(Anti-)deuteron production and anisotropic flow measured with ALICE at the LHC

(Anti-)deuteron production and anisotropic flow measured with ALICE at the LHC

Ramona Lea (for the ALICE Collaboration) Dipartimento Di Fisica, Università di Trieste e INFN, Sezione Trieste

The high abundance of (anti-)deuterons in the statistics gathered in Run 1 of the LHC and the excellent performance of the ALICE setup allow for the simultaneous measurement of the elliptic flow and the deuteron production rates with a large transverse momentum () reach. The (anti-) deuterons are identified using the specific energy loss in the time projection chamber and the velocity information in the time-of-flight detector. The elliptic flow of (anti-)deuterons can provide insight into the production mechanisms of particles in heavy-ion collisions. Quark coalescence is one of the approaches to describe the elliptic flow of hadrons, while the production of light nuclei can be also depicted as a coalescence of nucleons. In these proceedings, the measured  of deuterons produced in Pb–Pb collisions at  = 2.76 TeV will be compared to expectations from coalescence and hydrodynamic models.

Heavy-ion collisions, deuteron, elliptic flow

1 Introduction

Anisotropic flow Ollitrault:1992bk () studies can probe the nature of matter produced in heavy-ion collisions. These studies allow for the investigation of collective effects among produced particles. The angular distribution of all the reconstructed charged particles can be expanded into a Fourier series w.r.t. symmetry plane :


where is the energy of the particle, p the momentum,  the transverse momentum, the azimuthal angle, the rapidity, the symmetry plane angle and


The second term of the Fourier series () is called elliptic flow and is a parameter which may provide insight on initial conditions and particle production mechanisms. For identified hadrons  is sensitive to the partonic degrees of freedom in the early stages of the evolution of a heavy-ion collision Voloshin:1994mz (). The deuteron is a composite p+n bound state, whose binding energy ( 2.24 MeV) is much lower than the hadronization temperature. Thus, it is likely that it would suffer from medium induced breakup in the hadronic phase, even if it was produced at hadronization. The  measurements for d and  provide an important test for the universal scaling of elliptic flow Nonaka:2003ew () since its  should be additive both with respect to the  of its constituent hadrons and with respect to the  of the constituent quarks of these hadrons, i.e. .
In these proceedings, the measurements of (anti-)deuterons  are compared to other identified particles Abelev:2014pua () and to a model based on hydrodynamics Huovinen:2001cy (); Adler:2001nb ().

2 Analysis Details

For the analyses presented here, a sample of about 35 million Pb–Pb collisions at  = 2.76 TeV collected by ALICE Abelev:2014ffa () in 2011 were used. The events were classified into 6 different centrality intervals, which were determined using the forward V0 Abbas:2013taa () scintillator arrays. Particle tracking is done by means of the Time Projection Chamber (TPC) Alme:2010ke () and the Inner Tracking System (ITS) Aamodt:2010aa () with full azimuthal coverage for 0.8. The identification of deuteron (d) and anti-deuteron () was performed in the same way as done to extract deuteron spectra in Pb–Pb Adam:2015vda (). Specifically, for momenta up to 1 GeV the energy loss in the TPC gives a clean sample of (anti-)deuterons by requiring a maximum deviation of the specific energy loss of 3 with respect to the expected signal. Above 1 GeV a Time of Fight (TOF) Abelev:2014ffa () hit is required. The signal in the TOF detector is fitted with a function which is the sum of a Gaussian with an exponential tail, while the background is fitted with an exponential. As an example of the , where = -, for deuterons and anti-deuterons with    GeV and centrality interval 30-40% is shown in the left part of Figure 1.
The  was measured using the Scalar Product (SP) method Voloshin:2008dg (). The contribution to the measured elliptic flow () due to misidentified deuterons () was removed by studying the azimuthal correlations versus . This method is based on the observation that, since  is additive, candidate can be expressed as a sum of signal ((M)) and background ((M)) weighted by their relative yields


where N is the total number of candidates, N and N = N - N are the numbers of signal and background for a given mass and  interval. The yields N and N are extracted from fits to the distributions obtained with the TOF detector for each centrality and  interval. The vs for (d+) for 2.20  2.40 GeV in events with 30-40% centrality is shown in the middle panel of Figure 1. Points represent the measured , while the curve is the fit performed using equation 3. The was parametrized as a first degree polynomial (). The  range exploited in this analysis is 0.5  5 GeV. The measured  as a function of  for (d+) is shown in the right panel of Figure 1. Figure copyright CERN, reproduced with permission. The value of () increases progressively from central to semi-central collisions.

Fig. 1: Left: Distribution of for (d+) in the    GeV and centrality interval 30-40% fitted with a function which is the sum of Gaussian plus and exponential used to reproduce the signal and an exponential to reproduce the background. Middle: The vs for (d+) for 2.20 2.40 GeV in events with 30-40% centrality. Points represent the measured , while the curve is the fit performed using equation 3. Right: Measured  as a function of  for (d+) in Pb–Pb collisions at  2.76 TeV. Lines represent statistical errors, while boxes are systematic uncertainties. Figure copyright CERN, reproduced with permission.

2.1 Comparison with other identified particles

The measured (d+)  was compared with the  of other identified particles Abelev:2014pua (). Results for events with centrality in 30-40% are shown in Figure 2. In the top left panel the  vs  of (empty circles), (p+) (filled square) and (d+) (filled circles) as a function of  are shown. It is observed that at low  deuterons follow the mass ordering observed for lighter particles, which is attributed to the interplay between elliptic and radial flow. The second panel in the top part of the figure shows the /A vs /A of (p+) (filled squares) and (d+) (filled circles), while in the second panel of the lower part the ratios between the measured data and a 7 degree polynomial used to describe proton  are reported. For /A1 GeV a deviation from the /A scaling of the order of 20% is observed. A similar deviation is observed for all the measured centrality intervals. The third panels (upper and lower part) are used to test the n (number of constituent quark) scaling: both  and measured  were scaled by n. Also in this case a deviation for the n scaling of the order of 20% for each centrality interval is observed. Finally, in the upper fourth panel, the measured /n is shown as a function of of each particle: significant deviations from n scaling are seen in data for deuterons.

Fig. 2: Measured (d+)  compared with the  of other identified particles Abelev:2014pua () for events with centrality in 30-40% interval. A description of each panel can be found in the text. Figure copyright CERN, reproduced with permission.

2.2 Comparison with Blast-Wave model

Hydrodynamical calculations are able to reproduce the main features of  for 2 GeV Abelev:2014pua (). A simple model based on hydrodynamics is the Blast-Wave model Huovinen:2001cy (); Adler:2001nb (). The measured pions, kaons and protons  spectra and  () were fitted, and the parameters of the fit were used to predict deuteron (). The results for 30-40% mid-central events are shown in Figure 3. For deuterons a good description of the data in the measured  range and for all the measured centralities is observed.

Fig. 3: Left: Measured  for (empty squares), K (empty circles), p+ (filled squares) and d+ (filled circles) with Blast-Wave curves. The parameters of the Blast-Wave curve from , K and p+ were used to predict deuteron curve. Right: Ratio between data and model: each color corresponds to a different particle. Figure copyright CERN, reproduced with permission.

3 Conclusions

The  of deuterons produced in Pb–Pb collisions at  = 2.76 TeV was measured up to 5 GeV using the scalar product technique. At low  deuteron  follows mass ordering, indicating a more pronounced radial flow in the most central collisions as observed also for lighter particles. A deviation from A (number of mass) and n (number of constituent quark) scaling at the level of 20% is observed. The Blast-Wave model gives a good description of the measured elliptic flow of deuterons.



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