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Double Beta Decay GERDA UZH Contribution Outlook
The GERDA Experiment
Francis Froborg
University of Zurich
CHIPP 2008, Lausanne08. September 2008
Francis Froborg The GERDA Experiment
Double Beta Decay GERDA UZH Contribution Outlook
Double Beta Decay
2νββ
(Z ,A)→ (Z + 2,A) + 2e− + 2νe
∆L = 0˛T 2ν
1/2
˛−1= G2ν(Qββ ,Z) |M2ν |2 ∼ 1020y
3
!"
"
n
n p
p
e
e
!
W
W
"#
n
n p
p
e
eW
W
x
FIG. 2 Feynman Diagrams for !!(2") (left) and !!(0")(right).
where G0!(Q"" , Z) is the phase space factor for the emis-sion of the two electrons, M0! is another nuclear matrixelement, and !m""" is the “e!ective” Majorana mass ofthe electron neutrino:
!m""" # |!
k
mkU2ek| . (3)
Here the mk’s are the masses of the three light neutrinosand U is the matrix that transforms states with well-defined mass into states with well-defined flavor (e.g.,electron, mu, tau). Equation 2 gives the !!(0") rateif the exchange of light Majorana neutrinos with lefthanded interactions is responsible. Other mechanismsare possible (see Sections III and IV.D), but they requirethe existence of new particles and/or interactions in ad-dition to requiring that neutrinos be Majorana particles.Light-neutrino exchange is therefore, in some sense, the“minima” mechanism and the most commonly consid-ered.
That neutrinos mix and have mass is now acceptedwisdom. Oscillation experiments constrain U fairly well
— Table I summarizes our current knowledge — but theydetermine only the di!erences between the squares of themasses mk (e.g., m2
2 $m21) rather than the masses them-
selves. It will turn out that !!(0") is among the bestways of getting at the masses (along with cosmology and!-decay measurements), and the only practical way toestablish that neutrinos are Majorana particles.
To extract the e!ective mass from a measurement, itis customary to define a nuclear structure factor FN #G0!(Q"" , Z)|M0! |2m2
e, where me is the electron mass.(The quantity FN is sometimes written as Cmm.) Thee!ective mass !m""" can be written in terms of the cal-culated FN and the measured half life as
!m""" = me[FNT 0!1/2]
!1/2 . (4)
The range of mixing matrix values given below in Ta-ble I, combined with calculated values for FN , allow usto estimate the half-life a given experiment must be ableto measure in order to be sensitive to a particular valueof !m""". Published values of FN are typically between10!13 and 10!14 y!1. To reach a sensitivity of !m"""%0.1 eV, therefore, an experiment must be able to observea half life of 1026 $ 1027 y. As we discuss later, at thislevel of sensitivity an experiment can draw importantconclusions whether or not the decay is observed.
The most sensitive limits thus far are from theHeidelberg-Moscow experiment: T 0!
1/2(76Ge) & 1.9 '
1025 y (Baudis et al., 1999), the IGEX experiment:T 0!
1/2(76Ge) & 1.6 ' 1025 y (Aalseth et al., 2002a, 2004),
and the CUORICINO experiment T 0!1/2(
130Te) & 3.0 '1024 y (Arnaboldi et al., 2005, 2007). These experimentscontained 5 to 10 kg of the parent isotope and ran forseveral years. Hence, increasing the half-life sensitivityby a factor of about 100, the goal of the next generationof experiments, will require hundreds of kg of parent iso-tope and a significant decrease in background beyond thepresent state of the art (roughly 0.1 counts/(keV kg y).
It is straightforward to derive an approximate an-alytical expression for the half-life to which an ex-periment with a given level of background is sensi-tive (Avignone et al., 2005):
T 0!1/2(n#) =
4.16 ' 1026y
n#
" #a
W
#
$
Mt
b"(E). (5)
Here n# is the number of standard deviations correspond-ing to a given confidence level (C.L.) — a CL of 99.73%corresponds to n# = 3 — the quantity # is the event-detection and identification e#ciency, a is the isotopicabundance, W is the molecular weight of the source ma-terial, and M is the total mass of the source. The in-strumental spectral-width "(E), defining the signal re-gion, is related to the energy resolution at the energyof the expected !!(0") peak, and b is the specific back-ground rate in counts/(keV kg y), where the mass is that
0νββ
(Z ,A)→ (Z + 2,A) + 2e−
∆L = 2˛T 0ν
1/2
˛−1= G0ν(Qββ ,Z) |M0ν |2 〈m2
ββ〉 ∼ 1025y
3
!"
"
n
n p
p
e
e
!
W
W
"#
n
n p
p
e
eW
W
x
FIG. 2 Feynman Diagrams for !!(2") (left) and !!(0")(right).
where G0!(Q"" , Z) is the phase space factor for the emis-sion of the two electrons, M0! is another nuclear matrixelement, and !m""" is the “e!ective” Majorana mass ofthe electron neutrino:
!m""" # |!
k
mkU2ek| . (3)
Here the mk’s are the masses of the three light neutrinosand U is the matrix that transforms states with well-defined mass into states with well-defined flavor (e.g.,electron, mu, tau). Equation 2 gives the !!(0") rateif the exchange of light Majorana neutrinos with lefthanded interactions is responsible. Other mechanismsare possible (see Sections III and IV.D), but they requirethe existence of new particles and/or interactions in ad-dition to requiring that neutrinos be Majorana particles.Light-neutrino exchange is therefore, in some sense, the“minima” mechanism and the most commonly consid-ered.
That neutrinos mix and have mass is now acceptedwisdom. Oscillation experiments constrain U fairly well
— Table I summarizes our current knowledge — but theydetermine only the di!erences between the squares of themasses mk (e.g., m2
2 $m21) rather than the masses them-
selves. It will turn out that !!(0") is among the bestways of getting at the masses (along with cosmology and!-decay measurements), and the only practical way toestablish that neutrinos are Majorana particles.
To extract the e!ective mass from a measurement, itis customary to define a nuclear structure factor FN #G0!(Q"" , Z)|M0! |2m2
e, where me is the electron mass.(The quantity FN is sometimes written as Cmm.) Thee!ective mass !m""" can be written in terms of the cal-culated FN and the measured half life as
!m""" = me[FNT 0!1/2]
!1/2 . (4)
The range of mixing matrix values given below in Ta-ble I, combined with calculated values for FN , allow usto estimate the half-life a given experiment must be ableto measure in order to be sensitive to a particular valueof !m""". Published values of FN are typically between10!13 and 10!14 y!1. To reach a sensitivity of !m"""%0.1 eV, therefore, an experiment must be able to observea half life of 1026 $ 1027 y. As we discuss later, at thislevel of sensitivity an experiment can draw importantconclusions whether or not the decay is observed.
The most sensitive limits thus far are from theHeidelberg-Moscow experiment: T 0!
1/2(76Ge) & 1.9 '
1025 y (Baudis et al., 1999), the IGEX experiment:T 0!
1/2(76Ge) & 1.6 ' 1025 y (Aalseth et al., 2002a, 2004),
and the CUORICINO experiment T 0!1/2(
130Te) & 3.0 '1024 y (Arnaboldi et al., 2005, 2007). These experimentscontained 5 to 10 kg of the parent isotope and ran forseveral years. Hence, increasing the half-life sensitivityby a factor of about 100, the goal of the next generationof experiments, will require hundreds of kg of parent iso-tope and a significant decrease in background beyond thepresent state of the art (roughly 0.1 counts/(keV kg y).
It is straightforward to derive an approximate an-alytical expression for the half-life to which an ex-periment with a given level of background is sensi-tive (Avignone et al., 2005):
T 0!1/2(n#) =
4.16 ' 1026y
n#
" #a
W
#
$
Mt
b"(E). (5)
Here n# is the number of standard deviations correspond-ing to a given confidence level (C.L.) — a CL of 99.73%corresponds to n# = 3 — the quantity # is the event-detection and identification e#ciency, a is the isotopicabundance, W is the molecular weight of the source ma-terial, and M is the total mass of the source. The in-strumental spectral-width "(E), defining the signal re-gion, is related to the energy resolution at the energyof the expected !!(0") peak, and b is the specific back-ground rate in counts/(keV kg y), where the mass is that
Francis Froborg The GERDA Experiment
Double Beta Decay GERDA UZH Contribution Outlook
Signature
Measuring the energy of both electrons
2νββ: Continuous energy spectrum
0νββ: Sharp peak at Q value of decay
Q = Ee1 + Ee2 − 2me
Background reduction essential because of small half lives
Schechter & Valle (1982): Measuring 0νββ ⇒ ν Majorana particle
Francis Froborg The GERDA Experiment
Double Beta Decay GERDA UZH Contribution Outlook
Heidelberg-Moscow ExperimentThe Claim
5 HPGe crystals with 71.7 kg y
Peak at Q value:
T 0ν1/2 = 1.2× 1025y (4σ)
〈mββ〉 = 0.44 eV
Problem: Confidence depends on backgroundmodel and energy region selectedfor analysis
⇒ New experiments with higher sensitivityneeded
Evidenz für den Neutrinolosen Doppelbetazerfall?
• Peak beim Q-Wert des Zerfalls
• Periode 1990-2003: 28.8 ± 6.9 Ereignisse
• Periode 1995-2003: 23.0 ± 5.7 Ereignisse
! 4.1- 4.2 ! Evidenz
• ‘Evidenz’ unklar
! muss mit neuen, empfindlicheren Experimenten getestet werden
T1/2
0!= 1.2 "10
25yr
214Bi2010.7 keV 214Bi
2016.2 keV
2021.8 keV
214Bi2052.9 keV
0nußß decay?
?
H.V.Klapdor-Kleingrothaus et al., Phys. Lett. B 586 (2004) 198
m!e = 0.44 eV (0.3"1.24) eV
Francis Froborg The GERDA Experiment
Double Beta Decay GERDA UZH Contribution Outlook
The GERmanium Detector Array (GERDA)
Naked high purity 76Ge crystals placed in LAr
Phase I
8 Hd-Mo & IGEX crystals (15 kg y)
Background goal: 10−2 cts/kg/keV/y
⇒ T 0ν1/2
> 2.0× 1025 y
〈mββ〉< 0.33 eV
Phase II
Phase I + 14 new segmented crystals(100 kg y)
Background goal: 10−3 cts/kg/keV/y
⇒ T 0ν1/2
> 14× 1025 y
〈mββ〉< 0.13 eV
Empfindlichkeit von Doppelbetaexperimenten
90
% p
rob
. lo
we
r li
mit
T1/2
[1
025 y
r]Exposure [kg yr]
B = 10-2 Ereignisse/(kg keV yr)
B = 10-3 Ereignisse/(kg keV yr)
Klapdor-Kleingrothaus HM Signal
Francis Froborg The GERDA Experiment
Double Beta Decay GERDA UZH Contribution Outlook
The Collaboration
ITALYINFN LNGS, AssergiUniv. di Milano Biocca eINFNUniv, di Padova e INFN
RUSSIAINR, Moscow
ITEP Physics, MoscowKurchatov Institute,
MoscowJINR Dubna
GERMANYMPI HeidelbergMPI MunchenTU DresdenUniversitat Tubingen
POLANDJagiellonian University,
Cracow
BELGIUMIRMM, Geel
SWITZERLANDUniversity of Zurich
Francis Froborg The GERDA Experiment
Double Beta Decay GERDA UZH Contribution Outlook
The Calibration SystemUZH Contribution
Boundary Conditions
Fixed positions of the sources
Maximum radius ∼ 4cm
Minimum weight ∼ 4kg
Park position in the lock of the detector
Goals
Sort and strength of calibration sources
Collimator material and geometry
Efficiency of energy deposition in eachdetector
Efficiency of pulse shape analysis
Francis Froborg The GERDA Experiment
Double Beta Decay GERDA UZH Contribution Outlook
Progress & Plans
Status of Calibration
Monte Carlo Simulations with MaGe
Simulations with different collimator geometriesand the naked source in parking position running
Single detector analysis
Most promising: 228Th or 56Co as source and Cuor W as collimator material
Future Plans
Comparison of Monte Carlo results withmeasurements at a test facility in Zurich
Installing and testing system at LNGS
228-Th, W, new geometry
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1
Francis Froborg The GERDA Experiment
Double Beta Decay GERDA UZH Contribution Outlook
Phase II detectorsThe Zurich Test Facility
18 fold segmentation: 6(φ)× 3(z)
Possibility to distinguish betweensingle site events (signal) and multisite events (background)
First spectra taken with core and all18 segments
Test facility in Zurich underconstruction
GERDA PHASE II
• 14 76Ge, 18-fach segmentierte Detektoren + 8 Phase-I Detektoren, 40 kg
• Messzeit = 150 kg yr, Untergrund: B =10-3 Ereignisse/(kg keV yr)
!Erreichbare Empfindlichkeit:
T1/2
0!> 15 "10
25yr
m!e < 0.11eV
!! - Ereignis Gamma-Untegrund
MSE, " ~ cmSSE, " ~ mm
Segmentierung: 6 (#) x 3 (z)
durch Segmentierung:
Unterscheidung von Einzeln- (SSE) und
Mehrfach- (MSE) Wechselwirkungen
$
SSE
GERDA PHASE II
• 14 76Ge, 18-fach segmentierte Detektoren + 8 Phase-I Detektoren, 40 kg
• Messzeit = 150 kg yr, Untergrund: B =10-3 Ereignisse/(kg keV yr)
!Erreichbare Empfindlichkeit:
T1/2
0!> 15 "10
25yr
m!e < 0.11eV
!! - Ereignis Gamma-Untegrund
MSE, " ~ cmSSE, " ~ mm
Segmentierung: 6 (#) x 3 (z)
durch Segmentierung:
Unterscheidung von Einzeln- (SSE) und
Mehrfach- (MSE) Wechselwirkungen
$
MSE
GERDA PHASE II
• 14 76Ge, 18-fach segmentierte Detektoren + 8 Phase-I Detektoren, 40 kg
• Messzeit = 150 kg yr, Untergrund: B =10-3 Ereignisse/(kg keV yr)
!Erreichbare Empfindlichkeit:
T1/2
0!> 15 "10
25yr
m!e < 0.11eV
!! - Ereignis Gamma-Untegrund
MSE, " ~ cmSSE, " ~ mm
Segmentierung: 6 (#) x 3 (z)
durch Segmentierung:
Unterscheidung von Einzeln- (SSE) und
Mehrfach- (MSE) Wechselwirkungen
$
GERDA PHASE II
• 14 76Ge, 18-fach segmentierte Detektoren + 8 Phase-I Detektoren, 40 kg
• Messzeit = 150 kg yr, Untergrund: B =10-3 Ereignisse/(kg keV yr)
!Erreichbare Empfindlichkeit:
T1/2
0!> 15 "10
25yr
m!e < 0.11eV
!! - Ereignis Gamma-Untegrund
MSE, " ~ cmSSE, " ~ mm
Segmentierung: 6 (#) x 3 (z)
durch Segmentierung:
Unterscheidung von Einzeln- (SSE) und
Mehrfach- (MSE) Wechselwirkungen
$Francis Froborg The GERDA Experiment
Double Beta Decay GERDA UZH Contribution Outlook
Status of the Experiment!"#$ %$&'(')*+, -+$+.,/'0/+12/34567/4892(*:2;+/
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Francis Froborg The GERDA Experiment
Double Beta Decay GERDA UZH Contribution Outlook
Timetable
Phase I Phase II
September 2008 Purification of enriched GeJanuary 2009 Myon veto Tests for crystal growing
(IKZ, Berlin)April 2009 Clean room and lockJuli 2009 Start data taking Natural Ge test detectors
December 2009 Crystal growing of enriched GeMarch 2010 76Ge detectors (Canberra, France)
Juli 2010 Start data taking
Francis Froborg The GERDA Experiment