A liquid argon time projection chamber, constructed for the Argon Response to Ionization and Scintillation (ARIS) experiment, is exposed to the highly collimated and quasimonoenergetic LICORNE ...neutron beam at the Institut de Physique Nucléaire d’Orsay (IPNO) in order to study the scintillation response to nuclear and electronic recoils. An array of liquid scintillator detectors, arranged around the apparatus, tag scattered neutrons and select nuclear recoil energies in the 7, 120 keV energy range. The relative scintillation efficiency of nuclear recoils is measured to high precision at null field, and the ion-electron recombination probability is extracted for a range of applied electric fields. Single-scattered Compton electrons, produced by gammas emitted from the deexcitation of Li*7 in coincidence with the beam pulse, along with calibration gamma sources, are used to extract the recombination probability as a function of energy and electron drift field. The ARIS results are compared with three recombination probability parametrizations (Thomas-Imel, Doke-Birks, and PARIS), allowing for the definition of a fully comprehensive model of the liquid argon response to nuclear and electronic recoils down to the few-keV range. The constraints provided by ARIS to the liquid argon response at low energy allow the reduction of systematics affecting the sensitivity of dark matter search experiments based on liquid argon.
A
bstract
Single top-quark production in association with a
Z
boson, where the
Z
boson decays to a pair of charged leptons, is measured in the trilepton channel. The proton-proton collision data ...collected by the ATLAS experiment from 2015 to 2018 at a centre-of-mass energy of 13 TeV are used, corresponding to an integrated luminosity of 139 fb
−
1
. Events containing three isolated charged leptons (electrons or muons) and two or three jets, one of which is identified as containing a
b
-hadron, are selected. The main backgrounds are from
t
t
¯
Z
and diboson production. Neural networks are used to improve the background rejection and extract the signal. The measured cross-section for
t
ℓ
+
ℓ
−
q
production, including non-resonant dilepton pairs with
m
ℓ
+
ℓ
−
>
30 GeV, is 97
±
13 (stat.)
±
7 (syst.) fb, consistent with the Standard Model prediction.
A
bstract
A search for a chargino-neutralino pair decaying via the 125 GeV Higgs boson into photons is presented. The study is based on the data collected between 2015 and 2018 with the ATLAS ...detector at the LHC, corresponding to an integrated luminosity of 139 fb
−
1
of
pp
collisions at a centre-of-mass energy of 13 TeV. No significant excess over the expected background is observed. Upper limits at 95% confidence level for a massless
χ
˜
1
0
are set on several electroweakino production cross-sections and the visible cross-section for beyond the Standard Model processes. In the context of simplified supersymmetric models, 95% confidence-level limits of up to 310 GeV in
m
χ
˜
1
±
/
χ
˜
2
0
, where
m
χ
˜
1
0
= 0
.
5 GeV, are set. Limits at 95% confidence level are also set on the
χ
˜
1
±
χ
˜
2
0
cross-section in the mass plane of
m
χ
˜
1
±
/
χ
˜
2
0
and
m
χ
˜
1
0
, and on scenarios with gravitino as the lightest supersymmetric particle. Upper limits at the 95% confidence-level are set on the higgsino production cross-section. Higgsino masses below 380 GeV are excluded for the case of the higgsino fully decaying into a Higgs boson and a gravitino.
A measurement of the
B
s
0
→
J
/
ψ
ϕ
decay parameters using
80.5
fb
-
1
of integrated luminosity collected with the ATLAS detector from 13
Te
proton–proton collisions at the LHC is presented. The ...measured parameters include the
CP
-violating phase
ϕ
s
, the width difference
Δ
Γ
s
between the
B
s
0
meson mass eigenstates and the average decay width
Γ
s
. The values measured for the physical parameters are combined with those from
19.2
fb
-
1
of 7 and 8
Te
data, leading to the following:
ϕ
s
=
-
0.087
±
0.036
(
stat
.
)
±
0.021
(
syst
.
)
rad
Δ
Γ
s
=
0.0657
±
0.0043
(
stat
.
)
±
0.0037
(
syst
.
)
ps
-
1
Γ
s
=
0.6703
±
0.0014
(
stat
.
)
±
0.0018
(
syst
.
)
ps
-
1
Results for
ϕ
s
and
Δ
Γ
s
are also presented as 68% confidence level contours in the
ϕ
s
–
Δ
Γ
s
plane. Furthermore the transversity amplitudes and corresponding strong phases are measured.
ϕ
s
and
Δ
Γ
s
measurements are in agreement with the Standard Model predictions.
A
bstract
A measurement of the
t
-channel single-top-quark and single-top-antiquark production cross-sections in the lepton+jets channel is presented, using 3.2 fb
−1
of proton-proton collision data ...at a centre-of-mass energy of 13 TeV, recorded with the ATLAS detector at the LHC in 2015. Events are selected by requiring one charged lepton (electron or muon), missing transverse momentum, and two jets with high transverse momentum, exactly one of which is required to be
b
-tagged. Using a binned maximum-likelihood fit to the discriminant distribution of a neural network, the cross-sections are determined to be
σ
(
tq
) = 156 ± 5 (stat.) ± 27 (syst.) ± 3 (lumi.) pb for single top-quark production and
σ
t
¯
q
=
91
±
4
(stat.) ± 18 (syst.) ± 2 (lumi.) pb for single top-antiquark production, assuming a top-quark mass of 172.5 GeV. The cross-section ratio is measured to be
R
t
=
σ
t
q
/
σ
t
¯
q
=
1.72
±
0.09
(stat.) ± 0.18 (syst.). All results are in agreement with Standard Model predictions.
A
bstract
This paper presents a measurement of the production cross-section of a
Z
boson in association with
b
-jets, in proton-proton collisions at
s
= 13 TeV with the ATLAS experiment at the Large ...Hadron Collider using data corresponding to an integrated luminosity of 35.6 fb
−
1
. Inclusive and differential cross-sections are measured for events containing a
Z
boson decaying into electrons or muons and produced in association with at least one or at least two
b
-jets with transverse momentum
p
T
>
20 GeV and rapidity
|y| <
2
.
5. Predictions from several Monte Carlo generators based on leading-order (LO) or next-to-leading-order (NLO) matrix elements interfaced with a parton-shower simulation and testing different flavour schemes for the choice of initial-state partons are compared with measured cross-sections. The 5-flavour number scheme predictions at NLO accuracy agree better with data than 4-flavour number scheme ones. The 4-flavour number scheme predictions underestimate data in events with at least one b-jet.
A measurement of observables sensitive to spin correlations in
t
t
¯
production is presented, using 36.1
fb
-
1
of
pp
collision data at
s
=
13
TeV recorded with the ATLAS detector at the Large Hadron ...Collider. Differential cross-sections are measured in events with exactly one electron and one muon with opposite-sign electric charge as a function of the azimuthal opening angle and the absolute difference in pseudorapidity between the electron and muon candidates in the laboratory frame. The azimuthal opening angle is also measured as a function of the invariant mass of the
t
t
¯
system. The measured differential cross-sections are compared to predictions by several NLO Monte Carlo generators and fixed-order calculations. The observed degree of spin correlation is somewhat higher than predicted by the generators used. The data are consistent with the prediction of one of the fixed-order calculations at NLO, but agree less well with higher-order predictions. Using these leptonic observables, a search is performed for pair production of supersymmetric top squarks decaying into Standard Model top quarks and light neutralinos. Top squark masses between 170 and 230 GeV are largely excluded at the 95% confidence level for kinematically allowed values of the neutralino mass.
Many of the interesting physics processes to be measured at the LHC have a signature involving one or more isolated electrons. The electron reconstruction and identification efficiencies of the ATLAS ...detector at the LHC have been evaluated using proton–proton collision data collected in 2011 at
s
=
7
TeV and corresponding to an integrated luminosity of 4.7 fb
-
1
. Tag-and-probe methods using events with leptonic decays of
W
and
Z
bosons and
J
/
ψ
mesons are employed to benchmark these performance parameters. The combination of all measurements results in identification efficiencies determined with an accuracy at the few per mil level for electron transverse energy greater than 30 GeV.
A
bstract
Multi-particle azimuthal cumulants are measured as a function of centrality and transverse momentum using 470
μ
b
−
1
of Pb+Pb collisions at
s
NN
= 5
.
02 TeV with the ATLAS detector at the ...LHC. These cumulants provide information on the event-by-event fluctuations of harmonic flow coefficients
v
n
and correlated fluctuations between two harmonics
v
n
and
v
m
. For the first time, a non-zero four-particle cumulant is observed for dipolar flow,
v
1
. The four-particle cumulants for elliptic flow,
v
2
, and triangular flow,
v
3
, exhibit a strong centrality dependence and change sign in ultra-central collisions. This sign change is consistent with significant non-Gaussian fluctuations in
v
2
and
v
3
. The four-particle cumulant for quadrangular flow,
v
4
, is found to change sign in mid-central collisions. Correlations between two harmonics are studied with three- and four-particle mixed-harmonic cumulants, which indicate an anti-correlation between
v
2 and
v
3, and a positive correlation between
v
2
and
v
4
. These correlations decrease in strength towards central collisions and either approach zero or change sign in ultra-central collisions. To investigate the possible flow fluctuations arising from intrinsic centrality or volume fluctuations, the results are compared between two different event classes used for centrality definitions. In peripheral and mid-central collisions where the cumulant signals are large, only small differences are observed. In ultra-central collisions, the differences are much larger and transverse momentum dependent. These results provide new information to disentangle flow fluctuations from the initial and final states, as well as new insights on the influence of centrality fluctuations.
This paper describes precision measurements of the transverse momentum
p
T
ℓ
ℓ
(
ℓ
=
e
,
μ
) and of the angular variable
ϕ
η
∗
distributions of Drell–Yan lepton pairs in a mass range of 66–116 GeV. ...The analysis uses data from 36.1 fb
-
1
of proton–proton collisions at a centre-of-mass energy of
s
=
13
TeV collected by the ATLAS experiment at the LHC in 2015 and 2016. Measurements in electron-pair and muon-pair final states are performed in the same fiducial volumes, corrected for detector effects, and combined. Compared to previous measurements in proton–proton collisions at
s
=
7
and
8
TeV, these new measurements probe perturbative QCD at a higher centre-of-mass energy with a different composition of initial states. They reach a precision of 0.2
%
for the normalized spectra at low values of
p
T
ℓ
ℓ
. The data are compared with different QCD predictions, where it is found that predictions based on resummation approaches can describe the full spectrum within uncertainties.
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