alexander khanov, oklahoma state university physics seminar at the university of tulsa, 2/26/2010

HIGH ENERGY PHYSICS:

THE LHC ERA

Alexander Khanov, Oklahoma State University

Physics seminar at the University of Tulsa, 2/26/2010

Alexander Khanov, OSU 2

Outline High Energy Physics: the challenge The Large Hadron Collider: what we can do with it How we search for the Higgs boson and many

other fantastic things: what our group is doing

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Big picture Everything in the universe, from stars and planets,

to us is made from the same basic building blocks – particles of matter.

Some particles were last seen only billionths of a second after the Big Bang. Others form most of the matter around us today.

Particle physics studies these very small building block particles and works out how they interact to make the universe look and behave the way it does2/26/2010


Standard model: total success Our idea of the world around

us is based on SM, a theory of fundamental interactions and elementary particles which participate in these interactions

“Every high energy physics experiment carried out since the mid-20th century has eventually yielded findings consistent with the Standard Model.” (Wikipedia)

But there is a missing piece

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did you notice?


Higgs: a little bit of theory Electromagnetic interaction: mediated by massless

carriers (photons): interaction has infinite range, can be easily computed

Weak interaction: mediated by heavy carriers (W/Z bosons, or V-bosons): interaction is localized

Massive field carriers are a problem! Technically, the electroweak theory implies local gauge invariance (kind of

internal symmetry reflecting a redundancy in the field description), which seemingly fails to accommodate massive field quanta;

if the field carriers have a mass, the theory becomes non-renormalizable (the solution can’t be obtained as a converging infinite series)

Simply speaking, if V-bosons have mass, the theory does not compute2/26/2010


The Higgs boson The solution arrived from

superconductivity: we introduce a new (Higgs) field which is stable at =VEV0

If is replaced with effective field ’=VEV, the equations look like V-bosons have mass

This implies the existence of quanta of this field – Higgs bosons

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The Higgs boson, a mysterious particle which, according to SM, gives rise to vector boson masses, has not yet been observed


And there is more… There is a mounting evidence that SM is incomplete

we learned that neutrinos have mass, and SM didn’t know?

what is dark matter and dark energy?why there is the matter-antimatter asymmetry?

Half a century ago we got a lot of unexpected discoveriesmuons, tau-leptons, top and bottom quarks,…

By today we gave a deep thought about them, and realized that in order to make a consistent picture we need more discoveries!

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Alexander Khanov, OSU 82/26/2010

Search for Higgs at LEP Large Electron-Positron collider at CERN (1989-2000)

Max Higgs mass: beam energy (200 GeV) minus the Z mass

LEP did not find Higgs, but set important limits: direct observation (no Higgs seen): mH>114.4 GeV indirect limits (combination of electroweak data): mH<144 GeV

(without direct limit), mH<182 GeV (including direct limit)

e+ e

200 GeV

Four detectors: Aleph, Delphi, L3, Opal


Search for Higgs at the Tevatron Tevatron collider at Fermilab –

the former world highest energy collider

p p

2 TeV

Two detectors: CDF and D0

OSU is a member of D0


Touching the limit We haven’t seen the Higgs at the Tevatron. But we

touched the limit – for the first time since LEP!

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The TEVNPH Working Group, Nov 2009


We are one step from discovery We have a feeling that new discoveries are around

the corner, all we need is a big machineThe Higgs is needed to regulate divergences in theorySM (with Higgs!) is a great model which passed many

tests with enormous precision If we take out Higgs and calculate WWWW scattering,

its probability will exceed 1 at energies above 1 TeV! So we are confident we will see Higgs – or

whatever is playing its role

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Search for Higgs at the LHC Large Hadron Collider at CERN: discovery guaranteed

with the colliding beam energy and intensity available at the LHC, the whole mH range will be covered in 3 years

p p

14 TeV

Two detectors: CMS and ATLAS


LHC: a BIG machine 14 TeV (14000proton mass) energy 17 miles long, 570 ft below the surface 0.7 A proton currents

protons moving at 99.999999% of the speed of light

1,600 superconducting magnets96 tons of liquid helium

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LHC detectors ATLAS and CMS: general-purpose detectors ALICE: heavy ion collisions LHCb: b-physics

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ATLAS: a general-purpose detector 7000 Tons 15 years to build 500M$ in materials

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Physics potential: Higgs boson, supersymmetry, extra dimensions, and new unexpected physics!

ATLAS: a BIG collaboration 2900 Scientists 172 universities and laboratories from 37

countries 700 graduate students

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LHC status By the end of 2009, ATLAS recorded ~900k pp collisions

highest luminosity was 6.8x1026 cm2s1

most of collisions at 900 GeV for a short period LHC was running at 2.36 TeV – new world record

Currently we are in a shutdown, resume operation in 1—2 weeks The plan is to operate at 7 TeV (1/2 energy) for the rest of the year

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OSU experimental HEP group

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Babak Abi, Dr Flera Rizatdinova, Dr Alexander Khanov, Dmitri SidorovNot shown: Hatim Hegab


The OSU ATLAS program What are we doing in the ATLAS experiment?

working on the strategy to search for a heavy charged Higgs boson

preparing to measure the top quark pair production cross section in early ATLAS data

developing methods to evaluate the heavy flavor tagging performance

creating a pixel detector calibration data basedoing R&D on PiN diodes for the ATLAS tracker

upgrade I can’t talk about everything – let me pick one topic

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How one can see the Higgs boson? A short answer: by colliding the particles and

looking at the products of collisionswhen two protons (more exactly, quarks inside them)

collide, their kinetic energy gets transformed into the mass of new particles which are created during the collision

various particles are detected by various ATLAS subsystems – more on that on the next page

a special circuit (“trigger”) checks in real time what was produced and only records the most “interesting” events (typically those with many particles with large transverse momenta)

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ATLAS detector: the details A complex device aimed at detection of variety of particles

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ATLAS as a “typical” HEP detector usual collision products: pions, protons, neutrons,

electrons, muons, photons, neutrinos,…

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instead of neutral pions, see photons: 0

: can’t see them at all! Detect neutrinos as “missing energy”


But Higgs is not in the list? The Higgs boson is unstable, it decays before it

can be detected by any of the ATLAS subsystems it can only be observed through its decay products

To explain the details, let’s talk about another particle – Z bosonZ is routinely used at the Tevatron for detector

calibration, and will also be used so at the LHC like Higgs, Z immediately decays after it’s born let’s consider one of its decay modes: Ze+e

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How to see a Z? We select events which have two high transverse

momentum electrons of opposite charge We calculate invariant mass of these electrons:

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One event is not enough !Need many events to see a peak


What about Higgs? Like Z, the Higgs boson is unstable and quickly

decays into other particles Light SM Higgs (favored by theory) or SUSY Higgs

preferably decays to a pair of b-quarksnow that’s another trouble – quarks do not show up as

free particles, they undergo hadronizationwhat you see in the detector is a bunch of collimated

particles moving in a narrow cone – a jetwe need to detect events with jets, separate jets

produced by b-quarks, calculate their invariant mass, and get our hands on Higgs!

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Separating ore from gangue B-tagging is a technique which allows to

discriminate jets produced by b-quarks (b-jets) from other jets

In a regular multi-jet production which constitutes the majority of events at the LHC, the fraction of b-jets is small (2—3 %)

By simply requiring b-jets in the final state, the background from multi-jet and W+jets production can be suppressed by a factor of 30—50

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The basics of b-tagging B-jets are characterized by a presence of B-hadrons

(heavy particles containing a b-quark) B-hadrons are unstable and eventually decay into

lighter particles, usually into other hadrons, often accompanied by a low momentum lepton and neutrino

Before they decay, B-hadrons travel a significant distance – few mm, depending on their momentum

ATLAS inner tracker is able to reconstruct trajectories of B-decay products with spatial precision sufficient to locate their origin

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b-tagging methods (1) Begin by reconstructing the

primary vertex PV – a point in space where most of the particles in the event originate from

Impact parameter (IP) b-tagging: extrapolate trajectories of particles in the jet towards PV and look for cases when several tracks in the jet point away from PV. They are candidates for b-decay products2/26/2010


b-tagging methods (2) Secondary vertex (SV)

b-tagging: we construct the common point of origin for particles in the jet and see if this point is significantly displaced from PV

Soft lepton (SL) tagging: look for excess of muons and electrons from B-hadron decays2/26/2010


b-tagging performance Our group is working on

measurement of b-tagging efficiency (probability to identify a b-jet as such) and mistag rate (probability to misidentify a non-b-jet as a b-jet) in real data

It is not an easy task: in data, nobody knows the origin of jets!2/26/2010

b-jet

b-jet

l-jet

l-jet

Monte Carlo

?-jet

?-jet

?-jet

?-jet

Data


b-tagging efficiency The b-tagging efficiency

can be conveniently measured by applying two uncorrelated b-tagging algorithms simultaneously and looking at the numbers of jets tagged by both, one, or neither method

IP+SL and SV+SL are two good examples of such algorithm pairs

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Expected statistical error is 0.3% for 50 pb-1 and 0.2% for100 pb-1

llIP

lSLb

bIP

bSLIPSL

llIP

lSLb

bIP

bSLIPSL

llIPb

bIPIP

llIPb

bIPIP

llSLb

bSLSL

llSLb

bSLSL

lb

lb

ppp

nnn

ppp

nnn

ppp

nnn

ppp

nnn

System 8

measured and true b-tagging efficiency as a function of jet


Mistag rate Typical mistag rate is 103 to 104

at b-tagging efficiency of 50–60% even small admixture of b-jets spoils

the measurement! We explore two methods to

measure mistag rate: by measuring negative tag rate

(obtained by inverting IP or decay length sign): the negative part of IP/DL distribution is similar for all particles

by splitting the jet sample in two subsets with different b-fractions and measuring both mistag rate and b-fractions at the same time

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mistag rate uncertainty is dominated by systematics (~15%) due to presence of long-lived particles

measured and true b-tagging efficiency as a function of jet pT


Summary LHC has started to collect collision data – the new

HEP era has begun! The LHC physics program includes a lot of new

physics searches which can shed light on fundamental questions in physics

We are still understanding our detector and learning how to get the best performance

The OSU HEP group is part of this effort This is the very beginning of exciting times, and

we are looking forward to great discoveries!

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alexander khanov, oklahoma state university physics seminar at the university of tulsa, 2/26/2010

Documents

alexander khanov

field higgs bosons

new higgs field

max higgs mass

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