Proton structure at high Q 2 at HERA
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Transcript of Proton structure at high Q 2 at HERA
Proton structure at high Q2 at HERA
Yujin Ning
IntroductionExperiment setupHigh Q2 PDF from HERA INew PDF from HERA IIHigh x study in ZEUSSummary & outlook
Introduction
Deep inelastic scattering (DIS)
Lorentz-invariant variables:Q2=-q2=-(k-k’)2 Virtuality: scalex=-q2/2pq Parton momentum
fractiony=pq/pk Inelasticity: transferred
energy fraction W2=(p+q)2 Square of the invariant
mass of hadronic final states Square of center-of -mass
or Z0: neutral current (NC)ep->eXW: charged current (CC)e-p->X and e+p->X
Deep: small scaleInelastic: proton is broken
an important tool to study the proton structure
Neutral current cross sections and structure
functions
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d2σ Born (e± p)
dxdQ2=
2πα 2
xQ4Y+F2
NC (x,Q2) mY−xF3NC (x,Q2) − y 2FL
NC (x,Q2)[ ]
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Y± ≡1± (1− y)2
At Born level (Lowest order in QED)
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F2(x,Q2) = x A f (Q2)(q f (x,Q2) + q f (x,Q2))f
∑
xF3(x,Q2) = x B f (Q2)(q f (x,Q2) − q f (x,Q2))f
∑ valence quarks important at high Q2
valence + sea partons dominant
Parton density function(PDF)probability density finding parton q or q carry the momentum fraction x with Q
Longitudinal structure function
Charged current cross sections
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d2σ CC (e+ p)
dxdQ2=
GF2
2πx(
MW2
MW2 + Q2
)2[u + c + (1− y)2(d + s)]
d2σ CC (e− p)
dxdQ2=
GF2
2πx(
MW2
MW2 + Q2
)2[u + c + (1− y)2(d + s)]
sensitive to d quark
sensitive to u quark
MW=80.4GeV Cross sections are suppressed
CC good to disentangle flavor content of the proton (u or d by using e+/e-)
Experiment setup
HERA ep colliderEe=27.6 GeVEp=920(820 before year 98)GeV
ep collisions
Study the inner structure of proton and fundamental forces
xz
NC: Well isolated electron with high transverse momentum
Net transverse momentum is zero
ZEUS NC and CC events
CC: Large missing transverse momentum from the undetected neutrino
Scattered neutrino
HERA I PDF
ZEUS NC and CC cross sections
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˜ σ NCe ± p =
xQ4
2πα 2
1
Y+
d2σ NC (e± p)
dxdQ2
ZEUS-JETS(a new QCD fit using ZEUS inclusive cross section and jet production) describes data well
CCNC
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˜ σ NCe ± p =
2πx
GF2
[MW
2 + Q2
MW2
]2 d2σ CC (e± p)
dxdQ2
H1 structure functionsF2 contribution from exchange
xF3
FL from exchangeQ2<890GeV2
Good agreement with H1 PDF 2000
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˜ σ e − p − ˜ σ e + p =Y−
Y+
2xF3
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F2 =˜ σ e + p
1+ ΔF 2 + ΔxF3+ ΔFL
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FL =Y+
y 2(F2 − ˜ σ e ± p )
ZEUS-JETS PDF
ZEUS-JETS constrain the uncertainty on gluon density
Only ZEUS data used in ZEUS-JETS, well understood systematic uncertainty High Q2 NC/CC (94-00), constrain at high x DIS inclusive jet and dijet in photo production (PHP) (96-97), jet cross section is sensitive to gluon density First time HERA jet used in QCD fit
HERA II PDF
HERA update HERA IIEe=27.6 GeVEp=920 GeVsqrt(s)=318 GeV
• Longitudinally polarized lepton beam: sensitive to weak interaction• Increase specific luminosity: increase statistics at high Q2
e+ 85 pb-1
e- 205 pb-1
e- 62 pb-1
ZEUS NC cross section and xF3
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˜ σ e − p − ˜ σ e + p =Y−
Y+
2xF3
More precise measurement of xF3
with 120 pb-1 e- data, 6 times more than HERA IUnpolarized reduced cross section
for ep
xF3 valence quark
ZEUS CC e-p cross sections
Higher statistics, good agreement
ZEUS-pol QCD fit with HERA II polarized lepton beam
Uncertainties reduced, especially on u valence quark
ZEUS-JET+polarized e-pAll EM parameters fixed to SM value
Center value does not change compared to ZEUS-JETS
High x study in ZEUS
• DGLAP does not predict x-dependence
• PDFs decrease very quickly at high x, hard to measure because of low statistics and large migration
• Highest measured point x=0.75, BCDMS, data is available at higher x but at low W, need huge correction.
• HERA covers much larger phase space to high Q2 and low x, but not to high x
• ZEUS published x=0.65• The uncertainties on PDF grow
with x, might be infinite at x=1 New reconstruction methods are needed to reach the highest x
Motivation for high x study
• Double angle method (DA)– All the energy deposits in CAL: scattered electron and
all others->hadron– Electron polar angle– Spatial distribution hadronic deposits->hadronic polar
angle
• Insensitive to CAL overall energy scale• Sensitive to the accurate simulation of all effects
in CAL– Hadronic energy lost in beam hole at high x, no
accurate reconstruction on x – Highest published point, x=0.65
ZEUS published method
• Electron + Jet• Electron is well reconstructed for
Q2>450GeV2, very high acceptance in whole x range
• Define Q2 bins from Ee and e:
• In each Q2 bin, define x bins:– If leading jet is NOT near the beam
hole low x, jet is well reconstructed, x
from Ejet and jet, good resolution– If jet IS near beam hole
high x, jet is not well reconstructed no jet, count events without jet xedge<x<1 integral of cross section
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Q2 = 2 × Ee × Ee' × (1+ cosθe )
New method
In the ZUES published kinematic region, new method agrees well with published result
New method measures cross section to x=1
Compare with ZEUS published
Measured cross section
low x
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d2σ
dxdQ2
jet in FCAL
jet lost in beam hole
x
Agree well at measured low x regionMore information on high x
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dxd2σ
dxdQ2xedge
1
∫1− xedge
high x
Cross section ratio to theory
Theories describe data well at low x.Expectation is lower than data in the highest x bin, unmeasured before
With new method, the cross section at high x was measured as precisely as other x range.
Summary• A lot of work done on PDF: ZEUS-JETS and H1
PDF 2000. • A new method measure cross section to x=1.• The luminosity delivered by HERA II is more
than 350 pb-1.• NC and CC DIS cross section for e+p and e-p are
measured with HERA II and agree with the SM predictions.
• New fit including HERA II polarized data, the uncertainty is reduced
• HERA II will totally delivery 1fb-1, more precise measurement.