescope (LSST) will revolutionize the study of transient objects in the Universe by repeatedly imaging half the sky every few days using the largest digital camera in the world, generating 15 terabytes of new data each day.
Many objects in the Universe are transient, i.e. they produce light for a limited amount of time. This may be because the event itself is short-lived, like the supernova explosion of a dying star, or because the object is nearby so that its position on the sky changes quickly, like an asteroid. The 10 year movie that LSST produces will show these objects appearing, disappearing, and moving in the southern hemisphere’s night sky.
These observations will also produce deep images of the sky leading to catalogues of astronomical objects which are thousands of times larger than are currently available. First light with LSST is expected in 2019.
The Stockholm University Physics and Astronomy departments have joined the LSST collaboration. These researchers will use the observatory to help them better understand supernovae, to map the structure of the Universe, and to try to detect the optical counterparts to gravitational wave sources.
Physicists around the globe are working relentlessly to pin down the nature of dark matter. This enigmatic entity hides itself from our view as it does neither emit nor absorb any radiation. It only reveals itself through its gravitational interaction. With a new analysis of data from NASA’s gamma-ray large area telescope (LAT) on board the Fermi satellite, we have now come closer to test very light dark matter candidates.
Many regard the astrophysical evidence for dark matter as evidence for yet undiscovered fundamental particles. Well-motivated theories suggest that these particles should be quite heavy, probably a couple of dozen times as massive as the proton. If so, either their decay or the annihilation of two such particles could result in energetic light: gamma-ray emission that we could detect with the LAT. Also the LHC and direct detection experiments such as the recently inaugurated Xenon 1T are looking for traces of heavy dark-matter candidates.
However, dark-matter particles could also be extremely light, more than five hundred billion times lighter than the electron. They are often referred to as axion-like particles (ALPs). They are close relatives to the axion which was originally proposed in the late 70s to solve problem in the strong interactions. It was soon realized that this class of particles could also constitute the dark matter. Unfortunately, very light ALPs have lifetimes much longer than the age of the Universe and do not obliterate each other upon meeting. However, there’s a small chance that ALPs transform into ordinary light (and vice versa) in the presence of a magnetic field. This leads to oscillations between light and ALPs very much like neutrino oscillations.
Illustration of the photon-axionlike particle conversion: Light produced in the jet of the galaxy converts into ALPs (dashed lines) and vice-versa when interacting with the magnetic field (wiggled line). Credits: Aurore Simonnet, Sonoma State University and NASA/NOAA/GSFC/Suomi NPP/VIIRS/Norman Kuring.
We have used Fermi-LAT observations of an extremely bright gamma-ray source to search for traces of these oscillations: the central galaxy of the Perseus galaxy cluster. In its center, a spinning super-massive black hole powers a current stream of plasma that emits gamma-ray radiation. When the gamma rays propagate through the magnetic field present in the gas between the galaxies of the cluster, they could transform into ALPs and imprint distortions onto the galaxy’s spectrum.
We did not find any such features above the level of statistical fluctuations which enabled us to rule out ALP models that would contribute sub-dominantly to the overall dark-matter content. These are best constraints in the particular ALP mass region to date. Our limits turned out to be as sensitive as as the future “Any Light Particle Searches” (ALPS) experiment. In the ALPS experiment, it is planned to shoot a powerful laser beam immersed in a magnetic field onto a wall and look for light that might have made it through it in the form of ALPs.
Our analysis only marks the beginning for ALP searches with the Fermi-LAT. With the analysis of more sources we will be sensitive enough to probe ALPs that could make up all the dark matter in the Universe.
Caption for the picture in the cover: Composite image of the central galaxy in the Perseus cluster taken with the Hubble space telescope and the Chandra X-ray satellite.
Credits: X-ray: NASA/CXC/IoA/A.Fabian et al.; Radio: NRAO/VLA/G. Taylor; Optical: NASA/ESA/Hubble Heritage (STScI/AURA) & Univ. of Cambridge/IoA/A. Fabian
Dark matter is one of the basic ingredients of the Universe, and searches to detect it in laboratory-based experiments are being conducted since decades. However, until today dark matter has been observed via its gravitational interactions that govern the dynamics of the Cosmos at all length-scales. Dark matter is a big theme of the Oskar Klein Centre, both for theorists and experimentalists.
In 2014, with a grant of the Knut and Alice Wallenberg foundation, OKC has joined an international collaboration, called XENON, that builds and operates detectors to find the elusive dark matter particles in the laboratory.
The latest incarnation of the XENON detector, which is the most sensitive dark matter detector existing, has been inaugurated November 11, 2015 with a party attended by about 80 scientists, funding agency representatives and VIPs, e.g. the Swedish ambassador to Rome, Robert Rydberg. Why the ambassador in Rome? The detector at the Laboratory Nazionali del Gran Sasso (LNGS), one of the largest underground laboratories in the world. Here 1400 meters or solid rock shield the detectors from cosmic ray particles which would otherwise flood the detector with rubbish signal.
Principle of operation of a dual-phase Xenon TPC. A particle interacts with liquid xenon, the nucleus recoils and produces scintillation light and electrons. The elecons get drifted towards gas phase xenon, where a proportional scintillation signal is produces. The light is detected by photomultiplier tubes. The relative sizes of prompt and delayed signal can help to distinguish (electromagnetic) background from the nuclear recoils we are interested in.
How does it work?
As the name indicates, the detector is made of the noble gas xenon. The main idea is that dark matter particles, assumed to be stable relics of the Big Bang, will every once in a while interact with xenon nuclei. The nucleus will then recoil. A particle detector called Time Projection Chamber (TPC) can be used to detect this recoil. Our TPC consists of roughly 1 tonne of xenon, contained in a cylinder of 1 meter diameter and 1 meter height. In liquid phase the recoiling xenon atoms ionize and excite neighboring atoms. When these dexcite scintillation light is produced. The ionized electrons are drifted (by means of an electric field) to a gas-phase Xenon where they give rise to a secondary light signal, which is delayed by a few micro-seconds. The light is observed by 248 photosensors. The principle is illustrated in figure 1.
As the dark matter is elusive we only expect about five or so recoils due to dark matter particles in one tonne of xenon per year! Therefore, the number one enemy is a background of recoil signal sinduced by other particles or noise in the detector. Most of the work deals with reducing it. Luckily we have a few ways to reduce it.
The ratio and size of the prompt and delayed signal yield information on the type of interaction that happened, helping to distinguish precious signal from trash. The time between the prompt signal, as well as the patterns of signal in the PMT arrays can be used to reconstruct the position of the interaction in three dimensions, allowing only to consider recoils happening in the center of the TPC, which are less likely to come from background. The Xenon gas itself is highly purified, using cryogenic distillation techniques specially developed for the purpose. Similarly, all components of the detector have been screened for radioactive contamination using ultra-sensitive dedicated detectors. And also the TPC is situated in a huge water tank, about 10 meter in diameter, which is equipped with photosensors, detecting light produced by the Cherenkov effect of the relativistic particles inducing a possible background. Any event in the detector that has also a signal in this water tank is vetoed.
Such a complex detector can not be build by only a few physicists. Apart from us, the XENON Collaboration consists of 21 research groups from the US, Germany, Italy, Switzerland, Portugal, France, the Netherlands, Israel, and Abu Dhabi. In total about 130 physicists are involved.
OKC’s main responsibility is to build and operate the European analysis facility for performing the tricky analysis of finding the few interesting dark matter recoils in the ocean of fake signals, truly a task of finding a needle in a haystack. The facility will be hosted by the Parallel Data Centrum at KTH. Meanwhile, we tried to make ourselves useful: OKC researcher Alfredo Ferella has been the analysis coordinator and run coordinator for the 1T predecessor, XENON100, a similar but much smaller detector, which aside the comparable LUX detector in the US, provided the most stringent constraints on dark matter to date. We also helped to estimate XENON1Ts sensitivity and spend a few days to assist in the final construction steps of the TPC, see picture below.
OKC resarchers, Bart Pelssers, Alfredo Ferella and Jakob Calvén during the assembly of the TPC above ground in Gran Sasso
Now that the detector has been completed, we can start preparing for analysis. The next couple of month will be spent in getting the detector ready and fully understood to start taking data for the dark matter search. We expect XENON1T to be about 100 times more sensitive than the most sensitive detectors today, so as Elena Aprile, the founder of the XENON project, put it: we hope we will discover the damn thing.
Cover picture: The XENON1T detector. Left: the 10 meter high cylinder, which is the Cherenov muon veto. The cladding shows an illustration of the TPC inside it. Right: service building with cryogenic and purification systems on the top floor, data aquisition and slow control on the middle floor and storage and recovery on the ground floor
