1. DESY Hamburg and Zeuthen November 12th,13th 2007
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 nt
(g-2)m
|vus|
Mass measurement
Lifetime
Lepton Universality
Lepton Flavor Violation
Outlook
DESY Hamburg and Zeuthen November 12th,13th 2007
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

2. Excursion into Tau Physics, November 2007
How to Find a t Lepton
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 …
e+e l+l-
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

3. Characteristics of t Lepton Decays
The following is from Yung-Su Tsai (1971)
Assuming that the new lepton has the same electro-magnetic and weak interaction as e and m
Assuming it is heavy, so it will decay into hadrons
From p decays via CVC Tsai calculated the decay rates of the t into different hadrons:
t p nt , t K(*) nt , t r nt , t a1 nt , t cont. nt
and partial and total decay width, lifetime and cross section W- t- nt e- ne m- nm u dq
dq = cos qc d + sin qc s
qc the Cabibbo angle
p m-nm
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

4. Characteristics of t Lepton Decays
t-lifetime = 1/323 (1010 sec)-1 = 310 fsec
Br(te) ≈ Br(tm) ≈ 64.6(63)/323 ≈ 20%, Br(thad) ≈ 60%
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

5. How to Find the First t Pair Event
In 1973 MARK I was ready at SPEAR at SLAC
Look for events with only 2 charged tracks and missing energy W- t- nt e- ne m- nm u d s e+ e- t+ t-
M.L. Perl et al. 75 PRL
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

6. How to Find the First t Pair Event
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/Y was discovered  charm was born
2 families of leptons, 2 families of quarks
Nice SU(2) Ä U(1) symmetry
Who needs a 3rd generation!
…it worked out:
Martin Perl
Nobel Laureate of 1995
Have to give it a name:
triton “the third”
M. Perl, Frederick Reines, King Carl Gustaf
… 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. Excursion into Tau Physics, November 2007
Production of t 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. Experimental Luminosities
LEP
Clean event environment
Low systematic errors
Excellent for large branching fractions
B-Factories:
t cross section at U(4s) is nb ≈ 1M t pairs/fb
Very good for rare and forbidden decays
Working hard to get the systematic errors down
Number of t pairs per experiment:
ARGUS 0.5M
Cleo 14M
LEP 0.1M/experiment
Belle/BaBar 650M/440M
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

9. Reconstruction of t Pairs
Initial and final state radiation
Et ≤ ECMS/2
Taus are flying back to back in CMS
Each t decays to an odd number of charged particles (1, 3, 5, 7…)
plus neutral particles (p0,…)
Most (semi)hadronic decays proceed via resonances (a1, r, K*…)
Need to identify e, m, p, K, g, p0
Reconstruction of t decay vertex is always good
4p detector coverage, high efficiency, etc. e+ e- t+ t- g
g/Z0
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

10. Typical Pair: PEP II and BaBar
ECMS=U(4s) = GeV
Unfortunately with a boost ( E(e-)=9.0GeV and E(e+)=3.1GeV )
SVT, DCH: charged particle tracking Þ vertex &mom. resolution
EMC: electromagnetic calorimetry Þ g/p0/h
DIRC, IFR, DCH: charged particle ID Þ p/K/p,m
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. t Pair Event Reconstruction
Standard Event Selection of an 1- n topology:
In CMS: Demand 1 charged track vs. n charged tracks
Net charge of 0
Get rid of converted photons
Invent good ways to reduce background
Cuts on event thrust angle and magnitude
Cuts on photons in the event
Lepton identification on the tag side
Vertexing
Missing Energy (neutrinos)
Particle identification on signal side
Reconstruct (or get rid of) p0 e- e+
ttag
trec
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

12. How to: t to Three Hadrons
The following is an example of how to do a t analysis
Most of these decays are quite well measured, except for the “rarer” ones
t-  p-p-p+nt
t-  K-p-p+nt
t-  K-p-K+nt
t-  K-K-K+nt
t-  FK-(p-)nt
(Ian Nugent (BaBar) Tau 06 in Pisa, submitted to PRL in July 2007 )
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

13. How to: t to Three Hadrons II
Rely on superb particle identification of the B-Factory detectors
--+
K--+
K--K+
K-K-K+
97.40%
22.49%
4.73%
1.02%
1.42%
74.87%
16.43
6.38%
0.01%
0.49%
59.63%
25.54%
0.26%
50.87%
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

14. How to: t to Three Hadrons III
KKp
Kpp
ARGUS 1993
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

15. How to: t to Three Hadrons IV
All modes are related to each other (signal and background)
--+
K--+
K--K+
K-K-K+ e
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)%
<3.5x % CL
BaBar arXiv: [hep-ex] Submitted to PRL
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

16. How to: t 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. How to: t to Three Hadrons VI
First observation of tau to phi decays
Belle
Phys.Lett.B643:5-10,2006
Br (t-  FK-nt ) =(4.05±0.25±0.26) x 10-5 fK fp
Br (t-  Fp-nt ) =(3.49±0.55±0.32) x BaBar
Br (t-  FK-nt )=(3.48±0.20±0.26) x preliminary
Br (t-  Fp-nt ) =(3.42±0.55±0.25) x BaBar (July 2007)
Br (t-  FK-nt )=(3.39±0.20±0.28) x arXiv: v1 (acc. PRL)
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

18. Hadronic t Decays for QCD Tests
t heavy enough to decay into hadrons
probes the
hadronic V-A current
e+ e- H0 probes the
hadronic electromagnetic current
Isospin 
hadrons  W  e+
e –
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

19. Hadronic t Decays: t-  p-p0nt and (g-2)m
muon anomalous magnetic moment:
Hadronic vacuum polarisation is the largest uncertainty in calculating am
s(amhad)= a few x (others are only x )
Dominant part can be calculated from the 2p spectral functions
Isospin
Need to measure Br(t-p-p0nt) and the precise mass spectrum shape
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

20. Hadronic t Decays: t-  p-p0nt and (g-2)m
Fudjikava (Belle preliminary) Tau 06
BG subtraction
Detector unfolding
Radiative corrections
Fitting
 Pion form factor
Gounaris-Sakurai
parameterization
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

21. Hadronic t Decays: t-  p-p0nt and (g-2)m
Fudjikava (Belle preliminary) Tau 06
r(770)
r’(1400)
r’’(1700)
Interference between r’ and r”
am(2p) x 1010
Belle
459.8±0.5(stat.)±3.2(sys.)±2.3(Iso.)
Aleph,Cleo
464.0±3.2(exp.)±2.3(Iso.)
CMD2, Kloe
450.2±4.9(exp.)±1.6(rad.)
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

22. m Anomalous Magnetic Moment Summary
Davier/Pich
BNL - E821
Am = ( ±6.0) 10-10
1010 x amth =  QED Kinoshita-Nio, Passera
 EW Czarnecki-Marciano-Vainshtein
 hvp (711.0  5.8)t , (690.9  4.4)e+e- Davier et al
 hvp NLO Krause, Hagiwara et al
 light-by-light Melnikov-Vainshtein, Knech et al
=  (  6.8)t , (  5.6)e+e-
amexp - amth = s s s
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

23. |Vud|2 +|Vus|2 + |Vub|2 = 1 |Vud|2 = 0.97377 + 0.00027
Hadronic t 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 =
from nuclear b decays and neutron decays (PDG 2006)
|Vub|2 = ( ) x 10-3
from e.g. inclusive B Xuln decays
|Vus|2 =
Mike Roney, Lepton Photon 2007
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

24. Strange and non-Strange -Decays
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 t Decays: t-K-p0nt
I.Nugent: BaBar preliminary Tau 06 and updated in arXiv: [hep-ex] submitted PRD-RC
m+m-
qqbar
t BG
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

26. Hadronic t Decays: t-Ks0p-nt
B.Shwartz, Belle preliminary Tau 06 and updated in arXiv: [hep-ex] submitted to PLB
Fit with K*0(892) only
K*0(800)+ K*(892)+ K*(1410)
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

27. Hadronic t Decays: t-K*-(Ksp-)nt (cont.)
not in average
Br(-K0-) = (0.808  0.004stat 0.026syst)%
PDG 2007 (0.90  0.04) %
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

28. Hadronic t decays with h
Belle preliminary arXiv: [hep-ex]
BR t-  K-p0nt is dominated (100%) by t-  K*hnt
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

29. Hadronic t 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)
DHZ05, hep-ph/
Moments k,l=0
|Vus|=0.2204±0.0028exp±0.0003th±0.0001ms
No good solution
Need better data to clarify
Need B-factories to contribute
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

31. t Decay Branching Ratios
Br(-1-prong ³ 0 neutrals ) = (85.330.08)%
Br(-e-ve) = (17.840.05)%
Br(-m-vm) = (17.360.05)%
Br(-1h ³ 0 neutrals ) = (50.130.11)%
Br(-(3h)- ³ 0 neutrals ) = (14.590.08)%
Br(-(5h)- ³ 0 neutrals ) = (0.1020.004)%
Br(-(7h)- ³ 0 neutrals ) < 2.7 x 10-7
Sum = ( 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. Excursion into Tau Physics, November 2007
t Mass Measurement
MARK I
mt = 1.9±0.1 GeV
BES
mt = ±0.31 MeV
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

33. Excursion into Tau Physics, November 2007
t Mass Measurement
At threshold (BES 1996)
Need detailed knowledge of
production cross section, beam energy, luminosity
Ecm (GeV) B
r.c.
obs
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

34. m*t2=2(Ebeam – Ehad)(Ehad – Phad)+mhad2
t Mass Measurement
Pseudo-Mass Technique
Pseudo-mass was introduced by ARGUS in 1992
Assume neutrino is mass less
t direction is approximated by the reconstructed tracks
m*t2=2(Ebeam – Ehad)(Ehad – Phad)+mhad2
MC for input masses:
1.767 GeV GeV GeV
MC input mass
Fit result
BaBar
Belle
Mτ = ± 0.13(stat.) ± 0.32(sys.) MeV (prel.)
Mτ+ – Mτ– = 0.050.27 MeV
|Mτ+ – Mτ–|/Mτ < 2.8 · 90% CL (CPT-Test)
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

35. Excursion into Tau Physics, November 2007
t Mass Measurement
New preliminary t mass from KEDR Collaboration at the VEPP-4M Collider in Novosibirsk (hep-ex/ )
keV
enn,mnn,pn,Kn,rn
– vs –
enn t-pair events
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

36. Excursion into Tau Physics, November 2007
t Lifetime
Very precise results from all 4 LEP experiments are published
Will 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. Excursion into Tau Physics, November 2007
t 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. Excursion into Tau Physics, November 2007
t Lifetime
The Results
PDG ± 1.0 fs
DELPHI 2005
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

39. Test of Lepton Universality (charged current)
A.Pich
Tau06
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

40. Test of Lepton Universality
Now, let’s put everything together and see whether it all works:
mass (mt) life time(tt) leptonic branching ratios
Bm=(17.36±0.05)% LEP and
Be=(17.84±0.05)% CLEO
(Bm /Be)exp= ±
(Bm /Be)the= ±
Standard Model Prediction
mt= MeV
A.Pich Tau06
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

41. Lepton Flavor Violation
Lepton Flavor is conserved in the Standard Model
SM extended to include non zero n mass and n mixing predicts Lepton Flavor Violation
Quite tough for the current experimental sensitivity…
… but:
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

42. 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
Some examples:
Observation of LFV would be a clear signature of new physics
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

43. Typical Event Selectionh e+
m/e g
ttag
trec e-
Decay products of Taus are well separated in space (CM)
Signal side (Neutrinoless decays) lg (po,h), lhh (l = e, m h = p, K)
Mass reconstructed from particles = mt and their energy = Ebeam
Tag side (1-prong decays) B(t  p nt, rnt, e nent , m nmnt) ~ 85 %
(3-prong decays) B(t  hhh(p0) nt) ~ 15% (for mg)
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

44. t  mg Event Selection t  mg Signal simulation
MEC = t candidate invariant mass, with energy constrained to beam energy
Signal Region is blinded while developing the analysis
DE = Et – Ebeam = 0 smeared by resolution and radiation
DM = MEC –Mt ~ 0
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

45. t  mg Event Selection II
Signal from MC and background from Data (excluding ± 3s)
Common Input Variables:
Event Missing Mass
Event Missing PT
Tag side missing mass
Tag side momentum
m helicity angle
MC Optimized for different tagging modes
electron
electron + gamma
muon
hadron
hadron gamma
3 hadrons
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

46. t  mg Results B(t → mg) < 6.8  10-8 at 90% C.L.
207106 e+e- → t+t- events (L = 232 fb-1)
What a few 10-7 signal would look like
Estimate background from linear fit to DM sideband
Nbkg = 6.2 ± Nobs = e = 9.3 ± 0.6 %
BaBar
Phys. Rev. Lett. 95, (2005)
B(t → mg) < 6.8  at 90% C.L.
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

47. t  eg Results B(t → eg) < 1.1  10-7 at 90% C.L.
207106 e+e- → t+t- events
L = 232 fb-1
What a few 10-7 signal would look like
Estimate background from linear fit to DM sideband
1 event in a 2s2s signal box, Nbkg =1.9 ± 0.4, e = 4.7 ± 0.3%
BaBar
Phys. Rev. Lett. 96, (2006)
B(t → eg) < 1.1  at 90% C.L.
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

48. t  Lepton Flavor Violation Results (BaBar + Belle)
S.Banerjee Tau06
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

49. t  lhh Results (BaBar) B(t → lhh) < (0.7-4.8)  10-7 at 90% C.L.
197106 e+e- → t+t- events L = 221 fb-1
Estimate background from 2-dimensional fit
Total expected number of events: 11.3 , Nbkg = (depends on channel)
Total Observed: 10 events
BaBar
Phys. Rev. Lett. 95, (2005)
B(t → lhh) < ( )  at 90% C.L.
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

50. t  Lepton Flavor Violation Results (Belle)
Recent results on Belle’s LFV search
(Upper Limit on Br at 90% CL)
42 decay modes
535
401
281
158 86
154 87
This year’s publications
T. Ohshima Tau06
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

51. t  LFV new prelim. Belle tlll
Belle update to 535 fb-1 increase in lumi by 6.1 fold
Efficiency: % <Bkgnd>: events
Total <Bkgnd>=0.590.31 events Nobs=0 events
90%CL limits: BR (tlll)<( )x10-8
5-7 fold improvement over previous limit
Mike Roney, Lepton Photon 2007
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

52. t  LFV new prelim. BaBar tlll
BaBar update to 376fb-1 increase in lumi by 4.3 fold
Efficiency: % <Bkgnd>: events
Total <Bkgnd>=4.20.8 events Nobs=6 events
90%CL limits: BR (tlll)<( )x10-8
2-5 fold improvement over previous limit
Mike Roney, Lepton Photon 2007
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

53. Excursion into Tau Physics, November 2007
New Belle Results on tlV0
using 543fb-1
Efficiency: %
<Bkgnd>: events
Total <Bkgnd>=0.780.29ev
Nobs=3 events
90%CL limits:
BR(tlV0)<(6.1-18)x10-8
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

54. New BaBar Results t l p0/h/h’
Lafferty (BaBar preliminary) Tau 2006
… and more Violations …
New BaBar Results t l p0/h/h’
hep-ex/ PRL (2007)
expected background/channel ~
Total expected background=3.1, Observed=2
UL on Br. between 1.1 x 10-7 and 5.8 x @ 90% CL
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

55. Summary, Conclusions and Outlook
The t is a clean laboratory to test the standard model
Some examples of hadronic tau decay analysis
(g-2)m : 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 t to e/m 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
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 as (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 t decays: case for U(4s) running with polarized beams
Many thanks for your kind invitation!
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

57. Excursion into Tau Physics, November 2007
Additional
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

58. Excursion into Tau Physics, November 2007
BELLE (preliminary) Use -K-0h sample to get B(- K*-h ) through the K*-(892)K-0 mode using BR(K*-(892)K-0 )=1/3
-K*-h
BELLE
~1.9s lower than 
PDG 07
(CLEO) BR
(1.130.19 0.07)x10-4
(2.900.800.42)x10-4
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007

59. Some intepretations within benchmark models
compilation by S. Banerjee (Tau06,hep-ex/ )
Carsten Hast, SLAC
Excursion into Tau Physics, November 2007