{"id":15374,"date":"2021-11-03T15:33:08","date_gmt":"2021-11-03T15:33:08","guid":{"rendered":"https:\/\/www.innovationnewsnetwork.com\/?p=15374"},"modified":"2021-11-03T15:52:10","modified_gmt":"2021-11-03T15:52:10","slug":"expanding-knowledge-studying-electromagnetic-radiation","status":"publish","type":"post","link":"https:\/\/www.innovationnewsnetwork.com\/expanding-knowledge-studying-electromagnetic-radiation\/15374\/","title":{"rendered":"Expanding our knowledge by studying electromagnetic radiation"},"content":{"rendered":"

Tetyana Galatyuk asks what can we learn from electromagnetic radiation about the state of visible matter under extreme conditions?<\/h2>\n

One of the great challenges in modern physics is to understand the evolution of our Universe from the \u2018Big Bang\u2019 to the state we observe today. Of particular interest is to unravel the microscopic properties of the extreme states of strong-interaction matter that existed, on the one hand, almost 14 billion years ago in the early Universe and, on the other, is created when two neutron stars merge, as recently observed for the first time. The possibility to form and explore in the laboratory strong-interaction matter under conditions similar to those realised a few microseconds after the \u2018Big Bang\u2019, or to those in the interior of compact stellar objects, is truly fascinating. The physics of those extreme states of matter is of pivotal significance for understanding a fundamental aspect of nature.<\/p>\n

For most of human history, light was the only known part of the electromagnetic spectrum. Today, we know that visible light is just a small part of the electromagnetic spectrum and with dedicated detectors we can observe light over a large range of wavelengths.<\/p>\n

The photon spectrum of the Milky Way<\/a>, spanning over 15 orders of magnitude in wavelength, bears witness to this fact. The capability to detect electromagnetic radiation for such a large range in wavelengths is key to the enormous progress in astronomy and astrophysics. Indeed, the first detection of gravitational waves from a binary neutron star coalescence candidate (later designated GW170817) 1<\/sup> and the observation of electromagnetic radiation from the same event 2<\/sup> opened a new era in multi-messenger astronomy. The violent Universe can now be \u2018heard\u2019 through gravitational waves and \u2018seen\u2019 through electromagnetic radiation.<\/p>\n

\"\"
Simulation of the time evolution of the fireball in a high-energy heavy ion collision. Particle production in the initial collision (left) is followed by rapid thermalisation of matter, subsequent expansion through QGP (yellow-orange-red zone), transition and hadronic phases (with different hadrons sketched as spheres), and finally a freeze-out (right) at a temperature of about 100 MeV (about 1 trillion degrees Kelvin). During the whole time evolution of some 10-23 s real photons and dileptons are emitted (yellow diagrams)<\/figcaption><\/figure>\n

What can laboratory experiments tell us about extreme environments in the Universe?
\n<\/strong><\/h3>\n

Photons, the carriers of the electromagnetic force, have long been an essential tool to study the properties of \u2018matter\u2019. An illustrative example is the success story of mysterious electromagnetic radiation discovered by Wilhelm Conrad Roentgen, so-called \u2018X-rays\u2019. Since 1895, X-rays have allowed us to look through the human body. In 1900, Max Planck successfully described the thermal electromagnetic radiation emitted by a black body by introducing a quantisation concept.<\/p>\n

Albert Einstein\u2019s quantum theory of light proposed that light is composed of small packets of energy called photons that have wave-like properties. As one consequence, it was understood that the concept of photons could be generalised.<\/p>\n

As mediators of a force, virtual photons can carry any combination of energy and momentum but have to materialise after a short time by the formation of a pair of charged leptons, e.g. an electron and a positron. Such virtual photons can serve as ideal probes of matter under extreme conditions of temperature (> 1012<\/sup> K) and density (>280 Mt\/cm3<\/sup>) in analogy to the role X-rays played. The theory of the strong force, Quantum Chromodynamics (QCD), predicts that under such extreme conditions ordinary matter (protons and neutrons) breaks into elementary building blocks, quarks and gluons.<\/p>\n

To recreate such extreme states of matter in the laboratory, one collides heavy nuclei accelerated to more than 90% of the speed of light. For an extremely short period of time, of order 10-23<\/sup> seconds, transient states of QCD matter are produced in such collisions. Throughout the course of a heavy-ion collision, virtual photons are emitted (see Fig. 1) and offers a unique opportunity to directly obtain \u2018Roentgen images\u2019 of the hot and dense matter (in-medium electromagnetic spectral functions) and to measure its temperature by analysing \u2018Planck-like\u2019 spectral distributions.<\/p>\n

\"\"
Left: hadron production in e+ e- annihilation, relative to pure QED processes (R), characterising hadronic spectral functions in vacuum.5 Right: acceptance corrected dilepton spectra after subtraction of \u2018hadronic cocktail\u2019 measured by HADES in Au+Au collisions at 2.4 GeV6<\/figcaption><\/figure>\n

Electromagnetic probes of strongly interacting matter<\/h3>\n

The electromagnetic (EM) interaction provides powerful tools for identifying and characterising conjectured novel phases of strong-interaction matter. Thermal emission of dileptons (e+<\/sup> e–<\/sup><\/em> or \u03bc+<\/sup> \u03bc–<\/sup><\/em>) and photons from the locally thermalised medium occurs throughout the lifetime of the fireball. Unlike hadrons, leptons do not interact strongly, leading to large mean free paths in the medium, much larger than the spatial extent of the fireball created in a heavy-ion collision. Hence, dileptons carry important information about the entire space-time history of the collision. Dilepton invariant-mass spectra are the only observable which gives direct access to the in-medium modification of hadronic spectral functions. This is evident from the 8-fold differential thermal production rate:3, 4<\/sup><\/p>\n

\"\"<\/p>\n

which depends only on the in-medium electromagnetic spectral function, Im<\/em>\u03a0em<\/sub><\/em> (the imaginary part of the current-current correlation function), its thermal weight in terms of the Bose-Einstein distribution, fB<\/sup> (q\u00b7u;T)<\/em>, and a free virtual-photon propagator, 1\/M2<\/sup><\/em>.<\/p>\n

In the vacuum, Im<\/em>\u03a0em<\/sub><\/em> is accurately known from the inverse process of e+<\/sup> e–<\/sup><\/em> annihilation into hadrons.5<\/sup> As shown in Fig. 2 (left) it is characterised by two regions. At low masses (LMR), M<1.1 GeV\/c2<\/sup> its strength is concentrated in the light vector-mesons (\u03c1, \u03c9, \u03d5<\/em> [7]); the non-perturbative phenomena of mass generation and quark confinement manifest themselves in massive hadronic degrees of freedom.<\/p>\n

At masses M>1.5 GeV\/c2<\/sup> (IMR), the spectral function is in the perturbative regime, giving rise to a structure-less continuum of a weakly interacting quarks and anti-quarks (which subsequently fragment into multiple hadrons). The \u03c1<\/em>-meson has received most of the attention in this context as it has a dominant contribution to the low-mass EM correlators and outshines the \u03c9<\/em>-meson by a factor of about 10.<\/p>\n

\"\"
Left: excitation function of thermal dilepton radiation in central heavy-ion collisions in terms of the collision energy dependence of the integrated excess yield in the 0.3 < M < 0.7 GeV\/c2 region, normalised by number of charged pions. Right: excitation function of the invariant mass slope parameter, Tslope, and of the initial temperature, T_initial, as obtained from a fireball model8 and from a coarse-grained transport approach9 in comparison to temperatures extracted from dilepton spectra measured by HADES6 and NA6010<\/figcaption><\/figure>\n

Dileptons as spectrometer, thermometer, chronometer, barometer, polarimeter, and amperemeter of the fireball
\n<\/strong><\/h3>\n

The question is: how does the spectral function of the \u03c1<\/em>-meson change inside hot and dense nuclear matter and which fundamental questions can be addressed with comprehensive measurements of dilepton production in heavy-ion collisions?<\/p>\n

State-of-the-art measurements of dilepton \u2018excess spectra\u2019 (radiation from yellow-orange-red zone shown in Fig. 1) in nuclear collision from few GeV to few TeV energy and model predictions of thermal radiation from the QGP and hadronic phase that describe dilepton data available to date, show that the \u03c1<\/em> meson undergoes a strong broadening in dense hadronic matter, melting into a continuous emission spectrum that resembles quark-antiquark annihilation.11<\/sup> An example of dielectron \u2018excess spectrum\u2019 from baryon-dominated matter measured by the High Acceptance DiElectron Spectrometer (HADES) and theoretical analysis is shown in Fig. 2 (right).<\/p>\n

Model calculations predict that depending on the invariant mass (M<\/em>) and 3-momentum (q<\/em>) of the dilepton \u2018excess spectrum\u2019 we can:<\/p>\n