Clusters and Tubes in Multiparticle Production from Nuclear Targets

Abstract

Recent data on backward pion production in 0.8-4.89-GeV proton-nucleus collisions are analyzed within the scope of models where target nucleons react collectively. It is shown that the nuclear-size dependence in data is a sensitive probe of the effective target mass. The experimental results hint at a synthesis of the cluster ("fluctuon") model and the collective tube model. PACS numbers: 13.85. Kf, 12.40.Cc, 25.40.Rb, 24.10.Dp In a recent Lawrence Berkeley Laboratory experiment Schroeder etal. 1 measured the backward yield of pions inp +A-^(180°) +X at kinetic energies 7^ = 0.8-4.89 GeV and with nuclear targets having A = 12-207. Special attention was given to the so-called cumulative production discovered by Baldin etal. in 1971. 2 This occurrence of pions with T^>T^{NN^NNn) has inspired a vast amount of theoretical work. Many theories for high-energy collisions with nuclei are built on intuitive pictures of the space-time development of a hadronic collision. 3 Most, of these are expected to work only when the nucleus is Lorentz contracted to a thickness of less than around 1 fm in the projectile rest system, or alternatively, when a typical reaction time exceeds the time which the projectile needs to pass the whole nucleus. As this happens only at T P <;A 1/3 GeV, the data of Ref. 1 cover an interesting transition region in T p .The data demonstrate three clear energy dependences from 0.8 to 4.89 GeV. First, the ratio 7r"/7r + of produced pions grows from 0.3 to 1. Second, the pion spectrum obeys E da/d 3 p ~ exp(-T ff / T 0 ) with T 0 growing from 30 to 60 MeV. Third, the A dependence changes. One finds that Edo/ d 3 p ~A n {T v ) at fixed T\. At ^ = 0.8 GeV the exponent n~ 0.8 for all T v . At T p = 4.89 GeV there is, however, an increase in n{T^) with T".. For T\ «2.5T ff max one finds n* 1.2, which is a considerable enhancement compared to the 0.8-GeV data. All these trends indicate that there is a limiting situation at infinite energies which seems to be reached already in the 8.4-GeV data of Baldin et al. 4 and perhaps even in the 4.89-GeV data.The aim of this Letter is to show that the A dependence serves as a sensitive test of theoretical models of the nuclear response in high-energy collisions with nuclei. The slope parameter T 0 and the ratio 7r"Ar + , taken on a fixed nucleus, do not play this role. They instead probe the nature of the quark reaction responsible for cumulative production.There are two different ways of explaining the existence of cumulative pions: Either the effective target is not stationary or it is more massive than a single nucleon.The conclusion from models where nucleonic Fermi motion is held responsible for cumulative production is that "too many" fast nucleons are needed. I will not analyze these models, because any data can naturally be fitted with some Fermi motion. Data show that such a distribution would have to be strongly A dependent at 4.89 GeV but virtually A independent at 0.8 GeV. This is hard to understand since the Fermi motion depends only on the nuclear d...

Multiplicities of forward-backward particles in16O-emulsion interactions at 4.5AGeV/c

“…During de-excitation, pions are emitted in a manner similar to that in thermal models. This conclusion is also consistent with the model of Fredriksson [15] , in which the incident nucleons interact collectively with all matter within 1 fm (in the rest frame) at the line of collision. It is obvious from Fig.…”

“…They argued that the simple Fermi motion of the nucleons in the nucleus could not account for such backward production and stated that the dominant mechanism for producing such hadrons was the interaction between the incident nucleon and multi-nucleon clusters in the target, referring to this mechanism as cumulative production. Fredriksson [15] supported the model in which the nucleon interacts collectively with all matter within 1 fm in its rest frame at the time of collision. The LBL data [2,8] supported a model called the "effective target model" [4] .…”

System size dependence of final state hadron sources atElab= 3.7 A GeV

“…The emission in the backward hemisphere (BHS) is restricted. Pion emission beyond the kinematic limits at θ lab 90°was suggested to be due to target fragmentation system, regarding the limiting fragmentation hypothesis [11][12][13][14][15]. This was investigated and confirmed by our lab group [10,[16][17][18][19][20].…”

“…The intercept values are parameterized as a function of the projectile mass number in figure 9. The allometric fitting in figure 9 is presented by the power law equation (13). It has χ 2 /dof=0.…”

System size dependence in backward relativistic hadron production in pA and AA collisions

“…Accordingly, the pion production was investigated according to the directional zone of emission. Various experimental and theoretical studies had been focused at Dubna and Lawrence Berkeley National Laboratory (LBNL) to solve the problem [1][2][3][4]. El-Nadi et al [5][6][7] showed that the backward emitted pion multiplicity is strongly correlated with the target fragment multiplicity at Dubna energy.…”