Physics of Hadron Colliders Lecture 2: Top Physics at CDF Joseph Kroll University of Pennsylvania...
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Physics of Hadron CollidersLecture 2: Top Physics at CDF
Joseph Kroll
University of Pennsylvania
19 May 2004 J. Kroll University of Pennsylvania 2
Fermilab Collider Accelerator Complex
• There are 8 accelerators in the chain
• Proton source (3)– Cockroft-Walton
– Linac
– Booster
• Antiproton source (2)– Debuncher
– Accumulator
• Main Injector (1)
• Recycler (1)
• Tevatron (1)
see www-ad.fnal.gov/public/index.html
Drawing courtesy of Fermilab
19 May 2004 J. Kroll University of Pennsylvania 3
Fermilab Aerial View
Photo courtesy of Fermilab
Main Injector
Tevatron
DØCDF
Fixed Target
n.b., objects not to scale
19 May 2004 J. Kroll University of Pennsylvania 4
Fermilab Site (cont)
Photo courtesy of Fermilab
AccumulatorDebuncher
Booster
Linac
Cockroft-Walton
Photo courtesy of Fermilab
19 May 2004 J. Kroll University of Pennsylvania 5
Proton Source
Cockroft-Walton750 kV DCAccelerates H¯
Linac805 MHz130 m length3 MV/meter accel.400 MeV on outputalso H¯
Booster1 GHz475 m circumference400 MeV acc. to 8 GeVaccelerates protonsbeam to MiniBoone too
Beam from Boostergoes to Main Injectoraccelerated to 120 GeVto make antiprotons
Photo courtesy of Fermilab
Photo courtesy of Fermilab
Photo courtesy of Fermilab
19 May 2004 J. Kroll University of Pennsylvania 6
Antiproton Source – Three Components
Target• 120 GeV p’s hit Ni target• 106 p make » 20 8 GeV “pbars”• focused by Li lens• pbars filtered out by mag. spec.
Debuncher• trade E for t• pbars easier to accept in Accum.
Accumulator• 8 GeV pbars cooled (stacked)• Stochastic cooling• holds pbar “stack” for hours• large stack = 200mamps
Accumulator
DebuncherPhoto courtesy of Fermilab
19 May 2004 J. Kroll University of Pennsylvania 7
Stochastic Cooling
I said I wouldtry to find outthe answers to questions I couldnot answer.
Here’s an answerfrom Paul Derwent(FNAL) on why wesay “stochastic”
19 May 2004 J. Kroll University of Pennsylvania 8
Main Injector and Recycler
Main Injector
RecyclerMain Injector• Biggest change for Run II• 3.3 km circumference• replaced “Main Ring”• commissioned 98-99• increases pbar prod by 3• fixed target @ 120 GeV• collider @ 150 GeV• FT & collider simultaneous
Recycler• 8 GeV pbars from Acc• allows faster stacking• permanent magnets •1.5kG dipole, 3.7kG/m Quad
Photo courtesy of Fermilab
19 May 2004 J. Kroll University of Pennsylvania 9
Tevatron
After MIBefore MI
Tevatron
Main Ring
• 1st superconducting synchrotron• 6.28 km in circumference• commissioned 1983• 1000 4.4 T dipoles, 6.4 m long, 4.2o K• Run I: 900 GeV Run II: 980 GeV
1989 NASMedal ofTechnology:Alvin TollestrupHelen Edwardsof Fermilab
AT HE
Photo courtesy of Fermilab Photo courtesy of Fermilab
19 May 2004 J. Kroll University of Pennsylvania 10
Tevatron Collider Runs and CDF
• Run -2 (October 1985) “Engineering” Run– 28 collisions recorded with VTPC
• Run -1 (1987-1988)– first physics (» 30 nb^{-1}): W’s, jets, incl. particle distributions,…
• Run 0 (1988-1989)– » 5 pb-1 data: Z0 mass, limits on top, EWK, QCD, B’s, exotics,…
• Run 1 (1992-1996)– DØ’s first run
– peak L = 2.4 £ 1031 cm-2s-1, 6 £ 6 (3.6 sec)
– » 120 pb-1 data: top discovery, W mass, sin2,…
• Run 2 (2001-2009?)– to date: 500 pb-1 delivered, 400 recorded by CDF
– peak L = 7.2 £ 1031 cm-2s-1, 36 £ 36 (396 nsec)
19 May 2004 J. Kroll University of Pennsylvania 11
The CDF II Upgrade (1996-2001)*
• Actually there have been many CDF upgrades• CDF II refers to the detector built after Run I• Essentially the entire detector rebuilt
– Only the Central EM and HAD calorimeter remained
• new bunch separation (132 nsec) all new electronics• new drift chamber (COT) & silicon tracker (L00, SVXII, ISL)• Time of Flight for particle id• new end plug calorimetry & lumi monitor• more muon coverage• new fully digital trigger system with new capabilities (SVT)• new DAQ
* Actually the CDF II upgrade began well before 1996 and is still taking place
19 May 2004 J. Kroll University of Pennsylvania 12
Silicon trackingDrift chamber
Lumi monitor
Hadronic Calorimetry
Muon systems
Iron shielding
Solenoid and TOF
ElectromagneticCalorimetry
CDF II
19 May 2004 J. Kroll University of Pennsylvania 13
Running Conditions (Run I versus Run II)
Higher luminositysame number of bunches more interactions/crossing
Average number ofinteractions/crossingabout the same at Lmax
in Run 1 and Run 2
n.b.,We will not run with 99 bunches (132 nsec)
Example of a slide fromthe days before powerpoint
19 May 2004 J. Kroll University of Pennsylvania 14
CDF Run II Data Taking
Integrated luminosity deliveredand acquired by CDF
Integrated luminosityacquired by CDFper fiscal year (e.g., FY03 is Oct. 2002 to Sep. 2003)
Day since beginning of yearTevatron Store Number
19 May 2004 J. Kroll University of Pennsylvania 15
CDF Run II Data Taking (continued)
Initial Luminosity vs. Store Number CDF Data Taking Efficiecny vs. Store #
80%
Not at the LEP orB Factory Level (yet)
Blue is running averageover 20 Stores
CDF Averages and Records
19 May 2004 J. Kroll University of Pennsylvania 16
The CDF II Silicon Tracker
3 Parts:Layer 00SVX IIISL
19 May 2004 J. Kroll University of Pennsylvania 17
Insertion of the Central Outer Tracker
19 May 2004 J. Kroll University of Pennsylvania 18
Tracking System Assembly (continued)
19 May 2004 J. Kroll University of Pennsylvania 19
Roll-in of Central Detector
19 May 2004 J. Kroll University of Pennsylvania 20
CDF II Trigger System
Detector
L1 trigger
L2 trigger
L3 trigger
tape
46 L1buffers
1.7 MHz bunchcrossing rate
30 kHz L1 accept
300 Hz L2 accept
70 Hz L3 accept
Hardware tracking for pT 1.5 GeV
Muon-track matching
Electron-track matching
Missing ET, sum-ET
Silicon tracking
300 CPU’s
Jet finding
Full event reconstruction
Refined electron/photon finding
>100Hz with datacompression
4 L2 buffers
courtesy E. Thomson (OSU/Penn)
19 May 2004 J. Kroll University of Pennsylvania 21
Run II Physics @ CDF II
• Top physics (center piece of Run II physics program)– top is discovered measure properties (mass, production, decays)
– does it have non-standard model decays, production?
• Exotic Physics (new particle searches)– at the energy frontier until LHC turns on
• Electroweak Physics– W mass, Higgs search, WW, WZ, ZZ production
• QCD– inclusive jet cross-section, W/Z + jets, jet correlations, heavy flavor
– soft physics (diffraction, double pomeron scattering, etc.)
• Heavy Flavor Physics (b and c)– B hadron weak decays: B0
s flavor oscillations is the center piece
– Charm physics program (completely new for Run II – SVT)
19 May 2004 J. Kroll University of Pennsylvania 22
10 Years Ago
Aside: Have we directlyobserved all 12fundamental fermions?
19 May 2004 J. Kroll University of Pennsylvania 23
Top Pair Production
The dominant source of top is strong (QCD) production
quark-antiquarkannihilation
gluon fusion
top is massiveat Tevatronproducedcentrally
19 May 2004 J. Kroll University of Pennsylvania 24
Predicted Cross-section M. Cacciari et al., hep-ph/0303085
Central valueis “CTEQ6”structure fcns= 175 GeV
Variation from• pdf’s• s,
GeV cross-section in picobarns (pb)
30% increase s = 1.8 to 1.96 TeV
pdf uncertainty:from high-x g- see later slide
19 May 2004 J. Kroll University of Pennsylvania 25
Comment on Parton Distribution Functions
CTEQ is an acronym for a set of structure functions producedby a collaboration called :
There are others: e.g., MRST
“Coordinated Theoretical-Experimental Project on QCD” see www.phys.psu.edu/~cteq
Martins, Roberts, Stirling, Thorne, hep-ph/0207067
There are many sets of pdf’s based on global fits of various data
n.b., excellent summer schooltransparencies available there
check out www.durpdg.dur.ac.uk/hepdata/pdf3.html
19 May 2004 J. Kroll University of Pennsylvania 26
Which Process Dominates?
Depends on mt and s
Center of mass energy Ecm of partons 1 and 2
Parton 4-vectors are (E,pz):
Need at least
For x1 ¼ x2 x > 0.2 Tevatron x > 0.03 LHC
Aside: really need more (phase space)mean pT(top) » mt/2
19 May 2004 J. Kroll University of Pennsylvania 27
Quarks at Tevatron, Gluons at LHC
Tevatron
LHC
(uncertainty in g at high x ~ 10% uncertainty on
LHC
Tevatron
g
u
d
u
19 May 2004 J. Kroll University of Pennsylvania 28
Single Top Production
Electroweak top production also important
0.88 § 0.10 pb-1 1.98 § 0.24 pb-1 negligible @ Tevatron
Harder to observe than
Difficult background for SM Higgs search
See B.W. Harris et al.,Phys. Rev. D 66, 054024 (2002)
19 May 2004 J. Kroll University of Pennsylvania 29
Standard Model Top Anti-Top Signature
Vtb ¼ 1* BF(t ! Wb) = 100%
* assuming unitarity 0.9990 < Vtb < 0.9993 @ 90% CL (PDG2002)
mt = 175 GeV real W
t! Wb) = 1.5 GeV, t = 4£10-25 s no top hadrons
Classify topologies according to W decay
dilepton both W decays leptonic (means e or , not
lepton + jets One W decay leptonic, other hadronic
all hadronic both W decays hadronic
dilepton and lepton + jets are the discovery modes
19 May 2004 J. Kroll University of Pennsylvania 30
Three tt Signatures
Dilepton
2 high pT leptons, ET, 2 b-jets
Lepton + jets
1 high pT lepton, ET, 2 b-jets, 2 light quark jets
All hadronic
2 b-jets, 4 light quark jets
W!, ! hadrons treated separately
4.6% (6.4%)
33.6% (37.6%)
61.8% (56.0%)
Use W! e, 10.7% each(add W! 10.7%, !e, 35.2% total)
19 May 2004 J. Kroll University of Pennsylvania 31
Three Signatures (continued)
All hadronic
QCD multijet production several orders of magnitude higher ► need b-jet identification ► must reconstruct top invariant mass ► still very challenging (for trigger too)
Remember
most probable topology
Lepton+jet also a probable topology
Real W production still orders of magnitude higher ►several approaches – described later ►much cleaner than hadronic – especially with b-tag
see next slide
Dilepton least probably topology
Real WW production comparable see next slide
19 May 2004 J. Kroll University of Pennsylvania 32
Aside: W and Z Production at the Tevatron
Dilepton and Lepton+jets signature contain W! e,
(for s=1.96 TeV, about 10% lower for s=1.8 TeV)
WW production is comparable to top production
e.g., see J.M. Cambell & R.K. Ellis, Phys. Rev. D 60, 113006 (1999)
19 May 2004 J. Kroll University of Pennsylvania 33
Dilepton Signature
Pair of opp. charge, high pT leptons: e+e-, e+- & e-+, +-
Substantial ET from two ’s Two high pT b-jets
Background from real high pT lepton pairs (“physics backgrounds”)• Drell-Yan and Z0! ee, (no real ET)• Z0! (real ET too)• WW production (real ET too)
Jet requirement greatly reduces these backgrounds
Background from “fake” leptons too• W + jets, W! e, & jet! fake lepton
19 May 2004 J. Kroll University of Pennsylvania 34
Lepton + Jets Signature
High pT lepton: e or
Substantial ET from
Two high pT b-jets
Two high pT light quark jets
Physics background• W + jets
Reduce backgrounds with• kinematic criteria• identifying b jets (b-tags)
Run I results published in CDF Collaboration,T. Affolder et al., Phys. Rev. D 63, 072003 (2001)
19 May 2004 J. Kroll University of Pennsylvania 35
Digression: Jet Reconstruction at CDF
Quarks and gluons do not exist as “free” objects (colored)fragment or hadronize into “jets” of particles
Recall results from UA2 in Lecture 1
jet reconstruction get back to the parton (q or g) 4-vectorto compare to theory or reconstruct mass e.g., W! qq0
CDF: uses fixed cone algorithm in - space
Many possible approaches used at e+e- and hadron colliders
follows UA1
19 May 2004 J. Kroll University of Pennsylvania 36
CDF Jet - Cone Algorithm
• Start with calorimeter cells – define energy momentum vector– vector points from origin to centroid of cell: (px,py,pz,E), where E = |p|– ET
2 = px2 + py
2
• Select “seed” cells: ETcell > seed threshold (e.g., 1 GeV)
• Form seed “clusters”– Add vectors of all seed cells within R (ranked in ET)– typical values R: 0.4, 0.7, 1.0 top search uses R = 0.4– centroid of cluster determined by ET weighting– changes as seed clusters added - iterate
• After all seeds merged, add in remaining calorimeter cells– require E_T > noise threshold (e.g., 100 MeV)
• Jets defined this way have mass (partons are massless)
see CDF Collaboration F. Abe et al., Phys. Rev. D 45 1448 (1992)
19 May 2004 J. Kroll University of Pennsylvania 37
Azimuthal Energy Flow in 2 Jet Events
CDF Collaboration F. Abe et al., Phys. Rev. D 45 1448 (1992)From CDF two jet data
Distribution of calorimeter energyaround jet axes
leading jet has <ET> » 40 GeV
Cone size 0.4, 0.7, 1.0all reasonable
top: R=0.4 optimal for counting jets
leading jet leading jet
away jet
19 May 2004 J. Kroll University of Pennsylvania 38
Jet Energy CorrectionsJets formed from “raw”calorimeter energies
Detector Effects
Physics Effects
• nonlinear calorimeter response to low energy hadrons• B field bends low pT particles out of cone (or do not reach Cal)• cracks and transition regions of calorimeter• different response of EM & Had
• extra E from “underlying event” & multiple interactions• fragmentation effects & soft g rad.• and
Calibrate central calorimeter(||<1) in situ with spectrometer
Balance calorimeter responseout to ||=2.4 using dijet data
Check jet energy scalewith vs. jet data
Typical correction: increase raw ET by 30%
19 May 2004 J. Kroll University of Pennsylvania 39
CDF Jet Corrections Have Several Levels
Run II: use absolute E correction from Run I
Top analysis usesLevel 5 jets
19 May 2004 J. Kroll University of Pennsylvania 40
Focus 1st on Lepton + Jets Channel
• This was the discovery channel in Run I
• Used to measure top production and mass
• Signature is a leptonic W decay & four jets (2 are b’s)
• Jets not all in detector acceptance 3 or 4 jets
• Discovery relied on identifying at least one b jets– B hadron relatively long lived – use secondary vertex tag
– Exploit B semileptonic decays – soft lepton tag (soft ≠ W decay)
• Can also isolate top signal using kinematic criteria– top is heavy – harder more central jets than initial state parton radiation
– neural nets give even better discrimination
19 May 2004 J. Kroll University of Pennsylvania 41
W Selection
Start with event sample collected with inclusive high pT lepton trigger
see e.g., CDF Collaboration F. Abe et al.,Phys. Rev. D 50 p. 2966 (1994)
Lepton (e or ) must be isolated
Electron Selection
ET > 20 GeV, ||<1 (central)
Selection criteria based on• Ehad/EEM
• E/p• Shower shape in Calo.• track match with shower max det.• shape in shower max det.
Efficiency: 80%
Muon Selection
pT > 20 GeV/c, ||<1
Selection criteria based on• minimum ionizing in EM and Had• match chambers and track
Efficiency: 90%
ET > 20 GeV Reject dileptons, Z0! e+e-, +-
19 May 2004 J. Kroll University of Pennsylvania 42
Do Selected Events Look Like W’s?
Transverse Mass Distributions CDF Collaboration F. Abe et al.,Phys. Rev. D 50 p. 2966 (1994)
From ‘94 top “evidence” PRD, Run II selection very similar
19 May 2004 J. Kroll University of Pennsylvania 43
Jet Requirements and HT
Top signal sample: ≥ 3 Jets ET>15 GeV, ||<2.0
mt measurement: require 4th jet ET>8 GeV, ||<2.4
Run II: may add HT>200 GeV
HT ´ ET of lepton, jets, ET
@ this stage tt efficiency is » 8% 40-60 events in 100 pb-1, S:B»1:4
Top is heavy larger HT on average than W+jets background
Jets
HT
Simulation
R=0.4 contains 70% E
19 May 2004 J. Kroll University of Pennsylvania 44
Displaced tracks or secondary vertex
Identifying (tagging) b quark Jets
Semileptonic decays:
Not isolated and softer than W! l “soft lepton tag” or SLT
B hadron lifetime ~ 1.5 psecSignificant Lorentz boost measurable displacement
Simulation: top mass = 160 GeV
CDF Collaboration F. Abe et al.,Phys. Rev. D 50 p. 2966 (1994)
pT(b)
displacement intransverse plane
5 mm
19 May 2004 J. Kroll University of Pennsylvania 45
Illustration of Displaced Tracks and Vertex
19 May 2004 J. Kroll University of Pennsylvania 46
Illustration of Displaced Tracks and Vertex
d
Interaction point(primary vertex)
Secondary Vertex(displaced vertex)
B decay product
Underlying eventB fragmentation product
Jet Axis
d = impact parameter
in plane transverse to beam axis
19 May 2004 J. Kroll University of Pennsylvania 47
Definition of Terms
Impact parameter d: distance of closest approach of track to vertex
Primary vertex: pp interaction point
Primary vertex follows Gaussian distribution in x, y, & z
Unless specified otherwise, “impact parameter” meansin the plane transverse to the beam line (xy or rφ)
† at the most narrow point (waist) – transverse size varies with z
Secondary vertex: decay/interaction point displaced from theprimary vertex by a distance that is experimentally measurable.
19 May 2004 J. Kroll University of Pennsylvania 48
from J/ (pT>1.5 GeV)with SVXII inner-most layer
Methods of Identifying Long Lived Particles
• Associate Tracks with jety
– count number of significantly displaced tracks: d/d > parameter
• 1st used at Mark II (SLC) for Z0! bb (R. Jacobsen)
– determine probability tracks originated from PV: “jet probability”• developed at LEP (א – Dave Brown) – later used by CDF
– correlations between “d” and “φ” (used by CDF in “top evidence”)
– reconstruct secondary vertex
d should include• uncertainty in track parameters• uncertainty in vertex position
Rely on impact parameter d & error d
Impact parameter significance: d/d
† jet may be calorimeter jet or track based jet found with - algorithm
includes 30mfrom beamspot
19 May 2004 J. Kroll University of Pennsylvania 49
Decay Distance in plane transverse to beam axis
Lxy < 0
Lxy > 0
Generic jets: symmetric distribution around 0 in Lxy
b-jets: very asymmetric distribution, biased towards Lxy>0
Jet axis
19 May 2004 J. Kroll University of Pennsylvania 50
CDF Displaced Vertex Algorithm (SECVTX)
19 May 2004 J. Kroll University of Pennsylvania 51
CDF Top Discovery
CDF Collaboration, F. Abe et al., Phys. Rev. Lett. 74, p. 2626 (1995)
data sample: 67 pb-1
27 tags in 21 W + ≥ 3 jets evtsexp. # of tags from backgnd: 6.7 ± 2.1
b tag eff. 42§5%for ≥ 1 tag/evt
also found excess of SLTand dilepton events
19 May 2004 J. Kroll University of Pennsylvania 52
Run II: Preliminary Version of Same Analysis (162 pb-1)
19 May 2004 J. Kroll University of Pennsylvania 53
Run II Top Candidate
19 May 2004 J. Kroll University of Pennsylvania 54
Run II: Preliminary Version of Same Analysis (162 pb-1)
Dominantsystematicis background:motivates HT
19 May 2004 J. Kroll University of Pennsylvania 55
Run II: Preliminary Version of Same Analysis (162 pb-1)
Dominant systematic still background: but less so