Dilepton and $\phi$ meson production in elementary and nuclear collisions at the NICA fixed target experiment

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by  Gy. Wolf \

EPJ manuscript No.
(will be inserted by the editor)
Dilepton and φ meson production in elementary and nuclear
collisions at the NICA fixed target experiment
Gy¨orgy Wolf and Miklo´s Z´et´enyi
Institutefor Particle and Nuclear Physics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, P.O. Box 49,
H-1525 Budapest, Hungary
7
Received: date/ Revised version: date
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2 Abstract. We argue that the NICA fixed target experiment will be able to provide very important new
experimental data on dilepton and φ meson production in the basically undiscovered energy domain be-
n
tween theSISand SPSenergies. Experimentalinformation about elementary cross sections inthisenergy
a
J region is an essential ingredient of models of nuclear collisions in the same energy range.

1
PACS. 13.75.Cs Nucleon-nucleon interactions 25.75.DwParticle and resonance production
1

]
h Currently, the strongly interacting matter can be ac- in the first step of the collision a baryon resonance is ex-
t
- cessed experimentally at low density (RHIC/Brookhaven cited, which later decays – possibly in multiple steps –
cl andLHC/CERN)andatnormalnucleardensity(ordinary and creates the final state particles. These kinds of mod-
u nuclear physics). Its properties at high densities, where els were used to study the dilepton spectrum of nucleon-
n thecriticalendpointprobablysits,arenotknown,neither nucleon collisions in connection with the DLS data (see
[ experimentally nor theoretically. Therefore it is very im- Ref. [6]). However, a more or less satisfactory agreement

portantthatnewacceleratorsanddetectors(NICA/MPD between theory and experiment has been reached only
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v andFAIR/CBM))aregoingtobebuilttostudytheprop- with the HADES data and the new calculations in terms
7 erties of the dense, strongly interacting matter. of effective field theory (EFT) models [7]. A similar EFT
5 The chiral symmetry is expected to be restored at modelhasbeenrecentlyappliedtodileptonproductionin
4 high densities. The path to the restoration is not known, pion-nucleon collisions [8].
3 andmaybe observedbymeasuringmesonproperties,like The spectrum of baryon resonances is known only up
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masses and widths. to slightly above 2 GeV, with increasing uncertainty at
.
1 Dileptonsareanimportantprobeofrelativisticnuclear high masses. The resonance model cannot be applied to
0 collisions, because they leave the hot and dense phase of collisionsabovetheSISenergyrangenotonlybecausethe
7 the collision unaffected by strong final state interactions baryonresonance spectrum is unknown, but also because
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and can be used to observe vector meson properties in multiparticle final states become more and more impor-
:
v dense matter via their direct decays to dileptons. On the tant.Thecalculationofmultiparticleproductionprocesses
i otherhand,thedileptonspectrumobtainedfromanuclear in EFT models is complicated by the multi-dimensional
X
collision is a complicated superposition of many produc- phase-space integrals, and many different Feynman dia-
r tion channels coming from various stages of the process. gramscontributingtothesamefinalstate.Thesediagrams
a
Therefore, drawing any reliable conclusions based on the havetobeaddedcoherently,whichresultsinalargenum-
dileptonspectrumisdifficult,anddependsonunderstand- ber of interference terms.
ing many aspects of the underlying physics, like elemen- Heavy ion collisions in similar energy ranges are usu-
tary cross sections, in medium modification of particles, ally described by hadronic transport models. An ingre-
reaction dynamics, etc. dient of such models is a description of the elementary
Inrecentyearsdileptonproductionwasstudiedexper- hadronic reactions. Normally, a variant of the resonance
imentally at RHIC, SPS, and at much lower energies at dominance model serves this purpose in hadronic trans-
GSI SIS by the HADES experiment and theoretically by port codes. The reason of this is that EFT model cal-
severaltheoreticalgroupsaswell,afewofthemtomention culations contradict the philosophy of transport, where
here: [1,2,3,4,5].TherangebetweentheSISandSPSen- (baryonandmeson)resonancespropagateasordinarypar-
ergiesisbasicallyundiscovered,andtheNICAfixedtarget ticles. This prohibits the implementation of certain types
experiment will give a unique opportunity to study dilep- of interference terms that appear naturally in EFT mod-
ton production in this regime. els.
Atlowenergieselementaryhadroncollisionswereusu- Figure 1 shows the predictions of a BUU type trans-
ally described by the resonance dominance model, where port model (for some of the details see [1,2,9,10]) for the
2 Gy.Wolf, M. Z´et´enyi:Dilepton and φ meson production...
1e+06 Table 1. The probability of creating a φ meson for various
Au+Au 10 AGeV ωη DD channelsinaAu+Aucentralcollisonat10AGeVbombarding
pn energy (B and N denote a non-strange baryon and a nucleon,
ρ
100000 ω0 respectively).
V] R Dφ
Ge channel contribution
b/
µM [ 10000 BB → NN φ 8.910−3
σ/d π B → N φ 1.4410−3
d (ρ,ω) B → N φ 6.3010−4
1000 K+K− → φ 8.5710−2
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1
M [GeV]
Fig. 1. Contributions of various elementary channels to the
dilepton invariant mass spectrum of Au+Au collisions, at 10 spectral function can be studied on the dilepton invari-
AGeV laboratory kinetic energy, as predicted by a hadronic ant mass spectrum. Furthermore, as a particle contain-
transport model. inghidden strangeness,it decaysdominantlyto the kaon-
antikaon channel. The simultaneous study of both decay
channels can contribute to a deeper understanding of the
underlying physics. In medium modification of antikaon
variouscontributions to the dilepton invariantmassspec- mass can lead to a broadening of the φ. At low energy
trum of a central Au+Au collision in the NICA energy heavy ion reactions, collision of secondary particles plays
range of 10 AGeV. Above 0.6 GeV dilepton mass the animportantroleintheproductionofφmesons(see[11]),
spectrum is dominated by the direct decay of the neu- therefore it’s sensitive to the reaction dynamics, the EOS
tral vector mesons ρ, ω and φ. For lower dilepton masses etc.
the Dalitz decays are the most important source of dilep- Inahadronicfireballcreatedinmoderateenergyheavy
tons. In particular, around 0.4-0.5 GeV the Dalitz decay ion collisions, φ mesons can be created in baryon-baryon,
of baryon resonances is expected to be very important. meson-baryon,and in meson-meson collisions. Of the lat-
Thisis becauseatthese higherenergiesalargefractionof ter, obviously kaon-antikaon collisions are the most im-
the nucleons are excited to the resonance states. portant. The contributions of these channels to φ meson
Sincethespectrumofhigherbaryonresonancesispoor- production in central Au+Au collisions at 10 AGeV lab-
ly known,the predictions for contributions oftheir Dalitz oratory kinetic energyare shown in Table 1. The elemen-
decay are also rather uncertain (see [12]). These reso- tary cross sections are obtained from one boson exchange
nances also participate in the production of the vector models.Contributionsofhigherbaryonresonancesarein-
mesons,whichbringsinuncertaintiesinthe vectormeson cluded, too. They actually dominate the baryon-baryon
channelsofdileptonproduction.Furthermore,thehighest contributons.Theircrosssectionisnotknown,canonlybe
baryon densities (approximately 6 times the normal nu- extrapolatedfromnucleon-nucleoncrosssections.Mostof
clear density at 10 AGeV) are expected to be reached in theφmesonsoriginatesfromK+K− annihilations.There-
heavy ion collisions in the energy range studied at NICA. foreanyin-mediumeffectsonthekaonandantikaonprop-
Dilepton spectra will be influenced by the modificationof erties, eg. on the masses, heavily influences the dilepton
hadron spectral functions in the dense medium. In addi- spectra.
tion,multiparticlefinalstateswillcontributiontodilepton The energy at SIS was not high enough to study the
production as well. These are usually described in trans- φ meson in detail. In particular, it was not seen in the
port codes in terms of string fragmentation models. dileptonchannel.Onlyafewmeasurementshavebeenper-
This shows that there must be a transition in the ap- formedatSIStostudyφproductionviatheK+K− chan-
plied theoretical models just above the SIS energy range, nelinsubthresholdheavyioncollisions[13].Theseresults
with probably a pure EFT model (or in transport codes by the FOPI collaboration indicate a large φ production
a resonance dominace model) for lower energies, supple- crosssection,whichis notexplainedby the transportcal-
mented by a string fragmentation model the importance culations of Ref. [11].
of which is increasing with energy. It is clear that experi- At higher energies the φ meson peak in the dilepton
mentalinput aboutelementarycrosssectionsis neededin spectrum is expected to be more pronounced, which can
order to test and calibrate the theoretical models at this be seen in the BUU predictions of Fig. 1. This means,
regime of transition. The NICA fixed target experiment that the NICA fixed target experiment would be able to
is an ideal possibility to provide these important experi- study the φ meson in both decay channels and, therefore,
mental data. could contribute to a better understanding of φ produc-
Another reason why the energy range of the NICA tionaroundthe kinematicalthreshold.Since the φ meson
fixed target experiment is interesting is φ meson produc- productionisdominatedbyK+K−,experimentsatNICA
tion. φ,asaneutralvectormesonisinterestingbecauseit energies would enable us to study uniquely kaon and an-
decays directly to the dilepton channel and, therefore, its tikaonproperties ina verydensemedium (6–10times nu-
Gy.Wolf, M. Z´et´enyi:Dilepton and φ meson production... 3
clear matter density), which may help us understand the
interior of a neutron star [14].
References
1. Gy. Wolf, G. Batko, W. Cassing, U. Mosel, K. Niita, M.
Sch¨afer, Nucl.Phys.A517 615 (1990)
2. Gy. Wolf, W. Cassing, U. Mosel, Nucl.Phys.A552 549
(1993)
3. O. Linnyk, E. L. Bratkovskaya, and W. Cassing, Progress
in Particle and Nuclear Physics, 87, 50-115 (2016).
4. Stephan Endres, Hendrik van Hees, Marcus Bleicher,
arXiv:1512.06549.
5. H. van Hees, R.Rapp,Nucl.Phys.A827 341c-346c (2009)
6. E.L. Bratkovskaya, W. Cassing, M. Effenberger, and U.
Mosel, Nucl. Phys. A 653, 301 (1999); C. Ernst, S.A. Bass,
M. Belkacem, H. St¨ocker, and W. Greiner, Phys. Rev. C
58, 447 (1998); M. Z´et´enyi and Gy. Wolf, Phys. Rev. C 67,
044002 (2003).
7. R. Shyam and U. Mosel, Phys. Rev. C 67, 065202 (2003);
82, 062201(R) (2010); L.P. Kaptari and B. K¨ampfer, Phys.
Rev.C 80, 064003 (2009).
8. M. Z´et´enyi, Gy.Wolf, Phys.Rev. C86, 065209 (2012).
9. H. Schade,Gy.Wolf, B. Kampfer, Phys.Rev.C81, 034902
(2010).
10. H.W.Barz, B. Kampfer,Gy.Wolf, M. Zetenyi,The Open
Nuclear and Particle Physics Journal 3 (2010) 1-15.
11. W.S.Chung,G.Q.Li,andC.M.Ko,Nucl.Phys.A625347
(1997); H. W. Barz, M. Z´et´enyi, G. Wolf, and B. K¨ampfer,
Nucl.Phys. A 705, 223 (2002).
12. M. Z´et´enyi, Gy. Wolf, Heavy Ion Phys. Acta Phys. Hun.
17 27 (2003)
13. N. Herrmann, FOPI Collaboration, Nucl. Phys. A 610
(1996) 49c; R. Kotte, FOPI Collaboration, in: M. Buballa
et al. (Eds.), Proc. Int. Workshop XXVIII on Gross Prop-
erties of Nuclei and Nuclear Excitations, Hadrons in Dense
Matter, Hirschegg, Austria, January 1622, 2000, p. 112.
14. T. Klahn, D. Blaschke, F. Weber, Phys. Part. Nucl. Lett.
9 (2012) 484-487