Suppression of $\Lambda(1520)$ resonance production in central Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV

The production yield of the $\Lambda(1520)$ baryon resonance is measured at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV with the ALICE detector at the LHC. The measurement is performed in the $\Lambda(1520)\rightarrow {\rm pK}^{-}$ (and charge conjugate) hadronic decay channel as a function of the transverse momentum ($p_{\rm T}$) and collision centrality. The $p_{\rm T}$-integrated production rate of $\Lambda(1520)$ relative to $\Lambda$ in central collisions is suppressed by about a factor of 2 with respect to peripheral collisions. This is the first observation of the suppression of a baryonic resonance at LHC and the first evidence of $\Lambda(1520)$ suppression in heavy-ion collisions. The measured $\Lambda(1520)/\Lambda$ ratio in central collisions is smaller than the value predicted by the statistical hadronisation model calculations. The shape of the measured $p_{\rm T}$ distribution and the centrality dependence of the suppression are reproduced by the EPOS3 Monte Carlo event generator. The measurement adds further support to the formation of a dense hadronic phase in the final stages of the evolution of the fireball created in heavy-ion collisions, lasting long enough to cause a significant reduction in the observable yield of short-lived resonances.

√ s NN = 2.76 TeV [10] show that the production yields of the K * (892) 0 resonance are suppressed in central collisions with respect to peripheral collisions and are overestimated by SHM predictions. This phenomenon might be due to the short K * (892) 0 lifetime (τ ∼ 4 fm/c) and suggests the dominance of destructive re-scattering over re-generation processes in the hadronic phase. No suppression is observed for the longer-lived φ (1020) meson (τ ∼ 46 fm/c), indicating that it decays mostly outside the fireball. Both observations are in agreement with the calculations of EPOS3, a model that includes a microscopic description of the hadronic phase [11]. Within this model, the lifetime of the hadronic phase formed in central Pb-Pb collisions at √ s NN = 2.76 TeV is predicted to be ∼ 10 fm/c. The measurement of the production of the Λ(1520) baryonic resonance, owing to its characteristic lifetime (τ ∼ 12.6 fm/c), serves as an excellent probe to further constrain the formation, the evolution and the characteristics of the hadronic phase.
In this Letter, we present the first measurement of Λ(1520) production in Pb-Pb collisions at √ s NN = 2.76 TeV. The measurement is performed at midrapidity, |y| < 0.5, with the ALICE detector [12] at the LHC and is based on an analysis of about 15 million minimum-bias Pb-Pb collisions recorded in 2010. Previous results on Λ(1520) production in high-energy hadronic collisions have been reported by STAR in pp, d-Au and Au-Au collisions at √ s NN = 200 GeV at RHIC [13,14]. A detailed description of the ALICE experimental apparatus and its performance can be found in [15]. The relevant features of the main detectors utilised in this analysis are outlined here. The V0 detector is composed of two scintillator hodoscopes, placed on either side of the interaction point and covers the pseudorapidity regions 2.8 < η < 5.1 and −3.7 < η < −1.7, respectively. It is employed for triggering, background suppression and collision-centrality determination. The Inner Tracking System (ITS) and the Time-Projection Chamber (TPC) provide vertex reconstruction and charged-particle tracking in the central barrel, within a solenoidal magnetic field of 0.5 T. The ITS is a high-resolution tracker made of six cylindrical layers of silicon detectors. The TPC is a large cylindrical drift detector of radial and longitudinal dimensions of about 85 < r < 247 cm and −250 < z < 250 cm, respectively. Charged-hadron identification is performed by the TPC via specific ionisation energy-loss (dE/dx) and by the Time-Of-Flight (TOF) detector. The TOF is located at a radius of 370-399 cm and measures the particle time-of-flight with a resolution of   about 80 ps, allowing hadron identification at higher momenta. A minimum-bias trigger was configured to select hadronic events with high efficiency, requiring a combination of hits in the two innermost layers of the ITS and in the V0 detector. The contamination from beam-induced background is removed offline, as discussed in detail in [16,17]. The collision centrality is determined based on the signal amplitude of the V0 detector, whose response is proportional to the event multiplicity. A complete description of the event selection and centrality determination can be found in [18].
The Λ(1520) resonance is reconstructed via invariant-mass analysis of its decay daughters in the hadronic decay channel Λ(1520) → pK − (and charge conjugate, cc.), with a branching ratio of 22.5 ± 0.5% [19]. Particle and antiparticle states are combined to enhance the statistical significance of the reconstructed signal. Λ(1520) refers to their sum in the following, unless otherwise specified. Candidate daughters are selected from tracks reconstructed by the ITS and TPC, are required to have p T > 150 MeV/c and are restricted to the pseudorapidity range |η| < 0.8 for uniform acceptance and efficiency performance. Furthermore, a cut on the impact parameter to the primary vertex is applied to reduce contamination from secondary tracks emanating from weak decays or from interactions with the detector material. Details on the track selection can be found in [10]. Kaons and protons are identified from the combined information of the track dE/dx in the TPC and the time-of-flight measured by the TOF. The invariant-mass distribution of unlike-sign pairs of selected kaon and proton tracks is constructed for each centrality class and p T interval. The rapidity of the candidate Λ(1520) is required to be within |y| < 0.5. A large source of background from random combinations of uncorrelated pairs affects the invariant-mass spectrum. A mixed-event technique [20] is employed to estimate the combinatorial background, using unlike-sign proton and kaon tracks taken from different events with similar characteristics. The background is normalised and corrected for event-mixing distortions using a fit to the mixed/same-event ratio of like-sign pairs. Figure 1 shows examples of the reconstructed invariant-mass distribution after background subtraction. The Λ(1520) raw yield is then extracted by means of a global fit, where a Voigtian function (the convolution of the non-relativistic Breit-Wigner with the Gaussian detector resolution) is used to describe the signal. The shape of the residual background resembles that of a Maxwell-Boltzmann distribution, therefore the residual background is fitted with a similar functional form where B (normalization), m cutoff (low-mass cut-off), C and n are free parameters.
The raw yields are corrected for the decay branching fraction and for detector acceptance, reconstruction, track-selection and particle-identification efficiency, evaluated through a detailed Monte Carlo sim- (1520) + cc. The main sources of systematic uncertainty on the corrected yields are summarised in Tab. 1. They include the signal extraction procedures as well as the contributions related to the efficiency corrections (true p T distribution, track selection, particle identification, material budget and hadronic cross-section) and event normalisation. A significant fraction of this uncertainty, estimated to be about 12%, is common to all centrality classes. 0-20% 20-50% 50-80% Signal extraction 12 11 9 Global tracking efficiency 10 -10.5 10 -10.5 10 -10.5 TOF matching efficiency 1 -6.5 1 -6.5 0 -6. Uncorrelated 12 -11.5 11.5 -10.5 9.5 -9.5 Table 1: Main contributions to the systematic uncertainty of the Λ(1520) p T -differential yield in 0-20%, 20-50% and 50-80% centrality classes. The values are relative uncertainties (standard deviations expressed in %). When appropriate, they are reported for the lowest and highest measured p T bin. The total systematic uncertainty and the contribution uncorrelated across centralities (after removing the common uncertainties) are also reported.  are obtained using the best fit of the Blast-Wave function to the p T distributions. Several other fit functions (Maxwell-Boltzmann, Fermi-Dirac, m T -exponential, p T -exponential) are employed to estimate the systematic uncertainties. The fraction of the extrapolated yields are 6.2%, 6.2% and 10.4% for 0-20%, 20-50% and 50-80% centrality events. The total dN/dy systematic uncertainties are 17.2%, 16.5% and 15.6%, with a significant contribution common to all centrality classes of about 12%. The total systematic uncertainties on p T are 6.9%, 7.2% and 6.9%. The values of dN/dy and p T for Λ(1520) are reported in Tab 2.
The ratio of the p T -integrated yield of Λ(1520) to that of its stable counterpart, Λ, highlights the characteristics of resonance production directly related to the particle lifetime, as possible effects due to valence-quark composition cancel. The yields of Λ have been previously measured by ALICE in Pb-Pb collisions at √ s NN = 2.76 TeV [27]. Since the centrality classes for Λ(1520) are different from those of the measured Λ yields, the latter have been interpolated from the measured values fitting their dependence on the number of participating nucleons in the collisions (N part ) with the empirical parametrisation a + b N part c , where a, b and c are free parameters. Moreover, the Λ yield, which was not published in [27], has been assumed equal to that of Λ, as expected at LHC energies [24]. In the following, Λ refers to the sum of particle and antiparticle states, except for ALICE where it is defined as 2Λ. The yield ratios Λ(1520)/Λ are reported in Tab. 2. Figure 3 and 4, respectively, present the p T of Λ(1520) and the Λ(1520)/Λ yield ratios as a function of the cubic root of the charged-particle multiplicity density at mid-rapidity [17], dN ch /dη 1/3 . The latter is used as a proxy for the system radius to emphasise the system-size dependence of Λ(1520) production, as suggested by femtoscopic studies using Bose-Einstein correlations [28]. The p T increases significantly with increasing charged-particle multiplicity, hence with increasing collision centrality (Fig. 3). The value in central (0-20%) collisions is 23% higher than the one in peripheral (50-80%) collisions. The measured p T is compared to the prediction from the Blast-Wave model discussed previously [23, 24]. The good agreement further stresses the consistency of the Λ(1520) data with a common hydrodynamic evolution picture. The p T results also highlight the good agreement with the prediction from the EPOS3 [11] model. It is noteworthy that EPOS3 fails to describe the data when the UrQMD transport stage is disabled, underlining the importance of the latter in the description of the evolution of heavy-ion collisions.
A gradual decrease of the Λ(1520)/Λ yield ratio with increasing charged-particle multiplicity is observed from peripheral to central Pb-Pb collisions (Fig. 4). The Λ(1520) suppression in central Pb-Pb events with respect to peripheral events is measured as the double ratio where common uncertainties cancel. Λ(1520)/Λ in central collisions is about 45% lower than in peripheral collisions. The result provides the first evidence for Λ(1520) suppression in heavy-ion collisions, with a 3.1σ confidence level. The ratio is compared to grand-canonical equilibrium predictions from the GSI-Heidelberg [4], THERMUS [29] and SHARE3 [30] models, whose parameters have been determined from fits to stable particles [31]. The ratio is also compared to the non-equilibrium configuration implemented in SHARE3, where the under(over)-saturation parameters γ s (strange) and γ q (light quarks) are free [32]. All models describe the yield of stable hadrons well. Λ(1520)/Λ in central collisions is lower than SHM predictions by values ranging from 37% to 52%, depending on the reference model. Figure 4 also shows the data from the STAR Collaboration at RHIC in Au-Au, d-Au and pp collisions at √ s NN = 200 GeV [13,14]. The trend of the suppression is similar to the one seen from STAR data in Au-Au collisions at √ s NN = 200 GeV. The current measurement of Λ(1520) suppression has a higher precision at 3.1σ confidence level, as compared to the 1.8σ confidence level of STAR data in Au-Au collisions. Finally, the multiplicity-dependence of the Λ(1520)/Λ ratio is compared with the prediction from EPOS3 [11] (Fig. 4). It is important to note that the model, although it systematically overestimates the data, describes the trend of the suppression well. These observations highlight the relevance of the hadronic phase in the study of heavy-ion collisions and the importance of a microscopic description of the late hadronic interactions.
In conclusion, the first measurement of Λ(1520) production in Pb-Pb collisions at √ s NN = 2.76 TeV at the LHC has been presented. The spectral shapes and p T are consistent with the hydrodynamic evolution picture that describes pions, kaons and protons, indicating that the Λ(1520) experiences the same collective radial expansion, with a common transverse velocity which increases with collision centrality. The comparison of the p T results to EPOS3 predictions highlights the relevance of the hadronic phase in the study of heavy-ion collisions and the importance of a microscopic description of the late hadronic interactions. The p T -integrated ratio Λ(1520)/Λ is suppressed in central Pb-Pb collisions with respect to peripheral Pb-Pb collisions and is lower than the value predicted by statistical hadronisation models. The measurement adds further support to the formation of a dense hadronic phase in the latest stages of the evolution of the fireball created in high-energy heavy-ion collisions, lasting long enough to cause a significant reduction in the observable yield of short-lived resonances.

Acknowledgements
The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The  [11] A. G. Knospe, C. Markert [17] ALICE Collaboration, K. Aamodt et al., "Centrality dependence of the charged-particle multiplicity density at mid-rapidity in Pb-Pb collisions at √ s NN = 2.76 TeV," Phys. Rev. Lett. 106 (2011) 032301, arXiv:1012.1657 [nucl-ex].