A charge asymmetry measurement based on a boosted top quark selection?PCI, Valencia, January 2012

1

AC with boosted top quarks

A charge asymmetry measurement based on a boosted top quark selection?

PCI, Valencia, January 2012

Marcel Vos, IFIC Valencia

Based on work by Victoria, Amine, Miguel, Elena

and Eduardo Ros

Top production I

Theory x-sec @ 7 TeV



(MCFM) = 158

pb



= 163

(scale)

(PDF)

N. Kidonakis, Phys.Rev. D82 (2010) 114030,

arXiv:1009.4935. doi:10.1103/PhysRevD.82.114030.

Measurement Cross section [pb]

CMS l+jets+tag* 150  9 (stat)  17 (syst)  6(lumi)

CMS dilepton* 168  18 (stat)  14 (syst)  7(lumi)

CMS Combined* 158  10(unc.)  15(cor.)  6(lumi)

ATLAS Combined** 180  9 (stat.)  15 (syst.)  6 (lumi.) Full 2010 data set, 36 pb

, CMS PAS TOP-11-001

**Full 2010 data set, 5 channels, 35 pb

,ATLAS-CONF-2011-040

There is no super abundant exotic source of ttbar events

3

Top production II

Direct search for resonances

CMS 36 pb-1, combined e+jets and m+jets channels (TOP-10-007-PAS) ATLAS result approved

Extend in several directions:

- more reconstruction algorithms - more channels

- more models

- effective operators

(Zhang & Willenbrock '10, Aguilar-Saavedra '10, Degrande et al. '10)

ATLAS-CONF-2011-070

ATLAS-CONF-2011-070

Charge asymmetry

Charge asymmetry

CMS measurement:

AC = 0.06  0.134 (stat)  0.026 (syst)

complemented by same­sign top search

5

Single top

✔ The single top production mechanisms might display 

different/increased sensitivity to new physics. They moreover offer an  interesting field for studies of polarization.

✔ CMS: Selection optimized for the t­channel mode on 35.9 pb

 (TOP­

10­008­PAS)

Measured cross­section:  83.6   29.8(stat. + syst.)  3.3(lumi.) pb

✔ ATLAS (ATLAS­CONF­2011­027):

➔ T­channel 53 +46­36 pb (theory 66 pb)

➔ Wt channel < 158 pb (theory 15 pb) 

Top decay

W­boson polarization in top  decays 

ATLAS­CONF­2011­037

✔ Extract helicitiy fractions  (combining lepton and  muon channels):

➔ F

 = 0.59  0.12

➔ F

 = 0.42  0.12

✔ Constrain anomalous Wtb  couplings 

 +jets

 +jets

 +jets

7

ttbar+X

✔ Associated production ttbar+X, where X can be anything, from gluons to  bbar to ttbar to Higgs to SUSY sparticles. 

✔ Example: ATLAS ttbar + E

 study (ATLAS­PHYS­CONF­2011­036)

No excess found: limits on top partners T->tX, where X is

a neutral particle

Data!

Boosted objects form “fat” jets

At the LHC “heavy” particles like W, Z and top are produced

abundantly with p

>> m

at 7 TeV 17 % of tt pairs has p

> m

Rule of thumb for maximum jet size to resolve both partons in a two-body decay:

R < 2m X /p T X

R = 0.4 corresponds to W with 400 GeV

Reconstruct “boosted object”

as a single jet. Figure out it was indeed a “fat jet” by analyzing substructure

M. Seymour. Searches for new particles using cone and cluster jet algorithms:

A Comparative study. Z. Phys., C62:127–138, 1994.

9

Theory progress

More sophisticated taggers:

✔ Plehn, Spannowsky, Takeuchi, arXiv:1111.5034 [hep­ph] 

✔ Soper, Spannowsky, arXiv:1102.3480 [hep­ph]

✔ Barger, Huang, arXiv:1110.2214 [hep­ph] 

✔ Thaler, van Tilburg, arXiv:1102.3480 [hep­ph] 

   arXiv:1011.2268 [hep­ph]

✔ Reherman, Tweedie, arXiv:1007.2221 [hep­ph]

New sources of boosted tops:

✔ Plehn, Spannowsky, Takeuchi, arXiv:1102.0557 [hep­ph] 

✔ Perelstein, Spray, arXiv:1106.2171 [hep­ph]

✔ Berger et al., arXiv:1111.6594 [hep­ph]

Use top­tagging to do other things (like charge asymmetries):

✔ Hewett et al., Phys.Rev.D84:054005,2011, arXiv:1103.4618 [hep­ph]

With apologies to all people whose work I ommited. 

]

Substructure: experimental response

Limitations for jet substructure measurements:

→ limited energy resolution & granularity tracker >> calorimeter

→ systematic uncertainties in response hadronic shower model

detector description (dead material)

→ pile-up/underlying event

→ clustering

11

Measurements

Measurement of Jet Mass and Substructure for Inclusive Jets in √s = 7 TeV pp Collisions with the  ATLAS Experiment, ATLAS­CONF­2011­073, to be submitted to JHEP

A SM measurement, fully corrected to particle level

paper result (same data set, smaller errors, finer binning, some additional observables) will be on HEPDATA

Parton shower model is sufficiently precise

Detector response under control (using calo/tracker ratio)

We can use substructure!! (paper result has smaller errors)

Measurements

Measurement of Jet Mass and Substructure for Inclusive Jets in √s = 7 TeV pp Collisions with the  ATLAS Experiment, ATLAS­CONF­2011­073, to be submitted to JHEP

Filtered jets are more forgiving!

13

Measurements

The impact of pile­up  Measurement of Jet Mass and Substructure for  Inclusive Jets in √s = 7 TeV pp Collisions with the ATLAS Experiment ATLAS­CONF­2011­073, to be submitted to JHEP

Confirm the impact of pile-up

And come up with a solution Groomed jets are intrinsically pile-up resilient

ATLAS will support a “fat” groomed jet collection.

CMS is already using them

Boosted tops

Measurement of Jet Mass and Substructure for Inclusive Jets in √s = 7 TeV  pp Collisions with the ATLAS Experiment, ATLAS­CONF­2011­073

The world's first “boosted object”   BOOST2011. →

ATLAS certified

tt events with

m

> 700 GeV

15

Boosted tops

2010 data: the first five events of an excellent  control sample    → Use top and W mass peak to calibrate  jet mass response

ATLAS certified tt events with m

> 700 GeV

Recluster @ R=1 First observation of a

“boosted object”

When reclustered with R = 1.0 the three jets merge into a single jet with:

m

= 197 GeV (expected: m

) sqrt(d

) = 110 GeV (expected ~m

) sqrt(d

) = 40 GeV (expected …)

Matching small and large jets we can transfer the

small uncertainties of the well-known collection to

the less-well known one

Data!

ATLAS – boosted objects

Important information about the final state that's not  available for top quarks produced at rest

✔ Collimated top decay 

✔ Top and anti­top are approximately back­to­back

➔ Vicinity is a good criterion to pair objects

➔ No ambituities in jet assignment  

“At rest”

M tt < 500 GeV

“Transition region”

500 GeV < M “Transition region” tt < 700 GeV

500 GeV < M tt < 700 GeV “mono-jet”

M tt > 700 GeV

Top – three regimes

First boosted top quark

ATLAS-CONF-2011-073 m

> 1 TeV

ATLAS-CONF-2011-083 Early “l+jets” candidate

ATLAS-CONF-2010-063

Top – three regimes

Early “l+jets” candidate, ATLAS-CONF-2010-063

Top – three regimes

First boosted top quark, ATLAS-CONF-2011-073

Top – three regimes

m

> 1 TeV ATLAS-CONF-2011-083

2 TeV ttbar event!

ATLAS ttbar resonance searches

Better limits on benchmarks → proof of principle!!!

Note that “resolved” analysis accepts 3-jet events, while the “boosted” result is

obtained requiring isolated leptons

di-lepton: sensitivity is limited by primarily by small signal acceptance and poorer mass resolution

- does it add much when combined?

- does it compare better on even broader resonances?

lepton+jets:

- resolved approach has much better acceptance and

sensitivity at low mass (i.e. narrow resonance with small x-sec) - boosted approach clearly more promising in terms of mass reach,

Finally, a boosted selection has been implemented

(ATL-COM-PHYS-2011-1692)

23

TeV scale tops

Boosted “lepton + jets” → a pretty pure sample, up to 2 TeV

Excellent starting point for many

measurements!

Asymmetry measurement

High mass → better qqbar purity!

25

Asymmetry measurement

High mass → better sensitivity to BSM physics

Inclusive sample dominated

by tops at rest High mass selection

(m > 700 GeV) Ambiguities in assigning jets to top and anti-top quark candidate drive dominant systematic errors

Much reduced at high mass !!!

27

Summary

✔ The LHC needs to deal with boosted objects now  

➔ And more so at 8, 14 TeV...

✔ R&D on novel jet definitions and substructure observables help  improve our physics return

➔ Jet mass used already in several analyses

➔ Some techniques turn out to be useful in other ways than envisaged

✔ Proof­of­principle analysis about to be published. Time to think of 

what's next now (charge asymmetry, x­sec)

More...

Have a look at the Agenda of the workshop series: 

      SLAC 2009       Oxford 2010       Princeton 2011

+ events at Oregon, Washington, Manchester, SLAC, CERN LHCC Come to Valencia, July 23

 – 27

  2012

      Towards jetography, G. Salam

      Boosted objects ..., Eur.Phys.J.C71:1661,2011

      Boost 2011 report...