lhcb

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The sensitivity for the BS-mixing phase ?S at
LHCb
Peter Fauland (for the LHCb collaboration)
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Outline

• ?s in the standard model
• Measuring ?s at LHCb
• The LHCb MC simulation
• Results on the sensitivity to ?s (using Toy MC)

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?s in the standard model
Mixing Standard model box diagrams (?F2
transitions)
Mixing phase ?
If only SM box diagrams
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?s in the standard model
Wolfenstein parameterization of the CKM matrix
(??Vus, A, ?, ?)
Thus ? in the Bs system corresponds to ? in the
Bd system (therefore ? is also referred to as
?s )
Bd-system well measured, but Bs-system (?Ms, ??s,
?, p/q) not fully explored ?LHCb will have a
large statistics
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CP asymmetry

• CP eigenstates with eigenvalues ?f ?1
• CP interference in mixing and decay (no
  direct CP)
• is dominated by a single weak
  phase

(with ??q ? 0)
The time dependent CP asymmetry allows us to
measure ?s.
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Measuring ?s at LHCb
Thus measure the Bs mixing phase ?s and see if
it agrees with SM expectation from the box
diagrams (check if ?s ? -2? -2?2? ? -0.04)
The following Bs-decays have been used to
determine the LHCb sensitivity to ?s
Bs?J/?(?-?)?(KK-) CP-odd and CP-even eigenstates
Bs??c(h-hh-h)?(KK-) CP-even eigenstate
Bs?J/?(?-?) ?(??) CP-even eigenstate
Bs?J/?(?-?) ?(??-?0(??)) CP-even eigenstate
Bs?Ds (KK-?-) Ds (KK-?) CP-even eigenstate
Bs?Ds (KK-?-)? ?Ms determination, wrong tag fraction, Control Channel
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Angular analysis (CP-odd/CP-even separation)
Decay into two vector particles
Complication for
RT 0 ? CP even RT 0.5 ? maximum dilution (but
still some sensitivity to ?s since odd and even
contributions have different ?tr distribution)
Measurements ?RT ? 0.2
The angular distribution is used to separate odd
and even components
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The LHCb experiment

• LHCb
• Single arm spectrometer with
• good vertexing/tracking for reconstruction of the
  primary and B-decay vertex.
• good particle identification

LHCb under construction. Its real!
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The LHCb MC simulation
The results in the next slides have been obtained
with the LHCb Monte Carlo simulation.
Geant simulated event

• Detailed and realistic detector and material
  description
• Full pattern recognition, trigger simulation
  (also HLT), and offline event selection .
• Realistic detector inefficiencies, noise hits,
  and effects of events from the previous
  bunch crossings

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Results on the event selections
Yield (103/2 fb-1) B/S lt??gt (fs) ?mass (MeV/c2) wtag () ?tag ()
Bs?J/?(?-?)?(KK-) 131 0.12 36 14 33 57
Bs??c(h-hh-h)?(KK-) 3 0.6 30 12 31 66
Bs?J/?(?-?) ?(??) 8.5 2.0 37 34 35 63
Bs?Ds(KK-?-) Ds(KK-?) 4.0 0.3 56 6 34 57
Bs?Ds (KK-?-)? 120 0.4 40 14 31 63

• The sensitivity to ?s
• Bs?J/?? Large yield, but mixture of CP-odd
  and CP-even eigenstates.
• Bs?J/??() , Bs??c? Low yield, high background,
  but CP-even.
• Bs?DsDs Low yield, worse proper time
  resolution, but CP-even.

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Results on the event selections
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Sensitivity Studies using Toy MC
As input the results from the full LHCb MC are
used.
The CP parameters are extracted by performing a
likelihood fit to the mass and proper time
distributions (and to the transversity angle for
Bs?J/??).
The likelihood for the signal
transitions is simultaneously optimized with the
control sample (Bs?Ds?). The tagging performance
is assumed to be the same for the control and
signal sample. The statistical uncertainty of
control channel is thus included.
Perform 200 toy experiments, where each
experiment represents 2fb-1 of data taking (107 s
at 2?1032cm2s-1). The RMS of the ?s distribution
is given as the sensitivity.
Standard model values are used as input
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Modeling the mass and ?tr distributions
Projection of the likelihood on mass distribution
for Bs?J/??. The mass peak is modeled by an
Gaussian signal over an exponential background.
Projection of the likelihood on the transversity
angle distribution for Bs?J/??. Bluetotal, red
dotted CP-even, red dashed CP-odd,
blackbackground (is assumed to be independent on
?tr.
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Modeling the tagged proper time distributions
For illustration only !

• Sensitivity to ?s depends on D (tagging
  dilution factor) 1-2wtag
• But ? also sensitivity if we have untagged
  events (wtag0.5, D0) trough cos(?s) term

• Include
• Trigger and Selection bias on ?
• Proper time resolution

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Modeling the tagged proper time distributions
Projection of the likelihood on the proper time
distribution Blue solid Total Red dotted
Signal Black dashed Background
Bs??c? (Standard model parameters)
Bs??c? better proper time resolution ? wiggles
in the signal are visible Bs?J/?? higher
background ? flattens the wiggles
Bs?J/?? (Standard model parameters)

• Likelihood for the proper time distribution
  includes
• acceptance function (full MC)
• per-event-error for the proper time (full MC)
• tagging performance
• exponential background function

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Results on the sensitivity to ?s
The LHCb sensitivity for ?s with 2 fb-1
Channels
0.133 2.6 0.142 2.8 0.109 3.9 0.108 3.9
Combined sensitivity for pure CP eigenstates 0.059 13.2
0.023 86.8
Combined sensitivity for all CP eigenstates (?) 0.021 100.0
Additional studies ongoing
Total LHCb sensitivity with 10 fb-1 0.01 rad 0.6 degrees (but statistical uncertainty only)
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The sensitivity for ?s, ?Ms, ??s/?s, RT, wtag
with 2fb-1
Only Control sample used.
Input to the likelihood fit
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Results on the sensitivity to ?s
The effect of an improved or degraded proper time
resolution (??). And the effect of a larger B/S
Dependence of the ?s sensitivity on ?s (It has
been checked that the sensitivity does not depend
on the sign of ?s)
SM
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Possible improvements for the sensitivity at LHCb

• Include the J/??ee- events expect 20 increase
  of event yields
• Full angular analysis for Bs?J/??
• Perform a combined fit with all signal channels
• Study the systematic uncertainty (extract proper
  time resolution from data)
• Optimize the use of the control sample (Bs?Ds?)
  for the determination of the tagging performance
  of the signal samples (Bs?J/??, Bs??c?,
  Bs?J/??(), Bs?DsDs ) (define sub-samples with
  the similar tagging performance)

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Conclusions

• The value of ?s is not known very well !
• The LHCb sensitivity for ?s is 0.02 rad for 2
  fb-1
• Small dependence of the sensitivity on ??s/?s and
  ?s .
• After a few years of data LHCb will be able to
  measure also a SM ?s.
• Already with a small data sample (0.2 fb-1) we
  will have interesting results on ?s. We aim for a
  ?s result in 2008!

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Thank you ...
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Results on the sensitivity to ?s
Dependence of the ?s sensitivity on ??s/?s . The
world average is 0.14?0.06. ? not very sensitive
to ??s/?s.
nominal
Dependence of the ?s sensitivity on the CP-odd
fraction (RT) . The world average is
0.167?0.041. ? very sensitive to RT, but we still
have a reasonable sensitivity if maximum dilution
(i.e. RT0.5)
nominal
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