Dark matter particles, if sufficiently light, may be produced in decays of the Higgs boson. This Letter presents a statistical combination of searches for H→invisible decays where H is produced ...according to the standard model via vector boson fusion, Z(ℓℓ)H, and W/Z(had)H, all performed with the ATLAS detector using 36.1 fb^{-1} of pp collisions at a center-of-mass energy of sqrts=13 TeV at the LHC. In combination with the results at sqrts=7 and 8 TeV, an exclusion limit on the H→invisible branching ratio of 0.26(0.17_{-0.05}^{+0.07}) at 95% confidence level is observed (expected).
A
bstract
A search for new-physics resonances decaying into a lepton and a jet performed by the ATLAS experiment is presented. Scalar leptoquarks pair-produced in
pp
collisions at
s
= 13 TeV at the ...Large Hadron Collider are considered using an integrated luminosity of 139 fb
−
1
, corresponding to the full Run 2 dataset. They are searched for in events with two electrons or two muons and two or more jets, including jets identified as arising from the fragmentation of
c
- or
b
-quarks. The observed yield in each channel is consistent with the Standard Model background expectation. Leptoquarks with masses below 1.8 TeV and 1.7 TeV are excluded in the electron and muon channels, respectively, assuming a branching ratio into a charged lepton and a quark of 100%, with minimal dependence on the quark flavour. Upper limits on the aforementioned branching ratio are also given as a function of the leptoquark mass.
ATLAS has measured two-particle correlations as a function of the relative azimuthal angle, Deltavarphi, and pseudorapidity, Deltaeta, in s= 13 and 2.76 TeV pp collisions at the LHC using charged ...particles measured in the pseudorapidity interval eta<2.5. The correlation functions evaluated in different intervals of measured charged-particle multiplicity show a multiplicity-dependent enhancement at Deltavarphi 0 that extends over a wide range of Deltaeta, which has been referred to as the "ridge." Per-trigger-particle yields, Y(Deltavarphi), are measured over 2<Deltaeta<5. For both collision energies, the Y(Deltavarphi) distribution in all multiplicity intervals is found to be consistent with a linear combination of the per-trigger-particle yields measured in collisions with less than 20 reconstructed tracks, and a constant combinatoric contribution modulated by cos(2Deltavarphi). The fitted Fourier coefficient, v2,2, exhibits factorization, suggesting that the ridge results from per-event cos(2varphi) modulation of the single-particle distribution with Fourier coefficients upsilon sub(2). The upsilon sub(2) values are presented as a function of multiplicity and transverse momentum. They are found to be approximately constant as a function of multiplicity and to have a pT dependence similar to that measured in p+ Pb and Pb + Pb collisions. The upsilon sub(2) values in the 13 and 2.76 TeV data are consistent within uncertainties. These results suggest that the ridge in pp collisions arises from the same or similar underlying physics as observed in p+ Pb collisions, and that the dynamics responsible for the ridge has no strong s dependence.
Measurements of the Standard Model Higgs boson decaying into a
b
b
¯
pair and produced in association with a
W
or
Z
boson decaying into leptons, using proton–proton collision data collected between ...2015 and 2018 by the ATLAS detector, are presented. The measurements use collisions produced by the Large Hadron Collider at a centre-of-mass energy of
s
=
13
Te
, corresponding to an integrated luminosity of
139
fb
-
1
. The production of a Higgs boson in association with a
W
or
Z
boson is established with observed (expected) significances of 4.0 (4.1) and 5.3 (5.1) standard deviations, respectively. Cross-sections of associated production of a Higgs boson decaying into bottom quark pairs with an electroweak gauge boson,
W
or
Z
, decaying into leptons are measured as a function of the gauge boson transverse momentum in kinematic fiducial volumes. The cross-section measurements are all consistent with the Standard Model expectations, and the total uncertainties vary from 30% in the high gauge boson transverse momentum regions to 85% in the low regions. Limits are subsequently set on the parameters of an effective Lagrangian sensitive to modifications of the
WH
and
ZH
processes as well as the Higgs boson decay into
b
b
¯
.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The large rate of multiple simultaneous proton–proton interactions, or pile-up, generated by the Large Hadron Collider in Run 1 required the development of many new techniques to mitigate the adverse ...effects of these conditions. This paper describes the methods employed in the ATLAS experiment to correct for the impact of pile-up on jet energy and jet shapes, and for the presence of spurious additional jets, with a primary focus on the large 20.3
fb
-
1
data sample collected at a centre-of-mass energy of
s
=
8
TeV
. The energy correction techniques that incorporate sophisticated estimates of the average pile-up energy density and tracking information are presented. Jet-to-vertex association techniques are discussed and projections of performance for the future are considered. Lastly, the extension of these techniques to mitigate the effect of pile-up on jet shapes using subtraction and grooming procedures is presented.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A search for a heavy charged-boson resonance decaying into a charged lepton (electron or muon) and a neutrino is reported. A data sample of 139 fb−1 of proton-proton collisions at s=13 TeV ...collected with the ATLAS detector at the LHC during 2015–2018 is used in the search. The observed transverse mass distribution computed from the lepton and missing transverse momenta is consistent with the distribution expected from the Standard Model, and upper limits on the cross section for pp→W′→ℓν are extracted (ℓ=e or μ). These vary between 1.3 pb and 0.05 fb depending on the resonance mass in the range between 0.15 and 7.0 TeV at 95% confidence level for the electron and muon channels combined. Gauge bosons with a mass below 6.0 and 5.1 TeV are excluded in the electron and muon channels, respectively, in a model with a resonance that has couplings to fermions identical to those of the Standard Model W boson. Cross-section limits are also provided for resonances with several fixed Γ/m values in the range between 1% and 15%. Model-independent limits are derived in single-bin signal regions defined by a varying minimum transverse mass threshold. The resulting visible cross-section upper limits range between 4.6 (15) pb and 22 (22) ab as the threshold increases from 130 (110) GeV to 5.1 (5.1) TeV in the electron (muon) channel.
A measurement of production cross sections of the Higgs boson in proton-proton collisions is presented in the H→ττ decay channel. The analysis is performed using 36.1 fb−1 of data recorded by the ...ATLAS experiment at the Large Hadron Collider at a center-of-mass energy of s=13 TeV. All combinations of leptonic (τ→ℓvv¯ with ℓ=e,μ) and hadronic (τ→hadrons v) τ decays are considered. The H→ττ signal over the expected background from other Standard Model processes is established with an observed (expected) significance of 4.4 (4.1) standard deviations. Combined with results obtained using data taken at 7 and 8 TeV center-of-mass energies, the observed (expected) significance amounts to 6.4 (5.4) standard deviations and constitutes an observation of H→ττ decays. Using the data taken at s=13 TeV, the total cross section in the H→ττ decay channel is measured to be 3.77−0.59+0.60 (stat) −0.74+0.87 (syst) pb, for a Higgs boson of mass 125 GeV assuming the relative contributions of its production modes as predicted by the Standard Model. Total cross sections in the H→ττ decay channel are determined separately for vector-boson-fusion production and gluon-gluon-fusion production to be σH→ττVBF=0.28±0.09 (stat) −0.09+0.11 (syst) pb and σH→ττggF=3.1±1.0 (stat) −1.3+1.6 (syst) pb, respectively. Similarly, results of a fit are reported in the framework of simplified template cross sections. All measurements are in agreement with Standard Model expectations.
A search for heavy charged long-lived particles is performed using a data sample of 36.1 fb−1 of proton-proton collisions at √s=13 TeV collected by the ATLAS experiment at the Large Hadron Collider. ...The search is based on observables related to ionization energy loss and time of flight, which are sensitive to the velocity of heavy charged particles traveling significantly slower than the speed of light. Multiple search strategies for a wide range of lifetimes, corresponding to path lengths of a few meters, are defined as model independently as possible, by referencing several representative physics cases that yield long-lived particles within supersymmetric models, such as gluinos/squarks (R-hadrons), charginos and staus. No significant deviations from the expected Standard Model background are observed. Upper limits at 95% confidence level are provided on the production cross sections of long-lived R-hadrons as well as directly pair-produced staus and charginos. These results translate into lower limits on the masses of long-lived gluino, sbottom and stop R-hadrons, as well as staus and charginos of 2000, 1250, 1340, 430, and 1090 GeV, respectively.
A
bstract
Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses proton-proton collision data corresponding to ...an integrated luminosity of 36.1 fb
−1
at a centre-of-mass energy of 13 TeV collected in 2015 and 2016 with the ATLAS detector at the Large Hadron Collider. Events are required to have at least one jet with a transverse momentum above 250 GeV and no leptons (
e
or
μ
). Several signal regions are considered with increasing requirements on the missing transverse momentum above 250 GeV. Good agreement is observed between the number of events in data and Standard Model predictions. The results are translated into exclusion limits in models with pair-produced weakly interacting dark-matter candidates, large extra spatial dimensions, and supersymmetric particles in several compressed scenarios.
This Letter describes the observation of the light-by-light scattering process, γγ→γγ, in Pb+Pb collisions at sqrts_{NN}=5.02 TeV. The analysis is conducted using a data sample corresponding to an ...integrated luminosity of 1.73 nb^{-1}, collected in November 2018 by the ATLAS experiment at the LHC. Light-by-light scattering candidates are selected in events with two photons produced exclusively, each with transverse energy E_{T}^{γ}>3 GeV and pseudorapidity |η_{γ}|<2.4, diphoton invariant mass above 6 GeV, and small diphoton transverse momentum and acoplanarity. After applying all selection criteria, 59 candidate events are observed for a background expectation of 12±3 events. The observed excess of events over the expected background has a significance of 8.2 standard deviations. The measured fiducial cross section is 78±13(stat)±7(syst)±3(lumi) nb.
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