Midrapidity antiproton-to-proton ratio in pp collisions at $\sqrt{s} = 0.9$ and $7$~TeV measured by the ALICE experiment

arXiv:1006.5432v2 [hep-ex] 28 Sep 2017

Submitted to: PRL

Midrapidity antiproton-to-proton ratio in pp collisions at

√

s

= 0.9

and

7

TeV measured by the ALICE experiment

ALICE Collaboration

Abstract

The ratio of the yields of antiprotons to protons in pp collisions has been measured by the

AL-ICE experiment at

√

s = 0.9 and 7 TeV during the initial running periods of the Large Hadron

Col-lider(LHC). The measurement covers the transverse momentum interval

0.45 < p

t

< 1.05 GeV/c and

rapidity

_{|y| < 0.5. The ratio is measured to be R}

_|y|<0.5

_{= 0.957 ± 0.006(stat.) ± 0.014(syst.) at 0.9 TeV}

and

R

_|y|<0.5

_{= 0.991 ± 0.005(stat.) ± 0.014(syst.) at 7 TeV and it is independent of both rapidity and}

transverse momentum. The results are consistent with the conventional model of baryon-number

transport and set stringent limits on any additional contributions to baryon-number transfer over very

large rapidity intervals in pp collisions.

s

= 0.9 and 7 TeV measured by the ALICE experiment

(ALICE Collaboration)

K. Aamodt,1 _{N. Abel,}2 _{U. Abeysekara,}3 _{A. Abrahantes Quintana,}4 _{A. Abramyan,}5_{D. Adamov´a,}6_{M.M. Aggarwal,}7

G. Aglieri Rinella,8 _{A.G. Agocs,}9_{S. Aguilar Salazar,}10_{Z. Ahammed,}11 _{A. Ahmad,}12 _{N. Ahmad,}12_{S.U. Ahn,}13, a

R. Akimoto,14 _{A. Akindinov,}15 _{D. Aleksandrov,}16 _{B. Alessandro,}17 _{R. Alfaro Molina,}10 _{A. Alici,}18

E. Almar´az Avi˜na,10 _{J. Alme,}19 _{T. Alt,}2, b_{V. Altini,}20 _{S. Altinpinar,}21_{C. Andrei,}22 _{A. Andronic,}21 _{G. Anelli,}8

V. Angelov,2, b _{C. Anson,}23 _{T. Antiˇci´c,}24 _{F. Antinori,}8, c _{S. Antinori,}18 _{K. Antipin,}25 _{D. Anto´nczyk,}25

P. Antonioli,26 _{A. Anzo,}10 _{L. Aphecetche,}27 _{H. Appelsh¨auser,}25_{S. Arcelli,}18_{R. Arceo,}10 _{A. Arend,}25_{N. Armesto,}28

R. Arnaldi,17 _{T. Aronsson,}29 _{I.C. Arsene,}1, d _{A. Asryan,}30_{A. Augustinus,}8 _{R. Averbeck,}21_{T.C. Awes,}31_{J. ¨Ayst¨o,}32

M.D. Azmi,12 _{S. Bablok,}19_{M. Bach,}33 _{A. Badal`a,}34 _{Y.W. Baek,}13, a _{S. Bagnasco,}17 _{R. Bailhache,}21, e_{R. Bala,}35

A. Baldisseri,36_{A. Baldit,}37 _{J. B´an,}38 _{R. Barbera,}39 _{G.G. Barnaf¨oldi,}9 _{L. Barnby,}40_{V. Barret,}37 _{J. Bartke,}41

F. Barile,20_{M. Basile,}18 _{V. Basmanov,}42_{N. Bastid,}37 _{B. Bathen,}43_{G. Batigne,}27 _{B. Batyunya,}44 _{C. Baumann,}43, e

I.G. Bearden,45 _{B. Becker,}46, f _{I. Belikov,}47 _{R. Bellwied,}48_{E. Belmont-Moreno,}10 _{A. Belogianni,}49_{L. Benhabib,}27

S. Beole,35 _{I. Berceanu,}22 _{A. Bercuci,}21, g_{E. Berdermann,}21_{Y. Berdnikov,}50 _{L. Betev,}8 _{A. Bhasin,}51 _{A.K. Bhati,}7

L. Bianchi,35 _{N. Bianchi,}52 _{C. Bianchin,}53 _{J. Bielˇc´ık,}54_{J. Bielˇc´ıkov´a,}6_{A. Bilandzic,}55 _{L. Bimbot,}56 _{E. Biolcati,}35

A. Blanc,37 _{F. Blanco,}39, h _{F. Blanco,}57_{D. Blau,}16 _{C. Blume,}25 _{M. Boccioli,}8 _{N. Bock,}23 _{A. Bogdanov,}58

H. Bøggild,45 _{M. Bogolyubsky,}59_{J. Bohm,}60 _{L. Boldizs´ar,}9 _{M. Bombara,}61 _{C. Bombonati,}53, i _{M. Bondila,}32

H. Borel,36 _{A. Borisov,}62 _{C. Bortolin,}53, j _{S. Bose,}63 _{L. Bosisio,}64 _{F. Boss´u,}35 _{M. Botje,}55 _{S. B¨ottger,}2

G. Bourdaud,27 _{B. Boyer,}56 _{M. Braun,}30_{P. Braun-Munzinger,}21, 65, b _{L. Bravina,}1 _{M. Bregant,}64, k_{T. Breitner,}2

G. Bruckner,8 _{R. Brun,}8 _{E. Bruna,}29 _{G.E. Bruno,}20 _{D. Budnikov,}42_{H. Buesching,}25 _{P. Buncic,}8 _{O. Busch,}66

Z. Buthelezi,67 _{D. Caffarri,}53 _{X. Cai,}68 _{H. Caines,}29 _{E. Camacho,}69 _{P. Camerini,}64 _{M. Campbell,}8 _{V. Canoa}

Roman,8 _{G.P. Capitani,}52 _{G. Cara Romeo,}26 _{F. Carena,}8 _{W. Carena,}8_{F. Carminati,}8 _{A. Casanova D´ıaz,}52

M. Caselle,8 _{J. Castillo Castellanos,}36 _{J.F. Castillo Hernandez,}21 _{V. Catanescu,}22 _{E. Cattaruzza,}64

C. Cavicchioli,8_{P. Cerello,}17 _{V. Chambert,}56 _{B. Chang,}60 _{S. Chapeland,}8 _{A. Charpy,}56 _{J.L. Charvet,}36

S. Chattopadhyay,63 _{S. Chattopadhyay,}11 _{M. Cherney,}3 _{C. Cheshkov,}8_{B. Cheynis,}70 _{E. Chiavassa,}35

V. Chibante Barroso,8 _{D.D. Chinellato,}71 _{P. Chochula,}8_{K. Choi,}72 _{M. Chojnacki,}73 _{P. Christakoglou,}73

C.H. Christensen,45 _{P. Christiansen,}74 _{T. Chujo,}75 _{F. Chuman,}76 _{C. Cicalo,}46 _{L. Cifarelli,}18 _{F. Cindolo,}26

J. Cleymans,67 _{O. Cobanoglu,}35_{J.-P. Coffin,}47_{S. Coli,}17 _{A. Colla,}8_{G. Conesa Balbastre,}52 _{Z. Conesa del Valle,}27, l

E.S. Conner,77 _{P. Constantin,}66 _{G. Contin,}64, i _{J.G. Contreras,}69 _{Y. Corrales Morales,}35_{T.M. Cormier,}48

P. Cortese,78 _{I. Cort´es Maldonado,}79 _{M.R. Cosentino,}71_{F. Costa,}8 _{M.E. Cotallo,}57_{E. Crescio,}69_{P. Crochet,}37

E. Cuautle,80 _{L. Cunqueiro,}52 _{J. Cussonneau,}27_{A. Dainese,}81 _{H.H. Dalsgaard,}45_{A. Danu,}82 _{I. Das,}63 _{A. Dash,}83

S. Dash,83 _{G.O.V. de Barros,}84_{A. De Caro,}85 _{G. de Cataldo,}86 _{J. de Cuveland,}2, b _{A. De Falco,}87 _{M. De Gaspari,}66

J. de Groot,8 _{D. De Gruttola,}85 _{N. De Marco,}17 _{S. De Pasquale,}85 _{R. De Remigis,}17 _{R. de Rooij,}73 _{G. de Vaux,}67

H. Delagrange,27 _{G. Dellacasa,}78 _{A. Deloff,}88 _{V. Demanov,}42 _{E. D´enes,}9 _{A. Deppman,}84 _{G. D’Erasmo,}20

D. Derkach,30 _{A. Devaux,}37 _{D. Di Bari,}20_{C. Di Giglio,}20, i _{S. Di Liberto,}89 _{A. Di Mauro,}8 _{P. Di Nezza,}52

M. Dialinas,27 _{L. D´ıaz,}80 _{R. D´ıaz,}32_{T. Dietel,}43 _{R. Divi`a,}8 _{Ø. Djuvsland,}19 _{V. Dobretsov,}16 _{A. Dobrin,}74

T. Dobrowolski,88 _{B. D¨onigus,}21 _{I. Dom´ınguez,}80 _{D.M.M. Don,}90 _{O. Dordic,}1 _{A.K. Dubey,}11 _{J. Dubuisson,}8

L. Ducroux,70 _{P. Dupieux,}37 _{A.K. Dutta Majumdar,}63_{M.R. Dutta Majumdar,}11_{D. Elia,}86 _{D. Emschermann,}66, m

A. Enokizono,31 _{B. Espagnon,}56 _{M. Estienne,}27 _{S. Esumi,}75 _{D. Evans,}40 _{S. Evrard,}8 _{G. Eyyubova,}1

C.W. Fabjan,8, n _{D. Fabris,}81 _{J. Faivre,}91 _{D. Falchieri,}18 _{A. Fantoni,}52 _{M. Fasel,}21 _{O. Fateev,}44 _{R. Fearick,}67

A. Fedunov,44 _{D. Fehlker,}19 _{V. Fekete,}92 _{D. Felea,}82 _{B. Fenton-Olsen,}45, o _{G. Feofilov,}30 _{A. Fern´andez T´ellez,}79

E.G. Ferreiro,28 _{A. Ferretti,}35 _{R. Ferretti,}78, p _{M.A.S. Figueredo,}84 _{S. Filchagin,}42_{R. Fini,}86 _{F.M. Fionda,}20

E.M. Fiore,20_{M. Floris,}87, i _{Z. Fodor,}9 _{S. Foertsch,}67 _{P. Foka,}21 _{S. Fokin,}16 _{F. Formenti,}8 _{E. Fragiacomo,}93

M. Fragkiadakis,49 _{U. Frankenfeld,}21 _{A. Frolov,}94_{U. Fuchs,}8 _{F. Furano,}8 _{C. Furget,}91 _{M. Fusco Girard,}85

J.J. Gaardhøje,45 _{S. Gadrat,}91 _{M. Gagliardi,}35_{A. Gago,}95 _{M. Gallio,}35_{P. Ganoti,}49_{M.S. Ganti,}11_{C. Garabatos,}21

C. Garc´ıa Trapaga,35_{J. Gebelein,}2 _{R. Gemme,}78 _{M. Germain,}27 _{A. Gheata,}8_{M. Gheata,}8_{B. Ghidini,}20 _{P. Ghosh,}11

G. Giraudo,17 _{P. Giubellino,}17_{E. Gladysz-Dziadus,}41 _{R. Glasow,}43, q _{P. Gl¨assel,}66 _{A. Glenn,}96 _{R. G´omez Jim´enez,}97

H. Gonz´alez Santos,79 _{L.H. Gonz´alez-Trueba,}10 _{P. Gonz´alez-Zamora,}57 _{S. Gorbunov,}2, b _{Y. Gorbunov,}3

A. Grigoryan,5_{S. Grigoryan,}44 _{B. Grinyov,}62_{N. Grion,}93_{P. Gros,}74_{J.F. Grosse-Oetringhaus,}8 _{J.-Y. Grossiord,}70

R. Grosso,81 _{F. Guber,}99 _{R. Guernane,}91 _{B. Guerzoni,}18 _{K. Gulbrandsen,}45 _{H. Gulkanyan,}5 _{T. Gunji,}14

A. Gupta,51 _{R. Gupta,}51_{H.-A. Gustafsson,}74, q _{H. Gutbrod,}21 _{Ø. Haaland,}19_{C. Hadjidakis,}56 _{M. Haiduc,}82

H. Hamagaki,14_{G. Hamar,}9_{J. Hamblen,}100 _{B.H. Han,}101_{J.W. Harris,}29_{M. Hartig,}25_{A. Harutyunyan,}5_{D. Hasch,}52

D. Hasegan,82 _{D. Hatzifotiadou,}26 _{A. Hayrapetyan,}5_{M. Heide,}43 _{M. Heinz,}29 _{H. Helstrup,}102 _{A. Herghelegiu,}22

C. Hern´andez,21 _{G. Herrera Corral,}69 _{N. Herrmann,}66 _{K.F. Hetland,}102 _{B. Hicks,}29 _{A. Hiei,}76 _{P.T. Hille,}1, r

B. Hippolyte,47 _{T. Horaguchi,}76, s _{Y. Hori,}14 _{P. Hristov,}8 _{I. Hˇrivn´aˇcov´a,}56_{S. Hu,}103_{M. Huang,}19_{S. Huber,}21

T.J. Humanic,23 _{D. Hutter,}33 _{D.S. Hwang,}101 _{R. Ichou,}27 _{R. Ilkaev,}42 _{I. Ilkiv,}88 _{M. Inaba,}75_{P.G. Innocenti,}8

M. Ippolitov,16 _{M. Irfan,}12 _{C. Ivan,}73 _{A. Ivanov,}30 _{M. Ivanov,}21_{V. Ivanov,}50_{T. Iwasaki,}76 _{A. Jacho lkowski,}8

P. Jacobs,104_{L. Janˇcurov´a,}44_{S. Jangal,}47_{R. Janik,}92_{C. Jena,}83_{S. Jena,}105 _{L. Jirden,}8 _{G.T. Jones,}40 _{P.G. Jones,}40

P. Jovanovi´c,40_{H. Jung,}13 _{W. Jung,}13 _{A. Jusko,}40_{A.B. Kaidalov,}15 _{S. Kalcher,}2, b _{P. Kaliˇn´ak,}38_{M. Kalisky,}43

T. Kalliokoski,32_{A. Kalweit,}65_{A. Kamal,}12_{R. Kamermans,}73_{K. Kanaki,}19 _{E. Kang,}13_{J.H. Kang,}60 _{J. Kapitan,}6

V. Kaplin,58_{S. Kapusta,}8 _{O. Karavichev,}99_{T. Karavicheva,}99_{E. Karpechev,}99 _{A. Kazantsev,}16 _{U. Kebschull,}2

R. Keidel,77 _{M.M. Khan,}12 _{S.A. Khan,}11 _{A. Khanzadeev,}50 _{Y. Kharlov,}59_{D. Kikola,}106_{B. Kileng,}102_{D.J Kim,}32

D.S. Kim,13 _{D.W. Kim,}13 _{H.N. Kim,}13 _{J. Kim,}59 _{J.H. Kim,}101 _{J.S. Kim,}13 _{M. Kim,}13 _{M. Kim,}60 _{S.H. Kim,}13

S. Kim,101_{Y. Kim,}60_{S. Kirsch,}8_{I. Kisel,}2, d_{S. Kiselev,}15 _{A. Kisiel,}23, i_{J.L. Klay,}107_{J. Klein,}66_{C. Klein-B¨osing,}8, m

M. Kliemant,25 _{A. Klovning,}19 _{A. Kluge,}8_{M.L. Knichel,}21_{S. Kniege,}25_{K. Koch,}66_{R. Kolevatov,}1_{A. Kolojvari,}30

V. Kondratiev,30 _{N. Kondratyeva,}58 _{A. Konevskih,}99 _{E. Korna´s,}41 _{R. Kour,}40 _{M. Kowalski,}41 _{S. Kox,}91

K. Kozlov,16_{J. Kral,}54, k_{I. Kr´alik,}38 _{F. Kramer,}25_{I. Kraus,}65, d _{A. Kravˇc´akov´a,}61_{T. Krawutschke,}108_{M. Krivda,}40

D. Krumbhorn,66_{M. Krus,}54_{E. Kryshen,}50_{M. Krzewicki,}55_{Y. Kucheriaev,}16_{C. Kuhn,}47 _{P.G. Kuijer,}55_{L. Kumar,}7

N. Kumar,7_{R. Kupczak,}106_{P. Kurashvili,}88_{A. Kurepin,}99_{A.N. Kurepin,}99_{A. Kuryakin,}42_{S. Kushpil,}6_{V. Kushpil,}6

M. Kutouski,44_{H. Kvaerno,}1_{M.J. Kweon,}66 _{Y. Kwon,}60 _{P. La Rocca,}39, t _{F. Lackner,}8 _{P. Ladr´on de Guevara,}57

V. Lafage,56_{C. Lal,}51 _{C. Lara,}2 _{D.T. Larsen,}19_{G. Laurenti,}26 _{C. Lazzeroni,}40 _{Y. Le Bornec,}56 _{N. Le Bris,}27

H. Lee,72 _{K.S. Lee,}13 _{S.C. Lee,}13 _{F. Lef`evre,}27_{M. Lenhardt,}27 _{L. Leistam,}8 _{J. Lehnert,}25_{V. Lenti,}86_{H. Le´on,}10

I. Le´on Monz´on,97_{H. Le´on Vargas,}25 _{P. L´evai,}9 _{X. Li,}103_{Y. Li,}103_{R. Lietava,}40_{S. Lindal,}1_{V. Lindenstruth,}2, b

C. Lippmann,8 _{M.A. Lisa,}23_{L. Liu,}19 _{V. Loginov,}58_{S. Lohn,}8_{X. Lopez,}37 _{M. L´opez Noriega,}56_{R. L´opez-Ram´ırez,}79

E. L´opez Torres,4 _{G. Løvhøiden,}1_{A. Lozea Feijo Soares,}84 _{S. Lu,}103 _{M. Lunardon,}53 _{G. Luparello,}35_{L. Luquin,}27

J.-R. Lutz,47 _{K. Ma,}68 _{R. Ma,}29 _{D.M. Madagodahettige-Don,}90_{A. Maevskaya,}99_{M. Mager,}65, i _{D.P. Mahapatra,}83

A. Maire,47 _{I. Makhlyueva,}8 _{D. Mal’Kevich,}15 _{M. Malaev,}50 _{K.J. Malagalage,}3 _{I. Maldonado Cervantes,}80

M. Malek,56 _{T. Malkiewicz,}32 _{P. Malzacher,}21 _{A. Mamonov,}42 _{L. Manceau,}37 _{L. Mangotra,}51 _{V. Manko,}16

F. Manso,37 _{V. Manzari,}86 _{Y. Mao,}68, u _{J. Mareˇs,}109 _{G.V. Margagliotti,}64 _{A. Margotti,}26 _{A. Mar´ın,}21

I. Martashvili,100 _{P. Martinengo,}8_{M.I. Mart´ınez Hern´andez,}79_{A. Mart´ınez Davalos,}10_{G. Mart´ınez Garc´ıa,}27

Y. Maruyama,76_{A. Marzari Chiesa,}35 _{S. Masciocchi,}21_{M. Masera,}35_{M. Masetti,}18_{A. Masoni,}46 _{L. Massacrier,}70

M. Mastromarco,86 _{A. Mastroserio,}20, i _{Z.L. Matthews,}40 _{A. Matyja,}41, v_{D. Mayani,}80_{G. Mazza,}17_{M.A. Mazzoni,}89

F. Meddi,110 _{A. Menchaca-Rocha,}10 _{P. Mendez Lorenzo,}8 _{M. Meoni,}8 _{J. Mercado P´erez,}66_{P. Mereu,}17_{Y. Miake,}75

A. Michalon,47 _{N. Miftakhov,}50 _{L. Milano,}35_{J. Milosevic,}1 _{F. Minafra,}20_{A. Mischke,}73 _{D. Mi´skowiec,}21 _{C. Mitu,}82

K. Mizoguchi,76_{J. Mlynarz,}48 _{B. Mohanty,}11_{L. Molnar,}9, i _{M.M. Mondal,}11 _{L. Monta˜no Zetina,}69, w _{M. Monteno,}17

E. Montes,57_{M. Morando,}53 _{S. Moretto,}53 _{A. Morsch,}8 _{T. Moukhanova,}16 _{V. Muccifora,}52 _{E. Mudnic,}98

S. Muhuri,11 _{H. M¨uller,}8 _{M.G. Munhoz,}84 _{J. Munoz,}79 _{L. Musa,}8 _{A. Musso,}17_{B.K. Nandi,}105 _{R. Nania,}26

E. Nappi,86 _{F. Navach,}20_{S. Navin,}40_{T.K. Nayak,}11_{S. Nazarenko,}42_{G. Nazarov,}42 _{A. Nedosekin,}15_{F. Nendaz,}70

J. Newby,96 _{A. Nianine,}16 _{M. Nicassio,}86, i _{B.S. Nielsen,}45 _{S. Nikolaev,}16_{V. Nikolic,}24 _{S. Nikulin,}16_{V. Nikulin,}50

B.S. Nilsen,3 _{M.S. Nilsson,}1 _{F. Noferini,}26 _{P. Nomokonov,}44 _{G. Nooren,}73 _{N. Novitzky,}32 _{A. Nyatha,}105

C. Nygaard,45 _{A. Nyiri,}1 _{J. Nystrand,}19 _{A. Ochirov,}30 _{G. Odyniec,}104 _{H. Oeschler,}65 _{M. Oinonen,}32_{K. Okada,}14

Y. Okada,76_{M. Oldenburg,}8 _{J. Oleniacz,}106_{C. Oppedisano,}17_{F. Orsini,}36 _{A. Ortiz Velasquez,}80 _{G. Ortona,}35

A. Oskarsson,74_{F. Osmic,}8 _{L. ¨Osterman,}74 _{P. Ostrowski,}106_{I. Otterlund,}74 _{J. Otwinowski,}21 _{G. Øvrebekk,}19

K. Oyama,66 _{K. Ozawa,}14_{Y. Pachmayer,}66_{M. Pachr,}54_{F. Padilla,}35 _{P. Pagano,}85 _{G. Pai´c,}80_{F. Painke,}2

C. Pajares,28_{S. Pal,}63, x_{S.K. Pal,}11_{A. Palaha,}40 _{A. Palmeri,}34 _{R. Panse,}2_{V. Papikyan,}5 _{G.S. Pappalardo,}34

W.J. Park,21 _{B. Pastirˇc´ak,}38 _{C. Pastore,}86 _{V. Paticchio,}86 _{A. Pavlinov,}48_{T. Pawlak,}106 _{T. Peitzmann,}73

A. Pepato,81 _{H. Pereira,}36_{D. Peressounko,}16_{C. P´erez,}95 _{D. Perini,}8_{D. Perrino,}20, i_{W. Peryt,}106 _{J. Peschek,}2, b

A. Pesci,26_{V. Peskov,}80, i _{Y. Pestov,}94_{A.J. Peters,}8 _{V. Petr´aˇcek,}54_{A. Petridis,}49, q _{M. Petris,}22 _{P. Petrov,}40

L. Pinsky,90 _{N. Pitz,}25 _{F. Piuz,}8 _{R. Platt,}40 _{M. P losko´n,}104_{J. Pluta,}106_{T. Pocheptsov,}44, y _{S. Pochybova,}9

P.L.M. Podesta Lerma,97 _{F. Poggio,}35 _{M.G. Poghosyan,}35 _{K. Pol´ak,}109 _{B. Polichtchouk,}59 _{P. Polozov,}15

V. Polyakov,50_{B. Pommeresch,}19_{A. Pop,}22 _{F. Posa,}20_{V. Posp´ıˇsil,}54 _{B. Potukuchi,}51_{J. Pouthas,}56 _{S.K. Prasad,}11

R. Preghenella,18, t _{F. Prino,}17 _{C.A. Pruneau,}48 _{I. Pshenichnov,}99 _{G. Puddu,}87_{P. Pujahari,}105 _{A. Pulvirenti,}39

A. Punin,42 _{V. Punin,}42 _{M. Putiˇs,}61_{J. Putschke,}29 _{E. Quercigh,}8 _{A. Rachevski,}93_{A. Rademakers,}8_{S. Radomski,}66

T.S. R¨aih¨a,32 _{J. Rak,}32 _{A. Rakotozafindrabe,}36 _{L. Ramello,}78 _{A. Ram´ırez Reyes,}69 _{M. Rammler,}43

R. Raniwala,111 _{S. Raniwala,}111 _{S.S. R¨as¨anen,}32 _{I. Rashevskaya,}93 _{S. Rath,}83 _{K.F. Read,}100 _{J.S. Real,}91

K. Redlich,88, z _{R. Renfordt,}25 _{A.R. Reolon,}52 _{A. Reshetin,}99 _{F. Rettig,}2, b _{J.-P. Revol,}8 _{K. Reygers,}43, aa

H. Ricaud,65 _{L. Riccati,}17 _{R.A. Ricci,}112 _{M. Richter,}19 _{P. Riedler,}8 _{W. Riegler,}8 _{F. Riggi,}39_{A. Rivetti,}17

M. Rodriguez Cahuantzi,79 _{K. Røed,}102_{D. R¨ohrich,}8, bb _{S. Rom´an L´opez,}79_{R. Romita,}20, d _{F. Ronchetti,}52

P. Rosinsk´y,8 _{P. Rosnet,}37 _{S. Rossegger,}8 _{A. Rossi,}64 _{F. Roukoutakis,}8, cc _{S. Rousseau,}56_{C. Roy,}27, l_{P. Roy,}63

A.J. Rubio-Montero,57 _{R. Rui,}64 _{I. Rusanov,}66 _{G. Russo,}85 _{E. Ryabinkin,}16 _{A. Rybicki,}41 _{S. Sadovsky,}59

K. ˇSafaˇr´ık,8_{R. Sahoo,}53 _{J. Saini,}11_{P. Saiz,}8_{D. Sakata,}75 _{C.A. Salgado,}28 _{R. Salgueiro Domingues da Silva,}8

S. Salur,104_{T. Samanta,}11_{S. Sambyal,}51 _{V. Samsonov,}50 _{L. ˇS´andor,}38 _{A. Sandoval,}10 _{M. Sano,}75 _{S. Sano,}14

R. Santo,43 _{R. Santoro,}20_{J. Sarkamo,}32 _{P. Saturnini,}37 _{E. Scapparone,}26_{F. Scarlassara,}53_{R.P. Scharenberg,}113

C. Schiaua,22 _{R. Schicker,}66_{H. Schindler,}8 _{C. Schmidt,}21 _{H.R. Schmidt,}21 _{K. Schossmaier,}8 _{S. Schreiner,}8

S. Schuchmann,25_{J. Schukraft,}8_{Y. Schutz,}27 _{K. Schwarz,}21_{K. Schweda,}66 _{G. Scioli,}18 _{E. Scomparin,}17_{P.A. Scott,}40

G. Segato,53 _{D. Semenov,}30_{S. Senyukov,}78 _{J. Seo,}13 _{S. Serci,}87 _{L. Serkin,}80 _{E. Serradilla,}57 _{A. Sevcenco,}82

I. Sgura,20 _{G. Shabratova,}44 _{R. Shahoyan,}8 _{G. Sharkov,}15_{N. Sharma,}7 _{S. Sharma,}51 _{K. Shigaki,}76

M. Shimomura,75 _{K. Shtejer,}4 _{Y. Sibiriak,}16 _{M. Siciliano,}35 _{E. Sicking,}8, dd _{E. Siddi,}46 _{T. Siemiarczuk,}88

A. Silenzi,18 _{D. Silvermyr,}31_{E. Simili,}73 _{G. Simonetti,}20, i _{R. Singaraju,}11_{R. Singh,}51 _{V. Singhal,}11_{B.C. Sinha,}11

T. Sinha,63 _{B. Sitar,}92 _{M. Sitta,}78 _{T.B. Skaali,}1 _{K. Skjerdal,}19 _{R. Smakal,}54 _{N. Smirnov,}29 _{R. Snellings,}55

H. Snow,40 _{C. Søgaard,}45 _{A. Soloviev,}59 _{H.K. Soltveit,}66 _{R. Soltz,}96 _{W. Sommer,}25 _{C.W. Son,}72 _{H. Son,}101

M. Song,60 _{C. Soos,}8 _{F. Soramel,}53 _{D. Soyk,}21 _{M. Spyropoulou-Stassinaki,}49_{B.K. Srivastava,}113_{J. Stachel,}66

F. Staley,36 _{E. Stan,}82 _{G. Stefanek,}88 _{G. Stefanini,}8 _{T. Steinbeck,}2, b _{E. Stenlund,}74 _{G. Steyn,}67 _{D. Stocco,}35, v

R. Stock,25 _{P. Stolpovsky,}59 _{P. Strmen,}92 _{A.A.P. Suaide,}84 _{M.A. Subieta V´asquez,}35 _{T. Sugitate,}76 _{C. Suire,}56

M. ˇSumbera,6 _{T. Susa,}24 _{D. Swoboda,}8 _{J. Symons,}104 _{A. Szanto de Toledo,}84 _{I. Szarka,}92 _{A. Szostak,}46

M. Szuba,106_{M. Tadel,}8 _{C. Tagridis,}49 _{A. Takahara,}14 _{J. Takahashi,}71 _{R. Tanabe,}75 _{J.D. Tapia Takaki,}56

H. Taureg,8 _{A. Tauro,}8 _{M. Tavlet,}8_{G. Tejeda Mu˜noz,}79 _{A. Telesca,}8 _{C. Terrevoli,}20_{J. Th¨ader,}2, b _{R. Tieulent,}70

D. Tlusty,54 _{A. Toia,}8 _{T. Tolyhy,}9 _{C. Torcato de Matos,}8 _{H. Torii,}76 _{G. Torralba,}2 _{L. Toscano,}17_{F. Tosello,}17

A. Tournaire,27, ee _{T. Traczyk,}106 _{P. Tribedy,}11 _{G. Tr¨oger,}2_{D. Truesdale,}23 _{W.H. Trzaska,}32 _{G. Tsiledakis,}66

E. Tsilis,49_{T. Tsuji,}14 _{A. Tumkin,}42 _{R. Turrisi,}81_{A. Turvey,}3_{T.S. Tveter,}1 _{H. Tydesj¨o,}8_{K. Tywoniuk,}1_{J. Ulery,}25

K. Ullaland,19 _{A. Uras,}87 _{J. Urb´an,}61_{G.M. Urciuoli,}89 _{G.L. Usai,}87_{A. Vacchi,}93_{M. Vala,}44, ff_{L. Valencia Palomo,}10

S. Vallero,66 _{N. van der Kolk,}55 _{P. Vande Vyvre,}8_{M. van Leeuwen,}73 _{L. Vannucci,}112 _{A. Vargas,}79_{R. Varma,}105

A. Vasiliev,16 _{I. Vassiliev,}2, cc _{M. Vasileiou,}49 _{V. Vechernin,}30 _{M. Venaruzzo,}64 _{E. Vercellin,}35 _{S. Vergara,}79

R. Vernet,39, gg _{M. Verweij,}73 _{I. Vetlitskiy,}15 _{L. Vickovic,}98 _{G. Viesti,}53 _{O. Vikhlyantsev,}42 _{Z. Vilakazi,}67

O. Villalobos Baillie,40 _{A. Vinogradov,}16_{L. Vinogradov,}30 _{Y. Vinogradov,}42 _{T. Virgili,}85 _{Y.P. Viyogi,}11

A. Vodopianov,44_{K. Voloshin,}15 _{S. Voloshin,}48 _{G. Volpe,}20 _{B. von Haller,}8 _{D. Vranic,}21 _{J. Vrl´akov´a,}61

B. Vulpescu,37 _{B. Wagner,}19 _{V. Wagner,}54 _{L. Wallet,}8 _{R. Wan,}68, l _{D. Wang,}68 _{Y. Wang,}66 _{K. Watanabe,}75

Q. Wen,103 _{J. Wessels,}43 _{U. Westerhoff,}43 _{J. Wiechula,}66 _{J. Wikne,}1 _{A. Wilk,}43 _{G. Wilk,}88 _{M.C.S. Williams,}26

N. Willis,56 _{B. Windelband,}66 _{C. Xu,}68 _{C. Yang,}68 _{H. Yang,}66_{S. Yasnopolskiy,}16 _{F. Yermia,}27_{J. Yi,}72 _{Z. Yin,}68

H. Yokoyama,75 _{I-K. Yoo,}72 _{X. Yuan,}68, hh _{V. Yurevich,}44 _{I. Yushmanov,}16 _{E. Zabrodin,}1_{B. Zagreev,}15_{A. Zalite,}50

C. Zampolli,8, ii_{Yu. Zanevsky,}44 _{S. Zaporozhets,}44_{A. Zarochentsev,}30_{P. Z´avada,}109_{H. Zbroszczyk,}106_{P. Zelnicek,}2

A. Zenin,59 _{A. Zepeda,}69 _{I. Zgura,}82 _{M. Zhalov,}50 _{X. Zhang,}68, a _{D. Zhou,}68 _{S. Zhou,}103 _{J. Zhu,}68

A. Zichichi,18, t _{A. Zinchenko,}44 _{G. Zinovjev,}62 _{Y. Zoccarato,}70 _{V. Zych´aˇcek,}54 _{and M. Zynovyev}62

1_{Department of Physics, University of Oslo, Oslo, Norway}

2_{Kirchhoff-Institut f¨ur Physik, Ruprecht-Karls-Universit¨at Heidelberg, Heidelberg, Germany} 3_{Physics Department, Creighton University, Omaha, NE, United States}

4_{Centro de Aplicaciones Tecnol´ogicas y Desarrollo Nuclear (CEADEN), Havana, Cuba} 5_{Yerevan Physics Institute, Yerevan, Armenia}

6_{Nuclear Physics Institute, Academy of Sciences of the Czech Republic, ˇReˇz u Prahy, Czech Republic} 7_{Physics Department, Panjab University, Chandigarh, India}

9_{KFKI Research Institute for Particle and Nuclear Physics,} Hungarian Academy of Sciences, Budapest, Hungary

10_{Instituto de F´ısica, Universidad Nacional Aut´onoma de M´exico, Mexico City, Mexico} 11_{Variable Energy Cyclotron Centre, Kolkata, India}

12_{Department of Physics Aligarh Muslim University, Aligarh, India} 13_{Gangneung-Wonju National University, Gangneung, South Korea}

14_{University of Tokyo, Tokyo, Japan}

15_{Institute for Theoretical and Experimental Physics, Moscow, Russia} 16_{Russian Research Centre Kurchatov Institute, Moscow, Russia}

17_{Sezione INFN, Turin, Italy}

18_{Dipartimento di Fisica dell’Universit`a and Sezione INFN, Bologna, Italy} 19_{Department of Physics and Technology, University of Bergen, Bergen, Norway} 20_{Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy}

21_{Research Division and ExtreMe Matter Institute EMMI,} GSI Helmholtzzentrum f¨ur Schwerionenforschung, Darmstadt, Germany 22_{National Institute for Physics and Nuclear Engineering, Bucharest, Romania} 23_{Department of Physics, Ohio State University, Columbus, OH, United States}

24_{Rudjer Boˇskovi´c Institute, Zagreb, Croatia}

25_{Institut f¨ur Kernphysik, Johann Wolfgang Goethe-Universit¨at Frankfurt, Frankfurt, Germany} 26_{Sezione INFN, Bologna, Italy}

27_{SUBATECH, Ecole des Mines de Nantes, Universit´e de Nantes, CNRS-IN2P3, Nantes, France} 28_{Departamento de F´ısica de Part´ıculas and IGFAE,}

Universidad de Santiago de Compostela, Santiago de Compostela, Spain 29_{Yale University, New Haven, CT, United States}

30_{V. Fock Institute for Physics, St. Petersburg State University, St. Petersburg, Russia} 31_{Oak Ridge National Laboratory, Oak Ridge, TN, United States}

32_{Helsinki Institute of Physics (HIP) and University of Jyv¨askyl¨a, Jyv¨askyl¨a, Finland}

33_{Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universit¨at Frankfurt, Frankfurt, Germany} 34_{Sezione INFN, Catania, Italy}

35_{Dipartimento di Fisica Sperimentale dell’Universit`a and Sezione INFN, Turin, Italy</sub> 36<sub>Commissariat `a l’Energie Atomique, IRFU, Saclay, France}

37_{Laboratoire de Physique Corpusculaire (LPC), Clermont Universit´e,} Universit´e Blaise Pascal, CNRS–IN2P3, Clermont-Ferrand, France 38_{Institute of Experimental Physics, Slovak Academy of Sciences, Koˇsice, Slovakia} 39_{Dipartimento di Fisica e Astronomia dell’Universit`a and Sezione INFN, Catania, Italy} 40_{School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom} 41_{The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland}

42_{Russian Federal Nuclear Center (VNIIEF), Sarov, Russia}

43_{Institut f¨ur Kernphysik, Westf¨alische Wilhelms-Universit¨at M¨unster, M¨unster, Germany} 44_{Joint Institute for Nuclear Research (JINR), Dubna, Russia}

45_{Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark} 46_{Sezione INFN, Cagliari, Italy}

47_{Institut Pluridisciplinaire Hubert Curien (IPHC),} Universit´e de Strasbourg, CNRS-IN2P3, Strasbourg, France

48_{Wayne State University, Detroit, MI, United States} 49_{Physics Department, University of Athens, Athens, Greece}

50_{Petersburg Nuclear Physics Institute, Gatchina, Russia} 51_{Physics Department, University of Jammu, Jammu, India}

52_{Laboratori Nazionali di Frascati, INFN, Frascati, Italy}

53_{Dipartimento di Fisica dell’Universit`a and Sezione INFN, Padova, Italy} 54_{Faculty of Nuclear Sciences and Physical Engineering,}

Czech Technical University in Prague, Prague, Czech Republic 55_{Nikhef, National Institute for Subatomic Physics, Amsterdam, Netherlands}

56_{Institut de Physique Nucl´eaire d’Orsay (IPNO),} Universit´e Paris-Sud, CNRS-IN2P3, Orsay, France

57_{Centro de Investigaciones Energ´eticas Medioambientales y Tecnol´ogicas (CIEMAT), Madrid, Spain} 58_{Moscow Engineering Physics Institute, Moscow, Russia}

59_{Institute for High Energy Physics, Protvino, Russia} 60_{Yonsei University, Seoul, South Korea}

61_{Faculty of Science, P.J. ˇSaf´arik University, Koˇsice, Slovakia} 62_{Bogolyubov Institute for Theoretical Physics, Kiev, Ukraine}

63_{Saha Institute of Nuclear Physics, Kolkata, India}

64_{Dipartimento di Fisica dell’Universit`a and Sezione INFN, Trieste, Italy} 65_{Institut f¨ur Kernphysik, Technische Universit¨at Darmstadt, Darmstadt, Germany}

66_{Physikalisches Institut, Ruprecht-Karls-Universit¨at Heidelberg, Heidelberg, Germany} 67_{Physics Department, University of Cape Town,}

iThemba Laboratories, Cape Town, South Africa 68_{Hua-Zhong Normal University, Wuhan, China}

69_{Centro de Investigaci´on y de Estudios Avanzados (CINVESTAV), Mexico City and M´erida, Mexico} 70_{Universit´e de Lyon, Universit´e Lyon 1, CNRS/IN2P3, IPN-Lyon, Villeurbanne, France}

71_{Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil} 72_{Pusan National University, Pusan, South Korea}

73_{Nikhef, National Institute for Subatomic Physics and Institute} for Subatomic Physics of Utrecht University, Utrecht, Netherlands

74_{Division of Experimental High Energy Physics, University of Lund, Lund, Sweden} 75_{University of Tsukuba, Tsukuba, Japan}

76_{Hiroshima University, Hiroshima, Japan}

77_{Zentrum f¨ur Technologietransfer und Telekommunikation (ZTT), Fachhochschule Worms, Worms, Germany} 78_{Dipartimento di Scienze e Tecnologie Avanzate dell’Universit`a del}

Piemonte Orientale and Gruppo Collegato INFN, Alessandria, Italy 79_{Benem´erita Universidad Aut´onoma de Puebla, Puebla, Mexico}

80_{Instituto de Ciencias Nucleares, Universidad Nacional Aut´onoma de M´exico, Mexico City, Mexico} 81_{Sezione INFN, Padova, Italy}

82_{Institute of Space Sciences (ISS), Bucharest, Romania} 83_{Institute of Physics, Bhubaneswar, India} 84_{Universidade de S˜ao Paulo (USP), S˜ao Paulo, Brazil}

85_{Dipartimento di Fisica ‘E.R. Caianiello’ dell’Universit`a and Sezione INFN, Salerno, Italy} 86_{Sezione INFN, Bari, Italy}

87_{Dipartimento di Fisica dell’Universit`a and Sezione INFN, Cagliari, Italy} 88_{Soltan Institute for Nuclear Studies, Warsaw, Poland}

89_{Sezione INFN, Rome, Italy}

90_{University of Houston, Houston, TX, United States}

91_{Laboratoire de Physique Subatomique et de Cosmologie (LPSC), Universit´e Joseph Fourier,} CNRS-IN2P3, Institut Polytechnique de Grenoble, Grenoble, France

92_{Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, Slovakia} 93_{Sezione INFN, Trieste, Italy}

94_{Budker Institute for Nuclear Physics, Novosibirsk, Russia}

95_{Secci´on F´ısica, Departamento de Ciencias, Pontificia Universidad Cat´olica del Per´u, Lima, Peru} 96_{Lawrence Livermore National Laboratory, Livermore, CA, United States}

97_{Universidad Aut´onoma de Sinaloa, Culiac´an, Mexico} 98_{Technical University of Split FESB, Split, Croatia}

99_{Institute for Nuclear Research, Academy of Sciences, Moscow, Russia} 100_{University of Tennessee, Knoxville, TN, United States} 101_{Department of Physics, Sejong University, Seoul, South Korea} 102_{Faculty of Engineering, Bergen University College, Bergen, Norway}

103_{China Institute of Atomic Energy, Beijing, China}

104_{Lawrence Berkeley National Laboratory, Berkeley, CA, United States} 105_{Indian Institute of Technology, Mumbai, India}

106_{Warsaw University of Technology, Warsaw, Poland}

107_{California Polytechnic State University, San Luis Obispo, CA, United States} 108_{Fachhochschule K¨oln, K¨oln, Germany}

109_{Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic} 110_{Dipartimento di Fisica dell’Universit`a ‘La Sapienza’ and Sezione INFN, Rome, Italy}

111_{Physics Department, University of Rajasthan, Jaipur, India} 112_{Laboratori Nazionali di Legnaro, INFN, Legnaro, Italy}

113_{Purdue University, West Lafayette, IN, United States} (Dated: October 23, 2018)

The ratio of the yields of antiprotons to protons in pp collisions has been measured by the ALICE experiment at√s = 0.9 and 7 TeV during the initial running periods of the Large Hadron Collider(LHC). The measurement covers the transverse momentum interval 0.45 < pt< 1.05 GeV/c and rapidity |y| < 0.5. The ratio is measured to be R|y|<0.5= 0.957 ± 0.006(stat.) ± 0.014(syst.) at 0.9 TeV and R|y|<0.5 = 0.991 ± 0.005(stat.) ± 0.014(syst.) at 7 TeV and it is independent of both rapidity and transverse momentum. The results are consistent with the conventional model of

baryon-number transport and set stringent limits on any additional contributions to baryon-number transfer over very large rapidity intervals in pp collisions.

a _{Also at Laboratoire de Physique Corpusculaire (LPC), Clermont}

Universit´e, Universit´e Blaise Pascal, CNRS–IN2P3,

Clermont-Ferrand, France

b_{Also at Frankfurt Institute for Advanced Studies, Johann}

Wolf-gang Goethe-Universit¨at Frankfurt, Frankfurt, Germany

c_{Now at Sezione INFN, Padova, Italy}

d_{Now at Research Division and ExtreMe Matter Institute EMMI,}

GSI Helmholtzzentrum f¨ur Schwerionenforschung, Darmstadt,

Germany

e_{Now at Institut f¨ur Kernphysik, Johann Wolfgang}

Goethe-Universit¨at Frankfurt, Frankfurt, Germany

f _{Now at Physics Department, University of Cape Town, iThemba}

Laboratories, Cape Town, South Africa

g _{Now at National Institute for Physics and Nuclear Engineering,}

Bucharest, Romania

h_{Also at University of Houston, Houston, TX, United States}

i _{Now at European Organization for Nuclear Research (CERN),}

Geneva, Switzerland

j _{Also at Dipartimento di Fisica dell´Universit`a, Udine, Italy}

k_{Now at Helsinki Institute of Physics (HIP) and University of}

Jyv¨askyl¨a, Jyv¨askyl¨a, Finland

l _{Now at Institut Pluridisciplinaire Hubert Curien (IPHC),}

Uni-versit´e de Strasbourg, CNRS-IN2P3, Strasbourg, France

m_{Now at Institut f¨ur Kernphysik, Westf¨alische}

Wilhelms-Universit¨at M¨unster, M¨unster, Germany

n_{Now at : University of Technology and Austrian Academy of}

Sciences, Vienna, Austria

o _{Also at Lawrence Livermore National Laboratory, Livermore,}

CA, United States

p_{Also at European Organization for Nuclear Research (CERN),}

Geneva, Switzerland

q_Deceased

r _{Now at Yale University, New Haven, CT, United States}

s _{Now at University of Tsukuba, Tsukuba, Japan}

t _{Also at Centro Fermi – Centro Studi e Ricerche e Museo Storico}

della Fisica “Enrico Fermi”, Rome, Italy

u_{Also at Laboratoire de Physique Subatomique et de}

Cosmolo-gie (LPSC), Universit´e Joseph Fourier, CNRS-IN2P3, Institut

Polytechnique de Grenoble, Grenoble, France

v_{Now at SUBATECH, Ecole des Mines de Nantes, Universit´e de}

Nantes, CNRS-IN2P3, Nantes, France

w_{Now at Dipartimento di Fisica Sperimentale dell’Universit`a and}

Sezione INFN, Turin, Italy

x_{Now at Commissariat `a l’Energie Atomique, IRFU, Saclay,}

France

y_{Also at Department of Physics, University of Oslo, Oslo, Norway}

z_{Also at Wroc law University, Wroc law, Poland}

aa_{Now at Physikalisches Institut, Ruprecht-Karls-Universit¨at}

Hei-delberg, HeiHei-delberg, Germany

bb_{Now at Department of Physics and Technology, University of}

Bergen, Bergen, Norway

cc_{Now at Physics Department, University of Athens, Athens,}

Greece

dd_{Also at Institut f¨ur Kernphysik, Westf¨alische}

Wilhelms-Universit¨at M¨unster, M¨unster, Germany

ee_{Now at Universit´e de Lyon, Universit´e Lyon 1, CNRS/IN2P3,}

IPN-Lyon, Villeurbanne, France

ff_{Now at Faculty of Science, P.J. ˇSaf´arik University, Koˇsice,}

Slo-vakia

gg_{Now at : Centre de Calcul IN2P3, Lyon, France}

hh_{Also at Dipartimento di Fisica dell’Universit`a and Sezione INFN,}

Padova, Italy

In inelastic non-diffractive proton-proton collisions at

1

very high energy, the incoming projectile breaks up into

2

several hadrons which emerge after the collision in

gen-3

eral under small angles along the original beam

direc-4

tion. The deceleration of the incoming proton, or more

5

precisely of the conserved baryon number associated with

6

the beam particles, is often called “baryon-number

trans-7

port” and has been debated theoretically for some time

8

[1–7].

9

One mechanism responsible for baryon-number

trans-10

port is the break-up of the proton into a diquark–quark

11

configuration [2]. The diquark hadronizes after the

re-12

action with some longitudinal momentum pz into a new

13

particle, which carries the baryon number of the incoming

14

proton. This baryon-number transport is usually

quan-15

tified in terms of the rapidity loss ∆y = ybeam− ybaryon,

16

where ybeam (ybaryon) is the rapidity of the incoming

17

beam (outgoing baryon)1_.

18

However, diquarks in general retain a large fraction of

19

the proton momentum and therefore stay close to beam

20

rapidity, typically within one or two units. Therefore,

21

additional processes have been proposed to transport

22

the baryon number over larger distances in rapidity, in

23

particular via purely gluonic exchanges, where the

pro-24

ton breaks up into three quarks. The baryon number

25

resides with a non-perturbative configuration of gluon

26

fields, the so-called “baryon string junction”, which

con-27

nects the valence quarks [1, 3]. In this picture,

baryon-28

number transport is suppressed exponentially with the

29

rapidity interval ∆y, proportional to exp [(αJ− 1) ∆y],

30

where αJ is identified in the Regge model as the

inter-31

cept of the trajectory for the corresponding exchange in

32

the t-channel. If the string junction intercept is

approxi-33

mated with the one of the standard Reggeon (or meson),

34

αJ ≈ 0.5, baryon transport will approach zero with

in-35

creasing ∆y. If the intercept of the pure string junction

36

is αJ ≈ 1, as motivated by perturbative QCD [4], it will

37

approach a constant and finite value.

38

The LHC, being by far the highest energy proton–

39

proton collider, opens the possibility to investigate

40

baryon transport over very large rapidity intervals by

41

measuring the antiproton-to-proton production ratio at

42

midrapidity, R = Np/Np, or equivalently, the proton–

43

antiproton asymmetry, A = (Np− Np)/(Np+ Np). Most

44

ii_{Also at Sezione INFN, Bologna, Italy}

1_{The rapidity y is defined as y = 0.5 ln [(E + p}

z) / (E − pz)];

ra-pidity y = 0 corresponds to longitudinal momentum pz = 0 of

of the (anti)protons at midrapidity are created in baryon–

45

antibaryon pair production, implying equal yields. Any

46

excess of protons over antiprotons is therefore

associ-47

ated with the baryon-number transfer from the incoming

48

beam. Note that such a study has not been carried out

49

in high-energy proton–antiproton colliders (SppS,

Teva-50

tron) because of the symmetry of the initial system at

51

midrapidity. Model predictions for the ratio R at LHC

52

energies range from unity, i.e., no baryon-number

trans-53

fer to midrapidity, down to about 0.9 in models where

54

the string junction transfer is not suppressed with the

55

rapidity interval (αJ≈ 1).

56

In this letter, we describe the measurement of the

57

p/p ratio at midrapidity in non-diffractive pp collisions

58

at center-of-mass energies √s = 0.9 TeV and 7 TeV

59

(∆y ≈ 6.9–8.9), with the ALICE experiment at the LHC.

60

ALICE, which is the dedicated heavy-ion detector at

61

the LHC, consists of 18 detector sub-systems [8, 9]. The

62

central tracking systems used in the present analysis are

63

located inside a solenoidal magnet (B = 0.5 T); they

64

are optimized to provide good momentum resolution and

65

particle identification (PID) over a broad momentum

66

range, up to the highest multiplicities expected for heavy

67

ion collisions at the LHC. All detector systems were

com-68

missioned and aligned during several months of

cosmic-69

ray data-taking in 2008 and 2009 [10, 11].

70

Collisions occur inside a beryllium vacuum pipe (3 cm

71

in radius and 800 µm thick) at the center of the ALICE

72

detector. The tracking system in the ALICE central

bar-73

rel has full azimuth coverage within the pseudo-rapidity

74

window |η| < 0.9. The following detector sub-systems

75

were used in this analysis: the Inner Tracking System

76

(ITS) [11], the Time Projection Chamber (TPC) [12] and

77

the VZERO detector [8].

78

The ITS consists of six cylindrical layers of silicon

de-79

tectors with radii of 3.9/7.6 cm (Silicon Pixel Detectors–

80

SPD), 15.0/23.9 cm (Silicon Drift Detectors–SDD) and

81

38/43 cm (Silicon Strip Detectors–SSD). They provide

82

full azimuth coverage for tracks matching the acceptance

83

of the TPC (|η| < 0.9).

84

The TPC is the main tracking detector of the central

85

barrel. The detector is cylindrical in shape with an active

86

volume of inner radius 85 cm, outer radius of 250 cm and

87

an overall length along the beam direction of 500 cm.

88

Finally, the VZERO detector consists of two arrays of

89

32 scintillators each, which are placed around the beam

90

pipe on either side of the interaction region) at z = 3.3 m

91

and z = −0.9 m, covering the pseudorapidity ranges

92

2.8 < η < 5.1 and −3.7 < η < −1.7, respectively [13]. A

93

detailed description of the ALICE detectors, its

compo-94

nents, and their performance can be found in [8].

95

Data from 2.8 (√s = 0.9 TeV) and 4.2 (√s = 7 TeV)

96

million pp collisions, recorded during the first LHC runs

97

(December 2009, March–April 2010) were used for this

98

analysis. The events were recorded with both field

po-99 ] c p [GeV/ -1 10 1 10

dE/dx [arb. units]

2 10 3 10 e µ π K p [arb. units] exp. (dE/dx) 20 40 60 80 Entries 2 10 3 10 = 0.9 TeV s pp @ c 0.99 < p < 1.01 GeV/ ± ,K ± π p p,

FIG. 1. (Color online) The measured ionization per unit length as a function of particle momentum (both charges) in the TPC gas. The curves correspond to expected energy loss [14] for different particle types. The inset shows the mea-sured ionization for tracks with 0.99 < p < 1.01 GeV/c. The lines are Gaussian fits to the data.

larities for each energy. The trigger required a hit in

100

one of the VZERO counters or in the SPD detector, i.e.,

101

at least one charged particle anywhere in the 8 units of

102

pseudorapidity covered by these trigger detectors [13].

103

In addition, the trigger required a coincidence between

104

the signals from two beam pick-up counters, one on each

105

side of the interaction region, indicating the presence of

106

passing bunches.

107

Beam-induced background was reduced to a negligible

108

level (< 0.01%) with the help of the timing information

109

from the VZERO counters [13] and by requiring a

re-110

constructed primary vertex (calculated from the SPD)

111

within ±1 cm perpendicular to and ±10 cm along the

112

beam axis.

113

Measurements of momentum and particle

identifica-114

tion are performed using information from the TPC

de-115

tector, which measures the ionization in the TPC gas

116

and the particle trajectory with up to 159 space points.

117

In order to ensure a good track quality, a minimum of

118

80 clusters was required per track in the TPC and at

119

least two hits in the ITS of which at least one is in the

120

SPD. In order to reduce the contamination from

back-121

ground and secondary tracks (e.g. (anti)protons

originat-122

ing from weak hyperon decays or secondary interactions

123

in the material), a cut was imposed on the distance

124

of closest approach (dca) of the track to the primary

125

vertex in the xy (transverse) plane, which varied from

126

2.65 to 1.8 mm (2.33 to 1.5 mm for the 7 TeV data)

127

for the lowest (0.45 < pt < 0.55 GeV/c) and highest

128

(0.95 < pt < 1.05 GeV/c) pt bins, respectively. This

129

cut corresponds to 5σ of the measured dca resolution for

[cm] xy dca -2 -1 0 1 2 Counts 1 10 2 10 3 10 = 0.9 TeV s pp @ c < 0.55 GeV/ t 0.45 < p p p [cm] xy dca -2 -1 0 1 2 Counts 1 10 2 10 3 10 = 0.9 TeV s pp @ c < 1.05 GeV/ t 0.95 < p p p

FIG. 2. The distance of closest approach (dca) distributions of p and p for the lowest (left plot) and highest (right plot) transverse momentum bins. The broad background of protons at low momentum originates from secondary particles created in the detector material, whereas the tails for both p and p at high momentum (and for p at low momentum) arise from weak hyperon decays.

each momentum bin.

131

Particles are identified using their specific ionization

132

(dE/dx) in the TPC gas [12]. Figure 1 shows the

ioniza-133

tion (truncated mean) as a function of particle

momen-134

tum together with the expected curves [14] for different

135

particle species. The inset shows the measured dE/dx for

136

tracks in the momentum range 0.99 < p < 1.01 GeV/c

137

with clearly separated peaks for (anti)protons and lighter

138

particles. The dE/dx resolution of the TPC is 5,

de-139

pending slightly on the number of TPC clusters and the

140

track inclination angle. For this analysis, (anti)protons

141

were selected within a band of ±3σ around the expected

142

value.

143

In order to assure uniform geometrical acceptance,

144

high reconstruction efficiency and unambiguous proton

145

identification, we restrict the analysis to protons and

an-146

tiprotons in the rapidity range |y| < 0.5 and the

momen-147

tum range 0.45 < p < 1.05 GeV/c. The contamination

148

of the proton sample with electrons or pions and kaons is

149

negligible (< 0.1%) even at the highest momentum bins,

150

and in addition essentially charge symmetric.

151

Most instrumental effects associated with the

accep-152

tance, reconstruction efficiency, and resolution are

iden-153

tical for primary protons and anti-protons and therefore

154

cancel in the ratio. However, because of significant

dif-155

ferences in the relevant cross sections, anti-protons are

156

more likely than protons to be absorbed or elastically

157

scattered2_{within the detector, and a non negligible}

back-158

2 _{Particles undergoing elastic scattering in the inner detectors can}

ground in the proton sample arises from secondary

inter-159

actions in the beam pipe and inner layers of the detector.

160

In order to correct for the difference between p–A and

161

p–A elastic and inelastic reactions in the detector

mate-162

rial, detailed Monte Carlo simulations based on GEANT3

163

[15] and FLUKA [16] were performed. These corrections

164

rely in particular on the proper description of the

interac-165

tion cross sections used as input by the transport models.

166

These values were therefore compared with experimental

167

measurements [17, 18]. While p–A cross sections are

sim-168

ilar in both models and in agreement with existing data,

169

GEANT3 (as well as the current version of GEANT4)

170

significantly overestimates the measured inelastic cross

171

sections for antiprotons in the relevant momentum range

172

by about a factor of two, whereas FLUKA describes the

173

data very well. Concerning elastic scattering, where only

174

a limited data set is available for comparison, GEANT3

175

cross sections are about 25% above FLUKA, the latter

176

being again closer to the measurements. We therefore

177

used the FLUKA results to account for the difference of

178

p and p cross sections, which amount to a correction of

179

the p/p ratio by 8% and 3.5% for absorption and elastic

180

scattering, respectively.

181

The contamination of the proton sample due to

sec-182

ondaries originating from interactions with the detector

183

material was directly measured with the data and

sub-184

tracted. Most of these background tracks do not point

185

back to the interaction vertex and can therefore be

ex-186

cluded with a dca cut. Figure 2 shows the dca

distri-187

butions of p and p for the lowest (left panel) and the

188

highest (right panel) transverse momentum bins.

Sec-189

ondary protons are clearly visible in the left plot due to

190

their wide dca distribution. At higher momenta the

back-191

ground of secondary protons becomes very small. The

192

remaining tails visible in the dca distributions are due to

193

(anti)protons originating from weak decays. The

back-194

ground of secondary protons, which remains after the

195

dca cut under the peak of primaries, is subtracted by

de-196

termining its shape from Monte Carlo simulations and

197

adjusting the amount to the data at large values of the

198

dca. This correction is calculated and applied

differen-199

tially as a function of y and pt; it varies between 14% for

200

the lowest and less than 0.3% for the highest transverse

201

momentum bins.

202

The contamination coming from feed-down (i.e.,

203

(anti)protons originating from the weak decay of Λ and

204

¯

Λ) was subtracted in a similar way by parametrization

205

and fitting to the data of the respective simulated dca

dis-206

tributions. This correction ranges from 20% to 12% for

207

the lowest and highest pt bins, respectively.

208

still be reconstructed in the TPC but the corresponding ITS hits will in general not be associated to the track if the scattering angle is large.

TABLE I. Systematic uncertainties of the p/p ratio. Systematic Uncertainty

Material budget 0.5%

Absorption cross section 0.8% Elastic cross section 0.8%

Analysis cuts 0.4%

Corrections (secondaries/feed-down) 0.6%

Total 1.4%

The main sources of systematic uncertainties are the

209

detector material budget, the (anti)proton reaction cross

210

section, the subtraction of secondary protons and the

ac-211

curacy of the detector response simulations (see Table I).

212

The amount of material in the central part of ALICE

213

is very low, corresponding to about 10% of a radiation

214

length on average between the vertex and the active

vol-215

ume of the TPC. It has been studied with collision data

216

and adjusted in the simulation based on the analysis of

217

photon conversions. The current simulation reproduces

218

the amount and spatial distribution of reconstructed

con-219

version points in great detail, with a relative accuracy of

220

a few percent. Based on these studies, we assign a

sys-221

tematic uncertainty of 7% to the material budget. By

222

changing the material in the simulation by this amount,

223

we find a variation of the final ratio R of less than 0.5%.

224

The experimentally measured p–A reaction cross

sec-225

tions are determined with a typical accuracy better than

226

5% [17]. We assign a 10% uncertainty to the absorption

227

correction as calculated with FLUKA, which leads to a

228

0.8% uncertainty in the ratio R. By comparing GEANT3

229

with FLUKA and with the experimentally measured

elas-230

tic cross-sections, the corresponding uncertainty was

es-231

timated to be 0.8%, which corresponds to the difference

232

between the correction factors calculated with the two

233

models.

234

By changing the event selection, analysis cuts and

235

track quality requirements within reasonable ranges, we

236

find a maximum deviation of the results of 0.4%, which

237

we assign as systematic uncertainty to the accuracy of

238

the detector simulation and analysis corrections.

239

The uncertainty resulting from the subtraction of

sec-240

ondary protons and from the feed-down corrections was

241

estimated to be 0.6% by using different functional forms

242

for the background subtraction and for the contribution

243

of the hyperon decay products.

244

The contribution of diffractive reactions to our final

245

event sample was studied with different event generators

246

and was found to be less than 3%, resulting into a

negligi-247

ble contribution (< 0.1%) to the systematic uncertainty.

248

Finally, the complete analysis was repeated using only

249

TPC information (i.e., without using any of the ITS

de-250

tectors). The resulting difference was negligible at both

251

energies (< 0.1%).

252

Table I summarizes the contribution to the

system-253

atic uncertainty from all the different sources. The total

254 /p ratio p 0.8 0.9 1 = 0.9 TeV s pp @

]

c

[GeV/

t

p

0.4

0.6

0.8

1

0.8

0.9

1

= 7 TeV s pp @ Data PYTHIA 6.4: ATLAS-CSC PYTHIA 6.4: Perugia-SOFT HIJING/B

FIG. 3. (Color online) The ptdependence of the p/p ratio in-tegrated over |y| < 0.5 for pp collisions at√s = 0.9 TeV (top) and√s = 7 TeV (bottom). Only statistical errors are shown for the data; the width of the Monte Carlo bands indicates the statistical uncertainty of the simulation results.

systematic uncertainty is identical for both energies and

255

amounts to 1.4%.

256

The final, feed-down corrected p/p ratio R

inte-257

grated within our rapidity and pt acceptance rises from

258

R|y|<0.5 = 0.957 ± 0.006(stat.) ± 0.014(syst.) at √s =

259

0.9 TeV to R|y|<0.5= 0.991 ± 0.005(stat.) ± 0.014(syst.)

260

at√s = 7 TeV. The difference in the p/p ratio, 0.034 ±

261

0.008(stat.), is significant because the systematic errors

262

at both energies are fully correlated.

263

Within statistical errors, the measured ratio R shows

264

no dependence on transverse momentum (Fig. 3) or

ra-265

pidity (data not shown). The ratio is also independent of

266

momentum and rapidity for all generators in our

accep-267

tance, with the exception of HIJING/B, which predicts

y ∆ 2 3 4 5 6 7 8 9 10 /p ratio p 0.2 0.4 0.6 0.8 1 [GeV] s 10 102 103 104 ISR NA49 ALICE BRAHMS PHENIX PHOBOS STAR

FIG. 4. (Color online) Central rapidity p/p ratio as a function of the rapidity interval ∆y (lower axis) and center-of-mass energy (upper axis). Error bars correspond to the quadratic sum of statistical and systematic uncertainties for the RHIC and LHC measurements and to statistical errors otherwise.

a decrease with increasing transverse momentum for the

269

lower energy.

270

The data are compared with various model

predic-271

tions for pp collisions [6, 7, 19] in Table II (integrated

272

values) and Fig. 3. The analytical QGSM model does

273

not predict the pt dependence and is therefore not

in-274

cluded in Fig. 3. For both energies, two of the PYTHIA

275

tunes [19] (ATLAS-CSC and Perugia-0) as well as the

276

version of Quark–Gluon String Model (QGSM) with the

277

value of the string junction intercept αJ = 0.5 [6]

de-278

scribe the experimental values well, whereas QGSM

with-279

out string junctions (ǫ = 0, ǫ is a parameter

propor-280

tional to the probability of the string-junction exchange)

281

is slightly above the data. HIJING/B [7], unlike the

282

above models, includes a particular implementation of

283

gluonic string junctions to enhance baryon-number

trans-284

fer. This model underestimates the experimental results,

285

in particular at the lower LHC energy. Also, QGSM

286

with a value of the junction intercept αJ = 0.9 [6]

pre-287

dicts a smaller ratio, as does the Perugia-SOFT tune of

288

PYTHIA, which also includes enhanced baryon transfer3_.

289

Figure 4 shows a compilation of central rapidity

mea-290

surements of the ratio R in pp collisions as a function

291

of center-of-mass energy (upper axis) and the rapidity

292

interval ∆y (lower axis). The ALICE measurements

cor-293

respond to ∆y = 6.87 and ∆y = 8.92 for the two energies,

294

3 _{We have checked that baryon transfer is the main reason for the}

different p/p ratios predicted by the models; the absolute yield of (anti)protons in our acceptance, which is dominated by pair production, is reproduced by the models to within ±20%.

whereas the lower energy data points are taken from [20–

295

22]. The p/p ratio rises from 0.25 and 0.3 at the SPS and

296

the lowest ISR energy, respectively, to a value of about

297

0.8 at√s = 200 GeV, indicating that a substantial

frac-298

tion of the baryon number associated with the beam

par-299

ticles is transported over rapidity intervals of up to five

300

units.

301

Although our measured midrapidity ratio R at√s =

302

0.9 TeV is close to unity, there is still a small but

sig-303

nificant excess of protons over antiprotons

correspond-304

ing to a p–p asymmetry of A = 0.022± 0.003(stat.) ±

305

0.007(syst.). On the other hand, the ratio at√s = 7 TeV

306

is consistent with unity (A = 0.005 ± 0.003(stat.) ±

307

0.007(syst.)), which sets a stringent limit on the amount

308

of baryon transport over 9 units in rapidity. The

exis-309

tence of a large value for the asymmetry even at infinite

310

energy, which has been predicted to be A = 0.035 using

311

αJ= 1 [4], is therefore excluded.

312

A rough approximation of the ∆y dependence of the

313

ratio R can be derived in the Regge model, where

314

baryon pair production at very high energy is governed

315

by Pomeron exchange and baryon transport by

string-316

junction exchange [5]. In this case the p/¯p ratio takes

317

the simple form 1/R = 1 + C exp[(αJ − αP)∆y]. We

318

have fitted such a function to the data, using as value

319

for the Pomeron intercept αP = 1.2 [23] and αJ = 0.5,

320

whereas C, which determines the relative contributions of

321

the two diagrams, is adjusted to the measurements from

322

ISR, RHIC, and LHC. The fit, shown in Fig. 4, gives

323

a reasonable description of the data with only one free

324

parameter (C), except at lower energies, where

contribu-325

tions of other diagrams cannot be neglected [5]. Adding a

326

second string junction diagram with a larger intercept [4],

327

i.e., 1/R = 1+C exp[(αJ−αP)∆y]+C′exp[(αJ′−αP)∆y]

328

with αJ′ = 1, does not improve the quality of the fit 329

and its contribution is compatible with zero (C ≈ 10,

330

C′ _{≈ −0.1 ± 0.1). In a similar spirit, our data could}

331

also be used to constrain other Regge-model inspired

de-332

scriptions of baryon asymmetry, for example when the

333

string-junction exchange is replaced by the “odderon”,

334

which is the analogue of the Pomeron with odd C-parity;

335

see [6].

336

In summary, we have measured the ratio of

antipro-337

ton to proton production in the ALICE experiment at

338

the CERN LHC collider at√s = 0.9 and √s = 7 TeV.

339

Within our acceptance region (|y| < 0.5, 0.45 < pt <

340

1.05 GeV/c), the ratio of antiproton-to-proton yields

341

rises from R|y|<0.5= 0.957 ± 0.006(stat.) ± 0.014(syst.)

342

at 0.9 to a value close to unity R|y|<0.5 = 0.991 ±

343

0.005(stat.) ± 0.014(syst.) at 7 TeV. The p/p ratio is

344

independent of both rapidity and transverse

momen-345

tum. These results are consistent with standard models

346

of baryon-number transport and set tight limits on any

347

additional contributions to baryon-number transfer over

348

very large rapidity intervals in pp collisions.