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S. Pacetti

Publications and source records attributed to S. Pacetti.

35 records · Page 2Linked to original sources
arXiv2013

Modeling the elastic differential cross-section at LHC

An empirical model for the $pp$ elastic differential cross section is proposed. Inspired by early work by Barger and Phillips, we parametrize the scattering amplitude in building blocks, comprising of two exponentials with a relative phase, supplementing the dominant term at small $-t$ with the proton form factor. This model suitably applies to LHC7 and ISR data, enabling to make simple predictions for higher LHC energies and to check whether asymptotia might be achieved.

hep-ph↗
arXiv2013

Hunting for asymptotia at LHC

We discuss whether the behaviour of some hadronic quantities, such as the total cross-section, the ratio of the elastic to the total cross-section, are presently exhibiting the asymptotic behaviour expected at very large energies. We find phenomenological evidence that at LHC7 there is still space for further evolution.

hep-ph↗
arXiv2010

Nucleon form factors: the space-time connection

Analyticity of nucleon form factors allows to derive sum rules which, using space-like and time-like data as input, can give unique information about behaviors in energy regions not experimentally accessible. Taking advantage from new time-like data on proton-antiproton differential cross section and hence the possibility to separate electric and magnetic form factors also in the time-like region, we verify the consistency of the asymptotic behavior predicted by the perturbative QCD for the proton magnetic form factor.

hep-ph↗
arXiv2010

Proposal for taking data with the KLOE-2 detector at the DA$Φ$NE collider upgraded in energy

This document reviews the physics program of the KLOE-2 detector at DA$Φ$NE upgraded in energy and provides a simple solution to run the collider above the $ϕ$-peak (up to 2, possibly 2.5 GeV). It is shown how a precise measurement of the multihadronic cross section in the energy region up to 2 (possibly 2.5) GeV would have a major impact on the tests of the Standard Model through a precise determination of the anomalous magnetic moment of the muon and the effective fine-structure constant at the $M_Z$ scale. With a luminosity of about $10^{32}$cm$^{-2}$s$^{-1}$, DA$Φ$NE upgraded in energy can perform a scan in the region from 1 to 2.5 GeV in one year by collecting an integrated luminosity of 20 pb$^{-1}$ (corresponding to a few days of data taking) for single point, assuming an energy step of 25 MeV. A few years of data taking in this region would provide important tests of QCD and effective theories by $γγ$ physics with open thresholds for pseudo-scalar (like the $η'$), scalar ($f_0,f'_0$, etc...) and axial-vector ($a_1$, etc...) mesons; vector-mesons spectroscopy and baryon form factors; tests of CVC and searches for exotics. In the final part of the document a technical solution for the energy upgrade of DA$Φ$NE is proposed.

hep-ex↗
arXiv2009

Pointlike Baryons?

A peculiar feature, observed in the BaBar data on e+e- -> baryon-antibaryon cross sections, is the non-vanishing cross section at threshold for all these processes. This is the expectation due to the Coulomb enhancement factor acting on a charged fermion pair. Remarkably, in the case of e+e- -> p-pbar it is found that Coulomb final state interactions largely dominate the cross section at threshold and it turns out a form factor |G(4Mp^2)|~1, as a point-like fermion. Also in the case of e+e- -> Lambda_c-antiLambda_c, as recently measured by Belle for the first time, a point-like behavior is suggested for the charmed charged baryon, being the Lambda_c form factor at threshold |G|~1, even if within a large error. In the case of neutral strange baryons the non-vanishing cross section at threshold is interpreted as a remnant of quark pair Coulomb interaction before the hadronization, taking into account the asymmetry between attractive and repulsive Coulomb factors. Besides strange baryon cross sections are successfully compared to U-spin invariance relationships.

hep-ph↗
arXiv2008

A practical guide to unravel time-like transition form factors

A method to determine masses, widths and coupling constants of vector mesons, like phi(1020), omega(782) and rho0(770) recurrences is defined. Starting from data on decay rates and cross sections for the processes: phi -> M_I gamma, phi -> M_I e+e- and e+e- -> M_I phi, where M_I is a pseudoscalar or scalar meson with isospin I=0,1, the time-like transition form factors, which describe the vertex phi-gamma-M_I, are parametrized using a vector meson-propagators description in the low energy region <3-4 GeV, the quark-counting rule prescription for the high energy behavior, and the analyticity imposed by means of the dispersion relations.

hep-ph↗
arXiv2008

Parton fragmentation in the vacuum and in the medium

We present the mini-proceedings of the workshop on ``Parton fragmentation in the vacuum and in the medium'' held at the European Centre for Theoretical Studies in Nuclear Physics and Related Areas (ECT*, Trento) in February 2008. The workshop gathered both theorists and experimentalists to discuss the current status of investigations of quark and gluon fragmentation into hadrons at different accelerator facilities (LEP, B-factories, JLab, HERA, RHIC, and Tevatron) as well as preparations for extension of these studies at the LHC. The main physics topics covered were: (i) light-quark and gluon fragmentation in the vacuum including theoretical (global fits analyses and MLLA) and experimental (data from e+e-, p-p, e-p collisions) aspects, (ii) strange and heavy-quark fragmentation, (iii) parton fragmentation in cold QCD matter (nuclear DIS), and (iv) medium-modified fragmentation in hot and dense QCD matter (high-energy nucleus-nucleus collisions). These mini-proceedings consist of an introduction and short summaries of the talks presented at the meeting.

hep-ph↗
arXiv2008

Unexpected features of e+e-->ppbar and e+e-->lambda-lambdabar cross sections near threshold

Unexpected features of the BaBar data on e+e- in baryon-antibaryon cross sections are discussed. These data have been collected, with unprecedented accuracy, by means of the initial state radiation technique, which is particularly suitable in giving good acceptance and energy resolution at threshold. A striking feature observed in the BaBar data is the non-vanishing cross section at threshold for all these processes. This is the expectation due to the Coulomb enhancement factor acting on a charged fermion pair. In the case of e+e- in proton-antiproton it is found that Coulomb final state interactions largely dominate the cross section and the form factor is |G^p(4M^2_p)|~1, which could be a general feature for baryons. In the case of neutral baryons an interpretation of the non-vanishing cross section at threshold is suggested, based on quark electromagnetic interaction and taking into account the asymmetry between attractive and repulsive Coulomb factors. Besides strange baryon cross sections are compared to U-spin invariance predictions.

hep-ph↗
arXiv2008

Methods and Models for Hadron Physics

A round table held during the Hadron07 Conference focusing on experimental observations of new hadronic states, on theoretical perspectives for their description, and on the role of hadronic spectroscopy in furthering our knowledge of the fundamental theory of strong interactions.

hep-ph↗
arXiv2007

Search for two photon exchange from $e^+ +e^-\to p+\bar p +γ$ data

We look for asymmetries in the angular distributions of events from recent data on $e^+ +e^-\to p +\bar p +γ$ from BABAR collaboration. From first principles, as the C-invariance of the electromagnetic interaction and the crossing symmetry, the presence of two-photon exchange would create a forward backward asymmetry in the data. The analysis of the available data shows no asymmetry, within an error of 2%. This result is consistent with a structureless model for the proton, based on a calculation of $e^+ +e^-\to μ^+ + μ^- +γ$ with a proper replacement of the muon mass. As no systematic deviations are seen, we can conclude that these data do not give any hint of the presence of the two photon contribution, in all the considered kinematical range.

hep-ph↗
arXiv2007

Corrections to the generalized vector dominance due to diffractive rho_3 production

The idea of the vector dominance is still in use in various analyses of experimental data of photon-hadron reactions. It makes sense, therefore, to recast results of microscopic calculations of such reactions in this language. Here we present the diffractive DIS $ρ_3$ production as a specific correction to the generalized vector dominance. We perform a coupled channel analysis of spin-orbital excitations in diffractive photoproduction and reiterate the point that rho_3 in diffractive DIS will be sensitive to a novel aspect of diffraction.

hep-ph↗
arXiv2007

Prospects for e+e- physics at Frascati between the phi and the psi

We present a detailed study, done in the framework of the INFN 2006 Roadmap, of the prospects for e+e- physics at the Frascati National Laboratories. The physics case for an e+e- collider running at high luminosity at the phi resonance energy and also reaching a maximum center of mass energy of 2.5 GeV is discussed, together with the specific aspects of a very high luminosity tau-charm factory. Subjects connected to Kaon decay physics are not discussed here, being part of another INFN Roadmap working group. The significance of the project and the impact on INFN are also discussed. All the documentation related to the activities of the working group can be found in http://www.roma1.infn.it/people/bini/roadmap.html.

hep-ex↗
arXiv2001

Dispersive Techniques for $α_s$, $R_{had}$ and Instability of the Perturbative Vacuum

Recent dispersive techniques developed by us are applied to discuss three problems: 1. A long standing discrepan-cy between the measurements of $R(s)$ for $\sqrt{s} = (5÷7.5)GeV$ by Crystal Ball and MARK I has been analyzed and its consequences analyzed for the number of contributing quarks. 2. Noting that the perturbative $α_s$ has the wrong analyticity, analytic models consistent with asymptotic freedom (AF) and confinement have been constructed and applied to discuss $τ$ decay. 3. It is shown that AF leads to a wrong sign for $\im\big(α(s)\big)$ which signals an instability of the perturbative QCD vacuum.

hep-ph↗
arXiv2001

The Inverse Problem: Extracting Time-Like from Space-Like Data

A practical strategy is presented and successfully implemented to determine form factors in the time-like but unphysical (below threshold) region using dispersion relations, in a model independent way without any bias towards expected resonances. Space and time-like data have been employed along with a regularization scheme to unfold and solve the integral equations. Remarkably, resonance structures with peaks for the $ρ(770)$, $ρ'(1600)$ and a structure near the $N\bar N$ threshold are automatically generated. The $Φ$ peak is invisible thus refuting suggestions about any sizeable $s\bar s$ content in the nucleon.

hep-ph↗
arXiv2000

Resolution of a long standing discrepancy in R with spin zero quarks

A previously successful dispersive method has been applied to understand different values for $R(\sqrt{s} = 5 ÷7.5 GeV)$ obtained by MARK I and Crystal Ball collaborations. We compute $R$ in the disputed region with data from outside this region and asymptotic behavior given by the standard model with 5 quark flavors, and find agreement with the Crystal Ball result. On the other hand, the MARK I data are reproduced if we augment the asymptotic behavior with contributions from a single spin zero quark of charge ($ -1/3$). The visible hadronic fragments from such scalar quarks are not likely to produce predominantly pure 2-jet events at such low energies. Hence, such decay modes may have been removed by the Crystal Ball energy imbalance cuts in their definition of hadronic events but not in MARK I events, thus accounting for the discrepancy in the two results. Upper bounds on spin zero quark production at LEP through $Z$ decay data are used to estimate the mixing angle between $T_3\ = -1/2$ and $T_3 = 0$ scalar quarks. Recent negative results about spin zero quarks from CLEO are critically examined. We briefly discuss diquark production hypothesis and find it very unlikely to explain the discrepancy.

hep-ph↗