Studies of the three-pion system at COMPASS
3.1 Introduction to the COMPASS experiment
The COMPASS experiment is a fixed-target experiment located at CERN, aiming to develop a better understanding of the structure and dynamics of hadrons. The experiment operates with a pion or a muon beam with energies up to 190 GeV which is scattered off a liquid hydrogen or solid nuclear targets.
The hadronic program of COMPASS includes precision spectroscopy of light mesons, identification and studies of exotic states with gluonic degrees of freedom or multiquark configurations as well as tests of chiral dynamics. Tracking over a wide angular and momentum range and hermetic calorimetry allows for exclusive measurements of the mutli-hadron final states. Due to high beam energy the production reaction is clean: the excited meson system is boosted forward, it is kinematically separated from the baryon vertex, the recoil proton occupies a rapidity domain disjoint with the final state mesons.
The muon program is focused on the investigation of the spin structure of nucleon via measurements of the Deep Inelastic Scattering (DIS) processes.1
The setup is modified as per the demands of the physical program. Fig. 3.1 shows the setup for measurements with hadron beams starting in 2008, an extensive description of the COMPASS instrumentation can be found in Ref. [72], specifically for physics with hadron beams in Ref. [73].
The 50 m long apparatus starts with the beam section where the beam particle is tracked by silicon microstrip detectors and identified by a pair of differential Cherenkov counters (CEDAR). The Beam Momentum Station (BMS), dedicated to determination of the incident momentum for studies with the muon beam, is moved out of the beam line to minimize amount of material along the beam path.
The following target region contains the target filled with liquid hydrogen and the time-of-flight detector that is calledRecoil Proton Detector (RPD). TheRPDplays an important role in both the trigger formation and offline analyses to ensure exclusivity of the reactions under investigation. One unique feature of COMPASS is that the spectrometer has two stages which cover different ranges of momenta and scattering angles of the produced particles. Essentially, the classical sequence of particle detectors (dipole magnet surrounded by the tracking devices, also calorimetry, and muon ID systems) is replicated twice. The first stage, namedLarge Angle Spectrometer, serves to measure particles which leave the target region with large angles up to180mrad (mostly particles with low
1COMPASS experiment was upgraded in 2010-2012 years and named COMPASS-II [71]. In addition to the measurements of the DIS processes and tests of chiral dynamics, the physical program included studies of the Deeply Virtual Compton Scattering processes with the muon beam, and the Drell-Yan muon pair production processes with the pion beam.
ter3Studiesofthethree-pionsystematCOMP
Figure 3.1: A schematic view of the COMPASS experiment setup of 2008. The3D-model is exported from theGEANT4-based simulation program,TGeant described in details in Ref. [74, 75]. The panoramic photos can be found in the CERN PhotoLab, see Ref. [76].
3.1 Introduction to the COMPASS experiment
momenta). It is also equipped with the Ring-Imaging Cherenkov detector (RICH) where changed particles with the momentum up to 50 GeV can be identified as pions, kaons or antiproton. The calorimeters and muon filters of the first stage have large windows in the middle, so particles with small angles less than approximately70mrad can pass through. The second stage is calledSmall Angle Spectrometer, it covers the largest part of the experiment acceptance. The significant longitudinal scale of COMPASS facilitates a good momentum resolution for the charged particles. The calorimeters have a fine granularity: the central regions of theECAL1andECAL2contain the lead glass and the shashlik type modules, respectively, with the cell size,3.8 cm.
A large data set with a 190 GeV pion beam was collected during the summer of 2008. The 40 cm long liquid hydrogen target was installed used. A key advantage of COMPASS over the previous experiments is a high beam intensity of up to108particles per spill of 9.6 s. The physical events were recorded with a dedicatedDT0trigger which required: a beam signal determined by a coincidence of scintillating fiber detectors (SciFi) with a small scintillating disc (Beam Counter), a signal in the RPD(scintillator slabs surrounding the target), and a signal from Veto System. This system contains three components: three hodoscopes upstream the target, large lead-scintillator detector, and two scintillating counters, located exactly on the track of the non-interacting beam at the end of the experiment. With this trigger setup the recorded data was enriched with the diffractive scattering events. A rate of approximately 180 000 events per 10 s spill was reached [73]. The measured final states mostly contain charged pions, kaons, as well as photons from the decay of neutral pion and eta-mesons. The target recoil proton takes up the momentum transferred from the beam. In order to produce a trigger signal, this recoil proton needs to pass through the target material and the cryostat pipe and cross both scintillator rings ofRPD. It limits the momentum of observable recoil protons to values larger than 250 MeV, which is equivalent to a proton kinetic energy of 30 MeV. The associated transferred momentum squared,tis defined as the squared difference between four-momentum of the target protonptand recoil protonpr,t= (pr−pt)2. It is limited tot <−0.065 GeV2.
The elasticπ−pscattering has the highest cross section and dominates the collected data set. The diffractive productionπ−p→3π phas the second largest cross section. The COMPASS experiment has a large acceptance and a high efficiency for the charge configurationπ−π+π−: the efficiency for tracking and vertexing for particles with momentum above 3 GeV is above 94% as shown in Ref. [73], the averaged acceptance for the reactionπ−p→π−π+π−pis50%(see Ref. [77]). The pions are reconstructed using tracking detectors. The interaction vertex position is determined by track extrapolation to the target area. The recoil proton is measured in theRPD, however, since the reaction is over-constrained, theRPDmeasurements are used to suppress non-exclusive background.
The analysis is based on 109events recorded during four two-weeks periods of beam time in 2008.
A set of selection criteria is employed to isolate a clean sample of exclusiveπ−p → π−π+π−p events. The details of the event selection are described in Ref. [77] and just briefly listed below:
1. DT0trigger bit must be set.
2. The event must contain exactly three outgoing particles with a total charge of−1.
3. The position of the interaction vertex found by the reconstruction algorithm must be located inside the fiducial volume of the target (38 cm×3.2 cm).
4. The energy of the beam calculated from the energies and momenta of the outgoing particles is required to be within a window of ±3.78 GeV around the nominal beam momentum of
191 GeV, which corresponds to two standard deviations (see Fig. 3.2). 2
5. The event must have exactly one recoil particle detected by theRPD whose the azimuthal coordinate is required to be opposite to the one for the outgoing3πsystem within the azimuthal resolution of theRPD(In Ref. [77] it was found in the range from5◦ to9◦depending on the crossed slab of theRPDrings).
6. The event is rejected if the beam particle is identified as a kaon by the two beam Cherenkov detectors or if at least one of the outgoing particles is identified as a kaon, proton, electron, or the “noise” by the forward RICH detector [78].
7. In order to suppress the background from central production reactionsπ−p→π−fastπ+π−p the event is required to have a Feynman-xbelow0.9, wherex is defined by the ratio of the longitudinal momentum of the fast negatively charged pion in the beam-target rest frame to it maximal value approximated by√s0/2.
x= 2|(~pfast)(CMz 0)|
√s0 ,
where thez-direction in set by the beam particle.
Approximately50×106events passed all selection cuts and were used in the further analysis.