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Colin Wilkin University College London

From SATURNE-2 to COSY: What have we learned?. Colin Wilkin University College London. SPhN-Saclay le 21 septembre 2012. SATURNE-2.

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Colin Wilkin University College London

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  1. From SATURNE-2 to COSY: What have we learned? Colin Wilkin University College London SPhN-Saclay le 21 septembre 2012

  2. SATURNE-2 In view of several familiar faces, it is clear that some people in the audience will be very knowledgeable indeed about the SATURNE-2 accelerator and its long and distinguished physics programme. The seminar organisers have asked me to outline how many of the themes in this programme have continued, in particular at COSY-Jülich. SATURNE-2 was a proton synchrotron with a maximum energy of around 3 GeV that was capable of accelerating deuterons and other light ions to the same momentum per charge. Due to the pre-injector MIMAS, the intensity and quality of the polarised proton and deuteron beams was very high. The decision was taken in the mid-nineties to close SATURNE-2 at the end of 1997 and this was confirmed by an act of God when the snow fall collapsed the roof on the experimental hall in December of that year.

  3. COSY-Jülich Though commissioned in 1993, the first experimental results from COSY came only a little before the closure of SATURNE-2 in 1997. The energies of the two machines were very similar but the basic design was very different since the COoled SYncrotron (COSY) was both an accelerator and storage ring. Thus there were both internal experiments carried out inside the COSY ring as well experiments situated on external beam lines. Only protons and deuterons were accelerated and the cooling (stochastic or electron) of the beams improved significantly their quality. Some of the facilities available at COSY are shown in the layout plan 

  4. Layout of the COSY storage ring Circumference  183 m Tp(max) 2.9 GeV ANKE In a chicane WASA now here Big Karl programme completed EDDA physics programme completed COSY-11 finished

  5. I have selected about fifteen areas where the work at SATURNE-2 has significantly influenced the experiments that were subsequently carried out at COSY. This is not a complete list and I apologise if I do not discuss somebody’s most important experiment!

  6. Proton-proton elastic scattering The Nucleon-Nucleon (NN) interaction is fundamental to the whole of nuclear physics and any laboratory that could usefully add to the data base should do so as a service to the whole community. The NN group at Saclay worked throughout the whole life of SATURNE-2, measuring pp and pn cross sections, analysing powers, spin correlations, and spin transfers. Although the COSY-EDDA collaboration could never measure spin transfers, their results in the first decade of the 21st century now dominate much of the NN data base. This is because they could measure the directions of two charged particles at an internal target station of COSY during acceleration. They therefore studied a whole continuum of energies at the same time.

  7. The influence of the EDDA measurements can be seen in the abundance plots for the pp differential cross section and analysing power. The vast bulk of results in the solid blocks 0.5 < Tlab < 2.5 GeV, 35º < cm < 90º comes from the EDDA measurements. d/d Ay Nevertheless, there is a vast hole above 1 GeV for cm< 35º.

  8. New measurements have been carried out at COSY-ANKE at smaller angles. The fundamental question is “How does one establish the absolute normalisation in a storage ring when one cannot see what the overlap of the beam is with the target ?”. The trick is to measure the energy loss of the particles as they go through the target. This gives rise to a frequency change in the machine that can be measured to very high precision *. Depending upon the beam energy, the absolute luminosity can be determined to ≈ ±2-3%. d/d was measured at ANKE. Preliminary results at 2 GeV are compared with the SAID predictions. The analysing powers will be measured at the same energies in 2013. 2 GeV SAID ANKE * Stein, PRSTAB 11 (2008) 052801

  9. Neutron-proton elastic scattering One of the enormous advantages of the Saclay NN collaboration is that they had access to a quasi-monochromatic neutron beam up to ≈1.15 GeV produced by the breakup of the primary deuteron beam. This explains the cross section abundance plot. At higher energies they used quasi-free scattering of a proton on the neutron in the deuteron. At COSY there is no chance of making a useful secondary neutron beam from deuteron breakup. However, a proposal was accepted from ANKE to measure the differential cross section and proton analysing power in quasi-free pn scattering from a deuterium target up to ≈ 3 GeV. [Quasi-free scattering with a deuteron beam was already studied by detecting two fast particles in ANKE.]

  10. Deuteron charge exchange: the p(d,2p)n reaction David Bugg and I predicted that the reaction should have a large deuteron tensor analysing power provided that the excitation energy Epp in the final diproton is low. Under this condition the diproton is in the 1S0 state and there has to be a spin-isospin flip from the initial deuteron. This led to two independent programmes at SATURNE-2: The EMRIC group working at a few hundred MeV showed that the theoretical description of the reaction was adequate and used it to construct a working tensor polarimeter (POLDER) that was employed at JLab to measure the deuteron quadrupole form factor. The Franco-Scandinavian group detected both protons at high energies in SPESIV and so Epp< 1 MeV. The overall normalisation could not be fixed because of uncertainty in the beam profile but the data did agree quantitatively model at small momentum transfers with the Carbonell program predictions, based on the Bugg-Wilkin. There may be problems when q >> m.

  11. ANKE has a larger Epp acceptance (but poorer resolution) and data are shown with a 3 MeV cut. ANKE could also use smaller angular steps. The neutron-proton amplitudes are fairly reliable up to 1.0 GeV and so it is comforting that the Carbonell implementation of the B-W model reproduced so well the tensor observables at 600, 800, and 900 MeV. The np database at 1135 MeV is much poorer with essentially no information at all on the spin transfer parameter on the spin-transfer parameter. (d,2p) data at this energy can only be described by reducing the strength of the longitudinal spin-spin amplitude by 0.75. Td = 2.27 GeV Axx Modified SAID Ayy

  12. These observables depend upon the relative phases of the np elastic spin-spin amplitudes. The Cxx is well described by the standard SAID amplitudes at 600 MeV, though there remains a problem for Cyyfor q > 100 MeV/c. [This is probably of experimental origin.] At 1135 MeV, the model is only consistent with the data if the longitudinal spin-spin amplitude is reduced by the 0.75 factor. In PWIA Cyy should pass zero when the distorted single pion exchange amplitude vanishes. Double scattering could clearly modify the phases here. Reduction by 0.75

  13. Deuteron charge exchange: the p(d,2p)º reaction The SPESIV group also detected events where missing mass Mx > mn. High Mx corresponds to º production; what happens at ≈1150 MeV/c²? Similar features seen over wider energy range at ANKE. N* excitation much too small to explain the low mass data. Could be due to excitation in the projectile deuteron. This is analogous to αpαX, where the Roper is hidden by the  in the α-particle.

  14. The  revolution We measured the dp  3Hereaction with a polarised deuteron beam during one night with SPESIV at SATURNE. This experiment had a profound influence on the whole field and instigated detailed studies on the production and interaction of this meson. The surprise was the very large cross section near threshold and its anomalous energy dependence. Define a spin-averaged amplitude-squared by taking out the kinematic factors of initial (pd) and final (p) cm momenta: SPESIV and later SPESII data showed an | f|2 that changed rapidly with p. J.Berger, Phys.Rev.Lett. 61 (1988) 919

  15. SPESII results | f |2 falls by a factor of three for a change in pof about 38 MeV/c. In terms of excess energy Q = s - sthreshold p2/2mredwhere mred = 3He reduced mass, the change in Q is only about 6 MeV or less. f = fB /(1 - ipa) |Re{a}| = (3.8±0.6) fm Im{a} = (1.6±1.1) fm The effect is due to an 3He final state interaction (fsi) and the possible presence of an 3Hequasi-bound state. B.Mayer, Phys.Rev. C53(1996)2068

  16. Fits were made with a scattering-length formula: These gave|Re{a}| = (3.8±0.6) fm, Im{a} = (1.6±1.1) fm. [Note that the data are not sensitive to the sign of Re{a}.] The pole in the energy plane is at |Q| = (2.5±0.8) MeV, but it is not possible to determine on what sheet it is. Is it a bound state (like the deuteron) or an anti-bound state (like the S-wave singlet state in the pp system)? [More accurately we call it “quasi-bound” because it can still decay with the emission of pions and nucleons.]

  17. The COSY-ANKE dp  3He experiment Advantages of the COSY-ANKE setup • The ANKE spectrometer has 100% acceptance for 3He when Q < 20 MeV. • Energy loss in the hydrogen cluster-jet target is negligible. • Resolution is good. • The incident beam momentum could be changed in a continuous ramp so that it is possible to measure at MANY different energies in a single experimental run. • The linearity in the ramp allows one to evaluate the excess energy with high precision. • The ramping mode allows one to take data below threshold to be used for background subtraction. Needed in a missing-mass dp  3HeX experiment. T.Mersmann, PRL 98 (2007) 242301

  18. Identification of dp 3He events Q = - 4.75 MeV Q = - 0.60 MeV Difference Q = +6.95 MeV Difference All Q < 0 data

  19. The ramping mode: Excess energy versus time Slope agrees with rate of change of frequency of the machine The linear behaviour is quite evident ! Ramping much more efficient than the Beurtey/Saudinos wheel.

  20. Total cross section fordp 3He  Black points = ANKE Grey triangles = SPESII Grey squares = SPESIV Inverted triangles = COSY-11 Red line = ANKE fit Grey line = SPESII fit ANKE data compatible with SPESII but rise very quickly with Q.

  21. Near-threshold region SMEARING IS CRUCIAL Cross section  [nb] Smeared fit Unsmeared fit Excess energy Q [MeV] Fit function: f = fB/[(1 - p/p1)(1 - p/p2)] p1 = (4.1±12.5) ± i(17.6±7.0) MeV/c p2  (90 + i51)MeV/c [Poorly determined and unimportant] Q0 = (-0.2±0.4) ± i(0.16±0.52) MeV Clearly |Q0| < 1 MeV

  22. The low energy 3He system The energy dependence of T(dp 3He ) shows need for a pole with |Q| < 1 MeV. For all excess energies Q < 11 MeV the angular dependence is linear Q = 10.3 MeV s-p interference suggests that  should vary linearly with p but this is only true above  40 MeV/c. Similar behaviour noted at SPESII

  23. The s-p interference is influenced by the phase variation of the s-wave pole. Including this, the momentum dependence of the slope parameter can be easily understood. ANKE data and fit COSY-11 and fit There HAS to be a big phase variation of the s-wave amplitude with momentum. Is this seen also in 3He  3He  ? Neglect phase variation

  24. The 3He  3He  total cross section Impulse approximation The resolution is not as good as for dp 3He  but the cross section jumps to about 40% of its maximum in the first energy bin. Old data Dividing the result by the plane wave impulse approximation predictions shows the strong threshold enhancement. F.Pheron, Phys.Lett. B 709(2012)21

  25. The 3He  3He  differential cross section The slope of the cross section changes in the lowest Q bin, which indicates a rapidly varying s-wave amplitude. There seems to be a pole in the 3He  s-wave – but is the state “bound” or “unbound”?

  26. The dd 4Heη reaction The 4He system should be more bound than 3He because of the extra nucleon and because 4He is more compact. Thus, if |Q(4He)| > |Q(3He)|, then 4He is “bound”. GEM used a polarised deuteron beam but of much poorer quality than SATURNE. GEM Scattering length fit gave|Q|  4 MeV, which suggests that 4He is “bound”. Although the effects of higher partial waves were seen at COSY, the basic result |Q|  4 MeV was already known at SATURNE.

  27. The mass of the η meson There were two measurements of the η mass based upon studies of the kinematics of the dp 3He  reaction near threshold, where the meson was identified through a missing-mass peak. The energy Q above threshold can be found by looking at the 3He momentum ellipse. The beam energy at SATURNE was found through ingenious comparisons of the kinematics of dp 3He º, dp 3H +, and dp elastic scattering. They reported mη = 547.30 ± 0.15 MeV/c². The GEM collaboration at COSY also used a high resolution, small acceptance, magnetic spectrometer and calibrated the beam momentum by measuring also dp 3He º and dp 3H +. Their result was compatible with that of SPESIV: mη = 547.311 ± 0.028stat ± 0.032sys MeV/c². BUT all measurements that identify the meson through its decay find a result ≈ 0.5 MeV/c² larger.

  28. The ANKE measurement It was well known at SATURNE that an artificial depolarising resonance occurs at a well defined frequency that depends only upon a particle’s speed. Frequencies can be fixed to enormous precision and the momentum of the circulating COSY deuteron beam was measured to p/p ≈ 3×10-5. The background in the ANKE data can be well modelled using sub-threshold data (the Hibou method). However, in the evaluation of the 3He momentum, it was found that understanding the effects of momentum smearing was crucial and these could only be estimated using the 100% geometric acceptance of the ANKE spectrometer up to Q ≈ 15 MeV. If one only used collinear data then the result would have been ≈ 0.5 MeV/c² less. The result agrees with all the other modern measurements (except GEM): mη = 547.873 ± 0.005stat ± 0.027sys MeV/c². The error bar is possibly the best in the World. One big advantage is that there was almost no material in the target.

  29. Does one need to know the η mass with this precision? I am reminded of a quotation from the great German mathematician Felix Klein [Elementarmathematik vom höheren Standpunkte aus (Leipzig, 1908).] “The most elaborate result obtained was that of the Englishman Shanks, who calculated  to 707 places. One can ascribe this feat to a sportsmanlike interest in making a record, since no application could ever require such accuracy.” The real motivation for the proposal was to correct the misleading result presented by the COSY-GEM collaboration and show that the COSY beam momentum could be precisely calibrated.

  30. The p6Li  7Be η reaction The Pinot collaboration detected the meson through its decay and found about 7 events but spread over the first four states of 7Be. COSY-GEM worked closer to threshold, detecting directly the recoiling 7Be. Of the 8 detected events, 3 were believed to be background. GEM was only sensitive to the first two 7Be states and this was accounted for in the comparison shown. GEM The PINOT acceptance was miniscule and the η would be far better detected using the full WASA detector. However, one would never get good resolution on the 7Be levels there.

  31. The pp ppη reaction At PINOT and CELSIUS the meson was detected through its 2γ decay. At SPESIII the two protons were measured, which allowed the η' to be studied as well. The curves are Fäldt-Wilkin formula: PINOT η CELSIUS with  = 0.45 MeV and arbitrary scale factors C, adjusted to fit the overall scale. [This formula, though very useful, only takes into account the pp final state interaction.] η' SPESIII

  32. The pp ppη reaction At PINOT and CELSIUS the meson was detected through its 2γ decay. At SPESIII the two protons were measured, which allowed the η' to be studied as well. PINOT η η CELSIUS η' η' SPESIII All the extra data are from COSY11

  33. An η-meson factory Following the very positive results achieved on pp ppη, and especially dp 3He η, there was an initiative at Saclay to use these as sources of the meson in order to study its decays. This never came about and the challenge was taken up by the WASA detector, first in Uppsala and later at COSY. This detector can measure both charged hadrons as well as photons over a very large solid angle. BUT the hadronic background is very troublesome (FP & PF already worried about this) AND the competition from the electron machines makes life very difficult. The relative backgrounds are far less at Frascati, Mainz, or Bonn. This competition will be settled over the next few years.

  34. Pion production The total cross section for pp ppº was measured at SATURNE but at COSY-ANKE they studied pp {pp}sº, where the final diproton excitation energy was so low that it was in the 1S0 state. [I tried to get the Franco-Scandinavian collaboration to attempt this at SPESIV but without success.] Final spins do not then need to be measured and, by using the Watson theorem, it is possible to do a full amplitude analysis near threshold on the basis of analysing power and differential cross section data. Seven real parameters are sufficient to describe the pp {pp}sº and quasi-free pn {pp}s- data at 353 MeV. The partial wave amplitudes extracted are important ingredients in chiral perturbation theory evaluations of numerous low energy phenomena.

  35. pp {pp}sº pn {pp}s- ANKE ANKE CELSIUS TRIUMF Tsirkov, PLB 712(2012)370 Dymov, PLB 712(2012)375

  36. Inclusive pp K+X Siebert, NPA 567(1994)819 Budzanowski, PLB 687(2010)31 The improvement in the intervening fifteen years was only “quantitative”. The missing-mass resolution was better and this allowed the GEM collaboration to exclude a possible strange dibaryon at ≈2096 MeV/c². The dibaryon virus spread to COSY!

  37. Thepd 3Heφ reaction At SPESIV the cross section was measured in a missing-mass experiment. At COSY-MOMO the kaons (but not their charges) were measured in coincidence. θJ = angle between K+K- relative momentum and the proton beam direction. For s-wave kaon pairs, the distribution must be isotropic. For a vector meson like the φ, the distribution measures its polarisation. Data consistent with pure m=0 for φ production. For the ω, the production showed no alignment at all. Violation of OZI rule. ε = 35.1MeV QKK > 28MeV QKK < 28MeV Bellemann, PRC 75(2007)015204

  38. The pp ppφ reaction Φ production in pp collisions was studied at DISTO (one energy) and COSY-ANKE (three energies) by measuring the ppK+K- final state. The angular distributions at εφ = 83 MeV (DISTO) and 76 MeV (ANKE) are very similar. For a purely s-wave final state the distribution should behave like sin2. The ANKE data at 18.5 MeV are consistent with this but clearly not at 76 MeV. Model-independent proof for higher partial waves. The non-φ results are even more fascinating!

  39. The ratio of the differential cross sections in terms of the K-p and K+p invariant masses as well as those of K-pp and K+pp show that there is an enormous attraction between the antikaon and one or both protons. These are clearly above-threshold data but are not inconsistent with the existence of K-pp bound states. ANKE also has very nice data on the quasi-free pn dφ reaction. The Comité des Expériences rejected a proposal from DISTO to do a similar experiment! simulation

  40. New pp ppK+K- data at Q ≈ 25 MeV (still under analysis). By working closer to threshold the mass resolution is improved and there is no contamination from φ production. The acceptance-corrected data clearly show effects coming at the neutral kaon threshold – coupled channel phenomenon. Structure is sensitive to the isospin production amplitudes. Full results should be available by Christmas.

  41. The dd αº reaction Whether or not the ER54 group really did find a signal for the charge symmetry breaking dd αº reaction remains as controversial now as it was at Saclay 20+ years ago. The IUCF group measured cross sections at two near-threshold energies. One of the aims of COSY-WASA was to measure this with tensor polarised deuterons in order to find evidence for pion p-waves. So far there has been little or no success. Compared to SATURNE, the WASA collaboration has a much larger acceptance for the two photons from the º decay. However, unlike SPESIV used at SATURNE, there is no high resolution magnetic spectrometer at WASA but only a solenoid. There were suggestions of putting ANKE behind WASA to provide this tool but they seemed to have been impractical.

  42. Conclusions This has been a personal and very biased selection of some of the experiments carried out at SATURNE-2 and their continuation at COSY. Some were avoided for lack of time, such as inclusive K+ production in pA collisions. Others because COSY was not suited for the continuation – there could be no neutron beams at COSY, for example. Although the energies of the two machines were very similar, other characteristics were very different. The polarised proton and deuteron beams at SATURNE were almost as intense as the unpolarised ones; At COSY the polarised intensities were always a struggle! The external beams at SATURNE were very strong and this allowed a wide variety of magnetic spectrometers. On the other hand, at COSY we could work with both internal and external experiments.

  43. The crucial point though is that a large part of the SATURNE physics programme has continued and is still continuing at COSY. All machines have finite lives and COSY may only be doing hadronic physics for the next very few years. But what is important is that the physics ideas live on and this clearly happened with the death of SATURNE and the growing up of COSY. We hope that there is life after COSY, perhaps at FAIR at GSI. Time will tell! Good bye Au revoir cw@hep.ucl.ac.uk

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