Charmed strange vector meson production in antineutrino-nucleon interactions

Volume 156B, number 5,6 PHYSICS LETTERS 27 June 1985

CHARMED STRANGE VECTOR MESON PRODUCTION IN A N T I N E U T R I N O - N U C L E O N INTERACTIONS

A.E. ASRATYAN, A.V. FEDOTOV, P.A. GORITCHEV, S.P. K R U T C H I N I N , M.A. KUBANTSEV, I.V. MAKHLUEVA, V.I. SHEKELYAN, V.G. SHEVCHENKO

Institute of Theoretical and Experimental Physics, Moscow, USSR

V.V. AMMOSOV, V.S. BURTOVOY, A.G. DENISOV, G.S. GAPIENKO, V.A. GAPIENKO, V.I. KLYUKHIN, V.I. KORESHEV, P.V. P ITUKHIN, V.I. SIROTENKO, E.A. SLOBODYUK, Z.U. USUBOV and V.G. ZAETZ

Institute for High Energy Physics, Serpukhov, USSR

Received 19 February 1985

Evidence is reported for F*- antineutrino production with the subsequent radiative decay F* -~ y F-. We find 143 + 18 MeV for the F*-F mass difference.

The decays of the F* -+ meson, the vector counter- part of the pseudoscalar charmed strange meson F ±, are 100% radiative (F *± -+ 7F ±) since the strong zr0F ± decay is forbidden by isospin. The F* radiative decay was first observed by the DASP group at DESY [1]; the F * - F mass difference A was measured to be 110 -+ 46 MeV. The two recent measurements yield A = 144 -+ 11 MeV (e+e - , ARGUS group at DESY [2]) and A = 150 + 15 MeV (b~uN, Fermilab 15 ft bubble chamber [3] * 1).

The implications of the virtual transition W ± ~ F *± with subsequent strong rescattering (virtual F*)N (real F*)X (the vector meson dominance mechanism, or VMD) for the F* neutrino production were first theoretically explored by Arbusov et al. [4]. Accord- ing to the later calculations [5], the VMD F*- pro- duction might contribute as much as ~5% to the total charged current (CC) antineutrino-nucleon cross section in the energy region 50-200 GeV. Thus the F* - antineutrino production is likely to be domi- nated by the VMD rather than the deep inelastic mechanism. The VIVID contribution is concentrated at low Q2 values [4,5].

:1:1 A preliminary result by our group.

In this paper we report on a study of the F*- anti- neutrino production FuN ~/a+F*-X with subsequent F*- ~ 7 F - , F - -~ $~r-X ,2. Rather than single out a particular F decay mode we develop a method whereby to embrace all the $zr-X final states.

The approach is nearly the same as that used in our recent study of the D*- antineutrino production [7]. The simple underlying idea is that the subsystem $Tr- is massive enough so as to stay almost at rest in the F - (or $ n - X ) rest frame. Therefore the F*- decay photons which are strictly monoenergetic in the F- frame must be "almost monoenergetic" in the $rt-

t

frame: E, r ~ E, r. Indeed, the Monte Carlo phase space t calculation shows that the quantity E~/E7 is peaked

at unity with dispersion of ~0.15 for the decays F - -~ ~r-~r, ~r-(27r), ~- (31r) .

Analysed are the antineutrino data collected with the Fermilab 15 ft bubble chamber filled with heavy neon-hydrogen mixture (the data as well as the charged current sample are described at some length in ref. [8]). The heavy t]11 is instrumental for our relatively high gamma registration efficiency (~50%).

,2 The branching ratio for the F- -~ qm- channel alone is about 4% [6].

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Volume 156B, number 5,6 PHYSICS LETTERS 27 June 1985

For each 1-#+ event with E v > 10 GeV and P~ > 4 GeV we form the (K+K-)~r - combinations with mass below 2 GeV and m(K+K - ) in the interval 1010-1030 MeV (calculated mass resolution is as good as 6 MeV in the m~ region). The kaon mass is assigned to all tracks not positively identified as protons and electrons. For each combination thus formed, we plot the energies of the "odd" gamma's registered in the event.

The "odd" gamma's are those which survive the sim- ple 7r 0 rejection procedure: each gamma occurring in the interval t 15-155 MeV on the m(7'i) mass plot is dropped (the mass resolution is ~18 MeV in the 7r 0 area). The "odd" gamma's form 44% of the overall gamma sample.

To suppress the background, three additional cuts are imposed:

(i) u < 0.02, where o = Eu/Mp(1 - cos 0~) ~ 02/ (2MpEu). The motivation is that as noted above, the VMD contribution is concentrated at low Q2 values [4,5]. Within the model [6] ,3 the calculated accep- tance of the cut u < 0.02 for the elastic process ~ , N -~/a+F * - N is 0.39. Empirically, as much as 83% of the background (all CC events with no less than 2 negative tracks) are removed by this cut.

(ii) z ' > 0.7. Here z ' = ze,,~r.rrn(F*)/m(@lrT), where z¢~r~ stands for the fraction ofhadronic energy car- ded by the ( K + K - ) l r - 7 system ,4. Since this subsys- tem is massive enough so as to stay almost at rest in the decayed F * - frame, z ' is essentially the fractional hadronic energy of the entire F * - meson. Qualitative- ly, the cut selects the elastic (~ ,N ~ #+F* - N ) and "near elastic" F * - production events.

(iii) cos 0¢ > -0 .6 . Here 0¢ is the angle between the @ --- (K+K-)¢ and @zr- momenta in the @Tr- rest frame. The motivation is that while the decay @ me- sons are isotropic in the @It- frame, the "fake" @ formed by two pions is more likely to travel backwards because of the large wrongly assigned mass.

Plotted in fig. la are the "odd" gamma energies t E~ calculated in the (K+K-)¢zr - rest frame under the

cuts (i)-(iii) above (upper histogram: no 0¢ cut, shaded histogram: cos 0¢ > -0 .6) . For comparison,

.3 We employ the version ('tii) of the Q2 dependence taken by Cben et al. [6] as the best suited for VMD elastic F* produc- tion.

,4 Here, the F* mass is taken to be re(F*) = 2110 MeV (see refs. [6,2,3]).

~ s

a:i

o

1'01< mkk <l"03GeV + l"03"=mkk < 1'05 GeV t

C l e

I (K- K-}'n+ I f

c I ~r, r, ~rr" I g

d I (K~K3 Tr- I h

300 600 900 300 600 900

E~. , MeV

Fig. 1. Gamma momentum plotted in the (KK)~r rest frame (for details, see the text).

similar plots are given for other charge combinations: (K-K-)~zr +, (K+K-)~Tr +, and (K+K+)~zr - (see figs. lb, lc, ld). Shown in the fight-hand column (figs. le, If, lg, lh) are the analogous plots but with m(KK) shifted away from the me region (1030 < m(KK) < 1050 MeV).

The (K+K-)¢Ir - plot (a) displays an enhancement t

in the bin 120 < E. r < 180 MeV in contrast with the background plots (b, c, d). The gross disparity between the (K+K-)¢lr - (a) and (K-K-)¢Tr + (b) plots which have the same "charge content" is not observed in the non-@ region [compare (e) and (f)].

The peak is formed by 10 combinations belonging to 7 events (8 combinations and 6 events with the cut cos 0¢ > -0 .6) . Its width , s as well as the cos 0~ dis- tribution ,6 are consistent with the F * - ~ "IF- signal.

f The Ev averaged over the 8 combinations with cos 8~

,s The calculated dispersion is = 25 MeV (with the gamma momentum measurement error (Ap3,/p,y) = 0.08 taken into account).

.6 For cos 0¢ > -0.6, we have (cos O¢) = 0.19 _+ 0.15.

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Volume 156B, number 5,6 PHYSICS LETTERS 27 June 1985

t

> - 0 . 6 in the bin 120 < E 7 < 180 MeV (see fig. la) is 148 -+ 18 MeV. This corresponds to the F * - F mass difference of 143 -+ 18 MeV which agrees well with the ARGUS result [3] as well as with our preliminary measurement [4].

Ascribing the 6 peak events (cos 0~ > - 0 . 6 ) to the effect and folding in the ~ -* K+K - branching ratio, the 0~ angular acceptance (0.8), and relevant registra- tion efficiencies, one obtains the rate

N ( F * - ~ 7 F - -* 7(~zr-X), o < 0.02, z > 0.7)/N(1-# +)

= ( 5 + 2 ) X10 - 3 ,

relative to all the ~u-induced CC events with E v > 10 GeV. The experimentally observed sharp o dependence of the effect indicates the VMD rather than the deep inelastic F* - production mechanism * 7

We thank our colleagues at Fermilab and Michigan University for their invaluable contribution to our ex- periment at its early stage.

References

[1] R. Brandelik et al., Phys. Lett. 80B (1979) 412. [2] H. Albrecht et al., Preprint DESY 84-052 (June 1984). [3] A.E. Asratyan et al., Preprint ITEP-99 (June 1984). [4] B.A. Arbusov et al., Soy. J. Nucl. Phys. 21 (1975) 1322;

22 (1975) 173. [5] M.S. Chen et al., Nucl. Phys. Bl18 (1977) 345. [6] A. Chen et aL, Phys. Rev. Lett. 51 (1983) 634. [7] A.E. Asratyan et al., Phys. Lett. 132B (1983) 246. [8] V.V. Ammosov et al., Nucl. Phys. B177 (1981) 365.

,7 With the u < 0.02 cut lifted, the number of combinations with cos 0~ > -0.6 in the peak bin 120-180 MeV is in- creased by only four. Thus, the observed effect is 6 events for o < 0.02 and less than 8 (on the 90% level of confidence) for 0.02 < o < 1.

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