Some planetary systems harbour debris disks containing planetesimals such as asteroids and comets. Collisions between such bodies produce small dust particles, the spectral features of which reveal ...their composition and, hence, that of their parent bodies. A measurement of the composition of olivine crystals (Mg(2-2x)Fe(2x)SiO(4)) has been done for the protoplanetary disk HD 100546 (refs 3, 4) and for olivine crystals in the warm inner parts of planetary systems. The latter compares well with the iron-rich olivine in asteroids (x ≈ 0.29). In the cold outskirts of the β Pictoris system, an analogue to the young Solar System, olivine crystals were detected but their composition remained undetermined, leaving unknown how the composition of the bulk of Solar System cometary olivine grains compares with that of extrasolar comets. Here we report the detection of the 69-micrometre-wavelength band of olivine crystals in the spectrum of β Pictoris. Because the disk is optically thin, we can associate the crystals with an extrasolar proto-Kuiper belt a distance of 15-45 astronomical units from the star (one astronomical unit is the Sun-Earth distance), determine their magnesium-rich composition (x = 0.01 ± 0.001) and show that they make up 3.6 ± 1.0 per cent of the total dust mass. These values are strikingly similar to those for the dust emitted by the most primitive comets in the Solar System, even though β Pictoris is more massive and more luminous and has a different planetary system architecture.
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.
This Letter presents a measurement of γγ→μ+μ− production in Pb+Pb collisions recorded by the ATLAS detector at the Large Hadron Collider at sNN=5.02 TeV with an integrated luminosity of 0.49 nb−1. ...The azimuthal angle and transverse momentum correlations between the muons are measured as a function of collision centrality. The muon pairs are produced from γγ through the interaction of the large electromagnetic fields of the nuclei. The contribution from background sources of muon pairs is removed using a template fit method. In peripheral collisions, the muons exhibit a strong back-to-back correlation consistent with previous measurements of muon pair production in ultraperipheral collisions. The angular correlations are observed to broaden significantly in central collisions. The modifications are qualitatively consistent with rescattering of the muons while passing through the hot matter produced in the collision.
Full text
Available for:
CMK, CTK, FMFMET, NUK, UL
Differences in the behaviour of matter and antimatter have been observed in K and B meson decays, but not yet in any baryon decay. Such differences are associated with the non-invariance of ...fundamental interactions under the combined charge-conjugation and parity transformations, known as CP violation. Here, using data from the LHCb experiment at the Large Hadron Collider, we search for CP-violating asymmetries in the decay angle distributions of Λb0 baryons decaying to pπ-π+π- and pπ-K+K- final states. These four-body hadronic decays are a promising place to search for sources of CP violation both within and beyond the standard model of particle physics. We find evidence for CP violation in Λb0 to pπ-π+π- decays with a statistical significance corresponding to 3.3 standard deviations including systematic uncertainties. This represents the first evidence for CP violation in the baryon sector.
A
bstract
A search for new particles that decay into top quark pairs is reported. The search is performed with the ATLAS experiment at the LHC using an integrated luminosity of 20.3 fb
−1
of ...proton-proton collision data collected at a centre-of-mass energy of
s
=
8
TeV. The lepton-plus-jets final state is used, where the top pair decays to
W
+
b
W
−
b
¯
, with one
W
boson decaying leptonically and the other hadronically. The invariant mass spectrum of top quark pairs is examined for local excesses or deficits that are inconsistent with the Standard Model predictions. No evidence for a top quark pair resonance is found, and 95% confidence-level limits on the production rate are determined for massive states in benchmark models. The upper limits on the cross-section times branching ratio of a narrow
Z
′ boson decaying to top pairs range from 4.2 pb to 0.03 pb for resonance masses from 0.4 TeV to 3.0 TeV. A narrow leptophobic topcolour
Z
′ boson with mass below 1.8 TeV is excluded. Upper limits are set on the cross-section times branching ratio for a broad colour-octet resonance with Γ
/m
= 15% decaying to
t
t
¯
. These range from 4.8 pb to 0.03 pb for masses from 0.4 TeV to 3.0 TeV. A Kaluza-Klein excitation of the gluon in a Randall-Sundrum model is excluded for masses below 2.2 TeV.
This paper presents a measurement of the polarisation of
W
bosons from
t
t
¯
decays, reconstructed in events with one high-
p
T
lepton and at least four jets. Data from
pp
collisions at the LHC were ...collected at
s
= 8 TeV and correspond to an integrated luminosity of 20.2 fb
-
1
. The angle
θ
∗
between the
b
-quark from the top quark decay and a direct
W
boson decay product in the
W
boson rest frame is sensitive to the
W
boson polarisation. Two different
W
decay products are used as polarisation analysers: the charged lepton and the down-type quark for the leptonically and hadronically decaying
W
boson, respectively. The most precise measurement of the
W
boson polarisation via the distribution of
cos
θ
∗
is obtained using the leptonic analyser and events in which at least two of the jets are tagged as
b
-quark jets. The fitted fractions of longitudinal, left- and right-handed polarisation states are
F
0
=
0.709
±
0.019
,
F
L
=
0.299
±
0.015
and
F
R
=
-
0.008
±
0.014
, and are the most precisely measured
W
boson polarisation fractions to date. Limits on anomalous couplings of the
Wtb
vertex are set.
A
bstract
The angular distributions of Drell-Yan charged lepton pairs in the vicinity of the
Z
-boson mass peak probe the underlying QCD dynamics of
Z
-boson production. This paper presents a ...measurement of the complete set of angular coefficients
A
0−7
describing these distributions in the
Z
-boson Collins-Soper frame. The data analysed correspond to 20.3 fb
−1
of
pp
collisions at
s
=
8
TeV, collected by the ATLAS detector at the CERN LHC. The measurements are compared to the most precise fixed-order calculations currently available
O
α
s
2
and with theoretical predictions embedded in Monte Carlo generators. The measurements are precise enough to probe QCD corrections beyond the formal accuracy of these calculations and to provide discrimination between different parton-shower models. A significant deviation from the
O
α
s
2
predictions is observed for
A
0
−
A
2
. Evidence is found for non-zero
A
5,6,7
, consistent with expectations.
A search has been made for supersymmetry in a final state containing two photons and missing transverse momentum using the ATLAS detector at the Large Hadron Collider. The search makes use of
3.2
fb
...-
1
of proton-proton collision data collected at a centre-of-mass energy of 13 TeV in 2015. Using a combination of data-driven and Monte-Carlo-based approaches, the Standard Model background is estimated to be
0
.
27
-
0.10
+
0.22
events. No events are observed in the signal region; considering the expected background and its uncertainty, this observation implies a model-independent 95 % CL upper limit of 0.93 fb (3.0 events) on the visible cross section due to physics beyond the Standard Model. In the context of a generalized model of gauge-mediated supersymmetry breaking with a bino-like next-to-lightest supersymmetric particle, this leads to a lower limit of 1650 GeV on the mass of a degenerate octet of gluino states, independent of the mass of the lighter bino-like neutralino.
A
bstract
The production of
W
boson pairs in proton-proton collisions at
s
=
8
TeV is studied using data corresponding to 20.3 fb
−1
of integrated luminosity collected by the ATLAS detector during ...2012 at the CERN Large Hadron Collider. The
W
bosons are reconstructed using their leptonic decays into electrons or muons and neutrinos. Events with reconstructed jets are not included in the candidate event sample. A total of 6636
WW
candidate events are observed. Measurements are performed in fiducial regions closely approximating the detector acceptance. The integrated measurement is corrected for all acceptance effects and for the
W
branching fractions to leptons in order to obtain the total
WW
production cross section, which is found to be 71
.
1 ± 1
.
1(stat)
− 5.0
+ 5.7
(syst) ± 1.4(lumi) pb. This agrees with the next-to-next-to-leading-order Standard Model prediction of 63. 2
− 1.4
+ 1.6
(scale) ± 1.2(PDF) pb. Fiducial differential cross sections are measured as a function of each of six kinematic variables. The distribution of the transverse momentum of the leading lepton is used to set limits on anomalous triple-gauge-boson couplings.
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