A study of the centrally produced ppbar, ppbarpi, ppbarpipi and lambda lambda
channels has been performed in pp collisions using an incident beam momentum of
450 GeV/c. No significant new structures are observed in the mass spectra,
however, important new information on the production dynamics is obtained. A
systematic study of the production properties of these systems has been
performed and it is found that these systems are not produced dominantly by
double Pomeron exchange.Comment: 13 pages, Latex, 4 Figure

Barberis, D

Beusch, W

Binon, F.G

Blick, A.M

Close, F.E

Danielsen, K.M

Dolgopolov, A.V

Donskov, S.V

Earl, B.C

Evans, D

French, B.R

Hino, T

Inaba, S

Inyakin, A.V

Ishida, T

Jacholkowski, A

Jacobsen, T

Jones, G.T

Khaustov, G.V

Kinashi, T

Kinson, J.B

Kirk, A

Klempt, W

Kolosov, V

Kondashov, A.A

Lednev, A.A

Lenti, V

Maljukov, S

Martinengo, P

Minashvili, I

Nakagawa, T

Norman, K.L

Peigneux, J.P

Polovnikov, S.A

Polyakov, V.A

Romanovsky, V

Rotscheidt, H

Rumyantsev, V

Russakovich, N

Samoylenko, V.D

Semenov, A

Sené, M

Sené, R

Shagin, P.M

Shimizu, H

Singovsky, A.V

Sobol, A

Solovjev, A

Stassinaki, M

Stroot, J.P

Sugonyaev, V.P

Takamatsu, K

Tchlatchidze, G

Tsuru, T

Venables, M

Villalobos Baillie, O

Votruba, M.F

Yasu, Y

Physics Letters B

English

arXiv

A study of the centrally produced ppbar, ppbarpi, ppbarpipi and lambda lambda channels has been performed in pp collisions using an incident beam momentum of 450 GeV/c. No significant new structures are observed in the mass spectra, however, important new information on the production dynamics is obtained. A systematic study of the production properties of these systems has been performed and it is found that these systems are not produced dominantly by double Pomeron exchange.A study of the centrally produced ppbar, ppbarpi, ppbarpipi and lambda lambda channels has been performed in pp collisions using an incident beam momentum of 450 GeV/c. No significant new structures are observed in the mass spectra, however, important new information on the production dynamics is obtained. A systematic study of the production properties of these systems has been performed and it is found that these systems are not produced dominantly by double Pomeron exchange.A study of the centrally produced ppbar, ppbarpi, ppbarpipi and lambda lambda channels has been performed in pp collisions using an incident beam momentum of 450 GeV/c. No significant new structures are observed in the mass spectra, however, important new information on the production dynamics is obtained. A systematic study of the production properties of these systems has been performed and it is found that these systems are not produced dominantly by double Pomeron exchange.A study of the centrally produced ppbar, ppbarpi, ppbarpipi and lambda lambda channels has been performed in pp collisions using an incident beam momentum of 450 GeV/c. No significant new structures are observed in the mass spectra, however, important new information on the production dynamics is obtained. A systematic study of the production properties of these systems has been performed and it is found that these systems are not produced dominantly by double Pomeron exchange.A study of the centrally produced p p , p p π 0 , p p π + π − and Λ Λ channels has been performed in pp collisions using an incident beam momentum of 450 GeV/c. No significant new structures are observed in the mass spectra, however, important new information on the production dynamics is obtained. A systematic study of the production properties of these systems has been performed and it is found that these systems are not produced dominantly by double Pomeron exchange

CERN Document Server

3 December 1998A study of the centrally produced baryon-antibaryonsystems in pp interactions at 450 GeV/cThe WA102 CollaborationD.Barberis4, W.Beusch4, F.G.Binon6, A.M.Blick5, F.E.Close3,4, K.M.Danielsen10,A.V.Dolgopolov5, S.V.Donskov5, B.C. Earl3, D. Evans3, B.R. French4, T.Hino11, S. Inaba8,A.V. Inyakin5, T. Ishida8, A. Jacholkowski4, T. Jacobsen10, G.TJones3, G.V.Khaustov5,T.Kinashi12, J.B.Kinson3, A.Kirk3, W.Klempt4, V.Kolosov5, A.A.Kondashov5,A.A. Lednev5, V. Lenti4, S.Maljukov7, P.Martinengo4, I.Minashvili7, T.Nakagawa11,K.L.Norman3, J.P. Peigneux1, S.A. Polovnikov5, V.A.Polyakov5, V.Romanovsky7,H.Rotscheidt4, V.Rumyantsev7, N.Russakovich7, V.D. Samoylenko5, A. Semenov7, M. Sene4,R. Sene4, P.M. Shagin5, H. Shimizu12, A.V. Singovsky1,5, A. Sobol5, A. Solovjev7,M. Stassinaki2, J.P. Stroot6, V.P. Sugonyaev5, K.Takamatsu9, G.Tchlatchidze7, T.Tsuru8,M.Venables3, O.Villalobos Baillie3, M.F.Votruba3, Y.Yasu8.AbstractA study of the centrally produced pp , pp0 , pp+− and  channels has been performedin pp collisions using an incident beam momentum of 450 GeV/c. No signicant new structuresare observed in the mass spectra, however, important new information on the production dy-namics is obtained. A systematic study of the production properties of these systems has beenperformed and it is found that these systems are not produced dominantly by double Pomeronexchange.Submitted to Physics Letters1 LAPP-IN2P3, Annecy, France.2 Athens University, Physics Department, Athens, Greece.3 School of Physics and Astronomy, University of Birmingham, Birmingham, U.K.4 CERN - European Organization for Nuclear Research, Geneva, Switzerland.5 IHEP, Protvino, Russia.6 IISN, Belgium.7 JINR, Dubna, Russia.8 High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan.9 Faculty of Engineering, Miyazaki University, Miyazaki, Japan.10 Oslo University, Oslo, Norway.11 Faculty of Science, Tohoku University, Aoba-ku, Sendai 980, Japan.12 Faculty of Science, Yamagata University, Yamagata 990, Japan.1Experiment WA102 is designed to study exclusive nal states formed in the reactionpp! pf(X0)ps (1)at 450 GeV/c. The subscripts f and s indicate the fastest and slowest particles in the laboratoryrespectively and X0 represents the central system that is presumed to be produced by doubleexchange processes. The experiment has been performed using the CERN Omega Spectrometer,the layout of which is described in ref. [1]. In previous analyses of other channels it has beenobserved that when the centrally produced system has been analysed as a function of theparameter dPT , which is the dierence in the transverse momentum vectors of the two exchangeparticles [1, 2], all the undisputed qq states are suppressed at small dPT , in contrast to glueballcandidates.This paper presents a study of the pp , pp0 , pp+− and  nal states at a centre ofmass energy ofps = 29.1 GeV. The pp , pp+− and  channels have previously been studiedatps = 12.7 GeV [3]. In recent years there have been claims of the observation of two dierentresonant signals in the pp channel. The rst claim is for the observation of the (2220), with awidth of 20 MeV, in radiative J= decays made by the BES collaboration [4]. The (2220) isclaimed to be a good candidate for the tensor glueball. To date, every established JPC = 0++and 2++ glueball candidate has been observed in central pp collisions. Therefore, it is importantto look for these new states in central production in order to learn more about the nature and/orexistence of the (2220).The second claimed observation is for a state with a mass of 2.02 GeV and a width of lessthan 10 MeV observed in central baryon exchange by the WA56 experiment [5]. It is claimedthat this state could be interpreted as a baryonium candidate. Although the current experimentdoes not study baryon exchange it does study central production and hence a search for thisstate may be of interest.In addition to these searches, an analysis of the production kinematics of baryon-antibaryonproduction is presented which can give information on the mechanism of the formation of thesenal states in central production.The reactionpp! pf(pp)ps (2)has been isolated from the sample of events having four outgoing charged tracks, by rstimposing the following cuts on the components of the missing momentum: jmissing Pxj < 14:0GeV/c, jmissing Pyj < 0:16 GeV/c and jmissing Pzj < 0:08 GeV/c, where the x axis is along thebeam direction. A correlation between pulse-height and momentum obtained from a system ofscintillation counters was used to ensure that the slow particle was a proton.In order to select the pp system, information from the Cerenkov counter was used. Onecentrally produced charged particle was required to be identied as a p or an ambiguous K/p bythe Cerenkov counter and the other particle was required to be consistent with being a proton.The method of Ehrlich et al. [6], has been used to compute the mass squared of the two centrallyproduced particles assuming them to have equal mass. The resulting distribution is shown ing. 1a) where a clear peak can be seen at the proton mass squared. This distribution has beentted with Gaussians to represent the contributions from the +− , K+K− and pp channels.From this t the number of pp events is found to be 6256  220.2A cut on the Ehrlich mass squared of 0:65  M2X  1:15 GeV 2 has been used to select asample of pp events. The resulting pp eective mass distribution is shown in g. 1b). Thereare no signicant structures in the mass spectrum, in particular there is no evidence for the(2220) that has been claimed in the pp channel by the BES collaboration [4] nor is thereevidence for the narrow structure at 2.02 GeV claimed to have been observed in central baryonexchange reactions [5]. The mass resolution of the WA102 experiment in each region is betterthan 10 MeV. We can calculate an upper limit for the cross-sections for the production of theseclaimed resonances in central pp interactions to be ((2220)) < 1:6 nb and (2:02GeV ) < 1:4nb at 95 % condence level.The reactionpp! pf(pp0)ps (3)where the 0 has been observed decaying to γγ, has been isolated from the sample of eventshaving four outgoing charged tracks plus two γs each with energy greater than 0.5 GeV re-constructed in the electromagnetic calorimeter1, by rst imposing the following cuts on thecomponents of the missing momentum: jmissing Pxj < 17:0 GeV/c, jmissing Pyj < 0:16 GeV/cand jmissing Pzj < 0:12 GeV/c.One centrally produced charged particle was required to be identied as a p or an ambiguousK/p by the Cerenkov counter and the other particle was required to be consistent with beinga proton. The Ehrlich mass distribution is shown in g. 1c) where a clear peak can be seenat the proton mass squared. This distribution has been tted with Gaussians to represent thecontributions from the +−0 , K+K−0 and pp0 channels. From this t the number ofpp0 events is found to be 877  85. A cut on the Ehrlich mass squared of 0:65  M2X 1:15 GeV 2 has been used to select a sample of pp0 events. The resulting pp0 eective massdistribution is shown in g. 1d) where no signicant structure can be observed. Similarly, thepp, p0 and p0 mass spectra do not show any signicant structures (not shown).The reactionpp! pf(pp+−)ps (4)has been isolated from the sample of events having six outgoing charged tracks, by rst imposingthe following cuts on the components of the missing momentum: jmissing Pxj < 14:0 GeV/c,jmissing Pyj < 0:12 GeV/c and jmissing Pzj < 0:08 GeV/c.In order to select the pp+− system an event was accepted if a positive or negative particlewas identied as a p or a K/p by the Cerenkov system and the other particle with the samecharge was consistent with being a . A modied method of Ehrlich et al. [6], has been used tocompute the mass squared of the two highest momentum central particles assuming the othertwo particles to be pions. The resulting distribution is shown in g. 2a) where a clear peakcan be seen at the proton mass squared. This distribution has been tted with Gaussians torepresent the contributions from the +−+− , K+K−+− and pp+− channels. Fromthis t the number of pp+− events is found to be 2076  160. A cut on the Ehrlich masssquared of 0:65  M2X  1:15 GeV 2 has been used to select a sample of pp+− events. Theresulting pp+− eective mass spectrum is shown in g. 2b) and shows a broad distributionwith a maximum near threshold.1The showers associated with the impact of the charged tracks on the calorimeter have been removed fromthe event before the requirement of only two γs was made.3A study has been performed of the various two and three body subsystems but no structurehas been observed except ++ and −−in the p+ and p− mass spectra respectively. Fig. 2c)shows a scatter plot of M(p−) versus M(p+) where a clear accumulation of events can beobserved in the ++−−region. However, it is not possible to extract a reliable measure ofthe ++−−contribution due to the diculties in establishing the level of background.The reactionpp! pf()ps (5)has been isolated from the sample of events having two outgoing charged tracks plus two V 0s, byrst imposing the following cuts on the components of the missing momentum: jmissing Pxj <14:0 GeV/c, jmissing Pyj < 0:16 GeV/c and jmissing Pzj < 0:02 GeV/c. For each V 0 the valueof  =P+L −P−LP+L +P−L, was calculated, where P+L (P−L ) is the longitudinal momentum of the positive(negative) particle from the decay of the V 0 with respect to the V 0 momentum vector. Fora  ()  is positive (negative) and hence events which were compatible with being  wereselected by requiring that the product 1:2 was negative.The quantity , dened as  = MM2(pfps)−M2(), where MM2(pfps) is the missingmass squared of the two outgoing protons, was then calculated for each event and a cut ofjj  2.0 (GeV)2 was used to select the  channel. In order to study any possible residualK0S contamination a scatter plot of M() versus M(p) is shown in g. 3a) for the casewhen the other V 0 is compatible with being a  (1.09  M(p)  1.14 GeV). The resultingp− (p+) mass distribution is shown in g 3b) where a clear  () signal can be seen overlittle background, with negligible contribution from the K0S. The resulting  eective massspectrum is shown in g. 3c) and consists of 123 events.A study of the pp , pp0 , pp+− and  systems has been performed as a function ofthe parameter dPT , which is the dierence in the transverse momentum vectors of the twoexchanged particles [1, 2]. After acceptance corrections the results are shown in table 1 togetherwith the value of the ratio (R) of events at small dPT to large dPT . In previous studies [7]of the ratio R we have observed that all systems fall into three distinct classes. Firstly, thereare all the undisputed qq states which can be produced in Double Pomeron Exchange (DPE),namely those with positive G parity and I = 0, which have a small value for this ratio (< 0:1).Secondly, there are those states with I = 1 or G parity negative, which cannot be producedby DPE, which have a slightly higher value ( 0:25). Finally, there are the states which couldhave a gluonic component, which have a large value for this ratio (> 0:6). It is interesting tonote that the baryon-antibaryon systems have a value of R consistent with the second class, i.e.that they are not produced by DPE. This fact can be investigated by studying the cross-sectiondependence as a function of centre of mass energy.After correcting for geometrical acceptances, detector eciencies and losses due to selectioncuts, the cross-sections for the channels atps = 29.1 GeV in the xF interval jxF j  0:2 havebeen calculated and are shown in table 2. These can be compared, where possible, to thecross-sections found atps = 12.7 GeV which are also shown in table 2. As can be seen thecross-sections are decreasing with increasing centre of mass energy. This is not consistent withthem being produced dominantly by DPE and suggest that these systems are produced bydouble Regge or Regge-Pomeron exchanges [8].4The acceptance corrected azimuthal angle () between the pT vectors of the two protons(pf and ps) is shown in g. 4a), c), e) and g). The distributions in all cases are consistent withbeing flat. Although naively a flat distribution would be expected, this is the rst time that asystem or resonance has been observed to have a flat  distribution [9].Fig. 4b), d), f) and h) shows the four momentum transfer squared at one of the protonvertices. The distributions have been tted with a single exponential of the form exp(−bjtj)and the results are presented in table 2. The rst bin in the distributions has been excludedfrom the t due to the fact that the uncertainties in the acceptance correction are greatest inthis bin.In conclusion, a study of the centrally produced pp , pp0 , pp+− and  channels hasbeen performed. There is no evidence for resonance production with the exception of ++ and−−in the pp+− channel. In the pp channel there is no evidence for the (2220) and anupper limit on the cross-section for its production in central pp collisions has been calculatedto be 1.6 nb. A study of the centre of mass energy dependence for the production of centralbaryon-antibaryon systems shows that they are not produced dominantly by double Pomeronexchange.AcknowledgementsThis work is supported, in part, by grants from the British Particle Physics and AstronomyResearch Council, the British Royal Society, the Ministry of Education, Science, Sports andCulture of Japan (grants no. 04044159 and 07044098), the Programme International de Co-operation Scientique (grant no. 576) and the Russian Foundation for Basic Research (grants96-15-96633 and 98-02-22032).5References[1] D. Barberis et al., Phys. Lett. B397 (1997) 339.[2] F.E. Close and A. Kirk, Phys. Lett. B397 (1997) 333.[3] T.A. Armstrong et al., Zeit. Phys. C35 (1987) 167.[4] J.Z. Bai et al., Phys. Rev. Lett. 76 (1996) 3502;K.T. Chao, Comm. Theor. Phys. 24 (1995) 373;F.E. Close, G. Farrar and Z.P. Li, Phys. Rev. D55 (1997) 5749.[5] A. Ferrer et al., CERN/EP/98-66.[6] R. Ehrlich et al., Phys. Rev. Lett. 20 (1968) 686.[7] A. Kirk, hep-ex/9803024.[8] S.N. Ganguli and D.P. Roy, Phys. Rep. 67 (1980) 203.[9] D. Barberis et al., Phys. Lett. B427 (1998) 398;D. Barberis et al., Phys. Lett. B422 (1998) 399;D. Barberis et al., Phys. Lett. B432 (1998) 436;D. Barberis et al., Phys. Lett. B436 (1998) 204;D. Barberis et al., Phys. Lett. B440 (1998) 225.6Table 1: Production of the channels as a function of dPT expressed as a percentage of its totalcontribution and the ratio (R) of events produced at dPT  0.2 GeV to the events produced atdPT  0.5 GeV.dPT0.2 GeV 0.2dPT0.5 GeV dPT0.5 GeV R = dPT0.2GeVdPT0.5GeVpp 14.4 0.4 44.6  0.7 41.0  0.7 0.35  0.01pp0 14.5  1.0 43.3  1.6 42.2  1.6 0.34  0.03pp+− 13.9  0.5 43.3  0.9 42.8  0.9 0.32  0.01 15.0  3.5 39.4  5.6 45.6  6.1 0.33  0.097Table 2: The cross-sections and slope parameter of the four momentum transfer squared forthe pp , pp0 , pp+− and  channels.Channel Cross-sectionnnb Slopebps=12.7 GeVps=29.1 GeV GeV−2pp 400  60 186  19 5.5  0.3pp0 - 43  5 5.9  0.5pp+− 226  42 82  7 5.4  0.3 29  8 12  2 4.5  2.08FiguresFigure 1: a) The Ehrlich mass squared distribution and b) the pp mass spectrum for thepp channel. c) The Ehrlich mass squared distribution and d) the pp0 mass spectrum for thepp0 channel.Figure 2: For the pp+− channel a) the Ehrlich mass squared distribution, b) the pp+− massspectrum and c) the scatter plot of M(p−) versus M(p+).Figure 3: For the  channel a) the scatter plot of M(+−) versus M(p), b) the M(p) massdistribution and c) the  mass spectrum.Figure 4: a), c), e) and g) the azimuthal angle () between the two outgoing protons forthe pp , pp0 , pp+− and  channel respectively. b), d), f) and h) the four momentumtransfer squared (jtj) from one of the proton vertices for the pp , pp0 , pp+− and  channelrespectively.9Figure 110Figure 211Figure 312Figure 413

A study of the centrally produced baryon-antibaryon systems in pp interactions at 450 GeV/c

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A study of the centrally produced baryon-antibaryon systems in pp
  interactions at 450 GeV/c

A study of the centrally produced baryon-antibaryon systems in pp interactions at 450 GeV/c

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