Brian Foster - HQ physics @ HERA II 1 Heavy Quark Physics @ HERA II What’s new for HERA II and what does it mean for HQ physics – a reminder.. Brian Foster

Brian Foster - HQ physics @ HERA II

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Heavy Quark Physics@ HERA II

What’s new for HERA II and what does it meanfor HQ physics – a reminder..

Brian FosterOxford University

Cornell05/13/05

HQs in DIS, diffraction, photoproduction

Spectroscopy – is there a role for HERA II?

Production mechanisms

Single t production?

Summary


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HERA & experiments

HERA @DESY in Hamburg


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HERA II PhysicsBoth ZEUS & H1 have made major upgrades in order toutilise the increase in HERA luminosity to the full.


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Vertex RegionThe ZEUS MVD mostly 3 layers in barrel and 4forward wheels; > 200K readout channels.


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Vertex RegionH1 Si – 2 very thin CST layers; 5 disks covering 8 < < 17o


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Forward PhysicsZEUS major upgrade in forward direction replacement of TRD’s with two stations of straw-tube chambers, each with 3 stereo layers.

H1 have madeimprovementsto various partsof their trackingsystems.


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HERA II prospectsSo optimistically HERA II promised factor 5 increase inluminosity, with lepton polarisation, greatly improvedtracking, DAQ and triggering.

This implies generally order of magnitude improvementin statistics and increased kinematic range and coverage.

The reality has been somewhat different. Major problems with backgrounds delayed then achievement of the designgoals by almost 2 years - only now are we reaching, sometimes, 1pb-1 per day.

What does all this mean for the physics reach of HERA II?


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Low-x structure function datae(k) e'(k')

*(q)

p(P)

xPW2

Q2

s = k+P = energy in the ep c.m.s.

Q2 = -(k-k')2 = -q2 = virtuality of the exchanged

x = Q2/(2P • Q) = fraction of proton momentumcarried by the struck quark

y = (P• q)/(P• k) = fraction of beam lepton energytransferred to the photon

W 2 = ys ~ Q2/ x energy in the *p c.m.s.

Q2 = xys

Kinematics


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e p

Inside the proton

eq

e

pp remnant

e

e

qpp remnant


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Inside the proton

At higher and higherresolutions, the quarksemit gluons, which also emit gluons, which emit quarks, which…….

Heisenberg’s UPallows gluons, and qqpairs to be produced for a very short time.

Low Q2 (large Medium Q2 (medium

Large Q2 (short

At highest Q2, ~ 1/Q ~ 10-18 m


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Inside the proton


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F2 data

ZEUS has very precise F2 data over 6 orders of magnitude in (x, Q2).


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Inside the proton – the movie

QuickTime™ and aBMP decompressor

are needed to see this picture.


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Parton evolution

Factorization - hard processes can be regarded as convolutionof “sub-process” cross section with probability to find participating partons in target & probe - subsequenthadronisation ~ independent process

For DIS can (normally) consider virtual photon as -function=> f where is sub-process cross section f is parton dist. function, satisfying f/f P (renormalisation scale, P is a splitting function)


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Parton evolutionIn general Ps are perturbative expansions to particularorders, keeping terms most important for particular regions:

∑ ∑∞

= = ⎥⎥

⎦

⎤

⎢⎢

⎣

⎡+

⎩⎨⎧

⎭⎬⎫

⎟⎠⎞

⎜⎝⎛

⎟⎠

⎞⎜⎝

⎛=0 0

)()( )(1

ln2

),(n

n

m

nm

nm

ns

s xPxx

AxxPπ

αα

)(nPwhere are AP splitting functions

kts become comparable, losing factor ln Q2.

Leading ln Q2 terms come, in axial gauge, from evolution along parton chain strongly ordered in transverse momenta,

LO DGLAP sums up terms - NLO sums

terms which arise when two adjacent


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QCD evolutionIn small x region, leading terms in ln 1/x must be summed independent of Q2. This is done by the BFKL equation.LO (s ln 1/x)n terms arise from strong x ordering:

xn << x(n-1) << x(n-2) …Generally, however, QCD coherence angular ordering -work in unintegrated f(x,kt

2,m2) - 2 hard scales morecomplicated CCFM evolution equation. DGLAP/BFKL two limits of angular ordering. DGLAP, kt/kl, grows since kt grows; in BFKL, grows because kl x falls.


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Dipole ModelsMuch interest in dipole models & saturation.

prest Breit, mom.L.T.

In principle offers unification of inclusive DIS, diffraction

Inclusive F2

1 gexchange

2 g octetexchange

+2 g singletexchange

Diffraction


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HQ in DISSpecialise to HQ structure functions.

There are many theoretical issues with how to includeheavy quarks, in particular the mass thresholds;variations on the MVFN scheme a la Thorne&Robertsare currently favoured and have been examined at NNLO.


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HQ in DISWe will need substantial part of the HERA II statistics toresolve many of the interesting issues in charm in DIS, evenwith substantial improvement in tagging efficiency.

The mechanism to take charm mass into account can givesubstantial mods. to theoretical expectation as function of Q2 athigher x. Intrinsic charm can also modify these predictions but unlikely we can get useful info. on this at HERA II.

Thorne, Roberts1998


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HQ in DISThe D* tagging method in D* → Ks will still be usefulalthough in general multiple scattering increases.

However this will be more than offset by ability to useseparated vertex or large impact parameter tag, as alreadydemonstrated at HERA I by H1.


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HQ in DISHERA I has already told us a lot about the charm quarkdensity inside the proton; very nice new H1 results at DIS05.


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HQ in DISH1 now using the power of their silicon tracker which waspresent in a modified form for the last few years of HERA Irunning - uses impact parameter tagging in 2-D to tag long-lived quarks.


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HQ in DISH1 analysed 57 pb-1 of HERA I data using impact parameter.


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HQ in DIS

The fraction of c ismore or less independent of Q2,whereas that of b isstrongly rising.

Both agree well with the the predictions fromPDFs.

~0.2%~0.2%

~1%~1%

~4%~4%

~20%~20%

~30%~30%

~20%~20%


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HQ in DISSome of the open questions are obvious.


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HQ in DISSome were (much) more obscure.


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HQ in DISFortunately, this seems to have gone away at HERA II.


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HQ in DISThe promise of HERA II is great, since charm productionin DIS is sensitive probe to all sorts of dynamical models.

Saturation modelof Kowalski &Teaney.


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HQ in DIS - HERA IIFirst HERA II results starting to come through.


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HQ in DIS - HERA IIFirst HERA II results starting to come through.


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HQ in DIS

Very precise measurements of F2

C willbe possible for HERA II;also gives accurategluon determination andcross-check with the more global QCD fits.

Accurate b contributionto F2 will becomepossible – cross-check ofVFNS and clean test ofphoton-gluon fusionprocess.

500 pb-1500 pb-1


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HQ in DISHERA II should allow both collaborations to make a full flavour decomposition of the inclusive F2 structure function.For example, charm signal in charged currents (expect ~ 50Kevents) in principle measures the s-quark density (+ leading particles in NC etc.; but also competing non-s diagrams fromgluon splitting in CC).

At HERA II, both singlet & non-singlet pdfs - u,d,s (CC DIS)& c,b,g (NC DIS) - can be determined with good accuracy.

Constrain fit with s fromleading ?


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HQ & PolarisationPolarisation in DIS will be a unique tool @ HERA IIfor exploring EW couplings.

In principle the spin orientation of the struck quarkcan carry through a memory into the final-statehadron.

In c and b production, one can be sure that the struckquark is in one of the c or b hadrons and there is acorrelation between the detected heavy-quark hadronand the struck quark.

Using weak decay of c, can analyse for spin orientation of struck quark and hence disentangle spin-dependent s.


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HQ & Polarisation

But first signal reported forinclusive c productionfrom a substantial fraction of HERA Idata makes it difficult to believe that statistically significant results can be obtained via this technique.


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HQ in DiffractionThe structure of diffraction and its mechanism and explanation in the framework of pQCD is one of the mostfruitful areas of HERA I physics.

In the charm sector, verystrongly limited by statistics.

However, also very gooddiscrimination amongstmodels – one of the areaswhere we will mostbenefit from the statisticsof HERA II.


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HQ in DiffractionCharm production cf inclusive F2 will also be additionalconstraint on gluon pdf of diffractive exchange.In exclusive processes,the heavy c quarkgives clearly differentbehaviour to light quarks – opening the relationship of QCD models of diffraction.One of the areas whereone clearly sees the effectof mc as a hard scale. Again, in all of thesestudies, the statistics ofHERA II will be decisive.


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HQ in PhotoproductionIn general statistics not a problem with photoproductionresults, except where extra requirements on tagging – e.g.double jet tags.

Can use impact parameter tagging to isolate HQ samplein dijet events.

Measured quark fractions: Measured quark fractions: f fudsuds~58%, f~58%, fcc~35% , ~35% ,

ffbb~7%~7%

Visible range: pVisible range: pt t jet1(2)jet1(2) >11(8) GeV >11(8) GeV

Reach almost values expected Reach almost values expected for massless u,d,s,c,b !for massless u,d,s,c,b !


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HQ in PhotoproductionDifferential cross sections can be compared to NLO QCDpredictions.

Higher pHigher pTT reached than for D* reached than for D*

measurementsmeasurements

Excess Data/NLO in Excess Data/NLO in more forward direction more forward direction


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HQ in PhotoproductionStudy of angular distributions in dijet events where one ofthe jets contains a D* can disentangle the dynamics of thec-production process.

In principle, thismethod, with greatlyincreased statisticsat HERA II, can leadto determination ofc (and g) density in.


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HQ in PhotoproductionSince charm taggingworks over a wide rangeof momenta, has ratherhigh efficiency and highpurity, charm is a usefulmethod to look at fragmentation.The study of all thesecharm particles hasallowed the determinationof fragmentation probs.to u,d, ratios of P/V, strangeness suppression etc. to ~5 – 10%. This can certainly be improved at HERA II.


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HQ in Photoproduction

H1 data agrees well withthat from e+e- - notsurprisingly so dothe fragmentationparameters deduced.


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HQ in PhotoproductionZEUS published very similar results some time ago. Moreevidence that fragmentation factorises from the hard production process.


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HQ in PhotoproductionBy looking at eventscontaining leading neutrons and a charmtag, we can get a handle on the c (& g) density in the .

This is a classic “double-tag”experiment whichneeds the increasedstatistics of HERA II(and more!).


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B productionB production bothin photoproduction and DIS is really in its infancy at HERA I and willbe a major studyat HERA II.

Whether there is really a problem with the QCD prediction of (b) will surely have to wait for HERA II.


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Charm spectroscopy in p The large HERA I data sample gave the opportunity to makeuseful contributions to charm spectroscopy.

Rich spectrum - D1 ,D2* established; D*' seen by DELPHI, not OPAL/CLEO.


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Charm spectroscopyHowever HERA II can reach states that other machinescannot reach – e.g. states coupling strongly to gluons canbe copiously produced at HERA.

Not obvious why suchstates should want todecay to charm; but then we didn’t expectto see them in light-quark decayseither.

HERA II can do charmspectroscopy – we shouldlook in those places wherewe are competitive.


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Charm pentaquark?Many experiments have seen the strange pentaquark (1530)(1530)KK00

s s pp ; many more haven’t. Some who did now; many more haven’t. Some who did now

aren’t so sure. aren’t so sure.

H1 have signal fora charmedpentaquark.

Acceptance corr. event yields Acceptance corr. event yields

Unambiguously ruled out by ZEUS. Needs HERA II.


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Production mechanismsAs an electron-proton collider, HERA II has access tounique production mechanisms that can provide strongtests of QCD.


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Interplay with Tevatron can giveextra constraints onrelative importance ofCO & CS; large uncertainties in MEs mean not so obvious that HERA II statisticsvery helpful withoutsignificantly moretheoretical work.

Inelastic J/photoproduction


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One process at leastwhere we know HERA II will be vital, and which may well bea help in unraveling theJ/ problems, is Υ production – at HERA I we barely (?)saw elastic production,let alone inelastic.


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Single top production

Observation of events with high pt lepton, jet and high missing pt sensitive to this process – and lots of otherthings.

HERA cannot produce tt pairs and single productionis highly suppressed in the SM – FCNC process.


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Single top productionAs is well known, there is an excess of such events in H1,although not in ZEUS and not in either experimentin the hadron channel.


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Summary of events seen to date - ZEUS numbersconsistent with background.


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This is perhapsthe clearest andmost importantarea where justthe statistics gain(and the better b tagging) atHERA II willreally paydividends


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HERA II prospectsHeavy quark physics at HERA I has been exceptionallyrich. Not only have we measured production cross sectionsover wide range of Q2, we have made contributions toQCD understanding of heavy quarks, to spectroscopyand to diffraction.

HERA II promises much. There will be factors of ~5 inintegrated luminosity; the tagging efficiency and kinematicreach for heavy quark measurements will increase becauseof the improvements to the detectors in general and the vertexdetectors in particular.

Many areas will benefit – particularly DIS and heavy quarkstructure functions, and everything to do with B production.


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HERA II prospects

There have been problems with some of the upgraded parts of the detectors also.

The ZEUS Straw Tube Tracker is currently switched offbecause insufficient cooling available and the thermal loadon the superconducting coil was causing instabilities. Thismay be fixed during the shutdown coming in November.

The H1 silicon tracker also has problems. Backward and forward detectors removed in summer and being upgradedwith radhard chips. The FST wafers are also being replacedafter damage in a beam accident. They will be reinstalledin December.

The background problems are now solved and HERA isstarting to work well; however a series of bending magnets isprogressively failing and needs replacing in November.


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Summary

There are only 2 years left until HERA will be switched off tobecome a synchrotron radiation facility.

It’s going to be a race against time to achieve all the goals we set in HERA II - but things are now looking good.


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Dipole ModelsExample of this type of model: Golec-Biernat & Wuesthoff predicts

-Q20

Documents

Brian Foster - HQ physics @ HERA II 1 Heavy Quark Physics @ HERA II What’s new for HERA II and what does it mean for HQ physics – a reminder.. Brian Foster