Studies in many areas of particle and astroparticle physics require a good knowledge of hadron spectra produced at small angles to the primary particle direction, e.g. in the TeV energy range at the ...LHC. The present work is dedicated to development of a particle identification technique based on the transition radiation produced by highly relativistic particles. Dedicated experimental studies were carried out and simulation models were developed to reproduce experimental data obtained with different prototypes. On the basis of these studies, the possibility of making detectors able to provide hadron composition measurement with high accuracy and identify particles with high efficiency in the forward experiments at the LHC was demonstrated. Two concepts of large scale TRDs are proposed: the straw TRD and the GaAs/straw TRD. They allow the reconstruction of hadron spectra with accuracy about or below 1% and to identify particles with high efficiency in individual events. As an example, suppression of the combinatorial background was demonstrated for the D0→K−π+ and D̄0→K+π− decays.
Studying of hadron production in forward direction at the LHC energy has a great interest both for understanding of the fundamental QCD processes and also in applied areas such as the description of ...ultra-high energy cosmic particle interactions. The energies of secondary hadrons in such studies almost reach the maximum energy available at the LHC of ~6 TeV, which corresponds to a Lorentz γ-factor up to 104 and above. The only effective technique able to identify particles in this range is based on the transition radiation detectors (TRD). Prototypes of such kind of detector were built and tested at the CERN SPS accelerator. Some experimental results obtained in these tests are briefly presented here and compared with Monte Carlo (MC) simulations. MC model demonstrates a good agreement with the experiment. On this basis a concept of a full-scale TRD optimized for the hadron identification in the TeV energy region is proposed. Different particle identification techniques were considered and examined. The expected detector performance to reconstruct secondary hadrons produced in forward direction at the LHC is presented.
This paper presents the observation of four-top-quark (
t
t
¯
t
t
¯
) production in proton-proton collisions at the LHC. The analysis is performed using an integrated luminosity of 140
fb
-
1
at a ...centre-of-mass energy of 13 TeV collected using the ATLAS detector. Events containing two leptons with the same electric charge or at least three leptons (electrons or muons) are selected. Event kinematics are used to separate signal from background through a multivariate discriminant, and dedicated control regions are used to constrain the dominant backgrounds. The observed (expected) significance of the measured
t
t
¯
t
t
¯
signal with respect to the standard model (SM) background-only hypothesis is 6.1 (4.3) standard deviations. The
t
t
¯
t
t
¯
production cross section is measured to be
22
.
5
-
5.5
+
6.6
fb, consistent with the SM prediction of
12.0
±
2.4
fb within 1.8 standard deviations. Data are also used to set limits on the three-top-quark production cross section, being an irreducible background not measured previously, and to constrain the top-Higgs Yukawa coupling and effective field theory operator coefficients that affect
t
t
¯
t
t
¯
production.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A
bstract
Measurements of the production cross-sections of the Standard Model (SM) Higgs boson (
H
) decaying into a pair of
τ
-leptons are presented. The measurements use data collected with the ...ATLAS detector from
pp
collisions produced at the Large Hadron Collider at a centre-of-mass energy of
s
= 13 TeV, corresponding to an integrated luminosity of 139 fb
−
1
. Leptonic (
τ
→
ℓν
ℓ
ν
τ
) and hadronic (
τ
→ hadrons
ν
τ
) decays of the
τ
-lepton are considered. All measurements account for the branching ratio of
H → ττ
and are performed with a requirement
|y
H
| <
2
.
5, where
y
H
is the true Higgs boson rapidity. The cross-section of the
pp → H → ττ
process is measured to be 2
.
94
±
0.21
stat
−
0.32
+
0.37
(syst) pb, in agreement with the SM prediction of 3
.
17
±
0
.
09 pb. Inclusive cross-sections are determined separately for the four dominant production modes: 2
.
65
±
0.41
stat
−
0.67
+
0.91
(syst) pb for gluon-gluon fusion, 0
.
197
±
0.028
stat
−
0.026
+
0.032
(syst) pb for vector-boson fusion, 0
.
115
±
0.058
stat
−
0.040
+
0.042
(syst) pb for vector-boson associated production, and 0
.
033
±
0.031
stat
−
0.017
+
0.022
(syst) pb for top-quark pair associated production. Measurements in exclusive regions of the phase space, using the simplified template cross-section framework, are also performed. All results are in agreement with the SM predictions.
A
bstract
The associated production of a Higgs boson and a top-quark pair is measured in events characterised by the presence of one or two electrons or muons. The Higgs boson decay into a
b
-quark ...pair is used. The analysed data, corresponding to an integrated luminosity of 139 fb
−
1
, were collected in proton-proton collisions at the Large Hadron Collider between 2015 and 2018 at a centre-of-mass energy of
s
= 13 TeV. The measured signal strength, defined as the ratio of the measured signal yield to that predicted by the Standard Model, is
0.35
−
0.34
+
0.36
. This result is compatible with the Standard Model prediction and corresponds to an observed (expected) significance of 1.0 (2.7) standard deviations. The signal strength is also measured differentially in bins of the Higgs boson transverse momentum in the simplified template cross-section framework, including a bin for specially selected boosted Higgs bosons with transverse momentum above 300 GeV.
This paper presents the muon momentum calibration and performance studies for the ATLAS detector based on the
pp
collisions data sample produced at
s
= 13 TeV at the LHC during Run 2 and ...corresponding to an integrated luminosity of 139
fb
-
1
. An innovative approach is used to correct for potential charge-dependent momentum biases related to the knowledge of the detector geometry, using the
Z
→
μ
+
μ
-
resonance. The muon momentum scale and resolution are measured using samples of
J
/
ψ
→
μ
+
μ
-
and
Z
→
μ
+
μ
-
events. A calibration procedure is defined and applied to simulated data to match the performance measured in real data. The calibration is validated using an independent sample of
Υ
→
μ
+
μ
-
events. At the
Z
(
J
/
ψ
)
peak, the momentum scale is measured with an uncertainty at the 0.05% (0.1%) level, and the resolution is measured with an uncertainty at the 1.5% (2%) level. The charge-dependent bias is removed with a dedicated in situ correction for momenta up to 450 GeV with a precision better than 0.03
TeV
-
1
.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A
bstract
Measurements of Higgs boson production cross-sections are carried out in the diphoton decay channel using 139 fb
−
1
of
pp
collision data at
s
= 13 TeV collected by the ATLAS experiment at ...the LHC. The analysis is based on the definition of 101 distinct signal regions using machine-learning techniques. The inclusive Higgs boson signal strength in the diphoton channel is measured to be
1.04
−
0.09
+
0.10
. Cross-sections for gluon-gluon fusion, vector-boson fusion, associated production with a
W
or
Z
boson, and top associated production processes are reported. An upper limit of 10 times the Standard Model prediction is set for the associated production process of a Higgs boson with a single top quark, which has a unique sensitivity to the sign of the top quark Yukawa coupling. Higgs boson production is further characterized through measurements of Simplified Template Cross-Sections (STXS). In total, cross-sections of 28 STXS regions are measured. The measured STXS cross-sections are compatible with their Standard Model predictions, with a
p
-value of 93%. The measurements are also used to set constraints on Higgs boson coupling strengths, as well as on new interactions beyond the Standard Model in an effective field theory approach. No significant deviations from the Standard Model predictions are observed in these measurements, which provide significant sensitivity improvements compared to the previous ATLAS results.
A search is presented for the production of a single top quark via left-handed flavour-changing neutral-current (FCNC) interactions of a top quark, a gluon and an up or charm quark. Two production ...processes are considered:
u
+
g
→
t
and
c
+
g
→
t
. The analysis is based on proton–proton collision data taken at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC. The data set corresponds to an integrated luminosity of 139 fb
-
1
. Events with exactly one electron or muon, exactly one
b
-tagged jet and missing transverse momentum are selected, resembling the decay products of a singly produced top quark. Neural networks based on kinematic variables differentiate between events from the two signal processes and events from background processes. The measured data are consistent with the background-only hypothesis, and limits are set on the production cross-sections of the signal processes:
σ
(
u
+
g
→
t
)
×
B
(
t
→
W
b
)
×
B
(
W
→
ℓ
ν
)
<
3.0
pb and
σ
(
c
+
g
→
t
)
×
B
(
t
→
W
b
)
×
B
(
W
→
ℓ
ν
)
<
4.7
pb at the 95% confidence level, with
B
(
W
→
ℓ
ν
)
=
0.325
being the sum of branching ratios of all three leptonic decay modes of the
W
boson. Based on the framework of an effective field theory, the cross-section limits are translated into limits on the strengths of the
tug
and
tcg
couplings occurring in the theory:
|
C
uG
u
t
|
/
Λ
2
<
0.057
TeV
-
2
and
|
C
uG
c
t
|
/
Λ
2
<
0.14
TeV
-
2
. These bounds correspond to limits on the branching ratios of FCNC-induced top-quark decays:
B
(
t
→
u
+
g
)
<
0.61
×
10
-
4
and
B
(
t
→
c
+
g
)
<
3.7
×
10
-
4
.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A
bstract
Measurements of transverse energy-energy correlations and their associated azimuthal asymmetries in multijet events are presented. The analysis is performed using a data sample ...corresponding to 139 fb
−
1
of proton-proton collisions at a centre-of-mass energy of
s
= 13 TeV, collected with the ATLAS detector at the Large Hadron Collider. The measurements are presented in bins of the scalar sum of the transverse momenta of the two leading jets and unfolded to particle level. They are then compared to next-to-next-to-leading-order perturbative QCD calculations for the first time, which feature a significant reduction in the theoretical uncertainties estimated using variations of the renormalisation and factorisation scales. The agreement between data and theory is good, thus providing a precision test of QCD at large momentum transfers
Q
. The strong coupling constant
α
s
is extracted as a function of
Q
, showing a good agreement with the renormalisation group equation and with previous analyses. A simultaneous fit to all transverse energy-energy correlation distributions across different kinematic regions yields a value of
α
s
m
Z
=
0.1175
±
0.0006
exp
.
−
0.0017
+
0.0034
theo
.
, while the global fit to the asymmetry distributions yields
α
s
m
Z
=
0.1185
±
0.0009
exp
.
−
0.0012
+
0.0025
theo
.
.
This Letter presents the first study of Higgs boson production in association with a vector boson (V=W or Z) in the fully hadronic qqbb final state using data recorded by the ATLAS detector at the ...LHC in proton-proton collisions at sqrts=13 TeV and corresponding to an integrated luminosity of 137 fb^{-1}. The vector bosons and Higgs bosons are each reconstructed as large-radius jets and tagged using jet substructure techniques. Dedicated tagging algorithms exploiting b-tagging properties are used to identify jets consistent with Higgs bosons decaying into bbover ¯. Dominant backgrounds from multijet production are determined directly from the data, and a likelihood fit to the jet mass distribution of Higgs boson candidates is used to extract the number of signal events. The VH production cross section is measured inclusively and differentially in several ranges of Higgs boson transverse momentum: 250-450, 450-650, and greater than 650 GeV. The inclusive signal yield relative to the standard model expectation is observed to be μ=1.4_{-0.9}^{+1.0} and the corresponding cross section is 3.1±1.3(stat)_{-1.4}^{+1.8}(syst) pb.
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