B quark

Discovery of the Upsilon and B-mesons
24/11/14
Jordan Calcutt & Tom Webb
Outline
• Upsilon (ϒ)
-Background
-Fermilab experiment: theory, set-up & results
-ϒ & ϒ’: implications & properties
• CKM matrix and b-quark
• B-mesons
-The useful CKM matrix
-Experiments and decays
• Summary and conclusions
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Background
•
Incomplete fermion family
•
Following discovery of the charmed quark, Leon Lederman and team at
Fermilab extended the search for another quark
•
Discovered the ϒ by studying collisions of protons at Fermilab in mid1977 (just 3 years after J /ψ discovery)
•
Confirmed by PLUTO and DASP II at DESY, studying 𝒆+ 𝒆− annihilation in
mid-1988
•
Measurement of resonance width at DESY, led to discovery that the
upsilon consisted of a new quark, anti-quark pair, the bottom quark with
charge -1/3
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Fermilab E288 experiment
+ −
𝒑+𝑵 → 𝝁 𝝁 +𝑿
• Couldn’t directly observe the ϒ, as its lifetime is far too short.
• However, Lederman and his team at Fermilab analysed the decay
products of 400Gev proton-nucleus collisions. Searched for
resonant peaks in the 𝝁+ 𝝁− spectrum.
• Analogous with the J /ψ experiment, but with more energy.
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Fermilab E288 experiment
Figure 1; Sketch of the E288
experiment [Ref. 4]
• Two-armed
spectrometer set
to measure μ+μ−
pairs with invariant
TARGET
masses above 5
GeV with a
resolution of 2%.
Hadrons were
eliminated by using
long beryllium
filters in each arm.
• 1 ϒ produced for every billion protons that strike the target
• Beryllium absorbs background particles, tungsten minimises particle leakage
from outside aperture
• Magnets induce a transverse momentum kick
• Precise measurement of muons trajectory and momenta using; 11 proportional
wire chambers (PWCs), 7 scintillation hodoscopes, a drift chamber and a Čherenkov
counter
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Fermilab E288 experiment
Figure 2; Plan view of the
E288 experiment [Ref. 1]
• Čherenkov
counter on
each arm helps
prevent low
momentum
muon triggers
• Magnets
refocus muon
beam
• PWCs- filled with gas & wires under high voltage. Passing particle induces a discharge
between wires which can be measured.
• Scintillation hodoscopes- Scintillating material produces light from passing particle
which is converted into an electrical signal by a photomultiplier tube.
• PWCs and Scintillation hodoscopes determine trajectory and momentum of muons
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Fermilab E288 experiment
An aside…
Leon Lederman announced the ‘discovery’ of the Upsilon in 1976
with a mass of ~6 GeV, decaying into a positron and electron
However, after taking more data the signal turned out to be false: the
6 GeV particle actually never existed
The “discovery” was named the ‘Oops-Leon’… get it?
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Fermilab E288 experiment
An aside…
Leon Lederman announced the ‘discovery’ of the Upsilon in 1976
with a mass of ~6 GeV, decaying into a positron and electron
However, after taking more data the signal turned out to be false: the
6 GeV particle actually never existed
The “discovery” was named the ‘Oops-Leon’… get it?
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Fermilab E288 experiment
Results:
𝝁+ 𝝁− resonance peak
observed at ~ 9.5GeV
Figure 3; measured 𝜇+ 𝜇− production cross-section as a
function of the invariant mass of the muon pair [Ref. 1]
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ϒ and ϒ’ research at DESY
• A year later, European Scientists working at
Hamburg's Deutsches Elektronen-Synchrotron
(DESY) had upgraded the PLUTO and DASP II
detectors, in order to study the recently discovered
ϒ in greater detail
Figure 4 (above)
• Improved determination of ϒ mass 𝑴ϒ =9.46
±0.01 GeV
& Figure 5
(below); ϒ and ϒ’
resonant peaks
as discovered at
DESY [Ref. 3]
• Achieved via studying 𝒆+ 𝒆− annihilation (signals a
lot less cluttered than the massive μ+μ− pairs)
• Further research at DESY, following additional
cavities installed in DORIS, led to the discovery of
the ϒ’ state , with a mass of 𝑴ϒ′ = 10.02±0.02 GeV
• Analogous to the ψ and ψ’ discoveries a few years
previous, with ϒ’ – ϒ splitting found to be nearly the
same as ψ’ - ψ
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ϒ Properties
• Resonance width: 𝜞𝒆+ 𝒆− (ϒ)=1.3 ± 𝟎. 𝟒𝒌𝒆𝑽 as measured at DESY
• Using non-relativistic potential models derived from the ψ system &
assuming the potential was independent of the quark type, was possible
to predict the wavefunction at the origin and 𝛤𝑒 + 𝑒 − (ϒ) for cases of charge
-1/3 and +2/3
• Comparison indicated new quark had charge -1/3 and was dubbed the
bottom or “b”
• ϒ constituents: b𝒃
• b quark mass: 4.18±0.03 GeV/c²
• Further resonances ϒ, ϒ’, ϒ’’, ϒ’’’ corresponding to 𝟏𝟑 𝑺𝟏 , 𝟐𝟑 𝑺𝟏 , 𝟑𝟑 𝑺𝟏 ,
𝟒𝟑 𝑺𝟏 states observed by CLEO detector at Cornell university, New York
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B mesons
•
•
•
•
•
•
•
•
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What are B mesons?
𝒃𝑿
Analogous with D mesons
Discovery of the τ lepton (1975)[Ref. 5] lepton left an incomplete family
of fermions.
(𝑢, 𝑑, ν𝑒 , 𝑒)
(c, s, νμ , μ )
(??,??, ντ , τ)
Must be two new flavours!
Meson
Anti-Particle
Quark content
𝐵+
𝐵−
𝑢𝑏
𝐵0
𝐵0
𝑑𝑏
𝐵𝑠0
𝐵𝑠0
𝑠𝑏
𝐵𝑐+
𝐵𝑐−
𝑐𝑏
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Predictions for the b quark
• N. Cabibbo[ref. 6] described the weak interactions and flavour mixing as
a rotation frame.
• Initially just for u d and s before c had been discovered.
• On the discovery of the c, it was developed into a rotation matrix.
• However, CP-violation could not be explained by this model
• Kobayashi and Maskawa [Ref. 7] developed the model to a 3 x 3 matrix,
adding another quark, the b.
• Creates a complex term in the Hamiltonian of Fermi’s golden rule for the
weak interaction, providing for CP violation.
• V1,2 can be found from various measurements of decays (decay
times/cross sections) for example, nuclear beta decay and muon decay
for Vud.
[𝑢
𝑐]
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cos θ𝑐
−𝑠𝑖𝑛θ𝑐
𝑠𝑖𝑛θ𝑐 𝑑
𝑐𝑜𝑠θ𝑐 𝑠
[𝑢
𝑐
𝑉𝑢𝑑
𝑡 ] 𝑉𝑐𝑑
𝑉𝑡𝑑
𝑉𝑢𝑠
𝑉𝑐𝑠
𝑉𝑡𝑠
𝑉𝑢𝑏
𝑉𝑐𝑏
𝑉𝑡𝑏
𝑑
𝑠
𝑏
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Cabibbo angle
• Determined by measuring beta
decays in 0+ → 0+ and muon decay
• Vud and Vus are cosθc and sinθc
respectively.
• Results give cosθc ≈ 0.970-0.977
• Other transition rates are attainable
from studying other decays
• θc ≈ 13˚
• E.g. |Vcd|2 proportional to neutrino
nucleus scattering.
• cosθc defined as the decay rate of
down quarks to up quarks.
[𝑢
𝑐]
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cos θ𝑐
−𝑠𝑖𝑛θ𝑐
𝑠𝑖𝑛θ𝑐 𝑑
𝑐𝑜𝑠θ𝑐 𝑠
[𝑢
𝑐
𝑉𝑢𝑑
𝑡 ] 𝑉𝑐𝑑
𝑉𝑡𝑑
𝑉𝑢𝑠
𝑉𝑐𝑠
𝑉𝑡𝑠
𝑉𝑢𝑏
𝑉𝑐𝑏
𝑉𝑡𝑏
𝑑
𝑠
𝑏
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Importance of the CKM matrix
• Decay of mesons containing b quarks is
controlled by Vub (b→ulν) and Vcb (b→clν) .
• Experimental data from CUSB and CLEO (both
at CESR) show that (b→clν) is the dominant
mode of the two.
Figure 6. CUSB experiment, (A) b→ceν, (B) b→ueν
And (C) b→cX. Clearly shows more popular decay to
charm quark.
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Figure 7. data from CLEO experiment, also at
CESR. Upper plots are electronic decays
whereas lower is muonic. Solid line represents
decay predications without b→ulν, dotted
lines purely b→ulν decay predictions.
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Detecting the b quark
Behrends, S., et al (1983) “Observation of Exclusive Decay Modes of b-flavoured
Mesons”,
• ϒ(4S) resonances created through
e+e- annihilation
• Searching for charm containing
mesons, D0 and D* in particular due to
low-multiplicity.
• Detection of subsequent kaon
production.
• CLEO detector used at CESR.
• Consisted of 1T solenoid for momenta
measurement calculations
• Scintillation counters using ToF
method for mass calculations.
• Set to detect kaons at 0.45 < p < 1.00
GeV/c
• D0 → K- + π+
• D* → D0 + π+ ; D0 → K- + π+
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Detecting the b quark
Behrends, S., et al (1983) “Observation of Exclusive Decay Modes of b-flavoured
Mesons”,
• Assuming kaons were being detected, results of D0 decay collected, rejected
results not at ±40MeV mass of D0
• [D*,D0] mass difference found, rejected results outside of ±3MeV data book
value of 145.4MeV
• Sharp drop off proved D* production.
• Results then fitted to the postulated b meson decays:
• B- → D0 πFigure 8, data of
the (a) calculated
• B0 → D0 π+ πmass of D0 decay
and (b) the mass
• B0 → D*+ πdifference, [D*,D0]
• B- → D*+ π- π-
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Detecting the b quark
Behrends, S., et al (1983) “Observation of Exclusive Decay Modes of b-flavoured
Mesons”,
• Mass found to be 5274.2 ± 1.9±2.0 MeV = B0
• 5270.8 ±2.3 ±2.0MeV = B• Today, B0 = 5279.3±0.7 MeV, B- = 5279.1±0.5 MeV
• Average mass difference ΔM = 32.4±3.0±4.0 MeV
• Between ϒ(4S) mass and two times B – meson mass.
• Proof that ϒ(4S) exclusively decays to BB .
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Fig 9. Events of B – meson decays with
different masses attained
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Summary
• Upsilon discovered by Lederman et al. at Fermilab in 1977
• Confirmed and mass improved upon by group at DESY
• Upsilon consists of a b𝑏 pair, a new quark to the fermion
family
• B – meson discovered by Behrends et al at CESR in 1983
after postulation in 1973
• Consists of 𝑏X, where X is any other flavour quark
• CKM matrix used to determine what decays are easiest to
find
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References
1.
2.
3.
4.
5.
6.
7.
8.
9.
S.W. Herb et al., “Observation of a Dimuon Resonance at 9.5GeV in 400GeV Proton Nucleus
Collisions.” Phys. Rev. Lett., 39, 252 (1977)
PLUTO Collaboration, Ch. Berger et al., “Observation of a Narrow Resonance formed in 𝑒 + 𝑒 −
Annihilation at 9.46GeV.” Phys. Lett., 76B, 243 (1978)
J.K. Bienlein et al., “Observation of a Narrow Resonance at 10.02 GeV in 𝑒 + 𝑒 − annihilation .” Phys.
Lett., 78B, 360 (1978)
L. Lederman, “Observations in Particle Physics from Two Neutrinos to the Standard Model”,
http://history.fnal.gov/GoldenBooks/gb_lederman.html
Perl, M. L., et al, (1975) “Evidence for Anomalous Lepton Production in e+-e- Annihilation*”,
Physical Review Letters, vol. 35, no. 22, December, pp.1489-1492.
Cabibbo, N., (1963) “Unitary Symmetry and Leptonic Decays”, Phys. Rev. Lett., vol. 10, no. 12, June,
pp. 531-533.
Kobayashi, M., Maskawa, T., (1973) “CP-Violtation in the Renormalizable Theory of Weak
Interaction”, Progress of Theoretical Physics, vol. 49, no. 2, September, pp. 652-657.
Klopfenstein, C., et al, (1983) “Semileptonic decay of the B meson” Phy. Lett., vol. 130B, no. 6,
November, pp. 444-448.
Behrends, S., et al (1983) “Observation of Exclusive Decay Modes of b-flavoured Mesons”, Phys.
Rev. Lett., vol. 50, no. 12, March, pp. 881-884.
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