1. Ivan Belyaev LAPP/Annecy & ITEP/Moscow
Rare decays at LHCb
Ivan Belyaev
LAPP/Annecy & ITEP/Moscow

2. Loop induced rare decays
Outline
Loop-induced rare decays
Event Simulation
Event selection at LHCb
Annual Event yields
Background estimates
Summary
Loop induced rare decays
Radiative penguins
B → K*0g, Bs → fg, B → wg
EW-penguins
B → K*0m+m-
Gluonic penguins
Bs → ff , B → f KS
“Very rare”
B → m+m-
LHCb detector and its status is presented in detail in plenary talk by T.Nakada
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

3. Rare (=“loop-induced”) decays
Loop-induced decays are the perfect place to search for New Physics hints
SM model loops are suppressed
GIM cancellation
“rare decays”
Penguins
b →s(d) g,Z0,g
Boxes
Heavy particles are suppressed in trees
could appear in the loops
New particles in loops:
Enhancement in decay rates
New phases
New asymmetries
… ?
Ideal laboratory for New Physics search
But also some QCD tests
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

4. Radiative penguin decays
No so rare decays
PDG
Br(B→K*0g ) = (4.3±0.4)x10-5
Br(B-→K*-g ) =(3.8±0.5)x10-5
Isotopic asymmetries
~C6+C5/NC
~O(1%)
Suppressed by : as , 1/mb or |VCKM|
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

5. b→s(d)g : CP-asymmetries
1-amplitude dominance
strong phase appears at order of as or 1/mb
→“Direct” asymmetries are small (≤1%)
Bs → fg :
not CP-eigenstate!
V-A: g is circular polarized
“Wrong polarization”: ~ms(md)/mb
Both Amix and Adir are small
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

6. g Event Simulation p(K) K B(Bs)
PYTHIA as pp-event generator as √s= 14 TeV
QQ for weak-decays
GEANT 3.21
Realistic geometry & material description
The pile-up is included
“Realistic” digitization, reconstruction algorithms & L0/L1 trigger simulation
Background: “forward” bb̄-production in 400mrad cone
107 available events g
p(K) K
B(Bs)
[PV] PV z
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

7. Background suppression
Beauty particles:
mb ~ 5 GeV/c2
bgct ~O(1cm)
Particles from B-decays:
Large pT
L0 (hardware) trigger:
leptons (e±,m±,mm),
photons
hadrons
Large impact parameters
L1 (software) trigger
Background:
bb̄-production with at least one B within 400mrad cone
High Level Trigger and Off-line background suppression continues to utilize these properties
B-decay products do not point to reconstructed primary vertices
Exclusively reconstructed B-candidate does point to primary vertex
B-candidate is associated with primary vertex with minimal impact parameter (significance)
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

8. Selection of Bd→ K*0g and Bs →fg
p±,K± :
charged tracks consistent with PID
Inconsistent with any PV
c2IP > 16(4)
Two prong vertex
c2VX < 49
K*0 :
|DM| < 60 MeV/c2
f :
|DM| < 10 MeV/c2
g :
clusters in Ecal not associated with any reconstructed track
ET > 2.8 GeV
2.2(2.0) < ET* < 2.7 GeV
Bs →fg
bb̄
K*0 f
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

9. Selection of Bd→ K*0g and Bs →fg (II)
|qB| < 6 (15) mrad
Correlated feeddown with merged p0, wrongly reconstructed as single photon
B →K*0 p0 , Bs →fp0
opposite K*0(f) polarization
|cos q| < 0.75
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

10. Bd→ K*0g Bs →fg (III) B-mass window is defined as ±200 MeV/c2
s(MB) = 65 MeV/c2
The correlated feeddown is well under the control
Annual yield ( using 1012 bb̄ events/107 second)
Bd→ K*0g
Bs →fg
eREC [%]
4.5
4.3
eTRIG/REC [%] 19
eSEL/TRIG [%] 18 27
eTOT [%]
0.16
0.22
Bd→ K*0g
Bs →fg
N/year
35k
9.3k
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

11. Background
Background estimation is limited by the size of available sample of 107 forward bb̄ events and 3x107 minimum bias events
No background events are found in “wide” mass interval GeV/c2
only 90%CL upper limits can be set now from bb̄-background
We consider now forward bb̄ production as a major source of background
large pT, large impact parameters, secondary verticies, …
(This assumption need to be properly validated and proved)
Bd→ K*0g
Bs →fg
B/S
<0.7
<2.4
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

12. First look at Bd→ wg 40 b→dg transition
|Vtd| can be extracted without large theoretical uncertanty
also for large Dms
Br (B  K*g) /Br (B  wg ) ~ 65
reconstruction efficiency is low:
p0 need to be reconstructed
Background condition is difficult
3 neutral particles in final state
s = (63 ± 9) MeV/c2
eTOT [%]
N/year
0.012 40
B / S < 90 % CL
Br ( B0  w g ) = 0.5 × 10-6
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

13. EW penguins: Bd →K*0m+m-
Combination of b→sZ, b→sg penguins with the box diagram
Both G and dG/ds is very sensitive to New Physics as well as the forward-Backward AFB(s) asymmetry
q is angle between m+ and K*0 in dimuon restframe
dG/ds
AFB(s)
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

14. Selection of Bd→ K*0mm m±: charged tracks consistent with PID
pT > 500 MeV/c2
Two prong mm-vertex
c2VX < 8
J/y,y(2S) veto:
, GeV/c2
K,p
pT(p) > 200 MeV/c2
K*0
pT > 900 MeV/c2
|DM|<100 MeV/c2
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

15. Efficiencies, Event yields and B/S
eTOT = 0.7%, eTRIG = 74%
Annual yield : 4400 events
B/S for forward bb̄ events
[ ] at 90% CL
Various b→mX, mmX, J/yX channels were studied as sources of potential feeddown
B/S at90%CL
forward bb̄
[0.2, 2.0]
b→m(c→mX)X
<1.1
b→mX + c.c
0.5±0.2
B→J/yK*
<0.04
B→J/yKS
Bs→J/yf
<0.05
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

16. Gluonic penguins: Bd→ fKS Bs→ ff
SM: Channels with domination of 1-gluonic penguin amplitude
The contributions from EW-penguin amplitudes O(10%)
CP-violation for Bd→ fKS
ACP(Bd→ fKS ) = ACP(Bd→ J/yKS )
The accuracy: O(5%) →30%
Last summer Belle reports the value ACP(Bd→ fKS ) inconsistent with ACP(Bd→ J/yKS ) = - sin(2b)
Hints for New Physics in b→sg transitions ? or
The probe for FSI ?
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

17. Selection of Bd→ fKS and Bs →ff
charged tracks consistent with PID
Inconsistent with any PV
c2IP > 4
Two prong vertex
c2VX < 10(100)
f :
|DM| < 17(12) MeV/c2
Bs →ff
c2VX < 100
qB < 10 mrad
Decay angle: |cos q| < 0.75
|DM| < 24 MeV/c2
f →KK
Bs →ff
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

18. Selection of KS for Bd→ fKS
Secondary vertex from p+p- pair consistent with PID
c2VX < 20
Different track categories:
With and without track fragments measured in precise silicon vertex detector
|DM| < 15(25) MeV/c2
no ‘long’ tracks
KS →p+p-
2 ‘long’ tracks
1 ‘long’ track
1 ‘long’ track
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

19. Selection of Bd→ fKS B : Impact parameter to the primary vertex
IP < (250,200,100)mm
pT(KS) > 1100(500) MeV/c2
pT(f) > 1350 MeV/c2
qB < 10 mrad
|DM| < 55 MeV/c2
s(MB) = 16 MeV/c2
KS →p+p-
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

20. Efficiencies, Event Yields and B/S
Enlarged mass window
Bs →ff
4-7 GeV/c2
Bd→ fKS
4-6.6 GeV/c2
No background events from 107 forward bb̄ events
1 event from 106 b→fX sample (for Bd→ fKS)
effectively x3.5 statistics as forward bb̄
Bd→ fKS
Bs →ff
eTRIG[%] 19 23
eTOT[%]
0.074
0.45
N/year
800
1200
B/S
<1.1 (bb̄)
<0.3 (b→fX)
<0.2
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

21. Real rear decay: Bs →m+m-
SM: Br ~ 3x10-9
Many New Physics models predict enhancement m±
Compatible with m PID
pT > 1.3 GeV/c mm
c2VX < 4
DZ/sZ > 29
pT > 3 GeV/c
|DM| < 600 MeV/c2
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

22. Efficiencies, Event Yield and B/S
eTOT = 2.5%, eTRIG = 80%
N/year = 17 events
No background events neither from 107 forward bb̄ sample no from 107 b→mX,b̄→mX sample
5 years at LHCb
B/S at 90%
Forward bb̄
<440
b→mX,b̄→mX
<6
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna

23. Summary LHCb has a good physics potential for study of rare decays
N/year
B/S @90%CL
Bd → K*0g
35k
<0.7
Bs → fg
9.3k
<2.4
Bd → K*0m+m-
4.4k
[0.2, 2.0]
Bs → fKS
800
<1.1 (bb̄)
<0.3 (b→fX)
Bs → ff
1.2k
<0.2
Bs → mm 17
<440 (bb̄)
<6 (b→mX)
I.Belyaev "Rare decays at LHCb"
k+4 “Physics at LHC”, Vienna