A
bstract
We explore the phenomenology of models containing one Vector-Like Quark (VLQ),
t′
, which can decay into the Standard Model (SM) top quark,
t
, and a new spin-0 neutral boson,
S
, the ...latter being either a scalar or pseudoscalar state. We parametrise the underlying interactions in terms of a simplified model which enables us to capture possible Beyond the SM (BSM) scenarios. We discuss in particular three such scenarios: one where the SM state is supplemented by an additional scalar, one which builds upon a 2-Higgs Doublet Model (2HDM) framework and another which realises a Composite Higgs Model (CHM) through partial compositeness. Such exotic decays of the
t′
can be competitive with decays into SM particles, leading to new possible discovery channels at the Large Hadron Collider (LHC). Assuming
t′
pair production via strong interactions, we design signal regions optimised for one
t′ → S t
transition (while being inclusive on the other
t
¯
′
decay, and vice versa), followed by the decay of
S
into the two very clean experimental signatures
S → γ γ
and
S → Z
(
→ ℓ
+
ℓ
−
)
γ
. We perform a dedicated signal- to-background analysis in both channels, by using Monte Carlo (MC) event simulations modelling the dynamics from the proton-proton to the detector level. Under the assumption of BR(
t′ → S t
) = 100%, we are therefore able to realistically quantify the sensitivity of the LHC to both the
t′
and
S
masses, assuming both current and foreseen luminosities. This approach paves the way for the LHC experiments to surpass current VLQ search strategies based solely on
t′
decays into SM bosons (
W
±
, Z
,
h
).
Cross-sections for the production of a Z boson in association with two photons are measured in proton-proton collisions at a centre-of-mass energy of 13 TeV. The data used correspond to an integrated ...luminosity of 139 fb(-1) recorded by the ATLAS experiment during Run 2 of the LHC. The measurements use the electron and muon decay channels of the Z boson, and a fiducial phase-space region where the photons are not radiated from the leptons. The integrated Z(-> ll)gamma gamma cross-section is measured with a precision of 12% and differential cross-sections are measured as a function of six kinematic variables of the Z gamma gamma system. The data are compared with predictions from MC event generators which are accurate to up to next-to-leading order in QCD. The cross-section measurements are used to set limits on the coupling strengths of dimension-8 operators in the framework of an effective field theory.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
This letter documents a search for flavour-changing neutral currents (FCNCs), which are strongly sup-pressed in the Standard Model, in events with a photon and a top quark with the ATLAS detector. ...The analysis uses data collected in pp collisions at & RADIC;s =13 TeV during Run 2 of the LHC, corresponding to an integrated luminosity of 139 fb-1. Both FCNC top-quark production and decay are considered. The final state consists of a charged lepton, missing transverse momentum, a b-tagged jet, one high-momentum photon and possibly additional jets. A multiclass deep neural network is used to classify events either as signal in one of the two categories, FCNC production or decay, or as background. No significant ex-cess of events over the background prediction is observed and 95% CL upper limits are placed on the strength of left-and right-handed FCNC interactions. The 95% CL bounds on the branching fractions for the FCNC top-quark decays, estimated (expected) from both top-quark production and decay, are B(t & RARR; u & gamma; ) < 0.85 (0.88+0.37 -0.25) x 10-5 and B(t & RARR; c & gamma; ) < 4.2 (3.40+1.35-0.95) x 10-5 for a left-handed tq & gamma; cou-pling, and B(t & RARR; u & gamma; ) < 1.2 (1.20+0.50 -0.33) x10-5 and B(t & RARR; c & gamma; ) < 4.5 (3.70+1.47 -1.03) x10-5 for a right-handed coupling. & COPY; 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons .org /licenses /by /4 .0/). Funded by SCOAP3.
This Letter reports the observation of WZ gamma production and a measurement of its cross section using 140.1 +/- 1.2 fb(-1) of proton-proton collision data recorded at a center-of-mass energy of 13 ...TeV by the bosons decaying leptonically, pp -> WZ gamma -> l'(perpendicular to)nu l(+) l(-)gamma (l(')(+) = e, mu), is measured in a fiducial phasespace region defined such that the leptons and the photon have high transverse momentum and the photon is isolated. The cross section is found to be 2.01 +/- 0.30(stat) +/- 0.16(syst) fb. The corresponding standard model predicted cross section calculated at next-to-leading order in perturbative quantum chromodynamics and at leading order in the electroweak coupling constant is 1.50 +/- 0.06 fb. The observed significance of the WZ gamma signal is 6.3 sigma, compared with an expected significance of 5.0 sigma.
The 𝐶𝑃 properties of the coupling between the Higgs boson and the top quark are investigated using 139 fb −1 of proton–proton collision data recorded by the ATLAS experiment at the LHC at a ...centre-of-mass energy of √𝑠 = 13 TeV. The 𝐶𝑃 structure of the top quark–Higgs boson Yukawa coupling is probed in events with a Higgs boson decaying into a pair of 𝑏-quarks and produced in association with either a pair of top quarks, 𝑡̄𝑡𝐻,or a single top quark, 𝑡𝐻. Events containing one or two electrons or muons are used for the measurement. Multivariate techniques are used to select regions enriched in 𝑡̄𝑡𝐻 and 𝑡𝐻 events, where dedicated 𝐶𝑃 -sensitive observables are exploited. In an extension of the Standard Model (SM) with a 𝐶𝑃 -odd admixture in the top–Higgs Yukawa coupling, the mixing angle between 𝐶𝑃 -even and 𝐶𝑃 -odd couplings is measured to be 𝛼 = 11 ◦+52 ◦−73◦ , compatible with the SM prediction corresponding to 𝛼 = 0.
The inclusive top-quark-pair production cross section rrtt and its ratio to the Z-boson production cross section have been measured in proton-proton collisions at root s = 13.6 TeV, using 29 fb-1 of ...data collected in 2022 with the ATLAS experiment at the Large Hadron Collider. Using events with an opposite-charge electron-muon pair and b-tagged jets, and assuming Standard Model decays, the top-quark-pair production cross section is measured to be rrtt = 850 +/- 3(stat.) +/- 18(syst.) +/- 20(lumi.) pb. The ratio of the ttand the Z-boson production cross sections is also measured, where the Z-boson contribution is determined for inclusive e+e- and mu+mu- events in a fiducial phase space. The relative uncertainty on the ratio is reduced compared to the ttcross section, thanks to the cancellation of several systematic uncertainties. The result for the ratio, Rtt/Z = 1.145 +/- 0.003(stat.) +/- 0.021(syst.) +/- 0.002(lumi.) is consistent with the Standard Model prediction using the PDF4LHC21 PDF set.
The inclusive Higgs boson production crosssection is measured in the di-photon and the Z Z∗ → 4decay channels using 31.4 and 29.0 fb−1 of pp collision datarespectively, collected with the ATLAS ...detector at a centreof-mass energy of √s = 13.6 TeV. To reduce the modeldependence, the measurement in each channel is restrictedto a particle-level phase space that closely matches the channel’s detector-level kinematic selection, and it is correctedfor detector effects. These measured fiducial cross-sectionsare σfid,γ γ = 76+14−13 fb, and σfid,4 = 2.80 ± 0.74 fb, inagreement with the corresponding Standard Model predictions of 67.6±3.7 fb and 3.67±0.19 fb. Assuming StandardModel acceptances and branching fractions for the two channels, the fiducial measurements are extrapolated to the fullphase space yielding total cross-sections of σ (pp → H) =67+12−11 pb and 46±12 pb at 13.6 TeV from the di-photon andZ Z∗ → 4 measurements respectively. The two measurements are combined into a total cross-section measurement ofσ (pp → H) = 58.2±8.7 pb, to be compared with the Standard Model prediction of σ (pp → H)SM = 59.9 ± 2.6 pb.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A search for a long-lived, heavy neutral lepton (N) in 139 fb(-1) of vs = 13 TeV pp collision data collected by the ATLAS detector at the Large Hadron Collider is reported. The N is produced via W ?N ...mu or W -Ne and decays into two charged leptons and a neutrino, forming a displaced vertex. The N mass is used to discriminate between signal and background. No signal is observed, and limits are set on the squared mixing parameters of the N with the left-handed neutrino states for the N mass range 3 GeV < m(N) < 15 GeV. For the first time, limits are given for both single-flavor and multiflavor mixing scenarios motivated by neutrino flavor oscillation results for both the normal and inverted neutrino-mass hierarchies.
Higgs boson production via gluon-gluon fusion and vector-boson fusion in proton-proton collisions is measured in the H & RARR; WW* & RARR; ev & mu;v decay channel. The Large Hadron ...Collider delivered proton-proton collisions at a center-of-mass energy of 13 TeV between 2015 and 2018, which were recorded by the ATLAS detector, corresponding to an integrated luminosity of 139 fb-1. The total cross sections for Higgs boson production by gluon-gluon fusion and vector-boson fusion times the H & RARR; WW* branching ratio are measured to be 12.0 1 1.4 and 0.75 thorn 0.19 -0.16 pb, respectively, in agreement with the Standard Model predictions of 10.4 1 0.6 and 0.81 1 0.02 pb. Higgs boson production is further characterized through measurements of Simplified Template Cross Sections in a total of 11 kinematic fiducial regions.
The production of a W boson in association with a single charm quark is studied using 140 fb(-1) of vS = 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron ...Collider. ffiffis The charm quark is tagged by the presence of a charmed hadron reconstructed with a secondary-vertex fit. The W boson is reconstructed from the decay to either an electron or a muon and the missing transverse momentum present in the event. The charmed mesons reconstructed are D+ ?K-p+p+ and D*+ ? D0p+ ? (K-p+)p+ and the charge conjugate decays in the fiducial regions where pT(e; mu) > 30 GeV, l?(e; mu)l < 2.5, pT(D(*)) > 8 GeV, and l?(D(*))l < 2.2. The integrated and normalized differential cross sections as a function of the pseudorapidity of the lepton from the W boson decay, and of the transverse momentum of the charmed hadron, are extracted from the data using a profile likelihood fit. The measured total fiducial cross sections are sfidOS-SS(W- + D+) = 50.2 + 0.2(stat)+2.4 -2.3(syst) pb, s(OS-SS) (fid)(W- + D+) = 48.5 + 0.2(stat)+2.3-2.2(syst) pb, sfidOS-SS(W- + D*+) = 51.1 + 0.4(stat)+1.9 -1.8 (syst) pb, and s(OS-SS) (fid)(W+ + D*-) = 50.0 + 0.4(stat)+1.9 -1.8 (syst) pb. Results are compared with the predictions of next-to-leading-order quantum chromodynamics calculations performed using state-of-the-art parton distribution functions. Additionally, the ratio of charm to anticharm production cross sections is studied to probe the s -s- quark asymmetry. The ratio is found to be R+ c = 0.971 + 0.006(stat) + 0.011(syst). The ratio and cross-section measurements are consistent with the predictions obtained with parton distribution function sets that have a symmetric s -s- sea, indicating that any s -s- asymmetry in the Bjorken-x region relevant for this measurement is small.
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