Post on 23-Feb-2016
description
:
Chris Parkes
Part VCP violation and D Physics
Chris Parkes 2
Outline
PHENOMENOLOGY AND EXPERIMENTS
III. CP violation and Kaon physicsMixing in the neutral kaon system
Neutral kaon decays
Rare kaon decays
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 physicsMixing in neutral D mesons
Direct and Indirect CP Violation Searches
Future prospects
VI. Concluding remarks
D mesons
c
Nucl.Phys. B871 (2013) 1-20
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Mixing: D0 mixing slightly different from K0 and B0
(s) mixing only ‘up’ type quark system that can mix
D0 mixing only recently observed experimentally (2007 onwards) Beyond Standard Model (BSM) effects may enhance mixing (Accurate) SM and BSM theoretical calculations/predictions are difficult and
show a broad range of estimates why?: D-mesons are too light to be treated as heavy …
and too heavy to be treated as light!
CP violation: Predicted to be negligible in the SM
large effects would be evidence for New Physics … Experimental evidence for CP violation with D mesons not yet clear (CPV in Kaons got a Nobel prize, CPV in B got a Nobel prize…)
Why study mixing and CP violation in the D sector?
Mixing in neutral D mesonsBaBar and Belle find first evidence for D0 mixingWell established by combining multiple channels
Announcement at the Rencontres de Moriond Conference in March 2007
First single channel 5σ measurment D0 mixing
LHCb, November 2012
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Mixing in the neutral D0 – D0 system
c
ud, s, b W+
W-_
u
c_d, s, b
c
u d, s, bW- W+
_u
c_
d, s, b___
D0D0 -
Feynman (box) diagrams for D0 mixing: down type quarks in loops
D0 D0-
_ _ _
b loop CKM suppressed from Wolfenstein parametrisation is order (λ3λ2)2
and s,d loops GIM suppressed
Long distance effects important
– but even more difficult to calculate
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reminder x<<1Only small fraction of an oscillation before decay
suppressed
suppressed
suppressedsuppressed
DCS mixinginterference
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1. Flavour at productionKey points on D mixing
Hence, don’t need to use the flavour tagging procedures we discussed for B,
mistag rate will be low
2. Decay DiagramsRight sign – Cabibbo Favoured Wrong sign – Double Cabibbo Suppressed
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1. Wrong sign / right signKey points on D mixing
Consider events that are either wrong sign (rare and time independent)
or mixed (rare and number will increase with time).
to cancel systematics take ratio to right sign events
Flat with time? no mixing, increases with time? mixing
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1. Wrong sign / right signKey points on D mixing
Flat with time? no mixing, increases with time ? mixing
See just first part of oscillation curve
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yCP
Another way to measure D mixing
Measure lifetime of decays into CP eigenstates→ here CP even, e.g. K+K-
Assume D2 ≡ p|D0 - q|D̅⟩ 0 is CP even ⟩→ then this measures τCP+ = 1 / Г2
Compare to decay into flavour-specific final state, e.g. K-π+
→ this measures τ = 2 / (Г1+Г2) ≡ 1 / Г̅
Therefore τ / τCP+ - 1 = Г2 – Г1 / (Г1 + Г2) = ΔГ / (2Г) ≡ y
For non-zero CP violation D2 is not the physical state DH
→ measure yCP ≡ τ(Kπ) / τ(KK) – 1 = (1 – AM
2/8) y cos ϕ – AM/2 x sin ϕ
CP Violation Am Weak Phase ϕOur groupUpdate imminentwith ~100x more data
Indirect CPV
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AΓ : indirect CP violation *
* Actually it has a direct CP violation contribution if direct CPV is large→ see M. Gersabeck, C. Parkes et al., J.Phys. G39 (2012) 045005
Same concept as B or K measurements we saw before
But experimentally different since don’t see oscillations as x is so small
Measure from lifetimes of decay to CP eigenstate
Oscillation & Decay
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t=0 t
D0
D0
D0
D0
D0
D0
CP Violation Am Ad
Weak Phase ϕ
Prompt/Secondary Separation
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Experimental Complication
D come from both production in proton-proton (prompt)
And from decay of B to D (secondary)
Separate out to get prompt D for time dependent measurement
Use impact parameter that is the distance of flight vector of reconstructed D from the primary vertex (proton-proton interaction point)
World’s best measurement
– No sign of CP Violation yet….Phys.Rev.Lett. 112 (2014) 041801 (our group)
Direct CPV
ΔACP: Direct CP violation in two-body charm
Search for CP asymmetry
Use decays
where slow pion charge determines the D flavour at production
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What we measure
What we want
What we don’t want (1)
What we don’t want (2)
Note time-integrated not time dependent
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ΔACP: Direct CP violation in two-body charm
Measure difference of raw asymmetries for KK and ππ final states
Leads to difference of CP asymmetries (at first order)
Araw(KK) - Araw(ππ) ≈ ACP(KK) - ACP(ππ)
Expect similar magnitude and opposite sign for ACP(KK) and ACP(ππ)
→ ACP(KK) - ACP(ππ) ≈ 2 |ACP(KK)| ≈ 2 |ACP(ππ)|
Enhanced sensitivity in difference measurement!
Production Asymmetry of D*+
Technical Scale Drawing of LHC Collision
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Proton (Matter) Proton (Matter)
• Cancel by measuring difference of KK and ππ final states
+ve charge
-ve charge
Detection Asymmetry of D and of π+s
• Symmetric final statesno D detection asymmetry
• Difference of final statescancel slow π+
detection asym.• Periodic reversal of magnetic field
Phys. Rev. Lett. 108, 111602 (2012)
World Average Results
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ΔA
CP
: di
rect
CP
vio
latio
n
AΓ : indirect CP violation
(some tension more results needed )
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Mixing
Fully established - 5σ single channel measurement
CP violation: So far there is no clear evidence for CP violation with D mesons –
some discrepancy in direct, none in indirect Large CP Violation would be evidence for New Physics, as SM
prediction is small
Prospects: Enormous samples at LHCb under analysis
> 10 billion charm events recorded during LHC run 1! Picture changing very quickly
In addition to two-body decays discussed here, there is active work on 3-body (and 4-body) final state analyses using Dalitz plots
Status / Future prospects
(and a number of good LHCb PhD thesis topics…)
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Key Points – D section
• Neutral meson mixing • only ‘up’ type system
• x<<1, very slow oscillations
• Long distance effects important, not only box diagrams
• D0 -> K+π-, Wrong sign / right sign event ratio with time
• 5σ single channel discovery LHCb• No CP Violation observed (yet)
• Small in SM, could be larger due to new physics
• Search for indirect CP Violation
• Search for direct CP Violation