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Scientific Report 2007-2009
Particle physics
P7. Precision measurements of CP violation and rare decays of
B-hadrons at the CERN Large Hadron Collider LHC
CP violation, discovered in neutral kaon decays, is still
one of the outstanding mysteries of elementary particle
physics. In the weak interactions CP violation is generated
by the complex three by three unitary matrix known
as the CKM matrix. In cosmology CP violation is one
of the three ingredients required to explain the excess of
matter over antimatter observed in our universe, but the
level of CP violation that can be generated by the Standard
Model is insufficient to explain this excess. This
calls for new sources of CP violation beyond the Standard
Model.
CAVERN
and a 2 mm wire pitch. To check the long-term stability
of the MWPCs in a high radiation environment an ageing
test was performed [3] by exposing a few chambers
to a 800 TBq 60 Co source during one month. Moreover
the efficiency and the time resolution of a MWPC were
measured as a function of the anode HV. The effect of a
high radiation background on these two quantities was
tested by exposing a chamber to a muon beam superimposed
to the gamma flux of a 630 GBq 137 Cs radioactive
source [4]. The results reported in Fig. 2, show that
the MWPCs fulfill the requirements for HV larger than
∼ 2.6 kV.
100
6m
5m
4m RICH1
TT
Vertex
Locator
Magnet
HCAL M4
M3
M5
RICH2 ECAL M2
M1
T1 T2T3
Efficiency (%)
98
96
94
92
Source OFF
Source ON
6
2m
1m
0
5m
10m 15m 20m
Figure 1: LHCb setup. M1−M5 are the muon detectors.
time resolution (ns)
5
4
3
2
1
0
Source OFF
Source ON
2.4 2.5 2.6 2.7 2.8
High Voltage (kV)
To search for such possibilities the LHCb experiment
[1] at the CERN-LHC collider will precisely measure CPviolating
effects and rare decays of B d , B s and D mesons.
To reach these goals the LHCb detector (Fig. 1) must
provide an excellent vertex and momentum resolution
combined with very good particle identification.
Among the decay products of the B and D hadrons,
muons are present in many final states as for example in
the two CP-sensitive B decays, Bd
0 → J/ψ(µ+ µ − )KS
0
and Bs 0 → J/ψ(µ + µ − )ϕ. In addition, the observation
of the flavour-changing neutral current decays like
Bs 0 → µ + µ − and D 0 → µ + µ − may reveal new physics
beyond the Standard Model. Therefore a muon detector
combined with a muon trigger and an offline muon
identification are fundamental requirements of the experiment.
In the last years our laboratory has contributed to
the design of the muon system comprising 1368 multiwire
proportional chambers (MWPCs) and to check their
performance [2]. These chambers must have a time resolution
lower than 4 ns (rms) and an efficiency of at
least 99 % within a 25 ns time window. These stringent
requirements where obtained by using, in most of the
muon detector, four-gap MWPCs with a 5 mm gas gap
Figure 2: Efficiency and time resolution of a MWPC vs.
the anode HV. The effect of the 630 GBq 137 Cs source
is shown to be negligible.
In 2008 the entire setup was mounted on the p-p
collider. The tests with the first proton beam show
that the full setup and in particular the muon detector,
reach the desired performance and are ready for data
taking with the forthcoming machine runs.
References
1. A. Augusto Alves Jr. et al., JINST 3, S08005 (2008).
2. E. Dané et al., Nucl. Instrum. Methods Phys. Res.,
Sect. A 572, 682 (2007).
3. M. Anelli et al., Nucl. Instrum. Methods Phys. Res.,
Sect. A 599, 171 (2009).
4. M. Anelli et al., Nucl. Instrum. Methods Phys. Res.,
Sect. A 593, 319 (2008).
Authors
A. Augusto Alves Jr. 1 , G. Auriemma 1 , V. Bocci 1 , G.
Martellotti 1 , R. Nobrega 1 , G. Penso, D. Pinci 1 , R.
Santacesaria 1 .
http://lhcb.web.cern.ch/lhcb/
Sapienza Università di Roma 114 Dipartimento di Fisica