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The PANDA experiment at FAIR
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Nuclear Physics B (Proc. Suppl.) 245 (2013) 199–206 www.elsevier.com/locate/npbps
The PANDA experiment at FAIR M. Destefanis for the PANDA Collaboration INFN, Sezione di Torino
Abstract The PANDA (antiProton ANnihilation at DArmstadt) experiment is one of the major projects in preparation at the upcoming FAIR facility in Darmstadt, Germany. It will study interactions between antiprotons and protons or nuclei in the momentum range from 1.5 GeV/c to 15 GeV/c. The PANDA scientific program will address a wide range of topics, all aiming at improving our understanding of the strong interaction and hadron structure. The PANDA detector is a general-purpose spectrometer that will collect high quality and high statistics data in the fields of meson spectroscopy, baryon-antibaryon production, baryon spectroscopy, hypernuclear physics, hadron properties in the nuclear medium, and nucleon structure. This paper reviews some of the main physics topics of the experiment, together with a presentation of the detector. Keywords: PANDA, meson and baryon spectroscopy, strangeness, hypernuclei, electromagnetic processes
The future FAIR (Facility for Antiproton and Ion Research) facility, which is currently under construction at Darmstadt, Germany, will be an upgrade of the existing GSI laboratory (see Fig. 1), and it will provide high quality beams (protons, antiprotons, ions up to uranium, etc...). The facility will also be equipped with different storage rings. One of them, the High Energy Storage Ring (HESR), shown in Fig. 2, will be used to obtain extremely precise antiproton beams by means of stochastic and electron cooling systems. Those cooling systems will allow to operate at two different modes: a high resolution mode, characterized by L = 1031 cm−2 s−1 and δp/p = 10−5 , and a high luminosity mode, L = 2·1032 cm−2 s−1 and δp/p = 10−4 . The PANDA Collaboration [2] is planning to take advantage of these accessible high intensity, phase space cooled antiproton in a momentum range of 1.5 - 15 GeV/c, in order to address fundamental questions on QCD.
1. Panda Physics Program The physics program that PANDA wants to address is quite wide, and only few topics will be discussed here. 0920-5632/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nuclphysbps.2013.10.040
Figure 1: The new FAIR facility at GSI [1].
For a detailed description of all the physics measurements feasible in PANDA we refer to [2]. The main aim of the PANDA experiment is to improve our understanding of the strong interaction and hadron structure. By investigating processes characterized by a very low cross section, in some cases down to the nb region, taking advantage of the precision and high statistics available, important contributions are expected in the fields
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Figure 2: Layout of the HESR storage ring [2].
of: • Meson spectroscopy; • Charmed and multi-strange baryon spectroscopy; • Electromagnetic processes; • Properties of single and double hypernuclei; • Properties of hadrons in nuclear matter. During the past years, many new states in the charmonium region have been discovered [3] (see also the contribution of V. Mochalov in this volume). Unfortunately, due to the lack of statistics their quantum numbers still need a deeper investigation. In some cases, the resonance parameters as well as the nature of those states have still to be clarified. In this scenario, there is some disagreement between theory and experiment. The purpose of the PANDA Collaboration is to collect a high statistics in the resonance region in order to provide better measurements. The use of extremely precise beams is important to obtain an unprecedented resonance resolution. Since the production rate of a certain state can be calculated as the convolution of the Breit-Wigner cross section and the beam energy distribution function, the resonance mass, its width, and the product of the branching ratios of initial and final state can be extracted by measuring the formation rate for that resonance as a function of the center of mass energy. Therefore, a more precise beam implies a higher resolution in the resonance parameters. One of the main reasons to use pp ¯ interaction is that we could access mostly all the final states by direct valence quark annihilations. In e+ e− reactions, only PC −− states (J PC = 1−− ) are allowed, while in the PANDA scenario states with C-parity positive or negative can be formed. In order to access exotic quantum numbers, such as the ones expected for hybrids and glueballs, we plan to take advantage of production reactions, since those states cannot be directly created.
The glue tube (or flux-tube) adds degrees of freedom which may manifest in vibration of the tube and hence contribute to the quantum numbers of the state. In the simplest case, this corresponds to adding the quantum numbers of a gluon (J P = 1+ of 1− depending on the nature of the excitation, colour-electric or colour-magnetic respectively) to a simple qq¯ pair. Those states are called hybrids. If the final state is composed of only gluons, those states are identified as glueballs. The PANDA Collaboration plans to investigate the center of mass energy region between 3 and 5 GeV/c2 , where those states are expected to be seen. PANDA will offer the possibility to access also the strange hyperon sector. A satisfactory description of the hyperon states is still not available. Some of those states were predicted but not found and some others were not expected to be discovered. The PANDA experiment opens the possibility to access and study in detail all the so far known hyperons, as well as to search for new excited states. All those aspects of the PANDA physics are discussed in details in the same volume by V. Mochalov. 1.1. Electromagnetic Form Factors The structure of the nucleons can be investigated by means of electromagnetic probes. In reactions like pp ¯ → e+ e− , it will be possible to access the electric (GE ) and magnetic (G M ) form factors (FF), which parametrize the hadronic electromagnetic current. Those form factors are observables which can probe our understanding of the nucleon structure in both perturbative and nonperturbative regimes. In electron scattering, the space-like region (negative q2 ) can be addressed, while in annihilation processes, like the ones which will be available at PANDA, we can access the positive q2 values (time-like region). Space-like FF are real functions of q2 , and, on the contrary, the timelike FF are complex functions. Due to their analyticity, space-like and time-like form factors can be connected by applying the dispersion relations. So far, the modulus of G M was measured by almost all experiments by assuming GE = G M , a relation that is fulfilled strictly only at threshold. In Fig. 3, the world data available on the |G M | is shown, where the available data obtained in ¯ and e+ e− → γp p¯ reactions are pp ¯ → e+ e− , e+ e− → p p, included [4]. The prediction for PANDA are also presented [5] for an integrated luminosity of 2 fb−1 , corresponding to about 4 months of beam time using an extrapolation of the available cross sections. The magnetic form factor can be measured up to very large q2 , hence allowing for checks of pQCD predictions and asymptotic analytic properties of the form factors.
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Figure 4: Angular distributions of the expected yields in the pp ¯ → e+ e− reaction measurements with PANDA for different q2 values and different models [5], for an integrated luminosity of 2 fb−1 and assuming 100% efficiency and acceptance. Figure 3: q2 dependence of the world data on the effective proton TL FF, |G M |, extracted using |GE | = |G M | [5].
The purity of the lepton signal and the sensitivity to the shape of the angular distribution were investigated with detailed simulations. The pp ¯ → e+ e− integrated cross section was modelled [6] according to a G D , (1) |G M | = 2 1 + mq 2 a
where a = 22.5 is a normalization constant, and ma is a parameter which describes the deviation from a dipole (m2a = 3.6 ± 0.9 (GeV/c)2 ). G D indicates the usual dipole form factor and it can be expressed as 1
GD =
1+
q2 m2d
2 ,
(2)
where m2d = 0.71 (GeV/c)2 is the usual dipole-mass parameter. The e+ e− pairs angular distributions in the center of mass frame, presented in Fig. 4, were investigated for three different values of q2 (5.4, 8.2, and 13.8 (GeV/c)2 ), taking into account three different models: |GE | = 0 (full dots), |GE | = |G M | (triangular dots), and |GE | = 3|G M | (empty dots). Those studies showed that at higher q2 values, a lower sensitivity is expected. By measuring the angular distribution of the process pp ¯ → e+ e− , the PANDA experiment offers the unique possibility to determine the moduli of the complex form factors GE and G M . The investigation of the differential cross section values over a wide range of cos ϑ can lead to an independent determination of these two moduli. Moreover, in the same measurement, any sizable (above 5%) two-photon exchange contribution can be detected, because in this case a forward-backward asymmetry in the angular distribution, which is symmetric in onephoton exchange, should arise. In order to access an
otherwise inaccessible energy range below the threshold (i.e. q2 <4m2p , where m p is the proton mass) and measure the form factors down to lower q2 , the emission of a π by one of the pp ¯ in the initial state can be investigated [7]. As the initial state radiation (ISR) for e+ e− → γp p¯ processes, the pion emission would lower the q2 of the virtual photon at the annihilation vertex. The complete identification of the final state lepton pairs (e+ e− ) was required at the analysis stage. In the PANDA energy range, a very large background from hadronic processes is expected. The first source of background can be a misidentification of charged hadrons in binary reactions ( pp ¯ → π+ π− or K + K − ), which have a cross-section typically 106 times higher. A second source of background comes from reactions of the type pp ¯ → e+ e− X, where X can be any particle. In particular, the reaction pp ¯ → π0 π0 where one of the two π0 has a Dalitz decay or undergoes a photon conversion following the direct decay has a high probability but is easily suppressed by kinematical constraints. Special care was given to the case of the pp ¯ → π+ π− reaction, which is the most difficult to suppress. This background was simulated using event generators based on existing data [5]. Particle IDentification selections, using information from the different detectors (see section 2) and kinematical constraints could be optimized to reduce the π+ π− contribution to a level smaller than 0.1%. New simulations are on-going to check this result with an updated description of the detector geometry and an improved analysis software. The acceptance and reconstruction efficiency corrections were determined by means of an independent set of simulations in which the e+ e− were isotropically distributed. The global signal efficiency ranges from 0.4 to 0.15 depending on q2 . Those studies allowed to define the expected errors for the ratio
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R = |G E |/|G M | measurement. Figure 5 shows the expected error of R, in the case R = 1 (i.e. |GE | = |G M |), as function of q2 , in comparison with the data from PS170 (squares) and from the first BABAR publication (triangles) [5]. In the low q2 region, PANDA will be able to improve the error bars by an order of magnitude with respect to these data. In the most recent BABAR results, the error bars are reduced by about 30% [10]. In addition, it is expected that the BESIII experiment will produce results more precise than BABAR up to q2 = 9 (GeV/c)2 in the next years. It is however clear that PANDA will bring very useful measurements, especially in the high q2 region. The results in the case of the other models (|GE | = 0 and |GE | = 3|G M |) are similar.
The difficult interpretation of the experimental polarized cross sections in high energy hadron-hadron scattering suggests that other factors have to be taken into account [11]. The role of the intrinsic transverse momentum of the partons (kT ) in the hadron dynamics could be the key for this interpretation. In order to obtain a complete description of the nucleonic structure in a leading twist transverse momentum dependent (TMD) approach, the three PDFs above described are no more sufficient, and eight independent PDFs, functions of x and kT , are needed. Figure 6 shows such eight TMD ⊥ , and g1T are chirally even functions, PDFs: f1 , g1L , f1T ⊥ while h1 , h1L , h1T , and h⊥1T are chirally odd.
Figure 6: TMD PDF’s distributions. The arrows indicate the polarizations of the quark and of the parent hadron.
Figure 5: Expected error of the ratio R = |GE |/|G M | as function of q2 ; the band represents the expectations for PANDA measurements [5] in the pp ¯ → e+ e− reaction compared with PS170 (squares) [8] and BABAR (triangles) [9] data.
1.2. Drell-Yan Processes Both the parton distribution functions (PDF) and the fragmentation functions (FFs) are needed in order to provide a complete description of the nucleonic structure. The quark structure of a hadron is described at leading twist by: the unpolarized function f1 (x), the probability of finding a quark with a fraction x of the longitudinal momentum of the parent hadron regardless on the quarks spin orientation, the longitudinal polarization distribution g1 (x), the helicity distribution of the quarks, and the transverse polarization h1 (x), a.k.a. Transversity, which is the quarks transverse spin distribution inside a transversely polarized hadron. The Transversity has no probabilistic interpretation in the helicity basis; moreover it is chirally odd and hence it could not be extracted from the historical deep inelastic scattering (DIS) data [2].
Those functions have been and will be investigated in a wide energy range and with different beam/target configurations. Semi-Inclusive Deep Inelastic Scattering (SIDIS) experiments allow the extraction of the TMD PDFs convoluted with the FFs, posing hence experimental and theoretical challenges to their extraction. On the contrary, the Drell-Yan (DY) processes are giving a direct access to the TMD PDFs, since in such scenario experimental asymmetries depending on the TMD PDFs only can be defined. The DY is a process in which a quark-antiquark annihilation proceeds through a virtual photon into a final state containing a lepton pair: h1 h2 → γ∗ X → l+ l− X. For DY studies, the CollinsSoper frame [12] (shown in Fig. 7), i.e. the virtual photon rest frame, is usually adopted. In this frame the plane which contains the quark-antiquark momenta and the leptons production plane are considered. ϕ is defined as the angle between those two planes, and ϕS 1,2 is the angle between the nucleon spin (S 1,2 ) and the lepton plane. The differential cross sections of the completely unpolarized and single-polarized DY processes [13] show ⊥ , and h1T . The dependencies on h⊥1 , and on h⊥1 , f1T
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Figure 7: The virtual photon rest frame, so-called Collins-Soper frame [12], for the reaction h1 h2 → l+ l− X.
Boer-Mulders (BM) function h⊥1 describes the distribution of transversely polarized quarks in an unpolarized ⊥ is the so-called Sivers function, which dehadron. f1T scribes the distribution of unpolarized quarks inside a transversely polarized parent hadron. The density of transversely polarized quarks into a transversely polarized hadron is described by the Transversity function h1T . These functions can be obtained from experimental asymmetries weighted by the following azimuthal angular terms: cos 2ϕ, sin (ϕ − ϕS 2 ), and sin (ϕ + ϕS 2 ), respectively, as described in details in [2]. An effective test of QCD Universality could be provided by the comparison of the Sivers distribution function obtained in SIDIS and DY processes. The Wilsonlines, which ensure the colour gauge invariance, that are used to define the parton density are expected to lead ⊥ ) when exto opposite signs for the Sivers function ( f1T ⊥ ⊥ |S IDIS tracted from DY or from SIDIS data: f1T |DY = - f1T [14]. FAIR will √ give access to a unique kinematic region. At large s, as for example 200 GeV available at high energy experiments, one can access TMD PDFs mostly related to the sea quarks. On the contrary, the FAIR scenario is √ characterized by a lower center of mass energy range ( s ≈ 5.5 GeV) and by the availability of antiproton beams: each valence quark can take part to the DY diagram and hence the contributions of the valence quarks to the TMD PDFs could be directly accessed free of any convolution with sea-quark PDFs. The PANDA Collaboration is planning to investigate the unpolarized and, possibly, the single-polarized processes. In the PANDA scenario, DY processes of the type pp ¯ → l+ l− X have been investigated. The number of e+ e− pairs per event is supposed to be quite high, due not only to particle decays but also to the secondary interactions which can occur inside the spectrometer. Hence, the decay channel μ+ μ− is preferred in order to have a better identification of the Drell-Yan events. At s ≈ 30 GeV2 , the maximum provided center of mass energy, the expected cross section is of the order of 1 nb. The most relevant source of background are expected to be
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the pionic final states in reactions like pp ¯ → n(π+ π− )X, where the factor n indicates the number of charged pion pairs. In the considered energy regime, primary pions could reach the muon detector system and, hence, generate a fake signal. Moreover, since muons and pions have an almost similar mass, in case of pion decays into muons inside our magnetic field, the tilt angle between the pion and the decay muon momenta have to be big enough to allow to disentangle the two particles. At the center of mass energy of s ≈ 30 GeV2 , the pion pair production cross section is estimated to be ∼ 20-30 μb. A background rejection factor of the order of 107 is hence required. The dilepton mass range just above the J/ψ peak (from 4 to 9 GeV/c2 ) and showing no contribution from resonances is normally considered the “safe region” to investigate DY processes. Unfortunately, at the considered energies this region is characterized by an extremely low cross section. An alternative region characterized by a dilepton mass ranging from 1.5 to 2.5 GeV/c2 can be considered as a “safe region”, since free from resonances: in such a region the cross section is expected to be larger, and probing such a dilepton mass region as well allows to investigate larger kinematic region. The cross section of the two mentioned regions differs by 3 orders of magnitude. Moreover, simulations show that DY events accumulate at lower dilepton mass values. In order to investigate the production of DY muon pairs in PANDA, we used the event generator provided by A. Bianconi [15]. It provides final states containing muon pairs produced in p¯ and π− interactions with unpolarized or polarized nuclear targets making use of the data available in the literature. Simulations show that the DY events are boosted to the forward angles. Hence, most of the signal consists of two muons which should cross the end cap of the PANDA Target Spectrometer. In Fig. 8, the simulated Drell-Yan asymmetries for the unpolarized and singlepolarized cases are plotted as function of the longitudinal momentum fraction of the hadronic probe x p , and they are presented for two different muon pair transverse momenta intervals: 1 ≤ qT ≤ 2 GeV/c (square dots), and 2 ≤ qT ≤ 3 GeV/c (triangular dots). Efficiencies and acceptance corrections are still under investigation. Those plots are not intended to reflect a detailed expectation of the asymmetry itself, because the parton dynamics is roughly estimated and questionable; the key point is the size of the error bars, that allows to probe the feasibility of such a measurement. In the unpolarized case the asymmetries are expected to be small but not negligible; it should anyhow be possible to study the asymmetry de-
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pendency on the lepton pair transverse momentum. Taking advantage of a scan on the full transverse momentum range, a possible inversion of such dependence can be investigated; this would allow to probe the balance between soft and hard processes for DY production of lepton pairs. The simulated distributions for the singlepolarized case suggest that an investigation of the possible dependence of the experimental asymmetries on the transverse momentum of the lepton pair could be more difficult or eventually impossible, depending on the experimental asymmetry [16]. Background rejection studies based on the events kinematic and on a kinematically constrained refit of the track candidates are ongoing.
Figure 8: Simulated experimental asymmetry, from 5·105 simulated √ events, for the DY process for the reaction pp ¯ → μ+ μ− X at s = 5.5 GeV related to the cos 2ϕ (upper), sin (ϕ + ϕS 2 ) (middle), and sin (ϕ + ϕS 2 ) (lower) term plotted as a function of x p , longitudinal momentum fraction of the hadronic probe, presented for two different muon pair transverse momenta intervals: 1 ≤ qT ≤ 2 GeV/c (square dots), and 2 ≤ qT ≤ 3 GeV/c (triangular dots).
1.3. Hypernuclear Physics The PANDA Collaboration is also planning to investigate the binding of nucleons and nuclei. When a hyperon is bound inside a nucleus, it should offer a probe
of the hadronic many-body problem, since in populating all the possible nuclear states it is not restricted by the Pauli principle. At PANDA it will be possible to investigate the nuclear structure and its possible modification due to the presence of hyperons as well as to understand their properties, which may change, even dramatically, when implanted inside a nucleus. The presence of a Λ particle, which is not suffering from the Pauli blocking, may give rise to new nuclear structures that cannot be seen in normal nuclei. Moreover, the comparison between ordinary nuclei and hypernuclei could deliver new informations about key questions in nuclear physics, as the origin of the nuclear spin-orbit force. Hence, the key issue is to measure the hypernuclei level spectra and decay properties, in order to test the energies and the wave functions from microscopic structure models. Due to their short lifetimes, hyperon targets are not available. Moreover, there are few experimental data on Λ−N and Σ± −N scattering, while for Ξ−N or Ω−N essentially no data is available. In single hypernuclei, the strength of the Λ − N strong interaction can be extracted and the decomposition into different spin dependent contributions can be investigated as well. The PANDA experiment will also provide a unique approach to explore the hyperon-hyperon interaction. When bound in a nucleus, the Λ mesonic decay (Nπ) is disfavored by the Pauli principle, but still sizable for light nuclei. On the contrary, processes like ΛN → NN, ΛΛ → ΛN, and ΛΛ → ΣN are allowed, giving hence a unique access to the ΛΛK coupling, due to the dominance of kaon exchange processes. The use of Ξ− (S = -2) to form Ξ− atoms, Ξ− -hypernuclei and double ΛΛ hypernuclei can provide complementary informations about the baryonbaryon interaction. At PANDA the use of Ω− atoms is also foreseen: the interest on the Ω hyperon comes from the fact that it is the only elementary baryon with a non vanishing spectroscopic quadrupole moment. This behavior is determined by the one-gluon exchange contribution to the quark-quark interaction [2]. The production of Ξ hypernuclei proceeds through the reactions pp ¯ → Ξ− Ξ¯ + and pn ¯ → Ξ− Ξ¯ 0 followed − by rescattering of the Ξ within the primary target nucleus. The Ξ− is then stopped in an external secondary target to form the Ξ hypernucleus, which will be converted into a double Λ hypernucleus. The sequence of a double hypernucleus production in PANDA is depicted in Fig. 9. To perform those studies, the PANDA detector will be equipped with a primary carbon target at the entrance to the central tracking detector. For rescattering purposes, target nuclei with larger mass are more efficient,
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tions show that most of the produced charged and neutral particles are emitted into the forward region, which is not covered by the γ-rays detector. The particles emitted at backward angles will have a low kinetic energy and will be absorbed in the material surrounding the primary target. The most critical particles are the neutrons, which contribute to the radiation damage of the detector. 2. Panda Detector
Figure 9: Double hypernucleus production at PANDA [2].
but they have to be arranged to reduce the antiprotons losses due to the Coulomb scattering. At the incident momentum of 3 GeV/c, a cross section of around 2 μb is expected, and hence 7·105 Ξ− Ξ¯ pairs produced per hour. The secondary target will be composed of silicon detectors and 9 Be, 10,11 B, or 12,13 C absorbers arranged in a sandwich like structure. The task of the secondary detector is to slow down and stop the Ξ− and to identify the weak decay products. In order to limit the number of possible transitions, nuclei with low mass number were selected. In the 12 C scenario, per stopped Ξ− we expect a total double Λ hypernucleus production probability of around 4.7% [17]. For light nuclei, even a relatively small excitation energy can be comparable with their binding energy. The principal mechanism of de-excitation is an explosive decay of the excited nucleus into several smaller clusters. In our studies, the fragments masses in excited states were computed by adding the corresponding excitation energy to their ground state masses. According to the performed calculations, we expect a significant probability to produce double hypernuclei excited states. The γ emitted isotropically by the de-excitation of the double hypernuclei will be detected by a γ-rays detector placed close to the target region. If we consider only the mesonic decays, the momenta of the two emitted pions are strongly correlated, and their reconstruction will give the opportunity to tag the production of a double hypernucleus. The emitted pions will be tracked in the silicon strip detectors, characterized by a rather high efficiency, of the secondary target. Few weeks of data taking will be already enough to see the signals of the double hypernuclei states. Background studies are still ongoing. The simula-
In order to perform its wide physics program, the Panda Collaboration proposed to build a state-of-theart universal detector. The detector is designed to take advantage of the extraordinary physics potential which will be available utilizing high intensity, phase space cooled antiproton beams in a momentum range of 1.5 ÷ 15 GeV/c. The main achievements foreseen in the design of the spectrometer are an almost 4π acceptance, high resolution for tracking, particle identification (PID), and calorimetry, and high rate capabilities in order to reach the desired luminosity. In order to achieve a good momentum resolution, the detector, shown in Fig. 10, is composed of two spectrometers. The target spectrometer (TS) surrounds the interaction region, and the forward spectrometer (FS) with a second magnet provides angular coverage for the most forward angles. The target spectrometer covers angles larger than 5◦ and 10◦ in the vertical and horizontal plane respectively, and its basic concept is a shell-like arrangement of various detector systems surrounding the interaction point inside the field of a large solenoid.
Figure 10: 3D view of the PANDA spectrometer [18].
To reach the design luminosity of 2·1032 cm−2 s−1 , assuming 1011 stored antiprotons, a target thickness of about 4·1015 hydrogen atoms per cm2 is required. With this aim, two different types of targets will be used.
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The peculiarities of the pellet target, composed of a stream of frozen hydrogen micro-spheres (the so-called pellets), will allow to reach the expected luminosity. The cluster-jet target, characterized by a density of 1015 atoms/cm3 , acts as a very diluted target with an homogeneous density profile. The target spectrometer (TS) is an azimuthally symmetric system of detectors mostly contained inside the superconducting solenoid, characterized by a magnetic field of 2 T with homogeneity better than 2%, while the forward region (FS) will be equipped with a dipole with an integrated field of 2 Tm. The Micro Vertex Detector (MVD) is the detector closest to the interaction point. It is composed of 4 barrel layers and 6 forward disks. The construction of the detector implies the use of hybrid pixel technology, which is now under development, as well as silicon strips. It is optimized for the detection of secondary vertices coming from D and hyperon decays. The spatial and momentum resolutions are expected to be lower than 100 μm and around 2%, respectively. The Straw Tube Tracker (STT) is the central tracker of the experiment. They are composed of aluminized mylar tubes (straws) which are self supporting. The detector layers will be arranged in a honeycomb structure. The central layers will be placed with a skew angle of 3◦ to avoid ambiguities in the detection. The spatial resolution is expected to be σ xy ∼ 150 μm and σz ∼ 1 mm, and the momentum one on the percent level. In the forward region, for particle tracking purposes Gas Electron Multiplier (GEM) planes are foreseen due to their high rate capabilities. The charged particle identification is crucial to achieve all the physics goals of the experiment. The main part of the momentum spectrum above 1 GeV/c will be covered by Cherenkov detectors. At polar angles between 22◦ and 140◦ the PID will be performed by the detection of internally reflected Cherenkov (DIRC) light. The DIRC is composed of quartz slabs surrounding the central tracker. The light produced by a particle which crosses a quartz bar will be then collected by an array of photomultipliers. This detector will provide pion-kaon separation for momenta higher than 800 MeV/c. The energy loss (dE/dx) measurements will also be crucial to separate pions, kaons, protons, and electrons. RICH detectors will be used to identify the fast particles in the forward region. Another technique which will be exploited is the measurement of the Time of Flight (TOF) of the charged particles. TOF detectors will be placed after the DIRC in the TS region as well as in the forward part. For the muon identification, Iarocci tubes (Mini Drift Tubes, or MDT) operating in propor-
tional mode will be used. A fiberglass strip boards will allow for the read out of the second coordinate by collecting the induced charge. The MDT stations will be placed in the barrel segmented yoke as well as in the forward direction. The detection of the electrons and gammas will be performed by means of ElectroMagnetic Calorimeters (EMC) surrounding the interaction region and the most forward part. As scintillator material, the lead tungstate (PbWO4 ), a high density inorganic scintillator, was chosen for the TS region. It is characterized by a short decay time (< 10 ns) and a good radiation hardness. In order to increase its light yield, the 20 cm long (≈ 22 X0 ) scintillator crystals have to be cooled down to -25◦ C. In the FS the Collaboration is planning to use a shashlik calorimeter, composed of layers of lead and scintillating fibers. As hadronic calorimeter in the forward region, the Range System will be used. For the hypernuclear physics program, the plan is to remove the MVD and the backward end cap EMC in order to insert inside the apparatus the primary and the secondary targets needed, and the germanium-array detector. This detector will be composed of germaniumarray crystals grouped asymmetrically to form triple clusters and it will be devoted to γ-spectroscopy. A very reach and versatile program will be accomplished by the PANDA Collaboration by means of p¯ interactions. New detector concepts and technologies as well as detector materials have been developed. It offers clearly a unique opportunity to contribute to the hadron structure and dynamics understanding in the transition region between perturbative QCD and strong QCD. References [1] https://www.gsi.de/en/start/fair/aufbau der fair beschleunigeranlage. htm [2] W. Erni et al., Physics Performance Report for: PANDA, Strong Interaction Studies with Antiproton, (2009) arXiv:0903.3905. [3] N. Brambilla et al., hep-ph/0412158 (2004). [4] A. Denig, and G. Salm`e, Progress in Particle and Nuclear Physics 68 (2013) 113-157. [5] M. Sudol et al., EPJ A44 (2010) 373. [6] E. Tomasi-Gustafsson, and M. Rekalo, Phys. Lett. B 504 (2001) 291-295. [7] J. Boucher, PhD Thesis (2011). [8] G. Bardin et al., Nucl. Phys. B 411 (1994) 3. [9] B. Aubert et al., Phys. Rev. D 73 (2006) 012005. [10] J.P. Lees et al., Phys. Rev. D87 (2013) 092005. [11] C. Bourrely, and J. Soffer, EPJ C36 (2004) 371. [12] J.C. Collins, and D.E. Soper, Phys. Rev. D16 (1977) 2219. [13] R.D. Tangerman, and P.J. Mulders, Phys. Rev. D51 (1995) 3357. [14] J.C. Collins, Phys. Lett. B 536 (2002) 43-48. [15] A. Bianconi, NIM A593 (2008) 562-571. [16] M. Destefanis, PoS(BORMIO 2011) 012. [17] T. Yamada, and K. Ikeda, Phys. Rev. C 56 (1997) 3216. [18] PANDA Collaboration, TDR for the: PANDA Muon System
Nuclear Physics B (Proc. Suppl.) 245 (2013) 199–206 www.elsevier.com/locate/npbps
The PANDA experiment at FAIR M. Destefanis for the PANDA Collaboration INFN, Sezione di Torino
Abstract The PANDA (antiProton ANnihilation at DArmstadt) experiment is one of the major projects in preparation at the upcoming FAIR facility in Darmstadt, Germany. It will study interactions between antiprotons and protons or nuclei in the momentum range from 1.5 GeV/c to 15 GeV/c. The PANDA scientific program will address a wide range of topics, all aiming at improving our understanding of the strong interaction and hadron structure. The PANDA detector is a general-purpose spectrometer that will collect high quality and high statistics data in the fields of meson spectroscopy, baryon-antibaryon production, baryon spectroscopy, hypernuclear physics, hadron properties in the nuclear medium, and nucleon structure. This paper reviews some of the main physics topics of the experiment, together with a presentation of the detector. Keywords: PANDA, meson and baryon spectroscopy, strangeness, hypernuclei, electromagnetic processes
The future FAIR (Facility for Antiproton and Ion Research) facility, which is currently under construction at Darmstadt, Germany, will be an upgrade of the existing GSI laboratory (see Fig. 1), and it will provide high quality beams (protons, antiprotons, ions up to uranium, etc...). The facility will also be equipped with different storage rings. One of them, the High Energy Storage Ring (HESR), shown in Fig. 2, will be used to obtain extremely precise antiproton beams by means of stochastic and electron cooling systems. Those cooling systems will allow to operate at two different modes: a high resolution mode, characterized by L = 1031 cm−2 s−1 and δp/p = 10−5 , and a high luminosity mode, L = 2·1032 cm−2 s−1 and δp/p = 10−4 . The PANDA Collaboration [2] is planning to take advantage of these accessible high intensity, phase space cooled antiproton in a momentum range of 1.5 - 15 GeV/c, in order to address fundamental questions on QCD.
1. Panda Physics Program The physics program that PANDA wants to address is quite wide, and only few topics will be discussed here. 0920-5632/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nuclphysbps.2013.10.040
Figure 1: The new FAIR facility at GSI [1].
For a detailed description of all the physics measurements feasible in PANDA we refer to [2]. The main aim of the PANDA experiment is to improve our understanding of the strong interaction and hadron structure. By investigating processes characterized by a very low cross section, in some cases down to the nb region, taking advantage of the precision and high statistics available, important contributions are expected in the fields
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Figure 2: Layout of the HESR storage ring [2].
of: • Meson spectroscopy; • Charmed and multi-strange baryon spectroscopy; • Electromagnetic processes; • Properties of single and double hypernuclei; • Properties of hadrons in nuclear matter. During the past years, many new states in the charmonium region have been discovered [3] (see also the contribution of V. Mochalov in this volume). Unfortunately, due to the lack of statistics their quantum numbers still need a deeper investigation. In some cases, the resonance parameters as well as the nature of those states have still to be clarified. In this scenario, there is some disagreement between theory and experiment. The purpose of the PANDA Collaboration is to collect a high statistics in the resonance region in order to provide better measurements. The use of extremely precise beams is important to obtain an unprecedented resonance resolution. Since the production rate of a certain state can be calculated as the convolution of the Breit-Wigner cross section and the beam energy distribution function, the resonance mass, its width, and the product of the branching ratios of initial and final state can be extracted by measuring the formation rate for that resonance as a function of the center of mass energy. Therefore, a more precise beam implies a higher resolution in the resonance parameters. One of the main reasons to use pp ¯ interaction is that we could access mostly all the final states by direct valence quark annihilations. In e+ e− reactions, only PC −− states (J PC = 1−− ) are allowed, while in the PANDA scenario states with C-parity positive or negative can be formed. In order to access exotic quantum numbers, such as the ones expected for hybrids and glueballs, we plan to take advantage of production reactions, since those states cannot be directly created.
The glue tube (or flux-tube) adds degrees of freedom which may manifest in vibration of the tube and hence contribute to the quantum numbers of the state. In the simplest case, this corresponds to adding the quantum numbers of a gluon (J P = 1+ of 1− depending on the nature of the excitation, colour-electric or colour-magnetic respectively) to a simple qq¯ pair. Those states are called hybrids. If the final state is composed of only gluons, those states are identified as glueballs. The PANDA Collaboration plans to investigate the center of mass energy region between 3 and 5 GeV/c2 , where those states are expected to be seen. PANDA will offer the possibility to access also the strange hyperon sector. A satisfactory description of the hyperon states is still not available. Some of those states were predicted but not found and some others were not expected to be discovered. The PANDA experiment opens the possibility to access and study in detail all the so far known hyperons, as well as to search for new excited states. All those aspects of the PANDA physics are discussed in details in the same volume by V. Mochalov. 1.1. Electromagnetic Form Factors The structure of the nucleons can be investigated by means of electromagnetic probes. In reactions like pp ¯ → e+ e− , it will be possible to access the electric (GE ) and magnetic (G M ) form factors (FF), which parametrize the hadronic electromagnetic current. Those form factors are observables which can probe our understanding of the nucleon structure in both perturbative and nonperturbative regimes. In electron scattering, the space-like region (negative q2 ) can be addressed, while in annihilation processes, like the ones which will be available at PANDA, we can access the positive q2 values (time-like region). Space-like FF are real functions of q2 , and, on the contrary, the timelike FF are complex functions. Due to their analyticity, space-like and time-like form factors can be connected by applying the dispersion relations. So far, the modulus of G M was measured by almost all experiments by assuming GE = G M , a relation that is fulfilled strictly only at threshold. In Fig. 3, the world data available on the |G M | is shown, where the available data obtained in ¯ and e+ e− → γp p¯ reactions are pp ¯ → e+ e− , e+ e− → p p, included [4]. The prediction for PANDA are also presented [5] for an integrated luminosity of 2 fb−1 , corresponding to about 4 months of beam time using an extrapolation of the available cross sections. The magnetic form factor can be measured up to very large q2 , hence allowing for checks of pQCD predictions and asymptotic analytic properties of the form factors.
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Figure 4: Angular distributions of the expected yields in the pp ¯ → e+ e− reaction measurements with PANDA for different q2 values and different models [5], for an integrated luminosity of 2 fb−1 and assuming 100% efficiency and acceptance. Figure 3: q2 dependence of the world data on the effective proton TL FF, |G M |, extracted using |GE | = |G M | [5].
The purity of the lepton signal and the sensitivity to the shape of the angular distribution were investigated with detailed simulations. The pp ¯ → e+ e− integrated cross section was modelled [6] according to a G D , (1) |G M | = 2 1 + mq 2 a
where a = 22.5 is a normalization constant, and ma is a parameter which describes the deviation from a dipole (m2a = 3.6 ± 0.9 (GeV/c)2 ). G D indicates the usual dipole form factor and it can be expressed as 1
GD =
1+
q2 m2d
2 ,
(2)
where m2d = 0.71 (GeV/c)2 is the usual dipole-mass parameter. The e+ e− pairs angular distributions in the center of mass frame, presented in Fig. 4, were investigated for three different values of q2 (5.4, 8.2, and 13.8 (GeV/c)2 ), taking into account three different models: |GE | = 0 (full dots), |GE | = |G M | (triangular dots), and |GE | = 3|G M | (empty dots). Those studies showed that at higher q2 values, a lower sensitivity is expected. By measuring the angular distribution of the process pp ¯ → e+ e− , the PANDA experiment offers the unique possibility to determine the moduli of the complex form factors GE and G M . The investigation of the differential cross section values over a wide range of cos ϑ can lead to an independent determination of these two moduli. Moreover, in the same measurement, any sizable (above 5%) two-photon exchange contribution can be detected, because in this case a forward-backward asymmetry in the angular distribution, which is symmetric in onephoton exchange, should arise. In order to access an
otherwise inaccessible energy range below the threshold (i.e. q2 <4m2p , where m p is the proton mass) and measure the form factors down to lower q2 , the emission of a π by one of the pp ¯ in the initial state can be investigated [7]. As the initial state radiation (ISR) for e+ e− → γp p¯ processes, the pion emission would lower the q2 of the virtual photon at the annihilation vertex. The complete identification of the final state lepton pairs (e+ e− ) was required at the analysis stage. In the PANDA energy range, a very large background from hadronic processes is expected. The first source of background can be a misidentification of charged hadrons in binary reactions ( pp ¯ → π+ π− or K + K − ), which have a cross-section typically 106 times higher. A second source of background comes from reactions of the type pp ¯ → e+ e− X, where X can be any particle. In particular, the reaction pp ¯ → π0 π0 where one of the two π0 has a Dalitz decay or undergoes a photon conversion following the direct decay has a high probability but is easily suppressed by kinematical constraints. Special care was given to the case of the pp ¯ → π+ π− reaction, which is the most difficult to suppress. This background was simulated using event generators based on existing data [5]. Particle IDentification selections, using information from the different detectors (see section 2) and kinematical constraints could be optimized to reduce the π+ π− contribution to a level smaller than 0.1%. New simulations are on-going to check this result with an updated description of the detector geometry and an improved analysis software. The acceptance and reconstruction efficiency corrections were determined by means of an independent set of simulations in which the e+ e− were isotropically distributed. The global signal efficiency ranges from 0.4 to 0.15 depending on q2 . Those studies allowed to define the expected errors for the ratio
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R = |G E |/|G M | measurement. Figure 5 shows the expected error of R, in the case R = 1 (i.e. |GE | = |G M |), as function of q2 , in comparison with the data from PS170 (squares) and from the first BABAR publication (triangles) [5]. In the low q2 region, PANDA will be able to improve the error bars by an order of magnitude with respect to these data. In the most recent BABAR results, the error bars are reduced by about 30% [10]. In addition, it is expected that the BESIII experiment will produce results more precise than BABAR up to q2 = 9 (GeV/c)2 in the next years. It is however clear that PANDA will bring very useful measurements, especially in the high q2 region. The results in the case of the other models (|GE | = 0 and |GE | = 3|G M |) are similar.
The difficult interpretation of the experimental polarized cross sections in high energy hadron-hadron scattering suggests that other factors have to be taken into account [11]. The role of the intrinsic transverse momentum of the partons (kT ) in the hadron dynamics could be the key for this interpretation. In order to obtain a complete description of the nucleonic structure in a leading twist transverse momentum dependent (TMD) approach, the three PDFs above described are no more sufficient, and eight independent PDFs, functions of x and kT , are needed. Figure 6 shows such eight TMD ⊥ , and g1T are chirally even functions, PDFs: f1 , g1L , f1T ⊥ while h1 , h1L , h1T , and h⊥1T are chirally odd.
Figure 6: TMD PDF’s distributions. The arrows indicate the polarizations of the quark and of the parent hadron.
Figure 5: Expected error of the ratio R = |GE |/|G M | as function of q2 ; the band represents the expectations for PANDA measurements [5] in the pp ¯ → e+ e− reaction compared with PS170 (squares) [8] and BABAR (triangles) [9] data.
1.2. Drell-Yan Processes Both the parton distribution functions (PDF) and the fragmentation functions (FFs) are needed in order to provide a complete description of the nucleonic structure. The quark structure of a hadron is described at leading twist by: the unpolarized function f1 (x), the probability of finding a quark with a fraction x of the longitudinal momentum of the parent hadron regardless on the quarks spin orientation, the longitudinal polarization distribution g1 (x), the helicity distribution of the quarks, and the transverse polarization h1 (x), a.k.a. Transversity, which is the quarks transverse spin distribution inside a transversely polarized hadron. The Transversity has no probabilistic interpretation in the helicity basis; moreover it is chirally odd and hence it could not be extracted from the historical deep inelastic scattering (DIS) data [2].
Those functions have been and will be investigated in a wide energy range and with different beam/target configurations. Semi-Inclusive Deep Inelastic Scattering (SIDIS) experiments allow the extraction of the TMD PDFs convoluted with the FFs, posing hence experimental and theoretical challenges to their extraction. On the contrary, the Drell-Yan (DY) processes are giving a direct access to the TMD PDFs, since in such scenario experimental asymmetries depending on the TMD PDFs only can be defined. The DY is a process in which a quark-antiquark annihilation proceeds through a virtual photon into a final state containing a lepton pair: h1 h2 → γ∗ X → l+ l− X. For DY studies, the CollinsSoper frame [12] (shown in Fig. 7), i.e. the virtual photon rest frame, is usually adopted. In this frame the plane which contains the quark-antiquark momenta and the leptons production plane are considered. ϕ is defined as the angle between those two planes, and ϕS 1,2 is the angle between the nucleon spin (S 1,2 ) and the lepton plane. The differential cross sections of the completely unpolarized and single-polarized DY processes [13] show ⊥ , and h1T . The dependencies on h⊥1 , and on h⊥1 , f1T
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Figure 7: The virtual photon rest frame, so-called Collins-Soper frame [12], for the reaction h1 h2 → l+ l− X.
Boer-Mulders (BM) function h⊥1 describes the distribution of transversely polarized quarks in an unpolarized ⊥ is the so-called Sivers function, which dehadron. f1T scribes the distribution of unpolarized quarks inside a transversely polarized parent hadron. The density of transversely polarized quarks into a transversely polarized hadron is described by the Transversity function h1T . These functions can be obtained from experimental asymmetries weighted by the following azimuthal angular terms: cos 2ϕ, sin (ϕ − ϕS 2 ), and sin (ϕ + ϕS 2 ), respectively, as described in details in [2]. An effective test of QCD Universality could be provided by the comparison of the Sivers distribution function obtained in SIDIS and DY processes. The Wilsonlines, which ensure the colour gauge invariance, that are used to define the parton density are expected to lead ⊥ ) when exto opposite signs for the Sivers function ( f1T ⊥ ⊥ |S IDIS tracted from DY or from SIDIS data: f1T |DY = - f1T [14]. FAIR will √ give access to a unique kinematic region. At large s, as for example 200 GeV available at high energy experiments, one can access TMD PDFs mostly related to the sea quarks. On the contrary, the FAIR scenario is √ characterized by a lower center of mass energy range ( s ≈ 5.5 GeV) and by the availability of antiproton beams: each valence quark can take part to the DY diagram and hence the contributions of the valence quarks to the TMD PDFs could be directly accessed free of any convolution with sea-quark PDFs. The PANDA Collaboration is planning to investigate the unpolarized and, possibly, the single-polarized processes. In the PANDA scenario, DY processes of the type pp ¯ → l+ l− X have been investigated. The number of e+ e− pairs per event is supposed to be quite high, due not only to particle decays but also to the secondary interactions which can occur inside the spectrometer. Hence, the decay channel μ+ μ− is preferred in order to have a better identification of the Drell-Yan events. At s ≈ 30 GeV2 , the maximum provided center of mass energy, the expected cross section is of the order of 1 nb. The most relevant source of background are expected to be
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the pionic final states in reactions like pp ¯ → n(π+ π− )X, where the factor n indicates the number of charged pion pairs. In the considered energy regime, primary pions could reach the muon detector system and, hence, generate a fake signal. Moreover, since muons and pions have an almost similar mass, in case of pion decays into muons inside our magnetic field, the tilt angle between the pion and the decay muon momenta have to be big enough to allow to disentangle the two particles. At the center of mass energy of s ≈ 30 GeV2 , the pion pair production cross section is estimated to be ∼ 20-30 μb. A background rejection factor of the order of 107 is hence required. The dilepton mass range just above the J/ψ peak (from 4 to 9 GeV/c2 ) and showing no contribution from resonances is normally considered the “safe region” to investigate DY processes. Unfortunately, at the considered energies this region is characterized by an extremely low cross section. An alternative region characterized by a dilepton mass ranging from 1.5 to 2.5 GeV/c2 can be considered as a “safe region”, since free from resonances: in such a region the cross section is expected to be larger, and probing such a dilepton mass region as well allows to investigate larger kinematic region. The cross section of the two mentioned regions differs by 3 orders of magnitude. Moreover, simulations show that DY events accumulate at lower dilepton mass values. In order to investigate the production of DY muon pairs in PANDA, we used the event generator provided by A. Bianconi [15]. It provides final states containing muon pairs produced in p¯ and π− interactions with unpolarized or polarized nuclear targets making use of the data available in the literature. Simulations show that the DY events are boosted to the forward angles. Hence, most of the signal consists of two muons which should cross the end cap of the PANDA Target Spectrometer. In Fig. 8, the simulated Drell-Yan asymmetries for the unpolarized and singlepolarized cases are plotted as function of the longitudinal momentum fraction of the hadronic probe x p , and they are presented for two different muon pair transverse momenta intervals: 1 ≤ qT ≤ 2 GeV/c (square dots), and 2 ≤ qT ≤ 3 GeV/c (triangular dots). Efficiencies and acceptance corrections are still under investigation. Those plots are not intended to reflect a detailed expectation of the asymmetry itself, because the parton dynamics is roughly estimated and questionable; the key point is the size of the error bars, that allows to probe the feasibility of such a measurement. In the unpolarized case the asymmetries are expected to be small but not negligible; it should anyhow be possible to study the asymmetry de-
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pendency on the lepton pair transverse momentum. Taking advantage of a scan on the full transverse momentum range, a possible inversion of such dependence can be investigated; this would allow to probe the balance between soft and hard processes for DY production of lepton pairs. The simulated distributions for the singlepolarized case suggest that an investigation of the possible dependence of the experimental asymmetries on the transverse momentum of the lepton pair could be more difficult or eventually impossible, depending on the experimental asymmetry [16]. Background rejection studies based on the events kinematic and on a kinematically constrained refit of the track candidates are ongoing.
Figure 8: Simulated experimental asymmetry, from 5·105 simulated √ events, for the DY process for the reaction pp ¯ → μ+ μ− X at s = 5.5 GeV related to the cos 2ϕ (upper), sin (ϕ + ϕS 2 ) (middle), and sin (ϕ + ϕS 2 ) (lower) term plotted as a function of x p , longitudinal momentum fraction of the hadronic probe, presented for two different muon pair transverse momenta intervals: 1 ≤ qT ≤ 2 GeV/c (square dots), and 2 ≤ qT ≤ 3 GeV/c (triangular dots).
1.3. Hypernuclear Physics The PANDA Collaboration is also planning to investigate the binding of nucleons and nuclei. When a hyperon is bound inside a nucleus, it should offer a probe
of the hadronic many-body problem, since in populating all the possible nuclear states it is not restricted by the Pauli principle. At PANDA it will be possible to investigate the nuclear structure and its possible modification due to the presence of hyperons as well as to understand their properties, which may change, even dramatically, when implanted inside a nucleus. The presence of a Λ particle, which is not suffering from the Pauli blocking, may give rise to new nuclear structures that cannot be seen in normal nuclei. Moreover, the comparison between ordinary nuclei and hypernuclei could deliver new informations about key questions in nuclear physics, as the origin of the nuclear spin-orbit force. Hence, the key issue is to measure the hypernuclei level spectra and decay properties, in order to test the energies and the wave functions from microscopic structure models. Due to their short lifetimes, hyperon targets are not available. Moreover, there are few experimental data on Λ−N and Σ± −N scattering, while for Ξ−N or Ω−N essentially no data is available. In single hypernuclei, the strength of the Λ − N strong interaction can be extracted and the decomposition into different spin dependent contributions can be investigated as well. The PANDA experiment will also provide a unique approach to explore the hyperon-hyperon interaction. When bound in a nucleus, the Λ mesonic decay (Nπ) is disfavored by the Pauli principle, but still sizable for light nuclei. On the contrary, processes like ΛN → NN, ΛΛ → ΛN, and ΛΛ → ΣN are allowed, giving hence a unique access to the ΛΛK coupling, due to the dominance of kaon exchange processes. The use of Ξ− (S = -2) to form Ξ− atoms, Ξ− -hypernuclei and double ΛΛ hypernuclei can provide complementary informations about the baryonbaryon interaction. At PANDA the use of Ω− atoms is also foreseen: the interest on the Ω hyperon comes from the fact that it is the only elementary baryon with a non vanishing spectroscopic quadrupole moment. This behavior is determined by the one-gluon exchange contribution to the quark-quark interaction [2]. The production of Ξ hypernuclei proceeds through the reactions pp ¯ → Ξ− Ξ¯ + and pn ¯ → Ξ− Ξ¯ 0 followed − by rescattering of the Ξ within the primary target nucleus. The Ξ− is then stopped in an external secondary target to form the Ξ hypernucleus, which will be converted into a double Λ hypernucleus. The sequence of a double hypernucleus production in PANDA is depicted in Fig. 9. To perform those studies, the PANDA detector will be equipped with a primary carbon target at the entrance to the central tracking detector. For rescattering purposes, target nuclei with larger mass are more efficient,
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tions show that most of the produced charged and neutral particles are emitted into the forward region, which is not covered by the γ-rays detector. The particles emitted at backward angles will have a low kinetic energy and will be absorbed in the material surrounding the primary target. The most critical particles are the neutrons, which contribute to the radiation damage of the detector. 2. Panda Detector
Figure 9: Double hypernucleus production at PANDA [2].
but they have to be arranged to reduce the antiprotons losses due to the Coulomb scattering. At the incident momentum of 3 GeV/c, a cross section of around 2 μb is expected, and hence 7·105 Ξ− Ξ¯ pairs produced per hour. The secondary target will be composed of silicon detectors and 9 Be, 10,11 B, or 12,13 C absorbers arranged in a sandwich like structure. The task of the secondary detector is to slow down and stop the Ξ− and to identify the weak decay products. In order to limit the number of possible transitions, nuclei with low mass number were selected. In the 12 C scenario, per stopped Ξ− we expect a total double Λ hypernucleus production probability of around 4.7% [17]. For light nuclei, even a relatively small excitation energy can be comparable with their binding energy. The principal mechanism of de-excitation is an explosive decay of the excited nucleus into several smaller clusters. In our studies, the fragments masses in excited states were computed by adding the corresponding excitation energy to their ground state masses. According to the performed calculations, we expect a significant probability to produce double hypernuclei excited states. The γ emitted isotropically by the de-excitation of the double hypernuclei will be detected by a γ-rays detector placed close to the target region. If we consider only the mesonic decays, the momenta of the two emitted pions are strongly correlated, and their reconstruction will give the opportunity to tag the production of a double hypernucleus. The emitted pions will be tracked in the silicon strip detectors, characterized by a rather high efficiency, of the secondary target. Few weeks of data taking will be already enough to see the signals of the double hypernuclei states. Background studies are still ongoing. The simula-
In order to perform its wide physics program, the Panda Collaboration proposed to build a state-of-theart universal detector. The detector is designed to take advantage of the extraordinary physics potential which will be available utilizing high intensity, phase space cooled antiproton beams in a momentum range of 1.5 ÷ 15 GeV/c. The main achievements foreseen in the design of the spectrometer are an almost 4π acceptance, high resolution for tracking, particle identification (PID), and calorimetry, and high rate capabilities in order to reach the desired luminosity. In order to achieve a good momentum resolution, the detector, shown in Fig. 10, is composed of two spectrometers. The target spectrometer (TS) surrounds the interaction region, and the forward spectrometer (FS) with a second magnet provides angular coverage for the most forward angles. The target spectrometer covers angles larger than 5◦ and 10◦ in the vertical and horizontal plane respectively, and its basic concept is a shell-like arrangement of various detector systems surrounding the interaction point inside the field of a large solenoid.
Figure 10: 3D view of the PANDA spectrometer [18].
To reach the design luminosity of 2·1032 cm−2 s−1 , assuming 1011 stored antiprotons, a target thickness of about 4·1015 hydrogen atoms per cm2 is required. With this aim, two different types of targets will be used.
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The peculiarities of the pellet target, composed of a stream of frozen hydrogen micro-spheres (the so-called pellets), will allow to reach the expected luminosity. The cluster-jet target, characterized by a density of 1015 atoms/cm3 , acts as a very diluted target with an homogeneous density profile. The target spectrometer (TS) is an azimuthally symmetric system of detectors mostly contained inside the superconducting solenoid, characterized by a magnetic field of 2 T with homogeneity better than 2%, while the forward region (FS) will be equipped with a dipole with an integrated field of 2 Tm. The Micro Vertex Detector (MVD) is the detector closest to the interaction point. It is composed of 4 barrel layers and 6 forward disks. The construction of the detector implies the use of hybrid pixel technology, which is now under development, as well as silicon strips. It is optimized for the detection of secondary vertices coming from D and hyperon decays. The spatial and momentum resolutions are expected to be lower than 100 μm and around 2%, respectively. The Straw Tube Tracker (STT) is the central tracker of the experiment. They are composed of aluminized mylar tubes (straws) which are self supporting. The detector layers will be arranged in a honeycomb structure. The central layers will be placed with a skew angle of 3◦ to avoid ambiguities in the detection. The spatial resolution is expected to be σ xy ∼ 150 μm and σz ∼ 1 mm, and the momentum one on the percent level. In the forward region, for particle tracking purposes Gas Electron Multiplier (GEM) planes are foreseen due to their high rate capabilities. The charged particle identification is crucial to achieve all the physics goals of the experiment. The main part of the momentum spectrum above 1 GeV/c will be covered by Cherenkov detectors. At polar angles between 22◦ and 140◦ the PID will be performed by the detection of internally reflected Cherenkov (DIRC) light. The DIRC is composed of quartz slabs surrounding the central tracker. The light produced by a particle which crosses a quartz bar will be then collected by an array of photomultipliers. This detector will provide pion-kaon separation for momenta higher than 800 MeV/c. The energy loss (dE/dx) measurements will also be crucial to separate pions, kaons, protons, and electrons. RICH detectors will be used to identify the fast particles in the forward region. Another technique which will be exploited is the measurement of the Time of Flight (TOF) of the charged particles. TOF detectors will be placed after the DIRC in the TS region as well as in the forward part. For the muon identification, Iarocci tubes (Mini Drift Tubes, or MDT) operating in propor-
tional mode will be used. A fiberglass strip boards will allow for the read out of the second coordinate by collecting the induced charge. The MDT stations will be placed in the barrel segmented yoke as well as in the forward direction. The detection of the electrons and gammas will be performed by means of ElectroMagnetic Calorimeters (EMC) surrounding the interaction region and the most forward part. As scintillator material, the lead tungstate (PbWO4 ), a high density inorganic scintillator, was chosen for the TS region. It is characterized by a short decay time (< 10 ns) and a good radiation hardness. In order to increase its light yield, the 20 cm long (≈ 22 X0 ) scintillator crystals have to be cooled down to -25◦ C. In the FS the Collaboration is planning to use a shashlik calorimeter, composed of layers of lead and scintillating fibers. As hadronic calorimeter in the forward region, the Range System will be used. For the hypernuclear physics program, the plan is to remove the MVD and the backward end cap EMC in order to insert inside the apparatus the primary and the secondary targets needed, and the germanium-array detector. This detector will be composed of germaniumarray crystals grouped asymmetrically to form triple clusters and it will be devoted to γ-spectroscopy. A very reach and versatile program will be accomplished by the PANDA Collaboration by means of p¯ interactions. New detector concepts and technologies as well as detector materials have been developed. It offers clearly a unique opportunity to contribute to the hadron structure and dynamics understanding in the transition region between perturbative QCD and strong QCD. References [1] https://www.gsi.de/en/start/fair/aufbau der fair beschleunigeranlage. htm [2] W. Erni et al., Physics Performance Report for: PANDA, Strong Interaction Studies with Antiproton, (2009) arXiv:0903.3905. [3] N. Brambilla et al., hep-ph/0412158 (2004). [4] A. Denig, and G. Salm`e, Progress in Particle and Nuclear Physics 68 (2013) 113-157. [5] M. Sudol et al., EPJ A44 (2010) 373. [6] E. Tomasi-Gustafsson, and M. Rekalo, Phys. Lett. B 504 (2001) 291-295. [7] J. Boucher, PhD Thesis (2011). [8] G. Bardin et al., Nucl. Phys. B 411 (1994) 3. [9] B. Aubert et al., Phys. Rev. D 73 (2006) 012005. [10] J.P. Lees et al., Phys. Rev. D87 (2013) 092005. [11] C. Bourrely, and J. Soffer, EPJ C36 (2004) 371. [12] J.C. Collins, and D.E. Soper, Phys. Rev. D16 (1977) 2219. [13] R.D. Tangerman, and P.J. Mulders, Phys. Rev. D51 (1995) 3357. [14] J.C. Collins, Phys. Lett. B 536 (2002) 43-48. [15] A. Bianconi, NIM A593 (2008) 562-571. [16] M. Destefanis, PoS(BORMIO 2011) 012. [17] T. Yamada, and K. Ikeda, Phys. Rev. C 56 (1997) 3216. [18] PANDA Collaboration, TDR for the: PANDA Muon System