Directions in High Energy Physics

UCSB Physics Department Retreat

September 20, 2000

Jeffrey D. Richmanfor the Experimental HEP Group

Outline

• Big questions/overview – perspective– big questions

• UCSB HEP group– people– projects

• Prospects and conclusions

Particle physics in perspective

• We are entering a new era– Most objects so far studied have masses in the range

• 0 < M < 10 GeV• u, d, s, c, b (quarks); e, , (charged leptons); e, ,

(neutrinos); , gluons (gauge bosons)

– Important exceptions!• M(W) = 80.4 GeV M(Z) = 91.2 GeV M(t) = 174 GeV

– These heavy particles are indicative of a new mass scale that we are just beginning to explore:

• 0.1 TeV - 2 TeV

– Another mass frontier: very light neutrinos. Tiny differences in M

2 accessible via oscillations! Super-K has strong evidence for disappearance.

An experimental perspective• e+e- storage rings (recirculating beams stored 1-2 hours)

– CESR (10 GeV=>bbar) -- PEP-II, KEKB (10 GeV, 2 rings, multibunch op)– LEP/LEP2 (91->208 GeV; Z physics, W+W- production)

• e+e- linear colliders (need to reduce synchrotron rad., single pass collisions, tiny beam cross sections, polarized beams)– SLC (91 GeV=> Z physics) – NLC, JLC, TESLA (all under consideration) (>500 GeV)

• hadron colliders (“discovery machines” ?)– p pbar FNAL Tevatron + facilities for pbar production and cooling (2 TeV; start 4/01)– pp LHC at CERN (strong US participation) (14 TeV)

• neutrino sources– decay: new US experiments: MINOS (760 km), MiniBooNE (short baseline)– atmospheric, solar (Super-K,SNO,…)– muon storage ring (50 GeV beam; under consideration); get and e

• special underground labs-dark matter, proton decay

BaBar/PEP-II at SLAC

PEP-II: highest luminositystorage ring in the world.

-asymmetric energies-multibunch operation

TeV Scale Physics: CDF (Fermilab)

Big Questions

Big questions

• What is the origin of electroweak symmetry breaking? What sets the values of particle masses?

• Are there supersymmetric partners to the particles? Are there SUSY partners with masses below the 1 TeV scale?

• Can matter-antimatter asymmetries (CP violation) be explained within the framework of the SM? Is new physics required? What explains the matter-antimatter asymmetry of the universe?

• What is the pattern and origin of neutrino masses and mixings?• Are there sterile neutrinos?• Is there CP violation in neutrino interactions?• What makes up non-baryonic dark matter (= weakly interacting

massive particles?)• Are quarks and leptons composite particles?

…more big questions

• Are there additional, very heavy gauge bosons?• Are there observable extra dimensions? Is there gravitational radiation

in high-energy collisions?• Why is CP violation absent in the strong interactions?• What is the lifetime of the proton?

These questions, when translated into an actual research program, typically expand by a factor of 10-100.

Electroweak Symmetry Breaking

• What is the mechanism that gives weak gauge bosons (W, Z) masses of order 100 GeV, while leaving the photon massless? – EW interactions in the SM are based on SU(2)XU(1) gauge

symmetry, which all mass terms violate. Masses can appear because new interactions cause this symmetry to be spontaneously broken.

– What is the nature of the Higgs sector? Is SUSY involved?

– Minimal model: single Higgs boson (scalar), but actual sector

could well be much more complex.

– How many Higgs bosons are there? Is the Higgs composite?

– Do Higgs bosons couple to fermions in the expected pattern?

Supersymmetry• Supersymmetry protects the masses of scalar particles (Higgs)

from enormous loop corrections without the fine tuning of parameters (solving the “hierarchy problem”). Divergences are cancelled by presence of both fermions and bosons in loops.

• SUSY doubles the number of particles and leads to a complex phenomenology:– Spin 1/2 particles in the SM have spin 0 super-partners– Spin 1 particles in the SM have spin1/2 super-partners

• SUSY must be broken (superpartners have not been observed). The Higgs mass scale becomes proportional to SUSY mass scale rather than Planck scale.

An experimental SUSY program• Is it really SUSY?

– New particle quantum numbers, spin, statistics– identication of complete SU(2)XU(1) multiplets– SUSY relation of coupling constants

• Major spectrum parameters– gaugino/Higgsino mixing– gaugino mass ratios: m1: m2: m3

– flavor universality of q, lR, lL masses?– Q:lR:lL mass ratios– signatures of gauge- or anomaly-mediation– signatures of R-parity violation

• Third generation and EWSB– determination of , tan– mixing of L/R partners for t, b, – h0 mass– H0, A0, H+ masses and branching ratios

• Precision effects...

(From M. Peskin, Physics Goals of a Linear Collider)

HEP group: where are we now?

• We already had to examine our future--one year ago! Strong and clear group consensus on what we needed to do.

• Exploration of the TeV energy scale is the next major goal of HEP. Enormous potential for major discoveries.

• Successful senior-level hire: Joseph Incandela from Fermilab. Joe is a leader on two major experiments at the energy frontier, CDF and CMS.

• Our technological expertise in high-precision silicon tracking systems is highly relevant to LHC physics!– Large number of tracks=> need fine segmentation – b-quarks may be important in Higgs/SUSY processes

HEP group: where are we now?

• The group now has 4 faculty (Campagnari, Incandela, Nelson, and Richman)

• The group includes about 27 people total, including students, postdocs, staff physicists, engineers, and technicians.

• A junior-level search is approved for this fall; we should have no problem attracting top-quality person. Priority is to strengthen the TeV physics effort.

• We had very good support from UCSB on the Incandela search.

HEP Group: current program

Heavy-quark physics; CPviolation

BaBar experiment at SLAC;started 1999, operational until>2010 (?)

Campagnari, Richman

TeV scale: SUSY/Higgs

CDF experiment at Fermilab;starts 2001, operational until LHCexperiments overtake (2007-8).Ecm=2 TeV.

Incandela + assistant prof + ?Dark matter search: WIMPS

CDMS I at Stanford; CDMS II atSoudan deep underground sitenow under construction;operational until at least 2006

Nelson, Caldwell (emeritus)

TeV scale: SUSY/Higgs

CMS experiment at LHC; nowunder construction; operational2006-2020 (?). Ecm=14 TeV.

Incandela+assistant prof+ ?

Matter-Antimatter Asymmetry

• Can CP violation be explained within the framework of the standard model, or are these effects due to new physics?

– Some CP asymmetries in the B-meson system are expected to be of order unity in the SM! (Compare to 10-3 in kaon system.)

– This year: race between BaBar and Belle (Japan) to obtain first sensitive CP asymmetry measurements in B system. In reality, these are long-term programs.

È(B ! f )6=È(Bö ! fö)

CP Violation in the B meson system

Amplitudes can carry weak (CP violating) phases from the CKM matrix in the SM or from new physics.

Such phases change the sign of the interference for particle and antiparticle decays.

BaBar Silicon Vertex Tracker

BaBar/PEP-II Data Taking

BaBar Event Display (fisheye view)

Cryogenic Dark Matter Search (CDMS)• CDMS-I pilot experiment in Stanford Underground Facility• CDMS-II now under construction; to be installed in Soudan mine• Detectors from LBNL and Stanford; UCSB is providing DAQ

system, passive and active veto shields.

Expectations from CDMS-II

TeV Scale Physics at the LHC

How do experiments happen?• Many particle experiments today are capable of a very

broad range of physics studies.– In this sense they are like observatories, but the initial

conditions are controlled.– However, HEP experiments are built by their users, who also

calibrate, maintain, and operate the detectors. – The UCSB group is unusual in maintaining the ability to

construct sophisticated detectors.– It takes 2-3 years to fully understand a new detector. Some

physics results can be produced in the first year; others require a much more refined understanding of the detector..

– Although the collaborations are becoming very large, most physics results are produced by groups of 3-10 people.

Comments

• The department must be strong in all of our research areas.

• Many groups are near critical mass; relatively small downward fluctuations can create major problems.

• Growth of the Physics Dept from 30 to about 40 should have many benefits, including an even better departmental reputation.

Conclusions/Prospects

• The Incandela hire is almost optimally matched to our goals:– the new experiments at TeV scale diversify our program and address the

driving questions of HEP– the required hardware expertise is well matched to our group– we will establish very high profile efforts right from the start

• The main area in which we have no effort is neutrino physics.– This is clearly an exciting and rapidly developing area.

• However, we have just added two new experiments to our group, and we believe that our first priority should be to strengthen these new efforts.