Download - 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.
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QCD Tests in Lepton-Proton Collisions, DIS2000 2
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 phot
on () 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
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QCD Tests in Lepton-Proton Collisions, DIS2000 3
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
eesQ in 22giving the lengthof the current regionphase space
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QCD Tests in Lepton-Proton Collisions, DIS2000 4
• 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
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QCD Tests in Lepton-Proton Collisions, DIS2000 5
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.
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QCD Tests in Lepton-Proton Collisions, DIS2000 6
• 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 ET
2 !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
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QCD Tests in Lepton-Proton Collisions, DIS2000 7
• 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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• 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
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gluon density at high scalewhere the cross section is stable
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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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“Resolved” in virtual
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Forward jet
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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
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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
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• 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
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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
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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
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• Now with higher integrated luminositycross 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
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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
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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 pr
oduction – prompt photon p
roduction
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.
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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.
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• 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
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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 !
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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 sof
t 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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QCD Tests in Lepton-Proton Collisions, DIS2000 22
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
MVQ2
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QCD Tests in Lepton-Proton Collisions, DIS2000 23
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
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• 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
tVpp W
W
New result fromZEUS, talk in WG4
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QCD Tests in Lepton-Proton Collisions, DIS2000 25
• 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
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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
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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 ?
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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
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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
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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 ?
)(n larger thamuch )'( ,' ,)',( xgxgxxxxgA
Martin, Ryskin, Teubner Frankfurt, McDermott, Strikman
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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
e
p
e
p
New result from H1: cross section measurementH1+ZEUS talk WG4
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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 o
k.– Measuring excited states (', ', ')
• In general theories are advanced– We foresee greater precision in coming year thanks to HE
RA’s larger int. lumi– we should provide sensible measurements !
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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
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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
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• 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
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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
ee'
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
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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
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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 ?)
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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