arXiv subjects

Biagio Lucini

Publications and source records attributed to Biagio Lucini.

At least 145 records · Page 8Linked to original sources

arXiv2010

Confining vs. conformal scenario for SU(2) with adjoint fermions. Gluonic observables

Walking technicolor is a mechanism for electroweak symmetry breaking without Higgs field. The Higgs mechanism is provided by chiral symmetry breaking in the technicolor theory. An essential ingredient is the vicinity to an IR fixed point, which could reconcile technicolor with the electroweak precision tests. SU(2) gauge theory with two Dirac adjoint fermions has been proposed as a candidate for walking technicolor. Understanding whether this theory is confining or IR-conformal is a challenging problem, which can be addressed by means of numerical simulations. We have pointed out that a clean signal for the existence of an IR fixed point in this theory can be obtained by comparing the mesonic and gluonic sectors. We review some technical details of our calculations. Possible systematic errors are discussed.

hep-lat↗

arXiv2010

Improved Spectroscopy of Minimal Walking Technicolor

We present an improved study of spectroscopic observables in the $SU(2)$ Yang-Mills theory with two adjoint fermions. We make an improvement on the precision of previous results which clarify the scale of finite volume effects present. This analysis adds to the evidence for near-conformal dynamics of this theory, while indicating a preference for a low anomalous mass dimension of the massless theory.

hep-lat↗

arXiv2010

Orientifold Planar Equivalence: The Quenched Meson Spectrum

A numerical study of Orientifold Planar Equivalence is performed in SU(N) Yang-Mills theories for N=2,3,4,6. Quenched meson masses are extracted in the antisymmetric, symmetric and adjoint representations for the pseudoscalar and vector channels. An extrapolation of the vector mass as a function of the pseudoscalar mass to the large-N limit shows that the numerical results agree within errors for the three theories, as predicted by Orientifold Planar Equivalence. As a byproduct of the extrapolation, the size of the corrections up to O(1/N^3) are evaluated. A crucial prerequisite for the extrapolation is the determination of an analytical relationship between the corrections in the symmetric and in the antisymmetric representations, order by order in a 1/N expansion.

hep-lat↗

arXiv2010

The glueball spectrum at large N

The lowest-lying glueball masses are computed in SU($N$) gauge theory on a spacetime lattice for constant value of the lattice spacing $a$ and for $N$ ranging from 3 to 8. The lattice spacing is fixed using the deconfinement temperature at temporal extension of the lattice $N_T = 6$. The calculation is conducted employing in each channel a variational ansatz performed on a large basis of operators that includes also torelon and (for the lightest states) scattering trial functions. This basis is constructed using an automatic algorithm that allows us to build operators of any size and shape in any irreducible representation of the cubic group. A good signal is extracted for the ground state and the first excitation in several symmetry channels. It is shown that all the observed states are well described by their large $N$ values, with modest ${\cal O}(1/N^2)$ corrections. In addition spurious states are identified that couple to torelon and scattering operators.

hep-lat↗

arXiv2010

A numerical investigation of orientifold planar equivalence for quenched mesons

We measure on the lattice the quenched pseudoscalar and vector meson masses at a fixed value of the lattice spacing for SU(N) gauge theory with fermions in the adjoint, in the symmetric and in the antisymmetric representations of the gauge group. Simulations are performed for N=3,4,6 in all those representations, with the addition of N=2 for the adjoint representation. We illustrate a strategy for separating the even from the odd-power contributions in 1/N in the masses. Using this technique, we extrapolate the vector mass to the large-N limit in the chiral region and show that at N = infty this mass is the same within errors in all the three representations, as predicted by orientifold planar equivalence. Possible implications of our investigation for studying orientifold planar equivalence in the dynamical case are discussed.

hep-lat↗

arXiv2010

Mesonic spectroscopy of Minimal Walking Technicolor

We investigate the structure and the novel emerging features of the mesonic non-singlet spectrum of the Minimal Walking Technicolor (MWT) theory. Precision measurements in the nonsinglet pseudoscalar and vector channels are compared to the expectations for an IR-conformal field theory and a QCD-like theory. Our results favor a scenario in which MWT is (almost) conformal in the infrared, while spontaneous chiral symmetry breaking seems less plausible.

hep-lat↗

arXiv2010

The infrared dynamics of Minimal Walking Technicolor

We study the gauge sector of Minimal Walking Technicolor, which is an SU(2) gauge theory with nf=2 flavors of Wilson fermions in the adjoint representation. Numerical simulations are performed on lattices Nt x Ns^3, with Ns ranging from 8 to 16 and Nt=2Ns, at fixed β=2.25, and varying the fermion bare mass m0, so that our numerical results cover the full range of fermion masses from the quenched region to the chiral limit. We present results for the string tension and the glueball spectrum. A comparison of mesonic and gluonic observables leads to the conclusion that the infrared dynamics is given by an SU(2) pure Yang-Mills theory with a typical energy scale for the spectrum sliding to zero with the fermion mass. The typical mesonic mass scale is proportional to, and much larger than this gluonic scale. Our findings are compatible with a scenario in which the massless theory is conformal in the infrared. An analysis of the scaling of the string tension with the fermion mass towards the massless limit allows us to extract the chiral condensate anomalous dimension γ*, which is found to be γ*=0.22+-0.06.

hep-lat↗

arXiv2010

Strongly Interacting Dynamics beyond the Standard Model on a Spacetime Lattice

Strong theoretical arguments suggest that the Higgs sector of the Standard Model of the Electroweak interactions is an effective low-energy theory, with a more fundamental theory that is expected to emerge at an energy scale of the order of the TeV. One possibility is that the more fundamental theory be strongly interacting and the Higgs sector be given by the low-energy dynamics of the underlying theory. We review recent works aimed to determining observable quantities by numerical simulations of strongly interacting theories proposed in the literature for explaining the Electroweak symmetry breaking mechanism. These investigations are based on Monte Carlo simulations of the theory formulated on a spacetime lattice. We focus on the so-called Minimal Walking Technicolour scenario, a SU(2) gauge theory with two flavours of fermions in the adjoint representation. The emerging picture is that this theory has an infrared fixed point that dominates the large distance physics. We shall discuss the first numerical determinations of quantities of phenomenological interest for this theory and analyse future directions of quantitative studies of strongly interacting beyond the Standard Model theories with Lattice techniques. In particular, we report on a finite size scaling determination of the chiral condensate anomalous dimension $γ$, for which we find $0.05 \le γ\le 0.25$.

hep-ph↗

arXiv2009

Discrete symmetry breaking and baryon currents in U(N) and SU(N) gauge theories

In SU($N$) gauge theories with fermions in the fundamental or in a two-index (either symmetric or antisymmetric) representation formulated on a manifold with at least one compact dimension with non-trivial holonomy the discrete symmetries $C$, $P$ and $T$ are broken at small enough size of the compact direction(s) for certain values of $N$. We show that for those $N$ in the broken phase a non-zero baryon current wrapping in the compact direction exists, which provides a measurable observable for the breaking of $C$, $P$ and $T$. We prove that in all cases where the current is absent there is no breaking of those discrete symmetries. This includes the limit $N \to \infty$ of the SU($N$) gauge theory with symmetric or antisymmetric fermions and U($N$) gauge theory at any value of $N$. We then argue that the component of the baryon current in the compact direction is the physical order parameter for $C$, $P$ and $T$ breaking due to the breaking of Lorentz invariance.

hep-th↗

arXiv2008

Orientifold Planar Equivalence: The Chiral Condensate

The recently introduced orientifold planar equivalence is a promising tool for solving non-perturbative problems in QCD. One of the predictions of orientifold planar equivalence is that the chiral condensates of a theory with $N_f$ flavours of Dirac fermions in the symmetric (or antisymmetric) representation and $N_f$ flavours of Majorana fermions in the adjoint representation have the same large $N$ value for any value of the mass of the (degenerate) fermions. Assuming the invariance of the theory under charge conjugation, we prove this statement on the lattice for staggered quenched condensates in SU($N$) Yang-Mills in the large $N$ limit. Then, we compute numerically those quenched condensates for $N$ up to 8. After separating the even from the odd corrections in $1/N$, we are able to show that our data support the equivalence; however, unlike other quenched observables, subleading terms in $1/N$ are needed for describing the data for the symmetric and antisymmetric representation at $N$=3. Possible lessons for the unquenched case are discussed.

hep-lat↗

arXiv2008

The running of the coupling in SU(N) pure gauge theories

The running of the coupling is studied in SU(4) gauge theory using the Schrödinger functional technique. Up to energies of the order of the square root of the string tension $σ$, the running is found to agree with the two-loop perturbative formula. Relating the perturbative to the non-perturbative regime of the running and converting to the $\msbar$ scheme allows one to extract the ratio $Λ_{\msbar}/\sqrtσ$. The result is then used in combination with similar calculations present in the literature for SU(2) and SU(3) to extract $Λ_{\msbar}/\sqrtσ$ in the large $N$ limit. Our results for $N=3,4$ agree with a recent study of the same quantity performed using the Parisi mean field scheme as an intermediate scheme, while $Λ_{\msbar}/\sqrtσ$ in SU(2) turns out to differ by 2.5%. Possible explanations of this discrepancy are discussed.

hep-lat↗

arXiv2008

Lattice Study of Planar Equivalence: The Quark Condensate

We study quenched SU(N) gauge theories with fermions in the two-index symmetric, antisymmetric and the adjoint representations. Our main motivation is to check whether at large number of colours those theories become non-perturbatively equivalent. We prove the equivalence assuming that the charge-conjugation symmetry is not broken in pure Yang-Mills theory. We then carry out a quenched lattice simulation of the quark condensate in the symmetric, antisymmetric and the adjoint representations for SU(2), SU(3), SU(4), SU(6) and SU(8). We show that the data support the equivalence and discuss the size of subleading corrections.

hep-th↗

arXiv2008

Quenched mesonic spectrum at large N

We compute the masses of the $π$ and of the $ρ$ mesons in the quenched approximation on a lattice with fixed lattice spacing $a \simeq 0.145 \ \mathrm{fm}$ for SU($N$) gauge theory with $N = 2,3,4,6$. We find that a simple linear expression in $1/N^2$ correctly captures the features of the lowest-lying meson states at those values of $N$. This enables us to extrapolate to $N = \infty$ the behaviour of $m_π$ as a function of the quark mass and of $m_ρ$ as a function of $m_π$. Our results for the latter agree within 5% with recent predictions obtained in the AdS/CFT framework.

hep-th↗

arXiv2007

Confinement, Screening and the Center on S^3 x S^1

We compute the one-loop effective potential for the Polyakov loop on $S^3 times S^1$ for an asymptotically free gauge theory of arbitrary group $G$ and a generic matter content. We apply this result to study the phase structures of $G_2$, SO(N) and $Sp(2N)$ gauge theories which turn out to be in qualitative agreement with the results of lattice calculations. On $S^3 \times S^1$, at zero temperature, the Polyakov loop is zero for kinematical reasons. For small but non-zero temperature, the Polyakov loop is still zero if the gauge theory has an unbroken center, while it acquires a small vacuum expectation value for gauge theories whose center is trivial or explicitly broken by the presence of dynamical matter fields. At high temperatures, the saddle point structure of the effective potential is different from the low temperature case suggesting that the theory is in the deconfined phase. At finite $N$, the Polyakov loop is non-zero in the high temperature phase only in theories with no unbroken center symmetry, consistent with screening of the external charge introduced by the Polyakov loop and Gauss law on a compact space.

hep-th↗

arXiv2007

Determination of the running coupling in pure SU(4) Yang-Mills theory

The Schrodinger functional is used to define a renormalised coupling for pure SU(4) Yang-Mills theory, with Wilson action and suitably selected boundary conditions on the link field. The coupling, which runs with the size of the lattice, is then determined by a finite-size scaling technique through a large range of momenta, thereby allowing a connection to be made between the high energy regime and the low energy non-perturbative regime, where contact is made with the physical scale of the theory. Using data from previous SU(2) and SU(3) simulations obtained with the same technique, the running of the 't Hooft coupling defined through the Schrodinger functional is studied, and we check whether the large-N expectation that g^2*N is a universal function of the energy scale E holds down to energies of the order of the string tension. Finally, we determine Lambda_SF in units of the string tension as a function of N at leading order in 1/N^2.

hep-lat↗

arXiv2007

Confinement: $G_2$ group case

The gauge group being centreless, $G_2$ gauge theory is a good laboratory for studying the role of the centre of the group for colour confinement in Yang-Mills gauge theories. In this paper, we investigate $G_2$ pure gauge theory at finite temperature on the lattice. By studying the finite size scaling of the plaquette, the Polyakov loop and their susceptibilities, we show that a deconfinement phase transition takes place. The analysis of the pseudocritical exponents give strong evidence of the deconfinement transition being first order. Implications of our findings for scenarios of colour confinement are discussed.

hep-lat↗

arXiv2007

Baryon currents in the C-broken phase of QCD

In a space with some sufficiently small compact dimension (with non-trivial cycles) and with periodic boundary conditions for the fermions, the charge conjugation (C), spatial parity (P), time reversal (T) and CPT symmetries are spontaneously broken in QCD. We have investigated what are the physical consequences of the breaking of these discrete symmetries, that is what local observables can be used to detect it. We show that the breaking induces the generation of baryon currents, propagating along the compact dimensions.

hep-lat↗

arXiv2007

Spontaneous breaking of discrete symmetries in QCD on a small volume

In a compact space with non-trivial cycles, for sufficiently small values of the compact dimensions, charge conjugation (C), spatial reflection (P) and time reversal (T) are spontaneously broken in QCD. The order parameter for the symmetry breaking is the trace of the Wilson line wrapping around the compact dimension, which acquires an imaginary part in the broken phase. We show that a physical signature for the symmetry breaking is a persistent baryonic current wrapping in the compact directions. The existence of such a current is derived analytically at first order in perturbation theory and confirmed in the non-perturbative regime by lattice simulations.

hep-lat↗