1. in ATLAS and at Clermont-Ferrand
Heavy ions
in ATLAS and at Clermont-Ferrand
François Vazeille
ATLAS team meeting
(6 December 2010)
First comments
Short story in the LPC ATLAS team
First LHC results
First conclusions 1

2. First comments
What does happen during the last weeks and days?
A big communication action of CERN (and of the main responsible)
about the first HI results, particularly the ATLAS results on ″jet quenching″,
followed by a LPCC CERN seminar by December 2nd with talks of the 3
experiments (With finally ATLAS first to balance the CMS opposition to give
more time to ATLAS.)
Then the tendency of many people in the experiments and outside to minimize
these results, all the more ALICE is not concerned.
So, the question is: are these observations very important?
“QGP” is the main goal of the ions at LHC.
“Jet quenching” is the less ambiguous signature of it.
 Yes, these results are very important!
Moreover, they come at the right time closing the first LHC year. 2

3. Short story in the LPC ATLAS team
First official talk
in July 1992:
ATLAS
not yet created.
2 main channels are
Considered:
Dimuons and
Intermittency
but the
dimuon channels start
to be problematic.
(see next slides)
 1994
Our main interest goes towards the jets because of 2 reasons:
R&D on hadronic calorimetry.
Discovery on a possible curious jet behavior in QGP.
Next slide: selection of some talks (not everything). 3

4. Conseil Scientifique IN2P3
(February 1995)
SFP Lyon (November 1995)
jets
intermittency
DEA (July 1997)
IN2P3 (November 2002)
Our last action on HI: 2 reasons
No support from our laboratory
( in fact, since 1995: ″no ions in ATLAS″ ).
- Construction of Tilecal. 4

5. Low PT dimuons Jets Some words on the comparisons of the 3 experiments
(December 1996)
CMS is the best one
J/
with a limited interest
(next slides)
Jets
ATLAS is the best one
Yes
+ best radial
segmentation
Yes 5

6. ● Are dimuons the best signatures?
 Principle: search of a dimuon suppression with respect to increased centrality.
 J/
The possible suppression is balanced
by the abundant B decay in c c.
 
Not expected to be suppressed,
but for extreme temperature
conditions.
 ’ ’’
Expected to be suppressed,
but low rates.
In any case, as in the past (NA38, etc.),
any suppression will be a source of
controversy because of the role played
by the nuclear medium and other possible
explanations (comovers, etc.).
(From the first talk) 6

7.  In summary, our conclusions in the years 1994-1996 were:
- ALICE is well designed to access to soft processes (Low Magnetic field, TOF…)
and to some hard processes, mainly dimuons.
CMS is likely the best one for dimuons, and can access well to the jet quenching.
ATLAS is likely the best to access to the jet quenching.
The 3 experiments are well equipped to measure global variables
and are complementary in the search of QGP.
WARNING: these conclusions were not well understood …
 In the mean-time before the first LHC results
- Improved results from SPS (CERN) on dimuons and open charm particles.
Results from RHIC (USA) on dimuons,
but the main observations came from
the indirect measures of jet-quenching.
Example 7

8. That does confirm what was explained by us
First LHC results (December 2010)
ALICE  first papers on the multiplicity measures and on the elliptic flow.
CMS  first announcement of the first Z° in dimuons and also J/ and .
ATLAS  first paper on the direct measure of jet quenching.
That does confirm what was explained by us
about 15 years ago. 8

9. Soft physics 9

10. ALICE Very useful for the choice of the best generators in HI at LHC.
Information on the volume ( 2 times RHIC)
and energy density ( 3 times RHIC) 10

11. Dimuons 11

12. CMS
The most advanced results. 12

13. ALICE Only J/ shown. J/ regeneration is recalled by ALICE:
is it what I showed at the beginning?  to check.
Only J/ shown. 13

14. ATLAS Only Z° candidates shown,
but it was the choice to emphasis first the jet quenching in the talk. 14

15. Jet quenching The only and first indication so far on the QGP at LHC.
Direct comparison of experimental capabilities from the talks.
Experiments
Entire jet
 range
Jet 1 PT threshold
Jet 2 PT threshold
Background
subtraction
Asymmetry
measure
ATLAS
Yes
 2.8
100 GeV
25 GeV
Data/cell
R sampling
the first
CMS
 2.0
120 GeV
50 GeV
Data/MC
ALICE
Only
EM part
 1.0 No 15

16. ATLAS
The first and the best observation
Peripheral
Central 16

17. For information: good agreement of p-p and lead-lead data on missing ET17

18. CMS 18

19. ALICE 19

20. First conclusions
 From the true data, what was said 15 years can be wholly taken now.
- ALICE is well designed to access to soft processes (Low Magnetic field, TOF…)
and to some hard processes, mainly dimuons.
CMS is likely the best one for dimuons, and can access well to the jet quenching.
ATLAS is likely the best to access to the jet quenching.
The 3 experiments are well equipped to measure global variables
and are complementary in the search of QGP.
It is the first direct indications of the jet quenching from ATLAS/CMS:
- It is very important for physics at LHC and for LHC. 20

21. - It is too very important for our ATLAS team and for LPC:
 It was one of our major goal.
We demonstrate to everybody at LPC that ATLAS and our team play a role
in the HI at LHC.
+ 2 other facts:
Common communication of ALICE/ATLAS sent to LPC people (December 4th)
including the sentence
″Les particularités de chaque expérience étaient connues depuis longtemps ( )″.
 Next seminar of Peter Steinberg (BNL), ATLAS HI convener
Friday 10th at LPC: ″Heavy ions in ATLAS″. 21