The LHeC Project (Large Hadron-electron Collider) Cyrille Marquet Centre de Physique Théorique Ecole Polytechnique

Contents Accelerator design Detector considerations Small-x and e+A physics Other SM and BSM physics

Accelerator design (slides stolen from P. Newman)

Previously considered as `QCD explorer’ (also THERA) Main advantages: low interference with LHC, high and stageable E e, high lepton polarisation, LC relation? Main difficulties: obtaining high positron intensities, no previous experience exists First considered (as LEPxLHC) in 1984 ECFA workshop Main advantages: high peak lumi, tunnelling (mostly) exists Main difficulties: building round existing LHC, e beam energy and lifetime limited by synchrotron radiation LINAC-RING RING-RING How to do DIS using the LHC ? while allowing simultaneous ep(eA) and pp(AA) running

design constraint: power < 100 MW  E e = cm -2 s -1 Two 10 GeV linacs, 3 returns, 20 MV/m Energy recovery in same structures [CERN plans energy recovery prototype] ep Lumi ~ cm -2 s -1 corresponds to ~10 fb -1 per year (~ 100 fb -1 total) eD and eA collisions have always been integral to programme e-nucleon Lumi estimates ~ (10 32 ) cm -2 s -1 for eD (ePb) Baseline Design (electron Linac)

Design parameter summary electron beamRRLR e- energy at IP[GeV] luminosity [10 32 cm -2 s -1 ] polarization [%]4090 bunch population [10 9 ] e- bunch length [mm]100.3 bunch interval [ns]2550 transv. emit.  x,y [mm] 0.58, rms IP beam size  x,y [  m] 30, 1677 e- IP beta funct.  * x,y [m] 0.18, full crossing angle [mrad] geometric reduction H hg repetition rate [Hz]N/A 10 beam pulse length [ms]N/A 5 ER efficiencyN/A94%N/A average current [mA] tot. wall plug power[MW]100 proton beamRRLR bunch pop. [10 11 ]1.7 tr.emit.  x,y [  m] 3.75 spot size  x,y [  m] 30, 167  * x,y [m] 1.8, bunch spacing [ns]25 RR= Ring – Ring LR =Linac –Ring Include deuterons (new) and lead (exists) 10 fb -1 per year looks possible … ~ 100 fb -1 total

From 2012 Chamonix LHC performance workshop summary [See also NuPeCC long range plan] Current mandate from CERN is to aim for TDR by ~ … requires detailed further study and prototyping of accelerator components (including CERN ERL LHeC test facility), but also an experimental collaboration to develop the detector concept How and when might LHeC fit ?

Currently approved future of high-energy DIS

On IP2 after LS3 Rather it seems that LHeC is seen as a threat by ALICE (from the naïve perspective of a theorist) don’t rush into a decision, thoroughly consider both options When I first heard of it I thought: what a fantastic opportunity for the ALICE community! They will embrace this project and secure heavy-ion physics at the LHC for many years beyond LS3 I was naïve … this was not the predominant response and they will likely fight the project Embrace or fight the project ?

Detector considerations (slides stolen from P. Newman)

Access to Q 2 =1 GeV 2 in ep mode for all x > 5 x requires scattered electron acceptance to 179 o Similarly, need 1 o acceptance in outgoing proton direction to contain hadrons at high x (essential for good kinematic reconstruction) Detector acceptance requirements

Forward/backward asymmetry in energy deposited and thus in geometry and technology Present dimensions: LxD =14x9m 2 [CMS 21 x 15m 2, ATLAS 45 x 25 m 2 ] Taggers at -62m (e),100m (γ,LR), -22.4m (γ,RR), +100m (n), +420m (p) ep Overview (full acceptance version)

EM Calorimeter [encased in 3.5T solenoid field] Transverse momentum Δp t /p 2 t  GeV -1 transverse impact parameter  10μm Full angular coverage, long tracking region  1 o acceptance Several technologies under discussion Tracking region

Liquid Argon EM Calorimeter [accordion geometry, inside coil] Barrel: Pb, 20 X 0, 11m 3 FEC: Si -W, 30 X 0 BEC: Si -Pb, 25 X Hadronic Tile Calorimeter [modular, outside coil: flux return] Calorimeters

A GEANT4 simulated high-x event

In the absence of a detailed simulation set-up, simulated `pseudo-data’ produced with reasonable assumptions on systematics (typically 2x better than H1 and ZEUS at HERA). Assumed systematic presicion

Small-x and e+A physics

Deep inelastic scattering (DIS) LHeC  *A center-of-mass energy W 2 = (q+p) 2 photon virtuality Q 2 = - (k-k’) 2 = - q 2 > 0

What we know about small x fundamental consequence of QCD dynamics: at asymptotically small x: - QCD evolution becomes non-linear - particle production becomes non-linear - QCD stays weakly coupled the energy dependence of the saturation scale, and more generally of observables, can be computed from first principles although in practice, the predictivity will depend on the level of accuracy of the calculation (LO vs NLO, amount of non-perturbative inputs needed, …) both in terms of practical applicability and phenomenological success the Color Glass Condensate (CGC) has emerged as the best candidate to approximate QCD in the saturation regime

A big open question is this relevant at today’s colliders ? - for each of these observables, there are alternatives explanations - the applicability of the theory can be questioned when values of Q S start to drop below 1 GeV (e.g. p+p and peripheral d+Au at RHIC) the CGC is not widely accepted because in other words: can we get away with using such a gluon distribution (with ad hoc cutoff if necessary) ? or do we need to properly take into account the QCD dynamics at k T ~ Q S and below ? the CGC phenomenology is successful for every collider process that involves small-x partons and k T ~ Q S, i.e. for a broad range for high-energy observables: multiplicities in p+p, d+Au, Au+Au and Pb+Pb; forward spectra and correlations in p+p and d+Au; total, diffractive and exclusive cross sections in e+p and e+A, …

Bigger open question the impact parameter dependence of the gluon density and of Q S what is done in the most advanced CGC phenomenological studies, is to treat the nucleus as a collection of Woods-Saxon distributed CGCs, and to evolve (down in x) the resulting gluon density at different impact parameters independently but is this good enough ? (in principle not) this has always been the main non-perturbative input in CGC calculations modeling in the case of a proton, using an impact-parameter averaged saturation scale is enough most of the time, but in the case of a nucleus it is not

Why QGP physicists (should) care bulk observables in heavy-ion collisions reflect the properties of the initial state as much as those of the hydro evolution of the QGP new sources of uncertainties keep emerging, for instance even two CGC models predict different eccentricities the main source of error in the extraction of medium parameters (e.g. η/s) is our insufficient understanding of initial state fluctuations QGP properties cannot be precisely extracted from data without a proper understanding of the initial state; e+A collisions: access to a precise picture Schenke, Tribedy, Venugopalan

Inclusive structure functions NLO DGLAP cannot simultaneously accommodate F 2 and F L LHeC data if saturation sets in according to current models precisely measuring F L is crucial, and this requires an e+A energy ( ) scan Albacete measures quark distributionsgluon distribution

@ LHeC Exclusive Vector Meson production through a Fourier transformation, one can extract the spatial gluon distribution (and correlations), this is not feasible in p+A energy dependence momentum transfer dependence

Cold nuclear matter effects hard probes (esp. jets) in heavy-ion collisions need calibration what is the effect of cold nuclear matter on parton branching ? on hadronization ? what is the x,Q 2 dependence of nuclear quarks and gluons? answering these questions can help understanding jet suppression in HIC the complementarity of e+A with respect to p+A can help especially when cold matter effects in p+A collisions are “stranger than expected” recent PHENIX data

R i = Nuclear PDF i / (A*proton PDF i) Early LHC data (e.g. inclusive J/  ) suggest low x assumptions inadequate Nuclear parton densities don’t scale with A Nuclear pdfs: current knowledge

Simulated LHeC ePb F 2 measurement has huge impact on uncertainties Most striking effect for sea & gluons High x gluon uncertainty still large Valence Sea Glue [Example pseudo-data from single Q 2 Value] [Effects on EPS09 nPDF fit] Impact of eA F 2 LHeC data

Sample of other SM and BSM physics at LHeC

- Least constrained fundamental coupling by far (known to ~1%) - Do coupling constants unify (with a little help from SUSY)? - (Why) is DIS result historically low? Red = current world average Black = LHeC projected [MSSM ] - Simulated LHeC precision from fitting inclusive data  per-mille (experimental)  also requires improved theory Measuring α s

Gluon Sea d valence Full simulation of inclusive NC and CC DIS data, including systematics  NLO DGLAP fit using HERA technology… … impact at low x (kinematic range) and high x (luminosity) … precise light quark vector, axial couplings, weak mixing angle … full flavour decomposition PDF constraints at LHeC

Current uncertainties due to PDFs for particles on LHC rapidity plateau (NLO): - Most precise for quark initiated processes around EW scale - Gluon initiated processes less well known - All uncertainties explode for largest masses PDFs at LHC

Ancient history (HERA, Tevatron) - Apparent excess in large E T jets at Tevatron turned out to be explained by too low high x gluon density in PDF sets - Confirmation of (non-resonant) new physics near LHC kinematic limit relies on breakdown of factorisation between ep and pp PRL 77 (1996) 438 Searches near LHC kinematic boundary may ultimately be limited by knowledge of PDFs (especially gluon as x  1) Do we need to care ?

Summary: nothing on scale of 1 TeV … need to push sensitivity to higher masses (also non-SUSY searches) Status of LHC SUSY searches

- Both signal & background uncertainties driven by error on gluon density … Essentially unknown for masses much beyond 2 TeV - Similar conclusions for other non-resonant LHC signals involving high x partons (e.g. contact interactions signal in Drell-Yan) - Signature is large invariant mass - Expected SM background (e.g. gg  gg) poorly known for s-hat > 1 TeV. e.g. high mass gluino production

The (pp) LHC has much better discovery potential than LHeC (unless E e increases to ~500 GeV and Lumi to cm -2 s -1 ) e.g. Expected quark compositeness limits below m at LHeC … big improvement on HERA, but already beaten by LHC LHeC is competitive with LHC in cases where initial state lepton is an advantage and offers cleaner final states e q e q ~ 00 Direct sensitivity to new physics

Mass range of LQ sensitivity to ~ 2 TeV … similar to LHC Single production gives access to LQ quantum numbers: - fermion number (below) - spin (decay angular distributions) - chiral couplings (beam lepton polarisation asymmetry) Leptoquark quantum numbers

LHC is a totally new world of energy and luminosity, already making discoveries. LHeC proposal aims to exploit it for lepton-hadron scattering … ep complementing LHC and next generation ee facility for full Terascale exploration ECFA/CERN/NuPECC workshop gathered many accelerator, theory & experimental colleagues  Conceptual Design Report published. Moving to TDR phase  Awaiting outcome of European strategy exercise  Build collaboration for detector development [More at Summary

… with thanks to many colleagues working on LHeC …