| Lattice QCD(Quantum ChromoDynamics)predicts that quarks and gluons are deconfined from hadrons at extremely high temperatures or baryon densities,making it possible to form a new kind of matter called quark-gluon plasma(QGP).QGP is believed to exist in the early Universe shortly after the Big Bang.Such extreme conditions for creating QGP are reached in the laboratory with ultra-relativistic heavy-ion collisions.The Relativistic Heavy Ion Collider(RHIC),located at Brookhaven National Laboratory,is a dedicated machine to study the properties of the QGP and the QCD phase diagram in the laboratory.Searching for QGP and studying its properties is one of the primary goals of high-energy heavy ion collision experiments.In heavy-ion collisions,ultra-strong electromagnetic fields are induced by the ultra-relativistic heavy nucleus.The generated magnetic field reaches the order of 1014 Tesla,which is the strongest electromagnetic field in the known Universe.Strong electromagnetic fields in heavy-ion collisions generate various interesting phenomena in both the QED(Quantum ElectroDynamics)and QCD,such as the Chiral Magnetic Effect(CME),anisotropic charmonium production,spin-charge correlation for final-state hadrons,and so on.These phenomena have important implications for understanding the properties of QGP.Researches on these novel phenomena of QGP under extremely strong electromagnetic fields are hot topics and attract much experimental and theoretical attention in the field of relativistic heavy-ion collisions.However,there is a lack of experimental measurements of the properties of the initial strong electromagnetic fields.The strong electromagnetic field decays very fast.There has not yet been an evidence that it can survive to the QGP stage,and it also lacks of experimental methods.Dileptons can be produced by the collision of intense electromagnetic fields accompanying the relativistic heavy ions.Since dileptons are not involved in the strong interaction with the hot and dense medium,they are suggested as perfect "penetrating probes" to test the strong electromagnetic fields.The Lorentz-boosted Coulomb field created in heavy-ion collisions is perpendicular to the movement direction.It can be approximated as a transverse electromagnetic wave since the electric and magnetic fields are also perpendicular to each other with almost the same absolute amplitudes,which is similar to the electromagnetic fields of photons.Thus,the transverse electromagnetic field is quantized as a flux of quasi-real photons,the so-called Equivalent Photon Approximation(EPA).The photon-mediated processes occur when two fast-moving nuclei collide with each other.In the coherent photon-photon process,quasi-real photons emitted by two nuclei interact to produce dileptons,which is called the Breit-Wheeler process.The produced leptons are nearly back-to-back with small transverse momentum(pT),which makes them distinguishable from dileptons produced from other sources.Therefore,they can be used to study the properties of strong electromagnetic fields.In addition,when subjected to extremely strong electromagnetic fields,the vacuum exhibits an optical property of birefringence,resulting in the Δφ modulation of dileptons.Thus,measurements of the dileptons produced from the photon-photon process can be used to test vacuum birefringence and investigate the QED properties in the presence of strong electromagnetic fields.In this thesis,we report the measurements of photon-produced dimuon production in Au+Au collisions at(?)=200 GeV using the data taken in 2011 and 2014 with the Solenoidal Tracker At RHIC(STAR)experiment.The measured μ+μ-invariant mass spectra(0.44<Mμμ<0.56 GeV/c2)show significant enhancements with respect to the hadronic cocktails in both 60-80%and 40-60%centralities.Furthermore,the pair pT spectra show that the excesses concentrate below pT≈0.1 GeV/c consistent with the features of photon-photon interaction,while for pT>0.1 GeV/c can be described by the hadronic cocktails.The excesses(data-cocktail)are obtained after removing the hadronic cocktail contribution from the inclusive pair production.The EPA-QED calculations describe both the excess yields and shapes very well.These results show the enhancement at very low pT comes from the photon-photon interactions induced by the ultra-strong electromagnetic fields created in heavy-ion collisions.We also report the pT2 distributions within the STAR acceptance in 60-80%centrality.The(?),which characterizes the pT broadening,is also calculated.pT broadening depends on the impact parameter and may also be modified by the interaction of the dilepton with the electromagnetic fields trapped in QGP.EPA-QED calculations considering the impact parameter dependence are comparable with the data,and no additional pT broadening is observed.In addition,we also take the first measurement of the Δφ modulation for μ+μpairs produced in photon-photon collisions,which is related to the vacuum birefringence.The cos(4Δφ)amplitude is measured with a significance of about 3.5σ.This is the first measurement that shows the evidence of cos(4Δφ)modulation via the dimuon channel.It also first hints at the existence of cos(2Δφ)modulation with a significance of 2.3σ.The results are consistent with the EPA-QED calculations within uncertainties.Various observables provide experimental confirmation of the leading-order QED predictions.These results can be used to map the strength and spatial distribution of the initial EM field,which further provides the baseline for the research of the novel QCD phenomena in the presence of the strong electromagnetic field in QGP. |
