Comment on "On the subtleties of searching for dark matter with liquid xenon detectors"

In a recent manuscript (arXiv:1208.5046) Peter Sorensen claims that XENON100's upper limits on spin-independent WIMP-nucleon cross sections for WIMP masses below 10 GeV "may be understated by one order of magnitude or more". Having performed a similar, though more detailed analysis prior to the submission of our new result (arXiv:1207.5988), we do not confirm these findings. We point out the rationale for not considering the described effect in our final analysis and list several potential problems with his study.Comment: 3 pages, no figure

Lowering the radioactivity of the photomultiplier tubes for the XENON1T dark matter experiment

The low-background, VUV-sensitive 3-inch diameter photomultiplier tube R11410 has been developed by Hamamatsu for dark matter direct detection experiments using liquid xenon as the target material. We present the results from the joint effort between the XENON collaboration and the Hamamatsu company to produce a highly radio-pure photosensor (version R11410-21) for the XENON1T dark matter experiment. After introducing the photosensor and its components, we show the methods and results of the radioactive contamination measurements of the individual materials employed in the photomultiplier production. We then discuss the adopted strategies to reduce the radioactivity of the various PMT versions. Finally, we detail the results from screening 216 tubes with ultra-low background germanium detectors, as well as their implications for the expected electronic and nuclear recoil background of the XENON1T experiment.Comment: 10 pages, 5 figure

Search for Event Rate Modulation in XENON100 Electronic Recoil Data

We have searched for periodic variations of the electronic recoil event rate in the (2-6) keV energy range recorded between February 2011 and March 2012 with the XENON100 detector, adding up to 224.6 live days in total. Following a detailed study to establish the stability of the detector and its background contributions during this run, we performed an un-binned profile likelihood analysis to identify any periodicity up to 500 days. We find a global significance of less than 1 sigma for all periods suggesting no statistically significant modulation in the data. While the local significance for an annual modulation is 2.8 sigma, the analysis of a multiple-scatter control sample and the phase of the modulation disfavor a dark matter interpretation. The DAMA/LIBRA annual modulation interpreted as a dark matter signature with axial-vector coupling of WIMPs to electrons is excluded at 4.8 sigma.Comment: 6 pages, 4 figure

Search for Two-Neutrino Double Electron Capture of 124^{124}Xe with XENON100

Two-neutrino double electron capture is a rare nuclear decay where two electrons are simultaneously captured from the atomic shell. For $^{124}$Xe this process has not yet been observed and its detection would provide a new reference for nuclear matrix element calculations. We have conducted a search for two-neutrino double electron capture from the K-shell of $^{124}$Xe using 7636 kg$\cdot$d of data from the XENON100 dark matter detector. Using a Bayesian analysis we observed no significant excess above background, leading to a lower 90 % credibility limit on the half-life $T_{1/2}>6.5\times10^{20}$ yr. We also evaluated the sensitivity of the XENON1T experiment, which is currently being commissioned, and find a sensitivity of $T_{1/2}>6.1\times10^{22}$ yr after an exposure of 2 t$\cdot$yr.Comment: 6 pages, 4 figure

Removing krypton from xenon by cryogenic distillation to the ppq level

The XENON1T experiment aims for the direct detection of dark matter in a cryostat filled with 3.3 tons of liquid xenon. In order to achieve the desired sensitivity, the background induced by radioactive decays inside the detector has to be sufficiently low. One major contributor is the $\beta$-emitter $^{85}$Kr which is an intrinsic contamination of the xenon. For the XENON1T experiment a concentration of natural krypton in xenon $\rm{^{nat}}$Kr/Xe < 200 ppq (parts per quadrillion, 1 ppq = 10$^{-15}$ mol/mol) is required. In this work, the design of a novel cryogenic distillation column using the common McCabe-Thiele approach is described. The system demonstrated a krypton reduction factor of 6.4$\cdot$10$^5$ with thermodynamic stability at process speeds above 3 kg/h. The resulting concentration of $\rm{^{nat}}$Kr/Xe < 26 ppq is the lowest ever achieved, almost one order of magnitude below the requirements for XENON1T and even sufficient for future dark matter experiments using liquid xenon, such as XENONnT and DARWIN

Limits on spin-dependent WIMP-nucleon cross sections from 225 live days of XENON100 data

We present new experimental constraints on the elastic, spin-dependent WIMP-nucleon cross section using recent data from the XENON100 experiment, operated in the Laboratori Nazionali del Gran Sasso in Italy. An analysis of 224.6 live days x 34 kg of exposure acquired during 2011 and 2012 revealed no excess signal due to axial-vector WIMP interactions with 129-Xe and 131-Xe nuclei. This leads to the most stringent upper limits on WIMP-neutron cross sections for WIMP masses above 6 GeV, with a minimum cross section of 3.5 x 10^{-40} cm^2 at a WIMP mass of 45 GeV, at 90% confidence level

Observation and applications of single-electron charge signals in the XENON100 experiment

The XENON100 dark matter experiment uses liquid xenon in a time projection chamber (TPC) to measure xenon nuclear recoils resulting from the scattering of dark matter Weakly Interacting Massive Particles (WIMPs). In this paper, we report the observation of single-electron charge signals which are not related to WIMP interactions. These signals, which show the excellent sensitivity of the detector to small charge signals, are explained as being due to the photoionization of impurities in the liquid xenon and of the metal components inside the TPC. They are used as a unique calibration source to characterize the detector. We explain how we can infer crucial parameters for the XENON100 experiment: the secondary-scintillation gain, the extraction yield from the liquid to the gas phase and the electron drift velocity

Protein Kinase Cδ Stimulates Proteasome-Dependent Degradation of C/EBPα during Apoptosis Induction of Leukemic Cells

BACKGROUND:The precise regulation and maintenance of balance between cell proliferation, differentiation and death in metazoan are critical for tissue homeostasis. CCAAT/enhancer-binding protein alpha (C/EBPalpha) has been implicated as a key regulator of differentiation and proliferation in various cell types. Here we investigated the potential dynamic change and role of C/EBPalpha protein during apoptosis induction. METHODOLOGY/PRINCIPAL FINDINGS:Upon onset of apoptosis induced by various kinds of inducers such as NSC606985, etoposide and others, C/EBPalpha expression presented a profound down-regulation in leukemic cell lines and primary cells via induction of protein degradation and inhibition of transcription, as assessed respectively by cycloheximide inhibition test, real-time quantitative RT-PCR and luciferase reporter assay. Applying chemical inhibition, forced expression of dominant negative mutant and catalytic fragment (CF) of protein kinase Cdelta (PKCdelta), which was proteolytically activated during apoptosis induction tested, we showed that the active PKCdelta protein contributed to the increased degradation of C/EBPalpha protein. Three specific proteasome inhibitors antagonized C/EBPalpha degradation during apoptosis induction. More importantly, ectopic expression of PKCdelta-CF stimulated the ubiquitination of C/EBPalpha protein, while the chemical inhibition of PKCdelta action significantly inhibited the enhanced ubiquitination of C/EBPalpha protein under NSC606985 treatment. Additionally, silencing of C/EBPalpha expression by small interfering RNAs enhanced, while inducible expression of C/EBPalpha inhibited NSC606985/etoposide-induced apoptosis in leukemic cells. CONCLUSIONS/SIGNIFICANCE:These observations indicate that the activation of PKCdelta upon apoptosis results in the increased proteasome-dependent degradation of C/EBPalpha, which partially contributes to PKCdelta-mediated apoptosis

ULF geomagnetic pulsations at different latitudes in Antarctica

We present a study aimed to characterize the ULF (1–100 mHz) geomagnetic pulsation activity in the polar cap at different latitudes. We used magnetic measurements obtained through 2005–2007 in Antarctica, at Dome C (89&amp;deg; S corrected geomagnetic latitude) and at Terra Nova Bay (80&amp;deg; S corrected geomagnetic latitude). The results indicate a solar wind control of the wave activity, more important at larger distances from the cusp, as well as a significant role of the local ionospheric conditions. The different position of the two stations, with respect to the cusp and closed field lines, is responsible for the observed different pulsation characteristics. At Terra Nova Bay, due to the approaching of the station to the cusp and closed field lines in the daytime, the ULF power is characterized by a maximum around noon; daytime pulsation events in the Pc5 frequency band are related to the fundamental field line resonances occurring at lower latitudes, while higher harmonics of the fundamental may account for the characteristics of Pc3–4 pulsations. In the nighttime, at Pc3 frequencies, the results suggest waves propagating sunward, possibly due to the transmission of upstream waves from the magnetosheath via the magnetotail lobes. At Dome C, near the geomagnetic pole and very far from closed field lines, the ULF power in any frequency band only shows an enhancement in the postmidnight sector, more pronounced for Pc3 pulsations. The ULF activity appears to be driven by processes occurring in the magnetotail: in particular, nighttime Pc3 pulsation events may be originated from upstream wave penetration through the magnetotail lobes