LHCb实验揭示Λ0_b衰变中CP违反的新精确测量
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本文主要探讨了在LHCb（大型强子对撞机实验）上进行的一项关键实验，该研究利用pp（质子-质子）碰撞数据集，总集成亮度为3.0fb-1，来寻找Λb0（Lambda_b介子）在两种特定衰变模式下的CP违反（Charge-Parity Violation，即宇称和电荷共轭守恒的破坏）现象。CP违反是粒子物理学中的基本现象，对于理解标准模型的对称性和可能的新物理贡献具有重要意义。
研究对象是Λb0介子的两个衰变通道：Λb0→pK-（质子和反kaon）和Λb0→pπ-（质子和反π介子）。实验测量了这两个衰变过程中的CP违反不对称性，即ApK-CP和Apπ-CP，分别定义为衰变为对应反粒子和自身粒子的事件频率差。结果表明，ApK-CP的值为-0.020±0.013±0.019，而Apπ-CP的值为-0.035±0.017±0.020。两者之间的差异被测量为ApK-CP - Apπ-CP = 0.014±0.022±0.010。这些测量是迄今为止这类不对称性的最精确值，显示出CP违反效应在这些特定衰变路径中的存在或缺失。
1. 引言：
文章首先强调了CP违反的重要性，它揭示了弱相互作用的非对称性，这是标准模型预言之外的现象，可能暗示着新物理的存在。研究Λb0介子的CP违反有助于检验理论预测，并可能提供对CP violation机制的新见解。
2. 实验方法：
研究采用了LHCb实验的高精度数据，通过分析大量的pp碰撞事件，研究人员能够细致地测量衰变过程中CP相关参数的微小偏差。这种精密的测量技术包括了严格的统计处理和系统性误差控制，以确保结果的可靠性。
3. 结果与讨论：
论文结果显示的CP违反信号对当前理论模型构成了挑战，可能需要新的理论解释。这些测量结果为未来更深入的研究提供了基础，如进一步细化理论模型，或者在高能物理领域寻找新的物理效应。
4. 结论与未来方向：
LHCb的这一发现标志着在寻找CP违反方面取得了重要进展，但同时也提出了更多的未解之谜。未来的研究可能会继续探索其他Λb0衰变模式，以及寻找其他粒子系统中的CP违反证据，以深化我们对基本粒子性质的理解。
这篇发表在《物理快报B》的文章，通过对Λb0介子衰变的CP违反研究，不仅提供了实验观测数据，还促进了理论物理的革新思考，为粒子物理学领域的CP违反研究做出了重要贡献。
Physics Letters B 787 (2018) 124–133
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Search for CP violation in Λ
0
b
→ pK
−
and Λ
0
b
→ pπ
−
decays
.LHCb Collaboration
a r t i c l e i n f o a b s t r a c t
Article history:
Received
18 July 2018
Received
in revised form 21 September
2018
Accepted
19 October 2018
Available
online 24 October 2018
Editor:
L. Rolandi
A search for CP violation in Λ
0
b
→ pK
−
and Λ
0
b
→ pπ
−
decays is presented using a sample of pp
collisions
collected with the LHCb detector and corresponding to an integrated luminosity of 3.0 fb
−1
.
The CP-violating asymmetries are measured to be A
pK
−
CP
=−0.020 ± 0.013 ± 0.019 and A
pπ
−
CP
=−0.035 ±
0.017 ± 0.020, and their difference A
pK
−
CP
− A
pπ
−
CP
= 0.014 ± 0.022 ± 0.010, where the first uncertainties
are statistical and the second systematic. These are the most precise measurements of such asymmetries
to date.
© 2018 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
The non-invariance of weak interactions under the combined
application of charge conjugation (C ) and parity (P ) transforma-
tions
is accommodated within the Standard Model by the Cabibbo–
Kobayashi–Maskawa
mechanism [1,2]. The violation of the CP sym-
metry
was discovered in neutral-kaon decays [3], and later ob-
served
with B
0
[4–12], B
+
[13] and B
0
s
mesons [12,14]. First ev-
idence
for CP violation in the b-baryon sector was found more
recently [15]. The decays Λ
0
b
→ pK
−
and Λ
0
b
→ pπ
−
are medi-
ated
by the same quark-level transitions contributing to charmless
two-body B
0
and B
0
s
decays to charged pions and kaons, where
nonzero values of the CP asymmetries are well established [14].
The inclusion of charge-conjugate processes is implied throughout.
Predictions
for the CP asymmetries in the decays of the Λ
0
b
baryon to two-body charmless final states pK
−
or pπ
−
range
from a few percent in the generalised factorisation approach [16,
17]up
to approximately 30% within the perturbative quantum-
chromodynamics
formalism [18]. The only measurements of these
quantities available to date were performed by the CDF Collabora-
tion [12].
The asymmetries were found to be compatible with zero
within an uncertainty of 8to 9%.
This
Letter reports on a search for CP violation in Λ
0
b
→ pK
−
and Λ
0
b
→ pπ
−
decays, using pp -collision data collected with the
LHCb detector at centre-of-mass energies of 7 and 8 TeV and cor-
responding
to 3.0 fb
−1
of integrated luminosity. The CP asymmetry
is defined as
A
f
CP
≡
(Λ
0
b
→ f ) − (Λ
0
b
→ f )
(Λ
0
b
→ f ) + (Λ
0
b
→ f )
,
(1)
where is the partial width of the given decay, with f ≡
pK
−
(pπ
−
) and f ≡ pK
+
(pπ
+
). In addition, the difference of
the two CP asymmetries, A
CP
≡ A
pK
−
CP
− A
pπ
−
CP
, is also reported.
As the main systematic uncertainties cancel in the difference, this
quantity will become useful with the increasing size of the data
sample.
The
Letter is organised as follows. After a brief introduction on
the detector, trigger and simulation in Sec. 2, the formalism needed
to relate the physical CP asymmetries to the experimental mea-
surements
is presented in Sec. 3. Then, the event selection and the
invariant-mass fit are described in Secs. 4 and 5, respectively. The
determination of instrumental asymmetries and systematic uncer-
tainties
is discussed in Sec. 6. Finally, results are given and conclu-
sions
are drawn in Sec. 7.
2. Detector, trigger and simulation
The LHCb detector [19,20]is a single-arm forward spectrome-
ter
covering the pseudorapidity range 2 < η < 5, designed for the
study of particles containing b or c quarks. The detector includes
a high-precision tracking system consisting of a silicon-strip vertex
detector surrounding the pp interaction region [21], a large-area
silicon-strip detector located upstream of a dipole magnet with a
bending power of about 4 Tm, and three stations of silicon-strip
detectors and straw drift tubes [22]placed downstream of the
magnet. The tracking system provides a measurement of the mo-
mentum,
p, of charged particles with a relative uncertainty that
varies from 0.5% at low momentum to 1.0% at 200 GeV/c. The min-
imum
distance of a track to a primary vertex (PV), the impact
parameter (IP), is measured with a resolution of (15 + 29/p
T
) μm,
where p
T
is the component of the momentum transverse to the
beam, in GeV/c. Different types of charged hadrons are distin-
guished
using information from two ring-imaging Cherenkov de-
tectors [23].
Photons, electrons and hadrons are identified by a
calorimeter system consisting of scintillating-pad and preshower
https://doi.org/10.1016/j.physletb.2018.10.039
0370-2693/
© 2018 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.
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