recent results from ep -scattering at hera
DESCRIPTION
Recent results from ep -scattering at HERA. Universit ä t Siegen Physikalisches Kolloquium 8 Jun 2006 Yuji Yamazaki (KEK). Contents of the talk. Introduction: The ep -collider HERA and its objective Bread-and-butter physics at HERA: The structure of the proton - PowerPoint PPT PresentationTRANSCRIPT
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 3
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 4
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 5
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)
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 6
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 7
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 8
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 9
DESY and HERA in west of Hamburg, Germany
ZEUS
Hermes (fixed nucleus target exp’t)
H1
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 10
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 11
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 12
“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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 13
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 14
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 15
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 16
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 17
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 18
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 19
Heavy quark production: for understanding gluons
Heavy quark production is reproduced by HERA parton densities
Gluon density is double-checked
s
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 20
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 21
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 22
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
HERA physics relevant to the LHC
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 23
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 24
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 ,
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 25
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+)
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 26
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 27
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 28
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 29
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 30
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 31
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 32
Example from flavour violation: single-top production
Best or complementary limit from HERA
magnetic coupling
vect
or
coup
ling
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 33
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 34
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 35
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 36
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 37
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 38
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 39
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 …
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 40
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 41
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 42
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 43
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 44
Backup slides hereafter
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 45
HERA tunnel 2-story ringUpper: proton, lower: electron
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 46
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 47
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 48
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 49
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 50
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 51
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 52
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 53
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 54
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 55
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
8 Jun 2005 Recent results from ep-scattering at HERA / Y. Yamazaki 56
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