1,006 research outputs found
Observations of the Radiation Amplitude Zero effect and of the Diboson Longitudinal-Longitudinal Interactions in High Region using W±Z Production with the ATLAS Detector
No full textThis thesis focuses on the studies of di-boson polarization states in W±Z production, presenting two novel measurements using pp collisions at a center-of-mass energy of = 13 TeV. The WZ decaying to leptonic final states is considered. The data was collected at the Large Hadron Collider (LHC) with the ATLAS experiment, utilizing an integrated luminosity of 140.1 fb−1 . The first measurement pertains to the Radiation Amplitude Zero effect. By studying the depth of distributions involving the rapidity differences between the W lepton and the Z boson (∆Y3Z) and between the W boson and the Z boson (∆YWZ), an agreement is observed between the measured depth in the background-subtracted data consisting only of the transverse-transverse WZ contribution and the Monte Carlo (MC) prediction. The second measurement involves the determination of the longitudinal-longitudinal WZ polarization fraction. This measurement is performed in regions with predominantly s-channel WZ events, characterized by high values in two fiducial phase spaces: > 100 GeV or > 200 GeV. To extract the polarization fraction, a multivariate analysis based on a Boosted Decision Trees discriminant is employed. A maximum log-likelihood fit is then executed, yielding a longitudinal-longitudinal polarization fraction (00) of 0.25+0.06 −0.06 for the > 200 GeV region, with an observed (expected) significance of 3.4σ (4.6σ). In the p > 100 GeV region, the longitudinal-longitudinal polarization fraction is found to be 0.17+0.021 −0.022, with an observed (expected) significance of 7.9σ (8.8σ). Notably, this marks the first measurement of the longitudinal-longitudinal WZ production in the high- phase space. In addition to the analysis, this thesis covers the contribution of the University of Michigan ATLAS group to the ATLAS New Small Wheel Muon Spectrometer upgrade, particularly the integration and commissioning of the small-strip Thin Gap Chambers electronics. Detailed procedure regarding the integration work, especially the readout chain electronics validations, and detector noise studies is described in the thesis
Search for compressed scalar top quark pairs in p-p collisions at LHC at
No full textSupersymmetry (SUSY) is one of the most appealing theories, which tries to explain some of the experimental and theoretical shortcomings of the Standard Model. It tries to solve the Hierarchy problem by introducing new particles (supersymmetric partners of the Standard Model particles). The quantum corrections due to these particles cancel the divergent contribution from the SM particles to the Higgs mass calculation. The top quark couples to the Higgs maximally due to its higher mass. Therefore, particularly, the supersymmetric partner of the top quark- scalar top quark(stop) is interesting for the searches to stabilize the Higgs mass. It is not easy to single out a model in supersymmetry due to the large number of free parameters. So, theorists have come up with some simplified models, which can be tested experimentally. This thesis is based on one of such models, where a stop decays to a top quark and a neutralino, where neutralino is assumed to be the Lightest Supersymmetric Particle (LSP). It is weakly interacting and stable. Hence, it does not leave any trace in the detector contributing to the missingtransverse energy (MET) in the event. This additional source of MET differentiates the signature of background from the signature of the signal events. Some of the popular MET based analyses have been successful in excluding stop particles upto the mass of 900 GeV. But, the lower mass regions still have weaker exclusion limits. Especially,the region in the stop-LSP mass plane described by , contains some model with lower stop masses, which have not been excluded yet due to the limitations of the currentMET based searches. By selecting only the events with a hard ISR (Initial State Radiation) jet, the MET of the entire system can be improved and also the lower mass models can be probed. For the events produced using the Monte Carlo simulations, expected upper limits at 95% confidence level were calculated for the integrated luminosity L = 30 , 35.9 and 100 at of centre of mass energy. These limits exclude stop masses in the range 225 < 450. Forthe lower masses of LSP, another technique based on the fermionic and bosonic nature of the t and , respectively, is used to study the effect of the spin correlations between the decay products of the top or stop pair produced. A dileptonanalysis can be used to study (l1 , l2 ) distributions (l1and l2 are the two leptons) and calculating the limits using the events in (l1 , l2 ) bins, give betterexclusion potential for the models with < 225 GeV compared to that given by the full hadronic ISR tagging analysis
Author Correction: A detailed map of Higgs boson interactions by the ATLAS experiment ten years after the discovery
No full textIn the version of this article initially published, the ATLAS Collaboration
author names, affiliations and acknowledgements were omitted and
have now been included in the HTML and PDF versions of the article
Strong Constraints on Jet Quenching in Centrality-Dependent p plus Pb Collisions at 5.02 TeV from ATLAS
No full textJet quenching is the process of color-charged partons losing energy via interactions with quark-gluon plasma droplets created in heavy-ion collisions. The collective expansion of such droplets is well described by viscous hydrodynamics. Similar evidence of collectivity is consistently observed in smaller collision systems, including p p and p + Pb collisions. In contrast, while jet quenching is observed in Pb + Pb collisions, no evidence has been found in these small systems to date, raising fundamental questions about the nature of the system created in these collisions. The ATLAS experiment at the Large Hadron Collider has measured the yield of charged hadrons correlated with reconstructed jets in 0.36 nb - 1 of p + Pb and 3.6 pb - 1 of p p collisions at 5.02 TeV. The yields of charged hadrons with p T ch > 0.5 GeV near and opposite in azimuth to jets with p T jet > 30 or 60 GeV, and the ratios of these yields between p + Pb and p p collisions, I p Pb , are reported. The collision centrality of p + Pb events is categorized by the energy deposited by forward neutrons from the struck nucleus. The I p Pb values are consistent with unity within a few percent for hadrons with p T ch > 4 GeV at all centralities. These data provide new, strong constraints that preclude almost any parton energy loss in central p + Pb collisions
Search for new phenomena in collisions in final states with tau leptons, -jets, and missing transverse momentum with the ATLAS detector
No full textA search for new phenomena in final states with hadronically decaying tau leptons, -jets, and missing transverse momentum is presented. The analyzed dataset comprises collision data at a center-of-mass energy of 13 TeV with an integrated luminosity of 139/fb, delivered by the Large Hadron Collider and recorded with the ATLAS detector from 2015 to 2018. The observed data are compatible with the expected Standard Model background. The results are interpreted in simplified models for two different scenarios. The first model considers pair production of supersymmetric top squarks, each of which decays into a -quark, a neutrino and a supersymmetric tau slepton. Each tau slepton in turn decays into a tau lepton and a nearly massless supersymmetric gravitino. Within this model, top-squark masses of 1.4 TeV can be excluded at 95% confidence level over a wide range of tau-slepton masses. The second model considers pair production of scalar leptoquarks with decays into third-generation leptons and quarks. Depending on the branching fraction into charged leptons, leptoquarks with masses up to around 1.25 TeV can be excluded at 95% confidence level. In addition, model-independent upper limits are set on the cross section of processes beyond the Standard Model
Charged-hadron production in pp, p plus Pb, Pb plus Pb, and Xe plus Xe collisions at √sNN=5 TeV with the ATLAS detector at the LHC
No full textThis paper presents measurements of charged-hadron spectra obtained in pp, p+Pb, and Pb+Pb collisions at s or sNN = 5.02 TeV, and in Xe+Xe collisions at sNN = 5.44 TeV. The data recorded by the ATLAS detector at the LHC have total integrated luminosities of 25 pb −1, 28 nb −1, 0.50 nb −1, and 3 μb −1, respectively. The nuclear modification factors RpPb and R AA are obtained by comparing the spectra in heavy-ion and pp collisions in a wide range of charged-particle transverse momenta and pseudorapidity. The nuclear modification factor RpPb shows a moderate enhancement above unity with a maximum at p T ≈ 3 GeV; the enhancement is stronger in the Pb-going direction. The nuclear modification factors in both Pb+Pb and Xe+Xe collisions feature a significant, centrality-dependent suppression. They show a similar distinct p T-dependence with a local maximum at p T ≈ 2 GeV and a local minimum at p T ≈ 7 GeV. This dependence is more distinguishable in more central collisions. No significant |η|-dependence is found. A comprehensive comparison with several theoretical predictions is also provided. They typically describe R AA better in central collisions and in the p T range from about 10 to 100 GeV. [Figure not available: see fulltext.
Measurement of the production of a W boson in association with a charmed hadron in pp collisions at p s=13 TeV with the ATLAS detector
No full textThe production of a W boson in association with a single charm quark is studied using 140 fb - 1 of s = 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider. 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 - π + π + and D * + → D 0 π + → ( K - π + ) π + and the charge conjugate decays in the fiducial regions where p T ( e , μ ) > 30 GeV , | η ( e , μ ) | 8 GeV , and | η ( D ( * ) ) | < 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 σ fid OS - SS ( W - + D + ) = 50.2 ± 0.2 ( stat ) - 2.3 + 2.4 ( syst ) pb , σ fid OS - SS ( W + + D - ) = 48.5 ± 0.2 ( stat ) - 2.2 + 2.3 ( syst ) pb , σ fid OS - SS ( W - + D * + ) = 51.1 ± 0.4 ( stat ) - 1.8 + 1.9 ( syst ) pb , and σ fid OS - SS ( W + + D * - ) = 50.0 ± 0.4 ( stat ) - 1.8 + 1.9 ( 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
Author Correction: A detailed map of Higgs boson interactions by the ATLAS experiment ten years after the discovery
No full textMeasurement of the centrality dependence of the dijet yield in p+Pb collisions at √sNN = 8.16 TeV with the ATLAS detector
No full textATLAS measured the centrality dependence of the dijet yield using
165
nb
−
1
of
p
+
Pb
data collected at
√
s
NN
=
8.16
TeV
in 2016. The event centrality, which reflects the
p
+
Pb
impact parameter, is characterized by the total transverse energy registered in the Pb-going side of the forward calorimeter. The central-to-peripheral ratio of the scaled dijet yields,
R
CP
, is evaluated, and the results are presented as a function of variables that reflect the kinematics of the initial hard parton scattering process. The
R
CP
shows a scaling with the Bjorken
x
of the parton originating from the proton,
x
p
, while no such trend is observed as a function of
x
Pb
. This analysis provides unique input to understanding the role of small proton spatial configurations in
p
+
Pb
collisions by covering parton momentum fractions from the valence region down to
x
p
∼
10
−
3
and
x
Pb
∼
4
×
10
−
4
Search for a new pseudoscalar decaying into a pair of muons in events with a top-quark pair at √s = 13 TeV with the ATLAS detector
No full textA search for a new pseudoscalar
a
-boson produced in events with a top-quark pair, where the
a
-boson decays into a pair of muons, is performed using
√
s
=
13
TeV
p
p
collision data collected with the ATLAS detector at the LHC, corresponding to an integrated luminosity of
139
fb
−
1
. The search targets the final state where only one top quark decays to an electron or muon, resulting in a signature with three leptons
e
μ
μ
and
μ
μ
μ
. No significant excess of events above the Standard Model expectation is observed and upper limits are set on two signal models:
p
p
→
t
¯
t
a
and
p
p
→
t
¯
t
with
t
→
H
±
b
,
H
±
→
W
±
a
, where
a
→
μ
μ
, in the mass ranges
15
GeV
<
m
a
<
72
GeV
and
120
GeV
≤
m
H
±
≤
160
GeV
- …
