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Hadronic resonance production with ALICE at the LHC

SergeyKiselev for the ALICE collaboration1,∗

1Institute for Theoretical and Experimental Physics, 117218 Moscow, Russia

Abstract.

We present recent results on short-lived hadronic resonances obtained in the ALICE experiment at LHC energies, including results from the Xe-Xe run. The ALICE results on transverse momentum spectra, yields, their ratio to long-lived particles, and nuclear modification factors will be discussed. The results will be compared with model predictions and measurements at lower energies.

Hadronic resonances are an important probe for the properties of the medium formed in heavy-ion collisions. In heavy-ion collisions, since the lifetimes of short-lived resonances are comparable with the lifetime of the late hadronic phase, regeneration and rescattering effects become important and resonance ratios to longer lived particles can be used to estimate the time interval between the chemical and kinetic freeze-out [1]. The measurements in pp and p-Pb collisions constitute a reference for nuclear collisions and provide information for tuning event generators inspired by Quantum Chromodynamics.

Recent results on short-lived mesonic ρ(770)0, K∗(892)0, φ(1020) as well as baryonic

Λ(1520) and Ξ(1530)0 resonances (hereafter ρ0, K∗0, φ, Λ∗, Ξ∗0) obtained in the ALICE experiment are presented. Theρ0 has been measured in pp and Pb-Pb collisions at √sNN=

2.76 TeV and results were published recently in [2]. The K∗0andφhave been measured in pp collisions at √s=13 TeV, in Xe-Xe collisions at √sNN=5.44 TeV and in Pb-Pb collisions

at √sNN=5.02 TeV (results for the K∗0andφin pp at √

s=7 TeV, p-Pb at √sNN=5.02 TeV

and Pb-Pb at √sNN =2.76 TeV were published in [3], [4] and [5,6], respectively). The

Λ∗has been measured in Pb-Pb collisions ats

NN =2.76 TeV and results were published

recently in [7]. TheΞ∗0has been measured in Pb-Pb collisions ats

NN=2.76 TeV (results

for theΣ∗±andΞ∗0in pp ats=7 TeV and p-Pb ats

NN=5.02 TeV were published in [8]

and [9], respectively).

The resonances are reconstructed in their hadronic decay channels and have very different lifetimes as shown in Tab.1.

Table 1.Reconstructed decay mode, branching ratio and lifetime values for hadronic resonances

ρ0 K∗0 φ ΛΞ∗0

decay channel (B.R.) ππ(1.00) Kπ(0.67) KK(0.49) pK(0.22) Ξπ(0.67)

lifetime (fm/c) 1.3 4.2 46.2 12.6 21.7

Fig.1 presents the transverse momentum spectra for K∗0 andφ in Xe-Xe collisions at

sNN=5.44 TeV.

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) c (GeV/ T

p

0 2 4 6 8 10 12

-1 )

c

) (GeV/ T

p d y /(d N 2 d evt N 1/ 5 − 10 4 − 10 3 − 10 2 − 10 1 − 10 1 10 0-30% 50-70% 30-50% 70-90% ALICE Preliminary

= 5.44 TeV

NN

s

Xe-Xe

| < 0.5

y

| 0

K*

Uncertainties: stat. (bars) syst. (boxes)

ALI−PREL−148564

0 2 4 6 8 10

) c (GeV/ T p 5 − 10 4 − 10 3 − 10 2 − 10 1 − 10 1 10 2 10 3 10 -1) c ) (GeV/ T p d y /(d N 2 d ev N 1/ ALICE Preliminary

= 5.44 TeV

NN

s

Xe-Xe,

Uncertainties: stat.(bar), syst.(box)

(1020)

φ 0-10% × 2 10-30%

30-60% 60-90%

ALI−PREL−148421

Figure 1.(color online) Transverse momentum spectra for K∗0(left) andφ(right) in different centrality classes of Xe-Xe collisions at √sNN=5.44 TeV

The spectra have been measured for different centrality up topT=10 GeV/c.

The mean transverse momenta ofφand stable hadrons in Xe-Xe collisions as a function of the charged-particle multiplicity density are shown in Fig.2 (left). Results for Xe-Xe

2

10 103

| < 0.5 η | 〉 η /d ch N d 〈 0.5 1 1.5 2 ) c (GeV/ 〉T pALICE Preliminary

= 5.02 TeV (open markers)

NN

s Pb-Pb

= 5.44 TeV (full markers)

NN s Xe-Xe -π + + π -+K +

K p+p φ

ALI−PREL−158289

| < 0.5 lab η | 〉 lab η /d ch N d 〈

1 10 102 103

) c (GeV/ 〉 T p 〈 0.6 0.8 1 1.2 1.4 1.6 p 0 K*

| < 0.5 lab η | 〉 lab η /d ch N d 〈

1 10 2

10 3 10 0.6 0.8 1 1.2 1.4 1.6 p φ ALICE

= 2.76 TeV

NN s

Pb-Pb

= 5.02 TeV NN s p-Pb

= 7 TeV (Preliminary) s

pp

= 13 TeV (Preliminary) s

pp

ALI−PREL−156842

Figure 2. (color online) The mean transverse momentum as a function of the charged-particle multi-plicity density. (left) pion, kaon, proton andφ. (right) K∗0(left panel) andφ(right panel) compared to p (same data in both panels)

collisions confirm the trends observed in Pb-Pb collisions. In central collisions we observe mass ordering. Theφand p, which have similar masses, are observed to have similarhpTi

values, as expected if their spectral shape is dominated by radial flow.

Fig.2(right) presents thehpTiof K∗0,φand p as a function of the charged-particle

mul-tiplicity density measured for pp collisions at √s=13 TeV and compared with the results obtained in pp, p–Pb [4] and Pb–Pb [5] collisions at √sNN =7, 5.02 and 2.76 TeV,

respec-tively. In pp collisions thehpTiincrease with multiplicity increase at √

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to the one at √s=7 TeV. ThehpTivalues follow a similar trend with multiplicity increase

in pp and p-Pb collisions, where they rise faster with multiplicity increase than in Pb-Pb col-lisions. An analogous behavior has been observed in [10] for charged particles and can be understood as the effect of color reconnection between strings produced in multi-parton in-teractions. In central Pb–Pb collisions, thehpTivalues for these three particles are consistent

within uncertainties, as it is expected in the presence of a common radial flow. However in pp and p-Pb collisions thehpTivalues for the K∗0andφresonances are higher than for p. The

mass ordering observed in central Pb-Pb collisions, where particles with similar mass have similarhpTi, is not observed in pp and p-Pb collisions [4].

Fig.3shows thehpTias a function of the charged-particle multiplicity density forρ0[2]

andΛ∗ [7] in Pb-Pb collisions ats

NN =2.76 TeV. The results are in agreement with the

1/3

〉 η /d ch

N

d 〈

0 2 4 6 8 10 12

)

c

(GeV/

〉T

p

1.2 1.4 1.6 1.8 2

ALICE

= 2.76 TeV

NN s

Pb-Pb, EPOS v3.107 EPOS v3.107 (UrQMD OFF)

/K/p) π Blast-Wave (

(1520) + cc. Λ

| < 0.5

y

|

Figure 3.(color online) The mean transverse momentum ofρ0(left) [2] andΛ

(right) [7] as a function of the charged-particle multiplicity density. The measurements are also compared to model predictions: EPOS3 [11], Blast-Wave [12]

prediction from the EPOS3 generator with UrQMD [11], which includes modeling of rescat-tering and regeneration in the hadronic phase. The results forΛ∗are also in agreement with the average momentum extracted from the Blast-Wave model [12] with parameters obtained from the simultaneous fit to pion, kaon, and (anti)protonpTdistributions [13].

Fig.4presents pT-integrated yields of K∗0andφin pp, p-Pb [4], Xe-Xe and Pb-Pb

col-lisions as a function of the charged-particle multiplicity density. Yields are independent of collision system and appear to be driven by event multiplicity.

Fig. 5(top left) shows the particle yield ratios K∗0/K andφ/K in Xe-Xe collisions at

sNN =5.44 TeV. Results for p-Pb collisions at √

sNN =5.02 TeV [4], Pb-Pb collisions at √

sNN=2.76 TeV [6], pp collisions at √

s=13 TeV and Pb-Pb collisions at √sNN=5.02 TeV

are also shown. The K∗0/K ratio shows a significant suppression going from p-Pb and

periph-eral Xe-Xe/Pb-Pb collisions to the most central Xe-Xe/Pb-Pb collisions. This suppression is consistent with rescattering of K∗0 daughters in the hadronic phase of central collisions as

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|<0.5

lab η

|

η

/d

ch N

d 〈

0 10 20 30 40

y

/d

N

d

0 0.2 0.4 0.6

0.8 = 5.02 TeV (EPJC 76 (2016) 245)

NN

s

p-Pb,

ALICE Preliminary

= 13 TeV

s

pp, = 7 TeV

s

pp,

V0 Multiplicity Event Classes

0

K*

Uncertainties: stat.(bars), sys.(boxes), uncor. sys. (shaded boxes)

ALI−PREL−131517

η /d ch

N

d 〈

500 1000 1500

y

/d

N

d

0 5 10 15 20 25 30

ALICE Preliminary

= 5.02 TeV NN

s

Pb-Pb,

= 5.44 TeV NN

s

Xe-Xe,

| < 0.5

y

|

0

K*

Uncertainties: stat. (bars), sys. (boxes)

ALI−PREL−154863

| < 0.5

lab

η

|

lab

η

/d ch

N d

0 10 20 30 40 50

y

/d

N

d

0 0.1 0.2 0.3

0.4 pp s = 13 TeV

V0M Mult. Event Classes | < 0.5

CMS

y

|

= 5.02 TeV NN

s

p-Pb

V0A Mult. Event Classes < 0 CMS

y

-0.5 < stat. sys. uncorr. sys.

ALICE Preliminary

φ

ALI-PREL-132050

500 1000 1500 2000

〉 η

/d ch

N

d

5 10 15 20

y

/d

N

d

ALICE Preliminary = 5.02 TeV

NN

s

Pb-Pb

= 5.44 TeV

NN

s

Xe-Xe

| < 0.5

y

(1020), |

φ

ALI−PREL−155844

Figure 4.(color online)pT-integrated yields (dN/dy) of K∗0(top) andφ(bottom) as a function of the charged-particle multiplicity density in pp and p-Pb [4] (left) and Xe-Xe and Pb-Pb (right) collisions

sNN=2.76 TeV we observeρ0/π[2] andΛ∗/Λ[7] ratio suppression similar to the K∗0/K

ratio, see Fig.5(top right) and Fig.5(bottom left), respectively. TheΛ∗/Λsuppression con-firms the trend seen by STAR at √sNN=200 GeV [14]. For theΞ∗0/Ξratio, Fig.5(bottom

right), there is a hint of suppression, but systematics are to be reduced in peripheral Pb-Pb collisions before making any conclusive statement. Although predictions of the EPOS3 model with UrQMD [11] overestimate the data, the trend of the suppression is qualitatively reproduced for K∗0,ρ0, andΛ. Thermal model predictions [15–17] overestimate all particle ratios under study in central Pb-Pb collisions, except theφ/K ratio.

In central Pb-Pb collisions the nuclear modification factorRAAfor K∗0[6],φ[6] andρ0[2]

is consistent with light-flavored hadrons atpT>8 GeV/c, demonstrating strong suppression.

At low pT < 2 GeV/c, the K∗0 andρ0 are stronger suppressed than light-flavored hadrons

which is consistent with the hypothesis that flow and rescattering effects are important. Fig.6 (left) presentsRAAof K∗0for Pb-Pb collisions at

sNN=2.76 TeV [6] and √

sNN=5.02 TeV

in different centrality. We do not observe significant energy dependence ofRAA.RAAof K∗0

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1/3 〉 η /d ch N d 〈

0 2 4 6 8 10 12

Λ (1520) / Λ 0 0.05 0.1 0.15 ALICE

= 2.76 TeV NN

s

Pb-Pb,

= 200 GeV NN

s

STAR,

pp d-Au Au-Au EPOS v3.107

= 2.76 TeV NN

s

Pb-Pb,

= 156 MeV ch T THERMUS GSI-Heidelberg SHARE3 = 2.08 s γ = 1.63, q γ

= 138 MeV, ch

T

SHARE3

0 2 4 6 8 10 12 14

| < 0.5

lab η | 1/3 〉 lab η /d ch N d 〈 0 0.2 0.4 0.6 0.8 1 ) + Ξ + -Ξ )/( *0 Ξ + *0 Ξ (

Uncertainties: stat. (bars), sys. (boxes) uncorr. sys. (shaded boxes)

ALICE Preliminary

= 156 MeV

ch

T GSI-Heidelberg

= 156 MeV

ch

T THERMUS

EPOSv3.107 SHARE3 EPJ C77 (2017) 389

ALICE, V0A mult. classes

DPMJET EPJ C75 (2015) 1 ALICE, INEL

PYTHIA8 (Monash 2013)

= 2.76 TeV

NN

s Pb-Pb,

= 5.02 TeV

NN

s p-Pb,

= 7 TeV s pp,

ALI−PREL−128169

Figure 5. (color online) Particle yield ratios K∗0/K andφ/K (top left), ρ0(top right) [2],Λ/Λ (bottom left) [7] andΞ∗0/Ξ(bottom right) as a function of the charged-particle multiplicity density for various collision systems. STAR data from [14]. The measurements are also compared to model predictions: EPOS3 [11], THERMUS [15], GSI-Helderberg [16], SHARE [17]

0 5 10 15 20

AA R 0.5 1 20-30% ) c (GeV/ T p

0 5 10 15 20

0.5

1 30-40%

0 5 10 15 20

0.5 1 40-50% 0.5 1 0-10% 0.5 1 10-20% 0 * K + 0 K* Pb-Pb

= 2.76 TeV (empty markers)

NN

s

PRC 95 064606 (2017)

= 5.02 TeV (filled markers)

NN s ALICE Preliminary ALI−PREL−139804 ) c (GeV/ T p

0 2 4 6 8 10 12

AA R 0 0.2 0.4 0.6 0.8 1 1.2 ALICE Preliminary

= 5.44 TeV, 0-30%

NN s

Xe-Xe

= 5.02 TeV, 20-30%

NN s

Pb-Pb

| < 0.5

y

, |

0

K*

Uncertainties: stat. (bars), syst. (boxes)

ALI−PREL−148580

Figure 6. (color online) The nuclear modification factorRAAfor K∗0as a function of transverse mo-mentum. (left) Pb-Pb collisions at √sNN=2.76 TeV [6] and

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In summary, recent results on short-lived hadronic resonances obtained in the ALICE ex-periment in pp, p-Pb, Xe-Xe and Pb-Pb collisions at the LHC energies have been presented. In pp and p-Pb collisions thehpTivalues for the K∗0andφresonances rise faster with

mul-tiplicity increase than in Pb–Pb collisions. One possible explanation could be the effect of color reconnection between strings produced in multi-parton interactions. The mass ordering observed in central Xe-Xe and Pb–Pb collisions, where particles with similar masses (K∗0, φand p) have similarhpTi, is not observed in pp and p-Pb collisions. Yields of K∗0andφ

in pp, p-Pb, Xe-Xe and Pb-Pb collisions are independent of collision system and appear to be driven by event multiplicity. The K∗0/K,ρ0/πandΛ∗/Λratios exhibit a significant sup-pression going from peripheral to central Pb-Pb collisions, is consistent with rescattering of the decay products of the short-lived resonances in the hadronic phase. Theφ/K ratio is not suppressed due to the longer lifetime of theφ. Results for the K∗0/K andφ/K ratios in

Xe-Xe collisions confirm the trend observed in Pb-Pb collisions. There is a hint of suppression for theΞ∗0/Ξratio, but systematics are to be reduced in peripheral Pb-Pb collisions before

making any conclusive statement. Although predictions of the EPOS3 model with UrQMD overestimate the data, the trend of the suppression is qualitatively reproduced. Thermal model predictions overestimate all particle ratios under study in central Pb-Pb collisions, except the φ/K ratio. In central Pb-Pb collisions the nuclear modification factorRAAfor K∗0,φandρ0

consistent with light-flavored hadrons at pT >8 GeV/c. At lowpT <2 GeV/c, the K∗0and

ρ0 are stronger suppressed than light-flavored hadrons which would be consistent with the

hypothesis that flow and rescattering effects are important. We do not observe significant energy dependence ofRAAin Pb-Pb collisions.RAAof K∗0in Xe-Xe and Pb-Pb collisions are

consistent within uncertainties once compared at the same multiplicity.

References

[1] G. Torrieri and J. Rafelski, Phys. Lett. B509, 239 (2001)

[2] S. Acharya et al. (ALICE Collaboration), arXiv:1805.04365 [nucl-ex] [3] B. Abelev et al. (ALICE Collaboration), Eur. Phys. J. C72, 2183 (2012) [4] J. Adam et al. (ALICE Collaboration), Eur. Phys. J. C76, 245 (2016) [5] B. Abelev et al. (ALICE Collaboration), Phys. Rev. C91, 024609 (2015) [6] J. Adam et al. (ALICE Collaboration), Phys. Rev. C95, 064606 (2017) [7] S. Acharya et al. (ALICE Collaboration), arXiv:1805.04361 [nucl-ex] [8] B. Abelev et al. (ALICE Collaboration), Eur. Phys. J. C75, 1 (2015) [9] D. Adamova et al. (ALICE Collaboration), Eur. Phys. J. C77, 389 (2017) [10] B. Abelev et al. (ALICE Collaboration), Phys. Lett. B727, 371 (2013) [11] A.G. Knospe et al., Phys. Rev. C93, 014911 (2016)

[12] E. Schnedermann et al., Phys. Rev. C48, 2462 (1993)

[13] B. Abelev et al. (ALICE Collaboration), Phys. Rev. C88, 044910 (2013) [14] B. Abelev et al. (STAR Collaboration), Phys. Rev. C78, 044906 (2008) [15] S. Wheaton et al., Comput. Phys. Commun.180, 84 (2009)

Figure

Figure 1. (color online) Transverse momentum spectra for K∗0 (left) and φ (right) in different centralityclasses of Xe-Xe collisions at √sNN = 5.44 TeV
Figure 3. (color online) The mean transverse momentum ofof the charged-particle multiplicity density
Figure 4. (color online) pT-integrated yields (dN/dy) of K∗0 (top) and φ (bottom) as a function of thecharged-particle multiplicity density in pp and p-Pb [4] (left) and Xe-Xe and Pb-Pb (right) collisions
Figure 6. (color online) The nuclear modification factor RAA for K∗0 as a function of transverse mo-mentum

References

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