low x physics at the lhec: dis with e e =70gev and e p =7tev p.newman, birmingham [hep-ex/0603016,...
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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
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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!
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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 …
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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)
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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
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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
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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
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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!
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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?
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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
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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
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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
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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
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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
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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
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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
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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)
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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)
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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
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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
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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
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• 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
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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)
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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!
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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.
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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.
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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)
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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
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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!
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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!
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Back ups and Rejects follow
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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
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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!
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Overview of LHeC ParametersOverview of LHeC Parameters
e accelerator similar to LEP … FODO structure with 376 cells @ 60m (LEP 290 cells)
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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
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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!
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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
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F2F2
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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
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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!