Forschen am Large Hadron Collider am CERN ein anderer Blick auf das Higgs-Boson

Transcription

1 Forschen am Large Hadron Collider am CERN ein anderer Blick auf das Higgs-Boson TU Dresden DPT

2 The Standard Model of particle physics (SM) Based on 3 fundamental symmetries being origin of interactions between matter particles & mediators of the interactions Fundamental principle: local gauge invariance conservation laws Main ingredients: Matter: 6 quarks and 6 leptons in 3 generations Forces: electromagnetism (γ): between charged particles weak interaction (W±, Z): responsible for conversions of elementary particles strong interaction (g): between quarks Electroweak symmetry breaking (EWSB) via Brout-Englert-Higgs mechanism The heavy vector bosons W± and Z acquire their mass through EWSB 2

3 Standard Model processes Most frequent and very good understood WWV precisely measured at LEP H WW, ZZ, γγ and H ττ recently observed at the LHC Higgs self couplings not yet seen WWVV: limited experimental data accessible in (heavy) vector boson scattering (VBS) VV VV! (here: V = W,Z) LHC becomes sensitive to VBS 3

4 VV scattering and the role of the Higgs boson The mechanism responsible for EWSB must regulate the cross section σ(vv VV) to restore unitarity above 1 2 TeV A light SM Higgs boson cancels increase for large energies (exact cancellation for HWW coupling) arxiv: Higgs boson at work in vector boson scattering! 4

5 VV scattering and the role of the Higgs boson The mechanism responsible for EWSB must regulate the cross section σ(vv VV) to restore unitarity above 1 2 TeV A light SM Higgs boson cancels increase for large energies (exact cancellation for HWW coupling) arxiv: Higgs boson at work in vector boson scattering! 5

6 Mont Blanc LHC Alps Geneva, Switzerland Lac Leman Large Hadron Collider 27 km circumference 6

7 Proton collisions in the LHC LHC has stored beams of protons in a 16 µm diameter beam (~half the size of a human hair) Each proton bunch is ~20 cm long ~2000 bunches in each ring resulting in about 20 million crossings/sec! About 30 proton-proton collisions per bunch crossing Candidate Z μμ event with high pileup ATLAS Experiment c 2013 CERN 2011/2012: = 7/8 TeV from 2015 on: = 13/14 TeV 7

8 Detectors particle identification Detectors consist of tracking system, calorimeters and muon chambers Neutrinos escape detection, cause missing transverse energy 8

9 ATLAS collaboration 38 countries, 177 institutions ~ 3000 scientists (~ 15% women) Large project funds from science funding agencies of the various participating countries Anjaand Vestthe various individual university funds 9 Direct contributions from CERN

10 The ATLAS collaboration organization See and also talk by S. Lammers at the Ringvorlesung TU Dresden 2013 Management structure in place (similar to that in a company) top leaders are elected by the collaboration mid-level managers are appointed after calling for nominations from the authors How do important decisions get made? A consensus is usually formed through discussion (in rare cases the ATLAS management makes an executive decision) Key experts make proposals, their recommendations are discussed in the ATLAS Executive Board and presented in ATLAS plenary meetings in which all authors are invited to contribute their thoughts How does one apportion the tasks? One tries to match interests and resources of the participating team to the tasks However, everyone needs to share the less interesting but necessary tasks (works because the physicists are motivated by the prospect of the exciting results to be obtained) 10

11 The ATLAS collaboration authorship Who is author of ATLAS publications? all ~3000 collaboration members 5 page paper, 14 pages authorlist How does one become an ATLAS author? Every scientist should complete a task that benefits the entire experiment and requires 1/2 of their available research time for one year 11

12 The ATLAS collaboration collaboration members How does one collaborator get credit for his/her work and contributions? Internal publications within the collaboration document the individual contributions (typically written by small teams) Leading contributions often recognized by asking the person in question to present results at conference (presentations at conferences distributed centrally by ATLAS speakers committee ) Major results are possible only through the collective work of tens if not hundreds of people 12

13 Demographics Percentages of women in ATLAS by affiliation and nationality: 271 / 1952 people hired by German institutes: 13,9% 56 women / 271 people hired by German groups: 20,7% 41 German women / 269 Germans in ATLAS: 15,2% For more details see talk by P. Gagnon at DPT

14 Vector boson scattering at the LHC 14

15 Vector boson scattering at the LHC initial state: at the LHC we collide protons (q, q, g) source of vector boson beams 15

16 Vector boson scattering at the LHC initial state: at the LHC we collide protons (q, q, g) source of vector boson beams process of interest: scattering of vector bosons consider all possible diagrams in interaction 16

17 Vector boson scattering at the LHC final state: initial state: measurable signature in the detecors, defined by analysis: di-boson + 2 jets (VVjj), jets well separated in y with large mjj, bosons decaying e.g. leptonically at the LHC we collide protons (q, q, g) source of vector boson beams rapidity example topology 17

18 W±W±jj production at the LHC Signal: Eelectroweak W±W±jj self interactions, including quartic gauge vertex Higgs contribution non-resonant Strong W±W±jj can be separated from electroweak signal by topological cuts 18

19 W±W±jj candidate event Phys. Rev. Lett. 113,

20 W±W±jj production measurement at ATLAS Phys. Rev. Lett. 113, Invariant mass mjj of the 2 tagging jets: Signal: electroweak W±W±jj Important backgrounds: strong W±W±jj 3 or more prompt leptons, mainly from WZjj (one lepton is not measuered) veto events with any additional lepton with loose criteria Conversions/non-prompt, e.g. events with di-leptonic and semi-leptonic top pair decays veto events with any b-jets 20

21 W±W±jj production measurement at ATLAS Phys. Rev. Lett. 113, Rapidity difference yjj between the 2 tagging jets: To extract the electroweak W±W±jj component, require that the 2 tagging jets have a large rapidity difference ( y > 2.4) 7 contributors from Dresden, thereof 3 women! Measured fiducial cross section: σ = 1.3 ± 0.4(stat) ± 0.2(syst) fb (expected: 0.95 ± 0.06 fb) Significance: 3.6 σ (expected: 2.8 σ) 21

22 Overview of ATLAS SM cross sections 22

23 Conclusions Vector boson scattering processes provide a very important test of the electroweak theory and of the dynamics of electroweak symmetry breaking Higgs boson discovered, but still need to check whether this Higgs unitarizes the VBS process First results from LHC: W±W±jj production (statistically still limited) First evidence ever for a process dominated by VBS and containing a four boson vertex! Limits on anomalous quartic gauge couplings (not in this talk) Higgs boson glimpsed at work for first time (NewScientist, ) 23

24 Outlook Need to explore VBS at higher energies, complementary to studying Higgs properties 13/14 TeV (starting in 2015): vector boson scattering in the 500 GeV 2 TeV region allows tests on the unitarization of VV VV scattering amplitude and to probe the SM nature of EWSB look at same-sign W±W±jj, WZjj, ZZjj and other final states Beyond the LHC: fully explore EWSB, probing in particular unitarization of WW scattering at mww >> 1 TeV explore dynamics well above EWSB 24

25 Backup 25

26 The Standard Model of particle physics (SM) The SM is based on 3 fundamental symmetries being origin of interactions between matter particles & mediators of the interactions Fundamental principle: local gauge invariance conservation laws SU(2) symmetry forbids that elementary particles have mass Solution: spontaneous electroweak symmetry breaking (EWSB) via Brout-Englert-Higgs mechanism The heavy vector bosons W± and Z acquire their mass through EWSB 26

27 The Standard Model of particle physics (SM) EW theory is non-abelian EW gauge bosons carry weak charge their self-interactions should exist LWWVV contains the quartic self-couplings (QGC) no neutral gauge boson self-couplings in the SM 27

28 VVjj final states leading order cross sections at 8 TeV: final state W±W±jj W±W jj + ZZjj WZjj ZZjj σ(ew) 19.5 fb 93.7 fb 30.2 fb 1.5 fb σ(strong) 18.8 fb 3192 fb 687 fb 106 fb σ(ew)/σ(strong) ~1:1 ~ 1 : 35 ~ 1 : 20 ~ 1 : 70 (generator cuts: mll > 4 GeV, pt,l > 5 GeV, pt,j > 15 GeV) most promising measurable VVjj final states in terms of VBS: same-charge sign W±W±jj strong W±W±jj contributions very small (no LO gluon-gluon initial state) WZjj and ZZjj clean channel due to 3/4-lepton final states 28

29 CERN accelerator complex 29

30 The ATLAS detector 30