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Approval Presentation, 17.08.11

LHCb-ANA-

2011-062

Motivation for Measurement• The final state Ds

-K+ is accessible by both Bs and Bs:

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• Both diagrams are similar in magnitude, hence large interference between them is possible.

• Using flavour tagging, we can measure four decay rates – Bs or Bs to Ds

+K- or Ds-K+

• From these rates, γ can be extracted in an unambiguous and theoretically clean way.

• A review of the γ extraction can be found in e.g. LHCb note 2007-041.

Branching Ratio Strategy• A reliable extraction of γ requires a considerable amount of data

– Aim is to present first γ measurement from Bs→DsK at Moriond 2012.

• First step towards measuring γ is to observe Bs→DsK in our data, and measure its branching ratio (BR) relative to Bs→Dsπ.– Most systematics cancel in the ratio– Main differences between the modes are the bachelor PID requirements and

the smaller Bs→DsK yield.

• Analysis strategy for the relative BR measurement follows that used for the hadronic fd/fs measurement from 2010 data.

• Independently, the hadronic and semileptonic fd/fs measurements from 2010 data can be combined to extract BR(Bs→Dsπ). This is then used to measure BR(Bs→DsK) absolutely. 3

Current Experimental Status: DsK• PDG value is BR(Bs→DsK) = (3.0 ± 0.7)*10-4 (23% relative error)

– Calculated by rescaling Belle(*) measurement: BR(Bs→DsK)= (2.4 +1.2/-1.0(stat)± 0.3(syst) ± 0.3(fs) )*10-4. This uses 7±3 signal events.

– In addition, CDF(**) measures BR(Bs→DsK)/BR(Bs→Dsπ) = 0.097 ± 0.018 ± 0.009 with ~100 DsK candidates, using a combined mass-PID fit.

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Bs→DsπBs→DsK

(**) PRL 103 191802(*) PRL 102 021801

CDF

Belle

Current Experimental Status: Dsπ• BR(Bs→Dsπ) = (3.2 ± 0.5)*10-3 (16% relative error), from combining

– Belle(*): (3.67 ± 0.34(stat)± 0.43(syst) ± 0.49(fs) )*10-3 with 160 events

– CDF(**) : (3.03 ± 0.21(stat)± 0.45(syst) ± 0.46(fd/fs) )*10-4 with 500 events

5(*) PRL 102 021801

(**) PRL 98 061802, rescaled to new BR(Bd→Dπ)

CDF

Belle

What about LHCb?

• Today we are requesting approval of preliminary results for BR(Bs→DsK)/BR(Bs→Dsπ), BR(Bs→Dsπ) and BR(Bs→DsK).

• We plan to write a paper in the very near future.

• Plots for approval are marked with

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For Approval

Data Sample and Trigger/Stripping

• The analysis uses ~336pb-1 of 2011 data. • Trigger requirements are– L0: Hadron TOS or Global TIS – HLT: HLT1Track and HLT2 Topo BBDT TOS (2,3 or 4 body)

• Stripping lines from B2DX module (with D2hhh)– No PID is used, to allow inclusive selection of all the relevant modes

• Relative efficiency of reconstruction, trigger and stripping is checked on MC that has been reprocessed with a 2011 TCK (0x006d0032)– Results on later slides

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Offline Selection: BDTG• For the 2010 fd/fs analysis, TMVA was used to check the performance

of different provide classifiers as an offline selection, using kinematic and geometrical variables– Role of the MVA is to minimise combinatorics (not physics backgrounds)

• The best performing MVA was the Boosted Decision Tree with Gradient boosting (BDTG).

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• In the current analysis, the 2010 BDTG is retained, but optimal cut is re-evaluated– Optimal working point need not be the same, as the trigger has changed

• Re-optimisation for 2011 is performed using 10% of the Bs→Dsπ data (uniformly distributed in time)

BDTG (Re-)Optimisation• Figure of merit is

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• Here, B refers to combinatoric bkg only, and 1/14 is the expected Cabibbo suppression factor between Bs→Dsπ and Bs→DsK.

• Choose start of significance plateau (BDTG>0.1) as our working point.• This cut loses 6% of signal, for a background reduction of 45%.

PID Calibration• Correctly calibrating the PID cut efficiencies is crucial for this analysis.• Extensive use is made of the tools developed by the RICH group.• The D* (for K and π) and Λ (for proton) calibration samples are

binned in momentum and pT, and the resulting efficiency map is used to weight signal events.

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• Magnet Up and Magnet Down data are calibrated separately– PID performance is not constant

in time, as RICH calibration needs to be propagated to the more recent data

• Separation between K and π is poor above 100GeV, hence apply a cut of p<100GeV on the bachelor.

Example: DLL(K- π)>5 (1D binning for visualisation)

K eff

π misID

PID Cuts: D Daughters• For the moment, only the Ds→KKπ mode is considered

– Other modes could be added in the future

• To obtain clean samples of Bs→Dsh, PID cuts need to be applied to the D daughters to suppress Bd→D+h and Λb→ Λch.

• Hence on the Ds+→ K- K+ π+ candidate we require:

– DLL(K- π) > 0 for the K- and DLL(K- π) < 5 for the π+ (to suppress combinatorics)– DLL(K- π) > 5 for the K+ (to suppress D+ →K- π+ π+)– DLL(p-K) < 0 for the K+ (to suppress Λc

+ →K- p π+)

– K+ failing DLL(p-K) < 0 are retained if Kpπ mass is outside the Λc mass window

• Applying these cuts and a mass window of [1944,1990]MeV gives:– Efficiency of 78% for Bs→Dsπ (using momentum distributions from MC),

– MisID of 1.2% for Bd→D+π (using momentum distributions from data),

– MisID of 1.7% for Λb→ Λcπ (using momentum distributions from MC).11

Bs→Dsπ Purity after PID Cuts

• After these cuts, the Bs→Dsπ peak is rather pure

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PID Cuts: Bachelor

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• For the Dsπ fit, a cut of DLL(K-π)<0 is applied, to eliminate any residual contamination from DsK.

• A hard cut of DLL(K-π)>5 is applied before doing the DsK fit, to suppress the favoured Dsπ mode.

• As a cross-check, DsK fit is also done with a loose cut of DLL(K-π)>0, and a very tight cut of DLL(K-π)>10.

• The efficiencies of these cuts, applied after the p<100GeV cut, are:

Efficiency Ratios from MC• Ratio of generator level efficiencies is found to be 1.027±0.010. Until

the reasons for this are understood, the 1.027 is used as a correction factor, and a systematic of 2.7% is applied.

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• Ratio of efficiencies for reconstruction, trigger, BDT cut and upper momentum cut on the bachelor is 1.03±0.01. This correction factor is applied, and the associated systematic is conservatively set to 3%.

Signal Lineshapes

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• The B mass uses the D(s) mass constraint (improves resolution).

• Different shapes are tested on the MC signal samples.• Deafult shape is double Crystal Ball, with common mean & sigma• Radiative tail is smaller for modes with bachelor K than bachelor π.

Bs→Dsπ Bs→DsK

MisID Background Shapes • The physics bkgs to the different modes often involve misidentified

hadrons. So getting the misID’d mass shapes correct is important.• Example: the shape for Dsπ bkg to DsK is obtained as follows:

• Firstly, a clean sample of Dsπ is extracted from the Dsh data by applying DLL(K-π)<0 on the bachelor

• This cut biases the bachelor momentum, however the original momentum distribution can be recovered from the whole Dsh sample– This works because the Dsπ and DsK bachelor momenta are very similar

• Then the mass is recomputed under the DsK hypothesis

• Next, the shape is weighted according to the momentum spectrum of the misidentified bachelors– This from the original momentum distribution and the misID rate as a function

of momentum 16

MisID Background Shapes

• The shapes for Dsπ and Dπ under the DsK are sufficiently similar that in the fit only the Dsπ shape is used

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• The shape for Dπ bkg to Dsπ is obtained in a similar way, changing D daughter mass hypothesis instead of the bachelor.

• The shape for the DK bkg to DsK should be the same as the Dπ bkg to Dsπ.

Under the DsK mass hypothesis

An Incidental Discovery…• A bump was seen in the DsK fit at around 5500MeV, that was not

described by the misidentified Dsπ shape.

• The bump was investigated, and it turned out to be Λb→ Dsp!

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• A peak is seen at the Λb mass after switching to the Dsp mass hypothesis, applying extremely tight PID cuts (DLL(p-π)>10 and DLL(p-K)>15) on the bachelor, and tightening the BDT cut.

• In the future a measurement will be made of the BR of this mode, but for now…

A peak is also seen at lower mass, compatible with Λb→ Ds

*p

Λb→ Ds(*)p Shape Under DsK

• Cutting on DLL(p-K) would lose too much signal, so we must live with this background, and model its shape.

• The shape is taken from simulated events, after reweighting for the efficiency of the DLL(K-π)>5 cut as a function of momentum.

• The Λb→ Ds*p shape is obtained by shifting the Λb→ Dsp shape down

by 200MeV. As a baseline, the relative amount of Λb→ Dsp and Λb→ Ds

*p is assumed to be the same.

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Λb→ Dsp plus Λb→ Ds

*p • The amount of Λb→ Dsp in the DsK

fit is estimated by taking the 24 events from the previous slide, and correcting for the efficiency of the tight PID cuts and the BDTG cut.– This gives an expectation of ~150

events (Λb→ Dsp + Λb→ Ds*p)

Partially Reconstructed (and other) Bkgs• For partially reconstructed physics bkgs, the shapes are taken from

MC, with data-driven momentum reweighting applied where a misidentification is involved. PDFs are made using RooKeysPDF.

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• One final type of physics background needs to be considered: charmless modes such as Bs→K*KK

• These can appear if no cut is applied on the flight distance of the D from the B vertex– They can peak under the signal

• To remove such backgrounds, a soft cut of FDχ2(D from B) > 2 is applied. This will have the same efficiency for Bs→Dsπ and Bs→Dsπ, so will not affect the ratio of BR’s.

Combinatoric Background Shape• The slope of the combinatoric background can floated in the Dsπ fit.

• However it must be fixed in the DsK fit, due to the low statistics and the presence of the misidentified Bs→Dsπ in the right-hand sideband.

• Fitting to wrong-sign (same-sign D and bachelor) events passing the DsK selection and PID cuts, the slope is compatible with being flat.

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• As a cross-check, the wrong-sign events passing the Dsπ selection are also fitted, and the slope agrees well with that found in the Dsπ signal fit.

DsK wrong-sign

Splitting by Magnet Polarity• Since the PID efficiencies vary slightly between MagUp and

MagDown, the misID background shapes change.• In addition, the signal mean is found to shift by ~1MeV between

MagUp and MagDown.• So we split the data by polarity, and fit the two subsamples

independently. • About 55% (45%) of the data is MagDown (MagUp).

• In the following slides, the MagDown fit is on the left, and the MagUp on the right.

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Recipe for Dπ Fit• This fit is needed to estimate the amount of background Dπ to Dsπ,

and to check the mean and sigma of the signal shape with high statistics.

• The tails of the signal mass shape are fixed from the MC fit, but the mean and sigma are floated– Mean and sigma are allowed to be different for MagUp and MagDown

• The yields of all components are left free.• The slope of the combinatoric background is also floated in the fit.

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Fits: Dπ with PIDK<0

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Recipe for Dsπ Fit

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• The expected number of misID Bd→Dπ is calculated using the fitted Dπ yield, a mass window factor (from MC), and misID from the PID calibration tools. – It is constrained to be within 10% of this estimate

• The misID Bd→Dπ shape is also reweighted to take misID curve vs momentum into account.

• The signal width is fixed to that found in the Bd→Dπ fit, scaled by the ratio of widths for Bs→Dsπ and Bd→Dπ in the MC

• Signal mean and comb background slope are floating. • A Λb→ Λcπ component was allowed in the fit, but got fitted to zero.

• Some Bd→Dsπ can also be seen – re-use Bs→Dsπ mass shape, and constrain yield to {known BR ratio*fd/fs} = 1/35

relative to Bs→Dsπ yield.

Fits: Dsπ with PIDK<0

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For Approval

For Approval

Recipe for DsK Fits• The amount of misID Bs→Dsπ background is floated

– Provides x-check on misID rate estimate– Any Bd→Dπ should be taken care of by the Bs→Dsπ shape

• Treatment of signal shape is the same as for Bs→Dsπ

• Comb background slope is fixed to be flat (from wrong-sign)

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• Amount of Bd→DK is constrained from the Bd→Dπ under Dsπ, using the Bd→DK/Bd→Dπ BR ratio

• Relative yields of PartReco backgrounds are constrained using – Relative reconstruction efficiencies (from MC) when e.g. a charged track or

soft pion/photon is missed– Bs branching ratios from Bd branching ratios, using SU(3) symmetry– The yields can ove by 33% from these estimates

• The Bd→DsK yield is floated. The Bs→DsK and Bd→DsK shapes are the same.

• Amount of Λb→ Ds(*)p is constrained as detailed earlier.

Fits: DsK with PIDK>5 (default)

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For Approval

For Approval

Fits: DsK with PIDK>10 (cross-check)

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For Approval

For Approval

Fits: DsK with PIDK>0 (cross-check)

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For Approval

For Approval

Remark on Bd→DsK

• While this component is clearly visible in the DsK fits, the amount of background underneath it makes a reliable fit to its yield very difficult, at least with the current dataset.

• Hence we cannot make a competitive measurement of its BR (error in PDG is ~13%).

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Systematics Menu for BR Ratio• Ratio of trigger/stripping/(non-PID) selection efficiencies from MC • Fit model systematics will be evaluated using a large number of toy

fits (as was done for fd/fs analysis).

• But for the moment, we simply apply cross-checks on the data, and assign conservative systematics.

• For the PID, take systematic on efficiency curves quoted by the RICH group, evaluated at our cut values

• PID systematic can enter in three different ways:– Final PID efficiency correction to obtain BR(DsK)/BR(Dsπ)

– Shape of misID bkgs after reweighting – Expected number of Dπ/K under Dsπ/K (constrained in the fit)

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Systematics Budget for BR Ratio• The fit model systematic for DsK is the most involved part of the

systematics calculation.• The main contributions to this part are:

– The slope of the combinatoric is fixed to half of the Dsπ slope. This reduces the signal yield by 3%.

– The constraints on the partially reconstructed backgrounds are all varied by a factor of two. This changes the signal yield by ±4%.

– Also, the ratio of the Λb→ Ds*p component to the Λb→ Dsp component was

varied by a factor of two. The change to the signal yield is <0.5%.

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Results: BR(Bs→DsK)/BR(Bs→Dsπ)

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• Averaging MagUp and MagDown, we get• N(DsK) = 406±26, N(Dsπ) = 6038±105• εPID(DsK) = 83.4±0.2%, εPID(Dsπ) = 85.0±0.2%

• εsel(Dsπ)/ εsel(DsK) = 0.945± 0.014

• We obtain

Extraction of BR(Bs→Dsπ) • Basically we turn the 2010 fd/fs combination on its head, by

combining the ratio of yields of Bs→Dsπ and Bd→Dπ from the hadronic analysis, and the fd/fs value from the semileptonic analysis

35Output:

Input:

Results: BR(Bs→DsK)

• Combining these two results, we obtain

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• This agrees with the Belle result, but is significantly below the CDF result.

Conclusions• Using 2010 data we measure

• With 336pb-1 of 2011 data we measure

• These are combined to yield

• These are all World’s Best measurements.• Last but not least, we would like to thank our referees, Stefania

Vecchi and Stephane Monteil, for their quick work which has been very helpful in improving our analysis!

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Backups

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Extracting γ from DsK

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Strong phase difference

BDT Training (2010)

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• The MVA was trained for the fd/fs analysis using a small (2pb-1) subsample of the 2010 data

• Several MVAs were tried, the Boosted Decision Tree with Gradient boosting (BDTG) was found to have the best performance

CDF Measurement

41PID variable (uses de/dx)

Bachelor Momentum in MC

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PID Eff Curve for DLLK<0

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PID Efficiencies from Calib Tools

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These are for the bachelor momentum spectrum, after the p<100GeV has been applied.

Results of Signal Fit in MC

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Example of PartReco Bkg

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Shape for Bd→D*-π+ from 2010 MC, under pion (left) and kaon (right) mass hypothesis for the bachelor

Shape of Dπ bkg to Dsπ (from MC)

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Wrong Sign Fit for Dsπ

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Comparison to Theoretical Expectation

• As a side-product of the 2010 fd/fs analysis, we measured:

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• Whereas we now measure :

• Bd→DK Has only one tree diagram, while the Bs→DsK has two

• So our result suggests that the two different tree diagrams contributing to the DsK final state interfere destructively