Alexander HABER
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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.
| 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. |