Production of neutral mesons in relativistic heavy-ion collisions

NUCLEAR PHYSICS A ELSEVIER

Nuclear Physics A630 (1998) 23 lc-238c

Production of neutral mesons in relativistic heavy-ion collisions I R.Novotny II. Physics Institute, University Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany and for the TAPS-collaboration [1] The production of neutral mesons (n°jl,¢o) in relativistic heavy ion collisions over a wide dynamic range in energy and system size has been studied using the electromagnetic calorimeter TAPS (Two/Three Arm Photon Spectrometer). The detection of charged reaction products in scintillator hodoscopes allows exclusive measurements as a function of the number of participants. The inclusive production probabilities follow the established systematics. However, at fixed bombarding energies they decrease with system size due to absorption or increase of collective flow. Exclusive measurements comparing the systems Ar+Ca and Au+Au at 800 AMeV show strongly reduced meson multiplicities in the heavy system at the same number of participants. In contrast to n°-production, the observed ~-multiplicities increase quadratically with the number of participants which indicates multi-step processes as expected for subthreshold production. The mr-scaling can be established, but an enhancement at low m t values appears in the heavy systems. 1. P H Y S I C S M O T I V A T I O N

Relativistic heavy-ion collisions are considered to be an ideal tool to study nuclear matter under extreme conditions. At the highest bombarding energies as provided by the heavy-ion synchrotron SIS at GSI (Darmstadt) the reaction zone can be compressed to 2 - 3 times the normal nuclear density and heated up to temperatures of the order of 100 MeV according to theoretical calculations. The initial kinetic energy is converted into the excitation of thermal, collective and intrinsic degrees of freedom. During the different phases of the reaction a substantial fraction of the participating nucleons becomes excited into resonance states which subsequently decay via meson emission. Their investigation provides an insight into the reaction dynamics which is complementary to the determination of the full emission pattern of all involved hadrons as realized by 4n-detector systems like FOPI [2] installed at GSI. Pions are mainly produced in binary nucleon-nucleon collisions via the excitation and decay of the A(1232) resonance. However, due to rescattering and i supported by BMBF, DFG and GSI 0375-9474/98/$19 © 1998 Elsevier Science B.v: All rights reserved. Pll S0375-9474(97)00761-6

R. Nouotny/Nuclear Physics A630 (1998) 231c-238c

232c

absorption m o s t of the pions w h i c h are identified in the detector m i g h t h a v e left the collision zone at a late stage. The observed yield is expected to be very sensitive to the total m a s s of the colliding nuclei. In the investigated energy regime the observed ~ - m e s o n s originate primarily from the decay of the N*(1535) resonance. D u e to the high threshold in the free nucleon-nucleon collision (E~(T1) = 1255 MeV) the investigated TI-mesons are produced near or even subthreshold and their production is sensitive to the dynamics of multi-step processes and therefore to the size of the collision zone.

% >~

7T°

(a)

10 5

O g}

-E

10 4

o

2 AGeV C+C

"':-~

o

%

8O

1600

%

> •

7T°

60

~)

~6

4O

MeV/c2 52~ev/c:

FWHM

g oo 20

HM

1200

~

800

2~ ~

400

?,

% ~

0

0 100

35000

200

,

300

400

0.8AGeV

500

Au+Au

600

700

(b)

30000 25000 20000

15000 10000

8

5000 0 -5000 0

, , ,i .... I , 1O0 200

,,Ill, 300

II IT,,I ,, I' , , , I .... 400 500 600 700 800

invariant mass [MeV/c e]

Figure 1. Invariant m a s s distributions of neutral m e s o n s m e a s u r e d in the reactions 2 AGeV 1~C+1~C (a) and 800 AMeV 19VAu+lgVAu(b). The dashed line in the upper section of part (a) represents the combinatorial background determined by event mixing. The spectra presented on a linear scale s h o w the invariant m a s s distributions after background subtraction and d e m o n s t r a t e the quality of the background reconstruction and the achievable m a s s resolutions.

R. Novotny/Nuclear Physics A630 (1998) 231c 238c

233c

2. EXPERIMENTAL TECHNIQUE The detection of neutral mesons in relativistic heavy-ion collisions exploits t h e i r decay into 2 or 3 photons, which requires an electromagnetic (EM) calorimeter equipped with high spatial and energy resolution and good sensitivity for photon/particle discrimination. The photon spectrometer TAPS [3], as operated at GSI, consisted in the used experimental set-up of two movable towers positioned symmetrically with respect to the beam axis at mid-rapidity. Each spectrometer arm carries 3 detector blocks which contain 64 plastic/BaF~ scintillator telescopes pointing towards the t a r g e t location. The BaF2-crystals are of hexagonal shape with a length of 25 cm (~ 12 radiation lengths Xo). A plastic scintillator a r r a y of identical g r a n u l a r i t y mounted in front serves as an online charged particle veto (CPV). The time zero for the time-of-flight (TOF) m e a s u r e m e n t can be deduced either from an in-beam counter (NE102A-foil, 200 ~tm) operationable at projectile rates up to a few 106/s or from a plastic scintillator hodoscope comprising 48 modules which are arranged close to the t a r g e t area detecting fast charged reaction products (distance d = 10 cm, covered angle range 150 - 30 o with respect to the beam). The l a t t e r provides simultaneously a m e a s u r e of the centrality of the reaction based on the charged particle multiplicity. The small angle hodoscope of the KaoS-collaboration [4], consisting of 380 plastic scintillators (2.5 cm thick), covers polar angles between 0.5 o and 9 o in the laboratory system in full azimuth around the beam direction. The m e a s u r e m e n t of the energy-loss and TOF allows the identification of the spectator matter. In combination with the anticorrelated information of the reaction trigger, the centrality of the collision can be characterized by the n u m b e r of participating nucleons and allows exclusive m e a s u r e m e n t s on the meson production. The presented, mostly preliminary results concentrate on the systems 12C+12C (0.8,1.0 and 2.0 AGeV) [5], '°Ar+°a~Ca (0.8 AGeV) [6], ~SNi+58Ni (1.9 AGeV) [7] and 197Au+~97Au (0.8 AGeV) [8], respectively, investigated up to the highest projectile energies available at GSI. The neutral mesons are identified via an i n v a r i a n t mass analysis which requires the information on the individual energies E~ and the relative opening angle O12 between both photons reconstructed from the EM shower (for 2-photon decay: mrr =~]2E1Ee(1-COSO,2)).Charged and neutral particles are discriminated exploiting the information on TOF, the CPV system and the pulse-shape analysis of the BaF2-signal [9]. Figure l a illustrates as an example the i n v a r i a n t mass spectrum m e a s u r e d for the system 12C+12C at the highest bombarding energy of 2 AGeV. Structures assigned to rt°- and q-mesons, respectively, can be identified on top of a large combinatorial background which increases drastically in heavier systems due to the much higher photon multiplicity. Exploiting the mixed-event technique the combinatorial background can be reproduced with high accuracy even for individual event classes which is necessary in order to account for differences in the selected phase space. The lower part of Figure l a shows on a

234c

R. Nouotny/Nuclear Physics A630 (1998) 231c-238c

linear scale the i n v a r i a n t mass distributions after background subtraction d e m o n s t r a t i n g the achieved good experimental resolution Aml.v /mi. v - 5 - 10% (FWHM). Even in the heaviest investigated system a clean meson signal can be extracted as documented by results obtained for the system 0.8 AGeV 197Au+197Au (see Figure lb).

EM Vl -1

/%

10

.< V

10

A V II 13_

~o

280

'q w

1255 1900

-2

o -3

10

•

o

c+c Ne + AI

I

Ar + Co

Ni + Ni

-4

10

A

•

[] •

Kr + Zr Au + Au

C+Au / Au+C

-5

10

(E/A)/E,~r"

10

Figure 2. N e u t r a l meson production probability P per participant nucleon in heavy-ion collisions as a function of the b o m b a r d i n g energy normalized to the production threshold in free nucleon-nucleon collisions. The line d r a w n represents a fit given in [11]. The limited solid angle in the laboratory frame ( d ~ - 5%) at midrapidity translates to an average efficiency of a few times 10 .3 in the n o- or n-detection. However, due to the identification via an i n v a r i a n t mass analysis, transverse m o m e n t a Pt of the mesons can be m e a s u r e d completely down to values of p, = 0 MeV/c. The efficiency of TAPS is determined in detailed Monte-Carlo simulations u n d e r the assumption of a t h e r m a l and isotropic source of mesons located at midrapidity with t e m p e r a t u r e s t a k e n from the experimentally m e a s u r e d inverse slope p a r a m e t e r s of the pt-distributions. As a consequence, cross sections extrapolated into 4n become model dependent.

R. Novotny/Nuclear Physics A630 (1998) 231c-238c

235c

3. P H Y S I C S R E S U L T S The inclusive m e s o n production probability P per p a r t i c i p a n t nucleon, which is d e t e r m i n e d from the m e a s u r e d m e s o n multiplicity and the m i n i m u m bias v a l u e of p a r t i c i p a n t s t a k e n from a geometrical model by Cugnon et al. [10], provides a first overview on the reaction m e c h a n i s m . As proposed in [11], the production of particles w i t h different m a s s e s can be p r e s e n t e d as a function of b e a m energy per nucleon in units of the production threshold for free nucleonnucleon collisions. Differences in the Coulomb b a r r i e r are not significant at these energies. In Figure 2 the new results obtained by TAPS are p r e s e n t e d including for t h e first t i m e u p p e r limits for the detection of co-mesons. In general, all d a t a follows the smooth curve t a k e n from [11], which rises steeply within the i n v e s t i g a t e d r a n g e below and above threshold. The t r e n d indicates t h a t the m e s o n production depends in first order on the energy available in the collision. The more detailed s t u d y of the dependence on the individual s y s t e m m a s s will be discussed later.

10

~10

-2 0.8 AGeV -0.1 _
~ : ~ !

o

~10

i i iruCCAaui i iruC~a

-6

g .~E"10 -6

"O ~E10 -lO

10

-12 0

AuAu 76 ,,,i,,,,,

....

,

.... , ,.., .... ,., ,.,,,

1~ ,,,,',,[,,,x',,

100 200 300 400 500 600 700 800 9001000 transverse mass

[MeV/c 2]

Figure 3. T r a n s v e r s e m a s s spectra of n °- a n d Tl-mesons observed at m i d r a p i d i t y for the s y s t e m s 12C+ 12C, 40Ar+natCa and 197Au+~9~Aua t 800 AMeV projectile energy. T h e lines d r a w n r e p r e s e n t exponential fits to the data. T h e a v e r a g e t r a n s v e r s e m o m e n t u m < Pt > of x ° - m e s o n s increases also w i t h b o m b a r d i n g energy. The corresponding distributions for Tl-mesons, obtained w i t h

R. Nouotny /Nuclear Physics A630 (1998) 231c-238c

236c

larger e x p e r i m e n t a l uncertainties, show even l a r g e r m e a n values at the s a m e energy. This t r e n d reflects a considerable h e a t i n g of the collision zone. H o w e v e r in contrast to the energy dependence of the production probabilities, a n onset of a s a t u r a t i o n of < Pt > becomes visible at the highest energies, i.e. the fraction of available e n e r g y converted into m e s o n production becomes l a r g e r c o m p a r e d to the chaotic particle motion in the fireball region.

TAPS nKa°S

~

~ FOPI

9

~

8 7

5

I19/20A

i

,

,

V

i

i

i , i i

10

i

,

B

,

,

,

10 ,

2

< APART >

Figure 4. Pion production probabilities per p a r t i c i p a n t nucleon as a function of the s y s t e m size expressed by the a v e r a g e n u m b e r of p a r t i c i p a n t s t a k e n from a geometrical model [10]. The TAPS m e a s u r e m e n t s at various b o m b a r d i n g energies h a v e been completed by results obtained by the F O P I a n d KaoS collaborations for charged pions normalized according to the isobaryonic model. Figure 3 displays the inclusive cross sections m e a s u r e d at m i d r a p i d i t y in a r e p r e s e n t a t i o n of llm2d~ I dm, as a function of t r a n s v e r s e m a s s m, = 4m2m..... + p2. C o m p a r e d are spectra at 800 AMeV projectile energy obtained for t h r e e different s y s t e m sizes. As expected for a t h e r m a l and isotropic m e s o n source, the distributions show a p u r e exponential s h a p e for the lighter systems. However, in the '97Au+~97Au s y s t e m the s h a p e becomes concave leading to a strong e n h a c e m e n t at low m t values which is even more pronounced in p e r i p h e r a l collisions [8]. For m t > m n the =0 a n d TI cross sections exhibit not only a l m o s t the s a m e slope b u t even the absolute values fall on top of each other. The observation

R. Novotny/Nuclear Physics A630 (1998) 231c-238c

237c

of the so-called m,-scaling, which is well known in high-energy physics, indicates that the energy required to produce a given m t completely determines the relative abundance of the two meson species as expected for a source in thermal and hadrochemical equilibration. One should keep in mind that the two neutral mesons are produced via the excitation and decay of different baryon resonances. The systematic study of neutral and charged pion production at SIS energies including data for charged pions (scaled according to the isobaric model [12]) obtained with the detector systems FOPI and KaoS, respectively, allows to investigate effects on the inclusive meson production probabilities due to system size. Figure 4 documents the almost exponential decrease of the mean pion multiplicity < M~ > per participant (extrapolated into a solid angle of 4~) with the minimum bias number of participants < Ap~ > again deduced from a geometrical model. The same trend appears at all studied bombarding energies. The same behaviour is shown in the corresponding data for Tl-mesons which also drop by a factor of 2 comparing the systems ~C+~C and l~Au+~9~Au studied at 800 AMeV energy. The strong mass dependence of these inclusive data can be either addressed to pion rescattering and absportion effects, in particular due to cold spectator matter, or to the increase of the collective flow velocity with the total size of the system, as observed by preliminary results of the FOPI collaboration [13]. In the latter case, due to the increase of the collective energy a smaller fraction remains available for meson production and could account for the observed experimental finding. 8

6- I °

A.÷Au

/

/

i

~ 30,

~t~ 20

~ :i

~20.

"i4-

0

100

200

300

n u m b e r of participants Ap, n

400

100

200

2

M ~AZp,,

300

400

number of participants Ap,.

Figure 5. Measured exclusive neutral meson multiplicities [~o (lei~), ~l (right)] presented as a function of the number of participants for the systems '°Ar+"atCa and 197Au+~97Auat 800 AMeV energy. The lines are drawn to guide the eye. To study the influence of meson absorption in spectator matter, which surrounds the hot fireball region in the initial phase, meson yields have been studied in exclusive measurements at 800 AMeV projectile energy by comparing the symmetric systems 4°Ar+natCa and ~97Au+197Au, respectively. For both systems

238c

R. Novotny/Nuclear Physics A630 (1998) 231c-238c

the ~0_ and Tl-multiplicities have been determined as a function of impact parameter expressed by the number of participant nucleons Ap~ as shown in Figure 5. In case of neutral pions (Fig. 5 [lei~]), the multiplicity per A ~ remains constant for both systems with increasing centrality but the absolute value, i.e. the linear slope of the distribution, is significantly smaller in case of the heavy system. For better comparison, the value Ap~ ~ 50, which selects very central collisions in the 4°Ar+natCa system, corresponds to peripheral reactions in 197Au+197Au. In the latter case, large spectator masses can be present to be able to shadow the overlap region leading to strong pion absorption. A similar result can be observed for TI-production as well (see Fig. 5 [right]). In addition, the impact parameter dependence of the H-multiplicities indicates a clear difference in the involved production mechanism. The significantly strong non-linear increase (M~ ~ (Ap~)~) might reflect the importance of multi-step processes, which are expected at subthreshold energies and have been reproduced in BUU-calculations [14]. 4. SUMMARY The systematic study of neutral meson production in relativistic heavy-ion collisions has been completed in a second experimental phase of the TAPS spectrometer operating at SIS/GSI. Inclusive but, in particular, exclusive measurements for a large variety of system masses provide new insights into the reaction dynamics, the balance of collective, chaotic and intrinsic energies and the influence on meson yields due to absorption and rescattering processes. REFERENCES

1. The TAPS collaboration: NPI, Rez (Czech Republic), GANIL, Caen (France), GSI, Darmstadt, University Giessen, University Miinster (Germany), KVI, Groningen (The Netherlands), IFIC University Valencia (Spain) 2. A.Gobbi et al., Nucl. Instr. and Meth. in Phys. Res. A324 (1993) 156 3. R.Novotny, IEEE Trans. Nucl. Sci. 38 (1991) 379 A.Gabler et al., Nucl. Instr. and Meth. in Phys. Res. A346 (1994) 168 4. P.Senger et al., Nucl. Instr. and Meth. in Phys. Res. A327 (1993) 393 5. R.Averbeck et al., accepted for publication in Z. Phys. A (1997) 6. A.Marin, PhD Thesis, University of Valencia, Spain, 1996 7. M.Appenheimer, PhD Thesis, University Giessen, Germany, 1997 8. A.R.Wolf, PhD Thesis, University Giessen, Germany, 1997 9. R.Novotny et al., Nucl. Instr. and Meth. in Phys. Res. A262 (1987) 340 10.J.Cugnon et al., Nucl. Phys. A360 (1981) 444 ll.V.Metag, Prog. Part. Nucl. Phys. 30 (1993) 75 12.R.Stock, Phys. Rep. 135 (1986) 259 13.N.Hermann, Nucl. Phys. A 610 (1996) 49 14. Gy.Wolf, Nucl. Phys. A522 (1993) 549