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Ênergy frontieR: the There and back Again or Breese quinN University Mississippi of

neutron u u electron d d d . u proton electron neutrino The Standard modeL • up/down 1968 SLAC • strange 1964 BNL • charm 1974 SLAC/BNL • bottom 1977 Fermilab • top 1995 Fermilab D0/CDF • photon 1905 Planck/Einstein • gluon 1979 DESY • W/Z 1983 CERN • electron 1897 Thomson • e-neutrino 1956 Reactor • muon 1937 Cosmic Rays • mu-neutrino 1962 BNL • tau 1976 SLAC • tau-neutrino 2000 Fermilab EM Strong } Weak B. Quinn University of Mississippi

The Standard modeL • This model has successfully described or predicted almost everything we’ve seen in almost 100 years of particle physics. • However it does have some weaknesses • Too many free parameters • Predictive powers wane as we move past the Electroweak (EW) regime towards the TeV scale and beyond EM Strong } Weak B. Quinn University of Mississippi

The Energy FrontieR • It takes higher and higher energy to probe smaller and smaller distance scales • Higher energy = Earlier time • We’re looking back in time to the early universe • Accelerators have now gotten us to about 10-12 seconds, or equivalently, close to the TeV energy scale • This is the energy frontier! B. Quinn University of Mississippi

The Energy FrontieR • What might lie beyond the frontier? • What might lie beyond the frontier? • Where does mass come from? • Is the SM just a low-energy effective theory masking higher more fundamental symmetries? • Where does gravity fit in? B. Quinn University of Mississippi

how Do we get TherE? B. Quinn University of Mississippi

the Fermilab TevatroN • The world’s most powerful particle accelerator …until LHC at CERN, 14 TeV, 2008 Chicago  Run II 1.96 TeV 2001-? pp Run I 1.8 TeV 1992-1996 Booster CDF DØ Tevatron Linac p source Main Injector (new) B. Quinn University of Mississippi

A Generic detectoR • How do we “see” the events? • Observe, identify, and measure the decay particles Charged particle trajectories, vertices Charged particle momentum e,  energy < Infer missing energy () p, n, ,etc. energy  identification B. Quinn University of Mississippi

Muon Scintillators Muon Chambers A Specific detectoR: DØ Borg DØ separated at birth? Silicon Microstrip Tracker 900 silicon devices, 1M channels Central Fiber Tracker 16 layers, 80k channels, Calorimeter, 50k Channels Liquid argon calorimeterwith uranium absorber B. Quinn University of Mississippi

the DØ collectivE CollaboratioN • A collaboration of >650 physicists from more than 80 institutions in 19 countries B. Quinn University of Mississippi

24/7 Data collectioN • Proton-antiprotons collide at 7MHz or seven million times per second • Tiered electronics pick successively more interesting events • Level 1 2 kHz • Level 2 1 kHz • About 100 crates of electronics readout the detectors and send data to a Level 3 farm of 100+ CPUs that reconstruct the data • Level 3: 50 events or 12.5 Mbytes of data to tape per second • Per year: 500 million events B. Quinn University of Mississippi

A Common EvenT • A sample Ze+e- event • Collect many such events and measure the Z mass from the distribution of calculated masses for each event • Very important calibration sample B. Quinn University of Mississippi

Something a little more interestinG? • The top quark - it’s HUGE! B. Quinn University of Mississippi

t H W W W W Why Study the Top? In 1964, Peter Higgs postulated a physics mechanism which gives all particles their mass. This mechanism is a field which permeates the universe and is mediated by a particle called the Higgs boson. • The Standard Model (SM) predicts all top properties given mt • In the SM, the top and W mass constrain the mass of the Higgs Boson via EW radiative corrections B. Quinn University of Mississippi

Why Study the Top? • The Standard Model (SM) predicts all top properties given mt • In the SM, the top and W mass constrain the mass of the Higgs Boson via EW radiative corrections • Since the top is so heavy, it should have particularly strong coupling to the Higgs • Likely to play a significant role in EW Symmetry Breaking (mW, mZ) • Top decay time (τ~ 410-25 s) < hadronization time • Opportunity to study bare quarks • In general, quarks do not exist as free particles. qq pairs are pulled from the vacuum to produce stable hadrons with 2 quarks (mesons) or 3 quarks (baryons) • This process is called hadronization • Since the top quark decays before that happens, we can study quark properties such as spin polarization here and no where else B. Quinn University of Mississippi

Run II, the Top Quark & DØ • Most all that we knew about the top quark before Run II came from the DØ and CDF Run I data • 1992-1996: ~100 events / experiment • Top mass and production cross section at s = 1.8 TeV • mt= 174.3 ± 5.1GeV (DØ and CDF combined) • σtt= 5.7 ± 1.6 pb (DØ); σtt= 6.5+1.7-1.4 pb (CDF) • At this point in Run II we have 40 times more data, with a few thousand top quark events. Should yield: • mt ±3 GeV, along with mW ±40 MeV • Constrain the Higgs mass: mH 40%mH • At s = 1.96 TeV, σtt= 10%σtt • Standard Model: σtt= 8.8 pb • Larger than SM: new physics (resonance production, anomalous couplings) • Smaller than SM: non-SM decays, e.g. tHb B. Quinn University of Mississippi

Production: Cross Sections • Top events are very rare • One collision out of every 3109 produced a tt pair • To study processes with small cross sections, one needs • High luminosity to produce enough interesting events • Methods for distinguishing those events from more copious backgrounds B. Quinn University of Mississippi

t W+ l+ qq l+ qq b b b b b t W- l- l- qq qq b b b b b Top Physics at DØ: Decays • BR(tWb) ~ 100% • Significant deviation would indicate new physics • Top decays before hadronization • Can study polarization of a free quark 21% 44% 15% τ+X +jets { 15% 1.3% lepton+jets 2.6% 1.3% e+jets e+e { Jets: showers of particles produced from quarks dilepton e+ + all jets all hadronic B. Quinn University of Mississippi

Decay: What Does a Top Event Look Like? • BR(tWb) ~ 100% • W qq or W l • So top events should have either • 6 jets from quarks • 4 jets, 1 charged lepton, 1 neutrino • 2 jets, 2 charged leptons, 2 neutrinos • 2 jets must come from b quarks • These jets can be distinguished from light quark jets • Powerful background rejection tool B. Quinn University of Mississippi

B Decay Products Flight Length ~ few mm Collision Decay Vertex Impact Parameter b-tagging • Lifetime • 1.5 ps  c  0.5 mm • Displaced secondary vertex • Requires precise tracking near the primary collision point: silicon trackers, new for DØ in Run II! B. Quinn University of Mississippi

Run II: +jets Candidate m pt 48 GeV MEt 55 GeV W pt 51 GeV JetEt 154 GeV Apla 0.160 Ht 517 GeV ttjjjj candidate 2 jets are tagged as b -jets passes topological selection B. Quinn University of Mississippi

Run II: +jets Candidate 2 jets tagged with displaced secondary vertices Track in the calorimeter and muon tracker B. Quinn University of Mississippi

Run II, the Top Quark & DØ • What we’ve measured at this point: top mass B. Quinn University of Mississippi

Run II, the Top Quark & DØ • What we’ve measured at this point: cross section B. Quinn University of Mississippi

is there Something a bit more exotiC? • At the frontier will we find… • The origin of mass? The higgß ParticlE • Higher symmetry? SupersymmetrY - SUSY • A place for gravity? ExtrA dimensionß B. Quinn University of Mississippi

the Higgß • Peter Higgs showed that Electroweak unification implied the existence of a molasses-like field (the Higgs field), which gives particles their mass. • Predicts at least one Higgs particle – not yet discovered. • The strength of a particle’s interaction with the Higgs particle determines its mass. e H t g Higgs Field B. Quinn University of Mississippi

Past Higgß searches and current limitS • Over the last decade or so, experiments at LEP or the European e+e– collider have been searching for the Higgs. • Direct searches for Higgs production, similar to our Z mass measurement exclude mH < 114 GeV. • Precision measurements of electroweak parameters combined with DØ’s new* Run I top quark mass measurement, favor mH = 117 GeV with an upper limit of mH = 251 GeV. * Nature 10 June 2004 B. Quinn University of Mississippi

Search for hw productioN • One very striking and distinctive signature • Look for • an electron = track + EM calorimeter energy • neutrino = missing transverse energy • two b quarks = two jets each with a secondary vertex from the long lived quarks. e p W n q q’ W* b p H b B. Quinn University of Mississippi

a Higgß candidatE • Two events found, consistent with Standard Model Wbb production • 12.5 pb upper cross section limit for ppWH where Hbb mH=115 GeV. • By the end of Run II and combining all channels, we should have sensitivity to ~130 GeV. B. Quinn University of Mississippi

SupersymmetrY • SM is “ugly” – it requires too much fine tuning of too many free parameters. May only be an effective theory valid to some energy scale beyond which higher symmetries may manifest as new particles and forces. • Supersymmetry, or SUSY is the leading candidate. • Every particle and force carrier has a massive super-partner (squark, gluino, etc.) • Theoretically attractive: • SUSY closely approximates SM at low energies • Allows unification of forces at much higher energies • Provides a path to incorporate gravity and string theory: Local Supersymmetry = Supergravity • Lightest stable particle cosmic dark matter candidate • Masses expected to be 100 GeV - 1 TeV ? B. Quinn University of Mississippi

the Golden tri-leptoN SUSY SignaturÊ • In one popular model the charged and neutral partners of the gauge and Higgs bosons, the charginos and neutralinos, are produced in pairs • Decay into fermions and the Lightest Supersymmetric Particle (LSP), a candidate for dark matter. • The signature is particularly striking: • Three leptons = track + EM calorimeter energy or tracks + muon tracks (could be eee, eem, emm, mmm, eet , etc… ). • neutrino= missing transverse energy B. Quinn University of Mississippi

Tri-lepton search Resultß • In four tri-lepton channels three events total found. • Consistent with Standard Model expectation of 2.9 events. • Here is a like-sign muon candidate B. Quinn University of Mississippi

Extra dimensionS 4-D Universe • Gravity (in particular, its weakness) does not fit into the SM. It can be accommodated in a higher dimensional world (t + 3D + “n” extra spatial dim.). • Only gravity can propagate in the extra dimensions which are hidden to us because we are confined to the 4-D surface, or brane. • Gravity would therefore be as strong as the other forces, but since it spreads its influence in the other dimensions, it appears weakened or diluted from our viewpoint. q q graviton extra dimensions B. Quinn University of Mississippi

p e q q’ G p e Extra dimensionS • Gravitons could be produced in collisions, leave our 3-D world, go into the other dimensions… …come back and decay into two photons or electrons that appear to come from nowhere. • An unexpectedly high rate of such pairs at high energies would signal the presence of extra dimensions. B. Quinn University of Mississippi

High mass candidate eventß ee pair gg pair B. Quinn University of Mississippi

Extra dimension limitS • Di-electrogmagnetic objects are collected and the mass calculated (just as in our Z plot a few minutes ago). • The observed mass spectrum is compared to a linear combination of • SM signals • Instrumental backgrounds • Extra Dimension Signals • No evidence is found for hidden dimensions, @ 95%CL • n = 2, 170 mm • n = 3, 1.5 nm • n = 4, 5.7 pm • n = 5, 0.2 pm • n = 6, 21 fm • n = 7, 4.2 fm Note the long mass tail B. Quinn University of Mississippi

We’ve just begun toexplorE All of these results represent at most a quarter of our data B. Quinn University of Mississippi

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