New results from GridPix Detectors Bilevych, Y; Desch, K; Fransen, J-P ...
Journal of physics. Conference series,
04/2020, Letnik:
1498, Številka:
1
Journal Article
Recenzirano
Odprti dostop
GridPix structures combine the high resolution of a pixel readout chip with a Micromegas as gas amplification stage. This detector has shown excellent performance before with the Timepix ASIC, but ...has been improved by using the successor ASIC, the Timepix3. This new version of the GridPix detectors has been used in test beams at the ELSA accelerator at Bonn, where 2.5 GeV electrons are available for tracking studies. It could be demonstrated that the spatial resolution in both transverse and longitudinal direction follows the diffusion function. Structures made of four GridPixes have been designed and constructed to cover larger areas. Also these devices called quads have been successfully tested with the electron beam giving similarly good results as the single GridPix detector.
A small drift chamber was read out by means of a MediPix2 readout chip as a direct anode. A Micromegas foil was placed
50
μ
m
above the chip, and electron multiplication occurred in the gap. With a ...He/isobutane 80/20 mixture, gas multiplication factors up to tens of thousands were achieved, resulting in an efficiency for detecting single electrons of better than 90%. We recorded many frames containing 2D images with tracks from cosmic muons. Along these tracks, electron clusters were observed, as well as
δ
-rays.
A search for chargino–neutralino pair production in three-lepton final states with missing transverse momentum is presented. The study is based on a dataset of
s
=
13
TeV
pp
collisions recorded with ...the ATLAS detector at the LHC, corresponding to an integrated luminosity of 139
fb
-
1
. No significant excess relative to the Standard Model predictions is found in data. The results are interpreted in simplified models of supersymmetry, and statistically combined with results from a previous ATLAS search for compressed spectra in two-lepton final states. Various scenarios for the production and decay of charginos (
χ
~
1
±
) and neutralinos (
χ
~
2
0
) are considered. For pure higgsino
χ
~
1
±
χ
~
2
0
pair-production scenarios, exclusion limits at 95% confidence level are set on
χ
~
2
0
masses up to 210 GeV. Limits are also set for pure wino
χ
~
1
±
χ
~
2
0
pair production, on
χ
~
2
0
masses up to 640 GeV for decays via on-shell
W
and
Z
bosons, up to 300 GeV for decays via off-shell
W
and
Z
bosons, and up to 190 GeV for decays via
W
and Standard Model Higgs bosons.
A
bstract
This paper presents a search for direct electroweak gaugino or gluino pair production with a chargino nearly mass-degenerate with a stable neutralino. It is based on an integrated ...luminosity of 36.1 fb
−1
of
pp
collisions at
s
=
13
TeV collected by the ATLAS experiment at the LHC. The final state of interest is a disappearing track accompanied by at least one jet with high transverse momentum from initial-state radiation or by four jets from the gluino decay chain. The use of short track segments reconstructed from the innermost tracking layers significantly improves the sensitivity to short chargino lifetimes. The results are found to be consistent with Standard Model predictions. Exclusion limits are set at 95% confidence level on the mass of charginos and gluinos for different chargino lifetimes. For a pure wino with a lifetime of about 0.2 ns, chargino masses up to 460 GeV are excluded. For the strong production channel, gluino masses up to 1.65 TeV are excluded assuming a chargino mass of 460 GeV and lifetime of 0.2 ns.
A search for the electroweak production of charginos, neutralinos and sleptons decaying into final states involving two or three electrons or muons is presented. The analysis is based on 36.1 fb
of
... TeV proton-proton collisions recorded by the ATLAS detector at the Large Hadron Collider. Several scenarios based on simplified models are considered. These include the associated production of the next-to-lightest neutralino and the lightest chargino, followed by their decays into final states with leptons and the lightest neutralino via either sleptons or Standard Model gauge bosons; direct production of chargino pairs, which in turn decay into leptons and the lightest neutralino via intermediate sleptons; and slepton pair production, where each slepton decays directly into the lightest neutralino and a lepton. No significant deviations from the Standard Model expectation are observed and stringent limits at 95% confidence level are placed on the masses of relevant supersymmetric particles in each of these scenarios. For a massless lightest neutralino, masses up to 580 GeV are excluded for the associated production of the next-to-lightest neutralino and the lightest chargino, assuming gauge-boson mediated decays, whereas for slepton-pair production masses up to 500 GeV are excluded assuming three generations of mass-degenerate sleptons.
A search is presented for four-top-quark production using an integrated luminosity of 139 fb
-
1
of proton–proton collision data at a centre-of-mass energy of
13
TeV
collected by the ATLAS detector ...at the LHC. Events are selected if they contain a same-sign lepton pair or at least three leptons (electrons or muons). Jet multiplicity, jet flavour and event kinematics are used to separate signal from the background through a multivariate discriminant, and dedicated control regions are used to constrain the dominant backgrounds. The four-top-quark production cross section is measured to be
24
-
6
+
7
fb. This corresponds to an observed (expected) significance with respect to the background-only hypothesis of 4.3 (2.4) standard deviations and provides evidence for this process.
A
bstract
A search for new phenomena in events with two same-charge leptons or three leptons and jets identified as originating from
b
-quarks in a data sample of 36.1 fb
−1
of
pp
collisions at
s
=
...13
TeV recorded by the ATLAS detector at the Large Hadron Collider is reported. No significant excess is found and limits are set on vector-like quark, four-top-quark, and same-sign top-quark pair production. The observed (expected) 95% CL mass limits for a vector-like
T
- and
B
-quark singlet are
m
T
> 0.98 (0.99) TeV and
m
B
> 1.00 (1.01) TeV respectively. Limits on the production of the vector-like
T
5/3
-quark are also derived considering both pair and single production; in the former case the lower limit on the mass of the
T
5/3
-quark is (expected to be) 1.19 (1.21) TeV. The Standard Model four-top-quark production cross-section upper limit is (expected to be) 69 (29) fb. Constraints are also set on exotic four-top-quark production models. Finally, limits are set on same-sign top-quark pair production. The upper limit on
uu
→
tt
production is (expected to be) 89 (59) fb for a mediator mass of 1 TeV, and a dark-matter interpretation is also derived, excluding a mediator of 3 TeV with a dark-sector coupling of 1.0 and a coupling to ordinary matter above 0.31.
A
bstract
A search for charged Higgs bosons heavier than the top quark and decaying via
H
±
→
tb
is presented. The data analysed corresponds to 36.1 fb
−1
of
pp
collisions at
s
=
13
TeV and was ...recorded with the ATLAS detector at the LHC in 2015 and 2016. The production of a charged Higgs boson in association with a top quark and a bottom quark,
pp
→
tbH
±
, is explored in the mass range from
m
H
± = 200 to 2000 GeV using multi-jet final states with one or two electrons or muons. Events are categorised according to the multiplicity of jets and how likely these are to have originated from hadronisation of a bottom quark. Multivariate techniques are used to discriminate between signal and background events. No significant excess above the background-only hypothesis is observed and exclusion limits are derived for the production cross-section times branching ratio of a charged Higgs boson as a function of its mass, which range from 2.9 pb at
m
H
± = 200 GeV to 0.070 pb at
m
H
± = 2000 GeV. The results are interpreted in two benchmark scenarios of the Minimal Supersymmetric Standard Model.
A
bstract
A search for pair production of a scalar partner of the top quark in events with four or more jets plus missing transverse momentum is presented. An analysis of 36.1 fb
−1
of
s
=
13
TeV ...proton-proton collisions collected using the ATLAS detector at the LHC yields no significant excess over the expected Standard Model background. To interpret the results a simplified supersymmetric model is used where the top squark is assumed to decay via
t
˜
1
→
t
∗
χ
˜
1
0
and
t
˜
1
→
b
χ
˜
1
±
→
b
W
∗
χ
˜
1
0
, where χ
1
0
(χ
1
±
) denotes the lightest neutralino (chargino). Exclusion limits are placed in terms of the top-squark and neutralino masses. Assuming a branching ratio of 100% to
t
χ
˜
1
0
, top-squark masses in the range 450–1000 GeV are excluded for
χ
˜
1
0
masses below 160 GeV. In the case where
m
t
˜
1
∼
m
t
+
m
χ
˜
1
0
, top-squark masses in the range 235–590 GeV are excluded.
A
bstract
The coupling properties of the Higgs boson are studied in the four-lepton (
e
,
μ
) decay channel using 36.1 fb
−1
of
pp
collision data from the LHC at a centre-of-mass energy of 13 TeV ...collected by the ATLAS detector. Cross sections are measured for the main production modes in several exclusive regions of the Higgs boson production phase space and are interpreted in terms of coupling modifiers. The inclusive cross section times branching ratio for
H
→
ZZ
∗
decay and for a Higgs boson absolute rapidity below 2.5 is measured to be 1. 73
− 0.23
+ 0.24
(stat.)
− 0.08
+ 0.10
(exp.) ± 0.04(th.) pb compared to the Standard Model prediction of 1
.
34±0
.
09 pb. In addition, the tensor structure of the Higgs boson couplings is studied using an effective Lagrangian approach for the description of interactions beyond the Standard Model. Constraints are placed on the non-Standard-Model CP-even and CP-odd couplings to
Z
bosons and on the CP-odd coupling to gluons.
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