Browsing by Author "Nunes, Ana Sofia"
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- First results on $A_1^p$ and $g_1^p$ at low x and low $Q^2$ from COMPASSPublication . Nunes, Ana SofiaThe COMPASS experiment at CERN has collected a large sample of about 700 million events of quasi-real photoproduction in polarised m + scattering using a beam momentum of 160 GeV/ c in 2007 and 200 GeV/ c in 2011. The events have a Bjorken scaling variable in the range 4 10...
- $K^-/K^+$ multiplicity ratio for kaons produced in DIS with a large fraction of the virtual photon energyPublication . Nunes, Ana SofiaFor the first time, the $K^-/K^+$ multiplicity ratio is measured in deep-inelastic scattering 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. The regime of deep-inelastic scattering is ensured by requiring $Q^2>1$ (GeV/$c)^2$ for the photon virtuality and $W>5$ GeV/$c^2$ for the invariant mass of the produced hadronic system. The Bjorken scaling variable range is $0.010.75$. For very large values of $z$, {\it i.e.} $z>0.8$, the results contradict expectations obtained using the formalism of (next-to-)leading order perturbative quantum chromodynamics. Our studies suggest that, within this formalism, an additional correction may be required to take into account the phase space available for hadronisation.
- Odd and even partial waves of $ηπ^−$ and $η′π^−$ in $π^−p → η^{(′)}π^−p$ at 191 GeV/cPublication . COMPASS collaboration (207 authors); Bordalo, P.; Franco, C.; Nunes, Ana Sofia; Quaresma, M.; Quintans, C.; Ramos, S.; Silva, L.; Stolarski, M.Exclusive production of ηπ− and η′π− has been studied with a 191 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 GeV/c2 . 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 a2(1320) and a4(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- qq¯ quantum numbers. The L=1 intensity peaks at 1.7 GeV/c2 in η′π− and at 1.4 GeV/c2 in ηπ− , the corresponding phase motions with respect to L=2 are different.
- Search for exclusive photoproduction of Z$_c^{\pm}$(3900) at COMPASSPublication . COMPASS collaboration (209 authors); Bordalo, P; Franco, C; Nunes, Ana Sofia; Quaresma, M; Quintans, C; Ramos, S; Silva, L; Stolarski, MA search for the exclusive production of the Zc±(3900) hadron by virtual photons has been performed in the channel Zc±(3900)→J/ψπ± . 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(Zc±(3900)→J/ψπ±)×σγN→Zc±(3900)N/σγN→J/ψN of 3.7×10−3 has been established at the confidence level of 90%.
- The COMPASS Setup for Physics with Hadron BeamsPublication . COMPASS collaboration (232 authors); Bordalo, P.; Franco, C.; Nunes, Ana Sofia; Pires, C.; Quaresma, M.; Quintans, C.; Ramos, S.; Silva, L.; Stolarski, M.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 paper describes the new and upgraded detectors and auxiliary equipment, outlines the reconstruction procedures used, and summarises the general performance of the setup.