Results from the ATLAS experiment

Summer Institute SI2010 at Fuji Calm, Japan17/08/2010 Yuji Yamazaki (Kobe Univ.)

LHC (Large Hadron Collider) at CERN

Objective: origin of mass (Higgs in SM), physics beyond standard model The largest accelerator

with the highest energy

27km circumference (reusing the LEP tunnel)

7TeV proton-proton collisionsNominal CMS energy: 14 TeV Tevatron 7 2010-2011 run: collisions at 7 TeV

Construction started in 1996First physics run in March 2010

2

Mont Blanc

City of Geneva

CERN

Lac Leman

France

Switzerland

17/08/2010

ATLAS

Results from the ATLAS experiment

3

2010/3/30 First 7TeV collision ALSO in the ATLAS detector!

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and people are finally happy …

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… and busy days afterward:Luminosity vs time in ATLAS

Current lumi: 1.x pb–1 Gradual increase in intensity

for safty of the accelerator Linst ~ 4 1030 s–1 cm–2

2010/11 : 7 TeV 2010 ： aiming for

Linst ~ 4 1032 s–1 cm–2

2011 ： 1 fb–1 (Linst > 1032)

> 2013 : 14 TeV operation After repairing joints in SC

magnet17/08/2010 5Results from the ATLAS experiment

Where we are

Reviewed today:presented at ICHEP10-280 nb−1

We see: Collision/jets/b-quark W and Z some top quarks

Higgs is not even produced …

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7TeV

We are here

Today’s contents

Introduction Re-discovery of our old friends

W and Z top quarks (high-pT) jets

Understanding basics in hadron-hadron collisions Soft QCD

Status of searches and outlook

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* all results from ICHEP2010 or earlier release, see also: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/RESULTS/summer2010.html

ATLAS detector status

Active channel: 97-100% Central tracker (semiconductor) Calorimetry Muon system Trigger Understanding inactive material

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All working fineWorking for understanding in detail

Hard collisionsStandard-model processes: EW boson production, top, high-ET jets

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Hadron-hadron collisions

Hard collision: viewed as a parton-parton collision

More theoretically(QCD factorisation theorem)

Proton remnant may collide each other

a proton proton remnant

high-pT particles fromhard scattering

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gq

TTT FSpppxfpxf,

34122211

21

)34()(),(),(

)states final specific todecaying 4 ,3 ofy probabilit(

)3412 partons ofsection cross(

)densityparton ()density(parton

W/Z production: Drell-Yan process

s-channel production of W/Z à la e+e (best process for detector study) Sensitive to quark part of PDFs

(parton density functions) Higher order QCD process:

background for searches Possibly new resonance

decaying into dilepton

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Drell-YanW prod. O(100nb)

Z prod. O(10nb)

W+jets

W production: signal extraction

Selection criteria: Large missing from neutrino Transverse mass close to MW

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Example from electron channel – almost background free

W production cross section

consistent with NNLO Theoretical

uncertainty ~ 4%

W less than W+

as expected(more ud than du)

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using 17nb −1

e/μ, charge asymmetry

All consistent each othermore statistics → systematic error dominant (challenges!)

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.Asym

Z0 → ee, μμ observation

consistent with NLO

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using 225nb −1

Top quark at the LHC

LHC produces a lot of top quarks High energy collision

→ large effective gluon luminosity Much more events than Tevatron,

even at 7 TeV

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topthreshold

etc.

Top decays and physics

t → Wb (~ 100%) W → lν [1 b-jet, 1 charged lepton]

semileptonic decay W → cs, ud etc. [1 b-jet, 2 jets]

hadronic decay tt tagged by b-jets and

2 semileptonic or 1 semileptonic + 1 hadronic

Top: strong coupling to HiggsNew physics may emergee.g. t → H+b decay

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1-lepton sample

lepton + Emiss+ ≥ 3 jets (≥ b-tagged)

mjjj: clear peakfewer backgroundthan at Tevatron

Events are wherethey should be9 events observed

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electron channel muon channel

using 280nb −1

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2-lepton sample

One candidate forboth ee and eμ

No candidate for μμ

b-tagged jet notrequired, but foundin both events

Again events are foundwhere they should be2 events observed

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Statistically not conclusive, but ATLAS seems to find top quarks properly

QCD hard scattering – jet production

Cross section = (PDF1)(PDF2)(ME) Matrix element: Multijet event

Background for searches Effect on uncertainty in parton-density functions

Gluons in particularDGLAP evolution valid?

Other issues Heavy flavour production Effect from soft QCD

… will be covered later

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Inclusive jetcross sections

Algorithm: anti-kT

jet area: cone-like R = 0.4, 0.6

Comparison w/ MCgood agreement

Systematic error(purple band)mainly fromJet Energy Scaleuncertainty

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Comparison with NLO (R = 0.4)

Very good agreementalso on rapidity distribution

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Data / Theory

Main uncertainty: jet energy scale (JES)

large uncertainty in hadronic shower response simulation

Checked by track momentum vs

calorimeter cluster energy dijet balance

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JES uncertainty summary

7% for ET > 100 GeV ~ 10% at 20 GeV

Rome was not build in a day need more study/data to narrow down the uncertainty

Tentative goal : a few %

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How high in pT or Mjj are we?– up to 600GeV / ~ 2 TeV

Good agreement with NLO (no surprise yet …)

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Tentative conclusion on SM processes

Basic 2 → 2 processes investigated Drell-Yan W/Z High-ET jets

Successful re-discovery and quantitative confirmationon the standard model Statistically limited

ATLAS seems to be able to measure top quarks properly

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No apparent trouble in understanding SM eventsTheory (pQCD, in particular) is healthy also at 7 TeV

Softer QCD resultsb-quark, J/ψ, inclusive particle spectrum, and low-ET jets

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Heavy-flavour production

Not easy to calculate Important background for EM/searches QCD MC used in ATLAS seems

overestimating background in W study

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LO NLO

charm excitation

gluon splitting

bb pair not necessarilyback-to-back

Tevatron “excess” on B meson productionover NLO QCD, need resummation

First result through J/ψ production

Two production mechanism Direct J/ψ production through B-meson decays

Two component separatableby tagging b-meson decay vertex

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direct(prompt)

“breaching”colourby a gluon

+softemission

singlet

Octet

J/ψ yield

Shape well described, but PYTHIA with colour-octet diagram gives factor-10 too high

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B-decay/prompt ratio described by the model→ both may have less octet state?

Soft QCD study

Most of the hadron-hadron collisions are with low pT

αS very largeperturbative calculation of quark-gluon interaction not possible

hadronic object behaves like “quark-gluon matter”collision occurs when two objects overlap

partons not visible – no clear jetparticles emitted from “colour string”

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forward-goingproton remnant

proton remnant

central production

Soft QCD is still important at high energies

Proton remnant may collideeach other, or

more than one partons maycollide at a proton crossing because of high density of

partons in proton at high energies

changing particle flow between high-pT objects affecting measurements

Pile-up (more than one collision in a bunch crossing) One is high-pT (triggered)

Others would be low-pT (large cross section ~ 100mb)Need to know the nature of it

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Charged particle spectrum in “minimum bias” events

Minimum bias Requiring ≥ 1 charged track with ≥ 500 MeV close to inelastic collisions but not exactly the same

(marginally) more tracks at 7 TeV than 900 GeV collisions17/08/2010 Results from the ATLAS experiment 36

inelastic

single diffraction

Charged particle multiplicity in η:comparison with models (7 TeV)

Does not reproduce data some are quite close, though

Didn’t these models tuned to SpS/Tevatron minbias data?Answer: not only. also to particle flow between

hard jets in hard events showing deviation also

from 0.9 GeV and 2.36 GeV data models do not have infinite

number of free parameters!

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PT distribution,‹PT› vs. nch

Many models predict harder spectrum need to retune the models …

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After retuning (ATLAS MBT1)

Tuning mainly the parameters of multiple interactione.g. transverse distribution of partons in the proton

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Softer side of hard jets – fragmentation

Investigation on lowest possible ET jets ET > 4 GeV, using only charged particles

Should be described by pQCD (hard scattering)but how low?

Jet properties investigated Charged multiplicity nch

Fragmentation function f(z)

pTjet, track spectrum

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jetT

T

p

pz

particle) charged(

Charged multiplicity6 < pT

jet, track < 10 GeV

Less tracks in the model

pT spectrum, fragmentation function f(z)

pT spectrum softer thanmost of models

Fragmentation function is also softer → particle spectrum is softer than expected at 7 TeV

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Searches at 7 TeV LHC 2010-2011

Aiming for accumulating 1fb−1 of data

Rather difficult: SM Higgs

Sensitivity even at 7 TeV:to light particles of SUSY Extra dimension Z’ etc. (dilepton)

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SearchesDijet resonance and some initial studies for other channels

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Today reviews …

Dijet resonance with full data available before ICHEP (new limit!)

SUSY background study

Outlook for 2010/2011 with 7 TeV SUSY prospect Dilepton resonance Higgs prospect

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Dijet resonance – excited quark search

Search focused to q* (excited quark)

qg → q* → qg dijet resonance also sensitive to Technicolor,

extra dimension, GUT bosons, etc. etc.

produced at central rapidityapplying |η1|, |η2| < 1.3

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Limit on q*

Excluding < 1.2 TeV

Tevatron limit< 870 GeV (w/ 1.13fb−1)

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First limit alreadybetter than Tevatron!

Searching for contact interactionthrough dijet angular distribution χ

Contact interaction limit :> 900 GeV Quantum black hole may

decay to dijet, spherically Already excluded:

for nextra = 6 and M < 800 GeV

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Searching SUSY at the LHC

SUSY produced by squarks/gluinos Normally the heaviest Cascade decay to LSP + standard model

particles→ Multi-jet events with large total mass

Leptons and/or heavy flavour quarks may be produced

Most of the SUSY study assumes R-parity conservation Weakly interacting neutral LSP (dark

matter candidate)escaping the detector → missing ET

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Multijet + Missing ET

option: lepton(s), …

Background study (1) 4-jet 0-lepton channel

Distribution reproduced by QCD MC

High-mass event observed at 1.6 TeV, Emiss ~ 100 GeV

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Missing ET

Meff (scalar sum of pT’s, ETmiss)

ETmiss > 40 GeV, ET

miss/Meff > 0.2ET

miss vector not aligned to jets

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Background study (2)3-jets 1-lepton

Lepton + multijet (≥ 4) Background

W + jets, Z + jets, tt Same sign dilepton

small background

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Electron

Muon

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SUSY study prospect for 7 TeV, 1fb−1

If we are lucky, it would look like …

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Expected limit on SUSY: example

For mSUGRA, tanβ = 10, A0 = 0, μ > 0 mass range of 500 GeV – 1 TeV can be explored

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Limit on dilepton resonance Z’, W’

W’ exclusion up to 1.5 TeV with 50 pb−1

Z’ exclusion up to 1.3 TeV with 100 pb−1

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SM Higgs boson production @ LHC

four production processes gluon fusion and VBF give large σ

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gluon fusion

Vector-boson fusion (VBF)

Associated WH, ZH

Associated ttH, bbH

Higgs decay

mH > MW: WW, ZZ (, tt)

mH MW: WW(*) ~ 100%

mH < MW

bb, WW* ττ (< 10%) (gg 2 10-3)

Golden channel: ZZ4l , gg direct mass reconstruction possible

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200 500

Higgs yield and background at 7 and 14 TeV

Higgs cross section ~ factor 4 lower than 14 TeV

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Background:top: even lowerW and WW: not as low

as Higgs

7 TeV

ATLAS sensitivity to Higgs at 14TeV

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ZZ4l

WW, gg tt

ZZ

ATLAS is good at WW decay ZZ → 4 leptons

ττ and γγ important at low mass

7 TeV study withWW, ZZ and γγ

WW → lνlν channelZ → 4 leptons

W limit alone better thancurrent Tevatron limit

Z : small Z → ll BR …

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Z → 4l limit / SM

WW crosssection limit

WW limit / SM

Overall sensitivity to SM Higgs by 2012

γγ channel: not much sensitivity Overall: we will give quite better limit than Tevatron would do

135-190 GeVwould be excluded

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Charged Higgs H+ (e.g. MSSM)

Production: t → H+ b Subsequent decays:

H+ → cs for small tanβclear peak on dijet mass

H+ → τν for large tanβtau produced backward

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Chargeed Higgs limit

Competitive to Tevatron thanks to large cross section of tt

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H+ → τν H+ → cs

Summary

ATLAS is working – actively analysing data EW bosons and top: “observed” as expected Hard QCD: quite healthy Soft QCD: not perfectly modelled

Quite usual situation for any new collider Searches

Some new limit on strongly interacting particles Studying background, being ready for SUSY Do our best for Higgs

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Hope to see something new very soon!

Backup

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検出器の動作状況

順調に動作 97-100% channels active

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ジェットトリガーとジェット

トリガーも予想通りの動作

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中央飛跡検出器

Reconstructed mass のずれから検出器内の物質量を検証 |η| < 1.5:

シミュレーションが数％のオーダーで正しい

正しくないところも見つかる（超前方）

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6917/08/2010 Results from the ATLAS experiment

B-tagging

B meson が長寿命(cτ ~ 500μm)

実験の初期から検出器の位置が理解できている

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ビーム軸との最近接距離が2 番目に長いもの

Results from the ATLAS experiment

ミューオン検出器，トリガー

ほぼ予定通り動作を確認

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Barrel RPC

Endcap TGC

Results from the ATLAS experiment

J/ψ → μμ

Peak: 3.095 ± 0.004 GeV PDG value: 3.0969 GeV シリコン検出器の位置が

よくわかっている

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• 横運動量などの分布も再現

Results from the ATLAS experiment

Extra dimension （余剰次元）など

2-3 TeV 領域の余剰次元が見つかる可能性あり

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gGggqGqggGqq , ,

dilepton resonance からRS 余剰次元， Z’ など

Results from the ATLAS experiment