1. Study of the LFV channel t  3m
Silvia Ventura
Laboratori Nazionali di Frascati
studio di fattibilta’
19/05/ LNF – Spring school 2006

2. t  3m decay This decay is very sensitive to non-SM physics!
forbidden in the classical SM,
allowed but very small BR (BR<10-21)
considering neutrinos oscillations,
foreseen in several models beyond the SM (BR~ )
mSUGRA+seesaw
10-9
Phys. Rev. D 66, (2002)
SUSY+SO(10)
10-10
Phys. Rev. D 68, (2003)
SM+seesaw
Phys. Rev. D 66, (2002)
Non-Universal Z’
10-8
Phys. Lett. B 547,252 (2002)
MSSM
10-7
Phys. Lett. B 566,159 (2002)
This decay is very sensitive to non-SM physics!

3. Minimal Supersymmetric Standard Model
In the MSSM the Lepton Flavour Violating couplings of the Higgs bosons
can induce the decays (h0, H0, A)  mt with non-negligible rates.
DL and DR are dimensionless functions
of the MSSM mass parameters and take
into account one-loop diagrams, which involve the exchange of sleptons, gauginos and Higgsinos m t
h0,H0,A

4. Experimental results
CLEO: current upper limit in the PDG BR<1.9 x % C. L.
Big improvement from B-factory:
BABAR: BR< 1.9 x % C.L. on 91.5 fb-1
hep-ex/
BELLE: BR< 2.0 x % C.L. on 87.1 fb-1
Phys.Lett.B589: ,2004
Belle expectation:
Tau04 International workshop on t lepton physics
Belle has already collected 520fb-1
can reach now (3-5)x10-8 al 90% C.L.
2ab-1 are foreseen for 2010

5. LHC SIGNAL FEATURES At LHC the t will be produced in several ways:
(mesons D,B decays + Ztt + Wtn)
The most powerfull and clean signal source will be from the W decay,
produced t’s are:
 isolated
 high Pt
 high missing energy
With s(W)*BR(Wtn) = 19 nb and assuming the present limit
of BR (t  3m ) = 2 x 10-7 we expect:
38 events in 10 fb-1
corresponding to 1 year of LHC running at low luminosity 2x1033 cm-2s-1

6. CMS t3m 10fb-1 t from W decay
3 reconstructed muons with pt>3GeV in the barrel
total charge +/-1
common secondary vertex
isolation
missingEt > 20GeV
F-mass veto
SIGNAL + BACKGROUND
after 1 year of LHC with
the hypothesis of
BR=1.9E-6
Analysis of 2002:
0.6 background events
and 17 signal events
expected at the end
of the analysis in 10fb-1
with the hypothesis
of BR(t3m)=1.9x10-6
In case of NO signal events: 90% C.L.

7. THE ATLAS DETECTOR Muon spectrometer Elettromagnetic calorimeter
Forward
calorimeter
solenoid
Endcap
toroid
Inner detector
Barrel toroid
Hadronic
calorimeter
Shielding

8. SIGNAL: PRESELECTION and di-muon TRIGGER (Fast Simulation)
The t  3m decay has been inserted in PYTHIA forcing the two
neutrinos of tm n n to be muons with a uniform PHASE SPACE.
A modelling of the decay tried at the end to study systematics
on reconstruction efficiency
Reconstruction with fast simulation programs of ATLAS
Transverse momentum (MeV) distributions of the 3 muons (pt-ordered).
PRESELECTION = 3 muons with pt > 3GeV and |η|< 2.5 = 30%
Efficiency of tracks in low PT region (3-6 GeV) assumed = 100%

9. Signal: Atlfast M3m At preselection level: Mean 1777 ± 0.2 MeV
TRIGGER: L1/L2  2m6 (200 L=2x1033 cm-2s-1, DAQ limited 10 Hz )
HLT  specific requirements under study ex: 2m6 + Etmiss
3 muons invarinat mass distribution
At preselection level:
Mean 1777 ± 0.2 MeV
Sigma ± 0.16 MeV
MeV

10. BACKGROUNDS Main background source from ccbar and bbbar production,
with cascade decays producing light mesons as f, η,  ,η’
Real background
only from
processes of
type B,
same branch for
the 3 muons
Processes of
type A are no
to be considered:
topologically
different from
signal (large DR)
The preselection requirement of 3m with pt>3GeV in the same branch
is very hard on backgroud events.
Out of 106 ccbar produced events not one survives this preselection.
We produce ccbar and bbbar events imposing the c,b quark
to be generated by PYTHIA with pt>10GeV
s(ccbar) = 8 mb  pt(quark)>10GeV s(ccbar) = 3.76 x 10-2 mb
s(bbbar) = 470 mb  pt(quark)>10GeV s(bbbar) = 2.99 x 10-2 mb

11. PRODUCED BACKGROUNDS
From PDG table we select the most dangerous sources of background.
Four channels were considered:
1) Ds  mnf (3%) with fmm (BR=2.5 x10 -4) x103 events in 10 fb-1
2) Ds  mnη (2%) with η mmg (BR=3.1 x10 -4) x103 events in 10 fb-1
3) Bs  Ds mn (2%)
Ds* mn (5.5%) with Ds*  Ds g
with Ds Kf 0.5%
Ds pf 2.8%
Ds rf 5.2% fmm (BR=2.5x10-4)
4) Bs  Ds mn (2%)
with Ds Kη 0.5%
Ds K*η 0.5%
Ds pη 1.5%
Ds rη 7.9% ηmmg (BR=3.1x10-4)
75x103 events in 10 fb-1
113x103 events in 10 fb-1
Other channels give smaller contribute:
D+mnη (0.1%) con η mmg (3.1x10-4)
D+mnr (0.1%) con rmm (4.6x10-5)
D+mnη’ (0.1%) con η’ mmg (1.0x10-4)

12. Background PRESELECTION
Transverse momentum distributions of the 3 muons (pt-ordered)
for one of the four backgrounds.
PRESELECTION = 3 muons with pt > 3GeV and |η|< 2.5 = 2%

13. Missing Et
cut MEt > 15 GeV
SIGNAL
BACKGROUND
MeV
MeV
MeV
MeV

14. Spatial separation between the muons
cut: DRij < 0.2

15. F-veto mclosest < (mf- 30MeV) mclosest > (mf+30MeV)
For bcgk involving
a fmm decay.
Invariant mass
distribution of
di-muon combination
closest to the f mass.
mclosest < (mf- 30MeV)
mclosest > (mf+30MeV)

16. Signal-background comparison
Preselection
pt > 3GeV |eta|<2.5
Trigger
MissingEt DR
F-veto
t-window
SIGNAL
3075
(12)
2693
2550
2431
1997
1852
(7)
DSPHI
8846
(7600)
2412
270
227 24
DSETA
1663
(1225)
381 77 71 68
BSPHI
3758
(826)
872
147 40 2
BSETA
1047
(280)
185 44 17 16
(0.53)
(*) values in red are events normalized to 10fb-1
(**) for the normalization of the signal a BR of 2 x 10-7 is assumed
7 signal events and 0.53 bckg events are counted in the t mass window.
REMINDER 1: modelling of global efficiency (next slide)
REMINDER 2: the efficiency for low pt muons assumed to be 100%
both for signal and backgrounds!! -----> go to full simulation

17. Modelling of acceptance
Dalitz plot for PHASE SPACE decay
Dalitz plot for MSSM matrix element
tgb=50
a=b-p/2
Mh=MH=MA=100GeV
DL =
DR =
(MeV)
(GeV)
Signal selection efficiency as a function of muon pt
No effect on global efficiency!
MeV

18. SIGNAL: “first” comparison Fast /Full simulation
Start
Presel
Trigger
MissingEt DR
F-veto
t-window
eff tot
FAST
SIM
9959
(38)
3075 (30.8%)
2693(87.6%)
2550(94.6%)
2431(95.3%)
1997 (82%)
1852 (93%)
(7)
18.6%
FULL
7450
2525(34%)
2207(87.4%)
1998(90.4%)
1910(95.5%)
1568(82%)
1061(68%)
(5.4)
14.2%
CONCLUSIONS and OUTLOOK
Good efficiency for the signal expected… small dependence from
models.
Study of the backgrounds is crucial full simulation needed to add fake
muons (mostly from pions)
Estimate of reach for upper limits on BR in progress .. Results seem
to be promising with the fast simulation.
… FULL SIMULATION IN PROGRESS…

19. B-factory results on t3l analysis
BABAR: Analysis of 91fb-1
BELLE: Analysis of 87fb-1