HERA II Physics

Brian Foster - DIS01 - Bologna

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HERA II PhysicsBoth ZEUS & H1 have made major upgrades in order toutilise the increase in HERA luminosity to the full.


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HERA II PhysicsThe upgrades concentrate mainly on the following areas:

- Vertex region (MVD) - Studies of heavy quarks, exotics, etc.

- Forward region (STT) - Improved kinematic reconstruction of both jets and scattered electron.

- Luminosity monitor - Cope with greatly increased synchrotronbackground + likelihood of multipleoverlapping Brems. Photons + physics payoff for improved lumi. precision ~ 1%

- Trigger & electronics - Important for all physics. On ZEUS,triggering stage between Levels 1 & 2 -“Fast Clear” that has been available but never needed to be used; and including MVD in Level 2.

- Polarimeter - ZEUS involved with TPOL positionmeasurement.


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Vertex RegionThe ZEUS MVD is a silicon strip detector constructed from n-typesilicon with p+type implants and 20µ pitch strips, readout with 120µ pitch. There are 712 sensors and > 200K readout channels.


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Vertex PhysicsThe major physics topic will be the flavour composition breakdown of the proton (and indeed the photon). We already have quite precise measurements of the charm-quark structurefrom HERA I.

Very precise measurements will be possible for HERA II;also gives accurategluon determination.

Accurate b contributionto F2 will becomepossible

500 pb-1500 pb-1


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Vertex PhysicsA high-precision, high-acceptance vertex detector willallow both collaborations to make a full flavourdecomposition of the inclusive F2 structure function.

For example, charmsignal in charged currents in principal measures thes-quark density (+ leadingparticles in NC etc.; but alsocompeting non-s digrams in CC)At HERA II, both singlet & non-singlet pdfsu, d, s (CC DIS)c, b, g (NC DIS)can be determined with good accuracy.


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Forward PhysicsZEUS major upgrade in forward direction replacement of TRD’swith two stations of straw-tube chambers, each with 3 stereolayers.

The forward wheelsin the MVD also give improved resolution inforward direction.


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Forward PhysicsMajor payoff in increased accuracy for high-Q2 NC

The improvedaccuracy inforward directionwill also greatly improve resolutionon forward jets etc.and therefore“BFKL” non-DGLAPevolution studies.

1000 pb-1

x = 0.65

Q2 = 4 ×× 104

[ ]%3

003.0S

≈∆

≤∆

xg

xg

α

1000 pb-1

Q2 = 2 GeV2

Q2 = 20 GeV2

Q2 = 200 GeV2

Q2 = 2000 GeV2


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Active

FilterCalorimeter

ZEUS

6m Tagger

Spectrometer

e detector

Thin window

5.3 m

92 m

105 m

105.5 m

Spectrometer

dipole

95.2 m

ZEUSI.P.

γe

Lumi MonitorZEUS went for “belt & braces” approach: two devices withvery different systematics plus precision electron tagger.

“Standard” Pb/scintillatorcalorimeter plus “activefilter” of aerogel.

Dipole spectrometer tomeasure converting e+e- pairs.

“6m tagger” W/fibre tomeasure the energyafter the bremms.

Aim is to get to around 1% error in luminosity.


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PolarisationHERA II will give us access to a qualitatively new regionof physics via longitudinal polarisation.

Spin rotators will be commissionedaround H1 & ZEUS IRs.

Although thetheoreticalattainablepolarisation will fall somewhat because of the new lattice, we expect similar performance to that enjoyed until now by HERMES.

Laser

Laser

LPOL

TPOL

HERA ring

Spin rotator (new)

Spin rotator (new)

electrons

HERMES

ZEUSH1

HERA-B0 0.5 1 1.5 2


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PolarisationAccurate measurement of both transverse (TPOL) and longitudinal (LPOL) polarisation plus machine lattice simulationwill give us confidence in an accurate measurement at IP.

To do this we need accurate, short-time scale, determinationof the polarisation, ideally bunch-by-bunch. This is a challenge in high synchrotron radiation environment.

Key is to use back-scattered laser light and precisiondetectors and DAQ.


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PolarisationFor longitudinalpolarisation, consider situation in thephoton-electron CM frame

21=ZL

23−=ZL

Measure ENERGY asymmetry of back-scattered Compton photons:

Angular dependence ofscattering in CM systemdepends on degree ofpol. Boosting back tolab converts this angular asym. toan energy asymmetry.Measure with accurate calorimetry, preferably bunch-by-bunch.


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PolarisationFor transverse polarisation, the spin direction already definesa spatial axis wrt the beam direction, so that it is clear thatthere will be asymmetries in this axis depending on the relativefraction of up- and down-polarised electrons.

( ) ηΚ−= RL yyP

Requires high-precision radhard position detector to detect theup-down asymmetry.


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Polarisation

LPOL implementation (and overall DAQ) responsibility of French groups in H1; uses Fabry-Perot cavity to increase continuous laser power to allow operation in single-photon mode.

Advantages are in-situ calibration using Compton edge, high rateand thereby bunch-by-bunch measurement.

TPOL implementation - position-sensitive detector responsibility of IC London & Tokyo ZEUS groups.


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Polarisation physicsThe accurate measurement of polarisation opens up a completelynew domain of HERA physics at high Q2.

( )

onpolarisati

1CCpe

=

±∝±

P

Pσ

Resolution on MW

80 MeV

Exclusion limit

MW(R) > 400 GeV


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Polarisation physicsPolarisation dependence of cross-sections is a LARGE effect

For example,@ Q2 = 104 GeV2,

x = 0.2, σ(eL

-) / σ(eR-)

is almosta factor 2, andcan bemeasured toabout 10%tot. uncertainty


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Polarisation physicsQuark coupings can be accurately measured, e.g. light quarkcouplings by looking at differences between σ(L,R).

Great improvementover unpolarisedcase.

%17%17

%6%13

d

u

av

beamper pb 250

%70 ,

1-

RL, ±=± Pe


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Beyond SM & Pol.Beyond SM - maybe one will see new right-handedparticles!

For any new physics signal, polarisation is a very preciseand flexible tool - one can ~ “turn off” SM backgroundsby varying polarisation - if the new physics hasdifferent couplings to SM - which seems verylikely - that the S/B will be enhanced.

HERA is a unique facility for probing some aspectsbeyond the SM - it is our duty to ensure that we exhaust thephase-space it can reach - e.g. in CM energy, inpol. and charged lepton states - to maximise discovery potential.

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HERA II Physics