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Year:2016
PhysicsreachoftheXENON1Tdarkmatterexperiment
XENONCollaboration;Aprile,E;Agostini,F;Alfonsi,M;etal;Baudis,L;Franco,D;Galloway,
M;Kessler,G;Kish,A;Mayani,D;Pakarha,P;Piastra,F;Wei,Y;Wulf,J
Abstract:TheXENON1TexperimentiscurrentlyinthecommissioningphaseattheLaboratoriNazion-
alidelGran Sasso,Italy.Inthisarticlewestudytheexperiment’sexpectedsensitivitytothespin-
independentWIMP-nucleoninteractioncrosssection,basedonMonteCarlopredictionsoftheelectronic
andnuclearrecoilbackgrounds.Thetotalelectronicrecoilbackgroundin1tonneducialvolumeand(1,
12)keVelectronicrecoilequivalentenergyregion,beforeapplyinganyselectiontodiscriminatebetween
electronicandnuclearrecoils,is(1.80±0.15)·10(−)(4)(kg·day·keV)(−)(1),mainlyduetothedecay
of(222)Rndaughtersinsidethexenontarget.Thenuclearrecoilbackgroundinthecorrespondingnu-
clearrecoilequivalentenergyregion(4,50)keV,iscomposedof(0.6±0.1)(t·y)(−)(1)fromradiogenic
neutrons,(1.8±0.3)·10(−)(2)(t·y)(−)(1)fromcoherentscatteringofneutrinos,andlessthan0.01
(t·y)(−)(1)frommuon-inducedneutrons.ThesensitivityofXENON1TiscalculatedwiththeProle
LikelihoodRatiomethod,afterconvertingthedepositedenergyofelectronicandnuclearrecoilsintothe
scintillationandionizationsignalsseeninthedetector. Wetakeintoaccountthesystematicuncertain-
tiesonthephotonandelectronemissionmodel,andontheestimationofthebackgrounds,treatedas
nuisanceparameters.ThemaincontributioncomesfromtherelativescintillationeciencyScriptL(e),
whichaectsboththesignalfromWIMPsandthenuclearrecoilbackgrounds.Aftera2ymeasurement
in1tducialvolume,thesensitivityreachesaminimumcrosssectionof1.6·10(−)(47)cm(2)atm()
=50GeV/c(2).
DOI:https://doi.org/10.1088/1475-7516/2016/04/027
PostedattheZurichOpenRepositoryandArchive,UniversityofZurich
ZORAURL:https://doi.org/10.5167/uzh-129746
JournalArticle
Originallypublishedat:
XENONCollaboration;Aprile,E;Agostini,F;Alfonsi,M;etal; Baudis,L;Franco,D;Galloway,M;
Kessler,G;Kish,A;Mayani,D;Pakarha,P;Piastra,F;Wei,Y;Wulf,J(2016).Physicsreachofthe
XENON1Tdarkmatterexperiment.JournalofCosmologyandAstroparticlePhysics:1-36.
DOI:https://doi.org/10.1088/1475-7516/2016/04/027
Flux Modulations seen by the Muon Veto of the Gerda Experiment
Gerda collaboration
1,∗
,
M. Agostini
o
, M. Allardt
d
, A.M. Bakalyarov
m
, M. Balata
a
, I. Barabanov
k
, N. Barros
d,8
, L. Baudis
s
, C. Bauer
g
,
N. Becerici-Schmidt
n
, E. Bell otti
h,i
, S. Belogurov
l,k
, S.T. Belyaev
m
, G. Benato
s
, A. Bettini
p,q
, L. Bezrukov
k
, T. Bode
o
,
D. Borowicz
1,e
, V. Brudanin
e
, R. Brugnera
p,q
, A. Caldwell
n
, C. Cattadori
1
, A. Chernogorov
l
, V. D’Andrea
a
,
E.V. Demidova
l
, A. di Vacri
a
, A. Domula
d
, E. Doroshkevich
k
, V. Egorov
e
, R. Falkenstein
r
, O. Fedorova
k
, K. Freund
r
,
N. Frodyma
c
, A. Gangapshev
k,g
, A. Garfagnini
p,q
, P. Grabmayr
r
, V. Gurentsov
k
, K. Gusev
m,e,o
, A. Hegai
r
, M. Heisel
g
,
S. Hemmer
p,q
, W. Hofmann
g
, M. Hult
f
, L.V. Inzhechik
k,3
, L. Ioannucci
a
, J. Janicskó Csáthy
o
, J. Jochum
r
, M. Junker
a
,
V. Kazalov
k
, T. Kihm
g
, I.V. Kirpichnikov
l
, A. Kirsch
g
, A. Kli menko
g,e,4
, M. Knapp
r,5
, K.T. Knöpfle
g
, O. Kochetov
e
,
V.N. Kornoukhov
l,k
, V.V. Kuzminov
k
, M. Laubenstein
a
, A. Lazzaro
o
, V.I. Lebedev
m
, B. L ehnert
d
, H.Y. Liao
n
,
M. Lindner
g
, I. Lippi
q
, A. Lubashevskiy
g,e
, B. Lubsandorzhiev
k
, G. Lutter
f
, C. Macolino
a
, B. M ajorovits
n
,
W. Maneschg
g
, E. Medinaceli
p,q
, M. Misiaszek
c
, P. Moseev
k
, I. Nemchenok
e
, D. Palioselitis
n
, K. Panas
c
, L. Pandola
b
,
K. Pelczar
c
, A. Pullia
j
, S. Riboldi
j
, F. Ritter
r
, N. Rumyantseva
e
, C. Sada
p,q
, M. Salathe
g
, C. Schmitt
r
, B. Schneider
d
,
S. Schönert
o
, J. Schreiner
g
, A.-K. S chütz
r
, O. Schulz
n
, B. Schwingenheuer
g
, O. Selivanenko
k
, E. Sh evchik
e
,
M. Shirchenko
m,e
, H. Simgen
g
, A. Smolnikov
g
, L. Stanco
1
, M. Stepaniuk
g
, H. Strecker
g
, L. Vanhoefer
n
, A.A. Vasenko
l
,
A. Veresnikova
k
, K. von Sturm
p,q
, V. Wagner
g
, M. Walter
s
, A. Wegmann
g
, T. Wester
d
, H. Wilsenach
d
, M. Wojcik
c
,
E. Yanovich
k
, I. Zhitnikov
e
, S.V. Zhukov
m
, D. Zinatulina
e
, K. Zuber
d
, G. Zuzel
c
a
INFN Laboratori Nazionali del Gran Sasso, LNGS, and Gran Sasso Science Institute, GSSI, Assergi, Italy
b
INFN Laboratori Nazionali del Sud, Catania, Italy
c
Institute of Physics, Jagiellonian University, Cracow, Poland
d
Institut für Kern- und Teilchenphysik, Technische Universität Dresden, Dresden, Germany
e
Joint Institute for Nuclear Research, Dubna, Russia
f
Institute for Reference Materials and Measurements, Geel, Belgium
g
Max-Planck-Institut für Kernphysik, Heidelberg, Germany
h
Dipartimento di Fisica, Università Milano Bicocca, Milan, Italy
i
INFN Milano Bicocca, Milan, Italy
j
Dipartimento di Fisica, Università degli Studi di Milano e INFN Milano, Milan, Italy
k
Institute for Nuclear Research of the Russian Academy of Sciences, Moscow, Russia
l
Institute for Theoretical and Experimental Physics, Moscow, Russia
m
National Research Centre “Kurchatov Institute”, Moscow, Russia
n
Max-Planck-Institut für Physik, Munich, Germany
o
Physik Department and Excellence Cluster Universe, Technische Universität München, Munich, Germany
p
Dipartimento di Fisica e Astronomia dell‘Università di Padova, Padova, Italy
q
INFN Padova, Padova, Italy
r
Physikalisches Institut, Eberhard-Karls Universität Tübingen, Germany
s
Physik Institut der Universität Zürich, Zürich, Switzerland
Abstract
The Gerda experiment at Lngs of INFN is equipped with an active muon veto. The main part of the system is a
water Cherenkov veto with 66 PMTs in the water tank surrounding the Gerda cryostat. The muon flux recorded by
this veto shows a seasonal modulation. Two effects have been identified which are caused by secondary muons from the
Cngs neutrino beam (2.2 %) and a temperature modulation of the atmosphere (1.4 %). A mean cosmic muon rate of
I
0
µ
= (3.477 ± 0.002
stat
± 0.067
sys
) × 10
−4
/(s·m
2
) was found in good agreement with other experiments at Lngs at a
depth of 3500 meter water equivalent.
Keywords: water Cherenkov detector, underground experiment, cosmic rays, muon interaction,
∗
Correspondence: gerda-eb@mpi-hd.mpg.de
1
Laboratori Nazionali del Gran Sasso, Assergi, Italy
2
present address: maxment GmbH, Germany
3
also at: Moscow Inst. of Physics and Technology, Moscow, Rus-
sia
4
also at: Int. Univ. for Nature, Society and Man “Dubna”,
Dubna, Russia
5
present address: Areva, France
6
present address: LAL, Université Paris-Saclay, Orsay, France
7
present address: Bosch GmbH, Germany
8
present address: Dept. of Physics and Astronomy, U. of Penn-
sylvania, Philadelphia, Pennsylvania, USA
Preprint submitted to Astroparticle Physics January 25, 2016
arXiv:1601.06007v1 [physics.ins-det] 22 Jan 2016
1. Introduction
The Gerda (Germanium Detector Array) experiment
is searching for the neutrinoless double-beta (0νββ) decay
of
76
Ge [1, 2]. It is located in Hall A of the underground
laboratory Laboratori Nazionali del Gran Sasso (Lngs) of
Infn at a depth of 3500 meter water equivalent (m.w.e.).
In order to search effectively for such a rare process as
the 0νββ decay Gerda needs to be equipped with a ded-
icated veto systems which tags muons passing the exper-
iment [1, 3, 4]. Here we report about the observed rates
including an annual modulation of the latter during the
period 2010–2013 which encompasses Phase I of Gerda.
Other underground experiments have observed similar an-
nual modulations of their rates, either due to the muon
flux [5] or of other origin [6].
A full explanation of the muon rate is important to
assure that the systematics of the experiment are fully
understood, in particular when aiming for reduced back-
grounds in future phases. A particular problem could be
the generation of unstable isotopes by the muons directly
or through the secondary neutron flux. Thus, the more
obvious sources of backgrounds must be understood. The
present results serve also as cross checks to previous or fu-
ture data sets on muon fluxes in underground laboratories.
2. Modulations
The hardware of the muon veto worked very reliable
and stable. The overall muon rate of the veto is ob-
served to be modulated by two different sources. Firstly,
the majority of the detectable muons are produced cos-
mogenically [7]. Their spectrum and angular distribution
within the halls are both altered by the profile of the rock
overburden and have been measured for Lngs with high
precision [5]. These muons have an average energy of
hE
µ
i = 270 GeV. Due to seasonal temperature changes
in the atmosphere the mean muon energy changes over
the year and thus the muon flux at Lngs.
Secondly, an artificial source for muons was the Cern
Neutrinos to Gran Sasso (Cngs) neutrino beam [8] in the
period 2008–2012 serving the Opera experiment [9] for
the search of ν
µ
→ ν
τ
oscillations. This ν
µ
beam can cre-
ate muons by charged current reactions along its 730 km
long path. Thus, an additional muon flux is expected dur-
ing any Cngs beam line operation. As the beam is not
operated continuously but pauses in the winter months,
the additional flux takes the form of an annual modula-
tion.
Both effects can be described with high precision. The
parameters for the atmospheric muon generation are pre-
sented in this work which agree well with other experi-
ments at Lngs.
3. Instrumentation
The muon veto consists of two parts. The Gerda wa-
ter tank is instrumented with 66 photomultipliers of 8” size
Ge
detector
array
2m
water tank
cryostat
5m
66 PMT
Cherenkov
veto
neck
clean room
plastic scintillator veto
Figure 1: Sketch of the Gerda experiment [1, 3].
which detect the Cherenkov light of passing muons [1, 3]
(Fig. 1). The germanium crystals are lowered through the
“neck” from the clean room into the liquid argon (LAr)
cryostat for normal operation. This “neck” region is less
monitored by the Cherenkov veto. Thus, it is covered in
addition by a 4×3 m
2
layer of plastic scintillators equipped
with PMTs on top of the Gerda clean room. In the anal-
ysis shown here the standard Gerda muon veto analysis
cuts were performed. As described in Ref. [3] a 18 p.e.
(photo electron) cut on the Cherenkov data or a valid com-
bination of panels are needed to create a trigger. The sta-
bility of the rates and of the light output of veto system
was checked periodically.
This muon veto data set contains a period of 806 days
from November 2010 to July 2013 that includes also a pe-
riod before Phase I. Particularly during Phase I of the
Gerda experiment the veto system ran continuously sta-
ble and reliable.
4. Influence of the CNGS beam
The Cern SPS delivered proton bunches with an en-
ergy of 400 GeV that hit a carbon target in the Cngs
beam line [8]. Actually, each SPS extraction consists of
two proton bunches which are 10.5 µs wide and 50 ms
apart. Normally, the extraction is repeated every 6 s. Pi-
ons and kaons from the collision products are focused on
a decay line pointed towards Lngs. These particles can
decay according to π
+
/K
+
→ µ
+
+ ν
µ
. Muon detectors
at the end of the decay line record the µ
+
which can be
correlated with the ν
µ
intensity. The primary µ
+
will be
2
pot/d]
16
[10
CNGS
I
0
10
20
30
40
0
10
20
30
40
mean
pot/d
16
10×18.2
GERDA 15-12
[1/d]
coin
#
0
50
100
150
200
0
50
100
150
200
mean
2.5) /d±(71.5
Dec/2010 Jul/2011 Jan/2012 Jul/2012 Dec/2012 Jul/2013
pot)]
16
[1/( 10
coin
#
0
2
4
6
8
10
12
0
2
4
6
8
10
12
mean
pot)
16
0.16) / (10±(4.41
Figure 3: Cngs beam intensities in protons-on-target (POT) and rates over time with a binning of two days. Top: beam intensities
measured at Cern; middle: events correlated with the muon veto; bottom: ratio of the two. The grey hatched areas indicate breaks in the
muon data-taking.
t [ms] ∆
-10 -5 0 5 10
s)] µevents [1/(50
1
10
2
10
CPU clock
s] µt [∆
-5 0 5 10 15
×
events [1/(125 ns)]
1
10
2
10
3
10
GPS clock
GERDA 15-12
Figure 2: Time offset between Cngs and muon veto events. The
left plot shows the time resolution before the installation of the GPS
clock, the right one after. The origin in both plots is set to the rising
flank of the main feature. Note the different time scales.
stopped on the way towards Lngs while the ν
µ
travel al-
most unhindered. The ν
µ
however are able to produce
secondary muons of hE
µ
i = 17 GeV via ν
µ
+ d → µ
−
+ u
reactions upstream of Lngs. Thus, an additional muon
flux impinges horizontally into the Gerda setup.
Both systems, the Cngs beam at Cern and the muon
veto of the Gerda experiment, were operational at the
same time during 404 days in 2011 and 2012. In this pe-
riod ∼28800 coincident muons due to ∼7×10
19
protons-on-
target were detected. The events of both Cngs and muon
veto were correlated by using their respective time stamps.
In Fig. 2 the time differences of valid signals from both fa-
cilities are shown. The enhancement above the random
background shows that true coincidences are observed. In
the beginning, the muon veto was running with a Cpu
clock . However, prior to the start of Phase I a Gps clock
was installed [1]. A sharpening of the enhancement of the
correlated events is clearly visible from the time spectra of
both clock systems. Compared to the Cpu clock (Fig. 2,
left) the time resolution increased dramatically after the
installation of the Gps clock (Fig. 2, right). With the
Gps clock the 10.5 µs bunch length of the Cngs beam
can be reproduced. This shows that the recorded events
can be correlated in time with high accuracy when tested
against an external source like the Cngs beam. The ac-
curate timing will also b e of advantage when searching for
cosmogenic reaction products and their identification via
their half life.
In Fig. 3 time series of the daily beam intensities mea-
sured at Cern (top), the number of events correlated with
the muon veto per day (middle) and the number of coinci-
dent events per beam intensity (bottom) are shown. The
flat distribution in the bottom panel demonstrates the pro-
portionality between the beam intensity and muon events.
This nicely confirms the correct identification of coincident
events. The overall fraction of Cngs events in the muon
3
)]
2
/(s m
-3
[10
,corrµ
r
0.32
0.34
0.36
0.32
0.34
0.36
GERDA 15-12
[K]
eff,LNGS
T
215
220
225
230
215
220
225
230
ECMWF data
NASA AIRS data
Dec/2010 Jul/2011 Jan/2012 Jul/2012 Dec/2012 Jul/2013
)]
2
/(K s m
-6
ratio [10
1.5
1.6
1.7
1.5
1.6
1.7
mean
0.001±1.574
)]
2
/(K s m
-6
[10
Figure 4: Top: Muon flux measured by Gerda with a binning of two days corrected for the Cngs events. A cosine with a period of 365.25
days is fit to the data. Middle: The effective temperature T
eff
for muon production derived from data of Ecwmf [13] in red and from Airs [14]
in green. The black line is a fit to the Airs data. Bottom: the ratio of the muon rate and the T
eff
from the Ecwmf data set is shown to be
flat over the entire time.
data is of the order of 2.2 %.
Using the time stamps, coincidences between germa-
nium data and Cngs events can be found as well. A num-
ber of 45 coincident germanium events were identified in
Phase I of Gerda and 42 of these were accompanied by a
muon veto trigger and hence correctly discarded. With the
rates of the beam and the germanium detectors a number
of 4.9 ± 2.2 random coincident events are expected for this
period and thus the remaining three germanium events can
be attributed to random coincidences.
A single Cngs event was recorded in the 230 keV wide
interpolation region around Q
ββ
for the background-index
(BI) of Gerda. However, this event had a veto flag and
was hence excluded from the analysis and had no effect on
the BI. Since the Cngs has been decommissioned in 2013,
there will be no influence of this type in the next phase of
Gerda.
5. Atmospheric temperature modulation
The identified events due to the Cngs beam shown in
Fig. 3 are removed from the sample for a proper analysis
of the temperature dependence of the cosmic muon frac-
tion (see Fig. 4, top). The annual modulation of the muon
flux is a well-studied phenomenon [7, 10, 11, 12]. Due
to the shielding effect of atmosphere and ro ck overburden,
only cosmogenically produced muons with an energy above
a certain threshold E
thr
will be able to reach an experi-
ment located at a specific depth. Most muons are decay
products of pions and kaons in showers caused by cosmic
radiation. The amount of energy that can be transferred
to their decay products depends on the number of scatter-
ings of the mesons during their life time. The number of
scatterings of a meson is governed by its mean free path
which depends on the density of the air and ultimately
is influenced by the temperature. Hence the atmospheric
temperature and the subsequent density of air molecules
influence the muon energy spectrum and thus the flux at a
certain depth. Since the main temperature change is sea-
sonal, in first order approximation a cosine-like behavior
of the flux can be assumed that takes the form
I
µ
(t) = I
0
µ
+ δI
µ
cos
2π
T
(t − t
0
)
, (1)
where I
µ
(t) is the actual, I
0
µ
the mean muon flux, and δI
µ
the amplitude of the modulation; t
0
is the phase marking
the summer maximum.
The fit to the rate of the Gerda muon veto is shown
in Fig. 4 (top). The period of the fit was fixed to T =
365.25 d because only two maxima are covered up to now.
From the top panel it is obvious that a pure cosine-function
will not describe the rate modulation due to local weather
4
![Figure 1: Sketch of the Gerda experiment [1, 3].](/figures/figure-1-sketch-of-the-gerda-experiment-1-3-32ankx98.webp)
![Figure 4: Top: Muon flux measured by Gerda with a binning of two days corrected for the Cngs events. A cosine with a period of 365.25 days is fit to the data. Middle: The effective temperature Teff for muon production derived from data of Ecwmf [13] in red and from Airs [14] in green. The black line is a fit to the Airs data. Bottom: the ratio of the muon rate and the Teff from the Ecwmf data set is shown to be flat over the entire time.](/figures/figure-4-top-muon-flux-measured-by-gerda-with-a-binning-of-2if9fvmc.webp)
![Figure 6: Correlation coefficient αT as a function of depth. Experiments with different m.w.e. of rock overburden are listed such as Torino [21], Double Chooz [22], Amanda [23], IceCube [24], Minos far detector [11], Macro [18] and Gerda (this work). Gerda and macro are located at the same depth but are drawn slightly apart for better visualization. The curves show muon generation models based on either purely pionic (dashed) or only kaonic (dotted) processes. The full red line notes the literature value for the atmospheric kaon/pion ratio [7, 20].](/figures/figure-6-correlation-coefficient-at-as-a-function-of-depth-1cjrzgea.webp)