Net-Charge Fluctuations in Pb-Pb collisions at \sqrt(s)_NN = 2.76 TeV

arXiv:1207.6068v3 [nucl-ex] 22 Jun 2016

CERN-PH-EP-2012-212

Submitted to: PRL

Net–charge fluctuations

in Pb–Pb collisions at

√

_s

NN

= 2.76

TeV

ALICE Collaboration

Abstract

We report the first measurement of the net–charge fluctuations in Pb–Pb collisions at

√

s

NN

=

2.76

TeV, measured with the ALICE detector at the CERN Large Hadron Collider. The dynamical

fluctuations per unit entropy are observed to decrease when going from peripheral to central collisions.

An additional reduction in the amount of fluctuations is seen in comparison to the results from lower

energies. We examine the dependence of fluctuations on the pseudo–rapidity interval, which may

account for the dilution of fluctuations during the evolution of the system. We find that the fluctuations

at LHC are smaller compared to the measurements at the Relativistic heavy Ion Collider (RHIC),

and as such, closer to what has been theoretically predicted for the formation of the Quark–Gluon

Plasma (QGP).

(ALICE Collaboration)

B. Abelev,1 _{J. Adam,}2 _{D. Adamov´a,}3 _{A.M. Adare,}4 _{M.M. Aggarwal,}5 _{G. Aglieri Rinella,}6 _{A.G. Agocs,}7

A. Agostinelli,8 _{S. Aguilar Salazar,}9 _{Z. Ahammed,}10 _{A. Ahmad Masoodi,}11 _{N. Ahmad,}11 _{S.A. Ahn,}12

S.U. Ahn,13, 14 _{A. Akindinov,}15 _{D. Aleksandrov,}16_{B. Alessandro,}17 _{R. Alfaro Molina,}9 _{A. Alici,}18, 19 _{A. Alkin,}20

E. Almar´az Avi˜na,9 _{J. Alme,}21 _{T. Alt,}22 _{V. Altini,}23 _{S. Altinpinar,}24 _{I. Altsybeev,}25 _{C. Andrei,}26_{A. Andronic,}27

V. Anguelov,28 _{J. Anielski,}29 _{C. Anson,}30 _{T. Antiˇci´c,}31_{F. Antinori,}32 _{P. Antonioli,}18 _{L. Aphecetche,}33

H. Appelsh¨auser,34 _{N. Arbor,}35 _{S. Arcelli,}8_{A. Arend,}34 _{N. Armesto,}36 _{R. Arnaldi,}17 _{T. Aronsson,}4_{I.C. Arsene,}27

M. Arslandok,34 _{A. Asryan,}25_{A. Augustinus,}6 _{R. Averbeck,}27 _{T.C. Awes,}37_{J. ¨Ayst¨o,}38 _{M.D. Azmi,}11_{M. Bach,}22

A. Badal`a,39_{Y.W. Baek,}13, 14 _{R. Bailhache,}34_{R. Bala,}17 _{R. Baldini Ferroli,}19_{A. Baldisseri,}40 _{A. Baldit,}13

F. Baltasar Dos Santos Pedrosa,6J. B´an,41 R.C. Baral,42R. Barbera,43 F. Barile,23 G.G. Barnaf¨oldi,7 L.S. Barnby,44_{V. Barret,}13 _{J. Bartke,}45 _{M. Basile,}8 _{N. Bastid,}13 _{S. Basu,}10 _{B. Bathen,}29 _{G. Batigne,}33

B. Batyunya,46_{C. Baumann,}34 _{I.G. Bearden,}47 _{H. Beck,}34 _{N.K. Behera,}48_{I. Belikov,}49 _{F. Bellini,}8 _{R. Bellwied,}50

E. Belmont-Moreno,9 _{G. Bencedi,}7 _{S. Beole,}51 _{I. Berceanu,}26 _{A. Bercuci,}26 _{Y. Berdnikov,}52 _{D. Berenyi,}7

A.A.E. Bergognon,33_{D. Berzano,}17 _{L. Betev,}6 _{A. Bhasin,}53 _{A.K. Bhati,}5 _{J. Bhom,}54_{L. Bianchi,}51 _{N. Bianchi,}55

C. Bianchin,56 _{J. Bielˇc´ık,}2 _{J. Bielˇc´ıkov´a,}3_{A. Bilandzic,}57, 47 _{S. Bjelogrlic,}58 _{F. Blanco,}59 _{F. Blanco,}50_{D. Blau,}16

C. Blume,34 _{M. Boccioli,}6_{N. Bock,}30 _{S. B¨ottger,}60 _{A. Bogdanov,}61 _{H. Bøggild,}47 _{M. Bogolyubsky,}62 _{L. Boldizs´ar,}7

M. Bombara,63 _{J. Book,}34_{H. Borel,}40_{A. Borissov,}64_{S. Bose,}65_{F. Boss´u,}51 _{M. Botje,}57_{B. Boyer,}66 _{E. Braidot,}67

P. Braun-Munzinger,27 _{M. Bregant,}33 _{T. Breitner,}60 _{T.A. Browning,}68_{M. Broz,}69_{R. Brun,}6 _{E. Bruna,}51, 17

G.E. Bruno,23 _{D. Budnikov,}70 _{H. Buesching,}34_{S. Bufalino,}51, 17 _{K. Bugaiev,}20_{O. Busch,}28 _{Z. Buthelezi,}71

D. Caballero Orduna,4 _{D. Caffarri,}56_{X. Cai,}72 _{H. Caines,}4_{E. Calvo Villar,}73 _{P. Camerini,}74 _{V. Canoa Roman,}75, 76

G. Cara Romeo,18 _{F. Carena,}6 _{W. Carena,}6 _{N. Carlin Filho,}77 _{F. Carminati,}6 _{C.A. Carrillo Montoya,}6

A. Casanova D´ıaz,55 _{J. Castillo Castellanos,}40 _{J.F. Castillo Hernandez,}27 _{E.A.R. Casula,}78_{V. Catanescu,}26

C. Cavicchioli,6 _{C. Ceballos Sanchez,}79 _{J. Cepila,}2 _{P. Cerello,}17_{B. Chang,}38, 80 _{S. Chapeland,}6 _{J.L. Charvet,}40

S. Chattopadhyay,10 _{S. Chattopadhyay,}65 _{I. Chawla,}5 _{M. Cherney,}81 _{C. Cheshkov,}6, 82 _{B. Cheynis,}82

V. Chibante Barroso,6_{D.D. Chinellato,}83 _{P. Chochula,}6 _{M. Chojnacki,}58 _{S. Choudhury,}10_{P. Christakoglou,}57, 58

C.H. Christensen,47_{P. Christiansen,}84 _{T. Chujo,}54_{S.U. Chung,}85 _{C. Cicalo,}86 _{L. Cifarelli,}8, 6, 19 _{F. Cindolo,}18

J. Cleymans,71 _{F. Coccetti,}19 _{F. Colamaria,}23 _{D. Colella,}23 _{G. Conesa Balbastre,}35_{Z. Conesa del Valle,}6

P. Constantin,28 _{G. Contin,}74 _{J.G. Contreras,}75 _{T.M. Cormier,}64 _{Y. Corrales Morales,}51 _{P. Cortese,}87

I. Cort´es Maldonado,76_{M.R. Cosentino,}67 _{F. Costa,}6 _{M.E. Cotallo,}59_{E. Crescio,}75_{P. Crochet,}13 _{E. Cruz Alaniz,}9

E. Cuautle,88 _{L. Cunqueiro,}55 _{A. Dainese,}56, 32 _{H.H. Dalsgaard,}47 _{A. Danu,}89 _{D. Das,}65 _{I. Das,}66 _{K. Das,}65

S. Dash,48 _{A. Dash,}83 _{S. De,}10 _{G.O.V. de Barros,}77 _{A. De Caro,}90, 19 _{G. de Cataldo,}91 _{J. de Cuveland,}22

A. De Falco,78 _{D. De Gruttola,}90 _{H. Delagrange,}33 _{A. Deloff,}92 _{V. Demanov,}70 _{N. De Marco,}17 _{E. D´enes,}7

S. De Pasquale,90 _{A. Deppman,}77 _{G. D Erasmo,}23_{R. de Rooij,}58_{M.A. Diaz Corchero,}59_{D. Di Bari,}23 _{T. Dietel,}29

S. Di Liberto,93 _{A. Di Mauro,}6 _{P. Di Nezza,}55 _{R. Divi`a,}6 _{Ø. Djuvsland,}24 _{A. Dobrin,}64, 84 _{T. Dobrowolski,}92

I. Dom´ınguez,88 _{B. D¨onigus,}27 _{O. Dordic,}94 _{O. Driga,}33 _{A.K. Dubey,}10 _{L. Ducroux,}82 _{P. Dupieux,}13

M.R. Dutta Majumdar,10 _{A.K. Dutta Majumdar,}65 _{D. Elia,}91_{D. Emschermann,}29_{H. Engel,}60 _{H.A. Erdal,}21

B. Espagnon,66_{M. Estienne,}33_{S. Esumi,}54 _{D. Evans,}44_{G. Eyyubova,}94_{D. Fabris,}56, 32 _{J. Faivre,}35 _{D. Falchieri,}8

A. Fantoni,55_{M. Fasel,}27 _{R. Fearick,}71 _{A. Fedunov,}46 _{D. Fehlker,}24 _{L. Feldkamp,}29 _{D. Felea,}89 _{B. Fenton-Olsen,}67

G. Feofilov,25 _{A. Fern´andez T´ellez,}76_{A. Ferretti,}51 _{R. Ferretti,}87_{J. Figiel,}45 _{M.A.S. Figueredo,}77 _{S. Filchagin,}70

D. Finogeev,95_{F.M. Fionda,}23 _{E.M. Fiore,}23 _{M. Floris,}6_{S. Foertsch,}71 _{P. Foka,}27 _{S. Fokin,}16 _{E. Fragiacomo,}96

U. Frankenfeld,27_{U. Fuchs,}6 _{C. Furget,}35_{M. Fusco Girard,}90 _{J.J. Gaardhøje,}47 _{M. Gagliardi,}51 _{A. Gago,}73

M. Gallio,51 _{D.R. Gangadharan,}30 _{P. Ganoti,}37 _{C. Garabatos,}27 _{E. Garcia-Solis,}97_{I. Garishvili,}1_{J. Gerhard,}22

M. Germain,33 _{C. Geuna,}40 _{A. Gheata,}6 _{M. Gheata,}89, 6 _{B. Ghidini,}23_{P. Ghosh,}10_{C. Di Giglio,}23 _{P. Gianotti,}55

M.R. Girard,98 _{P. Giubellino,}6 _{E. Gladysz-Dziadus,}45 _{P. Gl¨assel,}28 _{R. Gomez,}99 _{A. Gonschior,}27 _{E.G. Ferreiro,}36

L.H. Gonz´alez-Trueba,9 _{P. Gonz´alez-Zamora,}59 _{S. Gorbunov,}22 _{A. Goswami,}100 _{S. Gotovac,}101 _{V. Grabski,}9

L.K. Graczykowski,98_{R. Grajcarek,}28_{A. Grelli,}58 _{C. Grigoras,}6 _{A. Grigoras,}6_{V. Grigoriev,}61 _{A. Grigoryan,}102

S. Grigoryan,46 _{B. Grinyov,}20 _{N. Grion,}96 _{P. Gros,}84 _{J.F. Grosse-Oetringhaus,}6 _{J.-Y. Grossiord,}82_{R. Grosso,}6

F. Guber,95 _{R. Guernane,}35 _{C. Guerra Gutierrez,}73 _{B. Guerzoni,}8 _{M. Guilbaud,}82_{K. Gulbrandsen,}47_{T. Gunji,}103

B.H. Han,104 _{L.D. Hanratty,}44 _{A. Hansen,}47 _{Z. Harmanova,}63 _{J.W. Harris,}4 _{M. Hartig,}34 _{D. Hasegan,}89

D. Hatzifotiadou,18 _{A. Hayrapetyan,}6, 102 _{S.T. Heckel,}34 _{M. Heide,}29 _{H. Helstrup,}21 _{A. Herghelegiu,}26

G. Herrera Corral,75_{N. Herrmann,}28 _{B.A. Hess,}105 _{K.F. Hetland,}21 _{B. Hicks,}4 _{P.T. Hille,}4 _{B. Hippolyte,}49

T. Horaguchi,54 _{Y. Hori,}103 _{P. Hristov,}6_{I. Hˇrivn´aˇcov´a,}66_{M. Huang,}24_{T.J. Humanic,}30_{D.S. Hwang,}104_{R. Ichou,}13

R. Ilkaev,70 _{I. Ilkiv,}92 _{M. Inaba,}54_{E. Incani,}78 _{G.M. Innocenti,}51 _{P.G. Innocenti,}6 _{M. Ippolitov,}16 _{M. Irfan,}11

C. Ivan,27 _{V. Ivanov,}52_{M. Ivanov,}27_{A. Ivanov,}25 _{O. Ivanytskyi,}20 _{A. Jacho lkowski,}6_{P. M. Jacobs,}67_{H.J. Jang,}12

S. Jangal,49 _{M.A. Janik,}98 _{R. Janik,}69 _{P.H.S.Y. Jayarathna,}50_{S. Jena,}48_{D.M. Jha,}64_{R.T. Jimenez Bustamante,}88

L. Jirden,6_{P.G. Jones,}44 _{H. Jung,}14 _{A. Jusko,}44_{A.B. Kaidalov,}15 _{V. Kakoyan,}102_{S. Kalcher,}22_{P. Kaliˇn´ak,}41

T. Kalliokoski,38 _{A. Kalweit,}106_{K. Kanaki,}24_{J.H. Kang,}80 _{V. Kaplin,}61 _{A. Karasu Uysal,}6, 107 _{O. Karavichev,}95

T. Karavicheva,95 _{E. Karpechev,}95 _{A. Kazantsev,}16 _{U. Kebschull,}60 _{R. Keidel,}108 _{P. Khan,}65 _{M.M. Khan,}11

S.A. Khan,10_{A. Khanzadeev,}52_{Y. Kharlov,}62 _{B. Kileng,}21 _{D.W. Kim,}14 _M.Kim,14 _{M. Kim,}80 _{S.H. Kim,}14

D.J. Kim,38 S. Kim,104 J.H. Kim,104 J.S. Kim,14 B. Kim,80 T. Kim,80 S. Kirsch,22 I. Kisel,22S. Kiselev,15 A. Kisiel,6, 98 _{J.L. Klay,}109 _{J. Klein,}28 _{C. Klein-B¨osing,}29 _{M. Kliemant,}34 _{A. Kluge,}6 _{M.L. Knichel,}27

A.G. Knospe,110_{K. Koch,}28 _{M.K. K¨ohler,}27 _{A. Kolojvari,}25_{V. Kondratiev,}25_{N. Kondratyeva,}61 _{A. Konevskikh,}95

A. Korneev,70 _{R. Kour,}44_{M. Kowalski,}45_{S. Kox,}35 _{G. Koyithatta Meethaleveedu,}48 _{J. Kral,}38 _{I. Kr´alik,}41

F. Kramer,34 _{I. Kraus,}27_{T. Krawutschke,}28, 111_{M. Krelina,}2_{M. Kretz,}22 _{M. Krivda,}44, 41_{F. Krizek,}38_{M. Krus,}2

E. Kryshen,52 _{M. Krzewicki,}27 _{Y. Kucheriaev,}16 _{C. Kuhn,}49 _{P.G. Kuijer,}57 _{I. Kulakov,}34_{J. Kumar,}48

P. Kurashvili,92 _{A.B. Kurepin,}95 _{A. Kurepin,}95 _{A. Kuryakin,}70_{V. Kushpil,}3 _{S. Kushpil,}3 _{H. Kvaerno,}94

M.J. Kweon,28 _{Y. Kwon,}80_{P. Ladr´on de Guevara,}88_{I. Lakomov,}66_{R. Langoy,}24_{S.L. La Pointe,}58_{C. Lara,}60

A. Lardeux,33 _{P. La Rocca,}43 _{C. Lazzeroni,}44 _{R. Lea,}74 _{Y. Le Bornec,}66 _{M. Lechman,}6 _{S.C. Lee,}14 _{K.S. Lee,}14

G.R. Lee,44 _{F. Lef`evre,}33 _{J. Lehnert,}34 _{L. Leistam,}6 _{M. Lenhardt,}33_{V. Lenti,}91 _{H. Le´on,}9 _{M. Leoncino,}17

I. Le´on Monz´on,99 _{H. Le´on Vargas,}34_{P. L´evai,}7 _{J. Lien,}24 _{R. Lietava,}44 _{S. Lindal,}94 _{V. Lindenstruth,}22

C. Lippmann,27, 6 _{M.A. Lisa,}30 _{L. Liu,}24 _{P.I. Loenne,}24 _{V.R. Loggins,}64 _{V. Loginov,}61_{S. Lohn,}6 _{D. Lohner,}28

C. Loizides,67_{K.K. Loo,}38_{X. Lopez,}13_{E. L´opez Torres,}79_{G. Løvhøiden,}94_{X.-G. Lu,}28_{P. Luettig,}34_{M. Lunardon,}56

J. Luo,72_{G. Luparello,}58_{L. Luquin,}33 _{C. Luzzi,}6 _{R. Ma,}4 _{K. Ma,}72 _{D.M. Madagodahettige-Don,}50_{A. Maevskaya,}95

M. Mager,106, 6 _{D.P. Mahapatra,}42 _{A. Maire,}28 _{M. Malaev,}52 _{I. Maldonado Cervantes,}88 _{L. Malinina,}46, a

D. Mal’Kevich,15 _{P. Malzacher,}27 _{A. Mamonov,}70 _{L. Manceau,}17 _{L. Mangotra,}53 _{V. Manko,}16 _{F. Manso,}13

V. Manzari,91 _{Y. Mao,}72_{M. Marchisone,}13, 51 _{J. Mareˇs,}112_{G.V. Margagliotti,}74, 96 _{A. Margotti,}18_{A. Mar´ın,}27

C.A. Marin Tobon,6 _{C. Markert,}110 _{I. Martashvili,}113 _{P. Martinengo,}6_{M.I. Mart´ınez,}76_{A. Mart´ınez Davalos,}9

G. Mart´ınez Garc´ıa,33_{Y. Martynov,}20_{A. Mas,}33 _{S. Masciocchi,}27_{M. Masera,}51_{A. Masoni,}86 _{L. Massacrier,}82, 33

M. Mastromarco,91 _{A. Mastroserio,}23, 6 _{Z.L. Matthews,}44 _{A. Matyja,}45, 33 _{D. Mayani,}88 _{C. Mayer,}45 _{J. Mazer,}113

M.A. Mazzoni,93 _{F. Meddi,}114 _{A. Menchaca-Rocha,}9 _{J. Mercado P´erez,}28 _{M. Meres,}69_{Y. Miake,}54_{L. Milano,}51

J. Milosevic,94, b _{A. Mischke,}58 _{A.N. Mishra,}100_{D. Mi´skowiec,}27, 6 _{C. Mitu,}89 _{J. Mlynarz,}64 _{B. Mohanty,}10

A.K. Mohanty,6 _{L. Molnar,}6 _{L. Monta˜no Zetina,}75 _{M. Monteno,}17 _{E. Montes,}59 _{T. Moon,}80 _{M. Morando,}56

D.A. Moreira De Godoy,77_{S. Moretto,}56 _{A. Morsch,}6_{V. Muccifora,}55_{E. Mudnic,}101 _{S. Muhuri,}10_{M. Mukherjee,}10

H. M¨uller,6 _{M.G. Munhoz,}77 _{L. Musa,}6 _{A. Musso,}17 _{B.K. Nandi,}48 _{R. Nania,}18_{E. Nappi,}91 _{C. Nattrass,}113

N.P. Naumov,70_{S. Navin,}44 _{T.K. Nayak,}10_{S. Nazarenko,}70 _{G. Nazarov,}70 _{A. Nedosekin,}15 _{M. Nicassio,}23

M.Niculescu,89, 6 _{B.S. Nielsen,}47 _{T. Niida,}54 _{S. Nikolaev,}16 _{V. Nikolic,}31 _{S. Nikulin,}16 _{V. Nikulin,}52

B.S. Nilsen,81 _{M.S. Nilsson,}94 _{F. Noferini,}18, 19 _{P. Nomokonov,}46 _{G. Nooren,}58 _{N. Novitzky,}38 _{A. Nyanin,}16

A. Nyatha,48 _{C. Nygaard,}47 _{J. Nystrand,}24_{A. Ochirov,}25_{H. Oeschler,}106, 6 _{S. Oh,}4 _{S.K. Oh,}14_{J. Oleniacz,}98

C. Oppedisano,17 _{A. Ortiz Velasquez,}84, 88 _{G. Ortona,}51_{A. Oskarsson,}84 _{P. Ostrowski,}98_{J. Otwinowski,}27

K. Oyama,28 _{K. Ozawa,}103_{Y. Pachmayer,}28_{M. Pachr,}2_{F. Padilla,}51 _{P. Pagano,}90 _{G. Pai´c,}88 _{F. Painke,}22

C. Pajares,36 _{S. Pal,}40 _{S.K. Pal,}10_{A. Palaha,}44_{A. Palmeri,}39_{V. Papikyan,}102 _{G.S. Pappalardo,}39 _{W.J. Park,}27

A. Passfeld,29_{B. Pastirˇc´ak,}41_{D.I. Patalakha,}62 _{V. Paticchio,}91_{A. Pavlinov,}64_{T. Pawlak,}98_{T. Peitzmann,}58

H. Pereira Da Costa,40 _{E. Pereira De Oliveira Filho,}77_{D. Peresunko,}16_{C.E. P´erez Lara,}57_{E. Perez Lezama,}88

D. Perini,6 _{D. Perrino,}23 _{W. Peryt,}98 _{A. Pesci,}18 _{V. Peskov,}6, 88 _{Y. Pestov,}115 _{V. Petr´aˇcek,}2_{M. Petran,}2

M. Petris,26 _{P. Petrov,}44_{M. Petrovici,}26_{C. Petta,}43_{S. Piano,}96_{A. Piccotti,}17_{M. Pikna,}69_{P. Pillot,}33 _{O. Pinazza,}6

L. Pinsky,50 _{N. Pitz,}34 _{D.B. Piyarathna,}50_{M. P losko´n,}67 _{J. Pluta,}98 _{T. Pocheptsov,}46 _{S. Pochybova,}7

P.L.M. Podesta-Lerma,99_{M.G. Poghosyan,}6, 51 _{K. Pol´ak,}112_{B. Polichtchouk,}62 _{A. Pop,}26_{S. Porteboeuf-Houssais,}13

V. Posp´ıˇsil,2_{B. Potukuchi,}53_{S.K. Prasad,}64 _{R. Preghenella,}18, 19 _{F. Prino,}17 _{C.A. Pruneau,}64_{I. Pshenichnov,}95

E. Quercigh,6 _{H. Qvigstad,}94 _{A. Rachevski,}96 _{A. Rademakers,}6_{S. Radomski,}28 _{T.S. R¨aih¨a,}38 _{J. Rak,}38

A. Rakotozafindrabe,40 _{L. Ramello,}87 _{A. Ram´ırez Reyes,}75 _{S. Raniwala,}100_{R. Raniwala,}100 _{S.S. R¨as¨anen,}38

B.T. Rascanu,34 _{D. Rathee,}5 _{K.F. Read,}113_{J.S. Real,}35 _{K. Redlich,}92, 116 _{P. Reichelt,}34_{M. Reicher,}58_{R. Renfordt,}34

A.R. Reolon,55 _{A. Reshetin,}95 _{F. Rettig,}22 _{J.-P. Revol,}6 _{K. Reygers,}28_{L. Riccati,}17 _{R.A. Ricci,}117 _{T. Richert,}84

M. Richter,94_{P. Riedler,}6 _{W. Riegler,}6 _{F. Riggi,}43, 39_{B. Rodrigues Fernandes Rabacal,}6 _{M. Rodr´ıguez Cahuantzi,}76

A. Rodriguez Manso,57 _{K. Røed,}24 _{D. Rohr,}22 _{D. R¨ohrich,}24 _{R. Romita,}27 _{F. Ronchetti,}55 _{P. Rosnet,}13

S. Rossegger,6 _{A. Rossi,}6, 56 _{C. Roy,}49 _{P. Roy,}65 _{A.J. Rubio Montero,}59_{R. Rui,}74 _{E. Ryabinkin,}16 _{A. Rybicki,}45

S. Sadovsky,62_{K. ˇSafaˇr´ık,}6_{R. Sahoo,}118 _{P.K. Sahu,}42 _{J. Saini,}10 _{H. Sakaguchi,}119 _{S. Sakai,}67 _{D. Sakata,}54

C.A. Salgado,36_{J. Salzwedel,}30 _{S. Sambyal,}53_{V. Samsonov,}52 _{X. Sanchez Castro,}49_{L. ˇS´andor,}41 _{A. Sandoval,}9

S. Sano,103 _{M. Sano,}54 _{R. Santo,}29 _{R. Santoro,}91, 6, 19 _{J. Sarkamo,}38 _{E. Scapparone,}18 _{F. Scarlassara,}56

R.P. Scharenberg,68_{C. Schiaua,}26 _{R. Schicker,}28 _{C. Schmidt,}27_{H.R. Schmidt,}105 _{S. Schreiner,}6_{S. Schuchmann,}34

J. Schukraft,6 Y. Schutz,6, 33 K. Schwarz,27K. Schweda,27, 28 G. Scioli,8 E. Scomparin,17 R. Scott,113 P.A. Scott,44 G. Segato,56_{I. Selyuzhenkov,}27_{S. Senyukov,}87, 49_{J. Seo,}85_{S. Serci,}78_{E. Serradilla,}59, 9_{A. Sevcenco,}89_{A. Shabetai,}33

G. Shabratova,46_{R. Shahoyan,}6_{N. Sharma,}5 _{S. Sharma,}53_{S. Rohni,}53_{K. Shigaki,}119_{M. Shimomura,}54_{K. Shtejer,}79

Y. Sibiriak,16 _{M. Siciliano,}51_{E. Sicking,}6 _{S. Siddhanta,}86 _{T. Siemiarczuk,}92 _{D. Silvermyr,}37 _{c. Silvestre,}35

G. Simatovic,88, 31 _{G. Simonetti,}6 _{R. Singaraju,}10_{R. Singh,}53 _{S. Singha,}10 _{V. Singhal,}10_{T. Sinha,}65 _{B.C. Sinha,}10

B. Sitar,69 _{M. Sitta,}87 _{T.B. Skaali,}94 _{K. Skjerdal,}24 _{R. Smakal,}2 _{N. Smirnov,}4_{R.J.M. Snellings,}58 _{C. Søgaard,}47

R. Soltz,1 _{H. Son,}104_{M. Song,}80 _{J. Song,}85 _{C. Soos,}6_{F. Soramel,}56 _{I. Sputowska,}45 _{M. Spyropoulou-Stassinaki,}120

B.K. Srivastava,68 _{J. Stachel,}28 _{I. Stan,}89 _{I. Stan,}89 _{G. Stefanek,}92 _{T. Steinbeck,}22 _{M. Steinpreis,}30 _{E. Stenlund,}84

G. Steyn,71 _{J.H. Stiller,}28 _{D. Stocco,}33 _{M. Stolpovskiy,}62_{K. Strabykin,}70_{P. Strmen,}69 _{A.A.P. Suaide,}77

M.A. Subieta V´asquez,51_{T. Sugitate,}119_{C. Suire,}66 _{M. Sukhorukov,}70_{R. Sultanov,}15 _{M. ˇSumbera,}3_{T. Susa,}31

A. Szanto de Toledo,77_{I. Szarka,}69 _{A. Szczepankiewicz,}45 _{A. Szostak,}24_{M. Szymanski,}98 _{J. Takahashi,}83

J.D. Tapia Takaki,66_{A. Tauro,}6 _{G. Tejeda Mu˜noz,}76 _{A. Telesca,}6 _{C. Terrevoli,}23 _{J. Th¨ader,}27 _{D. Thomas,}58

R. Tieulent,82 _{A.R. Timmins,}50_{D. Tlusty,}2_{A. Toia,}22, 6_{H. Torii,}103 _{L. Toscano,}17 _{D. Truesdale,}30_{W.H. Trzaska,}38

T. Tsuji,103 _{A. Tumkin,}70 _{R. Turrisi,}32 _{T.S. Tveter,}94 _{J. Ulery,}34 _{K. Ullaland,}24 _{J. Ulrich,}121, 60 _{A. Uras,}82

J. Urb´an,63 _{G.M. Urciuoli,}93 _{G.L. Usai,}78 _{M. Vajzer,}2, 3 _{M. Vala,}46, 41 _{L. Valencia Palomo,}66 _{S. Vallero,}28

N. van der Kolk,57_{P. Vande Vyvre,}6_{M. van Leeuwen,}58 _{L. Vannucci,}117_{A. Vargas,}76 _{R. Varma,}48_{M. Vasileiou,}120

A. Vasiliev,16 _{V. Vechernin,}25_{M. Veldhoen,}58 _{M. Venaruzzo,}74 _{E. Vercellin,}51 _{S. Vergara,}76 _{R. Vernet,}122

M. Verweij,58 _{L. Vickovic,}101_{G. Viesti,}56_{O. Vikhlyantsev,}70 _{Z. Vilakazi,}71_{O. Villalobos Baillie,}44 _{A. Vinogradov,}16

L. Vinogradov,25_{Y. Vinogradov,}70_{T. Virgili,}90 _{Y.P. Viyogi,}10 _{A. Vodopyanov,}46 _{K. Voloshin,}15 _{S. Voloshin,}64

G. Volpe,23, 6 _{B. von Haller,}6_{D. Vranic,}27 _{G. Øvrebekk,}24 _{J. Vrl´akov´a,}63 _{B. Vulpescu,}13 _{A. Vyushin,}70_{V. Wagner,}2

B. Wagner,24 _{R. Wan,}49, 72 _{M. Wang,}72 _{D. Wang,}72 _{Y. Wang,}28 _{Y. Wang,}72 _{K. Watanabe,}54 _{M. Weber,}50

J.P. Wessels,6, 29 _{U. Westerhoff,}29_{J. Wiechula,}105_{J. Wikne,}94 _{M. Wilde,}29 _{G. Wilk,}92_{A. Wilk,}29 _{M.C.S. Williams,}18

B. Windelband,28 _{L. Xaplanteris Karampatsos,}110_{C.G. Yaldo,}64 _{Y. Yamaguchi,}103_{H. Yang,}40 _{S. Yang,}24

S. Yasnopolskiy,16 _{J. Yi,}85 _{Z. Yin,}72 _{I.-K. Yoo,}85 _{J. Yoon,}80 _{W. Yu,}34 _{X. Yuan,}72 _{I. Yushmanov,}16 _{C. Zach,}2

C. Zampolli,18 _{S. Zaporozhets,}46_{A. Zarochentsev,}25_{P. Z´avada,}112 _{N. Zaviyalov,}70_{H. Zbroszczyk,}98_{P. Zelnicek,}60

I.S. Zgura,89_{M. Zhalov,}52_{X. Zhang,}13, 72 _{H. Zhang,}72_{F. Zhou,}72 _{D. Zhou,}72 _{Y. Zhou,}58 _{J. Zhu,}72_{J. Zhu,}72

X. Zhu,72 _{A. Zichichi,}8, 19 _{A. Zimmermann,}28 _{G. Zinovjev,}20_{Y. Zoccarato,}82_{M. Zynovyev,}20 _{and M. Zyzak}34

1_{Lawrence Livermore National Laboratory, Livermore, California, United States}

2_{Faculty of Nuclear Sciences and Physical Engineering,}

Czech Technical University in Prague, Prague, Czech Republic

3_{Nuclear Physics Institute, Academy of Sciences of the Czech Republic, ˇReˇz u Prahy, Czech Republic}

4_{Yale University, New Haven, Connecticut, United States}

5_{Physics Department, Panjab University, Chandigarh, India}

6_{European Organization for Nuclear Research (CERN), Geneva, Switzerland}

7_{KFKI Research Institute for Particle and Nuclear Physics,}

Hungarian Academy of Sciences, Budapest, Hungary

8_{Dipartimento di Fisica dell’Universit`a and Sezione INFN, Bologna, Italy}

9_{Instituto de F´ısica, Universidad Nacional Aut´onoma de M´exico, Mexico City, Mexico}

10_{Variable Energy Cyclotron Centre, Kolkata, India}

11_{Department of Physics Aligarh Muslim University, Aligarh, India}

12_{Korea Institute of Science and Technology Information, Daejeon, South Korea}

13_{Laboratoire de Physique Corpusculaire (LPC), Clermont Universit´e,}

14_{Gangneung-Wonju National University, Gangneung, South Korea}

15_{Institute for Theoretical and Experimental Physics, Moscow, Russia}

16_{Russian Research Centre Kurchatov Institute, Moscow, Russia}

17_{Sezione INFN, Turin, Italy}

18_{Sezione INFN, Bologna, Italy}

19_{Centro Fermi – Centro Studi e Ricerche e Museo Storico della Fisica “Enrico Fermi”, Rome, Italy}

20_{Bogolyubov Institute for Theoretical Physics, Kiev, Ukraine}

21_{Faculty of Engineering, Bergen University College, Bergen, Norway}

22_{Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universit¨at Frankfurt, Frankfurt, Germany}

23_{Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy}

24_{Department of Physics and Technology, University of Bergen, Bergen, Norway}

25_{V. Fock Institute for Physics, St. Petersburg State University, St. Petersburg, Russia}

26_{National Institute for Physics and Nuclear Engineering, Bucharest, Romania}

27_{Research Division and ExtreMe Matter Institute EMMI,}

GSI Helmholtzzentrum f¨ur Schwerionenforschung, Darmstadt, Germany

28_{Physikalisches Institut, Ruprecht-Karls-Universit¨at Heidelberg, Heidelberg, Germany}

29_{Institut f¨ur Kernphysik, Westf¨alische Wilhelms-Universit¨at M¨unster, M¨unster, Germany}

30_{Department of Physics, Ohio State University, Columbus, Ohio, United States}

31_{Rudjer Boˇskovi´c Institute, Zagreb, Croatia}

32_{Sezione INFN, Padova, Italy}

33_{SUBATECH, Ecole des Mines de Nantes, Universit´e de Nantes, CNRS-IN2P3, Nantes, France}

34_{Institut f¨ur Kernphysik, Johann Wolfgang Goethe-Universit¨at Frankfurt, Frankfurt, Germany}

35_{Laboratoire de Physique Subatomique et de Cosmologie (LPSC), Universit´e Joseph Fourier,}

CNRS-IN2P3, Institut Polytechnique de Grenoble, Grenoble, France

36_{Departamento de F´ısica de Part´ıculas and IGFAE,}

Universidad de Santiago de Compostela, Santiago de Compostela, Spain

37_{Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States}

38_{Helsinki Institute of Physics (HIP) and University of Jyv¨askyl¨a, Jyv¨askyl¨a, Finland}

39_{Sezione INFN, Catania, Italy}

40_{Commissariat `a l’Energie Atomique, IRFU, Saclay, France}

41_{Institute of Experimental Physics, Slovak Academy of Sciences, Koˇsice, Slovakia}

42_{Institute of Physics, Bhubaneswar, India}

43_{Dipartimento di Fisica e Astronomia dell’Universit`a and Sezione INFN, Catania, Italy}

44_{School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom}

45_{The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland}

46_{Joint Institute for Nuclear Research (JINR), Dubna, Russia}

47_{Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark}

48_{Indian Institute of Technology Bombay (IIT), Mumbai, India}

49_{Institut Pluridisciplinaire Hubert Curien (IPHC),}

Universit´e de Strasbourg, CNRS-IN2P3, Strasbourg, France

50_{University of Houston, Houston, Texas, United States}

51_{Dipartimento di Fisica Sperimentale dell’Universit`a and Sezione INFN, Turin, Italy}

52_{Petersburg Nuclear Physics Institute, Gatchina, Russia}

53_{Physics Department, University of Jammu, Jammu, India}

54_{University of Tsukuba, Tsukuba, Japan}

55_{Laboratori Nazionali di Frascati, INFN, Frascati, Italy}

56_{Dipartimento di Fisica dell’Universit`a and Sezione INFN, Padova, Italy}

57_{Nikhef, National Institute for Subatomic Physics, Amsterdam, Netherlands}

58_{Nikhef, National Institute for Subatomic Physics and Institute}

for Subatomic Physics of Utrecht University, Utrecht, Netherlands

59_{Centro de Investigaciones Energ´eticas Medioambientales y Tecnol´ogicas (CIEMAT), Madrid, Spain}

60_{Institut f¨ur Informatik, Johann Wolfgang Goethe-Universit¨at Frankfurt, Frankfurt, Germany}

61_{Moscow Engineering Physics Institute, Moscow, Russia}

62_{Institute for High Energy Physics, Protvino, Russia}

63_{Faculty of Science, P.J. ˇSaf´arik University, Koˇsice, Slovakia}

64_{Wayne State University, Detroit, Michigan, United States}

65_{Saha Institute of Nuclear Physics, Kolkata, India}

66_{Institut de Physique Nucl´eaire d’Orsay (IPNO),}

Universit´e Paris-Sud, CNRS-IN2P3, Orsay, France

67_{Lawrence Berkeley National Laboratory, Berkeley, California, United States}

68_{Purdue University, West Lafayette, Indiana, United States}

69_{Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, Slovakia}

70_{Russian Federal Nuclear Center (VNIIEF), Sarov, Russia}

72_{Hua-Zhong Normal University, Wuhan, China}

73_{Secci´on F´ısica, Departamento de Ciencias, Pontificia Universidad Cat´olica del Per´u, Lima, Peru}

74_{Dipartimento di Fisica dell’Universit`a and Sezione INFN, Trieste, Italy}

75_{Centro de Investigaci´on y de Estudios Avanzados (CINVESTAV), Mexico City and M´erida, Mexico}

76_{Benem´erita Universidad Aut´onoma de Puebla, Puebla, Mexico}

77_{Universidade de S˜ao Paulo (USP), S˜ao Paulo, Brazil}

78_{Dipartimento di Fisica dell’Universit`a and Sezione INFN, Cagliari, Italy}

79_{Centro de Aplicaciones Tecnol´ogicas y Desarrollo Nuclear (CEADEN), Havana, Cuba}

80_{Yonsei University, Seoul, South Korea}

81_{Physics Department, Creighton University, Omaha, Nebraska, United States}

82_{Universit´e de Lyon, Universit´e Lyon 1, CNRS/IN2P3, IPN-Lyon, Villeurbanne, France}

83_{Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil}

84_{Division of Experimental High Energy Physics, University of Lund, Lund, Sweden}

85_{Pusan National University, Pusan, South Korea}

86_{Sezione INFN, Cagliari, Italy}

87_{Dipartimento di Scienze e Innovazione Tecnologica dell’Universit`a del}

Piemonte Orientale and Gruppo Collegato INFN, Alessandria, Italy

88_{Instituto de Ciencias Nucleares, Universidad Nacional Aut´onoma de M´exico, Mexico City, Mexico}

89_{Institute of Space Sciences (ISS), Bucharest, Romania}

90_{Dipartimento di Fisica ‘E.R. Caianiello’ dell’Universit`a and Gruppo Collegato INFN, Salerno, Italy}

91_{Sezione INFN, Bari, Italy}

92_{Soltan Institute for Nuclear Studies, Warsaw, Poland}

93_{Sezione INFN, Rome, Italy}

94_{Department of Physics, University of Oslo, Oslo, Norway}

95_{Institute for Nuclear Research, Academy of Sciences, Moscow, Russia}

96_{Sezione INFN, Trieste, Italy}

97_{Chicago State University, Chicago, United States}

98_{Warsaw University of Technology, Warsaw, Poland}

99_{Universidad Aut´onoma de Sinaloa, Culiac´an, Mexico}

100_{Physics Department, University of Rajasthan, Jaipur, India}

101_{Technical University of Split FESB, Split, Croatia}

102_{Yerevan Physics Institute, Yerevan, Armenia}

103_{University of Tokyo, Tokyo, Japan}

104_{Department of Physics, Sejong University, Seoul, South Korea}

105_{Eberhard Karls Universit¨at T¨ubingen, T¨ubingen, Germany}

106_{Institut f¨ur Kernphysik, Technische Universit¨at Darmstadt, Darmstadt, Germany}

107_{Yildiz Technical University, Istanbul, Turkey}

108_{Zentrum f¨ur Technologietransfer und Telekommunikation (ZTT), Fachhochschule Worms, Worms, Germany}

109_{California Polytechnic State University, San Luis Obispo, California, United States}

110_{The University of Texas at Austin, Physics Department, Austin, TX, United States}

111_{Fachhochschule K¨oln, K¨oln, Germany}

112_{Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic}

113_{University of Tennessee, Knoxville, Tennessee, United States}

114_{Dipartimento di Fisica dell’Universit`a ‘La Sapienza’ and Sezione INFN, Rome, Italy}

115_{Budker Institute for Nuclear Physics, Novosibirsk, Russia}

116_{Institut of Theoretical Physics, University of Wroclaw}

117_{Laboratori Nazionali di Legnaro, INFN, Legnaro, Italy}

118_{Indian Institute of Technology Indore (IIT), Indore, India}

119_{Hiroshima University, Hiroshima, Japan}

120_{Physics Department, University of Athens, Athens, Greece}

121_{Kirchhoff-Institut f¨ur Physik, Ruprecht-Karls-Universit¨at Heidelberg, Heidelberg, Germany}

122_{Centre de Calcul de l’IN2P3, Villeurbanne, France}

We report the first measurement of the net–charge fluctuations in Pb–Pb collisions at√sNN =

2.76 TeV, measured with the ALICE detector at the CERN Large Hadron Collider. The dynamical fluctuations per unit entropy are observed to decrease when going from peripheral to central colli-sions. An additional reduction in the amount of fluctuations is seen in comparison to the results from lower energies. We examine the dependence of fluctuations on the pseudo–rapidity interval, which may account for the dilution of fluctuations during the evolution of the system. We find that the fluctuations at LHC are smaller compared to the measurements at the Relativistic heavy Ion Collider (RHIC), and as such, closer to what has been theoretically predicted for the formation of the Quark–Gluon Plasma (QGP).

The ALICE experiment [1] at the Large Hadron Col-lider (LHC) is a multi–purpose detector designed to study the formation and evolution of nuclear matter at high temperatures and energy densities. One of the major goals of the experiment is to explore as many signals as possible towards characterizing the properties of the Quark–Gluon Plasma (QGP), the deconfined state of quarks and gluons, produced in high energy heavy–ion collisions. The study of event–by–event fluctuations pro-vides a powerful tool to characterize the thermodynamic properties of the system. The fluctuations of conserved quantities in a finite phase space window, like net–charge of the system, are predicted to be one of the most sensi-tive signals of the QGP formation and phase transition, and may provide complementary understanding of strong interactions [2–9].

In the QGP phase, the charge carriers are quarks with fractional charges, whereas the particles in a hadron gas (HG) carry unit charge. The fluctuations in the net– charge depend on the squares of the charge states present in the system. Consequently, the net–charge fluctuations in the QGP phase are significantly smaller compared to that of a HG [2]. At the same time, if the initial QGP phase is strongly gluon dominated, the fluctuation per entropy may further be reduced as the hadronization of gluons increases the entropy [3]. Thus the net–charge fluctuations are strongly dependent on which phase they originate from. However, the net–charge fluctuations may get affected by uncertainties arising from volume fluctuations, so one considers the fluctuations of the ra-tio, R = N+/N−. Here N+ and N− are the numbers

of positive and negative particles respectively, measured in a specific transverse momentum (pT ) and pseudo–

rapidity (η) window. The parameter R is related to the fluctuations of the net–charge via the D–measure as per the following expression [2, 4, 5]:

D = hNchihδR2i ≈ 4hδQ 2_i

hNchi

, (1) which provides a measure of the charge fluctuations per unit entropy. Here the h...i denotes an average of the quantity over an ensemble of events. The term hδQ2_{i is}

the variance of net charge, Q = N+ − N− and Nch =

N++ N−. The D–measure has been estimated for

sev-eral different theoretical considerations including those of the lattice calculations. In a simple picture by neglect-ing quark–quark interactions, D is found to be approxi-mately 4 times smaller for a QGP compared to a HG [2]. Lattice calculations which include the quark–quark in-teractions give a quantitatively different estimate for a

a _{M.V.Lomonosov Moscow State University, D.V.Skobeltsyn}

Insti-tute of Nuclear Physics, Moscow, Russia

b_{University of Belgrade, Faculty of Physics and ”Vin˘ca” Institute}

of Nuclear Sciences, Belgrade, Serbia

QGP phase, still significantly smaller than for a HG. It has been shown that D = 4 for an uncorrelated pion gas, and after taking resonance yields into account, the value decreases to D ≃ 3. For a QGP, D is significantly lower and has been calculated to be D ≃ 1.0–1.5 where the uncertainty arises from the uncertainty of relating the entropy to the number of charged particles in the final state [5]. Thus, a measurement of D can be effectively used as a probe for distinguishing the two phases, the HG and the QGP. However in reality, these fluctuations may get diluted in the rapidly expanding medium due to diffusion of particles in rapidity space [8, 9]. Several other effects, such as collision dynamics, radial flow, res-onance decays and final state interactions may also affect the amount of measured fluctuations [2, 10–12].

In the experiment, the net–charge fluctuations are best studied [12–17] by calculating the quantity ν(+−,dyn)

de-fined as: ν(+−,dyn)= hN +(N+− 1)i hN+i2 +hN−(N−− 1)i hN−i2 −2 hN−N+i hN−ihN+i , (2) which is a measure of the relative correlation strength of particle pairs. A negative value of ν(+−,dyn)

signi-fies the dominant contribution from correlations between pairs of opposite charges. On the other hand, a positive value indicates the significance of the same charge pair correlations. The ν(+−,dyn) has been found to be robust

against random efficiency losses [17–19]. D–measure and ν(+−,dyn)are related to each other by [5]:

hNchiν(+−,dyn)≈ D − 4. (3)

The values of ν(+−,dyn) need to be corrected for global

charge conservation [17]. The predictions for the D– measure are based on the assumption of vanishing net– charge in the system. However, in a realistic situation, the system under consideration has a small but finite net– charge. A correction due to finite net–charge effect also needs to be applied [4].

In this letter, we report the first measurements of net– charge fluctuations, by calculating ν(+−,dyn)and the D–

measure, as a function of collision centrality in Pb–Pb collisions at√sNN= 2.76 TeV at the LHC with the

AL-ICE detector. We also make a comparison of the experi-mental results to the theoretical predictions.

Details of the ALICE experiment and its detectors can be found in [1]. For this analysis, we have used the Time Projection Chamber (TPC) [20] to reconstruct charged particle tracks. The detector provides a uniform ac-ceptance with an almost constant tracking efficiency of about 80% in the analyzed phase space (|η| < 0.8 and 0.2 GeV/c < pT< 5 GeV/c). The interaction vertex was

measured using the Silicon Pixel Detector (SPD), the in-nermost detector of the Inner Tracking System (ITS).

In the analysis, we have considered events with a ver-tex |vz| < 10 cm to ensure a uniform acceptance in the

central pseudo–rapidity region. The minimum bias trig-ger consisted of a coincidence of at least one hit on each of the two VZERO scintillator detectors, positioned on both sides of the interaction point, while at the startup of data taking period an additional requirement of having a coincidence with a signal from the SPD was also in-troduced. The background events coming from parasitic beam interactions are removed by a standard offline event selection procedure, which requires the VZERO timing information and hits in the SPD.

We present the results as a function of centrality that reflects the collision geometry. The collision centrality is determined by cuts on the VZERO multiplicity [21]. A study based on Glauber model fits [22–24] to the multi-plicity distribution in the region corresponding to 90% of the most central collisions, where the vertex reconstruc-tion is fully efficient, facilitates the determinareconstruc-tion of the cross section percentile and the number of participants. The resolution in centrality is found to be < 0.5% RMS for the most central (0-5%) collisions, increasing towards 2% RMS for peripheral (70-80%) collisions [21].

We require tracks in the TPC to have at least 80 re-constructed space points with a χ2 _{per TPC cluster of}

the momentum fit less than 4. We reject tracks with dis-tance of closest approach (dca) to the vertex larger than 3 cm both in the transverse plane and in the longitudi-nal direction. We have performed an alternative alongitudi-nalysis with tracks reconstructed using the combined tracking of ITS and TPC. In this case, the dca cuts were 0.3 cm in the transverse plane as well as in the longitudinal direc-tion. The results obtained with both tracking approaches are in agreement.

The data analysis has been performed for Pb–Pb colli-sions at√sNN= 2.76 TeV and pp collisions at the same

centre–of–mass energy. An identical analysis procedure has been followed for both the data sets. We calculate the ν(+−,dyn) from the experimental measurements of

posi-tive and negaposi-tive charged particles counted in ∆η win-dows, defined around mid–rapidity (for example, ∆η = 1 corresponds to −0.5 ≤ η ≤ 0.5) and in the pTrange from

0.2 to 5.0 GeV/c. Consistency checks had been per-formed for another pT window, viz., 0.3 GeV/c < pT <

1.5 GeV/c. The differences in the fluctuation results are small, and included in the systematic errors. In Figure 1, we present the ν(+−,dyn) as a function of centrality,

ex-pressed in terms of the number of participating nucleons. Moving from left to right along the x–axis of the figure corresponds to moving from peripheral to central colli-sions. The results are presented for ∆η = 1 and 1.6, for both Pb–Pb and pp collisions. In all cases, the magni-tude of ν(+−,dyn) is observed to be negative, indicating

the dominance of the correlation term in Eq. 2. The absolute values of ν(+−,dyn) for pp collisions are larger

compared to those measured for Pb–Pb collisions. When

(+-,dyn)

ν

_-0.04 -0.03 -0.02 -0.01 0 ∆η = 1.0 ∆η = 1.6 ∆η = 1.0 ∆η = 1.6 Pb-Pb pp (+-,dyn) ν

_{

(+-,dyn) corr ν

{

〉 part N 〈 0 50 100 150 200 250 300 350 400 -0.2 -0.1 0

FIG. 1. Dynamical net–charge fluctuations, ν(+−,dyn) and

their corrected values, νcorr

(+−,dyn), for charged particles

pro-duced in Pb–Pb collisions at√sNN= 2.76 TeV as a function

of centrality, expressed as the number of participating

nu-cleons. ν₍₊₋corr_,dyn) points are shifted along x-axis for better

representation. Superimposed are the results for pp

colli-sions at√s = 2.76 TeV. The statistical (bar) and systematic

(box) errors are plotted.

going from peripheral to central events, the absolute val-ues of ν(+−,dyn)are seen to decrease monotonically.

The values of ν(+−,dyn)have to be corrected for global

charge conservation and finite acceptance [17]. If all charges were accepted, the global charge conservation would lead to vanishing fluctuations. This will yield the minimum value of ν(+−,dyn) to be -4/hNtotali, where

hNtotali is the average total number of charged

par-ticles produced over full phase space. The corrected ν(+−,dyn)is

ν(+−,dyn)corr = ν(+−,dyn)+

4 hNtotali

. (4) The values of hNtotali for Pb–Pb collisions have been

es-timated from the experimental data [25], whereas for pp collisions, it is taken from PYTHIA [26] event genera-tor. As a reference, hNchi for ∆η=1 and hNtotali

val-ues are 1637±61 and 17165±772 for most central (0-5%) Pb–Pb collisions, and 4.8±0.2 and 36.0 for pp collisions. These are systematic errors, statistical errors are neg-ligible. The corrected values, νcorr

(+−,dyn), are plotted in

Figure 1 as a function of the number of participating nu-cleons for Pb–Pb and pp collisions. The absolute values of νcorr

(+−,dyn)are smaller compared to ν(+−,dyn)in all cases.

The differences are more apparent for pp and peripheral Pb–Pb collisions than for central collisions.

Taking the above corrections into account, we obtain, D′ _{= hN}

chiν(+−,dyn)corr + 4. (5)

ob-〉 part N 〈 0 50 100 150 200 250 300 350 400 (+-,dyn) corr _ν 〉 ch N 〈 -3 -2.5 -2 -1.5 -1 -0.5 0 1 1.5 2 2.5 3 3.5 4 D Pb-Pb pp HIJING PYTHIA _{∆η = 1.0} ∆η = 1.6 Hadron Gas QGP

FIG. 2. hNchiν_(+−,dyn)corr (left axis) and D (right axis) as a

function of the number of participants for ∆η=1 and 1.6 in

Pb–Pb at √sNN= 2.76 TeV and pp collisions at √s = 2.76

TeV. Corresponding results from the HIJING and PYTHIA

event generators are also presented. The data points are

shifted minimally along x-axis for clear view. Both statistical (error bar) and systematic (box) errors are plotted.

tained using [4]:

D′′= (hNchiν(+−,dyn)+ 4)/(CµCη), (6)

where Cµ(=hN+i 2 hN−i

2) corrects for the effects of the finite net

charge, and Cη (=1−_hNtotalihNchi ) accounts for finite bin size

in rapidity as well as global charge conservation. Differ-ences in the two corrected values, D′ _{and D}′′_{, are within}

4–9%, depending on ∆η. Subsequently, mean values of D′_{and D}′′_{are plotted in the figures as D, and differences}

of those have been included as systematic errors. The systematic uncertainties have additional contri-butions from the following sources: (a) uncertainty in the determination of the interaction vertex, (b) differ-ent magnetic field polarities, (c) contamination from secondary tracks (dca cuts), (d) centrality definition using different detectors, (e) selection criteria at the track level, (f) different tracking scenarios, and (g) two different pT windows. The total systematic error on

νcorr

(+−,dyn) amounts to 6–10% in going from peripheral to

central collisions. The error on the product of number of charged particles and νcorr

(+−,dyn) remains within 7–13%

at all centralities. The systematic and statistical uncer-tainties in all the figures are represented by boxes and error bars, respectively. The statistical errors are small and within the sizes of the symbols in most cases.

Figure 2 presents the values of hNchiν_(+−,dyn)corr and D in

the left and right axes, respectively, as a function of the number of participating nucleons. The hNchi values have

been measured for different centralities and ∆η windows, and corrected for detector inefficiencies [21]. Both the re-sults from the Pb–Pb and pp analyses are shown. The

η ∆ 0 0.5 1 1.5 2 2.5 3 3.5 4 (+-,dyn) corr _ν 〉 ch N 〈 -2 -1.5 -1 -0.5 0 2 2.5 3 3.5 4 D 0-5% 20-30% 40-50% f σ 8 η ∆ Function Erf Extrapolation

FIG. 3. hNchiν₍₊₋corr,dyn) (left axis) and D (right axis) as a

function of ∆η window for three different centrality bins in

Pb–Pb collisions at√sNN = 2.76 TeV. The data points are

fitted with the functional form, erf(∆η/√8σf). The dashed

lines correspond to the extrapolation of the fitted curves. The points are shifted minimally along x-axis for clear view. Both statistical (error bar) and systematic (box) errors are shown.

shaded bands in the figure indicate the predictions for a HG and a QGP. Results from the HIJING event genera-tor [27] at ∆η = 1 and 1.6 are observed to be close to the HG line and at the same time independent of centrality. The pp results agree well with the HG prediction. The experimental results for Pb–Pb for both the ∆η windows are observed to be below the HG predictions and above those of the QGP. The values of D for ∆η = 1.6 are lower compared to those for ∆η = 1 for all centralities.

A decreasing trend of D has been observed while going from peripheral to central collisions, as seen in Figure 2. This centrality dependence may arise partly because of the presence of radial flow [10]. The radial flow velocity could lead to the kinetic focussing of the produced par-ticles, causing a narrowing of the opening angles. This may affect the magnitude of net–charge fluctuations. The effect of radial flow on ν(+−,dyn) has been estimated by

using an afterburner [28] on the HIJING events where the particles get a boost in the transverse momenta be-cause of the radial flow velocity. We observe no signif-icant difference between the results from pure HIJING and HIJING with the afterburner. This indicates that the presence of radial flow may not be responsible for the centrality dependence of the D–measure.

The measured fluctuations may get diluted during the evolution of the system from hadronization to kinetic freeze-out because of the diffusion of charged hadrons in rapidity. This has been addressed in refs. [8, 9], where a diffusion equation has been proposed to study the de-pendence of net–charge fluctuations on the width of the rapidity window. Taking the dissipation into account, the asymptotic value of fluctuations may be close to the

primordial fluctuations. This has been explored for the ALICE data points by plotting hNchiν_(+−,dyn)corr and D as

a function of ∆η for three centrality bins, as shown in Figure 3. We observe that for a given centrality bin, the D–measure shows a strong decreasing trend with the increase of ∆η. In fact, the curvature of D has a decreasing slope with a flattening tendency at large ∆η windows. Following the prescriptions of [8, 9], we fit the data points with the functional form, erf(∆η/√8σf),

which represents the diffusion in rapidity space. Here, σf

characterizes the diffusion at freeze–out. The resulting values of σf are 0.41 ± 0.05, 0.44 ± 0.05 and 0.48 ± 0.07

for the 0-5%, 20-30% and 40-50% centralities, respec-tively. The fitted curves are shown as solid lines in Fig-ure 3. The dashed lines are extrapolations of the fitted curves to higher ∆η, which yield the asymptotic values of D. For the top 5% centrality, the measured values of D are 2.6 ± 0.02(stat.) ± 0.15(sys.) for ∆η = 1 and 2.3 ± 0.02(stat.) ± 0.21(sys.) for ∆η = 1.6. The extrapo-lated value of D is 2.24 ± 0.09(stat.) ± 0.21(sys.).

The evolution of the net–charge fluctuations with beam energy can be studied by combining the ALICE data with those of the STAR experiment [12] at RHIC. In Figure 4, we present the values of hNchiν(+−,dyn)corr (left

axis) and D (right axis) for top central collisions from ALICE at √sNN = 2.76 TeV and for STAR, Au–Au

collisions at four different energies. The ALICE data points correspond to ∆η = 1 and 1.6, whereas for STAR, the values for ∆η = 1 are shown. For the STAR data, (dNch/dη)ν_(+−,dyn)corr are plotted instead of hNchiν_(+−,dyn)corr ,

as dNch/dη values are approximately equal to hNchi for

∆η = 1 at central rapidity. The theoretical predictions for a HG and a QGP are indicated in the figure. In the absence of any dynamic model, these predictions do not have dependence on the beam energy.

Figure 4 shows a monotonic decrease in the magnitude of the net–charge fluctuations with increasing beam en-ergy. For the top RHIC energy of√sNN= 200 GeV, the

measured value of fluctuation is observed to be close to the HG prediction, whereas at lower energy, the results are above the HG value. At√sNN= 2.76 T eV , we

ob-serve significantly lower fluctuations compared to those of lower energies.

In summary, we have presented the first measurements of dynamic net–charge fluctuations at the LHC in Pb–Pb collisions at √sNN = 2.76 TeV in terms of ν(+−,dyn),

and their corrected values, ν_(+−,dyn)corr (corrected for charge conservation and finite acceptance effect). The results for pp collisions at the same center–of–mass energy are found to be in agreement with hadron gas prediction. The val-ues of ν(+−,dyn) and ν(+−,dyn)corr are seen to be negative in

all cases, indicating the dominance of the correlation of positive and negative charges. A decrease in fluctuations is observed while going from peripheral to central colli-sions. The D–measure, which gives the charge

(GeV) NN s 10 102 103 (+-,dyn) corr _ν 〉 ch N 〈 -3 -2.5 -2 -1.5 -1 -0.5 0 1 1.5 2 2.5 3 3.5 4 D Hadron Gas QGP STAR Au-Au = 1.0 η ∆ ALICE Pb-Pb = 1.6 η ∆ ALICE Pb-Pb

FIG. 4. Energy dependence of the net–charge fluctuations,

measured in terms of hNchiν₍₊₋corr,dyn)(left axis) and D (right

axis), for the top central collisions. The results from the

STAR [12] and ALICE experiments are presented for ∆η = 1 after the correction for charge conservation. The ALICE re-sult for ∆η = 1.6 is also shown. Both statistical (error bar) and systematic (box) errors are plotted.

tions per entropy, is calculated from νcorr

(+−,dyn) and from

the measured average charged particle multiplicity. A decreasing trend of D is observed in going from periph-eral to central collisions. Model studies indicate that the presence of radial flow may not be the cause of this de-crease. The dissipation of signal during the evolution of the fireball from the hadronization to freeze-out has been estimated by fitting D as a function of the ∆η window. The extrapolation of the fit function yields the asymp-totic value of D, which is not very different from the mea-surement at ∆η = 1.6. The beam energy dependence of charge fluctuations has been studied by comparing the ALICE data with those from the STAR experiment at RHIC for Au–Au collisions at four energies. A mono-tonic decrease in the value of D, measured at ∆η = 1, has been observed. The STAR data points at RHIC top energy are close to the prediction for a hadron gas. This may be due to the fact that the fluctuation may be not strong enough to be measured or because of the dilution of fluctuation during the evolution process. The fluctu-ations at√sNN= 2.76 T eV for ∆η = 1.0 are below the

measurements at RHIC. These data points show an ad-ditional decrease at ∆η = 1.6, where D turns out to be 2.3±0.02(stat.)±0.21(sys.) for top central collisions. The fluctuations at the LHC energy might also have been di-luted because of various effects as discussed earlier. As these fluctuations are smaller than the theoretical ex-pectations for a HG, and show for the first time at the LHC energy a clear tendency towards expectations from a QGP, we may infer that they have their origin in the QGP phase. Dynamical model calculations are needed

to better understand the results.

The ALICE collaboration would like to thank all its en-gineers and technicians for their invaluable contributions to the construction of the experiment and the CERN ac-celerator teams for the outstanding performance of the LHC complex.

The ALICE collaboration acknowledges the following funding agencies for their support in building and run-ning the ALICE detector:

State Committee of Science, Calouste Gulbenkian Foun-dation from Lisbon and Swiss Fonds Kidagan, Armenia; Conselho Nacional de Desenvolvimento Cient´ıfico e Tec-nol´ogico (CNPq), Financiadora de Estudos e Projetos (FINEP), Funda¸c˜ao de Amparo `a Pesquisa do Estado de S˜ao Paulo (FAPESP);

National Natural Science Foundation of China (NSFC), the Chinese Ministry of Education (CMOE) and the Min-istry of Science and Technology of China (MSTC); Ministry of Education and Youth of the Czech Republic; Danish Natural Science Research Council, the Carlsberg Foundation and the Danish National Research Founda-tion;

The European Research Council under the European Community’s Seventh Framework Programme;

Helsinki Institute of Physics and the Academy of Fin-land;

French CNRS-IN2P3, the ‘Region Pays de Loire’, ‘Re-gion Alsace’, ‘Re‘Re-gion Auvergne’ and CEA, France; German BMBF and the Helmholtz Association;

General Secretariat for Research and Technology, Min-istry of Development, Greece;

Hungarian OTKA and National Office for Research and Technology (NKTH);

Department of Atomic Energy and Department of Sci-ence and Technology of the Government of India; Istituto Nazionale di Fisica Nucleare (INFN) and Cen-tro Fermi - Museo Storico della Fisica e CenCen-tro Studi e Ricerche ”Enrico Fermi”, Italy;

MEXT Grant-in-Aid for Specially Promoted Research, Japan;

Joint Institute for Nuclear Research, Dubna; National Research Foundation of Korea (NRF);

CONACYT, DGAPA, M´exico, ALFA-EC and the HE-LEN Program (High-Energy physics Latin-American– European Network);

Stichting voor Fundamenteel Onderzoek der Materie (FOM) and the Nederlandse Organisatie voor Weten-schappelijk Onderzoek (NWO), Netherlands;

Research Council of Norway (NFR);

Polish Ministry of Science and Higher Education; National Authority for Scientific Research - NASR (Au-toritatea Nat¸ional˘a pentru Cercetare S¸tiint¸ific˘a - ANCS); Ministry of Education and Science of Russian Federation,

International Science and Technology Center, Russian Academy of Sciences, Russian Federal Agency of Atomic Energy, Russian Federal Agency for Science and Innova-tions and CERN-INTAS;

Ministry of Education of Slovakia;

Department of Science and Technology, South Africa; CIEMAT, EELA, Ministerio de Educaci´on y Ciencia of Spain, Xunta de Galicia (Conseller´ıa de Educaci´on), CEADEN, Cubaenerg´ıa, Cuba, and IAEA (International Atomic Energy Agency);

Swedish Research Council (VR) and Knut & Alice Wal-lenberg Foundation (KAW);

Ukraine Ministry of Education and Science;

United Kingdom Science and Technology Facilities Coun-cil (STFC);

The United States Department of Energy, the United States National Science Foundation, the State of Texas, and the State of Ohio.

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