In Search of Discovery: Results from the Tevatron Chris Hays, Oxford University UK HEP Forum, Coseners House, Abingdo n
Fundamental Particle Discoveries ~ 15 years since last co � ider > 30 years since last surpris e particle discovery accelerator particle discovery 1977 observation of Υ→μμ in proton - nucleus co � isions demonstrated the existence of a third generation of quarks top quark discovered in 1995 through tt productio n 2 8 May, 2009 Chris Hays, Oxford University
Making a Discovery Strategies Probe we �- motivated models Search for clear indications of new physics Study a � fi nal states Issues How do you know when it ’ s a discovery? How do you know you haven ’ t missed a discovery? 3 8 May, 2009 Chris Hays, Oxford University
Tevatron Searches W orld ’ s highest energy co � ider Emphasis on massive particles approaching the kinematic limi t pp co � isions complementary to e + e - High energy & rates, large cross sections for particles with color charg e > 6 � - 1 delivered per experimen t 4 8 May, 2009 Chris Hays, Oxford University
Tevatron Detectors Upgraded CDF & D Ø detectors have unique capabilities CDF: High resolution trackers, time - of -fl ight chamber D Ø : Broad muon coverage, fi nely segmented calorimeter Complementarity between detectors CDF D Ø 5 8 May, 2009 Chris Hays, Oxford University
Making a Discovery Strategies Probe we �- motivated models Search for clear indications of new physics Study a � fi nal states Issues How do you know when it ’ s a discovery? How do you know you haven ’ t missed a discovery? 6 8 May, 2009 Chris Hays, Oxford University
A W ell - Motivated Model Supersymmetry Regulates Hi � s boson mass Explains dark matter Predicts force uni fi catio n 7 8 May, 2009 Chris Hays, Oxford University
Supersymmetry at the Tevatron Candidates for discovery Sparticles with highest cross sections Squarks and gluinos Sparticles with lowest masses Charginos, neutralinos, stop squarks Final states depend on mass hierarchy Interpret results using a reference model mSUGRA most common ( 5 parameters ) Typica � y assume lightest sparticle stabl e 8 8 May, 2009 Chris Hays, Oxford University
Squark and Gluino Searches Search in fi nal states with 2, 3, or 4 jets plus E T ~ ~ ~~ 2 jets: qq → qq χ 10 χ 10 ( m q < m g ) ~ ~ ~ ~ ~~ 3 jets: qg → qqq χ 10 χ 10 ( m q ≈ m g ) ~ ~ ~ ~ ~ ~ 4 jets: � → qqqq χ 10 χ 10 ( m q > m g ) ~ ~ Cha � enging backgrounds Need to understand E T tails in multijet events At large E T tt and W/Z + jets dominat e 9 8 May, 2009 Chris Hays, Oxford University
Squark and Gluino Background V arious methods to estimate background MC - based predictio n Reduce QCD background with selectio n E T not aligned with a reconstructed je t Background predicted entirely with MC 10 8 May, 2009 Chris Hays, Oxford University
Squark and Gluino Background Data + MC predictio n Data - based predictio n Assume exponentia � y fa � ing Normalize W/Z + jets E T spectrum in QCD events prediction to measuremen t Negligible a fu er selectio n Background uncertainty: 6 % 11 8 May, 2009 Chris Hays, Oxford University
Squark and Gluino Limits No signi fi cant excess in 2 � - 1 of CDF or D Ø data Limits on squark & gluino production extended to masses of ~ 400 GeV Exclude m 0 below ~ 300 GeV for m 1/2 = 150 GeV in mSUGRA CDF Co � aboration, PRL 102, 121801 ( 2009 ) D Ø Co � aboration, PLB 660, 449 ( 2008 ) 12 8 May, 2009 Chris Hays, Oxford University
Squark and Gluino Searches Gluino decays to sbottom & Other fi nal states possibl e botto m through cascade decays 4 b - jets + E T Squark decays to gaugino & quark, gaugino decays to stau CDF Co � aboration, arXiv:0903.2618 & tau / neutrino ( 2009 ) 2 jets + τ + E T tan β = 15 13 8 May, 2009 Chris Hays, Oxford University
Stop Searches Large top mass results in large stop mass splitting One stop expected to be ligh t ~ ~ Final states depend on mass di ff erence m t - m χ ~ ~ m t > m W + m b or m χ ± + m b : l + l - + bb + E T ~ ~ m t > m χ ± + m LSP : 2 c - jets + E T ~ ~ m t < m χ ± + m LSP : two long - lived charged massive particles 14 8 May, 2009 Chris Hays, Oxford University
Stop Searches Dilepton + b - jets + E T fi nal state same as tt productio n Search for top - like production at lower mass D Ø Co � aboration, PLB 675, 289 ( 2009 ) 15 8 May, 2009 Chris Hays, Oxford University
Stop Searches T wo c - jets + E T fi nal state similar to generic squark search Charm ta � ing can reduce background D Ø Co � aboration, PLB 665, 1 ( 2008 ) 16 8 May, 2009 Chris Hays, Oxford University
Stop / Gaugino / Stau Search Use time - of -fl ight measurements to search for long - lived particles CDF : TOF and inner tracking detectors; D Ø : muon chambers CDF Co � aboration, arXiv:0902.1266 ( 2009 ) D Ø Co � aboration, PRL 102, 161802 ( 2009 ) 17 8 May, 2009 Chris Hays, Oxford University
Gaugino - Pair Searches Low background to chargino + neutralino productio n Decay through W, Z, or slepto n Final state: three leptons plus E T Separate leptons into high and low purity CDF Co � aboration, PRL 101, 251801 ( 2008 ) D Ø Co � aboration, arXiv:0901.0646 ( 2009 ) 18 8 May, 2009 Chris Hays, Oxford University
Gaugino - Pair Searches Results interpreted in the context of mSUGRA Limits depend on relative neutralino - slepton masses m χ 2 > m slepton increases branching ratio to e/ μ m χ 2 ≈ m slepton reduces acceptance to lowest p T lepto n 19 8 May, 2009 Chris Hays, Oxford University
Making a Discovery Strategies Probe we �- motivated models Search for clear indications of new physics Study a � fi nal states Issues How do you know when it ’ s a discovery? How do you know you haven ’ t missed a discovery? 20 8 May, 2009 Chris Hays, Oxford University
Indicators of New Physics Mass resonances & fi nal states with low SM background Fu � y reconstructed resonance an unambiguous sign of a new particl e Wide variety of possible resonances CDF Co � aboration, PRL 99, 271802 ( 2007 ) Neutral, charged, � actiona � y & doubly charged Decays to fermions and / or gauge bosons Strategies for resonance search: Calculate signi fi cance, accounting for fl uctuations over fu � spectru m Judiciously choose binning & variable for mass sca n Final states with little background o ff er unique discovery opportunity Can convincingly demonstrate new physics and study sample with high purity 21 8 May, 2009 Chris Hays, Oxford University
Neutral Resonances Many decays fu � y reconstructabl e Electrons, muons, light quarks, photons Constant - resolution variable simpli fi es narrow - resonance search Muons: 1/m σ pT ∝ p T2 , σ 1/pT = constan t Electrons & photons: log m σ ET ∝ E T , σ logET = constan t Jets : m 1/2 σ ET ∝ E T1/2 , σ sqrt ( ET ) = constan t 22 8 May, 2009 Chris Hays, Oxford University
Neutral Resonance Searches Resonances predicted by huge range of new physics Supersymmetry, extra dimensions, extra gauge groups and uni fi catio n axion / coloron mass > 1250 GeV CDF Co � aboration, PRL 102, 091805 ( 2009 ) CDF Co � aboration, arXiv:0812.4036 ( 2008 ) 23 8 May, 2009 Chris Hays, Oxford University
Neutral Resonances ZZ → �� , � jj Decays to gauge boson pairs WW / WZ → l ν jj Diboson mass: solve for p z ν with m W 24 8 May, 2009 Chris Hays, Oxford University
Low - Background Searches Most exciting Tevatron hints have been low - background events Look at both signature - based and model - based fi nal states l γγ scalar resulting in same - sign top quarks magnetic monopol e CDF Co � aboration, PRD 75, 112001 ( 2007 ) CDF Co � aboration, PRL 96, 201801 ( 2006 ) CDF Co � aboration, PRL 102, 041801 ( 2009 ) 25 8 May, 2009 Chris Hays, Oxford University
Making a Discovery Strategies Probe we �- motivated models Search for clear indications of new physics Study a � fi nal states Issues How do you know when it ’ s a discovery? How do you know you haven ’ t missed a discovery? 26 8 May, 2009 Chris Hays, Oxford University
Global Search Cover a � fi nal states with global data search Develop global SM prediction using MC, simulation, corrections Compare normalization and shapes of data - populated fi nal states Search fi nal states for mass resonances Combine fi nal states and search for excesses at large total p T 27 8 May, 2009 Chris Hays, Oxford University
Global Search Results Statistica � y signi fi cant shape and mass discrepancies observed CDF: Interpreted as mismode � ing of radiative jet events No excess at high total p T ( expect 8 % of experiments to observe more signi fi cant excess ) D Ø : Interpreted as mismode � ing of muon resolution tails CDF Co � aboration, PRD 79, 011101 ( 2009 ) 28 8 May, 2009 Chris Hays, Oxford University
Making a Discovery Strategies Probe we �- motivated models Search for clear indications of new physics Study a � fi nal states Issues How do you know when it ’ s a discovery? How do you know you haven ’ t missed a discovery? 29 8 May, 2009 Chris Hays, Oxford University
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