Measurements of ΔσL (np) between 500 and 800 MeV

Volume 258, number 1,2

PHYSICS LETTERS B

4 April 1991

Measurements of AtrL (np) between 500 and 800 MeV M. B e d d o a,~, G . B u r l e s o n a, J.A. F a u c e t t D . P . G r o s n i c k b, D. H i l l b, K . F . J o h n s o n H. S p i n k a b, R. S t a n e k b, D. U n d e r w o o d L. N o r t h c l i f f e c, J.J. J a r m e r d, S. Penttil~i P. K r o l l h

a,~, S. G a r d i n e r a, G . K y l e a, R. G a r n e t t b,29 b,2, D. L o p i a n o b, y . O h a s h i b, T. S h i m a b,3, b, A. Y o k o s a w a b, G . G l a s s c, R. K e n e f i c k c, S. N a t h c,2, o, R . H . J e p p e s e n e, G . T r i p a r d r, M. D e v e r e u x g,d a n d

a New Mexico State Univer.~ity, Las Cruces, NM 88003, USA t, Argonne National Laboratory, Argonne, IL 60439, USA Texas A&M University. College Station, 7")( 77843, USA J Los Alamos National Laboratory, Los Alamos, ,VM 87545, USA University of Montana, Missoula, MT59812, USA f Washington State University, Pullman. WA 99164, USA g Earlham College, Richmond, L¥ 47374, USA University of Wuppertal, W-5600 Wuppertal, FRG

Received 15 November 1990

A measuremcnt of AaL(np), the differcnce between neutron-proton total cross sections in pure longitudinal spin states, is described. Data were taken for five energies between 500 and 800 McV, with statistical errors of ~ 1.5 mb and an estimated normalization error of 6%. The data, combined with other results, show some evidence for an elastic l= 0 spin-singlet resonance with mass ~ 2213 McV and width ~ 74 MeV, or a coupled-triplet resonance with similar mass and width.

Measurements o f A a L ( n p ) , the difference in the total cross section for n e u t r o n - p r o t o n scattering with initial parallel and antiparallel longitudinal spin states, have been performed at the Clinton P. Anderson Meson Physics Facility ( L A M P F ) o f Los Alamos National Laboratory. These measurements were p r o m p t e d by the previous discovery [ 1 ] o f energyd e p e n d e n t structure in A~rL(pp), which has inspired much experimental and theoretical work on the question o f dibaryon resonances "~. A complete description o f elastic n u c l e o n - n u c l e o n ( N N ) scattering requires five complex a m p l i t u d e s cach for both 1 = 0 and I = l spin states. The I = 1 amplitudes are known [ 3 ] to above l GeV, but less d a t a Present address: Argonne National Laboratory, Argonne, IL 60439, USA. 2 Present address: Los Alamos National Laboratory, Los Alamos, NM 87545, USA. 3 Present address: Texas A&M University, College Station, TX 77843, USA. ~t See ref. [2 ] for a review of the status ofdiba~'ons. 24

are availablc for the I = 0 channel. Consequently, the I = 0 amplitudes are not as well determined above 500 MeV. D a t a for A a L ( n p ) are i m p o r t a n t because N N scattcring in the I - - 0 channel is mainly elastic over the range o f energies where structure is seen in AaL(pp, I = l ) and inelasticities are large. The d a t a from this experiment provide an i m p o r t a n t contribution towards the d e t e r m i n a t i o n o f the I = 0 amplitudes since they give information about the spin dependence o f elastic and inelastic np scattering near 0 °, a region not easily obtained in most other np measurements. The first d e t e r m i n a t i o n ofAo'L(np) was performed at Argonne National Laboratory, where the quantity [4] A a L ( p d ) was measured using a polarized proton b e a m and polarized deuteron target. However, the extraction o f AaL(Pn) from the A a L ( p d ) data was highly model d e p e n d e n t ~2. The AGE(l----0) results showed structure, but the model used was untested For footnote see next page.

0370-2693/91/$ 03.50 © 1991 - Elsevier Science Publishers B.V. ( North-Holland )

Volume 258, number 1,2

PHYSICS LETTERS B

for total cross sections with spin. Recently, SATURNE measurements of ACYL(np) using frcc polarized neutron beams were found to disagree with the Argonne values. The Glauber-typc corrections and the values of the real parts of the amplitudes in ref. [4] may be the source of the difference. A measurement with a free neutron beam was needed that did not rely on these corrections. The experiment reported in this letter measured AtYL(np) using a polarized neutron beam produccd by charge exchange of the LAMPF longitudinally-polarized proton beam on a liquid deuterium (LD2) target. Neutrons were polarized to ~ 0.5 through longitudinal spin transfer [5 ]. The energy distribution of the neutrons was characterized by' a narrow quasielastic peak (width ~ l0 MeV) just below the incident proton energy and a well-separated broad inelastic background associated with pion production [ 6 ]. The rejection of inelastic neutrons on the basis of time-of-flight information was facilitated using the new LAMPF "beam buncher", which concentrated about 90% of the incident proton beam current within a narrow pulse ( a ~ ~ ns) with each pulse spaced in time by 100 ns. Also, a new beam feedback steering system controlled the proton beam position at the L D 2 target to within 1 mm using small dipole magnets. The proton beam intensity was monitored by a secondary emission monitor (SEM), which gave a normalization consistent with that measured from scintillation counters viewing a C U 2 target in the neutron beam. The spins of the incident protons were flipped once every two minutes. The magnitude of the proton beam polarization was measured using the quench ratio method [ 7 ]. The neutrons produced at 0 ° were collimated by a 25 mm diameter hole within a 366 cm steel wall and passed through the magnetic fields of two spinprecession magnets, where the fields were adjusted to rotate the neutron spins to - L - t y p e (along beam momentum) in either a clockwise ( + ) or counterclockwise ( - ) sense. The field directions were in-

~2 The Argonne AaL(np) data have since been revised. The At7~ (pd) results arc unchanged, but new values for AOL(np) and A a ( l = 0 ) are available. The revision was necessitated by the neglect of one of the theoretical terms described in the formulas in rcf. [4].

4 April 1991

verted every" few hours to reverse the spin orientations. This allowed for a reduction of systematic effects in measuring AaL (np). The polarized proton target consisted of frozen propanediol beads contained within a cylindrical teflon cell 4.7 cm in diameter and 12.5 cm long. The target was cooled to about 0.5 K with a 3He-evaporation refrigerator. The target was polarized using the dynamic nuclear polarization technique in a superconducting solenoid field of 2.5 T. The target polarization was measured with the nuclear magnetic resonance (NMR) method using three NMR coils monitoring different parts of the target. The polarization direction was reversed after intervals of 6-8 h by re-tuning the polarizing microwaves frequency. Typical proton polarization was ~ _ 0.7. The neutrons were detected downstream of the polarized target by a segmented hodoscope [8] of scintillation counters, preceded by a charged-particle veto counter. The target-hodoscope distance was about 4 m. The hodoscope consisted of 24 counters arranged in four planes of six counters, with the counters in each plane oriented perpendicular to those in the preceeding plane. Two techniques were used to determine the positions of neutrons detected in the horoscope. Fast time-to-digital converters (FTDCs) found the time difference between photomultiplier signals at each end of a counter. This time difference was then used to determine the interaction points for detected neuIrons. The second method used signal coincidences between perpendicular counters in adjacent planes to determine the interaction points. The two methods gave consistent results for AaL(np), and the FTDC method was adopted because of the smaller statistical errors. In terms of the normalized yields N + and N - ( + / indicating parallcl/antiparallel orientations of beam and target polarizations), the asymmetry ~(t)=("V+-N-)/(N++N -) was calculated for each cell of the hodoscope, where t is the momentum transfer squared defined by the geometry of the hodoscope. The value of~(t) extrapolated to t = 0 is the asymmetry for the amount of beam transmitted through the polarized target at 0: for parallel and antiparallel spins, and is related to the total cross seclion difference AaL by -

25

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Pn Pt e(0)= -~--ao L.

(1)

Here, P. and PI represent the neutron beam and target polarizations, respectively, and ,4 is the target constant, (pFL) -~, with p~ being the number density office protons in the target and L its effective length. Table 1 shows the number of beam and target runs at each energy and thc resulting values of both AO'L(np) and A a , . ( l = 0 ) . Each data run lasted bctween one and two hours. The notations + + , + - , + and - - refer to the directions of the proton target polarization ( + or - ) and neutron beam precession ( + or - ) , respcctivcly. The I = 0 component of AaL was calculated by the relation -

Ao~.(I=0) = 2AcyL(np) - - A a L ( p p ) .

(2)

The values for AaL(pp) (corrected for Coulomb effects) were interpolated using data from refs. [ 1,9]. Correlations between the extrapolated spin asymmete" ~(0) and various experimental parameters were studied, such as the spin asymmetr3' eF in the fraction of proton beam bunched for both spin directions, fits were modelcd following the form e(0)=

P,,I~ ~-AcsL(np)+systematiceffects,

(3)

where the quantity AaL(np) was treated as an independent paramcter found by the least-squares methods. The square root o f t h c reduced Z 2 for the correlation fits ranged from about 0.9 to 1.6, and was typically about 1.4. This was attributed to the uncertainty in the SEM, which was not included in the error analysis for Aa~.(np). Therefore, the uncertainties reported in table 1 were increased by the square root of the reduced Z 2.

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The beam buncher was tbund to cause large effects, which were corrected by including a systematic term c~er in eq. (3). The parameter ot was determined to be ~ 1.0. An offset constant term was also found: however, the source of this term remains undetermined. No other statistically significant effects were found. The normalization error for Aoz(np) was estimated at 0.06lAal,(np) I at all energies ( ~0.5 mb). those systematic effects considered in the estimate for the normalization error included the uncertainties in the target constant (4.4%), the polarizations of the proton beam (1%) and the polarized target (3.8%), and the systematic error due to finite target size (1%). The beam profile at the polarized target was measured to estimate the magnitude of this latter effect and the error was taken as one-half the fraction of beam missing the target. The uncertainty in the polarization of the neutron beam ( ~ 10%) was not included in the calculation of the normalization error, as the spin transfer data arc dependent on existing np analyzing power data (see Chalmers ct al., ref. [ 5 ] ). The data are displayed in fig l a. Our data arc in reasonable agreement with published results from Saclay [ 10], and unpublished data from both S I N / PSI [11] and Saclay [12]. Fig. lb is a plot of A a L ( I = 0 ) calculated using eq. (2). The ACrL(I=0 ) data exhibit a clear, pronounced maximum at about 600 MeV. The phase shift prediction (dotted line) of Arndt et al. ~3 is also shown. The prediction is a re-

~3 T h e VPI N - N interactive d i a l - i n - p r o g r a m S A I l ) was used (rcf. [ 3 ] ). The p r e d i c t i o n for Aal.(1= 0 ) was taken from global energy s o l u t i o n S M 8 9 ( 1 9 8 9 ) .

Table 1 T h e n u m b e r o f a n a l y z e d r u n s at e a c h energy for v a r i o u s target a n d b e a m p o l a r i z a t i o n s , a n d c o r r e s p o n d i n g ACrLf o r n p a n d I = 0 are s h o w n . T h e n o t a t i o n s + + , + - , - + a n d - - refer to the d i r e c t i o n s o f the p r o t o n target p o l a r i z a t i o n ( + or - ) a n d n e u t r o n b e a m p r e c e s s i o n ( + or - ) for the d a t a runs. T h e I = 0 values were f o u n d u s i n g i n t e r p o l a t e d A a L ( p p ) values a n d eq. ( 2 ) . "/~b (MeV)

484 568 634 720 788

26

Number of runs

AO"L( m b )

++

+-

-+

24 12 21 30 34

18 14 20 27 28

23 11 20 17 27

28 11 20 28 22

np

1=0

- - 7 . 4 6 ± 1.41 - 4 . 6 0 + 1.64 - 2 . 6 2 ± 1.29 - 8.05 ± 1.36 -8.47+0.81

-3.77±2.82 - 0.05 + 3.28 6.83z2.58 0.32 _- 2.72 0.27 - 1.62

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I

I

I

I

10

AaL(np) 0 -10 -20

(a)

-30 o

m

o

10

~

0

...

A O ' L ( I ~

- 10

"i

-20 -30

. 1t i .....I 0

200

(b) 400

600

800

1000

1200

1400

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inelastic and total cross section data [ 17 ] following the methods described in ref. [ 14]. The best fit was found using a spin-singlet resonance (~P~, IF3 . . . . ) with mass 2213 MeV, width 1"= 74 MeV, and elasticity x = (2.1+ 1 )FJF= 0.913. F o r total spin J = 1 this I = 0 resonance would couple much m o r e strongly to the elastic channel than the I = 1 dibaryons. The fit is shown in fig. l b by the solid curve. The position of the peak is fixed by all data points. The d a t u m at 634 MeV, although it suggests a smaller resonance mass, contributes little to the overall Z 2 fit. Moreover, its influence is offset by the PSI d a t u m at 520 MeV. We cannot rule out the possibility o f a coupled-triplet resonance (3S,, 3Dr, ...), since the fits to the data are ver~' similar to the single case. Differentiation between the singtet and coupled-triplet probably requires A a [ data between 550 and 800 MeV. In conclusion, we note that recently two theoretical groups have predicted I = 0 d i b a r y o n resonances in our mass region. In ref. [18], positive-parity resonances ( ~S~, ~D~ ) are found that are likely to be rather broad, whereas in ref. [19], several narrow resonances in the ~P~ partial wave are predicted.

T1ob(MeV) Fig. I. Plot of AcrLfor np and I=0. The symbols ~ and A represent unpublished data from SIN/PSI (ref. [ I 1] ) and SACLAY (ref. [ 12i ), respectively; published SACLAY data (rcf. [ I0] ) are indicated by ,~'s; and the data from this experiment (LAMPF) are plotted using o's. The I=0 component shown in (b) was obtained using eq. (2). The dotted line is the phase shift prediction of Arndt et al. (see footnote 3), and the fitted solid curve, representing a resonance by a singlet partial wave, results from the analysis described in the text.

flection o f the small size o f the np database. None o f the conventional models can account for this structure, since there are no strong inelasticities for I = 0 over this range o f energies [ 13 ], as is the case for the I = 1 channel (see ref. [ 14] ). It is i m p o r t a n t to recognize that the structure in AOL for the I = 0 channel cannot be explained by threshold effects (such as NA or rid) or other nonrcsonant contributions from the inelastic channel. Note also that the thresholds for NN* and AA are at higher energies. The b u m p in A a L ( I = 0 ) m a y be interpreted as an I = 0 d i b a r y o n resonance. This possibility was investigated by fitting the B r e i t - W i g n e r resonance formula to AaL data ~4, Aav d a t a [ 16,15,10-12 ], and

We would like to thank the polarized-target, mechanical, cryogenic, computing, electronics, and accelerator operations groups at L A M P F for their assistance throughout the experiment. In particular, we thank M. McNaughton and O. van Dyck for their assistance with beam tuning and the b e a m feedback steering system, and J. Dawson (Argonne National L a b o r a t o r y ) for designing the F T D C s p r o v i d e d for the experiment. This work was s u p p o r t e d in part by the US D e p a r t m e n t o f Energy, Contracts No. W3 I109-ENG-38, No. DE-AS05-76ER-04449, and No. DE-AS04-76ER-03591, and G r a n t s No. D E - F G 0 5 88ER-40399 and No. DE-FG04-88ER40403. ~4 Our data were supplemented by refs. [ 10-12] and a few low energy data points from refs. [ 15,1.9 ].

References [ 1] I.P. Aucr et al., Phys. Left. B 67 (1977) 113; B 70 (1977) 475; Phys. Rcv. Lett. 41 (1978) 354. [2] E.g., M.P. Locher, M.E. Sainio and A. ~varc, Adv. Nucl. Phys. 17 (1986) 47; A. Yokosawa, Intern, J. Mod. Phys. A 5 ( 16 ) (1990) 3089. 27

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[3] R.A. Arndt, J.S. Hyslop and L.D. Roper, Phys. Rev. 1) 35 (1987) 128; J. Bystricky et al., J. Phys. 51 (1990) 2747. [4] I.P. Auer et al., Phys. Rev. Left. 46 ( 1981 ) 1177. [ 5 ] P.J. Riley et al., Phys. Lett. B 103 ( 1981 ) 313; J.S. Chalmers et al., Phys. Left. B 153 ( 1985 ) 235. [ 6 ] C.W. Bjork et al., Phys. Lctt. B 63 ( 1976 ) 31. [7] P.R. Bcvington et al., Phys. Rev. Lett. 41 (1978) 384. [ 8] R. Garnett et al., A neutron hodoscope for medium energy np scattering experiments, to lye submitted to Nucl. lnstrum. Methods. [ 9 ] E. Aprile-Giboni et al., Nucl. Phys. A 431 (1984) 637; I.P. Auer et al., Phys. Rev. D 29 ( 1984 ) 2435; J. Bystricky et al., Phys. Lctt. B 142 (1984) 130. [ 10] F. Lehar ct al., Nucl. Phys. A 478 (1988) 533c.

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[ 11 ] R. Binz et al., PSI preprint PSI-PR-90-36 (1990). [ 12] F. Lchar, private communication. [13]J. Bystricky et al., J. Phys. 48 (1987) 1901. [ 14] W. Grein, A. KiSnig and P. Kroll, Phys. Left. B 96 (1980) 176. [ 15 ] F. Lchar et al., Phys. Left. B 189 ( 1987 ) 241. [ 16] F. Perrot et al., Nucl. Phys. B 278 (1986) 881. [ 17 ] F. Shimizu ct al., Nucl. Phys. A 386 ( 1982 ) 571; P. Schwallcr et al., Nucl. Phys. A 316 (1979) 317; V. Grundies ct al., Phys. Lett. B 158 ( 1985 ) 15; P.W. Lisowski et al., Phys. Rev. Lctt. 49 (1982) 255. [18] H. Hofcst~idl, S. Merk and H.R. Perry, Z. Phys. A 326 (1987) 391. [ 19 ] K. Konno et al., in: Proc. 9th Intern. Symp. on High energy spin physics (Bonn, 1990), to be published.