prospects for the measurement of mixing induced cp violation in b s j/ at lhcb

Prospects for the Prospects for the measurement of measurement of

mixing induced CP violation mixing induced CP violation in Bin BssJ/J/ at LHCb at LHCb

1. Introduction, phenomenology2. The LHCb detector at LHC3. Trigger and selection4. Flavour tagging5. Fit and systematics6. Conclusion and prospects

Olivier Leroy

VIth Meeting on B physics, Ferrara, Italy, March 19-20, 2009

CPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille, France

Olivier Leroy Prospects for the measurement of mixing induced CP violation in Bs->JpsiPhi at LHCb, March 19th 2009

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Many thanks to my LHCb colleagues, in particular to:A. Carbone, M. Calvi, P. Clarke, T. du Pree, S. Hansmann-Menzemer, B. Khanji, G. Lanfranchi, C. Langenbruch, C. Linn, M. Musy, S. Poss, U. Uwer, S. Vecchi and Y. Xie.


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Introduction (1)Introduction (1) The interference between Bs decay to J/ either directly or via

mixing gives rise to a CP violating phase

is the sensitive probe of New Physics because:1. It is well predicted in the SM: SM -2s+P = (-0.0370.002+P) rad

2. New particles can contribute to the Bs-Bs box diagrams and significantly modify the SM prediction

Bs

Bs

f =J/

is observable, D and M are not


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Introduction (2)Introduction (2) In the SM, BsJ/ decay is dominated by a single weak phase:

Various penguin pollution estimates: P~10-4 [H. Boos et al., Phys.Rev. D70 (2004) 036006] P~10-3 [M. Gronau et al., arXiv:0812.4796] P up to ~0.1 [S. Faller et al., arXiv:0810.4248v1]


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Notations/conventionsNotations/conventions

1. s = the smallest b-s unitarity triangle angle

2. = the observable weak phase in BsJ/ = -2s + P No penguin pollution AND no NP = -2s

3. = the possible new physics phase in Bs-Bs mixing

4. The NP phase modify other measurements:

~10 times smaller than 2s

Standard Model:

2s=(0.0370.002) rad


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PPVV decays (1)VV decays (1)

B0s

B0s

J/ ψ Φ

μ+μ-K+K-

ℓ = 0

ℓ = 1

ℓ = 2

Bs is a pseudo scalar (spin=0), and J/ are vectors mesons (JPC = 1—) Total angular momentum conservation =>

in the Bs rest frame, and J/ have relative orbital momentum ℓ =0,1,2 Since CP|J/>= CP|J/> CP|> (-1)ℓ,

final state is mixture of CP even (ℓ=0,2) and CP odd (ℓ=1) Decompose decay amplitudes in term of linear polarization, when J/

and are: A0: longitudinally polarized (CP-even) A┴: transversely polarized and ┴ to each other (CP-odd) A||: transversely polarized and ║ to each other (CP-even)


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PPVV decays (2)VV decays (2) 3 angles , ϕ, ψ describe directions of final decay products

J/→ and →K+K Allow to separate statistically the 3 decay amplitudes

J/ rest frame (where the meson moves in the x direction, the z axis is perpendicular to the decay plane of →K+K−, and py(K+) ≥ 0), (θ, ϕ) are the polar and azimuthal angles of the μ+.

In the rest frame, ψ is the angle between p(K+) and −p(J/)


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Differential decay ratesDifferential decay rates


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Differential decay ratesDifferential decay ratesTime dependent part


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Differential decay ratesDifferential decay ratesAngular dependent part


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Time dependent BTime dependent Bss decay amplitudes decay amplitudes

Dependends on 8 physics parameters: , s, s, ms, R┴, R, δ┴, δ

Conventions: 0=0 |A┴(0)|2+|A(0)|2+|A0(0)|2 = 1

Moduli2

Interferences


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Time dependent BTime dependent Bss decay amplitudes decay amplitudes

Tagged analyses sensitive to both cos and sin Exact symmetry of the decay rates (2-fold ambiguity):


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Road map to measure Road map to measure at LHCbat LHCb The decay rates give the road map:

Trigger and select BsJ/ events Measure proper time Measure 3 transversity angles Tag initial Bs flavour Likelihood fit of time and angular decay rates

6 observables: proper time, 3 angles, q (=0,-1,+1 for untagged, Bs, Bs) and mass 8 physics parameters: , s, s, ms, R┴, R, δ┴, δ

+ many detector parameters (resolutions, acceptances, tagging, …)

We will increase sensitivity and complexity step by step: Take detector param. from MC, fit only phys. param. Measure detector param. in control channels, fit only phys. param. Fit all param. from BsJ/ alone

Control channels: B+J/K+, BdJ/K*, BsDs, …


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LHCb detector at LHC


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LHCb detector at LHCLHCb detector at LHC

(Pythia)

Designed to maximize B-acceptance (within cost and space constraints) Forward spectrometer, 1.9<<4.9 More b-hadrons produced

at low angles Single arm is OK as b-quarks are

correlated Rely low pT triggers, efficient also for

purely hadronic decays

Choose to run at L~2x1032cm-2s-1

Clean environment: easier event reconstruction

Less radiation damage bb cross section at 14TeV

= 500b 40% (Pythia) 1012 bb produced / 2fb-1

(nominal year)


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VELO:primary verteximpact parameterdisplaced vertex

Trigger Tracker: p for trigger and Ks reconstruction

Tracking Stations:p of charged particles

Calorimeters:Trigger andPID: h,e,, 0

Muon SystemTrigger and PID

RICHES:PID: K, separation

Interaction region

The LHCb detectorThe LHCb detector


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Trigger and selection


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BBJ/J/(()X unified trigger and selection )X unified trigger and selection (X= (X= ,, K K++, K*) , K*)

Coherent selection of the BsJ/ and two control channels: B+J/K+ and BdJ/K* in order to extract as much as possible information from data (rather than MC)

Selection criteria to obtain a reasonable statistical uncertainty on s while keeping systematics under control: Maximize signal yield while keeping background < few tens of

millions of events per nominal year (computing limit) Miminize proper time and angular acceptance distortions.

Do not cut on variables which bias the proper time b-hadrons have similar 4-momenta such that tagging properties

can be extracted from control channels and exported so signal channel without big corrections.Proper time resolution model can be cross-checked amongst channels

Other selections currently under investigation


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BBssJ/J/(()) trigger selection trigger selection

Trigger uses lifetime-unbiased di-muons: L0: pT>1.5GeV, ~94% HLT1: Distance of closest approach ()<0.5mm

and m>2.5GeV, ~80% HLT2: pT(both )>0.5GeV and |m()-m (J/)|<3

and 2(Vtx)<7, ~95%

Selection: J/,

PID, 2(Vtx)<6, pT(J/)>1GeV, |m()-m(J/)|<3 KK

PID, 2(Vtx)<20, pT()>1GeV, |m()-m()|<3 2(Bs Vtx)<5, |m(KK)-m(Bs)|<3

Event yield after trigger and selection (2fb-1) ~ 117k


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BBJ/J/X selection: backgroundsX selection: backgrounds

Yield (2fb-1) B(bb)/S B(prompt J/)/S

m.b. rate(300MeV mass window)

BsJ/ 117k 0.51 1.6 0.3 Hz

BJ/K* 489k 1.53 5.2 8.1 Hz

BJ/K+ 942k 0.29 1.6 1.4 Hz

Main background is prompt J/ but “easy” to isolate because ~0ps

Background studied: pp bb inclusive pp Buds J/ X Prompt pp J/ X Minimum bias (pp X)


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BBssJ/J/ mass mass

<>~16MeV/c


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BBssJ/J/ proper time resolution and proper time resolution and acceptanceacceptance

Average proper time resolution ~ 39 fs

Acceptance nearly flat


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BBssJ/J/ angular acceptance angular acceptance

Acceptance distortions at ~10% level, come from: Kinematical cuts on the Bs decay products LHCb geometry (10-400 mrad) Single particle reconstruction efficiency = function of

momentum Above plots are zero-suppressed and made with 17fb-1


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Angular resolutionsAngular resolutions

Resolution “good enough” to be ignored in likelihood fit (2fb-1) If resolutions two times worse:

|A┴,|||2 get biased s remains unbiased

Resolution (mrad) 0.020 0.027 0.027


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Prompt background mass and proper Prompt background mass and proper timetime

Main background is prompt J/ but “easy” to isolate because ~0ps


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Prompt background anglesPrompt background angles

Flat ? More statistics needed !


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Long lived background mass and proper Long lived background mass and proper timetime

Average proper time smaller than signal (72% of events have ~0.18 ps)


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Long lived background anglesLong lived background angles

Flat ? More statistics needed !


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Flavour tagging


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Same side

signal Bs

K-

K+

primary vertex

Opposite side opposite B

negative lepton taggers (e-, -) from b-quark

opposite

positive lepton taggers from bcl cascade

same side kaon tagger

vertex-charge taggerfrom inclusive vertexing

bb

s

u

s

u

Bs

K+

protonproton

kaon tagger (K-)

Flavour tagging at LHCb (1)Flavour tagging at LHCb (1)

+

-

BsJ/: efficiency ~ 55.7%, mistag rate ~33.3%, (1-2)2~6.2%


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Main control channels B+JK+ , B0JK*0 for opposite side taggers BsDsfor same side kaon

First results will probably come with opposite side taggers (higher statistics in Bu,d channels and proper time resolution less crucial)

Unified selection true opposite-side mistag compatible amongst BJX channels:

Flavour tagging (2)Flavour tagging (2)

Combined wOS

(%)

Combined OS (1-2w)2 (%)

BsJ/ 36.510.24 3.320.11

BdJ/K* 36.150.20 3.350.09

B+J/K+ 36.000.21 3.450.10

True

performance

(MC truth)

after L0


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Opposite-side mistag (wOpposite-side mistag (wOSOS) in ) in BBddJ/J/K* K*

BRvis[BsJ/()(K+K-)]= (2.710.96)x10-5

Measure wOS from mixing asymmetry in BdJ/K*

Toy MC including backgrounds Fit with 17 free parameters,

successful wOS/wOS)~0.3% with 2fb-1


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Sensitivity studies


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Baseline BBaseline BssJ/J/ parameters for toy MC parameters for toy MC sensitivity studies, extracted from full MCsensitivity studies, extracted from full MC

G means Gaussian: first parameter is the mean, second is the sigma.(t) is the Dirac function. “Flat Acc”, means a flat acceptance, “No Res” means a perfect resolution.The quoted yield is after L0, HLT and offline selection.

Signal Prompt background Long-lived background

Events in the 50MeV signal mass window B/S 117k B/S=1.8 B/S=0.5

Mass m (MeV)

f1G(m-MBs,1)+(1-f1)G(m-MBs,2) Exp(-1m) Exp(-2m)

MBs=5369.6

f1=0.74, 1=13.2; 2=22.5; (<=16.2)

1=0.0006 2=0.001

Proper time t (fs) (Flat Acc.)

S1(t)Res1 (t)Res2 {0.22 Exp(1=1114)+0.78 Exp(=161) }Res3

Res1 = f1G(0,1)+(1-f1)G(0,2)

f1=0.85, 1=31.5; 2=66.7 (<=39)

Res2= G(0,44) Res3 = G(0,66)

Angles (Flat Acc.)S2(angles) B1(angles) flat B2(angles) flat

No Res No Res No Res

Tagging parameters(Combined performance, assuming splitting in 5 categories !!)

wtag = 0.334±0.003tag = 0.564±0.002

tag=0.30±0.02 tag=0.62±0.06


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Baseline sensitivity studies (2fbBaseline sensitivity studies (2fb-1-1) )

Perform hundreds of toy MC, maximum likelihood fit with: 6 observables m, t, cos, , cos, q 7 free parameters , s, s, R┴, R0, δ1, δ2 (ms fixed, can be floated, no changes) All “detector parameters” fixed

Pulls of fitted quantities normally distributed With 2fb-1 (one nominal year), (2s)~0.03

5 NP discovery, if 2strue>0.2 !

Standard Model:

2s=0.0370.002


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Baseline sensitivity studies (2fbBaseline sensitivity studies (2fb-1-1) )

Sensitivity to other parameters is also good:

Similar results when all detector parameters are free (see backup)

Physics Parameters Input Value 2fb-1 sensitivity

s 0.049 [ps-1] 0.0095

s 0.680 [ps-1] 0.0028

2s 0.0368 0.030

R0 0.56 0.0031

R 0.233 0.0045

-2.91 0.082

-2.93 0.074


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Fit BFit BssJ/J/ in full MC in full MC

Angular and proper time acceptance included Angles and proper time well described


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BBssJ/J/KK S-wave (1)KK S-wave (1) The “simple” previous decay rates consider only the ->K+K- (P-wave) in

the vicinity of the KK and ignored any other resonant or non-resonant contributions in this region. Is this a problem ?

BJ/KK is a P → VPP process In principle the KK system can have any zero or positive integer angular

momentum A 5...10% S-wave contribution to Bs→J/K+K- cannot be excluded a

priori [arXiv:0812.2832]

The S wave has a time-dependent angular distribution different from that of any P-wave CP eigenvalue: -1 angular distribution

Its interferences with P waves introduce new terms Including S-wave contribution and its interference with P waves in the

signal pdf is necessary for correct extraction of s

22 cossin1


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BBssJ/J/KK S-wave (2)KK S-wave (2)

Toy MC studies with 5-10% S-wave:

If ignored, leads to 2s ~10-20% biased toward zero

Including it in the fit adds 2 parameters and leads to a reduction in 2s resolution of ~20%

If KK S-wave contribution large enough, it is promising to measure the S-P phase difference as a funtion of KK mass in order to measure sign of cos(2s); like BaBar in Bd->J/K*


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Preliminary systematics studiesPreliminary systematics studies

s/s (%)

Mistag =(341)% 7

Proper time resolution = (404)fs 5

Angular acceptance distorted by 5% 51

KK S wave of 10% under the 20

Doubly Cabbibo suppressed penguins ?

s measurement dominated by statistics at least up to 2fb-1 ?

Some systematics biases increases with s Work still ongoing


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Sensitivity versus integrated Sensitivity versus integrated luminosityluminosity


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Conclusions and prospectsConclusions and prospects

BsJ/ hot topic and promising way to discover NP at LHCb But also one of the most challenging !

Critical points: Need all LHCb detectors working and calibrated Flavour tagging Angular / proper time acceptance Proper time resolution Fit stability with low statistics

Tevatron expects (2s)~0.13 by the end of 2010 LHCb with 2fb-1

117k BsJ/(), (2s)~0.03 Could expect some deteriorations with more realistic MC and real data

However, if true 2s value is the current Tevatron one ~0.8 (NP-like), we should measure it with ~0.2fb-1 !

Standard Model: 2s=0.0370.002

prospects for the measurement of mixing induced cp violation in b s j/ at lhcb

Documents

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induced cp violation

bsbs box diagrams

cpj cp

cpevenolivier leroy

cp odd

mixture of cp

bs rest frame