Feasibility Study of ϕ (1020) Production at NICA/MPD
FFeasibility Study of φ (1020) Production at NICA/MPD L. S. Yordanova and V. I. Kolesnikovfor the MPD Collaboration
Veksler and Baldin Laboratory of High Energy Physics,Joint Institute for Nuclear Research,141980 Dubna, Moscow region, Russiae-mail: [email protected]
Abstract.
The goal of this article is to give information about the new accelerator complex NICA at JINR, to provide overviewof the MultiPurpose Detector (MPD) and its subdetectors and to present the current results of the MPD performance for φ -meson production. In our study we use the channel decay Φ → K + K − to detect the formation of the φ -meson. UrQMD eventgenerator is used and central events at √ s = 11 GeV are analyzed. The obtained peak from the invariant mass distribution isfitted by a Breit-Wigner function. The calculated values of the parameters are consistent with the values given in literature.This study shows that the measurement of φ -mesons is feasible at NICA/MPD. Keywords:
NICA/MPD, strange mesons, φ (1020) production PACS:
1. INTRODUCTION
The Nuclotron-based Ion Collider fAcility (NICA) is a new accelerator complex being constructed at JINR, Dubna,Russia (Fig. 1). The global scientific goal of the NICA/MPD project is to explore the phase diagram of stronglyinteracting matter in the region of highly compressed baryonic matter. The study of hot and dense baryonic matterprovides relevant information on the in-medium properties of hadrons and nuclear matter equation of state; allowsa search for deconfinement and/or chiral symmetry restoration, phase transition, mixed phase and critical end-point,possible strong P- and CP violation; gives information about the evolution of the Early Universe and the formation ofneutron stars [1].NICA’s aim is to provide collisions of heavy ions over a wide range of atomic masses, from Au+Au collisions at √ s NN = 11 GeV (for Au + ) and an average luminosity of L = cm − s − to proton-proton collisions with √ s pp = 20 GeV and L ∼ cm − s − . Two interaction points are foreseen at NICA which provide a possibility for twodetectors to operate simultaneously - MultiPurpose Detector (MPD) and Spin Physics Detector (SPD). This overviewis focused on the MPD detector.The MPD experimental program includes simultaneous measurements of observables that are presumably sensitiveto high nuclear density effects and phase transitions.The goal to start energy scan as soon as the first beams are availableand the present constraints in resources and manpower lead to the MPD staging. In the first stage of the project (startingin 2017) are considered - multiplicity and spectral characteristics of the identified hadrons including strange particles,multi-strange baryons and antibaryons; event-by-event fluctuations in multiplicity, charges and transverse momentum;collective flows (directed, elliptic and higher ones) for observed hadrons. In the second stage (starting in 2020) theelectromagnetic probes (photons and dileptons) will be measured. It is proposed that along with heavy ions NICA willprovide proton and light ion beams including the possibility to use polarized beams [2].The software of the MPD project is responsible for the design, evaluation and calibration of the detector; the storage,access, reconstruction and analysis of the data; and the support of a distributed computing infrastructure.The softwareframework for the MPD experiment (MpdRoot) is based on the object-orientated framework FairRoot (developed atGSI) and provides a powerful tool for detector performance studies, development of algorithms for reconstruction andphysics analysis of the data [3]. Extended set of event generators for heavy ion collisions is used (UrQMD, LAQGSM,HSD). a r X i v : . [ phy s i c s . i n s - d e t ] N ov IGURE 1.
NICA accelerator complex at JINR.
2. OVERVIEW OF THE MPD DETECTOR
The detector for exploring phase diagram of strongly interacting matter in a high track multiplicity environmenthas to cover a large phase space, be functional at high interaction rates and comprise high efficiency and excellentparticle identification capabilities. The MPD detector matches all these requirements (Fig. 2). It consists of centraldetector (CD) and two optional forward spectrometers (FS-A and FS-B). The central detector consists of a barrelpart and two end caps. The barrel part is a set of various subdetectors. The main tracker is the time projectionchamber (TPC) supplemented by the inner tracker (IT). IT and TPC have to provide precise tracking, momentumdetermination and vertex reconstruction. The high performance time of flight (TOF) system must be able to identifycharged hadrons and nuclear clusters in a broad pseudorapidity range. The electromagnetic calorimeter (ECAL) shouldidentify electrons, photons and measure their energy with high precision. The zero degree calorimeter (ZDC) shouldprovide event centrality and event plane determination, and also measurement of the energy deposited by spectators.There are also a straw-tube tracker (ECT) and a fast forward detector (FFD).The magnet of MPD is a solenoid with a thin superconducting NbTi winding and a flux return iron yoke. The magnetshould provide a homogeneous magnetic field of 0.5 T. The field inhomogeneity in the tracker area of the detector isabout 0.1%.The MPD time projection chamber (TPC) is the main tracking detector of the central barrel and has to providecharged particles momentum measurement with sufficient resolution (about 2% at pt=300 MeV/c), particle identi-fication and vertex determination, two track separation (with a resolution <1 cm) and dE/dx measurement (dE/dxresolution better than 8%) for hadronic and leptonic observables at pseudorapidities | η | < . ∼ µ m . ITS should also identify relatively rare events with production of hyperons. ITS is based on the SiliconStrip Detector (SSD) technology. It consists of a silicon barrel and discs which will register particles with largepseudorapidity | η | < . | η | < .
0, high combined geometrical anddetection efficiency (better than 80%), identification of pions and kaons with 0.1< pt < 2 GeV/c and (anti)protons with0.3 < pt < 3 GeV/c. The choice for the TOF system is Multigap Resistive Plate Counters (MRPC) which have good
IGURE 2.
General view of the MPD detector. time resolution of δ < ps . The barrel covers the pseudorapidity region | η | < . | η | <3.0.The fast forward detector (FFD) should provide fast determination of a nucleus-nucleus interaction in the center ofMPD, generation of a start pulse for TOF and production of L0-trigger signal. The proposed FFD design is a granulatedCherenkov detector which has a high efficiency for the high energy photons and for ultra-relativistic charged particlesas well. Its acceptance in pseudorapidity is 2.0 (cid:53) | η | (cid:53) | η | < 2.2. ECT has high track reconstruction efficiency and momentum resolution about 10%.ECT consists of two end cap parts which are made with modules, containing layers of straw tubes.The primary role of the electromagnetic calorimeter (ECAL) is to measure the spatial position and energy of elec-trons and photons. ECAL should have a high segmentation, should provide good space resolution, energy resolutionabout 3% and should allow a separation of overlapping showers. The Pb-scintillator ECAL of the "shashlyk" type willbe used.The zero degree calorimeter (ZDC) should provide a classification of events by centrality, event plane determinationand measurement of the energy deposited by spectators. ZDC consists of modules of 60 lead-scintillator tile "sand-wiches" with lead and scintillator plates. The light readout is provided by the wave-length shifting fibers (WLS-fibers)and the micropixel avalanche photodiodes (MAPDs).More detailed description of the MPD detector can be found in the ’MPD Conceptual Design Report’ [4].
3. MOTIVATION FOR STUDY OF THE Φ -MESON The φ vector meson is the lightest bound state of hidden strangeness, consisting of a quark-antiquark pair. Althoughit is a meson, it is heavy in comparison with mesons consisting of u and d quarks, having a mass ( m φ = . ± . MeV / c ) comparable to the proton and Λ baryons. The φ -meson is expected to have a very small cross-section forinteractions with non-strange hadrons. Its observables should remain largely undisturbed by the hadronic rescatteringphase of the system’s evolution. The φ -meson has a relatively long life-time of ∼
46 fm/c which means that it willmostly decay outside the fireball and its decay daughters won’t have enough time to rescatter in the hadronic phase.Previous experimental measurements of the φ /K- ratio as a function of centrality showed the possibility of φ productionvia K + + K − coalescence in the hadronic stage. The observed ratios are flat as a function of centrality. These propertiesmake the φ -meson an excellent probe of the hot and dense medium created in nucleus-nucleus collisions [5, 6, 7].easurements of the production of strange particles such as the φ -meson can provide important information on theproperties of the medium and particle production mechanisms in ultra-relativistic Au-Au collisions. Measurements ofthe φ -meson pT spectra and their dependence in terms of shape and normalization on centrality may shed light onthe constituents of the medium at the time of φ formation as well as the mechanism through which the φ -mesonsare formed. Further insight into mechanisms of particle production for strange particles compared to non-strangeparticles can be gained through measurement of the particle ratios of multistrange hadrons. The medium producedat NICA/MPD can be also probed by measuring the elliptic flow of the φ -meson. Since multistrange hadrons andparticles with hidden strangeness are assumed to freeze out early and undergo fewer interactions in the hadronic stage,their v2 signals should provide a clean signal from the early stage of the system’s evolution. Measurements of the φ -meson v2 can give information on the collectivity and possible deconfinement of the system in the early stage andcan serve to constrain different dynamical models of elliptic flow and particle production [8, 9].The φ -meson with its mass comparable to Λ and p and low interaction cross-section can be exploited as a verygood tool to probe the properties of the medium produced in nucleus-nucleus collisions at NICA/MPD. Φ -mesonobservables may help to distinguish further between different physical models of the dense medium and to enrich ourknowledge of the system created in ultra-relativistic heavy-ion collisions.
4. DATA SET
In our study we use the channel decay Φ → K + K − to detect the formation of the φ -meson. This channel is chosenbecause it has a high branching ratio (49.1%) and kaons are easy to detect. UrQMD event generator is used and gold-gold collisions are generated [10]. The number of the analyzed central events is 50 000 at √ s = 11 GeV. The detectorconfiguration used in the study includes the Time Projection Chamber (TPC) and the Time-Of-Flight system (TOF)for track reconstruction and particle identification in the pseudorapidity range | η | < 1.5.
5. RECONSTRUCTION OF Φ -MESONS5.1. Signal distribution The study of φ -meson production in central Au-Au collisions at √ s = 11 GeV is performed using the UrQMD eventgenerator. After the generation the events samples are transported through the MPD detector using the Geant 3 andGeant 4 transport packages.In this analysis, φ -mesons are measured through the channel decay Φ → K + K − . The kaon daughter particles areidentified through their ionisation energy loss (dE/dx ) in the Time Projection Chamber (TPC) and also by usingthe information from the Time-Of-Flight system (TOF). A selection of kaon pairs by track quality cuts and particleidentification (PID) is performed. The invariant mass of the kaon pairs is calculated by combination of all K + and K − pairs of the same event. Consequently, we obtain data which contain the signal of φ -meson with the background. It iscalled same-event or signal distribution. Since not all charged kaons in each event originate from φ -meson decays, the φ -meson signal extracted by this wayis above a large combinatorial background of uncorrelated pairs. This combinatorial background should be removedfrom the signal distribution. There are two methods for removing background events in case of mass reconstruction:mixed-event technique and same-event technique [11, 12]. In mixed-event technique to separate the signal from thebackground, pairs of K + and K − are generated from different events. Such pairs create the background and pairs of K + and K − from the same event give the signal. In same-event technique the background is generated from the pairsof K + K + and K − K − of the same event, while K + K − pairs of the same event give the signal. In our analysis we use themixed-event technique and the combinatorial background is estimated from the uncorrelated K + K − pairs. An invariantmass distribution is constructed using all positively charged kaon candidates from one event mixed with all negativelycharged kaon candidates from n other events where n can be chosen relatively high in order to reduce effects fromstatistical fluctuations. .3. Extraction and scaling of the combinatorial background The mixed-event background distribution is estimated by using a big number of events and it should be scaledbefore being subtracted from the signal distribution. The scaling can be performed by two different ways - scalingby the integral ratio and scaling iteratively. [5] In the scaling by the integral ration, the background distribution isscaled by the ratio of the integrals of the signal and the background distributions in a defined mass region. The massregion should not include the φ -meson mass peak. In the iterative scaling, the background distribution is scaled by theratio of the integrals of the signal and the background distributions in a fixed mass region including the φ -meson masspeak. The background distribution is then subtracted from the signal distribution and the remaining signal is fitted withBreit-Wigner function plus a straight line. The signal integral is the integral in the mass range minus the integral ofthe Breit-Wigner function. Then the ratio is recalculated and the background is rescaled. After scaling the backgrounddistribution, the raw φ -meson yields are obtained by subtracting the scaled mixed-event background distribution fromthe signal distribution. The obtained peak from the invariant mass distribution after the subtraction is then fitted by a Breit-Wigner functionand the characteristics of the φ -meson such as its mass and its width are found. The Breit-Wigner distribution isa continuous probability distribution. It is most often used to model resonances (unstable particles)in high energyphysics. The Breit-Wigner function has the following appearance: BW ( m inv ) = π AW ( m − m φ ) + ( W ) , (1)In this function A is the area of the distribution and W is the width. The measured values of the φ -meson mass andwidth are consistent, within the experimental resolution, with the particle data grop (PDG) values. The recent resultsof the φ -meson study are shown in the next section. This study shows that the measurement of φ -mesons is feasibleand we can expect detection of them when NICA/MPD will be put in operation. The recent results of the φ -meson study are shown in the following histograms (see Fig. 3, Fig. 4 and Fig. 5).In Fig. 3 an invariant mass signal distribution of the φ -meson is represented. The exact values of selection cuts forthe kaons used for the reconstruction of the φ -mesons are found by performing a scan over the whole set of selectioncriteria with a requirement to maximize the invariant mass peak significance. The invariant mass peak significance isdefined as S / (cid:112) ( S + B ) where S and B are total numbers of signal and background combinations inside ± σ intervalaround the peak position. For the current signal distribution it is - S / (cid:112) ( S + B ) = .
11. It is still seen that the peak ofthe signal is above a large combinatorial background, due to the uncorrelated kaon pairs.Fig. 4 shows a background estimation by using the mixed-event technique. The values of selection cuts correspondto those for the signal distribution. The background distribution has a smooth shape because of the relatively highnumber of events n chosen for calculating it.The final results for the raw yields of the φ -meson are shown in Fig. 5. The values of the parameters obtained by theBreit-Wigner fit (Width = 4.291 ± MeV / c and M inv = 1019.995 ± MeV / c ) are consistent with the valuesgiven in literature. All these preliminary results show that the measurement of φ -mesons is feasible at NICA/MPD.The next step of the φ -meson study is connected with detailed analyses of higher statistics and measurements of theelliptic flow. The properties of the φ -meson will be also studied in different types of collisions with different centrality. IGURE 3.
Signal distribution.
FIGURE 4.
Mixed-event background distribution.
FIGURE 5. Φ -meson invariant mass distribution after subtraction of the mixed-event background. . SUMMARY In conclusion, it should be said that the MPD detector has many advantages and meets all the ambitious physicsrequirements for exploring phase diagram of strongly interacting matter in a high track multiplicity environment. TheMPD detector’s advantages comprise coverage of a large phase space, functionality at high interaction rates, highefficiency and excellent particle identification capabilities.Measurements of the production of strange particles such as the φ -meson can provide important information onthe properties of the medium and particle production mechanisms in ultra-relativistic collisions. The φ -meson withits mass comparable to Λ and p and low interaction cross-section can be used to probe the properties of the mediumcreated at NICA/MPD. The study of the φ -meson, based on the current analyses, is feasible and gives quite goodresults. More detailed study of the φ -meson properties is included in the MPD physics programme. Therefore, NICAfacilities provide unique capabilities for studying fundamental properties of the theory of strong interactions (QCD). REFERENCES
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