Measurement of the parity-violating longitudinal single-spin asymmetry for W± boson production in polarized proton-proton collisions at sqrt[s] = 500 GeV.

ArticleinPhysical Review Letters 106(6):062002 · February 2011with38 Reads
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Abstract

We report the first measurement of the parity-violating single-spin asymmetries for midrapidity decay positrons and electrons from W+ and W- boson production in longitudinally polarized proton-proton collisions at sqrt[s] = 500 GeV by the STAR experiment at RHIC. The measured asymmetries, A(L)(W+) = -0.27 ± 0.10(stat.) ± 0.02(syst.) ± 0.03(norm.) and A(L)(W-) = 0.14 ± 0.19(stat.) ± 0.02(syst.) ± 0.01(norm.), are consistent with theory predictions, which are large and of opposite sign. These predictions are based on polarized quark and antiquark distribution functions constrained by polarized deep-inelastic scattering measurements.

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Measurement of the parity-violating longitudinal single-spin asymmetry for W
±
boson
production in polarized proton-proton collisions at
s = 500 GeV
M. M. Aggarwal,
29
Z. Ahammed,
21
A. V. Alakhverdyants,
17
I. Alekseev,
15
J. Alford,
18
B. D. Anderson,
18
C. D. Anson,
27
D. Arkhipkin,
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J. Balewski,
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D. R. Beavis,
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R. Bellwied,
49
M. J. Betancourt,
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R. R. Betts,
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A. Bhasin,
16
A. K. Bhati,
29
H. Bichsel,
48
J. Bielcik,
9
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B. Biritz,
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L. C. Bland,
2
W. Borowski,
40
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1
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arXiv:1009.0326v2 [hep-ex] 3 Sep 2010
Page 1
2
Z. P. Zhang,
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R. Zoulkarneev,
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and Y. Zoulkarneeva
17
(STAR Collaboration)
1
Argonne National Laboratory, Argonne, Illinois 60439, USA
2
Brookhaven National Laboratory, Upton, New York 11973, USA
3
University of California, Berkeley, California 94720, USA
4
University of California, Davis, California 95616, USA
5
University of California, Los Angeles, California 90095, USA
6
Universidade Estadual de Campinas, Sao Paulo, Brazil
7
University of Illinois at Chicago, Chicago, Illinois 60607, USA
8
Creighton University, Omaha, Nebraska 68178, USA
9
Czech Technical University in Prague, FNSPE, Prague, 115 19, Czech Republic
10
Nuclear Physics Institute AS CR, 250 68
ˇ
Reˇz/Prague, Czech Republic
11
University of Frankfurt, Frankfurt, Germany
12
Institute of Physics, Bhubaneswar 751005, India
13
Indian Institute of Technology, Mumbai, India
14
Indiana University, Bloomington, Indiana 47408, USA
15
Alikhanov Institute for Theoretical and Experimental Physics, Moscow, Russia
16
University of Jammu, Jammu 180001, India
17
Joint Institute for Nuclear Research, Dubna, 141 980, Russia
18
Kent State University, Kent, Ohio 44242, USA
19
University of Kentucky, Lexington, Kentucky, 40506-0055, USA
20
Institute of Modern Physics, Lanzhou, China
21
Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
22
Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA
23
Max-Planck-Institut f¨ur Physik, Munich, Germany
24
Michigan State University, East Lansing, Michigan 48824, USA
25
Moscow Engineering Physics Institute, Moscow Russia
26
NIKHEF and Utrecht University, Amsterdam, The Netherlands
27
Ohio State University, Columbus, Ohio 43210, USA
28
Old Dominion University, Norfolk, VA, 23529, USA
29
Panjab University, Chandigarh 160014, India
30
Pennsylvania State University, University Park, Pennsylvania 16802, USA
31
Institute of High Energy Physics, Protvino, Russia
32
Purdue University, West Lafayette, Indiana 47907, USA
33
Pusan National University, Pusan, Republic of Korea
34
University of Rajasthan, Jaipur 302004, India
35
Rice University, Houston, Texas 77251, USA
36
Universidade de Sao Paulo, Sao Paulo, Brazil
37
University of Science & Technology of China, Hefei 230026, China
38
Shandong University, Jinan, Shandong 250100, China
39
Shanghai Institute of Applied Physics, Shanghai 201800, China
40
SUBATECH, Nantes, France
41
Texas A&M University, College Station, Texas 77843, USA
42
University of Texas, Austin, Texas 78712, USA
43
Tsinghua University, Beijing 100084, China
44
United States Naval Academy, Annapolis, MD 21402, USA
45
Valparaiso University, Valparaiso, Indiana 46383, USA
46
Variable Energy Cyclotron Centre, Kolkata 700064, India
47
Warsaw University of Technology, Warsaw, Poland
48
University of Washington, Seattle, Washington 98195, USA
49
Wayne State University, Detroit, Michigan 48201, USA
50
Institute of Particle Physics, CCNU (HZNU), Wuhan 430079, China
51
Yale University, New Haven, Connecticut 06520, USA
52
University of Zagreb, Zagreb, HR-10002, Croatia
We report the first measurement of the parity violating single-spin asymmetries for midrapid-
ity decay positrons and electrons from W
+
and W
boson production in longitudinally polarized
proton-proton collisions at
s = 500 GeV by the STAR experiment at RHIC. The measured asym-
metries, A
W
+
L
= 0.27 ± 0.10 (stat.) ± 0.02 (syst.) ± 0.03 (norm.) and A
W
L
= 0.14 ± 0.19 (stat.) ±
0.02 (syst.) ± 0.01 (norm.), are consistent with theory predictions, which are large and of oppo-
site sign. These predictions are based on polarized quark and antiquark distribution functions
constrained by polarized DIS measurements.
Page 2
3
PACS numbers: 14.20.Dh, 13.88.+e, 13.38.Be, 13.85.Qk
Understanding the spin structure of the nucleon re-
mains a fundamental challenge in Quantum Chromody-
namics (QCD). Experimentally, polarized deep-inelastic
scattering (pDIS) measurements have shown that the
quark spins account for only 33% of the proton spin [1].
Semi-inclusive pDIS measurements [2–4] are sensitive to
the quark and antiquark spin contributions separated by
flavor [5, 6]. They rely on a quantitative understanding
of the fragmentation of quarks and antiquarks into ob-
servable final-state hadrons. While the sum of the con-
tributions from quark and antiquark parton distribution
functions (PDFs) of the same flavor is well constrained,
the uncertainties in the polarized antiquark PDFs sepa-
rated by flavor remain relatively large [5, 6].
High-energy polarized proton collisions at
s =
200 GeV and
s = 500 GeV at RHIC provide a unique
way to probe the proton spin structure and dynamics
using hard scattering processes [7]. The production of
W
±
bosons at
s = 500 GeV provides an ideal tool to
study the spin-flavor structure of sea quarks inside the
proton. W
+()
bosons are dominantly produced through
u +
¯
d (d + ¯u) interactions and can be detected through
their leptonic decays [8]. Quark and antiquark polarized
PDFs are probed directly in calculable leptonic W de-
cays at large scales set by the mass of the W boson.
The production of W bosons in polarized proton col-
lisions allows for the observation of purely weak inter-
actions, giving rise to large parity-violating longitudinal
single-spin asymmetries. A theoretical framework has
been developed to describe inclusive lepton production,
~p + p W
±
+ X l
±
+ X, that can be directly com-
pared with experimental measurements using constraints
on the transverse energy and pseudorapidity of the final-
state leptons [9, 10].
In this letter, we report the first measurement of the
parity violating single-spin asymmetries for midrapidity
decay positrons and electrons from W
+
and W
boson
production in longitudinally polarized ~p + p collisions at
s = 500 GeV by the STAR experiment at RHIC. The
asymmetry is defined as A
L
(σ
+
σ
)/(σ
+
+ σ
),
where σ
+()
is the cross section when the helicity of the
polarized proton beam is positive (negative).
The STAR detector systems [11] used in this mea-
surement are the Time Projection Chamber [12] (TPC)
and the Barrel [13] and Endcap [14] Electromagnetic
Calorimeters (BEMC, EEMC). The TPC provides
tracking for charged particles in a 0.5 T solenoidal
magnetic field for pseudorapidities |η| < 1.3 with
full azimuthal coverage. The BEMC and EEMC are
lead-scintillator sampling calorimeters providing full az-
imuthal coverage for |η| < 1 and 1.09 < η < 2, respec-
tively.
The data analyzed in this letter were collected in 2009
)V (Ge
e
T
E
20 40 60
)c
/
V balance (Ge
T
pSigned
−40
−20
0
20
40
60
(c)
Counts
0
3000
4
×4
T
E/
e
T
E(a)
Data
simulationW
X
T
E/
e
T
E
0 0.5 1
Counts
0
100
200
300
0.7<
R
T
E/
e
T
E(b)
FIG. 1: (a) Ratios of E
e
T
with respect to the 4 × 4 BEMC
E
T
sum, E
e
T
/E
X=4×4
T
, (b) the near-cone BEMC, EEMC
and TPC E
T
sum, E
e
T
/E
X=R<0.7
T
, and (c) correlation of
the signed p
T
balance variable and E
e
T
. MC shape distribu-
tions (arbitrary normalization) are shown in (a) and (b) for
W
±
e
±
+X as filled histograms in comparison to both data
distributions.
with colliding polarized proton beams at
s = 500 GeV
and an average luminosity of 55 × 10
30
cm
2
s
1
. The
polarization of each beam was measured using Coulomb-
Nuclear Interference proton-carbon polarimeters [15],
which were calibrated using a polarized hydrogen gas-jet
target [16]. Longitudinal polarization of proton beams
in the STAR interaction region (IR) was achieved by
spin rotator magnets upstream and downstream of the
IR that changed the proton spin orientation from its sta-
ble vertical direction to longitudinal. Non-longitudinal
beam polarization components were continuously moni-
tored with a local polarimeter system at STAR based on
the Zero-Degree Calorimeters with an upper limit on the
relative contribution of 15% for both polarized proton
beams. The longitudinal beam polarizations averaged
over all runs were P
1
= 0.38 and P
2
= 0.40 with cor-
related relative uncertainties of 8.3% and 12.1%, respec-
tively. Their sum P
1
+ P
2
= 0.78 is used in the analysis
and has a relative uncertainty of 9.2%.
Positrons (e
+
) and electrons (e
) from W
+
and W
boson production with |η
e
| < 1 are selected for this anal-
ysis. High-p
T
e
±
are charge-separated using the STAR
TPC. The BEMC is used to measure the transverse en-
ergy E
e
T
of e
+
and e
. The suppression of the QCD back-
ground is achieved with the TPC, BEMC, and EEMC.
The selection of W candidate events is based on kine-
matic and topological differences between leptonic W
±
decays and QCD background events. Events from W
±
decays contain a nearly isolated e
±
with a neutrino in
the opposite direction in azimuth. The neutrino escapes
detection leading to a large missing energy. Such events
exhibit a large imbalance in the vector p
T
sum of all re-
constructed final-state objects. In contrast, QCD events,
Page 3
4
)VGe
/c (
T
p/ 1×
Lepton charge sign
−0.05 0 0.05
)V (Ge
e
T
E
20
40
60
0
5
10
15
positivenegative
FIG. 2: E
e
T
as a function of the ratio of the TPC recon-
structed charge sign to the transverse momentum p
T
. The
black-solid and red-dashed lines indicate the selected kinematic
region used for the asymmetry analysis.
e.g. dijet events, are characterized by a small magnitude
of this vector sum imbalance.
Candidate W events were selected online by a two-step
energy requirement in the BEMC. Electrons or positrons
from W production at midrapidity are characterized by
large E
T
peaked at M
W
/2 (Jacobian peak). At the
hardware trigger level, a high tower calorimetric trig-
ger condition required E
T
> 7.3 GeV in a single BEMC
tower. At the software trigger level, a dedicated trig-
ger algorithm searched for a seed tower of E
T
> 5 GeV
and computed all four possible combinations of the 2 ×2
tower cluster E
T
sums and required at least one to be
above 13 GeV. A total of 1.4 × 10
6
events were recorded
for a data sample of 12 pb
1
. A Vernier scan was used
to determine the absolute luminosity [17].
An electron or positron candidate is defined to be any
TPC track with p
T
> 10 GeV/c that is associated with
a primary vertex with |z| < 100 cm, where z is measured
along the beam direction. A 2 × 2 BEMC tower cluster
E
T
sum E
e
T
, whose centroid is within 7 cm of the pro-
jected TPC track, is required to be larger than 15 GeV.
The excess BEMC E
T
sum in a 4 × 4 tower cluster cen-
tered around the 2 × 2 tower cluster is required to be
below 5%, as indicated by the vertical dashed line in
Fig. 1 (a). A cone, referred to as the near-side cone, is
formed around the e
±
candidate with a radius R = 0.7
in η-φ space. The excess BEMC, EEMC, and TPC E
T
sum in this cone is required to be less than 12% of the
2 × 2 cluster E
T
, as shown in Fig. 1 (b) by the vertical
dashed line. pythia 6.205 [18] Monte-Carlo (MC) shape
distributions (arbitrary normalization) for W
±
e
±
+ ν
passed through the geant [19] model of the STAR detec-
tor are shown in Figs. 1 (a) and 1 (b) as filled histograms
motivating both ratio cuts. The missing energy require-
ment is enforced by a cut on the p
T
balance vector, de-
fined as the vector sum of the e
±
candidate p
T
and the
p
T
vectors of all reconstructed jets, where the jet thrust
axis is required to be outside the near-side cone. Jets
are reconstructed using a standard mid-point cone algo-
rithm used in STAR jet measurements [20] based on the
TPC, BEMC, and EEMC. A scalar signed p
T
balance
variable is formed, given by the magnitude of the p
T
bal-
ance vector and the sign of the dot-product of the p
T
balance vector and the electron p
T
vector. This quantity
is required to be larger than 15 GeV/c. The correlation
of the signed p
T
balance variable and E
e
T
is shown in
Fig. 1 (c). The range for accepted W candidate events is
marked by red dashed lines. The lower cut is chosen to
suppress the contribution of background events whereas
the upper cut is mainly applied to ensure proper charge
sign reconstruction. Background events from Z
0
e
+
e
decays are suppressed by rejecting events with an addi-
tional electron/positron-like 2 × 2 cluster in the recon-
structed jet where the E
2×2
T
> p
jet
T
/2 and the invariant
mass of the two electron/positron-like clusters is within
70 to 140 GeV/c
2
. This avoids Z
0
contamination in the
data-driven QCD background described below.
Figure 2 shows E
e
T
as a function of the ratio of the TPC
reconstructed charge sign to the transverse momentum
p
T
for electron and positron candidates that pass all the
cuts described above. Two well-separated regions for pos-
itive (negative) charges are visible, identifying the W
+()
candidate events up to E
e
T
50 GeV. The range of E
e
T
for accepted W candidate events, 25 < E
e
T
< 50 GeV,
is marked by red dashed lines. Entries outside the black
solid lines in Fig. 2 were rejected due to false track re-
construction.
Figure 3 presents the charge-separated lepton E
e
T
dis-
tributions based on the selection criteria given above.
W candidate events are shown as the black histograms,
where the characteristic Jacobian peak can be seen at
M
W
/2. The total number of candidate events for
W
+()
is 462 (139) for 25 < E
e
T
< 50 GeV indicated
by vertical red dashed lines in Fig. 3. The number of
background events was estimated through a combination
of pythia 6.205 [18] MC simulations and a data-driven
procedure. The e
+()
background from W
+()
boson
induced τ
+()
decays and Z
0
e
+
+ e
decays was es-
timated using MC simulations to be 10.4 ±2.8 (0.7 ±0.7)
events and 8.5 ±2.0 events (identical for both e
+()
), re-
spectively. The remaining background is mostly due to
QCD dijet events where one of the jets missed the STAR
acceptance. We have developed a data-driven procedure
to evaluate this type of background. We excluded the
EEMC (1.09 < η < 2) as an active detector in our analy-
sis to estimate the background due to missing calorimeter
coverage for 2 < η < 1.09. The background contri-
bution due to missing calorimeter coverage along with τ
and Z
0
background contributions have been subtracted
from both W
+()
E
e
T
distributions. The remaining back-
ground, presumably due to missing jets outside the STAR
|η| < 2 window, is evaluated based on an extrapolation
from the region of E
e
T
< 19 GeV in both W
+()
E
e
T
dis-
tributions. The shape is determined from the E
e
T
distri-
bution in events previously rejected as background with
systematic variations of the signed p
T
balance cut be-
Page 4
5
10 20 30 40 50 60 70
Counts
0
10
20
30
40
50
60
70
1
<
|
e
η
|Electron
Candidates
Backg. est.
signal
W
X +
e X +
W
p
+
p
Software threshold
STAR
)V (Ge
e
T
E
10 20 30 40 50 60 70
Counts
0
10
20
30
40
50
60
70
1
<
|
e
η
|Positron
X +
+
e X +
+
W
p
+
p
FIG. 3: E
e
T
for W
+
(bottom) and W
(top) events showing
the candidate histograms in black, the full background esti-
mates in blue and the signal distributions in yellow.
low 15 GeV/c. This shape E
e
T
distribution is normalized
to both W
+()
E
e
T
distributions for E
e
T
< 19 GeV. The
total number of background events for e
+()
is 39 ± 9
(23 ± 6) for 25 < E
e
T
< 50 GeV shown in Fig. 3 as the
blue line. The errors on the total background are mostly
from the data-driven background events.
The leptonic asymmetry from W
±
decay, A
W
±
L
, was
obtained from:
A
W
±
L
=
1
β
±
2
P
1
+ P
2
R
++
N
W
±
++
R
−−
N
W
±
−−
Σ
i
R
i
N
W
±
i
α
±
β
±
(1)
where P
1,2
are the mean polarizations, N
W
±
i
are W
±
candidate yields for all four beam helicity configura-
tions i = {++, +, +, −−}, and R
i
are the respec-
tive relative luminosities. The longitudinal single-spin
asymmetry A
L
for Z
0
bosons has been estimated us-
ing a full next-to-leading (NLO) order framework [10].
With the W
±
selection criteria we estimated the Z
0
asymmetry to be A
Z
L
= 0.06. This value has been
used to determine the polarized background contribution
α
+()
= 0.002 ± 0.001 (0.005 ± 0.002). The unpo-
larized background correction for W
±
candidate events
is β
+()
= 0.938 ± 0.017 (0.838 ± 0.032). This dilution
factor is due to background events passing all W selec-
tion cuts and is determined by β = S/(S + B), where
S (B) is the number of signal (background) events for
25 < E
e
T
< 50 GeV.
The relative luminosities R
i
=
P
k
M
k
/(4M
i
) are de-
termined from the ratios of yields M
i
of QCD events, for
which parity conservation is expected. The M
i
are statis-
tically independent from N
W
±
i
because the isolation cut
on the 2 ×2 / 4 × 4 tower E
T
sum, shown in Fig. 1, was
reversed for those events. Additionally, an upper limit of
20 GeV was set on E
e
T
.
Figure 4 shows the measured leptonic asymmetries
A
W
+
L
= 0.27 ± 0.10 (stat.) ± 0.02 (syst.) and A
W
L
=
0.14 ± 0.19 (stat.) ± 0.02 (syst.) for |η
e
| < 1 and 25 <
E
e
T
< 50 GeV. The vertical black error bars include only
the statistical uncertainties. The systematic uncertain-
ties are indicated as grey bands. The statistical uncer-
tainties dominate over the systematic uncertainties. The
asymmetry A
L
observed in statistically independent sam-
ples of QCD dominated events was found to be 0.04±0.03
(0.00 ±0.04) for positive (negative) charged tracks and is
consistent with zero. We assumed the experimental limit
on the polarized background A
L
to be 0.02 as a system-
atic uncertainty of A
W
±
L
. This limit on polarized back-
ground and the uncertainty in unpolarized background
dilution have been added in quadrature to account for
the total systematic uncertainty of A
W
±
L
. The normal-
ization uncertainty of the measured asymmetries due to
the uncertainty for the polarization sum P
1
+ P
2
is 0.03
(0.01) for A
W
+()
L
. The normalization uncertainty is of
similar size as the systematic uncertainty of the asymme-
try measurement.
In Fig. 4, the measured asymmetries are compared to
predictions based on full resummed (rhicbos) [9] and
NLO (che) [10] calculations. The che calculations use
the DSSV08 polarized PDFs [5], whereas the rhicbos
calculations are shown in addition for the older DNS-K
and DNS-KKP [21] PDFs. The che and rhicbos re-
sults are in good agreement. The range spanned by the
DNS-K and DNS-KKP distributions for
¯
d and ¯u co-
incides, approximately, with the corresponding DSSV08
uncertainty estimates [5, 6]. The spread of predictions
for A
W
+()
L
is largest at forward (backward) η
e
and is
strongly correlated to the one found for the
¯
d (¯u) polar-
ized PDFs in the RHIC kinematic region in contrast to
the backward (forward) η
e
region dominated by the be-
havior of the well-known valence u (d) polarized PDFs
[10]. At midrapidity, W
+()
production probes a combi-
nation of the polarization of the u and
¯
d (d and ¯u) quarks,
and A
W
+()
L
is expected to be negative (positive) [5, 6].
The measured A
W
+
L
is indeed negative stressing the di-
rect connection to the u quark polarization. The central
value of A
W
L
is positive as expected with a larger sta-
tistical uncertainty. Our A
L
results are consistent with
predictions using polarized quark and antiquark PDFs
constrained by inclusive and semi-inclusive pDIS mea-
surements, as expected from the universality of polarized
PDFs. An independent measurement of W boson pro-
duction from RHIC is being reported by the PHENIX
collaboration [22].
Page 5
6
e
η
−2 −1 0 1 2
−0.4
−0.3
−0.2
−0.1
0
0.1
0.2
0.3
0.4
RHICBOS
+
W
W
K-DNS
KKP-DNS
08DSSV
08DSSV
CHE
Syst. uncertainty due to background,
w/o pol. norm. uncertainty of 9.2%
V500 Ge
=
s STAR
X +
±
e X +
±
W
p
+
p
V < 50 Ge
e
T
E
25 <
W
+
W
L
A
σ
+
+
σ
σ
+
σ
=
L
A
FIG. 4: Longitudinal single-spin asymmetry, A
L
, for W
±
events as a function of the leptonic pseudorapidity, η
e
, for
25 < E
e
T
< 50 GeV in comparison to theory predictions (See
text for details).
In summary, we report the first measurement of the
parity violating single-spin asymmetries for midrapid-
ity, |η
e
| < 1, decay positrons and electrons from W
+
and W
boson production in longitudinally polarized
~p + p collisions at
s = 500 GeV by the STAR exper-
iment at RHIC. This measurement establishes a new
and direct way to explore the spin structure of the pro-
ton using parity-violating weak interactions in polarized
~p + p collisions. The measured asymmetries probe the
polarized PDFs at much larger scales than in previous
and ongoing pDIS experiments and agree well with NLO
and resummed calculations using the polarized PDFs of
DSSV08. Future high-statistics measurements at midra-
pidity together with measurements at forward and back-
ward pseudorapidities will focus on constraining the po-
larization of
¯
d and ¯u quarks.
We thank the RHIC Operations Group and RCF at
BNL, the NERSC Center at LBNL and the Open Science
Grid consortium for providing resources and support. We
are grateful to D. de Florian, P. Nadolsky and W. Vogel-
sang for useful discussions. This work was supported in
part by the Offices of NP and HEP within the U.S. DOE
Office of Science, the U.S. NSF, the Sloan Foundation,
the DFG cluster of excellence ‘Origin and Structure of the
Universe’ of Germany, CNRS/IN2P3, FAPESP CNPq of
Brazil, Ministry of Ed. and Sci. of the Russian Fed-
eration, NNSFC, CAS, MoST, and MoE of China, GA
and MSMT of the Czech Republic, FOM and NWO of
the Netherlands, DAE, DST, and CSIR of India, Polish
Ministry of Sci. and Higher Ed., Korea Research Foun-
dation, Ministry of Sci., Ed. and Sports of the Rep. Of
Croatia, and RosAtom of Russia.
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