Azimuthal asymmetries in semi-inclusive production of positive (
h
+
) and negative hadrons (
h
−
) have been measured by scattering 160 GeV muons off longitudinally polarised deuterons at CERN. The ...asymmetries were decomposed in several terms according to their expected modulation in the azimuthal angle
φ
of the outgoing hadron. Each term receives contributions from one or several spin and transverse-momentum-dependent parton distribution and fragmentation functions. The amplitudes of all
φ
-modulation terms of the hadron asymmetries integrated over the kinematic variables are found to be consistent with zero within statistical errors, while the constant terms are nonzero and equal for
h
+
and
h
−
within the statistical errors. The dependencies of the
φ
-modulated terms versus the Bjorken momentum fraction
x
, the hadron fractional momentum
z
, and the hadron transverse momentum
were studied. The
x
dependence of the constant terms for both positive and negative hadrons is in agreement with the longitudinal double-spin hadron asymmetries, measured in semi-inclusive deep-inelastic scattering. The
x
dependence of the sin
φ
-modulation term is less pronounced than that in the corresponding HERMES data. All other dependencies of the
φ
-modulation amplitudes are consistent with zero within the statistical errors.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
New large-size Micromegas detectors are being developed for the future physics program of the COMPASS experiment at CERN. These detectors will have a pixelised readout in their center to detect ...particles in the beam region, where the particle flux can reach several MHz/cm super(2) in nominal conditions, and will have to handle high intensity hadron beams (up to a few 10 super(7) hadrons/s) with a discharge rate lower than 0.01 to 0.001 discharge/s. Several prototypes with two different discharge rate reduction technologies (preamplification stage with a GEM foil and resistive read-out with buried resistors) have been studied in the COMPASS beam since 2010. Four of them have been included in the spectrometer since 2012, and have been used for the track reconstruction. Their performance (detection efficiency, space and time resolutions, and discharge rates) for different beam intensities and magnetic fields environments are presented. These detectors play an important role in the track reconstruction at very small angle; their impact is presented, with a particular emphasis on the effect of the background reduction due to an improved cluster selection.
Large samples of
Λ
,
Σ
(1385) and
Ξ
(1321) hyperons produced in the deep-inelastic muon scattering off a
6
LiD target were collected with the COMPASS experimental setup at CERN. The relative yields ...of
Σ
(1385)
+
,
Σ
(1385)
−
,
,
,
Ξ
(1321)
−
, and
hyperons decaying into
were measured. The ratios of heavy-hyperon to
Λ
and heavy-antihyperon to
were found to be in the range 3.8 % to 5.6 % with a relative uncertainty of about 10 %. They were used to tune the parameters relevant for strange particle production of the LEPTO Monte Carlo generator.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The total cross sections for the three γp → Nππ reactions have been measured for photon energies from 400 to 800 MeV. The
γp → p
π
0
π
0 and
γp → n
π
+
π
0 cross sections have never been measured ...before while the
γp → p
π
+
π
− results are much improved compared to earlier data. These measurements were performed with the large acceptance hadronic detector DAPHNE, at the tagged photon beam facility of the MAMI microtron in Mainz.
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IJS, IMTLJ, KILJ, KISLJ, NUK, SBCE, SBJE, UL, UM, UPCLJ, UPUK
New Micromegas (Micro-mesh gaseous detectors) are being developed in view of the future physics projects planned by the COMPASS collaboration at CERN. Several major upgrades compared to present ...detectors are being studied: detectors standing five times higher luminosity with hadron beams, detection of beam particles (flux up to a few hundred of kHz/mm super(2), 10 times larger than for the present Micromegas detectors) with pixelized read-out in the central part, light and integrated electronics, and improved robustness. Two solutions for a reduction of the impact of discharges have been studied, with Micromegas detectors using resistive layers and using an additional GEM foil. The performance of such detectors has been measured during beam test periods. A large size prototype with nominal active area and pixelized read-out has been produced and installed in the COMPASS spectrometer in 2010. In 2011 prototypes featuring an additional GEM foil, as well as a resistive prototype, were tested in similar conditions and preliminary results from those detectors are very promising. We present here the project and report on its status, in particular the performance of large size prototypes with an additional GEM foil.
The COMPASS Collaboration experiment recently discovered a new isovector resonancelike signal with axial-vector quantum numbers, the a1( 1420 ) , decaying to f0( 980 ) π . With a mass too close to ...and a width smaller than the axial-vector ground state a1( 1260 ) , it was immediately interpreted as a new light exotic meson, similar to the X , Y , Z states in the hidden-charm sector. We show that a resonancelike signal fully matching the experimental data is produced by the decay of the a1( 1260 ) resonance into K∗( → K π ) ¯ K and subsequent rescattering through a triangle singularity into the coupled f0( 980 ) π channel. The amplitude for this process is calculated using a new approach based on dispersion relations. The triangle-singularity model is fitted to the partial-wave data of the COMPASS experiment. Despite having fewer parameters, this fit shows a slightly better quality than the one using a resonance hypothesis and thus eliminates the need for an additional resonance in order to describe the data. We thereby demonstrate for the first time in the light-meson sector that a resonancelike structure in the experimental data can be described by rescattering through a triangle singularity, providing evidence for a genuine three-body effect.
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