QCD Tests in Lepton-Proton Collisions

26/04/2000

QCD Tests in Lepton-Proton Collisions, DIS2000 1

QCD Tests in Lepton-Proton Collisions

• QCD tests in DIS and large pT photoproductio

n(non-diffractive processes)

– High-pT jets in DIS

– “Forward excess” and virtual photon structure

– Jets in photoproduction

– Heavy quark production

• QCD tests in diffractive processes

– Vector meson production

– Hadronic final state in diffraction

8th International Workshop on Deep Inelastic Scattering and QCD (DIS2000)

April 26th, 2000Yuji Yamazaki

KEK-IPNS (DESY F1J)

“Collisions”: Fixed target experiments are not covered.

26/04/2000


QCD tests in hard-scatteringep collisions

The QCD test “shopping list” in DIS and photoproduction (PHP) processes

• Scaling violation in the structure function – the ultimate QCD test at HERA.

More comes from Hadronic Final State (HFS) study.

• DIS with 2 jets – O(s) or higher s determination– Gluon density g(x, Q2) [especially in low Q2]– Studies of higher order QCD dynamics (jet shape,

3 jets etc.)– Forward jet as a signal of BFKL dynamics and/or virtual ph

oton () structure function

• Particle production, fragmentation (strange, charm)

• Heavy Quark (HQ) production

• Photoproudction jets and HQ production

– Low ET: Real photon pdf at low x

Soft Underlying Event (SUE)

– High ET: Photon pdf at high x, gluon density at high xp

Test of QCD dynamics

In Mike’s talk

26/04/2000


Introduction to DIS HFS

• Difference from e+e– : Treatment of the “ladder” from p to Multi-scale Q2, ET

• Development: Dijet definition infrared safe (not discussed)Inclusive-k algorithm in Breit frame (pp like)

Trivial Lowest Order (QPM)

In Breit frame

• Cross section F2 • Final state – quark dominated

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26/04/2000


• Inclusive-k algorithm in Breit frame –

longitudinal invariant cone-type, high ET, O(s) process

• Thus defined jets agree with NLO well for high ET

AND high Q2 region

Inclusive jet cross section and s

proceed to extract s

26/04/2000


S uncertainty from r

• Main uncertainty comes from renormalization scale ET , Q2

• Obtained s compatible

• The size of the uncertainty depends on the choice of the scale

smaller scale uncertainty for r = ET than Q2.

ET is the preferred theory here

Is this a good enough reason to choose ET ?

Let’s see the cross section behavior.

26/04/2000


• Dijet cross section (ET1 > 8 GeV, ET2 > 5 GeV in Breit frame) is measured as a function of ET

2/Q2 .

• ET2/Q2 dependence well reproduced by r = Q2

r = ET also OK for high ET2/Q2

• ET2 underestimates the cross section for large ET

2/Q2.

• The scale uncertainty is larger for Q2 than ET2 (not shown)

Dijet xsection: ET2/Q2 dependenc

e

The theoretically stable r may not reproduce the data

ZEUS preliminary

H1, ZEUS talk WG2

Here the natural

scale is ET2 !

NLO misses something in low Q2?

New measurement “phase space scanning” in HFS variables:See also ET, Q2 and jet for inclusive cross section H1 talk in WG2

26/04/2000


• Using jet cross sections for obtaining s: Need to estimate the uncertainty from pdf.

Here the propagationof input s to the outputis studied by H1.

• ZEUS has estimated the gluon density uncertainty estimated from a global F2 fitting using scaling violation.

s measurement: pdf uncertainty

H1+ZEUS talk inWG2, new measurementand progress report

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26/04/2000


• Dijet event: reconstruction of initial gluon momentum

through

• Benefit: gluon densityin relatively high x

• Again the choice of the scalemay give large difference.

• Low- excess = relation to the “forward excess”

– BFKL footprint

– virtual photon structure function

Measurement of g(x, Q2) using dijet events

)1( 22 QMxJJBj

gluon density at high scalewhere the cross section is stable

26/04/2000


Introduction to SF and BFKL effect on DIS HFS

DGLAP evolution “direct”

CCFM (LDCMC)

Q2 >> ET2

LEPTO, HERWIGNLO programs

Q2 ET2

Forward excess

• No MC available• No prediction with jet finder

Q2 << ET2 : QCD evolution

from photon• Forward excess• Low-x events

MC: RAPGAPNLO: JetViP

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BFKL evolution

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“Resolved” in virtual

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Forward jet

26/04/2000


The structure at high Q2

• “DIS” cross section is measured as a function of xOBS

• Clear evidence of resolved events over LO calculation(NLO not sufficient – not shown)

• contribution decreases as Q2 becomes higher

Hig

her Q

2

Higher ET2

SF exists even at high Q2 for the high ET events

H1 talk WG2

26/04/2000


SF as an explanation ofthe forward excess

• Forward region: sensitive to BFKL signal

• Plugging the SF in:increases the forward(= low xBj) cross sectionand explains the data

• Cross section as a function of ET

2/Q2:Both LO MC (RAPGAP)and NLO (JetViP)explain the shape

A solution for the forward excess.Is this entire story ?

“direct” “resolved”

BFKL

26/04/2000


• Forward (1.5 < < 3.1) 0 production at low-x (x ~ 10–

4)• For very low Q2 (2.0 < Q2 < 4.5) : SF estimates too l

ow• The modified BFKL (~ NLO) prediction is higher and clo

ser to the data

Virtual photon SF, is it enough ?

New measurement on jets, H1 talk in WG2

• NLO agrees with data for most of the phase space

• However: Large excess of data in low ET, low Q2, forwa

rd

• NNLO calculation (or BFKL, resolved photon?) awaited

26/04/2000


Azimuthal asymmetries in hadron production and FL

• The twice oscillation ( |cos 2 |) comes fromthe longitudinal contribution ( boson-gluon fusion)

Semi-direct measurement of FL at HERA, consistent with QCD expectation.

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ZEUS talk WG2

26/04/2000


Jets in Photoproduction

• Study on jets in photoproduction during first years

– Very large excess in low-ET cross section

– The excess mostly in the resolved process

– Large “pedestal” around jet in ET flow

Mainly attributed to Soft-Underlying-Events (SUE).

• Amount of the “jet pedestal” is measured

• The effect is expected to be smaller at higher ET …

talk WG2

26/04/2000


• Now with higher integrated luminosity

cross section measurement with higher ET is possible.

Very high ET PHP – test of QCD

NLO gives a good description of PHP dijets at high ET

26/04/2000


Very high ET PHP – photon SF

• Closer look to the cross section:A large cross section excess in central-forward rapidity (in Lab frame) over NLO.

• xOBS spectrum at high ET (> 2

5 GeV)The excess persists !

• Natural explanation: photon pdf in high x regionPHP at HERA is sensitive

to photon pdf in high x

Can’t we measure low x = gluon ?

talk WG1/2/3

26/04/2000


Low ET PHP: revisited. How we should proceed ?

• SUE bothers in measuring the “parton level” cross section

• One way:“subtract” the jet pedestal from SUE (estimated by MC)extract the LO parton density

• Another way: “less sensitive to SUE” method– single particle p

roduction – prompt photon

production

talk WG2

• Measuring the low-x pdf with large error

• Method indirect by subtracting the SUE effect.Any way out or we should forget ?

• For theorist: need also “uncorrected” xsectionas a starting point.

26/04/2000


Open charm production

• Now forward excess not related to SUE:

Charm shows also “Forward excess”.

• Both in DIS and PHP.

ZEUS 1996-97 ZEUS 1996-97

• For PHP: small x events resolved (virtual) photon

again ?“Massless” calculation inclu

de resolved effect (large ET/mc)

ZEUS talk WG2

• Comparison of xOBS with

massless NLO is awaited.• For DIS, associated jet x

O

BS should be checked.

26/04/2000


• Large excess is observed over LO/NLO

• Is this again due to “massless” effect ?(LO simulation: factor 2 increase)

• The reason is unknown, we need statistics to see the distribution differentially.

Open beauty production

talk WG2

26/04/2000


Summary on HFS in DIS/PHP

• Many issues are special for (*)p collisions

• Tremendous amount of understanding in “how to measure”

– Improved jet algorithm (inclusive-k in p or Breit)

– Infrared safe definition of the dijet cross section– Stable NLO calculations, improved MCs

• For most phase space the data agree with NLO

– High Q2 (> 100 GeV2), high ET (also in PHP), backward

– For these “good regions” : we can measure S, g(x), photon pdf in high x …

• Still data excess in many place: low Q2 and ET, forward jets.The NLO scale uncertainty also large (also choice of scale)– Important terms are missing in NLO

• Virtual photon SF, BFKL, NNLO ?– Similar effect in charm ? Massless solves this ? Necessary step to measure e.g. g(x, Q2) in low Q2

• Q2 = 0 suffers from SUE: any way out ?

We are near the completion, just need theoretical input !

26/04/2000


Introduction to QCD tests in diffractive processes

• Diffraction without a hard scale can be explained by an universal Pomeron Diffractive physics is traditionally classified as a so

ft phenomena. • The diffraction with a hard scale at Tevatron, LEP and

HERA has opened an wide range of QCD tests.

1. Soft hard: how is the transition of the underlying mechanism ?

2. How the hard scales play a role ?2 = f (Q2, t [, MV

2]) – are they similarly effective ?

Soft Pomeron

Hard scattering of“Hard Pomeron”and (virtual) photon 2-gluon exchange

and higher order

We could do only with these

pQCD modelof VM

Investigating by HFS and vector meson production

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26/04/2000


Introduction to quasi-elastic Vector Meson (VM) production

Soft production mechanism – VDM

Good description forno hard scale process(e.g. light VM PHP at HERA)Slow rise of the cross section

Hard process

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EM current conservation,SU(4) prediction

wave function

| g(x,Q2) |2 fast rise

Forming VMmuch after dipole collision

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26/04/2000


Elastic VM: W dependence in PHP

• Light VM (, , ): slow rise ~ 0.22 P(0) ~ 1.08

consistent with universal Pomeron• J/ : fast rise hard scale is

given by mc2 (= 2 GeV2, not so large!).

Now final (points not updated)

1

'4 0 b

26/04/2000


• A lot smaller shrinkage than the soft Pomeron if hard scale

In high-Q2, photon is small the blowup of the hadron size at high W cannot be detected by a small object– another evidence of being a hard process.

Shrinkage in J/ PHP

MV provides a hard scale.

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dependence From

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W

New result fromZEUS, talk in WG4

26/04/2000


• Q2 : expected to be another hard scale.• Light VM – no hard scale from mV : see p

• Result: – It rises more than soft Pomeron at Q2 > 1 GeV2

– Approaching to J/ only Q2 ~ 10 GeV2 ??? We don’t know.

– W slope slower than W2 (2 – 2) (Regge expectation)consistent with inclusive diffraction

VM: W dependence in *p

H1 PHPJ/

Need precise measurement

from P parametrization determined by inclusive cross section

H. Abramowicz W

softPomeron

mc2

26/04/2000


Q2 dependence: b-slope, L/T

• Expectation in b-slope:Soft: ~ 10 Ge

V–2

Hard: ~ 4 GeV–2

• b as a function of Q2:Slow approach, only at ~ 20 GeV–2 to the asymptotic value ?

L/T Q2/MV2 in naïv

e LO calculation

• Develops a lot slower than a linear rise

• Three models: all compatible to the data.

Need precise measurement

Again need precise measurement for distinguishing models

soft

hard

26/04/2000


Cross section ratio and SU(4) prediction – Q2 dependence

• The cross section ratio of the four VMs : : : J/ = 9:1:2:8

assuming EM current conservation (flavor independence).

• The ratio for / is badly broken for Q2 ~ 0• flat for

– mass effect ?

26/04/2000


VM ratio: is Q2 the right scale ?• Restoration of SU(4) at high Q2, what’s going on in low

Q2?• Universal behavior by taking Q2+MV

2 as a kinematical scale.

Compiled by B. Clerbaux

H1 talk: new result on in WG4 (also covers the topics for next pages)

Again: we want to havea bit more precision

26/04/2000


t-dependence and SU(4)

• Large t events (t > 1 GeV2) from proton dissociation.

• Light VM:no satisfactory explanation by soft+hard

• Charmonium is again described by pQCD – mc seems enough to give a hard scale

• The ratio of light VM follows SU(4) for t > 1 GeV2 t affects differently from Q2

26/04/2000


Heavy VM: cross section

• Surprisingly higher cross sections than LO theories

• Two theoretical models: Martin et al., Frankfurt et al.

– Both incorporate the skewed parton density(SPD) effect

– Many other effects can also raise the cross section [relativistic correction, real part of the amplitude, NLO corrections, Fermi momentum effect etc.]

• Seems we need SPD. Can’t we measure it directly ?

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Martin, Ryskin, Teubner Frankfurt, McDermott, Strikman

26/04/2000


DVCS

• DVCS – the cleanest VM production

• No uncertainty from the wave function of VMs• Large mass from the virtual photon, x1 x2

able to measure SPD• Interference with BH : the real part of the amplitude.• ZEUS has observed the signal …

x1 x2

LO diagram

Bethe-Heitler (BH) diagramAnalogy toVM production

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p

e

p

New result from H1: cross section measurementH1+ZEUS talk WG4

26/04/2000


Summary on Vector Mesons• Testing ground of pQCD by VM with a hard scale

• Current hypotheses are:

– Hard mechanism given by Q2, MV or t.

– A dipole scatters with p, then forms VM long time after– SPD seems necessary for heavy particles

• Are all these scenario true ? The status of measurement is– Data not precise enough to test models

[W dependence, b-slope, L/T]

– The role of Q2, MV or t as a hard scale seems different. Q2 may be MV , but MV in PHP in W dependence ? t seems different

• Way out – isolating non-perturbative wave function (WF) – DVCS: no uncertainty on WF effect, full pQCD

First glance: The data agrees with the predictionIdeas on DVCS mechanism seems

ok.– Measuring excited states (', ', ')

• In general theories are advanced– We foresee greater precision in coming year thanks to H

ERA’s larger int. lumi– we should provide sensible measurements !

26/04/2000


Detecting partonic mechanism in diffraction by HFS

• The transition from soft to hard Pomeron is seen in:– VM production [W, b-slope, t-distribution etc.] – Inclusive diffraction [W dependence steeper: howev

er the signal is not very clear]• Factorized Pomeron or pure pQCD ?

– Both models describe the F2D data well

Energy flow in P frame (LPS tagging)• Two jet structure in high MX, but not too aligned

– Indication of BGF diagram, gluonic Pomeron

Direct investigation by HFS and heavy flavor production

26/04/2000


HFS event shape in diffraction• Colourless system: analogy to e+e– final state ?

Thrust and sphericity

Diffraction final state is close to e+e–, but slightly broader

Transverse momentum in the “target region”

• Non-diffraction – soft spectrum in p

T

• Hard radiation from Pomeron

26/04/2000


• Charm production: diagrams limited, hard scale provided. Ideal measurement, but small statistics.

• Dijet production – partonic structure, gluon in P.Just Pomeron+resolved photon seems perfect ?

• Three jet structure: proving information ofpartonic dynamics.

HFS in diffraction: more to come

H1+ZEUS WG4

ZEUStalk WG4

H1 talk WG4

3jets

26/04/2000


A Remark on “Universal Pomeron”

• At HERA (Collins)Proton and photon vertex factorize

• At Tevatron: soft gluon exchangebetween two protons (SUE effect !)Reduction of gap survival probability

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X

Factorization

p

p

X

Soft gluon exchange

• Is the Pomeron flux universal ?• Is this the right explanation ? Do

we understand the soft exchange ?

An open question

• Large rapidity gap rate is a lot smaller in Tevatron

– 10-20 % at HERA, 1-3 % at Tevatron

26/04/2000


Remarks on the luminosity and detector issues on QCD tests

• After upgrade: we lose the forward detectors.

– Tagging diffraction by LRG will be limitedFatal for the most of diffraction study

– Very forward hadron measurement will be difficult for studying both diffraction and HFS

• However: ~ 100pb–1 data by Sep 2000 !Statistical error can be improved by factor 2

– Most of the studies so far are with 5-30 pb–1

e.g. at Q2 ~ 4 GeV2 are still statistically limited

• For diffractions after upgrade: H1 FPS in the cold sectionHigh acceptance at xL ~ 0.97 (talk in WG4)

Analysing current data can answer many questions

Data until September 2000: last chance for many analyses

26/04/2000


Conclusions

• The QCD study in ep collision in last years has been investigated as a collision of (*) and p

• Here the study is more complicated than we thought

– The incoming “hadron” = photon varies its size (Q2)

However this gives us a rich testing ground of pQCD.

Non-diffraction

• Tremendous amount of progress in understanding NLO calculations, jet algorithms etcFake problems are gone

• The excess of the cross section is being identified

(low Q2, low ET, forward jets etc.)

• Qusestion: how these excess can be explained ?

– Need a bit more investigation, especially in theoryInteresting by itself, but also for extracting importa

nt quantities at low Q2 and ET ( g(x, Q2), s ?)

26/04/2000


Conclusions(2)

Diffraction and Vector Mesons

• The observations in last years have set the direction of to go: we know what to measure for the moment.

• Q: Is the proposed pQCD mechanism of VM production

valid ? How the hard scales (Q2, t, mV) play role ?

– Need more precision measurements

• Q: Partonic mechanism of the inclusive diffraction ?

– New analyses on HFS and HQ production may give some hints, may not. Let’s see

• Large luminosity now precise data (hopefully)

Promising future, challenge for experimentalist

Documents

QCD Tests in Lepton-Proton Collisions