physics and physics prospects at hera - desy€¦ · 21 jun 2004 physics and physics prospects at...

Physics and Physics prospects at HERA

58th Extended Scientific Council / 130th Scientific Council21/22 June 2004

Yuji Yamazaki (KEK, ZEUS)

On behalf of the H1, ZEUS, HERMES and HERA-B collaborations

21 Jun 2004 Physics and physics prospects at HERA, 58th EWR / 130th WR 2

The “Super Electron Microscope” HERA

Explore the structure of the nucleon by electron beam

Deep-inelastic scattering (DIS)Scattering angle and energy of electron → momentum distribution of the quarks

High energy= short wave length

Scattering angle ~ Q2:mass of the exchanged particleCentre-of-mass energy 314 GeV≈ up to 10–18 m resolution

e p

Q2 ~ 11000 GeV2


HERA-I and HERA-II running

HERA-I: 1992-2000, Integrated Luminosity ~ 100pb−1

HERA-II: 2001-2007, Int. Lumi > 700 pb-1, with lepton longitudinally polarisedStartup problem as previously reported

Synchrotron radiation to detectorProton-beam and residual beamgas collisions producing

Standing current in chambersLoad to trigger / DAQ

All of them are solved !

Oct 03

22 May 2004


HERA-II running

H1 takes 42 pb-1 in 2004cf. HERA-I 2000: 51 pb-1 (8 months)ZEUS slightly behind, following up

HERA II has begun

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0.60.70.80.9

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-100 -50 0 50 100 150

Oct 2003 - Jun 2004days from 1.1.2004

( C

JC2

- I 0

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nRad

of

Dec

200

3 )

/ ( e

-gas

+ p

-gas

of

Feb

200

4)

Oct 2003 - Jun 2004

IeIp < 500 mA2

Oct 2003 - Jun 2004

IeIp < 1000 mA2

Oct 2003 - Jun 2004

IeIp < 2000 mA2

Oct 2003 - Jun 2004

IeIp > 4000 mA2

Oct 2003 - Jun 2004

IeIp < 4000 mA2

leak

NL

11m

for 5000 mA2

recentruns


HERA physics: Questions to be answered

Structure of the nucleonsHow is it composed by quarks and gluons ?Where the spin of the nucleon (½) come from ? HERMES

The nature of the interactionsStrong interactions – quantum chromo-dynamics (QCD)

Force between quarks, mediated by gluonsElectroweak interactions

The physics beyond standard modelIs the quark a fundamental particle ?New interactions between leptons, quarks and gluons

partons


Structure of the proton – simple view

More resolution: we see “small” partonsHow does the quarks behave at low-x ?

Increasing resolution (large angle scattering = large Q2)

gluon splitsinto quarks

Quark splitsinto gluon splitsinto quarks ...


“Structure function” ∝ quark density

Strong increase of sea quarks towards low x

F2(x) = e2 x(q(x)+q(x))

Density increase with Q2

More partons by magnified view

quark density

Dynamic creation of quarks at low-x


Evolution of thequark densities

Precise measurements in wide range of kinematics

except for high-xScaling violation in Q2

described well by “DGLAP” evolution equation (blue bands)

Dynamic production of quarks and gluonsunderstood by perturbative QCDExtraction of gluon density from F2 data

10-3

10-2

10-1

1

10

10 2

10 3

10 4

10 5

10 6

1 10 102

103

104

105

Q2 / GeV2

F 2 ⋅

2i

x = 0.65, i = 0

x = 0.40, i = 1

x = 0.25, i = 2

x = 0.18, i = 3

x = 0.13, i = 4

x = 0.080, i = 5

x = 0.050, i = 6

x = 0.032, i = 7

x = 0.020, i = 8

x = 0.013, i = 9

x = 0.0080, i = 10

x = 0.0050, i = 11

x = 0.0032, i = 12

x = 0.0020, i = 13

x = 0.0013, i = 14

x = 0.00080, i = 15

x = 0.00050, i = 16

x = 0.00032, i = 17

x = 0.00020, i = 18

x = 0.00013, i = 19

x = 0.000080, i = 20x = 0.000050, i = 21 H1 e+p

ZEUS e+p

BCDMS

NMC

H1 PDF 2000

extrapolation

H1

Col

labo

ratio

n

large x

small x


Extraction of parton densities in the proton

Gluon density obtained from the QCD fitPrecise determination of low-x parton densitiesGluon > quarkat low-xStill need precision:

High-x partons: constrained by fixed target dataLow-x: we can do better with more statistics

← homework for HERA-II

u valence

sea (x 0.05)

gluon (x 0.05)

d


Probing gluons by hadronic final state

Photon does not couple to gluons directly ... – hit a gluon by a quark instead At high energies, quark and gluon momentum can be reconstructed as jets

Scattered e

Jet 1

Jet 2


Gluon density from jets

10 15 20 25 30 35 40 45

10-3

10-2

10-1

1

10

102

103

104

105

106

107

(GeV)BT,jet

E (

pb/G

eV)

B T,je

t/d

Eσ

d

ZEUS 96-97 (DIS data) Jet Energy Scale Uncertainty

: 0Zδ × hadδ × NLO QCD

ZEUS-JETS (prel.) 94-00 total uncertainty

2 < 250 GeV2125 < Q

2 < 500 GeV2250 < Q

2 < 1000 GeV2500 < Q

2 < 2000 GeV21000 < Q

2 < 5000 GeV22000 < Q

2 > 5000 GeV2Q

)5 10×(

)4 10×(

)3 10×(

)2 10×(

10)×(

1)×(

10 15 20 25 30 35 40 45

10-3

10-2

10-1

1

10

102

103

104

105

106

107

ZEUS

gluon densityfrom DGLAP

Jet cross section well described→ Gluon density from

DGLAP is “confirmed”This means: we can predict cross section using the parton density (universality !)e.g. cross section for LHCDynamically produced partons are quite well understood


Particle production near the proton

Large number of particles produced between the struck quark and the proton remnantNo unique method to describe “multi-body decay”Large excess over DGLAP framework

Many idea coming out for explaining the “ordering” of particles

0

500

1000

1500

dσπ/

dx [n

b] 2 < Q2 < 4.5 GeV2

● H1 data DIR + RES

CCFM (CASCADE)

DIR CDM mod. LO BFKL

0

200

400

4.5 < Q2 < 15 GeV2

0

25

50

75

100

10-4

10-3

15 < Q2 < 70 GeV2

x

p*

T,π> 2.5 GeV

What ishappeninghere ?


Gluon density and heavy flavour production

Heavy flavour: only produced as a pair from a photon-gluon fusionData tend to be above NLO at low pT (transv. momentum)Important to understand the production mechanism

e.g. standard model background to Higgs at LHC

Statistics limited Continue towards HERA-II

0

2

4

4 6 8 10

H1 (Prel.)

ZEUS

-0.55 < ηµ < 1.1

-1.6 < ηµ < 2.3

NLO QCD ⊗ Had

ep → ebb−X → ejjµX

Beauty Production

pµt [GeV]

Dat

a/T

heo

ry

Q2 < 1 GeV2; 0.2 < y < 0.8

pjet1(2)t > 7(6) GeV; |ηjet| < 2.5


b-quark productionat HERA-B

Experiment: wire-target to the proton beamb-quark production cross section in pp collisions is also uncertain

Large difference in the past experiments

Reducing uncertainty by new HERA-B data5x data being analysed

1

10

10 2

400 500 600 700 800 900

R. Bonciani

N. Kidonakis

et al.

et al.E789E771

Preliminary Hera-B 2004

Proton energy (GeV)

σ(

) (

nb/n

ucle

on)

bb–


Measuring strong coupling constant αs

Measurement of probability to radiate a partonBut parton cannot be observed directly – worst precision among four forces2-jet probability etc. sensitive to αs

strong coupling const.


HERA measurements of αsRunning in single experimentPrecision competitive, thanks to the high energy of HERA

Theoretical uncertainty from hadronisation is small

0.1

0.15

0.2

0.25

10 102

Bethke 2002

ZEUS (82 pb-1) (inclusive jet γp - µ=ET jet )

ZEUS (38 pb-1) (dijet DIS - µ=Q) ZEUS (39 pb-1) (inclusive jet DIS - µ=ET

jet ) H1 (33 pb-1) (inclusive jet DIS - µ=ET

jet )

µ (GeV)

αs

th. uncert.

exp. uncert.

World average (S. Bethke, hep-ex/0211012)

Dijet cross sections in NC DIS ZEUS (Phys Lett B 507 (2001) 70)

Inclusive jet cross sections in NC DIS ZEUS (Phys Lett B 547 (2002) 164)

Inclusive jet cross sections in NC DIS H1 (Eur Phys J C 19 (2001) 289)

NLO QCD fit ZEUS (Phys Rev D 67 (2003) 012007)

NLO QCD fit H1 (Eur Phys J C 21 (2001) 33)

Jet shapes in NC DIS ZEUS (DESY 04-072 - hep-ex/0405065)

Subjet multiplicity in NC DIS ZEUS (Phys Lett B 558 (2003) 41)

Subjet multiplicity in CC DIS ZEUS (Eur Phys Jour C 31 (2003) 149)

Inclusive jet cross sections in γp ZEUS (Phys Lett B 560 (2003) 7)

0.1 0.12 0.14αs(MZ)


HERA-I study of proton structure and QCD(collider experiments)

HERA-I study have unveiled:The partonic structure of nucleons at low-xRapid increase of quarks and gluons towards low-xThe QCD description of the hadronic final state

gluon density checked by jetsprecise αs determination

Some subjects not completely understood : Forward production, heavy flavour → need more study with HERA-II

Most of the subjects at HERA cannot be covered heree.g. diffraction (proton stays intact)


HERA-LHC workshop

“Synergy” between running and future experimentsSubjects:

Parton densities at LHCMulti-jets, energy flowHeavy quarksDiffractionMonte-Carlo simulation

March 2004: 300 persons gatheredFinal report Jan 2005

Light Higgs ?


Nucleon spin by HERMES – motivation

Where the ½ spin of the nucleon comes from ?

Naive expectation: u↑u↑d↓

but measurement : only 12 % from valence + sea quarks (EMC, 1988)

From gluon ? From angular momentum of the quarks / gluons ?

( )gq LLG ++∆+∆Σ=21

21


How to measure the proton spin contributions

Longitudinally polarised nucleon (gas target) and electron beam

Observable: cross section asymmetry in spin parallel and anti-parallel

Photon can scatter with quarks with spin anti-parallel – photon spin absorbed by quark spin flip


0

0.02

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10-4

10-3

10-2

10-1

xg1

HERMESHERMES prel. rev.SMCE155E143

proton

HERMES ´ 00 prel.

deuteron

E142E154JLAB

neutron (3He)

x

Quark spin from polarised structure function

g1 – probability to find a quark with spin positivePrecise measurement by HERMES Flavour decomposition from p, n and deuteron target

u : ↑, d : ↓ polarisedFrom strange quark, sea quark ?

uud

uud+udd

udd

)()(

)()(

1

1

xqxqg

xqxqF

↓−↑∝

↓+↑∝


Flavour decomposition by semi-inclusive DIS

Tag the quark flavour by fastest hadronsSea quark spin is extracted without any assumption on the flavour symmetrySea quark spin slightly negativeValence is ~30 % where is the spin ?

0

0.2

x⋅∆u

-0.2

0x⋅∆d

-0.1

0

x⋅∆u–

Q2 = 2.5 GeV2

GRSV2000LO stdBB01 LO

-0.1

0

x⋅∆d–

-0.1

0

x⋅∆s

0.03 0.1 0.6

x

u

du

*γ

π+

(E, p )’ ’

N

e

q

π

h

h

(E, p)

ud ≈


Extracted gluon polarisation from world data

Using spin-dependent QCD evolution of the parton densitiesGluon, seems positive, but large uncertainty (small lever arm)Current decomposition: Valence+sea 21 ± 13 %, gluon 41 ± 27 %

Where is the rest ?

-0.4

-0.2

0

0.2

0.4

0.6

0.8

10-3

10-2

10-1

1x10-4

10-3

10-2

10-1

1

10

10 2

10 3

10 4

10-1

1 10 102

E143

Q2, GeV2

HERMES

E155

EMC

SMC


Now at HERMES: transversity h1(x)Unmeasured polarised structure function – world premier

through azimuthal asymmetry of the produced hadrons: semi-inclusive DIS

If quark is massless, h1 ~ g1Quark mass effect to spinValence/sea separation:

Expect (both mainly from u)First result:

largely negative: unexpectedDisfavoured fragmentation u-quark → π−?

Run until summer 2005Also addressing the angular momentum effect by measuring “Sivers” anglethen measuring DVCS

-1

-0.5

0

0.5

1

-1

-0.5

0

0.5

-1

-0.5

0

0.5

1

0.1 0.2 0.3

Pπ⊥/(zMπ) weighted

ACπ

π+

π-

x

π0

z0.3 0.4 0.5 0.6 0.7

beam

)(~1 qqxh δδ −||~|| pp AA −+ ππ

pA −π


HERA-II physics : objectives

Unique facility worldwide: ep collision with e± polarisationHigh luminosity for colliders

Nature of the valence quarks in the proton – density, flavour← high-x, high-Q2 cross sections are smallQCD study requiring large amount of data

b-quark production, jets at high-pT

Diffraction with jets, heavy flavourPolarisation also for the collider exp’t H1 and ZEUS

Electroweak interactionFlavour decomposition of the quarks

Searches: physics beyond standard modelHERMES: deeply virtual Compton scattering (also H1/ZEUS)

→ Next slides: Reviewing HERA-I status, prospect for HERA-II

monthly

↓yearly


Disentangling quark flavours by weak int’n

Charged current (CC) – exchanging W ±

Coupling to:up-type quarks for e−

down-type quarks for e+

spin polarisation: (1±Pz) dependence

High-Q2 neutral current (NC)exchanging Z0 ( )

Cross section difference

is sensitive toSpin polarisation gives similar effect

⎟⎠

⎞⎜⎝

⎛⎟⎠

⎞⎜⎝

⎛⎟⎠

⎞⎜⎝

⎛bt

sc

du

220ZMQ ≈

qq −)()( +− − ee σσ


Charged current and parton densities

Example : full HERA-I data(100pb-1), e+ beam

Different dependence in ydecompose ( ) and ( )→ distinguishing flavourSensitive to d

Precision limited by statistics

Increasing statistics is main goal for HERA-II

0.5

1

1.5

2

0.2

0.4

0.6

0.8

10-2

10-1

0.05

0.1

0.15

0.2

10-2

10-1

10-2

10-1

HERA e+p Charged Current

Q2 = 280 GeV2

σ∼

H1 e+p 94-00

ZEUS e+p 99-00

SM e+p (CTEQ6D)(1-y)2x (d+s)x (u

_+c

_)

Q2 = 530 GeV2 Q2 = 950 GeV2

Q2 = 1700 GeV2 Q2 = 3000 GeV2 Q2 = 5300 GeV2

Q2 = 9500 GeV2 Q2 = 17000 GeV2

x

sd +cu +


Precise measurement of the valence quark density at HERA-II

Very high-x valence density is not well known(d + u) valence quark by NCd valence by e+ CCxF3 ∝ σ(e+)−σ(e−) ∝

= SLAC = BCDMS = NMC

500 pb-1

qq −

x

xF3γZ

(x)

Q2=1500 GeV2

Simulation

0.01 0.1 10

0.2

0.4

0.6GRV94CTEQ6.1MRST03H1 PDF2000data

x

xF3γZ

(x)

Q2=1500 GeV2

Simulation

0.01 0.1 10

0.2

0.4

0.6GRV94CTEQ6.1MRST03H1 PDF2000data

500 pb−1 e±

100 pb−1 e±

NC simulation for HERA-II

xF3


NC/CC cross section and electroweak unification

Cross section of NC and CC becomes similar atQ2 ≈ MW

2, MZ2

Manifestation of the electroweak unification

Study at HERA-II – electroweak parameters in detail with polarisation e.g.

Electroweak coupling constants to u- and d-quarks(vu, au) and (vd, ad)

10-7

10-6

10-5

10-4

10-3

10-2

10-1

1

10

103

104

HERA

H1 e+p CC 94-00

H1 e-p CC

ZEUS e+p CC 99-00

ZEUS e-p CC 98-99

SM e+p CC (CTEQ6D)

SM e-p CC (CTEQ6D)

H1 e+p NC 94-00

H1 e-p NC

ZEUS e+p NC 99-00

ZEUS e-p NC 98-99

SM e+p NC (CTEQ6D)

SM e-p NC (CTEQ6D)

y < 0.9

Q2 (GeV2)

dσ/

dQ

2 (p

b/G

eV2 )


Physics with polarised positron beam: charged current from HERA-II

First measurement of CC with polarisationIf right-handed current exists, the cross section at 100 % RH polarisation is non-zeroConsistent with no right-handed current so far

0

10

20

30

40

50

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1Pe

σ CC (

pb)

H1e+p→ν

−X

Prelim. (L=15.3pb-1)

ZEUS

Prelim. (L=6.6pb-1)MRST

Q2>400 GeV2

y<0.9

Data with opposite pol. being collected


Size of the quark

Finite size of the quark→ cross section becomes smaller at high-Q2

Short wave cannot see a blur objectquark becomes transparent

Cross section agrees with SM: quark is point-like down toradius Rq < 0.7×10−18 m (< 1/1000 of proton – H1 and ZEUS)

Q2 (GeV2)N

/NC

TE

Q5D

Quark Radius Limits

ZEUSZEUS 94-00 e±pRq

2 = (0.85 ⋅10-16cm)2

Rq2 = -(1.06 ⋅10-16cm)2

1

10

103

104

0.8

0.9

1

1.1

1.2

103

104

point-like quark

large quark

Expectationwith finitequark size


Search for new particles and interactions

Only Tevatron (2 TeV ) and HERA are the high-energy colliders in operationGiving world best limits on:

Leptoquark (next slide)Flavour-changing neutral current

Lepton-flavour violationObserved in neutrino sector !Quark flavour violation Excited leptons e*, ν*

Super-symmetric particlesExtra-dimension etc.

e*e

p

γ / Z

e, ν

γ / Z

p, x

γ, Z, W

ppLeptoquark

LeptonFlavorViolation

Excited leptons


Leptoquark: quark-lepton resonance

Models from grand-unified theory, super-symmetry etc.Searching for a resonance in the eq = e+jet mass spectrumIn large region of parameters HERA gives best limitOther region: complementary to Tevatron and LEP

→ jet

E. Perez, Lepton-Photon 03


High-pT lepton + missing pT excess ?

SM: through W production

Not abundant at large pT

X

e�p� e��X

Event MUON-3

P�T � ��GeV� PXT � ��GeV� Pmiss

T � ��GeV

M�� GeV W�

� �� Candidate

H1


High-pT lepton + missing pT excess ?

SM: through W productionNot abundant at large pT

X

6 events observed/1.08 expectedfor e + µ decays in H1No excess in e + µ decays in ZEUSLess significant: 1 / 0.06 expected for τ channel in ZEUS

New particle decaying toa neutral + lepton ??


High-pT leptons in HERA-II

H1 analysis on high-pT leptons with HERA-II dataFor pT

X > 25 GeV2 events / 0.63 expected

agrees with the HERA-I yield

More luminosity eagerly awaited

Large pTX events


Deeply virtual Compton scattering (DVCS)

Process to take out a quark with (x1, kT1)then put back with (x2, kT2)

Correlation of two partons in a nucleonYou can rotate a proton – sensitive to the angular momentum of the quark

Amplitude through the interference with normal Compton events

Phase is also known – 3D reconstruction of proton structureChanging interference pattern by e+/e− beam, L or R polarisation

2Q

γ*

x1

p

γee’

x 2

p’

V. Belitsky,D. Müller,hep-ph/0206306

Tagscattered

proton


DVCS – measurement plan 2004~

H1, ZEUS studies: low-x, gluon sectorReconstructing kT1 and kT2:

Recoil proton angle+momentummeasured by new recoil detector at HERMES(summer 2005)H1 very forward proton detector(in operation from spring 2004) φ, rad

Ach

(φ)

e± p → e± p γ

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

-3 -2 -1 0 1 2 3

φ assymetry by e+/e- beam

1 x

p’p

2γ

e’γ∗e

2x

Q


Summary

HERA-I study being complete, understanding …The partonic structure of nucleons at low-xNucleon spin carried by quarksThe QCD description of the hadronic final state– but some needs more data for better understanding

HERA-II : unique machine world wide with e±, pol’nProton structure at high-x, high-Q2

Electroweakand maybe surprise …

The issue is the integrated luminosity for precisionWe need planned increase of order of magnitude in luminosity w.r.t. HERA-I

physics and physics prospects at hera - desy€¦ · 21 jun 2004 physics and physics prospects at...

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