discovery(?) of the higgs boson at the lhc - … 8, 2012 discovery of the higgs boson at the lhc s....

Discovery(?) of the Higgs boson at the LHC

OutlineWhat we do?How to take up a challenge?– A typical experiment

Have we found something new?Did we contribute from India?

August 8, 2012 Discovery of the Higgs Boson at the LHC S. Banerjee 2

High Energy Physics

Try to probe at short distances to study structure of matterBuild microscope with the highest resolving powerGo to the highest energy probeNeed particle accelerators


Finding so far

So far we have probed to a scale of 10-19 mBasic constituents of matters are all spin ½objects (Fermions)They interact through exchange of spin 1objects (Vector Bosons)The three types of interactions can be explained by gauge theories


Standard Model

Incorporates:– Electroweak theory by Glashow, Weinberg and Salam– Quantum Chromo Dynamics – theory of strong interactions.

Sheldon Glashow Steven Weinberg Abdus Salam

6


The Standard Model

Earlier experiments (particularly the experiments done at the LEP, CERN and Tevatron, Fermilab) have tested the predictions of the Standard Model to a high level of accuracy All measurements agree with the predictions which start with a few unknown parameters

The Standard Model is a beautiful theory and

arguably one that is most precisely tested

Measurement Fit |Omeas−Ofit|/σmeas

0 1 2 3

0 1 2 3

∆αhad(mZ)∆α(5) 0.02750 ± 0.00033 0.02759

mZ [GeV]mZ [GeV] 91.1875 ± 0.0021 91.1874

ΓZ [GeV]Γ Z [GeV] 2.4952 ± 0.0023 2.4959

σhad [nb]σ0 41.540 ± 0.037 41.478

RlRl 20.767 ± 0.025 20.742

AfbA0,l 0.01714 ± 0.00095 0.01646

Al(Pτ)Al(Pτ) 0.1465 ± 0.0032 0.1482

RbRb 0.21629 ± 0.00066 0.21579

RcRc 0.1721 ± 0.0030 0.1722

AfbA0,b 0.0992 ± 0.0016 0.1039

AfbA0,c 0.0707 ± 0.0035 0.0743

AbAb 0.923 ± 0.020 0.935

AcAc 0.670 ± 0.027 0.668

Al(SLD)Al(SLD) 0.1513 ± 0.0021 0.1482

sin2θeffsin2θlept(Qfb) 0.2324 ± 0.0012 0.2314

mW [GeV]mW [GeV] 80.399 ± 0.023 80.378

ΓW [GeV]Γ W [GeV] 2.085 ± 0.042 2.092

mt [GeV]mt [GeV] 173.20 ± 0.90 173.27

July 2011

But where is Higgs boson?


The Guralnik-Hagen-Kibble-Higgs-Englert-Broutfield…

The mass of particles is generated by interaction with a scalar field. This was predicted independently by three groups and published in 1964 in the same volume of Physical Review Letters.– All three papers were written from different perspectives, and each made

a distinct contributionPhysical Review Letters volume 13 (1964):– Guralnik, Hagen, Kibble, "Global Conservation Laws and Massless

Particles" – Higgs, "Broken Symmetries and the Masses of Gauge Bosons" – Englert, Brout, "Broken Symmetry and the Mass of Gauge Vector

Mesons" 15


Indirect hints

Possible due toprecision measurementsknown higher order electroweak corrections

Prediction for the Higgs mass

LEP: indirect determination of the top mass

t’ Hooft Veltman


Knowledge on HiggsCuurent knowledge on Higgs boson mass (if it exists) from precision measurements from earlier experiments within the framework of the Standard Model:

mH Range of mH 95% CL ExclusionStandard Fit 94.5 GeV [71,124] GeV 166.5 GeVComplete Fit 125.2 GeV [116,133] GeV 153.9 GeV(include direct limits from LEP & Tevatron)


How to produce Higgs boson

Higgs boson can be produced in pp collisions at sufficiently high energy (at √s>7 TeV, the rate of production could be reasonable)


100

ZZ-->2l2ν ZZ-->2l2q ZZ-->2l2τ

ZZ-->4l WW-->lνlν

γγ ττ bb

Higgs boson mass, GeV/ c2

How to look for Higgs

We have to look at the decay products and choose asuitable mode

High mass: WW, ZZLow mass: bb, ττ, WW(*), ZZ(*), γγ


CERN

CMS

CERN Site

ATLAS

GVA


Large Hadron Collider

Bunch Crossing 1.5×10 7 Hz

(3.5-4.0)×1012eV Beam Energy

6.6×1033 cm−2 s−1 Luminosity

1377 Bunches/Beam

1.4×1011 Protons/Bunch

(3.5-4.0) TeV Proton Protoncolliding beams

Proton Collisions 4.5×10 8 Hz

Parton Collisions

New Particle Production (Higgs, SUSY, ....)

p pH

µ +

µ -

µ +

µ -

Z

Z

p p

e-

νe

µ+

µ−

q

q

q

q

χ1

-

g~

~

χ2

0~

q~

χ1 0

~

5.5 m (50 ns)

Beam size ~ 5.5 cm ☓ 15 μm ☓ 15 μm

Hz0.1


Facts about LHC

Energy stored in magnets: 10 GJ = A380 at a cruise speed of 700 km/s. Can heat and melt 12 tons of copper!Energy stored in a single beam: 360MJ = 90 kg of TNT = 8 liters of gas = 15 kg of chocolateThe amount of liquid helium in the machine is 60 tons or 120 thousand gallonsLHC is the coldest place within the solar system with the temperature of 1.9KIt is also the emptiest place in the solar system with the vacuum in the pipe containing the beams at 10-13 atm.


CERN accelerator complex (not to scale)

CMS & ATLAS : General purpose, Higgs, SUSY ? ?

LHC-b : B-Physics, CP-violation

ALICE : Heavy Ion, QGP

50 MeV

1.4 GeV

25 GeV

450 GeV

4 TeV


CMSCMS

3.8T Solenoid

ECAL76k scintillating PbWO4 crystals

HCALScintillator/brassInterleaved ~7k ch

• Pixels (100x150 μm2) ~ 1 m2 ~66M ch

•Si Strips (80-180 μm)~200 m2 ~9.6M ch

Pixels & Tracker

MUON BARREL250 Drift Tubes (DT) and480 Resistive Plate Chambers (RPC)

473 Cathode Strip Chambers (CSC)432 Resistive Plate Chambers (RPC)

MUON ENDCAPS

Total weight 14000 tOverall diameter 15 mOverall length 28.7 m

IRON YOKE

YBOYB1-2

YE1-

3

PreshowerSi Strips ~16 m2

~137k ch

Foward CalSteel + quartz Fibers 2~k ch

31


The CMS collaboration

Slovak Republic

CERN

France

Italy

UK

Switzerland

USA

Austria

Finland

Greece

Hungary

Belgium

Poland

PortugalSpain

Pakistan

Georgia

Armenia

Ukraine

Uzbekistan

Cyprus

Croatia

China

TurkeyBelarus

Estonia

India

Germany

Korea

Russia

Bulgaria

China (Taiwan)

A large collaborative effort3600 Physicists, Engineers and students

38 Countries182 Institutions

Gradually increasing


The dataNeed highest possible luminosity to probe at small cross section of the production processLHC was generous for that– Provided excess of 6 fb-1 (~4x1014 interactions) of data during

2011 at 7 TeV cm energy and even more luminosity at 8 TeVduring the first half of 2012

– The experiments collected the provided luminosity with very highefficiency


What may happen

To get a reasonable rate of Higgs events need to produce large # of other type of events


What we seeBiggest problem: # of interactions in a bunch crossing


How much data is produced?Nearly 1 GB of data is recorded every second

– 15,000 TB/year = 15 PB/year– It’s like recording a DVD every 4 sec– Enough to fill your hard drive in 2 min

Processed all around the world via LHC Computing Grid


Understand the detector first

10


Candidate event (H→γγ)


Search for H→γγDivide the entire sample into many categories according to photon qualityFit each category of mγγ spectrum with a signal plus a background model optimized (with MC) to minimize biases


ATLAS search for H→γγ

Local p-value, local and global significance expected from SM Higgs:Max. Deviation Local Sig. Global Sig.

2011 126.0 GeV 3x10-4 3.4 σ 1.6 σ2012 127.0 GeV 3x10-4 3.2 σ 1.9 σ

2011+2012 126.5 GeV 2x10-6 4.5 σ 2.5 σ

[GeV]Hm

110 115 120 125 130 135 140 145 150

0Lo

cal p

-710

-610

-510

-410

-310

-210

-110

1

10

σ1

σ2

σ3

σ4

σ5

PreliminaryATLAS

-1 Ldt = 4.8 fb∫= 7 TeV, sData 2011,

-1 Ldt = 5.9 fb∫= 8 TeV, sData 2012,

2011+20120

Observed p 2011+2012

0 expected pγγ→SM H

2011+2012 (with energy scale uncertainty)0

Observed p 2011

0Observed p

20110

expected pγγ→SM H 2011 (with energy scale uncertainty)

0Observed p

20120

Observed p 2012

0 expected pγγ→SM H

2012 (with energy scale uncertainty)0

Observed p

Expected from SM Higgs at given mH

Fit to background only hypothesis

Consistent results from various categories within uncertainties


CMS search for H→γγ

Expected 95% CL exclusion 0.76 times SM at 125 GeVLarge range with expected exclusion below σSM

Largest excess is at a mass of 125 GeVMinimum local p-value is at 125 GeV with a local significance of 4.1 σSimilar excess in 2011 and 2012Independent cross check analyses give similar resultsGlobal significance in the full search range (110-150 GeV) 3.2 σ


Candidate event (H→ZZ(*)→2e+2µ)

8 TeV DATA

4-lepton Mass : 126.9 GeV

µ-(Z1) pT : 24 GeV

µ+(Z1) pT : 43 GeV

e-(Z2) pT : 10 GeV

e+(Z2) pT : 21 GeV


CMS search for H→4l164 events expected in [100, 800 GeV]172 events observed in [100, 800 GeV]Excess is seen at mass value around 126 GeVEnrich the signal content– Use a variable motivated by Higgs decay mechanism

Cut value chosen such that signal probability > background probability


ATLAS search for H→4lFor m(4l) > 160 GeV (dominated by ZZ background): 147 ± 11 events expected while 191 observed~1.3 times more ZZ events in data than SM prediction: in agreement with measured ZZ cross-section in 4l final states at √s = 8 TeVDiscrepancy has negligible impact on the low-mass region < 160 GeV


Final Results

ATLAS observes:– Maximum excess at 126.0 GeV at 5.9 σ CL – Probability of fluctuation 1.7x10-9

CMS observes:– Excess in 4 different channels at 125.3 GeV– Level of fluctuation at 5.0-5.1 σ CL (3x10-7)


Consistency?

Standard Model Higgs boson of mass around 125 GeV is expected to couple to a number of final statesThe yields are consistent with the predicted branching ratios


Indian contribution to the LHC machineRRCAT, BARC, VECC, IGCAR, ECIL, ATL, IGTR, BHEL contributed a number of critical components to LHCBARC, VECC, RRCAT contributed heavily to the GRID effort


Indian participation in CMSParticipants: Universities of Chandigarh, Delhi, Viswabharati + BARC (Mumbai), SINP (Kolkata), TIFR (Mumbai) (since 1994)Indian groups participated in – the design of the detectors, – building hardware components, – contributing to the software and detector performance studies– physics analysis leading to the papers for publication

Early design work includes– choice of material for the electromagnetic calorimeter– detector granularity to balance resolution vs particle identification


Hardware contributionTIFR, together with Panjab University constructed the outer hadron calorimeterHO covers central rapidity region |η|<1.3 occupied by the five muon rings to improve jet and MET resolution

Basic detector element maps tower granularity of 0.0873 ×0.0873 in η×Φ432 trays are built from 2730 tiles

Pseudorapidity, η = −loge(tan(θ/2))

SINP, TIFR are now contributing to the upgrade effort of HCAL


CMS preshower detector

End caps of CMS detector have 4300 silicon strip detectors covering area ofabout 17 m2

Under India-CMS collaboration, India has delivered more than 1000 detector modules

Higgs discovery : Good π0

rejection for H →γγ mode


Software contributionsOne of the main architects of the offline software project – starting from the very first version of simulation and reconstruction, graduating to object oriented software to the final version which is deployed from HLTto analysis. The first success of LHC experiments is how well and quickly the detectors are understood – largely due to work of a few task forces which were steered by Indian scientists.

First designer of web based GUI (Graphical User Interface) for data quality monitoring and coordinating DQM activities of the trackerPrototyping the DAQ system with testing of various high speed switchesDevelopment of GRID monitoring tools for CMSParticipation in calibration and overall performance of the hadroncalorimeter system


Physics studiesCarried out several analyses leading to public version of the analyses and a number of physics publications:– Single particle response in the calorimeter – Event shape distributions at a few CM energies– Studies of underlying events using jets reconstructed from tracks and

using Drell-Yan events– Direct photon production and constraints on parton density function– Measurement of subjet multiplicity in dijet events– Test of QCD in inclusive jet and multi-jet production– Measurement of W charge asymmetry and Wγ production– Search of Standard Model Higgs boson in a number of channels

involving leptons, τ’s and ν’s– Quarkonia production in heavy ion collisions– Search for excited lepton– Study of mono photon production in view of extra dimension– Search of Supersymmetry in all hadronic final state


Last Word

There is a very strong evidence of a new narrow boson of mass around 125-126 GeVThe search criteria of this object is motivated by Higgs boson within the Standard ModelThis is achieved by international collaboration of thousands of people working over two decadesWe, the Indians, have been a part of this from very early days


Back Up


Constituents of matter

Thomson Rutherford Chadwick SLAC 1897 1909 1932 1968


Nature of interactions

Weak → radioactivity Strong → binding of nucleus

Gravitational → solar system/galaxy Electromagnetic → photon


3D Viewpp collisions at 7-14 TeVPb+Pb at 1312 TeV

LHCf

totem

LuminosityFirst phase 3•1033 cm−2s−1

High lumi phase 1034 cm−2s−1


Construction of HO tiles

aa

Optical fibres are grouped together as a pigtail and inserted into the groovesSignal collected by WLS fibre, but carried by clear fibre


The Preshower DetectorPreshower silicon strip detector is used for π0/γ rejection in the ECAL, CMS

Strips of 1.80 mm width with a pitch of 1.9 mmArea - 63mm x 63mm

Detector specifications are very stringent as they are to be operated in a high radiation background of neutrons (2x1014 /cm2) and photons (10Mrad) for a long period of 10 years

Rad-hard front endelectronics (made at CERN)

First Silicon Strip Detector made in India


…usually known as the Higgs field

The Higgs mechanism– massless spin-1 particles

2 polarization states

– Higgs field

coupling to fermions quark and lepton masses

– massive spin-1 particles• 3 polarization states

massless field massive field

complexscalar field

Higgs boson

mix

only free parameter is mass of Higgs boson

(W+,W-,Z) (W+,W-,Z)

16


LHC/CMS Timeline1984 Workshop on a Large Hadron Collider in the LEP tunnel, Lausanne1987 Rubbia “Long-Range Planning Committee” recommends

Large Hadron Collider as the right choice for CERN’s future1990 ECFA LHC Workshop, Aachen

1992 General Meeting on LHC Physics and Detectors, Evian les Bains1993 Letters of Intent (ATLAS and CMS selected by LHCC)1994 Technical Proposals Approved1996 Approval to move to Construction (ceiling of 475 MCHF)1998 Memorandum of Understanding for Construction Signed

1998 Construction Begins (after approval of Technical Design Reports)2000 CMS assembly begins above ground. LEP closes2004 CMS Underground Caverns completed2008 CMS ready for First proton-proton Collisions, but…2009 CMS is ready again and sees collision at 900 GeV and at 2.36 TeV

August 8, 2012 Discovery of the Higgs Boson at the LHC S. Banerjee 46July

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discovery(?) of the higgs boson at the lhc - … 8, 2012 discovery of the higgs boson at the lhc s....

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