Bare propagator poles in coupled-channel. channel models (Possible link between microscopic theories and phenomenological models)

Transcription

1 Bare propagator poles in coupled-channel channel models (Possible link between microscopic theories and phenomenological models) Alfred Švarc varc Ruđer Boškovi ković Institute Croatia 1

2 The short history: In last few years I have been faced with two problems 1. Are the off-shell effects measurable? 2. How can we understand bare coupled-channel quantities? I have asked these questions at few workshops and conferences, and it turned out that these problems seem to be related. What is in common? 1. Both problems originate in an attempt to link microscopic to macroscopic effects 2. Both problems are controversial because basic field theoretical arguments forbid what seems to be very plausible on the macroscopic level 2

3 The question are: 1. Can we formulate the problem here exactly? 2. Can we make a step forwards towards giving a competent answer to the existing controversy? 3

4 A brief summary of the off-shell problem calculating processes with more then 2-nucleons requires an assumption about the off-shell behavior of the 2-body amplitude it has been widely accepted that off-shell behavior is a measurable quantity (like for instance in Nucleon-Nucleon Bremsstrahlung, pion photo- and electro production or real and virtual Compton scattering on the nucleon) many different models for the off-shell extrapolations have been suggested and the results compared A controversy has arisen when Fearing and Scherer declared that the offshell effects are unmeasurable because of first field-theoretical principles 4

5 Maybe the answer lies in this part of conclusions? My dilemma: we do need model off-shell form factors to calculate any observable in a more then 2-body process, and different models give different results if we can not establish the correctness of the off-shell form factors, that means that we in principle can not calculate anything at all 5

6 A brief summary of the bare propagator problem coupled-channel formalism has been known for decades, but (at least to my knowledge) no credible physical meaning to the bare quantities is given in spite of general agreement that bare quantities are obtained when self energy contributions are deducted (singled out, taken away) the idea to relate bare quantities to the quark-model-calculation ones has appeared (references follow) a controversy has arisen when the proposal has been criticized because of incompatibility with the first field-theory principles 6

7 From now on I will present some facts related to the possible understanding of bare quantities in coupled-channel models I will restrict my discussion to bare propagator pole values. Why poles? 7

8 The formulation of hadron spectroscopy program Höhler Landolt Bernstein A. Švarc, 2ndPWA Workshop, Zagreb

9 Most single channel theories recognize only one type of scattering matrix singularity scattering matrix pole. As nothing better has been offered quark model resonant states are up to now directly identified with the scattering matrix singularities obtained directly from the experiment. 9

10 Up to now: 10

11 However, coupled channel models, based on solving Dyson-Schwinger integral type equations having the general structure full = bare + bare * interaction* full G = G0 + G0 Σ G do offer two types of singularities: bare poles dressed poles Questions: 1. How do we extract bare and dressed propagator poles? 2. What kind of physical meaning can we assign to dressed and/or bare propagator poles? 11

12 According to my knowledge, no physical meaning to the bare propagator poles in the coupled-channel formalism has ever been given. Should be that done? 12

13 A tempting possibility has been suggested in by Sato and Lee within the framework of dynamical coupled-channel model, and elaborated for photoproduction of Δ-resonance (γn Δ): cc-model bare value quantities quark-model quantities Question: Can the idea be justified? Details given in 13

14 The idea has been repeated since: 14

15 15

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17 The controversy exists! Strong criticism of such an idea has been made by C. Hanhart and S. Sibirtsev at ETA07 in Peniscola The criticism is based on incompatibility of such an interpretation with some first principles originating in the field theory. 17

18 I will now give a short preview of the essential from: 18

19 So, in such a type of a model (as in any coupled-channel model) we have two type of quantities: bare and dressed ones bare: bare vertex interaction bare resonant state masses dressed: dressed vertex interaction defined by equation dressed resonant state masses defined by equation (when dressed propagator in resonant contribution is diagonalized) 19

20 UP TO NOW quark model resonant states scattering matrix poles Problems for transition amplitudes Proposed way out 20

21 Applied to Δ γn helicity amplitudes 21

22 Extension to the full N* resonance spectra is proposed in Matsuyama, Sato and Lee, Physics Reports 439 (2007): However it is not yet done: 22

23 So, let me give a short resume: 1. At our disposal we have two kind of singularities to be discussed: bare and dressed scattering amplitude poles. 2. The speculation to identify bare quantities in a cc model with quark model ones is introduced 3. The idea has not been proven (controversy in interpretation exists) 4. The idea is verified for γn Δ helicity amplitudes obtained when using bare and dressed interaction vertices, and the good agreement is found 5. The necessity to extent it to the full N* spectrum is stressed 6. No systematic results for coupled-channel models are given yet, but preliminary reports from a number of groups do exist 23

24 Let me give an example of one: A comparison of bare propagator poles with constituent quark-model predictions is for the whole N* spectrum given for a: coupled-channel model of CMB type where the interaction is effectively represented with an entirely phenomenological term. 24

25 Carnagie-Melon-Berkely (CMB) model Instead of solving Lipmann-Schwinger equation of the type: with microscopic description of interaction term we solve the equivalent Dyson-Schwinger equation for the Green function with representing the whole interaction term effectively. 25

26 We represent the full T-matrix in the form where the channel-resonance interaction is not calculated but effectively parameterized: channel propagator channel-resonance mixing matrix bare particle propagator 26

27 we obtain the full propagator G by solving Dyson-Schwinger equation G = G0 + G0 Σ G where we obtain the final expression 27

28 What should be identified with what? 28

29 Following the idea from photoproduction: bare propagator pole position imaginary part of the dressed propagator pole mass of a quark-model resonant state decay width 29

30 What is our aim? To establish if there is any regular pattern of behavior. 30

31 Results Model: 1. CMB model with three channels 2. Input: πn, ηn and π 2 N - effective 2-body channel πn elastic: VPI/GWU single energy solution πn ηn: Zagreb 1998 PWA data 3. Quark model quantities are taken from Capstick-Roberts constituent quark model 31

32 The intention is to ask for the absolute minimum! To see if the interpretation of bare propagator poles as quarkmodel resonant state is allowed for the used input data set. We perform a constrained fit with the bare propagator pole values fixed to the quark-model values! Of course, we shall investigate whether the solution is: unique best 32

33 The comparison is done for lowest partial waves S 11, P 11, P 13 and D 13 33

34 Let us show the two lowest parity odd states: S ( 1 ) ( 3 ) and D

35 ( ) S χ = R χ = R 35

36 ( ) S dressed pole PDG quark model resonant state constrained fit bare propagator mass free fit bare propagator mass 36

37 ( ) S dressed pole PDG quark model resonant state constrained fit bare propagator mass free fit bare propagator mass

38 ( ) D πn elastic πn ηn χ = R χ = R 38

39 ( ) D

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41 Let us show the two lowest parity even states states: P ( 1 ) ( 3 ) and P Problems appear 41

42 ( + ) P πn elastic πn ηn 42

43 ( + ) P

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45 P 11 ( + 1 ) 2 NOTORIOUSLY PROBLEMATIC ONE πn elastic πn ηn 45

46 ( + ) P

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48 Conclusions: 1. There is a certain level of resemblance between bare propagator poles in a CMB type coupled-channel model and constituent quark model resonant states 2. There is a certain level of resemblence between our bare propagator poles and Mainz group results. 3. The mechanism is established to distinguish between genuine scattering matrix pole generated by a nearby bare propagator pole and a dynamic scattering matrix pole which is generated by the interference effect among distant bare propagator poles 4. The Roper resonance is in this model consistent with being a dynamic scattering matrix pole 5. New partial wave data from other inelastic channels are required in order to further constrain the fit, and give a more confident answer about the precise position and nature of a scattering matrix resonant state under observation 48

49 Final question to be answered here: What is the correspondence between bare propagator poles in general and hadron structure calculations? 49