# Presentation: 948216

## A New  Round of DIS Experiments – HERA IIIA. Caldwell, Max-Planck-Institut f. Physik (WHI)/Columbia University

-  **Physics topics: QCD in the high energy limit**

**New Detector for HERA**
**H1 Upgraded Detector**

## Physics Motivation– Strong Interactions

-  **QCD is the most complex of the forces operating in the microworld  ******** expect many beautiful and strange effects**
** ****QCD is fundamental to the understanding of our universe: source of mass (relation to gravity ?), confinement of color, ...**
** ****We need to understand radiation processes in QCD, both at small distance scales and large.**
** ****small distance scales: understand parton splitting (DGLAP, BFKL, CCFM, ...)**
** ****larger distance scales: suppression of radiation, transition to non-perturbative regime (constituent quarks, ...)**
** ****Observation of the saturated gluon state (****color**** ****glass ****condensate****) ?  Expected to be a universal state of matter.**

## s=(k+P)2 = (320 GeV)2	CM energy squared

**Q****2****=-(k-k`)****2****	****	****	****virtualiy**
**W****2****=(q+P)****2****	****	****	**** *************P CM energy squared**
**Transverse distance scale probed: b ******** hc/Q**
**	****	****McAllister, Hofstadter****	****Ee=188 MeV****	****b****min****=0.4 fm**
**	****	****Bloom et al.****	****	****       10 GeV****	****         0.05 fm**
**	****	****CERN, FNAL fixed target****          500 GeV                   0.007 fm**
**	****	****HERA****	****	****	****       ****50 TeV****	****         0.0007 fm**

- HERA Kinematics

**E****e****=27.5 GeV**
**E****P****=920 GeV**

*****

**/**

## Proton inf mom frame

- Proton rest frame

**x=Q****2****/2P q       fraction on P momentum**
**                         ****carried by struck quark**

** ****= 1/2M****p****x****	****Lifetime of hadronic**
**   ****= W****2****/2M****P****Q****2**** ****	****fluctuations of photon**

**Radiation cloud surrounds both photon, proton ******** universal property of nature**

## Proton inf mom frame

- Proton rest frame

**d****2********/dxdQ****2****=2********2****/xQ****4****[(1+(1-y)****2****)F****2**** - y****2****F****L****]**
**F****2 ****= ********f**** e****2****f**** x {q(x,Q****2****) + q(x,Q****2****) }**
**e****f ****is quark charge**
**q(x,Q****2****) is quark density**
**F****L**** = 0 in LO (QPM), non-zero after gluon**
**radiation. Key test of our understanding**

**d****2********/dW dQ****2 ****= ********  (********T**** + ******** ********L****)**
** ****is flux of photons**
******T,L**** are cross sections for transversely,**
**        ****longitudinally polarized photons to**
**        ****scatter from proton**
******  ****is the relative flux **

**F****2**** = Q****2****/4********2********  (********T**** + ******** ********L****)**

- Rutherford

## Physics Picture in Proton Rest Frame

-  **r**** ****~ 0.2 fm/Q****  (0.02 – 2 fm for 100>Q****2****>0.01 GeV****2****) transverse size of probe**
** **** ****ct ****~ 0.2 fm (W****2****/2M****P****Q****2**** ****)****  (<1 fm to 1000‘s fm) – scale over which photon fluctuations survive**
**And, in ****exclusive process****es, can vary the**** impact parameter **
** ****b**** ****~ 0.2 fm/sqrt(t) ****	****	****t=(p-p‘)****2**
**   **** ****Can control these parameters experimentally !  Can scan the distribution of strongly interacting matter in hadrons.**

**r**

**b**

- *

## Hadron-hadron

**Hadron-hadron**
**scattering cross**
**section versus**
**CM energy**

***********P scattering cross section versus CM energy (Q****2********0).  Same energy dependence observed**
**	****s****0.08****  vs  W****2 0.08**

**Don’t see partons**

## Cross sections as a function of Q2 

**Cross sections as a function of Q****2**** **

**Equivalently, strongly rising *************P cross section with W at high Q****2**

## The behavior of the rise with Q2

**Below Q****2**** ********0.5 GeV****2****, see same energy dependence as observed in hadron-hadron interactions.  Observe transition from partons to hadrons (constituent quarks) in data. Distance scale ******** 0.3 fm ??**
**What physics causes this transition ?**

**Hadron-hadron scattering energy dependence (Donnachie-Landshoff)**

## NLO DGLAP fits can follow the data accurately, yield parton densities.  BUT:

**NLO DGLAP fits can follow the data accurately, yield parton densities.  BUT:**
** ****many free parameters (18-30)**
** ****form of parametrization fixed (not given by theory)**
**Constraints, e.g., d****sea****=u****sea**** put in by hand.  Is this correct ? Need more constraints to untangle parton densities.**

**Analysis of F****2**** in terms of parton densities (quarks and gluons)**

## See breakdown of pQCD approach ... 

**See breakdown of pQCD approach ...**_** **_

**Gluon density known with good precision at larger Q****2****. For Q****2****=1, gluons go negative.  NLO, so not impossible, ****BUT – cross sections such as ********L**** also negative !**

##  We need to test NkLO DGLAP fits and extraction of gluon densities. Crucial, since DGLAP is our standard tool for calculating PDF‘s in unmeasured regions.

-  **We need to test N****k****LO DGLAP fits and extraction of gluon densities. Crucial, since DGLAP is our standard tool for calculating PDF‘s in unmeasured regions.**

**Gluon densities not known at higher order, low Q****2****.  Need more precise measurements, additional observables (e.g., F****L****)**

- Thorne

## FL very sensitive observable – let’s measure it

**F****L**** shows tremendous variations when attempt to calculate at different orders.  But F****L**** is an observable – unique result.**
**Problem: F****2**** NLO DGLAP fits work well, but large number of free parameters.  Do we really know the gluon density ?  Need to show that we can make accurate predictions for cross sections.**
**F****L**** very sensitive observable – let’s measure it**

## Diffractive Surprises

- ‘Standard DIS event’

**Detector activity in proton direction**

- Diffractive event

**No activity in proton direction**

## Diffraction

**There is a large diffractive cross section, even in DIS (ca. 20 %)**
**The diffractive**** and**** total cross sections have similar energy dependences.  Data suggests simple physics – ****what is it**** ?**

##  Exclusive Processes (VM and DVCS)

-  Exclusive Processes (VM and DVCS)

- 

- VM

- 

- 

## Energy dependence of exclusive processes

**Rise similar again to that seen in total cross section.  **

**ep********eVp (V=********,********,********,J/********)       ep********e********p (as QCD process)**

- Summary of different
- Vector mesons

**Need bigger lever arm in W to see energy dependence more precisely****.**
**Need to distinguish elastic from proton dissociation events for small impact parameter scans of proton.**

## Dipole Model for DIS:

**Golec-Biernat.**
**Wuesthoff**

**Dipole Model for DIS:**

## Slide 19

## More recent advances: 

-  **add gluon evolution to cross section (DGLAP)**
** ****add impact parameter dependence**

**Profile function:**
**Gaussian example**

## Fix parameters of T(b) from exclusive J/ production

**Gluon density parameters fixed with F****2**** (or ************P****)**

## Model able to reproduce inclusive, diffractive, and exclusive differential cross sections with relatively few parameters.  Some examples:

**The model is in many respects quite simple but quite successful at reproducing the data.  Can we understand relationship of this approach to N****k****LO DGLAP, BFKL, CCFM, ... ?**

## Investigate this region

**Investigate this region**

**Large effects are expected in**
**Forward jet cross sections at high rapidities (also for forward particle production (strange, charm, ...)**

**More detailed tests of radiation in QCD: forward jets**

## Open Questions – Next Steps

** ****Measure the behavior of inclusive, diffractive and exclusive reactions in the region near Q****2****=1 GeV****2**** to understand parton to hadron transition.**
** ****Measure F****L**** over widest possible kinematic range, as this is a crucial observable for testing our understanding of radiation processes in QCD.**
** ****Measure exclusive processes (VM production, DVCS) over wide W range to precisely pin down energy dependence of cross section.  Need t-dependence of cross sections to get 3-D map of proton.**
** ****Measure forward jet cross sections over widest possible rapidity range, to study radiation processes over the full rapidity range from the proton to the scattered quark.**
** ****AND, do it all with nuclei ! **

## Precision eA measurements

-  **Enhancement of possible nonlinear effects (saturation)**

- b

- r

**At small x, the scattering is coherent over nucleus, so the diquark sees much larger # of partons:**
** ****xg(x****eff****,Q****2****) = A****1/3**** xg(x,Q****2****),****  at small-x, xg ******** x****- ********, so**
** **
** ****x****eff****-******** ****= A****1/3****x****- ********    so   x****eff**** ******** xA****-1/3 ******** ****= ****xA****-3 ****(Q****2****< 1 GeV****2****)**
**                                                       ****= ****xA****-1**** (Q****2**** ******** 100 GeV****2****)**

##  Parton densities in nuclei

**Early RHIC data is well described by the ****Color ****Glass ****Condensate**** model, which assumes a condensation of the gluon density at a saturation scale Q****S**** which is near (in ?) the perturbatively calculable regime.  Properties of such a color glass can be calculated from first principles (Mc Lerran-Venugopalan). Closely connected to dipole model approach.**

**The same basic measurements (F****2****, F****L****, dF****2****/d ln Q****2****, exclusive processes) are needed for understanding parton densities in nuclei.**

## What about HERA-2

-  **The goal of HERA-2 is to deliver 1 fb****-1****/expt, divided into e****-****,e****+**** and L,R handed lepton polarization.**
** ****The physics goal i****s**** the extraction of high-x,Q****2**** parton densities, measurement of EW parameters, high P****T**** processes, and searches for new physics.**
** ****The H1 and ZEUS detectors were designed for this**

**EW unification in one plot – measured with HERA-1.  Few K CC events.  The upgraded luminosity, and different polarizations, will yield precise tests of EW and flavor dependent valence quark densities.**

## HERA-III Program

_**HERA-III Program**_

- Two letters of intent were submitted to the DESY PRC (May 7,8)
-  H1 Collaboration
- 	Focus initially on eD: isospin symmetry of sea at small-x
- 				      uv/dv at large-x
- 				      diffraction on n, D
-       Discuss also interest in: eA, F2,FL at small Q2, forward jets,
-                                              spin structure
- New Collaboration
- 	Optimized detector for precision structure function measurements, forward jets and particle production over a
- 	large rapidity interval (eP, eD, eA)

## Possible upgrades of H1 detector

- Upgrade in electron direction for low Q2 F2 and FL measurements

- Upgrade in proton direction for forward particle, jets measurements

- + very forward proton, neutron detectors

## Precision eD measurements

-  Universality of PDF‘s at small-x, isospin symmetry

- Can measure F2p-F2n w/o nuclear corrections if can tag scattered nucleon

##  Flavor dependence at high-x

- Behavior of F2p/F2n as x  1
- SU(6) symmetry                    F2p/F2n = 2/3
- Dominant scalar diquark       F2p/F2n = 1/4
- Can measure this ratio w/o need to correct for nuclear effects if can tag scattered nucleon.

- Simulation with H1 based on 50 pb-1

## A new detector to study strong interaction physics

- e

- p

**Hadronic**
**Calorimeter**

**EM Calorimeter**

**Si tracking stations**

**Compact – fits in dipole magnet with inner radius of 80 cm.**
**Long - |z|********5 m**

## The focus of the detector is on providing complete acceptance in the low Q2 region where we want to probe the transition between partons and more complicated objects. 

- W=315 GeV

- W=0

- Q2=1

- Q2=0.1

- Q2=10

- Q2=100

- Tracking acceptance

## ZEUS,H1

**Tracking acceptance in proton**
**direction**

**This region covered by**
**calorimetry**

**Huge increase in tracking**
**acceptance compared to H1**
**And ZEUS.  Very important for forward jet, particle production, particle correlation studies.**

**Accepted ******** 4 Si stations crossed.**

## e/ separation study

- e/ separation study

**Aim for 2 GeV electron ID**

**Tracking detector: very wide rapidity acceptance, few % momentum resolution in ‘standard design’ over most of rapidity range.**

## Electron only

**Electron only**
**Q****2****=2E E`(1-cos ********)**
**y = 1-E`/2E(1+cos ********)**
**x = Q****2****/sy   dx/x ********1/y dp/p**

**Jet method****:**
**Jet energy yields x via **
**x = E****jet****/E****P**

**Mixed method in between – needs studies**

- 

- E`

- e

**Nominal beam energies**

## Fix x, Q2.  Use different beam energies to vary y.

**F****L**** can be measured precisely in the region of maximum interest.  This will be a strong test of our understanding of QCD radiation.**

**d****2********/dxdQ****2****=2********2****/xQ****4****[ (1+(1-y)****2****)F****2****(x,Q****2****) - y****2****F****L****(x,Q****2****)**** ****]**

**Fix x, Q****2****.  Use different beam energies to vary y.**
**Critical issue: e/******** separation**

## Very forward calorimeter allows measurement of high energy, forward jets, and access to high-x events at moderate Q2

**Cross sections calculated from ALLM**

**Integral of F****2****(x,Q****2****) up to x=1 known from electron information**

## Forward jet cross sections: see almost full cross section !

**Forward jet cross sections: see almost full cross section !**

**Range covered by H1, ZEUS**

- New region

## Very large gain also for vector meson, DVCS studies.  Can measure cross sections at small, large W, get much more precise determination of the energy dependence.

- HERA-1

- HERA-3

**Can also get rid of proton dissociation background by good choice of tagger:**
**FHD- hadron CAL around proton pipe at z=20m**
**FNC-neutron CAL at z=100m**

**W=0     50     100   150    200    250    300  GeV**

**0    5    10    15    20  **

## Summary

** ****Existing data (F****2**** fits, forward jets,+...)**** ****show limitations of pQCD calculations.  Transition region observed**.
-  **Exciting theoretical developments over the past few years.  We are approaching a much deeper understanding of the high energy limit of QCD.**
** ****Measure with more precision, over wider kinematical range, to see where/how breakdown takes place (high rapidities, high-t exclusive processes, expanded W, M****X**** range for diffraction, full coverage of transistion region)**
** ****Precision F****L ****measurement: key observable for pinning down pQCD.  Large differences in predictions at LO, NLO, NNLO, dipole model.**
** ****eD, eA measurements to probe high density gluon state, parton densities for nuclei.**
** ****Additional benefits: parton densities for particle, astroparticle and nuclear high energy physics experiments.  Crucial for cross section calculations.**

## Summary-continued

**The ZEUS and H1 detectors were not designed with this physics in mind.  An optimized detector would greatly enhance the sensitivity of the measurements to deviations from pQCD !**

**Experiment would focus on full acceptance in the small angle electron and proton directions.  Centered on precision tracking and EM calorimetry.**

**Let’s take advantage of the full potential of HERA to answer some fundamental questions about our universe !**

**Moderate machine requirements for eP program.  Nuclei need developments.**