Direct Photon Production with Meson Beams

A. Guskov, JINR, DubnaTrento, Italy

6.11.2017

Alexey Guskov, Joint Institute for Nuclear Research

Outline

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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

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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

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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)