Malcolm John 1/17

1
Early physics of LHCb Malcolm John On behalf of
the LHCb collaboration

1. Very brief introduction
2. Status of LHCb
3. A selection of the most promising results

2
At LHCb terms up to l5 must be considered
(r,h)
a
g
c
b
Major LHCb goals Weak phase,, g Bs mixing
phase fs -2c ? -2arg(Vts) B(Bs ? ??)
(0,0)
(0,1)
3
The LHCb detector status in a nut-shell

• All major sub-detector intrastructure is
  installed and instrumentation is well underway
• LHCb will be ready to space and time -align
  during the 2007 LHC engineering run
• 2008 Calibrate the complete detector and trigger
  for ?s 14TeV
• Expect 0.5fb-1 (50 billion b-quarks)
• 2009 Full physics data-taking
• Expect 2fb-1/year

4
LHCb at LHC - P8 Inset retracted HCAL muon
filter

5
VErtex LOcator

• 170 000 channels
• 8.1mm from beam
• (40ltpitchlt100)mm
• sZ(PV) lt 50mm
• st(Bs) lt 40fs

Beams eye view
6
Simulation

• Expectations are evaluated using the LHCb MC
  simulation software Pythia, EvtGen, GEANT4 and
  Gaudi-based reconstruction (2004 MC data)
• Detailed detector and material description
  (GEANT)
• Pattern recognition, trigger simulation and
  offline event selection
• Implemented detector inefficiencies, noise hits,
  effects of events from the previous bunch
  crossings

Slide by Peter Vankov
7
Bs ? Ds K B ? D0K()

• g

? (8220) (current direct measurements)
8
g from Bs ?DsK?
ch.c. diagrams

• Two tree decays (b?c and b?u), which interfere
  via Bs mixing
• can determine (?s ?), hence ? in a very clean
  way
• Fit 4 tagged, time-dependent rates
• Extract ?s ?, strong phase difference ?,
  amplitude ratio
• Bs? Ds? also used in the fit to constrain other
  parameters (w, ?ms, ??s)

• Expect 140 000 Dsp
• 98 suppression achieved with RICH PID system in
  the analysis
• Used to measure Dms
• 2 fb1 s(Dms) ? 0.012ps1

• Expect 6200 DsK events in 2 fb1
• B/S lt 0.5

• Study sensitivity by generating toy-experiments
  with experimental inputs derived from full MC
  (Decay time and mass resolution, reconstruction
  efficiency, tagging)
• Sensitivity with 2 fb-1 s(g) 13

9
g from Bu,d ?D0K

• Interplay of Bu and D0 decays where
  interferes with
• charged Bs only (time-independent, direct CPV)
• choose decay hierarchies in which large CP
  asymmetry is possible
• tree-level dominates. No penguins pollution

Colour favoured b?c amplitude
Colour suppressed b?u amplitude
? X
?
? X
Also known as ? X Yield / 2 fb1 s(g), 2 fb1
ADS, GLW Kp 700 (56k ) lt15º
ADS, GLW KK, pp 5k, 1.6k lt15º
D0-Dalitz (GGZS) KSpp 5.0k 8º
Benefit from CLEO-c

• A similar analyses possible with B0?D0K0 decays
• The b?c transition is also colour suppressed.
  Expect large CP-asymmetries
• self-tagging (i.e. the b-quark flavour is given
  by the sign of the prompt signal kaon)

B0 self-tagging variant Kp, KK, pp 530, 470, 130 (3.3k ) 8º10º
favoured decay (not sensitive to g)
10
Bs ? J/yf etc and Bs ? ff

• c

11
Bs mixing phase fs

• The equivalent of sin2b for Bs mesons
• In the standard model, fs is small -2arg(Vts)
  ? 0.036?0.003
• Could be larger if New Physics is present in the
  box diagram
• Recent D0 result ?s 0.79 0.56(stat)
  0.140.01(syst) with 1.1 fb1
• To resolve Bs oscillations, excellent proper time
  resolution is required
• Modes sensitive to fs
• CP-odd even
• Bs? J/y f
• CP-even only
• Bs? hc f
• Bs? J/y h
• Bs? Ds Ds
• Control channel (Dms)
• Bs? Ds p

12
Precision on a measurement of fs 0.04
Yield in 2fb-1 B/S s? (fs) smass (MeV/c2) Comment
Bs? J/y f 131k 0.12 36 14 Large yield but full angular analysis required 0.023
Bs? hc f 3k 0.6 30 12 Low yield High background 0.108
Bs? J/y h 11k lt3 35 30 Low yield High background 0.105
Bs? Ds Ds 4k 0.3 56 6 Poorest proper-time resolution 0.133
0.020
2fb-1
0.044
0.5fb-1
0.5
Arbtrary new physics parameterisation MNP MSM
(1hseis)
13
Bs ? ff

• FCNC gluonic penguin decay. Analogue of B0?fKs
  for the Bs
• Dependence on Vts in both the decay and Bs mixing
  amplitudes, phase cancels and leads to the SM
  CP-violation expectation lt 1
• Large CP asymmetry would be a signature of New
  Physics
• The P?VV decay requires a full angular,
  time-dependent CP analysis
• Expect 4000 events/2 fb-1 (based on a CDF B.F.
  measurement 1.4?0.9 x10-5)
• Early feasibility studies suggest LHCb
  statistical precision on a New Physics phase
  (defined at 0.2 for the purposes of this work) in
  2fb-1 is 0.10
• Current combined, B-factory measurement of sin 2ß
  in B0 ??K0S 0.39 0.18
• For comparison, the 2 fb-1 LHCb sensitivity in
  this mode is 0.32

14
Bs ? mm and Bs ? K0mm

• rare B-decays

15
Bs?mm expected sensitivity

• Very exciting possibility of sensitivity to New
  Physics enhancement in the early period
• Current upper limit from the Tevatron is around
  20 x SM prediction
• The dominate background is b?? , b??.
• Background analysis is currently limited by Monte
  Carlo statistics (generation)
• LHCbs superior Bs invariant mass resolution is
  crucial in the background rejection

16
NP model descrimination possible with B0 ? K0mm-

• Suppressed loop decay, BR 1.2?106
• Forward-backward asymmetry AFB(s) in the ??
  rest-frame is sensitive probe of New Physics
• Sensitivity (ignoring non-resonant K??? evts for
  the time being)
• 7.2k signal events/2fb1, Bbb/S 0.2 0.1
• After 2 fb1 zero of AFB(s) located to 0.52
  GeV2
• Other sensitive observables based on
• transversity angles accessible (under study)

17
Conclusion

• LHCb is a spectrometer experiment at the LHC
  which instruments the forward region of the LHC
  hadron collision
• The final assembly and commissioning is on
  schedule ready to take calibration and alignment
  data this autumn
• LHCb has a rich physics program and most analyses
  expect good results in the early period (lt2fb-1)
• Observation of Bs?mm
• s(g)LHCb ? 5 degrees
• s(fs)LHCb ? 0.02 radians
• Sensitivity to New Physics phase in Bs ? ff
• In addition,
• ?(Dms) ? 0.012ps1
• ?(sin(2?)) ? 0.02 (2x105/2fb1) final
  B-factory result s(sin(2?)) ? 0.017stat
• ?(?) ? 10 degrees
• ACP(K?) measured at level (ACP lt 1 in SM)
• Charm physics
• D0 mixing (expect 45k D0 candidates in final
  fit sample 5x B-factories combined yield)
• direct CPV in D0?KK
• D0???
• and Im sure Ive under-represented someone

18
Supplementary Slides

• c

19
Time-dependent analysis requires B flavour tagging

• We need to know the flavour of the B at a
  reference t0 (at the primary vertex)
• Tag (give best estimate of) the flavour by
  examining the rest of the event

Bs0
rec
t 0
Dt picoseconds after leaving the primary vertex,
the reconstructed B decays.
b-hadron
PV
20
RICH systems

• Particle ID p1-100 GeV provided by 2 RICH
  detectors

RICH2
Slide by Val Gibson
21
A successful trigger is crucial in LHCb

• Only 1 of inelastic collisions produces
  b-quarks.
• Branching fractions of interesting B decays are
  lt10-4
• Properties of minimum bias events ate similar to
  those containing B decays
• First Level Trigger (L0)
• Hardware (custom boards, 4ms latency)
• Largest ET hadron, e(g) and (di-)m
• Pile-up system (not for m trigger)
• Reduces 10 MHz inelastic rate to 1MHz
• High Level Triggers
• Software trigger run on CPU farm (1800 nodes)
• Access to all detector data
• Full event reconstruction inclusive and
  exclusive selections tuned to specific final
  states
• Output rate 2 kHz, 35 kB per event

Output rate Trigger Type Physics Use
200 Hz Exclusive B candidates Specific final states
600 Hz High Mass di-muons J/?, b?J/?X
300 Hz D Candidates Charm, calibrations
900 Hz Inclusive b (e.g. b?m) B data mining
Slide by Olivier Schneider
22
Expected tracking performance

• High multiplicity environment
• In a bb event, 30 charged particles traverse
  the whole spectrometer
• Track finding
• efficiency gt 95 for long tracks from B
  decays( 4 ghosts for pT gt 0.5 GeV/c)
• KS??? reconstruction 75 efficient for decay
  in the VELO, lower otherwise
• Average B-decay track resolutions
• Impact parameter 30 ?m
• Momentum 0.4
• Typical B resolutions
• Proper time 40 fs (essential for Bs physics)
• Mass 818 MeV/c2

Mass resolution
Bs ? ?? 18 MeV/c2
Bs ?Ds ? 14 MeV/c2
Bs ? J/? ? 16 MeV/c2
Bs ? J/? ? 8 MeV/c2

with J/? mass constraint
Slide by Olivier Schneider
23
Particle ID performance

• Average efficiency
• K id 88
• ? mis-id 3
• Good K/? separation in 2100 GeV/c range
• Low momentum
• kaon tagging
• High momentum
• clean separation of the different Bd,s?hh modes
• will be best performance ever achieved at a
  hadron collider

No PID
Slide by Olivier Schneider
24
At LHCb terms up to l5 must be considered

(r,h)
Vud Vtd
(1-l2/2)(r,h)

Vcd Vcb

a
Vub Vtb

a
Vcd Vcb
bc
g-c
g

b
Vus Vts
c

Vcd Vcb
(0,0)
(0,1)
(0,0)
Major LHCb goals Weak phase,, g Bs mixing
phase fs -2c ? 2arg(Vts)