# Presentation: 598947

## Study of SISCone  A Seedless Infrared-Safe Cone jet algorithm

**Manoj Jha (Delhi)**
**Anwar Bhatti (Rockfeller)**
**Marek Zielinski (Rochester)**
**Monica Acosta (CERN)**
**23****rd**** July, 2007**
**Jet Algorithm Subgroup**

## Outline

**Cone jet algorithm**
**Infrared-Safety issues**
**Why is this mandatory ?**
**IR unasfety of the midpoint algorithm**
**SIScone: A pratical solution**
**Results**
**High PT jets**
**Low PT jets**
**Dark energy towers**
**Effect of pileup**
**Conclusion & Next Steps**

## Cone jet algorithms

**Given:**** set of N particles with their 4-momentum**
**Goal:**** clustering those particles into jets**
**Idea:**** jets = cones around dominant energy flows for a cone of radius R in the (****η****,****φ****) plane, *****stable cones***** are such that center of the cone ≡ direction of the total momentum of its particles**
**Algorithm:****  Tevatron Run II**
**StepI:  find *****ALL***** stable cones of radius R**
**StepII: run a split-merge procedure with overlap fraction *****f***
**                     ****to deal with overlapping stable cones **

## Midpoint cone algorithm

**Usual seeded method to search stable cones: MP cone algorithm**
** ****For an initial seed**
** ****sum the momenta of all particles within the cone centered on the seed**
** ****use the direction of that momentum as new seed**
** ****repeat above steps until stable state cone reached**
** ****Sets of seeds:**
**All particles (above a p****T**** thresholds)**
**Midpoints between stable cones found in 1.**
_**Problems:**_
** ****the p****T**** threshold s is collinear unsafe**
** ****seeded approach ****→ stable cones missed → infrared unsafety**

## Infrared Safety: Why ?

**IR Safety:**** **
- 	Stability upon emission of soft particles, is required for perturbative computation to make sense !

- Cancellation of IR divergences between
- Real and virtual emissions of SOFT gluons.

- If Jet clustering is different in both cases, THEN the cancellation is
-   not done and the results, in principle, cannot  be compared to pQCD
- Stable cones must not change upon addition of soft particles

## SISCone: seedless solution

**Naive approach:**** check stability of each subset of particle.**
**    ****Complexity is ****Ο****(N2****N****) i.e. definitely unrealistic (10****17**** years for N = 100)**
**    ****Idea:**** all enclosures are defined by pair of points**

**Tricks:**** **
**Traversal order to avoid recomputation of the cone content**
**Complexity:**
**SISCone is ****Ο****(Nn ln n) ( with n ~ N the number pf points in **
**   ****a circle of Radius R **
**Midpoint standard implementation is ****Ο****(N****2****n)**
**For more information: see 0704.0292[hep-ph]**

## Data Samples

**CMSSW_1_5_2  QCD Jets in pT hat 120-135 GeV**
**Considered Calorimetry jets only**
**Parameters for SISCone jets**
**	**
** ****SISConeJetParameters = {****   ****double cone radius = 0.5 **
**	****	****	****         ****double coneOverlapThreshold = 0.75**
**	****	****          ****int32 maxPasses = 0**
**                              ****double protojetPtMin = 0.**
**} **
** ****Parameters MidPoint jets**
**    **
**  ****MidPointConeJetParameters = { ****       double coneRadius = 0.5**
**                ****double overlapThreshold = .75**
**                                                         ****double seedThreshold = 1.0**
**                ****double inputPtMin = 0.5**
**                ****int32 maxPairSize = 2**
**                ****int32 maxIterations = 100**
**}**
**No. of events = 7000**

## Results

## No. of jets for jets PT > 35 GeV

**Mean ~ 2.78**

**Mean ~ 2.76**

**MidPoint5**

**SISCone5**

## PT of Jets for PT > 35 GeV

**Mean ~ 85.46**

**Mean ~ 85.27**

**MidPoint5**

**SISCone5**

## ΣPT of Jets for PT > 35 GeV

**Mean ~ 192.0**

**Mean ~ 190.5**

**MidPoint5**

**SISCone5**

-  Good agreement between midPoint and sisCone algorithm for jets
-     having P**T** > 35 GeV.

## ΣPT of Jets for 0.1 < PT < 5 GeV

**Mean ~ 43.45**

**Mean ~ 95.65**

**MidPoint5**

**SISCone5**

## PT of Jets for 0.1 < PT,jet < 5 GeV

**Mean ~ 2.78**

**Mean ~ 1.87**

**MidPoint5**

**SISCone5**

## No. of jets for jets 0.1 < PT < 5 GeV

**Mean ~ 16.04**

**Mean ~ 51.06**

**MidPoint5**

**SISCone5**

-  SISCone algorithm produces more number of jets in low P**T** region
-    with respect to midPoint algorithm.

## Distribution of dark energy towers

**Mean ~ 0.307**

**Mean ~ 0.216**

**MidPoint5**

**SISCone5**

- Some of the energetic towers are being left by midPoint algorithm.
-  No problem of dark energy towers in case of sisCone algorithm.

## Σ Dark energy towers

**Mean ~ 147.5**

**Mean ~ 106.5**

**MidPoint5**

**SISCone5**

-  Midpoint algorithm is around ~28% energy (E**T**) deficient in comparison to SISCone algorithm.

## Effect of Pileup on jet PT

**Mean y ~ 0.858**

**Mean y ~ 0.524**

**MidPoint5**

**SISCone5**

## Effect of Pileup on jet PT

**Mean y ~ 0.858**

**Mean y ~ 0.524**

**MidPoint5**

**SISCone5**

- SISCone algorithm is more sensitive to pileup !!!
- Effect of pileup is as much as 35 GeV in some of the events.

## Conclusions & Next Steps

- Good agreement between jets from midpoint and SISCone algorithm for high PT jets > 35 GeV.
- SISCone algorithm produces more number of jets in low P**T** region with respect to midpoint algorithm.
- Some of the energetic towers are being left by midpoint algorithm.
- No problem of dark energy towers in case of SISCone algorithm.
- Midpoint algorithm is around ~28% energy (ET) deficient in comparison to SISCone algorithm.
- SISCone algorithm is more sensitive to pileup.
- Effect of pileup is as much as 35 GeV in some of the events.
- Need of pileup subtraction .
- Next Steps:
- Pileup subtraction using jet area
- Study hadronic top with and without pileup