arXiv subjects

T. G. Steele

Publications and source records attributed to T. G. Steele.

At least 55 records · Page 3Linked to original sources

arXiv2017

A QCD Sum-Rules Analysis of Vector ($1^{--}$) Heavy Quarkonium Meson-Hybrid Mixing

We use QCD Laplace sum-rules to study meson-hybrid mixing in vector ($1^{--}$) heavy quarkonium. We compute the QCD cross-correlator between a heavy meson current and a heavy hybrid current within the operator product expansion. In addition to leading-order perturbation theory, we include four- and six-dimensional gluon condensate contributions as well as a six-dimensional quark condensate contribution. We construct several single and multi-resonance models that take known hadron masses as inputs. We investigate which resonances couple to both currents and so exhibit meson-hybrid mixing. Compared to single resonance models that include only the ground state, we find that models that also include excited states lead to significantly improved agreement between QCD and experiment. In the charmonium sector, we find that meson-hybrid mixing is consistent with a two-resonance model consisting of the $J/ψ$ and a 4.3~GeV resonance. In the bottomonium sector, we find evidence for meson-hybrid mixing in the $Υ(1S)$, $Υ(2S)$, $Υ(3S)$, and $Υ(4S)$.

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arXiv2017

The Mass and Decay Properties of the $1^{-+}$ Light Hybrid Meson

We calculate the complete form of the dimension-8 condensate contributions in the two-point correlator of the ($1^{-+}$,$0^{++}$) light hybrid current considering the operator mixing under renormalization. We find the inclusion these higher power corrections as well as the update of $\langle g^3G^3\rangle$ increase the QCD sum rule mass prediction for the $1^{-+}$ light hybrid. The obtained conservative mass range 1.72--2.60 GeV does not favor the $π_1(1400)$ and the $π_1(1600)$ to be pure hybrid states and suggests the $π_1(2015)$ observed by E852 is more likely to have much of a hybrid constituent. We also study the $b_1π$ and $ρπ$ decay patterns of the $1^{-+}$ light hybrid with light-cone QCD sum rules. We obtain a relatively large partial decay width of the $b_1π$ mode, which is consistent with the predictions from the flux tube models and lattice QCD. More interestingly, using the tensor interpolating current we find the partial decay width of the $ρπ$ mode is small due to the absence of the leading twist contribution in the light-cone expansion of the correlation function.

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arXiv2017

Ground State Mass Predictions of Heavy-Light Hybrids from QCD Sum-Rule Analysis ($J^P=\left\{0^{\pm},\,1^{\pm}\right\}$)

We present QCD Laplace sum-rule predictions of ground state masses of heavy-light open-flavour hybrid mesons. Having computed leading-order diagonal correlation functions, including up to dimension six gluon condensate contributions, we extract hybrid mass predictions for all $J^P \in \{0^\pm, 1^\pm\}$, and explore possible mixing effects with conventional meson states. Similarities are found in the mass hierarchy in both charm and bottom systems with some exceptions that are discussed.

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arXiv2017

Heavy Quarkonium ($1^{--}$) Meson-Hybrid Mixing from QCD Sum Rules

We use QCD Laplace sum-rules to explore mixing between conventional mesons and hybrids in the heavy quarkonium vector $J^{PC}\!=\!1^{--}$ channel. Our cross-correlator includes perturbation theory and contributions proportional to the four-dimensional and six-dimensional gluon condensates. We input experimentally determined charmonium and bottomonium hadron masses into both single and multi-resonance models in order to test them for conventional meson and hybrid components. In the charmonium sector we find evidence for meson-hybrid mixing in the $J/ψ$ and a $\approx4.3$ GeV resonance. In the bottomonium sector, we find that the $Υ(1S)$, $Υ(2S)$, $Υ(3S)$, and $Υ(4S)$ all exhibit mixing.

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arXiv2017

Hunting for exotic doubly hidden-charm/bottom tetraquark states

We develop a moment QCD sum rule method augmented by fundamental inequalities to study the existence of exotic doubly hidden-charm/bottom tetraquark states made of four heavy quarks. Using the compact diquark-antidiquark configuration, we calculate the mass spectra of these tetraquark states. There are 18 hidden-charm $cc\bar c\bar c$ tetraquark currents with $J^{PC} = 0^{++}$, $0^{-+}$, $0^{--}$, $1^{++}$, $1^{+-}$, $1^{-+}$, $1^{--}$, and $2^{++}$. We use them to perform QCD sum rule analyses, and the obtained masses are all higher than the spontaneous dissociation thresholds of two charmonium mesons, which are thus their dominant decay modes. The masses of the corresponding hidden-bottom $bb\bar b\bar b$ tetraquarks are all below or very close to the thresholds of the $Υ(1S)Υ(1S)$ and $η_b(1S)η_b(1S)$, except one current of $J^{PC}=0^{++}$. Hence, we suggest to search for the doubly hidden-charm states in the $J/ψJ/ψ$ and $η_c(1S)η_c(1S)$ channels.

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arXiv2017

Constraint on the light quark mass $m_q$ from QCD Sum Rules in the $I=0$ scalar channel

In this paper, we reanalyze the $I=0$ scalar channel with the improved Monte-Carlo based QCD sum rules, which combines the rigorous Hölder-inequality-determined sum rule window and a two Breit-Wigner type resonances parametrization for the phenomenological spectral density that satisfies the the low-energy theorem for the scalar form factor. Considering the uncertainties of the QCD parameters and the experimental masses and widths of the scalar resonances $σ$ and $f_0(980)$, we obtain a prediction for light quark mass $m_q(2\,\textrm{GeV})$ = $\frac{1}{2}(m_u(2\,\textrm{GeV})$ + $m_d(2\,\textrm{GeV}))$ = $4.7^{+0.8}_{-0.7}\,\textrm{MeV}$, which is consistent with the PDG (Particle Data Group) value and QCD sum rule determinations in the pseudoscalar channel. This agreement provides a consistent framework connecting QCD sum rules and low-energy hadronic physics. We also obtain the decay constants of $σ$ and $f_0(980)$ at 2 GeV, which are approximately $0.64-0.83$ GeV and $0.40-0.48$ GeV respectively.

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arXiv2017

Mass spectra for $qc\bar q\bar c$, $sc\bar s\bar c$, $qb\bar q\bar b$, $sb\bar s\bar b$ tetraquark states with $J^{PC}=0^{++}$ and $2^{++}$

We have studied the mass spectra of the hidden-charm/bottom $qc\bar q\bar c$, $sc\bar s\bar c$ and $qb\bar q\bar b$, $sb\bar s\bar b$ tetraquark states with $J^{PC}=0^{++}$ and $2^{++}$ in the framework of QCD sum rules. We construct ten scalar and four tensor interpolating currents in a systematic way and calculate the mass spectra for these tetraquark states. The $X^\ast(3860)$ may be either an isoscalar tetraquark state or $χ_{c0}(2P)$. If the $X^\ast(3860)$ is a tetraquark candidate, our results prefer the $0^{++}$ option over the $2^{++}$ one. The $X(4160)$ may be classified as either the scalar or tensor $qc\bar q\bar c$ tetraquark state while the $X(3915)$ favors a $0^{++}$ $qc\bar q\bar c$ or $sc\bar s\bar c$ tetraquark assignment over the tensor one. The $X(4350)$ can not be interpreted as a $sc\bar s\bar c$ tetraquark with either $J^{PC}=0^{++}$ or $2^{++}$.

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arXiv2017

Open-flavor charmed/bottom $sq\bar q\bar Q$ and $qq\bar q\bar Q$ tetraquark states

We provide comprehensive investigations for the mass spectrum of exotic open-flavor charmed/bottom $sq\bar q\bar c$, $qq\bar q\bar c$, $sq\bar q\bar b$, $qq\bar q\bar b$ tetraquark states with various spin-parity assignments $J^P=0^+, 1^+, 2^+$ and $0^-, 1^-$ in the framework of QCD sum rules. In the diquark configuration, we construct the diquark-antidiquark interpolating tetraquark currents using the color-antisymmetric scalar and axial-vector diquark fields. The stable mass sum rules are established in reasonable parameter working ranges, which are used to give reliable mass predictions for these tetraquark states. We obtain the mass spectra for the open-flavor charmed/bottom $sq\bar q\bar c$, $qq\bar q\bar c$, $sq\bar q\bar b$, $qq\bar q\bar b$ tetraquark states with various spin-parity quantum numbers. In addition, we suggest searching for exotic doubly-charged tetraquarks, such as $[sd][\bar u\bar c]\to D_s^{(\ast)-}π^-$ in future experiments at facilities such as BESIII, BelleII, PANDA, LHCb and CMS, etc.

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arXiv2017

Masses of Open-Flavour Heavy-Light Hybrids from QCD Sum-Rules

We use QCD Laplace sum-rules to predict masses of open-flavour heavy-light hybrids where one of the hybrid's constituent quarks is a charm or bottom and the other is an up, down, or strange. We compute leading-order, diagonal correlation functions of several hybrid interpolating currents, taking into account QCD condensates up to dimension-six, and extract hybrid mass predictions for all $J^P\in\{0^{\pm},\,1^{\pm}\}$, as well as explore possible mixing effects with conventional quark-antiquark mesons. Within theoretical uncertainties, our results are consistent with a degeneracy between the heavy-nonstrange and heavy-strange hybrids in all $J^P$ channels. We find a similar mass hierarchy of $1^+$, $1^{-}$, and $0^+$ states (a $1^{+}$ state lighter than essentially degenerate $1^{-}$ and $0^{+}$ states) in both the charm and bottom sectors, and discuss an interpretation for the $0^-$ states. If conventional meson mixing is present the effect is an increase in the hybrid mass prediction, and we estimate an upper bound on this effect.

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arXiv2017

A comprehensive revisit of the $ρ$ meson with improved Monte-Carlo based QCD sum rules

We improve the Monte-Carlo based QCD sum rules by introducing the rigorous Hölder-inequality-determined sum rule window and a Breit-Wigner type parametrization for the phenomenological spectral function. In this improved sum rule analysis methodology, the sum rule analysis window can be determined without any assumptions on OPE convergence or the QCD continuum.Therefore an unbiased prediction can be obtained for the phenomenological parameters (the hadronic mass and width etc.). We test the new approach in the $ρ$ meson channel with re-examination and inclusion of $α_s$ corrections to dimension-4 condensates in the OPE. We obtain results highly consistent with experimental values. We also discuss the possible extension of this method to some other channels.

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arXiv2016

Conformal Complex Scalar Singlet Extensions of the Standard Model: Symmetry Breaking Patterns and Phenomenology

We consider a conformal complex singlet extension of the Standard Model with a Higgs portal interaction. Two different scenarios depending on whether the global U(1) symmetry is broken or not have been studied. In the unbroken phase, the decay of the complex singlet is protected by the global U(1) symmetry which leads to an ideal cold dark matter candidate. In the broken phase, we are able to provide a second Higgs at $554\,\rm{GeV}$. In addition, gauging the global U(1) symmetry, we can construct an asymptotically safe U(1)' leptophobic model. We combine the notion of asymptotic safety with conformal symmetry and use the renormalization group equations as a bridge to connect UV boundary conditions and Electroweak/ TeV scale physics. We also provide a detailed example to show that these boundary conditions will lead to phenomenological signatures such as diboson excesses which could be tested at the LHC.

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arXiv2016

Remarks on Top-philic $Z^\prime$ Boson Interactions with Nucleons

This article provides the calculation of an effective vertex function between a nucleon and a $Z^\prime$ boson that couples preferentially to either the top quark or the third generation of fermions, for the purpose of calculating vector-portal dark matter nuclear recoil cross sections. Mixing effects between the new gauge group $U(1)^\prime$ and the Standard Model hypercharge group $U(1)_Y$ are taken into account. Contributions to the $U(1)^\prime$ nucleon current from heavy quarks are quantified using the heavy quark expansion. Also taken into account are contributions from the 1-loop $Z^\prime$-gluon interactions and mixing-induced contributions from the light quarks in the nucleon. We find that, for reasonable values of the $U(1)^\prime$ gauge parameter, contributions from the light quarks dominate despite being mixing-suppressed. It is shown that this holds for most models even if mixing effects do not appear at tree level. Contributions from the heavy quarks and gluons are suppressed by $1/m_Q^2$ and possibly also by momentum in the low momentum transfer limits relevant for dark matter direct detection. We discuss under which conditions the subdominant terms become relevant.

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arXiv2016

QCD sum rule study of hidden-charm pentaquarks

We study the mass spectra of hidden-charm pentaquarks having spin $J = {1\over2}/{3\over2}/{5\over2}$ and quark contents $uud c \bar c$. We systematically construct all the relevant local hidden-charm pentaquark currents, and select some of them to perform QCD sum rule analyses. We find that the $P_c(4380)$ and $P_c(4450)$ can be identified as hidden-charm pentaquark states composed of an anti-charmed meson and a charmed baryon. Besides them, we also find a) the lowest-lying hidden-charm pentaquark state of $J^P = 1/2^-$ has the mass $4.33^{+0.17}_{-0.13}$ GeV, while the one of $J^P = 1/2^+$ is significantly higher, that is around $4.7-4.9$ GeV; b) the lowest-lying hidden-charm pentaquark state of $J^P = 3/2^-$ has the mass $4.37^{+0.18}_{-0.13}$ GeV, consistent with the $P_c(4380)$ of $J^P = 3/2^-$, while the one of $J^P = 3/2^+$ is also significantly higher, that is above $4.6$ GeV; c) the hidden-charm pentaquark state of $J^P = 5/2^-$ has a mass around $4.5-4.6$ GeV, slightly larger than the $P_c(4450)$ of $J^P = 5/2^+$.

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arXiv2016

Detection prospects for conformally constrained vector-portal dark matter

We work with a UV conformal U(1)' extension of the Standard Model, motivated by the hierarchy problem and recent collider anomalies. This model admits fermionic vector portal WIMP dark matter charged under the U(1)' gauge group. The asymptotically safe boundary conditions can be used to fix the coupling parameters, which allows the observed thermal relic abundance to constrain the mass of the dark matter particle. This highly restricts the parameter space, allowing strong predictions to be made. The parameter space of several UV conformal U(1)' scenarios will be explored, and both bounds and possible signals from direct and indirect detection observation methods will be discussed.

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arXiv2016

Investigation of the light four-quark states with exotic $J^{PC}=0^{--}$

We study the exotic $J^{PC}=0^{--}$ four-quark states in Laplace sum rules (LSR) and finite energy sum rules (FESR). We use the vector tetraquark-like currents as interpolating currents in the correlator, from which the $1^{+-}$ states are also studied. In the mass extraction, we use the standard stability criterion with respect to the Borel parameters and the QCD continuum thresholds and consider the effect of the violation of factorization in estimating the high dimensional condensates as a source of uncertainties. The obtained mass prediction $1.66\pm0.14$ GeV is much lower than the previous sum rule predictions obtained using the scalar currents. Our result favours the four-quark interpretation of the possible $ρπ$ dominance in the $D^0$ decay. We also discuss the possible decay patterns of these exotic four-quark states.

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arXiv2016

Revisiting the b1 pi and rho pi decay modes of the 1-+ light hybrid state with light-cone QCD sum rules

We study the $ρπ$ and $b_1π$ decay modes of the $1^{-+}$ light hybrid state within the framework of light-cone QCD sum rules. We use both the tensor current $\barψσ_{μν}ψ$ and the derivative current $\barψ\overleftrightarrow{D}_μγ_5ψ$ as interpolating currents to calculate the partial decay width of the $b_1π$ decay mode. Comparing the sum rules obtained by using different currents, we obtain $Γ(π_1\to b_1π)$ = 8--23, 32--86 and 52--151\,MeV for $m_{1^{-+}}$ = 1.6, 1.8 and 2.0\,GeV respectively, which favour the results from the flux tube models and lattice simulations. We also use the tensor current to study the $ρπ$ decay mode, and although an extended stability criterion is needed, our results suggest a small partial decay width.

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arXiv2016

Conformal Complex Singlet Extension of the Standard Model: Scenario for Dark Matter and a Second Higgs Boson

We consider a conformal complex singlet extension of the Standard Model with a Higgs portal interaction. The global $U(1)$ symmetry of the complex singlet can be either broken or unbroken and we study each scenario. In the unbroken case, the global $U(1)$ symmetry protects the complex singlet from decaying, leading to an ideal cold dark matter candidate with approximately 100 GeV mass along with a significant proportion of thermal relic dark matter abundance. In the broken case, we have developed a renormalization-scale optimization technique to significantly narrow the parameter space and in some situations, provide unique predictions for all the model's couplings and masses. We have found there exists a second Higgs boson with a mass of approximately $550\,\rm{GeV}$ that mixes with the known $125\,\rm{GeV}$ Higgs with a large mixing angle $\sinθ\approx 0.47$ consistent with current experimental limits. The imaginary part of the complex singlet in the broken case could provide axion dark matter for a wide range of models. Upon including interactions of the complex scalar with an additional vector-like fermion, we explore the possibility of a diphoton excess in both the unbroken and the broken cases. In the unbroken case, the model can provide a natural explanation for diphoton excess if extra terms are introduced providing extra contributions to the singlet mass. In the broken case, we find a set of coupling solutions that yield a second Higgs boson of mass $720\,\rm{GeV}$ and an $830\,\rm{GeV}$ extra vector-like fermion $F$, which is able to address the $750\,\rm{GeV}$ LHC diphoton excess. We also provide criteria to determine the symmetry breaking pattern in both the Higgs and hidden sectors.

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arXiv2016

Decoding the $X(5568)$ as a fully open-flavor $su\bar b\bar d$ tetraquark state

We investigate the recent evidence for a charged $X(5568)$ meson as an exotic open-flavor tetraquark state $su\bar b\bar d$ with $J^P=0^+/1^+$ in the framework of QCD sum rules. We use the color antisymmetric $[\mathbf{\bar 3_c}]_{su} \otimes [\mathbf{3_c}]_{\bar{b}\bar d}$ tetraquark currents in both scalar and axial-vector channels to perform evaluations and numerical analyses. Our results imply that the $X(5568)$ can be interpreted as both the scalar $su\bar b\bar d$ tetraquark state and the axial-vector one, which are in good agreement with the experimental measurement. We also discuss the possible decay patterns of the $X(5568)$ and suggest to search for its neutral partner in the radiative decay into $B_s^0 γ$ and $B_s^* γ$, which can be used to determine its spin-parity quantum numbers. Moreover, we predict its charmed partner state around $2.55$ GeV with the quark content $su\bar c\bar d$ and $J^P=0^+/1^+$.

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