# Presentation: 149458

## Physics at the Tevatron:Lecture I

- Beate Heinemann
- University of Liverpool

- CERN, May 15-19th 2006

- Physics at the TevatronLecture I

## Outline

- Lecture I
- The Tevatron, CDF and DØ
- Production Cross Section Measurements
- Lepton identification
- Lecture II
- The Top Quark and the Higgs Boson
- jet energy scale and b-tagging
- Lecture III
- Bs mixing and Bs rare decay
- Vertex resolution and particle identification
- Lecture IV
- Supersymmetry and High Mass Dilepton/Diphoton
- Missing ET

## The Tevatron

- pp collider:
- 6.5 km circumference
- Beam energy: 980 GeV
- √s=1.96 TeV
- 36 bunches:
- Time between bunchest=396 ns
- Main challenges:
- Anti-proton production and storage:
- Stochastic and electron cooling
- Irregular failures:
- Kicker prefires, Quenches
- CDF and DØ experiments:
- 700 physicists/experiment

- Chicago

## Tevatron Luminosity 

## Tevatron Performance

**∫ ****Ldt= 1.5 fb****-1**

- Integrated luminosity more than 1.5 fb-1 by now
- First years were difficult
- March’01-March’02 used for commissioning of detectors
- Physics started in March’02
- Luminosity doubles every year
- Currently in shutdown until early June:
- installed new silicon “Layer0” in D0

- 2006

- 2005

- 2004

- 2003

- 2002

## CDF

- Core detector operates since 1985:
- Central Calorimeters
- Central muon chambers
- Major upgrades for Run II:
- Drift chamber: COT
- Silicon: SVX, ISL, L00
- 8 layers
- 700k readout channels
- 6 m2
- material:15% X0
- Forward calorimeters
- Forward muon system
- Improved central too
- Time-of-flight
- Preshower detector
- Timing in EM calorimeter
- Trigger and DAQ

## CDF

## Some new CDF Subdetectors

## DØ Detector

- Retained from Run I
- Excellent muon coverage
- Compact high granularity LAr calorimeter
- New for run 2:
- 2 Tesla magnet !
- Silicon detector
- Fiber tracker
- Trigger
- Readout
- Forward roman pots

## DØ Detector

## Pictures of DØ Subdetectors

- new

**Layer 0**

## Detector Operation

- Data taking efficiency about 85%
- All components working very well:
- 93% of Silicon detector operates, 82-96% working well
- Expected to last up to 8 fb-1

**85%**

**85%**

## Detector Performances

**Good resolution for **
**track momenta**** **
**calorimeter energies**
**vertex**

|  | CDF | DØ |
| --- | --- | --- |
| M(J/+-) | 20 MeV | 60 MeV |
| M(Z+-) | 2.5 GeV | 6 GeV |
| M(Ze+e-) | 2.8-3.2 GeV | 3.1 GeV |
| (Etjet)/ Etjet | ~16% | ~14% |
| d0 | ~30 m | ~30 m  |

## Processes and Cross Sections

- Cross section:
- Total inelastic cross section is huge
- Used to measure luminosity
- Rates at e.g. L=1x1032 cm-2s-1:
- Total inelastic: 70 MHz
- bb: 42 kHz
- Jets with ET>40 GeV: 300 Hz
- W: 3 Hz
- Top: 25/hour
- Trigger needs to select the interesting events
- Mostly fighting generic jets!

- Jet ET>20 GeV

- Jet ET>40 GeV

- 7x103

-  3x103

- 2.5x105

- 2.8x106

- 108

- 109

- 1010

- 300

- 1012

- Events/fb-1

## Triggering at hadron colliders

- L1 trigger

- L2 trigger

- L3 farm

- disk/tape

*42 L1*
*buffers*

*4 L2*
*buffers*

- 1.7 MHz crossing rate

- 25 kHz L1 accept

- 500 Hz L2 accept

- 100 Hz L3 accept

- Hardware tracking for pT 1.5 GeV

- Muon-track matching

- Electron-track matching

- Missing ET, sum-ET

- Silicon tracking

- Dedicated
- hardware

- Hardware +
- Linux PC's

- Linux farm (200)

- Jet finding

- Full event reconstruction

- Refined electron/photon finding

- The trigger is the key at hadron colliders

- DØ trigger:
- L1: 1.6 kHz
- L2: 800 Hz
- L3: 50 Hz

## Typical Triggers and their Usage

- Unprescaled triggers for primary physics goals
- Examples:
- Inclusive electrons, muons pT>20 GeV:
- W, Z, top, WH, single top, SUSY, Z’,Z’
- Dileptons, pT>4 GeV:
- SUSY
- Lepton+tau, pT>8 GeV:
- MSSM Higgs, SUSY, Z
- Also have tau+MET: W->taunu
- Jets, ET>100 GeV
- Jet cross section, Monojet search
- Lepton and b-jet fake rates
- Photons, ET>25 GeV:
- Photon cross sections, Jet energy scale
- Searches (GMSB SUSY)
- Missing ET>45 GeV
- SUSY
- ZH->vvbb

- Prescale triggers because:
- Not possible to keep at highest luminosity
- Needed for monitoring
- Prescales depend often on Lumi
- Examples:
- Jets at ET>20, 50, 70 GeV
- Inclusive leptons >8 GeV
- B-physics triggers
- Backup triggers for any threshold, e.g. Met, jet ET, etc...
- At all trigger levels

- single electron trigger

- CDF

- Rate= 6 Hz at L=100x1030 cm-2/s-1

## Trigger Operation

- Aim to maximize physics at trigger level:
- Trigger cross section:
- Nevent/nb-1
- Independent of Luminosity
- Trigger Rate:
- Cross Section x Luminosity
- Luminosity falls within store
- Rate also falls within store
- 75% of data are taken at <2/3 of peak luminosity
- Use sophisticated prescale system to optimize bandwidth usage
- Trigger more physics!

|  | CDF | DØ |
| --- | --- | --- |
| L1 bits | 64 | 128 |
| L2 bits | 125 | >128 |
| L3 bits | 173 | 418 |

## The Proton

- It’s complicated:
- Valence quarks
- Gluons
- Sea quarks
- Exact mixture depends on:
- Q2: ~(M2+pT2)
- xBj: fractional momentum carried by parton
- Hard scatter process:

**X**

**p**

**p**

- xBj

- Q2

## Parton Kinematics

- For M=100 GeV: probe 10 times higher x at Tevatron than LHC

## Tevatron vs LHC

- Compare to LHC
- Cross sections of heavy objects rise much faster, e.g.
- top cross section
- Jet cross section ET>100 GeV
- Relative importance of processes changes
- Jet background to W’s and Z’s
- W background to top
- backgrounds to Higgs

## The Proton is Messy

- We don’t know
- which partons hit each other
- What their momentum was
- What the other partons did
- We know roughly: 2-30%
- The parton content of the proton
- The cross sections of processes

- higher-order pQCD corrections; accompanying radiation, jets

- Q /GeV

## Every Event is Complicated

- “Underlying event”:
- Initial state radiation
- Interactions of other partons in proton
- Many forward particles escape detection
- Transverse momentum ~0
- Longitudinal momentum >>0

## Kinematic Constraints and Variables

- Transverse momentum, pT
- Particles that escape detection (<3o) have pT≈0
- Visible transverse momentum conserved ∑I pTi≈0
- Very useful variable!
- Longitudinal momentum and energy, pz and E
- Particles that escape detection have large pz
- Visible pz is not conserved
- Not so useful variable
- Angle:
- Polar angle  is not Lorentz invariant
- Rapidity: y
- Pseudorapidity: 

- For M=0

## Standard Model Processes and Cross Sections

## Why Measure Cross Sections?

- They test QCD calculations
- They help us to find out content of proton:
- Gluons, light quarks, c- and b-quarks
- A cross section that disagrees with theoretical prediction could be first sign of new physics:
- E.g. quark substructure (highest jet ET)
- They force us to understand the detector
- Noone believes us anything without us showing we can measure cross sections

## Why Measure Cross Sections?

- They test QCD calculations
- They help us to find out content of proton:
- Gluons, light quarks, c- and b-quarks
- A cross section that disagrees with theoretical prediction could be first sign of new physics:
- E.g. quark substructure (highest jet ET)
- They force us to understand the detector
- Noone believes us anything without us showing we can measure cross sections

## Why Measure Cross Sections?

- They test QCD calculations
- They help us to find out content of proton:
- Gluons, light quarks, c- and b-quarks
- A cross section that disagrees with theoretical prediction could be first sign of new physics:
- E.g. quark substructure (highest jet ET)
- They force us to understand the detector
- Noone believes us anything without us showing we can measure cross sections

## Luminosity Measurement

- Measure events with 0 interactions
- Related to Rpp
- Normalize to measured inelastic pp cross section
- Measured by CDF and E710/E811
- Differ by 2.6 sigma
- For luminosity normalization we use the error weighted average
- CDF and DØ use the same
- Unlike in Run 1...

- CDF

- E710/E811

- pp (mb)

## Jet Cross Sections

- Inclusive jets: processes qq, qg, gg

- Measure at highest Q2 and x
- Testing new grounds
- High Q2
- new physics at TeV scale?

## Jet Cross Section History

- Run I (1996):
- Excess at high ET
- Could be signal for quark substructure?!?

- data/theory – 1, %

- Data/CTEQ3M

## Jet Cross Section History

- Since Run I:
- Revision of parton density functions
- Gluon is uncertain at high x
- Different jet algorithms
- MidPoint
- kT

- data/theory – 1, %

- Data/CTEQ3M

- Data/CTEQ4HJ

## Jet Cross Sections in Run II

- Excellent agreement with QCD calculation over 8 orders of magnitude!
- No excess any more at high ET
- Large pdf uncertainties will be constrained by these data

## Highest Mass Dijet Event: M=1.4 TeV

## W and Z Bosons

- Focus on leptonic decays:
- Hadronic decays impossible due to enormeous QCD dijet background
- Selection:
- Z:
- Two leptons ET>20 GeV
- Electron, muon, tau
- W:
- One lepton ET>20 GeV
- Large imbalance in transverse momentum
- Missing ET>20 GeV
- Signature of undetected particle (neutrino)
- Excellent calibration signal for many purposes:
- Electron energy scale
- Track momentum scale
- Lepton ID and trigger efficiencies
- Missing ET resolution
- Luminosity ...

## Lepton Identification

- Electrons:
- compact electromagnetic cluster in calorimeter
- Matched to track
- Muons:
- Track in the muon chambers
- Matched to track
- Taus:
- Narrow jet
- Matched to one or three tracks
- Neutrinos:
- Imbalance in transverse momentum
- Inferred from total transverse energy measured in detector
- More on this in Lecture 4

## Electrons and Jets

- Jets can look like electrons, e.g.:
- photon conversions from 0’s: ~13% of photons convert (in CDF)
- early showering charged pions
- And there are lots of jets!!!

- Electromagnetic Calorimeter Energy

- Hadronic Calorimeter Energy

## The Isolation Cut

- Isolation is very powerful for isolated leptons
- E.g. from W’s, Z’s
- Rejects background from leptons inside jets due to:
- b-decays
- photon conversions
- pions/kaons that punch through or decay in flight
- pions that shower only in EM calorimeter
- This is a physics cut!
- Efficiency depends on physics process
- The more jet activity the less efficient
- Depends on luminosity
- Extra interactions due to pileup

- Isolation cut:
- Draw cone of size 0.4 around object
- Sum up PT of objects inside cone
- Use calorimeter or tracks
- Typical cuts:
- <10% x ET
- <2-4 GeV

-               candidates
- Non-isolated          isolated

## Electron Identification

- Desire:
- High efficiency for isolated electrons
- Low misidentification of jets
- Cuts:
- Shower shape
- Low hadronic energy
- Track requirement
- Isolation
- Performance:
- Efficiency:
- Loose cuts: 86%
- Tight cuts: 80%
- Measured using Z’s
- Fall-off in forward region due to limited tracking efficiency

## Muon Identification

- Desire:
- High efficiency for isolated muons
- Rejection of background due to punch-through etc.
- Typical requirements:
- Signal in muon chamber
- Isolation
- Low hadronic and electromagnetic energy
- Consistent with MIP signal
- Efficiency 80-90%
- Coverage:
- DØ: Up to ||=2
- CDF: up to ||=1.5

## Jets faking Electrons

- Jets can pass electron ID cuts,
- Mostly due to
- early showering charged pions
- Conversions:0ee+X
- Semileptonic b-decays
- Difficult to model in MC
- Hard fragmentation
- Detailed simulation of calorimeter and tracking volume
- Measured in inclusive jet data at various ET thresholds
- Prompt electron content negligible:
- Njet~10 billion at 50 GeV!
- Fake rate per jet:
- Loose cuts: 5/100000
- Tight cuts: 1/100000
- Typical uncertainties 50%

- Jets faking “tight” electrons

- Fake Rate (‰)

## W’s and Z’s

- Z mass reconstruction
- Invariant mass of two leptons
- Sets electron energy scale by comparison to LEP measured value
- W mass reconstruction
- Do not know neutrino pZ
- No full mass resonstruction possible
- Transverse mass:

## W and Z Cross Section Results

******Th,NNLO**=2687±54pb

******Th,NNLO**=251.3±5.0pb

- W

- Z

- Experimental And theoretical errors:
- ~2%
- Luminosity uncertainty:
- ~6%
- Can use these processes to normalize luminosity absolutely
- Is theory reliable enough?

## More Differential W/Z Measurements

- d/dy

- d/dM

## WW Cross Section

- WW cross section
- Use W and We
- 2 leptons and missing ET
- Result:
- Data: =13.6+-3.1 pb
- Theory: =12.4+-0.8 pb
- Campbell, Ellis

## Summary of Boson Cross Sections

## Conclusion

- Tevatron is world’s highest energy collider today
- Large datasets with L=1.5 fb-1 now available in Run II
- 10 times more statistics than in Run I
- CDF and DØ detectors operate well
- Powerful tracking
- Good calorimeter coverage
- Good lepton identification
- A hadron machine provides a challenging environment
- Cross sections of jets, W’s and Z’s and other processes at √s=1.96 TeV well understood
- Excellent agreement with QCD calculations
- This is important before moving on to rarer processes, precision measurements and new physics searches
- See the next 3 lectures!

## Backup Slides

## Tevatron: Future

## Jets faking Muons

- Jets can pass muon ID cuts,
- Mostly due to
- Pions punching through
- Pions or kaons decaying in flight:
- K±±, ± ± 
- Semileptonic b-decays
- Difficult to model in MC
- Hard fragmentation
- Detailed simulation of calorimeter and tracking volume
- Measured in inclusive jet data at various ET thresholds
- Prompt muon content negligible
- Fake rate per loosely isolated track:
- Cannot measure per jet since isolated muon is usually not a jet!
- 2/1000
- Typical uncertainties 50%

- Tracks faking muons

- Fake Rate (%)

## A Few Comments on Monte Carlo

- Critical for understanding the acceptance and the backgrounds
- Speed: CDF ~10s per event, DØ ~ 3m per event
- Two important pieces:
- Physics process simulation:
- PYTHIA, HERWIG
- Working horses but limitations at high jet multiplicity
- “ME generators”: ALPGEN, MADGRAPH, SHERPA, COMPHEP,...
- Better modeling at high number of jets
- Some processes only available properly in dedicated MC programs
- e.g. W or single top
- NLO generators (_[MC@NLO](mailto:MC@NLO)_)
- Not widely used yet but often used for cross-checks
- Detector simulation:
- GEANT, fast parameterizations (e.g. GFLASH)
- Neither physics nor detector simulation can generally be trusted!
- Most experimental work goes into checking Monte Carlo is right

## Diphoton Cross Section

- Select 2 photons with ET>13 (14) GeV
- Statistical subtraction of background
- mostly 0
- Data agree well with NLO
- PYTHIA describes shape
- normalization off by factor two

## Z+jet Cross Section

- Select Z boson and jets
- Compare MC generators PYTHIA and SHERPA
- SHERPA gives better description