Nucleaz Physics B (Proc. Suppl.) 13 (1990) 247-250 247 North-Holland
AN EXPERIMENTAL REVIEW OF P-WAVE CHARMED MESONS
John A. PARSONS
I I - - ! . . . . . ?~ . . . . . A -
A review of the experimental information currently available concerning P-wave charmed mesons is presented. Recent results include the first observation of a candidate for an L=I D$ meson, the first measurement of an isospin mass splitting in the P-wave D meson sector, and a decay angular analysis attempting to determine the spin-parity of the D*(2420).
Much experimental progress has been made in the
study of P-wave c|zarmed mesons since the discovery by
ARGUS 1 of the D'(2420), the first candidate for such
a state. The spectroscopy of these states provides an
important means of exploring the spin-structure of ,~he
quark-antiquark potential at relatively large distances;
predictions of their mass spectra and ~ec~y propert;s
have been made with several different models 2.
Coupling a single unit of orbital angular momen-
tum to spin 0 or 1 yields four P-wave states, with spin-
parities (JP) 0+,1+,1 +, and 2 +. In order to extract
useful information from the theoretical models, it is es-
sential to determine the spin-parities of the observed
states. While spin-parity conservation in strong decays
serves to limit the possible JP values, in general a more
detailed angular analysis is needed to uniquely specify -,- jP. ,,,~ correct
In t;~e case of P-wave D mesons, the 2 + state can
decay to beth D*(2010)Tr and D~r, while the 1 + states
must decay to D'(2010)~r and the 0 + to DTr.
The DTr final state is simplest to interpret since all
models predict a 2 + - 0 + mass difference in excess of
100 MeV/c 2. Furthermore, the 2 + is expected to be
much narrower than the 0 +. E691 has reported 3 the
observation of a state of mass 2459 MeV/c 2, referred
to hencefor~.h as the D*(2459), decaying to D+Ir - (see
fig. 1). This observation has since been confirmed by
ARGUS 4 and by CLEO 5. The measurements are all in
*Dept. of Physics, Univ. of Toronto, 60 St. George St., Toronto, Ontario, Canada MSS 1A7.
good agreement, as shown in Table 1. The assignment
as the 2 + state, strongly suggested by the heavy mass
and narrow width, is further supported by a decay an-
gular analysis performed by ARGUS4; the signal was
fit in bins of cos O;, where 6; is defined as the angh
between the pion and the D*(2459) boost direction, as
measured in the D*(P.A59) rest frame. The evidence
for an anisotropic distribution (see fig. 2) would im-
ply polarized production, possible only for the 2 + state.
Superimposed on figure 2 are the results of fits to an
isotropic distribution (dotted line) and to the distribu-
tion for a 2 + decay where the 2 + meson is produced
with equal populations in helicity states 0 and 4-1, but
with population zero in helicity states 4-2 (solid fine).
Mass Width Rate Compared
(MeV/c z) (MeV/c 2) to D +
2459 4- 4 20 4- 11 .07 4- .03
2455 4- 6 .11 _+ .06
2463 4- 7
15 + 14
25 i5 t .07 4- . .
Table 1: Measured parameters of D*(2459).
ARGUS has now observed 6 a new meson decay-
ing to D% + (see fig. 3), which would seem to be the
charged isospin partner of the D*(2459). The natural
width and production rates are consistent with those
measured for the D*(2459), and the mass is measured
to be (2471 8) MeV/c ~t. This yields a measured
isospin mass splitting of
miD*+(2471)] - miD'0(2459)] = 16 4-9 MeV/c 2 t .
0920-5632/90/$03.50 Elsevier Science Publishers B.V. (North-lIolland)
248 J.A. Parsons~P-wave charmed mesons
0 ~ 60
~ 40 >
0.35 0 .45 0 .55 0 .65 0 .75 0 .85
m(D*~')-m(D *) GeV/c 2
Figure 1: m(D+Ir - ) - m(D +) mass difference spectrum
dN d(cos e.~)
. . . .
-1.( -O.5 O.O 0.5 1.0 COS ~;
Figure 2: Angular distribution from ARGUS for
N ~ ~ , , , i , . . , i , ' , , i , , ~ , .
20 MeV/cZl - ARGUS 160 L J- I Pre l iminar~
0 I . . . | i I I I I I I I , I . . i i
0.30 0.45 0.60 0.75 0.90 m(DOlT+)-m(D) (GeV/c ~)
Figure 3: Preliminary m(D%r +) - m(D ) mass difference
spectrum from ARGUS.
The enhancement seen in figure 3 towards lower mass
differences results from feed-down from the decay D* --~
D*(2007)Tr +, where D*(200T) --*D+ (Tr,'t), and the
neutrals bo undetected. This result was first seen by
E5913, with a mass difference corresponding to re(D*)
= 2443 MeV/c 2. E691 was unfortunately unable to
confirm the direct observation of the D'+(2471). As
will now be discussed in the case of the D*(2420), the
"peak" with mass 2443 MeV/c 2, arising as it does from
a D*~r final state, is very difficult to interpret directly in
terms of single P-wave states.
In many of the theoretical models, the 2 + -1 + mass
splittings are predicted to be very small. Furthermore,
predictions of the masses and decay properties of the
1 + P-wave states suffer from large uncertainties due to
the fact that the two states can mix with one another.
This means that the physical interpretation of experi-
mental observations in the D'Tr final state is much more
ambiguous than in the DTr case. Hence, although the
ARGUS observation of the D'(2420) has now been con-
firmed by CLEO 7 and by E6913, the physical interpre-
tation of the signal has proven very difficult. ARGUS
has now performed 6 a new decay angular analysis at-
tempting to disentangle this problem.
In the decay D*(J P) -~ D*(2010)lr1, the helicity
distribution of the D*(2010) can be measured through
the subsequent decay D'(2010) --~ D1r 2 in order to spin
analyze the mother particle. For a 2 + decay, spin-parity
conservation requires the D*(2010) and lrl be in a rel-
ative D-wave, and therefore that the D*(2010) have
helicity +1. A 1 + decay, on the other hand, could pro-
ceed Via either a D-wave, an S-wave, or a combination
of both. Defining ~ as the angle between 7rz and 1r2
as measured in the D*(2010) rest frame, the expected
angular distributions are then given by
sin 2 c~ ; 2 + decay dN
o( (1 + 3 cos 2 ~) ; 1 + pure D - wave d(cos ~) 1 ; 1 + pure S - wave
Due to the uncertainty in the admixture of the par-
tial waves in the 1 + decay, and also the uncertainty in
the polarization of the initial P-wave D meson, all we
J.A. Parsons/P.wave charmed mesons 249
can say a priori is that
dN [ sin 2a ; 2 + decay d(coso) ~ 0.75 2417.0 ~ 2.2 25.3 4- 9.2
0.50 2429.6 -I- 4.8 53.4 + 15.5
Table 2: Preliminary ARGUS measured mass and width
shifts of the D*(2420) as a function of I cos~l.
It is now possible to remove most of any 2 + contri-
bution by requiring I cos ~1 > 0.75. Fitting the resultant
signal with two Breit-Wigners convoluted with Gaus-
sians, one with mass and width fixed to the 2 + results
from the P_,~'~r- analysis, and the other with free mass
and width, then yields a 1 component of mass (24144-
2) MeV/ J f and natural width (13 4- 6) MeV/c 2t. The
results are quite insensitive to variations of the 2 + pa-
rameters, producing systematic uncertainties of about
3 and 6 MeV/c 2 on the mass and width respectively.
One can then fix both masses and widths, and fit
the total signal in bins of I cos~l in order to check the observed angular distributions against the expectations
outlined previously. The preliminary ARGUS results are
shown in figure 4. The 2 ~ distribution agrees nicely with
the sin s a expectation (solid line), while the mass 2414
MeV/c z component d,stribution looks quite similar to
the expected distribution for a 1 ~ pure D-wave decay;
a fit to the form B(l+Acos 2 (~), shown as the solid line, . . : - - I J . . . . . I . y,c,us a va,ue of A of approximately 2-3. Theoretically,
the mixing of the 1 + states is expected to result in
two physical states which, in the limit mc --* oo, decay
either via a pure D-wave or via a pure S-wave respec-
tively. Also shown in figure 4 are the fits to isotropic
distributions (dotted lines).
1 dN " " I " " I l I B Ndlcosa
0.0 : . . . . . . . " " I " "
,.5 __ (b )
0.5 . . .~ , . .~ .
0.0 ' ' ' ' " ' " ' . . . . 0.0 0.2 0A 0.6 0.8 1.0
I , . , ! . . , !
Figure 4: ARGUS angular distributions for D*+Tr - com-
ponents of masses (a) 2414 and (b) 2455 MeV/c 2.
Finally, in fig. 5 is shown the fit over all coso. As
a further check, theory tells us 2 that
R --- 1"(2+ -~ D':r) - 3 factors
~odel-dependent kinematics produce R values in
the range of 1.5 - 4. The ARGUS preliminary value is in
good agreement with this expectation, with a measured
value in the range 3 - 4.
250 J.A. Parsons~P-wave ci~armed mesons
I0 MeV/c ~ lO0
' ' I ' ' ' I ' ' " I ' ' ' ' '
~.0 2,.2 2.4 2.6 2.8 3.0
mlD*+~r -) (GeV/c')
Figure 5: D*+~r - mass spectrum including all cos,, from
N ' ' '
3 ~eV/c z
0 . , , i
| , , i | . , , ,
2.55 2.65 2.75 m(D**K~) (GeV/c ~)
Figure 6: D'+Ks 0 mass spectrum from ARGUS.
The P-wave Ds mesons are expected to lie approx-
imately 100 MeV/J above their D counterparts. For-
bidden to decay to Ds~ due to isospin conservation,
the/should instead decay strongly to final states such
as D'(2010)K and DK. ARGUS has very recently an-
nounced the first observation 8 of a state decaying to
D*+(2010)Ks 0 (see fig. 6). The signal of 16 + d events,
with a statistical significance in excess of 6.5r, yields a
fitted mass of (2535.6 + 0.9 -I- 2.0) MeV/c 2. The ob-
served width is consistent with the expected detector
resolution, yielding an upper limit on the natural width
of l ~