# Presentation: 108229

## New Particles and Phenomena

- Duke University
*for the CDF Collaboration*

- at CDF

- Searches for

**Notes:**
  Hello and welcome!  My name is Chris Hays, and today I will describe the results of searches for new particles using the first year of Run 2 data taken with the CDF detector.

## ( )

- 10-31 m

- Structure of Matter

- Distance Scale

- Interaction Time Scale

- 10-23 m

- 10-19 m

- 10-15 m

- 10-11 m

- Unitarity violation
- (WW scattering)

- SU(2)

- SU(3)

- Higgs?

- Grand Unification?

- Strong Gravity?

- Supersymmetry?

- Right-handed Weak Force?

- H0?

- Axigluons?

- Z'?

- H++?

- Leptoquarks?

- Monopole?

- Extra Dimensions?

- 10-40 sec

- 10-32 sec

- 10-28 sec

- 10-24 sec

- 10-20 sec

- u
- d

- c
- s

- t
- b

- e
- e

- 
- 

- 
- 

- g

- W,Z

- U(1)

******

- Electroweak Unification

- Compositeness?

- Technicolor?

- Chiral Color?

- e*?

- Strings?

- q*?

- Atom

- Nucleus

- particle

- 10-36 sec

- 10-27 m

- 10-44 sec

- 10-35 m

- string?

- History of the Universe

- 1 TeV

- 104 TeV

- 108 TeV

- 10-4 TeV

- 10-8 TeV

- Age of Universe

- Temperature of Universe

- 10-20 sec

- 10-36 sec

- 10-12 sec

- 104 sec

- 10-4 sec

- 10-28 sec

- 1012 TeV

- 1016 TeV

- 10-44 sec

**Notes:**
  I would like to start by providing a broad overview of what we are trying to accomplish, in order to give a context for our results. The Fermilab Tevatron is the world’s most powerful microscope, directly probing matter at the smallest scale.  To give a sense of the scale, as deep inside the atom as Rutherford had to probe to find the nucleus, we are probing equally deep inside the nucleus to study the properties of the fundamental particles.  
Our study of matter thus far has resulted in a tidy package of particles and interactions.  There are two sets of particles, quarks and fermions, each arranged in 3 generations of weak doublets.  The quarks interact via the strong force, and all particles interact via the unified electromagnetic and weak forces.  
  However, there is a dark cloud looming on the horizon of this “Standard Model.”  At a little over 1 TeV, the WW scattering cross section becomes unphysical, violating unitarity in a modern-day ultraviolet catastrophe.  Fortunately for us, this dark cloud over the Standard Model can mean a bright future for Fermilab, whose accelerator at the energy frontier is capable of discovering the kinds of new particles that could help explain this, and other, mysteries of the Standard Model.  The data taken in this decade will provide the best sensitivity in the world to the wide variety of new particles and interactions shown here.
  Of course, the holy grail on this list is strong gravity.  Although it appears to be out of the reach of accelerators, there is a distinct possibility that we could be studying strong gravity in the near future.  This possibility will be a reality if the apparent hierarchy between the Electroweak and Planck scales is just a mirage, and the existence of extra dimensions brings the two scales together, like a lense bringing an image into focus.
  Finally, an understanding of interactions at the highest energies translates into an understanding of the universe in its earliest moments, when it was so hot that these interactions were commonplace.  Understanding these interactions helps us understand how the universe evolved to its current state and answers questions about the relic matter in the universe today. 

## CDF

- CENTRAL HAD. CALORIMETER

- CENTRAL EM CALORIMETER

- CENTRAL MUON (CMU)

-  MUON EXTENSION  (CMX)

- STEEL

- MUON UPGRADE (CMP)

- 

- 

- q or g “jet”

******=1.0**

- Detector

- Reconstructed Objects

- e

**CDF**

**Notes:**
  We use the Fermilab accelerator microscope by measuring the particles that result from its 2 TeV proton-antiproton collisions.  At CDF, we use a detector that provides precision energy and momentum information through a tracking chamber in a magnetic field, and through electromagnetic and hadronic calorimeters.  The CDF detector also provides clean particle identification, distinguishing a range of particles from photons to electrons, and from muons to quarks and gluons.  We show here the signatures of each of these reconstructed objects.

## Excited Electrons

- Composite electrons have excited states

- Excited electron decays to e+

- Zdominant background

- Expect 1 event with mee= Z

- Observe 1 event (mee= Z),
- 0 events (mee = Z)

**Excited Electrons**

**Notes:**
  One of the basic questions we want to answer is whether we have found the most fundamental particles, or whether the known particles are merely bound states of more fundamental “preons.”  If this is the case, then these bound states will also have excited states, and we at CDF can observe the decay of these excited states.  For example, an excited electron will decay to an electron and a photon.
  We search for excited electrons in the 2 electron + photon data.  The dominant Standard Model background to these data comes from Z+photon production, where the Z decays to two electrons.  For observed electrons that reconstruct the Z mass, we expect 1 event, and we observe 1 event.  For observed electrons that reconstruct a mass outside the Z “window,” we expect a low background, less than a tenth of an event for each 10 GeV mass region.  We observe no events outside the Z window, and we set a limit on the mass of the excited state in the context of a model with a four-fermion contact interaction.  The limit in general depends on the ratio of the excited electron mass to the compositeness scale:  When the two are equal, we set a lower mass limit of 785 GeV; for other ratios, we exclude compositeness scales of up to 2 TeV.
  The detectors at HERA and LEP have also searched for excited electrons, and have set limits assuming a coupling of excited electrons to vector bosons.  We are attempting to put our results in the context of this model to allow a direct comparison.

## (  )

- X++/--

- Search mass windows

- e+/- e+/-

- L-R symmetric model:

- SU(2)LxSU(2)R

- Explains light neutrino masses

- m

****L

***l***

- m

**N**R

- ~

- m

**Doubly-charged Higgs**

- H0
- H+
**H****++**

- m

***l***

- Z

- ee

- W+jet

- 0 events observed

**Doubly-Charged Particles**

**Notes:**
  Another important question is whether we have discovered all of the basic interactions between the particles, or whether there are still new forces to be discovered.  The existence of new forces would help explain some of the mysteries of the Standard Model, such as why the weak force appears to couple only to left-handed particles.  A right-handed weak force would not only answer this question, it would also explain why the neutrinos are observed to be so light.  A new Higgs triplet in this left-right symmetric model would couple to fermions through a Majorana mass term, resulting in left-handed neutrino masses that are inversely proportional to the right-handed neutrino masses (this is the so-called “see-saw” model).  
  One of the components of the new Higgs triplet is a doubly-charged Higgs, which would only couple to leptons due to charge conservation.  The Fermilab accelerator can produce pairs of doubly-charged Higgs particles, which we look for in the CDF detector by studying same-sign lepton pairs.  We have searched the same-sign electron data, which has a low background of a few tenths of an event for each mass window.  We observe 0 events in the same-sign data, and set a cross section limit for doubly-charged Higgs particles decaying to electrons.  The limit does not exclude any mass region at the 95% confidence level, but including the data taken over the last 6 months will give us sensitivity in the 100 to 120 GeV mass range.

## Doubly-Charged Particles

- X++/--

- Cosmic rays dominant background

- +/- +/-

- Expect 0.9±0.2 events (mass>80 GeV)

- 2 events observed:  m=160, 760 GeV

- H++/-- mass > 110 GeV (LEP: > 100 GeV)

**Doubly-Charged Particles**

- W+jets

- dijets

- cosmics

- CDF RUN II PRELIMINARY

**Notes:**
  We have also searched the same-sign muon data for evidence of doubly-charged particles.  The same-sign muon data sample is dominated by high energy cosmic ray muons travelling through the detector.  At high momentum, muon tracks have an undetectable curvature through the tracking chamber, resulting in a potential sign mismeasurement.  The expected reconstructed mass distribution of cosmic rays is large, with events expected up to and above several TeV.  
  The background for a given mass region is small, less than a tenth of an event.  We expect a total of 0.9 events for masses greater than 80 GeV, and we observe two, with masses of 160 and 760 GeV.  We set a 95% confidence level limit of 110 GeV on doubly-charged Higgs particles decaying to muons.  This is the first run 2 result to extend the LEP sensitivity to a Higgs particle, and we expect many more to follow.

## Dijet Mass Search

- Sensitive to variety of new physics

- Excludes axigluon mass < 1.1 TeV

**Dijet Mass Search**

**Notes:**
   The bread and butter of the Fermilab Tevatron is quark and gluon jet production.  Studying the dijet mass distribution allows us to probe the highest energies available at the Tevatron, and is sensitive to a wide variety of new physics.  For example, it has been hypothesized that the strong force exhibits the same chiral nature as the weak force.  In such a scenario, new force particles called axigluons could be produced in quark-antiquark collisions.  There are also models that explain the Higgs mechanism (the mechanism that gives mass to particles) through a new strong force called technicolor.  In these models, a heavy new “coloron” could be produced by the Tevatron.  We can also look for excited quark states in the dijet mass distribution, and we can search for unusual particles called diquarks that arise in unified models inspired by string theory.  Any new narrow mass resonance that decays to quarks will appear in the dijet mass distribution.
  We search for new particles by fitting the dijet mass distribution to a parametric function and searching each bin using a maximum likelihood technique.  We find no evidence for new resonances decaying to dijets and set limits on a wide variety of hypothetical new particles, including the axigluon, which, with a mass limit of 1.1 TeV, is the first particle with a mass exclusion above a TeV set by an accelerator experiment.

## Dilepton Mass Search

- Search for neutral resonances

- Graviton

- Z’

- Run 1:
- > 690 GeV

- Theory with 1
- extra dimension

**Dilepton Mass Search**

**Notes:**
  A clean probe of high mass resonances is provided by the dilepton data.  These data are sensitive to any neutral resonance coupling to leptons.  Such resonances include high-mass gravitons, as predicted by the Randall-Sundrum model with one extra dimension, or Z’ particles, a potential carrier of the right-handed weak force.  
  The Standard Model contribution to the dilepton data comes primarily from the Z resonance and the Drell-Yan production of off-shell Z’s and photons.  The Drell-Yan cross section falls rapidly with increasing mass, making a high-mass resonance a clean signature of new physics.
  We do not observe evidence for a new resonance at high mass, and we set limits on Randall-Sundrum gravitons of up to 550 GeV, depending on the coupling.  We also set limits on the Z’ mass of 665 GeV, which is slightly lower than the CDF Run 1 limit of 690 GeV.  With the data taken in the last 6 months, we will be able to extend the Run 1 sensitivity into an unexplored region of phase space.

## Long-Lived Charged Particles

- Search for heavy, slow particles

*“**stable” supersymmetric top quarks*

- TOF>2.5 ns

*4th generation quarks*

- Tracks with long flight time (>d/*c*)

- Expect 2.9±3.1 events, observe 7

- stop mass > 108 GeV (LEP: > 95 GeV)

**Long-Lived Charged Particles**

**Notes:**
  A number of open questions in the Standard Model can be addressed by searching for long-lived charged particles.  For example, are there more than three generations of particles, or are there scalar partners to the fundamental matter particles, as predicted by supersymmetry?  The existence of supersymmetric particles would have profound consequences for cosmology, since it is likely that the lightest supersymmetric particle would be stable and contribute to the dark matter in the universe.
  We search for heavy, slow particles by using the Time of Flight detector to identify tracks with long flight times.  We maximize our sensitivity to stable supersymmetric top quarks by counting candidate tracks with a time difference of 2.5 ns relative to the interaction time.  We observe 7 such events, with an expected background of 2.9 ± 3.1 events.  We use this result to set a mass limit of 108 GeV for stable isolated stop particles.  This analysis provides the world’s best sensitivity to stable stop particles, extending the ALEPH mass limit of 95 GeV.

## Leptoquarks

- pp

- LQ

- LQ

- qe (q)

- qe (q)

- qqee

- qqe

- Expect 3.4±3.2 events, observe 0

- DØ Run 1: mLQ > 204 GeV

- CDF+DØ Run 1: mLQ > 242 GeV

- Expect 1.9±1.3 events, observe 1

- Directly couple quarks to leptons

- Predicted by Grand Unified Theories

**Leptoquarks**

- MLQ>230 GeV

- MLQ>177 GeV

- One LQ per generation at low mass

**Notes:**
   There are no Standard Model interactions that directly couple leptons to quarks, and there are many models that predict such interactions.  One example is a Grand Unified Theory, which unites the Electroweak and Strong forces.  At low mass, a given leptoquark must couple only to a single generation in order to remain consistent with bounds on Flavor Changing Neutral Currents.  
  We have performed a complete search for first generation leptoquarks in the run 2 data.  These leptoquarks can decay to a light quark and either an electron or a neutrino.  We have searched for pair production of leptoquarks that result in final states of two quarks and two electrons or two quarks, an electron, and missing energy.  In the 2 quark+2 electron final state, we expect 3.4 ± 3.2 events and observe 0; in the 2 quark + electron + missing energy final state, we expect 1.9 ± 1.3 events and observe 1.  We set a mass limit of 230 GeV for leptoquarks decaying exclusively to electrons and 177 GeV for leptoquarks decaying to electrons and neutrinos with equal probability.  The combined Run 1 mass limit from the 2 jet + 2 electron final state was 242, with each detector setting a limit of 225 GeV.  The new limit of 230 GeV is the highest limit so far produced by a single detector.

## Leptoquarks

- pp

- LQ

- LQ

- q

- q

- qq

- DØ Run 1:  mLQ > 98 GeV

- 2 jets + Missing Energy challenging experimental signature

**Leptoquarks**

- Many Standard Model, detector backgrounds

- Sensitive to leptoquarks of all 3 generations

- W, Z + jets
- QCD multijet production

- Requires understanding of calorimeter noise and cracks

**Notes:**
  We have also searched for leptoquarks decaying exclusively to quarks and neutrinos.  This signature is sensitive to leptoquarks of all three generations, since we do not identify the generation of the quarks or neutrinos.  Experimentally, the 2 jets plus missing energy signature is extremely challenging:  There are many Standard Model and detector backgrounds, which require a detailed understanding of calorimeter noise and cracks, as well as the effects of beam splash.  
  The dominant Standard Model backgrounds are W and Z + 2 jet production, as well as QCD multijet production, where one of the jets is mismeasured.  We expect a total of 42.5 events in this data sample, and we observe 42.  We use this result to set a mass limit of 107 GeV for leptoquarks decaying exclusively to quarks and neutrinos.  This is the best limit in the world for these types of particles, exceeding the DØ run 1 limit of 98 GeV.  

## Diphoton Search

- pp

- 

- 1

- G

- G

- Gauge-mediated supersymmetry breaking predicts ET signature

- Diphoton sample understood

**Diphoton Search**

- No excess at high ET for  ET> 25 GeV

- ...or for  ET> 13 GeV

## Still Searching...

**Still Searching...**

- Past year of data has provided increased sensitivity to new physics

- Compositeness

- Chiral color

- Technicolor

- Right-handed weak force

- Excited electrons

- ...and new particles

- Diquarks

- Supersymmetric top quarks

- Doubly-charged Higgs

- Gravitons

- Leptoquarks

- Coming year will provide even more sensitivity to new physics and particles

- stop, LQ (2 jets + ET, with heavy flavor tagging)

- chargino, neutralino (trileptons)

- SUGRA (Bs      )

- magnetic monopoles

**Notes:**
  We use the Fermilab accelerator microscope by measuring the particles that result from its 2 TeV proton-antiproton collisions.  At CDF, we use a detector that provides precision energy and momentum information through a tracking chamber in a magnetic field, and through electromagnetic and hadronic calorimeters.  The CDF detector also provides clean particle identification, distinguishing a range of particles from photons to electrons, and from muons to quarks and gluons.  We show here the signatures of each of these reconstructed objects.

## ET Search

- Variety of new physics with  ET signature

- Clean up ET

******E****T**** Search**

*Large Extra Dimensions*

*Supersymmetry*

- Clean up photons

- Observed events consistent
- with background prediction