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Sea ch o collec i i y wi h azimu hal J/ψ -had on co ela ions in high mul iplici y
p–Pb collisions a √sNN = 5.02 and 8.16 TeV
ALICE Collabo a ion
ALICE Collabo a ion. (2018). Sea ch o collec i i y wi h azimu hal J/ψ -had on
co ela ions in high mul iplici y p–Pb collisions a √sNN = 5.02 and 8.16 TeV. Physics
Le e s B, 780, 7-20. h ps://doi.o g/10.1016/j.physle b.2018.02.039
2018
Physics Le e s B 780 (2018) 7–20
Con en s lis s a ailable a ScienceDi ec
Physics Le e s B
www.else ie .com/loca e/physle b
Sea ch o collec i i y wi h azimu hal J/ψ-had on co ela ions in high
mul iplici y p–Pb collisions a √sNN =5.02 and 8.16 TeV
.ALICE Collabo a ion
a i c l e i n o a b s a c
A icle his o y:
Recei ed 27 Sep embe 2017
Recei ed in e ised o m 14 Feb ua y 2018
Accep ed 14 Feb ua y 2018
A ailable online xxxx
Edi o : M. Dose
We p esen a measu emen o azimu hal co ela ions be ween inclusi e J/ψand cha ged had ons in p–Pb
collisions eco ded wi h he ALICE de ec o a he CERN LHC. The J/ψa e econs uc ed a o wa d (p-
going, 2.03 <y <3.53) and backwa d (Pb-going, −4.46 <y <−2.96) apidi y ia hei μ+μ−decay
channel, while he cha ged had ons a e econs uc ed a mid- apidi y (|η| <1.8). The co ela ions a e ex-
p essed in e ms o associa ed cha ged-had on yields pe J/ψ igge . A apidi y gap o a leas 1.5 uni s is
equi ed be ween he igge J/ψand he associa ed cha ged had ons. Possible co ela ions due o collec-
i e e ec s a e assessed by sub ac ing he associa ed pe - igge yields in he low-mul iplici y collisions
om hose in he high-mul iplici y collisions. A e he sub ac ion, we obse e a s ong indica ion o e-
maining symme ic s uc u es a ϕ≈0and ϕ≈π, simila o hose p e iously ound in wo-pa icle
co ela ions a middle and o wa d apidi y. The co esponding second-o de Fou ie coefficien ( 2) in
he ans e se momen um in e al be ween 3 and 6 GeV/cis ound o be posi i e wi h a significance
o abou 5σ. The ob ained esul s a e simila o he J/ψ 2coefficien s measu ed in Pb–Pb collisions a
√sNN =5.02 TeV, sugges ing a common mechanism a he o igin o he J/ψ 2.
©2018 The Au ho . Published by Else ie B.V. This is an open access a icle unde he CC BY license
(h p://c ea i ecommons.o g/licenses/by/4.0/). Funded by SCOAP3.
1. In oduc ion
The measu emen o angula co ela ions be ween pa icles
p oduced in had on and nucleus collisions is a powe ul ool o
s udy he pa icle p oduc ion mechanisms. Usually he wo-pa icle
co ela ion unc ion is exp essed in e ms o di e ences in he
azimu hal angle (ϕ) and pseudo apidi y (η) o he emi ed
pa icles. In minimum-bias p o on–p o on (pp) collisions, he dom-
inan s uc u es in he co ela ion unc ion a e a nea -side peak
a (ϕ, η) ≈(0, 0)and an away-side idge loca ed a ϕ≈π
and elonga ed in η[1]. The nea -side peak o igina es om je
agmen a ion, esonance decays and em oscopic co ela ions. The
away-side idge esul s om agmen a ion o ecoil je s. In colli-
sions o hea y ions, he wo-pa icle co ela ion unc ion exhibi s
addi ional long- ange s uc u es elonga ed in η[2]. These s uc-
u es a e usually in e p e ed as signa u es o collec i e pa icle
flow p oduced du ing he hyd odynamic e olu ion o he fi eball.
They a e analyzed in e ms o he Fou ie coefficien s o he el-
a i e angle dis ibu ions. Assuming ac o iza ion, hese coefficien s
a e hen ela ed o he Fou ie coefficien s ( n) o he pa icle az-
imu hal dis ibu ion ela i e o he common symme y plane o
he colliding nuclei’s o e lap a ea.
E-mail add ess: alice -publica ions @ce n .ch.
The disco e y o a nea -side idge in high-mul iplici y pp [3]
and p–Pb [4] collisions has inc eased he in e es in wo-pa icle
angula co ela ions in small collision sys ems. These disco e ies
we e ollowed by he obse a ion ha he nea -side idge in p–Pb
collisions is accompanied by an away-side one [5,6]. Long- ange
s uc u es ha e also been epo ed in wo-pa icle co ela ions in
d–Au collisions a RHIC [7,8]. Fu he s udies using mul i-pa icle
co ela ions ha e p o en ha he obse ed long- ange co ela-
ions a e o a collec i e o igin [9–11]. Mo eo e , he ans e se-
momen um and pa icle-mass dependencies o he ncoefficien s
in p–Pb collisions ha e been ound o be simila o hose measu ed
in A–A collisions, sugges ing a common hyd odynamic o igin o he
obse ed co ela ions [12,13]. Al e na i e in e p e a ions, including
Colo -Glass Condensa e based models [14] and final-s a e pa on–
pa on sca e ing [15], ha e also been p oposed. Long- ange co e-
la ions o o wa d and backwa d muons wi h mid- apidi y had ons
ha e also been ound in p–Pb collisions a a cen e -o -mass en-
e gy pe nucleon pai √sNN =5.02 TeV [16]. The esul s show ha
hese co ela ions pe sis ac oss wide apidi y anges and ex end
in o he high muon ans e se-momen um in e al, which is dom-
ina ed by decays o hea y fla o s.
In pp collisions, he J/ψ esonance is o med mainly om pai s
o c and ¯
cqua ks p oduced in ha d sca e ing eac ions du ing he
ini ial s age o he collision. The heo e ical models desc ibing he
h ps://doi.o g/10.1016/j.physle b.2018.02.039
0370-2693/©2018 The Au ho . Published by Else ie B.V. This is an open access a icle unde he CC BY license (h p://c ea i ecommons.o g/licenses/by/4.0/). Funded by
SCOAP3.
8ALICE Collabo a ion / Physics Le e s B 780 (2018) 7–20
J/ψp oduc ion combine calcula ions o he p oduc ion o c¯
cpai s
wi hin a pe u ba i e Quan um Ch omodynamics app oach wi h
he subsequen non-pe u ba i e o ma ion o he c¯
cbound s a e
[17]. In p–Pb collisions, he p oduc ion is a ec ed by he modifi-
ca ion o pa on dis ibu ion unc ions inside he nucleus [18]as
well as possible ene gy loss and inelas ic sca e ing inside nuclea
ma e [19,20]. In A–A collisions, he e a e wo addi ional com-
pe ing phenomena ha influence he J/ψp oduc ion. Fi s is he
supp essed p oduc ion due o he dissocia ion o he c¯
cpai s in
he qua k–gluon plasma [21]. Second is he J/ψenhancemen ia
ecombina ion o cha m qua ks he malized in he medium [22,
23]. The ecombina ion is expec ed o become p e alen in cen al
collisions a he LHC ene gies.
Recen ly, he ALICE Collabo a ion has published a p ecise mea-
su emen o he second-o de Fou ie coefficien , 2, o he az-
imu hal dis ibu ion o he J/ψp oduc ion in Pb–Pb collisions a
√sNN =5.02 TeV [24]. The esul s show significan 2in cen al
and semi-cen al collisions. The measu ed J/ψ 2a low and in e -
media e ans e se momen um can be quali a i ely desc ibed by
a anspo model in which he J/ψazimu hal aniso opy is in-
he i ed om ha o ecombined cha m qua ks [25,26]. Howe e ,
a highe ans e se momen um he da a s ill indica es significan
2while he anspo model p edic s significan ly smalle al-
ues coming mos ly om pa h-leng h dependen supp ession in he
almond-shaped in e ac ion egion o he colliding nuclei and om
non-p omp J/ψp oduced om b-had on decays assuming he -
malized b qua ks. Gi en hese esul s in Pb–Pb collisions, i is o
in e es o s udy he J/ψ-had on azimu hal co ela ions also in he
smalle p–Pb sys em. The ecombina ion o cha m qua ks, i any,
should ha e much smalle impac , due o he smalle numbe o
ini ially p oduced cha m qua ks wi h espec o Pb–Pb collisions.
The small sys em size should no lead o a sizeable pa h-leng h
dependen supp ession. Ne e heless, he s udy o he J/ψ-had on
azimu hal co ela ions could allow o de e mine whene e J/ψ
p oduc ion is a ec ed by he medium possibly c ea ed in hese
collisions [27–29].
In his Le e , we p esen esul s o long- ange co ela ions
be ween o wa d (p-going, 2.03 <y <3.53) and backwa d (Pb-
going, −4.46 <y <−2.96) inclusi e J/ψand mid- apidi y cha ged
had ons in p–Pb collisions a √sNN =5.02 and 8.16 TeV. Inclu-
si e J/ψ e e s o bo h p omp J/ψ(di ec and decays om highe
mass cha monium s a es) and non-p omp J/ψ( eed down om
b-had on decays).
2. Expe imen al se up and da a samples
A de ailed desc ip ion o he ALICE appa a us can be ound in
Re . [30]. Below, we b iefly desc ibe he de ec o sys ems essen ial
o he p esen analysis.
In he ollowing, ηand ylab will deno e he pseudo apidi y and
apidi y in he ALICE labo a o y sys em. The muons a e econ-
s uc ed in he muon spec ome e co e ing he ange o −4 <η<
−2.5. The spec ome e con ains a on abso be loca ed be ween
0.9 and 5m om he nominal in e ac ion poin . The abso be is
ollowed by fi e acking s a ions, each made o wo planes o
Ca hode Pad Chambe s. The hi d s a ion is placed inside a dipole
magne wi h 3 Tm field in eg al. The acking s a ions a e ollowed
by an i on wall wi h a hickness o 7.2 in e ac ion leng hs and wo
igge s a ions, each one consis ing o wo planes o Resis i e Pla e
Chambe s.
The posi ion o he in e ac ion poin is ob ained using he clus-
e s econs uc ed in he Silicon Pixel De ec o (SPD) [31,32]. The
SPD is loca ed in he cen al ba el o he ALICE appa a us and op-
e a ed inside a la ge solenoidal magne p o iding a uni o m 0.5 T
magne ic field pa allel o he beam line. The SPD consis s o wo
cylind ical laye s which co e |η| <2.0 and |η| <1.4wi h espec
o he nominal in e ac ion-poin , o he inne and ou e laye ,
espec i ely. The associa ed cha ged had ons a mid- apidi y a e
econs uc ed ia he so-called SPD ackle s, sho ack segmen s
o med om he clus e s in he wo laye s o he SPD and he p i-
ma y e ex [32].
The V0 de ec o [33] consis s o wo ings o 32 scin illa o
coun e s each, co e ing 2.8 <η<5.1(V0-A) and −3.7 <η<−1.7
(V0-C), espec i ely. I is used o igge ing and e en -mul iplici y
es ima ion.
The da a samples p esen ed he e we e collec ed du ing he
2013 and 2016 p–Pb LHC uns. The collision ene gy was √sNN =
5.02 and 8.16 TeV o he 2013 and 2016 da a samples, espec-
i ely. Pa o he 5.02 TeV da a we e collec ed du ing he 2016
p–Pb un. Da a wi h bo h beam configu a ions, namely Pb–nucleus
momen um (deno ed as Pb–p collisions) o p o on momen um (de-
no ed as p–Pb collisions) o ien ed owa ds he muon spec ome e ,
ha e been analyzed. The asymme ic beam ene gies, imposed by
he wo-in-one LHC magne design, esul ed in collisions whose
nucleon–nucleon cen e -o -mass e e ence sys em is shi ed in a-
pidi y by 0.465 in he di ec ion o he p o on beam wi h espec
o he ALICE labo a o y sys em. The da a we e aken wi h a ig-
ge ha equi ed coincidence o minimum-bias (MB) and dimuon
igge s. The MB igge was p o ided by he V0 de ec o eques -
ing a signal in bo h V0-A and V0-C ings. I s efficiency is ound
o be abou 98% [34]. The dimuon igge equi ed a leas a pai
o opposi e-sign ack segmen s in he muon igge sys em, each
wi h a ans e se momen um (pT) abo e he h eshold o he on-
line igge algo i hm. This h eshold was se o p o ide 50% effi-
ciency o muon acks wi h pT=0.5GeV/c.
The collec ed da a samples o p–Pb and Pb–p collisions a
5.02 TeV (8.16 TeV) co espond o in eg a ed luminosi ies o 8.1
and 5.8 (8.7 and 12.9) nb−1, espec i ely. The maximum in e ac-
ion pile-up p obabili y anged up o 3% and 8% du ing 2013 and
2016 da a aking, espec i ely.
3. E en , ack and dimuon selec ion
The beam-induced backg ound is ejec ed by equi ing ha he
iming signals om bo h ings o he V0 de ec o a e compa i-
ble wi h pa icles coming om collision e en s. E en s con aining
mul iple collisions (pile-up) a e ejec ed by equi ing one single
in e ac ion e ex econs uc ed in he SPD and by exploi ing he
co ela ion be ween he numbe o clus e s in he wo laye s o
he SPD and he numbe o he econs uc ed SPD ackle s.
The longi udinal posi ion o he econs uc ed p ima y e ex
(z x) is equi ed o be wi hin ±10 cm om he nominal in e -
ac ion poin . The econs uc ed SPD ackle s a e selec ed by ap-
plying a z x-dependen pseudo apidi y cu . The cu is adjus ed
o exclude he con ibu ion om he edges o he SPD whe e
he de ec o accep ance is low. Fo example, we selec ackle s
wi hin −1.8 <η<0.5, −1.3 <η<1.3 and −0.5 <η<1.8 o
e en s wi h z x =10, 0 and −10 cm, espec i ely. The con ibu-
ion om ake and seconda y ackle s is educed by applying a
|| <5m ad cu on he di e ence be ween he azimu hal an-
gles o he clus e s in he wo laye s o he SPD wi h espec o
he p ima y e ex. Wi h his cu , he mean pTo he selec ed
cha ged had ons is ound o be app oxima ely 0.75 GeV/c[16].
The acks econs uc ed in he muon spec ome e a e equi ed
o eme ge a a adial ans e se posi ion be ween 17.6 and 89.5 cm
om he end o he on abso be in o de o a oid egions wi h
highe ma e ial budge . The acks econs uc ed in he acking
chambe s a e iden ified as muons by equi ing hei ma ching
wi h co esponding ack segmen s in he igge chambe s. Back-
g ound acks a e emo ed wi h a selec ion on he p oduc o
ALICE Collabo a ion / Physics Le e s B 780 (2018) 7–20 9
Fig. 1. The Mμμ dis ibu ion in he 3 <pμμ
T<6GeV/cin e al fi ed wi h a combina ion o a CB2 unc ion o he signal and a VWG unc ion o he backg ound, o
high-mul iplici y (le panel) and low-mul iplici y ( igh panel) p–Pb collisions a
√sNN =8.16 TeV.
he o al ack momen um and he dis ance o closes app oach
o he p ima y e ex in he ans e se plane [35]. The selec ed
dimuons a e defined as pai s o opposi e-sign muon acks ha -
ing −4 <yμμ
lab <−2.5, ans e se momen um pμμ
Tbe ween 0 and
12 GeV/cand in a ian mass Mμμ be ween 1 and 5GeV/c2. Only
e en s wi h a leas one dimuon sa is ying hese selec ion c i e ia
a e conside ed.
The da a samples a e spli in o mul iplici y classes based on he
o al cha ge deposi ed in he wo ings (V0-A and V0-C) o he V0
de ec o (V0M) [34]. The high-mul iplici y (low-mul iplici y) e en
class is defined as 0–20% (40–100%) o he MB igge e en sam-
ple.
4. Analysis
The Mμμ dis ibu ion in each e en -mul iplici y class and pμμ
T
bin is fi wi h he combina ion o an ex ended C ys al Ball (CB2)
unc ion o he J/ψsignal and a Va iable-Wid h Gaussian (VWG)
unc ion o he backg ound [36]. The ail pa ame e s o he CB2
unc ion we e fixed o he alues used in [37,38]. The J/ψpeak
posi ion and wid h we e ob ained om he fi in he 0–100%
e en class and fixed o hese alues in he o he e en -mul iplici y
classes. Examples o he Mμμ fi in he 0–20% and he 40–100%
e en classes in he 3 <pμμ
T<6GeV/cin e al a e shown in
Fig. 1.
The angula co ela ions be ween J/ψand cha ged had ons a e
ob ained om he associa ed-pa icle (SPD ackle s) yields pe
dimuon igge . The yields a e defined as
Yi(z x,Mμμ,pμμ
T,ϕ,η)
=1
Ni
ig(z x,Mμμ,pμμ
T)
d2Ni
assoc(z x,Mμμ,pμμ
T)
dϕdη
=1
Ni
ig(z x,Mμμ,pμμ
T)
SEi(z x,Mμμ,pμμ
T,ϕ,η)
MEi(z x,Mμμ,pμμ
T,ϕ,η),(1)
whe e Ni
ig(z x, Mμμ, pμμ
T)is he numbe o dimuons, Ni
assoc(z x,
Mμμ, pμμ
T)is he numbe o associa ed SPD ackle s co ec ed o
accep ance and combina o ial e ec s (as shown in he second line
o he equa ion and desc ibed below), ϕand η=yμμ
lab −η ackle
a e he azimu hal angle and (pseudo) apidi y di e ence be ween
he igge dimuon and he associa ed SPD ackle . The yields
a e calcula ed sepa a ely in each e en -mul iplici y class (index i)
and 1 cm-wide z x in e al. The dis ibu ion
SEi(z x,Mμμ,pμμ
T,ϕ,η)=d2Ni
same(z x,Mμμ,pμμ
T)
dϕdη
is he yield o associa ed SPD ackle s om he same e en . The
dis ibu ion
MEi(z x,Mμμ,pμμ
T,ϕ,η)
=αi(z x,Mμμ,pμμ
T)d2Ni
mixed(z x,Mμμ,pμμ
T)
dϕdη
is cons uc ed using he e en -mixing echnique, i.e. combining
dimuons om one e en wi h SPD ackle s om o he e en s
selec ed in he same e en -mul iplici y class and z x in e al. I
se es bo h o co ec o de ec o accep ance and efficiency and o
ake in o accoun he combina o ial backg ound. The no maliza ion
ac o αi(z x, Mμμ, pμμ
T)is defined as 1/(d2Ni
mixed(z x, Mμμ,
pμμ
T)/dϕdη)in he η egion co esponding o he maximal
accep ance [16].
Wi hin each e en -mul iplici y class and bin o Mμμ, pμμ
T, ϕ
and η, he yields Yia e aged o e z x a e ob ained by fi ing he
dis ibu ion YiN ig(z x)iMEi(z x) o he dis ibu ion SEi(z x).
A Poisson likelihood fi is used in o de o p ope ly deal wi h he
cases o low numbe o ackle s. Then, he a e age yields a e p o-
jec ed on he ϕaxis in he ange o 1.5 <|η| <5using he
me hod desc ibed in [16].
In o de o ex ac he yields pe J/ψ igge , he yields pe
dimuon igge in each e en -mul iplici y class, pμμ
Tand ϕbins
a e fi as a unc ion o Mμμ using he ollowing supe posi ion
Yi(Mμμ)=S
S+BYi
J/ψ +B
S+BYi
B(Mμμ), (2)
whe e Sand Ba e he numbe o J/ψand he backg ound
dimuons in each bin o Mμμ ob ained om he in a ian mass
fi (using a CB2 unc ion o he J/ψsignal and a VWG unc ion
o he backg ound) desc ibed abo e, YJ/ψ is he associa ed yield
co esponding o he J/ψ igge and YB(Mμμ)is a second-o de
polynomial unc ion aimed o desc ibe he associa ed yields co -
esponding o he backg ound. The fi ange is chosen be ween
1.5 and 4.5 GeV/c2. Examples o fi s in high-mul iplici y and low-
mul iplici y e en classes a e shown in Fig. 2.
Fig. 3shows he ob ained associa ed ackle yields pe J/ψ ig-
ge o p–Pb and Pb–p collisions a √sNN =5.02 and 8.16 TeV.
10 ALICE Collabo a ion / Physics Le e s B 780 (2018) 7–20
Fig. 2. Example o associa ed ackle yields pe dimuon igge in he 3 <pμμ
T<6GeV/cin e al o high-mul iplici y (le panel) and low-mul iplici y ( igh panel) p–Pb
collisions a
√sNN =8.16 TeV. The esul o he fi wi h he unc ion om Eq. (2)is ep esen ed wi h he blue solid line. The dashed ed line co esponds o he associa ed
ackle yields pe backg ound dimuon. (Fo in e p e a ion o he colo s in he figu e(s), he eade is e e ed o he web e sion o his a icle.)
As expec ed, in low-mul iplici y collisions we obse e a signifi-
can co ela ion s uc u e on he away side (Fig. 3, op panels),
p esumably o igina ing om he agmen a ion o ecoil je s. In
high-mul iplici y collisions (Fig. 3, middle panels), a possible en-
hancemen on bo h nea (ϕ≈0) and away (ϕ≈π) side can
be spo ed on op o he away-side s uc u e. In o de o isola e
possible co ela ions due o collec i e e ec s be ween he J/ψand
he associa ed ackle s, we apply he same sub ac ion me hod as
in p e ious measu emen s [5,6,12,16], namely sub ac ing he YJ/ψ
yields in low-mul iplici y collisions om hose in high-mul iplici y
collisions (Fig. 3, bo om panels). The sub ac ion me hod elies
on he assump ions ha he je co ela ions on he away side e-
main unmodified as a unc ion o he e en mul iplici y and ha
he e a e no significan co ela ions due o collec i e e ec s in
low-mul iplici y collisions (see discussion in Sec ion 6).
In o de o quan i y he emaining co ela ion s uc u es, he
sub ac ed yields Ysub
J/ψ (ϕ)a e fi wi h
a0+2a1cosϕ+2a2cos2ϕ.(3)
The second-o de Fou ie coefficien V2{J/ψ − ackle ,sub}o he
azimu hal co ela ion be ween he J/ψand he associa ed cha ged
had ons is finally calcula ed as a2/bhigh
0. The denomina o bhigh
0=
a0+blow
0co esponds o he combina o ial baseline o he high-
mul iplici y collisions, whe e he pa ame e blow
0is he combina-
o ial baseline o he low-mul iplici y collisions ob ained a he
minimum o he pe - igge yields, namely in ϕ<π/6. The pa-
ame e blow
0is he no maliza ion ac o used in Fig. 3. The pa am-
e e a1, which desc ibes he s eng h o he emaining away-side
co ela ion s uc u e, is ound o be compa ible wi h ze o in p ac-
ically all pJ/ψ
Tin e als, in bo h p–Pb and Pb–p collisions a bo h
5.02 and 8.16 TeV.
As an al e na i e ex ac ion me hod, he calcula ion o blow
0,
he sub ac ion o low-mul iplici y om high-mul iplici y collision
yields and he fi o Eq. (3)is done in each bin o Mμμ sepa a ely.
Then he V2{J/ψ − ackle ,sub}coefficien is ex ac ed by fi ing
V2{μμ − ackle ,sub}(Mμμ)wi h a supe posi ion simila o he
one defined in Eq. (2)
V2{μμ − ackle ,sub}(Mμμ)
=S
S+BV2{J/ψ − ackle ,sub}
+B
S+BVB
2{μμ − ackle ,sub}(Mμμ), (4)
whe e he VB
2{μμ − ackle ,sub}(Mμμ)is he second-o de
Fou ie coefficien o he azimu hal co ela ion be ween he back-
g ound dimuons and associa ed ackle s. The backg ound co-
efficien VB
2{μμ − ackle ,sub}(Mμμ)is pa ame e ized wi h a
second-o de polynomial unc ion. This pa ame e iza ion is cho-
sen since i ep oduces he dimuon 2(Mμμ)cons uc ed om
he measu ed muon 2coefficien [16] assuming ha he domi-
nan pa o he backg ound is combina o ial. An example o he
V2{μμ − ackle ,sub}(Mμμ)fi is shown in Fig. 4.
Following he p ocedu e used in Re s. [5,12,16], he V2{J/ψ −
ackle ,sub}coefficien is ac o ized in o a p oduc o J/ψand
cha ged-had on 2coefficien s. Thus, he J/ψsecond-o de Fou ie
azimu hal coefficien J/ψ
2{2,sub}is ob ained as
J/ψ
2{2,sub}=V2{J/ψ − ackle ,sub}/ ackle
2{2,sub},(5)
whe e he ackle
2{2,sub}is he ackle second-o de Fou ie az-
imu hal coefficien ob ained by pe o ming he analysis consid-
e ing SPD ackle s as bo h igge and associa ed pa icles. The
ob ained alues o ackle
2{2,sub}a e be ween 0.067 and 0.069
depending on he beam configu a ion and collision ene gy, wi h
1–2% ela i e s a is ical unce ain y and 5–6.5% ela i e sys ema ic
unce ain y.
5. Sys ema ic unce ain ies
The combined s a is ical and sys ema ic unce ain ies o he
measu ed ackle
2{2,sub}coefficien o each beam configu a ion
and collision ene gy a e aken as global sys ema ic unce ain ies o
he co esponding J/ψ
2{2,sub}coefficien s.
All he o he sys ema ic unce ain ies o he J/ψ
2{2,sub}coeffi-
cien s a e ob ained o each da a sample and pTin e al sepa a ely.
The ollowing sou ces a e conside ed.
A possible inaccu a e co ec ion o he SPD accep ance is as-
sessed by a ying he z x ange be ween ±8 and ±12 cm. Sys-
ema ic unce ain ies a e assigned only in he cases o a significan
change o he esul s. The significance is defined acco ding o he
p ocedu e desc ibed in Re . [39].
The sys ema ic e ec ela ed o he unce ain y o he shape o
he dimuon backg ound yields YB(Mμμ)is es ima ed by pe o m-
ing he fi wi h Eq. (2)using a linea unc ion o he backg ound
e m and a ying he fi ange. The sys ema ic e ec coming om
he unce ain y o he signal- o-backg ound a io S/Bis checked

ALICE Collabo a ion / Physics Le e s B 780 (2018) 7–20 11
Fig. 3. Associa ed ackle yields pe J/ψ igge in 3 <pJ/ψ
T<6GeV/cin p–Pb and Pb–p collisions a √sNN =5.02 TeV (le panels) and 8.16 TeV ( igh panels). The op
and he middle panels co espond o he low-mul iplici y and he high-mul iplici y e en classes, espec i ely. The bo om panels show he yields a e he sub ac ion o
he low-mul iplici y collision yields om he high-mul iplici y collision ones. The solid line ep esen he fi o he da a as desc ibed in he ex . The dashed, do -dashed
and do ed lines co espond o he indi idual e ms o he fi unc ion defined in Eq. (3). All he yields a e no malized o he alue in ϕ<π/6in he low-mul iplici y
(40–100%) e en class. Only he s a is ical unce ain ies a e shown. (Fo in e p e a ion o he colo s in he figu e(s), he eade is e e ed o he web e sion o his a icle.)
by employing a ious in a ian mass fi unc ions, bo h o he
backg ound and o he J/ψsignal. The maximal di e ence o he
esul s ob ained wi h he abo e checks wi h espec o he de aul
app oach is aken as he co esponding sys ema ic unce ain y.
The unce ain y a ising om he employed analysis app oach
is ob ained as he di e ence be ween he wo ex ac ion me hods
desc ibed in Sec ion 4.
As desc ibed in Sec ion 4, by de aul he mixed-e en dis ibu-
ion ME(ϕ, η)is no malized o uni y in he η egion co e-
sponding o he maximal accep ance. As an al e na i e app oach,
no malizing he in eg al o ME(ϕ, η) o uni y is used. No sig-
nifican e ec on he ob ained esul s is obse ed and hus no
sys ema ic unce ain y is assigned.
The used e en -mixing echnique can in oduce sys ema ic bi-
ases. The e en mul iplici y dis ibu ion o he selec ed dimuons
(1 <Mμμ <5GeV/c2) di e s om ha o he J/ψsignal. Since
he cha ged-had on spec a and he cha ged-had on densi y as a
unc ion o ηchange wi h e en mul iplici y [34], he non-uni o m
(bo h in he azimu hal and longi udinal di ec ions) SPD accep ance
can in oduce a bias. The co esponding sys ema ic unce ain y is
e alua ed by doing he e en mixing in fine e en -mul iplici y
bins.
12 ALICE Collabo a ion / Physics Le e s B 780 (2018) 7–20
Table 1
Summa y o absolu e sys ema ic unce ain ies o he J/ψ
2{2,sub}coefficien s. The unce ain ies a y wi hin he indica ed anges depending on pJ/ψ
T. The alues no p eceded
by a sign ep esen double-sided unce ain ies.
Sou ce o sys ema ics √sNN =5.02 TeV √sNN =8.16 TeV
p–Pb Pb–p p–Pb Pb–p
Accep anceco ec ion 0 o0.019 0 o0.057 0 o0.011 0 o0.007
Backg ound shape 0.007 o 0.013 0.015 o 0.056 0.011 o 0.013 0.003 o 0.012
Ex ac ion me hod 0.003 o 0.015 0.010 o 0.040 0.002 o 0.011 0.008 o 0.018
E en mixing 0.003 o 0.015 0.004 o 0.025 0.002 o 0.008 0.004 o 0.012
Residual away-side
je co ela ion
–−0.030 o 0 −0.018 o 0 –
To al +0.009 o +0.024 +0.024 o +0.084 +0.013 o +0.019 +0.015 o +0.021
−0.009 o −0.024 −0.024 o −0.090 −0.015 o −0.026 −0.015 o −0.021
Fig. 4. Example o he fi om Eq. (4)in he 3 <pμμ
T<6GeV/cin e al o
p–Pb collisions a
√sNN =8.16 TeV. The dashed line co esponds o he VB
2{μμ −
ackle ,sub}(Mμμ).
The non-uni o m accep ance o he muon spec ome e coupled
o sizeable co ela ions be ween he dimuons and SPD ackle s
can bias azimu hally he sample o SPD ackle s used o e en
mixing. In o de o check o possible e ec s on ou measu emen ,
he e en mixing is pe o med in in e als o azimu hal angle o
he selec ed dimuons. We obse e no significan sys ema ic e ec
as he ob ained esul s show negligible de ia ions wi h espec o
he esul s using he de aul e en -mixing echnique.
The e ec o a possible esidual nea -side peak is checked by
a ying he apidi y gap be ween he igge dimuons and asso-
cia ed cha ged-had ons om 1.0 o 2.0 uni s. We obse e no in-
dica ion o inc easing 2wi h educed gap and hus conside he
de aul gap o 1.5 uni s sufficien o elimina e any significan esid-
ual nea -side peak con ibu ion.
As shown in Sec ion 4, he ecoil-je away-side co ela ion
s uc u e in he high-mul iplici y e en class is g ea ly diminished
a e he sub ac ion o he low-mul iplici y e en class. By de-
aul , any emaining away-side s uc u e is supposed o be aken
in o accoun by he cosϕ e m in Eq. (3). In o de o check o
esidual e ec s we p oceed in he ollowing way. Fi s , he co -
ela ion unc ion in he low-mul iplici y e en class is fi wi h a
Gaussian unc ion cen e ed a ϕ=π. Then, he co ela ion unc-
ion in he high-mul iplici y e en class is fi wi h he unc ion
om Eq. (3), whe e he cosϕ e m is eplaced by a Gaussian
unc ion wi h a wid h fixed o he alue ob ained om he fi
in he low-mul iplici y collisions. No clea signa u e o sys ema ic
change o he esul s is seen, excep some hin s o a possible e ec
in he highes pJ/ψ
Tin e al. Conse a i ely, we assign sys ema ic
unce ain y as he di e ence wi h espec o he de aul analy-
sis app oach. Since he ypical alues o he Gaussian wid h a e
a ound 1 ad, one-sided (nega i e) sys ema ic unce ain y is as-
signed.
In Table 1we p esen a summa y o he assigned sys ema ic
unce ain ies o he J/ψ
2{2,sub}coefficien s. No sizeable co ela-
ions be ween he pJ/ψ
Tin e als a e obse ed and he e o e in he
ollowing he unce ain ies a e conside ed unco ela ed.
Ou measu emen is o inclusi e J/ψ. The ac ion o J/ψ om
decays o b-had ons eaches up o abou 15% a pJ/ψ
T≈6GeV/cin
p–Pb collisions a √sNN =5.02 [40] and 8.16 TeV [41]. The e o e
he eed-down con ibu ion is unlikely o influence significan ly
ou esul s. In p inciple, a possible s ong mul iplici y dependence
o he eed-down ac ion can po en ially a ec he sub ac ion
app oach. Howe e , no e idence o such a s ong dependence is
obse ed in pp collisions [42].
As addi ional c oss-checks he analysis is done using al e na-
i e e en -mul iplici y es ima o s, a ying he ackle ||cu ,
applying a cu on he asymme y o ans e se momen um o he
wo muon acks, emo ing he pile-up cu s and excluding he SPD
egions wi h non-uni o m accep ance in pseudo apidi y. The co e-
sponding esul s a e ound o be compa ible wi h hose ob ained
wi h he de aul analysis app oach and he e o e no u he sys-
ema ic unce ain ies a e assigned.
6. Resul s
In Fig. 5we epo he measu ed J/ψ
2{2,sub}coefficien s as
a unc ion o pJ/ψ
T o p–Pb and Pb–p collisions a √sNN =5.02
and 8.16 TeV. Up o pJ/ψ
To 3 GeV/c, no significan de ia ion om
ze o is obse ed o ei he p–Pb o Pb–p collisions a he wo colli-
sion ene gies. On he con a y, in he pJ/ψ
Tin e al be ween 3 and
6 GeV/c, he J/ψ
2{2,sub}is ound o be posi i e al hough wi h
la ge unce ain ies. As also shown in Fig. 5, he J/ψ
2coefficien s in
2.5 <y <4in cen al Pb–Pb collisions a √sNN =5.02 TeV each
maximal alues in he same pJ/ψ
Tin e al [24].
Two me hods a e employed in o de o ob ain he p obabili y
ha he J/ψ
2{2,sub}is ze o in he 3 <pJ/ψ
T<6 GeV/cin e al. In
he fi s me hod, he J/ψ
2{2,sub} alues in he wo pJ/ψ
Tin e als
(3 <pJ/ψ
T<4GeV/cand 4 <pJ/ψ
T<6GeV/c) a e combined in o
a weigh ed a e age o each apidi y and collision ene gy. The ob-
ained p obabili ies a e 0.13% and 0.13% (7.8% and 0.23%) o p–Pb
and Pb–p collisions, espec i ely, a √sNN =8.16 TeV (5.02 TeV).
Combining all eigh J/ψ
2{2,sub} alues yields a o al p obabil-
i y o 1.7 ×10−7. This co esponds o a 5.1σsignificance o he
measu ed posi i e J/ψ
2{2,sub}coefficien . The second me hod is
Fishe ’s combined p obabili y es [43]. Wi h his me hod one ob-
ains p obabili ies o 0.14% and 0.23% (10.3% and 0.41%) o p–Pb
and Pb–p collisions a √sNN =8.16 TeV (5.02 TeV), espec i ely.
ALICE Collabo a ion / Physics Le e s B 780 (2018) 7–20 13
Fig. 5. J/ψ
2{2,sub}in bins o pJ/ψ
T o p–Pb, 2.03 <y <3.53 (le panels), and Pb–p, −4.46 <y <−2.96 ( igh panels), collisions a √sNN =5.02 TeV ( op panels) and
8.16 TeV (bo om panels). The esul s a e compa ed o he J/ψ
2{EP}coefficien s measu ed in cen al Pb–Pb collisions a
√sNN =5.02 TeV in o wa d apidi y (2.5 <y <4)
using e en plane (EP) based me hods [24]. The s a is ical and unco ela ed sys ema ic unce ain ies a e ep esen ed by lines and boxes, espec i ely. The quo ed global
sys ema ic unce ain ies co espond o he combined s a is ical and sys ema ic unce ain ies o he measu ed ackle
2{2,sub}coefficien .
The o al p obabili y is 1.4 ×10−6which co esponds o a 4.7σ
significance. In he calcula ion o he abo e p obabili ies, bo h s a-
is ical and sys ema ic unce ain ies o he measu ed alues a e
aken in o accoun . The global sys ema ic unce ain y is no aken
in o accoun as i is i ele an in he case o he ze o hypo hesis.
The analysis me hod p esen ed in his Le e elies on he as-
sump ion ha he e a e no significan co ela ions due o collec i e
e ec s in he low-mul iplici y e en class. In case o a p esence o
such co ela ions, he measu ed V2{J/ψ − ackle ,sub}is equal o
V2{J/ψ − ackle ,high}−blow
0
bhigh
0
V2{J/ψ − ackle ,low},(6)
whe e V2{J/ψ − ackle ,high}and V2{J/ψ − ackle ,low}a e he
second-o de Fou ie coefficien s o he azimu hal co ela ion be-
ween he J/ψand he associa ed cha ged had ons in he high-
mul iplici y and he low-mul iplici y collisions, espec i ely, and
blow
0/bhigh
0≈1/3 is he a io o he combina o ial baseline in he
low-mul iplici y and high-mul iplici y collisions (see Fig. 3). As is
demons a ed in Re . [44], he assump ion o no significan collec-
i e co ela ions in he low-mul iplici y collisions is ce ainly ques-
ionable o ligh -fla o had ons. Ou da a indica es he same, as
we obse e a s a is ically significan inc ease o he measu ed al-
ues o ackle
2{2,sub}when sub ac ing a lowe e en -mul iplici y,
e.g. 60–100%, class. Ul ima ely, he alue o he ackle
2coefficien
is ound o be abou 17% highe in case no sub ac ion is applied.
The e o e, eplacing he sub ac ed ackle
2{2,sub}coefficien in
Eq. (5)by he non-sub ac ed coefficien would mean ha he J/ψ
2
coefficien s a e up o 17% lowe wi h espec o he measu ed
J/ψ
2{2,sub}coefficien s. Howe e , assuming ha he J/ψ
2coeffi-
cien s ollow he same end as a unc ion o e en mul iplici y as
he ackle
2coefficien , hey would be up o 17% highe wi h e-
spec o he measu ed J/ψ
2{2,sub}coefficien s. Sub ac ing lowe
e en -mul iplici y classes in he measu emen o he J/ψ
2{2,sub}
coefficien does no imp o e he p ecision o ou measu emen , be-
cause o he limi ed amoun o J/ψsignal in he low-mul iplici y
collisions.
The nuclea modifica ion ac o o J/ψin p–Pb and Pb–p colli-
sions [37,38]as well as he cha ged-pa icle 2coefficien [45–47]
in pp collisions show no significan √sNN dependence. As seen in
Fig. 5, he measu ed J/ψ
2{2,sub}coefficien s a √sNN =5.02 and
8.16 TeV also appea o be consis en wi h each o he . The la ges
absolu e di e ence be ween he esul s a he wo collision en-
e gies is obse ed in Pb–p collisions in he 3 <pJ/ψ
T<6GeV/c
in e al. The significance o his di e ence is a he low (below
1.5σ), because o he la ge unce ain ies o he measu emen a
√sNN =5.02 TeV. Hence, he da a o he wo collision ene gies
a e combined as a weigh ed a e age aking in o accoun bo h s a-
is ical and sys ema ic unce ain ies. In Fig. 6, we p esen hese
combined esul s o p–Pb and Pb–p collisions oge he wi h mea-
su emen s and model calcula ions o Pb–Pb collisions a √sNN =
5.02 TeV [25].
In Pb–Pb collisions, he posi i e J/ψ
2coefficien s a pJ/ψ
Tbe-
low 3–4 GeV/ca e belie ed o o igina e om he ecombina ion o
cha m qua ks he malized in he medium and a e desc ibed ai ly
well by he anspo model [25](see Fig. 6). In p–Pb collisions,
he amoun o p oduced cha m qua ks is small and he e o e he
con ibu ion om ecombina ion should be negligible. Ou mea-
su ed alues a pJ/ψ
T<3GeV/ca e compa ible wi h ze o, in line
wi h his expec a ion. The e is one publica ion [28] which sugges s
ha e en in p–Pb collisions a sizeable con ibu ion om ecom-
bina ion could occu due o canonical enhancemen e ec s. The
unce ain ies o ou esul s do no allow o confi m o o ule ou
his scena io.
In Pb–Pb collisions, he measu ed J/ψ
2coefficien s exceed sub-
s an ially he heo e ical p edic ions a pJ/ψ
T>4GeV/c, whe e he
14 ALICE Collabo a ion / Physics Le e s B 780 (2018) 7–20
Fig. 6. Combined J/ψ
2{2,sub}coefficien s in p–Pb and Pb–p collisions compa ed
o he esul s in cen al and semi-cen al Pb–Pb collisions a √sNN =5.02 TeV
[24]and he anspo model calcula ions o semi-cen al Pb–Pb collisions a
√sNN =5.02 TeV [25]. The solid line co esponds o he con ibu ion om pa h-
leng h dependen supp ession inside he medium. The band shows he esul ing
J/ψ
2including also he ecombina ion o he malized cha m qua ks and he eed-
down om b-had on decays assuming he maliza ion o b qua ks.
main con ibu ion o J/ψ
2is expec ed o come om pa h-leng h
dependen supp ession inside he medium [25](see Fig. 6). In
p–Pb collisions, he medium, i any, has a much smalle size [48]
and hence e y li le, i any, pa h-leng h dependen e ec s a e
expec ed. In p inciple, he eed-down om decays o b-had ons
can gi e a posi i e J/ψ
2a high ans e se momen um in case o
a posi i e b qua k 2. Howe e , he la e would ha e o each
un easonably high alues gi en he magni ude o he measu ed
J/ψ
2{2,sub}and he small eed-down ac ion. Despi e hese con-
side a ions, he measu ed posi i e J/ψ
2coefficien s would imply
ha he J/ψpa icipa es in he collec i e beha io o he p–Pb col-
lision sys em.
7. Summa y
We p esen ed a measu emen o he angula co ela ions
be ween o wa d and backwa d J/ψand mid- apidi y cha ged
had ons in p–Pb and Pb–p collisions a √sNN =5.02 and 8.16 TeV.
The da a indica e pe sis ing long- ange co ela ion s uc u es a
ϕ≈0 and ϕ≈π, eminiscen o he double idge p e iously
ound in cha ged-pa icle co ela ions a mid- and o wa d a-
pidi y. The co esponding J/ψ
2{2,sub}coefficien s in 3 <pJ/ψ
T<
6GeV/ca e ound o be posi i e wi h a o al significance o
4.7σ o 5.1σ. The ob ained alues, albei wi h la ge unce ain-
ies, a e compa able wi h hose measu ed in Pb–Pb collisions a
√sNN =5.02 TeV in o wa d apidi y. Al hough he unde lying
mechanism is no unde s ood, he compa able magni ude o he
J/ψ
2coefficien s a high ans e se momen um in p–Pb and Pb–Pb
collisions indica es ha his mechanism could be simila in bo h
collision sys ems.
Acknowledgemen 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 acknowledges he ollow-
ing unding agencies o hei suppo in building and unning
he ALICE de ec o : A.I. Alikhanyan Na ional Science Labo a o y
(Ye e an Physics Ins i u e) Founda ion (ANSL), S a e Commi ee
o Science and Wo ld Fede a ion o Scien is s (WFS), A menia;
Aus ian Academy o Sciences and Na ionals i ung ü Fo schung,
Technologie und En wicklung, Aus ia; Minis y o Communica-
ions and High Technologies, Na ional Nuclea Resea ch Cen e ,
Aze baijan; Conselho Nacional de Desen ol imen o Cien ífico e
Tecnológico (CNPq), Uni e sidade Fede al do Rio G ande do Sul
(UFRGS), Financiado a de Es udos e P oje os (Finep) and Fun-
dação de Ampa o à Pesquisa do Es ado de São Paulo (FAPESP),
B azil; Minis y o Science & Technology o China (MSTC), Na-
ional Na u al Science Founda ion o China (NSFC) and Minis y
o Educa ion o China (MOEC), China; Minis y o Science, Edu-
ca ion and Spo s and C oa ian Science Founda ion, C oa ia; Min-
is y o Educa ion, You h and Spo s o he Czech Republic, Czech
Republic; The Danish Council o Independen Resea ch – Na u-
al Sciences, he Ca lsbe g Founda ion and Danish Na ional Re-
sea ch Founda ion (DNRF), Denma k; Helsinki Ins i u e o Physics
(HIP), Finland; Commissa ia à l’Ene gie A omique (CEA) and Ins i-
u Na ional de Physique Nucléai e e de Physique des Pa icules
(IN2P3) and Cen e Na ional de la Reche che Scien ifique (CNRS),
F ance; Bundesminis e ium ü Bildung, Wissenscha , Fo schung
und Technologie (BMBF) and GSI Helmhol zzen um ü Schwe-
ionen o schung GmbH, Ge many; Gene al Sec e a ia o Resea ch
and Technology, Minis y o Educa ion, Resea ch and Religions,
G eece; Na ional Resea ch, De elopmen and Inno a ion Office,
Hunga y; Depa men o A omic Ene gy, Go e nmen o India
(DAE), Depa men o Science and Technology, Go e nmen o India
(DST), Uni e si y G an s Commission, Go e nmen o India (UGC)
and Council o Scien ific and Indus ial Resea ch (CSIR), India; In-
donesian Ins i u e o Science, Indonesia; Cen o Fe mi – Museo
S o ico della Fisica e Cen o S udi e Rice che En ico Fe mi and Is i-
u o Nazionale di Fisica Nuclea e (INFN), I aly; Ins i u e o Inno a-
i e Science and Technology, Nagasaki Ins i u e o Applied Science
(IIST), Japan Socie y o he P omo ion o Science (JSPS) KAKENHI
and Japanese Minis y o Educa ion, Cul u e, Spo s, Science and
Technology (MEXT), Japan; Consejo Nacional de Ciencia (CONACYT)
y Tecnología, h ough Fondo de Coope ación In e nacional en Cien-
cia y Tecnología (FONCICYT) and Di ección Gene al de Asun os
del Pe sonal Academico (DGAPA), Mexico; Nede landse O ganisa ie
oo We enschappelijk Onde zoek (NWO), Ne he lands; The Re-
sea ch Council o No way, No way; Commission on Science and
Technology o Sus ainable De elopmen in he Sou h (COMSATS),
Pakis an; Pon ificia Uni e sidad Ca ólica del Pe ú, Pe u; Minis y o
Science and Highe Educa ion and Na ional Science Cen e, Poland;
Ko ea Ins i u e o Science and Technology In o ma ion and Na ional
Resea ch Founda ion o Ko ea (NRF), Republic o Ko ea; Minis y o
Educa ion and Scien ific Resea ch, Ins i u e o A omic Physics and
Romanian Na ional Agency o Science, Technology and Inno a-
ion, Romania; Join Ins i u e o Nuclea Resea ch (JINR), Minis y
o Educa ion and Science o he Russian Fede a ion and Na ional
Resea ch Cen e Ku cha o Ins i u e, Russia; Minis y o Educa-
ion, Science, Resea ch and Spo o he Slo ak Republic, Slo akia;
Na ional Resea ch Founda ion o Sou h A ica, Sou h A ica; Cen-
o de Aplicaciones Tecnológicas y Desa ollo Nuclea (CEADEN),
Cubaene gía, Cuba, Minis e io de Ciencia e Inno acion and Cen o
de In es igaciones Ene gé icas, Medioambien ales y Tecnológicas
(CIEMAT), Spain; Swedish Resea ch Council (VR) and Knu & Alice
Wallenbe g Founda ion (KAW), Sweden; Eu opean O ganiza ion o
Nuclea Resea ch, Swi ze land; Na ional Science and Technology
De elopmen Agency (NSDTA), Su ana ee Uni e si y o Technol-