Helicity components of the cross section for double charged-pion production by real photons on protons

The helicity components σ1/2 and σ3/2 of the cross section for double charged-pion production by real photons on a nucleon are calculated within a phenomenological approach developed previously. A high sensitivity of the σ1/2–σ3/2 asymmetry to the contribution of nucleon resonances having strongly different electromagnetic helicity amplitudes A1/2 and A3/2 is demonstrated. This feature is of importance for seeking "missing" baryon states

Testing an Open Source installation and server provisioning tool for the INFN CNAF Tierl Storage system

In large computing centers, such as the INFN CNAF Tier1 [1], is essential to be able to configure all the machines, depending on use, in an automated way. For several years at the Tier1 has been used Quattor[2], a server provisioning tool, which is currently used in production. Nevertheless we have recently started a comparison study involving other tools able to provide specific server installation and configuration features and also offer a proper full customizable solution as an alternative to Quattor. Our choice at the moment fell on integration between two tools: Cobbler [3] for the installation phase and Puppet [4] for the server provisioning and management operation. The tool should provide the following properties in order to replicate and gradually improve the current system features: implement a system check for storage specific constraints such as kernel modules black list at boot time to avoid undesired SAN (Storage Area Network) access during disk partitioning; a simple and effective mechanism for kernel upgrade and downgrade; the ability of setting package provider using yum, rpm or apt; easy to use Virtual Machine installation support including bonding and specific Ethernet configuration; scalability for managing thousands of nodes and parallel installations. This paper describes the results of the comparison and the tests carried out to verify the requirements and the new system suitability in the INFN-T1 environment

Storage Infrastructure at the INFN LHC Tier-1

In this paper we will describe the Storage Infrastructure of the INFN-CNAF Tier-1, used to store data of High Energy Physics experiments, in particular those operating at the Large Hadron Collider

A self-configuring control system for storage and computing departments at INFN-CNAF Tierl

The storage and farming departments at the INFN-CNAF Tier1[1] manage approximately thousands of computing nodes and several hundreds of servers that provides access to the disk and tape storage. In particular, the storage server machines should provide the following services: an efficient access to about 15 petabytes of disk space with different cluster of GPFS file system, the data transfers between LHC Tiers sites (Tier0, Tier1 and Tier2) via GridFTP cluster and Xrootd protocol and finally the writing and reading data operations on magnetic tape backend. One of the most important and essential point in order to get a reliable service is a control system that can warn if problems arise and which is able to perform automatic recovery operations in case of service interruptions or major failures. Moreover, during daily operations the configurations can change, i.e. if the GPFS cluster nodes roles can be modified and therefore the obsolete nodes must be removed from the control system production, and the new servers should be added to the ones that are already present. The manual management of all these changes is an operation that can be somewhat difficult in case of several changes, it can also take a long time and is easily subject to human error or misconfiguration. For these reasons we have developed a control system with the feature of self-configure itself if any change occurs. Currently, this system has been in production for about a year at the INFN-CNAF Tier1 with good results and hardly any major drawback. There are three major key points in this system. The first is a software configurator service (e.g. Quattor or Puppet) for the servers machines that we want to monitor with the control system; this service must ensure the presence of appropriate sensors and custom scripts on the nodes to check and should be able to install and update software packages on them. The second key element is a database containing information, according to a suitable format, on all the machines in production and able to provide for each of them the principal information such as the type of hardware, the network switch to which the machine is connected, if the machine is real (physical) or virtual, the possible hypervisor to which it belongs and so on. The last key point is a control system software (in our implementation we choose the Nagios software), capable of assessing the status of the servers and services, and that can attempt to restore the working state, restart or inhibit software services and send suitable alarm messages to the site administrators. The integration of these three elements was made by appropriate scripts and custom implementation that allow the self-configuration of the system according to a decisional logic and the whole combination of all the above-mentioned components will be deeply discussed in this paper

A Phenomenological Description of (\pi^{-}\Delta^{++}) Photo- and Electroproduction in Nucleon Resonance Region

The (\pi^{-}\Delta^{++}) production on the nucleon by real and virtual photons is discussed as initial step in a simple model approach for the two pion photo- and electroproduction on the nucleon, with emphasis on nucleon resonance excitation which is of interest for new facilities like TJNAF. A calculation for (\pi^{-}\Delta^{++}) channel in resonance excitation region is presented and compared to existing experimental data along with a discussion of physical effects that we find to be of relevance. The calculation is proposed as a starting basis for the investigation of (N^{*}) electromagnetic form factors using experimental data about two pion production by real and virtual photons.Comment: 36 pages, 14 figures, to be published in Nucl. Phys.

Search for Θ+(1540)\Theta^+(1540) pentaquark in high statistics measurement of γp→Kˉ0K+n\gamma p \to \bar K^0 K^+ n at CLAS

The exclusive reaction $\gamma p \to \bar K^0 K^+ n$ was studied in the photon energy range between 1.6-3.8 GeV searching for evidence of the exotic baryon $\Theta^+(1540)\to nK^+$. The decay to $nK^+$ requires the assignment of strangeness $S=+1$ to any observed resonance. Data were collected with the CLAS detector at the Thomas Jefferson National Accelerator Facility corresponding to an integrated luminosity of 70 $pb^{-1}$. No evidence for the $\Theta^+$ pentaquark was found. Upper limits were set on the production cross section as function of center-of-mass angle and $nK^+$ mass. The 95% CL upper limit on the total cross section for a narrow resonance at 1540 MeV was found to be 0.8 nb.Comment: Submitted to Physical Review Letter

Measurement of the Polarized Structure Function σLT′\sigma_{LT^\prime} for p(e⃗,e′π+)np(\vec{e},e'\pi^+)n in the Δ(1232)\Delta(1232) Resonance Region

The polarized longitudinal-transverse structure function $\sigma_{LT^\prime}$ has been measured using the $p(\vec e,e'\pi^+)n$ reaction in the $\Delta(1232)$ resonance region at $Q^2=0.40$ and 0.65 GeV$^2$. No previous $\sigma_{LT^\prime}$ data exist for this reaction channel. The kinematically complete experiment was performed at Jefferson Lab with the CEBAF Large Acceptance Spectrometer (CLAS) using longitudinally polarized electrons at an energy of 1.515 GeV. A partial wave analysis of the data shows generally better agreement with recent phenomenological models of pion electroproduction compared to the previously measured $\pi^0 p$ channel. A fit to both $\pi^0 p$ and $\pi^+ n$ channels using a unitary isobar model suggests the unitarized Born terms provide a consistent description of the non-resonant background. The $t$-channel pion pole term is important in the $\pi^0 p$ channel through a rescattering correction, which could be model-dependent.Comment: 6 pages, LaTex, 5 eps figures: Submitted to PRC/Brief Reports v2: Updated referenc

Q^2 Dependence of the S_{11}(1535) Photocoupling and Evidence for a P-wave resonance in eta electroproduction

New cross sections for the reaction $ep \to e'\eta p$ are reported for total center of mass energy $W$=1.5--2.3 GeV and invariant squared momentum transfer $Q^2$=0.13--3.3 GeV$^2$. This large kinematic range allows extraction of new information about response functions, photocouplings, and $\eta N$ coupling strengths of baryon resonances. A sharp structure is seen at $W\sim$ 1.7 GeV. The shape of the differential cross section is indicative of the presence of a $P$-wave resonance that persists to high $Q^2$. Improved values are derived for the photon coupling amplitude for the $S_{11}$(1535) resonance. The new data greatly expands the $Q^2$ range covered and an interpretation of all data with a consistent parameterization is provided.Comment: 31 pages, 9 figure

Measurement of Deeply Virtual Compton Scattering with a Polarized Proton Target

The longitudinal target-spin asymmetry A_UL for the exclusive electroproduction of high energy photons was measured for the first time in p(e,e'p\gamma). The data have been accumulated at Jefferson Lab with the CLAS spectrometer using 5.7 GeV electrons and a longitudinally polarized NH_3 target. A significant azimuthal angular dependence was observed, resulting from the interference of the Deeply Virtual Compton Scattering and Bethe-Heitler processes. The amplitude of the sin(phi) moment is 0.252 +/- 0.042(stat) +/- 0.020(sys). Theoretical calculations are in good agreement with the magnitude and the kinematic dependence of the target-spin asymmetry, which is sensitive to the generalized parton distributions H and H-tilde.Comment: Modified text slightly, added reference

First measurement of coherent ϕ\phi-meson photoproduction on deuteron at low energies

The cross section and decay angular distributions for the coherent \phi meson photoproduction on the deuteron have been measured for the first time up to a squared four-momentum transfer t =(p_{\gamma}-p_{\phi})^2 =-2 GeV^2/c^2, using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The cross sections are compared with predictions from a re-scattering model. In a framework of vector meson dominance, the data are consistent with the total \phi-N cross section \sigma_{\phi N} at about 10 mb. If vector meson dominance is violated, a larger \sigma_{\phi N} is possible by introducing larger t-slope for the \phi N \to \phi N process than that for the \gamma N \to \phi N process. The decay angular distributions of the \phi are consistent with helicity conservation.Comment: 6 page