Transcript
Page 1: An experimental review of P-wave charmed mesons

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)

Page 2: An experimental review of P-wave charmed mesons

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

Page 3: An experimental review of P-wave charmed mesons

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.

Page 4: An experimental review of P-wave charmed mesons

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.


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