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LHCb
Eduardo RodriguesUniversity of Glasgow
SUPA Lectures, Glasgow, January 2011
CP VIOLATION
Part IV
CP violation and B Physics
Part IV
CP Violation and B Physics
Chris Parkes
Chris Parkes 2
Outline
PHENOMENOLOGY AND EXPERIMENTS
III. CP violation and Kaon physics
IV. CP violation and B physicsB factories, old and future experiments
Mixing in neutral B mesons
Benchmark B decays
Rare B decays
V. CP Violation and D physics
VI. Concluding remarksPresent status and future ahead
Chris Parkes 3
Overview of B (and D) physics CPV experiments
B factories (2000 2010): electron-positron at γ(4S) resonance BaBar (SLAC, USA), Belle (KEK, Japan)
Discovered CP Violation in B system, angle β Tested CKM mechanism D mixing established
BelleII for high luminosity Super KEK-B starts 2015 TeVatron run II (2001 2011):
Proton- anti-proton CDF, D0
Discovered Bs Mixing LHC (2009 )
LHCb (also ATLAS and CMS to some extent) Discovered Bsμμ
CP Violation in Bs system D mixing at 5σ
Chris Parkes 4
Of the 6 orthogonality relations the CKM matrix satisfies
the “bd” term is central in many B-meson decays:
CP violation studies with B mesons?
tbtstd
cbcscd
ubusud
CKM
VVV
VVV
VVV
Vikjkj
ijVV *
b t d,s
s,d t bW W
b
s,d
d,s
t tb
W
WcdcbVV *
udubVV *tdtbVV *
“The” unitarity triangle (“bd”)
0*** bdtdtbcdcbudub VVVVVV
butransitions
bctransitions
B0
mixing
Of the 6 orthogonality relations the CKM matrix satisfies
the “bd” term is central in many B-meson decays:
0*** bdtdtbcdcbudub VVVVVV
butransitions
bctransitions
B0
mixing
B factories,
old and future experiments
Chris Parkes 6
Ingredients of B physics experimentOscillations time dependent
measure time from distance (d=γct) travelled in experiment
hence B needs to be produced boosted
Symmetric e+e- won’t work ! p-p ok, partons different energies
B decays (lifetime=1.5ps) – observe decay products
Bs oscillations very fast
excellent Vertex Detector
Final state decay products (mostly) : pion, kaon; electron, muon,
Need excellent particle ID
B-hadrons are heavy and long-lived !
Chris Parkes 7
Idea of an asymmetric "B factory"
Oddone & Dorfan in PEP-II Tunnel, 2003
• ϒ(4s) since heavy enough to decay into BB
• Produce the (4S) with a strong boost in lab frame – different energies e-, e+
• BB in coherent state – oscillate together (EPR Paradox)
• Find if B or B at decay time from final state
• Deduce the t from the distance between the two B vertices along the boost axis
Chris Parkes 8
B factories PEP-II (BaBar) and KEKB (Belle)
• Asymmetric beams boosted B’s
• Time difference between B decays z
Chris Parkes 9
High rate– statistics limited channel
Why study CP violation at a hadron collider?
Clean environment– no additional tracks
Initial state– B0B0 or B+B-
B mesons ~ 20% stot
– simpler triggering Rich programme but messy environment
e+ e-
(BaBar)
pp(D0)
Production of all typesBs and b-hadrons
_
Chris Parkes 10
~ 6.23 Km long √s = 1.96 TeV
Started operation in 1987
Run I : collected about 100 pb-1 until 1996
Run II: between 2001 and 2011
(after long shutdown until 2000)
CDF and D0 @ TeVatron, Fermilab
Chris Parkes 11
LHC @ CERN and LHCb
9 km diameter
GenevaJura
CERN
Chris Parkes 12
LHCb environment
LHC environment pp collisions at ECM = 8 / 14 TeV
tbunch = 25/50 ns 40/20 MHz bunch crossing rate
<L> = 4.1032 cm-2 s-1 @ LHCb interaction region
Forward peaked, correlated production
~ 1 cm
Bp-p collision
Measure distance production
(primary vertex p-p) till decay
(B decay vertex) to get timeLHCb VErtex LOcator (VELO)
Silicon detector discs along beam direction
pp
Chris Parkes 13
The LHCb experiment @ the LHC – characteristics
Forward spectrometer
Acceptance: 1.9 < h < 4.9
Nr of B’s / year: 1012
Detector: excellent tracking excellent PID
Reconstruction: - muons: easy - hadronic tracks: fine - electrons: OK - p0’s: possible but difficult - neutrinos: no
p p
Tracking:Silicon & Straw tubesMagnetic field
Calorimeters:Electromagnetic &Hadronic calorimeters- Critical (with muons) for triggering
Vertexing:High precision silicon detectors (10μm position resolution) very close to collision point
B flight path of the order 5-10mm
RICH performance:Cherenkov radiation.Measures velocity, combine with momentum to get massParticle identification in p range 1-100 GeVp, K ID efficiency > 90%, misID<~10%
Mission statement
- Search for new physics probing the flavour structure of the SM- Study CP violation and rare decays with beauty & charm hadrons
Mission statement
- Search for new physics probing the flavour structure of the SM- Study CP violation and rare decays with beauty & charm hadrons
Mixing in neutral B mesons
Chris Parkes 15
Neutral B-mesons “identity card”: 2 types of neutral B mesons
24;72.0
8.17
5.0103.3
1.0,105
1
113
3
s
ss
d
dd
s
d
s
ss
d
dd
mx
mx
psm
psGeVm
yy
Neutral B system in nature
Oscillations parameter
Small lifetime differences
Large mass differences(~100 times larger in Bd
case compared to K system)
B0 = db
Bs = sb
B0 = db
Bs = sb
B=+1B=-1
Reminder of Natural Units, =c=1
Energy GeV Momentum GeV/c (abbreviated to GeV) Mass GeV/c2
Length (GeV/c)-1 c=0.197GeVfm=1 [1fm=1E-15m]– Natural unit of length 1GeV-1=0.197fm
Time (GeV/ )-1 =6.6E-25GeVs– Natural unit of time 1GeV-1=6.6E-25s
Cross-section (GeV/c)-2 1barn=10-28m2
– Natural unit of xsec =1GeV-2=0.389mb Charge - ‘Heavyside-Lorenz units’ ε0=1 Use dimensionless ‘fine structure constant’
137
1
44
2
0
2
e
c
e
Can quote mass
in seconds-1
Chris Parkes 17
b
du, c, t W-
W+_
d
b_u, c, t
b
d u, c, tW- W+
_
d
b_
u, c, t___
B0B0-
(and similarly for Bs)
Neutral B-mesons mixing
Feynman (box) diagrams for neutral B-meson mixing:
Dominated by top quark contribution :
Chris Parkes 18
Dominated by top quark contribution :
b
du, c, t W-
W+_
d
b_u, c, t
b
d u, c, tW- W+
_
d
b_
u, c, t___
B0B0-
12
*12
M
M
p
q
*
*
tdtb
tdtb
VV
VV
p
qFor B0
For B0s *
*
tstb
tstb
VV
VV
p
q
(and similarly for Bs)
Neutral B-mesons mixing
Feynman (box) diagrams for neutral B-meson mixing:
Sensitivity to a CKM triangle side and
angle b
Sensitivity to side and equivalent angle bs
Chris Parkes 19
Dominated by top quark contribution :
b
du, c, t W-
W+_
d
b_u, c, t
b
d u, c, tW- W+
_
d
b_
u, c, t___
B0B0-
(and similarly for Bs)
Neutral B-mesons mixing
Feynman (box) diagrams for neutral B-meson mixing:
Chris Parkes 20
ARGUS, 1987
Observed a fully reconstructed, mixed, event, with no possible background.
Measured the like-sign lepton fraction, and found that ~17% of B0 mesons mix before they decay tB~1.5 ps, Dm~0.5/ps
Phys. Lett. B 192, 245 (1987)
2
12
1
0.00002 psGeV
0.5ps
tB
mm
c
Discovery of B0 mixing
First hint of a really large top mass !
Chris Parkes 21
Belle: K. Abe et al., PRD 71, 072003 (2005) Babar: B. Aubert et al., PRD 73, 012004 (2006)
Belle: B0 lifetime BaBar: md
Some state-of-the-art B0 mixing measurements
B0 oscillates once every 8 decay times ! (2 /p Dm )t
Chris Parkes 22
Measuring Bs mixing – tagging & decay time
opposite-side K
jet charge
Decay modetags b flavorat decay
2nd B tags production flavor Proper decay timefrom displacement (L)and momentum (p)
Need to determine:– Flavour at production tagging– Flavour at decay, from final state– B decay length
Chris Parkes 23
Bs Mixing Measurement
CDF discovery 2006, LHCb measurement 2011
Oscillations occur at 3 trillion Hz !
Observed amplitude is not 1 as smeared
- Mistag (B or B) of events
- Resolution on time
Line is fitted oscillations
Points are data
Low background
Most precise measurement of |Vtd/Vts|
Δms= 17.768 ± 0.023 (stat) ± 0.006 (syst) ps−1
Chris Parkes 24
Key Points – B experiments & mixing
• Dedicated Experiments
• Asymmetric e+e- collider B Factories (Babar, Belle, Belle II)
• pp collider (LHCb)
• B needs to be boosted
• Excellent Vertexing and Particle ID
• Neutral systems: B0 and Bs
• Very different oscillation rates
• Very fast Bs oscillations (3 trillion Hz!)
• Mixing through box diagrams with top quark
• Flavour tagging at production
• Flavour tagging at decay
Benchmark B decays: α, β,ϒ
Chris Parkes 26
The CKM matrix in terms of the Wolfenstein parameters
B0 and Bs mixing phases sensitivity
CKM angle measurements with B decays
1ˆˆ12
1
21
423
22
52
32
iAAiA
AiA
iA
VCKM
iub eV
itd eV si
ts eV
cdcbVV *
udubVV *tdtbVV *
“The” unitarity triangle (“bd”)
a
g b
The standard techniques for the angles
b : B0 mixing (phase β) (+ single b c decay)a : B0 mixing (phase β) + single b u decay (phase γ)g : b u (phase γ) (interference with b c)
Chris Parkes 27
Measurement of sin(2b) – B0 J/Y Ks decay
Measurement type : time-dependent CP asymmetries of B decay to CP-eigenstate final state
The “golden mode” B0 J/Y Ks :
Theoretically clean way of measuring the b angle
Clean experimental signature (J/Yμ+μ-; Ks+-)
Large (for a B meson) branching ratio ~ 10-4
ff
ffCP tA
)(
The B-factories were built for the
measurement of b !c.f. CPLEAR K0 to π+π-
+
e-iφ
Amplitude 1 Amplitude 2
Process via interference with/without mixing
Chris Parkes 28
Angles – measured from interference
Both give same rate - Interference necessary but not sufficient
Two routes A1,A2 to same final state
- hence interference sensitive to phase
Chris Parkes 29
Angles – measured from interference
Additional phase κ that doesn’t flip under CP, allows ϕ to be measured
Oscillation & Decay
30
t=0 t
B0
B0
B0
B0
B0
B0
Amplitude
Amplitude
Rate
Rate
Measuring a CKM angle
31
But in B system and put
Gives: This extra i is the phase difference (here k=900) we need
1. Origin of extra phase k
2. Origin of weak phase ϕ
If and hence Lets assume we can write
Making these substitutionsThe two phase differences give terms
The rate difference is time dependent
( hence assumed i.e. no direct CP Violation)
Measuring a CKM angle
32
simplifying
Time dependent oscillations with amplitude of asymmetry given by phase ϕ
As x~1, only part of an oscillation seen
Chris Parkes 33
Aside on getting CKM phase or phase *
Feynman rules:
Vud if incoming d-quark or outgoing anti-d quark
Vud* if incoming u-quark or outgoing anti-u quark
Quantities to find:
Chris Parkes 34
Which CKM angle is measured ?
Chris Parkes 35
Showing that φ=2β from CKM elements
Chris Parkes 36
Chris Parkes 37
ff
ffCP tA
)(
β accurately
measured
β=21.5±0.80
(HFAG summer 2012)
Chris Parkes 38
Measurement of sin(2a) – B0 pp decay ?
Tree diagrams only:
Routes to final state
with and without mixing.
Interference of these gives angle.
mixing decay
Chris Parkes 39
Measurement of sin(2a) – B0 pp decay ?
But there is another route to this same final state with non-negligible
amplitudeHence not a clean measurement of α
Solutions: use channels with small penguin
contributiuons, or correct for penguin effect
Chris Parkes 40
Measurement of sin(2a) – B0 pp (and other hh) decays
No identification
Purity = 9.5%
With pion identification
Purity = 85%, Eff. =90%
LHCb:
particle identification is crucial !
From all channels α moderately well measured
α=85.4±4.00 (CKM fitter Aug. 2013)
Chris Parkes 41
B D0 K : - theoretically very clean way of measuring g
- sensitivity to g from interference between the 2 diagrams
- only requirement: D0 and D0 decay to common final state
- final state contains D - final state contains D-bar
Measurement of g – popular (family of) methods
u
bB 0D
u
c
Ku
s
u
b
B
Ku
s
0Dc
u
*uscbVV *
csubVV
Currently least well measured angle but LHCb changing this
Note – charged B here, so no mixing
Weak phase
But also relative strong phase (δ) between
the amplitudes of the two diagrams
- nuisance parameter
Chris Parkes 42
1ˆˆ12
1
21
423
22
52
32
iAAiA
AiA
iA
VCKM
In both cases only complex phase is in Vub element, so this measures γ
Measuring gamma
1. Why is this γ ?
2. How to get round strong phase
Interference of amplitudes sensitive to
Chris Parkes 43
1ˆˆ12
1
21
423
22
52
32
iAAiA
AiA
iA
VCKM
In both cases only complex phase is in Vub element, so this measures γ
Measuring gamma
1. Why is this γ ?
2. How to get round strong phase
Interference of amplitudes sensitive to
or
Hence using all four processes can get γ
Combining all channels
γ poorly measured yet
γ=68.0±8.30 (CKM fitter Aug. 2013)
Hot Topic -
Semi-leptonic B Asymmetry
CP Violation in mixing
Chris Parkes 45
Like sign dimuon asymmetryD0 Collab.
B0/B0s
B0/B0s
t=0 t
B0/B0s
B0/B0s
B0/B0s
B0/B0s
d d
bBo
c
μ-
ν
W-
D+
example decay:
• Produce BB pair (or Bs)
• If one oscillates before decaying
get two like sign leptons (++ or --)
• If no CP Violation in mixing get
N++ =N--
Chris Parkes 46
New Physics ? Situation unclear –improved measurements needed(excellent PhD project…)
Like sign dimuon asymmetry: current results
D0 – B and Bs decays inclusively
Tevatron: proton anti-proton – equal matter anti-matter
LHC proton proton – production asymmetry, makes analysis more tricky
but statistics higher
LHCb – Bs only:
first result compatible SM and D0 !
Asy
mm
etry
B0 s
Asymmetry B0
World average 2.9σ away from SM !
Direct CP Violation in B0/Bs
including discovery of
CP Violation in Bs system
Chris Parkes 48
Time-integrated measurement: Direct CP Violation
Direct CP Violation: two-body B0 & Bs decays
Chris Parkes 49
Time-integrated measurement: Direct CP Violation
Direct CP Violation: two-body B0 & Bs decays
Chris Parkes 50
Time-integrated measurement: Direct CP Violation
Direct CP Violation: two-body B0 & Bs decays
Use f
Chris Parkes 51
However several different two-body B decays
Separate with Particle ID and mass for B0/Bs
Direct CP Violation: two-body B0 & Bs decays
(also Λb, 3-body backgrounds)
Bhh, (h=K,π)
10.5σ
Asymmetry
Chris Parkes
52
PRL110, 221601 2013
B B
BsBs6.5σ
Asymmetry FIRST CP
Direct CP Violation: two-body B0 & Bs decays
Dalitz Plots – three body decays
Bhhh
Chris Parkes 54
Dalitz Plot – Visualize three body decays
Dalitz Plot: Scatter plot in mab2, mac
2
If no intermediate structure then uniformly populated
(inside kinematic bounds)
If intermediate resonances, r,
then plot will have internal structure
Shorter-lived resonances – larger widths
Richard Dalitz
• Energy Conservation sets boundaries of plot
• Q = TA+TB+TC,
• Q energy released in decay of P,
• Ti K.E. of product i
m2bc
Chris Parkes 55
CP Violation in B+hhh
• Make Dalitz plot for B+,B-
• Any difference is CP violation
Dalitz PlotACP in Dalitz plot bins
• Local regions of large CP violation(empty bands in plot are regions that have been cut-out as used as cross-checks)
ρ0(770), f0(980)
K*(890), K*(1430)
χc0
• Resonances seen in plot
Rare B decays
Chris Parkes 57
Rare Decay Loops
Chris Parkes 58
Rare B decays – All active research topics at LHCb
DECAY TYPE B.R. (approx.)
B0 K*0 g Bs f g
B0 w gRadiative penguin
4.0 x 10-5
2.1 x 10-5
4.6 x 10-7
B0 K*0 m+ m- Electroweak penguin 1.2 x 10-6
Bs f f
B0 f KS
Gluonic penguin1.3 x 10-6
1.4 x 10-6
Bs m+ m - Rare box diagram 3.5 x 10-9
Radiative penguin
Chris Parkes 59
The B(s) m+m- decay (1/2)
• Really really rare! But well predicted in SM
SM box SM Penguin
• Sensitive to New Physicsin SUSY models
• Unique Experimental signature• Easy to identify / trigger – good for ATLAS/CMS as well
60
25 year long search
Phys.Rev.Lett. 108 (2012) 231801
SM theory
Powerful constraint on SUSY
Chris Parkes 61
Key Points – B section
• CKM Angles
• Measured from interference of two routes to same final state
• sin(2b) – B0 J/Y Ks
• sin(2a) – B0 p+p- decay, and the problem of ‘’penguin pollution’’
• angle ϒ - B- D0 K-, and strong phases
• Semileptonic B asymmetry, D0 experiment discrepancy with SM
• Discovery of (Direct) CP Violation in Bs system, LHCb
• BsK- π+
• Dalitz Plots and use as tools for CP violation, LHCb
• B+hhh
• Rare B Decays
• Discovery of LHCb
