Performance Studies for
the LHCb Experiment
Marcel Merk
NIKHEF
Representing the LHCb collaboration
19 th International Workshop
on Weak Interactions and Neutrinos
Oct 6-11, Geneva, Wisconsin, USA
1
B Physics in 2007

Direct Measurement of angles:



Knowledge of the sides of unitary
triangle:
(Dominated by theoretical
uncertainties)




s(sin(2b)) ≈ 0.03 from J/y Ks in B
factories
Other angles not precisely known
s(|Vcb|) ≈ few % error
s(|Vub|) ≈ 5-10 % error
s(|Vtd|/|Vts|) ≈ 5-10% error
(assuming Dms < 40 ps-1)
In case new physics is present in
mixing, independent measurement of g
can reveal it…
2
B Physics @ LHC
√s
14 TeV
bb production:
(forward)
L (cm-2 s-2) 2x1032 cm-2 s-1
sbb
500 mb
sinel / sbb
160
qb
Large bottom production cross section:
1012 bb/year at 2x1032 cm-2s-1
 Triggering is an issue
 All b hadrons are produced:
Bu (40%), Bd(40%), Bs(10%), Bc and b-baryons (10%)
 Many tracks available for primary vertex
 Many particles not associated to b hadrons
 b hadrons are not coherent: mixing dilutes tagging

qb
B Decay eg.: Bs->Dsh
Bs
Ds
,K
K
K

LHCb: Forward Spectrometer with:
• Efficient trigger and selection of many
B decay final states
• Good tracking and Particle ID performance
• Excellent momentum and vertex resolution
• Adequate flavour tagging
3
Simulation and Reconstruction
No true MC info
used anywhere !
All trigger, reconstruction and selection studies are based on full
Pythia+GEANT simulations including LHC “pile-up” events and
full pattern recognition (tracking, RICH, etc…)
T1
T2
T3
TT
RICH1
VELO
Sensitivity studies are based on fast simulations using
efficiencies and resolutions and from the full simulation
4
Evolution since Technical Proposal
• Reduced
material
• Improved
level-1 trigger
5
Track finding strategy
T track
Upstream track
VELO
seeds
VELO track
Long track (forward)
Long track (matched)
T seeds
Downstream track
Long tracks
 highest quality for physics (good IP & p resolution)
Downstream tracks  needed for efficient KS finding (good p resolution)
Upstream tracks
 lower p, worse p resolution, but useful for RICH1 pattern recognition
T tracks
 useful for RICH2 pattern recognition
VELO tracks
 useful for primary vertex reconstruction (good IP resolution)
6
On average:
26 long tracks
11 upstream tracks
4 downstream tracks
5 T tracks
26 VELO tracks
Result of track finding
Typical event display:
Red = measurements (hits)
Blue = all reconstructed tracks
T1 T2
T3
TT
VELO
2050 hits assigned to a long track:
98.7% correctly assigned
Efficiency vs p :
Ghost rate vs pT :
Ghost rate = 3%
(for pT > 0.5 GeV)
Eff = 94%
(p > 10 GeV)
Ghosts:
Negligible effect on
b decay reconstruction
7
Experimental Resolution
Momentum resolution
Impact parameter resolution
sIP=
14m + 35 m/pT
dp/p =
0.35% – 0.55%
p spectrum B tracks
1/pT spectrum B tracks
8
RICH 1
Particle ID
RICH 2
e (K->K) = 88%
e (p->K) = 3%
Example:
B->hh decays:
9
m, e, h, g
Calorimeter
Muon system
Pile-up system
1 MHz
Vertex Locator
Trigger Tracker
Level 0 objects
40 kHz
HLT:
Final state
reconstruction
200 Hz output
L1
B->
ln IP/sIP
Level-1:
Impact parameter
Rough pT ~ 20%
Bs->DsK
ln IP/sIP
Level-0:
pT of
Trigger
pile-up
40 MHz
L0
Full detector
information
Signal
Min.
Bias
ln pT
ln pT
10
Flavour tag
Knowledge of flavour at birth is essential
for the majority of CP measurements






B0
tagging strategy:
opposite side lepton tag ( b → l )
opposite side kaon tag ( b → c → s )
(RICH, hadron trigger)
same side kaon tag (for Bs)
opposite B vertex charge tagging
B0
D
K-
l
b
b
sources for wrong tags:
Bd-Bd mixing (opposite side)
b → c → l (lepton tag)
conversions…
Combining tags
effective efficiency:
eeff = etag (1-2wtag
)2
Bd   
Bs  K K
tag
Bs 0
s
s
+
u K
u
Wtag
[%]
eff [%]
42
35
4
50
33
6
[%]
11
Efficiencies, event yields and Bbb/S ratios
Det.
eff.
(%)
B0   
12.2
Bs  K K
12.0
Bs  Ds 
5.4
Bs  Ds+ K
5.4
B0  D~0 (K)K*0
5.3
B0  J/y(mm) K0S
6.5
B0  J/y(ee) K0S
5.8
Bs  J/y(mm) 
7.6
Bs  J/y(ee) 
6.7
B0   
6.0
B0  K* g
9.5
Bs   g
9.7
+ few more channe ls in T DR
Rec.
eff.
(%)
91.6
92.5
80.6
82.0
81.8
66.5
60.8
82.5
76.5
65.5
86.8
86.3
Sel.
eff.
(%)
18.3
28.6
25.0
20.6
22.9
53.5
17.7
41.6
22.0
2.0
5.0
7.6
Trig.
eff.
(%)
33.6
36.7
31.1
29.5
35.4
60.5
26.5
64.0
28.0
36.0
37.8
34.3
Tot.
eff.
(%)
0.69
0.99
0.34
0.27
0.35
1.39
0.16
1.67
0.32
0.03
0.16
0.22
Vis. Annual
BR
signal
6
(10 )
yield
4.8
26k
18.5
37k
120.
80k
10.
5.4k
1.2
3.4k
20.
216k
20.
26k
31.
100k
31.
20k
20.
4.4k
29.
35k
21.
9.3k
B/S
from
bb bkg.
< 0.7
0.3
0.3
< 1.0
< 0.5
0.8
1.0
< 0.3
0.7
< 7.1
< 0.7
< 2.4
Nominal year = 1012 bb pairs produced (107 s at L=21032 cm2s1 with sbb=500 mb)
Yields include factor 2 from CP-conjugated decays
Branching ratios from PDG or SM predictions
12
CP Sensitivity studies
CP asymmetries due to interference of Tree, Mixing, Penguin, New Physics amplitudes:
tree
+
mix
+
pe
+ new
n
Measurements of Angle g:
Mixing phases:

Time dependent asymmetry in
Bd->J/y Ks decays


Sensitive to d
Time dependent asymmetry in
Bs->J/y  decays

Sensitive to s
1. Time dependent asymmetries in Bs->DsK
decays. Interference between b->u and b->c
tree diagrams due to Bs mixing

Sensitive to g
+ s
(Aleksan et al)
2. Time dependent asymmetries in B-> and
Bs->KK decays. Interference between b->u
tree and b->d(s) penguin diagrams

Sensitive to g, d, s (Fleischer)
3. Time Integrated asymmetries in B-> DK*
decays. Interference between b->u and b->c
tree diagrams due to D-D mixing

Sensitive to g
(Gronau-Wyler-Dunietz)
13
Bs oscillation frequency: Dms



Needed for the
observation of CP
asymmetries with
Bs decays
Use Bs Ds
If Dms= 20 ps1
Expected unmixed Bs Ds
sample in one year of data
taking.
s(Dms) = 0.011 ps1
Full MC

Can observe >5s
oscillation signal
if Dm < 68 ps1
s
well beyond SM
prediction
Proper-time resolution
plays a crucial role
14
Mixing Phases

Bd mixing phase
using B->J/y Ks
Background-subtracted
BJ/y(mm)KS CP asymmetry
after one year

Bs mixing phase
using Bs->J/y 
Angular analysis to separate
CP even and CP odd
Time resolution is important:
st = 38 fs
Proper time resolution (ps)
s(sin(d)) = 0.022
If Dms= 20 ps1:
NB: In the SM,
s = 2 ~ 0.04
s(DGs/Gs) = 0.018
s(sin(s)) = 0.058
15
1. Angle g from BsDsK
(2 Tree diagrams due to Bs mixing)
Simultaneous fit of Bs->Ds and Bs->DsK:
 Determination of mistag fraction
 Time dependence of background
Time dependent asymmetries:
Bs(Bs) ->Ds-K+: → DT1/T2 + (g+s)
Bs(Bs) ->Ds+K-: → DT1/T2 – (g+s)
ADs-K+
After one year, if Dms= 20 ps1,
DGs/Gs = 0.1, 55 < g < 105 deg,
20 < DT1/T2 < 20 deg:
s(g) = 1415 deg
ADs+K-
No theoretical uncertainty;
insensitive to new physics
in B mixing
(after 5 years of data)
16
2. Angle g from B and BsKK
(b->u processes, with large b->d(s) penguin contributions)

Measure time-dependent CP asymmetries in B
and BsKK decays:
ACP(t)=Adir cos(Dm t) + Amix sin(Dm t)

Method proposed by R. Fleischer:
SM predictions:
d exp(i) = function of tree and
Adir (B0 ) = f1(d, , g)
penguin amplitudes
0


Amix(B    ) = f2(d, , g, d)
in B0 
d’ exp(i’) = function of tree and
Adir (BsKK ) = f3(d’, ’, g)
penguin amplitudes


Amix(BsK K ) = f4(d’, ’, g, s)
in Bs  KK
 Assuming U-spin flavour symmetry
(interchange of d and s quarks):
d = d’ and  = ’
 4 measurements (CP asymmetries) and
3 unknown (g, d and )  can solve for g

17
2. Angle g from B and BsKK (cont.)


Extract mistags from BK and BsK
Use expected LHCb precision on d and s
blue bands from BsKK (95%CL)
red bands from B (95%CL)
ellipses are 68% and 95% CL regions
(for ginput = 65 deg)
pdf for g
If Dms= 20 ps1, DGs/Gs=0.1,
d =0.3, = 160 deg,
55 < g < 105 deg:
“fake”
solution
pdf for d
s(g) = 46 deg
U-spin symmetry assumed;
sensitive to new physics in
penguins
d vs g
18
3. Angle g from B DK* and B DK*
(Interference between 2 tree diagrams due to D0 mixing)

Application of Gronau-Wyler method to DK* (Dunietz):
√2 A2
A3
√2 A2
2g
A3
Dg
A1 = A1

Measure six rates (following three + CP-conjugates):
1) B D(K)K*, 2) B DCP(KK)K* , 3) B D (K) K*
 No proper time measurement or tagging required
 Rates = 3.4k, 0.6k, 0.5k respectively (CP-conj. included), with
B/S = 0.3, 1.4, 1.8, for g=65 degrees and D=0

s(g) = 78 deg
55 < g < 105 deg
20 < D < 20 deg
No theoretical uncertainty;
sensitive to new physics in D mixing
19
Measurement of angle g: New Physics?
1. Bs->DsK
g not affected by new
physics in loop diagrams

2. B->, Bs->KK
g affected by possible
new physics in penguin
Determine the CKM
parameters A,,h
independent of new physics

3. B->DK*
g affected by possible
new physics in
D-D mixing
Extract the contribution of
new physics to the
oscillations and penguins
20
Systematic Effects
Possible sources of systematic uncertainty in CP measurement:
 Asymmetry in b-b production rate
 Charge dependent detector efficiencies…



can bias tagging efficiencies
can fake CP asymmetries
CP asymmetries in background process
Experimental handles:
 Use of control samples:






Calibrate b-b production rate
Determine tagging dilution from the data:
e.g. Bs->Ds for Bs->DsK, B->K for B->, B->J/yK* for B->J/yKs, etc
Reversible B field in alternate runs
Charge dependent efficiencies cancel in most B/B asymmetries
Study CP asymmetry of backgrounds in B mass “sidebands”
Perform simultaneous fits for specific background signals:
e.g. Bs->Ds in Bs->DsK , Bs->K & Bs->KK, …
21
Conclusions

LHC offers great potential for B physics from “day 1”
LHC luminosity

LHCb experiment has been reoptimized:
 Less material in tracking volume
 Improved Level1 trigger

Realistic trigger simulation and full pattern recognition in
place

Promising potential for studying new physics
22