1. Measurement of jet properties with the ATLAS detector
Hugo Beauchemin
Tufts University
Put photo of team.
DIS2016, DESY, Germany, April 13th 2016

2. Introduction

3. The strong interaction intervenes in various ways and at various scales in every single event at the LHC
Matrix element of
interest
Gluon emission (ISR/FSR)
Proton structure
Fragmentation and hadronization
Underlying event
Long distance
physics
Short distance
physics
Long distance
physics
We can make
robust predictions!
Not so sure…

4. Factorization theorem:
The probabilities for short-distance and long-distance processes factorize
The long-distance factors are universal and can be empirically obtained from ancillary measurements.
Ok, at first order we are fine: even if there is a regime where QCD becomes non-perturbative, and even if every single events will involve such long distance physics effect, factorization theorem allows us to make robust predictions at the LHC for any processes of interest. There are nevertheless what I would call second order effects that could be quite annoying, and which requires a sophisticated measurement program at the LHC.
Evolution equations (e.g. DGLAP), analogous to b-functions for aS, account for transition from one scale to the other

5. These QCD predictions involve assumptions, approximations, and phenomenological modeling impacting final state selections and differential cross section predictions
Parton shower accounting for the effect of evolution on final states:
Soft and collinear approximations (where QCD radiation is enhanced)
Leading order kernel functions
Choice of ordering parameter
Parton distribution function (PDF):
Uncertainties on measurements used to extract structure functions
Modeling of structure functions at Q0
Fragmentation function:
Gaussian modeling of D(x,s) at small x
Supplemented by hadronization model

6. Predict different number of charged particle within jets and different distribution of the charged-particle energies in jets.
These effects implies possibly large uncertainties affecting the modeling or measurement of:
Observables likes Mjj, ETmiss-related, jet gap-related
Other infrared sensitive observables
Precision beyond 1/Qn for infrared safe observables

7. Measuring observables sensitive to these effects allows to:
Determine best model/calculations for predictions
Tune fragmentation and FSR parameters or PDF fit.
Assuming PDF, measuring observable sensitive to the transition from parton to jet can be used to:
Tag jet flavor
Distinguish between q-jet and g-jet
Distinguish between qq-resonance and QCD background

8. ATLAS measurements: Jet-charge and charged-particle multiplicity of jets
Phys. Rev. D93 (2016)
arXiv: [hep-ex]

9. Jet charged-particle multiplicity
An observable that is sensitive to longer-distance QCD effects
To be measured for various PT track threshold, thus regulating the sensitivity to soft particles
This observable depends on the parton that initiated the jet
Gluon jets contain more particles than quark jets because of larger color charge

10. Jet-charge Definition:
The momentum-weighted charge sum of particles in jets is sensitive to the electric charge of quarks and gluons from jets
Definition:
Measure mean mQJ and standard deviation sQJ vs PT jets
regulates the sensitivity of the jet charge to FSR and fragmentation
Low k enhances contribution from low-PT particles
k = 0.5 is the most sensitive value to parton charge initiating the jet

11. The average jet charge mQJ increases with jet-PT due to an increase in up-flavor from PDF at the origin of hard jets
Also more visible when the soft radiation is not suppressed
Distribution varies with rapidity due to higher up/down than gluon contributions in more forward regions
Discrepancies with predictions, meaning that large-distance QCD effects need to be better understood.

12. Similar observations about data to predictions agreement from measurements of the charged-particle multiplicity <ncgh> in jets

13. Sensitivity to PDF
Each PDF set has its own underlying event tune, factorizing out these effects from interpretation
Better agreement Pythia (LO generator) with LO PDF (CTEQ6L1) than with any NLO PDF, but still has wrong shape
Standard Deviation of jet charge sQJ well described by all.
Decoupled effects from FSR and fragmentation to a large extent

14. Sensitivity to parton emission (FSR), fragmentation and hadronization
Discrepancy between data and MC is not fully explained by PDF
Sensitivity to parton emission (FSR), fragmentation and hadronization
Compare data to different hadronization and FSR predictions :
Compare pythia 6 default tune with RadHi and RadLo tunes to quantify how the observable is sensitive to QCD FSR
The scale in as regulating FSR is changed by ½ and 2 for these two tunes
Realistic tune with some of the effects being absorbed in tuning
Compare Pythia 8 with Herwig++ to assess the sensitivity to fragmentation parameters and hadronization model

15. Significant sensitivity to fragmentation and hadronization
Average jet-charge mQJ distribution
The low k mQJ observable is insensitive to FSR
FSR effects are drown in the sensitivity of the observable to soft particle from infrared processes
Significant sensitivity to fragmentation and hadronization

16. Average jet-charge mQJ distribution
Sensitivity to FSR increases with k
Suppression of soft radiation makes the jet charge distribution more sensitive to the modeling of the energy fraction of leading emission

17. Jet charge standard deviation sQJ
Fluctuation decreases as soft radiation is suppressed
Because we see less charge
Significant sensitivity to FSR at large k, but not to fragmentation or hadronization.
Good for FSR tuning
Hard to disentangle FSR from fragmentation at low k

18. Multiplicity of charged particles <nchg> in jets
FSR: too much radiation in the main pythia P2012 tune
Insensitive to underlying event pythia tune (AU2 vs Monash), but not to Herwig corresponding tunes (EE3 vs EE5)
Best description at low jet-PT by Pytha 8 A14 tune, but at high jet-PT by Herwig EE5 tune
ATLAS charged-particle multiplicity in jets of PT<50 GeV included in A14 tune, and lower as FSR

19. mQJ differences between up and down
Using flavor-fraction information in PDF and matrix element calculation, we can extract mQJ and <nchg> for various parton flavor initiating the hard jets
mQJ differences between up and down
<ncharged> differences between quark and gluon

20. Conclusion

21. ATLAS performed a comprehensive study of jet-charge and jet charged-particle multiplicity sensitive to the transition between parton and hadrons
Sensitive to PDF, FSR, fragmentation and hadronization
Jet charge mean mQJ , and standard deviation sQJ, and jet charged-particle multiplicity <nchg> are not well described
LO PDFs provide a better description for LO generators, but cannot fully explain data to prediction discrepancies
Low k mQJ is good to test/tune fragmentation functions
High k and are good to test/tune QCD radiation
<nchg> infrared sensitive to underlying event modeling
These observables can be used to separate between jets of different parton origins

22. Back-up slides

23. Pythia (8.175) vs Herwig++ (2.63)
Hadronization:
Lund string (linear confinement)
Hadronization
Cluster
(preconfinement)
-Shower ordering: determine evolution of resolved QCD emission
-Linear confinement: color field stretching between quark-anti-quark pair like a string breaking spontaneously in segment of q-qbar pairs.
-Preconfinement: pair of color connected neighbor partons with a universal and Q2-independent asymptotic mass distribution suggesting particles.
Check ref 47 and 48 of jet-charge for the discussion on use of NLO PDF on LO ME.
pT-ordered showers
(natural for a shower partly based on dipole approach)
Showers ordering:
(nLL resummation)
angular-ordered showers
(deal with quantum interference)

24. Pythia (8.175) vs Herwig++ (2.63)
PDF:
Different can be used in generators, but as default:
CT10
(NLO)
-Shower ordering: determine evolution of resolved QCD emission
-Linear confinement: color field stretching between quark-anti-quark pair like a string breaking spontaneously in segment of q-qbar pairs.
-Preconfinement: pair of color connected neighbor partons with a universal and Q2-independent asymptotic mass distribution suggesting particles.
Check ref 47 and 48 of jet-charge for the discussion on use of NLO PDF on LO ME.
CTEQ6L1
(LO)
AU2 & A14 tunes
(Many parameters)
Multiple interactions:
(Each generator is tuned for different PDF on Run-1 ATLAS data)
EE3-EE5 tunes
(Fewer parameters)

25. Event topology I
Measure <ncgh>, mQJ and sQJ in dijet events with balanced PT
More sensitive to hard scatter jets and suppresses MPI
Measure impact of large-distance effects on short-distance processes
Less experimental and theoretical confusion with close-by jets
Hard-scattered quarks and gluons more cleanly matched to jet

26. Event topology II
Separate central from more forward jets because jet flavor depends on rapidity
Forward jets are less likely to come from gluon, than central jets
Forward jets are limited to |h| < 2.1 to detect all tracks
Define jet flavor from the highest energy parton within DR < 0.4 of particle-level jet

27. Unfolding Detector effects distort the underlying physics distribution
The average effect per jet-PT bin is about 10-20%
Reconstructed observable distributions are unfolded to produce results directly comparable to theoretical calculations
Unfolding accounts for fake, efficiency, and resolution effects
Unfold for <ncgh>, mQJ and sQJ and PT
Iterative Bayesian technique
Dominant source of uncertainty in low jet-PT bins

28. Variation of jet flavor origin with jet-PT and for different rapidity regions
Gluon and sea-quark fractions decrease with jet-PT
Up and down contributions increase with jet-PT
Up increases faster than down with Jet-PT, but this effect get attenuated in forward regions

29. Sensitivity to PDF does not depend on the amount of soft particles accounted for in the jet-charge definition:
PDFs impact leading jet charge mostly through flavor fraction
Decoupled effects from FSR and fragmentation to a large extent
Some systematic trend might be observed with Herwig, but not significant