Study of charmed vector meson production by antineutrinos

Volume 132B, number 1,2,3 PHYSICS LETTERS 24 November 1983

STUDY OF CHARMED VECTOR MESON PRODUCTION BY ANTINEUTRINOS

A.E. ASRATYAN, V.I. EFREMENKO, A.V. FEDOTOV, P.A. GORITCHEV, S.P. KRUTCHININ, M.A. KUBANTSEV, I.V. MAKHLUEVA, V.I. SHEKELYAN, V.G. SHEVCHENKO Institute for Theoretical and Experimental Physics, Moscow, USSR

and

V.V. AMMOSOV, A.G. DENISOV, G.S. GAPIENKO, V.A. GAPIENKO, V.I. KLYUKHIN, V.I. KORESHEV, P.V. PITUKHIN, V.I. SIROTENKO, E.A. SLOBODYUK, Z.U. USUBOV and V.G. ZAETZ Institute for High Energy Physics, Serpukhov, USSR

Received 18 August 1983

The two-stage decay D*- ~ D°lr-, D ° ~ K+~r-X of the D*- mesons produced by antineutrinos in the 15 ft bubble chamber is studied. For an antineutrino energy above 10 GeV the rate of D*- production relative to all charged current events is found to be 0.042 -+ 0.013.

The decay D*- ~ D°~r- (D *+ ~ DOn +) with its' vanishingly small phase space provides an efficient tool for the study of charged D* production in strong, electromagnetic and weak processes [ 1-3 ]. Qualita- tively, one chooses a particular ~0 decay mode (usual- ly K+Ir - , K+n+rr-n - or K0n+n-), selects the corresponding combination with a mass close to that of ~0 and looks for an additional n - which is almost at rest in the rest frame of the selected system. Insofar as D *÷ neutrino production is concemed, this method has first been applied to BEBC vup data (with D O ~ K - u +, K - n - n + n +, K°n+n - , collective absolute rate 7 + 4 events [2] ) and, more recently, to the 15 ft bubble chamber Pu Ne data (D O -+ K01r+n - , absolute rate 6 events [3] ). Such relatively small absolute rates are explained by the smallness of corresponding D O par- tial branching ratios [4]. The aim of the present study is to modify this approach so as to embrace a sizeable proportion of ~0 decays on a common basis.

~ualitatively, the idea is that once a massive enough subsystem X is selected among the ~0 decay products it is expected to stay almost at rest in the ~0 rest frame which is almost the same as the D*- decay pion frame. Thus, the quantity

A = m(Xn-) - re(X) - m(rr-) (1)

should not be much greater than m(D*-) - m(D 0) - m(n- ) ~ 6 MeV.

The choice X = (K+n - ) seems appropriate since thereby as much as (56 + 11)% of the ~0 decays are taken in ref. [4]. A simple phase space Monte Carlo calculation shows that A typically lies in the range 10 -20 MeV for all B 0 decays involving K +. The calculated acceptance of the cut A < 30 MeV is 0.80, 0.84, 0.92 for D*- ~ D0n- with subsequent B 0 -+ K+Tr-lr 0, K + n - 2n 0, K + n - 31r 0, respectively. For ~0

K+n+Tr-rr - two (K+lr - ) combinations can be formed; the calculated acceptance of A min < 30 MeV is 0.97. The figures quoted are not to be taken literal- ly since the assumption was made that the ~0 decay matrix element is constant over the phase space. With a less massive subsystem selected (X = n+lr-), the acceptance of the cut A < 30 MeV is around 0.6 for most decays.

Analysed along these lines are the antineutrino data from the 15 ft bubble chamber with heavy neon-hy- drogen Idling (see ref. [5] for details on the data as well as a description of the charged current sample iso- lation procedure). For the 1/a + events with E v > 10

246 0.031-9163/83/0000-0000/$ 03.00 © 1983 North-Holland


GeV and pu > 4 GeV the quantity A is calculated for all (K+n-)Tr - combinations with m(K+lr - ) ~< m(D0). The kaon mass is arbitrarily assigned to the positive meson. A furthe: angular cut cos 0 > - 0 . 2 is applied to these combinations where 0 is the angle between the K ÷ and X momenta in the X = (K+lr - ) rest frame. This cut serves as a kinematic K + selection criterium: while the decay kaons are produced isotropically in the X rest frame, an inclusive pion to which a kaon mass is assigned is more likely to travel backwards.

Of the surviving (K+Tr-)rr - combinations, a single (minimal) A is plotted for each event to reduce com- binatorics (see fig. 1). For comparison, A min is similar- ly plotted for "wrong sign" (K+rr-)Tr + combinations. The difference of the two plots is also displayed in fig. 1.

Indeed the (K+lr-)lr - plot is seen to be enhanced in the narrow region Amin < 30 MeV as compared to the background (K+~r-)Ir + plot. This enhancement is interpreted as a signal from D*- production with subsequent two-stage decay involving the D 0 * 1. To esti-

• 1 We believe tha t such a narrow enhancement cannot arise from the cascade decay o f any known hadron resonance.

SO

5O >

>

0

i i

q-~ cos8>-.2

1J~ 'L~ H ¢ K+~c-)qZ -

I i

20 ~ DIFFERENCE

-20 i I;0 200

A m i n MeV

Fig. 1. The quan t i ty Amin plot ted for the (K+Tr-)~r - and (K+Tr-)Tr + combinat ions. The difference o f the two plots l,, also shown; the dashed line is a linear fit (see text) .

mate the effect quantitatively we fit the difference with a linear function in the interval 3 0 - 4 0 0 MeV and then extrapolate the fit down to the region A min < 30 MeV. In this way the small-A excess is found to be 85 + 22 events.

In the assumptions that: (i) the acceptance of the cut A min < 30 MeV is equal to unity for all the D 0 decays with charged particles, (ii) for the same decays, the acceptance of the angular cut cos 0 > - 0 . 2 is equal to 0.6, (iii) the probability that the D 0 decays into neutral particles only is 0.1 [4] , the observed small-A excess corresponds to a relative rate of

N(D*- ~ [)Orr-)/N(lp +) = 0.027 + 0.007.

Folding in the D*- -+ D°lr - branching ratio of 0.64 + 0.11 [6] , for E v > 10 GeV one obtains

o(VuN ~ p+D*-X)/o(PuN ~ p+X)

= 0.042 +- 0.011 + 0 .008 ,

where the last error reflects the BR(D*- -+ D0rr - ) measurement error. This estimate is conservative insofar as the assumptions (i)-(ii) above tend to overesti- mate the acceptance.

The strangeness content and Bjorken x dependence of the small-A enhancement are studied next to test the assumption that its origin is indeed the "g -+ g transi- tion.

For the (K+Tr-)rr - histogram of fig. 1, the observed neutral strange vee (visible K 0 ~ ~r+Tr - , A ~ pTr- decay) rate per event is indeed slightly enhanced In the region A < 30 MeV (0.104 against 0.088 for the rest of the plot). Comparing the vee rates for Amin < 30 MeV and A min > 30 MeV one obtains 0.134 -+ 0.060

for the average vee multiplicity within the small-A enhancement. This is compatible with its D*- ~B0~r - origin, if not providing a compelling evidence.

The Bjorken x distribution of (K+n- ) r r - events is peaked towards small x B as compared to (K+n-)n ÷ in the region A min < 30 MeV (see fig. 2, left-handed plots). This points at an off-sea origin of the small-A enhancement, as is indeed expected of the "g ~ ~ transi- tion. Such a disparity is not observed in the adjoining interval of 30 MeV < A min < 60 MeV (fig. 2, right- hand column) where the (K+Ir-)Tr - and (K+Tr-)lr + x B distributions are seen to be almost identical.

247


_ 0 - A--30 HeY. 30~A~6'0 MeV _

( K+~')~ -

20

0

1 L L

2o L 0

DI FFERENEE

20 t ~ L

0 ~ ~ ' ~ ~

I I

0. o.5 0 0.5

XB

Fig. 2. Comparison between the (K+Ir-)Ir - and (K+rr-)Ir + Bjorken x distributions separately performed in two ~min in. tervals of fig. 1 :0 -30 MeV (left-hand column), and 30-60 MeV (right-hand column).

The alternative approach deals with D 0 decays into a visible K0; the appropriately "heavy" subsystem in this case is X = (K01r+Tr - ) [see eq. (1) ] . Using the old strategy, one again plots the quant i ty A min for X n - and X1r + combinations. With a similar angular cut

cos 0 > - 0 . 2 (where 0 again stands for the angle between the K 0 and X momenta in the X rest frame) there are 9 (K°Tr+lr-N - events with A min < 30 MeV against only 1 background (K0n+Ir - )n + event. The magnitude of the signal agrees well with that of fig. 1 once one takes into account the D 0 ~ K 0 inclusive branching ratio [4] as well as the K 0 registration effi- ciency [5].

In the likely assumption that D*- and D *0 production cross sections are equal the collective charmed

nonstrange vector meson product ion rate relative to all charged current events with E~ > 10 GeV appears to be as high as 0.084 + 0.026. This is fairly close to the overall charm product ion rate of 0.06 -+ 0.02 as earlier reported for the same data sample [5]. Thus one is led to believe that most (if not all) charm is produced by antineutrinos in spin-one form.

We wish to thank the physicists from Fermilab and Michigan State University for their invaluable contri- but ion to this experiment at its early stage. The efforts o f the accelerator personnel, as well as scanning and measuring staffs are also gratefully acknowledged.

References

[1] G.J. Feldman et al., Phys. Rev. Lett. 38 (1977) 1313; P. Avery et al., Phys. Rev. Lett. 44 (1980) 1309; V.L. Fitch et al., Phys. Rev. Lett. 46 (1981) 761.

[2] J. Blietschau et al., Phys. Lett. 86B (1979) 108. [3] C. Baltay, Proc. Intern. Conf. Neutrino-82 (Balatonfured,

Hungary, June 1982), Supplement, p. 109. [4] R.H. Schindler et al., Phys. Rev. D24 (1981) 78. [5] V.V. Ammosov et al., Nucl. Phys. B177 (1981) 365. [6] G.J. Feldman et al., Preprint SLAC-PUB-2068 (1977).

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Study of charmed vector meson production by antineutrinos