1. Ze+e- + Jets with CMS at the LHC
Christos Lazaridis
University of Wisconsin-Madison
Preliminary Examination
December 18, 2007
Christos Lazaridis
Preliminary Examination

2. Preliminary Examination
Outline
The Standard Model
Large Hadron Collider
Compact Muon Solenoid
Z + Jets
Previous results
Monte Carlo studies
Summary/Future Plans
December 18, 2007
Christos Lazaridis
Preliminary Examination

3. Preliminary Examination
The Standard Model
Quarks
u and d quark make nucleons in atom
Leptons
electrons complete the atoms
Force Carriers
γ: electromagnetism
W/Z : weak interaction
gluons: strong interaction
Higgs boson gives mass to particles
Not discovered yet
Particle Interactions:
In the SM, the elementary particles that make up matter are a set of fermions that interact primarily through the exchange of vector bosons. The elementary fermions include the leptons and the quarks whose bound states form particles (mesons and baryons) via strong interactions mediated by gluons.
December 18, 2007
Christos Lazaridis
Preliminary Examination

4. Preliminary Examination
Z + Jets
Verification of the Standard Model at high Q2
A QCD regime never probed before!
Events at scale where pQCD expected to be valid: Z mass scale
Due to high cross section, useful as a high-statistics detector calibration tool
Z + Jets are irreducible backgrounds
for interesting SM processes (e.g. top production)
for new physics searches (e.g. Higgs)
The colored partons produced through the hard scatter form colorless hadrons through fragmentation --> collimated spray or real particles (Jets)
December 18, 2007
Christos Lazaridis
Preliminary Examination

5. Preliminary Examination
New physics in Z+Jets
Resonances in the ZZ spectrum
Higgs Z’
: Final state jets
: Final state e+/e-
December 18, 2007
Christos Lazaridis
Preliminary Examination

6. Preliminary Examination
Large Hadron Collider
p-p collider
14 TeV total collision energy
27 km circumference
protons travel around circumference in 90 μsec
Bunches cross every 25 ns
8 μm bunch transverse radius
December 18, 2007
Christos Lazaridis
Preliminary Examination

7. Preliminary Examination
Magnets
Superconducting NbTi magnets
1232 dipoles bend proton beam around ring, B = 8T
Quadrupoles focus beam in transverse plane
Arrows show direction of magnetic field
December 18, 2007
Christos Lazaridis
Preliminary Examination

8. Compact Muon Solenoid Design
field: 4 Tesla
diameter: 6 m
length: 12.5 m
December 18, 2007
Christos Lazaridis
Preliminary Examination

9. Compact Muon Solenoid Built
← Surface assembly hall
Building of CMS is completed underground ↓
Solenoid
Endcap Discs: Designed, assembled & installed by Wisconsin
December 18, 2007
Christos Lazaridis
Preliminary Examination

10. Preliminary Examination
Tracker
Tracker coverage extends to || < 2.5
Silicon pixel detectors closest to interaction region
Silicon strip detectors in barrel and endcaps
cover an area of 210m2
Resolution:
Strong magnetic field
Very good tracker granularity
Pixels: Lower Occupancy (4 &7 cm – 7 & 11 cm – cell size 150 x 150 μmx)
SiStrips: Faster readout, easier to track with
Resolution gradually degrading to (60Pt )% as eta 2.5
210 m^2 silicon
66M channels SiPixels, 2.8M for SiStrip
Pixels provide 2-3 hits/track, precision ~10μm in transverse and 15μm in z.
SiStrips provide up to 14 hits per track 10 – 60 μm in transverse Some strips are double sided to provide similar accuracy in longitudinal measurements
Material thickness
0.35X0 central
1.4X0 at barrel/endcap transition
drops to .8X0 at |eta| = 2.5
Radiation ength It is both the mean distance over which a high-energy electron loses all but 1/e of its energy by bremsstrahlung, and 7/9 of the mean free path for pair production by a high-energy photon. It is also the appropriate scale length for describing high-energy electromagnetic cascades.
December 18, 2007
Christos Lazaridis
Preliminary Examination

11. Electromagnetic Calorimeter
Measures e/ energy and position to ||<3
~76,000 lead tungstate crystals
High density (8.2 gr/cm3)
Short radiation length (9 mm)
Small Moliere radius (22 mm)
Resolution:
36 supermodules / 1700 crystals each
Lead tungstate density
23 cm long 26 X0
Small moliere radius matches front face
• The electronic noise and pileup energy
• The shower containment and photostatictics
• The constant term (detector non-uniformity, calibration uncertainty)
The scintillation light from the crystals must be captured by a photodetector, amplified and digitized.
December 18, 2007
Christos Lazaridis
Preliminary Examination

12. Preliminary Examination
Hadronic Calorimeter
HCAL samples showers to measure their energy/position
Barrel/Endcap region (|| < 3)
Brass/scintillator layers Resolution:
Forward region (3 < || < 5)
Steel plates/quartz fibers Resolution:
Stochastic term/constant term
December 18, 2007
Christos Lazaridis
Preliminary Examination

13. Detecting electrons and jets
Leave a track
Deposit energy in the ECAL
Jets
Leave tracks
Deposit energy in the ECAL/HCAL
A collimated spray of high energy hadrons
December 18, 2007
Christos Lazaridis
Preliminary Examination

14. Proton-proton collisions at LHC
Proton-proton 2838 bunches/beam
Crossing Rate 40 MHz
Protons/bunch 1011
Bunch RT 8 μm
Luminosity cm-2s-1
Startup: – 1032 cm-2s-1
Interaction Rate:
f : revolution frequency
n : number of bunches/beam in the storage ring.
N : number of particles/bunch
A : cross section of the beam
December 18, 2007
Christos Lazaridis
Preliminary Examination

15. Preliminary Examination
CMS Trigger
40 MHz crossing frequency
Interaction rate ~1 GHz
Selects interesting events
Level 1 Trigger:
Hardwired processors
High Level Trigger:
Farm of processors
L1 Trigger Requirements:
Output: 100 kHz (50 kHz for initial running)
Latency: 3 μsec
Data collection, decision, propagation
HLT designed to output 100 Hz
Trigger Rejection ~4x105
December 18, 2007
Christos Lazaridis
Preliminary Examination

16. Preliminary Examination
Level 1 Trigger
Information from Calorimeters and Muon detectors
Electron/photon triggers
Jet/MET triggers
Muon triggers
Highly complex
Trigger primitives: ~5000 electronics boards
Regional/Global: 45 crates, 630 boards
Flexibility
Most algorithms implemented in reprogrammable FPGAs
***
7 microsec to readout tracker
Readout + processing: < 1 microsec
Signal collection & distribution: ≈ 2 microsec
*** The 18 crates of the RCT:
Receive input from the CMS calorimeters
Combined with the GCT outputs to the GT the 4 highest isolated/non-isolated electrons, jets, τ-jets, missing ET and total ET per event
December 18, 2007
Christos Lazaridis
Preliminary Examination

17. Calorimeter Trigger Towers  φ η
CRACK!
Eta: preferred over polar angle θ  particle production constant as a function of eta
December 18, 2007
Christos Lazaridis
Preliminary Examination

18. Electron Trigger Algorithms
Electron (Hit Tower + Max)
2-tower ET + Hit tower H/E
Hit tower 2x5-crystal strips >90% ET in 5x5 (Fine Grain)‏
Isolated Electron (3x3 Tower)‏
Quiet neighbors: all towers pass Fine Grain & H/E
One group of 5 EM ET < Threshold
December 18, 2007
Christos Lazaridis
Preliminary Examination

19. RCT Trigger Supervisor
An online framework to configure, test, operate and monitor the CMS Trigger
Each subsystem is represented by a “cell”
Cells communicate via XML- formatted commands
My contribution so far:
A control panel for the RCT that permits
Specific Crate/Card selection
Command execution
Crate Selection
Card Selection
Command Selection
December 18, 2007
Christos Lazaridis
Preliminary Examination

20. Z+Jets Characteristics
Total Z production cross section: 56 nb
Z → e+e- cross section: 1.5 nb
Total cross section x Branching Ratio ( ~ 3%)
105 events/100pb-1 at LHC startup luminosity (1032 cm-2s-1)
Looking for
2 electrons
N jets, N = 0...5
Within detector coverage
Reconstructing
Z Invariant Mass
Z PT
December 18, 2007
Christos Lazaridis
Preliminary Examination

21. Preliminary Examination
CDF Z+4 Jet events
CDF has found two Z+4 Jet
events in 1.7 fb-1 of p-pbar collisions at 1.96 TeV
CDF Public Note 8827
December 18, 2007
Christos Lazaridis
Preliminary Examination

22. Preliminary Examination
CDF Z→e+e- peak
Requirements:
Two electrons with ET > 25 GeV
At least one central electron: |ηe|<1
Second electron central or forward
|ηe|< 1 or 1.2 < |ηe|< 2.8
Z mass window: 66 < Mee < 116 GeV/c2
ΔR (e, jet) > 0.7
Jets found using a cone algorithm with ΔR = 0.7
pT,jet > 30 GeV/c and |yjet| < 2.1
Mee Invariant Mass
Events with at least one jet
CDF Public Note 8827
December 18, 2007
Christos Lazaridis
Preliminary Examination

23. Preliminary Examination
Z+Jets Cross Section
MCFM: Monte Carlo for FeMtobarn processes
Z + Jets Inclusive Cross Section
CMS will have large data samples at high jet pT. Will probe proton PDF and investigate scale choices
Data/Theory Ratio
MCFM: designed to calculate cross-sections for various femtobarn-level processes at
hadron-hadron colliders
Shaded bands show systematic uncertainties
Dashed lines: PDF uncertainties
Dash-dot lines: scale uncertainties
CDF Public Note 8827
December 18, 2007
Christos Lazaridis
Preliminary Examination

24. Monte Carlo Studies Dataset used: Z+N jets, N = 0…5 Generated with:
ALPGEN 2.12
PYTHIA 6.409
Detector simulation with GEANT4 8.2.p01
Detector electronics simulation and event reconstruction and physics objects provided by CMS framework
Matrix element generator: multi-parton processes in hadronic collisions
General purpose MC generator:
for hadronization and showering
Simulation of the passage of particles through matter
December 18, 2007
Christos Lazaridis
Preliminary Examination

25. Selecting Ze+e- events
# Jets
σ (pb)
Ze+e-
1.5x103
70K 1
3.2x102
15K 2
9.9x101 5K 3
6.9x101
3.5K 4
1.8x101
0.9K 5
9.3x100
0.4K
Events to be produced with 100pb-1 of data
Cross sections as computed by Alpgen
December 18, 2007
Christos Lazaridis
Preliminary Examination

26. Signal Identification Strategy
We will use the following cuts:
Starting with: 105 Z→e+e- events (with 100 pb-1)
Requirement
Reason
2 Electrons ET > 15 GeV, |η| < 2.4
Above trigger threshold:
Efficient for W and Z events
N Jets; N = 0...5, ET > 15 GeV, |η| < 2.4
Jet ET higher than QCD
80 GeV < Mee < 100 GeV
Mass of the e+e- pair close to Z mass
December 18, 2007
Christos Lazaridis
Preliminary Examination

27. Offline Electron Reconstruction
Create “super-clusters” from clusters of energy deposits using Level-1 E/M calorimeter information
In area specified by Level-1 trigger
ET > threshold
Match super-clusters to hits in pixel detector
Electrons leave a hit (track)
Photons do not
Combine with full tracking information
Track seeded with pixel hit
Final cuts made to isolate electrons ET γ e-
Tracker
Strips •
ET/pT cut
Pixels pT
December 18, 2007
Christos Lazaridis
Preliminary Examination

28. Trigger on Electrons for Z + 0-5 jets
Automatic L1 trigger for electrons above 63 GeV
Z+0 Jets
Z+1 Jets
Z+2 Jets
Z+3 Jets
x103
max value of 63 GeV (7 bits)
# electrons/100 pb-1
Isolated Electron Et
December 18, 2007
Christos Lazaridis
Preliminary Examination

29. Highest Et Reconstructed Electrons
Z+0 Jets
Z+1 Jets
Z+2 Jets
Z+3 Jets
x103
x103
# electrons/100 pb-1
# electrons/100 pb-1
electron
Electrons with:
ET > 15 GeV
-2.4 < η < 2.4
electron
December 18, 2007
Christos Lazaridis
Preliminary Examination

30. Highest Et Electrons η/φ
Z+0 Jets
Z+1 Jets
Z+2 Jets
Z+3 Jets
x103
η1st
Z+0 Jets
Z+1 Jets
Z+2 Jets
Z+3 Jets
x103
# electrons/100 pb-1
CRACK!
φ1st
# electrons/100 pb-1
Electron η
Electron φ
December 18, 2007
Christos Lazaridis
Preliminary Examination

31. ΔR Matching to Generated Electrons
Comparing the ΔR separation of the electrons generated and reconstructed
# electrons
No match for ΔR > 0.03
December 18, 2007
Christos Lazaridis
Preliminary Examination

32. Electron Finding Efficiency
Events where both electrons are fully reconstructed
Entries:
Overflows:
141530
135 Matched MCe
185 All MCe
December 18, 2007
Christos Lazaridis
Preliminary Examination

33. Z Invariant Mass
Reconstructed Z Invariant Mass
x103
With 100 pb-1 of data we should see ~100 Z + 5 Jets events!
# evts/100 pb-1
2 isolated electrons with pt > 25 GeV
-2.4 < η < 2.4
Z+0 Jets fit:
Range GeV
Z+0 Jets
Z+1 Jets
Z+2 Jets
Z+3 Jets
Z+4 Jets
Z+5 Jets
Mee
December 18, 2007
Christos Lazaridis
Preliminary Examination

34. Preliminary Examination
Reconstructed pt(Z)
x103
Measure differential cross section of Z production
Compare with MC predictions
# evts/100 pb-1
Z+0 Jets
Z+1 Jets
Z+2 Jets
Z+3 Jets
December 18, 2007
Christos Lazaridis
Preliminary Examination

35. Offline Jet Reconstruction
Iterative cone
Draw a cone ΔR = 0.5 around a seed with Et > threshold
The computed direction seeds a new cone
Iterate until the cone position is stable
Stable cone ≡ a jet; towers are removed from the list of input objects
no jet merging R
December 18, 2007
Christos Lazaridis
Preliminary Examination

36. Preliminary Examination
Highest Et Jets
Iterative Cone
Rcone = 0.5
Pt > 15 GeV
-2.4 < η < 2.4
Some jets (e.g. in Z+0 jets events) are underlying soft QCD radiation
Second Jet
Highest Jet
x102
x102
# jets/100 pb-1
# jets/100 pb-1
Z+0 Jets
Z+1 Jets
Z+2 Jets
Z+3 Jets
It's Z+0 partons according to Alpgen, but eventually we get a soft jet
December 18, 2007
Christos Lazaridis
Preliminary Examination

37. Signal Identification
Starting with: 105 Z→e+e- events (with 100 pb-1)
Requirement
Reason
Events
2 Electrons ET > 15 GeV, |η| < 2.4
Above trigger threshold:
Efficient for W and Z events
0.6 x 105 events
(60 % of initial)
N Jets; N = 0...5, ET > 15 GeV, |η| < 2.4
Jet ET higher than QCD
4.8 x 104 events (48 % of initial)
80 GeV < Mee < 100 GeV
Mass of the e+e- pair close to Z mass
4 x 104 events (40 % of initial)
December 18, 2007
Christos Lazaridis
Preliminary Examination

38. Preliminary Examination
Summary/Future Plans
Z + Jets is a high cross section channel
105 events/100pb-1 (LHC startup luminosity)
Measure Z + Jets total and differential cross sections
Can be used for detector calibration
Clear Z→e+e- peak
Plans:
Work on the RCT, key component in electrons triggering
Study/Improve reconstruction of electrons & jets
Take data
Tune Monte Carlo generators and PDFs
Discover new physics!
December 18, 2007
Christos Lazaridis
Preliminary Examination