lhcb eduardo rodrigues university of glasgow supa lectures, glasgow, january 2011 part iv cp...
TRANSCRIPT
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