1. Sparticle reconstruction at benchmark points
Alessia Tricomi
On behalf of ATLAS and CMS Collaboration
Dipartimento di Fisica e Astronomia and INFN Catania

2. QCD and Hadronic Interactions, 28 March - 4 April 2004
Outline
In the past years several inclusive studies done to understand the detector capabilities to discover SUSY
Counting excess of events over SM expectations
No explicit sparticle reconstruction done
Several different final states analysed
Etmiss + jets
No lepton + Etmiss + jets
1 l + Etmiss + jets
2 l OS + Etmiss + jets
2 l SS + Etmiss + jets
3 l + Etmiss + jets
Scan in mSUGRA (m0,m1/2) plane (also studies in p-MSSM scenario performed )
Fast MC simulation used: ATLFAST, CMSJET
Today I will focus on recent studies to reconstruct strongly interacting SUSY particles
S.Abdullin, F. Charles Nucl. Phys. B547 (1999) 60
S. Abdullin et al., J. Phys. G28 (2002) 469
M. Dzelalija et al., Mod. Phys. Lett. A15 (2000) 465
QCD and Hadronic Interactions, 28 March - 4 April 2004

3. Inclusive SUSY reach vs integrated luminosity
A0=0, tanb=35, m>0
If Supersymmetry exists, LHC will probably observe it
Cosmologically interesting region covered with 10 fb-1
Squark-gluino production dominates the total cross section at low mass scale
Squarks and Gluinos detectable up to 2 TeV mass with 100 fb-1
Expected squark-gluino mass reach
 1.5 – 1.8 TeV with 10 fb-1
 2.3 – 2.5 TeV with 100 fb-1
 2.6 – 3.0 Tev with 300 fb-1
 2.8 – 3.2 TeV with 1000 fb-1
S.Abdullin, F. Charles Nucl. Phys. B547 (1999) 60
S. Abdullin et al., J. Phys. G28 (2002) 469
M. Dzelalija et al., Mod. Phys. Lett. A15 (2000) 465
QCD and Hadronic Interactions, 28 March - 4 April 2004

4. But … It is not enough to observe the excess over the Standard Model…
DISCOVERY
SUSY SPECTROSCOPY
This requires a different approach…
Fix a set of points in the parameter space
Get information on the spectrum (i.e. end-points)
Reconstruct sparticles
QCD and Hadronic Interactions, 28 March - 4 April 2004

5. Decay chains p b b p SM bkg: tt, Z+jet, W+jet, ZZ, WW, ZW, QCD jets
Reconstruction of sbottoms, squarks and gluinos p b b p
 2 high pt isolated leptons OS
(leptons = e,m)
 2 high pt non-b jets
missing Et
 2 high pt isolated leptons OS
(leptons = e,m)
 2 high pt b jets
missing Et
SM bkg: tt, Z+jet, W+jet, ZZ, WW, ZW, QCD jets
QCD and Hadronic Interactions, 28 March - 4 April 2004

6. QCD and Hadronic Interactions, 28 March - 4 April 2004
Benchmark points
Proposed Post-LEP Benchmarks for Supersymmetry, M. Battaglia et al. (hep-ph/ )
Rather low m0 and m1/2 values in order to have high SUSY cross section
Three different tan b values (tan b = 10, 20, 35) since BR(c20  l+l- c10) depends on tan b
QCD and Hadronic Interactions, 28 March - 4 April 2004

7. QCD and Hadronic Interactions, 28 March - 4 April 2004
Point B spectra
95.6
c10 = LSP
173.8
c1±
136.2 lR
361.6
c2±
196.5 lL
174.4
c20
520 qR
339.9
c30
559 qL
361.1
c40
524.0 bR
575.9 tR
496.0 bL
392.9 tL
595.1 g
Point B
(17% bL, 10% bR)
(37 % bL, 25% bR)
(0.04 %)
(16.4 %)
(83.2 %)
Sbottom reconstruction
2 isolated leptons, pT>15 GeV, |h|<2.4
 2 b-jets, pT>20 GeV, |h|<2.4
QCD and Hadronic Interactions, 28 March - 4 April 2004

8. QCD and Hadronic Interactions, 28 March - 4 April 2004
First step: c20  l+l- c10
SUSY events
10 fb-1 p b
QCD and Hadronic Interactions, 28 March - 4 April 2004

9. QCD and Hadronic Interactions, 28 March - 4 April 2004
Bkg reduction
SUSY
ttbar
Z+jets
Edge = 79  2 GeV
Generated = 78.2 GeV
SM bkg can be strongly reduced cutting on ETmiss
QCD and Hadronic Interactions, 28 March - 4 April 2004

10. Second step: sbottom (squark) reconstruction
At the end-point:
Assuming M(c10) known
Selecting events “in edge”
Combining the c20 obtained from the two leptons with the most energetic b-jet in the event
QCD and Hadronic Interactions, 28 March - 4 April 2004

11. QCD and Hadronic Interactions, 28 March - 4 April 2004
Squark mass peak
Result of the fit:
1 fb-1
Generated values
QCD and Hadronic Interactions, 28 March - 4 April 2004

12. The peak should be considered as the superposition of two peaks
Sbottom mass peak
Result of the fit:
10 fb-1
Generated masses:
The peak should be considered as the superposition of two peaks
QCD and Hadronic Interactions, 28 March - 4 April 2004

13. Gluino reconstruction
10 fb-1
sbottom chain
10 fb-1
squark chain p b
Two separated gluino mass measurements, with two different samples
Generated value:
QCD and Hadronic Interactions, 28 March - 4 April 2004

14. Worse resolution, but model independent result
estimate
The reconstruction is performed assuming M(c10) known but …
The difference between the two masses is independent of M(c10)
Result of the fit:
Generated value:
Worse resolution, but model independent result
QCD and Hadronic Interactions, 28 March - 4 April 2004

15. QCD and Hadronic Interactions, 28 March - 4 April 2004
point B
Squark mass peak can be reconstructed in the first few weeks (resolution ~12%)
Sbottom and gluino in the first year (resolution ~6÷8%)
Two independent gluino mass measurements
The resolutions can be improved with larger statistics (~5÷6% at 300 fb-1)
300 fb-1
60 fb-1
60 fb-1
300 fb-1
300 fb-1
60 fb-1
Sbottom Chain
Squark Chain
10 fb-1
60 fb-1
300 fb-1
M(sbottom)
500±7
502±4
497±2
M(squark)
536±10
532±2
536±1
s(sbottom)
42±5
41±4
36±3
s(squark)
60±9
36±1
31±1
M(gluino)
594±7
592±4
591±3
592±7
595±2
590±2
s(gluino)
42±7
46±3
39±3
75±5
59±2
M(gl)-M(sb)
92±3
88±2
90±2
M(gl)-M(sq)
57±3
47±2
44±2
s(gl-sb)
17±4
20±2
23±2
s(gl-sq)
9±3
16±5
11±2
Errors are of the order of 1÷2 GeV (statistical) + 2÷3 GeV (energy scale of the calorimeters)
The main source of error is from the lack of knowledge on M(c10)
M(c10) assumed known for these reconstructions
No systematic errors evaluated
QCD and Hadronic Interactions, 28 March - 4 April 2004

16. Reconstruction @ point G
With respect to the Point B:
Lower total SUSY cross-section
Lower BR of useful decays
Higher BR’s of competitive decays
Lower BR of the last decay
300 fb-1
10 fb-1
sbottom chain
squark chain
QCD and Hadronic Interactions, 28 March - 4 April 2004

17. QCD and Hadronic Interactions, 28 March - 4 April 2004
Point G
sbottom
gluino
squark
Sbottom Chain
300 fb-1
M(sbottom)
720±26
s(sbottom)
81±18
M(gluino)
851±40
s(gluino)
130±43
M(sb)-M(gl)
127±10
s(sb-gl)
48±11
Squark Chain
M(squark)
774±9
s(squark)
84±9
853±11
126±11
M(sq)-M(gl)
82±3
s(sq-gl)
35±3
Repeating the same procedure as Point B:
QCD and Hadronic Interactions, 28 March - 4 April 2004

18. Reconstruction @ point IG
300 fb-1
Tau channel becomes predominant at large tan b
Tau-pair edge is not as sharp as in the e and m case, but could help to cover points in which the reconstruction is problematic
It could be exploited in regions with too low leptonic BR: work in progress both in ATLAS and in CMS B
Neutralino BR’s B G I
BR(c20→ll)
16.44
2.26
0.25
BR(c20→tt)
83.22
75.88
98.13
QCD and Hadronic Interactions, 28 March - 4 April 2004

19. QCD and Hadronic Interactions, 28 March - 4 April 2004
Di-tau lepton edge _
As t l n n identification is not possible, must rely on
hadronic decays – narrow, 1-prong jets (large QCD bkg though)
Can typically achieve t/jet ~100 for et ~50-60 %
ATLAS Physics TDR study (full GEANT simulation) example (“Point 6”) :
Narrow isolated jets selection :
Rjet = 0.2, Risol = 0.4
30 fb-1
Real t from SUSY
Require 0.8 GeV < Mjet < 3.6 GeV
(biased against 1-prong, but
improves di-tau mass resolution
- less neutrino momentum)
Fake t from SUSY
Di-tau efficiency = 41 %
Mvis = 0.66 Mtt
visible
expected
Additional cuts :
min. 4 jets : ET > 100 GeV,
ET > 50 GeV
missing ET > 100 GeV,
no e, m with pT > 20 GeV
SM bkg 1
2- 4
Recent results – even more encouraging…
QCD and Hadronic Interactions, 28 March - 4 April 2004

20. Why and how to measure the c10 mass…
Use as starting point for other sparticle mass measurements (sbottom, gluino, squark…)
Allanach et al., 2001
DAMA
10-3
10-4
10-5
10-6
Using mass of lightest neutralino and RH sleptons can discriminate between SUSY models differing only in slepton mass.
SUSY Dark Matter
Lightest Neutralino LSP excellent Dark Matter candidate.
Test of compatibility between LHC observations and signal observed in Dark Matter experiments.
QCD and Hadronic Interactions, 28 March - 4 April 2004

21. Why and how to measure the c10 mass…~ qL c 2 q 1 R
Long decay chains allow multiple measurements
Use kinematics to measure end-points (in general both maximum and minimum value due to 2-body decays)
Use dilepton signature to tag presence of c02 in event, then work back up decay chain constructing invariant mass distributions of combinations of leptons and jets c 2 ~ 1 h b -
ATLAS
Physics
TDR
1% error
(100 fb-1)
Point 5
e+e- + m+m-
30 fb-1
atlfast
Physics
TDR
ATLAS
Point 5
Starting point : di-lepton OS SF mass edge
Assume 2 hardest jets are from squarks,
combine each of these with leptons to from :
ATLAS
Physics
TDR
2% error
(100 fb-1)
Point 5
>
<
Mlq, Mlq
ATLAS
Physics
TDR
1% error
(100 fb-1)
Point 5
Endpoints:
Mllq
Threshold : Tllq, requiring Mll > Mll / 2
max
Also used endpoint from Mhq from h bb -
Turns out to have enough constraints to determine all masses involved (!)
Can measure mass relations to ~1%
as a function of LSP mass, determined to ~10 %
QCD and Hadronic Interactions, 28 March - 4 April 2004

22. QCD and Hadronic Interactions, 28 March - 4 April 2004
Mass Reconstruction
Combine measurements from edges from different jet/lepton combinations
Numerical solution of simultaneous edge position equations
c01 lR
c02 qL
Mass (GeV) ~
Allanach et al., 2001
ATLAS
Point 5
Mllq
High Mlq
Low Mlq
Mll
Mhq
Sparticle Expected precision (100 fb-1)
qL  3%
02  6%
lR  9%
01  12% ~
ATLAS Physics TDR
Point 5
Systematic uncertainties not evaluated
Main sources:
Th: theoretical distributions of edges
Ex: detector resolution (i.e. energy resolution, b/t-tagging efficiency, bias due to cuts applied…
Same order precision point SPS1A (similar to “CMS” point B)
Gives sensitivity to masses
Gjelsten et al., ATLAS-PHYS
QCD and Hadronic Interactions, 28 March - 4 April 2004

23. Lot of work before LHC start-up!!!
Conclusions
LHC experiments are expected to explore SUSY in a decisive way.
The plausible part of mSUGRA-MSSM parameter space will be explored in a number of characteristic signatures
Strongly interacting SUSY particles can be accessed up to 2TeV for 100 fb-1
Information on the SUSY spectrum achievable , with favourable SUSY parameters, already after the first months of data taking
Low tan b region (like point B, SPS1A, point 5):
first few weeks of LHC running period:
reconstruction of squark (resolution ~12%)
first year:
reconstruction of sbottom and gluino (resolutions ~6÷8%)
reconstruction of gluino in the squark chain (independent channel)
high integrated luminosity:
improvement on the resolutions
double fit of the sbottom peak
Intermediate tan b (i.e. point G):
first year:
dilepton edge hardly visible
no reconstruction possible in e,m channel
high integrated luminosity:
reconstruction of squarks, sbottom (~11%) and gluino (~15%)
High tan b (i.e point I):
no reconstruction possible in the e-m channel even with high accumulated statistics
Reconstruction in the tau channel exploited. Work is going on…
New analyses are going on:
New benchmark points to be analyzed
Full reconstruction studies in first priority
Multi-edge technique to be fully exploited
Deeper insight to the tau-tau channel
Systematic uncertainties to be evaluated
Lot of work before LHC start-up!!!
QCD and Hadronic Interactions, 28 March - 4 April 2004