![Casting a Wide Net
Kinematics matters more than spin or A-terms
Alwall, Le Lisanti, JW (0803.0019)
Q = mX − mχ ˜
X = g , q , ...
˜ ˜
Q=0
V. GLUINO EXCLUSION LIMITS
A. No Cascade Decays
˜
χ
m
For the remainder of the paper, we will discuss how model-independent jets + ET searches
can be used to set limits on the parameters in a particular theory. We will focus specifically
=
on the case of pair-produced gluinos at the Tevatron and begin by considering the simplified
scenario of a direct decay to the bino. The expected number of jets depends on the relative
mχ
mass difference between the gluino and bino. When the mass difference is small, the decay
jets are very soft and initial-state radiation is important; in this limit, the monojet search
˜ is best. When the mass difference is large, the decay jets are hard and well-defined, so
the multijet search is most effective. The dijet and threejet searches are important in the
transition between these two limits.
X
m
As an example, let us consider the model spectrum with a 340 GeV gluino decaying
directly into a 100 GeV bino. In this case, the gluino is heavy and its mass difference with
the bino is relatively large, so we expect the multijet search to be most effective. Table III
shows the differential cross section grids for the 1-4+ jet searches for this simulated signal
point. The colors indicate the significance of the signal over the limits presented in Table II;
the multijet search has the strongest excesses.
Tevatron
Previously [28], we obtained exclusion limits by optimizing the ET and HT cuts, which
involves simulating each mass point beforehand to determine which cuts are most appropri-
ate. This is effectively like dealing with a 1 × 1 grid, for which a 95% exclusion corresponds
250 GeV Sensitivity
150
mSugra
Bino Mass GeV
100
Out[27]=
50 X
0
100 200
Gluino Mass GeV
300 400 500
FIG. 4: The 95% exclusion region for DO at 4 fb−1 assuming 50% systematic error on background.
mX
250 GeV
The exclusion region for a directly decaying gluino is shown in light blue; the worst case scenario
500 GeV
for the cascade decay is shown in dark blue. The dashed line represents the CMSSM points and
the “X” is the current DO exclusion limit at 2 fb−1 .
15](https://image.slidesharecdn.com/newlhcjetsmetaspen-12640081265621-phpapp02/75/Jets-and-Missing-Energy-at-the-LHC-4-2048.jpg)
![542 than 20 GeV. We demand at least 4 jets with |η |
pT 100 GeV. The transverse sphericity ST should
Backgrounds
543
544 ETmiss should be larger than 100 GeV and larger th
545 add one more cut against the QCD background: the
Madgraph+Pythia 546
547
between the ET PGS
miss vector and the three highest-p
discussed in a dedicated note on QCD background e
(up to 2 → 4 )
548 3.2 Backgrounds in Monte Carlo
4+jetsFigure 16 shows the distributions of E
549
miss
T and Meff
SM 104
¯
Events / 1fb−1/ 50GeV
1000 SU3
tt± SM BG
tt
W 103 W
(fb/25 GeV)
100 Z0 Z
QCD
QCD 102
single top
ATLAS
10
10
dET
dσ
1
1
0 100 200 300 400 500 600 700 800 900 1000
Missing ET [GeV]
0.1
200 400 600 800 1000
ET (GeV)
Figure 16: The ET miss and effective mass distribut
for the no-lepton mode with an integrated luminosit
(SU3 point). The shaded histogram shows the sum o
Matching crucial to getting MET tail right
show the various components.](https://image.slidesharecdn.com/newlhcjetsmetaspen-12640081265621-phpapp02/75/Jets-and-Missing-Energy-at-the-LHC-6-2048.jpg)
![Radiation
Degenerate spectra require radiation to generate signal
V. GLUINO EXCLUSION LIMITS
A. No Cascade Decays
˜
mχ
For the remainder of the paper, we will discuss how model-independent jets + ET searches
can be used to set limits on the parameters in a particular theory. We will focus specifically
=
on the case of pair-produced gluinos at the Tevatron and begin by considering the simplified
scenario of a direct decay to the bino. The expected number of jets depends on the relative
mχ
mass difference between the gluino and bino. When the mass difference is small, the decay
jets are very soft and initial-state radiation is important; in this limit, the monojet search
˜
is best. When the mass difference is large, the decay jets are hard and well-defined, so
M
the multijet search is most effective. The dijet and threejet searches are important in the
transition between these two limits.
X
m
As an example, let us consider the model spectrum with a 340 GeV gluino decaying
directly into a 100 GeV bino. In this case, the gluino is heavy and its mass difference with
the bino is relatively large, so we expect the multijet search to be most effective. Table III
shows the differential cross section grids for the 1-4+ jet searches for this simulated signal
point. The colors indicate the significance of the signal over the limits presented in Table II;
the multijet search has the strongest excesses.
Previously [28], we obtained exclusion limits by optimizing the ET and HT cuts, which
involves simulating each mass point beforehand to determine which cuts are most appropri-
ate. This is effectively like dealing with a 1 × 1 grid, for which a 95% exclusion corresponds
150
Bino Mass GeV
···
100
˜
χ0 ˜
Out[27]=
50 X
g
mX
0
Gluino Mass GeV
100 200 300 400 500
FIG. 4: The 95% exclusion region for DO at 4 fb−1 assuming 50% systematic error on background.
The exclusion region for a directly decaying gluino is shown in light blue; the worst case scenario
for the cascade decay is shown in dark blue. The dashed line represents the CMSSM points and
the “X” is the current DO exclusion limit at 2 fb−1 .
15
UED-like Theories
Unbalanced momentum set by Q = mg − mχ
˜ ˜
j ˜
g j
˜
χ0 ˜
χ0
j ˜
g j
As pT of gluino increases,
Final state jets get harder (and eventually merge),
but won’t gain significant MET](https://image.slidesharecdn.com/newlhcjetsmetaspen-12640081265621-phpapp02/75/Jets-and-Missing-Energy-at-the-LHC-9-2048.jpg)
![Multijet Universality
Selection Criteria for Different Jet Multiplicity Searches
1j + ET
2j + ET
3j + ET 4+ j + ET
j1 ≥ 400 GeV ≥ 100 GeV ≥ 100 GeV ≥ 100 GeV
j2 50 GeV ≥ 50 GeV ≥ 50 GeV ≥ 50 GeV
j3 50 GeV 50 GeV ≥ 50 GeV ≥ 50 GeV
j4 50 GeV 50 GeV 50 GeV ≥ 50 GeV
Table 1: The selection criteria for the different jets plus missing energy samples. Jet pT cuts
in GeV for the different exclusive search channels. The following cuts are applied: |ηj | 2.5,
Never20 GeV, ∆φ(ET , ji) jet multiplicity to significantly enhance discovery
pT, found lower 0.2 rad. (i = 1, 2, 3).
Degenerate spectra
2.1 Classification of Search Channels
Light Squarks, Heavy Gluinos
Discovering new physics in an all-hadronic final state is a challenging prospect in the jet-
rich environment of the LHC. The QCD and instrumental backgrounds, particularly in
early running, are a major challenge. In shape variables and ensure that the events
Tried many different event order to both trigger and search strategies
are sufficiently distinctive compared to poorly understood backgrounds, the searches
considered in this article have tighter cuts than the ones considered in the published
search strategies from ATLAS and CMS. For instance, thethe Tevatron article were
Qualitatively different than cuts used in this
initially based on the benchmark ATLAS multijet + ET searches [3]. This article finds
that requiring a larger missing energy criteria is more effective at separating signal from
background
The first step in devising a model-independent, inclusive search strategy is to clas-](https://image.slidesharecdn.com/newlhcjetsmetaspen-12640081265621-phpapp02/75/Jets-and-Missing-Energy-at-the-LHC-16-2048.jpg)
![ariables, the sensitivity in multijet and ET searches will
ent shape variable, αRTS Alphaintroduced in [24]. αRTS was
The , first Variable
Introduced by L. Randall D. Tucker-Smith
pT j2 pT j2
α
αRTS =
mj1 j2
pT j1 (1 − cos θj1j2 )
are the transverse momentumto multijet leading two jets in t
Modified by CMS
of the searches
ant mass between the3 leading and subleading jets, and θj
j
j1
m. αRTS was introduced as an alternative to ET in eli
in dijet + ET searches; j2evaluating Eq. 2.3 for a topology
Group to minimize
al QCD scenario, gives αRT S = 0.5. Fig. 2 shows a αRTS
momentum imbalance
QCD sample and the electroweak 1 SM backgrounds with2
j Sj
hape suggests applying a cut αRTS ≥ 0.55 as an alternati
d ET searches. However, the region below 0.5 is where new](https://image.slidesharecdn.com/newlhcjetsmetaspen-12640081265621-phpapp02/75/Jets-and-Missing-Energy-at-the-LHC-19-2048.jpg)
Uploaded byJay Wacker
Jets and Missing Energy at the LHC
This document discusses techniques for discovering new physics at the LHC through jets and missing energy, focusing on model-independent searches for gluinos. It emphasizes the importance of mass differences between particles to distinguish signal from background and outlines optimal strategies for various jet multiplicities. The findings highlight that multijet searches are particularly effective for cases with significant mass differences in decay scenarios.
Jets and Missing Energy at the LHC
- 1. Casting a WideNet for New Physics at the LHC with Jets and Missing Energy Jay Wacker SLAC Aspen Winter Conference January 20, 2010 with Eder Izaguirre and Michael Manhart arXiv: 1002.XXXX “Well some sea monsters are shy”
- 2. Jets + MET Solution to Hierarchy Problem If the symmetry commutes with SU(3)C, new colored top partners (note twin Higgs exception) Dark Matter Wimp Miracle: DM a thermal relic if mass is 100 GeV to 1 TeV Usually requires a dark sector, frequently contains new colored particles Fewest requirements on spectroscopy Doesn’t require squeezing in additional states to decay chains
- 3. Spectrum in DifferentTheories MSSM UED High Cut-Off Low Cut-Off Large Mass Splittings Small Mass Splittings 2 g 2 g δm = m log Λ δm = Λ 16π 2 16π 2 ˜ g g1 w1 b1 ˜ w ˜ b
- 4. Casting a WideNet Kinematics matters more than spin or A-terms Alwall, Le Lisanti, JW (0803.0019) Q = mX − mχ ˜ X = g , q , ... ˜ ˜ Q=0 V. GLUINO EXCLUSION LIMITS A. No Cascade Decays ˜ χ m For the remainder of the paper, we will discuss how model-independent jets + ET searches can be used to set limits on the parameters in a particular theory. We will focus specifically = on the case of pair-produced gluinos at the Tevatron and begin by considering the simplified scenario of a direct decay to the bino. The expected number of jets depends on the relative mχ mass difference between the gluino and bino. When the mass difference is small, the decay jets are very soft and initial-state radiation is important; in this limit, the monojet search ˜ is best. When the mass difference is large, the decay jets are hard and well-defined, so the multijet search is most effective. The dijet and threejet searches are important in the transition between these two limits. X m As an example, let us consider the model spectrum with a 340 GeV gluino decaying directly into a 100 GeV bino. In this case, the gluino is heavy and its mass difference with the bino is relatively large, so we expect the multijet search to be most effective. Table III shows the differential cross section grids for the 1-4+ jet searches for this simulated signal point. The colors indicate the significance of the signal over the limits presented in Table II; the multijet search has the strongest excesses. Tevatron Previously [28], we obtained exclusion limits by optimizing the ET and HT cuts, which involves simulating each mass point beforehand to determine which cuts are most appropri- ate. This is effectively like dealing with a 1 × 1 grid, for which a 95% exclusion corresponds 250 GeV Sensitivity 150 mSugra Bino Mass GeV 100 Out[27]= 50 X 0 100 200 Gluino Mass GeV 300 400 500 FIG. 4: The 95% exclusion region for DO at 4 fb−1 assuming 50% systematic error on background. mX 250 GeV The exclusion region for a directly decaying gluino is shown in light blue; the worst case scenario 500 GeV for the cascade decay is shown in dark blue. The dashed line represents the CMSSM points and the “X” is the current DO exclusion limit at 2 fb−1 . 15
- 5. Plan of Talk Distinguishing Signal from Background * Where the LHC can get to (soon)
- 6. 542 than 20 GeV. We demand at least 4 jets with |η | pT 100 GeV. The transverse sphericity ST should Backgrounds 543 544 ETmiss should be larger than 100 GeV and larger th 545 add one more cut against the QCD background: the Madgraph+Pythia 546 547 between the ET PGS miss vector and the three highest-p discussed in a dedicated note on QCD background e (up to 2 → 4 ) 548 3.2 Backgrounds in Monte Carlo 4+jetsFigure 16 shows the distributions of E 549 miss T and Meff SM 104 ¯ Events / 1fb−1/ 50GeV 1000 SU3 tt± SM BG tt W 103 W (fb/25 GeV) 100 Z0 Z QCD QCD 102 single top ATLAS 10 10 dET dσ 1 1 0 100 200 300 400 500 600 700 800 900 1000 Missing ET [GeV] 0.1 200 400 600 800 1000 ET (GeV) Figure 16: The ET miss and effective mass distribut for the no-lepton mode with an integrated luminosit (SU3 point). The shaded histogram shows the sum o Matching crucial to getting MET tail right show the various components.
- 7. Signals Should use same methods for signals as backgrounds Signal may be in a different place than expected Parton Shower - Matrix Element Matching pp → XX + 0j pp → XX + 1j pp → XX + 2 j + Extra jets aren’t a nuisance, they can qualitatively alter signal
- 8. New Initial States Possible at higher order gg, q q → 2˜ + 0+ j ¯ g gq → 2˜ + 1+ j g qq → 2˜ + 2+ j g Parton Luminosities 1 µb gg dL qq dˆ s 1 nb qg ¯ qq 1 pb 3000 GeV 300 GeV 100 GeV 1000 GeV
- 9. Radiation Degenerate spectrarequire radiation to generate signal V. GLUINO EXCLUSION LIMITS A. No Cascade Decays ˜ mχ For the remainder of the paper, we will discuss how model-independent jets + ET searches can be used to set limits on the parameters in a particular theory. We will focus specifically = on the case of pair-produced gluinos at the Tevatron and begin by considering the simplified scenario of a direct decay to the bino. The expected number of jets depends on the relative mχ mass difference between the gluino and bino. When the mass difference is small, the decay jets are very soft and initial-state radiation is important; in this limit, the monojet search ˜ is best. When the mass difference is large, the decay jets are hard and well-defined, so M the multijet search is most effective. The dijet and threejet searches are important in the transition between these two limits. X m As an example, let us consider the model spectrum with a 340 GeV gluino decaying directly into a 100 GeV bino. In this case, the gluino is heavy and its mass difference with the bino is relatively large, so we expect the multijet search to be most effective. Table III shows the differential cross section grids for the 1-4+ jet searches for this simulated signal point. The colors indicate the significance of the signal over the limits presented in Table II; the multijet search has the strongest excesses. Previously [28], we obtained exclusion limits by optimizing the ET and HT cuts, which involves simulating each mass point beforehand to determine which cuts are most appropri- ate. This is effectively like dealing with a 1 × 1 grid, for which a 95% exclusion corresponds 150 Bino Mass GeV ··· 100 ˜ χ0 ˜ Out[27]= 50 X g mX 0 Gluino Mass GeV 100 200 300 400 500 FIG. 4: The 95% exclusion region for DO at 4 fb−1 assuming 50% systematic error on background. The exclusion region for a directly decaying gluino is shown in light blue; the worst case scenario for the cascade decay is shown in dark blue. The dashed line represents the CMSSM points and the “X” is the current DO exclusion limit at 2 fb−1 . 15 UED-like Theories Unbalanced momentum set by Q = mg − mχ ˜ ˜ j ˜ g j ˜ χ0 ˜ χ0 j ˜ g j As pT of gluino increases, Final state jets get harder (and eventually merge), but won’t gain significant MET
- 10. Radiation Radiation unbalancesLSP’s momentum ˜ χ0 j j ˜ χ0 j j ˜ ˜ g g j j MET is generated by radiated jets Without extra jets, signal is invisible
- 11. Spectrum of RadiationHarder for New Physics When LSP is heavy sS sBG (rest mass of LSP is unobservable) Cross section for background is large due to lower SM CM energy
- 12. Spectrum of RadiationHarder for New Physics Consider the additional cost of energy for radiating a jet off a particle, X, of mass m pµ = (Ej , Ej ) j pµ X = m2 + Ej , −Ej 2 √ Ej m Ej δ s(Ej ) = Ej + m2 + E2 − m = j 2Ej m Ej √ Identify m→ s
- 13. Spectrum of RadiationHarder for New Physics When LSP is heavy sS sBG (rest mass of LSP is unobservable) Cross section for background is large due to lower SM CM energy √ Radiate off a jet with Ej ∼ sS √ √ Ej Ej s δ s(Ej ) = √ 2Ej Ej s √ √ √ √ sS+j sS + Ej sBG+j sBG + 2Ej sS+j sBG+j Equalizes CM energy for signal and background Increases S/B significantly Works at the LHC because we’re beyond last SM threshold
- 14. An Example mg = 250 GeV ˜ mχ0 = 200 GeV ˜ 2000 1000 (fb/25 GeV) 500 High MET cut doesn’t kill signal 200 100 Matched dET 50 Unmatched dσ 20 10 0 200 400 600 800 1000 ET (GeV) Tail is enhance by a factor of 10!
- 15. Plan of Talk Distinguishing Signal from Background * Where the LHC can get to (soon)
- 16. Multijet Universality Selection Criteria for Different Jet Multiplicity Searches 1j + ET 2j + ET 3j + ET 4+ j + ET j1 ≥ 400 GeV ≥ 100 GeV ≥ 100 GeV ≥ 100 GeV j2 50 GeV ≥ 50 GeV ≥ 50 GeV ≥ 50 GeV j3 50 GeV 50 GeV ≥ 50 GeV ≥ 50 GeV j4 50 GeV 50 GeV 50 GeV ≥ 50 GeV Table 1: The selection criteria for the different jets plus missing energy samples. Jet pT cuts in GeV for the different exclusive search channels. The following cuts are applied: |ηj | 2.5, Never20 GeV, ∆φ(ET , ji) jet multiplicity to significantly enhance discovery pT, found lower 0.2 rad. (i = 1, 2, 3). Degenerate spectra 2.1 Classification of Search Channels Light Squarks, Heavy Gluinos Discovering new physics in an all-hadronic final state is a challenging prospect in the jet- rich environment of the LHC. The QCD and instrumental backgrounds, particularly in early running, are a major challenge. In shape variables and ensure that the events Tried many different event order to both trigger and search strategies are sufficiently distinctive compared to poorly understood backgrounds, the searches considered in this article have tighter cuts than the ones considered in the published search strategies from ATLAS and CMS. For instance, thethe Tevatron article were Qualitatively different than cuts used in this initially based on the benchmark ATLAS multijet + ET searches [3]. This article finds that requiring a larger missing energy criteria is more effective at separating signal from background The first step in devising a model-independent, inclusive search strategy is to clas-
- 17. 3 Searches 900 GeV HT High MET ET 600 GeV 1200 GeV Gluinos, Squarks Degenerate Spectra 600 GeV Alpha E T 400 GeV α 0.20 HT 900 GeV 1200 GeV Gluinos, Squarks 600 GeV Mid MET E T 400 GeV HT 900 GeV 1200 GeV Cascade Decays
- 18. The Searches pp → g g + X → (q q χ0 )(q q χ0 ) + X ˜˜ ¯˜ ¯˜ High MET Search ET 600 GeV HT 900 GeV, 1200 GeV 600 500 400 Bino Mass GeV 300 200 100 0 Gluino Mass GeV 200 400 600 800 1000
- 19. ariables, the sensitivityin multijet and ET searches will ent shape variable, αRTS Alphaintroduced in [24]. αRTS was The , first Variable Introduced by L. Randall D. Tucker-Smith pT j2 pT j2 α αRTS = mj1 j2 pT j1 (1 − cos θj1j2 ) are the transverse momentumto multijet leading two jets in t Modified by CMS of the searches ant mass between the3 leading and subleading jets, and θj j j1 m. αRTS was introduced as an alternative to ET in eli in dijet + ET searches; j2evaluating Eq. 2.3 for a topology Group to minimize al QCD scenario, gives αRT S = 0.5. Fig. 2 shows a αRTS momentum imbalance QCD sample and the electroweak 1 SM backgrounds with2 j Sj hape suggests applying a cut αRTS ≥ 0.55 as an alternati d ET searches. However, the region below 0.5 is where new
- 20. Discovery Reach Alpha Search E T 400 GeV HT 600 GeV, 900 GeV, 1200 GeV α 0.20 Directly decaying gluinos Directly decaying Squarks pp → g g + X → (q q χ0 )(q q χ0 ) + X ˜˜ ¯˜ ¯˜ pp → q q + X → (q χ0 )(¯χ0 ) + X ˜˜ ˜ q˜ 600 400 500 300 Bino Mass GeV 400 Bino Mass GeV Alpha 300 200 High MET High MET Alpha 200 100 100 0 Gluino Mass GeV 200 400 600 800 1000 0 Squark Mass GeV 100 200 300 400 500 600 Multijet still more effective than dije
- 21. Cascade Decays ˜ ¯ g → qqχ 0 → q q (q q χ ) ¯ ¯ 0 Lowers MET in Events Mid MET Search E T 400 GeV HT 600 GeV, 900 GeV, 1200 GeV 500 High MET 400 Bino Mass GeV 300 Mid MET 200 100 Alpha 0 Gluino Mass GeV 200 400 600 800
- 22. Extended Cascade Decays g→ q q χ → (q q )q q χ → (q q )(q q )q q χ ˜ ¯ ¯ ¯ ¯ ¯ ¯ LSP is great-granddaughter Momentum is divided many different ways Reduces MET dramatically Need a low MET search E T 200 GeV Worse case scenario ˜ g 400 GeV χ 200 GeV χ 100 GeV χ 50 GeV
- 23. Conclusion “Well some seamonsters are shy, but not our Walter.”
- 24. Conclusion Discovery at theLHC in Jets + MET much easier than at the Tevatron Matching gets the signal right and makes it more visible Simple search strategies will provide extensive coverage “Well some sea monsters are shy, but not our Walter.”