Quantum Chromodynamics Processes Research Articles

Scattering wave packets of hadrons in gauge theories: Preparation on a quantum computer

Quantum simulation holds promise of enabling a complete description of high-energy scattering processes rooted in gauge theories of the Standard Model. A first step in such simulations is preparation of interacting hadronic wave packets. To create the wave packets, one typically resorts to adiabatic evolution to bridge between wave packets in the free theory and those in the interacting theory, rendering the simulation resource intensive. In this work, we construct a wave-packet creation operator directly in the interacting theory to circumvent adiabatic evolution, taking advantage of resource-efficient schemes for ground-state preparation, such as variational quantum eigensolvers. By means of an ansatz for bound mesonic excitations in confining gauge theories, which is subsequently optimized using classical or quantum methods, we show that interacting mesonic wave packets can be created efficiently and accurately using digital quantum algorithms that we develop. Specifically, we obtain high-fidelity mesonic wave packets in the Z2 and U(1) lattice gauge theories coupled to fermionic matter in 1+1 dimensions. Our method is applicable to both perturbative and non-perturbative regimes of couplings. The wave-packet creation circuit for the case of the Z2 lattice gauge theory is built and implemented on the Quantinuum H1-1 trapped-ion quantum computer using 13 qubits and up to 308 entangling gates. The fidelities agree well with classical benchmark calculations after employing a simple symmetry-based noise-mitigation technique. This work serves as a step toward quantum computing scattering processes in quantum chromodynamics.

An overview of new measurements of flow, chirality and vorticity from STAR experiment

In relativistic heavy-ion collisions, the properties of quark–gluon plasma (QGP) and complex dynamics of multi-scale processes in Quantum Chromodynamics (QCD) are studied by analyzing the final state produced particles in a variety of different ways. In these proceedings, we present an overview of new detailed measurements of flow, chirality and vorticity by the STAR experiment at Relativistic Heavy Ion Collider (RHIC). Furthermore, STAR’s future opportunities for the precision measurements on small systems, fixed-target (FXT) mode, and Beam Energy Scan (BES-II) program are discussed.

Recent Jet Measurements in ALICE

The precise measurement of jets in pp collisions plays a critical role in the study of quantum chromodynamics (QCD). Measurements of jets in relativistic heavy-ion collisions are additionally useful for probing QCD matter in the regime of high densities and temperatures since the partons in the jet evolution interact strongly with the medium. This phenomenon is called jet quenching and is characterized by both jet energy loss and modification of the internal structure of the jet. The ALICE experiment is well suited for studies of jets and their corresponding internal structure due to the high precision tracking system. These proceedings report on an overview of recent ALICE jet results in both pp and Pb–Pb collisions. Results in pp collisions include measurements of generalized angularities of groomed and inclusive jets, a measurement of the Lund jet plane, and a measurement of jet axis differences. Results from Pb–Pb collisions include measurements of jet splittings, studies of subjet fragmentation, as well as a measurement of the jet nuclear modification factor made with machine learning techniques. These new results, when combined with the corresponding theoretical comparisons, provide information regarding the QCD processes of the parton shower and hadronization, as well as the underlying physics of jet quenching.

Quantum information with top quarks in QCD

Top quarks represent unique high-energy systems since their spin correlations can be measured, thus allowing to study fundamental aspects of quantum mechanics with qubits at high-energy colliders. We present here the general framework of the quantum state of a top-antitop (tt¯) quark pair produced through quantum chromodynamics (QCD) in a high-energy collider. We argue that, in general, the total quantum state that can be probed in a collider is given in terms of the production spin density matrix, which necessarily gives rise to a mixed state. We compute the quantum state of a tt¯ pair produced from the most elementary QCD processes, finding the presence of entanglement and CHSH violation in different regions of phase space. We show that any realistic hadronic production of a tt¯ pair is a statistical mixture of these elementary QCD processes. We focus on the experimentally relevant cases of proton-proton and proton-antiproton collisions, performed at the LHC and the Tevatron, analyzing the dependence of the quantum state with the energy of the collisions. We provide experimental observables for entanglement and CHSH-violation signatures. At the LHC, these signatures are given by the measurement of a single observable, which in the case of entanglement represents the violation of a Cauchy-Schwarz inequality. We extend the validity of the quantum tomography protocol for the tt¯ pair proposed in the literature to more general quantum states, and for any production mechanism. Finally, we argue that a CHSH violation measured in a collider is only a weak form of violation of Bell's theorem, necessarily containing a number of loopholes.

Parton physics from a heavy-quark operator product expansion: Lattice QCD calculation of the second moment of the pion distribution amplitude

The pion light-cone distribution amplitude (LCDA) is a central non-perturbative object of interest for high-energy exclusive processes in quantum chromodynamics. In this article, the second Mellin moment of the pion LCDA is determined as a proof-of-concept calculation for the first numerical implementation of the heavy-quark operator product expansion (HOPE) method. The resulting value for the second Mellin moment, determined in quenched QCD at a pion mass of $m_\pi=550$ MeV at a factorization scale of 2 GeV is $ \langle \xi^2 \rangle = 0.210 \pm 0.013\text{ (stat.)} \pm 0.034\text{ (sys.)}$. This result is compatible with those from previous determinations of this quantity.

QCD factorization and quantum mechanics.

It is unusual to find quantum chromodynamics (QCD) factorization explained in the language of quantum information science. However, we will discuss how the issue of factorization and its breaking in high-energy QCD processes relates to phenomena like decoherence and entanglement. We will elaborate with several examples and explain them in terms familiar from basic quantum mechanics and quantum information science. This article is part of the theme issue 'Quantum technologies in particle physics'.

Tag Z boson jets via convolutional neural networks

The jet tagging task in high-energy physics is to distinguish signals of interest from the background, which is of great importance for the discovery of new particles, or new processes, at the large hadron collider. The energy deposition generated in the calorimeter can be seen as a kind of picture. Based on this notion, tagging jets initiated by different processes becomes a classic image classification task in the computer vision field. We use jet images as the input built on high dimensional low-level information, energy-momentum four-vectors, to explore the potential of convolutional neural networks (CNNs). Four models of different depths are designed to make the best underlying useful features of jet images. Traditional multivariable method, boosted decision tree (BDT), is used as a baseline to determine the performance of networks. We introduce four observable quantities into BDTs: the mass, transverse momenta of fat jets, the distance between the leading and subleading jets, and N-subjettiness. Different tree numbers are adopted to build three kinds of BDTs, which is intended to have variable classifying abilities. After training and testing, the results show that the CNN 3 is the neatest and most efficient network under the design of stacking convolutional layers. Deepening the model could improve the performance to a certain extent but it is unable to work all the time. The performances of all BDTs are almost the same, which is possibly due to a small number of input observable types. The performance metrics show that the CNNs outperform the BDTs: the background rejection efficiency increases up to 150% at 50% signal efficiency. Besides, after inspecting the best and the worst samples, we conclude the characteristics of jets initiated by different processes: jets obtained by Z boson decays tend to concentrate in the center of jet images or have a clear differentiable substructure; the substructures of jets from general quantum chromodynamics processes have more random forms and not only just have two subjets. As the final step, the confusion matrix of the CNN 3 indicate that it comes to be kind of conservative. Exploring the way of keeping the balance between conservative and radical is our goal in the future work.

Factorisation and Local Subtraction of Infrared Divergences for QCD processes

We review a recently proposed subtraction procedure at next-to- and next-to-next- to-leading order in quantum chromodynamics (QCD), based on the introduction of a set of minimal, local and analytically integrable counterterms. We also provide an operator definition for the counterterms by investigating the factorisation properties of a generic massless virtual amplitude. Our local, analytic scheme is designed for massless final state radiation only, but its main features are extensible to initial state radiation.

A New Research about Pion Parton Distribution Function

A new fit of the pion parton distribution functions is provided. Only valence quark distributions are used at a low evolution scale and are evolved with the modified Dokshitzer-Gribov-Lipatov-Altarelli-Parisi equation which is briefly introduced in this work, and the sea quark and gluon distributions are only generated by the quantum chromodynamics processes. We find that the parton distributions can explain the pion-nucleon experiments data well, and it can also be compared with the data from the leading neutron data of experiments at HERA. The momentum distributions among the partons are discussed and our results are consistent with some models.

High-energy limit of quantum electrodynamics beyond Sudakov approximation

We study the high-energy behavior of the scattering amplitudes in quantum electrodynamics beyond the leading order of the small electron mass expansion in the leading logarithmic approximation. In contrast to the Sudakov logarithms, the mass-suppressed double-logarithmic radiative corrections are induced by a soft electron pair exchange and result in enhancement of the power-suppressed contribution, which dominates the amplitudes at extremely high energies. Possible applications of our result to the analysis of the high-energy processes in quantum chromodynamics is also discussed.

Recent results from the Tevatron

Four years after the shutdown of the Tevatron proton-antiproton collider, the two Tevatron experiments, CDF and DZero, continue producing important results that test the theory of the strong interaction, Quantum Chromodynamics (QCD). The experiments exploit the advantages of the data sample acquired during the Tevatron Run II, stemming from the unique pp initial state, the clean environment at the relatively low Tevatron instantaneous luminosities, and the good understanding of the data sample after many years of calibrations and optimizations. A summary of results using the full integrated luminosity is presented, focusing on measurements of prompt photon production, weak boson production associated with jets, and non-perturbative QCD processes.

Glueball spectrum and hadronic processes in low-energy QCD

Low-energy limit of quantum chromodynamics (QCD) is obtained using a mapping theorem recently proved. This theorem states that, classically, solutions of a massless quartic scalar field theory are approximate solutions of Yang-Mills equations in the limit of the gauge coupling going to infinity. Low-energy QCD is described by a Yukawa theory further reducible to a Nambu-Jona-Lasinio model. At the leading order one can compute glue-quark interactions and one is able to calculate the properties of the $\sigma$ and $\eta-\eta'$ mesons. Finally, it is seen that all the physics of strong interactions, both in the infrared and ultraviolet limit, is described by a single constant $\Lambda$ arising in the ultraviolet by dimensional transmutation and in the infrared as an integration constant.

Dynamical gluon mass in QCD processes

We perform phenomenological applications of modified gluon propagators and running coupling constants in scattering processes in Quantum Chromodynamics (QCD). The modified forms of propagators and running coupling constant are obtained by non-perturbative methods. The processes investigated includes the diffractive ones - proton-proton elastic scattering, light vector meson photo-production and double vector meson production in gamma-gamma scattering - as well as the pion and kaon meson form factors. The results are compared with experimental data (if available), showing a good agreement with a gluon with dynamical mass but do not indicate the correct gluon propagator functional form.

Monte Carlo Approach to Radiative Processes in e+e- Annihilation

We present a new approach to radiative processes in e+ e- annihilation based on a Monte Carlo method. This is nothing but the QED version of the parton shower model studied extensively in QCD. It is applicable to both initial and final state radiation but without the interference between these two. To examine the validity of the model, we compare it with the results of the exact calculations up to OCa) and OCa 2 ). Comparison of several distributions shows that the model can give reliable results for practical use. Study of radiative corrections in e+ e- collisions has a long history!) since its importance was recognized in the discovery of ]/CP.2) Until now, most efforts have concentrated on the calculation of O(a) corrections to weak interactions. Precise measurements of properties of the ZO boson, however, required the detailed study of higher order QED corrections. Techniques of calculating some contributions in all order have been developed, such as soft photon summation 3 ) and collinear photon summation. 4 ) Generally speaking, these are methods suitable only for inclusive distributions, and they can hardly be applied to exclusive processes or event genera­ tions. It is obvious that the collinear photon summation has an exact correspondence with the collinear parton summation based on the Altarelli-Parisi equation in pertur­ bative quantum chromodynamics (QCD). The parton shower model was originally proposed to deal with exclusive processes in QCD. 5 ) In this paper we apply this technique to QED and examine its validity for radiative processes. We shall call this new model QED parton shower (QEDPS). Although the development of the model is not difficult in the technical sense, study of its validity is a nontrivial task because a much higher accuracy of calculation is required than in QCD. Recently one of the authors calculated exact O(a 2 ) QED corrections to the initial state of e+ e- annihila­ tion. 6 ) With the help of this calculation, we are able to test the validity of QEDPS in a systematic and quantitative way. The purposes of this paper are to present the formulation of QEDPS in a Monte Carlo method and to show that it is really a practical method to describe radiative processes. Since the parton shower model is applied to the final q, q and gluon system in e+ e- annihilation, QEDPS can also be used to estimate QED radiative corrections to the charged particles in the final state. Thus it is equally applicable to both initial and final state radiation, though the interference between these two states cannot be included at present. In the next section we will give the algorithm of QEDPS. In § 3, comparison will

AsymptoticQ2for Exclusive Processes in Quantum Chromodynamics

It is found that at available ${Q}^{2}$ the calculable perturbative contributions to the pion electric form factor ${F}_{\ensuremath{\pi}}({Q}^{2})$ and the nucleon magnetic form factors ${G}_{M}^{N}({Q}^{2})$ are much smaller than the data, which can probably be explained by soft contributions. Both hard and soft effects are estimated from light-cone/infinite-momentum-frame wave functions suggested by quark models, but the main conclusions have a more general validity.

Application of the principle of minimal sensitivity to higher-order processes in quantum chromodynamics

We present the renormalization approach based on Stevenson's principle of minimal sensitivity (PMS) in a graphical form from which the PMS result can be read off for any next-to-leading-order calculation as input. Applications of this approach are made to several processes not previously investigated and a comparison between the PMS approach and other renormalization schemes is made. The PMS approach, when compared to data, yields a strong-interaction scale parameter ..lambda../sub M/S, which is significantly smaller than the one obtained using the modified minimal-subtraction scheme for the processes considered. For these processes, however, the PMS approach is practically identical to the one based on the requirement of fastest apparent convergence.

Higher-twist term in inclusive pion production at large transverse momentum

We calculate the absolute rate and kinematic dependence of the cross sections for the high-twist subprocesses $\mathrm{qG}\ensuremath{\rightarrow}\ensuremath{\pi}q$ and $q\overline{q}\ensuremath{\rightarrow}\ensuremath{\pi}G$ in perturbative quantum chromodynamics (QCD). These cross sections are used to estimate the high-twist contribution to large-transverse-momentum inclusive single-pion production in hadronic collisions. We compare the normalization of these terms with leading-order minimum-twist QCD processes such as $\mathrm{qG}\ensuremath{\rightarrow}\mathrm{qG}$. The high-twist contributions are shown to be quantitatively important for ${p}_{T}\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{l}6$ ${\mathrm{GeV}}^{2}$ and ${x}_{T}\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{g}0.5$, and they should be included in detailed analyses of data. We discuss cross sections and charge ratios, and we comment briefly about inclusive spin-correlation measurements at large ${p}_{T}$.

Transverse momenta of hadronically produced heavy-quark systems: 2→3 processes in quantum chromodynamics

Transverse-momentum spectra are calculated for heavy quarkonium states $Q\overline{Q}$ ($\frac{J}{\ensuremath{\psi}}$,$\ensuremath{\Upsilon}$) produced in $\mathrm{pp}$ and $\overline{p}p$ collisions. These transverse momenta result from the hard quark ($q$) and gluon ($g$) subprocesses $q\overline{q}\ensuremath{\rightarrow}Q\overline{Q}g$, $\mathrm{gq}\ensuremath{\rightarrow}Q\overline{Q}q$, and $\mathrm{gg}\ensuremath{\rightarrow}Q\overline{Q}g$. The ${p}_{T}$ distributions for $\frac{J}{\ensuremath{\psi}}$ production are expected to be significantly steeper than for the ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ continuum, whereas the two distributions become similar in the $\ensuremath{\Upsilon}$ mass region. These effects are partly confirmed by recent CERN ISR measurements. Predictions for $\overline{p}p$ collisions at $\sqrt{s}=540$ GeV are also given.

Exclusive Processes in Quantum Chromodynamics: The Form Factors of Baryons at Large Momentum Transfer

Exclusive Processes in Quantum Chromodynamics: The Form Factors of Baryons at Large Momentum Transfer

Exclusive processes in quantum chromodynamics: Evolution equations for hadronic wavefunctions and the form factors of mesons

The predictions of quantum chromodynamics for meson form factors at large momentum transfer are given. Evolution equations are derived which determine the structure of hadronic wavefunctions at short distances from their form at large distances. The eigenvalues of the evolution equations appear as exponents in anomalous logarithm corrections to the nominal power law of form factors determined by dimensional counting. The results lead to detailed tests of the spin and scaling structure of QCD at short distances. The predictions for the charged pion, kaon and rho form factors and the γ → π 0 transition form factor of the photon are absolutely normalized at asymptotic momentum transfer.