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Cohe en pho op oduc ion o ρ0 ec o mesons in ul a-pe iphe al Pb-Pb collisions a
√sNN = 5.02 TeV
© 2020 CERN
Published e sion
ALICE collabo a ion
ALICE collabo a ion. (2020). Cohe en pho op oduc ion o ρ0 ec o mesons in ul a-pe iphe al
Pb-Pb collisions a √sNN = 5.02 TeV. Jou nal o High Ene gy Physics, 2020(6), A icle 35.
h ps://doi.o g/10.1007/JHEP06(2020)035
2020
JHEP06(2020)035
Published o SISSA by Sp inge
Recei ed:Ma ch 1, 2020
Re ised:Ap il 2, 2020
Accep ed:May 11, 2020
Published:June 4, 2020
Cohe en pho op oduc ion o ρ0 ec o mesons in
ul a-pe iphe al Pb-Pb collisions a √sNN = 5.02 TeV
The ALICE collabo a ion
E-mail: [email p o ec ed]
Abs ac : C oss sec ions o he cohe en pho op oduc ion o ρ0 ec o mesons in ul a-
pe iphe al Pb-Pb collisions a √sNN = 5.02 TeV a e epo ed. The measu emen s, which
ely on he π+π−decay channel, a e p esen ed in h ee egions o apidi y co e ing he
ange |y|<0.8. Fo each apidi y in e al, c oss sec ions a e shown o di e en nuclea -
b eakup classes de ined acco ding o he p esence o neu ons measu ed in he ze o-deg ee
calo ime e s. The esul s a e compa ed wi h p edic ions based on di e en models o nu-
clea shadowing. Finally, he obse a ion o a cohe en ly p oduced esonance-like s uc u e
wi h a mass a ound 1.7 GeV/c2and a wid h o abou 140 MeV/c2is epo ed and compa ed
wi h simila obse a ions om o he expe imen s.
Keywo ds: Hea y Ion Expe imen s, Hea y-ion collision, Pa icle and esonance p oduc-
ion
A Xi eP in : 2002.10897
Open Access, Copy igh CERN,
o he bene i o he ALICE Collabo a ion.
A icle unded by SCOAP3.
h ps://doi.o g/10.1007/JHEP06(2020)035
JHEP06(2020)035
Con en s
1 In oduc ion 1
2 Expe imen al se -up 3
3 Analysis p ocedu e 4
3.1 E en selec ion 4
3.2 Backg ound sub ac ion and co ec ions o expe imen al e ec s 5
3.3 Signal ex ac ion 7
3.4 Signal ex ac ion a la ge in a ian masses 9
3.5 Sys ema ic unce ain ies 9
4 Resul s 12
4.1 Cohe en pho op oduc ion o ρ0 ec o mesons 12
4.2 Con ibu ions om con inuum p oduc ion 15
4.3 Obse a ion o a esonance-like s uc u e 15
5 Summa y and ou look 16
The ALICE collabo a ion 21
1 In oduc ion
The elec omagne ic ield o a as cha ged pa icle, such as hose ci cula ing in he La ge
Had on Collide (LHC), is s ongly Lo en z-con ac ed and i s s eng h is domina ed by
he componen pe pendicula o he di ec ion o mo ion, such ha i can be desc ibed as
a lux o quasi- eal pho ons. The in ensi y o his pho on lux is p opo ional o he squa e
o he elec ic cha ge o he pa icle; hus when lead ions ci cula e in he LHC he e a e, in
addi ion o he s anda d had onic collisions, also copious pho onuclea in e ac ions. Ul a-
pe iphe al collisions (UPC) a e de ined as hose o which he impac pa ame e is la ge
han he sum o he adii o he incoming pa icles, in which case he occu ence o had onic
p ocesses is s ongly supp essed due o he sho ange na u e o quan um ch omodynamics
(QCD), and pho on-induced p ocesses domina e he in e ac ion a e. The physics o UPC
and ecen esul s ob ained a he LHC a e e iewed in [1,2].
The pho onuclea p oduc ion o a ρ0 ec o meson in Pb-Pb UPC a he LHC is
pa icula ly in e es ing, because i s la ge c oss sec ion makes i a good ool o s udy he
app oach o he black-disk limi o QCD [3]. This p ocess can be pic u ed as ollows: a
quasi- eal pho on, emi ed by one o he Pb ions, luc ua es in o a QCD objec which
– 1 –
JHEP06(2020)035
hen in e ac s elas ically ei he wi h he o he lead nucleus (cohe en in e ac ion) o wi h
one o i s nucleons (incohe en in e ac ion) and p oduces a ρ0 ec o meson. The QCD
objec can be aken as a ec o meson [4], as a qua k-an iqua k colou dipole [5–7], o
one could conside in e media e di ac i e had onic s a es as done in he G ibo -Glaube
app oach [8]. In hese p ocesses, he mean ans e se momen um o he p oduced ec o
meson is ela ed o he size o he a ge in he impac pa ame e plane by a Fou ie
ans o ma ion; hence, i is es ic ed o be in he o de o 60 (500) MeV/c o cohe en
(incohe en ) in e ac ions. In he cohe en case he a ge nucleus emains in ac , bu
in UPC o hea y nuclei he pho on luxes a e so in ense ha u he pho on exchanges
be ween he same nuclei may occu independen ly o he p oduc ion o he ec o meson
and p oduce neu ons a beam apidi ies due o elec omagne ic exci a ion o one o bo h o
he incoming nuclei [9]. The expe imen al signa u e o cohe en ρ0pho onuclea p oduc ion
is hen he p esence o a single ρ0 ec o meson wi h ai ly low ans e se momen um in
he de ec o , accompanied some imes by one o ew neu ons a beam apidi ies.
The cohe en pho onuclea p oduc ion o a ρ0 ec o meson a mid apidi y was ex en-
si ely s udied in Au-Au UPC a he Rela i is ic Hea y Ion Collide (RHIC) a h ee di e -
en cen e-o -mass ene gies pe nucleon pai √sNN = 62.4 GeV [10], √sNN = 130 GeV [11],
and √sNN = 200 GeV [12,13]. I was also s udied by ALICE a he LHC in Pb-Pb UPC
a √sNN = 2.76 TeV [14].
A model based on a Glaube desc ip ion [3] p edic s c oss sec ions wice la ge han
hose measu ed a ene gies o 200 GeV [12] and 2.76 TeV [14] e en hough i is compa ible
wi h lowe -ene gy da a [10,11]. The STARligh model [15,16], which is also based on
a Glaube -like eikonal o malism, bu does no ake in o accoun he elas ic pa o he
elemen a y ρ0-nucleon c oss sec ion, success ully desc ibes all he da a men ioned abo e.
The inclusion o pho on inelas ic di ac ion in o la ge-mass in e media e had onic s a es
wi hin he G ibo -Glaube amewo k o nuclea shadowing p o ides a be e compa ison
wi h da a han he model based only on a Glaube desc ip ion [8]. None heless, he pho-
op oduc ion o ρ0o nuclei is no ye sa is ac o ily desc ibed in all o i s aspec s and new
measu emen s, pa icula ly a highe ene gies, a e needed o gain a be e unde s anding.
This a icle epo s he i s measu emen o cohe en pho onuclea p oduc ion o ρ0
ec o mesons in Pb-Pb UPC a √sNN = 5.02 TeV. The measu emen was pe o med by
he ALICE Collabo a ion wi h da a eco ded in he 2015 Pb-Pb un. The c oss sec ion o
his p ocess is measu ed as a unc ion o he apidi y o he ec o meson (y) in he ange
|y|<0.8. A each apidi y, he c oss sec ions a e epo ed o he ollowing nuclea -b eakup
classes de ined by he appea ance o neu ons a beam apidi ies: 0n0n (no neu ons), 0nXn
(neu ons a e measu ed only on one beam side, ei he a posi i e o nega i e apidi y), and
XnXn (neu ons a e de ec ed in bo h beam di ec ions). In he ollowing, hey a e deno ed
in gene al as o wa d-neu on classes. Fu he mo e, he obse a ion o a esonance-like
s uc u e in he π+π−in a ian mass spec um a a mass a ound 1.7 GeV/c2is epo ed
and compa ed wi h simila obse a ions om o he expe imen s.
– 2 –
JHEP06(2020)035
2 Expe imen al se -up
The analysed da a we e eco ded by ALICE owa ds he end o 2015 when he LHC p o-
ided Pb-Pb collisions a √sNN = 5.02 TeV. A ull desc ip ion o ALICE sys ems is gi en
in [17] and he pe o mance o he de ec o is discussed in [18]. He e, only he componen s
ele an o he analysis a e b ie ly desc ibed. The ρ0meson is econs uc ed h ough i s
decay in o a π+π−pai using he Inne T acking Sys em (ITS) and he Time P ojec ion
Chambe (TPC) o measu e he pion acks. Ve oes on he p esence o o he pa icles
o ensu e ha only he ρ0meson is p oduced a e imposed wi h he V0 and he ALICE
Di ac i e (AD) de ec o s. The neu ons a beam apidi ies a e measu ed wi h he Ze o
Deg ee Calo ime e s (ZDC).
The ITS [19] is he inne mos de ec o sys em o ALICE. I consis s o six cylind ical
laye s o silicon de ec o s, posi ioned coaxially wi h he di ec ion o he incoming beams,
which de ines he z-axis. This de ec o co e s he ull azimu hal angle and he pseudo-
apidi y ange |η|<0.9. All six laye s con ibu e o ack econs uc ion. The Silicon
Pixel De ec o (SPD) makes up he i s wo laye s o he ITS, closes o he beam, and
is pa icula ly impo an o his analysis because i pa icipa es in he igge de ini ion.
The SPD has 9.8×106pixels o e e se-biased silicon diodes, which a e ead ou by 400
(800) chips in he inne (ou e ) laye . Each o he eadou chips i es a igge i a leas
one o i s pixels has a signal. When p ojec ed in o he ans e se plane, he chips de ine
20 (40) azimu hal egions in he inne (ou e ) laye .
The TPC [20] is he main acking de ec o . I is a la ge cylind ical gas de ec o wi h a
cen al memb ane a high ol age and eadou planes, composed o mul i-wi e p opo ional
chambe s, a each o he wo end caps. I co e s he ull azimu hal ange and |η|<0.9 o
acks which ully a e se i . I p o ides up o 159 space poin s o ack econs uc ion
and o pa icle iden i ica ion by measu ing he ionisa ion ene gy loss. Bo h he ITS and
he TPC a e inside a la ge solenoid magne , which c ea es a uni o m 0.5 T magne ic ield
pa allel o he z-axis.
The V0 [21] is a se o wo segmen ed scin illa o coun e s, V0A and V0C. The V0A
co e s he ange 2.8< η < 5.1, while he V0C co e s −3.7< η < −1.7. The AD [22] is also
a se o wo a ays o scin illa o de ec o s, ADA and ADC, placed u he away om he
nominal in e ac ion poin and co e ing 4.7< η < 6.3 and −6.9< η < −4.9, espec i ely.
Bo h V0 and AD de ec o s pa icipa e in he i s le el igge , and bo h de ec o s ha e
iming esolu ion less han 1 ns.
The e a e wo ZDC de ec o s, ZNA and ZNC, dedica ed o he measu emen o neu-
ons a beam apidi y [23]. They a e loca ed a ei he side o he nominal in e ac ion
poin a ±112.5 m along he z-axis. These calo ime e s de e mine he a i al ime o he
pa icles allowing beam-beam and beam-gas in e ac ions o be sepa a ed. Fu he mo e,
hey ha e a good e iciency o de ec neu ons wi h |η|>8.8 and ha e a ela i e ene gy
esolu ion o a ound 20% o single neu ons, which allows o a clea sepa a ion o e en s
wi h ei he ze o o a ew neu ons a beam apidi ies. This is illus a ed in igu e 1, whe e
he concen a ion o e en s co esponds o he cases o ze o, one, wo o mo e, neu ons
de ec ed.
– 3 –

JHEP06(2020)035
1
10
2
10
3
10
2−0 2 4 6 8
ZNA ene gy (TeV)
2−
0
2
4
6
8
ZNC ene gy (TeV)
= 5.02 TeV
NN
sALICE Pb-Pb UPC
2−0 2 4 6 8 10 12
ZN ene gy (TeV)
1
10
2
10
3
10
4
10
E en s
ZNA ene gy
ZNC ene gy
= 5.02 TeV
NN
sALICE Pb-Pb UPC
Figu e 1. (Colou online). Co ela ion be ween he ene gy dis ibu ions o he ZNA and ZNC
de ec o s o e en s selec ed o he analysis (le ). Ene gy dis ibu ion in each single de ec o
( igh ).
The igge used o ob ain he da a sample o he measu emen s desc ibed below is
composed o i e signals. Fou o hem e o any ac i i y wi hin he ime windows o
nominal beam-beam in e ac ions in ADA, ADC, V0A and V0C. In addi ion, he SPD
p o ides a opological igge o med by ou SPD igge ed chips. These chips o m wo
pai s, each pai wi h wo chips alling in compa ible azimu hal egions, bu in di e en
SPD laye s. The igge selec s e en s wi h a leas wo pai s o chips ha ing an opening
angle in azimu h la ge han 153 deg ees. The eason o eques his opology is ha he
cohe en ly p oduced ρ0has e y small ans e se momen um, and hus he wo pions om
i s decay a e p oduced almos back- o-back in azimu h.
The in eg a ed luminosi y is de e mined using a e e ence igge based on he mul i-
plici y o he V0A and V0C de ec o s. The co esponding c oss sec ion is ob ained using
a Glaube model o had onic Pb-Pb collisions [24]. The in eg a ed luminosi y o he
measu emen s p esen ed below is 485 mb−1wi h a ela i e sys ema ic unce ain y o 5%.
3 Analysis p ocedu e
3.1 E en selec ion
E en s ha ul il he igge c i e ia desc ibed abo e a e selec ed o u he analysis i hey
con ain exac ly wo acks o opposi e elec ic cha ge. To ensu e a p ope measu emen ,
each ack is equi ed o ha e a leas 50 space poin s in he TPC and one associa ed hi
in each laye o he SPD. These SPD hi s ha e o be ma ched o a igge ed eadou chip.
Fu he mo e, each ack has o ha e a dis ance o closes app oach o he e en in e ac ion
e ex o less han 2 cm in he z-axis di ec ion and less han 0.0182 + 0.0350/(p k
T)1.01 cm
in he plane ans e se o he beam di ec ion. He e p k
Tdeno es he ans e se momen um
o he ack in GeV/c.
– 4 –
JHEP06(2020)035
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5
)
2
c (GeV/m
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
3
10×
2
cCoun s pe 5 MeV/
c < 0.2 GeV/
T
p
| < 0.8y|
Opposi e-sign pai s
Like-sign pai s
= 5.02 TeV
NN
sALICE Pb-Pb UPC
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
)c (GeV/
T
p
1
10
2
10
3
10
4
10
cCoun s pe 10 MeV/
2
c < 1.4 GeV/m <
2
c0.55 GeV/
| < 0.8y|
Opposi e-sign pai s
Like-sign pai s
= 5.02 TeV
NN
sALICE Pb-Pb UPC
Figu e 2. (Colou online). In a ian mass (le ) and ans e se momen um ( igh ) dis ibu ions
o opposi e-sign (blue) and like-sign ( ed) pai s.
The ene gy loss o each econs uc ed ack is measu ed in uni s o he s anda d de i-
a ion (σπ) wi h espec o Be he expec a ions o a pion passing he TPC. The ack pai
is accep ed i n2
σπ++n2
σπ−<52.
This c i e ion ejec s, in he conside ed mass ange, he con ibu ion om elec ons, while
he e emains a small backg ound om muon pai s which is discussed below.
The ou momen um o he ack pai is compu ed unde he assump ion o each ack
being a pion. A pai is accep ed i i s apidi y (y), ans e se momen um (pT) and mass (m)
a e wi hin |y|<0.8, pT<0.2 GeV/cand 0.55 <m<1.4 GeV/c2.
To e o ac i i y in he pseudo apidi y ange co e ed by he AD and V0 de ec o s, hei
o line signals a e s udied. The o line econs uc ion in hese de ec o s is mo e p ecise han
he online in o ma ion, because i uses la ge ime windows han he igge elec onics and
a mo e e ined algo i hm o quan i y he signal. E en s showing a econs uc ed signal in
any o ADA, ADC, V0A o V0C a e ejec ed.
The in a ian mass dis ibu ion o pT<0.2 GeV/cand ans e se momen um dis i-
bu ion o 0.55 <m<1.4 GeV/c2o he selec ed ack pai s a e shown in igu e 2. The
mass dis ibu ion shows he shape expec ed om a ρ0spec um, while a di ac ion dip is
clea ly seen in he ans e se momen um dis ibu ion. In o al, he signal sample con ains
almos 57 housand e en s which passed all selec ion c i e ia.
The signal sample is u he subdi ided in o wa d-neu on classes. The assignmen
o an e en o a class is based on he iming capabili ies o he ZNA and ZNC de ec o s.
E en s in which he iming o he ene gy deposi ion in he calo ime e is consis en wi hin
±2 ns wi h he neu on ha ing been p oduced in a beam-beam collision a e classi ied as
ha ing a o wa d neu on in he co esponding calo ime e .
3.2 Backg ound sub ac ion and co ec ions o expe imen al e ec s
In his sec ion he p ocedu e o de e mine he co ec ions used in he measu emen is
p esen ed. The co ec ion ac o s a e quo ed wi h hei co esponding unce ain ies, which
a e discussed in sec ion 3.5 and summa ised in ables 1 o 3.
– 5 –
JHEP06(2020)035
As a ool o quan i y some o he emaining backg ound con ibu ions a special sample
o e en s is selec ed ul illing all c i e ia men ioned in sec ion 3.1, excep ha bo h acks
ha e he same elec ic cha ge. The in a ian mass and ans e se momen um dis ibu ions
o his sample a e shown in igu e 2. The dis ibu ions o same-cha ge pai s a e used as
an es ima ion o he amoun and shape o he backg ound om e en s wi h a measu ed
opposi e-cha ge pai and o he cha ged acks ou side he accep ance o he de ec o . The
con ibu ion o same-cha ge pai s is a he le el o 1% and is s a is ically sub ac ed om
he signal sample.
Ano he po en ial backg ound comes om e en s wi h wo acks wi h opposi e elec ic
cha ge and a neu al pa icle. The main con ibu ion is expec ed om h ee-body decays o
he ω ec o meson. Dedica ed Mon e Ca lo (MC) simula ions o cohe en ωpho op oduc-
ion ollowed by he ω→π+π−π0decay demons a e ha he signal om such π+π−pai s
om h ee-body ωdecays concen a es a lowe masses and highe ans e se momen a
han hose conside ed o he signal-ex ac ion p ocedu e desc ibed below. This s udy, as
well as all s udies in ol ing MC, uses gene a ed MC e en s, in his case om STARligh ,
passed h ough a de ailed simula ion o he ALICE de ec o .
The con ibu ion om ρ0 ec o mesons p oduced in incohe en in e ac ions is es i-
ma ed by i ing a empla e p oduced by STARligh o he ans e se momen um dis ibu-
ion. The empla e is i ed in he egion o ans e se momen um 0.25 < pT<0.9 GeV/c
o ob ain i s p ope no malisa ion. The no malised empla e is used o es ima e his con-
ibu ion o pT<0.2 GeV/c. The inal yield o ρ0mesons is co ec ed by sub ac ing his
con ibu ion, which is (4 ±0.5)%.
The e iciency o he SPD eadou chips pa icipa ing in he igge is measu ed wi h
a da a-d i en app oach using a minimum bias igge . T acks selec ed wi hou equi ing
wo hi s in he di e en SPD laye s a e ma ched o he eadou chips hey c oss. A chip
ine iciency a ec s each ack, and hus each e en di e en ly. The e iciency maps ob ained
om da a a e inco po a ed in o he Mon e Ca lo simula ion o he signal and applied e en -
by-e en . The o e all e ec co esponds o a global co ec ion o abou (17 ±1)%.
The e iciency o he ZNA and ZNC o de ec neu ons is es ima ed wi h wo di e en
me hods. In he i s me hod, a sample o MC e en s gene a ed wi h he RELDIS p o-
g am [25,26] is used. The o he me hod elies on a simple p obabilis ic model [27] applied
di ec ly o he aw da a. Bo h me hods yield compa ible esul s, namely an e iciency o
abou (93 ±1)% each o he ZNA and ZNC o de ec neu on ac i i y. The p opaga ion
o his e ec , and he one discussed nex , in o he alue o he measu ed c oss sec ions is
discussed in sec ion 3.5.
Good e en s in he 0nXn and XnXn classes a e ejec ed when, in addi ion o he
o wa d neu ons, o he pa icles a e c ea ed a la ge apidi ies and lea e a signal ei he
in he AD o he V0 de ec o s. These ex a pa icles come om he di e en possibili ies
o dissocia ion o nuclei, e.g. neu on emission, mul i- agmen a ion o pion p oduc ion,
and he co esponding c oss sec ions a e expec ed o be la ge [28]. The amoun o good
e en s wi h neu ons which a e los due o e oes by AD and V0 is es ima ed using con ol
igge s. The co ec ions amoun o (26 ±4)% o e en s wi h a signal ei he in ZNA o in
ZNC, while i is (43 ±5)% o e en s wi h a signal in bo h ZNA and ZNC.
– 6 –
JHEP06(2020)035
Good e en s a e also ejec ed i ano he in e ac ion c ea es a signal in one o he e o
de ec o s, an e ec known as pile-up. The main pile-up comes om pu ely elec omagne ic
in e ac ions p oducing a low mass elec on-posi on pai . The p obabili y o he occu ence
o pile-up is co ela ed wi h he a e age numbe o inelas ic had onic collisions pe bunch
c ossing (µ), which o he da a used in his analysis a ied om µ= 0.0002 o µ= 0.0015.
The e ec o pile-up is es ima ed using wo di e en me hods. One me hod uses an e en
sample ob ained wi h an unbiased igge based only on he iming o bunches c ossing he
in e ac ion egion. This sample is sepa a ed in o pe iods wi h speci ic µ alues. The p ob-
abili y o a signal in each o he e o de ec o s is compu ed o each alue o µin o he wise
emp y e en s using he unbiased sample. This p obabili y exhibi s a linea beha iou as a
unc ion o µ. The e o ine iciencies a e de e mined by weigh ing he co esponding e o
ejec ion p obabili ies o e pe iods wi h di e en µ, aking he luminosi y o each pe iod
as a weigh . The co ela ion be ween he online and o line e oes is aken in o accoun .
The second me hod di ides he signal sample desc ibed in sec ion 3.1 in o subse s o e en s
wi h a speci ic ange o µ alues. Each one o hese sub-samples is subjec ed o he ull
analysis chain. The inal c oss sec ions show a linea dependence on µ. The in e cep a
µ= 0 is aken as he pile-up co ec ed c oss sec ion in his me hod. The wo app oaches
p oduce sligh ly di e en esul s. The a e age o bo h esul s is used as he inal co ec ion
ac o o (11.1±3.8)%.
Pile-up also a ec s he classi ica ion on o wa d-neu on classes. Elec omagne ic dis-
socia ion p ocesses [23] ha e a la ge c oss sec ion and p oduce neu ons a beam apidi ies.
Using he same unbiased sample as desc ibed abo e, he a e age pile-up p obabili y is
measu ed o be (3.3±0.3)% in bo h ZNA and ZNC.
Finally, he p oduc o he accep ance imes e iciency o measu e he cohe en ly p o-
duced ρ0 ec o meson is de e mined using e en samples gene a ed wi h STARligh . Two
di e en samples a e used: one o pu e cohe en ρ0pho op oduc ion and he o he p o-
duced wi h a la mass dis ibu ion. Bo h app oaches yield simila co ec ion unc ions
o he in a ian mass spec um. The accep ance imes e iciency ises smoo hly om 15%
o 19% in he mass ange om 0.6 GeV/c2 o 1.2 GeV/c2and emains cons an o la ge
masses.
3.3 Signal ex ac ion
The in a ian mass dis ibu ion, co ec ed by all e ec s desc ibed abo e and no malised by
he luminosi y o he sample, is i ed o he sum o a S¨oding o mula [29] and a e m M
o accoun o he con ibu ion o he γγ →µ+µ−p ocess:
dσ
dmdy=|A·BWρ+B|2+M, (3.1)
whe e Ais he no malisa ion ac o o he ρ0B ei -Wigne (BWρ) unc ion, and Bis he
non- esonan ampli ude. The ela i is ic B ei -Wigne unc ion o he ρ0 ec o meson is
BWρ=pm·mρ0·Γ(m)
m2−m2
ρ0+imρ0·Γ(m),(3.2)
– 7 –
JHEP06(2020)035
No o wa d-neu on selec ion C oss sec ion (mb) s a . (mb) sys . (mb)
|y|<0.2 537.0 4.6 +46.1
−42.0
0.2<|y|<0.45 538.6 4.4 +46.2
−42.1
0.45 <|y|<0.8 547.0 4.9 +46.9
−42.8
0n0n
|y|<0.2 431.1 4.0 +36.8
−33.6
0.2<|y|<0.45 433.8 3.8 +37.0
−33.8
0.45 <|y|<0.8 436.7 4.2 +37.3
−34.0
0nXn
|y|<0.2 90.2 1.9 +10.5
−9.5
0.2<|y|<0.45 87.7 1.8 +10.2
−9.3
0.45 <|y|<0.8 89.9 2.0 +10.4
−9.5
XnXn
|y|<0.2 24.4 1.3 +3.4
−2.9
0.2<|y|<0.45 24.5 1.2 +3.4
−3.0
0.45 <|y|<0.8 25.6 1.3 +3.5
−3.1
Table 4. Nume ical alues o he c oss sec ion o he cohe en pho op oduc ion o ρ0 ec o
mesons in Pb-Pb UPC a √sNN = 5.02 TeV. The sys ema ic unce ain ies a e ob ained by adding
in quad a u e he con ibu ions lis ed in ables 1 o 3.
The modi ica ion o he pho on lux due o he emission o he o wa d neu ons is
ca ied ou in he i s h ee models as p oposed in [9]. The ou h model uses he nO
On
a e bu ne desc ibed in [41].
Figu e 5shows ha he lowe limi o he GKZ model gi es a good desc ip ion o he
0n0n c oss sec ion and unde es ima es a li le bi he 0nXn and XnXn c oss sec ions while
he uppe limi o he same model o e es ima es he 0n0n, sligh ly unde es ima es he 0nXn
and desc ibes he XnXn c oss sec ions. The STARligh p edic ions unde es ima e all he
c oss sec ion a a ound he 2 sigma le el, excep XnXn whe e he di e ence is smalle . The
beha iou o he CCKT model based on ho spo s is qui e simila o he uppe limi o
GKZ; he CCKT (nuclea ) a ian o his model is some 10% la ge han he p edic ions
o he CCKT model wi h ho spo s. Finally, he GMMNS model p edic s c oss sec ions
la ge han STARligh , bu s ill unde es ima ing he measu emen s excep in he XnXn
class. Taking in o accoun he sp ead o he models and he unce ain ies o da a he
ag eemen be ween he models and he measu emen appea s in mos cases sa is ac o y,
pa icula ly o he p edic ions o he GKZ model. This o e all desc ip ion o da a by
models sugges s ha he me hod o ob ain he indi idual pho onuclea con ibu ions o
he cohe en p oduc ion o ρ0using o wa d-neu on classes [9,42] may be applied o he
– 14 –

JHEP06(2020)035
da a, specially once he unce ain ies in he measu emen s a e educed and he sp ead on
he heo e ical p edic ions is be e unde s ood.
4.2 Con ibu ions om con inuum p oduc ion
The |B/A| a io, see eq. (3.1), quan i ies he con ibu ion o he con inuum in ela ion o he
esonance p oduc ion c oss sec ion. The alue ound a mid apidi y o no o wa d-neu on
selec ion is 0.57 ±0.01 (s a .)±0.02 (sys .) (GeV/c2)−
1
2, whe e i has been checked ha
mos o he e ec s cancel in he a io and he only emaining con ibu ion o he sys ema ic
unce ain y a e he a ia ions in he i p ocedu e. The measu ed alue can be compa ed
wi h ha ound o he same p ocess a √sNN = 2.76 TeV: 0.50 ±0.04 (s a .)+0.10
−0.04 (sys .)
(GeV/c2)−
1
2[14]. Wi hin he cu en sys ema ic unce ain ies, he a io can be aken
as cons an bo h as a unc ion o apidi y and o he di e en o wa d-neu on classes.
None heless da a seems o indica e a small dec ease o he a io wi h apidi y o he no
o wa d selec ion case: |B/A|= 0.56 ±0.01 (s a .)±0.02 (sys .) (GeV/c2)−
1
2and |B/A|=
0.52 ±0.01 (s a .)±0.01 (sys .) (GeV/c2)−
1
2 o he 0.2<|y|<0.45 and 0.45 <|y|<0.8
in e als, espec i ely. I would be in e es ing i such a end is obse ed wi h he la ge
da a sample and he imp o ed p ecision, expec ed om he LHC Run 3 and 4 [43].
The co esponding a io in cohe en Au-Au UPC measu ed by STAR a √sNN =
200 GeV is 0.79 ±0.01 (s a .)±0.08 (sys .) (GeV/c2)−
1
2[13]. These esul s o p oduc ion
o hea y nuclea a ge s, can be compa ed wi h hose om exclusi e ρ0pho op oduc-
ion o p o ons. No e ha alue o |B/A|migh depend on he ange in | |selec ed
o pe o m he measu emen , whe e is he squa e o he ou momen um ans e a
he a ge e ex. The CMS Collabo a ion measu ed 0.50 ±0.06 (s a .) (GeV/c2)−
1
2in
p-Pb UPC a √sNN = 5.02 TeV [44] o | |<0.5 GeV2. The ZEUS Collabo a ion, us-
ing a sample o posi on-p o on collisions a a cen e-o -mass ene gy o 300 GeV, epo s
0.67 ±0.02 (s a .)±0.04 (sys .) (GeV/c2)−
1
2 o hei ull analysed sample, and ≈0.8
(GeV/c2)−
1
2 o alues simila o hose o cohe en ρ0p oduc ion in Pb-Pb UPC [45].
O e all, he a io o he con inuum o he esonance p oduc ion o π+π−pai s seems o
be sensi i e o bo h he kinema ics o he in e ac ion and he ype o a ge , bu no clea
pic u e has ye eme ged.
4.3 Obse a ion o a esonance-like s uc u e
As shown in igu e 4, he e seems o be a esonance-like s uc u e in he egion m >
1.2 GeV/c2. The model o eq. (3.4) yields a mass o (1725 ±17) MeV/c2and wid h (143 ±
21) MeV/c2, whe e he quo ed unce ain ies co espond o s a is ical luc ua ions only. As
shown in he same igu e, his esonance-like objec has e y low ans e se momen um as
expec ed om a cohe en -p oduc ion p ocess.
Such an objec is also seen by he STAR Collabo a ion [33] albei a a sligh ly lowe
mass o 1.65 GeV/c2, bu wi h a simila wid h. ZEUS epo s a peak a ound 1.8 GeV/c2
o exclusi e elec op oduc ion o π+π−pai s [46]. Mo e ecen ly, H1 epo s a peak a
1.6 GeV/c2in he exclusi e pho op oduc ion o he ρ0meson [47]. As sugges ed in [33],
his esonance is also compa ible wi h he ρ3(1690) lis ed in he PDG, which has a o al
angula momen um J= 3 [32].
– 15 –
JHEP06(2020)035
The la ge da a samples expec ed in Run 3 and Run 4 a he LHC [43] may help o
shed ligh on he o igin and s uc u e o his objec .
5 Summa y and ou look
The apidi y dependence o he cohe en ρ0 ec o meson p oduc ion c oss sec ion in Pb-Pb
UPC a √sNN = 5.02 TeV has been p esen ed. In each apidi y ange, he c oss sec ion is
measu ed o di e en classes o e en s de ined by he p esence o neu ons a beam apidi-
ies. The c oss sec ions a e compa ed wi h he main a ailable models o his p ocess. The
measu emen s o cohe en ρ0pho op oduc ion a e in good ag eemen bo h wi h models ol-
lowing he pa on-based colou -dipole app oach and wi h he amewo k o G ibo -Glaube
shadowing based on had onic deg ees o eedom. The models [9,41] o elec omagne ic
nuclea dissocia ion accompanying ec o meson pho op oduc ion p o ide a sa is ac o y
desc ip ion o he measu ed c oss sec ions o di e en neu on emission classes. This obse -
a ion sugges s ha he me hod p oposed in [42] o decouple he low-pho on-ene gy om
he high-pho on-ene gy con ibu ion o he UPC c oss sec ion using neu on-di e en ial
measu emen s migh also be applicable a o wa d apidi ies, which is specially impo an
in iew o he expec ed da a samples o be eco ded a he LHC du ing he Run 3 and 4 [43].
In addi ion, he cohe en pho op oduc ion o a esonance-like objec wi h a mass
a ound 1.7 GeV/c2which decays in o a π+π−pai is epo ed and compa ed wi h sim-
ila obse a ions om o he expe imen s.
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 : 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, Aus ian Science Fund (FWF): [M 2467-
N36] and Na ionals i ung ¨u 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 ´ı ico e Tecnol´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) and Uni e sidade Fede al do Rio G ande do Sul (UFRGS), B azil; Minis y o
Educa ion o China (MOEC) , Minis y o Science & Technology o China (MSTC) and
Na ional Na u al Science Founda ion o China (NSFC), China; Minis y o Science and
Educa ion and C oa ian Science Founda ion, C oa ia; Cen o de Aplicaciones Tecnol´ogicas
y Desa ollo Nuclea (CEADEN), Cubaene g´ıa, Cuba; Minis y o Educa ion, You h and
Spo s o he Czech Republic, Czech Republic; Czech Science Founda ion; The Danish
– 16 –
JHEP06(2020)035
Council o Independen Resea ch — Na u al Sciences, he VILLUM FONDEN and Danish
Na ional Resea ch Founda ion (DNRF), Denma k; Helsinki Ins i u e o Physics (HIP), Fin-
land; Commissa ia `a l’Ene gie A omique (CEA), Ins i u Na ional de Physique Nucl´eai e
e de Physique des Pa icules (IN2P3) and Cen e Na ional de la Reche che Scien i ique
(CNRS) and R´egion des Pays de la Loi e, F ance; Bundesminis e ium ¨u Bildung und
Fo schung (BMBF) and GSI Helmhol zzen um ¨u 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 O ice, 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 i ic and Indus ial Resea ch (CSIR), India; Indonesian
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),
Japanese Minis y o Educa ion, Cul u e, Spo s, Science and Technology (MEXT) and
Japan Socie y o he P omo ion o Science (JSPS) KAKENHI, Japan; Consejo Nacional de
Ciencia (CONACYT) y Tecnolog´ıa, h ough Fondo de Coope aci´on In e nacional en Cien-
cia y Tecnolog´ıa (FONCICYT) and Di ecci´on 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 Resea ch Council o No way, No way; Commission on Science and Tech-
nology o Sus ainable De elopmen in he Sou h (COMSATS), Pakis an; Pon i icia Uni-
e sidad Ca ´olica del Pe ´u, 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 i ic Resea ch, Ins i u e o A omic Physics and Minis y o Resea ch and Inno a ion
and Ins i u e o A omic Physics, Romania; Join Ins i u e o Nuclea Resea ch (JINR),
Minis y o Educa ion and Science o he Russian Fede a ion, Na ional Resea ch Cen e
Ku cha o Ins i u e, Russian Science Founda ion and Russian Founda ion o Basic Re-
sea ch, 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; 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; Su ana ee Uni e si y o Technology (SUT),
Na ional Science and Technology De elopmen Agency (NSDTA) and O ice o he Highe
Educa ion Commission unde NRU p ojec o Thailand, Thailand; Tu kish A omic Ene gy
Agency (TAEK), Tu key; Na ional Academy o Sciences o Uk aine, Uk aine; Science and
Technology Facili ies Council (STFC), Uni ed Kingdom; Na ional Science Founda ion o
he Uni ed S a es o Ame ica (NSF) and Uni ed S a es Depa men o Ene gy, O ice o
Nuclea Physics (DOE NP), Uni ed S a es o Ame ica.
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.
– 17 –
JHEP06(2020)035
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– 20 –
JHEP06(2020)035
The ALICE collabo a ion
S. Acha ya142, D. Adamo ´a95, A. Adle 74, J. Adol sson81, M.M. Agga wal100, G. Aglie i Rinella34,
M. Agnello30, N. Ag awal10,54, Z. Ahammed142, S. Ahmad16, S.U. Ahn76, A. Akindino 92,
M. Al-Tu any107, S.N. Alam142, D.S.D. Albuque que123, D. Aleksand o 88, B. Alessand o59,
H.M. Al anda6, R. Al a o Molina71, B. Ali16, Y. Ali14, A. Alici10,26,54, A. Alkin2, J. Alme21,
T. Al 68, L. Al enkampe 21, I. Al sybee 113, M.N. Anaam6, C. And ei48, D. And eou34,
H.A. And ews111, A. And onic145, M. Angele i34, V. Anguelo 104, C. Anson15, T. An iˇci´c108,
F. An ino i57, P. An onioli54, N. Apadula80, L. Aphece che115, H. Appelsh¨ause 68, S. A celli26,
R. A naldi59, M. A a ia80, I.C. A sene20, M. A slandok104, A. Augus inus34, R. A e beck107,
S. Aziz78, M.D. Azmi16, A. Badal`a56, Y.W. Baek41, S. Bagnasco59, X. Bai107, R. Bailhache68,
R. Bala101, A. Balbino30, A. Baldisse i138, M. Ball43, S. Balouza105, D. Bane jee3, R. Ba be a27,
L. Ba ioglio25, G.G. Ba na ¨oldi146, L.S. Ba nby94, V. Ba e 135, P. Ba alini6, K. Ba h34,
E. Ba sch68, F. Ba u aldi28, N. Bas id135, S. Basu144, G. Ba igne115, B. Ba yunya75, D. Bau i49,
J.L. Bazo Alba112, I.G. Bea den89, C. Bea ie147, C. Bedda63, N.K. Behe a61, I. Beliko 137,
A.D.C. Bell Hecha a ia145, F. Bellini34, R. Bellwied126, V. Belyae 93, G. Bencedi146, S. Beole25,
A. Be cuci48, Y. Be dniko 98, D. Be enyi146, R.A. Be ens131, D. Be zano59, M.G. Besoiu67,
L. Be e 34, A. Bhasin101, I.R. Bha 101, M.A. Bha 3, H. Bha 49, B. Bha acha jee42, A. Bianchi25,
L. Bianchi25, N. Bianchi52, J. Bielˇc´ık37, J. Bielˇc´ıko ´a95, A. Bilandzic105,118, G. Bi o146,
R. Biswas3, S. Biswas3, J.T. Blai 120, D. Blau88, C. Blume68, G. Boca140, F. Bock34,96,
A. Bogdano 93, S. Boi23, L. Boldizs´a 146, A. Bolozdynya93, M. Bomba a38, G. Bonomi141,
H. Bo el138, A. Bo isso 93, H. Bossi147, E. Bo a25, L. B a ud68, P. B aun-Munzinge 107,
M. B egan 122, M. B oz37, E. B una59, G.E. B uno106, M.D. Buckland128, D. Budniko 109,
H. Buesching68, S. Bu alino30, O. Bugnon115, P. Buhle 114, P. Buncic34, Z. Bu helezi72,132,
J.B. Bu 14, J.T. Bux on97, S.A. Bysiak119, D. Ca a i90, A. Cali a107, E. Cal o Villa 112,
R.S. Camacho45, P. Came ini24, A.A. Capon114, F. Ca nesecchi10,26, R. Ca on138, J. Cas illo
Cas ellanos138, A.J. Cas o131, E.A.R. Casula55, F. Ca alano30, C. Ceballos Sanchez53,
P. Chak abo y49, S. Chand a142, W. Chang6, S. Chapeland34, M. Cha ie 128,
S. Cha opadhyay142, S. Cha opadhyay110, A. Chau in23, C. Cheshko 136, B. Cheynis136,
V. Chiban e Ba oso34, D.D. Chinella o123, S. Cho61, P. Chochula34, T. Chowdhu y135,
P. Ch is akoglou90, C.H. Ch is ensen89, P. Ch is iansen81, T. Chujo134, C. Cicalo55,
L. Ci a elli10,26, F. Cindolo54, G. Clai54,ii, J. Cleymans125, F. Colama ia53, D. Colella53,
A. Collu80, M. Colocci26, M. Concas59,iii, G. Conesa Balbas e79, Z. Conesa del Valle78,
G. Con in24,60, J.G. Con e as37, T.M. Co mie 96, Y. Co ales Mo ales25, P. Co ese31,
M.R. Cosen ino124, F. Cos a34, S. Cos anza140, P. C oche 135, E. Cuau le69, P. Cui6,
L. Cunquei o96, D. Dab owski143, T. Dahms105,118, A. Dainese57, F.P.A. Damas115,138,
M.C. Danisch104, A. Danu67, D. Das110, I. Das110, P. Das86, P. Das3, S. Das3, A. Dash86,
S. Dash49, S. De86, A. De Ca o29, G. de Ca aldo53, J. de Cu eland39, A. De Falco23, D. De
G u ola10, N. De Ma co59, S. De Pasquale29, S. Deb50, H.F. Degenha d 122, K.R. Deja143,
A. Delo 85, S. Delsan o25,132, W. Deng6, D. De e ak107, P. Dhankhe 49, D. Di Ba i33, A. Di
Mau o34, R.A. Diaz8, T. Die el125, P. Dillensege 68, Y. Ding6, R. Di i`a34, D.U. Dixi 19,
Ø. Dju sland21, U. Dmi ie a62, A. Dob in67, B. D¨onigus68, O. Do dic20, A.K. Dubey142,
A. Dubla107, S. Dudi100, M. Dukhishyam86, P. Dupieux135, R.J. Ehle s96,147, V.N. Eikeland21,
D. Elia53, E. Epple147, B. E azmus115, F. E ha d 99, A. E okhin113, M.R. E sdal21,
B. Espagnon78, G. Eulisse34, D. E ans111, S. E dokimo 91, L. Fabbie i105,118, M. Faggin28,
J. Fai e79, F. Fan6, A. Fan oni52, M. Fasel96, P. Fecchio30, A. Feliciello59, G. Feo ilo 113,
A. Fe n´andez T´ellez45, A. Fe e o138, A. Fe e i25, A. Fes an i34, V.J.G. Feuilla d104, J. Figiel119,
S. Filchagin109, D. Finogee 62, F.M. Fionda21, G. Fio enza53, F. Flo 126, S. Foe sch72,
– 21 –
JHEP06(2020)035
P. Foka107, S. Fokin88, E. F agiacomo60, U. F anken eld107, U. Fuchs34, C. Fu ge 79, A. Fu s62,
M. Fusco Gi a d29, J.J. Gaa dhøje89, M. Gaglia di25, A.M. Gago112, A. Gal137, C.D. Gal an121,
P. Gano i84, C. Ga aba os107, E. Ga cia-Solis11, K. Ga g115, C. Ga giulo34, A. Ga ibli87,
K. Ga ne 145, P. Gasik105,118, E.F. Gauge 120, M.B. Gay Duca i70, M. Ge main115, J. Ghosh110,
P. Ghosh142, S.K. Ghosh3, M. Giacalone26, P. Giano i52, P. Giubellino59,107, P. Giubila o28,
P. Gl¨assel104, A. Gomez Rami ez74, V. Gonzalez107,144, L.H. Gonz´alez-T ueba71, S. Go buno 39,
L. G¨o lich119, A. Goswami49, S. Go o ac35, V. G abski71, L.K. G aczykowski143, K.L. G aham111,
L. G eine 80, A. G elli63, C. G igo as34, V. G igo ie 93, A. G igo yan1, S. G igo yan75,
O.S. G oe ik21, F. G osa30, J.F. G osse-Oe inghaus34, R. G osso107, R. Gue nane79,
M. Gui ie e115, K. Gulb andsen89, T. Gunji133, A. Gup a101, R. Gup a101, I.B. Guzman45,
R. Haake147, M.K. Habib107, C. Hadjidakis78, H. Hamagaki82, G. Hama 146, M. Hamid6,
R. Hannigan120, M.R. Haque63,86, A. Ha lende o a107, J.W. Ha is147, A. Ha on11,
J.A. Hasenbichle 34, H. Hassan96, D. Ha zi o iadou10,54, P. Haue 43, S. Hayashi133,
S.T. Heckel68,105, E. Hellb¨a 68, H. Hels up36, A. He ghelegiu48, T. He man37, E.G. He nandez45,
G. He e a Co al9, F. He mann145, K.F. He land36, H. Hillemanns34, C. Hills128,
B. Hippoly e137, B. Hohlwege 105, J. Hone mann145, D. Ho ak37, A. Ho nung68, S. Ho nung107,
R. Hosokawa15, P. H is o 34, C. Huang78, C. Hughes131, P. Huhn68, T.J. Humanic97,
H. Hushnud110, L.A. Huso a145, N. Hussain42, S.A. Hussain14, D. Hu e 39, J.P. Iddon34,128,
R. Ilkae 109, H. Ilyas14, M. Inaba134, G.M. Innocen i34, M. Ippoli o 88, A. Isako 95,
M.S. Islam110, M. I ano 107, V. I ano 98, V. Izuchee 91, B. Jacak80, N. Jacazio34, P.M. Jacobs80,
S. Jadlo ska117, J. Jadlo sky117, S. Jaelani63, C. Jahnke122, M.J. Jakubowska143, M.A. Janik143,
T. Janson74, M. Je cic99, O. Je ons111, M. Jin126, F. Jonas96,145, P.G. Jones111, J. Jung68,
M. Jung68, A. Jusko111, P. Kalinak64, A. Kalwei 34, V. Kaplin93, S. Ka 6, A. Ka asu Uysal77,
O. Ka a iche 62, T. Ka a iche a62, P. Ka czma czyk34, E. Ka peche 62, U. Kebschull74,
R. Keidel47, M. Keil34, B. Ke ze 43, Z. Khabano a90, A.M. Khan6, S. Khan16, S.A. Khan142,
A. Khanzadee 98, Y. Kha lo 91, A. Kha un16, A. Khun ia119, B. Kileng36, B. Kim61, B. Kim134,
D. Kim148, D.J. Kim127, E.J. Kim73, H. Kim17,148, J. Kim148, J.S. Kim41, J. Kim104, J. Kim148,
J. Kim73, M. Kim104, S. Kim18, T. Kim148, T. Kim148, S. Ki sch39,68, I. Kisel39, S. Kisele 92,
A. Kisiel143, J.L. Klay5, C. Klein68, J. Klein34,59, S. Klein80, C. Klein-B¨osing145, M. Kleine 68,
A. Kluge34, M.L. Knichel34, A.G. Knospe126, C. Kobdaj116, M.K. K¨ohle 104, T. Kollegge 107,
A. Kond a ye 75, N. Kond a ye a93, E. Kond a yuk91, J. Konig68, P.J. Konopka34, L. Koska117,
O. Ko alenko85, V. Ko alenko113, M. Kowalski119, I. K ´alik64, A. K a ˇc´ako ´a38, L. K eis107,
M. K i da64,111, F. K izek95, K. K izko a Gajdoso a37, M. K ¨uge 68, E. K yshen98,
M. K zewicki39, A.M. Kube a97, V. Kuˇce a34,61, C. Kuhn137, P.G. Kuije 90, L. Kuma 100,
S. Kundu86, P. Ku ash ili85, A. Ku epin62, A.B. Ku epin62, A. Ku yakin109, S. Kushpil95,
J. K apil111, M.J. Kweon61, J.Y. Kwon61, Y. Kwon148, S.L. La Poin e39, P. La Rocca27,
Y.S. Lai80, R. Langoy130, K. Lapidus34, A. La deux20, P. La iono 52, E. Laudi34, R. La icka37,
T. Laza e a113, R. Lea24, L. Lea dini104, J. Lee134, S. Lee148, F. Lehas90, S. Lehne 114,
J. Leh bach39, R.C. Lemmon94, I. Le´on Monz´on121, E.D. Lesse 19, M. Le ich34, P. L´e ai146,
X. Li12, X.L. Li6, J. Lien130, R. Lie a a111, B. Lim17, V. Lindens u h39, A. Lindne 48,
S.W. Lindsay128, C. Lippmann107, M.A. Lisa97, A. Liu19, J. Liu128, S. Liu97, W.J. Llope144,
I.M. Lo nes21, V. Logino 93, C. Loizides96, P. Lonca 35, J.A. Lopez104, X. Lopez135, E. L´opez
To es8, J.R. Luhde 145, M. Luna don28, G. Lupa ello60, Y.G. Ma40, A. Mae skaya62,
M. Mage 34, S.M. Mahmood20, T. Mahmoud43, A. Mai e137, R.D. Majka147,i, M. Malae 98,
Q.W. Malik20, L. Malinina75,i , D. Mal’Ke ich92, P. Malzache 107, G. Mandaglio32,56,
V. Manko88, F. Manso135, V. Manza i53, Y. Mao6, M. Ma chisone136, J. Ma eˇs66,
G.V. Ma gaglio i24, A. Ma go i54, J. Ma gu i63, A. Ma ´ın107, C. Ma ke 120, M. Ma qua d68,
C.D. Ma in24, N.A. Ma in104, P. Ma inengo34, J.L. Ma inez126, M.I. Ma ´ınez45, G. Ma ´ınez
– 22 –
JHEP06(2020)035
Ga c´ıa115, S. Masciocchi107, M. Mase a25, A. Masoni55, L. Massac ie 78, E. Masson115,
A. Mas ose io53,139, A.M. Ma his105,118, O. Ma onoha81, P.F.T. Ma uoka122, A. Ma yja119,
C. Maye 119, F. Mazzaschi25, M. Mazzilli53, M.A. Mazzoni58, A.F. Mechle 68, F. Meddi22,
Y. Melikyan62,93, A. Menchaca-Rocha71, C. Mengke6, E. Meninno29,114, M. Me es13,
S. Mhlanga125, Y. Miake134, L. Michele i25, D.L. Mihaylo 105, K. Mikhaylo 75,92, A.N. Mish a69,
D. Mi´skowiec107, A. Modak3, N. Mohammadi34, A.P. Mohan y63, B. Mohan y86, M. Mohisin
Khan16, , Z. Mo a co a89, C. Mo dasini105, D.A. Mo ei a De Godoy145, L.A.P. Mo eno45,
I. Mo ozo 62, A. Mo sch34, T. M nja ac34, V. Mucci o a52, E. Mudnic35, D. M¨uhlheim145,
S. Muhu i142, J.D. Mulligan80, M.G. Munhoz122, R.H. Munze 68, H. Mu akami133, S. Mu ay125,
L. Musa34, J. Musinsky64, C.J. Mye s126, J.W. My cha143, B. Naik49, R. Nai 85, B.K. Nandi49,
R. Nania10,54, E. Nappi53, M.U. Na u14, A.F. Nassi pou 81, C. Na ass131, R. Nayak49,
T.K. Nayak86, S. Naza enko109, A. Neagu20, R.A. Neg ao De Oli ei a68, L. Nellen69,
S.V. Nesbo36, G. Nesko ic39, D. Nes e o 113, L.T. Neumann143, B.S. Nielsen89, S. Nikolae 88,
S. Nikulin88, V. Nikulin98, F. No e ini10,54, P. Nomokono 75, J. No man79,128, N. No i zky134,
P. Nowakowski143, A. Nyanin88, J. Nys and21, M. Ogino82, A. Ohlson81,104, J. Oleniacz143,
A.C. Oli ei a Da Sil a131, M.H. Oli e 147, C. Oppedisano59, A. O iz Velasquez69,
A. Oska sson81, J. O winowski119, K. Oyama82, Y. Pachmaye 104, V. Pacik89, D. Pagano141,
G. Pai´c69, J. Pan144, S. Panebianco138, P. Pa eek50,142, J. Pa k61, J.E. Pa kkila127, S. Pa ma 100,
S.P. Pa hak126, B. Paul23, H. Pei6, T. Pei zmann63, X. Peng6, L.G. Pe ei a70, H. Pe ei a Da
Cos a138, D. Pe esunko88, G.M. Pe ez8, Y. Pes o 4, V. Pe ´aˇcek37, M. Pe o ici48, R.P. Pezzi70,
S. Piano60, M. Pikna13, P. Pillo 115, O. Pinazza34,54, L. Pinsky126, C. Pin o27, S. Pisano10,52,
D. Pis one56, M. P losko´n80, M. Planinic99, F. Plique 68, S. Pochybo a146,i, M.G. Poghosyan96,
B. Polich chouk91, N. Poljak99, A. Pop48, S. Po eboeu -Houssais135, V. Pozdniako 75,
S.K. P asad3, R. P eghenella54, F. P ino59, C.A. P uneau144, I. Pshenichno 62, M. Puccio34,
J. Pu schke144, L. Quaglia25, R.E. Quishpe126, S. Ragoni111, S. Raha3, S. Rajpu 101, J. Rak127,
A. Rako oza ind abe138, L. Ramello31, F. Rami137, S.A.R. Rami ez45, R. Raniwala102,
S. Raniwala102, S.S. R¨as¨anen44, R. Ra h50, V. Ra za43, I. Ra asenga90, K.F. Read96,131,
A.R. Redelbach39, K. Redlich85, i, A. Rehman21, P. Reichel 68, F. Reid 34, X. Ren6,
R. Ren o d 68, Z. Rescako a38, K. Reyge s104, V. Riabo 98, T. Riche 81,89, M. Rich e 20,
P. Riedle 34, W. Riegle 34, F. Riggi27, C. Ris ea67, S.P. Rode50, M. Rod ´ıguez Cahuan zi45,
K. Røed20, R. Rogale 91, E. Rogochaya75, D. Roh 34, D. R¨oh ich21, P.S. Roki a143,
F. Ronche i52, A. Rosano56, E.D. Rosas69, K. Roslon143, A. Rossi28,57, A. Ro ondi140, A. Roy50,
P. Roy110, O.V. Rueda81, R. Rui24, B. Rumyan se 75, A. Rus amo 87, E. Ryabinkin88,
Y. Ryabo 98, A. Rybicki119, H. Ry konen127, O.A.M. Saa imaki44, S. Sadhu142, S. Sado sky91,
K. ˇ
Sa aˇ ´ık37, S.K. Saha142, B. Sahoo49, P. Sahoo49, R. Sahoo50, S. Sahoo65, P.K. Sahu65,
J. Saini142, S. Sakai134, S. Sambyal101, V. Samsono 93,98, D. Sa ka 144, N. Sa ka 142, P. Sa ma42,
V.M. Sa i105, M.H.P. Sas63, E. Scappa one54, J. Schambach120, H.S. Scheid68, C. Schiaua48,
R. Schicke 104, A. Schmah104, C. Schmid 107, H.R. Schmid 103, M.O. Schmid 104, M. Schmid 103,
N.V. Schmid 68,96, A.R. Schmie 131, J. Schuk a 89, Y. Schu z34,137, K. Schwa z107,
K. Schweda107, G. Scioli26, E. Scompa in59, M. ˇ
Se ˇc´ık38, J.E. Sege 15, Y. Sekiguchi133,
D. Sekiha a133, I. Selyuzhenko 93,107, S. Senyuko 137, D. Se eb yako 62, A. Se cenco67,
A. Shabano 62, A. Shabe ai115, R. Shahoyan34, W. Shaikh110, A. Shanga ae 91, A. Sha ma100,
A. Sha ma101, H. Sha ma119, M. Sha ma101, N. Sha ma100, S. Sha ma101, A.I. Sheikh142,
K. Shigaki46, M. Shimomu a83, S. Shi inkin92, Q. Shou40, Y. Sibi iak88, S. Siddhan a55,
T. Siemia czuk85, D. Sil e my 81, G. Sima o ic90, G. Simone i34, B. Singh105, R. Singh86,
R. Singh101, R. Singh50, V.K. Singh142, V. Singhal142, T. Sinha110, B. Si a 13, M. Si a31,
T.B. Skaali20, M. Slupecki127, N. Smi no 147, R.J.M. Snellings63, C. Soncco112, J. Song126,
A. Songmoolnak116, F. So amel28, S. So ensen131, I. Spu owska119, J. S achel104, I. S an67,
– 23 –