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New Study Sheds Light on Elusive Dark Matter in Galaxies

Written by katrinlink on . Posted in

29 October 2014

Strong gravitational lensing as observed by the Hubble Space Telescope in the galaxy cluster Abell 1689 indicates the presence of dark matter (Image Credit: NASA, N. Benitez (JHU), T. Broadhurst (Racah Institute of Physics/The Hebrew University), H. Ford (JHU), M. Clampin (STScI),G. Hartig (STScI), G. Illingworth (UCO/Lick Observatory), the ACS Science Team and ESA).

The results of an indirect search for dark matter in dwarf spheroidal (dSph) galaxies reported at the 5th International Fermi Symposium held in Nagoya Japan, were anticipated by many scientists trying to understand the mysterious dark matter component in the Universe. The Fermi Large Area Telescope (LAT) “did not confirm a postulated gamma ray signal from annihilating dark matter” according to Brandon Anderson, a postdoctoral researcher at the Oskar Klein Centre at Stockholm University. Brandon is part of a team, also including US colleagues in Stanford and European colleagues in Montpellier, which are behind the Fermi-LAT’s Collaboration dSph study. The search results were particularly anticipated in the light of a recent possible detection of dark matter in the center of Milky Way.

The Fermi Large Area Telescope (LAT) is a NASA-led gamma-ray detector with strong European contributions that has been in orbit since 2008. Its energy range (about 100 MeV to 1 TeV) enables scientists to test a popular theoretical dark matter candidate, the Weakly Interacting Massive Particle (WIMP). In many versions of this model, when two WIMPs come together they can annihilate one another.  This means they transform their mass energy into conventional particles, including gamma rays, the most energetic form of radiation.

“WIMPs give with very few assumptions and well known physics the right amount of dark matter, within a factor of few, known from measurements for example of the cosmic microwave background,” says Jan Conrad, professor in Astroparticle Physics at Stockholm University. “We usually refer to this as the WIMP miracle. In addition there are theories, proposed for completely different reasons than the existence of dark matter, (in particular supersymmetry) that predict the existence of WIMPs”.

Brandon Anderson is a member of Jan Conrad’s group in Stockholm University, working in astrophysical searches for particle dark matter. “On Earth, the density of dark matter is so low that WIMPs annihilation almost never happens,” Anderson says. “But there are places where we know DM is very concentrated: locally, the densest regions are in the galactic center and nearby dwarf spheroidal galaxies”. Being an all-sky monitor, the LAT observes them all pretty equally. Gamma rays in excess to the expected background due to known gamma ray emitters coming from the dense regions are the signature that is looked for.

Although the galactic center is nearby and massive, it is astrophysically complex. So much so that interpretation of LAT data from the galactic center has still to be provided by the Fermi-LAT Collaboration. Undaunted, several groups now report that models of known sources do not account for all the gamma-ray emission, leaving what is blandly referred to as the galactic center excess (GCE). Despite the astrophysical uncertainty, all parties agree that the GCE is peaked at around 1 GeV (1 billion electron volts) and extends fairly symmetrically to about ten degrees (5,000 light years) from the centre of the Milky Way. Plausible conventional production mechanisms not requiring dark matter include pulsars and cosmic ray collisions on gas clouds. But the GCE can also be neatly accounted for by the existence of a 30 GeV WIMP which self-annihilates into b quarks. “Without completely ruling out alternatives, this possible dark matter explanation remains just that – possible,” Anderson says.

A large spiral galaxy (M51) colliding with its dwarf companion (NGC 5195) galaxy (Image Credit: NASA, Hubble Heritage Team, (STScI/AURA), ESA, S. Beckwith (STScI). Additional Processing: Robert Gendler)

What made this model so exciting was that it also happened to fit a slight gamma-ray excess seen in a previous dSphs analysis published this spring in the journal Physical Review D. Dwarf galaxies orbiting the Milky Way represent an independent data set, and one that is free of the uncertainties which plague the galactic centre data, because they lack other types of gamma-ray emitters. An excess signal of the same character in both the galactic centre and dSphs would be strong evidence of a dark matter detection. The spring dSph excess was so low that, if it was not just a fluctuation in the gamma-ray background, we would have to wait many years to gather enough photons for it to become significant.

“Fortunately, the LAT’s ground team just gave us a big push,” says Anderson. The LAT relies on extensively calibrated classification algorithms to reconstruct incoming gamma rays from their electronic signatures. These routines have been periodically overhauled throughout the mission as knowledge of the instrument continues to improve. The latest overhaul, known as Pass 8, marks the biggest advance yet, boosting the instrument’s effective area and sharpening its angular resolution. The resulting improved data set warranted a fresh look at the dSphs.

The announcement from Brandon Anderson and colleagues at the Fermi Symposium may have come as a disappointment for some scientists: For dSphs the significance of the GCE model dropped drastically, along with all other WIMP annihilation masses and channels. It dropped so far, in fact, that the limits now exclude the annihilation cross section (essentially the proximity at which they interact), which WIMPs should exhibit would they be the dark matter, up to particle masses of 100 GeV. “These are now the best WIMP model constraints in the world below 1 TeV, but what is more important is that if the simple WIMP paradigm would be correct, our results would require them to be heavier than 100 GeV,” Anderson says. “While these constraints do not conclusively rule out the dark matter interpretation of the GCE, they lend no support”.

Is there any other, unrelated evidence for the existence of WIMPs? “The direct detection experiment DAMA/LIBRA has reported evidence for WIMPs on high significance level, but this result is highly controversial and could not be confirmed by other experiments,” Conrad says. “Other direct detection experiments, e.g. CDMS, have reported hints that do not exceed the required significance to be called “detection” or “evidence”. So the simple answer is: no. ” he adds.

So, what are the alternatives to the standard WIMP picture? Apparently there is a myriad of particle candidates for dark matter. “From my point of view a good candidate has to a) give the right properties (e.g. the abundance) without too many additional assumptions and b) hopefully provide more solutions except the dark matter problem,” Conrad says. The WIMP fulfills both of these requirements. “If the WIMP would be ruled out, I would probably turn to axions, which could constitute dark matter but in addition solve another problem, the so called strong CP problem in Quantum Chromo Dynamics (QCD)”.

APPEC Communication Office:
Eleni Chatzichristou

Hands-on Experimental Underground Physics at LNGS

Written by katrinlink on . Posted in

27 October 2014

GRAN SASSO SUMMER INSTITUTE 2014

LNGS: A Unique Research Facility

The Gran Sasso d’Italia is a beautiful mountain located in the Abruzzo region of central Italy, immersed in a National Park of exceptional beauty. The Gran Sasso National Laboratory (Laboratori Nazionali del Gran Sasso, LNGS), the largest and most advanced underground laboratory in the world, is situated on one side of a 10-kilometre highway tunnel crossing the Gran Sasso massif. Its low background environment (provided by the 1400-metre rock coverage) is ideal for hosting experiments in the fields of astroparticle physics and nuclear astrophysics, which lie at the intersection of particle physics, astrophysics and cosmology. The main research topics currently carried out at LNGS are neutrino physics, cross-section measurements of rare nuclear processes of astrophysical relevance, neutrino-less double beta decay, dark matter searches, and low rate counting for radioactivity measurements.

LNGS is used as a worldwide facility by more than 900 scientists from 30 different countries, who are working on 20 experiments in different phases of realization: Neutrino experiments such as BOREXINO, ICARUS, LVD and OPERA are aimed at identifying and characterizing neutrinos from different natural sources, like the Sun, the Earth, the atmosphere, supernovae, or artificially produced by accelerators and other sources. The experiments CUORE and GERDA search for neutrinoless double beta decay in different isotopes. Dark matter and WIMP searches are performed through the experiments CRESST, DAMA, DARKSIDE and XENON, using detectors exploiting complementary techniques. Nuclear cross sections relevant in astrophysical and cosmological processes are directly measured by the LUNA experiment. Small effects due to general relativity are studied by the GINGER experiment. Innovative experimental technologies are being developed by the COBRA and LUCIFER projects, aiming at the discovery of neutrinoless double beta decay.

LNGS is a consortium member of APPEC and as one of its functional centres it carries three main functions: Networking among European institutions and projects; Organizing a “virtual theoretical astroparticle physics centre for the interpretation of experimental data; Organizing periodical international schools for graduate students.

Summer Institute 2014: An Educational Opportunity in Astroparticle Physics

The Gran Sasso Summer Institute 2014 was organized in LNGS after the positive experience of the ISAPP Summer Institute held at the Karlsruhe Institute of Technology (KIT). The objective was to get graduate students and young post-docs directly involved into the research carried out at LNGS. Under the supervision of Gran Sasso researchers the participants carried out hands-on activities related to the experiments currently underway. The Summer Institute took place between 22 September and 3 October 2014 and focused on the following topics:

  • Dark Matter (CREEST, DAMA, DARK SIDE, XENON)
  • Neutrinoless Double Beta Decay (CUORE, GERDA, LUCIFER)
  • Neutrino Oscillations (BOREXINO, ICARUS, OPERA)
  • Nuclear Astrophysics (LUNA)
  • Low Radioactivity Measurements (STELLA)
  • Geodesy and General Relativity (GINGER)

The school was attended by 26 participants (7 female and 19 male), associated with universities and research institutes active in astroparticle physics located in 10 different countries (China, Germany, India, Italy, Poland, South Korea, Spain, Sweden, Switzerland, USA). “The Institute had a strong appeal in the international scientific community” says Maddalena Antonello, researcher at LNGS, currently member of the ICARUS and LBNE collaborations and member of the Local Organizing Committee of the Gran Sasso Summer Institute. “Indeed the students were coming from major laboratories and Universities in the Astroparticle and Nuclear Astrophysics fields, such as Fermilab, Caltech, Princeton University, Seoul National University, IFIC Valencia, Max Plank Institute Heidelberg, Tokyo University” she adds.

The students first attended a series of lectures providing them with the background necessary to carry out the practical activities. The lectures, delivered by world-wide experts, focused on experimental topics, techniques and methods relevant for underground astroparticle physics.

The hands-on work was structured around 17 activities and carried out in small groups of 1-2 students to promote discussion and interaction in informal and constructive ways. At the end of the two weeks the students reported the results of their research projects in short seminars. These will also be published as proceeding papers in the peer-reviewed journal Proceedings of Science.

In addition to the educational activities, the students took part in excursions and social events and had the chance to discover the Gran Sasso National Park and its surroundings, including the top of “Gran Sasso” Mountain (Corno Grande) and the charming medieval villages nearby.

“The major success of the Institute was the positive feedback we got from the students about the peculiarity of this school: the possibility to actively learn an experimental technique or method, which will certainly be useful in their future scientific career” says the scientific secretary of the Summer Institute Carla Macolino, postdoctoral researcher working in the GERDA experiment at LNGS and Gran Sasso Science Institute.

The Summer Institute has aroused considerable interest and the organizers hope that the initiative will be repeated next year, providing a unique educational opportunity for doctoral students in astroparticle physics around the world: “Our feeling is that the Gran Sasso Summer Institute experience was very positive for everybody: students, organizers, tutors and lecturers. We believe that the success of this first edition will be a good starting point for further editions we wish to organize”, say Carla and Maddalena.

Comments of a student

”Gran Sasso Summer Institute 2014 was a great experience for me. It gave opportunity to know more about neutrino physics, the field in which I am working. The lectures covered topics about fields as interesting and exciting as neutrino oscillations, dark matter searches, nuclear astrophysics, neutrinoless double beta decay and general relativity.

The visit to the underground lab was very interesting since we got a chance to see all the famous experiments. It was quite exciting to do a summer project in a very prestigious institute like LNGS.

The hands-on session was very good. Our group worked on ICARUS data analysis which was very interesting. We got great encouragement and support from our tutors Maddalena and Iza who guided us very well.

Not only did we learn about our own hands on experiment, but in the student presentations, we got to know about what the other students did too, which was really great. We were given great hospitality and help by the organizers. I am grateful to the organizers for selecting me to be a part of this prestigious school. It was a great experience.“

The Gran Sasso Summer Institute 2014 was realized thanks to the amazing work of the LOC, composed by staff members at Gran Sasso (M. Antonello, S. Davini, A. D. Ferella, P. Gorla, A. Ianni (Chair), M. Junker, C. Macolino (Scientific Secretary), M. Mannarelli, L. Pilo, S. Ragazzi, F. Chiarizia) and with the support of the following institutions: Istituto Nazionale di Fisica Nucleare (INFN), International School on AstroParticle Physics (ISAPP), Astroparticle Physics European Consortium (APPEC), Gran Sasso Science Institute (GSSI), Department of Physics of L’Aquila University, Center for Astroparticle Physics (CFA).

The International Advisory Committee was composed by highest level scientists:  F. Avignone (University of South Carolina), F. Calaprice (Princeton University), E. Coccia (Gran Sasso Science Institute), K. Eitel (Karlsruhe Institute of Technology), F. Ferroni (Università di Roma La Sapienza & INFN), S. Katsanevas (Université Paris VII, Denis Diderot & CNRS), A. Masiero (Università di Padova & INFN), K. Peach (University of Oxford), F. Pröbst (Max Plank Institute für Physik, Munchen), S. Ragazzi (INFN LNGS), S. Schönert (Technische Universität, Munchen), G. Senjanovic (Gran Sasso Science Institute).

The Gran Sasso Summer Institute 2014 is included in the framework of the following EU Funded Project:  Progetto Speciale Multiasse “La Società della Conoscenza in Abruzzo” (PO FSE Abruzzo 2007-2013 – Piano Operativo 2009-2010-2011).

Fifth International Fermi Symposium

Written by katrinlink on . Posted in

20 October 2014

October 20-24 2014, Nagoya Japan

The fifth international Fermi Symposium starts today at Nagoya University in Japan. This meeting will focus on the new scientific investigations and results enabled by Fermi, the mission and instrument characteristics, future opportunities, and coordinated observations and analyses.

The two Fermi instruments have been surveying the high-energy sky since August 2008. The Large Area Telescope (LAT) has discovered more than a thousand new sources and many new source classes, bringing the importance of gamma-ray astrophysics to an ever-broadening community. Fermi LAT’s study of diffuse gamma-ray emission in our galaxy revealed giant bubbles shining in gamma rays. The direct measurement of a harder-than-expected cosmic-ray electron spectrum may imply the presence of nearby cosmic-ray accelerators. LAT data have provided stringent constraints on new phenomena such as supersymmetric dark-matter annihilations as well as tests of fundamental physics. The Gamma-ray Burst Monitor (GBM) continues to be a prolific detector of gamma-ray transients: magnetars, solar flares, terrestrial gamma-ray flashes and gamma-ray bursts at keV to MeV energies, complementing the higher energy LAT observations of those sources in addition to providing valuable science return in their own right.

For more information visit the Symposium Website.