The Neutron/WIMP Acceptance In XENON100

Transcription

1 The Neutron/WIMP Acceptance In XENON100 Symmetries and Fundamental Interactions September 2014 Chiemsee Fraueninsel Boris Bauermeister on behalf of the XENON collaboration Boris Bauermeister JG U

2 The Xenon Dark Matter Project: Laboratori Nationali del Gran Sasso (Italy) World wide XENON Collaboration Boris Bauermeister JG U 2

3 Motivation: (WIMP) Dark Matter Astrophysical hints for Dark Matter:... Rotation curves in galaxies Newest CMB result from Planck satellite (2013)1 What is Dark Matter? Known particles are ruled out WIMP Dark Matter: Weakly Interacting Massive Particles How to find such a mysterious particle? Direct Dark Matter search with liquid xenon Search for nuclear recoils (WIMP-nucleon interaction) Nuclear Recoil (NR) Electronic Recoil (ER) 1) preprint: arxiv: Boris Bauermeister JG U 3

4 Detection principle of a two phase time projection chamber (TPC) Amplitude [V] PMT top array S1: Photons (λ = 178 nm) from scintillation process Detected by PMTs (bottom array, mainly) Δt S2: Ionisation process frees electrons Electrons drift upwards Extracted in gaseous phase Proportional scintillation light Z position from time difference between S1 and S2 time [μs] Nuclear recoil (NR) (neutron) PMT bottom array Electronic recoil (ER) 3D Position reconstruction! X/Y position from top array Boris Bauermeister JG U 4

5 The XENON100 Time Projection Chamber (TPC) E. Aprile et al. (XENON100), Astroparticle Physics 35, 573 (2012) 242 (1 ) Photomultiplier (PMTs): 98 PMTs on the top array 80 PMTs on the bottom array 64 PMTs in the veto Top PMT array Bottom PMT array Detection material: 161 kg liquid xenon (-91 C) Target mass: ~ 62 kg TPC: 30 cm height / 30 cm diameter All used materials: Low radioactivity Multilayer passive shield: Cu, PE, Pb, H2O XENON100 Time Projection Chamber Boris Bauermeister JG U 5

6 232 Th/60Co data The XENON100 calibration: 60 Co (β-,γ - source) 232 Th (α-source) Once a week Electronic Recoil (ER) Response Neutron calibration 241 AmBe (n-source) Begin/End of a data taking period S2th Nuclear Recoil (NR) Response Calibration data: NR (red) and ER(blue) Further Calibrations 137 Cs (β-,γ - source) Once a week LED Once a week Electron lifetime PMT calibration Why is NR calibration necessary? WIMPs supposed to interact like neutrons! WIMPs single scatters Neutrons single, double,, scatters Neutron background (e.g. from rock) can not be switched off! Understand background! Neutron acceptance determined based on the threshold energy for S2 signals. Actual threshold: S2 > 150 pe (blue dashed line) Boris Bauermeister JG U 6

7 Neutron acceptance I: Z-dependence: Determine S2 acceptance for neutrons with a minimum recoil energy in dependence of S1 from the upper part of the detector: S2th > 150 pe t = 5 30 μs r = 145 mm Due to the impurities in LXe, electrons are absorbed while drifting in the TPC. In order to pass the S2 threshold, the amount of deposited energy has to be higher for higher drift lengths.! Charge loss: Upper TPC: Create same amount of electrons Drift-time Δt << Electron lifetime τ S2th de = Δt / τ dt e Gate mesh Data set: 241AmBe: April 2012: ~ 2 days May 2013: ~ 6 days e- ee- e- n' Correct S2 to Z-coordinate: n Electron lifetime influences the measured S2: April 2012: τ = 357 µs May 2013: τ = 533 µs S2 cs2 Δt e-ee-- e- ee e- e e- e- (Before/after AmBe measurement) radius n' n Electron clouds e- Boris Bauermeister JG U 7

8 Neutron acceptance II: NR only!!! Analysis method: Describe the cs2 spectra of each S1 slice by the expectation value of a Poisson distribution Fit with boundaries! Choose S1 slices of 1 pe: Example: S1-slice: 7 8 pe c Focus on low energies: 1 17 pe Describe each S1 slice by a Poisson distribution with two parameters: P(S2, α,β, λ )=α Poiss( S2,λ) β Lowest fit boundary: Apply electron loss correction to determine the fit boundary 159 pe (May 2013, red dashed line) Fit to free parameter λ by choosing a fix parameter for β. Minimize <χ2/ndf> with parameter β Determine the invisible cs2 part below 159 pe. Monte Carlo method Returns the expected loss (S1 dependent) Expectation value: λ Scale parameter: β Boris Bauermeister JG U 8

9 Neutron acceptance III: P(S 2, α,β, λ)=α Poiss( Expectation value: λ S2, λ) β Scale parameter: β Result from Minimization: β = 160 Linear approximation: λ(s1) = S1 x (β = 160, May, 2013) Linear approximation λ(s1) = S1 x (β = 175, April 2012) Extrapolate below 1 pe Move from bins to a continuous description Boris Bauermeister JG U 9

10 Monte Carlo Method: Determine acceptance Fiducial volume in current analysis: 34 kg ellipse (May, 2013) Choose random S1 [0, 17] λ(s1) Poisson distribution from λ Poiss(S2, λ, β) Random S2 Random drift time according the 34 kg volume in XENON100 Lowest electron life time Conservative estimation April 2012: τ = 374 µs May 2013: τ = 460 µs Conservative approach: Minimum electron lifetime Alternative: Choose a uniform time distribution: t = [0, 178] µs (April 2012) - Count how many events are accepted by the given S2 threshold for a random S1 137 Cs (β-,γ - source) Once a week Boris Bauermeister JG U 10

11 Latest results from neutron calibration: Pr e lim in ar y Take 34kg fiducial volume into account Reduce number of possible S2 events in the Monte Carlo procedure Increase in acceptance Higher minimum electron life time in May 2013 GO OD! Conclusion: Acceptance increased! Expected from the higher electron lifetime in May 2013 Tes t it! Question remains: Do we understand what we observe? Double check with an additional neutron simulation in XENON100 Boris Bauermeister JG U 11

12 Neutron simulation in XENON100 I: Model neutron interactions[1]: Input: Histograms of S2 for different S1 slices 241 AmBe spectrum Source measured at PTB (160 +/- 4 n/s) 1) Cuts: Radius < 145 mm Drift time: 5 µs to 30 µs Select: Single scatter neutrons Nuclear recoil cross-sections are calculated in a Geant4 simulation: XENON100 TPC and shield Resulting: S1/S2 from neutron interaction in the TPC Simulation shows good match with data down to 2 3 pe. 2) Analysis: 16 S1 slices: 1 17 pe with simulated S2 information Take τ from April 2012 Redo analysis chain: Poisson fit,... Important: Neutron simulations available for period April 2012! (for now) Verify previous used method to determine the S2 threshold and its acceptance with neutron simulation 1) E. Aprile et al. Phys. Rev. D 88, (2013) S2 Histogram from neutron simulation for S1 slice 7 8 pe Boris Bauermeister JG U 12

13 Pr e lim in a ry Neutron simulation in XENON100 II: Result Result: Neutron simulation shows good agreement with 241AmBe data set from April 2012! (< 0.4 %) If neutron simulation is prepared for later periods: Redo neutron simulation analysis to verify S2 acceptance for May 2013 data sets Boris Bauermeister JG U 13

14 What if... Calculate WIMP Acceptance I: Assumption: WIMPs interact like neutrons in XENON100: Neutrons: Single, Double, scatters are possible WIMPs: Only single scatters are assumed Boundary condition in the simulation: The detector threshold is valid for neutron and WIMP interactions 50 GeV WIMP recoil spectrum Apply Halo model WIMP distribution at earth Rate calculations based on J.D. Lewin, P.F. Smith[1] Single scatter WIMP spectrum: 200 WIMP masses are tested S1 energy range up to 30 pe (actual Profile Likelihood[2] analysis) WIMP simulation in XENON100 Estimate how many WIMPs are detected at a S2 threshold of 150 pe 1) Astroparticle Physics 6 (1998) ) E. Aprile et al., Astropart. Phys. 54 (2014) Boris Bauermeister JG U 14

15 What if... Calculate WIMP Acceptance II: Analysis: Count #WIMPs which pass S2 > 150 pe Normalize it to the total number of simulated WIMPs per WIMP mass WIMP mass dependent probability distributions Boris Bauermeister JG U 15

16 What if... Calculate WIMP Acceptance III: Evaluate WIMP acceptance: Take S1 analysis threshold into account: S1 = [3, 30] pe Test an ideal S1 measurement: S1 = [0, 30] pe WIMP mass dependent probability distributions Boris Bauermeister JG U 16

17 What if... Calculate WIMP Acceptance IV: Detector with a threshold of 3 pe has a maximum WIMP acceptance of ~ 80 % at mχ = 13 GeV Acceptances drops beyond maximum Higher WIMP mass interactions lead to S1 > 30 pe and hence higher acceptances are not taken into account here anymore Pr e lim in ar y The ideal detector has no S1 threshold 94 % WIMPs (mχ = 12 GeV) are detected WIMP acceptance developed from 2012 detector conditions Boris Bauermeister JG U 17

18 Pr e lim in ar y Pr e y in ar Pr el im lim in ar y Summary and Outlook: Compare: Simulated neutrons in XENON100 with April 2012 data WIMP acceptance developed from 2012 detector conditions Neutron simulation in XENON100 is in good agreement with the 241 AmBe data from April Energy threshold: S2th > 150 pe is applied! Method to estimate the neutron acceptance for low S1 energies for a given threshold is: Stable during different neutron calibrations Proven by a neutron simulation in XENON100 A WIMP simulation in XENON100 is testing the WIMP acceptance of the detector during Outlook: Neutron simulation with the detector configuration of 2013 Extend WIMP simulation to 2013 detector configuration Use WIMP acceptance in PL analysis Boris Bauermeister JG U 18

19 Backup I: Neutron acceptance 9 10 pe pe 5 6 pe 4 5 pe pe pe 8 9 pe pe 7 8 pe 3 4 pe pe 6 7 pe 2 3 pe 1 2 pe Overview S2 Histograms with Poisson fits for β = 160 (May, 2013): pe Boris Bauermeister JG U 19

20 Backup II: Neutron acceptance Expectation value: λ S2 P(S 2, α,β, λ)=α Poiss( β, λ) Scale parameter: β Mean value of χ2/ndf from 15 Poisson fits: Minimum β 60 1 = Boris Bauermeister JG U 20

21 Backup III: Neutron simulation in XENON100 Test β = [100, 200] in steps of 5: Compare <χ2/ndf> for each β Minimum β = 155 Boris Bauermeister JG U 21

22 Backup IV: Neutron simulation in XENON100 P(S 2, α,β, λ)=α Poiss( Expectation value: λ S2, λ) β Scale parameter: β Minimize χ2/ndf for parameter β Linear approximation: Neutron simulation: λ(s1) = S (β = 155) Data from April 2012: λ(s1) = S (β = 175) S2 loss with Monte Carlo Method Boris Bauermeister JG U 22

23 Backup V: AmBe souce/mc simulation: Data matching S2 spectrum: Best Fit Qy fits data Neutron calibration of XENON100 with AmBe Idea: Get a proper description of XENON100 by an improved simulation and test Ingredients: Measured AmBe source ( n/s) at the PTB/Germany Complete XENON100 description (detector + shield) Qy, Threshold, detection resolution and acceptance (S1) from XENON100 detector E= cs2 Y Qy How to do (I): Take direct measured Leff Reproduce S2 spectrum Best Fit Qy Conversion between Qy kevnr Qy conversion to deposited (NR) energy E. Aprile, M. Alfonsi, K. Arisaka et al., Phys. Rev. D 88, (2013) Boris Bauermeister JG U 23

24 Backup VI: AmBe souce/mc simulation: Data matching Best Fit Qy repreduce S1 spectrum How to do (II): Use Best Fit Qy Reproduce S1 spectrum Get a new Leff E= Fit the whole spectrum down to 2 PE (~5 kev) Leff from best fit matches the previous direct measurements Results of XENON100 remain unchanged using this Leff cs1 S ee ' LY L eff S nr Compare data and MC: Leff fits XENON100 E. Aprile, M. Alfonsi, K. Arisaka et al., Phys. Rev. D 88, (2013) Boris Bauermeister JG U 24