Low x Physics at the LHeC: DIS with E e =70GeV and E p =7TeV P.Newman, Birmingham [hep-ex/ , 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 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 Q 2 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  dipole. Just change the way in which the wavefunctions  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/ , … 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 Q 2 > 1GeV 2, x < 0.01 Only versions with saturation work for < Q 2 < 1 GeV 2 Similar conclusions from final state studies

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

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

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 (1 o ) … but luminosity less important, so dedicated alternative set-up possible?

Example F 2 with LHeC Data (Jeff Forshaw) (10 fb -1 ) (1 o acceptance) Precise data in THERA region - Cleanly establish Saturation at Q2 values where partonic language unquestionably applicable -Distinguish Between models of saturation Statistical precision <0.1%, systematics 1-3% HERA

Example 2: Interpreting Geometric Scaling Stasto, Golec-Biernat, Kwiecinski, hep-ph/   *p (  only),  = Q 2 R 0 2 (x) and R 0 2 (x) is “saturation radius” Change of behaviour near  =1 often cited as evidence For saturating But data below  =1 are very low Q 2 – 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

Geometric Scaling at the LHeC LHeC reaches  =0.15 for Q 2 =1 GeV 2 and  =0.4 for Q 2 =2 GeV 2 Some (though limited) Acceptance for Q 2 <Q 2 s with Q 2 “perturbative’’ HERA Limit for Q 2 >2 GeV 2

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 with Q 2 >1 become available (e.g. from LHeC) Alternative View (Avsar, Gustafson, Lonnblad) hep-ph/ ,

LHeC Comparison with Predictions 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 HERA

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

Example of DVCS at LHeC (Jeff Forshaw) (10 fb -1, stat errors only) HERA (1 o 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

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

Example HERA DPDF Results (linear z scale) Quark densities well Understood over a wide Range to z~ (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 Q 2 ) Known very poorly at high Z (q  qg dominates Q 2 Evolution)

LHeC Diffractive Kinematics Factorisation tests: DPDFs extracted at HERA predict LHeC cross section at moderate /large , higher Q 2. New dynamics: LHeC opens new low  region – parton saturation, BFKL etc showing up first in diffraction? Large Diff. Masses: Z production, studies of new 1 -- states DGLAP

LHeC Simulation Statistical precision Not an issue Big extensions to Lower x IP … cleaner Separation of The diffractive exchange Higher Q 2 at fixed , x IP  CC (and z in NC) allows flavour Decompositions of DPDFs Lower  at fixed Q 2, x IP

Example F 2 D with LHeC Large Rapidity Gap method assumed. Statistical precision ~0.1%, systematics ~5% (Jeff Forshaw) (10 fb -1 ) HERA (1 o acceptance) Diffractive Structure function Unknown for  <~ 0.01 … large Extrapolation Uncertainties. Plenty to learn from LHeC, Including the Proper way to Saturate a qqbarg dipole

Factorisation tests done at HERA with gluon initiated jet / charm processes… BUT … kinematically restricted to high  region where F 2 D is least sensitive to the gluon! kinematically restricted to low p T, where Scale uncertainties are large. Surprises and confusion in gp  what happens to gap survival at lower z … cf Totem etc? Final States in Diffraction Jets in  p Jets in DIS Charm in DIS

Final States in Diffraction at the LHeC At LHeC, diffractive masses M x up to hundreds of GeV can be produced with low x IP Low , low x IP 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 (x IP <0.05) (RAPGAP simulation) (ep  eXp)

Diffractive Detector Considerations Accessing x IP = 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 may provide recyclable technology. Dedicated studies needed!

Forward Jets 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  *… Conclusions limited by Kinematic restriction to High x (>~ ) and detector acceptance. LHeC can tackle both … see more emissions due to longer ladder, more instrumentation  lower x where predictions Really diverge.

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

With AA at LHC, LHeC is also an eA Collider LHeC extends by orders of magnitude towards lower x. With wide range of x, Q 2, 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 Rich physics of nuclear parton densities. Limited x and Q 2 range so far (unknown For x<~10 -2 and Q 2 > 1 GeV 2 )

Simple Model of Gluon Saturation Saturation point when xg(x) ~ Q 2 /  s (Q 2 ) Nuclear enhancement of gluon density a A 1/3 ccc Compare extrapolated (NLO) gluon density from HERA Saturation point reached in ep at LHeC for Q 2 <~ 5 GeV 2 Reached in eA for much higher Q 2

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!

Summary 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!

Back ups and Rejects follow

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, 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 e p

DPDFs and the LHC b, W H IP p’ p p IP b 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!

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

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

LHeC Context Combining the LHC protons with an electron beam is natural next step in pushing the frontiers of ep physics: small resolved dimensions, high Q 2 and low x Can be done without affecting pp running Latest of several proposals to take ep physics into the TeV energy range … … but with unprecedented lumi!

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

F2

Heavy Flavour Constraints for LHC At Q 2 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 … F 2 b from H1 Si

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