potential for searches & measurements: an overview for discussion

Jon Butterworth, UCL 1

Potential for searches & measurements:an overview for discussion

Jonathan ButterworthUCL

Boosted Boson Workshop,

CERN 25/3/2014

25/3/2014


Potential for searches & measurements:“I apologise in advance” Jonathan

ButterworthUCL

Boosted Boson

Workshop,CERN

25/3/2014

The core issue…• Final stage of jet structure is “soft” non-perturbative

QCD. • Formation of hadrons from gluons, 100 MeV energy scales (LQCD)

• Vast phase space between quark-gluon scatter (100’s GeV, few TeV) and LQCD

• Most of jet substructure can be analysed perturbatively• EWSB scale (~100 GeV) lies in this region

• Jets may contain objects with EW-scale mass (W,Z,H,t,?)

25/3/2014 Jon Butterworth, UCL 6

Standard Model Topologies

• Dibosons; especially WW/WZ/ZZ - inclusive and vector boson fusion/scattering.

• Inclusive: high pT tail is where anomalous couplings might show up

• VBS: Unitarity test of BEH mechanism at high diboson mass. High diboson masses high boosts (2002, JMB, Cox, Forshaw and others; recently Cui & Han 2014 (Schwartz 2011)).

• In both case, low statistics at most interesting area – recover hadronic decays of the bosons


• Top/tt• High pT tail… understand efficiencies for differential

cross section measurement.• Main power is in Higgs & BSM searches (?)


• Higgs … • H WW or ZZ decays (the “original” channel, Seymour

1994) . Boost will not come from Higgs mass. However there will be phase space where there are boosted (off-shell) hadronic decays of Ws and Zs. Any chance?

• VH(H bb) (the “classic” channel? BDRS 2008) High pT is the most sensitive region in Run I. Higher boosts in Run II likely to require jet substructure.

High pT Higgs and Vector Boson (reprise)• By requiring that the Higgs and Vector Boson have a high transverse

momentum, we lose a factor of ~20 in cross section • However, much of this would have failed other analysis cuts anyway• Background cross sections fall by a bigger factor (typically t-channel not s-channel)

• W/Z and H are all central• Better b-tagging, better jet resolution

• W/Z and H decay products collimated• Simpler topology, fewer combinatorials• Difficult for tops to fake this

• Z neutrinos becomes visible • High missing ET

25/3/2014 10Jon Butterworth, UCL

Hb

W

l

u

mH

“mono”-Jet

High pT Higgs and Vector Boson (reprise)


• Note excellent Z peak for calibration

• 5.9 s; potentially very competitive

• bb branching information critical for extracting Higgs properties

• “Measuring the Higgs sector” Lafaye, Plehn, Rauch, Zerwas, Duhrssen, arXiv:0904.3866 [hep-ph]

High pT Higgs and Vector Boson (reprise)


• Pre-data! • Included trigger, ATLAS b-tagging algorithm, detailed tracking &

calorimeter• Also include Wt background omitted from initial study.• Slight degradation w.r.t particle level, but still very promising


• Higgs … • gg H (Hbb) and others.

• Can we recover Hbb without the V tag? • Can we tag the hadronic V decays too?


• Higgs … • gg H (Hbb) and others.

• Can we recover Hbb without the V tag? • Can we tag the hadronic V decays too?

• ttH (Plehn, Salam, Spannowsky 2010) boost of top of Higgs can be useful and needs to be dealt with.

Standard Model Topologies• Combine techniques

for top and Higgs tagging to improve sensitivity to Higgs in ttH channel

25/3/2014 Jon Butterworth, UCL 15


• Higgs … • Hgg … I know we’re concentrating on hadrons, but…

isolation cone merging… similar issues and technologies?

BSM Topologies (all of the above?)

• Especially top: Enhanced BSM production at high pT in many models

• Kaluza-Klein graviton (Lillie, Randall, Wang, 2007)• t’ (Holdom, 2007)• + SUSY stuff…


Substructure in searches (boosted top, boosted W)

25/3/2014


• We have new boson to boost… HH (Kribs, Martin, Roy, Spannowsky, 2009; Cooper, Konstantinidis, Lambourne, Wardrope, 2013; ATLAS-CONF-2014-005)

BSM Topologies (all of the above?) • SUSY cascades to W,Z,H (JMB, Ellis, Raklev 2007)

top (Han, Katz, Son, Tweedie, 2012)• Scuffles over mass hierarchies/naturalness &c…• Models (e.g. CP violating) with low limits on hadronically

decaying Higgs partners (Bhattacherjee, Chakraborty, Ghosh, Raychaudhuri, 2012)


• R-Parity Violating SUSY (As above + Salam 2009, and more)

from talk by Minho Son


• …? • … Integrated into dijet/multijet searches?

Techniques

• Improve mass resolution• Dealing with pile-up, UE and soft radiation

• Identifying decays• Suppressing (hard) QCD background

Jon Butterworth, UCL 2625/3/2014


MANY MORE SINCE

MANY MORE SINCE

• Improve mass resolution• Dealing with pile-up, UE and soft radiation


Splitting and filtering (BDRS) and ATLAS


Filtering, Trimming and PruningCMS


Shower deconstruction (Soper & Spannowsky) and ATLAS-CONF-2014-003

Split/filter is an example of doing both1. Start with Higgs candidate jet (highest pT jet in acceptance) with mass m)

2. Undo last stage of clustering (reduce radius to R12)

J J1, J2

3. If max(m1,m2) < 2m/3Call this a “mass drop”. This fixes the optimal radius for reconstructing the Higgs decay. Keep the jet J and call it the

Higgs candidate.

Else, go back to 2

4. Require Y12 > 0.09Dimensionless rejection of asymmetric QCD splitting

Else reject the event

5. Require J1, J2 to each contain a b-tagElse reject the event


Split/filter is an example of doing both6. Define Rfilt = min(0.3, Rbb/2)

Make use event-by-event of the known Higgs decay radius

Angular ordering means this is the characteristic radius of QCD radiation from Higgs products

Stuff outside of this is likely to be underlying event and pileup.

7. Recluster, with Cambridge/Aachen, R = Rfilt

8. Take the 3 hardest subjets and combine to be the Higgsb, anti-b and leading order final state gluon radiation

9. Plot the mass


Improved subjet analysis


Learned since…

• y cut more powerful than mass drop (identifying decay. QCD suppression rather than UE/pile up.)

• Filtering, uses Higgs decay angle to optimise the zoom on “hard physics”/suppression of soft stuff. Other techniques also available, some more generic (n-subjettiness, shower deconstruction). Competition in this channel?

• Dijet R = 0.4 anti kT does a pretty good job (at least at 8 TeV).

Precision jets at the highest scales

• Highest transverse momentum jets; at the TeV scale

Jets at the highest scales• General agreement

with NLO QCD calculations (after soft corrections)

• Can comparisons be made more (or less?) precise if groomed jets are used?

42

Significant spread in “NLO” predictions. ME/PS matching? MC tune (UE)? PDFs?

Vector bosons and (b) jets

arXiv:1304.7098arxiv:1302:2929

• CMS b/anti-b correlations

• arXiv:1102.3194, JHEP 1103 (2011) 136

26 May 2011 Princeton, J.Butterworth

Limiting factors – b-tagging & gbb

• Bottom &/or charm production introduces another hard (or “semi-hard”) scale. How well is this understood?

14/11/2012 Jon Butterworth, HCP 45

arXiv:1210.0441


• B jet (and dijet) cross sections compared to NLO QCD

14/11/2012 Jon Butterworth, HCP 46 arXiv:1109.6833 arXiv:1202.4617


Good agreement with POWHEG, poorer with MC@NLO (seen by both ATLAS & CMS)



• Use properties of displaced decay vertices to distinguish light, c and b jets in dijet events

14/11/2012 Jon Butterworth, HCP 50

arXiv:1210.0441


Jet grooming and subjets

arXiv:1203.4606

Jet grooming and subjets• kT scale, N-subjettiness

arXiv:1203.4606


Shower deconstruction

25/3/2014



25/3/2014


Substructure in searches

25/3/2014


• Jet shapes, Q-jets, jet charge, jet pull, jet superstructure…

• Moving on from the “two goals” (tagging & grooming) to finding still more information; identifying jet properties in context, learning more about the short-distance physics.

… also

25/3/2014


• Gone from wacky new idea to obviously essential item in the toolkit (~6 years)

• Studies with data show: • modelling is adequate but can be improved in some cases• “Grooming” is generally robust against pile up and

important for controlling it in all jet measurements

• Not seen full potential yet in Higgs searches … will become essential in more places at 14 TeV

Subjets and measuring boosted objects

25/3/2014


• Already widely used in searches (mainly boosted tops, but more results on HH, W/Z coming…)

• Calibration work on of novel observables is intense & interesting

• Theoretical work on developing (analytic?) understanding

• Boost meetings 2009 Stanford 2010 Oxford 2011 Princeton 2012 Valencia 2013 Arizona 2014 UCL (18-22 August, www.hep.ucl.ac.uk/boost2014)

Subjets and measuring boosted objects

25/3/2014


Backup

25/3/2014

potential for searches & measurements: an overview for discussion

Documents

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vector boson repriseby

higgs bsm searches