The Enriched Xenon Observatory (EXO) is an experimental program searching for neutrino-less double beta decay in xenon-136. The first stage of this program, EXO-200, has been in operation since early 2011. I present here the latest physics results from the experiment

Tosi, D.

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DOI 10.1393/ncc/i2014-11783-5Colloquia: Pontecorvo100IL NUOVO CIMENTO Vol. 37 C, N. 3 Maggio-Giugno 2014Latest results from EXO-200D. Tosi, on behalf of the EXO CollaborationStanford University - 450 Serra Mall, Stanford, CA, USASummary. — The Enriched Xenon Observatory (EXO) is an experimental programsearching for neutrino-less double beta decay in xenon-136. The first stage of thisprogram, EXO-200, has been in operation since early 2011. I present here the latestphysics results from the experiment.Neutrino-less double-beta decay (0νββ) is an hypothetical process that can happenonly if neutrinos are Majorana particles. The signature for this decay is a peak in countsat the end point of the two-neutrino double-beta (2νββ) decay spectrum, an energy valuedependent on the isotope. The half-life of this process is linked to the neutrino Majoranaeffective mass [1].The search for 0νββ decay can be performed in those isotopes subject to 2νββ decay.One of these isotopes is xenon-136, and it has been the focus of the Enriched XenonObservatory (EXO) experimental program. EXO-200, the first stage of the program,is located in the Waste Isolation Pilot Plant (WIPP) near Carlsbad, New Mexico, at650 m depth. It features a time projection chamber (TPC) filled with about 175 kgliquid xenon (LXe). The energy resolution in xenon improves if both the charge andthe scintillation signal produced by a decay event are measured. To do so each side ofthe TPC is equipped with large area avalanche photodiodes (LAAPDs) and two sets ofwires (to sense the induction signal and to collect the charge) crossing at an angle. Fora detailed description of the experiment see Ref. [2].Since the beginning of operations (early 2011) EXO-200 has measured for the firsttime the 2νββ decay in Xe136 [3] and has improved the limit on the rate of 0νββ decay [4].Recently, the most precise measurement for the 2νββ decay half-life has been published[5]. Between the first and the second measurement several improvements to the systemwere implemented, both in hardware and analysis. We completed the external lead wall,which shields the EXO TPC from background (BG) radiation, we improved the front-endelectronics, and we obtained a reduction of the amount of electronegative impurities inxenon thanks to higher recirculation rate and more efficient purification. We also builta sophisticated suite of software tools to perform simulation and data analysis.Our Monte Carlo (MC) simulation is realized in GEANT4 from most of the realsystem’s 3-D CAD drawings for TPC, HFE, cryostat and lead wall. Some details aresimplified, but the total mass for each material closely matches the mass of the samec© Societa` Italiana di Fisica 197198 D. TOSI, ON BEHALF OF THE EXO COLLABORATIONmaterial in the real system. We simulate charge collection and induction signals assuminga 2-D simplified model for the electric field – this has been validated by comparingdistributions of relevant waveform parameters in simulation and in real data. We alsohave a light response function (generated by MC simulation) to determine the amountof light hitting an APD plane for a certain charge deposition at a particular location inthe detector.Once signals are produced, they undergo the same full processing chain as real data.The first step of this chain is the reconstruction of the event. We first look for signalsapplying matched filters which are specifically designed for each channel type. We thendeconvolve the signals and fit them to models to extract a set of waveform parameterswhich we use to eliminate mis-reconstructed or mis-identified events. All the eventspassing the cuts are then scanned for time coincidences: if more signals belong to the sameenergy deposition event, they are bundled in a cluster. At this point we classify eventsby topology, location, and energy. To assign the topology, we look at the distribution ofenergy deposition for each event. An energy deposit is single-site (SS) if it is confinedwithin 3 mm; this is expected to be the case for most β decay events in LXe. Most ofthe penetrating γ radiation on the other hand causes multiple energy deposits at wellseparated locations, therefore called multi-site (MS). To determine the location of anevent we measure the 3-D coordinates; we then evaluate the distance from the edges(standoff distance). Since most BG events originate outside the TPC or on TPC walls,their distribution falls moving towards the center of the LXe volume. In opposition,β events are distributed uniformly in the TPC. We discard events outside our fiducialvolume.To measure accurately the energy of an event, we need to calibrate our energy scale.Every few months we deploy different sources (137Cs, 60Co, 228Th) featuring γ emissionpeaks in the whole energy range of interest. We deploy 228Th every other day to measurethe electron lifetime; we use this information to select high purity data. We also needto apply corrections to take into account electronics gain variations. Each wire gain ismeasured daily with an internal pulser and using 228Th data. A light map is generatedusing multiple source runs to correct for the illumination of the APDs, which dependson the energy deposition location. Once both charge and scintillations are corrected, theTh energy spectrum is fitted to determine the time dependent rotation angle which givesthe optimal energy resolution. We apply this energy correction to all the low BG physicsdata.We compare source data and simulation to check for agreement in rate, shape, andfraction of single site events. With this we validate the probability density functions(PDFs) and estimate the systematic uncertainties which we use as input in the maximumlikelihood fit of the physics data; every BG which could be present is represented andfitted. Well-defined cuts are used to select data for this analysis [5]. To perform the fit,we let float all the parameters (normalization, SS fraction, number of events for eachPDF) and we maximize the likelihood to find the best fit for the number of events in the2νββ PDF. We perform a profile likelihood scan to determine the error on this parameter.Given the total 136Xe exposure of our data set and the detection efficiency, we obtain ahalf life of T 2νββ1/2 = 2.165±0.016(stat)±0.059(sys) ·1021 yr. This is the longest and alsomost precise 2νββ half-life ever measured.A much larger data set is currently being analyzed to search for 0νββ decay. Inparallel, the future 5-ton detector, called nEXO, is being designed.LATEST RESULTS FROM EXO-200 199∗ ∗ ∗EXO-200 is supported by DOE and NSF in the United States, NSERC in Canada,SNF in Switzerland, NRF in Korea, RFBR in Russia and DFG Cluster of Excellence“Universe” in Germany.REFERENCES[1] J.W.F. Valle, 2006, J. Phys. Conf. Ser., 53, 473[2] M. Auger et al. JINST 7 (2012) P05010.[3] N. Ackerman et al. Phys. Rev. Lett. 107 (2011) 212501.[4] M. Auger et al. Phys. Rev. Lett. 109, 032505 (2012).[5] J.B. Albert et al. 2013, arXiv:1306.6106

Latest results from EXO-200

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Latest results from EXO-200

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