arXiv subjects

Wangmei Zha

Publications and source records attributed to Wangmei Zha.

36 records · Page 2Linked to original sources

arXiv2023

Energy Dependence of the Breit-Wheeler process in Heavy-Ion Collisions and its Application to Nuclear Charge Radius Measurements

The collision energy dependence of the cross section and the transverse momentum distribution of dielectrons from the Breit-Wheeler process in heavy-ion collisions are computed in the lowest-order QED and found to be sensitive to the nuclear charge distribution and the infrared-divergence of the ultra-Lorentz boosted Coulomb field. Within a given experimental kinematic acceptance, the cross section is found to increase while the pair transverse momentum ($\sqrt{\langle p_{T}^{2} \rangle}$) decreases with increasing beam energy. We demonstrate that the transverse-momentum component of Weizsacker-Williams photons is due to the finite extent of the charge source and electric field component in the longitudinal direction. We further clarify the connection between the nuclear charge distribution and the kinematics of produced $e^+e^-$ from the Breit-Wheeler process, and propose a criterion for the validity of the Breit-Wheeler process in relativistic heavy-ion collisions. Following this approach we demonstrate that the experimental measurements of the Breit-Wheeler process in ultra-relativistic heavy-ion collisions can be used to quantitatively constrain the nuclear charge radius. The extracted parameters show sensitivity to the impact parameter dependence, and can be used to study the initial-state and final-state effects in hadronic interactions.

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arXiv2023

Reaction plane alignment with linearly polarized photon in heavy-ion collisions

The collective observables play critical roles in probing the properties of quark-gluon-plasma created in relativistic heavy-ion collisions, in which the information on initial collision geometry is crucial. However, the initial collision geometry, e.g., the reaction plane, cannot be directly extracted in the experiment. In this paper, we demonstrate the idea of determining the reaction plane via the feature of linear polarization of the coherent photoproduction process and discuss the advantages of the proposed approach in comparison with traditional methods.

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arXiv2022

Rescattering effect on the measurement of K* spin alignment in heavy-ion collisions

Spin alignment of vector mesons in noncentral relativistic heavy ion collisions provides a novel probe of the global quark polarization and hadronization mechanism. In this paper, we discuss the spin alignment of a short-lived vector meson, namely K*, arising from the hadronic rescattering process using the UrQMD model. This spin alignment is not related to global quark polarization but cannot be distinguished from those arising from global quark polarization in experiments. The spin alignment parameter $ρ_{00}$ is found to deviate from 1/3 by up to -0.008 (0.03) with respect to the reaction (production) plane. These deviations are much larger than the expected signal from all the theoretical models implementing the conventional global quark polarization mechanism as well as the current experimental precision, and should be considered seriously when comparing measurements with theoretical calculations.

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arXiv2022

Report on Progress in Physics: Observation of the Breit-Wheeler Process and Vacuum Birefringence in Heavy-Ion Collisions

This Report reviews the effort over several decades to observe the linear Breit-Wheeler process ($γγ\rightarrow e^+e^-$) and vacuum birefringence in high-energy particle and heavy-ion collider experiment. This Report, motivated by STAR's recent observations, attempts to summarize the key issues related to the interpretation of polarized $γγ\rightarrow l^+l^-$ measurements in high-energy experiments. To that end, we start by reviewing the historical context and essential theoretical developments, before focusing on the decades of progress made in high-energy collider experiments. Special attention is given to the evolution in experimental approaches in response to various challenges and the demanding detector capabilities required to unambiguously identify the linear Breit-Wheeler process and to detect the signatures of vacuum birefringence. We close the report with a discussion, followed by a look at near-future opportunities for utilizing these discoveries and for testing QED in previously unexplored regimes.

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arXiv2022

Exploring gluon tomography with polarization dependent diffractive J/$ψ$ production

We study azimuthal asymmetries in diffractive J/$ψ$ production in ultraperipheral heavy-ion collisions at RHIC and LHC energies using the color glass condensate effective theory. Our calculation successfully describes azimuthal averaged $J/ψ$ production cross section measured by STAR and ALICE. We further predict very large $\cos 2ϕ$ and $\cos 4ϕ$ azimuthal asymmetries for diffractive $J/ψ$ production both in UPCs at RHIC and LHC energies and in eA collisions at EIC energy. These novel polarization dependent observables may provide complementary information for constraining gluon transverse spatial distribution inside large nuclei. As compared to all previous analysis of diffractive $J/ψ$ production, the essential new elements integrated in our theoretical calculations are: the double-slit interference effect, the linear polarization of coherent photons, and the final state soft photon radiation effect.

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arXiv2021

Mapping the Electromagnetic Fields of Heavy-Ion Collisions with the Breit-Wheeler Process

Ultra-relativistic heavy-ion collisions are expected to produce the strongest electromagnetic fields in the known Universe. These highly-Lorentz contracted fields can manifest themselves as linearly polarized quasi-real photons that can interact via the Breit-Wheeler process to produce lepton anti-lepton pairs. The energy and momentum distribution of the produced dileptons carry information about the strength and spatial distribution of the colliding fields. Recently it has been demonstrated that photons from these fields can interact even in heavy-ion collisions with hadronic overlap, providing a purely electromagnetic probe of the produced medium. In this review we discuss the recent theoretical progress and experimental advances for mapping the ultra-strong electromagnetic fields produced in heavy-ion collisions via measurement of the Breit-Wheeler process.

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arXiv2021

An experimental review of open heavy flavor and quarkonium production at RHIC

Open heavy flavor and quarkonium are unique probes of the hot-dense medium produced in heavy-ion collisions. Their production in p+p collisions also provide important test of QCD. In this paper, we review the selected results on open heavy flavor and quarkonium in p+p and heavy-ion collisions achieved at RHIC. Physics implications are also discussed.

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arXiv2021

Discovery of higher order QED effect for the vacuum pair production

The higher order quantum electrodynamics (QED) effect for vacuum pair production has been searched without success since 1954. In this paper, we show that the lowest order QED calculations for lepton pair vacuum production in heavy-ion collisions are about 20$\%$ higher than the combined world-wide data with a seven sigma-level of significance and the corresponding higher order QED results are consistent with data. We claim the discovery of higher order effect for the QED pair production, which settles the dust of previous debates for several decades. The verification of higher order QED effect is a fundamental scientific problem, which is an important milestone towards the nonperturbative and nonlinear regime of QED vacuum.

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arXiv2020

Exploring the double-slit interference with linearly polarized photons

The linearly polarized quasi-real photons from the highly Lorentz-contracted Coulomb fields of relativistic heavy ions can fluctuate to quark-antiquark pairs, scatter off a target nucleus and emerge as vector mesons. In the process, the two colliding nuclei can switch roles to act as photon emitter or target, forming a double-slit interference pattern. The product from photoproduction inherits the photon polarization states, leading to the asymmetries of the decay angular distributions. In this letter, we study the interference effect in polarization dimension from the asymmetries of the decay angular distributions for photoprodution in heavy-ion collisions and find a periodic oscillation with the transverse momentum of vector meson, which could reasonably explain the transverse momentum dependence of the 2nd-order modulation in azimuth for the $ρ^{0}$ decay observed by the STAR collaboration.

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arXiv2020

Acoplanarity of QED pairs accompanied by nuclear dissociation in ultra-peripheral heavy ion collisions

This paper investigates the transverse momentum broadening effect for electromagnetic production of dileptons in ultra-peripheral heavy ion collisions accompanied by nuclear dissociation. The electromagnetic dissociation probability of nuclei for different neutron multiplicities is estimated, which could serve as a centrality definition (i.e. impact parameter estimate) in ultra-peripheral collisions. In the framework of lowest-order QED, the acoplanarity of dilepton pairs is calculated for different neutron emission scenarios in ultra-peripheral collisions, indicating significant impact-parameter dependence. The verification of impact-parameter dependence is crucially important to understand the broadening effect observed in hadronic heavy-ion collisions.

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arXiv2019

Initial transverse-momentum broadening of Breit-Wheeler process in relativistic heavy-ion collisions

We calculate the cross section and transverse-momentum ($P_{\bot}$) distribution of the Breit-Wheeler process in relativistic heavy-ion collisions and their dependence on collision impact parameter ($b$). To accomplish this, the Equivalent Photon Approximation (EPA) was generalized in a more differential way compared to the approach traditionally used for inclusive collisions. In addition, a lowest-order QED calculation with straightline assumption was performed as a standard baseline for comparison. The cross section as a function of $b$ is consistent with previous calculations using the equivalent one-photon distribution function. Most importantly, the $P_{\bot}$ shape from this model is strongly dependent on impact parameter and can quantitatively explain the $P_{\bot}$ broadening observed recently by RHIC and LHC experiments. This broadening effect from the initial QED field strength should be considered in studying possible trapped magnetic field and multiple scattering in a Quark-Gluon Plasma (QGP). The impact-parameter sensitive observable also provides a controllable tool for studying extreme electromagnetic fields.

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arXiv2018

$ψ(2S)$ photoproduction in high energy nuclear collisions

We study coherent photoproduction of $ψ'$ and the subsequent suppression in high energy nuclear collisions. The hot medium effect on charmonium photoproduction is controlled by the competition between the fireball size and nuclear size. When the former is smaller than the later, the photoproduction is only slightly affected by the hot medium. We calculate the yield ratio of $ψ'$ to $J/ψ$ in semi-central Pb-Pb collisions at LHC energy, while the high $p_T$ limit is characterized by the fireball, its value in low $p_T$ limit can still reach the significant enhancement by photoproduction.

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arXiv2018

Photoproduction of J/$ψ$ in non-single-diffractive p+p collisions

Recently, significant enhancements of J/$ψ$ production at very low transverse momenta were observed by the ALICE and STAR collaboration in peripheral hadronic A+A collisions. The anomaly excesses point to evidence of coherent photon-nucleus interactions in violent hadronic heavy-ion collisions, which were conventionally studied only in ultra-peripheral collisions. Assuming that the coherent photoproduction is the underlying mechanism which is responsible for the excess observed in peripheral A+A collisions, its contribution in p+p collisions with nuclear overlap, i.e. non-single-diffractive collisions, is of particular interest. In this paper, we perform a calculation of exclusive J/$ψ$ photoproduction in non-single-diffractive p+p collisions at RHIC and LHC energies base on the pQCD motivated parametrization from world-wide experimental data, which could be further employed to improve the precision of phenomenal calculations for photoproduction in A+A collisions. The differential rapidity and transverse momentum distributions of J/$ψ$ from photoproduction are presented. In comparison with the J/$ψ$ production from hadronic interactions, we find that the contribution of photoproduction is negligible.

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arXiv2018

Double-slit experiment at fermi scale: coherent photoproduction in heavy-ion collisions

The double-slit experiment has become a classic thought experiment, for its clarity in expressing the central puzzle of quantum mechanics -- wave-particle complementarity. Such wave-particle duality continues to be challenged and investigated in a broad range of entities with electrons, neutrons, helium atoms, C$_{60}$ fullerenes, Bose-Einstein condensates and biological molecules. All existing experiments are performed with the scale larger than angstrom. In this article, we present a double-slit scenario at fermi scale with new entities -- coherent photon products in heavy-ion collisions. Virtual photons from the electromagnetic fields of relativistic heavy ions can fluctuate to quark-antiquark pairs, scatter off a target nucleus and emerge as vector mesons. The two colliding nuclei can take turns to act as targets, forming a double-slit interference pattern. Furthermore, the `which-way' information can be partially solved by the violent strong interactions in the proposed scenario, which demonstrates a key concept of quantum mechanics -- complementary principle.

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arXiv2018

Coherent photo-produced J$/ψ$ and dielectron yields in isobaric collisions

Recently, significant enhancements of J/$ψ$ and $e^{+} e^{-}$ pair production at very low transverse momenta were observed by the STAR and ALICE collaboration in peripheral hadronic A+A collisions. The anomaly excesses point to evidence of coherent photon-nucleus and photon-photon interactions in violent hadronic heavy-ion collisions, which were conventionally studied only in ultra-peripheral collisions. The isobaric collisions performed at RHIC provides a unique opportunity to test the existence of coherent photon products in hadronic heavy-ion collisions. The idea is that the possible production of coherent photon products is significantly different in different collision systems due to the variations in their charge and nuclear density distributions. In this letter, we focus on the peripheral collisions and provide theoretical predictions for coherent production of J/$ψ$ and dielectron in isobaric collisions. We show that the expected yields differ significantly to perform the experimental test.

hep-ph↗

arXiv2017

Charmonium Coherent Photoproduction and Hadroproduction with Effects of Quark Gluon Plasma

We study the charmonium coherent photoproduction and hadroproduction consistently with modifications from both cold and hot nuclear matters. The strong electromagnetic fields from fast moving nucleus interact with the other target nucleus, producing abundant charmonium in the extremely low transverse momentum region $p_T<0.1$ GeV/c. This results in significative enhancement of $J/ψ$ nuclear modification factor in semi-central and peripheral collisions. In the middle $p_T$ region such as $p_T<3\sim 5$ GeV/c, $J/ψ$ final yield is dominated by the combination process of single charm and anti-charm quarks moving in the deconfined matter, $c+\bar c\rightarrow J/ψ+g$. In the higher $p_T$ region, $J/ψ$ production are mainly from parton initial hard scatterings at the beginning of nucleus-nucleus collisions and decay of B hadrons. We include all of these production mechanisms and explain the experimental data well in different colliding centralities and transverse momentum regions.

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arXiv2015

Systematic study of the experimental measurements on $J/ψ$ cross section and kinematic distribution in $p+p$ collisions at different energies

The world experimental data on cross section and kinematic distribution in $p+p$ and $p+A$ collisions at $\sqrt{s}$ = 6.8 - 7000 GeV are examined in systematic way. The $\sqrt{s}$ dependence of the inclusive cross section, rapidity and transverse momentum distributions are studied phenomenologically. We explore empirical formulas to obtain the total cross section, rapidity and transverse momentum ($p_{T}$) distribution. This is crucial for the interpretation of A$+$A $J/ψ$ results at RHIC when the $p+p$ reference data are not available. In addition, the cross section at mid-rapidity and transverse momentum distributions in $p+p$ collisions at $\sqrt{s}$ = 39 and 62.4 GeV are evaluated.

hep-ex↗

arXiv2013

Systematic study of the experimental measurements on ratios of different $Υ$ states

The world data on yields and ratios of different $Υ$ states created in hadron collisions at $\sqrt{s} = 19-8000$ GeV are examined in systematic way. We find that $Υ(2S)/Υ(1S)=0.275\pm0.005$ and $Υ(3S)/Υ(1S)=0.128\pm0.004$. No signficant energy dependence of these ratios are observed within the broad collision energies. In addition, the rapidity, transverse momentum, and transverse mass dependence of these ratios are also reported.

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