Yellow Report Physics Exclusive Reactions

Studies providing requirements on detector design

Deeply Virtual Compton Scattering (DVCS)

Conditions

• Study for: 5·10-5 < xBj < 0.7, 1 GeV2 < Q2 < 1000 GeV2, 0 < |t| < 1.6 GeV2
• GK GPD model, set of parameters corresponding to 'GK2016' parameterisation used in Phys. Lett. B 805 (2020) 135454
• tables of cross-sections produced with PARTONS (http://partons.cea.fr) and used in toy MC generator
• results cross-checked with MILOU MC generator for 3D tables (https://drf-gitlab.cea.fr/milou/milou)

Cross-sections

Electron beam energy [GeV]	Proton beam energy [GeV]	Integrated luminosity [fb-1]	Integrated x-sec. [nb]	Nb. of events
5	41	10	0.53	5.3e+06
5	100	10	0.64	6.4e+06
10	100	10	0.74	7.4e+06
18	275	10	0.95	9.5e+06

Kinematic plots

• mind different scales
• all for 10 fb^-1
• for distributions of energy see "DVMP of π⁰" section
• distributions for DVCS photons

Conclusions

• TO BE WRITTEN

Additional material

• talk at YR/Physics/Exclusive meeting by Paweł Sznajder (August, 14): https://indico.bnl.gov/event/9175
  

Contact person

• Salvatore Fazio (sfazio@bnl.gov)
• Jinlong Zhang (jinlong.zhang@stonybrook.edu)
• Paweł Sznajder (pawel.sznajder@ncbj.gov.pl)
• YR/Physics/Exclusive conveners

Deeply Virtual Meson Production (DVMP) of π⁰

Conditions

• Study for: 5·10-5 < xBj < 0.7, 1 GeV2 < Q2 < 1000 GeV2, 0 < |t| < 1.6 GeV2
• GK GPD model, set of parameters corresponding to 'GK2016' parameterisation used in Phys. Lett. B 805 (2020) 135454
• tables of cross-sections produced with PARTONS (http://partons.cea.fr) and used in toy MC generator
• DVMP π⁰ description of amplitudes in GK framework implemented in PARTONS

Cross-sections

Electron beam energy [GeV]	Proton beam energy [GeV]	Integrated luminosity [fb-1]	Integrated x-sec. [nb]	Nb. of events
5	41	10	2.4	2.4e+07
5	100	10	2.4	2.4e+07
10	100	10	2.4	2.4e+07
18	275	10	2.4	2.4e+07

Kinematic plots

• mind different scales
• all for 10 fb^-1
• distributions for exclusive π⁰

• distributions for photons coming from exclusive π⁰ decay

• distribution of energies (also for DVCS): DVCS by red, π⁰ by blue, photons coming from π⁰ decay by green

• distribution of opening angles between photons coming from π⁰ decay

Additional material

• talk at YR/Physics/Exclusive meeting by Kemal Tezgin (June, 10): https://indico.bnl.gov/event/8913
  
• talk at YR/Physics/Exclusive meeting by Paweł Sznajder (August, 14): https://indico.bnl.gov/event/9175
  

Contact person

• Salvatore Fazio (sfazio@bnl.gov)
• Paweł Sznajder (pawel.sznajder@ncbj.gov.pl)
• Kemal Tezgin(kemal.tezgin@uconn.edu)
• YR/Physics/Exclusive conveners

Conclusions

• Predominantly in hadron endcap, pseudorapidity: 1.8 - 3.6
• t_min limit: max π⁰ momentum for each x, Q² bin: affects angular res.
• High t (0.5, 1 GeV): energy decreases, must be detectable.
• For 10x100 GeV and 18x275 GeV at t_min, high Q², high x edge has π⁰ momentum > 80 GeV/c. Clusters start to merge. Low stats in this region.
• Calorimeter threshold affects the lower Q2 region, more so for low CM energies and for higher t: threshold will determine truncation in t: parts of low Q², high x missing.
• π⁰ x-sec in general lower than DVCS, except very high-xB region
• Min 2𝛾 angle depends on beam energy configuration, for 5/10x100 it's ~0.2 deg
• Exclusive π⁰ can reach high momentum/energy (but xsec decreases with meson’s energy)

Exclusive vector meson production in eA

Thomas Ullrich; Study of Diffractive Vector Meson Production and Requirements with Sartre generator. More details in this writeup. Feasible channels are J/Psi -> l+l-, phi -> K+K- and rho -> pi+ pi-. Focus on reconstructing t-distribution from decay particle transverse momenta. Two virtuality ranges: photoproduction and 1 < Q2 < 10GeV2

Main findings & detector requirements:

• Kinematic plots in pT,eta for J/Psi for phi and for rho
  

• For higher Q2 mid-rapidity barrel with -1 < eta < 1 coverage and backward scattered electron -3.5< eta <-2.5 is sufficient.
  
• Photoproduction for rho, phi requires detection at -3.5 < eta < -2 and and scattered electron very backward at eta < -5.5
• Photoproduction of rho, phi requires low pt: measuring decay kaons at momenta pT ~ 100MeV, decay pions at pT~350MeV
• t-resolution requires sigma_pT/pT (%) = 0.05 pT + 0.5 in barrel and sigma_pT/pT (%) = 0.1 pT + 0.5 inbackward region, significantly better than the Handbook Detector, so that the Fourier transform back to impact parameter b can resolve the nuclear geometry: Input vs. reconstructed b-dependence with handbook detector compared to required better resolution
  

  Handbook detector b-reconstruction
  

  b-reconstruction with MS-term smaller by factor of 2 (central)/4(scattered e-)
  
  
• Fourier-transform requires resolving up to the third diffractive minimum at |t| ~0.1GeV^2, which requires vetoing the incoherent background by a t-dependent factor up to ~1000
  
  

There is a separate study by Wan Chang (CCNU/BNL) on how to achieve this incoherent reduction. For an update, see slides from July 10, 2020

u-Channel Exclusive Electroproduction of π⁰

Study for 5x100 GeV beam energies, at s(𝛾^*p)=10 GeV², t=t_max, u=u_min.

• Kinematic plots in (momentum, eta) of the scattered electron, the recoil proton, and the decay photons from π⁰.:

• Detector requirements:

1- e Endcap: better than 90% efficiency in forward electron detection;

2- p Endcap: better than 90% efficiency in forward proton detection;

3- ZDC: centering neutral particle path through the center of magnet aperture onto the center of ZDC. Smaller than 3 cm granularity;

4- Momentum resolution: 0.5-1% for particle momentum > 20 GeV; less than 0.5% for particle momentum < 10 GeV.

Contact person

• Wenliang Li (wenliang.billlee@googlemail.com)

Charged current meson production

Motivation

• Complementary information on GPDs with smaller contamination by higher-twist effects.
• Sensitivity to gluon GPDs in pseudoscalar meson production (arXiv:1904.04252 [hep-ph])
• Flavour separation through pion / kaon production
• Greatest sensitivity to H_u and H_d

Cross-sections

• Goloskokov-Kroll parametrisation
• Loop corrections taken into account at NLO. Scale choice: μ_R = μ_F = Q
• Counting rates estimated at integrated luminosity of 100 fb^-1
• Cross-sections of strangeness production expected to be of the same order.

Kinematic plots

For the process e p -> ν_e p π^-

Backgrounds and challenges

• Main background is quasi-real photoproduction of multi-hadron final states, whose cross-section is enhanced by a kinematic factor of (Q² + M_W²)²/Q⁴ with respect to charged-current production (several orders of magnitude at EIC kinematics).
• Suppression of background relies on excellent acceptance, measurement of pion and proton momenta with outstanding precision and veto of other hadrons.
• Cut below missing mass of a pion (second pion required by charge conservation in photoproduction) will suppress this background but requires relative momentum resolution below 10-5.
• Mis-identification of electrons as pions yields a background from elastic scattering on the proton. Can be eliminated with a missing min energy cut of 0.5 GeV.
• Nuclear targets pose additional background challenges which cannot be effectively overcome at the EIC.

Contact persons

• Marat Siddikov (marat.siddikov@usm.cl)
• Ivan Schmidt (ivan.schmidt@usm.cl)

Deeply Virtual Compton Scattering (DVCS) on helium-4

Conditions

• Study for: Q² > 2 GeV², t_min < |t| < t_min+0.5 GeV²
• Model from S. Fucini, S. Scopetta, M. Viviani Phys.Rev.C 98 (2018) 1, 015203
• Monte Carlo generator built with the TFOAM library of ROOT -- S. Jadach, P. Sawicki Comput.Phys.Commun. 177 (2007) 441-458 physics/0506084 [physics]

Cross-sections

• The total cross section is estimated to 1260 nb for the 5 on 41 GeV setting (more to come on this)
  • Here all figures are made based on 100,000 events
• -t distributions

• Xb distributions

Xb distribution ( 5x 41)	Xb distribution (10x110)	Xb distribution (18x110)

Kinematic plots

• Electron kinematics

• Photon kinematics

• Helium kinematics (bins are rearranged to fit forward detection)

Conclusions

• Partial conclusions before full study with EIC smear :
  • Electrons and photons appear in easily accessible kinematics, with the possible exception of some low angle photons in the highest energy configuration.
  • Helium nuclei are scattered very close to the beam, careful study is needed to assess what kinematic will be detectable

Contact person

• Raphaël Dupré (dupre@ipno.in2p3.fr)
• Sara Fucini (Sara.Fucini@pg.infn.it)
• Sergio Scopetta (sergio.scopetta@pg.infn.it)

Deep Exclusive π⁺ production (DEMP)

Motivation

• Allows the extraction of the charged pion form factor up to Q²~35 GeV², as part of our better understanding of the DCSB mechanism of hadronic mass generation
  

Cross-sections

• Our generator is a parameterization of output from the Regge-based p(e,e'π⁺)n model of T.K. Choi, K.J. Kong, B.G. Yu, arXiv: 1508.00969
• dσ_L/dt and dσ_T/dt for: 5<Q²(GeV²)<35, 2.0<W(GeV)<10, 0<-t(GeV²)<1.2
  

Kinematic plots

• For the process e p → e' π⁺ n, different Q² bins, with 5x41 and 5x100 beam energy combinations.
• Note that the beam crossing angle has already been taken into account in these plots. Hence the neutron is centered around the outgoing proton beam line and will be detected in the ZDC.

pi+ 5x41

pi+ 5x100

electron 5x41 ​

electron 5x100 ​

neutron 5x41

neutron 5x100

Backgrounds and challenges

• The exclusive π⁺ channel cross-section is several orders of magnitude smaller than neighboring SIDIS background, but is distributed over a much narrower range of kinematics, and this is essential for the separation of the DEMP events from the background
  
• We assure the exclusivity of the p(e,e'π⁺n) reaction by detecting the forward-going high-momentum neutron, i.e. e-π⁺-n triple coincidences
• We investigate the effectiveness of kinematic cuts to isolate the exclusive π⁺ channel by comparison to a simulation of SIDIS events, including both detector acceptance and resolution smearing effects
• The outgoing neutron energy is 80-95% of the proton beam energy, so the neutron energy resolution in the ZDC is expected to be poor. Thus, we use the neutron hit in the ZDC only as a tag of the exclusive event, and the neutron energy is otherwise not used in the event reconstruction. The missing momentum is calculated as pmiss=(\vec{p_e}+\vec{p_p}-\vec{p_e'}-\vec{p_π})
• The most effective cuts are on the detected neutron angle (±0.7^o from the outgoing proton beam), a cut on reconstructed -t<0.5 GeV², and a cut on the missing momentum (Q²-dependent cut, as determined from the overlap of the SIDIS and DEMP distributions)
  
• After application of these cuts, the exclusive p(e,e'π⁺n) events are cleanly separated from the simulated SIDIS events

Additional Material

• Talk by Stephen Kay at June 19 Exclusive WG meeting

https://indico.bnl.gov/event/8315/contributions/37023/attachments/28561/44027/Kay_Stephen_CFNS2020.pdf

Contact persons

• Garth Huber (huberg@uregina.ca)
• Stephen Kay (stephen.kay@uregina.ca)