Low x Physics at the LHeC: DIS with E e =70GeV and E p =7TeV

Low x Physics at the Low x Physics at the LHeC: DIS withLHeC: DIS withEEee=70GeV and =70GeV and

EEpp=7TeV =7TeV

P.Newman, Birmingham

[hep-ex/0603016,JINST 1 (2006) P10001]

Thanks to E Avsar, J Dainton, M Diehl, M Klein, L Favart, J Forshaw, L Lonnblad,A Mehta, E Perez, G Shaw, F Willeke

Klein

Perez

Thistalk

ContentsContents

• What and Where is Low x Physics?

• The LHeC in overview

• Low x Detector Considerations

• Some first case studies:- Establishing new low x dynamics (F2, F2c, F2b, dipoles)- Diffractive DIS- DVCS- Forward Jets- eA- A long list of things I missed!

The Birth of Experiental Low x PhysicsThe Birth of Experiental Low x Physics

• Biggest HERA discovery: strong increase of quark density(F2) and gluon density (d F2 / d ln Q2) with decreasing x innewly explored regime.

Low x, `large’ Q2 is high density, low coupling limit of QCD …

Low x Physics ctdLow x Physics ctd

We have learned a lot about its properties…… from RHIC and Tevatron data as well as HERA …… but many questions are not fully answered…• Are non-DGLAP parton evolution dynamics Visible in the initial state parton cascade?• How and where is the parton growth with Decreasing x tamed as required by unitarity (parton saturation)?… barely(if at all) separated from confinement region• How is the large (~ constant?) fraction of diffraction related to the inclusive rate?

They are unanswered since low x is Kinematically correlated to low Q2, which brings problems (partonic pQCD language breaks down just where x values get interesting)

Offer a description in the most interesting low x region,Where Q2 is small and partons are not appropriate Degrees of freedom and the language of PDFs breaks down.

Added bonus: simple unified picture of many inclusive andDiffractive processes … all strong interaction physics liesIn the (universal) dipole cross section dipole. Just change the way in which the wavefunctions appear!

Reminder : Dipole modelsReminder : Dipole models

(qqbar-g dipolesAlso needed toExtend descriptionTo inclusive diffraction)

2, 2 2 , 2* *( , ) d d ( , , ) ( , , )T L T Lp dipolex Q z r z r Q x r z

An Example Dipole Approach to HERA DataAn Example Dipole Approach to HERA Data

Forshaw, Sandapen, Shawhep-ph/0411337,0608161… used for illustrationshere

Fit inclusive HERA dataWith dipole models Containing variousAssumptions.

FS04 Regge (~FKS): 2 pomeron model, no saturationFS04 Satn: Simple implementation of saturationCGC: Colour Glass Condensate version of saturation

All three models can describe data with Q2 > 1GeV2, x < 0.01Only versions with saturation work for 0.045 < Q2 < 1 GeV2

Similar conclusions from final state studies

LHeC Inclusive LHeC Inclusive KinematicsKinematics

s 1.4 V Te

710 at x 1.4 eV TW

70 GeV eE 7 TeVpE

2 2 1 GeVQ

(5 x HERA)

Unprecedented lumi = 1033 cm-2 s-1 !!!

• Extension to higherQ2 in x range coveredBy HERA

• Extension of low x(high W) frontier

LHeC Low x Kinematics and Electron LHeC Low x Kinematics and Electron DetectorsDetectors

• Without focusing,Acceptance to179o gives access to Q2=1 For all x… below 10-6!

2 modes considered:

• Focusing Magnet To optimise lumi … detector acceptance to 170o … not much Acceptance Below Q2=100

INCREDIBLELOW x

MACHINE!

Hadronic Final State Detector Hadronic Final State Detector ConsiderationsConsiderations

• Considerably more asymmetric than HERA!-Hadronic final state at the newly accessed lowest xValues goes central or backward in the detector - At x values typical of HERA (but large Q2), hadronic final state is boosted more in the forward direction.

• Full Study of low x / Q2 and of range overlapping withHERA, also of energy flow in outgoingproton direction require more (1o) … but luminosity less important, so dedicated alternative set-up possible?

Example FExample F22 with LHeC Data with LHeC Data

(Jeff Forshaw)(10 fb-1)

(1o acceptance)

Precise data in THERA region- Cleanly establishSaturation at Q2 values where partonic languageunquestionably applicable-DistinguishBetween models of saturation

Statistical precision <0.1%, systematics 1-3%

HERA

Example 2: Interpreting Geometric Scaling Example 2: Interpreting Geometric Scaling

Stasto, Golec-Biernat,Kwiecinski, hep-ph/0007192

*p( only), = Q2 R02(x) and

R02(x) is “saturation radius”

Change of behaviour near=1 often cited as evidenceFor saturating

But data below =1 are very low Q2 – various other effects and theoretical Difficultiesassociated with Confinement / change to hadronic dof’s

To reach a consensus, need to see transition in a Q2Region where we can unambiguously interpret partonically

Geometric Scaling at the LHeC Geometric Scaling at the LHeC

LHeC reaches =0.15 for Q2=1 GeV2

and =0.4 forQ2=2 GeV2

Some (thoughlimited)Acceptance forQ2<Q2

s with Q2

“perturbative’’

HERALimit for

Q2>2 GeV2

In framework of Linked Dipole Chain Model … Change in behaviour due To finite quark masses as well as saturation via multiple interactions and `swing’ mechanism’ (recouplings in dipole chain)Predict breaking of scaling for <1 if data withQ2>1 become available (e.g. from LHeC)

Alternative View (Avsar, Gustafson, Alternative View (Avsar, Gustafson, Lonnblad)Lonnblad)

hep-ph/0610157,0702087

LHeC Comparison with PredictionsLHeC Comparison with Predictions

HERA

‘1 pom only’ already ruled out at HERADistinguishing need for swing mechanism requires highest W and lowest Q2 at HERA. – Clean separation at LHeC

HERA

DVCS Kinematic DVCS Kinematic RangeRange

… can be tackled asAt HERA Through anInclusive Selection of Ep epand statistical Subtraction Of Bethe Heitler background

(Laurent Favart)

BH

DVCS

Example of DVCS at LHeCExample of DVCS at LHeC

(Jeff Forshaw)

(10 fb-1, stat errors only)

HERA

(1o acceptance)

StatisticalPrecision 1-4%

ClearlyDistinguishesDifferent modelsWhich containSaturation.

Interpretation inTerms of GPDsMuch cleaner atLarger Q2 valuesAccessed

VMs similar story

Diffractive DIS at HERADiffractive DIS at HERA

• Parton-level mechanism and relation to diffractive pp scattering, inclusive DIS, confinement still not settled

• Diffractive parton densities (DPDFs)Should be universal to diffractive DIS (i.e. apply to both HERA and LHeC) and can be used toPredict pp with additional `gapSurvival factors’

`discovery’ atHERA (~10% of low x events are ep -> eXp)

Example HERA DPDF Results (linear z Example HERA DPDF Results (linear z scale)scale)• Quark densities well

Understood over a wideRange to z~10-2. (known to ~5%)

• But most tests of Factorisation and requiredPredictions require theGluon density.

• Known to ~15% at low zFrom ln Q2 dependence,(small lever-arm in Q2)

• Known very poorly at highZ (qqg dominates Q2

Evolution)

LHeC Diffractive LHeC Diffractive KinematicsKinematics

• Factorisation tests: DPDFs extracted at HERA predict LHeC cross section at moderate /large , higher Q2.

• New dynamics: LHeC opens new low region – partonsaturation, BFKL etc showing up first in diffraction?

•Large Diff. Masses: Z production, studies of new 1-- states

DGLAP

LHeC LHeC SimulationSimulationStatistical precision

Not an issue

Big extensions toLower xIP … cleanerSeparation ofThe diffractiveexchange

Higher Q2 at fixed, xIP CC (andz in NC) allows flavourDecompositions of DPDFs

Lower at fixed Q2,xIP

Example FExample F22DD with LHeC with LHeC

Large Rapidity Gap method assumed.Statistical precision ~0.1%, systematics ~5%

(Jeff Forshaw)

(10 fb-1)

HERA

(1o acceptance)

DiffractiveStructure functionUnknown for<~ 0.01 … largeExtrapolationUncertainties.

Plenty to learn from LHeC, Including theProper way toSaturate a qqbarg dipole

• Factorisation tests done at HERA with gluon initiated jet / charm processes… BUT …• kinematically restricted to high region where F2

D is least sensitive to the gluon!• kinematically restricted to low pT, where Scale uncertainties are large.• Surprises and confusion in gp what happens to gap survival at lower z … cf Totem etc?

Final States in DiffractionFinal States in Diffraction

Jets in p

Jets in DIS

Charm in DIS

Final States in Diffraction at the LHeCFinal States in Diffraction at the LHeC• At LHeC, diffractive massesMx up to hundreds of GeVcan be produced with low xIP

• Low , low xIP region for jets and charm accessible

• Final state jets etc at higher pt … much more precise factorisationTests and DPDF studies (scale uncty)

• New diffractive channels …beauty, W / Z bosons

• Unfold quantum numbers /precisely measure exclusively produced new / exotic 1– states

(xIP<0.05)

(RAPGAP simulation)

(ep eXp)

Diffractive Detector ConsiderationsDiffractive Detector Considerations

• Accessing xIP = 0.01 with rapidity gap method requires max cut around 5 …forward instrumentation essential!

• Roman pots, FNC should clearly be an integral part

• The work going on in this community (Totem, FP420 …) already tells us a lot about what is (not) achievable and mayprovide recyclable technology. Dedicated studies needed!

Forward JetsForward JetsLong HERA program to understand parton cascade emissions by direct observation of jet patternin the forward direction. … DGLAP v BFKL v CCFM vresolved *…

Conclusions limited byKinematic restriction to High x (>~ 2.10-3) and detector acceptance.

LHeC can tackle both … see more emissions due to longer ladder, more instrumentation lower x where predictionsReally diverge.

Beauty as a Low x Observable!!!Beauty as a Low x Observable!!!

(Jeff Forshaw)

(10 fb-1)

HERA

(10o acceptance)

Statistical errors20-80%Systematics~5%

F2c and F2sAlso measured(to betterStatistical Precision) seeMax Klein’s talk.

With AA at LHC, LHeC is also an eA With AA at LHC, LHeC is also an eA ColliderCollider

• LHeC extends by orders of magnitude towards lower x.

• With wide range of x, Q2, A, opportunity to extract andunderstand nuclear parton densities in detail

• Symbiosis with ALICE, RHIC, EIC … disentangling Quark Gluon Plasma from shadowing or parton saturation effects

• Rich physics ofnuclear parton densities.

• Limited x and Q2

range so far (unknownFor x<~10-2 and Q2 > 1 GeV2)

Simple Model of Gluon SaturationSimple Model of Gluon Saturation• Saturation point when xg(x) ~ Q2 / s(Q2)• Nuclear enhancement of gluon density a A1/3 ccc• Compare extrapolated (NLO) gluon density from HERA

• Saturation point reached in ep at LHeC for Q2 <~ 5 GeV2

• Reached in eA for much higher Q2

Unmentioned TopicsUnmentioned TopicsThis talk contained an (embarrassingly) limited number ofStudies, which only scratches the surface of the low x Physics potential of the LHeC.

Some obvious omissions:- Lots of eA physics!- All sorts of low x jet measurements- All sorts of low x charm measurements- Prompt photons- Photoproduction and photon structure- Leading neutrons and other semi-inclusives- Exclusive vector meson production

… studies of these and many other topics are very welcome,In order fully to evaluate the physics case for such afacility!

SummarySummaryTo further pursue low x physics with unpolarised targets, the natural next step is an extension to lower x (i.e. higher energy)

For its relative theoretical cleanliness, ep should be a largeFeature of this.

For its enhanced sensitivity to high parton densities, eAShould also be a large part of the programme.

All of this is possible in the framework of the LHC - a totally new world of energy and luminosity! Why not exploit It for lepton-hadron scattering

First conceptual design exists … no show-stopper so farSome encouraging first physics studies shown here.

Much more to be done to fully evaluate physics potentialand determine optimum running scenarios!

Back ups and Rejects follow

LHeC Basic Principle LHeC Basic Principle • On timescale of LHC upgrades

• ep in parallel with standard pp operation

• Proton beam parameters fixed by LHC

• 70 GeV electron beam, compromisingbetween energy and synchrotron (0.7 GeV loss per turn)

Superconducting RF cavities then consume 50MW for Ie=70mA

New detector possibly replaces LHCb at end of their programme?

Electron beam by-passes other experiments via existing survey tunnels

e

p

DPDFs and the LHCDPDFs and the LHC

b, W

b, WH

IP

p’p

p p’

IP

bb

• It’s the gluon and S2 we need!

• At LHeC, DPDFs and theoreticalmodels can be tested in detail and possibly contribute to discoverypotential

e.g. Searches for `exclusive’Higgs production at the LHCrely on understanding backgroundfrom inclusive diffraction and of`gap survival probability’ inhadronic diffraction

Tested in inclusive diffractionand diffractive jet production at HERA! – LHeC goes way beyond!

Overview of LHeC ParametersOverview of LHeC Parameters

e accelerator similar to LEP … FODO structure with 376 cells @ 60m (LEP 290 cells)

Interaction RegionInteraction Region

Top view

2 mrad

Non-colliding p beamVertically displaced

• Matching electron and protonbeam shapes and sizes determines* x emittance for electron beam

• High luminosity requires low quadrupoles close to interactionpoint (1.2 m)

• Fast separation of beams withtolerable synchrotron power requires finite crossing angle

• 2 mrad angle gives 8 separation atfirst parasitic crossing

• Resulting loss of luminosity (factor 3.5)partially compensated by “crab cavities” … -> 1033 cm-2 s-1

LHeC ContextLHeC Context

• Combining the LHC protonswith an electron beam isnatural next step in pushingthe frontiers of ep physics:small resolved dimensions, high Q2 and low x • Can be done without affecting pp running

Latest of several proposals totake ep physics into the TeV energy range …… but with unprecedented lumi!

Overview of Physics MotivationsOverview of Physics Motivations-New Physics in the eq Sector leptoquarks, RP violating SUSY, quark compositeness

-The Low x Limit of Quantum Chromodynamics high parton densities with low coupling parton saturation, new evolution dynamics

-Quark-Gluon Dynamics and the Origin of Mass confinement and diffraction

-Precision Proton Structure for the LHC essential to know the initial state precisely! including heavy flavour (b), gluon

-Nuclear Parton Densities eA with AA -> partons in nuclei, Quark Gluon Plasma

F2F2

Heavy Flavour Constraints for LHCHeavy Flavour Constraints for LHC

• At Q2 values of LHC and LHeC, charm and beauty important

• Crucial for understanding initial state of many new processes (e.g. bbbar->H) and background rates.

• Precise knowledge available from ep …

F2b

fromH1 Si

Dipole Approach to HERA Data ctdDipole Approach to HERA Data ctd

Dipole formalism allows predictions of many other Observables …

DVCSF2c, vector meson production,F2D (with qqg dipole included),DVCS

Same story emerges throughout: no clear evidence forSaturation … so the story hinges on Q2<1 in the inclusiveData!