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Zhiguang Xiao

Publications and source records attributed to Zhiguang Xiao.

52 records · Page 3Linked to original sources
arXiv2013

Sakai-Sugimoto model in D0-D4 background

We add smeared D0 charges to the D4 background and discuss Sakai-Sugimoto model under this background. The corresponding gauge theory develops a glue condensate $< tr (F_{μν}\tilde F^{μν})> $. The D8 branes go less deep than in the original S-S model and massless Goldstones are still found in the spectrum. The effects of the condensate on the meson spectra, pion decay constant, and couplings of the vector mesons and Goldstones are then investigated.

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arXiv2012

Bubbles and Black Branes in Grand Canonical Ensemble

When the phase structure of the black brane in grand canonical ensemble is discussed, a new bubble phase with the same boundary data is found to exist in this structure. As such, the phase transitions among bubble, black brane and "hot flat space" are possible, therefore giving a much enriched phase structure. We argue that under some conditions, either the grand canonical ensemble itself is unstable or there are some unknown new phases.

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arXiv2011

The enriched phase structure of black branes in canonical ensemble

It is found that a necessary completion of phase structure of D-dimensional charged black p-brane ($p > 0$) in a cavity requires two additional thermodynamical phases, the so-called "bubble of nothing" and/or the extremal brane, in canonical ensemble. This finding resolves the puzzle about the missing phases which are needed for the underlying phase diagram when $\tilde d = D - p - 3 \leq 2$ and gives a new (bubble) phase which can become globally stable when $\tilde d > 2$. An analog of Hawking-Page transition is also found among other new phase transitions, giving a complete phase structure in this setup.

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arXiv2011

Phase structure of black branes in grand canonical ensemble

This is a companion paper of our previous work [1] where we studied the thermodynamics and phase structure of asymptotically flat black $p$-branes in a cavity in arbitrary dimensions $D$ in a canonical ensemble. In this work we study the thermodynamics and phase structure of the same in a grand canonical ensemble. Since the boundary data in two cases are different (for the grand canonical ensemble boundary potential is fixed instead of the charge as in canonical ensemble) the stability analysis and the phase structure in the two cases are quite different. In particular, we find that there exists an analog of one-variable analysis as in canonical ensemble, which gives the same stability condition as the rather complicated known (but generalized from black holes to the present case) two-variable analysis. When certain condition for the fixed potential is satisfied, the phase structure of charged black $p$-branes is in some sense similar to that of the zero charge black $p$-branes in canonical ensemble up to a certain temperature. The new feature in the present case is that above this temperature, unlike the zero-charge case, the stable brane phase no longer exists and `hot flat space' is the stable phase here. In the grand canonical ensemble there is an analog of Hawking-Page transition, even for the charged black $p$-brane, as opposed to the canonical ensemble. Our study applies to non-dilatonic as well as dilatonic black $p$-branes in $D$ space-time dimensions.

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arXiv2011

Hadron loops effect on mass shifts of the charmed and charmed-strange spectra

The hadron loop effect is conjectured to be important in understanding discrepancies between the observed states in experiments and the theoretical expectations of non-relativistic potential model. We present that, in an easily operable procedure, the hadron loop effect could shift the poles downwards to reduce the differences and provide better descriptions of both the masses and the total widths, at least, of the radial quantum number $n=1$ charmed and charmed-strange states. The $1^1P_1-1^3P_1$ mixing phenomena could be naturally explained due to their couplings with common channels. The newly observed $D$ states are also addressed, but there are still some problems remaining unclear.

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arXiv2011

The origin of light $0^{+}$ scalar resonances

We demonstrate how most of the light $J^{P}=0^{+}$ spectrum below $2.0\,\mathrm{GeV}$ and their decays can be consistently described by the unitarized quark model incorporating the chiral constraints of Adler zeros and taking SU(3) breaking effects into account. These resonances appear as poles in the complex $s$ plane in a unified picture as $q\bar{q}$ states strongly dressed by hadron loops. Through the large $N_c$ analysis, these resonances are found to naturally separate into two kinds: $σ, κ, f_0(980), a_0(980)$ are dynamically generated and run away from the real axis as $N_c$ increases, while the others move towards the $q\bar q $ seeds. In this picture, the line shape of $a_0(980)$ is produced by a broad pole below the $K\bar{K}$ threshold, and exhibits characteristics similar to the $σ$ and $κ$.

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arXiv2011

Phase transitions and critical behavior of black branes in canonical ensemble

We study the thermodynamics and phase structure of asymptotically flat non-dilatonic as well as dilatonic black branes in a cavity in arbitrary dimensions ($D$). We consider the canonical ensemble and so the charge inside the cavity and the temperature at the wall are fixed. We analyze the stability of the black brane equilibrium states and derive the phase structures. For the zero charge case we find an analog of Hawking-Page phase transition for these black branes in arbitrary dimensions. When the charge is non-zero, we find that below a critical value of the charge, the phase diagram has a line of first-order phase transition in a certain range of temperatures which ends up at a second order phase transition point (critical point) as the charge attains the critical value. We calculate the critical exponents at that critical point. Although our discussion is mainly concerned with the non-dilatonic branes, we show how it easily carries over to the dilatonic branes as well.

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arXiv2009

S-matrix equivalence restored

The canonical transformation that maps light-cone Yang-Mills theory to a Lagrangian description of the MHV rules is non-local, consequently the two sets of fields do not necessarily generate the same S-matrix. By deriving a new recursion relation for the canonical transformation expansion coefficients, we find a direct map between these coefficients and tree level light-cone diagrams. We use this to show that, at least up to one-loop with dimensionally regularised MHV vertices, the only difference is the omission of the one-loop amplitudes in which all gluons have positive helicity.

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arXiv2009

The canonical transformation and massive CSW vertices for MHV-SQCD

The similarity of massive CSW scalar vertices and quark vertices can be understood using a kind of light-cone SUSY transformation presented in this paper. We also show that the canonical transformation generating the MHV-SQCD lagrangian, can be fixed by applying this light-cone SUSY transformation to the canonical transformation for MHV-QCD obtained in paper arxiv:0805.0239. Most of the massive CSW vertices for SQCD can also be pinned down in this way.

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arXiv2008

The MHV QCD Lagrangian

We perform a canonical change of the field variables of light-cone gauge massless QCD to obtain a lagrangian whose terms are proportional up to polarisation factors to MHV amplitudes and continued off shell by the CSW prescription. We solve for this transformation as a series expansion to all orders in the new fields, and use this to prove that the resulting vertices are indeed MHV vertices as claimed. We also demonstrate how this works explicitly for the vertices with: two quarks and two gluons, four quarks, and a particular helicity configuration of two quarks and three gluons. Finally, we generalise the construction to massive QCD.

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arXiv2003

Further remarks on $ππ$ scattering dispersion relations

The naive use of higher order perturbation theory leads the left--hand cut integrals in $ππ$ dispersion relations~\cite{hjy,Xiao01} divergent. This problem is discussed and solved. Also we point out that the Adler zero condition imposes three constraints on the dispersion relations. The $σ$ pole position is determined using the improved method, $M_σ= 483\pm 13 {\rm MeV}$,$Γ_σ= 705 \pm 50{\rm MeV}$. The scattering length parameter is found to be in excellent agreement with the experimental result.

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arXiv2001

The Use of Dispersion Relations in the $ππ$ and $K\bar K$ Coupled-Channel System

Systematic and careful studies are made on the properties of the IJ=00 $ππ$ and $K\bar K$ coupled-channel system, using newly derived dispersion relations between the phase shifts and poles and cuts. The effects of nearby branch point singularities to the determination of the $f_0(980)$ resonance are estimated and and discussed.

hep-ph↗
arXiv2001

A Critical Examination to the Unitarized $ππ$ Scattering Chiral Amplitudes

We discuss the Padé approximation to the $ππ$ scattering amplitudes in 1--loop chiral perturbation theory. The approximation restores unitarity and can reproduce the correct resonance poles, but the approximation violates crossing symmetry and produce spurious poles on the complex $s$ plane and therefore plagues its predictions on physical quantities at quantitative level. However we find that one virtual state in the IJ=20 channel may have physical relevance.

hep-ph↗
arXiv2001

The Use of Analyticity in the $ππ$ and $K\bar K$ Coupled Channel System

Studies on the IJ=00 $ππ$ and $K\bar K$ coupled--channel system are made using newly derived dispersion relations between the phase shifts and poles and cuts. It is found that the $σ$ resonance must be introduced to explain the experimental phase shifts, after evaluating the cut contribution. The effects of nearby branch point singularities to the determination of the $f_0(980)$ resonance are also carefully clarified.

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arXiv2001

Left Hand Singularities, Hadron Form Factors and the Properties of the Sigma Meson

By applying analyticity and single channel unitarity we derive a new formula which is useful to analyze the role of the left-hand singularities in hadron form factors and in the determination of the resonance parameters. Chiral perturbation theory is used to estimate the left-hand cut effects in $ππ$ scattering processes. We find that in the IJ=11 channel the left-hand cut effect is negligible and in the IJ=20 channel the phase shift is dominated by the left-hand cut effect. In the IJ=00 channel the left-hand cut contribution to the phase shift has the wrong sign comparing with the experimental data and therefore it necessitates the $σ$ resonance. The new experimental results from the E865 collaboration is crucial in reducing the uncertainty in the determination of the mass and width of the $σ$ resonance within our scheme.

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