arXiv subjects

Ramona Vogt

Publications and source records attributed to Ramona Vogt.

At least 37 records · Page 2Linked to original sources

arXiv2019

Event-by-Event Multiplicity Correlations in $^{252}$Cf(sf)

Excited nuclear fragments are emitted during nuclear fission. The de-excitation of these fission fragments takes place as sequential emission of neutrons followed by photons. A correlation between neutron and photon multiplicities accompanying fission is thus expected. Fission event generators based on established statistical nuclear physics models predict a negative event-by-event correlation in neutron-photon multiplicity. A survey of published experimental results of an event-by-event covariance between the neutron and photon multiplicities emitted following the spontaneous fission of $^{252}$Cf is presented. Analytic unfolding expressions are developed in this work to determine the bias introduced by background sources, particle misclassification, pulse pileup, and inelastic photon production. The published experimental data are re-analyzed using these unfolding techniques and are found to be in qualitative agreement with the predictions of model-based calculations. In particular, we have concluded that there exists a significant event-by-event neutron-photon emission competition following the spontaneous fission of $^{252}$Cf.

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arXiv2019

Using excitation-energy dependent fission yields to identify key fissioning nuclei in r-process nucleosynthesis

We evaluate the impact of using sets of fission yields given by the GEF code for spontaneous (sf), neutron-induced ((n,f)), and beta-delayed (betadf) fission processes which take into account the approximate initial excitation energy of the fissioning compound nucleus. We further explore energy-dependent fission dynamics in the r process by considering the sensitivity of our results to the treatment of the energy sharing and de-excitation of the fission fragments using the FREYA code. We show that the asymmetric-to-symmetric yield trends predicted by GEF can reproduce the high-mass edge of the second r-process peak seen in solar data and examine the sensitivity of this result to the mass model and astrophysical conditions applied. We consider the effect of fission yields and barrier heights on the nuclear heating rates used to predict kilonova light curves. We find that fission barriers influence the contribution of 254Cf spontaneous fission to the heating at ~100 days, such that a light curve observation consistent with such late-time heating would both confirm that actinides were produced in the event and imply the fission barriers are relatively high along the 254Cf beta-feeding path. We lastly determine the key nuclei responsible for setting the r-process abundance pattern by averaging over thirty trajectories from a 1.2--1.4 M_odot neutron star merger simulation. We show it is largely the odd-N nuclei undergoing (Z,N)(n,f) and (Z,N)betadf that control the relative abundances near the second peak. We find the "hot spots" for beta-delayed and neutron-induced fission given all mass models considered and show most of these nuclei lie between the predicted N=184 shell closure and the location of currently available experimental decay data.

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arXiv2019

Parameter Optimization and Uncertainty Analysis of FREYA for Spontaneous Fission

Background: The complete event fission simulation code FREYA is used to study correlations in fission. To make the best possible simulations, FREYA one must find the optimized values of the five physics-based parameters which characterize the events produced by FREYA. So far this has been done only empirically or by brute force computational techniques. Purpose: We seek to develop a method of parameter optimization for FREYA that would include uncertainty quantification and an analysis of the correlations between the parameters. Methods: We focus on spontaneous fission as a simpler test case of the method. We first check the results of previous optimizations for $^{252}$Cf(sf) and then go on to develop a simulated annealing approach to optimize the parameters. Although $^{252}$Cf(sf) has the most measured observables, we are able to apply the technique to all spontaneously fissioning isotopes in the current version of FREYA. Results: We find optimal values of the five FREYA parameters, with uncertainties, for all isotopes studied. Our results are compared with available data. The parameter values are physically reasonable and in accord with intuition, even in cases where the data for optimization are sparse to non-existent. Conclusions: The simulated annealing method provides a way to determine the best parameters for the phenomenologically-successful fission model FREYA. We have developed an algorithm that can be updated to include additional isotopes or more data sets for the current isotopes as they become available.

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arXiv2018

Production and polarization of prompt $\varUpsilon$($n$S) in the improved color evaporation model using the $k_T$-factorization approach

We calculate the polarization of prompt $\varUpsilon$($n$S) production in the improved color evaporation model at leading order employing the $k_T$-factorization approach. We present the polarization parameter $λ_\vartheta$ of prompt $\varUpsilon$($n$S) as a function of transverse momentum in $p+p$ and $p+\bar{p}$ collisions to compare with data in the helicity, Collins-Soper and Gottfried-Jackson frames. We also present calculations of the bottomonium production cross sections as a function of transverse momentum and rapidity. This is the first $p_T$-dependent calculation of bottomonium production and polarization in the improved color evaporation model. We find agreement with both bottomonium cross sections and polarization measurements.

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arXiv2018

Production and polarization of prompt $J/ψ$ in the improved color evaporation model using the $k_T$-factorization approach

We calculate the polarization of prompt $J/ψ$ production in the improved color evaporation model at leading order employing the $k_T$-factorization approach. In this paper, we present the polarization parameter $λ_\vartheta$ of prompt $J/ψ$ as a function of transverse momentum in $p+p$ and $p$ + A collisions to compare with data in the helicity, Collins-Soper and Gottfried-Jackson frames. We also present calculations of the charmonium production cross sections as a function of rapidity and transverse momentum. This is the first $p_T$-dependent calculation of charmonium polarization in the improved color evaporation model. We find agreement with both charmonium cross sections and polarization measurements.

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arXiv2017

Polarized Heavy Quarkonium Production in the Color Evaporation Model

We explore polarized heavy quarkonium production using the color evaporation model at leading order. We present the polarized to total yield ratio as a function of center of mass energy and rapidity in $p+p$ collisions. At energies far above the $Q \overline Q$ production threshold, we find charmonium and bottomonium production to be longitudinally polarized ($J_z=0$). The quarkonium states are also longitudinally polarized at central rapidity, becoming transversely polarized ($J_z=\pm1$) at the most forward rapidities.

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arXiv2017

Polarization of prompt $J/ψ$ and $Υ$(1S) production in the color evaporation model

We calculate the polarization of prompt $J/ψ$ and $Υ$(1S) production using the color evaporation model at leading order. We present the polarization parameter $λ_\vartheta$ as a function of center of mass energy and rapidity in $p+p$ collisions. We also compare the $x_F$ dependence to experimental results in $p$+Cu and $π$+W collisions, and predict the $x_F$ dependence in $p$+Pb collisions at fixed-target energies. At energies far above the $Q\overline{Q}$ production threshold, we find the prompt $J/ψ$ and $Υ$(1S) production to be longitudinally polarized with $λ_\vartheta^{J/ψ}=-0.51^{+0.05}_{-0.16}$ and $λ_\vartheta^{Υ\rm{(1S)}}=-0.69^{+0.03}_{-0.02}$. Both prompt $J/ψ$ and prompt $Υ$(1S) are also longitudinally polarized at central rapidity, becoming transversely polarized at the most forward rapidities.

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arXiv2016

Quarkonium Production in an Improved Color Evaporation Model

We propose an improved version of the color evaporation model to describe heavy quarkonium production. In contrast to the traditional color evaporation model, we impose the constraint that the invariant mass of the intermediate heavy quark-antiquark pair to be larger than the mass of produced quarkonium. We also introduce a momentum shift between heavy quark-antiquark pair and the quarkonium. Numerical calculations show that our model can describe the charmonium yields as well as ratio of $ψ^\prime$ over $J/ψ$ better than the traditional color evaporation model.

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arXiv2015

Correlations of neutron multiplicity and gamma multiplicity with fragment mass and total kinetic energy in spontaneous fission of 252Cf

The dependence of correlations of neutron multiplicity ν and gamma-ray multiplicity Mg in spontaneous fission of 252Cf on fragment mass A* and total kinetic energy TKE has been investigated employing the ratio of Mg/ν and the form of Mg(ν). We show for the first time that Mg and ν have a complex correlation for heavy fragment masses, while there is a positive dependence of Mg(ν) for light fragment masses and for near-symmetric mass splits. The ratio Mg/ν exhibits strong shell effects for the neutron magic number N = 50 and near the doubly magic number shell closure at Z = 50 and N = 82. The gamma-ray multiplicity Mg has a maximum for TKE = 165-170 MeV. Above 170 MeV Mg(TKE) is approximately linear, while it deviates significantly from a linear dependence at lower TKE. The correlation between the average neutron and gamma-ray multiplicities can be partly reproduced by model calculations.

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arXiv2015

Quarkonia suppression in PbPb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV

We estimate the modification of quarkonia yields due to different processes in the medium produced in Pb+Pb collisions at LHC energy. The quarkonia and heavy flavour cross sections calculated up to next-to-leading order (NLO) are used in the study. Shadowing corrections are obtained with the NLO EPS09 parametrization. A kinetic model is employed which incorporates quarkonia suppression inside a QGP, suppression due to hadronic comovers, and regeneration from charm pairs. The quarkonia dissociation cross section due to gluon collisions has been considered and the regeneration rate has been obtained using the principle of detailed balance. The modification in quarkonia yields due to collisions with hadronic comovers has been estimated assuming that the comovers are pions. The manifestations of these effects on the nuclear modification factors for both $J/ψ$ and $Υ$ in different kinematic regions has been demonstrated for Pb+Pb collisions at $\sqrt{s_{_{NN}}}$ = 2.76 TeV in comparison with the measurements. Both the suppression and regeneration due to a deconfined medium strongly affect the low and intermediate $p_T$ range. The large observed suppression of $J/ψ$ at $p_T~>$ 10 GeV/$c$ exceeds the estimates of suppression by gluon dissociation.

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arXiv2011

Prospects for quarkonia production studies in U+U collisions

Collisions of deformed uranium nuclei provide a unique opportunity to study the spatial dependence of charmonium in-medium effects. By selecting the orientations of the colliding nuclei, different path lengths through the nuclear medium could be selected within the same experimental environment. In addition, higher energy densities can be achieved in U+U collisions relative to Au+Au collisions. In this paper, we investigate the prospects for charmonium studies with U+U collisions. We discuss the effects of shadowing and nuclear absorption on the J/ψ yield. We introduce a new observable which could help distinguish between different types of J/ψ interactions in hot and dense matter.

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arXiv2009

Energy dependence of J/psi absorption in proton-nucleus collisions

Charmonium states are expected to be considerably suppressed in the case of quark-gluon plasma formation in high-energy heavy-ion collisions. However, a robust identification of suppression patterns as signatures of a deconfined QCD medium requires a detailed understanding of the "normal nuclear absorption" already present in proton-nucleus collisions, where the charmonium production cross sections increase less than linearly with the number of target nucleons. We analyse the J/psi production cross sections measured in proton-nucleus collisions in fixed target experiments, with proton beam energies from 200 to 920 GeV, and in d-Au collisions at RHIC, at sqrt(s)=200 GeV, in the framework of the Glauber formalism, using several sets of parton distributions with and without nuclear modifications. The results reveal a significant dependence of the "absorption cross section" on the kinematics of the J/psi and on the collision energy. Extrapolating the observed patterns we derive the level of absorption expected at Elab=158 GeV, the energy at which the heavy-ion data sets were collected at the CERN SPS.

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arXiv2008

The theoretical top quark cross section at the Tevatron and the LHC

We present results for the top quark pair cross section at the Tevatron and the LHC. We use the resummed double differential cross section, employing the fully kinematics-dependent soft anomalous dimension matrices, to calculate the soft-gluon contributions at next-to-next-to-leading order (NNLO). We improve and update our previous estimates by refining our methods, including further subleading terms, and employing the most recent parton distribution function sets. The NNLO soft corrections significantly enhance the NLO cross section while considerably reducing the scale dependence. We provide a detailed discussion of all theoretical uncertainties in our calculation, including kinematics, scale, and parton distributions uncertainties and clarify the differences between our work and other approaches in the literature.

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arXiv2005

A High Energy Nuclear Database Proposal

We propose to develop a high-energy heavy-ion experimental database and make it accessible to the scientific community through an on-line interace. This database will be searchable and cross-indexed with relevant publications, including published detector descriptions. Since this database will be a community resource, it requires the high-energy nuclear physics community's financial and manpower support. This database should eventually contain all published data from the Bevalac, AGS and SPS to RHIC and LHC energies, proton-proton to nucleus-nucleus collisions as well as other relevant systems and all measured observables. Such a database would have tremendous scientific payoff as it makes systematic studies easier and allows simpler benchmarking of theoretical models to a broad range of old and new experiments. Furthermore, there is a growing need for compilations of high-energy nuclear data for applications including stockpile stewardship, technology development for intertial confinement fusion and target and source development for upcoming facilities such as the Next Linear Collider. To enhance the utility of this database, we propose periodically performing evaluations of the data and summarizing the results in topical reviews.

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arXiv2005

Probing small $x$ parton densities in ultraperipheral $AA$ and $pA$ collisions at the LHC

We calculate production rates for several hard processes in ultraperipheral proton-nucleus and nucleus-nucleus collisions at the LHC. The resulting high rates demonstrate that some key directions in small $x$ research proposed for HERA will be accessible at the LHC through these ultraperipheral processes. Indeed, these measurements can extend the HERA $x$ range by roughly a factor of 10 for similar virtualities. Nonlinear effects on the parton densities will thus be significantly more important in these collisions than at HERA.

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arXiv2005

Influence of Bottom Quark Jet Quenching on Single Electron Tomography of Au+Au

High transverse momentum single (non-photonic) electrons are shown to be sensitive to the stopping power of both bottom, b, and charm, c, quarks in AA collisions. We apply the DGLV theory of radiative energy loss to predict c and b quark jet quenching and compare the FONLL and PYTHIA heavy flavor fragmentation and decay schemes. We show that single electrons in the p_T=5-10 GeV range are dominated by the decay of b quarks rather than the more strongly quenched c quarks in Au+Au collisions at sqrt{s}=200 AGeV. The {smaller} b quark energy loss, even for extreme opacities with gluon rapidity densities up to 3500, is predicted to limit the nuclear modification factor, R_{AA}, of single electrons to the range R_{AA} ~ 0.5-0.6, in contrast to previous predictions of R_{AA} ~ 0.2-0.3 based on taking only c quark jet fragmentation into account.

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arXiv2005

QCD Predictions for Charm and Bottom Production at RHIC

We make up-to-date QCD predictions for open charm and bottom production at RHIC in nucleon-nucleon collisions at \sqrt{S} = 200 GeV. We also calculate the electron spectrum resulting from heavy flavor decays to allow direct comparison to the data. A rigorous benchmark, including the theoretical uncertainties, is established against which nuclear collision data can be compared to obtain evidence for nuclear effects.

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arXiv2004

Theoretical status of the top quark cross section

We discuss the most recent calculations of the top quark total cross section and transverse momentum distributions at the Tevatron and the LHC. These calculations include the soft-gluon corrections at next-to-next-to-leading order (NNLO). The soft NNLO corrections stabilize the scale dependence of the cross section.

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