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An Excursion into t Physics Carsten Hast Stanford Linear Accelerator Center. A rather abbreviated history of the t Reconstruction of hadronic t decays t 3 hadrons n t (g -2) m |v us | Mass measurement Lifetime Lepton Universality Lepton Flavor Violation Outlook. - PowerPoint PPT Presentation
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Carsten Hast, SLAC Excursion into Tau Physics, November 2007 1
• A rather abbreviated history of the • Reconstruction of hadronic decays
• 3 hadrons • (g-2)• |vus|
• Mass measurement• Lifetime• Lepton Universality• Lepton Flavor Violation• Outlook
An Excursion into Physics
Carsten HastStanford Linear Accelerator Center
DESY Hamburg and Zeuthen November 12th,13th 2007
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 2
• Back in the early 60’s there was no standard model. People were still trying to sort out why there are electrons and muons. Quarks were suggested in ’64. How to find more, for example more leptons?
• One way of doing that is pair production in e+e- scattering
• Accelerators want to be build, meaning:• need to know how, where, persuade people for money and
finally do it…• SPEAR was proposed in 1964 • MARK I was proposed in 1971• First events in 1974
• In between there was quite some time for people to make up, what the future standard model would be and what features a new, heavy lepton would have …
How to Find a Lepton
e+e- l+l-
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 3
The following is from Yung-Su Tsai (1971)• Assuming that the new lepton has the same electro-
magnetic and weak interaction as e and • Assuming it is heavy, so it will decay into hadrons
• From decays via CVC Tsai calculated the decay rates of the into different hadrons:
, K(*) , , a1 , cont. and partial and total decay width, lifetime and cross section
Characteristics of Lepton Decays
W
ee
u
d
d = cos c d + sin c sc the Cabibbo angle
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 4
Characteristics of Lepton Decays
lifetime sec)-1 = 310 fsec
re≈ r≈≈ rhad≈
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 5
In 1973 MARK I was ready at SPEAR at SLAC
• Look for events with only 2 charged tracks and missing energy
How to Find the First Pair Event
W
ee
u u
d se+
e-
M.L. Perl et al. 75 PRL 35 1489
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 6
…it worked out:Martin PerlNobel Laureate of 1995
Have to give it a name:“the third”
M. Perl, Frederick Reines, King Carl Gustaf
And then you have to convince yourself and your colleagues…
• In early 70th there were only 3 quarks (uds), but then in 1974 the J/ was discovered charm was born• 2 families of leptons, 2 families of quarks• Nice SU(2) U(1) symmetry• Who needs a 3rd generation!
How to Find the First Pair Event
… but it helps when you have nice colleagues who find strange electron-muon events as well, like Pluto in 1976, and then…
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 7
Production of Pairs
In the following let’s focus on e+e- machines
• CMS energy from threshold to the Z resonance and beyond
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 8
LEPClean event environmentLow systematic errorsExcellent for large branching fractions
B-Factories: cross section at (4s) is 0.919 nb ≈ 1M pairs/fbVery good for rare and forbidden decaysWorking hard to get the systematic errors down
Number of pairs per experiment:ARGUS 0.5MCleo 14MLEP 0.1M/experimentBelle/BaBar 650M/440M
Experimental Luminosities
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 9
• Initial and final state radiation
• E ≤ ECMS/2 • Taus are flying back to back in CMS• Each decays to an odd number of charged particles (1, 3, 5,
7…)plus neutral particles (0,…)
• Most (semi)hadronic decays proceed via resonances (a1, , K*…)• Need to identify e, , , K, , 0
• Reconstruction of decay vertex is always good• 4 detector coverage, high efficiency, etc.
Reconstruction of Pairs
e+
e-
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 10
• ECMS=(4s) = 10.46 GeV• Unfortunately with a boost ( E(e-)=9.0GeV and E(e+)=3.1GeV )
Typical Pair: PEP II and BaBar
SVT, DCH: charged particle tracking vertex &mom. resolution
EMC: electromagnetic calorimetry /0/DIRC, IFR, DCH: charged particle ID /K/p,
1.5 T Solenoid Electromagnetic Calorimeter
(EMC)Detector of Internally
Recflected Cherenkov
Light (DIRC)
Instrumented Flux Return
(IFR)
Silicon Vertex Tracker (SVT)
Drift Chamber (DCH)e- (9 GeV)
e+ (3.1 GeV)
“Typical Event”
Simulated
1-7 topology
event
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 11
Standard Event Selection of an 1- n topology:
1. In CMS: Demand 1 charged track vs. n charged tracks2. Net charge of 03. Get rid of converted photons4. Invent good ways to reduce background
1. Cuts on event thrust angle and magnitude2. Cuts on photons in the event3. Lepton identification on the tag side4. Vertexing5. Missing Energy (neutrinos)
5. Particle identification on signal side6. Reconstruct (or get rid of) 0
Pair Event Reconstruction
e-
e+
tag
rec
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 12
• The following is an example of how to do a analysis• Most of these decays are quite well measured, except
for the “rarer” ones
• • • • •
(Ian Nugent (BaBar) Tau 06 in Pisa, submitted to PRL in July 2007 )
How to: to Three Hadrons
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 13
Rely on superb particle identification of the B-Factory detectors
How to: to Three Hadrons II
--+ K--+ K--K+ K-K-K+--+ 97.40% 22.49% 4.73% 1.02%
K--+ 1.42% 74.87% 16.43 6.38%
K--K+ 0.01% 0.49% 59.63% 25.54%
K-K-K+ 0.26% 50.87%
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 14
How to: to Three Hadrons III ARGUS 1993
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 15
All modes are related to each other (signal and background)
How to: to Three Hadrons IV
2.8 3.1 3.5 3.9
BR (8.83±0.01±
0.13)%
(0.273±0.002±
0.009)%
(0.1346±0.0010±
0.0036)%
(1.58±0.10±
0.13) 10-5
BRDPG (9.02±0.08)% (0.333±0.35)% (0.153±0.10)% <3.5x10-5 90% CL
BaBar arXiv:0707.2981 [hep-ex] Submitted to PRL
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 16
How to: to Three Hadrons V--+
K--K+
K--+
K-K-K+
Mike Roney, Lepton Photon 2007
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 17
First observation of tau to phi decays
How to: to Three Hadrons VI
Br) =(3.49±0.55±0.32) x 10-5 BaBarBr)=(3.48±0.20±0.26) x 10-5
preliminary
Belle Phys.Lett.B643:5-10,2006
Br) =(4.05±0.25±0.26) x 10-5
Br) =(3.42±0.55±0.25) x 10-5 BaBar (July 2007)
Br)=(3.39±0.20±0.28) x 10-5 arXiv:0707.2981v1 (acc. PRL)
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 18
heavy enough to decay into hadrons
Hadronic Decays for QCD Tests
probes the hadronic V-A current
H
5(1 ) 0H d u
W
d
u
Hadrons
W
d
u
Hadrons
e
e
Hadronse
e
Hadrons
ee H0 probes thehadronic electromagnetic current
0 0q
qH Q q q
hadrons
Whadrons
e+
e –
Isospin
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 19
Hadronic vacuum polarisation is the largest uncertainty in calculating a
(ahad)= a few x 10-10 (others are only 0.3-04 x 10-10 )
Dominant part can be calculated from the 2 spectral functions
Hadronic Decays: and (g-2)
muonanomalous magnetic moment:
weakhadQED
22
aaaa
g
Need to measure Br) and the precise mass spectrum shape
Isospin
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 20
Fudjikava (Belle preliminary) Tau 06
Hadronic Decays: and (g-2)
BG subtractionDetector unfoldingRadiative correctionsFitting Pion form factor
Gounaris-Sakurai parameterization
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 21
Fudjikava (Belle preliminary) Tau 06
Hadronic Decays: and (g-2)
Interference between ’ and”
a2x 1010
Belle0.5(stat.)3.2(sys.)2.3(Iso.)
Aleph,Cleo3.2(exp.)2.3(Iso.)
CMD2, Kloe4.9(exp.)1.6(rad.)
(770)’(1400) ’’(1700)
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 22
1010 x ath = 11 658 471.81 0.02 QED Kinoshita-Nio, Passera 15.4 0.2 EW Czarnecki-Marciano-Vainshtein
698.2 9.7 hvp (711.0 5.8) , (690.9 4.4)eeDavier et al
9.8 0.1 hvp NLO Krause, Hagiwara et al
12.0 3.5 light-by-light Melnikov-Vainshtein, Knech et al
= 11 659 187.6 10.3 (11 659 200.4 6.8) , (11 659 180.3 5.6)ee
aexp - ath = 1.3
Davier/Pich Anomalous Magnetic Moment Summary
BNL - E821A= (11 659 208.06.0) 10-10
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 23
Hadronic Decays: |vus|
The route to world best measurements of |vus| and ms
Need detailed knowledge of all (strange) BRs and shapes of all (strange) mass spectra
|Vud|2 +|Vus|2 + |Vub|2 = 1
|Vud|2 = 0.97377 + 0.00027from nuclear decays and neutron decays (PDG 2006)
|Vub|2 = (4.40 + 0.34) x 10-3
from e.g. inclusive B Xul decays
|Vus|2 = 0.2275 + 0.0012Mike Roney, Lepton Photon 2007
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 24
1= e
ee
Br hadrons B BR
BBr e
1= e
ee
Br hadrons B BR
BBr e
Strange and non-Strange -Decays
Strangenon,Strange, RRR
0
,, 0
0
( ) ( ) ( )
s
ijwij
dRR s ds w s ij ud or us
ds
, 0 , 00 2 2
( ) ( )( )
| | | |
w ww ud us
ud us
R s R sR s
V V
From weighted spectral functions…
one can calculate the flavour breaking difference
Mike Roney, Lepton Photon 2007
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 25
Hadronic Decays: I.Nugent: BaBar preliminary Tau 06 and updated in arXiv:0707.2922 [hep-ex] submitted PRD-RC
qqbar
BG
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 26
Hadronic Decays: s0
B.Shwartz, Belle preliminary Tau 06 and updated in arXiv:0706.223 [hep-ex] submitted to PLB
K*0(800)+ K*(892)+ K*(1410)Fit with K*0(892) only
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 27
not in average
Br(-K0-) = (0.808 0.004stat 0.026syst)%
PDG 2007 (0.90 0.04) %
Hadronic Decays: s(cont.)
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 28
Belle preliminaryarXiv:0708.0733 [hep-ex]
Hadronic decays with
BR is dominated (100%) by
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 29
Hadronic Decays: |vus| Summary
Swagato Banerjee, Kaon 2007
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 30
Strange Spectral Functions: ALEPH (‘99) and OPAL (‘05)
|Vus|=0.2204±0.0028exp±0.0003th±0.0001msNo good solution Need better data to clarifyNeed B-factories to contribute
Moments k,l=0
DHZ05, hep-ph/0507078
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 31
Decay Branching Ratios
Br(-1-prong 0 neutrals ) = (85.330.08)%
Br(-e-ve) = (17.840.05)%
Br(--v) = (17.360.05)%
Br(-1h 0 neutrals ) = (50.130.11)%
Br(-(3h)- 0 neutrals ) = (14.590.08)%
Br(-(5h)- 0 neutrals ) = (0.1020.004)%
Br(-(7h)- 0 neutrals ) < 2.7 x 10-7
Sum = (100.02 0.11)%
148 allowed modes + 55 LF, or L, or B violating
Impressively well measured!
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 32
1977 MARK I m= 1.9±0.1 GeV
Mass Measurement
1996 BESm= 1776.96±0.31 MeV
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 33
Ecm (GeV)
B r.c.
obs
Mass Measurement
At threshold (BES 1996) Need detailed knowledge of production cross section, beam energy, luminosity
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 34
Mass Measurement
Pseudo-Mass Technique • Pseudo-mass was introduced by ARGUS in 1992• Assume neutrino is mass less • direction is approximated by the reconstructed tracks
m*2=2(Ebeam – Ehad)(Ehad – Phad)+mhad
2
MC for input masses:
1.767 GeV1.777 GeV1.787 GeV
MC input mass
Fit
res
ult
BaBar Belle
Mτ = 1776.77 ± 0.13(stat.) ± 0.32(sys.) MeV (prel.) Mτ+ – Mτ– = 0.050.27 MeV
|Mτ+ – Mτ–|/Mτ < 2.8 · 10-4 @ 90% CL (CPT-Test)
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 35
Mass Measurement
New preliminary mass from KEDR Collaboration at the VEPP-4M Collider in Novosibirsk (hep-ex/0611046)
0.29 20
0.25 20.23
.26177
1776.80 0.15
6.99 MeV/c (PDG06, BES dominated)
MeV/c (KEDR preliminary)
m
m
e,,,K,– vs –
e -pair events
keV
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 36
Lifetime
Very precise results from all 4 LEP experiments are publishedWill describe one measurement technique: “Decay Length Measurement”
A.Lusiani (BaBar preliminary) 80/fb• 1 vs. 3 topology • One prong tagged as lepton• Rigorous event selection criteria to reduce BG• Reconstruct the 3 prong vertex• Measure distance to beam spot
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 37
Lifetime
The needed precision makes this type of analysis all but trivial• Fighting detector effects in the real detector and in the simulation• Understanding BG sources and their bias towards the measurement
Decay Length
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 38
Lifetime
The Results
PDG 2006 290.6 ± 1.0 fs
DELPHI 2005
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 39
Test of Lepton Universality (charged current)
e
g
g
g
g
/ eg g
K K
W W
e
e
e
e
B B
B B
B B
B B
1.0000 0.0020
1.0017 0.0015
1.012 0.010
0.997 0.010
/g g
K K
W W
eB
B B
1.0004 0.0022
0.996 0.005
0.979 0.017
1.039 0.013
/ eg g
W W e
B
B B
1.0004 0.0023
1.036 0.014
A.PichTau06
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 40
Test of Lepton UniversalityNow, let’s put everything together and see whether it all works:
mass (m) life time() leptonic branching ratios
3 4 2( ) 1 8 8 12 logf x x x x x x
B=(17.36LEP
and
Be=(17.84CLEO
(B/Be)exp= 0.9725
(B/Be)the= 0.972564
A.Pich Tau06
Standard Model Prediction
m=1776.990.29-0.26 MeV
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 41
Quite tough for the current experimental sensitivity…
… but:
Lepton Flavor Violation
• Lepton Flavor is conserved in the Standard Model
• SM extended to include non zero mass and mixing predicts Lepton Flavor Violation
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 42
Observation of LFV would be a clear signature of new physics
Some examples:
Theoretical Predictions of SM Extensions
• Many SM extensions include LF violation
• Some models predict LFV up to existing experimental limits
• Neutrinoless 2 and 3 body decays have different sensitivity
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 43
Typical Event Selection
e-
e+
tag
rec
• Decay products of Taus are well separated in space (CM)
• Signal side (Neutrinoless decays) l, lhh (l = e, h = , K)
Mass reconstructed from particles = m and their energy = Ebeam
• Tag side (1-prong decays)
Bee
3-prong decays) hhhfor)
/e
ll
l
lh
h
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 44
Event Selection
E = E – Ebeam = 0 smeared by resolution and radiation
Signal simulation
MEC candidate invariant mass, with energy constrained to beam energy
M = MEC –M ~ 0
Signal Region is blinded while developing the analysis
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 45
• Signal from MC and background from Data (excluding ± 3)
• Common Input Variables:
• Event Missing Mass
• Event Missing PT
• Tag side missing mass
• Tag side momentum
• helicity angle
• MC Optimized for different tagging modes
• electron
• electron + gamma
• muon
• hadron
• hadron gamma
• 3 hadrons
Event Selection II
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 46
Results
207106 e+e→ events (L = 232 fb-1)
• Estimate background from linear fit to M sideband
• Nbkg = 6.2 ± 0.5 Nobs = 4 = 9.3 ± 0.6 %
B(→ ) < 6.8 10-8 at 90% C.L.BaBar
Phys. Rev. Lett. 95, 041802 (2005)
What a few 10-7 signal would look like
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 47
e Results207106 e+e→ eventsL = 232 fb-1 What a few 10-7 signal would look like
B(→ e) < 1.1 10-7 at 90% C.L.
Estimate background from linear fit to M sideband
1 event in a 22 signal box, Nbkg =1.9 ± 0.4, = 4.7 ± 0.3%
BaBar
Phys. Rev. Lett. 96, 041801 (2006)
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 48
Lepton Flavor Violation Results (BaBar + Belle)
S.Banerjee Tau06
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 49
Estimate background from 2-dimensional fit
Total expected number of events: 11.3 , Nbkg =0.1- 3.0 (depends on channel)
Total Observed: 10 events
197106 e+e→ events L = 221 fb-1
B(→ lhh) < (0.7-4.8) 10-7 at 90% C.L.
lhh Results (BaBar)
BaBar
Phys. Rev. Lett. 95, 191801 (2005)
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 50
Lepton Flavor Violation Results (Belle)Recent results on Belle’s LFV search (Upper Limit on Br at 90% CL)
42 decay modes
535
535
401
401
281
281
158
158158
86
86
154
154
87
This year’s publicationsT. Ohshima Tau06
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 51
LFV new prelim. Belle lll
Belle update to 535 fb-1 increase in lumi by 6.1 fold
Efficiency: 6.0-12.5% <Bkgnd>: 0.01-0.40 eventsTotal <Bkgnd>=0.590.31 events Nobs=0 events
90%CL limits: BR lll)<(2.0-4.1)x10-8
5-7 fold improvement over previous limit
Mike Roney, Lepton Photon 2007
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 52
Mike Roney, Lepton Photon 2007
LFV new prelim. BaBar lll
BaBar update to 376fb-1 increase in lumi by 4.3 fold
Efficiency: 5.5-12.4% <Bkgnd>: 0.3-1.3 eventsTotal <Bkgnd>=4.20.8 events Nobs=6 events
90%CL limits: BR (lll)<(3.7-8.0)x10-8
2-5 fold improvement over previous limit
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 53
New Belle Results on lV0 using 543fb-1
Efficiency:2.5-4.0%<Bkgnd>:0.0-0.2 eventsTotal <Bkgnd>=0.780.29evNobs=3 events
90%CL limits:BR(tlV0)<(6.1-18)x10-8
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 54
New BaBar Results l ’
hep-ex/0610067 PRL 98.061803 (2007)
expected background/channel ~ 0.1-0.3Total expected background=3.1, Observed=2
UL on Br. between 1.1 x 10-7 and 5.8 x 10-7 @ 90% CLLafferty (BaBar preliminary) Tau 2006
… and more Violations …
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 55
Summary, Conclusions and Outlook
The is a clean laboratory to test the standard model
• Some examples of hadronic tau decay analysis• (g-2) : resonance analysis of hadronic tau decays is
currently in a small contradiction with the production measurements
• More detailed analysis of the strange spectral functions at B-Factories is eagerly anticipated (|vus| and ms)
• Mass measurements are already very precise (0.1 per/mille) and will get even better
• Lifetime measurements from B-Factories are overdue• Lepton Universality is tested to the few per/mille level
but to e/ Br measurements of B-Factories are overdue • Lepton Flavor Violation: no signal at the 10-7 to -8 level
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 56
Summary, Conclusions and Outlook
Many thanks for your kind invitation!
B-Factories in 2008 will have (more/less) 1 ab-1 each• Lepton universality O(10-4) (maybe)• LFV O(10-8)• CPV and CPT test in more systematic fashion• |vus| and ms and s (even better)
Super-B Factory with 100 ab-1 • Lepton universality O(10-4) tough to get errors smaller• LFV O(10-10)• CPT test to O(10-4-5)• CPV in decays: case for (4s) running with polarized beams
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 57
Additional
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 58
BELLE (preliminary) Use -K-0sample to get- K*-) through the K*-(892)K-0modeusing BR(K*-(892)K-0
-K*- BELLE~1.9 lower than
PDG 07(CLEO)
BR (1.130.19 0.07)x10-4 (2.900.800.42)x10-4
Carsten Hast, SLAC Excursion into Tau Physics, November 2007 59
Some intepretations within benchmark models
compilation by S. Banerjee (Tau06,hep-ex/0702017)