A study of resonance production in central pp collisions is presented as a
function of several kinematical variables. In particular the difference in the
transverse momentum (dPT) of the exchanged particles shows that undisputed
qqbar mesons are suppressed at small dPT whereas glueball candidates are
enhanced and in addition, the azimuthal angle phi gives information on the
nature of the Pomeron.Comment: 14 pages, Latex, 5 Figure

A. Kirk

Adloff

Armstrong

Barberis

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Ganguli

Physics Letters B

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Resonance production in central pp collisions at the CERN Omega Spectrometer

A study of resonance production in central pp collisions is presented as a function of several kinematical variables. In particular the difference in the transverse momentum (dPT) of the exchanged particles shows that undisputed qqbar mesons are suppressed at small dPT whereas glueball candidates are enhanced and in addition, the azimuthal angle phi gives information on the nature of the Pomeron.A study of resonance production in central pp collisions is presented as a function of several kinematical variables. In particular the difference in the transverse momentum ( dP T ) of the exchanged particles shows that undisputed q q ̄ mesons are suppressed at small dP T whereas glueball candidates are enhanced and in addition, the azimuthal angle φ gives information on the nature of the Pomeron.A study of resonance production in central pp collisions is presented as a function of several kinematical variables. In particular the difference in the transverse momentum (dPT) of the exchanged particles shows that undisputed qqbar mesons are suppressed at small dPT whereas glueball candidates are enhanced and in addition, the azimuthal angle phi gives information on the nature of the Pomeron

Kirk, A.

CERN Document Server

7 August 2000Resonance production in central pp collisions at theCERN Omega SpectrometerA.KirkSchool of Physics and Astronomy, University of Birmingham, Birmingham, U.K.AbstractA study of resonance production in central pp collisions is presented as a function of severalkinematical variables. In particular the dierence in the transverse momentum (dPT ) of theexchanged particles shows that undisputed qq mesons are suppressed at small dPT whereasglueball candidates are enhanced and in addition, the azimuthal angle  gives information onthe nature of the Pomeron.Submitted to Physics LettersThe Omega central production experiments (WA76, WA91 and WA102) were designed tostudy exclusive nal states formed in the reactionpp−!pfX0ps,where the subscripts f and s refer to the fastest and slowest particles in the laboratory framerespectively and X0 represents the central system. Such reactions are expected to be mediatedby double exchange processes where both Pomeron and Reggeon exchange can occur. Thetrigger was designed to enhance double exchange processes with respect to single exchange andelastic processes. Details of the trigger conditions, the data processing and event selection havebeen given in previous publications [1].The experiments have been performed at incident beam momenta of 85, 300 and 450 GeV/c,corresponding to centre-of-mass energies ofps = 12:7, 23.8 and 29 GeV. Theoretical predictions[2] of the evolution of the dierent exchange mechanisms with centre of mass energy,ps, suggestthat(RR)  s−1,(RIP )  s−0.5,(IPIP )  constant,where RR, RIP and IPIP refer to Reggeon-Reggeon, Reggeon-Pomeron and Pomeron-Pomeronexchange respectively. Hence we expect Double Pomeron Exchange (DPE) to be more signif-icant at high energies, whereas the Reggeon-Reggeon and Reggeon-Pomeron mechanisms willbe of decreasing importance. The decrease of the non-DPE cross section with energy can beinferred from data taken by the WA76 collaboration using pp interactions atps of 12.7 GeVand 23.8 GeV [3]. The +− mass spectra for the two cases show that the signal-to-backgroundratio for the 0(770) is much lower at high energy, and the WA76 collaboration report that theratio of the 0(770) cross sections at 23.8 GeV and 12.7 GeV is 0.44  0.07. Since isospin 1states such as the 0(770) cannot be produced by DPE, the decrease of the 0(770) signal athighps is consistent with DPE becoming relatively more important with increasing energywith respect to other exchange processes.Due to the fact that there was no electromagnetic calorimeter in the WA76 experiment atps of 12.7 GeV, the s dependence of only a small number of states can be determined. Thestates for which the s dependence have been measured and whose compatibility with DPE havebeen determined are given in table 1. Although there was no calorimeter in the 85 GeV/c runof the WA76 experiment, it was able to reconstruct the +− mass spectrum using the decay ! +−(0)missing [4]. In this mass spectrum the 0 and f1(1285) are seen. The cross sectionof the f1(1285) at 85 GeV/c has been well measured through its all charged particle decaymodes and hence can be used to determine the cross section of the 0.After correcting for geometrical acceptances, detector eciencies, losses due to selectioncuts, and all known decay modes, the cross-sections for the production of all the resonances atps = 29.1 GeV observed in the WA102 experiment are shown in table 1. As can be seen, thereis evidence that in central pp collisions ss production is much weaker than nn production. Forexample, the cross section for the production of the f2(1270), whose production has been foundto be consistent with DPE, is more than 40 times greater than the cross section of the f 02(1525).Part of this suppression could be due to the fact that there is a kinematic suppression of M−2X02and some could be explained by the fact that the centre of mass energy dependence of thef2(1270) allows for up to a 30 % contribution from non-DPE processes. However, a suppressionof  20 still needs to be explained. Hence there could be some strong dependence on the massof the produced quarks in DPE. A similar eect is observed in electroproduction (γIP ) wherethe  becomes more suppressed relative to the  at low Q2 of the photon [5].The cross section as a function of mass has not previously been shown for the centrallyproduced system. This distribution could be determined in two ways. Either by measuring themissing mass from the two outgoing protons or by measuring the eective mass of the decayproducts of the central system. The latter method has the disadvantage of requiring a separateacceptance correction for each nal state. However, due to the measurement errors on theoutgoing fast proton, the second method has the overwhelming advantage of having more thanan order of magnitude better resolution in the determination of the mass. In order to determinethe d=dMX0 dierential cross section, starting with the  nal state, the acceptance correctedmass spectra of all the nal states observed in the WA102 experiment have been added togetherand the resulting dierential cross section, d=dMX0, is shown in g. 1a). Prominent featuresare the , !, 0, f1(1285), f1(1420) and a broad enhancement around 2 GeV.In addition to the cross section and s dependences, there are a number of dierent kine-matical variables that can be determined for each resonance. The rst is the Feynman x (xF )distribution of the central system. It should be noted that for resonances that decay into allcharged particle decay modes the xF distribution measured is symmetric about 0. However, forresonances that decay into nal states including γs, because of the acceptance of the calorime-ter, only events with xF > 0. are detected. Hence for consistency, only xF > 0 will be shown.All the resonances observed tend to fall into two distinct classes: g. 2a) those that are peakedat xF = 0 and g. 2b) those that are peaked away from 0. All the resonances that can beproduced by DPE (i.e. those with I = 0 and G parity positive) fall into the rst class whilethose with I = 1 and/or G parity negative fall into the second class which may be due to thefact that, in part, they are produced by diraction from the proton vertex rather than frombeing due to real central events. Fig. 1b) shows the d=dMX0 , distribution for jxF j < 0.1. Ascan be seen the !, which can not be produced by DPE and which has a xF distribution peakedaway from zero, is suppressed in the jxF j < 0.1 distribution relative to the total sample.Another variable that can be measured is the four momentum transfer squared (jtj) at oneof the proton vertices. The jtj distribution has been measured for all the resonances. All thedistributions fall into three distinct classes as summarised in g. 2c)-e). Fig. 2c) (type I) hasthe classic shape associated with  exchange [4] and has been tted byddt=  exp b1t + t exp b2twhere  and  are found to be similar in size. Fig. 2d) (type II) is similar to spin flip distribu-tions and can be tted byddt= t exp b2tFig. 2e) (type III) represents non-spin flip distributions and can be tted byddt=  exp b1t3The results of the t to d=dt and the t distributions types for all the resonances observed aregiven in table 1.In addition to the kinematical variables discussed above, in central production there arevariables that are specic to the central vertex at which the two exchanged particles interactto form the nal state X0. The WA91 collaboration published a paper [6] showing that therewas a dierence in the resonances observed when the angle between the outgoing slow and fastprotons was near to 0 degrees compared to when the angle was near to 180 degrees. In order totry to explain this eect in terms of a physical model, Close and Kirk [7] proposed that the databe analysed in terms of the parameter dPT , which is the dierence in transverse momentumbetween the particles exchanged from the fast and slow vertices.The WA102 collaboration has presented studies of how dierent resonances are producedas a function of the parameter dPT [8]. The fraction of each resonance has been calculatedfor dPT0.2 GeV, 0.2dPT0.5 GeV and dPT0.5 GeV. In addition, the ratio of productionat small dPT to large dPT has been calculated. The results are presented in table 2 for allthe resonances observed in the WA102 experiment. Figure 3 shows the ratio of the numberof events for dPT < 0.2 GeV to the number of events for dPT > 0.5 GeV for each resonanceconsidered. It can be observed that all the undisputed qq states which can be produced inDPE, namely those with positive G parity and I = 0, have a very small value for this ratio( 0:1). Some of the states with I = 1 or G parity negative, which can not be produced byDPE, have a slightly higher value ( 0:25). However, all of these states are suppressed relativeto the interesting states, which could have a gluonic component, which have a large value forthis ratio. Fig. 1c) shows the d=dMX0 distribution for dPT < 0.2 and g. 1d) the d=dMX0distribution for dPT > 0.5. The dominant feature of the distribution for dPT < 0.2 is the lowmass peak which comes from the  system. This  mass spectrum is dominated by theS-Wave although there is a 10 % D-wave contribution [9]. The S and D-waves both have thesame dPT distributions which are similar to those observed for the f0(980).In addition to the dPT dependencies, an interesting eect has been observed in the azimuthalangle  which is dened as the angle between the pT vectors of the two outgoing protons. His-torically it has been assumed that the Pomeron, with \vacuum quantum numbers", transformsas a scalar and hence that the  distribution would be flat for resonances produced by DPE. The dependences observed [8] are clearly not flat and considerable variation is observed amongthe resonances produced. The  distributions for all the resonances observed in the WA102experiment are shown in gs. 4, 5. These azimuthal dependences, as a function of JPC , are verystriking. For the mesons that can be produced by DPE the  distributions maximise around90o for resonances with JPC = 0−+, at 180o for those with JPC = 1++ and at 0o for thosewith JPC = 2−+. As can be seen from g. 5 the  distributions are not simply a J-dependenteect, since 0++ production peaks at 0o for some states whereas others are more evenly spread;moreover, 2++ established qq states peak at 180o whereas the f2(1910) and f2(1950) peak at0o.Several theoretical papers have been published on these eects [10, 11]. All agree that theexchanged particle must have J > 0 and that J = 1 is the simplest explanation for the observed distributions. Close and Schuler [11] have calculated the  dependences for the productionof resonances with dierent JPC for the case where the exchanged particle is a Pomeron thattransforms like a non-conserved vector current. In fact this is the only model in which detailed4calculations are made for resonances with spins other than JPC = 0−+. In order to gain insightinto the nature of the particles exchanged in central pp interactions Close, Kirk and Schuler [12]have compared the predictions of this model with the data for resonances with dierent JPCobserved in the WA102 experiment. They found that for the production of JPC = 0−+ mesonsthey could predict the  dependence and the vanishing cross section as t ! 0 absolutely andt the t slope in terms of one parameter, bT . For the JPC = 1++ mesons they could predictthe general form for the  distribution. By tting the t slope they obtained the parameter bL;this then gave a parameter free prediction for the variation of the  distribution as a functionof t which agrees with the data. In addition, they obtained absolute predictions for the t and dependences of the JPC = 2−+ mesons which were again in accord with the data when helicity 1dominance was imposed. In the 0++ and 2++ sector they were able to t all the  distributionswith one parameter (2) and they found that it is primarily the sign of this quantity that drivesthe  dependences. Understanding the dynamical origin of this sign is now a central issue inthe quest to distinguish qq states from glueballs or other exotic states.In a recent paper by Close and Kirk [13] studying the mixing of the scalar glueball with thenearby qq states it was noted that the  dependencies of the f0(1370), f0(1500) and f0(1710),and hence the sign of (2), could be explained if they were due to the relative phase betweenthe nn and gg in their wavefunctions.One interesting feature is that there is very little evidence for radially excited states incentral pp collisions. Therefore it is even more striking that there are three tensor states in theregion of 2 GeV which is where the tensor glueball is predicted to be by Lattice QCD [14]. Twoof these states have similar  dependencies and one the opposite. This eect is very similarto what was observed in the scalar sector. It is interesting to speculate whether the f2(1910),f2(1950) and f2(2150) are due to mixing between a tensor glueball and nearby qq states. Thismay explain their observation in central production as being through their gluonic component.In summary, a study of the production of resonances in central pp interactions has beenpresented. Cuts on the dPT variable select out known qq states from glueball candidates. Whythis works is still to be understood. The azimuthal angle  has given information on the natureof the Pomeron. This information may in turn be used in future to extract information onthe gluonic content of the observed mesons. It is interesting to note that although there isstrong evidence for all the ground state nonet states (except for those with JPC = 1+−) thereis little evidence for radially excited nonet members. This may give special signicance to theobservation of the f2(1910), f2(1950) and f2(2150).AcknowledgementsThis work is supported, in part, by grants from the British Particle Physics and AstronomyResearch Council and the British Royal Society. The author wishes to thank the WA76, WA91and WA102 collaborations for the use of their data.5References[1] T.A. Armstrong et al., Nucl. Instr. and Methods A274 (1989) 165;F. Antinori et al., Il Nuovo Cimento A107 (1994) 1857.[2] S.N. Ganguli and D.P. Roy, Phys. Rep. 67 (1980) 203.[3] T.A.Armstrong et al., Zeit. Phys. C 51 (1991) 351.[4] D.Barberis et al., Phys. Lett. B467 (1999) 165.[5] C. Adlo et al. (H1 Collaboration), Phys. Lett. B483 (2000) 360.[6] D.Barberis et al., Phys. Lett. B388 (1996) 853.[7] F.E. Close and A. Kirk, Phys. Lett. B397 (1997) 333.[8] D. Barberis et al., Phys. Lett. B422 (1998) 399;D. Barberis et al., Phys. Lett. B427 (1998) 398;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;D. Barberis et al., Phys. Lett. B462 (1999) 462;D. Barberis et al., Phys. Lett. B471 (2000) 435;D. Barberis et al., Phys. Lett. B474 (2000) 423;D. Barberis et al., Phys. Lett. B479 (2000) 59.[9] D. Barberis et al., Phys. Lett. B453 (1999) 316.[10] F.E. Close, Phys. Lett. B419 (1998) 387;P. Castoldi, R. Escribano and J.-M. Frere, Phys. Lett. B425 (1998) 359;N. I. Kochelev, hep-ph/9902203;N.I. Kochelev, T. Morii and A.V. Vinnikov, Phys. Lett. B457 (1999) 202;F.E. Close and G. Schuler, Phys. Lett. B458 (1999) 127.[11] F.E. Close and G. Schuler, Phys. Lett. B464 (1999) 279.[12] F.E. Close, A. Kirk and G. Schuler, Phys. Lett. B477 (2000) 13.[13] F.E. Close and A. Kirk, Phys. Lett. B483 (2000) 345.[14] G. Bali et al. (UKQCD), Phys. Lett. B309 (1993) 378;D. Weingarten, Nucl. Phys. Proc. Suppl. 53 (1997) 232;J. Sexton et al., Phys. Rev. Lett. 75 (1995) 4563;F.E. Close and M.J. Teper, \On the lightest Scalar Glueball" Rutherford Appleton Labo-ratory report no. RAL-96-040;W. Lee and D. Weingarten, hep-lat/9805029;G. Bali, hep-ph/00013126Table 1: Summary of resonance production. t type I, II and III refer to the shape of the jtjdistribution as depicted in g. 2c), d) and e) respectively. The error quoted represents thestatistical and systematic errors summed in quadrature.JPC Resonance  (nb) t type Value of b1 in Value of b2 in DPEatps=29.1 exp−b1t t exp−b2t CompatibleGeV (GeV−2) (GeV−2)0−+ 0 22 011  3 267 I 17.3  0.2 17.6  0.3 3 859 368 II 8.20.30 1 717  184 II 8.10.4 Yes0++ a0(980) 638  60 III 6.2  0.8f0(980) 5 711  450 III 5.3  0.2 Yesf0(1370) 1 753  580 III 6.7  0.2f0(1500) 2 914 301 III 5.2  0.5 Yesf0(1710) 245 65 III 6.5  0.5f0(2000) 3 139  480 III 5.6  0.41++ a1(1260) 10 011  900 III 6.9  0.2f1(1285) 6 857  1 306 III 6.3  0.3 Yesf1(1420) 1 080  385 III 5.6  0.5 Yes1−− (770) 3 102  250 III 5.8  0.1 No!(782) 7 440  553 I 3.8  0.2 24.5  0.6(1020) 60 21 III 7.8  1.0 No2−+ 2(1670) 1 505  145 I 4.3  0.2 22.3  1.02(1645) 1 907  152 II 7.3  1.32(1870) 1 940  185 II 8.3  2.02++ a2(1320) 1 684  134 III 7.8  0.3f2(1270) 3 275  422 II 8.7  0.4 Yesf 02(1520) 68  9 II 7.7  3.3f2(1910) 528  40 III 4.9  1.6f2(1950) 2 788  175 III 5.9  0.4 Yesf2(2150) 121  12 III 5.4  1.47Table 2: Resonance production as a function of dPT expressed as a percentage of its totalcontribution. The error quoted represents the statistical and systematic errors summed inquadrature.JPC Resonance dPT0.2 GeV 0.2dPT0.5 GeV dPT0.5 GeV R0−+ 0 12  2 45  2 43  2 0.27  0.05 6  2 34  2 60  3 0.10  0.030 3  2 32  2 64  3 0.05  0.030++ a0(980) 25  4 33  5 42  6 0.57  0.14f0(980) 23  2 51  3 26  3 0.88  0.12f0(1370) 18  4 32  2 50  3 0.36  0.08f0(1500) 24 2 54 3 22  4 1.05  0.18f0(1710) 26  2 46  2 28  2 0.95  0.10f0(2000) 12  2 38  3 50 4 0.23  0.041++ a1(1260) 13  3 51  4 36  3 0.36  0.09f1(1285) 3  1 35  2 61  4 0.05  0.02f1(1420) 2  2 38  2 60  4 0.03  0.031−− (770) 6  2 41  4 53  3 0.12  0.05!(782) 10  2 40  2 49  3 0.20  0.04(1020) 8 3 47  3 45  4 0.18  0.072−+ 2(1670) 11  2 48  4 40  4 0.27  0.062(1645) 9  1 32  3 59  5 0.15  0.022(1870) 8  1 29  3 63  6 0.13 0.022++ a2(1320) 10  2 38  5 52  6 0.19 0.05f2(1270) 8  1 29  1 63  2 0.12  0.02f 02(1520) 4  3 36  3 60  4 0.07  0.04f2(1910) 20  4 62  7 18  4 1.1  0.3f2(1950) 27  2 46  5 27  2 1.0  0.12f2(2150) 3  3 53  4 44  3 0.06  0.088FiguresFigure 1: The dierential cross section, d=dMX0 , of the centrally produced system X0 for a)all data, b) jxF j < 0.1, c) dPT < 0.2 GeV and d) dPT > 0.5 GeV.Figure 2: The jxF j distribution for a) the f2(1270) and b) the !. The jtj distributions for c)the 0, d) the 0 and e) the f1(1285).Figure 3: The ratio of the number of events with dPT  0.2 to the number of events withdPT  0.5 GeV for all the resonances observed grouped in JPC .Figure 4: The azimuthal angle  between the fast and slow protons for resonances withJPC = 0−+, 1−−, 1++ and 2−+.Figure 5: The azimuthal angle  between the fast and slow protons for resonances withJPC = 0++ and 2++.9Figure 110Figure 211Figure 312Figure 413Figure 514

Resonance production in central pp collisions at the CERN Omega
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Resonance production in central pp collisions at the CERN Omega Spectrometer

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