Prospects for time-dependent asymmetries at LHCb

0.26 MB·English

by  Frédéric Dupertuis

Prospects for time-dependent asymmetries at LHCb
Fr´ed´eric Dupertuis
Laboratoire de Physique des Hautes Energies (LPHE),
Ecole Polytechnique F´ed´erale de Lausanne (EPFL),
CH-1015 Lausanne, SWITZERLAND.
On behalf of the LHCb collaboration.
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1
0
2 1 Introduction

n
a
J LHCb [1] has been acquiring physics data since 2010 and recorded about 0.04 fb−1 in
√
3 2010 and 1.1 fb−1 in 2011 at center-of-mass energy ( s) of 7 TeV. In 2012, it is projected
√

to record 2.2 fb−1 at an energy of s = 8 TeV with a nominal instantaneous luminosity
]
x of L = 4×1032 cm−2s−1, before a long shutdown of almost two years. The data taking
e √
- is expected to resume by the end of 2014 at s = 13 − 14 TeV, before a second long
p
shutdown in 2018 when the upgraded LHCb detector components will be installed. Since
e √
h the bb cross section depends almost linearly on s, this will lead to an increase of about
[ √ √
100% in bb pairs yield at s = 14 compared to s = 7. By 2018, a data sample larger

1
than 8 fb−1 is expected to have been recorded, leading to an increase of about a factor
v √
6 four in statistical power with respect to the 1 fb−1 sample recorded at s = 7 TeV.
0
The LHCb upgrade [2] is designed to take data up to a luminosity of L = 2 ·
4 √
0 1033cm−2s−1 at s = 13 − 14 TeV, recording more than 5 fb−1 each year. In order
1. not to suffer from large pile-up, the 25 ns bunch spacing of the LHC will be required. The
0 detector readout will be upgraded to allow the full 40 MHz LHC interaction rate to be
3
read into a software trigger, improving the trigger efficiency on hadronic modes a factor
1
: 2 (Figure 1). With an operation time of 10 years starting from 2019, it is expected to
v
i record more than 50 fb−1. That will lead to a gain in statistical power by a factor ten with
X
respect to the 1 fb−1 of LHCb. This paper are based on the LHCb upgrade Letter-Of-
r
a Intent (LoI) [2], the LHCb upgrade Framework TDR [3] and the LHCb prospects paper
[4]. A summary of the status and the prospects of the time-dependent CP-observables
is provided with their statistical error expectations. Systematic errors are expected to be
kept below the statistical ones throughout the upgrade programme.
2 Weak phase in tree-dominant b → ccs transitions
B0/B0-mesons decaying to CP-eigenstates through tree-dominant b → ccs transitions
s
give rise to a weak phase, coming from the interference between the direct decay and
the decay after mixing. For B0 → J/ψφ and B0 → J/ψππ decays the weak phase
s s
1
LHCb 1.0 fb—1+ CDF 9.6 fb—1+ D 8 fb—1+ ATLAS 4.9 fb—1
0.25
HFAG
D
Fall 2012
0.20
68% CL contours
( )
0.15 LHCb
0.10 Combined
0.05 CDF SM
ATLAS
0
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Figure 1: Left: Trigger yield as function of the instantaneous luminosity for different
decay channels with the current LHCb trigger system. The trigger yield for hadronic
modes (ππ,φγ,D K) flatens compared to modes with muons in their final states (ψφ).
s
Right: Global fit of ∆Γ −φ results performed by HFAG including the preliminary results
s s
of LHCb with 1 fb−1 of data.
φ is equal to φ − 2φ where φ , φ are the weak phases rising in the mixing
s s,M s,D s,M s,D
and decay, respectively. Within the SM, φSM = −2β and φSM = φSM,penguin where
s,M s s,D s
β is the weak phase associated to the CKM element V , which is predictaed to be
s ts
small −2β = (−0.036 ± 0.002) rad [4], and φSM,penguin is the phase coming from the
s s
doubly Cabibbo suppressed penguin decay involving V which is expected to be even
ub
smaller than β . In the presence of New Physics (NP), the measured weak phase φ can
s s
be significantly enlarged with respect to the very small contribution from the SM. The
full tagged angular time-dependent analysis of B0 → J/ψφ and tagged time-dependent
s
analysis of B0 → J/ψππ allow a precision measurement of φ . With 1 fb−1 of data, results
s s
of φ from a preliminary analysis of B0 → J/ψφ decays and from B0 → J/ψππ decays
s s s
give:
φJ/ψφ = −0.001±0.101(stat)±0.027(syst)rad [5],
s
φJ/ψππ = −0.019+0.173(stat)+0.004(syst)rad [6].
s −0.174 −0.003
The combined result is φ = −0.002 ± 0.083(stat) ± 0.027(syst)rad [5] (Figure 1).
s
These results are compatible with the SM prediction and more data is required to pin
down possible small NP effects.
The expected precisions for measuring φ are 0.025 (0.045) rad with the full LHCb
s
dataset and 0.008 (0.014) rad with LHCb upgrade where the theoretical uncertainty of
0.003 (0.01) rad for B0 → J/ψφ (B0 → J/ψππ) will allow sensitivity to small NP
s s
phenomena. These results will be compared to measurements of φ in different decay
s
modes such as B0 → J/ψη, B0 → J/ψη(cid:48) and B0 → D+D− which have more significant
s s s s s
penguin contributions. With 1 fb−1, preliminary results yields 477 ± 23 B0 → D+D−
s s s
2
0 0
candidates (B(B → D+D−)/B(B → D+D−) = 0.508 ± 0.026(stat) ± 0.043(syst))
s s s s
[7] so the φ measurement in these decays will soon commence at LHCb. A clean
s
measurement of φ can be obtained by performing a tagged time-dependent Dalitz-
s
0
plot analysis of B0 → D (→ KK)KK penguin-free decays. With 0.62 fb−1, 104 ± 29
s
0
B0 → D (→ Kπ)KK candidates (B = (4.7±0.9(stat)±0.6(syst)±0.5(f /f ))·10−5) [8]
s s d
0
have been found which would yield to about 10 B0 → D (→ KK)KK. A measurement
s
of φ in this channel can be attempted with the LHCb upgrade.
s
In the B0 sector, a tagged time-dependent analysis of B0 → J/ψK0 decays allows
S
a measurement of the weak phase φ that gives within the SM almost the CKM angle
β. As for φ , a small pollution φSM,penguin from penguin diagrams is expected. New
s
NP particles entering the loop in penguin diagrams can give rise to new phase φNP such
that the measured φ = 2β + φSM,penguin + φNP. With 1 fb−1, the result of sin(2β) from
the LHCb analysis of B0 → J/ψK0 decays was presented at the CKM workshop [9],
S
sin(2β) = 0.73 ± 0.07(stat) ± 0.04(syst) [10]. By the end of 2012, LHCb will be able
to provide competitive results on the single measurement of sin(2β) with B0 → J/ψ(→
µ+µ−)K0(→ π+π−) and with the full LHCb dataset, the world’s best measurement is
S
expected. With the LHCb upgrade the statistical error on sin(2β) is expected to go down
to 0.0035. A clean measurement of sin(2β) can be obtained by performing a full tagged
0
time-dependent Dalitz-plot analysis of B0 → D (→ KK)ππ penguin-free decays. With
s
0
0.62 fb−1, 8060 ± 29 B0 → D (→ Kπ)ππ candidates [8] have been found which would
s
0
translate to about 800 B0 → D (→ KK)ππ.
s
The penguin pollution in b → ccs transitions is expected to be smaller than the weak
phase arising from the tree processes within the SM. But with the precision that LHCb
is aiming at for φ and sin(2β), and furthermore with the LHCb upgrade, it becomes
s
mandatory to have a handle on the weak phase coming from the penguin process. This
can be assessed by performing the analysis of SU(3)-related decays such as B0 → J/ψK∗,
s
B0 → J/ψρ0 or B0 → J/ψφ in the case of B0 → J/ψφ, and B0 → J/ψK0 or B0 → J/ψπ0
s s S
channels in the case of B0 → J/ψK0. With 0.37 fb−1, 116±13 B0 → J/ψK0 candidates
S s S
(B = (1.83 ± 0.21(stat) ± 0.10(syst) ± 0.14(f /f ) ± 0.07(B(B0 → J/ψK0))) · 10−5)
s d
[11] have been observed and with 0.41 fb−1, 114 ± 11 B0 → J/ψK∗ candidates (B =
s
(4.4+0.5(stat)±0.8(syst))·10−5) [12] have been observed. This means that with complete
−0.4
analysis of these channels, LHCb will be able to provide first constraints on the penguin
pollution size with the dataset recorded up to the end of 2012.
3 Weak phase in penguin-only b → sqq with q = s,d
transitions
In the B0 sector, b → sqq transitions with q = s,d are penguin-only processes. With the
s
dominant t-loop there is a cancellation between the weak phase in the mixing and in the
decay, so the measured weak phase φeff vanishes within the SM. φeff can be extracted from
s s
3
∗0
a tagged angular time-dependent analysis of B0 → φφ and B0 → K∗0K decays and a
s s
tagged time-dependent analysis of B0 → φf (980).
s 0
An untagged angular time-integrated analysis of B0 → φφ candidates has been per-
s
formed to measure the triple product asymmetries A ,A where 801±29 B0 → φφ have
U V s
been observed with a negligible S-wave fraction of (1.3±1.2)% [13]. Compatible with a
vanishing φeff they have been measured with:
s
A = −0.055±0.036(stat)±0.018(syst) [13],
U
A = +0.010±0.036(stat)±0.018(syst) [13].
V
∗0
With 36 pb−1, 50±7 B0 → K∗0K candidates (B = (2.81±0.46(stat)±0.45(syst)±
s
0.34(f /f )) · 10−5) have been found with a CP-averaged K∗0 longitudinal polarisation
s d
fraction of f = 0.31 ± 0.12(stat) ± 0.04(syst) [14]. A tagged angular time-dependent
L
analyses of these decays are expected with the 2011-2012 dataset and the precision on φeff
s
with 1 fb−1 has been estimated to be around 0.3-0.4 rad with the channel B0 → φφ and
s
0.03 rad from 50 fb−1 of data with the LHCb upgrade experiment.
In the B0 sector, the weak phase sin(2βeff) rising from b → sqq processes with q = s,d
is not far from sin(2β) within the SM, with |∆S| = |sin(2βeff)−sin(2β)| (cid:46) 0.1 [4]. The
current single best measurements of sin(2βeff) come from the B-factories with a precision
ofabout0.08-0.10usingB0 → η(cid:48)K0 decays. LHCbiscurrentlystudyingtotheB0 → φK0
S S
decays and the precision on sin(2βeff) is expected to go down to 0.06 with 50 fb−1.
4 Weak phase γ − 2β with B0 → D∓K±
s s s
Performing a tagged time-dependent analysis of B0 → D∓K± decays allows the weak
s s
phase γ−2β to be measured, where γ = arg[−V V∗/(V V∗)]. First preliminary results
s ud ub cd cb
of a fit to the CP-observables of B0 → D∓K± with 1 fb−1 have been presented at the
s s
CKM workshop [15, 16]. The precision on γ from B0 → D∓K± is expected to be about
s s
11◦ withthefullLHCbdatasetand2◦ withtheLHCbupgrade. B0 → D∓K±π+π− decays
s s
are also expected to provide constraints on γ.
5 Decay width difference ∆Γ and ∆Γ
s d
The best measurement of the decay width difference ∆Γ comes from the preliminary
s
results of the tagged angular time-dependent analysis of B0 → J/ψφ decays with 1 fb−1
s
whichgives∆Γ = 0.116±0.018(stat)±0.006(syst)ps−1 [5]. Theerroron∆Γ isexpected
s s
to go down to 0.003 ps−1 with 50 fb−1. It is possible to constrain ∆Γ −φ by combining
s s
the effective lifetimes of B0 → J/ψf (980) (CP-even) and B0 → K+K− (CP-odd). The
s 0 s
sign of ∆Γ has been resolved at 4.7σ to be positive using the running of the phase
s
difference between the P-wave and S-wave around the φ(1020) resonance [17].
LHCb expects to measure ∆Γ with a precision of 0.02 ps−1 with 1 fb−1 by comparing
d
the effective lifetimes of B0 → J/ψK0 and B0 → J/ψK∗, and 0.002 ps−1 with 50 fb−1.
S
4
6 Semileptonic asymmetries as and ad
sl sl
The flavour-specific asymmetry as can be mea-
sl
sured by computing the asymmetry between
B0 → D+µ−X and B0 → D−µ+X yields with
s s s s
D± → φ(→ K+K−)π± (use of other D decays
s s
is foreseen). ad can be similarly measured using
sl
B0 → D±µ∓X decayswithD± → K∓π±π±. The
first LHCb results of as with 1 fb−1 gives:
sl
as = (−0.24±0.54(stat)±0.33(syst))% [18].
sl
With the full LHCb dataset, a statistical preci-
sion of about 0.06% is expected and 0.02% with Figure 2: as −ad results including
sl sl
50 fb−1. the LHCb results with 1 fb−1 [18].
7 Conclusion
Table 1 summarises the current precision and the expectations of statistical accuracy of
the CP observables with the LHCb detector and the LHCb upgrade. LHCb results are
already providing world’s best measurements in the B0 sector. The LHCb upgrade will
s
provide high precision measurements that will stringently test the CKM paradigm and
contribute significantly to the search for possible small NP effects.
Parameter LHCb 2011 LHCb 2018 Upgrade Theory
(1 fb−1) (8 fb−1) (50 fb−1) uncertainty
φ (B0 → J/ψφ) [rad] 0.10 0.025 0.008 0.003
s s
φ (B0 → J/ψππ) [rad] 0.17 0.045 0.014 0.01
s s
sin(2β)(B0 → J/ψK0) [-] 0.07 - 0.0035 0.02
S
φeff(B0 → φφ) [rad] 0.3-0.4 (est.) - 0.03 < 0.02
s s
sin(2βeff)(B0 → φK0) [-] - - 0.06 0.02
S
γ(B0 → D∓K±) (t) [◦] - 11◦ 2◦ negligible
s s
∆Γ (B0 → J/ψφ) [ps−1] 0.018 - 0.003 0.02
s s
∆Γ (B0 → J/ψK0/K∗) [ps−1] 0.02 (est.) - 0.002 0.001
d S
as(B0 → D±µ∓X) [%] 0.54 0.06 0.02 0.003
sl s s
Table 1: Current and expected statistical accuracy of the relevent CP-observables mea-
surable with time-dependent analyses with LHCb and the LHCb upgrade. The current
theory uncertainties are also provided. [3, 4]
5
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