Nuclear Physics B Proceedings Supplement 00 (2014) 1–5
Supplement
The XENON Dark Matter project: from XENON100 to XENON1T
Matteo Alfonsia,1,∗, on behalf of XENON Collaboration
aNikhef, Science Park, Amsterdam, Netherlands
Abstract
The XENON Collaboration is searching for Dark Matter interactions in a liquid xenon target. The XENON100 detector, a dual phase xenon Time Projection Chamber employing 161 kg of liquid xenon, started the first science run at the Laboratori Nazionali del Gran Sasso in Italy in 2009. It provided limits on the spin-independent and spin-dependent interaction cross sections of Weakly Interacting Massive Particles (WIMPs), and the couplings of solar axions and galactic axion-like particles. We present these results and the status of the successor experiment, XENON1T. The new detector, currently under construction and starting data taking in 2015, will employ 3.3 tons of liquid xenon, reaching a sensitivity to spin-independent WIMP-nucleon cross section of the order of 10−47cm2. Keywords: Dark Matter, WIMP, Axions, dual-phase xenon Time Projection Chamber
1. Introduction
1
Astrophysical observations provide evidence that
2
Dark Matter constitutes the largest fraction of the mass
3
in the Universe, yet its nature is still unknown. Weakly
4
Interacting Massive Particles (WIMPs) are well moti-
5
vated candidate particles that can explain the observed
6
relic density and can be accommodated by several ex-
7
tensions of the Standard Model [1]. WIMPs can oc-
8
casionally interact elastically with nuclei and therefore
9
be “directly-detected” by observing the small recoils of
10
these nuclei in a target material. Since such interactions
11
exceedingly rare, the detection requires deep under-
12
ground experiments with low radioactive background
13
and high sensitivity to small energy deposits [2].
14
The XENON Collaboration pursues a phased ap-
15
proach to WIMP direct detection employing ultra-pure
16
liquid xenon as both target and detection medium. Liq-
17
uid xenon (LXe) has ideal properties as a dark mat-
18
ter target [3]. The high mass of the xenon nucleus
19
is favourable to scalar interactions and the presence
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∗Corresponding author on behalf of XENON Collaboration Email address:[email protected](Matteo Alfonsi)
1now at Institut f¨ur Physik & Exzellenzcluster PRISMA, Johannes Gutenberg-Universit¨at Mainz, Mainz, Germany
of two isotopes with unpaired neutrons (129Xe: spin-
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1/2, 26.4% and131Xe: spin-3/2, 21.2%) ensures sensi-
22
tivity to spin-dependent WIMP-nuclei couplings. The
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high density (3 g/cm3) and high atomic number (Z=54,
24
A=131.3) allow to build self-shielding, compact dark
25
matter detectors. As a detection medium, it has high
26
scintillation and ionisation yields because of its low ion-
27
isation potential.
28
The XENON detectors, located at the Laboratori
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Nazionali del Gran Sasso (LNGS) in Italy, are dual-
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phase (liquid and vapour xenon) Time Projection Cham-
31
bers (TPC) of increasing target mass and reduced back-
32
ground. The XENON100 detector is the current phase
33
of the programme. It has been recording data since
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2008, being the longest operated dual phase xenon TPC
35
so far. Its recent scientific production is discussed in
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Sec. 2. At the same time, the Collaboration is building
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the successor experiment at LNGS, XENON1T, which
38
will start data-taking in 2015. The new detector and the
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status of its construction is described in Sec. 3.
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2. Results from the XENON100 experiment
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The XENON100 detector is described in detail in [4].
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It is a cylindrical (30.6 cm diameter, 30.5 cm height)
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dual-phase TPC enclosing a LXe target mass of 62 kg.
44
An additional 99 kg of the same high-purity LXe, opti-
45
cally separated from the target volume, is instrumented
46
as an active scintillator veto. The prompt scintilla-
47
tion produced in the two volumes and the proportional
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scintillation from the ionisation electrons in the TPC
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are measured by a total of 224 low background VUV-
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sensitive PMTs. The dual phase TPC configuration al-
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lows for the rejection of the electronic recoils from the
52
ratio between the scintillation and the ionisation signal.
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In addition, the full three-dimensional reconstruction of
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the interaction vertex allows for multiple scattering re-
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jection and volume fiducialisation.
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The latest WIMP search results released in 2012 [5]
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were based on 34 kg fiducial volume and 224.6 live days
58
of exposure. From the same data sample and using the
59
same analysis methods [6] we extracted limits on the
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spin-dependent (SD) WIMP-nucleon couplings [7] and
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recently also limits on models with Axions and Axion-
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Like Particles [8].
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Spin-dependent WIMP-nucleon couplings
64
To derive limits on the SD WIMP-nucleon cross sec-
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tions, we assumed three different models for the nucleus
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structure functions: the choice of the model affects es-
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pecially the interpretation of the limit on the WIMP-
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proton coupling. However the odd isotopes of xenon
69
provide the best sensitivity on the WIMP-neutron cou-
70
pling, and the resulting limits in the three cases differ
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less than a factor of 2 [7]. Fig. 1 shows the limits in
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the case of the Menendezet al.[9] model. The limit on
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the WIMP-neutron coupling is still the most stringent to
74
date, also if the other two models are taken into account.
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Axions and Axion-Like Particles
76
Axions are subatomic particles initially proposed in
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order to find a solution to the strong CP problem of the
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Standard Model and are still considered in some exten-
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sions, for example the DFSZ and KSVZ models (see
80
references in [8]). Axion-Like Particles (ALPs) are sub-
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atomic particles proposed in several extensions of the
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Standard Model, though they do not necessarily relate
83
to the strong CP problem. The models foresee a cou-
84
pling of these particles to photons (gAγ), electrons (gAe)
85
and nuclei (gAN), which may give rise to observable sig-
86
natures in detectors. In a liquid xenon detector, the cou-
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plinggAemay be tested via scattering offthe electron of
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a xenon atom, the so-called axio-electric effect in anal-
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ogy to the photoelectric process.
90
The Sun can be a source of axions and ALPs with a
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production rate dictated by the same coupling constants.
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XENON10 CDMS
PICASSO
COUPP SIMPLE
KIMS
IceCube (b¯b)
IceCube W +W
−
WIMP Mass [GeV/c2] SD WIMP−proton cross section [cm2 ]
proton
101 102 103 104
10−40 10−39 10−38 10−37 10−36 10−35 10−34 10−33
XENON100 limit (2013)
± 2σ expected sensitivity
± 1σ expected sensitivity
XENON10
CDMS
ZEPLIN−III
WIMP Mass [GeV/c2] SD WIMP−neutron cross section [cm2]
neutron
101 102 103 104
10−40 10−39 10−38 10−37 10−36 10−35 10−34
XENON100 limit (2013)
± 2σ expected sensitivity
± 1σ expected sensitivity
Figure 1: XENON100 90% C.L. upper limits on the WIMP spin- dependent cross section on protons (top) and neutrons (bottom) using Menendezet al[9]. The 1σ(2σ) uncertainty on the expected sen- sitivity of this run is shown as a green (yellow) band. See [7] and references therein.
In this framework, detectors on Earth can infer the cou-
93
pling constant by observing this flux of “solar axions”.
94
Fig. 2 shows the upper limit on the couplinggAeset by
95
the search forsolar axionsin XENON100 science data,
96
as reported in [8].
97
ALPs can have also been generated via a non-thermal
98
production mechanism in the early universe and their
99
relic density can now constitute the observed Dark Mat-
100
ter. In this picture, the search in XENON100 data set
101
also a competitive limit on the axion-electron coupling
102
gAe, as shown in Fig. 3.
103
2] [keV/c mA
10-5 10-4 10-3 10-2 10-1 1
Aeg
10-13
10-12
10-11
10-10
10-9
ν Solar
Red giant
Si(Li)
XMASS
EDELWEISS DFSZ
KSVZ XENON100
Figure 2: The XENON100 limits (90% CL) on thegAecoupling of solar axionsis indicated by the blue line. The green (yellow) bands show the 1σ(2σ) expected sensitivity, based on the background hy- pothesis. See [8] and references therein.
2] [keV/c mA
1 2 3 4 5 6 7 8 910 20 30
Aeg
10-13
10-12
10-11
10-10
10-9
DFSZ
KSVZ ν Solar
CoGeNT CDMS EDELWEISS
XENON100
Figure 3: The blue line shows the XENON100 limit (90% CL) on ALP coupling to electrons as a function of the mass, under the as- sumption that ALPs constitute all the dark matter in our galaxy. The 1σ(2σ) expected sensitivity is shown by the green (yellow) bands.
See [8] and references therein.
Study of the detector response to nuclear recoils
104
A comparison between a Monte Carlo simulation of
105
the neutron exposure from an 241AmBe(n, α) neutron
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source and the equivalent calibration data is reported
107
in [10]. The generation of both S1 and S2 signals were
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modelled, respectively, taking into account all the recent
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measurements of the relative scintillation efficiencyLeff
110
and searching for the best fit value for the charge yield
111
Qy. An absolute spectral matching is achieved with a
112
source strength that has been independently measured at
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the Physikalisch-Technische Bundesanstalt (PTB), the
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German National Metrology Institute, after the use dur-
115
ing the science run.
116
Fig. 4 shows the comparison of the S2 spectrum be-
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Figure 4: Comparison between the S2 spectrum from data (black dots) and the Monte Carlo spectrum (blue histogram) as obtained from the optimisation ofQy. See [10] and references therein.
tween data and the Monte Carlo simulation after theQy
118
optimisation.
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In Fig. 5 the S1 spectrum as obtained from data is
120
compared with the spectra obtained by Monte Carlo
121
simulations with the best fit value forLeffand the most
122
recent measurements of this quantity.
123
3. The XENON1T detector
124
The construction of XENON1T in Hall B at LNGS
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started in mid-2013. The water tank that will house the
126
detector, the support structure inside and a service build-
127
ing next to the water tank have been completed (Fig. 6).
128
Most of the cryogenics instrumentation as well as the
129
xenon storage and purification systems have been al-
130
ready realised and they have been installed in the ser-
131
vice building in Summer 2014. In the same period the
132
Figure 5: Fit of the simulated S1 spectrum to data (black points). The blue Monte Carlo spectrum is obtained using theLefffrom the opti- misation process, while the grey dashed line shows the spectral shape using the most recent measurements ofLeff. See [10] and references therein.
cryostat and the UHV pipe already equipped with all the
133
services for the detector (HV and signal cables, xenon
134
cryogenics and recirculation pipes) have been success-
135
fully installed, while the installation of the TPC will
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take place in the first months of 2015. The commis-
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sioning and the beginning of the first science run of the
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new detector is expected for the second half of 2015.
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Figure 6: Status of the XENON1T construction as of Summer 2014.
(top) Photograph The XENON1T service building and water tank in Hall B at LNGS. (bottom) Photograph of the XENON1T cryostat hanging from the support structure inside the water tank: the inner vessel and the top flange of the outer vessel are visible. In the back- ground also the UHV pipe for all the detector services is visible, going from the cryostat to the hole in the water tank on the left side of the picture.
XENON1T will employ more than 3 tons of LXe,
140
with a fiducial mass of about 1 ton. The design is shown
141
in Fig. 7. The TPC has a diameter and a drift length
142
of about 1 m, and it is designed in order to achieve a
143
uniform electric field of 1.0 kV/cm. This is extremely
144
challenging and required a detailed calculation of the
145
electric field in all the regions of the TPC and special
146
attention in the production process of the meshes and
147
other electrodes. A custom-made feed-through has been
148
engineered to service the cathode with the necessary
149
high voltage (∼100 kV). The possibility to achieve a
150
stable condition with such a high electric field has been
151
proved with a Demonstrator TPC built and operated at
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Columbia University.
153
Figure 7: Drawing of the XENON1T detector. The TPC and the top and bottom PMT array are visible inside the inner vessel of the cryo- stat. The outer vessel is larger to allow for an upgrade, XENONnT, after two years of data-taking.
XENON1T is instrumented with 248 3”-diameter
154
Hamamatsu R11410-21 PMTs. These PMTs are a spe-
155
cial development from Hamamatsu optimised for the
156
detection of XENON scintillation light (QE between
157
36% and 40% at 178 nm wavelenght) and low radioac-
158
tivity. All the PMTs as well as all the components of
159
the detectors have been screened to verify that they ful-
160
fil the stringent low radioactivity requirement. Due to
161
the xenon self-shielding capability, the expected back-
162
ground rate in the fiducial volume and in the energy
163
range of WIMP searches, originated by the detector ma-
164
terials radioactivity, is less than 0.25 events per year. In
165
the fiducial volume additional intrinsic sources of back-
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ground are the impurities diluted in the liquid xenon.
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Most of the impurities species are efficiently removed
168
by the recirculation through a hot getter. Noble gasses
169
impurities, especially theβ-emitter85Kr isotope, are re-
170
moved by cryogenic distillation. The XENON1T dis-
171
tillation column will reduce the krypton concentration
172
in the liquid xenon below 0.5 ppt. This translates to a
173
background rate in the WIMP search region from intrin-
174
sic sources smaller than 0.15 events/year.
175
The cryostat is suspended within the 9.6 m diameter
176
water tank that is instrumented as a Cherenkov muon
177
veto. The role of the water tank is one the one hand
178
to shield neutron andγ-ray background from the under-
179
ground rock cavern and on the other hand to detect the
180
passage of muons in the proximity of the detector, in
181
order to reduce the background from muon-generated
182
neutrons. The background from this source is predicted
183
negligible (<0.01 events/year).
184
The muon veto, the extensive material screening cam-
185
paign, the reduction of the intrinsic background coming
186
from Kr and Rn contamination and the self-shielding
187
properties of liquid xenon allow to achieve the goal of
188
less than 1 background event during a 2-years exposure.
189
With this exposure the experiment will reach a sensitiv-
190
ity to spin-independent WIMP-nucleon cross sections
191
of∼2×10−47 cm2 at 90% CL in the case of a WIMP
192
mass of 50 GeV/c2.
193
4. Conclusions and prospects
194
The XENON100 experiment recently released new
195
limits of the spin-dependent WIMP-nucleon cross-
196
sections and, in the framework of Axion and Axion-
197
Like Particle models, set the constraints for the cou-
198
pling between these particle and electrons. The de-
199
tector is still taking data and a study of novel calibra-
200
tion techniques is ongoing. The successor experiment
201
XENON1T will improve the sensitivity by two order of
202
magnitude after two year of data-taking. The construc-
203
tion of XENON1T is well underway and the first science
204
run will start in 2015. The outer vessel of the cryostat
205
and all the systems already designed in view of an up-
206
grade, XENONnT. This upgrade will employs about 7
207
tons of liquid xenon and allow to achieve a sensitivity
208
on the spin-independent WIMP-nucleon cross section
209
of few times 10−48cm2.
210
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