# Presentation: 824110

## Neutrino Cross Sections

- Sam Zeller
- Columbia University
**Summer Student Lecture**
- July 29, 2005

-  what’s a cross section?
-  which  cross sections are important at MiniBooNE?
-     - why are they important?
-     - brand new cross section measurements
-  what does the future hold?

## What’s a Cross Section ()?

- detector

- 

-  tells you probability for a given interaction
-   is just as name suggests = effective area for a collision

-  the larger the , the more likely the  will interact
-   (or the smaller the , the less likely  will interact)

-  has _units of area_ (or length2)
-     - the larger the effective area, the greater the probability for a reaction
-      - convenient unit is the barn = 10-24 cm2
-          (square cm is tremendously large compared to effective area of a nucleus,
-            physicist once referred to this measure as being “as big as a barn”)

## Neutrinos Only Interact Weakly

-  cross sections for  interactions are extremely small,
-   (but not completely unattainable by experiments)
-  neutrinos only interact by the weak interaction

****

**...**

-  has good chance of traveling thru 200 earths w/o interacting
-   interacts 100,000,000,000 x less often than quarks!
-              (N)~10-36 cm2 compared to (pp)~10-26 cm2

-  this is why, in general, need lots of ’s & large detectors
-       (N =  * 

## Extreme Measures To OvercomeThis Weakness 

-  1930: Pauli says neutrinos exist!
-  takes nearly _30 years_ before we finally detect them
-     - with advent of very intense  source
-      - Reines/Cowan observe ’s emitted from reactor

- series of stacked detectors
- installed in basement
- of reactor building

## Neutrino Cross Sections

**QE**

**1******

**DIS**

- events observed at exps are a mixture

- low
- energy

- high
- energy

-  neutrino has several choices (depends on E)  ...

-  n  - p
- (target changes does not break up)

-  N  - N 
- (target goes to excited state)

-  N  - X
- (target breaks up)

- oscillation experiments

##  Scattering Kinematic Variables

- 

- 

- p

## Two Types Interactions    CC,NC

- this is how we
- detected neutrinos
- in the first place

- 1st discovered in 1972

- flavor of outgoing lepton tags
-  flavor of incoming neutrino

## Why Measure  Cross Sections?

** ****because we need them as input to oscillation experiments**
**  **(need to precisely predict both signal & background rates)
-     - QE necessary to accurately predict signal rates, spectra
-     - need to know NC 0 backgrounds for      e
-     - need to know CC + backgrounds for  disappearance
-     -  cross sections for CP violation searches

** ****because they’re interesting**
-  Jim Cronin: “if you can measure something 10 times
-                       better, you should just do it”

## Ok, So Let’s Talk Cross Sections

-  concentrate on QE, 1 production since those interactions
-   are what’s most important for MiniBooNE (& osc exps)

- QE

- 1

## Event Fractions at MiniBooNE

-   **40% QE**
** ****16% NC elastic**
** ****25% CC **********
-  **8% NC ********0**
-  **4% CC ********0**
-  **4% NC ********+/-**
-  **4% multi-******
- - 1% DIS

- MiniBooNE flux-averaged
-    event composition:

- *

## Ok, So Let’s Talk Cross Sections

-  concentrate on QE, 1 production since those interactions
-   are what’s most important for MiniBooNE (& osc exps)

- QE

- 1

-  why each is important (QE, NC 0, CC 1+)
-  past data (since this is what we use to constrain our MCs)
-  recent measurements shown this summer

## Quasi-Elastic (QE) Scattering 

-  n → - p

-  most copious interactions and most well
-   known  at low energy + simple two body
-   kinematics           signal samples

**Why important?**

-  QE interactions are oscillation signal:
-                e n  e- p
-  knowing details of analogous interaction:  n  - p
-   important for predicting oscillation rates & energy spectrum

## Previous QE  Measurements

-  n → - p

-  most of QE data comes from
-   old bubble chamber experiments
-  low statistics (~ 2500 events)
-  the target nucleon complicates
-   things; no free neutrons
-   implies nuclear corrections ...

## Things are More Complicated

-  oscillation experiments measure  reactions
-   on nuclear targets (16O, 12C, 56Fe, etc.)
-  nuclear effects:

-  nuclear effects impact rates/kinematics of
-    interactions as well as observed final states

## QE Scattering at MiniBooNE

-  n → - p

-   measure visible E
-   and  from mostly
-  Čerenkov () + some
-   scintillation light (*p*)

-  ~60,000  QE events collected so far
-  (already more data than all previous exps combined)

- p

## QE Scattering at MiniBooNE

**Q****2**** = m********************************p********cos**************

-  n → - p

-  interesting roll-over in data
-   not tracked by Monte Carlo
-  has received a lot of attention recently
-  could point to a common nuclear model deficiency? will come back to this

## K2K Near Detector

-  K2K has a suite of near detectors
-       - water Cerenkov and fine-grained SciFi, SciBar
-  can also make low energy cross section measurements
-  MiniBooNE’s only real “competition”

## QE Scattering at K2K

- p

-  in fine grained detector
-   can reconstruct proton track
-  enforce QE kinematics
-  if 2nd track not in expected
-   direction, then not QE

-  n → - p

## QE Scattering at NOMAD

- new QE cross
- section results
- from the NOMAD
- experiment just
- made public a few
- weeks ago
-  higher E than MB
-  new interest
- in these ’s

## Single Pion Production

-  in between elastic and deep inelastic scattering
-  above a certain energy, the target nucleon can
-   be excited to a , N* resonance which then decays:
-            N          
-                                         N’
-  called resonance or 1 production

## NC 0 Production

-  two mechanisms for NC 0 production (also true for CC +)

- resonant 0 production
-   (  N )

- coherent 0 production

-  not many absolute NC 0 RES meas
-     (two data points at E ~ 2 GeV)
-  GGM  from MiniBooNE reanalysis
-  Aachen-Padova  appearing
-          as footnote in their paper (!)

## Coherent Pion Production

- Paschos & Kartavtsev, hep-ex/0309148

-  A → ******NC)**
-  A → ******CC)**

-  coherently scatter off entire nucleus rather than
-   its constituents (glance off nucleus)
- * both NC and CC processes

- * distinct kinematics:
-    - negligible E transfer
-      to target (low Q2)
-    - forward scattered 
- (compared to resonantly prod ’s)

- * rate of production not
-   well known at low energy
-           (E ~ 1 GeV)

## NC 0 Production

-  two mechanisms for NC 0 production

- resonant 0 production
-   (  N )

- coherent 0 production

## NC 0 Production

-  N →  N 0

- Why
- important?

- 0→
- candidate event

-  can tell us about resonant vs. coherent 0 production
-        * in large volume Č detectors, 0’s easy to ID

## NC 0’s at MiniBooNE

-  N →  N 0

-     generic 2 ring fit
-   determine E, direction
-    of each Čerenkov ring
-     → decay kinematics

** ****~7,000 NC ********0**** events now**
- (we have the world’s largest data sample)

## NC Coherent 0 Production

- 0 angle (in lab) sensitive to mode of production

- ~ 80% **resonant** 0 contribution ()
-            - more isotropic
- ~ 20% **coherent**  contribution
-           - sharp forward peaked distribution

## New K2K Coherent  Results

## Single Pion Production

-  in between elastic and deep inelastic scattering
-  above a certain energy, the target nucleon can
-   be excited to a , N* resonance which then decays:
-            N         
-                                        N’
-  called resonance or 1 production
-  7 possible channels (3 CC and 4 NC):
-       p      - p +          p       n +
-       n      - n +          p       p 0
-       n      - p 0          n       n 0
-                                       n       p -

- NC 0

- CC +

## CC + Production

-         Why important?
-  useful in understanding event reconstruction
-   (3 final state particles)
-  largest background to QE samples
-   (large  and because + can be absorbed in nucleus)

-  p →  p +
-     n          n

## Why CC 1 Important for Osc Analyses 

-   disappearance; use QE sample
-  note CC 1 (non-QE) can look _just_
-   like this if  is absorbed before it
-   can make it out of nucleus

-  n → - p

-  p → - p  +

## CC 1+ Past Measurements 

-  CC + measurements
-   at bubble chamber exps
-       - typically not as
-         precise as QE
-       - low statistics
-       - combined data set
-         of all exps shown is
-         **~7,000 CC ********+**** events**

- MiniBooNE can measure !

## CC + Events at MiniBooNE

-  2nd most numerous
-   interaction at MiniBooNE
-  have lots of statistics

-  p →  p +
-     n          n

-  **~36,000 events !!****	**
-  5x more than all previous
-        bubble chamber data combined!

## CC + Events at MiniBooNE

-  let’s look at  Q2 again
-  interestingly, see larger
-    low Q2 problem in CC +
-    than in our QE data

## MiniBooNE CC + Cross Section

-  recall past measurements

## Deep Inelastic Scattering (DIS)

-  at even higher energies (Q2), the neutrino has the
-   resolution to “see” the constituent quarks inside nucleon

-  high statistics DIS samples provide precise measurements of
-   , nucleon structure (PDFs), electroweak parameters (sin2w)
-   (NuTeV, CCFR, NOMAD, CDHS, etc.)
-  however, small fraction of events at MiniBooNE (~1%)

-  N  - X

## Neutrino ’s: Past, Present, Future

## Future: MiniBooNE Running

## Future: Bringing SciBar to MiniBooNE

-  window of opportunity to bring fine-grained K2K SciBar to our beamline

- (M. Wascko)

## Future: MINERvA Experiment

-  approved proposal for additional near detector in NuMI beam line
-  high statistics neutrino samples
-   at a variety of beam energies

## Future: MINERvA Experiment

-  impressive laundry list of neutrino scattering measurements
-   can study in great detail ...

-  construction in late 2006
-  commissioning end of 2008

## Future: MINERvA Experiment

- recoil proton

- photon

-  with fine-grained detector have much improved
-   reconstruction of exclusive final states

- photon

-  p →  p 0,      0     

## Future: Even Lower Energy

-  also effort to extend  scattering activity down to even lower E’s (5-50 MeV)
-         - orders of magnitude lower than what we’ve been talking about
-         - valuable for SN modeling and constraining nuclear models

## Future: Liquid Argon Detectors

-  potential for vast improvement in  scattering measurements using
-   extremely high resolution lAr detectors ...

-  current detectors (12C, 16O)          40Ar
-  detailed studies of embedding inner water target
-   (T2K, to aid in relating O and Ar measurements)

-  more precise  measurements;
-   superior final state particle ID,
-   improvements in neutron detection,
-   +  mag moment (B. Fleming, Yale)

## Conclusions

-  low energy cross sections are what’s most important
-   for oscillation experiments
-      - QE and single  production
-      - we talked about QE, NC 0, CC + at MiniBooNE
-  future oscillation experiments will need even more
-   precise cross section knowledge to be successful
-  MiniBooNE, K2K, and MINERvA coming to the rescue
-  exciting time for neutrino cross sections! unprecedented
-   precise, new data already revealing some surprises ...