The present contribution reviews the principle of RF separation and explains its dependence on different parameters of beam optics and hardware. The first examination of potential showstoppers for ...the RF-separated beam implementation for Phase-2 of the AMBER experiment in the M2 beam line of the North Area is presented. Different beam optics settings have been examined, providing either focused or parallel beams inside the RF cavities. The separation and transmission capability of the different optics settings for realistic characteristics of RF cavities are discussed and the preliminary results of the potential purity and intensity of the RF-separated beam are presented. These show that a trade-off between the overall beam intensity and the share of the required particle type in the overall beam needs to be established. No showstoppers have been identified for achieving the beam parameters required for AMBER’s kaonic Primakoff reactions, kaon spectroscopy, prompt-photon production and kaon charge-radius programs. However, the high beam intensity requirements of the AMBER Drell–Yan programme cannot be satisfied with an RF-separated beam.
The transversely polarized target (PT) of the COMPASS (NA58) collaboration at CERN has been used for Drell–Yan measurements in 2015 and 2018. The transverse spin structure of the proton has been ...studied using a negative pion beam and a solid ammonia target. Employing the dynamic nuclear polarization (DNP) method, proton polarization values of more than 80% have been routinely achieved after one day, at a homogeneous magnetic field of 2.5 T and using a 3He/4He dilution refrigerator. During the data-taking the target operates in a transversely oriented magnetic dipole field at 0.6 T. This so-called frozen spin operation mode without the DNP pumping process leads to a slow depolarization of the target material, which is further accelerated by the heat input of the pion beam, produced secondary particles and radiation damage effects to the target material. Ammonia has the highest resistance against radiation-induced depolarization among known solid target materials. The proton polarization has been measured by the nuclear magnetic resonance (NMR). Relaxation times of about 1100 h have been observed for the proton polarization resulting in an average polarization between 68% and 76% during about two weeks long data-taking periods. To achieve a systematic uncertainty of the polarization ΔP/P as low as 3.2% and a statistical one of less than 1.8% two large target cells with appropriate positioning of the NMR-coils have been built.
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.
In this paper, we report the existence of a double helix folding a part of one of two cofocal parabolae. We observe the winding/unwinding of the double helix as temperature is changed and this ...mechanism is accompanied here by a change of the latus rectum p of the corresponding parabola. We propose a model of the layers inside and outside the double helix consistent with experimental observations.
We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive Formula omitted meson muoproduction on the proton at COMPASS using 160 GeV/c polarised Formula omitted and Formula ...omitted beams impinging on a liquid hydrogen target. The measurement covers the range 5.0 GeV/ Formula omitted Formula omitted 17.0 GeV/ Formula omitted, with the average kinematics Formula omitted 2.1 (GeV/c) Formula omitted, Formula omitted GeV/ Formula omitted, and Formula omitted (GeV/c) Formula omitted. Here, Formula omitted denotes the virtuality of the exchanged photon, W the mass of the final hadronic system and Formula omitted the transverse momentum of the Formula omitted meson with respect to the virtual-photon direction. The measured non-zero SDMEs for the transitions of transversely polarised virtual photons to longitudinally polarised vector mesons ( Formula omitted) indicate a violation of s-channel helicity conservation. Additionally, we observe a sizeable contribution of unnatural-parity-exchange (UPE) transitions that decreases with increasing W. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow to evaluate in a model-dependent way the contribution of UPE transitions and assess the role of parton helicity-flip GPDs in exclusive Formula omitted production.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive
ρ
0
meson muoproduction at COMPASS using 160 GeV/
c
polarised
μ
+
and
μ
-
beams impinging on a liquid hydrogen ...target. The measurement covers the kinematic range 5.0 GeV/
c
2
<
W
<
17.0 GeV/
c
2
, 1.0 (GeV/
c
)
2
<
Q
2
<
10.0 (GeV/
c
)
2
and 0.01 (GeV/
c
)
2
<
p
T
2
<
0.5 (GeV/
c
)
2
. Here,
W
denotes the mass of the final hadronic system,
Q
2
the virtuality of the exchanged photon, and
p
T
the transverse momentum of the
ρ
0
meson with respect to the virtual-photon direction. The measured non-zero SDMEs for the transitions of transversely polarised virtual photons to longitudinally polarised vector mesons (
γ
T
∗
→
V
L
) indicate a violation of
s
-channel helicity conservation. Additionally, we observe a dominant contribution of natural-parity-exchange transitions and a very small contribution of unnatural-parity-exchange transitions, which is compatible with zero within experimental uncertainties. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow one to evaluate in a model-dependent way the role of parton helicity-flip GPDs in exclusive
ρ
0
production.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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 -> pi(-)pi(-)pi(+) +p(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. The model contains the well-known resonances pi(1800), a(1)(1260), a(2)(1320), pi(2)(1670), pi(2)(1880), and a(4) (2040). In addition, it includes the disputed pi(1)(1600), the excited states a(1)(1640), a2(1700), and pi(2) (2005), as well as the resonancelike a(1)(1420). We measure the resonance parameters mass and width of these objects by combining the information from the PWA results obtained in the 11 t' bins. We extract the relative branching fractions of the rho(770)pi and f(2)(1270)pi decays of a(2)(1320) and a(4)(2040), where the former one is measured for the first time. In a novel approach, we extract the t' dependence of the intensity of the resonances and of their phases. The t' dependence of the intensities of most resonances differs distinctly from the t' dependence of the nonresonant components. For the first time, we determine the t' dependence of the phases of the production amplitudes and confirm that the production mechanism of the Pomeron exchange is common to all resonances. We have performed extensive systematic studies on the model dependence and correlations of the measured physical parameters.
Exclusive production of ηπ− and ηπ− has been studied with a 191 GeV/c π− beam impingingon a hydrogen target at COMPASS (CERN). Partial-wave analyses reveal different odd/even angularmomentum (L) ...characteristics in the inspected invariant mass range up to 3 GeV/c2. A striking similaritybetween the two systems is observed for the L = 2, 4, 6 intensities (scaled by kinematical factors) and therelative phases. The known resonances a2(1320) and a4(2040) are in line with this similarity. In contrast,a strong enhancement of ηπ− over ηπ− is found for the L = 1, 3, 5 waves, which carry non-q¯q quantumnumbers. The L = 1 intensity peaks at 1.7 GeV/c2 in ηπ− and at 1.4 GeV/c2 in ηπ−, the correspondingphase motions with respect to L =2 are different.
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