measurement(s) of ϕ s at lhcb

Measurement(s) of ϕs at LHCb

Yasmine AmhisOn behalf of the LHCb Collaboration

Yasmine Amhis - HQL 20122From Y. Nir

Lecture @ CERN April 2012

Yasmine Amhis - HQL 2012

The B0s neutral system

3

Time E

volu

tion

Diagonalizing this Hamiltonian leads to two masseigenstates with masses MH(L) and decay width ΓH(L)


ϕs in a box

• Measure relative phase difference : ϕs= ϕM − 2ϕD between two “legs”.

• In SM + Ignoring penguins : ϕD ~ 0

ϕsSM ~ ϕM

is predominantly determined by arg(Vts )

is predicted to be small ≈ − 0.04 [Charles et al. Phys. Rev. D84 (2011) 033005]

New Physics (NP) can add large phases:

ϕs = ϕs

SM + ϕsNP

Bs decay to a flavour eigenstate


ϕs

Ambiguity Measurement

Dramatis personae du jour

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ϕsAmbiguity Measurement

Dramatis personae du jour


Flavour Tagging

Currently use Opposite Side Tagger, fully optimized and calibrated on data

Effective tagging efficiency(2.29 ±0.07 ±0.26)%

arXiv:1204.5675

Time dependent CP asymmetry needs to identify the initial flavour of reconstructed Bs

0 mesons (initial state a b or b quark).


Decay time resolution

oWe measure from data using prompt J/ψ which decay at t = 0 ps.o Width is found to be about 40 - 45 fs. o Use per-event estimate of the decaytime resolution.


The Golden Channel


Theoretically :o b→ccs tree dominance leads to precise prediction of ϕs

in SM.o P → VV, admixture of CP-odd and CP-even states, measure also ΔΓs.

Experimentally : o Relatively large branching ratio.o Easy to trigger on muons from J/ψ → μ+μ-.

The Observableso 3 “P-wave” amplitudes of KK system.o 1 “S”-wave amplitude.o10 terms with all the interferences. o ϕs

, ΔΓs, , Γs …


There is a two-fold ambiguity in the solutions :

o ϕs and π - ϕs

and ΔΓs changes sign


o Full 2011 data set of 1 fb−1 is used. o Simple cut selections based on kinematics. o Most background removed by decay time cut t > 0.3 ps. o Very clean signal∼ 21200 signal candidates.

LHCb-CONF 2012-004


Fit

pro

ject

ion

s

CP-even

CP-odd

CP Content clearly seperated


Systematic uncertainties


Resu

lts

ΔΓs is the first observed non-zero value. All the measurements are compatible with SM.


Us and the rest of the world


Resolving the ambituity

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Resolving the ambiguous solution The sign of ΔΓs

Y. Xie et al., JHEP 09 (2009) 074]

K+K− P-wave :Phase of Breit-Wigner amplitudeincreases rapidly across φ(1020)mass region

K+K− S-wave:Phase of Flatté amplitude for f0(980)relatively flat (similar for non-resonance)

Phase difference between S- and P-wave amplitudesDecreases rapidly across φ(1020) mass region

Resolution method: choose the solution with decreasing trend of

δs −δP vs mKK in the φ(1020) mass region

Resolving the ambiguous solution The sign of ΔΓs

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Submitted PRL arXiv:1202.4717v2 o Use a sample of 0.37 fb -1 o Split data into 4 bins of mKK

o In each bin measure :• Fraction of S-wave• Measure relative strong

phase difference : δS – δperp

δS –

δp

erp ΔΓs < 0

ΔΓs > 0

Solution I is selected by the data


Previous analysis was Bs J/ψf0(980) PL B707 (2012) 497

All m(ππ) range found to be CP-odd :

97.7% @ 95% C.L [arXiv:1204.5643]

Now use wider m(ππ) range hence : Bs J/ψ ππ


o Selection using a Boosted Decision Tree.o Maximum likelihood fit to time and mass.o Approx. 7400 signal events in 1fb-1

Simpler decay rate expression since there is no angular structure


Result

ϕs = -0.02 ± 0.17(stat.) ± 0.02(syst.) rad.

arXiv1204.5675 Submitted to PLB

o Uses Γs and ΔΓs from the Bs J/ψΦ analysis (+ correlation) PRL 108 (2012) 101803o Uses Δms from Bs→ Ds (3)π PL B709 (2012) 177


Conclusion

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Γs = 0.6580 ± 0.0054(stat.) ± 0.0066(syst.) ps-1;

ΔΓs = 0.116 ± 0.018(stat.) ± 0.006(syst.) ps-1;

ϕs = -0.001 ± 0.101(stat.) ± 0.027(syst.) rad.

ϕs = -0.002 ± 0.083(stat.) ± 0.027(syst.) rad.

o LHCb has presented new preliminary results using the full 2011 data (1 fb-1)

o From an analysis of the J/ψϕ channel we find:

o From an analysis of the J/ψππ channel we find:

ϕs = -0.02 ± 0.17(stat.) ± 0.02(syst.) rad.

o Combining both results we find:

o We resolve the 2-fold ambiguity and find: ΔΓs > 0 .


Systematics


Conclusion

measurement(s) of ϕ s at lhcb

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