The Large Hadron electron Collider

Transcription

1 he Large Hadron electron Collider Brian Cole, Columbia University using (mostly) slides from previous talks by P. Newman and M. Klein Can we add ep and ea collisions to the existing LHC pp, AA and pa programme? towards a full understanding of QCD at high temperatures, baryon and parton densities 1

2 Deep Inelastic Scattering [eh! e X] eh! e + e -!! hh x = Q2 sy Q 2 = (k k') 2 y lab =1 E e ' E e s= 4E e E p 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 X Collider- HERA: y h =y e : Redundant kinematics HERA-LHeC-FCC-eh: finest microscopes with resolution varying like 1/ Q 2 Stanford SLAC Finite p Radius Quarks 100 years of lp scattering! 5 orders of magnitude deeper into matter FNAL CERN Quark Gluon Dynamics HERA LHeC electromagnetic radius? FCC-he

3 DIS and HERA Q 2 : exchanged boson resolving power xf x: fractional momentum of struck quark H1 and ZEUS HERA I+II PDF Fit xg ( 0.05) xs ( 0.05) HERAPDF1.5 NNLO (prel.) exp. uncert. model uncert. parametrization uncert Q = 10 GeV xu v xd v x HERAPDF Structure Function Working Group March 2011 HERA Proton parton densities in x range well matched to LHC rapidity plateau BU - Insufficient lumi for high x - Lack of Q 2 lever-arm for low x gluon - Assumptions on quark flavour decomposition - No deuterons or heavy 3ions

4 Conceptual Design Report (July 2012) 630 pages, summarising 5 year workshop commissioned by CERN, ECFA and NuPECC [arxiv: ] ~200 participants, 69 institutes Additional material in subsequent updates: A Large Hadron Electron Collider at CERN [arxiv: ] On the Relation of the LHeC and the LHC [arxiv: ] 4

5 LHeC Context Latest & most promising idea to take lepton-hadron physics to the ev centre-of-mass scale at high luminosity Designed to exploit intense hadron beams in high luminosity phase of LHC running from mid 2020s 5

6 High Q 2 Rutherford backscattering of dozens of ev e- energy FCC-he 175 GeV 60 GeV LHeC " ϑ h =1 o " HERA

7 Low x FCC-he 60 GeV LHeC Very low x reaches direct range of UHE neutrino physics! ß 179 o 180 GeV.. very low x requires not the maximum of E e For x < 10-3 no (average) energy deposition exceeding the electron beam energy x

8 Baseline Design (Electron Linac ) Design constraint: power consumption < 100 MW! E e = 60 GeV wo 10 GeV linacs, 3 returns, 20 MV/m Energy recovery in same structures [CERN plans energy recovery prototype] ep Lumi cm -2 s 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 ~ (10 32 ) cm -2 s -1 for ed (epb) Alternative designs based on electron ring and on higher energy, lower luminosity, linac also exist 8

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

10 DIS: Parton Microscopy Q 2 : exchanged boson resolving power x: fractional momentum of struck quark Only previously studied in collider mode and in ep at HERA ( ) Proton parton density at Q 2 = 10 GeV 2 from HERA data alone [HERAPDF1.5 NNLO] Low x saturation? 10

11 LHeC Strategy for making the target blacker LHeC delivers a 2-pronged approach: Enhance target `blackness by: 1) Probing lower x at fixed Q 2 in ep [evolution of a single source] 2) Increasing target matter in ea [overlapping many sources at fixed kinematics density ~ A 1/3 ~ 6 for Pb worth 2 orders of magnitude in x] Reaching saturated region in both ep & ea according to current models 11

12 Establishing and Characterising Saturation With 1 fb -1 (1 month at cm -2 s -1 ), F 2 stat. < 0.1%, syst, 1-3% F L measurement to 8% with 1 year of varying E e or E p LHeC can distinguish between different QCD-based models for the onset of non-linear dynamics Unambiguous observation of saturation will be based on tension between different observables e.g. F 2 v F L in ep or F 2 in ep v ea

13 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?

14 e.g. J/ψ Photoproduction e.g. b-sat Dipole model - eikonalised : with impact-parameter dependent saturation - 1 Pomeron : non-saturating Significant non-linear effects expected in LHeC kinematic range. [2 fb -1 ] Data shown are extrapolations of HERA power law fit for E e = 150 GeV! Sat n smoking gun?

15 LHeC as an Electron-ion Collider Four orders of magnitude increase in kinematic range over previous DIS experiments. ea Revolutionise our view of the partonic structure of nuclear matter. Study interactions of densely packed, but weakly coupled, partons Ultra-clean probe of passage of `struck partons through cold nuclear matter 15

16 LHeC-FCC_he: Electron Ion Collider FCC-he LHeC is part of NuPECCs long range plan since 2010 L en ~ cm -2 s -1 Extension of kinematic range in la by 4-5 orders of magnitude will change QCD view on nuclear structure and parton dynamics May lead to genuine surprises - No saturation of xg (x,q 2 )? - Small fraction of diffraction? - Broken isospin invariance? - Flavour dependent shadowing? Expect saturation of rise at Q 2 s xg α s c x-λ A 1/3 Precision QCD study of parton dynamics in nuclei Investigation of high density matter and QGP Gluon saturation at low x, in DIS region.

17 Relation to the Heavy Ion Programme 17

18 Current Status of Nuclear Parton Densities Complex nuclear effects, not yet fully understood [Existing DIS data] Quarks from DIS & DY Gluon mainly from dau single π 0 rates All partons poorly constrained for x < 10-2 R i = Nuclear PDF i / (A * proton PDF i) Valence Sea Gluon 18

19 Complementarity of pa and ea 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 19 - Clean final states / theoretical control to (N)NLO in QCD

20 Current Low x Understanding in LHC Ion Data Inclusive J/Ψ AA data Uncertainties in low-x nuclear PDFs preclude precision statements on medium produced in AA (e.g. extent of screening of c-cbar potential) Minimum Bias pa data η dependence of ppb charged particle spectra best described by shadowing-only models (saturation models too steep?) progress with ppb, but uncertainties still large, detailed situation far from clear Pb 20 p

21 Jet production in pa at LHC R ppb < y* < +2.8 ALAS Preliminary ppb R +1.2 < y* < +2.1 p+pb, 0-90% EPS09 calculation Ratio of inclusive p+pb jet yields to scaled p-p jet cross-sections < y* < +1.2 R ppb +0.3 < y* < +0.8 Compared to NLO calculations by Armesto using EPS09 PDFs < y* < +0.3 R ppb -0.8 < y* < -0.3 p [GeV] Good (but not perfect) agreement -- npdf effects are small < y* < -0.8 R ppb -2.1 < y* < -1.2 p [GeV] p +Pb s NN = 5.02 ev anti-k t, R = -1 L ppb dt = 27.8 nb -1 L pp dt = 4.0 pb p [GeV] p [GeV] 21

22 Jet production in p+a at LHC < y* < +4.4 ALAS Preliminary +2.1 < y* < < y* < < y* < +2.1 p [GeV] But ratios of jet crosssections between high- and lowmultiplicity events show unexpected behavior at high p, forward rapidities < y* < < y* < +0.8 p [GeV] < y* < < y* < -0.3 p [GeV] < y* < < y* < -1.2 p [GeV] -1 Ldt = 27.8 nb p+pb s NN = 5.02 ev anti-k t, R= 0-10%/60-90% 20-30%/60-90% 40-60%/60-90% p [GeV] p [GeV] p [GeV] 22

23 Jet production in p+a at LHC < y* < +4.4 ALAS Preliminary +2.1 < y* < < y* < < y* < < y* < < y* < +0.8 p [GeV] p [GeV] But ratios of jet crosssections between high- and lowmultiplicity events show unexpected behavior at high p, forward rapidities Scales with jet energy (x?) at forward rapidities < 1.4 y* < < y* < -0.3 p [GeV] 1.4 ALAS -1 Ldt = 27.8 nb Preliminary p+pb s NN = 5.02 ev anti-k t, R= ALAS Preliminary p+pb -1 Ldt = 27.8 nb s NN = 5.02 ev anti-k t, R= % -1.2 < y* < < y* < < y* < < y* < < y* < %/60-90% Ldt = 27.8 nb +1.2 < y* < %/60-90% p+pb s NN = 5.02 ev +0.8 < y* < +1.2 anti-k t, R= p [GeV] 40-60%/60-90% p [GeV] p [GeV] p [GeV] p cosh(<y*>) [GeV] % < y* < < y* y* < < y* y* < < y* y* < < y* y* < < y* < < y* < < y* < < y* < < y* < -1.2 p co

24 Jet production in p+a at LHC < y* < +4.4 ALAS Preliminary +2.1 < y* < < y* < < y* < < y* < < y* < +0.8 p [GeV] p [GeV] 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 < 1.4 y* < < y* < -0.3 p [GeV] 1.4 ALAS -1 Ldt = 27.8 nb Preliminary p+pb s NN = 5.02 ev anti-k t, R= ALAS Preliminary p+pb -1 Ldt = 27.8 nb s NN = 5.02 ev anti-k t, R= % -1.2 < y* < < y* < < y* < < y* < < y* < %/60-90% Ldt = 27.8 nb +1.2 < y* < %/60-90% p+pb s NN = 5.02 ev +0.8 < y* < +1.2 anti-k t, R= p [GeV] 40-60%/60-90% p [GeV] p [GeV] p [GeV] p cosh(<y*>) [GeV] % < y* < < y* y* < < y* y* < < y* y* < < y* y* < < y* < < y* < < y* < < y* < < y* < -1.2 p co

25 Inclusive charged particle production in p+a at LHC R ppb CMS Preliminary p+pb charged particle RpPb ppb N coll =6.9 = 5.02 ev s NN 1 10 p Puzzle from the Hard Probes 2013 conference: CMS observes unexpected enhancement in high-p charged particle yield in p+pb relative to p+p Seen by ALAS, not by ALICE Origin? CMS Charged Particles η <1 CM EPS09 fdss NLO π 0 y=0 Helenius et.al, JHEP 1207 (2012) 073 [GeV/c] 2 10 If the effect is real, likely due to poorly understood interplay between npdf modifications & FF. 25 R ppb ALAS Preliminary p+pb L s NN 0-90% 10 int =25 nb =5.02 ev -1-1<y*< p [GeV]

26 Further Surprises from pa Data Ridge observed in high multiplicity ppb collisions - due to saturation? 26

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? 27

28 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 28 than in p+a

29 Impact of ea F 2 LHeC data 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 29fit]

30 Exclusive Diffraction in ea Experimentally clear signatures and theoretically cleanly calculable saturation effects in coherent diffraction case (ea! eva) Experimental separation of incoherent diffraction based mainly on ZDC potential saturation 30 smoking gun?

31 In-medium radiation and hadronisation effects How do virtual parton probes lose Virtuality and colour to hadronise? Ratio of π 0 frag n functions Pb / p (Armesto et al.) Large ν: Hadronisation beyond medium. Partonic energy loss ν= struck parton energy in target rest frame Small ν: Hadron formation may be inside. Hadronic energy loss LHeC most sensitive to partonic loss.! Baseline `cold matter 31 input to use energy loss mechanisms to characterise QGP

32 Jet photoproduction Some other LHeC ea Studies Inclusive ep diffraction v nuclear shadowing Forward π 0 production & fragmentation Impact of ea charm & beauty data Many more processes and observables still to be investigated 32

33 Summary / Outlook [More at Many reasons that new, high-luminosity e+p and e+a measurements are needed. LHeC is high-energy option complementary to EIC 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 imeline?... Optimal impact by running in High Lumi LHC Phase 33

34 Clarification and radition Herwig Schopper (Chair IAC) at Chavannes in the Panel Discussion with the CERN Directorate

35 Summary of the LHeC Physics Programme CDR, arxiv: and New since CDR: Higgs discovered, 10 33!10 34, and the FCC horizon much to d ruth is stranger than fiction, but it is because fiction is obliged to stick to possibilities Mark wain, cited by Stan Brodsky at Chavannes

36 with thanks to Nestor Armesto, Max Klein, Anna Stasto and many experimentalist, theorist & accelerator scientist colleagues LHeC study group 36