Tag Archives: dark matter

Interview with Ankit Beniwal

Hej! My name is Ankit Beniwal, and I’m from Adelaide, Australia. I’m a short-term (6 months)
postdoc at the Oskar Klein Centre, Stockholm, Sweden. Before coming to OKC, I finished my PhD in theoretical particle physics at the University of Adelaide. I also did my first class Honours in theoretical and experimental particle physics at the same university.

Unlike many physicists who became passionate about physics at a young age, I wasn’t aware of physics in general when I was young. This completely changed when I took physics in year 11 and 12. I had an excellent (female) physics teacher named Ms Lindy Bartlett. She loves physics and encourages students by saying “Physics is gold!” By the time I finished high school, my love for physics had grown so much that I decided to undertake an undergraduate degree in physics at the University of Adelaide, and later went on to do a PhD.

I love being a scientist. Not only do we get paid to study the world around us, we are also trying to answer some of the fundamental questions in physics. However, in the particle physics community, there is a strong push for more publications; it’s ultimately not a bad thing, but it is hard for a postdoc applicant as he/she is partially judged on this basis. I solely believe in the quality of work rather than quantity. With that said, I also need to improve on the latter aspect of being a scientist.

What is your field of research?
My research interests include dark matter (DM) phenomenology, astroparticle and Higgs physics. In the past, I’ve studied the phenomenology of Higgs portal DM models where DM interacts with the Standard Model (SM) particles via the Higgs boson. This leads to a rich DM phenomenology at colliders, indirect and direct DM detection experiments. In addition, these models can also help in explaining the observed matter-antimatter asymmetry in our Universe.

At the OKC, I’m working under Prof. Joakim Edsjö on secluded DM models. In these models, the DM particle annihilates into metastable mediators which subsequently decays into SM particles. By having a weak coupling to SM particles, the models are difficult to detect directly. On the other hand, they offer much better indirect detection probes via gamma rays, charged particles and/or neutrinos. The main motivation behind these models is that a neutrino signal from DM annihilation in the Sun would generally be enhanced relative to the standard scenario, i.e., one where DM annihilates directly into SM particles via a short-lived mediator.

What are your research plans for your time in Sweden?
Although 6-months isn’t a long time, my plan is to understand the secluded DM models and their phenomenology at neutrino telescopes in more detail. In particular, I’d like to write up a paper with Joakim and others on this work.

Which of your skills are you most proud of? What new skills would you like to learn in the next year?
I’m particularly proud of the skills that I’ve acquired over the course of my PhD. These include computing skills (being proficient in multiple computing languages and high-performance computing), teaching, tutoring, mentoring and being able to learn new concepts in a short period of time.

There’s always room for more improvement. For instance, I’d like to get better at time management, writing more research papers, and initiating new collaborations.

What advances or new results are you excited about or looking forward to?
In the last few years, tremendous progress has been made in all areas of physics ranging from neutrinos, DM and gravitational waves (GW). Thus, it is an opportune moment to be involved in these areas.

Some future prospects that I’m looking forward to are as follows.
1. Many experiments are underway to better understand the neutrino properties (e.g., neutrino oscillations, CP-violating phase, absolute neutrino masses etc). This is exciting news!
2. Future direct DM search experiments will tell us if the particle description of DM is consistent or not. These experiments are very close to reaching the neutrino floor where they’ll also become sensitive to neutrinos. In addition, many planned experiments will try to either confirm or refute the long-standing annual modulation signal seen by the DAMA experiment.
3. Multiple GW signals have been detected. We are now entering a new era of GW astronomy. Future space-based GW experiments such as LISA will be able to observe GW signals from the electroweak phase transition, a simple mechanism that explains the matter-antimatter asymmetry in our Universe.

If I offered you unlimited funding right now, to be spent on something scientifically relevant, what would you use it for?
If I had unlimited funding, I’d spend it on new computing resources, in particular, on supercomputers. In recent years, it has become increasingly difficult to find new resources for performing multi-dimensional parameter space scans.

With no evidence of a DM signal, we must combine all available data from various DM searches and make statistical inferences on as many DM models as possible. This is the primary goal of the Global And Modular Beyond-the-standard-model Inference Tool (GAMBIT). To achieve these goals, we need a large number of computing resources.

What’s your favorite food? Why?
Being originally from India, I’m obviously biased towards Indian food. In particular, I love butter chicken and plain naan. It’s tasty and mouth-watering!

Why did you choose the OKC?
I enjoy working at the OKC. It has a good mixture of cosmologists, astronomers, experimentalists and particle physicists. Its status at the international level is outstanding. The research staff at OKC are also world-renowned scientists in their field of research.

How do you relax after a hard day of work?
From time to time, I try to explore the city. The weather is getting better day-by-day, so I’ll see more of what the city has to offer.

I’m also trying to learn swimming during my stay in Stockholm. This is an activity that has been on my bucket list for a long time. My goal is to become a proficient swimmer, so I can enjoy the beautiful beaches back home.

What do you hope to see accomplished scientifically in the next 50 years?
In the next 50 years, I hope that we can solve some of the biggest problems that are currently faced by mankind, e.g., global warming, fossil fuels, pollution, poverty, etc. Many people are trying to tackle these issues but more support is required from the government and public to solve them.

Scientifically, the most optimistic scenario in my case would be one where we have discovered DM non-gravitationally. Once we have a DM signal, we can hope to understand its properties, e.g., mass, spin, coupling to SM particles, etc.

Ankit is a postdoc in the Stockholm University Physics Department who joined the OKC in the Spring of 2018.
Thanks Ankit! Try the new Saravana Bhavan in Kista for great South Indian food.

Results released from first 34 days of the XENON1T experiment

Understanding all the detailed physics going on inside the world’s most sensitive dark matter detector is a great challenge to work on. — Bart Pelssers, PhD student, Stockholm University, Oskar Klein Centre

The first results have just been released from XENON1T (“Xenon One Ton”), the most sensitive dark matter detection experiment in the world. Dark matter has not yet been detected but these results push the limits for the detection of a specific kind of dark matter particle (Weakly Interacting Massive Particles) lower than those from previous experiments.  This is after collecting data for just 34 days.

This experiment consists of a liquid xenon central detector surrounded by ultra-pure water which shields the detector. When particles collide with the xenon nucleus it emits light and electrons that the experiment can detect. Xenon can also be made very pure and it is dense enough that the inner part of the xenon nucleus is almost completely isolated from radioactivity from the detector itself as well as outside.  The XENON1T experiment is located underneath a mountain in the Gran Sasso Underground Laboratory in Italy in order to shield the detector from cosmic rays.

The XENON collaboration contains scientists from 10 different countries, including a number of Oskar Klein Centre researchers. Given these results the dark matter community will be watching this experiment closely to see what it finds after analyzing a larger amount of data. In fact, more than 60 new days of data have already been recorded!

Note how quickly the sensitivity of these experiments increases — the last result from XENON100 was made with 100kg of xenon and a detector that could fit in a living room while the new one is suspended in a three-story tank of water. One should also take a second to look at the green band– that shows the region the upper limit would be in 68% of the time. It is huge! That is because we are looking for very rare events, and so any statistical fluctuation or misestimated background could move our result a lot. That we find a result like the one we show is a good indication that we already understand our detector quite well. — Knut Morå, PhD student, Stockholm University, Oskar Klein Centre

The spin-independent WIMP-nucleon cross section sensitivity limits as a function of WIMP mass at 90% confidence level for this run of XENON1T (in black). Results from previous experiments are shown.  The 1- and 2σ sensitivity bands are shown in green and yellow.
The spin-independent WIMP-nucleon cross section sensitivity limits as a function of WIMP mass at 90% confidence level for this run of XENON1T (in black). Results from previous experiments are shown. The 1- and 2σ sensitivity bands are shown in green and yellow.

Hunting light dark matter with gamma rays

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.
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.

– Manuel Meyer (manuel.meyer@fysik.su.se)

Article: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.161101
Preprint of the manuscript: http://arxiv.org/abs/1603.06978
Fermi satellite: http://fermi.gsfc.nasa.gov/
ALPS II experiment: https://alps.desy.de/

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

ATLAS Thesis Award goes to the Dark Side

Ruth Pöttgen is a postdoctoral researcher in the ATLAS group at Stockholm University. In 2015, she obtained her Ph.D. at the Johannes Gutenberg – University in Mainz, Germany, for her thesis on a “Search for Dark Matter in Events with a highly energetic jet and missing transverse momentum at a centre-of-mass energy of 8 TeV with the ATLAS Detector”. At the ATLAS collaboration meeting in February, Ruth was awarded one out of 4 ATLAS Thesis Awards for outstanding contributions to the ATLAS-Experiment in the context of a Ph.D. thesis; more than 100 theses were eligible.

Ruth Pottgen
Ruth Pottgen

During her Ph.D., Ruth was based at CERN within its doctoral student programme, funded by a Wolfgang-Gentner scholarship. She was active member of the CERN group responsible for the central trigger — a vital component of the ATLAS experiment — and contributed both to its day-to-day operation as well as its upgrade during the long LHC shutdown from 2013-2015.

In 2012, she joined one of her colleagues from the central trigger group in a search for weakly interacting massive particles (WIMPs) — a popular generic class of candidates for Dark Matter particles, for which also various searches are pursued within OKC.

While there is ample compelling evidence for the existence of Dark Matter at various cosmological scales, an unambiguous direct observation of Dark Matter in the laboratory is still pending. The Standard Modell (SM) of particle physics does not provide a viable candidate to account for all of the observed Dark Matter, which is estimated to amount to roughly 80% of the matter content of the universe. WIMPs could, in a natural way, account for the present day abundance of Dark Matter. Such particles feature in many theories for physics beyond the SM, e.g. the lightest supersymmetric particle in R-parity conserving super-symmetry models.

A number of experiments searches for evidence for WIMPs and can be grouped roughly into three different categories. Direct searches look for the nuclear recoil in a target material due to the scattering of WIMPs off nuclei, while indirect searches try to detect the products of WIMP annihilation into SM particles. In recent years, also the search for WIMP pair production at hadron colliders has gathered momentum. It is in many regards complementary to the other search approaches and has become the third pillar for the hunt for Dark Matter.

Like neutrinos, WIMPs do practically not interact with the detector material and are hence themselves ‘invisible’. If they are, however, produced together with some other, ideally highly energetic object, their presence can be inferred from the missing contribution to the momentum balance. One possibility is that the other object is a hadronic jet (a bundle of particles). Such events are commonly referred to as mono-jet events. The signal is then expected to manifest itself as an excess above the Standard Model prediction at large missing transverse energy. While the direct searches are essentially background-free experiments, one of the major challenges in the collider-based search is the careful estimation of irreducible backgrounds, dominated by the production of Z-bosons together with jets, where the former decay in neutrino-antineutrino pairs.

The preliminary ATLAS results based on half of the data set collected at a centre-of-mass energy of 8 TeV in 2012 had been presented by Ruth at the ISAPP summer school on Dark Matter, held in Stockholm and organised by OKC in 2013. The results were discussed with great interest and Ruth was awarded with one of the three poster awards.

For the analysis of the full 2012 proton-proton dataset (20 fb-1) in her thesis, Ruth optimised the event selection with respect to the sensitivity for a WIMP signal. The search was performed in eight signal regions of increasing missing transverse momentum and no significant excess was observed. Model-independent limits on the cross section for new physics as well as bounds on parameters of two different, but related models for Dark Matter were derived.

Together with Ruth, three members of the SU ATLAS group were and are still involved in the mono-jet analysis, but focussing on a different interpretation.

Ruth is currently based at CERN and coordinates the ATLAS search for first and second generation scalar leptoquarks, which is to be published for the Moriond conference in March. Afterwards, she’ll return to Stockholm and is eager to explore possibilities of a collaboration with Dark Matter experts at OKC.

The “Award ceremony” will take place on today, February 25, at CERN.

The XENON1T inauguration

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. The relative sizes of prompt and delayed signal can help to distinguish (electromagnetic) background from the nuclear recoils we are interested in.
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
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

Dark matter at the heart of the Galaxy

A new study is providing evidence for the presence of dark matter in the innermost part of the Milky Way, including in our own cosmic neighbourhood and the Earth’s location. The study demonstrates that large amounts of dark matter exist around us, and also between us and the Galactic centre. The result constitutes a fundamental step forward in the quest for the nature of dark matter.

The image displays the rotation curve tracers used in our paper over a spectacular photo of the disc of the Milky Way as seen from the Southern Hemisphere (credit to the photo below). The tracers are colour-coded in blue or red according to their relative motion with respect to the Sun. The spherically symmetric blue halo illustrates the dark matter distribution inferred from our analysis.
Background image credit: Brunier http://apod.nasa.gov/apod/ap080104.html

In the modern cosmological paradigm dark matter pervades the whole Universe and is the main component of galaxies. One of the emblematic cornerstones of this paradigm dates back from the 1970s, when Vera Rubin, Kent Ford and others measured the speed at which the gas revolves around the centre of spiral galaxies, thus deriving the so-called ‘rotation curve’ way beyond the extension of the luminous disc. This provided a way to trace the total gravitational potential and effectively ‘weigh’ the galaxies out to their periphery. The end of the story is well-known: the observed flat rotation curves indicated the presence of large amounts of dark matter.

This is (relatively!) straightforward for many external spiral galaxies, located along particularly convenient lines of sight and with particularly convenient inclination angles. But in the case of our own Galaxy, the mighty Milky Way, a spiral itself, there is not much we can do. We are sitting inside the stellar disc, about 8 kpc off-centre, and from this position it is very hard to measure the rotation of gas and stars with the needed precision. Therefore, it has been historically challenging to uncover the existence of dark matter in the Milky Way. Actually, in the outskirts of the Galaxy it was possible to ascertain the presence of dark matter with reasonable degree of confidence, but the same was never done in the innermost regions, where baryons (i.e. gas and stars) contribute the most to the total mass budget.

That is where our new work Evidence for dark matter in the inner Milky Way comes in. First, we set up a comprehensive compilation of published measurements of the motion of gas and stars in the Milky Way. This defines with unprecedented precision the rotation curve of our Galaxy, which tracks the total gravitational potential. Second, we constructed a wide array of data-based models for the visible components, namely stellar bulge, stellar disc and gas. This takes into account the current uncertainty in baryonic modelling, especially towards the inner part of the Galaxy, and allows us to estimate the baryonic contribution to the total gravitational potential. The discrepancy between these two components is striking and statistically significant already inside the solar circle, calling for the need of significant amounts of dark matter between us and the Galactic centre. In short, we obtained a direct observational proof of the presence of dark matter in the innermost part the Milky Way.

So what? Well, this result is a confirmation of long-standing theoretical expectations. In fact, simulations of galaxy formation do suggest the presence of dark matter in these inner regions of the Galaxy. However, the actual amount has been at the centre of a lasting debate in the community, mostly because even if dark matter is expected to be there it is not expected to be the major component. That is, baryons dominate the gravitational potential in the inner regions of the Galaxy, and so extracting the subdominant dark matter component is very challenging. Our findings pave the way for observational determinations of the quantity of dark matter in these regions with higher precision than ever before. This is of crucial importance for the worldwide experimental efforts in direct and indirect searches for dark matter particles.

– Miguel Pato, OKC member, miguel.pato@fysik.su.se and Fabio Iocco, former OKC member

Still no Dark Matter in the latest analysis of LHC data…

Last night the ATLAS Collaboration released its latest search for dark matter and other beyond the standard model theories [1] based on the full dataset from the LHC Run I (2010-2012).

By looking for proton-proton collisions where jets of hadronic particles are produced only in one direction (Figure 1), violating conservation of momentum only in appearance, we use ATLAS to search for  weakly interacting massive particles (WIMPs), such as dark matter particles. Because they are weakly interacting, the WIMPs escape ATLAS undetected and lead to what looks like missing momentum.

Fig1Figure 1: A proton-proton collision recorded by ATLAS in November 2012, viewed in the transverse plane perpendicular to the beam axis. It shows a 1.2 TeV jet of hadronic particles (tracks and green bars) on one side, one can note the absence of activity on the other side, mimicking a violation of momentum conservation by as much as 1.2 TeV.

Neutrinos do exist and are often produced in LHC proton-proton collisions. These collisions look exactly like dark matter candidates and make up most of the background. We need to precisely predict the expected number of background collisions with outgoing neutrinos and compare that with the observed number of events in ATLAS data. Only a significant excess over the expected background allows a claim for a new physics signal.

The new ATLAS result [1] explores collisions with very high missing momentum, beyond any previous LHC search and we observe a good agreement between the number of events actually detected and the background alone prediction.

How well do we know the backgrounds? This is the most central question of this work. The sensitivity to dark matter is dominated by systematic uncertainties on the background. This is why at Stockholm University we decided to focus on the background calculations.

In the fall of 2014 Olof Lundberg, PhD student at the Stockholm university physics department, defended his licentiate thesis [2] where in particular he presented the development of a new technique to compute the backgrounds.  All the gory details of the calculations are in his licentiat, all the work that went into defining and understanding our new background control region and the mechanics to extrapolate to the signal regions. This 1.5 year long effort has really made a difference. With our new control region we were able to significantly increase the sensitivity to dark matter and other exotic signals. Figure 2 shows our new limits on the WIMP-WIMP annihilation cross section as function of the WIMP mass for various explored models.

Figure2Figure 2: Exclusion upper limits on WIMP-WIMP annihilation cross section as function of the WIMP mass, from the ATLAS monojet analysis [1] in various signal scenarios. D5 corresponds to an effective field theory where the WIMP dark matter is a Dirac particle interacting via a massive spin-1 vector particle. D8 corresponds to a scenario where the WIMP would interact via an axial-vector interaction. This graph also illustrates the complementarity with astrophysical searches for WIMP-WIMP annihilation with HESS and FERMI-LAT.

The paper came out last night, but to tell the truth the data analysis has been ready and unblinded for almost six months, albeit embargoed at the time of Olofs licentiat thesis. The theoretical interpretation took a long time. To be able to translate ATLAS absence of new physics signal into WIMP-WIMP annihilation cross sections a specific model has to be used. Several models have been investigated, Figure 2 is based on an effective field theory which is only valid under certain assumptions. The validity of the approach depends on the momenta of the initial partons involved in the proton-proton collisions and the exchanged momentum. In the end we chose a very conservative approach and for Figure 2 we simply assumed that we had zero sensitivity to dark matter signals when the validity limit was broken. This weakened our limits but at least the limits feel more robust.

We are of course disappointed we did not find anything new. As Figure 2 shows, a WIMP with a mass below 20 GeV and interacting via a vector particle could no longer explain on its own the whole relic dark matter density provided to us by fits to cosmological data.

On the other hand our little Stockholm monojet analysis team: Gabriele Bertoli (grad student), Olof Lundberg (grad student), Valerio Rossetti (postdoc), Christophe Clément (faculty) is stronger than ever and we are already working on new developments for the upcoming 2015 ATLAS data. This spring LHC is restarting stronger than before, with a much higher center of mass energy and much more data. So we are certainly looking forward a very exciting second ride with the ATLAS data.

– Christophe Clément

[1] ATLAS Collaboration, Search for new phenomena in final states with an energetic jet and large missing transverse momentum in pp collisions at √s=8 TeV with the ATLAS detector; http://arxiv.org/abs/1502.01518 (submitted to Eur. Phys. J. C).

[2] Olof Lundberg, Searches for Dark Matter and Extra Dimensions in Monojet Final States with the ATLAS Experiment, Licentiat thesis  Stockholm University, October 2014.



Detecting dark matter in the lab with Xenon

View on Gran Sasso Mountain (courtesy: LNGS)
View on Gran Sasso Mountain (courtesy: LNGS)
In the beginning of November 2014, The Oskar Klein Centre officially joined the XENON dark matter project. The idea is to detect dark matter particles scattering of heavy nuclei.
Since one of the strongest limitation of dark matter detection is due to cosmic ray induced background, it is important to shield the detectors. For this reason XENON is situated in the Gran Sasso National Laboratory, on the side of a ten kilometer long freeway tunnel crossing the Gran Sasso mountain, about 120 km from Rome. Thus, there is about 1500 meters of rock protecting the laboratory from cosmic ray backgrounds.

The particular approach of the XENON project is to use XENON noble gas in liquid and gaseous phase. The noble element xenon has the right mass to be optimally sensitive to the heavy WIMP (Weakly Interacting Massive Particle) dark matter which constitute the current paradigm for dark matter particles. Interactions of WIMPs in the gaseous phase xenon yield scintillation light and electrons, which are drifted to the liquid phase xenon, where they are extracted to give a ionisation signal.

Signals read-out by arrays of photomultiplier tubes on the top and bottom of the detector volume, can be used to reject background coming from recoils of particles on the electrons of the xenon atom. Radioactive impurities in xenon and the light sensors are the main trouble makers. The collaboration has therefore developed new techniques for screening and purifiying all materials involved in the experiment.
Less than 100 kg of XENON were enough to provide the most stringent limit on WIMP existence so far, only superseeded recently by results of the LUX collaboration, employing the same principle. We are now in the process of building the next generation of this experiment, with 1 tonne of ultra-pure Xenon.

In the end of November, I was in Gran Sasso for my first collaboration meeting. It was a lot of fun, the collaboration consists of very dedicated and highly competent people. Discussions were vivid and boisterous at times, but always constructive and in a cordial astmosphere.

Right now I am the only person involved from Stockholm, thanks to a grant by the Wallenberg foundation, but from new year we welcome Alfredo Ferella, who has long experience in the direct detection business and will recruit a postdoc and a graduate student. At first, we are going to build a data analysis and calibration center in Stockholm and will meet with representatives of the Swedish National Infrastructure for Computing (SNIC) very soon. We are also very eager to get our hands on data to try out some new methods and prepare for the next phase. Later-on, we will get more involved in instrumentation, focusing on the behaviour of special photosensors with low radioactivity at low temperatures. Together with colleagues at Chalmers University in Gothenburg, we will also be working on the theoretical ingredients necessary to turn the experimental measurements into solid knowledge about WIMPs. The spokesperson of XENON, Elena Aprile from Columbia University, made quite clear what the goal is: “Now, I hope we will discover the damn thing”.

Do we see dark matter emission from dwarf spheroidal galaxies?

From a dark matter (DM) hunter’s perspective, this year’s Fermi Symposium was highly anticipated. In the six years since the launch of the Large Area Telescope (LAT), we’ve seen our share of ups and downs. An active community, both in and outside the Fermi Collaboration (FC), works hard to fit dark matter to or explain away every deviation in excess of what we expect from the gamma-ray sky. This year’s gathering got the answer to the latest burning question: do we see dark matter emission from dwarf spheroidal galaxies (dSphs)?

Image Credit: NASA, Hubble Heritage Team, (STScI/AURA), ESA, S. Beckwith (STScI). Additional Processing: Robert Gendler

Before we hear the answer, let’s review the mileposts of the LAT DM saga. The first big stir came from the electron/positron spectrum [1], which featured a weaker version of the bump-like feature already measured by the PAMELA experiment. Theorists rushed to explain this with special flavors of ‘leptophillic’ dark matter, tuned to enhance the local flux while producing nothing other detectors would have seen. After an avalanche of 700+ citations and one even more precise measurement (AMS-02), the excitement died, along with most of those models.

Next we had the (in)famous line feature at 135 GeV [2]. With a far more clear-cut interpretation, I held my breath while this result was seen first in the galactic center (GC), then in nearly every control region, and finally at lower and lower significance [3] until its appearance at the Symposium reported a mere 0.72 sigma. A dedicated observation from H.E.S.S. II may rescue it, but for now the line looks tenuous at best.

The latest big hint also comes from the galactic center, but in the continuum emission, rather than as a sharp feature. This makes some sense; the galactic center is nearby and massive, so any dark matter signal ought to be strongest there. On the other hand, it is astrophysically complex. So much so that interpretation of the data there seems to have been saved for last (the Fermi Collaboration has yet to weigh in with a publication). Undaunted, several groups (e.g. [4]) now report that models of known sources do not account for all the emission, leaving what is blandly referred to as the galactic center excess (GCE). Despite a wealth of systematic uncertainty, all parties agree that the galactic center excess is peaked at around 1 GeV and extends fairly symmetrically to about ten degrees from the center of the Milky Way.

Plausible conventional production mechanisms for the galactic center excess include pulsars and cosmic rays, but it can also be neatly accounted for by the existence of a weakly interacting massive particle (WIMP) of about 30 GeV which self-annihilates into b quarks. Without completely ruling out alternatives, this possible dark matter explanation remains just that — possible. What made this dark matter model so exciting was that it also happened to fit a slight excess (1.4 sigma) seen in the dwarf spheroidal galaxies (dSphs) orbiting the Milky Way [5]. As dSphs represent an independent data set, and a nearly background-free one at that, the coincidence was indeed tantalizing. One well-known physicist called the galactic center excess “the most compelling signal we’ve had for dark matter particles – ever.” [6]

The dSph excess was so low that, if it was not just a fluctuation of the background, we would have to wait many years to confirm the dark matter explanation of the galactic center excess. Fortunately, the LAT’s excellent ground team just gave us a big push. A pair-conversion telescope, Fermi 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 while lowering its point spread function like a pair of glasses. Upgrade in hand, the time was right to look again at the dSphs.

Dark matter searches in dwarf spheroidal galaxies have been a specialty of the Oskar Klein Centre. C. Farnier and M. Llena-Garde have both played lead roles in Collaboration papers on the subject ([7], [8]), and J. Conrad introduced the statistical technique now used to combine information from multiple targets [9]. For the latest publication, G. Martinez derived the dSph mass distributions by a nested Bayesian analysis of their hosted stellar populations [10]. Building on this success, yours truly, as part of the FC, took a peek at the Pass 8 data.

Preliminary Fermi-LAT Pass 8 constraints on the WIMP velocity-averaged cross section for one annihilation channel. Using five years of data for the combination of 15 dwarf spheroidal galaxies. Dashed line and bands represent limits expected from blank fields on the sky.” Credits Fermi-LAT Collaboration.

What we found was a whole lot of nothing. The significance of the GCE model dropped drastically, along with all other WIMP annihilation masses and channels. Dropped so far, in fact, that we can now set limits which exclude the annihilation cross section WIMPs need to make up all dark matter out to masses of 100 GeV (see Figure 1). These are now the best limits in the world below 1 TeV, and represent a big bite out of the parameter space left to the indirect dark matter detection field’s favorite class of models. While these constraints do not conclusively rule out the dark matter interpretation of the galactic center excess, they lend no support. “Tension” is the colloquial term.

So at this year’s Fermi Symposium, though debate still raged over the galactic center, I had to report that dSphs had pulled their support from the dark matter interpretation. Like all of us, I was disappointed to find we still have no answer to one of the greatest physics questions of the day, but as I said, dark matter hunters are used to highs and lows. Could gamma rays from annihilating dark matter still be buried in the LAT data? Of course, but there are not many coming from dSphs.

– Brandon Anderson, OKC fellow (brandon.anderson@fysik.su.se)

[1] Fermi-LAT Collaboration. Physical Review Letters, vol. 102, Issue 18, id. 181101
[2] C. Weniger. Journal of Cosmology and Astroparticle Physics, Issue 08, article id. 007 (2012)
[3] Fermi-LAT Collaboration. Physical Review D 88, 082002 (2013)
[4] D. Hooper, L. Goodenough. Physics Letters B, Volume 697, Issue 5, p. 412-428
[5] Fermi-LAT Collaboration. Phys. Rev. D 89, 042001 (2014)
[6] New Scientist, April 2014
[7] Fermi-LAT Collaboration. Astrophysical Journal, 712, (2010), 147-158
[8] Fermi-LAT Collaboration. Phys. Rev. Lett. 107, 241302 (2011)
[9] J. Conrad. Astroparticle Physics 62 (2015) 165-177
[10] G. Martinez. eprint arXiv:1309.2641

Interview with Katherine Freese

Katherine Freese is in Stockholm these days since she will be receiving a prestigious Honorary doctorate at Stockholm University on Friday, the 28th September. I met with her in one of the offices at the Oskar Klein Centre in front of cup of coffee to talk a bit with this energetic and fascinating scientist, and try to grab her secrets.

What was you reaction when you heard you will receive this title?
Oh I was very happy, I think it is really an honor to get this. First when Lars told me I was a candidate, and then when I got it, I was very excited. It is going to be great tomorrow. Although I am jetlegget I am not nervous at all, I am excited!

So let’s see if we can get to know a bit about your career then. Lets start from the beginning: how did you get the idea of studying physics and how you turned into a cosmologist?
My parents are both scientists. My father has been student of Heisenberg before turning into biology, and of course the role model I had in my family pushed me for a career in academics. But how did I get into physics, well, that was probably just by chance. I went to a summer school in relativity when I was 15 years old, and I though that was very interesting. But really, I though physics was a kind of broad field that could open many possibilities, and then I was good at it. I think this is the way many people choose what to study, by exclusion, if you are good at something and not every one else is, you have to go for it. I did not have passion for astronomy as a kid, not at all. Isn’t this how most people choose their field of work? I tried a number of different styles of physics (experimental high energy, solid state, etc) then I found my inspiration with cosmology. I was living in Japan for a while after I graduated from Princeton. I was only 20 e I was teaching English, and then I was hospitalized. I was so bored that I took a book called Spacetime Physics, by Taylor and Wheeler, and that really made an impression on me and really turned me on and made me wanting to keep on studying cosmology. And I also felt it was a challenge!

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