The experimental investigation of the missing N The experimental investigation of the missing N* - and D- resonances problem: current status and perspectives.  V.V.Sumachev, V.S.Bekrenev, Yu.A.Beloglazov, E.A.Filimonov, A.I.Kovalev, N.G.Kozlenko, S.P.Kruglov, A.A.Kulbardis, I.V.Lopatin, D.V.Novinsky, V.A.Shchedrov, V.Yu.Trautman. ( Petersburg Nuclear Physics Institute, Gatchina, Leningrad district, 188300, Russia; e-mail: sumachev@pnpi.spb.ru )   I.G.Alekseev, P.E.Budkovsky, V.P.Kanavets, L.I.Koroleva, B.V.Morozov, V.M.Nesterov, V.V.Ryltsov, A.D.Sulimov, D.N.Svirida. ( Institute for Theoretical and Experimental Physics, B. Cheremushkinskaya 25, Moscow, 117259, Russia; e-mail: Igor.Alekseev@itep.ru ) N.A.Bazhanov, E.I.Bunyatova. ( Joint Institute for Nuclear Research, Dubna, Moscow district, 141809, Russia ) 1

Abstract   Numerous attempts to create a model that would exactly reproduce the N* - and D - resonance spectrum that was presented in the Review of Particle Physics (RPP) failed. The existing models usually predicted considerably more resonances (twice or more in number) than were found in elastic pN scattering. This problem is known as the problem of “missing” resonances. The partial wave analysis (PWA) FA02 that was made at George Washington University (2003) and included the modern experimental data revealed considerably fewer ( approximately half) the N* - and D - resonances than those presented in the RPP tables (2002). This disagreement invites further experimental investigation of the pion-nucleon interactions. Recent experiments of the PNPI and PNPI-ITEP collaborations resolved a part of the twofold ambiguities of the PWA’s. These results were used in the recent PWA of the George Washington University groups . The proposals for the additional spin-rotation parameters R and A measurements and for the search of the narrow nucleon resonances in the resonance region below 2.0 GeV are motivated. Such additional experiments are necessary to resolve remaining twofold ambiguities in the partial wave amplitude reconstruction by the PWAs and estimate the real quantities of baryon N* - and D - resonances. 2

Comparison of the N* and D-resonance number predictions.   Comparison of the N* and D-resonance number predictions. References N* - resonance number D - resonance Rev. of Part. Phys. (1980) 26 19 Rev. of Part. Phys. (2004) 21 22 KH80 18 KA84 16 CMB (Phys.Rev.D 20 1979 ) 13 T.P.Vrana et al.( nucl-th/9910012 ) 14 SM95 (Phys.Rev.C 52 1995 ) 8 FA02 ( Phys.Rev.C 69, 2004 ) 10 7 SP06 ( nucl-th/0605082 ) 9 S.Capstick et al.(Phys.Rev.D 49,1994) 40 27 U.Loring et al.(hep-ph/0103289) 99 82 Skyrme model (Phys.Rev.D31,1985) J.Vijande et al.( hep-ph/0312165 ) 3  

Model predictions (Dalitz). At the next picture the prediction of the harmonic oscillator SU(6)xO(3) model is shown (R.H.Dalitz, L.J.Reinders - 1977). It can be seen that this model predictions are far from all PWA results. 4

Model predictions { SU(6) x O(3), Dalitz } 5

Model prediction comparison. 6

Model prediction comparisons for N*- resonances. 7

Model prediction comparisons for D - resonances. 8

PWAs predictions disagreement and twofold ambiguities. A closer inspection shows that the difference between results of three global PWAs (KA84, CMB and FA02) is due to twofold ambiguities, which are inherent to the PWA procedure . D.Atkinson et al. have shown that, in the inelastic region, unitarity is a rather weak constraint and there exist “islands of ambiguity” in the Argand diagrams of the partial waves. More profound explanations of discrete ambiguities can be found if one analyzes trajectories of zeros of the pN transverse amplitudes. It was shown that the discrepancy between the predictions of KA84 and CMB for the p+p spin rotation parameters, on the one hand, and values of these parameters recently measured by the PNPI-ITEP collaboration, on the other hand, has been associated with the behavior of zero trajectory of pN transverse amplitude at a “critical point” near a pion laboratory momentum of 0.8 GeV/c. The only kind of observable which, having been measured, can remove discrete PWA ambiguities and help to choose the unique correct PWA solution are the spin rotation parameters A and R in pp elastic scattering. The choice of the unique correct PWA solution is in urgent need of new additional spin rotation parameters A and R measurements in the resonance region. 9

Experimental setup for A and R parameters measurement. PT – polarized target; C1-C8 – scintillation counters; G1, G2 – beam hodoscopes; DC01 – DC18 – drift chambers; C – carbon scatterer. 10

PNPI-ITEP results for spin rotation parameter A. 11

Parameter A world’s data (1956-2006 year).

Parameter R world’s data (1956-2006 year).

Experimental search for a narrow baryon resonances Experimental search for a narrow baryon resonances. ( Proposal “EPECUR” ) °    The inspection of the DSG experimental data in the resonance region depicts a possibility some number of the narrow baryon resonances detection. I If such resonant state will be discovered in the production reaction, not only its mass can be determined but also the width, as well as all the quantum numbers. It’s also possible to extract the branching ratio for the decays to p– p and KΛ channels, providing the information about the amount of the strange component in, if the p– p - p– p and p– p -KL reactions will be investigated. °    Moreover the data on differential cross-section in both reactions and if no resonant effect is observed either in p– p - p– p or p– p - KL, such negative result is also very important for the checks and development of theoretical models. The data on DSG in both reactions and normal polarization in p– p -KL are of great self-importance, it will be readily used by partial wave analyses due to small expected errors and small momentum step of the data points. As a continuation of the experiment, one can imagine taking data in a wider energy and angular range, as well as measurements of p+p elastic scattering, aiming directly to provide excellent quality data for PWA. The general idea of the proposal is to look for a narrow ( 2 - 20 MeV) resonance effects in the cross-section in the “formation” type experiments using the elastic pion-nucleon scattering and reaction π-p→KΛ. The scan of the mass interval under investigation will be done by changing the initial pion momentum. Secondary pion beams with appropriate intensity and energy are available at ITEP from its 10 GeV proton synchrotron. Two-focused beamline optics provides the possibility to analyse the individual pion momentum with the accuracy up to (0.06-0.15)%, having the total momentum range of Δp/p=±2%. Wide energy range of (0.8–2.5) GeV/c can be covered by changing of the magnetic elements currents. The results of the measurements may be analyzed by the standard procedure of the partial-wave (PWA) analysis, which is the important advantage of the “formation” type experiments. In particular it means that all the quantum numbers of the resonance, if found, can be unambiguously determined. 14

p+p – p+p DSG world’s data (1956-2006 year). 15

p- p – p- p DSG world’s data (1956-2006 year). 16

p- p – p0n DSG world’s data (1956-2006 year). 17

Setup for resonance search in the p-p elastic scattering. The layout of the setup for search in πp→πp elastic scattering is shown. The setup consists of the following main elements: Ø       Proportional chambers PC1–PC3 located in first focus of the ITEP beamline 322 for the incident pion momentum measurement Ø       Proportional chambers PC4–PC6 for the tracking of the incident pions Ø       Liquid hydrogen target with 1 mm beryllium cover, 4 cm in diameter, 25 cm long Ø       Sets of drift chambers DC1–DC4 for the reaction products tracking Ø       Beam scintillation counters S1, S2, trigger hodoscopes H2,H3 with time of flight measurements features, anti- and beam TOF counter A1. 18

Experimental effect of the narrow resonance. Fig. illustrates how could be seen a narrow resonance in the elastic scattering assuming its full width equal 6 MeV, elasticity ra=5% and mass 1671 MeV. The insertion in the top right corner is the zoom of the area around the resonance. Error bars and the point density correspond to the proposed statistical accuracy and momentum resolution. It’s worth mentioning that the effect of the resonance can be either a minimum (as in the figure), a maximum or a bipolar structure, dependent on its unpredictable residue phase. . π–p elastic differential cross-section at 60o c.m. as a function of pion momentum in the presence of narrow resonances ; the upper scale is the c.m. energy; the most accurate of the existing data are at a different angle. 19

Conclusion. The experimental program of the missing resonance problem investigations includes as a part the measurement of the spin rotation parameters A and R at 7-8 meanings of the pion beam momentum, where the existence of discrete ambiguities is mostly probable. These data will be used in performing the new PWA for obtaining the unambiguous behavior of a zero trajectory at “critical points” and to exclude the twofold ambiguities from the pN-amplitudes. After this procedure, some additional measurements can be necessitated for a completion of the elastic channel investigation in the baryon resonance region. In the following, the main elements of SPIN-PMJ experimental set can be used for the investigation of the polarization parameters in the other channels of the pion-nucleon interactions in the resonance region. °    Pion beams at ITEP are ideally suitable for the experimental search of the non-strange narrow baryon resonances. At the moment there are no other pions beams in the world which may offer such possibility. °    The experimental conditions are extremely favorable for the suggested experiment: large cross-sections and high sensitivity to the effect, reasonable run time requirements. 20