Dr. Alexander Haber

Alexander HABER

Publications and Media Reports

For an up to date list of my publications see my profile on Google scholar or on Inspire.
Some code is available publicly on GitLab.
On this page you find a list of the projects I worked on and where you can read something about them.

General-relativistic radiation magnetohydrodynamics simulations of binary neutron star mergers: The influence of spin on the multi-messenger picture arXiv:2510.14850	The rich phenomenology of binary neutron star mergers offers a unique opportunity to test general relativity, investigate matter at supranuclear densities, and learn more about the origin of heavy elements. As multi-messenger sources, they emit both gravitational waves and electromagnetic radiation across several frequency bands. The interpretation of these signals relies heavily on accurate numerical-relativity simulations that incorporate the relevant microphysical processes. Using the latest updates of the BAM code, we perform general-relativistic radiation magnetohydrodynamic simulations of binary neutron star mergers with two different spin configurations. We adopt a state-of-the-art equation of state based on relativistic mean-field theory developed for dense matter in neutron star mergers. To capture both dynamical ejecta and secular outflows from magnetic and neutrino-driven winds, we evolve the systems up to 100 ms after the merger at considerably high resolution with a grid spacing of Δx≈93m across the neutron stars. Our results show that the non-spinning configuration undergoes a more violent merger, producing more ejecta with lower electron fraction and higher velocities, while the spinning configuration forms a larger disk due to its higher angular momentum. Although the initial magnetic field amplification within 10 ms after merger is similar in both systems, the non-spinning system reaches stronger magnetic fields and higher energies at later times. For a detailed view of the multi-messenger observables, we extract the gravitational-wave signal and compute nucleosynthesis yields, the expected kilonova and afterglow light curves from our ejecta profiles. This project was lead by Anna Neuweiler and other members of the group of Prof. Tim Dietrich.
Thermal and magnetic effects on viscosity in Binary Neutron Star mergers arXiv:2510.09104	Astrophysical scenarios such as binary neutron star mergers, proto-neutron stars and core-collapse supernovae involve finite temperatures and strong magnetic fields. Previous studies on the effect of magnetic fields on flavor-equilibration processes such as direct Urca relied on the Fermi surface approximation, which is not a reliable approximation in the neutrino-transparent regime of matter in supernovae or neutron star mergers. In a recent study, we went beyond the Fermi surface approximation, performing the full phase space integral to obtain direct Urca rates in background magnetic field. In this work, we extend these calculations to include collisional broadening effects by employing the recently developed nucleon width approximation. We demonstrate the impact of magnetic fields on the flavor-equilibrium condition for two finite-temperature equations of state with different direct Urca thresholds. We also study the impact of magnetic fields on the bulk viscous dissipation of density oscillations relevant in postmerger scenarios. This project was performed in collaboration with Pranjal Tambe and Debarati Chatterjee from IUCAA, and Mark Alford.
Neutrino absorption in two-flavor color-superconducting quark matter arXiv:2509.04240	We calculate the absorption mean free paths of electron and muon neutrinos in two-flavor color-superconducting (2SC) quark matter in the density and temperature range that is relevant to binary neutron star mergers. We model the strong interaction between quarks using a Nambu--Jona-Lasinio model, performing calculations self-consistently in the mean-field approximation. Since the 2SC gap is large we restrict our analysis to the contribution of unpaired quarks. We find that at low temperatures absorption by a down quark is kinematically not allowed, so absorption by a strange quark dominates the mean free path. As temperature or neutrino energy rises, the d quark absorption channel becomes active, and the mean free path shrinks. We find that in equilibrated 2SC matter with an electron lepton fraction YLe=0.1 the neutrinos form a degenerate gas with a mean free path of meters or less, independent of the temperature. This paper was written in collaboration with our collaborators in Darmstadt, most calculations were performed by Marco Hofmann.
Nonparametric extensions of nuclear equations of state: Probing the breakdown scale of relativistic mean-field theory arXiv:2505.07677	Phenomenological calculations of the properties of dense matter, such as relativistic mean-field theories, represent a pathway to predicting the microscopic and macroscopic properties of neutron stars. However, such theories do not generically have well-controlled uncertainties and may break down within neutron stars. To faithfully represent the uncertainty in this breakdown scale, we develop a hybrid representation of the dense-matter equation of state, which assumes the form of a relativistic mean-field theory at low densities, while remaining agnostic to any nuclear theory at high densities. To achieve this, we use a nonparametric equation of state model to incorporate the correlations of the underlying relativistic mean-field theory equation of state at low pressures and transition to more flexible correlations above some chosen pressure scale. We perform astrophysical inference under various choices of the transition pressure between the theory-informed and theory-agnostic models. We further study whether the chosen relativistic mean-field theory breaks down above some particular pressure and find no such evidence. Using simulated data for future astrophysical observations at about two-to-three times the precision of current constraints, we show that our method can identify the breakdown pressure associated with a potential strong phase transition. This paper was written in collaboration with our collaborators at CALTECH, CA (Legred, Prof. Chatziioannou) and CITA, Canada (Prof. Essick).
Suppression of composition g-modes in chemically-equilibrating warm neutron stars arXiv:2504.12230	We investigate the impact of chemical equilibration and the resulting bulk viscosity on non-radial oscillation modes of warm neutron stars at temperatures up to T ≈ 5, relevant for protoneutron stars and neutron-star post-merger remnants. In this regime, the relaxation rate of weak interactions becomes comparable to the characteristic frequencies of composition g-modes in the core, resulting in resonant damping. To capture this effect, we introduce the dynamical sound speed, a complex, frequency-dependent generalization of the adiabatic sound speed that encodes both the restoring force and the dissipative effects of bulk compression. Using realistic weak reaction rates and three representative equations of state, we compute the complex frequencies of composition g-modes with finite-temperature profiles. We find that bulk viscous damping becomes increasingly significant with temperature and can completely suppress composition g-modes. In contrast, the f-mode remains largely unaffected by bulk viscosity due to its nearly divergence-free character. Our results highlight the sensitivity of g-mode behavior to thermal structure, weak reaction rates, and the equation of state, and establish the dynamical sound speed as a valuable descriptor characterizing oscillation properties in dissipative neutron star matter. My collaborators on this project are Tianqi Zhao, Peter Rau, Steven Harris, Constantios Constantinou, and Sophia Han.
Building neutron stars with the MUSES calculation engine arXiv:2502.07902	Exploring the equation of state of dense matter is an essential part of interpreting the observable properties of neutron stars. We present here the first results for dense matter in the zero-temperature limit generated by the MUSES calculation engine, a composable workflow management system that orchestrates calculation and data processing stages comprising a collection of software modules designed within the MUSES framework. The modules presented in this work calculate equations of state using algorithms spanning three different theories/models: (1) crust density functional theory, valid starting at low densities, (2) chiral effective field theory, valid around saturation density, and (3) the chiral mean field model, valid beyond saturation density. Lepton contributions are added through the lepton module to each equation of state, ensuring charge neutrality and the possibility of Beta-equilibrium. Using the synthesis module, we match the three equations of state using different thermodynamic variables and different methods. We then couple the complete equation of state to a novel full-general-relativity solver (QLIMR) module that calculates neutron star properties. We find that the matching performed using different thermodynamic variables affects differently the range obtained for neutron star masses and radii (although never beyond a few percent difference). We also investigate the universality of equation of state-independent relations for our matched stars. Finally, for the first time, we use the flavor equilibration module to estimate bulk viscosity and flavor relaxation charge fraction and rates (at low temperature) for chiral effective field theory and the chiral mean field model. This paper has been published by the MUSES collaboration.
Effect of magnetic fields on Urca rates in neutron star mergers arXiv:2409.09423	Isospin-equilibrating weak processes, called “Urca” processes, are of fundamental importance in astrophysical environments like (proto-)neutron stars, neutron star mergers, and supernovae. In these environments, matter can reach high temperatures of tens of MeVs and be subject to large magnetic fields. We thus investigate Urca rates at different temperatures and field strengths by performing the full temperature and magnetic-fielddependent rate integrals for different equations of state. We find that the magnetic fields play an important role at temperatures of a few MeV, especially close to or below the direct Urca threshold, which is softened by the magnetic field. At higher temperatures, the effect of the magnetic fields can be overshadowed by the thermal effects. We observe that the magnetic field influences the neutron decay rates more strongly than the electron-capture rates, leading to a shift in the flavor equilibrium. This project was performed in collaboration with Pranjal Tambe and Debarati Chatterjee from IUCAA, and Mark Alford.
Beyond modified Urca: the nucleon width approximation for flavor-changing processes in dense matter arXiv:2406.13717	The paper that has hopefully killed modified Urca: Flavor-changing charged current ("Urca") processes are of central importance in the astrophysics of neutron stars. Standard calculations approximate the Urca rate as the sum of two contributions, direct Urca and modified Urca. Attempts to make modified Urca calculations more accurate have been impeded by an unphysical divergence at the direct Urca threshold density. In this paper we describe a systematically improvable approach where, in the simplest approximation, instead of modified Urca we include an imaginary part of the nucleon mass (nucleon width). The total Urca rate is then obtained via a straightforward generalization of the direct Urca calculation, yielding results that agree with both direct and modified Urca at the densities where those approximations are valid. At low densities, we observe an enhancement of the rate by more than an order of magnitude, with important ramifications for neutron star cooling and other transport properties. This paper was written in collaboration with Mark Alford and graduate student Ziyuan Zhang.
Finite-temperature expansion of the dense-matter equation of state arXiv:2404.01658	In this work we provide a new, well-controlled expansion of the equation of state of dense matter from zero to finite temperatures (T), while covering a wide range of charge fractions (YQ), from pure neutron to isospin symmetric nuclear matter. Our expansion can be used to describe neutron star mergers and core-collapse supernova explosions using as a starting point neutron star observations, while maintaining agreement with laboratory data, in a model independent way. We suggest new thermodynamic quantities of interest that can be calculated from theoretical models or directly inferred by experimental data that can help constrain the finite T equation of state. With our new method, we can quantify the uncertainty in our finite T and YQ expansions in a well-controlled manner without making assumptions about the underlying degrees of freedom. We can reproduce results from a microscopic equation of state up to T=100 MeV for baryon chemical potential μB≳1100 MeV (∼1−2 nsat) within 5% error, with even better results for larger μB and/or lower T. We investigate the sources of numerical and theoretical uncertainty and discuss future directions of study. This study was lead by UIUC graduate student Debora Mroczek in the group of Jacquelyn Noronha-Hostler, and I am happy that I was able to contribute our knowledge on finite T EOS calculations.
Isospin Equilibration in Neutron Star Mergers arXiv:2306.06180	We analyze the isospin equilibration properties of neutrino-transparent nuclear (npe) matter in the temperature and density range that is relevant to neutron star mergers. Our analysis incorporates neutrino-transparency corrections to the isospin ("beta") equilibrium condition which become noticeable at T>1 MeV. We find that the isospin relaxation rate rises rapidly as temperature rises, and at T=5 MeV it is comparable to the timescale of the density oscillations that occur immediately after the merger. This produces a resonant peak in the bulk viscosity at T∼5 MeV, which causes density oscillations to be damped on the timescale of the merger. We conclude that there is good reason to include isospin relaxation dynamics in merger simulations. This effort was lead by graduate student Ziyuan Zhang.
How Third Generation Gravitational Wave Detectors Can Unveil the True Degrees of Freedom of Dense Matter CE:P2300004-v1	Together with Elias Most and Mark Alford we wrote a Cosmic Explorer Science Letter that explains how third-generation gravitational wave detectors can unveil the true degrees of freedom of dense matter. We explain how our analysis of transport properties and isospin equilibration can help solve the "masquerade problem", which states that a purely nucleonic equation of state and a quark matter EOS can look identical if one only analyzes static stellar properties. Our science letter was cited in the official Cosmic Explorer white paper.
Tabulated Equations of State From Models Informed by Chiral Effective Field Theory arXiv:2304.07836	We construct four equation of state (EoS) tables, tabulated over a range of temperatures, densities, and charge fractions, relevant for neutron star applications such as simulations of neutron star mergers. The EoS are computed from a relativistic mean-field theory constrained by the pure neutron matter EoS from chiral effective field theory, inferred properties of isospin-symmetric nuclear matter, and astrophysical observations of neutron star structure. These models have been develope in arXiv:2205.10283. To model nuclear matter at low densities, we attach an EoS that models inhomogeneous nuclear matter at arbitrary temperatures and charge fractions called HS(IUF). The four EoS tables we develop are available from the CompOSE EoS repository and on my personal GitLab .
Nuclear and Hybrid Equations of State in Light of the Low-Mass Compact Star in HESS J1731-347 Phys. Rev. C 108, 025806 arXiv:2302.02989	Together with Liam Brodie we sample a wide range of relativistic mean-field theories (RMFTs) constrained by chiral effective field theory and properties of symmetric nuclear matter around saturation density and test them against known stellar structure constraints. This includes a relatively new mass and radius measurement of the central compact object in HESS J1731-347, which is reported to have an unusually low mass of 0.77 solar masses and a compact radius of roughly 10.4 km. We show that none of the sampled nuclear RMFTs meet all stellar structure constraints at the 68% credibility level, but that hybrid equations of state with a quark matter inner core and nuclear outer core fulfill all constraints at the 68% credibility level. This indicates a tension between astrophysical constraints and low-energy nuclear theory if future measurements confirm the central values reported above.
Theoretical and experimental constraints for the equation of state of dense and hot matter Living Rev.Rel. 27 (2024) 1, 3 arXiv:2303.17021	As part of the MUSES collaboration, I am happy that this extensive review has been published in "Living reviews of Relativity". This review aims at providing an extensive discussion of modern constraints relevant for dense and hot strongly interacting matter. It includes theoretical first-principle results from lattice and perturbative QCD, as well as chiral effective field theory results. From the experimental side, it includes heavy-ion collision and low-energy nuclear physics results, as well as observations from neutron stars and their mergers. The validity of different constraints, concerning specific conditions and ranges of applicability, is also provided.
Long Range Plan: Dense matter theory for heavy-ion collisions and neutron stars arXiv:2211.02224	I was recently invited to contribute to the "Long Range Plan" for dense matter theory for heavy-ion collions and neutron stars. Since the release of the 2015 Long Range Plan in Nuclear Physics, major events have occurred that reshaped our understanding of quantum chromodynamics (QCD) and nuclear matter at large densities, in and out of equilibrium. The US nuclear community has an opportunity to capitalize on advances in astrophysical observations and nuclear experiments and engage in an interdisciplinary effort in the theory of dense baryonic matter that connects low- and high-energy nuclear physics, astrophysics, gravitational waves physics, and data science.
Emergence of microphysical viscosity in binary neutron star post-merger dynamics arXiv:2207.00442	In nuclear matter in neutron stars the flavor content (e.g., proton fraction) is subject to weak interactions, establishing flavor equilibrium. During the merger of two neutron stars there can be deviations from this equilibrium. By incorporating Urca processes into general-relativistic hydrodynamics simulations, we study the resulting out-of-equilibrium dynamics during the collision. We provide the first direct evidence that microphysical transport effects at late times reach a hydrodynamic regime with a nonzero bulk viscosity, making neutron star collisions intrinsically viscous. Finally, we identify signatures of this process in the post-merger gravitational wave emission. This project was carried out in collaboration with Elias R. Most, Steven P. Harris, Ziyuan Zhang, Mark G. Alford and Jorge Noronha and is currently under review with PRL.
Relativistic mean-field theories for neutron-star physics based on chiral effective field theory Phys. Rev. C 106, 055804 arXiv:2205.10283	We describe and implement a procedure for determining the couplings of a Relativistic Mean Field Theory (RMFT) that is optimized for application to neutron star phenomenology. In the standard RMFT approach, the couplings are constrained by comparing the theory's predictions for symmetric matter at saturation density with measured nuclear properties. The theory is then applied to neutron stars which consist of neutron-rich matter at densities ranging up to several times saturation density, which allows for additional astrophysical constraints. In our approach, rather than using the RMFT to extrapolate from symmetric to neutron-rich matter and from finite-sized nuclei to uniform matter we fit the RMFT to properties of uniform pure neutron matter obtained from chiral effective field theory. Chiral effective field theory incorporates the experimental data for nuclei in the framework of a controlled expansion for nuclear forces valid at nuclear densities and enables us to account for theoretical uncertainties when fitting the RMFT. We construct four simple RMFTs that span the uncertainties provided by chiral effective field theory for neutron matter, and are consistent with current astrophysical constraints on the equation of state. Our new RMFTs can be used to model the properties of neutron-rich matter across the vast range of densities and temperatures encountered in simulations of neutron stars and their mergers. This project was carried out in collaboration with Liam Brodie and Mark G. Alford.
Beta Equilibrium Under Neutron Star Merger Conditions Universe 7 (2021) 11, 399 arXiv:2108.03324	We calculate the nonzero-temperature correction to the beta equilibrium condition in nuclear matter under neutron star merger conditions, in the temperature range between 1 and 5 MeV. We improve on previous work using a consistent description of nuclear matter based on the IUF and SFHo relativistic mean field models. This includes using relativistic dispersion relations for the nucleons, which we show is essential in these models. We find that the nonzero-temperature correction can be of order 10 to 20 MeV, and plays an important role in the correct calculation of Urca rates, which can be wrong by factors of 10 or more if it is neglected. This project was carried out in collaboration with Steven P. Harris, Ziyuan Zhang and Mark G. Alford.
Strangeness-changing Rates and Hyperonic Bulk Viscosity in Neutron Star Mergers Phys.Rev.C103, 045810 arXiv:2009.05181	In this paper we compute non-leptonic strangeness changing rates in hyperonic dense matter in neutron star merger environments. We find that the rates in the temperature range of a few MeV are very fast compared to the nucleonic direct Urca rates. In the context of bulk viscosity, the rates are too fast to match external oscillations with a typical merger frequency of 1 kHz, in order to yield sizeable dissipation. However, these rates are a paramount ingredient for other transport phenomena in binary neutron star merger, like phase conversion dissipation. In exceptionally cold areas of a merger or in the inspiral phase of an eccentric merger, hyperonic bulk viscosity might play a significant role in the merger dynamics.
Multicomponent Superfluids and Superconductors in Dense Nuclear and Quark Matter arXiv:1811.12533	My phd thesis is now available on the arXiv as well! It is a sligthly more detailed version of the three projects on multicomponent superfluids and superconductors cited below and includes a lengthy introduction into Ginzburg-Landau theory, as well as a short part which explains the connection of my research to the science of compact stars. In a rather lengthy appendix I show how my results are obtained analytically and numerically in much more detail than it is possible in a paper.
New color-magnetic defects in dense quark matter J.Phys. G45 (2018) no.6, 065001 arXiv:1712.08587 arXiv:1811.12302	In this project we used our machinery and knowledge of our flux tubes in nuclear matter project and applied it to dense quark matter. We found new magnetic defects in 2SC and in CFL color superconducting quark matter, namely flux tubes with minimized winding in the CFL case and domain walls in the 2SC phase. These new defects might be important for the evolution of magnetic fields in compact stars and can maybe even be linked to the emission of gravitational waves!
Critical magnetic fields in a superconductor coupled to a superfluid arXiv:1704.01575 Phys.Rev.D95 arXiv:1612.01865 EPJ Web Conf. 137 (2017) 0900	This project is the second one my supervisor Andreas Schmitt and I have been working on during my phd. We used the model we have introduced in our previous project but additionally gauged one of the scalar fields, having in mind the mixture of a proton superconductor and a neutron superfluid in neutron stars. In this setup we studied the influence of the superfluid on superconducting flux tubes. We computed the interaction energy between the flux tubes and found intreaging consequences of this interaction, like first order phase transitions and possible mixed phases.
Instabilities in relativistic two-component (super-)fluids arXiv:1510.01982 Phys.Rev.D93, 025011	This is the first project of my PhD carried out at the TU Wien together with my Supervisor Andreas Schmitt and his former student Stephan Stetina. We published our results on the arXiv, and in Physical Review D. You can read more about this project in the research section of this webpage.
Magnetized nuclear matter arXiv:1409.0425 Phys.Rev.D90, 125036 arXiv:1412.6282	This is the project I worked on during my master thesis, together with my supervisor Andreas Schmitt and with Florian Preis. We published our results on the arXiv and in Physical Review D. For a shorter version, there are also some conference proceedings available on the arXiv
Invariance property of wave scattering through disordered media PNAS 2014 111 (50) 17765-17770	This project was carried out under the supervision of Stefan Rotter and Sylvain Gigan during my internship in Paris. The paper was published in PNAS. Additionally it appeard as TU News article and in several newspapers and online magazines, including the austrian newspapers Der Standard and Die Presse. Laboratoire Kastler Brossel press release, CNRS press release, UPMC News, Austria Press Agency, Salzburg 24, Wirtschaftsblatt, Kleine Zeitung, Vienna.at, Unser Tirol, Paper suggestion on wavefrontshaping.net, HTSyndication, DO-CT.com, Technology.com, JuraForum.de, Extrem News, Neues Volksblatt, WebNewsWire.com, Opli.net, TeachersNews.net, Labo.de, National Society of Black Physicists, MatterNews.com, PestControlProductsPro.com, GPSdaily.com, Network54, Laborpraxis.de, Tiroler Tageszeitung, Nanowerk, Phys.org, Science Daily, TU Austria, Eurekalert, Science 2.0, Science Newsline, Österreich Journal, Noodls.com, WorldNews.com, Austria.com, InnovationsReport.de, Informationsdienst Wissenschaft, Industriemagazin, Science Codex, Ingenuity.be, Facebook Science and Technology News, NewsWalk.info, Red Orbit, Engineering.com, Suedtirol News, Life-Science.eu, MyScience.at, BillySpears.info, Institut Langevin News, Before It's News, IMEP CNRS News, Indian Monitor, Newsxz.com, One News Page, Space Daily, Science Seeker, Nano Magazine, Los Alamos National Lab Tweet, Nanotechnology World, Breaking News, Locker Dome, Technobahn, News Reality, Technology.org, Space War, TGTechno.com, Osttirol Online
Perturbative Analysis of the non-Perturbative Renomalization Group Equation	Bachelor thesis , carried out at the TU Wien under supervision of Andreas Ipp.