Aart Heijboer ● Observation of Bs oscilations ● Nikhef special seminar ● 01/08/07 ● slide 1...

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Aart Heijboer ● Observation of Bs oscilations ● Nikhef special seminar ● 01/08/07 ● slide 1 ● 03:44:44 PM

Aart HeijboerUniversity of Pennsylvania

previous result: PRL 97, 062003 (2006)current result: PRL 97, 242003 (2006)

PRD in preparation

outline: motivation what do we look for / previous results the measurement and how we improved it the observation

Observation of Bs oscillations at CDF

Introduction & motivation

What do we measure?There is a diagram which allows a B

s meson to

transform into its own anti-particle

mass difference

MH = M + M/2

ML = M - M/2

Because of a coupling between the two states, we get: two separate mass eigenstates oscillation between B

s and B

classical version

Energy eigenstates (normal modes)

By measuring the frequency,we are measuring the strength

of the coupling

motivation

)(2/14

cbcscd

ubusud

most uncertain parametersand

To get at Vtd: measure m

d oscillations)

hard to calculate (lattice QCD). uncertainty: 10%

1.210.05

q=s or d

If we measure ms, we

can form the ratio whichallows for much more accuratemeasurement of V

td(hep-lat/0510113)

pretty wellknown

parameterization assuming unitarity

motivation

2005 Limit on ms was

already helping to measureCKM matrix...

Plot Combines many different measurements to constrain and .

Check that all measurementsare consistent: i.e. that CKMmatrix is unitary.

Vtd=A3(1--i)

motivation

Effect of measuring ms to

the level of a few %=> shrinking the orange band

Vtd=A3(1--i)

motivation

Effect of measuring ms to

the level of a few %=> shrinking the orange band

Vtd=A3(1--i)

Turn the argument around: standard model prediction:

Go measure if SM is right. If the answer is no.....

motivation: new physics

New particles can enhance the Bs B

s transition

harnik et al. hep-ph/0212180

supersymmetry with largemixing between squarks

ms is sensitive to new physics

Phys.Lett.B596:229-239,2004

history

UA1 resultPLB 186 (1997) 247

1987 UA1: neutral B-mesons mix Argus measures fraction of B

d mesons

that mix (~18%) => ~half of the Bs mesons must mix => ms is large compared to the decay time => need to resolve rapid oscillations: hard!

1+ (m)-2

fraction of mixed events

Bs case:=0.5 -> no handleon ms

How do we look for mixing?

Make a Bs meson and measure

its flavor when it decays Identify the flavor at production (tagging) Look at oscillation probability vs proper

decay time.

Fourier component of asymmetry:Mixing amplitude

asymmetry=N

unminx-N

mixing frequency

history-II: looking for oscillations

LEP, SLD, CDF RUN-Ihave tried, but have not seen a convincingsignal: lower limits on m

2005 world average:compatible with mixingbut not significant enough

We know Bs mixes, now try tosearch for the right frequency m

history: recent results

D0 resultlarge amplitude around m

s = 19 ps-1

CDF's previous resultamplitude compatible with 1 around 17.3

history: recent results

D0 result A double sided 90% confidence

level interval. (17-20 ps-1) p-value = 5% (now 8%)

CDF's previous result p-value = 0.2% (3) linguistics “under the assumption that

this is signal, we measure:”

When are we sure we are not seeing a fluctuation?convention: 5: p-value = 5.7 x 10-7 => ~no doubt => publish “Observation”

PRL 97, 062003 (2006)

Upgrading the analysis

signal selectionuse neural networkuse particle-ID on kaon-candidatesadd partially reconstructed decaysadd some trigger paths

taggingkinematic information same side taggercombine opposite side taggers optimally

Previous analysis had: precise measurement of ms, but the significance

was not enough (0.2%).Goal for new analysis: remove all doubt and go for the conclusive, final, 'Observation'reblind the analysis & improve until we have a chance to see 5

most likely

contain 90% ofexperiments

previousresult

1) Getting the signals

The Tevatron

1.2 fb-1 collected,1 fb-1 in 'good' runs

~2/3 of the datacollected in 2005

no new data added since previous result

both are 1 fb-1

The CDF detector

momentum &dE/dx

decaytime

The signals we are looking for

lss lDB 0

Semileptonic

Hadronic

ss DB0

Bs Momentum is measured

Bs mass used for good S/N

Small branching ratio: low yield

Missing momentum ()Need to rely on D

s mass

Large branching ratio: high yield

Bs /Bs

A typical B event at a hadron collider

=> look for displaced tracks

Trigger on events with twodisplaced (d>120 m) tracks

dvery fast reconstruction of

silicon data at L2 (20s latency) by dedicated hardware: SVT

Displaced track trigger

Hadronic signals

Added partially reconstructed modesin the Ds-> mode.

undetected particles have smallmomentum => decay is

almost fully reconstructed

Hadronic signals

reflection of less severe because we checkthat the PID of the kaon. Allows looser veto based onM

KK - MD+

Added one new decay modeB

s -> six pions

hadronic signal selection

signal selection previously doneusing cuts on kinematic variables,vertex displacement, fit quality....

select signal using NNinputs: O(30) variables includingall of the above & P

T of tracks,

Ds's 's, K*, PID

particle-ID likelihoodsuse measurement of time-of-flightand dE/dx in the COT to distinguishkaons from pions.

careful to make sure NN is not trained to select mass. (e.g. R notused as input variable)

gain signal lose onlybackground

hadronic yields

8700 signal events (was 3700)improved S/B too

equivalent to an increase in sample size of a factor 2.5

Semileptonic signals

improvements: using particle ID to identify kaons

reduces combinatorial backgroundeliminates need for explicit D+ rejection

62k Bs signal events(was 37k)

Proper decay time and resolution

Proper time reconstruction: hadronic

piece of cake!

Bs mass from the PDG, PT from adding up the moment of all the decay tracks

Proper time: semileptonic

● k-factor PDF obtained from Monte Carlo● Take advantage of variation with l D

s mass.

● high mlDs

means small missing pT)

● can not measure pT for semileptonic

● large uncertainty in ct for high Lxy● correct for missing p

T on average:

Proper time resolution

Effect of non-zero ct error asymmetry: attenuation of the oscillation

Smearing of decay time causes attenuation of asymmetry signal: Have to know

ct to measure the mixing amplitude

How to measure ct?

Measuring the proper time resolution

Cannot measure the ct resolutiondirectly on data (no prompt peak in theB

s signal due to trigger)

Solution: construct events that look like a B but are known to come from the PV..

real D

some pionfrom the underlyingevent (i.e. from PV)

True <ct> must be zero; compare errorwith predicted error from the vertex fit.

And study dependence on kinematicvariables, isolation, 2 of fit etc..

proper time resolution

period for ms = 18 ps-1

● partially reconstructed hadronic almost as good as the fully rec.● time resolution = ¼ oscillation period

3) Flavour tagging

Flavor tagging

We need to know the flavor of the Bs at production.

Opposite side tag:look at the decay products of the other b quark in the event:

LeptonKaonCharge of the b-'jet'

the two b quarks fragment independently: can calibrate opposite side taggers with B

uother B often outside acceptance

Same side kaon tag: look at particles produced in B meson formation (K in case of Bs)

Very powerful (high acceptance)but cannot calibrate on B

uNeed to rely on MC

A tagger is characterized by: efficiencyD: dilution = 1-2 x mistag rate (large dilution is good)

Dampens asymmetry: Effective statistical power / figure of merit = D2

We must know what D is to measure A

D<1 dampens the oscillation signal

Flavor tagging II

OST: improvementprevious analysis:

In case multiple tagger gave a decision, used the best onenew analysis

combine all tagger decision, per-event predicted dilutions and kinematicvariables into a single Neural Network, producing tag decision & dilution.

●Fragmentation into Bs tends to

produce an additional kaon.●Charge of K identifies B

s flavor at production

●B0 and B+ mostly accompanied by pions●Use combined likelihood from time-of-flight detector and dE/dx in COT identify Kaons

Same side kaon tagging

tagger not optimized for these modes, but valuable checkthat MC predicts the right dilution

Same side kaon: improvementsSame side kaon: improvements

Previously based only on particle IDKinematic variables can also be used for

for selecting the tagging trackCombine in Neural network

PID likelihood, R, pT p

Trel, p

Lrel,...

improvement in D2 based on predicted dilutions for events: semileptonic data: +8% hadronic data : +0% monte carlo : +6%

1.8%OST

3.7(4.8)%

final tagger performance

Blind Amplitude Scan

(July 25)

With all the improvements:

A < 0.2 → good chance of seeing 5.

unblind!

(old value: 0.28)

Results

depth of likelihood dip=-17.26

what is the significance of such a dip?

old value

significance

“Observation”

Randomize the tagger decisions of the data events and repeat the likelihood scanmany many, many, many, many, many, many, many, many, many, many, many, many, many, many, many, many, many, many, many, many, many, many times

comparing taggers

Opposite side tagger Same side kaon tagger

Both taggers see the oscillations

Comparing datasetspartially reconstructed hadronic modes

semileptonic modes

fully reconstructed hadronic modes

all see the signal!

measurement of ms and |V

td|/|V

uncertainty of length scale ofdetector

limited by theoryerror

asymmetry

mtPPPP

Amixumix

mixumix cos

asymmetry

stack all oscillation periods on top of each otherand fit for A in bins of proper decay time...

data are well described by theexpected cosine curve.

hadronic data only

asymmetry

conclusions

We have 'Observed' Bs mixing p-value = 8x10-8, significance is 5.4

We measure

World average back in 1999justforfun

Abbaneo & Boix: JHEP 08(1999)004“p-value = 3%”

all data before 2004LEP + SLD

World average before Tevatron run II

Amplitude scan: fix ms and measure the amplitude

of the corresponding frequency component (Fourier)

Recent D0 result● 17 <ms <21 ps-1 at 90% CL● p-value = 5%

previous results

likelihood and amplitude scan

Amplitude: quantity constructed so thatA=1 if the data (i.e. the likelihood) consistent with mixingA=0 everywhere else

Amplitude is 'easy' to understand, but we use thelikelihood to make the measurement, namely:

signatureof a signal

●resolution ~ 100 ps●separates kaons from pions up to 1.5 GeV●crucial for SSKT

Time of flight detector

Expected CDF Sensitivity

With L00

Without L00

95% CL sensitivity

3 sensitivity

cdf-II error at 15.0/ps : 0.2

likelihood definition

the probability to measure an event as (un)mixed as a function of the measured decay time:

the probability that an event mixes (does not mix)as a function of the true decay time:

goodness of flavortagger (dilution)

time resolution

(sketch of)

-log likelihood of all events:

If the data are compatible with oscillations, -log(L) has a dip at the right value of ms

The big questions

How accurately can we measure ms?

determined by shape of the likelihood at minimum How sure are we that what we are seeing is

not just a random fluctuation?formally: What is the probability that an apparent signal at least as significant as the one we see in data would occur at any value of m

s in case there is no signal in reality?

probability that a dipso deep would come

from random fluctuation= 0.2 %

Opposite side taggers

Measure opposite side tagger dilutions Simultaneous fit to lD+, lD0

and lD* modes

tmDSeBt

)cos(1:/

predicteddilution

dilutionscale factor

Dilution predicted onevent-by-event basis(based on P

Trel, lepton-id

how to check/calibratethe prediction is correct?

Amplitude scan in sideband

A nice event

systematics on amplitude

systematics on ms

time resolution

(sketch of)

and cpv

0 1 2 3 4 5 6

Int Luminosity

DG/G stat. Err. a

DG/G (J/psi phi)

DG/G (BsKK)

DG/G (BsKK) +SVT imp

0 1 2 3 4 5 6

Int. Lum i (fb-1 )

sigma S(Psi phi)

J/ps i ph i

J/ps i+Ps i'

PSI + TTT

ed2=0.07

time resolution

(sketch of)

Aart Heijboer ● Observation of Bs oscilations ● Nikhef special seminar ● 01/08/07 ● slide 1...

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