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Subthreshold particle production

Subthreshold particle production. To produce a pion (mass ~ 140 MeV for charged pions, 135 MeV for neutral pions) in a NN collision, a lab energy of at least ~ 290 ( ~ 280) MeV is required.

florrie-morris
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Subthreshold particle production

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  1. Subthreshold particle production

  2. To produce a pion (mass ~140 MeV for charged pions, 135 MeV for neutral pions) in a NN collision, a lab energy of at least ~290 (~280) MeV is required. However, it has been known for a long time that in nucleus-nucleus collisions, pions may be created at energies significantly below 280 MeV/nucleon. First evidence for such pion production in AA collisions was obtained as early as 1948 with 300-380 MeV alpha particles. More recently (1982), CERN experiments with 12C at 60 MeV/nucleon showed evidence of positive and negative pion production. Since 1982, several groups have studied pion production between 25 and 100 MeV/nucleon. How to explain such “subthreshold” pion production?

  3. First explanations of such effects were given in terms of coupling the bound nucleon Fermi momenta to the momentum of relative motion between the two nuclei. Such mechanism is not expected to work for very low beam energies. On the basis of single nucleon-nucleon collision model (with realistic momentum distributions), threshold energies around 50 MeV/nucleon can be predicted. At lower energies and very close to the absolute threshold, one must invoke the presence of collective effects.

  4. In the low energy limit, close to the absolute threshold, the process of pion creation requires the transformation of most (or all) the projectile’s kinetic energy into a single degree of freedom (creation of a new particle). In contrast to processes where light particles and low energy photons are emitted in a nucleus-nucleus collision (where the statistical description of the collision plays a major role) here the coherent aspects of the collision should be evidenced.

  5. Lab Coulomb energy The absolute threshold for pion production in (symmetric) heavy ion collisions vs mass number of the two nuclei. Pion production close to the absolute threshold requires that many nucleons in the projectile and the target act cooperatively to convert their energy into the pion mass

  6. * = Exp. Data 250 MeV 0 MeV Miminum number of target nucleons required in a 14N-induced reaction to produce pions of different kinetic energies How many nucleons must cooperate to produce a pion of a given kinetic energy?

  7. Experimental methods for charged pions Since the mechanism is close to the absolute threshold, low energy pions are expected. For charged pions, basically two methods have been employed: Magnetic spectrometers Range telescopes

  8. Magnetic spectrometers A typical set-up includes a magnetic field (fixed or variable) and a focal plane detector, made by scintillators and/or drift chambers. Advantages: good resolution Disadvantages: small solid angles Pion flight paths may vary from 1-2 m to 30 m. CLAMSUD spectrometer (Catania), used in Moscow (1991-1995) and Uppsala (1995-2000) Note that the charged pion lifetime is 26 ns. Example: 93% of 100 MeV pions decay after 30 m flight path

  9. CLAMSUD spectrometer@Uppsala

  10. Range telescopes A multi-element scintillator telescope may be used to discriminate pions against protons At 100 MeV/nucleons, usually there are 10000 proton for each pion Advantage: Simple devices, large solid angles Disadvantages: Poor energy resolution, some contamination

  11. Next generation (exclusive) experiments employed multidetectors Example: GANIL Mur+Tonneau NP A519(1990)213

  12. Neutral pion detection Neutral pions decay (B.R. 98.8%) into two gammas, with 0.87 x 10-16 s lifetime γ1 π0 γ2 Detection of neutral pions require the coincidence detection of two gammas in a large background of low energy gammas (about 107-108 per neutral pion) from nuclear deexcitation. For a pion at rest, the two gammas have 70 MeV each.

  13. First set-up’s for neutral pion detection used small arrays of Pb-glass scintillators Sizes and granularity of scintillators determine the capability to reconstruct the electromagnetic shower initiated by the high-energy photon Energy resolution and angular resolution of the two gammas determine the overall resolution on the neutral pion From J.Stachel et al, PRC33(1986)1420

  14. Next generation experiments used large solid angle arrays, able to detect not only photons from the pion decay, but also coincident particles (exclusive experiments) One example is the MEDEA BaF2 crystal ball, which was used for the first time in GANIL at the end of 80’s, before being installed in Catania A 180 BaF2 array (from 30º to 140º, complemented by 120 phoswich detectors able to cover from 10º to 30º

  15. MEDEA

  16. Another multidetector widely used for photons and neutral mesons is TAPS, installed in several Laboratories (GANIL, GSI, Mainz, Groningen,..) Each BaF2 module has exagonal shape, with a charged particle veto in front of it.

  17. Various configurations for TAPS

  18. TAPS configuration at GSI, GANIL TAPS configuration at KVI

  19. Pion reconstruction When the energy resolution is not so good, it is better to extract the energy of pions from where the asymmetry parameter is given by With this choice the invariant mass is given by and the kinetic energy and pion emission angle by

  20. For simulation results using MEDEA as a neutral pion detector, see NIM A306(1991)283

  21. Main results from inclusive experiments on subthreshold pion production As early as 1987, several inclusive experiments on subthreshold pion (charged and neutral) production in heavy ion collisions were available MS: Magnetic Spectrometer RT: Range Telescope LG: Lead Glass scintillators

  22. Pion kinetic energy distributions All spectra exhibit a broad maximum at low pion kinetic energies (~10 MeV) and a nearly exponential decay dσ/dT = const exp(-T/E0) For the case reported here, the inverse slope parameter E0 is about 23 MeV N+Ni @35 MeV/A

  23. Angular distributions Energy integrated angular distributions are usually forward/backward symmetric. However, angular distributions are different for low energy and high energy pions, with a backward rise for the highest energies. Forward peaked distributions are expected for pion emission from a source moving in the beam direction, with a forward/backward symmetry in the source rest frame. Backward peaking difficult to explain: Role of reabsorption and source location? N+Ni @35 MeV/A

  24. Integrated pion cross sections The integrated cross sections increase with increasing target size At higher bombarding energies, data are in agreement with single nucleon-nucleon collisions. At lower energies, discrepancies observed. Possible interpretations: - Pion production implies a source extended more than just 2 nucleons - Pion reabsorption effects for large targets play a major role Solid line: (AT )2/3 Expected for pion production from single nucleon-nucleon collisions

  25. Invariant cross sections To investigate the pion production mechanism another relevant information is the invariant cross section, especially if large enough angular and energy ranges are explored. If the invariant cross section is plotted as a function of rapidity and transverse momentum, all the information of the pion-emitting source is contained in one variable (y), while the other variable (pt) is free from kinematical effects and is determined only by internal characteristics of the system emitting the pions.

  26. Invariant cross sections/2 N + Ni @35 MeV/A Contour plots of iso-invariant cross sections may be obtained if enough statistics is available For a single moving source emitting pions, the contour plot should be symmetric about the source rapidity For symmetric projectile-target systems, the source rapidity has been found to be one half of the projectile rapidity. For any system, a source rapidity intermediate between yb/2 and y(c.m.) is found. Here, average rapidity is very close to 0 (target), smaller than yb/2.

  27. For low bombarding energies, there are evidences that the projectile will stop in the c.m. system before traversing the target nucleus. This will lead to shadowing effects in the forward direction, and then to a downward shift in rapidity (even to negative values!) All data and conclusions are strongly affected by pion reabsorption effects!

  28. Reabsorption effects Experimental data on pion absorption are usually available at high pion energies ground state nuclear matter Here, we are dealing with low energy pions which are reabsorbed in excited nuclear matter

  29. Optical model calculations which reproduce pion absorbption data predict a maximum pion mean free path λabs around 3.6 fm for 25 MeV pions, with smaller values at lower and higher pion energies ( 3 fm at 10 MeV, 1.7 fm at 100 MeV). Consider that the combined system N+Ni has a radius of 7 fm at 50% density Pion reabsorption effects even in a light system cannot be neglected, and modify in a drastic way the primary energy/angular distributions

  30. Example of a Monte Carlo simulation of the pion reabsorption for different source location Z=+4 fm Z axis along the beam, with z=0 at the center of the combined projectile-target system Z=0 Z=-4 fm z Z=0

  31. Activity: Simulate the geometrical overlap of two nuclei (for simplicity assume them as discs in a 2D-plane), assume isotropic angular distribution and thermal energy distribution of emitted pions, introduce a mean free path of pions (dependent on pion kinetic energy?) and some distribution for the location of the source. Evaluate the modifications of angular and energy distributions due to pion reabsorption

  32. Systematics of cross section vs bombarding energy To compare cross sections measured for different target and projectile combinations, they are normalized to (Ap AT)2/3 For subthreshold energies, cross sections are strongly dependent on bombarding energy. (a factor 100000 between 20 and 90 MeV/A)

  33. So, what can be learned from inclusive experiments on subthreshold pion production? Different models have tried to explain the known data: Single nucleon-nucleon collision model Cooperative models Collective/coherent models

  34. Single nucleon-nucleon collision models The original idea that due to the Fermi motion of the nucleons inside the nuclei, in a single NN collision there could be enough relative energy to produce a pion, seems to work only at energies slightly below the threshold (100-150 MeV/A). This approach fails at low bombarding energies (20-100 MeV/A). At 84 MeV/A the predicted cross sections are already a factor 1000 lower than observed. Reabsorption effects still reduce the predicted cross sections. At 20-84 MeV/A the disagreement is even worse

  35. Single nucleon-nucleon collision models/2 Moreover: Inverse slope parameters predicted by such models are much smaller (a factor 2) than measured Source velocity in single NN collision models should be yb/2. Observed values are close to 0. For integrated cross sections, a (AT)2/3 dependence is predicted, whereas many data do not agree with this behaviour.

  36. Cooperative models Single nucleon-nucleon collision models fail at very low bombarding energies. One alternative is a model involving the cooperative action of several target and projectile nucleons. Within this model, two approaches gave been used: one is based on multiple off-shell collisions (Shiam and Knoll, NP A426(1984)606), the other on a thermal description (Aichelin and Bertsch, Phys.Lett.138B(1984)350). Both give comparable (and good) results at energies 50-100 MeV/A, but underpredict data at lower (25-50 MeV/A) energies.

  37. Collective or coherent models How much energy is left to the system, after the pion is created? In some cases, we are very close to the kinematical limit! Pion kinetic energy spectra, reported as a function of the energy available in the c.m. after pion emission. The zero on this scale marks the kinematical limit, where the pion carries out ALL the available energy.

  38. Near the kinematical limit, all spectra seem to merge together, and the differential cross section scales with total available energy in the c.m., rather than the energy per nucleon (or small number of nucleons). In other words, the relevant quantity seems the total c.m. energy, not the energy per nucleon. The process may be called pionic fusion. In such case, a fully coherent description is needed. Some evidence of pionic fusion has been observed especially for light systems (3He+A, A<10). No data exist for heavy systems.

  39. More recently (1991) some investigation has been done around 100 MeV/A with 12C+12C, to search for coherent pion production. The following process was considered where the projectile is excited to 15.1 MeV, decaying by photon emission. During the (peripheral) collision, the target is excited, with pion emission.

  40. The experiment was carried out by searching for coincidences between 12C, 15.1 MeV photons, and one high-energy photon originating from neutral pion decay (neutral pions not identified). Energy spectrum of photons in coincidence with 12C 9 events found!

  41. Another collective approach is the pionic bremmstrahlung. In this model, the projectile is coherently slowed down with pion emission. No clear evidence of such process exist, even though some experiments have searched for it. Example: peripheral pion production in projectile break-up at 95 MeV/A, Phys.Lett. B316(1993)240.

  42. The role of exclusive vs inclusive experiments on pion production at subthreshold energies Large arrays of detectors able to detect neutral pions may be used for fully exclusive experiments (not simply coincidence experiments between pions and something else). Two examples: MEDEA, TAPS

  43. Inclusive and exclusive experiments on subthreshold pion production were carried out in GANIL with 95 MeV/nucleon Ar beams. Neutral pions were measured in coincidence with charged particles. Invariant mass spectrum of neutral pions, Ar+Al @95 MeV/A

  44. Detailed inclusive differential cross sections of neutral pions were obtained for the first time in small angular bins, compared to microscopic BNV calculations Some results w/o reabsorption Ar+Al @95 MeV/A

  45. Where are pions created in heavy ion collisions at subthreshold energies? It is reasonable to assume that most of them are created in central collisions, but only after the first exclusive experiments, this could be experimentally demonstrated. Several global variables were used to determine the impact parameter in a quantitative way

  46. The charged multiplicity for pion events is strongly different from inclusive events! (Note the log scale) Ar + Al, 95 MeV/A

  47. Angular distributions are very different for low energy and high energy pions: Indication of different mechanisms and reabsorption effects

  48. Exclusive experiments on pion production at subthreshold energies allowed to investigate different aspects of the problem The importance of statistical processes through the correlation between pions and light fragments The role of nuclear stopping in pion production The reabsorption effect in nuclear matter (pion shadowing) The formation and decay of the Δ-resonance, through the correlation between pion and protons

  49. Subthreshold production of other particles In addition to pions, other particles may be created below the NN energy threshold by some of the above mechanisms Examples: Kaon and η production around 100 MeV/A

  50. Kaon production in AA collision around 100 MeV/A The process N+N -> K+ Λ N requires an energy of 670 MeV, which means 1.58 GeV in the lab system for pA K+ production is then a very unlikely process at 100 MeV/A! Some data exist for the Ar+C, Ti, Ta @92 MeV/A (Legrain et al., Phys.Rev. C59(1999)1464

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