In this contribution the latest results of dark matter direct detection obtained by the DarkSide Collaboration are discussed. New limits on the scattering cross-section between dark matter particles and baryonic matter have been set. The results have been reached using the DarkSide-50 detector, a double-phas

Agnes, P.

Albuquerque, I.F.M.

Kubankin, A.

Oleinik, A.

Shchagin, A.

DSpace at Belgorod State University

DOI 10.1393/ncc/i2020-20026-3Colloquia: IFAE 2019IL NUOVO CIMENTO 43 C (2020) 26Latest results of dark matter detection with the DarkSideexperimentE. Picciau(∗)INFN, Sezione di Cagliari and Dipartimento di Fisica, Università degli Studi di CagliariCagliari, Italyreceived 8 June 2020Summary. — In this contribution the latest results of dark matter direct detectionobtained by the DarkSide Collaboration are discussed. New limits on the scatteringcross-section between dark matter particles and baryonic matter have been set. Theresults have been reached using the DarkSide-50 detector, a double-phase TimeProjection Chamber (TPC) filled with 40Ar and installed at Laboratori Nazionalidel Gran Sasso (LNGS). In 2018, the DarkSide Collaboration has performed threedifferent types of analysis. The so-called high-mass analysis into the range between∼ 10 GeV and ∼ 1000GeV is discussed under the hypothesis of scattering between(∗) On behalf of P. Agnes, I. F. M. Albuquerque, A. K. Alton, M. Ave, H. O. Back,G. Batignani, K. Biery, V. Bocci, G. Bonfini, W. M. Bonivento, B. Bottino, S. Bussino,M. Cadeddu, M. Cadoni, F. Calaprice, A. Caminata, N. Canci, A. Candela, M. Caravati,M. Cariello, M. Carlini, M. Carpinelli, S. Catalanotti, V. Cataudella, P. Cavalcante, S. Cavuoti,A. Chepurnov, C. Cicalò, A. G. Cocco, G. Covone, D. D’Angelo, S. Davini, A. De Candia,S. De Cecco, M. De Deo, G. De Filippis, G. De Rosa, A. V. Derbin, A. Devoto, F. Di Eusanio,M. D’Incecco, G. Di Pietro, C. Dionisi, M. Downing, D. D’Urso, E. Edkins, A. Empl, G. Fiorillo,K. Fomenko, D. Franco, F. Gabriele, C. Galbiati, C. Ghiano, S. Giagu, C. Giganti, G. K.Giovanetti, O. Gorchakov, A. M. Goretti, F. Granato, A. Grobov, M. Gromov, M. Guan,Y. Guardincerri, M. Gulino, B. R. Hackett, K. Herner, B. Hosseini, D. Hughes, P. Humble,E. V. Hungerford, Al. Ianni, An. Ianni, V. Ippolito, T. N. Johnson, K. Keeter, C. L. Kendziora,I. Kochanek, G. Koh, D. Korablev, G. Korga, A. Kubankin, M. Kuss, M. La Commara, M. Lai,X. Li, M. Lissia, G. Longo, A. A. Machado, I. N. Machulin, A. Mandarano, L. Mapelli, S. M.Mari, J. Maricic, C. J. Martoff, A. Messina, P. D. Meyers, R. Milincic, A. Monte, M. Morrocchi,V. N. Muratova, P. Musico, A. Navrer Agasson, A. O. Nozdrina, A. Oleinik, M. Orsini, F. Ortica,L. Pagani, M. Pallavicini, L. Pandola, E. Pantic, E. Paoloni, K. Pelczar, N. Pelliccia, E. Picciau,A. Pocar, S. Pordes, S. S. Poudel, H. Qian, F. Ragusa, M. Razeti, A. Razeto, A. L. Renshaw,M. Rescigno, Q. Riffard, A. Romani, B. Rossi, N. Rossi, D. Sablone, O. Samoylov, W. Sands,C. Savarese, B. Schlitzer, E. Segreto, D. A. Semenov, A. Shchagin, A. Sheshukov, P. N. Singh,M. D. Skorokhvatov, O. Smirnov, A. Sotnikov, C. Stanford, S. Stracka, Y. Suvorov, R. Tartaglia,G. Testera, A. Tonazzo, P. Trinchese, E. V. Unzhakov, M. Verducci, A. Vishneva, R. B. Vogelaar,M. Wada, T. J. Waldrop, H. Wang, Y. Wang, A. W. Watson, S. Westerdale, M. M. Wojcik,X. Xiang, X. Xiao, C. Yang, Z. Ye, C. Zhu and G. Zuzel.Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 E. PICCIAUdark matter and Ar nuclei. The low-mass analysis, performed using the same hy-pothesis, extends the limit down to ∼1.8 GeV. Through a different hypothesis, thatpredicts dark matter scattering off the electrons inside of the Ar atom, it has beenpossible to set limits for sub-GeV dark matter masses.1. – IntroductionA good deal of past and recent astrophysical and cosmological evidence has indicatedthat the visible matter that makes up stars, galaxies and other objects that we have so fardetected is only a small fraction of the mass which makes up the Universe. The unknownmatter is named as dark matter, which is unseen by direct methods but whose presenceis detected by studying the evolution of the galaxies themselves as revealed, for example,by measuring their rotation as a function of distance from the center. Looking for directevidence of dark matter has been underway for many years, but so far no signal has beenobserved. Most of these studies have involved a search for Weakly Interacting MassiveParticles (WIMPs), whose presence would be indicated by the detection of a recoilingnucleus in a large volume detector. Among direct detection tecniques, dual-phase TPCsfilled up with liquified noble gases such as argon or xenon are the most promising formasses larger than few GeV. They allow to obtain large, homogeneous, low-background,and self-shielding detectors. In particular, the usage of Liquid Argon (LAr) guaranteesan excellent discrimination power (>108) in the scintillation pulse shape between nuclearand electron recoils. However, a disadvantage of natural argon is that it contains anamount of cosmogenic 39Ar, whose β− decays contribute in contamination inside of thedetector. This problem has been reduced using Underground Argon (UAr), in whichthe amount of 39Ar is 3 order of magnitude less. One of the current LAr detectors isDarkSide-50, located at Laboratori Nazionali del Gran Sasso (LNGS) and it consists of adouble-phase LAr TPC with an active mass of (46.4 ± 0.7) kg of low-radioactivity argondeployed in a Liquid-Scintillator Veto (LSV) and a Water Cherenkov Veto (WCV). Inthis paper, the latest results of DarkSide-50 obtained using data accumulated in 532.4live-days, from August 2015 to October 2017, are shown and discussed.2. – The DarkSide-50 detectorThe DarkSide-50 apparatus [1] is composed of three nested detectors (see fig. 1). Theoutermost is the WCD, a cylinder tank used to shield and to perform anti-coincidence toreject events caused by residual fluxes of cosmic muons. The WCD is an 11 m-diameter,10 m high cylindrical tank filled with high-purity water. More internally there is the LSV,that is a 4 m diameter stainless-steel sphere filled up with 30 tons of liquid scintillator [2].LSV serves as a shield and is useful to discriminate radiogenic and cosmogenic neutrons,γ-rays and cosmic muons. At the center of the LSV there is the DarkSide-50 TPC, whichis the effective dark matter detector. Inside of that, LAr is maintained at a temperatureof T ∼ 85K using an external circulation loop connected with the cryostat, while the gaspasses out of the detector system to the cryogenic purification system, located on thetop of the water tank, in the radon-suppressed clean room, which contains all equipmentinterfacing directly with the detectors. The detector system is located in Hall C of LNGSLATEST RESULTS OF DARK MATTER DETECTION ETC. 3Fig. 1. – DarkSide-50 detector apparatus. On the left, the water tank WCD, the LSV sphereand the liquid argon TPC are shown. On the right a more detailed view of the TPC is presented.at a depth of 3800 m.w.e. (meter water equivalent). The working principle of DarkSide-50 is based on the signals produced when the argon recoils. In the LAr phase, the recoilproduces a scintillation signal named S1. In addition to this, ionisation electrons areproduced and carried towards the upper part of the TPC thanks to an electric field of200 V/cm. Part of this energy is released in form of phonons and is not detected. Onceelectrons reach the gas phase, a second signal, called electroluminesce signal or S2, isproduced. Two arrays of 19 PMTs at the top and the bottom of the TPC collect thelight. The maximum drift time, corresponding to the height of the TPC (35.6 cm), is375 μs. From the time delay between S1 and S2 it is possible to obtain the z-position ofthe particle interaction. The spatial resolution is estimated to be 0.6 cm. Both signals, S1and S2, are measured by the same PMT array. This allows to exploit a three-dimensionalposition of the energy deposition. This helps to reject surface backgrounds and multi-sited events. A LAr detector has the possibility to exploit the technique of Pulse ShapeDiscrimination (PSD) that allows to indentify whether a signal is due to an ElectronRecoil (ER) process, such as β’s or γ’s, or to a Nuclear Recoil (NR), typical of α’sor neutrons. Since WIMPs are supposed to scatter producing a NR, the PSD is veryhelpful in decreasing the level of background in the detector. In fact, S1 is produced bya de-exitation of Ar from either singlet or triplet state, which have different decay times.The first one decays in ∼6 ns while the second one in ∼1500 ns, the ratio to singlet andtriplet state differs for NR and ER and this represents, definitely, the cornerstone of thePSD. This potentiality is used through a parameter called f90, defined as the fractionof S1 accumulated within the first 90 ns. Regarding the calibration of the system [3], ithas been done using neutron and gamma sources and 83mKr injected from outside andexploiting the presence of the 37Ar isotope into the total amount of LAr. To obtainthe best calibration of the response of S1 and S2 for nuclear recoils needed for theDarkSide program, members of the collaboration and others performed an experiment,called SCENE, that measured the intrinsic scintillation and ionization yield of recoilingnuclei in liquid argon as a function of applied electric field by exposing a small LAr TPC4 E. PICCIAUFig. 2. – On the left, the WIMP search box in the S1 vs. f90 region after all the cuts is shown.On the right, the black curve represents the DarkSide-50 90% CL limit on the spin-indipendentcross-section between WIMP and nucleons.to a low-energy pulsed narrowband neutron beam [4]. The S1 PhotoElectron (PE) yieldat the TPC center was measured at 7.0 ± 0.3 PE/keV with 200 V/cm drift field and atthe 83mKr peak energy of 41.5 keV. The amplification factor for the ionization signal isequal to 23 ± 1 PE per electron extracted in the gas phase with a resolution of about20%.3. – High-mass WIMP analysisThe analysis of DarkSide-50 in the high mass range was performed in a blind-mode onthe 532.4 live-day data set [5]. This means that candidate selection/background rejectionwas designed, and the background surviving cuts was estimated, without knowledge ofthe number or properties of events in the final search region. The data are reportedin the f90 vs. S1 region (fig. 2). During the analysis, the first operation consistedin opening sections of the blinded data outside of the WIMP search region to providesamples enriched in particular backgrounds for study, and later, when the backgroundpredictions would be mature, to test the predictions. The expected background wasclassified into three categories: surface events, neutrons (cosmogenic and radiogenic),and ERs. The number of surviving events expected using the entire statistics and afterall the background rejection is 0.09 ± 0.04, while the total acceptance after the cutshas become 72.5 ± 0.1(stat) ±0.50.4 (syst), whose impact is accounted for in consideringthe fiducial mass as (31.3 ± 0.5) kg. After data unblinding, no events were observedin the defined WIMP search region, as shown in fig. 2. The lack of observed eventsis consistent with up to the 2.3 WIMP-nucleon scattering expected at 90% CL, whichsets an upper limit on the spin-independent scattering cross-section corresponding to1.14 × 1044 cm2 (3.79 × 1044 cm2, 1.10 × 1044 cm2) for a WIMP mass of 100GeV/c2(1TeV/c2, 126GeV/c2), represented by the black curve shown in fig. 2. The limit iscalculated assuming the standard halo model and using vesc = 544 km/s, v0 = 220 km/s,vEarth = 232 km/s, and ρDM = 0.3GeV/c2 cm3.4. – Low-mass WIMP analysisThe very important achievement reached by DarkSide consists in the background-freecondition in the high-mass regime. However, relaxing the background-free requirement,it is possible to exploit the DarkSide-50 detector to perform analysis for lower energies,LATEST RESULTS OF DARK MATTER DETECTION ETC. 5Fig. 3. – On the left, the number of events after cuts as a function of the number of ionisationelectrons. On the top of the plot, the energy scale is also shown. On the right, the red curvesrepresent the DarkSide-50 90% CL limit on the spin-indipendent cross-section between low-massWIMP and nucleons under two hypotheses of quenching behaviour.associated to the search for lower masses of WIMPs [6]. In order to decrease the thresh-old only the S2 signal was used. On the other hand, not considering S1 means thatPSD is therefore not available. The efficiency of the software pulse finding algorithm isessentially 100% for S2 signals larger than 30 PE. The energy threshold for this analysiswas set at 4e−, which corresponds to ∼92 PE. Since S1 is not exploitable, neither thez-fiducialization is available. Moreover, at low energies the low statistics does not allowto use the xy algorithm for the radial fiducialization, thus the choice was to define afiducial region only accepting events where the largest S2 signal is recorded in one of theseven central top-array PMTs. This means that the detector acceptance for this analysisis 0.42±0.01, reducing the fiducial mass. The ionisation yield has been determined usingin situ calibration data from 241Am13C and 241AmBe neutron sources, combined withthe data from SCENE and ARIS [7]. The events collected during the live-time of the de-tector and after the cuts are shown in fig. 3. Precisely, three different ranges are defined.The first one is above 7 e−, where the observed rate of events (∼1.5/ keVee day kg) isvery well reproduced by the Monte Carlo simulation used. This consistence validates thesimulation in the range above 0.6 keVee. The second region to be considered is between4 and 7 e−, where there is small excess of events with respect to the Monte Carlo predic-tions, which is not understood and left for further studies. Finally, in the region below4 e− a huge excess of events is observed. However, these are observed to be correlatedin space and time with preceding events with large ionization, thus they are believedto be electrons trapped and subsequently released by impurities. Setting the thresholdat 4 e−, the low-energy excess is not taken into account, while the intermediate smallexcess limits the sensitivity in the WIMP mass region between 1.8GeV/c2 and 3GeV/c2.The uncertainty on the expected WIMP signal near the threshold is dominated by theaverage ionization yield, as extracted from calibrations, and its intrinsic fluctuations,modelled by applying binomial statistics to the ionization yield and the recombinationprocesses. Upper limits on the WIMP-nucleon scattering cross-section are extracted fromthe observed e spectrum using a binned profile likelihood method. Nowadays, this is thestrongest limit obtained by a dark matter detector in the low-mass region. Improvedionization yield measurement and assessment of a realistic ionization fluctuation model,which are left for future work, may be used to determine the actual sensitivity of thepresent experiment within the range indicated.6 E. PICCIAU5. – Sub-GeV dark-matter–electron analysisNowadays, since the phase space available for the different WIMP models has beensignificantly constrained by the very precise limits set by the different experiments, physi-cists are led to exploring new possibilities for dark matter. One of those is in the dark-sector framework, predicting sub-GeV particles with smaller coupling with respect tothe weak scale. For these models, a new mediator boson is assumed and dependingon its unknown mass, the predictions of the expected rate of events are differents. Inthis analysis two opposite limits are discussed, very heavy and very light mediator [8].Sub-GeV particles cannot be detected by the effects of the elastic nuclear recoils, sincetheir kinetic energy is too small. However, if dark matter scatters directly on the boundelectrons of the target, the kinematics is more favorable and is sufficent to ionize thetarget atom and to produce a visible signal in detectors. The interaction induces an ERsignal, whose response is therefore not subjected to the quenching uncertainty of the ion-ization signal induced by WIMP-nucleus interactions. Searching for these DM-electroninteractions requires the same data selection described for the low-mass WIMP analysisbut with a threshold set to 3 e−, equivalent to 0.05 keVee. A very important goal regard-ing the energy calibration is achieved exploiting the presence of the 37Ar isotope. Indeed,this disintegrates by 100% electron capture transition to the nuclear ground state of the37Cl nuclide. The recommended value for the 37Ar half-life is 35.01 ± 0.02 days. Thelines coming from the L-shell have an energy of 0.27 keV, while those from the K-shellhave an energy of about 2.81 keV and most of those (>90%) are produced by Auger elec-trons. DarkSide-50 observed that the sample of 37Ar was almost completely decayed after100 days. The resulting ionization spectra, shown in fig. 4, are then smeared consideringFig. 4. – On the left, the number of events obtained with the S2-only analysis compared withthe spectra for the interaction DM-electron. On the right, the DarkSide-50 90% CL limit in twodifferent regimes: heavy mediator, on top, and light mediator, on bottom.LATEST RESULTS OF DARK MATTER DETECTION ETC. 7the ionization yield and recombination processes and convolving them with the detectorresponse, measured from single-electron events. The resulting 90% CL limits are shownin fig. 4 for two different mediator assumptions. In the case of a light mediator, theconstraints from DarkSide-50 are not as stringent as those from XENON10, while inthe heavy-mediator regime this constraints improve the existing limits from XENON10and XENON100, for dark matter masses between 30MeV/c2 and 100MeV/c2, seeing afactor-of-3 improvement at 50MeV/c2.6. – ConclusionIn this contribution the latest results obtained from the analysis of 532.4 days ofdata collected with the DarkSide-50 detector have been discussed. In the high-masssearch, DarkSide-50 has demonstrated to be able to achieve a zero-background regimewith margins for further suppression. The results obtained are extremely promising andset the basis for future detectors, called DarkSide-20 k [9] and ARGO, which will providea multi-tonne argon target mass capable of collecting an exposure of hundreds tonneyear. Finally, exclusion limits at low-mass WIMPs and sub-GeV particles have beenpresented.REFERENCES[1] DarkSide Collaboration (Agnes P. et al.), Phys. Lett. B, 743 (2015) 456.[2] DarkSide Collaboration (Agnes P. et al.), JINST, 11 (2016) 03016.[3] DarkSide Collaboration (Agnes P. et al.), JINST, 12 (2017) T12004.[4] SCENE Collaboration (Cao H. et al.), Phys. Rev. D, 91 (2015) 092007.[5] DarkSide Collaboration (Agnes P. et al.), Phys. Rev. D, 98 (2018) 102006.[6] DarkSide Collaboration (Agnes P. et al.), Phys. Rev. Lett., 121 (2018) 081307.[7] ARIS Collaboration (Agnes P. et al.), arXiv:1801.06653 (2018).[8] DarkSide Collaboration (Agnes P. et al.), Phys. Rev. Lett., 121 (2018) 111303.[9] DarkSide Collaboration (Agnes P. et al.), Eur. Phys. J. Plus, 133 (2018) 131.

Latest results of dark matter detection with the DarkSide experiment

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Latest results of dark matter detection with the DarkSide experiment

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