Measurement of FL the longitudinal structure function at HERA

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Measurement of FLthe longitudinal structure
function at HERA

• Low x meeting, Lisbon, Portugal
• 28 June 2006

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Outlook

• Motivation
• Direct measurement of FL(x,Q2)
• FLD(x,Q2,xPom)
• Conclusion

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Physics motivations

• FL simply related to sL , the inclusive cross
  section of longitudinally polarised photons

Helicity conservation
In QPM
QCD radiation generates quark PT
In LO
At low x, the gluon density dominates. FL
is a clean probe of the gluon distribution at
HERA.
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At fixed beam energy high y (i.e. small x at
fixed Q2) data are sensitive to FL.
Q2s xy s(cm energy)2
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MRST, CTEQ have trouble fitting the H1 low Q2
data consistently at NLO
Lines fits Red points raw data points Green
pointsdata points shifted by optimal correlated
Sys Err.
Ex CTEQ6.1M (Tung, DIS 2006)
H1 96-97
MRST Fits similar problems ZEUS 96-97 data show
the same effects
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FL and gluon density at low x are poorly
constrained by present data.
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Direct measurement of FL(Q2,x)
Reduced cross section

• At fixed (Q2,x), measure cross sections at
  several beam energies (minimum 2), i.e. at
  different y.
• Perform straight line fit of sr to extract F2
  and FL
• FL is very sensitive to small relative shifts on
  cross sections

F2
F2-FL
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Direct measurement of FL(Q2,x)

• ZEUS and H1 have investigated the optimum
  beam energy for the low energy run.
• Requirements
• A large y difference to maximize lever arm 1/Ep
• Enough luminosity. Lumi scales as Ep2
• ? Reduce proton beam energy to lowest energy
  considered reasonable by HERA.
• Ep 920 GeV
  460 GeV
• Lumi 30 pb-1
  10 pb-1 Estimated time 3
  months.
• The highest possible y at low beam energies
  (error on FL 1/y2
• ? Go to lowest possible scattered electron energy

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Measurement with ZEUS
Daniel Kollar DIS2006
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Measurement with ZEUS
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Measurement with H1
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Measurement with H1
Measurement of electron scattered energy
demonstrated down to 3 GeV. Measurement of the
charge provides statistical substraction of
background
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Caldwell, DIS 2006
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Measurement with H1- Fast simulation
30 pb-1, Ep920 GeV 10 pb-1, Ep460 GeV
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Measurement with ZEUS detailed simulation
Daniel Kollar, DIS2006
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Attempt to put information in one number (ZEUS)
Daniel Kollar, DIS 2006
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xPQ2/(sy ß)
?vary y at fixed Q2, ß, x by changing s (i.e.
proton beam energy)
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H1 simulation
Encouraging result ! FLD extracted to 3 4 s
in 3 bins At ltQ2 gt 12 GeV2 and ltßgt 0.23

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FLD has never been measured FLD predicted from
QCD fits to be large at low ß! Inclusive
diffraction cannot be fully understood without
separating out FLD contribution.
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Conclusion

• A direct measurement of FL at x from 10-4 to
  10-3 is difficult but feasible by running HERA
  for a few weeks at low proton beam energy.
• Agreed by both H1 and ZEUS and supported by
  the PRC.
• The measurement of FL provides a very
  attractive added value to the legacy of HERA.
• It could also provide the first measurement of
  FLD at the 3-4 sigmas level.
• Lets do it.

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• EXTRAS

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Motivations
Q2 s xy s
(center of mass energy)2
Small Q2, xF3 terms neglected
The structure function FL is a basic Structure
Function.
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Why not using the radiative events?e p ? e ?
X

• We get radiative events fo free !
• Need a huge statistics (200 pb-1)
• Krasny, Placzek, Spiesberger,1991
• However
• For a fixed (Q2,x) bin, at different y,
• the overlap in the same part of the detector
  is quite small
• No access to very high y (Ee gt 3 GeV)
• Severe pile up of Bethe-Heitler events (e p?e p
  ?) in the gamma detector Favart, Maracek, 1996
• ?? huge errors on FL 50-100 !

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Ep 460 GeV
Ep 920 GeV
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5 pb-1 at Ep 460 GeV and 3.5 pb-1 at Ep 575
GeV
10 pb-1 at Ep460GeV
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