arXiv subjects

Sasa Prelovsek

Publications and source records attributed to Sasa Prelovsek.

At least 37 records · Page 2Linked to original sources

arXiv2022

Lattice spectroscopy with focus on exotics

Recent lattice QCD results on the hadron spectroscopy with beauty and charm quarks is reviewed. The focus of the review are exotic hadrons, while non-lattice approaches and conventional hadrons are reported as well. We discuss the recently discovered $Ω_c^*$, a charmonium state with spin three, $P_c$ pentaquarks, $B_c(2S)$ and $Λ_b^*$, the long-standing challenges for theory $Z_c$, $Z_b$ and $X(3872)$, and we review predictions for yet undiscovered states $bb\bar q\bar q$, $\bar bb\bar bb$, highly excited and hybrid $\bar bb$, baryons with bottom quarks and still-missing $B_{s}$ mesons.

hep-lat↗

arXiv2021

Tetraquark channels with $\bar b b$ pair in the static limit

Belle experiment discovered two hadrons with exotic quark content $Z_b^+\simeq \bar bb \bar du$. We present a lattice study of the $\bar bb\bar du$ systems with various quantum numbers using static bottom quarks. Only one set of quantum numbers that couples to $Z_b$ and $Υ\;π$ was explored on the lattice before: these studies found an attractive potential between $B$ and $\bar B^*$ which leads to a bound state below the threshold. In the present study, we consider the other three sets of quantum numbers. Eigen-energies of the $\bar bb \bar du$ system are extracted as a function of separation between $b$ and $\bar b$. The resulting eigen-energies do not show any sizable deviation from non-interacting energies of the systems $\bar bb+\bar du$ and $\bar bu+\bar db$, so no significant attraction or repulsion is found. A slight exception is a small attraction between $B$ and $\bar B^*$ at small distance for the quantum number that couples to $Z_b$ and $η_b\;ρ$.

hep-lat↗

arXiv2021

Scattering of Goldstone Bosons and resonance production in a Composite Higgs model on the lattice

We calculate the coupling between a vector resonance and two Goldstone bosons in $SU(2)$ gauge theory with $N_f=2$ Dirac fermions in the fundamental representation. The considered theory can be used to construct a minimal Composite Higgs models. The coupling is related to the width of the vector resonance and we determine it by simulating the scattering of two Goldstone bosons where the resonance is produced. The resulting coupling is $g_{\rm{VPP}}=7.8\pm 0.6$, not far from $g_{ρππ}\simeq 6$ in QCD. This is the first lattice calculation of the resonance properties for a minimal UV completion. This coupling controls the production cross section of the lightest expected resonance at the LHC and enters into other tests of the Standard Model, from Vector Boson Fusion to electroweak precision tests. Our prediction is crucial to constrain the model using lattice input and for understanding the behavior of the vector meson production cross section as a function of the underlying gauge theory. We also extract the coupling $g_{\rm{VPP}}^{\rm{KSRF}} =9.4 \pm 0.6$ assuming the vector-dominance and find that this phenomenological estimate slightly overestimates the value of the coupling.

hep-lat↗

arXiv2019

Resonance study of SU(2) model with 2 fundamental flavours of fermions

Composite Higgs models are promising candidate models to address the long-standing naturalness problem in the Standard Model. Among them, the most minimal one is the SU(2) with 2 flavours of fermions in the fundamental representation of the gauge group. An important prediction in these models is the existence of resonance spectrum in vector boson scattering. Here we study the lowest such resonance, which is the equivalent of rho resonance in QCD. We describe the scan of the parameter space using the clover-improved Wilson fermions with Symanzik improved gauge action and then show the first results for the mass and width of the rho resonance in this model.

hep-lat↗

arXiv2019

Nucleon-$J/ψ$ and nucleon-$η_{c}$ scattering in $P_{c}$ pentaquark channels from LQCD

The lattice QCD simulation of $NJ/ψ$ and $Nη_c$ scattering is performed at $m_π\simeq 266~$MeV in channels with all possible $J^P$. This includes $J^P=3/2^\pm$ and $5/2^\pm$ where LHCb discovered $P_c(4380)$ and $P_c(4450)$ pentaquark states in proton$-J/ψ$ decay. This is the first lattice simulation that reaches the energies 4.3-4.5 GeV where pentaquarks reside. Several decay channels are open in this energy region and we explore the fate of $P_c$ in the one-channel approximation in this work. Energies of eigenstates are extracted for the nucleon-charmonium system at zero total momentum for all quantum numbers, i.e. six lattice irreducible representations. No significant energy shifts are observed. The number of the observed lattice eigenstates agrees with the number expected for non-interacting charmonium and nucleon. Thus, we do not find any strong indication for a resonance or a bound state in these exotic channels within one-channel approximation. This possibly indicates that the coupling of $NJ/ψ$ channel with other two-hadron channels might be responsible for $P_c$ resonances in experiment. One of the challenges of this study is that up to six degenerate $J/ψ(p) N(-p)$ eigenstates are expected in the non-interacting limit due non-zero spins of $J/ψ$ and N, and we establish all of them in the spectra.

hep-lat↗

arXiv2019

Identifying spin and parity of charmonia in flight with lattice QCD

The spectrum of charmonium resonances contains a number of unanticipated states along with several conventional quark-model excitations. The hadrons of different quantum numbers $J^P$ appear in a fairly narrow energy band, where $J^P$ refers to the spin-parity of a hadron at rest. This poses a challenge for Lattice QCD studies of (coupled-channel) meson-meson scattering aimed at the determination of scattering amplitudes and resonance pole positions. A wealth of information for this purpose can be obtained from the lattice spectra in frames with nonzero total momentum. These are particularly dense since hadrons with different $J^P$ contribute to any given lattice irreducible representation. This is because $J^P$ is not a good quantum number in flight, and also because the continuum symmetry is reduced on the lattice. In this paper we address the assignment of the underlying continuum $J^P$ quantum numbers to charmonia in flight using a $N_f = 2 + 1$ CLS ensemble. As a first step, we apply the single-hadron approach, where only interpolating fields of quark-antiquark type are used. The approach follows techniques previously applied to the light meson spectrum by the Hadron Spectrum Collaboration. The resulting spectra of charmonia with assigned $J^P$ will provide valuable information for the parameterization of (resonant) amplitudes in future determinations of resonance properties with lattice QCD.

hep-lat↗

arXiv2018

Charmonium resonances from 2+1 flavor CLS lattices

Many exotic charmonium resonances have been identified recently in experiment, however their nature and properties are mostly unknown. Algorithmic and theoretical progress in lattice calculations has enabled reliable numerical investigation of the spectrum below the strong decay threshold, while the study of charmonium resonances remains an open challenge. The main difficulty to overcome is the presence of many open decay channels which are coupled together, resulting in a complex finite volume quantization condition. We report on our recent progress towards the determination of single-channel and coupled-channel scattering matrices in the scalar and vector channels on CLS ensembles. We also present an update concerning the study of the charmonium spectrum in moving frames.

hep-lat↗

arXiv2018

$J/ψ$ -nucleon scattering in $P_{c}^{+}$ pentaquarks channel

Two pentaquarks $P_{c}^{+}$ were discovered by LHCb collaboration as peaks in the proton-$J/ψ$ invariant mass. We perform the lattice QCD study of the scattering between $J/ψ$ meson and nucleon in the channels with $J^{P}=\frac{3}{2}^{+},\frac{3}{2}^{-}, \frac{5}{2}^{+}, \frac{5}{2}^{-}$, where $P_{c}^{+}$ was discovered. This is the first lattice simulation that reaches the energies $4.3-4.5~$GeV where pentaquarks reside. The higher partial waves $L>0$ are also explored for the first time. In this study we consider the single-channel approximation for scattering of $NJ/ψ$. Energies and eigenstates are extracted for the $NJ/ψ$ system at the zero total momentum for all six irreducible representations of the lattice irreducible representation. No significant energy shifts are observed. The number of eigenstates agrees with the number expected from non-interacting limit for scattering. This could possibly indicate that the $P_{c}$ resonances seen in experiment are a consequence of a coupling of the $NJ/ψ$ channel with other two-hadron channels.

hep-lat↗

arXiv2018

A lattice QCD study of pion-nucleon scattering in the Roper channel

We present a lattice QCD study of the puzzling positive-parity nucleon channel, where the Roper resonance $N^*(1440)$ resides in experiment. The study is based on an ensemble of gauge configurations with $N_f=2+1$ Wilson-clover fermions with a pion mass of $156$ MeV and lattice size $L=2.9$ fm. We use several $qqq$ interpolating fields combined with $Nπ$ and $Nσ$ two-hadron operators in calculating the energy spectrum in the rest frame. Combining experimental $Nπ$ phase shifts with elastic approximation and the Lüscher formalism suggests in the spectrum an additional energy level near the Roper mass $m_R=1.43$ GeV for our lattice. We do not observe any such additional energy level, which implies that $Nπ$ elastic scattering alone does not render a low-lying Roper resonance. The current status indicates that the $N^*(1440)$ might arise as dynamically generated resonance from coupling to other channels, most notably the $Nππ$.

hep-lat↗

arXiv2017

$Nπ$ scattering in the Roper channel

We present results from our recent lattice QCD study of $Nπ$ scattering in the positive-parity nucleon channel, where the puzzling Roper resonance $N^*(1440)$ resides in experiment. Using a variety of hadron operators, that include $qqq$-like, $Nπ$ in $p$-wave and $Nσ$ in $s$-wave, we systematically extract the excited lattice spectrum in the nucleon channel up to 1.65 GeV. Our lattice results indicate that N$π$ scattering in the elastic approximation alone does not describe a low-lying Roper. Coupled channel effects between $Nπ$ and $Nππ$ seem to be crucial to render a low-lying Roper in experiment, reinforcing the notion that this state could be a dynamically generated resonance. After giving a brief motivation for studying the Roper channel and the relevant technical details to this study, we will discuss the results and the conclusions based on our lattice investigation and in comparison with other lattice calculations.

hep-lat↗

arXiv2017

Degeneracy of vector-channel spatial correlators in high temperature QCD

We study spatial isovector meson correlators in $N_f=2$ QCD with dynamical domain-wall fermions on $32^3\times 8$ lattices at temperatures up to 380 MeV with various quark masses. We measure the correlators of spin-one isovector operators including vector, axial-vector, tensor and axial-tensor. At temperatures above $T_c$ we observe an approximate degeneracy of the correlators in these channels, which is unexpected because some of them are not related under $SU(2)_L \times SU(2)_R$ nor $U(1)_A$ symmetries. The observed approximate degeneracy suggests emergent $SU(2)_{CS}$ (chiral-spin) and $SU(4)$ symmetries at high $T$.

hep-lat↗

arXiv2017

Heavy Flavors on the Lattice

The lattice QCD results on the hadron spectrum and weak transitions between hadrons are briefly reviewed. Hadrons containing heavy quarks c or b are considered. The focus is on the recent simulations and some older results which are particularly relevant in view of the recent experimental discoveries.

hep-lat↗

arXiv2016

Prediction of positive parity $B_s$ mesons and search for the $X(5568)$

We use a combination of quark-antiquark and $B^{(*)}K$ interpolating fields to predict the mass of two QCD bound states below the $B^*K$ threshold in the quantum channels $J^P=0^+$ and $1^+$. The mesons correspond to the b-quark cousins of the $D_{s0}^*(2317)$ and $D_{s1}(2460)$ and have not yet been observed in experiment, even though they are expected to be found by LHCb. In addition to these predictions, we obtain excellent agreement of the remaining p-wave energy levels with the known $B_{s1}(5830)$ and $B_{s2}^*(5840)$ mesons. The results from our first principles calculation are compared to previous model-based estimates. More recently the D0 collaboration claimed the existence of an exotic resonance $X(5568)$ with exotic flavor content $\bar{b}s\bar{d}u$. If such a state with $J^P=0^+$ exists, only the decay into $B_sπ$ is open which makes a lattice search for this state much cleaner and simpler than for other exotic candidates involving heavy quarks. We conclude, however, that we do not find such a candidate in agreement with a recent LHCb result.

hep-lat↗

arXiv2016

Hadron spectroscopy and interactions from lattice QCD

Lattice QCD approach to study the hadronic resonances and exotic hadrons is described at an introductory level. The main challenge is that these states decay strongly via one or more decay channels, and they often lie near thersholds. Specific results for conventional and exotic hadrons are shown to illustrate the current status.

hep-ph↗

arXiv2016

Positive parity $D_s$ mesons

We study the positive parity charmed strange mesons using lattice QCD, the only reliable ab initio method to study QCD at low energies. Especially the experimentally observed $D_{s0}^*(2317)$ and $D_{s1}(2460)$ have challenged theory for quite some time. The dynamical lattice QCD simulations are performed at two distinct pion masses, $m_π$ = 266 MeV and 156 MeV, using both $\bar{c}s$ as well as $DK$ and $D^*K$ scattering operators in the construction of the correlation matrix in order to take into the account threshold effects. While the $J^P = 0^+$ channel benefited most from the inclusion of scattering operators, it was also crucial for the case of the $D_{s1}(2460)$. Using the Lüscher method, which relates the finite volume spectrum to the infinite volume scattering matrix, we were able to determine the near threshold behavior of the scattering amplitude. From it we extracted the binding momenta and the masses of the below-threshold bound states $D_{s0}^*(2317)$ and $D_{s1}(2460)$ by determining the pole positions of the scattering amplitudes. Our results compare well with experiment, resolving a long standing discrepancy between theory and experiment.

hep-lat↗

arXiv2015

Vector and scalar charmonium resonances with lattice QCD

We perform an exploratory lattice QCD simulation of $D \bar D$ scattering, aimed at determining the masses as well as the decay widths of charmonium resonances above open charm threshold. Neglecting coupling to other channels, the resulting phase shift for $D \bar D$ scattering in p-wave yields the well-known vector resonance $ψ(3770)$. For $m_π= 156$ MeV, the extracted resonance mass and the decay width agree with experiment within large statistical uncertainty. The scalar charmonium resonances present a puzzle, since only the ground state $χ_{c0}(1P)$ is well understood, while there is no commonly accepted candidate for its first excitation. We simulate $D \bar D$ scattering in s-wave in order to shed light on this puzzle. The resulting phase shift supports the existence of a yet-unobserved narrow resonance with a mass slightly below 4 GeV. A scenario with this narrow resonance and a pole at $χ_{c0}(1P)$ agrees with the energy-dependence of our phase shift. Further lattice QCD simulations and experimental efforts are needed to resolve the puzzle of the excited scalar charmonia.

hep-lat↗

arXiv2015

Predicting positive parity $B_{s}$ mesons from lattice QCD

We determine the spectrum of $B_s$ 1P states using lattice QCD. For the $B_{s1}(5830)$ and $B_{s2}^*(5840)$ mesons, the results are in good agreement with the experimental values. Two further mesons are expected in the quantum channels $J^P=0^+$ and $1^+$ near the $BK$ and $B^{*}K$ thresholds. A combination of quark-antiquark and $B^{(*)}$ meson-Kaon interpolating fields are used to determine the mass of two QCD bound states below the $B^{(*)}K$ threshold, with the assumption that mixing with $B_s^{(*)}η$ and isospin-violating decays to $B_s^{(*)}π$ are negligible. We predict a $J^P=0^+$ bound state $B_{s0}$ with mass $m_{B_{s0}}=5.711(13)(19)$ GeV. With further assumptions motivated theoretically by the heavy quark limit, a bound state with $m_{B_{s1}}= 5.750(17)(19)$ GeV is predicted in the $J^P=1^+$ channel. The results from our first principles calculation are compared to previous model-based estimates.

hep-lat↗

arXiv2015

Lattice studies of charmonia and exotics

The lattice QCD simulations of charmonia and exotic charmonium-like states are reviewed. I report on the first exploratory simulation of charmonium resonances above open charm threshold which takes into account the strong decay. The puzzles related to the first-excited scalar charmonia are discussed. Evidence for X(3872) is presented along with investigation of its Fock components. The $Z_c^+(3900)$ seems to emerge from the HALQCD approach as a result of the coupled channel effect $J/ψπ-D\bar D^*$. The indication for the pentaquark bound state $η_c N$ is presented.

hep-lat↗