SLAC-PUB-1709
January 1976
(T/E)
EVIDENCE FOR A NEW STRANGENESS-ONE PSEUDOSCALAR MESON*
G. W. Brandenburg?, R. K. Carnegie??, R. J. Cashmoref-tt,
M. Davier$, W. M. Dunwoodie, T. A. Lasinski, D. W. G. S. Leith,
J.A. J. Matthews$$, P. Walden$$$, and S. H. Williams
Stanford Linear Accelerator Center
Stanford University, Stanford, California 94305
ABSTRACT
The Jp = O- partial waves of the Kn7r system in the reactions
K&P
-+ K*r’n-p at 13 GeV are presented. Structure in intensities and
relative phase variations suggests the existence of a pseudoscalar reso-
nance, the K’, with a mass of -1400 MeV and a width of -250 MeV
decaying predominantly into EK.
(Submitted to Phys. Rev. Letters. )
“Work supported by U. S. ERDA.
SNOW at Phys. Dept. , MIT, Cambridge.
Tj-Now at Phys. Dept., Carleton U., Ottawa.
Ttj-Now at Phys. Dept. , Oxford U. , Oxford.
SNOW at Lab. de 1’Accel. Lineaire, Orsay.
f$Now at Phys. Dept., MSU, E. Lansing.
$$$Now at TRIUMF, U. of B. C., Vancouver.
-2-
The observation of pseudoscalar resonances is of fundamental importance
to our understanding of the meson spectrum. Within the quark model’ higher
mass recurrences of these states can occur only as radial excitations of the L=O
qS system.
Evidence2 for radial excitations in the meson system is provided by
the p1
and the $I,
although each of these states could have an alternate assign-
ment in the L=2 qs supermultiplet.
In this paper we present results which suggest
the existence of a pseudoscalar, strangeness-one resonance, the KY, with a mass
in the vicinity of 1400 MeV.
The data on which these results are based were obtained in a spectrometer
experiment studying the reactions K*p - K
*~++a-p at 13 GeV. The salient fea-
tures of the
experiment
3,495
are high statistics, good resolution, K/n identifica-
tion, and K* relative normalization uncertainty of 52%.
We have described
previously
394
the principal Jp=l+ and 2+ partial waves6 of the K*n+p- system.
In addition to isolating a small, but clear K*(1420) signal, we presented evidence
for the existence of two axial vector mesons, Ql with a mass of -1300 MeV and
Q2 at -1400 MeV.
As they are germane to our discussion of the O- waves, we briefly summarize
certain features of these Jp = 1’ states.
Although their total widths are compar-
able, they appear to have quite different production and decay properties.
The
production of Q, obeys approximate s-channel helicity conservation, and it seems
to decay principally to PK.
In contrast Q, production satisfies approximate
t-channel helicity conservation, while its dominant decay mode is K*n.
The
evidence for these different decay modes rests, in part, on the observed phase
motion of the If pK waves relative to l+O+K*?n a forward motion of
-70’ for
2.20 5 m(Knn) 5 1.35 GeV and a backward motion of -50’ for 1.35 2 rn(Krr)
5 1.45 GeV. In addition the l+
pK mass spectra peak only in the region of
L
/
-3-
1300 MeV. We infer from these observations that the absolute phase of the l+
oK waves has a Breit-Wigner variation in the 1300 MeV region but is slowly
moving by 1400 MeV.
On the other hand, the absolute phase of the l+O+K*n wave,
associated with the Q2 meson, would be rapidly moving in the 1400 MeV region.
Since we are discussing the possibility of a O- resonance in the vicinity of 1400
MeV, it is clearly preferable that the reference wave exhibit limited phase
motion in this region.
Thus, for the results presented in this letter, we use the
l+O+ pK wave as the reference wave.
In Figs. la,b we show the O-O+ EK cross section and relative phase as meas-
ured with respect to l+O+ PK.
3
Cross sections for ambiguous solutions, present
only in the K- data at low K7rr mass, are shown by crosses in those cases where
they differ significantly from the highest likelihood solutions.
For Knn masses
where either the displayed wave or the reference wave is small in intensity, the
relative phase measurement becomes unreliable and is not shown. The K’ and
K- data both exhibit a broad (-250 MeV) bump at about 1400 MeV.
The points
above 1.6 GeV were obtained with the wave set of Ref. 3.
While they indicate
that all the O- waves are decreasing in this region, the qualification must be made
that higher spin waves are becoming important beyond 1.6 GeV. Nevertheless a
preliminary study of our data in the L-region (1.6 < m(Krr) < 2.0 GeV) with an
extended wave set indicates that all the O- waves have decreased in intensity by
-1.7 Gev.
The corresponding results for the 2+1+K*7r wave are shown in Figs. lc, d.
The curves indicate the d-wave Breit-Wigner line shape and phase variation for
the K* (1420)) 2 and it is clear that the data are consistent with the expected be-
havior. In Figs. la, b the curves correspond to an s-wave Breit-Wigner param-
etrization with a mass of 1400 MeV and a width of 250 MeV.
7
Both the O-O+ EK
-4-
and the 2+lfK*n waves are well described by these parametrizations for intensity
and phase. This is strongly suggestive of a resonance interpretation for the
O-O+ EK wave.
We turn next to the other significant O- partial waves?
In contrast to O-O+ EK ,
the O-O+K*r wave (Figs. 2a, b) is already large by -1.23 GeV and reveals some
indication of a two-peak structure.
While the lower mass peak at -1.23 GeV is
of roughly the same strength in both K+ and K-, the K’ intensity is greater than
that for K- in the 1.3-1.5 GeV mass region. This two-peak structure is similar
to that for l+O+K*w,
3
except that for the latter the higher mass peak is stronger
for K- than for K’.
The relative phase measured with respect to l+O+ pK
decreases by
-60’ for 1.23 5 m(Knr) 5 1.35 GeV, where the l+
pK
system reso-
nates, and increases by -50’ for 1.35 5 rn(KrT) 5 1.56 GeV. We tentatively
interpret these observations in terms of a two-component description of the
O-O’K*n wave: a “Deck” background approximately constant in phase and peaking
in intensity near 1.23 GeV together with a resonance -contribution at -1.4 GeV.
The O-O+ pK wave (Fig. 2c, d) is about 30% the size of the O-O+ EK wave in
intensity and appears to peak at a somewhat higher KTM mass. However, since
its phase relative to l+O+ pK shows some forward motion in the 1.4 GeV region,
we would conclude that the O- resonance may couple weakly to PK.
The momentum transfer (t’) dependence of the O-O+K*n and O-O+ EK waves is
shown in Fig. 3.
The distributions for these waves are considerably steeper than
those for the l+O+K*n waves.
4
Parameter values from fits to the data with the
expression A exp (bt’) are given in Table I. The O-O+ EK waves are slightly larger
near t’= 0 than the O-O+K*r waves.
There is an apparent %-rang-sign” cross-
over (i.e.,
be > bK-)
in the O-O+K*r distributions, while none is evident for
the 0-O’ EK waves, even though the K- distribution is somewhat steeper than the
-5-
K+.
We note that the presence of a O-O+K*r cross-over is consistent with the
difference between the K+ and K- mass spectra of Fig. 2a, b in the region of
1.4 GeV. However, due to the steepness of these waves, the limited statistics
in the last two I? intervals, and certain technical considerationsY in our method
of analysis, we must at present consider this effect as an intriguing possibility.
The difference in the production properties of these waves is presumably related
to our previous observation that, whereas the O-O+ EK wave appears to be purely
resonant, the O-O+ K*n wave may require a two-component description.
We have presented evidence for the existence of a Knn pseudoscalar reso-
nance, the K’, of mass -1400 MeV and width -250 MeV.
The predominant decay
mode is EK with weaker couplings to K*n and, possibly, PK.
While it is con-
ceivable that non-r -exchange
71Deck7Y mechanisms could describe the O-O+ EK
mass spectrum, they are not expected to yield a phase variation such as that
shown in Fig.
1; it is then natural to associate such phase motion with the exist-
ence of a O- resonance.
In contrast “Deck” contributions appear to be significant
for the K*T system, so that a determination of the K’ coupling to K*‘n must await
a quantitative interpretation of the data in terms of the two-component descrip-
tion mentioned above.
Indeed a precise measurement of the K’ mass and width
also depends on how much the absolute phase of the l+O+ oK reference wave
changes in the 1.3 - 1.5 GeV mass region. Nevertheless, the evidence for a
O- KT~T resonance is already rather persuasive. In the quark model such a state
would result from a radial excitation of the L=O, qi system and would imply the
existence of other pseudoscalar mesons in the 1400 MeV mass region.