arXiv subjects

A. Filippi

Publications and source records attributed to A. Filippi.

At least 73 records · Page 4Linked to original sources

arXiv2018

First results on nucleon resonance photocouplings from the $γp \to π^+π^-p$ reaction

We report the first experimental measurements of the nine 1-fold differential cross sections for the $γp \to π^+π^-p$ reaction, obtained with the CLAS detector at Jefferson Laboratory. The measurements cover the invariant mass range of the final state hadrons from 1.6~GeV~$<W<$~2.0~GeV. For the first time the photocouplings of all prominent nucleon resonances in this mass range have been extracted from this exclusive channel. Photoproduction of two charged pions is of particular importance for the evaluation of the photocouplings for the $Δ(1620)1/2^-$, $Δ(1700)3/2^-$, $N(1720)3/2^+$, and $Δ(1905)5/2^+$ resonances, which have dominant decays into the $ππN$ final states rather than the more extensively studied single meson decay channels.

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arXiv2018

Beam-Target Helicity Asymmetry $E$ in $K^{0}Λ$ and $K^{0}Σ^0$ Photoproduction on the Neutron

We report the first measurements of the $E$ beam-target helicity asymmetry for the $\vecγ \vec{n} \to K^{0}Λ$, and $K^{0}Σ^{0}$ channels in the energy range 1.70$\leq W\leq$2.34 GeV. The CLAS system at Jefferson Lab uses a circularly polarized photon beam and a target consisting of longitudinally polarized solid molecular hydrogen deuteride with low background contamination for the measurements. The multivariate analysis method boosted decision trees was used to isolate the reactions of interest. Comparisons with predictions from the KaonMAID, SAID, and Bonn-Gatchina models are presented. These results will help separate the isospin $I=0$ and $I=1$ photo-coupling transition amplitudes in pseudoscalar meson photoproduction.

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arXiv2018

Photoproduction of $K^+K^-$ meson pairs on the proton

The exclusive reaction $γp \to p K^+ K^-$ was studied in the photon energy range $3.0 - 3.8 \mbox{ GeV}$ and momentum transfer range $0.6<-t<1.3 \mbox{ GeV}^2$. Data were collected with the CLAS detector at the Thomas Jefferson National Accelerator Facility. In this kinematic range the integrated luminosity was approximately 20 pb$^{-1}$. The reaction was isolated by detecting the $K^+$ and the proton in CLAS, and reconstructing the $K^-$ via the missing-mass technique. Moments of the di-kaon decay angular distributions were extracted from the experimental data. Besides the dominant contribution of the $ϕ$ meson in the $P$-wave, evidence for $S-P$ interference was found. The differential production cross sections $dσ/dt$ for individual waves in the mass range of the $ϕ$ resonance were extracted and compared to predictions of a Regge-inspired model. This is the first time the $t$-dependent cross section of the $S$-wave contribution to the elastic $K^+K^-$ photoproduction has been measured.

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arXiv2018

Search for a Dark Photon in Electro-Produced $e^{+}e^{-}$ Pairs with the Heavy Photon Search Experiment at JLab

The Heavy Photon Search experiment took its first data in a 2015 engineering run at the Thomas Jefferson National Accelerator Facility, searching for a prompt, electro-produced dark photon with a mass between 19 and 81 MeV/$c^2$. A search for a resonance in the $e^{+}e^{-}$ invariant mass distribution, using 1.7 days (1170 nb$^{-1}$) of data, showed no evidence of dark photon decays above the large QED background, confirming earlier searches and demonstrating the full functionality of the experiment. Upper limits on the square of the coupling of the dark photon to the Standard Model photon are set at the level of 6$\times10^{-6}$. Future runs with higher luminosity will explore new territory.

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arXiv2018

Search for $K^{+}\rightarrowπ^{+}ν\overlineν$ at NA62

$K^{+}\rightarrowπ^{+}ν\overlineν$ is one of the theoretically cleanest meson decay where to look for indirect effects of new physics complementary to LHC searches. The NA62 experiment at CERN SPS is designed to measure the branching ratio of this decay with 10\% precision. NA62 took data in pilot runs in 2014 and 2015 reaching the final designed beam intensity. The quality of 2015 data acquired, in view of the final measurement, will be presented.

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arXiv2018

Measurement of the beam asymmetry $Σ$ and the target asymmetry $T$ in the photoproduction of $ω$ mesons off the proton using CLAS at Jefferson Laboratory

The photoproduction of $ω$ mesons off the proton has been studied in the reaction $γp\to p\,ω$ using the CEBAF Large Acceptance Spectrometer (CLAS) and the frozen-spin target (FROST) in Hall B at the Thomas Jefferson National Accelerator Facility. For the first time, the target asymmetry, $T$, has been measured in photoproduction from the decay $ω\toπ^+π^-π^0$, using a transversely-polarized target with energies ranging from just above the reaction threshold up to 2.8 GeV. Significant non-zero values are observed for these asymmetries, reaching about 30-40% in the third-resonance region. New measurements for the photon-beam asymmetry, $Σ$, are also presented, which agree well with previous CLAS results and extend the world database up to 2.1 GeV. These data and additional $ω$-photoproduction observables from CLAS were included in a partial-wave analysis within the Bonn-Gatchina framework. Significant contributions from $s$-channel resonance production were found in addition to $t$-channel exchange processes.

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arXiv2018

Semi-Inclusive $π_0$ target and beam-target asymmetries from 6 GeV electron scattering with CLAS

We present precision measurements of the target and beam-target spin asymmetries from neutral pion electroproduction in deep-inelastic scattering (DIS) using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. We scattered 6-GeV, longitudinally polarized electrons off longitudinally polarized protons in a cryogenic $^{14}$NH$_3$ target, and extracted double and single target spin asymmetries for $ep\rightarrow e^\primeπ^0X$ in multidimensional bins in four-momentum transfer ($1.0<Q^2<3.2$ GeV$^2$), Bjorken-$x$ ($0.12<x<0.48$), hadron energy fraction ($0.4<z<0.7$), transverse pion momentum ($0<P_T<1.0$ GeV), and azimuthal angle $ϕ_h$ between the lepton scattering and hadron production planes. We extracted asymmetries as a function of both $x$ and $P_T$, which provide access to transverse-momentum distributions of longitudinally polarized quarks. The double spin asymmetries depend weakly on $P_T$. The $\sin 2ϕ_h$ moments are zero within uncertainties, which is consistent with the expected suppression of the Collins fragmentation function. The observed $\sinϕ_h$ moments suggest that quark gluon correlations are significant at large $x$.

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arXiv2018

Workshop on Pion-Kaon Interactions (PKI2018) Mini-Proceedings. Editors: M. Amaryan, Ulf-G. Meißner, C. Meyer, J. Ritman, and I. Strakovsky

This volume is a short summary of talks given at the PKI2018 Workshop organized to discuss current status and future prospects of pi-K interactions. The precise data on pi-K interaction will have a strong impact on strange meson spectroscopy and form factors that are important ingredients in the Dalitz plot analysis of a decays of heavy mesons as well as precision measurement of Vus matrix element and therefore on a test of unitarity in the first raw of the CKM matrix. The workshop has combined the efforts of experimentalists, Lattice QCD, and phenomenology communities. Experimental data relevant to the topic of the workshop were presented from the broad range of different collaborations like CLAS, GlueX, COMPASS, BaBar, BELLE, BESIII, VEPP-2000, and LHCb. One of the main goals of this workshop was to outline a need for a new high intensity and high precision secondary KL beam facility at JLab produced with the 12 GeV electron beam of CEBAF accelerator. This workshop is a successor of the workshops Physics with Neutral Kaon Beam at JLab [1] held at JLab, February, 2016; Excited Hyperons in QCD Thermodynamics at Freeze-Out [2] held at JLab, November, 2016; New Opportunities with High-Intensity Photon Sources [3] held at CUA, February, 2017. Further details about the PKI2018 Workshop can be found on the web page of the conference: http://www.jlab.org/conferences/pki2018/ .

hep-ph↗

arXiv2018

Dark matter search in a Beam-Dump eXperiment (BDX) at Jefferson Lab: an update on PR12-16-001

This document is an update to the proposal PR12-16-001 Dark matter search in a Beam-Dump eXperiment (BDX) at Jefferson Lab submitted to JLab-PAC44 in 2016 reporting progress in addressing questions raised regarding the beam-on backgrounds. The concerns are addressed by adopting a new simulation tool, FLUKA, and planning measurements of muon fluxes from the dump with its existing shielding around the dump. First, we have implemented the detailed BDX experimental geometry into a FLUKA simulation, in consultation with experts from the JLab Radiation Control Group. The FLUKA simulation has been compared directly to our GEANT4 simulations and shown to agree in regions of validity. The FLUKA interaction package, with a tuned set of biasing weights, is naturally able to generate reliable particle distributions with very small probabilities and therefore predict rates at the detector location beyond the planned shielding around the beam dump. Second, we have developed a plan to conduct measurements of the muon ux from the Hall-A dump in its current configuration to validate our simulations.

physics.ins-det↗

arXiv2018

Measurement of the helicity asymmetry $E$ in $ω\toπ^+π^-π^0$ photoproduction

The double-polarization observable $E$ was studied for the reaction $γp\to pω$ using the CEBAF Large Acceptance Spectrometer (CLAS) in Hall B at the Thomas Jefferson National Accelerator Facility and the longitudinally-polarized frozen-spin target (FROST). The observable was measured from the charged decay mode of the meson, $ω\toπ^+π^-π^0$, using a circularly-polarized tagged-photon beam with energies ranging from the $ω$ threshold at 1.1 to 2.3 GeV. A partial-wave analysis within the Bonn-Gatchina framework found dominant contributions from the $3/2^+$ partial wave near threshold, which is identified with the sub-threshold $N(1720)\,3/2^+$ nucleon resonance. To describe the entire data set, which consisted of $ω$ differential cross sections and a large variety of polarization observables, further contributions from other nucleon resonances were found to be necessary. With respect to non-resonant mechanisms, $π$ exchange in the $t$-channel was found to remain small across the analyzed energy range, while pomeron $t$-channel exchange gradually grew from the reaction threshold to dominate all other contributions above $W \approx 2$ GeV.

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arXiv2017

Double $K_S^0$ Photoproduction off the Proton at CLAS

The $f_0$(1500) meson resonance is one of several contenders to have significant mixing with the lightest glueball. This resonance is well established from several previous experiments. Here we present the first photoproduction data for the $f_0$(1500) via decay into the $K_S^0 K_S^0$ channel using the CLAS detector. The reaction $γp$ -> $f_0 p$ -> $K_S^0 K_S^0 p$, where J = 0, 2, was measured with photon energies from 2.7 to 5.1 GeV. A clear peak is seen at 1500 MeV in the background subtracted invariant mass spectra of the two kaons. This is enhanced if the measured 4-momentum transfer to the proton target is restricted to be less than 1.0 GeV2. By comparing data with simulations, it can be concluded that the peak at 1500 MeV is produced primarily at low t, which is consistent with a t-channel production mechanism.

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arXiv2017

Differential Cross Section Measurements for $γn\toπ^-p$ Above the First Nucleon Resonance Region

The quasi-free $γd\toπ^{-}p(p)$ differential cross section has been measured with CLAS at photon beam energies $E_γ$ from 0.445 GeV to 2.510 GeV (corresponding to $W$ from 1.311 GeV to 2.366 GeV) for pion center-of-mass angles $\cosθ_π^{c.m.}$ from -0.72 to 0.92. A correction for final state interactions has been applied to this data to extract the $γn\toπ^-p$ differential cross sections. These cross sections are quoted in 8428 $(E_γ,\cosθ_π^{c.m.})$ bins, a factor of nearly three increase in the world statistics for this channel in this kinematic range. These new data help to constrain coupled-channel analysis fits used to disentangle the spectrum of $N^*$ resonances and extract their properties. Selected photon decay amplitudes $N^* \to γn$ at the resonance poles are determined for the first time and are reported here.

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arXiv2017

First Exclusive Measurement of Deeply Virtual Compton Scattering off $^4$He: Toward the 3D Tomography of Nuclei

We report on the first measurement of the beam-spin asymmetry in the exclusive process of coherent deeply virtual Compton scattering off a nucleus. The experiment used the 6 GeV electron beam from the CEBAF accelerator at Jefferson Lab incident on a pressurized $^4$He gaseous target placed in front of the CEBAF Large Acceptance Spectrometer (CLAS). The scattered electron was detected by CLAS and the photon by a dedicated electromagnetic calorimeter at forward angles. To ensure the exclusivity of the process, a specially designed radial time projection chamber was used to detect the recoiling $^4$He nuclei. We measured beam-spin asymmetries larger than those observed on the free proton in the same kinematic domain. From these, we were able to extract, in a model-independent way, the real and imaginary parts of the only $^4$He Compton form factor, $\cal H_A$. This first measurement of coherent deeply virtual Compton scattering on the $^4$He nucleus, with a fully exclusive final state via nuclear recoil tagging, leads the way toward 3D imaging of the partonic structure of nuclei.

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arXiv2017

Measurement of two-photon exchange effect by comparing elastic $e^\pm p$ cross sections

[Background] The electromagnetic form factors of the proton measured by unpolarized and polarized electron scattering experiments show a significant disagreement that grows with the squared four momentum transfer ($Q^{2}$). Calculations have shown that the two measurements can be largely reconciled by accounting for the contributions of two-photon exchange (TPE). TPE effects are not typically included in the standard set of radiative corrections since theoretical calculations of the TPE effects are highly model dependent, and, until recently, no direct evidence of significant TPE effects has been observed. [Purpose] We measured the ratio of positron-proton to electron-proton elastic-scattering cross sections in order to determine the TPE contribution to elastic electron-proton scattering and thereby resolve the proton electric form factor discrepancy. [Methods] We produced a mixed simultaneous electron-positron beam in Jefferson Lab's Hall B by passing the 5.6 GeV primary electron beam through a radiator to produce a bremsstrahlung photon beam and then passing the photon beam through a convertor to produce electron/positron pairs. The mixed electron-positron (lepton) beam with useful energies from approximately 0.85 to 3.5 GeV then struck a 30-cm long liquid hydrogen (LH$_2$) target located within the CEBAF Large Acceptance Spectrometer (CLAS). By detecting both the scattered leptons and the recoiling protons we identified and reconstructed elastic scattering events and determined the incident lepton energy. A detailed description of the experiment is presented.

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arXiv2017

Photon beam asymmetry $Σ$ in the reaction $\vecγ p \to p ω$ for $E_γ$ = 1.152 to 1.876 GeV

Photon beam asymmetry $Σ$ measurements for $ω$ photoproduction in the reaction $\vecγ p \to ωp$ are reported for photon energies from 1.152 to 1.876 GeV. Data were taken using a linearly-polarized tagged photon beam, a cryogenic hydrogen target, and the CLAS spectrometer in Hall B at Jefferson Lab. The measurements obtained markedly increase the size of the database for this observable, extend coverage to higher energies, and resolve discrepancies in previously published data. Comparisons of these new results with predictions from a chiral-quark-based model and from a dynamical coupled-channels model indicate the importance of interferences between $t$-channel meson exchange and $s$- and $u$-channel contributions, underscoring sensitivity to the nucleon resonances included in those descriptions. Comparisons with the Bonn-Gatchina partial-wave analysis indicate the $Σ$ data reported here help to fix the magnitudes of the interference terms between the leading amplitudes in that calculation (Pomeron exchange and the resonant portion of the $J^P=3/2^+$ partial wave), as well as the resonant portions of the smaller partial waves with $J^P$= $1/2^-$, $3/2^-$, and $5/2^+$.

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arXiv2017

The Beam-Target Helicity Asymmetry for $\vecγ \vec{n} \rightarrow π^- p$ in the {\bf{$N^*$} Resonance Region

We report the first beam-target double-polarization asymmetries in the $γ+ n(p) \rightarrow π^- + p(p)$ reaction spanning the nucleon resonance region from invariant mass $W$= $1500$ to $2300$ MeV. Circularly polarized photons and longitudinally polarized deuterons in $H\!D$ have been used with the CLAS detector at Jefferson Lab. The exclusive final state has been extracted using three very different analyses that show excellent agreement, and these have been used to deduce the {\it{E}} polarization observable for an effective neutron target. These results have been incorporated into new partial wave analyses, and have led to significant revisions for several $γnN^*$ resonance photo-couplings.

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arXiv2017

Exclusive $η$ electroproduction at $W>2$ GeV with CLAS and transversity generalized parton distributions

The cross section of the exclusive $η$ electroproduction reaction $ep\to e^\prime p^\prime η$ was measured at Jefferson Lab with a 5.75-GeV electron beam and the CLAS detector. Differential cross sections $d^4σ/dtdQ^2dx_Bdϕ_η$ and structure functions $σ_U = σ_T+εσ_L, σ_{TT}$ and $σ_{LT}$, as functions of $t$ were obtained over a wide range of $Q^2$ and $x_B$. The $η$ structure functions are compared with those previously measured for $π^0$ at the same kinematics. At low $t$, both $π^0$ and $η$ are described reasonably well by generalized parton distributions (GPDs) in which chiral-odd transversity GPDs are dominant. The $π^0$ and $η$ data, when taken together, can facilitate the flavor decomposition of the transversity GPDs.

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arXiv2017

Target and Double Spin Asymmetries of Deeply Virtual $π^0$ Production with a Longitudinally Polarized Proton Target and CLAS

The target and double spin asymmetries of the exclusive pseudoscalar channel $\vec e\vec p\to epπ^0$ were measured for the first time in the deep-inelastic regime using a longitudinally polarized 5.9 GeV electron beam and a longitudinally polarized proton target at Jefferson Lab with the CEBAF Large Acceptance Spectrometer (CLAS). The data were collected over a large kinematic phase space and divided into 110 four-dimensional bins of $Q^2$, $x_B$, $-t$ and $ϕ$. Large values of asymmetry moments clearly indicate a substantial contribution to the polarized structure functions from transverse virtual photon amplitudes. The interpretation of experimental data in terms of generalized parton distributions (GPDs) provides the first insight on the chiral-odd GPDs $\tilde{H}_T$ and $E_T$, and complement previous measurements of unpolarized structure functions sensitive to the GPDs $H_T$ and $\bar E_T$. These data provide necessary constraints for chiral-odd GPD parametrizations and will strongly influence existing theoretical handbag models.

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