Robert Klanner Hamburg Precision QCD Tests at HERA 22. April 2005 - 1/36 Precision QCD Tests at HERA...

Robert Klanner Hamburg Precision QCD Tests at HERA 22. April 2005 - 1/36

Precision QCD Tests at HERA

- The Standard Model of Particle Physics

- HERA Machine and Experiments

- Precision QCD Tests at HERA

- Summary


Ch.1: Standard Model – Building Blocks

In the Standard Model of Particle Physics,which summarizes 50 years of experimen-tal and theoretical research:

Elementary building blocks of known matter:

Dunkle Energie

Dunkle Materie

Baryonen

Neutrinos

Sterne

mass/energy distribution in universe

all elementary particles of the SM have been

experimentally observed!

upup charm top

down strange bottom

ee ee

leptons(weak IA)

quarks(+strong IA)

Quarks und Leptonen come in 3 families - masses vary by 106 (1012)

mass

5%

missing element of the SM: generation of mass (of elementary particles):

Higgs mechanism


Standard Model: Forces

In SM forces mediated by the exchange of bosons, which couple to “charges”

The form of the interactions is derived from the symmetry of “local gauge invariance” (e.g. em: arbitrary rotation in U(1) space)

In Nature we know 4 fundamental forces:boson: photon

with spin 1, U(1), electrom. charge

iee '

bosons: W+ W- Z0 with spin 1, SU(2),

weak charge

bosons: 8 gluons with spin 1, SU(3),

colour charge

bosons: graviton with spin 2, mass, not part of SM !

unified GSW

SM is a beautiful theory – one of the greatest achievements of 20th century physics,but many open questions: many “arbitrary“ parameters, relation quarks – leptons, unification of forces, gravity, … limit of a more general theory


Standard Model: QED

Why do we believe in “correctness” of SM? precise predictions directly testing the struc-

ture of the theory, which have been verified experimentally to high accuracyQuantum-Electro-Dynamics:

- thanks to smallness of e.m. coupling

(E~0) =1/137 035 989 …

and the ingenuity of our theoretical colleagues perturbative calculations to high accuracy

½(g-2)e = (1 159 652 185.9±3.8) 10-12 meas.

½(g-2)e = (1 159 652 201.2±27.1) 10-12 theo.

½(g-2) = (1 165 920.80±.58) 10-9 meas.

(meas.-theory) (1.19±0.73) 10-9

((theoretical) dominated by had. contributions!)

Lamb shift, and many more


Standard Model: Electro-Weak Sector

Precision data from LEP,SLC,FNAL, etc test higher order e.w. predictions of SM

Status: Winter 2005

Open Question: Why does SM work so well ?!

demonstration of e.w. unification at HERA

GeVMNDF Higgs7348

2 126,13/3.18/

distance [m]ra

te


Predictive Power of SM (EW)

Quantum fluctuations (Heisenberg: E*t > h/2) allow access to higher masses and energies:

Mass of Top-Quarks was predicted by precision measurements within SM

e+

e-

l,q,W+

l,q,W-

virtual heavy particle influences reaction-rates and -properties

e.g. corrections of higher order in the electro-weak force:

)ln(,)( 2

W

Higgs

W

top

M

M

M

M


Gravity – Strong Force – Summary Ch. 1

Gravity not part of SM

- F(gravity)/F(em) ~ 10-43 (major puzzle !)

so far not relevant for experimental particle physics

- Newton’s law verified only down to ~ 0.1 mm possible deviations from 1/r2

extra dimensions to accommodate difference in strength of gravity compared to the other interactions (at low energy) ?

Precision tests of the strong force ch.3

Summary chapter 1:Standard Model of Particle Physics is an impressive intellectual achievement:

• a theory which makes precise and testable predictions

• so far all experimental results agree with predictions (sometimes to an incredible accuracy)

• but limitations + incomplete SM is only limiting case of a broader theory


Chapter 2: HERA and its Experiments

The world’s only electron/positron - proton storage ring

27.5 GeV polarised electrons (positrons)920 GeV protons ⇨ 320 GeV in cms

Four experiments H1, ZEUS, HERMES and {HERAb} (≈ 1000 scientists)

data taking: 1992 until mid 2007 when HERA data taking ends


HERA machine 2

HERA I (1992-2000): 110 pb-1 e+ and 15 pb-1 e- data for H1 and ZEUS + polarised e- on pol./unpol. gas target for HERMES + pN data for HERAb

HERAII ( 2007): polarised e+/e--p data for H1/ZEUS + data for HERMES upgrade in 2000/01, 2002/03 severe BG problems, now solved and (aging) HERA runs well

Lmax~ 4 1031cm-2s-1 ;Luminosity aims: ~0.7 fb-1 until end of data taking in mid 2007


HERA Collider Detectors - 1

- detectors conceived in 1985 to 1988

- first data in 1992

- HERA first collider with short (96 ns) time between bunches trigger/event pipe-lined

- 1-1.5% Lumi accuracy

- compared to LEP/D0/CDF: precision hadron calorimetry E(100GeV) ~ 3.5-5% calibration at 1% level

(essential for precision physics at HERA)

- HERAb with 20 MHz interaction rate: major progress in trigger + radiation tolerance

H1 Detector


HERA Collider Detectors 2

A (fairly typical) NC ep e+q(jet)+X event (Q2=3500GeV2, x=0.05)

pe

e

q

events clean precise reconstruction of kinematics (directly seen underlying Feynman graph)

q jet

e

p rest


Chapter 4: Tests of QCD at HERA

HERA: precision microscope shining (e.m. and weak) light onto the proton

0)'( 222 Qllq

resolution x~1/Q can be tuned at HERA down to 10-18m (1/1000 rp)

Q2 xGeV2 fm~0.5 1/3

~ 2 1/10

> 5 <1/10

>105 <1/1000

charge distr.

quarks scaling

scaling violation

sub-struct.?

qp

Qx

.2

2

p momentum fraction of parton (QPM)

resolution power can be tuned ⊕ for Q2 0 has a hadronic component (e.g. ) transition from point-like /Z-hadron to hadron-hadron scattering


Tests of QCD: Structure of photon vs Q2

Is the picture of the “photon structure” vs resolution – 1/Q2 correct?

2 jet production vs Q2

with increased Q2 – exchanged getting point-like

jets

jetTobs

E

eEx

jet

2,

“direct”x > 0.8

“resolved”x < 0.8

description not perfect qualitatively o.k!

~ yes


Tests of QCD: Properties of Exchanged Partons

Is the picture of the “exchange of virtual quarks and gluons” correct?

2 jet production vs Q2

yes

“direct”x > 0.8

“resolved”x < 0.8

angular distribution of jets in jet-jet cm system is sensitive to the spin of the exchanged particle:

Spin 1 (gluons):

(like Rutherford)

Spin ½ (quarks):

2** )cos1(

1

cos

d

dN

)cos1(

1

cos **

d

dN

g g

q q


Test of QCD: The running coupling constant S1973 Breakthrough in development of QCD: asymptotic freedom

strong coupling weak at small distances (high energies – scale ) perturbative calculations possible for precise predictions

dependence of S controlled by -function:

S() expressed via parameter [GeV]:

...,27

325

9

50332857

3

1951

3

211

...6442

)(2

32

2

1

0

43

232

120

ff

f

f

SSSSS

nn

n

n

...)/ln(

))/ln(ln(21[

)/ln(

4)(

)(ln

22

22

0

122

0

)(2

2

S

Sx

dx

< 0


Precision Tests of QCD: What are the Problems

S is small only at small distances (high Q2)

only*) domain for precision tests using perturbative expansion in S, e.g.

calculate Ai from Feynman graphs

divergences have to be regulated in some scheme

Ai depend explicitly on scale (infinite sum does not)

if wrong scale ’ is chosen terms ln(’/) appear

choice not obvious (if higher order terms small confidence that o.k.)

(NB there are also problems with two scales 1,2 terms ln(1/2))

2.) function changes when m passes quark mass M

some arbitrariness how to treat effect

3.) no free quarks non perturbative effects

“everywhere” – handled via models (pdfs,

Monte Carlo) use reactions where effects

not so important (e.g “total cross sections”)

...33

221 SSS AAA

*) Lattice Gauge Theory is start to produce precise results


Measurement of S – Why?

Besides masses, S [] only free parameter of QCD

Why precise measurements in many different ways?

• all strong interaction cross- sections depend on S,

• if QCD correct all measurements should agree

test QCDif disagree problem with QCD ??? (more probably large distance effects)

• unification of forces: precision at today’s energies is required for precise extrapolation to unification scale

and the physics on the way to there


Test of QCD: S from DIS Structure FunctionsQ2 evolution of structure functions q, g:

splitting functions P have been calculatedup to 2nd order – for unpol. 3rd order soon

- theoretically “clean” method,

- HERA alone: correlation gluon - S (resolved by fit SF + jets) - uncertainty on sea quarks: , - terms at large x, - terms at small x, - photon structure at low Q2.

improvements (experimental + theoretical) are under way later

g

qq

PP

PnP

g

qq

qqPqq

gggq

qgfqqS

qqS

2

2

)(

ln

)(2

)(

ln

)(

2

2

2

2

ss )1(log 12 xnn

S )/1(log 12 xnn

S

.)(.)(0017.01150.0)(:1

.),(.)(0048.01166.0)(:0009.00005.0

0018.00018.0

thexMH

thexMZEUS

ZS

ZS

(H1: 2000, ZEUS: 2002)


Test of QCD: S from Jet Production

Diagrams contributing to jet production: Example-1: H1 DIS in Breitframe:

deep inelastic scattering:

photoproduction:

- theoretically less clean than DIS- but measurement of dependence

running of S in agreement with QCD prediction observed in single experiment


Test of QCD: S from 2-Jet to 3- Jet Ratio

Example-2: ZEUS ratio of 3-jet to 2-jet production: :

- uncertainty of parton distribution functions cancel (in some approximation)



Test of QCD: S from Jet Sub-structure

Example-3: ZEUS from jet structure:

small experimental error

theoretical error ???

integrated jet shape:fraction jet transverseenergy within cone ofradius r in η, φ plane


ψ(r) comparison to theory

ψ(r=0.5) vs ET(jet)

S(ET)


Test of QCD: S Summary

many different S measurements give consistent results

“running of S as predicted by QCD major success QCD

measurements compatiblecombine results for each expt.


Test of QCD: Proton Structure (1)

- HERA !



major success of perturbative QCD

r ∝1/Q2



HERA has provided a precision determination of proton structure essential input for SM and BSM study at present and future accelerators

(use low Q2 fixed target data to constrain high x pdfs)


Proton Structure and Higgs Cross section at LHC

Impact on precision of parton functions on Higgs signal (+background) at LHC

Spread of existing pdf gives already up to 10% uncertainty improve !



HERA has provided a precision determination of proton structure essential input for SM and BSM study at present and future accelerators

Q2/GeV2

Use all (>100pb-1) HERAI data (neutral and charged current reaction):

e p data only (no nuclear data !)


Test of QCD: Jets in the Final State

QCD: parton functions universal the same parton function describes different reactions e.g.

jet production, heavy quark production, single photons, ….

Events with two jets:

- dominant graph (LO) “photon-gluon fusion”

(direct coupling to gluon – in DIS only derivative ∝ (gluon density in p)

- in two jet events momentum of gluon can be reconstructed


Test of QCD: Jets in the Final State

Combined fit of proton structure function and (jet)

data well described ⊕ improved precision at high x ⊕ no correlation between S and gluon ⊕ data from a single experiment

simultaneous description major success for QCD


Test of QCD: Heavy Flavour in the Final State

Production of heavy flavour particles: high quark mass provides large scale

cc /

tot=

F2cc

/F2

NLO QCD predicts charm correctly

first results become available for b-quarks


NLO QCD (MS) analysis/fit of spin data: Assumptions:

- Flavour symmetric spin dependent sea

- uv and dv constrained by F and D (SU(3) symmetry)

low-x extrapolation questionable !

results for Q02 = 4 GeV2:

uv 0.73 .....0.86 (0.10) dv -0.40...-0.46 (0.10) qs -0.04 ...-0.09

0.14 ...0.20 G 0.68 ...1.26information from high pT had. pairs (HER-MES, COMPASS) and charm (COMPASS)

Data described by NLO QCD fit(S(MZ) = 0.1182)

HERMES: Spin of Nucleon


QCD: Diffraction in Deep Inelastic Scattering

HERA 1992: ~ 10% of deep inelastic events have no particle flow between proton and jet (rapidity gap) - (same effect also seen in W-exchange)

Surprise:

- why does p remain intact - confinement?

- are there colour neutral objects in p?

- can process be described by QCD?

HERA microscope the ideal instru- ment for study

define and measure diffractive structure function: ),,,( 2

2 QtxF IPD

rap. gapto proton


QCD: Diffraction in Deep Inelastic Scattering

systematic measurementsusing different methods:

- rapidity gap, - fit to particle flow, - measurement of proton.

after hard work results of measurements in good agreement


QCD: Diffractive Structure Function

comparison diffractive ⇔ proton structure function:

diff. SF: positive gluon dominance; data well described by NLO QCD22

Q

F

H1DF2

2F


QCD: Diffractive Final States

using QCD jet and heavy flavour production in diffraction predicted:

good description by NLO QCD

charm structure function D* cross sections cDF ,2

high Q2 diffraction: well described by QCD – relation to basic QCD however unclear


Summary

The Standard Model of Particle Physics beautifully summarizes the present experimental results and knowledge of Particle Physics.

QCD, the theory of the strong force, is the least tested part of the SM.

Tests of QCD (and not of models) is so far (essentially) limited to the small distance predictions (perturbative) – but non-perturbative effects are present everywhere and have to be carefully understood.

HERA has put the tests of QCD to a new level of breadth and precision (scaling of structure functions, final states in photo-production and deep inelastic scattering, S) – sometimes reaching the 2% level.

So far QCD has very well passed the tests + lots has been learned on QCD

These results, interesting in themselves, are also important for understan-ding the much more complex situation at the LHC.

Robert Klanner Hamburg Precision QCD Tests at HERA 22. April 2005 - 1/36 Precision QCD Tests at HERA...

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Transcript of Robert Klanner Hamburg Precision QCD Tests at HERA 22. April 2005 - 1/36 Precision QCD Tests at HERA...