High Energy Physics: the LHC ERA

1. Alexander Khanov, Oklahoma State University Physics seminar at the University of Tulsa, 2/26/2010 High Energy Physics:the LHC ERA

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 Alexander Khanov, OSU

3. 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 does Alexander Khanov, OSU

4. 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 did you notice? Alexander Khanov, OSU

5. 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 compute Alexander Khanov, OSU

6. The Higgs boson • 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 The Higgs boson, a mysterious particle which, according to SM, gives rise to vector boson masses, has not yet been observed Alexander Khanov, OSU

7. 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 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! Alexander Khanov, OSU

8. Search for Higgs at LEP e+ e 200 GeV • 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) Four detectors: Aleph, Delphi, L3, Opal Alexander Khanov, OSU

9. Search for Higgs at the Tevatron p p 2 TeV • Tevatron collider at Fermilab – the former world highest energy collider Two detectors: CDF and D0 OSU is a member of D0 Alexander Khanov, OSU

10. Touching the limit • We haven’t seen the Higgs at the Tevatron. But we touched the limit – for the first time since LEP! The TEVNPH Working Group, Nov 2009 Alexander Khanov, OSU

11. We are one step from discovery • 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  Alexander Khanov, OSU

12. Search for Higgs at the LHC p p 14 TeV • 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 Two detectors: CMS and ATLAS Alexander Khanov, OSU

13. LHC: a BIG machine • 14 TeV (14000proton 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 magnets • 96 tons of liquid helium Alexander Khanov, OSU

14. LHC detectors • ATLAS and CMS: general-purpose detectors • ALICE: heavy ion collisions • LHCb: b-physics Alexander Khanov, OSU

15. ATLAS: a general-purpose detector • 7000 Tons • 15 years to build • 500M$ in materials Physics potential: Higgs boson, supersymmetry, extra dimensions, and new unexpected physics! Alexander Khanov, OSU

16. ATLAS: a BIG collaboration • 2900 Scientists • 172 universities and laboratories from 37 countries • 700 graduate students Alexander Khanov, OSU

17. LHC status • By the end of 2009, ATLAS recorded ~900k pp collisions • highest luminosity was 6.8x1026 cm2s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 Alexander Khanov, OSU

18. OSU experimental HEP group BabakAbi, Dr Flera Rizatdinova, Dr Alexander Khanov, Dmitri Sidorov Not shown: HatimHegab Alexander Khanov, OSU

19. 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 base • doing R&D on PiN diodes for the ATLAS tracker upgrade • I can’t talk about everything – let me pick one topic Alexander Khanov, OSU

20. How one can see the Higgs boson? • 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) Alexander Khanov, OSU

21. ATLAS detector: the details • A complex device aimed at detection of variety of particles Alexander Khanov, OSU

22. ATLAS as a “typical” HEP detector • usual collision products: pions, protons, neutrons, electrons, muons, photons, neutrinos,… instead of neutral pions, see photons: 0 : can’t see them at all! Detect neutrinos as “missing energy” Alexander Khanov, OSU

23. 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 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 Alexander Khanov, OSU

24. 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: One event is not enough ! Need many events to see a peak Alexander Khanov, OSU

25. 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-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! Alexander Khanov, OSU

26. 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 Alexander Khanov, OSU

27. 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 Alexander Khanov, OSU

28. 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 products Alexander Khanov, OSU

29. 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 decays Alexander Khanov, OSU

30. b-tagging performance b-jet • 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! l-jet l-jet b-jet Monte Carlo ?-jet ?-jet ?-jet ?-jet Data Alexander Khanov, OSU

31. b-tagging efficiency System 8 • 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 measured and true b-tagging efficiency as a function of jet  • Expected statistical error is 0.3% for 50 pb-1 and 0.2% for100 pb-1 Alexander Khanov, OSU

32. Mistag rate • 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 measured and true b-tagging efficiency as a function of jet pT • mistag rate uncertainty is dominated by systematics (~15%) due to presence of long-lived particles Alexander Khanov, OSU

33. 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! Alexander Khanov, OSU