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LBNL RPM

15 February 2007

Probing the Physics Frontier withRare B Decays at CDF

Cheng-Ju S. Lin(Fermilab)

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Interesting time in particle physics withmany exciting questions:

- Origin of EW symmetry breaking- Nature of cosmological dark matter- Nature of dark energy

- + …To get a consistent picture would requirephysics beyond the Standard Model

Tevatron could potentially uncover those mysteries. We can:

- Look for things directly (e.g. production of new particles)

- Look for deviations from the Standard Model predictions !!!

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Gold Mine for Heavy Flavor Physics

Mixing:Bs, Bd, D0

Lifetimes:b, Bs, Bc,

B+, Bd …

New particles:X(3872), Xb,

Cascade(b) …

Mass measurements:Bc, b, Bs, …

Rare decays:Bs, BK*

D0 , …

Production properties:(b), (J/), (D0), …

CP Violation:Acp(Bhh),

Acp(D0K), …

B and D Branching ratios

SURPRISES!?

Focus of today’s talk

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Indirect Search of New Physics : Bs

• Solid prediction from the Standard Model (SM)• In the SM, the decay of Bs +- is heavily suppressed

910)9.05.3()( sBBR

• Bd is further suppressed by another factor of ~20

• SM prediction is well below the sensitivity of current generation of experiments no observation yet

• New physics could significantly enhance the branching ratio Any signal would be a clear indication of NP

SM prediction

~ a few decays per 1 billion Bs produced

Bs =bs

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’i23 i22

b

s

R-parity violating SUSY

- MSSM: Br(B) is proportional to tan6. BR could be as large as ~100 times the SM prediction

- Tree level diagram is allowed in R-parity violating (RPV) SUSY models. Possible to observe decay even for low value of tan.

Some Scenarios of NP

Either discovery or null result could shed light on thethe structure of new physics !!!

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Collide proton (p) andanti-proton (p) at thec.m. energy of ~2 TeV

Chicago

Tevatron

CDF D0p

p

TEVATRON Collider

Lots of Bs producedin the collision debris !

Record luminosity: ~270E30 cm-2s-1

(design 300E30)

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Integrated Luminosity

Close to 2fb-1 of data collected by CDF

Analyses presenting today use from 780pb-1 to ~1fb-1 of data

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Flavor Creation (annihilation)

q b

q b

Flavor Creation (gluon fusion)

bg

g b

Flavor Excitationq q

b

g

b

Gluon Splitting

bg

g g

b

b’s produced via strong interaction

decay via weak interaction

Tevatron is great for heavy flavor:• Enormous b production cross-section, x1000 times larger than e+e- B factories• All B species are produced (B0, B+, Bs, b, b, etc…)

However,• Inelastic (QCD) background is about x1000 larger than b cross-section• Online triggering and reconstruction is a challenge: collision rate ~1MHz tape writing limit ~100Hz

Heavy Flavor Physics in Hadron Environment

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CDF II Detector

Significant detector, trigger, and DAQ upgrades in Run II

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Looking for Bs with CDF Detector

Look for Bs productionand decay vertices

Measure muontrack momentumand charge

CMU: ||<0.6

CMX 0.6<||<1.0

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• CDF has implemented a 3-tier trigger

• Level-1 is a synchronous hardware trigger - Can process one event every 132ns - Input rate = 1.7MHz (396ns 36x36 bunches) L1A rate ~ 30KHz (limited by L2)

• Level-2 is a combination of hardware and software trigger (asynchronous) - Average Level-2 processing time is ~30s - L2A rate ~1KHz (limited by event-builder)

• Level-3 is purely a software trigger - Massive PC farm - L3A rate ~ 100Hz (limited by tape writing)

• Data reduction rate (L1+L2+L3) 1 : 17000

CDF Trigger: Lifeline of B Physics Program

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+ (e+)

- (e-)

IP

(1) Dimuon trigger: For triggering on J/ and rare B decays (e.g. Bs and BX)

(3) Lepton+Displaced Track(SVT): For triggering on semileptonic B decays.

(2) Two-track trigger (SVT): For triggering on hadronic B and charm decays. Both tracks are required to have an impact parameter d0> 120m. (D0, Be, etc…)

X

}d0

X

IP

X

}d0

IP

+ (e+)

Three Classes of B Physics Triggers at CDF

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CDF is the first hadron collider experiment to be able to trigger on fully hadronic B events

Level-2 SVT Trigger

• SVT links drift chamber tracks from Level-1 with silicon hits to compute the impact parameter of the track.

SVT impact parameter (m)

-600 -300 0 300 600

• SVT d0 resolution is ~ 47m (35m beamline 33m resol).

• SVT revolutionized B and Charm physics at CDF.

Track impactparameter

Silicon Vertex Tracker (SVT)

SVT Performance

• Our physics program is dictated by what triggers we have

• In Run I, Bhh physics was a fantasy

• In Run II with SVT, we are making world class measurements

• First observations of:• BsK+K-

• BsK+-

• bp-

• bpK+

• Did I mention Bs mixing too?

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• Keeping trigger rates under control is a constant battle !!!

• Main issue: trigger rate blows up rapidly vs. luminosity

• For illustration, the following rare B dimuon triggers alone: - CMU-CMU pT>1.5GeV - CMU-CMX pT>1.5GeV would take up the entire level-2 trigger bandwidth at L=200E30 cm-2 s-1

• In the latest trigger table, there are more than 150 level-2 triggers that need to co-exist

RARE B TRIGGERS AT TEVATRON

~540 Hz @ 200 E30

~320 Hz @ 200 E30

CDF L2 Dimuon Trigger Cross Section

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KEEPING RARE B TRIGGERS ALIVE

• Handles to control rates: - Tighter selection cuts (e.g. pT of muon) - Apply prescales (DPS, FPS, UPS, etc.) - Improving trigger algorithm - Upgrading trigger hardware

• We’ve been using a combination of all four handles to control the trigger rate trading efficiency for purity

• It’s been a great challenge keeping B and high pT triggers alive at Tevatron

• It’ll be an even greater challenge at the LHC !!

Non-optimal for rare searches

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• Using 780pb-1 of dimuon trigger data: CMU-CMU trigger CMU-CMX trigger

• Use this inclusive sample to search for: Bs+- (Mass Bs=5.37GeV)

Bd+-

• Even if BR is x10 the SM value, only expect a hand full of signal events in the signal region

• Signal region is swamped with various kinds of background: both from SM processes and detector effect (fake muons)

B Data Sample

Effective backgroundrejection is the keyto this analysis!!

Search region

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Analysis Overview

Motto: reduce background and keep signal eff high

Step 1: pre-selection cuts to reject obvious background

Step 2: optimization (need to know signal efficiency and expected background)

Step 3: reconstruct B+ J/ K+ normalization mode (take into account Br of BJ/K and J/: >> 100million B+)

Step 4: open the box compute branching ratio or set limit

)/()/()(

JBRKJBBR

f

f

N

NBBR

Bsb

BbtotalBsBs

totalBB

B

Bss

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• Pre-Selection cuts:– 4.669 < m< 5.969 GeV/c2

– pT()>2.0 (2.2) GeV/c

CMU (CMX)

– pT(Bs cand.)>4.0 GeV/c

– Track, muon and vertex quality cuts

– 3D displacement L3D

between primary and secondary vertex

CDF Pre-selection

Bkg substantially reduced but stillsizeable at this stage

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Background Rejection:Bs Decay Characteristics

Bs-mass: MBs=5.37 GeV

Bs-flight distance

2 muons point to a“common vertex”

Background rejection cuts:

•muons add up to Bs mass

•muons are isolated and have common

vertex

•vertex is well separated from beam-spot

No other tracksfrom this vertex

pp

+

-

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– +- mass

(Eenergy, P momentum)

– B vertex displacement:

– Isolation (Iso):

(fraction of pT from B within R=(2+2)1/2 cone of 1)

– “pointing ()”:

(angle between Bs momentum and decay axis)

B Signal vs Background Discrimination

)(3

s

D

Bp

McL

i iiTsT

sT

RpBp

BpIso

)1()(

)(

))(( 3DLBp

+

-

L3D

primary vertex

di-muon vertex

PT()L3D )()( ss BpBEM

For each di-muon candidate, we computethe 4 variables:

• M• • Iso•

Distribution of the Discriminating Variables

ii

iR

xPxPxP

ibis

isL

)()(

)(

• Blue = Bs signal events from simulation (Monte Carlo)

• Black = expected bkg distributions

• Using and Iso to construct a new variable, likelihood ratio:

Ps(b)i is the probability distribution function for signal (background)

for variable i, with i loops over , , iso

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Likelihood Ratio (LR) Distribution

ii

iR

xPxPxP

ibis

isL

)()(

)(

Di-muon events withlarge LR (near 1) aremore likely to be signal

Events with LR near 0are more likely to be background

Signal distribution is based on simulation

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Background Estimate

• Use control samples to cross-check bkg estimate

• Assume linear background shape extrapolate # of background events in sidebands to signal region (± 60 MeV signal window)

1.) OS- : opposite-charge dimuon, < 02.) SS+ : same-charge dimuon, > 03.) SS- : same-charge dimuon, < 0

4.) FM : fake muon sample (at least one leg failed muon stub chi2 cut)

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LR CMU-CMU CMU-CMX cut pred obsv prob pred obsv prob

>0.50 489±12 483 41% 351±10 338 27% 28% OS- >0.90 62±4 73 12% 56±4 63 22% 7% >0.99 4.8±1.2 9 8% 3.9±1.1 8 7% 2%

>0.50 5.4±1.3 4 40% 3.3±1.0 2 39% 27%SS+ >0.90 <0.10 0 - 0.9±0.5 0 43% 43% >0.99 <0.10 0 - <0.10 0 - -

>0.50 6.6±1.4 7 49% 4.2±1.1 5 41% 40%SS- >0.90 0.6±0.4 1 45% 0.3±0.3 0 70% 57% >0.99 <0.10 0 - <0.10 0 - -

>0.50 188±8 159 3% 33±3 37 29% 7%FM >0.90 34±3 24 7% 6±1 5 46% 6% >0.99 4.5±1.0 9 6% 0.6±0.4 0 55% 12%

• Using a wider ± 150 MeV signal window for cross-check

(Probability factor in uncertainty on Poisson mean)

Combined prob

Cross Check BKG Estimate in Control Samples

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Bhh Background• CDF signal region is also contaminated by Bh+h- (e.g. BK+K-, K+, ) • K muon fake rates measured from data using D* sample

• Convolute fake rates with expected Bh+h- distributions to to obtain Bhh bkg

• Total bkg = Bhh + combinatorial

Decay Bhh Background

Combinatoric Background

Total Background

Bs 0.19±0.06 1.08±0.36 1.27±0.36

Bd 1.37±0.16 1.08±0.36 2.45±0.39

LR > 0.99

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LH(Bs) cut CMU-CMU CMU-CMX

LR>0.90 (70+/-1)% (66+/-1)%LR>0.92 (67+/-1)% (65+/-1)%LR>0.95 (61+/-1)% (60+/-1)%LR>0.98 (48+/-1)% (48+/-1)%LR>0.99 (38+/-1)% (39+/-1)%

• determined from Bs MC

• MC modeling checked by comparing LH(B+) between MC and sideband subtracted Data

(stat uncertainties only)

Likelihood Ratio Efficiency for Bs Signal

• Optimize analysis based on a-priori expected upper limit LR>0.99 !!!

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Now Look in the Bs and Bd Signal Windows

Bs Branching Ratio Limit:

Br(Bs)<1.0×10-7 @ 95%CL

Br(Bs)<0.8×10-7 @ 90%CLBd Branching Ratio Limit:

Br(Bd)<3.0×10-8 @ 95%CL

Number of observed events in the signal box is consistent with bkg expectation set branching ratio limit

Bs: Observed 1 candidate Expect ~1.3 background events

Bd: Observed 2 candidates Expect ~2.5 background events

Best limits in the world, but still no hints of new physics

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CDF Bs-> 176 pb-1 7.5×10-7 Published

DØ Bs-> 240 pb-1 5.1×10-7 Published

DØ Bs-> 300 pb-1 4.0×10-7 Prelim.

DØ <Bs-> 700 pb-1 <2.3×10-7>Prelim.

Sensitivity

CDF Bs-> 364 pb-1 2.0×10-7 Published

CDF Bs-> 780 pb-1 1.0×10-7 Prelim.

Branching Ratio Limits

• Evolution of limits (in 95%CL):

World’s best limits

Babar Bd-> 111 fb-1 8.3×10-8 Published

CDF Bd-> 364 pb-1 4.9×10-8 Published

CDF Bd-> 780 pb-1 3.0×10-8 Prelim.

90% CL

R. Dermisek et al., JHEP 0304 (2003) 037

SO(10) Grand Unification Model

tan()~50 constrained by unification of Yukawa couplings

Pink regions are excluded by either theory or experimentsGreen region is the WMAP preferred regionBlue dashed line is the Br(Bs) contourLight blue region excluded by old Bs analysis

R. Dermisek et al., hep-ph/0507233 (2005)

Red arrows indicate exclusionfrom this result!!!

Remaining white region is still not excluded by experiment

SUSY General Flavor Mixing (GFM) framework J. Foster et al.Hep-ph/0604121

xy parametersquantify variationsfrom MinimalFlavor violatingassumption

Bs, Bs, andBs mixing severelyconstrain non-MFV

Implications on SUSY Flavor Violation

TeVmm

GeVmA

gq

A

1

500

~~

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SUSY General Flavor Mixing (GFM) framework J. Foster et al.Hep-ph/0604121

2-D scan over xy space

non-MFV SUSY phase space is severely constrained

Implications on SUSY Flavor Violation

tan=40

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Sneak Preview of Bs

• 1fb-1 update is near completion• Implemented NN selection to enhance signal efficiency and bkg rejection:

NN contains additionalvariables: pT(Bs), pT(muon), etc.

For a given bkg level, NN signal eff is 15-20%higher than LR!!

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Sneak Preview of Bs

• Apply improved muon selection and particle ID to suppress fake muons and Bhh backgrounds

• Instead of single-bin counting experiment, use multi-bin parameterizations:

• 780pb-1 1fb-1, only about 30% increase in stat

Expect the sensitivity to increase by a factor of 2!!!

Neural Net Output

Bs Signal Eff

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CDF Projection

Conservative projectionbased on our current (780pb-1) performance

Improvement expectedfrom 1fb-1 analysis

Achievable in RunII

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• B Rare Decays B h :• B+ K+

• B0 K*

• Bs • b

• Penguin or box processes in the Standard Model:

• Rare processes: predicted BR(Bs )=16.1x10-7

observed at Babar, Belle

not seen

s

s

s

b

s

b

s

s

Bu,d,s K+/K*/

PRD 73, 092001 (2006)PRL 96, 251801 (2006)

C. Geng and C. Liu, J. Phys. G 29, 1103 (2003)

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• Probe various Wilson coefficients (bs)- Predicted in several new physics scenarios (e.g. SUSY)- Large forward backward asymmetry in B0 K* decay expected

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Standard Model

• Need better statistics In low q2 region

• CDF may be able to contribute to resolving this

Flipped signs

Probe of New Physics

1

1

2

1

1

2

2

cos),(

cos),()sgn(cos

)(

dqg

dqg

qAFB

cos/ 22 ddqdg

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• proper decay length () significance

• Pointing () | B – vtx|

• Isolation (Iso)

)(Bp

McL vtx

i iiTT

T

RpBp

BpIso

)0.1()(

)(

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Discriminating Variables For BK• Using similar discriminating variables and analysis strategy as Bs search

SIGNALSIDEBAND

SIGNALSIDEBAND

SIGNALSIDEBAND

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Bu,d,s K+/K*/Results with 1fb-1

• Using similar discriminating variables and strategy as Bs analysis

SignalSidebandExtrapolated fit

• Note: bins are counted. Gaussian is for illustration of expected width only

Bu K+Bd K*Bs

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• We see evidence for the B+ K+ B0 K* rare modes • We are homing in on the Bs rare mode

Bu,d,s K+/K*/Summary

With 1fb-1 of data:

Summary• Tevatron heavy flavor physics program is in full swing. What I’ve showed today is only the “tip of the iceberg”

• CDF could observe any modest enhancement of Bsin Run II. CDF is also on the verge of observing Bs decay

• For the next few years, Tevatron will continue to search for new physics with direct and indirect searches

• The ultimate frontier machine will be the LHC. We may finally have a first clean glimpse of the physics beyond the Standard Model

• Look forward to seeing what that new physics really is