We present the rst search for weakly interacting massive particles (WIMPs) using the back- ground rejection capabilities of SuperCDMS 1. A blind analysis of data from an exposure of 577 kg-days leads to an upper limit on the spin-independent WIMP-nucleon cross section of 1:2 10????42 cm2 at 8 GeV. This result probes new parameter space for WIMP-nucleon scat- tering cross section for light WIMPs with masses < 6 GeV/c2 and is in tension with WIMP interpretations of recent experiments

Augé, E.

Cerdeño, David G.

Dumarchez, J.

Thanh Vân, J.T.

English

Durham Research Online

SuperCDMS: Recent results on low-mass WIMPsDAVID G. CERDEÑODepartamento de F́ısica Teórica & Instituto de F́ısica Teórica IFT UAM/CSIC,Universidad Autónoma de Madrid, 28049 Madrid, SpainWe present the first search for weakly interacting massive particles (WIMPs) using the back-ground rejection capabilities of SuperCDMS 1. A blind analysis of data from an exposure of577 kg-days leads to an upper limit on the spin-independent WIMP-nucleon cross section of1.2 × 10−42 cm2 at 8 GeV. This result probes new parameter space for WIMP-nucleon scat-tering cross section for light WIMPs with masses < 6 GeV/c2 and is in tension with WIMPinterpretations of recent experiments.The detection and identification of the dark matter in the Universe constitutes one of thegreatest challenges in modern Physics. A generic weakly interacting massive particle (WIMP)is a well motivated candidate, as it can be produced in the right amount and searched for indirect and indirect detection experiments. Recent excesses of events reported by CDMS II (Si)2,CoGeNT 3, CRESST-II 4, DAMA 5, and possible indirect evidence from gamma rays from thegalactic centre 6, can be interpreted in terms of WIMPs with mass in the 6–30 GeV/c2 range.Various theoretical constructions also provide WIMP candidates in this mass range 7,8,9,10,11.The direct search for light WIMPs is extremely challenging, since they produce only low-energy nuclear recoils, forcing experiments to lower their analysis energy thresholds 12,13,14,15,16.Following this approach, low-energy recoils in the range 1.6–10 keVnr (nuclear-recoil equivalentenergy) from the SuperCDMS experiment at the Soudan Underground Laboratory 17 have beenanalysed, exploiting the excellent background discrimination capabilities of the new detectors 1.SuperCDMS at Soudan is an upgrade to the Cryogenic Dark Matter Search (CDMS II) 18with new detector hardware, operating in the same location with the same low-radioactivitysetup 19 since March 2012. The target consists of five towers, with three 0.6-kg cylindricalgermanium crystals each. These detectors (iZIPs) are instrumented with interleaved ionizationand phonon sensors on their flat faces. From the measured ionization and phonon energy,we derive the recoil energy and the “ionization yield,” the ratio between ionization and recoilenergy. This is a key quantity to discriminate nuclear recoils (expected from WIMP interactions)from electron recoils (expected from most backgrounds), since nuclear recoils exhibit a reducedionization yield compared to electron recoils. Moreover, the iZIP sensor layout greatly improvesthe determination of a fiducial volume in the bulk, designed to reject events in the peripheralregions of the detectors (which present a reduced ionization yield and pollute the signal region)20.The fraction of the total phonon or ionization energy measured by the guard sensors is encodedin “radial partition” parameter through which radial fiducialization is defined. Similarly, a “zpartition” parameter is constructed from the fraction measured by the sensors on each face andused to define the z fiducialization.The data presented here was recorded between October 2012 and June 2013 using the sevendetectors with the lowest trigger thresholds. The remaining detectors are used to reject eventswhich deposit energy in more than one detector (incompatible with WIMP interactions). Pe-riods of abnormal detector behavior and elevated noise are removed from the analysis. Afteraccounting for these losses, the exposure is 577 kg-days. To prevent bias when defining theevent-selection criteria, all single-detector hits with recoil energies between a time-dependentnoise threshold and 10 keVnr, and with ionization energy consistent with nuclear recoils, wereremoved from study, i.e. blinded. A distinct open set of data, containing 97 kg-days of exposure,was constructed from periods following 252Cf calibrations.The relation between the total phonon energy and the mean ionization energy of nuclearrecoils for each detector, as determined from 252Cf calibration data, is consistent with Lindhard’smodel. The nuclear-recoil band is defined by accepting events within 3σ of the mean ionizationenergy. The nuclear-recoil equivalent energy is constructed from the total phonon energy bysubtracting the contribution of Luke-Neganov phonons corresponding to the mean nuclear-recoilionization response for the respective total phonon energy.Hardware trigger thresholds for each detector were adjusted several times during the WIMPsearch. For each period of constant trigger threshold, the trigger efficiencies as functions of totalphonon energy were measured using 133Ba calibration data. Analysis thresholds are set to be1σ below the energy at which the detector trigger efficiency is 50% in periods of time for whichthis quantity is above the noise threshold used in the data blinding, and equal to such thresholdotherwise. The combined efficiency is an exposure-weighted sum of the measured efficiency foreach detector and period, and is shown in the left-hand side of Figure 1.Three levels of data-selection criteria have been used to identify triggered events as WIMPcandidates. A first data-quality cut rejects poorly reconstructed and noise-induced events, re-moving also periods of abnormal noise and spurious triggers (for which the pulse shape is in-consistent with that expected for real events). The second level (preselection) singles out eventconfigurations consistent with WIMPs, requiring single-scattered events that feature energieswithin the 3σ nuclear-recoil band and phonon partitions consistent with bulk nuclear recoils.A loose fiducial volume, constructed from the ionization partitions, further restricts events tobe consistent with bulk nuclear recoils. Similarly, events near the detectors’ sidewalls are re-jected by requiring the guard electrodes on both faces to be within 2σ from the mean of thebaseline noise. Events coincident with the muon veto are also rejected. The final level of event-selection (discrimination) uses a boosted decision tree (BDT) with four discriminators: thetotal phonon energy, ionization energy, phonon radial partition and phonon z partition. A BDTwas trained for each detector using simulated background events and nuclear recoils from 252Cfcalibration weighted to mimic a WIMP energy spectrum, accounting for the selection criteriaacceptance. The BDT discrimination thresholds for individual detectors were chosen simulta-neously to minimize the expected 90% confidence level (C.L.) Poisson upper limit of the rateof passing events per WIMP exposure. The BDT was trained and optimized separately for 5,7, 10, and 15 GeV/c2 WIMPs. Events that pass any of the four WIMP-mass optimizationsare accepted into the signal region as candidates. The left-hand side of Figure 1 shows thecumulative efficiency after applying each level of selection criteria and the analysis thresholds.A background model was developed that includes Compton recoils from the gamma-raybackground; 1.1–1.3 keV X-rays and Auger electrons from L-shell electron-capture (EC) decayof 65Zn, 68Ga, 68Ge and 71Ge; and decay products from 210Pb contamination on the detectorsand their copper housings. The right-hand side of Figure 1 shows the individual componentsof the background model as a function of the 10 GeV/c2 BDT discrimination parameter afterapplying the preselection criteria. This background model was finalized prior to unblinding andpredicted 6.1+1.1−0.8 (stat.+syst.) events passing the BDT selection. Simulations of radiogenic andcosmogenic neutrons predict an additional 0.098±0.015 (stat.) events. These estimates includedonly known systematic effects. Because the accuracy in background modeling required for a fulllikelihood analysis is difficult to achieve in a blind analysis of this type, the decision was madebefore unblinding to report an upper limit on the WIMP-nucleon cross section.2 4 6 8 10 12Total phonon energy [keV]0.00.20.40.60.81.0Efficiency2 3 4 5 6 7 8 9Lindhard nuclear-recoil energy [keVnr]BDT score-1 -0.5 0 0.5 1Number of events / 0.04110210DataWIMPPb206Sidewall Bi210Pb+210Sidewall Bi210Pb+210Face 1.3 keV lineComptonsBDT score -1 -0.5 0 0.5 1Residual -40-2002040Figure 1 – Left) Cumulative efficiencies after sequential application of each stage of event selection. From top tobottom, these are data-quality criteria, trigger and analysis thresholds, preselection criteria, and BDT discrim-ination with 68% C.L. (stat. + syst.) uncertainty band. Right) Components of the background model passingthe preselection criteria, summed over all detectors. For comparison, a 10 GeV/c2 WIMP with cross section6× 10−42 cm2 is shown on top of the total background. Events passing preselection criteria are overlaid showingthe statistical errors. The bottom plot shows the difference between the data and the background expectation.Tan bars indicate the systematic uncertainty (68% C.L.) on the background estimate.Upon unblinding, eleven candidates were observed, which are shown on the left-hand side ofFigure 2. This is consistent with the background prediction for most detectors, except for thethree high-energy events in detector T5Z3 (for which there is a probability of only 4× 10−4 toobserve this many background events). However, this detector has an ionization guard electrodeshorted to ground and, although the background model was developed to account for this, thealtered electric field may have affected the selection of background model templates, and thus thebackground estimate. Otherwise, the background model has been found to reproduce correctlymost features of the true background, as evidenced in the right-hand side of Figure 1. Thesystematic uncertainty is dominated by the uncertainty of the expected ionization of sidewallevents originating from 210Pb and 210Bi. P-value statistics comparing the data passing thepreselection criteria with the blind background model prediction range from 8–26% for theBDTs trained to each of the four WIMP masses.On the right-hand side of Figure 2 we show the 90% C.L. upper limit on the spin-independentWIMP-nucleon cross section, calculated using the optimum interval method without back-ground subtraction for standard halo assumptions. Statistical and systematic uncertaintiesin the fiducial-volume efficiency, the nuclear-recoil energy scale, and the trigger efficiency werepropagated into the limit by Monte Carlo and are represented by the narrow gray band aroundthe limit. The limit is consistent with the expected sensitivity for masses below 10 GeV/c2 asshown by the green band in Figure 2. The discrepancy above 10 GeV/c2 is due to the threehigh-energy events in T5Z3, which are in tension with the background expectation. These resultsstrongly disfavour a WIMP interpretation of the CoGeNT excess, which also uses a germaniumtarget. Similar tension exists with WIMP interpretations of several other experiments, includingCDMS II (Si), assuming spin-independent interactions and a standard halo model. New regionsof WIMP-nucleon scattering for WIMP masses below 6 GeV/c2 are excluded. This constitutesthe first search for WIMPs exploiting the full background rejection capability of SuperCDMS.AcknowledgmentsD.G.C. is supported by MultiDark CSD2009-00064, the Spanish MICINN under Grant No.FPA2012-34694, the Spanish MINECO under Grant No. SEV-2012-0249, and the Communityof Madrid under Grant No. HEPHACOS S2009/ESP-1473.Total phonon energy [keV]2 4 6 8 10 12Ionization energy [keV]-101234Lindhard nuclear-recoil energy [keVnr]2 3 4 5 6 7 8 9T2Z1T2Z2T5Z2T5Z33 4 5 6 7 8 9 10 20 30WIMP mass [GeV/c2]10-4410-4310-4210-4110-4010-39WIMP-nucleon cross section [cm2 ]10-810-710-610-510-410-3WIMP-nucleon cross section [pb]Figure 2 – Left) Veto-anticoincident single-scatter events within the ionization-partition fiducial volume that passthe data-quality selection criteria. Large encircled shapes are the 11 candidate events. Shaded regions (fromlight to dark) are the 95% confidence contours expected for 5, 7, 10 and 15 GeV/c2 WIMPs. The band ofevents above the expected signal contours corresponds to bulk electron recoils, including the 1.3 keV activationline at a total phonon energy of ∼3 keV. High-radius events near the detector sidewalls form the wide band ofevents with near-zero ionization energy. For illustrative purposes, an approximate nuclear-recoil energy scale isprovided. Right) The 90% confidence upper limit (solid black) based on all observed events is shown with 95%C.L. systematic uncertainty band (gray). The pre-unblinding expected sensitivity in the absence of a signal isshown as 68% (dark green) and 95% (light green) C.L. bands.References1. R. Agnese et al. [SuperCDMS Collaboration], Phys. Rev. Lett. 112 (2014) 241302.2. R. Agnese et al. [CDMS Collaboration], Phys. Rev. Lett. 111 (2013) 251301.3. C. E. Aalseth et al. [CoGeNT Collaboration], Phys. Rev. D 88 (2013) 1, 012002.4. G. Angloheret al. [CRESST-II Collaboration], Eur. Phys. J. C 72 (2012) 1971.5. R. Bernabei et al. 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SuperCDMS: Recent results on low-mass WIMPs

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SuperCDMS: Recent results on low-mass WIMPs

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