B-Physics at the LHC

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• B-Physics at the LHC
• P J Dornan
• Imperial College, London

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Why b-physics at the LHC

• Millions of bs
• With full luminosity, 1034, gives 5.1013 bb
  pairs per year
• But events much too difficult to analyse, 25
  interactions per crossing
• So - need to run at lower luminosities for most
  b-physics
• In the early period max luminosity expected
  1033 - ATLAS/CMS
• but will eventually be able to exploit full
  luminosity for certain rare decays
• LHCb currently plan to run at 2.1032 by detuning
  the beam
• Signal/Background improves with increasing energy
• sinel 80 mb, sbb 500 mb
• All b-species produced, B, B0, Bs, Bc, b-baryons
• At these energies bs are getting light
• Thus bb pairs produced dominantly forward -
  backward

-
-
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The Experiments

• General Purpose
• ATLAS, CMS 4p standard colliding beam
  detectors
• Main aim to search for new states - Higgs and
  those from BSM so will always aim to run at
  maximum luminosity, 1033 -gt 1034
• Specialised
• LHCb forward spectrometer (10 300 mrads),
  designed specifically for b-physics. Will always
  run at low luminosity, nominally 2.1032
• General Purpose and LHCb operate in complementary
  kinematic regions

b
qb
b
No
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Experimental Requirements

• An excellent vertex detector
• B-states identified by displaced secondary
  vertices
• Good K-p separation
• Difficult for general purpose detector - a
  weakness of ATLAS/CMS
• An essential feature of LHCb
• A good trigger for interesting b-physics
• Far too many bs produced to trigger on all of
  them. Therefore trigger must reject many b-states
  and concentrate on those from which CP/CKM
  physics will result
• This is probably the greatest challenge for a
  hadronic b-experiment
• -- and has caused failures in the past

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ATLAS
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ATLAS Tracker
Pixel Detector
Tracker
Pixel Detector designed for b-physics Radius of
inner layer 5 cm. 3 layers, but middle will not
be available at start-up
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ATLAS Pit Today
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CMS
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CMS Tracker
CMS Silicon Tracker
Pixel Detector
All Silicon 2 Pixel Layers Radii 4 and 7 cm Low
luminosity Radii 7 and 11 cm high luminosity
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CMS Today
Tracker being assembled
In the pit
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LHCb
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LHCB VELO
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LHCb - VELO
Proper time resolution 40 fs
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LHCb - RICH1
RICH1 detector Vertex locator
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LHCb RICH12
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LHCb RICH performance
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Triggers

• Vital - Still Evolving - Algorithms depend
  upon important physics channels
• Basic philosophy

LHCb much better for hadronic B-decays All
comparable for B -gt J/y decays ATLAS/CMS better
for Rare Decays -gtmm(X)
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What physics channels?

• Must be interesting and extractable at the
  trigger level

Unitarity Triangles
Rare Decays
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Flavour Tagging

• In many cases need to know the flavour of the B
  when produced
• Use Decays of the other B state - Opposite
  Side tag
• Lepton b -gt e, m
• Kaon b -gt c -gt s
• Or from the accompanying p/K with the signal B
  Same Side tag
• Or use vertex charge

Prelim. LHCb with Opposite side only Obtain eD2
6.4
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Bs Oscillation - Dms

• With Dmd yields Vtd
• Oscillation is fast (gt14.4 ps-1)
• Need excellent momentum and position resolution,
  i.e. a fully resconstructable final state and
  excellent vertex resolution
• Use Bs-gt Dsp
• (LHCb) Obtain 80,000 fully reconstructed/year,
  S/B 3. Proper time resolution 40fs

Expect to make a 5s measurement in 1 year to 68
ps-1
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sin2b - B0 -gt J/y Ks

• Classic channel for CP violation study
• Still important for LHC experiments to measure
  with the best possible precision.
• Measure time dependent asymmetry
• Amix yields sin2b
• Adir direct CP violation - BSM
• Assuming Adir 0
• ATLAS quote s(sin2b) 0.013 after 3 years at
  1033
• LHCb quote s(sin2b) 0.022 after 1 year at
  2.1032

LHCb
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Sin2a - B0 -gt pp

• Actually measure p-b-g
• pp a very good channel for LHCb
• High pT hadron to give trigger
• RICH is essential
• But Penguins complicate the analysis

Can reach 5 lt s(a) lt 10 in one year if P/T
known to 10
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Sin2a - B0 -gt rp

• Three final states
• rp-, r0p0, r-p ? pp-p0
• Requires time dependent Dalitz plot analysis
• But needs detailed understanding of the
  acceptance
• An analysis is being developed
• -- looks promising

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g - LHCb

• Will be a major result for LHCb - relies on the
  hadronic trigger
• Many ways
• But none are simple - involve measuring low
  decay rates, time dependent analyses in the Bs
  system, theoretical uncertainties
• Many approaches necessary to check consistency

4 time dep rates yields g-2dg
Relate with U-spin yields g
4 time dep rates yields 2bg
6 decay rates yields g
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Features of LHCb for g Determination

• Vertex Resolution - Time Dependent Bs
  Asymmetries
• Separation Bs-gt Dsp/Bs -gt DsK - RICH particle
  ID and mass cuts

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Bs? KK Separation With RICH
Bd? D0 K0 signal RICH minimises background
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LHCb Expected performance for g
2400 events per year 3 lt s(g-2dg) lt 16
5000 events per channel per year 3 lt s(g-2dg) lt
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500,000 events per year. s(2bg) 10
Some BRs very small, 10-7 -gt 10-8 s(g) 10
per year
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Bs Mixing phase fs -2dg

• Use Bs -gt J/y f
• Bs analogue of golden channel, B0 -gt J/y Ks
• Asymmetry very small in SM, fs -0.04
• so very sensitive to new physics
• But two vectors in final state
• therefore need a time dependent angular analysis

Sensitivity depends on Dms
For Dms 20, Expect s(dg) 2 per year
Analysis also yields Gs and DGs
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Bs,d -gt mmX

• Atlas/CMS can here use the high luminosity, so
  can do better than LHCb

Bs -gt mmX
Bs -gt mm
s 46 MeV
Full Tracker
ATLAS statistics with 30 fb-1
F-B Asymmetry sensitive to some SUSY scenarios
CMS Mass resolution Need 30 fb-1 for a 5s
observation
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Bc

• Production less peaked forward
• Better for ATLAS/CMS
• For Bc -gt J/y p

p (GeV)
Expect between 5 10K Bc -gt J/ y p for each of
ATLAS, CMS LHCb per year
ATLAS, s(M(Bc) 74 MeV
Also Bc -gt J/y mn gives Vbc
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An Event in LHCb
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The major problem - Triggering

• B-rates at the LHC are very high
• The final states of interest are a very small
  proportion
• For highest efficiency, the High Level Triggers
  (HLT)) must focus very directly on the predicted
  properties of the final states of interest and
  aim to distinguish them from the predicted
  backgrounds using the predicted properties of the
  detector
• Predictions in the forward area depend upon
  knowledge of the pdfs at very low x where they
  are least reliable
• The simulation will not be perfect!
• The performance of the trigger is key to the
  success of the experiment
• Planned on the simulation
• Too loose -gt low efficiency
• Too tight -gt potential bias

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Triggering LHCb

• Dimuon Triggers
• Much physics, J/y X decays, rare decays
• Strong signature, low rates
• Safe for LHCb and ATLAS CMS,
• Triggers for hadronic final states
• Much of the physics is here - quite probably
  any new physics will at the few level -
  probing this is the justification for LHCb
• But rates are low - or very low
• Need
• Statistical precision -gt highly efficient
  trigger
• Systematic precision -gt minimal biases and
  these must be accurately quantified
• Highly demanding for the trigger

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The Problem

• Crossing rate at LHC 40 MHz
• Running at 2.1032 and a 25 nsec bunch spacing
  expect crossings with interactions at 10 MHz
• - of which 200kHz will have bb pairs!
• But those useful for CP/CKM physics and having
  all decay products in the detector is very much
  less
• e.g. For B0 ?J/y(mm)Ks(pp-) it is 0.02
  Hz or 1 per minute.
• For Bs0 ? mm it is 1 per week

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Current LHCb Plan

• 3 Level Trigger
• Level 0 - reduce rate from 10 Mhz to 1 Mhz
• Pile-up veto, a high pt hadron, electron, muon,
  photon
• Increases b purity from 1 to 3
• Level 1 reduce rate from 1 Mhz to 40 kHz
• Demand tracks with finite impact parameter and
  high pt
• Divide bandwidth between generic and specific,
  cuts for special channels, electron, photon,
  dimuon
• b-purity now at 9
• High Level trigger reduce rate from 40 kHz to
  200 Hz to tape
• Fast reconstruction, using all detectors except
  RICH so far

Bandwidth Division at Level1
To maintain efficiency at this rate, HLT must use
tight offline type cuts. Efficiency for
channels not used to define HLT can be low
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Possible Improvement

• Keep present philosophy but add a new inclusive
  stream with simple cuts and a high output rate
• To be based on detection of just a single muon
  with minimal pt and impact parameter cuts -
  small modification of level 1 bandwidth
• This would be
• Inclusive - trigger on the other b
• Yields tagged events
• Robust
• Use to reduce/estimate systematic
• uncertainties
• Access to states not chosen for HLT optimisation
  ff, fKs .
• Output rate- whatever can be handled - would be
  2 5 kHz with 50 events with bb
• Under active investigation - Current 200 Hz
  stream would be preserved - now Hot Stream
• Inclusive stream to be reconstructed at many
  sites

Possible future Level 1 bandwidth
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The LHC

• Status
• Delays due to problems with the cryolines
• Poor quality control by the company charged with
  installation of work by its sub-contractors
• First collisions are still scheduled for summer
  2007
• Great pressure to maintain this date
• Components
• Almost all ontime
• Cryodipoles, which were a problem now stacking up
  on the surface waiting for the repair of the
  cryolines

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Summary

• The LHC has great potential to make major
  advances in precision CPV b-physics
• All species of b-hadron state are produced
• Thousands of events for many important channels
  with small branching ratios make measurements at
  the few level possible.
• But
• The hadronic environment will be difficult
• still a lot of background - mostly from
  uninteresting b-states
• Efficient, well understood triggering will be all
  important