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Statistical Modelling and Inference for XENON1T
Stockholm University, Faculty of Science, Department of Physics. OKC.ORCID iD: 0000-0002-2011-1889
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A great number of astrophysical observations suggests that of the matter in our universe, only a sixth is made up of known matter. The rest, named dark matter, has not been successfully identified. This thesis presents the analysis and statistical inference that was used by the XENON1T collaboration to conduct a search for a particular dark matter candidate; weakly interacting massive particles (WIMPs).

 

XENON1T is a dual-phase time projection chamber that can detect particles scattering in a 2 tonne target of liquid xenon with deposited recoil energies above ~3 keV. This is low enough to observe the elastic recoil between a WIMP and a xenon nucleus for WIMP masses >5 GeV c-2.

The results presented in this thesis use 278.8 days of data, with an analysis mass of 1.3 tonne.

 

XENON1T uses models for backgrounds and signals within this volume to construct a combined likelihood for two science data-taking periods as well as calibration data-sets. Fits to simulated data-sets were used to calibrate and validate the confidence interval construction. In addition, analysis choices were evaluated both to optimize the discovery power and expected sensitivity of the search, and to improve the robustness of the analysis.

 

No significant excess was observed in the search for a spin-independent WIMP-nucleon interaction for any WIMP masses between 6  GeV c-2 and 104 GeV c-2 for the 1 ton-year exposure. This analysis produced the strongest constraint on the spin-independent WIMP-nucleon cross-section so far, with a minimum of 4.1 10-47 cm2  for a 30 GeV c-2 WIMP.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2019. , p. 116
Keywords [en]
Dark matter, direct detection, statistical analysis
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-163390ISBN: 978-91-7797-566-3 (print)ISBN: 978-91-7797-567-0 (electronic)OAI: oai:DiVA.org:su-163390DiVA, id: diva2:1275477
Public defence
2019-02-25, FA31, Albanova, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript. Paper 5: Manuscript.

Available from: 2019-01-31 Created: 2019-01-06 Last updated: 2019-02-28Bibliographically approved
List of papers
1. First Dark Matter Search Results from the XENON1T Experiment
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2017 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 119, no 18, article id 181301Article in journal (Refereed) Published
Abstract [en]

We report the first dark matter search results from XENON1T, a similar to 2000-kg-target-mass dual-phase (liquid-gas) xenon time projection chamber in operation at the Laboratori Nazionali del Gran Sasso in Italy and the first ton-scale detector of this kind. The blinded search used 34.2 live days of data acquired between November 2016 and January 2017. Inside the (1042 +/- 12)-kg fiducial mass and in the [5, 40] keV(nr) energy range of interest for weakly interacting massive particle (WIMP) dark matter searches, the electronic recoil background was (1.93 +/- 0.25) x 10(-4) events/(kg x day x keV(ee)), the lowest ever achieved in such a dark matter detector. A profile likelihood analysis shows that the data are consistent with the background-only hypothesis. We derive the most stringent exclusion limits on the spin-independent WIMP-nucleon interaction cross section for WIMP masses above 10 GeV/c(2), with a minimum of 7.7 x 10(-47) cm(2) for 35-GeV/c(2) WIMPs at 90% C.L.

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-148981 (URN)10.1103/PhysRevLett.119.181301 (DOI)000413903200005 ()
Available from: 2017-12-04 Created: 2017-12-04 Last updated: 2019-01-07Bibliographically approved
2. Dark Matter Search Results from a One Ton-Year Exposure of XENON1T
Open this publication in new window or tab >>Dark Matter Search Results from a One Ton-Year Exposure of XENON1T
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2018 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 121, no 11, article id 111302Article in journal (Refereed) Published
Abstract [en]

We report on a search for weakly interacting massive particles (WIMPs) using 278.8 days of data collected with the XENON1T experiment at LNGS. XENON1T utilizes a liquid xenon time projection chamber with a fiducial mass of (1.30 +/- 0.01) ton, resulting in a 1.0 ton yr exposure. The energy region of interest, [1.4; 10.6] keV(ee) ([4.9; 40.9] keV(nr)), exhibits an ultralow electron recoil background rate of [82(-3)(+5) (syst) +/- 3 stat)] events/ton yr keV(ee)). No significant excess over background is found, and a profile likelihood analysis parametrized in spatial and energy dimensions excludes new parameter space for the WIMP-nucleon spin-independent elastic scatter cross section for WIMP masses above 6 GeV/c(2), with a minimum of 4.1 x 10(-47) cm(2) at 30 GeV/c(2) and a 90% confidence level.

Keywords
Dark matter, Particle dark matter, Weakly interacting massive particles, Dark matter detectors
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-161083 (URN)10.1103/PhysRevLett.121.111302 (DOI)000444588900003 ()30265108 (PubMedID)
Available from: 2018-10-31 Created: 2018-10-31 Last updated: 2019-01-07Bibliographically approved
3. Analysis of the XENON1T data for WIMP search: Background Models and Statistical Inference
Open this publication in new window or tab >>Analysis of the XENON1T data for WIMP search: Background Models and Statistical Inference
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The XENON1T experiment searches for dark matter recoils within a $2$ tonne liquid xenon target. The detector is operated as a dual-phase time projection chamber, and reconstructs the energy and position of interactions in the active volume. In the central volume of the target mass, the lowest background rate of a xenon-based direct detection experiment so far has been achieved. In this work we describe the detector response modelling, the background and signal models, and the statistical inference procedures used in a search for Weakly Interacting Massive Particles (WIMPs) using 1\,tonne$\times$year exposure of XENON1T data.

Keywords
WIMP dark matter, Statistical analysis
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-163389 (URN)
Note

By the XENON collaboration (127 authors). In internal XENON review.

Available from: 2018-12-30 Created: 2018-12-30 Last updated: 2019-01-06Bibliographically approved
4. First results on the scalar WIMP-pion coupling, using the XENON1T experiment
Open this publication in new window or tab >>First results on the scalar WIMP-pion coupling, using the XENON1T experiment
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We present first results on the scalar WIMP-pion coupling from 1 t×yr of exposure with the XENON1T experiment. This interaction is generated when the WIMP couples to a virtual pion exchanged between the nucleons in a nucleus. In contrast to most non-relativistic operators, these pion-exchange currents can be coherently enhanced by the total number of nucleons, and therefore may dominate in scenarios where spin-independent WIMP-nucleon interactions are suppressed. Moreover, for natural values of the couplings, they dominate over the spin-dependent channel due to their coherence in the nucleus. Using the signal model of this new WIMP-pion channel, no significant excess is found, leading to an upper limit cross section of 6.4×10−46 cm2 (90 % confidence level) at 30 GeV/c2 WIMP mass.

Keywords
Dark matter, wimps, direct detection
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-163387 (URN)
Note

131 total authors (XENON collaboration, M. Hoferichter, P. Klos, J. Menéndez and A. Schwenk).

Available from: 2018-12-30 Created: 2018-12-30 Last updated: 2019-01-06
5. Harmonizing discovery thresholds and reporting two-sided confidence intervals: a modified Feldman & Cousins method
Open this publication in new window or tab >>Harmonizing discovery thresholds and reporting two-sided confidence intervals: a modified Feldman & Cousins method
(English)Manuscript (preprint) (Other academic)
Abstract [en]

When searching for new physics effects, collaborations will often wish to publish upper limits and intervals with a lower confidence level than the threshold they would set to claim an excess or a discovery. However, confidence intervals are typically constructed to provide constant coverage, or probability to contain the true value, with possible overcoverage if the random parameter is discrete. In particular, that means that the confidence interval will contain the 0-signal case with the same frequency as the confidence level. This paper details a modification to the Feldman-Cousins method to allow a different, higher excess reporting significance than the interval confidence level.

Keywords
Data Analysis, Statistics and Probability
National Category
Other Physics Topics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-163388 (URN)
Note

Submitted to JINST.

Available from: 2018-12-30 Created: 2018-12-30 Last updated: 2019-01-06

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