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J B r o d s k y c, F B u d a n o op, S B u s s in o op, M C a d e d d u q,r, L C a d o n a ti6, M C a d o n iq,r, F C a la p r ic e c, N C a n c is’7, A C a n d e la 7, H C a o c,
M C a r ie llo m, M C a r lin i7, S C a ta la n o tti* -", P C a v a lc a n te v’7, A C h e p u r n o v w, A G C o c c o " , G C o v o n e * -", D D ’A n g e lo x n , M D ’I n c e c c o 7, S D e C e c c o 6, M D e D e o 7, M D e V in c e n z iop,
A D e r b in y, A D e v o t o q,r, F D i E u s a n io c, G D i P ie t r o 7”-, E E d k in s z , A E m p ls , A F a n h, G F io r illo * -", K Fom enko"", G F o s te r 6,7, D F ra n co" , F G abriele-7', C G a lb ia ti6,7, C G ig a n ti6, A M G o r e t ti7, F G ra n a to*’66, L G r a n d icc, M G r o m o v w, M G u a n dd, Y G u a r d in c e r r i7 , B R H a c k e ttz , K R H e r n e r 7 , E. V . H u n g e r fo r d s , A ld o I a n n i66’7, A n d r e a Ia n n ic, I J a m e s op, C J o lle t 77, K K e e t e r gg, C L K e n d z io r a 7 , V K o b y c h e v hh, G K o h c, D K o r a b le v ““, G K o r g a s j , A K u b a n k in M, X L ic, M L issia r, P L o m b a r d in, S L u itz 77, Y M a dd, I N M a c h u lin fcfc’H, A M a n d a r a n o k’7, S M M a r iop, J M a r ic ic z , L M a r in i1,m, C J M artoff*6, A M e r e g a g lia f f , P D M e y e r s c, T M ile tic 66, R M ilin c ic z , D M o n ta n a r if , A M o n t e 6, M M o n tu s c h i7, M E M o n z a n i77, P M o s te ir o c, B J M o u n tgg, V N M u r a to v a y, P M u s ic o m, J N a p o lita n o 66, M O rsin i7, F O r tic a mmnn, L P a g a n i1,m, M P a lla v ic in i1,m, E P a n t ic 00,
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T he D arkSide C ollaboration
a A PC , U niversite P aris D iderot, C N R S /IN 2P 3, C E A /Irfu , Obs de P aris, Sorbonne P aris C ite, 75205 P aris, France
b L PN H E P aris, U niversite P ierre et M arie Curie, U niversite P aris D iderot, C N R S /IN 2P 3, P aris 75252, France
c D ep artm en t of Physics, P rin c eto n University, P rinceton, N J 08544, USA d In stitu to de Flsica, U niversitade de Sao Paulo, Sao P aulo 05508-090, B razil
e A m herst C enter for F un d am en tal Interactio n s and D ep artm en t of Physics, U niversity of M assachusetts, A m herst, MA 01003, USA
f Ferm i N ational A ccelerator L aboratory, B atavia, IL 60510, USA
9 D ep artm en t of Physics, A u g u stan a University, Sioux Falls, SD 57197, USA
h D ep a rtm en t of Physics and A stronom y, U niversity of California, Los Angeles, CA 90095, USA
1 Pacific N orthw est N ational L aboratory, Richland, WA 99354, USA j L ab o rato ri N azionali del G ra n Sasso, A ssergi AQ 67010, Italy k G ran Sasso Science In stitu te , L ’A quila 67100, Italy
1 D ep artm en t of Physics, U niversita degli S tudi, Genova 16146, Italy
m Istitu to N azionale di Fisica N ucleare, Sezione di Genova, Genova 16146, Italy n Istitu to Nazionale di Fisica N ucleare, Sezione di M ilano, M ilano 20133, Italy 0 Istitu to N azionale di Fisica N ucleare, Sezione di R om a Tre, R om a 00146, Italy
p D ep artm en t of Physics and M athem atics, U niversita degli S tudi R om a Tre, R om a 00146, Italy
q D ep artm en t of Physics, U niversita degli S tudi, C agliari 09042, Italy
r Istitu to N azionale di Fisica N ucleare, Sezione di Cagliari, Cagliari 09042, Italy s D ep artm en t of Physics, U niversity of H ouston, H ouston, T X 77204, USA 1 Istitu to N azionale di Fisica N ucleare, Sezione di N apoli, N apoli 80126, Italy u D ep a rtm en t of Physics, U niversita degli Studi Federico II, N apoli 80126, Italy v D ep a rtm en t of Physics, V irginia Tech, B lacksburg, VA 24061, USA
w Skobeltsyn In stitu te of N uclear Physics, Lomonosov Moscow S tate University, Moscow 119991, R ussia
x D ep a rtm en t of Physics, U niversita degli S tudi, M ilano 20133, Italy
y St. P etersb u rg N uclear Physics In stitu te NRC K urchatov In stitu te , G atc h in a 188350, R ussia z D ep artm en t of Physics and A stronom y, U niversity of H aw ai’i, H onolulu, HI 96822, HI aa Joint In stitu te for N uclear Research, D u b n a 141980, R ussia
bb D ep a rtm en t of Physics, Tem ple University, P hiladelphia, PA 19122, USA
cc Kavli In stitu te , E nrico Ferm i In stitu te and D ept. of Physics, U niversity of Chicago, Chicago, IL 60637, USA
dd In stitu te for High Energy Physics, Beijing 100049, C hina
ee L aboratorio S ubterraneo de C anfranc, C anfranc E stacio n E-22880, Spain IPH C ,19 U niversite de S trasbourg, C N R S /IN 2P 3, S trasbourg 67037, France 99 School of N a tu ra l Sciences, Black Hills S ta te University, Spearfish, SD 57799, USA hh In stitu te for N ational Research, N ational A cadem y of Sciences of U kraine, K iev 03680, U kraine
R a d ia tio n Physics L aboratory, Belgorod N ational R esearch University, Belgorod 308007, R ussia
SLAC N ational A ccelerator L aboratory, M enlo P ark, CA 94025, USA kk N ational R esearch C entre K urchatov In stitu te , Moscow 123182, R ussia 11 N ational R esearch N uclear U niversity M E P hI, Moscow 115409, R ussia
mm D ep artm en t of Chem istry, Biology and Biotechnology, U niversita degli S tudi, P erugia 06123, Italy
nn Is titu to N azionale di F isica N ucleare, Sezione di P erugia, P erugia 06123, Italy oo D ep artm en t of Physics, U niversity of California, Davis, CA 95616, USA
pp Smoluchowski In stitu te of Physics, Jagiellonian University, Krakow 30059, Poland qq Lawrence Liverm ore N ational L aboratory, Liverm ore, CA 94550, USA
rr In stitu te of Physics Gleb W ataghin U niversidade E sta d u a l de C am pinas, Sao P aulo 13083-859, Brazil
ss School of Physics, U niversity of Chinese A cadem y of Sciences, Beijing 100049, C hina E-m ail: s t e f a n o . d a v i n i @ g s s i . i n f n . i t
A b s tr a c t.
T he D arkSide program a t LNGS aim s to perform background-free W IM P searches using two phase liquid argon tim e p rojection cham bers, w ith th e u ltim ate goal of covering all p aram eters down to th e so-called neu trin o floor. O ne of th e d istin ct features of th e program is th e use of underground argon w ith has a reduced content of th e radioactive 39A r com pared to atm ospheric argon. T he D arkSide C ollaboration is currently o p erating th e D arkSide-50 experim ent, th e first
such W IM P d etec to r using underground argon. O perations w ith underground argon indicate a suppression of 39 Ar by a factor (1.4 ± 0.2) x 103 relative to atm ospheric argon. T he new results obtained w ith D arkSide-50 and th e plans for th e next steps of th e D arkSide program , th e 2 0 1 fiducial m ass D arkSide-20k d etec to r and th e 20 0 1 fiducial Argo, are reviewed in this proceedings.
1. I n tr o d u c tio n
T he D arkSide p ro ject a t L ab o rato ri N azionali del G ran Sasso (LNGS) aim s to perform background-free W IM P searches using two phase liquid argon tim e pro jection cham bers (LAr T P C s). One of th e advantages of argon is th e powerful pulse-shape d iscrim ination (PSD) betw een electron recoils (such as fi and 7 decays) and nuclear recoils (such as an elastic scatterin g in teractio n betw een a W IM P and a nucleus). One of th e distin ctive features of th e program is th e use of underground argon (U A r), which has a lower content of th e radioactive 39A r com pared to atm ospheric argon (AAr).
2. U p d a t e s o n D a r k S id e -5 0 2.1. D etector description
T he D arkSide-50 d e te c to r system is described in Ref. [1, 2] and in several proceedings of th e collaboration. T he a p p a ra tu s consists of th re e nested detectors. Innerm ost is th e cylindrical L A r T P C , w ith an active U A r m ass of (46.4 ± 0.7) kg observed by 38 3” P M T s. T he design of th e D arkSide-50 T P C is based upon th e successful p ro to ty p e D arkSide-10 [3]. T he LA r T P C is m ounted and o p erated at th e center of a liquid scintillator veto (LSV), described in Ref. [4], consisting of a 4.0 m d iam eter stainless steel sphere in stru m en ted w ith 110 P M T s and filled w ith 30 tonnes of boron-loaded liquid scintillator. T h e scintillator is a solution of pseudocum ene (PC ), w ith 5 % by volum e trim e th y lb o ra te (T M B ). Surrounding th e LSV is a 1 kt w ater Cerenkov veto (W CV) in stru m en ted w ith 80 P M T s to veto th e residual cosm ic-ray m uons present a t th e LNGS d e p th [5]. Signals from th e LSV and W C V are used to reject events in th e LA r T P C caused by cosmogenic neutro ns [6, 7] or by neu tron s and 7-rays from radioactive co n tam in atio n in th e d e te c to r com ponents.
An in teractio n in th e LA r ta rg e t generates p rim ary scintillation light (S1 pulse) and ionization electrons. T he electrons escaping recom bination d rift in th e T P C electric field to th e surface of th e LAr, w here a stron g er electric field e x tra cts th em into th e gaseous region, w here th ey induce fu rth e r light em ission (S2 pulse) via p ro p ortional scintillation. T he S1 and S2 signals to g eth e r allow th e in teractio n v ertex to be localized in 3D. LA r T P C technology allows rejection of backgrounds from 7-ray- and fi-decay-induced events by using PSD . T he PSD p a ra m ete r used in th e W IM P searches is f90, th e fraction of S1 light in th e first 90 ns of th e scintillation pulse.
2.2. R u n with atm ospheric argon
A first ru n of DarkSide-50, w ith d a ta tak e n du rin g 2014 w ith a (1422 ± 6 7 )k g d exposure of AAr, produced a null result for th e W IM P search w ith zero backgrounds from 39A r decays [1].
A to ta l of 1.5 x 107 events in th e LA r T P C , m ostly originating from 39Ar, were collected. All b u t two of th e events falling w ithin th e W IM P region of interest were rejected using th e PSD . T he two rem aining events in th e W IM P search region had a signal in coincidence w ith th e LSV and were therefore discarded.
2.3. D etector calibrations
T he D arkSide-50 d etecto rs have been calib rated in situ durin g 2014 and 2015, using a series of Y and n e u tro n radioactive sources placed inside th e LSV next to th e T P C . D a ta tak en w ith
F ig u r e 1. Live-tim e-norm alized S1 pulse integral sp ectra obtain ed a t zero d rift field, w ith an A A r fill (black) and a U A r fill (blue).
Also shown are th e fit to th e U A r d a ta (red) and individual com ponents of 85K r (green) and 39A r (orange) e x tra cte d from th e fit. See Ref. [2] for a detailed description.
F ig u r e 2. Live-tim e norm alized S1 pulse integral sp ectra from single-scatter events in A A r (black) and U A r (blue) tak e n w ith
2 0 0V /c m drift field. Also shown are th e 85K r (green) and 39A r (orange) levels as inferred from a spectral fit. See Ref. [2] for a detailed description.
57Co, 133Ba, and 137Cs 7-ray sources were used to validate M onte C arlo (MC) sim ulations, and d a ta tak en w ith Am Be n eu tro n sources were used to validate th e tran sfer of th e nuclear recoil response from SC E N E [8] to D arkSide-50, as described in Ref. [2].
T he calib ratio n w ith Am Be n eu tro n sources allowed for th e m easurem ent of th e LSV response to n e u tro n captu res, which occurs predo m inantly on 10B and 1H, w ith estim ated probabilities for th e cu rren t TM B co n cen tratio n of ~ 9 2 % and ~8%, respectively. N eu tro n c a p tu re on 10B in th e TM B can occur th ro u g h two channels [9]:
T he scintillation light resulting from th e a and 7Li of th e ground s ta te channel is quenched to 25 to 35 P E , while th e 478 keV Y-ray accom panying th e 7Li channel gives 240 P E w hen th e Y-ray is fully absorbed. These signals are b o th well above th e LSV analysis th resh old of a few P E [4].
2.Ą.. Underground argon
A fter th e A A r run, th e T P C was drain ed and filled w ith low radio activity U A r in A pril 2015.
T he low radio activity U A r was e x tra cte d from th e Doe C anyon C O2 wells. It was tra n sp o rte d to Ferm ilab w here it was purified to < 10 ppm of C O 2, N 2, O 2, and He. A to ta l of 155 kg of low rad io activ ity U A r has been obtained . T he purified U A r was sent to LNGS for final g etterin g to d etector-grade argon [1 0, 1 1].
Fig. 1 com pares th e m easured zero-field sp ectra for th e U A r and A A r targ ets. T he background
Y-ray lines originate from identified levels of 238U, 232T h, 40K, and 60Co in th e d etecto r c o n stru ctio n m aterials, and are consistent w ith th e expectatio n s from m aterials screening. T he positions of th e Y-ray peaks in th e A A r and U A r sp ectra are th e same.
An unexpected co n tam in atio n of 85K r is present in th e U A r spectrum , which was not seen in th a t of th e AAr. T his is evident in Fig. 2 , which com pares th e U A r and A A r sp ectra for th e S1 pulse, w ith a 200 V /c m d rift field. T he 39A r activ ity of (0.73 ± 0.11) m B q /k g and 85K r activ ity of (2.05 ± 0.13) m B q /k g in th e U A r are determ ined by a com bined fit to th e sp e ctra of S1-late w ith field off (Fig. 1) , S1 w ith field on (Fig. 2) and th e z-position d istrib u tio n . T he 39A r activity of th e U A r corresponds to a depletion by a factor of (1.4 ± 0.2) x 103 relative to A A r [2].
10B + n ^ a (1775 keV) + 7Li (BR: 6.4 %) 10B + n ^ a (1471 keV) + 7Li* (BR: 93.6 %)
7Li* ^ 7Li + y (478 keV)
F ig u r e 4. Spin-independent W IM P-nucleon cross section 90% C.L. exclusion plot for D arkSide-50, com pared to o th er experim ents.
See Ref. [2] for a detailed description and references.
F ig u r e 3. D istrib u tio n of events in th e f90 vs S1 plane which survive all physics cuts.
Shaded blue w ith solid blue outline: W IM P search region. See Ref. [2] for a detailed description.
An in dependent estim ate of th e 85K r decay ra te is o b tain ed by m easuring its 0.43 % decay b ranch to m etastab le 85mRb, which gives a 7-ray w ith m ean lifetime of 1.46 ps following th e
^-decay. T he m easurem ent of th e decay ra te of 85K r via 85mR b gives (33.1 ± 0.9) e v e n ts/d , in agreem ent w ith th e value (35.3 ± 2.2) e v e n ts/d o b tained from th e known branching ratio and th e spectral fit result. T he presence of 85K r in U A r was unexpected, and so no a tte m p ts were m ade to remove it from th e U A r, which could have been done via cryogenic distillation . T h e 85K r in U A r could come from atm ospheric leaks or from deep underground n a tu ra l fission processes [1].
2.5. W IM P search with low radioactivity underground argon
T he first W IM P search in D arkSide-50 using U A r has been rep o rted in Ref. [2], w here it is shown th a t th e com bination of th e electron recoil background rejection observed in th e AAr run, and th e reduction of 39A r from th e use of U A r would allow D arkSide-50 to be free from 39A r background for several tens of years.
A non-blind physics analysis was perform ed. T he T P C and veto physics cu ts applied, as well as th e ir efficiency and acceptance, are described in Ref. [2 , 4]. T he d istrib u tio n of events in th e f90 vs. S1 plane, after all quality and physics cuts, is shown in th e left panel of Fig. 3. T here are 1.26 x 105 events in th e energy region of interest, defined as 2 0 P E to 4 60 P E (13keV nr to 201keV nr). T h e W IM P search region, shown in Fig. 3 , is defined as a region in th e f90 vs. S1 plane w ith known high acceptance for nuclear recoils and low leakage of single-scatter E R events, as described in Ref. [2].
No events are present w ithin th e W IM P search region. D ark m a tte r lim its from th e present exposure are d eterm ined from our W IM P search region using th e sta n d a rd isotherm al galactic W IM P halo p aram eters. W hen com bined w ith th e null result of th e previous A A r exposure in DarkSide-50, a 90% C.L. u p p e r lim it on th e W IM P-nucleon spin-independent cross section of 2.0 x 10- 4 4 cm2 (8 . 6 x 10- 4 4 cm 2, 8.0 x 10- 4 3 cm 2) for a W IM P m ass of 1 0 0 G e V /c2 (1 T e V /c 2, 1 0 T e V /c 2) is obtain ed . Fig. 4 com pares these lim its to those o b tain ed by o th er experim ents. A detailed description can be found in Ref. [2].
3. T h e fu tu r e s te p s o f t h e D a r k S id e p r o g r a m 3.1. D arkSide-20k and Argo
T he com bination of th e A A r and U A r results in D arkSide-50 leads to th e ex pectatio n th a t a background-free result can also be obtained from a m uch larger exposure w ith a m ulti-tonne d etector. O n th is basis, an enlarged D arkSide C ollaboration has proposed th e co n structio n of DarkSide-20k, a direct W IM P search using a LA r T P C w ith an active (fiducial) m ass of 2 3 1 (2 01) of depleted argon (D A r). D A r is U A r which has been fu rth e r depleted in 39A r by m eans of
isotopic separation. D arkSide-20k is designed to achieve a background-free exposure of 100 t y r accum ulated durin g a ru n of 5 years, giving a sensitivity to W IM P-nucleon in teractio n cross sections of 1 x 10-4 7cm 2(1 x 10- 4 6 cm 2) for W IM P s of 1 T e V /c2 (1 0 T e V /c 2) mass.
In th e longer term , th e aim of th e D arkSide collaboration is to develop a p a th tow ards a W IM P d e te c to r to be built and o p erated at LNGS w ith a 3 0 0 1 (20 01) D A r ta rg e t (fiducial) m ass. For now, th is u ltim a te experim ent is called Argo. Argo is planned to accum ulate an exposure of 1 0 0 0t y r , free of backgrounds o th er th a n those induced by coherent scatterin g of neutrinos, and th u s be sensitive to W IM P cross sections below th e n eu trin o floor.
3.2. Urania and A ria
P ro cu rem en t of th e necessary q u a n tity of low radioactivity U A r for D arkSide-20k is th e critical technical challenge for th e experim ent, and will be addressed w ithin th e fram ew ork of th e U rania and A ria projects. T he U rania p ro ject will provide a plan t capable of e x tra ctin g 100 k g /d of U A r from th e sam e wells th a t yielded th e U A r for D arkSide-50. T h e A ria p roject will provide a cryogenic d istillatio n p lan t capable of reducing th e residual 39A r in th e U A r by a facto r of 10 p er pass, at a ra te of 150 k g /d , by exploiting th e sm all difference in vapor pressure betw een 39Ar and 4 0Ar. T he U ran ia and A ria projects will u ltim ately supply th e D A r for th e D arkSide-20k experim ent.
4. C o n c lu s io n s a n d O u tlo o k
T he D arkSide collaboration rep o rted th e first W IM P search using low rad io activ ity U A r [2], resulting in th e best W IM P lim its o b tained w ith argon. T he com bination of PS D in argon and low radio activity U A r lead to th e ex p ectatio n th a t a background-free result can also be obtain ed w ith a m ulti-to n ne d etecto r. T h e D arkSide collaboration has proposed th e co n structio n of th e 20 t fiducial m ass D arkSide-20k detector.
R e fe r e n c e s
[1] Agnes P, et al. (The D arkSide C ollaboration) 2015 Phys. Lett. B 743 456-466
[2] Agnes P, et al. (The D arkSide C ollaboration) 2015 arXiv:1510.00702 [astro-ph, physics:hep-ex, physics:physics]
[3] A lexander T , et al. (The D arkSide C ollaboration) 2013 Astroparticle Physics 49 44-51
[4] Agnes P, et al. (The D arkSide C ollaboration) 2015 arXiv:1512.07896 [physics:ins-det] (Preprint 1512.07896) [5] Bellini G, et al. (The B orexino C ollaboration) 2012 J C A P 2 0 1 2 015-015
[6] E m pl A, H ungerford E V, Jasim R , and M osteiro P 2014 J C A P 2 0 1 4 064-064 [7] Bellini G, et al. (The B orexino C ollaboration) 2013 J C A P 2 0 1 3 049-049 [8] Cao H, et al. (The SC EN E C ollaboration) 2015 Phys. Rev. D 91 092007
[9] W right A, M osteiro P, Loer B, and C alaprice F P 2011 Nucl. Inst. Meth. A 6 4 4 18-26 [10] Back H O, et al. 2012 arXiv (Preprint 1204.6024v2)
[11] Back H O, et al. 2012 arXiv (Preprint 1204.6061v2)
