| 1. |
- Schad, A., et al.
(author)
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A UNIFIED APPROACH TO THE HELIOSEISMIC INVERSION PROBLEM OF THE SOLAR MERIDIONAL FLOW FROM GLOBAL OSCILLATIONS
- 2011
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In: Astrophysical Journal. - : University of Chicago Press. - 0004-637X .- 1538-4357. ; 734:2, s. 97-
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Journal article (peer-reviewed)abstract
- Measurements from tracers and local helioseismology indicate the existence of a meridional flow in the Sun with strength in the order of 15 m s(-1) near the solar surface. Different attempts were made to obtain information on the flow profile at depths up to 20 Mm below the solar surface. We propose a method using global helioseismic Doppler measurements with the prospect of inferring the meridional flow profile at greater depths. Our approach is based on the perturbation of the p-mode eigenfunctions of a solar model due to the presence of a flow. The distortion of the oscillation eigenfunctions is manifested in the mixing of p-modes, which may be measured from global solar oscillation time series. As a new helioseismic measurement quantity, we propose amplitude ratios between oscillations in the Fourier domain. We relate this quantity to the meridional flow and unify the concepts presented here for an inversion procedure to infer the meridional flow from global solar oscillations.
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| 2. |
- Aktar, Shaki, et al.
(author)
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Trends and risk of recurrent preterm birth in pregnancy cohorts in rural Bangladesh, 1990-2019
- 2023
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In: BMJ Global Health. - : BMJ Publishing Group Ltd. - 2059-7908. ; 8:11
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Journal article (peer-reviewed)abstract
- Introduction: A history of preterm birth reportedly increases the risk of subsequent preterm birth. This association has primarily been studied in high-income countries and not in low-income settings in transition with rapidly descending preterm birth figures. We evaluated the population-based trends of preterm births and recurrent preterm births and the risk of preterm birth recurrence in the second pregnancy based on prospectively studied pregnancy cohorts over three decades in Matlab, Bangladesh.Methods: A population-based cohort included 72 160 live births from 1990 to 2019. We calculated preterm birth and recurrent preterm birth trends. We assessed the odds of preterm birth recurrence based on a subsample of 14 567 women with live-born singletons in their first and second pregnancies. We used logistic regression and presented the associations by OR with a 95% CI.Results: The proportion of preterm births decreased from 25% in 1990 to 13% in 2019. The recurrent preterm births had a similar, falling pattern from 7.4% to 3.1% across the same period, contributing 27% of the total number of preterm births in the population. The odds of second pregnancy preterm birth were doubled (OR 2.18; 95% CI 1.96 to 2.43) in women with preterm birth compared with the women with term birth in their first pregnancies, remaining similar over the study period. The lower the gestational age at the first birth, the higher the odds of preterm birth in the subsequent pregnancy (test for trend p<0.001).Conclusion: In this rural Bangladeshi setting, recurrent preterm births contributed a sizeable proportion of the total number of preterm births at the population level. The increased risk of recurrence remained similar across three decades when the total proportion of preterm births was reduced from 25% to 13%.
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| 3. |
- Aktas, A., et al.
(author)
-
Elastic J/psi production at HERA
- 2006
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In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 46:3, s. 585-603
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Journal article (peer-reviewed)abstract
- Cross sections for elastic production of J/psi mesons in photoproduction and electroproduction are measured in electron proton collisions at HERA using an integrated luminosity of 55 pb(-1). Results are presented for photon virtualities Q(2) up to 80 GeV2. The dependence on the photon-proton centre of mass energy W-gamma p is analysed in the range 40 <= W-gamma p <= 305 GeV in photoproduction and 40 <= W-gamma p <= 160 GeV in electroproduction. The W-gamma p dependences of the cross sections do not change significantly with Q(2) and can be described by models based on perturbative QCD. Within such models, the data show a high sensitivity to the gluon density of the proton in the domain of low Bjorken x and low Q(2). Differential cross sections d sigma/dt, where t is the squared four-momentum transfer at the proton vertex, are measured in the range vertical bar t vertical bar < 1.2 GeV2 as functions of W-gamma p and Q(2). Effective Pomeron trajectories are determined for photoproduction and electroproduction. The J/psi production and decay angular distributions are consistent with s-channel helicity conservation. The ratio of the cross sections for longitudinally and transversely polarised photons is measured as a function of Q(2) and is found to be described by perturbative QCD based models.
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| 4. |
- Aktas, A., et al.
(author)
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Inclusive D-*+/- meson cross sections and D-*+/--jet correlations in photoproduction at HERA
- 2007
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In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 50:2, s. 251-267
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Journal article (peer-reviewed)abstract
- Differential photoproduction cross sections are measured for events containing D-*+/- mesons. The data were taken with the H1 detector at the ep collider HERA and correspond to an integrated luminosity of 51.1 pb(-1). The kinematic region covers small photon virtualities Q(2) < 0.01 GeV2 and photon-proton centre-of-mass energies of 171 < W gamma(Cep) < 256 GeV. The details of the heavy quark production process are further investigated in events with one or two jets in addition to the D-*+/- meson. Differential cross sections for D-* + jet production are determined and the correlations between the D-*+/- meson and the jet(s) are studied. The results are compared with perturbative QCD predictions applying collinear- or k(t)-factorisation.
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| 5. |
- Aktas, A., et al.
(author)
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Measurement and QCD analysis of the diffractive deep-inelastic scattering cross section at HERA
- 2006
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In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 48:3, s. 715-748
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Research review (peer-reviewed)abstract
- A detailed analysis is presented of the diffractive deep-inelastic scattering process ep -> eXY, where Y is a proton or a low mass proton excitation carrying a fraction 1 - x(IP) > 0.95 of the incident proton longitudinal momentum and the squared four-momentum transfer at the proton vertex satisfies |t| < 1 GeV2. Using data taken by the H1 experiment, the cross section is measured for photon virtualities in the range 3.5 <= Q(2) <= 1600 GeV2, triple differentially in x(IP), Q(2) and beta = x/x(P), where x is the Bjorken scaling variable. At low x(IP), the data are consistent with a factorisable x(IP) dependence, which can be described by the exchange of an effective pomeron trajectory with intercept alpha(IP)(0) = 1.118 +/- 0.008(exp.)(-0.010)(+0.029)(model). Diffractive parton distribution functions and their uncertainties are determined from a next-to-leading order DGLAP QCD analysis of the Q(2)and beta dependences of the cross section. The resulting gluon distribution carries an integrated fraction of around 70% of the exchanged momentum in the Q(2) range studied. Total and differential cross sections are also measured for the diffractive charged current process e(+) p -> (v) over bar eXY and are found to be well described by predictions based on the diffractive parton distributions. The ratio of the diffractive to the inclusive neutral current ep cross sections is studied. Over most of the kinematic range, this ratio shows no significant dependence on Q(2) at fixed x(P) and x or on x at fixed Q(2) and beta.
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| 6. |
- Aktas, A, et al.
(author)
-
Measurement of beauty production at HERA using events with muons and jets
- 2005
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In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 41:4, s. 453-467
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Journal article (peer-reviewed)abstract
- A measurement of the beauty production cross section in ep collisions at a centre-of-mass energy of 319 GeV is presented. The data were collected with the H1 detector at the HERA collider in the years 1999-2000. Events are selected by requiring the presence of jets and muons in the final state. Both the long lifetime and the large mass of b-flavoured hadrons are exploited to identify events containing beauty quarks. Differential cross sections are measured in photoproduction, with photon virtualities Q(2) < 1 GeV2, and in deep inelastic scattering, where 2 < Q(2) < 100 GeV2. The results are compared with perturbative QCD calculations to leading and next-to-leading order. The predictions are found to be somewhat lower than the data.
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| 7. |
- Aktas, A, et al.
(author)
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Measurement of charm and beauty photoproduction at HERA using D*mu correlations
- 2005
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In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 621:1-2, s. 56-71
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Journal article (peer-reviewed)abstract
- A measurement of charm and beauty photoproduction at the electron proton collider HERA is presented based on the simultaneous detection of a D*(+/-) meson and a muon. The correlation between the D* meson and the muon serves to separate the charm and beauty contributions and the analysis provides comparable sensitivity to both. The total and differential experimental cross sections are compared to LO and NLO QCD calculations. The measured charm cross section is in good agreement with QCD predictions including higher order effects while the beauty cross section is higher.
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| 8. |
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| 9. |
- Aktas, A, et al.
(author)
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Measurement of dijet production at low Q(2) at HERA
- 2004
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In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 37:2, s. 141-159
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Journal article (peer-reviewed)abstract
- Triple differential dijet cross sections in e(+/-)p interactions are presented in the region of photon virtualities 2 < Q(2) < 80 GeV2, inelasticities 0.1 < y < 0.85, jet transverse energies E*(T1) > 7GeV, E*(T2) > 5 GeV, and pseudorapidities - 2.5 < η(1)*, η(2)* < 0. The measurements are made in the gamma*p centre-of-mass frame, using an integrated luminosity of 57 pb(-1). The data are compared with NLO QCD calculations and LO Monte Carlo programs with and without a resolved virtual photon contribution. NLO QCD calculations fail to describe the region of low Q(2) and low jet transverse energies, in contrast to a LO Monte Carlo generator which includes direct and resolved photon interactions with both transversely and longitudinally polarised photons. Initial and final state parton showers are tested as a mechanism for including higher order QCD effects in low E-T jet production.
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| 10. |
- Aktas, A., et al.
(author)
-
Measurement of event shape variables in deep-inelastic scattering at HERA
- 2006
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In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 46:2, s. 343-356
-
Journal article (peer-reviewed)abstract
- Deep-inelastic ep scattering data taken with the H1 detector at HERA and corresponding to an integrated luminosity of 106 pb(-1) are used to study the differential distributions of event shape variables. These include thrust, jet broadening, jet mass and the C-parameter. The four-momentum transfer Q is taken to be the relevant energy scale and ranges between 14 GeV and 200 GeV. The event shape distributions are compared with perturbative QCD predictions, which include resummed contributions and analytical power law corrections, the latter accounting for non-perturbative hadronisation effects. The data clearly exhibit the running of the strong coupling alpha(s)(Q) and are consistent with a universal power correction parameter alpha(0) for all event shape variables. A combined QCD fit using all event shape variables yields alpha(s)(m(Z)) = 0.1198 +/- 0.0013 (+0.0056)(-0.0043) and alpha(0) = 0.476 +/- 0.008 (+0.018)(-0.059).
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