B0rp, B0rr and the measurement of a at LHCb

1
B0?rp, B0?rr and the measurement of a at LHCb
Patrick Robbe, LAL Orsay, 26 September 2006, for
the LHCb Collaboration
2
Introduction

• How to measure a
• a Measurements in LHCb
• Performances and sensitivity with B0 ? pp-p0
• Performances and sensitivity with B0 ? rr
• Combination and LHCb sensitivity to a

3
The a angle of the Unitarity Triangle
CKM Matrix
g
b
Vud
Vtb
Vtd
a
Vub
B0 decays to charmless CP final states are
sensitive to bg p-a, the relative weak phase
between tree and penguin contributions.
4
Current status of a measurements

• Direct measurements at B factories
• B ? pp
• B ? rr
• B ? rp
• Combined (with rp from BABAR only)
• Indirect (without direct a measurements)

rp
a
9
.
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)

5
.
91
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BABAR
-
2
.
18

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rp
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.
12
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5
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176
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5
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Belle
-
-
1
.
46
8
.
22
5
a measurements in LHCb

• In LHCb, a measurement is performed with
• Time dependant Dalitz analysis of B0 ? pp-p0
• SU(2) analysis of B0 ? rr-, B0 ? r0r0 and B ?
  rr0.
• Typical event in LHCb
• Challenge to reconstruct multi-track final
  states and final states with p0.

6
LHCb Calorimeter

• 4 devices Scintillator Pad Detector (SPD),
  Preshower (PRS), Electromagnetic Calorimeter
  (ECAL) and Hadronic Calorimeter (HCAL).
• Provides with acceptance 30 mrad to 300 (250)
  mrad
• Level-0 trigger information (high transverse
  momentum hadrons, electrons, photons and p0, and
  multiplicity)
• Kinematic measurements for g and p0 with sE/E
• Particle ID information for e, g, and p0.

10
? 1
?E
7
p0 reconstruction at LHCb

• Resolved p0 reconstructed from 2 isolated
  photons
• sm 10 MeV/c2
• Merged p0 pair of photons from high energy pion
  which forms a single ECAL cluster, where the 2
  showers are merged.
• The pair is reconstructed with a specific
  algorithm based on the expected shower shape.
• sm 15 MeV/c2
• Reconstruction efficiency ep0 53 for
  B0?pp-p0

Resolved p0
Merged p0
p0 mass (Mev/c²)
8
a with B ? r p (1)

• Assuming that the decay B0?pp-p0 proceeds
  through the r?pp resonance, 6 interfering decay
  modes contribute to the pp-p0 Dalitz plot
• B0?rp-, B0?r-p, and B0?r0p0
• B0?r-p, B0?rp-, and B0?r0p0
• Tree or Penguin transitions contribute to each
  decay mode. The time dependant analysis of the
  tagged Dalitz plot gives enough information to
  determine at the same time a and the relative
  amplitudes and strong phases between all
  processes. Snyder, Quinn, 1993

B0
s-
s-
s-
r0p0
rp-
s
s
s
B0
s-
s-
r-p
s-
s
s
s
t (ps)
0
2
6
10
9
a with B ? r p (2)
Maximize a Likelihood with 9 parameters a (
background fractions r )
Theoretical ingredients
Phenomenological ingredients
The ? line-shape
a ( a , T- , f- , T00 , f00 , P- , d- , P-
, d- )
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r
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bkg
B
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B
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Experimental acceptances
Experimental (mis)tagging tag 1/0/-1
Experimental resolutions
Background contamination
Event Yield
Experimental ingredients
10
B ? r p Selection

• Multivariate Selection based on
• Particle Identification Charged pion ID,
  neutral p0 clusters,
• Kinematical criteria transverse momenta,
• Vertexing criteria impact parameters, vertex
  isolation
• Combined PDF

Momenta sum
p identification
signal
e,µ,K,p
inclusive bb
p
SPt(po)/B (GeV)
?LL(p)
Inclusive bb
Signal
vertices-B-momentum. alignment
pp- vertex isolation

signal
inclusive bb
inclusive bb
signal
dmin (mm)
XPDF
T(PV-SV,PB)
11
B ? r p Results and Backgrounds

• 1 million of fully simulated B?rp events
• ? 10 days of LHCb at 2.1032 cm-2s-1
• ? 1300 events selected BABAR ?p statistics up
  to 2004, efficiency of 7x10-4
• ? 50 with merged p0s

• 33 millions of inclusive BB events
• ? 15 min of LHCb at 2.1032 cm-2s-1
• 3 signal events selected and passing the trigger
• 5 background events in side-bands (D(s)p,D(s)r)
  and rejected by the trigger

N3p 14x103 events / 2 fb-1
Consistent with B/S 20 (B/S lt 80 _at_ 90
CL)

• Few millions of specific charmless B decays

Assume B/S 1 in the following
12
B ? r p Fit Signal acceptance

• Acceptance in Dalitz plane

e ()
produced
selected
s-
s-
s
s
The lower corner of the Dalitz plot is highly
depopulated due to the cut on the p0
energy. However, the upper region of the Dalitz
figure contains enough information to allow the a
extraction.

• Proper time acceptance

e ()
Region of low lifetime depopulated due to
the large impact parameters required in the
selection
t (ps)
13
B ? r p Fit Resolution and Tagging
s 50 fs
s 60 MeV/c²

• Expected resolutions

Resolutions are dominated by calorimeter energy
resolution

• Flavour tagging

• Performance estimated from full MC simulation

? Tagging efficiency e 402 ? Wrong tag
fraction ? 312
eeff e (1-2?)² 62

• The tagging performance actually depends on the
  position in Dalitz plane.
• In the real experiment, the wrong tag fraction
  will be extracted from data (for example, using
  the self-tagged Kp-p0 decay)

• NB the untagged sample also enters in the
  global fit.

14
LHCb Sensitivity on a with B ? r p Method

• Assume a set of theoretical parameters agen
• Simulate a set of toy experiments accordingly

Yield 104 signal events 2 fb-1

• Simulate backgrounds according to rgen ratios

Background structure poorly known. Assume B/S 1
and use a mixture made of
The same proper time distribution, resolutions
and tagging dilution as signal are assumed. On
real data, information on background will be
extracted from the side-bands.

• Simulate the experimental effects (resolution,
  acceptance, wrong tag, ...)
• Maximize the likelihood with respect to afit
  and the background ratios rfit (12-D fit)

15
LHCb Sensitivity on a with B ? r p Results
70 toy experiments super-imposed ( L 2 fb-1)
15 converge to a pseudo-mirror
solution. Fraction decreases with increasing
luminosity (lt1 _at_ 10 fb-1)
Average
agen
Distribution of fit error
85 converge to the correct solution
The correct solution generally corresponds to a
deep (if not deepest) minimum.
90 of experiments with s? lt 10
16
LHCb Sensitivity on a
A typical LHCb toy experiment (2fb-1)
Current BABAR measurement (Stat Syst)
17
a Measurement Systematics Estimates
What is the impact of imperfect knowledge of
quantities included in the likelihood

• Extracting ? via the 3p Dalitz analysis requires
  an accurate control of the inputs
• The final analysis will be much more difficult
  than this prospective study
• ? Not likely to be a first year analysis for
  LHCb but very promising results

18
a with B ? r r

• Method Recent measurements showed that the
  B0?rr- decay is predominantly longitudinally
  polarized, and then a pure CP eigenstate.
• The time dependant asymmetry gives access to
  aeff a Da, shift due to penguin
  contributions
• Constraints on Da can be obtained measuring
  B?rr0 and B0?r0r0 branching fractions.
• Analysis similar to B0?pp- but with many
  advantages
• B(B?rr0) and B(B0?rr-) 5 times larger,
• B(B0?r0r0) is small (1.2?0.4?0.3)x10-6
• Time dependant analysis of B0?r0r0 could also
  provide additional information.

with
Aoo
A-/v2
?a
Aoo
A-/v2
Ao A-o
19
LHCb performances for B0?rr- and B?rr0

• Selection for B0 ? ??- and B ? ??0
• Multivariate selection as for B ? ?p
• 2 and 1 neutral pion(s) in the final state,
  respectively
• Overall efficiency 0.01 and 0.045
• B mass resolution dominated by ECAL resolution
  80 MeV/c² and 52 MeV/c²
• Proper time resolution 85 fs and 47 fs

• Expected annual yields (2 fb-1)

• B? ? ???0 9000 B/S 1
• B0 ? ??- 2000 B/S lt 5 _at_ 90CL

One year of LHCb probably not competitive with
current B factory performance. Will need several
years to provide a sizeable contribution to C-,
S- measurement.
The main contribution of LHCb to the B ? ??
analysis could be the improvement of the
measurement of the B0 ? ?0?0 mode
20
LHCb performances for B0?r0r0
s(mB) 16 MeV/c²

• Selection
• multivariate selection
• overall efficiency 0.16

Expected annual yield ( 2fb-1/year)
B mass
Background contamination
s(t) 32 fs
Proper time
21
a with B?rr LHCb sensitivity (1)
B00 1.2x10-6 sB00/B00 20

• Measuring B00

• Measuring C00 with time dependant analysis

C00 0.51 sC00 0.4

• Measuring S00

S00 0.30 sS00 0.4
With 2 fb-1
2006 WA
22
a with B?rr LHCb sensitivity (2)

• a resolution depends on the actual C00/S00
  central value

The current indirect measurement is weakly
constrained
e.g. large S00
S00 -0.61 sS00 0.4
23
rr and rp 2 fb-1 at LHCb
WA 2006

• LHCb a measurement with rp
• LHCb r0r0 (C00/S00) B factories rr- and r0r
  
• LHCb pp- (C-/S-) B factories pp0 and p0p0

LHCb 2 fb-1
24
Conclusions
At LHCb, 2 complementary analyses developed to
measure a

• The time-dependent B0?(?p)0 Dalitz plot
  analysis

• No ambiguity on ? in 0,p but pseudo-mirror
  solutions.
• With 2 fb-1 LHCb may achieve sstatlt 10 on ?
• Require an accurate control of the ?-lineshapes
  and the experimental efficiencies.
• Ambitious but promising.
• Probably several years to setup the analysis.

• The time-dependent B0?rr- asymmetry and SU(2)
  analysis

• 8-fold ambiguity on ? in 0,p.
• Several years of LHCb needed to improve the
  current B0???- measurement.
• With 2 fb-1 the main LHCb contribution could be
  the improved measurement of B0 ? ?0?0.
• Accessing the ?0?0 time-dependent asymmetry will
  reduce the degeneracy of mirror-solutions and
  improve the current ? determination.
• Performance strongly depends of the actual
  values of C00 and S00.

• During LHCb era the stat. error on ? could reach
  the few degrees level

SU(2) breaking effects, electroweak penguin
contributions could be an issue