arXiv subjects

A. Amoroso

Publications and source records attributed to A. Amoroso.

At least 847 records · Page 47Linked to original sources

arXiv2015

Observation of $e^+e^-\to π^0π^0 h_c$ and a neutral charmoniumlike structure $Z_c(4020)^0$

Using data collected with the BESIII detector operating at the Beijing Electron Positron Collider at center-of-mass energies of $\sqrt{s}=4.23$, 4.26, and 4.36~GeV, we observe $\EE\to \pphc$ for the first time. The Born cross sections are measured and found to be about half of those of $\EE\to π^+π^-h_c$ within less than 2$σ$. In the $π^0h_c$ mass spectrum, a structure at 4.02~GeV/$c^2$ is found. It is most likely to be the neutral isospin partner of the $\zcp^{\pm}$ observed in the process of $\EE\to π^+π^-h_c$ is found. A fit to the $π^0 h_c$ invariant mass spectrum, with the width of the $\zcpn$ fixed to that of its charged isospin partner and possible interferences with non-$\zcpn$ amplitudes neglected, gives a mass of ($4023.9\pm 2.2 \pm 3.8$)~MeV/$c^2$ for the $\zcpn$, where the first error is statistical and the second systematic.

hep-ex↗

arXiv2015

Search for the $Y(4140)$ via $e^{+} e^{-} \to γϕJ/ψ$ at $\sqrt{s}=$ 4.23, 4.26 and 4.36 GeV

Using data samples collected at center-of-mass energies $\sqrt{s} = 4.23$, 4.26, and 4.36 GeV with the BESIII detector operating at the BEPCII storage ring, we search for the production of the charmoniumlike state $Y(4140)$ through a radiative transition followed by its decay to $ϕJ/ψ$. No significant signal is observed and upper limits on $σ[e^{+} e^{-} \rightarrow γY(4140)] \cdot \mathcal{B}(Y(4140)\rightarrow ϕJ/ψ)$ at the $90\%$ confidence level are estimated as 0.35, 0.28, and 0.33 pb at $\sqrt{s} = 4.23$, 4.26, and 4.36 GeV, respectively.

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arXiv2015

Precision measurement of the $D^{*0}$ decay branching fractions

Using 482 pb$^{-1}$ of data taken at $\sqrt{s}=4.009$ GeV, we measure the branching fractions of the decays of $D^{*0}$ into $D^0π^0$ and $D^0γ$ to be $\BR(D^{*0} \to D^0π^0)=(65.5\pm 0.8\pm 0.5)%$ and $\BR(D^{*0} \to D^0γ)=(34.5\pm 0.8\pm 0.5)%$ respectively, by assuming that the $D^{*0}$ decays only into these two modes. The ratio of the two branching fractions is $\BR(D^{*0} \to D^0π^0)/\BR(D^{*0} \to D^0γ) =1.90\pm 0.07\pm 0.05$, which is independent of the assumption made above. The first uncertainties are statistical and the second ones systematic. The precision is improved by a factor of three compared to the present world average values.

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arXiv2015

Study of $e^+e^-\toωχ_{cJ}$ at center-of-mass energies from 4.21 to 4.42 GeV

Based on data samples collected with the BESIII detector at the BEPCII collider at 9 center-of-mass energies from 4.21 to 4.42 GeV, we search for the production of $e^+e^-\to ωχ_{cJ}$ ($J$=0, 1, 2). The process $e^+e^-\to ωχ_{c0}$ is observed for the first time, and the Born cross sections at $\sqrt{s}$=4.23 and 4.26 GeV are measured to be $(55.4\pm 6.0\pm 5.9)$ and $(23.7\pm 5.3\pm 3.5)$ pb, respectively, where the first uncertainties are statistical and the second are systematic. The $ωχ_{c0}$ signals at the other 7 energies and $e^+e^-\to ωχ_{c1}$ and $ωχ_{c2}$ signals are not significant, and the upper limits on the cross sections are determined. By examining the $ωχ_{c0}$ cross section as a function of center-of-mass energy, we find that it is inconsistent with the line shape of the $Y(4260)$ observed in $e^+ e^-\toπ^+π^-J/ψ$. Assuming the $ωχ_{c0}$ signals come from a single resonance, we extract mass and width of the resonance to be $(4230\pm8\pm6)$ MeV/$c^2$ and $(38\pm12\pm2)$ MeV, respectively, and the statistical significance is more than $9σ$.

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arXiv2015

Measurement of the charged-pion polarisability

The COMPASS collaboration at CERN has investigated pion Compton scattering, $π^-γ\rightarrow π^-γ$, at centre-of-mass energy below 3.5 pion masses. The process is embedded in the reaction $π^-\mathrm{Ni}\rightarrowπ^-γ\;\mathrm{Ni}$, which is initiated by 190\,GeV pions impinging on a nickel target. The exchange of quasi-real photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, $Q^2<0.0015$\,(GeV/$c$)$^2$. From a sample of 63\,000 events the pion electric polarisability is determined to be $α_π =\ (\,2.0\ \pm\ 0.6_{\mbox{\scriptsize stat}}\ \pm\ 0.7_{\mbox{\scriptsize syst}}\,) \times 10^{-4}\,\mbox{fm}^3$ under the assumption $α_π=-β_π$, which relates the electric and magnetic dipole polarisabilities. It is the most precise measurement of this fundamental low-energy parameter of strong interaction, that has been addressed since long by various methods with conflicting outcomes. While this result is in tension with previous dedicated measurements, it is found in agreement with the expectation from chiral perturbation theory. An additional measurement replacing pions by muons, for which the cross-section behavior is unambigiously known, was performed for an independent estimate of the systematic uncertainty.

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arXiv2015

Search for the weak decays $J/ψ\to D^{(*)}_{s}eν_{e}$

Using a sample of $2.25\times 10^8$ $J/ψ$ events collected with the BESIII detector at the BEPCII collider, we search for the $J/ψ$ semi-leptonic weak decay $J/ψ\to D^{-}_{s} e^{+}ν_{e}+c.c.$ with a much higher sensitivity than previous searches. We also perform the first search for $J/ψ\to D^{*-}_{s} e^{+}ν_{e}+c.c.$ No significant excess of a signal above background is observed in either channel. At the $90\%$ confidence level, the upper limits are determined to be $\mathcal{B}(J/ψ\to D^{-}_{s}e^{+}ν_{e}+c.c.)<1.3\times10^{-6}$ and $\mathcal{B}(J/ψ\to {D^{*}_{s}}^{-}e^{+}ν_{e}+c.c.)<1.8\times10^{-6}$, respectively. Both are consistent with Standard Model predictions.

hep-ex↗

arXiv2014

Study of J/psi->eta phi pi+pi- at BESIII

Based on a sample of $2.25\times 10^{8} J/ψ$ events taken with the BESIII detector at the BEPCII collider, we present the results of a study of the decay $J/ψ\to ηϕπ^{+}π^{-}$. The $Y(2175)$ resonance is observed in the invariant mass spectrum of $ϕf_{0}(980)$ with a statistical significance of greater than $10σ$. The corresponding mass and width are determined to be $M=2200\pm 6 \mathrm{(stat.)} \pm 5\mathrm{(syst.)} \mathrm{MeV}/c^{2}$ and $Γ=104\pm 15\mathrm{(stat.)}\pm 15\mathrm{(syst.)}$ MeV, respectively, and the product branching fraction is measured to be $\mathcal{B}(J/ψ\toηY(2175)$, $Y(2175)\to ϕf_{0}(980)$, $f_{0}(980)\to π^{+}π^{-})= (1.20\pm 0.14\mathrm{(stat.)}\pm 0.37 \mathrm{(syst.)})\times 10^{-4}$. The results are consistent within errors with those of previous experiments. We also measure the branching fraction of $J/ψ\to ϕf_1(1285)$ with $f_1(1285)\to ηπ^{+}π^{-}$ and set upper limits on the branching fractions for $J/ψ\to ϕη(1405)$/$ϕX(1835)$/$ϕX(1870)$ with $η(1405)$/$X(1835)$/$X(1870)\to ηπ^{+}π^{-}$ at the 90\% confidence level.

hep-ex↗

arXiv2014

Evidence for $e^+e^-\toγχ_{c1, 2}$ at center-of-mass energies from 4.009 to 4.360 GeV

Using data samples collected at center-of-mass energies of $\sqrt{s}$ = 4.009, 4.230, 4.260, and 4.360 GeV with the BESIII detector operating at the BEPCII collider, we perform a search for the process $e^+e^-\toγχ_{cJ}$ $(J = 0, 1, 2)$ and find evidence for $e^+e^-\toγχ_{c1}$ and $e^+e^-\toγχ_{c2}$ with statistical significances of 3.0$σ$ and 3.4$σ$, respectively. The Born cross sections $σ^{B}(e^+e^-\toγχ_{cJ})$, as well as their upper limits at the 90% confidence level are determined at each center-of-mass energy.

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arXiv2014

Search for C-parity violation in $J/ ψ\to γγ$ and $ γϕ$

Using $1.06\times10^8$ $ψ(3686)$ events recorded in $e^{+}e^{-}$ collisions at $\sqrt{s}=$ 3.686 GeV with the BESIII at the BEPCII collider, we present searches for C-parity violation in $J/ψ\to γγ$ and $ γϕ$ decays via $ψ(3686) \to J/ψπ^+π^-$. No significant signals are observed in either channel. Upper limits on the branching fractions are set to be $\mathcal{B}(J/ψ\to γγ) < 2.7 \times 10^{-7}$ and $\mathcal{B}(J/ψ\to γϕ) < 1.4 \times 10^{-6}$ at the 90\% confidence level. The former is one order of magnitude more stringent than the previous upper limit, and the latter represents the first limit on this decay channel.

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arXiv2014

Updated Measurement of the Single Top Quark Production Cross Section and $V{tb}$ in the Missing Transverse Energy Plus Jets Topology in $p\bar{p}$ Collisions at $\sqrt{s} = 1.96$ TeV

An updated measurement of the single top quark production cross section is presented using the full data set collected by the Collider Detector at Fermilab (CDF) and corresponding to 9.5 fb${}^{-1}$ of integrated luminosity from proton-antiproton collisions at 1.96 TeV center-of-mass energy. The events selected contain an imbalance in the total transverse energy, jets identified as originating from $b$ quarks, and no identified leptons. The sum of the $s$- and $t$-channel single top quark cross sections is measured to be $3.53_{-1.16}^{+1.25}$ pb and a lower limit on $V_{tb}$ of 0.63 is obtained at the 95% credibility level. These measurements are combined with previously reported CDF results obtained from events with an imbalance in total transverse energy, jets identified as originating from $b$ quarks, and exactly one identified lepton. The combined cross section is measured to be $3.02_{-0.48}^{+0.49}$ pb and a lower limit on $V{tb}$ of 0.84 is obtained at the 95% credibility level.

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arXiv2014

The COMPASS Setup for Physics with Hadron Beams

The main characteristics of the COMPASS experimental setup for physics with hadron beams are described. This setup was designed to perform exclusive measurements of processes with several charged and/or neutral particles in the final state. Making use of a large part of the apparatus that was previously built for spin structure studies with a muon beam, it also features a new target system as well as new or upgraded detectors. The hadron setup is able to operate at the high incident hadron flux available at CERN. It is characterised by large angular and momentum coverages, large and nearly flat acceptances, and good two and three-particle mass resolutions. In 2008 and 2009 it was successfully used with positive and negative hadron beams and with liquid hydrogen and solid nuclear targets. This article describes the new and upgraded detectors and auxiliary equipment, outlines the reconstruction procedures used, and summarises the general performance of the setup.

physics.ins-det↗

arXiv2014

Odd and Even Partial Waves of $ηπ^-$ and $η'π^-$ in $π^-p\toη^{(\prime)}π^-p$ at $191\,\textrm{GeV}/c$

Exclusive production of $ηπ^-$ and $η'π^-$ has been studied with a $191\,\textrm{GeV}/c$ $π^-$ beam impinging on a hydrogen target at COMPASS (CERN). Partial-wave analyses reveal different odd/even angular momentum ($L$) characteristics in the inspected invariant mass range up to $3\,\textrm{GeV}/c^2$. A striking similarity between the two systems is observed for the $L=2,4,6$ intensities (scaled by kinematical factors) and the relative phases. The known resonances $a_2(1320)$ and $a_4(2040)$ are in line with this similarity. In contrast, a strong enhancement of $η'π^-$ over $ηπ^-$ is found for the $L=1,3,5$ waves, which carry non-$q\bar q$ quantum numbers. The $L=1$ intensity peaks at $1.7\,\textrm{GeV}/c^2$ in $η'π^-$ and at $1.4\,\textrm{GeV}/c^2$ in $ηπ^-$, the corresponding phase motions with respect to $L=2$ are different.

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arXiv2014

Collins and Sivers asymmetries in muonproduction of pions and kaons off transversely polarised proton

Measurements of the Collins and Sivers asymmetries for charged pions and charged and neutral kaons produced in semi-inclusive deep-inelastic scattering of high energy muons off transversely polarised protons are presented. The results were obtained using all the available COMPASS proton data, which were taken in the years 2007 and 2010. The Collins asymmetries exhibit in the valence region a non-zero signal for pions and there are hints of non-zero signal also for kaons. The Sivers asymmetries are found to be positive for positive pions and kaons and compatible with zero otherwise.

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arXiv2014

Search for exclusive photoproduction of Z$_c^{\pm}$(3900) at COMPASS

A search for the exclusive production of the $Z_c^{\pm}(3900)$ hadron by virtual photons has been performed in the channel $Z_c^{\pm}(3900)\rightarrow J/ψπ^{\pm}$. The data cover the range from 7 GeV to 19 GeV in the centre-of-mass energy of the photon-nucleon system. The full set of the COMPASS data set collected with a muon beam between 2002 and 2011 has been used. An upper limit for the ratio $BR(Z_c^{\pm}(3900)\rightarrow J/ψπ^{\pm} )\times σ_{ γ~N \rightarrow Z_c^{\pm}(3900)~ N} /σ_{γ~N \rightarrow J/ψ~ N}$ of $3.7\times10^{-3}$ has been established at the confidence level of 90%.

hep-ex↗

arXiv2014

Spin alignment and violation of the OZI rule in exclusive $ω$ and $ϕ$ production in pp collisions

Exclusive production of the isoscalar vector mesons $ω$ and $ϕ$ is measured with a 190 GeV$/c$ proton beam impinging on a liquid hydrogen target. Cross section ratios are determined in three intervals of the Feynman variable $x_{F}$ of the fast proton. A significant violation of the OZI rule is found, confirming earlier findings. Its kinematic dependence on $x_{F}$ and on the invariant mass $M_{p\mathrm{V}}$ of the system formed by fast proton $p_\mathrm{fast}$ and vector meson $V$ is discussed in terms of diffractive production of $p_\mathrm{fast}V$ resonances in competition with central production. The measurement of the spin density matrix element $ρ_{00}$ of the vector mesons in different selected reference frames provides another handle to distinguish the contributions of these two major reaction types. Again, dependences of the alignment on $x_{F}$ and on $M_{p\mathrm{V}}$ are found. Most of the observations can be traced back to the existence of several excited baryon states contributing to $ω$ production which are absent in the case of the $ϕ$ meson. Removing the low-mass $M_{p\mathrm{V}}$ resonant region, the OZI rule is found to be violated by a factor of eight, independently of $x_\mathrm{F}$.

hep-ex↗

arXiv2014

Measurement of radiative widths of $a_2(1320)$ and $π_2(1670)$

The COMPASS Collaboration at CERN has investigated the reaction $π^- γ\rightarrow π^-π^-π^+$ embedded in the Primakoff reaction of $190~\textrm{GeV}$ pions scattering in the Coulomb field of a lead target, $π^- \text{Pb} \rightarrow π^-π^-π^+ \text{Pb}$. Exchange of quasi-real photons is selected by isolating the sharp Coulomb peak observed at momentum transfer below $0.001~(\text{GeV}/c)^2$. Using a partial-wave analysis the amplitudes and relative phases of the $a_2(1320)$ and $π_2(1670)$ mesons have been extracted, and the Coulomb and the diffractive contributions have been disentangled. Measuring absolute production cross sections we have determined the radiative width of the $a_2(1320)$ to be $Γ_0(a_2(1320) \rightarrow πγ) = (358 \pm 6_{\textrm{stat}} \pm 42_{\textrm{syst}})~\textrm{keV}$. As the first measurement, $Γ_0(π_2(1670) \rightarrow πγ) = (181 \pm 11_{\textrm{stat}} \pm 27_{\textrm{syst}})~\textrm{keV} \cdot (\textrm{BR}^{\textrm{PDG}}_{f_2 π}/\textrm{BR}_{f_2 π})$ is obtained for the radiative width of the $π_2(1670)$, where in this analysis the branching ratio $\textrm{BR}^{\textrm{PDG}}_{f_2 π}=0.56$ has been used. We compare these values to previous measurements and theoretical predictions.

hep-ex↗

arXiv2014

A high-statistics measurement of transverse spin effects in dihadron production from muon-proton semi-inclusive deep-inelastic scattering

A measurement of the azimuthal asymmetry in dihadron production in deep-inelastic scattering of muons on transversely polarised proton (NH$_{3}$) targets are presented. They provide independent access to the transversity distribution functions through the measurement of the Collins asymmetry in single hadron production. The data were taken in the year $2010$ with the COMPASS spectrometer using a $160\,\mbox{GeV}/c$ muon beam of the CERN SPS, increasing by a factor of about three the available statistics of the previously published data taken in the year $2007$. The measured sizeable asymmetry is in good agreement with the published data. An approximate equality of the Collins asymmetry and the dihadron asymmetry is observed, suggesting a common physical mechanism in the underlying fragmentation.

hep-ex↗

arXiv2014

Measurement of azimuthal hadron asymmetries in semi-inclusive deep inelastic scattering off unpolarised nucleons

Spin-averaged asymmetries in the azimuthal distributions of positive and negative hadrons produced in deep inelastic scattering were measured using the CERN SPS muon beam at $160$ GeV/c and a $^6$LiD target. The amplitudes of the three azimuthal modulations $\cosϕ_h$, $\cos2ϕ_h$ and $\sinϕ_h$ were obtained binning the data separately in each of the relevant kinematic variables $x$, $z$ or $p_T^{\,h}$ and binning in a three-dimensional grid of these three variables. The amplitudes of the $\cos ϕ_h$ and $\cos 2ϕ_h$ modulations show strong kinematic dependencies both for positive and negative hadrons.

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