Author Archives: Knut Morå

Modeling and statistical analysis of the XENON1T data

On May 31st 2018, XENON1T released the result of a search for dark matter interacting with xenon atoms using an exposure of 1 tonne-year. Papers presenting the scientific results are written to be brief, and communicate the most important information to the scientific community. Therefore, many details of the instrument, reconstruction of events and analysis work by the entire collaboration must be left out of the science papers. XENON1T has previously published a paper focusing on the operation of the detector itself. A new paper by XENON1T now goes into the details of the analysis of the XENON1T data, and another one, on the event reconstruction and calibration, is being prepared.

XENON1T detects the scintillation light and ionization electrons that energy depositions in the two tonne liquid xenon target produce. In addition to WIMPs, different background sources can produce an S1+S2 signal. The expected S1,S2 distribution may change depending on whether the energy deposition happens by a recoil on an electron of the xenon atom or the nucleus. This is one of the main methods XENON uses to discriminate against backgrounds, since WIMPs, which scatter on the xenon nucleus, have a mean S2 lower than 99.7% of the dominant background component, which is made up of scatters on electrons.

Modelling how an electronic or nuclear recoil will look like in the detector is crucial both to know the shape of a WIMP signal, and to model the backgrounds well. XENON1T uses a comprehensive fit to multiple calibration sources to constrain the distributions of backgrounds and signals in the analysis space; S1, S2 and the radius from the center axis of the detector.
Some background components are harder to model directly, and are estimated by using sidebands or other data samples. In the XENON1T analysis, coincidences between unrelated, lone S1 and S2 events were modeled this way, in addition to the surface background– events occurring close to or at the detector wall.

Models of various backgrounds and the expected WIMP signal in two of the parameters extracted from each even, scintillation S1 and ionization S2 signals.

The models of each background and the signal, for two separate science runs, are put together in a likelihood, which is a mathematical function of the WIMP signal strength as well as nuisance parameters. These are unknowns that could change the analysis, such as the true expectation value for each background component. The likelihood also contains multiple terms representing measurements of nuisance parameter, which constrain them when the likelihood is fitted to the data collected by XENON1T.

The value of the likelihood evaluated at a specific signal strength has a random distribution which is estimated using simulated realizations of the experimental outcome. The final statistical limits are computed by comparing the likelihood computed on the actual data with the distributions found from the simulations: 

Likelihood as function of the signal strength (measured by the WIMP-nucleon cross-section)
The gray area shows likelihoods that corresponds to a 90% exclusion. The confidence interval– the region of signal strength compatible with the observed data– is the region where the likelihood lies below the gray band.

The models and tools used in the XENON1T spin-independent analysis are also used to explore alternative models of dark matter, such as spin-independent interactions and scatterings between WIMPs and pions, with more to come!


The first limits on spin-independent WIMP-pion interactions

XENON1T was built to observe the recoil of xenon-atoms, which may be caused by the interaction of a Weakly Interacting Massive Particle (WIMP) as it passes through the detector. A recoiling xenon atom produces scintillation light and ionization that XENON1T detects as an S1 and S2 signal, which carry information of the recoil type, energy and position in the detector. The first results of the XENON1T were published on the spin-independent WIMP-nucleon interaction, which is expected to dominate the WIMP scattering rate. However, models of WIMPs exist where this contribution would be suppressed or vanish. XENON has therefore performed searches for alternative WIMP-recoil spectra, such as the one expected if the scattering depends on the nucleon spins.

A careful accounting of all the possible WIMP-nucleon interactions showed that WIMPs can also interact with pions— subatomic particles that contribute to the strong force that binds atoms together. The figure illustrates a WIMP (χ) scattering via a mediator line on a pion (π) exchanged between a proton and a neutron in the xenon nucleus. The xenon atom recoils from the interaction, which can be observed with our detector. Similarly to the spin-independent recoil, the wimp-pion interaction happens in a way where the WIMP scatters coherently, off the entire xenon atom together. 

A WIMP scattering on a pion exchanged within the xenon nucleus

The analysis was performed with the same tools as the main XENON1T spin-independent WIMP search, and 1 tonne-years of data. No significant evidence for a signal was observed, so we set the first limits on the spin-independent WIMP-pion interaction strength. An open access pre-print of the paper can be found on the arxiv.