DVCS and Exclusive Meson Production Measurements at the COMPASS Experiment - Po-Ju Lin on behalf of the COMPASS collaboration - CERN Indico

DVCS and Exclusive Meson
Production Measurements
 at
the COMPASS Experiment
 Po-Ju Lin
 CEA, Université Paris-Saclay
 on behalf of the COMPASS collaboration

 ICHEP 2020
 July 30, 2020

 1

Outline

 • The COMPASS Experiment
 • Deeply Virtual Compton Scattering (DVCS)
 • Hard Exclusive Meson Production (HEMP)
 • Summary and Outlook

 2

COMPASS Experiment
Versatile facility with hadron (, K, p …) & lepton
(polarized ) beams of energy 200 GeV

 COmmon
 LHC Muon and
 COMPASS Proton
 Apparatus for
 M2 Structure and
 SPS Spectroscopy

 220 physicists from 25 Institutes of 13 Countries

 3

COMPASS Setup for Hard Exclusive Measurements

➢ + & − beams with
 ~60m
 opposite polarisation
➢ A ± 80% polarisation
➢ Momentum: 160 GeV/c

 • 2012 pilot run with 4-week data taking
 • 2016-17 dedicated run. 2 x 6 months.

 CAMERA &
 Two-stage, large angle, and wide momentum range
 LH2 target spectrometer. PID including hadron absorbers, RICH,
 HCALs, ECALs, and muon filters.
 ❖ NIM A 577 (2007) & NIM A 779 (2015) 69 4

COMPASS Coverage for DVCS

 5

DVCS
 ′ ′
 DVCS: + → + + 
 To experimentally access the information about
 Generalized Parton Distributions (GPDs), DVCS is
 regarded as the golden channel and its
 interference with the well-known BH process
 gives access to more info.

 The variables measured in the experiment:
 
 Eℓ, Q2, xBj  2 /(1+ ), 
 t (or * ) and  (ℓℓ’ plane/* plane)
 6

DVCS
 ➢ The GPDs depend on the following variables:
 x: average longitudinal momentum frac.
 : longitudinal momentum diff.
 t: four momentum transfer
 (correlated to b⊥ via Fourier transform)
 Q2: virtuality of *

 Sensible to 4 GPDs, at COMPASS with small xB
 → focuses on H

CFF GPD REAL part Imaginary part
 +1 H( , , ) +1 H( , , )
 H =‫׬‬−1 =P ‫׬‬−1 − π H( =± , , )
 t, fixed − + − 
 Im H ( , )
 Re H ( , ) = P ‫ ׬‬ + d(t)
 − 7

Transverse Imaging and Pressure Dist.
 V. D. Burkert, L. Elouadrhiri, F. X. Girod
 Nature 557, 396-399 (2018)
 sea quarks Pion Valence
 and gluons cloud quarks

 b⊥ Pressure Distribution

 x = 0.01 x = 0.1 x = 0.3

CFF GPD REAL part Imaginary part
 +1 H( , , ) +1 H( , , )
 H =‫׬‬−1 =P ‫׬‬−1 − π H( =± , , )
 t, fixed − + − 
 Im H ( , )
 Re H ( , ) = P ‫ ׬‬ + d(t)
 − 8

Extraction of DVCS Events - 2012 data
 Increasing xBj

➢ Beam charge-spin sum
 
 SCS, U( ) ≡ ( +← ) + ( −→ ) = 2[ + + Im I ]
 = 2[ + 0 + 1 cos + 2 cos + 1 sin + 2 sin 2 ]

 c0DVCS  4 (Im H )2
 small xBj model dependent
 9

Tranverse extension of partons – 2012 data
 / ∝ − ⊥2 ≈ 2 At small xBj
 COMPASS 2012
 = 0.056 COMPASS PLB 793 (2019) 188-194
 2 = 1.8 (GeV/c)2
 = 5.8 GeV/c

 0.0025 COMPASS COVERAGE 0.15

 = 4.31 ± 0.62+0.09
 −0.25 (GeV/c)
 -2

➢ The transverse-size evolution as a function of 
 → Expect at least 3 bins from 2016-17 data With = 0.056
 10

2016 – 2017 Data First Insight
 μ’ * 
 μ Only 13% of 2016-17 data
 DVCS Bethe-Heitler (BH)  p
 0.005 < xBj < 0.01 0.01 < xBj < 0.03 xBj > 0.03

 No 0 subtraction.

 DVCS contribution at
 high will allow to
 perform re-analysis
 dDVCS /dt ∝ e-B’|t|
 = c0DVCS

 This research is part of the Blue Waters sustained-petascale computing project, which is supported by the National Science Foundation (awards OCI-
 0725070 and ACI-1238993) and the state of Illinois. Blue Waters is a joint effort of the University of Illinois at Urbana-Champaign and its National
 Center for Supercomputing Applications. This work is also part of the "Mapping Proton Quark Structure using Petabytes of COMPASS Data" PRAC
 11
 allocation supported by the National Science Foundation (award number OCI 1713684 ).

GPDs in Hard Exclusive Meson Production
Quark contribution 4 chiral-even GPDs: helicity of parton unchanged
 meson qq Hq(x, , t) Eq(x, , t) For Vector Meson
 γ*L Q2 q q
 H (x, , t) E (x, , t) For Pseudo-Scalar Meson
 x+ξ x-ξ + 4 chiral-odd or transversity GPDs: helicity of parton changed
 (not possible in DVCS)
 GPDs
 p p’ HTq(x, , t) ETq(x, , t) q
 q q EqT = 2 HT + EqT
 HT (x, , t) ET (x, , t)
Gluon contribution at the same order in S

 Vector meson qq
 ➢ Universality of GPDs, quark flavor filter
 γ*LL ➢ Ability to probe the chiral-odd GPDs.
 ➢ Additional non-perturbative term from meson wave
 function
 ➢ In addition to nuclear structure, provide insights into
 reaction mechanism
 12

0
2012 Exclusive Prod. on Unpolarized Proton
 2 1 
 μp→μ 0 p =
 2 
 + 
 
 + cos 2 
 
 + 2 (1 + ) cos 
 
 Chiral-odd GPDs

 A dip at small t would indicate a large impact of ET

 TT large ∝ ത 2 ?
 LT smaller but significantly positive

 COMPASS, PLB 805 (2020) 135454 13

2012 Exclusive  Prod. on Unpolarized Proton
(S-Channel Helicity Conservation)

 Exclusive 0,  production
 with trans. pol. target
 COMPASS, NPB865 (2012) 1-20
 COMPASS, PLB731 (2014) 19
 COMPASS, NPB915 (2017) 454-475

 Will be submitted
 to arXiv very soon!

 14

Summary and Outlook
 ➢ GPD by DVCS and HEMP in COMPASS
 DVCS x-sections with polarized + and -
 → Transverse extension of partons as a function of xBj
 → ImH (ξ,t) and ReH (ξ,t) for D-term and pressure distribution

 HEMP of 0, , , , J/
 → Universality of GPDs - Transverse GPDs - Flavor Decomposition

 On-going analysis on 2016-17 data.

 15

Backup Slides

 16

COMPASS Setup for GPD Measurement
 DVCS : μ p → μ’ p 
 μ μ’
 p 
 New equipements:
 ➢2.5m LH2 target
 ➢4m ToF Barrel CAMERA
 ➢ECAL0

 ECAL0

 CAMERA
 L=4m =2m ECAL0: 2 × 2 m2
 24 inner & outer Shashlyk modules + MAPD readout
 scintillators separated by about 1m one module is made of 9 cells (44 cm2)
 1 GHz SADC readout, 330ps ToF resolution = 194 modules or 1746 cells 17

ECAL2

 ECAL1

ECAL0

 CAMERA recoil proton detector
 surrounding the 2.5m long LH2 target

 |

 
 
 + SIDIS on unpolarized protons
 18

COMPASS++/AMBER
Letter of Intent - Draft 1.0: https://arXiv.org/abs/1808.0084

 Expected to start at 2022
 ➢ Unique beam line with polarised
 ± and high-intensity Pion beam
 ➢ Possible high-intensity antiproton
 and Kaon beams, provided by RF-
 separation technique
 ➢ With upgraded apparatus

 Proposed physics goals

 19

Possible RPD for COMPASS++/AMBER
A recoil proton detector (RPD) is mandatory to ensure the exclusivity. A Silicon detector is
included between the target surrounded by the modified MW cavity and the polarizing magnet

 A technology developed at JINR for NICA
 for the BM@N experiment
 No possibility for ToF ➔ PID of p/ with dE/dx
 Momentum and trajectory measurments
 |t|min  0.1 GeV
 20

 Dep. of BH+DVCS with Unpol Target
 γ* γ
 Well known
 x+ξ x-ξ
  
 μ’ * θ
  μ p
 GPDs
 p’
 Twist-2
   p
 γ* γ
  Twist-2 >>
 x+ξ x-ξ
  Twist-3,
   Twist-2
  double helicity flip GPDs
e-  p p’
 for gluons (NLO) Twist-3
s1I = Im F ~ at small xB γ* Q2
 F = F1H + ( F1 + F2 )H − t/4m F2E 
 2 F1H γ
c1I = Re F for proton x+ξ x-ξ
 at small xB
c0DVCS  4 (Im H) 2 GPDs
 p 21
 p’
 21
 Twist-2, NLO

Beam Charge-spin Difference
DCS, U( ) ≡ ( +← ) - −→ → 0 + 1 cos 

 1 = Re 1 H
 KM10
 VGG

➢ With Re 1 H and Im 1 H
 → Extraction of D-term

 Re H > 0 at H1
 < 0 at HERMES
 Value of xBj for the node?
 HERMES JLab

 COMPASS 2 years of data E= 160 GeV 1 < Q2 < 8 GeV2 22

2016 – 2017 Data First Insight
 DVCS : μ p → μ’ p 

 1)
 2)
 3)
 4)

 23

0
Exclusive Production on Unpolarized Proton
 e p → e 0 p

 Leading twist expected be dominant
 But measured as  only a few % of

 The other contributions arise from coupling between
 chiral-odd (quark helicity flip) GPDs to the twist-3 pion amplitude

 A large impact of ET can be identified:
 ➢ TT contribution
 ➢ The dip at small |t| of T
 24

Exclusive  Production on Unpolarized Proton
 experimental angular distributions

 Self analysis… with phi and cosTheta

 cos2

 25

COMPASS Acceptance of for DVCS

 Flux of transverse
 virtual photons

 26

GPD H Global Analysis

 Im H
 is better understood

 Re H linked to the d term
 is still poorly constrained

 KM15 K Kumericki and D Mueller arXiv:1512.09014v1
 GK S.V. Goloskokov, P. Kroll, EPJC53 (2008), EPJA47 (2011) 27

GPD E Global Analysis
 Figure made by D. Mueller and K. Kumericki

 Im E
 is rather unknown

 Re E
 is rather unknown

 KM15 K Kumericki and D Mueller arXiv:1512.09014v1
 GK S.V. Goloskokov, P. Kroll, EPJC53 (2008), EPJA47 (2011) 28