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Asimakopoulou, Eleni M.
Alternative names
Publications (10 of 117) 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., . . . Zhang, Z. (2019). Combination of Searches for Invisible Higgs Boson Decays with the ATLAS Experiment. Physical Review Letters, 122(23), Article ID 231801.
Open this publication in new window or tab >>Combination of Searches for Invisible Higgs Boson Decays with the ATLAS Experiment
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2019 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 122, no 23, article id 231801Article in journal (Refereed) Published
Abstract [en]

Dark matter particles, if sufficiently light, may be produced in decays of the Higgs boson. This Letter presents a statistical combination of searches for H -> invisible decays where H is produced according to the standard model via vector boson fusion, Z(ll)H, and W/Z(had)H, all performed with the ATLAS detector using 36.1 fb(-1) of pp collisions at a center-of-mass energy of root s = 13 TeV at the LHC. In combination with the results at root s = 7 and 8 TeV, an exclusion limit on the H -> invisible branching ratio of 0.26(0.17(-0.05)(+0.07)) at 95% confidence level is observed (expected).

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2019
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-390206 (URN)10.1103/PhysRevLett.122.231801 (DOI)000471988600003 ()31298882 (PubMedID)
Funder
Swedish Research CouncilEU, European Research CouncilEU, Horizon 2020Wallenberg Foundations
Note

For complete list of authors see http://dx.doi.org/10.1103/PhysRevLett.122.231801

Available from: 2019-08-09 Created: 2019-08-09 Last updated: 2019-08-09Bibliographically approved
Aaboud, M., Asimakopoulou, E. M., Bergeås Kuutmann, E., Bokan, P., Brenner, R., Ekelöf, T., . . . Woods, N. (2019). Combinations of single-top-quark production cross-section measurements and vertical bar f(LV)V(tb)vertical bar determinations at root s=7 and 8 TeV with the ATLAS and CMS experiments. Journal of High Energy Physics (JHEP) (5), Article ID 088.
Open this publication in new window or tab >>Combinations of single-top-quark production cross-section measurements and vertical bar f(LV)V(tb)vertical bar determinations at root s=7 and 8 TeV with the ATLAS and CMS experiments
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2019 (English)In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 5, article id 088Article in journal (Refereed) Published
Abstract [en]

This paper presents the combinations of single-top-quark production cross-section measurements by the ATLAS and CMS Collaborations, using data from LHC proton-proton collisions at = 7 and 8 TeV corresponding to integrated luminosities of 1.17 to 5.1 fb(-1) at = 7 TeV and 12.2 to 20.3 fb(-1) at = 8 TeV. These combinations are performed per centre-of-mass energy and for each production mode: t-channel, tW, and s-channel. The combined t-channel cross-sections are 67.5 +/- 5.7 pb and 87.7 +/- 5.8 pb at = 7 and 8 TeV respectively. The combined tW cross-sections are 16.3 +/- 4.1 pb and 23.1 +/- 3.6 pb at = 7 and 8 TeV respectively. For the s-channel cross-section, the combination yields 4.9 +/- 1.4 pb at = 8 TeV. The square of the magnitude of the CKM matrix element V-tb multiplied by a form factor f(LV) is determined for each production mode and centre-of-mass energy, using the ratio of the measured cross-section to its theoretical prediction. It is assumed that the top-quark-related CKM matrix elements obey the relation |V-td|, |V-ts| << |V-tb|. All the |f(LV)V(tb)|(2) determinations, extracted from individual ratios at = 7 and 8 TeV, are combined, resulting in |f(LV)V(tb)| = 1.02 +/- 0.04 (meas.) +/- 0.02 (theo.). All combined measurements are consistent with their corresponding Standard Model predictions.

Place, publisher, year, edition, pages
SPRINGER, 2019
Keywords
Hadron-Hadron scattering (experiments)
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-387959 (URN)10.1007/JHEP05(2019)088 (DOI)000468428300001 ()
Funder
Swedish Research CouncilEU, European Research CouncilEU, Horizon 2020EU, European Research CouncilEU, Horizon 2020, 675440EU, European Research CouncilWallenberg Foundations
Note

For complete list of authors see http://dx.doi.org/10.1007/JHEP05(2019)088

Available from: 2019-06-27 Created: 2019-06-27 Last updated: 2019-06-27Bibliographically approved
Aaboud, M., Asimakopoulou, E. M., Bergeås Kuutmann, E., Bokan, P., Brenner, R., Ekelöf, T., . . . Zwalinski, L. (2019). Comparison of Fragmentation Functions for Jets Dominated by Light Quarks and Gluons from pp and Pb plus Pb Collisions in ATLAS. Physical Review Letters, 123(4), Article ID 042001.
Open this publication in new window or tab >>Comparison of Fragmentation Functions for Jets Dominated by Light Quarks and Gluons from pp and Pb plus Pb Collisions in ATLAS
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2019 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 123, no 4, article id 042001Article in journal (Refereed) Published
Abstract [en]

Charged-particle fragmentation functions for jets azimuthally balanced by a high-transverse-momentum, prompt, isolated photon are measured in 25 pb(-1) of pp and 0.49 nb(-1) of Pb + Pb collision data at 5.02 TeV per nucleon pair recorded with the ATLAS detector at the Large Hadron Collider. The measurements are compared to predictions of Monte Carlo generators and to measurements of inclusively selected jets. In pp collisions, a different jet fragmentation function in photon-tagged events from that in inclusive jet events arises from the difference in fragmentation between light quarks and gluons. The ratios of the fragmentation functions in Pb + Pb events to that in pp events are used to explore the parton color-charge dependence of jet quenching in the hot medium. In relatively peripheral collisions, fragmentation functions exhibit a similar modification pattern for photon-tagged and inclusive jets. However, photon-tagged jets are observed to have larger modifications than inclusive jets in central Pb + Pb events.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2019
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-391373 (URN)10.1103/PhysRevLett.123.042001 (DOI)000476696300004 ()
Funder
EU, European Research CouncilEU, Horizon 2020Swedish Research Council
Note

For complete list of authors see http://dx.doi.org/10.1103/PhysRevLett.123.042001

Available from: 2019-09-25 Created: 2019-09-25 Last updated: 2019-09-25Bibliographically approved
Aaboud, M., Asimakopoulou, E. M., Bergeås Kuutmann, E., Bokan, P., Brenner, R., Ekelöf, T., . . . Zwalinski, L. (2019). Constraints on mediator-based dark matter and scalar dark energy models using root s= 13 TeV pp collision data collected by the ATLAS detector. Journal of High Energy Physics (JHEP) (5), Article ID 142.
Open this publication in new window or tab >>Constraints on mediator-based dark matter and scalar dark energy models using root s= 13 TeV pp collision data collected by the ATLAS detector
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2019 (English)In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 5, article id 142Article in journal (Refereed) Published
Abstract [en]

Constraints on selected mediator-based dark matter models and a scalar dark energy model using up to 37 fb(-1) = 13 TeV pp collision data collected by the ATLAS detector at the LHC during 2015-2016 are summarised in this paper. The results of experimental searches in a variety of final states are interpreted in terms of a set of spin-1 and spin-0 single-mediator dark matter simplified models and a second set of models involving an extended Higgs sector plus an additional vector or pseudo-scalar mediator. The searches considered in this paper constrain spin-1 leptophobic and leptophilic mediators, spin-0 colour-neutral and colour-charged mediators and vector or pseudo-scalar mediators embedded in extended Higgs sector models. In this case, also = 8 TeV pp collision data are used for the interpretation of the results. The results are also interpreted for the first time in terms of light scalar particles that could contribute to the accelerating expansion of the universe (dark energy).

Place, publisher, year, edition, pages
SPRINGER, 2019
Keywords
Dark matter, Hadron-Hadron scattering (experiments)
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-387956 (URN)10.1007/JHEP05(2019)142 (DOI)000469453900001 ()
Funder
Swedish Research CouncilEU, European Research CouncilEU, Horizon 2020Wallenberg Foundations
Note

For complete list of authors see http://dx.doi.org/10.1007/JHEP05(2019)142

Available from: 2019-06-27 Created: 2019-06-27 Last updated: 2019-06-27Bibliographically 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
Psaltis, A., Khaliel, A., Asimakopoulou, E. M., Kanellakopoulos, A., Lagald, V., Lykiardopoulou, M., . . . Mertzimekis, T. J. (2019). Cross-section measurements of radiative proton-capture reactions in Cd-112 at energies of astrophysical interest. Physical Review C: Covering Nuclear Physics, 99(6), Article ID 065807.
Open this publication in new window or tab >>Cross-section measurements of radiative proton-capture reactions in Cd-112 at energies of astrophysical interest
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2019 (English)In: Physical Review C: Covering Nuclear Physics, ISSN 2469-9985, E-ISSN 2469-9993, Vol. 99, no 6, article id 065807Article in journal (Refereed) Published
Abstract [en]

Reactions involving the group of nuclei commonly known as p nuclei are part of the nucleosynthetic mechanisms at astrophysical sites. The In-113 nucleus is such a case with several open questions regarding its origin at extreme stellar environments. In this paper, the experimental study of the cross sections of the radiative proton-capture reaction Cd-112(p, gamma)In-113 is attempted for the first time at energies lying inside the Gamow window with an isotopically enriched Cd-112 target. Two different techniques, the in-beam gamma-ray spectroscopy and the activation method, have been applied. The latter method is required to account for the presence of a low-lying In-113 isomer at 392 keV having a half-life of approximate to 100 min. From the cross sections, the astrophysical S factors and the isomeric ratios have been additionally deduced. The experimental results are compared to detailed Hauser-Feshbach theoretical calculations using TALYS and discussed in terms of their significance to the optical model potential involved.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2019
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-390202 (URN)10.1103/PhysRevC.99.065807 (DOI)000471984700006 ()
Available from: 2019-08-08 Created: 2019-08-08 Last updated: 2019-08-08Bibliographically 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
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