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Publications (10 of 459) Show all publications
Abulaiti, Y., Åkerstedt, H., Åsman, B., Bendtz, K., Bertoli, G., Bessidskaia Bylund, O., . . . Wallängen, V. (2017). Performance of algorithms that reconstruct missing transverse momentum in root s=8 TeV proton-proton collisions in the ATLAS detector. European Physical Journal C, 77(4), Article ID 241.
Open this publication in new window or tab >>Performance of algorithms that reconstruct missing transverse momentum in root s=8 TeV proton-proton collisions in the ATLAS detector
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2017 (English)In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 77, no 4, article id 241Article in journal (Refereed) Published
Abstract [en]

The reconstruction and calibration algorithms used to calculate missing transverse momentum (E-T(miss)) with the ATLAS detector exploit energy deposits in the calorimeter and tracks reconstructed in the inner detector as well as the muon spectrometer. Various strategies are used to suppress effects arising from additional proton-proton interactions, called pileup, concurrent with the hard-scatter processes. Tracking information is used to distinguish contributions from the pileup interactions using their vertex separation along the beam axis. The performance of the E-T(miss) reconstruction algorithms, especially with respect to the amount of pileup, is evaluated using data collected in proton-proton collisions at a centre-of-mass energy of 8 TeV during 2012, and results are shown for a data sample corresponding to an integrated luminosity of 20.3 fb(-1). The simulation and modelling of E-T(miss) in events containing a Z boson decaying to two charged leptons (electrons or muons) or a W boson decaying to a charged lepton and a neutrino are compared to data. The acceptance for different event topologies, with and without high transverse momentum neutrinos, is shown for a range of threshold criteria for E-T(miss), and estimates of the systematic uncertainties in the E-T(miss) measurements are presented.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-143589 (URN)10.1140/epjc/s10052-017-4780-2 (DOI)000399448800002 ()28515666 (PubMedID)
Available from: 2017-05-31 Created: 2017-05-31 Last updated: 2022-03-23Bibliographically approved
Abulaiti, Y., Åkerstedt, H., Åsman, B., Bendtz, K., Bertoli, G., Bessidskaia Bylund, O., . . . Wallängen, V. (2017). Search for lepton-flavour-violating decays of the Higgs and Z bosons with the ATLAS detector. European Physical Journal C, 77(2), Article ID 70.
Open this publication in new window or tab >>Search for lepton-flavour-violating decays of the Higgs and Z bosons with the ATLAS detector
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2017 (English)In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 77, no 2, article id 70Article in journal (Refereed) Published
Abstract [en]

Direct searches for lepton flavour violation in decays of the Higgs and Z bosons with the ATLAS detector at the LHC are presented. The following three decays are considered: H -> e tau, H -> mu tau, and Z -> mu tau. The searches are based on the data sample of proton-proton collisions collected by the ATLAS detector corresponding to an integrated luminosity of 20.3 fb(-1) at a centre-of-mass energy of root s = 8 TeV. No significant excess is observed, and upper limits on the lepton-flavour-violating branching ratios are set at the 95% confidence level: Br(H -> e tau) < 1.04%, Br(H -> mu tau) < 1.43%, and Br(Z -> mu tau) < 1.69 x 10(-5).

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-142515 (URN)10.1140/epjc/s10052-017-4624-0 (DOI)000394224700002 ()
Available from: 2017-05-10 Created: 2017-05-10 Last updated: 2022-03-23Bibliographically approved
Abulaiti, Y., Åkerstedt, H., Åsman, B., Bendtz, K., Bertoli, G., Bessidskaia Bylund, O., . . . Ughetto, M. (2017). Topological cell clustering in the ATLAS calorimeters and its performance in LHC Run 1. European Physical Journal C, 77(7), Article ID 490.
Open this publication in new window or tab >>Topological cell clustering in the ATLAS calorimeters and its performance in LHC Run 1
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2017 (English)In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 77, no 7, article id 490Article in journal (Refereed) Published
Abstract [en]

The reconstruction of the signal from hadrons and jets emerging from the proton-proton collisions at the Large Hadron Collider (LHC) and entering the ATLAS calorimeters is based on a three-dimensional topological clustering of individual calorimeter cell signals. The cluster formation follows cell signal-significance patterns generated by electromagnetic and hadronic showers. In this, the clustering algorithm implicitly performs a topological noise suppression by removing cells with insignificant signals which are not in close proximity to cells with significant signals. The resulting topological cell clusters have shape and location information, which is exploited to apply a local energy calibration and corrections depending on the nature of the cluster. Topological cell clustering is established as a well-performing calorimeter signal definition for jet and missing transverse momentum reconstruction in ATLAS.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-147166 (URN)10.1140/epjc/s10052-017-5004-5 (DOI)000406426400001 ()
Available from: 2017-09-22 Created: 2017-09-22 Last updated: 2022-03-23Bibliographically approved
Abulaiti, Y., Åkerstedt, H., Åsman, B., Bendtz, K., Bertoli, G., Bessidskaia Bylund, O., . . . Ughetto, M. (2016). A new method to distinguish hadronically decaying boosted Z bosons from W bosons using the ATLAS detector. European Physical Journal C, 76(5), Article ID 238.
Open this publication in new window or tab >>A new method to distinguish hadronically decaying boosted Z bosons from W bosons using the ATLAS detector
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2016 (English)In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, no 5, article id 238Article in journal (Refereed) Published
Abstract [en]

The distribution of particles inside hadronic jets produced in the decay of boosted W and Z bosons can be used to discriminate such jets from the continuum background. Given that a jet has been identified as likely resulting from the hadronic decay of a boosted W or Z boson, this paper presents a technique for further differentiating Z bosons from W bosons. The variables used are jet mass, jet charge, and a b-tagging discriminant. A likelihood tagger is constructed from these variables and tested in the simulation of W' -> WZ for bosons in the transverse momentum range 200 GeV < p(T) < 400 GeV in root s = 8 TeV pp collisions with the ATLAS detector at the LHC. For Z-boson tagging efficiencies of epsilon(Z) = 90, 50, and 10 %, one can achieve W+-boson tagging rejection factors (1/epsilon(W+)) of 1.7, 8.3 and 1000, respectively. It is not possible to measure these efficiencies in the data due to the lack of a pure sample of high p(T), hadronically decaying Z bosons. However, the modelling of the tagger inputs for boosted W bosons is studied in data using a t (t) over bar -enriched sample of events in 20.3 fb(-1) of data at root s = 8 TeV. The inputs are well modelled within uncertainties, which builds confidence in the expected tagger performance.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-144467 (URN)10.1140/epjc/s10052-016-4065-1 (DOI)000399466100001 ()
Available from: 2017-06-21 Created: 2017-06-21 Last updated: 2022-03-23Bibliographically approved
Abulaiti, Y., Åkerstedt, H., Åsman, B., Bendtz, K., Bertoli, G., Bessidskaia Bylund, O., . . . Wallängen, V. (2016). A search for an excited muon decaying to a muon and two jets in pp collisions at root s=8 TeV with the ATLAS detector. New Journal of Physics, 18, Article ID 073021.
Open this publication in new window or tab >>A search for an excited muon decaying to a muon and two jets in pp collisions at root s=8 TeV with the ATLAS detector
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2016 (English)In: New Journal of Physics, E-ISSN 1367-2630, Vol. 18, article id 073021Article in journal (Refereed) Published
Abstract [en]

Anew search signature for excited leptons is explored. Excited muons are sought in the channel pp -> mu mu* -> mu mu jet jet, assuming both the production and decay occur via a contact interaction. The analysis is based on 20.3 fb(-1) of pp collision data at a centre-of-mass energy of root s = 8 TeV taken with the ATLAS detector at the large hadron collider. No evidence of excited muons is found, and limits are set at the 95% confidence level on the cross section times branching ratio as a function of the excited-muon mass m(mu)*. For m(mu)* between 1.3 and 3.0 TeV, the upper limit on sigma B(mu* -> mu q (q) over bar) is between 0.6 and 1 fb. Limits on sB are converted to lower bounds on the compositeness scale Lambda. In the limiting case Lambda = m(mu)*, excited muons with a mass below 2.8 TeV are excluded. With the same model assumptions, these limits at larger mu* masses improve upon previous limits from traditional searches based on the gauge-mediated decay mu* -> mu gamma.

Keywords
excited leptons, lepton compositeness, leptoquarks
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-134289 (URN)10.1088/1367-2630/18/7/073021 (DOI)000381870700003 ()
Available from: 2016-10-05 Created: 2016-10-03 Last updated: 2024-01-17Bibliographically approved
Abulaiti, Y., Åkerstedt, H., Åsman, B., Bendtz, K., Bertoli, G., Bessidskaia Bylund, O., . . . Ughetto, M. (2016). A search for prompt lepton-jets in pp collisions at root s=8 TeV with the ATLAS detector. Journal of High Energy Physics (JHEP) (2), Article ID 062.
Open this publication in new window or tab >>A search for prompt lepton-jets in pp collisions at root s=8 TeV with the ATLAS detector
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2016 (English)In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 2, article id 062Article in journal (Refereed) Published
Abstract [en]

A search is presented for a new, light boson with a mass of about 1 GeV and decaying promptly to jets of collimated electrons and/or muons (lepton-jets). The analysis is performed with 20.3 fb(-1) of data collected by the ATLAS detector at the Large Hadron Collider in proton-proton collisions at a centre-of-mass energy of 8 TeV. Events are required to contain at least two lepton-jets. This study finds no statistically significant deviation from predictions of the Standard Model and places 95% confidence-level upper limits on the contribution of new phenomena beyond the SM, incuding SUSY-portal and Higgs-portal models, on the number of events with lepton-jets.

Keywords
Hadron-Hadron scattering
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-130985 (URN)10.1007/JHEP02(2016)062 (DOI)000375272300004 ()
Available from: 2016-06-13 Created: 2016-06-09 Last updated: 2022-03-23Bibliographically approved
Abulaiti, Y., Åkerstedt, H., Åsman, B., Bendtz, K., Bertoli, G., Bessidskaia Bylund, O., . . . Wallängen, V. (2016). A search for top squarks with R-parity-violating decays to all-hadronic final states with the ATLAS detector in root s=8 TeV proton-proton collisions. Journal of High Energy Physics (JHEP) (6), Article ID 067.
Open this publication in new window or tab >>A search for top squarks with R-parity-violating decays to all-hadronic final states with the ATLAS detector in root s=8 TeV proton-proton collisions
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2016 (English)In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 6, article id 067Article in journal (Refereed) Published
Abstract [en]

A search for the pair production of top squarks, each with R-parity-violating decays into two Standard Model quarks, is performed using 17.4 fb(-1) of root s = 8 TeV proton-proton collision data recorded by the ATLAS experiment at the LITC. Each top squark is assumed to decay to a b- and an 8-quark, leading to four quarks in the final state. Background discrimination is achieved with the use of b-tagging and selections on the mass and substructure of large-radius jets, providing sensitivity to top squark masses as low as 100 GeV. No evidence of an excess beyond the Standard Model background prediction is observed and top squalls decaying to bs are excluded for top squark masses in the range 100 <= m((t) over tilde) <= 315 GeV at 95% confidence level.

Keywords
Hadron-Hadron scattering (experiments)
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-132408 (URN)10.1007/JHEP06(2016)067 (DOI)000377999700001 ()
Available from: 2016-08-12 Created: 2016-08-11 Last updated: 2022-03-23Bibliographically approved
Abulaiti, Y., Åkerstedt, H., Åsman, B., Bendtz, K., Bertoli, G., Bessidskaia Bylund, O., . . . Wallängen, V. (2016). Beam-induced and cosmic-ray backgrounds observed in the ATLAS detector during the LHC 2012 proton-proton running period. Journal of Instrumentation, 11, Article ID P05013.
Open this publication in new window or tab >>Beam-induced and cosmic-ray backgrounds observed in the ATLAS detector during the LHC 2012 proton-proton running period
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2016 (English)In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 11, article id P05013Article in journal (Refereed) Published
Abstract [en]

This paper discusses various observations on beam-induced and cosmic-ray backgrounds in the ATLAS detector during the LHC 2012 proton-proton run. Building on published results based on 2011 data, the correlations between background and residual pressure of the beam vacuum are revisited. Ghost charge evolution over 2012 and its role for backgrounds are evaluated. New methods to monitor ghost charge with beam-gas rates are presented and observations of LHC abort gap population by ghost charge are discussed in detail. Fake jets from colliding bunches and from ghost charge are analysed with improved methods, showing that ghost charge in individual radio-frequency buckets of the LHC can be resolved. Some results of two short periods of dedicated cosmic-ray background data-taking are shown; in particular cosmic-ray muon induced fake jet rates are compared to Monte Carlo simulations and to the fake jet rates from beam background. A thorough analysis of a particular LHC fill, where abnormally high background was observed, is presented. Correlations between backgrounds and beam intensity losses in special fills with very high beta* are studied.

Keywords
Beam-line instrumentation (beam position and profile monitors, beam-intensity monitors, bunch length monitors), Data analysis, Performance of High Energy Physics Detectors
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-144465 (URN)10.1088/1748-0221/11/05/P05013 (DOI)000377851700036 ()
Available from: 2017-06-21 Created: 2017-06-21 Last updated: 2022-03-23Bibliographically approved
Abulaiti, Y., Åkerstedt, H., Åsman, B., Bendtz, K., Bertoli, G., Bessidskaia Bylund, O., . . . Tylmad, M. (2016). Centrality, rapidity, and transverse momentum dependence of isolated prompt photon production in lead-lead collisions at root S-NN=2.76 TeV measured with the ATLAS detector. Physical Review C, 93(3), Article ID 034914.
Open this publication in new window or tab >>Centrality, rapidity, and transverse momentum dependence of isolated prompt photon production in lead-lead collisions at root S-NN=2.76 TeV measured with the ATLAS detector
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2016 (English)In: Physical Review C, ISSN 2469-9985, Vol. 93, no 3, article id 034914Article in journal (Refereed) Published
Abstract [en]

Prompt photon production in root S-NN = 2.76-TeV Pb + Pb collisions has been measured by the ATLAS experiment at the Large Hadron Collider using data collected in 2011 with an integrated luminosity of 0.14 nb(-1). Inclusive photon yields, scaled by the mean nuclear thickness function, are presented as a function of collision centrality and transverse momentum in two pseudorapidity intervals, vertical bar eta vertical bar < 1.37 and 1.52 <= vertical bar eta vertical bar < 2.37. The scaled yields in the two pseudorapidity intervals, as well as the ratios of the forward yields to those at midrapidity, are compared to the expectations from next-to-leading-order perturbative QCD (pQCD) calculations. The measured cross sections agree well with the predictions for proton-proton collisions within statistical and systematic uncertainties. Both the yields and the ratios are also compared to two other pQCD calculations, one which uses the isospin content appropriate to colliding lead nuclei and another which includes nuclear modifications to the nucleon parton distribution functions.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-130143 (URN)10.1103/PhysRevC.93.034914 (DOI)000372800000003 ()
Available from: 2016-05-17 Created: 2016-05-09 Last updated: 2022-03-23Bibliographically approved
Abulaiti, Y., Åkerstedt, H., Åsman, B., Bendtz, K., Bertoli, G., Bessidskaia Bylund, O., . . . Wallängen, V. (2016). Charged-particle distributions in pp interactions at root s=8TeV measured with the ATLAS detector. European Physical Journal C, 76(7), Article ID 403.
Open this publication in new window or tab >>Charged-particle distributions in pp interactions at root s=8TeV measured with the ATLAS detector
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2016 (English)In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, no 7, article id 403Article in journal (Refereed) Published
Abstract [en]

This paper presents measurements of distributions of charged particles which are produced in proton-proton collisions at a centre-of-mass energy of root s = 8TeV and recorded by the ATLAS detector at the LHC. A special dataset recorded in 2012 with a small number of interactions per beam crossing (below 0.004) and corresponding to an integrated luminosity of 160 mu b(-1) was used. A minimumbias trigger was utilised to select a data sample of more than 9 million collision events. The multiplicity, pseudorapidity, and transverse momentum distributions of charged particles are shown in different regions of kinematics and charged-particle multiplicity, including measurements of final states at high multiplicity. The results are corrected for detector effects and are compared to the predictions of various Monte Carlo event generator models which simulate the full hadronic final state.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-134258 (URN)10.1140/epjc/s10052-016-4203-9 (DOI)000380090400001 ()
Available from: 2016-10-04 Created: 2016-10-03 Last updated: 2022-03-23Bibliographically approved
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