1. 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

2. B Physics in 2007 Direct Measurement of angles:
s(sin(2b)) ≈ 0.03 from J/y Ks in B factories
Other angles not precisely known
Knowledge of the sides of unitary triangle:
(Dominated by theoretical uncertainties)
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…

3. B Physics @ LHC ,K Bs K K Ds  qb qb bb production: (forward) √s
14 TeV
L (cm-2 s-2)
2x1032 cm-2 s-1
sbb
500 mb
sinel / sbb
160 qb 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
B Decay eg.: Bs->Dsh Bs K K
,K  Ds
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

4. Simulation and Reconstruction
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…)
No true MC info
used anywhere ! T3 T2 T1 TT
RICH1
VELO
Sensitivity studies are based on fast simulations using efficiencies and resolutions and from the full simulation

5. Evolution since Technical Proposal
Reduced
material
Improved
level-1 trigger

6. Track finding strategy
T track
Upstream track
VELO
seeds
Long track (forward)
Long track (matched)
VELO track
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)

7. Result of track finding
On average:
26 long tracks
11 upstream tracks
4 downstream tracks
5 T tracks
26 VELO tracks
Result of track finding T3 T2 T1
Typical event display:
Red = measurements (hits)
Blue = all reconstructed tracks 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

8. Experimental Resolution
Momentum resolution
Impact parameter resolution
sIP=
14m + 35 m/pT
dp/p =
0.35% – 0.55%
1/pT spectrum B tracks
p spectrum B tracks

9. Particle ID RICH 2 RICH 1 e (K->K) = 88% e (p->K) = 3% Example:
B->hh decays:

10. Trigger L0 pT of m, e, h, g L1 40 MHz Level-0: Level-1:
pile-up L0
Trigger
40 MHz
Calorimeter
Muon system
Pile-up system
Level-0:
pT of
m, e, h, g
1 MHz
Vertex Locator
Trigger Tracker
Level 0 objects
Level-1:
Impact parameter
Rough pT ~ 20% L1
B->pp
Bs->DsK
40 kHz
ln IP/IP
ln IP/IP
HLT:
Final state
reconstruction
Full detector
information
Signal
Min.
Bias
200 Hz output
ln pT
ln pT

11. Flavour tag εtag [%] εeff [%] eff = tag (1-2wtag )2 l Bd  p p
Knowledge of flavour at birth is essential
for the majority of CP measurements l B0 D p+ p- K- b s u
Bs0 K+
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
sources for wrong tags:
Bd-Bd mixing (opposite side)
b → c → l (lepton tag)
conversions… 4 35 42
εeff [%]
Wtag [%]
εtag [%] 6 33 50
Bd  p p
Bs  K K
Combining tags
effective efficiency:
eff = tag (1-2wtag )2

12. Efficiencies, event yields and Bbb/S ratios
Nominal year = 1012 bb pairs produced (107 s at L=21032 cm2s1 with bb=500 b)
Yields include factor 2 from CP-conjugated decays
Branching ratios from PDG or SM predictions

13. CP Sensitivity studies
CP asymmetries due to interference of Tree, Mixing, Penguin, New Physics amplitudes:
fnew + + +
fmix
ftree
fpen
Measurements of Angle g:
Mixing phases:
1. Time dependent asymmetries in Bs->DsK decays. Interference between b->u and b->c tree diagrams due to Bs mixing
Sensitive to g + fs (Aleksan et al)
2. Time dependent asymmetries in B->pp and Bs->KK decays. Interference between b->u tree and b->d(s) penguin diagrams
Sensitive to g, fd, fs (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)
Time dependent asymmetry in Bd->J/y Ks decays
Sensitive to fd
Time dependent asymmetry in Bs->J/y f decays
Sensitive to fs

14. Bs oscillation frequency: ms
Needed for the observation of CP asymmetries with Bs decays
Use Bs Ds
If ms= 20 ps1
Can observe >5 oscillation signal if well beyond SM prediction
Expected unmixed Bs Ds sample in one year of data taking.
(ms) = ps1
Full MC
ms < 68 ps1
Proper-time resolution
plays a crucial role

15. Mixing Phases Bd mixing phase using B->J/y Ks Bs mixing phase
using Bs->J/y f
Angular analysis to separate
CP even and CP odd
Background-subtracted
BJ/()KS CP asymmetry
after one year
Time resolution is important:
st = 38 fs
Proper time resolution (ps)
If ms= 20 ps1:
s(DGs/Gs) = 0.018
(sin(d)) = 0.022
NB: In the SM, s = 2 ~ 0.04
(sin(fs)) = 0.058

16. (2 Tree diagrams due to Bs mixing)
1. Angle  from BsDsK
(2 Tree diagrams due to Bs mixing)
Simultaneous fit of Bs->Dsp and Bs->DsK:
Determination of mistag fraction
Time dependence of background
Time dependent asymmetries:
Bs(Bs) ->Ds-K+: → DT1/T2 + (g+fs)
Bs(Bs) ->Ds+K-: → DT1/T2 – (g+fs)
ADs-K+
() = 1415 deg
After one year, if ms= 20 ps1, s/s = 0.1, 55 <  < 105 deg, 20 < T1/T2 < 20 deg:
No theoretical uncertainty;
insensitive to new physics in B mixing
ADs+K-
(after 5 years of data)

17. 2. Angle  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(m t) + Amix sin(m t)
Method proposed by R. Fleischer:
SM predictions:
Adir (B0 ) = f1(d, , )
Amix(B0 ) = f2(d, , , d)
Adir (BsKK ) = f3(d’, ’, )
Amix(BsKK ) = f4(d’, ’, , s)
Assuming U-spin flavour symmetry (interchange of d and s quarks): d = d’ and  = ’
4 measurements (CP asymmetries) and 3 unknown (, d and )  can solve for 
d exp(i) = function of tree and
penguin amplitudes
in B0 
d’ exp(i’) = function of tree and
in Bs  KK

18. 2. Angle  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 input = 65 deg)
“fake”
solution
d vs 
pdf for 
() = 46 deg
If ms= 20 ps1, s/s=0.1, d =0.3, = 160 deg,
55 <  < 105 deg:
U-spin symmetry assumed;
sensitive to new physics in penguins
pdf for d

19. 3. Angle  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):
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 =65 degrees and =0
A1 = A1
√2 A2 A3 2

55 <  < 105 deg
20 <  < 20 deg
() = 78 deg
No theoretical uncertainty;
sensitive to new physics in D mixing

20. Measurement of angle g: New Physics?
2. B->pp, Bs->KK
3. B->DK*
1. Bs->DsK
g not affected by new physics in loop diagrams
g affected by possible
new physics in penguin
g affected by possible
new physics in
D-D mixing
Determine the CKM parameters A,r,h independent of new physics
Extract the contribution of new physics to the oscillations and penguins

21. 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->Dsp for Bs->DsK, B->Kp for B->pp, 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->Dsp in Bs->DsK , Bs->Kp & Bs->KK, …

22. 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