Preparations for FAIR and related areas at FZJ and GSI
Hadron structure and dynamics at PANDA, PAX and BES-III
PANDA-activities at FZJ and GSI: The PANDA experiment is one essential pillar of FAIR. Its goal is to improve the understanding of Quantum Chromo Dynamics (QCD) in the charmonium energy regime. The main physics topics are precise spectroscopy of charmonium, the search for new multi quark states and glueballs, the study of the nucleon structure, the restoration of chiral symmetry for hadrons in nuclear matter and the measurement of hypernuclei. In order to pursue these goals brilliant beams of phase space cooled antiprotons will be provided by the High Energy Storage Ring (HESR). The PANDA detector is being built by an international collaboration of over 500 physicists from 19 countries. Within this context, the Helmholtz centers FZJ and GSI, as well as the Helmholtz Institute Mainz (HIM) are crucial to many of the developments connected to the charged particle tracking, particle identification and calorimetry, as well as computing. As discussed earlier, the resources from FZJ and GSI devoted to these tasks are exempted from this review, but nevertheless represent a large background providing massive leverage to the activities at HIM, which are subject to this review. Furthermore, in order to maximize the impact of the activities at HIM, a concurrent program with data from the running BES-III experiment is pursued.
At FZJ numerous activities related to PANDA are being pursued, generally centered on the reconstruction of charged particles. FZJ is a leading force in the efforts for the design, data readout and mechanical structure of the Micro-Vertex-Detector. After an external review, the PANDA collaboration selected the technology developed at FZJ for the central tracker, which consists of ultra lightweight, self-supporting straw tubes that additionally provides high resolution specific energy loss information. In addition to being responsible for the overall design, construction of the tubes and assembly of the device, FZJ is also developing a data readout system. FZJ together with HIM is developing the luminosity detector, which measures elastically scattered antiprotons in the nuclear Coulomb interference region. Directly connected to this, FZJ has built a high precision Si and Ge recoil detector in order to measure the elastic differential cross section with high precision and over a wider kinematic range to achieve an absolute precision in luminosity of better than 3%. Many computing aspects are driven by FZJ, for instance core developments of the software framework, online tracking for data filtering methods including GPU based acceleration techniques, GRID developments, and many more. FZJ will provide infrastructure and beam for the functional preassembly of PANDA.
GSI has a leading role in many development and construction aspects of PANDA and has the sole responsibility for the Detector for Internally Reflected Cherenkov Light (DIRC). This detector is mandatory for the identification of particles. The necessary work packages are shared with HIM and several partner universities. GSI focuses on the radiator material, mechanics and integration, while HIM is involved in electronics and read-out. In addition GSI is heavily involved in the PbWO4 electromagnet calorimeter (EMC). GSI is responsible for the mass production and screening of them and the customized ASIC preamplifier and a hit detection engine. GSI is responsible for the construction of the Forward GEM detectors in cooperation with international partners. Together with FAIR, GSI is responsible for the overall technical coordination and integration. GSI is leading a development for data concentration and new concepts for triggerless read-out in high background areas and cooperates with HIM on technical solutions. In addition core software activities like PANDAROOT developments (as implementation of FAIRROOT) and Grid-Coordination complement the work packages.
BESIII is the central detector at the electron-positron collider BEPC-II in Beijing/China, operating in the range of centre-of-mass energies between 2 and 5 GeV (tau-charm factory). The experiment has the world's largest data samples of J/psi, psi', psi(3770) decays. The collaboration is composed of about 350 physicists from 50 insitutions and 11 countries. The physics program includes the analysis of charmonium resonances, light quark spectroscopy, open charm decays as well as the measurement of time-like meson and baryon form factors. In late 2012 BESIII embarked on a program of research to produce large numbers of Y(4260) particles, which lead to a series of discoveries of charmonium-like particles, which are found to be of exotic nature. Both GSI and HIM are collaborating partners of BES-III. The results obtained in Beijing are an ideal preparation for future measurements at PANDA.
Polarized Antiprotons by Spin-filtering (PAX) The physics potential for studies with polarized antiprotons is enormous. The flag-ship experiment, Drell-Yan production in double polarized proton-antiproton scattering, will provide direct access to the transverse spin structure of the nucleon, transversity. So far no antiproton beams with sizeable polarisation could be produced. It is the aim of the PAX-collaboration to develop an efficient method for polarizing antiproton beams by in-situ build-up in a storage ring. The only viable method to do this effectively is through "spin-filtering" by the repeated interaction of an antiproton beam with a polarized hydrogen gas target in a cooler storage ring, thus selectively discarding more particles in one of the two spin states.
The PAX collaboration has performed a spin-filtering measurement with protons at COSY in order to build and to commission the equipment necessary for the antiproton measurement and confirmed earlier results of the FILTEX collaboration. The COSY measurement, actually a determination of the spin-dependent polarization build-up cross section for transverse polarisation, proves that spin-filtering can be adopted as a method to polarize a stored beam and that the present interpretation of the mechanism in terms of the proton-proton interaction is valid.
As the next step, PAX intends to transfer this method to longitudinal polarization. In order to access the longitudinal cross section a Siberian snake has been ordered from Cryogenic LTD (UK). After delivery (in 2013), it will be installed into the COSY ring, commissioned and subsequently used for the measurement during PoF 3. After that the PAX collaboration is ready to perform the corresponding experiments with antiprotons.
The only place worldwide where measurements of the spin-dependent antiproton-proton interactions are presently possible, is the Antiproton Decelerator (AD) at CERN, which covers the energy range of interest (50 to 500 MeV). A proposal to the CERN SPS committee has been submitted and all requested tests have been conducted, but the project has not been approved, because it would have "disruptive impact on the current antimatter physics programme". The PAX collaboration expects nonetheless that on a longer time scale these essential measurements will be possible at AD. Alternatively, these would have to wait until antiprotons will be available at a suitable storage ring of FAIR.
A next step will be to design and to build a dedicated antiproton polarizer ring (APR) with the future option of an implementation at FAIR: this would open new and unique research opportunities for spin-physics experiments in pp interactions. The PAX proposal suggests extending the HESR into an asymmetric proton-antiproton collider as a long-term upgrade. The overall machine complex consists of
- the APR built inside the HESR area with the crucial goal of polarizing antiprotons at a kinetic energy between 50 to 500 MeV, to be accelerated and injected into the other rings;
- A second Cooler Synchrotron Ring (CSR, COSY-like) in which antiprotons can be stored with momenta up to 3.5 GeV/c. Both rings shall have a straight section, where the PAX detector could be installed, and oriented parallel to the experimental straight section of HESR, and
- the HESR. By deflection of the HESR beam into the straight section of the CSR, either a collider or a fixed-target mode will be feasible. It is worthwhile to stress that, through the employment of the CSR, a second interaction point would be created with minimum interference with PANDA.
Nuclear quark matter at extreme conditions with ALICE, HADES and CBM
ALICE: (A Large Ion Collider Experiment) is the experiment at the CERN Large Hadron Collider (LHC) dedicated to the study of QGP in ultra-relativistic heavy-ion collisions. The ALICE group at GSI has established itself as one of the world-leading groups within this research field, taking a key role within ALICE in all aspects of the programme. During the first run from November 2009 until February 2013 ALICE reached all design specifications and demonstrated outstanding performance. The physics output already from the first run at the LHC (70 articles in refereed scientific journals, cited in total 3300 times; 9 of them have collected more than 100 citations) is remarkable. Among the important findings in the first running period are:
- Hadrons carrying charm quarks are found to exhibit collective flow, suggesting that these quarks equilibrate with the medium.
- The ALICE results on J/ψ "suppression" support a novel picture of charmonium production according to which charmonium is formed from thermalised charm quarks during hadronization. Due to the strongly increased charm production in hard initial processes at LHC energies over RHIC energies, this mechanism would turn a suppression pattern in the A-A to p-p ratio (RAA) into almost an enhancement at LHC. Such a trend in the J/ψ RAA is indeed observed.
- The temperature inferred from the slope of the direct photon transverse momentum spectrum measuredwith ALICE appears to be at the LHC almost 40% higher than at the Relativistic Heavy-Ion Collider (RHIC) at lower collision energies.
Measurements during Run 2 will focus on low-mass dielectrons, quarkonium and beauty hadrons at low transverse momentum, to gain an alternative access to the temperature of the QGP and to arrive at a complete understanding of heavy flavor production and propagation, respectively. The goals is to increase the statistics to about 100 million Pb-Pb collisions running at collision rates up to 10-20 kHz. In Run 3 (after 2018), LHC will provide Pb-Pb collisions at interaction rates as high as 50 kHz. The statistics available for data analysis will increase compared to Run 2 by up to a factor of 100 with the planned implementation of a continuous read-out. To meet this challenge, the ALICE Collaboration has presented a plan to upgrade parts of its detector systems as described in the Letter of Intent endorsed by the CERN LHC Scientific Committee and subsequently approved by the CERN Research Board. This is in line with the recent recommendation of the European Strategy Group for Particle Physics, which endorses the heavy-ion program at the LHC as a top priority for future European research in high-energy physics.
HADES: The High Acceptance Dielectron Spectrometer HADES was designed to measure electromagnetic radiation emitted from moderately hot and dense nuclear matter as it is formed in reactions of heavy ions in the few-GeV bombarding energy regime. It is also very well suited for investigating purely hadronic probes and in particular the production of open and hidden strangeness. Recently HADES has extended its systematic investigations of dielectron emission in relativistic collisions to measurements in the p+p and p+Nb reactions at 3.5 GeV and, following the successful upgrade of the hardware, to a high-statistics Au+Au run at 1.23 GeV/u in 2012. In total, HADES has meanwhile published 19 papers in refereed journals triggering more than 400 citations. Among the striking results are:
- A comparison of dilepton production in p+p and p+Nb signals a strong modification of the omega meson propagating through matter.
- The continuum in the dilepton low-mass region can be understood in terms of baryonic-resonace decays. The precise spectral distribution, however, is theoretically not understood. The analysis of pion production in p+p reactions (two-pion exclusive final states) provides guidance to further constrain the emissivity of baryonic matter.
- A comprehensive study of strangeness production in p+p, p+A and A+A reactions has shed new light on the production and propagation of strangeness. For example, production is not OZI-suppressed in A+A collisions, and the overall strangeness production can be explained by a statistical hadronization model.
More stringent tests of the theoretical framework for dilepton production are expected from the planned physics program exploiting secondary pion beams. The combination of a secondary pion beam with a high-acceptance dilepton spectrometer is a world-wide unique set-up for such investigations. The analysis of the Au+Au run is ongoing and will deliver a wealth of information on dilepton and strangeness production. A more comprehensive investigation of in-medium production and propagation of the light vector mesons is still impeded by the limited bombarding energy available from SIS18, allowing heavy-ion reactions only just at or even below production threshold. Here the move of HADES to SIS100 will open a more efficient approach to in-medium effects. The spectrometer will cover the low beam energies at SIS100 and will complement the CBM programme, thus providing a continuous excitation function of all relevant observables. It is evident that in the near future, the HADES detector will remain the state-of-the-art device able to address the relevant physics questions in the energy range up to of 2–4 GeV/u.
CBM: Hadronic and partonic matter at the highest baryon densities will be probed with unparalleled precision with the Compressed Baryonic Matter (CBM) experiment at FAIR. The goal of the CBM research programme is to explore in detail the QCD phase diagram in the region of large baryon-chemical potentials. This programme is complementary to the investigation of heavy-ion collisions at the highest energies where the net baryon density is negligible. The discovery potential of the CBM experiment includes a possible first-order phase transition between hadronic and quark-gluon matter, the related existence of the QCD critical endpoint, and signatures for chiral symmetry restoration. A modular detector concept has been developed which allows addressing all relevant observables. The challenge is to identify rare probes such as open charm, multi-strange baryons, anti-matter and virtual photons with high purity and to measure them with unprecedented statistics. To achieve this goal, the spectrometer has to tolerate very high interaction rates and the reconstruction has to provide very efficient background suppression capability. To cope with the shear amount of digital information produced by the freely streaming data acquisition system, CBM combines the detector systems with a High-Performance-Compute Cluster performing real time processing of the event information. This trigger-less concept will have to reconstruct all tracks online, and has to search for pattern characteristics of the observables under consideration. An important element of this strategy is the development of algorithms for pattern reconstruction, which provide robustness and selectivity and which have to make efficient use of features provided by modern many-core processor architectures. The realisation of the CBM experiment is well advanced. Technical Design Reports (TDR) on major experiment components have been submitted: the TDR on the superconducting dipole magnet, the Silicon Tracking System (STS), the Ring Imaging Cherenkov (RICH) detector, and the Projectile Spectator Detector (PSD). The TDRs on the time-of-flight (TOF) detector and the Muon Chambers (MUCH) will be submitted until the end of 2013. The TDRs on the Micro-vertex Detector (MVD), the Transition Radiation Detector (TDR), on the Data Acquisition (DAQ) system and on the First Level Event Selection (FLES) are in preparation and will be submitted in 2014. The activities at GSI are focused on the development of the following experimental components: The Silicon Tracking System including its read-out electronics, the DAQ and FLES system, the TOF read-out electronics, the simulation and analysis software, and the cave infrastructure.
Theory: The GSI activities on the theory of dense and hot strongly interacting matter are focused on three topics: understanding the QCD phase diagram and the properties of strongly interacting matter, modelling the dynamics of relativistic heavy-ion collisions and microscopic calculations of nuclear many-body systems at moderate densities and low temperatures. The properties of strongly interacting matter near the chiral and deconfinement phase transition are studied in effective chiral models as well as in lattice gauge theory. A focal point of the activities in the theory group has been critical fluctuations and the consequences for observables in nuclear collisions such as higher order cumulants of conserved charges. A qualitative agreement with RHIC data on cumulants of the net proton number is found. In recent work the nature of the deconfinement transition was explored in lattice QCD using fluctuations of the net baryon number and of the Polyakov loop. The goal of the microscopic calculations of the properties of bulk nuclear and neutron matter is to provide solid input for astrophysical applications in compact stars. An ab initio approach is pursued, using renormalization group techniques and finite-temperature perturbation theory with realistic two- and many-nucleon interactions as input. In order to obtain a deeper understanding of the deconfinement and chiral transitions of QCD, and to provide predictions for CBM@FAIR, detailed studies of fluctuations of net charges and of the Polyakov loop using effective models and lattice QCD will be continued. The results will be confronted with the results of the ongoing analysis of cumulants of net charges by the ALICE collaboration. The work on microscopic theory of nuclear many-body systems will be pursued, with the goal to obtain a systematic description of nuclear and neutron matter at low and moderate baryon densities.
Exotic Nuclei and Nuclear Astrophysics at NuSTAR
A large variety of forefront experiments has been carried out in the past years addressing modern questions of nuclear structure, nuclear astrophysics and reaction studies including superheavy element research. Together with theory, these achievements form the scientific and technical basis for the future experiments at GSI and FAIR. The following funding period will be governed by the transition from GSI to FAIR facilities. To fully devote all resources to the preparation and construction of the NuSTAR facility at FAIR, the experimental program at SIS-18 has been terminated at the end of 2012. Future beam times will be devoted to R&D efforts towards FAIR and, if beam time becomes available, concentrate on a small but high-level scientific program; its selection criteria are outstanding scientific impact and GSI-uniqueness (e.g. the need for intense 50Ti beams, kinetic energies above 400 MeV/u, high momentum resolution).
The main activities of GSI, in close connection with its strategic partners, will concentrate on the preparation and realization of the new NuSTAR experimental facility of the FAIR MSV. This comprises to provide scientific infrastructure activities, coordination and integration work as FAIR host laboratory, R&D contributions for and construction of the NuSTAR facility at FAIR, and pilot experiments using pre-existing instruments (such as the FRS as development platforms and test benches) and continuously implement and exploit the newly built experimental setups. The goal is to perform next-generation experiments on exotic nuclei with the Super-FRS and its instrumentation from 2020 on. The R&D and construction efforts focus primarily on the Super- FRS itself, which is the central device for all NuSTAR experiments at FAIR. Technical challenges are the construction of the high-power production target area, the high-power beam catchers, the remote-handling system in a high-radiation environment and the superconducting separator components. Also the R&D work for high-rate detectors (for tracking and particle identification) and data acquisition is demanding. Finally, simulation programs, slow controls and experimentspeci fic algorithms will be addressed. Tests with beam are foreseen wherever necessary. Besides the construction and procurement of individual components and sub-systems, the coordination, system integration, test and commissioning will be the main tasks of the laboratory. This applies also for the detector systems HISPEC/DESPEC, R3B and ILIMA, located at the three branches of the Super-FRS, which shall be ready for commissioning at the end of the forthcoming funding period. For NuSTAR@FAIR, with its specialized detector and distributed sub-systems, it is possible to combine R&D, prototyping, test and commissioning activities with pilot experiments in a synergetic manner, continuously transiting from GSI to FAIR. These pilot experiments will exploit the unique experimental capabilities of the novel equipment and beams available at GSI/FAIR. They go hand in hand with the theory activities outlined here.
Experiment and theory activities are widely synergetic and mutually stimulating. Nuclear structure and reactions for light nuclei in the p- and sd-shell will be studied in the framework of fermionic molecular dynamics (FMD). The flexibility of the Gaussian wave-packet basis allows a successful description of exotic features like clustering or halos. Three-body terms will be included in the effective interaction based on the symmetries of QCD providing a realistic interaction with predictive power. Observables like transition strengths, nucleon and cluster spectroscopic amplitudes, and form factors will provide a deeper understanding of exotic nuclei together with experiments at R3B and the low-energy branch. Reactions of astrophysical interest, like nucleonand alpha-capture, will be studied in the p-shell within the FMD framework. The FMD model can also be extendend to cover exotic hypernuclei.
Properties of the nuclear interaction can be studied by investigating short-range correlations in nuclei. Experimental efforts using quasi-free scattering will be complemented by calculated oneand two-nucleon momentum distributions and spectral functions. For this the power of unitary transformations will be combined with ab-initio many-body methods like the no-core shell model.
A generalized relativistic mean-field model with a non-trivial momentum dependence of the nucleon self-energies will be developed in order to improve the description of nuclei and nuclear matter. This feature is particularly important for the prediction of masses of exotic nuclei and their level densities, for optical potentials in (anti-)nucleon scattering on nuclei, the equation of state of dense matter and the simulation of heavy-ion collisions.