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各位老师和同学们好，
为加强本单位高能物理核物理及交叉学科等各研究方向研究生相互间的学术交流，拓宽研究生的学术视野，激励青年人才的成长，促进本单位科学研究的发展，第十一届粒子所“青年之星”年度学术论坛将于2025年11月28日如期举行。期待所有在本年度（2024/11-2025/11）发表或arXived文章（包括实验组文章 paper proposal approved）的在所研究生登台，一展风采。具体的会议活动细节、日程和安排会随后发出。欢迎同学们踊跃参加，具体事项通知如下:
1.论坛邀请粒子所全体师生参加。
2.申请报告需经导师同意，并在会议网站提交报告：
（1）注明博研或硕研身份。
（2）提交报告题目和摘要（中英文不限）。
（3）注明报告关联的已发表论文刊号（期刊或 arXiv 预印本号），论文数量不限。
3.论坛邀请全体博士后参加，汇报年度研究成果，可提交报告，但不参与“青年之星”评选。
4.注册截止时间为11月19日，报告时间暂定15+5分钟。
5.论坛组委会将在报告申请中，筛选15--25名报告人。论坛组委会将在论坛期间评选出2025年度“青年之星”，并设立奖项。
粒子物理研究所
夸克与轻子物理教育部重点实验室
夸克物质及探测技术国际联合研究中心
In the high-energy physics, particularly within the context of heavy-ion collisions conducted at the Large Hadron Collider (LHC), there exists a puzzle where viscous hydrodynamics fails to accurately depict the experimental data for the second and third flow harmonics, $v_{2}\{2 \}$ and $v_{3}\{2 \}$, in ultra-central Pb + Pb collisions at 2.76 TeV. This study proposes a novel approach to address this issue by reducing the initial state fluctuations through the augmentation of the minimum inter-nucleon distance within the nucleus. It was observed that increasing this minimum distance resulted in a reduction of the initial eccentricity, as compared to the predictions made by the Woods-Saxon model. Consequently, a lower ratio of shear viscosity to entropy density, $\eta / s$, is necessitated. Employing the TRENTo model, this research calculated the eccentricities within the $0–1 \%$ centrality class and subsequently determined the flow harmonic coefficients $v_2\{2 \}$ and $v_3 \{2\}$ using the (3+1) dimensional viscous hydrodynamics models CLVisc. By comparing various scenarios with different minimum distances between nucleons, this study discovered that a reduction in initial state fluctuations has a substantial impact on resolving the aforementioned puzzle within the nucleus of Pb. This conclusion not only addresses the specific issue of flow harmonic discrepancies but also contributes to a broader understanding of the initial state conditions in heavy-ion collisions. By refining our models to better reflect the true initial conditions, this study enhances the predictive capabilities of hydrodynamic simulations, thereby advancing the field of high-energy physics.
By analyzing the $e^{+}e^{-}$ collision data sample with an integrated luminosity of 8.0 fb$^{-1}$ collected with the BESIII detector at the center-of-mass energy of 3.773 GeV, we perform an amplitude analysis of $D^+\to K_S^0 K_S^0 \pi^+$ for the first time. Our amplitude model contains intermediate decays $D^+ \to K_S^0K^*(892)^+$ and $D^+\to K_S^0 (K_S^0\pi^+)_{\rm s-wave}$. The dominant intermediate process is $D^+\to K_S^0K^*(892)^+$, with a fit fraction of $(97.8 \pm 1.0_{\rm stat.}\pm 0.4_{\rm syst.})\%$, where the first uncertainty is statistical and the second uncertainty is systematic. In addition, with the detection efficiency obtained from the updated generic MC samples generated based on the amplitude analysis results, we obtain the absolute branching fraction of $\displaystyle\mathcal{B}(D^{+}\to K_{S}^{0}K_{S}^{0}\pi^{+}) = (2.97 \pm 0.09_{\rm stat.} \pm 0.05_{\rm syst.})\times 10^{-3}$. Using the branching fraction we measured, we obtain the branching fraction $\displaystyle\mathcal{B}(D^+ \to K_S^0 K^*(892)^+)$ = $(8.72 \pm 0.28_{\rm stat.}\pm 0.15_{\rm syst.})\times10^{-3}$.
In the NνDEx collaboration, a high-pressure gas TPC is being developed to search for the neutrinoless double beta decay. The use of electronegative $^{82}SeF_6$ gas mandates an ion-TPC. The reconstruction of z coordinate is to be realized exploiting the feature of multiple species of charge carriers. As the initial stage of the development, we studied the properties of the $SF_6$ gas, which is non-toxic and has similar molecular structure to $^{82}SeF_6$. In this work we present the measurement of drift velocities and mobilities of the majority and minority negative charge carriers found in SF6 at a pressure of 750 Torr, slightly higher than the local atmospheric pressure. The reduced fields range between 3.0 and 5.5 Td. It was performed using a laser beam to ionize the gas inside a small TPC, with a drift length of 3.7 cm. A customized charge sensitive amplifier was developed to read out the anode signals induced by the slowly drifting ions. The reconstruction of z coordinate using the difference in the velocities of the two carriers was also demonstrated.
The production cross section of inclusive isolated photons has been measured by the ALICE experiment at the CERN LHC in pp collisions at centre-of-momentum energy of $\sqrt{s}=13$ TeV collected during the LHC Run 2 data-taking period.
The measurement is performed by combining the measurements of the electromagnetic calorimeter EMCal and the central tracking detectors ITS and TPC, covering a pseudorapidity range of $|\eta^{\gamma}|<0.67$ and a transverse momentum range of $7
The measurement is compared with next-to-leading order perturbative QCD calculations and the results from the ATLAS and CMS experiments as well as with measurements at other collision energies. The measurement and theory prediction are in agreement with each other within the experimental and theoretical uncertainties.
Neutrinoless double beta (0νββ) decay is the most promising way to determine whether neutrinos are Majorana particles. There are many experiments based on different isotopes searching for 0νββ decay. Combining the searches of 0νββ decay in multiple isotopes provides a possible method to distinguish operators and different models. The contributions to 0νββ decay come from standard, long-range, and short-range mechanisms. We analyze the scenario in which the standard and short-range operators exist simultaneously within the framework of low-energy effective field theory. Five specific models are considered, which can realize neutrino mass and can contribute to 0νββ decay via multiple mechanisms. A criterion to evaluate the possibilities of future experiments to discriminate operators and models is built. We find that the complementary searches for 0νββ decay in different isotopes can distinguish the cases that contain the low-energy effective operators O1,2,5 and R-parity violating supersymmetry model. For other cases and models, the experimental searches within multiple isotopes can also more effectively constrain the parameter region than with only one isotope.
In this work, we make a study of the τ− → ωπ−ντ decay in the framework of low energy effective field theory.The JPG decompositions of the quark currents and the ωπ final state show that, besides the Standard Model vector interaction, only the non-standard tensor interaction can have a non-zero contribution to the decay. To discuss its effect, a reliable calculation of the ωπ tensor form factors is necessary. After constructing the Lagrangian of resonance chiral theory with external tensor sources, we calculate both the vector and tensor form factors with the relevant resonance couplings determined by combining the QCD short-distance constraints, the fit to the spectral function of τ− → ωπ−ντ decay, as well as the matching between the O(p4) odd-intrinsic-parity operators after integrating out the vector resonances and the O(p6) operators of chiral perturbation theory. The new physics effect is then investigated in the distributions of the spectral function and the forward-backward asymmetry of τ− → ωπ−ντ decay. We find that the spectral function is dominated by the Standard Model, and the non-standard tensor contribution is negligible. However, since the forward-backward asymmetry can be only generated with a non-zero tensor interaction, the observable is quite sensitive to this kind of new physics. A future measurement of the observable at the Belle II experiment as well as at the proposed Tera-Z and STCF facilities is, therefore, strongly called for to check the existence of such a non-standard tensor interaction.
Abstract: We investigate the chiral phase transition and the properties of the shear viscosity coefficient in a Nambu-Jona-Lasinio (NJL) model at finite temperature and chemical potential. We compare the contributions from the tensor spin polarization (TSP) and anomalous magnetic moment (AMM) of quarks in (2+1)-flavor NJL model. For light $u$ and $d$ quarks, when TSP and AMM are not considered, the magnetized system is characterized by magnetic catalysis. The introduction of TSP will further enhance the magnetic catalytic characteristics. On the other hand, when AMM is introduced, the phase-transition temperature decreases with the magnetic field, which is the feature of inverse magnetic catalysis.It is found that both the ratio $\eta/s$ of shear viscosity coefficient to entropy and the collision relaxation time $\tau$ show similar trend with temperature, both of which reach minima around the critical temperature. The shear viscosity coefficient of the dissipative fluid system can be decomposed into five different components as the strong magnetic field exists. The influences of the order of chiral phase transition and the critical end point on dissipative phenomena in such a magnetized medium are quantitatively investigated. It is found that ${\eta}_{1}$, ${\eta}_{2}$, ${\eta}_{3}$, and ${\eta}_{4}$ all increase with temperature. For first-order phase transitions, ${\eta}_{1}$, ${\eta}_{2}$, ${\eta}_{3}$, and ${\eta}_{4}$ exhibit discontinuous characteristics.
We study the universal dynamical relaxation behaviors of a quantum XY chain following a quench, paying special attention to the case in which the initial state is a critical ground state, or the postquenched Hamiltonian is at a critical point of equilibrium quantum phase transition, or both of them are critical. In such a “critical quench,”we find very interesting real-time dynamical scaling behaviors and we find crossover phenomena between them.For a quench from a noncritical point to a critical point, we find that, compared to the noncritical quench,the universal power-law scaling behavior does not change; however, there may be a crossover between theexponential decaying behavior and the power-law scaling. For a quench from a critical point to a noncritical point, the power-law scaling behaviors $t^{-3/2}$ and $t^{-3/4}$ in the noncritical quenches may be changed to $t^{-1}$ and $t^{-1/2}$, respectively. If the prequenched Hamiltonian is set to be a point that is close to but not exactly at a critical point, we find interesting crossover phenomena between different power-law scaling behaviors. We also study the quench from the vicinity of a multicritical point, we find crossover behaviors that are induced by a different mechanism, and we find another crossover exponent. All the results are related to the gap-closing properties of the energy spectrum of the critical points.
Abstract:
In this study, we explore the thermodynamics of heavy quarkonium within the context of a spinning black hole background. Specifically, we analyze the impact of angular momentum on various thermodynamic properties of heavy quarkonium, including interquark distance, free energy, binding energy, entropy, entropic force, and internal energy, based on the thermodynamic relationships.
Our results demonstrate that angular momentum diminishes the maximum interquark distance,thereby facilitating quarkonium dissociation. Furthermore, we note that angular momentum suppresses free energy. The analysis of binding energy reveals that angular momentum enhances the dissociation of mesons into free quarks and antiquarks. Additionally, our findings indicate that angular momentum augments entropy and entropic force, thereby accelerating quarkonium dissociation. Angular momentum also increases internal energy at extended interquark distances.Lastly,we observe that the effects of angular momentum on quarkonium are more significant when the axis of the quark pair is perpendicular to the direction of angular momentum.
In this talk, we present lattice results for the shear and bulk viscosities of gluon plasma, calculated in SU(3) Yang-Mills theory over the temperature range $0.76T_c \leq T \leq 2.25T_c $, where $T_c$ denotes the confinement/deconfinement transition temperature. Shear viscosity and bulk viscosity quantify the response of the energy-momentum tensor to shear flow and divergent flow, respectively. These viscosities serve as critical input parameters for phenomenological and transport models used to interpret the experimental data, e.g. the elliptic flow $v_2$. Using gradient flow, we achieve unprecedented precision for the temporal Euclidean two-point correlation functions of the energy-momentum tensor, from which the viscosities are extracted. We focus particularly on how the viscosities vary around $T_c$, with the goal of understanding the critical behavior of the pure-glue system. The methodology developed in this work also paves the way for extending the study to full QCD.
自2012年希格斯粒子发现以来，标准模型作为粒子物理理论框架取得了重大成功。然而，标准模型中的粒子仅能解释约4%的宇宙物质成分，而无法解释约25%的不可见物质（即暗物质）的来源，这成为当前粒子物理学的核心挑战之一。此现象表明可能存在超出标准模型的物理理论，其中弱相互作用大质量粒子（WIMP）是一个备受关注的暗物质候选者。假设宇宙起源于大爆炸，实验测量当前暗物质丰度得出 ( \Omega h^2 = 0.12 )。
天体物理观测结果（例如星系旋转曲线和子弹星系团的研究）支持暗物质的存在，并表明其参与引力相互作用，这是目前已知的暗物质唯一已确认的相互作用形式。在高能物理领域，诸多实验正致力于直接探测暗物质的性质，尽管在探索达到GeV量级的较重暗物质方面尚未取得突破。值得注意的是，GeV量级与B介子物理衰变的典型能标相符，并且去年实验首次测得了 ( B^+ \to K^+ \nu \bar{\nu} ) 衰变分支比的中心值，与标准模型的理论预言存在 ( 2.7\sigma ) 的偏差。因此，本研究旨在通过分析B介子衰变过程，探讨与GeV量级暗物质相关的潜在效应，以进一步加深对暗物质性质的理解。
We present the first lattice QCD results of quadratic fluctuations and correlations of conserved charges in (2+1)-flavor lattice QCD in the presence of a background magnetic field. The simulations were performed using the Highly Improved Staggered Quarks with physical pion mass $m_\pi$ = 135 MeV on $N_\tau=8$ and 12 lattices. We find that the correlation between net baryon number and electric charge, denoted as $\chi^{\rm BQ}_{11} $, can serve as a magnetometer of QCD. At pseudocritical temperatures the $\chi^{\rm BQ}_{11}$ starts to increase rapidly with magnetic field strength $eB > 2M^2_{\pi}$ and by a factor 2 at $eB\simeq 8 M^2_{\pi}$.
By comparing with the hadron resonance gas model, we find that the $eB$ dependence of $\chi^{\rm BQ}_{11}$ is mainly due to the doubly charged $\Delta$(1232) baryon. Although the doubly charged $\Delta$(1232) could not be detected experimentally, its decay products, protons and pions, retain the $eB$ dependence of $\Delta$(1232)’s contribution to $\chi^{\rm BQ}_{11}$.
Furthermore, the ratio of electric charge chemical potential to baryon chemical potential, $\mu_{\rm Q}/\mu_{\rm B}$, shows significant dependence on the magnetic field strength and varies with the ratio of electric charge to baryon number in the colliding nuclei in heavy ion collisions. These results provide baselines for effective theory and model studies, and both $\chi^{\rm BQ}_{11}$ and $\mu_{\rm Q}/\mu_{\rm B}$ could be useful probes for the detection of magnetic fields in relativistic heavy ion collision experiments as compared with corresponding results from the hadron resonance gas model.
Recently, there has been great interest in the phenomenon of severe violation of the Wiedemann-Franz law in graphene Dirac fluids around 75 K, due to the strong coupling relativistic plasma near the neutral point, where traditional perturbation theory fails. To explain this phenomenon, this article proposes a holographic dual two-current axion coupling model, describing the interaction between electrons and holes in graphene near the charge neutrality point (CNP) and revealing the related physical mechanism. The study shows that the holographic two-current model aligns with experimental results at 100μm^−2,and correctly predicts conductivity as temperature increases. Additionally, the article explores the behavior of β+γ and its impact on conductivity and thermal conductivity. The results suggest a frictional effect between electrons and holes. Consequently, this study provides us with a clearer understanding of the properties of graphene Dirac fluids and further confirms the reliability of the holographic duality method.
The impact of rotation on the deconfinement phase transition under the Einstein-Maxwell system of the soft and the hard wall models in holographic quantum chromodynamics is studied in this paper. The metric by cylindrical coordinates with rotation is introduced into the system to calculate the Hawking temperature. The first holographic study on the influence of the radius of a homogeneous rotating system on the phase diagram is proposed. It is found that the phase transition temperature hardly changes with the rotation angular velocity for a small rotation radius. Only with a larger rotation radius can the change in rotational angular velocity significantly alter the phase transition temperature. The phase transition temperature decreases rapidly with the increase of rotation angular velocity as the rotation radius increases.
We revisit the holographic Langevin diffusion coefficients of a heavy quark, when travelling through a strongly coupled anisotropic plasma in the presence of magnetic field $\mathcal{B}$.The Langevin diffusion coefficients are calculated within the membrane paradigm in the magnetic branes model which has been extensively studied to investigate the magnetic effects on various observables in strongly coupled QCD scenarios by holography. In addition to confirming some conventional conclusions, we also find several new interesting features among the five Langevin diffusion coefficients in the magnetic anisotropic plasma.
It is observed that the transverse Langevin diffusion coefficients depend more on the direction of motion rather than the directions of momentum diffusion at the ultra-fast limit, while one would find an opposite conclusion when the moving speed is sufficiently low. For the Longitudinal Langevin diffusion coefficient, we find that motion perpendicular to $\mathcal{B}$ affects the Langevin coefficients stronger at any fixed velocity. We should also emphasize that all five Langevin coefficients are becoming larger with increasing velocity. We find that the universal relation $\kappa^{\parallel}>\kappa^{\perp}$ in the isotropic background, is broken in a different new case that a quark moving paralleled to $\mathcal{B}$. This is one more particular example where the violation of the universal relation occurs for the anisotropic background.Further, we find the critical velocity of the violation will become larger with increasing $\mathcal{B}$.