the large hadron electron collider -...

The Large Hadron electron Collider

http://cern.ch/lhec

Can we add ep and eA collisions to the existing LHC pp, AA and pA programme?

… towards a full understandingof QCD at high temperatures, baryon and parton densities …

1

Brian Cole, Columbia University using (mostly) slides from previous talks

by P. Newman and M. Klein

Thursday, June 26, 14

Deep Inelastic Scattering [eh ! e’X]e+e-

hh

eh !

!

!

x = Q2

syQ2 = −(k− k ')2

ylab =1−Ee 'Ee

s= 4EeEp

Parton momentum fixed by electron kinematics

Incl. NC (γ,Z) and CC (W±) independent of hadronisation

Rigorous theory: Operator expansion (lightcone)

Parton momentum distributions to be measured in DIS

Collider- HERA: yh=ye : Redundant kinematics

HERA-LHeC-FCC-eh: finest microscopeswith resolution varying like 1/√Q2

X

electromagnetic radius

Finite p Radius

Quarks

QuarkGluonDynamics

?

Stanford

SLAC

FNAL

CERN

HERA

LHeC

FCC-he

100 years of lp scattering

!

5 orders of magnitude

deeper into matter


DIS and HERAQ2: exchanged boson resolving power

x: fractional momentum of struck quark

30

0.2

0.4

0.6

0.8

1

-410 -310 -210 -110 10

0.2

0.4

0.6

0.8

1

HERAPDF1.5 NNLO (prel.)

exp. uncert.

model uncert. parametrization uncert.

x

xf 2 = 10 GeV2Q

vxu

vxd

0.05)×xS (

0.05)×xg (

HER

APD

F St

ruct

ure

Func

tion

Wor

king

Gro

upM

arch

201

1

H1 and ZEUS HERA I+II PDF Fit

0

0.2

0.4

0.6

0.8

1

HERA Proton parton densities in x range well matched to LHC rapidity plateau … BUT…

- Insufficient lumi for high x

- Lack of Q2 lever-arm for low x gluon

- Assumptions on quarkflavour decomposition

- No deuterons or heavy ionsThursday, June 26, 14

Conceptual Design Report (July 2012)

630 pages, summarising 5 year workshop commissioned by CERN, ECFA and NuPECC

~200 participants, 69 institutes

Additional material in subsequentupdates:

“A Large Hadron Electron Collider at CERN” [arXiv:1211.4831] “On the Relation of the LHeC and the LHC” [arXiv:1211.5102]

[arXiv:1206.2913]

4


Latest & most promising idea to take lepton-hadron physics to the TeV centre-of-mass scale… at high luminosity

Designed to exploitintense hadron beams in high luminosity phase of LHC running from mid 2020s

LHeC Context

5


High Q2

" HERA

LHeC

FCC-he175 GeV 60 GeV

Rutherford backscatteringof dozens of TeV e- energy

" ϑh=1o


Low x

ß 179o

@ 180 GeV.. very low xrequires notthe maximumof Ee

-----------

For x < 10-3 no (average) energy deposition exceeding the electron beam energy

x

HERA

LHeC

FCC-he60 GeV

Very low xreaches directrange ofUHE neutrinophysics !


Baseline♯ Design (Electron “Linac”)Design constraint: power consumption < 100 MW ! Ee = 60 GeV

• Two 10 GeV linacs, • 3 returns, 20 MV/m• Energy recovery insame structures[CERN plans energy recovery prototype]

• ep Lumi 1033 – 1034 cm-2 s-1

➡ 10 - 100 fb-1 per year ➡ 100 fb-1 – 1 ab-1 total • eD and eA collisions have always been integral to programme• e-nucleon Lumi estimates ~ 1031 (1032) cm-2 s-1 for eD (ePb)

Alternative designs based on electron ring and on higher energy, lower luminosity, linac also exist

8


Detector Overview

e p

• Forward / backward asymmetry reflecting beam energies• Present size 14m x 9m (c.f. CMS 21m x 15m, ATLAS 45m x 25m)• ZDC, proton spectrometer integral to design from outset

9


DIS: Parton Microscopy

Q2: exchanged boson resolving power x: fractional momentum of struck quark

Only previously studied in collidermode and in ep at HERA (1992-2007)

Proton parton density at Q2 = 10 GeV2 from HERA data alone[HERAPDF1.5 NNLO]

… Low x saturation?10


LHeC Strategy for making the target blacker

Enhance target `blackness’ by: 1) Probing lower x at fixed Q2 in ep [evolution of a single source] 2) Increasing target matter in eA [overlapping many sources at fixed kinematics … density ~ A1/3 ~ 6 for Pb … worth 2 orders of magnitude in x]

LHeC delivers a 2-pronged approach:

11

… Reachingsaturatedregion in both ep & eA according tocurrent models


Establishing and Characterising SaturationWith 1 fb-1 (1 month at 1033 cm-2 s-1), F2 stat. < 0.1%, syst, 1-3%FL measurement to 8% with 1 year of varying Ee or Ep

• LHeC can distinguish between different QCD-based models for the onset of non-linear dynamics • Unambiguous observation of saturation will be based on tensionbetween different observables e.g. F2 v FL in ep or F2 in ep v eA


Exclusive / Diffractive Channels and Saturation

1) [Low-Nussinov] interpretation as 2 gluon exchange enhances sensitivity to low x gluon

1) Additional variable t gives access to impact parameter (b) dependent amplitudes

! Large t (small b) probes densest packed part of proton?


e.g. J/ψ Photoproductione.g. “b-Sat” Dipole model- “eikonalised”: with impact-parameter dependent saturation - “1 Pomeron”: non-saturating

• Significant non-linear effects expected in LHeC kinematic range.

• Data shown are extrapolations of HERA power law fit for Ee = 150 GeV… ! Satn smoking gun?

[2 fb-1]


eA

Four orders of magnitude increase in kinematic range over previous

DIS experiments.

LHeC as an Electron-ion Collider

• Revolutionise our view of the partonic structure of nuclear matter.

• Study interactions of densely packed, but weakly coupled, partons

• Ultra-clean probe ofpassage of `struck’partons through cold nuclear matter 15


LHeC-FCC_he: Electron Ion Collider

Precision QCD study of parton dynamics in nucleiInvestigation of high density matter and QGPGluon saturation at low x, in DIS region.

Extension of kinematic range in lAby 4-5 orders of magnitude willchange QCD view on nuclear structure and parton dynamics

May lead to genuine surprises…

- No saturation of xg (x,Q2) ?- Small fraction of diffraction ?- Broken isospin invariance ?- Flavour dependent shadowing ?

Expect saturation of rise at Q2

s ≈ xg αs ≈ c x-λA1/3

LHeC is part of NuPECCslong range plan since 2010LeN ~ 1032 cm-2 s-1

FCC-he


Relation to the Heavy Ion Programme

17


Current Status of Nuclear Parton Densities

[Existing DIS data]

Ri = Nuclear PDF i / (A * proton PDF i)

Valence Sea Gluon

• Complex nuclear effects,not yet fully understood

• Quarks from DIS & DY• Gluon mainly from dAusingle π0 rates

• All partons poorly constrained for x < 10-2

18


Complementarity of pA and eA

• New effects likely to be revealed in tensions between eA and pA, AA, ep (breakdown of factorisation)

• Detailed precision understanding likely to come from eA- LHeC offers access to lower x than is realistically achievable

in pA at the LHC- Clean final states / theoretical control to (N)NLO in QCD

eA

19


Current Low x Understanding in LHC Ion DataUncertainties in low-x nuclear PDFs preclude precision statements on medium produced in AA (e.g. extent of screeningof c-cbar potential)

Inclusive J/Ψ AA data

η dependence of pPb chargedparticle spectra best describedby shadowing-only models (saturation models too steep?)… progress with pPb, but uncertainties still large, detailedsituation far from clear

Minimum Bias pA data

20

Pb pThursday, June 26, 14

Jet production in pA at LHC

21

[GeV]T

p

pPb

R +2.1 < y* < +2.8

ATLAS Preliminary

[GeV]T

p

pPb

R +0.8 < y* < +1.2

[GeV]T

p

pPb

R -0.3 < y* < +0.3

[GeV]T

p

pPb

R -1.2 < y* < -0.8

= 5.02 TeVNNs+Pb p=0.4R, tkanti-

[GeV]T

p

pPb

R +1.2 < y* < +2.1

+Pb, 0-90%p

EPS09 calculation

[GeV]T

p

pPb

R +0.3 < y* < +0.8

[GeV]T

p

pPb

R -0.8 < y* < -0.3

[GeV]T

p

pPb

R -2.1 < y* < -1.2

∫ -1dt = 27.8 nbpPbL

∫ -1dt = 4.0 pbppL

1.6

1.0

0.4

1.6

1.0

0.4

1.6

1.0

0.4

1.6

1.0

0.4

pPb

R

40 100 1000 40 100 1000 [GeV]

Tp

• Ratio of inclusive p+Pb jet yields to scaled p-p jet cross-sections

• Compared to NLO calculations by Armesto using EPS09 PDFs

• Good (but not perfect) agreement -- nPDF effects are small


Jet production in p+A at LHC

22

• But ratios of jet cross-sections between high- and low-multiplicity events show unexpected behavior at high pT, forward rapidities

[GeV]T

p

CP

R +3.6 < y* < +4.4

ATLASPreliminary

[GeV]T

p

CP

R +2.1 < y* < +2.8

[GeV]T

p

CP

R +0.8 < y* < +1.2

[GeV]T

p

CP

R -0.3 < y* < +0.3

[GeV]T

p

CP

R -1.2 < y* < -0.8

∫ -1dt = 27.8 nbL = 5.02 TeVNNs+Pb p

=0.4R, tkanti-

[GeV]T

pC

P R +2.8 < y* < +3.6

[GeV]T

p

CP

R +1.2 < y* < +2.1

[GeV]T

p

CP

R +0.3 < y* < +0.8

[GeV]T

p

CP

R -0.8 < y* < -0.3

[GeV]T

p

CP

R -2.1 < y* < -1.2

0-10%/60-90%

20-30%/60-90%

40-60%/60-90%

1.4

1.0

0.4

1.4

1.0

0.4

1.4

1.0

0.4

1.4

1.0

0.4

1.4

1.0

0.4

CP

R

20 100 800 20 100 800 [GeV]

Tp



23

• But ratios of jet cross-sections between high- and low-multiplicity events show unexpected behavior at high pT, forward rapidities• Scales with jet energy (x?) at forward rapidities.

[GeV]T

p

CP

R +3.6 < y* < +4.4

ATLASPreliminary

[GeV]T

p

CP

R +2.1 < y* < +2.8

[GeV]T

p

CP

R +0.8 < y* < +1.2

[GeV]T

p

CP

R -0.3 < y* < +0.3

[GeV]T

p

CP

R -1.2 < y* < -0.8

∫ -1dt = 27.8 nbL = 5.02 TeVNNs+Pb p

=0.4R, tkanti-

[GeV]T

pC

P R +2.8 < y* < +3.6

[GeV]T

p

CP

R +1.2 < y* < +2.1

[GeV]T

p

CP

R +0.3 < y* < +0.8

[GeV]T

p

CP

R -0.8 < y* < -0.3

[GeV]T

p

CP

R -2.1 < y* < -1.2

0-10%/60-90%

20-30%/60-90%

40-60%/60-90%

1.4

1.0

0.4

1.4

1.0

0.4

1.4

1.0

0.4

1.4

1.0

0.4

1.4

1.0

0.4

CP

R

20 100 800 20 100 800 [GeV]

Tp

∫ -1dt = 27.8 nbL = 5.02 TeVNNs+Pb p

0-10%

=0.4R, tkanti-

ATLASPreliminary

+3.6 < y* < +4.4+2.8 < y* < +3.6+2.1 < y* < +2.8+1.2 < y* < +2.1+0.8 < y* < +1.2

+0.3 < y* < +0.8-0.3 < y* < +0.3-0.8 < y* < -0.3-1.2 < y* < -0.8-2.1 < y* < -1.2

1.4

1.0

0.4

CP

R

40 100 1000 40 100 1000 cosh(<y*>) [GeV]×

Tp

∫ -1dt = 27.8 nbL = 5.02 TeVNNs+Pb p

0-10%

=0.4R, tkanti-

ATLASPreliminary

+3.6 < y* < +4.4+2.8 < y* < +3.6+2.1 < y* < +2.8+1.2 < y* < +2.1+0.8 < y* < +1.2

+0.3 < y* < +0.8-0.3 < y* < +0.3-0.8 < y* < -0.3-1.2 < y* < -0.8-2.1 < y* < -1.2

1.4

1.0

0.4

CP

R

40 100 1000 40 100 1000 cosh(<y*>) [GeV]×

Tp



24

• Correlation between hard and soft production indicates breakdown of factorization• Likely due to correlation between parton x and proton configuration/size➡ need e+p measurements

[GeV]T

p

CP

R +3.6 < y* < +4.4

ATLASPreliminary

[GeV]T

p

CP

R +2.1 < y* < +2.8

[GeV]T

p

CP

R +0.8 < y* < +1.2

[GeV]T

p

CP

R -0.3 < y* < +0.3

[GeV]T

p

CP

R -1.2 < y* < -0.8

∫ -1dt = 27.8 nbL = 5.02 TeVNNs+Pb p

=0.4R, tkanti-

[GeV]T

pC

P R +2.8 < y* < +3.6

[GeV]T

p

CP

R +1.2 < y* < +2.1

[GeV]T

p

CP

R +0.3 < y* < +0.8

[GeV]T

p

CP

R -0.8 < y* < -0.3

[GeV]T

p

CP

R -2.1 < y* < -1.2

0-10%/60-90%

20-30%/60-90%

40-60%/60-90%

1.4

1.0

0.4

1.4

1.0

0.4

1.4

1.0

0.4

1.4

1.0

0.4

1.4

1.0

0.4

CP

R

20 100 800 20 100 800 [GeV]

Tp

∫ -1dt = 27.8 nbL = 5.02 TeVNNs+Pb p

0-10%

=0.4R, tkanti-

ATLASPreliminary

+3.6 < y* < +4.4+2.8 < y* < +3.6+2.1 < y* < +2.8+1.2 < y* < +2.1+0.8 < y* < +1.2

+0.3 < y* < +0.8-0.3 < y* < +0.3-0.8 < y* < -0.3-1.2 < y* < -0.8-2.1 < y* < -1.2

1.4

1.0

0.4

CP

R

40 100 1000 40 100 1000 cosh(<y*>) [GeV]×

Tp

∫ -1dt = 27.8 nbL = 5.02 TeVNNs+Pb p

0-10%

=0.4R, tkanti-

ATLASPreliminary

+3.6 < y* < +4.4+2.8 < y* < +3.6+2.1 < y* < +2.8+1.2 < y* < +2.1+0.8 < y* < +1.2

+0.3 < y* < +0.8-0.3 < y* < +0.3-0.8 < y* < -0.3-1.2 < y* < -0.8-2.1 < y* < -1.2

1.4

1.0

0.4

CP

R

40 100 1000 40 100 1000 cosh(<y*>) [GeV]×

Tp


p+Pb charged particle RpPb

• Puzzle from the Hard Probes 2013 conference:– CMS observes unexpected enhancement in high-pT charged

particle yield in p+Pb relative to p+p– Seen by ATLAS, not by ALICE

• Origin?– If the effect is real, likely due to poorly understood interplay

between nPDF modifications & FF. 25

[GeV/c]T

p1 10 210

pPb

R

0

0.2

0.4

0.60.8

1

1.2

1.4

1.6

1.82

|<1 CMηCMS Charged Particles |

y=00πEPS09 fDSS NLO

=6.9collN

CMS Preliminary = 5.02 TeVNNspPb

Helenius et.al, JHEP 1207 (2012) 073

[GeV]T

p10

210

pPb

R

0.8

1

1.2

1.4

1.6

1.8

2

2.2 PreliminaryATLAS

-1=25 nbint

p+Pb L=5.02 TeV

NNs

0-90%

-1<y*<1

Inclusive charged particle production in p+A at LHC


Further Surprises from pA Data• “Ridge” observed in high multiplicity pPb collisions - due to saturation?

26


Further Surprises from pA Data• “Ridge” observed in high multiplicity pPb collisions - due to saturation? • Or due to collective dynamics similar to that observed in Pb+Pb?

27


Further Surprises from pA Data• “Ridge” observed in high multiplicity pPb collisions - due to saturation? • Or due to collective dynamics similar to that observed in Pb+Pb?

• Data suggest the latter

• My prediction: we will observe effects of strong coupling in e+p/A final states and can study with more control than in p+A

28


• Simulated LHeC ePb F2 measurementhas huge impact on uncertainties

• Most striking effect for sea & gluons

• High x gluon uncertainty still large

Impact of eA F2 LHeC data

Valence

Sea

Glue [Example pseudo-datafrom single Q2 Value]

[Effects on EPS09nPDF fit] 29


Exclusive Diffraction in eAExperimentally clear signatures and theoretically cleanly calculable saturation effects in coherent diffraction case (eA ! eVA)

Experimentalseparation ofincoherent diffraction based mainlyon ZDC… potential saturation smoking gun?30


In-medium radiation and hadronisation effects

Ratio of π0 fragn functions Pb / p (Armesto et al.)

ν= struck parton energy in target rest frame

Small ν: Hadron formationmay be inside. Hadronic energy loss

Large ν: Hadronisation beyond medium. Partonic energy loss

How do virtual parton probes lose Virtuality and colour to hadronise?

31LHeC most sensitive to partonic loss. ! Baseline `cold matter’ input to use energy loss mechanisms to characterise QGP Thursday, June 26, 14

Some other LHeC eA Studies …

Impact of eA charm &beauty data

Forward π0

production &fragmentation

Jet photoproduction

Inclusive ep diffractionv nuclearshadowing

Many more processes and observables still to be investigated

32


Summary / Outlook

•Many reasons that new, high-luminosity e+p and e+A measurements are needed.

• LHeC is high-energy option complementary to EIC- 3-4 orders of magnitude in nPDF kinematic range

- New non-linear QCD dynamics of low x parton saturation? - Valuable input for heavy ion physics program at LHC

• Conceptual Design Report available.

• Ongoing work … - Further physics motivation - Detector / simulation, - Superconducting RF, ERL, machine

• Timeline?... Optimal impact by running in High Lumi LHC Phase

[More at http://cern.ch/lhec]

33


Herwig Schopper (Chair IAC) at Chavannes in the Panel Discussion with the CERN Directorate

Clarification and Tradition


Summary of the LHeC Physics ProgrammeCDR, arXiv:1211.4831 and 1211.5102 http://cern.ch/lhec

New since CDR: Higgs discovered, 1033 !1034, and the FCC horizon – much to do

Truth is stranger than fiction, but it is because fiction is obliged to stick to possibilitiesMark Twain, cited by Stan Brodsky at Chavannes


… with thanks to Nestor Armesto, Max Klein, Anna Stasto and many experimentalist, theorist & accelerator scientist colleagues …

LHeC study group …

36


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