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

[hep-ex/0603016, JINST 1 (2006) P10001] Low x Physics at the LHeC: DIS withEe=70GeV and Ep=7TeV P.Newman, Birmingham Perez 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 This talk

Contents • 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 Physics • Biggest HERA discovery: strong increase of quark density • (F2) and gluon density (d F2 / d ln Q2) with decreasing x in • newly explored regime. • Low x, `large’ Q2 is high density, low coupling limit of QCD …

Low 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 and Diffractive processes … all strong interaction physics lies In the (universal) dipole cross section sdipole. Just change the way in which the wavefunctions Yg appear! Reminder : Dipole models (qqbar-g dipoles Also needed to Extend description To inclusive diffraction)

An Example Dipole Approach to HERA Data Forshaw, Sandapen, Shaw hep-ph/0411337,0608161 … used for illustrations here Fit inclusive HERA data With dipole models Containing various Assumptions. FS04 Regge (~FKS): 2 pomeron model, no saturation FS04 Satn: Simple implementation of saturation CGC: Colour Glass Condensate version of saturation All three models can describe data with Q2 > 1GeV2, x < 0.01 Only versions with saturation work for 0.045 < Q2 < 1 GeV2 Similar conclusions from final state studies

LHeC Inclusive Kinematics (5 x HERA) • Extension to higher • Q2 in x range covered • By HERA • Extension of low x • (high W) frontier Unprecedented lumi = 1033 cm-2 s-1 !!!

LHeC Low x Kinematics and Electron Detectors • 2 modes • considered: • Focusing • Magnet To • optimise lumi • … detector acceptance to 170o … not much • Acceptance Below • Q2=100 INCREDIBLE LOW x MACHINE! • Without focusing, • Acceptance to • 179o gives access • to Q2=1 For all x • … below 10-6!

Hadronic Final State Detector Considerations • Considerably more asymmetric than HERA! • Hadronic final state at the newly accessed lowest x • Values 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 with • HERA, also of energy flow in outgoing • proton direction require more (1o) … but luminosity less important, so dedicated alternative set-up possible?

(1o acceptance) • Precise data in THERA region • Cleanly establish • Saturation at Q2 values where partonic language • unquestionably applicable • Distinguish • Between models of saturation Example F2 with LHeC Data HERA (10 fb-1) (Jeff Forshaw) Statistical precision <0.1%, systematics 1-3%

Example 2: Interpreting Geometric Scaling • sg*p(t only), t = Q2 R02(x) and R02(x) is “saturation radius” Change of behaviour near t=1 often cited as evidence For saturating But data below t=1 are very low Q2 – various other effects and theoretical Difficulties associated with Confinement / change to hadronic dof’s To reach a consensus, need to see transition in a Q2 Region where we can unambiguously interpret partonically Stasto, Golec-Biernat, Kwiecinski, hep-ph/0007192

Geometric Scaling at the LHeC LHeC reaches t=0.15 for Q2=1 GeV2 and t=0.4 for Q2=2 GeV2 Some (though limited) Acceptance for Q2<Q2s with Q2 “perturbative’’ HERA Limit for Q2>2 GeV2

Alternative View (Avsar, Gustafson, Lonnblad) 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 t<1 if data with Q2>1 become available (e.g. from LHeC) hep-ph/0610157,0702087

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

DVCS Kinematic Range … can be tackled as At HERA Through an Inclusive Selection of Ep  epgand statistical Subtraction Of Bethe Heitler background BH DVCS (Laurent Favart)

(1o acceptance) Statistical Precision 1-4% Clearly Distinguishes Different models Which contain Saturation. Interpretation in Terms of GPDs Much cleaner at Larger Q2 values Accessed VMs similar story Example of DVCS at LHeC (10 fb-1, stat errors only) HERA (Jeff Forshaw)

Diffractive DIS at HERA `discovery’ at HERA (~10% of low x events are ep -> eXp) • 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 to • Predict pp with additional `gap • Survival factors’

Quark densities well • Understood over a wide • Range to z~10-2. • (known to ~5%) • But most tests of • Factorisation and required • Predictions require the • Gluon density. • Known to ~15% at low z • From ln Q2 dependence, • (small lever-arm in Q2) • Known very poorly at high • Z (qqg dominates Q2 • Evolution) Example HERA DPDF Results (linear z scale)

DGLAP LHeC Diffractive Kinematics • Factorisation tests: • DPDFs extracted at HERA • predict LHeC cross section • at moderate /large b, • higher Q2. • New dynamics: LHeC opens • new low b region – parton • saturation, BFKL etc • showing up first in diffraction? • Large Diff. Masses: Z production, studies of new 1-- states

Statistical precision Not an issue Big extensions to Lower xIP … cleaner Separation of The diffractive exchange Higher Q2 at fixed b, xIP CC (and z in NC) allows flavour Decompositions of DPDFs Lower b at fixed Q2, xIP LHeC Simulation

(1o acceptance) Diffractive Structure function Unknown for b <~ 0.01 … large Extrapolation Uncertainties. Plenty to learn from LHeC, Including the Proper way to Saturate a qqbarg dipole Example F2D with LHeC (10 fb-1) HERA (Jeff Forshaw) Large Rapidity Gap method assumed. Statistical precision ~0.1%, systematics ~5%

Final States in Diffraction • Factorisation tests done at HERA with gluon • initiated jet / charm processes… BUT … • kinematically restricted to high b region where F2D 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? Charm in DIS Jets in DIS Jets in gp

Final States in Diffraction at the LHeC • At LHeC, diffractive masses • Mx up to hundreds of GeV • can be produced with low xIP • Low b, low xIP region for jets • and charm accessible • Final state jets etc at higher pt • … much more precise factorisation • Tests and DPDF studies (scale uncty) • New diffractive channels … • beauty, W / Z bosons • Unfold quantum numbers / • precisely measure exclusively produced new / exotic 1– states (RAPGAP simulation) (xIP<0.05) (ep  eXp)

Diffractive Detector Considerations • Accessing xIP = 0.01 with rapidity gap method • requires hmax 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 may • provide recyclable technology. Dedicated studies needed!

Long HERA program to understand parton cascade emissions by direct observation of jet pattern in the forward direction. … DGLAP v BFKL v CCFM v resolved g*… Conclusions limited by Kinematic 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 predictions Really diverge. Forward Jets

Beauty as a Low x Observable!!! (10o acceptance) Statistical errors 20-80% Systematics ~5% F2c and F2s Also measured (to better Statistical Precision) see Max Klein’s talk. (10 fb-1) HERA (Jeff Forshaw)

With AA at LHC, LHeC is also an eA Collider • Rich physics of • nuclear parton • densities. • Limited x and Q2 • range so far (unknown • For x<~10-2 and • Q2 > 1 GeV2) • LHeC extends by orders of magnitude towards lower x. • With wide range of x, Q2, A, opportunity to extract and • understand nuclear parton densities in detail • Symbiosis with ALICE, RHIC, EIC … disentangling Quark Gluon Plasma from shadowing or parton saturation effects

Saturation point when xg(x) ~ Q2 / as(Q2) • Nuclear enhancement of gluon density a A1/3 ccc • Compare extrapolated (NLO) gluon density from HERA Simple Model of Gluon Saturation • Saturation point reached in ep at LHeC for Q2 <~ 5 GeV2 • Reached in eA for much higher Q2

Unmentioned Topics This talk contained an (embarrassingly) limited number of Studies, 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 a facility!

To 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 large Feature of this. For its enhanced sensitivity to high parton densities, eA Should 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 far Some encouraging first physics studies shown here. Much more to be done to fully evaluate physics potential and determine optimum running scenarios! Summary

Back ups and Rejects follow

On timescale of LHC upgrades • ep in parallel with standard pp operation • Proton beam parameters fixed by LHC • 70 GeV electron beam, compromising • between 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 LHeC Basic Principle e p

p p’ IP IP p p’ H b, W b b, W b • It’s the gluon and S2 we need! • At LHeC, DPDFs and theoretical • models can be tested in detail and • possibly contribute to discovery • potential • e.g. Searches for `exclusive’ • Higgs production at the LHC • rely on understanding background • from inclusive diffraction and of • `gap survival probability’ in • hadronic diffraction • Tested in inclusive diffraction • and diffractive jet production at • HERA! – LHeC goes way beyond! DPDFs and the LHC

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

Matching electron and proton • beam shapes and sizes determines • * x emittance for electron beam • High luminosity requires low b • quadrupoles close to interaction • point (1.2 m) • Fast separation of beams with • tolerable synchrotron power • requires finite crossing angle • 2 mrad angle gives 8s separation at • first parasitic crossing • Resulting loss of luminosity (factor 3.5) • partially compensated by “crab cavities” … ->1033 cm-2 s-1 Interaction Region Top view Non-colliding p beam Vertically displaced 2 mrad

LHeC Context Latest of several proposals to take ep physics into the TeV energy range … … but with unprecedented lumi! • Combining the LHC protons • with an electron beam is • natural next step in pushing • the frontiers of ep physics: • small resolved dimensions, • high Q2 and low x • Can be done without • affecting pp running

Overview 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

Heavy Flavour Constraints for LHC F2b from H1 Si • 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 …

Dipole Approach to HERA Data ctd Dipole formalism allows predictions of many other Observables … F2c, vector meson production, F2D (with qqg dipole included), DVCS DVCS Same story emerges throughout: no clear evidence for Saturation … so the story hinges on Q2<1 in the inclusive Data!

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