Leading Particles

(1) V.A.Bodiagin, K.A.Chikin, A.M.Gribushin, A.I.Demianov, L.I.Sarycheva, I.N.Vardanian, A.A.Yershov
(2) A.D.Kirillov, A.I.Malakhov, P.A.Rukoyatkin

Spokesmen: A.I.Malakhov, L.I.Sarycheva

Contactmen from MSU: A.I.Demianov, A.A.Yershov

An ivestigation of leading particles from proton-nucleus interaction at the experimental apparatus SMS MSU ("Scintillation magnetic spectrometer MSU") to illuminate the mechanism of strong interaction at the energies to 10 GeV is proposed.

The experimental installation "Scintillation Magnetic Spectrometer" (SMS-MSU) [1] is a single-arm magnetic spectrometer with variable geometry of its spectrometric section (Fig.1). Differential production rates of leading hadrons L off nuclear targets are measured in a broad range of emission angles J_L and momenta P_L in the reactions hA ® L + X_h, where X_h means triggering on a final state with a preset kinematic configuration, equivalent to a certain effective mass M(h).

Fig.1.

The basic elements of the spectrometer are:

• a scintillation hodoscopic array with coordinate resolution ~ 1 mm, tracing the trajectories of a projectile h (sections P, B) and aleading secondary L (sections T, S, P);

• a complex of scintillation and solid Cherenckov detectors monitoring the beam (L, ) and "shaping" its profile on the target by means of anticoincidences (_1,2);

• a unit of automatic target positioning (Q);

• a triggering complex, combined of solid Cherenkov detectors (D₁, D₂) and scintillator-lead-scintillator sandwiches (E_x,y) covering the same solid angle, which allows selection of events with preset kinematics — i.e. emission angles h of secondaries X relative of leader trajectory;

• a movable analyzing magnet;

• a wide apparatus gas (threshold) Cherenkov counter (Č) at the downstream and of the apparatus for leading particle identification.

The apparatus SMS-MSU is used for research in two general directions:

1. Space-time structure of hadronic interaction.

The results obtained by now [2] are based on the statistical material of ~ 5.3·10⁵ interactions of protons of momentum P₀= 9GeV/c with the nuclei H, C, Al, Cu and Pb, and may by summarized as follows:

• The re-arrangement of quark structure in substantially inelastic "soft" processes does not entail observable effects at the level of real particle properties, at least on a time scale t ³ 0.01ħ / m_pc².

• Incoherent proton fragmentation on nuclei proceeds, most likely, via a bound baryonic state being absorbed in the nuclear matter like an ordinary proton and decaying into hadrons beyond the target-nucleus.

In the frameworks of direction 1, further research with the apparatus SMS-MSU will focus on those problems, which arose in course of the analysis of already obtained data:

• difference in production mechanism for leading p-mesons and protons in reactions pA®p_L/®p_L (+X_h) and pA®p_L/®p_L (+X_h): direct production « production via a bound (baryonic or mesonic) intermediate state ;

• nature of this intermediate state — resonance (as an isobar) or a non-resonant formation (type diffraction cluster) with continuous mass spectrum.

The resources required to implement this programme are presented in table 1.

In the frameworks of the programme of the investigation of interaction of polarized protons with nuclei (begun in 1992 – 1998) we propose the experimental investigation of single-spin asymmetries of polarized proton scattering on intranuclear nucleons to be compared with the analogous characteristics for the scattering on free nucleons in the region of energies 1 – 4 GeV. The main subject of measurements is vector analyzing power (a.p.) A(q), determined as normalized on left-right asymmetry

The reaction of the polarized proton scattering on carbon target is considered. Existing experimental data (including preliminary ones, obtained at SMS MSU experimental installation [3]) permit to assume an indication to significantly different reduction of a.p. for the scattering on intranuclear nucleons in comparison with the one on free nucleons, and strong dependence of this effect on energy. Proposed experiment has to confirm or to reject supposed phenomenon and to answer the question about its physical nature. As hypotheses of reduction mechanism the next ones are considered:

• relativistic effects for bounded state of target nucleon;

• nucleon clusterisation in nuclei;

• excitation of non-nucleon degrees of freedom.

Tested at the runs of 1992 – 1995 methodic of measurements is based on the measurement of double differential cross sections of polarized protons scattering for different directions of initial beam polarization. The initial beam is formed by the stripping of vector polarized deuterons [4,5]. To separate the quasielastic scattering the spectrometric analysis of leading particle under the detection of secondary particles (primary secondary pions and recoil proton) is used. The estimation of required resources by the mathematical simulation method is shown in the table 1.

The comparison of obtaining data with the different model calculations will allow to illuminate the picture of the relativistic proton interaction with nuclear matter, and it will be a source of information about nuclear structure indication in the spin interactions. At the same time this work is important for widely applicated carbon polarimetry of protons.

*) Required beam time at the HEP JINR synchrophasotron ("s") and nuclotron ("n").

lis@alex.npi.msu.su (L.I.Sarycheva)

e@lav1.npi.msu.su (A.A.Yershov)