Recent results from ep -scattering at HERA

8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki

1

Recent results fromep-scattering at HERA

Universität SiegenPhysikalisches Kolloquium

8 Jun 2006

Yuji Yamazaki (KEK)

8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 2

Contents of the talk

Introduction: The ep-collider HERA and its objective

Bread-and-butter physics at HERA: The structure of the proton

More on the quark-gluon interactions:The QCD (Quantum Chromo Dynamics) studies

The must for a high-energy collider:Electroweak force studySearch for particles beyond the Standard Model

HERA physics relevant to the LHC


HERA: The super electron-microscope for inside a protonOptical microscope

, -rays fromisotopes

Electron microscope

X-ray sourcesSynchrotron radiation

Electron beamHeavy-ion beam

Electron-protonscattering HERA

bird flu virus

euglena

Diffraction pattern of synchrotronradiation light from bio-moleculestaken at HASYLAB in DESY A modern implementation of

the Rutherford scattering at RAL

Exploring the nucleon spin:HERMES experiment at HERAwith electron beam and fixed target


How do we see the structure in practice

Longitudinal momentum fraction of the quark, x from the scattering

angle and energy The larger the scattering

angle is … the larger momentum

transfer (Q2) through the virtual photon (*)

the finer resolutionto 1018 m scattered e

scattered hadrons

e+ / e beam

protonbeam

Deep-inelastic scattering (DIS)

Q2

22 1

Q

Colourflow:manyhadrons


What we know about proton before HERA

Increasing resolution (large Q2)

gluon splitsinto quarks

early fixed target exp’ts Q ~ 1-3 GeV (101 fm)

proton diameter~ 1015 m (1fm)

recent fixed target + muonQ ~ 1-10 GeV (102 fm)

22 1

Q

Constituent of a proton is quarks and gluonsNamed as partons(= quarks + gluons)

The interaction between quarks and gluons:QCD (Quantum ChromoDynamics)


Quantum Chromo Dynamics (QCD) Describing interaction

between partons colour charges exchanged

by gluons The coupling constant

becomes smaller as the energy becomes higher Asymptotic freedom

Novel prize in physics 2004 This means: the force

between quarks is strongeras they are apart Quarks are confined

from Novelprize.org

s

S4

9

rr 1)(

rr ~)(

q

q

q

q

q

q

High energy in the potential field meson

meson

r


Structure function: status 1991

What is the observation ?

recent fixed target + muonQ ~ 1-10 GeV (102 fm)

HERAQ > 1-300 GeV (< 103 fm)

Courtesy C. Gwenlan, K. Nagano

Quarks emita gluon,which splitsinto quarks ...

Increasing resolution (large Q2)

Str

uctu

re f

unc

tion

qua

rk d

ensi

ty


HERA: the only electron-proton collider

Circumference: 6.3 km(similar size to the Tevatron at Fermilab)

Proton beam: 920 GeV Electron/positron beam:

27.5 GeV

centre-of-mass energy

Resolve structure down to 1018 m

220 bunch operation(96ns bunch spacing)

Operated from 1992

GeV 318s


DESY and HERA in west of Hamburg, Germany

ZEUS

Hermes (fixed nucleus target exp’t)

H1


HERA performance, luminosity upgrade HERA-I delivered 190pb-1

Luminosity upgrade (HERA-II) Production run from 2004 With final focusing magnets

3 specific luminosity Achieving ~ the design

Instantaneous luminosity 5.21031 cm2s1

370pb-1 delivered, ~250pb-1 collected

Longitudinal polarisation of e+/e– for colliding experiments


“Structure function” F2(x, Q2): observation 2000

F2(x) = e2 x(q(x)+q(x)) F2 is proportional to the

quark charge density q(x) at given x and Q2

Strong increase of sea quarks towards low x

22 ,QxF x

22 1

Q

rest of the proton

scattered quark

Deep-inelastic scattering (DIS)

Momentum fraction

“Saturated” behaviouris excluded


Number of quarks increases as we see closely

Sea quarks are dynamically produced at high-Q2

Predicted by the DGLAP (Dokschitzer-Gribov-Lipatov-Altarelli-Parisi) evolution equation Based on the perturbative QCD

Higher Q2

Higher Q2

p p

e

e'

evol

utio

n

You cansee gluonsindirectly

Quark density decreasing at high-x with Q2


DGLAP can predict the quark density vs Q2

at high-x : quarks radiate gluons momentum of quarks goes

down providing low-x gluons

at low-x : a gluon radiates

a qq pair gluon also radiates gluons driving a rapid increase of

the quark+gluon densityFixed target)(

log 2 xgQd

d


Parton density through the DGLAP analysis

Gluons from a DGLAP fit to the data Indirect extraction

Steep increase of gluons; gluon dominates at low-x Precise determination at HERA

BeforeHERA


Measuring gluons more “directly”

Photon does not couple to gluons directly ... hit a gluon by a quark instead Look at a high-pT qq pair production (jets)

and heavy-quark pair (QQ) production

Scattered e

Jet 1

Jet 2


The extracted gluon is universal

The extracted gluon + perturbative QCD calculation:agrees with the measurement

i x

fiT

fRsi

T

zfdE

zQddz

dE

Qd),(

),),(,(ˆ)( 22222

parton density:mainly gluons

parton-electroncross section

s

QCD calculationusing the partondensity extractedat HERA


Heavy quark production: for understanding gluons

Heavy quark production is reproduced by HERA parton densities

Gluon density is double-checked

s


The strong coupling constant s a fundamental parameter in the standard model Least well-determined qty

in the standard model Better measurement at high

energy: theoretical uncertainties are smaller

“See” parton’s behaviour by Using jets/HQs From the DGLAP analysis etc.

s

s

s

s

sDGLAP evolution


HERA measurement of s

s is running in single experiment Some of them give best precision among collider measurements

(LEP, Tevatron and HERA)

FromDGLAP

Some newest measurements not included in this summary


A small detour, back to the proton structure Introduction:

The ep-collider HERA and its objective Bread-and-butter physics at HERA:

The structure of the proton at high-x More on the quark-gluon interactions:

The QCD (Quantum Chromo Dynamics) studies The must for a high-energy collider:

Electroweak force studySearch for particles beyond the Standard Model



Disentangling valence quarks

Valence quarks: carrying the quantum number of the proton Fermion 3 quarks Charge +1 (Isospin +½) (uud)

Sea quarks:

Flavour decomposition by charged current (CC) up-type quarks with e

down-type quarks with e+

different scattering angle distribution for q and q

u

u

d

ss

b

t

s

c

d

u ee

ssdduu


Example of the CC measurement

Example: e+p CC produced

uniformly in angle d, s : neutrino produced

only in small scattering angle

Statistical error is largeHERA-II analysis ongoing

cu ,


Neutral current (NC) with e+ and e beam:disentangling valence and sea Difference of e and e+ scattering

gives amount of valence quarks (e)(e+)

Precision improved with large HERA-II e sample (130pb1) 16pb1 for HERA-I

qq From Z0 and Z0- interference

(e

)(

e+)


Valence quark from NC (cont’d)(

e)(

e+)

Now ~ 100pb1 for e+e each, will be 200pb1 each

frac

tiona

l unc

erta

inty

Valence quark uncertainty reduced,in particular for u-quark


Unification of the electromagnetic and weak force

Weak force appears weak because of the heavy

mass of the weak boson exchanged through

uncertainty principle at low energy

At high energy, the- neutral (, Z0) and- charged (W) currenthave similar cross sections

NC

CC

NC CC

HERA-I result


Polarisation dependence of the charged current cross sections

HERA-II: polarisation of electron for colliding exp’ts

Right-handed current is forbidden in SM V-A: vector and axial vector

coupling cancels Linear dependence on

polarisation

More right-handedMore left-handed

MW(Right-handed) > 180-190 GeV in 95% CL


Determination of the electroweak coupling constant to Z0 for u,d quarks (au,d, vu,d)

Standard model prediction to W:aq = vq = 1/2

to Z0: aq = 1/2 (u,c,t)

1/2 (d,s,b) vq = 1/2 2·(2/3)sin2w 0.19 (u,c,t)

1/2 2·(1/3)sin2w 0.34 (d,s,b)

LEP measured the (aq, vq) for heavy flavour precisely, but flavour decomposition is difficult

for light quarks Light quarks (u, d): HERA knows

the quark density well

Best measurement in the world vu improved thanks to

the polarisation


Searching for physics beyond Standard Model LEP/Tevatron

Exotic particles are often from pair creation througha fusion

Can produce (almost) any particles Initial state quantum number

vanishes

HERA Initial state has both e and quarks

They should not vanish Sensitive to flavour violation (flavour-

changing neutral current: FCNC) Best limit in many LFV combination

competition: B-factory, atomic parity v.

LEPTevatron

LFV(Lepton-flavourviolation)

Quark-flavourviolation


Example from flavour violation: single-top production

Best or complementary limit from HERA

magnetic coupling

vect

or

coup

ling


High-pT lepton + missing pT excess at H1To be or not to be?

The story started when … H1 found an event with a high-pT

muon in the final state It looked like an event with FCNC

More events found, revealing: With missing pT and missing longi

tudinal momentum not FCNC events

Excess pronounced if we enrich W-production

A ZEUS event with muon

muon


W production at HERA

W produced through q Wq Leptonic decay W l

Signal: Lepton, missing pT () and jet (q)

SM prediction: jet pT (pTX) tend to

be small H1: excess at large pT

X :only for e+p collisionsnot for ep collisions

positrons

electrons

p


and ZEUS?

The H1 excess on positron beam (e+ decays) is 3.4

ZEUS consistent with SM except fordecay (2 events)

H1 decay with large excess too, but all from the electron beam data

H1 claims the excess is not on electron beam data

PTX > 25 GeV e channel channel e+ channel

Electrons, 98-05

121 pb-1

2 / 2.4 0.5

0 / 2.0 0.3 2 / 4.4 0.7

3

/ 0.70.2

0Positrons, 94-04

158 pb-1

9 / 2.3 0.4

6 / 2.3 0.415 / 4.6 0.8

PTX > 25 GeV e channel channel e+* channel

Electrons, 98-05

143 pb-1

3 / 2.9 0.5

2 / 1.4 0.2

(126pb-1)

5 / 4.3 0.7

-

Positrons, 99-04

106 pb-1

1 / 1.5 0.1

To be analysed

- -

Single top search

130pb-1 (16 e- + 114 e+)

2 / 2.9 0.5

5 / 2.8 0.2 7 / 5.7 0.72 / 0.2

0.04

ZEUS result

H1 result

* Calculated by meMy personal conclusion: SM prediction is slightly wrong

Therefore, we see excess here and there

> 50pb-1 more e data to be analysed

>100pb-1 more e+ data by the end of HERA


From HERA to the LHC HERA is ep, the LHC is pp: all the physics are relevant

Advantage over Tevatron: the study “under control” by e beam “HERA and the LHC” workshop

2004-2005 meeting successful Continue to have one meeting per year (6-9 June 2006)

In particular: Parton densities – the luminosity function for the LHC HQ production – the (2nd) discovery channel for Higgs/SUSY Multijet final states and energy flow

– need 8 jet cross sections for the LHC ? Diffraction

Today: multi-parton phenomena


Parton luminosity in hadron colliders

Cross section prediction in hadron colliders:Factorisation formula Assuming only one parton

from each hadron But: parton density at low-x is very large

At x ~ 104: number of partons is O(102-3) Cross section at 10 GeV in parton-parton CMS comparable to the

total proton-proton cross section

2 interactions can occur at the same time Consequence: factorisation formula cannot predict cross se

ction any more


Multi-parton processes

1-parton: “normal” 2-partons

Multi-parton collisions Colourless exchange

2-gluons can formulate colour neutral state (diffraction)

3-partons Colourless state

destroyed by another coloured collision

LargeRapidityGap

LargeRapidityGap

destroyed

h l+l !!!

A jet overlapped:no longer an isolated lepton.Events rejected …


Example of factorisation breaking (1):Suppression of diffractive events in pp

CDF dijet rate is factor 3-10 lower than the prediction using the HERA event rate Rapidity gap seems destroyed

Various models developed to explain the “gap survival probability” Progress in understanding the

underlying events

Dijet cross section vsmomentum fraction of the partonin “pomeron”, the colourless object

Large Rapidity Gap

destroyed


Example of factorisation breaking (2):multi-parton interaction (MPI) in photoproduction

Ideal testing ground Direct (x ~ 1): point-like Resolved (x < 1): hadron-like

4-jets enhanced in resolved MPI can explain, but

Large uncertaintyMeasurements at HERA and the Tevatron are important

x < 1 x = 1

MC predictionsassuming factorisation

MPImodel1

MPImodel2

resolved direct

additionaljets


HERA is not yet over. Prime time now! HERA-II running until 2007

summer Already ~250pb-1/exp’t recorded

in 2004-2006 summer Further > 150pb-1/exp’t.

Focussing on low-stat studies High-x PDFs Heavy flavours Electroweak studies Search for exotics

Low-energy running for longitudinal structure function FL

Most pessimistic

More optimistic

End of HERA30 June 2007


Summary

HERA has measured low-x proton structureHigh-x is to be measured

HERA has tested QCD, and will continue HERA does EW studies and BSM searches

Currently the main subjects HERA measurements are relevant to the next generation

colliders, the LHC


Backup slides hereafter


HERA tunnel 2-story ringUpper: proton, lower: electron


So, what have we learned ?

In 1992 we planned to learn Proton structure Low-x parton densities

Jet production

Electroweak physics Search for physics beyond Standard Model


Yes, we have learned more than expected! In 2005 we have learned much more

Proton structure success of the DGLAP equation

Low-x parton densities very precise determination

Diffractive physics “forgotten”

Jet production and QCD, s very much in detail

Heavy flavour production became a major subject

Electroweak physics Search for physics beyond Standard Model

Promoted the QCD theory very much Having a high energy collider, we find what we expect.

But also something unexpected


Event with a large-rapidity gap (LRG) DIS: many hadrons between

proton and the scattered quark

Events with a large gap found in DIS The exchanged object is

colourless Called diffractive scattering

scattered e

Colourflow:manyhadrons

protonbeam

Scattered quark (with a colour charge)

No hadrons(LRG)No colour flow

scattered e

Photon diffractedinto hadrons

Colourlessexchange

Proton stays intact


Explaining diffraction by quarks+gluons

The colourless exchanged state is… Historically explained by a

hypothetical particle “Pomeron” Like a meson (, …) but has a

quantum number of vacuum Introduced at 60’s to explain the

behaviour of hadron-hadron scattering Modern trial: using perturbative QCD

2-quarks mesons: NG 2-gluons is colourless

Remember: many gluons in a proton! Internal structure of the “Pomeron” by

the super-microscope HERA

Colourless2-gluon

Colourless“Pomeron”

See Pomeronby microscope


Experimental result on diffraction

Gluon fraction in the “Pomeron” is determined Quark/gluon densities are

extracted through DGLAP 60-90 % gluons 2-gluon is a good

approximation


LeptoQuarks

A resonance state of a lepton and a quark Predicted in Grand Unified

Theory, Super Symmetry … Again HERA is best or

complementary

Many other search results

a peak in e-jet mass if there is a LQ(jet)

E. Perez in LP2003 conference


Size of quarks

Best electron microscopeBest place to see if quarks are elementary particles

Data agrees with the standard model predictionLimit on the quark radius: < 0.71018 m (< 1/1000 of proton)

Point-like quark

Large quark


Photoproduction (PHP) at HERA

Total cross section of ep scattering: dominated by very low-Q2 events Can be regarded as a

collision of a quasi-real photon and the proton

cf. events in e+e colliders Real photon acts as a hardon

Through vacuum polarisation to a qq-pair

Photon has a structure: F2

& the photon parton density

q

q

22 ,QxF

22 ,QxF

Gluonjet

Quark jet

e+e 2-photon

HERA photoprod.

22 ,QxF


Dijet in photoproduction and parton densities

A parton in proton can probe partons in photon Dijet cross section is consistent with prediction

using the photon parton densities measured at e+e

)( xq

22 ,QxF

22 ,QxF

Quark jet

Gluon jet


HERA-II: luminosity upgrade

HERA-I (before upgrade)was already a cutting-edge machine High beam current

(e: 45mA, p:100mA) Narrow bunch spacing (96ns)

cf. LEP 22s, Tevatron 4s

HERA-II upgrade No change in beam current & bunch spacing Increasing luminosity by final focusing magnets

like in e+e colliders Longitudinally polarised electron also for

colliding experiments


Luminosity upgrade and background

HERA: the only machine with e and p beams e+e :

vacuum in the beam pipe: bad (synchrotron radiation)

e-N (heavy ion) cross section: small pp :

p-N (heavy ion) cross section: large vacuum is good (no synch. rad)

ep : bad vacuum (synch. rad) high p-N cross section

Long time needed for conditioning vacuum (2002-2003)

Good condition+luminosity after 2004

Normalised background ratefrom proton-(residual gas) interaction

HERA-II

HERA-I

Recent results from ep -scattering at HERA

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