1. B Physics at the LHC
Neville Harnew
University of Oxford

2. OUTLINE Introduction The LHC experiments Physics prospects
Major changes to LHCb
News on detector optimization
ATLAS, CMS
Physics prospects
Particle ID and angle a
Measurement of angle g
Importance of high statistics, angle b
Rare decays
Summary and conclusions

3. Physics motivation Unitarity Triangles Standard Model
Bd0  p+ p-
Bd0  r p
Bd0  DK*0
BS0  DSK
Bd0  D* p, 3p
BS0  DS p
Bd0  J/y KS0
BS0  J/y f
Standard Model
predicts large CP violating asymmetries for B mesons
CP violation predicted in many (often v.rare) decays
need large samples of Bd, Bu, Bs mesons, B baryons
Need consistency checks

4. CP violation in 2007 s(sin2b) < 0.02 (world average) ~
s(sin2a) ~ 0.1
s( g ) ~ v.large
|Vtd/Vts| from DmS by CDF & D limited by theory
|Vub/Vcb| from bu by BaBar & Belle ~ a b g
|Vtd/Vts|
|Vub/Vcb|

5. bb angular distribution
Advantages of the LHC
bb angular distribution
LHCb makes advantage of:
bb production sharply peaked forward-backward
Low luminosity is sufficient (2.0 x 1032 cm-2s-1)
Vertex detector close to interaction region.

6. LHCb Experiment (“classic”)
Acceptance
10 – (250) 300 mrad (non) – bending plane
Particle ID
p-K separation 1<p<150 GeV/c
Vertexing
Proper time resolution
43 fs Bs -> Dsp (K)
30 fs Bs -> J/y f

7. LHCb Light No tracking stations in magnet region “Vertical” RICH-1
No magnetic shielding plate
RESULT : an improved detector

8. Major re-design of LHCb
LHCb material budget
Increased significantly since Technical Proposal.
Detector is being reoptimized.
Beampipe Al  Be-Al alloy
VELO reduced number of stations 25  21 & thinner Si
RICH-1 composite mirror and mirror support outside acceptance.
Tracking stations from 9  4
Trigger optimization
Include Level-0 information
B-field to provide PT information.
B-field to provide PT information s(1/p)~0.2/p  0.01 /GeV
New RICH-1 design required by presence of B-field.
2-mirror geometry
Magnetic shielding box
Retain Aerogel & C4F10
B  p+p- rate improved by factor 2
~0.7 X0 and ~0.2 l0 now reduced by factor ~2

9. ATLAS/CMS Experiments
Acceptance
|h| < 2.5
Particle ID
ATLAS v.limited p-K separation, ~0.8s from TRD
Vertexing
Special pixel B layer R~5cm
“Low” luminosity running
Lumi 1033 cm-2s-1
Only 30fb-1 (3 years)
Specialist B triggers
BUT : Possible staging of DAQ -need to wait and see
ATLAS

10. Power of LHCb particle ID
B0  h+h-
Mass resolution ~17 MeV
Purity = 84% ; Efficiency = 90%
B  p+p-

11. ATLAS particle ID ATLAS use event-by-event max likelihood using:
Proper time
Reconstructed mass
K/p separation variable (~0.8 s)
Takes into account asymmetry only in:
B0  p+p-
BS  K+K-
Not easy !
B0  h+h-
Mass resolution ~ 70 MeV

12. ATLAS : Measurement of a
After 30 fb-1 (3 years)
s(a)
LHCb (1 year) 2a

13. Measurement of g at LHCb
Importance of redundancy
Importance of particle ID
Importance of BS modes
Re-optimization of LHCb currently in progress. All performance figures are pre - detector optimization.

14. Measurement of angle g (1)
Measurement of angle g (1)
Expect 2400 events in 1 year of data taking
s(g-2dg) = 30 160
Depends on (g-2dg) strong phase diff and xs .
( )
Bs -> Ds K + -
4 Rate asymmetries measure angle g-2dg
Theoretically clean

15.  Measurement of g (2) From Bd  +- , Bs  K+K-
ACP = AdircosDmt + AmixsinDmt
~5k events per year in each channel
Invoke U-spin symmetry & relate pp and KK coefficients to extract g
(Fleischer CERN-TH/ )
s(g) ~ 5 – 10o [theory]

16. Assumes perfect knowledge (blue) and 10% uncertainty (red) in |h|
Measurement of g (3)
Bd  D*-+ , D*+-
Measures 2b+g
( )
4 Time-dependent decay rates
Relies on efficient hadron trigger
CP asymmetry very small (need large statistics)
s (2b + g) in degrees
1 year
5 years
- Inclusive D* reconstruction
~ 500 k events/year with
S/B~5
- Add D*a1 channels
~ 360 k events/year
- Get b from B->J/y Ks
(2b + g) in degrees
Assumes perfect knowledge (blue) and 10% uncertainty (red) in |h|

17. LHCb sensitivity per year : s(g) ~ 10O
Measurement of g (4)
Bd D0 K*0 signal
From Bd0 D0 K*0
Determination of g from the measurement
of 6 time-integrated decay rates :
Bd D0 K*0 , Bd D0 K*0 , Bd D0CP=+1 K*0
K+p -
K- p +
K+K-,
p+p-
Visible BR’s ~ 10-8  10-7
Measurement only possible with forward
detector with particle ID
LHCb sensitivity per year : s(g) ~ 10O

18. Bd0  J/y Ks sworld(sin 2b) ~ 0.02 or better by 2006
What will LHC bring to this topic ?
STATISTICS !
Each LHC experiment gives s(sin 2b) ~ 0.02 in 1 year
True precision measurement of this parameter
Eg. Fit for direct CP-violating contribution

19. Rare decays Bs0  mm Standard Model BR ~ 4x10-9
Here the General Purpose Detectors
have an advantage : high pT di-muon
triggering at high (1x1034) luminosity.
CMS : 100 fb-1 (107s at 1034 cm-2s-1) ~26 signal events
6.4 events background
Muon trigger :
2 m’s with pT > 4 GeV
| h| < 2.4
Search also for Bd0  mm
Standard Model BR ~ 1x10-10

20. LHCb efficiencies & yields
Performance figures are for 1 year’s running.
Trigger efficiencies for reconstructable events
Event yields are for tagged events
Performance figures are currently being re-evaluated.
45%
Preliminary
64%
35%

21. Performance figures (1 year)

22. After 1 year of LHC (2008)

23. or maybe …
… maybe g will provide a surprise

24. Summary
A precision study of CP violation will be performed at the LHC. Physics beyond the SM will be probed.
Redundancy of measurements in many channels. LHCb provides good particle ID, vertexing. An efficient & flexible trigger is essential.
ATLAS/CMS measure lepton channels very well, but are not competitive in hadronic modes.
Detector construction of all 3 expts progressing well.
LHC will be ready for data-taking in ATLAS, CMS and LHCb will all be there on day 1.