Cosmogenic Backgrounds Prompt and delayed signals from muons���

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Cosmogenic Backgrounds for EXO-200 and nEXO Joshua Albert, on behalf of the EXO-200 collaboration

Neutrino 2014, Boston, U.S.A.

Neutrino-less Double Beta Decay (0νββ) Best chance to probe the nature of the neutrino

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Latest 0νββ Results Fit signal and background PDFs simultaneously in standoff���

distance and SS/MS energy to set limits on 0νββ

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References [1] J.B. Albert, et al, The EXO-200 Collaboration. Search for Majorana neutrinos with the first two

years of EXO-200 data. doi:10.1038/nature13432, also arXiv:1402.6956 [nucl-ex] [2] Stanley G. Prussin, et al. Gamma rays from thermal neutron capture in Xe-136. Phys.Rev.,

C16:1001–1009, 1977. ���63Cu and 65Cu data extracted from the ENSDF database, http://www.nndc.bnl.gov.

[3] S. Miyake. Rapporteur Paper On Muons And Neutrinos. 1973. 13th International Cosmic Ray Conference, Denver, CO, vol. 5, 1973, p. 3638. ���J. Beringer et al. Review of Particle Physics (RPP). Phys.Rev., D86:010001, 2012.

Muon Veto-tagged Dataset Neutron-enriched dataset in coincidence with muon veto panels

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Utilizing this technique in EXO-200 and beyond Independent 137Xe measurement, veto future cosmogenic backgrounds

Neutron Capture Gammas Modeling and Identifying Prompt Capture Signals���

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EXO-200 Signals and Backgrounds

EXO-200 Detector

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•  Look in muon veto rejected data. •  Select data with:���

•  Event sample is highly “neutron-

enriched”. •  Neutron capture γ lines visible. •  Use this data to validate

understanding of neutron captures and cosmogenic backgrounds.

•  4990 µs per veto, corresponds to ~0.14% of runtime.

Comparing Veto-tagged Data and MC Good shape/rate agreement based only on model and measured muon rate!

Comparing the veto-tagged data and MC spectra, we see remarkably good agreement. The MC PDFs are based on neutron capture PDFs, and normalized to the neutron capture predictions from our FLUKA simulation. The MC is normalized to the data livetime and measured muon flux at WIPP. No fitting was used in this comparison. The shape and rate agreement validates our MC model and demonstrates that EXO-200 can perform as a neutron detector.

2

Double-beta decay (ȕȕ)

Our favorite channels:

0Ȟȕȕ2Ȟȕȕ

Half-life (t1/2) can be relate to the effective Majorana mass (mȕȕ) that is a function of the light neutrino masses (mi)

First observed by EXO-200. This was the smallest NME ever directly observed.

2

Double-beta decay (ȕȕ)

Our favorite channels:

0Ȟȕȕ2Ȟȕȕ

Half-life (t1/2) can be relate to the effective Majorana mass (mȕȕ) that is a function of the light neutrino masses (mi)

First observed by EXO-200. This was the smallest NME ever directly observed.

(simple 0νββ mechanism)

2

Double-beta decay (ȕȕ)

Our favorite channels:

0Ȟȕȕ2Ȟȕȕ

Half-life (t1/2) can be relate to the effective Majorana mass (mȕȕ) that is a function of the light neutrino masses (mi)

First observed by EXO-200. This was the smallest NME ever directly observed.

2

Double-beta decay (ȕȕ)

Our favorite channels:

0Ȟȕȕ2Ȟȕȕ

Half-life (t1/2) can be relate to the effective Majorana mass (mȕȕ) that is a function of the light neutrino masses (mi)

First observed by EXO-200. This was the smallest NME ever directly observed.

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Background series

Best fit counts in ���

2-! SS ROI

232Th 16.0 238U 8.1 137Xe 7.0

90% limit from ML fit

t0⌫��1/2 > 1.9 · 1025yr90% sensitivity

t0⌫��1/2 > 1.1 · 1025yrMajorana mass limit

hmi�� < 190� 450 meV

Cluster multiplicity helps discriminate between βs and γs

VETO PANELS

DOUBLE-WALLED

CRYOSTAT

LXe VESSEL

LEAD SHIELDING

JACK AND FOOT

VACUUM PUMPS

FRONT END

ELECTRONICS

HV FILTER AND

FEEDTHROUGH

Figure 2. Cutaway view of the EXO-200 setup, with the primary subassemblies identified.

The outermost shielding layer, outside the outer vessel of the cryostat, consists of 25 cm oflead. The low-noise front end electronics are located outside of the lead shielding and are connectedto the detector through thin polyimide cables. This choice trades some increased noise for thesimplicity and accessibility of room temperature, conventional construction electronics.

A cosmic-ray veto counter made of plastic scintillators surrounds the cleanroom housing therest of the detector. EXO-200 is located at a depth of 1585 m water equivalent [21] in the WasteIsolation Pilot Plant (WIPP) near Carlsbad, New Mexico (32�22’30”N 103�47’34”W).

2.2 Design sensitivity and estimated backgrounds

While the measured performance of EXO-200 utilizing substantial low-background and calibrationdata sets will be the subject of a future paper, here we provide sensitivity figures assuming designparameters for the detector performance and background. Initial data taking roughly confirms thevalidity of such parameters. Using the expected energy resolution of sE/E = 1.6% at the 136Xeend point, EXO-200 was designed to reach a sensitivity of T 0nbb

1/2 = 6.4⇥ 1025 yr (90% C.L.) intwo years of live time, should the 0nbb be beyond reach. 0nbb is defined by a ±2s windowaround the end-point and 40 background events are expected to accumulate in such a window intwo years. This estimate was made using a fiducial mass of 140 kg (200 kg with 70% efficiency),while the final detector design has 110 kg of active Xe, requiring a longer time to reach the samesensitivity. The T1/2 limit above corresponds to a 90% C.L. Majorana mass sensitivity of 109 meV(135 meV) using the QRPA [22] (NSM [23]) matrix element calculation.

– 5 –

Liquid xenon time projection chamber

enriched to 80% 136Xe

Th calibration source data

s)µTime since veto (0 5000 10000 15000 20000 25000

Ener

gy (M

eV)

1

2

3

4

5

6

7

8

9

10

single site

multi site

1H n-capture (2.22 MeV)

136Xe n-capture (4.025 MeV)

Non-cosmogenicevents

(232Th, 2νββ, etc.)

Energy (keV)2000 3000 4000 5000 6000 7000 8000

Even

ts/1

00 k

eV

1

10

210DataXenonHFECopperSum

Multi-Site Veto-Tagged Events

Energy (keV)2000 3000 4000 5000 6000 7000 8000

Even

ts/1

00 k

eV-110

1

10

DataXenonHFECopperSum

Single-Site Veto-Tagged Events•  Analytic µ energy

and angle distributions [3]

•  Muon rate from TPC muon measurement (unpublished so far)

•  FLUKA MC package for neutron production, transport, & capture

•  Geant4 for capture gammas and detector response

Energy (keV)2000 3000 4000 5000 6000 7000 8000

SS C

ount

s/50

keV

-210

-110

1

10 DataCu + HFE + LXeCu

HFELXe

Energy (keV)2000 3000 4000 5000 6000 7000 8000

MS

Cou

nts/

50 k

eV

-110

1

10

DataCu + HFE + LXeCu

HFELXe

•  Veto-tagged data fits to 0.46 ± 0.15 136Xe captures/day/FV after efficiency and livetime corrections.

•  Corresponds to 6.5 ± 2.1 137Xe events in 2-! SS ROI. Consistent with low background fit of 7.0 events.

46 cm

130 cm

nEXO NSAC subcommittee, Washington 24 Feb 2014 38

nEXO- 5 tonnes of enrXe: entirely cover inverted

hierarchy (more later)- LXe TPC “as similar to EXO-200 as possible”- Provide access ports for a possible later

upgrade to Ba tagging

Î A unique combination of conservative and aggressivedesign with important upgrade paths as desirablefor a large experiment

nEXO: ~5 ton LXe, next-gen R&D + design phase

•  Coincidence of muon veto and 136Xe ���n-capture signal can be used to start a long (~20 min.) veto to reject 137Xe decay.

•  Requires good understanding of capture signal to reduce livetime loss.

•  Applicable for both EXO-200 and nEXO. •  Important for nEXO depth requirements. •  Passive xenon self-shielding in nEXO

greatly improves γ rejection for inner volume, but more penetrating neutrons require this active technique.

EXO-200: ���175 kg LXe

136Xe + n → 137Xe → 137Cs + e- + νe���

t1/2 = 3.8 min Qβ = 4.2 MeV

Beta decay with ���Qβ > Q0νββ : critical background!

One chance to tag this background: identify the prompt capture signal!

Nuclear de-excitation by gamma emission from excited capture state to ground state.

E (keV)0 1000 2000 3000 4000 5000 6000 7000 8000

Cou

nts/

20 k

eV

0

0.5

1

1.5

2

2.5

3

3.5

Total PDFs MS veto-taggedTotal PDFs MS veto-tagged

MC PDFs for different neutron captures

Cu TPC/Cryostat (63,65Cu) HFE Cryogenic Fluid (1H)

Liquid Xenon (136Xe)

Excited state 137Xe* from n-capture

…Ground ���state 137Xe

Gamma cascade to…

Reduce cosmic ray backgrounds with: •  Depth (1585 m.w.e. at WIPP) •  Passive shielding (stop e-, p, γ) •  Active muon veto (reject prompt signals) Muon-induced neutrons can still capture���and produce long-lived radioisotopes.

Observation of 0νββ would: •  Determine the Dirac/Majorana nature of the neutrino

•  •  Demonstrate non-conservation ���

of lepton number (beyond SM)

•  •  Reveal the absolute ν mass

•  ���

Signal of 0νββ is a beta-like���mono-energetic peak at the Q-value (2458 keV for 136Xe). ���

Non-observation can be used to set limits on Majorana ���neutrino mass.

⌫ = ⌫?

mν

2∝ T1/2

0νββ( )−1

�L 6= 0?

Single-site events (SS)

Multiple-site events (MS)

β-like γ-like

SS MS

Developed custom generators in Geant4 based on nuclear structure data (ENSDF, others) [2].

see Nature or arXiv [1]

•  Maximum likelihood fit to 99.8 kg-yr 136Xe exposure.

•  137Xe background (highlighted in dark red) fits to ~22.5% of total background in ���2-! SS region of interest.