Soft physics and collective phenomena in p-Pb collisions from ALICE

E. Leogrande, on behalf of the ALICE Collaboration Universiteit Utrecht, The Netherlands

emilia. leogrande@cern.ch

New ALICE results concerning soft physics and collective phenomena in p-Pb collisions at yBiiN = 5.02 TeV are briefly discussed. First, the particle-multiplicity dependence of the flow coefficients v2 and V3 derived via multiparticle curnulants is reviewed. Then, results on the multiplicity dependence of jet-like two-particle correlation structures are shown. Finally, p-Pb femtoscopic radii of the pion-emitting source are compared with different colliding systems, such as pp and Pb--Pb.

1 Introduction

Heavy-ion collisions have provided striking evidences for the existence of collective phenomena involved in particle production 1 . Sources of collectivity can be several: a common source of produced particles, rescattering between constituents and/or hydrodynamics. The latter seems rather successful in describing heavy-ion results. Lately, collective phenomena have been investigated also in p-Pb collisions. Given the multiplic­ity reached in this collision system (high multiplicity p-Pb is comparable to peripheral Pb-Pb), the "fireball" (if present) is expected to be small. Initial state fluctuations, then, are foreseen to play an important role. Surprisingly, p-Pb and Pb-Pb collisions have revealed interesting similarities. As first astonish­ing results, the double ridge z,3,4 and the mass ordering of the v2 of pions, kaons and protons 5

from two-particle correlations have raised the question whether collective effects are also present in such small systems. New measurements from ALICE investigating the multiplicity dependence of soft physics ob­servables in p-Pb collisions are presented in these proceedings: flow coefficients v2 and V3 from multiparticle cumulants 6 ( 2), jet-like two-particle correlations 8 ( 3) and femtoscopic radii of the pion-emitting source 13•14 (4) .

2 Multiplicity dependence of flow coefficients v2 and v3

As collectivity is a correlation between many particles, multi-particle correlation methods are used to investigate it. Multi-particle azimuthal correlations, though, also contain contributions from jets and resonances. An effective approach to reduce such contributions is the use of multi-particle cumulants en{ m} a, which can be qualitatively depicted as standard correlators averaged over all the events. The effectiveness of this approach increases with a higher number of correlated particles m, since all non-flow contributions up to m - 1 are removed, and/or with a larger pseudo-rapidity gap between them.

"n is the order, m is the number of correlated particles.

0�1 2 AUGE p-Pb \ s,.,. = 5.02 TeV

• Vd2. �·\111 > 1 4)

0.1 0 V2j4)

0.08

..... . : e e • 0.06 0 0.04 ?00 0.02

0.2 < p_ < 3.0 GeV1c

0 o 50 100

• . • ¢ 0

150 200 NcM(frll:ibl < 1 )

>' 0.12

Centrality (%} 82 65 52 43 31 17 7

• vJ2 !.1111 > 1 4}

0.1 0 v;{4}

ALICE Pb Pb \SN,.. = 2 76 TeV �J • v,{6) · ·�

0.08

0.06

0.04

0.02

· = · ii� . ":.

0 2 < p1 < 3.0 GeV/c

.. •

lllt

!'I o�����������

1 0 1 o'

�0.04r � �0.03 ; :::��:,:�:::::v g, 0.2 < p1 < 3.0GeV'G , r "'--S:'--' U_,!) - ..., .-' ( Y1• � f

0.02 •

. :, .� · ' . . . · · ····· . • 0.01-

0\L_ -1 0 102 103 NcM(/rllabl < 1 )

Figure 1 - Midrapidity multiplicity dependence of v2 in p-Pb (left) and in Pb-Pb (middle) and vs (right) in p-Pb and Pb-Pb.6

From the cumulants, one can directly calculate the flow coefficients 7: for instance, the relation­ship between the second-order two-particle cumulant and the elliptic flow is v2{2} = Jc2{2}.

Figure 1 shows the v2 in p--Pb (left panel) and Pb-Pb (middle panel) and the V3 (right panel) in p-Pb and Pb-Pb as a function of midrapidity ( l77labl < 1 ) charged particle multiplicity in the range 0.2 < PT < 3.0 GeV/c. As expected, the v2{4} values are systen1atically lower than the

v2{2} ones, since non-flow contributions up to the third order are removed. A first remarkable observation is the agreement between v2{ 4} and v2{6} in Pb-Pb collisions, supporting the picture of collectivity. We then observe that v2{2} and v2{4} in p-Pb are smaller that v2{2} and v2{ 4} in Pb-Pb at similar multiplicities. On the contrary, v3 does not differ between p--Pb and Pb-Pb in the overlapping multiplicity region. This would suggest that the V3 could be mainly dominated by a common physics process in the two collision systems.

3 Multiplicity dependence of jet-like two-particle correlations

The long-range two-particle correlations in p-Pb collisions have been extensively studied 2•5 . However, useful information for the physical interpretation of the double-ridge structure comes also from the jet-like structure, which emerges in both long- and short-range correlations. Jet­like two-particle correlations create a peak in the near side (A(/)=0,.6.77=0) and an elongated structure in .6.77 in the away side (Ll'P = 1f) . Therefore, to extract these jet-like contributions in the near side it is enough to apply a .6.77 cut and to subtract the long-range (1 .2 < J.6.771 < 1 .8) from the short-range ( l.6.771 < 1 .2) correlations. On the away side, though, the jet- and ridge-like contributions are mixed together. However, one can mirror the near-side long-range correlations in the away-side and subtract them from the total correlations in the away-side. The underlying assumption is that the double ridge is symmetric around A(/) = 1f /2. The effects due to the non-symmetric v3 affect the away side only. The yields of the jet-like peaks extracted using this technique are then studied as a function of multiplicity. A useful way of characterizing the multiplicity is by classifying events according to multiplicity classes, which are defined as percentile intervals of the multiplicity distribution measured with a specific detector 9 . The 0 - 53 class corresponds to the highest multiplicity, while the 95 - 1003 to the lowest one. In Fig. 2 the near-side per-trigger yield evaluated in 0.7 < PT,assoc < PT,trig < 5 GeV /c is presented as a function of multiplicity class measured with the forward detector VO-A lO. The red points are obtained after the aforementioned subtraction of long-range correlations is performed. Their flatness in 0 - 603 can be interpreted as if higher multiplicity events are not built up by a higher number of associated particles per trigger particle (i.e. per minijef), but rather by a

•Minijets are jets in the low-pr region, where QCD is non perturbative.

�� 0.5

!. �!

0.1

ALICE p-Pb \ S,.. = 5.02 TeV

0.7 < PT.assoc < Pr,tng < 5.0 GeV/c

���2�0,..._�40���60,.,.....��80,.,.....��1�00'""' VOA multiplicity class (%)

Figure 2 - Multiplicity dependence of per-trigger jet-yields with (red points) and without (black points) long-range correlation subtraction�

(kr) = 0.25 GeV/c

• �. t ...

•' , 'o

. .. 1. ';

�· 1 0

{ dN_·'d17 ':-1 '.•

ST AA Ai��J.,_, {i:;. � ZOO G<iV ' Gf..\A CtJ .. Ctqfi1: "' �G6V " STA.RNAv.:. '�-, e<! '3.;iV

� STAACu.C•.: {�,::; "' l?.2 G"J\'

10

"' CERES Pl:i-Ai., � = 1 r 2 G'f\' • A.I.ICE P-b-·PQ ($,;,-" 2700 Gr::V • ALICE cxi fS :o 7G!:;QG.;.1;' c Al..ICE n::i G � -oco �·1 ST,o\Rro f$ .,,, 200 G0V • At,ICE p.fl'b �-� ,, 5020 �·1

Figure 3 - Femtoscopic radii in three directions (out, side, long) as a function of the cube root of the measured charged-particle multiplicity density for many collision energies and systems.13

higher number of minijets per event. This is compatible with a model of minijets stemming from incoherent fragmentation of multiple parton interactions (MPis) . The black points, instead, are obtained if the long-range correlations are also included and they show an increasing trend with multiplicity. Thus, this is a possible hint of a different physical origin for jet- and ridge-like phenomena.

4 Femtoscopic radii of the pion-emitting source

One of the observables characterizing the bulk collective system is the size of the particle-emitting region at freeze-out, which can be extracted for pions from femtoscopic techniques 11•12 . Two-pion correlation functions are extracted 13 as a function of the momentum difference of the pair q in three different directions (long along the beam axis, out along the pair transverse momentum, side perpendicular to the other two). Femtoscopic effects become important for q below 0.4 GeV/c.

The radii extracted with a Gaussian fit of the correlation functions in the three directions are shown in Fig. 3 as a function of (dNch/dry)113 , since it has been observed that they scale roughly

with the cube root of the measured charged-particle multiplicity density for many collision energies and initial system sizes. In fact, the pp, p-Pb and Pb-Pb radii evolve linearly with it, but with a significantly different scaling. The p-Pb radii agree with the pp ones at low multiplicities, but they diverge at higher multiplicities. Consistent results are found with a three-pion cumulants analysis 14 , where radii in p-Pb are observed to be 5-15% higher than in pp and 35-55% lower than in Pb-Pb.

5 Conclusions

The soft physics observables in p-Pb collisions measured by ALICE add new information for the interpretation of results such as the double ridge in two-particle correlations, and in general for addressing the question whether collectivity is formed in such small systems. The multiparticle cumulants method has proven to be very effective in isolating genuine correlations among many particles. At similar multiplicity, the elliptic flow v2 obtained in p-Pb is observed to be smaller than in Pb-Pb, while the triangular flow v3 overlap, suggesting for the latter a common physics process in the two collision systems. A study of the jet-like correlations above the double ridge and of the minijet-yield evolution with multiplicity has shown that jet- and ridge-like phenomena likely arise from different physical mechanisms and are additive. This analysis also allows for a better characterization of the particle production in terms of minijets and multiple parton interactions. Finally, femtoscopic techniques grant the possibility of estimating the size of the source of pion emission and of comparing it among different collision energies and systems. In p-Pb, it is found to be larger than in pp, though comparable at low multiplicities, and smaller than in Pb-Pb. The combination of these results provide a strong challenge and constraints for theoretical models.

References

1. U. W. Heinz and R. Snellings, Annu. Rev. Part. Sci. 63 (2013) 123-151. 2. ALICE Collaboration, Phys. Lett. B 719, 29 (2013). 3. CMS Collaboration, Phys. Lett. B 718, 795 (2013). 4. ATLAS Collaboration, Phys. Rev. Lett. 110, 182302 (2013). 5. ALICE Collaboration, Phys. Lett. B 726, 164 (2013). 6. ALICE Collaboration, Phys. Rev. C 90, 054901 (2014). 7. A. Bilandzic, R. Snellings and S. Voloshin, Phys. Rev. C 83, 044913 (2011) . 8. ALICE Collaboration, Phys. Lett. B 741, 38 (2014). 9. ALICE Collaboration, arXiv:1412.6828 (2014).

10. ALICE Collaboration, JINST 3, S08002 (2008). 1 1 . R. Lednicky, Nucl. Phys. A 774, 189 (2006). 12. M. Lisa, S. Pratt, R. Soltz, U. Wiedemann, Annu. Rev. Part. Sci. 55 (2005) 311. 1 3. ALICE Collaboration, Phys. Rev. C 91, 034906 (2015). 14. ALICE Collaboration, Phys. Lett. B 739, 139 (2014).

")fi)