The modification of the production of
J
/
ψ
,
ψ
(
2
S
)
, and
Υ
(
n
S
)
(
n
=
1
,
2
,
3
) in
p
+Pb collisions with respect to their production in
pp
collisions has been studied. The
p
+Pb and
pp
...datasets used in this paper correspond to integrated luminosities of
28
nb
-
1
and
25
pb
-
1
respectively, collected in 2013 and 2015 by the ATLAS detector at the LHC, both at a centre-of-mass energy per nucleon pair of 5.02 TeV. The quarkonium states are reconstructed in the dimuon decay channel. The yields of
J
/
ψ
and
ψ
(
2
S
)
are separated into prompt and non-prompt sources. The measured quarkonium differential cross sections are presented as a function of rapidity and transverse momentum, as is the nuclear modification factor,
R
p
Pb
for
J
/
ψ
and
Υ
(
n
S
)
. No significant modification of the
J
/
ψ
production is observed while
Υ
(
n
S
)
production is found to be suppressed at low transverse momentum in
p
+Pb collisions relative to
pp
collisions. The production of excited charmonium and bottomonium states is found to be suppressed relative to that of the ground states in central
p
+Pb collisions.
A measurement of
J
/
ψ
and
ψ
(
2
S
)
production is presented. It is based on a data sample from Pb+Pb collisions at
s
NN
=
5.02
TeV
and
pp
collisions at
s
=
5.02
TeV
recorded by the ATLAS detector at ...the LHC in 2015, corresponding to an integrated luminosity of
0.42
nb
-
1
and
25
pb
-
1
in Pb+Pb and
pp
, respectively. The measurements of per-event yields, nuclear modification factors, and non-prompt fractions are performed in the dimuon decay channel for
9
<
p
T
μ
μ
<
40
GeV in dimuon transverse momentum, and
-
2
<
y
μ
μ
<
2
in rapidity. Strong suppression is found in Pb+Pb collisions for both prompt and non-prompt
J
/
ψ
, increasing with event centrality. The suppression of prompt
ψ
(
2
S
)
is observed to be stronger than that of
J
/
ψ
, while the suppression of non-prompt
ψ
(
2
S
)
is equal to that of the non-prompt
J
/
ψ
within uncertainties, consistent with the expectation that both arise from
b
-quarks propagating through the medium. Despite prompt and non-prompt
J
/
ψ
arising from different mechanisms, the dependence of their nuclear modification factors on centrality is found to be quite similar.
A search for charged leptons with large impact parameters using 139 fb−1 of √s = 13 TeV pp collision data from the ATLAS detector at the LHC is presented, addressing a long-standing gap in coverage ...of possible new physics signatures. Results are consistent with the background prediction. This search provides unique sensitivity to long-lived scalar supersymmetric lepton partners (sleptons). For lifetimes of 0.1 ns, selectron, smuon, and stau masses up to 720, 680, and 340 GeV, respectively, are excluded at 95% confidence level, drastically improving on the previous best limits from LEP.
Results of a search for physics beyond the Standard Model in events containing an energetic photon and large missing transverse momentum with the ATLAS detector at the Large Hadron Collider are ...reported. As the number of events observed in data, corresponding to an integrated luminosity of 36.1 fb
-
1
of proton–proton collisions at a centre-of-mass energy of
13
TeV
, is in agreement with the Standard Model expectations, model-independent limits are set on the fiducial cross section for the production of events in this final state. Exclusion limits are also placed in models where dark-matter candidates are pair-produced. For dark-matter production via an axial-vector or a vector mediator in the
s
-channel, this search excludes mediator masses below 750–
1200
GeV
for dark-matter candidate masses below 230–
480
GeV
at 95% confidence level, depending on the couplings. In an effective theory of dark-matter production, the limits restrict the value of the suppression scale
M
∗
to be above
790
GeV
at 95% confidence level. A limit is also reported on the production of a high-mass scalar resonance by processes beyond the Standard Model, in which the resonance decays to
Z
γ
and the
Z
boson subsequently decays into neutrinos.
Measurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark,
t
t
¯
system and event-level kinematic observables in proton–proton ...collisions at a centre-of-mass energy of
s
=
8
TeV
. The observables have been chosen to emphasize the
t
t
¯
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 fb
-
1
, 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. Predictions beyond NLO accuracy improve the agreement with data at high top-quark transverse momenta. 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 are made of differential cross-sections of highly boosted pair-produced top quarks as a function of top-quark and tt¯ system kinematic observables using proton-proton collisions at a ...center-of-mass energy of s=13 TeV. The data set corresponds to an integrated luminosity of 36.1 fb−1, recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Events with two large-radius jets in the final state, one with transverse momentum pT>500 GeV and a second with pT>350 GeV, are used for the measurement. The top-quark candidates are separated from the multijet background using jet substructure information and association with a b-tagged jet. The measured spectra are corrected for detector effects to a particle-level fiducial phase space and a parton-level limited phase space, and are compared to several Monte Carlo simulations by means of calculated χ2 values. The cross-section for tt¯ production in the fiducial phase-space region is 292±7(stat)±71(syst) fb, to be compared to the theoretical prediction of 384±36 fb.
A
bstract
A search for heavy Majorana neutrinos in events containing a pair of high-
p
T
leptons of the same charge and high-
p
T
jets is presented. The search uses 20.3 fb
−1
of
pp
collision data ...collected with the ATLAS detector at the CERN Large Hadron Collider with a centre-of-mass energy of
s
=
8
TeV. The data are found to be consistent with the background-only hypothesis based on the Standard Model expectation. In the context of a Type-I seesaw mechanism, limits are set on the production cross-section times branching ratio for production of heavy Majorana neutrinos in the mass range between 100 and 500 GeV. The limits are subsequently interpreted as limits on the mixing between the heavy Majorana neutrinos and the Standard Model neutrinos. In the context of a left-right symmetric model, limits on the production cross-section times branching ratio are set with respect to the masses of heavy Majorana neutrinos and heavy gauge bosons
W
R
and
Z
′.
PDF
