The Panda experiment
|Klaus Peters and Klaus Goetzen (2011), Scholarpedia, 6(4):10210.||doi:10.4249/scholarpedia.10210||revision #91871 [link to/cite this article]|
The PANDA Experiment will be one of the future experiments at the Facility for Antiproton and Ion Research (FAIR), which has been officially founded in October 2010. The facility will be build on the area of the GSI Helmholtzzentrum für Schwerionenforschung GmbH (GSI) in Darmstadt, Germany. Further large experiments planned at this facility, which is dedicated to do fundamental research on the structure of matter are for example the CBM experiment , the FLAIR experiment  and the Super-FRS project.
The central part of FAIR is a synchrotron complex providing intense pulsed ion beams (from p to U). Antiprotons produced by a primary proton beam will then be filled into the High Energy Storage Ring which collide with the fixed target inside the PANDA Detector.
The main focus of the experiment are investigations related to the strong force at medium energy. The PANDA Collaboration with more than 450 scientist from 17 countries intends to do basic research on various topics comprising hadron spectroscopy, search for exotic hadrons, hadrons in media, nucleon structure and exotic nuclei.
In order to gather all the necessary information from the antiproton-proton collisions a versatile detector will be build being able to provide precise trajectory reconstruction, energy and momentum measurements and an efficient particle identification system.
The name PANDA itself is an acronym which stands for anti-Proton ANnihilations at DArmstadt.
Accelerator and Storage Ring
The FAIR accelerator project ( Figure 2 ) at GSI will increase the intensity of primary proton and heavy ion beams by up to two orders of magnitude, relative to the existing GSI facility. In addition to the design of the new synchrotron SIS- 100 and the storage rings, the intensity upgrade of the SIS- 18 synchrotron plays a key role for the FAIR project.
One of the four major scientific pillars of FAIR will be the physics with anti-protons. The anti-protons created by colliding high intensity protons with a copper target will be collected, pre-cooled and finally stored in the FAIR High Energy Storage Ring (HESR). The covered momentum range for the stored anti-protons will be 1.5 GeV/c up to 15 GeV/c.
Two experimental modes are envisaged: a high luminosity mode with peak luminosities up to 2⋅1032 cm-2s-1, and a high resolution mode with a maximum relative momentum resolution of Δp/p≤10-5. The number of stored anti-protons will be of the order of N≈1011.
For the envisaged experimental program a nearly full coverage of the solid angle together with good particle identification and high energy and angular resolutions for charged particles and photons are mandatory.
The proposed detector is subdivided into the target spectrometer (TS) consisting of a solenoid around the interaction region and a forward spectrometer (FS) based on a dipole to momentum-analyze the forward-going particles ( Figure 3 ).
The combination of two spectrometers allows a full angular coverage, it takes into account the wide range of energies and it still has sufficient flexibility, so that individual components can be exchanged or added for specific experiments.
Since the PANDA experiment is addressing very different physics problems there are also different requirement for the used target system. Currently two kinds of targets are under discussion:
- Pellet beam target. A pellet target provides a regular stream of frozen Hydrogen microspheres (pellets), vertically traversing the accelerator beam. These pellets typically have sizes between 10 μm and 40 μm depending on the size of the injection nozzle. The pellet stream has a low angular divergence, thus leading to a position uncertainty of its interaction with the beam of about ±1mm. The pellets fall with a speed of 60 m/s and at a flow rate of 10000 pellets/s.
- Cluster beam target. Cluster-jet beams for internal storage ring experiments are commonly produced by expansion of pre-cooled gases in micron-sized throat convergent divergent Laval-type nozzles into vacuum. During the passage of the gas through such a nozzle the gas cools down and forms a supersonic beam. Under appropriate conditions, depending on the type of gas, condensation can take place and nano-particles are created, the so-called clusters. Being dependent of the experimental conditions the size of clusters amounts to typically 103 - 105 atoms per cluster.
The PANDA detector will operate in two different magnetic fields: a 2T solenoidal field in the target spectrometer and a 1T dipole field in the forward region.
To provide measurement of charged particle trajectories with high resolution over the complete solid angle four different tracking systems are foreseen, which are, ordered from the inside outwards
- Micro Vertex Detector. The Micro-Vertex-Detector (MVD) is situated in the target spectrometer and is the closest detector part with respect to the primary interaction vertex. The MVD is a tracking device for charged particles and thus essential for a very precise determination of secondary decay vertices of short-lived particles such as hyperons or mesons with charm or strangeness content. In the current MVD design four barrel layers and eight disks are foreseen. Being equipped with either fine granular silicon pixel detectors or double sided silicon strip detectors the envisaged spatial resolution for secondary vertices is about 100 μm.
- Central Tracking Detector. The cylindrical central detector will be the key device for momentum measurement of charged particles. Currently there are two different options under discussion: a time projection chamber (TPC) or a straw tube tracking system (STT). Although the concept of track reconstruction will be rather different both options will be able to fulfill the requirement of providing a momentum resolution Δp/p of about 1-2%. In addition both devices will contribute to particle identification via a dE/dx measurement.
- Gas Electron Multiplier Stations. A set of large area planar Gaseous Electron Multipliers (GEM) detectors forms a GEM-Tracker, which will be used as a first forward tracking detector after the central tracker. Located in the middle of each GEM-Disc is a double-sided read-out pad plane which allows particle track position measurement in four projections. The high number of projections per GEM-Disc allows unambiguous determination of the particle trajectory position with a resolution of better than 100 μm.
- Forward Mini Drift Chamber System. The forward tracker (FT) is designed for momentum analysis of charged particles deflected in the field of the PANDA dipole magnet. The FT covers angular acceptance defined by the aperture of the magnet equal to ±10° horizontally and ±5° vertically with respect the beam direction. The FT consists of three pairs of planar tracking stations. With expected position resolution of σ=0.1 mm per detection layer and the material budget in one tracking station of 0.3%⋅X0, the momentum resolution is better than 1%.
High count rates together with the proposed compact design of the target spectrometer require a fast scintillator with a small radiation length for the electromagnetic calorimeter. In recent years PbWO4 has been investigated as a high density inorganic scintillator with good energy resolution for photon and electron detection even at intermediate energies. For high energy physics it has been chosen by the CMS collaboration  at CERN . Besides the short decay time of less than 20 ns a good radiation hardness has been achieved. It is proposed to use crystals with a length of 20 cm (i.e. 22 X0) achieving an energy resolution for photons and electrons of about 1.54% / √E[GeV]+0.3%.
These crystals allow a pion/electron discrimination of 103 for momenta above 0.5 GeV/c and as a result a gas Cherenkov inside the TS can be omitted. This results in a smaller detector and a lower price. The readout of the crystals will be done by large area avalanche photodiodes. With an inner radius of 57 cm the barrel part of the calorimeter requires 11360 crystals with a frontface of 2cm × 2cm.
The backward and forward endcaps require 592 and 3600 crystals, respectively. A total of 15552 PbWO4 crystals will thus be required ( Figure 4 ). A fast signal will be derived for the first level trigger.
The identification of charged particles with extreme accuracy is one of the key requirements for unveiling many aspects of the physics program envisaged by the PANDA experiment.
Therefore the PANDA Detector will be equipped with various dedicated high developed particle identification (PID) systems providing the ability of classifying particle species over the whole kinematic range in addition to dE/dx measurements from tracking and information from the electromagnetic calorimetry.
The envisaged PID systems are
- Detection of Internally Reflected Cherenkov light (DIRC) System. The basic concept of a DIRC detector is given by a barrel consisting of rather thin radiators which guide Cherenkov photons created by incident charged particles to a readout system by internal total reflection. The typical radiator material is ultraclean and polished fused silica. This kind of detector concept has been introduced by the BaBar experiment for the first time. The current setup involves 96 bars with dimensions 3 × 1.7 × 250 cm3. As a substantial new development the same technology applied to a disc like geometry for forward going particles in the TS is currently in R & D phase.
- Time Of Flight System. For the particle identification of slow charged particles, for the DIRC detector, and for track deconvolution of the central tracker a time of flight (TOF) barrel in the target spectrometer is desirable. In order to achieve low-energy thresholds, the barrel is located in front of the radiator barrel of the DIRC and could also become mechanically combined with the latter. For the envisaged time resolution of σ=100ps, one expects a three standard deviation π/K separation up to 430 MeV/c for 90 degree tracks. Furthermore the precise timing information itself might also be a crucial input to the online event selection as well as the event building process.
- Muon Detection System. The most suitable technology for detecting the muons in PANDA is a Range System (RS) with proper granularity close to muon's straggling in the iron absorber. The stopping power of iron is about 1.5 GeV per metre of absorber for the relativistic muons with dE/dx = 2 MeV/g. The muon system consists of a barrel and an endcap (EC) in the target spectrometer and a muon filter (MF) in the forward spectrometer. In the barrel part the granularity of the iron absorber (sampling) is 3 cm, in the EC and MF the 6 cm sampling is selected for better detection of muons with higher energies.
- Ring Imaging Cherenkov Detector. Aerogel Cherenkov Counters (ACC) with a refractive index of about n = 1.02, located in the endcap of the TS between polar angles of 5° - 22°, are envisaged in order to provide information for higher level triggers and PID. They are specially suited for π/K separation. The projected design involves the usage of mirrors to reflect the Cherenkov photons onto photomultipliers or hybrid photon detectors outside of the magnetic field region.
Protons and neutrons - collectively called nucleons - belong to the family of hadrons. They are built of quarks and bound by the strong force that is mediated via gluons. The force is acting between quarks and has an unusual behavior: It is very small when the quarks are at close distance and increases as the distance grows and then remains constant even if the quarks are removed further and further from each other.
If one attempts to separate a quark-antiquark pair, the energy of the gluon field becomes larger and larger, until a new quark-antiquark pair can be created. As a result, one does not end up with two isolated quarks but with new quark-antiquark pairs instead. This absolute imprisonment of quarks is called confinement. One of the greatest intellectual challenges of modern physics is to understand confinement not just as a phenomenon but to comprehend it quantitatively from the theory of the strong force.
Another puzzle of hadron physics addresses the origin of the hadron masses, i.e. of the particles composed of quarks. In the nucleon, less than 2% of the mass can be accounted for by the three valence quarks. Obviously, the bulk of the nucleon mass results from the kinetic energy and the interaction energy of the quarks confined in the nucleon. Physicists believe that new experiments exploiting high-energy antiproton and ion beams will also elucidate the generation of hadronic masses.
Furthermore the question will be addressed, whether there are existing exotic states of hadronic matter, neither being mesons nor baryons. These kind of states can consist of more than two or three quarks or anti-quarks, contain additional gluonic degrees of freedom or even solely consist of gluons. In past experiments a few states have been observed which have an intrinsic exotic structure, but the unambiguous proof has not yet been found.
Hadron Spectroscopy and Exotic Hadrons
One of the main challenges of hadron physics is the search for exotic excitations. This can either be represented by hadrons in which the gluons can act as principal components or hadronic configurations with more than two or three valence quarks or anti-quarks called multiquarks. The gluonic hadrons fall into two main categories: glueballs, i.e. states where only gluons contribute to the overall quantum numbers, and hybrids, which consist of valence quarks and antiquarks as hadrons plus one or more excited gluons which contribute to the overall quantum numbers. The additional degrees of freedom carried by gluons allow these hybrids and glueballs to have JPC exotic quantum numbers. In this case mixing effects with nearby qq states are excluded and this makes their experimental identification easier. Antiproton-proton annihilations provide a very favourable environment to search for exotic hadrons.
Many of the recent observations have been made in the charmonium energy regime. In particular the states X(3872), Y(4260) and Z(4430)+ exhibit unexpected, exotic properties. While the X(3872) and Z(4430)+ are mainly discussed to be multiquark states, the Y(4260) is a candidate for being ccg charmonium hybrid .
Concerning open charm and charm strange systems for many of the recent observations like e.g. the Ds0(2317) or Ds1(2460) the internal structure is still unknown.
At full luminosity PANDA will be able to collect several thousand charmonium and open charm states per day. By means of fine scans it will be possible to measure masses with an accuracy of the order of 100 keV and widths to 10% or better.
Hadrons in Matter
The study of medium modifications of hadrons embedded in hadronic matter aims at understanding the origin of hadron masses in the context of spontaneous chiral symmetry breaking in QCD and its partial restoration in a hadronic environment. So far experiments have been focused on the light quark sector. The high-intensity p beam of up to 15 GeV/c will allow an extension of this program to the charm sector both for hadrons with hidden and open charm. The in-medium masses of these states are expected to be affected primarily by the gluon condensate.
Another study which can be carried out in PANDA is the measurement of J/ψ and D meson production cross sections in p annihilation on a series of nuclear targets. The comparison of the resonant J/ψ yield obtained from p annihilation on protons and different nuclear targets allows to deduce the J/ψ-nucleus dissociation cross section, a fundamental parameter to understand J/ψ suppression in relativistic heavy ion collisions interpreted as a signal for quark-gluon plasma formation.
One aspect concerning nucleon structure is the consideration of so called Generalized Parton Distributions (GPDs). The theoretical framework of GPDs has recently been developed and caused excitement in the field of understanding the structure of the nucleon. It has recently been shown that exclusive pp annihilation into two photons at large s and t can be described in terms of GPDs. Estimates of the expected count rates based on a simple model predict a few thousand γγ events per month for a luminosity of 2⋅1032 cm-2s-1 at an energy of √s = 3.2GeV/c2.
Other estimates, based on cross section measurements of the inverse process γγ → pp, predict count rates up to a factor 50 above the later estimate of but still are consistent with the handbag ansatz. The comparison of the differential cross sections for the various processes and the comparison with GPD based models will allow new insights into the annihilation process in terms of quark models and QCD.
Another aspect is the electromagnetic form factor of the proton in the time-like region. This can be extracted from the cross sections for the process pp → e+ e-. The proton time-like form factors have been measured by several experiments in the low Q2 region down to threshold. At high Q2 the only measurements have been achieved by E760 and E835 at Fermilab up to Q2 of 15GeV2/c2 under the assumption |GE| = |GM| due to limited statistics. In PANDA, it will be possible to determine the form factors |GM| and |GE| separately over the widest Q2 range with a single experiment, from threshold to 20 GeV2/c4 and above.
Replacing an up or a down quark with a strange quark in a nucleon, which is bound in a nucleus, leads to the formation of a hypernucleus. A new quantum number, strangeness, is introduced into the nucleus, adding a third axis to the nuclear chart. Due to experimental limitations the third dimension has only scarcely been explored in the past. Single and double Λ-hypernuclei were discovered 50 and 40 years ago, respectively.
However, only 6 double Λ-hypernuclei are presently known, in spite of a considerable experimental effort during the last 10 years. Thanks to the use of p-beams and the skilful combination of experimental techniques, copious production at PANDA is expected, with even higher numbers than at (planned) dedicated facilities. A new chapter of strange nuclear physics will be opened whose first result will be the determination of the ΛΛ strong interaction strength, not feasible with direct scattering experiments.
- PANDA Collaboration, PANDA Technical Progress Report (2005), 
- PANDA Collaboration, PANDA Physics Performance Report (2008), arXiv:0903.3905v1
- PANDA Collaboration, EMC Technical Design Report (2008), arXiv:0810.1216v1
- PANDA Collaboration, Magnets Technical Design Report (2009), arXiv:0907.0169