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Alexander Khanov, Oklahoma State University Physics seminar at the University of Tulsa, 2/26/2010.

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Presentation on theme: "Alexander Khanov, Oklahoma State University Physics seminar at the University of Tulsa, 2/26/2010."— Presentation transcript:

1 Alexander Khanov, Oklahoma State University Physics seminar at the University of Tulsa, 2/26/2010

2  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 2/26/2010Alexander Khanov, OSU2

3  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 does 2/26/2010Alexander Khanov, OSU3

4  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 2/26/2010Alexander Khanov, OSU4 did you notice?

5  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 compute 2/26/2010Alexander Khanov, OSU5

6  The solution arrived from superconductivity: we introduce a new (Higgs) field  which is stable at  =  VEV  0  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 2/26/2010Alexander Khanov, OSU6 The Higgs boson, a mysterious particle which, according to SM, gives rise to vector boson masses, has not yet been observed

7  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 discoveries  muons, 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! 2/26/2010Alexander Khanov, OSU7

8 2/26/2010Alexander Khanov, OSU8  Large Electron-Positron collider at CERN ( )  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): m H >114.4 GeV  indirect limits (combination of electroweak data): m H <144 GeV (without direct limit), m H <182 GeV (including direct limit) e+e+ ee 200 GeV Four detectors: Aleph, Delphi, L3, Opal

9 2/26/2010Alexander Khanov, OSU9  Tevatron collider at Fermilab – the former world highest energy collider p pp 2 TeV Two detectors: CDF and D0 OSU is a member of D0

10  We haven’t seen the Higgs at the Tevatron. But we touched the limit – for the first time since LEP! 2/26/2010Alexander Khanov, OSU10 The TEVNPH Working Group, Nov 2009

11  We have a feeling that new discoveries are around the corner, all we need is a big machine  The Higgs is needed to regulate divergences in theory  SM (with Higgs!) is a great model which passed many tests with enormous precision  If we take out Higgs and calculate WW  WW 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 2/26/2010Alexander Khanov, OSU11

12 2/26/2010Alexander Khanov, OSU12  Large Hadron Collider at CERN: discovery guaranteed  with the colliding beam energy and intensity available at the LHC, the whole m H range will be covered in 3 years pp 14 TeV Two detectors: CMS and ATLAS

13  14 TeV (14000  proton mass) energy  17 miles long, 570 ft below the surface  0.7 A proton currents  protons moving at % of the speed of light  1,600 superconducting magnets  96 tons of liquid helium 2/26/2010Alexander Khanov, OSU13

14  ATLAS and CMS: general-purpose detectors  ALICE: heavy ion collisions  LHCb: b-physics 2/26/2010Alexander Khanov, OSU14

15  7000 Tons  15 years to build  500M$ in materials 2/26/2010Alexander Khanov, OSU15 Physics potential: Higgs boson, supersymmetry, extra dimensions, and new unexpected physics!

16  2900 Scientists  172 universities and laboratories from 37 countries  700 graduate students 2/26/2010 Alexander Khanov, OSU 16

17  By the end of 2009, ATLAS recorded ~900k pp collisions  highest luminosity was 6.8x10 26 cm  2 s  1  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 2/26/2010Alexander Khanov, OSU17

18 2/26/2010Alexander Khanov, OSU18 Babak Abi, Dr Flera Rizatdinova, Dr Alexander Khanov, Dmitri Sidorov Not shown: Hatim Hegab

19  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 base  doing R&D on PiN diodes for the ATLAS tracker upgrade  I can’t talk about everything – let me pick one topic 2/26/2010Alexander Khanov, OSU19

20  A short answer: by colliding the particles and looking at the products of collisions  when 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) 2/26/2010Alexander Khanov, OSU20

21  A complex device aimed at detection of variety of particles 2/26/2010Alexander Khanov, OSU21

22  usual collision products: pions, protons, neutrons, electrons, muons, photons, neutrinos,… 2/26/2010Alexander Khanov, OSU22 instead of neutral pions, see photons:  0   : can’t see them at all! Detect neutrinos as “missing energy”

23  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 boson  Z 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: Z  e + e  2/26/2010Alexander Khanov, OSU23

24  We select events which have two high transverse momentum electrons of opposite charge  We calculate invariant mass of these electrons: 2/26/2010Alexander Khanov, OSU24 One event is not enough ! Need many events to see a peak

25  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-quarks  now that’s another trouble – quarks do not show up as free particles, they undergo hadronization  what you see in the detector is a bunch of collimated particles moving in a narrow cone – a jet  we need to detect events with jets, separate jets produced by b-quarks, calculate their invariant mass, and get our hands on Higgs! 2/26/2010Alexander Khanov, OSU25

26  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 2/26/2010Alexander Khanov, OSU26

27  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 2/26/2010Alexander Khanov, OSU27

28  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 products 2/26/2010Alexander Khanov, OSU28

29  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 decays 2/26/2010Alexander Khanov, OSU29

30  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! 2/26/2010Alexander Khanov, OSU30 b-jet l-jet Monte Carlo ?-jet Data

31  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 2/26/2010Alexander Khanov, OSU31 Expected statistical error is 0.3% for 50 pb -1 and 0.2% for100 pb -1 System 8 measured and true b-tagging efficiency as a function of jet 

32  Typical mistag rate is 10  3 to 10  4 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 2/26/2010Alexander Khanov, OSU32 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

33  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! 2/26/2010Alexander Khanov, OSU33


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