John Womersley

QCD at the TevatronQCD at the TevatronCurrent results and future prospects Current results and future prospects

John WomersleyFermilab

Fifth International Symposium on Radiative Corrections (RADCOR 2000)Carmel, CA, September 2000

http://d0server1.fnal.gov/users/womersley/radcor2000.ppt

John Womersley

OutlineOutline

• 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

John Womersley

The Fermilab Tevatron ColliderThe Fermilab Tevatron Collider

1992-95 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

Main Injector(new)

Tevatron

DØCDF

Chicago

p source

Booster

CDF DØ

John Womersley

Hadron-hadron collisionsHadron-hadron collisions

• 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

fragmentation

partondistribution

partondistribution

Jet

Underlyingevent

Photon, W, Z etc.

Hard scattering

ISR FSR

John Womersley

A high-EA high-ETT event at CDF event at CDF

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John Womersley

1pb 87

Ldt1

pb 87

Ldt

0.1 jet 0.7

1pb 20

Ldt1

pb 20

Ldt

Jet cross sections at Jet cross sections at s = 1.8 TeVs = 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

DØjet 0.5

John Womersley

What’s happening at high EWhat’s happening at high ETT??

NB Systematic errors not plotted

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”

Figure 1: “The horse is dead”

John Womersley

The DØ and CDF data agreeThe 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)

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John Womersley

Jet data with latest CTEQ5 PDF’sJet data with latest CTEQ5 PDF’s

• CDF data • DØ data

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John Womersley

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

John Womersley

Forward JetsForward Jets

• DØ inclusive cross sections up to || = 3.0

• Comparison with JETRAD usingCTEQ3M, = ET

max/2

0.5 1.0

0.0 0.5

1.0 1.5

ET (GeV)

DØ PreliminaryDØ Preliminary

DØ PreliminaryDØ Preliminary

DØ PreliminaryDØ Preliminary

1.5 2.0

2.0 3.0

DØ PreliminaryDØ Preliminary

DØ PreliminaryDØ Preliminary

ET (GeV)

Data

- T

heory

/ T

heory

ET (GeV)

d2 (

dET

d) (

fb/G

eV) 0.0 0.5

0.5 1.0 1.0 1.5 1.5 2.0 2.0 3.0

DØ PreliminaryDØ Preliminary

John Womersley

Triple differential dijet cross sectionTriple differential dijet cross section

Can be used to extract or constrain PDF’s

At high ET, the same behaviour as the inclusivecross section, presumably because largely the same events

211

3

dddE

dT

Beam line

Trigger Jet0.1<||<0.7

Probe Jet ET>10 GeV0.1<||<0.7, 0.7<||<1.4, 1.4<||<2.1, 2.1<||<3.0

1

2

John Womersley

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

( D

ata

- T

heor y

) /

Th

eor y

SS, 0.0 0.5 OS, 0.0 0.5

SS, 0.5 1.0 OS, 0.5 1.0

SS, 1.0 1.5 OS, 1.0 1.5

SS, 1.5 2.0 OS, 1.5 2.0

Beam line

John Womersley

Tevatron jet data can constrain PDF’sTevatron jet data can constrain PDF’s

Tevatron

HERA

FixedTarget

John Womersley

Highest EHighest ETT jet event in DØ jet event in DØ

ET1 = 475 GeV, 1 = -0.69, x1=0.66ET2 = 472 GeV, 2 = 0.69, x2=0.66

MJJ = 1.2 TeVQ2 = 2.2x105 GeV2

John Womersley

Extracting Extracting ss from from the jet cross sectionthe jet cross section

)(),()()( 3,

22

TFRSTFRST

EBEAddE

d

CDF parametrize the NLO cross-section:

Nice demonstration of the evolution of s

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

Obtained from JETRAD

2TE

FR

Measured by CDF

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0089.0

0078.00001.01129.0)M( ZS

John Womersley

Jet cross section ratio 630/1800 GeVJet cross section ratio 630/1800 GeV

• DØ and CDF both measure the ratio of scale invariant cross sections ET

3/2 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

John Womersley

Suggested explanationsSuggested 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?)

John Womersley

Ratio of 3-jet/2-jet events at DØRatio of 3-jet/2-jet events at DØ

• Plot ratio for various third jet thresholds as a function of HT = ET

jets

• 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

DØ

John Womersley

Ratio of 3-jet/2-jet events at DØRatio of 3-jet/2-jet events at DØ

• 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 HT is best as ET3

DØ

’or

John Womersley

Quark jets and gluon jetsQuark 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

~ CF = 4/3q qg 2

ggg ~ CA= 3

2

49

CC

~tymultiplicijet quarktymultiplicijet gluon

n

nr

F

A

q

g

John Womersley

Quark and Gluon Subjet MultiplicitiesQuark and Gluon Subjet Multiplicities

04.091.11M

1MR

q

g

DØ Data

04.086.1R HERWIG 5.9

Quark Jets

Gluon Jets

Subjet Multiplicity

jetsjets dN

dM

N

1 kT algorithmD=0.5, ycut= 10-3

55 < ET(jet) < 100 GeV|jet| < 0.5

DØ Preliminary

1 2 3 4 5

0.1

0.2

0.3

0.4

0.5

Dominant uncertainties come from g jet fraction and jet ET

scale

• measure M630 = fg630

Mg + (1 – fg630) Mq

M1800 = fg1800

Mg + (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

John Womersley

Weak Boson ProductionWeak 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

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Note: CDF luminosity normalization is 6.2% higher than DØ (divide CDF cross

sections by 1.062 to compare with DØ)

John Womersley

DØ pDØ pTTZZ measurement measurement

• Phys. Rev. D61, 032004 (2000)

Data–Theory/TheoryFixed Order

NLO QCD

Data–Theory/TheoryResummed

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Ellis & Veseli andDavies, Webber & Stirling(Resummed)

not quite as good adescription of the data

Low pT (< 10 GeV)resum large logarithms of mW

2/pT2 and include

nonperturbative parameters extracted from the data

Large pT (> 30 GeV)perturbative calculation

John Womersley

W + jet measurementsW + 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

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W+1 jet/W vs. NLO QCD

W+n jetsvs. LO QCD(various scales)

John Womersley

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Isolated photon cross sectionsIsolated photon cross sections• New DØ PRL 84 (2000) 2786

QCD prediction is NLO by Owens et al.

±14% normalizationstatistical errors only

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John Womersley

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What’s happeningWhat’s happening at low Eat low ETT??

• 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

PYTHIA

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

John Womersley

ResummationResummation

• 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

Catani et al. hep-ph/9903436

Threshold resummation: doesnot model E706 data very well

Threshold + recoilresummation:looks promising

Thresholdresummation

Fixed Order

Laenen, Sterman, Vogelsang, hep-ph/0002078

John Womersley

Contrary viewpointsContrary 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”

E706

Ouch!

Aurenche et al.vs.

E706

John Womersley

Is it just the PDF?Is 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)

New

Blue = Giele/Keller setGreen = MRS99 setOrange = CTEQ5M and L

John Womersley

Photons: final remarksPhotons: 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?

John Womersley

b production at the Tevatronb production at the Tevatron

• b cross section at CDF and at DØ

• Data continue to lie ~ 2 central band of theory

b

B

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central forward

Cross section vs. |y|pT > 5 GeV/c

pT > 8 GeV/c

John Womersley

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bb correlationsbb 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

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John Womersley

DØ b-jet cross section at higher pDØ b-jet cross section at higher pTT

Differential cross section Integrated pT > pTmin

varying the scale from 2μO to μO/2, where μO = (pT

2 + mb2)1/2

New

John Womersley

b-jet and photon production b-jet and photon production comparedcompared

0

- 0.5

0.5

1.0

1.5

DØ b-jets (using highest QCD prediction)CDF photons 1.33DØ photons

Data

– T

heory

/Th

eory

Photon or b-jet pT (GeV/c)

DØ b-jets

John Womersley

b production summaryb 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)

John Womersley

t production at the Tevatront 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

John Womersley

Things we can look forward toThings 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

John Womersley

Jet AlgorithmsJet 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 “Midpoint cone”

Additional seed

DØ MC

Effect of pileup on ThrustkT algorithm jets, ET > 30 GeV

John Womersley

Some other things I would likeSome 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”

John Womersley

ConclusionsConclusions

• 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?)