Amsterdam-Paris-Stockholm 5th meeting

21 Sept 2015, 07:00 → 23 Sept 2015, 16:30 Europe/Stockholm

Djurönäset

Doug Spolyar, Joakim Edsjo (Stockholm University), Lars Bergstrom (Stockholm University), Timur Delahaye (Oskar Klein Centre)

We are very happy to welcome you in Stockholm for the 5th meeting of our network. Please register as soon as possible to help us organise the workshop. You should also book your flight as soon as possible if you want to benefit from the shuttle from Arlanda airport to the conference centre.

Monday 21 September

Breakfast

08:30 Arrival Coffee Break

Direct Detection

1 Welcome address and presentation of the OKC

Speaker: Lars Bergstrom (Stockholm University)

2 New Directions in Direct Dark Matter Searches

Direct searches for Dark Matter (DM) aim at detecting the nuclear recoils arising from a scattering between DM particles and target nuclei in underground detectors. Since the physics that describes the collision between DM particles and target nuclei is deeply non-relativistic, in the first part of this seminar I’ll review a different and more general approach to study signal in direct DM searches based on the formalism of non-relativistic operators. Then, I’ll present the main observables and the experimental landscape pointing out all the uncertainties that enter in this field. Finally, I’ll show that a Dirac DM particle interacting with ordinary matter via the exchange of a light pseudo-scalar can accommodate the DAMA data while being compatible with all null direct DM searches.

Speaker: Paolo Panci (Institut d'Astrophysique de Paris)

3 A halo-independent lower bound on the dark matter capture rate in the Sun from a direct detection signal

We show that a positive signal in a dark matter (DM) direct detection experiment can be used to place a lower bound on the DM capture rate in the Sun, independent of the DM halo. For a given particle physics model and DM mass we obtain a lower bound on the capture rate independent of the local DM density, velocity distribution, galactic escape velocity, as well as the scattering cross section. We illustrate this lower bound on the capture rate by assuming that upcoming direct detection experiments will soon obtain a significant signal. When comparing the lower bound on the capture rate with limits on the high-energy neutrino flux from the Sun from neutrino telescopes, we can place upper limits on the branching fraction of DM annihilation channels leading to neutrinos. With current data from IceCube and Super-Kamiokande non-trivial limits can be obtained for spin-dependent interactions and direct annihilations into neutrinos. In some cases also annihilations into ττ or b ̄b start getting constrained. For spin-independent interactions current constraints are weak, but they may become interesting for data from future neutrino telescopes.

Speaker: Dr Juan Herrero-Garcia (KTH)

Direct Detection

4 XENON100 and XENON1T

I will review recent XENON100 results and XENON1T status.

Speaker: Jan Conrad (KTH Royal Institute of Technology (SE))

5 Halo-independent tests of direct detection signals

From an assumed signal in a Dark Matter (DM) direct detection experi- ment a lower bound on the product of the DM–nucleon scattering cross section and the local DM density is derived, which is independent of the local DM velocity distribu- tion. This can be combined with astrophysical determinations of the local DM density. Within a given particle physics model the bound also allows a robust comparison of a direct detection signal with limits from the LHC. Furthermore, the bound can be used to formulate a condition which has to be fulfilled if the particle responsible for the direct detection signal is a thermal relic, regardless of whether it constitutes all DM or only part of it. We illustrate the arguments by adopting a simplified DM model with a Z mediator and assuming a signal in a future xenon direct detection experiment.

Speaker: Stefan Vogl (University of Stockholm)

6 New directional signatures from the non-relativistic effective field theory of dark matter

The framework of non-relativistic effective field theory (NREFT) aims to generalise the standard analysis of direct detection experiments in terms of spin-dependent (SD) and spin-independent (SI) interactions. I will show that a number of NREFT operators lead to distinctive new directional signatures, such as prominent ring-like features in the directional recoil rate, even for relatively low mass WIMPs. I will discuss these signatures and how they could affect the interpretation of future results from directional detectors. In particular, I will show that for certain NREFT operators, directional sensitivity provides the only method of distinguishing them from the standard SI/SD operators, highlighting the importance of directional detectors in probing the particle physics of dark matter.

Speaker: Bradley Kavanagh (IPhT - CEA/Saclay)

12:45 Lunch

Direct Detection

7 Discussion

Particle Physics

8 Global scans of combined EFT operators

An effective field theory (EFT) approach allows us to describe WIMP interactions as point-like, regardless of the specific fundamental theory at higher energies. We consider simultaneously all phenomenologically relevant EFT operators and perform a global bayesian analysis. We identify the most probable regions of the theory parameter space compatible with experimental constraints on the relic density as well as with direct and indirect detection experiments.

Speaker: Sebastian Liem (GRAPPA, University of Amsterdam)

9 On the determination of the leptonic CP phase

The combination of data from long-baseline and reactor oscillation experiments leads to a preference of the leptonic CP phase $\delta_{\rm CP}$ in the range between $\pi$ and $2\pi$. We study the statistical significance of this hint by performing a Monte Carlo simulation of the relevant data. We find that the distribution of the standard test statistic used to derive confidence intervals for $\delta_{\rm CP}$ is highly non-Gaussian and depends on the unknown true values of $\theta_{23}$ and the neutrino mass ordering. Values of $\delta_{\rm CP}$ around $\pi/2$ are disfavored at between $2\sigma$ and $3\sigma$, depending on the unknown true values of $\theta_{23}$ and the mass ordering. Typically the standard $\chi^2$ approximation leads to over-coverage of the confidence intervals for $\delta_{\rm CP}$. For the 2-dimensional confidence region in the ($\delta_{\rm CP},\theta_{23}$) plane the usual $\chi^2$ approximation is better justified. The 2-dimensional region does not include the value $\delta_{\rm CP} = \pi/2$ up to the 86.3 % (89.2 %) CL assuming a true normal (inverted) mass ordering. Furthermore, we study the sensitivity to $\delta_{\rm CP}$ and $\theta_{23}$ of an increased exposure of the T2K experiment, roughly a factor 12 larger than the current exposure and including also anti-neutrino data. Also in this case deviations from Gaussianity may be significant, especially if the mass ordering is unknown.

Speaker: Ms Jessica Elevant (OKC, Stockholm University)

10 Next-generation BSM global fitting with GAMBIT

Is model X a viable candidate for physics "Beyond the Standard Model", in the face of the combined weight of all currently known experimental data? If so, what limits exist on its parameter space? These simple questions are the first that one would like to answer when examining a particular theoretical model, however answering them comprehensively is a huge task, requiring a wide array of computational tools to produce theoretical predictions for a wide array of experiments, as well as smart algorithms to sample these predictions across theory parameter spaces with many dimensions in a feasible amount of CPU time, while remaining statistically valid. To coordinate all these tasks in a flexible way that allows new data and new models to be analysed using new sampling and statistical techniques, while making as few changes to the code base as possible, requires an extremely flexible and modular tool. GAMBIT is such a tool, as well as the name of the collaboration developing it, which is comprised of nearly 30 theorists and experimentalists, from 15 institutions, 9 countries, and 8 experiments. In this talk I will give an overview of the GAMBIT project, from its goals, to the code and its present status.

Speaker: Dr Benjamin Farmer (Oskar Klein Centre)

BF_APSV.pdf

11 Discussion

16:00 Coffee Break

Indirect Detection

12 Dark Matter searches with Cosmic Rays

We study the possibility of improving the constraints on the lifetime of gravitino dark matter in scenarios with bilinear R-parity violation by estimating the amount of cosmic-ray antideuterons that can be produced in gravitino decays. Taking into account all different sources of theoretical uncertainties, we find that the margin of improvement beyond the limits already set by cosmic-ray antiproton data are quite narrow and unachievable for the next generation of experiments. However, we also identify more promising energy ranges for future experiments.

Speaker: Timur Delahaye (Oskar Klein Centre)

13 Gamma ray tests of Minimal Dark Matter

We reconsider the model of Minimal Dark Matter (a fermionic, hypercharge-less quintuplet of the EW interactions) and compute its gamma ray signatures. We compare them with a number of gamma ray probes: the galactic halo diffuse measurements, the galactic center line searches and recent dwarf galaxies observations. We find that the original minimal model, whose mass is fixed at 9.4 TeV by the relic abundance requirement, is constrained by the line searches from the Galactic Center: it is ruled out if the Milky Way possesses a cuspy profile such as NFW but it is still allowed if it has a cored one. Observations of dwarf spheroidal galaxies are also relevant (in particular searches for lines), and ongoing astrophysical progresses on these systems have the potential to eventually rule out the model. We also explore a wider mass range, which applies to the case in which the relic abundance requirement is relaxed. Most of our results can be safely extended to the larger class of multi-TeV WIMP DM annihilating into massive gauge bosons.

Speaker: Filippo Sala (CEA/Saclay and CNRS)

FilippoSala_APS15.pdf

14 Self-consistent velocity distributions for WIMPs explaining the Galactic Centre GeV excess

Complementarity in the search for Dark Matter (DM) is realised when, for example, you use indirect detection gamma-ray data for infer information on the expected event rate in a direct detection experiment. However, one should take care of adopting the same assumptions when combining different detection strategies. I will assume that the excess measured by Fermi-LAT from the Galactic Centre is due to DM and I will use the information on its morphology to infer the DM distribution in the inner Galaxy. This will be combined with other kinematics information at larger distances to generate a mass model of the Milky Way that is compatible with the Galactic Centre excess. The velocity distribution consistently determined from this mass model is what should be used to predict the expected recoil rate. I will show the impact of such a consistent modelling of the DM halo in the determination of the upper limits from LUX.

Speaker: Mattia Fornasa (GRAPPA Institute (University of Amsterdam))

APS_2015.pdf

Supernovæ

15 High redshift supernova rates measured with a gravitational telescope

Supernovae (SNe) are one the most energetic phenomena in the Universe and have been extensively used for astrophysical and cosmological applications. For example, type Ia SNe as distance indicators tools have revealed the accelerating expansion of the universe, while core-collapse (CC) being directly related to the death of massive stars, they trace the star formation history. One of the challenges of SN research is the measurements of the SN rates, particularly at high-z where not many measurements exist. Possible approach to this problem is to use the magnification power of gravitational telescopes such as galaxy clusters. I will present our ground based near-infrared/optical search for gravitationally magnified supernovae behind the galaxy cluster A1689. Our search resulted in the discovery of five highly magnified candidates at high-z classified as CC SNe. We measure the first volumetric CC SN rates in the redshift bins 0.6<z<1.2 and 1.2<z<1.8 and add the first upper limit on the CC SN rate in the range 1.8 < z < 2.4. We attempt to compare the CC SN rate with the cosmic star formation rate.

Speaker: Tanja Petrushevska (OKC)

Indirect Detection: Discussion

20:00 Dinner

Tuesday 22 September

Breakfast

Indirect Detection

16 Modelling the flux distribution function of the extragalactic gamma-ray background from dark matter annihilation

The one-point function (i.e., the isotropic flux distribution) is a complementary method to (anisotropic) two-point correlations in searches for a gamma-ray dark matter annihilation signature. Using analytical models of structure formation and dark matter halo properties, we compute the gamma-ray flux distribution due to annihilations in extragalactic dark matter halos, as it would be observed by the Fermi Large Area Telescope. Combining the central limit theorem and Monte Carlo sampling, we show that the flux distribution takes the form of a narrow Gaussian of ‘diffuse’ light, with an ‘unresolved point source’ power-law tail as a result of bright halos. We argue that this background due to dark matter constitutes an irreducible and significant background component for point-source annihilation searches with galaxy clusters and dwarf spheroidal galaxies, modifying the predicted signal-to-noise ratio. A study of astrophysical backgrounds to this signal reveals that the shape of the total gamma-ray flux distribution is very sensitive to the contribution of a dark matter component, allowing us to forecast promising one-point upper limits on the annihilation cross section. We show that by using the flux distribution at only one energy bin, one can probe the canonical cross section required for explaining the relic density, for dark matter of masses around tens of GeV.

Speaker: Michael Feyereisen (University of Amsterdam)

17 Towards the first gamma rays from Galaxy Clusters: Searches for Cosmic Rays and Dark Matter

Galaxy clusters are the most massive bound systems known in the universe and are believed to have formed through large scale structure formation. As such they host relativistic cosmic-ray (CR) populations and are gravitationally bound by large amounts of Dark Matter (DM). Both provide conditions in which high-energy gamma rays may be produced either via CR interactions with the intracluster medium or through the annihilation or decay of DM particles. Prior to the launch of the Fermi satellite, predictions were optimistic that Galaxy clusters would be established as gamma-ray-bright objects by observations through its prime instrument, the Large Area Telescope (LAT). Yet, despite numerous efforts, even a single cluster detection is still pending. I will review the most recent methodological searches and investigate the implications that these non-detections yield, both for the extended mission of Fermi and future experiments at gamma-ray wavelengths.

Speaker: Mr Stephan Zimmer (OKC/ Stockholm University)

18 Angular power spectrum of sterile neutrino decay lines: the role of eROSITA

Taking as reference the performance of the soon-to-be-launched eROSITA satellite, I will discuss the potential of angular auto- and cross-correlation power spectra in identifying sterile neutrino dark matter in the cosmic X-ray background. While sterile neutrino decays are always sub-dominant in the auto-correlation power spectra with respect to other background sources, they can be efficiently enhanced when cross-correlating with tracers of dark matter distribution. The planned eROSITA all-sky survey will potentially yield very stringent constraints on the sterile neutrino decay lifetime, enabling to test the recently claimed 3.56-keV X-ray line.

Speaker: Fabio Zandanel (University of Amsterdam)

10:00 Coffee Break

Indirect Detection

19 The search for cosmological annihilation signals with the Fermi LAT

We search for evidence of dark matter (DM) annihilation in the isotropic gamma-ray background (IGRB) measured with 50 months of Fermi Large Area Telescope (LAT) observations. An improved theoretical description of the cosmological DM annihilation signal, based on two complementary techniques and assuming generic WIMP properties, renders more precise predictions compared to previous work. We consistently include both the Galactic and extragalactic signals under the same theoretical framework, and study the impact of the former on the IGRB spectrum derivation. We find no evidence for a DM signal and we set limits on the DM-induced isotropic gamma-ray signal. Our limits are competitive for DM particle masses up to tens of TeV and, indeed, are the strongest limits derived from Fermi-LAT data at TeV energies. We quantify uncertainties in detail and show the potential this type of search offers for testing the WIMP paradigm with a complementary and truly cosmological probe of DM particle signals.

Speaker: Miguel Sánchez-Conde (Oskar Klein Centre, Stockholm University)

20 Strong support for the millisecond pulsar origin of the Galactic center GeV excess

Using gamma-ray data from the Fermi Large Area Telescope, various groups have identified a clear excess emission in the inner Galaxy, at energies around a few GeV. This excess resembles remarkably well a signal from dark matter annihilation. One of the most compelling astrophysical interpretations is that the excess is caused by the combined effect of a previously undetected population of dim gamma-ray sources. Due to their spectral similarity, the best candidates are millisecond pulsars. Here, we search for this hypothetical source population, using a novel approach based on wavelet decomposition of the gamma-ray sky and the statistics of Gaussian random fields. Using almost seven years of Fermi-LAT data, we detect a clustering of photons as predicted for the hypothetical population of millisecond pulsar, with very high statistical significance. For plausible values of the luminosity function, this population can explain 100% of the observed excess emission. We show that other extragalactic or Galactic sources, a mis-modeling of Galactic diffuse emission, or the thick disk population of pulsars are unlikely to account for this observation.

Speaker: Christoph Weniger (University of Amsterdam)

21 Discussion

12:45 Lunch

Outdoor activities

Feel free to go swimming, enjoy the sauna or paddle in the archipelago

19:00 Dinner

Wednesday 23 September

Breakfast

Dark Matter density

22 Boosting the Annihilation Boost: Tidal Stripping and the Subhalo Luminosity

In the cold dark matter paradigm, structures form hierarchically, implying that large structures contain smaller substructures. These subhalos will enhance signatures of dark matter annihilation such as gamma rays. In the literature typical estimates of this boost factor assume a concentration- mass relation for field halos, to calculate the luminosity of subhalos. However, since subhalos accreted in the gravitational potential of their host lose mass through tidal stripping and dynamical friction, they have a quite characteristic density profile, different from that of the field halos of the same mass. We attempt to quantify the effect of tidal stripping on the boost factor, by developing a semi-analytic model that combines mass-accretion history of both the host and subhalos as well as subhalo accretion rates. We find that when subhalo luminosities are treated consistently, the boost factor increases by a factor 2–3, compared to the typical calculation assuming a field-halo concentration. This holds for host halos ranging from sub-galaxy to cluster masses and is independent of the subhalo-mass function or specific concentration-mass relation.

Speaker: Richard Bartels (University of Amsterdam)

23 Towards a Maximum Likelihood estimate of the dwarf spheroidal galaxies J factor

The dwarf spheroidal satellite galaxies of the Milky Way appear to be the most dark matter (DM) dominated objects in the near Universe. Their very low astrophysical background in the $\gamma$-ray energy range makes them ideal targets for DM indirect detection, which can be achieved by searching for its decay or annihilation signals. The latter approach requires the calculation of the $\textit{J-factor}$, which quantifies the amount of DM along the line of observation. This quantity has been previously derived with bayesian techniques, thereby subjecting the results to the effects of priors. We report here the development of a new fully frequentist approach based on the Maximum Likelihood procedure, which thus allows to build the profile likelihood for $\textit{J}$ and from it the calculation of confidence intervals. The new results are in most cases consistent with the previously derived ones, being compatible to the 1-$\sigma$ level; their uncertainties are also compatible and consistently scale with the dataset size, being smaller for the larger samples; the largest discrepancies are restricted to the systems with the smallest dataset. We also present possible improvements and extensions to this technique.

Speaker: Mr Andrea Chiappo (The Oskar Klein Center, Stockholm University)

24 Implications of hydrodynamic simulations for dark matter direct detection

There is significant astrophysical uncertainty in the interpretation of data from dark matter direct detection experiments, due to the poorly known dark matter distribution at the position of the Sun. I will discuss the local dark matter density and velocity distribution of Milky Way-like galaxies obtained from the high-resolution EAGLE hydrodynamic simulations. To make reliable predictions for direct detection searches, we identify simulated haloes which satisfy the Milky Way observational constraints. Using the local dark matter distribution obtained for the selected Milky Way-like simulated haloes, I will present an analysis of current direct detection data.

Speaker: Nassim Bozorgnia (GRAPPA, University of Amsterdam)

10:00 Coffee Break

Dark Matter density

25 A New Method for Determining the Local Dark Matter Density

Determination of the Dark Matter (DM) density at the solar position is critical to direct and indirect dark matter searches. Additionally, it is important to make this determination with as few assumptions as possible, as results from direct detection searches are used to explore a wide variety of theoretical models, and hidden astrophysical assumptions could bias theoretical searches. Here we present a Jeans analysis based method for the determination of the local DM density which allows us to limit the number of assumptions we need to make. We fit vertical profiles of baryon and DM density to tracer density and velocity dispersion data via integrated Jeans equations, and from these derive the local DM density. We present tests on mock data and demonstrate the importance of the 'tilt term' which links radial and vertical motions.

Speaker: Hamish Silverwood (University of Amsterdam)

26 Discussion

12:00 Lunch

Sandwitches served before the shuttle to the Airport leaves at 15:00