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FUTURE LOW X PHYSICS AND F ACILITIES M.KLEIN DESY Zeuthen E-mail:klein@ifh.de A brief overview is given on the physics and future lepton-nucleon collider facilities to explore the domain of high parton densities at low Bjorken x ,with HERA,the electron-ion collider (EIC)and with THERA -in ep ,polarised ?→e ?→N and eA mode.1HERA-2The only existing lepton-nucleon collider which accesses the region of low Bjorken x is HERA.During 2001the detectors were upgraded and the ma-chine was modi?ed to signi?cantly increase the luminosity.Spin rotators were installed enabling to run the collider experiments H1and ZEUS with longitudinally polarised positrons and electrons.It is now expected to collect over the coming ?ve years an integrated luminosity of 1fb ?1,which repre-sents about ten times the luminosity recorded in HERA’s ?rst running period.This luminosity upgrade primarily is dedicated to the exploration of the low cross section region at high momentum transfers,Q 2?M 2Z .Yet,a wealth of precise low x data can still be expected from HERA which are necessary to further develop low x theory.These comprise for example:i)a precision measurement of αs from scaling violations of F 2,ii)the accurate measurement of the x dependence of the longitudinal structure function F L from a series of runs with lowered proton beam energy,iii)?rst measurements of F b 2and precision data on F c 2in an extended x range using new Silicon detectors,iv)data on the di?ractive structure functions F D 32and F D 42with a new,very forward proton spectrometer,v)accurate data on vector meson production,on ?nal states and on charm in di?raction.An exciting decade of low x ,high precision ep physics is ahead.Key questions in low x physics,however,will remain unanswered even after completion of the HERA-2run because it is

restricted to electron-proton scattering at energy s =4E e E p <105GeV 2,and because the low Q 2acceptance range,around 1GeV 2,in H1and ZEUS is now obstructed by focusing magnets.

2Low x Physics Beyond HERA-2

Future low x physics,which can only be sketched here,regards the unknown behaviour of the neutron,photon and nuclear structure,of the nucleon spin lxdatong2:submitted to World Scienti?c on February 1,20081

composition in this domain and it needs to clarify experimentally the question of saturation.Super-high-energy neutrino scattering 1and the LHC data cannot be reliably understood without extending the x range considerably beyond the region accessed in ep scattering at HERA.

After completion of the currently approved HERA programme there re-main precision measurements to be performed in the low x region at HERA.These regard in particular the measurement of jets very close to the pro-ton beam direction,to understand the emission of gluons,and the low Q 2~1GeV 2region,in particular the longitudinal structure function F L and the di?ractive structure function F D,422.Such a programme needs the forward and backward regions very close to the beam pipe at HERA to be reopened and upgraded.E?cient proton tagging and a small beam divergence are required as for electron-deuteron scattering.

Deuterons are a source of quasi-free neutrons,apart from nuclear shadow-ing e?ects at low x <0.1.These are related to the di?ractive parton densities which provide enough constraint to control shadowing to better than 1-2%of the cross section 3.The asymmetry of sea quarks,d ,can thus be accurately measured from the di?erence F p 2?F n 2,which is free of Pomeron exchange.Furthermore,ed data are essential in the unfolding of parton dis-tributions,such as s ?c or the d/u ratio,in determining charged current structure functions and in precision tests of the Q 2evolution in perturba-tive QCD.Electron-deuteron scattering appears to be the natural next step beyond ep scattering.Investigation of the nucleon spin structure with ?→e ?→N colliders will ex-plore spin phenomena at high Q 2and at low x for the ?rst time.This opens a completely new ?eld for testing QCD,4,5making use of the complete ?nal state reconstruction capabilities.For example,dijet signatures can be used to measure the spin distribution ?G .The behaviour of the spin structure function g 1(x,Q 2)at low x is unknown,but expected to change even more dramatically than F 2.The Q 2dependence of g 1determines the gluon spin distribution ?G .The integral of ?G enters the proton spin decomposition,1/2=J g +1/2·?Σ+L q ,together with the quark orbital angular momentum and the quark spin distributions,?Σ= i ?q i ,which are similarly unde-termined at low x .Here J g = ?Gdx +L g .Semi-inclusive measurements explore the ?avour structure of spin and DVCS is sensitive to o?diagonal parton distributions.Photoproduction gives insight into the polarised gluon structure of the photon.The kinematic region of polarised eN scattering is a vast terra incognita beyond the ?xed target experiment domain.The ?rst ?→e ?→N collider experiment,extending also to high Q 2,may change radically our view on nucleon spin structure since it accesses the sea and the gluon.

lxdatong2:submitted to World Scienti?c on February 1,20082

Figure1.Extrapolation of the gluon distribution towards much lower x.This qualitatively illustrates the range accessible with di?erent colliders and eA scattering by comparing the possible further rise of xg with unitarity limits.The behaviour of xg at low x may yet di?er very much from this bold extrapolation for various reasons,see text.

The rise of the proton structure function F2towards low x in the DIS region is due to a high sea quark density related to a gluon distribution xg∝?F2/?ln Q2which also rises towards low x as x.?λSince xg is roughly expected to be enhanced∝A1/3in nuclei of atomic number A,in electron-nucleus(eA)scattering an equivalent Bjorken x=x N/(A1/3)1/λis accessed: nuclei can thus be utilized to preview a kinematic region which in ep scattering requires a considerable increase of energy s.This is illustrated by extrapo-lating xg and its experimental uncertainty,using the NLO QCD analysis by the H1Collaboration6,much beyond its range of validity towards extremely low x in Fig.1.At a certain value of x,the rise of the gluon distribution is expected to contradict unitarity limits7which,within an uncertainty of about a factor of two,limit xg by Q2/αs(Q2).Depending on higher order corrections,contributions∝ln1/x,shadowing e?ects,the chosen parameter-isation of xg at minimum Q2,the heavy?avour treatment and for di?erent A than Ca(A=40),this picture may be drawn di?erently.Yet,Fig.1illustrates the expected density ampli?cation e?ect of nuclei.Notice in particular the great extension of kinematic range with THERA,7which is the eN collider using HERA(N)and TESLA(e±).With nuclei,THERA directly accesses the range of super-high-energy neutrino astrophysics with x as small as10?8. Di?raction in nuclei may constitute up to50%of the inclusive cross section, and since xg enters squared into the di?ractive cross section,non-linear damp-ing e?ects may possibly be seen in di?raction?rst.

lxdatong2:submitted to World Scienti?c on February1,20083

3Lepton-Nucleon Collider Prospects

HERA is scheduled to run with upgraded luminosity of7·1031cm?2s?1until the end of2006.8The ultimate value is estimated to be1.3·1032cm?2s?1which

corresponds to an annual luminosity of about500pb?1.With such high lumi-

nosity the measurement of small asymmetries at low x in polarised?→e?→p scat-tering is statistically feasible.Polarised electron-proton interactions require a

high proton polarisation transferred through the accelerator chain,polarime-

ters and Siberian snakes for spin rotation.Because of the small anomalous magnetic deuteron moment,polarised?→e?→d collisions may possibly be realised more easily than?→e?→p.Ions can be accelerated at HERA with some modi?-cations regarding the source,electron cooling in PETRA for heavier nuclei to counter intrabeam scattering(IBS)e?ects,longer bunch trains and ramp cy-cles.The luminosity is then estimated to scale as L A?L p/A.For deuterons the IBS time exceeds2h and a luminosity of3.5·1031cm?2s?1can be achieved without cooling.Thus a high luminosity ed run can be done right after com-pletion of HERA-2without any major modi?cation beyond the source.

The EIC9is a project to be proposed in about2005as an intense polarised

electron-ion collider using electron-beam-cooled ions in RHIC(of up to E p= 250GeV proton and E Au=100GeV/A gold energy)colliding with electrons

from a ring or linear accelerator of10GeV maximum energy.Thus the EIC

has10times less energy

THERA DETECTOR

5m0?5m

Figure2.Feasibility design of a detector to operate at the ep collider THERA in the TeV range of energy.The detector is equipped with silicon tracking and plug calorimeters in p and e beam direction,in order to measure forward going jets and backward scattered low x electrons very closely to the beam axis.It can be supplemented with tagging detectors for forward p,n and backward e,γ.The radial dimensions are determined by the proton beam energy.Thus a THERA detector resembles H1or ZEUS with tracking,calorimetry, solenoid magnetic?eld and muon detection.

precooling in PETRA and dynamic focusing.Much of the low x physics,how-ever,can be done with less than10pb?1which opens the exciting possibility to perform the low x ep programme simultaneously with precision electroweak e+e?physics.

THERA would complement the e+e?and pp colliders in the TeV energy range and cover a huge?eld of physics as well as accessing the smallest Bjorken x values.A superconducting linac with a bunch crossing time of a few100ns could also be combined with the Tevatron.An electron ring of60GeV energy on top of the LHC would access about the same kinematic region as THERA with an estimated luminosity10of≥1032cm?2s?1.

lxdatong2:submitted to World Scienti?c on February1,20085

4Summary

Deep inelastic lepton-nucleon scattering has been a most exciting?eld of re-search over three decades.HERA has contributed enormously to the devel-opment of strong interaction theory and it will continue to do so with high precision data for quite some time.While many new e?ects have been ob-served,they still give rise to basic,yet-unanswered questions in the?eld of low x physics.These regard the saturation of the rise of F2and xg,the cross-section behaviour down to x~10?8,the mechanism of gluon emis-sion,the nature of di?raction,the theory of heavy quarks in the proton,the unknown spin structure at low x,etc.Con?nement is not understood,and nearly all mass is carried by nucleons,the deep structure of which has only just been addressed experimentally.Future experiments have to aim at max-imum accuracy,to extend the kinematic range,to reach regions of higher quark and gluon density,to accelerate nuclei in colliding eA mode as well as to study polarised lepton-nucleon interactions at much higher energy than hitherto.High-luminosity DIS has a discovery potential which requires con-tinuing e?orts.These successfully focus on three projects,HERA-3,the EIC and THERA,which all deserve to be realised,since they cover di?erent re-gions in phase space at di?erent times.One hundred years after Rutherford, lepton-nucleon scattering experiments exploring the deep structure of matter will continue to attract our attention.

References

1.A.Z.Gazizov and S.I.Yanush,astro-hep/0105368(2001),and ref.cited.

2.A.Caldwell,A New Detector for Low x P hysics at HERA,Inv.Talk

at the Durham Workshop on the Future of Lepton-Nucleon Scattering, https://www.wendangku.net/doc/4f873085.html,/,2001.

3.L.Frankfurt and M.Strikman,Eur.P hys.J A5,293(1999);

M.Strikman,private communication.

4.S.Bass and A.DeRoeck,hep-ph/0111377and ref.cited therein.

5.T.Sloan,Spin P hysics at HERA,Inv.Talk,Durham Workshop,ibid.

6.H1Collaboration,C.Adlo?et al.,Eur.P hys.J C21,33(2001).

7.The THERA book,DESY01-123F,vol.4,http://www.ifh.de/thera.

8.D.Barber,G.Ho?staetter and F.Willeke,Inv.Talks,Durham Work-

shop,ibid.

9.I.Ben Zvi,T he EIC,Inv.Talk,Durham Workshop,ibid.

10.E.Keil,Talk at Snowmass2001,M5Working Group,unpublished. lxdatong2:submitted to World Scienti?c on February1,20086

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高中物理学史人物大全

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