The luminosity determination for the ATLAS detector at the LHC during Run 2 is presented, with
pp
collisions at a centre-of-mass energy
s
=
13
TeV. The absolute luminosity scale is determined using ...van der Meer beam separation scans during dedicated running periods in each year, and extrapolated to the physics data-taking regime using complementary measurements from several luminosity-sensitive detectors. The total uncertainties in the integrated luminosity for each individual year of data-taking range from 0.9% to 1.1%, and are partially correlated between years. After standard data-quality selections, the full Run 2
pp
data sample corresponds to an integrated luminosity of
140.1
±
1.2
fb
-
1
, i.e. an uncertainty of 0.83%. A dedicated sample of low-pileup data recorded in 2017–2018 for precision Standard Model physics measurements is analysed separately, and has an integrated luminosity of
338.1
±
3.1
pb
-
1
.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A
bstract
A determination of the top-quark mass is presented using 20.2 fb
−
1
of 8 TeV proton-proton collision data produced by the Large Hadron Collider and collected by the ATLAS experiment. The ...normalised differential cross section of top-quark pair production in association with an energetic jet is measured in the lepton+jets final state and unfolded to parton and particle levels. The unfolded distribution at parton level can be described using next-to-leading-order QCD predictions in terms of either the top-quark pole mass or the running mass as defined in the (modified) minimal subtraction scheme. A comparison between the experimental distribution and the theoretical prediction allows the top-quark mass to be extracted in the two schemes. The value obtained for the pole-mass scheme is:
m
t
pole
=
171.1
±
0.4
stat
±
0.9
syst
−
0.3
+
0.7
theo
GeV
The extracted value in the running-mass scheme is:
m
t
m
t
=
162.9
±
0.5
stat
±
1.0
syst
−
1.2
+
2.1
theo
GeV
.
The results for the top-quark mass using the two schemes are consistent, when translated from one scheme to the other.
A
bstract
A direct search for Higgs bosons produced via vector-boson fusion and subsequently decaying into invisible particles is reported. The analysis uses 139 fb
−
1
of
pp
collision data at a ...centre-of-mass energy of
s
= 13 TeV recorded by the ATLAS detector at the LHC. The observed numbers of events are found to be in agreement with the background expectation from Standard Model processes. For a scalar Higgs boson with a mass of 125 GeV and a Standard Model production cross section, an observed upper limit of 0
.
145 is placed on the branching fraction of its decay into invisible particles at 95% confidence level, with an expected limit of 0
.
103. These results are interpreted in the context of models where the Higgs boson acts as a portal to dark matter, and limits are set on the scattering cross section of weakly interacting massive particles and nucleons. Invisible decays of additional scalar bosons with masses from 50 GeV to 2 TeV are also studied, and the derived upper limits on the cross section times branching fraction decrease with increasing mass from 1
.
0 pb for a scalar boson mass of 50 GeV to 0
.
1 pb at a mass of 2 TeV.
A
bstract
A search for decays of pair-produced neutral long-lived particles (LLPs) is presented using 139 fb
−
1
of proton-proton collision data collected by the ATLAS detector at the LHC in ...2015–2018 at a centre-of-mass energy of 13 TeV. Dedicated techniques were developed for the reconstruction of displaced jets produced by LLPs decaying hadronically in the ATLAS hadronic calorimeter. Two search regions are defined for different LLP kinematic regimes. The observed numbers of events are consistent with the expected background, and limits for several benchmark signals are determined. For a SM Higgs boson with a mass of 125 GeV, branching ratios above 10% are excluded at 95% confidence level for values of
c
times LLP mean proper lifetime in the range between 20 mm and 10 m depending on the model. Upper limits are also set on the cross-section times branching ratio for scalars with a mass of 60 GeV and for masses between 200 GeV and 1 TeV.
A
bstract
Differential and double-differential distributions of kinematic variables of leptons from decays of top-quark pairs (
t
t
¯
) are measured using the full LHC Run 2 data sample collected ...with the ATLAS detector. The data were collected at a
pp
collision energy of
s
= 13 TeV and correspond to an integrated luminosity of 140 fb
−
1
. The measurements use events containing an oppositely charged
eμ
pair and
b
-tagged jets. The results are compared with predictions from several Monte Carlo generators. While no prediction is found to be consistent with all distributions, a better agreement with measurements of the lepton
p
T
distributions is obtained by reweighting the
t
t
¯
sample so as to reproduce the top-quark
p
T
distribution from an NNLO calculation. The inclusive top-quark pair production cross-section is measured as well, both in a fiducial region and in the full phase-space. The total inclusive cross-section is found to be
σ
t
t
¯
=
829
±
1
stat
±
13
syst
±
8
lumi
±
2
beam
pb
,
where the uncertainties are due to statistics, systematic effects, the integrated luminosity and the beam energy. This is in excellent agreement with the theoretical expectation.
A
bstract
This paper presents a search for hypothetical massive, charged, long-lived particles with the ATLAS detector at the LHC using an integrated luminosity of 139 fb
−
1
of proton–proton ...collisions at
s
= 13 TeV. These particles are expected to move significantly slower than the speed of light and should be identifiable by their high transverse momenta and anomalously large specific ionisation losses, d
E/
d
x
. Trajectories reconstructed solely by the inner tracking system and a d
E/
d
x
measurement in the pixel detector layers provide sensitivity to particles with lifetimes down to
O
(1) ns with a mass, measured using the Bethe–Bloch relation, ranging from 100 GeV to 3 TeV. Interpretations for pair-production of
R
-hadrons, charginos and staus in scenarios of supersymmetry compatible with these particles being long-lived are presented, with mass limits extending considerably beyond those from previous searches in broad ranges of lifetime.
A
bstract
Cross-section measurements of top-quark pair production where the hadronically decaying top quark has transverse momentum greater than 355 GeV and the other top quark decays into
ℓνb
are ...presented using 139 fb
−
1
of data collected by the ATLAS experiment during proton-proton collisions at the LHC. The fiducial cross-section at
s
= 13 TeV is measured to be
σ
= 1
.
267
±
0
.
005
±
0
.
053 pb, where the uncertainties reflect the limited number of data events and the systematic uncertainties, giving a total uncertainty of 4
.
2%. The cross-section is measured differentially as a function of variables characterising the
t
t
¯
system and additional radiation in the events. The results are compared with various Monte Carlo generators, including comparisons where the generators are reweighted to match a parton-level calculation at next-to-next-to-leading order. The reweighting improves the agreement between data and theory. The measured distribution of the top-quark transverse momentum is used to search for new physics in the context of the effective field theory framework. No significant deviation from the Standard Model is observed and limits are set on the Wilson coefficients of the dimension-six operators
O
tG
and
O
tq
8
, where the limits on the latter are the most stringent to date.
A search for long-lived charginos produced either directly or in the cascade decay of heavy prompt gluino states is presented. The search is based on proton–proton collision data collected at a ...centre-of-mass energy of
s
= 13 T
eV
between 2015 and 2018 with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 136 fb
-
1
. Long-lived charginos are characterised by a distinct signature of a short and then disappearing track, and are reconstructed using at least four measurements in the ATLAS pixel detector, with no subsequent measurements in the silicon-microstrip tracking volume nor any associated energy deposits in the calorimeter. The final state is complemented by a large missing transverse-momentum requirement for triggering purposes and at least one high-transverse-momentum jet. No excess above the expected backgrounds is observed. Exclusion limits are set at 95% confidence level on the masses of the chargino and gluino for different chargino lifetimes. Chargino masses up to 660 (210) G
eV
are excluded in scenarios where the chargino is a pure wino (higgsino). For charginos produced during the cascade decay of a heavy gluino, gluinos with masses below 2.1 T
eV
are excluded for a chargino mass of 300 G
eV
and a lifetime of 0.2 ns.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A search for the pair production of heavy leptons as predicted by the type-III seesaw mechanism is presented. The search uses proton–proton collision data at a centre-of-mass energy of 13 TeV, ...corresponding to
139
fb
-
1
of integrated luminosity recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis focuses on final states with three or four electrons or muons from the possible decays of new heavy leptons via intermediate electroweak bosons. No significant deviations above the Standard Model expectation are observed; upper and lower limits on the heavy lepton production cross-section and masses are derived respectively. These results are then combined for the first time with the ones already published by ATLAS using the channel with two leptons in the final state. The observed lower limit on the mass of the type-III seesaw heavy leptons combining two, three and four lepton channels together is 910 GeV at the 95% confidence level.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A
bstract
A search for flavour-changing neutral-current decays of a top quark into an up-type quark (either up or charm) and a light scalar particle
X
decaying into a bottom anti-bottom quark pair is ...presented. The search focuses on top-quark pair production where one top quark decays to
qX
, with
X
→
b
b
¯
, and the other top quark decays according to the Standard Model, with the
W
boson decaying leptonically. The final state is thus characterised by an isolated electron or muon and at least four jets. Events are categorised according to the multiplicity of jets and jets tagged as originating from
b
-quarks, and a neural network is used to discriminate between signal and background processes. The data analysed correspond to 139 fb
−
1
of proton–proton collisions at a centre-of-mass energy of 13 TeV, recorded with the ATLAS detector at the LHC. The 95% confidence-level upper limits between 0.019% and 0.062% are derived for the branching fraction
B
(
t
→
uX
) and between 0.018% and 0.078% for the branching fraction
B
(
t
→
cX
), for masses of the scalar particle
X
between 20 and 160 GeV.
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