QCD at the Tevatron

1
QCD at the Tevatron

• Iain Bertram
• Lancaster University
• DØ Experiment
• 6 June 2001 - Bristol

2
Outline

• Introduction to Tevatron Kinematics
• DØ CDF Detectors
• Jet Cross Sections
• Jet Structure
• Photons
• Run II _at_ DØ
• Conclusions
• Caveat This is not the entire Tevatron QCD
  program that is impossible in one hour. I have
  chosen the topics I consider to be most important.

3
Hadron-hadron collisions
Photon, W, Z etc.
parton distribution
Hard scattering
Underlying event
FSR
ISR
parton distribution
fragmentation
Jet
4
Measured Event Variables

• In a event the following are measured

Jet, electron 1 ET1, h1, ?1
Jet, electron 2 ET2, h2, ?2
h 0
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The Detectors (Run I 1993-95)
?s 1.8 TeV W/Zs 40k/3k for mass
Luminosity 2?1031 ttbar 10-100 events
?Ldt 100 pb-1 Higgs negligible
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DØ Calorimeter Run I
Full Coverage ? lt 4.1 Segmentation
?????0.1?0.1 (?????0.05?0.05 EM shower max)
Single Electron Response 0.15/?E
0.003 Single Hadron Response 0.5/?E
0.004 Depth ???
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Jets at the Tevatron

• Mostly Fixed cone-size jets
• Add up towers
• Iterative process
• Jet quantities
• ET, ?, ?
• Correct to Particles
• Do not include underlying event.
• Model underlying event with Minimum bias event
  (inelastic scattering)

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Typical DØ Dijet Event
ET,1 475 GeV, h1 -0.69, x10.66 ET,2 472
GeV, h2 0.69, x20.66
MJJ 1.18 TeV Q2 ET,1ET,22.2x105 GeV2
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Tevatron x-Q2 Reach
Overlaps and extends reach of HERA
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Inclusive Jet Cross Section

• How well do we know proton structure (PDFs) ?
• Is NLO (?s3) QCD sufficient ?
• Are quarks composite ?

pQCD, PDFs, substructure,..?
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Inclusive Jet Cross Section
CDF hep-ph/0102074 (acc PRD)
DØ PRL82 2451 (1999) , hep-ex/0012046 (acc PRD)
CDF 0.1 lt ? lt 0.7
Good Agreement with NLO QCD
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Theory Uncertainties

• NLO pQCD predictions (?s3) - Ellis, et al.,
  Phys. Rev. D, 64, (1990) EKS- Aversa, et al.,
  Phys. Rev. Lett., 65, (1990)- Giele, et al.,
  Phys. Rev. Lett., 73, (1994) JETRAD
• Choices (hep-ph/9801285, EPJ C5, 687, 1998)
• Renormalization Scale (10)
• PDFs (20 with ET dependence)
• Clustering Alg. (5 with ET dependence)

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Comparisons with CDF

• DØ analyzed 0.1 lt ? lt 0.7 to compare with CDF
• Good Agreement
• If we calculate ?2 between DØ data and fit to CDF
  and its uncertainties
• ?2 30.8 (0.16)

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DØ Comparisons to NLO Theory

• No indication of an excess above 350 GeV.
• Good agreement quantitatively
  as indicated by c2 test
• Di and Ti data and theory, Cij covariance
  matrix.

c2 S (Di-Ti) C-1ij (Dj-Tj)
? lt 0.5 0.1 lt? lt 0.7
CTEQ 3M 25.3 (0.39) 32.7 (0.11)
CTEQ4M 20.1 (0.69) 26.8 (0.31)
CTEQ4HJ 16.8 (0.86) 22.4 (0.56)
MRS(A) 20.4 (0.67) 28.5 (0.24)
MRST 25.3 (0.39) 29.6 (0.20)
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CDF Comparisons to NLO Theory

• No indication of an excess above 350 GeV.
• Good agreement quantitatively as indicated by c2
  test

c2 c2stat ?S2
S2 is a contribution depending on the best fit
to the uncertainties
33 bins 0.1 lt? lt 0.7
CTEQ4M 63.4
CTEQ4HJ 46.8
MRST 49.5
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DØ Forward Jets
QCD?JETRAD
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Comparison to Theory
Closed CTEQ4HJ Open CTEQ4M
Closed MRTSg? Open MRST
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Quantitative Comparison

• Discriminates between PDF
• Now being used by CTEQ and MRST to restrict
  gluons to 20 level (see DIS 2001
  http//dis2001.bo.infn.it/wg/sfwg.html)

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What have we learned?

• These results extend significantly the kinematic
  reach of previous studies and are consistent with
  pQCD calculations over the large dynamic range
  accessible (? lt 3).
• Once incorporated into revised modern PDFs, these
  measurements will greatly improve our
  understanding of the structure of the proton at
  large x and Q2.
• Are gluon distributions at large x enhanced?
• factor 20 more data in Run II, starting summer
  2001, will extend the reach to higher ET and
  should make the asymptotic behaviour clearer

20
KT Jet Algorithm
Fixed Cone Algorithm
KT Jet Algorithm
resolution parameter
Jet ET
21
Jet Cross Section using KT

• KT with D1.0, equals NLO cross section with Cone
  R0.7
• Energy difference between KT and cone causes
  difference in cross section
• 1-2 GeV Difference caused by
• Hadronic Showering effects (parton to particle)
• Underlying Event
• Showering
• Difference with theory most at low ET.

?227 (31)
24 d.o.f.
?231/24 (31)
?227 (31)
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Ratio of Cross Sections

• Express Inclusive Jet Cross Section as
  dimensionless quantityas a function of
• Theory uncertainties could be reduced to 10
• Experimental Uncertainties Cancel

Naive Parton Model 1
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Ratio of Cross Sections

• Phys.Rev.Lett.86,2523 (2001) hep-ex/0012046
• Data 10-15 below NLO QCD
• No obvious problemInteresting!

Agreement Probability (from c2 test) with CTEQ4M,
CTEQ4HJ, MRST, MRSTGU 25-80
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Comparison with CDF

• Consistent at high xT, possible discrepancy at
  low values

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Suggested explanations

• Different renormalizationscales at the two
  energies
• OK, so its allowed, but . . .

• Mangano proposes an O(3 GeV)non-perturbative
  shift in jet energy
• losses out of cone
• underlying event
• intrinsic kT
• could be under or overcorrecting the data (or
  even different between theexperiments DØ?)

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Dijet Mass Spectrum
Nevents L DM Dh1 Dh2
Experiments in excellent agreement. Different
rapidity ranges and very different analysis
techniques. Reasonable agreement with
predictions.
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Dijet Mass Cross Section Ratio
s (h1,2 lt 0.5 ) / s ( 0.5 lt h1,2lt 1 )
(?s1800 GeV ) PRL 82, 2457-2462, 1999
Theory uncertainty 6 (m) , 3 (PDF)
Systematic Uncertainty 8
NLO QCD in good agreement with data
? ? 2.4 TeV (95 confidence level)
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BFKL
BFKL (Balitsky-Fadin-Kuraev-Lipatov) evolution
1/x
PDF
DGLAP (Dokshitser-Gribov-Lipatov-Altarelli-Parisi)
Q2

• In hadron-hadron collisions

DGLAP (as in PYTHIA) gluon radiation strongly
ordered
BFKL many gluons all same pT
One way to realise this situation is jets widely
separated in rapidity the total energy is then
much greater than the jet pT scale, and one can
have many gluons of comparable pT emitted between
the jets Note BFKL provides a way to resum the
contribution of these gluons it doesnt predict
how many there are, and there is no BFKL event
generator yet
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BFKL

• Phys.Rev.Lett. 84, 5722 (2000)
• Another attempt to find anobservable which
  displays BFKLbehaviour
• DØ measurement of 630/1800 GeV cross section
  ratio at large rapidity separations
• use bins such that x and Q2 arethe same in the
  two datasets(but different ??)
• Whats going on here?
• data behave qualitatively like BFKL (but also
  like HERWIG)
• given that we cant predict the 630/1800 GeV
  ratio of inclusive cross sections, how much can
  we really infer?

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Subjet Multiplicity in Quark Gluon Jets

• Method
• Select quark enriched gluon enriched jet
  sample
• Compare jets at same ( ET , h ) produced at
  different and assume relative q/g
  content is known

• Motivation
• Test of QCD ( Q G jets are different)
• Separate Q jets from G jets (top, Higgs, WJets
  events)
• Measure the subjet multiplicity in quark and
  gluon jets

s
630GeV
Contributions of different initial states to the
cross section for fixed Jet ET vary with
qq
gg
qg
100 200 300
Jet ET
Jet ET
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Jet Structure at the Tevatron

• Subjets inside jets perturbative part of
  fragmentation
• DØ compares 630 to 1800 GeV data at same ET and
  ?, and infers q and g jet differences

kT algorithm D0.5, ycut 10-3 55 lt ET(jet) lt 100
GeV ?jet lt 0.5
DØ Preliminary
1
2
3
4
5
Subjet Multiplicity
DØ Data
HERWIG 5.9
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Direct Photon Production
DØ PRL 84 2786 (2000)
CDF Preliminary
QCD prediction is NLO Owens et al. Note ET range
probed with photons is lower than with jets
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Direct Photon Production
DØ PRL 84 2786 (2000)
CDF Preliminary
Low ET excess Theory QCD NLO Owens et al.
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kT Smearing???

• Gaussian smearing of the transverse momenta by a
  few GeV can model the rise of cross section at
  low ET (hep-ph/9808467)

Motivated by observed pT(??)
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Alternative Viewpoint
Also note that Vogelsang et al. get closer to
the observed shape than the Owens prediction
with no extra kT (using NLO fragmentation and
setting ?R??F)
36
Photons at ?s 630 GeV

• CDF data, compared with UA2 data show deficit at
  high ET compared with the theory.
• Relation to jets?

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DØ Photons at 630 GeV
?2 Comparison (7 bins) CC 11.4 -- 12 EC 4.6
-- 71
First measurement at forward rapidities
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Ratio of Photons at 630 and 1800

• Slight excess at low XT
• Insignificant statistically
• Run II wont help!
• Good overall agreement with NLO
• No significant high ET deficit.
• No statistics at high ET (gt 40 GeV)

?2 Comparison (7 bins) CC 6.5 -- 49 EC 3.0
-- 89
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Photons Comments

• Situation is complicated and not easily
  explained.
• Low ET kT effects?
• High ET deficits at 630 GeV
• Lack of statistics in both regions
• Challenge for both theory and experiment in Run
  II.

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Run II Coming March 2001
?s 2.0 TeV W/Zs gt1M/gt50k
Luminosity 2?1032 ttbar gt1k events
?Ldt 2-30fb-1 Higgs Possible
Upgrades to both detectors Silicon, Tracking
(Drift-CDF, Fibres-DØ) Preshowers, Calorimeter
Upgrade CDF, Muon upgrades, new trigger
electronics (bunch spacing), C OO code
development
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Case Study Dijet Mass Spectrum
Dramatic increase in high pT cross sections Large
gains in statistics
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Looking ahead Run II
Run II 100 events ET gt 490 GeV 1K events ET gt
400 GeV Run I 16 Events ETgt 410 GeV Great
reach at high x and Q2, the place to look for
new physics!
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Jet Algorithms

• Fermilab Run II workshop
• Various species of kT
• New Cone Algorithm
• Theoretical desires
• infrared safety is not a joke!
• avoid ad hoc parameters like Rsep
• Cone algorithm improved by
• e.g. by modification of seed choices - midpoints
• seedless algorithm? In development.
• Experimental desires
• sensitivity to noise, pileup, negative energies

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Requirements for Progress

• Quantitative Theoretical Uncertainties
• pdf and renormalization scale uncertainties
• Full understanding of correlations of
  experimental uncertainties
• Statistical Uncertainties in most cases will be
  negligible
• Underlying event
• Require a better understanding and treatment of
  the surrounding event

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Status of DØ detector

• Run commenced March 2001
• Two data taking periods completed of a few weeks.
• Detector Commissioning taking place!
• Physics data in autumn
• First results in early 2002

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First?pp collisions of Run 2 at DØ April 3, 2001
Antiproton halo

• Luminosity counters
• timing

Luminosity ( coincidence)
5 ? 1027 cm-2 sec-1
Proton halo
Vertex distribution along z of min bias events
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Global track finding
Track with 5 fiber tracker hits, 5 3D silicon hits
Relative alignment of silicon and fiber
trackers verified to 40 ?m level
36 ? 36 Store Run 119679, Event 232931Level 3
(software trigger) Global Tracking
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Event with 6 tracks pointing to same vertex
36 ? 36 Store Run 119679, Event 231653Level 3
(software trigger) Silicon-only Tracking
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Vertex finding

• Primary vertices reconstructed from tracks in
  silicon

z
z
y
x
y
A. Schwartzman Buenos Aires
x
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Tracking in Silicon

• 3D event display showing hits and reconstructed
  track

E. Barberis Y. Kulik
? Offline track finding
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Silicon Detector
H1
B1
H4
B4
B6
North
F1
South
F12
F11
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Tracks in the Silicon Fiber Tracker

• Offline track finding from?pp events

8 or more points on a track (5 fiber hits, 3 or
more Silicon)
?2
1/pT
Since B0 for this run, real tracks should be
found with 1/pT 0
D. Adams Fermilab
Data taken in 36 x 36 store (end April) Plot
posted 5/18/01
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Hits in Central Preshower Detector
Preshower hits
Fiber tracker hits
D. Coppage, Kansas
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L1 muon triggered event
zy view, looking East
xy view, looking North
xz view, looking down
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First?pp collisions of Run 2 at DØ

• Calorimeter and forward muon hits in a minimum
  bias event

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The work of many people...
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Closing Remarks

• Run II has started 1 March 2001BIG Opportunity
  for QCD
• In most cases QCD predictions work exceptionally
  well.
• Exceptions Low pT processes are problematic
• low pT jets and photons
• Underlying Event and Event characteristics