Exclusive Meson Production with EIC Tanja Horn (JLab) Antje Bruell (JLab) Garth Huber (University of...
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Transcript of Exclusive Meson Production with EIC Tanja Horn (JLab) Antje Bruell (JLab) Garth Huber (University of...
Exclusive Meson Production with EIC
Tanja Horn (JLab)Antje Bruell (JLab)
Garth Huber (University of Regina)Christian Weiss (JLab)
EIC Collaboration Meeting, Hampton University, 19-23 May 2008
OutlineOutline
• Exclusive processes: physics motivation
• Cross section parameterization
• Monte Carlo simulations: input for detector design
• L/T separations and the pion form factor
Exclusive Processes: Physics Exclusive Processes: Physics motivationmotivation
• Experimental challenge– Small cross sections, σ(meson+N) ~1/Q8
– Detection of the recoil nucleon– Differential measurements in x, Q2, t
[cf. GPD White Paper for NSAC Long-Range Plan, presented at Rutgers Town Meeting Jan-07]
• Study of high-Q2 exclusive processes essential part of physics program for ep collider
– Reaction mechanism: QCD factorization– Information about GPDs, meson wave functions
(baryon/meson structure)
pγe'pe
Nmesone'
222 GeV1Q,W 2GeV1|| t
Exclusive Processes: Collider Exclusive Processes: Collider EnergiesEnergies
Exclusive Processes: EIC Potential Exclusive Processes: EIC Potential and Simulationsand Simulations
11H(e,e’H(e,e’ππ++)n at EIC: Cross Section )n at EIC: Cross Section ParameterizationParameterization
MC Simulations
• Rate predictions including simulations of the detector restrictions
• Input for detector design• Momentum and angular distributions for various particles
• Case studies:• H(e,e’π+)n• H(e,e’π°)p• H(e,e’K) Λ
Exclusive MC Generator
• Exclusive EIC Monte Carlo:• Based on HERMES GMC • New event generator using
standard cernlib functions• Includes cross section model
by Ch. Weiss model for π+ production
• Can be easily extended to other channels, e.g. π°, KΛ etc.
• MC agrees with fixed target data from Jlab
1H(e,e’π+)n Momentum and Angular Distributions
• Kinematically, electrons and pions are separated
• The neutron is the highest energy particle and is emitted in the direction of the proton beam
neutrons
π+ n
electrons
Ee=5 GeVEp=50 GeV
π+
Q2>1 GeV2
1H(e,e’π+)n – Scattered Electron
• Most electrons scatter at angles <25° • BUT access to the high Q2 region of interest for GPD studies requires
larger electron angles
Electron Lab Angle (deg)
Q2
(GeV
2)
Minimum angle for Q2=40 GeV2 is ~70°
P (G
eV)
Electron Lab Angle (deg)
Q2=40 GeV2 can be reached for electron momenta < 7 GeV
Ee=5 GeVEp=50 GeV
1H(e,e’π+)n – Scattered Neutron
Neutron Lab Angle (deg)-t
(GeV
2)• Low –t neutrons are emitted at very small angles with respect to the
beam line, outside the main detector acceptance
• A separate detector placed tangent to the proton beam line away from the intersection region is required
P (G
eV)
Neutron Lab Angle (deg)
1H(e,e’π+)n – Scattered Pion
Pion Lab Angle (deg) Pion Lab Angle (deg)P
(GeV
)
Q2
(GeV
2)
• The pion cross section is peaked in the direction of the proton• At larger Q2 pion angles and momenta are smaller
• within the capability of the detector (pπ and Q2 are uncorrelated)• provide good missing mass resolution
Ee=5 GeVEp=50 GeV
Event Topologies
Q2, x
t, φ
e e’
p n
π
• The most straightforward way to assure exclusivity of the 1H(e,e’π+)n reaction is by detecting the recoil neutron
• The neutron acceptance is limited to <0.27° by a magnet aperture close to the interaction point
• Alternatively, the neutron can be reconstructed from missing momentum
• Missing mass resolution has to be good enough to exclude additional pions
Rates and coverage in different Event Topologies
-t (GeV2)
Γ dσ
/dt (
ub/G
eV2)
-t (GeV2)
Γ dσ
/dt (
ub/G
eV2)
Detect the neutron Missing mass reconstruction
• Neutron acceptance limits the t-coverage• The missing mass method gives full t-coverage for x<0.2
Assume dp/p=1% (pπ<5 GeV)
Ee=5 GeVEp=50 GeV
0.01<x<0.02 0.02<x<0.05 0.05<x<0.1
10<Q2<1515<Q2<2035<Q2<40
10<Q2<1515<Q2<2035<Q2<40
0.05<x<0.1
Assume: 100 days, Luminosity=10E34
• At higher energies, the missing mass resolution deteriorates, so need to detect the neutron
• At lower energies, the missing mass reconstruction works well, but neutron detection is more difficult
• With Ee=5 GeV and Ep= 50 GeV can ensure exclusivity over the full region in (x,-t, Q2) using a combination of the two methods:• Overlap region between the two methods allows for cross checks
Systematic uncertainty on the rate estimate
• Data rates obtained using two different approaches are in reasonable agreement:
• Ch. Weiss: Regge model• T. Horn: π+ empirical
parameterization
10<Q2<1515<Q2<2035<Q2<40
0.01<x<0.02 0.02<x<0.05 0.05<x<0.1
Assume: 100 days, Luminosity=10E34
Statistical uncertainty in the measurement
Luminosity= 1031
Γ dσ
/dt (
ub/G
eV2)
• High luminosity is essential to achieve the experimental goals
Ee=5 GeVEp=50 GeV Assume: 100 days
1H(e,e’π°)p Momentum and Angular Distributions
• Similar to π+, but additional complication due to photons from π° decay
• π° decay photon opening angle places a constraint on the calorimetry
electrons π° protons
Photon from π° decay
π°
2γ opening angle
Q2>1 GeV2
Ee=5 GeVEp=50 GeV t<1GeV2
1H(e,e’π°)p – π° Decay PhotonsEe=5 GeVEp=50 GeV
• Opening angle is small and requires fine calorimeter granularity• JLab/BigCal: 38x38mm, H1 forward calorimeter: 35x35mm
• High energy photons at large angles can be detected • At high momentum, charged particles are difficult to measure
1° → 35mm / 2m
electrons K Λ
protonπ-
1H(e,e’K)Λ Momentum and Angle Distributions
• Kinematics overall similar to the pion case
• Some π- from Λ decay might be detected in an outbending toroidal field
Λ
Assume: 100 days, Luminosity=10E34
Rate estimate for KΛ
• Using an empirical fit to kaon electroproduction data from DESY and JLab
10<Q2<1515<Q2<20
35<Q2<40
0.01<x<0.02 0.02<x<0.05 0.05<x<0.1
1H(e,e’π+)n L/T Separation Experiments
1. Pion Form Factor, Fπ(Q2)– Excellent opportunity for studying the QCD transition from effective degrees of
freedom to quarks and gluons.i.e. from the strong QCD regime to the hard QCD regime.
2. Longitudinal Photon, Transverse Nucleon Single-Spin Asymmetry, A┴π
Especially sensitive to spin-flip GPD which can only be probed via hard exclusive pseudoscalar meson production.
3. QCD and GPD scaling tests– Scan vs Q2 at fixed xB to test Hard QCD scaling predictions
σL~1/Q6, σT~1/Q8
1. Scan σL vs xB at fixed Q2 to distinguish pole and axial contributions in GPD framework.
E~
To access higher Q2, one must employ the p(e,e’+)n reaction.
• the t-channel process dominates L at small –t<0.02 GeV2.
At low Q2<0.3 GeV2, the + form factor can be measuredexactly using high energy + scattering from atomic electrons.
F determined by the pion charge radius 0.657±0.012 fm.
Determination of F via Pion Electroproduction
In the actual analysis, a model incorporating the +
production mechanism and the `spectator’ nucleon is used to extract F from L.
πNNg
),()()(
2222
2
tQFtgmttQ
dtd
NNL
• Cross Section Extraction– Determine σT+ ε σL for high and low ε– Isolate σL, by varying photon
polarization, ε
dφdtdσεdφdt
σ2d T dφdtdσL
22
-121
-1εε
dσL )ε(R)ε(Rdσ
21
L/T separations in exclusive π+ production
ε=0.64
ε=0.40
• Requires special low energies for at least one ε point and cannot be done with the standard EIC
• L/T separations require sufficiently large Δε to avoid magnification of the systematic uncertainty in the separation
Ee=5 GeVEp=2 GeV
Ee=3 GeVEp=5 GeV
)( lossenergy fractional the where
)1(1)1(2
2
2
2Ntot Msx
Qyyy
Different accelerator mode
• The ability to use 5-15 GeV protons will allow many high priority L/T-separation experiments which are otherwise not possible.
• The proton accelerator needs a mode where the injector is not run to its full energy.– This beam is injected into the main proton accelerator, which is
used as a storage ring.
• The costs to implement this low energy mode will be reduced if this flexibility is included at the planning stage.– Achieving the high luminosity required for this experiment may
not be possible
Recoil Polarization Technique
1
ε1hP
ε1
σσR
2z
T
L
• In parallel kinematics can relate σL/σT to recoil polarization observables
εσσR10
L
• From R and the simultaneous measurement of σ0 one can obtain σL
dRσdσ 0L
• Requires only one epsilon setting• Polarized proton beam• Additional model assumptions needed in general if the reaction is not elastic
27
Kinematic Reach (Pion Form Factor)
Assumptions:• High High εε:: 5(e-) on 50(p).• Low Low εε proton energies as
noted.• Δε~0.22.• Scattered electron detection
over 4π.• Recoil neutrons detected at Recoil neutrons detected at
θθ<0.35<0.35oo with high efficiency. with high efficiency.• Statistical unc: ΔσL/σL~5%• Systematic unc: 6%/Δε.• Approximately one year at Approximately one year at
LL=10=103434..
Excellent potential to study the QCD transition nearly over the whole range from the strong QCDstrong QCD regime to the hard QCDhard QCD regime.
Preliminary
Projected uncertainties for Q-n scaling
• Transition region 5-15 GeV2 well mapped out even with narrow fixed x and t • careful with detector requirements
EIC: Ee=5 GeV, Ep=50 GeV
Preliminary
Outlook
• Extend studies to vector mesons
• Resolution studies
• Test additional requirements from e.g. π˚ and KΛ• At high energies, calorimeter granularity needs to be better than
35x35mm• Requirements on magnets, e.g. toroidal fields for KL
Summary
• High Q2 studies of exclusive processes are an essential part of the physics program for an ep collider
• For beam energy 5 on 50 two methods are available to ensure exclusivity over the full range in (x,-t,Q2):• At high energies, need a separate detector tangent to proton
direction to detect the exclusive final state – limited acceptance• At low energies, missing mass reconstruction works well • Overlap in certain kinematic regions allows for cross checks
between the two methods
• High luminosity (10E34) is essential for these studies
Other
1H(e,e’π+)n Momentum and Angular Distributions
• Kinematically, electrons and pions are separated
Ee/Ep
(GeV)pelectron
(GeV)pπ
(GeV)θelectron
(deg)θπ
(deg)
3/30 1-4 1-18 18-45 140-175
5/50 2-6 1-30 11-30 150-177
10/250 8-11 1-145 5-15 173-179
• The neutron is the highest energy particle and is emitted in the direction of the proton beam
E(GeV)
pneutron (GeV)
Θneutron
(deg)
3/30 12-30 >177.8
5 /50 20-50 >178.6
10/250 95-250 >179.7
neutrons
π+ n
electrons
Ee=5 GeVEp=50 GeV
π+
Q2>1 GeV2
1H(e,e’π°)p – π° Decay Photons
π° Lab Angle (deg)O
peni
ng A
ngle
(de
g)
Ope
ning
Ang
le (
deg)
6 on 15
3 on 30
5 on 50
10 on 250
• Separating the π° decay photons is getting more difficult as the energy increases, but recall that pion momenta are low at high Q2
Systematic uncertainty on the π° rate estimate
Ee=5 GeVEp=50 GeV
• Data rates obtained using two different approaches are in reasonable agreement:
• Ch. Weiss: σT from Regge model
• T. Horn: σT from π+ empirical parameterization
15<Q2<2010<Q2<15
Missing Mass Resolution
Assume dp/p=0.5%
36
Longitudinal Photon, Transverse Nucleon Single-Spin Asymmetry, A┴
π
• Measure A┴π to access the spin-flip
GPD • Requires a transversely polarized
proton beam, and an L/T-separation.• The asymmetry vanishes in parallel
kinematics, so the π+ must be detected at θπq>0, -t up to 0.2Q2.
E~
12 2
0 0
L L Ld d dA d d dd d d
where dσ is the exclusive p(e,e’π+)n cross section using longitudinal photonsβ is the angle between the proton polarization vector and the reaction plane.
A┴π vs xB
-LO-Q2=4-Q2=10
A.V
.Bel
itsky
, hep
-ph/
0307
256
QCD Scaling TestsQCD Scaling Tests• To access physics contained in GPDs, one is limited to the
kinematic regime where hard-soft factorization applies– No single criterion for the applicability, but tests of necessary conditions can
provide evidence that the Q2 scaling regime (partonic picture) has been reached
• One of the most stringent tests of factorization is the Q2 dependence of the π electroproduction cross section– σL scales to leading order as Q-6
– σT scales as Q-8
– As Q2 becomes large: σL >> σT
Factorization
H H~ E E~
• Factorization theorems for meson electroproduction have been proven rigorously only for longitudinal photons [Collins, Frankfurt, Strikman, 1997]
Q2 ?
Low ε data from Jlab12?
• L/T separations at EIC will benefit from Jlab12 measurements
JLAB: Ee=12 EIC: Ee=5 GeV, Ep=50 GeV
ε=0.99
ε=0.3-0.7