8,953 research outputs found
Search for new phenomena in high-mass diphoton final states using 37 fb(-1) of proton-proton collisions collected at root(s) = 13 TeV with the ATLAS detector
Searches for new phenomena in high-mass diphoton final states with the
ATLAS experiment at the LHC are presented. The analysis is based on pp
collision data corresponding to an integrated luminosity of 36.7 fb(-1)
at a centre-of-mass energy root s = 13 TeV recorded in 2015 and 2016.
Searches are performed for resonances with spin 0, as predicted by
theories with an extended Higgs sector, and for resonances with spin 2,
using a warped extra-dimension model as a benchmark model, as well as
for non-resonant signals, assuming a large extra-dimension scenario. No
significant deviation from the Standard Model is observed. Upper limits
are placed on the production cross section times branching ratio to two
photons as a function of the resonance mass. In addition, lower limits
are set on the ultraviolet cutoff scale in the large extra-dimensions
model. (C) 2017 The Author(s). Published by Elsevier B.V
Measurement of jet activity produced in top-quark events with an electron, a muon and two b-tagged jets in the final state in pp collisions root s=13TeV with the ATLAS detector
Measurements of jet activity in top-quark pair events produced in
proton-proton collisions are presented, using 3.2 fb of pp collision
data at a centre-of-mass energy of 13 TeV collected by the ATLAS
experiment at the Large Hadron Collider. Events are chosen by requiring
an opposite-charge pair and two b-tagged jets in the final state. The
normalised differential cross-sections of top-quark pair production are
presented as functions of additional-jet multiplicity and transverse
momentum, . The fraction of signal events that do not contain additional
jet activity in a given rapidity region, the gap fraction, is measured
as a function of the threshold for additional jets, and is also
presented for different invariant mass regions of the system. All
measurements are corrected for detector effects and presented as
particle-level distributions compared to predictions with different
theoretical approaches for QCD radiation. While the kinematics of the
jets from top-quark decays are described well, the generators show
differing levels of agreement with the measurements of observables that
depend on the production of additional jets
Search for new phenomena in dijet events using 37 fb(-1) of pp collision data collected at root s=13 TeV with the ATLAS detector
Dijet events are studied in the proton-proton collision dataset recorded
at root s = 13 TeV with the ATLAS detector at the Large Hadron Collider
in 2015 and 2016, corresponding to integrated luminosities of 3.5 fb(-1)
and 33.5 fb(-1) respectively. Invariant mass and angular distributions
are compared to background predictions and no significant deviation is
observed. For resonance searches, a new method for fitting the
background component of the invariant mass distribution is employed. The
dataset is then used to set upper limits at a 95\% confidence level on a
range of new physics scenarios. Excited quarks with masses below 6.0 TeV
are excluded, and limits are set on quantum black holes, heavy W'
bosons, W* bosons, and a range of masses and couplings in a Z' dark
matter mediator model. Model-independent limits on signals with a
Gaussian shape are also set, using a new approach allowing factorization
of physics and detector effects. From the angular distributions, a scale
of new physics in contact interaction models is excluded for scenarios
with either constructive or destructive interference. These results
represent a substantial improvement over those obtained previously with
lower integrated luminosity
Reconstruction of primary vertices at the ATLAS experiment in Run 1 proton-proton collisions at the LHC
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 root 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 mu m is
achieved for events with high multiplicity of reconstructed tracks. The
transverse position resolution is better than 20 mu 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
Search for new phenomena in events containing a same-flavour opposite-sign dilepton pair, jets, and large missing transverse momentum in root s=13 TeV pp collisions with the ATLAS detector
Two searches for new phenomena in final states containing a same-flavour
opposite-sign lepton (electron or muon) pair, jets, and large missing
transverse momentum are presented. These searches make use of
proton-proton collision data, collected during 2015 and 2016 at a
centre-of-mass energy root s = 13 TeV by the ATLAS detector at the large
hadron collider, which correspond to an integrated luminosity of 14.7
fb(-1). Both searches target the pair production of supersymmetric
particles, squarks or gluinos, which decay to final states containing a
same-flavour opposite-sign lepton pair via one of two mechanisms: a
leptonically decaying Z boson in the final state, leading to a peak in
the dilepton invariant-mass distribution around the Z boson mass; and
decays of neutralinos (e.g.. (chi) over tilde (0)(2) -> l(+)l(-1)(chi)
over tilde (0)(1)), yielding a kinematic endpoint in the dilepton
invariant-mass spectrum. The data are found to be consistent with the
Standard Model expectation. Results are interpreted in simplified models
of gluino-pair (squark-pair) production, and provide sensitivity to
gluinos (squarks) with masses as large as 1.70 TeV (980 GeV)
Measurements of top-quark pair differential cross-sections in the lepton+jets channel in pp collisions at sqrt(s) = 8 TeV using the ATLAS detector
Measurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark, ttÂŻ system and event-level kinematic observables in proton-proton collisions at a centre-of-mass energy of sâ=8 TeV. The observables have been chosen to emphasize the ttÂŻ production process and to be sensitive to effects of initial- and final-state radiation, to the different parton distribution functions, and to non-resonant processes and higher-order corrections. The dataset corresponds to an integrated luminosity of 20.3 fbâ1, recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of the jets tagged as originating from a b-quark. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations. The results are in fair agreement with the predictions over a wide kinematic range. Nevertheless, most generators predict a harder top-quark transverse momentum distribution at high values than what is observed in the data. The agreement for this observable improves when next-to-next-to-leading order corrections are taken into account. Using the current settings and parton distribution functions, the rapidity distributions are not well modelled by any generator under consideration. However, the level of agreement is improved when more recent sets of parton distribution functions are used
Measurements of top quark spin observables in tt events using dilepton final states in root s=8 TeV pp collisions with the ATLAS detector
Measurements of top quark spin observables in tt events are presented
based on 20.2 fb(-1) of root s = 8TeV proton-proton collisions recorded
with the ATLAS detector at the LHC. The analysis is performed in the
dilepton final state, characterised by the presence of two isolated
leptons ( electrons or muons). There are 15 observables, each sensitive
to a different coefficient of the spin density matrix of tt production,
which are measured independently. Ten of these observables are measured
for the first time. All of them are corrected for detector resolution
and acceptance effects back to the parton and stable-particle levels.
The measured values of the observables at parton level are compared to
Standard Model predictions at next-to-leading order in QCD. The
corrected distributions at stable-particle level are presented and the
means of the distributions are compared to Monte Carlo predictions. No
significant deviation from the Standard Model is observed for any
observable
Search for supersymmetry in final states with two same-sign or three leptons and jets using 36 fbâ1 of sqrt(s) = 13 TeV pp collision data with the ATLAS detector
A search for strongly produced supersymmetric particles using signatures involving multiple energetic jets and either two isolated same-sign leptons (e or ÎŒ), or at least three isolated leptons, is presented. The analysis relies on the identification of b-jets and high missing transverse momentum to achieve good sensitivity. A data sample of proton-proton collisions at sqrt(s) = 13 TeV recorded with the ATLAS detector at the Large Hadron Collider in 2015 and 2016, corresponding to a total integrated luminosity of 36.1 fbâ1, is used for the search. No significant excess over the Standard Model prediction is observed. The results are interpreted in several simplified supersymmetric models featuring R-parity conservation or R-parity violation, extending the exclusion limits from previous searches. In models considering gluino pair production, gluino masses are excluded up to 1.87 TeV at 95% confidence level. When bottom squarks are pair-produced and decay to a chargino and a top quark, models with bottom squark masses below 700 GeV and light neutralinos are excluded at 95% confidence level. In addition, model-independent limits are set on a possible contribution of new phenomena to the signal region yields
Search for pair production of heavy vector-like quarks decaying to high-pT W bosons and b quarks in the lepton-plus-jets final state in pp collisions at sqrt(s) = 13 TeV with the ATLAS detector
A search is presented for the pair production of heavy vector-like T quarks, primarily targeting the T quark decays to a W boson and a b-quark. The search is based on 36.1 fbâ1 of pp collisions at sqrt(s) = 13 TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Data are analysed in the lepton-plus-jets final state, including at least one b-tagged jet and a large-radius jet identified as originating from the hadronic decay of a high-momentum W boson. No significant deviation from the Standard Model expectation is observed in the reconstructed T mass distribution. The observed 95% confidence level lower limit on the T mass are 1350 GeV assuming 100% branching ratio to Wb. In the SU(2) singlet scenario, the lower mass limit is 1170 GeV. This search is also sensitive to a heavy vector-like B quark decaying to Wt and other final states. The results are thus reinterpreted to provide a 95% confidence level lower limit on the B quark mass at 1250 GeV assuming 100% branching ratio to Wt; in the SU(2) singlet scenario, the limit is 1080 GeV. Mass limits on both T and B production are also set as a function of the decay branching ratios. The 100% branching ratio limits are found to be applicable to heavy vector-like Y and X production that decay to Wb and Wt, respectively
ATLAS RPC Quality Assurance results at INFN Lecce
The main results of the quality assurance tests performed on the Resistive
Plate Chamber used by the ATLAS experiment at LHC as muon trigger chambers are
reported and discussed.
Since July 2004, about 270 RPC units has been certified at INFN Lecce site
and delivered to CERN, for being integrated in the final muon station of the
ATLAS barrel region.
We show the key RPC characteristics which qualify the performance of this
detector technology as muon trigger chamber in the harsh LHC enviroments.
These are dark current, chamber efficiency, noise rate, gas volume
tomography, and gas leakage.Comment: Comments: 6 pages, 1 table, 9 figures Proceedings of XXV Physics in
Collision-Prague, Czech Republic, 6-9 July 200
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