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COMPASS Papers

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2020-09-06
15:46
Spin Density Matrix Elements in Exclusive $\omega$ Meson Muoproduction $^*$ / COMPASS Collaboration
We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive $\omega$ meson muoproduction on the proton at COMPASS using 160 GeV/$c$ polarised $ \mu ^{+}$ and $ \mu ^{-}$ beams impinging on a liquid hydrogen target. The measurement covers the range 5.0 GeV/$c^2$ $< W <$ 17.0 GeV/$c^2$, with the average kinematics $\langle Q^{2} \rangle=$ 2.1 (GeV/$c$)$^2$, $\langle W \rangle= 7.6$ GeV/$c^2$, and $\langle p^{2}_{\rm T} \rangle = 0.16$ (GeV/$c$)$^2$. [...]
arXiv:2009.03271; CERN-EP-2020-169.- Geneva : CERN, 2021-02-05 - 22 p. - Published in : Eur. Phys. J. C 81 (2021) 126 Article from SCOAP3: PDF; Draft (restricted): PDF; Fulltext: 2009.03271 - PDF; CERN-EP-2020-169 - PDF;

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2020-06-03
07:55
A Triangle Singularity as the Origin of the $a_1(1420)$ / Alexeev, M.G. (Turin U. ; INFN, Turin) ; Alexeev, G.D. (Dubna, JINR) ; Amoroso, A. (Turin U. ; INFN, Turin) ; Andrieux, V. (CERN ; Illinois U., Urbana) ; Anosov, V. (Dubna, JINR) ; Antoshkin, A. (Dubna, JINR) ; Augsten, K. (Dubna, JINR ; CTU, Prague) ; Augustyniak, W. (NCBJ, Warsaw) ; Azevedo, C.D.R. (Aveiro U.) ; Badełek, B. (Warsaw U.) et al.
The COMPASS experiment recently discovered a new isovector resonance-like signal with axial-vector quantum numbers, the $a_1(1420)$, decaying to $f_0(980)\pi$. With a mass too close to and a width smaller than the axial-vector ground state $a_1(1260)$, it was immediately interpreted as a new light exotic meson, similar to the $X$, $Y$, $Z$ states in the hidden-charm sector. [...]
arXiv:2006.05342; CERN-EP-2020-104.- Geneva : CERN, 2020 - 19 p. Draft (restricted): PDF; Fulltext: CERN-EP-2020-104 - PDF; 2006.05342 - PDF;

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2020-03-20
09:52
Antiproton over proton and K$^-$ over K$^+$ multiplicity ratios at high $z$ in DIS / COMPASS Collaboration
The $\bar{\rm p} $ over p multiplicity ratio is measured in deep-inelastic scattering for the first time using (anti-) protons carrying a large fraction of the virtual-photon energy, $z>0.5$. The data were obtained by the COMPASS Collaboration using a 160 GeV muon beam impinging on an isoscalar $^6$LiD target. [...]
arXiv:2003.11791; CERN-EP-2020-041.- Geneva : CERN, 2020-08-10 - 19 p. - Published in : Phys. Lett. B 807 (2020) 135600 Article from SCOAP3: PDF; Draft (restricted): PDF; Fulltext: CERN-EP-2020-041 - PDF; 2003.11791 - PDF; Fulltext from publisher: PDF;

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2019-12-18
08:40
Contribution of exclusive diffractive processes to the measured azimuthal asymmetries in SIDIS / COMPASS Collaboration
Hadron leptoproduction in Semi-Inclusive measurements of Deep-Inelastic Scattering (SIDIS) on unpolarised nucleons allows one to get information on the intrinsic transverse momentum of quarks in a nucleon and on the Boer-Mulders function through the measurement of azimuthal modulations in the cross section. These modulations were recently measured by the HERMES experiment at DESY on proton and deuteron targets, and by the COMPASS experiment using the CERN SPS muon beam and a $^6$LiD target. [...]
arXiv:1912.10322; CERN-EP-2019-286.- Geneva : CERN, 2020-07 - 13 p. - Published in : Nucl. Phys. B 956 (2020) 115039 Article from SCOAP3: PDF; Draft (restricted): PDF; Fulltext: 1912.10322 - PDF; CERN-EP-2019-286 - PDF;

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2018-09-02
17:21
Measurement of $P_T$-weighted Sivers asymmetries in leptoproduction of hadrons / COMPASS Collaboration
The transverse spin asymmetries measured in semi-inclusive leptoproduction of hadrons, when weighted with the hadron transverse momentum $P_T$, allow for the extraction of important transverse-momentum-dependent distribution functions. In particular, the weighted Sivers asymmetries provide direct information on the Sivers function, which is a leading-twist distribution that arises from a correlation between the transverse momentum of an unpolarised quark in a transversely polarised nucleon and the spin of the nucleon. [...]
arXiv:1809.02936; CERN-EP-2018-242.- Geneva : CERN, 2019-03 - 20 p. - Published in : Nucl. Phys. B 940 (2019) 34-53 Article from SCOAP3: PDF; Draft (restricted): PDF; Fulltext: CERN-EP-2018-242 - PDF; 1809.02936 - PDF;

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2018-02-14
11:43
Light isovector resonances in $\pi^- p \to \pi^-\pi^-\pi^+ p$ at 190 GeV/${\it c}$ / COMPASS Collaboration
We have performed the most comprehensive resonance-model fit of $\pi^-\pi^-\pi^+$ states using the results of our previously published partial-wave analysis (PWA) of a large data set of diffractive-dissociation events from the reaction $\pi^- + p \to \pi^-\pi^-\pi^+ + p_\text{recoil}$ with a 190 GeV/$c$ pion beam. The PWA results, which were obtained in 100 bins of three-pion mass, $0.5 < m_{3\pi} < 2.5$ GeV/$c^2$, and simultaneously in 11 bins of the reduced four-momentum transfer squared, $0.1 < t' < 1.0$ $($GeV$/c)^2$, are subjected to a resonance-model fit using Breit-Wigner amplitudes to simultaneously describe a subset of 14 selected waves using 11 isovector light-meson states with $J^{PC} = 0^{-+}$, $1^{++}$, $2^{++}$, $2^{-+}$, $4^{++}$, and spin-exotic $1^{-+}$ quantum numbers. [...]
arXiv:1802.05913; CERN-EP-2018-021.- Geneva : CERN, 2018-11-02 - 72 p. - Published in : Phys. Rev. D 98 (2018) 092003 Article from SCOAP3: PDF; Draft (restricted): PDF; Fulltext: CERN-EP-2018-021 - PDF; 1802.05913 - PDF;

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2018-02-07
15:23
Transverse extension of partons in the proton probed in the sea-quark range by measuring the DVCS cross section / COMPASS Collaboration
We report on the first measurement of exclusive single-photon muoproduction on the proton by COMPASS using 160 GeV/$c$ polarized $\mu^+$ and $\mu^-$ beams of the CERN SPS impinging on a liquid hydrogen target. We determine the dependence of the average of the measured $\mu^+$ and $\mu^-$ cross sections for deeply virtual Compton scattering on the squared four-momentum transfer $t$ from the initial to the final final proton. [...]
CERN-EP-2018-016; arXiv:1802.02739.- Geneva : CERN, 2019-06-10 - 7 p. - Published in : Phys. Lett. B 793 (2019) 188-194 Article from SCOAP3: PDF; Draft (restricted): PDF; Fulltext: 1802.02739 - PDF; CERN-EP-2018-016 - PDF; Fulltext from publisher: 1-s2.0-S0370269319302710-main - PDF; j.physletb.2019.04.038 - PDF;

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2018-01-30
07:15
K$^{-}$ over K$^{+}$ multiplicity ratio for kaons produced in DIS with a large fraction of the virtual-photon energy / COMPASS Collaboration
The K$^{-}$ over K$^{+}$ multiplicity ratio is measured in deep-inelastic scattering, for the first time for kaons carrying a large fraction $z$ of the virtual-photon energy. The data were obtained by the COMPASS collaboration using a 160 GeV muon beam and an isoscalar $^6$LiD target. [...]
CERN-EP-2018-012; arXiv:1802.00584.- Geneva : CERN, 2018-11-10 - 9 p. - Published in : Phys. Lett. B 786 (2018) 390-398 Article from SCOAP3: PDF; Draft (restricted): PDF; Fulltext: PDF; Preprint: PDF;

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2017-09-28
10:30
Longitudinal double-spin asymmetry $A_1^{\rm p}$ and spin-dependent structure function $g_1^{\rm p}$ of the proton at small values of $x$ and $Q^2$ / COMPASS Collaboration
We present a precise measurement of the proton longitudinal double-spin asymmetry $A_1^{\rm p}$ and the proton spin-dependent structure function $g_1^{\rm p}$ at photon virtualities $0.006~({\rm GeV}/c)^2<Q^2 < 1~({\rm GeV}/c)^2$ in the Bjorken $x$ range of $4 \times 10^{-5} < x < 4 \times 10^{-2}$. The results are based on data collected by the COMPASS Collaboration at CERN using muon beam energies of $160~{\rm GeV}$ and $200~{\rm GeV}$. [...]
CERN-EP-2017-263; arXiv:1710.01014.- Geneva : CERN, 2018-06-10 - 9 p. - Published in : Phys. Lett. B 781 (2018) 464-472 Article from SCOAP3: PDF; Draft (restricted): PDF; Fulltext: PDF; Preprint: PDF;

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2017-09-19
08:11
Transverse-momentum-dependent Multiplicities of Charged Hadrons in Muon-Deuteron Deep Inelastic Scattering / COMPASS Collaboration
A semi-inclusive measurement of charged hadron multiplicities in deep inelastic muon scattering off an isoscalar target was performed using data collected by the COMPASS Collaboration at CERN. The following kinematic domain is covered by the data: photon virtuality $Q^{2}>1$ (GeV/$c$)$^2$, invariant mass of the hadronic system $W > 5$ GeV/$c^2$, Bjorken scaling variable in the range $0.003 < x < 0.4$, fraction of the virtual photon energy carried by the hadron in the range $0.2 < z < 0.8$, square of the hadron transverse momentum with respect to the virtual photon direction in the range 0.02 (GeV/$c)^2 < P_{\rm{hT}}^{2} < 3$ (GeV/$c$)$^2$. [...]
CERN-EP-2017-253; arXiv:1709.07374.- Geneva : CERN, 2018-02-08 - 23 p. - Published in : Phys. Rev. D 97 (2018) 032006 Article from SCOAP3: PDF; Draft (restricted): PDF; Fulltext: PDF; Preprint: PDF;

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