Laurence Carson, Beauty 20091 Charmless Hadronic B Decays at LHCb Laurence Carson, University of Glasgow (on behalf of the LHCb collaboration) 12 th International

Laurence Carson, Beauty 2009 1

Charmless Hadronic B Decays at LHCbLaurence Carson, University of Glasgow

(on behalf of the LHCb collaboration)

12th International Conference on B Physics at Hadron Machines

Heidelberg, 7th – 11th September 2009


Outline

• Overview of LHCb detector

• Physics opportunities in charmless hadronic B decays at LHCb– Measurement of γ in decays involving penguins

• Using Bd→π+π- and Bs→ K+K-

• Using B+/-→K+/-π+π- and Bd→KSπ+π-

– Search for rare decay modes– Many other analyses not covered here

• e.g. α measurement with Bd→ρπ, ρρ and ππ


• Take advantage of B production at LHC– pairs in 2fb-1

(nominal year @ 2*1032cm-2s-1)

– Produce all B species: B+,Bd,Bs,Bc,Λb,…

• Use trigger(*) to cope with high event rate– Level 0: hardware trigger on high pT/ET

leptons or hadrons

– High Level Trigger: matches L0 object to track with displaced vertex, then runs inclusive & exclusive selections

The LHCb Detector

bbbbb

1210500

Unique angular coverage: 1.9 < η < 4.9

Single forward arm spectrometer

(*) see talk on Friday by L. de Paula


bkg

Hadronic Modes at LHCb• Vertex Locator (VeLo): Si strip

detector giving στ ≈ 40fs; allows displaced vertex trigger and time-dependent Bs studies

• Cherenkov Counters (RICH): Excellent p/K/π separation for 2<p<100GeV; can separate out final states

Without RICH PID

With RICH PID

PID example: Bs→K+K-

Bd→π+π- τ

residualBs→K+K-

oscillations with 2fb-1 width = 39fs

Mass resoln ≈ 17MeV


Current Status of γ• Least well constrained parameter of the “db” unitarity triangle.

• Indirect measurement from fit to apex gives γ = (66.8 +5.4-3.8)°.

• Direct measurements from time-independent analyses of B+/- → D0(*)K+/-(*) (“tree-only” decays) give γ = (70+27

-29)°

– Analysis method (ADS/GLW/GGSZ) depends on D decay mode.

• Such tree-only decays expected to give SM value for γ.

Following talk (S. Ricciardi) will discuss how LHCb can improve on current γ measurements using tree-only decays.


LHCb γ Strategy

• Measurements of γ from decays involving both tree and penguin amplitudes are sensitive to New Physics effects– Compare to SM “standard candle” γ value from tree-only decays.

– Significant disagreement would be unambiguous sign of NP.

• Prospects for two such measurements have been studied by LHCb– Measurement of time-dependent CP asymmetries in Bd → π+π- and Bs

→ K+K- (method: R. Fleischer, Phys. Lett. B 459 (306) 1999).

– Combined Dalitz analysis of B+/- → K+/-π+π- and Bd → KSπ+π- (method:

I. Bediaga et al, Phys. Rev. D 76 (073011) 2007).

Gluonic penguin in SM

Gluonic penguin in MSSM


Measuring γ with Bd → π+π- and Bs → K+K-

Method first proposed by R. Fleischer, Phys. Lett. B 459 (306) 1999


• εgeo acceptance~21%, εselection(no PID)~18%, εtrigger~36% εtotal~1.4%

B→hh at LHCb• Studies of two-body charmless hadronic (“B→hh”) decays at LHCb

summarised as part of forthcoming “physics book”.– Including selection/bkg studies, calibration/systematics, and γ extraction.

Expected total mass distribution (ππ hypothesis) with 0.2fb-1

Channel 2fb-1 Yield (no PID)

Bd → π+π- 59k

Bd → K+π- 217k

Bs → K-π+ 15k

Bs → K+K- 72k

Λb → pπ- 7k

Λb → pK- 11k

3-body bkg [76k,186k]@68% C.L.

Combinatorial bkg [106k,226k]@68% C.L.


Some Calibrations for B→hh

• Proper time resolution model is Gaussian with mean and width depending on per-event proper time error στ

– Model can be determined on data using Bs → K-π+ (if mistag is known).

• Proper time acceptance distorted by IP cuts in trigger and selection – Can be determined on data by “swimming” the B to build up event-by-event

acceptance (top hat).

mean = GM.στ width = GS.στ

swimB→hh acceptance after: offline selection offline selection + trigger

• PID likelihood calibrated using D*+→ D0(→K-π+)π+ and Λ → pπ-.


γ from Bd → π+π-, Bs → K+K-

• Direct and mixing induced CP asymmetries measured using tagged, time-dependent analysis:

)2/sinh()2/cosh(

)sin()cos(

)()(

)()(

tAt

mtAmtA

fBfB

fBfB mixCP

dirCP

• Have 4 asymmetries depending on 7 physics parameters: ,

, , .

),,(1 dfAdir ),,,(2 d

mix dfA ),','(3 dfAdirKK ),,','(4 s

mixKK dfA

• Assume U-Spin (d↔s) symmetry d=d’ and θ=θ’

• Take φd from J/ψKS system becomes soluble.

• d (d’) is related to the ratio of the magnitudes of the penguin and tree contributions for the Bd (Bs) decay, while θ (θ’) is related to the difference in their strong phases.


LHCb B→hh Fit Approach• Fit for branching ratios, lifetimes, CP asymmetries, and detector parameters

is made in all B→hh channels simultaneously (68 free parameters)– Mass and PID likelihoods used to separate modes.

• With 2fb-1 of data, each of the four CP asymmetries from Bd→π+π- and Bs→K+K- will be measured with a statistical uncertainty of ≈0.04.

• For the extraction of γ, Adir(KK) can be replaced with ACP(Bd→K+π-), since they are expected to be equal given U-spin symmetry, and the uncertainty on ACP(Bd→K+π-) with 2fb-1 will be <0.01 (systematics dominated).

Fit to Bs→K+K- mass distribution under ππ hypothesis (full MC)

PID distribution for 0.2fb-1 toy MC sample

π K p


LHCb Sensitivity to γ• Extraction of γ allows for some U-spin breaking, with 0.8 < d’/d < 1.2 and

-20° < θ’ - θ < 20°.• SM solution for γ is isolated by imposing the condition θ>90°.

68% C.L.95% C.L.

γ (input 70°) φs (input -0.04 rad)

• With 2fb-1, find σγ = 7°, σφs = 0.05 rad , σd = 0.13 and σθ = 7°.

Contours in (d,γ) plane without imposing θ >90°

• Alternative strategy would be to constrain φs (expect σφs≈ 0.03 rad from Bs→J/ψφ with 2fb-1) and

release one of the U-spin constraints d=d’ or θ=θ’.


Measuring γ with B+/-→K+/-π+π- and Bd→KSπ+π-

Method first proposed by I. Bediaga et al, Phys. Rev. D 76 (073011) 2007


• B+/-→K*0π+/- penguin only; Bd→K*+π- penguin and tree

– Extract penguin amplitude (VtbV*tsP) via standard Dalitz analysis of B+/- →K+/-π+π-

– Use this as input to untagged Dalitz analysis of Bd→KSπ+π- to extract tree amplitude (VubV*usT)

– Tagging not required as resonance interference regions for Bd and Bd are distinct

– Parameters extracted relative to contribution from χc0K (common contribution to Bd and B+/-)

γ from B+/-→K+/-π+π-, Bd→KSπ+π-

Combined: two distinct regions of resonance interference

Bd→KSπ+π- Bd→KSπ+π-

• Tree amplitudes in three-body charmless hadronic decays involve Vub

– Probe γ using interference between intermediate resonances

Resonances: K*(892)+/-, K*(1430)+/-, ρ(770)0, f0(980), χc0

m2(KSπ+) m2(KSπ+) m2(KSπ+)

m2 (

KSπ- )

m2 (

KSπ- )

m2(K

Sπ- )


B+/-→K+/-π+π-, Bd→KSπ+π- at LHCb

• Lower efficiency for Bd→KSπ+π- due to lower KS reconstruction efficiency– 25% decay in VeLo good efficiency/resolution– 50% decay between VeLo and TT Stations poorer efficiency/resolution– 25% decay downstream of TT Stations track lost

Channel B.R. εselection 2fb-1 yield (no HLT)

B+→K+π+π- 5.5*10-5 1.1% 494k

Bd→KSπ+π- 2.2*10-5 0.5% 90k

• Selection: want to avoid tight cuts on pT as this lowers efficiency in corner of Dalitz plot, where some of the resonances lie– Trade-off between minimising bias and reducing

combinatoric bkg

B+→K+π+π- with pT >200 MeV for

all daughters

• Sensitivity to γ : idealised toy MC study (no background or detector effects)

estimates σγ ~5° (stat. only) with 2fb-1.


• Baryonic mode Bd → pp unobserved SM predictions close to being ruled out

– If true B.R. close to current limit (1.1*10-7), expect evidence with ~0.2fb-1 (2010 run)

– Also expect to observe Bd → K+K- and Bs → π+π- (penguin annihilation and W exchange only) with ~1fb-1 (if SM B.R.s).

Rare Hadronic B Decays at LHCb

Specific bkgs to Bd→pp suppressed by RICH PID

• B+ → K-π+π+ and B+ → K+K+π- highly suppressed in SM– B.R.s O(10-14) and O(10-11) respectively.

• Can be strongly enhanced by NP effects– e.g. B.R.(B+→K-π+π+)~10-6 (≈ current B.R. limit) with Z’.

• If such enhancements are present, LHCb may be able to make an observation.


• Search for undiscovered rare decays– Expect to observe Bs → π+π-, Bd → K+K- and Bd → pp with early data, whether SM

branching ratio or not

– Can observe B+ → K-π+π+, B+ → K+K+π- if B.R. strongly enhanced by NP.

• Measure α using Bd → ρπ, ρρ, and ππ

– Expect σα(stat) ~ 5° after 2fb-1 (dominated by ρπ).

Conclusions• LHCb has a rich physics programme using charmless hadronic decays:• Measure γ from decays with penguins and compare to SM value from tree-

only decays– Expect σγ = 7° after 2fb-1 from analysis using Bd → π+π-, Bs → K+K-

– Measurement with B+/- → K+/-π+π- and Bd → KSπ+π- should be competitive (effect

• We are awaiting the first collisions…

of backgrounds and systematics to be checked).


Backup Slides


More Calibrations for B→hh

• PID calibrated using PID-unbiased samples of D*+→ D0(→K-π+)π+ and Λ → pπ-

– Dedicated stream in trigger

– Re-weight likelihood distribution for B using p and pT distributions.

• Tagging performance for Bd measured using flavour-specific Bd → K+π-

– For Bs tagging, Bs → K-π+ has less statistics and is sensitive to proper time resolution

– Can study Bd-Bs difference in other modes (e.g. Bd → D-π+ and Bs → Dsπ+)

– Can also calibrate using per-event mistag.

DLL distributions: Bd →π+π-, π from D → Kπ before reweighting, π from D → Kπ after reweighting.

Per-event mistag for Bd → K+π-


Measuring α with Bd → π+π- π0

• α can be measured via a tagged Dalitz analysis of Bd → π+π- π0

– Dominant submodes are ρ+π-, ρ-π+ and ρ0π0

– Isospin symmetry used to extract α, tree/penguin amplitudes and strong phases.

• Multivariate selection used, εtotal~0.07%. Expected 2fb-1 yield is 14k, with B/S~1.

Resolved π0

Merged π0

• 2fb-1 toy experiment gives α = (97+9-4)° (stat. only)

Sum of Dalitz coords

Effect of ET cut on π0

Time evolution of tagged Dalitz plotsbefore

after

Documents

Laurence Carson, Beauty 20091 Charmless Hadronic B Decays at LHCb Laurence Carson, University of Glasgow (on behalf of the LHCb collaboration) 12 th International