Post on 24-Feb-2016
description
Salvatore Fazio BNL
for the BNL EIC Science Task Force
XX International Conference on Deep-Inelastic Scattering and Related SubjectsUniversity of Bonn, Germany
March 26-30, 2012
GPDs at an EIC
p p
gg*
Plan of the talk The eRHIC accelerator and an EIC detector
capabilities for exclusive diffraction
GPDs and DVCS
Bethe-Heitler subtraction
|t|-differential cross sections
Charge and spin asymmetries
Imaging with an EIC (the impact!)
J/y
Summary
2March. 29, 2012 S. Fazio: DIS2012 - Bonn, Germany
See also: D. Mueller’s andM. Diehl’s talks
S. Fazio: DIS2012 - Bonn, Germany 3March. 29, 2012
The eRHIC collider
5 - 20 GeV electrons on 100-250 (130) GeV protons (nuclei). Polarization of electrons and protons (nuclei)
Lumi: 0.62 x 1033 cm-2s-1 (stage 1–ep pol)
3.1 x 1033 cm-2s-1 (stage 1–ep unp)
9.7 x 1033 cm-2s-1 (stage 2-ep)
Important for exclusive DIS:• Dedicated forward instrumentation• High tracker coverage • Very High lumi!
eSTAR
ePHENIX
6 pass 2.5 GeV ERL
Coherent
e-cooler
Beam-dump
Polarized e-guneRHIC
detector
EIC/eRHIC EIC detector
An electron ring will be built at the RHIC facility
The current experiments can be upgraded for ap(A) physics
A new dedicated detector will be built
S. Fazio: DIS2012 - Bonn, Germany 4
Important for exclusive diffraction:• Hermetic Central Tracking Detector (Si pixels)• Good em calorimeter resolution with fine granularity (fibres)• Very forward calorimetry• Roman pots from the early beginning (and with excellent acceptance)
The EIC detectorep/N
FORW
ARDREAR
General properties:• Hermetic• Asymmetric
March. 29, 2012
S. Fazio: DIS2012 - Bonn, Germany 5March. 29, 2012
20x250 GeV 5x50 GeV
Accepted in“Roman Pot”(example) at s=20mPlots from J-H Lee
L = 27.77 pb-1
55 events (DVCS + BH)
Roman Pots at HERA
Quadrupoles acceptance
10s from the beam-pipe
• high t acceptance mainly limited by magnet aperture‐• low t acceptance limited by beam envelop (~10σ)‐• t resolution limited by‐
– beam angular divergence ~100μrad for small t– uncertainties in vertex (x,y,z) and transport– ~<5-10% resolution in t (RP at STAR)
Direct |t| measurement @ eRHIC
S. Fazio: DIS2012 - Bonn, Germany 6
DVCS (g): H, E, H, E VM (r, w, f): H E Info on quark flavors via
PS mesons (p, h): H E ~
~ ~
~
quantum number of final state selects different GPDs:
Accessing the GPDs
March. 29, 2012
S. Fazio: DIS2012 - Bonn, Germany 7
€
ALU ∝ y F1 t( )H ξ,ξ,t,Q2( ) −t
4M 2 F2 t( )E ξ,ξ, t,Q2( ) + ... ⎡ ⎣ ⎢
⎤ ⎦ ⎥
March. 29, 2012
€
φ = φN − φl
€
f = ΦT − φN
Angle btw the production and scattering planes
Angle btw the scattering plane and the transverse pol. vector
Accessing the GPDs
€
AC =dσ + − dσ −
dσ + +dσ − ∝ Re ADVCS( ) Requires a positron beam at eRHIC
€
AUT ∝−t
4M 2 F2 t( )H ξ,ξ, t,Q2( ) − F1 t( )E ξ,ξ,t,Q2( ) + ...[ ]
Dominated by H
sin(FT-fN) governed by E and H
€
dσdt
~ A0 H 2 x,t,Q2( ) −t
4M p2 E 2 x,t,Q2( )
⎡
⎣ ⎢
⎤
⎦ ⎥ Dominated by H
slightly dependent on E
slightly dependent on E
S. Fazio: DIS2012 - Bonn, Germany 8
p p
γγ*
Deeply Virtual Compton ScatteringVM (ρ, ω, φ, J/ψ, Υ) DVCS (γ)
Q2Q2 + M2Scale:
DVCS properties:• Similar to VM production, but γ instead of VM in the final state• Very clean experimental signature• Not affected by VM wave-function uncertainty• Hard scale provided by Q2
• Sensitive to both quarks and gluons
IPp p
Vγ*
March. 29, 2012
A GOLDEN MEASUREMENT!
S. Fazio: DIS2012 - Bonn, Germany 9
Scanning the phase space…
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EIC will provide sufficient lumi to bin in multi-dimensions
wide x and Q2 range needed to extract GPDs
EIC lumi: ~10 fb-1/year @ stage 1 – 5x100~10 fb-1/month @ stage 2 – 20x250
5 X 100
… we can do a fine binning in Q2 and W… and even in |t|
20 X 250
DVCS |t|-slope @ HERA
S. Fazio: DIS2012 - Bonn, Germany 10March. 29, 2012
DVCS phase-space
Stage 1
Stage 2
S. Fazio: DIS2012 - Bonn, Germany 11
MC simulation
GPDs, evolved at NLO by an indipendent code which provides tables of CFF - at LO, the CFFs are just a convolution of GPDs:
provide the real and imaginary parts of Compton form factors (CFFs), used to calculate cross sections for DVCS and DVCS-BH interference.
-> The B slope is allowed to be costant or to vary with Q2:
Proton dissociation (ep → eγY) can be includedOther non-GPD based models are implemented like FFS, DD
€
H (ξ,Q2,t) =ei
2
1− x ξ − iε± ξ →−ξ{ }
⎡ ⎣ ⎢
⎤ ⎦ ⎥Hi x,ξ,Q2,t( )dx
−1
1∫u,d,s∑
The code MILOU is Based on a GPDs convolution by: A. Freund and M. McDermott [All ref.s in: http://durpdg.dur.ac.uk/hepdata/dvcs.html]
Written by E. Perez, L Schoeffel, L. Favart [arXiv:hep-ph/0411389v1]
March. 29, 2012
€
dσdxdyd | t | dφdϕ
= α 3xB y
16π 2Q2 1+ε 2
Ie3
€
φ = φN − φl
€
f = ΦT − φN
€
ε ≡ 2xmN
Q
€
I BH
2=
e6
x 2y 2(1+ε 2)2 Δ2P1(φ)P2(φ)c0
BH + cnBH cos nφ( ) + s1
BH sin φ( )n =1
2
∑ ⎧ ⎨ ⎩
⎫ ⎬ ⎭
I DVCS
2=
e6
y 2Q2 c0DVCS + cn
DVCS cos nφ( ) + snDVCS sin nφ( )[ ]
n =1
2
∑ ⎧ ⎨ ⎩
⎫ ⎬ ⎭
I 2 =±e6
xy3Δ2P1(φ)P2(φ)c0
I + cnI cos nφ( ) + s1
I sin φ( )n =1
3
∑ ⎧ ⎨ ⎩
⎫ ⎬ ⎭
€
dσd t
= exp B Q2( )t( )
S. Fazio: DIS2012 - Bonn, Germany 12
BHDVCS
p p
gg*
ee
Wrong-sign sample: a negative track match to the second candidate
g sample: no tracks matching to the second candidate
e sample: a track match to the second candidate
(DVCS+BH)
(BH+ bkgd)
(bkgd)
DVCS - BH
March. 29, 2012
BH is “precisely” known but to certain level! Uncertainty on proton form factor uncertainty on BH xsec ~ 3% (at LO)
S. Fazio: DIS2012 - Bonn, Germany 13
BH rejection
March. 29, 2012
Eel
EγEel
Eel
Eel
Eγ
Eγ
Eγ
1. BH electron has very low energy (often below 1 GeV)2. Photon for BH (ISR) goes often forward (trough the
beam pipe)Important: em Cal must discriminate clusters above noise down to 1 GeV
S. Fazio: DIS2012 - Bonn, Germany 14March. 29, 2012
BH rejection
In DVCS most of the photon are less “rear”Than the electrons:(θel-θg) > 0 rejects most of the BH
BH and DVCSBH dominated
S. Fazio: DIS2012 - Bonn, Germany 15March. 29, 2012
BH subtraction will be not an issue for y<0.6
Stage 2
BH fraction
S. Fazio: DIS2012 - Bonn, Germany 16March. 29, 2012
BH subtraction will be relevant in stage 1, at large y, depending on the x-Q2 bin
BUT…
Stage 1
BH fraction
Stage 1-2 overlapping:x-sec. measurements in stage 2 at low-y can cross-check the BH subtrac. made in stage 1
S. Fazio: DIS2012 - Bonn, Germany 17
Stage 1: 5 X 100 GeV ~10 fb-1 (~ a year)
Stage 2: 20 X 250 GeV ~100 fb-1 (~ a year)
Data simulation & selection
March. 29, 2012
0.01 < |t| < 0.85 GeV2
(Low-|t| sample)• Very high statistics• Systematics will dominate!• Within Roman pots acceptance
1.0 < |t| < 2.0 GeV2
(Large-|t| sample)• Xsec goes down exponentially• requires a year of data taking @
20x250 GeV (stage 2)• Main detector can be used in
measuring |t|
Acceptance criteria• for Roman pots: 0.03< |t| < 0.88 GeV2
• for |t| > 1GeV2 detect recoil proton in main detector
• 0.01 < y < 0.85 GeV2
• h < 5
BH rejection criteria (applied to x-sec. measurements)• y < 0.6• (θel-θg) > 0 • Eel>1GeV2; Eel>1GeV2
Events smeared for expected resolution in t, Q2, x
Systematic uncertainty assumed to be ~5% (having in mind experience from HERA)
Overall systematic uncertainty from luminosity measurement not taken into account
S. Fazio: DIS2012 - Bonn, Germany 18March. 29, 2012
(~ 10 months EIC)
€
dσd t
=# evt
Δbin • A • L
€
~ e−bt
b=5.6Specifications:
• Statistical error down to 1%• It uses smeared t values (5%
momentum resol.)• |t|-binning –> 3 * resolution (or higher)
dσ/d|t|- Stage 1
Smearing of momenta and energies accordig to…
S. Fazio: DIS2012 - Bonn, Germany 19March. 29, 2012
Asymmetries
Plots from D. Mueller
S. Fazio: DIS2012 - Bonn, Germany 20March. 29, 2012
€
q x,r b ,μ 2( ) =
14π
d t0
∞∫ J0
r b t( )H x,η = 0, t,μ 2( )
Imaging
Errors are extrapolated for:|t| 0 ; |t| > 1.5 GeV2
…FIT…Plots from D. Mueller
S. Fazio: DIS2012 - Bonn, Germany 21March. 29, 2012
See also: D. Mueller’s talk for details M. Diehl’s talk for an overview
Plots from D. Mueller
Imaging
A global fit over all mock data was done, based on the GPDs-based model:
[K. Kumerički, D Müller, K. Passek-Kumerički 2007]
Known values q(x), g(x) are assumed for Hq, Hg (at =0, t=0 forward limits Eq, Eg are unknown)
Excellent reconstruction of Hsea, Hsea and good reconstruction of Hg (from dσ/dt)
S. Fazio: DIS2012 - Bonn, Germany 22March. 29, 2012
g*p -> J/y p
pseudo-data generated using a version of Pythia tuned to J/y data from HERA
wave function uncert. (non-relativistic approximation)
mass provides hard scale• Sensitive to gluons• Both photo- and electro-production
can be computed
Plots from E. Aschenauer
J/ψ
S. Fazio: DIS2012 - Bonn, Germany 23
Summary A lot of experience carried over from HERA Simulation shows how an eRHIC can much improve our knowledge of GPDs
a.r.o. HERA A fine binning of x-sec and symmetries will be possible, uncertainties mostly
dominated by systematics Large potential for an accurate 2+1D imaging of the polarized and unpolarized
quarks and gluons inside the hadrons (and nuclei!)
March. 29, 2012
S. Fazio: DIS2012 - Bonn, Germany 24
Back up
March. 29, 2012
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Q2 dependence for the W slope not clear within the uncertainties!
ZEUS: JHEP05(2009)108H1: Phys.Lett.B659:796-806,2008
Fit: σ ~ Wδ
DVCS @ HERA
€
t ~ PTγ2 +PTe
2 ⎛ ⎝ ⎜ ⎞
⎠ ⎟2
t measured indirectly:
€
Fit :dσdt
∝ e−b |t |
No evidence for W dependence of bby roman pots!
March. 29, 2012
S. Fazio: DIS2012 - Bonn, Germany 26
qg vs Eg
20 X 250
March. 29, 2012
S. Fazio: DIS2012 - Bonn, Germany 27
qg vs Eg
10 X 100
March. 29, 2012
S. Fazio: DIS2012 - Bonn, Germany 28March. 29, 2012
BH
DVCS
BH
DVCS
• BH dominates at large y. • DVCS dropes with y
S. Fazio: DIS2012 - Bonn, Germany 29March. 29, 2012
Effect of the cuts
The effect of the cut for the 20x250 conf. is that BH never exceeds 70% of the sample
BH fraction - overall
S. Fazio: DIS2012 - Bonn, Germany 30March. 29, 2012
for the 5x100 conf. is that BH can be a problem at large y and large t, depending on the bin
BH fraction - overall
S. Fazio: DIS2012 - Bonn, Germany 31
t-xsec (ep -> gp)
March. 29, 2012
Selection criteria:
0.01 < y < 0.6 θγ < 2x10-2 rad θel< 2x10-2 rad Eγ > 1 GeV Eel > 1 GeV
b=5.6 GeV-2
10 x-bins ; 5 Q2-bins€
~ e−btHERA eRHIC
S. Fazio: DIS2012 - Bonn, Germany 32March. 29, 2012
(~ 3.5 weeks EIC)
€
dσd t
=# evt
Δbin • A • L
€
~ e−bt
b=5.6
dσ/d|t|- Stage 2
S. Fazio: DIS2012 - Bonn, Germany 33March. 29, 2012
(~ 1 year EIC)
€
dσd t
=# evt
Δbin • A • L
€
~ e−bt
b=5.6• Resolution: uniform assumed ~20%
• Large-|t| -> |t| is reconstructed using the main detector
dσ/d|t|- Stage 2
S. Fazio: DIS2012 - Bonn, Germany 34March. 29, 2012
• Electron and photon clusters are ofter very close!
• can affect the phi distribution reconstruction!
Crucial!Our em cal must distinguish two clusters within the order of 1 deg
More studies on-going…
S. Fazio: DIS2012 - Bonn, Germany 35
How does the nuclear environment modify parton-parton correlations? How do nucleon properties change in the nuclear medium?
(Bethe-Heitler) Nuclear GPDs ≠ GPDs of free nucleon Enhancement of effect when leaving forward limit?
caused by transverse motion of partons in nuclei?
important role of mesonic degrees of freedom? manifest in strong increase of real part of τDVCS
with atomic mass number A?
DVCS in coherent region: new insights into ‘generalized EMC effect’?
DVCS on nuclear targets
MC simulation for DVCS on nuclei coming soon thanks to an updated version of MILOU code
March. 29, 2012