nonappearance of dehnen-shahin resonances in the positronium continuum

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  • Physics Letters B 301 (1993) 1-5 PHYSICS LETTERS B North-Holland

    Nonappearance of Dehnen-Shahin resonances in the positronium continuum

    Chun Wa Wong Department of Physics, University of California, Los Angeles, CA 90024-1547, USA

    and

    Cheuk-Yin Wong Oak Ridge National Laboratory, Oak Ridge, TN 37831-6373, USA

    Received 27 October 1992; revised manuscript received 16 December 1992

    The Dehnen-Shahin relativistic equations for the positronium are found to have no resonant solutions in both the ~So and the 3P o positronium continua. A certain pole singularity appearing in the ~So potential gives rise to resonances, but only if the electro- magnetic interaction strength is increased 160-fold or more. These resonances are found to have zero width. However, the same singnlarity, even in a weak potential, gives unacceptable nonphysical bound states near zero energy. The strange similarity of Spence-Vary 3P 0 resonances to resonances in an infinite square-well potential is noted.

    The possible appearance of resonant states in the positronium (Ps) continuum has been a question of considerable interest in the past. Their appearance would indicate a need to modify or fine-tune our un- derstanding of QED. Interest in this question has been enhanced in recent years by persistent reports of res- onance-like structures or composite states in heavy- ion collisions where ultra-large Coulomb fields are generated [ 1,2 ]. Groups of electrons and positrons of identical energies have been seen in coincidence. The total energies of these experimental structures are in the range of 1.5-1.8 MeV. They might represent structures in the Ps continuum.

    One of the ideas studied in the past in this connec- tion is the appearance of magnetic resonances in the 3Po state [ 3-5 ]. But a more recent reexamination [6 ] has shown that the potential is not strong enough to hold a resonance.

    Three related, but technically different, ideas have recently appeared. First, Arbuzov et al. [7 ] have solved a two-body relativistic wave equation in sca- lar QED (the Wick-Cutkosky model) in momentum space and found resonances not only in the Ps con- t inuum, but also in the e - -e - and proton-proton

    continua. However, a similar equation has been solved by Walet, Klein and Dreizler [ 8 ] who can find no resonance. They believe that the results ofref. [ 7 ] are the spurious effects coming from the chosen mo- mentum basis used in the calculation. Similarly, Horbatsch [9] cannot find resonances in similar equations.

    Secondly, Dehnen and Shahin have solved the Breit equation for the Ps (DS1 [ 10] ) and a similar equa- tion (DS2 [ 11 ] ) proposed by Barut et al. [ 12 ], both in coordinate space. They find a number of reso- nances of finite widths in the Ps continuum in at least the 3po and ~So states beginning at the total energy of 1.3 MeV, i.e. the same energy region as the experi- mental structures.

    Finally, zero-width resonances rather similar to those of DS have been found by Spence and Vary (SV 1 [ 13 ] and SV2 [ 14 ] ) in three momentum-space relativistic wave equations derived from QED. These resonances have been criticized by Horbatsch [9 ] who finds no resonances in similar momentum-space equations. Horbatsch further states that scattering solutions which do not include the on-shell momen- tum as a basis point will introduce spurious narrow

    0370-2693/93/$ 06.00 1993 Elsevier Science Publishers B.V. All fights reserved. 1

  • Volume 301, number I PHYSICS LETTERS B 25 February 1993

    structures at the arbitrarily chosen momentum basis and that the treatment of the infinite-range Coulomb potential in momentum space might not be simple.

    We should add that the physical origin of these claimed resonances have not been studied or identi- fied in refs. [ 7,10,1 l, 13,14 ]. DS do mention a pole singularity on a spherical shell of finite radius in the effective potential for the ~So state of parity ( - 1 )J+ (misidentified as the 3P o state of parity ( - 1 )Sin both ref. [ 10] and ref. [ 11 ] ) when the e+-e - separation is near the classical electron radius. However, they have not shown that their resonances come specifi- cally from this singularity. Furthermore, this singu- larity is absent in the 3Po state (misidentified as the IS o state in both ref. [10] and ref. [ l l ] ) , and yet similar resonances are found in this state in ref. [ 10 ].

    In view of the continued interest concerning the possible appearance of resonances in the Ps contin- uum in both experiment and theory, it would be of interest to clarify the theoretical situation with re- spect to the DS resonances. Furthermore, the DS wave equations involve only local potentials in coordinate space; they are therefore much easier to solve than the momentum-spaced equations mentioned previ- ously. Finally, it is an interesting question whether the DS pole singularity could give rise to resonances at all.

    We would like to prove here that the DS pole sin- gularity does not give rise to a resonance in the Ps continuum. Resonances appear only when the inter- action is made much stronger. They have zero width and are similar to resonances in an infinite square- well potential. We find, furthermore, that the pole singularity gives rise to unphysical bound states near zero energy. Hence equations containing such poles cannot give an acceptable description of the Ps.

    In the 3P o state where the pole singularity is absent, our phase-shift solutions for the DS equations show no resonant behavior, only the very small effects nor- mally expected of the weak e+-e - interaction.

    In common with many other relativistic wave functions (w.f.s) for the Ps, the DS w.f.s have 16 spi- nor components. The resulting 16 coupled first-order differential equations separate into two disjoint groups of eight, with parity ( - 1 )J+~ and ( - 1 )J, re- spectively. Four of the eight equations in each group can be written as algebraic equations for properly chosen w.f.s. They can be used to eliminate four of

    the w.f.s, leaving four coupled, first-order, differen- tial equations with effective potentials arising from the algebraic elimination. We shall show explicitly that these effective potentials could have unusual singularities where certain w.f.s acquire unusual boundary conditions (b. c. s ) because of the algebraic elimination.

    For the ~So states, only three of the eight DS w.f.s (denoted us, Vs andfa in DS2) are nonzero, so that the particle number density takes on the form

    C~u=l( lu212+ Ivs 12+ If31 z) . (1)

    These w.f.s are actually related to one another:

    2m u2= E_2e2/rvs, (2)

    10~V2, (3) A=-~

    (E+-~)v2+2mu2-2(Or+2)f3=O, (4)

    where m is the electronic mass and E= 2~- - /+p 2 is the total Ps energy (with h=c=l ) . It is therefore necessary to study only one of these w.f.s, say vz.

    The wave equation satisfied by v2 is

    0~+20r+V2-r -e+E_--ffe~/r)Jvs=o. (5) The effective potential contains an unusual term with a pole at rpole= 2eS/E coming from the elimination of u2. This is the pole singularity mentioned earlier in the paper.

    The pole term is attractive inside rpo~e, and repul- sive outside, being Coulomb-like on each side of rw~,. It therefore gives rise to an infinite bowl-shape po- tential in which resonances could appear. However, rpol~ itself shrinks to zero as the total energy E in- creases to infinity. We shall now show that the situa- tion is not unlike resonances in an infinite square- well potential, but that because of the shrinking ra- dius, no resonance is trapped inside a weak potential.

    The mathematical analysis for a shrinking radius is facilitated by using the dimensionless (but energy- dependent) distance x= (E/ot)r, where ot = e s is the fine structure constant. The wave equation for the ra- dial w.f. F(x) =xvs is then

    F"+ (k s - V )F=0, (6)

  • Volume 301, number 1 PHYSICS LETTERS B 25 February 1993

    where

    k2 =~a2( 1_ 4~ p2~,

    A B V(x) =

    x 2 -x '

    E m

    (7)

    (8)

    2o 2 B= g''T-, A=Ot2" (9)

    Eq. (6) is just eq. (3.11 a) of ref. [ 1 ]. It represents the starting point of our investigation.

    In terms of x, the pole appears at x--2 at all ener- gies. It is the dimensionless wave number k which now has an unusual energy dependence: It is always small and reaches a maximum value of only lot at infinite energy! Thus the shrinking radius has the same effect as the limitation ofk in eq. (6) to small values.

    Eq. (6) is to be solved subject to the b.c.s:

    F (x )=0, a tx=0, x=2. (10)

    The condition at x= 2, needed to ensure that u2 is fi- nite there, reduces the problem to that of an infinite square-well potential, plus a very weak inside poten- tial V(x). We therefore perturb from the discrete states of the infinite square-well potential to obtain the following first-order discrete spectrum:

    k 2 = (nit) 2 - (A+B)Cn, (11 )

    where 2

    ,12, 0

    Since A and B are both very small ( ~ 10-4), kn is very close to nrc. These are resonances of zero width embedded in the k-continuum 0 < k < ~.

    The first nontrivial (nonzero) solution in the in- side region (x

  • Volume 301, number 1 PHYSICS LETTERS B 25 February 1993

    (6) consistent with the stated b.c.s at all physical energies are the nonresonant solutions in which all w.f.s (F, u2 and f3) are completely excluded from the inside region. The fractional excluded volume in the normal 23po Ps state can be shown to be of the order 4 10-5ot6__ 10-,7. But is even this very small effect real?

    To answer this question, we reexamine eq. (6). The pole term in the potential is proportional to e -2, where e=E/m; it therefore becomes infinitely strong as e (or E) approaches zero. We find

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