Why Higgs Boson Searches?

Transcription

1 Why Higgs Boson Searches? Not just a search for a new particle... The missing piece in a spectacularly successful theory construct: the Standard Model of the fundamental interactions A massive experimental program to validate our (50 year old) picture of the Electroweak Symmetry Breaking (EWSB) spanning four generations of large colliders and their associated detectors Arthur Maciel, C. Jordão, SP (Jan. 2013) 0

2 Quote: Lev Okun closing talk Lepton-Photon 1981 Instead of giving a general overview of the prospects, I decided to choose and discuss in some details just one problem, which could be considered as problem No. 1 in particle physics. To be No. 1 this problem has to be theoretically advanced and urgent. It should also be experimentally accessible. It seems to me that the problem No. 1 of high energy physics are scalar particles. Painstaking search for light scalars should be considered as the highest priority for the existing machines... and even more so for the next generation of accelerators... Arthur Maciel, C. Jordão, SP (Jan. 2013) 1

3 Quote: M. Peskin closing talk Lepton-Photon 2011 A project of the magnitude of the LHC can only be created by an organization that goes beyond the usual human scale. Of course, it takes billions of dollars, huge technological expertise, and much effort in construction. But it requires more. The LHC was imagined in the early 1980s, and not realized until the 2010s. It required an institution whose goals could be coherent over that time period more than a generation constantly working with governments and the scientific community to move the project forward. It is a unique achievement. Our whole community must be grateful to CERN as an institution for making it possible. Arthur Maciel, C. Jordão, SP (Jan. 2013) 2

4 4 July 2012 non-vector It is a boson... (a first!) Is it the Higgs boson? Needs identity tests, e.g. spin, parity, couplings to fermions... Arthur Maciel, C. Jordão, SP (Jan. 2013) 3

5 14 December 2012 The United Nations General Assembly in New York today adopted a resolution granting CERN observer status. This status gives the Organization the right to participate in the work of the General Assembly and to attend its sessions as an observer. It s a great honour for CERN to accede to the status of observer at the UN General Assembly, said CERN Director-General, Rolf Heuer. CERN has a long tradition of close cooperation with the United Nations and its agencies, which dates back to 1954 when the Laboratory was founded under the auspices of UNESCO. Arthur Maciel, C. Jordão, SP (Jan. 2013) 4

6 The Higgs Boson Role on µ 2 φ φ + λ ( φ φ ) 2 Unitarity v φ plane References for calculations: C. Q. book chapter 6 P & S book page 750 Arthur Maciel, C. Jordão, SP (Jan. 2013) 5

7 A Bit of History (W.I.) And, jumping over to the late 90 s at LEP, finally, observation of e + e W γ Z e e + e e + (a) (b) The Standard Model view e ν e + H... and trilinear gauge couplings e at work e +... (c) (d) Arthur Maciel, C. Jordão, SP (Jan. 2013) 6

8 The Gauge Cancelation Consider e + e W, suppose m e = 0 three gauge interaction diagrams (pure J=1 amplitudes) all three with divergent high energy behavior γ Z but the divergences cancel out in the sum a major experimental test for (non-abelian) trilinear gauge couplings at work... Compare this process with ν e ν e W e e + e e + (a) (b) H ν The massless neutrino case is calculated in C.Q. sec.6.2 If you consider e s (with mass), the weird e R and e+ will add L to the NC interactions, and disturb the gauge cancellation e (c) e + e (d) e + Arthur Maciel, C. Jordão, SP (Jan. 2013) 7

9 And it Works Beautifully... σ WW (pb) LEP PRELIMINARY (c)-only add (a) add (b) 11/07/2003 γ Z e e + e e + (a) (b) 10 0 YFSWW/RacoonWW no ZWW vertex (Gentle) only ν e exchange (Gentle) s (GeV) e ν (c) e + H But... the e original e problem still + persists... and is connected (d) with MASS Arthur Maciel, C. Jordão, SP (Jan. 2013) 8

10 The S-Wave Amplitudes With gauge interactions only, and m e 0, J = 0 amplitudes also exist (e + L e R W+ l W l ) smaller, and vanishing in the limm e 0 but with unacceptable H.E. behavior M (J=0) diverges s γ Z This is the amplitude associated with the production of longitudinally polarized W s e e + e e + (a) (b) This residual divergence is canceled exactly by the H e + e and H W vertices. Note the m e dependence in f ν H f m f /v e e + e e + (c) (d) h Arthur Maciel, C. Jordão, SP (Jan. 2013) 9

11 The hidden symmetry at work Because electrons have mass (and therefore exist in mixtures of (e L ) and (e R ) components), a J = 0 partial wave exists which diverges as s for the production of longitudinally polarized boson pairs (e + e l W l ), and again will break the unitarity bounds, only slower, at much higher energies ( a few TeV ). This divergence is precisely cancelled by an added neutral scalar boson exchange (d), provided its couplings are proportional to the masses of the particles to which it couples. e γ Z e e + e e + (a) (b) H ν e + e e + In other words, it must be a Higgs-like scalar boson. (c) (d) Arthur Maciel, C. Jordão, SP (Jan. 2013) 10

12 The Higgs Boson is the Cure Fermion helicity flips (mixings) are proportional to the fermion masses and it is therefore necessary that the S-wave cure also be proportional to mass. Electroweak Symmetry Breaking (EWSB) is such that the potential problems created by the addition of a scalar Higgs boson are cured by its own coupling properties. γ Z e e + e e + (a) (b) This is a general result: the only theories of massive vector bosons with acceptable high energy limit behaviour are those that result from spontaneously broken gauge theories. See the full e + e W calculation in Peskin&Schroeder, p.750 e ν (c) e + e (d) H e + Arthur Maciel, C. Jordão, SP (Jan. 2013) 11

13 Where is this ptcle? What is its Mass?...we need a search strategy......we need all the hints we can get... Arthur Maciel, C. Jordão, SP (Jan. 2013) 12

14 Indirect SM Higgs Mass Constraints Tevatron (DØ+CDF, March 2012) M W = ± 15 MeV M t = ± 0.9 GeV M H < 152 GeV at 95% C.L. Arthur Maciel, C. Jordão, SP (Jan. 2013) 13

15 The W Boson Mass The (SM) W boson mass can be written as m 2 W = π α em 2GF sin 2 θ W (1 R) sin 2 θ W = 1 m2 W m 2 Z where three of the parameters are very highly constrained G F = (1) 10 5 GeV 2 α em (m 2 Z) = 1/ (18) m Z = (21)GeV where R stands for radiative corrections dominated by top and Higgs loops W b W W W W R m 2 t t H R lnm H = Within the SM the top and W masses drive a strict constraint on M H Arthur Maciel, C. Jordão, SP (Jan. 2013) 14

16 Tevatron Precision Measurements Precise knowledge of m W and m t will indirectly constrain M H These measurements have been at the forefront of the Tevatron program Relative sensitivity: M H m W 170 M H m t For equivalent constraints you want m W m t m W [GeV] March 2012 LEP EWWG 2012 LHC excluded LEP2 and Tevatron LEP1 and SLD 68% CL m W is the limiting factor in constraining M H, and the observable is m T W = [ 2 p l T pν T (1 cos θ lν) ] 1/ m H [GeV] α m t [GeV] Arthur Maciel, C. Jordão, SP (Jan. 2013) 15

17 Measurements of m W Time evolution of measurements ( TeV EWWG ) CDF Run 0/I ± D0 Run I ± CDF Run II ± Tevatron ± D0 Run II ± Tevatron ± LEP2 average ± World average ± m W (GeV) July 09 March 2012: CDF: m W = ± 19 MeV (arxiv: ) DØ: m W = ± 23 MeV (arxiv: ) World: m W = ± 15 MeV (TeVEWWG-prelim.) mass known to 0.02%! Arthur Maciel, C. Jordão, SP (Jan. 2013) 16

18 Measurements of m t 15 Mass of the Top Quark July 2011 (* preliminary) Tevatron, July 2011 Results GeV/c 2 M t δ(m t )stat ±0.56 δ(m t )syst ±0.76 δ(m t )JES ±0.49 δ(m t )total ±0.94 mass known to 0.54%! 0 CDF-I dilepton ± 11.4 (±10.3 ± 4.9) DØ-I dilepton ± 12.8 (±12.3 ± 3.6) CDF-II dilepton ± 3.8 (± 2.2 ± 3.1) DØ-II dilepton ± 3.1 (± 1.8 ± 2.5) CDF-I lepton+jets ± 7.4 (± 5.1 ± 5.3) DØ-I lepton+jets ± 5.3 (± 3.9 ± 3.6) CDF-II lepton+jets ± 1.2 (± 0.6 ± 1.1) DØ-II lepton+jets ± 1.5 (± 0.8 ± 1.2) CDF-I alljets ± 11.5 (±10.0 ± 5.7) CDF-II alljets * ± 2.1 (± 1.4 ± 1.5) CDF-II track ± 9.5 (± 9.0 ± 2.9) CDF-II MET+Jets * ± 2.6 (± 1.8 ± 1.8) Tevatron combination * ± 0.9 (± 0.6 ± 0.8) CDF March 07 χ /dof ± = /11 (± (68.5%) 1.5 ± 2.2) (GeV/c ) m top (± stat ± syst) Arthur Maciel, C. Jordão, SP (Jan. 2013) 17

19 m t m W Status, ICHEP 2012 Electroweak consistency check on whether the new boson is the Standard Model Higgs boson (thin blue line) or a minimal supersymmetric (MSSM) one (green band). The blue ellipse shows the current knowledge on mt and mw, whereas the black ellipse depicts what will happen if mw becomes known to 5 MeV. CERN Courier Sept Arthur Maciel, C. Jordão, SP (Jan. 2013) 18

20 M H Global Fit in the SM sin M Z Γ Z 0 σ had 0 Rlep 0,l A FB A l (LEP) A l (SLD) 2 lept Θ eff (Q ) FB 0,c A FB 0,b A FB A c A b 0 Rc 0 Rb (5) 2 α had (M ) Z M W Γ W m c m b m t G fitter SM AUG which mass best fits most precision measurements? sin M Z Γ Z 0 σ had 0 Rlep 0,l A FB A l (LEP) A l (SLD) 2 lept Θ eff (Q ) FB 0,c A FB 0,b A FB A c A b 0 Rc 0 Rb (5) 2 α had (M ) Z M W Γ W m c m b m t G fitter SM AUG M H [GeV] (O fit - O meas ) / σ meas Model/ (2011) Arthur Maciel, C. Jordão, SP (Jan. 2013) 19

21 Preferred Mass (SM) vs LHC Exclusion (March 2012) LEPEWWG: March 2012 at 68% C.L.( χ 2 = 1) M H = GeV Precision EW: M H < 152 GeV/c 2 at 95% C.L. χ March 2012 Theory uncertainty α had = α (5) ± ± incl. low Q 2 data m Limit = 152 GeV Pr.EW+LEPlim: M H < 171 GeV/c 2 at 95% C.L. 1 0 LEP excluded m H [GeV] LHC excluded In yellow, the experimentally excluded regions at 95% C.L. Arthur Maciel, C. Jordão, SP (Jan. 2013) 20

22 Higgs Boson Status (circa 2010) Manifest internal consistency in the SM at a very rigorous and precise level, over various independent experimental tests Indicating the theoretical need for a scalar boson (elementary?) Indicating the (virtual) existence of a scalar boson But... where is it? On to the search... Developing a strategy Arthur Maciel, C. Jordão, SP (Jan. 2013) 21

23 Developing a Strategy The various search strategies must necessarily come from a detailed analysis of the various production modes coupled with the various decay modes Generate a list of distinct final states according to the choice of observables [ These are: light jets, heavy jets, photons, leptons (e,µ, τ), E/ T (ν), H T ] all mutually exclusive for combining search channels in the final result... Arthur Maciel, C. Jordão, SP (Jan. 2013) 22

24 Detectors and Event Reconstruction Arthur Maciel, C. Jordão, SP (Jan. 2013) 23

25 Detectors (multipurpose) Arthur Maciel, C. Jordão, SP (Jan. 2013) 24

26 CMS, transverse sector Arthur Maciel, C. Jordão, SP (Jan. 2013) 25

27 Detectors Event Reconstruction Example reconstruction hierarchy: 1 st stage 2 nd stage 3 rd stage 4 th stage 5 th stage 6 th stage em e c W t physics jet γ b Z H E/ T ν τ hadrons µ prim.vtx. sec.v tx. tracks Arthur Maciel, C. Jordão, SP (Jan. 2013) 26

28 W ( )! ` 0 0 Example Signal/Bkgnd Discrimination (H W, W lν): dilepton events use [V-A] properties on scalar W system (l + l strong angular correlation differentiates higgs decays from background physics) ν ν = W = l + l charged leptons align up with their parent W bosons Other discriminants: dilepton invariant mass event missing E T lepton isolation etc... etc... fed to a multivariate technique g H 0! `+ Events/0.16 rad 10 4 data DO Preliminary L = 9.7 fb eµ + MET Z+jets Diboson 3 W+jets 10 Multijet 1 ttbar t φ eµ Arthur Maciel, C. Jordão, SP (Jan. 2013) Signal 1 (M = 165 GeV) H g