HydroX : Hydrogen-doped Liquid Xenon to Search for Sub-GeV/c 2 Dark Matter Particles Alden Fan Stanford University / SLAC National Accelerator Laboratory CPAD 2019 Madison, WI 8-10 Dec 2019
Low mass dark matter From Cosmic Visions (1707.04591) �� - �� �� - � � � ��������� �� ���� � � - � �� - �� �� - � � � � � � � � � � � � - � � � � � � ���� ������ - ������� ����� ������� [ �� � ] ���� ������ - ������� ����� ������� [ �� ] � � � � � ��������� �� ���� - Many new proposed � � �� - �� �� - � � � � � � Existing liquid noble � � � experiments aimed at ����� ������ ( � ) � � � �������� �� � searches are in � <5 GeV/c 2 �� - �� �� - � � ) � ( � � � � � � � � � � � � 10s-100s GeV/c 2 � ������ �� - �� �� � � � �� - �� �� - � � � � - � ��� �� ( ������ ) ��� �� ( ����� ) � �� - �� � �� - � � � - � � � � � � � � � � Unconstrained � � � � � �� - �� �� - � � � ������� � � � � � ��� - �� �� - �� �� - � Neutrino floor ������� ������ � �� - �� �� - � � � � � � � �� - �� ��� ��� � � �� ���� ������ ���� [ ��� / � � ] A. Fan (SLAC) CPAD 2019 HydroX 2
Low mass dark matter rate R(cts/10kg/yr) for 10 -45 cm 2 , 10 GeV R(cts/100kg/yr) for 10 -46 cm 2 , 100 GeV ⇧ year ⇥ ⇧ year ⇥ Xe 100 GeV 10 GeV Xe Typical Xe threshold Ge 1.00 Ge 1.00 Knowing your energy scale Ar Ar 0.50 0.50 and efficiency at threshold Si Si are crucial! Ne Ne 0.10 0.10 0.05 0.05 40 Ethresh 40 Ethresh 0 10 20 30 0 10 20 30 Energy threshold (keV) Energy threshold (keV) For low mass sensitivity, need: (1) low threshold (2) lighter target for better kinematic match to DM mass A. Fan (SLAC) CPAD 2019 HydroX 3
Low mass dark matter detectors Challenge Solution Kinematics Match target-DM mass Low energy threshold Low energy depositions Extremely rare interaction Large/scalable target mass Environmental backgrounds Underground / shielding Detector backgrounds Self-shielding, discrimination, radiopurity Impurities Purification Unknown particle physics Sensitivity to multiple interaction types A. Fan (SLAC) CPAD 2019 HydroX 4
Low mass dark matter detectors Challenge Solution Kinematics Match target-DM mass Low energy threshold Low energy depositions Already achieved in LZ (and other G2 DM experiments) Extremely rare interaction Large/scalable target mass Environmental backgrounds Underground / shielding Detector backgrounds Self-shielding, discrimination, radiopurity Impurities Purification Unknown particle physics Sensitivity to multiple interaction types A. Fan (SLAC) CPAD 2019 HydroX 5
Low mass dark matter detectors Challenge Solution Kinematics Match target-DM mass Low energy threshold Low energy depositions Already achieved in LZ (and other G2 DM experiments) Extremely rare interaction Large/scalable target mass Environmental backgrounds Underground / shielding But LZ has a heavy target (Xe) Detector backgrounds Self-shielding, discrimination, radiopurity Impurities Purification Unknown particle physics Sensitivity to multiple interaction types A. Fan (SLAC) CPAD 2019 HydroX 5
Low mass dark matter detectors Challenge Solution Kinematics Match target-DM mass Low energy threshold Low energy depositions Already achieved in LZ (and other G2 DM experiments) Extremely rare interaction Large/scalable target mass Environmental backgrounds Underground / shielding But LZ has a heavy target (Xe) Detector backgrounds Self-shielding, discrimination, radiopurity Impurities Purification Put a low-Z target in LZ, Unknown particle physics Sensitivity to multiple interaction while retaining benefits of Xe types A. Fan (SLAC) CPAD 2019 HydroX 5
HydroX : Hydro gen-doped X enon 1. Dissolve H 2 into LXe 2. Look for recoiling proton LZ H Xe 𝛙 A. Fan (SLAC) CPAD 2019 HydroX 6
HydroX advantages: signal yield Xe recoil: m Xe =m Xe → energy lost to heat (Lindhard) → O(20%) of energy is observable • H 2 recoil: m p ≪ m Xe → all electronic excitations → ~100% of energy is observable • H Total Quanta [e+ph] 10 2 He ~5x more signal Xe vs. from H relative to Xe 10 1 𝛙 SRIM H SRIM He NEST He NEST Xe 0 1 2 3 4 5 Nuclear recoil energy [keV] 𝛙 A. Fan (SLAC) CPAD 2019 HydroX 7
HydroX advantages: BG mitigation Retain self-shielding of LXe • Vetoes, water tank, intensive radio-cleanliness of LZ • Fully characterized BG model from LZ • LZ ER+NR backgrounds (external) 10 3 Interaction length [cm] 10 2 10 1 Liquid H 2 10 0 Liquid xenon 10 -1 10 -1 10 0 10 1 Gamma energy [MeV] LZ TDR (1703.09144) A. Fan (SLAC) CPAD 2019 HydroX 8
HydroX advantages: SD sensitivity 1 H nat Xe For equivalent masses of H and Xe: unpa unpaired ed neutr neutron n spi pin unpaired neutron spin unpaired proton spin 1 H has 820x more SD sensitivity per kg than nat Xe In addition, use deuterium : gives both DM-p and DM-n sensitivity A. Fan (SLAC) CPAD 2019 HydroX 9
HydroX sensitivity �� - �� �� � ���� ������ - ������� σ �� [ �� � ] ���� ������ - ������� σ �� [ �� ] LZ H2 �� - �� �� � LZ H2 S1/S2 S2-only, 5e- �� - �� �� - � �� - �� �� - � NEWS-G NEWS-G �� - �� �� - � LZ H2 S2-only, 3e- �� - �� �� - � CRESST-II Assumptions: �� - �� �� - � CDMSLite DS-50 �� - �� �� - � Superfluid S2-only • Signal yields from SRIM + LZ detector LHe proposal �� - �� �� - � SuperCDMS SI Si+Ge HV �� - �� �� - � model LUX LZ Xe �� - �� ���� ���� � �� • 2.2 kg of H 2 in LXe (2.6% mol fraction) ���� ������ ���� [ ��� / � � ] • Proton recoil S2/S1 is ER-like ���� ������ - ������� σ �� � [ �� � ] �� - �� �� � ���� ������ - ������� σ �� � [ �� ] (no discrimination) LZ H2 �� - �� �� � LZ H2 S1/S2 S2-only, 5e- �� - �� �� - � PICASSO PICASSO • 250 live-day exposure �� - �� �� - � LZ H2 �� - �� �� - � S2-only, 3e- SuperK SuperK �� - �� �� - � �� - �� �� - � PICO �� - �� �� - � �� - �� �� - � SD SD sensitivity at low mass is unique �� - �� �� - � �� - �� ���� ���� � �� ���� ������ ���� [ ��� / � � ] A. Fan (SLAC) CPAD 2019 HydroX 10
R&D • Will it work? • What is Henry coe ffi cient? • E ff ect on signal generation (light and charge) • Circulation and cryogenics • Purification removes H 2 • Ti embrittlement • H 2 leakage into PMTs • How do we calibrate? • Ultra low energy proton recoils in LXe • E ff ect on discrimination • How do we make it work in LZ? A. Fan (SLAC) CPAD 2019 HydroX 11
Injecting H 2 into LXe • XELDA: small TPC constructed at Fermilab • Originally for measuring ER discrimination for inner shell e-, now for H 2 -doping • One 3” PMT facing four 1” PMTs • Gas phase circulation, inject H 2 at the condenser A. Fan (SLAC) CPAD 2019 HydroX 12
RGA scans Injecting H 2 into LXe 1e-5 [arb] before mixing • Is H 2 in the liquid? • YES, though hard to say how much • Measure H 2 in gas space after injection, before and after inducing mixing (circulating) • H 2 level in gas space goes down, 3e-6 [arb] after mixing (by factor 2-3) → H 2 is in the LXe A. Fan (SLAC) CPAD 2019 HydroX 13
Injecting H 2 into LXe • TPC still working! • S1s and S2s still being produced and can see them • Loss of S2 yield (as predicted) • Possible decrease in S1 yield (~10%) Xe only Xe + H 2 S2 yield shifted down A. Fan (SLAC) CPAD 2019 HydroX 14
SLAC System Test Immediate next steps S2 measurement setup XELDA Run 2 • Improved gas analysis • Inject more H 2 • S1-only mode to measure S1 loss more carefully • S2s di ffi cult to measure well in XELDA setup with H 2 H 2 +GXe at SLAC • Use SLAC System Test in room temperature gas-only mode • Used extensively for electron emission studies (see R. Mannino’s talk) • Measure e ff ect on S2 yield more carefully A. Fan (SLAC) CPAD 2019 HydroX 15
Low energy recoil calibration • Classic neutron scattering setup: scattering angle gives recoil energy • Low energy neutron source: 24 keV neutrons from 124 Sb- 9 Be source • TPCs for both target and neutron tagger Poly Fe Sb-Be γ -n source ~24 keV neutrons n 10” keV Iron Neutron source (keVIN) A. Fan (SLAC) CPAD 2019 HydroX 16
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