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IIT October 2002M. Oreglia 1 Searching for the Origin of Mass IIT 31 October 2002 Outline: The Standard Model; Higgs Boson ElectroWeak Corrections: a.

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Presentation on theme: "IIT October 2002M. Oreglia 1 Searching for the Origin of Mass IIT 31 October 2002 Outline: The Standard Model; Higgs Boson ElectroWeak Corrections: a."— Presentation transcript:


2 IIT October 2002M. Oreglia 1 Searching for the Origin of Mass IIT 31 October 2002 Outline: The Standard Model; Higgs Boson ElectroWeak Corrections: a Signal! Higgs Production Hints of a Direct Signal Future Prospects Mark Oreglia The Enrico Fermi Institute, The University of Chicago

3 IIT October 2002M. Oreglia 2 Thats because the Higgs particle is a manifestation of a field which could explain the mechanism by which all particles get mass. One of IITs illustrious faculty feels the Higgs boson is so important it deserves the monikerGod particle. It is not yet clear that the Higgs boson exists.

4 IIT October 2002M. Oreglia 3

5 IIT October 2002M. Oreglia 4 Explaining the Standard Model (thanks to Claus Grupen!)

6 IIT October 2002M. Oreglia 5 Electro-Weak Physics We had a great theory : QED … now add W,Z WW cross section alone shows the need for W, Z:

7 IIT October 2002M. Oreglia 6 The Standard Model It was so simple before 1968: –QED: massive fermions –massless photon; no because it violates Gauge invariance … –…which we like because it gives rise naturally to conserved charges Attempt to unify QED w/ Weak Force: –additional bosons known to exist –but cant just add boson mass terms –The trick of Weinberg and Salam: add scalar field –Two cases: 2 > 0 : not interesting 2 < 0 remarkable!

8 IIT October 2002M. Oreglia 7 Symmetry Breaking For <0, min V at = v Expanding about this minimum gives rise to new structure: –bosons: 1 massless neutral ( ) 1 massive neutral (Z 0 ) 2 massive charged (W –and a left-over scalar: H –boson masses show the sym. breaking! Lagrangian now has mass terms: –M 2 W = ¼ g 2 v 2 –M 2 Z = ¼ (g 2 +g 2 ) v 2 –M –M 2 H = 2 v 2 –… and v = 246 GeV Fermion masses i v (Yukawa couplings) H-fermion coupling v -1 m f h –Higgs field is a mass generator!

9 IIT October 2002M. Oreglia 8 Philip Anderson On occasional forays outside CMT I worked on pulsar glitches with David Pines and invented the "Higgs" boson in 1962. We really should credit this Condensed Matter physicist with the discovery of the basic SSB mechanism. From his Princeton Research Description: … but he has often been an outspoken opponent of HEP programs

10 IIT October 2002M. Oreglia 9 Things are even better in a theory having 2 Higgs fields ( and with SUSY!) … MSSM –with SUSY, get exact cancellations without fine tuning problem –family of 5 Higgs particles or more The MSSM model, e.g., gives masses and rates in terms of a fairly small set of fundamental parameters –5 or >100, depending on symmetry! H field couples to mass creates a mechanism for generating masses in the theory H boson cancels divergences SM without Higgs boson has WW scattering cross section violating unitarity Unifies EM and weak forces

11 IIT October 2002M. Oreglia 10 The Higgs Mass Not specified explicitly … but restrictions are imposed by the structure of gauge couplings is a running coupling (i.e., function of the energy scale Q) –large- : –and so can be calculated if you fix a high energy cutoff scale – finite gives an upper bound on m H (Landau Pole) –small- : – the t-quark Yukawa coupling is large –quartic coupling positive gives a lower bound on m H Thus m H = 180 GeV at unless: –there is new physics on the way

12 IIT October 2002M. Oreglia 11 Theoretical Mass Bounds At current energy scale, m H can cover a large range Current limit

13 IIT October 2002M. Oreglia 12 … because there is a very solid body of circumstantial evidence that a Higgs-like object exists and is perturbing some high- precision measurements we have recently made

14 IIT October 2002M. Oreglia 13 Indirect Evidence of the Higgs The existence of a Higgs-like object with mass on the order of 100 GeV is suggested by several precision electroweak measurements Higgs boson modifies the Z propagator … and decay vertices small correction (~ 0.1%) ~ ln(m) largest effect in angular distributions of Z-decay products The WW cross section also requires the existence of a Higgs-like object We are psyched for a Higgs discovery!

15 IIT October 2002M. Oreglia 14 Electroweak Observables W mass Z widthTau Asymmetry Electron asymmetry

16 IIT October 2002M. Oreglia 15 Direct and Indirect Limits Radiative corrections imply m H = 94 GeV … or m H < 210 GeV at 95% CL

17 IIT October 2002M. Oreglia 16 How to Search for it The Higgs particle(s) couple to fermion pairs … strength goes like the fermion mass … So collide the heaviest particles you can and look for appearance of the new bosons In a perfect world we would have a top- or bottom-quark collider … this would have the highest production rate –But proton colliders are the only practical quark machines, and there are few b quarks in the proton Leptons are clean and simple –A muon collider would make a pretty nice Higgs factory … –But it is a difficult machine to build and we are waiting on Dan Kaplan … –Settle for electron colliders

18 IIT October 2002M. Oreglia 17 Accelerators We can Consider LEP – Large Electron Positron Collider –up to E cm = 209 GeV Fermilab TeVatron –proton-antiprotons @ 1 TeV LHC – Large Hadron Collider –proton-proton @ 14 TeV –starts 2008 LC – electron Linear Collider –electron-positron 0.5-1.3 TeV –conceivable 2012ish MSR – Muon Storage Ring –a tremendous technical challenge –perhaps staged as a neutrino factory first … towards end of next decade

19 IIT October 2002M. Oreglia 18 LEP --The Large Electron Positron Collider 27 km circumference electron synchrotron supported 4 experiments simultaneously initiated in 1989 at E cm = 91 GeV … upgraded over the years with additional superconducting RF cavities... reached 209 GeV in summer of 2000 … and is now dismantled On mange bien ici

20 IIT October 2002M. Oreglia 19 The 4 Logos ALEPH fine grained DELPHI high tech L3 hi-res photon OPAL general purpose, conservative group quite superb, really

21 IIT October 2002M. Oreglia 20 Higgs Production at LEP Higgs mass (GeV)

22 IIT October 2002M. Oreglia 21 MSM Higgs Decays Remember … H couples to mass! It predominantly decays to the heaviest particles

23 IIT October 2002M. Oreglia 22 Decay Topologies

24 IIT October 2002M. Oreglia 23 Most Common Event Expected

25 IIT October 2002M. Oreglia 24 B-Tagging The heavy b decay products have long lifetime, and can travel centimeters before decaying

26 IIT October 2002M. Oreglia 25 4-fermion Backgrounds is an irreducible background for m H near m Z, and it caused trouble in 1998. Beyond m=100 GeV, backgrounds are low.

27 IIT October 2002M. Oreglia 26 LEP Has Benefited from Many Higgs Discoveries! ALEPH, 1992, E cm = 90 GeV –M h =58 GeV L3, 1992, E cm = 90 GeV –M h =60 GeV ALEPH, 1996, E cm = 133 GeV –M h +M A =105 GeV ALEPH, 1998, E cm = 189 GeV –M h =102 GeV ALEPH, 1999, E cm =196 GeV –M h =105 L3, 2000, E cm =206 GeV –M H +=68 GeV

28 IIT October 2002M. Oreglia 27 Even OPAL Should Know Better

29 IIT October 2002M. Oreglia 28 Y2K LEP Performance 2000: the maximal LEP upgrade done Two run modes: –Maximum Energy Mode 209.14 GeV attained but no klystrons in reserve; if a klystron trips (every 15-60 minutes), the beam is lost refill time: 20-90 minutes –Maximum Production Mode keep 1 klystron in reserve 205-207 GeV fills last 3 hours yields 2-4 pb -1 per week HE running stresses LEP: –severe pitting in RF cavities –… but reconditioning worked

30 IIT October 2002M. Oreglia 29 SM Higgs Cross Section For m=115 GeV, L=150 pb -1 : about 6 events/expt for 100% efficiency. More like 3 really! Higgs Mass (GeV)

31 IIT October 2002M. Oreglia 30 How We Spent Summer 2000 The LEP accelerator was scheduled to close down at beginning of September run extended to 30 Sept after a LEPC meeting held on 20 July a meeting of the LEPC was scheduled for 5 Sept to decide if any hint of new physics could justify an extension. 4 events (3 from ALEPH, 1 from DELPHI ) were recorded between 20 July and 5 Sept. Three events are perfect candidates for Higgs boson decays. Run extended to 2 Nov. 10 Oct LEPC: No new candidates –LEP shuts 2 Nov … for this year. 3 Nov LEPC: Big decision time –hoped that HE data would double –… but -1 new candidates –DG declares LEP will be dismantled

32 IIT October 2002M. Oreglia 31 Best ALEPH event e + e - Z H, Z qq, H bb (85%) m H = 115 GeV

33 IIT October 2002M. Oreglia 32 What ALEPH saw Sept 5 (one representative plot) The mass plot does not tell the whole story … … as I will now elucidate s/b = 2.0 m>109 GeV: 0.3 bkg 0.6 signal 3 observed c b significance= 3.5

34 IIT October 2002M. Oreglia 33 The Next Steps The events are only in one channel (4-jet) –ok... low statistics Mostly from one experiment –again, low statistics Weighted by the significance of the b-tagging –2 of the ALEPH events are gold plated At this high mass, very little background is expected, but just how well modeled is it? –more to the point, have enough MC events been processed to get smooth background and signal curves ? It was hoped that we could double the data at E cm =207 by running an additional month The LEP-wide HIGGS working group was instructed to create a team which could combine the data from 4 experiments rapidly, and cross-check results

35 IIT October 2002M. Oreglia 34 How E cm Helps Shifting from 205 to 206 GeV boosts the production rate for m H =115 considerably Higgs Mass (GeV)

36 IIT October 2002M. Oreglia 35 Analysis Procedure The plot of candidate masses only shows a part of the total information In searching for the H 0 in the context of the Standard Model theory, we also have distributions in the various decay channels Each channel has different: –backgrounds –mass resolutions Individual events should be weighted for quality and systematic uncertainty –the likelihood of the b-tag quantified –mass reconstruction quality –likelihood of kinematic cuts –… and the S/B All this info should go into CL But everybody wants to see mass plots, so we show them for variations of the cuts which yield different minimum S/B

37 IIT October 2002M. Oreglia 36 ADLO Mass Plot S/B > 0.3 S/B > 1.0 S/B > 2.0 Reconstructed mass (GeV)

38 IIT October 2002M. Oreglia 37 Even Some ALEPH Skeptics! (Thanks again, Claus Grupen!)

39 IIT October 2002M. Oreglia 38 Statistical Procedures We are looking for the Higgs within a very specific theory The mass plot does not convey all information (i.e., separate decay channels, amount of background, etc) So we need a statistical treatment

40 IIT October 2002M. Oreglia 39 Signal, Background Hypotheses Construct a parameter that orders outcomes as more signal-like, or less signal-like For hi-stat, -2lnQ is distributed like 2 (From Alex Read)

41 IIT October 2002M. Oreglia 40 For some test masses: M test = 110 GeV M test = 116 GeV M test = 120 GeV

42 IIT October 2002M. Oreglia 41 In Terms of Test Statistic: Bckgrnd- like Signal-like The data, expected background, and signal are binned in reconstructed mass channels for the likelihood computation include: each decay channel each experiment each signal mass hypothesis NB: analyses, cuts may be m H dependent

43 IIT October 2002M. Oreglia 42 For M test = 116 GeV: blue CL(s+b) area: compatibility of LEP combined measurement with s+b hypothesis define CL(s) as CL(s+b)/CL(b); rule out signals at the 95% confidence limit if CL(s)<0.05

44 IIT October 2002M. Oreglia 43 So we set: Upper Limit

45 IIT October 2002M. Oreglia 44 Where Do We Go From Here? Next experiments: –Fermilab TeVatron run II higher luminosity than before –15 inverse fb by 2004? More like 2006 now upgraded detectors (2) –CERN Large Hadron Collider high field magnets in LEP tunnel pp collisions at E cm = 14 TeV Turn-on in 2006 2007 2008 1 inverse femtobarn first year x10 luminosity in 2007 The CERN director decided there would NOT be LEP running in 2001 VERY expensive: LHC contractor penalties The Large Hadron Collider is delayed by more than 1 year

46 IIT October 2002M. Oreglia 45 LEPs Difficult Legacy A 115 GeV Higgs is difficult to see Fermilab TeVatron and LHC: large backgrounds 2-3 years of good running for CDF/D0 Not so easy for LHC either … 2009?

47 IIT October 2002M. Oreglia 46 TeVatron Best bet While gg =1pb, the QCD background is 10 6 The hW and hZ allow mass-tag of W,Z Using all channels, they can probably just get up to 115 GeV mass reach in 4-5 years ??? So far…not off to a good start… not meeting luminosity goals

48 IIT October 2002M. Oreglia 47 Hadron Collider Dominant Process

49 IIT October 2002M. Oreglia 48 Highest energy and rates yet Expect solid sighting of Higgs

50 IIT October 2002M. Oreglia 49 LHC Golden Process The rates at LHCs higher energy are larger, but so are the backgrounds Can afford to tag on very clean Z decays In this way, mass reach is up to 1 TeV

51 IIT October 2002M. Oreglia 50 LHC m=115 Clean Process The low 115 GeV Higgs mass is a problem for LHC (below ZZ threshold). Make some use of the relactively clean diphoton decays of the Higgs But there is background from QCD here too

52 IIT October 2002M. Oreglia 51 What LHC Can Do 2006-2007: 20-40 fb -1

53 IIT October 2002M. Oreglia 52 But to Really Clinch SSB: LHC will have the mass reach for discoveries, but to be truly convinced, we will need precision measurements –Higgs spin –HH and other unusual couplings –Higgs width A TeV-scale electron machine –must be linear…cant afford to bend –such a Linear Collider was selected by HEPAP as the next big project –really needs to operate while LHC is still running there is a complementarity of the two methods LC can help LHC through the parameter space, and LHC can tell LC where to focus

54 IIT October 2002M. Oreglia 53

55 IIT October 2002M. Oreglia 54 Conclusions We might be seeing the Higgs …but not at a discovery level Indirect evidence strongly suggests new physics in the energy range 100- 200 GeV FNAL has just started, LHC is on the horizon I am confident that this new energy scale of 100-200 GeV will be revealed by the end of this decade! This decade will see tremendous excitement in High Energy Physics!

56 IIT October 2002M. Oreglia 55 And I Hope You Arent Thinking …

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