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Observation of a spin 4 neutral meson with 2 GeV mass decaying in π° π°
Volume 57B, number 4
PHYSICS LETTERS
21 July 1975
OBSERVATION OF A SPIN 4 NEUTRAL MESON WITH 2 GeV MASS DECAYING IN n° o W.D. APEL l , K. AUGENSTEIN l , E. BERTOLUCCI2 , S.V. DONSKOV a , A.V. INYAKIN 3 , V.A. KACHANOV a , W. KITTENBERGER4 , R.N. KRASNOKUTSKYa , M. KRUGER 1 , G. LEDER 4, A.A. LEDNEV a , I. MANNELLI 2 , Yu.V. MIKHAILOVa , H. MI]-LLER l , G.M. PIERAZZINI2 , Yu.D. PROKOSHKIN 3 , M. QUAGLIA2 , H. SCHNEIDER 1 , A. SCRIBANO 2 , F. SERGIAMHETRI 2 , R.S. SHUVALOV a , G. SIGURDSSON s , M. STEUER 4 and M.L. VINCELLI 2 Institute for High Energy Physics, Serpukhov, USSR and the European Organization for Nuclear Research, Geneva, Switzerland Recewed 30June1975 The invariant mass spectrum of neutral meson states from ~r-p interactions at 40 GeV/c incident momentum has been investigated in a high statistics experiment performed at the 70 GeV IHEP accelerator. To detect the high energy photons coming from the produced neutral states, a horoscope spectrometer with a computer onqine was used. A clear structure on the mass spectrum of dipions produced in the reaction 7r-p ~ ~r°~r°nis observed at 2 GeV. The decay angular distributions show in this mass region the variation with mass typical of a state with a spin J = 4. The mass of the observed meson is found to be M= (2020 ± 30) MeV and the estimate of the full width is (180 ± 60) MeV.
We have performed an experiment which studies the mass spectrum of neutral meson states from 7r- p. interactions at 40 GeV/c incident momentum. The reactions investigated are of the type ~- +p~M°+n
(1)
with the M° meson states decaying in 3"s, zr°'s and other neutral particles whose final decay products are 3"s. The invariant mass region above 2 GeV has been investigated in a previous work [ 1]. In the following we report the results of a further search for heavy neutral mesons in the mass region up to 3 GeV. The experiment has been carried out in a negative secondary beam at the 70 GeV IHEP accelerator with the NICE set-up described elsewhere [2]. The H 2 liquid target was surrounded by a set of veto counters selecting neutral final states of the type (1), with •),rays going in a forward cone. These high energy 3'rays from the decay M° ~ m3' (m = 2, 3 .... ) were de1 Institut fiir Experimentelle Kernphysik, Universit~itund Kernforschungszentrum, Karlsruhe, BRD. 2 Istituto di Fisica dell'UniversitY,Pisa and INEN, Sezione di Pisa, Italy. 3 1HEP, Serpukhov, USSR. 4 Institut fiir Hochenergiephysik, Wien (3AdW,Austria. s Cern, Geneva, Switzerland. 398
tected in a 648-channel hodoscope spectrometer [3], on line with a computer. The distance between the spectrometer and the liquid hydrogen target was L = 3 m, which corresponds to a kinematical mass acceptance for m3'-systems in reaction (1) up to about 5 GeV. During all the running time about 107 events were collected. Here we report results based on 1/3 of the total statistics corresponding to a sensitivity of 10 -35 cm2/event for the reaction (1). The analysis of the m3,-systems has been done with the help of an automatic shower recognition program which scans the pulse height distributions in the 3 projections of the spectrometer looking for peaks that could be associated as belonging to the same shower [2, 4]. The events thus found with 2, 3 or 4 showers have been kinematically reconstructed and invariant mass spactra have been produced. Before discussing these spectra some comments are necessary about the procedure to determine the multiplicity m of showers. Due to the small distance between the target and the spectrometer, gammas coming from fast 7r°'s are not sufficiently separated (~< 3 cm) to give two distinct showers. Thus an energetic shower found by program could mean a 3' detected as well as a zt°. Therefore a significant fraction
Volume 57B, number 4
PHYSICS LETTERS
21 July 1975
7r-p ~ lr°Tr° n
of the decays M° ~ ~r°rr°, namely those decaying asymmetrically, so having a forward ~r° with momentum near the maximum are detected as 3 shower events, the 3 showers being one unresolved lr° plus 2 resolved 7's from the other lr°. We will indicate these decays as M° ~ 0r°lr°)as where the subscript "as" stands for asymmetrical. Fig. 1a shows the mass spectrum for 3 shower events (m = 3). For the reason previously mentioned this spectrum is a mixture of M° -* rr°3, and M° ~ 0r°rr°)a s events. This explains the simultaneous presence of the 60 and f peaks in the spectrum with 6o ~ rr°7 and f ~ (rr°lr°)as. Besides these well established states, a new peak is dearly visible at higher mass, around M = 2 GeV, which can be either a resonant state or a strong dynamical effect (cusp etc.). The mass spectrum for events with shower multiplicity m = 4, for those identified by program as
(2)
is shown in fig. lb. Here too a shoulder is visible in the mass region around 2 GeV. This indicates that the events in the peak at 2 GeV in fig. 2 do not decay mostly into lr°? (such events should disappear in the spectrum in fig. l b , as is clearly illustrated by the co case), but rather into (rr°lr°)as, so they belong to reaction (2). Some indications about a possible structure in this mass region have been reported for K K and 7r+Tr- states [5]. It should be remarked that the structure at 2 GeV in the mass spectrum with m = 3 (fig. la) is more pronounced than the one visible in the spectrum with m = 4 (fig. lb); the number of events in the h-region being 10% of the f events in the former spectrum and 3% in the latter. This indicates that the decays M° 7r°Tr°, in the region of the observed structure, are concentrated at small c.m. angles as it should be expected for the decay of a high spin particle. In order to understand the structure observed in the mass spectra better, an angular analysis for 3 and
AN/AM
f AN/AM
W
'fOOG I000
h SO0 SO0
a~
I~
15
2.5
2D
3.0
M.GeV
Fig. la. lnvariant mass spectrum for 3-shower events, m = 3, i.e. 7rO-ror 0rOTrO)asdecays. The peaks correspond to the decays of ~(783), f(1270) and h(2020). The solid curve is the result of a 1.s. fit to the data described in the text. The dashed line describes the continuous part of the spectrum by polinomium. The dotted curve shows the h-peak as it comes out from the fit. An attempt to fit the spectrum above f meson with non-resonant curve gives the probability value P(x2) < 10"~.
0
O.S
tO
15
210
2S
3.0
M.GelV
Fig. lb. Invariant mass spectrum for 4-shower events, m = 4,
i.e. 7rOTrOstates produced in reaction (2). The peak corresponds to the decay f ~ rr°Tro. The solid curve is the result of a 1.s. fit. --399
Volume
57B,
4
number
21 July 1975
PHYSICS L E T T E R S
events
events !
0.g*ID Gt~¢
1.0o1.1 c~v
IN
u
I,?-I~
"L.
/ 1]-12 GeV
S~
'100
i
200 100
5O ,
/ -~
150
~J~
,00
100
50
5(t
2,0-2JG¢¢
t J-4 I
1 2 i ~ 3 0 1.& 0-1.5r..'eV 200
100 0
u
0 200
200
100
'L5-1,6~
1,5-1.? Gt'V
I
~
[3-~.4 ~
. - ,
10 -
02
0A.
06
0.6
COS~Gj
1.0
-
50 02
0.4
0.6
0,B
1.0
COS~Gj
02
OY.
0~i
08
COSOGJ
t0
0-2
0&
0.6
0.8
H)
COS~Gj
Fig. 2. Experimental ~r°Tr° angular distributions, evaluated in the Gottfried-Jackson frame (not corrected for efficiency) for various mass intervals. The dotted curves show the ideal behaviour for decaying dipions with spin J = 2 and J = 4, fully polarized and detected with 100% efficiency. 4 shower events added together has been made. For the events used in this analysis a cut in the four-momentum transfer - t < 0.16 (GeV/c) 2 has been applied in order to favour events produced by OPE mechanism. In fig. 2 we present the experimental lr°lr ° angular distributions evaluated in the Gottfried-Jackson frame (not corrected for efficiency) for masses from 0.9 to 2.5 GeV. The general behaviour of the low mass distributions in fig. 2 is as expected. The isotropic angular distribution below 1 GeV gradually gets distorted by the growing amount of D-wave scattering. In the f region S- and D-wave produce the familiar pattern with a dip at c o s 0Gj ~ 0.57 typical for spin J = 2. At masses higher than f mass the rise of the angular distributions above this dip gradually disappears. 400
The striking feature of the distributions at higher masses is the reappearance of a second maximum at COS 0Gj ~ 0.65 but now the dip is shifted to lower cos 0Gj around 0.35, for the h-peak mass region 1 . 9 2.0 GeV. This behaviour cannot be explained in terms of S- and D-wave ~r~rscattering alone. A strong contribution of G-wave spin 4 rr~rscattering must be assumed. Above 2.2 GeV mass of the lr°rt°-system the structure of the angular distributions again changes substantially: the dip is filled up, the second maximum flattens out. The spin 4 contribution to 7rlr scattering is decreasing again. In conclusion the peak we observe in the invariant mass spectrum combined with the dip at c o s 0Gj ~. 0.35 that we see in the angular distributions between 1.9 and 2.2 GeV gives strong evidence for the observa-
Volume 57B, n u m b e r 4
PHYSICS LETTERS
tion of the expected spin 4 neutral h-meson, produced in our experiment in the reaction It- + p - ~ h + n
(3)
and decaying in two lr°: h ~ rr°rr °.
(4)
The q u a n t u m numbers which have to be assigned to the h meson are spin parity JP = 4 +, positive C- and G-parity and isospin 1 = 0 or 2. In order to evaluate the mass and the width of the observed h-meson a 1.s. fit to the 3 shower mass spectrum has been done using a third degree polynomium to describe the background and gaussian curves for the peaks (the width of the peaks are determined essentially by the mass resolution of the apparatus). The results of the fit are shown in the first column of table 1. The mass of the co is found to be (792.6 + 1.2) MeV, which is in agreement with the table value 782.7 MeV within 1.5%. The difference does not exceed the systematical error which is due mainly to the uncertainty in the incident beam momentum. The table value of the ~ mass is used to normalize the mass scale in our spectrum. The linearity of the scale is proved by the very good agreement of the ratios Mf/Mto for measured and table mass values: (Mf/M~)meas/(Mf/Mto)table = 1.00l 5 -+0.0025. The Table 1 Parameters of the fits for mass spectra with shower multiplicities m = 3 and 4. Spectrum co
f
m = 3 (fig. 2)
m = 4 (fig. 3) -
A* g
782.7** (792 -+ 2) 85 -+5 70-+3
M
1269 -+2 228 -+5 258 -+4
1267"* (1300 -+2) 240 -~ 5) 302 ± 5
2020 -+ 15 250 -+ 30 26-+2
2010 -+ 20 120 -+60 7+-2
0.11 ± 0.01
0.03 -+0.01
M*
A
g h
M A
g
gh/gf
---
21 July 1975
fit gives as a mass value for the h meson M = (2020 +-30) MeV. The quoted error includes the scale uncertainty. The shape of the h peak is well described by a gaussian curve. The intrinsic width calculated taking into account the mass resolution of the apparatus (AM/M = +- 5%) is 1-'= (180 -+60) MeV. A fit similar to the one described for the m = 3 mass spectrum has been done also for the m = 4 mass spectrum (fig. lb). The results are shown in the second column of the table and it can be seen that the mass value for the h peak is equal to the one obtained for m=3. A preliminary evaluation of the angular moments illustrates the presence of such a spin 4 state. Using simply the geometrical acceptance of our apparatus, but not taking into account the shower recognition efficiencies, we fitted the contribution of the 8th unnormalized spherical harmonic moment N ( Y ~ ) to our data (fig. 3). We find it consistent with zero for masses of the lr°n ° system below 1.7 GeV, rising above this value and reaching its maximum at 2 GeV, then decreasing again to low values around 2.5 GeV. Reaction (3), associated with the decay (4), is a very low rate process. Our estimated production cross
N 6O0
400
200
0
,J
,I
J
M,GeV * Masses M, a n d widths A(FWHM), are given in MeV. g is a normalization factor. ** Table mass value, used for scale normalization. T h e measured mass value (not corrected) is given in brackets.
Fig. 3. Preliminary evaluation o f the 8th unnormalized spherical harmonic moment as a function of ~rTrmass. The arrow
shows the position of the peak maximum in fig. 2. 401
Volume 57B, number 4
PHYSICS LETTERS
section, the u n k n o w n branching ratio for the decay (4) included, is only some tens of nanobarns. The analysis Of the data is continuing in order to study further characteristics of this new particle.
References [1] W.D. Apel ef"al., IHEP preprint 72-28, Serpukhov (1975); Phys. L~tt. 56B (1975) 190. [2] W.D. Apel et al., IHEP preprint 74-117, Serpukhov (1974); XVII Intern. Conf. on High energy physics, paper 536; Proceedings, 1-35, London, 1974.
402
21 July 1975
[3] Yu.B. Bushnin et al., IHEP preprints 72-34 and 74-21, Serpukhov (1974); Nucl. Instr. Meth. 106 (1973) 497 and 120 (1974) 391. [4] W.D. Apel et al., IHEP preprint 74-118, Serpukhov (1974); XVII Intern. Conf. on High energy physics, paper 537; Proceedings, 1-35, London, 1974. [5] R. Wagner, XVII Intern. Conf. on High energy physics, Proceedings, 11-35, London, 1974; Dowel et al., Experimental meson spectroscopy conference, Boston, 1974; Maener et al., Experimental meson spectroscopy conference, Boston, 1974.
PHYSICS LETTERS
21 July 1975
OBSERVATION OF A SPIN 4 NEUTRAL MESON WITH 2 GeV MASS DECAYING IN n° o W.D. APEL l , K. AUGENSTEIN l , E. BERTOLUCCI2 , S.V. DONSKOV a , A.V. INYAKIN 3 , V.A. KACHANOV a , W. KITTENBERGER4 , R.N. KRASNOKUTSKYa , M. KRUGER 1 , G. LEDER 4, A.A. LEDNEV a , I. MANNELLI 2 , Yu.V. MIKHAILOVa , H. MI]-LLER l , G.M. PIERAZZINI2 , Yu.D. PROKOSHKIN 3 , M. QUAGLIA2 , H. SCHNEIDER 1 , A. SCRIBANO 2 , F. SERGIAMHETRI 2 , R.S. SHUVALOV a , G. SIGURDSSON s , M. STEUER 4 and M.L. VINCELLI 2 Institute for High Energy Physics, Serpukhov, USSR and the European Organization for Nuclear Research, Geneva, Switzerland Recewed 30June1975 The invariant mass spectrum of neutral meson states from ~r-p interactions at 40 GeV/c incident momentum has been investigated in a high statistics experiment performed at the 70 GeV IHEP accelerator. To detect the high energy photons coming from the produced neutral states, a horoscope spectrometer with a computer onqine was used. A clear structure on the mass spectrum of dipions produced in the reaction 7r-p ~ ~r°~r°nis observed at 2 GeV. The decay angular distributions show in this mass region the variation with mass typical of a state with a spin J = 4. The mass of the observed meson is found to be M= (2020 ± 30) MeV and the estimate of the full width is (180 ± 60) MeV.
We have performed an experiment which studies the mass spectrum of neutral meson states from 7r- p. interactions at 40 GeV/c incident momentum. The reactions investigated are of the type ~- +p~M°+n
(1)
with the M° meson states decaying in 3"s, zr°'s and other neutral particles whose final decay products are 3"s. The invariant mass region above 2 GeV has been investigated in a previous work [ 1]. In the following we report the results of a further search for heavy neutral mesons in the mass region up to 3 GeV. The experiment has been carried out in a negative secondary beam at the 70 GeV IHEP accelerator with the NICE set-up described elsewhere [2]. The H 2 liquid target was surrounded by a set of veto counters selecting neutral final states of the type (1), with •),rays going in a forward cone. These high energy 3'rays from the decay M° ~ m3' (m = 2, 3 .... ) were de1 Institut fiir Experimentelle Kernphysik, Universit~itund Kernforschungszentrum, Karlsruhe, BRD. 2 Istituto di Fisica dell'UniversitY,Pisa and INEN, Sezione di Pisa, Italy. 3 1HEP, Serpukhov, USSR. 4 Institut fiir Hochenergiephysik, Wien (3AdW,Austria. s Cern, Geneva, Switzerland. 398
tected in a 648-channel hodoscope spectrometer [3], on line with a computer. The distance between the spectrometer and the liquid hydrogen target was L = 3 m, which corresponds to a kinematical mass acceptance for m3'-systems in reaction (1) up to about 5 GeV. During all the running time about 107 events were collected. Here we report results based on 1/3 of the total statistics corresponding to a sensitivity of 10 -35 cm2/event for the reaction (1). The analysis of the m3,-systems has been done with the help of an automatic shower recognition program which scans the pulse height distributions in the 3 projections of the spectrometer looking for peaks that could be associated as belonging to the same shower [2, 4]. The events thus found with 2, 3 or 4 showers have been kinematically reconstructed and invariant mass spactra have been produced. Before discussing these spectra some comments are necessary about the procedure to determine the multiplicity m of showers. Due to the small distance between the target and the spectrometer, gammas coming from fast 7r°'s are not sufficiently separated (~< 3 cm) to give two distinct showers. Thus an energetic shower found by program could mean a 3' detected as well as a zt°. Therefore a significant fraction
Volume 57B, number 4
PHYSICS LETTERS
21 July 1975
7r-p ~ lr°Tr° n
of the decays M° ~ ~r°rr°, namely those decaying asymmetrically, so having a forward ~r° with momentum near the maximum are detected as 3 shower events, the 3 showers being one unresolved lr° plus 2 resolved 7's from the other lr°. We will indicate these decays as M° ~ 0r°lr°)as where the subscript "as" stands for asymmetrical. Fig. 1a shows the mass spectrum for 3 shower events (m = 3). For the reason previously mentioned this spectrum is a mixture of M° -* rr°3, and M° ~ 0r°rr°)a s events. This explains the simultaneous presence of the 60 and f peaks in the spectrum with 6o ~ rr°7 and f ~ (rr°lr°)as. Besides these well established states, a new peak is dearly visible at higher mass, around M = 2 GeV, which can be either a resonant state or a strong dynamical effect (cusp etc.). The mass spectrum for events with shower multiplicity m = 4, for those identified by program as
(2)
is shown in fig. lb. Here too a shoulder is visible in the mass region around 2 GeV. This indicates that the events in the peak at 2 GeV in fig. 2 do not decay mostly into lr°? (such events should disappear in the spectrum in fig. l b , as is clearly illustrated by the co case), but rather into (rr°lr°)as, so they belong to reaction (2). Some indications about a possible structure in this mass region have been reported for K K and 7r+Tr- states [5]. It should be remarked that the structure at 2 GeV in the mass spectrum with m = 3 (fig. la) is more pronounced than the one visible in the spectrum with m = 4 (fig. lb); the number of events in the h-region being 10% of the f events in the former spectrum and 3% in the latter. This indicates that the decays M° 7r°Tr°, in the region of the observed structure, are concentrated at small c.m. angles as it should be expected for the decay of a high spin particle. In order to understand the structure observed in the mass spectra better, an angular analysis for 3 and
AN/AM
f AN/AM
W
'fOOG I000
h SO0 SO0
a~
I~
15
2.5
2D
3.0
M.GeV
Fig. la. lnvariant mass spectrum for 3-shower events, m = 3, i.e. 7rO-ror 0rOTrO)asdecays. The peaks correspond to the decays of ~(783), f(1270) and h(2020). The solid curve is the result of a 1.s. fit to the data described in the text. The dashed line describes the continuous part of the spectrum by polinomium. The dotted curve shows the h-peak as it comes out from the fit. An attempt to fit the spectrum above f meson with non-resonant curve gives the probability value P(x2) < 10"~.
0
O.S
tO
15
210
2S
3.0
M.GelV
Fig. lb. Invariant mass spectrum for 4-shower events, m = 4,
i.e. 7rOTrOstates produced in reaction (2). The peak corresponds to the decay f ~ rr°Tro. The solid curve is the result of a 1.s. fit. --399
Volume
57B,
4
number
21 July 1975
PHYSICS L E T T E R S
events
events !
0.g*ID Gt~¢
1.0o1.1 c~v
IN
u
I,?-I~
"L.
/ 1]-12 GeV
S~
'100
i
200 100
5O ,
/ -~
150
~J~
,00
100
50
5(t
2,0-2JG¢¢
t J-4 I
1 2 i ~ 3 0 1.& 0-1.5r..'eV 200
100 0
u
0 200
200
100
'L5-1,6~
1,5-1.? Gt'V
I
~
[3-~.4 ~
. - ,
10 -
02
0A.
06
0.6
COS~Gj
1.0
-
50 02
0.4
0.6
0,B
1.0
COS~Gj
02
OY.
0~i
08
COSOGJ
t0
0-2
0&
0.6
0.8
H)
COS~Gj
Fig. 2. Experimental ~r°Tr° angular distributions, evaluated in the Gottfried-Jackson frame (not corrected for efficiency) for various mass intervals. The dotted curves show the ideal behaviour for decaying dipions with spin J = 2 and J = 4, fully polarized and detected with 100% efficiency. 4 shower events added together has been made. For the events used in this analysis a cut in the four-momentum transfer - t < 0.16 (GeV/c) 2 has been applied in order to favour events produced by OPE mechanism. In fig. 2 we present the experimental lr°lr ° angular distributions evaluated in the Gottfried-Jackson frame (not corrected for efficiency) for masses from 0.9 to 2.5 GeV. The general behaviour of the low mass distributions in fig. 2 is as expected. The isotropic angular distribution below 1 GeV gradually gets distorted by the growing amount of D-wave scattering. In the f region S- and D-wave produce the familiar pattern with a dip at c o s 0Gj ~ 0.57 typical for spin J = 2. At masses higher than f mass the rise of the angular distributions above this dip gradually disappears. 400
The striking feature of the distributions at higher masses is the reappearance of a second maximum at COS 0Gj ~ 0.65 but now the dip is shifted to lower cos 0Gj around 0.35, for the h-peak mass region 1 . 9 2.0 GeV. This behaviour cannot be explained in terms of S- and D-wave ~r~rscattering alone. A strong contribution of G-wave spin 4 rr~rscattering must be assumed. Above 2.2 GeV mass of the lr°rt°-system the structure of the angular distributions again changes substantially: the dip is filled up, the second maximum flattens out. The spin 4 contribution to 7rlr scattering is decreasing again. In conclusion the peak we observe in the invariant mass spectrum combined with the dip at c o s 0Gj ~. 0.35 that we see in the angular distributions between 1.9 and 2.2 GeV gives strong evidence for the observa-
Volume 57B, n u m b e r 4
PHYSICS LETTERS
tion of the expected spin 4 neutral h-meson, produced in our experiment in the reaction It- + p - ~ h + n
(3)
and decaying in two lr°: h ~ rr°rr °.
(4)
The q u a n t u m numbers which have to be assigned to the h meson are spin parity JP = 4 +, positive C- and G-parity and isospin 1 = 0 or 2. In order to evaluate the mass and the width of the observed h-meson a 1.s. fit to the 3 shower mass spectrum has been done using a third degree polynomium to describe the background and gaussian curves for the peaks (the width of the peaks are determined essentially by the mass resolution of the apparatus). The results of the fit are shown in the first column of table 1. The mass of the co is found to be (792.6 + 1.2) MeV, which is in agreement with the table value 782.7 MeV within 1.5%. The difference does not exceed the systematical error which is due mainly to the uncertainty in the incident beam momentum. The table value of the ~ mass is used to normalize the mass scale in our spectrum. The linearity of the scale is proved by the very good agreement of the ratios Mf/Mto for measured and table mass values: (Mf/M~)meas/(Mf/Mto)table = 1.00l 5 -+0.0025. The Table 1 Parameters of the fits for mass spectra with shower multiplicities m = 3 and 4. Spectrum co
f
m = 3 (fig. 2)
m = 4 (fig. 3) -
A* g
782.7** (792 -+ 2) 85 -+5 70-+3
M
1269 -+2 228 -+5 258 -+4
1267"* (1300 -+2) 240 -~ 5) 302 ± 5
2020 -+ 15 250 -+ 30 26-+2
2010 -+ 20 120 -+60 7+-2
0.11 ± 0.01
0.03 -+0.01
M*
A
g h
M A
g
gh/gf
---
21 July 1975
fit gives as a mass value for the h meson M = (2020 +-30) MeV. The quoted error includes the scale uncertainty. The shape of the h peak is well described by a gaussian curve. The intrinsic width calculated taking into account the mass resolution of the apparatus (AM/M = +- 5%) is 1-'= (180 -+60) MeV. A fit similar to the one described for the m = 3 mass spectrum has been done also for the m = 4 mass spectrum (fig. lb). The results are shown in the second column of the table and it can be seen that the mass value for the h peak is equal to the one obtained for m=3. A preliminary evaluation of the angular moments illustrates the presence of such a spin 4 state. Using simply the geometrical acceptance of our apparatus, but not taking into account the shower recognition efficiencies, we fitted the contribution of the 8th unnormalized spherical harmonic moment N ( Y ~ ) to our data (fig. 3). We find it consistent with zero for masses of the lr°n ° system below 1.7 GeV, rising above this value and reaching its maximum at 2 GeV, then decreasing again to low values around 2.5 GeV. Reaction (3), associated with the decay (4), is a very low rate process. Our estimated production cross
N
400
200
0
,J
,I
J
M,GeV * Masses M, a n d widths A(FWHM), are given in MeV. g is a normalization factor. ** Table mass value, used for scale normalization. T h e measured mass value (not corrected) is given in brackets.
Fig. 3. Preliminary evaluation o f the 8th unnormalized spherical harmonic moment as a function of ~rTrmass. The arrow
shows the position of the peak maximum in fig. 2. 401
Volume 57B, number 4
PHYSICS LETTERS
section, the u n k n o w n branching ratio for the decay (4) included, is only some tens of nanobarns. The analysis Of the data is continuing in order to study further characteristics of this new particle.
References [1] W.D. Apel ef"al., IHEP preprint 72-28, Serpukhov (1975); Phys. L~tt. 56B (1975) 190. [2] W.D. Apel et al., IHEP preprint 74-117, Serpukhov (1974); XVII Intern. Conf. on High energy physics, paper 536; Proceedings, 1-35, London, 1974.
402
21 July 1975
[3] Yu.B. Bushnin et al., IHEP preprints 72-34 and 74-21, Serpukhov (1974); Nucl. Instr. Meth. 106 (1973) 497 and 120 (1974) 391. [4] W.D. Apel et al., IHEP preprint 74-118, Serpukhov (1974); XVII Intern. Conf. on High energy physics, paper 537; Proceedings, 1-35, London, 1974. [5] R. Wagner, XVII Intern. Conf. on High energy physics, Proceedings, 11-35, London, 1974; Dowel et al., Experimental meson spectroscopy conference, Boston, 1974; Maener et al., Experimental meson spectroscopy conference, Boston, 1974.