Eu . Phys. J. C (2015) 75:152
DOI 10.1140/epjc/s10052-015-3344-6
Regula A icle - Expe imen al Physics
Sea ch o long-li ed pa icles decaying o je pai s
LHCb Collabo a ion
CERN, 1211 Gene a 23, Swi ze land
Recei ed: 10 Decembe 2014 / Accep ed: 2 Ma ch 2015 / Published online: 17 Ap il 2015
© CERN o he bene i o he LHCb collabo a ion 2015. This a icle is published wi h open access a Sp inge link.com
Abs ac A sea ch is p esen ed o long-li ed pa icles
wi h a mass be ween 25 and 50 GeV/c2and a li e ime
be ween 1 and 200 ps in a sample o p o on–p o on collisions
a a cen e-o -mass ene gy o √s=7 TeV, co esponding o
an in eg a ed luminosi y o 0.62 b−1, collec ed by he LHCb
de ec o . The pa icles a e assumed o be pai -p oduced by he
decay o a s anda d model-like Higgs boson. The expe imen-
al signa u e o he long-li ed pa icle is a displaced e ex
wi h wo associa ed je s. No excess abo e he backg ound is
obse ed and limi s a e se on he p oduc ion c oss-sec ion
as a unc ion o he long-li ed pa icle mass and li e ime.
1 In oduc ion
A a ie y o models o physics beyond he s anda d model
(SM) ea u e he exis ence o new massi e pa icles whose
coupling o ligh e pa icles is su icien ly small ha hey
a e long-li ed. I hese massi e pa icles decay o SM pa -
icles and ha e a li e ime be ween app oxima ely 1 ps and
1 ns, cha ac e is ic o weak decays, hey can be iden i ied by
hei displaced decay e ex. Examples o such pa icles a e
he ligh es supe symme ic pa icle in SUSY models wi h
ba yon o lep on numbe iola ion [1–4], he nex - o-ligh es
supe symme ic pa icle in g a i y media ed SUSY [5] and
he neu al π pa icle in hidden alley (HV) models wi h a
non-abelian gauge symme y [6–8]. The la e model is pa -
icula ly in e es ing as i p edic s ha expe imen al s udies
ha e sensi i i y o he p oduc ion o long-li ed pa icles in
SM Higgs decays.
This pape epo s on a sea ch o π pa icles, pai -
p oduced in he decay o a SM-like Higgs pa icle wi h
a mass o 120 GeV/c2, close o he mass o he scala boson
e-mail: [email p o ec ed]
disco e ed by he ATLAS and CMS expe imen s [9,10].1
The π candida es a e iden i ied by wo had onic je s o ig-
ina ing om a displaced e ex. The e ex is equi ed o
be displaced om he p o on–p o on collision axis by mo e
han 0.4 mm and less han 4.8 mm. The lowe bound is cho-
sen o ejec mos o he backg ound om hea y la ou
decays. The uppe bound ensu es ha e ices a e inside he
LHCb beam pipe, which gene a es a sizeable backg ound o
had onic in e ac ion e ices. The signal is ex ac ed om a
i o he di-je in a ian mass dis ibu ion. The analysis is sen-
si i e o a π pa icle wi h a mass be ween 25 and 50 GeV/c2
and a li e ime be ween 1 and 200 ps. The lowe bounda y on
he mass ange a ises om he equi emen o iden i y wo
had onic je s while he uppe bounda y is mos ly due o he
geome ic accep ance o he LHCb de ec o .
This analysis uses da a collec ed in p o on–p o on (pp)
collisions a a cen e-o -mass ene gy o √s=7 TeV. The
da a co espond o an in eg a ed luminosi y o 0.62 b−1,
collec ed du ing he second hal o he yea 2011 when
an analysis-speci ic igge selec ion was implemen ed.
Al hough simila sea ches ha e been epo ed by he CDF [11],
D0 [12], ATLAS [13] and CMS [14] expe imen s, LHCb
has a unique co e age o long-li ed pa icles wi h ela i ely
small mass and li e ime, because i s igge makes only mod-
es equi emen s on ans e se momen um.
2 De ec o desc ip ion
The LHCb de ec o [15] is a single-a m o wa d spec ome-
e co e ing he pseudo apidi y ange 2 <η<5, designed
o he s udy o pa icles con aining bo cqua ks qua ks. The
de ec o includes a high-p ecision acking sys em consis ing
o a silicon-s ip e ex de ec o su ounding he pp in e ac-
ion egion [16], a la ge-a ea silicon-s ip de ec o loca ed
ups eam o a dipole magne wi h a bending powe o abou
1The esul s a e equally alid o a Higgs pa icle wi h a mass up o
126 GeV/c2wi hin a ew pe cen .
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152 Page 2 o 12 Eu . Phys. J. C (2015) 75 :152
4Tm, and h ee s a ions o silicon-s ip de ec o s and s aw
d i ubes [17] placed downs eam o he magne . The ack-
ing sys em p o ides a measu emen o momen um, p, wi h a
ela i e unce ain y ha a ies om 0.4 % a low momen um
o 0.6 % a 100 GeV/c. The minimum dis ance o a ack
o a p ima y e ex, he impac pa ame e , is measu ed wi h
a esolu ion o (15 +29/pT)µm, whe e pTis he compo-
nen o p ans e se o he beam, in GeV/c. Di e en ypes
o cha ged had ons a e dis inguished using in o ma ion om
wo ing-imaging Che enko de ec o s [18]. Pho on, elec on
and had on candida es a e iden i ied by a calo ime e sys-
em consis ing o scin illa ing-pad and p eshowe de ec o s,
an elec omagne ic calo ime e and a had onic calo ime e .
Muons a e iden i ied by a sys em composed o al e na ing
laye s o i on and mul iwi e p opo ional chambe s [19].
3 E en simula ion
Fo he e en simula ion, pp collisions a e gene a ed using
Py hia 6.4 [20] wi h a speci ic LHCb con igu a ion [21]
using CTEQ6L [22] pa on densi y unc ions. Decays o
had onic pa icles a e desc ibed by E Gen [23], in which
inal-s a e adia ion is gene a ed using Pho os [24]. The
in e ac ion o he gene a ed pa icles wi h he de ec o and i s
esponse a e implemen ed using he Gean 4 oolki [25,26]
as desc ibed in Re . [27].
To simula e a signal e en , a SM-like scala Higgs boson
wi h a mass o 120 GeV/c2is gene a ed wi h Py hia h ough
he gluon–gluon usion mechanism, and is o ced o decay
in o wo spin-ze o π pa icles, each o which decays o b¯
b.
Assuming he decay occu s ia a ec o o axial- ec o cou-
pling, he b¯
b inal s a e is p e e ed o ligh qua ks, due o
helici y conse a ion [6–8]. The a e age ack mul iplici y
o he π decay, including acks om seconda y band c
decays, a ies om abou 15 o a π mass o 25 GeV/c2 o
abou 20 o la ge masses. Simula ed e en s a e e ained i
a leas ou cha ged acks om he decay o he gene a ed
π pa icles a e wi hin he LHCb accep ance, which co e-
sponds o abou 30 % o he cases. Fo π pa icles wi hin he
accep ance on a e age abou en acks can be econs uc ed.
Simula ed samples wi h π li e imes o 10 ps and 100 ps
and π masses o 25, 35, 43 and 50 GeV/c2a e gene a ed;
o he π li e imes a e s udied by eweigh ing hese samples.
Two addi ional samples a e gene a ed in which π pa icles
wi h a li e ime o 10 ps and a mass o 35 GeV/c2decay o
ei he c¯co s¯squa k pai s.
4 E en selec ion and signal ex ac ion
The selec ion o candida es s a s wi h he LHCb igge [28],
which consis s o a ha dwa e s age, based on in o ma ion
om he calo ime e and muon sys ems, ollowed by a so -
wa e s age, which applies a ull e en econs uc ion. The
ha dwa e igge (L0) equi es a single high-pThad on, elec-
on, muon o pho on signa u e. The h esholds ange om
pT>1.48 GeV/c o muons, o ans e se ene gy la ge han
3.5 GeV o had ons. The o al L0 e iciency, domina ed by
he had on igge selec ion, depends on he mass and inal
s a e o he π pa icle and is ypically 20 %, including he
de ec o accep ance.
The so wa e igge is di ided in o wo s ages and con-
sis s o algo i hms ha un a simpli ied e sion o he o line
ack econs uc ion, which allows iden i ica ion o displaced
acks and e ices. Fo his analysis he p ima y signa u e in
he i s so wa e s age (HLT1) is a single high-quali y dis-
placed ack wi h high pT. The e iciency o HLT1 ela i e
o L0 accep ed e en s is ypically 60 %. Howe e , his e i-
ciency educes apidly o e ices ha a e displaced by mo e
han abou 5 mm om he beamline due o limi a ions in he
ack econs uc ion in he e ex de ec o .
In he inal igge s age (HLT2) wo di e en signa u es
a e exploi ed. The i s o hese elies on he gene ic econ-
s uc ion o a displaced e ex, using an algo i hm simila
o ha used o he p ima y e ex (PV) econs uc ion [29].
Seconda y e ices a e dis inguished om PVs using he dis-
ance o he in e ac ion egion in he ans e se plane (Rxy).
To elimina e con ibu ions om in e ac ions wi h ma e ial, a
so-called ‘ma e ial e o’ emo es e ices in a egion de ined
as an en elope a ound he de ec o ma e ial [30]. E en s a e
selec ed when hey ha e a displaced e ex wi h a leas ou
acks, a sum o he scala pTo all acks ha is la ge han
3GeV/c, a dis ance Rxy la ge han 0.4 mm and an in a i-
an mass o he pa icles associa ed wi h his e ex m x
abo e 4.5 GeV/c2. To u he e ine he selec ion, e ices a e
equi ed o ha e ei he Rxy >2mmo m x >10 GeV/c2.
The second HLT2 signa u e is designed o iden i y wo-,
h ee- and ou -body exclusi e b-had on decays [31]. A mul-
i a ia e algo i hm is used o he iden i ica ion o seconda y
e ices consis en wi h he decay o a bhad on. The com-
bined e iciency o he wo HLT2 selec ions ela i e o e en s
accep ed by L0 and HLT1 is abou 60 %.
The o line candida e econs uc ion s a s om a gene ic
seconda y e ex sea ch, simila o ha applied in he igge ,
bu using acks om he o line econs uc ion as inpu . A
his s age a leas six acks pe e ex a e equi ed and he
sum o he scala pTo all acks mus be abo e 3 GeV/c. The
e ex is equi ed o ha e ei he Rxy >0.4 mm and m x >
9.7GeV/c2,o Rxy >2.5 mm and m x >8.5GeV/c2,o
Rxy >4 mm and m x >6.5GeV/c2.
The e ex econs uc ion is ollowed by a je econs uc-
ion p ocedu e. Inpu s o he je clus e ing a e ob ained using
a pa icle low app oach [32] ha selec s cha ged pa icles,
neu al calo ime e deposi s and a small con ibu ion om
K0
sand Λ0decays. To educe con amina ion om pa icles
ha do no o igina e om he displaced e ex, only cha ged
pa icles ha ha e a smalle dis ance o closes app oach el-
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Eu . Phys. J. C (2015) 75 :152 Page 3 o 12 152
a i e o he displaced e ex han o any PV in he e en a e
e ained. Fu he mo e, he dis ance o he displaced e ex is
equi ed o be less han 2 mm, which also allows acks om
displaced band c e ices in he π →b¯
bdecay chain o be
accep ed.
The je clus e ing uses he an i-kTalgo i hm [33] wi h a
cone size o 0.7. Only je s wi h a pTabo e 5 GeV/c a e used.
Addi ional equi emen s a e made o enhance he ac ion o
well- econs uc ed had onic je s: i s , he cha ged pa icle
wi h he la ges pTin he je mus ha e a pTabo e 0.9 GeV/c,
ye ca y no mo e han 70 % o he pTo he je . Second, o
emo e je s whose ene gy is domina ed by neu al pa icles,
which canno be unambiguously associa ed wi h a e ex, a
leas 10 % o he pTo he je mus be ca ied by cha ged
pa icles.
The di-je in a ian mass is compu ed om he econ-
s uc ed ou -momen a o he wo je s. Co ec ion ac o s o
he je ene gy a e de e mined om he simula ion and pa am-
e e ised as a unc ion o he numbe o econs uc ed PVs in
he e en , o accoun o e ec s due o mul iple in e ac ions
and he unde lying e en [32].
Two u he equi emen s a e made o enhance signal
pu i y. Fi s , a co ec ed mass is compu ed as
mco =m2+(psin θ)2+psin θ, (1)
whe e mis he di-je in a ian mass and θis he poin ing
angle be ween he di-je momen um ec o pand i s dis-
placemen ec o d=xDV −xPV, whe e xDV is he posi ion
o he displaced e ex and xPV he posi ion o he PV. To
selec candida es poin ing back o a PV, only e en s wi h
m/mco >0.7 a e e ained. A equi emen on his a io is
p e e ed o e a equi emen on he poin ing angle i sel ,
since i s e iciency depends less s ongly on he boos and
he mass o he candida e.
Second, a equi emen is made on he dis ance R=
φ2+η2be ween he wo je s, whe e φis he azimu hal
angle and η he pseudo apidi y. A backg ound consis ing o
back- o-back je candida es, o example di-je b¯
b-e en s,
appea s mainly a la ge alues o econs uc ed mass, and is
cha ac e ised by a la ge di e ence be ween he je s in bo h
φand η. Only candida es wi h R<2.2 a e accep ed.
Finally, in o de o acili a e a eliable es ima e o he ig-
ge e iciency, only candida es igge ed by pa icles belong-
ing o one o he je s a e kep . Table 1shows he e iciency
o selec a π pa icle, o an illus a i e mass o 35 GeV/c2
and li e ime o 10 ps, oge he wi h he yield in he da a a e
he mos impo an selec ion s eps. The o al e iciency o
o he masses and li e imes, as well as o he decays o ligh
qua k je s, is shown in Table 2. The e iciencies lis ed in
Tables 1and 2 ep esen he numbe o selec ed candida es
di ided by he numbe o gene a ed e en s. As he selec ion
e iciencies o he wo π pa icles in an e en a e p ac ically
Table 1 A e age numbe o selec ed candida es pe e en (e iciency)
in % o he main s ages o he o line selec ion o simula ed H0→
π π e en s wi h π →b¯
b,mH0=120 GeV/c2,mπ =35 GeV/c2
and τπ =10 ps. The p e-selec ion consis s o he accep ance, igge
and o line e ex econs uc ion. I ep esen s he i s s age in which
he candida e yield on he o al da a sample, shown in he igh column,
can be coun ed. The epo ed unce ain y on he e iciency is only he
s a is ical unce ain y om he ini e sample size
Selec ion s ep Signal e iciency Yield in da a
P e-selec ion 2.125 ±0.018 2,555,377
Je econs uc ion 1.207 ±0.014 117,054
m/mco and R0.873 ±0.012 58,163
T igge on candida e 0.778 ±0.012 29,921
Table 2 A e age numbe o selec ed candida es pe e en (e iciency)
in % o di e en π masses, li e imes and decay modes. The epo ed
unce ain y is only he s a is ical unce ain y om he ini e sample
size. No simula ed samples we e gene a ed o he 100 ps decay o ligh
qua ks
Decay mπ [GeV/c2] Signal e iciency
τπ =10 ps τπ =100 ps
π →b¯
b25 0.373 ±0.008 0.0805 ±0.0019
35 0.778 ±0.012 0.181 ±0.005
43 0.743 ±0.011 0.183 ±0.003
50 0.573 ±0.015 0.154 ±0.004
π →cc 35 2.18 ±0.05 –
π →ss 35 2.06 ±0.04 –
independen , he ac ion o selec ed e en s wi h mo e han
one candida e is less han a ew pe cen in simula ed signal.
In da a no e en s wi h mo e han one π candida e a e ound.
Figu e 1shows he mass and pTdis ibu ions o selec ed
di-je candida es in da a and in simula ed signal e en s,
assuming a π pa icle wi h a mass o 25, 35 o 50 GeV/c2.
The u n-on a low alues in he mass dis ibu ion o e en s
obse ed in da a (Fig. 1a) is caused by he minimum pT
equi emen on he je s. The es o he dis ibu ion alls o
exponen ially. The pTdis ibu ion shown in Fig. 1b illus-
a es ha long-li ed pa icles wi h a highe mass ha e lowe
pTas he e is less momen um a ailable in he Higgs decay.
This a ec s he selec ion e iciency since o a gi en decay
ime he ans e se decay leng h is p opo ional o pT.
S udies on simula ed e en s ha e shown ha bo h he
shape and he no malisa ion o he mass dis ibu ion in da a
a e compa ible wi h he expec ed backg ound om b¯
bp o-
duc ion. I is no possible o gene a e su icien ly la ge sam-
ples o b¯
be en s o use hese o a quan i a i e es ima e o
he backg ound a e he inal selec ion. The e o e, he signal
yield is ex ac ed by a i o he in a ian mass dis ibu ion
assuming a smoo h shape o he backg ound, as discussed
in Sec . 6.
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152 Page 4 o 12 Eu . Phys. J. C (2015) 75 :152
]
2
cmass [GeV/
0 20 40 60 80 100
)
2
cCandida es / (1 GeV/
0
1000
2000
3000
4000
5000
6000
Da a
2
c = 25 GeV/
π
m
2
c = 35 GeV/
π
m
2
c = 50 GeV/
π
m
LHCb
(a)
]c [GeV/
T
p
0 20406080100
)c
Candida es / (1 GeV/
0
500
1000
1500
2000
2500
3000
3500
4000
4500
Da a
2
c = 25 GeV/
π
m
2
c = 35 GeV/
π
m
2
c = 50 GeV/
π
m
LHCb
(b)
Fig. 1 In a ian mass (a)andpTdis ibu ion (b) o di-je candida es
in da a and in hidden alley models wi h 25, 35 and 50 GeV/c2π
masses and 10 ps li e ime. Fo isibili y, he simula ed signal is scaled
o 0.62 b−1assuming a Higgs c oss-sec ion o 10 nb and b anching
ac ions o 100 % o B(H→π π )and B(π →b¯
b)
[mm]
xy
R
02468
Candida es / (0.1 mm)
1
10
2
10
3
10
4
10
5
10
Da a
2
c = 35 GeV/
π
m
LHCb
Fig. 2 Dis ibu ion o he dis ance o he displaced e ex o he in e -
ac ion egion in he ans e se plane o da a and o a hidden alley
model wi h mπ =35 GeV/c2and τπ =10 ps a e he ull selec-
ion. Fo isibili y, he simula ed signal is scaled o 0.62 b−1assuming
a Higgs c oss-sec ion o 10 nb and b anching ac ions o 100 % o
B(H→π π )and B(π →b¯
b). The bounda ies o he in e als used
in he i a e indica ed by he do ed lines. The gene a ed Rxy dis ibu ion
is app oxima ely exponen ial wi h an a e age o abou 2 mm
Since he backg ound yield, he shape o he back-
g ound in a ian mass dis ibu ion and he selec ion e i-
ciency s ongly depend on he adial displacemen Rxy, limi s
a e ex ac ed om a simul aneous maximum likelihood i o
he di-je in a ian mass dis ibu ion in i e bins o Rxy.The
in e als a e chosen in he mos sensi i e egion, be ween
0.4 and 4.8 mm. The e en s a la ge adii a e no used as
hey con ibu e only ma ginally o he sensi i i y. Figu e 2
shows he dis ibu ion o Rxy o selec ed displaced e ices
o da a and simula ed signal e en s, oge he wi h he bin
bounda ies. The e ec o he educ ion in e iciency a la ge
adii due o he ma e ial e o and he HLT1 igge is isible,
as is he e ec o equi emen s on Rxy in he igge . The
igge e ec s a e mo e p onounced in da a han in simula ed
signal, because signal e en s a e less a ec ed by cu s on he
e ex in a ian mass.
The backg ound di-je in a ian mass dis ibu ion is cha -
ac e ised by an exponen ial allo , wi h a low-mass h esh-
old de e mined mos ly by he minimum pT equi emen o
he je s. I is modelled by a single-sided exponen ial unc ion
con olu ed wi h a bi u ca ed Gaussian unc ion. The pa ame-
e s o he backg ound model a e i ed o da a, independen ly
in each Rxy bin. The signal is modelled by a bi u ca ed Gaus-
sian unc ion, whose pa ame e s a e de e mined om simu-
la ed e en s in bins o Rxy. The e ec o he unce ain y on
he je -ene gy scale is included by a scale pa ame e o he
mass, which is common o all bins and cons ained using a
sample o Z+je e en s, as explained in Sec . 5. Addi ional
nuisance pa ame e s a e added o accoun o he ini e s a is-
ics o he simula ed samples and he sys ema ic unce ain ies
on he signal e iciency and he luminosi y. The i model is
implemen ed using he RooFi [34] package. Figu e 3shows
he i esul in he i e adial bins o a signal model wi h
mπ =35 GeV/c2and τπ =10 ps.
5 Sys ema ic unce ain ies
Se e al sou ces o sys ema ic unce ain ies ha e been con-
side ed. The unce ain ies depend on he π mass and a e
summa ised in Table 3. The unce ain y on he e ex ind-
ing e iciency is assessed by compa ing he e iciency o
he e exing algo i hm on a sample o B0→j/ψ K∗0
wi h K∗0→K+π−e en s in da a and simula ion as a
unc ion o Rxy. The e iciency di e ence is abou 7.5 %
a la ge Rxy, which is aken as an es ima e o he unce -
ain y on he e ex inding algo i hm e iciency. Since he
B0 e ices ha e only ou acks, and he π decays s ud-
ied in his pape ha e ypically mo e acks, his is consid-
e ed a conse a i e es ima e. The unce ain y on he ack
123
Eu . Phys. J. C (2015) 75 :152 Page 5 o 12 152
0 10 20 30 40 50 60 70 80
1
10
102
103
0 10 20 30 40 50 60 70 80
1
10
102
103
0 10 20 30 40 50 60 70 80
1
10
102
103
0 10 20 30 40 50 60 70 80
1
10
102
103
0 10 20 30 40 50 60 70 80
1
10
102
103
Fig. 3 Di-je in a ian mass dis ibu ions o each o he i e Rxy bins,
supe imposed wi h he i s o a hidden alley model wi h mπ =
35 GeV/c2and τπ =10 ps. The blue line indica es he esul o he o al
i o he da a. The black sho -dashed line is he backg ound-only con-
ibu ion, and he ed long-dashed line is he i ed signal con ibu ion.
Fo illus a ion, he g een dash-do ed line shows he signal scaled o a
c oss-sec ion o 17 pb, which co esponds o he SM Higgs p oduc ion
c oss-sec ion a 7 TeV [35]
inding e iciency o p omp acks in LHCb is 1.4 % pe
ack, wi h a small dependence on ack kinema ics [36]. The
unce ain y o displaced acks was e alua ed in he con ex
o a ecen LHCb measu emen o b-had on li e imes [37]
and ex apola ed o la ge Rxy, leading o a pe - ack unce -
ain y o 2 %. Due o equi emen s on he minimal num-
be o acks in he e ex, his ansla es in o an unce ain y
on he e ex inding e iciency, which is es ima ed o be
2 % o signal e en s. Adding in quad a u e he ack e i-
ciency and he e ex inding algo i hm e iciency unce ain-
ies leads o a o al unce ain y o 7.9 % on he e ex econ-
s uc ion. The selec ion on he e ex sum-pTand mass is
a ec ed by he ack inding e iciency as well. P opaga ing
he pe - ack unce ain y leads o an unce ain y on he e -
ex selec ion e iciency o up o 2.9 %, depending on he π
mass.
The unce ain ies ela ed o he je selec ion a e de e -
mined by compa ing je s in da a and simula ion on a sample
o Z+je e en s, analogously o a ecen LHCb measu emen
o Z+je p oduc ion [32]. The Zcandida e is econs uc ed
in he μ+μ− inal s a e om wo opposi ely cha ged acks,
iden i ied as muons, ha o m a good e ex and ha e an
in a ian mass in he ange 60–120 GeV/c2. Je s a e econ-
Table 3 Sys ema ic unce ain ies on he selec ion e iciency and lumi-
nosi y o simula ed hidden alley e en s wi h a li e ime o 10 ps and
a ious π masses
Sou ce Rela i e unce ain y (%)
π Mass [GeV/c2] 25354350
Ve ex econs uc ion 7.97.97.97.9
Ve ex scala -pTand mass 2.92.32.01.7
Je econs uc ion 1.30.60.40.3
Je iden i ica ion 2.93.03.23.2
Je poin ing 4.62.92.62.0
L0 igge 4.64.54.54.4
HLT1 igge 4.14.04.04.3
HLT2 igge 5.95.96.16.3
Luminosi y 1.71.71.71.7
To al 13.312.712.612.6
s uc ed using he same selec ion o inpu pa icles as in he
econs uc ion o je s o long-li ed pa icles, excep ha he
o igin e ex is in his case he PV consis en wi h he Z
e ex. The di e ences be ween da a and simula ion in he
Z+je sample a e pa ame e ised as unc ion o he je pT
123
152 Page 6 o 12 Eu . Phys. J. C (2015) 75 :152
and subsequen ly p opaga ed o he simula ed hidden alley
signal samples.
The unce ain y on he je ene gy scale is de i ed om
he a io o ans e se momen a o he je and he Z, which
a e expec ed o ha e a back- o-back opology, and co e-
la ed ans e se momen a. Da a and simula ion ag ee wi hin
abou 2 %, esul ing in an unce ain y on he di-je in a i-
an mass scale o 4 %. This unce ain y on he signal shape
is aken in o accoun in he i ing p ocedu e. The unce -
ain y on he je -ene gy scale also a ec s he je econs uc-
ion e iciency due o he equi emen on he minimum je
pT. I leads o an unce ain y on he e iciency be ween 0.3
and 1.3 %, depending on he assumed π pa icle mass. The
unce ain y on he had onic je iden i ica ion equi emen s
a e assessed using he Z+je sample as well and amoun o
abou 3 %.
The esolu ions on he poin ing angle θand on Ra e
domina ed by he esolu ion on he di ec ion o he π candi-
da e, which in u n is de e mined by he je angula esolu ion.
The la e is es ima ed om he di e ence be ween da a and
simula ion in he esolu ion o he azimu hal angle be ween
he je and he Z. Due o he limi ed s a is ics in he Z+je
sample a ela i ely la ge unce ain y be ween 2.0 and 4.6 %
is ob ained, depending on he π mass.
The igge selec ion e iciency on signal is de e mined
om he simula ion. The igge e iciencies in da a and sim-
ula ion a e compa ed using a sample o gene ic B→J/ψ X
e en s ha con ain an o line econs uc ed displaced e ex,
bu a e igge ed independen ly o he displaced e ex ig-
ge lines. The in eg a ed e iciency di e ence o he igge
s ages L0, HLT1 and HLT2 amoun s o sys ema ic unce ain-
ies o a mos 4.6, 4.3 and 6.3 % espec i ely. This is a con-
se a i e es ima e since he igge e iciencies o he sample
o displaced J/ψ e ices a e smalle han he e iciencies o
he signal, which consis s o hea ie , mo e displaced objec s
wi h a la ge numbe o acks. Finally, he unce ain y on he
luminosi y a he LHCb in e ac ion poin is 1.7 % [38].
Se e al al e na i es ha e been conside ed o he back-
g ound mass model, in pa icula wi h an addi ional expo-
nen ial componen , o a componen ha is independen o
he mass. Wi h hese models he es ima ed backg ound yield
a highe mass is la ge han wi h he nominal backg ound
model, leading o igh e limi s on he signal. As he nom-
inal model gi es he mos conse a i e limi , no addi ional
sys ema ic unce ain y is assigned.
6 Resul s
The i p ocedu e is pe o med o a π mass o 25, 35, 43 and
50 GeV/c2and o se e al alues o he li e ime in be ween
1 and 200 ps. No signi ican signal is obse ed o any com-
bina ion o π mass and li e ime. Uppe limi s a e ex ac ed
using he CLsme hod [39] wi h a equen is ea men o he
nuisance pa ame e s desc ibed abo e, as implemen ed in he
RooS a s [40] package.
Limi s a e se on he Higgs p oduc ion c oss-sec ion mul-
iplied by he b anching ac ion in o long-li ed pa icles
σ(H)×B(H→π π ). In he simula ion i is assumed ha
bo h π pa icles decay o he same inal s a e. I he decay
wid h o he π pa icle is domina ed by inal s a es o he han
q¯q, he limi s scale as 1/(Bq¯q(2−Bq¯q)) whe e Bq¯qis he
π →q¯qb anching ac ion. The ob ained 95 % CL uppe
limi s on σ(H)×B(H→π π ), unde he assump ion o a
100 % b anching ac ion o b¯
b, a e shown in Table 4and in
Fig. 4. As he backg ound dec eases wi h he obse ed di-je
in a ian mass, he limi s become s onge wi h inc easing
π mass. The sensi i i y has an op imal alue a a li e ime o
abou 5 ps.
Addi ional limi s a e se on models wi h a π pa icle
decaying o c¯cand o s¯s. The limi s o π decay o u¯u
and d¯
da e expec ed o be he same as o s¯s. The ligh
qua k decays esul in a highe displaced e ex ack mul-
iplici y, and ligh e je s, leading o a highe selec ion e i-
ciency. Consequen ly, he limi s o decays o ligh qua k
je s a e mo e s ingen han hose o decays o b-qua k
je s.
Table 4 Obse ed 95 % CL c oss-sec ion uppe limi s on σ(H)×B(H→π π )(in pb) on a hidden alley [6–8] model o a ious π masses
and li e imes. Bo h π pa icles a e assumed o decay in o b¯
b, unless speci ied o he wise
π Mass [GeV/c2]π Li e ime [ps ]
1 2 5 10 20 50 100 200
25 106.354.643.854.280.0 164.1 285.7 588.5
35 19.010.48.08.913.325.446.589.8
43 10.55.64.44.76.712.422.742.8
50 10.65.13.73.84.89.316.229.3
35 (π →c¯c)3.72.42.12.43.46.712.524.1
35 (π →s¯s)3.42.11.92.23.36.411.622.0
123
Eu . Phys. J. C (2015) 75 :152 Page 7 o 12 152
1 10 102
1
10
102
103
104
Fig. 4 Obse ed 95 % CL c oss-sec ion uppe limi s on a hidden alley
model [6–8] o a ious π masses, as a unc ion o π li e ime. Bo h
π pa icles a e assumed o decay in o b¯
b, unless speci ied o he wise
7 Conclusion
A sea ch has been p esen ed o massi e, long-li ed pa icles
in a sample o pp collisions a √s=7 TeV, co esponding o
an in eg a ed luminosi y o 0.62 b−1, collec ed by he LHCb
expe imen . The long-li ed spin-ze o pa icles a e assumed
o be pai -p oduced in he decay o a 120 GeV/c2SM Higgs,
and o decay o wo had onic je s. They appea o ins ance
as π pa icles in hidden alley models. A single π pa icle
is iden i ied by a displaced e ex and wo associa ed je s.
No signi ican signal o π pa icles wi h a mass be ween
25 and 50 GeV/c2and a li e ime be ween 1 and 200 ps is
obse ed. Assuming a 100% b anching ac ion o b-qua k
je s, he 95 % CL uppe limi s on he p oduc ion c oss-sec ion
σ(H)×B(H→π π )a e in he ange 4–600 pb.
The esul s co e a egion in mass and li e ime ha so a
has been unexplo ed a he LHC. The ob ained uppe limi s
a e mo e es ic i e han esul s om he Te a on expe i-
men s in he same mass and li e ime egion. The bes sensi-
i i y is ob ained o π pa icles wi h a li e ime o abou 5 ps
and a mass abo e app oxima ely 40 GeV/c2. The SM Higgs
c oss-sec ion a 7 TeV is abou 17 pb [35]. The measu emen s
in he mos sensi i e egion exclude b anching ac ions o
g ea e han 25 % o a SM Higgs boson o pai p oduce π
pa icles ha decay o wo had onic je s.
Acknowledgmen s We exp ess ou g a i ude o ou colleagues in he
CERN accele a o depa men s o he excellen pe o mance o he
LHC. We hank he echnical and adminis a i e s a a he LHCb
ins i u es. We acknowledge suppo om CERN and om he na ional
agencies: CAPES, CNPq, FAPERJ and FINEP (B azil); NSFC (China);
CNRS/IN2P3 (F ance); BMBF, DFG, HGF and MPG (Ge many); INFN
(I aly); FOM and NWO (The Ne he lands); MNiSW and NCN (Poland);
MEN/IFA (Romania); MinES and FANO (Russia); MinECo (Spain);
SNSF and SER (Swi ze land); NASU (Uk aine); STFC (Uni ed King-
dom); NSF (USA). The Tie 1 compu ing cen es a e suppo ed by IN2P3
(F ance), KIT and BMBF (Ge many), INFN (I aly), NWO and SURF
(The Ne he lands), PIC (Spain), G idPP (Uni ed Kingdom). We a e
indeb ed o he communi ies behind he mul iple open sou ce so wa e
packages on which we depend. We a e also hank ul o he compu ing
esou ces and he access o so wa e R&D ools p o ided by Yandex
LLC (Russia). Indi idual g oups o membe s ha e ecei ed suppo
om EPLANET, Ma ie Skłodowska-Cu ie Ac ions and ERC (Eu o-
pean Union), Conseil géné al de Hau e-Sa oie, Labex ENIGMASS
and OCEVU, Région Au e gne (F ance), RFBR (Russia), Xun aGal
and GENCAT (Spain), Royal Socie y and Royal Commission o he
Exhibi ion o 1851 (Uni ed Kingdom).
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E. Ge sabeck11, M. Ge sabeck54, T. Ge shon48, Ph. Ghez4, A. Gianelle22, S. Gianì39,V.Gibson
47, L. Giubega29,
V. V. Gl igo o 38, C. Göbel60, D. Golubko 31, A. Golu in31,38,53, A. Gomes1,a, C. Go i20,k, M. G abalosa Gánda a5,
R. G aciani Diaz36, L. A. G anado Ca doso38, E. G augés36, E. G a e ini40, G. G aziani17, A. G ecu29, E. G eening55,
S. G egson47,P.G i i h
45, L. G illo11, O. G ünbe g63,B.Gui
59, E. Gushchin33,Yu.Guz
35,38,T.Gys
38, C. Hadji asiliou59,
G. Hae eli39, C. Haen38, S. C. Haines47,S.Hall
53, B. Hamil on58, T. Hampson46,X.Han
11, S. Hansmann-Menzeme 11,
N. Ha new55, S. T. Ha new46,J.Ha ison
54,J.He
38, T. Head39, V. Heijne41, K. Hennessy52, P. Hen a d5, L. Hen y8,
J. A. He nando Mo a a37, E. an He wijnen38,M.Heß
63, A. Hicheu 2, D. Hill55, M. Hoballah5, C. Hombach54,
W. Hulsbe gen41, N. Hussain55, D. Hu chc o 52, D. Hynds51, M. Idzik27, P. Il en56, R. Jacobsson38, A. Jaege 11,
J. Jalocha55, E. Jans41, A. Jawahe y58,F.Jing
3, M. John55, D. Johnson38, C. R. Jones47, C. Jo am38,B.Jos
38,
N. Ju ik59, S. Kandybei43, W. Kanso6, M. Ka acson38, T. M. Ka bach38, S. Ka odia51,M.Kelsey
59, I. R. Kenyon45,
T. Ke el42, B. Khanji20,38,k, C. Khu ewa hanakul39,S.Kla e
54, K. Klimaszewski28, O. Kochebina7, M. Kolpin11,
I. Koma o 39, R. F. Koopman42, P. Koppenbu g41,38, M. Ko ole 32, L. K a chuk33, K. K eplin11, M. K eps48, G. K ocke 11,
P. K oko ny34,F.K use
9, W. Kucewicz26,o, M. Kucha czyk20,26,k, V. Kud ya se 34, K. Ku ek28, T. K a a skheliya31,
V. N. La Thi39, D. Laca e e38, G. La e y54,A.Lai
15, D. Lambe 50, R. W. Lambe 42, G. Lan anchi18, C. Langenb uch48,
B. Langhans38, T. La ham48, C. Lazze oni45,R.LeGac
6, J. an Lee dam41, J.-P. Lees4,R.Le è e
5, A. Le la 32,
J. Le ançois7,O.Le oy
6,T.Lesiak
26, B. Le e ing on11,Y.Li
3, T. Likhomanenko64, M. Liles52, R. Lindne 38,
C. Linn38, F. Lione o40,B.Liu
15, S. Lohn38, I. Longs a 51, J. H. Lopes2, P. Lowdon40, D. Lucchesi22, ,H.Luo
50,
A. Lupa o22, E. Luppi16, , O. Lup on55, F. Mache e 7, I. V. Machikhiliyan31, F. Maciuc29,O.Mae
30, S. Malde55,
A. Malinin64, G. Manca15,e, G. Mancinelli6, A. Mapelli38, J. Ma a as5, J.F. Ma chand4, U. Ma coni14, C. Ma in Beni o36,
P. Ma ino23, , R. Mä ki39, J. Ma ks11, G. Ma ello i25, M. Ma inelli39, D. Ma inez San os42, F. Ma inez Vidal65,
D. Ma ins Tos es2, A. Massa e i1,R.Ma e
38, Z. Ma he38, C. Ma euzzi20, A. Mazu o 45, M. McCann53, J. McCa hy45,
A. McNab54, R. McNul y12, B. McSkelly52, B. Meadows57, F. Meie 9, M. Meissne 11,M.Me k
41, D. A. Milanes62,
M.-N. Mina d4, N. Moggi14, J. Molina Rod iguez60, S. Mon eil5, M. Mo andin22,P.Mo awski
27, A. Mo dà6,
M. J. Mo ello23, , J. Mo on27, A.-B. Mo is50, R. Moun ain59, F. Muheim50, K. Mülle 40, M. Mussini14,B.Mus e
39,
P. Naik46, T. Nakada39, R. Nandakuma 49, I. Nas e a2, M. Needham50,N.Ne i
21, S. Neube 38, N. Neu eld38,
M. Neune 11, A. D. Nguyen39, T. D. Nguyen39, C. Nguyen-Mau39,q, M. Nicol7,V.Niess
5,R.Nie
9, N. Niki in32,
T. Nikodem11, A. No oselo 35, D. P. O’Hanlon48, A. Oblakowska-Mucha27, V. Ob az so 35, S. Ogil y51, O. Okh imenko44,
R. Oldeman15,e, C. J. G. Onde wa e 66, M. O landea29, B. Oso io Rod igues1, J. M. O alo a Goicochea2, A. O o38,
P. Owen53, A. Oyangu en65,B.K.Pal
59, A. Palano13,c, F. Palombo21,u, M. Palu an18, J. Panman38, A. Papanes is38,49,
M. Pappagallo51, L. L. Pappala do16, ,C.Pa kes
54, C. J. Pa kinson9,45, G. Passale a17, G.D.Pa el
52, M. Pa el53,
C. Pa ignani19,j, A. Pea ce54,49, A. Pelleg ino41, G. Penso25,m, M. Pepe Al a elli38, S. Pe azzini14,d, P. Pe e 5,
L. Pesca o e45, E. Pesen67, K. Pe idis53, A. Pe olini19,j, E. Pica os e Olloqui36, B. Pie zyk4, T. Pilaˇ 48, D. Pinci25,
A. Pis one19, S. Play e 50, M. Plo Casasus37, F. Polci8, A. Poluek o 34,48, I. Polyako 31, E. Polyca po2, A. Popo 35,
D. Popo 10, B. Popo ici29, C. Po e a 2,E.P ice
46,J.D.P ice
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A. Puig Na a o39, G. Punzi23,s,W.Qian
4, B. Rachwal26, J. H. Rademacke 46, B. Rako omia amanana39,M.Rama
23,
M. S. Rangel2, I. Raniuk43, N. Rauschmay 38,G.Ra en
42, F. Redi53, S. Reiche 54, M.M.Reid
48, A. C. dos Reis1,
S. Riccia di49, S. Richa ds46,M.Rihl
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P. Rod iguez Pe ez54,S.Roise
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38,
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C. Sanchez Mayo domo65, B. Sanma in Sedes37, R. San acesa ia25, C. San ama ina Rios37, E. San o e i24,l, A. Sa i18,m,
C. Sa iano25,n, A. Sa a24, D.M. Saunde s46,D.Sa ina
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B. Schmid 38, O. Schneide 39, A. Schoppe 38, M.-H. Schune7, R. Schwemme 38, B. Sciascia18, A. Sciubba25,m,
A. Semenniko 31, I. Sepp53, N. Se a40, J. Se ano6, L. Ses ini22,P.Sey e
11, M. Shapkin35, I. Shapo al16,43, ,
Y. Shcheglo 30, T. Shea s52, L. Shekh man34, V. She chenko64, A. Shi es9, R. Sil a Cou inho48,G.Simi
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N. Skidmo e46, I. Skillico n51, T. Skwa nicki59, N.A.Smi h
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H. Snoek41, M. D. Sokolo 57, F. J. P. Sole 51, F. Soom o39, D. Souza46, B. Souza De Paula2, B. Spaan9, P. Sp adlin51,
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