We have measured the reaction γp → pπ+π− in the DESY 1 m Streamer Chamber. The dominant ϱo production is analyzed in terms of various models

Aachen-Hamburg-Heidelberg-Munich Collaboration

Brandt, S.

Derado, I.

Dittmann, P.

Eckardt, V.

Grimm, M.

Joos, P.

Knobloch, J.

Kronseder, G.

Ladage, A.

Maier-Reimer, E.

Meyer, H.

Naroska, B.

Notz, D.

Pollmann, D.

Rabe, E.

Schacht, P.

Struczinski, W.

Nuclear Physics B

DESY Publication Database

Photoproduction of $\mathrm{\rho^o}$ on hydrogen with tagged photons between 4 and 6 GeV

DESY

MPG.PuRe

~ ]  Nuclear Physics B47 (1972) 436-444. North-Holland Publishing Company PHOTOPRODUCTION OF p o ON HYDROGEN WITH TAGGED PHOTONS BETWEEN 4 AND 6 GeV W. STRUCZINSKI III. Physikalisches Institut der Technischen Hochschule Aachen P. DITTMANN, V. ECKARDT *, P. JOOS, A. LADAGE, H. MEYER, B. NAROSKA and D. NOTZ Deutsches Elektronen-Synchrotron DESY, Hamburg J. KNOBLOCH, E. RABE and E. MAIER-REIMER IL Institut fiir Experimentalphysik der Universitiit Hamburg S. BRANDT, M. GRIMM and D. POLLMANN lnstitut fffr Hochenergiephysik, Heidelberg I. DERADO, G. KRONSEDER and P. SCHACHT Max-Planck Institut f[Jr Physik und A strophysik, Miinchen Received 11 July 1972 Abstract: We have measured the reaction "rP ~ P ~r+n- in the DESY 1 m Streamer Chamber. The dominant pO production isanalyzed in terms of various models. 1. INTRODUCTION Previous studies [1-3]  o fp  ° production in the reaction TP -* P rr+rr- , (1) have shown that the rr+rr - mass distribution is skewed relative to a p-wave Breit- Wigner resonance shape and that the slope of the t-distribution (t is the four-mo- mentum transfer squared from incident o outgoing proton) depends trongly on the effective di-pion mass. The skewing of the pO shape requires model-dependent background subtraction, thus introducing a significant uncertainty into the deter- mination of the cross section for the channel 3,19 -~ ppO. (2) * Now at Max-Planck Institut fiir Physik und Astrophysik, Miinchen. W. Struczinski et aL, p ° photoproduction 437 We have used three phenomenological models, namely a parametrization f the Breit-Wigner curve [3, 4], an interference model [5] and a simple pomeron exchange model [6]. The resulting cross sections are given in table 2. 2. EXPERIMENTAL SET-UP A monochromatic e ÷beam (with energy settings of 6.5, 4.3, 3.5 and 2.9 GeV) crossing a 1.2 mm A1 radiator produced abremsstrahlung beam. For each photon in the selected energy region the recoil positron was bent onto a hodoscope of 12 over- lapping scintillation counters. This defined 23 energy channels with widths between -+60 MeV and -+30 MeV. These channels were calibrated by measuring the energy of e+e - pairs produced by the tagged photons in the streamer chamber. The double-gap streamer chamber [7] was filled with a helium-neon gas mixture at 1 arm and was operated with a 500 kV, 10 nsec pulse. The pulse was supplied by a 14 stage Marx generator and formed by a 0.8 m co-axial Blumlein system. Built into the 22 kG magnet of the DESY bubble chamber, the chamber had a sensitive volume of 100 X 60 × (2 X 16) cm 3. A liquid hydrogen target (4 cm in length, 2.6 cm in diameter) was surrounded by a cylindric scintillation counter of 0.3 cm thickness and placed inside the chamber. Two counters in the plane perpendicular to the magnetic field recognized electron pair production. A shower counter was used as flux monitor. 3. ANALYSIS OF THE FILM Approximately 800 000 photographs were taken, showing a total of 40 000 events on hydrogen and 100 000 events on scintillation material. Part of the film was scanned twice, and the combined scanning efficiency was found to be greater than 99%. In this paper, we are concerned only with the data in the energy region between 4.1 and 6.2 GeV. In this energy region we measured 1701 events of type (1). After three measurements about 5% of the events remained unmeasurable. For the geometric and kinematic reconstruction f the events, the CERN program chain (THRESH, GRIND) was used with minor changes. The vertex reconstruction al-lowed a complete separation of events produced in hydrogen and the scintillation material. The measurement resolution in space was 250/1m. 4. CORRECTIONS A detailed escription of the corrections for our triggering biases will be pub- fished later. The following major corrections to the data were made: (a) The effi- ciency of the flux monitor was intensity dependent. Therefore our data were nor- 438 w. Struczinski et aL, o o photoproduc tion malized to the total 7P cross section [3] of 124.5/~b at 5 GeV; (b) the cylindrical target counter was not fully efficient. The loss is topology dependent and amounts to (12 -+ 2)%; (c) hadronic events in which one particle hits one of the pair counters were vetoed. This loss has an average value of (29 -+ 3)%. 5. RESULTS The cross section for reaction (1) was found to be 20.9 -+ 1.7 ~b, in agreement with previous experiments [ 1-3]. The dominant feature of reaction (1) is the pro- duction ofp  °. Fig. la shows the 7r+Tr mass distribution and fig. lb the t-distribu- tion for all events. The double differential cross section d2o/d,M~r~ dt for reaction (1) is given in table 1. In order to obtain the cross section for/9 ° production we used three models: (a) Modification [3, 4] of the p-wave Breit-Wigner distribution by a factor (Mo/MTr~r)n (t), where n(t) is a free parameter. (b) An interference model, where the pO production amplitude interferes with a Drell-type background [5]. (c) A pomeron-exchange model with s-channel 7-t9 helicity conservation [6]. In cases (a) and (b) maximum likelihood fits were made to the events of reaction (1) in 9 t-intervals between I tlmin and I tl = 0.54 GeV 2, taking into account also &(1236) production and a phase-space background. The results of the fits are given in table 2. The values of the slope and the forward cross sections were obtained as- suming an exp (At) dependence of the #- production cross section. In case (c) the two free parameters of the model, to be determined from experi- mental data, are the overall normalization g and a mass-independent t-distribution slope a (see ref. [6]). In fitting these parameters we used only the region 0.7 GeV <Mn7 r< 0.8 GeV and I tl < 0.3 GeV 2. In this interval about 2% background were subtracted using the prism-plot method [8]. From g and a one can calculate within the pomeron exchange model all interesting quantities of reaction (2). They are given in table 2c. The quoted errors are from the statistical uncertainties of the two fitted parameters a and g only. The extrapolation to the unphysical point t = 0 was done by fitting the function exp (At +Bt 2) in the region Itl < 0.4 GeV 2 to the cal- culated differential pO cross sections do/dt. The A(1236) cross section was obtained by using the prism-plot method [8]. To study the helicity of the 7r+Tr system in reaction (1) the angular distribution of one pion in the 7r+Tr - rest system was investigated, with the c.m.s, direction of flight of the 7r+Tr - system as polar axis. Fig. 2a shows the helicity density matrix elements for Mn7 r< 1.0 GeV, which were obtained from a maximum likelihood fit assuming p-wave dominance. They are consistent with s-channel helicity conserva- tion (P00 = #1 1 = Re Pl0 = 0) for Itl values below 0.3 GeV 2. In addition fig. 2b shows ~Y~(g2i) and EY,~(f2i) as a function ofM~r ~. The sum is taken over all events, and g2 is the decay angle in the s-channel helicity system. The Y~ moment shows the same (skewed) #-shape as the cross section, again consistent with a helicity W. Struczinski et al., p ° photoproduction 439 )~ 0 .36-0 .40  O.4O 0.44 0.44 0.48 0 .48-0 .52  0 .52-0 .56  0 .56-0 .60  0 .60-0 .64  0 .64-0 .68  0 .68-0 .72  0 .72-0 .76  0 .76-0 .80  0 .80-0 .84  0 .84-0 .88  0 .88-0 .92  O.92 0.96 0 .96-1 .00  1 .00-1 .04  1 .04-1 .08  1 .08-1 .12  1 .12-1 .16  1 .16-1 .20  1 .20-1 .28  1 .28-1 .36  1.36 1.44 1 .44-1 .52  1 .52-1 .60  1 .60-1 .68  1 .68-1 .76  1 .76-1 .84  1 .84-1 .92  1.92 2.00 Table 1 React ion2p ~ pn+n - ,  4.1 < E 7 < 6.2 GeV. 1tim± n 0.03 0 .03-0 .06  0 65 ,+ 65 77,+ 77 40 ,+32 150,+ 94 39+31 177 ,+ 111 0 138,+ 77 79+-35 200 ,+ 99 106 ,+ 42 177,+ 73 91-+35 177_+ 69 144 ,+42 237 +- 82 300 ,+ 67 418 ,+ 128 405 ,+ 73 585 ,+ 146 499 ,+ 81 490 _+ 145 415 ,+ 72 250 +- 91 242 ,+ 48 30-+ 19 104+-34 72,+ 43 81 ,+28 7 +- 7 14 ,+ 10 0 7 -+7 0 8 -+6 0 9 -+9 0 0 0 16,+11 0 7 -+7 0 5 -+5 0 4 -+4 0 0 0 4+-4  0 0 0 0 0 0 0 0 0 0 0 0 0 .06-0 .09  0 .09-0 .12  0.12 0.15 0 0 22 +- 22 0 18+- 18 18,+ 14 16 ,+ 16 12 ,+ 12 35 + 22 91 ,+49 25 + 19 6 + 6 117 + 48 10 + 10 22 ,+ 22 111 ,+39 34+22 32 ,+20 171 +- 53 80 ,+ 35 26 -+ 16 204 ,+ 57 120 + 41 59 + 25 200 +- 51 201 ,+ 50 68 ,+ 29 368 + 73 188 + 51 125 -+ 38 477 + 87 348 ,+ 70 245 ,+ 56 326 ,+ 62 263 + 58 202 ,+ 48 199 + 44 150 ,+ 43 133 ,+ 38 63-+24 83-+29 51-+23 32 ,+ 17 79 ,+ 30 30 ,+ 16 8 + - 8 15,+11 32 ,+17 8+ 8 6 -  + 6 0 33 ,+ 21 31 ,+ 16 8 +- 8 33-+26 9 + - 9 8-+ 8 9 + - 9 17 ,+12 0 0 9 -+9 0 0 0 0 7 -  + 5 4+- 4 5 -  + 5 4-+ 4 0 9+- 9 8-+ 6 9_+ 9 0 5-  + 5 12-+10 0 0 9 -  + 6 10+- 7 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 do/dt(ub/GeV 2) 127 -+ 23 108 -+ 11 100 ,+ 11 71 +- 8 47 -+ 6 Double di f ferent ia l  cross sect ion da/dMTrrr dtpp in ub /GeV 3. I+ I+ I+ t~ I+ t~ I+ t~ I+ t~ I+ t~ l+ I+ I+ I+ I+ I+ l+ ~ ~ ~ I+ I+ I+ I+ ~ I+ I+ I+ I+ I+ I+ I+ I+ I+ I+ I+ I+ ~+ J+ 1+ I+ I+ I+ I+ I+ I+ I+ ~+ ~ I+ I+ I+ I+ I+ I+ I+ I+ ~ I+ ~ ~ I+ I+ I+ ~ I+ I+ I+ ~+ ~ I+ ~ ~ I+ I+ I+ I+ I+ I+ t+ I+ L+ ~ I+ I+ ~ I+ I+ I+ ~+ ~ ~ ~ I+ I+ ~ ~ ~ L+ l+ ~ I+ t+ I+ I+ I+ I+ ~ S+ ~ I+ I+ I+ I+ I+ I+ I+ 1+ I+ 1+ I+ I+ I+ I+ I+ I+ I+ L+ ~+ I+ I+ I+ I+ I+ I+ I+ I+ I+ I+ I+ I+ I+ ~+ C~ Oc~ L~ C~ i ~O C~ I C~ C~ 0 =. c~ C~ 0 W. Struczinski et al., pO photoproduction 441 (.b .,Q ::1. "1o lOO 80 60 ~o 20 o o)  0.8 1.2 7.6 2.0 Mrr [  GeV] (LJ (D b 1oo I÷, 50 I0 5 ÷÷ ÷+ +++++ + i I l i 0 0.2 O Z b) i I I 0.6 0.8 it~ [ c~v 2] Fig. 1. Reaction ~/p -~ Wr+Tr - ,  4.1 < E7 < 6.2 GeV. a. do/dMTr~r i n2~b/GeV. The curve is a fit with model (a). b. do/dtpp in ~b/GeV . conserving p-wave. The structure in the Y~ moment at M~r n < Mp is suggestive of an interference between diffractive pO production and a Drell-type background ampli- tude [5]. It was checked that this structure is not a statistical effect induced by the strongly peaking Y~ moment. .Ix t~ Model o(Tp ~ PO °) A (pb) (GeV -2) Table 2 Reaction 3'P ~ P n+Tr-, 4.1 < E 7 < 6.2 GeV. B do(Tp --" po °) Mp (G eV-4) dt t=0 (GeV) (#b/GeV 2) Fp (GeV) o(~p -~ ~++~r-) (ub) o(Tp ~ ~on+) (#b) (a) 18.1±1.5 8.9±0.4 (b) 15.2±1.4 8.0±0.5 (c) 17.0±2.0 9.7±0.8 2.3±0.2 156 + 15 0.762 0.133 135 ± 19 0.762 0.133 159 ± 20 0.765 0.155 1.1±0.2 0.9±0.3 1.0±0.15 0.4±0.1 0.5±0.1 © pO and A production cross section in terms of three models. W. Struczinski et al., p o photoproduction 443 a) b) 0.2 0.1 0 -01 -0  2 ~oo 0.2 0.I 0 -0.1 -0.2 0.2 0.1 0 -0.1 -0.2 Re ~to 0.2 0.4 0.6 0.8 I~1 [GevZ] 0 -20 ,° l-50 20 ,% -I-~ t t f ÷ ff 10 0 -10 I 0.2 0,6 1.0 t.4 1.8 t4~ r [ Gev] Fig. 2. 4.1 < E~, < 6.2 GeV. a. Reaction "rP -~ PP °,  density matr ix  e lements  o fo  ° as a funct ion of tpp. b. Reaction "rP ~ P n+n-, moments  ZY~(S2i) and ~Y~(~i) as a funct ion of M~n for • tpp < 0.5 GeV 2 (the sum is taken over all events). We are indebted to G. Horlitz and his group for continuous upport of our ex- periment while the streamer chamber was inside the bubble chamber magnet. We want to thank our scanning and measuring staff for their skillful work. We are grate- ful to P. SiSding and N. Schmitz for many discussions. 444 W. Strucz insk i  et al., 0 ° photoproduct ion  REFERENCES [ 1 ] Aachen-Berlin-Bonn-Hamburg-Heidelberg-Miinchen Collaboration, Phys. Rev. 175 (1968) 1669. [2] Y. Eisenberg et al., Phys. Rev. D5 (1972) 15. [3] J. Ballam et al., Phys. Rev. D5 (1972) 545. [4] M. Ross and L. Stodolsky, Phys. Rev. 149 (1966) 1172. [5] P. SSding, Phys. Letters 19 (1965) 702. [6] I. Derado et al., Nucl. Phys. B38 (1972) 541. [7] V. Eckardt and A. Ladage, Proc. of the Syrup. on nuclear electronics, Versailles, III, 10-1, 1968; Proc. of the Int. Conf. on instrumentation for high energy physics, Dubna, 1970. [8] J.E. Brau et al., Phys. Rev. Letters 27 (1971) 1481. 

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