This Letter reports on a search for narrow high-mass resonances decaying into dilepton final states. The data were recorded by the ATLAS experiment in pp collisions at √s=7 TeV at the Large Hadron ...Collider and correspond to a total integrated luminosity of 1.08 (1.21) fb(-1) in the e(+)e(-) (μ(+)μ(-)) channel. No statistically significant excess above the standard model expectation is observed and upper limits are set at the 95% C.L. on the cross section times branching fraction of Z' resonances and Randall-Sundrum gravitons decaying into dileptons as a function of the resonance mass. A lower mass limit of 1.83 TeV on the sequential standard model Z' boson is set. A Randall-Sundrum graviton with coupling k/M(Pl)=0.1 is excluded at 95% C.L. for masses below 1.63 TeV.
A
bstract
A measurement of
b
-hadron pair production is presented, based on a data set corresponding to an integrated luminosity of 11.4 fb
−1
of proton-proton collisions recorded at
s
=
8
TeV with ...the ATLAS detector at the LHC. Events are selected in which a
b
-hadron is reconstructed in a decay channel containing
J/ψ
→
μμ
, and a second
b
-hadron is reconstructed in a decay channel containing a muon. Results are presented in a fiducial volume defined by kinematic requirements on three muons based on those used in the analysis. The fiducial cross section is measured to be 17.7 ± 0.1(stat.) ± 2.0(syst.) nb. A number of normalised differential cross sections are also measured, and compared to predictions from the P
ythia
8, H
erwig
++, M
ad
G
raph
5_
a
MC@NLO+P
ythia
8 and S
herpa
event generators, providing new constraints on heavy flavour production.
This Letter describes a search for narrowly resonant new physics using a machine-learning anomaly detection procedure that does not rely on signal simulations for developing the analysis selection. ...Weakly supervised learning is used to train classifiers directly on data to enhance potential signals. The targeted topology is dijet events and the features used for machine learning are the masses of the two jets. The resulting analysis is essentially a three-dimensional search A→BC, for mA∼O(TeV), mB,mC∼O(100 GeV) and B, C are reconstructed as large-radius jets, without paying a penalty associated with a large trials factor in the scan of the masses of the two jets. The full run 2 s=13 TeV pp collision dataset of 139 fb-1 recorded by the ATLAS detector at the Large Hadron Collider is used for the search. There is no significant evidence of a localized excess in the dijet invariant mass spectrum between 1.8 and 8.2 TeV. Cross-section limits for narrow-width A, B, and C particles vary with mA, mB, and mC. For example, when mA=3 TeV and mB≳200 GeV, a production cross section between 1 and 5 fb is excluded at 95% confidence level, depending on mC. For certain masses, these limits are up to 10 times more sensitive than those obtained by the inclusive dijet search. These results are complementary to the dedicated searches for the case that B and C are standard model bosons.
This Letter presents a search for quantum black-hole production using 20.3 fb-1 of data collected with the ATLAS detector in pp collisions at the LHC at √s = 8 TeV. The quantum black holes are ...assumed to decay into a final state characterized by a lepton (electron or muon) and a jet. In either channel, no event with a lepton-jet invariant mass of 3.5 TeV or more is observed, consistent with the expected background. Limits are set on the product of cross sections and branching fractions for the lepton+jet final states of quantum black holes produced in a search region for invariant masses above 1 TeV. The combined 95% confidence level upper limit on this product for quantum black holes with threshold mass above 3.5 TeV is 0.18 fb. This limit constrains the threshold quantum black-hole mass to be above 5.3 TeV in the model considered.
A study of the charge conjugation and parity (CP) properties of the interaction between the Higgs boson and top quarks is presented. Higgs bosons are identified via the diphoton decay channel (H → ...γγ), and their production in association with a top quark pair (tt H) or single top quark (tH) is studied. The analysis uses 139 fb−1 of proton–proton collision data recorded at a center-of-mass energy of √ s = 13 TeV with the ATLAS detector at the Large Hadron Collider. Assuming a CP-even coupling, the tt H process is observed with a significance of 5.2 standard deviations. The measured cross section times H → γ γ branching ratio is 1.64+0.38−0.36 (stat)+0.17−0.14 (sys) fb, and the measured rate for tt H is 1.43+0.33−0.31 (stat)+0.21−0.15 (sys) times the Standard Model expectation. The t H production process is not observed and an upper limit on its rate of 12 times the Standard Model expectation is set. A C P -mixing angle greater (less) than 43 (−43) ° is excluded at 95% confidence level.
Measurements of
K
S
0
and
Λ
0
production in
t
t
¯
final states have been performed. They are based on a data sample with integrated luminosity of 4.6
fb
-
1
from proton–proton collisions at a ...centre-of-mass energy of 7 TeV, collected in 2011 with the ATLAS detector at the Large Hadron Collider. Neutral strange particles are separated into three classes, depending on whether they are contained in a jet, with or without a
b
-tag, or not associated with a selected jet. The aim is to look for differences in their main kinematic distributions. A comparison of data with several Monte Carlo simulations using different hadronisation and fragmentation schemes, colour reconnection models and different tunes for the underlying event has been made. The production of neutral strange particles in
t
t
¯
dileptonic events is found to be well described by current Monte Carlo models for
K
S
0
and
Λ
0
production within jets, but not for those produced outside jets.
A search for heavy neutral Higgs bosons is performed using the LHC Run 2 data, corresponding to an integrated luminosity of 139 fb-1 of proton-proton collisions at s=13 TeV recorded with the ATLAS ...detector. The search for heavy resonances is performed over the mass range 0.2–2.5 TeV for the τ+τ- decay with at least one τ-lepton decaying into final states with hadrons. The data are in good agreement with the background prediction of the standard model. In the Mh125 scenario of the minimal supersymmetric standard model, values of tanβ>8 and tanβ>21 are excluded at the 95% confidence level for neutral Higgs boson masses of 1.0 and 1.5 TeV, respectively, where tanβ is the ratio of the vacuum expectation values of the two Higgs doublets.
(ProQuest: ... denotes formulae and/or non-USASCII text omitted; see image) Abstract The ATLAS experiment at the LHC has measured the production cross section of events with two isolated photons in ...the final state, in proton-proton collisions at ... The full data set collected in 2011, corresponding to an integrated luminosity of 4.9 fb^sup -1^, is used. The amount of background, from hadronic jets and isolated electrons, is estimated with data-driven techniques and subtracted. The total cross section, for two isolated photons with transverse energies above 25 GeV and 22 GeV respectively, in the acceptance of the electromagnetic calorimeter (|eta|<1.37 and 1.52<|eta|<2.37) and with an angular separation DeltaR>0.4, is ... pb. The differential cross sections as a function of the di-photon invariant mass, transverse momentum, azimuthal separation, and cosine of the polar angle of the largest transverse energy photon in the Collins-Soper di-photon rest frame are also measured. The results are compared to the prediction of leading-order parton-shower and next-to-leading-order and next-to-next-to-leading-order parton-level generators.Figure not available: see fulltext.
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