Measurements of CP violation and mixing in two body charm decays

Measurements of CP violation and mixing in two body charm decays

International Conference on High Energy Physics, Valencia

Mark Smith on behalf of the LHCb collaboration

The University of Manchester

4 July 2014

Introduction

Mixing:

Charm mixing discovered through a combination of measurements in 2007:

BABAR (PRL 98:211802 (2007)), Belle (PRL 98:211803 (2007)), CDF (PRL 100, 121802 (2008))

First individual > 5σ result for charm mixing released by LHCb in 2012. Phys.Rev.Lett., 110:101802 (2013)

CP Violation:

CP violation in charm not yet observed.

LHCb has collected very large samples of charm decays to look for CP violation in mixing and decay.

Introduction

Charm mixing: |D1,2i=p|D⁰i ∓q|D⁰i x =^∆m_Γ y = ^∆Γ_2Γ CP violation:

CPV in mixing

q p

6= 1 q p

±2

≈1±Am

a^ind_CP =−Am

2 ycosφ+xsinφ

CPV in decay

A¯_f Af

±2

≈1±A_d 6= 1 a^dir_CP ≈ −¹₂A_d

CPV in interference

λf = q p

A¯f

A_f

e^iφ φ6= 0

In the Standard Model CP violation in charm is expected to be small.

Significant enhancements are an indication of New Physics.

Prompt and semi-leptonic

We can tag the initialD⁰flavour in two ways:

Prompt:

D^∗+ produced at the interaction point.

Look for the decayD^∗+→D⁰π⁺_s. Background fromB decays.

Fit difference betweenD^∗+ andD⁰mass,

∆m, to ascertain correctly tagged candidates.

LHCb primarily used prompt decays thus far.

Semi-leptonic:

Search for the decayB→D⁰µ⁻X. Negligible mis-tag.

prompt

K^- D⁰

IP ~ 0

K⁺

π_s⁺ D*⁺

semi-leptonic

K^- D⁰

μ^- B

large IP

X K⁺

LHCb

Forward spectrometer.

Acceptance 2< η <5

Excellent vertex resolution Excellent PID

3 level trigger:

L0 hardware selects events with highpT particles.

Two layers of software triggers.

Charm output at∼2kHz

Data set

2011: 1fb⁻¹at 7TeV 2012: 2fb⁻¹at 8TeV

Charm

σ_bb,acc¯ = 75.3±14.1µb at 7TeV Phys. Lett. B694 209-216 σ_c¯c,acc = 1419±134µb at 7TeV

Nucl. Phys. B871, 1-20

Wrong sign I

Study the time dependence of the ratio of D⁰→K⁻π⁺(right-sign) and D⁰→K⁺π⁻ (wrong-sign) decays.

R(t) = N_WS(t)

NRS(t) =R_D+p

R_Dy⁰t+x⁰²+y⁰²

4 t

x⁰=xcosδ+ysinδ y⁰ =ycosδ−xsinδ δis the strong phase between CF and DCS decays,RD is the ratio of the decay amplitude magnitudes.

D⁰ K^-π⁺

mix

D⁰ RS:

CF DCS

D⁰ K⁺π^- CF DCS

mix

D⁰ WS:

Analysis of 2011 data: PRL 110, 101802 (2013) No mixing hypothesis excluded at 9.1σ.

Update with full 3fb⁻¹data set. PRL 111, 251801 (2013)

MeasureD⁰(R_D⁺) and D⁰(R_D⁻) to look for CP violation in mixing and in the decay amplitude:

AD =R_D⁺−R_D⁻ R_D⁺+R_D⁻

Wrong sign II

PRL 111, 251801 (2013) FitR⁺,R⁻ and plot the difference.

τ / t

0 2 4 6 20

]-3[10r−ε/−R−r+ε/+R-0.2

0 0.2 (c) ]-3[10−R

4 5 6

CPV allowed No direct CPV No CPV (b)

]-3[10+R

4 5 6

LHCb (a)

LHCb Preliminary

Fit to 3fb⁻¹of 2011 + 2012 data.

Consistent with no CP violation.

A_D = (−1.3±1.9)%

0.75<

q p

<1.24 at 68.3% confidence level.

-0.1 0 0.1 0.2

]-3 [10y'

0 2 4 6 8 10

LHCb (a) CPV allowed

68.3% CL D0

68.3% CL D0

-3] 2 [10 x'

-0.1 0 0.1 0.2

0 2 4 6 8 10

No direct CPV (b)

68.3% CL D0

68.3% CL D0

-0.1 0 0.1 0.2

0 2 4 6 8 10

No CPV (c)

99.7% CL 95.5% CL 68.3% CL

∼factor 2.5 improvement on previous mixing measurement (no CPV).

y⁰= (4.81±0.85±0.53)×10⁻³ x⁰²= (5.5±4.2±2.6)×10⁻⁵

A

I

Asymmetry ofD⁰ andD⁰decay rates to aCP eigenstate,K⁺K⁻ orπ⁺π⁻: AΓ(KK) =

ˆΓ D⁰→K⁺K⁻

−Γˆ D⁰→K⁺K⁻

ˆΓ (D⁰→K⁺K⁻) + ˆΓ D⁰→K⁺K⁻ ≈Am+Ad

2 ycosφ−xsinφ In the SM:

Roughly final state independent

∆A_Γ=A_Γ(KK)−A_Γ(ππ)≈∆A_dycosφ+ (A_m+A_d)y∆ cosφ−x∆ sinφ LargeA_Γ or final state dependence is indicative of New Physics.

A

II

1fb⁻¹ofpp collisions collected in 2011.

Measure effective lifetime ofD⁰ decaying toK⁻ K⁺andπ⁺π⁻.

Fit toD⁰ mass and ∆mto determine signal and backgrounds.

Simultaneous fit toD⁰decay time and ln(IPχ²) to extract the prompt signal mean lifetime.

D⁰→K⁺K⁻

2] KK mass [MeV/c )2Entries / (0.33 MeV/c

10 102

103

104 Data

FitSignal πs Rnd.

π0 π+ K-

→ D0

π+ K- K+

→ + Ds Comb. bkg

LHCb

2] m(KK) [MeV/c

1820 1840 1860 1880 1900

Pull

-5 0

5 KK deltam [MeV/c²]

)2Entries / (0.02 MeV/c

102

103

104 ^Data_Fit

Signal πs Rnd.

π0 π+ K-

→ D0

π+ K- K+

→ + Ds Comb. bkg

LHCb

2] m [MeV/c

140 145 ∆150

Pull

-5 0 5

PRL 112 (2014) 041801

A

III

D⁰→K⁺K⁻ D⁰→π⁺π⁻

Entries / (0.02 ps)

1 10 102

103

104 ^Data_Fit

Signal Secondary

πs Prompt rnd.

πs Sec. rnd.

π0 π+ K-

→ D0

π+ K- K+

→ + Ds Comb. bkg

LHCb

t [ps]

2 4 6

Pull

-5 0 5

Entries / (0.02 ps)

1 10 102

103

Data Fit Signal Secondary

πs Prompt rnd.

πs Sec. rnd.

Comb. bkg

LHCb

t [ps]

2 4 6

Pull

-5 0 5

t [ps]

1 2 3

)0)/n(D0Dn(

0.8 0.9 1 1.1

1.2 Data

FitPrompt signal

LHCb

t [ps]

1 2 3

)0)/n(D0Dn(

0.8 0.9 1 1.1

1.2 Data

FitPrompt signal

LHCb

AΓ(KK) = (−0.35±0.62stat±0.12syst)×10⁻³ AΓ(ππ) = (0.33±1.06stat±0.14syst)×10⁻³

PRL 112 (2014) 041801

A

- HFAG average

-0.2 -0.1 -0 0.1 0.2 0.3 A_Γ (%)

World average -0.014 ± 0.052 %

LHCb 2013 ππ 0.033 ± 0.106 ± 0.014 % LHCb 2013 KK -0.035 ± 0.062 ± 0.012 %

BaBar 2012 0.088 ± 0.255 ± 0.058 %

Belle 2012 -0.030 ± 0.200 ± 0.080 %

HFAG-charm CHARM 2013

HFAG fit results

|q/p|

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Arg(q/p) [deg.]

−60

−40

−20 0 20 40 60

σ 1 σ 2 σ 3 σ 4 σ 5 HFAG-charm

April 2013

no CPV

|q/p|

0.6 0.8 1 1.2 1.4 1.6

Arg(q/p) [deg.]

−60

−40

−20 0 20 40 60

σ 1 σ 2 σ 3 σ 4 σ 5 HFAG-charm

FPCP 2014

no CPV

∆A

Time-integrated asymmetries for final statesf: K⁺K⁻ andπ⁺π⁻. ACP(f) = Γ(D⁰→f)−Γ(D⁰→f)

Γ(D⁰→f) + Γ(D⁰→f) Measured asymmetry:

Araw =N(D⁰→f)−N(D⁰→f) N(D⁰→f) +N(D⁰→f)

The measured quantity includes production and measurement asymmetries:

Araw =ACP+A^prod+A^det

Taking the difference cancels detection and production asymmetries.

To a good approximation this is a measure of direct CP violation.

∆A_CP =A_KK −A_ππ≈∆a^dir_CP 1 +y_CP hti¯

τ

!

+a^ind_CP∆hti τ LHCb has both prompt and semi-leptonic analyses - they are completely independent.

∆A

- prompt analysis

LHCb-CONF-2013-003 Full 1fb⁻¹2011 data set.

Fit ∆mto extract signal yield;K⁺K⁻ on left,π⁺π⁻ on right.

5 10

-5 -4 -3 -2 -1 0 1 2 3 4 5

2) m (MeV/c δ ¹⁰ 5

)2Events / ( 0.1 MeV/c

0 10000 20000 30000 40000

50000 Fit (χ²/ndof = 1.24)

Background Preliminary Data

LHCb D0→KK

5 10

-5 -4 -3 -2 -1 0 1 2 3 4 5

2) m (MeV/c δ¹⁰ 5

)2Events / ( 0.1 MeV/c

0 2000 4000 6000 8000 10000 12000 14000 16000

Preliminary

LHCb Fit (χ²/ndof = 1.00)

Background

D⁰→ππ Data

∆A

= (−0.34 ± 0.15

± 0.10

)%

Preliminary

∆A

- semi-leptonic analysis I

Submitted to JHEP: arXiv:1405.2797

Tag the initialD⁰ flavour from the charge of the muon in the decay of aB meson.

A_raw =A_CP+A_prod(B) +A_det(µ) Full 3fb⁻¹ data set - this result supersedes the previous semi-leptonic analysis on 1fb⁻¹.

semi-leptonic

K^- D⁰

μ^- B

large IP

X K⁺

Make two measurements:

Measure ∆ACP.

Extract the individual asymmetriesA_CP(KK) andA_CP(π⁺π⁻) using CF control channels.

B →D⁰ (K⁻π⁺) µ⁻X D⁺→K⁻π⁺π⁺ D⁺→K⁰π⁺

Aprod(B) +Adet(µ) ACP(K⁰)

∆A

- semi-leptonic analysis II

Submitted to JHEP: arXiv:1405.2797

Fit mass distributions to ascertain yields of each decay mode:

2] [MeV/c

+)

−K M(K

1850 1900

)2cCandidates / ( 1.1 MeV/

0 20 40 60 80 100 120 140 160

103

×

LHCb Data Total Signal Comb. bkg.

2] [MeV/c

+)

−π M(π

1800 1850 1900

)2cCandidates / ( 1.45 MeV/

0 10 20 30 40 50 60

103

×

LHCb Data Total Signal Comb. bkg.

π+ K− 0→ D

D⁰→K⁻K⁺ D⁰→π⁺π⁻

∆A

= (+0.14 ± 0.16

± 0.08

)%

A

(K

K

) = (−0.06 ± 0.15

± 0.10

)%

A

(π

π

) = (−0.20 ± 0.19

± 0.10

)%

Consistent with CP conservation. First individual charm asymmetry measurement at LHCb; most precise to date.

HFAG fit results

ind

aCP

-0.02 -0.015 -0.01 -0.005 0 0.005 0.01 0.015 0.02

dir CPa∆

-0.02 -0.015 -0.01 -0.005 0 0.005 0.01 0.015 0.02

BaBar ACP

∆ Belle prel.

ACP

∆ CP CDF

∆A

LHCb prompt prel.

ACP

∆ LHCb semil.

ACP

∆ LHCb AΓ

BaBar AΓ

Belle prel.

AΓ

HFAG-charm

March 2013

a

= (0.010 ± 0.162)%

∆a

= (0.329 ± 0.121)%

HFAG fit results

ind

aCP

-0.02 -0.015 -0.01 -0.005 0 0.005 0.01 0.015 0.02

dir CPa∆

-0.02 -0.015 -0.01 -0.005

0 0.005 0.01 0.015 0.02

BaBar ACP

∆ Belle prel.

ACP

∆ CP CDF A

∆

LHCb prompt prel.

ACP

∆ LHCb semil.

ACP

∆ LHCb 2010 AΓ

BaBar AΓ

Belle prel.

AΓ LHCb KK AΓ

π LHCb π AΓ

HFAG-^charm

May 2014

a

= (0.013 ± 0.052)%

∆a

= (0.253 ± 0.104)%

D

→ K

h

I

Submitted to JHEP: arXiv:1406.2624

Look for CP violation in the SCS decaysD^±→K_S⁰K^± andD_s^± →K_S⁰π^±. A^D

± (s)→K_S⁰h^±

CP ≡ Γ(D_(s)⁺ →K_S⁰h⁺)−Γ(D_(s)⁻ →K_S⁰h⁻) Γ(D_(s)⁺ →K_S⁰h⁺) + Γ(D_(s)⁻ →K_S⁰h⁻) The measurement includes nuisance asymmetries:

A^D

± (s)→K_S⁰h^±

meas ≈A_CP+A^D

± (s)

prod+A^h_det^± +A_K0/K⁰

Use CF modesD^± →K_S⁰π^± andD_s^±→K_S⁰K^± and construct thedouble difference:

A^DD_CP =h A^D

± s →K_S⁰π^± meas −A^D

± s →K_S⁰K^± meas

i−h A^D

±→K_S⁰π^± meas −A^D

±→K_S⁰K^± meas

i−2A_K0

=A^D

±→K_S⁰K^±

CP +A^D

± s →K_S⁰π^± CP

Then use a combination of CF modes to extract the individual asymmetries.

D

→ K

h

II

Submitted to JHEP: arXiv:1406.2624 Extract yields from fit to invariant mass:

D_(s)⁺ →K_S⁰π⁺ D_(s)⁺ →K_S⁰K⁺

A

= +0.41 ± 0.49 ± 0.26 A

±→K_S⁰K^±

CP

= +0.03 ± 0.17 ± 0.14 A

± s →K_S⁰π^±

CP

= +0.38 ± 0.46 ± 0.17

Consistent with CP conservation.

NEW! NEW!

Conclusion

LHCb made the first individual measurement of charm mixing. More data has improved the precision further.

Presented the most accurate measurement ofAΓ. Analysis of 2012 data set is to follow.

Semi-leptonic analysis is to follow.

Presented the most precise measurements of the time-integrated CP asymmetriesA_CP(K⁻K⁺) andA_CP(π⁺π⁻).

First measurement of the individual asymmetries at LHCb.

Improved the constraints on charm CP violation parameters.

Presented most precise measurement of CP violation inD_(s)⁺ →K_S⁰h^± decays.

Expect further improvements in precision with data from run 2.

All results so far are consistent with CP conservation in

charm.

Backup

Wrong sign - CP violation

The ratio of DCS to CF decays is:

R(t) =NWS(t)

NRS(t) =RD+p

RDy⁰t+x⁰²+y⁰²

4 t

with

x⁰=xcosδ+ysinδ y⁰=ycosδ−xsinδ whereδis the strong phase between CF and DCS decays.

If we include CP violation in the mixing and consider D⁰(R⁺) and D⁰(R⁻) separately the ratio becomes:

R⁺(t) = N(D⁰)_WS(t)

N(D⁰)RS(t) =R_D⁺+ q

R_D⁺y⁰⁺t+(x⁰⁺)²+ (y⁰⁺)²

4 t

with

x^0±=

1±A_M 1∓AM

¹₄

(x⁰cosφ±y⁰sinφ) y^0±=

1±A_M 1∓AM

¹₄

(y⁰cosφ∓x⁰sinφ)

Wrong sign - CP violation

Perform the mixing measurements for D⁰and D⁰, extractR_D^±,x^02± andy^0±

and look for differences to find CP violation in mixing.

Can extract the direct CP violation in the decay amplitude:

A

= R

− R

R

+ R

Extracting CP asymmetries

The raw asymmetry measured using semi-leptonic decays is:

Araw =ACP+Aprod(B) +Adet(µ)

RemoveA_prod(B) +A_det(µ) via:

Araw(B→D⁰(K⁻π⁺)µX) =Aprod(B) +Adet(µ) +Adet(K⁻π⁺) UseD⁺→K⁻π⁺π⁺andD⁺→K⁰π⁺withA_CP(K⁰) to deal with A_det(K⁻π⁺).

Adet(K⁻π⁺) =Araw(K⁻π⁺π⁺)−Araw(K⁰π⁺)−AD(K⁰) Then:

ACP(K⁻K⁺) =Araw(K⁻K⁺)−Araw(K⁻π⁺) +Adet(K⁻π⁺)

D

→ K

h

The double difference:

A^DD_CP =h

A^Dmeas^{±s →KS0π±}−A^Dmeas^s±→KS0K±

i−h

A^Dmeas^±→KS0π±−A^Dmeas^±→KS0K±

i−2A_K0

Because of the cancellation this is the sum of the CP asymmetries:

A^DD_CP =A^D

±→K_S⁰K^±

CP +A^D

± s →K_S⁰π^± CP

Using the CF decayD_s^±→φπ^± the individual asymmetries are extracted:

A^D

±→K_S⁰K^±

CP =h

A^D

±→K_S⁰K^± meas −A^D

± s →K_S⁰K^± meas

i−h A^D

±→K_S⁰π^± meas −A^D

±

s →φπ^±

meas

i−A_K0

A^D

± s →K_S⁰π^±

CP =A^D

± s →K_S⁰π^± meas −A^D

±

s →φπ^±

meas −A_K0