Direct Photon Production with Meson Beams
A. Guskov, JINR, DubnaTrento, Italy
6.11.2017
Alexey Guskov, Joint Institute for Nuclear Research
Outline
2
Gluon structure of hadrons Photons in hadron collisions Prompt photons Previous prompt photon experiments with pion beams Possible future measurements with kaon beam Summary
Alexey Guskov, Joint Institute for Nuclear Research
Gluon component: π vs. p
3
x0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1xg(x)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
protonpion (WA70)
2 = 2 (GeV/c)2Q
Gluon contribution at high x in pion is much larger than
in proton
Alexey Guskov, Joint Institute for Nuclear Research
g(x) for pion
4
prompt photons
quarconia production jet production
xg(x) ⇠ (1� x)⌘, ⌘ ⇡ 2
Alexey Guskov, Joint Institute for Nuclear Research 5
No direct experimental data!
(based on yield of D-mesons in NA32)
G(x) for kaon
Not too many theoretical works
Phys. Rev. D93 (7) (2016) 074021
Gluon content of kaon is ~1/6 in respect to pion)!
Gluon content of kaon is ~1.5 larger in respect to pion)!
Sov.J.Nucl.Phys. 49 (1989) 346Yad.Fiz. 49 (1989) 554-558
IFVE-88-67
Alexey Guskov, Joint Institute for Nuclear Research
Production of photons in hadron collisions
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Photons in A+A
Direct Photons Decay Photons
hardthermal hard+thermal
QGP Hadron gasprompt
fragmentation
Preequilibrium photons
jet-γ-conv.
Medium inducedγ bremsstr.
collective effects, not important for hA collisions
Alexey Guskov, Joint Institute for Nuclear Research
Prompt photons
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LO
signal
NLO
� ⇠ (gB · q(q̄)T + gT · q(q̄)B + q(q̄)B · q̄(q)T +NLO)⇥K-factor
access to gluon distributions in hadrons
Alexey Guskov, Joint Institute for Nuclear Research
Gluon Compton Scattering (GCS)
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y3− 2− 1− 0 1 2 30
20
40
60
80
100
310×
GCS
GCS π
with g
>0.5 GeV/cT
pπ+ p→γ X
Eπ=100 GeV
The region of negative y (or xF) is the most sensitive for
gluon content of beam meson
Alexey Guskov, Joint Institute for Nuclear Research
Decay photons
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, GeV/cT
p1 2 3 4 5 6
4−10
3−10
2−10
1−10
1
10
210
310
410
510
610
GCS
decay photons
, GeV/cT
p1 2 3 4 5 6
1
10
210
310
410
510
610
decay0π
decayη
other decays
π+ p→γ X Eπ=100 GeV
Decay photons dominate over prompt photons in the full range of transverse momentum pT. Even at high pT
signal-to-background ratio in much below 1
LO
Alexey Guskov, Joint Institute for Nuclear Research
Fragmentation photons
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γ γ
Relative contribution of fragmentation
photons is below 15% even at much higher
energies.
It can be calculated in LO and NLO
Alexey Guskov, Joint Institute for Nuclear Research
Previous studies at our energies
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Fixed target measurements
xT=2pT/√s
A lot of measurements with proton beams at much higher energies
(Tevatron, LHC)
Alexey Guskov, Joint Institute for Nuclear Research
NA24 — typical fixed-target experiment for prompt photon studies
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300 GeV beam
Alexey Guskov, Joint Institute for Nuclear Research
Signal vs background
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Much higher rate of background photons mainly from π0 and η
mesons decay!
N�prompt = N�detected � N�bkg. found
✏
NA24
from MC!
ηπ0
π0→2γ→1 cluster
At high E (and pT) we should expect background from merging of 2γ into
a single cluster!
Alexey Guskov, Joint Institute for Nuclear Research
WA70-E706 puzzle
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π/p ratio for γ
production of π0
WA70 E706
Alexey Guskov, Joint Institute for Nuclear Research
Previous results: pp(pbar)
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pp(pbar)
Alexey Guskov, Joint Institute for Nuclear Research 16
Two approaches were proposed to fix disagreement between data ant predictions:
kT- smearing:
<kT> =1.5 GeV for E706 and up to 3-4 GeV for ~ √s~1 TeV This approach modifies only low-pT part
ISR of soft gluons: also an impact to transverse momenta of partons
WA70-E706 puzzle
Alexey Guskov, Joint Institute for Nuclear Research
2γ data
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Alexey Guskov, Joint Institute for Nuclear Research
gK(x) with RF-separated kaon beam…
18, GeVBEAME
50 100 150 200 250 300
, nb
σ
0
100
200
300
400
500
600
700
800
900
γ q →q g − , K+for K
− for Kγ g → qq
+ for Kγ g → qq
It would be nice to have K+ beam with momentum as large as
possible (>100 GeV/c) and intensity of about ~5e6 s-1 for 1 yearK− would be nice for study of
systematics
, GeVBEAME50 100 150 200 250 3000
0.1
0.2
0.3
0.4
0.5
0.6
0.7 interactionγq→ per 1 qgγ background 610
See Johannes’s talk
Alexey Guskov, Joint Institute for Nuclear Research
… at COMPASS ?
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System of 3 precise electromagnetic calorimeters of high aperture for prompt photons detection in wide kinematic range. They can be used also in trigger.
Threshold Cherenkov detectors on the beam line to identify incoming hadron. Nice tracking to separate neutral and charged clusters. Pions as a reference.
COMPASS
1 year of data taking with kaon beam — ~106 events with
pT>2.5 GeV/c, comparable with E706 (gK(x)=gπ(x))
Alexey Guskov, Joint Institute for Nuclear Research
Prompt photos vs quarkonia production
20Complimentary approaches!
Transparent theory
Experimental difficulties with strong background
subtraction
Nice signal
Difficulties to treat the signal
Different mechanisms of J/ψ production:
gg→J/ψgqqbar→J/ψ
…→χc012→J/ψγdiffractive production of J/ψ
…?
See Jen-Chieh’s talk for more details
Alexey Guskov, Joint Institute for Nuclear Research
Prompt photons and DY
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— two orders of magnitude smaller cross section
— possibility to achieve low-pT region
Phys.Lett. B209 (1988) 397-406 (1988)
γ*
Production of low-mass dimuon pairs is a process very similar to
prompt photon production
ppbar collisions
mμμ<2.5 GeV/c
Alexey Guskov, Joint Institute for Nuclear Research
γ→μ+μ− in matter
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σN,E=10-100 GeV⋍5 μb
For ~1m of the COMPASS ammonia target probability for secondary photon to convert into dimuon is ~1×10-5 So, the effective cross section of dimuon pair production via real secondary photons conversion in the target material is ~1 μb that is a few orders of magnitude larget than the cross section of low-mass dimuon production via virual photon.
Prompt photon - low-mass dimuons duality could be used in the collider experiments but not in the experiments with a thick fixed
target
Alexey Guskov, Joint Institute for Nuclear Research
Summary
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Prompt photon production is a unique instrument for study of gluon content in hadrons. All the measurements at energy scale ~20 GeV were
performed with pion and proton beams only 20-30 years ago It is a good time to come back with new level of experimental techniques and theoretical understanding Gluon structure of kaon is unknown. Prompt photon
production with kaon beam could provide a unique opportunity to access it. Opportunity to have RF-separated kaon beam at CERN can be
used to study of pion and kaon gluon structure at fixed target experiments like COMPASS.
Alexey Guskov, Joint Institute for Nuclear Research
Lets look inside kaon!
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K+
gK(x)