Strange hadron production at SIS energies : an update from HADES

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15th International Conference on Strangeness in Quark Matter (SQM2015) IOP Publishing Journal of Physics: Conference Series 668 (2016) 012022 doi:10.1088/1742-6596/668/1/012022

S tra n g e h ad ron p r o d u c tio n at SIS en ergies: an u p d a te from H A D E S

M . L o r e n z 8,f,J. A d a m c z e w s k i-M u s c h 4, O . A r n o ld 10,9, E .T . A t o m s s a 15, C . B e h n k e 8, J .C . B e r g e r - C h e n 10,9, J. B ie r n a t 3, A . B la n c o 2,

C . B lu m e 8, M . B o h m e r 10, P. B o r d a lo 2, S. C h e r n e n k o 7, C . D e v e a u x 11, A . D y b c z a k 3, E. E p p le 10,9, L. F a b b ie t t i10,9, O. F a t e e v 7, P. F o n te 2,a, C . F r a n c o 2, J. F r ie s e 10, I. F r o h lic h 8, T . G a la ty u k 5,6, J. A . G a r z o n 17, K . G ill8, M . G o lu b e v a 12, F. G u b e r 12, M . G u m b e r id z e 5,6,

S. H a r a b a sz 5,3, T . H e n n in o 15, S. H la v a c 1, C . H o h n e 11, R . H o lz m a n n 4, A . I e r u s a lim o v 7, A . I v a s h k in 12, M . J u r k o v ic 10, B . K a m p fe r 6,c,

T . K a r a v ic h e v a 12, B . K a r d a n 8, I. K o e n ig 4, W . K o e n ig 4, B . W . K o lb 4, G . K o r c y l3, G . K o r n a k o v 5, R . K o t t e 6, A . K r a s a 16, E . K r e b s 8,

H . K u c 3,15, A . K u g le r 16, T . K u n z 10, A . K u r e p in 12, A . K u r ilk in 7, P . K u r ilk in 7, V . L a d y g in 7, R . L a lik 10,9, K . L a p id u s 10,9, A . L e b e d e v 13, L. L o p e s 2, T . M a h m o u d 11, L. M a ie r 10, A . M a n g ia r o t t i2, J. M a r k e r t8, V . M e t a g 11, J. M ic h e l8, C . M u n tz 8, R . M u n z e r 10,9, L. N a u m a n n 6, M . P a lk a 3, Y . P a r p o t t a s 14,d, V . P e c h e n o v 4, O . P e c h e n o v a 8,

V . P e t o u s is 14, J. P ie tr a s z k o 4, W . P r z y g o d a 3, B . R a m s t e in 15,

L. R e h n is c h 8, A . R e s h e t in 12, A . R o s t 5, A . R u s t a m o v 8, A . S a d o v s k y 12, P . S a la b u r a 3, T . S c h e ib 8, K . S c h m id t-S o m m e r fe ld 10, H . S c h u ld e s 8, P . S e llh e im 8, J. S ie b e n s o n 10, L. S ilv a 2, Y u .G . S o b o le v 16, S. S p a ta r o e, H . S t r o b e le 8, J. S t r o th 8,4, P. S t r z e m p e k 3, C . S tu r m 4, O. S v o b o d a 16, A . T a r a n to la 8, K . T e ila b 8, P. T lu s t y 16, M . T r a x le r 4, H . T s e r t o s 14, T . V a s ilie v 7, V . W a g n e r 16, C . W e n d is c h 4, J. W i r t h 10,9,

J. W u s t e n f e ld 6, Y . Z a n e v s k y 7, P. Z u m b r u c h 4

(HADES collaboration)

1I n s titu te of Physics, Slovak A cadem y of Sciences, 84228 B ratislava, Slovakia

2L IP -L aboratório de In stru m en taca o e Fisica E xperim ental de P artic u las , Coim bra, P o rtu g a l 3Smoluchowski In stitu te of Physics, Jagiellonian U niversity of Cracow, 30-059 K rakow , Poland 4GSI H elm holtzzentrum fair Schw erionenforschung Gm bH, 64291 D arm sta d t, G erm any 5Technische U niversitat D arm sta d t, 64289 D arm sta d t, G erm any

6In s titu t fur S trahlenphysik, H elm holtz-Z entrum D resden-Rossendorf, 01314 D resden, G erm any J o i n t In stitu te of N uclear Research, 141980 D ubna, R ussia

8In s titu t fur K ernphysik, G oethe-U niversitat, 60438 F rankfurt, G erm any

9Excellence C luster ’O rigin and S tru c tu re of th e U niverse’ , 85748 G arching, G erm any 10Physik D ep a rtm en t E12, Technische U niversitat M unchen, 85748 G arching, G erm any

11II.Physikalisches In stitu t, Ju stu s Liebig U niversitat Giessen, 35392 Giessen, G erm any 12In stitu te for N uclear Research, R ussian Academ y of Science, 117312 Moscow, R ussia 13In stitu te of T heoretical and E xperim ental Physics, 117218 Moscow, R ussia

14D ep a rtm en t of Physics, U niversity of C yprus, 1678 Nicosia, C yprus 15In s titu t de P hysique N ucleaire (UM R 8608), F-91406 O rsay Cedex, France

16N uclear Physics In stitu te , A cadem y of Sciences of Czech Republic, 25068 Rez, Czech Republic

17L abC A F. F. Fisica, Univ. de S antiago de C om postela, 15706 Santiago de C om postela, Spain

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b also a t E xtreM e M a tte r In stitu te EM M I, 64291 D arm sta d t, G erm any c also a t Technische U niversitat D resden, 01062 D resden, G erm any d also a t Frederick University, 1036 Nicosia, C yprus

e also a t D ipartim ento di Fisica and IN FN , U niversita di Torino, 10125 Torino, Italy f also a t U trecht University, 3584 CC U trecht, T he N etherlands

E-m ail: m .lo re n z @ g si.d e

A b s t r a c t. We present and discuss recent experim ental activities of th e H ADES collaboration on open and hidden strangeness pro d u ctio n close or below th e elem entary NN threshold. Special em phasis is p u t on th e feed-down from 0 m esons to antikaons, th e presence of th e S - excess in cold nuclear m a tte r and th e com parison of sta tistic a l m odel ra te s to elem entary p + p d a ta . T he im plications for th e in te rp re ta tio n of heavy-ion d a ta are discussed as well.

1. I n tr o d u c tio n

In th e last two decades stran ge hadrons have been considered to represent p a rticu la rly suitable probes of th e high d ensity phase of nuclear m a tte r produced in relativistic heavy-ion collisions.

For exam ple, th e relatio n of subthreshold-p ro du ced K + in light and heavy system s has been connected to th e nuclear equ atio n of s ta te (EOS) a t up to th re e tim es nuclear sa tu ra tio n [1, 2, 3], while phase space d istrib u tio n s and flow p a tte rn s are considered to be sensitive to th e in-m edium kaon-nucleon p o ten tial [4, 5, 6, 7].

Various aspects of strangeness p ro d uctio n a t energies available th e Schwerionen Synchrotron (SIS) a t GSI D a rm sta d t (up to kinetic beam energies of 2 A GeV) have been investigated by th e F O P I and KaoS experim ents and lately also by HA D ES (for reviews see [8, 9]). In co n trast to stran g e particle pro d uctio n in elem entary N N collisions, in heavy ion reactions m ulti-step processes as well as strangeness-exchange reactions like nA ^ N K - are possible. Hence for any detailed u n d erstan d in g of strangeness p ro d uction one has also to u n d e rsta n d strangeness dynam ics in th e hot and dense m edium . It is therefore im p o rta n t to m easure as m any particles w ith open or hidden strangeness as possible. O nly recently d a ta from F O P I [10] and HA DES [11] has come close to fulfilling th is requirem ent, and several still puzzling aspects have been revealed, e.g.:

• T he stre n g th of th e kaon-nucleon p o tential, as derived from th e com parison of d a ta to models, yields values varying by ab o u t 50% for different d a ta sam ples [12, 13, 14].

• T he discovery of n eu tro n stars w ith masses larger th a n 1.5 solar masses [15, 16] challenges th e previously e x tra cte d co n strain ts on th e softness of th e EO S [17].

• T he large 0 / K - ratio rep o rted in [18, 19] and th e resulting feed-down which affects significantly th e slope of th e K - questions th e com m on in te rp re ta tio n of a late r freeze- out of th e K - com pared to K + [20]. In addition, new m echanism s seem to be required to reproduce m icroscopically th e enhanced 0 prod uction [21, 22].

• A lthough tra n s p o rt codes do not reach chem ical equilibrium w ithin th e lifetime of such collisions [9], th e yields of produced hadrons can be well described w ithin th e fram ew ork of sta tistic a l m odels [11, 23], except for th e 2 - (see below).

• T he deep-subthreshold p ro d u ctio n of th e 2 - which exceeds th erm al m odel predictions by an o rder of m agn itu d e [24, 25], can up to now, only be reproduced by one m odel [26].

In th is p ap er we will review and investigate th e s ta tu s of th e last th re e aspects.

2. A n tik a o n s: s lo p e s , y ie ld s a n d fr e e z e -o u t

System atic investigations of th e KaoS collaboration in th e nineties revealed a sim ilar rise of kaon and antikaon yields w ith increasing cen trality of th e collision. In addition, th e inverse slope p aram eters of th e antikaons obtained from transverse sp ectra are system atically lower

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F ig u r e 1. Left panel: N orm alized rap id ity density d istrib u tio n s of th e th erm al K - and th e ones resulting of a 0 according to th e m easured yields in [19] . T he black solid line shows th e sum of th e two d istribu tio n s. R ight panel: Sim ulated transverse m ass sp ectra for K - coming from a th erm al source w ith a te m p e ra tu re of 89 MeV and those which stem from a 0 decay.

T he resulting cocktail sp ectrum is shown as a black solid line.

th a n those observed for th e kaons [8]. These findings have been in terp reted w ith th e help of tra n s p o rt m odels and th e following conclusion has been draw n: th e p ro d uction of kaons and antikaons is coupled via strangeness exchange reactions, e.g. n - + A ^ p + K - . As a consequence, th e antikaons experience a late r freeze-out th a n th e kaons and hence have steeper transverse spectra. F u rth erm o re people concluded th a t strangeness p ro duction close or below th e elem entary nucleon-nucleon (NN) threshold is very different to th e one in elem entary reactions [9]. T he antikaon ab u nd an ce has also been connected to th e reduction of th e effective antikaon m ass via th e stre n g th of th e strangeness exchange channels w ith an antikaon in th e final sta te in [27].

Newer d a ta from F O P I and HA D ES showed th a t for energies around j ^sn n = 2.6 GeV th e fraction of antikaons originating from a decay of th e 0 m eson is ab o u t 20% and is hence not negligible [18, 19]. T he resulting effect of th e 0 m eson feed-down on th e antikaon slopes has first been investigated in our previous work [20]. Sim ulating a cocktail of th erm al and antikaons resulting from 0 m eson feed-down, according to th e m easured co n tribu tion s in th e A r+ K C l system , we found th a t w ithin errors th e inverse slope of th e resulting K - sp ectra agrees w ith th e experim entally observed one, see Fig. 1. Hence th e different slopes of th e kaon and antikaon sp e ctra do not necessarily call for a sequential freeze-out of th e two kaon species b u t can be explained solely by 0 feed-down. M eanwhile this issue has been investigated in g reater d etail by th e F O P I co llaboration [28].

P relim in ary H A D ES A u + A u d a ta tak en a t j s NN = 2.4 GeV, w here th e com plete set of particles carrying open and hidden strangeness is produced below th e free nucleon-nucleon threshold, indicate a strong rise of th e 0 / K - ratio tow ards lower energies [29], as predicted in a statistical m odel calculation [19]. For m inim ization of system atic errors due to efficiency corrections and e x trap o latio n s in rapidity, ratios of th e corrected yields a t m id -rapid ity are used in th is analysis.

T he resulting K - / K + ratio can be directly com pared to th e previously o b tained system atics a t sim ilar energy. T he m easured ratio K - / K + fits into th e tre n d observed a t higher energies and ex tra p o la ted down to th e b eam energy of 1.23 A GeV, see left side of Fig.2: T he 0 / K -

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tow ards lower energies. T his rise can be reproduced in th e fram ew ork of a sta tistic a l model, if th e suppression of strangeness is handled by introducing a strangeness correlation radius R c w ithin which strangeness has to be exactly conserved [30]. In th is context it is im p o rta n t to realize th a t, as th e ^ conserves strangeness by definition, it is not suppressed by th e strangeness correlation p a ra m ete r in c o n trast to th e o th er particles containing strange quarks.

Recently, feed down from higher lying baryonic resonances have been included in U rQ M D in order to be able to describe th e energy dependence of th e - ratio also in a tra n s p o rt code [31].

T he au th o rs tu n ed th e m ass depending branching ratio of high lying baryon resonances, nam ely th e N *(1990), N *(2080), N *(2190), N*(2220) and N *(2250), in a tra n s p o rt code to m atch

elem entary d a ta on (f) meson p roduction. As a result, th e - in A r+ K C l is successfully reproduced, as well as th e shape of th e excitatio n function. A t higher energies th e m odel

und ersho o ts th e d a ta and th e p relim inary A u + A u d a ta point is also not fully reproduced.

It will be very interesting, as soon as th e final d a ta becomes available, to com pare besides th e pure yields, th e shape of th e transv erse particle spectra, as th e y are strongly depen den t on the co n trib u tio n from resonances as investigated in [32] for kaons w ith respect to th e sensitivity of th e sp ectral shape to th e kaon nucleon potential.

F ig u r e 2. Left: K inetic beam energy excitation function of th e K - / K + ratio for various colliding system s. D a ta is tak en from [8, 19]. T he prelim inary ratio a t m id rap id ity ex tra cte d in th is work, fits nicely to th e tren d . Right: T he 0 / K - ratio shows a flat tre n d a t high energies and a sh arp rise tow ards lower energies, which can be explained w ithin th e sta tistic a l m odel fram ew ork using a pro p er strangeness correlation radius R c. T he d a ta is tak en from [19].

3. S t a tis tic a l e q u ilib r iu m r a te s

In th e last 30 years sta tistic a l had ro nization m odels have been established as a successful tool to fit particle yields or yield ratios from relativistic and u ltra re la tiv istic heavy-ion collisions [33, 34, 35] w ith only a few p aram eters. T he e x tra cte d freeze-out p aram eters show a striking regularity, lining up on a curve in th e tem p eratu re-baryoch em ical p o ten tia l plane, connecting sm oothly d a ta from th e GeV to th e TeV regim e [36] . These findings have been widely in ter

preted as a h int th a t (local) chem ical equilibrium is achieved in such collisions.

Since the days of H agedorn [37], sta tistic a l m ethods have also been used to predict particle p ro d u ctio n in elem entary reactions. R ecently two groups applied th e sam e model, which suc

cessfully describes hadro n yields in heavy-ion collisions, also for elem entary reactions b u t reached very different conclusions. W hile th e a u th o rs of [38, 39] found good agreem ent for yields and even transv erse m om entum sp e ctra o b tain ed in elem entary e+ + e - , th e a u th o rs of [40] found significantly worse agreem ent betw een m odel and experim en tal yields.

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F ig u r e 3. T he up

p er plot shows th e yields of secondary hadrons in p + p re

actions (filled red circles) and th e cor

responding T H E R - MUS fit values (blue bars) as well as th e obtained fit p a ra m eters. T he lower plot shows th e ratio of th e experim ental value and th e s ta tis tical m odel value.

In order to investigate th is issue, we use th e yields of hadrons produced in elem entary p + p collisions a t j ^{s n n}= 3.2 GeV m easured w ith HA DES, which have recently become available [41, 42, 43]. We apply a sim ilar fit as in our previous work [11] b u t use an u p d a te d version of TH ER M U S (v3.0) [44]. We use th e m ixed canonical ensem ble w here strangeness is exactly conserved while all o th er q u a n tu m num bers are calculated grand canonically and co n strain th e charge chem ical p o ten tia l ^^qusing th e ratio of th e baryon and charge num bers of th e collision system .

We sta te th a t th e yield of th e 0 m eson is of p a rticu la r interest, because of its different sensi

tiv ity to th e strangeness suppression p aram eters ys and R c. As th e 0 conserves strangeness by definition as a ^{s s}s ta te its yield is not suppressed in th e R c form alism , while strongly suppressed w hen Y^sis used. We found in [11] th a t th e 0 yield is well described using R c and therefore stick to this way of suppressing strang e particle yields in our sta tistic a l m odel calculations.

We sim ultaneously fit th e yields of th e n e u tra l pions, th e n, th e w and th e kaons, as well as th e m ean num ber of p a rticip a n ts (A^{p a r t}) and co n strain th e charge chem ical p o ten tial ^^qby th e initial charge to baryon ratio. We find th e following values for chem ical freeze-out param eters:

T^{c h e m} = (100 ± 6) MeV, ^^b = (560 ± 29) MeV, th e strangeness correlation radius results as R c = (0.8 ± 0.4) fm and th e radius of th e whole fireball R = (2.0 ± 0.4) fm w ith %2/d .o .f.= 2 .9 . A detailed com parison of th e d a ta w ith our th e sta tistic a l m odel fit is shown in th e u p p e r p a rt of Fig. 3, while th e lower p a rt of th is figure depicts th e ratio of d a ta . T he overall agreem ent betw een m odel and d a ta is com parable to th e outcom e of our previous work w here we fitted h ad ro n yields obtain ed from A r+ K C l collisions at j s NN = 2.6 GeV. T his is ra th e r surprising as one naively expects a larger am ount of th erm alizatio n in th e larger A r+ K C l system and hence less deviation from sta tistic a l equilibrium values. F urtherm ore, th e p + p freeze-out point fits quite well to th e previously observed regularity of freeze-out points in th e T — plane, displayed in Fig. 4, w here th e e x tra cte d point of this work are displayed to g eth er w ith sim ilar points ex tra cte d in [45] and [36].

T his brings us back to th e in tro d u ctio n ; while th e success of th e sta tistic a l m odel in describing particle rates from heavy-ion collision is often im plicitly connected to a th erm alizatio n of th e created system , th e success of th e m odel in describing th e p + p d a ta at j s NN = 3.2 GeV strongly questions th is connection.

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F ig u r e 4 . Freeze-out po ints in th e T — plane o b tain ed from sta tistic a l m odel fits to h adron yields or yield ratio s [45] (filled sym bols) an d [36] (open sym bols).

In additio n , th e HA DES points for A r+ K C l [11] (blue triangle) and the p + p point (cyan triangle) from this work are displayed. T he d o tte d line corresponds to a com m on freeze- o u t condition of 1 GeV fixed energy p er baryon.

F ig u r e 5. Invariant m ass spec

tru m of A hyperons and negative pions o b tain ed in p + N b collisions a t y / s NN = 3.2 GeV. A clear peak corresponding to th e s - signal is visible.

R ecent d a ta of HA DES revealed th a t th e excess of th e S - over several s ta te of th e a rt m odels shows up also in p + N b reactions a t j s n n = 3.2 GeV [46]. T he e x tra cte d experim ental signal in th e invariant m ass sp e ctru m is displayed in Fig. 5. In ad ditio n, th e energy ex citatio n function of th e S - L am bd a ra tio for various colliding system s is displayed in Fig. 6. W hile th e open sym bols correspond to heavy-ion d a ta , th e filled sym bols represent p ro to n induced collisions on nuclei. T he black line corresponds to a sim ple p aram eterizatio n fitted to th e p ro to n induced d a ta of th e form

t r i g\ h

f ( j s n n ) = C x ^ - [ j

^J

^j ^. ⁽¹⁾

T he inlay shows a zoom to th e a rea of th e HA DES p + N b d a ta p o in t an d th e com parison to three different models. T h e u p p er sta r sym bol corresponds to th e yield o b tain ed from a TH ER M U S (v2.3) fit to p + N b d a ta sim ilar to th e one discussed in th is work. W hile th e lower s ta r sym bol corresponds to th e yield o b tain ed via th e UrQ M D (v3.4) tra n s p o rt code, th e in term ediate point results from a GiBUU tra n s p o rt calculation.

T he presence of th e s - excess in cold nuclear m a tte r has several interesting im plications for th e in te rp re ta tio n of th e heavy-ion d a ta as its origin seem s to be alread y in th e elem entary channels w ith o u t th e involvem ent of m any b o d y effects in th e m edium [26, 47]. Therefore, th e increased cross sections of strangeness exchange reactions, which were found to be sufficient to explain the high yield in, seem to be questionable as th e y are highly unlikely to play an im p o rta n t role in

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p + N b reactions. Also, th e invoked [23] c ataly tic strangeness p ro d uctio n by secondary processes like n + Y ^ 2 + K are strongly suppressed in cold nuclear m atte r.

F ig u r e 6. E x c ita tio n function of th e 5 - /A ratio for various colliding system s.

T he open symbols correspond to heavy-ion d a ta , th e filled sym bols represent d a ta from p + A collisions. T he black line corresponds to a sim ple p aram eterizatio n fitted to th e p + A d a ta , see te x t for details. T he inlay shows a zoom to th e area of th e HA DES p + N b d a ta point and th e com parison to th re e different m odels. T he u p p e r sta r sym bol corresponds to a th e yield obtain ed from a TH E R M U S (v2.3) fit to p + N b d a ta . T he lower s ta r symbol corresponds to th e yield obtained via th e U rQ M D (v3.4) tra n s p o rt code and th e in term ed iate point a GiBU U tra n s p o rt calculation.

R ecently th e sam e m echanism of feed-down of th e high invariant m ass tails of baryonic res

onances used to explain th e 0 m eson yield by th e U rQ M D group was also used for th e 2 - hyperon. D ue to th e lack of elem entary d a ta th e m odel is tu n e d to m atch our p + N b d a ta [32].

A fter th is tu n in g it is not too surprising th a t th e m odel is able to describe th e excess observed in A r+ K C l w ith o u t fu rth e r tuning.

T he technique of m ass d ep end ent branching ratios for broad resonances has been successfully applied in order to describe th e dilepton sp e ctra a t low energies as pointed o ut in several p u b lications [48, 49]. Note, how even th a t tu n ed branching ratio s are still consistent w ith th e OZI rule th ere is yet no experim ental evidence for th e decay of th e N * resonance to final states containing a 0 m eson or a 2 hyperon.

5. S u m m a r y

We showed th a t for all th re e topics discussed here elem entary and cold nuclear m a tte r d a ta are needed for th e in te rp re ta tio n of heavy-ion d a ta . In p articular:

• T he observed large 0 / K - ratio and th e resulting feed-down lowers significantly th e slope of th e K - . Hence a late r freeze-out of antikaons com pared to kaons is not necessarily needed to explain th e different slopes of b o th kaon species. F u rtherm o re, th e rise tow ards lower energies can also be reproduced, a t least to some ex ten t, in tra n s p o rt codes after tu n in g of th e 0 cross sections on elem entary d a ta .

• We find th a t a sta tistic a l m odel fit to elem entary p + p d a ta is able to describe th e yields w ith th e sam e param eters as used for A r+ K C l w ith com parable accuracy and th is questions th e often im plicitly used connection to a therm alized system .

• T he presence of th e 2 - excess also in cold nuclear m a tte r rules o ut m ore exotic explanations involving m ultistep processes in heavy-ion collisions. Feed-dow n from high m ass tails of baryonic resonances rem ains a possible explan ation b u t a t th e m om ent it is not very well constrain ed by b o th th e experim ental d a ta and th e th eo retical tre a tm e n t.

T he collaboration g ratefully acknowledges su p p o rt by P T D C /F IS /1 1 3 3 3 9 /2 0 0 9 (LIP C oim bra), N C N g ran t 2 0 1 3 /1 0 /M /S T 2 /0 0 0 4 2 (SIP JU C Cracow), H elm holtz Alliance

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D a rm sta d t), 283286, 05P12C R G H E (H ZD R), H elm holtz Alliance H A 216/E M M I, HIC for FA IR (L O E W E ), (GSI F & E G oethe-U niversity F ran k fu rt) VH-NG-330, B M B F 06M T7180 (T U M unchen), G arching B M B F:05P12R G G H M (JL U Giessen, Giessen) U CY /3411-23100 (U niversity Cyprus) C N R S /IN 2 P 3, (IP N O rsay), O rsay M SM T LG 12007, AS C R M100481202, G A C R 13-06759S N P I AS CR, Rez, E U C o n tract No. HP3-283286. One of us (M.L.) acknowledges th e su p p o rt of th e H um boldt Foundation.

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