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Ohm, Christian
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Publications (10 of 348) Show all publications
Alimena, J., Beacham, J., Borsato, M., Cheng, Y., Cid Vidal, X., Cottin, G., . . . Zhang, Y. (2020). Searching for long-lived particles beyond the Standard Model at the Large Hadron Collider. Journal of Physics G: Nuclear and Particle Physics, 47(9), Article ID 090501.
Open this publication in new window or tab >>Searching for long-lived particles beyond the Standard Model at the Large Hadron Collider
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2020 (English)In: Journal of Physics G: Nuclear and Particle Physics, ISSN 0954-3899, E-ISSN 1361-6471, Vol. 47, no 9, article id 090501Article in journal (Refereed) Published
Abstract [en]

Particles beyond the Standard Model (SM) can generically have lifetimes that are long compared to SM particles at the weak scale. When produced at experiments such as the Large Hadron Collider (LHC) at CERN, these long-lived particles (LLPs) can decay far from the interaction vertex of the primary proton-proton collision. Such LLP signatures are distinct from those of promptly decaying particles that are targeted by the majority of searches for new physics at the LHC, often requiring customized techniques to identify, for example, significantly displaced decay vertices, tracks with atypical properties, and short track segments. Given their non-standard nature, a comprehensive overview of LLP signatures at the LHC is beneficial to ensure that possible avenues of the discovery of new physics are not overlooked. Here we report on the joint work of a community of theorists and experimentalists with the ATLAS, CMS, and LHCb experiments-as well as those working on dedicated experiments such as MoEDAL, milliQan, MATHUSLA, CODEX-b, and FASER-to survey the current state of LLP searches at the LHC, and to chart a path for the development of LLP searches into the future, both in the upcoming Run 3 and at the high-luminosity LHC. The work is organized around the current and future potential capabilities of LHC experiments to generally discover new LLPs, and takes a signature-based approach to surveying classes of models that give rise to LLPs rather than emphasizing any particular theory motivation. We develop a set of simplified models; assess the coverage of current searches; document known, often unexpected backgrounds; explore the capabilities of proposed detector upgrades; provide recommendations for the presentation of search results; and look towards the newest frontiers, namely high-multiplicity 'dark showers', highlighting opportunities for expanding the LHC reach for these signals.

Keywords
beyond the Standard Model, long-lived particles, Large Hadron Collider, high-luminosity LHC, collider phenomenology, high-energy collider experiments
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-186459 (URN)10.1088/1361-6471/ab4574 (DOI)000570614200001 ()
Available from: 2020-11-11 Created: 2020-11-11 Last updated: 2022-02-25Bibliographically approved
Åsman, B., Bendtz, K., Bohm, C., Hellman, S., Johansen, M., Lundberg, J., . . . Sjölin, J. (2016). The ATLAS Data Acquisition and High Level Trigger system. Journal of Instrumentation, 11, Article ID P06008.
Open this publication in new window or tab >>The ATLAS Data Acquisition and High Level Trigger system
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2016 (English)In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 11, article id P06008Article in journal (Refereed) Published
Abstract [en]

This paper describes the data acquisition and high level trigger system of the ATLAS experiment at the Large Hadron Collider at CERN, as deployed during Run 1. Data flow as well as control, configuration and monitoring aspects are addressed. An overview of the functionality of the system and of its performance is presented and design choices are discussed.

Keywords
Control and monitor systems online, Data acquisition concepts, Online farms and, online filtering, Trigger concepts and systems (hardware and software)
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-132414 (URN)10.1088/1748-0221/11/06/P06008 (DOI)000379239700020 ()
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., . . . Tylmad, M. (2015). Evidence for the Higgs-boson Yukawa coupling to tau leptons with the ATLAS detector. Journal of High Energy Physics (JHEP) (4), Article ID 117.
Open this publication in new window or tab >>Evidence for the Higgs-boson Yukawa coupling to tau leptons with the ATLAS detector
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2015 (English)In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 4, article id 117Article in journal (Refereed) Published
Abstract [en]

Results of a search for H -> tau tau decays are presented, based on the full set of proton-proton collision data recorded by the ATLAS experiment at the LHC during 2011 and 2012. The data correspond to integrated luminosities of 4.5 fb(-1) and 20.3 fb(-1) at centre-of-mass energies of root s = 7TeV and root s = 8 TeV respectively. All combinations of leptonic (tau -> l nu(nu) over bar with l = e, mu) and hadronic (tau -> hadrons nu) tau decays are considered. An excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.5 (3.4) standard deviations. This excess provides evidence for the direct coupling of the recently discovered Higgs boson to fermions. The measured signal strength, normalised to the Standard Model expectation, of mu = 1.43(-0.37)(+0.43) is consistent with the predicted Yukawa coupling strength in the Standard Model.

Keywords
Hadron-Hadron Scattering
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-117448 (URN)10.1007/JHEP04(2015)117 (DOI)000353557400001 ()
Note

AuthorCount:2885;

Available from: 2015-05-19 Created: 2015-05-19 Last updated: 2022-03-23Bibliographically approved
Abulaiti, Y., Åsman, B., Bendtz, K., Bessidskaia, O., Bohm, C., Clément, C., . . . Tylmad, M. (2015). Jet energy measurement and its systematic uncertainty in proton-proton collisions at root s=7 TeV with the ATLAS detector. European Physical Journal C, 75(1), Article ID 17.
Open this publication in new window or tab >>Jet energy measurement and its systematic uncertainty in proton-proton collisions at root s=7 TeV with the ATLAS detector
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2015 (English)In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 75, no 1, article id 17Article in journal (Refereed) Published
Abstract [en]

The jet energy scale (JES) and its systematic uncertainty are determined for jetsmeasured with the ATLAS detector using proton-proton collision data with a centre-of-mass energy of root s = 7 TeV corresponding to an integrated luminosity of 4.7 fb(-1). Jets are reconstructed from energy deposits forming topological clusters of calorimeter cells using the anti-kt algorithmwith distance parameters R = 0.4 or R = 0.6, and are calibrated using MC simulations. A residual JES correction is applied to account for differences between data and MC simulations. This correction and its systematic uncertainty are estimated using a combination of in situ techniques exploiting the transverse momentum balance between a jet and a reference object such as a photon or a Z boson, for 20 <= p(T)(jet) < 1000 GeV and pseudorapidities vertical bar eta vertical bar < 4.5. The effect of multiple proton-proton interactions is corrected for, and an uncertainty is evaluated using in situ techniques. The smallest JES uncertainty of less than 1% is found in the central calorimeter region (vertical bar eta vertical bar| < 1.2) for jets with 55 = p(T)(jet) < 500 GeV. For central jets at lower p(T), the uncertainty is about 3%. A consistent JES estimate is found using measurements of the calorimeter response of single hadrons in proton-proton collisions and test-beam data, which also provide the estimate for p(T)(jet) > 1 TeV. The calibration of forward jets is derived from dijet p(T) balance measurements. The resulting uncertainty reaches its largest value of 6% for low-p(T) jets at vertical bar eta vertical bar| = 4.5. Additional JES uncertainties due to specific event topologies, such as close-by jets or selections of event samples with an enhanced content of jets originating from light quarks or gluons, are also discussed. The magnitude of these uncertainties depends on the event sample used in a given physics analysis, but typically amounts to 0.5-3%.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-115458 (URN)10.1140/epjc/s10052-014-3190-y (DOI)000348991900001 ()
Note

AuthorCount:2934;

Available from: 2015-03-26 Created: 2015-03-24 Last updated: 2022-03-23Bibliographically approved
Abulaiti, Y., Åkerstedt, H., Åsman, B., Bendtz, K., Bertoli, G., Bessidskaia, O., . . . Tylmad, M. (2015). Measurement of the production and lepton charge asymmetry of W bosons in Pb plus Pb collisions at root s(NN)=2.76 TeV with the ATLAS detector. European Physical Journal C, 75(1), Article ID 23.
Open this publication in new window or tab >>Measurement of the production and lepton charge asymmetry of W bosons in Pb plus Pb collisions at root s(NN)=2.76 TeV with the ATLAS detector
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2015 (English)In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 75, no 1, article id 23Article in journal (Refereed) Published
Abstract [en]

A measurement of W boson production in lead-lead collisions at root s(NN) = 2.76 TeV is presented. It is based on the analysis of data collected with the ATLAS detector at the LHC in 2011 corresponding to an integrated luminosity of 0.14 nb(-1) and 0.15 nb(-1) in the muon and electron decay channels, respectively. The differential production yields and lepton charge asymmetry are each measured as a function of the average number of participating nucleons < N-part > and absolute pseudorapidity of the charged lepton. The results are compared to predictions based on next-to-leading-order QCD calculations. These measurements are, in principle, sensitive to possible nuclear modifications to the parton distribution functions and also provide information on scaling of W boson production in multi-nucleon systems.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-115445 (URN)10.1140/epjc/s10052-014-3231-6 (DOI)000348992600002 ()
Note

AuthorCount:2891;

Available from: 2015-03-27 Created: 2015-03-24 Last updated: 2022-03-23Bibliographically approved
Abulaiti, Y., Åkerstedt, H., Åsman, B., Bendtz, K., Bertoli, G., Bessidskaia Bylund, O., . . . Tylmad, M. (2015). Measurement of the transverse polarization of Lambda and (Lambda)over-bar hyperons produced in proton-proton collisions at root s=7 TeV using the ATLAS detector. Physical Review D, 91(3)
Open this publication in new window or tab >>Measurement of the transverse polarization of Lambda and (Lambda)over-bar hyperons produced in proton-proton collisions at root s=7 TeV using the ATLAS detector
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2015 (English)In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 91, no 3Article in journal (Refereed) Published
Abstract [en]

The transverse polarization of Lambda and (Lambda) over bar hyperons produced in proton-proton collisions at a center-of-mass energy of 7 TeV is measured. The analysis uses 760 mu b(-1) of minimum bias data collected by the ATLAS detector at the LHC in the year 2010. The measured transverse polarization averaged over Feynman x(F) from 5 x 10(-5) to 0.01 and transverse momentum p(T) from 0.8 to 15 GeV is -0.010 +/- 0.005(stat) +/- 0.004(syst) for Lambda and 0.002 +/- 0.006(stat) +/- 0.004(syst) for (Lambda) over bar. It is also measured as a function of x(F) and p(T), but no significant dependence on these variables is observed. Prior to this measurement, the polarization was measured at fixed-target experiments with center-of-mass energies up to about 40 GeV. The ATLAS results are compatible with the extrapolation of a fit from previous measurements to the x(F) range covered by this measurement.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-115685 (URN)10.1103/PhysRevD.91.032004 (DOI)000349487000001 ()
Note

AuthorCount:2875;

Available from: 2015-03-31 Created: 2015-03-27 Last updated: 2022-03-23Bibliographically approved
Abulaiti, Y., Åkerstedt, H., Åsman, B., Bendtz, K., Bertoli, G., Bessidskaia, O., . . . Tylmad, M. (2015). Measurements of Higgs boson production and couplings in the four-lepton channel in pp collisions at center-of-mass energies of 7 and 8 TeV with the ATLAS detector. Physical Review D, 91(1), Article ID 012006.
Open this publication in new window or tab >>Measurements of Higgs boson production and couplings in the four-lepton channel in pp collisions at center-of-mass energies of 7 and 8 TeV with the ATLAS detector
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2015 (English)In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 91, no 1, article id 012006Article in journal (Refereed) Published
Abstract [en]

The final ATLAS Run 1 measurements of Higgs boson production and couplings in the decay channel H -> ZZ* -> l(+)l(-)l(+)l'(-), where l, l' = e or mu, are presented. These measurements were performed using pp collision data corresponding to integrated luminosities of 4.5 and 20.3 fb(-1) at center-of-mass energies of 7 and 8 TeV, respectively, recorded with the ATLAS detector at the LHC. The H -> ZZ* -> 4l signal is observed with a significance of 8.1 standard deviations, with an expectation of 6.2 standard deviations, at m(H) = 125.36 GeV, the combined ATLAS measurement of the Higgs boson mass from the H -> gamma gamma and H -> ZZ* -> 4l channels. The production rate relative to the Standard Model expectation, the signal strength, is measured in four different production categories in the H -> ZZ* -> 4l channel. The measured signal strength, at this mass, and with all categories combined, is 1.44(-0.33)(+0.40). The signal strength for Higgs boson production in gluon fusion or in association with (tt) over bar or (bb) over bar pairs is found to be 1.7(-0.4)(+0.5), while the signal strength for vector-boson fusion combined with WH/ZH associated production is found to be 0.3(-0.9)(+1.6).

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-115305 (URN)10.1103/PhysRevD.91.012006 (DOI)000348678700001 ()
Note

AuthorCount:2899;

Available from: 2015-04-01 Created: 2015-03-18 Last updated: 2022-03-23Bibliographically approved
Abulaiti, Y., Åkerstedt, H., Åsman, B., Bendtz, K., Bertoli, G., Bessidskaia Bylund, O., . . . Tylmad, M. (2015). Search for anomalous production of prompt same-sign lepton pairs and pair-produced doubly charged Higgs bosons with root s=8 TeV pp collisions using the ATLAS detector. Journal of High Energy Physics (JHEP) (3), Article ID 041.
Open this publication in new window or tab >>Search for anomalous production of prompt same-sign lepton pairs and pair-produced doubly charged Higgs bosons with root s=8 TeV pp collisions using the ATLAS detector
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2015 (English)In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 3, article id 041Article in journal (Refereed) Published
Abstract [en]

A low-background inclusive search for new physics in events with same-sign dileptons is presented. The search uses proton-proton collisions corresponding to 20.3 fb(-1) of integrated luminosity taken in 2012 at a centre-of-mass energy of 8TeV with the ATLAS detector at the LHC. Pairs of isolated leptons with the same electric charge and large transverse momenta of the type e(+/-) e(+/-), e(+/-) mu(+/-), and mu(+/-) mu(+/-) are selected and their invariant mass distribution is examined. No excess of events above the expected level of Standard Model background is found. The results are used to set upper limits on the cross-sections for processes beyond the Standard Model. Limits are placed as a function of the dilepton invariant mass within a fiducial region corresponding to the signal event selection criteria. Exclusion limits are also derived for a specific model of doubly charged Higgs boson production.

Keywords
Hadron-Hadron Scattering, Beyond Standard Model, Particle and resonance production
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-117106 (URN)10.1007/JHEP03(2015)041 (DOI)000351313700001 ()
Note

AuthorCount:2892;

Available from: 2015-05-07 Created: 2015-05-07 Last updated: 2022-03-23Bibliographically approved
Abulaiti, Y., Åkerstedt, H., Åsman, B., Bendtz, K., Bertoli, G., Bessidskaia Bylund, O., . . . Tylmad, M. (2015). Search for charged Higgs bosons decaying via H-+/- -> tau(+/-)nu in fully hadronic final states using pp collision data at root s=8 TeV with the ATLAS detector. Journal of High Energy Physics (JHEP) (3), Article ID 088.
Open this publication in new window or tab >>Search for charged Higgs bosons decaying via H-+/- -> tau(+/-)nu in fully hadronic final states using pp collision data at root s=8 TeV with the ATLAS detector
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2015 (English)In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 3, article id 088Article in journal (Refereed) Published
Abstract [en]

The results of a search for charged Higgs bosons decaying to a T lepton and a neutrino. H-+/- -> T-+/-nu, are presented. The analysis is based on 19.5 fb(-1) of proton-proton collision data at root s = 8 TeV collected by the ATLAS experiment at the Large Hadron Collider. Charged Higgs bosons are searched for in events consistent with top-quark pair production or in associated production with a top quark, depending on the considered H-+/- mass. The final state is characterised by the presence of a hadronic T decay, missing transverse momentum, b-tagged jets, a hadronically decaying W boson, and the absence of any isolated electrons or unions with high transverse momenta. The data are consistent with the expected background from Standard Model processes. A statistical analysis leads to 95% confidence-level upper limits on the product of branching ratios B(t -> bH(+/-)) x B(H-+/- -> T-+/-nu), between 0.23% and 1.3% for charged Higgs boson masses in the range 80 160 GeV. lit also leads to 95% confidence-level upper limits on the production cross section times branching ratio, sigma(pp -> tH(+/-) + X) x B(H-+/--> T(+/-)v), between 0.76 pb and 4.5 fb, for charged Higgs boson masses ranging from 180 GeV to 1000 GeV. In the context of different scenarios of the Minimal Supersymnietric Standard Model, these results exclude nearly all values of tan beta above one for charged Higgs boson masses between 80 GeV and 160 GeV, and exclude a region of parameter space with high tan beta for H-+/- masses between 200 GeV and 250 GeV.

Keywords
Supersymmetry, Hadron-Hadron Scattering, Beyond Standard Model, Higgs physics
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-117332 (URN)10.1007/JHEP03(2015)088 (DOI)000351369000008 ()
Note

AuthorCount:2881;

Available from: 2015-05-18 Created: 2015-05-18 Last updated: 2022-03-23Bibliographically approved
Abulaiti, Y., Åkerstedt, H., Åsman, B., Bendtz, K., Bertoli, G., Bessidskaia Bylund, O., . . . Tylmad, M. (2015). Search for direct pair production of a chargino and a neutralino decaying to the 125 GeV Higgs boson in root s=8 TeV pp collisions with the ATLAS detector. European Physical Journal C, 75(5)
Open this publication in new window or tab >>Search for direct pair production of a chargino and a neutralino decaying to the 125 GeV Higgs boson in root s=8 TeV pp collisions with the ATLAS detector
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2015 (English)In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 75, no 5Article in journal (Refereed) Published
Abstract [en]

A search is presented for the direct pair production of a chargino and a neutralino pp -> (X) over tilde (+/-)(1)(X) over tilde (0)(2), where the chargino decays to the lightest neutralino and the W boson, (X) over tilde (+/-)(1) -> (X) over tilde (0)(1) (W-+/- -> l(+/-)nu), while the neutralino decays to the lightest neutralino and the 125 GeV Higgs boson, (X) over tilde (0)(2) -> (X) over tilde (0)(1) (h -> bb/gamma gamma/l(+/-)nu qq). The final states considered for the search have large missing transverse momentum, an isolated electron or muon, and one of the following: either two jets identified as originating from bottom quarks, or two photons, or a second electron or muon with the same electric charge. The analysis is based on 20.3 fb(-1) of root s = 8 TeV proton-proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with the Standard Model expectations, and limits are set in the context of a simplified supersymmetric model.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-118925 (URN)10.1140/epjc/s10052-015-3408-7 (DOI)000355677700004 ()
Available from: 2015-07-21 Created: 2015-07-21 Last updated: 2022-03-23Bibliographically approved
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