Color-octet States Research Articles

Azimuthal asymmetries in J/ψ -photon production at the EIC

We calculate azimuthal asymmetries in back-to-back production of $J/\ensuremath{\psi}$ and a photon in electron-proton scattering process at the future electron-ion collider using the transverse momentum dependent parton distribution (TMD) factorization framework. We consider the cases where the proton is unpolarized or transversely polarized. For the formation of $J/\ensuremath{\psi}$, nonrelativistic QCD is used. We find that the cross section gets contributions from only one color octet state. As a result, the azimuthal modulations become independent of the long-distance matrix elements, and thus can be used to probe, in particular the gluon TMDs which are dominant in this kinematics. We show estimates of the upper bounds of different azimuthal asymmetries using the positivity bounds on TMDs. In addition, we show an estimate of asymmetries using the Gaussian parametrization of the TMDs.

Higgs boson decay to charmonia via c-quark fragmentation

We calculate the decay branching fractions of the Higgs boson to J/ψ and ηc via the charm-quark fragmentation mechanism for the color-singlet and color-octet states in the framework of non-relativistic QCD. The decay rates are governed by the charm-quark Yukawa coupling, unlike the decay H → J/ψ + γ, which is dominated by the γ∗-J/ψ mixing. We find that the decay branching fractions can be about 2×10−5 for Hto coverline{c}+J/psi , and 6 × 10−5 for Hto coverline{c}+{eta}_c . We comment on the perspective of searching for the Higgs boson to J/ψ transition at the High-Luminosity LHC for testing the charm-quark Yukawa coupling.

Process dependence of the gluon Sivers function in p↑p→J/ψ+X within a TMD scheme in NRQCD

We consider the transverse single-spin asymmetry (SSA) for $J/\psi$ production in $p^\uparrow p \to J/\psi + X$ within a TMD approach in non-relativistic QCD. Extending a previous study [1], we employ here the color-gauge invariant generalized parton model (CGI-GPM), in which spin and intrinsic transverse momentum effects are taken into account, together with leading-order initial- and final-state interactions (ISIs and FSIs). We find that, even when the heavy-quark pair is produced in a color-octet state, ISIs and FSIs lead to a nonvanishing SSA, allowing, in principle, to test the process dependence of the gluon Sivers function (GSF). We show that of the two independent contributions, due to the so-called $f$- and $d$-type GSFs, appearing in the CGI-GPM, the $d$-type one turns out to be dynamically suppressed. Therefore, as already found adopting the Color-Singlet Model approach for the $J/\psi$ formation [2], only the $f$-type GSF could play a role in phenomenology. A comparison with the corresponding results obtained in the generalized parton model, without the inclusion of ISIs and FSIs, is also carried out.

Nuclear p\u22a5-broadening of Drell-Yan and quarkonium production from SPS to LHC

The nuclear p⊥-broadening of Drell-Yan and quarkonium (J/ψ, Υ) production in πA and pA collisions is investigated. The world data follow a simple scaling from SPS to LHC energies, once the process-dependent color factors are properly taken into account, which allows for the extraction of the transport coefficient in cold nuclear matter. We find that hat{q}(x)propto {x}^{-alpha } with α = 0.25–0.30. The magnitude of the transport coefficient at x = 10−2 is {hat{q}}_0=0.051;{mathrm{GeV}}^2/mathrm{fm} and {hat{q}}_0=0.075;{mathrm{GeV}}^2/mathrm{fm} qˆ0 = 0.075 GeV2/fm, whether Qoverline{Q} pairs are assumed to be produced as color octet or color singlet states, respectively. The relation between nuclear broadening data and the (CT14) gluon density is also investigated.

Covariant calculation of a two-loop test of nonrelativistic QCD factorization

We test the nonrelativistic QCD factorization conjecture for inclusive quarkonium production at two loops by carrying out a covariant calculation of the nonrelativistic quantum chromodynamics (NRQCD) long-distance matrix element (LDME) for a heavy-quark pair in an $S$-wave, color-octet state to fragment into a heavy-quark pair in a color-singlet state of arbitrary orbital angular momentum. The NRQCD factorization conjecture for the universality of the LDME requires that infrared divergences that it contains be independent of the direction of the Wilson line that appears in its definition. We find this to be the case in our calculation. The results of our calculation differ in some respects from those of a previous calculation that was carried out by Nayak, Qiu, and Sterman using light-cone methods. We have identified the sources of some of these differences. The results of both calculations are consistent with the NRQCD factorization conjecture. However, the general principle that underlies this confirmation of NRQCD factorization at two-loop order has yet to be revealed.

Sivers asymmetry in the photoproduction of a J/ψ and a jet at the EIC

We calculate the Sivers asymmetry in the photoproduction of almost back-to-back $J/\ensuremath{\psi}$-jet pair in the process $e{p}^{\ensuremath{\uparrow}}\ensuremath{\rightarrow}J/\ensuremath{\psi}+\text{jet}+X$, which will be possible at the future planned Electron-Ion Collider (EIC). We use the framework of a generalized parton model and nonrelativistic QCD for calculating the $J/\ensuremath{\psi}$ production rate. We include contributions from both color singlet and color octet states in the asymmetry. We obtain sizable Sivers asymmetry that can be promising to determine the gluon Sivers function. We also investigate the effect of transverse momentum dependent evolution on the asymmetry.

Hidden-charm pentaquarks with color-octet substructure in QCD sum rules

We study the hidden-charm pentaquark states $udsc\bar{c}$ with spins 1/2, 3/2, and 5/2 within the QCD sum-rule approach. First, we construct the currents for the particular configuration of pentaquark states that consist of the flavor singlet three-quark cluster $uds$ of spins 1/2 and 3/2 and the two-quark cluster $\bar cc$ of spin 1, where both clusters are in a color-octet state. From the QCD sum rules obtained by the operator product expansion up to dimension-10 condensates, the extracted masses for the pentaquark states $uds$-$\bar{c}c$ are about 4.6 GeV (5.6 GeV) for spin 1/2$^\pm$, about 5.1 GeV (6.0 GeV) for spin 3/2$^\pm$, about 6.1 GeV (5.9 GeV) for spin 5/2$^\pm$, where the masses of the positive parity states are given in parentheses. Additionally, based on the flavor singlet pentaquark states, it is also shown that other pentaquark states of clusters like $udc$-$\bar{c}s$ and $usc$-$\bar cd$ lead to masses similar to the $uds$-$\bar cc$ case within error bars. Furthermore, in order to see whether any of the states, observed by the LHCb Collaboration, could be understood as the pentaquark of two clusters in the color-octet state, we study the pentaquark formed by the two clusters $udc$-$\bar cu$, where the three-quark cluster is assumed to have the same flavor structure as the above $uds$ cluster. We come to the conclusion that if the observed pentaquark will be found to have spin 1/2 and negative parity, then it could be described as a state of two color-octet clusters.

Sivers effect in inelastic J/ψ photoproduction in ep↑ collision in color octet model

The prediction of single-spin asymmetry in inelastic photoproduction of $J/\psi$ in $ep^\uparrow$ collision is presented. At next-to-leading order, the dominating process is photon-gluon fusion, $\gamma+g\rightarrow J/\psi+g$ for the production of $J/\psi$ in $e+p^\uparrow\rightarrow J/\psi+X$, which directly probes the gluon Sivers function. Using the non-relativistic QCD based color octet model, the color octet states $\leftidx{^{3}}{S}{_1}^{(8)}$, $\leftidx{^{1}}{S}{_0}^{(8)}$ and $\leftidx{^{3}}{P}{_{J(0,1,2)}}^{(8)}$ contribution to $J/\psi$ production is calculated. Sizable asymmetry is estimated as a function of transverse momentum $P_T$ and energy fraction $z$ of $J/\psi$ in the range $0<P_T\leq 1$ GeV and $0.3<z\leq 0.9$. The unpolarized differential cross section of inelastic $J/\psi$ photoproduction is found to be in good agreement with H1 and ZEUS data.

Constraining scalar resonances with top-quark pair production at the LHC

Constraints on models which predict resonant top-quark pair production at the LHC are provided via a reinterpretation of the Standard Model (SM) particle level measurement of the top-anti-top invariant mass distribution, mleft(toverline{t}right) . We make use of state-of-the-art Monte Carlo event simulation to perform a direct comparison with measurements of mleft(toverline{t}right) in the semi-leptonic channels, considering both the boosted and the resolved regime of the hadronic top decays. A simplified model to describe various scalar resonances decaying into top-quarks is considered, including CP-even and CP-odd, color-singlet and color-octet states, and the excluded regions in the respective parameter spaces are provided.

Flavor-singlet hidden charm pentaquark

A new type of charm pentaquark $P_{cs}$ with quark content $c\bar{c}uds$ in light-flavor singlet state is studied in the quark model. This state is analogous to the $P_{c}$ with $c\bar{c}uud$ in light-flavor octet, which was observed in LHC in 2015. Considering various combinations of color, spin and light flavor as internal quantum numbers in $P_{cs}$, we investigate the mass ordering of the $P_{cs}$'s by adopting both the one-gluon exchange interaction and the instanton-induced interaction in the quark model. The most stable configuration of $P_{cs}$ is identified to be total spin $1/2$ in which the $c\bar{c}$ is combined to be color octet and spin $1$, while the $uds$ cluster is in a color octet state. The other color octet configurations, the total spin $1/2$ state with the $c\bar{c}$ spin 0 and the state with total spin $3/2$ and $c\bar{c}$ spin 1, are found as excited states. We also discuss possible decay modes of these charm pentaquarks.

Probing a new strongly interacting sector via composite diboson resonances

Diphoton resonance was a crucial discovery mode for the 125 GeV SM Higgs boson at the LHC. This mode or the more general diboson modes may also play an important role in probing for new physics beyond the SM. In this paper, we consider the possibility that a diphoton resonance is due to a composite (pseudo)scalar boson, whose constituents are either new hyperquarks Q or scalar hyperquarks tilde{Q} confined by a new hypercolor force at a confinement scale Lambda_h. Assuming the mass m_Q (or m_{tilde Q}) >> Lambda_h, a diphoton resonance could be interpreted as either a Q bar{Q} state eta_Q with J^{PC} = 0^{-+} or a tilde{Q} tilde{Q}^dagger state eta_{tilde Q} with J^{PC}=0^{++}. For the Q bar{Q} scenario, there will be a spin-triplet partner psi_Q which is slightly heavier than eta_Q due to the hyperfine interactions mediated by hypercolor gluon exchange; while for the tilde{Q} tilde{Q}^dagger scenario, the spin-triplet partner chi_{tilde Q} arises from higher radial excitation with nonzero orbital angular momentum. We consider productions and decays of eta_Q, eta_{tilde Q}, psi_Q, and chi_{tilde Q} at the LHC using the NRQCD factorization approach. We discuss how to test these scenarios by using the DY process and the forward dijet azimuthal angular distributions to determine the J^{PC} quantum number of the diphoton resonance. Constraints on the parameter space can be obtained by interpreting some of the small diphoton excesses reported by the LHC as the composite scalar or pseudoscalar of the model. Another important test of the model is the presence of a nearby hypercolor-singlet but color-octet state like the eta^8_Q or eta^8_{tilde Q}, which can also be constrained by dijet or monojet+monophoton data. Both possibilities of a large or small width of the resonance can be accommodated, depending on whether the hyper-glueball states are kinematically allowed in the final state or not.

Production of heavy quarkonia in hadronic experiments

Processes of single and pair production of heavy quarkonia under LHC conditions have been studied within nonrelativistic quantum chromodynamics. Constraints on the matrix elements of color-singlet and color-octet states have been obtained by analyzing the existing experimental data. It has been shown that the leading contribution to the cross sections for these processes comes from the color singlet mechanism with the singlet matrix elements exceeding phenomenological values obtained from the solutions of potential models or experimental decay widths of the corresponding mesons. It has also been found that the contribution from color-octet states should be taken into account to describe the ratio of the cross sections for the production of tensor and axial charmonia. These results have been used to analyze the single production of bottomonia, the pair production of heavy quarkonia, and the production of vector charmonium in jets. The resulting theoretical predictions are in good agreement with experimental data.

Hidden-charm pentaquarks and their hidden-bottom and Bc -like partner states

In the framework of the color-magnetic interaction, we have systematically studied the mass splittings of the possible hidden-charm pentaquarks $qqqc\overline{c}$ ($q=u$, $d$, $s$) where the three light quarks are in a color-octet state. We find that (i) the LHCb ${P}_{c}$ states fall in the mass region of the studied system, (ii) most pentaquarks should be broad states since their $S$-wave open-charm decays are allowed while the lowest state is the ${J}^{P}={\frac{1}{2}}^{\ensuremath{-}}$ $\mathrm{\ensuremath{\Lambda}}$-like pentaquark with probably the suppressed ${\ensuremath{\eta}}_{c}\mathrm{\ensuremath{\Lambda}}$ decay mode only, and (iii) the ${J}^{P}={\frac{5}{2}}^{\ensuremath{-}}$ states do not decay through the $S$ wave and their widths are not so broad. The masses and widths of the two LHCb ${P}_{c}$ baryons are compatible with such pentaquark states. We also explore the hidden-bottom and ${B}_{c}$-like partners of the hidden-charm states and find the possible existence of the pentaquarks which are lower than the relevant hadronic molecules.

Constraints on color-octet companions of a 750 GeV heavy pion from dijet and photon plus jet resonance searches

Color octet bosons are a universal prediction of models in which the 750 GeV diphoton resonance corresponds to a pion of a QCD-like composite sector. We show that the existing searches for dijet and photon plus jet resonances at the LHC constrain single productions of color octet states and can be translated into stringent limits on the 750 GeV diphoton rate. For a minimal 5 + 5bar model, the 750 GeV diphoton signal cross section at the 13 TeV LHC is constrained to be below around 5 fb. Future LHC searches for the photon plus jet resonances can establish evidence of a new color-octet state with 20/fb and validate a pion-like explanation for the 750 GeV resonance.

Production of ϒJ/ψ pairs at the LHC

The inclusive production of ϒJ/ψ pairs in proton–proton interaction is studied under conditions of the LHCb experiment. In the leading order of perturbation theory, the production of a pair of color-singlet vector quarkonia is forbidden in this reaction, so that channels such as double parton scattering and χbχc pair production followed by radiative decays of P-wave quarkonia are considered in detail in the present study along with the contribution of color-octet states and next-to-leading-order (NLO) corrections. For all these channels, we obtain theoretical predictions for total cross sections in the case of measurements with the LHCb detector and for distributions of various kinematical variables. According to the results obtained in the present study, a dominant contribution to the reaction cross section comes from double parton interaction.

Simultaneous production of charmonium and bottomonium mesons at the LHC

Inclusive production of $\Upsilon J/\psi$ pair in proton-proton interation at LHCb is considered. This process is forbidden at leading order of perturbation theory, so such channels as double parton scattering, $\chi_b\chi_c$ pair production with subsequent radiative decays of $P$-wave quarkonia, contributions of color-octet states, and NLO corrections are studied in details. For all these channels we present theoretical predictions of total cross sections at LHCb and distributions over different kinematical variables. According to presented in the paper results, double parton interaction gives main contribution to the cross section of the considered reaction.

Heavy color-octet particles at the LHC

Many new-physics models, especially those with a color-triplet top-quark partner, contain a heavy color-octet state. The “naturalness” argument for a light Higgs boson requires that the color-octet state be not much heavier than a TeV, and thus it can be pair-produced with large cross sections at high-energy hadron colliders. It may decay preferentially to a top quark plus a top partner, which subsequently decays to a top quark plus a color-singlet state. This singlet can serve as a WIMP dark-matter candidate. Such decay chains lead to a spectacular signal of four top quarks plus missing energy. We pursue a general categorization of the color-octet states and their decay products according to their spin and gauge quantum numbers. We review the current bounds on the new states at the LHC and study the expected discovery reach at the 8-TeV and 14-TeV runs. We also present the production rates at a future 100-TeV hadron collider, where the cross sections will be many orders of magnitude greater than at the 14-TeV LHC. Furthermore, we explore the extent to which one can determine the color octet’s mass, spin, and chiral couplings. Finally, we propose a test to determine whether the fermionic color octet is a Majorana particle.

The soft function for color octet production at threshold

We evaluate the next-to-next-to-leading order soft function for the production of a massive color octet state at rest in the collision of two massless colored partons in either the fundamental or the adjoint representation. The main application of our result is the determination of the threshold expansion of the heavy-quark pair-production cross sections in the quark annihilation and gluon fusion channels. We discuss the factorization necessary for this purpose and explain the relationship between hard functions and virtual amplitudes.

Relativistic correction to J/ψ and $ \varUpsilon $ pair production

The relativistic corrections to the productions of double J/psi, double Upsilon, and J/psi+Upsilon at the Tevatron and the LHC were investigated within the frame of nonrelativistic QCD. The ratios of short distance coefficients between relativistic correction and leading order result for color singlet and color octet states in large p_T limit are approximately -1,-11/3, respectively, for pair production. And for J/psi+Upsilon process the ratio is -11/6 for the CO channel. The K factors of relativistic corrections for color singlet and color octet double J/psi production are -0.23 and -0.84 with adopting v^2=0.23. The relativistic corrections significantly dilute the difference between the shape of color-singlet differential cross sections and the color-octet's at leading order. Also our results show that the single parton scattering model may be enough to explain the LHCb data for the J/psi pair production.

High transverse momentum quarkonium production and dissociation in heavy ion collisions

We calculate the yields of quarkonia in heavy ion collisions at the BNL Relativistic Heavy Ion Collider (RHIC) and the CERN Large Hadron Collider (LHC) as a function of their transverse momentum. Based upon nonrelativistic quantum chromodynamics, our results include both color-singlet and color-octet contributions and feed-down effects from excited states. In reactions with ultrarelativistic nuclei, we focus on the consistent implementation of dynamically calculated nuclear matter effects, such as coherent power corrections, cold nuclear matter energy loss, and the Cronin effect in the initial state. In the final state, we consider radiative energy loss for the color-octet state and collisional dissociation of quarkonia as they traverse through the QGP. Theoretical results are presented for $J/\ensuremath{\psi}$ and $\ensuremath{\Upsilon}$ and compared to experimental data where applicable. At RHIC, a good description of the high-${p}_{T}$ $J/\ensuremath{\psi}$ modification observed in central $\mathrm{Cu}+\mathrm{Cu}$ and $\mathrm{Au}+\mathrm{Au}$ collisions can be achieved within the model uncertainties. We find that measurements of $J/\ensuremath{\psi}$ yields in proton-nucleus reactions are needed to constrain the magnitude of cold nuclear matter effects. At the LHC, a good description of the experimental data can be achieved only in midcentral and peripheral $\mathrm{Pb}+\mathrm{Pb}$ collisions. The large fivefold suppression of prompt $J/\ensuremath{\psi}$ in the most central nuclear reactions may indicate for the first time possible thermal effects at the level of the quarkonium wave function at large transverse momenta.