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TAU LEPTONS IN THE QUEST FOR NEW PHYSICS Alexei Safonov Texas A&M University LIFE OF A TAU Fairly typical life of a celebrity: Michael Jackson Tau Lepton Unnoticed at birth 1958 14B yrs ago Instant celebrity when discovered 1970’s on: awards and best selling albums 1975: discovery …Nobel Prize by Martin Perl (1995) People digging through your personal life A lot! e.g. hundreds publications in Enquirer and such 1982-2004 measurements of tau lifetime, branchings …no lawsuits, though Greedy and heavy exploration for profit after death e.g. $250M deal signed for music distribution rights A tool for new great discoveries 2 …heavily used to get jobs and tenures WHAT DOES A TAU LOOK LIKE? Unstable, Lifetime: ct~87 mm Decay undergoes weak decays ne channels: Leptonic: t→enent, t →μnmnt (~36%) W nt t Hadronic: t →πnt, t→ππ0nt, t→πππnt, t→ππ0π0nt ... (~64%) Nomenclature: 1-prong, 3-prong etc. t e Kp,Np0,… W nt 3 TAU DISCOVERY (1975) Discovered at MARK I using e+e- beams at SLAC SPEAR Stanford Positron Electron Accelerating Ring E<4 GeV per beam The most “cost effective” collider ever built 4 MARK I DETECTOR Compared to CMS, almost a table-top experiment And not a very good one First hard evidence was the anomalous em events Electron ID: The main background was some other meson or hadron production Pulses in 24 leadscintillator counters extending full length with PMTs on each end Muon ID: Spark chambers behind a 24 cm absorber 5 PROPERTIES OF ANOMALOUS EVENTS Rate of events vs Ecm Simple mass estimate m(t)=1.9±0.1 GeV/c2 A candidate em event Event displays seem to have made a much bigger progress since 1975 than the rest of our field 6 LEPTON OR BOSON? Essentially trying to distinguish between: e+e-MMemnn e+e- LL(enn)(mnn) Still a lot of disbelief until in 1977 Pluto and DASP (DORIS @ DESY) confirm the discovery The new lepton was named t (triton = third) Data used to measure mass and B(tenn)≈ B(tmnn) ≈ 18% Fraction of Ecm energy carried by visible lepton Data follows the 3-body pattern consistent with a lepton decay 7 SIGNIFICANCE OF TAU DISCOVERY First evidence of the third generation Many hoped this is just another one in a series of new generations Statistically significant confirmation of “V-A” versus “V+A” nature of weak interactions First hints at large disparity in masses between generations m(t)=1.77 GeV/c2 vs m(e)=0.000511 m(m)=0.1057 GeV/c2 Also an amusing equality - Yoshio Koide (1981): me mm mt 2 0.666659 3 me mm mt 8 LIFE AFTER DISCOVERY Lifetime measurements required better detectors SLD decay length measurement (1995) using pixel vertex 9 detector LEP: END OF TAU’S STORY OF LIFE Ideal for high precision measurements: Ultra low backgrounds Fairly large boosts Precise reconstruction of momentum and di-tau mass via energy conservation LEP performed exhaustive studies of branching ratios, rare decay modes, lifetime etc. Tau: ready to be boxed and put next to e and m 10 IS THERE LIFE AFTER DEATH? 11 TAUS IN SEARCHES FOR NEW PHYSICS Two main reasons Many implications Higgs boson: Coupling to fermions hff~mf Tau is the heaviest lepton Supersymmetry: Third generation SUSY particles could be the lightest Even more Higgs signatures with taus 12 HIGGS LIKES TAUS Low mass Higgs: Taus: second highest leptonic Branching fractions after b’s Much cleaner signatures – can potentially use ggH process Low mass Higgs non-tau signatures Tevatron: relies on WH/ZH 5 times lower production rate compared to gluon fusion LHC: h: Tiny branching fraction Taus can come very handy: Also we won’t know what we found w/ just one measurement 13 SUPERSYMMETRY (SUSY) New symmetry: fermions bosons New “mirror” particles Particle e,n,u,d ,W,Z,h Dark Matter Candidate SUSY partner ~ ~ ~ ~ e ,n , u , d ~ , ~ , 1 2 ~10 ...~40 14 HOW SUSY HELPS In SM, Higgs mass acquires huge mass corrections H Fine tuning needed (10-30) SUSY: exact cancellation of diagrams with particles and sparticles H f mH2 | f |2 16p 2 [22UV ...] Unification of interactions f Resolves hierarchy problem Similar to EW unification Can include strong interactions Dark Matter candidate 15 AMUSING SUSY PREDICTIONS Top quark mass: 1980’s: Top quark mass was thought to be mt<~30 GeV, Tristan collider is built to find top - no luck… SUSY prediction: top has to be heavy: mt>mW! 1995: Mixing sin2qW = mW/mZ - arbitrary in SM: 1980’s: Tevatron discovers top: mt~175 GeV SUSY predicts sin2qW =mW/mZ= 0.231 1990: LEP sin2qW ~0.2309+/-0.0009 Could be a coincidence, but SUSY seems just too good to not be true 16 SEARCHES FOR SUSY While we have been setting boring limits, the strongest constraints on SUSY came from some place else WMAP measurements of dark matter density A handful of preferred regions in SUSY parameter space giving the right amount of dark matter 17 SUSY often over-produces the dark matter To solve it, need a mechanism to destroy extra neutralinos Stau co-annihilation: If stau is slightly heavier than lightest neutralino: mutual annihilation Can get the relic density right ~10 t t~ ~10 1 t ~10 t~ 1 t t Mass of Squarks and Sleptons SUSY: STAU CO-ANNIHILATION REGION Mass of Gauginos 18 FINDING SUSY At the end, convincing discovery of SUSY will likely require direct detection at colliders SUSY in stau co-annihilation region may be difficult to discover Complex cascades lead to busy events Can easily disguise as other SUSY species: If taus in the final state are not recognized, you will discover “wrong” SUSY 19 HIGGS IN SUPERSYMMETRY MSSM: A more complex Higgs hierarchy: Three neutral higgs bosons h/H/A Charged H+: Another good use for taus SUSY with Left-Right Symmetry: Often enhanced cross-section Doubly charged H++tt alongside right-handed W’s and neutrinos Next-to-MSSM: More complex higgs sector, new light CP-odd higgs a1 Can avoid standard searches via h1a1a1 (2t) (2t) 20 SUSY: NEUTRAL HIGGS PRODUCTION Additional diagrams and modified couplings to quarks Can be right around the corner Top row leads to enhanced production at large tanb: s(ggh/H/A)~tanb2 21 HIGGS IN DI-TAUS AT THE TEVATRON MSSM Htt WHttjj Also an interesting interplay with the CDMS results 22 CHARGED HIGGS If light enough, can be produced in top decays Will modify top branching fractions due to preference for taus Or else can be searched directly Production reduced by the coupling to light quarks 23 CHARGED HIGGS AT THE TEVATRON 24 DOUBLY CHARGED HIGGS t t t t 25 NEXT-TO-MSSM Adds a new singlet field to MSSM New decay mode for light higgs haa For a large range of m(a) dominant B(att) May explain the tension Sound as an abstract theoretical exercise, but has its between direct and merits: indirect higgs searches Solves the ”m problem” in SUSY (it is now generated by the new field) Resolves many of the “naturalness” problems in SUSY “Hiding” Higgs weakens LEP limits Experimental nightmare at a hadron collider! 26 SUMMARY FOR TODAY Many compelling arguments to look for new physics in final states with taus Almost always, taus are indispensible in understanding the nature of the discovered phenomenon Frequently, taus hold keys to discoveries Sometimes, an “incorrect” discovery can be made if not paying attention to taus The bad news is that taus are challenging in hadron collider environment You saw some examples showing high backgrounds and similar shapes Tomorrow we will talk about experimental techniques and challenges in searches for new physics with taus 27