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S. Piemonte

Publications and source records attributed to S. Piemonte.

15 recordsLinked to original sources
arXiv2022

One flavour adjoint QCD with overlap fermions

The infrared effective theory of adjoint QCD with one Dirac flavour is still under debate. The theory could be confining, conformal, or with a massless fermion in the infrared. The study of chiral symmetry seems to be important to answer this question. While previous investigations have considered Wilson fermions, we present here the first results for this theory based on overlap fermions to avoid explicit chiral symmetry breaking. These indicate spontaneous chiral symmetry breaking by the formation of a fermion condensate. We have also investigated the running coupling of the theory, which indicates no infrared conformality in the energy region we have explored.

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arXiv2021

Charmonium-like resonances in coupled $D \bar D$-$D_s \bar D_s$ scattering

Charmonium-like resonances and bound states with isospin zero and $J^{PC}=0^{++},~1^{--},~2^{++},~3^{--}$ are extracted on the lattice. Coupled $D\bar D$ and $D_s\bar D_s$ scattering suggests three charmonium-like states with $J^{PC}=0^{++}$ in addition to $\chi_{c0}(1P)$: a so far unobserved $D\bar D$ bound state just below threshold, a conventional resonance likely related to $\chi_{c0}(3860)/\chi_{c0}(2P)$ and a narrow resonance just below the $D_s\bar D_s$ threshold with a large coupling to $D_s\bar D_s$ likely related to $X(3915)/\chi_{c0}(3930)$. One-channel $D\bar D$ scattering renders resonances and bound states with $J^{PC}= 1^{--},~2^{++},~3^{--}$ related to the observed conventional charmonia. Lattice QCD ensembles from the CLS consortium with $m_{\pi}\simeq 280$ MeV are utilized.

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arXiv2020

Nonperturbative Renormalization in Lattice QCD with three Flavors of Clover Fermions: Using Periodic and Open Boundary Conditions

We present the nonperturbative computation of renormalization factors in the RI'-(S)MOM schemes for the QCD gauge field ensembles generated by the CLS (coordinated lattice simulations) effort with three flavors of nonperturbatively improved Wilson (clover) quarks. We use ensembles with the standard (anti-)periodic boundary conditions in the time direction as well as gauge field configurations with open boundary conditions. Besides flavor-nonsinglet quark-antiquark operators with up to two derivatives we also consider three-quark operators with up to one derivative. For the RI'-SMOM scheme results we make use of the recently calculated three-loop conversion factors to the modified minimal subtraction scheme. The present version of the paper contains an Addendum with additional analytical expressions and updated results.

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arXiv2020

Charmonium-like resonances with $J^{PC}=0^{++},2^{++}$ in coupled $D\bar D$, $D_s\bar D_s$ scattering on the lattice

We present the first lattice investigation of coupled-channel $D\bar D$ and $D_s\bar D_s$ scattering in the $J^{PC}=0^{++}$ and $2^{++}$ channels. The scattering matrix for partial waves $l=0,2$ and isospin zero is determined using multiple volumes and inertial frames via L\"uscher's formalism. Lattice QCD ensembles from the CLS consortium with $m_{\pi}\simeq280$ MeV, $a \simeq 0.09 $ fm and $L/a=24,~32$ are utilized. The resulting scattering matrix suggests the existence of three charmonium-like states with $J^{PC}=0^{++}$ in the energy region ranging from slightly below $2m_D$ up to 4.13 GeV. We find a so far unobserved $D\bar D$ bound state just below threshold and a $D\bar D$ resonance likely related to $\chi_{c0}(3860)$, which is believed to be $\chi_{c0}(2P)$. In addition, there is an indication for a narrow $0^{++}$ resonance just below the $D_s\bar D_s$ threshold with a large coupling to $D_s\bar D_s$ and a very small coupling to $D\bar D$. This resonance is possibly related to the narrow $X(3915)$/$\chi_{c0}(3930)$ observed in experiment also just below $D_s\bar D_s$. The partial wave $l=2$ features a resonance likely related to $\chi_{c2}(3930)$. We work with several assumptions, such as the omission of $J/\psi\omega$, $\eta_c\eta$ and three-particle channels. Only statistical uncertainties are quantified, while the extrapolations to the physical quark-masses and the continuum limit are challenges for the future.

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arXiv2020

Monte-Carlo simulations of overlap Majorana fermions

Supersymmetric Yang-Mills (SYM) theories in four dimensions exhibit many interesting non-perturbative phenomena that can be studied by means of Monte Carlo lattice simulations. However, the lattice regularization breaks supersymmetry explicitly, and in general a fine tuning of a large number of parameters is required to correctly extrapolate the theory to the continuum limit. From this perspective, it is important to preserve on the lattice as many symmetries of the original continuum action as possible. Chiral symmetry for instance prevents an additive renormalization of the fermion mass. A (modified) version of chiral symmetry can be preserved exactly if the Dirac operator fulfills the Ginsparg-Wilson relation. In this contribution, we present an exploratory non-perturbative study of N=1 supersymmetric Yang-Mills theory using the overlap formalism to preserve chiral symmetry at non-zero lattice spacings. N=1 SYM is an ideal benchmark toward the extension of our studies to more complex supersymmetric theories, as the only parameter to be tuned is the gluino mass. Overlap fermions allow therefore to simulate the theory without fine-tuning. We compare our approach to previous investigations of the same theory, and we present clear evidences for gluino condensation.

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arXiv2019

Charmonium resonances with $J^{PC}=1^{--}$ and $3^{--}$ from $\bar DD$ scattering on the lattice

We present a lattice QCD study of charmonium resonances and bound states with $J^{PC}=1^{--}$ and $3^{--}$ near the open-charm threshold, taking into account their strong transitions to $\bar DD$. Vector charmonia are the most abundant in the experimentally established charmonium spectrum, while recently LHCb reported also the first discovery of a charmonium with likely spin three. The $\bar DD$ scattering amplitudes for partial waves $l=1$ and $l=3$ are extracted on the lattice by means of the L\"uscher formalism, using multiple volumes and inertial frames. Parameterizations of the scattering amplitudes provide masses and widths of the resonances, as well as the masses of bound states. CLS ensembles with 2+1 dynamical flavors of non-perturbatively $O(a)$ improved Wilson quarks are employed with $m_\pi\simeq 280$ MeV, a single lattice spacing of $a\simeq0.086$ fm and two lattice spatial extents of $L=24$ and $32$. Two values of the charm quark mass are considered to examine the influence of the position of the $\bar{D}D$ threshold on the hadron masses. For the lighter charm quark mass we find the vector resonance $\psi(3770)$ with mass $m=3780(7)$ MeV and coupling $g=16.0(^{+2.1}_{-0.2})$ (related to the width), both consistent with their experimental values. The vector $\psi(2S)$ appears as a bound state with $m=3666(10)$ MeV. The charmonium resonance with $J^{PC}=3^{--}$ is found at $m=3831(^{+10}_{-16})$ MeV, consistent with the $X(3842)$ recently discovered by LHCb. At our heavier charm-quark mass the $\psi(2S)$ as well as the $\psi(3770)$ are bound states and the $X(3842)$ remains a resonance. We stress that all quoted uncertainties are only statistical, while lattice spacing effects and the approach to the physical point still need to be explored. This study of conventional charmonia sets the stage for more challenging future studies of unconventional charmonium-like states.

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arXiv2018

Charmonium resonances on the lattice

The nature of resonances and excited states near decay thresholds is encoded in scattering amplitudes, which can be extracted from single-particle and multiparticle correlators in finite volumes. Lattice calculations have only recently reached the precision required for a reliable study of such correlators. The distillation method represents a significant improvement insofar as it simplifies quark contractions and allows one to easily extend the operator basis used to construct interpolators. We present preliminary results on charmonium bound states and resonances on the Nf=2+1 CLS ensembles. The long term goal of our investigation is to understand the properties of the X resonances that do not fit into conventional models of quark-antiquark mesons. We tune various parameters of the distillation method and the charm quark mass. As a first result, we present the masses of the ground and excited states in the 0++ and 1-- channels.

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arXiv2017

Charmonia in moving frames

Lattice simulation of charmonium resonances with non-zero momentum provides additional information on the two-meson scattering matrices. However, the reduced rotational symmetry in a moving frame renders a number of states with different $J^P$ in the same lattice irreducible representation. The identification of $J^P$ for these states is particularly important, since quarkonium spectra contain a number of states with different $J^P$ in a relatively narrow energy region. Preliminary results concerning spin-identification are presented in relation to our study of charmonium resonances in flight on the Nf=2+1 CLS ensembles.

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arXiv2017

Non-perturbative renormalization of flavor singlet quark bilinear operators in lattice QCD

We report on our studies of the renormalization of flavor singlet quark bilinear operators in lattice QCD. The renormalization constants are determined non-perturbatively using gauge field ensembles with Nf=2 dynamical clover improved fermions. The renormalization is performed in the RI'-MOM scheme. The difference between flavor singlet and non-singlet quark bilinear operators is a disconnected contribution, which has to be evaluated by stochastic estimators. We compare our results for the running with perturbation theory.

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arXiv2015

Witten index and phase diagram of compactified N=1 supersymmetric Yang-Mills theory on the lattice

Owing to confinement, the fundamental particles of N=1 Supersymmetric Yang-Mills (SYM) theory, gluons and gluinos, appear only in colourless bound states at zero temperature. Compactifying the Euclidean time dimension with periodic boundary conditions for fermions preserves supersymmetry, and confinement is predicted to persist independently of the length of the compactified dimension. This scenario can be tested non-perturbatively with Monte-Carlo simulations on a lattice. SUSY is, however, broken on the lattice and can be recovered only in the continuum limit. The partition function of compactified N=1 SYM theory with periodic fermion boundary conditions corresponds to the Witten index. Therefore it can be used to test whether supersymmetry is realized on the lattice. Results of our recent numerical simulations are presented, supporting the disappearance of the deconfinement transition in the supersymmetric limit and the restoration of SUSY at low energies.

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arXiv2015

First studies of the phase diagram of N=1 supersymmetric Yang-Mills theory

The behavior of supersymmetric theories at finite temperatures differs from that of other theories in certain aspects. Due to the different thermal statistics of bosons and fermions, supersymmetry is explicitly broken for any non-zero value of the temperature. We study N=1 supersymmetric Yang-Mills theory on the lattice at finite temperatures. This model is the simplest supersymmetric extension of the pure gauge sector of QCD, describing the interactions between gluons and their fermionic superpartners, the gluinos. At zero temperature the theory confines like QCD, and chiral symmetry is spontaneously broken. At high temperatures, deconfinement and chiral symmetry restoration are expected to take place, but it is not known whether these two phase transitions coincide or not. First results on this topic, obtained in numerical simulations on the lattice, will be presented and discussed.

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arXiv2014

Compactified N=1 supersymmetric Yang-Mills theory on the lattice: Continuity and the disappearance of the deconfinement transition

Fermion boundary conditions play a relevant role in revealing the confinement mechanism of N=1 supersymmetric Yang-Mills theory with one compactified space-time dimension. A deconfinement phase transition occurs for a sufficiently small compactification radius, equivalent to a high temperature in the thermal theory where antiperiodic fermion boundary conditions are applied. Periodic fermion boundary conditions, on the other hand, are related to the Witten index and confinement is expected to persist independently of the length of the compactified dimension. We study this aspect with lattice Monte Carlo simulations for different values of the fermion mass parameter that breaks supersymmetry softly. We find a deconfined region that shrinks when the fermion mass is lowered. Deconfinement takes place between two confined regions at large and small compactification radii, that would correspond to low and high temperatures in the thermal theory. At the smallest fermion masses we find no indication of a deconfinement transition. These results are a first signal for the predicted continuity in the compactification of supersymmetric Yang-Mills theory.

hep-lat↗
arXiv2014

Phase structure of the N=1 supersymmetric Yang-Mills theory at finite temperature

Supersymmetry (SUSY) has been proposed to be a central concept for the physics beyond the standard model and for a description of the strong interactions in the context of the AdS/CFT correspondence. A deeper understanding of these developments requires the knowledge of the properties of supersymmetric models at finite temperatures. We present a Monte Carlo investigation of the finite temperature phase diagram of the N=1 supersymmetric Yang-Mills theory (SYM) regularised on a space-time lattice. The model is in many aspects similar to QCD: quark confinement and fermion condensation occur in the low temperature regime of both theories. A comparison to QCD is therefore possible. The simulations show that for N=1 SYM the deconfinement temperature has a mild dependence on the fermion mass. The analysis of the chiral condensate susceptibility supports the possibility that chiral symmetry is restored near the deconfinement phase transition.

hep-lat↗
arXiv2013

Clover fermions in the adjoint representation and simulations of supersymmetric Yang-Mills theory

Clover improvement is the standard choice for lattice simulations of QCD, when the lattice artefacts coming from Wilson fermions have to be reduced. However, the clover improvement is not limited to QCD, but can be applied to a wider range of theories with fermions in higher representations of the gauge group SU(N), like the adjoint fermions required by supersymmetry or by technicolor theories. We present the calculation of the clover coefficient up to one loop order with standard perturbation theory for these models. Applications of clover fermions to supersymmetric Yang-Mills theory are also discussed.

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