arXiv subjects

Philipp Gubler

Publications and source records attributed to Philipp Gubler.

At least 55 records · Page 3Linked to original sources

arXiv2017

Flavor structure of $Λ$ baryons from lattice QCD: From strange to charm quarks

We study $Λ$ baryons of spin-parity $\frac{1}{2}^{\pm}$ with either a strange or charm valence quark in full 2+1 flavor lattice QCD. Multiple $SU(3)$ singlet and octet operators are employed to generate the desired single baryon states on the lattice. Via the variational method, the couplings of these states to the different operators provide information about the flavor structure of the $Λ$ baryons. We make use of the gauge configurations of the PACS-CS Collaboration and chirally extrapolate the results for the masses and $SU(3)$ flavor components to the physical point. We furthermore gradually change the hopping parameter of the heaviest quark from strange to charm to study how the properties of the $Λ$ baryons evolve as a function of the heavy quark mass. It is found that the baryon energy levels increase almost linearly with the quark mass. Meanwhile, the flavor structure of most of the states remains stable, with the exception of the lowest $\frac{1}{2}^{-}$ state, which changes from a flavor singlet $Λ$ to a $Λ_c$ state with singlet and octet components of comparable size. Finally, we discuss whether our findings can be interpreted with the help of a simple quark model and find that the negative-parity $Λ_c$ states can be naturally explained as diquark excitations of the light $u$ and $d$ quarks. On the other hand, the quark-model picture does not appear to be adequate for the negative-parity $Λ$ states, suggesting the importance of other degrees of freedom to describe them.

hep-lat↗

arXiv2016

Exact vector channel sum rules at finite temperature and their applications to lattice QCD data analysis

We derive three exact sum rules for the spectral function of the electromagnetic current with zero spatial momentum at finite temperature. Two of them are derived in this paper for the first time. We explicitly check that these sum rules are satisfied in the weak coupling regime and examine which sum rule is sensitive to the transport peak in the spectral function at low energy or the continuum at high energy. Possible applications of the three sum rules to lattice computations of the spectral function and transport coefficients are also discussed: We propose an ansatz for the spectral function that can be applied to all three sum rules and fit it to available lattice data of the Euclidean vector correlator above the critical temperature. As a result, we obtain estimates for both the electrical conductivity $σ$ and the second order transport coefficient $τ_J$.

hep-ph↗

arXiv2016

Negative-parity nucleon excited state in nuclear matter

Spectral functions of the nucleon and its negative parity excited state in nuclear matter are studied using QCD sum rules and the maximum entropy method (MEM). It is found that in-medium modifications of the spectral functions are attributed mainly to density dependencies of the $\langle \bar{q}q \rangle $ and $\langle q^{\dagger}q \rangle $ condensates. The MEM reproduces the lowest-energy peaks of both the positive and negative parity nucleon states at finite density up to $ρ\sim ρ_N$ (normal nuclear matter density). As the density grows, the residue of the nucleon ground state decreases gradually while the residue of the lowest negative parity excited state increases slightly. On the other hand, the positions of the peaks, which correspond to the total energies of these states, are almost density independent for both parity states. The density dependencies of the effective masses and vector self-energies are also extracted by assuming the mean-field green functions for the peak states. We find that, as the density increases, the nucleon effective mass decreases while the vector self-energy increases. The density dependence of these quantities for the negative parity state on the other hand turns out to be relatively weak.

hep-ph↗

arXiv2016

Phi meson spectral moments and QCD condensates in nuclear matter

A detailed analysis of the lowest two moments of the $ϕ$ meson spectral function in vacuum and nuclear matter is performed. The consistency is examined between the constraints derived from finite energy QCD sum rules and the spectra computed within an improved vector dominance model, incorporating the coupling of kanonic degrees of freedom with the bare $ϕ$ meson. In the vacuum, recent accurate measurements of the $e^+ e^- \to K^+ K^-$ cross section allow us to determine the spectral function with high precision. In nuclear matter, the modification of the spectral function can be described by the interactions of the kaons from $ϕ\rightarrow K\bar{K}$ with the surrounding nuclear medium. This leads primarily to a strong broadening and an asymmetric deformation of the $ϕ$ meson peak structure. We confirm that, both in vacuum and nuclear matter, the zeroth and first moments of the corresponding spectral functions satisfy the requirements of the finite energy sum rules to a remarkable degree of accuracy. Limits on the strangeness sigma term of the nucleon are examined in this context. Applying our results to the second moment of the spectrum, we furthermore discuss constraints on four-quark condensates and the validity of the commonly used ground state saturation approximation.

hep-ph↗

arXiv2016

D meson mass increase by restoration of chiral symmetry in nuclear matter

Spectral functions of the pseudoscalar $D$ meson in the nuclear medium are analyzed using QCD sum rules and the maximum entropy method. This approach enables us to extract the spectral functions without any phenomenological assumption, and thus to visualize in-medium modification of the spectral functions directly. It is found that the reduction of the chiral condensates of dimension 3 and 5 causes the masses of both $D^+$ and $D^-$ mesons to grow gradually at finite density. Additionally, we construct charge-conjugate-projected sum rules and find a $D^+$-$D^-$ mass splitting of about -15 MeV at nuclear saturation density.

hep-ph↗

arXiv2016

D mesons in a magnetic field

We investigate the mass spectra of open heavy flavor mesons in an external constant magnetic field within QCD sum rules. Spectral ansätze on the phenomenological side are proposed in order to properly take into account mixing effects between the pseudoscalar and vector channels, and the Landau levels of charged mesons. The operator product expansion is implemented up to dimension-5 operators. As a result, we find for neutral D mesons a significant positive mass shift that goes beyond simple mixing effects. In contrast, charged D mesons are further subject to Landau level effects, which together with the mixing effects almost completely saturate the mass shifts obtained in our sum rule analysis.

hep-ph↗

arXiv2016

The mass of heavy-light mesons in a constituent quark picture with partially restored chiral symmetry

We probe effects of the partial chiral symmetry restoration to the mass of heavy-light mesons in a constituent quark model by changing the constituent quark mass of the light quark. Due to the competing effect between the quark mass and the linearly rising potential, whose contribution to the energy increases as the quark mass decreases, the heavy-light meson mass has a minimum value near the constituent quark mass typically used in the vacuum. Hence, the meson mass increases as one decreases the constituent quark mass consistent with recent QCD sum rule analyses, which show an increasing $D$ meson mass as the chiral order parameter decreases.

nucl-th↗

arXiv2015

Moments of $ϕ$ meson spectral functions in vacuum and nuclear matter

Moments of the $ϕ$ meson spectral function in vacuum and in nuclear matter are analyzed, combining a model based on chiral SU(3) effective field theory (with kaonic degrees of freedom) and finite-energy QCD sum rules. For the vacuum we show that the spectral density is strongly constrained by a recent accurate measurement of the $e^+ e^- \to K^+ K^-$ cross section. In nuclear matter the $ϕ$ spectrum is modified by interactions of the decay kaons with the surrounding nuclear medium, leading to a significant broadening and an asymmetric deformation of the $ϕ$ meson peak. We demonstrate that both in vacuum and nuclear matter, the first two moments of the spectral function are compatible with finite-energy QCD sum rules. A brief discussion of the next-higher spectral moment involving strange four-quark condensates is also presented.

hep-ph↗

arXiv2015

New determination of $\mathcal{S} \mathcal{T} \langle N| \overline{q} D_μ D_ν q |N \rangle$ based on recent experimental constraints

The symmetric and traceless part of the matrix element $\mathcal{S} \mathcal{T} \langle N| \overline{q} D_μ D_ν q |N \rangle$ can be determined from the second moment of the twist-3 parton distribution function $e(x)$. Recently, novel experimental data on $e(x)$ have become available, which enables us to evaluate the magnitude of the above matrix element with considerably reduced systematic uncertainties. Based on the new experimental data, we show that $\mathcal{S} \mathcal{T} \langle N| \overline{q} D_μ D_ν q |N \rangle$ is likely to be at least an order of magnitude smaller than what previous model-based estimates have so far suggested. We discuss the consequences of this observation for the analysis of deep inelastic scattering and QCD sum rules studies at finite density for the vector meson and the nucleon, in which this matrix element is being used as an input parameter.

hep-ph↗

arXiv2015

Single-particle spectral density of the unitary Fermi gas: Novel approach based on the operator product expansion, sum rules and the maximum entropy method

Making use of the operator product expansion, we derive a general class of sum rules for the imaginary part of the single-particle self-energy of the unitary Fermi gas. The sum rules are analyzed numerically with the help of the maximum entropy method, which allows us to extract the single-particle spectral density as a function of both energy and momentum. These spectral densities contain basic information on the properties of the unitary Fermi gas, such as the dispersion relation and the superfluid pairing gap, for which we obtain reasonable agreement with the available results based on quantum Monte-Carlo simulations.

cond-mat.quant-gas↗

arXiv2015

Constraining the strangeness content of the nucleon by measuring the $ϕ$ meson mass shift in nuclear matter

The behavior of the $ϕ$ meson at finite density is studied, making use of a QCD sum rule approach in combination with the maximum entropy method. It is demonstrated that a possible mass shift of the $ϕ$ in nuclear matter is strongly correlated to the strangeness content of the nucleon, which is proportional to the strange sigma term, $σ_{sN} = m_s \langle N | \overline{s}s | N \rangle$. Our results furthermore show that, depending on the value of $σ_{sN}$, the $ϕ$ meson could receive both a positive or negative mass shift at nuclear matter density. We find that these results depend only weakly on potential modifications of the width of the $ϕ$ meson peak and on assumptions made on the behavior of four-quark condensates at finite density. To check the stability of our findings, we take into account several higher order corrections to the operator product expansion, including $α_s$-corrections, terms of higher order in the strange quark mass and terms of higher twist that have not been considered in earlier works.

hep-ph↗

arXiv2014

Revisting the boiling of quark nuggets at nonzero chemical potential

The boiling of possible quark nuggets during the quark-hadron phase transition of the Universe at nonzero chemical potential is revisited within the microscopic Brueckner-Hartree-Fock approach employed for the hadron phase, using two kinds of baryon interactions as fundamental inputs. To describe the deconfined phase of quark matter, we use a recently developed quark mass density-dependent model with a fully self-consistent thermodynamic treatment of confinement. We study the baryon number limit $A_{\rm boil}$ (above which boiling may be important) with three typical values for the confinement parameter $D$. It is firstly found that the baryon interaction with a softer equation of state for the hadron phase would only lead to a small increase of $A_{\rm boil}$. However, results depend sensitively on the confinement parameter in the quark model. Specifically, boiling might be important during the Universe cooling for a limited parameter range around $D^{1/2} = 170$ MeV, a value satisfying recent lattice QCD calculations of the vacuum chiral condensate, while for other choices of this parameter, boiling might not happen and cosmological quark nuggets of $10^2 < A < 10^{50}$ could survive.

astro-ph.CO↗

arXiv2014

QCD sum rules on the complex Borel plane

Borel transformed QCD sum rules conventionally use a real valued parameter (the Borel mass) for specifying the exponential weight over which hadronic spectral functions are averaged. In this paper, it is shown that the Borel mass can be generalized to have complex values and that new classes of sum rules can be derived from the resulting averages over the spectral functions. The real and imaginary parts of these novel sum rules turn out to have damped oscillating kernels and potentially contain a larger amount of information on the hadronic spectrum than the real valued QCD sum rules. As a first practical test, we have formulated the complex Borel sum rules for the phi meson channel and have analyzed them using the maximum entropy method, by which we can extract the most probable spectral function from the sum rules without strong assumptions on its functional form. As a result, it is demonstrated that, compared to earlier studies, the complex valued sum rules allow us to extract the spectral function with a significantly improved resolution and thus to study more detailed structures of the hadronic spectrum than previously possible.

hep-ph↗

arXiv2013

Parity projection of QCD sum rules for the nucleon

The nucleon and its negative-parity excited states are examined in a maximum entropy method analysis of QCD sum rules. First, we rederive the parity projected sum rules for baryons using forward correlation functions. Doing this, the method is generalized so that higher order operator product expansion (OPE) terms can be calculated unambiguously. We then apply this approach to the nucleon channel taking into account all known first order alpha_s corrections to the Wilson coefficients of the OPE. As these corrections have turned out to be large, we suppress them by using a phase-rotated Gaussian kernel. Simultaneously, this phase rotation strongly suppresses the continuum contribution and improves the convergence of the OPE. The resulting sum rule has the interesting feature that it is dominated by the term containing the chiral condensate of dimension 3. Analyzing this sum rule by the maximum entropy method, we are able to extract information of both the positive and negative parity states.

hep-ph↗

arXiv2012

Modification of hadronic spectral functions under extreme conditions: An approach based on QCD sum rules and the maximum entropy method

Studies of quarkonium spectral functions at finite temperature, based on an approach combining QCD sum rules and the maximum entropy method are briefly reviewed. QCD sum rules for heavy quarkonia incorporate finite temperature effects in form of changing values of gluonic condensates that appear in the operator product expansion. These changes depend on the energy density and pressure at finite temperature, which we extract from quenched lattice QCD calculations. The maximum entropy method then allows us to obtain the most probable spectral function from the sum rules, without having to introduce any specific assumption about its functional form. Our findings suggest that the charmonium ground states of both S-wave and P-wave channels dissolve into the continuum already at temperatures around or slightly above the critical temperature T_c, while the bottomonium states are less influenced by temperature effects, surviving up to about 2.5 T_c or higher for S-wave and up to about 2.0 T_c for P-wave states.

hep-ph↗

arXiv2012

Thermal modification of bottomonium spectra from QCD sum rules with the maximum entropy method

The bottomonium spectral functions at finite temperature are analyzed by employing QCD sum rules with the maximum entropy method. This approach enables us to extract the spectral functions without any phenomenological parametrization, and thus to visualize deformation of the spectral functions due to temperature effects estimated from quenched lattice QCD data. As a result, it is found that Υand η_b survive in hot matter of temperature up to at least 2.3T_c and 2.1T_c, respectively, while χ_{b0} and χ_{b1} will disappear at T<2.5T_c. Furthermore, a detailed analysis of the vector channel shows that the spectral function in the region of the lowest peak at T=0 contains contributions from the excited states, Υ(2S) and Υ(3S), as well as the ground states Υ(1S). Our results at finite T are consistent with the picture that the excited states of bottomonia dissociate at lower temperature than that of the ground state. Assuming this picture, we find that Υ(2S) and Υ(3S) disappear at T=1.5-2.0T_c.

hep-ph↗

arXiv2011

A Bayesian analysis of the nucleon QCD sum rules

QCD sum rules of the nucleon channel are reanalyzed, using the maximum entropy method (MEM). This new approach, based on the Bayesian probability theory, does not restrict the spectral function to the usual "pole + continuum"-form, allowing a more flexible investigation of the nucleon spectral function. Making use of this flexibility, we are able to investigate the spectral functions of various interpolating fields, finding that the nucleon ground state mainly couples to an operator containing a scalar diquark. Moreover, we formulate the Gaussian sum rule for the nucleon channel and find that it is more suitable for the MEM analysis to extract the nucleon pole in the region of its experimental value, while the Borel sum rule does not contain enough information to clearly separate the nucleon pole from the continuum.

hep-ph↗

arXiv2011

Charmonium spectra at finite temperature from QCD sum rules with the maximum entropy method

Charmonia spectral functions at finite temperature are studied using QCD sum rules in combination with the maximum entropy method. This approach enables us to directly obtain the spectral function from the sum rules, without having to introduce any specific assumption about its functional form. As a result, it is found that while J/psi and eta_c manifest themselves as significant peaks in the spectral function below the deconfinement temperature T_c, they quickly dissolve into the continuum and almost completely disappear at temperatures between 1.0 T_c and 1.1 T_c.

hep-ph↗