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).
Group
E691
ARGUS
CLEO
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
2 5 i 5 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
120
lOO
) e0
0 ~ 6 0
~ 4 0 >
ZO
0 . 3 5 0 . 4 5 0 . 5 5 0 . 6 5 0 . 7 5 0 . 8 5
m(D*~')-m(D *) GeV/c 2
Figure 1: m(D+Ir - ) - m(D +) mass difference spectrum
from E691.
dN d(cos e.~)
1.5
1.0
0.5
0.0
. . . .
-1.( -O.5 O.O 0.5 1.0 COS ~ ;
Figure 2: Angular distribution from ARGUS for
D'(2459) decay.
N ~ ~ , , , i , . . , i , ' , , i , , ~ , .
20 MeV/cZl - ARGUS • 160 L J- I Preliminar~
5O
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) ~ <( ( l + A c o s 2 a ) ; 1 + decay
where A lies in the range between -1 and 3 inclusively.
Note that a superposition of 2 + and 1 + states could
lead to an isotropic distribution, such as reported by
CLEO 7,
Assuming that the D*(2420) were a single reso-
nance, f i t t ing the signal in different bins of cos ~, should
always yield the same mass and width. On the other
hand, if it were an overlap of two or more resonances,
one might expect such an analysis to see shifts in the
fitted mass and width as a function of angle as differ-
ent relative weights of the underlying resonances were
averaged. In Table 2 are shown the preliminary results
of such an analysis by ARGUS, clearly exhibiting that
the "D*(2420)" is indeed not a single resonance.
!cosal Fitted Masst Fitted Widtht
Range (MeV/c ~) (MeV/c 2)
> 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) M e V / 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
N d l c o s a
1.5
1.0 .......
0.5
0.0 : . . . . . . . • " " I " " •
,.5 _ _ ( b )
1.0
0.5 . . . ~ , . . ~ .
0.0 ' ' ' ' " ' " ' . . . . 0.0 0.2 0A 0.6 0.8 1.0
ICOS al
• 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
50
• ' ' I ' ' ' I ' ' " I ' ' ' ' '
ARGUS
~.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
ARGUS.
N ' ' ' °
3 ~ e V / c z
6
3
0 . , , i
2.45
| , , 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 6.5¢r, 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 ~ <~ 6.2 MeV/c ~ (90% confidence level). The frag-
mentation function of the signal is hard, as expected for
a leading particle from non-resonant c~ production.
[n order to constrain the spin-parity, a search for the
same state has been performed in the final state D+Ks O.
No signal has been found, yielding the upper limit
Br(Ds(2536 ) -~ D+Ks 0) Br(D~(2536)-~ D'+Ks °) < 79% (90% C.L.).
This would imply that the most likely interpretation is
that the D~(2536) is one of the 1 + L = I Ds mesons.
In conclusion, a great deal of progress has re-
cently been made in the experimental study of P-wave
charmed mesons. All of the currently available informa-
tion would tend to imply that we have thus far observed,
using the new PDG naming conventions, both neutral
and charged 2 + P-wave D mesons, the D~°(2459) and
the D~+(2471), one of the neutral 1 + D mesons, the
D°i(2414), and one of the 1 + Ds mesons, the D+1(2536 ).
t Statistical error only.
1. H. Albrecht, st al. (ARGUS), Phys. Rev. Lett. s49 (1986).
. See the review in J.L. Rosner, Comments on Nu- clear and Particle Physics 16, 109 (1986), and references therein.
3. J.C. Anjos, et ah (Tagged Photon Collaboration), Fermilab-PU B-88-155E.
4. H. Albrecht, et al. (ARGUS), Phys. Lett. B221, 422 (1989).
. CLEO Collaboration, "Report to the CESR Pro- gram Advisory Committee", Internal Report, 1989, unpublished.
6. J.A. Parsons, Ph.D. thesis, University of Toronto, Toronto, Canada; In preparation.
. C. Bebek, et al. (CLEO), Paper submitted to the 1987 International Symposium on Lepton and Photon Interactions at High Energies, Hamburg, 1987.
. R. Kutschke, Presented at the Lake Louise Win- ter Institute, Frontiers in Physics from Colliders to Cosmology, Lake Louise, Canada (1989); To be published in the proceedings. R. Kutschke, Ph.D. thesis, University of Toronto, Toronto, Canada; In preparation.