Download - LBNL RPM 15 February 2007
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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