Gluon Contribution Research Articles

Higgs boson CP-properties of the gluonic contributions in Higgs plus three jet production via gluon fusion at the LHC

In high energy hadronic collisions, a general CP-violating Higgs boson $\Phi$ with accompanying jets can be efficiently produced via gluon fusion, which is mediated by heavy quark loops. In this letter we study the dominant sub-channel $ gg \to \Phi ggg$ of the gluon fusion production process with triple real emission corrections at order $\alpha_s^5$. We go beyond the heavy top limit approximation and include the full mass dependence of the top- and bottom-quark contributions. Furthermore, we show within a toy-model scenario that bottom-quark loop contributions in combination with large values of $\tan \beta$ can modify visibly the differential distributions sensitive to $\mathcal{CP}$-measurements of the Higgs boson particle.

Drell-Yan Production at Threshold to Third Order in QCD

The recent computation on the full threshold contributions to Higgs boson production at next to next to next to leading order (N^{3}LO) in QCD contains valuable information on the soft gluons resulting from virtual and real emission partonic subprocesses. We use those from the real emissions to obtain the corresponding soft gluon contributions to Drell-Yan production and determine the missing δ(1-z) part of the N^{3}LO. The numerical impact of threshold effects demonstrates the importance of our results in the precision study with the Drell-Yan process at the LHC.

Recent results of gluon and sea quark polarization measurements in polarized proton-proton collisions at STAR

The STAR experiment at RHIC is carrying out a comprehensive high-energy spin physics program to understand the internal structure and dynamics of the proton in polarized proton-proton collisions at = 200 GeV and = 500/510 GeV. STAR has the capability, with nearly full azimuthal coverage, to reconstruct leptons, hadrons and jets in the mid-rapidity region (|η| < 1). The results for inclusive jet longitudinal double spin asymmetries taken during the 2009 RHIC run indicate the first non-zero gluon contribution (Δg(x,Q2)/g(x,Q2)) to the proton spin for 0.05 < x < 1 (Bjorken-x: momentum fraction of partons). Recent longitudinal single-spin asymmetry measurements of W+/− bosons at = 500/510 GeV in polarized proton-proton collisions provide a direct probe of the polarized anti-u and anti-d quark distributions (Δū(x, Q2), Δd̄(x, Q2)). These results better constrain the polarized gluon and sea quark distributions of the proton in the RHIC sensitive kinematic region. Future measurements with continuing high energy polarized proton-proton run at RHIC and detector upgrade will explore the gluonic contribution to the proton spin in extended range.

Complex saddle points in QCD at finite temperature and density

The sign problem in QCD at finite temperature and density leads naturally to the consideration of complex saddle points of the action or effective action. The global symmetry $\mathcal{CK}$ of the finite-density action, where $\mathcal{C}$ is charge conjugation and $\mathcal{K}$ is complex conjugation, constrains the eigenvalues of the Polyakov loop operator $P$ at a saddle point in such a way that the action is real at a saddle point, and net color charge is zero. The values of ${\mathrm{Tr}}_{F}P$ and ${\mathrm{Tr}}_{F}{P}^{\ifmmode\dagger\else\textdagger\fi{}}$ at the saddle point are real but not identical, indicating the different free energy cost associated with inserting a heavy quark versus an antiquark into the system. At such complex saddle points, the mass matrix associated with Polyakov loops may have complex eigenvalues, reflecting oscillatory behavior in color-charge densities. We illustrate these properties with a simple model which includes the one-loop contribution of gluons and two flavors of massless quarks moving in a constant Polyakov loop background. Confinement-deconfinement effects are modeled phenomenologically via an added potential term depending on the Polyakov loop eigenvalues. For sufficiently large temperature $T$ and quark chemical potential $\ensuremath{\mu}$, the results obtained reduce to those of perturbation theory at the complex saddle point. These results may be experimentally relevant for the compressed baryonic matter experiment at FAIR.

Leading large-x logarithms of the quark–gluon contributions to inclusive Higgs-boson and lepton-pair production

We present all-order expressions for the leading double-logarithmic threshold contributions to the quark–gluon coefficient functions for inclusive Higgs-boson production in the heavy top-quark limit and for Drell–Yan lepton-pair production. These results have been derived using the structure of the unfactorized cross sections in dimensional regularization and the large-x resummation of the gluon–quark and quark–gluon splitting functions. The resummed coefficient functions, which are identical up to colour factor replacements, are similar to their counterparts in deep-inelastic scattering but slightly more complicated.

QCD effective potential with strongU(1)emmagnetic fields

We derive the analytic expression for the one-loop $SU({N}_{c})$ QCD effective potential including ${N}_{f}$ flavor quarks which nonlinearly interact with the chromomagnetic background field and the external $U(1{)}_{\mathrm{em}}$ magnetic field. After the renormalization of couplings and fields, we obtain the correct one-loop $\ensuremath{\beta}$ functions of both QCD and QED, and the resulting effective potential satisfies the renormalization group equation. We investigate the effect of the magnetic field on the QCD vacuum by using the effective potential, in particular for the color $SU(3)$ case with the three flavors ($u,d,s$). Our result shows that the chromomagnetic field prefers to be parallel to the external magnetic field. Furthermore, quark loop contributions to the effective potential with the magnetic field enhance gluonic contributions, and then the chromomagnetic condensate increases with an increasing magnetic field. This result supports the recent observed gluonic magnetic catalysis at zero-temperature in lattice QCD.

On Deeply Virtual Compton Scattering at Next-to-Leading Order

Deeply Virtual Compton Scattering in the near forward kinematic region is the golden access to Generalized Parton Distributions. We studied the O(alphaS) corrections to the scattering amplitude for both spacelike and timelike kinematics relevant respectively to the leptoproduction of a real photon and to the photoproduction of a lepton pair. It turns out that these corrections are phenomenologically important and that the gluonic contributions are by no means negligible, even in the moderate energy range of JLab12 and of the COMPASS-II experiment at CERN.

Sea-quark and gluonic contributions to strange baryons and its properties

We treat the hadrons as an ensemble of quark-gluon Fock states where contributions from sea-quarks and gluons can be studied in detail for the properties of low lying baryons. Statistical model is applied to calculate individual probabilities from various scalar, vector and tensor sea components in flavour, spin and colour subspaces for each quark-gluon Fock state. The mass of strange quark is imposed in terms of constraints to the availability of strange quarks in the sea and free energy of gluons in conformity with experimental indications. We calculate multiplicities for different sets of Fock states to compute the role of strange sea for cascade doublet Ξ+ and Ξ0. The low energy properties like magnetic moment, weak decay matrix elements and axial coupling constant ratios have been studied. The incorporation of strange quark gluon sea is discussed and they are found to affect the results by almost 46%.The results provide deeper understanding for baryon structure thereby motivating experiments for further inspection, especially the spin distribution among the quarks and gluons.

Unbiased spin-dependent Parton Distribution Functions

We present the first unbiased determination of spin-dependent, or polarized, Parton Distribution Functions (PDFs) of the proton. A statistically sound representation of the corresponding uncertainties is achieved by means of the NNPDF methodology: this was formerly developed for unpolarized distributions and is now generalized to the polarized here for the first time. The features of the procedure, based on robust statistical tools (Monte Carlo sampling for error propagation, neural networks for PDF parametrization, genetic algorithm for their minimization, and possibly reweighting for including new data samples without refitting), are illustrated in detail. Different sets of polarized PDFs are obtained at next-to-leading order accuracy in perturbative quantum chromodynamics, based on both fixed-target inclusive deeply-inelastic scattering data and the most recent polarized collider data. A quantitative appraisal on the potential role of future measurements at an Electron-Ion Collider is also presented. We study the stability of our results upon the variation of several theoretical and methodological assumptions and we present a detailed investigation of the first moments of our polarized PDFs, compared to other recent analyses. We find that the uncertainty on the gluon distribution from available data was substantially underestimated in previous determinations; in particular, we emphasize that a large contribution to the gluon may arise from the unmeasured small-x region, against the common belief that this is actually rather small. We demonstrate that an Electron-Ion Collider would provide evidence for a possible large gluon contribution to the nucleon spin, though with a sizable residual uncertainty.

Next to leading order analysis of DVCS and TCS

The study of O (αs ) QCD contributions to the timelike and spacelike virtual Compton scattering amplitudes in the generalized Bjorken scaling regime demonstrates that gluonic contributions are by no means negligible even in the medium energy range which will be studied intensely at JLab12 and in the COMPASS-II experiment at CERN.

Study of the proton structure by measurements of polarization transfers in real Compton scattering at JLab

A preliminary analysis of polarization-transfer data at large scattering angle (70 o ), obtained in an experiment of real Compton scattering on proton, performed in Hall-C of Je erson Lab, is presented. It is also discussed the relevance of this kind of experiments for shedding light on the non-perturbative structure of the proton, at low energy, and on the transition from the non-perturbative regime to the perturbative one, that occurs at high energy. Moreover, the possibility to extract Compton form factors and the Generalized Parton Distributions, one of the most promising theoretical tool to determine the total angular momentum contribution of quarks and gluons to nucleon spin, is emphasized.

Eta Prime Gluonic Contribution to the Nucleon Self-Energy in an Effective Theory

We estimate a possibleη′gluonic contribution to the self-energy of a nucleon in an effective theory. The couplings of the topological charge density to nucleons give rise to OZI violatingη′-nucleon interactions. The one-loop self-energy of nucleon arising due to these interactions is studied using a heavy baryon chiral perturbation theory. The divergences have been removed using appropriate form factors. The nontrivial structure of the QCD vacuum has also been taken into account. The numerical results are sensitive to the choice of the regulator to a nonnegligible extent. We get the total contribution to the nucleon mass coming from its interaction with the topological charge densityδmtot≅-(2.5–7.5)% of the nucleon mass.

MUONIC HYDROGEN AS A QUANTUM GRAVIMETER

High precision spectroscopy of muonic hydrogen has recently led to an anomaly in the Lamb shift, which has been parametrized in terms of a proton charge radius differing by seven standard deviations from the CODATA value. We show how this anomaly may be explained, within about a factor of three, in the framework of an effective Yukawian gravitational potential related to charged weak interactions, without additional free parameters with respect to the ones of the standard model. The residual discrepancy from the experimental result in this model should be attributable to the approximations introduced in the calculation, the uncertainty in the exact value of the Fermi scale relevant to the model and the lack of detailed knowledge on the gravitational radius of the proton. The latter cannot be inferred with electromagnetic probes due to the unknown gluonic contribution to the proton mass distribution. In this context, we argue that muonic hydrogen acts like a microscopic gravimeter suitable for testing a possible scenario for the reciprocal morphing between macroscopic gravitation and weak interactions, with the latter seen as the quantum, microscopic counterpart of the former.

Polarized parton distributions at an electron–ion collider

We study the potential impact of inclusive deep-inelastic scattering data from a future electron–ion collider (EIC) on longitudinally polarized parton distributions (PDFs). We perform a PDF determination using the NNPDF methodology, based on sets of deep-inelastic EIC pseudodata, for different realistic choices of the electron and proton beam energies. We compare the results to our current polarized PDF set, NNPDFpol1.0, based on a fit to fixed-target inclusive DIS data. We show that the uncertainties on the first moments of the polarized quark singlet and gluon distributions are substantially reduced in comparison to NNPDFpol1.0, but also that more measurements may be needed to ultimately pin down the size of the gluon contribution to the nucleon spin.

PROBING THE ANOMALOUS COUPLINGS OF THE TOP QUARK WITH GLUON AT THE LHC AND TEVATRON

In this paper, we study the sensitivity of the fraction of [Formula: see text] events arising from gluon–gluon fusion to the chromoelectric and chromomagnetic dipole moments (CEDM and CMDM) as well as the total and differential [Formula: see text] cross-sections at the LHC and Tevatron. The sensitivity of measured charged asymmetry at the LHC to CEDM and CMDM is also studied. We find that at the Tevatron and the LHC, nonzero values of CMDM could suppress the [Formula: see text] production rate. It is shown that the ratio of [Formula: see text] at the Tevatron is more sensitive to CEDM and CMDM than the LHC case. The presence of CEDM always increases the contribution of gluon–gluon fusion process in top pair rate at the Tevatron and LHC. Except for a small range of CMDM, the presence of CEDM and CMDM can increase the fraction of gluon–gluon fusion at the Tevatron and LHC. The measured ratio of [Formula: see text] at the Tevatron is used to derive bounds on the chromoelectric and chromomagnetic dipole moments as well as the total and differential [Formula: see text] cross-sections at the LHC and Tevatron, and the measured charged asymmetry at the LHC. The combination of [Formula: see text] and σ LHC provides stringent limits on CMDM and CEDM.

Higgs-boson production in association with three jets via gluon fusion at the LHC: Gluonic contributions

Higgs production in association with three jets via gluon fusion is an important channel for the measurement of the $\mathcal{C}\mathcal{P}$ properties of the Higgs particle at the LHC. In this paper, we go beyond the heavy top effective theory approximation and include at LO the full mass dependence of the top- and bottom-quark contributions. We consider the dominant subchannel $gg\ensuremath{\rightarrow}Hggg$ which involves the manipulation of massive rank-5 hexagon integrals. Furthermore, we present results for several differential distributions and show deviations from the effective theory as large as 100% at high ${p}_{T}$ for light Higgs masses.

Light-cone sum rules for theD(s)→η(′)lνlform factor

We present an improved light-cone sum rule analysis of the decay form factors of $D$ and $D_s$ into $\eta$ and $\eta^{\prime}$ and argue that these decays offer a very promissing possibility to determine the leading Fock-state gluonic contribution of the $\eta'$ at future experimental facilities as FAIR or Super-KEKB. We also give the corresponding branching ratios for B decays.

Double-spin asymmetryALTin open charm production

In the collinear twist-three approach, we calculate for the first time the gluon contribution to double- (longitudinal-transverse) spin asymmetry ${A}_{LT}$ for open charm production in proton-proton collisions measurable at RHIC. Utilizing the Ward-Takahashi identity for the nonpole part of the hard scattering amplitude, we derive a factorized, gauge invariant formula for the asymmetry. The result may be combined with the previous studies of single-spin asymmetry in the same channel and allows for a systematic analysis of three-gluon correlations inside a transversely polarized nucleon.

A study of the diffusion pattern in [formula omitted] SYM at high energies

In the context of evolution equations and scattering amplitudes in the high energy limit of the N=4 super Yang–Mills theory we investigate in some detail the BFKL gluon Green function at next-to-leading order. In particular, we study its collinear behavior in terms of an expansion in different angular components. We also perform a Monte Carlo simulation of the different final states contributing to such a Green function and construct the diffusion pattern into infrared and ultraviolet modes and multiplicity distributions, making emphasis in separating the gluon contributions from those of scalars and gluinos. We find that the combined role of the non-gluonic degrees of freedom is to improve the collinear behavior and reduce the diffusion into ultraviolet regions while not having any effect on the average multiplicities or diffusion into the infrared. In terms of growth with energy, the non-zero conformal spin components are mainly driven by the gluon terms in the BFKL kernel. For zero conformal spin (Pomeron) the effect of the scalar and gluino sectors is to dramatically push the Green function towards higher values.

Excited spectroscopy of charmed mesons from lattice QCD

We present spectra of highly excited D and Ds mesons up to around 3.8 GeV determined using dynamical lattice QCD. We employ novel computational techniques and the variational method with a large basis of carefully constructed operators in order to extract and reliably identify the continuum spin of an extensive set of excited states. These include states with high spin and states identified as having an explicit gluonic contribution. Calculations were performed on two volumes, both with a pion mass of approximately 400 MeV, achieving a high statistical precision for both ground and excited states. We discuss our results in light of experimental observations, comment on the phenomenological implications and identify the lightest `supermultiplet' of hybrid mesons in each sector.