An experimental review of P-wave charmed mesons

  • Published on
    25-Aug-2016

  • View
    214

  • Download
    0

Transcript

  • 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

    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

    120

    lOO

    ) e0

    0 ~ 60

    ~ 40 >

    ZO

    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

    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 Pre l iminar~

    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) ~ 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

    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 ~eV/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 6.5r, 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 ~