The Search for New Physics in Hyperon Decays

1
The Search for New Physics in Hyperon Decays

• E. Craig Dukes
• University of Virginia
• HyperCP Collaboration
• 28 March 2007
• Carnegie Mellon University

2
The Standard Model

• one of the greatest scientific achievements of
  all time
• allows us to understand nature at the most
  fundamental level
• allows us to look back into the beginning of time
  itself

3
Motivation for New Physics

• Theory
• EW symmetry breaking ? Higgs?
• Quantum theory of gravity ? strings?
• Hierarchy problem ? SUSY?
• Cosmology
• Non-zero baryon number of the universe ? baryon
  nonconservation, new sources of CP violation
• Dark matter ? new particles, SUSY?
• Dark energy
• Experiment
• Neutrino mass
• Other hints, g-2, LSND, NuTeV, etc.

4
Why Search for New Physics using Hyperons?

• Hyperons can be particularly sensitive to New
  Physics
• They are copiously produced
• They are cheap
• Their decays are simple and easily detected by
  simple spectrometers

5
HyperCP Fermilab experiment searching for rare
and forbidden charged hyperon decays

• Primary Goal
• Search for exotic sources of CP violation in
  X?Lp?ppp decays
• Secondary Goals
• Search for CP violation in W?LK
• Make precision measurement of hyperon decay
  parameters
• a, b, and g decay parameters in X-?Lp- decays
• a decay parameter in W?LK
• Search for rare and forbidden charged kaon and
  hyperon decays
• lepton-number violation X-?pm-m-
• FCNC K?pmm-
• DS 1 decays X-?pp-p-, W-?Lp-
• Search for q pentaquark
• ??p??-

6
New hyperon beam and high-rate spectrometer built

• Charged Secondary Beam
• 800 GeV protons on 2x2mm2 target
• mean momentum 167 GeV/c
• rate 10-15 MHz
• alternate /- beam polarity

• High-Rate Magnetic Spectrometer
• 8 high-rate, narrow-pitch MWPCs
• left and right muon stations only pid
• simple hyperon decay trigger
• dimuon and single-muon triggers
• very high-rate DAQ 100,000 evts/s to tape

7
Spectrometer Performance

• Spectrometer has large acceptance
• Good momentum resolution
• Excellent mass resolution

8
HyperCP Yields

• In 12 months of data taking HyperCP recorded one
  of the largest event sample ever
• 231 billion events
• 29,401 tapes
• 120 TB
• Entire WWW as of end of data taking (Jan 2000) 5
  TB

9
Why Search for CP Violation in Hyperon Decays?

• After 40 years of intense effort and many
  beautiful experiments we still no little about
  CP violation the origin of CP violation remains
  unknown and there is little hard evidence that it
  is explained by the Standard Model.
• The importance of CP violation to our
  understanding of particle physics, indeed the
  universe, cannot be overstated
• The asymmetry can be relatively large up to
  O(10-2)
• The price is modest
• No new accelerators needed
• Apparatus is modest in scope and cost
• Hyperons are sensitive to sources of CP violation
  that, for example, kaons are not
• Almost all scenarios for New Physics produce
  large CP asymmetries

We are willing to stake our reputation on the
prediction that dedicated and comprehensive
studies of CP violation will reveal the presence
of New Physics Big and Sanda, CP Violation
10
Are Microscopic and Macroscopic Asymmetries
Related?
DNA Left Right

• Many examples of macroscopic asymmetries in
  nature
• Many attempts have been made to relate the two
• No one has yet succeeded in doing so without
  seeding the bath
• Successes are occasionally reported, but never
  verified.

the single most important finding since chemists
discovered the chiral carbon atom itself
It was just too good to be true.
Amino Acids Left Right
11
Symmetric Universe

• The very early universe was composed of equal
  amounts of matter and antimatter.
• After 10-6 s (T1013K) baryons (neutrons and
  protons, and their antipartners) formed as the
  universe cooled.
• The universe was sufficiently dense that by 10-3
  s (T1012K) all the baryons and antibaryons
  annihilated, leaving only photons the
  annihilation catastrophe.

12
What Actually Happened

• What we actually observe is quite different
• There is no compelling evidence for any
  antimatter in the Universe
• The observed ratio of baryons to photons is nine
  orders of magnitude too large!

13
Good thing there is no antimatter!
14
Sakharovs Ingredients

• Sakharov in 1967 first elucidated the three
  needed ingredients
• Baryon number must be violated. Need a way to
  get rid of matter (or antimatter) that doesnt
  involve annihilation. Grand Unified Theories
  (GUTs) do this.
• Violation of both C and CP. This produces
  different decay rates for particles and
  antiparticles.
• A departure from thermal equilibrium when the
  antimatter was turning into matter. Otherwise,
  if in thermal equilibrium the reverse processes
  occur with an equal rate.

SM too small
15
How to Search for CP Violation in L Decays

• Due to parity violation the proton likes to go in
  the direction of the L spin

Under CP violation that antiproton prefers the go
opposite to the direction of the anti-L spin
16
Problem Producing Ls of Known Polarization

• Ls and anti-Ls of known polarization can be
  produced through X decays

If the X is produced unpolarized which can
simply be done by targetting at 0 then the L
is produced in a helicity state.
If CP is good the slopes of the proton and
antiproton cosq distributions are identical.
17
We are Sensitive to CP in both X and L Decays
where

• What we experimentally measure is the slope of
  the proton (antiproton) cosq distribution in the
  rest frame of the L (L).
• We do this in a special L rest frame called the
  Lambda Helicity Frame in which the L direction in
  the X rest frame defines the polar axis.

18
Theory
What have theorists contributed in the course of
25 years toward a quantitative understanding of
CP violation? A. Pais Blois 1989
Nothing!
19
Phenomenology of CP Violation in Hyperon Decays

• CP violation is manifestly direct with DS 1
• Three ingredients needed to get a non-zero
  asymmetry
• At least two channels in the final state S- and
  P-wave amplitudes
• The CP-violating weak phases must be different
  for the two channels
• There must be unequal final-state strong phase
  shifts
• Asymmetry greatly reduced by small strong phase
  shifts
• the pp phase shifts have been measured to about
  1
• the Lp phase shifts cannot be directly measured
  theory predictions disagree

strong phases
weak phases
20
Measuring the L-p Phase Shift

• Done by measuring one of the transverse
  components of the L polarization from polarized
  X- decays
• Daughter L polarization given by
• In the absence of CP violation
• Difficult measurement to make! From 144 million
  polarized X- decays we find

Confirms expected small size from recent cPT
calculations
21
Comparison of AX, AL with e?/e
AX, AL
e?/e

• Thought to be due to Penguin diagram in Standard
  Model
• Expressed through a different CP-violating phase
  in S- and P-wave amplitudes
• Probes parity-violating and parity-conserving
  amplitudes

• Thought to be due to Penguin diagram in Standard
  Model
• Expressed through a different CP-violating phase
  in I0 and I2 amplitudes
• Probes parity-violating amplitudes

Our results suggest that this measurement is
complementary to the measurement of e?/e, in that
it probes potential sources of CP violation at a
level that has not been probed by the kaon
experiments. He and Valencia, PRD 52 (1995),
5257.
22
Bad News SM Theoretical Predictions Small

• Much enthusiasm a decade ago as theory
  predictions were relatively large and
  experimentally accessible
• Standard Model predictions have slowly fallen
  since then to

• At same time there was concern that accidental
  cancellation would cause e?/e ? 0
• The expected SM asymmetry is out of reach of any
  experiment, planned or otherwise

Valencia, (1991)
(Tandean and Valencia, 2003)
Note no unambiguous connection between dCKM ?
AX, AL
23
Good News SM Theory Predictions are Small

• Most beyond-the-standard-model theories predict
  new and large CP-violating phases
• These predictions are often not well constrained
  by kaon CP measurements as hyperon CP violation
  probes both parity conserving and partiy
  violating amplitudes
• A recent paper by Tandean (2004) shows that the
  upper bound on AXL from e?/e and e measurements
  is O(10-2).
• For example, some supersymmetric models that do
  not generate e?/e, can lead to AL of O(10-3).
• Other BSM theories, such as Left-Right mixing
  models (Chang, He, Pakvasa, 1994), also have
  enhanced asymmetries.

It is clear that hyperon decays are much more
sensitive to new physics than e?/e. Sandip
Pakvasa
He, Murayama, Pakvasa, Valencia, PRD 61, 071701
(2000)
Any CP-violation signal will almost certainly
indicate New Physics
24
What is the Experimental Situation?

• To date, there are only upper limits
• AL has been measured to 2?10-2
• There is a measurement of AXL to the same level
  using the HyperCP technique
• Measurements of AXL can be used with AL to infer
  AX
• None of the previous measurements is in the realm
  of testing theory

HyperCP is pushing to improve the measurements by
two orders of magnitude
25
Extracting the CP Asymmetry

• If CP is good then the proton and antiproton cosq
  distributions are identical

• Take the ratio of the two distributions to
  extract AXL if not flat CP is violated

Note No Monte Carlo used in measurement!
26
Equalize X- and X Acceptances by Weighting

• Problem acceptances for X- and Xdecays not the
  same due to different production dynamics
• Solution weight the X- and X momentum
  distributions to force them to be identical
• only 3 momentum dependent parameters weighted
• 100x100x100 106 bins

27
Proton, L-pion, X-pion before/after Weighting
28
Monte Carlo Tests

• Monte Carlo only used to
• verify algorithm and implementation
• check that weighting procedure doesnt wash out
  asymmetry
• study a few systematic errors
• Problem how to generate 1 billion MC events?
• Solution Hybrid Monte Carlo
• We get the input asymmetry back

Important Final result has no Monte Carlo
dependence!
29
Controlling Biases to the 10-4 Level

• Targets changed to equalize secondary beam rates
• polarity 2 mm Cu
• - polarity 6 mm Cu

• Little difference in PWC effieciencies between
  and polarity running
• - data solid line
• data dashed line

• Two important features of HyperCP allow biases to
  be controlled to 10-4 level
• Fact that the same spectrometer is used for both
  X- and X proton/antiproton cosq measurements
• Need to make sure that magnetic fields were
  exactly reversed
• Need to make sure that there was no temporal
  dependence of spectrometer efficiencies
• Measuring the proton/antiproton cosq slope in the
  Lambda Helicity Frame
• Localized acceptance differences to not map to
  any part of the cosq plot

• When flipping polarity field magnitude kept to
  within 2x10-4
• This corresponds to a 0.3 mm deflection at 10 m
  for the lowest momentum (10 GeV/c) pions

Important overall acceptance differences do not
cause any bias!
30
Systematic Uncertainties

• Most estimated from data, a few from Monte Carlo
• Most systematic uncertainties can be reduced in
  analysis of full data set

31
The Raw CP Asymmetry

• Data broken up into 18 Analysis Sets of roughly
  equal size, each with and - polarity data
• 10 of data sample 119 million X-, 42 million
  X
• No acceptance corrections
• No efficiency corrections
• No background subtraction

Raw (non-background subtracted) CP asymmetry AXL
from all 18 Analysis Sets
Weighted average of all 18 Analysis Sets
32
Background Subtracted Asymmetry

• Background subtraction
• No efficiency or acceptance corrections.
• Factor of 20 improvement in sensitivity over
  previous limit
• Null result constraining allowed SUSY effects

Expect to have full data set analyzed by end of
year with factor of 3 improvement in sensitivity
33
Hints of New Physics in the Decay S?pmm-
34
Motivation
In SM S?pmm- highly suppressed ? leading
diagrams FCNC and WR decays
Hence sensitive to New Physics
FCNC at tree level
35
Basic event selection cuts

• Striking topology low Q decay (40 MeV) with two
  unlike-sign muons and same-sign high momentum
  proton eminating from common vertex
• Hits in 2/3 muon PWCs and hodoscopes
• Good three-track vertex
• DCA
• Decay vertex well within Vacuum Decay Region
• Target pointing R

36
Two further Kaon removal cuts

• Huge K background
• 0.5 billion K?pp-p
• 1,000 K?pmm-
• In S?pmm- proton carries most momentum ?
  require
• After fhadron cut, MC studies show K-decay
  background negligible
• Second cut removes events with pmm- mass within
  10 MeV (3s) of K mass

37
Three S?pmm- decays survive all cuts
Basic selection cuts
Basic kaon removal cuts
7 events, 3 signal, 4 bkgd

• Observe 3 events with 1s of the S mass (1189 MeV)
• Backgrounds 20s away from S mass
• fhadron and K?pmm- cuts get rid of all but 7
  events

38
Backgrounds other hyperon decays

• No other positively charged hyperon
• Anti-hyperon decays have a different topology
• an opposite-sign highest momentum track
• decays-in-flight that produce like-sign dimuons
• Negligible

39
Backgrounds other S decays

• Only possible problems come from radiative decays
  with gamma conversion g?mm-
• Probability of a photon conversion to mm- in the
  vacuum pipe window (0.21 X0 upstream, 0.15 X0
  downstream) 10-7
• Monte-Carlo studies of S?pp0 and S?pg
  conversion backgrounds, with 100-1,000 times the
  expected level, show no background
• No evidence of much larger S?pee- rate if
  photon conversions were a problem (g?ee- 105x
  g?mm-)
• Proton momentum not consistent with S?pg
  two-body decay

40
Backgrounds Look at Data

• MC indicates backgrounds not a problem but with
  1x1010 K decays non-gaussian tails can be a
  problem
• Look at data
• Negative polarity data sample
• about ½ size of positive polarity sample
• Note anti-S production down by 10X
• no events below 1230 MeV
• Single muon trigger sample
• 30X larger more background
• prescaled by 10X expect no events
• no events below 1205 MeV

41
Extracting the branching ratio

• Since our acceptance is not perfect we need to
  know the form factors
• Form factors cannot be calculated ab initio
• Four form factors a, b, c, d
• a and b (at q2 0) come from WR decay S?pg
• c and d limited by on B(S?pee-)
• smallest branching ratio ever measured for a
  baryon

42
What does theory say?

• Bergström, Safadi and Singer, ZPC 37, 281 (1988)
• B(S?pee-) B(S?pg)ae 10-6
• B(S?pmm-) 1/100 B(S?pee-) 10-8
• Updated calculation by He, Tandean, and Valencia,
  PRD 72, 074003 (2005)
• 9.110-6 B(S?pee-) 10.110-6
• 1.610-8 B(S?pmm-) 9.010-8
• HyperCP result consistent with theory, albeit a
  bit high.

PDG
43
So far nothing terribly exciting about this result
Until you look at the mm- mass

• The dimuon masses of all 3 events are within 1
  MeV of each other!
• This is the mass resolution of the HyperCP
  spectrometer
• Suggests that the decay proceeds via an
  intermediate state, X0

44
Can this be real?

• Is it a statistical fluctuation?
• Probability of 3 events having mm- mass within 1
  MeV of each other anywhere in the kinematically
  allowed range is 1
• Form factors cant be fudged to increase that
  probability by much
• Is it muonium? Not likely it is 3.0 MeV (6.0
  s) above 2mm
• Is it S?pg, g? mm-? Probability is negligible
  and if true we would see loads of events in the
  ee- mode.
• Is it another hyperon decay? All of the charged
  hyperons have the wrong decay topology
• Is it the analysis or detector? Weve looked
  hard, but we dont see anything wrong.

45
What about S?pee-?

• Much more difficult than S?pmm-
• no electron identification
• trigger prescaled by 100X, although SM BR
  expected to be 100X larger
• WR decay background a problem S?pg, gZ? ee-
• Clear S peak
• Most of the events appear to be S?pee- , but
  extracting S?pg, gZ? ee- background difficult
• No sign of X0, but we dont expect to see it

46
Suppose X0 is real

• What properties does it have?
• HyperCP can say little on this subject
• know its mass
• lifetime not a resonance and not super
  long-lived
• dont know its spin
• dont know branching ratios however X0?mm- must
  be large
• Why wasnt it seen before?
• Is there any theoretical context for such a
  particle?
• Where else can we look for it?

47
Kaon searches eliminate all but a
parity-conserving pseudoscaler or axial vector
K?pmm-

• If either parity violating, or scalar or vector,
  HyperCP and others would have seen it in
  K?pmm- at several orders of magnitude more
  than the (8.11.4)?10-8 BR or KL?gmm-
• However existing constraints on
  parity-conserving pseudoscalar or axial vector of
  this mass are weak

KL?gmm-
48
Who ordered that? Supersymmetry

• Sgoldstino superpartner to the goldstino, the
  longitudinal component of the gravitino
• Properties Spin 0, all other properties
  ill-determined
• Mass 0 at tree level obtains mass from Kähler
  potential, however expected to be light
• Lifetime can be long or short lived
• Should be two scalar (S) and pseudoscalar (P)
• Can have flavor conserving and flavor violating
  interactions
• Interactions with quarks may or may not conserve
  parity however theoretical motivation for
  parity-conserving interaction
• Branching ratio to dimuons can be large if light
  (
• Hyperons a good place to search for
  parity-conserving pseudoscalar sgoldstino
• B(S?pX0)

Parity conservation in sgoldstino interactions
with quarks and gluons may not be accidentalIt
is likely that sgoldstino interaction will
conserve parity in supersymmetric versions of
other models designed to solve the strong CP
problem without introducing light
axion. Gorbunav and Rubakov,PRD 64, 054008
(2001)
vF 1 TeV (SUSY breaking scale) Mgg 100 GeV
(order of photino mass) Al soft-mass term
if the sgoldstino is sufficiently light, the
hyperon decays into baryon and sgoldstino are
kinematically allowed and searches for these
decays are very sensitive to sgoldstino couplings
in models with light pseudoscalar sgoldstino and
parity conservation. D.S. Gorbunov, Proc.
Quarks-2004
49
Other Explanations
50
More exotic explanations!
Diether and Inopin, physics/0601110
51
Where to look for parity-conserving pseudoscalar
Expected BRs based on HyperCP BR (with X0?mm-)

• Four-body kaon decay limits weak
• No data on K?ppmm
• KL? pp-ee- 3.110-7
• KL? p0p0ee-
• If X0 sgoldstino then X0?mm- X0?ee-
• B and D limits getting to upper range of
  predictions

KTeV analyzing KL? p0p0mm-
K ? pp0X0 10-12 KL ? p0p0X0 10-8 ? pp-X0
10-13 - 10-9 KS ? p0p0X0 10-11 ? pp-X0
10-16 - 10-12 W-?X-X0 10-6 D ? rX0 10-9
- 10-6 B ? KX0 10-9 - 10-6 t ? mX0 10-7
Only other hyperon mode accessible. SM BR 10-8
Gorbunov and Rubakov, He, Tandean and Valencia,
Deshpande, Eilam and Jiang, Gorbunov and Demidov,
Chen and Geng
HyperCP is looking for W-?X-mm-?Lp-mm-?pp-p-mm-
At best we expect 1 or 2 events
52
Fermilab pp-bar Source

• Produce W, not S, pairs
• Should get 40-120 events with 50 acceptance, L
  2 x 1032

53
Conclusions and Outlook

• With one of the largest data samples ever taken
  HyperCP has pioneered high-sensitivity searches
  in hyperon decays
• This program is complementary to those carried
  out in other sectors, and often more sensitive
• Our CP-violation search is probing limits not
  constrained by Kaon, B, or EDM measurements
• Our S?pmm- result is intriguing and begs to be
  confirmed
• Mounting an experiment with 10X the statistics of
  HyperCP would be easy
• Unfortunately, the Tevatron no longer available
  for fixed-target physics at Fermilab, so the only
  options for hyperon decay searches appears to be
  the antiproton accumulator or the CERN SPS

we can then conclude that the available
preliminary measurement by HyperCP has already
begun to probe the parity even contributions
better than e does. Tandean, 2004
54
Backup Slides
55
Systematic Errors
56
Backgrounds Relaxed Cuts

• Relax all basic selection cuts by 1s
• No events within 8s

57
Searches for light boson

• Many experiments have searched for a light boson
• Few searches for a short-lived boson of mass 200
  MeV/c2

58
Search for the q Pentaquark

• Much excitement last year with reports of
  observation of a five-quark state predicted by
  Diakonov et al in 1997
• Three states in Anti-decuplet have exotic quantum
  numbers q(1530), X-3/2(2070), X3/2(2070)
• Many reports of pentaquarks in 2003-2004
• HyperCP Search
• pN?qL, q?Ksp, Ks?pp-, L?pp-
• HyperCP has excellent mass resolution and largest
  KS sample ever taken
• No evidence of any particle around 1.530 GeV/c2
• One of first of now many negative results
• PRD 70, 111101(R) (2004)

59
Search for Lepton Number Violation
X-?pm-m-

• Lepton-number-violating decay could imply
  existence of Majorana neutrino
• Not constrained by limits on neutrinoless beta
  decay
• Previous limit (PDG)
• HyperCP limit

Before Cuts
After Cuts
60
Search for DS2 Decays

• SM branching ratio
• window for New Physics
• Important! Limits from K decays to not preclude
  an observable effect
• Search done through
• Nothing found

61
Observation of Parity Violation in W-?LK-

• The only hyperons in which parity violation has
  not yet been observed are the W- and W
• HyperCP has 4.5 million W-?LK- ?pK-p- and 1.5
  million W?LK ?pKp decays
• Measure product aWaL in W-?LK-?pK-p- in the
  Lambda Helicity Frame using Hybrid MC method
• P violation observed, no CP violation
• Most precisely known alpha parameter