научная статья по теме STUDY OF IN-MEDIUM -MESON PROPERTIES IN AND COLLISIONS Физика

Текст научной статьи на тему «STUDY OF IN-MEDIUM -MESON PROPERTIES IN AND COLLISIONS»

ЯДЕРНАЯ ФИЗИКА, 2010, том 73, № 1, с. 154-165

= ЭЛЕМЕНТАРНЫЕ ЧАСТИЦЫ И ПОЛЯ

STUDY OF IN-MEDIUM ^-MESON PROPERTIES IN Ap AND pA COLLISIONS

©2010 Yu. T. Kiselev*, S. M. Kiselev, M. M. Chumakov

Institute for Theoretical and Experimental Physics, Moscow, Russia Received November 12, 2008; in final form, June 11, 2009

The possibility to investigate the in-medium properties of the vector w mesons at normal nuclear density is considered. The folding model and simulations with the RQMD generator have been used for studying of the w-resonance production in Ap and pA reactions and its w ^ n0Y ^ З7 decay. We show that measurements in the inverse Ap kinematics is an effective way to get information about the w-meson mass modification especially in not yet explored range of small meson momenta relative to the projectile nuclei where the strength of the effect is expected to be most strong. The traditional pA kinematics appears to be more preferable for the investigation of the in-medium w-meson width. Using of compact electromagnetic calorimeter provides the possibility to collect large statistics and study the momentum dependencies of both the w-meson mass and width at the density of normal nuclei.

1. INTRODUCTION

The modification of the hadron properties in baryon environment is one of the important topics of contemporary strong interaction physics. This phenomenon has been predicted within various theoretical approaches such as QCD sum rules [1], chiral dynamics [2], relativistic mean-field [3], and quark—meson coupling model [4] (for a recent review see [5]). A hadron can change its properties such as mass and width once it is embedded into a baryon matter. This change is connected to the many-body interactions of a hadron with surrounding nucleons. Whether a hadron is also affected by QCD condensates and their in-medium change [1, 2, 6] is still a matter of debate. Nevertheless, the great interest in study of in-medium hadron properties is caused by the expectation to find the evidences of the chiral symmetry restoration. The investigation of the vector mesons is of special interest in this context. Theoretically, the possibility of the decrease in the mass of light vector mesons in matter was first pointed in [7] and later in [2]. Recently, the in-medium spectral change of the u mesons was proposed as a probe of higher order QCD four-quark condensate [8].

An evidence for a decrease of the p-meson mass in heavy-ion collisions was obtained by the CERES Collaboration at CERN [9] and by the STAR Collaboration at RHIC [10]. The above results have found an explanation in terms of a shift p-meson spectral

E-mail: yurikis@itep.ru

function to a lower mass, however, even the calculations that just used the free radiation rates with their experimental uncertainties are compatible with the observation. The interpretation of experimental data on nucleus—nucleus collisions is far from being simple because heavy-ion interaction is a complicated process in which the temperature and baryon density vary dramatically with time. Therefore, it is useful to explore the reactions with elementary probes (7, n, p) since sizable — about 20% — medium effects were predicted already at the density of ordinary nuclei [2, 11, 12]. The advantage of the reactions on nuclei is related to the fact that they proceed in the nearly cold static nuclear matter and thus the colliding system is much better under control. First attempt to estimate the in-medium mass of the u meson produced in pXe interactions had been made in [13]. Recently, the signals for lowering of the u-meson mass at normal nuclear matter density were observed in the yA [14] and pA reactions [15]. However, the critical analysis [16] shows that data of the experiment [14] are compatible with normal u mass and an enlarged width. In contrast to the conclusion [15] the preliminary results of CLAS Collaboration on the photoproduction of p/u mesons [17] also evidence for no shift in the mass. Now there are only a few experimental estimates of the u-meson width in matter [14, 18]. Thus, the available now experimental information does not allow to draw the decisive conclusion about the change of the u-meson properties even in nuclear matter of normal density. It should be stressed that the indications for a decreasing of the u-meson mass in both experiments [14, 15] have been found for the mesons with relatively low momenta with respect

to the surrounding nuclear matter. Therefore, next generation of experiments need to address the issue of momentum dependence of medium effects. In the present investigation we show that measurements of the w-meson production in nucleus—proton and proton—nucleus collisions at 4 A GeV provide the possibility to study the momentum dependencies of both w-meson mass and width in nuclear matter.

2. THEORETICAL PREDICTIONS

All information about the intrinsic properties of a meson is encoded in its spectral function S(M) which can be written in nonrelativistic Breit—Wigner form. In free space

S(M) = (To/2)2/[(M - Mo)2 + (ro/2)2], (1)

where r0 and M0 stand for a meson width and pole mass, correspondingly.

Due to the interaction with surrounding nuclear medium the meson acquires a self-energy £ which is related to the nuclear optical potential U as [19]:

£/2E = U = ReU + iImU, (2)

where E is the total meson energy.

The meson spectral function in nuclear medium is read:

S (M) = (3)

= [(ro/2) + (r*/2)]2/[(M - (Mo + M*)]2 + + [(ro/2) + (r*/2)]2.

Two extra terms, M* and r*/2, which describe the shift of the meson pole mass and the increase of its width in matter, are related to the nuclear optical potential U as follows [19]:

M * = ReU, r*/2 = —ImU. (4)

The pole mass and width of the w meson in free space (vacuum) are M0 = 782 MeV and r0 = = 8.4 MeV, correspondingly. Most theoretical investigations predict the dropping of the in-medium w-meson mass by 20—140 MeV [20] at normal nuclear density. However, there have also been suggestions for a rising mass [21] or even a structure with several peaks [22]. At the same time there seems to be a general agreement that in-medium w width is within the range from 20 to 60 MeV [23] at the density p = = p0. Thus, it is expected that the w meson in matter survives as a quasiparticle and can be observed as a structure in the w-mass spectrum. In principle, both dilepton and n0j invariant mass spectra can be used for the study of modification effects. The advantage of the dilepton decay channel is related to the fact that leptons are almost undistorted by the final state interactions. However, the w signal in the dilepton

mode is rather weak (BR(w ^ e+e ) w 7.1 x 10 5) and is always accompanied by a comparatively large background from p0 ^ e+e" decays. The w ^ decay has a branching ratio 8.9 x 10"2 what is three orders of magnitude higher. Furthermore, the competing p ^ channel has a branching ratio which is a factor of 102 smaller. By these reasons the w ^ n0Y decay mode can be considered as a promising probe to study the w-meson properties in matter. The disadvantage of this channel is possible rescattering of the n0 within the nuclear medium which would distort the deduced w invariant mass distribution. However, as was shown in [24, 25] the above distortion effect can be significantly decreased by rejecting the low-energy pions.

3. DIRECT AND INVERSE KINEMATICS

As will be shown below the in-medium w-meson width can be deduced from the analysis of the production of relatively fast mesons in the nucleus rest frame. The direct pA kinematics appears to be more preferable for such an investigation. In contrast with in-medium width the effect of the w-meson mass shift is expected to be strong for slow mesons relative to the nuclear matter. The study of low-momentum w-meson production in the inverse Ap kinematics [26] has important advantages over the study in the direct pA kinematics. As it follows from the Lorentz transformation, slow particles in a projectile nucleus system appear to be fast in the laboratory (in the target proton rest frame). At beam energy of 4 A GeV all w mesons produced in full solid angle with momenta of less than 0.3 GeV/c relative to the projectile nucleus rest frame will be concentrated in the laboratory inside narrow cone of less than 5° and the momentum range from 2.8 till 5.9 GeV/c. The mesons which are almost at rest inside the incident nucleus ("comovers") have the laboratory momenta around 4 GeV/c. Due to the decrease of the production cross section with the w-meson momentum the main contribution to the w yield comes from the momentum interval of 2.8—4.0 GeV/c. These events will be observed in small phase space dPd cos d in the laboratory resulting in significant increase in forward production cross section as compared to the one in pA reactions. The photons from the w ^ ^ 3j decay are distributed inside a more wide cone as compared to parent mesons, however the coverage of the angular interval 5°—25° — which corresponds to the solid angle of less than 9% of 4n — permits to collect significant part of the signal events (see Section 5).

The mean free pass of the proton in nuclear matter is as small as 2 fm and therefore the w mesons

are predominantly created inside the front layers of a projectile nucleus. Since the u's produced in forward direction within the momentum from 2.8 to 4.0 GeV/c have the laboratory velocities which are less than the ones of the surrounding nucleons, in the rest frame of the projectile nucleus these mesons move into the interior of the nucleus and then decay in its more dense inner layers. That is of great importance because the strength of the medium effects increases with the nuclear density.

Another important advantage of the inverse kinematics is sizable increase of the detected photons' energies because they are emitted by relativistic u and n0. For example, the decay in transverse direction of the u carrying the momentum of 4 GeV/c results in emissi

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