STATUS OF XMASS Y. KISIMOTO forXMASS collaboration Kamioka Observatory, Institute for Cosmic Ray Research, the University of To kyo, Higashi-Mozumi, Kamioka, Hida, Gifu 506-1205, JAPAN, Kavli Institute for Physics and Mathematics of the Universe (WPI) , the University of Tokyo, Kashiwa, Chiba 217-8582, JA PAN The XMASS experiment is a multi-purpose detector for rare events, such as direct detection of dark matter, using single-phase liquid xenon. The current phase of the XMASS(XMASS­ I) is focused on direct dark-matter detection with the largest target mass (835kg). In this paper, we report results of searches carried out with commissioning runs of the XMASS-I and current status of XMASS-I after hardware modification to reduce background followed by the commissioning runs. 1 The XMASS experiment 1.1 The XMASS project The XMASS project was proposed to observe rare events such as elastic scattering of electron by pp solar neutrinos, neutrinoless double beta decay, and elastic scattering of nuclei by dark matter particles with a large liquid-Xe detector 1. In the XMASS detector, scintillation lights from liquid xenon are observed by photo-multiplier tubes PMTs arranged around the liquid xenon volume. This simple configuration is very much suitable( for) observing the rare events because liquid xenon is an efficient scintillator and attenuation of scintillation light is quite small and also because liquid xenon has hight atomic number and high density and so it is worked as shield material against radiation from outside. We can, therefor, search for the rate events by extracting events in a low-background volume. Other important advantages of the liquid xenon scintillator are existence of no long-lived isotopes and capability to reduce contamination. Distillation 2 and active charcoal 3 can be used to remove krypton and radon, respectively. 1.2 XMASS-I The first stage of the XMASS project concentrates on the search for the dark matter. One of the candidates for the dark matter is weakly interacting massive particles WIMPs . The XMASS-1 detector was constructed in 2010 with 835kg of liquid xenon lOOkg of( fiducial) target mass in the Kamioka underground site, and commissioning runs were (conducted from 2010 to 2012.) During the commissioning, we performed calibrations and tested its performance. The detailed results of these are presented in a reference4• 2 Results of XMASS-I During the commissioning period, we conducted several searches using the acquired data. These physics results are presented in this section. 2.1 Search for light WIMPs Since deposited energy by WIMPs is small, the lower energy threshold of XMASS is advantageous for detecting them. Some of the commissioning runs were taken in a low threshold setting. The threshold was set to four hits of PMTs and it corresponds to 300 eVee· The data set we used is total 6.7 days with 835 kg liquid-xenon in this low-threshold data. A simple and robust cut is applied to reduce Cerenkov events that occur in quartz windows of the PMTs. To constrain light WIMPs, we compare each expected energy spectrum of a certain mass of WIMPs with the observed one as in Fig. 1. Upper limits of cross section foreach WIMP's mass are set. (See a reference 5 fordeta ils.) + ++ + : - ++ ++ +++ ++ + ++ '·. ++ + ------ XENON10 + ++ + + + +++ + ++ CRESST-11 EDELWEI S + + CDMS-ll S1 . CDMS-t1 G� . COMS-11 Ge lov.< thr ····--·· ........ .. .... + . EDELWEISS-It 2012 CDMSll-Ge XENON100 2012 + L...,.um;ertainty c::::..::______ -XEN.QN100 XMASS90%CL -- 0 4--l"±.._._��-'-���-'-���ic_���__._j 0 0.5 1.5 9 10 12 14 16 18 20 energy{keVee] Mx GeV Figure 1 Energy spectrum left : The observed spectrum after the cut is shown by the black crosses. Solid - ( ) green, dotted blue and dashed red lines are expectation for 7, 12 and 18 GeV WIMPs with cross section indicated in the figure, respectively. Upper limit of cross section for light WIMPs right : The red line shows our limit and the red band shows the uncertainty comes from ( ) Leff. 2.2 Seasonal modulation analysis We also searched for WIMPs signal with the seasonal moderation analysis. We selected runs which were taken in the same run-condition from the commissioning runs with high statistics. The data we used here is 835 kg x 136.1 days-livetime from Dec. 2010 to May 2012. The analysis threshold was set to E h keVee· We do not observe any seasonal moderation t = LO components and so set limits of cross-section as shown in Fig. 2. 2.3 Inelastic scattering 129 Xe by WIMPs WIMPs are expected to cause inelastic scattering off nuclei as well as elastic scattering. 129Xe has an exited states at 39.58 keV. This state is low enough to be excited by spin-dependent coupling between WIMPs and nucleons 6. The lifetime of the exited states is short ( � 1 ns), so we searched for a peak at 38.59 keV with high energy tail by the nuclear recoil. For sensitive search for the events, we applied various cuts to reduce the background as shown in the left panel of Fig. 3. These include the standard cut to remove noise and Cerenkov events in the quartz windows of PMTs, one based on reconstructed vertex positions, ones based on timing information and on a hit patterns to reject surface events between PMTs. Because we do not find peak structure in the final spectrum, we set an upper limit on the cross section of inelastic scattering as plotted in the right panel of Fig. 3. 37 � io- ,�..........,-----------r;:=:::::::====::::?835kg===== lkeVee N - XMASS (ann. mod.) (2009) CDMS 11 Al! l0-38 - XENONlOO (2012) .- UE . - LUX300 (2013) § 10-39 � � 10-•0 l/l 10-•1 �0 u 10-42 c 43 � 10 10-•4 c� lff45 Q_:2: � 10-46 L__._--..L.----�-�--"-----���...._..J 102 103 WIMP Mass[GeV/c2 ] Figure Limit by seasonal modulation analysis in XMASS-I: The black line shows the upper limit as afunction of WIMPs'2 - mass. w1J� mass [GeV] Figure 3 The spectrum after each cut (left) and the exclusion plot (right): The spectra after the cuts are presented -in the left. The black line is the spectrum after the standard cut. The dotted red and green are ones after the radius cut and after the timing cut, respectively. The blue solid is the final one after the cut by the hit pattern. In the right panel, the red line shows the upper limit on the asymptotic cross-section for inelastic scattering on 129Xe using the same form factor used as DAMA(black dashed line). The hatched region represents the uncertainty in the analysis. 2.4 Search for solar axions Axion is a hypothetical particle introduced by Pecci and Quinn to solve one long standing problem in the quantum chromodynamics, the strong GP problem. The search for axion, as well as axion-like particles (ALPs), can be performed by considering the Sun as a strong source of these hypothetical particles. As axions and ALPs are generated in the core of the Sun, their typical energy would be a few keV corresponding to the temperature of the Sun's core. They can couple to electrons and cause the axio-electric effect to deposit their total energy in the XMASS-1 detector. To carry out search for them, we uses the data for light WIMPs search. The data spectrum and expected ones for various axion masses are shown in the left of Fig. 4. In the right of Fig. 4, the limit on the axion-electron-electron coupling constant, gaee, is also 1 Ql drawn. The limit is calculated by requiring the expected spectrum of each axion mass is not exceed any observed data points. ..... ····· ··-······ ···I )i 3�·5 - --i-- J 3�5 · · · ... �4. f,- - .. .. .. ... · 4. ' · . OkeV . · · · SkeV. ..• · . · . · . 2 � · � ·· .···· ···· ·· : .. ..... 2 ··• ...·.· ···· ·· · ···· ······· ····· ···· ·. · · . �...... 2 . 2' . · J� .5 �-VI , L . · 1 . ... ..... ..• -;;;- .\.. .. -.....4.1 . · . \ . · · . 1.5 r .. �o�· ,� o�r ...... .... .... .. 0 ·· . • -· 4 · or 10 °o 20 - · o � '.i ss"' 7.5 Maximum allowed Figure 4 Spectra of various axion masses(O, 5, 10, and 50 keV) together with the observed data (The left four) and upper- limit on the axion-electron-electron coupling constant, (The right). 9aee 2.5 Search for bosonic super-WIMPs There remains issues in the cold dark matter (CDM) models of the Universe: the galaxy structure of the Universe in the CDM model is too complicated comparing with the actual structure. Lighter dark-matter particles in keV to MeV range are motivated in the literatures 7 8. One of such hypothetical particle is discussed as "bosonic super-WIMPs" 7. In particular, the vector super-WIMPs in this mass range are not experimentally constrained and are a good candidate for the dark matter in the Universe. When the bosonic super-WIMPs are absorbed in a target material, all of their energy including their rest mass is deposited. With this nature of the bosonic super-WIMPs, we can expect their energy spectrum to have a monochromatic peak at their rest mass. The same data set and the same reduction method as the inelastic scattering from 129Xe are used in this search. We optimized cut parameters in the reduction steps in order to obtain the best sensitivity for the various rest masses of the vector super-WIMPs. After applying the optimized reduction steps, both of Monte Carlo spectra and the observed one are compared. We found no signal of the super-WIMPs to set a stringent limit on the coupling constant (see Fig. 5). This is the firstexperimental constraint on the electron coupling of bosonic super-WIMPs. 3 Refurbishment of XMASS-I and the current status of XMASS-I During the commissioning we conducted detailed studies on XMASS-I and foundthat the alu­ minum seal of the PMT windows was the major origin of observed events.