Seeking the Higgs Boson, the Giver of Mass

1. Seeking the Higgs Boson, the Giver of Mass Paul Grannis Ohio University colloquium June 12, 2009

2. 34 The Standard Model of Particle Physics The SM is common framework for describing the fundamental forces and the matter particles on which they operate QCD (quantum chromodynamics – the strong nuclear force) Electromagnetic Weak + Gravity (still no quantum theory, and left out of the SM) } become the unified Electroweak(EW) force at ~ 250 GeV QCD and EW are not unified in the SM, but are pasted together in ad hoc fashion. Both QCD & EW are Yang Mills gauge theories, based on a local invariance (at every space-time point) to generalized phase transformations of the matter fields, leading to forces mediated by massless spin-1 (vector) bosons. • The microscopic forces have a common nature

3. 33 Exchange Forces A familiar every day example: two skaters ( ) and ( )passing a puck ( ) between them. Yellowskater recoils when she throws the puck.Greenskater recoils when he catches the puck. The skaters are the matter particles; the puck is the force transmitting boson. The particle trajectories form Feynman diagrams

4. 32 Matter Particles and bosons - QCD u c t QCD matter particles are quarks – 6 ‘flavors’ in 3 SU(2) doublets. d s b

5. g 31 Matter Particles and bosons - QCD u c t QCD matter particles are quarks – 6 ‘flavors’ in 3 SU(2) doublets. d s b + the other 2 color sets The massless gluon is the carrier of QCD, coupling to a ‘charge’ called color. Quarks come in 3 colors and gluons in 8 colors (the color – anticolor combinations). + the other color gluon states

6. u c t d s b ne nm nt e m t g W± Z 30 Matter Particles and bosons - Weak QCD matter particles are quarks – 6 ‘flavors’ in 3 SU(2) doublets. gluon is the carrier of QCD, coupling to a ‘charge’ called color. Quarks come in 3 colors and gluons in 8 colors. The weak interaction is carried by W± and Z0. All quarks and leptons carry weak charges and participate in the weak interaction.

7. g g W± Z 29 Matter Particles and bosons - EM u c t QCD matter particles are quarks – 6 ‘flavors’ in 3 SU(2) doublets. d s b ne nm nt + the other 2 color sets e m t gluon is the carrier of QCD, coupling to a ‘charge’ called color. Quarks come in 3 colors and gluons in 8 colors. + the other color gluon states The weak interaction is carried by W± and Z0. All quarks and leptons carry weak charges and participate in the weak interaction. The electromagnetic interaction is mediated by the photon, coupling to electric charge. All charged particles respond to the electromagnetic force.

8. quark; e,m, t photon,g quark; e,m, t Neutral current Charged current n, u quark n, lepton, quark w0,b0 w± n, lepton, quark e, d quark 28 Electroweak force EM – mediated by massless photons coupling to electric charge. The underlying WEAK interaction is similar – charged and neutral currents are mediated by massless isovector ‘w’ tripletand isoscalar ‘b’ singlet bosons that couple to ‘weak isospin’ and ‘weak hypercharge’. (Need massless gauge bosons for gauge invariance and renormalizability). But the observed weak interaction has very short range (fm), so the real world weak bosons cannot be massless. Measurements give MW = 80.399 ± 0.025 GeV and MZ = 91.1875 ± 0.0021 GeV. The apparent incompatibility between Yang-Mills theory and reality, brings the need to break the EM and Weak symmetry (give W and Z mass).

9. ( ) b0 w- w0 w+ 27 EW symmetry breaking The SM invokes spontaneous symmetry breaking of EW force. Introduce complex doublet of scalar Higgs fields – 2 neutral degrees of freedom and 2 charged. The symmetry point (f=0) is not the minimum of the potential: V(F) = l(F2 – ½ v2) (v ~ 246 GeV) massless g W- W+ Z0 ( ) In symmetry limit, we have 4 massless gauge bosons (2 polarization states each) EWSB massive f+f0 ( ) + Higgs field doublet (4 d.o.f) Symmetry breaking and mixing The mixing of gauge bosons and Higgs fields gives rise to massive W± and Z0, massless g. 3 Higgs d.o.f. go to provide the missing longitudinal polarization states required for massive (W+ W- Z). The remaining degree of freedom gives rise to a physical spin 0 Higgs boson.

10. Unphysical self coupling Allowed Unstable vacuum 26 SM Higgs boson Remaining Higgs field should be observed as a new spin 0 particle: MH = 4 l v2 , but its mass is not known (lis not known). Higgs gives mass to W and Z The Higgs also generates the quark masses. The larger the (Yukawa) coupling gHqq, the more massive the quark. Higgs e.g. W boson W boson g g Without incredible fine tuning of parameters, loops like these drive the W, Z, H masses in the SM to the Planck scale – the Hierarchy Problem There are some theoretical constraints on the Higgs mass. If it is too large, its coupling to another Higgs diverges above a scale L. If too small, the vacuum destabilizes above scale L. But for 150 < MH < 180, the Higgs mass can be as large as the Planck mass (1019 GeV).

11. Courtesy D.J. Miller 25 Higgs as giver of mass Before the arrival of the star physicist, the room is filled with quietly chattering students (i.e. the vacuum) The star (Prof. Higgs) arrives and moves across the room ... attracting questioners, thus gaining mass (e.g. slowing down). The stronger the appeal (the coupling), the larger the mass. Prof. Higgs gets more mass from physics groupies than punk rock afficionados.

12. 24 Experimental constraints on Higgs mass Many observables of the Z and W bosons, top quarks etc. depend on the mass of the Higgs boson. These measurements indirectly constrain the Higgs to be <163 GeV (at95% C.L., in the context of the SM). The set of all indirect measurements give the blue c2 curve. The yellow region below MH=114 GeV is ruled out from direct Higgs searches at the LEP e+e- collider. Recent measurements of the W boson and top quark masses require Higgs mass to be low. In fact MW – Mtop and direct Higgs searches are now pushing toward disagreement with SM

13. 23 The Standard Model is flawed • The SM does not protect the Higgs (or W,Z) mass from becoming very large (Planck scale); without extreme fine tuning of parameters, need some new physics at TeV scale. • The SM contains 26 arbitrary unexplained parameters – particle masses, force constants, mixing matrix of quark and lepton states. • The SM does not allow for unification of QCD and EW interactions; postulated new symmetries at TeV scale would allow this. • The SM does not have sufficient CP violation (symmetry of particles and antiparticles and mirror inversion symmetry) to explain the observed dominance of matter over antimatter in the universe. • The SM does not have a candidate for dark matter observed in the universe; new physics can provide this. • The SM does not incorporate gravity, and says nothing about dark energy Thus, despite the 100’s of measurements that agree with the SM, we strongly expect new physics beyond the SM. The new physics should occur at the TeV energy scale.

14. 22 The Tevatron Counter-rotating p and p, colliding at 1.96 TeV. Protons are bags of quarks and gluons, so in effect the Tevatron is a quark-antiquark collider ( 15% of collisions are gg/ gq) with qq energy up to ≈ 1 TeV. Two experiments CDF and DØ will operate until the LHC program is producing physics results (2011, 2012?) CDF DØ 2 km Collision rate = luminosity x s Peak luminosity is ~300 mb-1/s-1. Expect ∫L dt ~10 fb-1 by the end of the program, so for Higgs production with s≈100 fb, will produce ~1000 events.

15. 21 DØExperiment Operating since 1992; major upgrade in 2001.

16. 20 The LHC The Large Hadron Collider (LHC) at CERN will collide protons with protons at 14 TeV. This will provide collisions of the constituent quarks and gluons to about 5 TeV (dominantly gg collisions). General purpose experiments ATLAS and CMS. LHCb will focus on b-quark physics; ALICE on heavy ions; TOTEM on small angle scattering. First collisions are now expected in fall 2009; first physics results in 2011?? Mt. Blanc Lake Geneva ATLAS 8.6 km CMS Luminosity should reach 10,000mb-1 s-1 (~30x Tevatron) The LHC will reach the energy scale where current experiments tell us that new physics should surely exist – LHC is the primary discovery vessel.

17. electron hadron (p+, p etc.) photon muon particles quark 19 Identifying particles calorimeter track chambers • electron – seen in track chamber; calorimeter shower builds and dies quickly. • chargedhadron – seen in track chamber; calorimeter shower builds and dies slowly • photon – not seen in track chambers (no charge); calorimeter shower like electron • muon – seen in track chamber; observe muon chambers behind calorimeter • neutrino – undetected, but inferred from lack of momentum balance in the event • jets – quarks and gluons do not emerge freely, but ‘fragment’ into sprays of particles within a small cone

18. q W/Z W/Z q H Tevatron: pp at 2 TeV (dominantly qq collisions) g t H g q’ q W+ H W- q q’ 18 Higgs production 3 main production mechanisms at hadron colliders: gluon-gluon fusion (gg) associated VH production (V=W/Z) vector boson fusion (VBF) Higgs cross sections are small (~ 100 fb) compared to other processes (the total inelastic pp collision XS is 1015 x larger). This gives rise to a severe needle in the haystack problem! Must reject most of the 105 collisions in a bunch crossing in real time, and then dig out the few Higgs events offline. 1 pb 

19. 17 Higgs decay Higgs couples to particles in proportion to their mass, so the heaviest kinematically accessible pair dominates. At MH < 135 GeV, 90% of decays to bb. Above 135 GeV, HWW takes over. In gg fusion, the H  bb decay is swamped by background from copious QCD production of jets, so for low mass searches must use the associated VH production with V (W or Z) decaying to leptons – infrequently produced by QCD. 80 200 MH (GeV) For MH>135 GeV, use HWW For MH<135 GeV, use Hbb When H  WW decay becomes possible, the W’s can decay to leptons (e, m, t) and neutrinos and the gg fusion (and vector boson fusion) process with larger XS can be used for the high mass search.

20. 16 Higgs search at the Tevatron Low mass channels VH production WH: W→en, mn, tn and H→bb WH: W→qq and H→tt ZH: Z→ee, mm, tt or nn and H→bb ZH: Z→qq and H→tt At the upper end of the low mass region WH : W→ln and H→WW* (both W’s to ln), so 3 lepton final states. Some sensitivity from ttH: H→bb (4 b final state) gg fusion the low background H→gg & Htt +2jets VBF Htt+2 jets. For the low mass (MH<135 GeV) search: The most sensitive channels are Z(nn)H(bb) and W(ln) H(bb). But to gain sensitivity, use all these channels (and more to come) and combine limits from them all. For high mass (135<MH<180 GeV), where the H → WW* branching ratio dominates, we become sensitive to gg fusion reaction with leptonic decays of both W’s (so far, ee, em, mm).

21. jet1 MET jet2 15 A real world Higgs search Take as a case study, a DØ search for associated ZH production with Z  nn, H bb (the highest yield single channel for low mass Higgs). Final state is 2 b-jets, missing ET (MET) together with other particles produced from spectator quarks, extra interactions, gluon radiation etc. (beam’s eye view) • The primary backgrounds are : • W+2 b jets with W  ln and the lepton not identified • W+2 light jets with W  ln, light jets faking b-jets and unseen lepton • t anti-t quark pairs • Z+2 jets (Znn) • di-boson production; e.g. ZZ with one Znn and one Zbb • multijet QCD production with mismeasured jets to give fake MET

22. 14 Z(nn)H(bb) search Triggering: To keep bandwidth to tape at ~100 Hz, DØ must select one interesting event in 105 collisions on each beam-crossing in real time. Typically use dedicated electronics to make primitive recognition of electrons, muons, jets, etc. These primitives are collected in successive stages for more inclusive event recognition. For the ZH search, trigger on events with large MET and 2 acolinear jets. • Offline event pre-selection: • no observed e orm • 2 or 3 jets pT > 20 GeV; leading 2 jets not back to back in f. Jets must have enough reconstructed tracks to allow for b-tagging in final selection. • MET> 50 GeV; require MET in calorimeter to be roughly aligned with MET from tracks (suppress bad measurement of jet in calorimeter).

23. secondary vertex secondary vertex primary vertex 13 Z(nn)H(bb) search Final event selection: b quarks fragment to long lived hadrons and can be identified by seeing a displaced vertex within the jets. Require at least two b-tagged jets using precise measurement in the with inner silicon vertex detector. Higgs signal and primary physics backgrounds are simulated using theoretical cross sections and Monte Carlo (MC) event generators. These events are passed through a simulation of the detector to give the observed energies and angles of particles, and then through the same event reconstruction programs as used for data. The backgrounds arising from mismeasurement of multi-jet events produced by QCD interactions are difficult to simulate, and are estimated directly from the data using events in which MET in the calorimeter does not agree with that seen in the tracking detectors.

24. 12 Z(nn)H(bb) search Compare data and MC for many kinematic distributions to validate analysis Before b-tagging (Higgs signal X500) After b-tagging (Higgs signal X10) Dijet invariant mass MET • The data is well modelled by MC • Signal is small compared to backgrounds (x500 and x10 in plots) • Dominant background before b-tagging is W+light quark jets • Dominant background after b-tagging is W/Z+bb and ttbar • Simple cuts to eliminate background will throw out much of the signal

25. 11 Multivariate classifiers Cuts on single variables to distinguish signal and background reduce yield too much. Thus use a multivariate technique where a large number of variables, each with some degree of signal/background discrimination, are fed to an artificial intelligence learning algorithm that recognizes correlations of the variables for signal and background samples. Neural Networks are often used; this analysis uses a Boosted Decision Tree (BDT). In BDT, create a tree structure using MC signal and background. Split sample into signal and background-like at each node (leaf) based on best discriminating variable. Iterate by reweighting events to help reclassify mis-assigned signal and background. Final output is the probability for being signal-like at the final leaf. Decision tree output for 115 GeV Higgs (red-line =signal x25), data (points) and background expectations (histogram). Signal and background are reasonably well separated in this variable.

26. Our real exp’t Signal like Bknd like 10 Calculating limits Use the BDT output to form a negative log likelihood ratio (LLR) that compares data (D) to S+B hypothesis and background-only hypothesis Simulate 1000’s of MC experiments (Poisson statistics for D, S, B), varying all the parameters (e.g. jet energy scale, XS uncertainty) within Gaussian errors. Using the ensemble distribution of simulated experiments assuming (a) B only and (b) S+B, determine the expected LLRS+B and LLRB. Calculate the confidence levels (CL) for data to conform to the LLRB and LLRS+B distributions. CLS = 1 – CLSB/CLBgives the confidence level that a signal is present. The CLB term guards against cases where the result resembles neither background only or signal plus background. In example, CLSB ~ ¼ ; CLB ~ ¾ ; CLS ~2/3 corresponding to ~1s evidence.

27. 9 Z(nn)H(bb) result Typically the distinction of S and S+B hypotheses is weak, so to get limits, scale up the signal artificially to reach CLS = 0.95. We quote the result as the ratio of this 95% CL excluded cross section to that expected in the SM. Limit obtained from the Z(nn)H(bb) channel as a function of MH, relative to the SM cross section. At MH=115 GeV, set a limit at about 7.5XsSM. Sensitivity worsens as MH increases due to sHiggs decrease. Get 95% exclusion when ratio reaches 1 (This analysis is based on ~20% of final expected data sample)

28. 8 Sanity check How can we verify that all this multivariate analysis, limit setting etc. work properly? We use the same machinery to search for production of WW and WZ in a final state ln qq (similar to the WH going to ln bb). WV production is a SM process with a known small (16 pb) cross section and in this channel, large background. The red portion shows the expected signal on top of the background (dominantly W or Z plus jets). The bottom panel shows the remaining signal after subtracting background (data points), the expected SM signal (red) and the ±1s band on background uncertainty (blue). The measured result, s = 20.2±4.5 pb is in good agreement with the SM, and with previous measurements in the much cleaner four lepton final state.

29. 7 Tevatron combined result For both the DØ and CDF experiments, combine all the distinct Higgs search channels to get a combined Tevatron limit. Plot shows the ratio of the 95% CL excluded Higgs cross section to the predicted SM value. Dotted line is expected limit ratio from MC experiments. Green/yellow bands are 1 & 2s uncertainties in expected limit. Solid line is observed limit ratio. Tevatron now excludes Higgs boson for 160<MH<170 GeV.

30. 3s exclusion 6 New Higgs boson constraint The Tevatron is closing in the upper end of the previously allowed window on the Higgs boson mass from indirect measurements. For the more favored lower masses, the current Tevatron exclusion limits are still a factor 2 – 4 above the SM prediction, so more data and improved analyses should be able to exclude any Higgs mass (if not chosen by Nature) from the current LEP limit of 114 GeV up to 180 GeV. Newly excluded SM Higgs region from Tevatron

31. CDF+D0 5 Prospects at the Tevatron If no improvements are made, we would expect the limit to scale as square root of integrated luminosity. In fact, the limits have improved faster than this – using refined analysis techniques, more production and decay channels. Tevatron will run through 2010 (~9 fb-1 of data) and may run in 2011 (12 fb-1). Luminosity needed for 95% exclusion and for 3s evidence vs. MH Red line shows the luminosity needed for 95% CL exclusion as a function of MH. With 9 fb-1 (blue line), can exclude SM Higgs up to 190 GeV. The black line shows luminosity needed for 3s evidence of Higgs as a function of MH. Can find evidence for MH≈115 GeV and 150<MH<180 GeV. 9 fb-1

32. Luminosity needed for – 5s discovery – 95% CL exclusion 4 Prospects at the LHC The Large Hadron Collider at CERN will turn on later this year. For low mass Higgs, the favored Hbb decay is swamped by background at the LHC, so must resort to the rare Hgg decay. At higher (disfavored in SM) Higgs mass, the LHC can more readily discover the Higgs through its decays to WW or ZZ. 10 fb-1 The LHC detectors will find (or rule out) the SM (or more complex) Higgs if it exists at any mass below 1000 GeV. But the favored low mass region is the most difficult at LHC, so if Higgs is there (≈115 GeV), a combination of Tevatron and LHC may be needed. 1 fb-1 CMS and ATLAS 0.1 fb-1 MH

33. 3 What if we don’t find Higgs? If the Tevatron and LHC fail to discover the Higgs boson in the mass region indicated in the SM framework, we will have ruled out the Standard Model and new physics is assured. Something else will then be needed to provide the symmetry breaking that takes the Electroweak force into its separate EM and Weak components. Even if we find the Higgs, the flaws of the SM indicate the need for new physics. Candidate theories Beyond the SM exist: Supersymmetry, new strong interactions (new QCD like forces), or extra spatial dimensions curled up to be small and so far unobserved. Manifestation of the new physics should be seen at LHC. Each of these new theories predicts observable particles and observables that will signal which is at work. What kind of new physics is there is not predicted theoretically, and each model type has many variants. Only experiment can make the observations that tell us what Nature has chosen.

34. 2 The voyage of discovery Grand Unification Gondwandaland EWSB- land Bay of SUSY Cliffs of Dark Matter Planck Dragon Beagle n Oscillations Dark Energy Maelstrom Quark mixing Mare SM nCP B≠B e→m Muon g-2 The Flavor Archipeligo GZK Atolls Gravitational Waves Quark-gluon plasma perfect liquid Map fragment reconstructed from demented theorist wanderings in the new world

35. Conclusion • Our Standard Model of subnuclear particle interactions requires a Higgs boson to break the Electroweak symmetry and give mass to all elementary particles. • Searches for the SM Higgs are nearing full coverage of the allowed region, and the Tevatron or LHC should sight it. • Failure to find the Higgs boson will tell us that the SM is incorrect. We have indications already that this may be so. • The next several years of experiments will map the Terascale (TeV energies) and tell us what the new terrain looks like.