LHC HI WG: Kickoff - HI Weekly Update Dennis Perepelitsa & Dominik Derendarz June 24, 2021 - CERN Indico

‫בס"ד‬

LHC HI WG: Kickoff (!)
 ATLAS perspective, wishes (!!),
 proposals, and views on the working
 group

 HI Weekly Update
 Zvi Citron

 Dennis Perepelitsa & Dominik Derendarz
 LHC HI WG: 7 July 2021

Getting Our House in Order
• Most basic function of such a forum is resolving tensions within the
 community
• Place to systematically and comprehensively review
 • Tensions (or disagreements) in data/results
 • Tensions (or disagreements) in planning priorities with shared implications

 Zvi Citron
 2
 LHC HI WG: 7 July 2021

EW Bosons as Standard Candles (Have
 We/They Failed?)
 • Tension in results is clear
 CMS 1.7 nb-1 (5.02 TeV PbPb)
 0.6
 • Probably not some ‘trivial’ Z boson
R AA

 ATLAS
 1.6
 0.55
 60 < mll < 120 GeV
 Pb+Pb, sNN=5.02 TeV, 0.49 nb-1

 1.4
 pp , s=5.02 TeV, 25 pb-1
 0.5
 |yZ| < 2.1
 measurement issue

 1 1 N (nb)
 1.2 0.45 • Origin not yet understood

 Nevt TAA Z
 0.4 -
 1 Z/γ * → l+l
 0.35
 0.8 0-90% data
 0.3
 Data HG-Pythia (isospin corrected)
 0.6
 W- 0.25 HG-PYTHIA scaled by 0-90%
 Z
 +
 0.4 W
 Model

 1.2 0 20 40 60 80 100
 Data

 0 20 40 60 80 100 1
 0.8
 Centrality [%] 0 20 40 60 80 0−90
 Centrality (%)

 Zvi Citron
 3
 LHC HI WG: 7 July 2021

inel = 70m
 ATLAS W ATLAS W + ATLAS Z data

 inel = 41.5
 1.2 1.2 Fitted inel
 uncertainty
 nn
 RPbPb with nn

 RPbPb with nn
 1.0 1.0

 Broad Implications
 EPPS16 EPPS16
 min.bias min.bias

 0.8 0.8
 ATLAS W ATLAS W + ATLAS Z data

 4
 0.6 K.Eskola AA
 et al [PRL 125, 212301 (2020)]
 Original T uncertainty
 0.6

 • ATLAS results mean HG-Pythia model
 p p
 1.4 s = 5.02 TeV 1.4 s = 5.02 TeV

 inel = 41.5mb
 inel = 70mb

 ATLAS W ATLAS W + ATLAS Z data
 1.2 1.2 Fitted inel
 nn uncertainty is incomplete/wrong
 • May suggest modification of σNN
 EPPS16 EPPS16
 RPbPb with nn

 RPbPb with nn
 min.bias min.bias
 1.0 1.0
 EPPS16 EPPS16

 • CMS results mean strong
 min.bias min.bias

 0.8 0.8
 ATLAS W ATLAS W + ATLAS Z data

 confirmation of HG-Pythia model
 EPPS16 EPPS16
 min.bias min.bias
 Original TAA uncertainty
 0.6 0.6

 1.4
 0 ps10 20 30
 = 5.02 TeV
 40 50 60 70 80 90 100
 1.4
 0 ps10 20 30
 = 5.02 TeV
 40 50 60 70 80 90 100 • à This is not a Z boson issue, this is
 inel = 41.5mb

 Centrality [%] Centrality [%]
 a baseline for HI interpretation issue
 inel = 70mb

 The centrality-dependent nuclear modification ratios for W ± and
FIG. 3. 1.2 1.2 Z boson production in Pb+Pb collisions from ATLAS
 inel
[37, 38] compared to NNLO pQCD calculation with EPPS16 nuclear modification with the nominal value of nn = 70.0 mb
 inel
(left) and with the nuclear-suppressed value nn = EPPS16
 41.5 mb (right). EPPS16
 RPbPb with nn

 RPbPb with nn

 min.bias min.bias
 1.0 1.0

(close-to) minimum-bias collisions. The impact, however, IV. MINIJETS WITH SHADOWING
 0.8
grows towards more peripheral centrality classes, see Ta- 0.8

 exp ALICE
ble I. To illustrate this, Fig. 3 compares the centrality
dependent RPbPb before and after rescaling the data by
 EPPS16
 min.bias To study the plausibility of the obtained suppression
 inel 0.6
 EPPS16
 min.bias in
 0.6 , we calculate its value in an eikonal model for minijet
hTAA ( pp https://doi.org/10.1
 inel
 )i/hTAA ( nninel
 )i using the fitted nninel
 . The left- nn
 production with nuclear shadowing. The model is based
hand panels016/j.physletb.2019.
 show the original ATLAS data including the
 0 10 20 30 40 50 60 70 80 90 100 on a similar0 10setup
 20as 30
 in Ref.40 [57]
 50 but60 in70
 the 80
 eikonal
 90 func-
 100
quoted hT 04.047
 AA i uncertainties, and in the right-hand panels tion we include only the contribution from the hard mini-
the data have been rescaled Centrality [%]
 and the uncertainties follow pCentrality [%]
 inel
 jet cross section jet ( snn , p0 , [Q]), calculated at lead-
from the nn fit. The striking e↵ect is that the mysteri- ing order in pQCD. The transverse-momentum cuto↵ p
FIG. 3. The
ous rise centrality-dependent
 towards more peripheralnuclear modification
 collisions ratios for W ± and Z boson production
 in the original p in Pb+Pb collisions from ATLAS0
[37,
data38] compared
 becomes to NNLOwith
 compatible pQCD calculationcentrality
 a negligible with EPPS16 (which
 de- nuclear
 depends on
 modification with thes , scale choice
 nn nominal value of Q and
 inel the pro-
 nn = 70.0 mb Zvi Citron
(left) and with inel ton thickness) and the width of the assumed Gaussian
pendence, the the nuclear-suppressed
 central values indicatingvalue nn = a
 perhaps 41.5 mb (right).
 mildly proton thickness function we fix so that the model repro- 4
decreasing trend towards peripheral bins. As discussed LHC HI WG: 7 July 2021
 duces inel = 70 mb matching the COMPETE analysis pp

because of the worse energy
 eightresolution of electrons
 measurements compared
 per track. Theseto muons.
 silicon In simulated
 detectors events
 are comple- in Ref. [23].
these selections correspond to 90%bybackground
 mented rejection, andtracker,
 the transition-radiation result inwhich
 a small efficiency
 enables loss
 radially Events with a Z boson candidate are selected by requiring
for Z bosons, which is taken care oftrack
 extended withreconstruction
 the applied corrections.
 up to |η| =Candidates
 2.0. resulting from exactly two opposite-charge muons or electrons, at least one of
this background account for 0.6
 The(0.7)% of the dimuon
 calorimeter system (dielectron)
 covers the yield before subtraction.
 pseudorapidity range |η|The
 < which is matched to a lepton selected at trigger level. The dilepton

 Centrality – Cart or Horse?
other backgrounds considered are Z ! t + t , tt production, and the production of W bosons
 4.9. Within the region |η| < 3.2, electromagnetic (EM) calorime- invariant mass must satisfy the requirement 66 < m$$ < 116 GeV
decaying to a lepton thattry
 is combined
 is provided with another
 by barrel lepton
 and originating
 endcap from a hadron
 high-granularity decay.
 liquid-argon consistent with previous ATLAS measurements. A total of 5347 Z
 Z
The expected yields are calculated as functions
 (LAr) sampling of centrality,
 calorimeters, with anpT ,additional
 or yZ using appropriate
 thin LAr presam- MC boson candidates are found in the muon channel and 4047 in the
samples. These backgrounds
 plercontribute
 covering |less
 η|

Compatible (Unified?) Centrality Treatment
• Whether or not my guess is correct, points to a need for a compatible
 or even unified centrality treatment
• Clear definitions and goals for centrality classification
 • (I don’t claim this will be trivial)
• Once ‘vocabulary’ is shared well positioned for more possibilities:
 • pA collisions
 • Light ion collisions
 • EW boson based centrality (?)

 Zvi Citron
 6
 LHC HI WG: 7 July 2021

➤ ATLAS uses full Run 2 (2015+2018) run of Pb+Pb at 5.02 TeV
➤ CMS has a measurement based on 2015 data only
 ➤ Important: difference in the kinematic phase space: CMS: pT>2 GeV, ATLAS: pT>2.5 GeV will

 Go Out for a Stroll …
 require an extrapolation correction
➤ About 2 sigma difference with Standard Model predictions
➤ Combination analysis is ongoing as part of the HonexComb project [more]
 ➤ Could be improved with the unpublished CMS 2018 data
 ➤ Opens a door for the combination of differential cross sections
 • Once our house is in
 order we can go out for a
 walk
 à Clear target is data
 combination, e.g. LbyL
 Combination analysis is
 ongoing as part of the
 HonexComb project

 [JHEP 03 (2021) 243] [Phys. Lett. B 797 (2019) 134826]

 Zvi Citron
 7
 LHC HI WG: 7 July 2021

Go Out for a Stroll …5
 [JHEP 03 (2021) 243]
 • Once our house is in
 order we can go out for a
 walk
 à Clear target is data
 combination, e.g. LbyL
 Also set limits on ALP

s Zvi Citron
 8
 LHC HI WG: 7 July 2021

A Combination Paradigm?
• LbyL is too good of an example!
• [HonexComb exists to focus on finding these - LbyL is 2nd case, total charm
 was 1st ]
• Examples ripe for combination or in tension from ATLAS measurements
 next slides
• Some wishlist items :
 • Top quark pairs
 • Tau pair production in UPC
• Should also consider to what extent this changes how we approach and
 chose the analyses that further the HI program
 • There are of course downsides to too much cooperation between independent
 experiments

 Zvi Citron
 9
 LHC HI WG: 7 July 2021

Exclusive Di-Leptons: → + −
 • Baseline measurement
 for UPC
 • Statistical uncertainties
 dominate at high m and
 high aco-planarity

 Zvi Citron
 10
 LHC HI WG: 7 July 2021

Inclusive Jets v2, central collisions 2

ATLAS, ATLAS-CONF-2020-019
Inclusive Jet v2 in central PbPb
 Tension observed between
 Tension observed between
 ALICE results and the other
 ALICE results and the other
 experiments
 experiments, as those results are
 Would bedata,
 used to constraint greatitto clarify
 will be …
 great to clarified the tension with
 ALICE and other experiments.

 Zvi Citron
 11
 LHC HI WG: 7 July 2021

A Shared Interpretation-Oriented Paradigm?
• Besides the cases of direct comparison there is room for
 improvement in how we go from measurement , and specifically
 multiple measurements, to interpretation
• Centrality as discussed
• Bulk property measurements typically have plenty of statistics, no
 large blatant tensions between experiments, “ahead of theory”,
 where do we go forward? Can we develop common interpretation-
 oriented approach
 • Important example: Template method for 2PC used only by ATLAS
• Similar measurements are sometimes still difficult/impossible to
 directly compare
 Zvi Citron
 12
 LHC HI WG: 7 July 2021

Photon-Jet Asymmetry
Analysis approach, in PbPb, pp
 procedure and observables 3

 • CMS compares reco-level
 PbPb w/smeared pp
 • ATLAS compares fully
 unfolded
 • Similar story elsewhere
 (e.g. di-jets)
 • It would be great if the
 philosophy behind the
 analysis could be discussed,
 à find common ground to
 make the comparison
 possible/easier.
 Zvi Citron
 13
 LHC HI WG: 7 July 2021

Light Ions (sooner)
• Many of these concerns/opportunities converge for upcoming Oxygen
 running
• [See Anne’s nice talk at OO workshop for review of ATLAS perspective]
• Small system flow and related physics questions, require good handle
 on geometry description, inter-experiment clarity on centrality and
 related crucial
• Similar issues for understanding onset of jet quenching + need for
 compatibility in places where we haven’t always had it

 Zvi Citron
 14
 LHC HI WG: 7 July 2021

Light Ions (later)
 p
• It’s distant but post Run
 Year
 2021
 4
 Systems, sNN
 Time
 Pb–Pb 5.5 TeV
 3 weeks
 running
 L
 2.3 nb
 (and any changes of course before
 int
 1

 then) will need some level
 1 week of
 pp 5.5 TeV common
 3 pb (ALICE), 300buy
 1
 in from
 pb (ATLAS,
 1
 CMS),the
 25 pb community
 1
 (LHCb) 1

 2022 Pb–Pb 5.5 TeV
 5 weeks 3.9 nb
• Requires common physics1 week case/questions
 O–O, p–O 500 µb and 200 µb and forum to express them
 1 1

 2023 p–Pb 8.8 TeV 3 weeks 0.6 pb 1 (ATLAS, CMS), 0.3 pb 1 (ALICE, LHCb)
 pp 8.8 TeV few days 1.5 pb 1 (ALICE), 100 pb 1 (ATLAS, CMS, LHCb)
 2027 Pb–Pb 5.5 TeV 5 weeks 3.8 nb 1
 pp 5.5 TeV 1 week 3 pb 1 (ALICE), 300 pb 1 (ATLAS, CMS), 25 pb 1 (LHCb)
 2028 p–Pb 8.8 TeV 3 weeks 0.6 pb 1 (ATLAS, CMS), 0.3 pb 1 (ALICE, LHCb)
 pp 8.8 TeV few days 1.5 pb 1 (ALICE), 100 pb 1 (ATLAS, CMS, LHCb)
 2029 Pb–Pb 5.5 TeV 4 weeks 3 nb 1
 Run-5 Intermediate AA 11 weeks e.g. Ar–Ar 3–9 pb 1 (optimal species to be defined)
 pp reference 1 week

 Zvi Citron
 15
 LHC HI WG: 7 July 2021

Summary
• Some existing tensions linger and understanding them matters!
• Data combination can help us with important measurements
• We should consider where coordinating in general approach (i.e.
 beyond one particular measurement) can help us, especially for
 reaching meaningful intepretations
• Upcoming (and future) light ion running at the LHC is an especially
 good opportunity to implement these ideas and make use of this
 forum

 Zvi Citron
 16
 LHC HI WG: 7 July 2021

Backups

 Zvi Citron
 17
 LHC HI WG: 7 July 2021

ATLAS-ALICE Tension in PbPb v2 from 4-
cumulants
v2{4}

 ATLAS
 0.08 Pb+Pb sNN = 2.76 TeV

 0.06

 ATLAS
 ALICE
 0.04 0.2 < p < 3 GeV, |η| < 1
 T

 50 100 150 200 250 300
 〈Nch(|η| < 1)〉
 Zvi Citron
 18
 LHC HI WG: 7 July 2021

Inclusive JetR RAA
 Inclusive Jets R=0.4 jets
 AA 1

 CMS arXiv:2102.13080
 ATLAS arXiv:1805.05635

 Hint of a tension, but still
 consistent within uncertainties.

 Zvi Citron
 19
 LHC HI WG: 7 July 2021