CERN Accelerating science

Production mechanisms employed in COMPASS for (left) diffractive dissociation, (middle) central production, (right) photo-production by quasi-real photons $\gamma$, with $\pi$ denoting the beam particle (can be also $p$, $K$), and $N$ the target nucleon or nucleus.

Production mechanisms employed in COMPASS for (left) diffractive dissociation, (middle) central production, (right) photo-production by quasi-real photons $\gamma$, with $\pi$ denoting the beam particle (can be also $p$, $K$), and $N$ the target nucleon or nucleus.

Production mechanisms employed in COMPASS for (left) diffractive dissociation, (middle) central production, (right) photo-production by quasi-real photons $\gamma$, with $\pi$ denoting the beam particle (can be also $p$, $K$), and $N$ the target nucleon or nucleus.

Three-dimensional view of the COMPASS setup for measurements with hadron beams. The beam comes from the left side. The upstream part of the setup (beam line) is not shown here. The different colours indicate different detector types.

Top view of the COMPASS setup for data taking with hadron beams. The labels indicate the various detectors, as referenced throughout this paper. The vertical scale is only indicative of the relative detector sizes. The colour code follows that of Fig.~\ref{fig:layout.3d}.

Side view of the target region with the liquid hydrogen target system.

The CERN M2 beam line.

The basic principle of a CEDAR counter. Two particles with the same momentum but with different masses (here red and green lines) radiate Cherenkov photons at different angles, resulting in rings with different radii. A diaphragm selects the rings from the required particle type.

A cut through one of the CEDAR detectors.

Pressure scan with CEDAR1 for a positive hadron beam with at least 4, 6 or 8 PMTs in coincidence. The kaon peak cannot be distinguished from the pion peak.

Pressure scan with CEDAR1 for a negative hadron beam with at least 6, or with 8 PMTs in coincidence.

Horizontal (left) and vertical (right) track angles at the CEDARs. The angles for all tracks measured by the Silicon beam telescope and propagated back to the CEDAR positions are compared to the angles of the tracks accepted by CEDAR1 or CEDAR2. The acceptance of the CEDARs is reduced significantly for very divergent beam tracks.

Count rate of coincident events recorded with CEDAR1 and CEDAR2. The pressure of CEDAR2 was scanned while CEDAR1 was set to detect kaons.

Side view of the liquid hydrogen target system. A closer view of the cylindrical Mylar cell and hydrogen piping is shown in the inset.

Schematic view of the target holder used for measurements with nuclear targets.

\small Momentum resolution of the RPD for protons detected at an angle of $70^\circ$ relative to the beam axis.

\small Energy loss $\Delta E$ in the outer ring of the RPD as a function of the velocity of the particle in elastic pp scattering.

Sketch of the Sandwich veto detector. The active area of the detector (depicted in grey) has dimensions of 200$\times 200\,\Cm^2$.

The conical cryostat with the upstream beam window dismounted. The height of the (green) PCB frame that holds the detector (sensor) is about $100\,\mm$, the length of the full cryostat about $400\,\mm$. The bent cooling capillary is fixed to the PCB close to the sensitive area of the detector. Inside the cryostat, the readout cables are directly soldered to the detector module and plugged to vacuum-sealed feedthrough connectors also visible on the outer surface of the cryostat.

Block diagram of the valve box and the first upstream cryostat labelled SI01. The other two upstream cryostats SI02 and SI03 are equipped analogously. The downstream conical cryostat (CC) is shown in Fig.~\ref{fig:tracking.SI_CC_schematics}. The phase separators are integrated in the cryostats near the detectors, but outside the acceptance.

Block diagram of the conical cryostat (CC), symbols as in Fig.~\ref{fig:tracking.SI_valvebox_schematics}. The phase separator is mounted in an extra housing outside the spectrometer acceptance with a vacuum connection to the cryostat.

Spatial resolution as determined for a single Silicon detector plane. ``RMS1'' and ``RMS2'' refer to the cases of clusters with one and two hit strips, respectively.

Time resolution of a single Silicon detector projection.

Two-dimensional efficiency distribution for a plane in the beam telescope. The stereo-angle tilt of the sensitive area is visible.

Schematic view of the pixel and strip region of the readout circuit. Note that the pixel region consists of $32\times 32$ pixels of $1\,\mm^2$ size each, while only $4\times 4$ are shown for clarity. Figure not to scale.

The PixelGEM read-out foil. The inner $10\times 10\,\Cm^2$ darkest part is the active area. The symmetric wires connecting the pads and the strips to the read-out electronics surround this part.

A fully assembled PixelGEM detector, equipped with 16 APV front-end cards. The digitisation of the analog signals from the APVs is done at an external ADC card, which is connected via the grey cables.

Front-end card carrying (from top to bottom) the 130-pin connector, the protection network, a ceramic pitch adaptor, and the APV25-S1 ASIC for analog sampling of the signals induced on the readout electrodes.

Residual distribution (difference between measured cluster position and track penetration point) in $x$-direction for (left) the pixel region and (right) the strip region of a PixelGEM detector. The quoted residual widths are obtained from fits of a sum of two Gaussians. When corrected for the track uncertainties, spatial resolutions of $106\,\mum$ (pixels) and $54\,\mum$ (strips) are obtained for this particular detector.

Residual distribution (difference between measured cluster position and track penetration point) in $x$-direction for (left) the pixel region and (right) the strip region of a PixelGEM detector. The quoted residual widths are obtained from fits of a sum of two Gaussians. When corrected for the track uncertainties, spatial resolutions of $106\,\mum$ (pixels) and $54\,\mum$ (strips) are obtained for this particular detector.

Time residual distribution (difference between measured cluster time and track time) for (left) the pixel region and (right) the strip region ($x$-direction) of a PixelGEM detector.

Time residual distribution (difference between measured cluster time and track time) for (left) the pixel region and (right) the strip region ($x$-direction) of a PixelGEM detector.

Efficiency of one of the PixelGEM detectors, measured in a high-intensity hadron beam. The horizontal lines with reduced efficiency correspond to boundaries between GEM sectors. In the white region not enough tracks are reconstructed when this particular detector is excluded from the tracking.

Two-dimensional efficiency of a Micromegas detector. The empty region in the middle is the $5\,\Cm$ central dead zone.

Space residual distribution of a Micromegas detector. The quoted residual width is obtained from a fit of a sum of two Gaussians.

Two-dimensional representation of the efficiency for one of DC4 layers. The half horizontal lines with reduced efficiency indicate the position of the power supply lines of the beam killer.

Double residual (see text) distribution of the DC4 chamber for one of its doublets. The quoted width is from the fit of a simple Gaussian.

Sketch of a Mini Drift Tube module.

Front view of an $X$-plane of the Rich Wall detector. The large-size numbers correspond to the number of MDT modules in each sector, the small numbers indicate the dimensions in units of $\mm$.

Schematic view of the Rich Wall readout chain.

Rich Wall residual distribution, showing the difference between reconstructed cluster position and extrapolated track position along the axis perpendicular to the wire layer. The quoted sigma is extracted by fitting a sum of two Gaussians.

Resolution of the reconstructed Cherenkov ring for pions as a function of the track angle. The two different trends in the curve below and above $\sim 175\,\mrad$ are due to the different RICH-1 photon detector types (see \secref{sec:pid.rich1}).

A typical event display during hadron data taking. The 16 squares represent the detector areas; the four central ones are equipped with MAPMTs. The small squares represent the hits detected in the photon detectors.

Two-dimensional hit distributions in the central part of the RICH-1 photon detectors for (left) data taken with a muon beam and (right) data taken with a positive hadron beam.

Two-dimensional hit distributions in the central part of the RICH-1 photon detectors for (left) data taken with a muon beam and (right) data taken with a positive hadron beam.

Horizontal axis projection of the integrated hit distributions for the lower photon detectors. Both central and peripheral parts of RICH-1 are included. The shaded histogram refers to the muon environment, the open to the hadron one. The small dips in the hit distributions correspond to the dead zones between the detector parts equipped with MAPMTs and with MWPCs.

Mean number of detected photons per reconstructed ring as a function of the corresponding Cherenkov angle $\theta_\mathrm{Ch}$ in the central region of the RICH-1 detector for track angles $\theta$ between $30\,\mrad$ and $90\,\mrad$. The line is a fit with the functional form $N=N_0$sin$^2(\theta_\mathrm{Ch})$.

Configuration of ECAL1. The central area is equipped with GAMS modules. The MAINZ modules are installed above and below the GAMS area. The OLGA modules cover the outer left and right regions.

Schematic view of the LASER monitoring system for ECAL1. The laser beam is distributed to the ECAL1 modules using one primary (D1) and eight secondary (D2) light diffusion spheres. For clarity, only one of the 8 primary fibres dispatching the light to D2, only one of the secondary 1500 fibres transmitting it to the LG modules, and only one of the 8 front-end-monitoring (FEM) modules are explicitly shown.

ECAL1 module responses as monitored during a period of one week for (left) a stable module and (right) an unstable module. The vertical scale is normalised to the SADC charge measured in the beginning of the period.

ECAL1 module responses as monitored during a period of one week for (left) a stable module and (right) an unstable module. The vertical scale is normalised to the SADC charge measured in the beginning of the period.

Configuration of ECAL2. The outer and intermediate regions are equipped with GAMS and radiation-hardened GAMS modules respectively. The inner region is equipped with Shashlik sampling modules. The transverse sizes of all three types of modules are identical. The central hole of $2\times 2$ modules can be seen as a white spot.

Photographs of a Shashlik-type calorimeter module. Left part: the upstream face of the module with its four central rods and 16 light fibres. Right part: the module itself with the fibres guide at the downstream face.

VME carrier card with four mounted MSADC modules.

Standard deviation $\sigma$ for the ECAL2 time resolution as a function of the photon energy E. The solid curve is a fit to the data points using the expression: $\sigma^{2}(E)=1.13/E+0.22/E^{2}+0.39$.

ECAL2 module responses as monitored during a period of one week for (left) a stable module and (right) an unstable module. The vertical scale is normalised to the SADC charge measured in the beginning of the period.

ECAL2 module responses as monitored during a period of one week for (left) a stable module and (right) an unstable module. The vertical scale is normalised to the SADC charge measured in the beginning of the period.

Arrangement of trigger elements in the spectrometer (schematic side view, not to scale).

Beam counter efficiency distribution in transverse coordinates.

Time residual of the beam trigger.

Allowed combinations for target pointing in the RPD part of the proton trigger.

Correlation between the energy losses of protons and pions traversing ring A and stopping (or traversing) ring B of the RPD. For each particle type the minimum and the maximum polar angles ($50^\circ$ and $90^\circ$) are shown. The shaded area corresponds to the region rejected by the trigger logic.

The multiplicity counter. All dimensions are in mm.

The active area of the ECAL2 trigger (shown in blue). The cells shown in orange are rejected due to high rates.

Efficiency of the ECAL2 trigger as a function of the energy. The solid line is a fit to the data with an error function.

Time resolution of the CFD algorithm for a representative cell in the centre and signal amplitudes above 800 MeV.

\small Trigger rate versus event size. The COMPASS DAQ system is compared to several large-scale experiments. The comparison is done for first-level (L1) triggers or their equivalent.

\small Overview of the COMPASS DAQ system. Data coming from the detectors are first digitised in the front-end cards and then merged in the concentrator modules, either CATCH or GeSiCA(HotGeSiCA). The data from the concentrator modules are first sent to the Readout Buffers and then transmitted to the Event Builders. The data are temporarily saved on disk, before being migrated to the Central Data Recording facility.

\small Data acquisition dead time for three different TCS settings, as measured as a function of the attempted trigger rate. The settings used in 2008/2009 are shown in red triangles.

Implementation of supervision, front-end and device layers of the Detector Control System.

Efficiency of tracking and vertexing as a function of momentum with the efficiency of tracking software (red, solid line), setup efficiency (hatched, green area), and overall efficiency (crossed, blue area).

Relative momentum resolution as a function of track momentum. The standard deviation of the reconstruction error is shown for tracks deflected by the SM2 magnet alone or by both SM1 and SM2 (squares), by the SM1 magnet alone (circles) and for those deflected by the fringe field of SM1 only (triangles, right scale).

Run-by-run alignment correction applied to the silicon detector positions and correlation with ambient temperature.

Distribution of scattering angle of the outgoing pion vs the position of primary vertex along the beam axis from Primakoff data, illustrating the improvement of the vertex resolution between (left) standard alignment and (right) run-by-run alignment. The structures correspond to interactions in the different targets used in the measurement (see Table~\ref{tab:target.overview}) and in the first Silicon station downstream of the targets.

Distribution of scattering angle of the outgoing pion vs the position of primary vertex along the beam axis from Primakoff data, illustrating the improvement of the vertex resolution between (left) standard alignment and (right) run-by-run alignment. The structures correspond to interactions in the different targets used in the measurement (see Table~\ref{tab:target.overview}) and in the first Silicon station downstream of the targets.

Distribution of reconstructed interaction vertices with three outgoing charged particles along the beam direction for exclusive events. For each solid state target the thickness is indicated (in $\mum$).

Vertex distributions for the liquid hydrogen target ($xy$ projection) for events with three charged tracks.

Vertex distributions for the liquid hydrogen target ($xz$ projection) for events with three charged tracks. For the explanation of the structures, see also \Figref{fig:targets.lh2.pic2}.

Correlation between the longitudinal vertex position $z$ determined with the RPD and the one determined with the spectrometer.

Correlation between the azimuthal angles of the recoil proton detected in the RPD and the scattered proton detected in the spectrometer.

Momentum transfer correlation between the recoil proton detected in the RPD and the scattered proton detected in the spectrometer.

Cherenkov angle for reconstructed rings as a function of the particle momentum for the C$_4$F$_{10}$ radiator.

Cherenkov angle for reconstructed rings as a function of the particle momentum for the N$_2$ radiator.

Identification efficiency and mis-identification probabilities as a function of the particle momentum for (left) a pion sample and (right) a kaon sample.

Identification efficiency and mis-identification probabilities as a function of the particle momentum for (left) a pion sample and (right) a kaon sample.

Dependence of $P(\text{signal}|\pi)$ on $\theta_x$ (horizontal) and $\theta_y$ (vertical) for the eight PMTs of CEDAR 2 (arranged according to the CEDAR geometry). The range for both angles is from $-250\,\murad$ to $250\,\murad$. The insets in the centre illustrate the position of a pion (dashed, red) and a kaon (green) ring relative to the PMT positions for $\theta_x=0$ and $\theta_y=0$ (left inset) and for $\theta_x>0$ and $\theta_y=0$ (right inset).

pion sample

Values for the log-likelihoods function for different samples obtained from CEDAR 2 calculated for (a) the kaon sample, (b) the pion sample and (c) an unbiased beam sample. The red line indicates $\log L(\pi) = \log L(K)$.

Shower reconstruction: (left) fraction of total shower energy collected from $-\infty$ up to a particular distance from the shower center (Eq.~\ref{eq:reconstruction.ecal.showerprofile1D}); (right) fraction of the total energy deposited in a column as a function of its distance from the shower center.

Shower reconstruction: (left) fraction of total shower energy collected from $-\infty$ up to a particular distance from the shower center (Eq.~\ref{eq:reconstruction.ecal.showerprofile1D}); (right) fraction of the total energy deposited in a column as a function of its distance from the shower center.

ECAL2 fit results for (left) number of modules per cluster and (right) number of fitted showers per cluster.

ECAL2 fit results for (left) number of modules per cluster and (right) number of fitted showers per cluster.

Energy deposition in two ECAL2 modules as a function of the difference between reconstructed and nominal $\pi^0$ mass for (left) a module with typical behaviour and (right) a module with an unusual behaviour.

Energy deposition in two ECAL2 modules as a function of the difference between reconstructed and nominal $\pi^0$ mass for (left) a module with typical behaviour and (right) a module with an unusual behaviour.

Difference $\Delta M$ between reconstructed and nominal $\pi^0$ masses in ECAL2 for (left) before calibration and (right) after calibration.

Difference $\Delta M$ between reconstructed and nominal $\pi^0$ masses in ECAL2 for (left) before calibration and (right) after calibration.

Difference between reconstructed and nominal $\pi^0$ masses as a function of the impact position for the ECAL2 modules for (left) before $\pi^0$ calibration and (right) after $\pi^0$ calibration. The difference is calculated using the mean value of the fitted (with a Gaussian) X-projection of $E_{\gamma}$ vs $(M_{\gamma \gamma}-M_{\pi^0})$ histograms. The grey rows at the top and bottom ends and on the right side of ECAL2 are located beyond the angular acceptance for photons coming from the target (see \secref{sec:pid.ecal.ecal2}.)

Difference between reconstructed and nominal $\pi^0$ masses as a function of the impact position for the ECAL2 modules for (left) before $\pi^0$ calibration and (right) after $\pi^0$ calibration. The difference is calculated using the mean value of the fitted (with a Gaussian) X-projection of $E_{\gamma}$ vs $(M_{\gamma \gamma}-M_{\pi^0})$ histograms. The grey rows at the top and bottom ends and on the right side of ECAL2 are located beyond the angular acceptance for photons coming from the target (see \secref{sec:pid.ecal.ecal2}.)

Ratio of track momentum over calorimeter energy as a function of the impact position in a Shashlik module relative to its centre. The four central spots with a ratio larger than one correspond to the four module rods.

Difference between the beam energy and the total measured energy as a function of the photon energy.

Intra-cell energy variation as a function of the distance to the cell centre.

Difference between beam and measured energies (energy balance) for Primakoff-Compton scattering (left) with a muon beam and (right) with a pion beam. The distributions are displayed with the standard $\pi^0$ calibration only (dashed curve) and with linearity and intra-cell position corrections (solid curve); the corresponding RMS$_1$ and RMS$_2$ values are indicated.

Difference between beam and measured energies (energy balance) for Primakoff-Compton scattering (left) with a muon beam and (right) with a pion beam. The distributions are displayed with the standard $\pi^0$ calibration only (dashed curve) and with linearity and intra-cell position corrections (solid curve); the corresponding RMS$_1$ and RMS$_2$ values are indicated.

Simulated photon efficiency (left) as a function of the photon energy and (right) as a function of the photon direction in the laboratory system.

Simulated photon efficiency (left) as a function of the photon energy and (right) as a function of the photon direction in the laboratory system.

Monte Carlo simulation of the Primakoff-Compton reaction, showing the reconstructed position of the primary vertex along the beam direction as a function of the scattering angle of the outgoing pion. Note that interactions outside the target material are not simulated.

Acceptance for the diffractively produced $\pi^-\pi^+\pi^-$ final state (left)~as a function of the $3\pi$ invariant mass and (right)~as a function of the polar angle of the $\pi^+\pi^-$ isobar in the Gottfried-Jackson frame.

Acceptance for the diffractively produced $\pi^-\pi^+\pi^-$ final state (left)~as a function of the $3\pi$ invariant mass and (right)~as a function of the polar angle of the $\pi^+\pi^-$ isobar in the Gottfried-Jackson frame.

Acceptance for the diffractively produced $K^-\pi^+\pi^-$ final state (left)~as a function of the $K\pi\pi$ invariant mass, and (right)~as a function of the polar angle of the $\pi^+K^-$ isobar in the Gottfried-Jackson frame.

Acceptance for the diffractively produced $K^-\pi^+\pi^-$ final state (left)~as a function of the $K\pi\pi$ invariant mass, and (right)~as a function of the polar angle of the $\pi^+K^-$ isobar in the Gottfried-Jackson frame.

Acceptance for the diffractively produced $\pi^-\pi^0\pi^0$ final state (left)~as a function of the $3\pi$ invariant mass and (right)~as a function of the polar angle of the $\pi^0\pi^0$ isobar in the Gottfried-Jackson frame.

Acceptance for the diffractively produced $\pi^-\pi^0\pi^0$ final state (left)~as a function of the $3\pi$ invariant mass and (right)~as a function of the polar angle of the $\pi^0\pi^0$ isobar in the Gottfried-Jackson frame.

Energy balance between outgoing and incoming particles for (left) diffractive dissociation with three charged pions in the final state and (right) for Primakoff scattering.

Energy balance between outgoing and incoming particles for (left) diffractive dissociation with three charged pions in the final state and (right) for Primakoff scattering.

Squared four-momentum transfer for $\pi^-\pi^+\pi^-$ events produced by a pion beam impinging on a liquid hydrogen target, and selected by the DT0 trigger.

Squared four-momentum transfer for $\pi^-\pi^+\pi^-$ events produced by pions hitting a lead target, and selected by the multiplicity trigger.

Squared four-momentum transfer of reconstructed beam kaons (data points) compared to the Monte Carlo simulation of purely electromagnetic interaction (solid lines). The dashed line is an exponential fit, used to determine the resolution.

Momentum transfer distributions for exclusive (left) $\pi^-\gamma$ and (right) $\mu^-\gamma$ events. The data (dotted lines) are compared to the MC simulation (solid lines).

Momentum transfer distributions for exclusive (left) $\pi^-\gamma$ and (right) $\mu^-\gamma$ events. The data (dotted lines) are compared to the MC simulation (solid lines).

Two-photon invariant mass distribution as measured in ECAL2, in the (left) $\pi^0$ mass region and (right) $\eta$ mass region. The solid curves are fits to the signal and to the background. The values of the resolution achieved are indicated in each plot.

Two-photon invariant mass distribution as measured in ECAL2, in the (left) $\pi^0$ mass region and (right) $\eta$ mass region. The solid curves are fits to the signal and to the background. The values of the resolution achieved are indicated in each plot.

Reconstructed invariant masses for charged particles in the final state. The peaks shown are for (top left) $K^0_S(498)$, (top right) $\phi(1020)$, (bottom left) $\Lambda(1115)$, and (bottom right) $\Xi^\pm$. The $K^0_S$, $\Lambda$, and $\Xi^\pm$ particles are produced in inclusive reactions. The dashed curve in the $\phi(1020)$ plot is a fit to the background.

Reconstructed invariant masses for charged particles in the final state. The peaks shown are for (top left) $K^0_S(498)$, (top right) $\phi(1020)$, (bottom left) $\Lambda(1115)$, and (bottom right) $\Xi^\pm$. The $K^0_S$, $\Lambda$, and $\Xi^\pm$ particles are produced in inclusive reactions. The dashed curve in the $\phi(1020)$ plot is a fit to the background.

Reconstructed invariant masses for charged particles in the final state. The peaks shown are for (top left) $K^0_S(498)$, (top right) $\phi(1020)$, (bottom left) $\Lambda(1115)$, and (bottom right) $\Xi^\pm$. The $K^0_S$, $\Lambda$, and $\Xi^\pm$ particles are produced in inclusive reactions. The dashed curve in the $\phi(1020)$ plot is a fit to the background.

Reconstructed invariant masses for charged particles in the final state. The peaks shown are for (top left) $K^0_S(498)$, (top right) $\phi(1020)$, (bottom left) $\Lambda(1115)$, and (bottom right) $\Xi^\pm$. The $K^0_S$, $\Lambda$, and $\Xi^\pm$ particles are produced in inclusive reactions. The dashed curve in the $\phi(1020)$ plot is a fit to the background.

Invariant mass spectra for (left) $\pi^-\pi^+\pi^0$ and (right) $\pi^-\pi^+\eta$ systems. The full line in the left panel is a fit to the $\omega$ peak only; the dashed line includes also the background.

Invariant mass spectra for (left) $\pi^-\pi^+\pi^0$ and (right) $\pi^-\pi^+\eta$ systems. The full line in the left panel is a fit to the $\omega$ peak only; the dashed line includes also the background.

Dalitz plot for three diffractively produced charged pions after a cut of $\pm 130\,\MeV/c^2$ around the $\pi_2(1670)$ mass.