JHEP06(2015)055
Published o SISSA by Sp inge
Recei ed:Ma ch 30, 2015
Accep ed:May 11, 2015
Published:June 9, 2015
Rapidi y and ans e se-momen um dependence o
he inclusi e J/ψnuclea modi ica ion ac o in p-Pb
collisions a √sNN = 5.02 TeV
The ALICE collabo a ion
E-mail: [email p o ec ed]
Abs ac : We ha e s udied he ans e se-momen um (pT) dependence o he inclusi e
J/ψp oduc ion in p-Pb collisions a √sNN = 5.02 TeV, in h ee cen e -o -mass apidi y
(ycms) egions, down o ze o pT. Resul s in he o wa d and backwa d apidi y anges
(2.03 < ycms <3.53 and −4.46 < ycms <−2.96) a e ob ained by s udying he J/ψdecay o
µ+µ−, while he mid- apidi y egion (−1.37 < ycms <0.43) is in es iga ed by measu ing
he e+e−decay channel. The pTdependence o he J/ψp oduc ion c oss sec ion and
nuclea modi ica ion ac o a e p esen ed o each o he apidi y in e als, as well as he
J/ψmean pT alues. Fo wa d and mid- apidi y esul s show a supp ession o he J/ψyield,
wi h espec o pp collisions, which dec eases wi h inc easing pT. A backwa d apidi y no
signi ican J/ψsupp ession is obse ed. Theo e ical models including a combina ion o cold
nuclea ma e e ec s such as shadowing and pa onic ene gy loss, a e in ai ag eemen
wi h he da a, excep a o wa d apidi y and low ans e se momen um. The implica ions
o he p-Pb esul s o he e alua ion o cold nuclea ma e e ec s on J/ψp oduc ion in
Pb-Pb collisions a e also discussed.
Keywo ds: Had on-Had on Sca e ing, Rela i is ic hea y ion physics, Hea y ions, Cha m
physics
A Xi eP in : 1503.07179
Open Access, Copy igh CERN,
o he bene i o he ALICE Collabo a ion.
A icle unded by SCOAP3.
doi:10.1007/JHEP06(2015)055
JHEP06(2015)055
The supp ession o cha monia, bound s a es o cand ¯cqua ks, and in pa icula o he
J/ψs a e, has long been p oposed as a signa u e o he o ma ion o a plasma o qua ks
and gluons (QGP) [1] in ul a ela i is ic nucleus-nucleus collisions. Howe e , i was soon
ealized ha cha monium p oduc ion can also be modi ied by nuclea e ec s no necessa ily
ela ed o QGP o ma ion [2]. These so-called cold nuclea ma e (CNM) e ec s can
be in es iga ed by s udying cha monium p oduc ion in p o on-nucleus (p-A) collisions as
con i med by he analysis o esul s ob ained by se e al ixed- a ge (SPS [3,4], HERA [5]
and Te a on [6]) and collide (RHIC [7] and LHC [8,9]) expe imen s.
Theo e ical models ha e s udied he p oduc ion o cha monium in p-A collisions and
he e ec s o he su ounding cold nuclea medium by in oducing a ious mechanisms
which include nuclea shadowing, gluon sa u a ion, ene gy loss and nuclea abso p ion.
Models [10–12] inspi ed by Quan um Ch omoDynamics (QCD) desc ibe cha monium p o-
duc ion as a wo-s ep p ocess, wi h he cc pai c ea ed in a ha d pa on sca e ing, ollowed
by i s e olu ion in o a bound s a e wi h speci ic quan um numbe s. The pai c ea ion is
sensi i e o he Pa on Dis ibu ion Func ions (PDFs) in bo h colliding pa ne s and, a
high ene gy, occu s mainly ia gluon usion. Al hough PDFs a e known o be modi ied in a
nuclea en i onmen , in o ma ion on he dependence o such modi ica ions on he ac ion
x(Bjo ken-x) o he nucleon momen um ca ied by he gluons and on he ou -momen um
squa ed Q2 ans e ed in he sca e ing is s ill limi ed [13–15]. Cha monium p oduc ion
measu emen s can he e o e p o ide insigh in o he so-called nuclea shadowing, i.e., on
how he nucleon gluon PDFs a e modi ied in a nucleus.
Modi ica ions o he ini ial s a e o he nucleus a e also add essed by app oaches as-
suming ha a su icien ly high ene gies, when he qua k pai is p oduced om a dense
gluon sys em ca ying small x- alues in he nuclea a ge , a cohe en e ec known as gluon
sa u a ion se s in. Such an e ec can be desc ibed by he Colo Glass Condensa e (CGC)
e ec i e heo y, which is cha ac e ized by a sa u a ion momen um scale (Q2
s). When com-
bined wi h a speci ic qua konium p oduc ion model [16,17], i is able o p o ide p edic ions
o cha monium p oduc ion in p-A collisions. In he con ex o shadowing and CGC mod-
els, a measu emen o he cha monium yield as a unc ion o ans e se momen um (pT)
and apidi y (y) is impo an as i gi es access o speci ic anges o alues o he gluon x
and/o Q2.
In addi ion o hese pu ely ini ial s a e e ec s, bo h he incoming pa ons and he cc
pai p opaga ing h ough he nucleus may lose ene gy by gluon adia ion a he a ious
s ages o he cha monium o ma ion p ocess [18]. The in e e ence o gluons adia ed be o e
and a e he ha d p oduc ion e ex can lead o cohe en ene gy loss e ec s, expec ed o
induce a modi ica ion o he cha monium kinema ic dis ibu ions [19].
Finally, while a elling h ough nuclea ma e , he e ol ing cc pai o , i c ossing
imes a e su icien ly la ge, he ully o med esonance, may b eak-up in o open cha m me-
son pai s. Al hough his mechanism, known as nuclea abso p ion, plays an impo an ole
a lowe collision ene gies [4], a he LHC he con ibu ion o his e ec o he p oduc ion
c oss sec ion is expec ed o be small, due o he e y sho c ossing ime o he pai h ough
he nuclea en i onmen .
Unde s anding he ole o he cold nuclea ma e e ec s ou lined abo e is essen ial
o u he ou knowledge o a ious aspec s o he physics o s ong in e ac ions, and i
– 2 –
JHEP06(2015)055
is c ucial o he in e p e a ion o he esul s on cha monium p oduc ion in hea y-ion
collisions, whe e he o ma ion o a QGP is expec ed. In such a ho and dense decon ined
medium he colo sc eening mechanism ( he QCD analogue o he Debye sc eening in
QED) can p e en he o ma ion o he hea y-qua k bound s a es, leading o a supp ession
o qua konium p oduc ion [1]. In addi ion, a LHC ene gies, he la ge cha m qua k densi y
may lead o a ( e)gene a ion o cha monium by ( e)combina ion o cha m qua ks [20,21]
in he QGP phase and/o when he sys em cools down and he o ma ion o had ons
occu s. This e ec enhances cha monium p oduc ion and is expec ed o be pa icula ly
sizeable a low pT. In hea y-ion collisions, a supe posi ion o ho and cold nuclea ma e
e ec s is expec ed, and a quan i a i e e alua ion o he la e is an impo an p e equisi e
o a de ailed unde s anding o he o me . A lowe ene gy, bo h a SPS [22–24] and
RHIC [25,26], a supp ession o J/ψp oduc ion, in addi ion o he CNM e ec s es ima ed
om p-A(d-A) collisions, was indeed obse ed.
A supp ession o J/ψp oduc ion has been measu ed in Pb-Pb collisions a he LHC [27–
31]. I was quan i ied ia he nuclea modi ica ion ac o , i.e., he a io o he Pb-Pb yields
wi h espec o hose measu ed in pp a he same ene gy, scaled by he numbe o bi-
na y nucleon-nucleon collisions. The supp ession has been ound o be s onge a o wa d
apidi y and a high pT[30,31], in ag eemen wi h expec a ions om ( e)combina ion
models. Simila o he lowe ene gy expe imen s, accu a e measu emen s in p-A collisions
a e needed o quan i a i ely assess he con ibu ion o ho and cold nuclea ma e e ec s
in Pb-Pb.
The i s measu emen s o inclusi e J/ψp oduc ion in p-Pb collisions a he LHC a
√sNN = 5.02 TeV [8,9] ha e shown a sizeable supp ession, wi h espec o bina y-scaled
pp collisions, a o wa d apidi y (p-going side) and no supp ession a backwa d apidi y
(Pb-going side). The nuclea modi ica ion ac o s a e in ai ag eemen wi h models based
on nuclea shadowing [32,33]. Calcula ions including a con ibu ion om cohe en ene gy
loss [19] also ep oduce he da a. Co esponding measu emen s o he less s ongly bound
ψ(2S) cha monium s a e a e p esen ed in [34]. In addi ion, an ex apola ion o Pb-Pb
collisions o he J/ψsupp ession measu ed in p-Pb showed ha he e ec s obse ed in
Pb-Pb canno be asc ibed only o CNM [8].
In his si ua ion, a s udy o he ans e se-momen um dependence o J/ψp oduc ion a
LHC ene gies o a ious apidi y egions is pa icula ly in e es ing in o de o: (i) each a
deepe unde s anding and be e quan i y he complica ed in e play o CNM e ec s, which
a e expec ed o exhibi a well-de ined kinema ical dependence [33,35,36]; (ii) de e mine i
he di e en ial ea u es o he Pb-Pb esul s ha sugges he p esence o ( e)combina ion
e ec s a e s ill p esen when he con ibu ion o CNM is conside ed.
In his pape , we p esen ALICE esul s on he ans e se-momen um dependence o
he inclusi e J/ψp oduc ion in p-Pb collisions a √sNN = 5.02 TeV, measu ed in h ee
cen e -o -mass apidi y (ycms) anges: backwa d (−4.46 < ycms <−2.96), mid- (−1.37 <
ycms <0.43) and o wa d (2.03 < ycms <3.53). The da a a e om he 2013 LHC p-Pb un.
A mid- apidi y, J/ψa e econs uc ed in he e+e−decay channel wi h he ALICE
cen al ba el de ec o s, co e ing he pseudo apidi y ange |ηlab|<0.9. Fo he backwa d
and o wa d apidi y analysis, J/ψa e de ec ed, h ough hei µ+µ−decay channel in he
muon spec ome e , in he pseudo apidi y ange −4< ηlab <−2.5.
– 3 –
JHEP06(2015)055
Due o he ene gy asymme y o he LHC beams (Ep= 4 TeV and EPb = 1.58 ·
APb TeV, whe e APb= 208 is he Pb a omic mass numbe ), he nucleon-nucleon cen e -o -
mass is shi ed, wi h espec o he labo a o y ame, by ∆y= 0.465 in he di ec ion o he
p o on beam. Since da a we e collec ed in wo con igu a ions, in e changing he di ec ion
o he p o on and he Pb beams in he LHC, he muon spec ome e accep ance co e s he
o wa d and backwa d ycms egions quo ed abo e, whe e posi i e (nega i e) apidi ies e e
o he di ec ion o he p o on (Pb) beam. In he ollowing, he no a ion p-Pb (Pb-p) will
e e o he i s (second) con igu a ion.
Fo he dielec on analysis, he cen al ba el de ec o s used o he J/ψ econs uc ion
a e he Inne T acking Sys em (ITS) [37] and he Time P ojec ion Chambe (TPC) [38].
The ITS con ains six cylind ical laye s o silicon de ec o s, wi h he inne mos laye a a
adius o 3.9 cm wi h espec o he beam axis and he ou e mos laye a 43 cm. This
de ec o is used o econs uc ing he p ima y in e ac ion e ex as well as e ices om
di e en in e ac ions and seconda y e ices om decays o hea y- la o ed pa icles. The
TPC has a cylind ical geome y wi h an ac i e olume ha ex ends om 85 o 247 cm
in he adial di ec ion and 500 cm longi udinally. I is he main cen al ba el acking
de ec o and also p o ides pa icle iden i ica ion ia he measu emen o he speci ic ene gy
loss (dE/dx) in he de ec o gas.
The muon spec ome e [39] is he main de ec o used in he dimuon analysis. I con-
sis s o a 3 T·m dipole magne , coupled wi h a acking and a igge ing sys em. Be ween
he in e ac ion poin and he muon spec ome e , a en in e ac ion-leng h (λI) on ab-
so be il e s ou he had ons p oduced in he in e ac ion. Muon acking is pe o med
by means o i e acking s a ions, each one made o wo planes o Ca hode Pad Cham-
be s. A 7.2 λIi on wall, which s ops seconda y had ons escaping he on abso be and
low momen um muons, is placed a e he acking s a ions. I is ollowed by a muon
igge sys em, based on wo s a ions equipped wi h Resis i e Pla e Chambe s. A conical
abso be made o ungs en, lead and s eel p o ec s he spec ome e agains seconda y
pa icles p oduced by he in e ac ion o la ge-ηp ima y pa icles in he beam pipe. In
he dimuon analysis, he de e mina ion o he in e ac ion e ex is p o ided by he wo
inne mos Si-pixel laye s o he ITS (Silicon Pixel De ec o , SPD).
Fo bo h analyses, iming in o ma ion om he Ze o Deg ee Calo ime e s [40], placed
symme ically a 112.5 m wi h espec o he in e ac ion poin , is used o emo e de-
bunched p o on-lead collisions. Fu he mo e, wo scin illa o hodoscopes (VZERO) [41],
wi h pseudo apidi y co e age 2.8< ηlab <5.1 and −3.7< ηlab <−1.7, a e used o emo e
beam-induced backg ound. Mo e de ails on he ALICE appa a us can be ound in [39].
A coincidence o signals in he wo VZERO de ec o s p o ides he minimum bias (MB)
igge , which has a >99% e iciency o selec ing non single-di ac i e p-Pb collisions [42].
While he dielec on analysis is based on MB- igge ed e en s, he s udy o J/ψin he µ+µ−
decay channel elies on a dimuon igge which equi es, in addi ion o he MB condi ion,
he de ec ion o wo opposi e-sign acks in he igge sys em. The dimuon igge selec s
wo muon candida es wi h ans e se momen a pT,µ la ge han 0.5 GeV/c. The igge
h eshold is no sha p, and he single muon igge e iciency eaches i s pla eau alue
(∼96%) a pT,µ ∼1.5 GeV/c. The dielec on analysis was pe o med on a da a sample
– 4 –
JHEP06(2015)055
co esponding o he p-Pb con igu a ion, wi h an in eg a ed luminosi y Lin = 51.4±1.9
µb−1, while o he dimuon analysis he co esponding alues a e 5.01±0.19 nb−1 o p-Pb
and 5.81 ±0.20 nb−1 o Pb-p ( he quo ed unce ain ies a e sys ema ic) [43].
The dielec on analysis is based on 1.07×108e en s, collec ed wi h a low MB in e ac-
ion a e (∼10 kHz), wi h a negligible amoun o e en s ha ing mo e han one in e ac ion
pe bunch c ossing (pile-up e en s). The in e ac ion e ex is equi ed o lie wi hin ±10
cm om he nominal collision poin along he beam axis, in o de o ob ain a uni o m
accep ance o he cen al ba el de ec o sys em in he iducial ange |ηlab|<0.9. Elec on
candida es a e selec ed wi h c i e ia e y simila o hose used in p e ious analyses o pp
collisions a √s=7 TeV [44] and Pb-Pb collisions a √sNN = 2.76 TeV [30]. To ensu e a
uni o m acking e iciency and pa icle iden i ica ion esolu ion in he TPC, only acks
wi hin |ηlab|<0.9 a e used. Elec on iden i ica ion is pe o med using he TPC, as shown
in igu e 1, by equi ing he dE/dxsignal o be compa ible wi h he elec on assump ion
wi hin 3σ, whe e σdeno es he esolu ion o he dE/dxmeasu emen . Fu he mo e, he
TPC acks ha a e compa ible wi h he pion and p o on assump ions wi hin 3.5σa e
ejec ed. A sligh ly loose ejec ion condi ion (3σ) is applied when conside ing acks co -
esponding o dielec on candida es wi h pT>5 GeV/cin o de o enhance he s a is ics.
A cu on he ans e se momen um (pT,e>1.0 GeV/c) is applied o emo e combina o ial
backg ound om low-momen um elec ons. The e iciency loss induced by his cu amoun s
o only ∼20%, due o he ela i ely la ge momen um o he J/ψdecay p oduc s. The elec-
on candida es mus ha e a leas one hi in he inne mos wo laye s o he ITS, hus
ejec ing a la ge ac ion o backg ound elec ons om pho on con e sions. Fo dielec ons
wi h pT<3 GeV/c he elec on candida es a e equi ed o ha e a hi in he i s laye , o
u he educe backg ound. The acks a e equi ed o ha e a leas 70 ou o a maximum
o 159 clus e s in he TPC and a χ2no malized o he numbe o clus e s a ached o he
ack smalle han 4.
The J/ψyields a e ob ained by coun ing he numbe o en ies in he in a ian mass
ange 2.92 < me+e−<3.16 GeV/c2a e backg ound sub ac ion. The J/ψ adia i e
decay channel and he ene gy loss o he elec ons due o b emss ahlung in he de ec o
ma e ial p oduce a long ail owa ds low in a ian masses. A i using a C ys al Ball
(CB) [45] unc ion o he J/ψsignal gi es compa ible alues in Mon e-Ca lo (MC) and
da a (∼20 MeV/c2 o he wid h o he Gaussian componen o he CB). Taking in o accoun
such a mass esolu ion and he p esence o he b emss ahlung ail, 67 −73% o he signal,
depending on pT, alls wi hin he coun ing window. The backg ound shape is ob ained om
e en mixing. E en mixing is pe o med by pai ing lep ons om di e en e en s ha ing
simila global cha ac e is ics such as he p ima y- e ex posi ion and he ack mul iplici y
( he esul being qui e insensi i e o he apidi y ange, ei he o wa d o cen al, chosen o
he mul iplici y measu emen s). The mixed-e en backg ound is hen scaled o ma ch he
same-e en opposi e-sign dis ibu ion in he mass anges 2.0< me+e−<2.5 GeV/c2and
3.2< me+e−<3.7 GeV/c2( he con ibu ion o he b emss ahlung ail in he o me ange
and o he ψ(2S) in he la e a e negligible). Consis en esul s a e ound when he same-
e en like-sign dis ibu ions a e used, ins ead o e en mixing, o es ima e he backg ound.
The sys ema ic unce ain y on he signal ex ac ion comes om he a ia ion o he mass
– 5 –
JHEP06(2015)055
)c (GeV/p
1 10
in TPC (a b. uni s)x/dEd
20
40
60
80
100
120
ALICE
= 5.02 TeV
NN
s
p-Pb
p d
e
K
π
Figu e 1. Cha ged pa icle speci ic ene gy loss (dE/dx) as a unc ion o momen um, as measu ed
in he TPC in p-Pb collisions. The black lines a e he co esponding Be he-Bloch pa ame iza ions
o he a ious pa icle species.
2.5 3 3.5 4 4.5
2
cCoun s pe 40 MeV/
20
40
60
80
= 5.02 TeV
NN
sALICE, p-Pb
<0.43
cms
y -1.37<
c < 1.3 GeV/
T
p 0 <
2.5 3 3.5 4 4.5
20
40
60
80
c < 3 GeV/
T
p1.3 <
2.5 3 3.5 4 4.5
20
40
60
80
c < 5 GeV/
T
p3 <
2.5 3 3.5 4 4.5
20
40
60
80
c < 7 GeV/
T
p5 <
)
2
c (GeV/
-
e
+
e
m
2.5 3 3.5 4 4.5
20
40
60
80
c < 10 GeV/
T
p7 <
Figu e 2. Opposi e-sign dielec on in a ian mass spec a (blue symbols) o a ious pTin e als,
compa ed o he backg ound (black cu e) es ima ed h ough mixed e en s. The backg ound is
scaled o ma ch he da a in he mass anges 2.0< me+e−<2.5 GeV/c2and 3.2< me+e−<
3.7 GeV/c2.
ange whe e he no maliza ion o he mixed-e en backg ound shape is pe o med and
om he choice o he mass window whe e he signal is coun ed. The signal ex ac ion has
been pe o med in i e ans e se-momen um bins, pT<1.3, 1.3< pT<3, 3 < pT<5,
5< pT<7 and 7 < pT<10 GeV/c. The J/ψcoun s in hese bins a y om 25 o 132, wi h
a signi icance, compu ed in he 2.92 < me+e−<3.16 GeV/c2mass egion, anging om 4.6
o 8.7. An analysis o he pT-in eg a ed da a sample, using he p ocedu e de ailed abo e,
gi es 465±37(s a .)±16(sys .) J/ψsignal coun s. The sys ema ic unce ain y on he signal
ex ac ion is la ges a low pT(10% o pT<1.3 GeV/cand 12% o 1.3< pT<3 GeV/c),
– 6 –
JHEP06(2015)055
due o a less a o able signal o e backg ound a io, and dec eases o ∼5.5–8.4% in he
o he h ee pTbins. Figu e 2shows he in a ian mass dis ibu ions o he opposi e-sign
dielec ons compa ed wi h he mixed-e en backg ound o he di e en in e als o pT.
The dimuon analysis is pe o med as de ailed in [8], and is sho ly summa ized he e-
a e . Da a we e collec ed wi h he dimuon igge , and he MB in e ac ion a e (up o 200
kHz) was much highe han in he sample used o he dielec on analysis. This leads o
a∼2% in e ac ion pile-up p obabili y. Howe e , he p obabili y o ha ing mo e han one
dimuon in he same bunch c ossing sa is ying he igge condi ion is negligible. Muon can-
dida e acks a e econs uc ed in he acking sys em by using he s anda d econs uc ion
algo i hm [44]. The quali y o he acks is ensu ed by equi ing he single muon pseudo-
apidi y o be in he ange −4< ηlab,µ <−2.5, in o de o emo e pa icles a he edges o
he muon spec ome e accep ance. In addi ion, a cu on he adial coo dina e o he ack
a he end o he on abso be (17.6< Rabs <89.5 cm) is pe o med, ensu ing ejec ion o
muons c ossing i s high-densi y pa , whe e ene gy loss and mul iple sca e ing e ec s a e
mo e impo an . The acks econs uc ed in he acking sys em ha a e no ma ched o
a co esponding ack in he igge ing sys em a e ejec ed [44]. Finally, he econs uc ed
dimuons a e equi ed o be in 2.03 < ycms <3.53 (−4.46 < ycms <−2.96) o he o wa d
(backwa d) apidi y analysis. The numbe o J/ψis ex ac ed in ans e se-momen um
bins, in he ange pT<15 GeV/c, h ough i s o he in a ian mass spec a o opposi e-sign
dimuons. The spec a a e i ed wi h a supe posi ion o backg ound and esonance shapes.
The backg ound is desc ibed wi h a Gaussian unc ion wi h a mass-dependen wid h o ,
al e na i ely, wi h an exponen ial unc ion imes a ou h-o de polynomial unc ion. Fo
he J/ψshape an ex ended C ys al Ball unc ion, which accommoda es a non-Gaussian
ail bo h on he igh and on he le side o he esonance peak, is adop ed. Al e na i ely,
a pseudo-Gaussian unc ion [46] is used, co esponding o a Gaussian co e a ound he J/ψ
pole, and ails on he igh and le side o i , pa ame e ized by a ying he wid h o he
Gaussian as a unc ion o he mass. The alue o he J/ψmass and i s wid h (σ) a he pole
posi ion a e ee pa ame e s o he i . The mass coincides wi h he PDG alue wi hin less
han 5 MeV/c2and he wid h is ∼70 MeV/c2, sligh ly inc easing wi h pT, due o a small
ela i e dec ease in he acking esolu ion o ha de muons. Al hough he signal o e
backg ound a ios, calcula ed o a ±3σin e al a ound he esonance peak, a e ela i ely
la ge ( anging om 1.4 o ∼6 mo ing om low o high pT), he pa ame e s o he ails
o he J/ψdis ibu ions canno be eliably uned on he da a (in pa icula a la ge pT,
whe e s a is ics is limi ed), bu a e ixed, o each pTbin, o he alues ex ac ed om i s
o econs uc ed samples om a signal-only MC gene a ion. The con ibu ion o he ψ(2S)
esonance is also included in he i ing p ocedu e, e en i i s in luence on he de e mina ion
o he J/ψyield is negligible. Finally, all he i s a e pe o med in wo di e en in a ian
mass anges, ei he 2 < mµµ <5 GeV/c2o 2.2< mµµ <4.5 GeV/c2. Examples o i s o
he in a ian mass spec a, in he pTbins unde s udy, a e shown in igu e 3.
Fo each pTbin, he numbe o J/ψis e alua ed as he a e age o he in eg als o
he esonance unc ions ob ained in he a ious i s. The RMS o he co esponding yield
dis ibu ions (0.2−3%, depending on pT) p o ides he sys ema ic unce ain y on he signal
ex ac ion. Addi ional se s o ails, ob ained om he MC, bu e e ing o o he ycms and
– 7 –
JHEP06(2015)055
2
Coun s pe 50 MeV/c
2000
4000 < 1 GeV/c
T
p0 <
= 5.02 TeV
NN
s
ALICE, p-Pb
<3.53
cms
y2.03<
2000
4000
< 2 GeV/c
T
p1 <
2000
4000
< 3 GeV/c
T
p2 <
2000
4000
< 4 GeV/c
T
p3 <
2000
4000
< 5 GeV/c
T
p4 <
2.5 3 3.5 4 4.5
2
Coun s pe 50 MeV/c
500
1000
< 6 GeV/c
T
p5 <
2.5 3 3.5 4 4.5
500
1000
< 7 GeV/c
T
p6 <
2.5 3 3.5 4 4.5
500
1000
< 8 GeV/c
T
p7 <
2.5 3 3.5 4 4.5
500
1000
< 10 GeV/c
T
p8 <
)
2
c (GeV/
-
µ
+
µ
m
2.5 3 3.5 4 4.5
500
1000
< 15 GeV/c
T
p10 <
Figu e 3. The opposi e-sign dimuon in a ian mass spec a o he a ious pTbins, ela i e o
he p-Pb da a sample (blue symbols). The i s shown in his igu e (blue cu es) we e pe o med
by using he sum o ex ended C ys al Ball unc ions o he J/ψand ψ(2S) signals, and a a iable
wid h Gaussian o he backg ound. The signal and backg ound componen s a e shown sepa a ely
as ed cu es.
pTphase space egions, ha e also been es ed and he dependence o he ex ac ed yields
on he a ia ion o he ails (2%) is included in he sys ema ic unce ain y on he signal
ex ac ion. As a unc ion o pT, he numbe o J/ψin he p-Pb (Pb-p) con igu a ion anges
be ween ∼16100 (∼16000) in he mos popula ed bin (1 < pT<2 GeV/c) and less han
∼900 (∼300) in he highes pTbin (10 < pT<15 GeV/c).
The J/ψyields a e hen co ec ed o he p oduc o accep ance imes e iciency (A×ε),
e alua ed by means o a MC simula ion. J/ψp oduc ion is assumed o be unpola ized, as
mo i a ed by he small deg ee o pola iza ion measu ed in pp collisions a √s= 7 TeV [47–
49]. In he e+e−decay channel, A×εis calcula ed using a MC simula ion whe e J/ψ
a e injec ed in o p-Pb collisions simula ed wi h HIJING [50]. The decay p oduc s o he
J/ψa e hen p opaga ed h ough a ealis ic desc ip ion o he ALICE se -up, based on
GEANT3.21 [51], aking in o accoun he ime e olu ion o he de ec o pe o mance. Fi-
nally, J/ψcandida es a e econs uc ed wi h he same p ocedu e applied o da a. The pT-
in eg a ed A×ε ac o amoun s o 8.9%. I s pT-dependence exhibi s a minimum (∼7.5%)
a ound pT= 2 GeV/c, due o he kinema ical accep ance, and i eaches ∼12% a high
pT. The in eg a ed alue o A×εis a ec ed by a 3% sys ema ic unce ain y ela ed o
he choice o he J/ψ pT- and y-dis ibu ions used in he MC simula ion. This alue is ob-
ained using as inpu se e al dis ibu ions, de e mined by a ying wi hin unce ain ies he
di e en ial spec a ex ac ed om he ALICE p-Pb da a hemsel es. Fo pT-di e en ial
s udies, he alues o A×εa e ound o be sensi i e only a a sub-pe cen le el o he
adop ed inpu pT- and y-dis ibu ions. A u he small sys ema ic unce ain y eaching
1.5% in he highes pTin e al and ela ed o he s a is ical unce ain y o he MC sample
is also in oduced. The sys ema ic unce ain y on he dielec on econs uc ion e iciency
– 8 –
JHEP06(2015)055
is s ongly domina ed by he pa icle iden i ica ion unce ain y and amoun s o 4%. I was
ob ained by compa ing he single ack econs uc ion e iciency o opologically iden i ied
posi ons and elec ons om pho on con e sions wi h he co esponding MC quan i ies.
In he dimuon analysis, he J/ψ A ×εis ob ained wi h a MC simula ion, by gene a ing
signal-only samples, acking hem in he expe imen al se -up modeled wi h GEANT3.21
and using he same econs uc ion p ocedu e applied o da a. The use o a pu e signal
MC is jus i ied, since he acking e iciency does no show a dependence on he had onic
mul iplici y o he collision. A ealis ic desc ip ion o he se -up is adop ed, including he
ime e olu ion o he e iciencies o acking and igge ing de ec o s. As o he dielec on
analysis, he di e en ial dis ibu ions used as an inpu o he MC a e uned di ec ly on he
da a. The J/ψ A ×ε alues, in eg a ed o e pT, a e 25.4% and 17.1% o p-Pb and Pb-p
espec i ely [8], and exhibi a dependence on ans e se momen um, being o he o de o
∼24% (∼16%) o p-Pb (Pb-p) a low pTand eaches ∼50% (∼35%) in he highes pTbin
(10 < pT<15 GeV/c). The sys ema ically lowe A×ε alues in Pb-p e lec he smalle
de ec o e iciency in he co esponding da a aking pe iod. The sys ema ic unce ain y
on he in eg a ed A×εdue o he inpu shapes is 1.5% o bo h p-Pb and Pb-p, and
has been es ima ed using a ious dis ibu ions ob ained om da a and co esponding o
smalle in e als in y,pTand cen ali y (see [8] o de ails). Fo pT-di e en ial s udies,
he co esponding unce ain ies a e below 1.5%. The unce ain y on he dimuon acking
e iciency amoun s o 4% (6%) o p-Pb (Pb-p) and is aken as cons an o he ull pT
ange. I is e alua ed by combining he unce ain ies on single muon acking e iciencies,
conside ed as unco ela ed. The e iciency o each acking plane is ob ained using he
edundancy o he acking sys em ( wo independen planes pe s a ion) and hen single
muon e iciencies o he ull acking sys em a e calcula ed acco ding o he acking al-
go i hm [52]. Thei unce ain y is de e mined by compa ing he e iciency ob ained wi h
acks om MC and eal da a. The sys ema ic unce ain y on he dimuon igge e iciency
includes: (i) a con ibu ion due o he unce ain y in he e alua ion o he igge de ec o
e iciency (∼2%, independen o pT); (ii) a 0.5−3% pT-dependen con ibu ion (2% o he
in eg a ed e iciency), ela ed o small di e ences in he igge esponse unc ion be ween
da a and MC in he egion close o he igge h eshold; (iii) a 0.5−3.5% pT-dependen
con ibu ion due o a small ac ion o opposi e-sign pai s which we e misiden i ied as like-
sign by he igge sys em. Finally, a ∼1% unce ain y, independen o pT, is included, due
o he choice o he alue o he χ2cu applied o he ma ching o acks econs uc ed in
he muon acking and igge ing sys ems.
The di e en ial c oss sec ion o inclusi e J/ψp oduc ion is de ined as:
d2σJ/ψ
pPb
dydpT
=NJ/ψ(∆y, ∆pT)
LpPb
in ·(A×ε)(∆y,∆pT)·B.R.(J/ψ →l+l−)·∆y·∆pT
(1)
whe e NJ/ψ(∆y, ∆pT) is he numbe o J/ψ o a gi en ∆yand ∆pTin e al. The b anching
a io o dilep ons, B.R.(J/ψ →l+l−), is 5.94 ±0.06% (5.93 ±0.06%) o he dielec on
(dimuon) decay [53]. The in eg a ed luminosi y, LpPb
in , is he a io be ween NMB, he
numbe o MB collisions, and σMB
pPb, he co esponding c oss sec ion, measu ed in a an de
– 9 –
JHEP06(2015)055
o he combines cohe en ene gy loss wi h EPS09 shadowing. The unce ain y bands in-
clude, o he cohe en ene gy loss mechanism, a a ia ion o bo h he q0pa ame e (gluon
anspo coe icien e alua ed a x= 0.01) and he pa ame iza ion o he p oduc ion
c oss sec ion. A o wa d apidi y he pu e ene gy loss scena io p edic s a much s eepe
pT-dependence, while be e ag eemen is ound when he EPS09 con ibu ion is included.
Howe e , a low pT, a disc epancy be ween da a and bo h calcula ions is obse ed. Also
a mid- apidi y he cohe en ene gy loss model including he EPS09 con ibu ion be e
desc ibes he da a, al hough he la ge unce ain ies p e en a i m conclusion. The same
ea u es can be obse ed a backwa d apidi y, whe e he calcula ion including cohe en
ene gy loss and shadowing ag ees wi h he da a in showing weak nuclea e ec s on J/ψ
p oduc ion. Finally, he esul s a cen al and o wa d apidi ies a e compa ed wi h a p e-
dic ion based on he CGC amewo k and using CEM o he p omp J/ψp oduc ion [36].
In he backwa d apidi y egion, highe gluon xin he nucleus a e p obed and he CGC
model is ou o i s ange o applicabili y. The quo ed unce ain ies a e ela ed o he choices
o Q2
sand o he cha m qua k mass. While he model is in ai ag eemen wi h mid- apidi y
da a, i clea ly unde p edic s he J/ψ RpPb in he ull pT ange a o wa d apidi y.
The heo e ical calcula ions discussed abo e a e ca ied ou o p omp J/ψ(i.e., di-
ec J/ψand he con ibu ion om χcand ψ(2S) decays), while he measu emen s a e o
inclusi e J/ψwhich include a non-p omp con ibu ion om B-had on decays. The con-
ibu ion o he la e sou ce o Rincl
pPb can be e alua ed om he measu ed ac ion Bo
non-p omp o p omp J/ψp oduc ion in pp collisions and on he supp ession Rnon−p omp
pPb
o non-p omp J/ψin p-Pb collisions. Mo e in de ail, in he ange 2 < ycms <4.5, he ac-
ion Bmeasu ed by LHCb in pp collisions a √s= 7 TeV, inc eases om 0.08 o 0.22 om
pT= 0 o 8 GeV/c[48]. This quan i y has a small a ia ion wi hin he ycms ange co e ed
and is also no s ongly √s-dependen (simila alues a e ob ained o √s= 8 TeV [58]).
A mid- apidi y, Bwas measu ed by ALICE in pp collisions a √s= 7 TeV and anges
om 0.10 o 0.44 o pTinc easing om 1.3 o 10 GeV/c[61]. Rnon−p omp
pPb was measu ed
a √sNN = 5.02 TeV by LHCb, in eg a ed o e pT, ob aining 0.83 ±0.02 ±0.08 o
2.5< ycms <4 and 0.98 ±0.06 ±0.10 o −4< ycms <−2.5 [9]. Assuming o each pT-bin
a a ia ion o Rnon−p omp
pPb be ween 0.6 and 1.3, a conse a i e choice due o he una ail-
abili y o a pT-di e en ial esul , and conside ing he pT-dependence o Ba √s= 7 TeV,
one can ex ac Rp omp
pPb as Rp omp
pPb =Rincl
pPb + B·(Rincl
pPb −Rnon−p omp
pPb ). The maximum
di e ences be ween he inclusi e and p omp RpPb ob ained in his way a e, o low and
high pT: (i) 3 and 10% a backwa d apidi y; (ii) 11 and 16% a cen al apidi y; (iii) 10
and 8% a o wa d apidi y. These a ia ions a e, a mos , o he same o de o magni ude
as he quo ed unce ain ies on inclusi e RpPb.
The RpPb esul s shown in his pape can be conside ed as a aluable ool o imp o e
ou unde s anding o he con ibu ion o CNM o he supp ession o he J/ψyields obse ed
in Pb-Pb [30,31]. Indeed, as e i ied in [8] o he dimuon analysis, in Pb-Pb collisions
he Bjo ken-x anges p obed by he J/ψp oduc ion p ocess in he wo colliding nuclei,
assuming a gg →J/ψ (2→1) [62] mechanism, a e shi ed by only ∼10% wi h espec o he
co esponding in e als o p-Pb and Pb-p, despi e he di e en ene gy (√sNN = 2.76 TeV)
– 16 –
JHEP06(2015)055
and he sligh ly di e en ycms ange (2.5< y < 4) o Pb-Pb. A simila conclusion holds
a mid- apidi y, whe e he co e ed x-in e als, calcula ed o pT=hpTi, a e 6.1×10−4<
x < 3.0×10−3and 7.0×10−4<x<3.5×10−3 o p-Pb and Pb-Pb collisions, espec i ely.
Unde he assump ion ha shadowing is he main CNM- ela ed mechanism ha plays a
ole in he J/ψp oduc ion and ha i s e ec on he wo colliding nuclei in Pb-Pb collisions
can be ac o ized, he p oduc RpPb ×RPbp (R2
pPb) can be conside ed as an es ima e o
CNM e ec s in Pb-Pb collisions a o wa d (cen al) apidi y [63,64]. This conclusion
holds no only o he 2 →1 p oduc ion p ocess bu also when he mo e gene al 2 →2
mechanism (gg →J/ψg) is conside ed.
In igu e 6 he compa ison o he measu ed RPbPb wi h he quan i ies de ined abo e is
ca ied ou . Such a compa ison should be conside ed as quali a i e, in iew o he sligh x-
misma ch de ailed abo e and o he ac ha , a mid- apidi y, he cen ali y anges p obed
in p-Pb and Pb-Pb a e no he same (0-100% and 0-50%, espec i ely). In bo h apidi y
egions, he ex apola ion o CNM e ec s shows a clea pT-dependence, co esponding o
a s ong supp ession a low pT, which anishes o la ge ans e se momen a. A low
pTand cen al apidi y, he e migh be an indica ion o a Pb-Pb supp ession smalle
han he CNM ex apola ion, consis en wi h he p esence o a con ibu ion ela ed o he
( e)combina ion o c¯cpai s [30], aking place in he ho medium. A simila e ec can be
seen a o wa d apidi y. A la ge pTand o wa d apidi y, he obse ed supp ession in
Pb-Pb collisions is much la ge han CNM ex apola ions, showing ha , in his ans e se-
momen um egion, supp ession e ec s in ho ma e , possibly ela ed o colo sc eening,
become dominan .
Finally, a mo e di ec compa ison o Pb-Pb esul s wi h he CNM ex apola ion can be
ob ained by de ining he a io SJ/ψ =RPbPb/(RpPb ×RPbp). Such a quan i y, o o wa d
apidi y esul s, is shown in igu e 7and con i ms he main ea u es de ailed abo e, i.e., a
s ong supp ession o J/ψa la ge pT, and a hin o an enhancemen a low pT. A cen al
apidi y, due o he sizeable unce ain ies on bo h p-Pb and Pb-Pb esul s, only he pT-
in eg a ed a io can be ob ained. Using he RPbPb in he 0-90% cen ali y ange [30], and
he in eg a ed RpPb gi en abo e, one ge s 1.43 ±0.26(s a ) ±0.56(sys ). Mo e p ecise
measu emen s a e needed o d aw a i m conclusion in his apidi y ange.
In summa y, we ha e p esen ed esul s on he inclusi e J/ψp oduc ion in p-Pb col-
lisions a √sNN = 5.02 TeV. The pT-di e en ial c oss sec ions, he hpTiand he nuclea
modi ica ion ac o s ha e been e alua ed in h ee apidi y egions: −4.46 < ycms <−2.96,
−1.37 < ycms <0.43 and 2.03 < ycms <3.53. A o wa d and mid- apidi y a signi ican
supp ession is obse ed a low pT, wi h a anishing end a high pT. A backwa d apidi y
no signi ican supp ession o enhancemen is isible. Compa isons wi h heo e ical models
based on a combina ion o nuclea shadowing and cohe en ene gy loss e ec s p o ide a
ai desc ip ion o he obse ed pa e ns, excep a o wa d apidi y and low ans e se
momen um. These esul s can be used o p o ide a quali a i e es ima e o he in luence
o cold nuclea ma e e ec s on he J/ψsupp ession obse ed in Pb-Pb collisions. Unde
he assump ion ha shadowing ep esen s he main CNM con ibu ion, we ind ha i
canno accoun o he obse ed supp ession in Pb-Pb a high pT. A low pT, he obse ed
CNM e ec s alone may sugges a supp ession la ge han ha obse ed in Pb-Pb, which
– 17 –
JHEP06(2015)055
)c (GeV/
T
p
0 2 4 6 8 10
PbPb
R,
2
pPb
R
0
0.2
0.4
0.6
0.8
1
1.2
1.4 -
e
+
e→ ψALICE inclusi e J/
= 5.02 TeV
NN
s
<0.43),
cms
y(-1.37<
2
pPb
R
= 2.76 TeV, 0-50 %
NN
s|<0.8),
cms
y(|
PbPb
R
)c (GeV/
T
p
0 1 2 3 4 5 6 7 8
PbPb
R,
backw
pPb
R x
o w
pPb
R
0
0.2
0.4
0.6
0.8
1
1.2
1.4 -
µ
+
µ→ψALICE inclusi e J/
= 5.02 TeV
NN
s
<-2.96),
cms
y (-4.46<
backw
pPb
R<3.53) x
cms
y (2.03<
o w
pPb
R
= 2.76 TeV, 0-90%
NN
s<4),
cms
y (2.5<
PbPb
R
(Phys. Le . B734 (2014) 314)
Figu e 6. The es ima e o he pT-dependence o CNM e ec s in Pb-Pb, calcula ed as R2
pPb o mid-
apidi y da a ( op) and as RpPb ×RPbp (bo om) a o wa d apidi y. The quan i ies a e compa ed
o RPbPb measu ed in Pb-Pb collisions in he (app oxima ely) co esponding y- anges [30,31].
The e ical e o ba s co espond o he s a is ical unce ain ies, he open boxes (shaded a eas)
ep esen pT-unco ela ed (pa ially co ela ed) sys ema ic unce ain ies, while he boxes a ound
RpPb = 1 show he size o he co ela ed unce ain ies. The ho izon al ba s co espond o he
wid hs o he pTbins. The Pb-Pb poin s in he bo om panel we e sligh ly displaced in pT, o
imp o e isibili y.
– 18 –
JHEP06(2015)055
)c(GeV/
T
p
0 1 2 3 4 5 6 7 8
ψJ/
S
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8 -
µ
+
µ→ψALICE inclusi e J/
<3.53
cms
y = 5.02 TeV, 2.03<
NN
s:
o w
pPb
R
<-2.96
cms
y = 5.02 TeV, -4.46<
NN
s
:
backw
pPb
R
<4, 0-90%
cms
y = 2.76 TeV, 2.5<
NN
s
:
PbPb
R
(Phys. Le . B734 (2014) 314)
backw
pPb
R ×
o w
pPb
R
PbPb
R
=
ψJ/
S
Figu e 7. The a io be ween RPbPb o inclusi e J/ψa o wa d apidi y and he p oduc
RpPb ×RPbp o he nuclea modi ica ion ac o s a o wa d and backwa d apidi y. None o he
unce ain ies cancels ou in he a io. S a is ical unce ain ies a e shown as e ical e o ba s, while
he boxes a ound he poin s ep esen a quad a ic combina ion o unco ela ed and pa ially co e-
la ed sys ema ic unce ain ies. The box a ound SJ/ψ = 1 co esponds o co ela ed unce ain ies.
The ho izon al ba s coincide wi h he wid hs o he pTbins.
is consis en wi h he p esence o a cha m qua k ( e)combina ion componen o he J/ψ
p oduc ion in nucleus-nucleus collisions.
Acknowledgmen s
The ALICE Collabo a ion would like o hank all i s enginee s and echnicians o hei
in aluable con ibu ions o he cons uc ion o he expe imen and he CERN accele a o
eams o he ou s anding pe o mance o he LHC complex. The ALICE Collabo a ion
g a e ully acknowledges he esou ces and suppo p o ided by all G id cen es and he
Wo ldwide LHC Compu ing G id (WLCG) collabo a ion. The ALICE Collabo a ion ac-
knowledges he ollowing unding agencies o hei suppo in building and unning he
ALICE de ec o : S a e Commi ee o Science, Wo ld Fede a ion o Scien is s (WFS) and
Swiss Fonds Kidagan, A menia, Conselho Nacional de Desen ol imen o Cien ´ı ico e Tec-
nol´ogico (CNPq), Financiado a de Es udos e P oje os (FINEP), Funda¸c˜ao de Ampa o `a
Pesquisa do Es ado de S˜ao Paulo (FAPESP); Na ional Na u al Science Founda ion o China
(NSFC), he Chinese Minis y o Educa ion (CMOE) and he Minis y o Science and Tech-
nology o China (MSTC); Minis y o Educa ion and You h o he Czech Republic; Danish
Na u al Science Resea ch Council, he Ca lsbe g Founda ion and he Danish Na ional Re-
sea ch Founda ion; The Eu opean Resea ch Council unde he Eu opean Communi y’s
Se en h F amewo k P og amme; Helsinki Ins i u e o Physics and he Academy o Fin-
land; F ench CNRS-IN2P3, he ‘Region Pays de Loi e’, ‘Region Alsace’, ‘Region Au e gne’
– 19 –
JHEP06(2015)055
and CEA, F ance; Ge man Bundesminis e ium u Bildung, Wissenscha , Fo schung und
Technologie (BMBF) and he Helmhol z Associa ion; Gene al Sec e a ia o Resea ch and
Technology, Minis y o De elopmen , G eece; Hunga ian O szagos Tudomanyos Ku a asi
Alappg ammok (OTKA) and Na ional O ice o Resea ch and Technology (NKTH); De-
pa men o A omic Ene gy and Depa men o Science and Technology o he Go e nmen
o India; Is i u o Nazionale di Fisica Nuclea e (INFN) and Cen o Fe mi — Museo S o ico
della Fisica e Cen o S udi e Rice che “En ico Fe mi”, I aly; MEXT G an -in-Aid o
Specially P omo ed Resea ch, Japan; Join Ins i u e o Nuclea Resea ch, Dubna; Na-
ional Resea ch Founda ion o Ko ea (NRF); Consejo Nacional de Cienca y Tecnologia
(CONACYT), Di eccion Gene al de Asun os del Pe sonal Academico (DGAPA), M´exico;
Ame ique La ine Fo ma ion academique — Eu opean Commission (ALFA-EC) and he
EPLANET P og am (Eu opean Pa icle Physics La in Ame ican Ne wo k) S ich ing oo
Fundamen eel Onde zoek de Ma e ie (FOM) and he Nede landse O ganisa ie oo We en-
schappelijk Onde zoek (NWO), Ne he lands; Resea ch Council o No way (NFR); Na ional
Science Cen e, Poland; Minis y o Na ional Educa ion/Ins i u e o A omic Physics and
Consiliul Na
,ional al Ce ce ˘a ii S¸ iin
,i ice — Execu i e Agency o Highe Educa ion Re-
sea ch De elopmen and Inno a ion Funding (CNCS-UEFISCDI) — Romania; Minis y
o Educa ion and Science o Russian Fede a ion, Russian Academy o Sciences, Russian
Fede al Agency o A omic Ene gy, Russian Fede al Agency o Science and Inno a ions
and The Russian Founda ion o Basic Resea ch; Minis y o Educa ion o Slo akia; De-
pa men o Science and Technology, Sou h A ica; Cen o de In es igaciones Ene ge icas,
Medioambien ales y Tecnologicas (CIEMAT), E-In as uc u e sha ed be ween Eu ope and
La in Ame ica (EELA), Minis e io de Econom´ıa y Compe i i idad (MINECO) o Spain,
Xun a de Galicia (Conselle ´ıa de Educaci´on), Cen o de Aplicaciones Tecnol´ogicas y De-
sa ollo Nuclea (CEADEN), Cubaene g´ıa, Cuba, and IAEA (In e na ional A omic En-
e gy Agency); Swedish Resea ch Council (VR) and Knu & Alice Wallenbe g Founda ion
(KAW); Uk aine Minis y o Educa ion and Science; Uni ed Kingdom Science and Tech-
nology Facili ies Council (STFC); The Uni ed S a es Depa men o Ene gy, he Uni ed
S a es Na ional Science Founda ion, he S a e o Texas, and he S a e o Ohio; Minis y
o Science, Educa ion and Spo s o C oa ia and Uni y h ough Knowledge Fund, C oa ia.
Council o Scien i ic and Indus ial Resea ch (CSIR), New Delhi, India.
Open Access. This a icle is dis ibu ed unde he e ms o he C ea i e Commons
A ibu ion License (CC-BY 4.0), which pe mi s any use, dis ibu ion and ep oduc ion in
any medium, p o ided he o iginal au ho (s) and sou ce a e c edi ed.
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– 25 –
