1. Prospects for New Physics at the LHC
Michael Schmitt
Northwestern University
PANIC 08
Eilat, ISRAEL
10 – Nov – 2008
10-Nov-2008
New Physics at the LHC (Schmitt)

2. New Physics at the LHC (Schmitt)
This is a vast topic.
And I have just 20 minutes!
I will discuss just three analyses, and hopefully - give some real content…
Z' → m+m-
H → gg
jets and missing energy
intro: CMS and ATLAS
concluding remarks
10-Nov-2008
New Physics at the LHC (Schmitt)

3. New Physics at the LHC (Schmitt)
CMS
LHC-b
ALICE
ATLAS
Drs. Funk and Morettini
will have discussed this
in the prior session.
10-Nov-2008
New Physics at the LHC (Schmitt)

4. New Physics at the LHC (Schmitt)
CMS
10-Nov-2008
New Physics at the LHC (Schmitt)

5. New Physics at the LHC (Schmitt)
ATLAS
10-Nov-2008
New Physics at the LHC (Schmitt)

6. New Physics at the LHC (Schmitt)
Z' → m+m-
Many theories of new physics predict a new gauge boson
which will decay to fermion pairs, like the usual Z boson.
Also, in extra-dimensional theories, spin-2 excited states
of SM gauge bosons can decay to fermion pairs.
Masses, couplings, cross sections, branching ratios are all
unknown a priori (there are some theory constraints).
We can observe these new bosons as resonances
in di-fermion invariant mass distributions.
The best result comes when the peak is narrow.
This means electrons and muons are best.
10-Nov-2008
New Physics at the LHC (Schmitt)

7. New Physics at the LHC (Schmitt)
Elements of the Analysis:
ATLAS & CMS
have endcap
muon systems –
good acceptance.
select a pair of high-pT opposite-charge leptons
make sure kinematics are well measured
Drell-Yan continuum, boson pairs, top quarks, fake leptons in jets
none of these gives a peak in di-lepton invariant mass
fake leptons can be strongly suppressed by demanding that the selected leptons not fall inside a jet – “isolation”
Main background will be
continuum di-lepton pairs.
Look for a peak on top
of a smooth background.
CMS
10-Nov-2008
New Physics at the LHC (Schmitt)

8. New Physics at the LHC (Schmitt)
Results / Sensitivity :
Assume resonance is relatively narrow.
Test: are the data compatible with a peak at an arbitrary mass?
Since couplings are indeterminate, take some benchmarks.
CMS
What can we see with 100 pb-1?
Surpass the Tevatron…
10-Nov-2008
New Physics at the LHC (Schmitt)

9. New Physics at the LHC (Schmitt)
Discussion :
Assume resonance is relatively narrow.
→ Then the width of the bump comes from detector resolution;
a narrower peak means better signal/background.
linear
For muons, this means primarily alignment: point errors translate
into curvature, ie, pT errors.
“Prospects for Discovery”
→ getting a good alignment early!
10-Nov-2008
New Physics at the LHC (Schmitt)

10. New Physics at the LHC (Schmitt)
Alignment Methodology:
0) use photogrammetry to get
excellent starting point.
use lasers and sensors to refine this, and to track movements of gross structures
2) use reconstructed tracks for ultimate precision
cosmic rays
ATLAS
10-Nov-2008
New Physics at the LHC (Schmitt)

11. New Physics at the LHC (Schmitt)
H → gg
This is a SM process – that has never been observed…
Theoretically novel – decay proceeds only through a loop,
which could reveal new particles indirectly (in principle).
In the SM, the rate is firmly predicted (as function of MH). And in SUSY, variations are small – lightest Higgs is SM-like.
Main production is from gluons (another loop!) with important
contributions from VBF and ttH production.
BR, however, is very small, typically (MH < 140 GeV)
eff. cross-section = (30 – 50 pb) x = 60 – 100 fb.
10-Nov-2008
New Physics at the LHC (Schmitt)

12. New Physics at the LHC (Schmitt)
Elements of the Analysis:
As before, look for a narrow peak on top of a continuum.
two well-reconstructed photons, high ET > 25, 40 GeV
The background is huge!!
irreducible – direct production of 2 photons w/o Higgs
reducible – one of the photons is from p0 or is fake
Use isolation and shower shape to suppress bad photons.
photon
ATLAS
jet
fraction outside core
strip width
10-Nov-2008
New Physics at the LHC (Schmitt)

13. New Physics at the LHC (Schmitt)
Interesting Extension:
Really try to remove all background – pull out all the stops!
Combined kinematic quantities in an artificial neural network:
polar decay angle
scaled photon energies
difference in h
longitudinal momentum of di-photon pair
Standard “cut-based” analysis simply looks for a peak; - hence, is lett model-dependent;
This approach assumes the model and exploits all features wisely.
→ sensitivity improved by about 30%
The two approaches are complimentary.
10-Nov-2008
New Physics at the LHC (Schmitt)

14. New Physics at the LHC (Schmitt)
Results / Sensitivity :
Although signal is fairly well “known” the background is not,
so discovery potential estimates comes with a lot of uncertainty.
CMS NN
Fortunately, sidebands are a ready source of background events
→ directly measure the rate, and kinematic characteristics
10-Nov-2008
New Physics at the LHC (Schmitt)

15. New Physics at the LHC (Schmitt)
Energy Resolution :
CMS comparison
test-beam and CR:
1.5% (need 0.5%)
Again – we want as narrow a peak as possible.
EM calorimeter must be well-calibrated.
basic calibration done with test beam
inter-channel calibration done with inclusive p0’s and muons (cosmic rays!)
absolute scale will be set from W → en and Z peak
2) The amount of material in front of calorimeter is crucial
more than one X0 !
40% of the photons in H → gg convert.
quality degrades when shower is spread out laterally
recuperation is possible – not fully developed yet
divide samples according to photon quality.
10-Nov-2008
New Physics at the LHC (Schmitt)

16. Jets and Missing Energy
Supersymmetry = theorist’s best guess about BSM.
LHC might turn out to be the machine that finds SUSY.
Characteristic feature: LSP is weakly-interacting & neutral.
Best place to find SUSY is probably in a sample of Jets+MET.
Signatures vary widely, difficult to specify and hence most analyses pick one possibility and optimize for it:
two jets and MET
three or more jets, and MET
jets and one lepton, and MET
etc.
10-Nov-2008
New Physics at the LHC (Schmitt)

17. New Physics at the LHC (Schmitt)
Elements of the Analysis:
Reconstruct jets from calorimeter energy clusters.
demand at least 3: ET > 30 GeV and | h | < 3
MET > 200 GeV
ET1 > 180 GeV, ET2 > 110 GeV, HT > 500 GeV
Event Cleanup:
vertex reconstructed from charged tracks
fractions of “electromagnetic” and “charged” energy not too large.
CMS cosmic ray data!
10-Nov-2008
New Physics at the LHC (Schmitt)

18. New Physics at the LHC (Schmitt)
Veto events with leptons (loose criteria).
QCD 3-jet events have large MET usually due to one badly mis-measured jet:
MET vector should not point close to any jet.
CMS
SUSY
QCD
10-Nov-2008
New Physics at the LHC (Schmitt)

19. New Physics at the LHC (Schmitt)
Results / Sensitivity :
Even though the selection is fairly straight-forward,
the expected signal varies greatly.
Here are two “benchmarks scenarios” :
10-Nov-2008
New Physics at the LHC (Schmitt)

20. New Physics at the LHC (Schmitt)
Interesting Model-Independent Approach :
Promising ATLAS study* looks for statistical excess anywhere in
a predefined kinematic space.
Kinematic quantities should be
well suited to the generic channel.
Construct a sphere around a given
data point and compare number of
data events to expectation (simul’n).
No use of signal simulation at all.
Performance appears to be as good
as “targeted” analyses, yet much less model-dependent.
10-Nov-2008
New Physics at the LHC (Schmitt)
(* E. Duchovni et al., Weizmann Institute)

21. New Physics at the LHC (Schmitt)
Concluding Remarks
You have seen a brief
overview of three selected
analysis – they are already
quite advanced; others, too.
Prospects for discoveries
will be good since so much
is being done in advance
of collisions…
10-Nov-2008
New Physics at the LHC (Schmitt)

22. New Physics at the LHC (Schmitt)
10-Nov-2008
New Physics at the LHC (Schmitt)