A 6.8 nb -1 sample of pp collision data collected under low-luminosity conditions at s=7TeV by the ATLAS detector at the Large Hadron Collider is used to study diffractive dijet production. Events ...containing at least two jets with p T > 20GeV are selected and analysed in terms of variables which discriminate between diffractive and non-diffractive processes. Cross sections are measured differentially in δη F , the size of the observable forward region of pseudorapidity which is devoid of hadronic activity, and in an estimator, ξ~, of the fractional momentum loss of the proton assuming single diffractive dissociation (pp→pX). Model comparisons indicate a dominant non-diffractive contribution up to moderately large δη F and small ξ~, with a diffractive contribution which is significant at the highest δη F and the lowest ξ~. The rapidity-gap survival probability is estimated from comparisons of the data in this latter region with predictions based on diffractive parton distribution functions.
The ATLAS experiment has observed 1995 Z boson candidates in data corresponding to 0.15 nb-1 of integrated luminosity obtained in the 2011 LHC Pb + Pb run at √ s N N = 2.76 TeV . The Z bosons are ...reconstructed via dielectron and dimuon decay channels, with a background contamination of less than 3%. Results from the two channels are consistent and are combined. Within the statistical and systematic uncertainties, the per-event Z boson yield is proportional to the number of binary collisions estimated by the Glauber model. The elliptic anisotropy of the azimuthal distribution of the Z boson with respect to the event plane is found to be consistent with zero.
The result of a search for pair production of the supersymmetric partner of the Standard Model bottom quark (
b
~
1
) is reported. The search uses 3.2 fb
-
1
of
pp
collisions at
s
=
13
TeV ...collected by the ATLAS experiment at the Large Hadron Collider in 2015. Bottom squarks are searched for in events containing large missing transverse momentum and exactly two jets identified as originating from
b
-quarks. No excess above the expected Standard Model background yield is observed. Exclusion limits at 95 % confidence level on the mass of the bottom squark are derived in phenomenological supersymmetric
R
-parity-conserving models in which the
b
~
1
is the lightest squark and is assumed to decay exclusively via
b
~
1
→
b
χ
~
1
0
, where
χ
~
1
0
is the lightest neutralino. The limits significantly extend previous results; bottom squark masses up to 800 (840) GeV are excluded for the
χ
~
1
0
mass below 360 (100) GeV whilst differences in mass above 100 GeV between the
b
~
1
and the
χ
~
1
0
are excluded up to a
b
~
1
mass of 500 GeV.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A search for heavy long-lived multi-charged particles is performed using the ATLAS detector at the LHC. Data collected in 2012 at
s
=
8
TeV from
pp
collisions corresponding to an integrated ...luminosity of 20.3 fb
-
1
are examined. Particles producing anomalously high ionisation, consistent with long-lived massive particles with electric charges from
|
q
|
=
2
e
to
|
q
|
=
6
e
are searched for. No signal candidate events are observed, and 95 % confidence level cross-section upper limits are interpreted as lower mass limits for a Drell–Yan production model. The mass limits range between 660 and 785 GeV.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A
bstract
This paper presents a search for new particles in events with one lepton (electron or muon) and missing transverse momentum using 20.3 fb
−1
of proton-proton collision data at
s
= 8 TeV ...recorded by the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. A
W
′ with Sequential Standard Model couplings is excluded at the 95% confidence level for masses up to 3.24 TeV. Excited chiral bosons (
W
*
) with equivalent coupling strengths are excluded for masses up to 3.21 TeV. In the framework of an effective field theory limits are also set on the dark matter-nucleon scattering cross-section as well as the mass scale
M
*
of the unknown mediating interaction for dark matter pair production in association with a leptonically decaying
W
.
A
bstract
A search for flavour-changing neutral current decays of a top quark to an uptype quark (
q
=
u
,
c
) and the Standard Model Higgs boson, where the Higgs boson decays to
b
b
¯
, is ...presented. The analysis searches for top quark pair events in which one top quark decays to
Wb
, with the
W
boson decaying leptonically, and the other top quark decays to
Hq
. The search is based on
pp
collisions at
s
=
8
TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider and uses an integrated luminosity of 20.3 fb
−1
. Data are analysed in the lepton-plus-jets final state, characterised by an isolated electron or muon and at least four jets. The search exploits the high multiplicity of
b
-quark jets characteristic of signal events, and employs a likelihood discriminant that uses the kinematic differences between the signal and the background, which is dominated by
t
t
¯
→
WbWb
decays. No significant excess of events above the background expectation is found, and observed (expected) 95% CL upper limits of 0.56% (0.42%) and 0.61% (0.64%) are derived for the
t
→
Hc
and
t
→
Hu
branching ratios respectively. The combination of this search with other ATLAS searches in the
H
→
γγ
and
H
→
WW
*
,
ττ
decay modes significantly improves the sensitivity, yielding observed (expected) 95% CL upper limits on the
t
→
Hc
and
t
→
Hu
branching ratios of 0.46% (0.25%) and 0.45% (0.29%) respectively. The corresponding combined observed (expected) upper limits on the |
λ
tcH
| and |
λ
tuH
| couplings are 0.13 (0.10) and 0.13 (0.10) respectively. These are the most restrictive direct bounds on
tqH
interactions measured so far.
A
bstract
The inclusive jet cross-section is measured in proton-proton collisions at a centre-of-mass energy of 7 TeV using a data set corresponding to an integrated luminosity of 4.5 fb
−1
collected ...with the ATLAS detector at the Large Hadron Collider in 2011. Jets are identified using the anti-
k
t
algorithm with radius parameter values of 0.4 and 0.6. The double-differential cross-sections are presented as a function of the jet transverse momentum and the jet rapidity, covering jet transverse momenta from 100 GeV to 2 TeV. Next-to-leading-order QCD calculations corrected for non-perturbative effects and electroweak effects, as well as Monte Carlo simulations with next-to-leading-order matrix elements interfaced to parton showering, are compared to the measured cross-sections. A quantitative comparison of the measured cross-sections to the QCD calculations using several sets of parton distribution functions is performed.
A
bstract
A search is conducted for new physics in multijet final states using 3.6 inverse femtobarns of data from proton-proton collisions at
s
=
13
TeV taken at the CERN Large Hadron Collider with ...the ATLAS detector. Events are selected containing at least three jets with scalar sum of jet transverse momenta (
H
T
) greater than 1 TeV. No excess is seen at large
H
T
and limits are presented on new physics: models which produce final states containing at least three jets and having cross sections larger than 1.6 fb with
H
T
>
5.8 TeV are excluded. Limits are also given in terms of new physics models of strong gravity that hypothesize additional space-time dimensions.
A search for the production of single-top-quarks in association with missing energy is performed in proton–proton collisions at a centre-of-mass energy of √s = 8 Tev with the ATLAS experiment at the ...large hadron collider using data collected in 2012, corresponding to an integrated luminosity of 20.3 fb -1 . In this search, the W boson from the top quark is required to decay into an electron or a muon and a neutrino. No deviation from the standard model prediction is observed, and upper limits are set on the production cross-section for resonant and non-resonant production of an invisible exotic state in association with a right-handed top quark. In the case of resonant production, for a spin-0 resonance with a mass of 500 GeV, an effective coupling strength above 0.15 confidence level for the top quark and an invisible spin-1/2 state with mass between 0 and 100 GeV. In the case of non-resonant production, an effective coupling strength above 0.2 is excluded at 95% confidence level for the top quark and an invisible spin-1 state with mass between 0 and 657 GeV.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
PDF
