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QCD Tests in Lepton-Proton Collisions. QCD tests in DIS and large p T photoproduction (non-diffractive processes) High- p T jets in DIS “Forward excess” and virtual photon structure Jets in photoproduction Heavy quark production QCD tests in diffractive processes Vector meson production

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QCD Tests in Lepton-Proton Collisions


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qcd tests in lepton proton collisions
QCD Tests in Lepton-Proton Collisions
  • QCD tests in DIS and large pT photoproduction(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, 2000

Yuji Yamazaki

KEK-IPNS (DESY F1J)

“Collisions”: Fixed target experiments are not covered.

QCD Tests in Lepton-Proton Collisions, DIS2000

qcd tests in hard scattering ep collisions
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 photon () 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 xpTest of QCD dynamics

In Mike’s talk

QCD Tests in Lepton-Proton Collisions, DIS2000

introduction to dis hfs
Introduction to DIS HFS

Trivial Lowest Order (QPM)

In Breit frame

  • 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)

giving the length

of the current regionphase space

  • Cross section  F2
  • Final state – quark dominated

BGF

gluondensity

QCDC

quarkdensity

QCD Tests in Lepton-Proton Collisions, DIS2000

inclusive jet cross section and s
Inclusive jet cross section and s
  • Inclusive-k algorithm in Breit frame – longitudinal invariant cone-type, high ET, O(s) process
  • Thus defined jets agree with NLO well for high ETAND high Q2 region

 proceed to extract s

QCD Tests in Lepton-Proton Collisions, DIS2000

s uncertainty from r
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 scalesmaller 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.

QCD Tests in Lepton-Proton Collisions, DIS2000

dijet xsection e t 2 q 2 dependence
Dijet xsection: ET2/Q2 dependence
  • Dijet cross section (ET1 > 8 GeV, ET2 > 5 GeV in Breit frame) is measured as a function of ET2/Q2 .
  • ET2/Q2 dependence well reproduced by r = Q2r = ET also OK for high ET2/Q2
  • ET2 underestimates the cross section for large ET2/Q2.
  • The scale uncertainty is larger for Q2 than ET2 (not shown)

 H1, ZEUS talk WG2

ZEUS preliminary

Here the natural scale is ET2 !NLO misses something

in low Q2?

The theoretically stable r may not reproduce the data

  • New measurement “phase space scanning” in HFS variables:

See also ET, Q2 and jetfor inclusive cross section H1 talk in WG2

QCD Tests in Lepton-Proton Collisions, DIS2000

s measurement pdf uncertainty
s measurement: pdf uncertainty
  • 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 F2fitting using scaling violation.
  • H1+ZEUS talk in

WG2, new measurement

and progress report

QCD Tests in Lepton-Proton Collisions, DIS2000

measurement of g x q 2 using dijet events
Measurement of g(x, Q2) using dijet events
  • 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 

gluon density at high scalewhere the cross section is stable

QCD Tests in Lepton-Proton Collisions, DIS2000

introduction to sf and bfkl effect on dis hfs
Introduction to  SF and BFKL effect on DIS HFS

DGLAP evolution “direct”

Q2 >> ET2

LEPTO, HERWIG

NLO programs

CCFM (LDCMC)

BFKL evolution

  • Q2ET2
  • Forward excess
  • No MC available
  • No prediction with jet finder

“Resolved” in virtual 

  • Q2 << ET2 : QCD evolutionfrom photon
  • Forward excess
  • Low-x events
  • MC: RAPGAP
  • NLO: JetViP

QCD Tests in Lepton-Proton Collisions, DIS2000

the structure at high q 2

Higher Q2

Higher ET2

The  structure at high Q2
  • “DIS” cross section is measured as a function of xOBS
  • Clear evidence of resolved events over LO calculation(NLO not sufficient – not shown)
  • contribution decreases as Q2 becomes higher

 H1 talk WG2

 SF exists even at high Q2 for the high ET events

QCD Tests in Lepton-Proton Collisions, DIS2000

sf as an explanation of the forward excess

“direct”

“resolved”

BFKL

 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 ofET2/Q2:Both LO MC (RAPGAP)and NLO (JetViP)explain the shape

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

QCD Tests in Lepton-Proton Collisions, DIS2000

virtual photon sf is it enough
Virtual photon SF, is it enough ?
  • Forward (1.5 <  < 3.1) 0production at low-x (x ~ 10–4)
  • For very low Q2 (2.0 < Q2 < 4.5) :  SF estimates too low
  • The modified BFKL (~ NLO) prediction is higher and closer to the data

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

QCD Tests in Lepton-Proton Collisions, DIS2000

azimuthal asymmetries in hadron production and f l
Azimuthal asymmetries in hadron production and FL
  • The twice oscillation ( |cos 2 |) comes fromthe longitudinal contribution ( boson-gluon fusion)

 ZEUS talk WG2

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

QCD Tests in Lepton-Proton Collisions, DIS2000

jets in photoproduction
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

QCD Tests in Lepton-Proton Collisions, DIS2000

very high e t php test of qcd
Very high ET PHP – test of QCD
  • Now with higher integrated luminositycross section measurement with higher ET is possible.

NLO gives a good description of PHP dijets at high ET

QCD Tests in Lepton-Proton Collisions, DIS2000

very high e t php photon sf
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.
  • xOBS spectrum at high ET (> 25 GeV)The excess persists !
  • Natural explanation: photon pdf in high xregion

PHP at HERA is sensitive to photon pdf in high x

Can’t we measure low x = gluon ?

 talk WG1/2/3

QCD Tests in Lepton-Proton Collisions, DIS2000

low e t php revisited how we should proceed
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 production
    • prompt photon production
  • 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.

 talk WG2

QCD Tests in Lepton-Proton Collisions, DIS2000

open charm production
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 include resolved effect (large ET/mc)

  • Comparison of xOBS with massless NLO is awaited.
  • For DIS, associated jet xOBS should be checked.

 ZEUS talk WG2

QCD Tests in Lepton-Proton Collisions, DIS2000

open beauty production
Open beauty production
  • 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.

 talk WG2

QCD Tests in Lepton-Proton Collisions, DIS2000

summary on hfs in dis php
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 !

QCD Tests in Lepton-Proton Collisions, DIS2000

introduction to qcd tests in diffractive processes
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 soft phenomena.

  • The diffraction with a hard scale at Tevatron, LEP and HERA has opened an wide range of QCD tests.
  • Soft  hard: how is the transition of the underlying mechanism ?
  • How the hard scales play a role ?2 = f (Q2, t [, MV2]) – are they similarly effective ?

Hard scattering of“Hard Pomeron”

and (virtual) photon

2-gluon exchangeand higher order

Soft Pomeron

pQCD modelof VM

We could do only with these

Investigating by HFS and vector meson production

QCD Tests in Lepton-Proton Collisions, DIS2000

introduction to quasi elastic vector meson vm production

EM current conservation,SU(4) prediction

wave function

MV

Q2

Forming VMmuch after dipole collision

t

| g(x,Q2) |2 fast rise

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

QCD Tests in Lepton-Proton Collisions, DIS2000

elastic vm w dependence in php
Elastic VM: W dependence in PHP
  • Light VM (, , ): slow rise  ~ 0.22  P(0) ~ 1.08consistent with universal Pomeron
  • J/ : fast rise hard scale is given by mc2 (= 2 GeV2, not so large!).

Now final (points not updated)

QCD Tests in Lepton-Proton Collisions, DIS2000

shrinkage in j php
Shrinkage in J/ PHP
  • A lot smaller shrinkage than the soft Pomeron if hard scaleIn 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.
  • New result from

ZEUS, talk in WG4

MV provides a hard scale.

QCD Tests in Lepton-Proton Collisions, DIS2000

vm w dependence in p
VM: W dependence in *p
  • 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

from P parametrization determined by inclusive cross section

H. Abramowicz

H1 PHP

J/

soft

Pomeron

mc2

Need precise measurement

QCD Tests in Lepton-Proton Collisions, DIS2000

q 2 dependence b slope l t
Q2 dependence: b-slope, L/T
  • Expectation in b-slope:

Soft: ~ 10 GeV–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 naïve LO calculation
  • Develops a lot slower than a linear rise
  • Three models: all compatible to the data.

soft

hard

Need precise measurement

Again need precise measurement for distinguishing models

QCD Tests in Lepton-Proton Collisions, DIS2000

cross section ratio and su 4 prediction q 2 dependence
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 ?

QCD Tests in Lepton-Proton Collisions, DIS2000

vm ratio is q 2 the right scale
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+MV2 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

QCD Tests in Lepton-Proton Collisions, DIS2000

t dependence and su 4
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

QCD Tests in Lepton-Proton Collisions, DIS2000

heavy vm cross section
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 ?

Martin, Ryskin, Teubner

Frankfurt, McDermott, Strikman

QCD Tests in Lepton-Proton Collisions, DIS2000

slide31
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 …

Bethe-Heitler (BH) diagram

Analogy to

VM production

LO diagram

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

QCD Tests in Lepton-Proton Collisions, DIS2000

summary on vector mesons
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 prediction Ideas on DVCS mechanism seems ok.
    • Measuring excited states (', ', ')
  • In general theories are advanced
    • We foresee greater precision in coming year thanks to HERA’s larger int. lumi
    • we should provide sensible measurements !

QCD Tests in Lepton-Proton Collisions, DIS2000

detecting partonic mechanism in diffraction by hfs
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: however the signal is not very clear]
  • Factorized Pomeron or pure pQCD ?
    • Both models describe the F2Ddata well

Direct investigation by HFS and heavy flavor production

  • Energy flow in P frame (LPS tagging)
  • Two jet structure in high MX, but not too aligned
    • Indication of BGF diagram, gluonic Pomeron

QCD Tests in Lepton-Proton Collisions, DIS2000

hfs event shape in diffraction
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 pT
  • Hard radiation from Pomeron

QCD Tests in Lepton-Proton Collisions, DIS2000

hfs in diffraction more to come
HFS in diffraction: more to come
  • 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.

 H1+ZEUS WG4

 H1 talk WG4

3jets

 ZEUStalk WG4

QCD Tests in Lepton-Proton Collisions, DIS2000

a remark on universal pomeron
A Remark on “Universal Pomeron”
  • Large rapidity gap rate is a lot smaller in Tevatron
    • 10-20 % at HERA, 1-3 % at Tevatron
  • At HERA (Collins)Proton and photon vertex factorize
  • At Tevatron: soft gluon exchangebetween two protons (SUE effect !)Reduction of gap survival probability

p

  • Is the Pomeron flux universal ?
  • Is this the right explanation ? Do we understand the soft exchange ?
  • An open question

X

p

Soft gluon exchange

QCD Tests in Lepton-Proton Collisions, DIS2000

remarks on the luminosity and detector issues on qcd tests
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–1e.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)

Data until September 2000: last chance for many analyses

Analysing current data can answer many questions

QCD Tests in Lepton-Proton Collisions, DIS2000

conclusions
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 important quantities at low Q2 and ET ( g(x, Q2), s ?)

QCD Tests in Lepton-Proton Collisions, DIS2000

conclusions 2
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

QCD Tests in Lepton-Proton Collisions, DIS2000