We report new results from the Cryogenic Dark Matter Search (CDMS II) at the Soudan Underground Laboratory. Two towers, each consisting of six detectors, were operated for 74.5 live days, giving spectrum-weighted exposures of 34 kg-d for germanium and 12 kg-d for silicon targets after cuts, averaged over recoil energies 10-100 keV for a WIMP mass of 60GeV/c{sup 2}. A blind analysis was conducted, incorporating improved techniques for rejecting surface events. No WIMP signal exceeding expected backgrounds was observed. When combined with our previous results from Soudan, the 90% C.L. upper limit on the spin-independent WIMP-nucleon cross section is 1.6 x 10{sup -43} cm{sup 2} from Ge, and 3 x 10{sup -42} cm{sup 2} from Si, for a WIMP mass of 60GeV/c{sup 2}. The combined limit from Ge (Si) is a factor of 2.5 (10) lower than our previous results, and constrains predictions of supersymmetric models

Akerib, D. S.

Attisha, M. J.

Bailey, C. N.

Baudis, L.

Bauer, Daniel A.

Brink, P. L.

Brusov, P. P.

Bunker, R.

Cabrera, B.

Caldwell, D. O.

Chang, C. L.

Cooley, J.

Crisler, M. B.

Cushman, P.

Daal, M.

Dixon, R.

Dragowsky, M. R.

Driscoll, D. D.

Duong, L.

Ferril, R.

Filippini, J.

UNT Digital Library

arXiv:astro-ph/0509259 v1   9 Sep 2005Limits on spin-independent WIMP-nucleon interactions from the two-tower run of theCryogenic Dark Matter SearchD.S. Akerib,2 M.J. Attisha,1 C.N. Bailey,2 L. Baudis,11 D.A. Bauer,3 P.L. Brink,7 P.P. Brusov,2 R. Bunker,9B. Cabrera,7 D.O. Caldwell,9 C.L. Chang,7 J. Cooley,7 M.B. Crisler,3 P. Cushman,6 M. Daal,8 R. Dixon,3M.R. Dragowsky,2 D.D. Driscoll,2 L. Duong,6 R. Ferril,9 J. Filippini,8 R.J. Gaitskell,1 S.R. Golwala,12 D.R. Grant,2R. Hennings-Yeomans,2 D. Holmgren,3 M.E. Huber,10 S. Kamat,2 S. Leclercq,11 A. Lu,8 R. Mahapatra,9V. Mandic,8 P. Meunier,8 N. Mirabolfathi,8 H. Nelson,9 R. Nelson,9 R.W. Ogburn,7 T.A. Perera,2 M. Pyle,7E. Ramberg,3 W. Rau,8 A. Reisetter,6 R.R. Ross,4, 8, ∗ B. Sadoulet,4, 8 J. Sander,9 C. Savage,9 R.W. Schnee,2D.N. Seitz,8 B. Serfass,8 K.M. Sundqvist,8 J-P.F. Thompson,1 G. Wang,12, 2 S. Yellin,7, 9 J. Yoo,3 and B.A. Young5(CDMS Collaboration)1Department of Physics, Brown University, Providence, RI 02912, USA2Department of Physics, Case Western Reserve University, Cleveland, OH 44106, USA3Fermi National Accelerator Laboratory, Batavia, IL 60510, USA4Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA5Department of Physics, Santa Clara University, Santa Clara, CA 95053, USA6School of Physics & Astronomy, University of Minnesota, Minneapolis, MN 55455, USA7Department of Physics, Stanford University, Stanford, CA 94305, USA8Department of Physics, University of California, Berkeley, CA 94720, USA9Department of Physics, University of California, Santa Barbara, CA 93106, USA10Department of Physics, University of Colorado at Denver and Health Sciences Center, Denver, CO 80217, USA11Department of Physics, University of Florida, Gainesville, FL 32611, USA12Department of Physics, California Institute of Technology, Pasadena, CA 91125, USAWe report new results from the Cryogenic Dark Matter Search (CDMS II) at the Soudan Under-ground Laboratory. Two towers, each consisting of six detectors, were operated for 74.5 live days,giving spectrum-weighted exposures of 34 kg-d for germanium and 12 kg-d for silicon targets aftercuts, averaged over recoil energies 10–100 keV for a WIMP mass of 60 GeV/c2. A blind analysis wasconducted, incorporating improved techniques for rejecting surface events. No WIMP signal exceed-ing expected backgrounds was observed. When combined with our previous results from Soudan, the90% C.L. upper limit on the spin-independent WIMP-nucleon cross section is 1.6×10−43 cm2 fromGe, and 3×10−42 cm2 from Si, for a WIMP mass of 60GeV/c2. The combined limit from Ge (Si)is a factor of 2.5 (10) lower than our previous results, and constrains predictions of supersymmetricmodels.PACS numbers: 14.80.Ly, 95.35.+d, 95.30.Cq, 95.30.-k, 85.25.Oj, 29.40.WkOne quarter of the energy density of the universe con-sists of non-baryonic dark matter [1], which is gravita-tionally clustered in halos surrounding visible galaxies [2].The Weakly Interacting Massive Particle (WIMP) [3], adark matter candidate, arises independently from con-siderations of Big Bang cosmology and from supersym-metric phenomenology, where the neutralino can be aWIMP [4, 5]. A WIMP has a scattering cross section withan atomic nucleus characteristic of the weak interactionand a mass comparable to that of an atomic nucleus.The CDMS II experiment [6, 7] is designed to detectrecoils of atomic nuclei that have been scattered by in-cident WIMPs in germanium (Ge) and silicon (Si) crys-tals. Events with recoil energies of a few tens of keVand rates .1 event kg−1 d−1 are expected [5, 8]. TheCDMS II search is most sensitive to spin-independent(SI) WIMP-nucleon scattering amplitudes [9]. Coherentsuperposition of SI amplitudes gives Ge better sensitivitythan Si, except for small WIMP masses, where kinemat-ics increase the Si sensitivity. In this report we interpretour data with the SI ansatz. Another report describes aspin-dependent interpretation of our data [10].The CDMS II apparatus in the Soudan UndergroundLaboratory has been described elsewhere [6, 7]. Atthe experiment’s core, Z(depth)-sensitive Ionization andPhonon detectors (ZIPs) measure the ionization andathermal phonon signals caused by recoiling particles inGe and Si crystals [7, 11]. We report new results fromthe most recent CDMS data run collected between March25 and August 8, 2004, consisting of 74.5 live days. SixGe (250 g each) and six Si (100 g each) ZIPs were oper-ated in two vertical stacks (“towers”) at a temperature of50mK. This report includes the first data from Tower 2,which contains four Si and two Ge ZIPs. Improvementsmade since the previous run [6, 7] include a reduction ofelectrical noise and more frequent infrared illuminationto clear the crystals of space-trapped charge.The ZIP detector provides event-by-event discrimina-tion of nuclear recoils from the dominant background ofelectron recoils. The ratio of ionization energy to phononenergy (“ionization yield”) is ∼ 0.3 for nuclear recoils,normalized to electron recoils (see yield∼1 in Fig. 1).FERMILAB-PUB-05-455-E2Electron recoils near the detector surface suffer from poorionization collection and can mimic the ionization yieldof nuclear recoils that occur throughout the detector.Each ZIP’s phonon sensors are divided into four quad-rants. Timing and signal amplitude comparisons amongthe quadrants provide discrimination against electron re-coils, particularly those near the surface. Also, surfaceelectron recoils often deposit energy in adjacent ZIPswithin a tower, causing multiple-detector events. Energyfrom a WIMP recoil would be contained in one detector.To avoid bias, we performed blind analyses. Eventsin and near the signal region were removed from WIMP-search data sets, or “masked” [12]. The cuts that define asignal were determined using calibration data from 133Baand 252Cf sources and from the non-masked WIMP-search data. Seven million electron recoils were collectedusing a 133Ba source of gamma rays, exceeding the com-parable WIMP-search data by a factor of twenty. Half ofthe 133Ba data were used to define analysis cuts and theother half to test cut definitions and estimate expectedbackgrounds. The detector response to nuclear recoilswas characterized with 104 neutrons from a 252Cf source,collected in four separate, several-hour periods during therun.Data from the 133Ba source were used to monitor de-tector stability and to characterize detector performance.One Ge detector in Tower 2, ZIP 5 (T2Z5(Ge)), had aspatial region of abnormal ionization response which weexcluded from analysis. The Si detector T1Z6, known tobe contaminated with 14C, a beta emitter, was excluded,as were detectors T1Z1(Ge) and T2Z1(Si) due to poorphonon sensor performance. We report results from theremaining 5 Ge and 4 Si ZIPs, chosen before unmaskingthe WIMP signal region.Most event selection criteria, based on quantities fromthe four phonon and two ionization pulses from each de-tector, are similar to those described in our previous re-ports [6, 7]. However, we completed five distinct analyses[13] focused on improving existing methods and devel-oping new techniques to reject surface electron recoils.Events with low ionization yield in the 133Ba calibrationdata, which are from surface electron recoils, were usedto develop rejection criteria. All cuts were frozen priorto unmasking the signal region.Phonon pulses from surface recoils are more promptthan those from recoils in the detector bulk. Two tim-ing quantities in the quadrant with the largest phononsignal or “local quadrant” are particularly powerful: thetime delay of the phonon signal relative to the fast ion-ization signal, and the phonon pulse risetime [6, 14]. Forthe first and simplest of the five analysis techniques, thesum of delay and risetime forms a timing parameter, af-ter energy corrections to delay and risetime are appliedto achieve an energy-independent distribution. Figure 1shows the ionization yield versus this timing parameterfor a typical Ge detector. The 133Ba calibration events−4 −2 0 2 4 6 8 10 120  0.20.40.60.81  1.2Ionization YieldTiming Parameter (µs)FIG. 1: Ionization yield versus timing parameter (see text)for calibration data in T2Z3(Ge), with recoil energies inthe range 10–100 keV. Typical bulk-electron recoils from the133Ba source of gamma rays are dots (red), with yield nearunity. Low-yield 133Ba events (+, black), attributed to sur-face electron recoils, have small values of timing parameter,allowing discrimination from neutron-induced nuclear recoilsfrom 252Cf (◦, blue), which show a wide range of timing pa-rameter values. The vertical dashed line shows the minimumtiming parameter allowed for WIMP candidates, and the boxshows the approximate signal region, which is in fact weaklyenergy dependent. (Color online.)with yields of 0.1 to 0.8, identified as surface electron re-coils, show a smaller timing parameter than most nuclearrecoils induced by the 252Cf source.For data from the Ge detectors, we require that can-didates for WIMP-induced nuclear recoil exceed a mini-mum value for this timing parameter (see Fig. 2 upper-right). Because this “timing parameter” analysis is sim-ple and robust, we agreed before unmasking to report itsresults. The expected sensitivities computed for the fourmore advanced analyses described below were all within±20% of that computed for this technique.Two of the four advanced analysis techniques evaluatethe goodness of fit, χ2, for surface electron versus bulknuclear recoil hypotheses. These methods use three vari-ables including their correlations: time delay, risetime,and “partition”. The partition parameter, a measure ofenergy distribution between the phonon quadrants, is de-fined as the ratio of phonon energy in the local quadrantto that in the diagonal quadrant. The difference in χ2between surface electron and bulk nuclear recoil hypothe-ses, ∆χ2, is used as the principal discrimination parame-ter in these analyses. One method uses energy-dependentvariances and the other energy-independent variances.For the Si detectors, we decided after unmasking to usethe energy-dependent χ2 technique, which has the bestexpected sensitivity to a nuclear recoil signal from low-mass WIMPs for any of our five methods. This analysis3technique is four times as sensitive as the timing param-eter analysis for low-mass WIMPs. Before unmasking,we set the minimum requirement on the ∆χ2 for surfaceelectron recoil rejection (see Fig. 2 lower-left). These cri-teria are used for our WIMP-search results for Si.The two remaining analysis techniques confirm the ro-bustness of our Ge and Si results. One technique com-bines delay, risetime and partition in a neural net analy-sis, and the other technique exploits additional informa-tion from the fitted signal pulses to reconstruct recoil po-sition and type. These two and the χ2 techniques promisefurther improvements in sensitivity for the larger expo-sures planned in our future runs, beyond the improve-ments already demonstrated in Fig. 2.−6 −4 −2 0 2 4 6 8 100101102Timing Parameter (µs)CountSi−20 0  20 40 60 100101102∆χ2CountSi−6 −4 −2 0 2 4 6 8 10Ge−20 0  50 100 150GeFIG. 2: Variables used to reject surface electron recoils (7-100 keV), for data from T2Z4 (Si/left) and T1Z2 (Ge/right),for the timing parameter (upper) and ∆χ2 (lower). Light(red) lines show distributions of low-yield electron recoils fromthe 133Ba source, while dark (blue) lines show distributions ofnuclear recoils from the 252Cf source. Dashed lines indicatethe minimum values for acceptable WIMP candidates. A cuton the timing parameter is used for the Ge detectors, whilea requirement on ∆χ2 is used for the Si detectors. (Coloronline.)Upon unmasking the Ge data, one candidate with10.5 keV recoil energy was found to pass all cuts in thetiming parameter analysis. Unmasking the Si data re-vealed that no events passed all cuts. Figure 3 shows theunmasked data in ionization yield versus recoil energy;the data before and after application of the cut intendedto reject surface electron recoils are shown. All analysistechniques showed consistent results.After unmasking the Ge data, we realized that the can-didate occurred in a detector during an interval of timewhen that detector suffered inefficient ionization collec-tion. This defect by itself would prevent us from claimingthe event was evidence for a WIMP-induced nuclear re-coil. The candidate is also consistent with the rate ofexpected background. Although we do not claim thisevent as a nuclear recoil from a WIMP, we do include it00.20.40.6Ionization Yield5  10 20 30 40 50 60 70 80 90 10000.20.40.6Recoil Energy (keV)Ionization YieldGeSiFIG. 3: Ionization yield versus recoil energy for events in allGe detectors (upper) and all Si detectors (lower) passing ini-tial data selection cuts prior to applying the surface electronrecoil rejection cut. The signal region consists of recoil ener-gies exceeding 7 keV, shown with a vertical dashed line, andyields between the curved lines defined with recoils induced bythe 252Cf source while WIMP-search data were still masked.Below 7 keV, separation between nuclear and electron recoilsbecomes poor. Events passing the surface electron recoil cutare a star (red) inside the signal region and dark filled circles(blue) outside. Bulk-electron recoils with yield near unity areabove the vertical scale limits. (Color online.)in setting limits on the WIMP-nucleon cross section.The expected surface-event backgrounds are estimatedby multiplying two factors. The first factor is the num-ber of events in the signal region before surface-eventcuts as obtained upon unmasking. The second factoris the fraction of surface events expected to pass thesurface-event cut. This fraction may be estimated withthe actual passing fraction of low-yield events similar tothe single-detector event background. An estimate usingthe passing fraction from 133Ba Ge (Si) data indicates asurface-event background of 0.13±0.05 (0.90±0.4) events.However, we decided before unmasking to base ourbackground estimate on the passing fraction of WIMP-search multiple-detector events in a wide low-yield re-gion. The number of surface events expected to pass thesurface-event cuts is 0.4±0.2 (stat) ±0.2 (sys) between10–100keV in Ge and 1.2±0.6 (stat) ±0.2 (sys) between7–100keV in Si. Evidence suggests that beta decaysof radioactive nuclides distributed across the detectorscause most surface electron recoils in the WIMP-searchdata. The spatial distribution of these contaminants dif-fers from that of surface electron recoils from the 133Basource, which might explain the difference in estimatedbackground levels. From simulation methods reported in[7], the expected background due to cosmogenic neutronsthat escape our muon veto is 0.06 events in Ge and 0.05events in Si.4WIMP Mass [GeV/c2]Cross−section [cm2 ] (normalized to the nucleon)CDMS (Ge) combinedCDMS (Ge) 2−TowerZeplin−ICDMS (Si)Edelweiss 2003DAMA 19965 10 50 100 50010−4310−4210−4110−4010−3910−38FIG. 4: WIMP-nucleon cross section upper limits (90% C.L.)versus WIMP mass. The upper CDMS Ge curve also usesdata from the current run, while the lower Ge curve in-cludes the previous run [6]. Supersymmetric models allowthe largest shaded (light-blue) region [15], and the smallershaded (green) region [17]. The shaded region in the upperleft (see text) is from DAMA [18], and experimental limitsare from DAMA [16], EDELWEISS [19], and ZEPLIN [20].(Color online.)Figure 4 shows the upper limits on WIMP-nucleoncross sections calculated from the Ge and Si analysesreported here using standard assumptions for the galac-tic halo [8]. For the upper Ge limit, data between 10–100keV from this run are used. Also shown is the com-bined limit obtained from this report and our earlier work[6, 7]. For the combined Ge limit, we have included datain the 7–10keV interval of recoil energy from the runreported here [21]. The combined result for Ge limitsthe WIMP-nucleon cross-section to < 1.6 × 10−43 cm2at the 90% C.L. at a WIMP mass of 60GeV/c2, a fac-tor of 2.5 below our previously published limits. Thisnew Ge limit constrains some minimal supersymmet-ric (MSSM) parameter space [15] and for the first timeexcludes some parameter space relevant to constrainedmodels (CMSSM) [17].The Si limit in Fig. 4 is based on standard halo as-sumptions using Si data from 7–100keV in this run.The Si result limits the WIMP-nucleon cross-section to<3×10−42 cm2 at the 90% C.L. at a WIMP mass of60GeV/c2. This Si result excludes new parameter spacefor low-mass WIMPs, including a region compatible withinterpretation of the DAMA signal (2–6 and 6–14keVeebins) as scattering on Na [18].This work is supported by the National Science Foun-dation under Grant Nos. AST-9978911 and PHY-9722414, by the Department of Energy under contractsDE-AC03-76SF00098, DE-FG03-90ER40569, DE-FG03-91ER40618, DE-FG02-94ER40823, and by Fermilab, op-erated by the Universities Research Association, Inc., un-der Contract No. DE-AC02-76CH03000 with the Depart-ment of Energy. The ZIP detectors were fabricated in theStanford Nanofabrication Facility operated under NSF.∗ Deceased[1] D. N. Spergel et al., (WMAP Collab.), Astrophys. J.Suppl. 148, 175 (2003); M. Tegmark et al., (SDSS Col-lab.), Phys. Rev. D 69, 103501 (2004).[2] P. Salucci and A. Borriello, Lect. Notes Phys. 616, 66(2003); T. Broadhurst et al., Astrophys. J. 621, 53(2005).[3] G. Steigman and M.S. Turner, Nucl. Phys. B253, 375(1985).[4] B.W. Lee and S. Weinberg, Phys. Rev. Lett. 39, 165(1977); S. Weinberg, Phys. Rev. Lett. 48, 1303 (1982).[5] G. Jungman, M. Kamionkowski, and K. Griest, Phys.Rep. 267, 195 (1996); G. Bertone, D. Hooper, and J. Silk,Phys. Rep. 405, 279 (2005).[6] D.S. Akerib et al., (CDMS Collab.) Phys. Rev. Lett. 93,211301 (2004).[7] D.S. Akerib et al., (CDMS Collab.), accepted for PRD.arXiv:astro-ph/0507190 (2005).[8] J.D. Lewin and P.F. Smith, Astropart. Phys. 6, 87(1996).[9] A Majorana neutralino undergoes a scalar interactionof roughly equal strength for neutrons and protons, seeK. Griest Phys. Rev. D 38, 2357 (1988). The SI scatter-ing amplitude is generally sensitive to scalar, vector, andtensor interactions of a spin-1/2 WIMP, see A. Kurylovand M. Kamionkowski, Phys. Rev. D 68, 103509 (2003).[10] D.S. Akerib et al., (CDMS Collab.), submitted to Phys.Rev. Lett.[11] K.D. Irwin et al., Rev. Sci. Instr. 66, 5322 (1995);T. Saab et al., AIP Proc. 605, 497 (2002).[12] Although software safeguards to enforce the blindingscheme did not work as intended, the blinding policy it-self remained in effect, and we exercised care not to ob-tain any information from inside the WIMP-search signalregion. We believe the cuts were developed fully blind.[13] CDMS collaboration, in preparation forPhys. Rev. D; theses and notes athttp://cdms.berkeley.edu/Dissertations.[14] R.M. Clarke et al., Appl. Phys. Lett. 76, 2958 (2000).[15] A. Bottino et al., Phys. Rev. D 69, 037302 (2004).[16] R. Bernabei et al., Phys. Lett. B389, 757 (1996).[17] J. Ellis, et al., Phys. Rev. D 71, 095007 (2005).[18] We show the 90% allowed region from Fig. 2a of P. Gon-dolo and G. Gelmini, Phys. Rev. D 71, 123520 (2005).[19] V. Sanglard et al., (EDELWEISS Collab.), Phys. Rev.D 71, 122002 (2005).[20] G.J. Alner et al., (UK Dark Matter Collab.), Astropart.Phys. 23, 444 (2005).[21] Cuts for 7–10 keV interval of recoil energy were frozenbefore data were unmasked. The decision to report thesedata was made after the unmasking. Inclusion of thisinterval increases the expected background by 0.1 ±0.02(stat) events, and lowers the limit on WIMP-nucleoncross section only for WIMP masses in the 8–11 GeV/c2interval. No candidate events were found in the 7–10 keVinterval of recoil energy.

Limits on spin-independent wimp-nucleon interactions from the two-tower run of the cryogenic dark matter search

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Limits on spin-independent wimp-nucleon interactions from the two-tower run of the cryogenic dark matter search

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