Understanding Of QCD Research Articles

Spinning protons and gluons in the η′

The proton spin puzzle has inspired a vast programme of experiments and theoretical work challenging our understanding of QCD and its role in the structure of hadrons. The proton’s internal spin structure is connected to chiral symmetry and, through gluon degrees of freedom in the flavor singlet channel, to the physics of the [Formula: see text] meson. Why do quarks contribute just about one third of the proton’s spin? Why are [Formula: see text] mesons and their interactions so sensitive to gluonic degrees of freedom? We review the status of these topics and some key observables for forthcoming experiments.

Issues with the search for critical point in QCD with relativistic heavy ion collisions

A systematic search for a critical point in the phase diagram of QCD matter is underway at the Relativistic Heavy Ion Collider (RHIC) and is planned at several future facilities. Its existence, if confirmed, and its location will greatly enhance our understanding of QCD. In this note we emphasize several important issues that are often not fully recognized in theoretical interpretations of experimental results relevant to the critical point search. We discuss ways in which our understanding on these issues can be improved.

Heavy Flavor Physics at ATLAS and CMS

In this talk I report recent results on heavy flavor physics from ATLAS and CMS. It includes two topics: spectroscopy of open or hidden beauty hadrons and studies of rare B decays of the type b → sμ+μ−. The former one is aimed to collect new information on the properties of beauty hadrons which helps to gain better understanding of QCD. And the latter one provides a good opportunity of searching for New Physics, the effects beyond the Standard Model. The discussed results were obtained with the Run 1 data or Run 2 data .

Imaginary Chemical Potential, NJL-Type Model and Confinement–Deconfinement Transition

In this review, we present of an overview of several interesting properties of QCD at finite imaginary chemical potential and those applications to exploring the QCD phase diagram. The most important properties of QCD at a finite imaginary chemical potential are the Roberge–Weiss periodicity and the transition. We summarize how these properties play a crucial role in understanding QCD properties at finite temperature and density. This review covers several topics in the investigation of the QCD phase diagram based on the imaginary chemical potential.

X, Y, Z search at Belle II

The Belle II experiment has successfully concluded Phase-2 of data taking in July 2018, and just started Phase-3, with the complete detector setup. Great perspectives and unique physics cases are enhanced in the Belle II physics program. In the sector of charmonium and spectroscopy, Belle II will investigate several physics processes: ISR physics for the vector states, bottomonium search and the study of the X(3872) line shape are some examples where the contribution of the Belle II experiment will help in better understanding QCD, for which several open issues still remain unsolved. First simulations and data analyses will be shown, with the first available dataset (2018), and the plan for spectroscopy search at Belle II will be discussed.

The sPHENIX Experiment

Our understanding of QCD under extreme conditions has advanced tremendously in the last 20 years with the discovery of the Quark Gluon Plasma and its characterisation in heavy ion collisions at RHIC and LHC. The sPHENIX detector planned at RHIC is designed to further study the microscopic nature of the QGP through precision measurements of jet, upsilon and open heavy flavor probes over a broad pT range. The multi-year sPHENIX physics program will commence in early 2023, using state-of-the art detector technologies to fully exploit the highest RHIC luminosities. The experiment incorporates the 1.4 T former BaBar solenoid magnet, and will feature high precision tracking and vertexing capabilities, provided by a compact TPC, Si-strip intermediate tracker and MAPS vertex detector. This is complemented by highly granular electromagnetic and hadronic calorimetry with full azimuthal coverage. In this document I describe the sPHENIX detector design and physics program, with particular emphasis on the comprehensive open heavy flavour program enabled by the experiment’s large coverage, high rate capability and precision vertexing.

Measurement of hadron form factors at BESIII

The BESIII experiment, operated at the BEPCII e+e- collider in Beijing, has acquired large data sets at center-of-mass energies between 2.0 GeV and 4.6 GeV. One of the key aspects of the physics program of the BESIII collaboration is to test the understanding of QCD at intermediate energies. Applying different experimental techniques, form factors of hadrons are measured. Among these are the pion form factor, as an important input to the (g - 2)μ puzzle, and the electro-magnetic form factors of nucleons and hyperons in the time-like regime. An overview of the recent results and some ongoing studies at BESIII is provided.

Measurement of the Transverse Single-Spin Asymmetry in p^{↑}+p→W^{±}/Z^{0} at RHIC.

We present the measurement of the transverse single-spin asymmetry of weak boson production in transversely polarized proton-proton collisions at sqrt[s]=500 GeV by the STAR experiment at RHIC. The measured observable is sensitive to the Sivers function, one of the transverse-momentum-dependent parton distribution functions, which is predicted to have the opposite sign in proton-proton collisions from that observed in deep inelastic lepton-proton scattering. These data provide the first experimental investigation of the nonuniversality of the Sivers function, fundamental to our understanding of QCD.

A Measurement of g2p at Low Q2

Jefferson Lab has been at the forefront of a program to study the polarized structure of nucleons using electron scattering. Measurements of the spin dependent structure functions, [Formula: see text] and [Formula: see text], have proven to be powerful tools in testing and understanding QCD. The neutron structure function [Formula: see text] has been measured extensively in Hall A at Jefferson Lab over a wide range of [Formula: see text], but data for [Formula: see text] remains scarce. This docment will discuss the [Formula: see text] experiment, which ran in Hall A at Jefferson Lab in the spring of 2012, and will provide the first measurement of [Formula: see text] in the resonance region; covering [Formula: see text] GeV2. The 0[Formula: see text] moment of [Formula: see text] provides a test of the Burkhardt-Cottingham sum rule, which states that the integral of [Formula: see text] over the Bjorken scaling variable [Formula: see text] goes to zero. This sum rule, valid for all values of [Formula: see text], has been satisfied for the neutron, but a violation is suggested for the proton at high [Formula: see text]. The 2[Formula: see text] moment allows for a benchmark test of [Formula: see text]PT at low [Formula: see text]. Specifically, the behavior of the longitudinally-transverse spin polarizability ([Formula: see text]), as [Formula: see text]PT calculations of this quantity deviate significantly from the measured neutron data. This document will discuss the current status of the analysis along with preliminary results.

MESON SPECTROSCOPY AT JLAB AT 12 GEV

The 12 GeV upgrade to the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab will enable a new generation of experiments in hadronic nuclear physics, seeking to address fundamental questions in our understanding of QCD. The existence of exotic states, suggested by both quark models and lattice calculations, would allow gluonic degrees of freedom to be explored, and may help explain the role played by gluons in the QCD interaction. This article will review the meson spectroscopy program being planned at the lab following the 12 GeV upgrade, utilising real and quasi-real photon beams in two of the lab's four experimental halls, whose distinct capabilities will enable an extensive set of spectroscopy experiments to be performed at the same facility.

Quarkonium production and polarization in pp collisions with the CMS detector

Studies of the production of heavy quarkonium states are very important to improve our understanding of QCD and hadron formation, given that the heavy quark masses allow the application of theoretical tools less sensitive to nonperturbative effects. Thanks to a dedicated dimuon trigger strategy, combined with the record-level energy and luminosity provided by the LHC, the CMS experiment could collect large samples of pp collisions at 7 and 8 TeV, including quarkonium states decaying in the dimuon channel. This allowed the CMS collaboration to perform a series of systematic measurements in quarkonium production physics, including double-differential cross sections and polar- izations, as a function of rapidity and pT , for five S-wave quarkonia: J/ψ, ψ(2S), Y(1S), Y(2S), and Y(3S). Some of these measurements extend well above pT � 100 GeV, prob- ing regions of very high pT /mass, where the theory calculations are supposed to be the most reliable. Thanks to its high-granularity silicon tracker, CMS can reconstruct low- energy photons through their conversions to e + e − pairs, thereby accessing the radiative decays of the P-wave quarkonium states, with an extremely good mass resolution, so that the J=1 and J=2 1P states can be resolved. This allows CMS to determine cross-section ratios and feed-down decay fractions involving the χ states, in both the charmonium and bottomonium families. This talk presents some of the most recent CMS quarkonium production results, in pp collisions, in particular the production of Y(1S,2S,3S), the cross-section ratio of charmo- nium and bottomonium P-wave states and the polarization of S-wave c¯ c and b¯ b states.

Quarkonium production in pp collisions with the CMS detector

The studies of the inclusive production of heavy quarkonium states at LHC are very important to improve our understanding of QCD and hadron formation, given that the heavy-quark masses allow the application of theoretical tools less sensitive to nonperturbative effects. The prompt cross sections and polarizations measured by CMS and the other LHC experiments are presented for the five S-wave states J/ψ, ψ ( 2S ) and ϒ( nS ) ( n = 1, 2, 3) and discussed especially in comparison to the theoretical predictions provided by Non Relativistic QCD.

Partial wave analysis at BESIII

The BESIII experiment in Beijing takes data in τ-charm domain since 2009. For the moment the world largest samples of J/ψ, ψ(3686), ψ(3770) and ψ(4040) data have been collected. Hadron spectroscopy is a unique way to access QCD, which is one of the most important physics goals of BESIII. Experimental search of new forms of hadrons and subsequent investigation of their properties would provide validation of and valuable input to the quantitative understanding of QCD. The key to success lies in high levels of precision during the measurement and high statistics in the recorded data set complemented with sophisticated analysis methods. Partial wave analysis (PWA) is a powerful tool to study the hadron spectroscopy, that allows one to extract the resonance's spin-parity, mass, width and decay properties with high sensitivity and accuracy. In this poster, we present the working PWA framework of BESIII – GPUPWA and the recent results of PWA of J/ψ → γηη. GPUPWA is a PWA framework for high statistics partial wave analyses harnessing the GPU parallel computing.

Jet and photon measurements from ATLAS

Di erential cross-section measurements of inclusive-jet and di-jet production provide stringent tests of perturbative QCD predictions and provide inputs for determination of parton density functions. Ratios of jet multiplicities are sensitive to s and have reduced theoretical uncertainties. Measurements of the inclusive isolated-photon and di-photon cross-sections provide a direct probe of short-distance physics, complementary to that from measurements of jets or vector-bosons and are sensitive to the gluon density in the proton. The measurements are compared to next-to-leading-order or higher-order QCD calculations. 1 Introduction ATLAS is a general purpose detector operating within the LHC experiment. The main goals of ATLAS are searches of the Higgs boson, study of the properties of the top quark and search for new physics. High-precision measurements to make novel tests of perturbative QCD (pQCD) will also be possible due to the higher centre-of-mass energy ( p s) available at LHC than in previous accelerators. In this pa- per, measurements of jets and photons at ATLAS are pre- sented. These measurements are one of the tools that can be used at LHC to increase the understanding of QCD in- teractions. In particular, they allow the testing of pQCD down to the shortest accessible distances. Their sensitivity to the proton parton distribution functions (PDFs) make them adequate to measure the strong coupling constant s and to the improvement of the determination of the PDFs. Photons are colorless probes to QCD and can be used to constrain the proton PDFs. Di-photons processes are in ad- dition an important background for the identification of the Higgs boson in the H ! channel. Comparison of data and next-to-leading-order (NLO) and next-to-next-leading- order (NNLO) QCD theoretical calculations are the main focus of this paper.

Heavy Flavour Hadron Spectroscopy: Challenges and Future Prospects

During the last few years, wealth of new experimental results in the heavy flavor hadron sector has become available. The diversity, quantity and accuracy of the data are impressive and include many surprising spectroscopic results. Following the discovery of ηb states and the excited ηc(2S) states many excited states of open charm and beauty mesons and plethora of exotic states are reported. Discoveries of many new hadrons at B-factories have shed light on a new class of hadrons beyond the ordinary mesons. Many of these states awaits for its right identification. Though we have the theory (QCD) for the strong interaction, we are still far from extracting the major part of the hadron properties from it. These properties at the hadronic scale obviously play relevant role in many searches for new physics and new phenomenon. For obvious reasons, heavy flavour sector offers unique opportunities in this case. For example, the quarkonium systems are crucially important to improve our understanding of QCD as it falls in the low energy region where the non-perturbative effects dominate. Thus the heavy quarks / quark-antiquark bound states are ideal laboratory where our understanding of non-perturbative QCD and its interplay with perturbative QCD may be tested. A comparative review of different model predictions for example in the case of heavy flavor hadronic systems will be highlighted. The quarkonium studies may be used as a benchmark for our understanding of QCD and for the precise determination of standard model strong interaction parameters such as the constituent quark masses, αs, the confinement strength (string tension) etc.

A RICH detector for hadron identification at JLab

The "standard" Hall A apparatus at Jefferson Lab (TOF and aerogel threshold Cherenkov detectors) does not provide complete identification for proton, kaon and pion. To this aim, a proximity focusing C(6)F(14)/CsI RICH (Ring Image CHerenkov) detector has been designed, built, tested and operated to separate kaons from pions with a pion contamination of a few percent up to 2.4GeV/c. Two quite different experimental investigations have benefitted of the RICH identification: on one side, the high-resolution hypernuclear spectroscopy series of experiments on carbon, beryllium and oxygen, devoted to the study of the lambda-nucleon potential. On the other side, the measurements of the single spin asymmetries of pion and kaon on a transversely polarized (3)He target are of utmost interest in understanding QCD dynamics in the nucleon. We present the technical features of such a RICH detector and comment on the presently achieved performance in hadron identification.

A quasi-real photon tagger for CLAS12

The planned energy upgrade of the Jefferson Lab electron beam to 12 GeV will enable a host of different physics topics to be explored. The CLAS detector will undergo a corresponding upgrade to focus the detector systems in the forward angle region. As part of this CLAS12 project, a forward “quasi-real” photon tagger is being developed. This device will measure the scattered electrons with kinematics consistent with low Q 2 virtual photons. Such a system will enable a full reconstruction of the photoproduction of many poorly understood hadronic states, for example hybrid meson, cascade and Ω − states. Investigation of these states will provide a much needed examination of our understanding of QCD in the nonpeturbative regime.

Visualization as a tool for understanding QCD evolution algorithms

In this paper we present a project of visualizing topological charge in Lattice QCD in order to understand its evolution under the Markov Chain Monte Carlo algorithms and how to maximize its equilibration. Lattice QCD is a very computationally expensive technology for computing properties of composite particles from a few fundamental parameters such as quark masses. Our research plays an important role in validating these computations by showing that the autocorrelation length is small and it can eventually lead to even more efficient algorithms.

AdS/CFT aspects of the cosmological QCD phase transition

Recently, deeper understanding of QCD emerges from the study of the AdS/CFT correspondence. New results include the properties of quark-gluon plasma and the confinement/deconfinement phase transition, which are both very important for the scenario of the QCD phase transition in the early universe. In this paper, we study some aspects of how the new results may affect the old calculations of the cosmological QCD phase transition, which are mainly based on the studies of perturbative QCD, lattice QCD, and the MIT bag model.

Heavy-ion physics prospects with the ATLAS detector at the LHC

The next great energy frontier in relativistic heavy-ion collisions is quickly approaching with the completion of the large hadron collider and the ATLAS experiment is poised to make important contributions in understanding QCD matter in extreme conditions. While designed for high-pT measurements in high-energy p+p collisions, the detector is well suited to study many aspects of heavy-ion collisions from bulk phenomena to high-pT and heavy-flavor physics. With its large and finely segmented electromagnetic and hadronic calorimeters, the ATLAS detector excels in measurements of photons and jets, observables of great interest at the LHC. In this paper, we highlight the performance of the ATLAS detector for Pb+Pb collisions at the LHC with special emphasis on a key feature of the ATLAS physics program: jet and direct photon measurements.