Production of hadronic resonances measured with ALICE at the LHC

Measurements of short-lived hadronic resonances are used to probe the properties of the hadronic phase in ultra-relativistic heavy-ion collisions. Since these resonances have lifetimes comparable to that of the produced fireball, they are sensitive to the competitive rescattering and regeneration effects in the hadronic gas, which modify the observed particle momentum distributions and yields after hadronization. Having different masses, quantum numbers, and quark content, hadronic resonances can provide insight into processes that determine the shapes of particle momentum spectra, strangeness production, and the possible onset of collective effects in small systems. We here present the latest results on ρ(770)0, K*(892), φ(1020), Σ(1385)±, Λ(1520), Ξ(1530)0 and Ξ(1820) production in pp, p–Pb, Pb–Pb and Xe–Xe collisions at different LHC energies. Results include system-size and collision-energy evolution of transverse momentum spectra, integrated yields, mean transverse momenta, particle ratios, and nuclear modification factors.


Introduction
Hadronic resonances with various lifetimes are valuable probes to study the properties of the hadronic medium formed in ultra-relativistic heavy-ion collisions, since the yield ratios of resonances to stable hadrons provide information about the re-scattering and regeneration effects in the hadronic medium. If the loss of resonances due to elastic or pseudo-elastic scattering of their decay products (re-scattering) is dominant over regeneration, the resonance yield after kinetic freeze-out will be smaller than the one originally produced at the chemical freeze out. Considering the expected lifetime of the hadronic phase (∼ 10 fm/c), the measurement of the production of a comprehensive set of resonances with different lifetimes can be used to study the interplay of particle re-scattering and regeneration.
Studies of strangeness production play an important role in understanding the matter produced in heavy-ion collisions. Measurements of strange and non-strange particle yields can be described by grand-canonical thermal models in heavy-ion collisions [1], while canonical suppression is expected in small systems [2,3]. Enhancement of strangeness production has been observed in high energy nucleus-nucleus (A-A) collisions with respect to pp collisions at RHIC (Relativistic Heavy-Ion Collider) and LHC (Large Hadron Collider) energies [4]. The study of strangeness production as a function of the charged particle multiplicity produced in the collision systems from pp to A-A, therefore, allows one to investigate the origin of the enhancement.

Experiment
The ALICE experiment [5] is particularly well suited to study the production of both identified and unidentified charged particles thanks to its excellent tracking performance coupled with extensive particle identification (PID) capabilities over a wide range of transverse momentum. In the central barrel, the Inner Tracking System (ITS) and the Time Projection Chamber (TPC) are used for charged-particle tracking and primary collision vertex reconstruction. The ITS consists of three sub-detectors of two layers each, covering a central pseudorapidity range |η| < 0.9: Silicon Pixel Detector (SPD), Silicon Drift Detector (SDD) and Silicon Strip Detector (SSD). The TPC is the main charged particle tracking detector, and has full azimuthal coverage in the pseudorapidity range |η| <0.9. Along with track reconstruction, it also provides a measurement of the momentum and excellent particle identification. The TPC provides the measured specific energy loss (dE/dx) to identify the particles, especially in low momentum range (p < 1 GeV/c) where the dE/dx of particles are well separated. To extend the particle identification to higher p T , the Time of Flight (TOF) detector is used in addition to the TPC information. It covers a pseudorapidity range |η| < 0.9 and provides excellent PID capabilities in the intermediate p T range by exploiting the time-of-flight information. The forward V0 detector, a scintillator detector with a timing resolution less than 1 ns, is used for centrality selection, triggering and beam-induced background rejection. The V0 consists of two sub-detectors, V0A and V0C, placed at asymmetric positions, one on each side of the interaction point with full azimuthal acceptance and cover the pseudorapidity ranges 2.8 < η < 5.1 and -3.7 < η < -1.7, respectively. The year and different collision energies for accumulated data in different collision systems are summarized in Table 1. The list of resonances that we report is provided in Table 2 with their quark content, decay modes exploited for the measurements, and branching ratios. 3. Results and discussion 3.1. Transverse momentum spectra The p T spectra for K * 0 and φ in the various multiplicity classes in pp collisions at √ s = 13 TeV are shown in Figure 1 with the ratios of these spectra to the inclusive INEL>0 spectrum [7]. Note that, the INEL>0 event class is defined as the set of inelastic collisions with at least one charged particle in the range |η| < 1. For p T ≤ 5 GeV/c the evolution of spectral shape from low to high multiplicity has been observed. For higher p T , the spectra in different multiplicity classes all have similar shape, indicating that the processes that change the shape of the p T spectra in different multiplicity classes are dominant at low p T .
ALI-PUB-338922 ALI-PUB-338935 Figure 1. p T spectra of K * 0 and φ in pp collisions at √ s = 13 TeV for different multiplicity classes, scaled by factors as indicated [7]. The lower panels show the ratios of the multiplicitydependent p T spectra to the multiplicity-integrated INEL>0 spectra (with both linear and logarithmic vertical scales) 3.2. p T integrated yields dN/dy and resonance particle ratios The p T -integrated yields dN /dy of K * 0 and φ are extracted from the p T spectra in different multiplicity classes and presented in Figure 2 as functions of dN ch /dη lab |η|<0.5 . For both particles, dN /dy exhibits an approximately linear increase with increasing dN ch /dη lab |η|<0.5 . Results for pp collisions at √ s = 7 and 13 TeV and for p-Pb collisions at √ s NN = 5.02 TeV follow approximately the same trends. This indicates that, for a given multiplicity, K * 0 and φ production does not depend on the collision system or energy. The measured dN /dy values are compared with five different model calculations: PYTHIA6 (Perugia 2011 tune) [15], PYTHIA8 (Monash 2013 tune, both with and without color reconnection) [16], EPOS-LHC [17], and DIPSY [18]. The EPOS-LHC and PYTHIA8 without color reconnection give the best description for the the K * 0 while the other PYTHIA calculations exhibit fair agreement with the measured data, and DIPSY overestimates the K * 0 yields. The φ yields tend to be slightly overestimated  Figure 2. p T -integrated yields dN /dy of K * 0 (average of the particle and antiparticle) and φ as functions of dN ch /dη lab |η|<0.5 . Results are shown for pp collisions at s = 13 and 7 TeV [7,3], as well as for p-Pb collisions at √ s NN = 5.02 TeV [8]. The measurements in pp collisions at s = 13 TeV are also compared with values from common event generators [18,17,15,16]. Bars represent statistical uncertainties, open boxes represent total systematic uncertainties, and shaded boxes show the systematic uncertainties that are uncorrelated between multiplicity classes.   ( dN ch /dη |η lab |<0.5 ) in Figure 3. The scaled integrated yield is flat with multiplicity and independent of collision energy and systems for pp and p-Pb collisions within the uncertainties. This confirms that the particle production is mainly driven by charged particle multiplicity irrespectively of collision systems and energies for pp and p-Pb collisions.  The ratios of p T -integrated yields as a function of multiplicity are shown in Figure 4 for pp, p-Pb, Pb-Pb and Xe-Xe collisions, for ρ 0 /π, K * 0 /K, Σ * ± /Λ, Λ(1520)/Λ, Ξ * 0 /Ξ and φ/K. Short-lived resonances show a sizable dependence on the multiplicity. A clear suppression is observed for ρ 0 /π going from pp to Pb-Pb collisions at √ s NN = 2.76 TeV [9]. The EPOS3 event generator [10], which includes UrQMD [11] for the late stage hadronic cascade is able to describe the evolution with multiplicity. The same behavior is observed in the K * 0 /K ratio, where the suppression can be observed in a wider multiplicity range with data from pp at √ s = 7 TeV, Pb-Pb at √ s NN = 5.02 TeV and Xe-Xe at √ s NN = 5.44 TeV.
The ratios of Σ * ± /Λ and Λ(1520)/Λ show flat behavior in small systems from pp and p-Pb collisions, and the results in Pb-Pb collisions show that Σ * ± and Λ(1520) are suppressed with respect to the Λ production [13]. Finally, there is no significant centrality dependence of the Ξ * 0 /Ξ and φ/K ratios with multiplicity for all measured systems. This is expected in the context of re-scattering, considering that the Ξ * 0 baryon and the φ meson live longer than the expected fireball lifetime and therefore their decay daughters will not undergo re-scattering.
Further, to quantify the p T -dependence of the rescattering effect observed in Pb-Pb collisions, p T -differential yield ratios were studied as shown in Figure 5 [14]. At low p T , the K * 0 /K ratio for central collisions is lower than in peripheral (pp) collisions whereas the φ/K ratio is comparable within the uncertainties. This observation is consistent with the suppression of K * 0 yields due It demonstrates that fragmentation is the dominant hadron production mechanism in this p T region.

Mean transverse momentum
The mean transverse momentum ( p T ) values for each multiplicity event class are obtained by integrating the p T -spectra in the measured range and by using a fit function to extrapolate the yields in the unmeasured p T region. Figure 6 shows the p T of proton, K * 0 and φ as a function of the average charged particle multiplicity density ( dN ch /dη ) measured at mid-rapidity (|η| For all the particles, an increase in p T from low to high multiplicity classes is observed. The same increasing trend of the p T as a function of the multiplicity is observed in pp collisions at √ s = 7 TeV and 13 TeV and a mass ordering of p T is found to be followed in central and semi-central Pb-Pb collisions as expected from the hydrodynamic expansion of the system [22]. However, this breaks down for smaller systems. The increase in p T is steeper for smaller systems.
different collision systems and different energies. The ratios of all strange particle yields to pions increase with dN ch /dη |η|<0.5 and saturate in larger collision systems such as Pb-Pb and Xe-Xe collisions. The magnitude of the increase depends on strangeness content. It is observed that the ratios are consistent at similar multiplicities in all collision systems at all LHC energies. In order to investigate the behavior of hidden strangeness, p T -integrated yield ratios of φ (|S|=0) to K (|S|=1) and Ξ (|S|=2) to φ (|S|=0) are compared as shown in Figure 8. The φ/K ratio shows flat or slightly increasing trend at lower multiplicities that suggests φ behaves like a S ≥ 1 particle. The ratio of Ξ/φ increases with increasing dN ch /dη lab |η|<0.5 for low-multiplicity collisions and is then fairly constant for a wide range of multiplicities from high-multiplicity pp and p-Pb collisions to central Pb-Pb collisions. The multiplicity evolution of φ/K and Ξ/φ ratios suggests that the φ has effective strangeness between 1 and 2 units.

Nuclear modification factors
The nuclear modification factor R AA is used to study medium-induced effects in heavy-ion collisions. The R AA is the ratio of the yield of a particle in nucleus-nucleus collisions to its yield in pp collisions. This ratio is scaled by the number of binary nucleon-nucleon collisions in each centrality class, which is estimated from Glauber model calculations. For each p T bin, The nuclear modification factors measured in 0-10% central Pb-Pb collisions at √ s NN = 5.02 TeV for for charged pions, charged kaons, (anti)protons, K * 0 and φ are presented on the left panel in Figure 9. And the R AA of ρ 0 meson [9] are reported with charged pions, charged kaons, (anti)protons in 0-20% (middle panel) and 60-80% (right panel) in Figure 9. At high transverse momenta (p T > 8 GeV/c), the production of all hadrons is suppressed by a similar amount and there is no dependence of the suppression on particle mass or quark content within uncertainties.   There is a species dependence of R AA at intermediate transverse momentum (2< p T <8 GeV/c), which is likely to be a result of an interplay between different effects such as radial flow, low-p T suppression, species dependent p T shapes of the pp reference spectra.

Reconstruction of Ξ(1820)
The first measurement of Ξ(1820) from collider experiments is shown in Figure 10. The calculation from FASTSUM Collaboration [25] shows potential parity doubling of strange resonant states, which could be a signature of chiral symmetry restoration in heavy-ion collisions. The positive-parity masses are largely temperature-independent while negative-parity masses drop and become near-degenerate with the corresponding positive-parity mass close to critical temperature. Because the Ξ(1820) has negative-parity, the comparison of mass shift, width broadening or change in yield to the positive-parity Ξ(1530) can be one of the key measurements to study chiral symmetry restoration.  10

Summary
A comprehensive set of resonance and strangeness production measurments in all available collision systems is presented. The p T -integrated yields normalized by the average charged particle multiplicity are observed to be constant as a function of charged particle multiplicity. The ratios of p T -integrated particle yields for various resonances are measured and ρ 0 /π, K * 0 /K and Λ(1520)/Λ exhibit a decrease from pp and peripheral Pb-Pb to central Pb-Pb collisions. This behavior can be explained by the dominance of re-scattering of decay daughters over regeneration in the hadronic phase. The increase of p T is observed from low to high multiplicity classes and a mass ordering of p T is found in central Pb-Pb collisions. The φ meson with hidden strangeness seems to have an effective strangeness between 1 and 2 units. The nuclear modification factors for resonances are suppressed at high p T for Pb-Pb collisions and the amount of suppression is similar to charged pions, charged kaons, (anti)protons. The first measurement of Ξ(1820) is presented and will be compared to the corresponding positive-parity particle to study chiral symmetry restoration.