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BETA
Bokan, Petar
Publications (10 of 214) Show all publications
Aaboud, M., Asimakopoulou, E. M., Bergeås Kuutmann, E., Bokan, P., Brenner, R., Ekelöf, T., . . . Zwalinski, L. (2019). A strategy for a general search for new phenomena using data-derived signal regions and its application within the ATLAS experiment. European Physical Journal C, 79(2), Article ID 120.
Open this publication in new window or tab >>A strategy for a general search for new phenomena using data-derived signal regions and its application within the ATLAS experiment
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2019 (English)In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 79, no 2, article id 120Article in journal (Refereed) Published
Abstract [en]

This paper describes a strategy for a general search used by the ATLAS Collaboration to find potential indications of new physics. Events are classified according to their final state into many event classes. For each event class an automated search algorithm tests whether the data are compatible with the Monte Carlo simulated expectation in several distributions sensitive to the effects of new physics. The significance of a deviation is quantified using pseudo-experiments. A data selection with a significant deviation defines a signal region for a dedicated follow-up analysis with an improved background expectation. The analysis of the data-derived signal regions on a new dataset allows a statistical interpretation without the large look-elsewhere effect. The sensitivity of the approach is discussed using Standard Model processes and benchmark signals of new physics. As an example, results are shown for 3.2fb-1 of proton-proton collision data at a centre-of-mass energy of 13TeV collected with the ATLAS detector at the LHC in 2015, in which more than 700 event classes and more than 105 regions have been analysed. No significant deviations are found and consequently no data-derived signal regions for a follow-up analysis have been defined.

Place, publisher, year, edition, pages
SPRINGER, 2019
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-378533 (URN)10.1140/epjc/s10052-019-6540-y (DOI)000457998500007 ()
Funder
Swedish Research Council
Note

For complete list of authors see http://dx.doi.org/10.1140/epjc/s10052-019-6540-y

Available from: 2019-03-22 Created: 2019-03-22 Last updated: 2019-03-22Bibliographically approved
Balek, P., Asimakopoulou, E. M., Bergeås Kuutmann, E., Bokan, P., Brenner, R., Ekelöf, T., . . . Zwalinski, L. (2019). Charged-hadron suppression in Pb plus Pb and Xe plus Xe collisions measured with the ATLAS detector. Nuclear Physics A, 982, 571-574
Open this publication in new window or tab >>Charged-hadron suppression in Pb plus Pb and Xe plus Xe collisions measured with the ATLAS detector
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2019 (English)In: Nuclear Physics A, ISSN 0375-9474, E-ISSN 1873-1554, Vol. 982, p. 571-574Article in journal (Refereed) Published
Abstract [en]

The ATLAS detector at the LHC recorded 0.49 nb(-1) of Pb+Pb collisions and 25 of pp(-1) collisions, both at the center-of-mass energy 5.02 TeV per nucleon pair. Recently, ATLAS also recorded 30 mu b(-1) of Xe+Xe collisions at the center-of-mass energy 5.44 TeV, which provides a new opportunity to study the system-size dependence of the charged-hadron production in heavy-ion collisions. The large acceptance of the ATLAS detector allows to measure the spectra of charged hadrons in a wide range of pseudorapidity and transverse momentum. The nuclear modification factors R-AA are constructed as a ratio of the spectra measured in Pb+Pb or Xe+Xe collisions to that measured in pp collisions. The R-AA obtained in the two systems are presented for different centrality intervals and the results are discussed.

Keywords
xenon-xenon collisions, lead-lead collisions, charged-hadron production, nuclear modification factor
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-378203 (URN)10.1016/j.nuclphysa.2018.10.079 (DOI)000457515500128 ()
Note

For complete list of authors see http://dx.doi.org/10.1016/j.nuclphysa.2018.10.079

Available from: 2019-03-05 Created: 2019-03-05 Last updated: 2019-03-05Bibliographically approved
Aaboud, M., Asimakopoulou, E. M., Bergeås Kuutmann, E., Bokan, P., Brenner, R., Ekelöf, T., . . . Zwalinski, L. (2019). Correlated long-range mixed-harmonic fluctuations measured in pp, p plus Pb and low-multiplicity Pb plus Pb collisions with the ATLAS detector. Physics Letters B, 789, 444-471
Open this publication in new window or tab >>Correlated long-range mixed-harmonic fluctuations measured in pp, p plus Pb and low-multiplicity Pb plus Pb collisions with the ATLAS detector
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2019 (English)In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 789, p. 444-471Article in journal (Refereed) Published
Abstract [en]

Correlations of two flow harmonics v(n) and v(m) via three- and four-particle cumulants are measured in 13 TeV pp, 5.02 TeV p+Pb, and 2.76 TeV peripheral Pb+Pb collisions with the ATLAS detector at the LHC. The goal is to understand the multi-particle nature of the long-range collective phenomenon in these collision systems. The large non-flow background from dijet production present in the standard cumulant method is suppressed using a method of subevent cumulants involving two, three and four subevents separated in pseudorapidity. The results show a negative correlation between v(2) and v(3) and a positive correlation between v(2) and v(4) for all collision systems and over the full multiplicity range. However, the magnitudes of the correlations are found to depend on the event multiplicity, the choice of transverse momentum range and collision system. The relative correlation strength, obtained by normalisation of the cumulants with the < v(n)(2)> from a two-particle correlation analysis, is similar in the three collision systems and depends weakly on the event multiplicity and transverse momentum. These results based on the subevent methods provide strong evidence of a similar long-range multi-particle collectivity in pp, p+Pb and peripheral Pb+Pb collisions. (C) 2019 The Author. Published by Elsevier B.V.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2019
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-377205 (URN)10.1016/j.physletb.2018.11.065 (DOI)000457165400062 ()
Funder
Swedish Research CouncilEU, European Research CouncilEU, FP7, Seventh Framework ProgrammeEU, Horizon 2020
Note

For complete list of authors see http://dx.doi.org/10.1016/j.physletb.2018.11.065

Available from: 2019-02-15 Created: 2019-02-15 Last updated: 2019-02-15Bibliographically approved
Aaboud, M., Asimakopoulou, E. M., Bergeås Kuutmann, E., Bokan, P., Brenner, R., Ekelöf, T., . . . Zwalinski, L. (2019). Cross-section measurements of the Higgs boson decaying into a pair of τ-leptons in proton-proton collisions at √s=13 TeV with the ATLAS detector. Physical Review D: covering particles, fields, gravitation, and cosmology, 99(7), Article ID 072001.
Open this publication in new window or tab >>Cross-section measurements of the Higgs boson decaying into a pair of τ-leptons in proton-proton collisions at √s=13 TeV with the ATLAS detector
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2019 (English)In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 99, no 7, article id 072001Article in journal (Refereed) Published
Abstract [en]

A measurement of production cross sections of the Higgs boson in proton-proton collisions is presented in the H → τ τ decay channel. The analysis is performed using 36.1 fb-1 of data recorded by the ATLAS experiment at the Large Hadron Collider at a center-of-mass energy of √s = 13 TeV. All combinations of leptonic (τ → l v v‾ with l = e, μ) and hadronic (τ→ hadrons v) τ decays arc considered. The H → τ τ signal over the expected background from other Standard Model processes is established with an observed (expected) significance of 4.4 (4.1) standard deviations. Combined with results obtained using data taken at 7 and 8 TeV center-of-mass energies, the observed (expected) significance amounts to 6.4 (5.4) standard deviations and constitutes an observation of H → τ τ decays. Using the data taken at √s = 13 TeV, the total cross section in the H → τ τ decay channel is measured to be 3.77+0.60-0.59(stat)+0.87-0.74 (syst) pb, for a Higgs boson of mass 125 GeV assuming the relative contributions of its production modes as predicted by the Standard Model. Total cross sections in the H → τ τ decay channel are determined separately for vector-boson-fusion production and gluon-gluon-fusion production to be σVBFH → τ τ = 0.28 ± 0.09 (stat)+0.11-0.09 (syst) pb and σggFH → τ τ = 3.1 ± 1.0 (stat)+1.6-1.3 (syst) pb, respectively. Similarly, results of a fit are reported in the framework of simplified template cross sections. All measurements are in agreement with Standard Model expectations.

National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-382859 (URN)10.1103/PhysRevD.99.072001 (DOI)000464740700002 ()
Funder
Swedish Research CouncilEU, European Research CouncilEU, Horizon 2020Danish National Research FoundationThe Research Council of NorwayWallenberg FoundationsGerman Research Foundation (DFG)
Note

For complete list of authors see http://dx.doi.org/10.1103/PhysRevD.99.072001

Available from: 2019-05-13 Created: 2019-05-13 Last updated: 2019-05-13Bibliographically approved
Steinberg, P., Asimakopoulou, E. M., Bergeås Kuutmann, E., Bokan, P., Brenner, R., Ekelöf, T., . . . Zwalinski, L. (2019). Electromagnetic processes with quasireal photons in Pb+Pb collisions: QED, QCD, and the QGP. Paper presented at XXVIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2018). Nuclear Physics A, 982, 259-262
Open this publication in new window or tab >>Electromagnetic processes with quasireal photons in Pb+Pb collisions: QED, QCD, and the QGP
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2019 (English)In: Nuclear Physics A, ISSN 0375-9474, E-ISSN 1873-1554, Vol. 982, p. 259-262Article in journal (Refereed) Published
Abstract [en]

Electromagnetic processes, both photon-photon and photon-nucleus, are shown to be useful in studying aspects of QED, QCD, and potentially the QGP. Using lead-lead collisions at root s(NN) = 5.02 TeV, the ATLAS detector has performed measurements of exclusive dimuon production, light-by-light scattering (via exclusive diphoton production), and photo-nuclear dijet production. These are all important examples of ultraperipheral collisions, where the nuclei do not interact hadronically. A recent study of the opening angles of dimuons produced in hadronic heavy-ion collisions, after subtracting heavy-flavor backgrounds, demonstrates that the dimuons carry information correlated with the overlap geometry, potentially about the density of charges in the QGP itself.

National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-378199 (URN)10.1016/j.nuclphysa.2018.10.087 (DOI)000457515500050 ()
Conference
XXVIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2018)
Note

For complete list of authors see http://dx.doi.org/10.1016/j.nuclphysa.2018.10.087

Available from: 2019-03-06 Created: 2019-03-06 Last updated: 2019-03-06Bibliographically approved
Aaboud, M., Asimakopoulou, E. M., Bergeås Kuutmann, E., Bokan, P., Brenner, R., Ekelöf, T., . . . Zwalinski, L. (2019). Electron and photon energy calibration with the ATLAS detector using 2015-2016 LHC proton-proton collision data. Journal of Instrumentation, 14, Article ID P03017.
Open this publication in new window or tab >>Electron and photon energy calibration with the ATLAS detector using 2015-2016 LHC proton-proton collision data
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2019 (English)In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 14, article id P03017Article in journal (Refereed) Published
Abstract [en]

This paper presents the electron and photon energy calibration obtained with the ATLAS detector using about 36 fb(-1) of LHC proton-proton collision data recorded at root s = 13 TeV in 2015 and 2016. The different calibration steps applied to the data and the optimization of the reconstruction of electron and photon energies are discussed. The absolute energy scale is set using a large sample of Z boson decays into electron-positron pairs. The systematic uncertainty in the energy scale calibration varies between 0.03% to 0.2% in most of the detector acceptance for electrons with transverse momentum close to 45 GeV. For electrons with transverse momentum of 10 GeV the typical uncertainty is 0.3% to 0.8% and it varies between 0.25% and 1% for photons with transverse momentum around 60 GeV. Validations of the energy calibration with J/psi -> e(+)e(-) decays and radiative Z boson decays are also presented.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2019
Keywords
Calorimeter methods, Pattern recognition, cluster finding, calibration and fitting methods, Performance of High Energy Physics Detectors
National Category
Subatomic Physics Accelerator Physics and Instrumentation
Identifiers
urn:nbn:se:uu:diva-382467 (URN)10.1088/1748-0221/14/03/P03017 (DOI)000463330900005 ()
Funder
EU, European Research Council
Note

For complete list of authors see http://dx.doi.org/10.1088/1748-0221/14/03/P03017

Available from: 2019-05-02 Created: 2019-05-02 Last updated: 2019-05-02Bibliographically approved
Citron, Z., Asimakopoulou, E. M., Bergeås Kuutmann, E., Bokan, P., Brenner, R., Ekelöf, T., . . . Zwalinski, L. (2019). Electroweak probes of small and large systems with the ATLAS detector. Nuclear Physics A, 982, 603-606
Open this publication in new window or tab >>Electroweak probes of small and large systems with the ATLAS detector
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2019 (English)In: Nuclear Physics A, ISSN 0375-9474, E-ISSN 1873-1554, Vol. 982, p. 603-606Article in journal (Refereed) Published
Abstract [en]

Measurements of isolated prompt photon and massive electroweak (W and Z) boson production in different collision systems are of great interest to understand the partonic structure of heavy nuclei, and serve as a constraint on the initial state in larger collision systems. These channels are sensitive to a variety of effects such as the modification of the parton densities in nuclei in certain kinematic regions, and the energy loss of partons as they undergo multiple interactions in the nucleus before the hard parton-parton scattering. High-statistics samples of lead-lead and proton-lead collision data at root s(NN)=5.02 TeV and 8.16 TeV, respectively, taken by the ATLAS experiment at the LHC, as well as proton-proton comparison data at analogous collision energies, allow for a detailed study of these phenomena in data and comprehensive comparisons to the predictions of a variety of theoretical approaches. This paper presents the latest ATLAS results in these topics, including updated results on inclusive prompt photon production in proton-lead collisions over a broad kinematic range and high-precision W boson results in lead lead collisions.

Keywords
Lead-lead collisions, Proton-lead collisions, Proton-proton collisions, Photons, W boson, Z boson
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-378205 (URN)10.1016/j.nuclphysa.2018.09.029 (DOI)000457515500136 ()
Note

For complete list of authors see http://dx.doi.org/10.1016/j.nuclphysa.2018.09.029

Available from: 2019-03-05 Created: 2019-03-05 Last updated: 2019-03-05Bibliographically approved
Zhou, M., Asimakopoulou, E. M., Bergeås Kuutmann, E., Bokan, P., Brenner, R., Ekelöf, T., . . . Zwalinski, L. (2019). Flow fluctuations in Pb plus Pb collisions at √sNN=5.02 TeV with the ATLAS detector. Paper presented at XXVIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2018). Nuclear Physics A, 982, 323-326
Open this publication in new window or tab >>Flow fluctuations in Pb plus Pb collisions at √sNN=5.02 TeV with the ATLAS detector
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2019 (English)In: Nuclear Physics A, ISSN 0375-9474, E-ISSN 1873-1554, Vol. 982, p. 323-326Article in journal (Refereed) Published
Abstract [en]

Measurements of four-particle cumulants c(n){4} for n = 1, 2, 3, 4 are presented using 470 mu b(-1) of Pb+Pb collisions at root s(NN) = 5.02 TeV with the ATLAS detector at the LHC. These cumulants provide information on the event-by-event fluctuations of single harmonics p(v(n)). For the first time, a negative c(1){4} is observed. The c(4){4} is found to be negative in central collisions but changes sign around 20-25% centrality. This behavior is consistent with a nonlinear contribution to v(4) that is proportional to v(2)(2). c(2){4} and c(3){4} are calculated using two reference event classes in order to investigate the influence of volume fluctuations. Over most of the centrality range, c(2){4} and c(3){4} are found to be negative, while in the ultra-central collisions, c(2){4} changes sign and becomes positive, suggesting a deviation from Gaussian behavior in the event-by-event fluctuation of v(2). The magnitudes of the sign change are also found to be dependent of the event class definition .

Keywords
Multi-Particle Correlation, Cumulants, Heavy-ion, Flow Fluctuation
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-378200 (URN)10.1016/j.nuclphysa.2018.09.012 (DOI)000457515500066 ()
Conference
XXVIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2018)
Note

Title in WoS: Flow fluctuations in Pb plus Pb collisions at root s(NN)=5.02 TeV with the ATLAS detector

For complete list of authors see http://dx.doi.org/10.1016/j.nuclphysa.2018.09.012

Available from: 2019-03-05 Created: 2019-03-05 Last updated: 2019-03-05Bibliographically approved
Grabowska-Bold, I., Asimakopoulou, E. M., Bergeås Kuutmann, E., Bokan, P., Brenner, R., Ekelöf, T., . . . Zwalinski, L. (2019). Highlights from the ATLAS experiment. Paper presented at XXVIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2018). Nuclear Physics A, 982, 8-14
Open this publication in new window or tab >>Highlights from the ATLAS experiment
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2019 (English)In: Nuclear Physics A, ISSN 0375-9474, E-ISSN 1873-1554, Vol. 982, p. 8-14Article in journal (Refereed) Published
Abstract [en]

This report provides an overview of the new results obtained by the ATLAS Collaboration at the LHC, which were presented at the Quark Matter 2018 conference. These measurements were covered in 12 parallel talks, one flash talk and 11 posters. In this document, a discussion of results is grouped into four areas: electromagnetic interactions, jet quenching, quarkonia and heavy-flavour production, and collectivity in small and larger systems. Measurements from the xenon-xenon collisions based on a short run collected in October 2017 are reported for the first time.

Keywords
ATLAS experiment, quark-gluon plasma, photon-induced processes, jet quenching, quarkonia production, heavy-flavour production, collectivity in small systems, xenon-xenon collisions
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-378197 (URN)10.1016/j.nuclphysa.2018.08.024 (DOI)000457515500003 ()
Conference
XXVIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2018)
Note

For complete list of authors see http://dx.doi.org/10.1016/j.nuclphysa.2018.08.024

Available from: 2019-03-07 Created: 2019-03-07 Last updated: 2019-03-07Bibliographically approved
Aaboud, M., Asimakopoulou, E. M., Bergeås Kuutmann, E., Bokan, P., Brenner, R., Ekelöf, T., . . . Zwalinski, L. (2019). In situ calibration of large-radius jet energy and mass in 13 TeV proton-proton collisions with the ATLAS detector. European Physical Journal C, 79(2), Article ID 135.
Open this publication in new window or tab >>In situ calibration of large-radius jet energy and mass in 13 TeV proton-proton collisions with the ATLAS detector
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2019 (English)In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 79, no 2, article id 135Article in journal (Refereed) Published
Abstract [en]

The response of the ATLAS detector to large-radius jets is measured in situ using 36.2 fb(-1) of root s = 13TeV proton-proton collisions provided by the LHC and recorded by the ATLAS experiment during 2015 and 2016. The jet energy scale is measured in events where the jet recoils against a reference object, which can be either a calibrated photon, a reconstructed Z boson, or a system of well-measured small-radius jets. The jet energy resolution and a calibration of forward jets are derived using dijet balance measurements. The jet mass response is measured with two methods: using mass peaks formed by W bosons and top quarks with large transverse momenta and by comparing the jet mass measured using the energy deposited in the calorimeter with that using the momenta of charged-particle tracks. The transverse momentum and mass responses in simulations are found to be about 2-3% higher than in data. This difference is adjusted for with a correction factor. The results of the different methods are combined to yield a calibration over a large range of transverse momenta (p(T)). The precision of the relative jet energy scale is 1-2% for 200 GeV < p(T) < TeV, while that of the mass scale is 2-10%. The ratio of the energy resolutions in data and simulation is measured to a precision of 10-15% over the same p(T) range.

Place, publisher, year, edition, pages
SPRINGER, 2019
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-378376 (URN)10.1140/epjc/s10052-019-6632-8 (DOI)000458691000004 ()
Funder
Swedish Research CouncilEU, European Research CouncilEU, Horizon 2020Wallenberg Foundations
Note

For complete list of authors see http://dx.doi.org/10.1140/epjc/s10052-019-6632-8

Available from: 2019-03-07 Created: 2019-03-07 Last updated: 2019-03-07Bibliographically approved
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