arXiv subjects

Giancarlo Rossi

Publications and source records attributed to Giancarlo Rossi.

At least 19 recordsLinked to original sources

arXiv2025

A model calculation of the CKM matrix

We propose a strategy to compute the CKM matrix based on the conjecture, recently put forward in the literature, according to which elementary particle masses are not generated like in the standard Higgs scenario, but emerge from a non-perturbative mechanism triggered by the presence in the fundamental Lagrangian of ``irrelevant'' chiral breaking operators of the Wilson type of dimension $d\geq 6$ scaled by $d-4$ powers of the UV cutoff. Non-perturbatively generated quark masses have the form $m_q\sim C_q(α) Λ_{RGI}$ where $Λ_{RGI}$ is the RGI scale of the theory and $C_q(α)$ is a function of the gauge couplings. For the (elementary) fermion $q$ the $C_q(α)$ leading behaviour is $C_q(α)={\mbox{O}}(α^{1+(d_q-4)/2})$. The dependence of the gauge coupling power behaviour from the dimension $d_q$ of the Wilson-like operators associated with the fermion $q$ can be exploited to construct hierarchically organized up and down ''proto-mass matrices'' for ''proto-flavours'', the diagonalization of which yields flavoured quarks with definite masses and a first principle construction of the CKM matrix.

hep-ph↗

arXiv2024

Baryon-number -flavor separation in the topological expansion of QCD

Gauge invariance of QCD dictates the presence of string junctions in the wave functions of baryons. In high-energy inclusive processes, these baryon junctions have been predicted to induce the separation of the flows of baryon number and flavor. In this paper we describe this phenomenon using the analog-gas model of multiparticle production proposed long time ago by Feynman and Wilson and adapted here to accommodate the topological expansion in QCD. In this framework, duality arguments suggest the existence of two degenerate junction-antijunction glueball Regge trajectories of opposite $\cal{C}$-parity with intercept close to 1/2. The corresponding results for the energy and rapidity dependence of baryon stopping are in reasonably good agreement with recent experimental findings from STAR and ALICE experiments. We show that accounting for correlations between the fragmenting strings further improves agreement with the data, and outline additional experimental tests of our picture at the existing (RHIC, LHC, JLab) and future (EIC) facilities.

hep-ph↗

arXiv2023

A road to an elementary particle physics model with no Higgs -- I

This is the first of two companion papers where we prove that the recently discovered non perturbative mechanism capable of giving mass to elementary fermions, in the presence of weak interactions can also generate a mass for the $W$, and can thus be used as a viable alternative to the Higgs scenario. The non perturbative fermion and $W$ masses have the form $m_f\sim C_f(α)Λ_{RGI}$ and $M_W\sim g_wc_w(α)Λ_{RGI}$ with $C_f(α)$ and $c_w(α)$ functions of the gauge couplings, $g_w$ the weak coupling and $Λ_{RGI}$ the RGI scale of the theory. These parametric structures imply that a realistic model must include a new sector of massive fermions (Tera-fermions) subjected, besides Standard Model interactions, to some kind of super-strong gauge interactions (Tera-interactions) so that the RGI scale of the full theory, $Λ_T$, will be in the few TeV region. The extension of the model by introducing hypercharge and particles singlets under strong interactions (leptons and Tera-leptons) is the focus of the companion paper, where we also discuss some phenomenological implications of this approach. One can show that, upon integrating out the (heavy) Tera-degrees of freedom, the resulting low energy effective Lagrangian closely resembles the Standard Model Lagrangian. The argument rests on the conjecture that the 125 GeV resonance detected at LHC is a $W^+W^-/ZZ$ composite state, bound by Tera-particle exchanges, and not an elementary object. Although we restrict to the one family case, neglecting weak isospin splitting, this scheme has a certain number of merits with respect to the Standard Model. It offers a radical solution of the Higgs mass tuning problem as there is no Higgs. It allows identifying $Λ_T$ as the electroweak scale. It helps reducing the number of Standard Model parameters as elementary particle masses are determined by the dynamics.

hep-ph↗

arXiv2023

A road to an elementary particle physics model with no Higgs -- II

This is the second of two companion papers in which we continue developing the construction of an elementary particle model with no Higgs. Here we show that the recently identified non-perturbative field-theoretical feature, alternative to the Higgs mechanism and capable of giving masses to quarks, Tera-quarks and $W$, can also provide mass to leptons and Tera-leptons when the model is extended to include, besides strong, Tera-strong and weak interactions, also hypercharge. In the present approach elementary fermion masses are not free parameters but are determined by the dynamics of the theory. We derive parametric formulae for elementary particle masses from which we can ``predict'' the order of magnitude of the scale of the new Tera-interaction and get crude numerical estimates for mass ratios in fair agreement with phenomenology. The interest of considering elementary particle models endowed with this kind of non-perturbative mass generation mechanism is that they allow solving some of the conceptual problems of the present formulation of the Standard Model, namely origin of the electroweak scale and naturalness.

hep-ph↗

arXiv2022

Elementary particle non-perturbative mass generation. A step towards a beyond-the-Standard-Model model

We show that a recently discovered non-perturbative field-theoretical mechanism giving mass to elementary fermions, is also capable of generating a mass for the electro-weak bosons and can thus be used as a viable alternative to the Higgs scenario. A detailed analysis of this remarkable feature shows that the non-perturbatively generated fermion and $W$ masses have the parametric form $m_{f}\sim C_f(α)Λ_{RGI}$ and $M_W\sim g_w c_w(α)Λ_{RGI}$, respectively, where the coefficients $C_f(α)$ and $c_w(α)$ are functions of the gauge couplings, $g_w$ is the weak coupling and $Λ_{\rm RGI}$ is the RGI scale of the theory. In view of these expressions, we see that to match the experimental values of the top quark and $W$ masses, we are led to conjecture the existence of a yet unobserved sector of massive fermions (that we denote Tera-fermions) subjected, besides ordinary Standard Model interactions, to some kind of super-strong gauge interactions (Tera-interactions), so that the full theory (SM plus Tera-particles) will have an RGI scale $Λ_{\rm RGI}\equiv Λ_T$ in the TeV region. This approach offers a solution of the mass naturalness problem (there is no fundamental Higgs), an understanding of the fermion mass hierarchy and a physical interpretation of the electro-weak scale as a fraction of $Λ_T$.

hep-ph↗

arXiv2022

Twisted mass gauge ensembles at physical values of the light, strange and charm quark masses

Lattice QCD simulations directly at physical masses of dynamical light, strange and charm quarks are highly desirable especially to remove systematic errors due to chiral extrapolations. However such simulations are still challenging. We discuss the adaption of efficient algorithms, like multi-grid methods or higher order integrators, within the molecular dynamic steps of the Hybrid Monte Carlo algorithm, that are enabling simulations of a new set of gauge ensembles by the Extended Twisted Mass collaboration (ETMC). We present the status of the on-going ETMC simulation effort that aims to enabling studies of finite size and discretization effects. We work within the twisted mass discretization which is free of odd-discretization effects at maximal twist and present our tuning procedure.

hep-lat↗

arXiv2020

Tetra-quarks, penta-quarks and the like: old and new views

In this talk, after a short overview of the history of the discovery of tetra-quarks and penta-quarks, we will discuss a possible interpretation of such states in the framework of a 40-years-old "string junction" picture that allows a unified QCD description of ordinary mesons and baryons as well as multi-quark resonances.

hep-ph↗

arXiv2019

$P_c$ Photo-production And Decay

The 2015 LHCb discovery of a structure (denoted by $P_c^+$) decaying in $J/ψ\,p$ and conjectured to be a penta-quark state, has triggered a renewed interest in the question of possible existence of multi-quark states not predicted by the naive quark model. In this talk we present some considerations on $P_c$ photo-production experiments, aimed at testing its multi-quark interpretation in the framework of a 40-years-old string-junction picture that allows a unified description of baryons, tetra-, and penta-quark states.

hep-ph↗

arXiv2019

Dynamical Generation of Elementary Fermion Mass: First Lattice Evidence

Using lattice simulations we demonstrate from first principles the existence of a non-perturbative mechanism for elementary particle mass generation in models with gauge fields, fermions and scalars, if an exact invariance forbids power divergent fermion masses and fermionic chiral symmetries broken at UV scale are maximally restored. We show that in the Nambu-Goldstone phase a fermion mass term, unrelated to the Yukawa operator, is dynamically generated. In models with electro-weak interactions weak boson masses are also generated opening new scenarios for beyond the Standard Model physics.

hep-th↗

arXiv2018

Euclidean partons?

In this talk we reexamine the possibility of evaluating parton distribution functions from lattice simulations. We show that, while in principle individual moments can be extracted from lattice data, in all cases the process of renormalization, hindered by lattice momenta limitation, represents an obstruction to a direct calculation of the full parton distribution function from QCD simulations. We discuss the case of the Ji quasi-parton distribution functions, the possibility of using the reduced Ioffe-time distributions and the more recent proposal of directly subtracting power divergent mixings in perturbation theory.

hep-lat↗

arXiv2018

Towards models with a unified dynamical mechanism for elementary particle masses

Numerical evidence for a new dynamical mechanism of elementary particle mass generation has been found by lattice simulation in a simple, yet highly non-trivial SU(3) gauge model where a SU(2) doublet of strongly interacting fermions is coupled to a complex scalar field doublet via a Yukawa and a Wilson-like term. We point out that if, as a next step towards the construction of a realistic beyond-the-Standard-Model model, weak interactions are introduced, then also weak bosons get a mass by the very same non-perturbative mechanism. In this scenario fermion mass hierarchy can be naturally understood owing to the peculiar gauge coupling dependence of the non-perturbatively generated masses. Hence, if the phenomenological value of the mass of the top quark or the weak bosons has to be reproduced, the RGI scale of the theory must be much larger than $Λ_{QCD}$. This feature hints at the existence of new strong interactions and particles at a scale $Λ_T$ of a few TeV. In such a speculative framework the electroweak scale can be derived from the basic scale $Λ_T$ and the Higgs boson should arise as a bound state in the $WW+ZZ$ channel.

hep-lat↗

arXiv2018

Non-perturbative generation of elementary fermion masses: a numerical study

In this talk we present a numerical lattice study of an SU(3) gauge model where an SU(2) doublet of non-Abelian strongly interacting fermions is coupled to a complex scalar field doublet via a Yukawa and a Wilson-like term. The model enjoys an exact symmetry, acting on all fields, which prevents UV power divergent fermion mass corrections, despite the presence of these two chiral breaking operators in the Lagrangian. In the phase where the scalar potential is non-degenerate and fermions are massless, the bare Yukawa coupling can be set at a critical value at which chiral fermion transformations become symmetries of the theory. Numerical simulations in the Nambu-Goldstone phase of the critical theory, for which the renormalized Yukawa coupling by construction vanishes, give evidence for non-perturbative generation of a UV finite fermion mass term in the effective action.

hep-lat↗

arXiv2018

A first-principle calculation of the XANES spectrum of Cu$^{2+}$ in water

The progress in high performance computing we are witnessing today offers the possibility of accurate electron density calculations of systems in realistic physico-chemical conditions. In this paper, we present a strategy aimed at performing a first-principle computation of the low energy part of the X-ray Absorption Spectroscopy (XAS) spectrum based on the density functional theory calculation of the electronic potential. To test its effectiveness we apply the method to the computation of the X-ray Absorption Near Edge Structure part of the XAS spectrum in the paradigmatic, but simple case of Cu2+ in water. In order to keep into account the effect of the metal site structure fluctuations in determining the experimental signal, the theoretical spectrum is evaluated as the average over the computed spectra of a statistically significant number of simulated metal site configurations. The comparison of experimental data with theoretical calculations suggests that Cu2+ lives preferentially in a square-pyramidal geometry. The remarkable success of this approach in the interpretation of XAS data makes us optimistic about the possibility of extending the computational strategy we have outlined to the more interesting case of molecules of biological relevance bound to transition metal ions.

cond-mat.soft↗

arXiv2018

Multi-scale theoretical approach to X-ray absorption spectra in disordered systems: an application to the study of Zn(II) in water

We develop a multi-scale theoretical approach aimed at calculating from first principles X-ray absorption spectra of liquid solutions and disordered systems. We test the method by considering the paradigmatic case of Zn(II) in water which, besides being relevant in itself, is also of interest for biology. With the help of classical molecular dynamics simulations we start by producing bunches of configurations differing for the Zn(II)-water coordination mode. Different coordination modes are obtained by making use of the so-called dummy atoms method. From the collected molecular dynamics trajectories, snapshots of a more manageable subsystem encompassing the metal site and two solvation layers are cut out. Density functional theory is used to optimize and relax these reduced system configurations employing a uniform dielectric to mimic the surrounding bulk liquid water. On the resulting structures, fully quantum mechanical X-ray absorption spectra calculations are performed by including core-hole effects and core-level shifts. The proposed approach does not rely on any guessing or fitting of the force field or of the atomic positions of the system. The comparison of the theoretically computed spectrum with the experimental Zn K-edge XANES data unambiguously demonstrates that among the different a priori possible geometries, Zn(II) in water lives in an octahedral coordination mode.

cond-mat.soft↗

arXiv2018

Euclidean versus Minkowski short distance

In this note we reexamine the possibility of extracting parton distribution functions from lattice simulations. We discuss the case of quasi-parton distribution functions, the possibility of using the reduced Ioffe-time distributions and the more recent proposal of directly making reference to the computation of the current-current $T$-product. We show that in all cases the process of renormalization hindered by lattice momenta limitation represents an obstruction to a direct Euclidean calculation of the parton distribution function.

hep-lat↗

arXiv2018

Simulating twisted mass fermions at physical light, strange and charm quark masses

We present the QCD simulation of the first gauge ensemble of two degenerate light quarks, a strange and a charm quark with all quark masses tuned to their physical values within the twisted mass fermion formulation. Results for the pseudoscalar masses and decay constants confirm that the produced ensemble is indeed at the physical parameters of the theory. This conclusion is corroborated by a complementary analysis in the baryon sector. We examine cutoff and isospin breaking effects and demonstrate that they are suppressed through the presence of a clover term in the action.

hep-lat↗

arXiv2017

Simulation of an ensemble of $N_f=2+1+1$ twisted mass clover-improved fermions at physical quark masses

We present a general strategy aimed at generating $N_f=2+1+1$ configurations with quarks at their physical mass using maximally twisted mass fermions to ensure automatic $O(a)$ improvement, in the presence of a clover term tuned to reduce the charged to neutral pion mass difference. The target system, for the moment, is a lattice of size $64^3 \times 128$ with a lattice spacing $a\sim 0.08$ fm. We show preliminary results on the pion and kaon mass and decay constants.

hep-lat↗

arXiv2017

Spontaneous $CP$ breaking in QCD and the axion potential: an effective Lagrangian approach

Using the well-known low-energy effective Lagrangian of QCD --valid for small (non-vanishing) quark masses and a large number of colors-- we study in detail the regions of parameter space where $CP$ is spontaneously broken/unbroken for a vacuum angle $θ= π$. In the $CP$-broken region there are first order phase transitions as one crosses $θ=π$, while on the (hyper)surface separating the two regions, there are second order phase transitions signaled by the vanishing of the mass of a pseudo Nambu-Goldstone boson and by a divergent QCD topological susceptibility. The second order point sits at the end of a first order line associated with the $CP$ spontaneous breaking, in the appropriate complex parameter plane. When the effective Lagrangian is extended by the inclusion of an axion these features of QCD imply that standard calculations of the axion potential have to be revised when the QCD parameters fall in the above mentioned $CP$-broken region, in spite of the fact that the axion solves the strong-$CP$ problem. These latter results could be of interest for axionic dark matter calculations if the topological susceptibility of pure Yang-Mills theory falls off sufficiently fast when temperature is increased towards the QCD deconfining transition.

hep-th↗