Research 2: P Allfrey

1. Heavy-Ion Physics with the
CMS experiment at the
Large Hadron Collider
Philip Allfrey
University of Auckland,
for the CMS Collaboration

2. Outline
• Large Hadron Collider and
Compact Muon Solenoid Experiment
• Quark-Gluon Plasma
• Jet Quenching
• Upsilon suppression
• Grid computing

3. Large Hadron Collider

4. CMS Detector
EM Calorimeter (ECAL) Hadron Calorimeter (HCAL)
Beam Scintillator Counters (BSC)
Forward Calorimeter
(HF)
TRACKER
(Pixels and Strips)
MUON
MUON
(Barrel)
(Endcaps) 4

5. Quarks and Gluons
• Protons and neutrons made up
of quarks and gluons
• Quarks and gluons normally
bound within a nucleon, can’t
exist in isolation
• If enough matter with enough energy
compressed into small enough volume, get a
‘soup’ of unbound quarks and gluons
• This is known as a quark-gluon plasma (QGP)
5

6. Heavy Ion Physics
• Goal of high-energy heavy-ion
physics to produce and study a
quark-gluon plasma
• Allows experimental study of
QCD in extreme temperature/energy density
• Various probes/signatures – jet quenching,
heavy quark production, elliptic flow,
correlations…
6

7. Heavy Ion Collisions
Lorentz- Nuclei collide. Quarks and Plasma
contracted Binary collisions gluons freed, expands and
nuclei between quarks plasma formed cools, quarks
approach and gluons can and gluons
produce jets or form bound
heavy quarks states
(particles)

8. Jet quenching
• Pair of jets must be formed
back-to-back to conserve
momentum
• Jets not necessarily formed
in centre of colliding nuclei,
so have different path
lengths in the medium
• Therefore back-to-back
jets with different
energies imply energy loss
• Signature of QGP

9. Examples of Jets in CMS
Balanced Unbalanced
Energy Energy

10. Dijet energy imbalance
Pb Pb PbPb Pb
Pb
Semi-Peripheral Semi-Central Central
E E
j1 j2
Jet energy
AJ  T T
Phys Rev C 84
asymmetry E E
T
j1
T
j2
(2011) 024906

11. Quarkonia
• Bound state of quark-antiquark pair is
called quarkonium
• Quarks and gluons carry colour charge,
can move around in QGP => screening
• If screening radius drops below binding
radius, quarkonia should melt
• Screening radius decreases with increasing
temperature, therefore suppression of
quarkonia acts as thermometer of QGP
• Look for suppression of Upsilon
(ϒ, b anti-b pair) in both heavy-ion and
proton-proton collisions
11

12. ϒ→μ+μ- mass spectrum
proton-proton Pb-Pb
ϒ(1S) ϒ(3S)
ϒ(2S) 12 Phys. Rev. Lett., 107 (2011) 052302

13. Double Ratio
• Compare ratios of ϒ(2S+3S) relative to ϒ(1S) in PbPb & pp
• Benefits from cancellation of possible acceptance and
efficiency differences
PbPb (2S + 3S) (1S) PbPb  0.24 0.12 (stat) ± 0.02 (sys)
+0.13
pp (2S + 3S) (1S) pp  0.78 +0.16 (stat) ± 0.02 (sys)
0.14
(2S + 3S) (1S) PbPb
 0.31+0.19 (stat) ± 0.03 (sys)
0.15
(2S + 3S) (1S) pp
First observation of suppression of excited ϒ states 13

14. Grid Overview
• Computing and data storage requirements of
LHC experiments too large for single site to
handle
• Distributed among computing centres
worldwide
• Classed as Tier-0,1,2 or 3 depending on
resources and responsibilities
• Based on OSG or gLite middleware
14

15. CMS Grid Sites by Tier
Tier 0 Tier 1 Tier 2 Tier 3 15

16. Architecture
Non-trivial to set up, even if documented! 16

17. Architecture
Computing Interface
CMS-
Specific
Storage Monitoring
17

18. Performance
• Certified by Asia-Pacific ROC
• 50k CPU-hours over past 12 months
• Users from 15 different countries
18

19. Summary
• Jet quenching and suppression of excited
upsilon states observed at CMS
• Made possible by greater energy at LHC
• Interpreted as signatures of QGP formation
• NZ contributing to analysis of LHC data with
Tier-3 Grid site