Исследование множественных и корреляционных явлений в мезон-ядерных и мезон-протонных взаимодействиях при 250 ГэВ/с тема автореферата и диссертации по физике, 01.04.16 ВАК РФ

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ЕРЕВАНСКИЙ ФИЗИЧЕСКИЙ ИНСТИТУТ

¿кегля Рафаел Шаликоезич

ИССЛЕЛОВ*ИИЕ МНОЖЕСТВЕННЫХ И КОРРЕЛЯЦИОННЫХ ЯВЛЕНИ-'4 В МЕЭОН-ЯДКРНЫХ И МЕЗОН-ПРОТОННЬК БЗАЖОЛГЛСТВИЯХ ПРИ 250 Г'-В/с

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ЕРЕВАН 1997

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■-Г'-ев'чн^-ом физическом институте кандил&г^физика-матвиатич^ских -наук. Г.Р. Íулканян

доктор физико-математических наук академик НАН РА Г.А. Еартапетян СЕрФк доктор фиэико-матепатич^ских наук В.М. Хапкочян (ЕрФй)

Ереванский государственный университет (Кафедра ядерной физики) Защита состоится 1 Т~ июня 1S97 г. в /4 часов на заседании Специализированного совета Ü'<íA при Ереванской физическом институте (375036, Ереван, ул- Братьев Алиханян ) г -ч-чнчей можно ознакомиться в библиотеке ЕрФИ-

Автореч разослан мая 1997 г -

Официальные оппоненты: Ведущая организация;

/7

Ученый секретарь

Специализированного совета hlñf t^Of'Oi^

_А.Т. Маргарян

GENERAL CHARACTERISTICS

Actuality: The multiparticle production is the most characteristic feature of the high-energy strong interaction physics. The underlying mechanisms of this process, which relates to the strong-coupling long-distance regime of Quantum Chromodynamics, at present are not well understood. The present semiphenomenological models of hadronic and nuclear interactions are not able to reproduce satisfactorily the whole complexity of the multiparticle production. A more and detailed experimental data, especially on the correlation, semi-inclusive and collective characteristics of produced hadrons in hadronic and nuclear interactions, are needed to obtain new information about the space-time structure and mechanisms of the multiparticle production.

The experiments on nuclcar targets provide unique conditions for the study of the multiple interactions between hadrons (including non-assymptotical hadron states), not present in elementary interactions on nucleons. and are expected to add new insight into the nature of the liadronization process.

A powerful tool for the investigation of the dynamics and space-time structure of the particle-production process is offered by the indentical pion interferometry. The inteTfero-metric correlations of the two identical pions (Bose-Einstein correlations) reflect both geometrical and dynamical properties of the particle-radiating source. The experimental study of thise correlations provides a measurement of the size, shape and radiation time of the source, contains an information on its non-static properties and underlying mechanisms of multiparticle dynamics.

Purpose: The main goals of the thesis are:

• The experimental investigation of the semi-inclusive, collective and correlation characteristics of the multiparticle production processes in interactions of and mesons with nuclei (Al,Au) and proton at 250 GeV/c, in order to obtain new information on the mechanisms of these processes.

• Investigation of Bose-Einstein correlations (one-, two- and three- dimensional analysis) between two identical pions, in order to obtain new information on the space-time evolution of the multiparticle production processes in meson-proton and meson-nucleus interactions at 250 GeV/c.

The experiment is performed with the help of Europian Hybrid Spectrometer (experiment NA22) at the CERN Superproton Synchrotron.

Scientific novelity:

1. First experimental data on the collective characteristics of hadrons produced in the beam fragmentation region in meson-nucleus interactions are obtained.

• The average inelasticity coefficient of the leading hadron (cluster) intranuclear interactions (or the nuclear stopping power) is extracted. It is shown, that in the

intranuclear multiple collision process, the first and the following inelastic collisions occur with approximately the same inelasticity.

• The average total number of intranuclear collisions is extracted and it is shown that their dominant part is caused by interactions of particles produced in the target fragmentaion.

2. New experimental data on backward proton production in meson-nucleus interactions are obtained.

• First time for middle and heavy nuclei, the contribution of the secondary pion intranuclear absorption to the inclusive cross section of backward proton production is estimated.

• First time an upper limit of the cross section for the double colour-charge-exchange mechanism of backward proton production is estimated.

3. A detailed study of Bose-Einstein correlation in meson-proton and meson-nucleus interactions is performed.

• First time two- and three- dimensional analysis of Bose-Einstein correlations have been done in meson-proton interactions. It is shown, that meson-proton interactions can be described in the framework of the hydrodynamically expanding source model, and the parameters, characterising the space-time pattern of the multiparticle production are extracted.

• First time it is shown, that the pion source in meson-nucleus interactions is elongated along the collision axis.

Practical usefulness: The results of this thesis can be used for improvement of models of multihadron production processes, the undelying mechanisms of which are not well estabished-yet, particularly those concerning space-time evolution of the strong interacting matter, intranuclear collision effects and the nucleón production from the nucleus. The results could be useful also for planning; future experiments devoted to the study of the multihadron production in hadronic and nuclear collisions at superhigh energies.

Structure of thesis: The thesis consists of an Introduction, five Chapters and Conclusions. The thesis is presented in 123 pages, which includes 50 figures, 23 Tables and 124 references.

CONTENTS OF THESIS

In Introduction the actuality of performed exploration is shown, the purpose is formulated and the structure and brief contents of thesis are presented.

In Chapter one the experimental set-up is described. The central part of the experimental apparatus is the Rapid Cycling Bubble Chamber (RCBC), 80 cm in diameter and

filled with hydrogen, was used as vertex and track detector, as well as an ionization device. A 2T magnetic field was applied along the cylinder axis. The RCBC was equipped with two nuclear targets consisting of an aluminium and a gold foil of thickness 2.5 mm and 0.64 mm, respectively, corresponding to 0.5 % of an interaction length. The foils were placed side by side, orthogonally to the beam, 15.5 cm behind the entrance window of the chamber.

Tracks of secondary charged particles, leaving RCBC through the exit window, are reconstructed from tuts in the wire and drift chambers of the spectrometer and from measurement in RCBC. The backward tracks are reconstructed from measurement in RCBC. The average momentum resolution for charged particles varies from 1 to 2% for tracks reconstructed in the bubble chamber and from 1 to 2.5% for those reconstucted in the spectrometer. For reconstructed tracks with lab momentum pub <1.2 GeV/c, the ionization strength was estimated visually on the scanning table.

The following selection criteria are used for the meson-nucleus events: the interaction vertex is within one of the nuclear foils; the event is satisfactorily measured and reconstructed and is not a candidate for a quasi-elastic or coherent interaction with the nucleus. For the meson-proton interactions elastic and single-diffractive events are excluded.

The events are weighted with a multiplicity dependent weight in order to compensate for the loss of events due to badly reconstructed tracks.

In Chapter two new results on multiplicities, energy and angular distributions of protons, on correlations and on collective characteristics of the multiparticle production processes in K+Al, K+Au, tt+AI and 7T+Au interactions are presented. The distributions are corrected for ionization losses in the nuclear foils.

It is obtained, that the proton energy distributions for A1 and Au nuclei are essentially similar, but the angular distributions demonstrate a strong A-dependence: the parameter a in the parametrization Aa for the proton average differential multiplicity is a=0.42±0.02 at very small angles and increases with increasing angle up to a=0.73±0.03 in the backward hemisphere at cos9 < -0.3, where its value becomes practically independent of 9.

A detailed study of the multiplication ratio of negative particles is performed. The dependence of the average value of R- on the average number of leading hadron (cluster) intranuclear collisions is obtained:

__________________- R- = a + b(vA), - (1)

with a=0.70±0.14, 6=0.57±0.07 and a=0.64±0.12, b=0.50±0.06 using the average (n_)fcp in inelastic and in non-single diffractive hp-interactions, respectively.

It is shown that, in the target fragmentaion region , depends strongly on the number rig of "grey" protons and reaches up to R_ ~ 30-50 for (tt"1"/K+)Au interactions at small mpact parameters. In the beam fragmentation region, the dependence on ng disappears, md, at the largest rapidities, the multiplication ratio becomes less than one (ii_=0.37±0.06 :or 7r+Au and -R_=0.71±0.08 for ir+Al interactions).

By using the experimentally observed distributions of the total charge Q of the secondary >articles, a model independent estimate for the average total number of intranuclear collisions s extracted and it is shown that their dominant part 60% for A1 and 2; 80% for Au) is :aused by interactions of the non-leading particles (see Table 1). The simplified calculations n the framework of Glauber theory agree with the measured value of (Q) for meson-A\ nteractions, but underestimate the (Q) for meson-Au interactions by about 25%.

New information on the mechanisms of multiparticle production processes can be obtained rom an investigation of another collective characteristic, i.e. the distribution of the total ongitudinal momentum XI of charged particles in the beam fragmentation region (y >5).

These distributions are shown in Fig.l; their first point iy=0)_corresponds to events which do not contain charged particles with y >5. The mean values of the quantity £ P|| are:

C||Wi=81.5±1.2GeV7c, (EPll)K+M = 7T.4±2.1GeV/c, (EPllh+Au =68.1il.lGeV/c, <E^||)K^ = 68.8±2.0GeV/c.

As expected, the average momentum losses for Au nuclei are larger than for Al:

= <£ p||)x+a«/(e ^-mí = 0-84 ± 0.02 , rK+ = (zp\\)k+aj(zp\\)k+ai = 0.89 ±0.04 ,

From the comparision of the data, on Al and Au targets, the average inelasticity of the leading hadron (cluster) intranuclear collisions (or the nuclear stopping power) is extracted in the framework of the Glauber model: fc=0.17±0.03. This estimation is consistent with the value fc=0.2±0.1 found for jr+p-interactions. Thus, in the intranuclear multiple collision process, the first and the following inelastic collisions occur with approximately the same inelasticity, which is similar also for non-strange and strange leading clusters.

In Chapter three new experimental data on backward proton production (BPP) are obtained in (x+/A'+) Al and (7T+ ¡K*) Au interactions. Multiplicity distributions and inclusive spectra of backward protons are measured. The 4-dependence of the backward proton yield is consistent with ocA0'7. The slope parameter of the p,2aS-spectruni^vithin statistical errors, independent of A, being equal, respectively, to b = 10.2±0.6 and 9.7±0.3 (GeV/c)-2.

An interesting two-step mechanism for BPP in hadron-nucleus interactions has been proposed by Kopeliovich and Nidermayer. The incident hadron exchanges colour (by gluon exchange) consecutively with two nucleons in a nucleus and becomes white again. A colour flux tube is stretched between the two coloured "nucleons". As a result, one of them acquires a momentum directed backward. This process is similar to beam diffraction (elastic or inelastic). The colour charge exchange (CCE) model predicts that the momentum spectrum of the backward proton should be harder than in inclusive BPP and should have a maximum at p¡„í.~500 to 600 MeV/c.

In older to look for CCE, we select diffractive-like events

----------M+ -F A —**p + M* -t- A' " (2)

with at least one backward proton, no particle produced in the target fragmentation region (except for recoil protons with p¡aj < 1.2 GeV/c and other nuclear fragments labelled ^4') and only fast products (with rapidity y" > 0 in the meson-nucleon c.m.) from the fragmentation of the excited mesonie state M* (the summed charge of which is required to be Q=+l). An upper limit of the cross section for the CCE mechanism is estimated; the contribution of this mechanism is less than 1% of the high-momentum tail (p;„¡, > 0.55 GeV/c) of the inclusive spectrum.

Another two-step mechanism of BPP is the absorption of a secondary pion, SPA, on a correlated nucleón pair or quasi-deuteron within the nucleus,

*(NN)-*rp • (3)

The momentum dependence of the cross section for jr+ scattering on a free deuteron, <ra (■x+d—*pp), has resonance character with a maximum of <T„«12mb at pT~0.25 GeV/c, dropping to ~4mb at 0.5 GeV/c and ~2mb at 0.75 GeV/c. Therefore, one can expect that,

in the SPA mechanism, the dominant role is that of low-energy secondary pions (with j), < 0.5 GeV/c).

In order to search for the SPA mechanism, we use the method of kinematicai analysis. As a variable describing the kinematicai correlation of the secondary protons in subprocess (3) we use

/'2 = (T\ + T?)1 - (p, + p2)2, (4)

where 7\, T-2 and p,, p2 are the kinetic energies and momenta of the secondary protons. For the reaction on a free deuteron, 7r—'pp, the relation p2=m2 would hold. One can show that the binding of the nucleoli pair in the nucleus and its Fermi motion lead to a shift of the peak in the /12 distribution from /12 = rnj towards smaller values by a quantity ¿z:2á.E(T-¡ + T2) - p2 and to its broadening by 7%±2|p(((pI + p2)|, where AE is the average binding energy of the nucleón pair and p¿ is the average magnitude of its Fermi momentum.

We look for the SPA mechanism in the reaction

M+ + /1 — *p + p + X, (Ó)

where the second proton can be produced either in the forward or backward direction. Note, that in subprocess (3) on a free deuteron, the minimal momentum of a secondary proton would be pmm = 0.37 GeV/c. However, due to the Fermi motion of the nuclear pair (NN) in the nucleus, this boundary can decrease to 0.2 GeV/c. In the selection of our sample for reaction (5) we, therefore, vary pm,„ from 0.25 to 0.35 GeV/c.

Furthermore, it should be taken into account that rescattering of secondary protons in the nuclear matter would result in an additional broadening of the distribution, mainly towards smaller values of /i2.

The histograms in Fig. 2 show the p.2 distribution for reaction (5) with pm,„ = 0.25,0.30 and 0.35 GeV/c, respectively. The full curves correspond to a "background" distribution, obtained by combining protons randomly chosen from different events and normalized to the experimental distribution in the region fi2 < -0.25 (GeV/c2)2 where the contribution of mechanism (3) is expected to be negligible.

The dashed curve in Fig. 2a demonstrates the result of a Monte Carlo simulation of subprocess (3) for Al, The following values are used : AE = 60 MeV [26] and (p2d) = (220 MeV/c)2. The experimental error in measurement of the final proton momentum ((Ap/p} = 2%) is also taken into account. This simulated /^-distribution has a peak shifted from p,2 = m2 to the left by about 0.1 (GeV/c2)2 and a tail falling below 5% of the total content at H2 « -0.25 (GeV/c2)2. The dashed line is normalized to the difference between histogram and solid line of Fig. 2a in the region p2 > —0.25 (GeV/c2)2. One can see that after subtraction of the background distribution (full line) from the histogram, the shape of the resulting distribution would be similar to that of the simulated one (dashed).

The number of combinations in Fig. 2 above background at p.2 > —0.25 (GeV/c2)2 is used as an estimate of the contribution of the SPA mechanism to inclusive BPP, r = cSPA(*p'p) as presented in Tables 2. Within the errors, r is independent of pmtn for Al. However, our data indicate a possible decrease of r with increasing pm¡„ for Au.

The /l-dependence of the ratio r at pmin = 0.25 GeV/c is obtained. The share of the SPA increases with increasing A to become one of the main sources of BPP for heavy nuclei. It can be fitted by a power dependence « with /3 = 0.27 ±0.05.

In Chapter four Bose-Einstein correlations have been studied in two and three dimensions for pairs of negative pions in (jr+/K+)p-interactions.

In this analysis all negative particles are assumed to have pion mass. The contamination from other particles is estimated to be (7±3)%. Each accepted track is required to he in the region of Feynman variable < 0.5, in order to reduce possible correlations due to phase space restriction, as well as biases due to violation of energy and momentum violation imminent to the mixed-event technique.

The following pairs or triplets of variables are used for the two- and three-dimensional analyses:

a) |q| versus go, where q = Pi — P2 and (¡0 = |£i — -E2I are, respectively, the momentum and energy difference of the two identical pious (in the CMS).

b) The Lorentz-invariant variables versus Qy, where QT is the component of q perpendicular to the collision axis, and Ql = </2 — <Zo, where q£ is the component of q parallel to the collision axis.

c) ?l(= I?lI) versus Qx(= IQtI) and </l versus Qt„ and Qts, where Qt0 is the 'out' component of QT parallel to pair transverse momentum pT = pT1 -I- pT2 and Qts is the 'side' component of QT perpendicular to pT:

d) n{— M) versus qo, where qT is the component of q perpendicular to p = P! + p2 (in the CMS).

The following parametrizations are used for the normalized two-particle density: Mr, P(Pi.Pi)

1. The Gaussian form

K(q2,9o) = 7[l+ Aexp(-/9,q2-/32?g)](H-iq2 + f7?) • (6)

In (1), A < 1 is the coherence parameter, 7 is an overall normalization and (1 -f ¿q2 + is introduced to account for a possible slow variation of R outside the interference peak.

At /3i,/32 > 0, these two parameters are related, respectively, to the mean radius and the mean radiation time of a fireball-like (volume emitting) source with a Gaussian space-time distribution. At negative 02, an(l 0 = 0\ = —02, (6) reduces to the Goldhaber parametriza-tion

ft(q2,?o) = 7[l + Aexp(-/JQ2)](l + <5£?2) (7)

with the Lorentz-invariant variable Q2 = q2 — and a parameter /} related to the r.m.s. radius (/3 = r2/3) of a source being of a spherically symmetric Gaussian form in the dipion rest frame.

2. The Bowler parametrizations ¡or a string-like source:

ml Qt) = 7[1 + A exp{-Ml - /?tQt)](1 + SQl+eQ\) (8)

and

R{Qt,Q\)^i

1 + 2[(/3LQ2)2-i]lll(/?Ll^l)exp(-/?T0^

(l + Wtfl + dft) . (9)

where /?l and f3j correspond to the longitudinal and transverse size of the string segment radiating the BE correlated pions, respectively. The parameter A has the definition of A = 1 /|Утах|> where ymix is the maximum rapidity y, above which the у distribution drops rapidly.

3. The two- and three-dimensional Gaussian parametrizations used for a hydrodynamically expanding cylindrical source:

R(4L, Qt) = 7[1 4 a exp(-\rlql ~ \t^Q2t)]{\ + 4 £<3t) (10)

H(«íl,Qt.,QT.) = 7[14Aexp(-ir^-ir№0-irs2Q^5)]x (11)

x(I + iijb + fQxo 4 íQts) ,

where ix, гт, To, т, ате , respectively, the longitudinal (along the cylinder axis), transverse, 'out' and 'side' effective dimensions of the source segment radiating the BE correlated pion pairs.

4. The Kopylov-Podgoretskii parametrization:

R(n,1 o) = 7[1 4 A pfog^]' 0 4 r2g02)-'](l + ¿gT + eg0) , (12)

where is the first-order Bessel function and where nc is the radius of a surface-emitting spherical source decaying exponentially with the mean time r.

In the variables q2 and our data exclude a volume-emitting fireball-like spherically symmetric source with a Gaussian space-time distribution (parametrization (6)). Contrary to data from e+e- and ¿¿N collisions, our data, furthermore, cannot be described as a function of the single variable Q2 = q2 — qfi and are thus inconsistent with parametrization (7). The space-time evolution of multiparticle production, therefore, is different in hadron- and lepton-induced reactions.

_ Our data do not confirm the expectation from the string-type model, which predicts an exponential rise of the BE correlation function in the region of negative (parametrization (8)). Comparatively more successful is the description in the variables |Q£| aI*d Qt uset* 'n the framework of the string model predicting parametrization (9).

A good description of our data is, however, achieved in the framework of the hydrodynamics! expanding source model.

The evolution pattern of hydrodynamical models includes the thermalization of the hadron matter, its longitudinal expansion, and final breakup at a freeze-out time tj and temperature Tj. For a centially produced pion pair with an average rapidity |j/| < Y (below the value Y = 1.5 in the CMS is nsed) the BEC can be approximately parameterized as (10). In contrast with the case of a non-expanding source the parameter rt in (10) is not a constant, but depends on y and the average transverse mass of the two pions, mi-

(13)

The dependence of the ratio R on ql and Qt is plotted in Fig. 3. The fit (with CL=63%) according to parameterization (10) results in ti, = 1.26 ± 0.06 fm and tt = 0.89 ± 0.04 fm.

The effective longitudinal size at different y, shown in Fig. 4, demonstrates a behavior consistent with the (cosht/)-1 dependence predicted by (13).

The observed dependence r¿(jí) is expected to disappear in the so-called longitudinal CMS (LCMS), in which the longitudinal momentum sum is zero. This expectation is confirmed by the results of our analysis in the LCMS (Fig. 4b).

The data were analyzed in the LCMS in two different regions, m-p < 0.35 GeV (with < mT >= 0.26 ± 0.05 GeV) and mT = 0.35 -f 1 GeV (with < mT >= 0.45 ± 0.09 GeV). We find, that the variation of rj and rj, is consistent with the 1 ¡^fmj dependence predicted (for tl) by the formula (13). The quoted values are: rly/mf a 0.67 ± 0.06 GeV1/2 fin and rj-ymj « 0.48 ± 0.04 GeV1/2 fm. Inserting the latter value into Eq.13 and assuming tj ~ ss 140 MeV, one can estimate the parameter t¡ = 1.3 ± 0.2 fm. Note, that the freeze-out temperature Tj could be estimated from the single-particle rapidity distribution at fixed transverse mass mj. For thermally emitting sources the Boltzmann distribution predicts a specific mj--dependence of the width of rapidity distribution:

(Ayf = (A,,)2+—, (14)

niT

where the parameter Aj; is the width of the space-time rapidity distribution of the emitting source. We verified that our data on the x- meson rapidity spectra at fixed irar and in the range |5/| < 1.5 could be fitted by Gaussian distribution. The observed (l/mx)-dependence of (Aj/)2 corresponds to (AÍ7)2 = 1.91 ±0.12 and T¡ = 159±38 MeV, the latter being consistent with Tj ~ mr.

A more general scenario of the hydrodynamical evolution of hadronic matter includes the transverse expansion of the hydrodynamical tube and a non-vanishing duration time A Tj of the pion emission at the freeze-out temperature Tj. In this case, the interference pattern can be described by the three-dimensional dependence (11), where the radii r0 and rs may differ. For example, in the case of non-relativistic transverse hydrodynamical flow, the effective radius ra is sensitive to Ar/ and exceeds the radius r, which measures the geometrical transverse size of the hydrodynamical tube. Under the assumption that the freeze-out time tj obeys a Gaussian distribution of width At¡ < Tj, the following relation holds:

rl = rj •: 2(i."/Ar<)2, . ________________(15)

where vt is the transverse pion-pair velocity in the LCMS, the average value of which is estimated from our data as vj = 0.484c.

The three-dimensional fit (with CL=7%) according to parameterization (11) results in tl> r0> TV Tl = 1.75 ± 0.20 fm, r0 = 1.06 ± 0.23 fm and r, = 0.76 ± 0.10 fm. Projections onto the three axes are shown in Fig. 5, -using 40 MeV cuts on the non-projected components. Note, that the quoted radius r, is close to the proton radius. From the difference between r0 and r, (Eq. 15) one obtains: cAry = 1.3 ± 0.3 fm. The quoted value of A Tj does not satisfy the condition Ar/ < r/, which is necessary for the validity of (15). Nevertheless, if A Tj could be accepted as a rough estimate of the duration time of pion radiation, then a possible interpretation of Ar/ ~ t¡ might be that the radiation process occurs during almost all the hydrodynamical evolution of the hadronic matter. This pattern is in contrast with that observed in nuclear collisions for which the duration time is found to be much shorter than the freeze-out time.

Another distinction between mterferometric data in hadronic and nuclear collisions is revealed when comparing the ratio of the freeze-out volume and the density of pions p(y) in the central rapidity region. In nuclear'collisions, this ratio is found to be k = r2íri¡p{y) =

!.00±0.15fm3 for all considered combinations of colliding nuclei. The constancy of fc indicates hat the hadronic matter breaks up and radiates pions at constant particle density. In our experiment, the ratio k =1.1±0.3 fm3 is smaller than that for the nuclear data, indicating hat in hadronic collisions the pion radiation occurs during earlier stages of matter evolution md at higher densities than in nuclear collisions.

In Chapter five one- and two- dimentional analysis and the angular dependence of 3ose-Einstein correlations are investigated for (jr+/A"+)Al and (jt+/A'+)Au interactions.

The mean radii and average radiation time of the pion emitting source are estimated in the ramework of the Kopylov-Podgoretzki 1 (12) model: r^i = 5.23 ± 0.50 fm, r,4„ = 7.54 ± 1.35 m and ctai = 1.82 ± 0.48 fm, ctau = 2.46 ± 0.72 fm, for Al and Au nuclei respectively (see •"ig. 6), indicating that the average geometrical size of the pion source significantly increases vith the target atomic weight A, while the average radiation time depends on A more weakly.

It is known, that pion interferometry not only allows to measure the average radius of a lion source, but also to determine its shape. The latter can be obtained from the dependence if the size on a direction given by the angle № of the c.m.s. momentum difference q = Pi — p2 vith iespect to the collision axis.

Recently, a method for direct determination of the ratio a = tj/tl from the angular iistribution of the vector q itself has been proposed by Podgoretskii and Cheplakov. As a ninimal assumption on the form of the pion source, rotational symmetry is used around the nteraction axis.

In general, the angular distribution of q for pion pairs with | q |< qcut and very small :.m.s. energy difference q0 =| E\ - E? \ is given by

2

v>(cosfl) =---rr . (16)

' 2[a2 + (1 — a2) cos2 0]

The ratio a = rj/ri can be determined by fitting distribution (16) to the experimental mgular distribution obtained after subtraction of a background (reference) distribution for vliich like-pion interference effects are absent. More simply, it can be determined from the isymmetry parameter A = (iVi - iV^/t^i + N2), where N\ and Nj are the numbers of :orrelated pion pairs (i.e. pairs after subtraction of the reference distribution) with | cos#| < 1/2 and | cos0| > 1/2, respectively, as

^S^-1»' (17)

The advantages of the method described above are that it does not require a fit to any particular form of the spatial distribution of the source, it is insensitive to the strength of ;he correlation, and it can be based on a smaller statistics than that required for separate neasurement of rj and r/>

Figs. 7a,b show the angular distribution of the vector q for (7r_7r~) pairs from the same :vents (points) and for mixed pairs from different events (histogram). The result of subtrac-;ion of these distributions is presented in Figs. 7c,d.).

At (qo)cut = 0.05 GeV, the ratio a obtained from (17) is equal to 0.65±0.13 for meson-iluminium and 0.50£0.21 for meson-gold interactions, within errors equal to the numbers jbtained from the fit according to-(16) in Fig. 7. The values of parameter a, extrapolated ;o (<7o)cui=0, are equal, respectively, to 0.53±0.15 and 0.33±0.21. The quoted values of the ratio ry/rt show that the nuclear source is elongated along the interaction axis in the c.m.s. if meson-nucleon collision.

In Conclusion the main results of the thesis are summarized.

1. Multiparticle production process are studied in K+A1, K+Au, Al and 7r+Au interactions,

• By using the experimentally observed distributions of the total charge Q of the secondary particles, a model independent estimate for the average total number of intranuclear collisions is extracted and it is shown that their dominant part

60% for Al and ~ 80% for Au) is caused by interactions of the non-leading particles produced in the target fragmentation.

• In the intranuclear multiple collision process, the first and the following inelastic collisions of the leading hadron (cluster) occur with approximately the same inelasticity, which is similar also for non-strange and strange leading hadron clusters: k = 0.17 ±0.03.

2. New experimental data on backward proton production are presented in (7r+ / A'+) Al and (7r+ / K'+) Au interactions at 250 GeV/c.

• The A-dependence of the backward proton yield is consistent with cxA0-7. For Au, the slope parameter of the pfal)-spectrum increases with increasing emission angle Qlabi but it is, within statistical errors, independent of A.

• Semi-inclusive spectra and correlation characteristics of BPP are investigated. From two-proton correlations it is found that a significant part of BPP at $iai > 100° and pM > 0.25 GeV/c, (26±4)% for Al and (44±6)% for Au, can be attributed to secondary pion absorption by a nucleón pair in the nuclear medium, 7r+(pn) —> *pp and ir°(pp) -* *pp-

• From fitting the contributions of SPA for different nuclei (C,Ne,Al,Au), it is found that the share of the SPA can be described by a power dependence a. A", with ¡3 = 0.27 ± 0.05.

• The upper limit of the cross section for the double colour charge exchange mechanism is estimated to be 1% at the high momentum tail (P¡0), > 0.5 GeV/c) of the backward proton inclusive spectrum.

3. Bose-Einstein correlations have been studied in two and three dimensions for pairs of negative pions in (7r+/K+^-interactions at 250 GeV/c.

• Our data ^ontrary to the data from lepton-induced reactions, do not confirm the expectation from the string type models.

• A good description of our data is, however, achieved in the framework of the hydrodynamical expanding source model. The two-dimensional analysis reveals the y- and mj-dependence of the longitudinal interferometric radius ji, predicted by the model.

• The proper freeze-out time of hadronic matter is estimated to be cr¡ = 1.3±0.2 fm (at T( = 140 MeV).

• From the m^-dependeiice of the width of the single ir~ rapidity distribution the freeze-out temperature is estimated to be T¡ = 159 ± 38 MeV.

• The throe-dimensional analysis leads to the relation rj, > r0 > rs, with the 'longitudinal' radius Tx=1.70±0.20im, the 'out'radius ro=1.18±0.13 fmtand the 'side' radius (assumed to measure the geometrical transverse size of the hydrodynamical tube) rs = 0.76 ± 0.10 fin.

• An indication is obtained that in meson-proton collisions the pion radiation occurs during earlier stages of matter evolution and at higher densities than in nuclear collisions.

4. One- and two- dimentional analysis of Bose-Einstein correlations are investigated for (7r+/A'+)Al and (jr+/A'+)Au interactions.

• The mean radii and average radiation time of the pion emitting source aie estimated in the framework of the Kopylov-Podgoretzkii model: r,u = 5.23 ± 0.50 fill, ran = 7.51 ± 1.35 fill and ctm = l.S2± 0.48 fm, ctau = 2.46 ±0.72 fin, for Al and Ah nuclei respectively, indicating that the average geometrical size of the pion source significantly increases with the target atomic weight A, while the average radiation time depends on A more weakly.

• An angular dependence of Bose-Einstein correlations has been observed. Within the framework of static model, the ratio a = t/Vl of transverse and longitudinal radii of the pion source is estimated to be a = 0.53±0.15 for (jt+/K^)Al and a = 0.33±0.'21 for (x+/K+)Au interactions, showing that the source is significantly elongated along the collision axis.

The main contents of the thesis has been presented in the following publications:

1. N.M.Agababyan,...,R.Sh.Hakobyanet al.: Z.Phys. C56, 1992, p.371

2. N.M.Agababyan,...,R.Sh.Hakobyanet al.: Z.Phys. C66, 1995, p.385

3. N.M.Agababyan,...,R.Sh.Hakobvanet al.: Z.Phys. C66, 1995, p.409

4. N.M.Agababyan,...,R.Sh.Hakobvanet al.: Z.Phys. C71, 1996, p.405

5. H.Gulkanyan,R.Sh.Hakobyan and W.Kittel: Proc. 7th Int. Workshop on Multiparticle Production "Correlations and Fluctuations", Nijmegen, The Netherlands 1996, Eds: R.C.Hwa et al. (World Scientific, Singapore, 1997) p. 26 (in press).

Table 1 Average total charge (Q), average total number of intranuclear collisions {vr) of leading hadron collisions (1/4) and of cascade collisions {vk)-

(Q) (vT) M («*>

K+Al Tr+Al K+Au x+Al 3.07±0.06 3.22±0.04 6.15±0.17 6.75±0.11 4.30±0.12 4.61±0.08 12.84±0.42 14.34±0.27 1.65±0.06 1.73±0.05 2.61±0.11 2.86±0.12 2.65±0.10 2.88±0.06 10.23±0.41 11.48±0.24

Table 2 Contribution from the SPA mechanism.

%

target pmin = 0.25 C'eV/c pmin = 0.30 GeV/c pmin = 0.35 GeV/c

Al 26.4 ±4.0 27.0 ±4.3 24.5 ± 4.4

Au 44.4 ± 5.5 40.2 ±6.0 34.5 ±5.7

,o2

SIC"

oi o

a)

• K'AI

• jr'Al

U

't

' ■ ■ . . I . . . ......1

a 0 50 100 150 200 250 W

xi 2

o 50 100 150 200 250 Zp„ (GeV/c)

'ig. la,b The distributions of the total longitudinal momentum £ />„ of charged particles in the beam fragmentation region (y)o).

M+AI

§ 200

-1 0 M+Au

H2 (GeV/c2)2

Fig. 2 The distribution in ¡j.2 at different pm;„ values. The solid curves refer to the bac ground and the dashed curve demonstrates the result of the Monte Carlo simulation subprocess (3) for Al (see text).

qL, GeV qL, GeV

Fig. 3 a) Lego plot for the ratio at 0 < < 1 GeV and 0 < QT < 1 GeV and (b-h)

its slices in different intervals of Qj. The curves are the fit results obtained according to parametrization (10).

Fig. 4 The a-dependence of rLin the a) CMS and b) LCMS frames, obtained with parametriza tion (10). The dashed curve is the function rL(0)/chy. -

0.4 0.6 QTo. GeV

QT=. GeV

Fig. "i 'Die projections of the ratio /?.(%, Qt0, Qt,) onto the three axes with 10 MeY cur-; on the non-projected components. The curves are the fit results ohtained accoiding to parametrization (11).

ст с?

о: 2

i

0.00<qo^Q.03 GeV

AL

j—' ' I ' ' ' I ' '_i I ' ' '_iii'

0.03<q0^0.06 GeV

AL

J—i i 1 i i ' I _i_i I

0.06<q0^0.09 GeV

AL

СГ

cr

ccf

J—I I I I I I I I I I I_I_I_I_I_I 1 I

0.09<q0=iC).12 GeV

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I I I I I I I I I I I_I I I_I_I I I

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AL

■ ' ' I ' ' i i ' ' ' '_i i i_I_' ' i

0 0.2 0.4 0.6 0.8 1 qT (GeV/c)

i

0.00<q0^0.03 GeV

A

J_I_I I I I_I_I_I_I_I I I_I ' I_L_

0.03<q0^0.06 GeV

A

-J_!_I_I_I_I_I_I_L_J_I_Ll_I_l_J_L

0.06<q0=-0.09 GeV

A

J_I_1_I I I_I_I_I_I_I_I_I_I_l_l_L_

0.09<qo^0.12 GeV

A

I I I I I I I I I I I I I ' I

0.12<q0^0.15 GeV

A

t i \ \ \ \ i i i \ i \ i i i \ i

0 0.2 0.4 0.6 0.8 Чг (Ge\

Fig. 6 a) Lego plot for the ratio R(qy,4o) at 0 < дт < 1 GeV and 0 < q0 < 0.15 GeV M+Al and M-f Au interactions and (b-f) its slices in different intervals of qo- The cu are the fit results obtained according to parametrization (12).

__

2

^ 4000

m

icosei

Fig. 7 (a,b) The angular distribution of the vector q for pion pairs in meson-nucleus interactions with % < 0-05 GeV, ]q| < 0.55 GeV/c. Points: experimentally observed distribution; histograms: reference distribution. (c,d) The result of the subtraction of the distributions presented in a,b (curves: the result of the fits by expression (16^)-

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ИССЛЕДОВАНИЕ МНОЖЕСТВЕННЫХ И КОРРЕЛЯЦИОННЫХ ЯВЛЕНИЙ В МЕЗОН-ЯДЕРНЫХ И МЕЗОН-ПРОТОННЫХ ВЗАИМОДЕЙСТВИЯХ ПРИ 250 Гэв/С

РЕЗЮМЕ

Диссертация состоит из введения, пяти глав и заключения. Работа :ожена на 123 страницах, включая 50 рисунков, 23 таблицы, 12.4-менования цитируемой литературы.

Диссертация посвящена изучению полуинклюзивных. коллективных и 'реляционных характеристик процесса множественного рождения адронов в он-ядерных и мезон-протонных взаимодействиях при 250 Гэв/с. перикентальные данные получены в рамках эксперимента ЫА22, при о щи установки Европейский гибридный спектрометр ДЕРН {Женева). Впервые в мезон-ядерных взаимодействиях исследованы коллективные актеристики адронов, рожденных в области фрагментации лучка, учено значение коэффициента неупругости лидирующего адронного стера во внутриядерных столкновениях', показано, что он пр.>иеркг. наков для первого и последующих внутриядерных столкновений. Впервые измерено среднее число внутриядерных столкновен;*. л и азано, что их большая часть С605Е для ядра А1 и ~80Х для ядра Йи) словлена вторичными взаимодействиями нелидирующих адронов. Получены новые экспериментальные данные по образованию кумулятивных тонов. Впервые для средних и тяжелых ядер оценен вклад механизма гриядерного поглощения вторичных пионов в инклюзивное сечение ззования кумулятивных протонов.

Впервые в мезон-протонных взаимодействиях детально изучены «мерные и трехмерные корреляции Бозе-Эйнштейна. Показано, что ученные экспериментальные данные описываются в рамках -■•о динамической модели, оценены параметры характеризующие :транственно-временную картину множественного, рождения. Зпервые показано, что в мезон-ядерных' взаимодействиях область /чения пионов имеет вытянутую форму вдоль оси взаимодействия.