View
219
Download
3
Category
Preview:
Citation preview
Ênergy frontieR:Ênergy frontieR:thethe
There and back There and back AgainAgain
oror
Breese quinNBreese quinNUniversity University MMississippiississippiofof
Quarknet Workshop 2008B. Quinn University of Mississippi
2
TheThe Standard modeL Standard modeL
EM
Strong
Weak}
up/down1968 SLACstrange 1964 BNLcharm 1974 SLAC/BNLbottom 1977 Fermilabtop 1995 Fermilab
D0/CDFphoton 1905 Planck/Einsteingluon 1979 DESYW/Z 1983 CERN
electron 1897 Thomsone-neutrino 1956 Reactor muon 1937 Cosmic Raysmu-neutrino 1962 BNLtau 1976 SLACtau-neutrino 2000 Fermilab
ud d
d
u
u
electronneutron
proton..electron neutrino
Quarknet Workshop 2008B. Quinn University of Mississippi
3
The The Standard modeLStandard 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}
Quarknet Workshop 2008B. Quinn University of Mississippi
4
TheThe Energy FrontieR Energy FrontieRIt takes higher and higher energy to probe smaller and smaller distance scalesHigher 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!
Quarknet Workshop 2008B. Quinn University of Mississippi
5
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?
TheThe Energy FrontieR Energy FrontieR
What might lie beyond the frontier?
Quarknet Workshop 2008B. Quinn University of Mississippi
6
how Do we get how Do we get TherE?TherE?
Quarknet Workshop 2008B. Quinn University of Mississippi
7
the the Fermilab TevatroNFermilab TevatroNThe world’s most powerful particle accelerator
…until LHC at CERN, 14 TeV, 2008
Main Injector(new)
Tevatron
DØCDF
Chicago
p source
Booster
Run I1.8 TeV
1992-1996
Run II1.96 TeV2001-?
ppLinac
Quarknet Workshop 2008B. Quinn University of Mississippi
8
AA Generic detectoR Generic detectoRHow do we “see” the events?
Observe, identify, and measure the decay particles
Charged particle trajectories,vertices
Charged particle momentum
e, energy
p, n, ,etc. energy
identification
Infer missing energy ()<
Quarknet Workshop 2008B. Quinn University of Mississippi
9
AA Specific detectoR: Specific detectoR: DDØØ
Silicon Microstrip Tracker900 silicon devices, 1M channelsCentral Fiber Tracker
16 layers, 80k channels, Calorimeter, 50k ChannelsLiquid argon calorimeterwith uranium absorber
Muon Scintillators
Muon Chambers
Borg DØBorg DØ
separated at birth?separated at birth?
Quarknet Workshop 2008B. Quinn University of Mississippi
10
thethe D DØ collectivEØ collectivE
A collaboration of >650 physicists from more than 80 institutions in 19 countries
CollaboratioNCollaboratioN
Quarknet Workshop 2008B. Quinn University of Mississippi
11
24/7 Data collectioN24/7 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
Quarknet Workshop 2008B. Quinn University of Mississippi
12
A A Common EvenTCommon EvenTA sample Ze+e- event
Collect many such events and measure the Z mass from the distribution of calculated masses for each event
Very important calibration sample
Quarknet Workshop 2008B. Quinn University of Mississippi
13
Something Something a littlea little
more more interestinG?interestinG?The top quark - it’s HUGE!
Quarknet Workshop 2008B. Quinn University of Mississippi
14
Why Study the Top?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
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.
22 / WtW mmm
W Wt
W WH
)/ln( WHW mmm
Quarknet Workshop 2008B. Quinn University of Mississippi
15
Why Study the Top?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 ~ 410-25 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
Quarknet Workshop 2008B. Quinn University of Mississippi
16
Run II, the Top Quark & Run II, the Top Quark & DDØØ
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 / experimentTop mass and production cross section at s = 1.8 TeV
mt = 174.3 ± 5.1 GeV (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 MeVConstrain the Higgs mass: mH 40%mH
At s = 1.96 TeV, σtt = 10%σtt Standard Model: σtt = 8.8 pbLarger than SM: new physics (resonance production, anomalous couplings)Smaller than SM: non-SM decays, e.g. tHb
Quarknet Workshop 2008B. Quinn University of Mississippi
17
Production: Cross SectionsProduction: Cross Sections
Top events are very rareOne collision out of every 3109 produced a tt pair
To study processes with small cross sections, one needs
High luminosity to produce enough interesting eventsMethods for distinguishing those events from more copious backgrounds
Quarknet Workshop 2008B. Quinn University of Mississippi
18
Top Physics at DTop Physics at DØØ: : DecaysDecays
BR(tWb) ~ 100%Significant deviation would indicate new physics
Top decays before hadronization
Can study polarization of a free quark
21%
15%
15%1.3%2.6%1.3%
44%
τ+X+jetse+jetse+ee++all hadronic
Jets: showers of particles produced from quarks
bbbbb
qqqql-l-W-
t
bbbbb
qql+qql+W+
t
lepton+jets{
dilepton {all jets
Quarknet Workshop 2008B. Quinn University of Mississippi
19
Decay: What Does Decay: What Does a Top Event Look Like?a Top Event Look Like?
BR(tWb) ~ 100%W qq or W lSo top events should have either
6 jets from quarks4 jets, 1 charged lepton, 1 4 jets, 1 charged lepton, 1 neutrinoneutrino2 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
Quarknet Workshop 2008B. Quinn University of Mississippi
20
b-taggingb-tagging
Lifetime1.5 ps c 0.5 mm
Displaced secondary vertex
Requires precise tracking near the primary collision point: silicon trackers, new for DØ in Run II!
Flight Length ~ few mm
Collision
Impact Parameter
Decay Vertex
B Decay Products
Quarknet Workshop 2008B. Quinn University of Mississippi
21
pt 48 GeVMEt 55 GeVW pt 51 GeVJetEt 154 GeVApla 0.160Ht 517 GeV
Run II: Run II: +jets +jets CandidateCandidate
2 jets are tagged as b -jets2 jets are tagged as b -jets
passes topological selectionpasses topological selection
ttttjjjjjjjjcandidatecandidate
Quarknet Workshop 2008B. Quinn University of Mississippi
22
Run II: Run II: +jets +jets CandidateCandidate
2 jets tagged with displaced secondary vertices
Track in the calorimeterand muon tracker
Quarknet Workshop 2008B. Quinn University of Mississippi
23
Run II, the Top Quark & Run II, the Top Quark & DDØØ
What we’ve measured at this point: top mass
Quarknet Workshop 2008B. Quinn University of Mississippi
24
Run II, the Top Quark & Run II, the Top Quark & DDØØ
What we’ve measured at this point: cross section
Quarknet Workshop 2008B. Quinn University of Mississippi
25
is thereis there Something Something a bita bit more exotiC?more exotiC?
At the frontier will we find…At the frontier will we find…The origin of mass?The origin of mass? The higgß The higgß
ParticlEParticlE
Higher symmetry?Higher symmetry? SupersymmetrY SupersymmetrY - SUSY- SUSY
A place for gravity?A place for gravity? ExtrA ExtrA
dimensionßdimensionß
Quarknet Workshop 2008B. Quinn University of Mississippi
26
the the HiggßHiggß
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
t
Higgs Field
HH
Quarknet Workshop 2008B. Quinn University of Mississippi
27
Past Higgß searches and Past Higgß searches and current limitS 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
Quarknet Workshop 2008B. Quinn University of Mississippi
28
Search for Search for hwhw productioN 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.
p
pW*
qq’
H
We
b
b
Quarknet Workshop 2008B. Quinn University of Mississippi
29
a Higgß candidatEa Higgß candidatETwo events found, consistent with Standard Model Wbb production
12.5 pb upper cross section limit for ppWH where Hbb mH=115 GeV.
By the end of Run II and combining all channels, we should have sensitivity to ~130 GeV.
Quarknet Workshop 2008B. Quinn University of Mississippi
30
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 energiesAllows unification of forces at much higher energiesProvides a path to incorporate gravity and string theory: Local Supersymmetry = SupergravityLightest stable particle cosmic dark matter candidate
Masses expected to be 100 GeV - 1 TeV
SupersymmetrYSupersymmetrY
??
Quarknet Workshop 2008B. Quinn University of Mississippi
31
the the Golden tri-leptoN Golden tri-leptoN SUSY SUSY
SignaturÊSignaturÊIn one popular model the charged and neutral partners of the gauge and Higgs bosons, the charginos and neutralinos, are produced in pairsDecay 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, ee, e, , ee , etc… ).neutrino= missing transverse energy
Quarknet Workshop 2008B. Quinn University of Mississippi
32
Tri-lepton search ResultßTri-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
Quarknet Workshop 2008B. Quinn University of Mississippi
33
Extra dimensionSExtra dimensionS
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.
4-D Universe4-D Universe
extra extra dimensionsdimensions
q
graviton
graviton
q
Quarknet Workshop 2008B. Quinn University of Mississippi
34
Extra dimensionSExtra 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.
p
pG
qq’
e
e
Quarknet Workshop 2008B. Quinn University of Mississippi
35
High mass candidate High mass candidate eventßeventß
ee pair pair
Quarknet Workshop 2008B. Quinn University of Mississippi
36
Extra dimension Extra dimension limitSlimitS
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 m
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
Quarknet Workshop 2008B. Quinn University of Mississippi
37
We’ve just begun We’ve just begun toto explorEexplorE
All of these results represent at most a quarter of our data
Recommended