# Presentation: 130120

## HERA and the ZEUS experiment

- OUTLINE
- A short history of HERA and ZEUS.
- Search for new physics.
- QCD: structure functions, strong coupling constant, jets, charm and beauty
- Hadronic physics
- ZEUS + H1 combined F2
- Outlook

- R. Yoshida, Argonne National Laboratory

- November 30, 2007, FNAL

## The SLAC-MIT Experiment 

- The SLAC-MIT Experiment

- Under the leadership of Taylor, Friedman, Kendall

- partons!

- scaling!

- 1969

## HERA electron-proton collider

- electrons

**protons**

**ZEUS**

**H1**

- HERMES

- HERA-b

- 2 collider experiments
-    ZEUS and H1
- 2 fixed target experiments
-    HERMES and HERA-b

- HERA construction
- approved 1984

- DESY laboratory in Hamburg, Germany

- 5.12x1031cm2s-1 after upgrade

## 1984-1987: Civil Construction

##  Zeus Detector

-  Zeus Detector

**Complete 4****π**** detector **
**Tracking:**
**  ****- central tracking detector**
**  ****- Silicon ****μ****-Vtx**
** ****(operate in a B field of 1.43 T)**
**Calorimeters:**
**  ****- uranium-scintillator (CAL)**
**        ****σ****(E)/E=0.18/√E  [emc]**** **
**        ****σ****(E)/E=0.35/√E  [had]**
**  ****- instrumented-iron   (BAC)**
**Muon chambers**

**18 countries,  ~400 members**

- Experiment approved
- 1986

## U-Sci compensating calorimeter

**U-Sci compensating calorimeter**

## 96 ns bunch spacing:

- ZEUS Trigger DAQ

- Fully “pipelined” readout
- and trigger—for
- “deadtime-less”
- trigger.   We had <1%
- deadtime normally.

- Similar to LHC readout
- systems:  as far as I know,
- the first time such a system
- was used on a large scale.

## Slide 8

## Slide 9

## First luminosity on May 31, 1992

-            -first paper that September

- Control room in the mid-90’s

## HERA I: 1992-2000

-   ~180 pb-1/experiment
-    delivered. (mostly e+)

- HERA II: 2002-2007
-    ~580 pb-1/experiment
-    delivered. (e+ and e-)

- Upgrade: 2001-2002

- For HERA II:
-    - Luminosity ~x3 (low-β insertion)
-    - Long. polarized leptons
-    - Some running at lower proton
-      energy:  460 and 575 GeV

## Slide 12

## June 30, 2007  ZEUS End-of-run party

## Physics at HERA

- 920 GeV

- 27.5 GeV

## 1997

## 2003

- After ~100 pb-1 of data
- --no excess.

- (similar results for H1)

- Limits were set:

- contact interactions

## Summary from ~2004

- 2-7 TeV range limits from all three colliders

- Contact Interaction Limits

## Extra-dimensions and quark radius limits (2007 results)

## Channels where excesses at HERA were reported

- previously:

- No compelling indications of BSM in the total data set

- lepton + missing PT

- multi-leptons

## Introduction: Deep Inelastic Scattering

***x***

- = Q2/2p.q

- Described by 2 kinematic variables

- p

- Mostly about
- F2

## In the early 90’s, HERA was

- HERA Kinematic Limit

- about to push the
- proton structure
- function measurements
- by 2 orders of magnitude
- in x and Q2.

- Clearly, the first
- measurements would be
- in relatively low Q2
- and would extend to low x.

- What were the expectations?
- What would be the proton structure at low x?

## 0

- HERA Kinematic Limit

- 1

- “pQCD”

- “Hadronic”

- Predictions of F2

- Donnachie & Landshoff

- Gluck, Reya
- and Vogt

- Why such different predictions?

- Fixed
- target
- data

## Quarks are asymptotically free!

- Quarks are asymptotically free!

- Proton is a beam of partons
- whose behavior can be
- understood using
- perturbative QCD!

- OR

- Protons are hadrons—whose
- constituents are confined.  The
- behavior of hadrons is not
- understood from the first principles
- of QCD: however we have relatively
- good phenomenology to describe them.

- pQCD

- Hadronic

- Two ways to think about the problem

## Before HERA: hadronic view of the proton and F2

- F2 at low x is simply related to the total γ*P  cross-section.
- x ≈ Q2/W2 so as x falls W rises
- small x limit of DIS is a large energy limit of the γ*p cross-section.
- at HERA W goes up to ~300 GeV.
- Large energy limit of total cross-sections is where the Pomeron trajectory dominates in Regge phenomenology: slow rise of the cross-section.

- 10

- 1000

- W(GeV)

- γp

- γγ

- hadron-hadron

- W: γ*P cms energy

## Hadronic view of F2

- We do not understand how hadrons are formed and behave from first principles.
- We do, however, have a phenomenology that describes most of the properties of hadron-hadron collisions. (Regge)  This is somehow the result of QCD in the strong coupling limit.
- Virtual-photon proton cross section (or F2 at low x) is yet another total cross-section which should be dominated by the properties of the proton as a hadron  governed by the same Pomeron trajectory as other hadronic cross-sections: slow power rise with W (or 1/x) .

- Donnachie and Landshoff (1993)

- 1

- 0

- F2

## pQCD view of F2

- Asymptotic freedom!   A proton at a high energy collider is a beam of partons.
- A proton knows itself as a proton only to the extent that the non-perturbative “initial distributions of partons” are somehow determined by the hadronic properties of the proton.
- The parton distributions at any Q2 can be calculated via perturbative QCD given enough data to determine it at some Q2.

***Factorization:****** ***

**(universal) parton densities**

**PQCD cross-sec.**

- x momentum frac. of parton
- Q2resolving power of probe

## pQCD view of F2

***P*****'s are splitting functions:**** **

- pQCD view of F2

- Q2

- resolution

- smaller
- x

- 0

- 1

- F2

- Evolution in Q2
- many gluons at low x

- Predict:

## Measurements at HERA

- 0

- 1

- F2

- Early ZEUS data

## HERA measurements and pQCD

**smaller x**

**resolution ( Q2  )**

**HERA measurements and pQCD**

**Incr.**

- Lines are pQCD fits to
- parton distributions.

## Scaling violation of F2

- Scaling violation of F2

- To LO:

## Additional information: jets at HERA

- Jets can probe the gluon

- Jet measurements are consistent with NLO QCD fits from F2
- Can be used to further constrain the gluon and/or αs

## as measurement at HERA are as precise as those from LEP

- as measurement at HERA are as precise as those from LEP

- S. Bethke
- hep-ex/0606035

- HERA measurement
- (green triangles)

## Charm and beauty

## Parton densities from ZEUS data

- Proton is mostly gluons at low x

- W+ production at LHC

- Predicts
- e.g.

## Triumph of perturbative QCD

- A part of Wilczek’s comments upon the Nobel Prize announcement

- gluon

- resolution

## On the other hand...

- Has the hadronic proton completely vanished
- (only manifestation in the parton densities) ?

- Look for leading protons in the final state

- carries most
- of the beam
- momentum

- Peak!

- t is small

- If proton carries most
- of the beam momentum
- and t is small 

## Mass of X, Mx, must be small

- with respect to W

- X is far away in rapidity from
- the proton a rapidity gap

- ...then

- Not

- color
- connection
- leads to
- particles
- in “gap”.

- color singlet

- No particles in the proton direction

## ~10% of DIS events are “rapidity gap” events

- ηmax, the most forward energy deposit

- 1993

## In the simplest interpretation 2 gluons in a 

- color singlet state are exchanged:

- gluon “taken out”

- gluon “put back”

- small perturbation

- Proton stays intact: this
- process carries information
- about the proton wave
- function.

- Here, proton is behaving as a hadron!

- This is “diffraction” familiar from hadronic physics:
- however, with some peculiarities

## Sizable part of F2 even at high Q2 (~10% at 30 GeV2).  High Q2 means interpretable in terms of pQCD(?)  
Ratio to total cross section is flat with W (or x).  How is this possible? If 
σtot ~ gluon density
σdiff~(gluon density)2
(Naively...) 

- Sizable part of F2 even at high Q2 (~10% at 30 GeV2).  High Q2 means interpretable in terms of pQCD(?)
- Ratio to total cross section is flat with W (or x).  How is this possible? If
- σtot ~ gluon density
- σdiff~(gluon density)2
- (Naively...)

## At high Mx, 

- At high Mx,
- diffractive DIS is
- not vanishing
- at high Q2.—”leading
- twist” in pQCD
- language.

- Expectation
- from hadron-hadron
- data.

- Energy dependence
- of diffractive DIS
- is not like the usual
- hadronic diffraction.

## Proton as a hadron

- In DIS diffraction we have:
- A phenomenon that is clearly related to the hadronic nature of the proton—i.e. that of confined color.
- that exists at 10% level at high Q2—where perturbative QCD should be usable.
- that does not conform to the expectation from the hadronic phenomenology.
- that does not conform to the naïve expectation of 2 gluon exchange.
- Plenty of mysteries:
- We observe protons as hadrons clearly in the kinematic region where asymptotic freedom+partons appears to give a good description of data.
- Do we, then, truly understand the evolution of partons in the proton—especially at low x?
- Is diffractive DIS the opportunity to finally begin to unravel confinement from a perturbative point of view?

- A lot of high precision data from HERA exists

## So far, no true understanding of this phenomenon

- Diffractive DIS cross-section

- Elastic VM production

## Combining H1 and ZEUS F2 results

- Some understood differences
- between H1 and ZEUS PDF’s

- ZEUS uses jet-data to constrain
- mid-x gluon smaller uncertainty

- H1 uses BCDMS data to pin down
- the high-x behavior smaller uncertainty

- Still, there appears to be some
- systematic differences.

- H1-ZEUS Working Group: A. Cooper-Sarkar, K. Nagano, J. Ferrando
- Y. Ri, A. Glazov, M. Klein, V. Shekelian, Z. Zhang, E. Rivzi, U. Martyn

- H1-prelim-07-007,ZEUS-prel-07-026

## H1 QCD fit

- ZEUS QCD fit

- ZEUS data

- H1 data

- Fixed target data

- Global QCD fit
- CTEQ, MRST etc.

- Somewhat dissimilar

- ZEUS, H1 data (also other data sets) “have tension”,
- “not compatible”, must redefine chi2 requirement... etc.

- Are there problems with one or the other, or both
- data sets??

- Tevatron data

## NLO pQCD is an approximation.  To which kinematic range is it applicable? How low in Q2, how low in x?  If not using higher twist terms, how high in x?  How much difference does NNLO terms make?
What is the mass of c and b quarks used in the fit? How is the transition from massive to massless quarks handled?
Poorly known parameters; e.g. what is the fraction of strange and charm in the sea at the chosen input scale?
Parameterizations of parton distributions: modern QCD fit has 20-30 parameters.  We have very little first principle knowledge of the shape of the distributions.  Which parameters should be constrained in which way?  e.g.  is ubar/dbar =1 as x  0?   How quickly do parton distributions go to 0 as x  1?

- NLO pQCD is an approximation.  To which kinematic range is it applicable? How low in Q2, how low in x?  If not using higher twist terms, how high in x?  How much difference does NNLO terms make?
- What is the mass of c and b quarks used in the fit? How is the transition from massive to massless quarks handled?
- Poorly known parameters; e.g. what is the fraction of strange and charm in the sea at the chosen input scale?
- Parameterizations of parton distributions: modern QCD fit has 20-30 parameters.  We have very little first principle knowledge of the shape of the distributions.  Which parameters should be constrained in which way?  e.g.  is ubar/dbar =1 as x  0?   How quickly do parton distributions go to 0 as x  1?

- Before doing a QCD fit, ask a simpler question

## Are the ZEUS and H1 results consistent with having

- measured the same physical quantity?

- Is this
- kind of
- difference
- within
- uncertainties?

- Correlated systematic uncertainties

## Q2 = 2• EeE’e(1-cosθe)

- example 1:

- Luminosity uncertainty:
-   shift the whole data set by some amount (~1.5% for HERA)

- If you reconstruct Q2 using the energy, E’e, and angle
- θe of the scattered electron:

- mis-measurement of E’e by n% will be correlated across
- the entire measured kinematic plain.

- This kind of thing may be no problem

- example 2:

- But, of course all data points need
- to be checked simultaneously.

## 1.5 GeV2 < Q2 < 105 GeV2

- 10-6< x < 0.65

- 554 data points each from ZEUS and H1

- Fit for data points (554 of them)

- And j systematic uncertainties

## χ2/dof = 510/599

- χ2/dof = 510/599

- H1 and ZEUS measurements
- are consistent.

- Of course, this procedure also produces a combined F2

## The improvement is (much) better than √2 !!!

**Notes:**
Make new fig

## How is this possible?

- Ep=920 GeV

- Systematics correlated across the kinematic plain,
- but uncorrelated between experiments cancel.

- E’e

- θe

- “hadrons”

- Hadron measurement:
- minimize the influence
- of loss in the beampipe

- ∑(E-Pz)h

- ∑PTh

- Z

- Use:

- Four measurements
- to determine x and Q2

## Additional kinematic constraints:

- ∑(E-Pz)h + (E-Pz)e = 2•Ee = 55 GeV

- ∑PTh = PTe

- HERA detectors over-constrain the kinematics.
- How ZEUS and H1 actually reconstruct x and Q2
- depend on the particular characteristics of each detector.
- The procedures are, in practice, quite different, and
- relatively complicated.  Roughly speaking:
- H1:   θe and E’e at high y, and use ∑(E-Pz)h  at low y.
- ZEUS:   θe  and  θh  { tan(θh/2) = ∑(E-Pz)h / ∑PTh }

## ZEUS and H1 have similarly sized uncertainties.
ZEUS and H1 have differently “shaped” uncertainty correlations.
ZEUS and H1 have different best measured regions.
 You win big from the fit

- log y

- log y

- E’e shifted +δ

- E’e shifted +δ

- This is what happens schematically

- E’e shifted -δ

- E’e shifted -δ

- ΔF2

- ΔF2

- “ZEUS”

- “H1”

- ZEUS and H1 have similarly sized uncertainties.
- ZEUS and H1 have differently “shaped” uncertainty correlations.
- ZEUS and H1 have different best measured regions.
-  You win big from the fit

## ZEUS data only

- H1+ZEUS data

- Leads to (very) significant improvements in parton densities

- Note:  These plots are taken from the proceedings of
-   HERA-LHC workshop hep-ph/0601012 and uses an earlier
-   version of the combined data.

- Only HERA I data have been combined so far

## HERA ep collider has ceased operations as of July 2007.
The experiments H1 and ZEUS have together recorded about 1 fb-1 of data. 
The rise of F2 at low x and diffractive DIS are two of the most important discoveries at HERA.  The quantitative understanding of the latter phenomena is still missing.
I’ve tried to cover a relatively small number of topics in a little more detail than is usually given.
I certainly did not cover all (or even a large portion) of the new results from ZEUS.  For those interested, the talks at this summer’s Lepton-Photon in Korea (by Gwenlan, Vallee, Rostovsev and Diehl) are good starting points.
Although data taking is over, there is a lot more to come 

- The experiments H1 and ZEUS have together recorded about 1 fb-1 of data.
- The rise of F2 at low x and diffractive DIS are two of the most important discoveries at HERA.  The quantitative understanding of the latter phenomena is still missing.
- I’ve tried to cover a relatively small number of topics in a little more detail than is usually given.
- I certainly did not cover all (or even a large portion) of the new results from ZEUS.  For those interested, the talks at this summer’s Lepton-Photon in Korea (by Gwenlan, Vallee, Rostovsev and Diehl) are good starting points.
- Although data taking is over, there is a lot more to come 

- Concluding remarks and outlook

## Outlook

- Full analysis of HERA II data—will probably take 2-3 years more.
- ~30-50% improvement in statistically limited measurements (high Q2, ET), <1% experimental precision in alpha_s.
- Factor x(2?) improvement in systematics in F2 (improved understanding and H1+ZEUS combination)
- Charm and beauty cross-section to 5 and 20% respectively—micro-vertex detectors.
- Electroweak measurements.
- FL measurement for the first time at HERA.