QCD Physics Research Articles

Beta functions of 2D adjoint QCD

We discuss the long-distance physics of 2D adjoint QCD when it is viewed as an effective field theory and determine the β functions for its two classically marginal four-Fermi operators. These four-fermion terms preserve the invertible symmetries of the kinetic terms, and they have important implications at long distances if they are generated at short distances. Our results are likely to be important for future numerical lattice Monte Carlo studies of 2D adjoint QCD. Published by the American Physical Society 2024

STAR forward detector upgrade status and performance

An upgrade to the STAR detector system at forward rapidities has been completed before the RHIC RUN 2022. It consists of the Forward Tracking System (FTS) and the Forward Calorimeter System (FCS). The Forward Tracking System is composed of a Silicon Tracker and a small-strip Thin Gap Chamber Tracker. The Forward Calorimeter System contains an Electromagnetic Calorimeter and a Hadronic Calorimeter. These systems cover the pseudorapidity region of 2.5 < η < 4, providing detection capabilities for neutral pions, photons, electrons, jets, and charged hadrons. This enables the STAR experiment to study cold QCD physics at very high and low regions of Bjorken x and to explore the longitudinal structure of the initial state in relativistic heavyion collisions by measuring the decorrelations at large η. This proceeding will introduce the STAR forward upgrade, its current status, and its performance during the STAR Run22.

Impact of Multiple Phase Transitions in Dense QCD on Compact Stars

This review covers several recent developments in the physics of dense QCD with an emphasis on the impact of multiple phase transitions on astrophysical manifestations of compact stars. To motivate the multi-phase modeling of dense QCD and delineate the perspectives, we start with a discussion of the structure of its phase diagram and the arrangement of possible color-superconducting and other phases. It is conjectured that pair-correlated quark matter in β-equilibrium is within the same universality class as spin-imbalanced cold atoms and the isospin asymmetrical nucleonic matter. This then implies the emergence of phases with broken space symmetries and tri-critical (Lifshitz) points. The beyond-mean-field structure of the quark propagator and its non-trivial implications are discussed in the cases of two- and three-flavor quark matter within the Eliashberg theory, which takes into account the frequency dependence (retardation) of the gap function. We then construct an equation of state (EoS) that extends the two-phase EoS of dense quark matter within the constant speed of sound parameterization by adding a conformal fluid with a speed of sound cconf.=1/3 at densities ≥10nsat, where nsat is the saturation density. With this input, we construct static, spherically symmetrical compact hybrid stars in the mass–radius diagram, recover such features as the twins and triplets, and show that the transition to conformal fluid leads to the spiraling-in of the tracks in this diagram. Stars on the spirals are classically unstable with respect to the radial oscillations but can be stabilized if the conversion timescale between quark and nucleonic phases at their interface is larger than the oscillation period. Finally, we review the impact of a transition from high-temperature gapped to low-temperature gapless two-flavor phase on the thermal evolution of hybrid stars.

Signatures of a High Temperature QCD Transition in the Early Universe.

Beyond-Standard-Model extensions of QCD could result in quark and gluon confinement occurring well above at temperature around the GeV scale. These models can also alter the order of the QCD phase transition. Therefore, the enhanced production of primordial black holes (PBHs) that can accompany the change in relativistic degrees of freedom at the QCD transition could favor the production of PBHs with mass scales smaller than the Standard Model QCD horizon scale. Consequently, and unlike PBHs associated with a standard GeV-scale QCD transition, such PBHs can account for all the dark matter abundance in the unconstrained asteroid-mass window. This links beyond-Standard-Model modifications of QCD physics over a broad range of unexplored temperature regimes (around 10-10^{3} TeV) with microlensing surveys searching for PBHs. Additionally, we discuss implications of these models for gravitational wave experiments. We show that a first-order QCD phase transition at around 7TeV is consistent with the Subaru Hyper-Suprime Cam candidate event, while a transition of around 70GeV is consistent with OGLE candidate events and could also account for the claimed NANOGrav gravitational wave signal.

Vector meson-nucleon scattering length $$|\alpha _{VN}|$$ and trace anomalous energy contribution to the nucleon mass $$T_{A}$$

Low-energy scattering processes of vector meson and nucleon are an important window for studying non-perturbative QCD. The interaction of vector meson with nucleon, vector meson-nucleon scattering length $|\alpha_{VN}|$, is an important component of the study of hadronic interactions. Nowadays many scattering length values $|\alpha_{VN}|$ have been reported using the recent photoproduction experiment data or quasi data. In addition, the study of trace anomalous energy contribution to the proton mass is also a hot topic in non-perturbative QCD and hadron physics. However, it is difficult to measure proton trace anomalous energy experimentally, and the study of the trace anomaly of proton is still inconclusive. In this study, we established the relationship between the scattering length of the vector meson-proton $|\alpha_{Vp}|$ and the trace anomaly contribution of the proton mass $T_{A}$. With the scattering length values extracted by using the Vector Meson Dominance model, we obtained the trace anomaly contribution of the proton mass $T_{A}$ = (22.8$\%$ $\pm$ 1.2$\%$), which is of similar order of magnitude as the 23$\%$ given by Lattice QCD calculation. We conjecture that the trace anomaly contribution of nucleon is independent of the type of vector meson probe. We hope that high precision measurements of vector meson-nucleon scattering length could give us a better chance to explore the origin of the nucleon mass.

Determination of alpha _s(m_Z) by the non-perturbative decoupling method

We present the details and first results of a new strategy for the determination of alpha _s(m_Z) (ALPHA Collaboration et al. in Phys. Lett. B 807:135571, 2020). By simultaneously decoupling 3 fictitious heavy quarks we establish a relation between the Lambda -parameters of three-flavor QCD and pure gauge theory. Very precise recent results in the pure gauge theory (Dalla Brida and Ramos in Eur. Phys. J. C 79(8):720, 2019; Nada and Ramos in Eur Phys J C 81(1):1, 2021) can thus be leveraged to obtain the three-flavour Lambda -parameter in units of a common decoupling scale. Connecting this scale to hadronic physics in 3-flavour QCD leads to our result in physical units, Lambda ^{(3)}_{overline{textrm{MS}}} = 336(12), {textrm{MeV}}, which translates to alpha _s(m_Z) = 0.11823(84). This is compatible with both the FLAG average (Aoki et al. in FLAG review 2021. arXiv:2111.09849 [hep-lat]) and the previous ALPHA result (ALPHA Collaboration et al., Phys. Rev. Lett. 119(10):102001, 2017), with a comparable, yet still statistics dominated, error. This constitutes a highly non-trivial check, as the decoupling strategy is conceptually very different from the 3-flavour QCD step-scaling method, and so are their systematic errors. These include the uncertainties of the combined decoupling and continuum limits, which we discuss in some detail. We also quantify the correlation between both results, due to some common elements, such as the scale determination in physical units and the definition of the energy scale where we apply decoupling.

The θ-angle and axion physics of two-color QCD at fixed baryon charge

We analyze the impact of the θ-angle and axion dynamics for two-color (in fact any Sp(2N )) QCD at nonzero baryon charge and as a function of the number of matter fields on the vacuum properties, the pattern of chiral symmetry breaking as well as the spectrum of the theory. We show that the vacuum acquires a rich structure when the underlying CP violating topological operator is added to the theory. We discover novel phases and analyse the order of their transitions characterizing the dynamics of the odd and even number of flavours. We further determine the critical chemical potential as function of the θ angle separating the normal from the superfluid phase of the theory. Our results will guide numerical simulations and novel tests of the model’s dynamics. The results are also expected to better inform phenomenological applications of the model ranging from composite Higgs physics to strongly interacting massive dark matter models featuring number changing interactions. In the companion work [1] we repurpose and upgrade the approach to determine the impact of the θ-angle and axion physics on non-perturbative near conformal dynamics related to the fixed baryon charge sector.

Diquark induced short-range nucleon-nucleon correlations & the EMC effect

Diquark formation across a short-range nucleon-nucleon pair is proposed as the underlying QCD physics of short-range correlations (SRC) in nuclei. SRC pairs have been proposed as the cause of distorted quark behavior in nuclei; experimentally observed quark momentum distribution distortions termed the EMC effect. The strong spatial overlap of SRC pairs brings nucleon constituents within range of inter-nucleon QCD potentials and any bonds formed - such as the diquark bond - affects their distributions. In this SRC model, diquarks form in the 3C⊗3C→3¯C channel of SU(3)C acting on valence quarks from highly overlapping nucleon wavefunctions. The most energetically favorable diquark is a valence u quark from one nucleon with a valence d quark from the other in a spin-0 state bound together via continual single gluon exchange and an attractive quantum chromodynamics short-range potential. Formation of a new scalar isospin-singlet [ud] diquark across a NN pair is proposed as the primary QCD-level theoretical foundation for SRC models of distorted structure functions in A≥3 nuclei. Contributions from the higher mass spin-1 isospin triplet states (ud), (uu) and (dd) are possible, with the spin-1 (ud) diquark proposed as a higher mass but viable structure function distortion mechanism for the spin-1 ground state deuteron. Predictions are made for lepton scattering experiments on H3 and He3 nuclear targets, with implications for the coefficients of the 3-valence quark Fock states in the nucleon wavefunction.

Electroweak physics at LHCb

The Electroweak (EW) Theory is one of the most important and well-known pillars of the Standard Model (SM) of particle physics. Possible signs of New Physics could appear in the EW sector, therefore requiring the SM to be extended. EW physics can be probed at the LHCb experiment, which is a general purpose forward detector that offers unique opportunities to do studies in the forward region. Its excellent vertex and track reconstruction system allows to perform several measurements of perturbative QCD physics in a region unexplored by other experiments. In this paper the latest EW physics analyses performed at LHCb studying pp collisions during Run 1 and Run 2 are presented.

Extraction of proton trace anomaly energy from near-threshold $$\phi $$ and $$J/\psi $$ photo-productions

The trace anomalous energy contribution to the proton mass is a very important topic in non-perturbative QCD and hadron physics. In experiments, it is under the hot discussions on how to measure the trace anomalous energy.The QCD interpretation of proton trace anomaly is still not clear.To connect the theory with the experiment,we extract the trace anomaly by analyzing the near-threshold photo-production data of $\phi$ and J/$\psi$ vector mesons. Based on the vector-meson-dominance model and QCD Van der Waals representation, we find that the percentage of trace anomaly in the proton mass ranges from $16\%$ to $24\%$, which is of similar order of magnitude as the 23$\%$ given by Lattice QCD. We also provide the approximate magnitudes of the systematic uncertainties of the extracted results from the model assumptions as well as the data fitting procedures. We give relative statistical uncertainties of 17.2$\%$, 17.7$\%$, 3.6$\%$, and 8.2$\%$, total relative systematic uncertainties of 21.4$\%$, 54.2$\%$, 37.2$\%$, and 25.7$\%$, for the analyses of the GlueX, LEPS, CLAS, and SAPHIR data, respectively. We argue that the near-threshold $\Upsilon$ photo-production experiments are more beneficial for the measurement of trace anomaly in proton mass in the future.

Dimuons in neutrino telescopes: New predictions and first search in IceCube

Neutrino telescopes allow powerful probes of high-energy astrophysics and particle physics. Their power is increased when they can isolate different event classes, e.g., by flavor, though that is not the only possibility. Here we focus on a new event class for neutrino telescopes: dimuons, two energetic muons from one neutrino interaction. We make new theoretical and observational contributions. For the theoretical part, we calculate dimuon-production cross sections and detection prospects via deep-inelastic scattering (DIS; where we greatly improve upon prior work) and $W$-boson production (WBP; where we present first results). We show that IceCube should have $\simeq 130$ dimuons ($\simeq 6$ from WBP) in its current data and that IceCube-Gen2, with a higher threshold but a larger exposure, could detect $\simeq 620$ dimuons ($\simeq 30$ from WBP) in 10 years. These dimuons are almost all produced by atmospheric neutrinos. For the observational part, we perform a simple but conservative analysis of IceCube public data, finding 19 candidate dimuon events. Subsequent to our paper appearing, visual inspection of these events by the IceCube Collaboration revealed that they are not real dimuons, but instead arise from an internal reconstruction error that identifies some single muons crossing the dust layer as two separate muons. To help IceCube and the broader community with future dimuon searches, we include the updated full details of our analysis. Together, these theoretical and observational contributions help open a valuable new direction for neutrino telescopes, one especially important for probing high-energy QCD and new physics.

Tsallis statistics, fractals and QCD

We study the non-extensive Tsallis statistics and its applications to QCD and high energy physics, and analyze the possible connections of this statistics with a fractal structure of hadrons. Then, we describe how scaling properties of Yang-Mills theories allow the appearance of self-similar structures in gauge fields, which actually behave as fractals. The Tsallis entropic index, q, is deduced in terms of the field theory parameters, resulting in a good agreement with the value obtained experimentally.

Gauge/gravity dual dynamics for the strongly coupled sector of composite Higgs models

A holographic model of chiral symmetry breaking is used to study the dynamics plus the meson and baryon spectrum of the underlying strong dynamics in composite Higgs models. The model is inspired by top-down D-brane constructions. We introduce this model by applying it to Nf = 2 QCD. We compute meson masses, decay constants and the nucleon mass. The spectrum is improved by including higher dimensional operators to reflect the UV physics of QCD. Moving to composite Higgs models, we impose perturbative running for the anomalous dimension of the quark condensate in a variety of theories with varying number of colors and flavours. We compare our results in detail to lattice simulations for the following theories: SU(2) gauge theory with two Dirac fundamentals; Sp(4) gauge theory with fundamental and sextet matter; and SU(4) gauge theory with fundamental and sextet quarks. In each case, the holographic results are encouraging since they are close to lattice results for masses and decay constants. Moreover, our models allow us to compute additional observables not yet computed on the lattice, to relax the quenched approximation and move to the precise fermion content of more realistic composite Higgs models not possible on the lattice. We also provide a new holographic description of the top partners including their masses and structure functions. With the addition of higher dimension operators, we show the top Yukawa coupling can be made of order one, to generate the observed top mass. Finally, we predict the spectrum for the full set of models with top partners proposed by Ferretti and Karateev.

Towards grounding nuclear physics in QCD

Towards grounding nuclear physics in QCD

Theory for quarkonium: from NRQCD factorization to soft gluon factorization * *Supported by the National Natural Science Foundation of China (11875071, 11975029) and the China Postdoctoral Science Foundation (2018M631234)

We demonstrate that the recently proposed soft gluon factorization (SGF) is equivalent to the nonrelativistic QCD (NRQCD) factorization for heavy quarkonium production or decay, which means that, for any given process, these two factorization theories are either both valid or both violated. We use two methods to arrive at this conclusion. In the first method, we apply the two factorization theories to the physical process . Our explicit calculation shows that both SGF and NRQCD can correctly reproduce the low energy physics of full QCD, and the two factorizations are thus equivalent. In the second method, by using equations of motion, we successfully deduce SGF from NRQCD effective field theory. By identifying SGF with NRQCD factorization, we establish relations between the two factorization theories and prove the generalized Gremm-Kapustin relation as a byproduct. Compared with the NRQCD factorization, the advantage of SGF is that it resums the series of relativistic corrections originating from kinematic effects to all powers, yielding better convergence of the relativistic expansion.

WITHDRAWN: Towards grounding nuclear physics in QCD

Abstract Exascale computing could soon enable a predictive theory of nuclear structure and reactions rooted in the Standard Model, with quantifiable and systematically improvable uncertainties. Such a predictive theory will help exploit experiments that use nucleons and nuclei as laboratories for testing the Standard Model and its limitations. Examples include direct dark matter detection, neutrinoless double beta decay, and searches for permanent electric dipole moments of the neutron and atoms. It will also help connect QCD to the properties of cold neutron stars and hot supernova cores. We discuss how a quantitative bridge between QCD and the properties of nuclei and nuclear matter will require a synthesis of lattice QCD (especially as applied to two- and three-nucleon interactions), effective field theory, and ab initio methods for solving the nuclear many-body problem. While there are significant challenges that must be addressed in developing this triad of theoretical tools, the rapid advance of computing is accelerating progress. In particular, we focus this review on the anticipated advances from lattice QCD and how these advances will impact few-body effective theories of nuclear physics by providing critical input, such as constraints on unknown low-energy constants of the effective (field) theories. We also review particular challenges that must be overcome for the successful application of lattice QCD for low-energy nuclear physics. We describe progress in developing few-body effective (field) theories of nuclear physics, with an emphasis on HOBET, a non-relativistic effective theory of nuclear physics, which is less common in the literature. We use the examples of neutrinoless double beta decay and the nuclear-matter equation of state to illustrate how the coupling of lattice QCD to effective theory might impact our understanding of symmetries and exotic astrophysical environments.

The STAR Detector Upgrades for the BES-II and at Forward Rapidity

The Beam Energy Scan Phase II program (BES-II) at the Relativistic Heavy Ion Collider (RHIC) is examining the center-of-mass collision energy region from 7.7 GeV to 19.6 GeV which was determined from the results of BES-I. Key measurements such as the net proton kurtosis, the directed flow and the dilepton production are possible during BES-II with an order of magnitude better statistics due to the Low Energy RHIC electron Cooling (LEReC) upgrade and several STAR detector upgrades. Beyond BES-II, the STAR Collaboration is currently designing, constructing, and installing a suite of new detectors in the forward rapidity region (2.5 < η < 4), enabling a program of novel measurements in pp, pA and AA collisions. To fully explore this physics, the forward rapidity region needs superior detection capability for neutral pions, photons, electrons, jets and leading hadrons by adding charged particle tracking and electromagnetic and hadronic calorimetry to STAR's capabilities at forward rapidity. In this proceedings, we will present the details on the STAR detector upgrades for the BES-II and at forward rapidity. The Cold QCD physics opportunities will also be discussed.

N* Experiments and what they tell us about Strong QCD Physics

I give an overview on experimental studies of the spectrum and the structure of the excited states of the nucleon and what we can learn about their in ternal structure. One focus is on the efforts to obtain a more complete picture of the light-quark baryon exci tation spectrum employing electromagnetic beams that will allow us to draw some conclusions on the symme tries underlying the spectrum. For the higher mass ex citations, the full employment of coupled channel ap proaches is essential when searching for new excited states in the large amounts of data already accumulated in different channels involving a variety of polarization observables. The other focus is on the study of transition form factors and helicity amplitudes and their de pendences on Q2, especially on some of the more promi nent resonances, especially Δ(1232)3/2+, N(1440)1/2+, and negative parity states N(1535)1/2-, and N(1675)5/2-.These were obtained in pion and eta electroproduction experi ments off proton targets and have already led to further insights in the active degrees-of-freedom as a function of the distance scale involved.

Holographic QCD in the Veneziano limit and neutron stars

We use the holographic V-QCD models to analyse the physics of dense QCD and neutron stars. Accommodating lattice results for thermodynamics of QCD enables us to make generic predictions for the Equation of State (EoS) of the quark matter phase in the cold and dense regime. We demonstrate that the resulting pressure in V-QCD matches well with a family of neutron-star-matter EoSs that interpolate between state-of-the-art theoretical results for low and high density QCD. After implementing the astrophysical constraints, i.e., the largest known neutron star mass and the recent LIGO/Virgo results for the tidal deformability, we analyse the phase transition between the baryonic and quark matter phases. We find that the baryon density nB at the transition is at least 2.9 times the nuclear saturation density ns. The transition is of strongly first order at low and intermediate densities, i.e., for nB/ns ≲ 7.5.

The STAR BES-II and Forward Rapidity Physics and Upgrades

The second phase of the Beam Energy Scan at RHIC, BES-II, is scheduled to start in 2019 and will explore the high baryon density region of the QCD phase diagram with precision measurements. The detector upgrades will enhance some of the key measurements by extending STAR's kinematic reach, like the kurtosis of net-protons distribution which could pinpoint the position of a critical point. The upgrades currently under way comprise: the replacement of the inner TPC sectors, the Event Plane Detector (EPD) and the end-cap TOF. Building on these upgrades STAR is planning to further enhance its detector capabilities by installing a Forward Calorimeter System (FCS) integrating an electromagnetic and hadronic calorimeter and a Forward Tracking System (FTS) combining 3 Silicon mini-strip disks and 4 Small-Strip Thin Gap Chamber (sTGC) wheels. The forward upgrades are motivated by studying the initial state of nucleons and nuclei and the exploration of cold QCD physics in the very high and low x regions. This contribution will highlight these upgrades and some of the physics opportunities that they allow.