arXiv subjects

Elisabetta Pallante

Publications and source records attributed to Elisabetta Pallante.

At least 37 records · Page 2Linked to original sources

arXiv2011

Bulk transitions of twelve flavor QCD and $U_A(1)$ symmetry

We present an update on our ongoing study on the nature of the bulk transition observed at strong coupling in the SU(3) gauge theory with N_f = 12 flavors in the fundamental representation. We show evidence that there is a first order chiral symmetry breaking bulk transition separating a region at weak coupling where chiral symmetry is restored from a region at strong coupling where chiral symmetry is broken. We also discuss hints of a separate partial restoration of U_A(1) at weaker coupling. The results are in agreement with restoration of conformality in non abelian gauge theories as the number of flavors is increased.

hep-lat↗

arXiv2011

Thermodynamic Study for Conformal Phase in Large Nf Gauge Theory

We investigate the chiral phase transition at finite temperature (T) in colour SU(3) Quantum Chromodynamics (QCD) with six species of fermions (Nf = 6) in the fundamental representation. The simulations have been performed by using lattice QCD with improved staggered fermions. The critical couplings (bc) for the chiral phase transition are observed for several temporal extensions Nt, and the two-loop asymptotic scaling of the dimensionless ratio Tc/Lambda_L (Lambda_L = Lattice Lambda-parameter) is found to be achieved for Nt >= 6. Further, we collect bc at Nf = 0 (quenched), and Nf = 4 at a fixed Nt = 6 as well as Nf = 8 at Nt = 6 and 12, the latter relying on our earlier study. The results are consistent with enhanced fermionic screening at larger Nf. The ratio Tc/Lambda_L depends very mildly on Nf in the Nf = 0-4 region, begins increasing at Nf = 6, and significantly grows up at Nf = 8, as Nf reaches to the edge of the conformal window. We discuss the interrelation of the results with preconformal dynamics in the light of a functional renormalization group analysis.

hep-lat↗

arXiv2011

N=1 SQCD-like theories with N_f massive flavors from AdS/CFT and beta functions

We study new supergravity solutions related to large-$N_c$ ${\cal N}=1$ supersymmetric gauge field theories with a large number $N_f$ of massive flavors. We use a recently proposed framework based on configurations with $N_c$ color D5 branes and a distribution of $N_f$ flavor D5 branes, governed by a function $N_f S(r)$. Although the system admits many solutions, under plausible physical assumptions the relevant solution is uniquely determined for each value of $x\equiv N_f/N_c$. In the IR region, the solution smoothly approaches the deformed Maldacena-Núñez solution. In the UV region it approaches a linear dilaton solution. For $x<2$ the gauge coupling $β_g $ function computed holographically is negative definite, in the UV approaching the NSVZ $β$ function with anomalous dimension $γ_0= -1/2$ (approaching $-3/(32π^2)(2N_c-N_f)g^3$)), and with $β_g \to-\infty $ in the IR. For $x=2$, $β_g$ has a UV fixed point at strong coupling, suggesting the existence of an IR fixed point at a lower value of the coupling. We argue that the solutions with $x>2$ describe a "Seiberg dual" picture where $N_f-2N_c$ flips sign.

hep-th↗

arXiv2011

CP violation and FCNC in a warped A4 flavor model

We recently proposed a spontaneous A4 flavor symmetry breaking scheme implemented in a warped extra dimensional setup to explain the observed pattern of quark and lepton masses and mixings. The quark mixing is induced by bulk A4 flavons mediating "cross-brane" interactions and a "cross-talk" between the quark and neutrino sectors. In this work we explore the phenomenology of RS-A4 and systematically obtain bounds on the Kaluza-Klein (KK) mass scale implied by flavor changing neutral current (FCNC) processes. In particular, we study the constraints arising from Re(ε'/ε_K), b->sγ, the neutron EDM and Higgs mediated FCNCs, while the tree level contribution to ε_K through a KK gluon exchange vanishes. We find an overall lower bound on the Kaluza-Klein mass scale M_{KK} > 1.3 TeV from FCNCs, induced by b->sγ, differently from flavor anarchic models. This bound is still weaker than the bound M_{KK} > 4.6 TeV induced by Zb_L\bar{b}_L in RS-A4. The little CP problem, related to the largely enhanced new physics contributions to the neutron EDM in flavor anarchic models, is absent. The subtleties of having the Higgs and flavons in the bulk are taken into account and final predictions are derived in the complete three-generation case.

hep-ph↗

arXiv2010

The bulk transition of many-flavour QCD and the search for a UVFP at strong coupling

We explore the nature of the bulk transition observed at strong coupling in the SU(3) gauge theory with Nf=12 fermions in the fundamental representation. The transition separates a weak coupling chirally symmetric phase from a strong coupling chirally broken phase and is compatible with the scenario where conformality is restored by increasing the flavour content of a non abelian gauge theory. We explore the intriguing possibility that the observed bulk transition is associated with the occurrence of an ultraviolet fixed point (UVFP) at strong coupling, where a new theory emerges in the continuum.

hep-lat↗

arXiv2010

Kaon and D meson masses with N_f = 2+1+1 twisted mass lattice QCD

We discuss the computation of the kaon and D meson masses in the N_f = 2+1+1 twisted mass lattice QCD setup, where explicit heavy flavor and parity breaking occurs at finite lattice spacing. We present three methods suitable in this context and verify their consistency.

hep-lat↗

arXiv2010

Computing K and D meson masses with N_f = 2+1+1 twisted mass lattice QCD

We discuss the computation of the mass of the K and D mesons within the framework of N_f = 2+1+1 twisted mass lattice QCD from a technical point of view. These quantities are essential, already at the level of generating gauge configurations, being obvious candidates to tune the strange and charm quark masses to their physical values. In particular, we address the problems related to the twisted mass flavor and parity symmetry breaking, which arise when considering a non-degenerate (c,s) doublet. We propose and verify the consistency of three methods to extract the K and D meson masses in this framework.

hep-lat↗

arXiv2010

Light hadrons from lattice QCD with light (u,d), strange and charm dynamical quarks

We present results of lattice QCD simulations with mass-degenerate up and down and mass-split strange and charm (N_f = 2+1+1) dynamical quarks using Wilson twisted mass fermions at maximal twist. The tuning of the strange and charm quark masses is performed at two values of the lattice spacing a~0.078 fm and a~0.086 fm with lattice sizes ranging from L~1.9 fm to L~2.8 fm. We measure with high statistical precision the light pseudoscalar mass m_PS and decay constant f_PS in a range 270 < m_PS < 510 MeV and determine the low energy parameters f_0, l_3 and l_4 of SU(2) chiral perturbation theory. We use the two values of the lattice spacing, several lattice sizes as well as different values of the light, strange and charm quark masses to explore the systematic effects. A first study of discretisation effects in light-quark observables and a comparison to N_f=2 results are performed.

hep-lat↗

arXiv2009

Traces of a fixed point: Unravelling the phase diagram at large Nf

With a sufficiently high number of fundamental fermionic flavours present, Yang-Mills theory develops an infrared fixed point and becomes (quasi-)conformal in nature. The range of flavour numbers for which this occurs defines the conformal window, the lower limit of which has yet to be determined. We studied the phase diagram of SU(3) Yang-Mills theory with twelve flavours of staggered fermions. Here we present evidence for the occurrence of a bulk transition towards a chirally broken phase and the existence of a Coulomb phase on the symmetric side of this transition, using results from the measurements of the chiral condensate and spectrum, leading to the determination of a positive sign of the beta function. Assuming the validity of the Appelquist-Miransky-Yamawaki scenario, this implies the existence of a conformal window that comprises the theory under investigation.

hep-lat↗

arXiv2008

Status of ETMC simulations with Nf=2+1+1 twisted mass fermions

We present the status of runs performed in the twisted mass formalism with $N_{\rm f}=2+1+1$ flavours of dynamical fermions: a degenerate light doublet and a mass split heavy doublet. The procedure for tuning to maximal twist will be described as well as the current status of the runs using both thin and stout links. Preliminary results for a few observables obtained on ensembles at maximal twist will be given. Finally, a reweighting procedure to tune to maximal twist will be described.

hep-lat↗

arXiv2008

The physics of eight flavours

When the flavour content of QCD is increased sufficiently, the theory develops a non-trivial infra red fixed point. Thus, for a number of flavours above a certain critical value, but not yet so high that asymptotic freedom is lost, QCD becomes a conformal field theory. The location of the lower limit of this conformal window has not yet been unequivocally determined. Using an improved lattice action, and exploiting modern algorithms allowing for larger lattices and lower quark masses, we have shown that the theory of QCD with eight flavours breaks chiral symmetry in the continuum. We present proof that the accompanying transition is thermal in nature and as a consequence, the conformal window of QCD can only start afterwards, corroborating recent analytical studies at the expense of older results.

hep-lat↗

arXiv2008

Majorana neutrinos from warped extra dimensions

Field theory in the presence of extra dimensions has offered interesting solutions to the gauge hierarchy problem, and inspired many attempts to understand the fermion mass generation mechanism. We study the behaviour of a bulk fermion field subject to non-standard boundary conditions under Z2 symmetry in a slice of (5D) AdS. These pseudo-Majorana boundary conditions relate the 5d spinor fields to their 5d charge conjugate and generate Majorana spinors in four-dimensions. There is a zero mode localized on the visible brane and the masses of Kaluza-Klein excitations are naturally of order the weak-scale. We make connection to neutrino phenomenology. We also demonstrate that, contrary to previous claims, a bulk singlet scalar field cannot provide a successful mechanism to generate a suppressed Majorana mass for Standard Model neutrinos localized on the visible brane.

hep-ph↗

arXiv2008

The physics of eight flavours

We study Quantum Chromodynamics with eight flavours by use of lattice simulations and present evidence that the theory still breaks chiral symmetry in the zero temperature, continuum limit. This confirms that the lower end of the conformal window of QCD lies above Nf = 8.

hep-lat↗

arXiv2008

The Many Uses of Chiral Effective Theories

I review basic concepts of chiral effective field theories guided by an historical perspective: from the first ideas to the merging with other effective frameworks, and to the interplay with lattice field theory. The impact of recent theoretical developments on phenomenological predictions is reviewed with attention for rare decays, and charm physics. I conclude with a critical look at future applications.

hep-ph↗

arXiv2006

Hyperon-Nucleon Scattering from Fully-Dynamical Lattice QCD

We present results of the first fully-dynamical lattice QCD determination of hyperon-nucleon scattering. One s-wave phase shift was determined for n Lambda scattering in both spin-channels at pion masses of 350, 490, and 590 MeV, and for n Sigma- scattering in both spin channels at pion masses of 490, and 590 MeV. The calculations were performed with domain-wall valence quarks on dynamical, staggered gauge configurations with a lattice spacing of b ~ 0.125 fm.

hep-lat↗

arXiv2006

Quenched penguins, the Delta I=1/2 rule, and epsilon'/epsilon

The transformation properties of strong penguin operators under the action of the flavor group change when they are considered as operators in (partially) quenched QCD instead of the unquenched theory. An ambiguity arises, which is parameterized by new low-energy constants in the effective theory describing non-leptonic kaon decays in the (partially) quenched setting. Here we summarize results of the analysis for the complete set of three-flavor strong penguin operators, consisting of products of two left-handed flavor currents, or a left- and a right-handed current. Our results imply that (partially) quenched lattice computations of the Delta I=1/2 rule and epsilon'/epsilon are both affected by ambiguities intrinsic to the use of the quenched approximation at leading order in the chiral expansion. The only exception is the partially quenched case with three light sea quarks, consistent with general expectations. We also address the issue of quenched ambiguities in the case of an active charm, correcting and extending that in Phys. Rev. D 74, 014509 (2006).

hep-lat↗

arXiv2006

Pi-K Scattering in Full QCD with Domain-Wall Valence Quarks

We calculate the pi+ K+ scattering length in fully-dynamical lattice QCD with domain-wall valence quarks on MILC lattices with rooted staggered sea-quarks at a lattice spacing of b=0.125 fm, lattice spatial size of L =2.5 fm and at pion masses of m_pi=290, 350, 490 and 600 MeV. The lattice data, analyzed at next-to-leading order in chiral perturbation theory, allows an extraction of the full pi K scattering amplitude at threshold. Extrapolating to the physical point gives m_pi a_3/2 = -0.0574 (+- 0.0016)(+0.0024 -0.0058) and m_pi a_1/2 = 0.1725 (+- 0.0017)(+0.0023 -0.0156) for the I=3/2 and I=1/2 scattering lengths, respectively, where the first error is statistical and the second error is an estimate of the systematic due to truncation of the chiral expansion.

hep-lat↗

arXiv2003

A note on the power divergence in lattice calculations of $ΔI = 1/2$ $K\toππ$ amplitudes at $M_{K}=M_π$

In this note, we clarify a point concerning the power divergence in lattice calculations of $ΔI = 1/2$ $K\toππ$ decay amplitudes. There have been worries that this divergence might show up in the Minkowski amplitudes at $M_{K}=M_π$ with all the mesons at rest. Here we demonstrate, via an explicit calculation in leading-order Chiral Perturbation Theory, that the power divergence is absent at the above kinematic point, as predicted by CPS symmetry.

hep-lat↗