Antiquark Annihilation Research Articles

Study of double J∕ψ production mechanisms at COMPASS

During the past 40 years, the production of pairs of [Formula: see text] mesons in high-energy hadron collisions has been studied by several experiments. Despite the experimental and theoretical efforts, the origin of the process and the relative weight of different production mechanisms remain unknown. Depending on the energy scale, the double [Formula: see text] production can be described by single- and/or double-parton scattering sub-processes and gluon–gluon fusion or quark–antiquark annihilation mechanisms. The process can also be related to the hypothesis of the intrinsic charm of hadrons and the existence of exotic tetraquark states which were predicted by various theoretical models and have recently been observed by the LHCb experiment. To study dimuon pair production, the COMPASS experiment at CERN uses a 190[Formula: see text]GeV/[Formula: see text] negative pion beam impinging on different nuclear targets. First preliminary COMPASS results on [Formula: see text] pair production will be presented. The search for possible signals from exotic resonances and the study of double [Formula: see text] production mechanisms will be discussed.

Virtual photon polarization and dilepton anisotropy in relativistic nucleus–nucleus collisions

The polarization of virtual photons produced in relativistic nucleus–nucleus collisions provides information on the conditions in the emitting medium. In a hydrodynamic framework, the resulting angular anisotropy of the dilepton final state depends on the flow as well as on the transverse momentum and invariant mass of the photon. We illustrate these effects in dilepton production from quark–antiquark annihilation in the QGP phase and π+π− annihilation in the hadronic phase for a static medium in global equilibrium and for a longitudinally expanding system.

Interpreting 750 GeV diphoton excess with R-parity violating supersymmetry

We propose a supersymmetric explanation of the diphoton excess in the Minimal Supersymmetric Standard Model (MSSM) with the leptonic R-parity violation. In our model, sneutrino serves as the 750 GeV resonance and produced through quark–antiquark annihilation. With introducing appropriate trilinear soft parameters, we show that the diphoton branching ratio is significantly enhanced compared with the conventional MSSM. For current dijet and W-pair LHC constraints, we can successfully fit the observed diphoton signal rate in sizeable parameter regions, the resulting parameter space strongly favor the masses of light smuon and stau within the range from 375–500 GeV, which depends on the choice of electroweakino masses and soft trilinear terms. While after taking into account the compatibility of diphoton excess between the 8 TeV and 13 TeV LHC, only the coupling involved with the second generation quarks is survived. In this case, the corresponding parameter space favors a narrow mass range of smuon and stau with [Formula: see text]. Even if the 750 GeV diphoton excesses were not confirmed by the ATLAS and CMS experiments, we point out that our proposal can still be used to explain the current and future tentative diphoton excesses.

Charged Higgs pair production at the LHC as a probe of the top-seesaw assisted technicolor model

The top-seesaw assisted technicolor (TC) model, which was proposed recently to accommodate the 126 GeV Higgs mass discovered by the Large Hadron Colliders (LHC), predicts light and heavy charged Higgs bosons in addition to the neutral Higgses. In this paper, we will study the pair productions of the charged Higgs, proceeding through gluon–gluon fusion and quark–antiquark annihilation, at the LHC in the frame of the top-seesaw assisted TC model. We find that in a large part of parameter space the production cross-sections of the light charged Higgs pair at the LHC can be quite large compared with the low standard model backgrounds, while it is impossible for the pair production of the heavy ones to be detected with the strong final mass suppression. Therefore, the light charged Higgs pair production may be served as a probe of this new TC model at the LHC.

Photon production from gluon-mediated quark–anti-quark annihilation at confinement

Heavy ion collisions at the BNL Relativistic Heavy Ion Collider produce direct photons at low transverse momentum ${p}_{T}$ from 1--3 $\mathrm{GeV}/c$, in excess of the $p+p$ spectra scaled by the nuclear overlap factor ${T}_{AA}$. These low-${p}_{T}$ photons have a large azimuthal anisotropy ${v}_{2}$. Theoretical models, including hydrodynamic models, struggle to quantitatively reproduce the large low-${p}_{T}$ direct photon excess and ${v}_{2}$ in a self-consistent manner. This paper presents a description of the low-${p}_{T}$ photon flow as the result of increased photon production from soft-gluon-mediated $q\ensuremath{-}\overline{q}$ interactions as the system becomes color neutral. This production mechanism will generate photons that follow constituent quark number, ${n}_{q}$, scaling of ${v}_{2}$ with an ${n}_{q}$ value of 2 for direct photons. ${\ensuremath{\chi}}^{2}$ comparisons of the published PHENIX direct photon and identified particle ${v}_{2}$ measurements finds that ${n}_{q}$ scaling applied to the direct photon ${v}_{2}$ data prefers the value ${n}_{q}=1.8$ and agrees with ${n}_{q}=2$ within errors in most cases. The 0--20% and 20--40% Au+Au direct photon data are compared to a coalescence-like Monte Carlo simulation that calculates the direct photon ${v}_{2}$ while describing the shape of the direct photon ${p}_{T}$ spectra in a consistent manner. The simulation, while systematically low compared to the data, is in agreement with the Au+Au measurement at ${p}_{T}$ less than 3 $\mathrm{GeV}/c$ in both centrality bins. Furthermore, this production mechanism predicts that higher order flow harmonics ${v}_{n}$ in direct photons will follow the modified ${n}_{q}$-scaling laws seen in identified hadron ${v}_{n}$ with an ${n}_{q}$ value of 2.

Dilepton production in thermal-dependent baryonic quark–gluon plasma

We evolute a fireball of quark–gluon plasma (QGP) at thermal-dependent chemical potential (TDCP) through a statistical model in the pionic medium. The evolution of the fireball is explained through the free energy created in the pionic medium. We study the dilepton production at TDCP from such a fireball of QGP and hadronic phase. In this model, we take a finite quark mass dependence on temperature and parametrization factor. The temperature and factor enhance in the growth of the droplet formation of quarks and gluons as well as in the dilepton production rates. The production rate shows dilepton spectrum in the low mass region of the lepton pair as 0–1.2 GeV and in the intermediate mass region of 1.0–4.0 GeV. The rate of production is observed to be a strong increasing function of the TDCP for quark and antiquark annihilation. We compare the result of dilepton production at this TDCP with the production rate of the recent dilepton productions at zero and finite baryonic chemical potential and found the result far ahead in the production rates of dilepton at TDCP.

Dilepton Emission at Temperature Dependent Baryonic Quark-Gluon Plasma

Dilepton emission is calculated from quark-gluon plasma (QGP) with the consideration of temperature dependent chemical potential (TDCP). The model is considered to be quasi-model and hence take the dynamical quark mass as a finite value dependence on temperature. The temperature in quark mass enhances in the growth of dilepton emission rates. The emission rate of dilepton spectrum is shown in the intermediate mass region (IMR) of 1.0 - 4.0 GeV and its rate is observed to be a strong increasing function of the temperature dependent chemical potential for quark and anti-quark annihilation.

Dilepton production in finite baryonic quark–gluon plasma

We study the evolution of hot plasma through a statistical model in the hadronic medium. Evolution of the plasma can be expressed by the free energy at finite temperature and quark chemical potential of the constituent particles in the system. In this study, the dynamical quark mass is dependent on momentum and temperature. The evolution is explained through thermodynamic variables like free energy and entropy curve. These variables show the behaviour of the system for the different chemical potentials, μ, at these transition temperatures T = 150–170 MeV. Moreover, the study of the dilepton production at these finite temperatures and quark chemical potentials from the fireball of quark–gluon plasma shows a specific structure of dilepton spectrum in the intermediate mass region of 1.0–4.0 GeV and its production rate is observed to be a strong increasing function of quark chemical potential for quark and antiquark annihilation. We further observe lepton spectra coming from the hadronic phase in the low mass M = 0–1.2 GeV region.

Testing for [formula omitted]-factorization with inclusive prompt photon production at LHC

We study the possibility to analyse the data on inclusive prompt photon production at first LHC runs in the framework of the kT-factorization QCD approach. Our consideration is based on the amplitude for the production of a single photon associated with a quark pair in the fusion of two off-shell gluons. The quark component is taken into account separately using the quark–gluon Compton scattering and quark–antiquark annihilation QCD subprocesses. The unintegrated parton densities in a proton are determined using the Kimber–Martin–Ryskin (KMR) prescription as well as the CCFM evolution equation.

Long Range Structure of the Nucleon*

Many physicists have the following picture of the structure of the nucleon: In the inner region at “short range” reside quarks bound by gluons, in the outer region at “large distances” live mesons and in particular pions. The nucleon consists of constituents (i.e., “constituent quarks” and pions), as the atom consists of a nucleus and electrons, and the nucleus of protons and neutrons. The range of the inner “colored” region, frequently called “confinement radius,” is rather elusive. It is a model parameter in the old bag model or models for the nucleon resonances based on the constituent quarks. One can estimate it from experiment by identifying it with the “annihilation radius” of the antiproton–proton system. Only quark–antiquark annihilation in the overlap region of color can contribute to annihilation [2]. It amounts to approximately 0.8 fm, in reasonable agreement with the mentioned model parameters. However, it cannot be easily identified with the root-mean-square (rms) radius of the electric charge of the proton, since the pion cloud will contribute to the charge distribution. A rough idea of the range of this contribution may be gotten by the Compton wave length of the pion which is of the order of 1.4 fm.

Charge asymmetry ratio as a probe of quark flavour couplings of resonant particles at the LHC

We show how a precise knowledge of parton distribution functions, in particular those of the u and d quarks, can be used to constrain a certain class of New Physics models in which new heavy charged resonances couple to quarks and leptons. We illustrate the method by considering a left–right symmetric model with a W′ from a SU(2) R gauge sector produced in quark–antiquark annihilation and decaying into a charged lepton and a heavy Majorana neutrino. We discuss a number of quark and lepton mixing scenarios, and simulate both signals and backgrounds in order to determine the size of the expected charge asymmetry. We show that various quark–W′ mixing scenarios can indeed be constrained by charge asymmetry measurements at the LHC, particularly at $\sqrt{s}= 14$ TeV.

QCD transverse-momentum resummation in gluon fusion processes

We consider the production of a generic system of non-strongly interacting particles with a high total invariant mass M in hadron collisions. We examine the transverse-momentum ( q T ) distribution of the system in the small- q T region ( q T ≪ M ), and we present a study of the perturbative QCD contributions that are enhanced by powers of large logarithmic terms of the type ln ( M 2 / q T 2 ) . These terms can be resummed to all orders in QCD perturbation theory. The partonic production mechanism of the final-state system can be controlled by quark–antiquark ( q q ¯ ) annihilation and/or by gluon fusion. The resummation formalism for the q q ¯ annihilation subprocess is well established, and it is usually extrapolated to the gluon fusion subprocess. We point out that this naïve extrapolation is not correct, and we present the all-order resummation formula for the q T distribution in gluon fusion processes. The gluon fusion resummation formula has a richer structure than the resummation formula in q q ¯ annihilation. The additional structure originates from collinear correlations that are a specific feature of the evolution of the colliding hadrons into gluon partonic states. In the q T cross section at small values of q T , these gluon collinear correlations produce coherent spin correlations between the helicity states of the initial-state gluons and definite azimuthal-angle correlations between the final-state particles of the observed high-mass system.

Direct production of lightest Regge resonances

We discuss direct production of Regge excitations in the collisions of massless four-dimensional superstring states, focusing on the first excited level of open strings ending on D-branes extending into higher dimensions. We construct covariant vertex operators and identify “universal” Regge states with the internal parts either trivial or determined by the world-sheet SCFT describing superstrings propagating on an arbitrary Calabi–Yau manifold. We evaluate the amplitudes involving one such massive state and up to three massless ones and express them in the helicity basis. The most important phenomenological applications of our results are in the context of low-mass string (and large extra dimensions) scenarios in which excited string states are expected to be produced at the LHC as soon as the string mass threshold is reached in the center-of-mass energies of the colliding partons. In order to facilitate the use of partonic cross sections, we evaluate them and tabulate for all production processes: gluon fusion, quark absorbing a gluon, quark–antiquark annihilation and quark–quark scattering.

Heavy-quark production from relativistic heavy-ion collisions

We study the production of heavy quarks, charm at BNL-RHIC ( = 200 GeV/nucleon) and CERN-LHC ( = 5.5 TeV/nucleon) and bottom at CERN-LHC, from heavy ions colliding at relativistic energies. We consider initial fusion of gluons (and quark–anti-quark annihilation), pre-thermal parton interactions and interactions in a thermalized quark–gluon plasma. We also consider free-streaming partons as another extreme and compare the results with those from a thermalized plasma of partons. The pre-thermal contribution is calculated by considering the interaction among partons having large transverse momenta (jet–partons) after the initial interaction and from the passage of these partons through a thermalized quark–gluon plasma. Charm production from pre-thermal processes is found to be comparable to that from prompt (initial) interactions at LHC. It is suggested that this may have important implications for the study of the nuclear modification factor, RAA, as well as for the back-to-back correlation of heavy quarks and the production of dileptons having a large mass.

On the quark component in prompt photon and electroweak gauge boson production at high energies

In the framework of the kT-factorization approach, we study the production of prompt photons and electroweak gauge bosons in high-energy proton–(anti)proton collisions at modern colliders. Our consideration is based on the amplitude for the production of a single photon or W±/Z0 boson associated with a quark pair in the fusion of two off-shell gluons. The quark component is taken into account separately using the quark–gluon scattering and quark–antiquark annihilation QCD subprocesses. Special attention is given to the contributions from the quarks involved in the earlier steps of the evolution cascade. Using the Kimber–Martin–Ryskin formalism, we simulate this component and demonstrate that it plays an important role at both the Tevatron and LHC energies. Our theoretical results are compared with recent experimental data taken by the D∅ and CDF Collaborations at the Tevatron.

Dileptons from a Chemically Equilibrating Quark–Gluon Plasma at Finite Baryon Density

We perform a complete calculation for the dilepton production from the processes qq̄→ll̄, Compton-like (qg→qll̄, q̄g→q̄ll̄), qq̄→gll̄, gluon fusion gḡ→cc̄, annihilation qq̄→cc̄ as well as multiple scattering of quarks in a chemically equilibrating quark–gluon plasma system at finite baryon density. It is found that quark–antiquark annihilation, Compton-like, gluon fusion and multiple scattering of quarks give important contribution. Moreover, the increase of the quark phase life-time with increasing initial quark chemical potential makes the dilepton yield as an increasing function of the initial quark chemical potential.

Single spin asymmetries in heavy quark and antiquark productions

The single transverse spin asymmetries in heavy quark and antiquark production from the quark–antiquark annihilation channel contribution is studied by taking into account the initial and final state interactions effects. Because of the different color charges, the final state interaction effects lead to about a factor of 3 difference in the spin asymmetry for heavy quark over that for the antiquark in the valence region of low energy pp collisions. The experimental study of this model-independent prediction shall provide a crucial test for the underlying mechanism for the single spin asymmetry phenomena.

NLO QCD corrections to tt-barbb-bar production at the LHC: 1. quark-antiquark annihilation

The process pp -> top anti-top bottom anti-bottom + X represents a very important background reaction to searches at the LHC, in particular to top anti-top H production where the Higgs boson decays into a bottom anti-bottom pair. A successful analysis of top anti-top H at the LHC requires the knowledge of direct top anti-top bottom anti-bottom production at next-to-leading order in QCD. We take the first step in this direction upon calculating the next-to-leading-order QCD corrections to the subprocess initiated by quark anti-quark annihilation. We devote an appendix to the general issue of rational terms resulting from ultraviolet or infrared (soft or collinear) singularities within dimensional regularization. There we show that, for arbitrary processes, in the Feynman gauge, rational terms of infrared origin cancel in truncated one-loop diagrams and result only from trivial self-energy corrections.

Tops from light quarks: Full mass dependence at two-loops in QCD

I present the two-loop QCD corrections to the production of a massive quark–anti-quark pair in the massless quark–anti-quark annihilation channel. The result is obtained as a combination of a deep expansion in the mass around the high energy limit and of a numerical integration of a system of differential equations. The primary application of the outcome and developed methods is top quark pair production at the Large Hadron Collider.

Kinematical Constraints on QCD Factorization in the Drell-Yan Process

We study factorization schemes for parton shower models in hadron-hadron collisions. As an example, we calculate lepton pair production mediated by a virtual photon in quark–antiquark annihilation, and we compare factorized cross sections obtained in the conventional MS scheme with those obtained in a factorization scheme in which a kinematical constraint due to parton radiation is taken into account. We discuss some properties of factorized cross sections.