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Dark Matter 2014 MarcSchumann1,a 1AlbertEinsteinCenterforFundamentalPhysics,UniversityofBern,Switzerland 5 Abstract. This article gives an overview on the status of experimental searches for dark matter at the 1 end of 2014. The main focus is on direct searches for weakly interacting massive particles (WIMPs) using 0 underground-based low-background detectors, especially on the new results published in 2014. WIMPs are 2 excellentdarkmattercandidates, predictedbymanytheoriesbeyondthestandardmodelofparticlephysics, n andareexpectedtointeractwiththetargetnucleieitherviaspin-independent(scalar)orspin-dependent(axial- a vector)couplings.Non-WIMPdarkmattercandidates,especiallyaxionsandaxion-likeparticlesarealsobriefly J discussed. 6 ] O 1 Introduction newresultspublishedduringtheyear. Wedonotintendto C provide detailed descriptions of the various experiments, Numerous indirect observations at astronomical and cos- butmainlyfocusontheunderlyingconceptsandtherecent . h mologicalscales[1], aswellasresultsfromcomplexnu- results,andreferthereadertothereferencesforfurtherin- p mericalN-bodysimulations[2],indicatethepresenceofa formation.Thisreviewisapersonallybiasedselectionand - o new form of matter in the Universe, which only interacts otherauthorsmightplacetheirfocusdifferently. r significantly via gravity. The existence of this dark mat- A short review on the formalism to describe WIMPs t s ter is one of the strongest indications for physics beyond scattering in lab-based detectors and the relevant back- a thestandardmodelofparticlephysics,asnoknownparti- groundsisgiveninSection2. Sections3and4summarize [ clecanbeattributedtodarkmatter. Recently,resultsfrom thestatusandthemainnewresultsonWIMPandimpor- 1 the Planck satellite mission [3] on precise measurements tantnon-WIMPsearches,respectively,followedbyabrief v of the cosmic microwave background (CMB) have been conclusion. 0 published. TheseagreewiththepredictionsoftheΛCDM 0 model,describingacosmosdominatedbydarkenergy(Λ) 2 2 Principles of Direct Detection 1 andcolddarkmatter(CDM).AccordingtoPlanck,68.3% 0 of the Universe’s energy density is from dark energy and If theWIMP darkmatter particleinteracts notonly grav- 1. from26.8%darkmatter. itationallywithordinarymatter, butalsowithweak-scale 0 Theparticle(s)constitutingthedarkmatterremainun- cross sections, the signature of WIMPs present in the lo- 5 known as of today, even though dark matter outnumbers cal solar neighborhood might be directly detectable in 1 ’ordinary’ baryonic matter by a factor 5. Neutral, cold : sensitive Earth-based detectors [8]. This path towards a v (i.e. non-relativistic), and stable particles (or with half- WIMP observation is called direct detection. Alternative i lifes longer than the age of the Universe), representing X approachesaretheindirectdetectionofWIMPsbyobserv- viabledarkmattercandidates,arepredictedbymanythe- r ingtheirannihilation(ordecay)products(eventuallylead- a ories beyond the standard model, especially if their de- ing to γ, ν, e+, p, etc.) in satellite experiments and the cay is inhibited by a new symmetry. Important exam- productionofdarkmatterincolliderexperiments. ples are the lightest supersymmetric particle (LSP) in in Inthisarticle,weconcentrateondirectdetection. supersymmetric theories [4], such as the neutralino χ , 0 the lightest T-odd particle in little Higgs theories [5], or 2.1 EventRatesandEnergies the lightest Kaluza-Klein particle (LKP) in models with extra-dimensions [6]. This class of dark matter candi- NeutralWIMPsareexpectedtoscatteroffthetargetnuclei dates are referred to as weakly interacting massive parti- ofadirectdetectionexperimentofmassM.Theeventrate cles (WIMPs) [7], covering a largely unconstrained mass pernuclearrecoil(NR)energyE isthengivenby r rangefrom1GeV/c2toseveralTeV/c2. This article attempts to provide an overview of the dR = ρ0M (cid:90) vescvf(v)dσ dv. (1) experimental status of direct WIMP searches at the end dEr mNmχ vmin dEr of2014,focusingontheprojectsleadingthefieldandon m and m are the masses of target nucleus and WIMP N χ ae-mail:[email protected] particle, respectively, and f(v) is the normalized WIMP velocity distribution. All velocities are defined in the de- totalnuclearspinJofthetargetnucleusaswellasitsspin- tector’sreferenceframe,with structurefunctionS(|(cid:126)q|),whichdependsonthemomentum transfer (cid:126)q. While heavy nuclei are generally more sensi- (cid:115) (cid:115) E m (m +m )2 E m 1 tivetoSI-interactions(ignoringimportantdetectordetails v = r N N χ = r N (2) min 2 (m m )2 2 µ2 suchasthresholdandbackground),thisisnottrueforthe N χ SD-case. As neutrons and protons in the target can con- being the the minimal velocity required to induce a nu- tributedifferentlytothetotalspin,oneusuallyquotesthe clearrecoilEr. Theescapevelocityvesc =544kms−1[10] SD-resultsassumingaWIMP-couplingtoprotons(an =0) isthemaximumvelocityforWIMPsboundinthepoten- andneutrons(ap =0)only. tial well of the galaxy. The canonical value for the local Whiletheshapeoftheexpectedrecoilspectrumisto- WIMPdensityusedfortheinterpretationofmeasurements tally determined by kinematics (f(v), mN, mχ), the total is ρ = 0.3GeV/c2/cm3. The observed number of events expected rate depends on the cross section (for a given 0 inanexperimentrunningforalive-timeT isobtainedby ρ0). Spin-independent rates are of <1event/kg/year have integrating Eq. (1) from the threshold energy E to the already been excluded, corresponding to WIMP-nucleon low upperboundaryEhigh: cross sections σn ≈ 10−45cm2 for mχ ∼ 50GeV/c2. In order to be sensitive to such cross sections, the optimal (cid:90) Ehigh dR WIMP detector should therefore have a large total mass N =T dE (cid:15)(E ) , (3) Elow r r dEr aMn,ualtrhai-glhowmbaascskngurmoubnedraAn,dathloewabeinlietyrgtyotdhirsetisnhgouldishElboew-, with the detector efficiency (cid:15). As dR/dE is a steeply tweensignalandbackgroundevents. r falling exponential function with E = O(10)keVr only, r Ehigh ismuchlessrelevantthantheenergythresholdElow. 2.2 Backgrounds ThenuclearrecoilenergyisgiveninkeVr(nuclearrecoil equivalent), which is different from the electronic recoil Most current WIMP searches are dominated by γ-back- scale (keVee) due to quenchingeffects caused by the dif- grounds from the environment or the experimental setup itselfandbyβ-particlesatthesurfacesorinthebulkofthe ferentenergy-lossmechanisms. detector. Theygenerateelectronicrecoils(ER)byelectro- BecauseofitslargedeBrogliewavelength,theWIMP magnetic interactions withthe atomic electrons. The dif- interactscoherentlywithallnucleonsinthetargetnucleus. ferent ionization density of ERs compared to the WIMP- The WIMP-nucleus scattering cross section in Eq. (1) is inducednuclearrecoils(NR)isoftenusedtodiscriminate velocityandrecoil-energydependentandgivenby background (ER) from signal (NR). α-contamination in dσ m (cid:16) (cid:17) the detector materials is usually uncritical and only be- = N σ F2 (E )+σ F2 (E ) . (4) dE 2v2µ2 SI SI r SD SD r comesrelevantifthemajorpartoftheα-energyislostin r insensitivedetectorregions. (Anotableexceptionarethe The loss of coherence is accounted for by the finite form bubblechambersdescribedinSection3.2.) factors Fi, which are only relevant for heavy WIMP tar- The most dangerous background for direct detection getssuchasXeorI.SincetheinteractionofWIMPswith experiment are single-scatter neutron-induced nuclear re- baryonicmatterisaprioriunknown,thecrosssectioncon- coilsfrom(α,n)andspontaneousfissionreactions, orin- sists of two terms, for spin-independent (SI, scalar) and duced by muons, as such events cannot be distinguished spin-dependent(SD,axial-vector)couplings. Thefirstone fromaWIMPsignal. Theeventmultiplicityiscrucialas reads WIMPswillonlyscatteronceinthedetectorduetotheir tiny cross section, while neutrons have a shorter mean- µ2(f Z+ f (A−Z))2 µ2 σ =σ p n =σ A2, (5) free path and will often generate multiple-scatter signa- SI nµ2n fn2 nµ2n tures. Effective WIMP detectors can therefore identify (and reject) multi-scatter events. If the interaction ver- wherethe f describetheWIMPcouplingstoprotonsand p,n neutrons,andthesecondequalityassumes f = f ,leading texcanbeadditionallymeasuredwithsomeprecision,the p n backgroundcanbefurtherreducedbyexploitingtheself- to a A2 dependence of the cross section. µ is the WIMP- shielding capability of the target material, as background nucleus reduced mass, see Eq. (2), and µ the one of the n eventspredominantlyoccurclosetothedetectorsurfaces. WIMP-nucleon system. It is used to relate the WIMP- This fiducialization is especially effective for high-Z ma- nucleus cross section σ to the WIMP-nucleon cross section σ (which allows for comparisons between different terials. n target nuclei). The differential spin-dependent cross sec- Massive shields around the detector, which itself is made from selected low-background materials, are used tionreads to suppress most backgrounds: high-Z materials such as dσSD = 8G2F (cid:104)a (cid:104)S (cid:105)+a (cid:104)S (cid:105)(cid:105)2 J+1S(|(cid:126)q|). (6) lead and copper or large amounts of water are efficient d|(cid:126)q|2 πv2 p p n n J S(0) against external γ-rays, polyethylene and water against neutrons. Inordertoreducemuon-inducedneutrons,dark The(cid:104)S (cid:105)aretheexpectationvaluesofthetotalspinoper- matterdetectorsareinstalledindeep-undergroundlabora- p,n atorsinthethenucleus.Thespin-dependentcaseshowsno tories: theirtypicalrockoverburdenof1-2kmsuppresses A2 enhancement of the cross section, but is related to the themuonfluxby5-7ordersofmagnitude. Figure1. Resultsonspin-independent(scalar)WIMP-nucleoninteractionsasderivedfromdirectdetectionexperiments. Theresults fromDAMA/LIBRA[16,21]andCDMS-Si[19](closedcontours,seetext),whichcouldbeinterpretedasbeinginducedbyinteractions oflight-massWIMPdarkmatter,arechallengedbytheexclusionlimits(at90%CL,lines)frommanyotherexperiments.Theparameter spaceiscurrentlydominatedbythedual-phaseliquidxenontimeprojectionchambers(TPCs)XENON100[11]andLUX[12]. Some resultsdiscussedinthetextarenotplottedtoincreasethereadabilityoftheplot. 3 Status direct WIMP Searches Low-massregion,scintillatorsandcryogenicdetectors TheDAMA/LIBRAexperimentrunningattheGranSasso In this Section we present the current status of direct National Laboratory (LNGS) in Italy searches for a dark searches for WIMP dark matter. The discussion is sep- matter-inducedannualmodulation[20]ofthebackground arated into spin-independent (scalar) and spin-dependent rate in a massive high-purity NaI(Tl) scintillator target. (axial-vector) interactions. No convincing sign of WIMP Thecollaborationobservedsuchasignalovernow14an- darkmatterhasbeenobservedsofar. nual cycles at 9.3σ significance, exploiting a cumula- tive exposure of 1.33t×y [16]. The measured phase is is agreement with the expectation of a standard galac- 3.1 Spin-independentInteractions tic WIMP halo. If this observation is interpreted as being due to WIMP scatters [21], it leads to two preferred regions in the spin-independent parameter space, around Thecurrentsituationconcerningspin-independentWIMP- 10GeV/c2 and 70GeV/c2 for interactions with Na or I, nucleon interactions is summarized in Figure 1. The respectively. EventhoughtheNaI-crystalsareofunprece- most sensitive exclusion limits from 6GeV/c2 up to dentedradio-purity,theDAMA/LIBRAbackgroundissig- >10TeV/c2 stillcomefromtheliquidxenonexperiments nificantlyhighercomparedtootherexperiments,andthere XENON100[11]andLUX[12]. AtlowerWIMPmasses, isnodiscriminationbetweenERandNRsignals. the best limits are new results from SuperCDMS (3.5– 6GeV/c2) [13] and CRESST-II (below 3.5GeV/c2) [14]. The preferred region derived from data taken with A few more competitive results from new players in the the silicon detectors of the CDMS-II experiment [19], field have been added as well and will be discussed be- acquired 2007/8 at the Soudan mine (USA), also cov- low,however,themoststrikingdifferencecomparedtothe ers low WIMP masses, however, at considerably lower statusof2013[15]isthatthenumberof“anomalies”has crosssectionscomparedtoDAMA/LIBRA.Threeevents beenreduced. were observed in this measurement, with an expectation Out of the previously reported four anomalies from of∼0.5backgroundeventsforthe140kg×dexposure. A DAMA/LIBRA [16], CoGeNT [17], CRESST-II [18] profile likelihood analysis yields only a small probabil- and CDMS-Si (silicon detectors of the CDMS-II exper- ity of 0.2% for the background-only hypothesis and ob- iment) [19], which could all be interpreted as possible tains the highest likelihood for a 8.6 GeV/c2 WIMP at hints for the detection of WIMPs with masses around σ = 1.9×10−41cm2. BothhintsforaWIMPsignalare n 10GeV/c2,onlyDAMA/LIBRAandCDMS-Siremain. inconflictwithvariousotherresults. Oneoftheseisfromthesamecollaboration,usingthe scintillation light and the heat deposited in the crystal by newSuperCDMSdetector. Thisinstrument,basedonger- particle interactions. Comparing the size of the two sig- maniumcrystalsoperatedatcryogenictemperatures, fea- nals is used to discriminate between ER background and tures excellent background rejection capabilities by com- NRsignal,whilethecompositetargetincludinglight(O), paringionizationandphonon(heat)signal,whichareboth intermediate (Ca) and heavy (W) nuclei provides a good simultaneously measured for each event [22]. 11events sensitivity from lowest to highest WIMP masses. After wereobservedina577kg×dexposure,inagreementwith the results from 2012 [18], where a significant excess of thebackgroundpredictionaftertakingintoaccountsome eventsabovealargebackgroundexpectation(withacon- peculiarities. This leads to an 90% CL upper limit of siderable contribution from degraded α-events) was ob- 1.2×10−42cm2 at 8GeV/c2, almost one order of magni- served,thecollaborationhasrecentlyimprovedthecrystal tudebelowtheCDMS-Sibest-fitpoint[13]. purityandtheexperimentaldesign.Anewresult,basedon During the last years, the result from the CoGeNT p- asingleupgradeddetectormoduleandasmallexposureof typegermaniumdetectorinstalledatSoudanhasreceived 29kg×d,doesnotconfirmthepreviouslyreportedexcess. lots of attention, as the observed excess of events could ItalsoexcludesparameterspaceatlowWIMPmassesbe- be interpreted as being induced by low-mass WIMPs as low3GeV/c2,whichwaspreviouslynotcoveredbydirect well. Insuchdetectors,thepulserise-timeallowsthedis- detectionWIMPsearches. tinction of surface from bulk events to efficiently reduce the background level, however, ER vs. NR discrimina- Liquidnoblegasdetectors tion is not possible. Three new analyses of the data have The most sensitive dark matter results so far have been beenpresentedin2014,andallleadtotheconclusionthat published by the XENON100 and LUX collaborations. the excess is most likely caused by background surface- Bothgroupsoperatedual-phasetimeprojectionchambers events. The first one was published by the CoGeNT col- (TPCs)[29]filledwiththethenoblegasxenon, whichis laboration[23]: thedarkmattersignalextractedfromthe cooleddowntoabout−90◦Cat2barpressuresuchthatit datainamaximumlikelihoodanalysisremainsbelowthe liquefies(ρ ≈ 3g/cm3). Theprincipleofsuchaposition- 3σlevelwhensystematicuncertaintieshowtomodelthe sensitivedetectorisdetailedinFigure2(right). pulserise-timeareincluded. AnotheranalysisfromDavis Using liquid xenon as WIMP target combines sev- etal. [24]concludes thatthesignificance ofthe excessis eral advantages [30]: the high atomic number A ≈ 131 evenbelow1σ,ifthea-prioriunknownshapesoftherise- for an excellent WIMP sensitivity; the high Z = 54 for time distributions for bulk and surface are systematically dense, compactdetectorsprovidingself-shieldingagainst varied. Finally, Kelso et al. [25] presented a maximum externalbackgrounds;theabsenceofanylong-livedxenon likelihood analysis in which the best fit to the public Co- radio-isotopes (besides 136Xe which undergoes double- GeNTdatawasachievedbyamodelwithoutaWIMPsig- beta decay with T = 2.1×1021y); the high light out- nal. The fit also gets worse if a dark matter stream is in- 1/2 put at 178nm (comparable to NaI(Tl)); and the fact that cludedintheanalysisinsteadofthestandardhalomodel. the target mass can be increased more easily compared The WIMP interpretation of the CoGeNT excess has to detectors using crystals. Threshold energies of 3keVr also been questioned by two other experiments, CDEX have already been achieved [12] and the combination of and MALBEK, which essentially use the same technol- externalshields,radio-puredetectormaterials,fiducializa- ogy based on p-type, low-threshold, high purity germa- tionandmultiple-scatterrejectionsleadstodemonstrated nium(HPGe)detectors. TheCDEX-1detectorattheJin- backgroundratesof≤0.005events/keV/kg/day,dominated pingLaboratory(China)isembeddedinaNaI(Tl)crystal by decays of the target intrinsic isotopes 85Kr and 222Rn. operated in anti-Compton mode. The background spec- ThedifferentenergylossdE/dxforelectronicandnuclear trum above 475eV observed in 54kg×d agrees with the recoils leads to different charge-to-light ratios in dual- backgroundmodelandchallengestheCoGeNTexcessas phase TPCs, which is used to reject ∼99.5% of ERs at beingduetodarkmatter[26]. UsingtheirCDEX-0appa- ∼50%NRacceptance[31]. ratus,thecollaborationpublishedalimitfromarunfeatur- XENON100isadual-phaseTPCusing62kgofliquid ing an even lower threshold of Elow = 144eV [27]. This xenonasWIMPtarget[32].Itstotalxenonmassis161kg, result is not yet competitive, however, is could explore withthexenonoutsideoftheTPCbeinginstrumentedas the region from 1.5–10GeV/c2 at cross-sections around active veto to reject background events. The instrument 10−41...42 in the future. MALBEK is an R&D project for is running at LNGS (Italy) since 2009 and has reached the MAJORANA experiment, installed at the Kimballton its design goal by excluding spin-independent WIMP- UndergroundResearchFacility(USA).Thisbroad-energy nucleoncrosssectionsof2×10−45cm2 atWIMPmasses low-backgroundHPGedetectorisverysimilartoCoGeNT of50GeV/c2 withanexposureof34kg×225d[11]. The andhasalsobeenusedtosetlimitsonlow-massWIMPs same low-background dataset has also been used to de- intheparameterspacepreferredbythepreviousCoGeNT rive constraints on spin-dependent interactions (see Sec- analysis[28]. tion3.2)andaxions(seeSection4). The last anomaly reported in the previous years was This result has been recently confirmed and super- fromtheCRESST-IIexperiment,measuringwithCaWO sededbythelargerLUXdetectorlocatedatSURF(USA), 4 crystals at mK-temperatures at LNGS. These operating which features 250kg of liquid xenon inside the TPC conditionsallowforthesimultaneousmeasurementofthe (370kg total). No excess above the background expectation has been observed in a 118kg×85d run, leading achievablebyusingthecharge-to-lightratio.DarkSideex- to the strongest spin-independent limits which have been ploitsthedifferentscintillationpulse-shapefromERsand publishedsofarforWIMPmassesof>6GeV/c2 [12]. NRs[35]toefficientlyrejectERs. Ina1422kg×dexpo- A new dual-phase TPC has published its first science sure of atmospheric argon, less than 0.1event from 39Ar results in 2014: PandaX-I is installed in the Chinese Jin- isexpectedtoleakintotheWIMPsearchregionfrom38- pingLaboratoryandhasaliquidxenontargetof120kg.In 206keVr. Thisratherhighthresholdisrequiredasasize- contrasttoXENONandLUX,whichhaveanaspectratio able amount of photoelectrons (here: 80) needs to be de- close to unity, the PandaX TPC is pancake-shaped in or- tected in order to be able to trace the pulse shape. This dertooptimizethedetectorforahighlightyield,whichin leads to a considerably reduced WIMP sensitivity below turnleadstoalowthreshold. Consequently,thenewlim- ∼50GeV/c2. No excess of events were observed, the ex- itsfromameasurementof37kg×17dgobeyondtheones clusionlimitisshowninFigure1aswell. ofXENON100andLUXatverylowWIMPmasses[33]. The drawback is a larger background due to the reduced Future: Upcomingresultsandlargedetectors fiducializationpower. An alternative way to build WIMP detectors using lique- First results have also been reported from the fied noble gases is illustrated in Figure 2 (left): single- DarkSide-50detector,adual-phaseTPCfilledwith46kg phase detectors only detect the prompt scintillation light. ofliquidargon[34]. Itisinstalledinsidea30tliquidscin- Compared to TPCs, this has some advantages: the target tillatorveto,whichitselfislocatedinsidealargeC˘erenkov can be surrounded by light sensors in 4π; the light yield muonveto. Comparedtotheveryexpensivexenon,argon increases due to the absence of quenching effects in the ismuchcheaperduetoitslargeratmosphericabundance. electric field; and the operation of a detector without his The major challenge is the high contamination of ∼1Bq bias-voltagesisfacilitated. Thedrawbackisthelowerpo- of 39Ar per kg of natAr, which requires ER discrimina- sitionresolutionand–incaseofaxenontarget–thatthere tionefficiencieswhicharemuchbetterthanthe10−3levels arevirtuallynomeanstodiscriminateERsfromNRs. XMASS, located in the Kamioka mine (Japan), is an operational single-phase detector, where 835kg of liquid xenon are viewed by 642 photomultipliers [36]. First results on WIMPs have already been published [37], but were not competitive due to an increased background. This has been improved in the meantime and new results are expected soon. The large single phase project DEAP-3600, with a 3.6t liquid argon target, is currently being commissioned at SNOLAB (Canada). First results areexpectedfor2015,withanultimatesensitivityaround 1×10−46cm2[38]. In July 2014, the US funding agencies DOE and NSF announced a long-awaited decision regarding their joint program for second-generation dark matter detectors [39]. The agencies will support three projects: LZ (LUX-Zeplin) [40], a dual-phase TPC with a 7t liquid xenontarget,isthesuccessoroftheLUXexperimentand Figure2. Conceptsusedfordarkmatterdetectorsbasedonliq- will be installed in the SURF laboratory (USA) as well. uidnoblegases. (Left)Singlephasedetectorsfilledwithliquid WhileLZwillmainlysearchforWIMPdarkmatterabove xenon,argonorneonrecordthescintillationlight(S1)bymeans 10GeV/c2,withanoptimalsensitivitytospin-independent ofphotodetectors(usuallyphotomultipliers)surroundingthetar- cross-sectionsofafew10−48cm2,thesecondexperiment, getin4π.Thehitpatternonthesensorsallowsthereconstruction SuperCDMS [41], will focus on the low mass region. It oftheeventvertex,whichisemployedforfiducialization.Inde- willinitiallyoperate∼50kgofhighpuritygermaniumand tectorsfilledwithliquidargon,thelightpulseshapecanbeused to distinguish ER background from NR signals. (Right) Liq- silicon crystals at SNOLAB (Canada), and will be de- uidxenonandargoncanbeeasilyionizedwhatisexploitedin signed such that an upgrade to more mass is possible at dual-phasetimeprojectionchambers.Particleinteractionscreate a later stage. Both experiments should be in the com- promptscintillationlight(S1),detectedbytwoarraysofphoto- missioning phase by 2018, and will take several years sensorsaboveandbelowthetarget,andfreeionizationelectrons. of data in order to reach their ultimate sensitivity. The Thesearemovedtowardsthegasphaseabovetheliquidtarget third project in the US program is ADMX-Gen2 [42]. It byanelectricfield.Asecondfieldextractstheelectronsintothe doesnotsearchforWIMPdarkmatterbutoperatesami- gas phase, where they generate a secondary scintillation signal crowave cavity in order to look for axions, an alternative (S2),proportionaltothechargesignal. Thehitpatternonthetop darkmatterparticlewhicharisesinapossiblesolutionto array(xy)combinedwiththetimedifferencebetweenS1andS2 thestrong-CPproblem[43],seealsoSection4. signal (z) is used to precisely determine the event position and multiplicity. TheratioS2/S1isusedasadiscriminantbetween There are more next generation dark matter projects ERandNRevents. which are currently in the initial design phase, such as DarkSide-G2 [44], a dual-phase TPC filled with 3.6t of liquidargon, ortheupgradesofthePandaXliquidxenon experiment[45]:itfirstaimsata500kgdetector,followed byanevenlargerstage. The next phase of the XENON program is currently being installed underground at LNGS (Italy): the XENON1T detector [46] is a dual-phase TPC with a tar- getmassof2.0tofliquidxenon(dimensions:∼1mheight anddiameter, totalmass: 3.3t), instrumentedby248low background photomultipliers [47]. The background goal is<1eventfora1t×2yexposureandwillbeachievedby careful selection of low-background materials, shielding by a 9.6m diameter water shield operated as muon veto as well as by liquid xenon, and by using the charge-to- light ratio for discrimination. Detector commissioning is planned for the second half of 2015, the sensitivity goal of2×10−47cm2 form ∼ 50GeV/c2 canbeachievedaf- χ ter 2years of operation. All major detector components of XENON1T are designed such that an upgrade to a total xenon mass of ∼7t is straightforward. This phase, XENONnT,willincreasethesensitivitybyalmostanother factorof10. 3.2 Spin-dependentInteractions IftheWIMPcouplestotheunpairednuclearspinsofthe target nucleus via an axial-vector current, the cross section does not simply scale with A2 as for coherent spin- independent interactions, but depends on a factor λ2 = J/(J+1)(a (cid:104)S (cid:105)+a (cid:104)S (cid:105))2,seeEq.(6).Thisfactorisnon- p p n n zeroonlyfornucleiwithanoddnumberofprotonsorneu- trons,andismaximalfor19F(λ2 =0.86),followedby7Li (λ2 =0.11),whichbothhaveunpairedproton-spins.Some of the experiments described in Section 3.1 contain iso- Figure 3. Results on spin-dependent WIMP-nucleon scatter- ingcrosssections,presentedassumingthatWIMPswouldcou- topes which are sensitive to spin-dependent interactions, ple only to proton- or to neutron-spins. (Top) The proton- even though to a lesser extent than 19F. These are 23Na onlycaseisdominatedbyresultsfromexperimentswhichem- and 127I(unpaired protons)as presentin DAMA/LIBRA, ploy a target containing 19F (COUPP [48], SIMPLE [49], PI- 29Si and 73Ge (unpaired neutrons) in CDMS, and 129Xe CASSO[50]). ThenewresultsfromthedirectionalDRIFTde- and131Xe(unpairedneutrons)inXENON. tector[52]andfromindirectWIMPsearchesbyIceCube[53]are Theparameterspaceofspin-dependentWIMP-proton alsoshown. (Bottom)Thebestlimitonneutron-onlycouplings couplings (assuming that a =0 in Eq. (6)) is therefore isfromXENON100[54]using129Xeand131Xeastargetnuclei. n dominated by experiments using targets which contain Figuresadaptedfrom[54],seemorereferencesthere. 19F, see Figure 3 (top). The tightest constraints on the cross section come from COUPP [48], a bubble cham- ber filled with CF I, as well as SIMPLE [49] and PI- 3 CASSO [50]. These consist of superheated droplets of An interesting new result comes from the DRIFT-IId C CIF and C F , respectively, embedded in a gel. The detector. Whileallprojectsdiscussedsofarmeasureonly 2 5 4 10 dropletsworkas“mini”bubblechambers,whereincident theenergyofaninteraction,aswellastheparticletypeand radiation causes the formation of bubbles, which are de- multiplicity in some cases, DRIFT also detects the direc- tected acoustically and – in case of COUPP – also opti- tion of the recoil in a low-pressure gas TPC. This allows cally. The advantage of this technology is that the detec- thedistinctionofWIMP-inducedrecoils,whosedirection torscanbemadealmostinsensitivetoERbackgroundra- isexpectedtobecorrelatedwiththerotationoftheEarth, diationbychoosingtherightdetectorparameters(temper- frombackgrounds.Thedetectorwasfilledwithagasmix- atureandpressure),whilethecharacteristicsofthesound tureofCS2:CF4:O2 atapressureratio30:10:1, searching signal can be used to partially discriminate between NRs forspin-dependentWIMPinteractionswith19F.Noevent and α-particles [51]. In order to keep this forefront po- was observed in a background-free run observing 33g of sition also in the future, PICASSO and COUPP recently fluorinegasover46d[52]. mergedtoformthePICOcollaboration,aimingtowardsa About 50% of the naturally abundant xenon isotopes ton-scalebubblechamber,operatedwitheitheraCF Iora are the neutron-odd 129Xe and 131Xe, which are sensi- 3 C F target. tive targets for spin-dependent neutron-only interactions 3 8 Figure 4. Limits on the axio-electric coupling constant g for (a) interactions of solar axions and (b) galactic axion-like particles Ae obtainedbydarkmatterdetectorsusinggermanium(EDELWEISS,CoGeNT,CDMS)andliquidxenontargets(XMASS,XENON100). Allconstraintscutsignificantlyintothedarkmatteraxionregime,asindicatedbytheDFSZandKSVZmodellines. Figureadapted from[59],seereferencestotheindividualresultsthere. (assuming a = 0). The most sensitive published ex- WIMP search and requires a very low background even p clusion limit is from the XENON100 dual-phase liquid without ER discrimination. Several experiments have re- xenonTPC,seeFigure3(bottom),whichdidnotobserve centlyperformedsuchananalysis,amongthemthesingle- aWIMPsignalina34kg×225dexposure[54]. phase liquid xenon detector XMASS [58]; the HPGe detectors EDELWEISS [56], CDMS-II and CoGeNT; and thedual-phaseliquidxenonTPCXENON100[59]. Their 4 Other Dark Matter Channels limitsareshowninFigure4fortwodifferentaxioncandi- dates. Thefirstsearchfocusesonsolaraxionsemittedby The discussion so far has focused on weakly interacting the Sun, where they are expected to be produced in large massiveparticles(WIMPs)ascandidatesfordarkmatter. amounts. The second one places constraints on galactic Due to their very low radioactive backgrounds, many of axion-likeparticles(ALPs),assumingthattheyconstitute the WIMP detectors mentioned so far can also be em- theentireamountoftheobserveddarkmatter.TheseALPs ployed to search for other rare events as well, even neu- donotsolvethestrongCP-problemandaremoremassive trinochannelswilleventuallybeaccessiblewithmultiton- thantheclassicalaxion. scaleliquidxenondetectors[55]. Asomewhatsimilarsearchforbosonicsuper-WIMPs Axions are very light dark matter candidates [43], has been performed by XMASS [60]. These warm dark which arise naturally in the Peccei-Quinn solution to the matter candidates with masses of a few keV/c2 could be “strong” CP-problem in QCD, manifesting itself in the absorbed by the xenon atoms, depositing their rest mass absenceofanelectricdipolemomentoftheneutron. The energy in the single-phase detector. Due to its very low mostcommonwaytosearchforaxionsisviathePrimakov effect, where an axion is converted into a photon of the ERbackgroundof∼10−4events/keV/kg/d,XMASSplaces tightlimitsonsuper-WIMPswithmassesbetween40and sameenergyinamagneticfieldperpendiculartotheaxion 120keV/c2,asnoexcessofeventswasobservedinamea- momentum. Dedicated instruments for axion-searches, surement of 41kg×166d. In particular, the possibility suchasthemicrowavecavityADMX[42],relyentirelyon this effect and are installed inside strong magnets. Low- thatalldarkmatterismadeupfromvectorsuper-WIMPs iscompletelyexcludedbythisresult. background detectors optimized for WIMP searches do notemployexternalmagneticfields,hencethiseffectcan onlybeexploitedincrystalswithaknownorientationaxis 5 Conclusions with respect to a possible axion source, e.g., the Sun, re- lyingonthefieldsbetweenthenuclei. Limitsong ,the Eventhoughthethebestconstraintsonspin-independent Aγ axio-photoncouplingconstant,havebeenderivedbysev- (Figure 1) and spin-dependent (Figure 3) WIMP-nucleon eralgermaniumexperiments(e.g.,EDELWEISS[56])and interactionswerenotimprovedin2014,ithasbeenavery NaI(Tl)-crystals(DAMA[57]). interestingyearintermsofdirectsearchesforWIMPdark Non-crystal targets, as used in the massive liquid matter,asseveralnewdetectorscameonlineorpresented xenondetectors, areinsensitivetog duetotheabsence firstscienceresults. Especiallyinterestingistheapparent Aγ of a well-oriented magnetic field. 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