Background and sensitivity predictions for XENON1T

Background and sensitivity predictions for XENON1T Marco Selvi INFN - Sezione di Bologna (on behalf of the XENON collaboration) Feb 19 th 016, UCLA Dark Matter 016 1

Outline Description of the detector; Prediction of the ER and NR backgrounds through a GEANT4 simulation of the experiment;! Conversion of the energy released in the TPC by signal and background events into the S1 and S signals seen in the detector;! Study the sensitivity to WIMP-nucleus spin-independent interactions.! E. Aprile et al. (the XENON collaboration) Physics reach of the XENON1T dark matter experiment, arxiv:151.07501, submitted to JCAP

The XENON1T experiment 3

The XENON1T experiment χ 97 cm 96 cm χ 4

Main Backgrounds NR ER Electron recoils (ER): low energy Compton scatters from the radioactive contaminants in the detector components: U and Th chains, 40 K, 60 Co, 137 Cs. Intrinsic contaminants: β decays of Rn daughters, 85 Kr, 136 Xe. Elastic scattering of solar neutrinos off electrons. Nuclear Recoils (NR): Radiogenic neutrons: spontaneous fission and (α, n) reaction from the U and Th chains in the detector components. Muon-induced neutrons. Coherent scattering of neutrinos (mostly solar) off the Xe nuclei. 5

MC simulation in GEANT4 Top PMT array The whole TPC Diving bell PTFE pillars Field shaping electrodes A single PMT Supports Bottom PMT array Cathode PMTs PTFE reflector PTFE bottom plate Copper plate PMT bases 6

ER from the materials ] -1 ER Rate [ ( kg day kev ) 1 3 4 5 6 7 8 14 Pb 08 Tl Total 14 Bi 08 Tl 1 Bi 137 Cs 14 Bi Cryostat Flanges PMT and Bases 8 Ac 08 Tl 8 14 Bi Ac 14 Bi 60 Co 14 Bi 0 500 00 1500 000 500 60 Co 14 Bi Cryostat Shells SS in the TPC 40 K 14 Bi 8 Ac 14 14 Bi Electronic Recoil Energy [kev] Bi PTFE and Cu in the TPC 14 Bi 14 Bi 08 Tl Extensive screening campaign using gamma (Ge ) and mass (ICP-MS) spectrometry.! Eur.Phys.J. C75 (015) 546 ER background! from the materials! in 1 t Fiducial Volume,! in [1, 1] kev:! 7.3-6 (kg day kev) -1 Z [mm] 400 300 00 0 0-0 -00-300 -400 3 0 0 40 60 80 0 10 140 160 180 00 0 R [mm ] -1 - -3-4 -5-6 ] -1 kev ) day Rate [ ( kg 7

Total ER background Assumptions on the intrinsic backgrounds:! 0. ppt of nat Kr (already achieved in XENON1T distillation column tests), µbq/kg of Rn (conservative estimation based on Rn emanation meas.). ] -1 kev ) day 1 3 4 ] -1 kev) day 3 4 Total Rn solar ν 136 Xe 85 Kr Total except materials materials ER Rate [ ( kg 5 6 7 8 Total materials 136 Xe Rn 85 Kr solar ν 0 500 00 1500 000 500 Electronic Recoil Energy [kev] ER Rate [ (kg 5 6 7 00 400 600 800 00 100 1400 1600 1800 Fiducial Mass [kg] Rn (mainly from 14 Pb β-decay) is the most relevant source of ER background in most of the TPC. 8

Total ER background The numbers above are before any ER/NR discrimination. In 1 t FV, the background from the material is at the same level as the one from solar neutrinos and from Kr 9

NR from radiogenic neutrons Z [mm] ] -1 Neutron Yield per Parent Decay [MeV 400 300 00 0 0-0 -00-300 -5-6 -7-8 -9 - -11-1 -13-14 -15-16 Neutron yield from! SOURCES-4C 38 U - 35 U - 0 4 6 8 Neutron Energy [MeV] -400 3 0 0 40 60 80 0 10 140 160 180 00 0 R [mm ] 6 3 8 Ra - Th - Th - 30 Th 07 06 8 08 Pb Pb Ac Pb -5-6 -7-8 -9 ] -1 kev) day NR Rate [ ( kg 00 400 600 800 00 100 1400 1600 1800 Fiducial Mass [kg] NR Rate [ y -1 ] 1-1 - In 1t FV, [4, 50] kev:! 0.6 events / y! (with no discrimination) [3, 50] kev [4, 50] kev [5, 50] kev

Total NR background ] -1 kev) day NR Rate [(kg - -3-4 -5-6 -7-8 -9 - -11 CNNS Radiogenic neutrons Muon-induced neutrons 0 0 30 40 50 60 70 80 90 0 Nuclear Recoil Energy [kev] Single ScaBer, 1 t Fiducial Volume, [4, 50] kevr, 0% NR acceptance Source Background (ev/y) Radiogenic neutrons 0.6 ± 0% Muon- induced neutrons < 0.01 (muon veto ON) CNNS 0.0 ± 0% Total NR 0.6 ± 0.1 Given the very steep spectrum of NR from CNNS, its contribuqon will become more relevant aser the conversion into the S1, S signals, considering the detector response and energy resoluqon. 11

! Light & charge emission model ER: NEST model! M. Szydagis et al., JINST 6 (011) P00, [arxiv:16.1613] NR: direct measurements of L eff,! Q y measured by XENON0! E. Aprile et al., Phys.Rev.D88 (013) 01006, [arxiv:1304.147] NEST is based at low energies on the direct! measurement at Columbia and Zurich:! Phys. Rev. D 86, 11004 (01)! Phys. Rev. D 87, 115015 (013)! 1

Light Collection EfGiciency We generate optical photons uniformly inside the TPC, and follow their propagation inside it with all the reflections, refractions, absorptions, etc. Ly at 500 V/cm Mesh Mesh Mesh Wires Mesh 4.6 PE/keV @1 kev LCE [%] 40 35 30 5 0 PTFE with 99% reflectivity PTFE with 95% reflectivity PTFE with 90% reflectivity Our assumption 0 30 40 50 60 70 80 90 0 Absorption length [m] 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 Ly [PE/keV] Z [mm] 0 00 300 400 0 0 40 60 80 0 10 140 160 180 00 0 [mm 400 300 00 0 0 R ] 44 4 40 38 36 34 3 30 8 6 3 LCE [%] 13

Signal and backgrounds in S1 ] -1 PE) day Event Rate [ (kg -4-5 -6-7 -8-9 - -11 Total (ER+NR) background ER background NR background from neutrons NR background from ν WIMP, m = GeV/c, σ = WIMP, m = 0 GeV/c, σ = WIMP, m = 00 GeV/c, σ = 0 0 30 40 50 60 70 80 S1 [PE] -46 cm -47 cm cm -46 - Electric field in the TPC: 500 V/cm - Lower threshold of the region of interest:! 3 PE (lowest one used so far by XENON0) - Higher end: 70 PE, corresponding on average to 50 kevr! (include >90% of NR spectrum for 0 GeV WIMP) - S > 8 electrons ER background is the most relevant one,! apart from the low S1 region where CNNS becomes dominant. 14

Statistical model D statistical model:! S1 and discrimination.! Profile Likelihood method,! exclusion test statistic, with CLs! (Eur. Phys. J. C71 (011) 1554).! ER Background events! NR Background events! (NR) Signal events! Systematic uncertainties are included in the model! as nuisance parameters, and profiled out.!!! The most relevant uncertainty is related to L eff, which rules the light output of nuclear recoils in LXe. Thus it affects, in a correlated way, both the WIMP signal and the NR backgrounds.! We consider also other systematics: Q y, and the overall uncertainty on ER and NR background (assumed % and 0%, respectively).! We study the sensitivity bands via generation of MC toy experiments. 15

XENON1T sensitivity Dotted blue line shows the XENON1T sensitivity assuming a cutoff below 3 kev ] WIMP-nucleon Cross Section [cm 39 40 41 4 43 44 45 46 47 48 49 DAMA/Na DAMA/I CDMS-Si (013) XENON (013) SuperCDMS (014) XENON0 (01) PandaX (014) XENON1T ( t y) 6 0 30 0 00 00 000 000 WIMP mass [GeV/c XENON1T Sensitivity in t y Expected limit (90% CL) ± 1 σ expected ± σ expected ] DarkSide-50 (015) LUX (013) XENONnT (0 t y) Billard et al., Neutrino Discovery limit (013) With a t y exposure, with XENON1T we ll reach a sensitivity to spin-independent WIMP-nucleon interactions of 1.6-47 cm for a 50 GeV/c WIMP. 16

Sensitivity VS time [cm ] spin-independent WIMP-nucleon σ -44-45 -46-47 -48 1 XENON0 (01) m χ = 50 GeV/c LUX (013) LUX (013) LUX (015) XENON1T ( year exposure) Livetime [days] In less than days we can reach the sensitivity! of the currently running experiments 3 17

LUX015 emission model Photon Yield [ph/kev] 14 1 8 6 4 XENON0 model LUX015 model /kev] - Electron Yield [e 1 8 6 4 XENON0 model LUX015 model 0 1 Nuclear Recoil Energy [kev] 0 1 Nuclear Recoil Energy [kev] With the recent LUX measurement (arxiv:151.03506): Larger photon and electron yield, in particular at low energy (below 3 kev), Much smaller systematic uncertainties. 18

XENON1T sensitivity Assuming the LUX015 emission model Potential to detect CNNS! from solar neutrinos. Significant improvement in sensitivity to WIMPs! at low masses, below GeV/c. 19

Summary & Conclusions ER and NR backgrounds from the detector materials are reduced to a subdominant level in the inner 1 t FV.! The most relevant background comes from Rn in LXe, (assuming a uniform contamination of µbq/kg). Total ER background: 1.8-4 (kg day kev) -1. With a t y exposure, with XENON1T we ll reach a sensitivity to spinindependent WIMP-nucleon interaction of 1.6-47 cm for a 50 GeV/c WIMP.! Good potential to detect for the first time coherent scattering of neutrinos from the Sun, in particular assuming the LUX015 emission model. Improved sensitivity at low mass WIMPs too. 0

Backups 1

MC simulation in GEANT4 Diving bell PTFE pillars Field shaping electrodes Supports Cathode PMTs PTFE reflector PTFE bottom plate Copper plate PMT bases

ER from the materials Extensive screening campaign using Ge gamma spectrometry and mass spectrometry (ICP-MS) Screening of PMTs: E. Aprile et al., Eur.Phys.J. C75 (015) 546, [arxiv:1503.07698] Dedicated paper on screening of XENON1T materials in preparation 3

NR from µ- induced n Negligible, thanks to the water shield and Cherenkov muon veto surrounding the detector 4

NR from CNNS (Coherent neutrino-! nucleus scattering) Total rate above 1 kev: 90 (t y) -1 ] -1 kev) y Rate [(t 1 1 3 Total 8 B hep DSN Atm Total rate above 4 kev: 1.8 - (t y) -1 4 5 1 Nuclear Recoil Energy [kev] 5

WIMP signal Halo model: v 0 = 0 km/s, v Sun = 50 km/s,! v esc = 544 km/s, ρ=0.3 GeV/cm 3 6

d statistical model S1 distributions Recoil energy spectra converted into S1 distributions (n PE detected) through Light Yield L Y (n γ produced/kev) Charge Yield Q Y (n e - produced/kev) Detector performance (LCE, PMTs QE and CE)!! t y exposure m χ =0 GeV/c μ s =50 (enhanced)! μ ber =1300" μ bnr =.3" Y distributions Y is an idealized version of the discrimination parameter log (S/S1) ER events Gauss(0,1) NR events Gauss(-.58,0.9) These distributions reproduce the XENON0 ER/NR discrimination and NR acceptance performance NO discrimination cut on ER events (full data set used) Total NR recoils ER recoils ] -1 PE) day Event Rate [ (kg -4-5 -6-7 -8-9 - -11 Total (ER+NR) background ER background NR background from neutrons NR background from ν -46 WIMP, m = GeV/c, σ = cm -47 WIMP, m = 0 GeV/c, σ = cm -46 WIMP, m = 00 GeV/c, σ = cm 0 0 30 40 50 60 70 80 S1 [PE] ER Background events! NR Background events! (NR) Signal events! 7

" Systematic uncertainty on L eff The RELATIVE SCINTILLATION EFFICIENCY L eff rules the light output of nuclear recoils in Xenon L eff is the major systematic uncertainty for XENON1T; no direct experimental measurements below 3 kev. Conservatively extrapolated down to zero at 1 kev IMPACT OF L eff VARIATIONS m χ =0 GeV/c " S1 SPECTRAL SHAPES of signal and backgrounds change very slightly under different L eff! We keep fixed the shape of S1 spectra"! NUMBER of EXPECTED EVENTS in [3,70] PE Parameterization of the uncertainty on L eff Gaussian nuisance parameter t CNNS neutrinos behave like 6 GeV/c WIMP Neutrons behave like 30 GeV/c WIMP The number of expected events from CNNS and low mass WIMPs is highly affected by L eff variations μ s = μ s (t) and μ bnr = μ bnr (t) " "! 8

ProGile likelihood method Likelihood function (unbinned, extended) Parameter of interest μ s! The uncertainty on L eff, affecting both WIMP signal and NR background in a correlated way, is included in the likelihood as a nuisance parameter and profiled out. Exclusion test statistic Profile Likelihood ratio Compute test statistic P.D.F. under signal hypothesis H! under bkg-only hypothesis H 0 Run hypotheses tests" Generated 4 MC toy experiments under H 0 Rejection test of each signal hypothesis H! (different! s ) using med[q μ H μ ] as observed test statistic q μ obs" The significance of each test is given by the p-value μ s = " μ s =5 " Modified p-value CL s method! 9