download report - Sapienza

Scientific Report 2007-2009
Particle physics
P11. Heavy Flavor and Spectroscopy with the CDF experiment
The Tevatron collider at Fermilab provides protonantiproton
collisions at √ s = 1.96 TeV. The integrated
luminosity collected until the end of 2009 was about 6
fb −1 corresponding to approximately 3 · 10 10 b-quarks
produced in the acceptance of the CDF detector. The b-
quarks hadronize in all sort of b-mesons and b-baryons,
in contrast to e + e − machines operating at the Y(4S)
which can only study B + and B d mesons, allowing thus
the discovery and detailed spectroscopy of previously unseen
b-baryons or other exotic states and the search for
rare decays of the Bs 0 meson. Moreover the sophisticated
trigger capability of the CDF detector allowed for
the first time the online selection of events characterized
by the presence of charged particles that do not point
back to the primary collision point, a clear signature of
the long lifetime of b-hadrons. The latter option has
been made possible by the development of the Silicon
Vertex Trigger (SVT), a custom hardware processor for
fast pattern reconstruction and track fitting in the silicon
vertex detectors (< 20 µs) at the trigger level. The CDF
group in Roma gave important contributions to the SVT
deployment and operation. The sample collected by the
SVT allowed the observation of several hadronic decay
modes of the Bs 0 meson, like Bs 0 → ϕϕ that has been discovered
by a team from Roma. Subsequently a detailed
investigation of the polarization amplitudes in this channel
has been performed in order to check the calculations
made in the QCDf and pQCD theoretical frameworks.
Other two body decay modes of the Bs 0 meson have been
Candidates per 20 MeV/c
2
1000
800
600
400
200
-1
CDF Run II Preliminary L =1 fb
int
0
B → K + π -
0
B → K - π +
0 0 + -
Bs/Bs
→ K K
0 0
+ -
B /B → π π
0
0 + -
B → K - s π + +Bs
→ K π
0 - 0
Λ → pπ + Λ → pπ +
b
b
0 - 0 +
Λb
→ pK + Λb
→ pK
Combinatorial backg.
Three-body B decays
0
5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8
2
Invariant ππ-mass[GeV/c
]
Figure 1: Invariant mass of two body B 0 d and B 0 s candidates
showing the overlapping mass distribution of the four
identified decay modes.
observed for the first time [1]. The Bs 0 → K + K − and
Bs 0 → K − π + signals, shown in Figure 1, have to be disentangled
from the overlapping two body modes of the
B d meson by a multidimensional fit to the mass, kinematics
and specific ionization of the two tracks. The first
measurement of direct CP violation in the B s system has
been performed using the Bs 0 → K − π + decay.
The rare decay mode Bs
0 → µ + µ − has a very suppressed
Branching Ratio (BR) in the Standard Model
due to helicity suppression. However, in many theories
beyond the Standard Model this suppression is lifted and
BR larger by factors as large as 100 are predicted. The
CDF collaboration published[2] the most stringent upper
limit on the Bs 0 → µ + µ − BR (< 4.3 · 10 −8 @95%C.L.),
severely constraining models of new physics.
The large yield of B-meson and the excellent momentum
resolution provided by the central drift chamber
and silicon detectors has allowed CDF to study
in detail the resonance structure of the rare decay
B + → J/ψϕK + [3]. In particular evidence for a near
threshold resonant structure in the J/ψϕ mass spectrum
has been shown, Figure 2. The significance of
the peak is 3.8σ and its mass and width have been
measured M = 4143.0 ± 2.9(stat.) ± 1.2(syst.) MeV,
Γ = 11.7 +8.3
−5.0 (stat.) ± 3.7(syst.) MeV. Awaiting further
confirmation speculations about this new exotic state include
a DsD ∗ s ∗ molecule or a 4 quark [cs][¯c¯s].
2
Candidates/10 MeV/c
CDF II Preliminary, 2.7 fb
9
8
7
6
5
4
3
2
1
0
1 1.1 1.2 1.3 1.4 1.5
2
∆M (GeV/c )
Figure 2: Invariant mass of J/ψϕ system (minus J/ψ mass)
in B + → J/ψϕK + decays showing the Y(4140) resonance
over a phase space background.
CDF has discovered in the last three years several
new b-baryons. In particular the Σ ± b
(with [uud] and
[ddb] quark content respectively) have been discovered
in the SVT samples by looking for their decay to
Λ b π. Additionally Ξ b ([dsb]) and Ω b ([ssb]) have been
identified respectively in the channel Ξ b → J/ψΞ − , with
Ξ − → Λπ − and Ω b → J/ψΩ − with Ω − → Λk − [4].
References
1. T. Aaltonen et al., Phys. Rev. Lett. 103, 031801 (2009).
2. T. Aaltonen et al., Phys. Rev. Lett. 100, 101802 (2008).
3. T. Aaltonen et al., Phys. Rev. Lett. 102, 242002 (2009).
4. T. Aaltonen et al., Phys. Rev. Lett. 99, 052002 (2007).
Authors
S. De Cecco 1 , C. Dionisi, S. Giagu, M. Iori, C. Luci, P.
Mastrandrea 1 , M. Rescigno 1 , L. Zanello
http://www.roma1.infn.it/exp/cdf/
-1
Sapienza Università di Roma 118 Dipartimento di Fisica