Overview of direct photon
and neutral mesons measurementswith ALICE at the LHC
Dmitry Blau for the ALICE collaboration
NRC “Kurchatov Institute”
23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

1QGP studied through e-m probes (heavy-ion collisions)Study parton energy loss and collective effects in hot medium via 𝜋0 and 𝜂
spectra modification relative to pp collisions 𝑅𝐴𝐴
Study thermal and bulk properties of medium with direct photon spectrum
measurements. High 𝑝T – test of initial conditions and 𝑁coll scaling.
,Study collective flow formation and evolution through 𝑣
Measurements of fully reconstructed jets provide possibility to further explore
parton energy loss

23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

2Study of direct photons and neutral mesons in pp and p-Pb

High precision and wide 𝑝T neutral meson spectra in pp collisions provide test ofpQCD predictions, and constraints of parton distribution functions PDFs andfragmentation functions FFs
Direct photon spectra in pp collisions also test QCD
Provides baseline for AA collisionsp-Pb collisions: study of modifications due to cold nuclear effects modificationof nucleon structure functions in nuclei, isospin effects, test scaling ofproduction with 𝑁coll𝑑𝜎 ,𝑑𝑝T𝑑η𝐹/ ⊗𝜎⊗𝐷 /𝐹 / – nucleon structure function
𝜎 – cross-section of the elementaryprocess
𝐷 / – fragmentation function

23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

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Direct photons – a probe to study QGP
Photons in heavy-ion collisions are produced bydifferent mechanisms and don't interact strongly:
carry out information about the dynamic of thecollision.

~ 2-3 GeV/c23 August 2021

timeDirectphotons

High 𝑝T: test of initial conditions:- 𝑁coll scaling- PDF modification
Low 𝑝T: test of hot matter evolution:- spectrum- collective flow

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

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Photon detection in ALICE
EMCal calorimeter
Pb/scintillator
Sampling calorimeter
distance to interaction
point IP : 4.28 mcell size 6 6 cm2
𝐸𝑀𝐶𝑎𝑙:
𝜂0.7,80°
𝜑187°
𝐷𝐶𝑎𝑙:
𝜂0.7,0.22
260°
𝜑320°
𝐷𝐶𝑎𝑙:
𝜂0.7,320°
𝜑327°

ALICE is an experiment at the LHC designed tostudy the quark-gluon plasma QGP

Complementary techniques: result inexcellent precision and 𝑝T range!

23 August 2021

Photon conversion
method PCM
ITS and TPC|𝜂|0.9,0° 𝜑 360°
Conversion in detectormaterial
11.4
0.5 %
𝑋/𝑋
Conv. probability ~ 8%

PHOS calorimeter
PbWO4 crystals
distance to IP: 4.6 m
cell size 2.2 2.2 cm2|𝜂|
0.12,260° 𝜑 320°

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

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0 meson measurements in pp
Neutral meson measurements over wide 𝑝T range provide data for FF and PDF global fits,
and are also needed for γdir measurements.
Wide range of measurements shown only 𝑠 2.76 and 8 TeV, but also √𝑠 0.9, 5.02, 7
TeV published earlier * .
PYTHIA 8.2 with Monash 2013 tune reproduce data well, but for larger √𝑠 the difference
increases. NLO pQCD calculations generally overestimates data.
* PLB 717 (2012) 162-172
Eur. Phys. J. C 74 (2014) 3108

Eur. Phys. J. C 78 (2018) 263
Eur. Phys. J. C 77 (2017) 339
23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS6and ω meson measurements in pp𝜂 and ω spectra are well reproduced by PYTHIA 8.2.
NLO pQCD calculations overestimate data.
Need to update FF for 𝜂 meson.

Eur. Phys. J. C 78 (2018) 263
Eur. Phys. J. C 77 (2017) 339
Eur. Phys. J. C 80 (2020) 1130
23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

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Neutral meson measurements in p-Pb

New method of 𝜋0 reconstruction in EMCal merged clusters allows to extend 𝑝Trange up to 200 GeV/c.
NLO pQCD calculations overestimate data. PYTHIA 8 generally reproduces 𝜋0 and 𝜂spectra, but has problems describing the shape low 𝑝T, high 𝑝T region . 𝜂/𝜋0 ratio isreproduced well.
RpPb is consistent with unity above 10 GeV/c and shows suppression below. This isconsistent with theory predictions

arxiv:2104.03116
23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

8Neutral meson measurements in Pb-Pb

Study parton energy loss in hot medium via 𝜋0 and 𝜂spectra modification relative to pp collisions RAA
New results on 𝜋0 RAA from Run 2 Pb-Pb collisions at
5.02 TeV:

Similar magnitude of suppression as at 2.76 TeV
𝑝T range 0.6 to 30 GeV/c Pb-Pb .

23 August 2021

Eur. Phys. J. C 74 (2014) 3108

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

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Direct photons in pp collisions
𝑅γ

High 𝑝T
Low 𝑝T𝛾inc𝛾decay𝛾inc/𝜋0𝛾decay/𝜋0param𝛾dir𝛾inc – 𝛾decay

11–𝛾
𝑅γ inc4 GeV/𝑐 – in agreement with pQCD
2-3 GeV/𝑐 – no thermal radiation excess visible within uncertainties

Phys. Rev. C 99 (2019) 024912
23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

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Direct photons in pp collisions
NLO pQCD calculations are able to reproduce measurements

Phys. Rev. C 99 (2019) 024912
23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

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Direct photons in p-Pb collisions


Direct photon spectrum was calculated in wide 𝑝T range up to 30 GeV/𝑐, several
NLO pQCD calculations are able to reproduce results
No thermal radiation fraction is visible at low 𝑝T within uncertainties, even in mostcentral 0-20% centrality

23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

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Direct photons in Pb-Pb collisions
 At low 𝑝T 2-3 GeV/𝑐∼ 8%-15% excess in 0-20% ;∼ 8%-9% in 20-40%
 At high 𝑝T above ~5 GeV/𝑐 inagreement with NLO pQCD and JETPHOX

Comparison with hydro models:
Underpredicted, but within
uncertainties

Phys. Lett. B 754 (2016) 235
23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

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Direct photons in Pb-Pb collisions


Effective temperature can be extracted from the low-𝑝T part of the spectrum
Both absolute yield of direct photons and effective slope increase with increasing
the collision energy

Phys. Lett. B 754 (2016) 235
23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

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Direct photon flow in Pb-Pb collisions
𝑅 𝑣





,,𝑣
,𝑣
𝑅
1
Large 𝑣2 for 𝑝T 3 GeV/𝑐, comparable to hadron flow for 20-40% - too large
uncertainties for conclusions
Hydro models underpredict direct photon low → models need further
development, hint for late direct photons production, and early flow formationPhys. Lett. B 789 (2019) 308-322
23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

15Conclusions

Neutral meson spectra in pp, p-Pb and Pb-Pb collisions measured to large 𝑝T constrainmodels
Direct photons confirm creation of hot matter in Pb-Pb collisions with significant
collective expansion

This work was supported by the Russian Science Foundation grant 17-72-20234

23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

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Backup slides

23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS17The ALICE experiment at the LHCALICE is designed to study the quark-gluon plasma QGP
Good tracking and PID capabilities are supplemented with electromagnetic-probe
measurements with the help of EMCal and PHOS
Two long data taking periods: 2010-2013 Run 1 and 2015-2018 Run 2 .
Comprehensive studies on hot QCD matter in pp, pA and AA collisions at LHC energies

23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

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Physics performance: neutral meson measurements
EMCal



PHOS, EMCAL, PCM are able to reconstructneutral mesons in low and high multiplicity
environments down to low 𝑝T
Excellent precision and 𝑝T range thanks tocombination of all methods

PCM
23 August 2021

PHOS
TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

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Direct photons in pp collisions
Proton-proton collisions at √𝑠 2.76 and 8 TeV are analyzed
PCM, EMCal and PCM-EMC methods are used

Systematic uncertainties of individual meas.
are dominated by 𝑝T-independent ones:
material budget unc. of 4.5% PCM, 2.8% EMC

Phys. Rev. C 99 (2019) 024912
23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

20Comparison of methods
Different measurements produce consistent results
Direct photons in Pb-Pb
Neutral mesons

23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

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Isolated photons in pp collisions

Access to direct prompt photons through isolation
techniques
Lower 𝑝T 10 GeV/𝑐 reach compared to other LHC
experiments
The measurements are consistent with NLO pQCD
predictions𝑝𝑝𝑝

2 GeV/𝑐

Eur. Phys. J. C (2019) 79: 896
23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

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Isolated photons in p-Pb collisions


Isolated photons production
was also measured in p-Pb at√𝑠NN 5.02 TeV
The spectrum is also consistent
with NLO pQCD predictions𝑝UE

2 GeV/𝑐arXiv:1906.01371
23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

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Direct photon flow – an unsolved puzzle?

Collective expansion transforms initial
spatial asymmetry of fireball toasymmetry in momentum space

Thermal photons, emitted early fromhotter fireball carry smaller collective
flow than those, emitted at later stages=> one can test development of collective
flow with direct photons

23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

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Direct photon flow in Pb-Pb collisions
Measurements are done with PCM and PHOS
Inclusive gamma 𝑣2:
𝑣2γ inc𝑣2γ dec ⇒ Either no contribution of 𝛾dir or 𝑣2γ dirTheory predicts ∼ 30 40% higher low

𝑣2γ dec

Phys. Lett. B 789 (2019) 308-322
23 August 2021

TWENTIETH LOMONOSOV CONFERENCE ON ELEMENTARY PARTICLE PHYSICS

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