ATLAS Masterclass - W and Z path physics and presentation of the Z path measurement

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1 ATLAS Masterclass - W and Z path physics and presentation of the Z path measurement University of Oslo International Conference on New Frontiers in Physics September 2nd 2013 Kolymbari, Crete, Greece

2 Outline Physics W and Z path common physics - proton-proton collisions ATLAS W path and the structure of the proton - Dresden University ATLAS Z path and the invariant mass technique for particle indentification and discovery - Oslo University ATLAS Z path Layout of a typical Masterclass day Identification of events in HYPATIA with examples The Oslo Plotting Tool (OPloT) and example results

3 W and Z path physics - proton-proton collisions The proton consists of three (valence) quarks bound together by the strong force These valence quarks are two up-quarks and one down-quark, giving the proton a total electric charge of +1 However, in high energy collisions, the proton seems to consist of both quarks and anti-quarks as well as gluons

4 W and Z path physics - proton-proton collisions In high energy collisions, such as at the LHC, we observe collisions between individual quarks, anti-quarks, and gluons from within the proton Such a collision is referred to as a hard scattering

5 W and Z path physics - proton-proton collisions The parton distribution functions (PDFs) describe how often the various proton constituents will go into a hard scattering with a given momentum fraction x (the fraction of the proton momentum carried by the constituent) They summarize the proton structure as seen on a given energy scale

6 W and Z path physics - the W and Z bosons The W and Z bosons are the massive mediators of the weak force in the Standard Model They can be produced in hard scatterings between quarks and anti-quarks in proton-proton collisions They are both frequently produced at the LHC, and give characteristic signatures Two oppositely charged leptons from Z One lepton and missing momentum (neutrino) from W

7 W and Z path physics - Higgs The Higgs boson is in the Standard Model responsible for giving mass to gauge bosons and fermions

8 W path physics - proton structure and W charge The charge of a W boson produced at the LHC depends on which quarks and anti-quarks collided to form it Naively, we expect more W + than W because there are two up-quarks and only one down-quark in the proton The relative frequencies of W + and W depends strongly on the PDFs, and is an excellent probe of the proton structure Naive ratio: N W +/N W = 2 ATLAS result (uncorrected): N W +/N W = 1.50 ± 0.03 MC 2013 result: N W +/N W = 1.25

9 W path physics - proton structure and W charge ATLAS result on the W µν charge asymmetry A µ = dσ W µ +/dη µ dσ W µ /dη µ dσ W µ +/dη µ + dσ W µ /dη µ The data are compared to different PDFs

10 W path physics - Higgs search in WW events The Higgs boson can decay into two charged leptons and two neutrinos through an intermediate state of two W bosons The azimuthal angle between the two charged leptons, φ ll, can be used as search variable to discover the Higgs Higgs signal accumulates at low values of φ ll

11 Z path physics - invariant mass and discovery Short lived particles can only be observed indirectly through their decay products Their properties, such as the mass, must be inferred through measurements of the decay products Einstein s famous relation between energy (E), momentum (p), and mass (m) is useful E 2 = p 2 c 2 + m 2 c 4 or m = E 2 c 4 p2 c 2

12 Z path physics - invariant mass and discovery We find the energy and momentum of the short lived particle by measuring and summing up the decay products (E1 + E 2 ) m 12 = 2 c 4 (p 1 + p 2 ) 2 c 2 The mass of the short lived particle (Z, H,...) can be inferred m Z = m l + l = (El + + E l ) 2 c 4 (p l + + p l )2 c 2 (Eγ1 + E γ2 ) m H = m γ1 γ 2 = 2 c 4 (p γ 1 + p γ2 ) 2 c 2

13 Z path physics - invariant mass and discovery In addition to being useful for measuring the mass of a short lived particle, the invariant mass is useful for the observation/discovery of such a particle since invariant mass values will cluster and form a peak in an invariant mass histogram

14 W and Z path physics - the measurements Z path - developed by Oslo University In the Z path exercise, students identify events with two leptons, two photons, or four leptons The students calculate invariant masses, and the resulting two-lepton invariant mass distribution is used to look for evidence of the Z 0 boson as well as the J/ψ and Υ mesons (+ possibly new particles?) The two photon and four lepton invariant mass distributions are used to provide insight into the process of discovering the Higgs boson at the LHC W path - developed by Dresden University (see talk by Uta Bilow) In the W path exercise, students identify events with one lepton and missing transverse energy (W events) and events with two leptons and missing transverse energy (WW events) The W events are used to measure the W charge ratio The WW events are used to teach the students about Higgs searches

15 Z path Masterclass - typical Masterclass day Morning lectures Introduction to particle physics theory - the Standard Model (and beyond) Introduction to experimental particle physics - accelerators and detectors Lunch Measurement Introduction to the measurement including repetition of the most important points from morning lectures - some example events are analyzed in detail The students go to various computer rooms where they analyze (in groups of 2) their own dataset - several (as many as possible) supervisors are available in each room for questions and guidance Break Results session Discussion of results locally at the institute Videoconference with all institutes on the given day

16 Z path Masterclass - the measurement Students go through each event in their datasample using the event display program HYPATIA in a specially customized Masterclass version They look for events with: 2 leptons (e + e or µ + µ ) 2 photons (γγ) 4 leptons (e + e e + e, µ + µ µ + µ, or e + e µ + µ ) For each identified event, the invariant mass of the particles selected by the student is automatically calculated by HYPATIA All the invariant masses can finally be written to a file which is subsequently uploaded to an online plotting tool by the student

17 Z path Masterclass - example events (e + e )

18 Z path Masterclass - example events (µ + µ )

19 Z path Masterclass - example events (γγ)

20 Z path Masterclass - example events (e + e )

21 Z path Masterclass - example events (e + e µ + µ )

22 Z path Masterclass - OPloT The Oslo Plotting Tool - OPloT is an online plotting tool where each stundent group can upload their results Each student group can view their own results in the form of invariant mass histograms The tool can also show the combined histograms obtained by combining all data on a given institute on a given day all data on all institutes on a given day The combination histograms are used when results are discussed locally on each institute and when they are discussed in the video conference at the end of the day

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33 Status and outlook 70 institutes from around the world took part in the Z path MasterClass this year The Z path started out focusing only on dilepton events, the observation of a few well-known particles, and a discussion of the potential for new discoveries After the Higgs-discovery at the LHC, the Z path was extended to include Higgs related searches (four lepton and diphoton events) Future LHC publications may reveal what the Z -path MasterClass could look like in the future (discovery of supersymmetry, exotic particles,...?)

34 Links and MasterClass: ATLAS MasterClass: Z path: W path: Hypatia: OPloT: If you have any questions or requests, do not hesitate to contact the Oslo Z path team at epf-mc@fys.uio.no