arXiv subjects

Feng Yuan

Publications and source records attributed to Feng Yuan.

At least 289 records · Page 16Linked to original sources

arXiv2011

Gluon Distribution Functions and Higgs Boson Production at Moderate Transverse Momentum

We investigate the gluon distribution functions and their contributions to the Higgs boson production in pp collisions in the transverse momentum dependent factorization formalism. In addition to the usual azimuthal symmetric transverse momentum dependent gluon distribution, we find that the azimuthal correlated gluon distribution also contributes to the Higgs boson production. This explains recent findings on the additional contribution in the transverse momentum resummation for the Higgs boson production as compared to that for electroweak boson production processes. We further examine the small-x naive k_t-factorization in the dilute region and find that the azimuthal correlated gluon distribution contribution is consistently taken into account, and the result agrees with the transverse momentum dependent factorization formalism. We comment on the possible breakdown of the naive k_t-factorization in the dense medium region, due to the unique behaviors for the gluon distributions.

hep-ph↗

arXiv2011

Single Spin Asymmetry Scaling in the Forward Rapidity Region at RHIC

We investigate the scaling properties in inclusive hadron production and the associated single transverse spin asymmetry in the forward rapidity region at RHIC. We find that the spin-averaged experimental data in both $pp$ and $dAu$ collisions demonstrates a transverse-momentum-dependent geometric scaling. We introduce the transverse momentum dependent Collins fragmentation function to study the scaling of the single transverse spin asymmetries. The general feature of the scaling analysis is consistent with the experimental observations, in particular, for the transverse momentum dependence of the spin asymmetries at RHIC energy. We further propose to probe the saturation scale of nucleus by measuring the spin asymmetry normalized by that in $pp$ scattering at low transverse momentum.

hep-ph↗

arXiv2011

QCD Resummation for Single Spin Asymmetries

We study the transverse momentum dependent factorization for single spin asymmetries in Drell-Yan and semi-inclusive deep inelastic scattering processes at one-loop order. The next-to-leading order hard factors are calculated in the Ji-Ma-Yuan factorization scheme. We further derive the QCD resummation formalisms for these observables following the Collins-Soper-Sterman method. The results are expressed in terms of the collinear correlation functions from initial and/or final state hadrons coupled with the Sudakov form factor containing all order soft-gluon resummation effects. The scheme independent coefficients are calculated up to one-loop order.

hep-ph↗

arXiv2011

Observational evidence for AGNs feedback at parsec scale

In a hot accretion flow, the radiation from the innermost region of the flow propagates outward and heats the electrons at large radii via Compton scattering. It has been shown in previous works that if the radiation is strong enough, $L\ga 2%L_{\rm Edd}$, the electrons at the Bondi radius ($r_B\sim 10^5 r_s$) will be heated to be above the virial temperature thus the accretion will be stopped. The accretion will recover after the gas cools down. This results in the oscillation of the black hole activity. In this paper we show that this mechanism is the origin of the intermittent activity of some compact young radio sources. Such intermittency is required to explain the population of these sources. We calculate the timescales of the black hole oscillation and find that the durations of active and inactive phases are $3\times 10^4 (0.1/α)(M/10^8\msun) (L/2%L_{\rm Edd})^{-1/2} {\rm yr}$ and $10^5(α/0.1)(M/10^8\msun) {\rm yr}$, respectively, consistent with those required to explain observations. Such kind of feedback occurring at parsec scale should be common in low-luminosity AGNs and should be considered when we consider their matter and energy output.

astro-ph.CO↗

arXiv2011

Universality of Unintegrated Gluon Distributions at small x

We systematically study dijet production in various processes in the small-$x$ limit and establish an effective $k_t$-factorization for hard processes in a system with dilute probes scattering on a dense target. We find that the well-known Weizsäcker-Williams gluon distribution can be directly probed in the quark-antiquark jet correlation in deep inelastic scattering and the dipole gluon distribution can be directly measured in the direct photon-jet correlation in $pA$ collisions. In the large-$N_c$ limit, the unintegrated gluon distributions involved in other different dijet channels in pA collisions are shown to be related to two widely proposed ones: the Weizsäcker-Williams gluon distribution and the dipole gluon distribution.

hep-ph↗

arXiv2011

An Observation Concerning the Process Dependence of the Sivers Functions

The $k_\perp$-moment of a quark's Sivers function is known to be related to the corresponding twist-three quark-gluon correlation function $T_{q,F}(x, x)$. The two functions have been extracted from data for single-spin asymmetries in semi-inclusive deep inelastic scattering and in single-inclusive hadron production in $pp$ collisions, respectively. Performing a consistent comparison of the extracted functions, we find that they show a "sign mismatch": while the magnitude of the functions is roughly consistent, the $k_\perp$-moment of the Sivers function has opposite sign from that of $T_{q,F}(x, x)$, both for up and for down quarks. Barring any inconsistencies in our theoretical understanding of the Sivers functions and their process dependence, the implication of this mismatch is that either, the Sivers effect is not dominantly responsible for the observed single-spin asymmetries in $pp$ collisions or, the current semi-inclusive lepton scattering data do not sufficiently constrain the $k_\perp$-moment of the quark Sivers functions. Both possibilities strengthen the case for further experimental investigations of single-spin asymmetries in high-energy $pp$ and $ep$ scattering.

hep-ph↗

arXiv2011

Revisiting the Thermal Stability of Radiation-dominated Thin Disks

The standard thin disk model predicts that when the accretion rate is over a small fraction of the Eddington rate, which corresponds to $L \ga 0.06 L_{Edd}$, the inner region of the disk is radiation-pressure-dominated and thermally unstable. However, observations of the high/soft state of black hole X-ray binaries with luminosity well within this regime ($0.01L_{Edd} \la L \la 0.5L_{Edd}$) indicate that the disk has very little variability, i.e., quite stable. Recent radiation magnetohydrodynamic simulations of a vertically stratified shearing box have confirmed the absence of the thermal instability. In this paper, we revisit the thermal stability by linear analysis, taking into account the role of magnetic field in the accretion flow. By assuming that the field responses negatively to a positive temperature perturbation, we find that the threshold of accretion rate above which the disk becomes thermally unstable increases significantly compared with the case of not considering the role of magnetic field. This accounts for the stability of the observed sources with high luminosities. Our model also presents a possible explanation as to why only GRS 1915+105 seems to show thermally unstable behavior. This peculiar source holds the highest accretion rate (or luminosity) among the known high state sources, which is well above the accretion rate threshold of the instability.

astro-ph.HE↗

arXiv2011

kt-factorization for Hard Processes in Nuclei

Two widely proposed kt-dependent gluon distributions in the small-x saturation regime are investigated using two particle back-to-back correlations in high energy scattering processes. The Weizsacker-Williams gluon distribution, interpreted as the number density of gluon inside the nucleus, is studied in the quark-antiquark jet correlation in deep inelastic scattering. On the other hand, the unintegrated gluon distribution, defined as the Fourier transform of the color-dipole cross section, is probed in the direct photon-jet correlation in pA collisions. Dijet-correlation in pA collisions depends on both gluon distributions through combination and convolution in the large Nc limit.

hep-ph↗

arXiv2011

Magnetothermal and magnetorotational instabilities in hot accretion flows

In a hot, dilute, magnetized accretion flow, the electron mean-free path can be much greater than the Larmor radius, thus thermal conduction is anisotropic and along magnetic field lines. In this case, if the temperature decreases outward, the flow may be subject to a buoyancy instability (the magnetothermal instability, or MTI). The MTI amplifies the magnetic field, and aligns field lines with the radial direction. If the accretion flow is differentially rotating, the magnetorotational instability (MRI) may also be present. Using two-dimensional, time-dependent magnetohydrodynamic simulations, we investigate the interaction between these two instabilities. We use global simulations that span over two orders of magnitude in radius, centered on the region around the Bondi radius where the infall time of gas is longer than the growth time of both the MTI and MRI. Significant amplification of the magnetic field is produced by both instabilities, although we find that the MTI primarily amplifies the radial component, and the MRI primarily the toroidal component, of the field, respectively. Most importantly, we find that if the MTI can amplify the magnetic energy by a factor $F_t$, and the MRI by a factor $F_r$, then when the MTI and MRI are both present, the magnetic energy can be amplified by a factor of $F_t \cdot F_r$. We therefore conclude that amplification of the magnetic energy by the MTI and MRI operates independently. We also find that the MTI contributes to the transport of angular momentum, because radial motions induced by the MTI increase the Maxwell (by amplifying the magnetic field) and Reynolds stresses. Finally, we find that thermal conduction decreases the slope of the radial temperature profile. The increased temperature near the Bondi radius decreases the mass accretion rate.

astro-ph.HE↗

arXiv2010

Quark splitting in non-trivial θ-vacuum

Quark splitting in non-trivial $θ$-vacuum with a given helicity is investigated in pQCD with a modified quark propagator. We found that the quark splitting functions were modified by the presence of a topologically non-trivial QCD background field, though there is no explicit helicity flip associated with the radiative processes. The interaction with the topological non-trivial field leads to the degeneracy of the quark splitting functions for left- and right-handed quarks. Such degeneracy can lead to imbalance of left- and right-handed quarks in quark jet showers. We also discuss phenomenological consequences of such imbalance if there exists non-trivial topological gluon field configuration in heavy-ion collisions.

hep-ph↗

arXiv2010

On the Origin of Ultraviolet Emission and the Accretion Model of Low-luminosity AGNs

Low-luminosity active galactic nuclei (LLAGNs) are generally believed to be powered by an inner radiatively inefficient, advection-dominated accretion flow (ADAF), an outer truncated thin disk, and a jet. Maoz (2007) recently challenged this picture based on the observation that the strength of ultraviolet emission relative to the X-ray and radio bands does not depart from empirical trends defined by more luminous sources. He advocates that AGNs across all luminosities have essentially the same accretion and radiative processes, which in luminous sources are described by a standard optically thick, geometrically thin disk. We calculate ADAF models and demonstrate that they can successfully fit the observed spectral energy distributions of the LLAGNs in Maoz's sample. Our model naturally accommodates the radio and X-ray emission, and the ultraviolet flux is well explained by a combination of the first-order Compton scattering in the ADAF, synchrotron emission in the jet, and black body emission in the truncated thin disk. It is premature to dismiss the ADAF model for LLAGNs. The UV data can be fit equally well using a standard thin disk, but an additional corona and jet would be required to account for the X-ray and radio emission. We argue that there are strong theoretical reasons to prefer the ADAF model over the thin disk scenario. We discuss testable predictions that can potentially discriminate between the two accretion models.

astro-ph.HE↗

arXiv2010

Initial and Final State Interaction Effects in Small-x Quark Distributions

We study the initial and final state interaction effects in the transverse momentum dependent parton distributions in the small-$x$ saturation region. In particular, we discuss the quark distributions in the semi-inclusive deep inelastic scattering, Drell-Yan lepton pair production and dijet-correlation processes in $pA$ collisions. We calculate the quark distributions in the scalar-QED model and then extend to the color glass condensate formalism in QCD. The quark distributions are found universal between the DIS and Drell-Yan processes. On the other hand, the quark distribution from the $qq'\to qq'$ channel contribution to the dijet-correlation process is not universal. However, we find that it can be related to the quark distribution in DIS process by a convolution with the normalized unintegrated gluon distribution in the color glass condensate formalism in the large $N_c$ limit.

hep-ph↗

arXiv2010

Test of the Universality of Naive-time-reversal-odd Fragmentation Functions

We investigate the ''spontaneous'' hyperon transverse polarization in $e^+e^-$ annihilation and semi-inclusive deep inelastic scattering processes as a test of the universality of the naive-time-reversal-odd transverse momentum dependent fragmentation functions. We find that universality implies definite sign relations among various observables. This provides a unique opportunity to study initial/final state interaction effects in the fragmentation process and test the associated factorization.

hep-ph↗

arXiv2010

On the convective instability of hot radiative accretion flows

How many fraction of gas available at the outer boundary can finally fall onto the black hole is an important question. It determines the observational appearance of accretion flows, and is also related with the evolution of black hole mass and spin. Previous two-dimensional hydrodynamical simulations of hot accretion flows find that the flow is convectively unstable because of its inward increase of entropy. As a result, the mass accretion rate decreases inward, i.e., only a small fraction of accretion gas can fall onto the black hole, while the rest circulates in the convective eddies or lost in convective outflows. Radiation is usually neglected in these simulations. In many cases, however, radiative cooling is important. In the regime of the luminous hot accretion flow (LHAF), radiative cooling is even stronger than the viscous dissipation. In the one dimensional case, this implies that the inward increase of entropy will become slower or the entropy even decreases inward in the case of an LHAF. We therefore expect that convective instability becomes weaker or completely disappears when radiative cooling is important. To examine the validity of this expectation, in this paper we perform two-dimensional hydrodynamical simulations of hot accretion flows with strong radiative cooling. We find that compared to the case of negligible radiation, convection only becomes slightly weaker. Even an LHAF is still strongly convectively unstable, its radial profile of accretion rate correspondingly changes little. We find the reason is that the entropy still increases inward in the two-dimensional case.

astro-ph.HE↗

arXiv2010

On the convective instability of hot radiative accretion flows

astro-ph.HE↗

arXiv2010

Modeling the hard states of three black hole candidates

Simultaneous multiwavelength observations were recently performed for three black hole candidates --- SWIFT J1753.5-0127, GRO J1655-40 and XTE J1720-318. In this paper we test the accretion-jet model originally proposed to XTE J1118+480 by investigating the hard state of these three sources using this model. The accretion flow in the model is composed of an inner hot accretion flow and an outer truncated thin disk. We find that the model satisfactorily explains the spectrum ranging from radio to X-rays, with the radio and X-ray spectra dominated by the synchrotron and thermal Comptonization emissions in the jet and the hot accretion flow respectively, while the infrared and optical being the sum of the emissions from the jet, hot accretion flow, and the truncated thin disk. Similar to the case of XTE J1118+480, the model can also explain, although only qualitatively in some cases, the observed timing features including QPO, and positive and negative time lags between the optical and X-ray emissions detected in SWIFT J1753.5-0127. The origin of the ejection events detected in XTE J1720-318 is also briefly discussed.

astro-ph.HE↗

arXiv2010

Non-Universality of Transverse Momentum Dependent Parton Distributions at Small-x

We study the universality issue of the transverse momentum dependent parton distributions at small-x, by comparing the initial/final state interaction effects in dijet-correlation in pA collisions with those in deep inelastic lepton nucleus scattering. We demonstrate the non-universality by performing an explicit calculation in a particular model where the multiple gauge boson exchange contributions are summed up to all orders. We comment on the implications of our results on the theoretical interpretation of di-hadron correlation in dA collisions in terms of the saturation phenomena in deep inelastic lepton nucleus scattering.

hep-ph↗

arXiv2010

Accretion and ejection in Sgr A*

We review our current understanding to the accretion and ejection processes in Sgr A*. Roughly speaking, they correspond to the quiescent and flare states of the source respectively. The high-resolution {\it Chandra} observations to the gas at the Bondi radius combined with the Bondi accretion theory, the spectral energy distribution from radio to X-ray, and the radio polarization provide us strict constraints and abundant information to the theory of accretion. We review these observational results and describe how the advection-dominated accretion flow model explains these observations. Recently more attentions have been paid to flares in Sgr A*. Many simultaneous multi-wavelength campaigns have been conducted, aiming at uncovering the nature of flares. The main observational properties of flares are briefly reviewed. Especially, the time lag between the peaks of flare at two radio frequencies strongly indicates that the flare is associated with ejection of radio-emitting blobs from the underlying accretion flow. Such kind of episodic jets is distinctive from the continuous jets and are quite common in black hole systems. We introduce the magnetohydrodynamical model for the formation of episodic jets recently proposed based on the analogy with the theory of coronal mass ejection in the Sun. We point out that the various observational appearances of flares should be explained in the framework of this model, since ejection and flare originate from the same physical process.

astro-ph.HE↗