The top quark at LHC status and prospects

The top quark at LHC: status and prospects Marina Cobal-Grassmann “Journee ATLAS France” Londe Les Maure, 3 -5 May, 2004 1

2 Motivations for Top Physics studies Sensitivity through radiative corrections F Scrutinize SM by precise determination top mass ¡ Summer 2003 result n Beyond SM: New Physics? ¡ ¡ Many heavy particles decay in tt Handle on new physics by detailed properties of top UD ED direct CL n Top quark exists and will be produced abundantly! In SM: top- and W-mass constrain Higgs mass n Experiment: Top quark useful to calibrate the detector n Beyond Top: Top quarks will be a major source of background for almost every search for physics beyond the SM EX n indirect Marina Cobal - Londe Les Maure 2004

3 What we know. . LEP+SLD: VCKM (4) UA 2+Tevatron: s(1) GF (1) SM mfermions (9) mbosons (2) m. H predictions (down to 0. 1% level) Nu. Te. V: APV: ee qq l. e. : No observable directly related to m. H. However the dependence can appear through radiative corrections tree level quantities changed , r = f [ln(m. H/m. W), mt 2] The uncertainties on mt, m. W are the dominating ones in the electroweak fit By making precision measurements (already interesting per se): • one can get information on the missing parameter m. H • one can test the validity of the Standard Model Marina Cobal - Londe Les Maure 2004

4 Top mass: Where we are Marina Cobal - Londe Les Maure 2004

5 Near future of Mtop Tevatron only (di-lepton events or lepton+jet ) from W decays Status of inputs (preliminary): mt=(178. 0 2. 7 (stat) 3. 3 (syst)) Ge. V/c 2 (latest Tevatron updated combination – Run. I data) mt=(175 17 (stat) 8 (syst)) Ge. V/c 2 (CDF di-leptons – Run. II data) mt=(178+13 -9 (stat) 7 (syst)) Ge. V/c 2 (CDF lepton+jets – Run. II data) Matter of statistics (also for the main systematics) and optimized use of the available information. Each experiment expects 500 b-tagged tt l+jets events/fb Mtop ~ 2 -3 Ge. V/c 2 for the Tevatron combined (2 -4/fb) mt 2. 5 Ge. V ; m. W 30 Me. V m. H/m. H 35% In 2009 (if upgrade is respected) from Tevatron: DMtop = 1. 5 Ge. V !! Marina Cobal - Londe Les Maure 2004

6 What can we do at LHC? √s [Te. V] Luminosity [cm-2 s-1] ∫L [fb-1/y] 2 <1032 0. 3 LHC (low lum) 14 1033 10 LHC (high lum) 14 1034 100 Te. Vatron (pb) Events/s Events/y bb 5 108 106 1013 Z ee 1. 5 103 ~3 107 W ℓ (ℓ=e, μ) 3 104 ~60 108 WW e X 6 10 -2 105 tt 830 ~1. 7 107 H(700 Ge. V/c 2) 1 2 10 -3 104 process - - LHC Te. Vatron Marina Cobal - Londe Les Maure 2004

7 Top production at LHC Cross section determined to NLO precision n ¡ ¡ Total NLO(tt) = 834 ± 100 pb Largest uncertainty from scale variation Compare to other production processes: n Low lumi Process N/year Total collected before start LHC W e 15 108 104 LEP / 107 FNAL Z ee 1. 5 107 LEP tt 1 107 104 Tevatron bb 106 1012 -13 109 Belle/Ba. Bar ? H (130) 0. 02 105 ? LHC is a top factory! ~90% gg ~10% qq ¡ Opposite @ FNAL Top production cross section approximately 100 x Tevatron Marina Cobal - Londe Les Maure 2004

8 Top decay n In the SM the top decays to W+b 1. Di-leptons (e/ ) ¡ ¡ ¡ 2. n All decay channels investigated ¡ ¡ Single Lepton (e/ ) ¡ Using ‘fast parameterized’ detector response Checks with detailed simulations ¡ ¡ 3. BR≈4. 9% 0. 4 x 106 ev/y No top reconstructed Clean sample BR=29. 6% 2. 5 x 106 ev/y One top reconstructed Clean sample Fully Hadronic ¡ ¡ BR≈45% 3. 5 x 106 ev/y Two tops reconstructed Huge QCD background Large combinatorial bckgnd Marina Cobal - Londe Les Maure 2004

9 MTop from lepton+jet Lepton side n Golden channel ¡ Br(tt bbjjl )=30% for electron + muon n The reconstruction starts with the W mass: ¡ ¡ n Hadron side Typical selection efficiency: ~5 -10%: • Isolated lepton PT>20 Ge. V • ETmiss>20 Ge. V Clean trigger from isolated lepton different ways to pair the right jets to form the W jet energies calibrated using m. W Important to tag the b-jets: ¡ ¡ enormously reduces background (physics and combinatorial) clean up the reconstruction • 4 jets with ET>40 Ge. V Background: <2% • >1 b-jet ( b 40%, uds 10 -3, c 10 -2) W/Z+jets, WW/ZZ/WZ Marina Cobal - Londe Les Maure 2004

10 Lepton + jet: reconstruct top n Hadronic side ¡ ¡ n W from jet pair with closest invariant mass to MW n Require |MW-Mjj|<20 Ge. V Assign a b-jet to the W to reconstruct Mtop Kinematic fit ¡ Using remaining l+b-jet, the leptonic part is reconstructed n |ml b -<mjjb>| < 35 Ge. V j 1 n Kinematic fit to the tt hypothesis, using MW constraints n j 2 Selection efficiency 5 -10% b-jet t Marina Cobal - Londe Les Maure 2004

11 Top mass systematics ¡ Method works: n n ¡ Biggest uncertainties: n n n ¡ Linear with input Mtop Largely independent on Top PT Jet energy calibration FSR: ‘out of cone’ give large variations in mass B-fragmentation Verified with detailed detector simulation and realistic calibration Challenge: determine the mass of the top around 1 Ge. V accuracy in one year of LHC Source of uncertainty Hadronic Mtop (Ge. V) Fitted Mtop (Ge. V) Light jet scale 0. 9 0. 2 b-jet scale 0. 7 b-quark fragm 0. 1 ISR 0. 1 FSR 1. 9 0. 5 Comb bkg 0. 4 0. 1 Total Marina Cobal 2. 3 - Londe Les 0. 9 Maure 2004

12 Alternative mass determination n Select high PT back-to-back top events: ¡ ¡ n Use the events where both W’s decay leptonically (Br~5%) ¡ ¡ n Hemisphere separation (bckgnd reduction, much less combinatorial) Higher probability for jet overlapping Much cleaner environment Less information available from two ’s Mtop Use events where both W’s decay hadronically (Br~45%) ¡ ¡ Difficult ‘jet’ environment Select PT>200 Ge. V Various methods all have different systematics Marina Cobal - Londe Les Maure 2004

13 Jet scale calibration n Calibration demands: ¡ Ultimately jet energy scale calibrated within 1% n ¡ Uncertainty on b-jet scale dominates Mtop: light jet scale constrained by m. W At startup jet-energy scale known to lesser precision ± 10% MTop Scale light-jet energy Uncertainty on light jet scale: 1% 10% Hadronic Mt < 0. 7 Ge. V Mt = 3 Ge. V Scale b-jet energy Uncertainty On b-jet scale: 1% 5% 10% Hadronic Mt = 0. 7 Ge. V Mt = 3. 5 Ge. V Mt = 7. 0 Ge. V Marina Cobal - Londe Les Maure 2004

14 Alternative methods n Determining Mtop from (tt)? ¡ n Luminosity uncertainty then plays the game (5%? ) huge statistics, totally different systematics But: Theory uncertainty on the pdfs kills the idea ¡ ¡ ¡ n n 10% th. uncertainty mt 4 Ge. V Constraining the pdf would be very precious… (up to a few % might not be a dream !!!) Luminosity uncertainty then plays the game (5%? ) Continuous jet algorithm n n ¡ ¡ Reduce dependence on MC Reduce jet scale uncertainty Repeat analysis for many cone sizes R Sum all determined top mass: robust estimator top-mass Marina Cobal - Londe Les Maure 2004

15 Top mass from J/ n Use exclusive b-decays with high mass products (J/ ) ¡ ¡ Higher correlation with Mtop Clean reconstruction (background free) BR(tt qqb +J/ ) 5 10 -5 ~ 30% 103 ev. /100 fb-1 (need high lumi) Ml. J/ Different systematics (almost no sensitivity to FSR) Uncertainty on the bquark fragmentation function becomes the dominant error M(J/ +l) Pttop Marina Cobal - Londe Les Maure 2004

16 Search for resonances Many theoretical models include the existence of resonances decaying to top-topbar ¡ ¡ ¡ n SM Higgs (but BR smaller with respect to the WW and ZZ decays) MSSM Higgs (H/A, if m. H, m. A>2 mt, BR(H/A→tt)≈1 for tanβ≈1) Technicolor Models, strong Electro. Weak Symmetry Breaking, Topcolor, “colorons” production, […] Study of a resonance Χ once known σΧ, ΓΧ and BR(Χ→tt) ¡ Reconstruction efficiency for semileptonic channel: ¡ ¡ 1. 6 Te. V resonance Mtt 20% mtt=400 Ge. V 15% mtt=2 Te. V σx. BR [fb] n 830 fb x. BR required for a discovery 30 fb-1 300 fb-1 1 Te. V m [Ge. V/c 2] Marina Cobal - Londe Lestt Maure 2004

17 Couplings and decays n Does the top quark behaves as expected in the SM? n n n According to the SM: ¡ n Yukawa coupling to Higgs from ttbar. H events Electric charge Top spin polarization CP violation Br(t Wb) 99. 9%, Br(t Ws) 0. 1%, Br(t Wd) 0. 01% (difficult to measure) Can probe t W[non-b] by measuring ratio of double b-tag to single b-tag ¡ ¡ Statistics more than sufficient to be sensitive to SM expectation for Br(t W + s/d) need excellent understanding of b-tagging efficiency/purity Marina Cobal - Londe Les Maure 2004

18 Rare decays: FCNC n In the SM the FCNC decays are highly suppressed (Br<10 -13 -10 -10) ¡ Any observation would be sign of new physics n Sensitivity according to ATLAS and CMS studies : Ø t Zq (CDF Br<0. 137, ALEPH Br<17%, OPAL Br<13. 7%) ¡ ¡ Ø t q ¡ Ø Reconstruct t Zq (l+l-)j Sensitivity to Br(t Zq) = 1 X 10 -4 (100 fb-1) (CDF Br<0. 032) Sensitivity to Br(t q) = 1 X 10 -4 (100 fb-1) t gq ¡ ¡ ¡ Difficult identification because of the huge QCD bakground One looks for “like-sign” top production (ie. tt) Sensitivity to Br(t gq) = 7 X 10 -3 (100 fb-1) Marina Cobal - Londe Les Maure 2004

19 Top Charge determination n Can we establish Qtop=2/3? ¡ Currently cannot exclude exotic possibility Qtop=-4/3 n Assign the ‘wrong’ W to the b-quark in top decays ¡ n t W-b with Qtop=-4/3 instead of t W+b with Qtop=2/3 ? Technique: ¡ Hard radiation from top quarks n n Radiative top production, pp tt cross section proportional to Q 2 top Radiative top decay, t Wb ¡ n On-mass approach for decaying top: two processes treated independently Matrix elements have been calculated and fed into Pythia MC Radiative top production Radiative top decay Marina Cobal - Londe Les Maure 2004

20 Top Charge determination n Yield of radiative photons allows to distinguish top charge Q=2/3 Q=-4/3 pp tt 101 ± 10 295 ± 17 pp tt ; t Wb 6. 2 ± 2. 5 2. 4 ± 1. 5 Total background n Determine charge of b-jet and combine with lepton ¡ ¡ Use di-lepton sample Investigate ‘wrong’ combination b-jet charge and lepton charge 38 ± 6 10 fb-1 One year low lumi ¡ ¡ Effective separation b and b-bar possible in first year LHC Study systematics in progress events p. T( ) Marina Cobal - Londe Les Maure 2004

21 Top spin correlations n In SM with Mtop 175 Ge. V, (t) 1. 4 Ge. V » QCD ¡ ¡ n Top decays before hadronization, and so can study the decay of ‘bare quark’ Substantial ttbar spin correlations predicted in pair production Can study polarization effects through helicity analysis of daughters ¡ ¡ Study with di-lepton events Correlation between helicity angles + and for e+/ + and e-/ - <CosΘ+ · CosΘ-> No helicity correlation <CosΘ+ · CosΘ-> With helicity correlation e+/ + + top Marina Cobal - Londe Les Maure 2004

22 Top spin correlations n Also study spin correlations in hadronic decays (single lepton events) ¡ <CosΘ+ · CosΘ-> n Least energetic jet from W decay: ~ 0. 5 30 fb-1 Able to observe spin correlations in asymmetry C ¡ 30 fb-1 of data: n n ¡ ± 0, 035 statistical error ± 0, 028 systematic error 10 statistical significance for a non-zero value with 10 fb-1 n Ratio between ‘with’ and ‘without’ correlations Marina Cobal - Londe Les Maure 2004

23 Single top production 1) Determination of Vtb 2) Independent mass measurement Three production mechanisms: Wg fusion: 245± 27 pb S. Willenbrock et al. , Phys. Rev. D 56, 5919 n +16. 6 Wt: 62. 2 -3. 7 pb A. Belyaev, E. Boos, Phys. Rev. D 63, 034012 Main Background [ x. BR(W→ℓ ), ℓ=e, μ]: ¡ ¡ ¡ tt Wbb Wjj σ=833 pb [ 246 pb] σ=300 pb [ 66. 7 pb] σ=18· 103 pb [4· 103 pb] Wg Wt W* W* 10. 2± 0. 7 pb M. Smith et al. , Phys. Rev. D 54, 6696 [54. 2 pb] [17. 8 pb] [2. 2 pb] Marina Cobal - Londe Les Maure 2004

24 Single top results n Detector performance critical to observe signal ¡ ¡ n Fake lepton rate b and fake rate id Reconstruction and vetoing of low energy jets Identification of forward jets n n Each of the processes have different systematic errors for Vtb and are sensitive to different new physics ¡ ¡ heavy W’ increase in the s-channel W* FCNC gu t increase in the W-gluon fusion channel Signal unambiguous, after 30 fb-1: Process Signal Bckgnd S/B Wg fusion 27 k 8. 5 k 3. 1 Wt 6. 8 k 30 k 0. 22 W* 1. 1 k 2. 4 k 0. 46 Complementary methods to extract Vtb (stat) Vtb (theory) Wg fusion 0. 4% 6% Wt 1. 4% 6% W* 2. 7% 5% Process n fb-1 With 30 of data, Vtb can be determined to %-level or better (experimentally) Marina Cobal - Londe Les Maure 2004

25 Undergoing analyses n n n CP violation in top events (K. Martens, University of Toronto ) Top spin polarization in di-lepton events (V. Simak et al. , Prague) Top spin polarization in single lepton events (E. Monnier, P. Pralavorio, F. Hubaut, CPPM) Single top studies (M. Barisonzi, NIKHEF) Optimization of kinematic reconstruction in the single lepton channel (V. Kostioukhine, University of Genova) Commissioning studies (S. Bentvelsen, NIKHEF) New MC validation (S. Bentvelsen, E. Monnier, P. Pralavorio) Full simulation studies of detector effects (A. Etienvre, J. Schwindling, JP Meyer, Saclay) Full simulation studies of b-tagging (S. Moed, University of Geneva) Top mass and calibration studies (D. Pallin, F. Binet, Clermont-Ferrand) Ttbar resonances (E. Cogneras, Clermont-Ferrand) Marina Cobal - Londe Les Maure 2004

26 What is left before the LHC starts? ü ü Cover topics still open: cross section, couplings, exotic, resonances, Define a strategy for validation of the MC input models (e. g: UE modeling and subtraction, jet fragmentation properties, jet energy profiles, b-fragmentation functions. . ) see M. Mangano talk at IFAE 2004 ü ü Explore the effects of changing detector parameters in evaluating the top mass. Perform commissioning studies with top events Contribute to simulation validation … Marina Cobal - Londe Les Maure 2004

27 Commissioning the detectors n Determination MTop in initial phase ¡ n Period Stat Mtop (Ge. V) Stat / 1 year 0. 1 0. 2% 1 month 0. 2 0. 4% 1 week 0. 4 2. 5% No background included Selection: ¡ ¡ n Use ‘Golden plated’ lepton+jet Calibrating detector in comissioning phase Assume pessimistic scenario: -) No b-tagging -) No jet calibration -) But: Good lepton identification Isolated lepton with PT>20 Ge. V Exactly 4 jets ( R=0. 4) with PT>40 Ge. V Reconstruction: ¡ Select 3 jets with maximal resulting PT ¡ Signal can be improved by kinematic constrained fit n Assuming MW 1=MW 2 and MT 1=MT 2 Marina Cobal - Londe Les Maure 2004

28 Commissioning the detectors n Most important background for top: W+4 jets ¡ Leptonic decay of W, with 4 extra ‘light’ jets F Alpgen, Monte Carlo has ‘hard’ matrix element for 4 extra jets (not available in Pythia/Herwig) n Signal plus background at initial phase of LHC L = 150 pb-1 (2/3 days low lumi) ALPGEN: W+4 extra light jets Jet: PT>10, | |<2. 5, R>0. 4 No lepton cuts Effective : ~2400 pb With extreme simple selection and reconstruction the toppeak should be visible at LHC measure top mass (to 5 -7 Ge. V) give feedback on detector performance Marina Cobal - Londe Les Maure 2004

29 Top in DC 2 Tier test n The 10 M tier 1 events in light of top: ¡ Generation/simulation of 106 top events, inclusive decays, using MC@NLO n n ¡ Simulation 500 K top events with displaced ID n n ¡ Same truth generated top events as above 1 cm displacement of ID – check tracking performance Simulation of 106 W+jet events MC@NLO n ¡ Using Herwig for MC + UE Simulation with full geometry For W+2 jet background Simulation of 250 K W+4 jet events with Alp. Gen n p. T>15 Ge. V approximately Marina Cobal - Londe Les Maure 2004

30 What is still missing? n Top production is ‘over-weighted’ in 10 M sample ¡ n One of priorities in ‘post-production’: ¡ ¡ n Unrealistic to ask for more in this sample Regenerate half of the top MC@NLO sample Now using Jimmy UE (much more activity), after tuning Still on the wish-list: ¡ Top events with spin correlations n ¡ Single top events n ¡ ¡ Top. Rex available, perhaps Acer. MC? also Top. Rex Dedicated samples single- and dilepton top events Top events with PYTHIA (cross check with DC 1) Marina Cobal - Londe Les Maure 2004

31 Conclusions n Precise determination of Mtop is waiting… ¡ n Measure Vtb, charge, CP, spin, decays Top quarks for commissioning the detectors ¡ n Challenge to get Mtop ~ 1 Ge. V Confirmation that top-quark is SM particle ¡ n LHC is top factory (tt)~830 pb-1 107 events in first year Top peak should be visible with eyes closed Today’s signal, tomorrow’s background ¡ Top quarks as main background for many new physics channels Marina Cobal - Londe Les Maure 2004

32 Rare SM top decays n Direct measurement of Vts, Vtd via decays t s. W, t d. W n Decay t b. WZ is near threshold (mt~MW+ MZ+mb) BRcut(t b. WZ) 6 10 -7 (cut on m(ee) is 0. 8 MW) n Decay t c. WW suppressed by GIM factor BR(t c. WW) ~ 1 10 -13 n If Higgs boson is light: t b. WH n FCNC decays: t cg, t c. Z (BR: 5 10 -11 , 5 10 -13 , 1. 3 10 -13 ) n Semi-exclusive t-decays t b. M (final state 1 hadron recoiling against a jet: BR(t b ) 4 10 -8, BR(t b. Ds) 2 10 -7) Marina Cobal - Londe Les Maure 2004

33 Top mass from di-leptons n Use the events where both W’s decay leptonically (Br~5%) ¡ ¡ n Much cleaner environment Less information available due to two neutrino’s Sophisticated procedure for fitting the whole event, i. e. all kinematical info taken into account (cf D 0/CDF) ¡ Compute mean probability as function of top mass hypothesis n Maximal probability corresponds to top mass Mean probability 80000 events (tt) = 20 % S/B = 10 mass Selection: 2 isolated opposite sign leptons Pt>35 and Pt>25 Ge. V 2 b-tagged jets Source of uncertainty Di-lepton Mtop (Ge. V) statistics 0. 3 b-jet scale 0. 6 b-quark fragm 0. 7 ISR 0. 4 FSR 0. 6 pdf 1. 2 Total 1. 7 Marina Cobal - Londe Les Maure 2004

34 Top mass from hadronic decay n n Use events where both W’s decay hadronically (Br~45%) Difficult ‘jet’ environment ¡ ¡ n Perform kinematic fit on whole event ¡ n (QCD, Pt>100) ~ 1. 73 mb (signal) ~ 370 pb b-jet to W assignment for combination that minimize top mass difference Selection 6 jets ( R=0. 4), Pt>40 Ge. V 2 b-tagged jets Note: Event shape variables like HT, A, S, C, etc not effective at LHC (contrast to Tevatron) Increase S/B: ¡ Require p. T(tops)>200 Ge. V 3300 events selected: (tt) = 0. 63 % (QCD)= 2· 10 -5 % S/B = 18 Source of uncertainty Hadronic Mtop (Ge. V) Statistics 0. 2 Light jet scale 0. 8 b-jet scale 0. 7 b-quark fragm 0. 3 ISR 0. 4 FSR 2. 8 Total. Marina Cobal - Londe 3. 0 Les Maure 2004

35 High Pt sample n The high p. T selected sample deserves independent analysis: ¡ ¡ n Hemisphere separation (bckgnd reduction, much less combinatorial) Higher probability for jet overlapping Use all clusters in a large cone R=[0. 8 -1. 2] around the reconstructed top- direction ¡ ¡ Less prone to QCD, FSR, calibration UE can be subtracted j 2 j 1 b-jet t Mtop Statistics seems OK and syst. under control R Marina Cobal - Londe Les Maure 2004

36 Jet scale calibration n Calibration demands: ¡ Ultimately jet energy scale calibrated within 1% n ¡ Uncertainty on b-jet scale dominates Mtop: light jet scale constrained by m. W At startup jet-energy scale known to lesser precision ± 10% MTop Scale light-jet energy Uncertainty on light jet scale: 1% 10% Hadronic Mt < 0. 7 Ge. V Mt = 3 Ge. V Scale b-jet energy Uncertainty On b-jet scale: 1% 5% 10% Hadronic Mt = 0. 7 Ge. V Mt = 3. 5 Ge. V Mt = 7. 0 Ge. V

37 Rare decays: top Wb. Z ¡ ¡ Since Mtop~MW+Mb+MZ With present error mt 5 Ge. V, BR varies over a factor 3 B-jet too soft to be efficiently identified “semi-inclusive” study for a WZ near threshold, with Z l+l- and W ->jj n ¡ G. Mahlon hep-ph/9810485 (t Wb. Z)/ (t Wb) Interesting: branching ratio depends strongly on Mtop Requiring 3 leptons reduces the Z+jets background M(top) (Ge. V) n Sensitivity to Br(t Wb. Z) 10 -3 for 1 year at low lumi. ¡ Even at high L can’t reach Sm predictions ( 10 -7 - 10 -6) Marina Cobal - Londe Les Maure 2004

38 top Hq Signal tt. H n Various approaches studied ¡ ¡ n n Previously: ttbar Hq Wb (b-bbar)j(l b) for m(H) = 115 Ge. V Sensitivity to Br(t Hq) = 4. 5 X 10 -3 (100 fb-1) tt New results for: t tbar Hq Wb WW*q Wb (l l j) (l b) ≥ 3 isolated lepton with p. T(lep) > 30 Ge. V ¡ p. Tmiss > 45 Ge. V ¡ ≥ 2 jets with p. T(j) > 30 Ge. V, incl. ≥ 1 jet con b-tag ¡ Kinematical cuts making use of angular correlations ¡ ¡ Signal tt. H tt Sensitive to Br(t Hq) = 2. 4 X 10 -3 for m(H) = 160 Ge. V (100 fb-1) Marina Cobal - Londe Les Maure 2004

39 Non-SM Decays of Top n n 4 thfermion family Constraints on Vtq relaxed: Supersymmetry (MSSM) ¡ Observed bosons and fermions would have superpartners 2 -body decays into squarks and gauginos (t H+ b ) Big impact on 1 loop FCNC two Higgs doublets ¡ H LEP limit 77. 4 Ge. V (LEP WG 2000) ¡ Decay t H+ b can compete with t W+ b ¡ 5 states (h 0, H 0, A 0, H+, H-) survive after giving W & Z masses ¡ H couples to heaviest fermions detection through breakdown of e / m / t universality in t t production ¡ n Marina Cobal - Londe Les Maure 2004