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QCD at the Tevatron Current results and future prospects. John Womersley Fermilab Fifth International Symposium on Radiative Corrections (RADCOR 2000) Carmel, CA, September 2000 http://d0server1.fnal.gov/users/womersley/radcor2000.ppt. Outline.

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qcd at the tevatron current results and future prospects

QCD at the TevatronCurrent results and future prospects

John Womersley


Fifth International Symposium on Radiative Corrections (RADCOR 2000)

Carmel, CA, September 2000


  • It is over four years since we completed data taking in Run I, so there are rather few new results
  • This presentation will therefore be more of a review of the current state of knowledge, highlighting unresolved issues and prospects for Run 2:
    • jets
    • vector bosons
    • photons
    • heavy flavor
  • Since this is a review, what you will hear are generally my personal opinions and not necessarily the party line of the experiments
the fermilab tevatron collider




p source

Main Injector


The Fermilab Tevatron Collider


Run 1: 100 pb-1, 1.8TeV

Major detector upgrades  now

2001-03 Run 2a: 2 fb-1, 1.96 TeV

Short shutdown to install new silicon

2003-07(?) Run 2b: ~ 15 fb-1


hadron hadron collisions
Hadron-hadron collisions

Photon, W, Z etc.

  • Complicated by
    • parton distributions — a hadron collider is really a broad-band quark and gluon collider
    • both the initial and final states can be colored and can radiate gluons
    • underlying event from proton remnants





Hard scattering







jet cross sections at s 1 8 tev
Jet cross sections at s = 1.8 TeV

R = 0.7 cone jets

  • Cross section falls by seven orders of magnitude from 50 to 450 GeV
  • Pretty good agreement with NLO QCD over the whole range

jet  0.5

0.1  jet  0.7

what s happening at high e t
What’s happening at high ET?

CDF 0.1<||<0.7

DØ ||<0.5

  • So much has been said about the high-ET behaviour of the cross section that it is hard to know what can usefully be added:

Figure 1: “The Horse is Dead”

NB Systematic errors not plotted

Figure 1: “The horse is dead”

the d and cdf data agree
The DØ and CDF data agree
  • DØ analyzed 0.1 <||< 0.7 to compare with CDF
  • One can (e.g. CTEQ4HJ distributions shown above) boost the gluon distribution at high-x without violating experimental constraints*; results are more compatible with CDF data points

*except maybe fixed-target photons, which require big kT corrections before they can be made to agree with QCD (see later)

what have we learned from all this
What have we learned from all this?
  • Do the CDF data show a real or just a “visual” excess at high ET?
    • depends critically on understanding the systematic errors and their correlations as a function of ET
  • Whether nature has actually exploited the “freedom” to enhance gluon distributions at large x will only be clear with the addition of more data
    • with 2fb-1 in Run II, the reach will extend a further 50-100 GeV in ET which should make the asymptotic behavior clearer
  • whatever the Run II data show, this has been a useful lesson:
    • parton distributions have uncertainties, whether made explicit or not
    • we should aim for a full understanding of experimental systematics and their correlations

It’s a good thing

forward jets
DØ inclusive cross sections up to || = 3.0

Comparison with JETRAD usingCTEQ3M,  = ETmax/2

  • 0.0    0.5
  • 0.5    1.0
  • 1.0    1.5
  • 1.5    2.0
  • 2.0    3.0

d2 (dET d) (fb/GeV)

  • 0.0    0.5

DØ Preliminary

ET (GeV)

  • 1.5    2.0
  • 0.5    1.0

Data - Theory / Theory

DØ Preliminary

DØ Preliminary

  • 1.0    1.5
  • 2.0    3.0

DØ Preliminary

DØ Preliminary

ET (GeV)

ET (GeV)

Forward Jets

DØ Preliminary

triple differential dijet cross section
Triple differential dijet cross section


Trigger Jet


Can be used to extract or constrain PDF’s

Beam line


Probe Jet ET>10 GeV

0.1<|h|<0.7, 0.7<|h|<1.4, 1.4<|h|<2.1, 2.1<|h|<3.0

At high ET, the same behaviour as the inclusive

cross section, presumably because largely the same events

DØ: same side (1 ~2) and opposite side (1 ~–2) topologies measured up to || = 2.0

Beam line

OS, 0.0    0.5

SS, 0.0    0.5

SS, 1.0    1.5

OS, 1.0    1.5

( Data - Theory ) / Theory

SS, 0.5    1.0

OS, 0.5    1.0

SS, 1.5    2.0

OS, 1.5    2.0

highest e t jet event in d
Highest ET jet event in DØ

ET1 = 475 GeV, h1 = -0.69, x1=0.66

ET2 = 472 GeV, h2 = 0.69, x2=0.66

MJJ = 1.2 TeV

Q2 = 2.2x105 GeV2

extracting s from the jet cross section
Extracting s from the jet cross section

CDF parametrize the NLO cross-section:

Nice demonstration of the evolution of s

But rather large sensitivityto choice of PDF’s and to input s: more of a consistency check than a measurement

Obtained from JETRAD

Measured by CDF

jet cross section ratio 630 1800 gev
Jet cross section ratio 630/1800 GeV
  • DØ and CDF both measure the ratio of scale invariant cross sections ET3/2 d2/dETd vs. xT=ET/s/2 ( 1 in pure parton model)

Not obviously consistent with each other (at low xT) . . . or with NLO QCD (at any xT)

various PDF’s

various scales

suggested explanations
Suggested explanations
  • Different renormalization scales at the two energies
    • OK, so it’s 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 thedata (or even different between theexperiments — DØ?)
ratio of 3 jet 2 jet events at d
Ratio of 3-jet/2-jet events at DØ
  • Plot ratio for various third jet thresholds as a function of HT =  ETjets
  • Note how large the ratio is:

70% of high ET jet events have a third jet above 20 GeV,50% have a third jet above 40 GeV

  • Insensitive to PDF’s

ratio of 3 jet 2 jet events at d1
Can this ratio be predicted by QCD?

Yes, reasonably well even by JETRAD (a leading order prediction of R32)

Can any information be extracted on the best renormalization scale for the emission of the third jet?

Same scale as the first two jets seems better than a scale tied to ET3

 = 0.6 ETmax is pretty good

 = 0.3 HTis best as ET3

Ratio of 3-jet/2-jet events at DØ



quark jets and gluon jets


~ CF = 4/3





~ CA= 3



Quark jets and gluon jets
  • Probability to radiate proportional to color factors:
  • We might then naively expect
  • Instead of counting tracks, look at energy flow:use kT algorithm to find subjets inside jets
    • subjets separated by y = 0.001
  • Compare jets of same (ET,) produced at different  s
    • assume relative q/g content is as given by MC (= 33% g at 630 GeV, 59% g at 1800 GeV) and q/g jet multiplicities do not depend on  s
quark and gluon subjet multiplicities
DØ Preliminary

kT algorithm

D=0.5, ycut= 10-3

55 < ET(jet) < 100 GeV

|jet| < 0.5


Gluon Jets


Quark Jets









Subjet Multiplicity

Quark and Gluon Subjet Multiplicities
  • measure M630 = fg630 Mg + (1 – fg630) Mq M1800 = fg1800Mg + (1 – fg1800) Mq
  • Have we glimpsed the holy grail (quark/gluon jet separation)?
    • The real test will be to use subjet multiplicity in (for example) the top  all jets analysis, but unfortunately this will probably have to wait for Run II

Dominant uncertainties come from g jet fraction and jet ET scale

DØ Data


weak boson production
Weak Boson Production
  • O(2) QCD predictions for W/Z production
    • (pp  W + X) B(W  )
    • (pp  Z + X) B(Z  )
  • QCD in excellent agreement with data
    • so much so that it has been seriously suggested to use W as the absolute luminosity normalization in future

Note: CDF luminosity normalization is 6.2% higher than DØ (divide CDF cross sections by 1.062 to compare with DØ)

d p t z measurement
DØ pTZ measurement
  • Phys. Rev. D61, 032004 (2000)

Low pT (< 10 GeV)

resum large logarithms

of mW2/pT2 and include

nonperturbative parameters

extracted from the data


Fixed Order




Ladinsky & Yuan

Ellis & Veseli and

Davies, Webber & Stirling


not quite as good a

description of the data

Large pT (> 30 GeV)

perturbative calculation

w jet measurements
W + jet measurements
  • DØ used to show a W+1jet/W+0jet ratio badly in disagreement with QCD. This is no longer shown (the data were basically correct, but there was a bug in the DØ version of the DYRAD theory program).
  • CDF measurements of W+jets cross sections agree well with QCD:
  • alas, no sensitivity to s

W+1 jet/W


W+n jets

vs. LO QCD

(various scales)

isolated photon cross sections
New DØ PRL 84 (2000) 2786

New CDF preliminary

±14% normalization

statistical errors only

Isolated photon cross sections

QCD prediction is NLO by Owens et al.

what s happening at low e t
What’s happeningat low ET?
  • Gaussian smearing of the transverse momenta by a few GeV can model the rise of cross section at low ET (hep-ph/9808467)

“kT” from soft gluon emission

kT = 3.5 GeV

Even larger deviations from QCD

observed in fixed target (E706)

Again, Gaussian smearing (~1.2 GeV here) can account for the data


3.5 GeV

  • Predictive power of Gaussian smearing is small
    • e.g. what happens at LHC? At forward rapidities?
  • The “right way” to do this should be resummation of soft gluons
    • as we have seen, this works nicely for W/Z pT

Laenen, Sterman, Vogelsang,


Catani et al. hep-ph/9903436

Threshold + recoil


looks promising



Fixed Order

Threshold resummation: does

not model E706 data very well

contrary viewpoints
Aurenche et al., hep-ph/9811382: NLO QCD (sans kT) can fit all the data with the sole exception of E706“It does not appear very instructive to hide this problem by introducing an extra parameter fitted to the data at each energy”


Contrary viewpoints


Aurenche et al.



is it just the pdf
NewIs it just the PDF?
  • New PDF’s from Walter Giele can describe the observed photon cross section at the Tevatron without any kT:

CDF (central)

DØ (forward)

Blue = Giele/Keller set

Green = MRS99 set

Orange = CTEQ5M and L

photons final remarks
Photons: final remarks
  • For many years it was hoped that direct photon production could be used to pin down the gluon distribution through the dominant process:
  • Theorist’s viewpoint (Giele):
    • “... discrepancies between data and theory for a wide range of experiments have cast a dark spell on this once promising cross section … now drowning in a swamp of non-perturbative fixes”
  • Experimenter’s viewpoint: an interesting puzzle
    • kT remains a controversial topic
    • experiments may not all be consistent
    • resummation has proved disappointing so far (though the latest results look better)
    • new results only increase the mystery
      • is it all just the PDF’s?
b production at the tevatron
b production at the Tevatron
  • b cross section at CDF and at DØ
  • Data continue to lie ~ 2  central band of theory




Cross section vs. |y|

pT > 5 GeV/c

pT > 8 GeV/c


bb correlations
bb correlations
  • CDF rapidity correlations DØ angular correlations
  • NLO QCD does a good job of predicting the shapes of inclusive distributions and correlations, hence it’s unlikely that any exotic new production mechanism is responsible for the higher than expected cross section
d b jet cross section at higher p t
DØ b-jet cross section at higher pT

Differential cross section Integrated pT > pTmin


varying the scale from 2μO toμO/2, where μO = (pT2 + mb2)1/2





DØ b-jets (using highest QCD prediction)

- 0.5

CDF photons  1.33

DØ photons

b-jet and photon production compared

DØ b-jets

Data – Theory/Theory

Photon or b-jet pT (GeV/c)

b production summary
b production summary
  • Experimental measurements at Tevatron are all consistent and are all several times higher than the QCD prediction
    • factor of ~ 2 at low rapidity
    • factor of ~ 4 at high rapidity
  • Note that the same magnitude of excess is now seen in b-production at HERA and in  collisions at LEP2
  • Modifications to theory improve agreement but do not fix
  • New measurement at higher pT: jets from DØ
    • better agreement above about 50 GeV
    • shape of data–theory/theory is similar to photons
  • The same story? (whatever that is)
t production at the tevatron
t production at the Tevatron
  • CDF 1999 result: (175 GeV) = 6.5 +1.7 –1.4 pb
  • DØ PRD 60 (1999) 012001: (172 GeV) = 5.9 ± 1.7 pb
  • Excellent agreement between data and theory
    • let no one say that we can’t calculate heavy quark production (provided the quark is heavy enough!)
  • In Run II, top could become a nice laboratory for QCD
things we can look forward to
Things we can look forward to
  • More data — the next decade belongs to the hadron colliders
  • Improved calculations (NNLO calculations, resummations...)
  • PDF’s with uncertainties (or a technique for the propagation of PDF uncertainties) as implemented by Giele, Keller, and Kosower
    • see pdf.fnal.gov and Walter’s presentation
    • we won’t get excited unnecessarily by things like the high ET jet excess (if there is one)
    • but imposes significant work on the experiments
      • understand and publish all the errors and their correlations
  • Better jet algorithms
    • CDF and DØ accord for Run II from recent workshop
    • kT will be used from the start
jet algorithms
Effect of pileup on Thrust

kT algorithm jets, ET > 30 GeV


Jet Algorithms
  • Experimental desires
    • high efficiency, low biases
    • minimize sensitivity to noise, pileup, negative energies
    • computationally efficient (may be an issue for kT)
  • Theoretical desires
    • “infrared safety is not a joke!”
    • avoid ad hoc parameters like Rsep
  • Can the cone algorithm be made acceptable?
    • e.g. by modification of seed choices
    • or with a seedless algorithm?
  • Many variations of kT exist — choose one and fully define it

Additional seed

“Midpoint cone”

some other things i would like
Some other things I would like
  • Theoretical and experimental effort to understand the underlying event
    • don’t subtract it out from jet energies
      • it’s an inconsistent treatment of the event
      • the 1800/630 GeV jet data may indicate problems with our usual assumption that the underlying event is ~ a minbias event
    • would also allow a consistent treatment of double parton scattering (where more than one pair of partons in the same two colliding nucleons undergoes a hard interaction)
    • There are very nice new results from CDF on the underlying event
  • A consistent approach to hard diffraction
    • a high ET jet production process: should be amenable to perturbative calculation
    • we need to break down the walls of the “pomeron ghetto”
  • Tevatron QCD measurements have become precision measurements
    • no longer testing QCD, now testing our ability to make precise predictions within the framework of QCD
    • the state of the art is NNLO calculations, NLL resummations
  • … but this level of precision demands considerable care both from the experimentalists and the phenomenologists, in understanding —
    • jet algorithms
    • jet calibrations
    • all the experimental errors and their correlations
    • the level of uncertainty in PDF’s
  • In general our calculational tools are working very well; the open issues generally relate to
    • pushing calculations closer to the few GeV scale (b’s? low-ET photons?)
    • PDF uncertainties (high ET jets, photons?)