béla majorovits for the gerda collaboration ichep 2012, melbourne, australia, july 4-11 2012 1...

Béla Majorovits for the GERDA collaboration

1ICHEP 2012, Melbourne, Australia, July 4-11 2012


Status and plans with the GERDA experiment to probe the nature of neutrinos

Max-Planck-Institut für Physik, München, Germany

O U T L I N E

• 0νββ decay as a probe for neutrino mass and neutrino nature

• The GERDA experiment: Design and construction

• First results with enriched detectors

• Status and plans for phase II: new detectors

~100 physicists 19 Institutes 7 Countries



Neutrino oscillation experiments:Neutrinos have non vanishing rest mass!

invertednormal orhierarchy ?

what is the absolute mass scale?

The quest for the mass of neutrinos



DIRAC: ν=ν Majorana: ν=ν

Neutrinos could be their own anti-particles:

The quest for the nature of neutrinos



DIRAC: ν=ν Majorana: ν=ν

Massive Majorana neutrinos could be responsible for the Baryon asymmetry in the universe (via leptogenesis)

Neutrinos could be their own anti-particles:

The quest for the nature of neutrinos



• Neutrino is a Majorana particle

• Helicity flip can occur in the vertex

1/ = G(Q5,Z) |Mnucl|2 <mee>2

0νββ decay-rate

Phase space-factor

Matrix element

Effective Majorana Neutrino mass

Neutrinoless mode of double beta decay only possible if:

0νββ decay:

n

n

p

pW-

e-

e-

W-

n

n

p

pW-

e-

e-

W-

νe

νe νe= νe νl νr

Lepton flavor

violating: ΔL=2

Effective Majorana neutrino mass contributes to 0νββ decay rate:

2νββ decay:2nd order weak

standard modell

process

Neutrinoless double beta decay



Signature: Sharp peak at Q-value of the decay

2 neutrinos escape the detector undetected: continuous spectrum

Total energy of decay is deposited

within detector:

sharp peak

Peak height not to scale!

Non vanishing CP

violating Majorana

phases βi can supress

<mee> even to zero!

Effective Majorana neutrino mass:

<mee> = |Σ|Uei|2eiβimi|

Neutrinoless double beta decay


7ICHEP 2012, Melbourne, Australia, July 4-11 2012 Béla Majorovits 7

Mass of lightest neutrino [eV]

Eff

ecti

ve M

ajor

ana

neut

rion

mas

s [e

V] 1

10-1

10-2

10-3

10-4

10-4 10-3 10-2 10-1 10-4

Bilenky and Giunti arxiv:1203.5250

0νββ decay as a probe for neutrino mass and neutrino nature

Observation of 0νββ decay:

• Neutrino must have Majorana nature!

• Determination of absolute mass scale

• Mass hierarchy of Neutrinos

• Information on CP violating phases?

CP violating Majorana phases could be responsible

for Baryogenesis via Leptogenesis



Source = 76Ge = Detector High signal detection efficiency

Detector material very pure (zone refinement, Czochralski growth)

Very low intrinsic internal background

Very good energy resolution Background due to 2νββ decay negligible

Considerable experience Industrial production, improvements possible

Natural abundance of 76Ge 7,83% Enrichment necessary

The quest for the mass of neutrinos



Location: Hall A of LNGS, Assergi, Italy3500 mwe

9

Phase I: Use HdM and IGEX detectors

Phase II: Convert 37.5 kg of enriched germanium (87% 76Ge) into detectors

GERDA : design and construction



Clean room

Water tankwith HP waterand -veto

Detector array

Lock system

HP liquid Ar

Cryostat with internal Cu shield

10




phase I Detectors (from HdM and IGEX) after dismounting from cryostats:

ANG2: 2833g ANG3: 2391g ANG4: 2372g

ANG5: 2746g RG1: 2110g RG2: 2166g

Tot

al m

ass:

14.

63 k

gGERDA : design and construction

Detectors ANG1 and

RG3 started drawing leakage

current after installation



Full phase I infrastructure for deployment of 12 detectors (all HdM and IGEX plus reference detectors) completed in May 2011




GTF 112

ANG 2

ANG 1

ANG 3

ANG 5

RG 3

ANG 4

RG 1

RG 2

Installation of phase I detectors :

GERDA : Status of phase I



GERDA phase I of started on 1.11.11 !

Data in Energy RoI is blinded since

9th of Jan. 2012

Installation of phase I detectors :





Energy calibration of all detectors:

AN

G1 and R

G3 are N

OT

includ

ed



GERDA : Status of phase I Live time: 152.49 days

Enriched exposure: 6.104 kg∙yDuty cycle Nov. 2011 – May 2012: 78.3%




PRELIMINARY



GERDA : Status of phase I Background data for enriched detectors (red) and control

detectors with natural germanium abundance (blue):

Background index in energy RoI for enriched detectors: (2.0+0.6

-0.4) ∙ 10-2 cts/(kg yr keV)



Very pronounced structures for individual energy deposits

Improved multi site recognition efficiency by A/E parameter

BEGe for improved background recognition

we

igh

ing

po

ten

tial

e− h+

D. Budjas et al., JINST 4 P10007 (2009)

GERDA : Status and plans for phase II

curr

en

t

single site event: SSE

time [ns]

multi siteevent: MSE

curr

en

t

time [ns]

Background like event:

Signal like event:




5 working HPenrGe detectors available at LNGS

Transports in shielded

container, storage

underground

The voyage of the enriched germanium



Signal: Background:

To light detector

Liquid Argon

128nm scintillation light

Background rejection by detection of LAr scintillation light




Background rejection by detection of LAr scintillation light


Two solutions (supported by MC with light tracking):

SiPMs connected to fibres Low background PMTssimulations show: reduction of background to 0.001 cts/(kg yr kev) realistic



Background rejection by detection of LAr scintillation lightGERDA : Status and plans for phase II



Conclusions:• Observation of 0νββ decay would have far

reaching consequences

• GERDA phase I started on 1.11.11

• Design sensitivity of phase I nearly reached

• Observed 2νββ decay with unprecedented signal to noise ratio

• PSA on BEGe detectors and LAr instrumentation will reduce phase II background by order of magnitude

• First phase II BEenrGe detector crystals available at LNGS



BACKUP



EXO




Uncertainty of EXO 200 90% exclusion limit on 76Ge

due different to matrix element calculations



Plans for phase II: new detectorsBackground recognition powers of BEGes

standard

signals

n+

p+

n+ surface slow

pulses

Data taken with 90Y β-source n+ surface events

Identify surface events:

Low E-fields in “partially” dead layer

Slow pulses

Decrease A/E parameter

90Y source

n+ surfacepulse: NSP

curr

en

t

time [ns]



amplified current amplitude A

PCP

MSE

SSE band

BEGe

Plans for phase II: new detectorsBackground recognition powers of BEGes

p+

At p+ contact also e- are “visible”

Amax/E is increased

D. Budjas et al., JINST 4 P10007 (2009)

M. Agostini et al., JINST 6P03005 (2011)

228Th source

γ

béla majorovits for the gerda collaboration ichep 2012, melbourne, australia, july 4-11 2012 1...

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