1. Tau leptons as an exciting probe for New Physics in ATLAS
Anna Kaczmarska
NZ14, Institute of Nuclear Physics PAN, Cracow
ATLAS experiment
Tau decays’ properties
Tau signatures of New Physics
Taus in Standard Model channels:
Z→ττ, W→τν, ttbar
Conclusions
Results based on Monte-Carlo simulations at 14 TeV pp centre of mass energy

2. ATLAS (A Toroidal LHC ApparatuS)
Muon Detectors: fast response for trigger, good p resolution
Energy-scale:
e/γ ~0.1%
muons ~0.1%
Jets ~1%
Inner Detector: high
efficiency tracking, good
impact parameter resolution
40MHz beam crossing
Readout: 160M channels
(3000 km cables)
Raw data = 320Mbyte/sec
(1TB/hour)
Electromagnetic Calorimeters: excellent e/γ identification, E and angular resolution, response uniformity
Hadron Calorimeters: Good jet and ET miss performance

3. Tau reconstruction in ATLAS
IFJ PAN:
Anna Kaczmarska
Elżbieta Richter-Wąs
Tau reconstruction in ATLAS
Decay modes
Branching ratio
leptonic
35.2%
1-prong (τ→nπ0π±ν)
49.5%
3-prong (τ→nπ0π±ππ±ν)
14.6%
τ decay π0 π+ π-
τ jet
Tau hadronic decays in detector:
●Decay products very collimated
Low track multiplicity
Displaced secondary vertex
Main sources of fake taus: QCD jets, electrons, muons
Two ways for reconstruction of taus in ATLAS
Using tracks as seeds and energy from Energy Flow method (using Tracker + EM Calorimeter)
Using calorimeter clusters as seeds and energy only from calorimeters
Reconstruction efficiency for candidates reconstructed with both seeds (1P/3P):
τ’s in signal events ~60%
fake τ’s from QCD jets ~4-8%

4. Tau identification in ATLAS
IFJ PAN:
Marcin Wolter
Andrzej Zemła
Tau identification in ATLAS
Tau ID Algorithms:
Cut analysis, Projected Likelihood Probability Density Estimator with Range Searches, Neural Network, Boosted Decision Tree
Both reconstruction algorithms provide variables discriminating between taus and fakes from QCD background
~1000
Neural Network
Rejection of fakes from QCD jets
~600
Radius of EM shower
ATLAS
30%
Dedicated algorithms for rejection of fake candidates from electrons
Efficiency (reco τ) = 95%
Rejection (e reco as τ) = 60 +
Efficiency of tau identification

5. Higgs and New Physics signatures with tau leptons
Higgs Processes
Standard Model
VBF production, H→ττ
MSSM Higgs
bbH/A , H/A→ττ
H+ in tt→H+b Wb,
gb→bH+, H+→τν
SUSY signatures with τ’s in final states
discovery
tau polarization measurement to constrain the underlying SUSY model
Extra dimensions… exotic physics…

6. Standard Model signatures with tau leptons
Looking for New Physics is very exciting but first we have to
… rediscover Standard Model
To understand performance of complex detector
calibration, particle reconstruction algorithms, triggers… etc
To measure basic SM processes (cross-sections…) and compare them to theory and various Monte-Carlo tools
SM processes are background for New Physics
Direct sensitivity new physics (e.g. rare top decays .. )
LHC will be a factory of W, Z and top
Large statistics even with low luminosity
- analysis possible already with 100 pb-1 of integrated luminosity
All of those channels have tau signatures: Z→ττ, W→τν, ttbar→W(τ)W(l/jj)bb

7. Z→τ(lepton)τ(had) events
IFJ PAN:
Anna Kaczmarska
Elżbieta Richter-Wąs
Z→τ(lepton)τ(had) events
Z→τ(e/μ)τ(had) will provide a statistically significant sample of τ leptons for 100 pb-1
measurement of the cross-section and the τ-ID efficiency
selected by lepton trigger -> unbiased sample of τ(had) decays
measurement of the τ trigger efficiencies
detector calibration: ETmiss and τ-jet energy scale determination (precision ~3%)
Cross check between τ(e)τ(had) and
τ(μ)τ(had) final states
Use of opposite-sign and same-sign
e/μ-τ(had) events to control background
Dominant backgrounds
QCD jets
We
Signal-to-background ratio 6:1
Expected events in 100 pb-1
Visible mass (GeV)

8. W→τν events Signal-to-background ratio 3:1
IFJ PAN:
Paweł Malecki
Elżbieta Richter-Wąs
W→τν events
The most abundant source of τ‘s in SM processes
measurement of the cross-section
branching ratios for leptonic W decays: BR(W→τν)/BR(W→eν)
ratio of the τ and electron electroweak coupling to charged current: gτ/ge
Triggered on single tau + ETmiss
Dominant backgrounds
QCD jets
We
Signal purity and cross-section estimated on the basis of characteristic double peak structure in the track multiplicity distribution.
Signal-to-background ratio 3:1
Expected events in 100 pb-1
ATLAS
Track multiplicity of tau candidates

9. IFJ PAN:
Tadeusz Szymocha
Involvement starting now
ttbar events
Gives larger pT range of τ’s complementary to that from the W and Z processes
studies of τ trigger and tau ID efficiency
to complete the scan of the ttbar channels for cross-section measurements
to search for the effects beyond the SM in the top quark decay:
ttbar→W(τν )W( qq‘ )bb channel
about 300 signal events with S:B of 20:1 are expected for 100 pb-1
ttbar →W(τν )W(lν)bb channel
dominated by W→lν+3jets events.
about 67 events can be observed with S:B of 1:1 for 100 pb-1 t t τ

10. We hope to have early measurements with tau data this year!
Summary
Events with τ’s from Standard Model will be observed with the first data of LHC
excellent possibility of understanding detector performance
Tau leptons will be the key to the Higgs and New Physics discovery:
SM Higgs, MSSM Higgs, SUSY, extra dimensions, new theories…
ATLAS group in IFJ PAN strongly involved in:
τ leptons reconstruction and identification algorithms
Standard Model physics with τ’s in final states
Important for our success are strong links with expertise of IFJ PAN Theory group (NO4), MC package Tauola for tau decays
Leading role of IFJ PAN group in preparation of 9 Internal Notes documenting expected tau performance  ( ); E. Richter-Was convener of the ATLAS Tau WG in years
External funding:
EU Marie Curie reintegration grant ; Polish Government grant (supplement to EU MC grant); Polish Government grant N ; IN2P3 Collaboration IFJ PAN- LAPP Annecy
We hope to have early measurements with tau data this year!

11. Backup Slides

12. Why are tau leptons so interesting?
massive particles
measurable lifetime
undergo electroweak interaction only
production and decay well separated in time
potential for measurement of the polarisation, spin correlations, parity
excellent knowledge about tau decay modes from low-energy experiments
Ideal signature to probe “New Physics” !
however .....
several decay modes possible
jet-like signature
huge background from QCD multi-jet production
Make it quite difficult for being observed in the experimental environment of pp collisions at ATLAS
Why are tau leptons so interesting? Tau leptons are massive particles with measurable lifetime undergoing electroweek interactions only.
The production and the decay of the tau letons are well separated in time providing unbiased potential for measurements of the polarisation, spin correlations and the parity of resonances decaying into tau leptons. Also the excellent knowledge of tau decay modes from low energy experiments makes them an ideal signature to probe new physics.
However tau leptons have possible several decay modes. …..
1.0 min

13. The very first beam-splash event in ATLAS
10:19, 10th September 2008
Online display
Offline display
The LHC has been fully installed and it started operation with single beams on Now it is delayed until summer 2009 after an incident that happened on 13