HREPHAH 0H3HKA, 2004, moM 67, № 1, c. 108-113
VERY HIGH MULTIPLICITY PHYSICS
PROTON-PROTON INTERACTION WITH HIGH MULTIPLICITY AT ENERGY 70 GeV (PROPOSAL)
© 2004 P. F. Ermolov1), E. A. Kuraev, J. Manjavidze, A. P. Meschanin2), V. A. Nikitin*. I. A. Rufanov, A. N. Sissakian
Joint Institute for Nuclear Research, Dubna, Russia Received May 21, 2003
The goal of the proposed experiment is to investigate the collective behavior of particles in the process of multiple hadron production in pp interaction pp ^ nnn + 2N at the beam energy E\ab = 70 GeV. The domain of high multiplicity nn = 30—40 or z = n/n = 4—6, will be studied. Near the threshold of reaction nn ^ 69, z ^ zth = 8.2, all particles get a small relative momentum Aq < 1/R, where R is the dimension of the particles production region. As a consequence of multiboson interference a number of collective effects may show up: (a) drastic increase of partial cross section a(n) of n identical particles production is expected, comparing with commonly accepted extrapolation; (b) the jets formation consisting of identical particles may occur as a result of multiboson Bose—Einstein correlation (BEC) effect; (c) large fluctuation of charged n(n+ ,n-) and neutral n(n0) components, onset of centauros or chiral condensate effects are anticipated; (d) increase of the rate of direct 7's as a result of the bremsstrahlung in partonic cascade and annihilation of n+n- ^ nj in dense and cold pionic gas or condensate is expected. In the domain of high multiplicity z > 5, the major part of the centre-of-mass energy a/s = 11.6 GeV is materialized leading to the high density thermalized hadronic system. Under this condition a phase transition to cold quark—gluon plasma (QGP) may occur. The search for QGP signatures like large intermittency in the phase-space particle distribution, an enhanced rate of direct photons will be performed. The experimental setup is designed for detection of rare high-multiplicity events. The experiment is carried out at the extracted proton beam of IHEP U-70 accelerator. The required beam intensity is ~107 s-1. Assuming the partial cross section a(nn = 35) = 10—1 nb the anticipated counting rate is 10—1 events/h. The multiboson BEC enhancement may drastically increase the counting rate.
1. INTRODUCTION
The investigation of multiple particles production at high energy is one of the fundamental problems of hadron hysics. It is essentially a nonperturbative process. QCD gives only a qualitative picture of this phenomenon: hadron collision initiates a partonic cascade. The gluon strings that arise between colore partons eventually break and produce quark—atiquark pairs. At the final stage of the cascade development when the energy is exhausted the partons join together creating hadrons. The mechanism of the colore confinement is unknown. As a consequence at present it is impossible to calculate theoretically even the main parameters of the process: multiplicity distribution, the energy and mass spectra of particles. Some features of the reaction are described by different models: thermodynamic, hydrodynamic, partonic
1)Institute of Nuclear Physics of Moscow State University, Russia.
2)State Science Center Institute of High Energy Physics, Protvino, Russia.
E-mail: nikitin@sunse.jinr.ru
cascade, Redge poles, and so on. But none of these approaches are complete and their substantiation is far from being rigorous.
There exists an extensive literature on this subject. The theoretical approaches based on statistics and thermodynamics are reviewed in [1, 2]. The grounds for the phase transition search are presented in [3, 4]. The importance of the investigation of mul-tiparticle production for understanding the hadron matter properties under extreme conditions, like high density and temperature, is stressed in [5]. Complete survey of the situation with particles correlation and intermittency can be find in [6]. Experimental data on multiplicity distribution and its phenomenological comprehension is given in [7].
The purpose of the proposed experiment "Ther-malization" is to investigate the collective behavior of particles in the process of multiparticle production in pp (or pN) interactions
pp ^ nnn + 2N (1)
at the proton energy Elab = 70 GeV. At present the multiplicity distribution at this energy is measured
up to the number of charged particles nch = 20 [8]. The corresponding scaling variable z = nch/nch = = 3.5. The kinematics limit is nn,th = 69, zth = 8.2. Here, nn,th is the maximal number of charged and neutral pions allowed by energy—momentum conservation. We plan to study the events with multiplicity nn = 30—45, z = 5—6. At large multiplicity and near the threshold of the process (1), where all particles have a small relative momentum, the high particle density f = (2n)3d6n/dp3dr3 & n3/2N/(VpVr) in the 6-dimensional phase space is reached. Here, N is number of particles in momentum—space volume VpVr. Note that in the system h = c = 1 the value f is dimensionless. The authors of [9—11] argue that the parameter f indicates the importance of multiparticle effects. The value f is the mean number of pions that interfere with one given pion and build the Bose—Einstein (BE) enhancement in the two particle correlation function. So, if f < 1, only the two particle correlation may be observed. Typically f & 0.1 for the mid-rapidity region and p± & p±. Even at LHC energy in Pb—Pb collisions f is expected to be small in spite of huge multiplicity. This is due to large phase-space volume VpVr & (4/3)2p3r3 occupied by secondary particles: p & 0.5 GeV/c, r & & 10 fm. In contrast to high energy A— A collision in our case of the pp collision the volume VpVr is by three order of magnitude less, since we expect to have p & 0.07 GeV/c, r & 2—3 fm. So we anticipate to reach very high particle phase-space density f » 1. As a consequence, one expects to observe the collective effects connected with multiboson interference: broadening of the multiplicity distribution, anomalous fluctuation of charged and neutral components, the jets formation and so on.
In the region z > 4 the major part of center-of-mass energy y/s = 11.6 GeV is transferred into mass of produced particles. The density of the created hadron system may be rather high, p/p0 & 5—10. Here, po is the density of nuclear matter in the ground state. According to the common notion the system under such condition supposed to be quark— gluon plasma (QGP). Figure 1 is taken from [12] to illustrate this statement.
The onset of QGP manifests itself, at least, by two signatures: large particles intermittency in phase space of rapidity—transverse momentum and an excess of the direct photons and lepton pairs. We plan to search for both of these signatures. The unique feature of QGP in our case is its low temperature. The closer we approach to the reaction threshold, the lower is the temperature and higher is the density of the system. The lower is the temperature the longer is lifetime of the system. The latter is especially
T, MeV
P/Po
Fig. 1. Phase diagram of nuclear matter.
important: the system must come to the equilibrium in order to reveal the QGP features.
The further progress in understanding the dynamics of the multiparticle production process will come from further development of the experiment.
For the purposes of this experiment we plan to improve the setup Spectrometer with Vertex Detector (SVD) installed at the extracted proton beam of IHEP (Protvino) U-70 accelerator. The beam intensity is 107 s"1. It incidents on hydrogen (or light nuclei) target and generates 104 s"1 pp (or pN) interaction. One should mention that it is difficult to make a reliable extrapolation of experimentally measured multiplicity distribution from the region 0 < < z < 3.5 to the region z > 5. We expect a partial cross section in the interval 4 < z < 5 is about 10— 1 nb. Then we can collect about 1 —10 events/h. However, there are theoretical arguments favoring BE enhancement of the identical pions production. Then the counting rate will be much higher.
2. PHYSICS PROGRAM
2.1. Multiparticle Process Near Kinematics Limit
Let us consider high enough multiplicity ncrit when in c.m.s. no energy remains for formation of the leading particles. Then all secondaries have equal
energy \Jp]_ +1of + m2 which we determine from the
mean transverse momentum of pion p& 0.3 GeV/c as it was measured in soft hadron reactions. The
longitudinal momentum is pi = —=p±. The critical
10
10
10'
,0
. 10 10
10-10 10
10-2 10-4 10
-O
,-6 S
i 10-
1 10-
0 1 2 3 4 5 6 7
Fig. 2. Multiplicity distribution (partial cross section) in pp interaction. For details see text.
multiplicity is determined by relation \Jp']_ + m'i- =
= (s/s — 2mjv)/(«Crit + kn)- Here, iin = 2 is the nucleon multiplicity. We get ncrit = 23. In the region n > ncrit the particles in c.m.s. should have isotropic angular distribution and their energy distribution is Maxwell or BE. The corresponding temperature is T = f^idn; Skin = (\fs ~ 1mN - nn>mn)/(njr + nN). Here, nn is pion multiplicity. T depends on pion multiplicity nn and vanishes when nn ^ nth. On this basis we develop the Monte Carlo (MC) event generator and calculated the angular and momentum distribution of the pion and nucleons. These data are necessary for the experiment planing. An example: at multiplicity nn = 50 mean c.m. energy of all particles is 50 MeV and mean lab. emission angle 0n = = 90 mrad, 0N = 40 mrad. These numbers indicate remarkable feature of apparatus: having very modest angular acceptance 0 = 20n = 200 mrad it will detect 95% of all secondary products.
There is the experimental indication on onset of the thermalization regime at multiplicity nch = 18 [13].
2.2. Multiplicity Distribution
Topological cross section a(nch) in pp interaction at 70 GeV at U-70 accelerator have been measured in two experiments [8], see Fig. 2. The calculation by the MC PYTHIA code is shown. One can
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