Search for exclusive photoproduction of Z ± c ( 3900 ) at COMPASS

C. Adolph h, R. Akhunzyanov g, M.G. Alexeev aa, G.D. Alexeev g, A. Amoroso aa,ac, V. Andrieux v, V. Anosov g, A. Austregesilo j,q, B. Badełek ae, F. Balestra aa,ac, J. Barth d, G. Baum a, R. Beck c, Y. Bedfer v, A. Berlin b, J. Bernhard m, K. Bicker j,q, E.R. Bielert j, J. Bieling d, R. Birsa y, J. Bisplinghoff c, M. Bodlak s, M. Boer v, P. Bordalo l,1, F. Bradamante x,y, C. Braun h, A. Bressan x,y,∗, M. Büchele i, E. Burtin v, L. Capozza v, M. Chiosso aa,ac, S.U. Chung q,2, A. Cicuttin z,y, M.L. Crespo z,y, Q. Curiel v, S. Dalla Torre y, S.S. Dasgupta f, S. Dasgupta y, O.Yu. Denisov ac, S.V. Donskov u, N. Doshita ag, V. Duic x, W. Dünnweber p, M. Dziewiecki af, A. Efremov g, C. Elia x,y, P.D. Eversheim c, W. Eyrich h, M. Faessler p, A. Ferrero v, A. Filin u, M. Finger s, M. Finger Jr. s, H. Fischer i, C. Franco l, N. du Fresne von Hohenesche m,j, J.M. Friedrich q, V. Frolov j, F. Gautheron b, O.P. Gavrichtchouk g, S. Gerassimov o,q, R. Geyer p, I. Gnesi aa,ac, B. Gobbo y, S. Goertz d, M. Gorzellik i, S. Grabmüller q, A. Grasso aa,ac, B. Grube q, T. Grussenmeyer i, A. Guskov g,∗, F. Haas q, D. von Harrach m, D. Hahne d, R. Hashimoto ag, F.H. Heinsius i, F. Herrmann i, F. Hinterberger c, Ch. Höppner q, N. Horikawa r,4, N. d’Hose v, S. Huber q, S. Ishimoto ag,5, A. Ivanov g, Yu. Ivanshin g, T. Iwata ag, R. Jahn c, V. Jary t, P. Jasinski m, P. Jörg i, R. Joosten c, E. Kabuß m, B. Ketzer q,6, G.V. Khaustov u, Yu.A. Khokhlov u,7, Yu. Kisselev g, F. Klein d, K. Klimaszewski ad, J.H. Koivuniemi b, V.N. Kolosov u, K. Kondo ag, K. Königsmann i, I. Konorov o,q, V.F. Konstantinov u, A.M. Kotzinian aa,ac, O. Kouznetsov g, M. Krämer q, Z.V. Kroumchtein g, N. Kuchinski g, F. Kunne v,∗, K. Kurek ad, R.P. Kurjata af, A.A. Lednev u, A. Lehmann h, M. Levillain v, S. Levorato y, J. Lichtenstadt w, A. Maggiora ac, A. Magnon v, N. Makke x,y, G.K. Mallot j, C. Marchand v, A. Martin x,y, J. Marzec af, J. Matousek s, H. Matsuda ag, T. Matsuda n, G. Meshcheryakov g, W. Meyer b, T. Michigami ag, Yu.V. Mikhailov u, Y. Miyachi ag, A. Nagaytsev g, T. Nagel q, F. Nerling m, S. Neubert q, D. Neyret v, V.I. Nikolaenko u, J. Novy t, W.-D. Nowak i, A.S. Nunes l, A.G. Olshevsky g, I. Orlov g, M. Ostrick m, R. Panknin d, D. Panzieri ab,ac, B. Parsamyan aa,ac, S. Paul q, D.V. Peshekhonov g, S. Platchkov v, J. Pochodzalla m, V.A. Polyakov u, J. Pretz d,8, M. Quaresma l, C. Quintans l, S. Ramos l,1, C. Regali i, G. Reicherz b, E. Rocco j, N.S. Rossiyskaya g, D.I. Ryabchikov u, A. Rychter af, V.D. Samoylenko u, A. Sandacz ad, S. Sarkar f, I.A. Savin g, G. Sbrizzai x,y, P. Schiavon x,y, C. Schill i, T. Schlüter p, K. Schmidt i,3, H. Schmieden d, K. Schönning j, S. Schopferer i, M. Schott j, O.Yu. Shevchenko g,19, L. Silva l, L. Sinha f, S. Sirtl i, M. Slunecka g, S. Sosio aa,ac, F. Sozzi y, A. Srnka e, L. Steiger y, M. Stolarski l, M. Sulc k, R. Sulej ad, H. Suzuki ag,4, A. Szabelski ad, T. Szameitat i,3, P. Sznajder ad, S. Takekawa aa,ac, J. ter Wolbeek i,3, S. Tessaro y, F. Tessarotto y, F. Thibaud v, S. Uhl q, I. Uman p, M. Virius t, L. Wang b, T. Weisrock m, M. Wilfert m, R. Windmolders d, H. Wollny v, K. Zaremba af, M. Zavertyaev o, E. Zemlyanichkina g, M. Ziembicki af, A. Zink h

It has been interpreted as a tetraquark state [3][4][5][6], although other explanations like a molecular state [7][8][9][10][11], a cusp effect [12] and an initial-single-pion-emission mechanism [13] were also proposed. According to the vector meson dominance (VMD) model, a photon may behave like a J /ψ so that a Z ± c (3900) can be produced by the interaction of an incoming photon with a virtual charged pion provided by the target nucleon The corresponding diagram is shown in Fig. 1a.
Based on the VMD model, the authors of Ref. [14] predict a sizable cross section of the reaction in Eq. (2) for Under the assumption that the decay channel of Eq. (1) is dominant and that the total width Γ tot of the Z ± c (3900) particle is 46 MeV/c 2 , as measured by BES-III, the cross section reaches a maximum value of 50 nb to 100 nb at production in photon-nucleon interactions at COMPASS covers the range √ s γ N from 7 GeV to 19 GeV and thus can be used to also study Z ± c (3900) production and to estimate the partial width The COMPASS experiment [15] is Polarization effects were canceled out by combining data with opposite polarization orientations. Particle tracking and identification were performed in a two-stage spectrometer, covering a wide kinematic range. The trigger system comprises hodoscope counters and hadron calorimeters. Beam halo was rejected by veto counters upstream of the target.
In the analysis presented in this Letter, the reaction was searched for. In order to select samples of exclusive μ + J /ψπ ± events, a reconstructed vertex in the target region with an incoming beam track and three outgoing muon tracks (two positive and one negative) is required. Tracks are attributed to muons if they cross more than 15 radiation lengths of material. Only the events with exactly three muons and one pion in the final state were selected. A pair of muons is treated as a J /ψ candidate if the difference between its reconstructed mass M μ + μ − (Fig. 2a) and the nominal J /ψ mass is less than 150 MeV/c 2 that is 3 times larger than the mass resolution. In case both μ + μ − combinations satisfy this condition, the event is rejected. Except for the tiny recoil of the target nucleon, the sum of the scattered muon energy, E μ , and the energies of produced J /ψ and π ± mesons, E J /ψ and E π ± , should be equal to the beam energy E b for the exclusive reaction of Eq. (3). The distribution of events as a function of the Fig. 2b.
With the experimental energy resolution of about 3 GeV, the energy balance is required to be | E| < 10 GeV. The distribution of the negative squared four-momentum transfer Q 2 = −(P b − P μ ) 2 is shown in Fig. 3a. Here P μ and P b are four-momenta of the scattered and incident muons, respectively. The momentum of the produced pion is required to be larger than 2 GeV/c in order to reduce the background of exclusive events with a J /ψ and a π ± in the final state produced via pomeron exchange (Fig. 1b). The total number of selected μ + J /ψπ + and μ + J /ψπ − events is 565 and 405, respectively. The distribution of the centre-of-mass energy of the photon-nucleon system √ s γ N is shown in Fig. 3b.
The mass spectrum for J /ψπ ± events is shown in Fig. 4a.
It does not exhibit any statistically significant resonant structure around 3.9 GeV/c 2 . In order to quantify possible contribution from the Z c decay we define the signal range 3.84 GeV/c 2 < M J /ψπ ± < 3.96 GeV/c 2 . It is selected according to the measured mass and width of Z c , their uncertainties, observed in the previous experiments, and the COMPASS setup resolution for M J /ψπ ± of about 15 MeV/c 2 . The observed number of events N J /ψπ in this range is treated as consisting of an a priori unknown Z ± c (3900) signal N Z c and a background contribution N bkg . According to the method described in Ref. [16], the probability density function g(N Z c ) is given by (4) where n is a normalization constant and the probability density    For the absolute normalization of the Z ± c (3900) production rate we estimated for the same data sample the number of exclusively produced J /ψ mesons from incoherent exclusive production in the cross section of which is known for our range of √ s γ N [17].
The same selection criteria are applied for the exclusive production of the J /ψ mesons and Z ± c (3900) hadrons. To separate J /ψ production and nonresonant production of dimuons, the dimuon mass spectrum is fitted by a function consisting of three Gaussians (two to describe the J /ψ peak and one for the ψ(2S) peak) and an exponential background under the peaks (see Fig. 2a). Finally 18.2 × 10 3 events of exclusive J /ψ production remain in the sample. The distribution of the squared transverse momentum p 2 T of the J /ψ (Fig. 4b) for the exclusive sample is fitted by a sum of two exponential functions in order to separate the contributions from exclusive coherent production on the target nuclei and exclusive production on (quasi-)free target nucleons. The contribution from coherent production is found to be 30.3% for the 6 The upper limits for the ratio of the cross sections in intervals of √ s γ N are presented in Table 1.
The main contribution to the systematic uncertainty of the result shown in Eq. (8) comes from the background description in the signal range of the J /ψπ spectrum. Changes of the fitting function and the fitting ranges shift the result within ±15%.
The absolute normalization is performed with a relative accuracy of about 25% that includes our limited knowledge of the ratio R a = 0.5 ± 0.1 syst. and systematic errors in the estimation of the nonexclusive contamination in the reference J /ψ sample (15%), determined from the p T dependence of the energy balance E.