Studies of the spin and parity quantum numbers of the Higgs boson are presented, based on proton–proton collision data collected by the ATLAS experiment at the LHC. The Standard Model spin–parity ...JP=0+ hypothesis is compared with alternative hypotheses using the Higgs boson decays H→γγ, H→ZZ⁎→4ℓ and H→WW⁎→ℓνℓν, as well as the combination of these channels. The analysed dataset corresponds to an integrated luminosity of 20.7 fb−1 collected at a centre-of-mass energy of s=8TeV. For the H→ZZ⁎→4ℓ decay mode the dataset corresponding to an integrated luminosity of 4.6 fb−1 collected at s=7TeV is included. The data are compatible with the Standard Model JP=0+ quantum numbers for the Higgs boson, whereas all alternative hypotheses studied in this Letter, namely some specific JP=0−,1+,1−,2+ models, are excluded at confidence levels above 97.8%. This exclusion holds independently of the assumptions on the coupling strengths to the Standard Model particles and in the case of the JP=2+ model, of the relative fractions of gluon-fusion and quark–antiquark production of the spin-2 particle. The data thus provide evidence for the spin-0 nature of the Higgs boson, with positive parity being strongly preferred.
The Compact Linear Collider (CLIC) is an option for a future
e
+
e
-
collider operating at centre-of-mass energies up to
3
TeV
, providing sensitivity to a wide range of new physics phenomena and ...precision physics measurements at the energy frontier. This paper is the first comprehensive presentation of the Higgs physics reach of CLIC operating at three energy stages:
s
=
350
GeV
, 1.4 and
3
TeV
. The initial stage of operation allows the study of Higgs boson production in Higgsstrahlung (
e
+
e
-
→
Z
H
) and
W
W
-fusion (
e
+
e
-
→
H
ν
e
ν
¯
e
), resulting in precise measurements of the production cross sections, the Higgs total decay width
Γ
H
, and model-independent determinations of the Higgs couplings. Operation at
s
>
1
TeV
provides high-statistics samples of Higgs bosons produced through
W
W
-fusion, enabling tight constraints on the Higgs boson couplings. Studies of the rarer processes
e
+
e
-
→
t
t
¯
H
and
e
+
e
-
→
H
H
ν
e
ν
¯
e
allow measurements of the top Yukawa coupling and the Higgs boson self-coupling. This paper presents detailed studies of the precision achievable with Higgs measurements at CLIC and describes the interpretation of these measurements in a global fit.
A
bstract
The Compact Linear Collider (CLIC) is a proposed future high-luminosity linear electron-positron collider operating at three energy stages, with nominal centre-of-mass energies
s
= 380 GeV, ...1
.
5 TeV, and 3 TeV. Its aim is to explore the energy frontier, providing sensitivity to physics beyond the Standard Model (BSM) and precision measurements of Standard Model processes with an emphasis on Higgs boson and top-quark physics. The opportunities for top-quark physics at CLIC are discussed in this paper. The initial stage of operation focuses on top-quark pair production measurements, as well as the search for rare flavour-changing neutral current (FCNC) top-quark decays. It also includes a top-quark pair production threshold scan around 350 GeV which provides a precise measurement of the top-quark mass in a well-defined theoretical framework. At the higher-energy stages, studies are made of top-quark pairs produced in association with other particles. A study of t
̄
tH production including the extraction of the top Yukawa coupling is presented as well as a study of vector boson fusion (VBF) production, which gives direct access to high-energy electroweak interactions. Operation above 1 TeV leads to more highly collimated jet environments where dedicated methods are used to analyse the jet constituents. These techniques enable studies of the top-quark pair production, and hence the sensitivity to BSM physics, to be extended to higher energies. This paper also includes phenomenological interpretations that may be performed using the results from the extensive top-quark physics programme at CLIC.
A
bstract
The factor of four increase in the LHC luminosity, from 0
.
5
×
10
34
cm
−
2
s
−
1
to 2
.
0
×
10
34
cm
−
2
s
−
1
, and the corresponding increase in pile-up collisions during the 2015–2018 ...data-taking period, presented a challenge for the ATLAS trigger, particularly for those algorithms that select events with missing transverse momentum. The output data rate at fixed threshold typically increases exponentially with the number of pile-up collisions, so the legacy algorithms from previous LHC data-taking periods had to be tuned and new approaches developed to maintain the high trigger efficiency achieved in earlier operations. A study of the trigger performance and comparisons with simulations show that these changes resulted in event selection efficiencies of
>
98% for this period, meeting and in some cases exceeding the performance of similar triggers in earlier run periods, while at the same time keeping the necessary bandwidth within acceptable limits.
With the increase in energy of the Large Hadron Collider to a centre-of-mass energy of 13
TeV
for Run 2, events with dense environments, such as in the cores of high-energy jets, became a focus for ...new physics searches as well as measurements of the Standard Model. These environments are characterized by charged-particle separations of the order of the tracking detectors sensor granularity. Basic track quantities are compared between 3.2 fb
-
1
of data collected by the ATLAS experiment and simulation of proton–proton collisions producing high-transverse-momentum jets at a centre-of-mass energy of 13
TeV
. The impact of charged-particle separations and multiplicities on the track reconstruction performance is discussed. The track reconstruction efficiency in the cores of jets with transverse momenta between 200 and 1600
GeV
is quantified using a novel, data-driven, method. The method uses the energy loss,
d
E
/
d
x
, to identify pixel clusters originating from two charged particles. Of the charged particles creating these clusters, the measured fraction that fail to be reconstructed is
0.061
±
0.006
(stat.)
±
0.014
(syst.)
and
0.093
±
0.017
(stat.)
±
0.021
(syst.)
for jet transverse momenta of 200–400
GeV
and 1400–1600
GeV
, respectively.
This paper presents the performance of the ATLAS muon reconstruction during the LHC run with
p
p
collisions at
s
=
7
–8 TeV in 2011–2012, focusing mainly on data collected in 2012. Measurements of ...the reconstruction efficiency and of the momentum scale and resolution, based on large reference samples of
J
/
ψ
→
μ
μ
,
Z
→
μ
μ
and
Υ
→
μ
μ
decays, are presented and compared to Monte Carlo simulations. Corrections to the simulation, to be used in physics analysis, are provided. Over most of the covered phase space (muon
|
η
|
<
2.7
and
5
≲
p
T
≲
100
GeV) the efficiency is above
99
%
and is measured with per-mille precision. The momentum resolution ranges from
1.7
%
at central rapidity and for transverse momentum
p
T
≃
10
GeV, to
4
%
at large rapidity and
p
T
≃
100
GeV. The momentum scale is known with an uncertainty of
0.05
%
to
0.2
%
depending on rapidity. A method for the recovery of final state radiation from the muons is also presented.
A
bstract
A summary of the constraints from the ATLAS experiment on
R
-parity-conserving supersymmetry is presented. Results from 22 separate ATLAS searches are considered, each based on analysis of ...up to 20.3 fb
−1
of proton-proton collision data at centre-of-mass energies of
s
=
7
and 8 TeV at the Large Hadron Collider. The results are interpreted in the context of the 19-parameter phenomenological minimal supersymmetric standard model, in which the lightest supersymmetric particle is a neutralino, taking into account constraints from previous precision electroweak and flavour measurements as well as from dark matter related measurements. The results are presented in terms of constraints on supersymmetric particle masses and are compared to limits from simplified models. The impact of ATLAS searches on parameters such as the dark matter relic density, the couplings of the observed Higgs boson, and the degree of electroweak fine-tuning is also shown. Spectra for surviving supersymmetry model points with low fine-tunings are presented.
This paper presents the method and performance of primary vertex reconstruction in proton–proton collision data recorded by the ATLAS experiment during Run 1 of the LHC. The studies presented focus ...on data taken during 2012 at a centre-of-mass energy of
s
=
8
TeV. The performance has been measured as a function of the number of interactions per bunch crossing over a wide range, from one to seventy. The measurement of the position and size of the luminous region and its use as a constraint to improve the primary vertex resolution are discussed. A longitudinal vertex position resolution of about
30
μ
m
is achieved for events with high multiplicity of reconstructed tracks. The transverse position resolution is better than
20
μ
m
and is dominated by the precision on the size of the luminous region. An analytical model is proposed to describe the primary vertex reconstruction efficiency as a function of the number of interactions per bunch crossing and of the longitudinal size of the luminous region. Agreement between the data and the predictions of this model is better than 3% up to seventy interactions per bunch crossing.
The SuperB project is an asymmetric e+e− accelerator of 1036 cm−2s−1 luminosity, capable of collecting a 50–75 ab−1 data sample in five years of running. The SuperB electromagnetic calorimeter (EMC) ...provides energy and direction measurement of photons and electrons and identification versus other charged particles for electrons. A matrix of 25 LYSO crystals has been tested at the Beam Test Facility at Frascati in May 2011 at energies between 100 MeV and 500 MeV. Results from this test will be presented. Design and Monte Carlo studies for the general EMC system will also be presented.
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