An Introduction to HERA
Physics
DESY Summer Student Program
16/17 August, 2005
Tobias Haas
DESY, Hamburg
Tobias Haas: Introduction to HERA
Part 1:
 What is HERA?
 Structure Function Formalism
 Kinematics
 Structure Function Evolution
(RGE/DGLAP)
Part 2:
 Selected HERA I Results:



© G. Larson: “The Far Side”
Overview
Structure Functions
High Q2 and EW
Jets and the strong coupling αS
Part 3:
 Why HERA II:


High lumi/polarization
EW precision measurements
Tobias Haas: Introduction to HERA
Part 1
Tobias Haas: Introduction to HERA
What is HERA?
© G. Larson: “The Far Side”
Tobias Haas: Introduction to HERA
Physics @ Colliders
+ Simple initial state
+ Clean final states
+ Little background
- Limited energy
• e.g. LEP(200 GeV), ILC(1 TeV)
e+
e-
p
p
+ High energy (no synch rad)
- Complicated initial state
- Large and complicated backgrounds
- e.g TEVATRON(2 TeV), LHC(14 TeV)
p+
e
+ Unique initial state
- two accelerators
- HERA (300 GeV)
Tobias Haas: Introduction to HERA
Tobias Haas: Introduction to HERA
HERA: The only ep Collider on
the Planet
s  320 GeV
Tobias Haas: Introduction to HERA
HERA

Highlights:


Started operation in 1992
4 Experiments:








H1 and ZEUS (ep)
HERMES (e)
HERA-B (p) (until 2003)
s = 300 GeV (1997)
s = 318 GeV (1998  )
e+ and e- beams
up to 60% lepton
polarization
> 400 Mio ep collisions
recorded per experiment
Tobias Haas: Introduction to HERA
Physics Topics of HERA

Proton Structure

Parton densities








gluon density (xg(x))
Valence quark distributions
QCD evolution
Different evolution schemes
(e.g. BFKL)
Strangeness and charm





S


Perturbative QCD




Jets


Gluon density
S


Multiparticle Observables:



Border between pQCD and
non-perturbative QCD
Photon Structure
Diffraction
EW
BSM and Exotics
Multiplicity distributions
Event chapess
Multiparticle Correlations

Leptoquarks
Excited Quarks and Fermions
FCNC
MSSM Searches
R-parity violation SUSY
Contact Interactions
…
Spectroscopy
Tobias Haas: Introduction to HERA
Structure
Function
Formalism
See e.g. David J Griffiths: “Introduction to elementary particles”, New York, 1987
Tobias Haas: Introduction to HERA
“Bj”
(aka James Daniel Bjorken)
Tobias Haas: Introduction to HERA
Kinematics
… or a guided tour
around the HERA
phase space …
Tobias Haas: Introduction to HERA
Current Jet
Scattered eScattered ee-
P
27.5 GeV
920 GeV
Current Jet
Tobias Haas: Introduction to HERA
HERA I Kinematic Range

Huge extension of
kinematic reach:



xBj: 6 orders
Q2: 6 orders
Overlap with previous
(fixed target)
experiments
Tobias Haas: Introduction to HERA
1
Tobias Haas: Introduction to HERA
Very High
2
Q
Q 2  20000 GeV 2 , x  0.6

Current Jet
e-

P

920 GeV 
27.5 GeV

Scattered e-
Very clean events
Very high energy
electron (> Ebeam )
Very collimated jet
Electron forward
Activity around the
beam pipe forward
(proton remnant)
Tobias Haas: Introduction to HERA
2
Tobias Haas: Introduction to HERA
Very Low Q2
Little activity in main
detector
 Electron backward – seen
in special beampipe
calorimeter
 Scattered
Electroneenergy
close to
Ebeam
 No jet structure
 Activity around the beam
pipe forward (proton
remnant)

e27.5 GeV
P
920 GeV
Tobias Haas: Introduction to HERA
3
Tobias Haas: Introduction to HERA
Medium Q2




Jetin main
Jet and electron
detector
Well isolated electron
Well collimated jet
eActivity around the beam27.5 GeV
pipe forward (proton
remnant)
Jet
P
920 GeV
Scattered e-
Scattered eTobias Haas: Introduction to HERA
~ xF2

Deep inelastic:


W >> MP
xF2  const.
Tobias Haas: Introduction to HERA
Tobias Haas: Introduction to HERA

xF2 x    ei2 qi ( x)  q i ( x)

Tobias Haas: Introduction to HERA
HERA Results for F2
Sample F2 data
Dramatic Scaling Violations!
Tobias Haas: Introduction to HERA
NC Cross Section and Structure
Functions
Q2=q2=(kk’)2
x: momentum fraction
of the struck parton
y=Q2/xs
NC Reduced cross section:
2
~
(
x
,
)
Q
 NC
NC Cross Section:
2

2
(
p
)
2

y

d  NC e
Y

[


xF3]
Y
F
F

2
L
2
4
dxd Q
xQ
Y
Y
2
Dominant contribution
Sizeable only at high y (y>~0.6)
2

1

(
1

y
)
Y
Contribution only important at high Q2
Tobias Haas: Introduction to HERA
F2 vs

2
Q
Note:



Enormous range
of data (5 orders
in Q2 and 8 orders
in x)
Approximate
scaling at high Q2
Scaling violations
at low Q2
Tobias Haas: Introduction to HERA
F2 vs xBj


Dramatic rise a low xBj
Note previous picture:
Tobias Haas: Introduction to HERA
Structure Function
Evolution
Tobias Haas: Introduction to HERA
Reminder:
Nomenclature/Kinematics
= 0 in the QPM
Z0 Exchange
Tobias Haas: Introduction to HERA
What QCD tells about
F2(x,Q2) ?
Splitting functions:
Can be calculated
in pQCD
DGLAP Equation:
• Integral-Differential equation for the
dependence of q(x,Q2), g(x,Q2) on Q2
• Need an initial condition!
 x  b( x )
(a  b)( x)   dya 
 y y
0
1
Tobias Haas: Introduction to HERA
QCD Fits

Make an ansatz at a fixed value of Q2 = Q02

Write F2 simpler:
_
_
F2 ~ 4/9 (U+U) + 1/9 (D+D)
with
_ _ _
_ _ _
D = d+s
U = u+c
U = u+c
D = d+s

Ignore F3 and FL (for the moment)
Tobias Haas: Introduction to HERA
Parton Distribution Functions
QCD fits to structure functions:
_
_
F2 ~ 4/9 (U+U) + 1/9 (D+D)
_
_
Valence quarks:
2 (U-U) +
(D -D)
_ _ _
_ _ _
D = d+s
U = u+c
U = u+c
D = d+s
% precision except for gluon :
Tobias Haas: Introduction to HERA